W.D. Teng

Consolidation of nanocrystalline hydroxyapatite powder

Abstract The effect of sintering temperature on the sinterability of synthesized nanocrystalline hydroxyapatite (HA) was investigated. The starting powder was synthesized via a novel wet chemical route. HA green compacts were prepared and sintered in atmospheric condition at various temperatures ranging from 900–1300°1C. The results revealed that the thermal stability of HA phase was not disrupted throughout the sintering regime employed. In general, the results showed that above 98% of theoretical density coupled with hardness of 7.21 GPa, fracture toughness of 1.17 MPa m1/2 and Young’s modulus of above 110 GPa were obtained for HA sintered at temperature as low as 1050 1C. Although the Young’s modulus increased with increasing bulk density, the hardness and fracture toughness of the sintered material started to decline when the temperature was increased beyond 1000–1050 °C despite exhibiting high densities > 98% of theoretical value. The occurrence of this phenomenon is believed to be associated with a thermal-activated grain growth process.

The effect of manganese oxide on the sinterability of hydroxyapatite

Abstract The sinterability of manganese oxide (MnO2) doped hydroxyapatite (HA) ranging from 0.05 to 1 wt% was investigated. Green samples were prepared and sintered in air at temperatures ranging from 1000 to 1400°C. Sintered bodies were characterized to determine the phase stability, grain size, bulk density, hardness, fracture toughness and Young’s modulus. XRD analysis revealed that the HA phase stability was not disrupted throughout the sintering regime employed. In general, samples containing less than 0.5 wt% MnO2 and when sintered at lower temperatures exhibited higher mechanical properties than the undoped HA. The study revealed that all the MnO2-doped HA achieved > 99% relative density when sintered at 1100–1250 °C as compared to the undoped HA which could only attained highest value of 98.9% at 1150 °C. The addition of 0.05 wt% MnO2 was found to be most beneficial as the samples exhibited the highest hardness of 7.58 GPa and fracture toughness of 1.65 MPam1/2 as compared to 5.72 GPa and 1.22 MPam1/2 for the undoped HA when sintered at 1000 °C. Additionally, it was found that the MnO2-doped samples attained E values above 110 GPa when sintered at temperature as low as 1000 °C if compared to 1050 °C for the undoped HA.

The influence of Ca/P ratio on the properties of hydroxyapatite bioceramics

The paper reports on the effect of Ca/P ratio (1.57, 1.67 and 1.87) on the densification behaviour of nanocrystalline hydroxyapatite (HA) prepared by a chemical precipitation method. Green compacts were prepared and sintered at temperatures ranging from 1000°C to 1350°C. The sintered samples were characterized to determine the HA phase stability, bulk density, hardness, fracture toughness and Youngs modulus. XRD analysis revealed that the phase stability was not disrupted throughout the sintering regime employed for HA having Ca/P ratio of 1.57 and 1.67. However, secondary phases were observed for HA having a Ca/P ratio of 1.87 when sintered at high temperatures. In general, regardless of Ca/P ratio, the HA bodies achieved > 95% relative density when sintered at 1100°C-1250°C. The results indicated that the stoichiometric HA (Ca/P ratio = 1.67) exhibited the overall best properties, with the highest hardness of 7.23 GPa and fracture toughness of 1.28 MPam1/2 being attained when sintered at 1000°C-1050°C.

THE EFFECT OF COLD ISOSTATIC PRESSING ON THE SINTERABILITY OF SYNTHESIZED HA

The effect of cold isostatic pressing (CIP) on the sinterability of a laboratory synthesized hydroxyapatite (HA) powder was investigated. The starting powder was initially uniaxially diepressed at about 6 kN force to form disk and rectangular green compacts. Two batches of green samples were prepared, i.e. one batch was in the as-compacted state (Un-CIP) and another batch samples was subjected to cold isostatic press at 200 MPa (CIP). The latter samples exhibited alinear shrinkage of ∼16% prior to sintering. All the samples were sintered in air at temperatures ranging from 700°C to 1400°C. The densification behaviour of HA was evaluated in terms of linear shrinkage, phase stability, bulk density, Vickers hardness and Youngs modulus. The results revealed that green samples subjected to cold isostatic pressing exhibited better sinterability and possessed excellent mechanical properties. This effect is more pronounced particularly for the low temperature sintering regime i.e. < 1100°C.

Effect of sintering holding time on low-temperature degradation of yttria stabilised zirconia ceramics

Abstract Effect of sintering holding time on low-temperature degradation of 3 mol-% yttria stabilised zirconia was investigated. Yttria stabilised zirconia samples were sintered using a two-step sintering cycle. The samples were initially heated to 1500°C with the heating rate of 10°C min−1, held for 1 minute before cooling down to 1200°C and kept at this temperature for varying holding times from 0 to 10 hours. The phase contents of the zirconia samples were measured using X-ray diffractometer, and the microstructural evaluation was evaluated by field emission scanning electron microscope. All the as-sintered samples exhibited 100% tetragonal phase and bulk density of about 6 g cm−3. The low-temperature degradation study was conducted under autoclave condition containing superheated steam at 180°C and 10 bar vapour pressure for periods up to 24 hours. The results showed that the tetragonal to monoclinic phase transformation decreased with increasing holding time up to 3 hours and thereafter it increased. The sample sintered with 3-hour holding time showed better aging resistance than other samples. It was found that the grain size of this sample was the lowest (0·29 μm) when compared to other samples.

Sintering of Hydroxyapatite Ceramic Produced by Wet Chemical Method

In the present work, densification of synthesised hydroxyapatite (HA) bioceramic prepared via chemical precipitation method was investigated. HA samples was prepared by compaction at 200 MPa and sintered at temperatures ranging from 800°C to 1400°C. The results revealed that the HA phase was stable for up to sintering temperature of 1250°C. However, decomposition of HA was observed in samples sintered at 1300°C with the formation of tetra-calcium phosphate (TTCP) and CaO. Samples sintered above 1400°C were found to melt into glassy phases. The bulk density increases with increasing temperature and attained a maximum value of 3.14 gcm-3 at 1150°C whereas maximum hardness value of 6.64 GPa was measured in HA sintered at 1050°C. These results are discussed in terms of the role of grain size.

Effect of Copper Oxide on the Sintering of Alumina Ceramics

The effect of adding small amounts of copper oxide (CuO) on the sintering and mechanical properties of alumina ceramic was studied. Samples were prepared and fired in air atmosphere at temperatures ranging from 1400oC to 1600oC. Sintered samples were characterized to determine phase present, bulk density, hardness and grain size. The results indicated that all the doped samples could be sintered to high density > 3.85 Mgm−3 when compared to the undoped alumina. According to the XRD analysis, the α-Al2O3 phase was not disrupted by the dopant addition. Although the hardness of the CuO-doped material was higher when sintered below 1550°C, the maximum hardness of 21 GPa was measured for the undoped ceramics when sintered at 1600°C. The lower hardness of the doped samples could be attributed to the increased in grain size with increasing sintering temperature.

Oxygen Vacancy Comparisons for 3Y - TZP Sintered in Air and Argon Gas Atmosphere

In this study oxygen vacancy concentration on 3Y TZP ceramics was compared for sintered and aged samples in air and argon gas atmosphere. The sintering study was conducted over the temperature range 1250°C - 1500°C. Photoluminescence (PL) spectrums of the sintered samples were taken by laser with excitation wavelength 325nm. Two distinctive sharp and broad peaks were observed for the wavelength ~ 615 nm and ~ 580 600nm. The sample sintered at 1250 °C showed higher oxygen vacancy in both atmospheres. At the same time, argon gas sintered sample showed higher oxygen vacancy than air sintered sample. The emission band blue shifted after 1 h ageing for the sample sintered in 1500 °C in both atmospheres. XRD results on 1 hour aged sample sintered in argon gas atmosphere showed higher monoclinic phase than air sintered sample.

Dependence of the Fracture Toughness on the Sintering Time of Dense Hydroxyapatite Bioceramics

Fracture toughness dependence of sintered hydroxyapatite (HA) bioceramics on the sintering time was studied. The nanocrystalline and highly pure Hydroxyapatite powders produced by wet chemical precipitation method were used as starting material. After uniaxial and cold isostatic pressing, the green HA samples sintered at temperatures ranging from 1000 °C to 1300 °C with different sintering time. Dense compacts with grain sizes in the nanometer to micrometer range were processed. The average grain size of HA compact sintered at 1000 °C was around 500 nm. Grain size increased to 3 µm when the compacts were sintered at higher temperature. The average microhardness value of sintered HA decreased with an increased in grain size. Indentation fracture toughness for HA compacts of 700 nm grain size was 1.41±0.4 MPa.m1/2 which is similar to fracture toughness of human cortical bone.

Sintering Behavior of Nanocrystalline Hydroxyapatite Produced by Wet chemical Method

The sintering behavior of synthesized nanocrystalline hydroxyapatite (HA) powder was investigated in terms of phase stability and mechanical properties. A wet chemical precipitation method was successfully employed to synthesize a high purity and single phase HA powder. After shaping, HA powder compacts have been sintered over the temperature range of 1000 degrees C to 1300 degrees C. Two different sintering holding times of 1 minute and 120 minutes were investigated. The results revealed that the 1 minute holding time profile was effective in suppressing grain growth and producing a HA body with improved densification. Additionally, higher mechanical properties such as Youngs modulus of 119 GPa, high fracture toughness of 1.41 MPa.m(1/2) and hardness of 9.5 GPa were obtained for this sample as compared to HA bodies when sintered using the 120 minutes holding time. The study revealed for the first time that HA could be sintered using a 1 minute holding time without compromising on HA phase stability and mechanical properties.