High Temperature Tribology of Aluminium : Effect of Lubrication and Surface Engineering on Friction and Material Transfer

Abstract: Lightweight design for automotive applications has been pursued for several decades and continues to increase. The main driving forces are new and increasingly stringent emission regulations as well as the increasing popularity of electric, or hybrid, vehicles where the increased weight of the batteries need to be compensated by light weight structures. It is also critical to maintain or improve passenger safety while creating components and structures with lower weight. Materials exhibiting a high strength-to-weight ratio, such as high strength aluminium alloys, are highly interesting to realise the next generation of lightweight vehicle structures.The high strength aluminium alloys include the 5XXX, 6XXX and 7XXX series. In order to increase their formability and minimise springback induced during forming at room temperature, these alloys are preferentially formed at elevated temperatures. Different forming processes such as warm forming and hot stamping (e.g. hot forming and quenching) have been developed to enable forming of components with high geometrical complexity and mechanical properties. However, hot forming of aluminium alloys leads to a challenging tribological interface. Aluminium alloys are ductile and reactive metals, prone to severe adhesion (also termed as seizure or galling) when sliding against a harder metallic counter surface. Aluminium transfer to the forming dies affect the tool lifetime and impacts the quality of the formed component which leads to significant maintenance costs and reduced productivity. These are the main limitations that hinder the implementation of hot aluminium forming for mass production.Lubrication as well as surface engineering strategies are potential methods to control friction and wear in the hot aluminium-tool steel interface. Solid lubricants such as graphite and hexagonal boron nitride (hBN) have been studied for aluminium forming. Polymer-based lubricants are also increasingly evaluated for high-temperature applications. Surface engineering techniques includes both the control of the tool topography and the use of protective coatings. Surface roughness has been observed as a crucial parameter in the initiation of aluminium transfer to the counter surface. PVD and CVD thin coatings are increasingly studied as ways to alleviate galling. Among others, CrN and DLC coatings are known to reduce adhesion when sliding against aluminium. Despite the research efforts in this field, there is still lack of systematic studies where synergistic effects of lubrication, surface topography and coatings are explored in the context of hot aluminium forming.The aim of this research is to enhance the understanding of the tribological behaviour of aluminium sliding against tool steel at elevated temperatures. The effect of tool steel composition, surface roughness (as-received and post-polished), and PVD surface coating  composition (CrTiN, CrAlN, CrN and DLC ta-C) has been evaluated under dry and lubricated conditions (hBN-based and polymer-based).High temperature tribological tests were carried out in a reciprocating sliding flat-on-flat configuration. In dry conditions, the aluminium-tool steel tribosystem is characterised by severe adhesive wear and high friction. Effective control of friction and wear was found to be highly dependent on the ability of the lubricant to remain in the contact zone. The combined use of a polymer-based lubricant with post-polished surface topography on a PVD coated tool led to the best improvements in terms of frictional stability and reduced material transfer. This was mainly attributed to prevention of direct contact between the tool material and aluminium together with minimised mechanically initiated material transfer. Post-polished uncoated tool steels resulted in the development of a protective tribolayer in the contact and together with flattening of the aluminium surface, led to friction and wear reduction. In case of post-polished PVD coatings, the lubricant entrapment in the contact zone as well as the development of mechanically mixed layers on the aluminium lowered friction and wear.