Tribological and mechanical properties of high power laser surface-treated metallic glasses

The processing power of high power Nd:YAG laser has been utilised to achieve the inherently high cooling rates required to form many of today's bulk metallic glasses (BMGs). The production of thick (≥250 μm) amorphous surface layers has been considered. Microstructural and chemical observation techniques including scanning electron microscopy (SEM) and transmission electron microscopy (both with energy-dispersive X-ray spectrometry, EDS), and X-ray diffraction (XRD), reveal that fully amorphous layers are attainable. Coating-to-substrate adherence is functionally graded by virtue of an amorphous matrix interlayer around 50 μm in depth. Actual cladding and remelting to Ti substrates indicate that the process of laser cladding is a suitable technique for the application of metallic glasses as surface layers. Hardness and nanoindentation profiles reveal hardnesses up to 13 GPa over the full depth of a coating, coupled with elastic modulus around 150 GPa, which are comparable with bulk metallic glass melt-spun ribbons. Tribological tests have also been conducted which reveal good wear properties are attainable and shear banding has been seen in the contact region. Scratch testing shows the layers may exhibit extremely low coefficients of friction, and again shear band formation is witnessed.