Russian scientists have developed new superhard metal ceramics
Tungsten carbide-based hard alloys with increased (6-10%) cobalt content are widely used in the machine tool industry. These materials are often referred to as sintered carbides - the basic materials for milling cutters and cutting tool inserts which are used for turning or milling operations to manufacture parts with the required geometry and dimensions. The world leader in carbide development is the Swedish company Sandvik Coromant, whose products currently dominate the Russian machine tool market. The company's nanocomposite-coated tools are used by many Russian machine-building companies for milling and turning metal products.
"The introduction of a highly ductile cobalt phase into initially brittle tungsten carbide can improve its crack resistance (fracture toughness), but reduces its strength, corrosion resistance and the maximum temperature at which these materials can be used. Therefore, one of the challenges in the development of hard alloys is to reduce the concentration of cobalt," notes Professor Alexei Nokhrin, head of the Materials Diagnostics Laboratory at the UNN Physics and Technology Research Institute (PTRI).
Researchers of Lobachevsky University's PTRI and the Baikov Institute of Metallurgy and Materials Science (IMET) of the Russian Academy of Sciences (Moscow) have developed new superhard ultrafine-grained ceramics based on pure tungsten carbide and hard alloys with ultra-low cobalt content (0.3-1% Co), which can be used for making wear-resistant metal-cutting tools. The ceramics combining high hardness and crack resistance were fabricated at the UNN PTRI by high-speed spark plasma sintering. Tungsten carbide nanopowders for the ceramics were produced in the Baikov Institute of Metallurgy and Materials Science using a unique technology of direct current (DC) arc plasma synthesis.
"An important element of the manufacturing process is a new method for introducing cobalt into nanopowders. In order to produce hard alloys with high crack resistance, each tungsten carbide particle must be surrounded by a ductile cobalt phase during sintering. Otherwise cracks can easily form at the boundary under load, destroying the tool's cutting edge and resulting in the flaking of the coatings," continues Alexei Nokhrin, one of the authors of this study.
Traditionally, cobalt is introduced into tungsten carbide by stirring in special mills and then sintering at high temperatures above the melting point of the cobalt. However, this method is not applicable to nanopowders because they form agglomerates and it is almost impossible to distribute cobalt at such small concentrations evenly in the bulk of the sample. In addition, liquid-phase sintering at elevated temperatures would result in abnormal growth of tungsten carbide nanoparticles and the formation of undesirable brittle phases in the structure, reducing the crack resistance of the ceramics.
Therefore, an ultra-thin (nanoscale) layer of cobalt was deposited on the surface of the synthesized tungsten carbide nanopowders by chemical-metallurgical methods and then the obtained powders with the structure "WC core - Co shell" were sintered under pressure at very high heating rates (100-300° C/min) at moderate temperatures. The combination of the unique capabilities of the new technologies made it possible to obtain samples of cutting tool inserts made of hard alloys with a very low cobalt content (not more than 1%) and with a very small grain size. The materials
developed by UNN PTRI and IMET RAS researchers have demonstrated unique hardness and crack resistance characteristics which are unattainable using traditional powder metallurgy technologies.
The results of this research were published in Ceramics International (impact factor: 3.83) and Journal of Alloys and Compounds (impact factor: 4.650).