Thermal spray coating applications require precise control of substrate temperature during deposition in order to achieve optimum performance of the final part. Applications such as high-velocity oxy-fuel (HVOF) hardfacing of aircraft landing gear and plasma spraying of thick sputtering targets are but a few examples where control of substrate temperature throughout the entire coating cycle is critical. A broad effort to develop a versatile and cost-effective cryogenic gas cooling system for controlling part temperature in high-energy thermal spray operations is described. Results show that the new liquid nitrogen cooling technology can significantly improve productivity of many conventional operations and provide corresponding powder and process gas savings by completely eliminating the need for interpass cooling breaks. Optimized temperature control and minimization of coating oxidation during the spraying process, resulting from the use of the new system, were also found to: (1) preserve substrate properties, (2) decrease residual stress gradients at coating interfaces, (3) enable applications of soft masking materials, and (4) increase the deposition efficiency (DE) of WC-CoCr coatings while minimizing decarburization of the WC phase. This work focused on HVOF spraying of WC-CoCr coatings which offer a performance and cost alternative to toxic chromium (Cr 6+ ) plating but yet to be published, recent experimentation with the other coating materials and deposition systems was consistent with the observations reported here.

High production rate and feed powder efficiency are critical in the HVOF hardfacing of aircraft landing gear, turbine, and actuator components traditionally electroplated with carcinogenic hard chromium. Desired improvements are hindered by rapid heat build-up in substrate component and thermal expansion mismatch between the carbide coating and steel, titanium or aluminum substrate. A new, cryogenic nitrogen gas (-195°C) cooling system has been developed which limits the thermal expansion and substrate softening problems, and enables a non-stop, gun-on-target spraying. Fully automated, the operation of the new AP LIN-Cooling System is based on thermal imaging of the entire substrate and multi-zone cooling with novel, cryofluidic nozzles. Thermal logs and images of components processed are saved by the system for quality auditing purposes. This paper presents results of industrial tests of the system during WC-10Co4Cr coating of Boeing 737 landing gear, demonstrating a 50% reduction in spraying time, corresponding reductions in the consumption of powder and HVOF gases, and additional labor savings due to the use of flexible masking, unfeasible with the traditional cooling methods. Analysis of residual stresses, structures and properties of the coating and AISI 4340 steel substrate shows that the cryogenic nitrogen cooling results in high-quality products.