PVD refractory metal based coatings for tribological applications

The need for materials with special and often unique functionalities is particularly felt in forefront domains, particularly in areas where materials are subjected to extreme erosion and/or corrosion. Coating technologies are becoming more widespread since they help to improve existing materials and products separating the structural performances from the surface-protection performances [1]. Refractory metals and their alloys constitute a class of particularly interesting materials because, thanks to their outstanding properties, they can be used in hazardous environments and extreme conditions. In this work, we present an investigation on W and Ta based coatings on stainless steel substrates. Among many different applications, tungsten is used in electronic industry for x-ray optics and in harsh environments such as thermonuclear fusion reactors (tokamaks) where it is generally considered to be the most promising plasma facing material (PFM). This is due to its high cohesive energy, which makes it resistant to erosion and ensures an high sputtering threshold under hydrogen bombardment, its high melting point (the highest one when considering refractory metals), low coefficient of thermal expansion, good electrical conductivity, low affinity with hydrogen and excellent thermal conductivity. Unfortunately tungsten finds just a few applications in the standard industrial fields because of the low stability of its oxides (WO3, WO2). Tantalum has properties that make it useful for many applications, from electronics to mechanical and chemical systems. Its high melting point, toughness, low ductile-to-brittle transition temperature and exceptional resistance to chemical attack make it an attractive coating material for components exposed to high temperature, wear, and harsh chemical environments. Its excellent resistance against corrosion is ensured by the formation of a stable passive oxide film (Ta2O5). Tantalum is also used to produce a variety of alloys that have high melting points, are strong and have good ductility. In particular W is alloyed with Ta to lower brittle-to-ductile transition temperature [2,3]. W rich alloys (W-Ta) are not very well known and, to the authors' knowledge, there are just a few papers in literature. On the contrary Ta rich alloys (Ta-W) are well known materials and they excel due to their good mechanical properties and excellent corrosion resistance. With the 10 % of W the resulting alloy is 1.4 times stronger than pure tantalum but it remains easy to work even at high temperature (up to 1 600 °C). Physical vapor deposition (PVD) is one of the most promising coating technology and it is widely employed to improve mechanical, wear and corrosion properties of materials. Moreover it is possible to tailor coating features (crystallinity, grain orientation, etc.) to obtain properties that can be far apart from the bulk material ones. In this work, different films, with different W and Ta contents, have been produced via PVD using Direct Current Magnetron Sputtering (DCMS) and Pulsed Laser Deposition (PLD) [4]. Moreover pure W and Ta samples have been deposited by High-Power Impulse Magnetron Sputtering (HiPIMS) [5,6]. For these samples the various deposition parameters have not been fully optimized. However, the preliminary results look very promising. DC Magnetron Sputtering is a traditional PVD technique extensively used in industry. PLD is a relatively new technique that permits to deposit films with very complex stoichiometry (e.g. YBCO). Moreover it is able to deposit nanostructured materials tailoring film features at the nanoscale. HiPIMS is an innovative technique which utilizes magnetron sputtering cathodes and high peak power density of up to 3 kW cm-2 on the target. The plasma produces a particle flux with high degree of ionization. HiPIMS has been successfully used to enhance coating adhesion. It produces high-density microstructure films with smooth surfaces. Moreover, it has a few industrial applications in hard, electronic, and optical coatings. The produced refractory metal based coatings have been extensively analyzed. Microstructural characterization activities consisted of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The nano-mechanical properties of the films (hardness and elastic modulus) have been analyzed by nanoindentation testing. Adhesion has been finally evaluated by scratch tests [7], using a fully-computerized UMT apparatus. The electrochemical properties of thin films have been also evaluated in 3.5 wt.% NaCl aqueous solution [8]. The goal of this work was to evaluate the properties of high tech nanostructured coatings of refractory metals. Moreover it was possible to stress how to drive the film characteristics needed for the specific application using three different techniques.