Cold spray deposition behavior, characterization, and erosion performance of polyetheretherketone-nickel composites using ball milling for deposition on fiber reinforced plastics
Open Access
- Author:
- Littmann, Tyler
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 18, 2024
- Committee Members:
- Douglas Edward Wolfe, Thesis Advisor/Co-Advisor
Namiko Yamamoto, Thesis Advisor/Co-Advisor
John Mauro, Program Head/Chair
Timothy John Eden, Committee Member - Keywords:
- Cold Spray
Erosion Resistance
Composite Coatings
Ball Milling - Abstract:
- Rotor blades are exposed to particulates such as sand, rain, or ice, within their operating environments. This exposure causes significant damage to the materials, notably at the high tip speeds (>500mph). In order to extend component life to over 3000 flight hours, the carbon fiber reinforced plastic (CFRP) portion of the blade is currently protected with thin foils of stainless steel or nickel. Direct deposited coatings perform better than these foils at resisting erosion, but coatings cannot be directly deposited to CFRP’s without damaging the integrity of the CFRP components. High particle bombardment and elevated temperatures inherent in other spray techniques degrade the CFRP and fail to provide good adhesion of the coating due to dissimilarities in material properties used for the coating and in the substrate. Attempts at promoting adhesion through chemical functionalization or surface texturing of the substrate have, to date, been unsuccessful. This thesis is a subset of a larger Vertical Lift Research –Center of Excellence program, where the above challenge is being addressed by studying the preparation of polymer/metal composites that can be deposited via cold spray deposition onto CFRP substrates without substrate damage. Good adhesion and less substrate damage are achieved by using the soft polymer’s ability to deform at lower temperatures as a cushion on the substrate and using the metal particles to provide erosion resistance. Previously, the Ni-PEEK composite powder preparation by ball milling was studied. This thesis covers 1) scaling up of the composite powder preparation by transitioning from planetary to horizontal ball milling, 2) cold spray deposition of the composite powders, and 3) preliminary characterization of the cold-sprayed layers about their erosion performance. First, the fabrication of powders was scaled up from previous works investigating small-scale production in order to mass-produce quantities of usable powders. Two nickel powder sizes (5-15µm and 5-45µm) were ball milled with as-received PEEK powder (450PF, ≈50µm), using YSZ ball media. Ball-to-powder (BPR) ratio was held at 9:1 with a 5:1 by mass ratio of 1mm:5mm YSZ. Nickel mass percent was varied from 50% Ni to 90% Ni. Morphology as a function of milling time is investigated, with optimal milling times varying from 6-9 hours depending on composition. The ductility of PEEK promoted adhesion between the polymer and Ni particles, but the milling process is sensitive to deformation, agglomeration, and fracturing. The powder processing was scaled up to produce 1kg of powders every 6 hours on current lab equipment, a 200x improvement over prior planetary setups. Second, continuous coatings were achieved by multiple varying cold spray depositions to achieve adhesion but without substrate damage. Depositions occurred at gas pressures between 175 and 250 psi, and standoff distances between 10 and 25.4mm. Cross sectional analysis revealed no apparent substrate damage, and little to no porosity within the coatings. Nickel volume fraction within the deposited layers is calculated both by density approximations as well as image analysis programs and software. This determined no change in nickel volume as a function of spray parameters, and a 12-15% loss of Ni by volume in the deposited coating versus the milled powders. Coatings contained approximately 20-25% Ni by volume, confirmed by both aforementioned methods. Deposited coatings were characterized in order to determine the deposition mechanisms. Third, the preliminary erosion resistance was conducted on the cold-sprayed samples. Erosion data concluded that the cold sprayed coatings perform worse than bulk metals such as Ni at a calibrated 500mph erodent velocity. Coatings exhibited metallic erosion characteristics, and erosion rate improved at low angles of incidence. The eroded surfaces were inspected to study the erosion behaviors. The erosion testing determined that the deposited coatings eroded layer by layer. Erosion resistance exhibited a high correlation with the deposition parameters and impingement angle, where the lower impingement angles performed higher than high angles of impingement. Cold spray parameters that increase the deposition velocity, in this case, high gas flow pressure and low standoff distance, produced the highest performing coatings out of those studied. This study provides a scalable method of producing composite powders suitable for cold spray. Most importantly, the depositions of the powders via the cold spray process resulted in continuous coatings without substrate damage. Challenges remain in this work, most prominently relating to the improvement of the erosion resistance of the coatings. The next steps of this work will consist of additional investigation into suitable deposition parameters in order to further improve the coating inter-layer adhesion. To this end, higher deposition pressures with low standoff distances will be conducted to determine the substrate damage threshold. In addition, annealing of the coatings to reduce stresses and the resulting impact on erosion resistance will be determined. Furthermore, functional grading of the coatings would potentially improve erosion resistance while keeping substrate damage minimal. This would be accomplished by having an initial layer of less Ni by mass deposited first to prevent substrate damage and then gradually building up to a pure Ni layer on the surface for maximum erosion resistance. Ultimately, the goal remains to create a field-applicable deposited coating with comparable erosion resistance with the ability for localized, in-situ repairs.