Atmospheric pressure plasma assisted deposition of zirconia/silica conversion coatings

For decades, conversion coatings based on hexavalent chromium (Cr6+) were used to protect steel from corrosion in infrastructure, manufacturing, and transportation. However, Cr6+ is carcinogenic, leading to many restrictions being put in place since the 1980s. Since then, the automotive and aerospace industries have migrated to zinc phosphate-based systems. These systems tend to be porous which can negatively affect the corrosion mitigation performance. To remedy this, a chromatic sealing step is used to further improve its barrier properties but increases the environmental and health hazards in the process. More recently, zirconium oxide-based pretreatments have become more appealing as they have been shown to reduce the environmental impact while providing corrosion performance similar to phosphate-based coatings on certain substrates. However, these are immersion-based processes that require the treatment of substantial amounts of hazardous waste, large costs associated to protect staff, and substantial amounts of energy needed to heat and continuously stir the precursor baths. Plasma-based processes can serve as a wet chemical waste-free surface modification alternative to the current state-of-the-art. Recent advancements in atmospheric pressure plasma (APP) technology have made it possible to modify surfaces without the need for any vacuum components. These systems have also shown that they can clean heavily contaminated surfaces and increase their wettability. Furthermore, many studies have shown that APP processes can be used to deposit high quality coatings allowing for surface cleaning and deposition to be conducted using the same source. Despite these rapid advancements, the scope of APPs is still untested in their use towards replacing existing coating technologies. This limitation stems from the lack of variety in coatings which can be synthesized using these systems. This work aims to explore the feasibility of a scalable AP-PECVD process to synthesize a zirconia-silica composite coatings using a single plasma source. Due to the lack of any existing work on APP assisted deposition of zirconia, a new PECVD processes with various zirconia precursors are explored. Three different zirconium precursors are investigated for their efficacy. These include zirconium tert-butoxide (ZTB), zirconium acetylacetonate (Zr(acac)4) and zirconium oxynitrate hydrate. The work described highlights various challenges associated with each of these precursors and explores strategies to realize a suitable zirconia deposition process. In addition, the possibility of cerium doping to facilitate additional functionalities in these coatings is briefly investigated. The deposition of silica, on the other hand, is very straightforward and has already been shown in many APP systems. Two different commonly used silicon precursors were put to test – Hexamethyldisilane (HMDSO) and tetraethyl orthosilicate (TEOS). The compositional and morphological properties of coatings fabricated using these precursors are studied to find a suitable process window for a suitable coating deposition. Upon identifying the process parameters for deposition of zirconia and silica individually, a full coating stack comprised of both zirconia and silica is synthesized. Two possible configurations of these coatings are studied in this work. These include a simple 2-layered zirconia-silica coating stack and a gradient in composition coating. A bilayer coating is achieved by depositing zirconia and silica coatings in succession. The gradient in composition coating, on the other hand, is achieved by depositing both zirconia and silica simultaneously. While these coatings leave room for further optimization, this work demonstrates that complex coatings are achievable without needing any additional wet-chemical processing.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Nuclear, Plasma, Radiolgc Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2023

Author dc:creator

• Patel, Dhruval Kamlesh

Contributors dc:contributor

• Ruzic, David N

• Zirconium Oxide
• Silicon Oxide
• Plasma Enhanced Chemical Vapor Deposition
• Atmospheric Pressure Plasma Jet

Statement dc:rights

• Copyright 2023 Dhruval Patel

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/121568

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F121568