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 ZrO 2 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 ZrO 2 . It was found that the best applicable equation was: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 ZrO 2 . 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 ¼ 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.

Development in laser peening of advanced ceramics

“…Typically, laser beam absorption also depends on the particular wavelength applied at the material surface. Particularly for ceramics, laser in the range of 1µm wavelength and under, tends to produce sufficient absorption [14][15][16][17] . …”

Fracture toughness modification by using a fibre laser surface treatment of a silicon nitride engineering ceramic

“…Measured hardness and the crack lengths from the Vickers indentation test are placed into an empirical equation to calculate the materials K 1c [7,8,13,28]. The results from the Vickers indentation test can then be applicable to the empirical equations which were derived by Ponton [7,8], Chicot [31] and Liang et al [32].…”

“…It can be predicted that the outcome for most of the crack profiles in this study would be of median half-penny shape. For cracks that are of median half-penny shape the applicable equations differ (see Equations 1-15) [7,8,13]. …”

“…All tested samples were investigated for their top surface hardness, generated crack profiles from the diamond indentations and micro-structural changes before and after the fibre laser treatment. Shukla and Lawrence [13] recently investigated the effects on the K 1c using a fibre laser to surface treat a ZrO 2 engineering ceramic which showed changes in the K 1c of the ZrO 2 ceramic, however, the effects of the fibre laser interaction with ZrO 2 are different to that of the Si 3 N 4 in comparison. This why the effects of the fibre laser treatment on the K 1c of the Si 3 N 4 ceramic was explored in this study.…”

“…Despite the use of industrial lasers such as CO 2 , Nd:YAG, HPDL and an excimer to process various technical ceramics (see Table 1 for further details); no other investigation except the one of Shukla and Lawrence [13] has been done hitherto by employing a fibre laser to process engineering ceramics.…”