Difussion-controlled in-situ surface modification of cemented carbonitrides for fabrication of gradient microstructures

As much as 70% of the sintered cemented carbides tool inserts which are used for cutting operations (mainly steel) are surface modified mainly by chemical vapor deposition (CVD) methods in order to enhance the tool lifetime. The deposited layers consist (of a sequence) of hard constituents (e.g. TiN, TiC, TiCN, Al2 O3 ) which have a higher resistance towards interaction with the machined material (mainly steel) and air resulting in much better cutting properties than uncoated inserts. The costs for the coating of the cemented carbides which is performed in a completely separated process reach 25% of the production costs of the whole toot insert. From the view of a materials engineer the most alternative promising way in the fabrication of such materials is the modification of the sintering cycle in connection with a modification of the gas phase leading to a so-called functional gradient at the surface. Such processes (e.g. addition of nitrogen at a specific point in the time-temperature sintering cycle) are performed within the sintering cycle of the cemented carbides - in situ - and the occurrence of a modified surface structure is controlled by diffusion of the various constituents in turn dependent on the thermodynamics of the material system. This process modification may increase the sintering time and complicate the sintering process (with some increase of production costs) but a much more expensive separated second production step such as a coating process could be avoided. A potential of the in situ surface modification is that it is also possible to create of a much flatter gradient leading to a smooth variation of surface properties to bulk properties. Because of this smooth variation the compatibility of substrate material (cemented carbide) with layer material (pure carbides, nitrides,..) could be increased so that the functional-gradient hardmetals could even outperform coated materials. Diffusion-controlled in-situ surface modification can cover the whole range of steepness of gradients and indeed result in surface layers which are similar to coated hardmetals as well as in layers with a very smooth variation of composition and properties. In the present project a variety of sintering experiments with near-commercial as well as model alloys will be carried out at low pressures as well as at higher-than-ambient pressures (sinter hipping). In addition, the application of microwaves on the surface modification process will be investigated. The microwaves will be used for both, the heating of the samples together accelerated sintering of the compacts as well as for an activation of reactive species in gas phase. The aim of the project is to get insight into the diffusional behaviour of the various species as a function of the process parameters in order to gain knowledge on the phenomena which establish a surface- modified multi-component material covering a hard phase at the surface. According to literature this has hardly been investigated nor have the metallurgical processes been elucidated. It is one of the most promising fields in modern cemented carbonitride science and technology.