Preparation and Characterization of Compositionally Graded Multilayered Hydroxyapatite/Zirconia Ceramics

Preparation and characterization of multilayered ceramics with graded structure of hydroxyapatite (HAP)/partially stabilized zirconia (PSZ) was studied. It was difficult to prepare two-layered bodies only consisting a layer of HAP and a layer of PSZ, because these layers could not bond to each other by conventional sintering procedure. Due to peeling off, it was also difficult to prepare three-layered bodies consisting of a monolithic HAP layer, a layer of HAP and PSZ with weight ratio 1/1, and a PSZ layer. Then, five- and seven-layered HAP/PSZ bodies with graded structure were prepared by atmospheric heating (1450°C, 2h) in an electric furnace. In this case, HAP reacted with PSZ at high temperature, and the resulting product was calcium zirconium oxide (Ca0.15Zr0.85O1.85). Seven- and eight-layered HAP/PSZ bodies with graded structure were also prepared by hot-pressing (1150°C, 1350°C). In this case, the layers bonded very firmly to each other and no peeling off was observed even upon cutting and grinding operations. When the sintering temperature was 1350°C, small cracks parallel to the press direction were generated in the layer of monolithic HAP. The cracks were not detected in any of the remaining layers after sintering at 1150°C. No chemical reaction between HAP and PSZ was found by ordinary XRD analysis in the multilayered body prepared by hot-pressing. The graded structure was confirmed by elemental analytical results of Ca and Zr. The bending strength of the seven-layered HAP/PSZ body was much higher than that of monolithic HAP. The Vickers hardness value of the layer consisting of monolithic HAP was 4.3GPa, increasing with increasing the fraction of PSZ to reach 12.0GPa in the layer of monolithic PSZ.