ABSTRACT
In Chapter 6 we discussed the bulk or macromechanical properties of HAp-based coatings. Now, beside its bonding strength, shear strength, and fatigue behavior, it is also important to know the behavior of HAp-based coatings at microstructural length scale because any failure or disintegration of material would primarily initiate at its microstructural length scale. Further, if we look into the actual application of HAp coatings, they would face mainly surface contacts rather than bulk contacts. In other words, for such bio application, surface contacts is expected to play a role more dominant than that of the bulk contacts. Further, nanoindentation is one of the potential tools to evaluate mechanical behavior at the microstructural length scale. Therefore, in this chapter, more importance has been given to understanding the nano indentation process and how it is useful for measuring nanomechanical properties like nanohardness and elastic modulus of HAp coatings. Moreover, it will be shown that prior to the present effort by the author and coworkers, the work reported on especially the micro/ nano-level characterizations of the surface mechanical properties for HAp and HAp composite coatings is far from significant, presumably because of the difficulties associated with the evaluation of the aforesaid properties for an extremely heterogeneous microstructure, such as that of the plasma sprayed HAp coatings. It is also a matter of big concern for prosthetic applications of the HAp coatings that the coating lifetime may be and indeed is often limited by premature failure due to contact or local delamination-induced fracture. That is why evaluation and investigation of mechanical properties at the local microstructural length scale of HAp-based coatings is so important and pertinent for further development of this biomaterial of tremendous scientific and technological importance. We start our journey by presenting next the basic theory of nanoindentation.
