International Journal of Refractory Metals and Hard Materials
Liquid phase sintering of tough coated hard particles
Introduction
This paper introduces the TCHP concept and shows the theoretical basis for new performance combinations via various powder, coating, and matrix phase combinations. The goals are to provide a hard, wear resistant composite with high fracture toughness via a homogeneous microstructure. Further, microstructure design and consolidation protocols are targeted at minimized intergrain ligaments to maximize hardness and fracture toughness. Finally, these coated hard particle systems are designed for consolidation by liquid phase sintering without grain coarsening. However, rapid densification is required to minimize dissolution or attack of the coating on the hard particles.
Early concepts for coated hard particles realized that outstanding property combinations were possible [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Indeed, the use of coated hard powders has been an area of much prior research. Table 1 summarizes several studies to show the range of material systems, core particles chemistries, core particle sizes, coatings chemistries, coating volume percentages, and consolidation conditions. There is no pattern to these prior studies—the coating contents range from 9 vol.% in a liquid phase sintered system to over 65 vol.% in a hot pressed system. The average is 40 vol.% coating or matrix. But properties are not totally controlled by composition. The highest property gains come from systems that do not undergo coarsening during consolidation. For example, 6 μm diamond coated with titanium nitride and alumina [9] gives a very hard composite, but likewise proves very resistant to consolidation. Thus, the most impressive property gains are from hard core particles, thin and tough coatings, in systems that do not undergo significant coarsening during consolidation. As the coating thickness or content increases then property gains are generally less impressive. For example, 5 μm tungsten coated with nickel and iron coarsens so rapidly in liquid phase sintering that there were no property gains [1]; the microstructure is the same as obtained from mixed powders so in this case there was no benefit from the coated precursor particles.
Section snippets
Goals in TCHP materials
Underpinning the TCHP concept are the calculations by Louat [18] showing the properties of a particulate composite change as the coating layer becomes very thin. Effectively the coating properties are lost in the ambience of the particles they coat. This happens as dislocation image forces project hard phase properties into the coating. When the microstructure scale reaches the size of the dislocation image forces, the latter dominate the composite properties. In such a case, the mechanical
Microstructure goals
Thus, the goals for production of TCHP materials are summarized succinctly as follows:
Core material. Diamond, boron carbide, silicon carbide, tungsten carbide, zirconium carbide, niobium carbide, titanium carbide, silicon nitride, alumina, zirconia, and various composites or compounds from these materials.
Coating. Homogeneous, uniform coating of osmium, rhenium, silicon carbide composites, nickel-base superalloys, tungsten–rhenium alloys, heat treated steels, tool steels, hard stainless steels,
Realization
For the early demonstrations, the hard particles are alumina, the coatings are layers of tungsten carbide and cobalt. One key development was to layer the coating such that during consolidation the chemistry gradient in the coating prevented liquid attack of the hard particles. Fig. 2 shows an image taken from such a material after consolidation (digitally reconstructed to enhance contrast). Unlike conventional hardmetals [29], [30], chemical vapor deposition gives an ideal starting
Conclusions
Liquid phase sintering is the most desirable and anticipated consolidation route for tough coated hard particles. Hot pressing has been demonstrated for densification at lower temperatures to avoid attack of the coating, but is not a desirable long-term consolidation process. Consequently, early TCHP systems are being designed for liquid phase sintering. A new coating process, with a composition gradient from interior to surface, has been developed to avoid liquid phase attack of the core hard
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