Friction and wear of chromium and nickel coatings

Wear, 129 (1989)

FRICTION

123

123 - 142

AND WEAR OF CHROMIUM AND NICKEL COATINGS

D. T. GAWNE and U. MA Department of Materials Technology, Ux bridge (U.K.) (Received November 23,1987;revised

Brunel, The University of West London, July 22,1988;

accepted September 5,1988)

Summary Conventional chromium plating has a considerably higher wear resistance than electroless nickel, heat-treated electroless nickel, electroplated nickel, heat-treated chromium and crack-free chromium coatings in all tests, even though it does not have the highest hardness. The ranking order of the other coatings depends upon the dominant material removal mechanism and can change from test to test. Transitions between severe and mild wear occur in electroless nickel and chromium owing to changes in phase, brittleness and scratch indentation size effects. Heat treatment effects the transition by crystallizing the amorphous electroless nickel, variation of the electroplating conditions alters the crystal structure of chromium and reduction of the scratch size changes the dominant material removal mechanism from fracture to plastic deformation. The relative merits of each coating and the role of laboratory data in coatings selection and design are discussed.

1. Introduction Electroplating and electroless deposition are the most economic processes for applying metallic coatings of thicknesses between approximately 10 and 500 pm on many engineering components. This is primarily because their rates of deposition can provide the required product quality in acceptable process times at relatively low capital and operating costs. Electroless nickel deposition, chromium and nickel electroplating are major processes within this category. Whereas electroplating has been well established for a considerable period, electroless deposition has only been used extensively over the last 10 years and has not yet reached its potential. A substantial proportion of the increasing usage of electroless nickel has been at the expense of electroplated chromium, because of a number of inherent advantages. Firstly, electroless nickel deposition produces a uniform coating thickness over intricate parts, which avoids the expensive jigging, overplating and machining that is often necessary for electroplated components. Secondly, electroless nickel offers effective barrier properties QQ43-1648/89/$3.50

0 Elsevier Sequoia/Printed

in The Netherlands

124

and corrosion resistance, owing to its amorphous structure and the absence of the crack networks as found in chromium plating. Thirdly, electroless nickel in the heat-treated condition exhibits a higher hardness than electroplated chromium and it is claimed that this provides a correspondingly high wear resistance. This paper is directed specifically at investigating this claim by evaluating the wear behaviour of three types of conventional electroplated chromium, crack-free chromium, heat-treated chromium together with two types of electroplated nickel and comparing the results with those previously obtained by the authors for electroless nickel [ 1,2]. The work is aimed at characterizing the wear performance of the coatings, underst~d~g the mechanism of wear and clarifying their positions in the range of protective coatings available to the design engineer. 2. Materials All the coatings investigated were 40 pm in thicknss and deposited on plain carbon steel substrates (substrate compositions given in Section 3). Three types of hard chromium plating were tested: Cr A, Cr B and Cr C. Cr A was produced from a conventional sulphate-ion~at~ys~ chromic acid bath (CR 710 manufactured by M & T Chemicals Ltd., Birmingham) with a deposition rate of 25 pm h-l and a current efficiency of 15%. Cr B was deposited from a high speed mixed catalyst solution (CR 840, M & T Chemicals) with a 50 pm h-l deposition rate and 25% current efficiency. This solution has the advantage of a. high deposition rate and current efficiency but in certain cases (e.g. low current density areas in complex shapes) can cause serious etching or pitting of steel substrates. Cr C (HEEF, M & T Chemicals) was designed to overcome this etching problem while still providing a high deposition rate and current efficiency. Crack-free chromium coatings (Cr D) do not contain the crack networks found in conventions chromium plating. These coatings were included in the investigation since the absence of cracks may benefit their fracture toughness, wear and corrosion resistance. Deposition was undertaken in a solution containing 250 g Cr03 dmV3 and 2.5 g H,S04 dms3 operating at 72 “C under a current density of 16 A dmm2. The electroless nickel coating contained 8.5 wt.% P and was prepared from a conventional acid-hypophosphite solution (Nichem CS, M & T Chemicals). The coating was wear tested in the as-deposited condition, EN, and after heat treatment at 400 “C, EN400, and 600 “C, EN600. The heat treatments were undertaken in a tube furnace with a soak period of 1 h in air at 400 “C and in argon at 600 “C. Two types of electroplated nickel coating, Ni A and Ni B, were deposited from a Watts solution (nickel sulphate, nickel chloride and boric acid) and a sulphamate solution (nickel sulphamate and boric acid) solutions respectively. The Watts solution produces a conventional, low cost deposit whereas the sulphamate bath provides a much faster deposition process but at a significantly higher cost.

3. Mechanical testing Wear performance was assessed under dry unlubricated conditions using the Taber Abraser, Falex machine, reciprocating diamond scratch technique and pm-on-flat tests illustrated schematically in Fig, 1. The Taber Abraser and Falex machines are widely used and accepted in the metal finishing industry. The Taber Abraser is employed to represent conditions of light abrasive wear, the Falex machine for metal-metal contact in a unidirectional action, the reciprocating diamond scratch test for heavy abrasive wear and the pin-on-flat confi~mtion for metal-metal contact in a ~ci~rocat~g action. Tke tests were operated under a mean relative humidity and ambient temperature of 60% and 20 “C respectivelyly. load

ioad

11 eels

‘“q-J)“-

specimen

(--(jgd

?

rev min-1

ibl

c9

290 rev min-1

Fig. 1. Wear apparakw (a) Tabs Abraser, (b) Fakx machine, fc) reciprocating diamond scratch tester, fdf pin-m-Rat maehim.

The Taber Abraser (Fig. l(a)) consists of two rotating abrading wheels, each under a 1 kgf load, which produce a circular wear track of 12.5 mm width and 60 mm inside diameter on the coated flat test panel. The test panel rotates about a vertical axis at a constant speed of 60 rev min-r against the sliding rotation of the two abrading wheels. The wheels are driven by the test sample about a horizontal axis displaced tangentially from the axis of the sample, which produces a rubbing action on the material under investigation. Wear was quantified by weight loss, which was measured every 1000 cycles (one cycle is one revolution of the coated test panel). Teledyne CSlO wheels, consisting of rubber impregnated with fine silicon carbide particles, were used for all tests. The wheels were cleaned with abrasive paper

126

every 1000 cycles to obviate clogging and contamination. The substrate for the test panel was SAE 1010 grade, a plain carbon steel containing 0.1 wt.% C and 0.3 wt.% Mn, and possessing a hardness of 110 HV 0.1. The reciprocating scratch test (Fig. l(c)) involved the tip of a Rockwell diamond indenter sliding under a 2 kgf load against a flat coated specimen. The reciprocating action of the diamond was fixed at 40 cycles min-’ with a single track length of 30 mm (giving a total distance travelled by the diamond per cycle of 60 mm). The substrate of the coated specimen was 080M40 grade, which is a plain carbon steel containing 0.4 wt.% C and 0.8 wt.% Mn. The steel was heat treated to give a hardness of 560 HV 0.1. Prior to plating, the substrate was surface ground at 45” to the sliding direction with a surface finish of R, value in the range of 0.5 - 0.6 pm. The extent of wear was assessed using a calibrated linear variable-differential transducer on the diamond holder. The Falex machine (Fig. l(b)) was used to measure the wear of a cylindrical coated test pin, 6.35 mm in diameter, rotating at 290 rev min-’ between two stationary uncoated V-grooved blocks under a load of 22.5 kgf. The pin consisted of 080M40 grade steel with a hardness of 170 HV 0.1 and the V-block, heat-treated 080M40 grade steel with a hardness of 250 HV 0.1; both parts were machined to a 0.25 pm R, surface finish. The tests were carried out in the non-lubricated condition. Compressed air jets were applied to the chuck, pin and block assembly during testing for temperature control and debris removal. There is no standard Falex procedure to determine the wear behaviour of coatings and so the following procedure was developed. The coated pin was run against the two V-blocks for a 30 s running-in period with a 13.5 kgf load and the load was then increased to 22.5 kgf for the remainder of the test. Test specimens were removed at regular intervals and their weight loss determined. The pin-on-flat test (Fig. l(d)) was carried out using a reciprocating machine in which the flat end of a cylindrical steel pin (30 mm X 6.35 mm) slides under a 10 kgf load against a flat coated specimen (75 mm X 35 mm X 10 mm). The pin consisted of 080M40 grade steel with a hardness of 170 HV 0.1. The flat substrate was a heat-treated 080M40 grade steel of hardness 560 HV 0.1 and surface ground at 45” to the sliding direction to give a surface finish with an R, value in the range 0.5 - 0.6 pm prior to plating. The reciprocating action of the pin was fixed at 150 cycles min-’ over the wear track length of 30 mm (giving a total distance travelled per cycle of 60 mm). The extent of wear of the electroless nickel coating was determined by measuring the mean depth of the wear track with a profilometer. Hardness measurements were performed on through-thickness sections of the coatings using a Vickers M55 microhardness tester with a Vickers indenter under an applied load of 100 gf. 4. Results Table 1 shows that each type of coating exhibits a wide range of hardness values depending upon the processing conditions. Chromium plating

127 TABLE 1 Hardness values of the coatings Coating

Hardness

(HV 0.1)

As-deposited

Electroplated Cr CrA Electroplated Cr Cr B Electroplated Cr Cr C Electroplated Cr Cr D Electroplated Ni Ni A Electroplated Ni Ni B Electroless Ni EN

Heat treated at 400 “C

1100

840

Heat treated at 600 “C 470

1055

-

-

940

-

-

560

-

-

275

-

-

360

-

-

630

1120

630

gives 470 - 1100 HV 0.1, electroless nickel 630 - 1120 HV 0.1 and electroplated nickel 275 - 360 HV 0.1. The influence of heat treatment on the hardness of chromium plating and electroless nickel is presented in Fig. 2. The hardness of chromium plating is more than halved, while that of electroless nickel is almost doubled. The hardness of chromium plating monotonically decreases from 1100 to 470 HV 0.1 after heat treatment at 600 “C, whereas that of electroless nickel rises from 630 to a peak at 1120 HV 0.1 after heat treatment at 400 “C and diminishes at higher heat treatment temperatures. Conventional chromium plating (Cr A, B, C) had substantially lower wear rates than the electroless and electroplated nickel coatings under all test conditions (Figs. 3 - 6). The conventional chromium coatings generally showed the lowest friction coefficients during wear. For example, the friction coefficient for the chromium coatings (Cr A, B, C) was 0.8 - 0.9 compared with 0.9 - 1.0 for electroless nickel in the Falex test using a plain carbon steel counter-face. Heat treatment of the electroless nickel coatings resulted in an increase in wear rate in the reciprocating diamond scratch test (Fig. 3) but a decrease in wear in the Falex, pin-on-flat and Taber tests (Figs. 4 - 6). Coefficients of friction were measured continuously throughout the wear tests and mean values taken. The data in Table 2 give the friction coefficients against a diamond counterface from the reciprocating scratch test, a plain carbon steel counterface from the Falex test and a stainless steel counterface also from the Falex test. The salient points from Table 2 are that the conventional chromium coatings (Cr A, B, C) give consistently lower friction coefficients than electroless nickel and that the diamond

L

0

I

I

I

100

200

300

I 400

I

I

500

600

TEMPERATURE,

OC

Fig. 2. Effect of heat treatment temperature (1 h soak) on the hardnessof conventional chromium plating (Cr A) and electrobss nickel (EN) coatings.

0

100

300

200 NUMBER

OF

400

CYCLES

Fig. 3. Wear of conventional chromium plating (Cr A), 600 “C heat-treated chromium plating (Cr A 600), crack-free chromium (Cr D), electroleas nickel (EN), 400 “C heattreated electroless nickel (EN 400) and 600 “C heat-treated electroless nickel (EN 600) in the reciprocating diamond scratch test.

counterface gives considerably lower friction coefficients than the other counterfaces. The crack-free chromium coating (Cr ID) exhibited the highest friction coefficient against diamond, while the highest value of all was obtained between asdeposited electroless nickel and stainless steel.

129

0

1000

2000

4000

3000 NUMBER

OF

CYCLES

Fig. 4. Wear of conventional hard chromium plating grades (Cr A, Cr B, Cr C), electroless nickel (EN), 400 “C heat-treated electroless nickel (EN 400), 600 “C heat-treated electroless nickel (EN 600), Watts electroplated nickel (EP-W) and sulphamate electroplated nickel (EP-S) in the Falex test.

40

CrA l

I, 0

10000

20000

30000 NUMBER

40000

OF

50000

CYCLES

Fig. 6. Wear of conventional chromium plating (Cr A), electroless nickel (EN) and 400 “C heat-treated electroless nickel (EN 400) in the pin-on-flat test.

0

1000 ~000 3000 4000 5000 6000 7000 0000 9000 loo00 lumberof Cycles

Fig. 6. Wear of conventional chromium plating grades (Cr A, Cr B, Cr C), electroless nickel (EN), 400 “C heat-treated electroless nickel (EN 400), 600 Oc heat-treated electroless nickel (EN 600) aud the uncoated steel substrate {Fe) in the Taber Abraser. TABLE 2 Coefficients of friction Coating

Coefficient

0 f friction

Cow terface diamond

Counterface plain carbon steel (OSOM40)

Counterface stainless 8 tee1 (302)

CrA cr A600 Cr B CrC Cr D

0.040 0.035 0.035 0.030 0.500

0.88 0.82 0.81 -

0.68 -

EN EN400

0.180 0.300

0.96 0.95

1.20 -

EN600

0.060

0.90

5. Discussion 5.1.Hardness Table 1 shows markedly large variations in hardness within and between the different types of coating. The chromium coatings in the as-deposited

131 TABLE 3 Properties of bulk chromium

and nickel [ 1 -4 ]

Material

Fracture Tensile Yield Density Young’s strain strength strength (Mg mw3) modulus (GN me2) (MN mw2) (MN mm2)

Hardness

7.2 8.9

180 90

Melting point

("C)

Latent heat of fusion

(HV 0.1)

(kJmol_') cr Ni

1660 1455

20.9 17.7

289 214

100

415 350

0.40 0.60

state have hardnesses between 560 and 1100 HV 0.1 depending upon the deposition parameters, while heat treatment reduces the hardness from 1100 to 430 WV 0.1. Bulk chromium has quite different properties as shown in Table 3: high purity chromium produced by melting has hardness values of 120 - 230 HV 0.1 and tensile elongations up to 40% [ 3 - 61. Transmission electron microscope observations [7,8] on as-deposited chromium coatings revealed body-centred cubic crystalline structures, high dislocation densities with dislocation cell sizes of the order of 0.1 pm and the absence of fine precipitates. Heat treatment at 800 “C for 1 h reduced the hardness of chromium plating to 350 HV 0.1 and gave a fine equiaxed grain structure virtually free from dislocations [7], which indicates that softening is due to rec~st~lization. The high hardness of the as-deposited chromium plating is attributed primarily to its small grain size and high dislocation density, which raise the flow stress by obstructing the movement of dislocations. The nickel coatings were also found to be much harder than bulk nickel (Tables 1 and 2). The electroplated coatings gave hardness values of 275 360 HV 0.1 compared with 90 HV 0.1 for bulk nickel. In comparison, 70% cold work raises the hardness of bulk nickel from 90 to 240 HV 0.1 [6]. Transmission electron microscopy observations on electroplated nickel 19, 103 deposited under similar conditions as Ni A revealed a face-centred cubic st~cture with a low dislocation density, substantial twinning and a grain size of approx~ately 1 ym. The higher hardness of the ele~tropla~d coatings over bulk nickel are attributed to their fine grain structures and high twin densities. Electroless nickel in the asdeposited state is roughly double the hardness of electroplated nickel (Table 1). X-ray diffraction on the asdeposited electroless nickel produced a broad single peak (Fig. 7) in agreement with the published data [ 111 and indicative of an amorphous structure. There is no periodic lattice in amorphous structures and so deformation cannot occur by dislocation mechanisms. Amorphous metals have been shown to deform plastically by the flow of a thin fluid layer in the plane of shear with the material outside this shear band only deforming elastically [ 12 - 141. Shear banding is a less energetically favourable mode of deformation than dislocation motion and the high hardness of the amorphous elec-

132

Fig. 7. X-ray diffraction traces for as-deposited and heat-treated electroless nickel.

troless nickel relative to the crystalline electroplated nickel is a reflection of the high stress required to initiate shear banding. Electroless nickel doubles in hardness on heat treatment and then softens back to its original level (Fig. 2). This is consistent with the data in the published literature [ 15 - 18 1. X-ray diffraction data show that heat treatment induces the decomposition of the amorphous structure to facecentred cubic nickel and tetragonal nickel phosphide (NiJP) (Fig. 7). Hardening on heat treatment is due to the precipitation of the hard nickel phosphide particles and subsequent softening to coarsening of the nickel phosphide particles, which ahows dislocation movement in the more ductile nickel matrix to take place at lower stresses [ 191.

133

It has been claimed that electroless nickel has a higher hot hardness than chromium plating and this is apparently confirmed by Fig. 2. However, both coatings are met&able and will gradually transform under thermal activation to the state of minimum free energy, which in both cases corresponds to a soft condition. The hardness advantage of electroless nickel over chromium will, therefore, only persist for at most the order of hours at elevated temperatures. 5.2. Wear Although the intrinsic properties of the materials (e.g. hardness, ductility) will change in the surface layers during wear, it is assumed that steady state conditions will be reached and that the initial (unworn) properties rather than the instantaneous properties will generally be adequate for a qualitative interpretation of wear behaviour. The results show that not only are there major differences in the relative wear rates between the coatings but also the ranking order of coatings can change between wear tests. The discussion centres on relative wear rates since the absolute rates clearly depend upon the mechanics and counter-face (which differ from test to test) in addition to the material properties. 5.2.1. Diamond coun terface Wear in the reciprocating diamond scratch test is quantified by the depth of the scratch groove, which is produced by two mechanisms: plastic deformation and fracture. The scratch test gave relatively low wear rates for the electroless nickel and conventional chromium coatings (EN, Cr A, Cr A 600) but high wear rates for the heat-treated electroless nickel and crack-free chromium coatings (EN400, EN600, Cr D) as shown in Fig. 3. Scanning electron microscope observations on the wear tracks of the low wear coatings show smearing of the material along the sliding direction, plastic flow over the edge of the wear track and ductile tearing (Figs. 8(a) and 9(a)). This indicates appreciable ductility and that the necessary material displacement in the wear track has taken place predominantly by plastic deformation. Microstructural examination of the high wear coatings revealed significantly different behaviour. The material at the base of the wear track showed thin sheets of material slightly lifted up from the surface (Figs. 8(b) and 9(b)) indicative of delamination fracture due to the propagation of subsurface cracks parallel to the sliding surface [20]. The material at the edge of the wear track experiences higher deviatoric stresses and exhibits more brittle behaviour as evidenced by large scale chipping (Fig. 9(b)). The material displacement required for groove formation in these high wear coatings therefore relies much more on fracture mechanisms. Fracture processes generally remove substantially more material than plastic deformation during wear. For instance, Moore and King [21] estimate that fracture can cause an order of magnitude more wear than plastic deformation under abrasion. The data in the current paper are clearly

Fig. 8. Scanning electron micrographs of wear tracks from the reciprocating diamond scratch test on (a) conventionai chromium plating (Cr A) and (b) crack-free chromium plating (Cr D). The sliding direction is horizontal.

tb)

(4

Fig. 9, Scanning electron micrographs of wear tracks from the reciprocating diamond scratch test on (a) as-deposited electroless nickel coating and (b) heat-treated electroless nickel coating. The sliding direction is vertical in (a) and horizontal in (b).

consistent with this association between the incidence of fracture and relatively high wear rates. Low and high wear behaviour can occur in the same type of material depending upon the processing conditions (Fig. 3), implying competition between deformation and fracture processes. The parameters governing the transition between ductile and qu~i-bottle behaviour will exert a major influence on wear. Lawn and Marshall [22] have shown from indentation fracture mechanics that the critical indentation size o, at which cracking first occurs is given by a,=q-

Kc2 H

where R, is the fracture toughness, H is the hardness and n is a constant.

(1)

135

The minimum size for fracture decreases as the fracture toughness decreases, the hardness increases and the sharpness of the indenter increases. a, represents the threshold size above which the mechanical behaviour is dominated by fracture and below which it is dominated by plastic deformation. The principles of static indentation also apply qualitatively to sliding indenters, although fracture occurs at reduced loads [23]. The idea of a critical scratch width is thus useful in the interpretation of scratch test data. On the basis of eqn. (l), Lawn and Marshall [22] have proposed that the ratio of hardness to fracture toughness, H/K,, be used as an index of brittleness. Values of K, are not available for the coatings but there are data on the ductility of electroless nickel coatings in terms of the strain et at which cracking is first observed on a coated tensile specimen [ 151. The ratio H/ef may be taken as an indication of brittleness since ef and K, are related (e.g. a high value of ef gives appreciable plasticity and energy absorption at the crack tip resulting in a high K, value). Table 4 shows that H/et gives the same ranking order as wear rate, which suggests that this ratio may indicate the transition between the two modes of material removal. TABLE 4 Properties and scratch wear rates of the coatings Coating

Hardness, H (GN rn”)

CrA Cr A 600 Cr D EN EN 400 EN 600

10.8 4.6 5.5 6.2 11.0 6.2

Fracture strain,

Friction coefficient,

ef (%I

P

2.1 0.25 1.75

0.040 0.035 0.500 0.18 0.30 0.06

H

(1 + 9$)1’2

Average wear rate (pm cycle-l)

1.01 1.00 1.80 1.14 1.35 1.02

0.01 0.02 0.71 0.14 0.46 0.33

Ef

2.9 44.0 3.5

The friction coefficients of the materials tested vary by over an order of magnitude and need to be considered in the interpretation of the wear data. The first-order theory of wear for ductile materials [24] gives the wear rate dV/dl as dV -= dl

k?;

where V is the wear volume, 1 is the sliding distance, k is the wear coefficient, W is the load and H is the hardness of the softer material. The analyses of Bowden and Tabor [25] and Rowe [26] incorporate the effect of friction by assuming that plastic flow takes place as a result of the combined action of the normal applied load and the tangential frictional force, which effectively leads to the use of a scratch hardness H, rather than the conventional static hardness H

(3) where /.Lis the coefficient of friction. Substituting iY, for H in eqn. (2) gives the wear rate as dV W = k--(1 dl

+ gpy

Equation (4) is unlikely to be adequate for the more brittle materials for which an upper limit [27] on the wear rate is given by dV

-=

dl

w5/4 h

K

c

314~112

where X is a constant. Frictional effects can be incorporated in eqn. (5) by similar means as in eqn. (4). In practice, material removal takes place by a combination of plastic deformation and fracture so that eqns. (4) and (5) represent the lower and upper limits of wear rate respectively. Table 4 shows that the term (1 + 9112) 1’2 has the highest values for the crack-free chromium (Cr D) and the 400 “C heat-treated electroless nickel (EN400), suggesting that their high friction coefficients contribute significantly to their high wear rates (Fig. 3). The low wear of the heat-treated chromium (Cr A 600) compared with the harder electroless nickel (EN) is attributed to its lower friction coefficient and its high strain-hardening rate. The much higher hardness of Cr A relative to Cr A 600 is mainly due to strengthening by dislocations [ 81, which implies that Cr A 600 will strain harden considerably during wear. The high strain-hardening rate will benefit wear resistance by raising the hardness and distributing the plastic deformation over a large volume, which delays plastic instability and the localization of strain [ 281. Crack-free chromium, Cr D, behaves quite differently from the other chromium deposits and has the highest wear rate of all the coatings tested. Its uniquely high friction coefficient (Table 2) and possibly its crystal structure will contribute to the poor wear resistance. Crack-free chromium has a predominantly hexagonal close-packed crystal structure [29] whereas conventional chromium is body-centred cubic. Unlike b.c.c. structures, h.c.p. metals tend to slip on only one family of slip planes, those parallel to the basal plane, which results in less dislocation interactions, larger strains at a given stress level and a low strain-hardening rate. The low strain-hardening rate will give rapid localization of deformation [28], early fracture and an increased wear rate. 5.2.2. Plain carbon steel counterface The electroplated chromium, electroless nickel and electroplated nickel were worn against plain carbon steel (08OM40 grade) in the Falex and pin-on-flat tests. The as-deposited electroless nickel and electroplated nickel coatings exhibited severe wear, as shown by high wear rates, rough

137

and heavily damaged surfaces (Figs. 4 and 5), which is due to the high chemical ~ompatib~ty of their surfaces with plain carbon steel giving rise to adhesive wear [19]. The low hardness values of the electroplated nickel coatings contribute towards their particularly high wear rates. Heat treatment effects a transition from the severe to the mild wear regime as shown by low wear rates, smooth and polished surfaces. It changes the structure of electroless nickel from amorphous nickel to predominantly nickel phosphide, which presents an ~~ompatible surface to plain carbon steel and suppresses adhesive transfer [ 191. The Falex and pm-on-flat tests placed electroless nickel in a different ranking order to that given by the diamond scratch test: heat treatment reduced the wear rate of electroless nickel in former tests but increased it in the scratch test as summarized in Table 5. This is due to the fact that the dominant wear mech~ism changes from adhesive transfer to abrasive wear

TABLE 5 Effect of test method on relative we= rate Test method

Relative CrA

(1) Reciprocating scratch (2) Falex (3) Pin-on-flat (4) Taber

diamond

600

wear ratea CrD

2

71

-

-

EN 14 165 38 5.0

EN 400

EN 600

46

33

32 6.2 4.1

19 3.3

*Relative wear rate equals wear rate of coating under specificed test divided by wear rate of conventional chromium plating under 8ame test. Chromium plating is used a8 the standard because it8 ranking order in term8 of wear amongst the other coating8 doe8 not change with the test method.

Chromium plating showed much lower wear rates than even the heattreated electroless nickel coatings and in some cases showed slight increases in mass owing to adhesive transfer of steel. The low wear rates and low friction coefficients of the chromium coatings are attributed to the thin passivating, self-healing oxide layer, CrZ03, on their surfaces, which efficiently covers the underlying metal. The Cr203 film acts as a barrier layer preventing contact between the two metals and suppressing adhesive transfer by presenting an incompatible surface to the plain carbon steel counterface. 5.2.3. Stainless steel cow terface Previous work has shown that as-deposited electroless nickel is particularly prone to severe wear against stainless steel [ 1,301. The results given in

CYCLES

Fig. IQ. Wear of conventional chromium plating (CT A) and as-deposited electroless nickel (EN) pins against stainless steel (SS), plain carbon steel (Fe) and conventional chromium-plated (Cr A) blocks in the Falex test. The first material quoted in the diagram refers to the pin and the seeortd, to the black. 10 ahow that chromium plating is a useful wear~resistantsurface to run against stainless steel. The low wear rate of this material couple is accompanied by a low friction coefficient (Table 2), which suggests that the underlying cause of the favourabfe performwee is the Cr$& p~~at~g film on the chromium surface. Fig.

5.2.4. Silicon carbide coun terfke The counterface in the Taber test consists of fine angular particles of silicon carbide. The WCISX data in Fig. 6 show that heat treatment reduces

139

the wear rate of electroless nickel. This is unexpected since it is contrary to the behaviour in the diamond scratch test (Table 5) and there are clear similarities between the mechanical action of the silicon carbide abrasive particles in the Taber test and that of the diamond in the scratch test. Hardness does not play a major role since the 600 “C heat-treated electroless nickel has a lower wear rate but almost half the hardness of the 400 “C heat-treated electroless nickel. The chromium coatings have considerably lower wear rates than the 400 “C heat-treated electroless nickel but again have lower hardness values. The first-order theory of wear based on hardness (eqn. (2)) does not therefore hold for the wear rates of these materials in the Taber test. Consideration of the relative brittleness of the materials in terms of H/ef (Table 4) also does not provide an explanation, e.g. the brittle EN 400 is more wear resistant than the ductile EN. Scanning electron microscopy examination of the surfaces of the worn Taber samples showed numerous fine grooves, each of which is produced by an individual silicon carbide particle. There was no evidence of brittle fracture at the grooves as there was found after wear with the scratch test (Fig. 9(b)). The width of the grooves is approximately 1 pm as compared with groove widths of about 200 pm produced in the diamond scratch test. The results imply that the grooves generated by the Taber test are well below the critical width for fracture (Section 5.2.1) whereas they are above it for certain materials in the scratch test. Wear in the Taber test therefore involves plastic deformation for ah samples and may be considered to be broadly similar to a polishing operation. Nevertheless, the relative wear rates do not follow the simple theory of wear based on eqn. (2) and so other factors must also apply. The ranking order in the Taber test is the same as that in the Falex and pin-on-flat tests, which suggests that adhesive transfer is the controlling mechanism. The extent of adhesive transfer will be promoted by the plastic deformation occurring in the Taber test, since plastic flow causes an increase in the true contact area and junction growth. The low wear rates of chromium and heat-treated electroless nickel are thus attributed to the fact that their respective surfaces (Crz03 and predominantly Ni$’ respectively) are incompatible with silicon carbide [ 191. 5.2.5. Wear rate transitions The data show that transitions between severe and mild wear occur in electroless nickel and chromium owing to changes in phase, brittleness or scratch indentation size effects. Heat treatment effects a transition from the severe to mild wear regime by crystallizing the amorphous electroless nickel to predominantly Ni,P, variation of the deposition conditions alters the crystal structure of chromium from h.c.p. to b.c.c., and reduction of the scratch width changes the dominant material removal mechanism from fracture to plastic deformation. Transitions between mild and severe wear as a result of changes in load and sliding velocity have been widely observed in steels and are generally

140

related to the ~owth-~pture of surface oxide films or the formation of martensitic surface layers [ 31,321. A fuller understanding of the current coatings may possibly be achieved by determining wear as a function of load and velocity. However, the wear transitions in the coatings are due to changes in their bulk structure rather than alterations in only the surface layers (e.g. heat treating electroless nickel in air or argon did not significantly affect the wear properties) and their wear performance may consequently be less sensitive to load and velocity than that of steel.

6. En~ee~~

~plications

An important feature of the work is that the relative wear rates of materials depend upon the type of wear test method used, as summarized in Table 5. It is primarily because the dominant material removal mechanism can change from test to test. This serves to emphasize the importance of wear diagnosis in material selection and design, in which an essential initial stage is the examination of the worn components in a particular application in order to identify the predominant wear mechanism. A range of materials is then tested on laboratory apparatus chosen to reveal the relevant wear mechanism and on the basis of the results, a provisional material combination is selected for prototype testing and in-service trials. The laboratory work serves as compatib~ty tests in which the number of material combinations can be reduced to manageable proportions and as the initial stage in the optimization of the use of existing materials and the development of new materials. The hardness and wear resistance of electroplated and electroless coatings are often assumed to be correlated. In particular, heat-treated electroless nickel is frequently taken to have at least the same wear resistance as chromium plating because of its higher hardness. The results in this paper indicate that this assumption is incorrect: electroless nickel has a substantially inferior wear resistance to chromium plating under all the conditions tested. Heat-treaty electroless nickel is, nevertheless, a moderately wearresistant coating that provides appreciably higher wear resistance than electroplated nickel and may be adequate for many engineering applications. Heat treatment of electroless nickel produces a major increase in wear ressistance in applications requiring protection against adhesive wear, such as in metal-to-metal contact. However, heat treatment results in a deterioration in the performance of electroless nickel in applications requiring flexibility and ductility. The wear of electroless and electroplated coatings is very dependent upon the material used as a counter-face. Stainless steel is a particularly problematic material, even against heat-treated electroless nickel [ 301. The work has shown, however, that the severe wear of stai&ss steel can be eliminated by electroplating its counter-face with chromium. Elec~opla~d and electroless coatings have different attributes and should be regarded as complement rather than competitive materials,

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Chromium has advantages in terms of wear resistance, wettability to lubricating oils [ 331 and a moderately low unit cost for simply shaped parts. Chromium plating is, for example, likely to be the preferred coating for simply shaped parts in lubricated systems where a high wear resistance, but not a high corrosion resistance, is needed. Electroless nickel provides uniform coverage for intricately shaped parts, corrosion resistance or moderate wear resistance, but generally incurs higher chemical costs than chromium. Electroplated nickel offers rapid deposition rates, very thick coatings, corrosion resistance and low unit costs (usually lower than chromium), and is particularly useful as an undercoat for chromium in corrosive conditions and for building up worn or ove~ach~~ parts.

7. Conclusions (1) Chromium plating, electroless and electroplated nickel coatings show large differences in hardness and wear rate, but no correlation between these two properties is observed. (2) Conventional chromium plating has appreciably lower wear rates than the other coatings in all the tests. In particular, its wear rate is much lower than heat-treated electroless nickel despite its lower hardness. The principal reason for the high wear resistance of chromium plating is the effectiveness of its surface oxide film as a barrier layer in suppressing adhesive transfer. (3) The wear rate of electroless nickel in the reciprocating diamond scratch test is dependent upon the brittleness of the material under test as expressed by the ratio of hardness to fracture toughness. (4) The dominant mechanism in the Falex, pin-on-flat and Taber tests is adhesive transfer and the difference in wear mechanism between the latter tests and the reciprocating scratch test alters the ranking order of the coatings: heat treatment increases the wear rate of electroless nickel in the scratch test but reduces it in the other three tests. (5) Crack-free chromium plating behaves markedly differently than conventional electroplated chromium and has the highest wear rate of all the coatings tested. (6) The severe wear of stainless steel can be avoided by electroplating its counterface with chromium.

Acknowledgment The work was sponsored authors would like to thank this paper.

by M & T Chemicals-Elf Aquitaine and the the movement for permission to publish

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