Abstract
The influence of grain growth inhibitors and powder size on the microstructure and mechanical properties of ultrafine and nanostructured cemented carbides was researched. Three different WC powders, with an addition of different type and content of grain growth inhibitors GGIs, VC and Cr3C2 and with dBET grain sizes in the range from 95 to 150 nm were selected as starting powders. Four different mixtures with 6 and 9 wt. % Co were prepared. The consolidated samples are characterized by different microstructural and mechanical properties with respect to the characteristics of starting powders. Increased sintering temperatures led to microstructural irregularities in the form of a discontinuous WC growth, carbide agglomerates and abnormal grain growth as a consequence of coalescence via grain boundary elimination. The addition of 0.45% Cr3C2 contributed to microstructure homogeneity, reduced discontinuous and continuous grain growth, and increased Vickers hardness by approximately 70 HV and fracture toughness by approximately 0.15 MN/m3/2. The reduction of the starting powder to a real nanosize of 95 nm resulted in lower densities, and significant hardness increase, with a simultaneously small increase in fracture toughness. The consolidation of real nanopowders (dBET < 100 nm) solely by conventional sintering in hydrogen without isostatic pressing is not preferred.
Highlights
Ultrafine cemented carbides with a dWC grain size in the range from 0.2 to 0.500 μm and nanostructured cemented carbides with a dWC grain size < 0.200 μm are consolidated from ultrafine and nanosized WC starting powders, which are characterized by a big surface area and a very high sintering activity
Continuous growth is characterized by uniform growth of WC grains, while discontinuous growth characterizes growth of isolated WC grains or groups of WC grains, which grow faster and larger compared to the surrounding grains [8,9,10]
Both growth processes must be avoided in order to obtain satisfactory properties in the sintered products since mechanical properties of ultrafine and nanostructured cemented carbides are directly dependent on the developed microstructure: WC grain size, mean free path of Co and contiguity of WC grains [2,10]
Summary
Ultrafine cemented carbides with a dWC grain size in the range from 0.2 to 0.500 μm and nanostructured cemented carbides with a dWC grain size < 0.200 μm are consolidated from ultrafine and nanosized WC starting powders, which are characterized by a big surface area and a very high sintering activity. Continuous growth is characterized by uniform growth of WC grains, while discontinuous growth characterizes growth of isolated WC grains or groups of WC grains, which grow faster and larger compared to the surrounding grains [8,9,10] Both growth processes must be avoided in order to obtain satisfactory properties in the sintered products since mechanical properties of ultrafine and nanostructured cemented carbides are directly dependent on the developed microstructure: WC grain size, mean free path of Co and contiguity of WC grains [2,10]. Microstructure characteristics and mechanical properties of ultrafine and nanostructured cemented carbides were developed by conventional liquid phase sintering in hydrogen with respect to characteristics of the starting powder, and GGIs and the grain size of starting powders were researched. The research was performed to examine the possibility of consolidation by sintering in hydrogen atmosphere
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