Author ORCID Identifier:

https://orcid.org/0009-0006-7435-2166

Date of Graduation

12-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Mechanical Engineering

Advisor/Mentor

Zou, Min

Committee Member

Chen, Jingyi

Second Committee Member

Millet, Paul

Third Committee Member

Fleming, Robert

Fourth Committee Member

Ghosh, Sujan

Keywords

coating performance; coefficient of friction; dry film lubricant; durability; solid lubrication; wear mechanism

Abstract

Graphite is a widely studied solid lubricant valued for its low friction and layered structure. However, its application as a durable coating remains constrained by poor adhesion to metal substrates and limited wear resistance under sliding. This dissertation presents a systematic approach to overcoming these limitations by integrating deposition process optimization with surface chemistry modification to develop scalable, high-performance graphite coatings. Unlike many prior studies that focus on polished or idealized surfaces, this work targets steel substrates with elevated roughness levels (Sa ≥ 1 μm), aligning with conditions found in practical engineering components. The first phase of research begins by addressing the influence of deposition conditions, using a full-factorial design to investigate how spray flow rate and graphite concentration impact coating morphology, thickness, and tribological properties. This study revealed that spray parameters govern not only surface roughness and coating density but also the formation and behavior of transfer films during sliding. Optimized coatings achieved coefficients of friction as low as 0.09, a reduction of 86% relative to uncoated steel, highlighting the potential for spray deposition to produce effective, scalable solid lubricants. Building on this foundation, the next phase introduces a polydopamine (PDA) adhesive underlayer to address the challenge of weak adhesion between graphite and metal substrates. PDA is known for its ability to improve interfacial strength in various coating systems, and its inclusion here resulted in coatings that demonstrated significantly enhanced mechanical robustness. In progressive load scratch tests, the PDA-modified coatings withstood contact pressures exceeding 1.6 GPa without delamination. The presence of PDA also promoted better flake compaction and reduced coating porosity, leading to improved adhesion and structural integrity. The final phase examines the long-term durability of PDA/graphite coatings under dry reciprocating sliding, capturing the evolution of wear mechanisms across short, mid, and extended durations. Compared to graphite-only coatings, PDA-modified systems exhibited a sevenfold increase in wear life and sustained low-friction performance. Detailed surface and structural analyses using SEM, EDS, and Raman spectroscopy showed that PDA delays substrate exposure, stabilizes transfer films, and influences the evolution of carbon disorder, shifting the dominant wear mode from ploughing to compaction and retention. Together, these three studies establish a cohesive strategy for advancing dry-film lubrication technologies. By optimizing spray deposition parameters, reinforcing interfacial adhesion with a bioinspired underlayer, and tracking wear evolution across different sliding durations, this work demonstrates how graphite coatings can be engineered for long-lasting, real-world performance. The findings offer scalable solutions for fabricating solid lubricants capable of operating under high contact stresses in oil-free environments. In parallel, this research deepens the mechanistic understanding of transfer film formation, interfacial bonding, and the role of microstructure in wear resistance. These insights provide actionable design guidelines for developing robust solid lubricant coatings for applications in aerospace, industrial manufacturing, and dry-running mechanical systems.

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