Problems associated with manufacturing Si,N,/SiCwhisker composites have been overcome by developing selfreinforced Si,N, with elongated P-Si,N, grains formed in situ from oxynitride glass. This Si,N,-Y,O,-MgO-SO,-CaO-based material has a flexure strength >lo00 MPa and fracture toughness >8 M P a -d 2 . The optimum combination of mechanical properties has been obtained with Y,O,:MgO ratios ranging from 3:l to 1:2, CaO contents ranging from 0.1 to 0.5 wt%, and Si,N, contents between 90 and 96 wt%.

B,C/AI offers a family of engineering materials in which a range of properties can be developed by postdensification heat treatment. In applications where hardness and high modulus are required, heat treatment above 600°C provides a multiphase ceramic material containing only a small amount of residual metal. Heat treatment between 600" and 700°C produces mainly AIB,; 700" and 900°C results in a mixture of AIB, and AI,BC; 900" and 980°C produces primarily AI,BC; and 1000" to 1050°C results in AIB,,C, with small amounts of AI,C, if the heating does not exceed 5 h. Deleterious AI,C, is avoided by processing below 1000°C. All of these phases tend to form large clusters of grains and result in lower strength regardless of which phase forms. Toughness is also reduced; the least determinal phase is AIB,. The highest hardness (88 Rockwell A) and Young's modulus (310 GPa) are obtained in A1,BC-rich samples. AIB,-containing samples exhibit lower hardness and Young's modulus but higher fracture toughness. While the modulus, Poisson's ratio, and hardness of multiphase B,C/AI composites containing 5-10 vol% free metal are comparable to ceramics, the unique advantage of this family of materials is low density (<2.7 g/cm3) and higher than 7 MPa.rn"' fracture toughness.305 306

The carbothermal nitridation synthesis of ␣-Si 3 N 4 is studied using electron microscopy techniques (FEG/SEM and TEM) and chemical composition analysis to characterize the reaction at various degrees of conversion. The reaction follows a nucleation-growth mechanism. Without ''seed'' ␣-Si 3 N 4 in the precursor, the reaction rate is controlled by the formation of nuclei which are associated with a Si-O-C intermediate phase. In the presence of ''seed,'' the limiting step is growth of ␣-Si 3 N 4 onto the ''seed'' nuclei. Growth appears to follow a gas-phase route and is characterized by an irregular porous layer which grows onto the ''seed.'' The porous structure is the result of reaction around carbon particles which are consumed during the process. The presence of admixed ''seed'' Si 3 N 4 in the precursor formulation increases the reaction rate since the nucleation step is eliminated. An activation energy of E = 457 ± 55 kJ/mol for the overall reaction closely approximates that previously reported for the formation of SiO. This result, along with the finding that residual crystalline SiO 2 is present at all stages of the reaction, indicates that the overall reaction rate is controlled by the reduction of SiO 2 . Since reaction at the carbon and SiO 2 contact points is fast, the rate-limiting step is most likely the gas-phase carbon reduction of SiO 2 with CO.

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The possibility of having Sr as an interstitial metal cation in α‐SiAION has been investigated in two systems: a single‐cation system (Si3N4‐SrO‐AlN) and a multication system (Si3N4‐(Y2O3/SrO/CaO)‐AlN). It was found that Sr alone does not form α‐SiAlON and that Sr could only be accommodated in α ‐SiAION in conjunction with Y and Ca. The Sr content of α‐SiAION increased as the total content of (Y + Ca) increased and appeared to reach a limit at 0.5 at.%, or 0.15 atom per α‐SiAlON. Unexpectedly, some of the α‐SiAlON that contained (Sr + Y + Ca) was present as laths or fibers with the c‐axis perpendicular to the hot‐press direction.