AVS 66 Session SE-ThA: New Challenges and Opportunities in Surface Engineering

Machines with rotating components usually rely on bearings to reduce friction in moving its parts around a fixed axis. The increasing demand for more precise bearings to lower power consumption and heat generation, while simultaneously support increasing applied loads and/or higher speeds, has given place to the use of surface engineering processes.

In the case of bearings, it is widely accepted the advantages of using coatings as the surface process to improve its performance. During the last three decades, advanced coatings have enjoyed a growing interest in several industrial applications because they can be engineered to provide different properties like electrical insulation, low friction, and resistance to corrosion, surface initiated rolling contact fatigue, abrasive wear, and plastic deformation. The main surface engineering processes to deposit these coatings include traditional technologies such as dipping and liquid spraying, chemical conversion, galvanizing and electroless processes, as well as more sophisticated technologies such as thermal spraying, physical vapor deposition, diffusion, and ion implantation. However, the special characteristics of the bearing steel and the need to limit the deposition costs reduce the number of methods that can be practically used.

In this talk I will first introduce the four main areas where coatings can contribute to improve the performance of bearings made of standard bearing steel: lower friction, decreased wear, corrosion resistance, and electrical insulation. For each area I will review which coatings are industrially used, their possible industrial deposition methods, and their main mechanical and tribological properties. Examples of SKF coatings used to extend maintenance and life expectancy of specialized bearings will be described, like NoWear® (carbon-based nanostructured coating), Black Oxide (iron oxide conversion film), INSOCOAT® (aluminum oxide coating), and manganese phosphate films. I will finish the presentation visioning which the (near)-future needs of specialty surface-treatment coatings are in response to bearing application challenges, including a novel fifth area of sensorized coatings.

Here, we report on the room-temperature mechanical behavior of 100-oriented NbC single-crystalline pillars of diameters between 330 and 830 nm. We prepared the pillars via focused ion beam milling of bulk, commercially-available, NbC single-crystals. We carried out uniaxial compression of the pillars using Hysitron PI-85 picoindenter in situ in a scanning electron microscope . We find that all the pillars exhibit plastic deformation with strains up to 26%. Load-displacement curves obtained during the compression tests reveal multiple displacement bursts, indicative of sustained slip. Interestingly, yield strengths vary non-monotonically with pillar diameter between 8 GPa and 12 GPa and by up to 40% among the pillars of the same diameter. From the post-compression images of the pillars, we identify {110}<110> and {111}<110> as the two likely slip systems operating within these pillars. We suggest that the observed size-dependence in NbC(100) pillars is a consequence of the activation of these two slip systems. We observe a similar size-dependence in VC(100) pillars based upon which we suggest that the observed mechanical behavior is characteristic of group 5 TMCs.

Within the current work, the thermal stability of refractory MoNbTaVW HEA thin films was studied up to an annealing temperature of 1600 °C. The thin films with a thickness of about 1 µm were synthesized on sapphire substrates by cathodic arc deposition. The samples were annealed in a vacuum furnace for 1 hour at temperatures ranging from 1000 to 1600 °C in steps of 100 °C. After annealing, scanning electron microscopy images were recorded indicating changes in the film morphology at 1200 °C and above. Analysis by X-ray diffraction revealed the formation of new phases at 1500 °C. Nanoindentation tests were performed to assess possible changes in the mechanical properties of the films. A decrease from about 20 GPa for the as-deposited films to about 9 GPa after annealing at 1600 °C was noticed. The electrical conductivity of the MoNbTaVW thin film slightly decreased due to annealing as the measured resistivity increased from 5·10^-7 Ωm to 1.5·10^-6 Ωm.

There is an ever-growing interest in the use of functional coatings to protect surfaces of materials and workpieces against harsh environments such as corrosion, abrasion or solid particle erosion (SPE), making surface engineering solutions a very attractive balance between performance and cost. Numerous vapor-based fabrication techniques have been developed, namely PVD, CVD and PECVD, that can be used to achieve high hardness and high wear resistance, while being compatible with substrate materials such as metals, and different substrate shapes. This is increasingly important in the case where there is a need for protective coating solutions for inner surfaces of tubular components, such as parts of aircraft engines, oil pipelines, mining components, and numerous others. Specifically, certain aircraft diffusers are designed to conduct the air to the combustion chamber by means of many narrow gas inlets arranged around a circular frame. In such case, SPE arising from dust particles and volcanic ashes present in the air can result in an increase of the gas inlets diameter, leading to back streaming of air into the compressor (known as the compressor surge), which can give rise to significant aircraft engine damage and catastrophic consequences.

In response, we propose a novel Non-Line-Of-Sight (NLOS) technique to coat the inner parts of non-linear surfaces and cavities with hard, wear- and erosion-resistant coatings possessing high erosion resistance, hardness significantly higher than the hardness of the particles impacting the surface, as well as a large thickness (preferably 8 µm and more).

Specifically, we review, study and demonstrate the fabrication process of hard SPE-resistant TiN protective coatings on the inner surfaces of narrow tubes using a non-obvious NLOS approach yielding a uniform film thickness and properties along the tube axis (better than 20%). The deposition process indicates the importance of applying pulsed-DC PECVD, when uniform hard TiN films are prepared at low-frequency in the several kHz range. The TiN films (about 12 µm thick), exhibit high hardness and Young’s modulus (25 and 225 GPa, respectively), corresponding to the (111) preferred crystallographic orientation. We show that the SPE resistance on the inner surface decreased by a factor of more than 15 compared to the bare substrate, and that the process is well suited for the protection of aerospace, manufacturing, 3D printed and other critical components with a complex shape of inner surfaces.