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Graded tribological materials formed by electrophoresis


Different ceramic powders were characterised with respect to their electrophoretic behaviour in different organic suspensions. Based on these results and the preferences of the industrial partners, priority was given to the development of functionally graded materials (FGMs) based on WC-Co with additives of VC, TiC, TiN and Ti(C,N) on one hand and ZrO2-based ceramics on the other hand.
Hardmetal FGMs were deposited in a newly developed deposition set-up for graded materials. FGMs with a cobalt gradient varying from 6 wt% on the hard site up to 9-18 wt% on the soft side were produced. After CIPing and sintering at 1290°C, no open porosity was found. Different WC-Co/WC-Co-TiC FGMs with a TiC gradient were engineered by the EPD process. Sintering experiments showed that precise control of the TiC content in the gradient during EPD is essential in order to control the Co content after sintering.
Because of the need to engineer the functionally graded region in the FGM plates very accurately, it was absolutely essential to establish a kinetic model of the EPD process allowing to directly correlate EPD, powder and suspension specific parameters with the gradient profile in the FGM plate. The developed 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 asymmetric FGM plates was verified for the deposition of WC-Co with a gradient in TiC.
In order to be able to deposit symmetrical WC-Co-TiC/WC-Co/WC-Co-TiC FGM plates, the model for asymmetric plates was extended to allow the prediction of symmetric plates. Flat symmetrical WC-Co-TiC/WC-Co/WC-Co-TiC plates could be processed. A relatively smooth gradient interlayer with a thickness of minimum 0.5 mm, between the homogeneous core and outer layers was essential in order to avoid transverse cracks in the outer layers after sintering. A very good correlation was observed between the experimental and predicted profiles in the symmetrical plates.
With respect to the electrophoretic deposition of hard metal graded rods, a layer of about 150 µm can be deposited on a green homogeneous hard metal 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 on the hard metal electrode was noted by such a dipping technique, providing electrodes were used containing a binder and the same ingredients in the suspension. A sintered coating thickness of 600 µm could be obtained.
Cylindrical ZrO2-based FGMs with a continuous gradient were developed. Tribologically interesting functionally graded composites of Al2O3/ZrO2 and Y-ZrO2/Ce-ZrO2 were prepared by electrophoretic deposition and pressureless sintered in air. The obtained composites have excellent mechanical properties. The most challenging problems in the FGM processing were found to be the cracking of the deposit during drying and the cracking of the sintered body during heating.

A new EPD technique was developed for making ZrO2-based FGM drill blanks by 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. Moreover it is possible with this technique to produce a deposit without an inner hole. By moving the inner electrode upwards during EPD, the central hole can be filled and a fully dense cylinder can in principle be obtained. It was also shown that electric field calculations are necessary to obtain electrode geometries, which give a uniform electric field distribution on the deposition electrode. In order to upscale this technique to produce a rod with a length of 100 mm, a 3-D rigid positioning device was constructed. In this way, the vertical positioning and displacement of the electrode can be controlled very precisely. Homogeneous Al2O3/ZrO2 cylinders with a hemispherical end and a total sintered length of about 50 mm could be obtained successfully. Within the constraints of this project it was not possible to produce longer cylinders.
A cylindrical single mode microwave furnace was modified into a hybrid-sintering furnace. The microwave furnace consists of a 2.45 GHz microwave generator with a continuously adjustable power output from 0-2 kW, a cylindrical single-mode tuneable applicator, and a computer control system. 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 ZrO2, ceria-stabilised ZrO2 and Al2O3, as well as Al2O3/ZrO2 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/Co6 samples could be reproduced by hybrid MW sintering. The results revealed that near full density hardmetal samples with a homogeneous microstructure could be obtained, without free carbon or eta-phase.
Another application realised with the hybrid heating concept is the densification of hardmetal powder layers deposited on steel substrates by means of electrophoretic deposition. The possibility to coat different steel substrates by means of electrophoretic deposition (EPD) with a WC-Co layer was investigated. 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.
For simulation of the sintering process, a numerical model and FEM programs were developed. The collection of reliable data was found to be the most difficult and time-consuming task. Many data at high temperatures are not available. Nevertheless, the respective correlation equations have been obtained. A micromechanical model was used for calculation of the basic mechanical and thermal properties. This model was successfully used in the sintering simulations. With respect to the initial materials evaluation and properties calculation, room temperature properties of most materials under investigation were collected from literature sources. The variation of these properties with temperature however was found to be documented on a very limited scale. Nevertheless, the data collected were used in the calculations and the respective correlation equations have been obtained.
Thermodynamic evaluation of WC-Co-TiC system has been performed with THERMOCALC software. Optimised modelling equation for migration pressure of liquid phase in the WC-TiC-Co system has been obtained and pressure values have been calculated. Using assessed data for surface energy and angles, it is possible to calculate apparent migration pressure. This allows predicting resulting gradient of cobalt etc. concentration after sintering vs. carbide grain size, TiC additions and initial cobalt content for FGM manufacturing. The grain size of the WC is the most critical factor for liquid migration, which in the most cases overcomes influence of TiC additions (e.g. wetting effect and thermodynamics of the phases equilibrium). The results of the experimental feedback evaluation for FGM specimen show rather good correlation for liquid re-distribution and TiC final concentration with the experimental data. It is possible to evaluate any suitable type of WC-TiC-Co FGMs in respect to their final composition prediction.
A high-resolution optical dilatometry method was developed to measure dimensional changes of specimens in 2-D simultaneously. This gives a unique opportunity to quantitatively observe the sintering behaviour of graded specimens.
Objectives and content At present, there is a tremendous industrial need for materials with major engineering applications in these situations where tribology is important, exemplified by machining and metal forming. The majority of these applications are currently served by HSS and hard metals with or without surface coatings. New innovative materials or improved existing materials that combine high hardness, high toughness and excellent strength with an excellent wear resistance under operation conditions however are needed. The goal of this basic research project is to investigate the possibility to manufacture materials with a functional gradient of mechanical properties such as hardness, toughness and wear resistance. For example, complex shape materials with a high toughness core and a hard rim that can be used as wear parts or for the fabrication of cutting tools with complex geometry's such as drills. These innovative graded materials will be engineered in order to achieve improvements in mechanical properties and wear resistance of the final component and are distinguished from isotropic materials by gradients of composition, phase distribution texture, and related functional properties.

The broad objectives of this basic research proposal are:
- To develop, refine and optimise the processing route for graded materials, shaped by electrophoretic deposition (EPD) and densities by classical pressureless sintering and sinter HIPping and by microwave heating.

Graded materials with complex geometry will be developed for three material classes: WC-Co hardmetals, TiCN-based cermets and ZrO2-based ceramic matrix composites. To establish a fundamental base for the design and processing of graded materials incorporating data on material properties and processing characteristics aiming in particular at facilitating the processing of graded innovative materials. To assess the performance of selected graded microstructures in specific industrial application conditions for rotationally symmetric cutting tools, punching tools for metalworking and rock drilling tools. Although this is a proposal for basic research, it may be viewed as the demonstration of performance potential of a new generation of electrophoretic deposited tribological materials for 21st Century Engineering. BE97-4176


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