ICMCTF 2022 Session B3-ThA: Deposition Technologies and Applications for Carbon-based Coatings

Recently, high power impulse magnetron sputtering (HiPIMS) was reported to be a suitable method to deposit dense hydrogen-free amorphous carbon (a-C) coatings. In this work, we report on the coating growth and properties of smooth and hard carbon coatings produced by the S3p™ (Scalable Pulsed Power Plasma) method in an industrial deposition plant. S3p™ technology enables scalability of the pulse power density and pulse length in a wide range and expands significantly the choice of the deposition parameters and process stability as compared to conventional HiPIMS technology. Smooth and hard hydrogen-free carbon layers were produced using graphite targets in Ar ambient. The thermal stability of the coatings in air and their tribological behavior in dry and lubricated environments were investigated and compared to the results of standard a-C:H and ta-C coatings. Furthermore, the influence of adding a C₂H₂ precursor in an Ar ambient atmosphere on the film properties and S3p™ reactive process was investigated.

Finally, selected applications where carbon coatings deposited by S3p™ outperformed common DLC coatings will be presented.

Nanostructured and amorphous coatings play an important role in today’s automotive, industrial and tool applications. There are huge areas for application for these kinds of coatings – first of all the automotive market, but also traditional applications like cutting or forming tools.

Today the focus is on the growing customer demand for coating properties like higher temperature stability and increased wear resistance in combination with low viscosity lubricants and fuels. In the last years therefore development work shifted to the group of hydrogen free DLC coatings like a-C and ta-C coatings.

To industrialize these coatings different developments routs were carried out in the last years, to ensure an economic and reliable way of deposition. The equipment design and even the selection of most suitable process technology are however also strongly determined by the productivity and the coating properties. In this regard we will demonstrate by using the different Hauzer high energetic pulsed technologies HIPIMS as well as new developed pulsed arc, that coatings can be more tuned and tailormade towards dedicated properties. We will also show that these technologies can be easy upscaled on different machine sizes and deliver here reliable industrial processes with a good ratio of coating cost per coated part and performance.

Carbon-based coatings, because of their unique combination of low friction, low wear and chemical inertness, have a long-proven potential as coating materials. Both cutting and forming tools are coated with diamond-like carbon (DLC) coatings, as are components in the automotive sector, both for combustions engines and xEV components. Besides, its appealing dark black color renders DLC useful for decorative purposes.

For different applications different types of DLC coatings have been established, where hydrogen-stabilized DLC, despite its relatively low hardness, is the material of choice for components such as sliding or rolling bearings, or decorative coatings. On the high-performance end, tetrahedral amorphous carbon (ta-C) withstands tough conditions and provides superior hardness of up to about 60 GPa.

While the temperature- respectively, oxidation sensitivity limits the application range of carbon-based coatings, new applications keep being added and progress in deposition technology allows for (relatively) low-cost coating even of large-area parts with DLCs.

A recent example for the successful use of amorphous carbon as protective coating for stainless steel are metallic bipolar plates for proton-exchange membrane (PEM) fuel cells. These thin sheets of below 100 µm, often made from 316L (1.4404) steel, need to be protected from corrosion in order to achieve the desired stack lifetimes. For this purpose, chemically passive carbon-based coatings have been used with success.

Recently, precision forming processes have been developed for the economical mass production of these bipolar plates. In production-near tests, DLC coatings combined with an advanced surface treatment process for the steel tools have been shown to perform very well, compared to alternative PVD coatings

DLC coatings are also commonly applied as black color layer for e.g. household appliances. However, because of the imminent exposure to abrasive wear of those surfaces, the supporting properties of the otherwise soft stainless steel substrate need to be enhanced by mechanical of diffusion treatment.

Progress has been made in deposition technology, too. A novel hybrid deposition technique combining a microwave plasma with sputtering allows for the fabrication of high-hardness, non-hydrogenated diamond-like carbon coatings. The properties and morphology of the fabricated DLC films are comparedto those from pulsed-direct current magnetron sputtering (DCMS) or high-power impulse magnetron sputtering (HiPIMS), using techniques such as Raman spectroscopy, nanoindentation, X-ray reflectometry and scanning electron microscopy.

Dimond-like Carbon (DLC) coatings have unceasingly absorbed overwhelming interest from industry as well as academic research institutions within last years. High hardness and elastic modulus, chemical inertness, superior tribological properties and good corrosion resistance as well as high biocompatibility and resistance to bacterial colonization make DLC an engrossing coating system. Owed to the unique and broad range of properties, DLC coatings are constantly employed in new applications from cutting and forming tools to components; saw blades, end mills, micro-tools, punches, injection and extrusion molds and dies, automotive, decorative, medical applications are just some raised examples here. DLC coatings consist of a mixture of sp³ (diamond) and sp² (graphite)bonds. The higher sp³ bond fraction results in a higher density, hardness (at ambient and elevated temperature), thermal stability, oxidation resistance, higher residual stress and lower thermal conductivity. Hence, comprehending the correlation between DLC coating properties and industrial application requirements is a crucial prerequisite to a performance increase in industry. An attempt is made here to cover the range from DLC synthesis to its implementation in industrial applications and the obtained performance results thereof.

Tetrahedral amorphous carbon coatings (ta-C) are characterized by their very high wear resistance and low friction under most conditions. However, the requirements in the various application areas differ significantly and there is a need for application-specific adaptations of the coatings in some cases.

Current investigations show that the Laser-Arc process can be used to produce doped carbon coatings via the evaporation of composite graphite cathodes. Cathodes with 5 at% dopant content were used for this purpose. Iron and molybdenum were investigated as metal dopants and boron and silicon as non-metal dopants. The processing conditions were kept identical for all doped cathodes, and an undoped ta-C reference was also prepared. It can be observed that the defect density of the resulting coatings are significantly lower than the reference, especially for boron and molybdenum doping. For example, a 5 µm thick boron-doped coating shows lower roughness than 1 µm ta-C. Significant differences are seen in the morphology of these coatings, with the defect density being significantly reduced. Excitation of boron in the plasma is similar to carbon, and amorphous coatings are formed with comparable hardness to the ta-C reference. In contrast, the very highly excited molybdenum reduces the hardness significantly. In the case of iron and molybdenum doping the coatings are then no longer completely amorphous, as crystalline clusters are formed. Under some lubricated tribological conditions, the ta-C:B and a-C:Mo show clear advantages over ta-C reference coatings with comparable hardnesses.

For many applications in cutting tools or watchmaking industries, the need to develop new solutions in order to improve the lifetime and performances of the components requires new approaches and solutions. In a joint project between the (Berner Fachhochschule) BFH and the (Haute Ecole ARC) HE-ARC, promising results have been obtained with nanotextured graphite layers produced using a two step hybrid production technology. First, a graphite coating is deposited using magnetron sputtering technology. Second, the coating is treated at with high power pulsed laser at high repetition rate.

As pin-on-disc measurements show, laser nanotexturing not only lowers the friction coefficient of the graphite coating, but also eliminates the run-in phase and significantly reduces wear in dry conditions. In addition, the topography induced by the laser treatment generates an optical effect of iridescence which adds a decorative function.

These first result open possibilities to develop a new type of graphite coating modified by laser pulses for application on 2D or 3D products with sizes in the order of mm2 to a few cm2.

These are first results, further improvement are expected via appropriate texture patterns and doping of the carbon target. Under wet condition, microchannel with heights in the nanometer range between are expected to be beneficial for storing lubricant.