ICMCTF2016 Session G2: Components Coatings

Both high wear resistance and low friction coefficient is strongly demanded in improving life time and eco-friendliness of different automotive components. In early stage of PVD coating for automotive application, CrN was used and achieved a great success in piston ring market. CrN can be deposited up to several tens of microns without failure and suitable for piston rings for heavy duty diesel engines. Meanwhile, as the various emission regulations becomes more strict in each continent, although CrN still represents majority of coated piston rings in these days, carbon based coating such as Diamond-like carbon (DLC) coatings have been drawing more attention for this application due to combination of low friction coefficient, high hardness, and high chemical inertness.

A variety of deposition technique for DLC coatings including arc evaporation, sputtering and PECVD had been developed since 80’s, commercial application of DLC coating for mass production auto motive components was not realized soon, mainly due to lack of reliable adhesion. Almost 20 years have passed since then, reliability of DLC coating has been improved through various deposition techniques and coating architecture optimization and DLC coating is now commonly adapted to various automotive components.

Our PVD business has been growing as the coating application to automotive components is expanded and has been supplying numbers of AIP and UBMS coater including standard coaters and purpose-built coaters such as piston ring coater, UBMS coater which has PECVD capability, and so on.

This paper will include a brief summary of our history in PVD business and two new technologies towards next stage of DLC coating will be highlighted, one is MF-AC CVD process which is capable of stable depositing thick DLC with good sliding property. The other one is round bar type carbon cathodic arc source for stable deposition of smooth ta-C coating with a simple system configuration.

Due to their unique combination of superhardness and low friction properties tetrahedral amorphous carbon films (ta-C) are very attractive as tribological coatings e.g. on automotive sliding components like piston pins, piston rings, and valve-train components. In contrast to conventional DLC films (a-C:H), ta-C cannot be deposited by PECVD or magnetron sputtering techniques. The only effective technique for mass production of ta-C coatings is arc evaporation of graphite. Several specific arc-evaporation technologies including dc-, pulsed, or laser-triggered arc discharge have been developed to obtain a stable and efficient arc evaporation source. A common and unwanted feature of all these techniques is a more or less pronounced co-evaporation of macroparticles from the graphite source and hence, a considerable amount of defects in the growing ta-C film resulting in a significant surface roughness. The most effective way to suppress the particle problem is to use a plasma filtering unit in order to separate the particles from the plasma. The quality of ta-C coatings deposited by filtered arc technique is improved, but the efficiency of the deposition process is significantly reduced compared to an unfiltered process. Therefore, post treatment of unfiltered ta-C coatings is desirable. Due to the superhardness of ta-C conventional polishing techniques do not work or are very costly.

With the aim to fulfill legal regulations concerning energy efficiency and greenhouse gas emissions diamond-like carbon (DLC) coatings are increasingly used on highly stressed components of internal combustion engines in order to reduce friction and prolong component lifetime. The valve train is a key system of the combustion engine considerably contributing to frictional losses especially under boundary and mixed friction conditions at lower crankshaft speeds. In this regard, the tribological contact bucket tappet/camshaft offers high potential for friction reduction but places high demands on DLC coatings due to highly complex kinematics and repetitive loads depending on the cam contours and the camshaft angle and speed. The aim of this work was to analyze the influence of DLC coatings on the frictional and wear behavior within the contact bucket tappet/camshaft. This tribological contact was analyzed in a one tappet/one cam friction test-rig using series-production tappets (16MnCr5, AISI 5115), cams (100Cr6, AISI 52100) and valve springs ensuring high transferability of the results into the real application. Two different amorphous hydrogen containing carbon based coatings were deposited on series-production bucket tappets. The zirconium based DLC coating graded zirconium carbide ZrC_g (a-C:H/ZrC_g) was compared to an under industrial conditions deposited DLC coating system (a-C:H:X). Here two various deposition techniques, middle frequency magnetron sputtering (mfMS) for the ZrC_g coating and plasma assisted chemical vapor deposition (PACVD) for the DLC coating were applied. A mineral engine oil and two synthetic engine oils formulated with different additive packages containing the anti-wear (AW)/extreme pressure (EP) additive zinc dialkyl dithiophosphate (ZDDP) and the friction modifier (FM) additives molybdenum dialkyl dithiocarbamat (MoDTC) and glycerol mono-oleat (GMO) were used in the one tappet/one cam friction test-rig. The influence of the additive packages on the frictional and wear behavior was tested as a function of engine oil temperature T_oil=50 °C and T_oil=80 °C and camshaft speeds between n=500 min^-1 and n=2,000 min^-1 by time-resolved measurement of friction forces. The wear of DLC coated functional surfaces was analyzed by means of confocal laser scanning microscopy (CLSM). Chemical interactions between the DLC coatings and the additive packages of the formulated engine oils were investigated by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The results were correlated with the frictional and wear behavior observed in the one tappet/one cam friction test-rig.

Thin films improve the quality of everyday life in all areas of living. In line with development in industry, like industry 4.0 additional functionalities are required. In former days, it was sufficient to realize low friction, wear reduction, non-sticking, or any other direct function related to the interaction of the coated piece with the surrounding. Today, additional functions for security, of supervision of the status, or any kind of sensing, i.e. direct interaction with the environment becomes more and more important. In many cases, sensors or sensing functions can fulfill these requirements. Instead of assembling different sensors and somehow attaching them to the workpiece a full integration, using thin film technology is desired.

Using tailored layer stacks different sensing functions with respect to temperature, force, loading, position, and many more become available. Instead of gluing strain gauges on workpieces, thin film strain gauges can be sputtered. Additionally a temperature sensor might be integrated. Using magnetic encoding a positioning system can be realized that can operate even under dirty environment. Since most technical surfaces are metallic, an insulating layer is essential, regardless the final sensor function. This insulating layer has to be free of defects, like voids and cracks and of course free of particles. On top of this layer an electrical or magnetic active layer can be applied. For protection against external forces finally, a protective layer, either insulating, or wear resistant, or even anti-sticking can finish the layer stack.

The presentation will cover the deposition of the insulating and the sensing layer and the application of surface integrated sensors. For the case of the insulating layer different deposition techniques as PVD and PECVD will be presented. As sensing layer different materials like metal-doped DLC, ITO, and NiCr will be discussed. Finally, different application of the layer systems as weighing element, pressure sensors, and machine components will be reported.

Tungsten is the plasma-facing material for next fusion reactors (e.g. ITER divertor) due to its high melting temperature, high thermal conductivity and low erosion yield. As a drawback, tungsten has a strong chemical affinity with oxygen and native oxide is naturally present on tungsten surfaces, which leads to the formation of tungsten oxide layers. In order to study the effect of oxidation on tungsten properties, the behavior of WO_3-x layers under deuterium/helium bombardment and thermal cycling effect in divertor-like conditions, we have produced, by thermal oxidation, thin layers of WO_3-x on W substrates which mimic the possible oxidation of tungsten plasma facing components. The produced tungsten oxide layers were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray diffraction (XRD) techniques. The thickness of the colored oxide thin layer δ, measured by SEM using focused ion beam cross-section (FIB), follows a parabolic law as a function of the oxidation time (figure 1). A set of those oxide tungsten thin film samples were separately exposed, at PIIM laboratory (Marseille-France) and in collaboration with the University of Tennessee UT (Knoxville-USA), to D and He plasma beams with E = ~ 20 eV and total fluence ~ 4.10²¹ m^-2. Due to D implantation (which has high affinity to bond formation) followed by its deep diffusion [1], preliminary results show a phase transition in the WO_3-x, change in the layer color as well as formation of tungsten bronze (D_xWO₃) have been observed. However, the He implantation (that has high affinity to induce the creation of bubbles, holes and nanostructure morphology on W [2]) neither causes surface morphological change on the oxide of tungsten nor changes in its color. Deep analysis on the process of structural damage in surface/bulk will be described for both exposures using the coupling of Raman spectroscopy and SEM approaches.