JOURNAL OF MATERIALS SCIENCE 39 (2 0 0 4 ) 4855 – 4860 Dopant-dependent oxidation behavior of α-SiAlON ceramics JAY YU, HENRY DU Department of Chemical, Biomedical and Material Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, USA ROMAN SHUBA, I-WEI CHEN Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA Oxidation behavior of α-SiAlON ceramics doped with Yb, Y, Nd, Ca and Li to the same general molecular formulation was studied at 1000 ◦ –1300 ◦ C in dry oxygen. Oxidation resistance of α-SiAlON ceramics increased in order of Li, Ca, Nd, Y, and Yb dopants. Oxide layers grown on Nd-, Y-, and Yb-doped samples were dense and crack-free whereas those formed on Ca- and Li-doped samples were porous. The dopant-dependent oxidation behavior of α-SiAlON ceramics can be attributed to the effect of the dopants on refractoriness of grain boundary phases and oxide layers as well as to the associated changes in the rate of grain boundary diffusion of dopants and oxygen diffusion in the oxide layers. C  2004 Kluwer Academic Publishers 1. Introduction SiAlON ceramics, the solid solutions of Si 3 N 4 with oxygen, aluminum, and other select metal dopants, con- tinue to attract considerable attention in both the re- search as well as the user community due to their ex- cellent mechanical properties and thermal stability at high temperatures [1–7]. They are easier to sinter rela- tive to Si 3 N 4 , thus rendering them significant cost ad- vantage. Recent advance in making intrinsically hard α-SiAlON ceramics with whisker-like or elongated grain microstructures has led to the development of highly promising α-SiAlON ceramics with combined hardness, strength and fracture toughness [8]. The race is on to further improve the microstructure and grain boundary crystallinity of α-SiAlON ceramics, all of which will drive and accelerate their practical utiliza- tion in applications such as aircraft bearings, cutting tools, and turbine engine components [8, 9]. Silicon-based ceramics, e.g., SiAlON, Si 3 N 4 , and SiC, all share a common problem of oxidation degrada- tion at high temperatures due to their thermodynamic instability in an oxidizing environment. A large body of literature exists on oxidation of Si 3 N 4 . While it is broadly recognized that pure and dense Si 3 N 4 exhibits oxidation resistance superior to all other silicon-based ceramics, the oxidation resistance of hot-pressed or reaction-sintered Si 3 N 4 is not as robust and is a strong function of the type and the amount of additive and impurity cations [10]. Of particular interest are recent oxidation studies of Si 3 N 4 surface-alloyed with alu- minum by implantation [11–13]. It was reported that aluminum implantation significantly reduced outward diffusion of Mg cation from the bulk ceramic to the oxide layer during oxidation in dry O 2 , thus enhancing the oxidation resistance of Si 3 N 4 sintered with MgO [11]. Similarly, incorporation of aluminum in Si 3 N 4 markedly suppressed the adverse effect of sodium on the oxidation resistance of Si 3 N 4 in O 2 enriched with NaNO 3 [12, 13]. Oxidation behavior of Si 3 N 4 contain- ing additives, impurities, as well as alloying elements (e.g., Al) are clearly dictated by how these species alter the characteristics of the oxide layers as well as ceramic grain boundaries. Such alteration significantly changes the rate of transport of oxygen in the oxide layer and of additive/impurity cations in the grain boundaries. Oxidation investigation of SiAlON ceramics has been sparse, compared to their Si 3 N 4 counterparts. Sev- eral comprehensive studies, particularly those by Ny- gren’s team of Sweden, have yielded significant insights into the oxidation behavior of SiAlONs [14–18]. This ceramic system, with a large variety of choice dopants such as Nd, Sm, Gd, Y, Er, Y, Ca, and Li is far more complex in composition, phase types and structure. The complexity of the oxidation process for SiAlON ceram- ics has been reflected in the earlier oxidation studies [15–18]. The rate of oxidation of α-SiAlON ceram- ics has been shown to vary by orders of magnitude due to vastly different dopants that these ceramics can accommodate [15, 16, 18]. For instance, doping with Nd, Sm, Y, and Yb led to increased oxidation resis- tance of α-SiAlON ceramics in the same dopant order [15]. In general, the more refractory the grain bound- ary phases and oxide layers are the better the oxida- tion resistance of α-SiAlON ceramics. This family of ceramics is not yet competitive compared with Si 3 N 4 from high-temperature oxidation stability viewpoint due mainly to high concentration of additive species. This gap is narrowing with continued proactive control 0022–2461 C  2004 Kluwer Academic Publishers 4855