Low temperature MOCVD process for fast aluminium deposition on metallic substrates

Abstract

A CVD pilot plant, designed and built in INTA, is presently being used to deposit aluminium coatings with applications in the fields of industrial and aeronautic turbines, as well as on the protection of components employed in the chemical industry, waste incinerators, fuel cells, and for the replacement of Cd coatings in aeronautic components.

The industrial process currently used to coat aeronautic and industrial turbine components employs AlCl3 as precursor at 700–1100 °C and requires more than 12 h per batch (including loading, heating, coating and cooling) due to the relatively low deposition rates and the long heating and cooling cycles. The new process carried out at INTA employs an organometallic precursor, which results in higher deposition rates, at 280–350 °C with a total processing time lower than 5 h per batch. As in any other CVD process, this one allows deposition of coatings in complex geometry components such as on the inner surfaces of turbine blades and heat exchangers tubes. Other important advantages of this particular process are the possibility of recovering and re-utilising the unreacted precursor as well as the high purity of the produced coatings in comparison with those produced by other commercially available technologies. It is well known that the higher the contamination degree, the lower the useful life of this type of coatings. The pilot plant has a deposition chamber with a useful coating zone of 30 cm in length and 18 cm in diameter, heated by a three zone furnace equipped with a pumping system that allows working pressures of 0.1–100 mbar. The system can be manually or automatically controlled and can be easily adapted to deposit other materials. By heat treating the pure Al coatings deposited on Ni base superalloys, Ni aluminide coatings have been obtained and excellent cyclic oxidation behaviour has been observed at 1000 °C. Al has also been deposited on ferritic steels (P91 and 92) and after a suitable heat treatment Fe aluminide coatings with excellent steam oxidation resistance have been obtained. Another potential important use of this process is the deposition of dense aluminium coatings for cadmium replacement in several industrial applications.