ABSTRACT
The mode I fracture toughness (KIc) is the most important property that governs the microplasma-sprayed hydroxyapatite (MIPS-HAp) coating’s intrinsic resistance against catastrophic failure. Therefore, it is important to understand the KIc behavior of these coatings for prospective biomedical implant applications. A critical survey of pertinent literature data [1-11] reveals that most of the KIc data (average ≈ 0.5 MPa⋅m0.5) reported are evaluated by Vickers microindentation across the cross sections of dense MAPS (macroplasma-sprayed) and high-velocity oxy fuel (HVOF) sprayed HAp coatings on Ti6Al4V substrates and, indeed, very rarely by nanoindentation on MIPS-HAp coatings [7]. Therefore, in this chapter we shall try to show how we can measure fracture toughness and its load dependency by using the nanoindentation technique with a Berkovich indenter such that the measured value of toughness pertains to the microscale, compatible with the local microstructure of the cross section of highly porous MIPS-HAp coatings on SS316L substrates. The relevant details of the MIPS-HAp coatings preparation and the measurement of KIc by nanoindentation on these MIPSHAp coatings are given elsewhere [7] and also discussed briefly in Chapter 9.
The major experimental finding [7] was that the KIc data of the present MIPSHAp coatings were as good as or even slightly higher than those reported (≈0.6 MPa⋅m0.5) [1-6] for dense HAp coatings (Figure 35.1a). Further, they had almost negligible (i.e., very slight) degradation with load (Figure 35.1b). The slight degradation can be explained in terms of competitive dependencies of
