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 (K_1c). 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 K_1c. 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 K_1c for both engineering ceramics. A rise in K_1c 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.