CP Dof JA Hawk/wear 20J-20(/992)267-277 monotic and composite m yy. 8 Aly those provided by the manufacturer to identical matenals, N-a+sic Sic whiskersresults in norealchange in the toughness of Si,Na-B. 3.3 Ab this 99 5% material is 3 der than in 99.8% AL-O,, and grains as large as and also a glass-bonded ceramic ness, the 150-grit alumina abrasive is the least aggressive for mately 2 wol. of a -matrix [23] is ohserve Such aa size within this composite ave Measured values for hardness and fracture toughness, 日9g monolithic SiN NN-B an increase In the hardness of the composite in all monolith SiN4 mater270 C.P. Do&n, J.A. Hawk/ Wear203-204 (1997) 267-277 the monolithic and composite Si,N,-B ceramics The micro￾structure of the composite is similar [Fig. 1 (b) 1, except for the presence of randomly oriented Sic whiskers. which have an average diameter of 0.75 pm and a variable aspect ratio. In this case, the whiskers are locatedboth intra8ranuldy (i.e. either partially orcntrrely encapse!a~~withinasihcon nitride grain) and inrergranularly within the microstructure. For the alumina-based ceramics, the microstructums vary quite a lot between the two monolithic materials and between the monolithic and composite materials. 99.8% AlsOs [Fig. 1 (c) ] is carbon bonded, with graphite detected at most alumina-aluminaboundarics. Inaddition,elongated “whisk￾ers” of a potassium-modified &Also, phase are also occa￾sionally observed at the grain boundaries. Significant stresses are apparent at the a-AlsO&AlsOs interfaces; however, because the population of such interfaces is relatively small within this material, the presence of this stress is unlikely to influence the tribological properties of the bulk material. The alumina grain size in this material averages around 2 pm, although there arc pockets of much smaller. sub-micron, grains within the microstructure. In monolithic 99.5% AlsOs, on the other hand, tha microstructum is typical of that of a liquid-phase sintered ceramic, with the alumina grains bonded by an amorphous calcium aluminosilicate phase that is continuous in this material. The average alumina grabt size in this 99.5% material is 3 p.m. but the grain sire distribution is much wider than in 99.8% A120sr and grains as large as 10 Pm are not unusual. The alumina-based composite, AlsOs+SiC,, is also a glass-bonded ceramic [Fig. l(d)], containirtg approxi￾mately 2 vol.% of an amorphous magnesium altinosilicate phase that is located at all three- and four-grain junctions and along most two-gram boundaries. This amorphous phase is also observed as a thin layer ( <SO nm) at whisker-matrix interfaces. The Sic whiskers. with an average diameter of 0.75 pm and a vartablc aspect ratio, are distributed randomly throughout the alumina matrix and occur both inter- and intragranularly. In amorphous pockets adjacent to the SE whiskers, small crystals of graphite and an iron-nickel inter￾metallic are often observed [as in Fig. 1 (d) 1. Alumina grain size within this composite averages around 4 pm. 3.2. Hardness and fiactam toughness Measured values for hardness and fracture toughness, along withreportedvaluesforYoung’smodulus (asprovided by the manufacturers), are listed in Table 2 for all of the ceramic materials examined in this study. The alumina-based materials, Also, + SiC, and 99.8% Also,, tend to have the highest hardness, with values of 23.8 and 19.2 GPa, respec￾tively: whereas unreinforced S&N.-B and 99.5% AlsOs have the lowest hardness, with values of 15.0 and 15.2GPa, respectively. As expected, the addition of silicon carbide whiskers to the silicon nitride and alumina matrices results in an increase in the hardness of the composite in all cases. Indentation fracture toughness measurements do not always provide the most accurate measure of the bulk fracture toughness of a ceramic, often resulting in values lower than those obtained by other measurement ~&uilques 1 i9 ] ; how￾ewr, the indentation technique is selected here because it is believed to provide the value most representative of the near￾surface regions of a material exposed to an abrasive wear environment. As predicted, the measured values for fracture toughness obtained in this study are somewhat lower than thosa provided by the manufacturer for identical materials, or quoted in the literature for similar materials (theexception being the 99.5% A1203). Nonetheless, it is apparent from Table 2 that as a class of materials, the alumina-based ceram￾ics are not as tough as the silicon nitride.-based ceramics, and that 99.8% AlsOs has the lowest toughness of all of the matc￾vials tested in this study. The addition of SE whiskers to S&N.-A results in a 29% increase in toughness, making S&N.-A + SE, the highest toughness material of this study, whemas the addition of SE whiskers results in no teal change in the toughness of S&N.-B. 3.3. Abrasive wear behavior The measured specific wear rates and wear constants for all of the ceramics and ceramic composites tested against alumina and silicon carbide abrasives are listed in Table 3. and the wear constants for the Si&A and S&N.-A + Sic, materials tested against ISO-grit SE as a function of load are listed in Table 4. As expected from its relatively lower hard￾ness, the 150-grit alumbra abrasive is the least aggressive for all of the ceramics tested except for the finest SE abrasive against the 99.5% Also,. Similarly. the expected increase in abrasive wear rate with increase in Sic abrasive particle size [ 231 is observed for all of tha ceramics and ceramic com￾posites over the abrasive size range of 37-100 pm (Fig. 2). An interesting aspect of Lis data is the large increase ( lSO- 388%) in wear rate with increase in abrasive particle size from 37 pm (400-g&) to 58 pm (240-g&). Such a large difference in wear rate over a relatively small change in abra￾sive particle size suggests a change in wear mechanisms, and in fact such a change is observed. Following wear against the 4OOgrit Sic abrasive, examination of the wear surfaces indi￾cates that the response of these materials is primarily ona of plastic deformation, with only minimal fracture observed. Bxamination of the wear surfaces following the test against the 24~grit Sic, on the other hand, indicates that fracture is now a signi8cant material response to the wear environment. Under the various abrasive wear conditions of thls study, monolithic SisN,-A is consistently a better performer than is monolithic SisN,-B, particularly against the “softer” alu￾mina abrasive. This result is interesting since examination of the wear surfaces of these materials after abrasion against 15Ogrit alumina suggests that both materials are in the mild wear regime 1241, where fracture toughness is expected to dominate wear behavior. Yet there is no real difference in the fracture toughness of the two monolithic SisN, materials