The ceramic-metal composites (CERMETS) are still very often used because of their good mechanical properties and chemical stability as complementary materials for construction of cutting tools parallel to the relatively modern protecting coatings. In comparison with the thin films, CERMETS show better properties at extremely high temperatures, whereas the production of thin coatings is substantially easier and cheaper. One of the principal problems that have still to be solved is the kinetics of the microstructure evolution as well as the processes running at the sintering of materials at high temperatures.
Our study is concerning the phase stability of the metal components that were the quarternary mixtures of (Ti,Mo)(C,N) and (Ti,W)(C,N), respectively, and the dependence of the phase stability on both the annealing temperature and the starting stoichiometry of the compounds. Generally two effects can take place at the sintering and/or at the annealing of the material. Firstly, the material can be homogenized during the annealing. In such a case, a diffusion process is running in the material. It causes the decrease of the concentration gradients in the sample. The second case is characterized by the phase separation. As a product of the phase separation, the metal-rich substances (Ti,Mo)C and (Ti,W)C and the metalloid-rich substance Ti(C,N) usually arise. All phases are face centred cubic. The phase separation is, in addition, a good indication for the miscibility gap.
For the estimation of homogeneity as well as for the measurements of concentration profiles the electron probe microanalysis with the wavelength or energy dispersion analysis of the excited X-ray spectrum is commonly used because of its good lateral resolution and high sensitivity to the changes in the concentration. Nevertheless, this method can fail if the size of the particles that are to be investigated is below 5mm. The grains in our specimens was even smaller than 1mm. In such a case, the X-ray diffraction is a very useful tool for investigation of the phase homogeneity.
Taking the X-ray diffraction pattern, we used the conventional Bragg-Brentano diffractometer (XRD-7 produced by Seifert - FPM) operating with the Nickel filtered Cu Ka radiation. The powder samples were covered on a glass substrate. The measurements have been carried out with a step size of 0.01 2q and with a basic counting time of 5 sec per step. To reduce the noise in the measured intensities, the step scan in the respective range was repeated until the quality of data was sufficient.
The experimental data were reduced by using a deconvolution procedure based on the least-squares method with optional smoothing of data. The deconvoluted diffraction patterns offer directly a survey on the distribution of d-spacing, which corresponds to the distribution of lattice parameters and concentrations in the cubic materials. The instrumental line broadening was approximated both by the diffraction profile obtained with the powder sample of LaB6 (NIST standard sample for line broadening) and by using the diffraction profile measured at the homogeneous ternary mixtures.
After sintering, both materials consist of two main components. The first one is the respective ternary mixture of (Ti,Mo)C and (Ti,W)C with a small amount of nitrogen atoms that occupy the metalloid positions in the fcc structure. The second component is the Ti(C,N) in both cases. During the successive annealing, both materials absorb nitrogen into the carbon-rich phase, but the difference between the mixtures containing tungsten and the samples containing molybdenum is striking. The absorption of nitrogen is much faster in (Ti,W)(C,N) than in (Ti,Mo)(C,N). Moreover, whereas the (Ti,W)(C,N) consists of two main components also after the annealing, a strong redistribution of species between individual grains was observed in the (Ti,Mo)(C,N).