Alternative routes to submicron hardmetals

Hardmetals (cemented carbides) are materials composed in principle of a hard and wear-resistant compound WC and a ductile Co binder phase. These materials are used for a variety of high-loaded parts such as micro-drills for production of printed circuit boards or of inserts for milling, drilling and cutting. For these materials there is a strong trend towards finer microstructures, also because of ongoing miniaturisation and the increasing interest for hardmetals for use in micro-machines. For the establishment of fine microstructures not only finest powders have to be employed but also grain-growth inhibitors have to be added in order to prevent the WC particles from growth. The grain-growth inhibitors such as VC, Cr3 C2 or TaC are either added to the WC+Co starting powder mixture or pre-doped WC is used for which the grain-growth inhibitor is added upon powder production. It is extremely important that the grain-growth inhibitor is homogeneously distributed in the starting mixture before pressing the green bodies. Because of the strong challenge in increasing productivity and uniformity of the sinter charges the achieved homogeneity is still not sufficient and flaws still exist in the hardmetal such as inhomogeneously grown WC particles, representative of sources which shorten the lifetime due to breakage. Thus there is strong effort to design new recipes and techniques for enhancing the homogeneity. The main idea of this project is the application and proof of a new concept of adding the grain-growth inhibitor for increased homogeneity. This grain-growth inhibitor will not be added as a separate carbide to WC, which always represents a physical mixture - irrespective whether it is pre-alloyed or not - but will be alloyed together with tungsten in a W-Co-C-phase. This procedure is chosen, because WC does not dissolve any grain-growth inhibitor. In own research work is has been found that not only one but all three mainly grain-growth inhibitors VC, TaC and Cr3 C2 are soluble to an appropriate extent. If such an alloyed W-Co-C-phase is reacted with C it forms a high- quality hardmetal, whereas the grain-growth inhibitor is right at the position in the lattice where WC forms, i.e. in the position where the action of grain-growth inhibitor is best. The formation of high-quality hardmetals by such reactive sintering has been proven by Japanese researches, but without the new concept of grain-growth inhibitor distribution. In the research work the solubilities of the grain-growth inhibitors in the W-Co-C-phases should be investigated in detail first. Then the preparation of the various alloyed powders has to be investigated and third, sub-micrometer hardmetals have to be sintered from these powders. The hardmetals will be characterised in detail for their microstructure as well as for their properties for establishing the appropriate sintering condition. The phenomena occurring within sintering will be investigated by a variety of thermo-chemical investigations. The goal is to establish a new technique for obtaining more uniform hardmetals with a lesser number of flaws capable of extending the application field. All experimental techniques are industry-near so that in case of successful realisation of the ideas a scale up of the concept into industrial dimensions seems feasible. Because of the existing cooperation with several industry partners in Austria, Europe and USA not only several costly characterisation steps can be made for free but also a industry cooperation on the basis of these ideas seems possible.

Hardmetals (cemented carbides) are materials composed in principle of a hard and wear-resistant compound WC and a ductile Co binder phase. These materials are used for a variety of high-loaded parts such as micro-drills for production of printed circuit boards or of inserts for milling, drilling and cutting. For these materials there is a strong trend towards finer microstructures, also because of ongoing miniaturisation and the increasing interest for hardmetals for use in micro-machines. For the establishment of fine microstructures not only finest powders have to be employed but also grain-growth inhibitors have to be added in order to prevent the WC particles from growth. The grain-growth inhibitors such as VC, Cr3 C2 or TaC are either added to the WC+Co starting powder mixture or pre-doped WC is used for which the grain-growth inhibitor is added upon powder production. It is extremely important that the grain-growth inhibitor is homogeneously distributed in the starting mixture before pressing the green bodies. Because of the strong challenge in increasing productivity and uniformity of the sinter charges the achieved homogeneity is still not sufficient and flaws still exist in the hardmetal such as inhomogeneously grown WC particles, representative of sources which shorten the lifetime due to breakage. Thus there is strong effort to design new recipes and techniques for enhancing the homogeneity. The main idea of this project is the application and proof of a new concept of adding the grain-growth inhibitor for increased homogeneity. This grain-growth inhibitor will not be added as a separate carbide to WC, which always represents a physical mixture - irrespective whether it is pre-alloyed or not - but will be alloyed together with tungsten in a W-Co-C-phase. This procedure is chosen, because WC does not dissolve any grain-growth inhibitor. In own research work is has been found that not only one but all three mainly grain-growth inhibitors VC, TaC and Cr3 C2 are soluble to an appropriate extent. If such an alloyed W-Co-C-phase is reacted with C it forms a high- quality hardmetal, whereas the grain-growth inhibitor is right at the position in the lattice where WC forms, i.e. in the position where the action of grain-growth inhibitor is best. The formation of high-quality hardmetals by such reactive sintering has been proven by Japanese researches, but without the new concept of grain-growth inhibitor distribution. In the research work the solubilities of the grain-growth inhibitors in the W-Co-C-phases should be investigated in detail first. Then the preparation of the various alloyed powders has to be investigated and third, sub-micrometer hardmetals have to be sintered from these powders. The hardmetals will be characterised in detail for their microstructure as well as for their properties for establishing the appropriate sintering condition. The phenomena occurring within sintering will be investigated by a variety of thermo-chemical investigations. The goal is to establish a new technique for obtaining more uniform hardmetals with a lesser number of flaws capable of extending the application field. All experimental techniques are industry-near so that in case of successful realisation of the ideas a scale up of the concept into industrial dimensions seems feasible. Because of the existing cooperation with several industry partners in Austria, Europe and USA not only several costly characterisation steps can be made for free but also a industry cooperation on the basis of these ideas seems possible.