Pack-cementation diffusion coatings

Chemical vapor deposition (CVD) involves the formation of a coating by the reaction of the coating substance with the substrate. The coating species can come from a gas or gases or from contact with a solid as in the pack-cementation diffusion process described in Chapter 5. TTie process is more precisely defined as the deposition of a solid on a heated surface by a chemical reaction from the vapor or gas phase (Ref 54). In general, three processing steps are involved in any CVD reaction (1) the production of a volatile carrier compound, (2) the transport of the gas to the deposition... 

Each type of metallic coating process has some sort of hazard, whether it is thermal energy, the reactivity of molten salt or metal baths, particulates in the air from spray processes, poisonous gases from pack cementation and diffusion, or electrical hazards associated with arc spray or ion implantation. 

Cockeram and Rapp have evaluated the kinetics of silicide coatings on Ti [103] and have used a halide-activated pack-cementation method to form boron- and germanium-doped silicide coatings on orthorhombic alloy substrates [104]. The coatings greatly decreased the cyclic oxidation kinetics and microhardness measurements did not indicate diffusion of oxygen into the substrate. 

Fig. 5-11 shows an example of a platinum-modified, )3-NiAl diffusion coating on an Ni-base superalloy. The platinum is in the P solution and was added by electroplating a 5-10 pm platinum layer on to the superalloy before diffusion aluminizing via pack cementation. It is now well established that the addition of platinum improves the oxidation behavior of / -based coatings the mechanisms of this beneficial effect are, however, presently not well understood (Streiff and Boone, 1985 National Research Council, 1996). Proposed mecha-... 

As mentioned in the preceding text, pack aluminizing is commonly carried out on nickel- and cobalt-base superalloys. Diffusion-coated superalloys develop an aluminide (NiAl or CoAl) outer layer with enhanced corrosion resistance. It is estimated that more than 90% of all coated gas turbine engine hot section blades and vanes made from superalloys are coated by pack cementation and related processes. Detailed information on protective diffusion coatings for superalloys can be found in Ref 24. 

M.A. Harper and R.A. Rapp, Codeposition of Chromium and Silicon in Diffusion Coatings for Iron-Base Alloys Using Pack Cementation, Surface Modification Technologies TV, T.S. Sudarshan, D.G. Bhat, and M. Jeandin, Ed., TMS, 1991, p 415... 

Pack cementation is the most widely used process for making diffusion aluminide coatings. Diffusion coatings are primarily aluminide coatings composed of aluminum and the base metal. A nickel-based superalloy forms a nickel-aluminide, which is a chemical compound with the formula NiAl. A cobalt-based superalloy forms a cobalt-aluminide, which is a chemical compoimd with the formula CoAl. It is common to incorporate platinum into the coating to improve the corrosion and oxidation resistance. This is called a platinum-aluminide coating. Diffusion chrome coatings are also available. 

The aim of this study is to present the microstructures of the coatings developed during the first two years of the SUNASPO project. These include single element aluminides as well as silicon and boron modified aluminides developed at low temperature by pack cementation or FBCVD on 9-12% Cr steels. Also, first attempts at the combination of the heat treatment with the coating process in order to allow the diffusion of Cr are reported. 

Pack cementation as well as fluidised bed chemical vapour deposition have been extended to temperatures below 715°C. Homogeneous diffusion aluminide Fe2Al5 coatings of up to 50 pm thick have been obtained by pack cementation. FBCVD has shown a higher success for the Al-Si co-deposition. Further improvement is, though, required for the Si deposition as well as the pack cementation codiffusion at low temperatures. 

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... 

Pack cementation (CVD) A CVD-type process where the part to be coated is placed in a mixture (pack) of inert powder and powder of the material to be deposited. The mixture is heated and a reactive gas reacts with the coating powder to form a chemical vapor precursor that decomposes and diffuses into the surface of the part. Used to carburize, aluminize, and chromize surfaces. 

Aluminide diffusion coating is applied by the classical pack cementation process. This technique has been used since the last century [255-258], even though its industrial and technical applications are more recent [259,260]. It... 

Hack Diffusion. Pack diffusion or cementation processes are similar 10 pack carburizing, anil are used to coat iron, nickel, cuhali. and coppci with chromium, boron, zinc (Sheradi/ingl. aluminum, silicon, titanium, molybdenum, and other metals. 

Ecord [37]. The base metal is packed into a powdered mixture composed of the metal to be diffused, an inert filler, and an activator (usually a halide salt). The use of an activator allows that the temperature to be used is lower than that which would be required by a conventional cementation process. The sealed pack is then held at a given temperature in a furnace for an appropriate time. Parts to be coated by the fluidized bed method are suspended in a bed of the coating metal, which is fluidized by a halogen gas stream mixed with an inert carrier gas. This is similar to the flowing gaseous methods in which the gaseous metal halide (of the coating metal) is streamed over the metal to be coated. 

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