When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Surface engineering is of utmost importance in ensuring the efficient and long-lasting performance of components across diverse industries, such as automotive, aerospace, mining, transportation, and biomedical applications. Conventional Ni-based alloys (e.g., Mar-M-247, PWA1484 and PW1480) commonly utilized in these sectors often face limitations and failures under harsh operating conditions, attributed to factors like friction, wear, oxidation, and corrosion. To address these challenges, this Ph.D. thesis investigates the potential of high entropy alloys (HEAs) as a viable solution for enhancing tribological performance under extreme conditions. HEAs are unique alloys with five or more principal elements in near-equal atomic percentages, offering exceptional mechanical and thermal properties.
The study investigates four HEA systems: CrMnFeCoNi, Al0.5FeCrMnCoNi, AlFeCrMnCoNi, and AlCoCrFeMo, particularly for their use as coatings. The research involves producing HEAs using solid-state reactions and utilizing various thermal spray techniques such as low-pressure cold spraying (LPCS), flame spraying (FS) and high velocity oxygen fuel (HVOF) for coating deposition. The study meticulously investigates how the deposition process and various spraying parameters influence coating composition and microstructure. Additionally, a novel transverse scratch test is employed to evaluate the cohesion and adhesion of the coatings. Comprehensive characterization techniques, such as high-resolution scanning electron microscopy (SEM), electron channeling contrast imaging (ECCI), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman analysis, and Electron Backscatter Diffraction (EBSD), are employed to evaluate the microstructure.
Results show the formation of solid solution phases in coatings with minor oxide formation. CrMnFeCoNi coatings exhibit a single solid solution with a face-centered cubic (FCC) structure, while AlFeCrMnCoNi coatings consist of a combination of body-centered cubic (BCC) and minor FCC phases. AlCoCrFeMo HEA coatings exhibit a BCC/B2 phase structure. LPCS coatings, in particular, stand out for not exhibiting oxide formation and retaining the feedstock phases. When Al was added to the CrMnFeCoNi HEA system, it resulted in increased hardness but reduced cohesive strength in the AlFeCrMnCoNi coatings.
All thermally sprayed high entropy coatings (HECs) were tested on a ball-on-disc tribometer under dry sliding reciprocating conditions up to 350°C, using alumina counterballs. The tribological testing revealed that the HVOF-sprayed AlCoCrFeMo coatings outperformed all other tested HECs across all temperature conditions. This superior wear resistance can be attributed to several key factors, including the presence of finer splats, controlled oxide formation within the coating, higher hardness due to the influence of the BCC phase, and the development of a protective Co-based oxide film at the contact region.
Overall, this Ph.D. dissertation has proposed innovative strategies to enhance the wear resistance of high entropy coatings (HECs) and has identified critical parameters affecting wear performance. The research significantly contributes to materials design by elucidating the relationship between interfacial processes and tribological behavior. It establishes a strong foundation for the future development of HEA-based coatings, emphasizing their potential as next-generation tribological interfaces for demanding operating conditions.