NOVELTY - Hydroelectric turbine component comprises an erosion resistance coating of cobalt chromium matrix containing cobalt (4 - 12 wt.%) and chromium (2 - 5 wt.%); and tungsten carbide grains having diameter of less than 2 microns, embedded in the matrix. The coating is formed on a surface of the turbine component by a high velocity air fuel process. The cobalt and chromium provide a total amount of 6 - 14 wt.%, based on the total weight of the coating.    USE - As a hydroelectric turbine component (e.g. Francis runners, Francis guide vanes, Francis check plates, Francis rotating and stationary seals, Francis draft tube, Pelton needles, Pelton seats, Pelton beaks, Kaplan blades and Kaplan discharge rings), that is exposed to slit during operation (claimed), and useful in wide variety of industrial applications.    ADVANTAGE - Hydroelectric turbine component exhibits improved slit erosion resistance from the coating. The high velocity air fuel (HVAF) spraying deposits an extremely dense (minimal porosity) and non-oxidized coating. The increased thicknesses can be obtained relative to other thermal plasma spray processes, resulting in turbine components exhibiting superior erosion resistance properties. Since the HVAF process ensures a high particle velocity of 600 - 800 m/s and lower particle temperature, the coatings produced thus have lower levels of oxidation and decarburization as well as lower residual stresses. The HVAF thermal spray processes employ higher temperatures of 1500 - 2200degreesC, which deleteriously results in oxidation and deterioration of spray material upon deposition of the coating. The high velocity and lower temperatures employed reduce decarburization of primary carbides and enable thicker and denser coatings due to the lower residual stresses associated with the process. As such, high percentage primary carbide coatings can be applied at thicknesses that were previously unattainable, thus providing improved life of coatings in erosion prone environments. The HVAF process provides high velocity during spraying that results in a dense well bonded coating; high velocity and lower flame temperatures resulting in a coating with low thermal degradation of the hard phase, and limited dissolution of the hard phase which produces coatings with the desired high primary hard phase content for better erosion resistance and better toughness; coatings with low residual stresses because of lower flame temperature; and coatings with high thickness because of lower residual stresses. The HVAF process can provide coating thicknesses in excess of 500 microns, with thicknesses greater than about 2,000 microns attainable, thereby providing erosion resistant coatings that can withstand prolonged contact in silt containing environments. The process ensures higher retention of the primary tungsten carbide (WC) phase. The erosion resistant coating has relative lower amounts of cobalt chromium compared to prior art compositions and this reduces the mean free distance between WC grains, which promotes erosion resistance. The nano-sized and/or micron sized WC grains generally did not crack and did not raise stress levels in the surrounding metal cobalt chromium binder. The process ensures minimal decomposition, dissolution, or oxidation of the WC particles and ensures coatings with high primary WC content.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for improving erosion resistance of a surface of a metal substrate, involving thermally spraying a powder comprised of tungsten carbide and cobalt chromium by high velocity air fuel process to form grains of tungsten carbide in cobalt chromium matrix.