Structure and mechanical properties of ''thick'' tungsten plasma coatings

It is shown that the formation of thick (2 mm) plasma coatings is affected by the physicochemical properties of the substrate material as well as by spraying-process parameters. Unlike thin (0.1-0.2 mm) plasma coatings, in the formation of which the thermal action of the plasma jet and particles is of short duration, thick coatings are formed under conditions of prolonged thermal action. As a result of this, in the case of a titanium alloy substrate partial recrystallization of the tungsten in the vicinity of the substrate and more intense spreading of drops sprayed from a distance as large as 175 mm are observed. These circumstances, combined with a growing amount of oxides, adversely affect the strength characteristics of coatings. Electron probe microanalyses revealed the following: In the titanium-tungsten transition zone is formed a substitution solid solution of variable tungsten concentration; under these conditions the coating becomes the most vulnerable zone in mechanical shear testing. In the case of a bronze substrate the adhesion of a tungsten coating is determined chiefly by mechanical interlocking between the coating and the substrate. The character of formation of thick plasma coatings explains the fundamental difference in shear rupture behavior between titanium and bronze substrate specimens coated with tungsten