TRIBOLOGICAL BEHAVIOUR OF PVD COATED TOOL STEELS IN HOT FORMING OF ALUMINIUM AL- LOYS
Justine Decrozant-Triquenaux
1, Leonardo Pelcastre
1, Cédric Courbon
2, Braham Prakash
1, Jens Hardell
1Abstract
Aluminium alloys are commonly used as light-weight materials in the automotive industry. This non-ferrous family of metal alloys offers high versatility of properties and designs. In order to reduce weight and improve safety, grades with high strength-to-weight ratio, such as the 6XXX and 7XXX alloy series, are increasingly implemented in vehicles. These alloys, however, exhibit low formabil- ity and experience considerable springback when formed at low temperatures, and thus have to be formed at elevated temperatures. Severe adhesion and galling are known to be critical tribological challenges in hot forming of aluminium. During the forming operation, adhesion and transfer of aluminium onto the die surface take place. This phenomenon has a detrimental effect on the surface properties, geometrical tolerances of the formed parts and maintenance of the dies. The influence of surface engineering as well as lubricant composition on adhesion and galling has not been suffi- ciently investigated. Diamond-like-Carbon and Chromium Nitride PVD coatings applied on the tool steel have shown promising results for reducing aluminium transfer at high-temperatures, especially in the presence of a lubricant. However, the interaction between lubricants and PVD coatings during hot forming of aluminium alloys is not yet fully understood. The present study thus aims at charac- terising the high temperature tribological behaviour of selected PVD coatings and a lubricant during sliding against an aluminium alloy. The objectives are to select promising lubricant-coating combi- nations for the given application and to study their tribological response in a high-temperature re- ciprocating friction and wear tester. The tests were carried out at 300°C, under dry and lubricated conditions, in order to study the friction and wear performance. Uncoated tool steel reference tests were performed under dry and lubricated conditions and lubricated tests using DLC, CrN, CrTiN and CrAlN coated tool steel were performed and compared to the reference results. The initial and worn surfaces were analysed with white light 3D optical interferometry, scanning electron micros- copy (SEM) and energy dispersive spectroscopy (EDS) with a view to understand the wear mecha- nisms.
1 Introduction
The use of light-weight materials is continuously increasing in many industries. In particular, alu-
minium alloys, such as the 6XXX and 7XXX series, are implemented as structural components in
cars. These materials offer great opportunities in terms of weight reduction while still providing
strength levels that comply with the passenger safety regulations.
Hot forming, and press-hardening in particular, are commonly employed methods for manufacturing of aluminium sheet metal components with high strength [1]. The process involves heating of an aluminium sheet to the solubilisation temperature, and thereafter formed in the dies at elevated tem- peratures (around 0,6Tm – Tm being the aluminium melting temperature [1]) and subsequently quenched [2]. These techniques offer a solution to the poor room-temperature formability of these alloys [2], the considerable springback [3] and the need for high geometrical tolerances of the final components.
However, forming aluminium at elevated temperatures leads to challenging tribological phenomena that impact the tool lifetime, the surface quality as well as the mechanical properties of the compo- nents [1] [3] [4] [5]. High adhesion and severe galling are critical wear mechanisms known to occur when forming aluminium at high temperatures [6] [7] [8].
In order to address these tribological challenges and ensure high production rates, different wear reduction strategies are employed. These methods commonly include the use of lubricants and sur- face engineering techniques [9]. High-temperature lubrication, however, has many limitations, as lubricants decompose at elevated temperatures [9]. Different lubricants, such as oil-based [10], emulsions and additives [11] [12], polymers [13] and ionic liquids [14], with specific purposes, are hence developed to resist these severe conditions. The surface engineering route combines the con- trol of the topography and roughness of the dies as well as the use of coatings [15]. Recently, studies on wear that highlight the potential beneficial effect of using DLC, CrN, TiCN and TiAlN PVD coatings against aluminium, have been published [16] [17] [18]. However, the combined effect of such coatings, their roughness and their interaction with suitable lubricants on the tribological re- sponse is not fully understood to date.
The aim of the present work is thus to study the potential of using PVD coatings with and without lubricant in order to reduce aluminium transfer at high temperature as well as to evaluate the effect of surface topography on lubrication and material transfer initiation.
2 Experimental work
2.1 Materials and specimens
A commercially available Cr-Mo-V-alloyed hot work tool steel (further referred to as tool steel O) was used in uncoated and PVD coated condition with four different coatings (CrTiN, CrAlN, DLC ta-C and CrN). The counter surface was an AA6016 aluminium alloy. The chemical composition of the materials, as provided by the supplier, are shown in Table 1.
Table 1: Chemical composition (mass contents in %) of the investigated materials (Fe for the steel and Al for the alu- minium make up the balance)
Material C Si Mn Cr Mo V Mg Zn Cu
Tool Steel O
0.39 1.0 0.4 5.2 1.4 0.9 - - -Al6016
- 1.0– 1.5 ≤ 0.2 ≤ 0.1 - - 0.25– 0.6 ≤ 0.2 ≤ 0.2The tribological tests were carried out under dry and lubricated conditions, using a warm forming
commercially available lubricant consisting of an aqueous emulsion of polymer and siloxanes (fur-
ther referred to as polymer).
To investigate the impact of roughness on friction and wear, the CrN and CrTiN coated tool steel specimens were mirror polished. The tests using mirror polished specimens and as-received topog- raphies will be referred to as MP and AR, respectively, in the figures. The roughness values of the different surfaces are given in Table 2.
Table 2: Average roughness parameters of the different specimens [ISO 25178, λc 0,25µm]
Specimen AR Sa [µm] MP Sa [µm]
Ø4mm Uncoated tool steel O
0,28±0.02
CrTiN coated tool steel O
0,11±0.03 0,02 ±0.01
CrAlN coated tool steel O
0,17±0.01 DLC coated tool steel O
0,40±0.02
CrN coated tool steel O
0,34±0.07 0,02 ±0.00
AA6016
0,93±0.00
The test configuration was a flat-on-flat pin on plate contact, using specimens supplied as Ø4mm flat end tool steel cylinders and 20x20x1mm aluminium plates.
2.2 Test equipment and procedure
The set-up was adapted from a standard Optimol SRV
®high temperature reciprocating friction and wear tester configuration, to a flat-on-flat pin on plate contact. This specific contact configuration required accurate specimen alignment, using pressure sensitive paper, of the samples before starting the heating cycle. The test configuration and conditions can be seen in Figure 1. The average friction levels were computed from the last 25s of test in order to avoid the influence of the initial friction peak.
Figure 1: Sketch of the SRV
®test set-up, applied heat cycle (not to scale) to the lower specimen and test parameters
The aluminium plates underwent a heat treatment within the tribometer, according to the industrial process requirements, before the initiation of the tests. This heat cycle is described in Figure 1. The lubricant was applied on the pin surface before the beginning of the heat cycle and the contact be- tween the pin and the plate was not made until the beginning of the tribological test (i.e. only few seconds before sliding started).
The surfaces of both pin and plate samples were analysed before and after the tests using a white light 3D optical interferometer, for the topographical analyses; and a scanning electron microscope (SEM) incorporating energy dispersive spectroscopy (EDS), in order to assess the wear mechanisms.
Heat cycle
Test duration [s] 30
Load [N] 10
Contact pressure [MPa] 0.8 Test temperature [°C] 300 Sliding frequency [Hz] 12.5 Sliding speed [mm/s] 100 10min
3 Results and Discussion
3.1 Friction
Dry sliding tests were performed using only the uncoated tool steel O, the DLC ta-C and the CrN PVD coated specimens. The friction level was high (as can be seen in Figure 2), and the friction behaviour showed a rapid increase from the very beginning in all three cases.
Figure 2: Average friction levels over the last 25s of tribotest for all specimen combinations (3 repetitions for each con- figuration), showing the improvements due to the lubricant and the reduced surface roughness
The use of the lubricant significantly decreased the friction for all the specimen combinations. For the tests involving the as-received topographies, friction was observed to decrease by at least half as compared to the dry conditions. Its stability, however, was fluctuating depending on the material combination: the best results were obtained for the DLC ta-C and the CrN PVD coatings, for which a reduction of the friction by three times and the best stability over the entire duration were observed (COF≈0.4±0.2). This reduction is quite significant, as aluminium at elevated temperatures generally exhibit high friction levels and significant instability due to adhesive wear, detrimentally affecting the contact interface [4] [5] [19].
It has been observed in the literature that controlling the roughness of the materials in contact with aluminium has a great influence on both friction and wear under lubricated conditions, though not being enough to fully prevent adhesion to take place [15] [20]. The same trend regarding the effect of surface roughness was also observed in this study, as the greatest reduction and stability of the friction was obtained from the tests using mirror-polished CrN PVD coated specimens under lubri- cated conditions. The CoF was reduced by more than three times compared to the as-received spec- imens. The manual mirror polishing of the CrTiN PVD coated specimens however only improved the friction value during the initial part of sliding, followed by a sharp rise in friction towards the end of the test, leading to a high standard deviation (Figure 2). This was due to the waviness of those samples and their different chemical affinity towards aluminium. This suggests that both the chem- istry and the topography of the coatings play an important role in the reduction of friction.
3.2 Wear mechanisms
The wear mechanisms were found to be quite different when comparing the as-received dry, as-
received lubricated and mirror-polished lubricated tests. Figure 3 shows the worn surfaces of the
CrN PVD coated tool steel after the dry sliding test (Figure 3a), the lubricated tests with as-received
surface topography (Figure 3b) and the lubricated test with the polished surface (Figure 3c).
The critical need for lubrication under these conditions, as seen in the current industrial practices and state of the art [1] [20], was confirmed: in the present study, the dry sliding tests showed similar immediate and severe adhesion whether using uncoated or PVD coated tool steel. Adhesion was observed on the pins in the form of thick lumps of aluminium, which covered most of the pins’
surface (as seen in Figure 3a). Material transfer was found to initiate from mechanical pick-up by the roughness asperities of the pin specimens (when used in their as-received state) and continuously growing.
Figure 3: SEM micrographs of the worn CrN coated tool steel after the a) as-received dry sliding test, b) as-received lubricated test and c) mirror-polished lubricated test.
The presence of a lubricant led to the formation of a carbon- and silicon-rich protective tribofilm on all the as-received PVD coated pin specimens. However, those tribofilms were found to differ in their durability in two distinct groups. The uncoated, CrAlN and CrTiN PVD coated specimens showed the development of a heterogeneous and granular tribofilm, which did not resist the entire test duration. The tribofilm did not fully cover the contact area and showed signs of detachment, which led to a progressive build-up of aluminium transfer onto the pins, correlating to the observed unstable friction. On the other hand, the DLC ta-C and CrN PVD coated specimens showed more compact and protective tribofilms on the surface in contact (Figure 3b). These were more resilient and covered larger areas on the pin surface than in the other cases, enabling a lower and more stable frictional behaviour (see Figure 2).
The best improvements in terms of friction and wear were found for the DLC ta-C and the CrN PVD coated specimens, which motivated to select these specimens for polishing. However, the low thick- ness of the DLC ta-C PVD coating did not permit polishing without damaging the coating. Focus was therefore put onto studying the behaviour of the CrN PVD coating and one of the other coatings (CrTiN) to investigate the influence of PVD coating surface roughness on the tribological behaviour.
In the case of the mirror-polished CrN and CrTiN PVD coatings, both friction and wear were found to be significantly decreased compared to the as-received topography. In the case of CrTiN PVD coated specimens, the average friction is not representative of the significant improvement which was still observed for few seconds. The improvement due to the reduced roughness can be explained by the fact that the as-received topographies exhibit large valleys, whose height exceeds the lubri- cant –and later tribofilm– thickness. These valleys will therefore cause scraping and removal of the protective tribofilms. They will also facilitate ploughing of the aluminium counter surface, resulting in more unstable and more severe wear than in the mirror-polished case. Extremely thin and only sporadic wear/material transfer was found on the polished PVD coated pins (as exemplified in Fig- ure 3c for the CrN PVD coating as a very similar appearance was found for the CrTiN PVD coating)
CrN Tribofilm
a b c
Aluminium heavy transfer
Only traces of C; Si; O on the pin
100μm
100μm 100μm
which correlate well with the low and stable friction (shown in Figure 2). Only traces of a tribofilm –from the presence of C, Si and O– were found on the pins’ surface. It thus appears that in the case of the rougher as-delivered specimens (as exemplified in Figure 3b), the tribofilm was retained in the contact due to its mechanical anchorage in the roughness grooves.
When analysing the aluminium counter surfaces, the effect of polishing the PVD coatings on the formation of tribofilms became even more pronounced (exemplified in Figure 4 after sliding against the CrN PVD coating and a similar appearance was found for the CrTiN PVD coating). Firstly, the aluminium counter surfaces sliding against the as-received DLC ta-C and CrN PVD coatings exhibit mild grooves, parallel to the sliding direction, characteristic from mild abrasive wear (as shown in Figure 4a). This wear phenomenon is different compared to the other as-received cases. For the uncoated, CrAlN and CrTiN PVD coated tests, clear adhesive wear features (in the form of smearing and deformation) were found on the aluminium counter surfaces. The abrasive wear behaviour of DLC and CrN PVD coatings, when used against aluminium, has been reported in the literature [20], [21]. Due to the low chemical affinity between these coatings and aluminium, severe adhesion is inhibited. Hence, this phenomenon results in the hard asperities of the coating ploughing the alu- minium which is thermally softened at elevated temperatures.
Figure 4: SEM micrographs of the worn aluminium samples used under lubrication against a) as-received and b) mirror- polished CrN coated tool steel.
When analysing the aluminium surfaces after sliding against the mirror-polished PVD coatings, chemical and topographical changes were found (Figure 4b for the CrN PVD coating and similar to the CrTiN PVD coating). A tribofilm, rich in carbon and silicon, was observed to form in the initial valleys on the aluminium surface (darker areas in the SEM micrograph in Figure 4b). These valleys are likely to act as reservoirs for the lubricant during the sliding action. Simultaneously, the initial highest and protruding features of the aluminium surface (i.e. the tips of the shingles and scales obtained from the hot rolling process [1]) were levelled out to load-bearing plateaus in the contact area. This occurred through plastic deformation of the thermally softened aluminium by the smooth surface of the mirror-polished PVD coated pin. The flat load-bearing areas showed the presence of sharp and relatively large silicon particles (white particles in Figure 4b,). The particles originate from the tribofilm itself, which is solidified and present a specific chemical composition at the outer edges of the wear track. At those locations, the tribofilm also exhibits brittle fracture marks, which are matching the shape of the embedded particles. These sharp fragments detach under the tribolog- ical stresses and act as third-body particles in the contact zone and are progressively embedded in the soft aluminium matrix.
20μm 20μm
a b
AlSi Si
O
C Visibly abraded aluminium
Tribofilm in valleys
