Fracture Toughness of a Zirconia Engineering Ceramic and the Effects Thereon of Surface Processing with Fibre Laser Radiation

Abstract

Vickers hardness indentation tests were employed to investigate the near-surface changes in the hardness of a fibre laser-treated and an as-received ZrO₂ engineering ceramic. Indents were created using 5, 20, and 30 kg loads to obtain the hardness. Optical microscopy, white-light interferometry, and a coordinate measuring machine were then used to observe the crack lengths and crack geometry. Palmqvist and half-penny median crack profiles were found, which dictated the selection of the group of equations used herein. Computational and analytical approaches were then adapted to determine the K_1c of ZrO₂. It was found that the best applicable equation was: K_1c = 0.016 (E/H)^1/2 (P/c^3/2), which was confirmed to be 42 per cent accurate in producing K_1c values within the range of 8 to 12 MPa m^1/2 for ZrO₂. Fibre laser surface treatment reduced the surface hardness and produced smaller crack lengths in comparison with the as-received surface. The surface crack lengths, hardness, and indentation loads were found to be important, particularly the crack length, which significantly influenced the end K_1c value when K_1c = 0.016 (E/H)^1/2 (P/c^3/2) was used. This is because, the longer the crack lengths, the lower the ceramic's resistance to indentation. This, in turn, increased the end K_1c value. Also, the hardness influences the K_1c, and a softer surface was produced by the fibre laser treatment; this resulted in higher resistance to crack propagation and enhanced the ceramic's K_1c. Increasing the indentation load also varied the end K_1c value, as higher indentation loads resulted in a bigger diamond footprint, and the ceramic exhibited longer crack lengths.