HVOF Low Oxide Coatings - An Alternative to LPPS 931050
In recent years, many attempts have been made to modify conventional thermal spray technologies to produce low cost alternatives to LPPS (Low Pressure Plasma System) and EBPVD (Electron Beam Physical Vapor Depostion). Gas shrouded plasma as developed by General Plasma and Praxair has had reasonable success. Gas shrouded plasma MCrAlYs, for instance have demonstrated capabilities in meeting certain turbine OEM LPPS specifications in terms of oxide content. However, there still remains concerns of gas shrouded plasma's reproducibility especially on complex geometric shapes like airfoils where turbulence can disturb the gas shroud and induce unacceptable levels of oxides. Studies at General Plasma in conjunction with several turbine OEMs suggest that HVOF may provide the solution.
The highly stable and collimated HVOF flame, when coupled with a fuel-rich flame chemistry, is capable of producing low oxide MCrAlYs of high density and bond strength. Although not identical in structure to LPPS and EBPVD coatings, HVOF MCrAlYs have performed well in actual field tests on land-based turbine vanes. HVOF coatings are not oxide free, but the process tends to create highly isolated oxides as opposed to oxide stringers typical of the air plasma process. There is some speculation that these isolated oxides may have beneficial effects in the coating by inhibiting high temperature fatigue and hot extrusion. Two gas turbine OEMs shave have already adopted HVOF MCrAlYs for 1st stage vanes. Evaluation is underway to utilize HVOF coatings for turbine blades.