Variations in the microstructure and the dominant fretting wear mechanisms of carbon steel alloy in oscillatory sliding contact against stainless steel in a dry atmosphere were evaluated by various mechanical testing and microanalytical methods. These included scanning electron microscopy and energy dispersive spectrometry with corresponding elemental maps of the wear tracks, in conjunction with cross-sectional transmission electron microscopy of samples prepared by focused ion beam machining to assess subsurface and through-thickness changes in microstructure, all as a function of applied load and sliding time. Heavily dislocated layered microstructures were observed below the wear tracks to vary with both the load and sliding time. During the accumulation of fretting cycles, the subsurface microstructure evolved into stable dislocation cells with cell walls aligned parallel to the surface and the sliding direction. The thickness of the damaged subsurface region increased with the load, consistent with the depth distribution of the maximum shear stress. The primary surface oxide evolved as Fe₂O₃ and Fe₃O₄ with increasing sliding time, leading to the formation of a uniform oxide scale at the sliding surface. It is possible that the development of the dislocation cell structure in the subsurface also enhanced oxidation by pipe diffusion along dislocation cores. The results of this study reveal complex phase changes affecting the wear resistance of steels undergoing fretting wear, which involve a synergy between oxidative wear, crack initiation, and crack growth along dislocation cell walls due to the high strains accumulating under high loads and/or prolonged surface sliding.

We perform a gas gun experiment by shock loading tantalum samples of varying grain structures to assess the suitability of a numerical model for simulating spall behavior. The observed differences in spall strength, as well spallation and re-compression history, are not captured in uncalibrated hydrodynamic simulations. An optimization is performed on the Johnson spall model to determine the best parameters that fit the observed trends. Linear stability analysis is employed to motivate bounds on those parameters. Herein, optimized simulations agree well with the experimental results, reproducing pullback depth and recompression timescales across the different samples tested. Further, the observed pullback time of the single crystal sample was found to imply, via the stability analysis, a percolation threshold in good agreement with the theoretical value for a body centered cubic lattice. Therefore, the combined linear stability and percolation analysis shows promise and may be applied to other materials with diverse microstructures. Collectively, the findings demonstrate that the model is suitable for reproducing spall-induced free surface behavior across various microstructures, but also points to caution in using model coefficients for uncalibrated microstructures.

Vanadium oxides (VOxs) are technologically important materials because of their versatile properties, including a reversible metal-insulator transition. However, the complex vanadium-oxygen phase diagram includes numerous stable stoichiometries and metastable polymorphs, making material identification challenging. This work presents a data-driven, nondestructive characterization workflow that combines Raman spectroscopy and principal component analysis (PCA) to provide rapid and intelligent feedback during VOx thin-film fabrication. Although Raman spectroscopy provides ultrasensitive chemical information, conventional spectral interpretation requires specialized expertise and extensive reference data. An in-house fabrication protocol based on spin-casting chemical precursors and thermal annealing was developed to produce thin-film standards of VO₂, V₂O₅, V₃O₇, and V₆O₁₃. Raman fingerprints from these standards were used to construct two- and three-dimensional PCA models that distinguish stoichiometric variation and provide interpretable feedback during fabrication-process development. The standards also serve as references for evaluating thermally corroded samples. These results demonstrate that PCA-enabled Raman analysis can serve as an adaptive sensing approach for rapid assessment of VOx stoichiometry, simplified spectral interpretation, and fabrication optimization.