The influence of cold isostatic pressing on compaction and properties of Mg-PSZ ceramics

Abstract

Magnesia partially stabilized zirconia (Mg-PSZ) exhibits high fracture toughness with peak values up to 20 MPa m1/2 [1]. The toughening mechanism is associated with the stress-induced tetragonal → monoclinic phase transformation. High fracture toughness is achieved for materials containing transformable tetragonal precipitates generated by eutectoid or subeutectoid aging treatments and controlled cooling of sintered specimens [2, 3]. However, coarse-grained microstructure resulting from a high sintering temperature and subsequent aging as well as generation of surface flaws during the transformation impair the strength of Mg-PSZ ceramics [4, 5]. Refinement of the structure and hence the decrease of the effective defect size (elimination of the large pores) improves the strength of zirconia ceramics [6]. Recently, hot isostatic pressing was used for preparation of fine-grained, high-strength (peak strength 900 MPa) and tough (Mg, Y)-PSZ [7]. The application of high pressure during the green body forming can contribute to refinement of the microstructure of the sintered body by means of increasing a green density and shortening the necessary sintering time. The elimination of macrodefects is also expected. The aim of the present work is therefore to evaluate the influence of high pressures (up to 1500 MPa), applied during the green body forming, on densification and microstructure of sintered Mg-PSZ. The influence of high pressure on mechanical properties is also studied. The ZrO2 powder PMG035 partially stabilized with 3.5 wt % (10 mol %) of MgO (United Ceramics, Stafford, UK) was axially pressed in a steel dye at 80 MPa. Cylindrical pellets 10 mm in diameter and 5 mm high were coated with a layer of natural latex and cold isostatically pressed at 600, 900, 1200 or 1500 MPa. The pressed specimens were sintered in ambient atmosphere at 1300, 1400, 1500 or 1600 ◦C, dwell time 1 h, heating rate 600 ◦C h−1 and cooled to room temperature in 1 h. The density was determined by the mercury immersion method. The volume fraction of open pores and the pore size distribution was measured by mercury porosimetry (Micromeritics Poresizer 9320). The morphology of the pores in sintered specimens was observed by an optical microscope on the polished surfaces (1 μm finish) and evaluated by the image analysis and processing system Quantimet 500 (Leica, Cambridge, UK). The hardness and fracture toughness was measured by Vickers indentation [8] (indenter LECO M400G2, 5 s dwell time at the maximum load 20 N). Bend strength was determined by four point bending (6 and 18 mm inner and outer span, respectively) of bars with dimensions 4×3×25 mm on a Mayes SM50 universal mechanical tester. The faces of the bars were diamond polished to 6μm finish and the tensile edges bevelled to eliminate edge defects. A minimum of 15 bars were measured for each sample. The phase composition was determined by X-ray diffraction (XRD) analysis on a SIEMENS 5000 diffractometer operating at 25 kV (CuKα radiation). The weight fraction of the monoclinic phase in the specimens was calculated from integrated intensities of monoclinic (111), (111) and tetragonal/cubic (111) diffraction peaks [9]. Polished specimens were etched chemically (concentrated HF, 5 min) and gold coated. Microstructure was studied by scanning electron microscopy (SEM) JEOL JSM35. The mean grain size was determined by the lineal intercept method [10]. The density of Mg-PSZ green bodies increased significantly with the pressure applied during the compaction. The green density of axially pressed bodies was 2.99 g cm−3. Subsequent isostatic pressing at 600