Tough to brittle transition with increasing grain size in 3Yb-TZP ceramics manufactured from stabilizer coated nanopowder

Tetragonal zirconia polycrystals (TZP) are ceramics combining high strength and fracture resistance which are applied in biomedical and engineering applications. In this study, a nanoscale monoclinic zirconia nanopowder was coated with 3 mol-% ytterbia via a wet chemical route. The powders obtained after calcination and milling were consolidated into dense samples by hot pressing at 1275–1500°C for 1 h at 50 MPa axial pressure. Microstructure phase composition and mechanical properties were determined. The materials were initially very fine grained and showed grain growth starting at sintering temperatures exceeding 1350°C. At low sintering temperatures bending strength values of 1000–1100 MPa were recorded along with high fracture resistance of >10 MPa√m. In the sintering temperature range between 1350–1375°C, a steep decline of toughness to a level of 6 MPa√m was observed. This decline in fracture resistance is accompanied by a drop in transformability of the tetragonal phase and grain growth. While TZP from coprecipitated powders tend to become tougher with sintering temperature and increasing grain size, materials from stabilizer coated powders showed an adverse tendency. The tough to brittle transition in the 3Yb-TZP studied was attributed to the elimination of the initial stabilizer concentration gradient in the tetragonal grains.