Meaning
Carbon stoichiometry in tungsten carbide powder mixtures defines the thermodynamic phase balance during liquid-phase sintering. Decarburization below the narrow two-phase region causes eta phase embrittlement through the formation of complex ternary iron-group tungsten carbides. Eta phase embrittlement precipitates carbon-deficient phases such as Co3W3C or Co6W6C within the binder network.
Microstructural degradation of this type consumes metallic cobalt, drastically reducing fracture toughness and transverse rupture strength in hard metal components. The boundary of this embrittlement phenomenon stops at the carbon saturation threshold, above which free carbon or stoichiometric tungsten carbide forms instead.
Phase Transformation
Decarburization reactions during vacuum sintering remove carbon from the tungsten carbide matrix. When carbon activity drops below equilibrium levels, cobalt reacts with tungsten and carbon to form hard, brittle complex ternary carbides. These ternary structures grow as irregular dendritic networks or angular particles dispersed throughout the hard metal body.
Toughness Reduction
Brittle ternary carbide structures consume ductile cobalt binder, reducing impact resistance and lowering transverse rupture strength below operational thresholds.
Sintering Atmosphere
Atmospheric control inside sintering furnaces regulates carbon activity to prevent phase degradation. Hydrogen reducing atmospheres or dew point variations accelerate carbon loss from the compact surface. Methane or carbon monoxide gas additions restore carbon potential during the high-temperature soak phase.
Hard metal components suffering from carbon deficiency exhibit extreme brittleness, making slitting blades prone to catastrophic edge chipping during initial contact with battery foils. Quality assurance procedures verify phase purity through magnetic coercivity measurements and metallurgical cross-section etching. Optical examination under light microscopy reveals eta phase structures as dark-etching angular features within the metallic binder matrix.
Maintaining strict carbon balance in raw materials eliminates phase transformation risks prior to final tool consolidation.
