PULSED LASER ANNEALING AND ION BEAM MIXING OF NI-COATED ALPHA SiC AND THE SUBSEQUENT EFFECT ON FRACTURE STRENGTH.

Polycrystalline alpha -SiC possesses excellent mechanical properties and is best known for its high temperature, high strength applications. Surface modification techniques are currently being investigated which can increase the fracture strength. Two such surface modification techniques are ion beam mixing and pulsed laser annealing. Thin Ni over-layers (20-100 nm) are evaporated onto the SiC surface. The specimens are subsequently irradiated with pulses of a ruby or excimer laser or bombarded with high energy ions. As a result of each of the surface modifications, non-equilibrium conditions are established at the specimen surface and under optimum processing conditions, the fracture strength can be significantly increased. Following laser irradiation, the fracture strength of the SiC is increased by as much as 50%, but after ion beam mixing, there is no observed strength increase. Laser annealing of uncoated SiC, a process known as laser glazing, results in a considerable decrease in strength. Scanning electron microscopy (SEM) cross-section and high resolution transmission electron microscopy (X-TEM and HREM, respectively), and Rutherford backscattering (RBS) techniques are being used to chemically and structurally characterize the modified surfaces following processing. The complex processes involved in the two modification techniques and the mechanisms responsible for the observed changes in fracture strength are emphasized.