Microhardness studies on calcium hydrogen phosphate (brushite) crystals

Vickerʹs and Knoop microhardness studies were carried out on grown calcium hydrogen phosphate dihydrate (CaHPO4·2H2O) crystals over a load range of 10–50 g. The Vickers (HV) and Knoop (HK) microhardness numbers for the above loads were found to be in the range of 94–170 kg/mm2 and 28–35 kg/mm2 respectively. It was also found that these numbers increased with increase in load. The Mayerʹs index (n) was found to be greater than 1.6 showing soft-material characteristics. The fracture toughness values (Kc), determined from measurements of crack length, were estimated to be 6 ± 0.5 × 103 kg m−3/2 and 4.5 ± 0.5 × 103 kg m−3/2 at 25 g and 50 g respectively. The brittleness indices (Bi) were found as 2.3 ± 0.1 × 104 m−1/2 for 25 g and 3.7 ± 0.1 × 104 m−1/2 for 50 g. Using Woosterʹs empirical relation, the elastic stiffness coefficient (c11) has been calculated from Vickerʹs hardness values as 4.8 ± 0.5 × 1015 Pa for 10 g, 9.7 ± 0.5 × 1015 Pa for 25 g and 13.3 ± 0.5 × 1015 Pa for 50 g. The Youngʹs modulus was calculated as 1.5 ± 0.1 × 1010 N m−2 from Knoop microhardness values.