Synthesis, processing and mechanical characterization of ti(c,n)-based cermets through the combination of colloidal and powder metallurgy techniques
- Autores: Miguel de Dios Pérez
- Directores de la Tesis: Elena Gordo Odériz (dir. tes.), Begoña Ferrari Fernández (codir. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2017
- Idioma: inglés
- Tribunal Calificador de la Tesis: Antonio Javier Sánchez Herencia (presid.), Sophia Alexandra Tsipas (secret.), Raquel De Oro Calderón (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: e-Archivo
- Resumen
Although 90 years have passed since its invention, cemented carbide is still used today. This composite material consists of at least one hard and wear resistant phase; being mainly tungsten carbide (WC) embedded into a ductile and softer metallic binder phase from the iron group of metals (mainly cobalt, Co). Although WC is not the hardest carbide nor Co is the toughest metal, the combination of both brings to the composite material a high hardness value that is maintained at high temperature as well as high wear resistance and toughness. The reasons for the dominant role of Co are some unique properties of this binder phase as well as it is ternary system with tungsten and carbon. It is well known that the solubility of WC in Co is not only high but also strongly varies depending upon the temperature. The metallic phase is an alloy Co-W-C where the W and C are dissolved in a solid solution of Co. This two aspects are closely connected to the excellent wetting between WC and molten Co during liquid phase sintering (LPS) and their consequent mechanical properties such as high hardness, yield stress, toughness and strength.
However, different perspectives condition future developments in the field of hard materials. These investigations are motivated not only by economical factors due to the temporarily high and strongly fluctuating prices of Co and W metal powders but also due to technological and strategic aspects such as the low resistance to corrosion and oxidation of the WC-Co composite materials, the undesirable allotropic transformation of Co that causes a worsening of the mechanical properties and the strategic character of these raw materials. In addition to all the aspects discussed above, another consideration comes from the health perspective, which has increased the research activities to replace or substitute WC-Co totally or partly. In Europe, Registration, Evaluation, Authorization and Restriction of Chemical substances (REACH), so far has classified cobalt as very toxic for the human health. Also, the U.S. National Toxicology Program, NTP, states that the tungsten carbide-cobalt hard metal dust has been shown to be more toxic in combination than both pure cobalt and tungsten carbide by themselves in vitro studies. By the reasons pointed out before and the increasing demand of materials with optimized properties, one of the main topics of the actual research in the field of hard metals concerns the development of new metal-ceramic composites having comparable or superior properties than the commercial ones using more sustainable compositions and processing methods.
In this sense, Titanium Carbonitride based cermets have drawn great attention in an attempt to replace the traditional WC-Co hardmetals in specific applications. Notwithstanding their relatively lower bending strength as compared to hardmetals, Ti(C,N)-based cermets have an excellent and unique combination of physical properties such as high melting point, hardness at high temperature, wear and oxidation resistance and thermal conductivity. All these characteristics make cermets well adapted to the requirements for high performance wear parts and semi-finishing or finishing cutting tools. This fact offers the opportunity to use relatively cheap and abundant raw materials, thereby reducing the dependence on critical raw materials such as W or Co.
In most Ti(C,N)-based cermets, the binder phase is mainly composed of Ni, Co or a combination of both. However, there is an interest to find alternative binder compositions to replace them totally or partially, due to its classification as critical raw materials and the health risks. As it has been explained at the introduction, there are several studies that propose the use of Fe as a metal matrix for cermets, as it is non-toxic and cheaper than the other routes as well as strengthened by heat treatment. However, despite the considerable amount of research carried out, the application of such cermet system is limited, mainly due to two major issues: the poor wettability on Ti(C,N) particles during the liquid phase sintering (LPS) and the low toughness values resulting from the coarsening of the carbides. To overcome such limitations, the addition of alloying elements and compounds such as WC, VC, Mo2C, TaC, Cr, Mo, Ni has been proposed aiming to improve the wetting behavior between both phases during the liquid phase sintering as well as hinder the ceramic particle growth. However, as a result of these additions a typical core-rim microstructure can be formed and a careful selection of the amount of carbides should be done to avoid the harmful effects of brittle phases formed at the rim. In this work instead of adding secondary carbides, the addition of Ni does improve notably the wetting and solubility behavior of Fe on the Ti(C,N) particles.
Although the most common route used for the preparation of cermets is still the traditional mechanical milling, cermets have been also processed through spark plasma sintering (SPS), hot pressing (HP), self-propagating high-temperature synthesis (SHS), mechanical induced self-sustaining reaction (MSR), hot isostatic pressing (HIP) or a combination of sintering and HIP, under vacuum, nitrogen or argon atmosphere. In an attempt to obtain a well-controlled microstructure and a better dispersion of phases with lower energy consumption than the rest of the conventional techniques, colloidal processing emerges as an alternative for the processing of cermets. The advantages offered by the colloidal route have provided controlled microstructures through a very intimate mixture of the ceramic and metal phases, using particles in the micrometric range and avoiding mechanical milling.
The colloidal processing implies the formation of slurries in aqueous media, which is a drawback in the case of both-non-oxide ceramic and metal particles, due to the elevated reactivity of these materials with the suspension media. From an electrochemical point of view, water is one of the more complex liquid media to work with. It has a very high polar moment which requires tight control of the colloidal and chemical stability of the slurries to prevent the particle oxidation, while maintaining the interparticle repulsion networks and thus the slurry stability. However, in addition to the superficial stability of the particles within the dispersion, their flow conditions are another factor of vital importance for the colloidal shaping. The rheological behavior of the high solid content slurries must provide a homogeneous mixture of phases, as well as avoid risks of segregation that may arise during the processing of the bulk pieces.
Among the wide range of forming methods to obtain bulk specimens with low defect population, high density and well-controlled microstructure, colloidal and powder metallurgical techniques has emerged as an attractive and affordable approach to be successfully implemented for the fabrication of cermets. But also, the reduction in both, content and size of the binder phase, can be also achieved by bottom-up approaches, mainly profiting from the chemical routes such as electroless processes. In this sense, the present work is aimed to develop an alternative route to the processing of Ti(C,N)-based cermets. Different activations have been studied to synthesize Ni NPs onto the surface of micrometric Ti(C,N) particles, producing core-shell composited to explore all the aspects that this one-pot procedure can offer to the synthesis of metal-ceramic materials.
