Toughening of complete solid solution cermets by graphite addition

Abstract

Abstract(Ti 0.95 Ta 0.05 )(C 0.5 N 0.5 )-Co complete solid solution cermets (CSCs) were developed by a mechanochemical synthesis process and a pressureless sintering method. The effect of different percentages of graphite used as a sintering additive on the nature of the binder phase and the mechanical properties of the cermets was investigated. Microstructural and mechanical characterisations were carried out by X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, energydispersive X-ray spectroscopy, Vickers hardness, indentation fracture toughness and nanoindentation. The addition of graphite modified the carbon activity during sintering, reducing the dissolution of carbonitride ceramic particles into the molten binder. The amount of Ti and Ta remaining in the binder after sintering gradually decreased as the amount of graphite added increased, which induced a change in the nature of the binder phase. When no graphite was added, the binder consisted of the brittle Ti x Ta 1-x Co 2 intermetallic phase. With the increase in the amount of graphite added, the formation of more ductile phases, such as Ti x Ta 1-x Co 3 and -Co, was observed, causing a significant improvement in the toughness of the cermets.

Hot-pressing of (Ti, Mt)(C, N)–Co–Mo 2 C (Mt = Ta, Nb) powdered cermets synthesized by a mechanically induced self-sustaining reaction

“…The hardness for the carbonitride phases was consistent with the values reported for the following binary ceramic phases: TiC, TiN, NbC, NbN, TaC and TaN [45]. However, the nanohardness obtained for the binder phases composed of intermetallic compounds was, as expected, significantly higher than the values found for constrained Co in hardmetals and cermets [14,54,55]. The values observed were more characteristic of brittle intermetallic compounds.…”

“…The nano-Young's modulus determined for the ceramic phase, between ~340 GPa and ~400 GPa, was also close to those reported for the binary ceramic phases [45]. Moreover, the values for the binder, between ~240 GPa and ~320 GPa, were also substantially higher that those corresponding to Co (209 GPa) or α-Co alloy [14] due again to the presence of the intermetallic compounds.…”

“…Moreover, the introduction of Nb or Ta into cermets significantly decreased the strength, although an increase in this property was expected according to the decreased size of the ceramic particles observed when these elements were added (table 3). Therefore, it is clear that the greater embrittlement of the intermetallic compounds with Ta, Nb and Mo in their structures [14,31] Table 5 shows the nanohardness and nano-Young's modulus of both components. The ceramic component showed nanohardness values ranging from ~26 GPa to ~29 GPa, whereas the binder phase had values from ~14 GPa to ~16 GPa.…”

“…A mechanically induced self-sustaining reaction (MSR) has been proven to be an attractive and affordable alternative to the conventional methods for producing advanced ceramics [7][8][9], ceramic matrix composites [10][11][12], ceramic-metal composites [13][14][15] and intermetallic compounds [16][17][18]. An MSR is a simple and low-energy-cost procedure that produces fine and homogeneous powders.…”

“…More precisely, homogeneous titanium-based carbonitride solid solutions have already been synthesized and successfully employed in the fabrication of cermets [14,23].…”