Consolidation of Cermet Coatings by Cold Gas Dynamic Spraying

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Title: Consolidation of Cermet Coatings by Cold Gas Dynamic Spraying
Authors: Fernandez-Urrutia, Ruben
Date: 2017
Embargo: 2019-05-18
Abstract: Metal-ceramic (Cermets) materials that combine properties of both: high hardness, high wear resistance, and high working temperatures of ceramics and the ductility, toughness, and heat conductance of metals. Cold gas dynamic spraying, or simply cold spray, is a solid state thermal spray process that has been in development for the last 25 years. In the cold spray process, ductile materials are accelerated in a supersonic flow. These particles impact a substrate and adhere by plastic deformation. The continuous accumulation of these particles covers the substrate and creates a dense coating. The cold spray process is beginning to become a popular method to consolidate some select cermet materials into coatings. This technique can be advantageous when an erosion and wear resistant coating is required. During the deposition of these coatings, researchers have shown that the ceramic particles have a dramatic influence on the deposition behavior by causing an increase in deposition efficiency and coating adhesion. These effects have been presented in several experiments but have yet to be thoroughly explained. The goal of this investigation is to increase the knowledge, on a fundamental level, with regards to the deposition behavior of metal-ceramic blending and cermet powders. Ultimately, the focus is to prove the feasibility of these coatings for the requirements needed in the engineering industry. The first part of the investigation is a fundamental study on the deposition behavior of metal-ceramic blends with different compositions. Three theories that aim to explain the increase in deposition efficiency were proposed in the literature and further investigated in this study. One proposed mechanism for the increase in deposition efficiency was established by probability analysis to be too unlikely to contribute to the increment in deposition efficiency. The other two proposed mechanism, the presence of asperities caused by ceramic particles, and the oxide removal produced by the impact of ceramic particles, shown to play a major role in increasing the deposition efficiency. The effect of the ceramic particle morphology on the deposition behavior of metal-ceramic blending was studied in the second part of the investigation. This study greatly complements the previous one adding more depth to the investigation and confirming results. The increment in deposition efficiency normally seen with the addition of small amounts of angular alumina was not seen when spherical alumina was added instead. The creation of asperities during deposition was explored for the two morphologies and was determined that spherical alumina does not produce the same asperities at the surface. In addition, the coating sprayed with spherical alumina showed very little ceramic retention compared with the ones sprayed with angular alumina. These results have a direct impact on the mechanical properties of the coatings. Wear resistance for coatings sprayed with spherical alumina showed no improvement compared with pure aluminum coating due to the low ceramic content. Hardness was lower in coatings sprayed with spherical alumina for the same feedstock powder composition but was harder when the final coating composition was considered. Adhesion strength significantly increases with the addition of ceramic content in the feedstock powder; this increase was greater for coating sprayed with spherical alumina. The third part of the investigation focuses on understanding the mechanism of deposition for cermet particles with various morphologies. Six commercially available CrC-NiCr powders were studied, varying in morphology and metal/ceramic ratio. Spherical powders led to the erosion of the substrate and no coating was formed. Porous agglomerated and sintered powder lead to severely cracked coatings. For dense agglomerated and sintered powders, the outcome of powder depended on the initial metal/ceramic ratio, powders with 25%wt.NiCr led to erosion while 35%wt.NiCr powders led to a dense coating. Finally blended ceramic metal powders also lead to a successful coatings. All coatings obtained had lower ceramic content than the initial feedstock powder. Interrupted deposition tests, FEA analysis, and SEM observation were used to draw conclusions on the deposition behavior and explain the results. Finally, the last part of this investigation aims to apply the knowledge learned to an applied engineering problem. The problem that is targeted is the replacement of chrome plating for the aerospace industry. A commercially available cermet powder CrC-NiCr (65/35) was proposed as a replacement of chromium plating as well as a restoration for this coating and its alternatives (electroless nickel-plating, and WC-Co-Cr HVOF). The coatings and restoration were analyzed by SEM and tested by strip rupture rest, neutral salt spray fog, and fluid immersion testing. The adhesion strength, porosity, and hardness of the cold spray coating was also tested. The deposition and restoration of coatings were successful; a hard and dense coating was obtained with good adhesion strength. The process of restoration chromium-plating and its alternatives was also developed with a clean interface was achieved in each case. Coatings and restoration passed strip to rupture rest as well as fluid immersion test in two selected industry fluids. Neutral salt spray fog test revealed that the cold spray coating and repairs may have a path that allows the solution to penetrate the substrate and start the corrosion process. This behavior was found in a few select spots and should be further investigated. Overall, the coating proved to have potential as an alternative of chromium-plating or to restore damaged hard coatings.
URL: http://hdl.handle.net/10393/36103
http://dx.doi.org/10.20381/ruor-20383
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