Zirconia (ZrO2) offers three advantages over alumina: it is more refractory, having a melting point some 500oC above alumina. It is readily machined and gives a high quality surface finish and is a useful oxygen anion conductor for use in sensors and fuel cells. However, it is considerably more expensive than alumina.
Zirconia has the complication of being metastable and needs to be stabilised before it can be put to use. The addition of small quantities of stabilising oxides such as CaO, MgO and best of all Y2O3 allow the high temperature cubic phase to stabilise itself. Partially stabilised zirconia can also be used as a toughening agent in alumina. The zirconia-toughened alumina (ZTA) shows a considerable improvement in strength and more importantly toughness. As a result, these ceramics can be used in areas of extreme mechanical abrasion and thermal shock.
Fully stabilised zirconia (FSZ) produces a high-density ceramic on sintering. The grain size is relatively large at 10-40m m and the ceramics are often translucent in appearance. Recent optimisation of zirconia powders have resulted in a readily sinterable high purity product.
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