Hydroxyapatite – TZP Composites: Processing, Mechanical Properties,Microstructure and in Vitro Bioactivity

Abstract

The present study describes the results on hydroxyapatite (HA)-TZP (tetragonal zirconia polycrystal) composites containing low weight percent TZP (2, 5, 7.5 and 10 wt%). The work could be divided into two parts. In the first part, hydroxyapatite was synthesized by two different chemical precipitation route; normal precipitation (NP) and reverse strike precipitation (RS) using Ca(NO3)2.4H2O and (NH4)2HPO4 as the precursor for hydroxyapatite.The as precipitated amorphous powders were studied for thermal decomposition behavior (DSC/TG), phase evolution and phase stability of the calcined powder, particle size distribution and non-isothermal densification behaviour.The TEM of the calcined hydroxyapatite powder shows that HA particles prepared from both the routes have elongated morphology. The particle size distribution of 850OC calcined
powder is multimodal for NP and monomodal for RS route. The sintered density of RS route HA is higher than that of NP route for the entire sintering range studied. The results also indicate that while RS-HA remains phase stabile until 1250OC, NP-HA decomposes to a mixture of HA and TCP at that temperature.Based on the above observations, the RS route was adopted for synthesis of HA-3YTZP composite containing 2, 5, 7.5 and 10 wt% of TZP respectively. The composite powders were also characterized by FTIR, DSC/TG, XRD and TEM. The raw HA-TZP powders crystallized between 650OC- 050OC and the calcined powder (850OC/2hrs ) had phase pure HA and TZP. The composites were sintered by pressureless sintering at 1250OC/4hrs. XRD of the sintered composites revealed that at higher TZP content (HZ5, HZ7 and HZ10), β-TCP and α-TCP along with HA and t-ZrO2 were present. The TEM of calcined powder showed dark coloured TZP particles (5-10 nm) uniformly distributed in light colored HA (50-150 nm) matrix. The highest sintered density (> 99%) was obtained for HA- 2wt% TZP composite (HZ2). The density for 5, 7.5 and 10 wt% TZP containing composites were 96%, 92% and 90% respectively. The fracture strength of the composites appeared to be controlled by the size of the largest flaw present in the sintered sample. The highest three point bending strength was 72MPa for HZ2 composite while 30MPa was lowest for HZ5 composite. The decrease in strength at higher TZP additions could be due to the incomplete densification of the composite due to the constraining effect of TZP. The highest fracture toughness (0.97MPam1/2) was recorded for HA-5 wt% TZP (HZ5). SEM microstructure showed a bimodal grain size distribution with presence of very fine and coarse grains. A grain size reduction of HA was observed with TZP addition. It may be pointed out here that at a lower TZP calcination temperature (850OC), the fine TZP particles forms agglomerates thereby hampering the final stage of densification. The Vickers hardness exhibited also porosity
dependence. The maximum hardness (3.83GPa) was for HZ2 and the lowest (2.43GPa) was for HZ10 composites. Thus, it was anticipated that an effort should be made to increase the sintered density particularly at higher wt% TZP (i.e. 5, 7.5 and 10) which could effectively improve the strength, toughness and hardness of the composite.Therefore, in the next stage of work, an effort was made to observe the effect of TZP calcination temperature on the agglomeration behaviour of TZP and thereby on sintered density, phases, mechanical properties and microstructure of HA-TZP composites.