High temperature tribological behaviour of PVD coated tool steel and aluminium under dry and lubricated conditions

Aluminium alloys are commonly used as lightweight materials in the automotive industry.This non-ferrous family of metallic alloys offers a high versatility of properties and designs.To reduce weight and improve safety,high strength-to-weight ratio alloys(e.g.6XXX and 7XXX),are increasingly implemented in vehicles.However,these alloys exhibit low formability and experience considerable springback during cold forming,and are therefore hot formed.During forming,severe adhesion(i.e.galling)of aluminium onto the die surface takes place.This phenomenon has a detrimental effect on the surface properties,geometrical tolerances of the formed parts and maintenance of the dies.The effect of surface engineering as well as lubricant chemistry on galling has not been sufficiently investigated.Diamond-like carbon(DLC)and CrN physical vapour deposition(PVD)coated steel have been studied to reduce aluminium transfer.However,the interaction between lubricants and PVD coatings during hot forming of aluminium alloys is not yet fully understood.The present study thus aims to characterise the high temperature tribological behaviour of selected PVD coatings and lubricants during sliding against aluminium alloy.The objectives are to first select promising lubricant-coating combinations and then to study their tribological response in a high-temperature reciprocating friction and wear tester.Dry and lubricated tests were carried out at 300℃ using a commercial polymer lubricant.Tests using DLC,CrN,CrTiN,and CrAIN coated tool steel were compared to uncoated tool steel reference tests.The initial and worn test specimen surfaces were analysed with a 3-dimensional(3D)optical profiler,scanning electron microscope(SEM)and energy dispersive X-ray spectroscope(EDS)as to understand the wear mechanisms.The results showed formation of tribolayers in the contact zone,reducing both friction and wear.The stability of these layers highly depends on both the coatings'roughness and chemical affinity towards aluminium.The DLC and CrN coatings combined with the polymer lubricant were the most effective in reducing aluminium transfer.

Influence of lubrication,tool steel composition,and topography on the high temperature tribological behaviour of aluminium

The use of high strength aluminium alloys,such as 6 XXX and 7 XXX series,is continuously increasing for automotive applications in view of their good strength-to-weight ratio.Their formability at room temperature is limited and they are thus often formed at high temperatures to enable production of complex geometries.Critical challenges during hot forming of aluminium are the occurrence of severe adhesion and material transfer onto the forming tools.This negatively affects the tool life and the quality of the produced parts.In general,the main mechanisms involved in the occurrence of material transfer of aluminium alloys at high temperature are still not clearly understood.Therefore,this study is focussed on understanding of the friction and wear behaviour during interaction of Al6016 alloy and three different tool steels in as-received and polished state.The tribotests were carried out under dry and lubricated conditions,with two distinct lubricants,using a reciprocating friction and wear tester.The worn surfaces were analysed using scanning electron microscopy(SEM)and energy dispersive X-ray spectroscopy(EDS).The results showed a high dependence of friction and wear behaviour on the tool steel roughness as well as on the stability of the lubricant films.Tribolayers were found to develop in the contact zone and their capacity to improve the tribological behaviour is seen to be drastically impacted by the surface roughness of the tool steel.When the tribolayers failed,severe adhesion took place and led to high and unstable friction as well as material transfer to the tool steel.