Thesis of farzin arianpour

Effects of Nano and Micron Sized Additives on the Properties and Microstructure of TaC-HfC System

By Farzin Arianpour

Supervisors: Prof. Farhad Golestanifard, Prof. Hamid Reza Rezaie

Iran University of Science and Technology

Advisors: Prof. Ferhat Kara, Anadolu University, Turkey, Prof. Gilbert Fantozzi, INSA Lyon University, France

January 2016

Abstract: Ultra high temperature ceramics (UHTCs) are a class of refractory ceramics showing superior stability at temperatures higher than 2500°C. They are mainly classified as borides, carbides and nitrides of transition metals of groups IV-VI of periodic table of elements. Most of the efforts in this field have focused on commonly used borides (HfB2, ZrB2), nitrides (ZrN, TiN), carbides (TiC, TaC) and their composites. Tantalum carbide (TaC) and hafnium carbide (HfC)  are binary compounds of metallic tantalum and hafnium with carbon which are extremely hard, brittle, refractory (melting points over 3900°C) with metallic electrical conductivity and cubic crystal structure.Binary 4:1 mole ratio TaC-HfC solid solution (Ta4HfC5) is considered as the most refractory material with the melting point over 4000°C and has superior physical and mechanical properties.

 In this research, Ta4HfC5 based samples were synthesized and consolidated using carbide materials and different amounts of nano and micron sized additives such as MoSi2, B4C, SiC and CNT by means of spark plasma sintering (SPS) at 2000°C. The nearly full dense monophase Ta4HfC5 basedspecimens were fabricated via MoSi2 as additive with a relative density value higher than 99%. Mechanical tests revealed values of 18-19 GPa and 4-4.3 MPa.m1/2, for average Vickers hardness and fracture toughness of the composites, respectively. Densification behavior of the samples during sintering was investigated by interpretation of SPS data. Formation of solid solution was analyzed via X-ray diffraction and scanning electron microscope (SEM). It is proposed that at the intermediate stage of sintering, mass transfer can be accelerated by liquid phase and viscous flow mechanisms. The binary TaC-HfC solid solution formation enhanced the densification process at the final stage of sintering. Also the effects of micron and nano sized SiC additives were compared in this system. It revealed that nano SiC improves the densification of the system up to more than 98% via solid state mechanism in the amounts of less than 3 wt.%. The B4C additives also show interesting solid solution binary boride phase formation during sintering. The CNTs containing samples are investigated via Raman spectroscopy analysis showed the well survivability of CNTs during SPS sintering condition.