Response of silicon nitride ceramics subject to laser shock treatment

Abstract

A comprehensive and novel investigation on multiple-layer, square-beam laser shock treatment (“laser peening”) of Si 3 N 4 ceramics is reported in this work. Surface topography, hardness, fracture toughness (K Ic ), residual stresses, and microstructural changes were investigated. The evaluation of fracture toughness via the Vickers hardness indentation method revealed a reduction in crack lengths produced by the indenter after laser shock treatment (LST). Upon appropriate calculation, this revealed an increase in K IC of 60%. This being attributed to a near-surface (50 μm depth) compressive residual stress measured at −289 MPa. Multiple layer LST also induced beneficial residual stresses to a maximum measured depth of 512 μm. Oxidation was evident, only on the top surface of the ceramic, post LST (<5 μm depth) and was postulated to be due to hydrolyzation . The surface enhancement in K IC and flaw-size reduction was assigned to an elemental change on the surface, whereby, Si 3 N 4 was transformed to SiO 2 , particularly, with multiple layers of LST. Compressive residual stresses measured in the sub-surface were attributed to mechanical effects (below sub-surface elastic constraint) and corresponding shock-wave response of the Si 3 N 4 . This work has led to a new mechanistic understanding regarding the response of Si 3 N 4 ceramics subject to the LST deployed in this resesrch. The findings are significant because inducing deep compressive residual stresses and corresponding enhancement in surface K IC are important for the enhanced durability in many applications of this ceramic, including cutting tools, hip and knee implants, dental replacements, bullet-proof vests and rocket nozzles in automotive, aerospace, space and biomedical industries.