0536-1-193

The content and chemical state of oxygen in nanosized powders of WC and related substances - the oxidation behaviour of powders during synthesis, processing and storage

Keywords

  • materials
  • ceramics
  • nanosized powders
  • powder metallurgy
  • hard metals
  • oxygen determination
  • hesis
  • gaseous impurities
Nanosized (ns) powders of tungsten monocarbide WC are believed to be a promising material in such key industries as hard metals production, mechanical engineering and catalysis. Some of the high-tech applications involve production of nanostructured cemented carbides WC-Co for micro-cutting tools for electronic industry and deposition of nanostructured WC-Co coatings to enhance the wear and erosion resistance of engineering components. Nowadays, the major problem in manufacturing of WC-based nanostructured materials is related to the poor reproducibility of their microstructure and, hence, of their performance. The microstructure is, in turn, very sensitive to the powder feedstock characteristics. The high quality ns powders are required which have high specific area, narrow size distribution of particles, their loose agglomeration, strictly controlled stoichiometry, uncontaminated surface and high purity. The synthesis of high quality ns-powders still presents a considerable challenge.

Of a particular importance is the content and chemical state of oxygen, which becomes a most remarkable impurity in ns WC powders. Its content depends greatly on the conditions of the synthesis, processing, storage and varies in a wide range from 0.2 mass% up to percent and higher. In most cases the physical and chemical properties of powders as well as their processing behaviour are greatly affected by oxygen-containing sorbates. The most detrimental effect is uncontrolled decarburization of carbide on sintering or thermal spraying. The decarburization causes appearance in the microstructure of brittle phases such as W2C and ?-carbides, resulting in a sharp degradation of the fracture toughness and wear resistance of a material. Besides, adsorbed oxygen strongly modifies the catalytic properties of the fresh WC surface by introducing WOx (x < 2) surface species. This results in a so called bifunctional catalytic activity of WC. The object of the present project is to gain knowledge of the content and chemical state of oxygen in nanosized WC powders and some related pure metals such as W and Co.

The tasks to be performed within this project are:
  • adaptation and testing of the analytical procedures for oxygen/nitrogen and water/carbon determination in selected materials;
  • comparative study of oxygen in commercial and laboratory-made ns-powders, produced with different methods such as carbothermal reduction, plasma-chemical, sol-gel;
  • tracking of the powder oxidation behaviour at different stages of the synthesis, processing and storage.

In our laboratories there are a number of methods for ns-powders preparation. Their characteristics are given in Table 1. The analytical methods available in our laboratories are summarized in Table 2. Besides, the powder characterisation with BET, TEM and X-ray analysis can be carried out. All of thermo-analytical procedures are performed on commercially available instruments and can be directed transferred into industrial laboratory.

Table 1: The methods for nanosized powders preparation
MethodDescriptionMaterialParticle size, nm
Plasma-chemicalEvaporation of oxide feedstock in high-temperature gas stream; In case of WC preparation plasma-chemical synthesis is followed by low-temperature annealing to obtain single phase compositionWC W, Co, Ni, ...5-200 nm
Sol-gelPrecipitation of hydroxides from salts solutions, low-temperature hydrogen reduction of hydroxides to obtain metals powdersCo, Ni50-200 nm
Low-temperature compounds decompositionLow-temperature decomposition of unstable compounds in hydrogenCo, Ni50-200 nm


Table 2: The analytical methods for the determination of gas-forming impurities in nanosized powders
MethodFunctionInstrument
inert gas fusion analysis (GFA) in graphite crucible under helium stream with simultaneous IR detection of CO/CO2 gases evolvedelemental analysis for O, N content; fractionation of O species using temperature programmingTC-600 (Leco)
carrier gas hot extraction in quartz boat under nitrogen stream with simultaneous IR detection of H2O/CO2 evolvedelemental analysis for water content; fractionation of water species using temperature programmingRC-412 (Leco)
combustion of powders in quartz boat under oxygen stream with simultaneous IR detection of H2O/CO2 specieselemental analysis for carbon content; differentiation between organic, carbidic and probably free carbon using temperature programming; study of powder reactivityRC-412 (Leco)