Dynamic fatigue and degradation in solution of hydroxyapatite ceramics

Polycrystalline hydroxyapatite was densified by hot pressing. The dissolution process in aqueous solution and the effects of environment on dynamic fatigue resistance of the resulting HAP ceramics were investigated. Pure water or Ringer's solution strongly enhances subcritical crack growth. The crack propagation exponent decreases from 22.5+/-2 in air to 10+/-4 in Ringer's solution for materials densified at 98% of the theoretical value. The residual porosity ratio is also very detrimental for the mechanical reliability. Both fatigue resistance and immediate fracture strength are decreased, with values of only 14+/-4 for the propagation exponent and 40 MPa for the tensile strength (compared to 90 MPa at 98% relative density) for materials densified at 94% tested in air. The degradation in solution appears to be governed by uniform physico-chemical dissolution of crystalline HAP phase at the surface of the material. This dissolution is accompanied by a decohesion of grains located around residual pores which leads to the growth of local surface defects.