ABSTRACT
In the present chapter, we will be describing the nanoindentation behavior of a tough ceramic, i.e., zirconia-toughened alumina (ZTA). Generally, zirconia is known to be a strong ceramic. It has failure strength (≈650 MPa) nearly 1.5 times that of alumina. Similarly, it has a critical strain energy release rate (≈153 J⋅m−2) about 7.5 times as high as that of alumina (≈20 J⋅m−2). To obtain high toughness for structural application purposes, tetragonal zirconia is incorporated in an alumina matrix that gets toughened [1-3]. When the alumina matrix contains the tetragonal phase of appropriate amount, what we get are the ZTA ceramics. These ZTA ceramics often show an R-curve (crack resistance curve) behavior, which means that the material has an intrinsic capacity to exhibit an increase in fracture toughness with indentation crack length [4]. Transformation toughening occurs when the retained metastable tetragonal-ZrO2 (t-ZrO2) transforms to the stable monoclinic-ZrO2 (m-ZrO2) phase in the tensile stress field around a propagating crack. The volume expansion (4%–5%) characteristic of the t → m transformation introduces a net compressive stress in the process zone around the crack tip. This reduces the local crack-tip stress intensity and hence the driving force for crack propagation, thereby increasing the effective toughness of the material. The related issues were discussed in detail in an excellent review by Basu [5].
