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An investigation has been carried out into the effects of three common oxide impurities, TiO2, A12O3, and SiO2/ on the properties and behaviour of Y-T.Z.P. These impurities are present in varying amounts in almost all commercially available Y-T.Z.P. materials, and substantial costs are incurred in removing them in the purest systems. However, the effects of these impurities, both individually and in combination have received relatively little study in the published literature, and it has not been made clear to what degree these impurities influence the properties and behaviour of the material. To carry out the investigation it has been necessary to develop a novel technique for introducing the impurities as dopants into a high purity, commercially available Y-T.Z.P., whilst retaining a high degree of chemical homogeneity in the material. The technique developed uses a variant on the alkoxide sol-gel process to coat the individual powder particles with a thin layer of dopant atoms and offers a number of advantadges over other doping techniques. The process could be exploited to solve a variety of ceramic processing problems. The results obtained from impurity doped materials showed that alumina and silica reduced the sintering temperature and promoted enhanced densification at lower sintering temperatures, whilst titania impaired the sintering at lower temperatures. Alumina additions resulted in pronounced grain growth and associated destabilisation of the tetragonal phase of zirconia, particularly for higher sintering temperatures. A factorial experiment was carried out to obtain additional, and previously unreported information. This showed that there were significant interactions occurring between all of the additives investigated some of which appeared to be beneficial. An investigation into the effect of the additives on the mechanical properties (hardness and fracture toughness) was carried out for a range of sintering temperatures. The results of these experiments suggested that the impurities had very limited direct effects on the transformation toughening mechanism, although there were differences in properties associated with the effects of the impurity additions on the microstructures of the sintered materials.

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Laser surface treatment of engineering ceramics offers various advantages in comparison with conventional processing techniques and much research has been conducted to develop applications. Even so, there still remains a considerable gap in knowledge that needs to be filled to establish the process. By employing a fibre laser for the first time to process silicon nitride (Si3N4) and zirconia (ZrO2) engineering ceramics, a comparison with the CO2 and a Nd:YAG lasers was conducted to provide fundamental understanding of various aspects of the laser beam-material interaction. Changes in the morphology, microstructure, surface finish, fracture toughness parameter (K1c) were investigated, followed by thermal finite element modelling (FEM) of the laser surface treatment and the phase transformation of the two ceramics, as well as the effects of the fibre laser beam parameter - brightness (radiance). Fibre and CO2 laser surface treatment of both Si3N4 and ZrO2 engineering ceramics was performed by using various processing gases. Changes in the surface roughness, material removal, surface morphology and microstructure were observed. But the effect was particularly more remarkable when applying the reactive gases with both lasers and less significant when using the inert gases. Microcracking was also observed when the reactive gases were applied. This was due to an exothermic reaction produced during the laser-ceramic interaction which would have resulted to an increased surface temperature leading to thermal shocks. Moreover, the composition of the ceramics was modified with both laser irradiated surfaces as the ZrO2 transformed to zirconia carbides (ZrC) and Si3N4 to silicon dioxide (SiO2) respectively. The most appropriate equation identified for the determination of the fracture toughness parameter K1c of the as-received, CO2 and the fibre laser surface treated Si3N4 and ZrO2 was K1c=0.016 (E/Hv) 1/2 (P/c3/2). Surfaces of both ceramics treated with CO2 and the fibre laser irradiation produced an increased K1c under the measured conditions, but with different effects. The CO2 laser surface treatment produced a thicker and softer layer whereas the fibre laser surface treatment increased the hardness by only 4%. This is inconsiderable but a reduction in the crack lengths increased the K1c value under the applied conditions. This was through a possible transformation hardening which occurred within both engineering ceramics. Experimental findings validated the generated thermal FEM of the CO2 and the fibre laser surface treatment and showed good agreement. However, a temperature difference was found between the CO2 and fibre laser surface treatment due to the difference in absorption of the near infra-red (NIR) wavelength of the fibre laser being higher than the mid infra-red (MIR) wavelength of the CO2 laser. This in turn, generated a larger interaction zone on the surface that was not induced further into the bulk, as was the case with the fibre laser irradiation. The MIR wavelength is therefore suitable for Viability and Characterization of the Laser Surface Treatment of Engineering Ceramics 3 the surface processing of mainly oxide ceramics and surface treatments which do not require deep penetration. Phase transformation of the two ceramics occurred at various stages during the fibre laser surface treatment. The ZrO2 was transformed from the monoclinic (M) state to a mixture of tetragonal + cubic (T+C) during fibre laser irradiation and from T+C to T and then a partially liquid (L) phase followed by a possible reverse transformation to the M state during solidification. The Si3N4 transformed to a mixture of α-phase and β-phase (α→ α+β) followed by α+β and fully transforms from α+β → β-phase. What is more, is a comparison of the fibre laser-beam brightness parameter with that of the Nd:YAG laser. In particular, physical and microstructural changes due to the difference in the laser-beam brightness were observed. This research has identified the broader effects of various laser processing conditions, as well as characterization techniques, assessment and identification of a method to determine the K1c and the thermal FEM of laser surface treated engineering ceramics. Also, the contributions of laser-beam brightness as a parameter of laser processing and the influence thereof on the engineering ceramics have been identified from a fundamental viewpoint. The findings of this research can now be adopted to develop ceramic fuel cell joining techniques and applications where laser beam surface modification and characterization of engineering ceramics are necessary.