Photodeflection and photoacoustic microscopy of cracks and residual stresses induced by Vickers indentation in silicon nitride ceramic

The feasibility of using photodeflection and photoacoustic microscopy to detect subsurface lateral and vertical cracks, as well as residual internal stresses induced by Vickers indentation in silicon nitride ceramic has been investigated. It is shown that the normal component of the photodeflection signal can be used to detect lateral subsurface cracks and the tangential component to detect vertical cracks. It is established that the sensitivity of the photoacoustic method to residual internal stresses arises from the dependence of the elastic parameters or the coefficient of thermal expansion of the silicon nitride ceramic on the internal stresses. (C) 1997 American Institute of Physics.