Nanostructured CVD W/WC Coating Protects Steam and Gas Turbine Blades Against Water Droplet Erosion

Abstract

Abstract Gas turbine compressor blades operating with air inlet fogging can suffer from Water Droplet Erosion (WDE). WDE can also affect the last rows of steam turbines where expanding steam produces water condensation, especially under start-up and low load conditions. WDE damages the blades’ leading and sometimes trailing edges, increasing turbine rotation drag, reducing efficiency and leading to costly maintenance. This paper reports the testing of Hardide® nano-structured W/WC metal matrix composite coating as a protection against WDE. The Chemical Vapor Deposition (CVD) technology crystallizes the coating atom-by-atom from the gas phase and produces a uniform pore-free coating on complex shaped parts like turbine blades, vanes and pump impellers, including non-line-of-sight areas. Two variants of CVD W/WC coatings were tested: “A” type is 50–100 microns thick and has a hardness range of 800–1200 Hv and “T” type is 35–65 microns thick with a higher hardness of 1100–1600 Hv. Both coating types are made of Tungsten Carbide nanoparticles dispersed in metal Tungsten matrix. This composition and structure produce a combination of enhanced fracture toughness with high hardness and enables the deposition of exceptionally thick hard CVD coatings to provide durable protection against WDE and solid particle erosion. The coatings are pore-free thus also provide an effective barrier against corrosion. The coatings were tested for WDE resistance by the UK National Physics Laboratory (NPL) using 350 μm water droplets at 300 m/sec velocity. Uncoated 410 SS control samples suffered from a major loss of material after just 7-hours of exposure to WDE, forming a 200 μm deep scar across the whole tested area. After a much longer exposure of 90 hours, the coating samples showed negligible WDE damage, only measurable on the samples’ edges. The coating also outperformed Stellite, which is widely used as WDE protection in the form of welded overlay or plates brazed to the blade’s leading edge. The thicker and less hard type “A” CVD coating showed better performance when compared to the thinner, harder type “T”. The effects of the coatings’ thickness, hardness, and residual stresses on the WDE resistance are discussed. The rig testing showed that the CVD WC/W coating can protect steam and gas turbine blades against WDE thus increasing the service life of equipment and maintaining its optimal performance for longer, reducing CO2 emissions and cutting the life-cycle costs. Hardide coatings are used by major oil service companies, pump and valve producers to improve durability in abrasive and corrosive environments. Airbus has approved Hardide-A coating as a REACH-compliant replacement for Hard Chrome plating on aircraft components. Other customers include BAE Systems, EDF Energy, Leonardo Helicopters and Lockheed Martin. The Hardide coating service is provided from state-of-the-art coating facilities near Oxford (UK) and in Virginia (US). Production and quality control are accredited to ISO9001, AS9100 and NADCAP standards.