Cryogenic Nitrogen Gas Cooling for Thermal Spray Coatings

Current Process HVOF thermal spray is widely used for coating critical wear parts like landing gear, bearing races, valves and turbine components. Generally, fully or partially molten metal, composite, or ceramic droplets are propelled from a gun or torch onto the workpiece. Multiple passes are required to build up the coating, since each pass only deposits ~ 0.0002 in 0.0005 in thickness of material. A significant amount of thermo-kinetic energy is required to deposit a dense coating onto the work surface—and a portion of this energy, manifested as heat, is absorbed by the workpiece. Improper temperature control during thermal deposition frequently leads to coating and workpiece overheating, thermal deformation and degradation of substrate material, and damaging stresses due to a mismatch of thermal expansion coefficients between the coating and substrate. When damage occurs through overheating and thermal stress, delamination of the coatings can occur in service. Consequently, thermal management of the workpiece is extremely critical during the spray deposition process. Compressed air jets are the primary cooling method for most HVOF thermal spray operations; however, air cooling is usually insufficient and the oxygen, residual moisture and hydrocarbons present in the cooling air are often detrimental to the coating quality. In spray operations involving tighter temperature control, air cooling is usually inadequate and manufacturers are often forced to introduce breaks in the process cycle so that the accumulated heat is dissipated to the ambient environment. The spray gun is moved away from the part but continues firing during the inter-pass cooling breaks, resulting in wasted feed powder, process gas, and booth time. Another issue that affects productivity in spray coating operations is the set-up time for masking and de-masking. It is important to mask certain areas of the part, where the coating might not be needed. The coating of these areas might be undesirable (it may interfere with the mechanical working of the component), unneeded, or simply, uneconomical. In these cases, it is critical to provide an effective barrier to coating for these areas. Metal plates (shadow plates) are often used to protect these areas, as are masking tapes. The desirable aspects of a masking tape are flexibility, ease of application and removal, quick clean-up and extended useful life. There is a wide variety of masking tapes available today with materials of construction ranging from fiberglass and metals to polymer and silicone rubbers. Metal tapes are usually difficult to make and install, while the fiberglass and polymer tapes are easy to install but difficult to remove and require extensive post-spray cleaning. Inadequacy of air cooling and build-up of temperature is the primary reason for tape degradation, e.g. thermal decomposition, hardening, or tape embrittlement.