DE2327395

1433137 Spray producers; coatings UNION CARBIDE UK Ltd 29 May 1973 [30 May 1972] 25179/72 Headings B2E and B2F [Also in Divisions C7 and H5] In plasma spraying, wherein an arc is maintained between first and second electrodes 3 and 5, a gas is passed into said arc (through block 2) to produce a hot gas effluent which passes through constricting nozzle 4, and together with a coating powder entrained therein is directed at a substrate to be coated, said substrate being >7.5 cm from said nozzle, and the hot gas effluent and entrained coating powder are passed through a chamber in the form of a tube 6 between the nozzle and the substrate in which tube a zone of sub-atmospheric pressure is maintained by providing a sheath of gas moving in a helical path through said tube between the inner surface 12 thereof and said hot gas effluent having powder entrained therein. Said tube 6 may be cylindrical as shown, or tapered outwardly toward the substrate, or may first taper inwardly and then outwardly; the means for supplying the helical gas flow may be an inlet 9 leading to a spiral channel 10 in an insert 7 as shown, or may be a series of tangetial inlets; various forms are described referring to Figs. 2-6 (not shown). There may be an air-gap between the said nozzle and the tube 6. The powder feed rate may be 25-60 g/min and the surface speed less than 25 cm/sec; the zone of reduced pressure preferably extends beyond tube 6 as far as the substrate. Coating materials referred to are Cr 2 O 3 , WC-Co, Al 2 O 3 , Ni, Co-Cr-Ni-W and Ta.