Hardness and the nature of microplasticity of hydroxyapatite

Structural transformations in hydroxyapatite ceramics as a result of severe plastic deformation

“…The hardness of the initial HA ceramics was 9.5 GPa and the hardness of the ceramics after SPD became 4.0 GPa, i.e. close to the hardness of amorphous calcium phosphate coatings (the coatings deposited by rf-magnetron sputtering [2]). …”

“…Fig. 8 shows loading diagrams "the load (P) versus the indentation depth (h)" representing deformation process during initial HA ceramics nanoindentation (1) and HA ceramics after SPD (2). The diagrams correspond to elastic-plastic deformations.…”

“…In Ref. [2] it was noted that the hardness of nanocrystalline samples corresponds to the average values of hardness of HA microcrystals, and that the highest plasticity was observed in amorphous calcium phosphate (ACP). However, there is no classified data describing HA the plastic deformation mechanism.…”

“…During investigation of submicrocrystalline ceramics intra-grain structure of nanocrystalline and microcrystalline HA coatings with equilibrium grain structure [2][3][4][5][6][7][8][9] and of the coatings regions upon nanoindentation [2] growth-induced and strain-induced dislocations in the grains bulk have not been found. Frank dislocations, related to stacking faults in indentation deformed regions of fluorine-substituted HA single crystal have been revealed [1].…”

“…It was shown that during indentation of crystalline HA its grain size decreases as a result of plastic deformation [1,2]. It is known that deformations of microcrystals [3], HA ceramics [4], human dental enamel [5] and HA-based coatings of different structures [6][7][8] are elastic-plastic.…”