Laser surface treatment of engineering ceramics and the effects thereof on fracture toughness

Surface treatment of Si3N4 and ZrO2 engineering ceramics with a CO2 laser and a fibre laser was conducted to identify changes in the fracture toughness (K1c). Vickers macro hardness indentation tests were employed prior to and after the laser treatment to investigate the near surface changes in the hardness of the engineering ceramics. Optical microscopy was then used to observe the near surface integrity, crack lengths and crack geometry within the engineering ceramics. A co-ordinate measuring machine was used to observe the diamond indentations and to measure the lengths of the cracks in the ceramics. Thereafter, computational and analytical methods were employed to determine the K1c. A decrease in the near surface hardness and the resulting crack lengths was found with both materials after the laser treatment. This in turn led to increase in the K1c for both engineering ceramics. A rise in K1c of 64 % for Si3N4 and 40 % was obtained for ZrO2 ceramics using the CO2 laser treatment. Fibre laser treatment induced 50 % increase with ZrO2 and 51 % with Si3N4 ceramics. The likely cause of this increase is the softening of the near (top) surface layer through some degree of melting and redistribution of the melt zone, along with surface oxidation that changed the engineering ceramics composition.