Effect of ultra-fine powders on the microstructure of Ti(CN)–xWC–Ni cermets

Cermets obtained by sintering powders of TiCN and Mo 2 C with cobalt or nickel binder are studied in this paper. Different TiCN/Mo 2 C ratios are used in order to vary the grain size. A characteristic core–rim structure of the ceramic phase is formed by precipitation of TiMoCN cubic carbide around undissolved TiCN cores. The morphological parameters of the materials such as grain size and the respective volume of rims and cores are determined by image analysis. The ceramic phase and the binder composition are measured by analytical scanning and transmission microscopy. The chemical composition of the phases, obtained after sintering, is modelled by a second order interface reaction controlled by molybdenum precipitation.

TiMoCN based cermets: Part I. Morphology and phase composition

Two main families of hardmetals for cutting tool applications can be defined according to the ceramic matrix: WC and TiMoCN based hardmetals with either cobalt or nickel binder. In this paper, we illustrate an original approach to study this kind of materials combining different techniques, macroscopic and microscopic. The morphology of hardmetals can be well described by a structure made of two interpenetrated skeletons: the ceramic and the metal ones. This morphology makes it easy to remove the metal by chemical etching. The comparison between the properties of the ceramic skeleton and those of the whole material allow to locate the mechanisms responsible for a given mechanical behaviour. The macroscopic mechanical behaviour is studied by bending tests made as a function of temperature and composition. Generally, three distinguished domains can be defined while increasing the temperature: brittle, limited plasticity and plasticity. A transition from bulk deformation in the limited plasticity region to grain boundary sliding in the plasticity region is observed in cobalt base hardmetals. Only bulk deformation of TiMoCN and nickel is observed in nickel base hardmetals. The kinetics of the high temperature deformation is studied by temperature jumps leading to better stability of the microstructure compared to traditional creep tests. However, apparent activation energies are often obtained due to changes in the internal composition of the binder. A finer tool to study the dynamics of defects responsible for deformation and their kinetics is the mechanical spectroscopy. The analysis of the thermal activation of damping peaks can be made isothermally as a function of frequency which means without structural changes: this method provides true activation energies with respect to deformation tests. Mechanical tests are supported by electron microscopy imaging.

Experimental strategy to study the mechanical behaviour of hardmetals for cutting tools

Two series of Ti (C, N)-based cermets, one with TiC addition and the other with TiN addition, were fabricated by conventional powder metallurgy technique. The initial powder particle size of the main hard phase components (Ti (C, N), TiC and TiN) was nano/submicron-sized, in order to achieve an ultra-fine grade final microstructure. The TiC and TiN addition can improve the mechanical properties of Ti (C, N)-based cermets to some degree. Ultra-fine grade Ti (C, N)-based cermets present a typical core/rim (black core and grayish rim) as well as a new kind of bright core and grayish rim structure. The average metallic constituent of this bright core is determined to be 62 at% Ti, 25 at% Mo, and 13 at% W by SEM–EDX. The bright core structure is believed to be formed during the solid state sintering stage, as extremely small Ti (C, N)/TiC/TiN particles are completely consumed by surrounding large WC and Mo2C particles. Low carbon activity in the binder phase will result in the formation (Ni2Mo2W)Cx intermetallic phase, and the presence of this phase plays a very important role in determining the mechanical properties of TiN addition cermets. © 2005 Elsevier Ltd. All rights reserved.

Effects of TiC/TiN addition on the microstructure and mechanical properties of ultra-fine grade Ti (C, N)–Ni cermets

TiC-Ni-Mo cermet specimens were prepared by using a mixture of fine (1.5 [mu]m) and coarse (30 [mu]m) TiC powders. When the fraction of fine TiC particles was 80%, a (Ti,Mo,Ni)C complex carbide phase was observed deposited on the coarse TiC particles and resulted in a typical cored structure. As the fraction of fine TiC particles decreased, the coarse TiC particles exhibited a unique microstructural evolution with the development of a concave interface. This microstructural change of the coarse TiC grains can be explained in terms of the coherency strain energy.

Osamu Terada

Papers

TiC-Mo2C-Ni合金における周辺組織形成機構