Development of cermet microstructures during sintering
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CERMETS are Ti(C,N) based powder metallurgical products manufactured by liquid phase sintering. Generally, sintering of cermet materials involves several different raw material powders, giving rise to a complex sintering process and microstructure. Several authors have reported results concerned with microstructural evolution during sintering of different cermet materials.[1–6] However, the mechanisms behind the formation of cermet microstructures during sintering are not yet clear. Previous work presents X-ray diffraction data on the dissolution of the different starting powders of cermet materials interrupted at different stages of the sintering cycle.[3,7] It was found that some powders start to dissolve before a melt is formed, which indicates that parts of the cermet microstructure are formed during solid state sintering. The microstructure of a fully sintered cermet material consists of hard carbonitride grains in a tough metallic binder phase. The hard grains are small (⬇1 m), whereas the binder phase has a substanially larger grain size and embeds a large number of hard grains. Often, the carbonitride grains have a core-rim structure,[8,9] where the cores are undissolved titanium carbonitride powder particles.[10] The rim is also a titanium based carbonitride phase where some of the titanium sites in the lattice have been occupied by heavier elements such as tungsten, tantalum, niobium, vanadium or molybdenum. It is divided into two parts, an inner and an outer rim, where the inner rim lies between the core and the outer rim. The composition of the inner and outer rim depends on the starting powder mixture and the sintering parameters, but the inner rim is always enriched in heavier elements compared with the outer rim.[11] The core and the rim have the same crystal structure and the core-rim boundary is epitaxial. Dr. J. ZACKRISSON and Prof. H.-O. ANDRE´N are with the Department of Experimental Physics, Chalmers University of Technology, SE-412 96 Goteborg, Sweden. Dr. U. ROLANDER is with AB Sandvik Coromant, SE-126 80 Stockholm, Sweden. Manuscript submitted June 19, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
This work deals with the formation of cermet microstructures during sintering. In particular, the inner rim formation and solid-state sintering were of interest. The investigated materials were sintered from a Ti(C, N)-TiN-WC-Co powder mixture and sintering was interrupted at different stages of the sintering cycle. The small carbonitride grain size and the importance of localizing carbon and nitrogen in the different microstructural features implied that most of the microstructural characterization was performed using transmission electron microscopy (TEM) and energy-filtered transmission electron microscopy (EFTEM). Energy-dispersive X-ray analysis (EDX) and electron energy-loss spectroscopy (EELS) in combination with TEM were used for quantitative microanalysis. Scanning electron microscopy (SEM) was utilized to obtain an overview of the microstructures. II. EXPERIMENTAL DETAILS A.
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