MULTIDIMENSIONAL COMPOSITION SPACE EXPLORATION: HIGH-ENTROPY CARBIDES SYNTHESIS, STRUCTURE AND PROPERTIES
Open Access
- Author:
- Hossain, Mohammad Delower
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 17, 2021
- Committee Members:
- Jon-Paul Maria, Chair & Dissertation Advisor
Brian Foley, Outside Unit & Field Member
Allison Beese, Major Field Member
Stefano Curtarolo, Special Member
Ismaila Dabo, Major Field Member
John Mauro, Program Head/Chair - Keywords:
- High entropy carbides
Valence electron concentration
Ultra-high temperature ceramics
Sputtering
Density functional theory - Abstract:
- This dissertation investigate the synthesis, structure and properties of high entropy carbides, a new class of ultra-high temperature ceramics (UHTC). In the last decade, multiple space missions and resurrection of hypersonic vehicles and missile defense revived research activity in UHTC. Managing extreme environments at hypersonic speed requires innovative advanced materials with multifunctionality. Last five years or so, high entropy based ceramic materials are leading the research dynamics as new UHTC materials for hypersonic system with innumerable possibilities. As new materials understanding the thermodynamic and kinetic boundary conditions is critical with regard to materials synthesis and high temperature applications. The first principles calculations illustrate that high entropy carbides are only stable at high temperature and solid solution phase results at temperature > 2000 K. At that high temperature the configurational entropy dominates the energy landscape and counterbalance enthalpically favourable phases formation with negligible contributions from vibrational and electronic entropy. Thesis further explores materials chemistry effect on predicting boundary conditions toward solid solution phase formation and higher valence electron high entropy carbides are more resilient to go into solid solution. Valence electron concentration (VEC) effectively describes high entropy carbides mechanical properties: hardness, bulk modulus, shear modulus, and poisson’s ratio etc. High entropy carbides with VEC 8.4, show the highest hardness due to filling of strong _ bonding states which resist shear deformation. Below or above that VEC point, hardness decreases due to filling or emptying of energy orbitals that facilitate shear deformation. The experimental nanoindentation hardness measurements corroborate theoretical prediction and highest hardness of 30 GPa is achieved at VEC 8.4. Furthermore, based on electronic population one can fine tune plastic deformation characteristics of materials and produce ceramics with high hardness and ductility. The pugh modulus which is the ratio of shear modulus to bulk modulus act as benchmarking parameter to describe ductile-brittle transition of carbides. As VEC controls the elastic properties of materials and in turn pugh modulus varies as a function of VEC. Thereby a high entropy carbides (VNbTaMoW)C designed based on VEC and pugh modulus calculation reveals ductile characteristics. This multicomponent carbide synthesized with high-power impulse magnetron sputtering (HiPIMS) and measured hardness is comparable to binary TaC but tolerates three time more stress before fracture. VEC further captures transition metals affinity for carbon as such different high entropy carbide synthesizability with HiPIMS can be predicted based on VEC. The reactive gas flow rate during HiPIMS deposition produce three different types multicomponent carbides: a metallic carbon deficient carbide; a stoichiometric ceramic carbides; and nanocomposite carbide contains carbide plus excess carbon. The ceramic transition zone between metallic and nanocomposite carbide depends on materials chemistry. The lower VEC carbides with higher affinity for carbon show a comprehensively larger transition zone whereas for higher VEC carbides the transition zone shrinks. In addition, HiPIMS deposition allows stoichiometric crystal growth with close regulation over microstructure transformation which is elusive in case of regular rf and DC magnetron sputtering. This dissertation establishes VEC is a simple but powerful descriptor to predict synthesizability, thermodynamic, mechanical and structural properties of high entropy carbides. Based on the electron population and periodic properties of elements, it is possible to predict and synthesize a new high entropy carbides with enhanced physicochemical properties.