Thermochemical Basis of the Preparation of Well-Defined Transition Metal Carbide, Nitride and Carbonitride Reference Materials for Electron-Probe Microanalysis (EPMA)

Abstract

A common and straightforward method for the standardisation in electron-probe microanalysis (EPMA) is the use of homogeneous reference materials prepared by various techniques such as by melting, sintering, high-temperature annealing and hot-pressing. The reference materials have to be analysed by independent methods accurately in order to define their "true" composition. For some compounds the preparation techniques are difficult because of their specific thermo-chemical properties (e.g. low diffusivities, high equilibrium nitrogen pressure, incongruent melting). In addition, many compounds show large homogeneity ranges with an a priori existing uncertainty in composition, contrary to what is generally preferred: to use compounds with a narrow homogeneity range ("line compounds"). For the latter, diffusional preparation techniques can be applied to yield diffusion layers instead of massive samples for standardisation. However, also single-phase samples with narrow homogeneity ranges can be prepared by diffusion, depending on the phase equilibria in the corresponding system. The presentation summarises efforts that have been made in order to prepare various reference materials for carbon and nitrogen standardisation of EPMA by various techniques. The boundary conditions such as phase stabilities, phase compositions and diffusion kinetics, which are important for their preparation to obtain well-defined reference samples are discussed. These samples were applied to various WDS/EPMA-based studies of phase diagrams and diffusion kinetics by means of Cameca SX 50 and SX 100 microprobes.