GB913301A - Improvements in or relating to the formation of firmly adherent coatings of refractory materials on metal - Google Patents

Abstract

A method of forming a coating of particles of a "refractory material" on a surface of a metal body having a melting point above 600 DEG C. comprises applying to said surface a coating comprising said material in particle form and an "active metal" or compound thereof, heating said coated surface at a temperature below the melting point of the metal of the body to cause said active metal or compound thereof to form a solder which flows around said particles joining them together and to the metal body, and thereafter removing the solder from the outer surface of the coating so as to expose the outermost refractory particles. The "refractory material" is composed of carbides and/or oxides of W, Ta, Nb, Ti, Zr, Hf and/or Cr, and the "active metal" is one which preserves its wetting properties at temperatures at which it alloys with other metals, examples of active metals being Ti, Zr and Hf. The refractory materials used have low secondary electron emissions and the coatings are thus applied to electrodes in electron discharge devices (see Group XL(a)). The metal body which may be a tube may be composed of Al, Cu, Ag, Au, Ni, Fe or Ni-Fe alloy and may initially be sand-blasted. The coating may be applied as a single layer composed of a mixture of refractory material with the active metal or compound thereof (e.g. titanium hydride which is reduced to titanium during the heating). Thus a suspension of titanium hydride powder and tantalum carbide powder, both of a particle size of 10 n or less, in ethyl acetate-butyl acetate with a nitro-cellulose binder may be applied and heated. Alternatively, a layer of active metal may be applied first as by evaporation, electroplating or by applying a hydride suspension, and a layer of refractory material in suspension applied thereover. Quoted temperatures of heating vary between 900 DEG and 1100 DEG C. The outermost layer of active metal may be removed by blasting with Al2O3, or by etching with e.g. HCl, HNO3 or aqua regia. Specification 875,814 is referred to.ALSO:A method of forming a coating of particles of a "refractory material" on a surface of a metal body having a melting-point above 600 DEG C. comprises applying to said surface a coating comprising said material in particle form and an "active metal" or compound thereof, heating said coated surface at a temperature below the melting-point of the metal of the body to cause said active metal or compound thereof to form a solder which flows around said particles joining them together and to the metal body, and thereafter removing the solder from the outer surface of the coating so as to expose the outermost refractory particles. The "refractory material" is composed of carbides and/or oxides of p W, Ta, Nb, Ti, Zr, Hf and/or Cr, and the "active metal" is one which preserves its wetting properties at temperatures at which it alloys with other metals, examples of active metals being Ti, Zr and Hf. The refractory materials used have low secondary electron emissions and the coatings are thus applied to electrodes in electron discharge devices (see Group XL (a)). The metal body may be composed of Al, Cu, Ag, Au, Ni, Fe or Ni-Fe alloy. The coating may be applied as a single layer composed of a mixture of refractory material with the active metal or compound thereof (e.g. titanium hydride which is reduced to titanium during the heating). Thus a suspension of titanium hydride powder and tantalum carbide powder, both of a particle size of 10m or less, in ethyl acetate-butyl acetate with a nitro-cellulose binder may be applied and heated. Alternatively, a layer of active metal may be applied first as by evaporation, electroplating or by applying a hydride suspension, and a layer of refractory material in suspension applied thereover. Quoted temperatures of heating vary between 900 DEG and 1100 DEG C. The outermost layer of active metal may be removed by blasting with Al2O3, or by etching with, e.g., HCl, HNO3 or aqua regia. Specification 875,814 is referred to.