In-situ optical oblique observation of scratch testing

In situ observation of scratching has been previously limited to two methods: optical observation of transparent materials from beneath the scratching indenter and oblique observation of using the electron microscope. In this work, a third option is explored: using a long working distance optical microscope alongside a commercial scratch tester to allow oblique observation of scratch deformation with sub-micrometer resolution during scratching. The advantages and disadvantages of optical vs. scanning electron microscopy techniques are discussed, and the observational power and limitations of the technique are illustrated for a range of different samples: metals, glasses, ceramics, and coatings. The failure and deformation sequence is characterized for each type of material, and both surface and sub-surface cracks are observed for transparent glasses and ceramics.