《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_Al2O3-SiC-21.pdf

ELSEVIER WEAR Wear 203-204 ( 1997) 267-277 Influence of whisker reinforcement on the abrasive wear behavior of silicon nitride- and alumina-based composites C.P. Dogan, J.A. Hawk Albany Research Cenrrr. Deportmmr of Energy Albany. OR 97321. USA The abrasive wear of brittle materials, while not a tme property of materials, is generally modeled as an inverse limction of both the bulk hardness and frashue toughness. According to these models. an increase in the hardness and/or fmcture tott@ of a material will thereforr enhance its wear resistance. In ceramic mat&Is, the addition of whisker reinforcement is a pmven method ofe&ancing long-crack fmcmm toughness via such mechanisms as crack bridging attd wbiskerdcbondittg. However, less is known about bow wbiskerreinforcementingtzmes the properties that arc dependent upon the shott-ctack oughttess. such as abrasive wear. The results of this study indicate th8t while the addition of randomly oriented SE whiskers can dramatically improve the abrasive wear resistance of an abmdna-based ceramic. the arkiition of Sic reinforcement to silicon nitride ceramics does not always result in improved wear resistance. This wtriation in the intluence of whisker ninforcetnent on wear behavior is attributed to the residual stress state created in the composites as a result of the addition of the second phase whiskers. Keywordc Composites; Abrasive wear. Silicon nitride; Alumina; Micmsmxture 1. lntroductioa Because of the complexity of wear processes, a detailed understanding of how ceramic materials react in trihological environments continues to elude both materials and design engineers. For the abrasive wear of brittle materials. mathematical models generally express volume wear as an inverse function of the hardness and the fracture toughness of the material [ 1,2]. However, these. models fail to adequately describe the abrasive wear behavior of most advanced ceramic materials in at least one important way: they assume that bulk hardness and fracture toughness measurements are sufficient to describe the deformation and fracturecharacteristics of the test material in an abrasive wear environment. A number of studies on a wide variety of ceramic materials have indicated that this is not the case [3-S]. In particular, it has been noted that in ceramic materials which exhibit increasing fracture toughness with increasing crack length (called Rcurve or T-curve behavior), such as alumina and zirconia ceramics and many ceramic-based composites, the measured bulk fracture toughness may not describe the fracture tougbness of the ceramic at the micmstructural scale where abrasive wearmechanismsareactive [6-g]. Ceramic-reinforced ceramic composites represent one of the best developments so far in the race to produce tough, yet 0043.1648/97/$17.OO Q1997 Elsevier Science .%A AU tights reserved PllSOO43.1648(96)07348-6 mechanically reliable, ceramic materials for advanced sttttcturalapplications,andp~cular)yforapplicationsatelevated temperatures. Over the course of the last decade, much research has gone into the tmderstandhtg of precisely how the addition of reinforcement phases can intloence the strength and toughness of the bulk ceramic material. For whisker-reinforced ceramic materials, enhanced toughness can occur through any one. or several, of the followhtgmechanisms: whisker &bonding, whii pullout, crack bridging. and/or crack deflection [S-14]. However, in almost eveq case, :arge-scaie toughening requires that these mechanisms be activated over some distance behind the tip of a propagating crack. Thus, while the long-crack toughness of the bulk material may be enhanced by the pmsence of the whisker reinforcement, the short-crack toughness of the bulk material may remain ttnchanged, or even ba degraded, by the preaenca of these whiskers. Studies of how the preseneo of whisker reinforcement influences the ttibological pmpetties of ceramic materials are largely absent, although sevaral studies have addressed the nlative performance of whiia-reinforced ceramic matrix composites in sling and abrasive wear environments [45.15-111. In thii study. we begin to examine how randomly oriented SE whiskers ittflucnce the abrasive wear behavior of several silicon nitride- and ahtmina-based ceramic materials by comparing the micmsbuc-

268 C.P. Do&m. J.A. Hawk/ Wear 203-204 (1997) 267-277 mres, hardness, fracture toughness and wear behavior of both the reinforced and unreinforced materials. 2. Experimental Three commercially available, Sic-whisker reinforced composite materials were selected for this study, along with several chemically similar but unreinforced matix materials for comparison. The first pair of silicon nitride-based materials, S&N,-A and S&N,-A + SIC,, were processed in an identical manner, except for the addition of 15 vol.% SIC whiskers to the composite material. The second series of silimn _..L...C nrAA^-5ised Y ceramics, S&N.-B and S&N.,-B +SiC,, were processed somewhat differently than the hrst, in order to produce a different matrix microstructure; the composite material also contained i5 vol.% SE whiskers. For the alumina-based materials, the composite, A120~+SiC,,., consisted of 34 vol.% SE whiskers in a high-purity alumina matrix. Because no similarly processed monolithic alumina was available, two high purity ahuninas-a 99.8% AlzO, of relatively high hardness, and a 99.5% A1203 of intermediate hardness-were selected for comparison with tbe composite. Microstructural characterization of these materials was accomplished primarily by transmission electron microscopy (TEM), in combination with chemical analysis by X-ray energy dispersive spectroscopy (XBDS). Analysis of the materials’ microstructural response to the various abrasive wear tests was by scanning electron microscopy (SBM) in secondary electron imaging mode. Sample preparation for TBM analysis followed traditional ceramographic techniques, including ion milling to electron transparency. Sample preparation for SEM analysis was limited to ultrasonic removal of the wear debris from the wear surfaces, followed by coating with u Au-Pd alloy to prevent charging in the microscope Hardness and fracture toughness were measured for each material on surfaces mechanically polished to a 1 km diamond 8nish. Vickers hardness was determined under a load of 1 kg, with a dwell time of 15 s. Ten separate hardness measurements were taken for each material, with the results averaged. Fracture toughness was measured utilizing the indentation technique described by Anstis et al. [ 191 with the indenting load varied between 10 and 20 kg. depending on what was required to develop a well-defined crack pattern with a measured crack length that was at least three times the diagonal diameter. A minimum of five fracture toughness me‘asurements were made for each material and the results averaged. Material response to two-body abrasive wear was measured utilizing a pin abrasion test designed at me Albany Research Center to simulate abrasion mat occurs during crushing and grinding operations. This test is described in detail elsewhere [ 201, and will not be discussed here-; however, it is important to recognize that during this test the sample was continually exposed to fresh abrasive. Theceram- 3. Results 3.1. Microstructure Micrographs illustrating the general microstructural features of the three composite materials, and &he monolithic 99.8% A&O,, can be found in Fig. 1, and an outline of the microstructural characteristics of all of the ceramics examined in this study can be. found in Table I. With the exception of the 99.8% Al,O,, all ceramics and ceramic composites are liquid-phase sintered materials in which the matrix grams are bonded by a continuous crystalline and/or amorphous grain boundary phase. The composition and nature of these gram boundary phases depend upon the additives and impurities

270 C.P. Do&n, J.A. Hawk/ Wear203-204 (1997) 267-277 the monolithic and composite Si,N,-B ceramics The microstructure 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 “whiskers” of a potassium-modified &Also, phase are also occasionally 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 approximately 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 intermetallic 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, respectively: 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 ] ; howewr, the indentation technique is selected here because it is believed to provide the value most representative of the nearsurface 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 ceramics are not as tough as the silicon nitride.-based ceramics, and that 99.8% AlsOs has the lowest toughness of all of the matcvials 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 hardness, 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 composites 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 abrasive 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 indicates 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” alumina 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

C.P. Do&m. JA. Hawk/ Wear203-204 (1997) 267- 277 Young’s modulus (GPa)’ Hardners (GPa) Sifl.-A 310 15.6 5.4 SiIN.-A + SiC. 335 19.0 6.4 S&N,-B 303 15.0 5.5 Si,N,-B + SiC, 335 16.5 5.4 99.8% ~~0~ 400 19.3 3.4 99.5% Ai@> 386 15.2 3.9 AlzO, + SiC. 395 23.8 4.6 ’ Dntn provided by the rcspcaive manufactums. TPble 3 Abrasive war rawa for ceramics and ceramic composites (66.7 N nomnl load; 16.0 m sliding distance) 100 pm SIC 58 pm Sic 37pmsiC SisN.-A 0.9 (1.8) 9.1 (19.1) 6.6 (13.9) 2.5 (5.2) Si,N,-A + SiC, 1.0 (2.2) 9.9 (20.8) 6.8 (142) 1.8 (3.9) Z&N.-B 1.7 (3.7) 12.4 (26.1) 8.8 (18.6) 2.9 (6.2) Si3N.-B + SiC, 1.9 (4.0) 14.0 (Z9.4) 9.6 (20.2) 3.4 (7.1) 99.8% Al,o, 1.5 (3.2) 8.6 (18.0) 5.9 (i2.4) 2.1 (4.5) 99.5% Al,o, 6.4 (13.4) 23.6 (49.7) 22.1 (46.6) 5.3 (11.2) Al,O,+SiC_ 0.7 (0.9) 4.8(101) 3.3 (6.9) 0.9 (1.8) (Table 2). For the monolithic aluminas, 99.8% A&O, is much more V.VP -&ant than is the !XW& A!$, urder all conditions tested. This large variation in abrasive wearbehavior between the two high purity ahuninas is most likely the result of the difference in grain size and grain s&distribution between the two materials (a smaller grain size and narrower grain sizedistribution is known to enhance wear performance in alumina ceramics [ 25-291). although the difference in grain boundary microstructure also plays a contributing role by influencing the residual stress state of Ihe ceramic [ 6,301. A comparison of the top performing silicon nitride (S&N.- A) with the top performing alumina (99.8% A&O,), shows that the alumina, with the higher ha&e88 but lower fracture tOughIh?SS, possesses a lower wear rate for all SIC abrasive environments. Against the softer aluminabrasive, however, Si,N4-A has a 75% lower wear rate than the 99.8% A1203. thanks to its higher fracture toughness. Perhaps the most interesting result of this study is the observation that tbe addition of 15 vol.% SE whiskers to a silicon nitride matrix either does not affect the abrasive wear behavior of the bulk material, or degrades it slightly in some instances (Figs. 2 and 3). This cccur8 in spite of the fact that the addition of SiC whiskers increases the hardness of both Si,N,materials and increases the fracture toughness of S&N,- A. The only exception to this rule is in the SisN,-A ceramics tested against 400-grit Sic, where the composite outperforms the monolith In the alumina-based ceramics, on the other hand, the addition of Sic whiskers leads to a dramatic improvement io the abrasive wear resistance under all test conditions of this study. III fact, with it8 high ha&e88 and respectable fracture toughness, A&O, + Sii is by far the most wear resistant material examined in thii study. This is in stark contrast to the 99.5% A1203 which, with it8 low hardness and propensity for fracture in all abrasive wearenvironments. is clearly the worst performer. 3.4. Microstructural response to the wear environment In addition to the measured response to various abrasive wear environmenta listed in Tablea 3 and 4. examination of the wear sorf8ces immediately following the wear tests can give clues to the influence of micro8hucture and whisker reinforcement on the wear behavior of a ceramic material. A8 an example, micrograph of the wear surface8 following abrasion against tbe h8rder U&grit SE are provided in Pig. 4, and examples of wear surfaces produced by tests against tbe softer HO-grit Al,Os arc given in Fig. 5. Comparison of tbe micrograpbs in Figs. 4 and 5 clearly indicate varhtions in the microstructural response ofthedifferentccram&totheabrasive wear environments. For the A-silicon nitride materials, the principal re8ponse to the Sic wear en vironment [Fig. 4(a) and (b)] is plastic deformation, although fmcture at wear groove peripheries and delamination within the grooves is also rexlily rppraent Because of its relatively lower hardness. tbe Sii abrasive particles can penetrate more deeply into the surfa~x of the

《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_Al2O3-SiC-21

《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_Al2O3-SiC-21