PhD Oral Exam - Parastoo Fallah, Chemical Engineering

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.

Abstract

Due to their exceptional tribological performance (i.e., low friction and wear), MoS2 solid lubricants have been extensively used in aerospace/space applications. Since the effectiveness of pure MoS2 tends to deteriorate in humid and oxygen-containing environments, it is traditionally co-deposited with hazardous additives such as lead-based compounds and Sb2O3 to improve its oxidation resistance and tribological properties. However, increasing environmental and regulatory concerns have driven the search for greener additives capable of maintaining the tribological performance of MoS2-based solid lubricants. Moreover, despite the widespread use of spray-bonded solid lubricants in the aerospace industry due to their low cost and ease of application, most published studies have focused on physical vapor deposition (PVD) coatings, leaving spray-bonded systems comparatively underexplored. Furthermore, a fundamental understanding of the interfacial processes governing friction and wear under realistic service conditions is essential for designing next-generation aerospace solid lubricants. Thus, this study focuses on improving the environmental sustainability of bonded MoS2-based solid lubricants through the reformulation of a commercial product and evaluating their tribological performance. MoS2-based solid lubricants containing Sb2O3 and lead phosphite, which have been used commercially for decades, served as the baseline formulations. As a short-term strategy, lead phosphite was eliminated and MoS2 was co-deposited with a combination of Sb2O3 and Ag2O, resulting in a more sustainable coating. In addition, the commercial Everlube 620C formulation was obtained by substituting lead phosphite with additional Sb2O3, resulting in a MoS2-Sb2O3 solid film lubricant. Based on this formulation, Sb2O3 was subsequently replaced with the environmentally friendly oxides (i.e., Bi2O3 and TiO2) as medium-term strategies. As a long-term strategy, non-oxide materials such as hexagonal boron nitride (hBN) and polyether ether ketone (PEEK) were explored as environmentally friendly alternatives to Sb2O3 in Everlube 620C. The tribological performance and interfacial processes of the developed coatings were evaluated using a reciprocating ball-on-flat tribometer under application-relevant conditions. Subsequently, the unworn and worn surfaces of the coatings were characterized using ex-situ techniques including scanning electron microscopy (SEM) and field-emission scanning electron microscopy (FESEM), both coupled with energy-dispersive spectroscopy (EDS), as well as focused ion beam (FIB), Raman spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM). The results demonstrated that Ag2O is a promising replacement for lead phosphite, maintaining friction performance comparable to the commercial MoS2-Sb2O3-lead phosphite formulation, although with reduced wear resistance. Among the environmentally friendly alternatives to Sb2O3 in the MoS2-Sb2O3 formulation, Bi2O3 emerged as the most promising candidate, while TiO2 showed the poorest tribological performance. The friction and wear behavior of MoS2-hBN and MoS2-PEEK coatings were strongly dependent on additive concentration, with optimum compositions providing the best friction and wear performance. Furthermore, all coatings exhibited substantially lower steady-state friction at −50 °C than at room temperature, highlighting the dominant role of an interfacial ice layer in low-temperature lubrication. Overall, this work establishes practical design guidelines for developing environmentally sustainable bonded MoS2-based solid lubricants by correlating additive chemistry, concentration, and operating environment with the governing friction and wear mechanisms.