Coatings process examples

In bulk coating processes, bulk materials are joined to the substrate either by a surface melt process or by attachment of the soHd material. An example of the latter is the appHcation of heat-resistant tiles of sHica-type material to the aluminum alloy skin of a space shuttle vehicle, enabling the vehicle to withstand the reentry heat. 

In the CVD coating process, the tools are heated in a sealed reactor with gaseous hydrogen at atmospheric or lower pressure volatile compounds are added to the hydrogen to supply the metallic and nonmetaUic constituents of the coating. For example, TiC coatings are produced by reaction of TiCl vapors with methane (CH and hydrogen (H2) at 900 to 1100°C. The reaction is... 

It is claimed that the cured materials may be used continuously in air up to 300°C and in oxygen-free environments to 400°C. The materials are of interest as heat- and corrosion-resistant coatings, for example in geothermal wells, high-temperature sodium and lithium batteries and high-temperature polymer- and metal-processing equipment. 

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. 

Even small traces of certain corrosion stimulants, notably soluble chlorides and sulphates, can maintain a continuing corrosion process under a paint film because the salts accelerate the initial dissolution of ferrous iron (and other metal ions) but are not immobilised in the hydrated oxide corrosion products. Filiform corrosion is the most spectacular example of this phenomenon, but progressive spread, preceded by blistering, is also observed from scratches or other breaks in a coating, for example during salt spray tests. 

In the example of Fig. 7.15 [43j rhodium particles have been deposited by spin coat impregnation of a Si02/Si substrate with an aqueous solution of rhodium trichloride. After drying, the particles were reduced in hydrogen. The images show samples prepared at three different rotation speeds in the spin coating process, but with concentrations adjusted such that each sample contains about the same amount of rhodium atoms. The particles prepared at high rotation speeds are smaller, which... 

Other coating processes involving fluoridated apatite have been investigated to improve the long-term adhesion and promote osteointegration of cementless titanium-based metal implants pulsed laser deposition, electron beam deposition and ion beam sputter deposition techniques, and sol-gel methods, for example. They lead to fluor-containing calcium phosphates (apatites in most cases) with different compositions and crystallinity states. 

One important aspect of such coating processes is the generality of the procedure. It would appear that specific surface characteristics of the preformed particles are not necessarily essential for the successful deposition of the new layer. For example, yttrium basic carbonate coatings were produced on zirconium basic sulfate... 

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