21Metals and Ceramics

21.1 Introduction21.2 Pure Metals21.3 Soft Metals and Soft Bearing Alloys21.4 Copper-based Alloys21.5 Cast Irons21.6 Steels21.7 Ceramics21.8 Special Alloys21.9 Comparisons Between Metals and Ceramics21.10 Concluding Remarks

21.1 Introduction

Friction and wear can be kept low if the contact interface is well-lubricated. Even when the contactinterface is not supplied with lubricants, friction and wear are changed by adsorbed gasses (Bowden et al.,1954) or by frictional repetition. This means that tribological properties are responses of a tribosystemthat is lubricated on purpose or is under the effects of surroundings. Therefore, material properties ofonly one of two contacting bodies cannot be independently related to the tribological properties in adirect way.

At the frictional contact surfaces, there exist frictional heating, high flash temperature, severe plasticshear deformation under contact pressure, and the agglomeration of wear particles to form the tribolayer(Rigney et al., 1977). These produce new surface properties that are different from the bulk materialproperties. Friction and wear take place at the contact interface between such unsteady surfaces.

Nevertheless, metal and ceramic materials can be classified into groups for different applicationalpurposes, and the tribological usefulness of each group in practice can be qualitatively explained, to acertain extent, by the bulk material properties.

These explanations are described in the following sections, which can be guides in the first step ofmaterial selection for tribo-elements.

21.2 Pure Metals

Pure metals are generally soft and ductile. Therefore, the contact junctions of asperities between themshow large amounts of junction growth in sliding if the contact interface is not lubricated.

Table 21.1 shows the friction coefficients µ observed with eight pure metals sliding on themselves indifferent atmospheres (Bowden et al., 1954). In air, oxygen, or water vapor, the friction coefficients ofthese eight frictional pairs vary from 0.8 to 3.0. Gold, nickel, platinum, and silver show relatively largevalues, which means that the adhesion is relatively strong at the contact interfaces of these metals andcontact junctions grow sufficiently to generate such large values. In hydrogen or nitrogen, copper, gold,

Koji KatoTohoku University

Koshi AdachiTohoku University

and nickel show large µ values (between 4 and 5), which means adhesion is stronger in the inert gasesthan in air. The high friction of pure metals shown in Table 21.1 is applied in friction bonding of noblemetals such as gold.

Abrasive wear resistance of such pure metals linearly increases with hardness as shown in Figure 21.1(Khruschov, 1957). On the other hand, adhesive wear does not show a clear relationship with hardness.

21.3 Soft Metals and Soft Bearing Alloys

When hard metals such as steels slide on themselves without lubricants, high friction, gross seizure, andsevere wear take place in air or vacuum. A soft-metal thin film at the sliding interface between hardmaterials can reduce friction to the level of µ = 0.1 to 0.2. Gold, silver, lead, and indium are representativesoft metals whose hardness values vary from about 0.3 GPa to about 0.5 GPa. In practical cases of softmetal-lubricated tribosystems, sliding velocities are relatively small and soft metals are not expected towork in the molten state.

TABLE 21.1 Friction of Metals (Spectroscopically Pure) Outgassed in Vacuum (When clean, there is gross seizure.)

Coefficient of Friction after Admitting

Metals H2 or N2 Air or O2 Water Vapor

Aluminum on aluminum — 1.9 1.1Copper on copper 4 1.6 1.6Gold on gold 4 2.8 2.5Iron on iron — 1.2 1.2Molybdenum on molybdenum — 0.8 0.8Nickel on nickel 5 3 1.6Platinum on platinum — 3 3Silver on silver — 1.5 1.5

Data from Bowden, F.P. and Tabor, D. (1954), Friction and Lubricationof Solids, I, Clarendon Press, Oxford.

FIGURE 21.1 Effect of hardness on the relative wear resistance of pure metals. (From Khruschov, M.M. (1957),Resistance of metals to wear by abrasion as related to hardness, Proc. Conf. Lubrication and Wear, Inst. Mech. Engr.,655-659. With permission.)

Figure 21.2 shows the lubricating properties of Ag, Au, Bi, In, Pb, Sb, and Sn observed with the frictionpair of an Si3N4 pin against an SUS440C stainless steel disk in a vacuum of 10–6 Pa (Kato et al., 1996). Itis recognized that the soft-film thickness has its optimum value for the minimum friction coefficient(Bowden et al., 1954), but such optimum film thickness can be held during running only when the softmetal is supplied continuously to repair the worn parts of the film (Kato et al., 1990). When a soft-metalfilm of a certain thickness is precoated on a hard material substrate, the life of the tribocomponent isdetermined by the wear life of the film. Soft-metal film lubrication is, therefore, convenient for relativelysmall and replaceable tribocomponents such as ball bearings.

When a bearing system is expected to run in a state of hydrodynamic lubrication with oil, an unexpectedsolid contact is generated by the introduction of hard abrasive particles, misalignment, high load, or slowspeed at the sliding contact interface. Soft alloys such as lead- or tin-based babbitts and aluminum-basedalloys work well as bearing materials in such contact conditions.

Lead-based babbitts contain a high percentage (>80 wt%) of lead with 1 to 10 wt% tin and 10 to15 wt% antimony, and have a hardness value of about 0.2 GPa. An Sb-Sn phase is distributed as finecubes throughout the structure. This material has the weakness of low fatigue strength because ofsegregation of the Sb-Sn phase during solidification. Tin-based babbitts contain a high percentage(>85 wt%) of tin with 5 to 8 wt% antimony and 4 to 8 wt% copper, and have hardness values of about0.2 GPa. They have the phase of Sb-Sn or Cu6Sn5, and the presence of either or both of these intermetallicphases increases fatigue strength below 130°C (Glaesure, 1992).

These babbitts are soft enough to embed dirt or hard particles, but also provide good conformingunder misalignment or high load. Even when the supply of oil is interrupted, babbitts flow or melt toprotect the shaft from damage. The dry friction coefficient against steel remains at ~0.55 to 0.80 (Bowdenet al., 1954). They are used below the contact pressure of 30 to 40 MPa and their fatigue strength is ~20 to30 MPa.

Aluminum-based alloys are used for bearings that require large fatigue strength and higher operatingtemperature than babbitt bearings. Aluminum-tin alloys show a fatigue strength three times larger thantin- or lead-based babbitts (Pratt, 1969) and provide better compatibility with steels. Aluminum-20 wt%lead alloy is less expensive and has fatigue strength almost equal to that of aluminum-20 wt% tin alloyand better wear resistance (Bierlein et al., 1969). Although these aluminum-based alloys exhibit betterfatigue strength, corrosion resistance, wear resistance, and compatibility with steel than babbitts, theirembeddability and seizure resistance are not as good as that of babbitts and a thin overlay of lead-tinbecomes necessary.

By considering all the tribological properties of babbitts and aluminum-based alloys, as well as thematerial costs, soft alloys are used for the oil-lubricated bearings. Because these alloys have relatively

FIGURE 21.2 Thin film lubrication of soft pure metals in sliding of an Si3N4 pin on SUS440C stainless steel diskin high vacuum. (From Kato, K., Kim, H., Adachi, K., and Furuyama, H. (1996), Basic study of lubrication by tribo-coating for space machines, Trans. Japan Soc. Mech. Eng., 62(600), 3237-3243. With permission.)

small strength and elasticity, they are usually overlayed on strips of steel or copper alloys, which providestrength.

Bearings for the crankshafts of internal combustion engines and reciprocating compressors, enginecamshafts, and railroad car axles are good examples of the applications of these alloys.

21.4 Copper-based Alloys

When journal bearings are oil lubricated and run under heavy load, bearing materials must have hardnessand strength higher than that of babbitts or aluminum-based alloys. Copper-based alloys, known asbronze, have been used for such bearings for their strength and good compatibility with steels.

Tin bronzes contain 5 to 13 wt% tin and 1 to 5 wt% zinc. The existence of a hard intermetalliccompound of Cu31Sn8 increases the strength of the alloys, and gives a hardness value of about 0.75 GPa,which is useful for heavy loads. Leaded bronzes contain 4 to 6 wt% tin, 4 to 6 wt% zinc, and 1 to 10 wt%lead. Lead is insoluble in copper and forms free globules in the copper matrix. These free lead globuleswork as solid lubricants that transfer to the mating surface and form a coating of low shear strength.This lead coating reduces friction and seizure in boundary lubrication. The alloys have hardness valuesof ~0.65 GPa, and show good conforming against roughness and misalignment of the shaft.

In addition to copper bronzes, aluminum bronzes, manganese bronze, silicon bronze, and phosphatebronze have been developed. Hardness values of some of these alloys are shown in Table 21.2. They areused for gears, seals, nonsparking contacts, and heavily loaded, slow-moving journal bearings.

21.5 Cast Irons

Wheels, rails, brakes, clutches, crushers, piston rings, gears, and rollers are examples of machine elementsthat require higher bulk strength than bronzes to support heavy loads with small deformation and highwear resistance, even in unlubricated sliding. A hardness greater than 2 GPa is generally required forthese purposes. Cast irons with a carbon content between 2.5 and 4.0 wt% are inexpensive structuralmaterials with the required properties. There are five representative cast irons: grey iron, white iron,nodular iron, malleable iron, and high-alloy iron. They have hardness values between 2 and 6 GPa, asshown in Table 21.3.

TABLE 21.2 Copper-based Alloys and Hardness Values

Alloy Cu Pb Sn P Al BeHardness

BHN, GPa

Copper-lead alloy >60 25–35 <1 — — — 280Lead bronze >80 1–10 4–6 — — — 650Tin bronze >80 — 5–15 — — — 750Phosphor bronze >85 — 9–15 0.1–0.2 — — 800Aluminum bronze >78 — — — 8–10 1.8 1700Beryllium copper 99.5 — — — — — 3000

TABLE 21.3 Hardness and Carbon Content of Cast Irons

Cast IronHardness Hv

(GPa)Carbon Content

(%)

Grey iron Flake graphite 2.00 <3.0White iron Iron carbides 4.25 <3.0Nodular or ductile iron Graphite nodules 3.50 2.5 ~ 4.0Malleable iron Graphite nodules 2.48 2.5 ~ 3.2High-alloy iron Chromium carbides 5.60 ~3.0

Grey cast iron has flake graphite in a matrix of bainite or martensite. White iron has iron carbides inthe matrix. Malleable iron is formed from white iron by heat treatment and has nodular graphite in thematrix. Nodular iron also has nodular graphite, which is formed by the addition of small amounts ofmagnesium or cerium for spheroidizing. These graphites in the matrix form a graphite layer at the slidinginterface and reduce friction and wear. Steel shows a friction coefficient of 0.8 in sliding against steel;and cast iron shows a friction coefficient of 0.4 in sliding against the steel (Bowden et al., 1954). It is theresult of graphite film formation at the sliding interface with graphite in the cast iron.

In the case of rolling contact, fatigue wear is expected to take place, and the positive contribution offree graphite to fatigue is considered from the viewpoint of friction reduction. Nodular iron is preferableto grey iron from this viewpoint because it has more fatigue resistance than grey iron.

White iron, on the other hand, contains large iron carbides and no free graphite, which gives it highabrasive resistance. When high wear resistance is required under heavy abrasion or high-temperaturecorrosion, high-phosphorous irons or high-chromium irons are used. High-silicon irons are used forcombined abrasion and corrosion conditions. In the case of high-phosphorous irons, a hard phosphideeutectic network is formed and provides high abrasive resistance.

When especially high abrasive wear resistance is required, cast irons of harder matrix structure andharder inclusions become necessary because abrasive wear resistance is proportional to the hardness ofthe wear material, as shown in Figure 21.1. Figure 21.3 shows the weight loss of a high-chromium pin inrelation to its hardness HRC observed in an abrasive wear test (Diesburg et al., 1974). The hard matrix ofaustenite or martensite yields smaller amounts of wear. Hard chromium carbides in the matrix also workas abrasive wear-resistant parts. High phosphorous (>3 wt% phosphor) irons used for railroad cars includea hard phosphide eutectic network that gives high abrasive wear resistance by a similar mechanism.

In general, the hardness of cast iron increases with increasing volume of carbides in the matrix.Figure 21.4 shows that the volume loss of high-chromium-molybdenum white irons decreases withincreasing volume of carbides Cr7C3 in abrasive sliding against garnet, whose hardness is less than thatof the carbides. But this protective property of carbides disappears when the abrasive is silicon carbide,which is much harder than carbides (Zum Gahr et al., 1980). This result is caused by the increasedbrittleness of carbides at higher content and their brittle fracture in abrasion against the harder siliconcarbide.

FIGURE 21.3 Abrasive wear weight loss of high-chromium iron pairs in abrasive wear tests. (From Diesburg, D.E.and Borik, F. (1974), Optimizing abrasion resistance and toughness in steels and irons for the mining industry, Symp.Materials for the Mining Industry, Climax Molybdenum Co., 15-41. With permission.)

21.6 Steels

Steels are representative structural materials for almost all kinds of machines and structures because oftheir high strength, good machinability, and low cost. Their tribological applications are journals, gears,ball and roller bearings, tools, wheels, rails, fasteners, etc., which need to support heavy load withminimum elastic deformation and minimum clearance over a long period of time. The hardness of steelscan be controlled by carbon content and heat treatment from ~1.0 GPa to ~3.0 GPa, which gives a choiceof steels for each tribo-element.

In many steel-made tribosystems, contact interfaces are lubricated with liquid lubricants and theproperties of friction and wear are under the strong influence of operating conditions, which determinethe state of lubrication.

In the state of hydrodynamic lubrication, friction depends mainly on the viscosity of the liquid andthe shape of clearance for the liquid film. In the state of boundary or mixed lubrication, lubrication filmis penetrated by surface asperities in contact, and the friction coefficient is increased to about 0.1 andwear is generated as a result.

Hard asperities act as abrasives to cause breakdown of the lubricant film, because the real contactpressure at each asperity contact is nearly equal to the hardness value of the asperity material.

On the other hand, breakdown of lubricant film at the contact interface is caused more easily at surfacesof large roughness because real contacts of asperities at the interface are localized and the contact pressuresthere tend to be larger than those at smooth surface contacts. Therefore, the choice of an acceptablehardness of steel must be made in relation to the surface roughness of the tribo-elements and lubricantproperties, especially in contact between steels.

When steels have contacts with abrasive particles or abrasive surfaces without lubricants, their hardnessis a good measure of abrasive wear resistance. The effect of hardness on abrasive wear of steels is shownin Figure 21.5, where the amount of abrasive wear of low alloy steels decreases with increasing originalsteel hardness (Borik, 1976). This result is similar to those in Figure 21.3. Thus, one can generally expecthigher abrasive wear resistance with harder steels so long as brittle phases are not introduced into thematrix.

There are some steel-made tribo-elements that are used without lubricants, such as wheels and rails,bolts and nuts, and fasteners. In such cases of unlubricated sliding between steels, work hardening,frictional heating, phase transition, and oxidization take place at the same time in a complicated way,and the original bulk hardness of the steels is not simply related to friction and wear, as in the case ofabrasive wear.

FIGURE 21.4 Volume loss (pin abrasion on 150 mesh garnet and 180 mesh SiC) of high-chromium-molybdenumwhite irons as a function of the volume % of massive carbides. (From Zum Gahr, K.H. and Eldis, G. (1980), Abrasivewear of white cast iron, Wear, 64, 175-194. With permission.)

Figure 21.6 shows the wear rate of a pin of annealed plain carbon steel rubbing against a ring of thesame material (Archard, 1980). Below the transition load T1 in Figure 21.6, the wear rate is below about10–5 mm3/m (10–6 mm3/Nm), and the wear mode is called “mild wear.” The dominant wear mechanism

FIGURE 21.5 Relationship between the weight loss of various low alloy steels caused by abrasion of alumina particlesand their hardness changed by carbon contents in the material. Numbers at data points indicate the carbon contentin percent. (From Borik, F. (1976), Testing for abrasive wear, Selection and Use of Wear Test for Metals, Paper No. 3,ASTM Special Technical Publication 615, 30-40.)

FIGURE 21.6 Unlubricated wear rates of annealed plain carbon steel (0.52%, 268 DPN) as a function of the load.(From Archard, J.F. (1980), Wear theory and mechanisms, Wear Control Handbook, ASME, Peterson, M.B. and Winer,W.O. (Eds.), 35-80. With permission.)

is oxidational wear. There is a change in the wear rate by more than 2 orders of magnitude above T1,where the wear mode is called “severe wear” and the dominant wear mechanism is adhesive wear. Abovethe load T2, the wear rate is between those in mild wear and severe wear, and the wear mechanism isdominated again by the oxidational wear on the work-hardened wear surfaces.

In the regime of oxidational wear, the wear rate changes within 1 order of magnitude, depending onthe combination of load and sliding velocity. In Figure 21.7, mild oxidational wear is observed below thecritical load of about 60 N at the sliding velocity of 1.0, 2.0, or 3.3 m/s; and severe oxidational wear isobserved above the critical load or at the sliding velocity of 0.23 m/s (Queen, 1992). Wear rates obtainedwith these results are shown in Table 21.4, where it is confirmed that the change of specific wear rate iswithin the order of 10–5 mm3/Nm. Further comprehensive expressions about oxidational wear are intro-duced in the chapter on wear maps (Chapter 9).

21.7 Ceramics

Ceramics such as Si3N4, SiC, Al2O3, and ZrO2 have high hardness between 15 and 20 GPa at roomtemperature and such high hardness is maintained up to high temperature. However, their toughnessvalues are between 4 and 8 MPa(m)1/2, which are much lower than those of steels. These materialproperties suppress junction growth at asperity contacts in friction, and the friction coefficient in self-mated sliding is not increased to a value above 1.0 in many cases. Friction coefficients of these ceramics

FIGURE 21.7 Wear patterns for high-chromium ferritic steel pins sliding on austenitic stainless steel disks at 0.23, 1.00,2.00, and 3.30 m/s. (From Quinn, T.F.J. (1992), Oxidational wear modelling. I, Wear, 153, 179-200. With permission.)

TABLE 21.4 Specific Wear Rates for High-Chromium Ferrite Steel Pins

Sliding Velocity(m/s)

Specific Wear Rate(×10–5 mm3/Nm)

Below Transition Above Transition

0.23 Not applicable 4.41.00 1.3 1.72.00 1.1 2.83.00 0.8 1.1

Data from Quinn, T.F.J. (1992), Oxidational wear mod-elling. I, Wear, 153, 179-200.

in self-mated sliding are between 0.7 and 0.9 in air and between 0.8 and 1.0 in vacuum, as shown inTable 21.5 (Kato, 1990).

The high hardness of ceramics does not work positively for high wear resistance because their lowtoughness reduces wear resistance by easy introduction of microfracture at contact surfaces in sliding.The combined effect of hardness and toughness on abrasive wear resistance, which is given by thereciprocal of material removal rate, is shown for six different ceramics in Figures 23 and 24 in Evans andMarshall (1981), where abrasive wear resistance increases linearly with the parameter of (Hardness)5/8 ×(Toughness)1/2 (Evans and Marshall, 1981).

Although ceramics are considered chemically inert, Si3N4 and SiC react with oxygen and water insliding in air and form soft surface layers of SiO2 and its hydride via tribochemical reaction. Hence, thewear rate of Si3N4 or SiC is sensitive to humidity in air and is reduced drastically from about 10–4 mm3/Nmto about 10–6 mm3/Nm by increasing the humidity, as shown in Figure 21.8 for Si3N4 (Fischer et al., 1985).

A similar effect of humidity on wear is observed with Al2O3 sliding on itself, as shown in Figure 21.9where aluminum hydroxide is formed by tribochemical reaction at higher humidity (Gee, 1992), and thewear rate is reduced from about 10–5 mm3/Nm to about 10–7 mm3/Nm. The friction coefficient, on theother hand, is insensitive to humidity and remains at ~0.7 in the case of Figure 21.9.

TABLE 21.5 Friction Coefficients of Ceramics in Self-mated Sliding

Friction Coefficient

Material Vacuum Air

Si3N4 0.85 0.75SiC 0.84 0.80Al2O3 0.98 0.70PSZ — 0.90

Data from Kato, K., Furuyama, H.,and Mizumoto, M. (1990), The funda-mental properties of tribo-coating filmsin ultra high vacuum, in Proc. Japan Int.Tribology Conf., Nagoya, I, 261-266.

FIGURE 21.8 Effect of humidity on wear rate of Si3N4 sliding against itself. (From Fischer, T.E. and Tomizawa, H.(1985), Interaction of tribochemistry and microfracture in the friction and wear of silicon nitride, Wear, 105, 29-45.With permission.)

Because of the brittleness of ceramics, their wear rate in sliding is increased by 1 or 2 orders ofmagnitude at a certain contact load, above which surface cracks are propagated drastically by the tensilestress in the contact region. The tensile stress at the contact region is further increased by the thermalstress induced by frictional heating. Figure 21.10 shows these effects of load and sliding speed on thetransition of wear from mild to severe wear observed with Al2O3 in self-mated sliding, where wearincreases by an order of magnitude at a certain critical load and the critical load becomes smaller atlarger sliding speed (Hsu et al., 1989). The effect of sliding speed on wear mode transition is more clearlyshown in Figure 21.11, where the wear rate increases from a low value of about 1 × 10–5 mm3/m (5 ×10–7 mm3/Nm) to a high value of about 4 × 10–4 mm3/m (2 × 10–5 mm3/Nm) at about 2.5 m/s (Blomberget al., 1994).

The tribochemical wear observed in Figures 21.8 and 21.9 with silicon nitride and alumina in humidair promises smooth wear surfaces in water and the possibility of useful water lubrication (Tomizawaet al., 1987). Figure 21.12 shows the Stribeck curves in water lubrication of SiC, Si3N4, and Al2O3 in self-mated sliding. The hydrodynamic lubrication observed in ηN/Pm > 0.2 × 10–7 means that the contactsurfaces of ceramics are very smooth as a result of tribochemical wear, and a thin water film of lowviscosity works well because of the high hardness of contact materials (Wong et al., 1995). It is impossibleto get similar curves with steels in water.

Ceramics with such tribological properties are used for seals and bearings in water pumps, ball bearingsfor high-speed guides, and rollers in heavy-duty manufacturing processes, and cylinder liners and valvesof engines and cutting tools.

FIGURE 21.9 Effect of humidity on wear of Al2O3 sliding against itself. (From Gee, M. (1992), The formation ofaluminium hydroxide in the sliding wear of alumina, Wear, 153, 201- 227. With permission.)

FIGURE 21.10 Load dependent wear mode transition in unlubricated sliding contact of Al2O3/Al2O3. (From Hsu,S.M., Wang, Y.S., and Munro, R.G. (1989), Quantitative wear maps as a visualization of wear mechanism transitionsin ceramic materials, Wear, 134, 1-11. With permission.)

21.8 Special Alloys

For usage in heavy-duty tribo-elements at high temperatures and/or highly corrosive environments,materials of both high hardness and high strength are required. Cermets are materials for such require-ments. They have hard ceramic particles in a matrix of metals, which gives high toughness. The repre-sentative materials of hard ceramic particles are WC, TiC, TiN, Cr2C, and Al2O3, and those of bindersare Co, Ni, and Cr. Different kinds of cermets are formed by combining some of these constituents.

WC cermets have tungsten carbide bonded with cobalt. They have hardness values from 10 to 15 GPaand tensile strengths from 3 to 4 GPa, depending on the amount of cobalt (3 to 25 wt%). They are usedas cutting tools for brittle metals such as cast irons. TiC cermets have titanium carbide bonded withcobalt and nickel, and have hardness values similar to those of WC cermets. Some TiC cermets havebinders of cobalt, nickel, or chromium, which are extremely resistant to high-temperature oxidation. TheWC-TiC cermet bonded with cobalt, which has about 15 wt% TiC, shows high wear resistance at hightemperature and works well in machining steels. Cr3C2 cermets have chromium carbides bonded withnickel (10 to 15 wt%) and tungsten (–2 wt%), which are resistant to corrosion and oxidation.

Figure 21.13 shows the specific wear rates of three kinds of cermets in relation to those of originalceramics and binding metals (Tsuya et al., 1989). It is obvious that high wear resistances of ceramics arewell maintained in cermets, despite the existence of soft binding materials.

FIGURE 21.11 Velocity-dependent wear mode transition in unlubricated sliding contact of Al2O3/Al2O3. (FromBlomberg, A., Olsson, M., and Hogmark, S. (1994), Wear mechanisms and tribo mapping of Al2O3 and SiC in drysliding, Wear, 171, 77-89. With permission.)

FIGURE 21.12 Stribeck curve in water lubrication of three different ceramics in self-mated sliding. η: viscosity, N:rotational speed, Pm: contact pressure. (From Wong, H.C., Umehara, N., Kato, K., and Nii, K. (1995), Fundamentalstudy of water-lubricated ceramic bearings, Trans. Japan Soc. Mech. Eng., 61(509), C, 4027-4032. With permission.)

21.9 Comparisons Between Metals and Ceramics

It is more useful to see the differences in tribological properties of metals and ceramics under the samecontact conditions than to see them individually under different contact conditions. To see the differencesin wear resistances of materials, fatigue wear and abrasive wear are appropriate wear modes because theycan be compared without the strong influence of lubricants.

In repeated rolling contact, silicon nitride balls show much longer fatigue life than bearing steel balls,as shown in Figure 21.14 (Fujiwara et al., 1988). If one remembers that silicon nitride has larger hardnessand smaller toughness than steel, it is obvious that fatigue strength in rolling contact is determined byother functions.

Figure 21.15 shows the effect of fracture toughness on abrasive wear resistance, which shows a maxi-mum value at a certain fracture toughness (Diesburg et al., 1974). Abrasive wear resistance of ceramics

FIGURE 21.13 The specific wear rates of cermets, and their original ceramics and binding metals observed in pin-on-disk sliding. (From Tsuya, Y. and Ishii, S. (1989), Wear characteristics of ceramics and cermets, J. Japanese Soc.Tribologists, 34(5), 348-351. With permission.)

FIGURE 21.14 Fatigue strength of bearing steel and silicon nitride observed by rolling fatigue test. (From Fujiwara,T., Yoshioka, T., Kitahara, T., Koizumi, S., Takebayashi, H., and Tada, T. (1988), Study on load rating property ofsilicon nitride for rolling bearing material, J. Japan Soc. Lubrication Engineers, 33(4), 301-308. With permission.)

increases with fracture toughness below ~10 MPa(m)1/2, and that of metals decreases with fracturetoughness above ~15 MPa(m)1/2. This means that optimum material of optimum toughness must becarefully selected for high abrasive resistance. It is noted at the same time that hardness is not a measureto compare abrasive wear resistance between metals and ceramics. It is available only for the group ofmetals as shown in Figure 21.1 or for the group of ceramics as shown in Figures 23 and 24 in Evans andMarshall (1981).

In modern, advanced machine systems, metal/ceramic combinations are being used for tribo-elements.It is clearly shown in Figures 21.16(a) and (b) that metal/ceramic combinations always show smaller wearrate than combinations of metal/metal or ceramic/ceramic (Iwasa, 1985). A similar effect is shown inFigure 21.17, where the largest value of critical contact pressure against seizure initiation is given by thecombination of SiC/FC25, where FC25 is a cast iron (Asanabe, 1987).

The excellent performances of metal/ceramic combinations in wear and seizure indicate the importanceof material combination of elements. Therefore, it must be recognized that individual evaluation of amaterial for tribological needs can be useful only in some special cases.

21.10 Concluding Remarks

It has been generally recognized for a long time that dissimilar materials combination shows bettertribological properties than similar materials combination for tribo-elements. Bearings of babbitts orbronzes for harder steel shafts with oil lubricants are traditionally established examples of this meaning.Figures 21.16 and 21.17 confirm the same empirical law for smaller wear and higher seizure resistancewith metal/ceramic combinations.

On the other hand, the Stribeck curves in Figure 21.12 and the successful applications of the SiC/SiCcombination for water-lubricated journal bearings of a water pump (Kimura et al., 1996) suggest thathigher performances of tribo-elements are attainable using hard similar materials combination withcareful treatments.

It is important, therefore, to enlarge the materials combination matrix for both dissimilar and similarmaterials combinations in selecting materials for tribo-elements.

FIGURE 21.15 Effect of material toughness on abrasive wear resistance. (From Diesburg, D.E. and Borik, F. (1974),Optimizing abrasion resistance and toughness in steels and irons for the mining industry, Symp. Materials for theMining Industry, Climax Molybdenum Co., 15-41. With permission.)

FIGURE 21.16 The effect of materials combination on wear rate in unlubricated sliding; (a) wear rates of Si3N4,SiC, AlN, and SUJ2 against an Si3N4 disk; (b) wear rates of Si3N4, SiC, AlN, and SUJ2 against an SUJ2 disk, whereSUJ2 is bearing steel. (From Iwasa, M. (1985), Frictional properties of ceramics and measuring methods, Science ofMachine, 12, 45-52. With permission.)

FIGURE 21.17 The effect of materials combination on critical contact pressure for seizure initiation in turbine oil-lubricated sliding. (From Asanabe, S. (1987), Applications of ceramics for tribological components, Tribol. Int., 20(6),355-364. With permission.)

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