Investigation On Various Hard Coating Technologies

Abstract For high head hydropower applications, the predominant coating material is 86% tungsten carbide embedded in a cobalt chromium matrix. This material is applied using a high velocity spraying application, which can be powered by either gas or liquid fuels. This paper outlines the application and wear characteristics for three different scenarios: HVOF (high velocity oxygen fuel) with gas or liquid fuel, and HVAF (high velocity air fuel) with propane as the fuel source. After providing an overview of the distinct technologies and their corresponding application equipment, the discussion delves into the observed coating efficiency during standardized applications with varying coating angles. To assess the wear resistance of the coatings applied by the different systems, a water jet test rig was developed, that allows the variation of the impingement angle and velocity of the sediment. In the test rig laboratory samples were analyzed against standard sediment with different grain size impacting at predefined angles on the probe. The results clearly demonstrate the superiority of HVAF technology over both HVOF systems. Even at lower coating angles, enhanced wear resistance is evident when utilizing HVAF. Further laboratory investigations on porosity and hardness in the coating support the analysis and the test rig findings. However, it is worth noting that the deposition rate of HVAF with coating angles smaller than 90° is significant lower compared to the HVOF systems as confirmed by measurements during the application. Consequently, the economic feasibility of the HVAF system is limited to geometries where the coating alignment can be maintained close to perpendicular. Given that the coating system necessitates robotic manipulation, only hydraulic components with sufficient space for the access are suitable for HVAF, while others are more suited to HVOF systems.