Origin of unusual sintering phenomena in compacts of chloride-derived 3Y-TZP nanopowders

After evaluating three alternative possibilities, the present study shows that seemingly minor amounts (at least as low as 0.06 wt%) of chlorine impurities are responsible for the poor sintering behavior observed in chloride-derived 3 mol% yttria stabilized zirconia (3Y-TZP) nanopowders. Models and quantitative estimates are used to explain the role of evolved HCl and ZrCl4 gases in such anomalous behaviors as reduced sintered densities for higher green densities, de-densification, improved sintering in nitrogen over oxygen, and formation of a dense shell microstructure. Two solutions to problematic residual chlorides are compared: 1) a thermal treatment composed of an extended hold at 1000°C to allow HCl gas removal before the onset of closed porosity, and 2) a chemical treatment performed by washing bisque-fired samples at room temperature using a concentrated ammonium hydroxide solution to remove chlorides. The thermal treatment was found to be superior.