N-on-1 Damage Testing of Single Crystal Metal Surfaces at 1.06 μm

We examined the accumulation dependence of laser damage on single crystal metal surfaces under Q-switched Nd:YAG laser irradiation (1.064μm, 20 nsec pulses with a 200μm spot diameter). The samples were chemically polished, electropolished, and single point diamond machined Al, Cu and Ni single crystal surfaces of several crystallographic orientations. The single point diamond machined samples did not have single crystal surfaces as detected by electron channeling patterns, whereas the other samples had good single crystal surfaces. Damage probability curves and accumulation curves were plotted to investigate N-on-1 damage behavior. The slope of the damage probability curve increased with the number of pulses required for damage. Accumulation curves, derived from 50% damage fluences, often had two regions showing different slopes. We interpret these measurements to indicate two different damage mechanisms, a defect related local melting at low pulse numbers and plastic deformation damage at high pulse numbers. These observations correlate with typical damage morphologies obtained by a Nomarski optical microscope and by SEM.