Year

2001

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

Tungsten carbide-based hardmetals with iron aluminide intermetallic binder have been processed using uniaxial hot pressing. The new binder, a binary B2-iron aluminide alloy with a composition of 40 at% aluminium, was used in stead of conventional cobalt binder. Following optimisation of the sintering process, the microstructure and properties of these new materials were assessed and compared with those of the conventional tungsten carbide/cobalt hardmetals. Sintering temperatures were above the melting point of the aluminide binder and all sintering was performed in the presence of liquid phase.

The tungsten carbide/iron aluminide composites exhibited low densification at high volume fractions of the carbide phase. Pressure was essential for the elimination of porosity in composites containing higher than 31% volume fraction of carbide phase. The poor densification is attributed to the combination of low solubility of carbide phase in the aluminide binder and the surface alumina layer present with the binder material. None of the sintered tungsten carbide/iron aluminide composites, of varying carbide contents and carbide grain sizes showed evidence of development of new phases in sintered microstructures.

The mechanical properties of tungsten carbide/iron aluminide composites were comparable with those of conventional hardmetals of tungsten carbide/cobalt. Based on the hard carbide content, tungsten carbide/iron aluminide composites exhibited significantly higher hardnesses and abrasive wear resistance than tungsten carbide/cobalt hardmetals. Iron-40 at% aluminium alloy was found to be a superior binder in abrasive wear environment to cobalt, since significantly lower volume fraction of the hard carbide phase was required in tungsten carbide/iron aluminide composites to achieve the same wear rate as that of the tungsten carbide/cobalt hardmetals.

A reduction in grain size of precursor tungsten carbide powder was associated with increases in composite hardness. A reduction of grain size of precursor powders of tungsten carbide and iron aluminide to sizes in the nanometer range was achieved by both ball milling and ring grinding. However, retention of the fine structures of these powders was not possible by hot pressing. Rapid grain growth of the tungsten carbide phase took place, even when short sintering times were employed. This growth is attributed to the high sintering temperatures used in processing of these composites. The increase in composite hardness with decrease in carbide grain size was valid in the micron and nanometer range of grain sizes tested in this work.

The results of this work have important implications for the development of alternative binders to cobalt in tungsten carbide-based hardmetals. The uniaxial hot pressing technique allowed the processing of composites with high carbide content, similar to those used in conventional materials. The use of high content of hard carbide phase in the tungsten carbide/iron aluminide composites resulted in materials with hardnesses and abrasive wear resistance, suitable for many cutting and wear applications. High hardness and wear resistance exhibited by these materials must be accompanied by acceptable fracture toughnesses (higher than the values reported in this work) before alloy, iron-40 at% aluminium, can partly or completely replace cobalt binder. The solution to the fracture toughness question is mainly dependent on further improvements to the mechanical properties of binder.

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