Abstract:
Turbine blades in aero-engines experience very severe environments like high service temperatures and erosion. Protective coatings are effective solution to avoid the damage of the components and to improve their life time. Nickel aluminide (NiAl), which is used as a protective coating on turbine blades, is an ordered intermetallic with low density, high temperature resistance (melting point of 1638℃), good oxidation and corrosion resistance.Thermal spraying of NiAl poses challenges due oxidation. Pack cementation is the conventional method used for producing these coatings but this process suffers from solid state diffusion assisted deposition and high temperature post-treatments. Cold Gas Dynamic Spray (CGDS) is an emerging solid state coating technique, which can be an efficient technique to produce coatings of various materials, in which metal powders (typically of the size range 5µm - 50µm) are accelerated to very high velocities ranging from 300 m/s to 1200 m/s and made to impinge on a substrate to form coatings. Since, melting of particles does not take place, problems like oxidation, phase transformation, segregation of elements in the final coating, etc., which are associated with conventional thermal spray are eliminated.NiAl and Ni matrix composites with NiAl and Ni3Al as reinforcement possesses good low temperature toughness and are good candidates for high temperature applications.Cold sprayed coatings of Ni-Al mixtures have been recently reported and there is a lot of scope in the area. These studies have not focused on the co-deposition efficiency of Ni and Al and the effect of spray parameters on the composition of the coating. NiAl or NiAl matrix composite coating have not been studied which leaves the property evaluation of these coatings as an open area to explore.
In this present research, Ni and Al powder mixturesare sprayed using CGDSto form Ni-Al mixed coatings which can further be heat treated to obtain Ni matrix composite coatings with Ni3Al/NiAl phase as reinforcement. Nickel and aluminium powders are mixed in atomic ratios of 20:80, 40:60, 60:40 and 80:20 and cold sprayed on Al substrate using N2 gas at various gas pressures (3MPa, 5MPa and 6MPa) and gas temperatures (400℃, 500℃ and 600℃). The effect of cold spray process parameters and feedstock composition on the composition of the coating, overall deposition efficiency (DE) and co-deposition efficiency (CDE) of Ni/Al are analysed. Increasing process temperature and pressure increases the velocity of the particles which in turn improves the overall DE of the feed powders.Increasing Ni content in the feed mixture resulted in lower coating thickness, lower overall DE and lower CDE of Ni. Optimum spray parameters and composition of the feed powder were determined for obtaining compositions closer to NiAl phase in the coating. Coatings of mixtures rich in Ni showed good bonding at the interface. Macro-hardness of the coatings increased with increasing Ni content in the feed mixture but not much variation was observed with increasing T and P.Post spray heat treatment will be optimized to obtain NiAl and Ni-matrix composite coatings. Wear and oxidation behaviour of the composite coatings will be studied.