The influence of machining induced residual stress and phase transformation on the measurement of subsurface mechanical behavior using nanoindentation

Surface integrity of machined components is critical for product performance in service. Due to the very small scale of the subsurface, nanoindentation can be used to determine the surface/subsurface mechanical properties. However, the test data may be significantly influenced by machining induced residual stresses, strain hardening, size effect, and microstructures. A fundamental relationship between these factors and mechanical behavior in the subsurface is yet to be understood. Furthermore, it is not clear if residual stress in the subsurface can be determined from nanoindentation data. This study focuses on the basic relationships between nanohardness, residual stress, and microstructure. A series of nanoindentation tests were conducted on the cross-sections of the machined samples with distinct surface integrity induced by hard turning, grinding, and honing. It was found that white layer increases nanohardness and dark layer decreases nanohardness in subsurface, while strain hardening only slightly increases subsurface hardness. The research results indicate that residual stress nature can be qualitatively determined by the shape of a load-displacement curve and the characteristic parameters such as slope at initial loading, total depth, residual depth, and the ratio of residual depth to total depth. Residual stress affects the load-displacement curve shape at the onset of material yielding. Microstructure changes make a significant difference on the characteristics of a load-displacement curve, while strain hardening exerts slight influence. The effect of residual stress influence on indentation depth can be explained based on the Mohr's circle.