Zirconia ceramic dental restorations: Laser machining and optical testing (invited presentation)

Zirconia ceramic, or more precisely yttria-stabilised tetragonal zirconia polycrystal (Y-TZP) is a high toughness engineering ceramic which is extremely difficult to machine in its final state. It is increasingly used in dental restorations, and here it is essential to machine the required shape from solid (pre-sintered) billets rather than to produce customized moulds, as every restoration is of course different. We have therefore developed laser machining processes to replace the slow mechanical processes using diamond-tipped grinding and drilling tools, and these processes will be described in this presentation. Unfortunately, both these laser processes and the mechanical grinding processes may introduce cracking, resulting in reduced strength in the manufactured parts. Uncertainty of the exact shape, size and distribution of flaws introduced during manufacturing or machining require a reliable nondestructive testing technique. We have therefore developed a novel mid-infrared transmission technique for flaw detection in Y-TZP and similar ceramics, based on the reduced optical scattering at these wavelengths. The light from a broadband infrared source illuminates the sample which is observed by an infrared detector.