Diffusion heat treatment coatings

Selective Carburi ng. In most components, it is desirable to carburize only parts of the surface. To prevent other regions from carburizing, they must be protected. For holes, simple plugs of copper may be used. In some cases, copper plating can be appHed, but diffusion into the steel must be considered, and the copper may have to be machined off later. Coatings (qv), which can be appHed as a paste and then removed after heat treatment, are also available and include copper plating, ceramic coatings, and copper and tin pastes. 

In metallurgical practice, sodium uses include preparation of powdered metals removal of antimony, tin, and sulfur from lead modification of the stmcture of siHcon—aluminum alloys appHcation of diffusion alloy coatings to substrate metals (162,163) cleaning and desulfurizing alloy steels via NaH (164) nodularization of graphite in cast iron deoxidation of molten metals heat treatment and the coating of steel using aluminum or zinc. 

It must always be remembered that diffusion coatings are produced by a form of heat treatment and that, with the exception of low-temperature zinc diffusion (sherardising), the treated ferrous materials are usually in the annealed condition. Whenever the mechanical properties of the parts must be restored to their original level, a subsequent heat treatment is necessary . This does not as a rule present any difficulty with chromised or boronised steels. In order to prevent undue distortion and internal stresses during treatment and subsequent hardening, it is recommended that high-carbon and alloy steels should be processed in the normalised condition. 

Another path to alloy deposition is via diffusion. In this case different coatings are deposited alternately, and then heat treatment is applied to promote mutual diffusion, thus ending up with an alloy. As a specific example, an alloy of 80% Ni and 20% Cr can be produced by the deposition of alternating layers of 19- m-thick Ni and 6- m-thick Cr. Subsequent heating to 1000°C for 4 to 5 h produces completely diffused alloys of rather high quality as far as corrosion is concerned. Brass can also be produced by interdiffusion of Cu and Zn under suitable conditions. 

Other perfluoroalkyl polymers have been produced with sufficient thermal stability to withstand the temperatures of nylon melts from which carpet fihers are spun. During the subsequent heat treatment of the fiher the perfluoroalkyl groups diffuse to the surface to form a soil-resistant coating. Although such coatings add to the initial cost of the carpet they eventually save money by increasing the time intervals between cleanings. 

The surface of bottom disc is covered with a layer of Re/W materiales, which is formed through heat treatment after two step CVD coatings. The Re surface, having preferred orientations with (0001) plane, shows higher work function than a surface of conventional tjrpe. The FGM layer pre-coated with W prevents the diffusion at high temperature. 

Luo, Cui and Li (1999) addressed the problem of temperature sensitivity of IBAD of ACPs and their subsequent crystallisation forming hydroxyapatite. Post-depositional annealing temperatures were decreased to 400 °C. The crystallisation of calcium phosphate coating is a hydroxyl ion diffusion-controlled process, thought to be the mechanism responsible for the decrease of the crystallisation temperature. The detailed study of the crystallisation process of calcium phosphate coatings shows that the crystallinity of the hydroxyapatite coating can be well controlled by adjusting the post-heat-treatment time. 

Fig. 10. Schematic diagram showing the as-deposited structure of a low activity (left) and a high activity (right) diffusion aluminide coating on a nickel-base superalloy. The high activity coating would receive a heat treatment to convert the Ni2Al3 to NiAl.

After the application of the aluminum coating to the steel substrate, a heat treatment is often applied. Coatings resulting from vacuum deposition, electroplating, electrophoretic application, cladding, or powder coating techniques are annealed to provide adhesion by diffusion alloying [125],... 

The alloy layer of coatings formed by a secondary heat treatment to provide an interfacial bond is similar to that obtained by hot dipping, but much thinner, and is basically FcjAlj. Where a heat treatment is applied to increase the heat resistance of the system, the aim is to produce a single-phase layer of a-Al(Fe) solid solution a diffused hot-dip coating is therefore similar to a calorized coating, but thinner [127,128]. 

As shown in Fig. 9, the metal A is first plated or coated (by any process) on a substrate. Then the metal B is plated/coated on top of film A. Next a heat treatment is applied. If the melting temperature of metal A would be lower than that of metal B, the temperature should be slightly over the melting point of metal A. Then some of the metal A would melt, and the atoms of metal B would diffuse into the metal A at a very high speed, since one of the phases (phase of metal A) is a melted form. To enhance the antibacterial effect for tin plating used in food industries, tin was chosen as metal A. And as metal B, silver and copper were chosen independently. As a result, the process was used to produce tin-silver or tin-copper intermetallic compounds. The evaluation tests for the antibacterial effect (ISO 22196) confirmed that tin film did not show any antibacterial effect. On the other hand, the intermetallic compounds for the alloy films clearly showed antibacterial effects. This suggests that silver or copper could dissolve into the environment at a low concentration level, and that the silver or copper ions could make the antibacterial effect appear on materials surfaces. Fortunately, these well-known antibacterial metals could show antibacterial effects at very low levels [7]. 

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