R. Tolouei

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.

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.

Effect of Grain Size on Vickers Microhardness and Fracture Toughness in Calcium Phosphate Bioceramics

Grain size dependences of Vickers microhardness and indentation fracture toughness in fully dense hydroxyapatite bioceramics without additives were studied. The nanostructure 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 one minute holding 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 a 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 properties of zirconia-based ceramic composite

Abstract This study examines the effects of different ZrB2 content on various mechanical properties and electrical conductivity of ZrB2/Y-TZP composite. Composites with ZrB2 content of up to 20 wt-% were particularly beneficial at the lower sintering temperature range by achieving greater densification and better hardness than Y-TZP monolith. In contrast to the trends estimated from rule of mixture, the increment of ZrB2 content did not result in any significant improvement in the elastic modulus and hardness of the zirconia composites. Nevertheless, all composites showed tremendous improvement in fracture toughness compared with monolithic Y-TZP and thus, suggested that other toughening mechanisms were operative besides transformation toughening of zirconia. Incorporation of ZrB2 up to mass fraction of 20 wt-% into Y-TZP generally did not affect the tetragonal phase stability of zirconia. Significant reduction of electrical resistivity of the composites was achieved with ZrB2 content of 20 wt-% and sintering temperature of 1400°C.

Manufacturing of High Toughness Hydroxyapatite Produced by Wet Chemical Method

Hydroxyapatite (HA) is among the leading ceramic materials for hard tissue replacement implants. Despite the excellent bioactivity of HA, low toughness has limited the application of these materials to non-load bearing areas. The sinterability of nanocrystalline hydroxyapatite (HA) powder via new heating profile for conventional pressureless sintering was studied. The starting nanocrystalline HA powder was synthesized by wet chemical precipitation method. After uniaxial pressing followed by isostatic pressing, HA powder compacts are sintered over the temperature range of 1000°C to 1300°C. Different holding time of 1 minute and 120 minutes was applied as a heating profile of HA samples. The results revealed that new heating profile was effective in producing a HA body with high density of 98% when sintered at 1200°C. Subsequently, mechanical properties such as fracture toughness and hardness, of HA compacts increased with decrease in grain size. HA showed the highest hardness of 9.51 GPa and fracture toughness of 1.41 MPa.m1/2 when sintered at 1100 °C. XRD analysis indicated that decomposition of HA phase during sintering at high temperatures do not occur. Short holding time leads to finer microstructure of HA and subsequently better mechanical properties.

Influence of Magnesium Doping in Hydroxyapatite Bioceramics Sintered by Short Holding Time

In the present research, nano hydroxyapatite (HA) powder doped with magnesia (MgO) was studied. The dopant was added to pure HA powder and ball milling was done for 1 hour. Green samples, in the form of discs and rectangular bars, were prepared and consolidated in air at temperatures ranging from 1000°C to 1300°C. The sintered samples were characterized to determine the phase stability, relative density, hardness, fracture toughness and Young’s modulus. The phase analysis revealed that the HA phase was not disrupted regardless of dopant additions and sintering temperature. It has been revealed that all HA samples achieved > 98% relative density when sintered between 1100oC–1300oC. However, the addition of 0.5 wt% MgO when sintered at 1100°C was found to be most beneficial in aiding sintering with samples exhibiting the highest Young’s modulus of 122.15 GPa and fracture toughness of 1.64 MPam1/2 as compared to 116.57 GPa and 1.18 MPam1/2 for the undoped HA.

Calcination Effects on the Sinterability of Hydroxyapatite Bioceramics

The sinterability of calcined synthesized HA (700oC to 1000oC) was investigated over the temperature range of 1050oC to 1350oC in terms of phase stability, bulk density, Young’s modulus and Vickers hardness. Calcination has resulted in higher crystallinity of the starting synthesized HA powder. Decomposition of HA phase to form secondary phases was not observed in the present work for the calcined powders. The results also indicated that calcination of the HA powder prior to sintering has negligible effect on the sinterability of the HA compacts (up to 900oC). Further treatment at 1000oC was found to be detrimental to the properties of sintered HA.

Pressureless Sintering of Electro-Conductive Zirconia Composites

In the present work, 3 mol% Yttria-stabilized tetragonal zirconia (Y-TZP) composite containing 25 wt.% of zirconium diboride (ZrB2) was prepared via pressureless sintering method in an inert atmosphere over the temperature range of 1350-1550°C for one hour. The effect of zirconium diboride content in the zirconia matrix, as well as the sintering temperature on densification, phase stability and electrical properties of sintered samples have been studied. The results revealed that there was a significant increased in electrical conductivity of sintered samples when 25 wt.% of ZrB2 is incorporated into Y-TZP matrix.

Synthesis of High Fracture Toughness of Hydroxyapatite Bioceramics

The sinterability of magnesium oxide (MgO) doped hydroxyapatite (HA) ranging from 1 to 10 wt% when sintered at 1150°C was investigated in terms of phase stability, bulk density, Young’s modulus, Vickers hardness and fracture toughness. The addition of up to 1 wt% MgO as sintering additive was found to be beneficial in promoting the densification of HA. Further addition of MgO in the HA matrix would deteriorate its densification properties. Similar results were observed for its stiffness and Vickers hardness. Nevertheless, the fracture toughness of HA was greatly enhanced by the incorporation of 5 wt% MgO. An increased toughness of up to 35% was obtained for the MgO-doped HA when compared to the undoped HA. This improvement is associated to the smaller grain size of the doped sample as compared to the undoped HA.