Micro- and macro-scale strength properties of c-axis aligned hydroxyapatite ceramics

In this study we first investigated the strength of grains, i.e., single crystals, and grain boundaries (micro-scale strength) of hydroxyapatite ceramics in different crystal orientations, using microcantilever bending tests. The specimens were prepared from two types of hydroxyapatite ceramics; one is that with uniaxially aligned c-axes of grains and the other with randomly oriented ones. Both the grain and grain boundary strengths were slightly higher in tensile stress application parallel to c-axis than in the perpendicular in the aligned material. These strengths, however, were substantially higher than those of the randomly oriented one. Then, three-point bending tests were conducted to determine the polycrystalline strength (macro-scale strength) for the above two types of hydroxyapatite ceramics. While the strength values were comparable in tensile stress applications parallel and perpendicular to c-axis in the aligned material, that of the randomly oriented one is again significantly lower than them. In particular, the strength of the grain boundary was higher when the angle between the c-axis of the two hydroxyapatite grains divided by the boundary was close to 0 or 90 degrees. The effects of the grain orientation and alignment on the mechanical properties were discussed on the basis of both the micro-scale and macro-scale results. These results indicate that selective formation of high-strength grain boundaries due to c-axis alignment of hydroxyapatite grains is effective in improving the mechanical properties of bulk ceramics, such as macroscopic strength and fracture toughness.