Deformation of Si(100) in spherical contacts — Comparison of nano-fretting and nano-scratch tests with nano-indentation

Nano-indentation, nano-scratch and nano-fretting tests were performed on highly polished Si(100) using a commercial ultra-low drift nano-mechanical test system (NanoTest, Micro Materials Ltd.) fitted with a 4.6 μm sphero-conical diamond indenter. In addition to plastic deformation, the contact deformation of Silicon is strongly affected by phase transformation and micro-/nano-fracture at room temperature. The objective of this work was to investigate the influence of tangential loading by comparison of deformation in the idealised loading in the nano-indentation test with the more complex tribo-contact situations. A novel method is introduced allowing quantitative comparison of deformation during loading in the nano-indentation, nano-scratch and nano-fretting tests. The loading curves in all three tests were almost identical at very low load. Tangential loading in the nano-scratch and nano-fretting tests promotes yield resulting in greater penetration depths at higher load than in nano-indentation. Contact damage assessment by in situ measurements of probe displacement during nano-fretting was supported by post-test SEM imaging and wear scar measurement by confocal microscopy. The nano-fretting crack morphology was related to the normal load applied and resulting track length. A brittle/ductile response of the material was observed, characterised by brittle micro-chips around wear track and ductile wear debris observed in the centre of the contact. The critical loads in the nano-scratch test show a subtle dependence on scan speed and loading rate. Pronounced lateral cracking observed in high load nano-scratch and nano-indentation tests is absent in the nano-fretting tests, which is consistent with the fretting wear process minimising the accumulation of strain.