AbstractsEngineering

Bulk metallic glass (BMG) is a promising class of materials in many industries where tribological performances are important considerations, for example, mechanical, automobile, aerospace, defence as well as in biomedical applications. However, the amorphous atomic structure of a BMG often experiences microstructural transformations during contact sliding, leading to a variation of its tribological behaviour. Several studies demonstrate that the sliding-induced microstructural transformation varies across cases, and the mechanism of phase transformation and its effects on friction and wear of BMG is still unclear. The aim of the present research is to dig into the details of the tribological properties of BMG by exploring the microstructure change mechanism. This thesis has conducted a systematic and thorough analysis of the tribological behaviour of BMGs with the aid of various techniques, including statistical modelling, pin-on-disc testing, scanning interferometry, X-ray diffraction, and secondary electron and transmission electron microscopies. A novel approach for estimating the interface temperature during contact sliding of BMG was developed that incorporates the microstructural analysis and statistical analysis of surface parameters. This thesis has also developed a novel experimental technique for direct measurement of the interface temperature during the contact sliding of two solids. The relationship between the estimated temperature and that measured experimentally were also studied by FE modelling. The major findings of the thesis are as follows: (1) Frictional heating plays a central role in the variation of tribological properties of a BMG during contact sliding. The BMG deforms superplastically or becomes nanocrystalline depending on the amount of frictional heating. Under this heating, the wear mode of the materials experiences transitions of (brittle)(ductile)(brittle–ductile). However, a BMG remains amorphous when frictional heating is insufficient even when the environmental temperature is close to the material’s glass transition temperature (Tg). (2) When frictional heating is not uniform on the wear track (under a lower contact stress and higher sliding speed), superplastic deformation and phase transformation take place in some areas of the wear track. Under these conditions, the wear resistance of the BMG decreases slightly. This is the result of (i) the decrease in the material’s strength and hardness, (ii) the increase in its ductility or toughness and (iii) the improvement of the mechanical properties by the emergence of nanocrystals. (3) The wear rate of a BMG decreases with increasing the environmental temperature, due to the reduced stresses on the contacting asperities and trapped wear debris (either rolling or adhered) at elevated temperatures. The wear debris also alters the wear mechanism from two-body abrasion to three-body abrasion at a certain elevated temperature. Conversely, the friction coefficient does not change significantly with environmental temperatures. (4) The…