C.P. DoBan, /A Hawk/Wew 20j-204459971267-27 the prin. the local fracture toughness (31 cipal microstructural response to the wear environm Si,N,B. The addition of SiC whiskers to these matrices 4. Discussion in the case of S: N A, an increase in bulk fracture toug sker-matrix interface that a whisk parent at the wear surfaces of both Si incrense in the hardness and a 35% increas n than dod of whisker reinforcer mics [6, 7]. In the composite ear environment have on the wear behavior of the debonding and pullout of the SiC 4. 1. influence of microstructure retained the SiC whiskers with their higher hardiess are mate. composit t the s race to enhance t the wear resistance of the ence the tribological per this ds to a difference 1a havior. In Si,N A, the grain boundary hig d in the previous secton Becaise aterial performs equally well under bot ough the ther- tred. it is believed that sults in an increase in the coefficient of thermal expansion compo ot the boundary regons, leading to a concomitant increase n particle sIze, see Hg. 2), t is only hen the abrasive partcleC.P. Uo&n, J.A. Hawk/ Wear 203-204 (I 997) 267-277 99.5% AIrOr [Fig. 5(g) 1, fracture continues to be the prin￾cipal microstructural response to the wear environment. 4. Discussion It is clear from the results of this study that a simple increase in the hardness and/or toughness of aceramic-basedmaterial through the addition of randomly oriented whiskerreinforcc￾ment does not necessarily translate into improved perfonn￾ante in an abrasive wear environment. For example, the addition of IS vol.% silicon carbide whiskers to the S&N.,-A matrix results in a 22% increase in the hardness of the material and a 20% increase in the fracture toughness. Yet, for the most part, the abrasive wear behavior is degraded by the presence of these whiskers. On the other hand, the addition of SIC whiskers to a high-Al,O, matrix resuhs in a 23% increase in the hardness and a 35% increase in the fracture toughness (in comparison to the 99.8% AI*O,) In this case, the addition of SIC whiskers provides a dramatic improve￾ment in the abrasive wear resistance of the ceramic. A clue to this variation in the effect of whisker reinforcement on the wear behavior of ceramic-based comeosites resides in the micr~structures of these materials, and in the residual stresses created by the addition of second phases to the ceramic. A second important aspect is the influence that the variabics of the wear environment have on the wear behavior of the composite. 4. I. hjluence of microstructure Variations in the microstructures of the monolithic mate￾rials. as well as in the microstructures of the composites, can be directly linked to variations in the abrasive wear behavior, primarily through their role in the creation of internal stress. In ceramics it is well known that differences in the thermal expansion between phases of a multiphase material, as well as anisotropic thermal expansion in a single phase material, can result in the creation of residual tensile and compressive stresses at hetero- and homophase boundaries. The magnitude of such residual stresses, which are proportional to both the expansion mismatch (A a) and the temperature range over which the stresses develop (AT). can be sufficient todirectly influence the tribological performance of the bulk ceramic [6-8,25-291. In the unreinforced silicon nitrides of this study, a difference in the grain boundary microstructures leads to a difference in the residual stress state at the grain boundaries of the two materials, and therefore, to a difference in abrasive wear behavior. In S&N,-A, the grain boundary regions have nearly completely crystallized to a-Y,SizO,. whereas in S&X.+-B the grain noundary regions consist of an amorphous yttrium aluminosilicate phase. Although the ther￾mal expansion coefficients of the boundary phases have not heen measured, it is believed that ctystallizationofa-Y&O, results in an increase in the coefficient of thermal expansion of the boundary regions, leading to a concomitant increase in the local fracture toughness [ 311. As a result, the abrasive wear behavior of S&N.-A is improved relative to that of Si3N4-B. The addition of SIC whiskers to these matrices results in an increase in the hardness of the composites and, in the case of Si$$-A, au increase m bulk fracture toughness. However, the narrow glass phase which wets the SE whisk￾ers creates a relatively weak whisker-matrix interface that leads to easy debonding of the whisker during fracturetype events (the origin of enhanced long-crack fracture toughness m this composite). As a result of this debonding, the SIC whiskers are easily pulled from the surface of the composite in an abrasive wear environment, leading to enhanced mab￾rial removal, and degradation of the wear properties when compared to the monolithic SiJ$. Regions of whisker debonding are apparent at the wear surfaces of both S&N,- based composite materials. In the alumina-baaed ceramics, matrix gram size plays a decisive role in determining the abrasive wear behavior [6- 8.25-291. Thus, the 99.8% AlaOs. with a smaller gram size and narrower grain size distribution, has a much lower wear rate than does the 99.5% AlaOs. Enhancing the grain size effectistheinRuenceofgrainbcumdaryphasesontheresidual stress state of the alumina ceramics [6,7] _ In the composite material, tbe addition of the relatively lower expansion Sic whiskers to the alumina matrix creates residual compressive stresses at the whisker-matrix interfaces. As a result, the whisker-matrix interfaces are locally tougher, resisting the debonding and pullout of the SE whiskets in the ahtasive wearenvironmentunderalltestconditionsofthisstudy.Thus retained, the Sic whiskers with their higher hardness are avai!abb at me surtace to enhance the wear resistance. of the composite material. 4.2. Injluence of the wear environment Variables in the wear environment combine with variables in the material properties to determine the wear resistance of a composite material. Under the relatively “softer” abrasive wearconditionsofthealuminaabrasive,theprimaryresponse of all composites is plastic deformation with only limited fracture. However, the presence of whisker reinforcement in the silicon nitrides tends to increase the amount of fracture observed at the wear surface relative to that of the monolithic materials. This results in a higher measured wear rate and suggests that the relatively weaker SisN&iC interfaces are the origin of this increased surface hnchtre. Under the more aggressive condttions of the harder SE abrasive, SIC whisker debonding and pullout in the S&N.+ composites results in a higher wear rate, as described in the previous w&m. Bccatts~ of tbc stronger matrix-whisker interfaces in the AlaO, + SIC, composite, this material performs equally well under both “soft” and “hard” abrasive conditions. While abrasive particle size does influence the measured wear rate of both the monolithic ceramics and the ceramic composites (increasing the abrasive wear rate withincreasing particle size, see Fig. 2), it is only when the abrasive particle