Sihcides oxidation

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. 

Advanced Structural and Heating Materials. Molybdenum siHcide [12136-78-6] and composites of MoSi2 and siHcon carbide, SiC, have properties that allow use as high temperature stmctural materials that are stable in oxidizing environments (see Composite materials Metal-matrix composites). Molybdenum disiHcide also finds use in resistance heating elements (87,88). 

By far the most common iadustrial refractories are those composed of single or mixed oxides of Al, Ca, Cr, Mg, Si, and Zr (see Tables 1, 4, and 6). These oxides exhibit relatively high degrees of stabiHty under both reduciag and oxidizing conditions. Carbon, graphite, and siHcon carbide have been used both alone and ia combination with the oxides. Refractories made from these materials are used ia toa-lot quantities, whereas siHcides are used ia relatively small quantities for specialty appHcation ia the auclear, electronic, and aerospace iadustries. 

Sihcon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between sihcon carbide and a variety of compounds at relatively high temperatures. Sodium sihcate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal sihcide. Sihcon carbide decomposes in fused alkahes such as potassium chromate or sodium chromate and in fused borax or cryohte, and reacts with carbon dioxide, hydrogen, ak, and steam. Sihcon carbide, resistant to chlorine below 700°C, reacts to form carbon and sihcon tetrachloride at high temperature. SiC dissociates in molten kon and the sihcon reacts with oxides present in the melt, a reaction of use in the metallurgy of kon and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new sihcon nitride-bonded type exhibits improved resistance to cryohte. 

Chemical formula Ba/Sr carbonate with ferric oxide, NiZn, MnZn, CuZn, iron sihcide, BaO-Fe Os, Sr0-6Fe203, (Mn,Zn)y(Fe203)2.a BaPb, BaSrPb, Nd2Fei4B... 

Liquid phase assisted sintering has also been observed with few non-oxide sintering additives such as MoSi [Basu, 2006 Fahrenholtz, 2007 Murthy, 2006 Mukhopadhyay, 2009 Raju, 2009 Biswas, 2006 Bellosi, 2006 Silvestroni, 2011) and TaSij (Fahrenholtz, 2007 Silvestroni, 2011 Sciti, 2008). Such addition has led to improved densification for most of the refractory borides (Basu, 2006 Fahrenholtz, 2007 Murthy, 2006 Mukhopadhyay, 2009 Raju, 2009 Biswas, 2006 Bellosi, 2006 Silvestroni, 2011 Sciti, 2008) as well as carbides (Raju, 2007). While TiSi (Basu, 2006 Murthy, 2006 Mukhopadhyay, 2009 Raju, 2009 Biswas, 2006) has been observed to form due to sintering reaction between the borides and MoSi (see Equation 8 [Biswas, 2006]), addition of TaSi leads to the formation of complex phases consisting of Ta-Si-B-C-0, both of which are supposed to be in the liquid state during the final sintering temperatures. The sihcides have also been reported to react with the surface oxide phases (such as B Oj) to form SiO (see Equation 9 [Sciti, 2008]), which is transient liquid above 1850 C. 

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