Gradients in elastic modulus for improved contact-damage resistance. Part I: The silicon nitride–oxynitride glass system Original Research Article

Silicon nitride (Si3N4)-based graded materials were fabricated with controlled, unidirectional gradients in elastic modulus from the surface to the interior. This was accomplished by infiltrating a low modulus silicon oxynitride glass into a dense, higher modulus, Si3N4 ceramic. Elastic Hertzian indentation (spherical indenter) experiments were performed on both the graded and the monolithic Si3N4. While Hertzian indentation of the monolithic ceramic resulted in classical cone cracks, such cracks were completely suppressed in the graded materials at comparable load levels, despite the lower strength and lower toughness of the surface layer comprising glass. Finite element analysis (FEA) of the stresses associated with the indentation was also performed to gain insight into the mechanism for the enhanced contact damage resistance in the graded materials. The computational analysis revealed that the maximum tensile stresses outside the Hertzian contact circle, which drive the cone-cracks, are reduced by approximately 30% relative to those present in the monolithic silicon nitride. This reduction in the tensile stresses more than compensates for the lower toughness at the graded material surfaces, relative to the monolithic Si3N4. The FEA also allowed us to develop some strategies for elastic–modulus-gradients that would lead to further improvements in the cone-crack suppression characteristics of graded materials in general.