Comparative sintering of zirconia and hydroxyapatite-zirconia composites

Hydroxyapatite [Ca10(PO4)6OH2, HA] is the major constituent of the mineral phase of bone and exhibits desirable properties as a bone graft, such as biocompatibility, bioactivity, osteoconductivity and direct bonding to bone tissue. However, due to its limited mechanical properties, in particular low fracture toughness (KIC) and tensile strength, HA is limited to non-load bearing applications and metal implant surface coatings. Zirconia (ZrO2) based materials exhibit high fracture toughness due to the stress induced martensitic transformation of tetragonal to monoclinic ZrO2. In theory, a composite material composed of HA and a reinforcing ZrO2 phase, combining the inherent bioactivity of HA with reinforcement from the ZrO2 is an attractive proposition in terms of load bearing bone replacement. The work contained herein attempts to improve the mechanical properties of HA through the addition of a ZrO2 phase and consolidation through two comparative sintering techniques, microwave and conventional sintering. The first objective was to investigate the effect of microwave sintering on physical, mechanical and chemical properties of Y2O3 (2-5 mol%) doped ZrO2 compositions compared with conventional sintering. Y2O3 content was altered through the attrition milling of commercial undoped monoclinic ZrO2 powders with co-precipitated Y2O3-doped ZrO2 powders and these were characterized using X-ray diffraction (XRD), particle size analysis (PSA), transmission electron microscopy (TEM) and BET. Comparative sintering was performed at temperatures of 1100, 1200 and 1300°C. Post sintering characterization included phase analysis (XRD), microstructural analysis using scanning electron microscopy (SEM), density measurements and mechanical testing, which included Young?s modulus, biaxial flexural testing and microhardness. Compositions containing 2 mol% Y2O3 microwave sintered at a temperature of 1300°C exhibited the greatest improvement, due to retention of tetragonal ZrO2, with a 5% increase in relative density compared with conventional sintering. Grain size analysis indicated that there was significant grain growth in microwave sintered samples (to d = 353nm) compared to their conventionally sintered counterparts (d = 200nm). Associated with this was a 22% increase in Young?s modulus to 220GPa, a 77% increase in Vicker?s Hardness up to 11.5GPa and a 165% increase in biaxial flexural strength up to 800MPa. Based on these observations, a reinforcing ZrO2 phase containing 2 and 3 mol % Y2O3 was decided upon for addition to HA. The second objective was to investigate the effect of microwave sintering on HA-ZrO2 composites containing a commercial HA powder and HA synthesized using a co-precipitation technique and ZrO2 with the optimum Y2O3 content for Y-TZP found in the initial stage. HA was precipitation synthesized at temperatures of 25 and 45°C and characterized using XRD, X-ray fluorescence (XRF), TEM and BET. Commercial HA composites containing 0, 5, 10, 20 & 40wt % ZrO2 additions and synthesized HA (S-HA) composites containing 0, 5 and 10 wt% ZrO2 were comparatively sintered. The post sintering characterization was similar to the previous step. In terms of phase stability it was found that the decomposition of HA to a and/or ß-TCP increased with increasing amounts of ZrO2 irrespective of the HA powder used. This decomposition increased with increasing temperature and was found to increase in microwave sintered samples at 1300°C. It was also found that CaO released through the decomposition of HA reacted with the ZrO2 reinforcing phase to form either a c-ZrO2 solid solution and/or CaZrO3 above temperatures of 1100°C. Microwave sintering resulted in increased densification at lower temperatures of 1000 & 1100°C. It was observed, however, that there was a reduction in the relative densities of ZrO2 composites compared to unfilled HA as the sintering temperatures increased to 1200°C. It was suggested that H2O formed through the decomposition of HA led to formation of pores and lower densities. The increase in porosity was observed to be detrimental to the mechanical properties of the HA-ZrO2 composites. The highest stiffness, biaxial flexural strengths and microhardness of 112 GPa, 164MPa and 5.7GPa were observed in unfilled HA conventionally sintered at a temperature of 1300°C with a corresponding relative density of 96.7%. Comparative values of 109GPa, 138MPa and 4.9GPa were achieved in microwave sintered samples sintered at 1200°C. Subsequent increases in ZrO2 reduced the mechanical properties. The addition of 2.5 wt % CaF2 to microwave sintered synthesized HA-ZrO2 composites resulted in a significant increase in the relative density. The substitution of F- for OH- resulted in the partial formation of Fluoro-HA, increasing the thermal stability, limiting the formation of H2O, and thus increasing density. While the addition of CaF2 increased the density of composites, CaO formed in this case from the substitution of F- for OH- reacts with t-ZrO2 again forming CaZrO3 and/or c-ZrO2. The increased density led to an increase in the Young?s modulus and the hardness. However, the biaxial flexural strengths of composites were not increased. While there was an increase in mechanical properties compared to HA-ZrO2 composites without CaF2, the Young?s modulus and the hardness only exhibited values comparable to unfilled HA. The strengths were inferior to unfilled HA. The biocompatibilities of HA-ZrO2 composites were determined using In vivo & In vitro evaluation. In vitro evaluation indicated that the materials induced a favourable response in rat osteosarcoma cells. In vivo evaluation indicated that the material was osteoconductive and biocompatible in the rat tibia model.