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Well-founded selection of materials for improved wear resistance
WEAR ELSEVIER
Wear 194 (1996) 238-245
Case Study
Well-founded
selection of materials for improved wear resistance A. Fischer NUTECHGtnbH. Ilscrhl5. 24536 Neurnuensrer, Germuny Received 28 April 1994; accepted
11 July 1995
Abstract Wear-resistant bulk or layer materials have to be chosen by designers, but tribological stresses are complex and the properties of materials cannot be taken from a list. Thus, a well-founded selection requires a strategy. which is suitable for al1 engineers in production, design, and research. Such a strategy is presented in this paper. It makes use of the acting wear mechanisms as a link to materials behaviour. This strategy has been achieved during the development of new wear-resistant materials. It is successfully used in consulting researchers, designers, and production engineers as wel1 as for the analysis of failures. Kqvwords: Wear resistance; Materials selection; Wear mechanisms
1. Introduction
Machine parts and tools, which are subjected to tribological stresses, should be designed to achieve constant wear rate for their entire life time after a short running-in period. In order to reduce costs appropriate bulk and layer materials should be chosen during development and design. In tribology the problem arises that beneficial properties of materials are not governed solely by the material but by the structure of the entire tribological system [ 11. This tribosystem consists of four elements: body, counter body, interfacial medium, and surrounding medium [ 21 (Fig. 1) . They undergo certain tribological stresses (load, speed, temperature, time), while the type of interaction between body and counterbody may be sliding, rolling, impact, or flowing. Thus, the structure of a tribosystem and the characteristics of interaction define the
Tribological
Stresses Surrounding
Medium
type of wear, which is identified by a certain combination of acting wear mechanisms [ 1,2] (Fig. 2). Four major wear mechanisms can be separated: abrasion, surface fatigue, adhesion, and tribochemical reactions. Abrasion and surface fatigue are dominated by mechanica1 interactions, while adhesion and tribochemical reactions are governed by additional chemical effects [ 31. The knowledge of the acting wear mechanisms is essential for a well-founded selection of materials. Under sliding wear for example adhesion or surface fatigue might be predominant. But, properties of materials against adhesion differ distinctly from those against surface fatigue. A well-founded selection of the appropriate materials will, therefore, not be possible just by knowing the type of wear. In this paper the abrasive wear behaviour of metallic materials under both two-body abrasion [4-61 and three-body abrasion (sliding abrasion) [ 7-111 is shown and discussed. The similarity of these types of wear often brings about the selection of similar materials. But, considering the acting wear mechanisms, the criteria for the selection of materials are different. Afterwards a general strategy for the wellfounded selection of materials for improved wear resistance is presented. 2. Experimental
procedures
2.1. Materials tested wear
appearances
loss of material
Fig. 1. Basic structure of a tribological
system.
0043-1648/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO43-1648(95)06738-8
Wear-resistant metallic materials usually contain hard phases such as carbides, borides, or nitrides. In this investi-
A. Fischer / Wear 194 (1996) 238-245
239
Salod - (luid with pW
I
r SOM - c*
bvith pr
SOM -
Fig. 2. Classification
tluid
of types of wear with respect to the structure of tribosystems,
the type of tribological
action, and the acting wear mechanisms.
Table 1 Groups of materials tested Base metal
Fe
Ni
Co
Other metals
Metal matrix with precipitations ( < 1 pm) (MM)
50CrV4 X2Cr 11 X2CrNiMo 18 10 X7OCrNi23 2 X155CrVMo12 1 56NiCrMoV7 + CrB,
Ni NiCROAlTi
CoCr25Ni9Mo
Al, Cu
_
CoCr27Mo CoCr29W CoCrlO+CB,
_
Metal matrix withfine hard phases (5-15 Metal matrix withcoarse
km) (FHP)
hard phases ( > 30 km) CHP
NiCRO + CrBZ
tates smaller than 1 km. They do not influence the wear behaviour, although ihey change hardness. The fine hard phases (FHP) are eutectic carbides of M,C,-type in the tast iron X70CrNiMo23 2, the cold-work tool steel X155CrVMo12 1, and the Co-base alloy CoCr29W, while those of M,C-type are found in CoCr27Mo. CHP materials consist of hipped powders of 56NiCrMoV7, NiCr20, and CoCrlO mixed with 5 and 10 vol.% CrB,. During hipping a rim of borides and carbides grows around CrB, generating coarse hard phases of 70-120 Frn diameter [ 121 (Fig. 3).
56NiCrMoV
7 < 100 urn, CrB2 < 100Ym
Fig. 3. Microstructure
gation different
materials
of hipped 56NiCrMoV7+
with and without
2.2. Wear tests
IS%CrB,.
hard phases
were
chosen for wear testing. With respect to the hard phases there are three groups of interest. 1. Metal matrix with or without precipitates ( < 1 Pm) designated as MM. 2. Metal matrix with fine (5-15 Pm) hard phases (FHP). 3. Metal matrix with coarse ( > 30 cm) hard phases (CHP) . These are divided into nine groups as to their appearance in a light microscope and as to their base materials (Fe, Ni, Co) (Table 1) . Standard heat treatments were used in order to change the properties of the metal matrix; they are specified as follows: annealed (G) , hardened (H), hardened and tempered (H + A), solution annealed (L). The MM alloys 50CrV4, NiCr20AlTi, and CoCr25Ni9Mo contain precipi-
The tribological systems which were chosen for the laboratory wear tests are described in Table 2. In order to determine the wear rate the weight loss of the specimens (pin AG,, disc AG,, ring AG,) was measured and related to the density (p) of the material, the nomina1 areas of contact A, and the length of the wear path L. This brings about the dimensionless wear rate for two-body abrasive wear W,, w
2tl
(1)
=’ PA& and for three-body
abrasive wear (sliding
abrasion)
W,,.
(2)
240
Table 2 Parameters
A. Fischer / Wear 194 (1996) 238-245
of laboratory
wear tests
Structure of tribosystem
Pin-on-disc
Type of wear
Abrasive wear (2.body
Elements of tribosystem Body Counter body Interfacial medium Average grain size of Aint particles Surrounding medium Tribological Nomina1 Nomina1 Relative Ambient
Ring-on-disc abrasion)
Pin Fhnt grinding paper _ (km)
stresses area of contact (mm’) contact pressure ( MPa) velocity (mm s- i) temperature (“C)
Sliding abrasion
(3-body abrasion)
220 Air
Disc Ring Flint abrasive particles, wear debris 80 Air
28.27 1.32 6 2s
207 0.82 28.3 25
Table 3 Hardness prior to testing of the Fe-base alloys Material German (US)
Hardness
Wear rates X IOmh
Heat treatment
HVx,
SOCrV4 (AIS1 6150) X70CrNiMo23 2 Xl55CrVMo 12 1 (AIS1 D2) X2Crll (-AIS1410S) XZCrNiMol8 10 (-AIS1
316L)
270 680 280 190 620 160 170
W,b
WS,
57 53 21 46 6.4 57 40
5.6 0.39 0.27 1.4 0.22 6.5 1.9
G H As tast G H+A L L
Table 4 Hardness prior to testing of the pure metals and the Ni- and Co-base alloys Material German (US)
Hardness
Wear rateX IO-”
Heat treatment
HV,, W,,
Pure metals Cu Al Ni-base alloys Ni NiCROAITi (Nimonic 80A ) Co-base alloys CoCr25Ni9Mo (Ultimet 1233) CoCr27Mo (Stekte 21) CoCr29W (Stellite 6)
130 100
146 215
120 280
69 47
1.9 2.9
L
320 330 430
44 40 25
0.47 0.52 0.31
L As tast As tast
wear rates given in Tables 3-5 are average values of three (two-body) to five (three-body) measurements. The standard deviation is less than 5%.
The
2.3. Microscopic
analysis
The wear appearances were investigated on the worn surfaces as wel1 as in the subsurface regions using a scanning electron microscope (SEM) with an attached energy-dispersive X-ray (EDX) analyzer. In order to remove the loose
15 42
wear debris al1 specimens were cleaned ultrasonic cleaning process.
intensively
by an
3. Results and discussion The measured wear rates are listed in Tables 3-5. The results of wear tests are not discussed in detail here, they are published elsewhere [ 12-151. It should be noticed that flint particles (900 HV,.,,) are harder than al1 metal matrices ( 10&700 HVo,os) and softer than al1 hard phases ( 1 200-
A. Fischer / Weur 194 (1996) 238-245
241
Table 5 Hardness prior to testing, wear rates, and heat treatment of hiped Fe-, Ni- and Co-base materials Material German (US)
Fe-base 56NiCrMoV7
Hardness
Wear rates X IO-”
Heat treatment
HV,, W,,
WS,
+
0% CrBZ 5% CrBZ 10% CrB2 15% CrB2 20% CrB, Ni-base NiCRO +
660 640
38 8.8 4.4 2.9 2.8
0.44 0.2 1 0.19 0.15 0.14
H+A
5% CrBZ 10% CrB, Co-base CoCr 10 +
240 280
49 20
3.4 0.28
L
330 390
52 14
0.74 0.29
L
0% CrBZ 10% CrBZ
-t
t ----1-1
200
300
Hardness
400
500
600
700
HV30 urior to Testine
Fig. 4. Abrasive wear rates of materials tested.
56NiCrMoV7
+ 15 vol-%
hard phase
Cr62
Fig. 5. Abrasive wear; Bint (900 HVa,,) wears the softer metal matrix (640 HVO,OS).while the hard phases ( > 1700 HV,,OS) cannot be scratched.
2 400 HV,,,,) . With respect to the aim of this paper a comparison of the wear behaviour under these well-known wear tests wil1 be drawn focusing on the acting mechanisms. After-
wards criteria for a well-founded selection of materials wil1 be introduced and discussed. Under two-body abrasion fixed Flint particles scratch the surfaces and abrasion is the only acting wear mechanism. The wear behaviour is governed by its submechanisms [ 2,161. For soft materials such as pure Cu, Ni or Al microploughing is most likely, whereas with increasing hardness the depth of the scratch decreases and microcutting prevails [ 17,181. The work hardening capability of the metal matrix has another strong influence on the wear behaviour. Obviously, there is no direct correlation between hardness and wear rate (Fig. 4, MM). W,, values of MM materials are reduced by one order of magnitude making use of a sufficient combination of hardness, toughness and work hardening capability. The hard phases that produce a further decrease of the wear rate are shown in Fig. 5. Their effectiveness depends on the volume fraction, size, shape, and distribution and on the capability of the metal matrix to support them. Thus, an adequate volume
242
A. Fischer/
200
Weur 194 (1996) 238-245
300
400
500
600
Hardness HV30 prior to Testing Fig. 6. Sliding abrasian wear rates of materials tested
Flint.80
vm, 250 C, Air
680
t
Fig. 7. Sliding abrasion; soft metal matrices (270 HVYo) are worn by abrasion (opper micrograph). hard ones (670 HV,,,) by indentation (lower micrograph).
two orders of magnitude just for the metal matrices, less for the others. This is due to the fact that adhesion might appear with soft materials giving rise to high wear rates [ 1.51. In addition the wear mechanism changes with increasing hardness from abrasion (Fig. 7, upper micrograph, and Fig. 8) by abrasive particles, which are embedded within the surface of the opposite body, to indentation by rolling ones (Fig. 7, lower micrograph, and Fig. 8) [ 15,221. Abrasion is not observed with materials containing hard phases. The metal matrices are only removed by indentation (Fig. 9). The hard phases loose their support and are torn off the surfaces by cracking. Thus, the wear rate is governed by the volume fraction of al1 constituents that are harder than Flint. Indentation, which has a low efficiency of removing material from surfaces 1231, is the only acting mechanism and the wear rates of FHP and CHP materials differ by a factor of only about three. Fig. 10 shows the wear rates of both tests plotted versus each other. Obviously, the influence of microstructure on the tribological behaviour strongly depends on the type of wear. Hence, the selection of appropriate materials with low wear rate provokes different results, even though both types of wear seem to be similar. Under two-body condition abrasion is the only mechanism acting. One can achieve the lowest wear rates by selecting materials with a high ( > 30%) volAbrasion
fraction of coarse hard phases renders the lowest wear rates (Fig. 4, CHP). Microcracking is not observed and, hence, a detrimental effect of coarse hard phases on the wearresistance is not found. This is also reported for other tribosystems [ 16.19-2 1] Within this tribosystem the wear rate is reduced by two orders of magnitude over the range of metal matrices from pure Ni to hardened 56NiCrMoV7 and additional 20 vol.% coarse hard phases of CrB? type. Under three-body abrasion (sliding abrasion) the ranking of these materials is similar (Fig. 6). but, W,, ranges over
Indentation
abrasive particle Fig. 8. Schematic model of wear mechanisms body abrasion.)
under sliding abrasion (three-
A. Fischer / Wear 194 (1996) 238-245
243
might cause microcracking, which has a detrimental effect on the wear behaviour. It is important to notice that materials with fine hard phases have better production (casting, forging) and handling properties (heat treatment, machining, grinding) compared with those with coarse hard phases. This reduces the costs of a product, markedly.
4. Basic principle of the well-founded selection of materials As shown in the previous section the use of acting wear mechanisms as a link to the wear behaviour brings about certain criteria for appropriate materials to be chosen against wear. This strategy should not only be used after wear testing, but earlier in the state of development and design of a new product. In both cases the procedures are similar and should be presented in succession. 4.1. Development RT, Air, Flint, 80 urn, 0.83 MPa. 28 mm/s Fig. 9. Sliding abrasion; the metal matrix is removed by indentation coarse hard phases protrude from the surface.
and
urne fraction of coarse hard phases, which are embedded in a hard (700 HV,,,,) metal matrix. This is correct provided microcracking does not appear. These materials can be produced by casting, hardfacing welding or powder metallurgy. But due to their distinct brittleness the handling properties like for heat treatment, machining etc. are detrimental. The mechanisms acting under the three-body condition are adhesion, abrasion, and indentation, while the latter brings about the lowest wear rates. Consequently, microstructures are chosen, which are prone to wear by indentation. Adequate materials have a medium ( > 550 HV,,,,) hardness and volume ( > 12%) fraction of fine hard phases. Coarse hard phases
Step 1. Analysis of the structure of the tribosystem, of its stresses, and of the type of interaction between the elements. Step 2. Determination of the type of wear. Assumption of acting wear mechanisms (Fig. 2). Step 3. Selection of appropriate materials (bulk, layer, deposit) . If there is no material with the desired properties, the design has to be changed as to shape, stresses, or surface characteristics. The selection can be carried out by carefully using existing models. But, although there are many models published, most of them are only valid for some specific tribological system. Unfortunately, only a few authors define their tribosystems exactly and even fewer show the acting wear mechanisms. In addition these models do not consider the important effect of third bodies, which are likely to govern many technical tribosystems [ 24-3 11. Most models make use of bulk proper-
10
100
Wear Rate Wab x Fig. 10. Comparison
of abrasive wear (two-body
and design
abrasion)
1000
lö6
and sliding abrasion
(three-body
abrasion)
244
A. Fischer / Wear 194
Table 6 Correlation
between wear appearances
and wear mechanisms
Wear mechanism
Wear appearances
Fig.
Abrasion Surface fatigue Adhesion
Grooves, scratches, scores, striations Cracks, pits Scuffs, holes, bumps, flakes, material transfer Layers, particles
5 12
Tribochemical
reactions
wear appearance: wear mechanism:
11
flakes, pits
adhesion
(1996)238-245 at the wear appearances, which are attributed to a certain mechanism, (Table 6, Fig. 11 and Fig. 12) with some limitations. Several wear mechanisms might act at the same time and their interaction has to be taken into account. Submechanisms such as microploughing, microcutting, and microcracking should also be included. They might require conflicting preventive measures. Step 6. Comparison between assumed and examined wear mechanisms. If the assumed wear mechanisms are the same as those seen on the worn surface and if the wear rate is acceptable, the material has been properly selected. 4.2. Failure in production
sourrounding medium: oil counter body: 42CrMo4,52
wear appearance: wear mechanism:
or application
Step 1. Analyses of the structure of the tribosystem, of its stresses, and of the type of interaction between its elements. Step 2. Examination of the worn part as to the acting wear mechanisms. Steps 1 and 2 must lead to the same type of wear. Otherwise Step 1 or 2 have not been carried out, correctly. One reason for steps 1 and 2 leading to different types of wear might arise by the failure sequence. Worn parts often show adhesion being the predominant wear mechanism (Fig. 1 l), but, if the starting type of wear is different, e.g. cavitation, surface fatigue is the predominant mechanism (Fig. 12). Special attention must also be paid to failures starting by other mechanica1 or chemical attacks. Step 3. Selection of appropriate materials with respect to: (a) the starting mechanism, and (b) the interaction of mechanisms (see Section 4.1). Step 4. Practica1 Application. It should be noted that the above strategy does not always imply a need to change of material. The design and the stresses should be optimized, too.
HRC
pits
surface fatigue
5. Conclusion surrounding
medium:
42CrMo4,46 HRC
oil
Fig. 12. Cavitation;
appearance
of surface fatigue.
ties instead of those of surface or subsurface areas. Even if surface properties are introduced, neither the influence of high deformation rates nor additional chemical or physical effects are considered. Obviously, we are far away from any quantitative model, which allows US to predict the life time under tribological stresses. This strategy, therefore, yields only a qualitative conclusion. Step 4. Wear test in laboratory or practica1 application. There is no value in wear tests which are just apparently similar to the desired practica1 application. The laboratory wear test must simulate of the acting wear mechanisms. This has to be evaluated in every single case. Thus, practica1 application is stil1 the best wear test. Step 5. Investigation of the worn surface as to wear appearances (Table 6). Determination of acting wear mechanisms. The acting wear mechanisms can be investigated by looking
A strategy for the well-founded selection of materials in order to improve wear resistance is presented. It makes use of the acting wear mechanisms as a link to materials behaviour. This strategy is valuable for the well-founded development of new wear-resistant materials, for consulting researchers, designers, and production engineers as wel1 as for the analysis of failures. As an example the wear behaviour of metallic materials under, the wel1 known, two-body and three-body abrasion is shown. The similarity of these types of wear [ 321 often brings about the selection of similarmaterials. But the analyses of the acting wear mechanisms renders different criteria for the selection of appropriate materials. Acknowledgements
The author would like leading him into the field for his steady willingness the author would like to
to thank Prof. Dr.-Ing. H. Berns for of wear of multiphase materials and for helpful discussions. In addition thank al1 employees of the Institute
Wear 194 (1996) 238-245
Case Study
Well-founded
selection of materials for improved wear resistance A. Fischer NUTECHGtnbH. Ilscrhl5. 24536 Neurnuensrer, Germuny Received 28 April 1994; accepted
11 July 1995
Abstract Wear-resistant bulk or layer materials have to be chosen by designers, but tribological stresses are complex and the properties of materials cannot be taken from a list. Thus, a well-founded selection requires a strategy. which is suitable for al1 engineers in production, design, and research. Such a strategy is presented in this paper. It makes use of the acting wear mechanisms as a link to materials behaviour. This strategy has been achieved during the development of new wear-resistant materials. It is successfully used in consulting researchers, designers, and production engineers as wel1 as for the analysis of failures. Kqvwords: Wear resistance; Materials selection; Wear mechanisms
1. Introduction
Machine parts and tools, which are subjected to tribological stresses, should be designed to achieve constant wear rate for their entire life time after a short running-in period. In order to reduce costs appropriate bulk and layer materials should be chosen during development and design. In tribology the problem arises that beneficial properties of materials are not governed solely by the material but by the structure of the entire tribological system [ 11. This tribosystem consists of four elements: body, counter body, interfacial medium, and surrounding medium [ 21 (Fig. 1) . They undergo certain tribological stresses (load, speed, temperature, time), while the type of interaction between body and counterbody may be sliding, rolling, impact, or flowing. Thus, the structure of a tribosystem and the characteristics of interaction define the
Tribological
Stresses Surrounding
Medium
type of wear, which is identified by a certain combination of acting wear mechanisms [ 1,2] (Fig. 2). Four major wear mechanisms can be separated: abrasion, surface fatigue, adhesion, and tribochemical reactions. Abrasion and surface fatigue are dominated by mechanica1 interactions, while adhesion and tribochemical reactions are governed by additional chemical effects [ 31. The knowledge of the acting wear mechanisms is essential for a well-founded selection of materials. Under sliding wear for example adhesion or surface fatigue might be predominant. But, properties of materials against adhesion differ distinctly from those against surface fatigue. A well-founded selection of the appropriate materials will, therefore, not be possible just by knowing the type of wear. In this paper the abrasive wear behaviour of metallic materials under both two-body abrasion [4-61 and three-body abrasion (sliding abrasion) [ 7-111 is shown and discussed. The similarity of these types of wear often brings about the selection of similar materials. But, considering the acting wear mechanisms, the criteria for the selection of materials are different. Afterwards a general strategy for the wellfounded selection of materials for improved wear resistance is presented. 2. Experimental
procedures
2.1. Materials tested wear
appearances
loss of material
Fig. 1. Basic structure of a tribological
system.
0043-1648/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDIOO43-1648(95)06738-8
Wear-resistant metallic materials usually contain hard phases such as carbides, borides, or nitrides. In this investi-
A. Fischer / Wear 194 (1996) 238-245
239
Salod - (luid with pW
I
r SOM - c*
bvith pr
SOM -
Fig. 2. Classification
tluid
of types of wear with respect to the structure of tribosystems,
the type of tribological
action, and the acting wear mechanisms.
Table 1 Groups of materials tested Base metal
Fe
Ni
Co
Other metals
Metal matrix with precipitations ( < 1 pm) (MM)
50CrV4 X2Cr 11 X2CrNiMo 18 10 X7OCrNi23 2 X155CrVMo12 1 56NiCrMoV7 + CrB,
Ni NiCROAlTi
CoCr25Ni9Mo
Al, Cu
_
CoCr27Mo CoCr29W CoCrlO+CB,
_
Metal matrix withfine hard phases (5-15 Metal matrix withcoarse
km) (FHP)
hard phases ( > 30 km) CHP
NiCRO + CrBZ
tates smaller than 1 km. They do not influence the wear behaviour, although ihey change hardness. The fine hard phases (FHP) are eutectic carbides of M,C,-type in the tast iron X70CrNiMo23 2, the cold-work tool steel X155CrVMo12 1, and the Co-base alloy CoCr29W, while those of M,C-type are found in CoCr27Mo. CHP materials consist of hipped powders of 56NiCrMoV7, NiCr20, and CoCrlO mixed with 5 and 10 vol.% CrB,. During hipping a rim of borides and carbides grows around CrB, generating coarse hard phases of 70-120 Frn diameter [ 121 (Fig. 3).
56NiCrMoV
7 < 100 urn, CrB2 < 100Ym
Fig. 3. Microstructure
gation different
materials
of hipped 56NiCrMoV7+
with and without
2.2. Wear tests
IS%CrB,.
hard phases
were
chosen for wear testing. With respect to the hard phases there are three groups of interest. 1. Metal matrix with or without precipitates ( < 1 Pm) designated as MM. 2. Metal matrix with fine (5-15 Pm) hard phases (FHP). 3. Metal matrix with coarse ( > 30 cm) hard phases (CHP) . These are divided into nine groups as to their appearance in a light microscope and as to their base materials (Fe, Ni, Co) (Table 1) . Standard heat treatments were used in order to change the properties of the metal matrix; they are specified as follows: annealed (G) , hardened (H), hardened and tempered (H + A), solution annealed (L). The MM alloys 50CrV4, NiCr20AlTi, and CoCr25Ni9Mo contain precipi-
The tribological systems which were chosen for the laboratory wear tests are described in Table 2. In order to determine the wear rate the weight loss of the specimens (pin AG,, disc AG,, ring AG,) was measured and related to the density (p) of the material, the nomina1 areas of contact A, and the length of the wear path L. This brings about the dimensionless wear rate for two-body abrasive wear W,, w
2tl
(1)
=’ PA& and for three-body
abrasive wear (sliding
abrasion)
W,,.
(2)
240
Table 2 Parameters
A. Fischer / Wear 194 (1996) 238-245
of laboratory
wear tests
Structure of tribosystem
Pin-on-disc
Type of wear
Abrasive wear (2.body
Elements of tribosystem Body Counter body Interfacial medium Average grain size of Aint particles Surrounding medium Tribological Nomina1 Nomina1 Relative Ambient
Ring-on-disc abrasion)
Pin Fhnt grinding paper _ (km)
stresses area of contact (mm’) contact pressure ( MPa) velocity (mm s- i) temperature (“C)
Sliding abrasion
(3-body abrasion)
220 Air
Disc Ring Flint abrasive particles, wear debris 80 Air
28.27 1.32 6 2s
207 0.82 28.3 25
Table 3 Hardness prior to testing of the Fe-base alloys Material German (US)
Hardness
Wear rates X IOmh
Heat treatment
HVx,
SOCrV4 (AIS1 6150) X70CrNiMo23 2 Xl55CrVMo 12 1 (AIS1 D2) X2Crll (-AIS1410S) XZCrNiMol8 10 (-AIS1
316L)
270 680 280 190 620 160 170
W,b
WS,
57 53 21 46 6.4 57 40
5.6 0.39 0.27 1.4 0.22 6.5 1.9
G H As tast G H+A L L
Table 4 Hardness prior to testing of the pure metals and the Ni- and Co-base alloys Material German (US)
Hardness
Wear rateX IO-”
Heat treatment
HV,, W,,
Pure metals Cu Al Ni-base alloys Ni NiCROAITi (Nimonic 80A ) Co-base alloys CoCr25Ni9Mo (Ultimet 1233) CoCr27Mo (Stekte 21) CoCr29W (Stellite 6)
130 100
146 215
120 280
69 47
1.9 2.9
L
320 330 430
44 40 25
0.47 0.52 0.31
L As tast As tast
wear rates given in Tables 3-5 are average values of three (two-body) to five (three-body) measurements. The standard deviation is less than 5%.
The
2.3. Microscopic
analysis
The wear appearances were investigated on the worn surfaces as wel1 as in the subsurface regions using a scanning electron microscope (SEM) with an attached energy-dispersive X-ray (EDX) analyzer. In order to remove the loose
15 42
wear debris al1 specimens were cleaned ultrasonic cleaning process.
intensively
by an
3. Results and discussion The measured wear rates are listed in Tables 3-5. The results of wear tests are not discussed in detail here, they are published elsewhere [ 12-151. It should be noticed that flint particles (900 HV,.,,) are harder than al1 metal matrices ( 10&700 HVo,os) and softer than al1 hard phases ( 1 200-
A. Fischer / Weur 194 (1996) 238-245
241
Table 5 Hardness prior to testing, wear rates, and heat treatment of hiped Fe-, Ni- and Co-base materials Material German (US)
Fe-base 56NiCrMoV7
Hardness
Wear rates X IO-”
Heat treatment
HV,, W,,
WS,
+
0% CrBZ 5% CrBZ 10% CrB2 15% CrB2 20% CrB, Ni-base NiCRO +
660 640
38 8.8 4.4 2.9 2.8
0.44 0.2 1 0.19 0.15 0.14
H+A
5% CrBZ 10% CrB, Co-base CoCr 10 +
240 280
49 20
3.4 0.28
L
330 390
52 14
0.74 0.29
L
0% CrBZ 10% CrBZ
-t
t ----1-1
200
300
Hardness
400
500
600
700
HV30 urior to Testine
Fig. 4. Abrasive wear rates of materials tested.
56NiCrMoV7
+ 15 vol-%
hard phase
Cr62
Fig. 5. Abrasive wear; Bint (900 HVa,,) wears the softer metal matrix (640 HVO,OS).while the hard phases ( > 1700 HV,,OS) cannot be scratched.
2 400 HV,,,,) . With respect to the aim of this paper a comparison of the wear behaviour under these well-known wear tests wil1 be drawn focusing on the acting mechanisms. After-
wards criteria for a well-founded selection of materials wil1 be introduced and discussed. Under two-body abrasion fixed Flint particles scratch the surfaces and abrasion is the only acting wear mechanism. The wear behaviour is governed by its submechanisms [ 2,161. For soft materials such as pure Cu, Ni or Al microploughing is most likely, whereas with increasing hardness the depth of the scratch decreases and microcutting prevails [ 17,181. The work hardening capability of the metal matrix has another strong influence on the wear behaviour. Obviously, there is no direct correlation between hardness and wear rate (Fig. 4, MM). W,, values of MM materials are reduced by one order of magnitude making use of a sufficient combination of hardness, toughness and work hardening capability. The hard phases that produce a further decrease of the wear rate are shown in Fig. 5. Their effectiveness depends on the volume fraction, size, shape, and distribution and on the capability of the metal matrix to support them. Thus, an adequate volume
242
A. Fischer/
200
Weur 194 (1996) 238-245
300
400
500
600
Hardness HV30 prior to Testing Fig. 6. Sliding abrasian wear rates of materials tested
Flint.80
vm, 250 C, Air
680
t
Fig. 7. Sliding abrasion; soft metal matrices (270 HVYo) are worn by abrasion (opper micrograph). hard ones (670 HV,,,) by indentation (lower micrograph).
two orders of magnitude just for the metal matrices, less for the others. This is due to the fact that adhesion might appear with soft materials giving rise to high wear rates [ 1.51. In addition the wear mechanism changes with increasing hardness from abrasion (Fig. 7, upper micrograph, and Fig. 8) by abrasive particles, which are embedded within the surface of the opposite body, to indentation by rolling ones (Fig. 7, lower micrograph, and Fig. 8) [ 15,221. Abrasion is not observed with materials containing hard phases. The metal matrices are only removed by indentation (Fig. 9). The hard phases loose their support and are torn off the surfaces by cracking. Thus, the wear rate is governed by the volume fraction of al1 constituents that are harder than Flint. Indentation, which has a low efficiency of removing material from surfaces 1231, is the only acting mechanism and the wear rates of FHP and CHP materials differ by a factor of only about three. Fig. 10 shows the wear rates of both tests plotted versus each other. Obviously, the influence of microstructure on the tribological behaviour strongly depends on the type of wear. Hence, the selection of appropriate materials with low wear rate provokes different results, even though both types of wear seem to be similar. Under two-body condition abrasion is the only mechanism acting. One can achieve the lowest wear rates by selecting materials with a high ( > 30%) volAbrasion
fraction of coarse hard phases renders the lowest wear rates (Fig. 4, CHP). Microcracking is not observed and, hence, a detrimental effect of coarse hard phases on the wearresistance is not found. This is also reported for other tribosystems [ 16.19-2 1] Within this tribosystem the wear rate is reduced by two orders of magnitude over the range of metal matrices from pure Ni to hardened 56NiCrMoV7 and additional 20 vol.% coarse hard phases of CrB? type. Under three-body abrasion (sliding abrasion) the ranking of these materials is similar (Fig. 6). but, W,, ranges over
Indentation
abrasive particle Fig. 8. Schematic model of wear mechanisms body abrasion.)
under sliding abrasion (three-
A. Fischer / Wear 194 (1996) 238-245
243
might cause microcracking, which has a detrimental effect on the wear behaviour. It is important to notice that materials with fine hard phases have better production (casting, forging) and handling properties (heat treatment, machining, grinding) compared with those with coarse hard phases. This reduces the costs of a product, markedly.
4. Basic principle of the well-founded selection of materials As shown in the previous section the use of acting wear mechanisms as a link to the wear behaviour brings about certain criteria for appropriate materials to be chosen against wear. This strategy should not only be used after wear testing, but earlier in the state of development and design of a new product. In both cases the procedures are similar and should be presented in succession. 4.1. Development RT, Air, Flint, 80 urn, 0.83 MPa. 28 mm/s Fig. 9. Sliding abrasion; the metal matrix is removed by indentation coarse hard phases protrude from the surface.
and
urne fraction of coarse hard phases, which are embedded in a hard (700 HV,,,,) metal matrix. This is correct provided microcracking does not appear. These materials can be produced by casting, hardfacing welding or powder metallurgy. But due to their distinct brittleness the handling properties like for heat treatment, machining etc. are detrimental. The mechanisms acting under the three-body condition are adhesion, abrasion, and indentation, while the latter brings about the lowest wear rates. Consequently, microstructures are chosen, which are prone to wear by indentation. Adequate materials have a medium ( > 550 HV,,,,) hardness and volume ( > 12%) fraction of fine hard phases. Coarse hard phases
Step 1. Analysis of the structure of the tribosystem, of its stresses, and of the type of interaction between the elements. Step 2. Determination of the type of wear. Assumption of acting wear mechanisms (Fig. 2). Step 3. Selection of appropriate materials (bulk, layer, deposit) . If there is no material with the desired properties, the design has to be changed as to shape, stresses, or surface characteristics. The selection can be carried out by carefully using existing models. But, although there are many models published, most of them are only valid for some specific tribological system. Unfortunately, only a few authors define their tribosystems exactly and even fewer show the acting wear mechanisms. In addition these models do not consider the important effect of third bodies, which are likely to govern many technical tribosystems [ 24-3 11. Most models make use of bulk proper-
10
100
Wear Rate Wab x Fig. 10. Comparison
of abrasive wear (two-body
and design
abrasion)
1000
lö6
and sliding abrasion
(three-body
abrasion)
244
A. Fischer / Wear 194
Table 6 Correlation
between wear appearances
and wear mechanisms
Wear mechanism
Wear appearances
Fig.
Abrasion Surface fatigue Adhesion
Grooves, scratches, scores, striations Cracks, pits Scuffs, holes, bumps, flakes, material transfer Layers, particles
5 12
Tribochemical
reactions
wear appearance: wear mechanism:
11
flakes, pits
adhesion
(1996)238-245 at the wear appearances, which are attributed to a certain mechanism, (Table 6, Fig. 11 and Fig. 12) with some limitations. Several wear mechanisms might act at the same time and their interaction has to be taken into account. Submechanisms such as microploughing, microcutting, and microcracking should also be included. They might require conflicting preventive measures. Step 6. Comparison between assumed and examined wear mechanisms. If the assumed wear mechanisms are the same as those seen on the worn surface and if the wear rate is acceptable, the material has been properly selected. 4.2. Failure in production
sourrounding medium: oil counter body: 42CrMo4,52
wear appearance: wear mechanism:
or application
Step 1. Analyses of the structure of the tribosystem, of its stresses, and of the type of interaction between its elements. Step 2. Examination of the worn part as to the acting wear mechanisms. Steps 1 and 2 must lead to the same type of wear. Otherwise Step 1 or 2 have not been carried out, correctly. One reason for steps 1 and 2 leading to different types of wear might arise by the failure sequence. Worn parts often show adhesion being the predominant wear mechanism (Fig. 1 l), but, if the starting type of wear is different, e.g. cavitation, surface fatigue is the predominant mechanism (Fig. 12). Special attention must also be paid to failures starting by other mechanica1 or chemical attacks. Step 3. Selection of appropriate materials with respect to: (a) the starting mechanism, and (b) the interaction of mechanisms (see Section 4.1). Step 4. Practica1 Application. It should be noted that the above strategy does not always imply a need to change of material. The design and the stresses should be optimized, too.
HRC
pits
surface fatigue
5. Conclusion surrounding
medium:
42CrMo4,46 HRC
oil
Fig. 12. Cavitation;
appearance
of surface fatigue.
ties instead of those of surface or subsurface areas. Even if surface properties are introduced, neither the influence of high deformation rates nor additional chemical or physical effects are considered. Obviously, we are far away from any quantitative model, which allows US to predict the life time under tribological stresses. This strategy, therefore, yields only a qualitative conclusion. Step 4. Wear test in laboratory or practica1 application. There is no value in wear tests which are just apparently similar to the desired practica1 application. The laboratory wear test must simulate of the acting wear mechanisms. This has to be evaluated in every single case. Thus, practica1 application is stil1 the best wear test. Step 5. Investigation of the worn surface as to wear appearances (Table 6). Determination of acting wear mechanisms. The acting wear mechanisms can be investigated by looking
A strategy for the well-founded selection of materials in order to improve wear resistance is presented. It makes use of the acting wear mechanisms as a link to materials behaviour. This strategy is valuable for the well-founded development of new wear-resistant materials, for consulting researchers, designers, and production engineers as wel1 as for the analysis of failures. As an example the wear behaviour of metallic materials under, the wel1 known, two-body and three-body abrasion is shown. The similarity of these types of wear [ 321 often brings about the selection of similarmaterials. But the analyses of the acting wear mechanisms renders different criteria for the selection of appropriate materials. Acknowledgements
The author would like leading him into the field for his steady willingness the author would like to
to thank Prof. Dr.-Ing. H. Berns for of wear of multiphase materials and for helpful discussions. In addition thank al1 employees of the Institute