Silicate Bond Coats

Studies by Lamy (1993) and Lamy, Pierre and Heimann (1996) confirmed the feasibility of applying calcium silicate-based bond coats to enhance considerably 

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For example, Tsuya and Kitamura studied the change in wear rate of two bonded coatings with temperature. They found that the wear rate increased with increase in temperature for an epoxy-bonded coating, and decreased with increase in temperature for a silicate-bonded coating. On the other hand Hopkins and Campbell found that coatings bonded with polyimide, silicate or aluminium phosphate all had decreased wear life with increasing temperature. The relationships... 

It is less clear why there has been very little use of other types of inorganic matrices. Some inorganic solids would seem to have useful properties for this purpose, but the only one which has been described in any detail is sodium silicate. This was the binder for the bonded coating developed by the US Naval Aircraft Materials Laboratory which was described in Chapter 11, but the same material was used ° as a bulk composite in certain components in a dry-lubricated engine. It was used to provide transfer lubrication, and presumably it was not capable of use in load-bearing situations. 

FIGURE 61.15 Resin-silicate bonded automobile body core, coated ready for use. 

One disadvantage of all silica-based stationary phases is their instability against hydrolysis. At neutral pH and room temperature the saturation concentration of silicate in water amounts to lOOppm. Solubility increases with surface area, decreasing particle diameter drastically with pH above 7.5. This leads also to a reduction of the carbon content. Hydrolysis can be recognized during the use of columns by a loss in efficiency and/or loss of retention. Bulky silanes [32], polymer coating [33], or polymeric encapsulation [34] have been used in the preparation of bonded phases to reduce hydrolytic instability, but most of the RPs in use are prepared in the classical way, by surface silanization. Figure 2.3 schematically shows these different types of stationary phases. 

PORCELAIN ENAMEL. A substantially vitreous inorganic coating bonded to metal by fusion above 426 C (ASTM), Composed of various blends of low-sodium frit, clay, feldspar, and other silicates ground in a ball mill and sprayed onto a metal surface (steel, iron, or aluminum), to which it bonds firmly after firing, giving a glasslike fire-polished surface. 

We will next deal with the hydrolysis reaction. Traditionally, most industrial processes used to make products containing the siloxane bond, such as silicones and silicates, involve the hydrolysis of chlorosilanes. This causes hydrochloric acid to be produced as a by-product. Hydrochloric acid is, however, an unacceptable by-product in products such as zinc-rich silicate coatings and glasses and glass ceramics made by the sol-gel process. The alternative is to use alkoxysilanes, which when hydrolyzed, have alcohols as a by-product. 

The tetraalkoxysilanes (tetraalkyl silicate) are a special category of low molecular weight silicones. Masonry products contain tetraalkoxysilanes, which in the presence of air and humidity will form a bond with the masonry and crosslink to a hydrophobic insoluble coating. 

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