Ductile Regime Machining of SiC J. Patten (PI), Western Michigan University, DMR We have previously demonstrated ductile regime machining of SiC experimentally. Recently we have successfully theoretically studied this behavior through the use of FEA. The figure to the right demonstrates that pressures comparable to the hardness of SiC are generated in the cutting-chip formation zone. This “red zone” of material is believed to undergo a high pressure phase transformation (HPPT) to a more ductile/plastic state allowing for machining to occur without brittle fracture. The lower figure compares the experimental and simulation cutting force results, which for a 100 nm depth are in good agreement. The difference at the greater depths (300 & 500 nm), are due to the brittle nature of the actual experiments, which reduces the forces, whereas the simulations currently assume complete ductile behavior. Current work is addressing 3-D Experiments & simulations of ductile scratches.

TEM and Raman Spectroscopic Analysis of High Pressure Phase Transformations in Machined Single Crystal Silicon TEM and Raman Spectroscopic Analysis of High Pressure Phase Transformations in Machined Single Crystal Silicon; R. O. Scattergood (co-PI), Precision Engineering Center, North Carolina State University, DMR (A) The ductile behavior of diamond turned silicon is thought to be driven by a high pressure phase transformation. TEM micrographs and Raman spectra are presented to show the characteristics of the surface and subsurface of the machined silicon. The spectra in (A) shows the relationship between feedrate and the amount of amorphous silicon (520 cm -1 ) that is formed. Amorphous silicon is believed to be an indicator that a high pressure phase transformation has occurred. As feedrate increases the amount of amorphous silicon decreases. The TEM micrographs in (B) and (C) show the subsurface microstructure of the silicon which was machined at 5um/rev. The amorphous silicon is ~50 nm deep while an accompanying damage layer extends ~600 nm below. (111) slip planes can be seen in (C) since they are at a ~54° angle from the (100) surface. This microstructure has been observed by other research groups, but the cutting mechanics creating the layer remain unclear.

Critical resolved shear stress of single crystal  -Si 3 N 4 High Pressure Phase Transformations of Silicon, Germanium, and Silicon Nitride, George M. Pharr (co-PI), University of Tennessee, DMR The deformation behavior of  -Si 3 N 4 and 6H-SiC single crystals, as they relate to precision machining, were explored by nanoindentation using 2 spherical indenters with different radii and 6 pyramidal indenters with different centerline-to-face angles. We measured, for the first time, the yield strength of single crystalline  -Si 3 N 4 and found that the critical resolved shear stress for yielding in this super-hard material (E = ~365 GPa; H = ~34.5 GPa) is about GPa, which is close to the theoretical shear strength (~ G/2  ). We also established the compressive stress-strain behavior of 6H-SiC by combining nanoindentation experiments using different indenters with finite element simulations. This is first time the strain hardening behavior of this important material has been measured. The behavior cannot be measured by conventional compression or tensile testing due to the brittle nature of the material. Predicted stress-strain behavior of single crystal 6H-SiC

High Pressure Phase Transformations (HPPT) of Si, Ge, and Si 3 N 4 J. Patten, Western Michigan University, DMR Our Focused Research Grant (FRG) has been fortunate to support a female student at each one of our participating institutions (WMU, UNCC, NCSU, UT-K); including two PhD students, one maters and one undergraduate student. Their combined work resulted in three paper awards. Their projects include: micro Raman and TEM analyses, SEM and AFM characterization, IR laser heating, scratching and nanoindentating, and polishing. To date we have also sponsored three undergraduate students during the conduct of the research program. Additionally, we have engaged the support of numerous companies that have contributed to research projects.