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

Silicon nitride (Si3N4)-based graded materials were fabricated with control led, unidirectional gradients in elastic modulus from the surface to the in terior. This was accomplished by infiltrating a low modulus silicon oxynitr ide glass into a dense, higher modulus, Si3N4 ceramic, Elastic Hertzian ind entation (spherical indenter) experiments were performed on both the graded and the monolithic Si3N4. While Hertzian indentation of the monolithic cer amic resulted in classical cone cracks, such cracks were completely suppres sed in the graded materials at comparable load levels, despite the lower st rength and lower toughness of the surface layer comprising glass. Finite el ement analysis (FEA) of the stresses associated with the indentation was al so performed to gain insight into the mechanism for the enhanced contact da mage resistance in the graded materials, The computational analysis reveale d that the maximum tensile stresses outside the Hertzian contact circle, wh ich drive the cone-cracks, are reduced by approximately 30% relative to tho se present in the monolithic silicon nitride. This reduction in the tensile stresses more than compensates for the lower toughness at the graded mater ial surfaces, relative to the monolithic Si3N4. The FEA also allowed us to develop some strategies for elastic-modulus-gradients that would lead to fu rther improvements in the cone-crack suppression characteristics of graded materials in general. (C) 2001 Acta Materialia Inc. Published by Elsevier S cience Ltd. All rights reserved.