1.

It has been established that molybdenum and tungsten borides oxidize during storage in air and generally contain a significant amount of oxides. A method is proposed for ridding molybdenum and tungsten borides of oxides, involving treatment with 2% hydrochloric acid and 1% oxalic acid, respectively.




2.

A study was made of the chemical stability of molybdenum and tungsten borides in some acids, mixtures of acids with oxidizing and complexing agents, oxidizing mixtures, and alkaline solutions.

In this study, the conditions of producing tungsten boride powder from calcium tungstate (CaWO 4 ) by self-propagating high-temperature synthesis (SHS) followed by HCl leaching techniques were investigated. In the first stage of the experimental study, the SHS products consisting of borides and other compounds were obtained starting with different initial molar ratios of CaWO 4 , Mg and B 2 O 3 . It was found that increasing B 2 O 3 content in the initial mixture resulted in an increase of W 2 B 5 /WB ratio along with Mg 3 B 2 O 6 in the SHS product. In the second step, Mg and Ca containing byproducts (i.e. MgO, Mg 3 B 2 O 6 and Ca 3 B 2 O 6 ) found in the SHS product were leached out by using aqueous HCl solution to obtain a clean tungsten boride product. The effects of particle size, temperature, time, acid concentration and solid/liquid ratio were selected as leaching parameters. The acid leaching experiments of the SHS product synthesized at 1:8:2.5 initial molar ratio of CaWO 4 :Mg:B 2 O 3 showed that optimum leaching conditions could be achieved by using 2.85 M HCl at 1/10 S/L ratio and the temperature of 80 °C for 60 min.

Chemical stability of molybdenum and tungsten borides

Production of tungsten boride from CaWO4 by self-propagating high-temperature synthesis followed by HCl leaching

In this study, molybdenum plates (99·5% purity) were subjected to electrochemical boriding in a molten borax electrolyte in order to synthesise molybdenum boride phases on exposed surfaces. Electrochemical boriding was carried out at temperatures of 900, 950 and 1000°C for a duration of 30–180 min at a current density of 0·5 A cm−2. Cross-sectional microscopic examination of the plates indicated formation of 20 to 50 μm thick boride layers, depending on process temperature and duration. These layers consisted of two distinct boride phases: Mo2B4·027 and Mo2B5. The boride layer cross-sectional hardness was in the range of 1900–3250 HV with 50 g load. The adhesion of the boride coating to the molybdenum substrate was determined by a Rockwell C indentation adhesion test machine. The boride layers produced due to the electrochemical boriding technique were thick, hard, dense, and homogeneous.

Chemical stability of molybdenum and tungsten borides

Citations

Production of tungsten boride from CaWO4 by self-propagating high-temperature synthesis followed by HCl leaching

Electrochemical boriding of molybdenum in molten borax

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