Nano-indentation of scratched poly(methyl methacrylate) surfaces

Abstract:

The paper describes an experimental study of the nano-indentation hardness profiling of scratches, of various depths, produced upon the surface of a poly(methyl methacrylate). The method has limited spatial resolution and typically the indent size, and hence the volume of deformed material sensed, was in the worst cases one-tenth of the scratch width; in exceptional cases the resolution was ten times better than this figure. Additionally, the various topographical and morphological features are evaluated using laser profilometry and optical and scanning electron microscopy. The data described indicate that the scratching process introduces damage in subsurface regions, which may be tentatively ascribed to the formation of subsurface crazing or voidage in certain regions of the scratch damage area; in particular within the base of the scratch formation. Such structures could potentially develop in the substrate tensile fields developed at the rear of the indenter. In addition, there is evidence for densification of the polymer adjacent to the primary damage zone—on the “shoulder” areas adjacent to the main scratch path. An analysis of the periodic fluctuations of the nanohardness data, in certain zones, allows a speculative description of the topology of the subsurface damage in terms of the induced voidage and its distribution. Such features would naturally lead to a “positive” deformation volume; an analysis of an orthogonal topographical trace of a scratch allows a simulation of volume with respect to the unpurturbed surface—a “positive” comparison infers an additional volume and vice versa. In the current study, such “positive” volume deformation regions were not unequivocally identified with the proposed craze zones using the topographical estimation procedures employed.