Graded tribological materials formed by electrophoresis

Technical and scientific knowledge for the colloidal processing of plate shaped components was established. ZrO(2)/Al(2)O(3) and WC-Co hard-metal graded plates are processed by electrophoretic deposition and sintering. ZrO(2)/Al(2)O(3) FGM plates with a ZrO(2)-rich core were successfully produced. The composition of the intermediate gradient layer could be changed by continuously adding a suspension with different composition to the start suspension with a peristaltic pump. In this way, the length, slope and composition of the gradient could be freely engineered. For the hardmetal FGM plates, the goal was to investigate the possibility to manufacture materials with a gradient in cobalt and/or TiC, Ti(C, N) content. Different Functionally Graded Hard-metals were prepared: - FGMs with a cobalt content varying from 6wt% on the hard side to 9-18 wt% on the soft side. The length scales of the gradients can differ from each other resulting in a shallow gradient, an intermediate gradient and a steep gradient. - FGMs with a gradient in TiC or Ti(C,N) from 0 up to 5-20 wt%. Finally, flat symmetrical WC-Co-TiC/WC-Co/WC-Co-TiC plates could be successfully processed providing that the TiC was added from a pre-mixed WC-Co-TiC suspension. A relatively smooth gradient interlayer with a thickness of minimum 0.5mm, between the homogeneous core and outer layers, was essential in order to avoid transverse cracks in the outer layers after sintering. In the next year, SNUN 1204 type cutting inserts with homogeneous outer layers with a thickness of 0.5mm containing 5wt% TiC or TiCN will be produced by EPD and sintering.

The most important prerequisite for the prediction of final shape and properties of FGM after sintering is the basic (generic) model development. A thermo-elasto-viscoplastic model was developed and adapted for the present problem of sintering of FGM. With the model distribution of temperature, density, strain rates and displacements, normal (radial and axial) stresses and shear stresses can be obtained. The model was coupled with heat transfer and grain growth process. Heating non-uniformity has been taken into account and time-slice (instant) temperature distribution was also obtained. The model was implemented for ABAQUS/Standard software, where quadric elements have been chosen for the meshing for fully coupled thermal displacement analysis. The materials properties have been collected over different sources and integrated into a special user subroutine UMAT.

During fabrication of FGM hardmetals (WC-MeC-Co), liquid phase migration occurs during sintering which may destroy the originally implemented gradient of composition or create other gradients of chemical composition and particle size distribution due to grain growth. Thus to control the process of liquid phase sintering of hard metals, the direction of cobalt (liquid phase) flow and quantitative parameters of this flow to evaluate the final spatial composition of the FGM component must be known. For these data a modelling equation for migration pressure of liquid phase in the WC-Co system has been obtained and pressure values have been calculated. The equations relating liquid volume fraction and chemical potentials at 1573-1673ºK have been calculated. Full set of surface energies, wetting, groove and dihedral angles of this system was calculated and the specific surface energies and angles have been calculated for WC-Co and WC-Co-TiC systems. In the new method, the only starting data required are temperature, cobalt and titanium carbide concentration. It is possible to calculate apparent migration pressure using e.g. an Excel file. This would allow predicting the resulting gradient of cobalt after sintering vs. carbide grain size, TiC additions and initial cobalt content for FGM manufacturing.

Electrophoretic deposition of hardmetal graded rods is carried out by immersing a green hardmetal rod shaped electrode in a suspension containing hardmetal powder. A layer of about 150µm can be deposited on a green homogeneous hardmetal rod that was used as core electrode, without cracking during drying. After CIPing and careful degassing and sintering, a crack free cylinder with a hard layer of about 100µm thick could be processed successfully. In order to deposit layers with a larger thickness, a dipping technique has been developed. This method consists of depositing layer by layer, subsequently immersing the electrode in the suspension for a few seconds and removing it from the suspension for drying during a longer period, using an automated dipping set-up. No crack formation or delamination on the hardmetal electrode was noted when using the dipping technique, providing electrodes with a suitable binder were used in a suitable suspension. A sintered coating thickness of 600µm could be obtained. This dipping technique can be used to deposit thick WC-Co-TiCN graded layers on green-machined WC-Co milling cutter blanks. Demo samples are available.

Scientific and technical know-how for the deposition of FGM materials on the inner surface of an outer electrode was established. A new EPD technique was developed for depositing on the inner surface of an outer electrode. This was done because the main problem with deposition on an inner electrode is that the deposit has to be removed from the electrode after EPD and this has to be done without damaging. In normal atmospheric conditions, the deposit dries very quickly due to the fast evaporation of acetone, shrinks and causes cracking. When depositing on the inner surface of an outer electrode, the outer cylinder, on which the powder is deposited, is closed at the bottom. The deposit will shrink during drying towards the centre electrode, thus away from the deposition electrode. Hence, cracking of the deposit during drying is easier to control since the deposit can shrink freely. Moreover, it is possible with this technique to produce a deposit without inner hole, by moving the inner electrode upward during EPD. In order to upscale this technique to produce rods with a sintered length of 100mm (needed for drill blanks) a 3-D rigid positioning device was constructed. In this way, the vertical positioning and displacement of the electrode can be controlled very precisely. This device enables to control the vertical displacement of the electrode, maintaining an excellent alignment between inner and outer electrodes. Moreover, an electrode displacement of 15cm is possible. Within the constraints of the project defect free sintered rods up to 50mm in length were produced using this technique.

Different steel substrates were coated with a WC-Co layer by means of electrophoretic deposition (EPD). After EPD, the hardmetal powder deposit on the steel was densified by means of hybrid microwave sintering. A densified and strongly adhering WC-Co layer was formed on HSS, stainless steel as well as low and medium carbon steel substrates by means of fast hybrid microwave heating. The optimum sintering time and temperature are strongly related to the steel grade. The EPD of WC-Co coatings is a very rapid processing technique with a typical EPD coating time between 3-20 seconds and a typical heat treatment time of 1000 seconds. In the up-coming months, thread former blanks will be covered with a hardmetal coating and subsequently hybrid microwave sintered. Small demo samples are available.

Because of the need to engineer the functionally graded region in the FGM plates very accurately, it is absolutely essential to establish a model of the EPD process allowing to directly correlate EPD, powder or suspension specific parameters with the gradient profile in the FGM plate. K.U.Leuven successfully processed plate-shaped functionally graded materials (FGM) by depositing from a circulating powder suspension to which a second suspension is continuously added during the process. The presented model enables to calculate the composition gradient in the FGM material from the starting composition of the suspensions, the EPD operating parameters and the powder-specific EPD characteristics. The model for FGM plates was verified for the deposition of WC-Co with a gradient in TiC. The EPD model is not restricted to the cemented carbide system, but can also be applied to the Al(2)O(3)-ZrO(2) system for example.

A cylindrical single mode microwave furnace was modified into a hybrid-sintering furnace with a SiC tube susceptor. The microwave furnace was designed and fabricated by MEAC. The microwave furnace consists of a 2.45GHz microwave generator with a continuously adjustable power output from 0-2kW, a cylindrical single-mode tuneable applicator, and a computer control system. Temperature control is performed by means of a two-colour pyrometer, directly focussed on the sample surface. The onset temperature of the pyrometer used is 700°C. The furnace can be operated in vacuum, air, nitrogen, argon or gas mixtures. Experimental results with oxide ceramics as well as cemented carbides proved the innovative and successful approach of the hybrid-heating concept in establishing well controllable and fully reproducible microwave-sintering cycles. Moreover, well-defined heating ramps can be established when using hybrid sintering. A variety of homogeneous CIPed ceramic powder compacts, including yttria-stabilised ZrO(2), ceria-stabilised ZrO(2) and Al(2)O(3), as well as Al(2)O(3)/ZrO(2) FGMs were hybrid sintered in air. The mechanical properties of hybrid sintered and conventionally sintered materials are comparable, but the sintering times during microwave hybrid sintering however are significantly shorter. Preliminary experiments with hardmetals were performed in order to find out whether the properties and microstructure of conventionally sintered (graphite furnace) WC/Co[6] samples could be reproduced by hybrid MW sintering. The results revealed that near fully dense samples with a homogeneous microstructure could be obtained, without free carbon or eta-phase.

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