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Title: Spray Pyrolysis‐Aerosol Deposition for the Production of Thick Yttria‐Stabilized Zirconia Coatings
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Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
Advanced Engineering Materials
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    Aerosol deposition with gas phase‐synthesized chain‐like nanoaggregates can yield dense coatings from the impaction of particles on a substrate; however, dense coating formation is not well understood. Here, we study coating consolidation at the single nanoaggregate level. Flame spray pyrolysis‐made tin oxide nanoaggregates are mobility (size) filtered, accelerated through a de Laval nozzle, and impacted on alumina substrates. TEM images obtained from low velocity collection and supersonic deposition are compared via quantitative image analysis, which reveals that upon supersonic impact nanoaggregates fragment into smaller aggregates. This suggests that fragmentation is a key step in producing coatings denser than the depositing nanoaggregates themselves. We supplement experiments with detailed particle trajectory calculations, which show that the impact energies per atom during nanoaggregate deposition are below 0.2 eV/molecule. These results suggest that fragmentation can only occur at locations where nanoaggregates bonded by van der Waals and capillary interactions.

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  4. Abstract

    During drying, liquid‐applied particulate coatings develop stress and are consequently prone to stress‐induced defects, such as cracking, curling, and delamination. In this work, the stress development and cracking of coatings, prepared from aqueous silica and zinc oxide particle suspensions, were characterized using cantilever beam deflection with simultaneous imaging of the coating surface. Drying uniformity was improved and lateral or edge‐in drying was discouraged by using thin silicone walls around the perimeter of the cantilever. Coatings prepared from larger monodisperse silica particles (D50∼ 0.9 µm) dried uniformly but had a high critical cracking thickness (>150 µm) that prevented simultaneous study of stress development and cracking. Coatings prepared from smaller silica particles (D50∼ 0.3 µm) cracked readily at low thicknesses but exhibited edge‐in drying that complicated the stress measurement data. This drying nonuniformity was connected to the potential for these small particles to accumulate at the coating surface during drying. Hence, the selection of particle size and density was critical to drying uniformity when characterizing stress development and cracking. Coatings prepared from suspensions of zinc oxide particles (D50∼ 0.4 µm) were well‐suited for these studies, with uniform drying stress peaking at ∼1 MPa. Characteristic features in the stress development data above and below the critical cracking thickness (53 µm) were identified, demonstrating that cantilever beam deflection is a useful tool for studying the effectiveness of crack mitigation methods and the fundamentals of coating fracture during drying.

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    The successful processing of bilayer protective coatings on plastics using a combined spray and atmospheric plasma deposition method is shown. The base layer is a spray deposited coating with high adhesion using (3‐glycidyloxypropyl) trimethoxysilane and tetrapropyl zirconate (TPOZ) precursors. The top dense layer is deposited by atmospheric plasma deposition with a tetraethyl orthosilicate precursor. The coating deposition rate, chemical composition, elastic modulus, hardness, and adhesion to poly(methyl methacrylate) (PMMA) substrates are investigated. The adhesion to the polymer substrate is found to decrease with increasing TPOZ content in the precursor solution, while the elastic modulus and hardness of the base layer increase. A silane surface pretreatment of the PMMA substrate is shown to significantly increase the coating adhesion. The adhesion of the optimized coating is so high that it forces the debond interface change from adhesive failure at the coating/PMMA interface to cohesive failure within the PMMA substrate. The combined bilayer structure exhibits a >90% transparency in the visible wavelengths, eightfold increase in adhesion energy and fourfold increase in Young's modulus compared to commercial sol–gel polysiloxane coatings. The approach provides a strategy for an unprecedented combination of adhesion and mechanical properties.

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