Residual Stress Distribution in the Sub-Surface Region of Shot-Peened Ceramics

Abstract

The surface of silicon nitride ceramics was modified by fine particles bombarding (FPB) of high strength steel with 50 μm in diameter and by ultrasonic shot peening (USP) of tungsten carbide shots with 1.1 mm in diameter. The residual stress in the sub-surface region measured by the X-ray diffraction method was compression. For FPB surfaces, the sin² ψ diagram was nonlinear, indicating a steep gradient of the residual stress distribution in the sub-surface region. The distribution of the residual stress was determined by the constant-penetration-depth method. The stress on the surface was a large compression of about -1.5 GPa and diminished 5 μm below the surface. The compressive stress increased with the shot pressure for FPB. For USP surfaces, the compressive stress was nearly constant with in the layer of 10 μm and increased up to -1.5 GPa with the peening time. The half value breadth of diffraction profiles increased nearly proportionally with the magnitude of the compressive residual stress. The fracture toughness measured by the Vickers indentation method also increased with the compressive residual stress.