Aqueous colloidal processing of w and wc-based composites, sintering and mechanical properties at high temperature
- Autores: Esperanza Macarena García Ayala
- Directores de la Tesis: Jose Ygnacio Pastor Caño (dir. tes.), Antonio Javier Sánchez Herencia (dir. tes.)
- Lectura: En la Universidad Politécnica de Madrid ( España ) en 2021
- Idioma: español
- Tribunal Calificador de la Tesis: Luis Llanes Pitarch (presid.), Elena María Tejado Garrido (secret.), Ana Maria de Oliveira e Rocha Senos (voc.), Ana Ferrández Montero (voc.), Luis Fernando García (voc.)
- Programa de doctorado: Programa de Doctorado en Ingeniería de Estructuras, Cimentaciones y Materiales por la Universidad Politécnica de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: Archivo Digital UPM
- Resumen
There have been many advanced technologies that operates at high temperature under severe conditions, such as those over the inner walls of the future fusion nuclear reactor (ITER, DEMO, etc.). Refractory metals such as tungsten are proposed as optimum materials to face these conditions. Pure ceramics, such as WC and metal ceramics are proposed also as alternative materials for the components that require operate at high temperature under high wear conditions or neutronic damage. For that reason, the design of complex compositions and microstructures is necessary to achieve reliable materials under these conditions. That makes this work investigate on colloidal processing W and WC materials, as well as their composites with nickel for softening of the sintering conditions.
Aqueous colloidal processing is proposed as an alternative to the solvent-less techniques. By the study of zeta potential and chemical analysis of powders suspended in water the optimal conditions for high solid content suspension preparation were determined at different pH conditions and considering the use a cationic dispersant (polyethyleneimine, PEI). The rheological studies have determined the maximum solid content through the Krieger-Dougherty model adjustment, so the time spent removing water after powder preparation can be diminished. Three different types of composite powders were prepared: WC-W, WC-Ni and W-Ni. For WC-W composites, the amount of metallic phase was fixed in 10, 20 and 50 vol.%, while for WC-Ni and W-Ni mixtures, the amount of metallic nickel phase was 5 vol.% (3 wt.%).
Three different sintering methods are used: Spark Plasma Sintering (SPS), Hot-Press Sintering (HPS) and Vacuum Sintering (VS). WC-W materials were sintered by SPS at 1700 and 1800 oC for 10 min and 60 MPa. The crystalline and chemical studies determined the high reactivity between W and WC during the thermal cycle, which gives place to the presence of a third species: tungsten semicarbide (W2C), which provoked all the W consumption. Mechanical tests at high temperatures for the W2C/WC composites demonstrated their excellent properties and performance at conditions similar to the ITER divertor walls. A self-reinforcement behaviour due to the appearing of metallic tungsten in the microstructure as the temperature approach to 1200 ºC is described.
On the other hand, mixture of WC with Ni and W with Ni, obtained by aqueous colloidal processing were sintered by the three mentioned sintering techniques and the differences between them studied. Two types of Ni powders were used: a commercial nickel (cNi) and a nanometric nickel synthesized in the lab (nNi). WC-Ni sintered by HPS achieved a density higher than 98%, which was not the case of SPS and VS sintered samples. SEM/EDX observations demonstrated that sintering was by a solution-precipitation mechanism that leaved the nickel located in the grain junctions, while there were slight carbon precipitations favoured by the thermodynamical conditions during the thermal cycle. For W-Ni alloys, successful results were obtained from VS process, achieving 96% density and a fully dense microstructure. In this case, W and Ni reacts while thermal treatment, giving an intermetallic species: NiW. For both WC-Ni and W-Ni materials, the presence of small amounts of Ni showed a clear softening of sintering conditions, which make the sintering of W and WC an affordable method for wide productions in terms of both technology and economic cost. Mechanical tests of these composites exhibited a ductilisation of the materials due to the presence of the nickel phase, which matches to the data found in the literature for this type of composites and alloys.
This thesis is framed within the national plan Project (MAT2015-70780-C4-1-P), entitled “Aqueous colloidal processing of W and WC-based composites, sintering and mechanical properties at high temperature”. This work has been developed combining the main lines of research of the two groups in which the experimental work has been carried out: the colloidal chemistry developed by the Tailoring through Colloidal Processing group (ICV-CSIC) and the mechanical properties knowledge of the CIME group (UPM)
