Effect of sintering additives on the densification and mechanical properties of alumina toughened zirconia ceramic composite / Mohamed Kamaleldin Gaffar Abbas

Mohamed Kamaleldin , Gaffar Abbas (2022) Effect of sintering additives on the densification and mechanical properties of alumina toughened zirconia ceramic composite / Mohamed Kamaleldin Gaffar Abbas. PhD thesis, Universiti Malaya.

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Abstract

In this research, the influence of transition metal oxide and stainless steel on the sintering behavior of alumina toughened zirconia (ATZ: 20 wt% Al2O3 + 80% Y-TZP) was studied. In Phase 1, selective dopants i.e. 0.2 wt% stainless steel, 0.5 wt% manganese oxide and 0.2 wt% copper oxide were doped with the ATZ. The prepared ceramic composites were subjected to conventional sintering (CS) at 1250 °C, 1350 °C and 1500 °C, using different holding times of 1, 12, 120 min. The tetragonal phase stability, microstructural evolution, relative density, Vickers hardness and the fracture toughness of the samples were assessed. In Phase 2, the sintering behaviour of these ATZs was compared with that obtained by using microwave (MW) sintering at the same temperature regimes with a holding time of 5 min. The results revealed that all the dopants were beneficial in aiding the densification of ATZ, particularly at low temperatures and short holding times. In particular, the addition of copper oxide (CuO) was found to be most effective in enhancing the properties of ATZ when sintered at 1250 °C and 1350 °C using a short holding time. The results indicated that sintering at 1500 °C was unnecessary for the copper-doped ATZ to attain a maximum relative density of > 98%, Vickers hardness of > 12 GPa and fracture toughness of > 5 MPam1/2. In contrast, the undoped ATZ required a longer holding time and high sintering temperature to achieve similar results as the doped samples, but this scenario could be overcome through microwave sintering. Phase 3 of this research focused on the effect of doping various amounts of CuO (0.05 wt% to 1 wt%) on the densification behaviour of ATZ using both the CS and MW methods. It was found that the tetragonal phase stability was not disrupted for addition up to 0.2 wt% CuO. However, a spontaneous t to m phase transformation upon cooling from sintering was inevitable for the 0.5 wt% and 1 wt% CuO doping, with amounts exceeding 30% being measured in these samples. The m phase development in the ATZ matrix was accompanied by low density and poor mechanical properties. The addition of 0.2 wt% CuO was found to be optimum amount for doping and the sintered composite exhibited overall best performance i.e. a relative density of about 92% was obtained at 1250 C, about 97% dense at 1350 C and above 99% dense at 1450-1500 C. This was also accompanied by high Vickers hardness of 12.7 GPa and fracture toughness of 6.94 MPam1/2 when sintered at 1450 °C/12 min. The results also showed that in general the Vickers hardness increased linearly with increasing relative density regardless of sintering methods. Finally, in Phase 4, the effect of low-temperature degradation (LTD) or hydrothermal ageing on the undoped ATZ and 0.2 wt% CuO-doped ATZ was investigated by exposing the samples to autoclave conditions containing superheated steam at 180 ℃/10 bar for periods up to 100 h. It was found that the tetragonal phase stability was disrupted for samples sintered above 1400 C and that the CuO dopant was not effective in suppressing the ageing-induced phase transformation. A critical tetragonal grain size limit of about 0.23 m was determined for the present ATZ, above which resulted in LTD.

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