Diffusion Coating

a process based on the diffusion saturation of the surface layers of objects made of metals and alloys of various metals. It is used to impart special physicochemical and mechanical properties to metal articles. A distinction is made among the following operations, depending on the diffusing element: aluminizing, diffusion chrome plating, molybdenization, manganizing, chrome calorization, and chrome-titanium plating. Diffusion saturation is possible from various phases: solid, gaseous, vapor, and liquid.

Saturation from the solid phase is used with iron, nickel, cobalt, titanium, and other metals. In this case, diffusion coating is performed using various refractory metals, including molybdenum, tungsten, niobium, and uranium, whose vapor pressure is lower than the vapor pressure of the basic metal. The process is performed in a hermetically sealed container, in which the parts to be treated are covered with the powdered metal and heated either under a vacuum or in an inert gas atmosphere at 1000°-1500° C. Saturation from the vapor phase is used for alloys based on iron, nickel, molybdenum, and titanium with elements that have higher vapor pressures than the metal to be saturated, such as zinc, aluminum, chromium, and titanium. The process is performed in hermetically sealed containers under a vacuum of ~10¹-10⁻² newtons per sq m (N/m²), or 10¹-10⁻⁴ mm of mercury (mm Hg), and at 850°-1600°C, by the contact or noncontact method. In the former case, the vapor phase forms because of the sublimation of the metal and is generated near the contact locations between the powdered or granular metal and the surface being processed. In the second case, generation of the vapor phase takes place at some distance from the surface.

Saturation from the gaseous phase is performed during diffusion coating of various metals with aluminum, chromium, manganese, molybdenum, tungsten, niobium, and titanium. The diffusion of the metal is preceded by the reactions of gaseous chemical compounds of the diffusing element with the basic metal. The gas phase consists of the halides of the diffusing metals. Gaseous saturation is performed in muffle furnaces or in specially designed furnaces at 700°-1000° C. The gaseous phase may be generated at a distance from the surface being saturated (noncontact method), as well as in the contact zone of the source of the active phase and the metal surface (contact method).

Saturation from the liquid phase is used in calorization and chrome, zinc, and copper plating. The process takes place in tank furnaces, in which the melt of the diffusing metal or its salt interacts with the surface of the objects being treated at 800°-1300° C. This method is also used for complex diffusion coating, such as chrome calorization and chrome-titanium and chrome-nickel plating.

A diffusion layer 10 microns to 3 mm thick may be produced by diffusion coating. The diffusion processes make possible an increase in the resistance of alloys to oxidation (for example, calorized steel is oxidation-resistant up to 900°C), abrasion (for example, chromium-plated U12 steel is six times more abrasion-resistant than the base metal), heat shock, rapid temperature changes, corrosion, and acid, as well as the improvement of other properties of metals and alloys.