Coatings refractory metals

But they all oxidise very rapidly indeed (see Table 21.2), and are utterly useless without coatings. The problem with coated refractory metals is, that if a break occurs in the coating (e.g. by thermal fatigue, or erosion by dust particles, etc.), catastrophic oxidation of the underlying metal will take place, leading to rapid failure. The unsafeness of this situation is a major problem that has to be solved before we can use these on-other-counts potentially excellent materials. 

The Zond VD - 96 is intended for the detection of cracks, precipitates, inclusions etc. and corrosive defects with the preliminary estimation of their depth in various objects of magnetic and non - magnetic steels, non-ferrous and refractory metals and welds including coatings. 

In the case of refractory metals, coatings generaHy are sHicides, appHed by pack cementation or slurry processes. Typical sHicide compositions are... 

Siliconizing is yet another process used especially for coating of the refractory metals Ti, Nb, Ta, Cr, Mo, and W (see Refractories). These metals form siHcides which have a surface oxidation protection layer of Si02. Siliconizing is especially effective on molybdenum against air oxidation up to 1700°C. 

Sihcide coatings of refractory metals may contain as much as three to five coating components other than siUcon. A mixture of haUde carriers is selected containing the best carrier for each component. 

Cemented carbides belong to a class of hard, wear-resistant, refractory materials ia which the hard carbides of Group 4—6 (IVB—VIB) metals are bound together or cemented by a soft and ductile metal biader, usually cobalt or nickel. Although the term cemented carbide is widely used ia the United States, these materials are better known iatemationally as hard metals (see also Refractories Refractory coatings Refractory fibers). 

Ceramics are joined to metals by metal eoating and brazing, and by the use of adhesives. In metal coating, the mating face of the ceramic part is coated in a thin film of a refractory metal such as molybdenum (usually applied as a powder and then heated). 

Hallowell, J.B., Maykuth, D.J. and Ogden, H.R., Silicide Coatings for Tantalum and Tantalum-Base Alloys, in Refractory Metals and Alloys III Applied Aspects, 30, Part 2, American Institute of Mining, Metallurgical and Petroleum Engineers, Dec. (1963)... 

Silicon diffusion (Ihrigising) is not commonly applied to steels, but is increasingly used to protect the refractory metals - (disilicide coatings). 

Molybdenum is a high-strength refractory metal, although recrystallizes above 950°C with accompanying reduction in mechanical properties. It is easily fabricated. Its properties are summarized in Table 6.6. CVD is commonly used for the production of molybdenum coatings and free-standing shapes. 

Niobium (also known as columbium) is a soft, ductile, refractory metal with good strength retenti on at high temperature, and a low capture cross-section for thermal neutrons. Itis readily attacked by oxygen and other elements above 200°C. CVD is used to produce coatings or free standing shapes. The properties of niobium are summarized in Table 6.8. 

The coating material, usually in the form of powder, is metered into a compressed-gas stream and fed into the heat source where it is heated to its melting point and proj ected onto the substrate. In the case of refractory metals and compounds which have high melting points, spraying is carried out in an inert atmosphere to avoid detrimental chemical reactions such as oxidation. 

The advantage of this procedure is that a spongy coating of metallic iron is obtained on the refractory iron oxide, with the result that the iron and the resulting magnetic oxide tend to be held together, and so keep the material open, and therefore free from back pressure to tile passage of the steam and water gas. 

Electrical contact between the electrode and connecting wires can be made with solder if the electrode is a refractory metal, while lower-melting-point metals such as lead, and reactive metals such as magnesium, should be joined to a connection lead with commercially available conductive silver paint . Contact to ITO-coated electrodes will similarly require this conductive paint. 

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