Gradients in elastic modulus for improved contact-damage resistance. part ii: the silicon nitride–silicon carbide system Original Research Article

In an effort to create elastic-modulus (E) graded materials for contact-damage resistance—free of substantial amounts of glass—silicon nitride (Si3N4)-silicon carbide (SiC) graded materials were processed. The structure of these graded materials is such that Si3N4 (E=300 GPa) is at the contact surface 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 technique, was used to achieve this structure. The liquid phase used was yttrium aluminum garnet (YAG). Under spherical indentation, cone-cracks did not form in the graded material, but some inelastic shear deformation was observed. Cone cracks formed in both the monolithic Si3N4 and the monolithic SiC end member materials under identical indentation conditions. Finite element analysis (FEA) of the stresses associated with indentation revealed that the maximum principal tensile stresses outside the Hertzian contact circle, which drive the classical cone-cracks, are reduced by approximately 12% in the graded material relative to the monolithic silicon nitride case. This 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 mm depth in Si3N4-SiC, as compared with the power-law, steeper E-gradient over 0.4 mm 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 residual stresses may be present in the Si3N4-SiC graded material, which are also likely to aid in the suppression of cone-cracks.