Dual-phase magnesia-zirconia ceramics with strength retention at elevated temperatures

Abstract

Two-phase polycrystalline ceramics containing MgO and ZrO₂ were fabricated by pressureless sintering powder compacts in air to near theoretical density. MnO was added as a densification aid in most compositions. For samples fabricated with 20 vol% ZrO₂ and 80 vol% MgO (which actually contained ∼23 vol% ZrO₂(ss) after sintering because some of the MgO dissolved in zirconia), densities in excess of 98% theoretical were achieved at temperatures as low as about 1250° C. However, most of the samples were typically sintered at 1420±10° C. The grain sizes of the two phases, ZrO₂(ss) and MgO(ss), were of the order of 1.4μm. Thermal etching of the specimens showed the presence of very uniform sized domains (approximately 240 nm in size) in zirconia grains. Some samples were also fabricated in which 8 mol% CaO was added in order to stabilize the high-temperature cubic polymorph of zirconia to room temperature. The grain sizes of the two phases in this composition were also of the order of 1.4μm. No domains were observed in zirconia grains in CaO-doped samples. Fracture strength was measured as a function of volume fraction of zirconia. Strength values in excess of 500 MPa have been measured on samples fabricated with 40 vol% zirconia (the amount of zirconia (ss) is ∼43 vol%). Samples of similar composition but with CaO doping exhibited strength of the order of 300 MPa despite an essentially identical grain size and density. Fracture toughness of samples containing CaO was 3.0 MPa m^1/2 while that of the samples without CaO was 5.2 MPam^1/2. No monoclinic phase was observed on either the fracture or the ground surfaces of CaO-doped and undoped samples. Fracture strength and toughness, measured as a function of temperature up to 1000° C, were found to be nearly independent of temperature. The temperature independence of the strength suggests that strengthening and toughening in this material does not occur by transformation toughening.