Effect of Spark Plasma Sintering Pressure on the Properties of Functionally Graded Implant and its Biocompatibility

The coexistence of Ti and hydroxyapatite (HAP) makes HAP decomposed at the sintering process heated at more than 1000°C in vacuum or Ar atmosphere. Spark plasma sintering (SPS) method could make the stable functionally graded materials (FGM) with the gradual change of concentration in the longitudinal direction from pure Ti to 100% HAP (Ti/100HAP) at the sintering temperature as low as 850 °C. However, sintering is insufficient. To increase the degree of sintering the dependence of mechanical properties on sintering pressure was investigated. By increase of sintering pressure from 40 to 80 MPa compression strength was increased from 48 to 88 MPa, and flexural strength from 6 to 36 MPa. Brinel hardness decreased monotonically from Ti region to HAP inside FGM specimen. This decreasing gradience inside implant contributes to stress relaxation in the implanted region of bone where the compatibility with tissue is essential in biomechanical properties. Animal implantation tests in hard tissue showed that the tissue reacted gradiently in response to the graded composition of FGM implant. The maturation of new bone occurred from th earlier stage in the HAP rich region. Functionally graded implants could thus satisfy the biocompatibility in both mechanical properties and biochemical affinity to osteogenesis.