Plasma-sprayed molybdenum

The Tribaloy aUoy T-800, is from an aUoy family developed by DuPont in the eady 1970s, in the search for resistance to abrasion and corrosion. Excessive amounts of molybdenum and sUicon were aUoyed to induce the formation during solidifica tion of hard and corrosion-resistant intermetaUic compounds, known as Laves phase. The Laves precipitates confer outstanding resistance to abrasion, but limit ductUity. As a result of this limited ductUity the aUoy is not generaUy used in the form of plasma-sprayed coatings. 

Flame sprayed molybdenum articles have poor corrosion resistance, no doubt owing to the porosity of the coating. However, modern plasma spraying techniques produce a dense coating and this should lead to more widespread use of clad materials such as molybdenum clad steel where the clad product should have the same corrosion resistance as the solid material. 

Plasma spraying Metals, or refractory materials or composites are applied by melting in an ionised inert gas M, tantalum, molybdenum, alumina, zirconia M2 a variety of metallic substrates... 

Properties of the deposits Almost any material which can be melted is suitable for plasma spraying, giving a vast range of possible coatings of single or mixed metallic or non-metallic substances. It is often possible to produce types of coatings which are not obtainable in any other way. Typical of the materials which are plasma sprayed are copper, nickel, tantalum, molybdenum. Stellites, alumina, zirconia, tungsten and boron carbides, and stainless steels. 

A plasma spray can therefore be used to apply sintered coatings to substrates which would normally be adversely affected by the sintering temperature of the coating. However, the powders carried by the plasma are exposed to greater thermal stress than the substrate, and some experimenting with specific powders is usually necessary to define the best application conditions. Where molybdenum disulphide in a metal, resin or ceramic binder is applied by this technique, the optimum conditions will usually be different for the two materials. This difficulty has been overcome by the use of two separate entry ports into the nozzle for the two components. The use of this technique has even been applied to molybdenum disulphide in a polyethylene binder. 

Pretreatment of the substrate for plasma spraying consists only of cleaning and mild grit-blasting. If the bulk substrate is not capable of withstanding the temperatures involved, it may be necessary to apply a film of a metal such as nickel or molybdenum to provide a stable substrate. Plasma spraying has not been widely used, and for many applications has probably been superseded by sputtering. 

Kirner reported the successful use of plasma spraying for niobium diselenide, niobium ditelluride, and a mixture of tungsten disulphide and silver, but the performances in high vacuum and high temperature were inferior to those obtained with molybdenum disulphide. There has been a great deal of Russian work on the... 

Lu H, Jia W, Ma R. Titanium diboride and molybdenum silicide composite coating on cathode carbon blocks in aluminium electrolysis cells by atmospheric plasma spraying. Light Met. 2005 134 785-8. 

Thompson and Zao [170] have described a solvent extraction-inductively coupled plasma atomic emission spectrometric method for the determination of down to 0.02 - 0.03 xg/g of molybdenum in soils. The soil sample is pressure-leached with 6 M hydrochloric acid and at 120 °C for 15 minutes. The digest is then extracted with heptan-2-one to separate molybdenum from potentially interfering elements such as iron, aluminium, calcium and magnesium. This organic extract is then directly sprayed into an inductively coupled plasma atomic emission spectrometer operated at 1.65 to 1.7 kW power. 

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