Evaluation of fracture toughness of ZrO2 and Si3N4 engineering ceramics following CO2 and fibre laser surface treatment

The fracture toughness property (K1C) of Si3N4 and ZrO2 engineering ceramics was investigated by means of CO2 and a fibre laser surface treatment. Near surface modifications in the hardness were investigated by employing the Vickers indentation method. Crack lengths and the crack geometry were then measured by using the optical microscopy. A co-ordinate measuring machine was used to investigate the diamond indentations and to measure the lengths of the cracks. Thereafter, computational and analytical methods were employed to determine the K1C. An increase in the K1C of both ceramics was found by the CO2 and the fibre laser surface treatment in comparison to the as-received surfaces. The K1C of the CO2 laser radiated surface of the Si3N4 was over 3% higher in comparison to that of the fibre laser treated surface. This was by softening of the near surface layer of the Si3N4 which comprised of lowering of hardness, which in turn increased the crack resistance. The effects were not similar in ZrO2 ceramic to that of the Si3N4 as the fibre laser radiation in this case had produced an increase of 34% compared to that of the CO2 laser radiation. This occurred due to propagation of lower crack resulting from the Vickers indentation test during the fibre laser surface treatment which inherently affected the end K1C through an induced compressive stress layer. The K1C modification of the two ceramics treated by the CO2 and the fibre laser was also believed to be influenced by the different laser wavelength and its absorption co-efficient, the beam delivery system as well as the differences in the brightness of the two lasers used.