Development of Conventional and Nanocrystalline Bond Coats by Cold Gas Dynamic Spraying for Aerospace Thermal Barrier Coatings

Gas turbine engines are considered to be among the most hostile operating environments for conventional material systems. Increasing demands for higher engine performance and durability of components have led to the development of thermal barrier coating (TBC) systems. Typical TBC systems consist of two coating layers: an insulating ceramic top coat for thermal protection and an underlying metallic bond coat for improved adhesion of the top coat and better chemical protection against high temperature oxidation and hot corrosion. However, current use of these coating systems is limited due to their premature failure which is associated to cracking and delaminating of the ceramic top coat. It is generally accepted that the primary mechanism responsible for TBC failure is attributed to oxidation of the bond coat which results in the formation of an oxide scale at the interface between the bond coat and ceramic top coat and eventually causes cracking and delamination of the top coat. Better understanding and control of the bond coat oxidation dynamics is therefore of primary importance for the development of TBC systems with improved performance. The bond coat is commonly manufactured by thermal spray techniques: the bond coat material, initially in powder form, is heated beyond its melting point, projected onto the surface to be coated and finally re-solidified upon cooling to form a coating. It has been demonstrated that certain microstructural features of the bond coat that are detrimental to its oxidation behaviour originate from thermally induced effects encountered during thermal spraying. Therefore, it is expected that improved bond coat oxidation behaviour could be achieved if the deposition process did not involve significant heating of the material. Recent developments in the surface and coatings industry have given rise to a new coating technology known as Cold Gas Dynamic Spraying (CGDS). As its name implies, this process does not rely on thermal energy for the formation of coatings, but rather on kinetic energy: particles are accelerated above a critical velocity and plastically deform upon impact on the substrate to adhere and form a coating. Due to the absence of significant heating of the sprayed material, this work aims to manufacture bond coats using the CGDS deposition technique and investigate whether improved oxidation behaviour can be achieved. The present thesis provides a description of the experimental approaches considered for the development of CGDS bond coats with improved oxidation behaviour. Given the complexity of TBC systems due to the various interactions between the multiple coating layers, this work strictly concentrates on the bond coat layer without the presence of the superalloy substrate or ceramic top coat, thereby allowing the thorough characterization of the bond coat oxidation behaviour as a function of the initial powder microstructure and different deposition techniques. As such, the objectives of this work are to demonstrate the feasibility of manufacturing bond coats using the CGDS technique, optimize the deposition process for the materials considered, verify whether the CGDS process induces microstructural changes in the deposited material and finally evaluate and compare the oxidation behaviour of CGDS bond coats with those of thermal sprayed bond coats. Results of this work show that bond coatings with conventional and nanocrystalline microstructures were successfully manufactured by the CGDS system developed at the University of Ottawa Cold Spray Laboratory. Optimal spraying parameters were also identified and coatings with low levels of porosity were successfully deposited using this technique. Investigation of the original feedstock powder and resulting coating microstructures revealed that significant microstructural transformations had occurred throughout the CGDS deposition process as a result of extensive plastic deformation of the particles. Isothermal oxidation testing was also carried out on both the conventional and nanocrystalline CGDS coatings. For comparison purposes, thermal spray coatings were also manufactured (using the air plasma spray and high velocity oxy-fuel processes) and subjected to oxidation testing. Results showed that low temperature processing of bond coat materials is beneficial to their oxidation behaviour as it results in coatings with low porosity and limited oxide content, thereby leading to lower oxide growth rates. Furthermore, the CGDS process was observed to produce coatings characterized with fine grain structures (either by means of a grain refinement process of the conventional material or by preserving the fine grain structure of the nanocrystalline material) which was also shown to be beneficial to the oxidation behaviour. Results from this work therefore demonstrate that potentially significant improvements to TBC performance could be achieved by manufacturing bond coats using the CGDS deposition technique.

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