Coatings silicide

Fig. 5.1 Protection offered by three commercial silicide coatings overlap broadly in this performance band (after Prod. Engng., June 25, 63 (1962))...

Protection of niobium and its alloys from oxidation in air is accomplished by coating, e.g. with zinc deposited by holding in zinc vapour at 865°C or coating with a layer of chemically stable oxide, nitride or silicide. Silicide coatings applied by pack cementation, fused slurry or by electrolytic methods have been found to be one of the most effective means of preventing oxidation of the metal. 

Priceman, C. and Soma, L., Development of Fused Slurry Silicide Coatings for the Elevated Temperature Oxidation Protection of Niobium and Tantalum Alloys, Report AFML-TR-68-210, Sylvania Electric Products Inc., Dec. (1968)... 

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)... 

Figure 1. Photocurrent-photovoltage characteristics of the cell n-Si (Pt silicide coated)/1.0 M FeClg, 0.1 M FeCU, 1 M HCl/Pt at 65 nW/cm2 illumination. Pt thickness deposited 40 A and annealing temperature 400 °C at 10 6 torr for 10 min. Key a, before long-term stability test and b, after long-term stability test.

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

The borides are extremely hard (9.8-29 GPa (1000-3000 kgf/mm2) Knoop) and, in the case of molybdenum, >39 GPa (4000 kfg/mm2) (see Hardness). However, oxidation resistance is usually poor unless a subsequent coating is formed, such as silicidizing or chromizing, which imparts oxidation resistance. Silicides are generally very oxidation resistant, but not as hard as borides. Silicide coatings formed on molybdenum (51 Jim in 3 h) at 675°C have superior oxidation resistance. At these low temperatures, the molybdenum substrate does not embritde and the coatings are quite flexible. 

Diffusion aluminide and silicide coatings on external and internal surfaces for high temperature corrosion protection in parts such as gas-turbine blades is estimated at 40 x 106/yr in North America and about 50 x 106 worldwide. 

Satellite thruster combustion chambers manufactured by EADS Space Transportation are machined from massive Pt based alloys, which are capable of operating at about 1600°C in continuous mode. Main inconvenient of such chambers is the elevated raw material cost of about 30 /gr. Other qualified materials for the application are silicide coated Nb,... 

Cockeram and Rapp have evaluated the kinetics of silicide coatings on Ti [103] and have used a halide-activated pack-cementation method to form boron- and germanium-doped silicide coatings on orthorhombic alloy substrates [104]. The coatings greatly decreased the cyclic oxidation kinetics and microhardness measurements did not indicate diffusion of oxygen into the substrate. 

B. V. Cockeram, R. A. Rapp Oxidation Resistant Boron- and Germanium-Doped Silicide Coatings for Refractory Metals at High Temperature to appear in Mater. Sci. and Eng. 

FI. Lavendel, R. A. Perkins, A. G. Elliot, J. Ong investigation of Modified Silicide Coatings for Refractory Metal Alloys with Improved Low pressure Oxidation Behavior AFML-TR-65-344.1965. 

There are no structural alloys which rely on silica formation for protection primarily because Si tends to embrittle most alloys. Some intermetallic compounds (e.g., MoSi2), silicide coatings, and ceramics (e.g., SiC) are silica formers. The following is a brief discussion of the oxidation of model Fe-Si alloys. 

Stern K.H. and Carlton E.W., Electrodeposition of Tantalum Silicide Coatings from molten salts, (1986),/. Electrochem. Soc., 133, 2157-60. 

Babu, N.S., Ramesh Narayanan, P., Natarajan, A., 1995. Metallographic characterisation of silicide coatings on niobium alloys. Prakt. Metallogr. 32, 260. 

Kim, I.C. Byun, D. Lee, S. Lee, J.K (2006). Electrochemical Characteristics of Copper Silicide-Coated Graphite as an Anode Material of Lithium Secondary Batteries. Electrochim. Acta, Vol. 52, pp. 1532-1537... 

Liu G M, Li M S, Zhang Y M, Zhou Y C, Oxidation behaviors of silicide coatings on Xi3SiC2-based ceramic . Mater Res Innov, 2002 6 226-231. 

Cockeram B and Rapp R A (1996), Isothermal and cyclic oxidation resistance of boron-modified and germanium-doped silicide coatings for titanium alloys , Oxid Met, 45 (5/6), 427 68. 

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