Z. Zurecki, C. A. Ward, Air Products & Chemicals, Inc., Allentown, PA
Applied to landing gear, turbine and actuator components, the WC-Co, High-Velocity Oxy-Fuel sprayed coatings gradually replace the EPA/OSHA-objectionable chromium plating. However, the optimum wear, fatigue, and corrosion performance of the sprayed coatings is reached only when the substrate temperature is fully controlled during spraying. Maintaining a narrow temperature range of 50oC—150oC is typically required while coating shot-peened, aerospace-grade 4340 steel components, and a further reduced top limit is applied during coating of aluminum components. The use of interpass cooling breaks (ICBs) is therefore necessary, even in the HVOF operations involving the conventional, forced air or CO2 cooling. Further improvements in the substrate temperature control are needed since ICBs lead to significant time, feed powder and process gas losses. This paper presents results of industrial tests of a newly developed, nitrogen cryovapor (-195oC) substrate cooling system for thermal spray coating. Comprising novel, cryo-fluidic spray-cooling nozzles and a multi-zone, infra-red thermal imaging control algorithm, the system enabled halving the spraying time and consumption of feed powder, hydrogen, and oxygen during the HVOF coating of a Boeing 737 landing gear with WC-CoCr. The operation of the tested system will be illustrated with the thermal logs of the component, recorded by automation software for QC/QA and archiving purposes. SEM/X-ray coating examination and further improvements in the structure and properties of N2 cryovapor-cooled coatings will be discussed.
Summary: This presentation concludes a broad effort to develop a versatile and cost-effective, cryo-gas cooling method and system for HVOF, plasma, and other thermal spray (TS) coated substrate components. Thermal imaging camera and IR-sensors, the backbone of the system’s diagnostics, are integrated with cryofluid controller by an algorithm reducing temperature sampling errors and calculating standard deviation in order to estimate thermal stresses developing on component surface; collected data is saved for post-inspection and playback. A novel type of proportionally controlled cryogenic nozzles is used to assure cooling and cleaning of coating surface without an undesired, condensed-phase wetting. Beta-site tested at a major airline’s HVOF facility, the system has shown potential for radical savings by improving production rate, feed powder utilization efficiency, and enabling alternative masking methods.
