Effect of additives on sintering of zirconia ceramics

The effect of two types of additives on the sintering processes of zirconium ceramics was investigated. The impact of both polyvinyl alcohol (12 mass%) and bismuth oxide (1 mass%) on compaction of ultradisperse powders of stabilized zirconia produced by plasma-chemical method and on densification of the compacts during sintering was studied. In the initial state, plasma-chemical powders demonstrate hindered compactibility and sinterability. The additives were incorporated through mechanical stirring with intermediate sieving using a sieve. It was found that introduction of polyvinyl alcohol significantly reduces wall friction and hence facilitates the compaction process. However, no effect on densification and sintering kinetics was observed. Incorporation of bismuth oxide microadditives decreases the temperature of the peak compacting rate. In one-step sintering, the compacts are found to markedly expand at the isothermal ageing stage. The expansion is more distinct at increased compaction pressure. It is shown that this expansion is associated with an increase in the volume of pores surrounded by the liquid phase of bismuth oxide. Two-step sintering with an intermediate stage of isothermal ageing at 1200 °C shows a 100 °C decrease in the sintering temperature, no expansion of the samples occurs at the final ageing stage at sintering temperature, and the ceramics produced exhibits good operational parameters. Recommendations are given on the use of low-melting microadditives in sintering ultradisperse powders obtained by plasma-chemical method.