Surface engineering of ultra-hard polycrystalline structures using a nanosecond Yb fibre laser: Effect of process parameters on microstructure, hardness and surface finish

The use of lasers for near-net shape manufacturing of cutting tools, made of ultra-hard materials such as polycrystalline diamonds, is recently becoming a standard processing step for cutting tool manufacturers. Due to the different machinability exhibited by microstructurally different composites, the laser processing parameters and their effects need to be investigated systematically when changing the material. In this context, the present paper investigates the effects of a fibre laser milling process (nanosecond pulse duration) on surface topography, roughness, microstructure and microhardness of two microstructurally different polycrystalline diamond composites. Pockets were first milled using a pulsed ytterbium-doped fibre laser (1064 nm wavelength) at different fluences, feed speeds and pulse durations, and finally characterised using a combination of Scanning Electron Microscopy, White Light Interferometry, Energy Dispersive using X-Ray (EDX) and micro hardness analyses. For laser feed speed in the region of 1000 mm/s, micro-indentation tests revealed an improvement of hardness from 75 GPa to 240 GPa at a depth of 350 nm, and to 258 GPa at a depth of 650 nm below which the microstructure is preserved as confirmed by microscopy images of the analysed cross sections. For fluences in the region of 11.34 Jcm−2 a variation of cobalt binder volume between the two composites causes a change in milling mechanism. At fluences below 20 Jcm−2, the proposed milling process for CTM302 resulted in a microstructural change (ultra-hard grain size and Cobalt binder weight), better surface integrity (140 nm) and improvement of micro hardness (up to 258 GPa). The properties achieved through the proposed process achieve better hardness and roughness when compared to laser shock processing. To the best of authors’ knowledge, it is reported for the first time that an increase of hardness accompanied by improved surface roughness can be achieved on polycrystalline diamond through low-energy laser processing.