Novel methods for in-situ characterisation and simulation of processes during sintering of hardmetals

Kinetic analysis is a novel approach for understanding and improving processes during sintering of hard metals or other hard materials. With the determination of the kinetics using experimental results of property changes during heating, it is possible to obtain information about the specific mechanisms and to optimise the time-temperature atmosphere-profiles. Other new developments in the field of in-situ characterisation of processes during sintering are the application of macro devices for investigation of components instead of small material samples and the advancement of simultaneous techniques. The following processes are discussed: (1) desorption or vaporisation (and cracking respectively) of air, water, residues of milling liquids and lubricants; (2) melting of lubricants and WC-Co eutectics; (3) decomposition of residual hydrocarbons related to milling liquids and lubricants; (4) reduction of oxidic impurities and (5) mass transport phenomena and densification during sintering. These reactions result in length changes, mass changes, thermal effects and gas formation. They are characterised by means of thermoanalytical methods (Thermodilatometry, Differentia! Thermal Analysis, and Mass Spectrometry). Examples for the kinetic analysis of the results are presented. Kinetic analysis is a tool to model complex thermal behaviour with few parameters. The model can then be used for predictions and for process optimization. Simultaneous complex thermal analysis of outgassing, melting of the binder phase and sintering gives accurate insights into the relationship between these processes. This knowledge is important because with increasing temperatures of outgassing the sensitivity of product quality to process parameters also increases. If the outgassing takes place at unfavourable temperatures (where the shrinkage rates are too high if active surfaces are created or gasses are entrapped) pores or disturbances of the carbon balance may occur. This knowledge may also be used for the optimisation of technical processes. The thermoanalytical data are a key to understanding critical temperature ranges for product quality (initiation of reactions and ranges with high reaction rates). Thermoanalytical methods are particularly powerful for the determination of the influence of changes in material or process parameters.