Nanosecond pulsed excimer laser machining of chemical vapour deposited diamond and highly oriented pyrolytic graphite: Part I An experimental investigation

Abstract

A laser beam offers the benefits of high precision, contamination-free, high speed, and low bulk temperature for machining of chemically vapour deposited (CVD) diamond thin films that in turn enable ultrafine finishing of diamond coated cutting tool inserts and drills, and for finishing and drilling of diamond coated multichip module applications. In this work, laser hole drilling and polishing of CVD diamond (free-standing diamond and coated tool inserts) and HOPG (highly oriented pyrolytic graphite) using a 248 nm wavelength, 23 ns pulsed excimer laser were conducted. The threshold energy fluence required for ablation of diamond and graphite was nearly the same but the material removal rate rapidly increases with the energy fluence for the graphite compared to diamond. At an energy fluence of 10 J cm-2, the depth removed per pulse was 0.05 μm and 0.30 μm for diamond and graphite respectively. Raman microprobe analysis indicates that the laser machining induced the transformation of diamond to disordered forms of carbon in CVD diamond and some transformation of graphite to diamond in HOPG. The experimental data indicates that the transformation of diamond to graphite requires an energy input of 1.44 × 107 J per mole. For a given set of laser parameters, the depth per pulse was substantially higher for diamond coated tool inserts compared to the free-standing diamond. The surface roughness of CVD diamond was reduced by 0.25 μm per pulse at an energy fluence of 16 J cm-2