Investigating the effects of configurational entropy on the hardening of transition metal-based carbonitride ceramics
Restricted (Penn State Only)
- Author:
- Ryan, Caillin
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 12, 2025
- Committee Members:
- John Mauro, Program Head/Chair
Jon-Paul Maria, Major Field Member
John Mauro, Major Field Member
Md Haque, Outside Unit & Field Member
Timothy Eden, Chair & Dissertation Advisor - Keywords:
- Hardness
Ceramics
High Entropy Carbonitrides (HECNs)
TiCN
Configurational Entropy
Microindentation
Nanoindentation
Entropy-Hardening
Alloys
FAST
Single-phase
Multiphase
Cations
Anions
Synthesizability
Hardness-Standardization
Physiochemical
Elastoplasticity - Abstract:
- The demand for hard materials capable of withstanding extreme environments and mechanical loads has always been necessary to advance technological areas, such as industrial abrasives, cutting tools, bearings, rotary/hammer bits, turbomachinery, ballistic protection, and hypersonic-related applications. The hardest materials currently available are diamond, cubic/wurtzite boron nitride, and boron carbide; however, their use is limited due to high cost and processing challenges. Hence, finding suitable, less-expensive alternatives that demonstrate superior hardness is highly desirable. Ceramics have been the premier choice for developing hard materials to satiate this demand. Specifically, transition metal-based ceramics (i.e., carbides, nitrides, borides) have been the most widely developed and employed hard material systems for commercial use. However, their performance has plateaued to a point still well below the threshold of their harder/less-affordable counterparts. To exceed the intrinsic limits of these conventional ceramics, a new innovative approach to enhance the hardness of these materials is needed. In recent years, one of the most promising and disruptive materials design approaches has been the use of so-called “high entropy” alloys. These high entropy alloys, which contain 5+ components in representative amounts (~5-35 mol%) and configurational entropies greater than ~12.5 J/mol·K (>1.5R), have been a growing trend in across a wide variety of material systems, including ceramics. This dissertation covers the research in studying the effects of configurational entropy on the hardness of transition metal-based ceramics to understand the viability of applying high entropy-based methodologies as an effective hardening strategy. The predominate focus is on transition-metal based carbonitrides, which are a unique class of carbon/nitrogen-based hard ceramics. In the first study (Chapter 3), the assessment of a baseline titanium carbonitride produced via Field-Assisted Sintering Technology (FAST) was performed to understand the intrinsic effects associated with processing on a simple system with alloying of anions (i.e., carbon, nitrogen) without the influence of multiple cations. It was found that processing variables such as soak temperature and time substantially influenced the hardness through various mechanisms (e.g., solid solution strengthening, consolidation, carbon segregation) via microstructural evolution differences. The processing insights developed from homogenizing these low entropy titanium carbonitride (TiCN) alloys were then applied and further optimized for higher entropy materials. In the second study (Chapter 4), a total of nine new high entropy carbonitrides (HECNs) were developed and validated new computational prediction methods for improving the synthesizability of high entropy alloys across many materials. These discoveries enabled greater insights into the phase stability of these alloys, such as accurately predicting which compositions form homogenous single-phase and heterogenous multiphase microstructures. Lastly in the third study (Chapter 5), cumulative assessments of these carbonitrides and various additional ceramics were analyzed in standardized hardness comparisons to determine the effect of configurational entropy on hardening. It was found that regardless of composition or hardness type/statistic, high entropy carbonitrides are mostly harder than lower entropy ones. Both the anion and cation configurational entropy affected the extent of hardening in different ways and many examples of direct hardness/alloying effects controlled by the number and molar fraction of these species were observed. The entropy-hardening mechanism itself derives from the structural consequences imposed by higher physiochemical uncertainty (e.g., lattice distortions, local bonding variations) from increased configurational entropy in disordered alloys. Overall, the high entropy approach does serve as a viable materials design strategy for systematically and intrinsically hardening most carbonitrides ceramic alloys for further applications that demand hard materials for extreme environments and mechanical loads.