Inducing phase transformations using depth-sensing indentation

Phase transformations in solids occur during a number of common contact loading situations including polishing, cutting, and grinding. To understand and optimize material response to such processes or to engineer a surface with specific properties (i.e. electrical, optical, mechanical) at a micro-scale, depth-sensing indentation is used to study point loading situations and simulate dynamic processes. Sometimes, evidence of phase transformation can be found from load-displacement curves. Raman spectroscopy or other techniques (i.e. transmission electron microscopy, atomic force microscopy, or x-ray diffraction) on residual impressions often provides useful information to characterize transformations which take place during indentation. In this work, depth-sensing indentation and Raman spectroscopy are used as the primary techniques to identify phase transformations under different maximum applied loads, loading rates and unloading rates. Single-crystal silicon and lead zirconate titanatemodified by niobium (PNZT) are used as exemplary materials for study because of their widespread industrial importance. A general technique for studying phase transformations induced by indentation is developed. Silicon undergoes a transformation from the cubic-diamond (Si-I) to [beta]-tin phase (Si-II) on loading around 12 GPa, and from Si-II to the r8 (Si-XII), bc8 (Si-III) or amorphous silicon on unloading. Using a number of different loading conditions, the stability of Si-II was found to range between 4 and 12 GPa, depending on maximum applied load, unloading rate and indenter tool geometry. 95/5 PNZT undergoes a ferroelectric to antiferroelectric phase change around 300 MPa of contact pressure. Both sharp (Berkovich) and spherical indentation are used to study different stress states for each material, and its influence on phase changes. On silicon, over 4500 indentations are made to determine the statistical nature of phase changes. Differentiation of loaddisplacement curves is developed to help detect the precise point in the curve where transformation begins to occur. Raman spectroscopy is used to correlate distinctive curve shapes with the type of phase change occurring. Cyclic indentation and indentation stress-strain curves provide additional insight. The collective data is used to establish understanding of the two materials' pressure-induced phase transformations on a microscale, and may be extended to other materials and dynamic loading situations.