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

Abstract

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 12751500°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 10001100 MPa were recorded along with high fracture resistance of >10 MPa√m. In the sintering temperature range between 13501375°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.

Evidence of Phase Transitions and Their Role in the Transient Behavior of Mechanical Properties and Low Temperature Degradation of 3Y-TZP Made from Stabilizer-Coated Powder

“…The inverse grain size toughness correlation in Y-TZP ceramics made from monoclinic starting powders either very homogeneously mixed or coated with the stabilizer oxide was shown in many previous publications by different authors [16,17,[22][23][24]. The experimental evidence however stands in sharp contrast to proven findings on Y-TZP materials made from co-precipitated powders which show a positive grain size -toughness correlation.…”

“…In such a situation self-accomodation of tetragonal domains is favored and the shear dependent part of the t-m phase transformation can be exploited. This was already supposed in an earlier publication by the author however only supported by an a priori assumption and a misfit of experimental data to the accepted theory [23]. The "real" transformation toughness increments are however difficult to calculate.…”

“…The manufacturing of the stabilizer-coated powders by the nitrate route was first described by Yuan [20]. The modified procedure applied here has been shown by the authors in detail elsewhere [23,24]. The starting powder was a nanoscale monoclinic powder (S BET = 15-17 m 2 /g).…”

“…It is believed that the grains initially have an yttria-rich and non-transformable (by either stress or LTD) shell and a low yttria content core which is very transformable (and tough). Early separation of cubic phase is believed to be favored as the stabilizer does not need to diffuse against the concentration gradient [22,23].…”

Yttria Ceria Co-Stabilized Zirconia Reinforced with Alumina and Strontium Hexaaluminate

“…The starting powders for the study were monoclinic zirconia (d 50 = 300 nm, S BET = 7 m 2 /g), α-alumina (d 50 = 300 nm, S BET = 8 m 2 /g), yttria (purity 99.9%), ceria (purity 99.9%), and strontium zirconate (d 50 = 5 µm, purity 99%). In a first powder technological step, the monoclinic zirconia was coated with 1 mol% of yttria according to the nitrate route, as described in detail elsewhere [26,27]. The coating process was chosen to avoid inhomogeneous yttria distribution.…”