Influence of laser beam brightness during surface treatment of a ZrO2 engineering ceramic

A comparative study between fibre and Nd:YAG (neodymium, yttrium, aluminium, garnet) laser surface treatment on a cold isostatic pressed (CIP) ZrO2 engineering ceramic was conducted to investigate the individual differences of laser brightness (radiance) produced by the two laser sources. The effects of brightness exhibited by the two lasers were investigated in respect to the change in the hardness, dimensional size of the laser radiated zones and the microstructure of the ZrO2 engineering ceramic. The results showed that the hardness of the ZrO2 engineering ceramic was reduced by 36% for the Nd:YAG laser in comparison to the as-received surface. However, only 4% reduction in the surface hardness was found from employing the fibre laser surface treatment which was not significant as much as the results of the Nd:YAG laser radiation. The change in hardness occurred due to softening of the laser radiated surface of the ZrO2 with a changed composition which was softer than the laser unaffected surface. The dimensional size of the fibre laser radiated track was also found to produce broader surface profiles in comparison to that of the Nd:YAG laser. The fibre laser radiated surface track was 32% larger in width and 51.5% longer in depth of penetration in comparison to that of the Nd:YAG laser. Change in microstructure of the ZrO2 engineering ceramic radiated by both lasers was found as opposed to the ground and polished untreated surface with the fibre laser affecting the grain morphology to a greater extent in comparison to that of the Nd:YAG laser radiation. The physical and micro-structural effects from applying the two laser types to the ZrO2 engineering ceramic differed as deep penetration and broader laser radiated track as well as larger grains were produced by the fibre laser, despite using identical laser processing parameters such as spot size, power density, traverse speed, gas flow rate, wavelength and the Gaussian beam profile. This occurred due to the high brightness exhibited by the fibre laser radiation which generated larger power per unit area which in turn induced into the ZrO2 engineering ceramic and resulted to producing high processing temperature, larger fibre laser-ceramic-interaction zone and melt-pool at the laser-ZrO2 interface in comparison to that of the Nd:YAG laser which intrinsically resulted to a change in physical attributes of the ceramic.