Optimized Processing of High Density Ternary Hafnium-Tantalum Carbides and Molybdenum Alloys Via Field Assisted Sintering Technology
Open Access
- Author:
- Albert, Patrick
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 24, 2025
- Committee Members:
- John Mauro, Program Head/Chair
Jay Keist, Major Field Member
Timothy Eden, Chair & Dissertation Advisor
John Mauro, Major Field Member
Mark Traband, Outside Unit & Field Member - Keywords:
- Field Assisted Sintering Technology (FAST)
Spark plasma Sintering (SPS)
refractory materials
ultrahigh temperature ceramics (UHTC) - Abstract:
- Current manufacturing processes strive to meet demands for materials with properties suitable for application in harsh thermal, chemical, and/or mechanical environments. Such materials are often challenging to manufacture at high quality and large scale. Global needs in both the defense and private sectors demand the development of disruptive advances in materials for extreme environments for applications including: creating turbine engines that can cycle at higher operating temperatures for efficiently and power generation, next-generation burning-plasma magnetic thermonuclear fusion devices and energy producing fusion devices that can generate power for our growing strain on the electrical grid, and vehicles that can travel at high Mach numbers to serve national security efforts. All of these areas require revisiting materials engineering design of systems. A solution to most of the aforementioned technologies, are looking to classes of ultra-high temperature (UHT) materials that can serve the harsh environments. To this end, it is necessary to develop cost effective manufacturing processes capable to consistently producing parts with the desired performance characteristics. Ultra-high temperature ceramics (UHTCs) and refractory alloys present great opportunities for aerospace, nuclear, and hypersonic applications, but densification via conventional sintering is challenging and often requires sintering aids whose instability at UHT jeopardizes the materials for UHT applications. UHTCs are refractory transition-metal carbides, nitrides, and borides with the highest known melting points of any known materials, making them prime candidates along with refractory alloys for applications in aerospace and hypersonic vehicles. .However, Field-assisted sintering technology (FAST) has demonstrated it can produce dense, mechanically-robust components without the need for sintering aids. Adoption of FAST as a manufacturing process is growing due to its ability to consolidate many UHT materials to theoretical density for simple and complex geometries the order of minutes to hours, rather than hours to days for conventional processes. The FAST process leverages high uniaxial loads and rapidly heating components to rapidly densify UHT materials (>2400°C) on the order of 100s of degrees a second thanks to the applied high current, low voltage wave form passed through the part. In this work, we have developed an optimized set of FAST processing conditions without sintering aids for various Compositions of a familty of UHTCs and a family of refractory alloys (Mo). Of the UHTCs, tantalum carbide (TaC) and hafnium carbide (HfC) feature the highest melting temperatures, therefore we explored the (Hf,Ta)C ternary system. We also examined Mo and Mo + HfC compositions. These material systems all present various manufacturing challenges, but are attractive for applications with extreme thermomechanical requirements such as nuclear energy systems, electronics, aerospace vehicles, and hypersonic vehicles. We investigated the binderless consolidation of HfC/TaC powder blends using Field Assisted Sintering Technology (FAST). Powders consisting of 90/10, 50/50, and 10/90 vol% HfC:TaC were sintered to high densities (>94%). The novel processing approach yields high-density ceramics with minimal grain growth. It was found that 50 vol% HfC (~55 mol%) demonstrated record-breaking nanohardness (41.45 ± 1.37 GPa), Vickers microhardness (30.2 ± 3.1 GPa), and elastic (indentation) modulus (590.12 ± 10.64 GPa). These peak mechanical properties arose from the balance of two underlying structure-property relationships: solid solution strengthening and the Hall-Petch effect. The interplay of these compositionally-linked phenomena yields an optimal regime of superior mechanical properties. Bulk and nanomechanical, chemical, and microstructural characterization revealed substantially greater strength, hardness, and stiffness for ternary alloys. Mechanical properties correlated with physiochemical analysis indicated trace oxygen phases, solid solution strengthening, and nonstoichiometric carbon were the key mechanisms driving the peak property enhancement of the 50 vol% solid-solution sample, despite lower densities. This study provides insight into optimization of the compositional design of HfC-TaC alloys by balancing influences from solid solution strengthening and the thermodynamic effects of oxygen/carbon stoichiometry. Combining this interplay with optimized FAST parameters, superior ternary HfC-TaC ceramics can be realized for next-generation hypersonic applications. Pure molybdenum and samples with added hafnium carbide (HfC) grain refiners were produced using field assisted sintering technology (FAST). The molybdenum and HfC reacted with oxygen to produce MoO2 and HfO2, and increased HfC content from 1 wt% to 5 wt% decreased grain size while increasing the microhardness. Room temperature three-point bending tests were conducted, and finite element modeling was used to define HfC-dependent bilinear material models. The presence of oxygen most severely affected pure molybdenum, which exhibited little strength and limited ductility, whereas for samples with added HfC, HfO2 was present, resulting in increased toughness hypothesized to be due to microcrack toughening. The samples with 1 wt% added HfC had the greatest energy absorption capability.