Environment effect on the strength of hydroxy- and fluorohydroxyapatite ceramics

The effect of partial fluoride-ion substitution for hydroxy groups on the stability of hydroxyapatite ceramics toward delayed fracture is studied in three media: air, water, and a solution modeling saliva. The stability toward delayed fracture is assessed in dynamic fatigue tests. The results are used to evaluate the exponent in the relation between the rate of subcritical crack propagation and the stress intensity factor, which characterizes the stability of the material toward delayed fracture. The composition of the environment is found to have a significant effect on the resistance to delayed fracture and the average strength of the material. The introduction of fluorine into hydroxyapatite ceramics slightly enhances their stability to delayed fracture and, in addition, notably increases their average strength.