Elsevier

Surface and Coatings Technology

Volume 357, 15 January 2019, Pages 1048-1059
Surface and Coatings Technology

Friction and wear performance of different carbon coatings for use in dry aluminium forming processes

https://doi.org/10.1016/j.surfcoat.2018.10.103Get rights and content

Highlights

  • Dry oscillating ball-on-plate tests (99,900 cycles) of carbon coatings against aluminium with Hertzian pressure about 700 MPa

  • Hydrogenated amorphous carbon and polished diamond coatings result in low wear and coefficient of friction against aluminium

  • Polished polycrystalline diamond coatings resulted in wear rates of 4 · 10−9 mm3/Nm (Al ball) and 1 · 10−8 mm3/Nm (coating)

  • For hydrogenated amorphous carbon and polished diamond coatings a transfer film formation is detected.

Abstract

Lubricants are commonly used in metal forming processes to reduce the friction between the workpiece and the forming tool, to protect semi-finished products and goods against corrosion and to reduce the load on the tool. One aim of environmentally friendly production technologies is to achieve dry forming without the use of lubricants. An oscillating ball-on-plate tribometer (load: 10 N, frequency: 2.5 Hz, ball diameter: 10 mm, 99,900 cycles) was used to compare the suitability of different carbon coatings for dry contact against the aluminium alloy EN AW 5083. As candidates a graded and multilayered hydrogenated amorphous carbon coating (a-C:H) system by physical vapour deposition (PVD) as well as a polycrystalline diamond coating by chemical vapour deposition (CVD) were studied. The CVD diamond coatings were tested in two surface conditions, a rough as-deposited (roughness Sa 1.19 μm) and a subsequent polished (Sa 0.01 μm) variant. An uncoated, hardened steel plate of ledeburitic cold working steel (X153CrMoV12-1) was used as reference material. Worn surfaces were examined using laser scanning and atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy to determine their wear rates and to characterize the degree of adhesion, the change in nano-topography and the type and quantitative change of chemical bindings. The tests showed that polishing of CVD-diamond coatings is mandatory for the use in regards to dry forming. Sliding against the hydrogenated amorphous carbon (a-C:H) coating and polished polycrystalline CVD-diamond coating (pCVDD) resulted in the lowest sliding friction coefficients (COF) of 0.13 respectively 0.12. The polished polycrystalline diamond coating showed the lowest wear rates at the round-ended aluminium pin of 4 · 10−9 mm3/Nm and at the coating of 1 · 10−8 mm3/Nm. Cycle-dependent friction coefficient transitions were exhibited by the a-C:H-coating. For the pCVDD-coating as well as for the a-C:H-coating the formation of a transfer film was detected, which explains the small COFs. Due to the nearly negligible wear rate the pCVDD-coating is the most promising one of the tested carbon coatings to enable dry aluminium forming with a long lifetime of the coated tool.

Introduction

Under the tribological conditions between the active surfaces of the tool and workpiece during dry forming, the application of hard coatings can have a positive effect on tool wear [1]. In order to significantly reduce tool wear, a good adhesion strength to the substrate material and a high hardness of the applied coating is not sufficient if the coating has a high tendency towards adhesion with the counter material. Normally, nitride or carbide based coatings such as titanium nitride (TiN), titanium carbide (TiC), titanium aluminium nitride (TiAlN), aluminium titanium carbonitride (AlTiCN) or chromium nitride (CrN) are used for cold forming operations [2]. These types of coatings offer very good wear protection, but show high coefficients of friction (COF) and a strong tendency to adhesion to the workpiece material [3].

Although they show promising friction and wear characteristics, diamond-like carbon (DLC) coatings are not widely used in metal forming processes or are restricted to low contact pressure applications [4]. Qi et al. [5] tested non‑hydrogenated amorphous carbon (a-C) coatings against Al alloy 319 (AlSi5Cu3) in different gas environments. The average sliding coefficient of friction was 0.72 in nitrogen, 0.28 in dry air (0% relative humidity (RH)), 0.12 in ambient air (40% RH), and 0.015 in hydrogen. Horiuchi et al. [6] tested hydrogenated amorphous carbon (a-C:H) coated tools in dry deep drawing processes of aluminium. Compared to uncoated tools in the lubricated state, these can reduce both the required process forces during dry forming and the formability of the deep-drawn EN AW-5052 sheet components. Abraham et al. [7] have carried out pin-on-disc and strip drawing tests of uncoated and a-C:H(:Si)-coated specimens made of cold working steel 1.2379. They achieved a significant reduction of the cold welding phenomena of the aluminium counter materials by the application of an a-C:H(:Si) coating under unlubricated conditions. Hasselbruch et al. [8] showed in dry ambient pin-on-disc tests against the aluminium alloy EN AW 6060, that an a-C:H-coating reduced the COF by a factor of three, and the wear of the pin and disc by several times, compared to the uncoated steel substrate.

Schade et al. [9] showed that diamond coatings with a lower roughness induce less wear than coatings with a higher roughness. They used a dry sliding tribological test with a vertical ring-on-ring configuration to test the sliding friction of diamond on diamond with different grain sizes. They measured the smallest COF of 0.27 and lowest wear for fine-grained diamond coatings with a surface roughness of between Sa 0.10 μm and Sa 0.13 μm. Diamond coatings with a roughness between Sa 0.32 μm and Sa 0.51 μm resulted in a similar COF but with a much higher wear rate. Chakravarthy et al. [10] conducted pin-on-disc tests of WC-Co, TiN and chemical vapour deposition (CVD) diamond coatings against Al-Si alloys. They demonstrated that applied CVD-diamond coatings enhance the service life of the WC-Co based machining tool. Also a better surface finishing at the counter material was realized due to a low COF of about 0.19. The low COF of 0.1 in the run-in phase is attributed to the formation of a fine graphite rich transfer layer at the interface, due to high stress values at local contact points. After 15 min adhesive wear of Al-Si fragments onto the diamond coated pins started that result after 30 min in local spallation of the diamond coating. Chattopadhyay et al. [11] studied the wetting characteristics of liquid aluminium on various surfaces. They discovered that CVD-diamond coatings exhibit a near non-wetting behaviour with a wetting angle of 174°. They concluded from their wetting and machining tests that diamond is largely chemically inert towards aluminium. Podgursky et al. [12] discovered a ripple formation in the wear scar surfaces of nano-crystalline diamond coatings after reciprocating sliding tests against various ceramic balls (normal load 2 N, frequency 5 Hz, ball diameter 3 mm). The resulting ripple formation was independent of the used ball material (Al2O3, Si3N4 and ZrO2). Scharf et al. [13] observed the build-up and attachment of a transfer film during sliding of sapphire and steel against amorphous carbon coatings in dry and ambient air. They concluded that the formation of a transfer film is traced back to shear and extrusion of loose debris out of the ball and coating material.

In tribological tests without lubricants, further effects arise which influence the behaviour of the tribological system. Tröber et al. [14] have found a correlation between thermoelectricity and wear in a dry blanking process by isolating the sheet metal and punch. They could demonstrate that resulting temperature gradients in the shear zone during blanking generate thermoelectric currents which strongly influence the adhesive wear.

In this paper microcrystalline CVD-diamond as well as hydrogenated amorphous carbon (a-C:H) coatings are tested for their ability to enable dry forming of aluminium alloys. Both coatings are especially designed for dry aluminium forming. The CVD-diamond coating is deposited by an atmospheric process based on a plasma flame, which allows the deposition of local coatings on three dimensional surfaces without limiting the tool size. Especially the high hardness and chemically inertness against aluminium are the advantages. To investigate the impact of the surface roughness, the CVD-diamond coating is tested as-deposited (roughness Sa 1.19 μm) as well as polished (roughness Sa 0.01 μm). The a-C:H-multi-layered coating system is particular designed to combine a good substrate adhesion strength, a high hardness and a high fracture toughness. A cold working tool steel 1.2379 (X155CrMoV12) is used as reference. An oscillating ball-on-plate tribometer test is used in order to simulate the dry contact conditions between coating and aluminium alloy at a Hertzian contact pressure of around 700 MPa. Long-term tests of approximately 105 cycles were carried out in order to derive conclusions about the expectable tool wear and resulting workpiece quality. The determined wear rate by the use of laser confocal microscopy on the plate specimen serves as an indicator of the expected tool wear. The wear rate at the counter material can be used to estimate the workpiece quality. The oscillating ball-on-plate test exhibits phases of alternating and static sliding friction which simulates the processing of single parts as for example in sheet metal forming. The aim of this study is to discover which of the designed coatings is promising to enable the production of a large number of aluminium parts by dry forming. Therefore the dry sliding friction of the coatings against round-ended aluminium pins as well as the worn surfaces of plates and counter bodies are intensively analysed.

Up to now challenges remain in dry forming of aluminium alloys, primarily due to the strong tendency to adhesion, see e.g. the above mentioned work. Therefore we explore in the current study novel designed coatings, a polished CVD-diamond coating as well as a coating system with hydrogenated amorphous carbon as top layer, regarding their behaviour in dry contact with aluminium.

Section snippets

Methods

Dry tribological oscillating ball-on-plate tests were performed using a CETR Universal Mechanical Tester UMT-3MT, see Fig. 1. All tests were carried out according to DIN EN 1071–12 standards [15]. A surrounding climate chamber kept the environmental conditions constant at a temperature of 24 °C ± 1 °C and 40% ± 1% relative humidity. The DFM-2-0984 sensor head can control forces within the range of 0.2 N to 20 N. An R35HW rotating drive unit was used to apply a sinusoidal speed profile to avoid

Results

Fig. 4 shows the running-in behaviour of all tribological systems in the dry oscillating ball-on-plate tribometer test within the first 15 min, equivalent to 4500 cycles. The uncoated steel sample (curve 1) shows no running-in behaviour and is characterized by the highest coefficient of friction (COF) against aluminium which increases from approx. 0.4 to about 0.6. The running-in duration of the CVDD (curve 2) and a-C:H (curve 3) coatings are both about 3 min. The COF of the CVDD-coating

Discussion

The tribological behaviour of different diamond and diamond like coatings against the aluminium alloy EN-AW-5083 (AlMg4.5Mn0.7) was tested. As reference, also an uncoated and polished steel X153CrMoV12-1 plate was tested and analysed. The observed high wear rate at the steel plate (6 · 10−6 mm3/Nm) and the round-ended pin (5 · 10−5 mm3/Nm) are explained by excessive abrasive and adhesive wear, also leading to the high COF of about 0.62. Pronounced and dispersed adhesion of aluminium and iron

Conclusion

With respect to the performed dry tribological tests of specially designed different carbon coatings for their suitability in terms of enabling dry forming of the aluminium alloy EN AW 5083 and subsequent analyses, the following conclusions could be derived:

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    Uncoated and polished steel X153CrMoV12–1 shows high abrasive and adhesive wear by dry sliding against a round-ended aluminium pin and results in a high COF of 0.62. That means that a surface coating is mandatory for forming tools out of

Conflicts of interest

None.

Acknowledgements

The authors would like to thank the German Research Foundation (DFG Deutsche Forschungsgemeinschaft) for funding this work of the priority program SPP 1676 “Dry metal forming – sustainable production through dry processing in metal forming”. This publication is the result of a cooperation between the sub-projects “Lubricant free deep drawing of aluminium sheets for body construction” (contract no. DFG VO 530/75-2) and “Potentials of Dry Swaging” (contract no. DFG ZO 140/13-2) within the

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