Enhanced sintering properties of yttria face coat by addition of B₂O₃, YF₃, Al₂O₃ and ZrO₂

Yttria is widely used as a key material in the manufacture of the mould face coat for the investment casting of Ti or titanium aluminide alloys, as it has the required thermal and chemical inertness. Because of the high melting temperature of yttria, moulds manufactured using an yttria face coat need high sintering temperatures to achieve a good surface finish, which increases the production cost of the mould. In this research, small amounts of sintering aids, B₂O₃, YF₃, Al₂O₃ and ZrO₂, were added into a simple yttria face coat and sintered at temperatures from 1000 to 1400°C to investigate the influence on the sintering properties of yttria. These experiments found that the sintering additive B₂O₃ showed the best performance in enhancing yttria face coat sintering over the test temperatures. Sintering aids such as YF₃ only slightly altered the sintering properties of yttria at temperatures around 1000–1200°C, and powders such as YAZ were seen to be more suitable for sintering at temperatures around 1300–1400°C. It may be possible to use these sintering aids for the production of investment casting moulds.