Microstructural and micro-compositional analysis of WC/Co and Fe-Cu-C powder materials, prepared by a microwave sintering method
Recently, significant developments and advances have taken place in the field of microwave processing of ceramics and metals. Microwave processing of materials mostly has been confined to ceramics, inorganic and polymeric materials. There are few detailed reports on microwave processing of metallic materials. The main reason for this is due to the misconception that all metals reflect microwave and/or cause plasma formation, and hence cannot be heated. Only recently has microwave sintering been effectively and efficiently applied to powder metals (Roy 1999). Here, in this study we have conducted a microstructural investigation using Scanning Transmission Electron Microscopy (STEM) of powder metals and cemented carbides prepared by conventional and microwave sintering methods to examine the differences between two kinds of samples. Microwave heating of materials is fundamentally different from conventional conduction-convection heating. It is a sensitive function of the material being processed. Microwaves are electromagnetic radiation with wavelengths ranging from about 1 mm to 1 m in free space and frequencies between 300 GHz to 300 MHz, respectively. However, most research and industrial activities involve microwaves only at 2.45 GHz and 915 MHz frequencies. Microwave processing offers several advantages: rapid heating, shorter processing times, uniform heating and fine microstructures (Clark 1996). Tungsten Carbide - Cobalt (WC/Co) based hard metal composites due to their unique combination of hardness, toughness and strength, are universally used as cutting and polishing tools, and in drilling operations underground. The production of WC/Co components involves the mixing of WC powders with cobalt binder, and then compacting the mass into a pre-sintered green body followed by the sintering. Conventional methods for sintering of WC/Co green bodies involve high temperature and lengthy sintering cycles, and consequently undesirable grain growth occurs in the presence of liquid Co phase. This has an adverse effect on the mechanical properties of the tool. Microwave sintering offers an alternative method of sintering and providing finer microstructures without using grain growth inhibitors. In 1991, J. P. Cheng in a Ph.D. thesis (1991) first showed that WC/Co composites could be sintered in a microwave field. Gerdes and Willert-Porada (1996) also reported the microwave sintering of WC with improved mechanical properties. In another independent work, Cheng et al.