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Nanodyne advances ultrafine WC-Co powders

Ultrafine and nanoscaled hardmetals show mechanical properties like hardness and bending strength which lie way above conventional fine or submicron grained hardmetals. To achieve such fine microstructures very fine WC starting powders as well as grain growth inhibitors such as Cr₃C₂ or VC are needed. To study the grain growth inhibition in ultrafine hardmetals investigations on samples made from nearly nanoscaled WC and Co starting powders with and without the addition of Cr₃C₂ were done. For studying the dissolution behaviour of Cr₃C₂ and the evolution of density, magnetic properties and lattice parameters of WC during sintering, interrupted sintering experiments were carried out. Thermal analysis techniques including TG-MS and DSC were used, to link the observed changes to expected reactions. The results show that grain growth inhibitors greatly influence the sintering behaviour already way below the eutectic melting of WC-Co. Especially dissolution of Cr₃C₂ and homogenous distribution of Cr within the samples already starts below 800 °C with the reduction of W surface oxides and the creation and spreading of Cr oxides. The findings are relevant for optimising sintering regime, composition (amount of grain growth inhibitors) as well as the microstructure and mechanical properties.

In this work, a core–shell structured precursor was used to prepare WC–Co composite powders, and then the bulk materials has been fabricated using rapid hot–pressing sintering (HP) technique. The microstructure and phase composition were characterized by XRD, TG–DSC, SEM and TEM. The Vickers hardness, transverse rupture strength and relative density were also measured. Owing to the special precursor, the pure phase composite powders with an average grain size of 80 nm were obtained in short–term at mild condition. The as–prepared bulk sample exhibits homogeneous microstructure and good mechanical properties. Compared with the conventional methods, both the time and temperature in present route of in–situ reaction and HP sintering are apparently reduced.

A nanostructured WC–12Co powder was sprayed using three different HVOF gun-fuel combinations in which the fuels were kerosene, hydrogen and propylene. Using particle diagnostics, significant differences were observed in the particle temperatures and velocities generated using these various combinations. Important differences were identified when comparing various aspects of coatings produced using either hydrogen or propylene as the fuel. When using hydrogen, deposition efficiencies were higher, the coatings were harder and the extent of carbide degradation was less than when propylene was employed as the fuel.

The concept of strengthening a binder cobalt phase in P30 and P20 hardmetals by means of heat treatment in a salt bath and in an argon atmosphere has been used. The physicochemical modifications of the binder cobalt phase in hardmetal tools after heat treatment have effected an improvement of strength, tribological properties and cutting abilities. The results of cutting tests using PVD and carbon vapor deposition (CVD) coated inserts were compared. Technical and economical analysis confirmed the numerous advantages of the presented heat treatment technology.

The paper reports the processing of a homogenous cobalt tungstate salt synthesised by co-precipitation starting from ammonium paratungstate and cobalt hydroxide. Different processing parameters such as temperatures and times of reduction and carburization and the composition of the carburization gas were studied. Powders of W and Co were obtained after the reduction of the precursor at 600 °C, 650 °C and 700 °C. The specific surface area of W-Co powders increased with decreasing reduction temperature. The carburization was carried out at 700 °C for 3 hours in different CO/CO₂ — mixtures. The extent of the reaction was found to depend on the CO/CO₂ — ratio. A nanophase WC-Co powder agglomerated at the micrometer scale was obtained after carburization in a 90%CO/10%CO₂ gas-mixture.