Pack-cementation diffusion processes

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... 

The more specialized pack-cementation diffusion processes, such as aluminizing, chromizing, siliconizing, and boronizing, are usually carried out at companies that specialize in these processes. Some of these processes are also performed by aerospace companies, for example, aluminizing of jet engine turbine components. 

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. 

The most common method of aluminizing is pack cementation, which has been a commercially viable process for many years. This process, which is shown schematically in Figure 10.1, involves immersing the substrate in a mixture of powders. The mixture contains a powder source of Al, either A1 metal or a suitable master alloy, a halide activator, and a filler, which is usually alumina and taken to be inert. A pack will usually contain 2-5% activator, 25% source and the rest filler. The purpose of the filler is to support the component and to provide a porous diffusion path for the gases generated by reaction between the source and activator. 

Sherardizing is a diffusion process in which the steel parts are heated in the presence of zinc dust or powder in inert medium. Aluminium oxide or sand in amount of 20 % is added to the zinc powder as inert filler and 1 to 2 % halide salts are used as activator. The thermochemical treatment can be carried out in retort, rotated drum or as a pack-cementation process at 350 to 500 °C for three to twelve hours. The structure of the obtained layer is the same as the structure on steel surface after hot-dip galvanizing with a thickness about 50 - 400 pm... 

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. 

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 (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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