Sliding-Wear-Resistant Liquid-Phase-Sintered SiC Processed Using a-SiC Starting Powders Oscar Borrero-Lo´ pez, Angel L. Ortiz,* ,w and Fernando Guiberteau Departamento de Electro´ nica e Ingenierı´a Electromeca´ nica, Escuela de Ingenierı´as Industriales, Universidad de Extremadura, 06071 Badajoz, Spain Nitin P. Padture** Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210 Low-cost a-silicon carbide (SiC) starting powder, instead of the more expensive b-SiC starting powder, has been used to process liquid-phase-sintered (LPS) SiC ceramics with different micro- structures: (i) elongated SiC grains (in situ toughened LPS SiC), (ii) fine equiaxed SiC grains, and (iii) coarse equiaxed SiC grains. The effects of microstructure on the sliding-wear prop- erties of these LPS SiC ceramics have been studied. The sliding- wear resistance of the in situ toughened LPS SiC ceramic is found to be significantly better than that of two equiaxed-grain LPS SiC ceramics. This has been attributed to the existence of a hard, interlocking network of elongated SiC grains and the isol- ated nature of the yttrium aluminum garnet (YAG) second phase in the in situ toughened LPS SiC ceramic. This is in contrast to the equiaxed-grain LPS SiC ceramics, where the equiaxed grains are embedded within a continuous YAG phase matrix. The use of the a-SiC starting powder allows the processing of low-cost LPS SiC ceramics that are both sliding-wear resistant and tough. I. Introduction S ILICON carbide (SiC) ceramics are used in tribological appli- cations due to their attractive mechanical, thermal, and physical properties. 1–7 However, solid-state-sintered or hot-pressed SiC ceramics are brittle, limiting their utility in load-bearing applications. Also, the high temperatures and/or pressures required for the processing of SiC ceramics make them expensive. In this context, pressureless liquid-phase sintering (LPS) has been used to densify SiC ceramics, thereby reducing the processing temperatures and eliminating the need for appli- cation of external pressure. 8–10 It has also been shown that LPS SiC ceramics with elongated-grain microstructures can be toughened significantly, the so-called in situ toughened SiC. 11–14 More recently, we have shown that in situ toughened SiC can be highly resistant to sliding wear. 15 It appears that the improved wear resistance in these in situ toughened SiC ceramics is most likely due to the formation of interlocking networks of elong- ated SiC grains, preventing massive grain pullout during the severe-wear phase of sliding wear. 15 Typically, b-SiC starting powders are used for processing in situ toughened SiC ceramics, because the b-a transformation during sintering promotes the growth of highly anisotropic SiC grains. 8,9 The b-SiC powders are manufactured commercially using the Acheson process through a carbothermic reduction of silicon dioxide (SiO 2 ) powder by carbon (C) powder at high temperatures under ambient air. 16 The synthesis of b-SiC pow- ders requires the use of temperatures lower than 17001C because the b polytype (cubic) is metastable, and it tends to transform to a polytypes (hexagonal or rhombohedral) at higher tempera- tures. However, these temperatures are generally insufficient to complete the synthesis reaction, resulting in b-SiC powders that are contaminated with SiO 2 and C reactants. Therefore, exten- sive purification procedures are required to obtain high-purity b-SiC powders. This makes b-SiC powders about two to three times more expensive than a-SiC powders. b-SiC powders can be synthesized directly by chemical-vapor methods, but these powders are even more expensive. Thus, the objective of this study is to explore the use of low- cost a-SiC starting powders in fabricating LPS SiC with micro- structures akin to in situ toughened SiC by making use of the 6H-4H polytypic transformation (both 6H and 4H polytypes are classified as a-SiC), and to study the effects of microstructure on the sliding-wear properties of the resulting ceramics with the intention of making conceptual advancements in the design of highly sliding-wear-resistant ceramics. Here, we show that in situ toughened LPS SiC ceramics fabricated from low-cost a-SiC starting powders are much more resistant to sliding wear than their equiaxed-grain LPS SiC counterparts. II. Experimental Procedure (1) Processing A powder batch was prepared, containing 90 wt% a-SiC pow- der (UF-15, H.C. Starck Inc., Newton, MA), 4.29 wt% Al 2 O 3 (AKP-30, Sumitomo Chemical Company, New York, NY), and 5.71 wt% Y 2 O 3 (Fine Grade, H.C. Starck Inc.). The Al 2 O 3 and Y 2 O 3 powders were combined in the molar ratio Y 2 O 3 :Al 2 O 3 ::3:5 to result in a 7.3 vol% yttrium aluminum garnet (Y 3 Al 5 O 12 (YAG)) liquid phase during the sintering of a-SiC. After successive steps of powder mixing, drying, and deagglomeration, already described in detail elsewhere, 8–11 the powder blend was uniaxially pressed into pellets (25 mm diam- eter, 4 mm thickness) at a pressure of 50 MPa (Model C, Carver Inc., Indianapolis, IN), which were subsequently cold isostati- cally pressed (CP360, AIP, Columbus, OH) at a pressure of 350 MPa. Individual pellets were embedded into powder beds (coarse SiC plus Al 2 O 3 powders) inside graphite crucibles with screwable lids. The pellets were pressureless sintered in a graph- ite furnace (1000–3560-FP-20, Thermal Technology Corp., Santa Rosa, CA) at 19501C for 1, 2, or 7 h in flowing Ar-gas atmosphere, and they will be referred to hereafter as SiC-1, SiC- 2, and SiC-7, respectively. The sintered pellets were cleaned and lightly ground to remove the adhered packing powder. Cross M. Hoffman—contributing editor This work was supported by the Ministerio de Ciencia y Tecnologı´a (Government of Spain) and the Fondo Europeo de Desarrollo Regional (FEDER) under Grant Nos. CI- CYT MAT 2004-05971, UNEX00-23-013, and UNEX05-23-037. *Member, American Ceramic Society. **Fellow, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: alortiz@unex.es Manuscript No. 21957. Received June 29, 2006; approved September 19, 2006. J ournal J. Am. Ceram. Soc., 90 [2] 541–545 (2007) DOI: 10.1111/j.1551-2916.2006.01421.x r 2006 The American Ceramic Society 541