Topic
SiC-bonded diamond materials – a newly developed class of materials – cover an exceptional range of properties, making them ideal for a wide variety of applications in wear protection and thermal management. In many of these areas, new technical requirements are emerging that conventional materials can no longer fulfill. This is where components based on SiC-bonded diamond materials can become a game changer.
Wear-resistant materials are of interest for a variety of applications where increased service life is required. Above all, they contribute to increasing the efficiency of production processes, improving reliability and minimizing safety risks. As typical diamond-based wear-resistant materials are manufactured using high pressures, components based on them are limited in terms of size, geometry and cost. SiC-bonded diamond materials, on the other hand, can be produced cost-effectively and in complex geometries using conventional ceramic technologies. The manufacturing process is similar to that of silicon infiltrated silicon carbide (see production process). Mill components, classifier vanes or sandblasting nozzles have already been realized and successfully tested in applications. Mechanical seals and plain bearings are also under development.
Wear protection components made of SiC-bonded diamond material:
The wear resistance of the diamond ceramics was tested using a wear test in accordance with ASTM G65, whereby a rubberized steel wheel was used in combination with an aqueous standard sand suspension to achieve a higher wear effect. The results show that the wear resistance of diamond ceramics is even much higher than that of other structural ceramics such as silicon nitride (Si3N4) or boron carbide (B4C). The surface analysis of the diamond ceramics after the wear tests showed that only a part of the silicon carbide phase was removed and that the diamond grains hardly showed any friction marks. The wear resistance in the sandblasting test was thus 10 times higher than that of dense commercial B4C materials. The measurement of tribological properties using an oscillating friction wear test with steel and Si3N4 balls showed the great potential of this new material for tribological applications. Although the new diamond ceramics contain a maximum of 60 % diamond by volume, they have a coefficient of friction under dry running conditions of only 0.1 to 0.2. These values and the wear resistance are similar to those of polycrystalline diamond materials (PCD) and CVD diamond coatings.
Super-hard materials compared to typical wear materials. SiC-bonded diamond is the only superhard material that does not require high-pressure technology in component manufacture (*High-pressure process HPHT, **Coating); own illustration according to McMillan, 2002).
Results of sandblasting wear tests of Al2O3, SSiC, B4C bonded diamond materials (diamond size (50/5 μm)).
Due to their low thermal expansion, high thermal conductivity and high rigidity, SiC-bonded diamond materials can also be used in thermal management, for example for heat sinks. Compared to standard materials such as silicon, copper, aluminum, aluminum nitride or silver, they have a higher thermal conductivity: With a diamond content of 60% by volume and a diamond grain size of 100 μm, thermal conductivities of up to 640 W/m*K can be achieved. The coefficient of thermal expansion is less than 5*10-6 1/K. Even at application temperatures of 200 °C, the thermal conductivity is between 400 and 450 W/m*K.
SiC-bonded diamond materials can also be used as a substrate in high-end power electronics, where extreme power dissipation occurs in the smallest of spaces at elevated temperatures or where a high degree of functional compression and miniaturization is required with the same performance. They are also interesting for power converters with direct cooling. Due to the very good corrosion resistance of SiC-bonded diamond materials, converter components can be structured and brought into direct contact with coolants.
At the beginning of the production process, diamond powder is mixed with the appropriate binders and solvents and granulated. The mixture is then pressed or converted into a molded body using other shaping technologies. During the subsequent pyrolysis in an inert atmosphere, parts of the binder are converted into carbon, so that the molded body now consists of diamond and carbon. This porous molded body is then infiltrated with liquid silicon at 1450 to 1600 °C. During infiltration, the silicon reacts with the carbon and partly with the diamond grains. In the resulting structure, the diamond grains are firmly embedded in a three-dimensional SiC lattice. There is a direct chemical bond between the diamond and the SiC. Depending on the diamond powder used, different microstructures can be produced and thus properties can be specifically modified. So far, diamond contents of up to 60 % by volume and residual silicon contents of less than 5 % have been achieved. The diamond ceramics therefore exhibit outstanding mechanical and corrosive stability.
Components can be produced both as compact components and as SiC components with 300 to 500 µm thick layers in highly stressed areas. As the green parts can be green-machined after pyrolysis and the volume change during silicon infiltration is almost zero, diamond ceramic components can be produced in a wide variety of geometries – including complex geometries – close to the final shape and therefore very cost-efficiently. Due to the high wear resistance of diamond ceramics, finishing by cutting or grinding is limited. Finishing by lapping or polishing the surface is possible, but time-consuming. Laser cutting and electrical discharge machining (EDM), on the other hand, are attractive alternatives for machining diamond ceramics.
Production of SiC-bonded diamond ceramics.
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Fraunhofer Institute for Ceramic Technologies and Systems IKTS