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

Justine Decrozant-Triquenaux,Braham Prakash,Cédric Courbon,Leonardo Pelcastre,Jens Hardell

doi:10.1007/s40544-020-0435-7

Abstract

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 °C using a commercial polymer lubricant. Tests using DLC, CrN, CrTiN, and CrAlN 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.

Highlights

Lightweight aluminium makes it a material of interest in many sectors and, the automotive industry
The manual post-polishing of the physical vapour deposition (PVD) coated samples greatly reduced the surface roughness
The surface features on the aluminium samples are oxidised shingles (composed of lower gorges and protruding tongueshighlighted in Fig. 4(e)), gorges and rolling grooves, which are typical from the hot sheet rolling process [3]

Summary

Introduction

Lightweight aluminium makes it a material of interest in many sectors and, the automotive industry. The 6XXX and 7XXX aluminium series, due to their superior strength-to-weight ratio. These alloys are promising candidates to enable the compliance of new vehicles with modern environmental energy consumption as well as passenger safety regulations. Hot forming, and hot stamping in particular, are common manufacturing techniques for aluminium alloy components. These processes involve solubilisation of the aluminium work-piece, and subsequent forming in the dies at elevated temperatures (at around 0.6 Tm, where Tm is the melting temperature of aluminium [3]) and a final quenching step within the dies [2]

Objectives

Results

Conclusion