Strength and fracture toughness of isostatically hot-pressed (Y,Nb)-TZP/Al2O3 composites

Abstract

Among transformation-toughened ceramics, tetragonal zirconia polycrystals (TZPs) have been considered as the biomaterials for dental and orthopedic prostheses due to their excellent biocompatibility and mechanical properties [1, 2]. Although yttria-stabilized TZPs (Y-TZPs) possess high strength and toughness at room temperature, they suffer low-temperature strength degradation (LTD) because of the spontaneous tetragonal (t) to monoclinic (m) phase transformation when annealed at temperatures from 100 to 500◦C in air [3–5]. The LTD is accelerated under hydrothermal environments due to the reaction between Zr-O-Zr bonds and H2O [4, 5]. The phase stability of TZPs under hydrothermal conditions is a critical requirement for application of TZPs for use in medical devices because TZPs should pass the autoclave sterilization for 20– 30 min at temperatures from 120 to 135◦C prior to use [1]. Recently, Lee et al. [6] examined the t-phase stability composition region in the ZrO2-Y2O3-Nb2O5 system and showed that Nb2O5 doping to Y-TZP with a composition of 90.24 mol% ZrO2-5.31 mol% Y2O3-4.45 mol% Nb2O5 ((Y,Nb)-TZP) influenced remarkably the fracture toughness and LTD. The phase stability of TZPs in the ternary system is likely due to the Y-Nb ordering in t-ZrO2 lattice into a scheelite-like arrangement, which resulted in a relief of the internal strain in the t-ZrO2 lattice since the internal stress was known to assist the degradation under aging environments [5–7]. Even though (Y,Nb)-TZPs, having a large grain size of above 4μm, showed excellent phase stability, a relatively large grain size caused lower fracture strength of 450 MPa, as compared with that (800 MPa) of the commercial 3Y-TZP [6, 8]. The strength of (Y,Nb)-TZP was further enhanced by the addition of Al2O3 due to the combined effects of phase transformation, grain bridging toughening, and grain growth inhibition [8, 9]. The strength of (Y,Nb)-TZP/Al2O3 composites containing 20 vol% of 2.8μm Al2O3, sintered for 2 h at 1550◦C in air, was 700 MPa [9], which was still lower than that of 3Y-TZP. Masaki reported that greater strength of Y-TZP was achieved by hot-pressing (HPing) or hot-isostatic pressing (HIPing) [10, 11]. To improve fracture strength, (Y,Nb)-TZP was hot-pressed for 1 h at 1400◦C under 30 MPa. (Y,Nb)-TZP exhibits the strength of about 1 GPa, the fracture toughness of 7.5 MPa ·m1/2, and no LTD behavior at temperatures lower than 400◦C [12]. However, TZPs, aged at temperatures higher than 400◦C, suffered from high-temperature strength degradation due to the extensive cavitation caused by graphite oxidation. X-ray photoelectron spectroscopy results of the as-HPed (Y,Nb)-TZP demonstrated that the carbon incorporated in (Y,Nb)-TZP was either an ether-type C-O or a carbonyl-type C=O and the oxidation of CO to CO2 during aging at temperatures from 400◦C to 1000◦C was attributed to strength degradation [12]. (Y,Nb)-TZP was greatly strengthened by the addition of Al2O3 and the application of sinter-HIPing. The green compacts were sintered for 2 h at 1500◦C with heating rates of 6◦C min−1 to 900◦C and 3◦C min−1 up to the sintering temperature, and then furnace cooled to room temperature. The presintered (Y,Nb)-TZPs and (Y,Nb)-TZP/Al2O3 composites were placed into the hot zone of a high pressure vessel and isostatically hot-pressed for 0.5 h at 1500◦C under 100 MPa using Ar gas as the pressure-transmitting medium. The pressure was applied to HIPing temperature with a rate of 27 MPa h−1 and the temperature was with a rate of 400◦C h−1. The purpose of the present study is to investigate the mechanical properties of (Y,Nb)-TZPs and (Y,Nb)-TZP/Al2O3 composites prepared by HIPing. The powder preparation procedures of (Y,Nb)-TZP and (Y,Nb)-TZP/Al2O3 composite were reported elsewhere [6, 8]. The bulk density of both sintered and HIPed specimens was determined by the Archimedes method. For mechanical property measurements, the disk specimens were polished to a 1μm diamond finish and subsequently annealed for 2 h at 1200◦C to remove the possible residual stress. A hydrothermal stability of both sintered and HIPed specimens was evaluated after aging for 20 h at 180◦C under 3.5 MPa water vapor pressure in an autoclave. The extent of m-ZrO2 was estimated from the X-ray diffraction diffractometry (XRD) after Garvie and Nicholson [13]. The dimension of the specimens after polishing was 18 mm in diameter and 1.8 mm in thickness. A flaton-three ball biaxial-fixture was used for the determinations of strength and toughness with a stress rate of 23 MPa · s−1 that was determined from a correlation between the rate and the specimen thickness in the ASTM standard [14]. The fracture toughness was determined