Unprecedented hardness of polycrystalline diamond via laser surface engineering

The use of polycrystalline diamond (PCD) as a tool material for machining intractable alloys has increased due to their excellent abrasive and wear properties. In this study, for the first time, a novel low-energy/high-frequency laser shock peening technique (LE/HFLSP) is proposed to control the mechanical and surface properties of PCD. This technique utilises laser energies below the ablation threshold (10-100 mJ), high frequencies (35-70 kHz) and short pulse durations (in the region of ns) and is applied to the target material with/without sacrificial coatings. The results of the samples processed using this novel technique without coatings revealed a 14% increase of micro-hardness in comparison to the commercially available PCDs. In addition, this technique showed a reduction in cobalt volume by 18.42% compared to the case using coatings increasing in the range of 16.4 to 66.3%. It was also observed that using vinyl & quartz as sacrificial coatings, improved the hardness from 150 GPa to 230 GPa. On the other hand, fluence above 102.29 J/cm² processed with vinyl and soda-lime glass as ablative and transparent overlays showed softening of the material. A transition between tensile to compressive stress has been observed when processed with vinyl and quartz. Graphitisation and deformation of cobalt have been observed from the results obtained from XRD and Raman spectroscopy. The results showed a correlation between the surface modification, micro-hardness and phase changes for PCDs revealing a relationship between the phase transition and hardness to the laser fluence. It is believed that a suitable range of coatings with a variety of acoustic impedances can be effective in controlling the mechanical properties of PCD.