Evaluation of adhesive strength of chemical vapor deposition diamond films by laser spallation

The adhesive strength of polycrystalline chemical vapor deposition (CVD)-synthesized diamond films deposited on sintered SiC substrates was estimated using laser spallation. A strong pulse expansion wave was produced on the surface opposite the diamond film by the pulse laser breakdown of silicone grease confined by a silica plate and used to cause Mode-I fracture at the interface between the diamond and SiC. The amplitude of the expansion wave was monitored by a fast laser interferometer as a function of laser characteristics and the confining method. The critical laser energy for causing film spallation changed depending on the diameter of the laser beam and the thickness of the energy-absorbing layer (grease) confined by the silica plate. The driving force of the spallation was estimated to be the tensile stress of the expansion wave following the first compression wave, and it was changed significantly by the dynamics of dielectric breakdown. This was demonstrated by the waveform simulation of the out-of-plane displacement using a theoretical Green's function of the second kind for the dipole source. A pulse dipole source with short rise and decading times generates strong expansion waves.