ICMCTF2019 Session G2-FrM: Component Coatings for Automotive, Aerospace, Medical, and Manufacturing Applications

Alumina coatings prepared by PEO (plasma electrolytic oxidation) process have been proposed and in a validation process to replace heavy cast iron liners for internal combustion engines. The bipolar current mode in the PEO process is known as an excellent coating preparation condition. However, the industrial production would require an optimized as well as cost-efficient process. The unipolar current mode can si gnificantly reduce the investment on coating facilities. Thus, this research was to use the unipolar mode to prepare oxide ceramic coatings on Al-Si alloy samples with different durations, i.e., 20%, 30% and 40% durations of 1000Hz. One of the samples was used with a normal bipolar mode for comparison. The influence of duty ratios on the coating’s deposition rate, porosity and wear resistance was investigated. At a similar coulomb charge input (I × t), the change of duty ratios from 0.8 to 0.2 had an insignificant effect on the coating deposition rates, but the coating’s porosities decreased and wear resistance increased after the coated samples were polished to Ra around 25 microns and 20 microns. Dry and lubricated pin-on-disc tests indicated that the coatings produced by unipolar current mode at the lower duty ratio had comparable properties of the one prepared with the bipolar current mode. The tribological behavior of the polished surfaces was analyzed based on their surface morphology characteristics (Ra, Rpk, Vo, skewness and kurtosis ). The work provides a new perspective for the optimization of PEO process for automotive application.

We will give an overview of uncoated and PVD coated polycrystalline or p-cBN cutting tools that are used in finish turning on hardened steel. Examples include turning of automotive braking disks and precision shafts. Interestingly, PVD coatings can improve wear behavior in many applications. As the hardness of such thin films is comparable to the substrate values the improvement is explained by thermal and chemical protection. As many of the different p-cBN substrate materials show reduced electrical conductivity the deposition process has to be adapted. To improve adhesion and overall performance different interlayers and plasma etching cycles were applied in deposition of AlTiN arc ion plated coatings. Difference in adhesion were determined by indentation and scratch testing. Tool life was compared in turning tests of hardened steel (60 HRc) without coolant. The resulting wear patterns will be discussed.

Efficiency during operation is besides reliability and cost effective production one of the most important demands on machines and components. Especially within the automotive sector, components need to fulfill these requirements. Since the demands for high efficiency and reliability cannot be met solely by typical base materials such as case hardened steels, physical vapor deposition (PVD) coatings for the application on highly loaded components gain increasing importance. A possible approach to reduce friction and wear in tribological systems are triboactive and tribocatalytic coatings which contain triboactive elements such as Mo and Cu which can interact with lubricants and lead to the formation of friction and wear reducing tribochemical reaction layers. Besides coating development, also the design of lubricants is in the focus of research activities to reach friction reductions. Therefore increasing interest gains towards low viscosity lubricants e.g. for e-mobility applications.

Within the current work, triboactive (Cr,Al,Mo)N and (Cr,Al,Cu)N coatings were deposited by means of cathodic arc evaporation (Arc PVD) in an industrial scale coating unit. The contents of Mo and Cu were varied. As substrate material the case hardened gear steel AISI 5115 (16MnCr5E) was used. The effects of Mo and Cu on the phase formation were investigated by means of X-ray diffraction (XRD). Analysis of the mechanical properties was conducted by nanoindentation (NI) measurements. Tribological behavior of the coatings was analyzed under continuous sliding conditions in pin on disc (PoD) tribometer under minimum quantity lubrication with lubricant amounts of V = 0.05 ml at a temperature T = 80 °C. As lubricants a low viscosity lubrication oil and a conventional mineral oil were used. Both lubricants contained Sulphur and Phosphorous (S-P) additives. Initial Hertzian contact pressure was set to p_H ≈ 1,600 MPa. In order to investigate the influence of the counter body material on the tribological behavior, inert Si₃N₄ balls and 100Cr6 steel balls were used. Wear was analyzed by confocal laserscanning microscopy (CLSM) and scanning electron microscopy (SEM). Tribochemical interactions between the coatings and lubricants were studied by Raman spectroscopy. It was found that tribo chemical interactions between Mo of the coating and S of the lubricant can lead to the in situ formation of MoS₂ in tribological contact. Triboactive (Cr,Al,Mo)N and (Cr,Al,Cu)N coatings are a promising approach to achieve reduced friction and wear in tribological systems.

Presently, various kinds of hard film coatings are used in combination with cemented carbide (WC-Co) and high speed steel (HSS) cutting tools for increased longevity. For further improvement of the coated tools' life-span, their regrinding and recoating are commonly implemented. Successful recoating requires stripping off of a previous coating in order to provide sufficient adhesion. Wet electro-chemical etching (ECE) process is commonly applied for the stripping purpose. Although well established, the ECE method has some known issues such as cobalt binder leaching from the WC-Co tools, HSS corrosion, large amount of chemical waste produced. Moreover, the ECE stripping method cannot be applied to the carbon-based thin films, such as diamond and diamond like carbon (DLC), due to their chemical inertness. In this context new competitive stripping approaches are of high applicational importance.

In the present work we introduce a vacuum stripping process based on ion beam etching (IBE) of the hard films. The IBE method utilizes low energy (~1keV) high current (~0.1A) broad (~10cm) ion beams in order to create sufficient ion and radical fluxes on the tools’ surface. In order to etch the metal based films, such as TiN, TiAlCrN, TiCN argon gas was introduced, while carbon based DLC and diamond films were IBE processed utilizing pure oxygen as a working gas for improved erosion rate and selectivity against the carbide material.

IBE erosion rates of commercial PVD and CVD hard coatings on various WC-Co cutting tools were experimentally measured and plasma-chemical aspects of diamond decomposition were discussed. Effect of tool geometry on the IBE uniformity was experimentally studied by measuring the local erosion rate over the tools’ surface. Moreover, mathematical model of the IBE process, describing the erosion rate distribution was suggested. Surface condition of the WC-Co substrates treated by IBE were evaluated and no significant deterioration was found. Overall, it was shown that IBE provides sufficient erosion rate and uniformity without significant damage to the tool material for virtually any kind of hard coating used in today’s cutting tools industry.