Materials for wear-resistant surfaces

NUCLEAR ENGINEERING AND DESIGN 17 (1971) 205-246. NORTH-HOLLAND PUBLISHING COMPANY

MATERIALS

FOR WEAR-RESISTANT

SURFACES

*

J. FOSTER Oak Ridge National Laboratory, Oak Rtdge, Tennessee 37830, USA Recewed 10 November 1969

The basic nature of the wearing process is reviewed in terms of macroscopic phenomena and related bnefly to microscopic considerations such as crystal structure and hardness. Problems confrontmg the designer are examined, and alternatives dependent upon design conditions are discussed. Major emphasis is on four groups of metals and alloys and on cermets, ceramics, graphite, and plastics. Applications are related to the desirable properties of each of these materials. Composition, property, and application data are presented in tables and figures. Reference sources were intentionally drawn largely from journal literature and reports on recently developed property and application data current at the time this material was written. A brief list of supplemental references is included relating largely to wear problems in aerospace and nuclear environments.

1. Introduction

2. Basic considerations

Although there are important benefits to be derived from increased wear rates in such areas as rock drilling, grinding, polishing, and many others, this article is concerned with resistance to wear. It is m this area, more than in high wear rate apphcatlons, that modern technology makes its most difficult demands for improved performance. This is so for several reasons. Achieving ever lower wear rates is a process that always approaches yet never reaches zero. The advances that require improved wear resistance frequently impose severe new wear environments. Thermodynamic considerations, space and weight limitations, demands for high performance, a broadening spectrum o f environmental condmons, and applications requiring long service life in devices not accessible for maintenance all combine to create increasingly stringent requirements for wear-resistant materials.

The lack of a direct and systematic set o f relationships, based upon cataloged material properties, for the design of surfaces in wearing contact is not to be taken as evidence that past methods have been poorly concewed but rather as evidence of the complex nature of the wear problem. The variables that relate directly to the serviceability of a gwen pair o f materials in wearing contact include the physical properties of the wearing materials, design conditions such as load and rubbing velocity, lubrication, chemical environment, and physical environmental conditions such as temperature and exposure to foreign particles. The relatwe Importance of these variables differs from one application to another. Lubrication is a particularly important consideration as evidenced by the fact that design problems are substantially less difficult in essentially all applications m which good lubrication can be provided. When only limited lubrication can be supplied, it should be used to the fullest extent possible with the realization that sealed systems with small lubricant inventorlesand hmited provxslons for replemshlng the lubricant ~upply at the wearing surfaces will in time lose effectiveness through loss or removal of lubricant from the wear zones. The consequent di-

* Chapter 67 of the projected third edition of the USAEC Reactor Materials Handbook.

206

J.Foster, Materials for wear-resistant surfaces

rect surface-to-surface contact is the condition that results in accelerated wear rates and eventual failure. Because the wear rate experienced by a given pair of materials in wearing contact is strongly influenced by the conditions of lubrication and because it is the direct surface-to-surface contact resulting from lubrication failure that causes wear, the frictional and wear characteristics of two materials operating in contact are generally reported for clean unlubricated surfaces unless otherwise specified. When testing a pair of materials for unlubrlcated application in a very clean, inert, or vacuum environment, great care must be ex erslsed to duplicate the actual service conditions. Very thin surface layers of oxide film or adsorbed gas on clean metal surfaces have been found to exhibit lubricating properties. Even solvent cleaned metal surfaces must be subjected to a fine abrasive cleaning to ensure removal of residual solvent from the surfaces. Nuclear systems frequently impose difficult material selection problems for surfaces in wearing contact. In some cases it IS the nuclear problems themselves that impose restrictions. Materials with high neutron absorption or actwatlon cross sections may be unacceptable. Cobalt, for example, is a constituent of a number of excellent wear-resistant alloys which, except for the high activity level induced in the cobalt by thermal neutron irradiation, might otherwise be ideally suited to a given design condition. In other cases it is the reactor coolant environment that limits the choice of materials. Such mechanisms as pumps, control rod systems, and fuel handling machinery are sometimes required to operate submerged in high temperature reactor coolant fluids such as liquid metals, molten salts, helium, carbon dioxide, and water. Contamination limitations, environmental conditions, and compatibility considerations generally preclude the use of lubricant materials, and the moving parts must use the coolant fluid to the best possible advantage as a lubricant The less than ideal lubricating quahtles of the usual coolant fluids make the selection of materials for these wearing surfaces particularly important. Mechanlslns in gas-cooled systems can sometimes be treated with solid film lubricants to extend their useful life and reliability. When tins is done, care should be taken to avoid an overapphcatlon. Very thin residual solid film layers are generally better than thicker deposits which can form and eject unwanted loose masses of surplus material, often with detnmen-

tal results. Some devices with relatively low operating velocities and loads and which are required to c)perate only at Infrequent intervals can be designed to give satisfactory service for long periods even when operated in a completely dry or unlubricated condition. For apphcations such as liquid metal pump bearlngs, tungsten carbide compositions have been found to give good performance. In such applications, once a good bearing material has been found, the mechanical details of the design to accomodate such effects as thermal expansion becomes an important part of the component design work. Materials selection for wearing components is very important, but the ultimate success of the operating device depends also upon load, velocity, duty cycle, environment, and other factors. In all cases, permissible forms of lubrication should be used to the fullest extent possible, and wearing mechanisms should be designed with the best possible access for inspection, maintenance, and replacement. There are, of course, those devices, both nuclear and nonnuclear, that are not accessible for inspection and maintenance in the usual sense. Some satellite, undersea, and remote site devices are designed for entirely unattended operation, and the valuable assurance derivable from maintenance and replacement access is lost. Even In these systems, however, moderate design loads and low sliding velocities can often be used to good advantage. The designers' problems are further simplified in those systems with relatively short design or mission life. Many systems of this sort, however, introduce new design problems arising not from continued operation and wear but rather from prolonged periods of dormancy after which a brief period of mechanical operation must be assured without failures due to deterioration dunng inactive storage and environmental exposurt.'. Under such conditions materials selection may be governed more by resistance to fretting, if vibration is present, ~,r to environmental attack than by the usual wearing conditions. The materials most commonly used for surface~ in sliding contact are metals or metal alloys. As design requirements have become Increasingly severe, however, even the most resistant metals and alloys have become unserviceable, and other classes of material have been examined and brought into use. These include cermets, ceramics, cemented carbides, graphite, and plastics

J.Foster, Materialsfor wear-resistant surfaces

207

Table 1 Typical values of coefficient of wear k [ 1]. NO LUBRICANT IN AIR

Metal on metal

FAIR LUBRICANT PURE MINERAL OIL MOLTEN GLASS WETTING LIQUID METAL

Non-metal on metal

Condition Like

Unlike

Clean

5× 10 -3

2× 10-4

5x 10-6

Poorly lubrtcated

2× 10-4

2× 10 -4

5x 10-6

Average lubrication

2X 10-s

2X 10 -s

5X 10 -6

Execellent lubrication

2X10-6-10 -7

2X10-6-10 -7

2X10-6

10-2

10-t ii0-2 u.

-~

SIMILARMETALS

t0-3 o

t0 .3

t0-* o

==

w I0 -4 10-S

E ~U10-5

AINST METAL

0 6U 10- ¢

~t0-6 t0-?

When lubrication is limited, or indeed under most conditions, hard materials of demonstrated compatibility generally give the best service under wearing conditions, provided proper attention is given to elastic properties to avoid brittle fracture in extremely hard materials. The question arises whether two hard materials in rubbing contact should be similar or dissimilar. Severe frictional behavior is generally observed when two sliding surfaces are of the same or closely similar metals [1]. The criteria by which similarity is governed are the ability of the two metals to form alloys, or the substantial solubility of one metal in the lattice of the other. Tables 1 and 2 indicate the higher wear rates expected when similar materials are used in sliding contact. It is apparent that the presence of even limited lubrication markedly reduces the wear problem when like metal surfaces are in rubbing contact. The coefficient of wear, k, is a dimensionless parameter and is discussed further in section 3.3. Values of k for both particle transfer and loose particle sepa-

Table 2 Wear coefficient k of various sliding combinations [ 1] Combination

Coefficient of wear k

Zinc on zinc Low carbon steel on low carbon steel Copper on copper Stainless steel on stainless steel Copper on low carbon steel Low carbon steel on copper Bakelite on bakelite

160 45 32 21 1.5 0.5 0.02

x × x x x x

10 -3 10 -3 10 -3 10 -3 10 -3 10 -3 x 10 -3

CLEAN HIGH VACUUM

POOR LUBRICANT WATER GASOLINE NON-WETTING LIQUID METAL

GOOD LUBRICANT MINERAL OIL WITH LUBRICITY AODITIVE FATTY OIL GOOD SYNTHETIC LUBRICANT

Fig. 1. Values of the wear coefficient for transfer for similar and dissimilar materials in sliding contact as a function of lubrication conditions [2].

ration are shown in fig. 1 for similar and dissimilar materials in sliding contact as a function of lubrication conditions.

3. Friction and wear

3.1. Wear mechanisms The processes of friction and wear between two solid objects are the results of surface interactions. The customary smooth finishes on the surfaces of two bodies designed to operate in sliding or rolling contact are in reahty quite rough when examined under high magnification. Physical contact occurs at small local areas governed by the peak-and-valley topography of the mated surfaces. The normal force between surfaces is not uniformly distributed over the apparent area of contact but rather gives rise to numerous small contact areas carrying locally higher loads than the apparent uniformly distributed load. Friction effects originate from tangential forces transmitted across the interface and can arise as a consequence not only of interlocking surface asperities b u t also of interfacml adhesion. When adhesion occurs, it is at points where

208

J.Foster, Materials for wear-reszstant surfaces

the two surfaces have contacted, and it is evidenced by a tendency of the two contacting surfaces to resist tensile as well as shearing forces. The phenomena of friction and wear are not necessarily concomitant effects. Theoretically at least, friction could occur without wear. Friction is evidenced as a resistance at the sliding or rolling surfaces to the tangential driving force. It might be due solely to the viscous drag of a lubricant film between the sliding surfaces. It is the process of wear, more than friction, that requires study to develop an understanding not only of the mechanisms by which the wear process takes place but also some aspects of the mechanism of friction. The phenomenon of wear represents the removal of material from the surfaces of the contacting bodies. The five principal wear mechanisms are adhesive wear, abrasive wear, corrosive wear, surface fatigue, and fretting wear. Adhesive wear occurs between two surfaces in contact when fragments are pulled off of one surface to adhere to the other. Such fragments may also separate as loose particles. Abrasive wear occurs when a relatively hard rough surface slides over a softer surface and cuts loose particles from the softer surface creating grooves. Corrosive wear occurs in a corrosive environment which, in the absence of sliding motion, might form a surface film that would retard further corrosion. The sliding action wears away the film and exposes new surface from which additional corrosion products are removed as wear fragments Surface fatigue wear occurs as a result of successive loading and unloading of the contact surface such as occurs at a point on a track over which rollers pass many times or on the races of ball bearings. The repeated load cycling may induce surface or subsurfag¢ crackang with the eventual result that surface particles are loosened and removed. Fretting wear occurs when a small and often unintentional tangential oscillatory displacement takes place between the surfaces with the result that the wear particles formed are not readily removed from the fretted area and remain to act as abrasive particles. Fretting wear may be thought of as combined adhesive and abrasive wear. 3.2. Material properties related to wearing charactertstics The particular wear mechanisms that are important for a given application depend upon the specified en-

vlronmental and mechanical conditions. Because there is httle problem encountered in the selection of materials for the design of surfaces in rolling or shding contact under normal environmental conditions with conventional lubrication, the major design problems arise in specifying materials for application under severe environmental conditions and/or with limited or no lubrication. Under these conditions, the two wear mechanisms of most general concern are adhesion and abrasion. Corrosive wear is usually eliminated by proper selection of materials, and fretting wear is minimized or eliminated by proper design If fretting condltions cannot be entirely eliminated, the most effectwe steps for reduction of wear are those taken to minimize adhesion and abrasion. Wear resulting from surface fatigue can be reduced by sizing and shaping load-bearing surfaces to prevent high local stresses and by good materials selection. Tests and experience have established some general guides for materials selection based upon fundamental physical properties. The most important volume propertles of materials relating to their wear characteristics are the plastic strength properties of yield strength and hardness Following these in Importance are the elastic properties such as Young's modulus of elasticity, the shear modulus, and the stored elastic energy or hysteresis characteristic. Surface energy, chemical properties, and the tendency to adsorb surface films from the environment can also have important effects upon wear characteristics. Many of the volume physical properties are related, and a correlation of one with wear properties will often be paralleled by a similar correlation between wear and another mechanical property This is so because properties such as yield strength, hardness, elastic moduh, and even coefficients of thermal expansion and melting points are fundamentally related to the strengths of the bonds between atoms in the solid structure. Hence a metal or other solid material having strong interatomic bonds will be characterized by high values for all the parameters named above except the coefflc~ent of thermal expansion which wall be small. Hardness is one of the most valuable and commonly used properties in selecting materials for wear resistance. The classical lubricated sleeve bearing uses a hard shaft or journal turning In a relatively soft bearing Foreign particles of sufficient hardness to cause wear are either removed by the lubricant stream or

209

J.Foster, Materials for wear-resistant surfaces

embedded in the softer bearing matenal where they are harmless. Ball and roller bearings for normally lubricated applications, on the other hand, use hard materials for both inner and outer races and for the rolling elements with relatively softer material used only for the rolling element cage or separator. In rubbing or rolling contact applications under less conventional conditions, with respect to either enwronment or lubrication, both contacting surfaces are usually of relatwely hard materials. Once the chemical and physical environmental condmons have been determined, however, selection of matenals for surfaces in wearing contact cannot be made solely on the basis of chemical resistance and hardness. Elastic and toughness properties must be considered, and good hot hardness as well as room temperature hardness is reqmred for applications at elevated temperatures.

3.4. Hardness Hardness is among the most widely used material properties for characterizing and selecting materials for service m wearing contact. Most hardness testing of metals lS done by the penetration method, i.e. the Brinell, Vickers diamond pyramid, and the Rockwell tests. In general, hardness measured by one indentaZ t0,000 DIAMOND- '10 --

5000

--

2000

1000

3.3. Estimation o f wear rates Analytical expressions have been developed for the prediction of adhesive wear rates between two unlubncated surfaces m sliding contact [ 1]. The results, as the volume of material worn away in the course of sliding through some distance, x, are expressed in terms of a dimensionless wear constant, k, which is the probabdity that a gwen contact junction will produce a wear particle. This expression is of the form,

CUTTING TOOLS

I

60--

--

500 110

-

40--

QUARTZ

tO0

--

20

200

0 ROCKWELL

80--

--

FILE HARD ORTHOCLASE -APATITE

v = kLx/3p ,

EASILY MACHINED STEELS

--

FLUORITE -CALCITE --

C --

where k = adhesive wear constant for the given pair of materials, dimensionless (see tables 1 and 2), L = normal force between surfaces, p = hardness of the softer of the two materials [force/areal, x = total sliding distance. If k values for abrasive wear conditions are substituted for the adhesive wear constants, the same formula predicts abraswe wear rates. This analytical expression appears to predict reasonably well the amounts of wear to be expected for a gwen set of test materials under the relatwely simple and controlled condmons of comparatwe tests. its mare value hes in the comparisons It makes possible and m the relationships it demonstrates between wear, operating conditions, and hardness. The more complex conditions attending a specific design application or the possible use of some lubricant material will frequently prevent use of this relationship for the solution of a given design problem in a simple manner.

80 l

CORUNDUM OR SAPPHIRE -NITRIDED STEELS TOPAZ -

100

60-40

--

2O

--

O--

--

50

ROCKWELL B

140

--

BRASSES AND ALUMINUM ALLOYS

120

100

--

80---

20 60--

GYPSUM -- 2 MOST PLASTICS

40--

t0 f

120 t30

1

t00 5 BRINELL HARDNESS

80 60 40 ROCKWELL R

20

ROCKWELL M

TALC I MOHS HARDNESS

Fig. 2. Comparison of hardness scales (approximate) [ 3].

Diamond pyramid hardness no.

940 900 865 832 800 772 746 720 697 674 653 633 613 595 577 560 544 528 513 498 484 471 458 446 434 423 412 402 392 382 372 363

Rockwell C-scale hardness no.

68 67 66 65 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37

Hultgren ball

Tungsten carbide ball

. . . . . . . . . ... .. . . . . . 739 .. 722 .. 705 ... 688 670 .. 613 654 599 634 587 615 575 595 .. 561 577 ... 546 560 ... 534 543 519 525 500 508 512 487 494 496 475 481 481 464 469 469 451 455 455 442 443 443 432 432 432 421 421 421 409 409 409 400 400 400 390 390 390 381 381 381 371 371 371 362 362 362 353 353 353 344 344 344

Standard ball 85.6 85.0 84.5 83.9 83.4 82.8 82.3 81.8 81.2 80.7 80.1 79.6 79.0 78.5 78.0 77.4 76.8 76.3 75.9 75.2 74 7 74 1 73.6 73.1 72.5 72.0 71.5 70.9 70.4 69 9 69.4 68.9

A-scale, 60-kg load brale penetrator

10-mm ball. 3000-kg load

B-scale, 100-kg 1 load. Yg m -dlam ball

Rockwell hardness no.

Brinell hardness n o .

15-N scale, 15 kg load 93.2 92.9 92 5 92.2 91.8 91.4 91.1 90.7 90.2 89.8 89.3 88 9 88.3 87.9 87.4 86.9 86.4 85.9 85 5 85.0 84.5 83.9 83.5 83 0 82.5 82.0 81,5 80 9 80.4 79 9 794 78 8

D-scale 100-kg load. brale penetrator 76.9 76.1 75 4 74.5 73.8 73.0 72.2 71.5 70.7 69.9 69.2 68.5 67.7 66.9 66.1 65.4 64 6 63.8 63 1 62.1 61.4 60.8 60.0 59 2 58.5 57.7 56.9 56.2 55.4 54.6 53.8 53.1

84 4 83.6 82 8 81.9 81.1 80.1 79 3 78.4 77.5 76 6 75.7 74.8 73.9 73.0 72.0 71.2 70.2 69.4 68 5 67.6 66.7 65.8 64.8 64.0 63.1 62.2 61.3 60 4 59.5 58.6 57.7 56.8

30-N scale, 30 kg load 75.4 74.2 73.3 72.0 71.0 69.9 68.8 67.7 66.6 65.5 64.3 63 2 62.0 60.9 59.8 58.6 57.4 56.1 55 0 53.8 52.5 51.4 50.3 49.0 47 8 46.7 45.5 44.3 43.1 41.9 40.8 39.6

45-N scale, 45-kg load

Superficial brale penetrator

Rockwell superficial hardness no.

97 95 92 91 88 87 85 83 81 80 78 76 75 74 72 71 69 68 67 66 64 63 62 60 58 57 56 55 54 52 51 50

hardness no

Shore scleroscope

Table 3 Approximate equivalent hardness numbers for steel (a) (reprinted from Metals Handbook)

Rockwell hardness

326 315 305 295 287 278 269 262 253 245 239 232 225 219 212 206 201 196 191 186 181 176 172

...

...

68 67 66 65 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37

(approx.) C-scale [1000 psi] no.

Tensile strength

~ ~" ,~

~

va

354 345 336 327 318 310 302 294 286 279 272 266 260 254 248 243 238 230 222 213 204 196 188 180 173 166 160

336 327 319 311 301 294 286 279 271 264 258 253 247 243 237 231 226 219 212 203 194 187 179 171 165 158 152

336 327 319 311 301 294 286 279 271 264 258 253 247 243 237 231 226 219 212 203 194 187 179 171 165 158 152

336 327 319 311 301 294 286 279 271 264 258 253 247 243 237 231 226 219 212 203 194 187 179 171 165 158 152 ... ... ... ... ...

68.4 67.9 67.4 66.8 66.3 65.8 65.3 64.7 64.3 63.8 63.3 62.8 62.4 62.0 61.5 61.0 60.5 ... ... ... ... (109.0) (108.5) (108.0) (107.5) (107.0) (106.0) (105.5) (104.5) (104.0) (103.0) (102.5) (101.5) (101.0) 100.0 99.0 98.5 97.8 96.7 95.5 93.9 92.3 90.7 89.5 87.1 85.5 83.5 81.7

52.3 51.5 50.8 50.0 49.2 48.4 47.7 47.0 46.1 45.2 44.6 43.8 43.1 42.1 41.6 40.9 40.1 ... ... ... ... ... ... .. ... .. ...

78.3 77.7 77.2 76.6 76.1 75.6 75.0 74.5 73.9 73.3 72.8 72.2 71.6 71.0 70.5 69.9 69.4

55.9 55.0 54.2 53.3 52.1 51.3 50.4 49.5 48.6 47.7 46.8 45.9 45.0 44.0 43.2 42.3 41.5

38.4 37.2 36.1 34.9 33.7 32.5 31.3 30.1 28.9 27.8 26.7 25.5 24.3 23.1 22.0 20.7 19.6 49 48 47 46 44 43 42 41 41 40 38 38 37 36 35 35 34 33 32 31 29 28 27 26 25 24 24

168 163 159 154 150 146 142 138 134 131 127 124 121 118 115 113 110 106 102 98 94 90 87 84 80 77 75

(a) The values m cader correspond to the values m the joint SAE-ASM-ASTM hardness conversions as printed in ASTM E 140, table 2. Values for Rockwell hardness m parentheses are beyond normal range and are given for information only.

(6) (4) (2) (0)

(8)

(14) (12) (10)

(18) (16)

36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 (18) (16) (14) (12) (10) (8) (6) (4) (2) (0)

tO

212

J.Foster, Materialsfor wear-resistantsurfaces

tlon method cannot be accurately converted to a corresponding number on another scale. However, for a particular material, a fairly reliable conversion between scales can be made up empirically. An approximate conversion table for hardness readings for steel using several scales appears in table 3. Fig. 2 shows the approximate relationship among the Mho, Rockwell, and Brlnell scales. Although both the surface energy, ), and the hardness, p, of a material increase numerically with the strengths of the bonds between atoms, the surface energy is proportional only to about the 1/3 power of the hardness Thus a low value of the parameter, 7/P, which has units of length, is generally indicative of a material with good wear resistance [1]. It is interestIng that non-metals such as oxides and carbides have generally lower ~/p values than metals with the same hardness, and experience shows that materials with low 7/P values such as A12O3 or TiC do often perform better than metals in applications involving unlubrv cated sliding surfaces.

3 5 Effects of crystal structure As In most other lines of investigation, progress in the study of wear has led from the inltlal empirical examination of the macroscopic aspects of the problem to a more detailed examination of the slgmficant microscopic material properties. These microscopic properties are related to the crystal structure and crystal shear plane orientations at the sliding interface. Studies run in vacuum (to separate the observed effects of crystal structure from the lubricating effects of surface oxides and adsorbed gas films) have demonstrated a correlation between the crystal structures of the contacting surfaces and their tendency to form adhesion bonds [4]. Surfaces contacting in air seldom exhibit friction coefficients greater than 1.0, whereas clean metal surfaces in sliding contact in ultra-high vacuum under load often exhibit very high friction coefficlents.-For some pure metals friction coefficients from 3.0 to 20 have been observed. Similar behavior has been observed with more complex alloy structures such as 52100 bearing steel, for which friction coefficients as high as 5.0 have been observed. Examination of rolhng or shdlng contact surface pairs suggests a tendency to develop a preferred surface-structure orientation. Adhesion involves attractive forces between atoms and usually occurs under

conditions that produce plastic deformation. These observations led naturally to the examination of crystal structure. Studies and tests with single crystals permit observations under known conditions of crystal lattice orientation and give information on the effects of crystal plane orientation upon the friction, adhesion, and wear of metal surfaces. In particular, it can be stated [4] that 1. Crystal structure markedly influences the friction and wear behavior of metals in shding contact. With cubic metals, cold welding readily occurs while with hexagonal metals It does not. 2 Friction characteristics of hexagonal metals are dictated by operating shp systems and the lattice parameters. Those metals exhibiting predominantly basal shp have the lowest coefficients of friction. 3. Crystallographic plane and direction for metals also influence friction behavior Friction is lowest for the greatest atomic density planes and when sliding in the preferred or greatest atomic density direction. 4. Order-disorder reactions for systems such as Cu3Au also influence friction behavior in vacuum. Lower friction coefficients are observed for these alloys in the ordered state while high friction and complete welding are observed for the disordered state

4. Materials

4 1. Metals and alloys 4.1.1 Low alloy steels The low alloy steels are specified by a numerical indexing system that identifies them by composition. The AISI and SAE numbering systems are essentially identical and were Initially conceived as a four digit numbering system in which the first digit indicates the type of steel; "1" indicates a carbon steel, "2" indicates a mckel steel, and so on. For simple alloys the second digit generally indicates the approximate percentage of the predominant alloying element, and the last two ( or sometimes three) digits, represented by "x" m table 4, Indicate the average carbon content in points or hundredths of one percent. The maximum content of any one alloying element generally does not exceed 5% in these low alloy steels. The presence of carbon in these steels, even in relatively small quantities, makes possible heat treat-

213

J.Foster, Materials for wear-resistan t surfaces

Table 4 Low alloy steel numbers and composlhons AISI

%C

%Mn

0.1 -1.0 0.1 -0.5 0 18-0.43 0 15-043 0.09-0.17 0.13-0.53 0.20-0.35 0.28-0.53 0.17-0.43 0.17-0.43 0 1 7 - 0 53 0.95-1.10 0.17-0.53 0 1 7 - 0 54 0 1 7 - 0 59 0.13-0.18 0 17-0.45

06 - 1 6 1.60-1.90 0.40-0.90 0.40-0.60 0.40-0.90 0.70-0.90 0 40-1.00 0.45-0.80 0.45-0.80 0.70-0.90 0.25-0.45 0 7 0 - 0 90 0.60-1.05 0 60-1.05 040-0.65 0.80-1.25

% N1

% Cr

% Mo

%V

Type

no.

10xx llxx 13xx 23xx 25xx 31xx 40xx 41xx 43xx 46xx 51xx 52100 61xx 86xx 87xx 93xx 94xx

3.25-3.75 4.75-5.25 1.10-1.40 1.65-2.00 1.65-2.00 0.35-0.75 0.35-0.75 3.00-3 50 0.25-0.65

ments in which phase changes take place that alter the basic iron propertms dramatically. The f u n d a m e n t a l process in h e a t treating steels to increase hardness and other plastic strength parameters is the conversion to austenlte at elevated t e m p e r a t u r e followed by a rapid cooling to convert the austenite to martenslte which is a very hard solution o f carbon in iron having a bodycentered tetragonal crystal structure. The m a x i m u m hardness obtainable in the low alloy carbon steels is largely governed by the hardness o f the martensite structure. This in turn is d e p e n d e n t u p o n the carbon c o n t e n t as indmated generally by fig. 3 [5]. Excesses o f carbon above a b o u t 0.6 to 0.7 w / o f o r m undissolved carhides w i t h alloying elements such as chrom i u m or m o l y b d e n u m , and in this f o r m generally provide increased resistance to b o t h wear and shock. The properties o f these standard steel grades are readily available [6]. The 5 2 1 0 0 c o m p o s i t i o n has been highly developed for use m ball and roller bearings and is used e x t e n s w e l y for this purpose in m o t o r vehicles and in other machines. This steel is usually made by the electric m e l t process, to minimize impurities. Its c o m p o s i t i o n is given in table 5, and its properties in table 6. The 5 2 1 0 0 bearing steel c o m p o s i t i o n is n o t serviceable at very high temperatures c o m p a r e d w i t h some o t h e r materials, as shown in fig. 4. Ball or roller

0.55-0.90 0.80-1.10 0.40-0.90 0.70-0.90 1.30-1.60 0.70-1.10 0.35-0.65 0.35-0.65 1.00-1.40 0.25-0 55

0.20-0.30 0.15-0.25 0.20-0.30 0.20-0.30 0.15-0.25 0.20-0.30 0.08-0.15 0 08-0.15

Plato carbon steels Free cutting steels Mn steels 3% N1 steels 5% Nl steels Ni-Cr steels Mo steels Cr-Mo steels Ni-Cr-Mo steels NI-Mo steels Cr steel Cr steel Cr-V steels Low Ni-Cr-Mo steels

0.10 mm -

900

800

~n 700

o 600

/

(1. 500 o

u~

~ 400

/

/

j

p --

65

-- 6O

w --

55~ Q:

-5o~

--

45~

--

40

--

35

300

3O --

25

--

20

ZOO 0

0.2

0 4 0"6 CARBON (~t % )

0.8

4.0

Fig. 3. Hardness of the martensite structure versus carbon content.

w z

./.Foster, Materials for wear.resistant surfaces

214

Table 5 Composmon, [w/o] of AISI 52100 steel [6] C

Mn

P

S

S1

Cr

0.95-1 10

0.25-0.45

0.025 max.

0.025 max

0.20-0.35

1 30-1.60

Table 6 Mechamcal properties of AISI 52100 steel [ 6 ] Draw temperature [°F]

Tensile strength [1000 psi]

Elongation I%1

Hot rolled bars, annealed

100

25

Hot rolled bars, normalized

185

13

Form or conditmn

Quenched and tempered Heat to 1525 to 1550 F (830 to 840 C) and off quench Temper at 300 to 350 °F (150 to I 8 0 C ) Rockwell C hardness 60 to 63 is normal for bearings

200 300 400 500 600 700

Tensile properties for other non-bearmg applications

o o N

I

. . . . . .

800 900 1000 1100 1200

i

. . . . . .

238 190 170 146 120

4.0 4.5 4.3 14.1

80

~6 g

60

\ \ \,,,

40

STE ~'L~ TOOL STEEL ~CASTALLOY~

2o

o

0

200

400

600

I

I

800

4000

I 4200 t 4 0 0

TEMPERATURE('F)

[ 4600

Reduction of area [%1

Hardness brmnel

Hardness Rockwell C

81

57

192

-

139

20

363

-

. . . . . .

. . . . . . 228 189 143 131 96

63-64 62-63 60-61 58-60 55-57 50-54 30.0 30.0 36.0 48.2

-

-

bearings o f 5 2 1 0 0 steel s h o u l d n o t n o r m a l l y b e used abo~e 2 5 0 F ( 1 2 5 C). T h i s is j u s t b e l o w t h e usual t e m p e r i n g t e m p e r a t u r e , a n d at h i g h e r t e m p e r a t u r e s a decrease in h o t h a r d n e s s is e x p e r i e n c e d . T h e p r o b l e m o f d i m e n s i o n a l stability also arises w h e n t h e o p e r a t i n g t e m p e r a t u r e a p p r o a c h e s the h e a t t r e a t m e n t t e m p e r a ture at w h i c h metallurgical t r a n s f o r m a t i o n s w i t h i n the steel cause p e r m a n e n t increases in d i m e n s i o n s . T h e r m a l e x p a n s i o n as given m table 7 m u s t also be t a k e n i n t o a c c o u n t in designing for p r o p e r clear-

I

f440 C . . . . . . _. ~/_ STAINLESSSTEEL K- 162B J~lO0 .... " ~ . , ~ - - ~ q ~ - - - [. - TITANIUM - _ _ CARBI . . DE E

. . . . . .

Yield point [ 1000 psi]

1800

3

Rg. 4. Load capacity of several antlfnction bearing materials 171.

ances. Surface t r e a t m e n t s o f several t y p e s can b e used to increase the h a r d n e s s a n d wear resistance o f these low alloy steel surfaces. T h e s e include c a r b u n z i n g , mtriding, a n d e l e c t r o p l a t i n g w i t h c h r o m i u m . I t is also possible t o a p p l y metallic h a r d surfacing m a t e r i a l s b y welding p r o c e d u r e s a n d m e t a l h c or c e r a m i c coatings w i t h a p l a s m a t o r c h . S u c h c o a t i n g s are usually finished

J.Foster, Materials for wear-resistant surfaces Table 8 Surface hardness of various materials [ 1].

Table 7 Coefficients of thermal expansion for 52100 steel [ 8] Temperature range, [°F]

Averagecoefficient of thermal expansion, [10-6/°F]

70-200 70-400 70-600 70-800 70-1000 70-1200

215

63 6.8 7.2 75 7.8 8.1

by grinding. It is necessary to match coefficients of thermal expansion or total expansion of ceramic and metal when such coatings are used. The surface hardness of a variety of materials, some of which are useful as surface coatings, is listed in table 8. Carburizlng and nitriding processes are heat treatments m which the surface of the treated part is enriched m either carbon or nitrogen by exposure In a controlled atmosphere furnace. Carburlzing requires only a few hours of exposure whereas nitndlng requires as much as 90 hr in an atmosphere of cracked NHa gas. Case hardening by carbunzing may be done on essentmlly any of the low alloy carbon steels. Nitriding is frequently done with standard steel compositions as well as with special alloys developed for the purpose. Some of these special alloys are designated as Nitralloy compositions. For example, AMS 6470 is

Material

Comp OSltlon

Hardness [kg/mm2 ]

Diamond

C

8000

Boron carbide

B4C

2750

Carborundum, silicon carbide

SIC

2500

Titanium carbide

TiC

2450

Corundum, alumina

A1203

2100

Zirconium carbide

ZrC

2100

Tungsten carbide

WC

1900

Garnet

AI203.3FeO 0"3S102

1350

Zlrcoma

ZrO

1159

Quartz, sihca

SiO2

Glass

Sihcate

Bearing steel

-

700- 950

Tool steel

-

700-1000

Chrommm, electroplated

-

900

Carburized steel

-

900

Nitnded steel

-

900-1250

Tungsten carbide, cobalt binder

800 ~500

1400-1800

-

Table 9 Effect of aluminum content on hardness of nltnded AMS 6740 steel [9].

[%]

Core hardness, Rockwell C

R c 50 (mln) Max. case depth hardness, Rockwell C [in]

R c 60 (mm) depth [In]

R c 60 (rain)/ Rc 50 (mm) ratio [%}

Etched case depth [m]

High aluminum, proper treatment

2.t0

33.5

70.8

0.0207

0.0136

65 6

0.0226

Low aluminum, proper treatment

0 82

33.5

68.3

0.0238

0.0076

31.9

0.024

Low aluminum, overtempered

0.82

18.0

68.0

0.0163

0.0081

49.7

0.028

ReJected gear, slightly overtempered

0.67 to 0.73

28.5

67.0

0 0194

0.0072

37.1

0.0273

Normal aluminum, proper treatment

1.10

35.0

70.0

0.0207

0.0118

57.0

0.0207

Characteristics

Aluminum

216

J.Foster, Materials for wear-resistant surfaces

referred to as Nitralloy G and analyzes 0.38 to 0.45% C, 0.40 to 0.70% Mn, 0.95 to 1.35% A1, 1.40 to 1.80% Cr, and 0.30 to 0.45% Mo. Aluminum is a strong nitride former and as usually present in nitnding steels, but many other compositions are also nitnded. The effect of aluminum content upon nitrlded AMS 6470 is shown m table 9. The H-11 class of 5% chromium hot work die steels is nitrided to give case hardness o f R c 65 to 70. The standard 4140, 4340, and 6140 compositions are also frequently nitrided to R c 50 to 60 with core hardness o f R c 25 to 35. Chromium plating can be controlled to deposit surfaces of high hardness, as indicated in table 8. The thickness o f the chrommm deposit for wear surfaces Is substantially greater than that used for decorative chrome surfaces. Chromium deposits even over large surfaces can be held to very close dimensional limits

ll0]. Flame plating using an oxy-acetylene detonation gun permits surface coatings o f a wide variety of materials to be apphed to almost any materml with a hardness below R c 60. Refractory materials can be apphed without heating the base material above 300 F (150 C) according to McGeary and Koffskey [11]. Some avadable coating materials are listed m table 10. The properties and characteristics of some flame plated coatings appear m table 37. In general, the low-alloy steels offer a wide range of properties that meet many requirements for wearresistant surfaces. These Inexpenswe materials should not be overlooked for applications at temperatures up to a few hundred degrees F and m chemical environments with which they are compatible 4.1.2. Stainless steels Some stainless steel compositions have serviceable properties for applications as sliding surfaces whereas others have poor properties for such use. The most commonly encountered 300 series stainless steels are generally in the latter group. In all grades the stainless property requires that at least 12 w/o chromium be m solution in the structure, whether as austemte, martensite, or femte. The 300 series stainless steels contain from 16 to 26 w/o chrommm and no austemte is formed m the binary F e - C r system above 16 w/o chrommm. These 300 series steels are made austenltlC by additions of at least 7 w/o nickel, are not hardenable b y heat treatment, and are known as the aus-

Table 10 Flame plating materials [ 11 ]. Designation

Compos~tmn

LW-1

Tungsten carbide + 9% Co

LW-IN30

Tungsten carbide + 13% Co

LW-IN40

Tungsten carbide + 15% Co

LW-5

25% WC + 5% N1 + mixed W-Cr carbides

LC-1B

80% chrommm carbide + 20% Nlchrome (80N1-20Cr)

LC-1D

70% Cr3C2 + 30% Nlchrome

LC-5

80% Cr203 + 20% A1203

LZ-1

Zlrcomum oxide

LA-2

99% aluminum oxide

LA-7

60% aluminum oxide + 40% titanmm oxxde

LC-4

Chromium oxide

LC-7

Chrommm oxide + 5% Nichrome

LAL-1C

Low temperature abradable coat (aluminum + graphite)

LNC-1B

High temperature abradable coat (Nlchrome + boron mtnde)

LH-1

Hysteresis metal (specml magnetic proper ties)

LCN-1

Copper-mckel-mdmm

LAL-2

Aluminum

LM-6

Molybdenum

LN-2

Nickel

LT-1

Tantalum

LW-6

Tungsten

tenitlc steels. Their austenitic structure xs not entirely stable at room temperature and tends to transform to harder forms when deformed. This work-hardening is rather pronounced and is accompanied by a tendency for the steel to become strongly magnetic. The 400 series stainless steels are chromium steels and, with the exception of a few ferritic compositions, are martensitlc in structure. These steels lack the austemte stabilizing effect of nickel which is present only in grade 431. The martensitic grades are hardenable by heat treatment and can be used for wearing surfaces where corrosion resistance is required. The compositions of these steels are listed m table 11.

J.Foster, Materials for wear-resistant surfaces

217

Table 11 AISI c o m p o s i t i o n limits o f wrought c h r o m i u m stainless steels [ 12]. Composition [wt %] Type number

Classa

403

Cr limits

C lnnits

Mn (max)

Sx (max)

M

11.5-13

0 15 m a x

1.00

0.50

Other elements b

405

F

11.5-13.5

0.08 m a x

1.00

1.00

410

M

11.5-13.5

0.15 m a x

1.00

1.00

0.10 to 0.30 A1

416

M

12-14

0.15 m a x

1.25

1.00

420

M

12-14

0 15 m m

1 00

1.00

430

F

14-18

0.12 m a x

1 00

1.00

430F

F

14-18

0 12 m a x

1.25

1.00

0.07 P or Se min; 0.60 Zr or Mo max

431

M

15-17

0 20 m a x

1.00

1.00

1.25 to 2.50 N1

440A

M

16-18

0 . 6 0 - 0 175

1.00

1.00

0.75 Mo m a x

440B

M

16-18

0.75-0.95

1.00

1.00

0.75 Mo m a x

440C

M

16-18

0.95-1.20

1.00

1.00

0.75 Mo m a x

446

F

23-27

0.35 m a x

1 50

1.00

0.25 N m a x

0.07 P or Se min; 0.60 Zr or Mo max

a M - martensitic, hardenable, magnetic; 4 to 18 Cr, 2.50 Ni max, 1.20C max. F - f e m t l c , nonhardenable, magnetic; 14 to 25 Cr, no Ni, 0.35 C max. b 0.040 P m a x and 0.030 S m a x , except as s h o w n for types 416 and 4 3 0 F

Table 12 Physical properties o f Martensltic stainless steels [6 ]. Thermal conductivity AISI type

403 403 410 410 416 416 420 420 440A, B, C 440A, B, C

Coefficient of thermal expansion

Specific heat at 70 ° F

[Btu ft/hr ft 2 °F]

lemp. [OF]

[ 106/OF]

T[OF] e m p . range

[Btu/lb.OF]

14 4 16.6 14.4 16.6 14.4 16.6 14.4 14.0 -

212 932 212 932 212 932 212 212 -

5.5 5.6 5.5-6 la 5.6 5.5 5.6 5.7 6.0 5.6 5.9

32-212 32-600 32-212 32-600 32-212 32-600 32-212 32-600 32-212 32-600

0.11 0 11 0 11 0.11 0.11 -

218

J.Foster, Materials for wear-resistant surfaces Table 13 Room-temperature mechanical propertms of Martensltac stainless steels [6,121

AISI type

Condition

Tensile strength [ 103 psi]

403 403 410 410 416 416 420 420 440A, B, C 440A, B, C

Annealed Heat treated '~nnealed Heat treatment Annealed Heat treated Annealed Heat treated Annealed Heat treated

65

Yield strength [ 103 psi]

38 35-40 85 40 85 50

65-75 110 mm 75 110 95 105-110 -

30 1

Hardness

R b 95 R c 35-45 R b 95 R c 35-45 R b 90 R c 35-45 R b 100 Rc 45-55 R b 100-105 Rc 58-62

-

29 29 29 29 29 29 31 8 -

60-65

T h e t h e r m a l physical p r o p e r t i e s o f s o m e o f these c o m p o s i t i o n s are listed in table 12 T h e range o f propertles t h a t can be p r o d u c e d m these steels b y h e a t t r e a t m e n t is r a t h e r b r o a d [5, 12, 14]. T h e 4 4 0 compositions with varying carbon contents designated by suffices A, B, a n d C are p a r t i c u l a r l y suitable for wear a p p l i c a t i o n s a n d are f r e q u e n t l y used for b e a r i n g rolh n g e l e m e n t s T h e tensile p r o p e r t i e s o f t h e 4 4 0 grades In the fully h a r d e n e d c o n d i t i o n are s o m e w h a t variable and d o n o t a p p e a r w i t h the o t h e r martensltlC stainless steel p r o p e r t i e s listed in table l 3. T h e hardness a t t a i n a b l e in the 4 4 0 C grade is h i g h e r t h a n for

Young's modulus of elastcltlty-tenslon [ 106 pSl]

the o t h e r m a r t e n s i t l c stainless steels. A n o t h e r g r o u p o f stainless steels f o u n d useful for surfaces in sliding c o n t a c t Includes the s e m l a u s t e n t t i c p r e c i p i t a t i o n - h a r d e n i n g stainless steels [ 14-17] T h e s e grades have n o t y e t b e e n given s t a n d a r d AISI designations b u t are n e v e r t h e l e s s receiving wide use T h e y are h a r d e n a b l e b y h e a t t r e a t m e n t a n d are i d e n t i f i e d b y the grade codes u n d e r w h i c h t h e y are listed in the tables t h a t follow. Five p r e c i p i t a t i o n - h a r d e n i n g stareless steels are listed in table 14. Physical a n d m e c h a n i cal p r o p e r t i e s o f s o m e o f these alloys are listed m tables 15 t h r o u g h 22.

Table 14 ComposlUons of preclplattion-hardenmg stainless steels [ 1 4 - 1 6 ] . Composition [w/o] Element

Stainless

17-7PH

17-4PH

PH15-7 Mo

PH14-8 Mo

0.9 16.0-18.0 6 5 - 7 75 . . 0 75-1.50 1.0 . . 1.0 -

0.07 15 5 - 1 7 5 3 0-5 0 . 3.05- 5.0 1.0 . 1.0 0 2 5 - 0 45

0.09 15 0 70

0 05 13 5-15.5 7 5-9 5

10

1 5 max

25 10

2.0-3 0 l 0

10

1.0

~,W ~

Carbon, max Chrommm Nickel Tltanmm Aluminum Copper Molybdenum Manganese, max Nitrogen Sflmon, max Cb + Ta

0.12 16.0-18.0 6.0-8 0 1 0 max 1 0 max 1.0 0 2 max 10 -

. -

. -

J.Foster, Materials for wear.resistant surfaces

219

Table 15 Phy~cal properties of stainless "W" [ 15] Stamless "W"

Stamless "W" Property

Property AnneXed 7.65

Specific gravity' [g/cm 3 ]

7.65

Specific electrical resistance [/zohm/cm 3] at 20 o C

100

85

Structure

Ferritlc

Ferntic

Thermal conductwity [Cal/cm-sec-° C] At 100 °C At 200 °C At 400 °C

0.045 0 048

0.050 0.053

0.056

0.059

[Btu/ft-hr °F]

At 212 °F At 392 °F At 752 °F

10.8 11.6 13 5

Annealed

Aged

12.1 13.0 14.2

Aged

Mean coefficient of thermal expansion: [ 10-6/°C] 0 to 100 °C 0 to 400 °C 0 to 500 °C [ 10-6/°F] 32 to 212 °F 32 to 572 °F 32 to 932 °F

99 112 11.3

Magnetic permeability (H = 100)

81

Modulus of elasticity (tension) [ 106 psi]

28.0

5.5 6.2 6.3 101

Modulus of elasticity (torsion) [ 106 pSl]

28.0 11.3

Table 16 Room-temperature tensile properties of stainless "W" [ 15 ]. Minimum elongation in 2 mches [%]

Yield strength (0.2% offset) [ 1000 psi]

Tensile strength [1000 psi]

Sheets and strips

Plates and bars

0.030 in 0.030 to Over and less 0.060 in 0.060 in

|

Solution annealed at 1850 to 1950 °F, air cooled

75-115

120-150

3

4

5

8

10

22-28

No. 1 plus aged at 950°F, 1 5. hr, atr cooled

180-210

195-225

3

4

5

8

10

39-47

No. 1 plus aged at 1000 °F, hr, air cooled

170-210

190-220

3

4

5

8

10

38-46

4

No. 1 plus aged at 1050°F, l hr, air cooled

150-185

170-210

4

5

7

8

10

35-43

5

No 1 plus solution anneal at 1300 °F, air cooled

70-110

120-150

5

6

7

10

12

23-29

No. 5 plus aged at 950 °F, I hr, air cooled

135-175

155-185

5

6

7

10

12

35-41

No. 5 plus aged at 1000 °F, t ~- hr, atr cooled

125-165

145-175

5

6

7

10

12

34-40

No. 5 plus aged at 1050 °F, 1 5. hr, air cooled

110-145

135-170

5

6

8

10

12

31-37

Item

1 2 3

6 7 8

Treatment

.

5. m and Over 1. less 5" m

Hardness, Rc

J.Foster, Materials for wear-resistant surfaces

220

Table 17 Tensde modulus of elasticity of stainless "W" [15] Temp [°F]

Mod~u s of elasticity [10 6 pSl]

80 400 800 1000 1200

29 29 27 20 14

Table 18 Physical properties of 17-7PH [ 15]. Condluon Property A (a) Density [g/cm 3 ] Electrical reslstwxty [tz~2-cm] Magnetic permeability At 100 Oe At 200 Oe Maximum Mean coefficient of thermal expansion [ 10 -6 in/in/°F] 7 0 - 2 0 0 °F 7 0 - 4 0 0 oF 70-600 ° F 7 0 - 8 0 0 oF Thermal conductavlty, [ B tu/hr/ft 2/in/° F] At 300 °F At 500 °F At 840 °F At 900 °F Polsson's ratio (Longitudinal, transverse, and 45 degrees) (a) (b) (c) (d)

781 81

TH 950(b) 7.65 87

3.9 34

85-100 50 130-165

8.5 9.0 9.5 9.6

5.6 60 6.1 6.2

-

117 128 146 146

0.28

After annealing at 1950 °F and air coohng As annealed at 1950 °F, treated at 1400 °F, and aged at 950 °F for 1 hr As annealed at 1950 °F, treated at 1400 °F, and aged at 1050 °F for 1½ hr As cold worked and aged at 900 ° F for 1 hr.

TH 1050(c) 7 65 85 90-110 53-63 175- 260

5~ 5.8 59 61

CH 900(d) 7.65 85 70 43 125

61 6.3 6.4 6.6

221

ZFoster, Materials for wear-resistant surfaces Table 19 Transverse tenstle properUes of 17-7PH [ 14]. Yield strength (0.2% offset), [ 1000 psq

Condition

Ultimate tenstle strength [ 1000 psi]

Elongation In 2 inches [%]

Rockwell hardness

A (annealed) Minimum or maxtmum Typical

55 max 40

150 max 130

20 mm 30

B 92max B 85

A 1750 Typical

Modulus of elasticity [106 psi]

29

42

135

19

B87

T (treated at 1400 o F) Minimum Typical

75 100

125 145

4 9

C 26 C31

R 100 (1750 °F + - 1 0 0 ° F ) Typical

115

175

12

C37

TH 950 (1400 + 950 °F) Minimum Typical

165 200

185 215

6 8

C41 C45

29

TH 1050 (1400+ 1050°F) Minimum Typical

150 185

180 200

6 9

C39 C43

29

RH 950 (1750 °F + - 1 0 0 ° F ÷ 950 °F) Typical

215

230

6

C47

C (cold worked) Minimum Typical

175 185

200 215

1 2

C41 C43

29 (longitudinal)

CH 900 (C + 900 ° F) Minimum Typical

230 240

240 250

1 1

C46 C49

29.5 (longitudinal) 32 (transverse)

29

Table 20 Elevated-temperature tensile properties of 17-7PH [ 15]. Condition TH 950(a) Test temperature [°F]

80 300 500 600 700 900

Condition TH 1050(b)

Ymld strength (0.2% offset), [ 1000 psi]

Ultimate tensile strength [ 1000 psq

Elongation m 2 inches [%]

Yield strength (0.2% offset) [ 1000 psi]

180 177 170 163 152 112

198 195 182 175 166 128

11.5 5 4 5 6 6

150

180

11.5

146 137 132 126 102

172 163 158 150 124

9.5 6 6 6 6

(a) As aged at 9 5 0 ° F for 1 hr. 1 (b) As aged at 1050 °F for 15 hr.

Ultimate tensile strength [ 1000 psq

Elongation m 2 inches [%]

J Foster, Materials for wear.resistant surfaces

222

Table 21 Physical propertms of 17-4PH [ 15]. Condilaon

Condition

Property

Property A (a)

H 900 (b)

7.78

Density [g/cm 3 ] Electrical resistwlty [~sX-cm]

77

Magnetm permeabihty At 100 Oe At 200 Oe Maxtrnum

74 48 95

100 60 151

6.0 6.0

7 0 - 6 0 0 oF 7 0 - 800 o F

7.80

98

Mean coefficmnt of thermal expansion, [ 10 -6 m/m/°F] 7 0 - 2 0 0 °F 70-400 ° F

A (a) 6.2 6.3

Thermal conduotwtty [Btu/hr/ft 2/m/°F] At 300 °F At 500 °F At 900 °F

H 900 (b) 6.3 6.5

124 135 157

Note. Contraction on aging at 900 °F = 0.0004 to 0.0006 in/in (a) As annealed at 1900 °F. (b) As aged at 9 0 0 ° F for 1 hr.

6.0 6 1

Table 22 Tensile properties of 17-4PH [ 15].

Condmon

Tensile strength, [ 1000 psl]

Yield strength (0.2% offset) [1000 psi]

Elongalaon in 2 inches(a) or 4.5A or

Reduclaon of area [%]

Hardness Rockwell Brinell

4D [%]

A (annealed 1900 °F ~1 hr, oll quench) Mmtmum Typmal

135 150

95 110

6 12

30 45

H 900 (hardened 900 °F 1 hr, air cool) Minimum Typical

180 195

165 180

8 or 6(b) 13

30 46

C 40 C 43

387 415

H 1000 (harneded 1000 °F 1 hr, mr coold) Typical

180

171

14

54

C 40

387

H 1000 (hardened 1000 °F 4 hr, air cool) Typical

170

16 0

14

55

C 37

363

H 1100 (hameded 1100 °F i hr, air cool) Typical

161

155

15

56

C 36

352

H 1100 (harneded 1100°F 4 hr, atr cool) Typmal

150

140

17

58

C 35

341

H 1200 (hardened 1200 °F 4 hr, atr cool) Typical

145

95

17

60

C 30

302

1

(a) 2-m gage length applicable only on sizes ~- m and larger. (b) 6% for sizes over 3 m.

Modulus of elasticity, [ 106 psi]

285

J.Foster, Materials for wear-resistant sur:aces

4.1.3. Tool steels The tool steels comprise a large family of compositions which are classified in groups such as air-hardening, oil-hardening, shock resisting, hot work, and other types [18, 19]. A further distinction is made to identify two particular types as high-speed tool steels. These are the tungsten and molybdenum tool steels which retain high hardness and good cutting edges at temperatures up to 1000 F. Table 23 lists a number of tool steels by type and composition and indicates generally tlae major uses for each type. Tool steels, by the very nature of the conditions of their use, must be highly wear-resistant materials. The uses of these steels are steadily expanding into many applications outside the tool and die field. The very high strengths that have been developed are being matched in some compositions with increasing ductility to permit applications where the possibility of brittle failure formerly precluded their use. Modified H 11 steel is used in large aircraft structural parts as well as in landing gear, hydraulic, and hinge parts where its high strength-to-weight ratio is used to good advantage. This steel reaches room temperature tensile strengths of 220,000 to 310,000 psi and has a ductibihty corresponding to a 15% reduction in area. Laboratory work with H 11 steel has produced specimens with 430,000 psi tensile and 415,000 psi yield strengths with elongation and reduction in area of 4.5% and 24%, respectively. Both hardness and hardenability are important in tool steels. The hardenabllity of a steel represents the depth to which hardening in a relatively thick section takes place when the heat-treated piece is quenched. This is largely a function of the time-temperaturetransformation behawor of a given steel as governed by the amounts and types of alloying constituents. The plain carbon water-hardening tool steels do not have high hardenabIlity although they do harden at the surface to high hardness values. Many of the other more highly alloyed tool steels harden to greater depths or thoughout their entire volume even though their interior cooling rates upon quenching are not any faster than those for the carbon steels. These are said to have high hardenability. The high speed tool steels have the special property of retaining high hardness at elevated temperatures. The first standard high speed steel was developed at the beglnmg of the twentieth century and had the

223

composition 18-4-1, which corresponds to AISI grade T1 steel, as shown in table 23. The addition of molybdenum has created the newer and lower cost group of molybdenum tool steels which comprises about 85% of all the cutting tools used in the United States. A large number of the applications of tool steels to special gear, bearing, and other mechanical wear surfaces have made use of tool steels from this group with the M1, M2, M10, and M50 compositions appearing frequently in such applications. It should be noted that MS0 is not a standard AISI tool steel composition, but it is a frequently used material for high temperature bearings. A typical M50 composition IS Fe-0.81 C 0.25 Mn-0.30 Si-4.10 C r - l . 0 0 V-4.25 Mo. These materials have been selected for high temperature apphcatlons because of their high room-temperature and hot hardness Table 24 shows the hardness of a number of high speed tool steels after full hardening and then reheating in salt baths for five hours at the three temperatures listed. These data indicate how well these compositions retain hardness after heating at elevated temperatures. The rate at which several of these steels lose hardness with increasing temperature is shown in fig. 5. Hardnesses at 800°F of some of these same steels following exposure at 800°F for up to 500 hr are shown in table 25. These data show the effects of secondary hardening which occurs In high speed steels, generally between 700 and 1050°F. The hardness developed in a given high speed tool steel depends upon the austenitizing temperature and time. These steels are heated well above 1600°F, at which carbon steels easily form austenlte, and temperatures above 2000°F are required to bring the alloy carbides into solution in the austenlte so the quenched Martensite structure will contain the alloy carbides required to give red hardness. Fig. 6 illustrates this effect for M2 tool steel. In recent years modifications of some of the basic tool steel compositions have been examined, and modified alloys with superior properties have been developed. A modified Ml composition with 8 w/o cobalt has been studied [22] and found to give a hardness of R c 70. Both the carbon and cobalt contents affect the maximum hardness of this modified M1 steel, as shown in fgs. 7 and 8. The impact strength of this modified M1 steel is compared with that of several other tool steels in fig. 9. The hot

ZFoster, Materials for wear-resistant surfaces

224

Table 23 Classification of p r m o p a l types of tool steels [5]. AISI-SAE designation

%C

Wl W2

0.6-1.4 0.6-1.4

SI $5

0.5 05

0.8

O1 02

0.9 0.9

1.0 1.6

A2 A5

1.0 10

Composltmn '1 yF,~a_t u s e s

%Mn

%Cr

9~V

%W

%Mo

%Co

Water hardenmg grades Cold-heading dies, woodworking tools, etc

0.25

Shock-resisting tool steels 1.5

..

25 ..

... 0.4

(2.0 $1 )

Chisels, hammers, rwet sets, etc

Od-hardenmg cold-work tool steels 05

0.5

..

Short-run coldforming dies, cutting tools

Air-hardening medmm-alloy cold-work tool steels 5.0

.. . . . .

3 0

1.0 1.0

Thread rolhng and s h t t m g dies, mt n c a t e die shapes

High-carbon M g h - c h r o m m m cold-work steels D2 D3 D4

1.5 2 25 2.25

12 0 120 12 0

H12 H13 H16

0.35 0.35 0.55

5.0 50 70

H21 H23

0.35 0.30

3.5 12.0

T1 T15

0.70 1.50

4.0 40

..

1.0

Uses under 900 °F, gages, long-run formmgand b l a n k m g dies

.. 1.0

C h r o m m m hot-work steels

0.4 1.0

15

1.5 15

A1 or Mg extrusion dies, die-casting dies, mandrels, h o t shears, forgrog dles

70

T u n g s t e n hot-work steels .. ...

9.5 12 0

Hot extrusion dies for brass, nickel, and steel, h o t forging dies

T u n g s t e n high-speed steels 10 5 0

180 12.0

5.0

Original high-speed cutting steel Most wear reslsstant grade

M o l y b d e n u m high-speed steels M1 M2 M3 MIO M15

0.80 0.85 1.0 0.85 1.50

4.0 4 0 4.0 4 0 4.0

10 2.0 2.4 2.0 5.0

1.5 6 25 6 0 ... 6.5

8.5 5.0 5.0 8.0 3.5

50

85% of all cutting tools m U m t e d States made from this group Most wear-resistant grade

225

J.Foster, Materials for wear-resistant surfaces Table 24 Comparative red hardnesses o f some tool steels [20]. Rockwell hardness Grade

C

V

Co Room

temperature* M1 M2 M10 M3 M35 Rex 49t" T1 T2 T9 T4 T8 T15 T5 T6

0.80% 0.80 0.85 1.0 0.80 1.1 0.70 0.80 1.20 0.75 0.75 1.5 0.80 0.80

1.0% 2.0 2.0 2.7 2.0 2.0 1.0 2.0 4.0 10 20 5.0 2.0 1.5

5.0% 5.0 5.0 5.0 5.0 8.0 12.0

C-66 65.5 65.5 66 66.5 68 66 65.5 66 66 66 67 65.5 65.5

llO0°F

l150°F

1175 °F

C-63 62.5 62 62.5 63 66 62.5 63 64 64 63.5 63.5 63.0 63.0

C57.5 58 57 56.5 57.5 61.5 57.5 59.5 61.5 57.5 60.5 60 60 61 5

C-56.5 61 59 60 59 59 59.5 60

* As double tempered. ¢ Balance of composition. 4.0 Cr, 6.75 W, 3 75 Mo.

1000 900

I

.=. \

800

I • T45 o M4 v M2

67

• M50 • 52100

64~

700

so

600

55 =

A

O

,,,x,

%

u~

500

< r

Table 25 Hot hardness of several tool steels after long exposure at 800 °F [21].

69

\

\

lhr

10hr

C-65.0 65.0 65.5 65.0 62.5

C 57.0 58.5 57.0 56.5 62.5

C-57.5 58.0 57.5 58.0 56.5

100hr

500hr

C-57.5 C-58.5 57.5 59.0 58.0 56.0 57.5 58.0 56.0 56.5

44 * Before and after testing. %

300

Hardness after exposure at 800 °F

-r

'\

400

ILl

49 ~ z

T1 M2 M1 M10 M50

Hardness at r o o m temperature *

30 70

200

~65

400

---....

j 200

400

600 800 1000 4200 4400 4600 TEMPERATURE (*F)

Fig. 5. Hot hardness of several tool steels used for bearings

[191.

~"

~ 60

.L.-......

"'--

~

2250°F "~"~""~ 2,,.~IO0"F ' ~ , ,

55

N 5o a

~ 45 Fig. 6. Hardness o f steel M2 austemtized at three different temperatures and tempered at 1050°F for two 2-hr periods

[211.

40 RT

200

400

600 800 TEMPERATURE (*F)

1000

J.Foster, Matertals for wear-reststant surfaces

226

71

70 70

J

j

6,

68

d

J

J

66

J

jJ 68

(1:

67

w za

,/

'~ 6 4 I

62

o

ct:

<= 66

f---_.

65

/

J

64

60 0.8

t .o

0.9

65

1.2

t

CARBON{wt%)

Fig. 7. Effect of carbon content on peak hardness of modlf]ed M1 at three tempering temperatures [22].

0

J

2

J i I

4

6 8 COBALT (wt %)

I0

t2

Fig. 8. Effect of cobalt content on peak hardness of modzfzed M1 at three tempering temperatures [22].

6O

2.5 M2

Mt\\

50

\~

•,

40

N%

:30

\\

/

E

.,,,,

J

t.5

o

""~'%--~ . , ~

~

I f M1+8 Co

3 t.0

Tt5-

2o

CONVENT~

2.0

o

b

t025'='F

.~M2-<

0 >

n-

--

t0

T 6 "--- .

.

.

.

.

.

0.5 ~..~

0

0 55

57

59

61 63 65 67 HARDNESS (ROCKWELL C)

69

Fig. 9. Toughness of tool steels as measured by unnotched izod tests [22].

71

0

2

4 CYLINDER REVOLUTIONS

8(x~O5)

Fzg. 10. Wear resistance of M2 composiUons wzth varying carbon content [23].

227

./.Foster, Materials for wear-resistant surfaces Table 26 Properties of M2 steels [ 23]. Rockwell C hardness Steel As quenched

Aftertempenng atl050°F

After add]laonal heatin~ for 3 hr

2+2hr

2+2+2 hr

llO°F

lI50°F

1200 °F

1250°F

1300 °F

M2

65.0 65.5 65.5

65.8 65.6 64.8

65.1 64.8 64.4

63.9 63.5 62.8

60.8 60.1 59.6

56.9 55.7 55.5

49.5 -

43.1 -

M2, 1.0% C

63.5 65.0

66.9 66.4

66.2 66.0

65.5 66.5

62.0 61.8

60.2 59.0

50.5 49.0

44.5 43.0

hardness at 1,000°F of this M1 with 8 w/o cobalt is 655 dph (diamond pyramid or Vlckers hardness) and may be compared with values for other tool steels in fig. 5. A modified M2 composition containing 1 w/o carbon has been shown [23] to have a higher tempered hardness than standard M2 steel. The hardness prop-

erties of the standard and modified M2 compositions are shown in table 26. The hardness of the modified M2 measured at elevated temperatures is shown in table 27. The wear behavior of the M2 steels, measured by rotating a tungsten carbide cylinder against a static cyhndrical test specimen, is shown in fig. 10. The high strength charactensUc of tool steels rela-

Table 27 Hot hardness of M2 with 1% C [ 23 ]. Temperature [°F] 600 800 1000 1200

350

Rockwell C hardness TOOL STEEL

300 M2

M2-1 C

62 60 57 47

63 60.5 58 49

250 Q. 0 0

9 200 Table 28 Coefficient of linear thermal expansion for M2 steel [24].

z

u~

Room t e m p e r a ~ m (68 °F) To

Average coefficient [ 10-6/°F]

212 392 572 752 932 1112 1292 1382 1472 1562

5.58 6.16 6 46 6.63 6.78 6.83 6.89

150

i. ,Ji >-

~oo %ALUMINA

50

500

I000 1500 2000 TEMPERATURE (°F)

2500

6.98 Fig. 1 I. Yield strengths of wear-resistant materials [7].

3000

228

J.Foster, Materials for wear-reststant surfaces

twe to other materials is displayed in fig 11. On this chart, graphite would be represented by a horizontal region just below the lower end of the dense alumina lines and extending out to about 4000°F. The values for 52100 steel correspond closely with those shown for 17-4 PH steel Plastics would be represented by a small area up to roughly 500°F with yield strengths below 10 000 psi. The coefficient of linear thermal expansion for M2 tool steel is hsted in table 28 4.1.4. Refractory metals and superalloys The refractory metals and most of the superalloys are more commonly used for applications requiring high strength at high temperature than as wear-resistant materials. The refractory metals are usually consldered to include the twelve metals with melting points equal to or greater than the melting point of chromium (1875°C), Hf, V, Nb, Ta, Cr, Mo, W, Re, Ru, Os, Rh, and Ir Of these twelve, the six in groups V and Vl In the periodic table are generally more available than the remaining six, and for this reason V through W generally receive the greatest attention as refractory metals. The refractory metals oxidize m aa~ at elevated temperatures, a s~gnificant d~sadvantage They are generally used in forms containing at least small amounts of alloying constituents, and the four most commonly used, Cr, Mo, W, and Nb, appear in superalloy compositions, which are classified as ~ron, nickel, or cobalt base superalloys [25]. Most of the compositions have been developed for use in such devices as jet engines, gas turbines, high-temperature valves and tubing, and where high strength, toughness, and corrosion resistance are reqmred at temperatures In the range of 1200 to 1800°F or higher. There are, however, some refractory metal compositions with excellent wear-resisting properties. These Include ceramic forms such as carbides, described m section 4.2 2. and some metal alloys m the iron and cobalt base superalloy groups. The nickel base superalloys are most often used for their corrosion resistance I261. Cobalt base alloys that offer excellent wearing qualities are listed in table 9.9 These composmons are xdentxfled using the manufacturer's trade names and numbers by which these materials are easily identified Composmons 1 and 21 m table 29 are supplied as hard facing alloys ~n the form of rods. The others are

o

I

I

I

I

I I I I I I

o

ZZZZZ~

I

I

I

J.Foster, Materials for wear-resistant surfaces

229

Table 30 Static friction of Haynes Stellite 6B [ 27] (tangent of angle of respose of dry surfaces with better than 120 grit finish).

supplied in such forms as sheet, bar, and castings and also, in some cases, as hard facing welding rod. The manufacturer supplies bulletins with complete information on compositions, properties, and uses for these and other compositions [27, 28]. The cobalt base alloys retain high hardness even at red heat, and recover full original hardness when cooled to room temperature. They resist galling and seizing and exhibit low coefficients of sliding friction with other metals as do other cobalt base wear resistant alloys. Properties appear in tables 30 to 33. The Stellite 1 composition typically yields a surface hardness of

Haynes Stellite 6B

Gast

Haynes Stellite 6B

0.119

0 . 1 2 3 0.125

0.138

0.119

Cast tron

0.123

0.199

0.245

0.213

0.225

Bronze

0.125

0.245

0.231

0.257

0.249

Aluminum

0.138

0.213

0.257

0.213

0.328

Bronze

Aluminum Lead

iron

Table 31 Thermal properties of cobalt base alloys [27]. Haynes Stelhte compositions 3 Mean coefficient of thermal expansion [ 10-6/°F] 32-212°F 32-572°F 32-1112 °F 32-1832 °F

6B

6.8 7.1 7.3 8.9

Thermal condutivity [Btu in/hr-ft2-°F] at 72 °F

-

Specific heat [Btu/lb-°F]

-

Melting range [°F]

2255-2408

12

7.7 8.0 8.5 9.7

Star J

6.7 7.2 7.8 8.8

102.7

6.7 6.8 7.1 8.4

-

0.101 2310-2470

m

0.098

0.0925

2306

2130-2430

Table 32 Typical mechanical properttes of sand cast cobalt base alloys at room temperature [27 ]. Haynes SteUite compositmns

Ulttmate tensile strength [psi] Yield strength [psi] Elongation [%] Reduction of area [%] Compressive strength [psi] Transverse break strength(a) load [ lb] lzod input strength [ft lb] (b) Modulus of rupture [psi] Elastic Modulus [psi X 10 -e ] 1

.

(a) 4 m span, T m square bars. Co) Unnotehed. (c) Sheet material.

3

6B

12

Star J

55 000 near UTS 0-1 Nil

121 000 91 600(c) 1.5 1.4

76 000 near UTS 0-1

62 000 near UTS 0-1 Nil

1775 2.5 85 000 36.1

7050(c) 338 000(c) 30.4(c)

193 000 4470 7 215 000 32.9

1755 2.5 84 000 3.75

230

J.Foster, Materials for wear-resistant surfaces Table 33 Hardness of cobalt base alloys [ 27 ] Haynes Stelhte c ompositlon 3 3 3 3 3 3 3 3

Test temperature [° F ] 68 752 932 1292 1472 1652 1832 Rm temp recovery

6B 6B 6B 6B 6B

68 1000 1200 1400 1600

12 12 12 12 12 12 12

68 752 932 1292 1472 1652 Rm. temp. recovery

Star Star Star Star Star Star Star Star

J J J J J J J J

68 752 932 1292 1472 1652 1832 Rm. temp recovery

R c 4 6 w h e n d e p o s i t e d b y m e t a l l i c arc welding, whereas S t e l h t e 21 yields surface h a r d n e s s values o f R c 35 to 4 8 , the latter figure b e i n g p r o d u c e d b y w o r k - h a r d e ning. O x y a c e t y l e n e deposits o f S t e l h t e 1 yield surface h a r d n e s s values o f R c 54. Surfaces h a r d faced wxth Stellite 1 gwe g o o d p e r f o r m a n c e in a p p l i c a t i o n s revolving m e t a l - t o - m e t a l wearing surface c o n t a c t s . Tests o f gears for service at I O 0 0 ° F at 15 0 0 0 r p m were r u n w i t h t o o t h loads u p to 1000 lb p e r linear inch o f t o o t h face w i d t h a n d a l u b r i c a n t o f m i x e d graphite a n d c a d m m m oxide [ 2 9 ] . T w o c o b a l t base superalloys, H a y n e s alloy 151 and S t e l h t e 6B, and one m c k e l base superalloy, R e n e ' 4 1 , were tested.

Brmell hardness Chdl cast

Sand cast

Form unspecified

580 520 475 400 270 160 55 550-600

496 416 400 365 280 175 90 496-545

-

-

-

3 9

-

-

-

-

-

-

226 203 167 102

-

444

-

-

344 344

-

2 8 8

-

2 1 6

-

-

1 3 0

-

-

-

600 520 510 435 395 245 70 600

Rc

-

-

436-444

-

528 436 221 528-572

-

T h e nickel base alloy did n o t s t a n d up. A f t e r 49 h r o f test o p e r a t i o n the 6B gears were still intact, t h o u g h s h o w i n g signs o f i m p e n d i n g failure. The w e a r - r e s i s t a n t Iron-base superalloys m a y be c h a r a c t e r i z e d b y H a y n e s Stellites 9 0 a n d 93. Propertles o f these alloys a p p e a r m tables 29 and 34 to 36. T h e r e are m a n y superalloy c o m p o s i t i o n s [27] m a d d i t i o n to those for w h i c h p r o p e r t i e s have b e e n gwen. T h e r e are also m a n y c o m m e r c i a l a n d e x p e r i m e n t a l alloys o f the r e f r a c t o r y m e t a l s [ 3 0 ] , some o f w h i c h e x h i b i t significant tensile s t r e n g t h s u p to 3 0 0 0 ° F . M a n y o f the superalloy a n d r e f r a c t o r y m e t a l c o m p o sitions d o n o t develop as h i g h a h a r d n e s s as some o f

J.Foster, Materialsfor wear-resistant surfaces

231

Table 35 Mechanical properties of iron base alloys at room temperature [27].

Table 34 Thermal properties of iron base alloys [ 27]. Haynes Stelhte composition

Haynes Stelhte composition c 90

93 93

90 Mean coefficient of thermal expansion [ 10-6/°F] 73-212°F 73-392 °F 73-572 °F 73-932 °F 73-1292 ° F

6.10 6.45 7.04 7.62 8.81

5.50 6.11 6.52 6.99 7.09

Thermal conductivity [Btu in/ft 2 hr °F] at 72 °F

78.5

Specific heat [Btu/lb °F]

0.110 at 68°F

0.11 at 77-212°F

Melting range [° F]

2390

2080-2220

Sand cast

Sand cast

95 000

50 000

Yield strength [] sa]

near UTS

near UTS

near UTS

Elongation [ %]

0-1

0-1

Ntl

Reduction of area [%]

Nfl

Nfl

Nd

Transverse break strength [psi] a

2300

2200

-

I zod ~mpact strength [ft lb] b

3

< 1

-

Modulus of rupture [psi]

110 500

105 000

-

32.6

-

-

Ultimate tensile strength [ps~]

Elastic modulus [106 psi] |

Test temperature [°F]

Chill cast

Sand cast 476

68

540

90

752

400

90

932

338

90

1112

228

90

1292

164

90

1652

4.2. Cermets, ceramics, and graphite

70 440-520

440-520 788

93

752

612

93

1112

476

93

1652

113

Rm. temp. recovery

the alloys hsted for wear-resistant service, b u t the con&tions o f a given apphcation will govern the selection of materials.

247

68

93

.

Brmell hardness at temperatu re

90

Rm. temp. recovery

90 000

a 4 in span ~- m squar bars. b Unnotched. c See table 29.

Table 36 Hardness of iron base alloys [27]. Haynes Stelhte composition

Investment cast

664-720

4.2.1. Cermets Cermets are composites o f ceramic and metallic materials usually made by milling, forming, and sintermg. F o r m i n g is done b y extrusion, in presses, or b y exploswe forming methods. Ceramic materials include A1203, C r 2 0 3 , TIO a , CraCk, TiC, WC, WTiC2, and others. The metallic constRuents include Co, W, Cr, NI, and others. C e r m e t compositions are generally characterized by high hardness and good o x i d a t i o n resistance at b o t h r o o m and elevated temperatures. It Is these properties that make the c e r m e t materials attractive for temperatures above those at which m o s t

232

J.Foster, Materials for wear.resistant surfaces Table 37 Coating composmon (wt. %) and designation 60% AlaO3 + 40% TiO3 (LA-7)

Properties

WC + 9%Co (LW-1, AMS 2435)

WC + 13%Co (LW-IN30)

WC + 15%Co (LW-IN40)

Vickers hardness (300 kg load)

1300

1150

1050

950

Maximum temperature m oxidizing atmosphere [°F]

1000

1000

1000

1300

Coefficient of thermal expansion [10-6/°F]

4.5 (70

Modulus of rupture [ 106 psi] Modulus of elasticity [ 106 psi] Porosity [%] Specific gravity Thermal conductwlty, [Btu/ft-hr-° F] at 200 °F at 500 °F Charactenstacs of flame plated coatings

4.7 to

1000 °F)

--

( 7 0 to I O 0 0 ° F

-

80

90

100

19

31

31

31

11

1

1

1

1

14.2

13.2

3.7 5.3

3.7 53

Extreme wear resistance

metals retain serviceable properties. High temperature bearing study work displays in fig. 12 the superior hot hardness of smtered non-metallic materials at Ingh temperature. Cermet materials can generally be fimshed to very smooth surfaces b u t their brittleness has limited their use in some applications for which their other properties appear excellent. Bearing apphcatlons have been among those limited by brittleness, but Glaeser [7] has reported that a cermet composition demgnated as LT-1B has performed well in bearings and has oxidation resistance to 2200°F (1205°C), has good thermal shock resistance, and retains adequate yield strength above 1800°F (980°C). Tins composition contains 59 w/o chromium, 19 w/o A1203, and 20 w/o molyb-

-

37 5.3

Excellent wear resistance, increased resistance to mechamcal and thermal shock

~//"////~/~

/

-

Greatest resistance to mechamcal and thermal shock Excellent wear resistance

SINTERED CARBIDES ( C E R M E T S ) [

t 85

Coating Is a true semiconductor. Good wear resistance and mating properties Excellent textile Material

l_

I

~

8O 75 7O

200

400

600

800 1000 t200 TEMPERATURE (*F)

t400

t600

Fig. 12. Hot hardness of several types of bearing materials [31l.

233

ZFoster, Materialsfor wear-resistantsurfaces Properties and chaxacteristics of flame-plated coatings [ 11 ].

25% WC + 5% Ni + mixed W-Cr carbides (LW-5)

80% Cr3C2 + 20% Nichrome (LC-1B)

70% Cr3C2 30% Nichrome (LC-1D)

80% Cr203 + 20% A1203 (LC-5)

99% AI20 3 (LA-2, AMS 2436)

1075

700

625

925

1100

1400

1800

1800

1600

1800

4.6 (70 to 1400 °F)

6.4 (70 to 1800 °F)

-

-

3.8 (70 to 1830 °F)

40

70

95

15

20

17

18

21

8

12

1

1

1

10.1 3.1 2.8 Resistance to wear at higher temperatures, Corrosion resistant

6.5 Resists flame impingement, Good resistance at high temperature or in corrosive medm

denum, and has been used for liquid metal-lubricated bearings. Two modifications of this composition are reported to have good properties; 77 C r - 2 3 A12 Oa has superior oxidation resistance, and 25 C r - 6 0 W - 1 5 A12 Oa has optimum erosion and abrasion resistance. Wearing parts need not always be made entirely o f a cermet material. Flame plating a thin cermet f'dm on a base material may be accomplished using a detonation gun. Although the gas and cermet powder in the flame are very hot, the base material need not exceed 300°F (150°C). A wide variety o f materials, usually metals or graphite, have been successfully flame plated

1.5

2

-

-

3.5

-

-

1.2 0.9

Excellent resistance to mechanical and thermal shock at high temperatures

Good selfmating characteristics; resists wear and chemical attack

Excellent resistance to wear, chemical attack, and high temperature deterioration

with cermet, ceramic, and metallic materials. The process and materials have been described b y McGeary and Koffskey [11]. Table 10 lists a number of materials used b y these investigators for flame plating, and table 37 lists the properties o f several cermet and ceramic flame plated coatings. Solid cermet parts are also available from kennametal for rubbing seal surfaces in a wide variety o f compositions [32, 33] as listed in table 38. These compositions are also available from suppliers o f tool steels and other high temperature materials.

145 Wear, impact

Common application

12.0

Corrosion, wear

675

10

680

88.5

83.5

. 2.93 3.18 3.40 94.0

.

38.7 35.0

TaC, WC, No Binder

K 601

89.0

2.80 2.99 3.34

590

.

52.0 41.1

WC,Co Binder

Abrasaon resistance factors [ 1/vok loss]

Impact resistance [inches fall]

Compressive strength [ 103 psi]

Modulus of elasticity [ 106 psi]

Hardness-Rockwell A Rm. temp. At 1400 °F

Mean coeff, thermal expansion [10-6/°F] - I 0 0 to 35 °F Rm. temp. to 400 °F Rm. temp. to 750 °F Ran. temp. to 1200°F

Thermal conductivity [Btu/ft-hr-° F] 212°F 842 °F

Main consutuents

K9

77.3

92.0

3.45 3.76 3.90

Corrosion, wear

825

20

680

.

31.2 29.7

WC, Cr and Co Binder

K 701

Corrosion, wear

110

7.0

610

89.6

89.5

2.66 2.87 3.10

47.4 38.7

WC, N1 Binder

K 801

Gall resastance

55

9.0

610

70.1

91.5

30 3.38 3.70 3.90

16.4 17.9

WT1C2, Co Binder

K 86

Grade number

Table 38 Properties of cermet materials [32]

Wear resistant

165

8.0

690

91.6

92.0 84.0

2.52 2.77 3.01

57.8 44.5

WC, Co Binder

K96

94.0

93.0

Wear, metal cuttmg

300

4.0

600

-

2.75 2.75 2.75

68.9 52.0

WC, Co Binder

K 11

58.5

89.0

High temp

30

2.0

520

-

3.70 4.15 4.40

15.2 17.9

TIC, Co Binder

K 138A

High temp

20

5.5

450

59 0

89.0 74 0

3.70 4.30 4.60

11.1 13 1

TIC, NICMo Bmder

K 162B

bo

J.Foster, Materials for wear-resistant surfaces

4.2.2. Ceramics Ceramics were originally thought of as Inorganic materials derived from clay or associated mineral origins. In modern technology the term extends to cover a broader range of inorganic compounds or mixtures which exhibit the oxidation res:stance, dimensional stability, high elastic moduli, and hardness that geneally characterize ceramic materials. Their retention of good properties at temperatures above those at which most metals become unserviceable is one of the major advantages they offer. The disadvantage that has limited thetr use in many applications ~s their brittleness. Many can be finished to close tolerances and to high surface finishes. However, because of their hardness, the machining or grlndmg operations are expensive and tedious; it is advantageous to form and sinter the required shape to conform as closely as possible with the required finished dimensions. Advances in ceramic technology have produced a wide variety of materials, including oxides, borldes, carbides and nitrldes. A tabulation of properties [34-36] of materials with melting points above 4000°F consists largely of boride, carbide, nitride, oxide, and slhcide ceramics. The oxides are among the most important heatresistant ceramics. They exhibit a greater resistance to oxidation at elevated temperatures than any other material. A1203 is one of the most widely used oxide ceramics for industrial purposes. Alpha alumina (table 39) is the most common and useful form. The addttional physical data on Alz O3 that follow are for polycrystalline alumina, and vary as average grain size, test specimen density, and other variables. Figs. 13 through 19 present thermal and mechanical property data for single-crystal and polycrystalline A12 Oa test specimens. PolycrystaUine A12Oa has a maxtmum Vickers or Diamond Pyramid hardness of 3000 kg/mm 2 (Rock-

235

0.4

..........,--,-,--~

~o3

f

o

000

°

2000 3000 TEMPERATURE(°F)

4000

Fig. 13. Specific heat of polycrystalline alumina [36].

well C 79). The hardness of single crystals is anisotroplC and hardness values as low as 2000 kg/mm ~ can be found. Thus AlzO3 is a very hard material. Solid alumina rollers with journals operating in alumina bearing blocks have been used successfully at temperatures up to 2300°F to convey materials through heat treatment furnaces [37]. Ball bearings with alumina balls and races have also been made for extreme environments. A12O3 coatings on metal shafts were found to operate well in carbon-graphite journals In tests to satisfy design conditions for small nuclear power systems [38]. Studies have also been

24 t A ,u_ 20 =

~ /

A SINGLECRYSTAL B POLYCRYSTALLINE, t00% DENSE

I

~ 12: -Z

Table 39 Properties of alumina [ 35, 36 ]. Material

c=-AI20 a 7-A120 3

Crystal system

Hexagonal Cubic

Theoretical density

[g/cm 3 ]

3.98 +- 0.02 3.65

Melting point

[°F l

3270 ± 40 -

0

0

tO00

2000 TI[I/I~IUTU~ ('F)

3000

Fig. 14. Thermal conductivity of alumina [36].

4000

236

&Foster, Materials for wear-resistant surfaces

2.0

--B

A POLYCRYSTALLINE SINGLE CRYSTAL, c - A X I S C SINGLE CRYSTAL, a - A X I S

I z 0 m

/

o3 z

x w .J ~ 0 n~

LIJ 3" l'n,' <

2//

'1I,-

O3 UJ

0

I000

0 C)

-J

o

0

Q-

~"

2000 3000 TEMPERATURE (*F)

4000

Fig. 17. Compressive strength of polycrystalline alumina as affected by temperature [36].

CURVE A al, iO-6/°F

iO00

\

z w 200

~0.5

o/

,oo

o o 0

44 5.0

( t000 -- ~8000F )

( 70-- IO00*F )

6.6

('1800--3200°F)

2000

_

6O

I

3000

o o

4000

£ 40

TEMPERATURE (*F)

._.___

Fig. 15. Linear thermal expansion of alumina [36]. 0%

made of the frictional behavior of smgle and polycrystalline A1203 against itself and against metal surfaces in a high vacuum [39]. Among the other ceramic oxides for which physical data are available [35, 36], beryllium oxide has an unusual combination of properties that give it a growing importance. It has a melting point of above 4658°F, and good chemical stability even in reducing environments. Its thermal conductiwty exceeds that of most metals and its electrical resistivity is higher than that

20

z w 0

t000

0

CURVE

MATERIAL SINGLE CRYSTAL POLYCRYSTALLINE SINGLE CRYSTAL

100 % DYNAMIC 9 9 . 8 % DYNAMIC '100 % DYNAMIC

D

POLYCRYSTALLINE

98 %

A 60 "''" B

'~ ''--'FC_ __

o

4O

_J o

\o

20

40

20

tOOO

STATIC

I

do

z

DENSITY METHOD

A B C

g. o o o

/

4000

Fig. 18. Tensile strength of polycrystallinealumina as affected by temperature [36].

t00

A SINGLE CRYSTAL -- B POLYCRYSTALLINE - - '

2000 3000 TEMPERATURE (°F)

2000

3000

TEMPERATURE(*F) Fig. 16. Bend strength of alumina [36].

4000

1000

2000 3000 TEMPERATURE (°F)

Fig. 19. Young's modulus of alumina [36].

4000

J.Foster, Materials for wear-resistant surfaces of most oxide ceramics. BeO has the highest specific heat of all the refractory oxides. These properties make it attractive where heat must be absorbed and dissipated. The Knoop microhardness of BeO is reported as 1100 to 1300 kg/mm 2 but may reach 1500 ' kg/mm 2 . The Knoop hardness corresponds closely to the Vickers hardness [40] but makes a smaller indentatlon in the test specimen. Physical property data for beryllia appear in figs. 20 through 26. Carbides have the highest melting temperatures known and also very high hardness. Carbides in general lack oxidation resistance at elevated temperatures, particularly in the presence of water vapor, except for silicon carbide, which is serviceable in high temperature oxidizing environments. The carbide ceramics are most frequently used with a metallic binder such as cobalt, and for this reason most of the carbides in common use are cermets. Some binderless carbide compositions' such as Kennametal K601 are in use. This tantalum-tungsten carbide exhibits good resistance to both corrosion and wear, and

~

>.

~ o

O

I000

2000

60

o 0

40

I

20

- B C

8eO + 4/2 % egO 8eO 1

0

-r 0 . 2

"<'-

A "ANION-FREE"BeO

(n

i

[

1000

2000

3000 (*F)

40(

--_

/

TEMPERATURE

.5000

Fig. 22. Thermal conductivity of beryllia [36].

(h

1000

..._.

TEMPERATURE (*F)

a::

0

I

_j

m

w I1.

I

\

Q z

o

I

r

(.9 Z w

0.4

I

80

:~ tic ~ ~-

0.6

~

I

POLYCRYSTALLINE, tOO % DENSE _

> (..) a = z 40 o

I

LL

|

~- 12o I ~:

~" 0.8 o

Iu.I

237

4000

2000 3000 TEMPERATURE('F)

4000

Fig. 23. Bend strength of beryllia [ 36].

Fig. 20. Specific heat of beryllia [36].

~ 500

~ 3.0

a,10-6/°F

z

_o ~ 2.0 x uJ --

la-B T ' RANST I'O I Nla ~"

9

i

i

I

I

I

A HOT PRESSED, 98 % DENSE B PRESSEDAND SINTERED,95 %DENSE

r

200

4.10 (70-800"F) 5.7t 7.50

I

~,~

0 0

/

(800-2200*F) (2200-58000F)

/

/

z

/

t.0

\

m t00

-r

A bd Z

5

w

0 0

1000

2000 TEMPERATURE

5000

4000

(*F)

8

o

0

4000

2000

TEMPERATURE

5000

(*F)

4000

Fig. 21. Linear thermal expansion of polycrystalHne beryllia [36].

Fig. 24. Compressive strength of polycrystalline beryllia [ 36].

238

o0 o

J.Foster, Materials for wear-reststan t surfaces

A

9 0 - 9 4 °/o DENSE, TESTED

B

95-98

IN B E N D I N G

20

A N D TORSION

% DENSE, TESTED

Carbide

IN T E N S I O N

T

Table 40 Physical properties of tungsten and tantalum carbides [36] Crystal system

Theoretical density [g/cm3 }

Melting point [OF]

Hexagonal Hexagonal Cubic Hexagonal

17 3 15.8 14.5 15.0

5030 5030 7080

~o

:~

0 1000

0

2000 TEMPERATURE (°F)

3000

4000

W2C WC TaC Ta2C

-

1

Table 4 Mechamcal properties of WC and TaC [ 36].

Fig. 25. Tensile strength of polycrystalhne berylha [36]. 60

Reported data

Temperature [° F ]

Property

WC -J 4 0 O

z

20

A

>98 % DENSE, DYNAMIC TEST

B

o I O

> 9 0 % DENSE, STATIC TEST I I I

I

t000

2000

3000

4000

TEMPERATURE (°F)

Jig. 26. Young's modulus beryllia [36].

X

10-6 [psi] for polycrystalhne

Bend strength [10 a psi]

70

Bend strength [ 103 psi]

70-1830

Youngs modulus [106 psa]

70

Youngs modulus [106 psi]

70

Hardness [kg/mm2 ]

70

70-121 -

TaC 40-45

100 -

52 8

1700-2400

-

02

has been used in seal rings running against itself, in nozzles, bushings, valve parts, and high pressure dies. It has a compressive yield strength at 1600°F of 600000 psi. The properties of tungsten and tantalum carbides appear in tables 40 and 41 and in fig. 27 through 30. Boron and silicon nitrldes, BN and SiaN4, are the two most common commercial nltrlde ceramics [36, 45]. Boron nltride is similar to graphite in some properties but at 2 on the Mohs scale, it is too soft for most rubbing applications. Silicon nitride has a high hardness, good dimensional stability, and oxidation resistance up to 2550°F (1400°C) in air. Disadvantages are low impact strength and bend strength. The boride ceramics have only limited commercial apphcatlons, but refractory dlborides are being studied as potential aerospace structural materials because of their oxidation resistance and strength retention at high temperature.

- -

TUNGSTEN CARBIDE

WC - '

¢D F-- 0 1

bin

0

(L 03

0

1000

2000

3000

4000

TEMPERATURE (*F)

Fig. 27. Specific heat of tungsten carbide [36]. z20 0

x uJ

TUNGSTEN CARBIDE/~/~ , ~ ' " 1.0

/

ud

0

~ooo

I-

~, I0-61OF z.9 ( 7 0 . 4 0 0 0 °,F ) 2ooo

3oo0

4oo0

TEMPERATURE (°F) _1

Fig. 28. Linear thermal expansion of tungsten carbide [ 36].

LFoster, Materials for wear-resistant surfaces

bonded with pitch, and electrographites produced by graphitizmg baked carbons at high temperatures. The

,~ 0.2 .oI

TANTALUM CARBIDE, ToC

¢n

if_ 0

lO00

4000

2000 3000 TEMPERATURE (*F)

Fig. 29. Spedfic heat o f t a n t a l u m carbide [36].

z20 _o --TANTALUM

CARBIDE T o C -

/

x

w 1.0 ¢Y w I

~/

o o< : b.I 7

239

o

/ I000

=, ~O- 6/*F 3.7 (70--40000F)

2000 3000 TEMPERATURE (°F}

4000

..J

Fig. 30. Linear thermal expansion of tantalum carbide [36].

4.2.3. Graphite The lubricating qualities o f graphite were for a long time attributed to its lamellar structure and the low shear strength between loosely bound layers. Numerous recent tests have shown that the presence or absence of adsorbed condensible vapors on the graphite surfaces strongly affect both their friction and wear behavior [41,44]. A water vapor pressure of 3 m m Hg is sufficient to reduce wear to a negligible rate and to reduce the coefficient of friction from 0.8 to 0.18. Hydrogen substituted for the water vapor gave no significant improvement even at pressures up to 600 mm Hg. Nitrogen and carbon monoxide showed no lubricant effects up to 600 mm Hg. Oxygen showed a lubricating effect similar to that o f water but at pressures from 200 to 400 mm Hg. In general, easily condensible vapors such as acetone, benzene, ethanol, ammonia, and others all reduced friction and wear at pressures below 5 mm Hg. Carbon materials can be classified as natural graphite, often bonded with resin or pitch, baked carbons consisting largely o f petroleum or pitch coke and

natural graphites are mined and are characterized by larger individual graphite crystals than in the electrographites. All forms develop relatively soft surface layers from surface degradation during rubbing contact [42]. The low friction of these surfaces is due to the low strength of these surface layers. The development o f a preferred orientation at these surfaces to align the low friction graphite crystallite planes with the sliding shear plane is unlikely to occur until the macroscopic surface grain structure is broken down to produce particles of the same general size as the indwldual graphite crystallites. These are very small in electrographites, and effective development of a preferred orientation for low friction in these materials would therefore require a more extensive surface structure alteration than a composition containing larger natural graphite crystals. Lamellar solids tested for sliding friction on a copper disc in a vacuum [43], shown in table 42, suggest that strengths of the chemical bonds between atoms in adjacent layers have an important effect upon frictional behavior. The many carbon composiUons make it unreasonable to list particular compositions as having typical properties. Rubbing seal rings and brushes m motors and generators are typical carbon uses for wear surfaces. Table 42 Vacuum friction data for lamellar solids on copper [43].

Solid

1 Natural graphite 2 Pyrolytlcgraphite 3 Boron mtnde 4 MoS2 5 WS2 6 BiI2 7 LiOH 8 NiI2 9 CdC12 10 CdI2 11 CdBr 2 12 SnS2 13 Phthaloeyanine

Crystal system Graphite Graphite Graphite MoS2 MoS2 AsI3 L~OH CdCI2 CdCI~ CdI 2 CdI2 Cdl 2 -

Coefficient of friction, f air

vac

0 10 0.18 0.25 0.18 0.17 0.34 0.37 0.48 0.35 0.24 0.22 0.40 0.35

0.44 0.50 0.70 0.07 0.13 0.39 0.21 0.44 0.16 0.18 0.15 1.0 0.33

Pressure [ torr] 6 × 10 -9 2 X 10 -9 2 X 10 -9 2 × 10 -9 3 X 10 -9 5 X 10 -7 2 x 10 -9 2 X 10 -s 2 X 10 -9 2 × 10 -9 2 × 10 -9 2 × 10 -9 1 X 10 -5

240

J.Foster, Materials for wear-resistant surfaces

4.2.4. Plastics Plastic materials are widely used in sleeve and journal bearings, seals, sliding assemblies, and also m ball and roller bearings. Relatively few plastic compositions are suitable for sliding surface applications, but they have excellent properties for the purpose, such as low coefficients of friction, good unlubricated performance, good corrosion resistance, low noise level, light weight, low cost, and easy fabrication to intrtcate shapes (table 43). Plastic bearings are frequently lubricated where the application permits even though they offer good unlubricated life. Nylon is tough and abrasion resistant and operates well with no lubricant. It is compatible with mineral oil lubricants. It also tends to absorb water and expand, which can be reduced by conditionmg in boiling water followed by a slow drying period. Nylon bearings can be operated contmuously to above 250°F for short periods at light loads. Nylon has the disadvantage of low thermal conductivity and high ther-

mal expansion, but a thin nylon section with metal backing helps solving these problems. Generous clearance/diameter ratios of 0.010 to 0.015 are used. In general, nylon running against an acetal provides the lowest friction and wear rate obtainable with plastics. Nylon on brass is the poorest nylon-metal combination, and nylon-steel is as good as nylon on nylon (table 44). Acetal plastics are good bearing matertals with low coefficients of friction, good resilience, high strength, and good resistance to water and most solvents [48]. They can be used continuously to 180°F or intermittently to 250°F or more. The design capability of a given plastic bearing material is usually expressed in the form of a pressurevelocity or P V product of the load in psi on the projected area and the velocity m fpm. The allowable P V for a given material and application usually increases as the velocity decreases. Table 45 lists P V limits for unlubricated acetal. Higher

Table 43 Types and properties of plastic bearing materials [46]. Material

Advantages

Dlsadvanatges

Nylon

High abrasion resistance, toughness and reliability ; good conformability; reduces noise and vabration

Absorbs water and has low thermal conducUvity and high thermal expansion, however, problems can be minimized by preconditioning and proper design

Acetal

Very low friction coefficients which are same for both static and dynamic conditions; inexpensive; resists water and most solvents

Low heat resistance; generous clearances required

Fluorocarbon

Excellent resistance to heat, water and chemicals; very low friction coefficients; easdy filled to provide special properties

Relatively expensive; low thermal con. ductivity and high expansion; some types cannot be molded

Phenolic

High resdtency and resistance to severe shock and imoact; low wear rates

Absorbs water; low thermal conductivity in large sizes

Chlorinated polyether

Excellent dimensional stability;high water and chemical resistance

Bearing properties not as good as most other plastics

Polycarbonate

Oose tolerances and good dimensaonal stability; good resistance to heat and impact

High coefficient of friction

High-molecular weight polyethylene

Excellent impact resistance and good wear properties

Relatively difficult to fabricate

J.Foster, Materials ]'or wear-resistant surfaces P V may be used when acetal is lubricated or contains a filler such as tetrafluoroethylene (TFE). Acetal can be lubricated with most common otis, greases, and hydrocarbons, with the possible exception of hot recirculating motor oil. Unlubricated acetal bearings should have a clearance/diameter ratio (c/d) of about 0.015, but when lubricated can be operated with c/d's of 0.002 to 0.004 for diameters up to 3 in. Tests show that an acetal-steel bearing combination provides the lowest plastic-to-steel friction coefficient and wear rate (table 46).

Table 44 Coefficient of friction of nylon [ 16 ]. Nylon-steel

Nylon-nylon

Static Kinetic

Static

Kinetic

0.37 0.2 0.23 0.15 0.3 0.19 0.23

0.46 0.58 0.36 0.29 0.58 0.43 0.62

0.37 0.19 0.19 0.13 0.24 0.17 0.33

Lubticant Dry Ethylene glycol Glycerol Oleic acid Perfluorolube oil Polymethyl siloxane Water

0.34 0.16 0.18 0.08 0.19 0.12 0.19

Fluorocarbon plastics have low friction coefficients and are the most resistant to high temperature and chemical attack. They are used up to 500°F and are tough with good abrasion resistance. The fluorocarbons do not absorb water but have relatively high thermal expansion and low thermal conductivity. These undesirable properties can be largely overcome by filler materials or by using thin layers on metal backing (fig. 31). Sleeve bearings of TFE fibers on metal backing have been tested successfully to 550°F and in liquid oxygen pumps at - 3 2 0 ° F . Fluorocarbon bearings are sometimes oil lubricated in spite of their good dry performance. The clearances required are dependent upon the plastic thickness, filler content, and bearing size. Tetrafluoroethylene (TFE) has a relatively high thermal expansion and m the unfilled condition can expand as much as 2.8% from room temperature to 400°F. Normally unfilled TFE bearings are not specified with clearances less than 0.003 m for 1Ain shafts. TFE surfaces often exhibit lower static friction coefficients than thetr dynamic values. FEP is the copolymer of TFE and hexa-

O08

Table 45 Approximate PV hmits for unlubricated acetal bearings [46]. Speed, fpm

Maximum PV value

0-1 O0 100-200 200-400

3000 2600 2200

400-600

2100

600-1000

1900

241

-,..,~COPPER-FILLED 0 06

=

~ 004 E

Table 46 Coefficient of friction of acetal on steel [46 ].

G.A,TE-F,LLED ~

Friction coefficmnt a

Oil lubricated b

0.05-0.1

Water lubricated

0.1 - 0 . 2

Dry

0.1 -0.3

-

3o% ASBESTOSF,LLED I

oo~

Condition

~

()

~"~

~

30% MOLYBDENUM-DISULFIDE -FILLED

~ _

UNFILLED

0 750

t500

2250

5000

a Inclined plane method. Friction values are same for both static and dynamic conditions. Values established over a range of 0.5 to 2500 psi, 8 to 367 fpm, and 73 to 250°F.

Fig. 31. Coefficients of friction of filled and unfilled TFE-

b Viscosity 58 SSU at 100 °F.

fluorocarbon beating against polished steel [46].

LOAD (ps=)

242

J.Foster, Materials for wear-resistant surfaces Table 47 P V Limits of T F E - c a r b o n bearings [46].

Table 48 P V limits of FEP-fluorocarbon bearings [46]

P V hmlt at Material

Unfilled 30 v/o bronze filled 35 v/o graphite filled 25 v/o glass-fiberf'flled Metal-backed TFE-impregnated bronze fabric TFE-impregnated graphite fabric TFE-impregnated glass fabric TFE fabric

P V ltrnit at

10 [Fpm]

100 [Fpm]

1000 [Fpm]

1 200 12 000 9 000 10 000 17 000 23 000 10 000 20 000 50 000

1 800 28 000 19 000 13 000 17 000 60 000 30 000 30 000 32 000

2 500 45 000 35 000 17 000 17 000 35 000 30 000 10 000 5 000

Material

Unfilled 30 v/o bronze-filled 20 v/o graphite-filled 10 v/o glass-fiber-filled FEP-lmpregnated graphite fabric

10 [Fpm]

100 [Fpm]

600 9 000 8 000 4 500 10 000

12 10 10 18

1000 [Fpm]

800 900 000 10 000 000 8 000 000 8 000 000 12 000

Table 50 Engineering properties of materials [47]

o

Thermal conductivity, [Btu/(hr) (sq ft) (°F/m)]

Maximum allowable service temp [°C]

13

330

600-700

B 75-90

0.9

150

800-900

10.6

B 70

38

850

450

1.3

M 120

25

20

10 000

1.2

M 105

2.5

2.0

150

R 118

3.0

2.9

450

Material

Specific gravity [g/cm 3 ]

Tensile strength [psl]

Young's modulus [ 106 psi]

hardness

Steel SAE 1020

7.85

55 0 0 0 - 6 5 000

28.5

B 78

Stainless steel type 316

7 98

80 0 0 0 - 9 0 000

28.5

Aluminum alloy 2024

2.77

68 000

8 000

Urea formaldehyde formaldehyde ~ (cellulose) 15 § ~ Phenol ~ E formaldehyde (cellulosic)

1.7

Rockwell

Linear coeff, of expansion [lO-S/°Fl

130

Polyimlde

14

11 000

0 46

Nylon 66

1.14

11 800

0.41

R 118

55

1.7

120

Nylon 6

1 12 1.14

9 000

0.30

R 107

10.0

1.7

100

Acetal resin

1.4

10 000

0 40

R 120

4.5

16

85

2.2

280

1.9

70

o ~

PTFE

22

2 500

0.06

R 21

.~ [..,

Polyethylene

0.9

1 800

0 015

R 26

5.5 -

J.Foster, Materials for wear-resistant surfaces

f l u o r o p r o p y l e n e ; P V hmits o f b o t h are given in tables

Table 49 Properties of fabric-base phenolic laminate bearing materials (tubing) [46]. Property Ten. str. [psi] Axial compr, str. [psi] Mod. of elast, in ten. [10 '1 psl] Water absorp. [%] 1 Tg- in wall 1

in wall

l

~- m wall Coeff. of thee exp. [ 10-S/°F] Ther. con. [Btu/hr/sq ft/°F/ft] Coeff. of friction a Relalave wear res.

243

Without graphite

With graphite

9 000 19 000 80

8 000 18 000 70

47 and 48. Phenohc laminate bearings, in use for m a n y years, exhibit low wear rates and low friction coefficients and have good resistance to impact and shock. T h e y are f r e q u e n t l y used w i t h water cooling and lubrication as propeller shafts on ships, in steel rolling mills, and m pumps, or w i t h regular grease lubricants. Properties

3.5

4

2.2

2.8

1 1.11 0.17 0.32 Good

1.2 1.11 0.17 0.31 Superior

are listed in table 49. Chlorinated p o l y e t h e r does n o t have as good bearing properties, b u t has good &mensional stability and good wear resistance with close tolerances in corrosive environments. It can be filled with materials such as graphite, b u t fillers should be selected to avoid degradmg its resistance to water and o t h e r materials. Polycarbonate plastics are relatively n e w [49].

a Ground finish against steel. Table 51 Coefficients of frictaon for plastics vs. plastics [47]. Sliders Flat ring

Stamlessstee1440C Polyoxymethylene ("Delrm") Nylon 6.6 ("Zytel" 101) Nylon 6.10 ("Zytel" 211) ABS-resin FFFE ("Teflon")

Polystyrene Phenolformaldehyde Nylon 6

Nylon 6.10 Polyoxymethylene PCTFE

0.46-0.62 0.38-0.42 0.18-0.20 0.31-0.32 0.'11

0.21-0.23 0.15-0.16 0.26-0.30 0.24-0.25 0.19-0.20 0.12-0.13

0.71-0.74 0.35-0.42 0.15-0.17 0.34-0.36 0.30-0 32 0.14-0.15

0.36-0.38 0.32-0.42 0.17-0.23 0.15-0.16 0.30-0.3 0.15-0.16

0.36-0.37 0.40-0.42 0.21-0.29 0.22-0.27 0.30-0.32 0.12-0.14

0.43-0.49 0.29-0.37 0.61-0 66 0.40-0.45 0.42-0.55 0.13-0.15

Table 52 Bearing characteristics of "Teflon" [47]. Bem-ing material

o = 10 [ft/min]

o = 100 [ft/min] v = 1000 [ft/min]

K[inS-mm/lb-ft-hr X 10 - l ° ]

Virgin "Teflon" TFE 30 v/o bronze-filled TFE 35 v/o graphite-filled TFE 25 v/o glass-fiber-filled TFE 25 v/o glass-fiber TFE, metal-backed TFE impregnated bronze fabric TFE impregnated graphite fabric TFE impregnated glass fabric TFE fabric Virgin "Teflon" FEP 30 v/o bronze-filled FEP 30 v/o bronze-filled FEP 20 v/o graphite-filled FEP 10 v/o glass-fiber-filled FEP FEP impregnated graphite fabric

1 200 12 000 9 000 10 000 17 000 23 000 10 000 20 000 50 000 600 9 000 9 000 8 000 4 500 10 000

1 800 28 000 19 000 13 000 17 000 60 000 30 000 30 000 32 000 800 12 000 12 000 10 000 10 000 18 000

10 000 to 20 000 3-6 10-20 15 - 3 0 15 - 3 0 3-6 15 - 3 0 20 - 4 0 15 -25 25 000 to 50 000 X-20 X-20 X-60 20-40 X-80

2 500 45 000 35 000 17 000 17 000 35 000 30 000 10 000 5 000 900 10 000 10 000 8 000 8 000 12 000

200-400

1000

400

0.20-0.40

Speed, continuous operation (5-1b load), max fpm

Po for continuous service, 0.005 in wear in 1000 hr

Limiting Pv at 100 fpm

Coefficient of frictmn

Good

1.2

1.4

Chemicals

Density [g/cm 3 ]

Cost index for base material

* Exceeds 1Lmiting P V

Fair

400

0.3

Resistance to humidity

Critmal temperature at bearing surface [°F]

Elastic modulus, bending [psi x 106 ]

50

4900

Maxtmum load projected area (zero speed) [psi]

Wear factor, K X 10 -1°, [cu in/min/ftIb-hr]

Nylon

Performance eharacteru stic or property

1

1.43

Good

Good

300

0.4

50

0.15-0.30

3000

1000

500

5200

Acetal

Unmodified polymers

100 000

300

1000

10 000

Polyimide

Good

Good

600

0.45

150

5

15

2 15-2 20 1.42

Excellent

Excellent

500

0.08

2500

0.04-0 13 0.1-0.3

1800

200

100

1000

Fluorocarbon

1.4

Good

Good

300-400

250 2000

0.90-1.1

5000

100

1000

4000

Phenohc

15

12

Good

Fair

400

04

50

0 . 1 - 0 25

4000

1000

200-400

1000

Nylon graphite rifled

1.54

Good

Good

300

0.4

20

0 05-0.15

5500

2500

800

1800

Acetal, TFE fiber fdled

Modffmd polymers

Table 53 Performance data for self-lubricating plastm bearmg materials [47].

2.15-2 25

Excellent

Excellent

500

04

1-20

0.04-0.25

30 000

2500 50 000"

1000

2000

Fluorocarbon, wide range of Idlers

15

1.49

Good

Good

600

0.63

15

0.1-0.3

100 000

300

1000

10 000

1.4

Good

Good

300-400

5

10

0.05-0.45

40 000

5000

1000

4000

Polytmlde, Phenolic, graphite TFE frilled f'dled

?

to 4~ -~

J.Foster, Materials for wear-resistant surfaces

They offer good dimensional stability and resistance to heat and impact, but tend to develop increased friction coefficients and even to gall when run against metals at high PV. Normal gear lubricants improve their dry performance properties markedly. Regular polyethylene is not usually regarded as a bearing material, but the high molecular weight grades with MW about 2 000 000 and higher have better bearing properties. They resist wear and impact better than almost any other thermoplastic material. Aromatic polyimides are strong temperature-resistant polymers with tensile strengths in excess of 5000 psi at 600°F and 10 500 psi at room temperature. They have good wear resistance, excellent sliding properties, relatively low thermal expansion, and good radiation resistance. Graphite filled polyimides serve as ball bearing retainers up to 700°F and 10 000 rpm. They also exhibit good behavior in liquid hydrogen at --423°F. Friction and wear of polylmides against 440C stainless steel in a vacuum of 10 -9 to 10 -6 torr are lower than for TFE. Summaries of properties for bearings o f plastic materials appear m tables 50 through 53.

References [ 1] E. Rabinowicz, Friction and Wear of Materials (John Wiley, New York) 1965, 30 and 73. [2] E. Rabinowicz, Friction and Wear of Metals, Metals Engineering Quarterly, 7(2), (May 1967). [3] G.F. Kinney, Engineering Properties and Applications of Plastics (John Wiley, New York) 1957, 202. [4] D.H. Buekley, The Influence of Various Physical Properties of Metals on Their Friction and Wear Behavior in Vacuum, Metals Engineering Quarterly, 7(2) (1967) 44. [5] R.M. Brick, R.B. Gordon and A. Phillips, Structure and Properties of Alloys (McGraw Hill, New York) 1965, 251. [6] S.L. Hoyt (e&), Metals Properties-ASME Handbook (McGraw Hill, New York) 1954. [7] W.A. Glaeser, High-Temperature Bearing Materials, Metals Engineering Quarterly, 7(2) (1967) 54. [8] E.A. Eldridge and H.W. Deem, ASTM-Report on Physical Properties of Metals and Alloys from Cryogenic to Elevated Temperatures, Am. Soc. for Testing Materials, 1961, 44. [9] D.A. Dashfield, Nitriding Problems and Their Solutions, Metal Progress (February 1964) 88. [10] J.C. Merriam, Quality Finish: Hard Chromium's Aim, Iron Age (June 9, 1966) 63.

245

[11] T.C. McGeary and J.M. Koffskey, Engineering Applications for Flame Plating, Metal Progress (January 1965) 80. [12] C.R. Tipron, Jr. (Ed.), Reactor Handbook, Vol. 1-2nd ed., (lnterscienee Publishers, Inc., New York) 1960. [13] Reactor Handbook, 1st ed., VoL 3-Section 1, (General Properties of Materials, 1955.) [14] Stainless Steel Handbook (Allegheny Ludlum Steel Corp., Pittsburgh). [15] D.B. Roach and A.M. Hall, The Engineering of Precipitation-Hardening Stainless Steels, TML Report No. 48, Battelle Memorial Institute (July 20, 1956) 5. [16] Materials In Design Engineering, Materials Selector Issue, October 1966. [ 17] D.C. Perry, H. Tanczyn and W.C. Clarke, Jr., New PH Steel Matches Strength and Toughness, Metal Progress (June 1963) 101. [18] Metal Progress, Metal Proeress Data Sheet (June 1961) 96B. [ 19] Metal Progress, What Tool Steel Shall I Use for Applications in Structures and Components? (October 1961). [20] Peter Payson, High Speed Steels Meet Tool Needs-Part II, Metal Progress (July 1961). [21] David P. Hughes, How Hot Hardness Varies in Tool Steels, Metal Progress (June 1961). [22] James R. Handyside, John C. Hamaker, Jr., and Daniel H. Yates, Rc70 High Speed Steel-lts Development, Properties, and Performance, Metal Progress (June 1963). [23] Gary Steven and Stanley J. Smialek, What Modified M2 Offers The User, Metal Progress, May 1964. [24] Carpenter Matched Tool and Die Steel Manual, Revised Edition, The Carpenter Steel Company, Reading, Pa. [25] Ward F. Simmons, What Alloy Shall I Use For High Temperature Applications (Above 1200"F)?, Metal Progress (October 1961). [26] Corrosion Resistance of Union Carbide Alloys, Union Carbide Corporation Bulletin F-30, 313, Union Carbide Corp., Kokomo, Indiana. [27] Haynes Wear-Resistant Alloys, Bulletin F-30, 133A, Stellite Division, Union Carbide Corp., Kokomo, Indiana. [28] Haynes Hard-Facing Alloys, Bulletin F-30, 216, Stelllte Division, Union Carbide Corp., Kokomo, Indiana. [29] Stanley Wallerstein, Gear Material For Operation at 1000°F, Metal Progress (May 1967). [30] D.J. Maykuth and H.R. Ogden, What Refractory Metal Shall I Use for Ultrahigh-Temperature Applications (Above 1800°F)?, Metal Progress (October 1961). [311 Carl R. Weymueller, Bearings For High Temperatures Use, Metal Progress (July 1964). [32] Properties of Kennametal Hard Carbide Alloys, Kennametal, Inc., Latrobe, Pa., Form 800. [ 33 ] Kennametal and Kentanium-Proven Materials for Rotary Mechanical Seals, Kennametal, Inc., Latrobe, Pa., Form B-900. [34] Samuel Skalrew and M.J. Albem, Properties of Solids That Melt Above 40000F, Metal Progress, February 1965, Data Sheet.

246

J. Foster, Materials for wear-resistant surfaces

[35] Refractory Ceramics For Aerospace-A materials Selection Handbook, Compiled by Battelle Memorial Institute, The American Ceramic Society (Publisher), Columbus, Ohio, 1964. [36] Engineering Properties of Selected Ceramic Materials, Compiled by Battelle Memorial Institute, Published by the American Ceramic Society, Columbus, Ohio, 1966. [37] Robert E. Lintner, V.E. Wolkodoff, L.E. Ferretra and L.C. Hageman, New Horizons For Industrial Ceramics, Metal Progress (November 1963). [38] L.G. Kellog and W.G. Dewart, SNAP 8 Reactor Bearing Development, Transactions of the American Society of Lubricating Engineers (1966) 325. [39] Donald H. Buckley, Frictions Characteristics m Vacuum of Single and Polycrystalline Aluminum Oxide in Contact With Themselves and With Various Metals; Transactions of the American Society of Lubrication Engineers (1967) 134. [40] D. Tabor, The Hardness of Metals, The Clarendon Press, 1951. [41] Robert H. Savage, Graphite Lubrication, J. Appl. Physics (January 1948). [42] W.T. Clark and J.K. Lancaster, Breakdown and Surface Fatigue of Carbons During Repeated Shding, Wear, (1963) 6. [43] A.J. Haltner, Sliding Behavior of Some Layer Lattice Compounds in ultrahigh Vacuum, ASLE Transactions (1966) 9. [44] J.P. Bryant, P.L. Gusthall and L.H. Taylor, A study of Mechanisms of Graphite Friction and Wear, Wear 7 (1964). [45] R. Thompson, Borides: Their Chemistry and Applications, Lecture Series No. 5, 1965, The Royal Institute of Chemistry, 30 Russell Square, London. [46] Robert J. Fabian, Plastics Bearing Materials, Matermls m Design (March 1964). [47] R.P. Steijn, Friction and Wear of Plastics, Metals Engineermg Quarterly (May 1967). [48] John D. Young, What Engineers Should Know About Acetal Resins, Metal Progress (October 1963). [49] Robert J. Kunze, What Engineers Should Know About Polycarbonate Plastics, Metal Progress (August 1963). [50] J. Harry DuBois and Frederick W. John, Plastics (Reinhold Publishing Corp., New York).

[51] N.B. Dewees, Sliding-Wear Performance of 135 Material Couples m High Purity Water at 300°F and 500°F, WAPD-314, Bettis Atomic Power Laboratory. [52] Lubrication, Corrosion, and Wear-A Continuing Bibliography, National Aeronautics and Space Administration, NASA SP-7020. [53] F.P. Bowden and D. Tabor, Friction, Lubrication and Wear: A Survey of Work Dunng the Last Decade, Brit. J. Appl. Phys. 17 (1966). [54] H.E. Sliney, Self-Lubricating Composites of Porous Nickel and Nickel-Chromium Alloy Impregnated with Barium Fluoride - Calcium Fluoride Eutectic, NASA Technical Note NASA TN D-3484. [55] E.E. Bisson and W.J. Anderson, Advanced Bearing Technology, National Aeronautics and Space Admimstration, NASA SP-38. [56] J.T. Dunn and H.S. Rawcliffe, Literature Survey of Water Lubricated Bearing Materials, Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, EDI-123. [57] J.T. Dunn, Comparison of Wear Rates in Water From Different Test Machines, Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, AECL2603. [58] W.J. Freede, L. Newcomb and R.S. Kennedy, Static and Sliding Contact Behavior of Materials m Sodium Environments at Elevated Temperatures, Liqmd Metal Engineering Center, Atomlcs International, NAA-SR12446. [59] M.J. Wallace, Wear and Compatibility Behavior of Liquid Metal Beating Materials, Trans. ASME, J. Basic Engng., Paper No. 66-WA/Lub-3. [60] W.H. Roberts, Friction and Wear Behavior of Shding Bearing Materials in Sodium Environments at Temperatures up to 600°C, United Kingdom Atomic Energy Authority, TRG Report 1269(R). [61] R.G. Frank (Ed.), Materials for Potassium Lubricated Journal Bearings, Quarterly Progr. Rept. No. 10, General Electric Space Power and Propulsion Section, NASACR-72027. [62] D.J. Depaul (Ed.), Corrosion and Wear Handbook for Water Cooled Reactors, Report No. TID-7006, Westinghouse Electric Corporation.