Abstract：Cold spray is an effective technique to deposit pure metallic coatings on a variety of substrates, especially for those metals that are apt to oxidation at high temperature.During cold spray, the spray particles are fed into a supersonic gas stream and accelerated to a high velocity. The coating is deposited through intensive plastic deformation of spray material and substrate on impact of the particles in a solid state.A typical of feature of cold spraying is the relatively low temperature of driving gas compared with the conventional thermal spray techniques.
Titanium coatings had been deposited on 1Cr13 stainless steel substrate by cold spray technique at different operating parameters. The microstructural features of coatings were analyzed using Environment Scanning Electron Microscopy (ESEM),X-ray diffraction (XRD), microhardness of depth profiles and tensile tests. With the ESEM experiments, it is found that as the gas temperature increased, the porosity decreased while the oxygen content increased. A higher gas pressure yielded a lower porosity. And a little higher oxygen content was discovered in air process than in nitrogen gas. But no phase change was observed in all the coatings via XRD analysis.The hardness of coatings was relative to the particle impact velocity and the oxygen content of coatings. Higher impact velocity and higher oxygen content produced harder coatings. Tensile tests illustrated that with the increasing of gas temperature and gas pressure, the adhesion strength increased. Stronger adhesion was obtained in nitrogen gas process than in air.
Potentiodynamic polarization experiments were performed to understand the corrosion behavior of the coatings. SEM characterization of the coatings was also performed after the electrochemical experiments. The result indicated that the coatings which had lower porosity and lower oxygen content had lower corrosion current density. The polishing treatment peeled the outer layer including the small pores as well as decreasing of the actual surface area of the coating, leading to the considerable improvement of corrosion resistance.
The change of galvanic current density of stainless steel versus time was measured in order to study the galvanic corrosion behavior between stainless steel and titanium coating. To interpret the mechanism, potentiodynamic polarization experiments were performed and open circuit potentials (OCP) of titanium coatings,titanium metal bulk and stainless steel were observed. It is found that the OCP of titanium coatings was much smaller than titanium metal bulk, and close to stainless steel. Therefore, the galvanic corrosion behavior was not obvious. And this behavior was also due to the higher polarization slope of titanium coatings.
In order to reduce the porosity of the surface coating, the coating was tamped by Al2O3 powders using the same cold-sprayed equipment. The features of coatings before and after cold spray tamping process were compared. It is found that after tamping, the porosity near the surface of coating was evidently reduced, and the hardness and corrosion resistance of coatings were improved. Calculated by Scherrer formula, crystal dimension of the surface of temped coating was less than 100 nanometer.
Key Words: cold spray, 1Cr13 Stainless steel, titanium coating,microstructure, corrosion behavior, compacting
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