Analysis of mechanical properties of cold spray coatings for tribological and vacuum applications

Abstract

Cold Gas Dynamic Spray (CGDS) is a process in which solid particle are accelerated in a de Laval nozzle toward a substrate. At the impact, if the particle velocity exceeds a critical value, i.e. the critical velocity, particles undergo plastic deformation and the consequent adiabatic shear instability provides material flow and heat for bonding. This phenomenon is the main driving force for the adhesion of the coating to the substrate. Compared other thermal spray techniques, since it utilizes kinetic rather than thermal energy for deposition, it offers several technological advantages; in fact, residual stresses, oxidation and chemical reactions can be avoided. Materials such as metals, ceramics, composites and polymers can be deposited using cold gas dynamic spray, creating new opportunities in order to obtain particular properties; in fact, good coatings using hard metals can be obtained with the purpose to enhance the tribological properties of such mechanical components. Therefore, the expected qualitative leap in using CGDS is magnified in harsh environments with great benefits predicted in design flexibility gains, precision improvements, production time reductions, cost reduction, integration of additional embedded functionalities. The purpose of this thesis is to demonstrate the possibility to obtain good deposits of Stellite-6 with the aim to enhance the surface resistance during sliding condition, while mechanical and vacuum properties of pure titanium coatings were analyzed for possible application in ultra high vacuum (UHV) systems at the European Organization for Nuclear Research (CERN). This thesis, firstly, analyzes the effects of process parameters on mechanical and tribological properties of Stellite-6 coatings. The gas pressure and temperature as well as the traverse speed of the deposition torch were considered as significant process parameters. The aim is to overcome some technical issues arising in the cold spray deposition of hard anti-wear metallic coatings, such as Stellite-6, due to their high strength and melting point. A High-Pressure CGDS equipment was used and systematic studies were carried out for a deeper understanding of the effects of all investigated process parameters. A particular focus has been put on the substrate temperature, that can be regarded as an indirect process parameter. This latter, in fact, was monitored in-situ during deposition by infrared thermography (𝐼𝐼𝐼𝐼). The microstructure was analyzed by both optical and scanning electron microscopic observations. Mechanical properties were analyzed by instrumented micro- and nano-indentation measurements. Hardness (𝐻𝐻) and Young's modulus (𝐸𝐸) were considered as affective parameters to estimate the inter-particle cohesion strength and the work hardening of the coating. Results revealed that the substrate temperature, that is affected by the process parameters, plays a fundamental role in the coating formation process, and, both mechanical and tribological properties, of CGDS Stellite-6 coatings are mainly affected by the impact temperature of the particle-substrate system. It is also well-known that this alloy undergoes several physical changes at the interface during dry sliding while is sensitive to the loading conditions and environment. Due to these micro-structural alterations, the wear behavior of the alloy is modified, which linear Archard-like wear models could not capture. To better understand the wear performance a Stellite-6 coatings in-situ, a mechanistic model of wear would be desirable, so a systematic experimental study was performed. Tests were done under combinations of sliding speed (0.1–0.5 𝑚𝑚/𝑠𝑠) and contact pressure (0,5–5 𝑀𝑀𝑀𝑀𝑀𝑀). Platelet wear and subsurface cracking was seen in high speed tests, as well as evidence of plastic deformation at the wear surface. These results suggest the platelet wear observed is more likely a consequence of adhesive wear. On the other hand, in low speed conditions detachment and pull-out phenomena mainly affect the worn surface of coatings leading to a type of fatigue wear known as “nano-grain wear” that does not allow to use the wear model proposed by Archard. Unique to this study, the cross-sectional nano-indentation study showed the stiffness of material at and below wear interface to drop significantly. The last section was aimed by the necessity to overcome some technical issues, usually experienced during pure titanium deposition. These latter are mainly related to poor coating compactness and adhesion to the substrates. These technical issues become even more stringent when dealing with vacuum systems as they could affect the leak tightness and gas release in UHV. Preliminary micrographic observations were carried out to select the optimal values of the process parameters, that are pressure (𝑝𝑝) and temperature (𝑇𝑇) of the propellent gas. Mechanical properties of deposits were subsequently analyzed at the nano/micro and macro scale by instrumented indentations and adhesion tensile tests respectively. Vacuum properties were analyzed by outgassing rate measurements, thermal desorption spectroscopy (TDS) and helium tightness tests. Indentation results revealed that compact and homogeneous coatings can be obtained if high energy deposition parameters (𝑝𝑝~4 𝑀𝑀𝑀𝑀𝑀𝑀,𝑇𝑇~1000° 𝐶𝐶) are applied. However, a limited adhesion strength on stainless steel substrates is the main technical issue of the coating process. Outgassing and TDS tests revealed an abnormal nitrogen release that is attributed to gas entrapped during deposition or during the production stage of the Ti powders as N2 is used in the gas atomization process. Finally, helium leak rates were found to be incompatible with UHV requirements applied in modern particle accelerators. Much higher helium leak rates were detected along the interface between the coating and the substrate than through the thickness. These results confirm that the interface represents the weakest point of the bi-material system. Further studies are needed to solve this technical issue.