Gradients in elastic modulus for improved contact-damage resistance. Part II: The silicon nitride-silicon carbide system

In an effort to create elastic-modulus (E) graded materials for contact-dam age resistance-free of substantial amounts of glass-silicon nitride (Si3N4) -silicon carbide (SiC) graded materials were processed. The structure of th ese graded materials is such that Si3N4 (E = 300 GPa) is at the contact sur face and SiC (E = 400 GPa) is in the interior, with a stepwise gradient in composition existing between the two over a depth of 1.6 mm. A pressureless , liquid-phase co-sintering method, in conjunction with a powder-layering t echnique, was used to achieve this structure. The liquid phase used was ytt rium aluminum garnet (YAG). Under spherical indentation, cone-cracks did no t form in the graded material, but some inelastic shear deformation was obs erved. Cone cracks formed in both the monolithic Si3N4, and the monolithic SiC end member materials under identical indentation conditions. Finite ele ment analysis (FEA) of the stresses associated with indentation revealed th at the maximum principal tensile stresses outside the Hertzian contact circ le, which drive the classical cone-cracks, are reduced by approximately 12% in the graded material relative to the monolithic silicon nitride case. Th is reduction is significantly lower than what was calculated for the Si3N4- glass case (Part I), owing to the shallower, linear E-gradient over a 1.6 m m depth in Si3N4-SiC, as compared with the power-law, steeper E-gradient ov er 0.4 nun depth in the Si3N4-glass. It appears that, in addition to the E- gradient, the inelastic deformation contributes to the suppression of cone cracks in the Si3N4-SiC graded material. It is suggested that compressive r esidual stresses may be present in the Si3N4-SiC graded material, which are also likely to aid in the suppression of cone-cracks. (C) 2001 Acta Materi alia Inc. Published by Elsevier Science Ltd. All rights reserved.