D. Sciti

Indentation grid analysis of nanoindentation bulk and in situ properties of ceramic phases

Composites properties are often derived from the proper-ties of the constituent phases measured in bulk forms.However, in situ properties can be different from thosemeasured in bulk as a consequence of material processing[1–3]. In ceramic composites, for example, spurious phasescan form due to the chemical interaction of differentpowders. The knowledge of in situ properties would allowa better characterization and tailoring of composites per-formances. Many ceramic composites are particle-reinforced composites so that the evaluation of in situproperties involves measurements in very small volumes.For some mechanical properties, this can be accomplishedby nanoindentation tests. By nanoindentation, singlemicrostructural elements can be tested as grains in poly-crystals [4, 5] or single phases in composites [3, 6–8]. Inthis work, a comparison between nanoindentation bulk andin situ properties of some ceramic phases will be presented.Generally, in situ properties are evaluated by imaging theindentation marks, for example using a scanning electronmicroscope (SEM), to detect which phase was indented.Besides this traditional technique, which can be timeconsuming especially when indentations are tiny as ithappens in hard phases like advanced ceramics, in situproperties will be estimated applying a new type of anal-ysis to nanoindentation data [9–11]. According to this newanalysis, the mechanical properties of the constituentphases of a composite can be easily derived from a sta-tistical analysis of nanoindentation results without havingto image where indentation marks were placed. Basically,the analysis consists in fitting the experimental dataaccording to a proper number of statistical distributionswhose central values correspond to the specific propertiesof each phase. Constantinides et al. [10] called this analysisindentation grid (IG). In this work, IG will be applied toparticle-reinforced ceramic composites, some based onwell-known phases, such as MoSi

Effect of MoSi2 Particles on the Fracture Toughness of AlN-, SiC-, and Si3N4-based Ceramics

The effect of 30 vol% MoSi2 addition on the fracture toughness of AlN-, SiC-, and Si3N4-based composites is investigated. To isolate the contribution of MoSi2 particulate, the toughness of the composite is compared to that of the reference AlN, SiC, and Si 3N4 matrixes. It turns out that the main toughening mechanisms are residual stress and crack deflection. The residual stress is evaluated through the strain measure in the reinforcing particles by X-ray diffraction, and the crack deflection by analysis of the crack paths. The overall toughening effect is calculated as the sum of these two contributions compared to the experimental values of the toughness of the composites.

Analysis of nanoindentation tests on SiC-based ceramics

Two silicon carbide-based ceramics, with very different mean grain size, and a standard fused silica sample were characterized by nanoindentation. The values of hardness and Youngs modulus were measured at several peak loads and calculated using the models developed by Oliver and Pharr (O&P) and by Cheng et al. (C&C). For all the materials, the values of hardness and Youngs modulus were strongly dependent on the adopted model. In the silica specimen, the C&C Youngs modulus and hardness were lower than those calculated by the O&P model. In the SiC ceramics, the differences between the two models were both qualitative and quantitative. The C&C Youngs modulus values were lower than those calculated by the O&P model but the hardness values were higher. For most of the peak loads, the O&P model distinguished between the two SiC specimens while the C&C model did not.