Nanoindentation response of novel hydroxyapatite–mullite composites

In this contribution, the experimental results of the nanoindentation response of the newly developed hydroxyapatite–mullite composites are presented and analyzed. Three different composite compositions i.e. HAp-10 wt% mullite, HAp-20 wt% mullite and HAp-30 wt% mullite were selected for the present study and for comparison, the measurements on pure HAp and pure mullite were also carried out. For nanoindentation experiments, a maximum load of 100 mN and loading/unloading rate of about 0.2 N/min were used. Also, the multiple indentation experiments were carried out at random locations on polished surfaces and an attempt has been made to explain the difference in the localized mechanical response in terms of the microstructural differences. The obtained results reveal lower hardness of 3–4 GPa for HAp–mullite composites irrespective of mullite content; whereas pure HAp and mullite exhibit higher hardness values of ∼4.5 GPa and ∼9 GPa, respectively. Considerable decrease in E-modulus with mullite addition was also noticed. The composites have E-modulus values of ∼80 GPa, whereas higher values of ∼125 GPa and 230 GPa were recorded for pure HAp and pure mullite, respectively. Work of indentation for plastic work as well as elastic work are calculated and correlated with hardness and E-modulus values of various samples.