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Cold Spray
A mention is made of "Cold Spray" although the name seems to contradict the concept of "Thermal (heat) Spraying"; however, the method has garnered significant research interest over the last five years. Developed in the former Soviet Union in the mid 1980s by Papyrin (4), the process is, however, now being commercialized in both Europe and the United States. A typical system is shown above. The basis of the cold spray process is the gas-dynamic acceleration of particulates to supersonic velocities (300-1200 m/sec-1), and hence high kinetic energies, so that solid-state plastic deformation and fusion occur on impact to produce dense coatings without the feedstock material being significantly heated. This is achieved using convergent-divergent, de Laval nozzles, high pressures (up to 500 psi [3.5 MPa]) and flow rates (up to 90 m3/hr) of gases such as helium or nitrogen. The gases are pre-heated to about 800°C (1472°F), or below the melting point of many metals, to increase the velocity. Pre-heating also aids in particle deformation. The spray pattern is roughly 20 to 60 mm2 (0.031 to 0.093 sq in.); spray rates - 3-5 kg/hr (6.5 to 11 lb/hr), with build ups of about 250 µm (10 mils) per pass and DEs of 70 wt %. Feedstock particle sizes are typically of the order of 1-50 µm. The advantage of cold spray versus the "hot" spray processes, which melt or soften the feedstock, is a significantly reduced level of coating oxidation. Electrical conductivity of cold sprayed copper has been reported at about 90% of wrought material - a significant increase over the <50% typical for other sprayed copper deposits.Cold spray coatings also exhibit improved adhesion, reduced material loss by vaporization, low gas entrapment, insignificant grain growth and recrystallization, low residual stress, phase and compositional stability, reduced masking requirements and improved surface finishes (5) (6). Cold spray, owing to its principle of impact-fusion coating build-up, is limited to the deposition of ductile metals and alloys (Zn, Sn, Ag, Cu, Al, Ti, Nb, Mo, NiCr, Cu-Al, nickel alloys and MCrAlYs) and polymers, or blends of >50 vol % ductile materials with brittle metals or ceramics. The absence of a heated jet also yields a low heat input to the substrate. Obvious disadvantages to the cold spray process include the use of high gas flows, increased gas costs, especially in the case of helium, recycling would be needed. Consequently, lower cost gases as nitrogen are being investigated as alternatives. Also, high gas pressures have required the development and modification of powder feeders. Solid materials traveling at high velocities are abrasive, so the lifetime and dimensional stability of key components are emphasized. Nozzle lifetimes in excess of 100 hours have been reported (7). Applications for cold spray coatings include corrosion protection, where the absence of process-induced oxidation may offer improved performance; deposition of electrical conductors and solders; and, the application of metallic coatings to ceramic and glass substrates.
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