Silicones high-temperature coating

The pyrolysis products from 9.13 can give essentially pure silicon carbide in 89% yield. Intermediates that contain from 5 to 25% allyl functionalized units give ceramics with progressively larger amounts of carbon beyond the 1 1 Si C ratio. This process has been developed into a manufacturing sequence for the production of reinforced composites for aircraft brakes and high temperature coatings. 

A brief summary of the silicones in high-temperature coating applications follows ... 

Silicones high temperature endurance, water repellant HT coatings, elastomers... 

The most simple methods for preparation of hafnium carbide and its composites are the follows. The powder of HfO was thermally treated with Mg in molar ratio 5 4 under a CH flow ranging from 800 to 950 °C [4]. The effective high temperature coating for carbon fiber reinforced carbon and carbon fibre reinforced silicon carbide was prepared with use of HfC [5]. For this purpose hafnium carbide layers were obtained in a thermally simulated chemical vapor deposition (CVD) reactor on nonporous substrates by reaction of hafnium tetrachloride, methane and addition of hydrogen (Eq. 10.1) ... 

Specialized alloys are used for high temperature appHcations on turbine blades, furnace parts, thermocouples, etc. These coatings can be as simple as iron—silicon—chromium or as exotic as chromium—aluminum—hafnium (36,41,52). 

Budden, G., High temperature properties of silicone elastomers. J. Coated Fabr., 27, 294-308 (1998). 

The bath is normally at a temperature in the range 620-710°C, depending on whether the coating material is an aluminium-silicon alloy (for use in high-temperature conditions) or pure aluminium (for corrosion prevention). It is heated by inductors, by resistance heaters or by an external flame. The pot will usually be refractory lined unless cast-iron pots are needed to ensure adequate heat transfer from an external flame. As molten aluminium is extremely aggressive towards ferrous metals, replacement of cast-iron pots is fairly frequent. Refractory-lined pots obviously do not have this drawback, although the bath hardware, in particular the sinker roll and support mechanism, will still be attacked and need replacement at intervals. 

As a future alternative to glassed steel there is ceramics-coated steel which is resistant to abrasion, corrosion and high temperatures. The base metal is coated with silicon nitride formed in situ. Silicon nitride has resistance to both acid and alkali and it is durable at temperatures up to 1 000°C, suggesting a promising future coating in aggressive operating environments. 

Fluidized-bed CVD was developed in the late 1950s for a specific application the coating of nuclear-fuel particles for high temperature gas-cooled reactors. PI The particles are uranium-thorium carbide coated with pyrolytic carbon and silicon carbide for the purpose of containing the products of nuclear fission. The carbon is obtained from the decomposition of propane (C3H8) or propylene... 

If the composite is left only partially densified, it can be used as a filter for high temperature filtering systems with high collection efficiency as required in direct coal-fired gas and steam turbines. Similar systems are considered for particulate filtering in diesel engines by a carbon foam or felt coated with silicon carbide by CVI. 

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

In order to overcome these problems, hybridization of both materials (C and Si) in one electrode material by HTC seemed to be a promising option [75]. For this purpose, pre-formed silicon nanoparticles were dispersed into a dilute solution of glucose followed by hydrothermal treatment at 180 °C. The carbon-coated particles were then further treated at 750 °C in order to improve the conductivity and structural order of the carbon layer. It was shown that the hydrothermal treatment, following by high temperature carbonization, resulted in formation of a few nanometer thin layer of SiOx layer on the Si nanoparticles, effectively leading to a Si/SiOx/C nanocomposite. Some TEM micrographs of these materials are shown in Fig. 7.8. 

Silicon nitride (Si N ), resistant to oxidation, is an excellent coating for metals, as well as an adhesive and abrasive, and it is used in high-temperature crucibles. It has proven useful as heat-resistant substance for the nozzles on rocket engines. 

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