Compatibility of graphite, hBN and graphene with self-lubricating coatings and tool steel for high temperature aluminium forming

Abstract

In recent years, the hot forming of aluminium has raised significant interest in industrial sectors such as the automobile and aerospace. However, such processes are not without difficulties, as the high adhesive wear characteristic of aluminium alloys is exacerbated at high temperatures and detrimentally affects the quality of the finished product. In an effort to improve this situation, different solid lubricant additives including hBN, graphite and graphene have been mixed with a commercial-grade lithium grease and evaluated under conditions representative of the high temperature forming of aluminium alloys, using an innovative single-pass, high temperature tribometer.Two different alloys have been chosen for the present study, including a nickel-based, self-lubricating laser cladding previously studied by the authors in the context of high temperature applications in addition to a reference tool steel widely used in metal forming industry. Experimental results showed a detrimental role of hBN as a solid lubricant, providing insufficient protection against adhesive wear and even leading to noticeable increases in the observed friction compared to other solid lubricants such as graphite. It has also been found that graphene excels at decreasing friction and material transfer from the aluminium counter body, suggesting a great potential as a lubricant additive in high temperature forming processes. However, the results of the present study indicate that graphene concentrations of 1 wt% are not enough for effective lubrication at high temperatures.