Diffusion coatings atmospheres

The reactions work both ways, and constituents of the alloy being coated may be removed by the halide atmosphere even when interchange does not occur. For example, a nickel-chromium alloy may be superficially dechro-mised by nickel powder in a chloride atmosphere. Thus loss of important alloying constituents may have to be controlled during diffusion coating processes. 

G. H. Meier, C. Cheng, R. A. Perkins, and W. T. Bakker, Formation of chromium diffusion coatings on low alloy steels for use in coal conversion atmospheres. In Materials For Coal Gasification, eds. W. T. Bakker, S. Dapkunas, and V. Hill, Metals Park, OH, ASM International, 1988, p. 159. 

Diffusion can result in the presence of voids and porosity [43]. There are, however, instances where inherent porosity (e.g., the formation of a diffusion coating on a sintered metal substrate) has been decreased by the formation of a diffusion coating. Some metals are sensitive to nonmetallic impurities in that they are hardened and embrittled by small amounts of carbon, oxygen, nitrogen, and so forth. The formation of compounds containing these elements during diffusion can result in a poorly bonded interlayer, a hard or brittle layer, or even a barrier to diffusion. Since these elements are present in the base metal and sometimes in the process atmosphere, diffusion coatings... 

The photopolymerizable formulation containing between 1 and 3 wt % photoinitiator was coated on a polypropylene film or on a silicon wafer, at a typical thickness of 24 pm. In some experiments a second polypropylene film was laminated on top of the liquid resin to prevent the diffusion of atmospheric oxygen. The sample was placed in the compartment of an infrared spectrophotometer where it was exposed for a few seconds to the UV-radiation of a medium pressure mercury lamp. The light intensity at the sample position could be varied between 5 and 100 mW cm, as measured by radiometry (International Light radiometer IL-390). 

Cementation consists of tumbling the work in a mixture of metal powder and a flux at elevated temperatures, allowing the metal to diffuse into the base metal. Aluminum and zinc coatings can be prepared in this way. Diffusion coatings of chromium, nickel, titanium, aluminum, and so on, can also be prepared by immersing metal parts, under an inert atmosphere, in a bath of molten calcium containing some of the coating metal in solution [6]. 

Diffusion coatings can also be formed by pack cementation. In this technique, the diffusion coatings are formed by heating the surface in contact with the material to be diffused (i.e. solid state diffusion) or by heating in a reactive atmosphere where the reactive gas reacts with the solid material to be diffused, thus forming a vapor (vapor precursor) that decomposes on the heated surface and provides the material that diffuses into the surface (similar to CVD... 

Aluminide diffusion coating performance in oxidising atmospheres... 

Yoshihara M, Suzuki T and Tanaka R (1991), Improvement of oxidation resistance for TiAl by surface treatment under a low partial pressure oxygen atmosphere and aluminum diffusion coating , ISIJInter, 31 (10), 1201-1206. 

Zinc diffusion sherardisingY " is mainly used for protection of ferrous metals against atmospheric corrosion. It has, in some respects, properties related to other types of zinc coating such as galvanising, but owing to the small dimensional change involved, it is of particular value for the treatment of machined parts, bolts, nuts, etc. 

Gas-phase deposition In this process, a halide of the solute metal is passed in vapour form over the surface of the metal to be coated, which is heated to a temperature at which diffusion can take place. Temperatures of 500-1 300°C or more can be used, depending on the particular system considered. Generally, filler atmospheres are provided to carry the halide vapour these atmospheres are usually reducing gases such as hydrogen, cracked ammonia, etc. or inert gases (helium, argon). 

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