Approaches for Linking the High Kinetic Thermal Spray Process, Residual Stresses and Coating Performance by Utilizing In-situ Monitoring

Thermally sprayed hardmetal coatings have been successfully used in many critical applications including hydraulic cylinders, landing gear, paper machine rolls, ball and gate valves, and several other parts, which require wear resistance. Currently, due to variations in the spray processes, feedstock material, and spray parameters, there might exist a wide range of properties for the same coating material. Perhaps the most important factor for the coating properties is the feedstock powder and its quality. The size distribution of the powder needs to be suitable for the process; in addition to this the particle density, carbide size, and powder homogeneity affect the properties of the coating. Furthermore, the coating properties for a selected powder are related to the particle state, more precisely the particle thermal and kinetic energy at the impact. Today, the particle state can be monitored by in-situ diagnostics with devices that measure the temperature (T) and velocity (v) of the particles during flight. The particle state can be further linked to the coating properties and performance by so-called process mapping methodology. At present, many thermal spray processes and equipment exist, each having their own specific characteristics of particle temperature and velocity. For example, the newest thermal spray processes, such as High Velocity Air Fuel (HVAF), provides about a 1000 °C lower flame temperature and 30-40% higher particle velocity compared to more conventional High Velocity Oxygen Fuel (HVOF) thermal spray processes. HVAF thus produces very dense coating structures and reduces the brittleness caused by excessive particle heating.

Coating formation also induces stresses caused by the rapid solidification of the spray droplets (quenching) and thermal mismatch stresses during cooling. The thermal history will have a major impact on the residual stresses and it may influence the performance of the coating by affecting the mechanical properties of the coating as well. In high-kinetic-energy thermal spray processes, e.g. the HVOF, HighPressure High Velocity Oxygen Fuel (HP-HVOF), HVAF, and cold spray (CS) processes, the compressive stress component also known as peening stress, intensifies during the manufacturing process. Peening stresses act on the substrate or on the previously deposited layer.

Insufficient attention has been paid so far to the factors arising from the manufacturing process. Thus the effect of the thermal history and residual stresses on the properties of coatings is largely unknown. Moreover, there is generally a lack of knowledge on the property variation in coatings produced by various devices from the same material, as coating properties are managed largely by the trial and error approach. Consequently, insufficient understanding and/or information on the relationship between the manufacturing process and coating properties makes it significantly more difficult to set property targets for the applications.

This work focuses on the approaches to provide a link between processstructureproperty correlations in high kinetic thermal spraying by utilizing in-situ monitoring tools, which enable reliable manufacturing of thermal spray coating. These tools include inflight particle temperature and velocity measurements and an in-situ coating property sensor (ICP). The ICP measures the substrate curvature during spraying, enabling the monitoring of information on the coating formation process and residual stresses. First, the role of gas flows and process conditions on the particle state was evaluated by mapping the particle temperature and velocity resulting from different conditions and how they are linked to coating properties. Further, the in-situ curvature technique and progressive deposition model of Tsui and Clyne [1,2] were used in order to understand how thermal spray processes and parameters affect the residual stresses of coatings made by the HVOF, HP-HVOF, HVAF, and CS processes. Materials focused on in relation to HVOF and HVAF were WC-CoCr and Cr3C2-NiCr, whereas Al, Ti, and Cu were used in the CS case. Studies showed that high compressive residual stresses controlled by the particle molten state, velocity, and substrate temperature can develop in high kinetic thermal sprayed carbide coatings. The role of compressive stresses proved to be significant for the cavitation erosion resistance and fatigue life performance of the coatings. It was shown that the residual stress of cold spray coatings, mostly controlled by impact pressure and thus in most cases developing into compressive stress, may develop into tensile stress in conditions with low impact pressure and relatively high thermal energy.