Xigeng Miao

Laminated and functionally graded hydroxyapatite/yttria stabilized tetragonal zirconia composites fabricated by spark plasma sintering.

Hydroxyapatite (HA) ceramics have been conventionally strengthened and toughened in the form of composites and coatings. New microstructural designs and processing methodologies are still needed for the improvement of the mechanical properties of HA-based ceramics. This study was to prepare laminated and functionally graded HA/yttria stabilized tetragonal zirconia (Y-TZP) composites by the relatively new process of spark plasma sintering (SPS). The microstructure and the mechanical properties of the laminated and functionally graded composites were studied for possible orthopedic applications. It was found that the laminated and functionally graded HA/Y-TZP composites could be densified at 1200 degrees C within 5 min by the SPS process and the average HA grain size in the composite layers was reduced by half due to the well-dispersed Y-TZP second phase. The HA phase in the composite layers was stable up to 1200 degrees C and the Y-TZP second phase remained the tetragonal zirconia (t-ZrO(2)) phase after being processed at the highest temperature of 1250 degrees C. The laminated and functionally graded HA/Y-TZP composites exhibited much improved mechanical properties compared with the pure HA ceramics; the bending strength of the composites reached about 200 MPa, double the strength of the pure HA ceramics.

Characterization of hydroxyapatite– and bioglass–316L fibre composites prepared by spark plasma sintering

To improve the mechanical properties of pure hydroxyapatite (HA) ceramics and pure 45S5 bioglasses, HA-316L fibre composites and bioglass 45S5-316L fibre composites were produced by spark plasma sintering (SPS) at 950 oC and 850 oC respectively. While the HA phase in the HA-316L fibre composites did not decompose after the SPS process, microcracks were found around the 316L fibres in the composites. Consequently, the HA-316L fibre composites could not effectively improve the mechanical properties of the pure HA ceramics. In contrast, the bioglass 45S5-316L fibre composites showed no microcracks around the 316L fibres and thus exhibited bending strengths of up to 115 MPa.

Observation of microcracks formed in HA-316L composites

Hydroxyapatite-stainless steel 316L fiber composites were prepared using as-supplied, 900oC calcined, and 15 vol% yttria stabilised zirconia added hydroxyapatite powders. The reinforcements were 20 vol% chopped 316L fibres of length 1 mm and diameter 55 mm. Both hot isostatic pressing and spark plasma sintering were used to densify the composites with the processing temperatures varying from 825 to 950 degrees Celsius, pressures from 20 to 140 MPa, and time from 10 to 180 min. The obtained composites were subjected to microstructural analysis using a scanning electron microscope. It was found that microcracking took place invariably in the hydroxyapatite matrices but near and around the 316L fibres. Such patterns of microcracks resulted from the thermal residual stresses developed during the cooling from the high temperatures and the intrinsic low mechanical strengths of the hydroxyapatite ceramics. Such microcracks may play important roles in the mechanical behaviour of the HA-316L fibre composites.

Highly Interconnected and Functionally Graded Porous Bioceramics

Highly interconnected porous yttria stabilised zirconia ceramics were prepared by slurry infiltration of expanded polystyrene powder compacts, followed by sintering at 1500 C. The porous zirconia ceramics exhibited a high porosity (60%) with uniform or graded pore sizes ranging from 300 μm to 1000 μm. The compressive strength of the uniform porous ceramics decreas d with the increase of pore size, whereas the compressive strength of the graded porous cer amics was anisotropic, depending on the direction of measurement. Compressive strengths from 8 MPa to 36 MPa were obtained for the porous zirconia ceramics.