A computational fluid dynamics model for understanding the HVAF process and the influence of the process parameters on the particle flight properties is investigated. Achieving this objective involves a novel approach to modeling the HVAF process with pressure inlet boundary conditions and integration of the mixing chamber. The study comprises the prediction of the flow fields described by a set of equations consisting of continuity, momentum, energy, and species transport. These equations are then solved with realizable k-ε turbulence model, a two-step chemistry model and eddy dissipation model to simulate the combustion reaction. Consequently, the interaction between the CoNiCrAlY alloy particles and the flow is modeled using a Lagrangian approach considering the forces acting on the particles and the heat transfer. The results show that the combustion chamber pressure is mainly affected by the compressed air and propane parameters. Furthermore, the flight behavior of the smaller particles is significantly influenced by the gas flow, while the larger particles tend to maintain their momentum and energy. Through the simulation model, an in-depth process understanding of the HVAF process can be achieved. More importantly, the model can be used as a tool for efficient process development.

This paper evaluates the performance of a new single-cathode cascaded arc spray gun developed for low-pressure plasma spraying (LPPS). It describes key design features, explaining how they contribute to arc and voltage stability, improved thermal efficiency, higher throughput, and extended equipment life. It assesses the effect of nozzle geometry on spray spot morphology and examines the microstructure of CoNiCrAlY and YSZ coatings deposited on different substrates, including a turbine blade, using the new gun. Cross-sectional images show the uniformity of the CoNiCrAlY coatings in different locations on the turbine blade, including platform, fillet, and airfoil surface.

The plasma torch design affects the particle-plasma interaction, in-flight properties and the coating microstructure. When spraying metallic powders, the in-flight oxidation as well as the particle velocity and temperature determine the mechanical, corrosion and oxidation properties, which have a major impact on the in-service degradation of bond coats. This study aims to determine the microstructural and mechanical properties of as-sprayed CoNiCrAlY coatings deposited on the Inconel 718 alloy. Depositions were made using a High Velocity Plasma Spray Process (HVPS), which is based on a special plasma torch design. In-flight particle characteristics were determined to elucidate the kinetic and thermal regime of HVPS process.

The erosion behaviour at room temperature (RT) of as-deposited SPS, EB-PVD and APS YSZ-based TBCs was investigated. All coatings were deposited on Inconel 625 alloy coupons. The same APS CoNiCrAlY bond coat was employed for all SPS and APS TBCs. The erodent material was 50 μm alumina and the impact angles were 15° and 90°. A total of 4 different types of SPS YSZ-based TBCs were tested, which consisted of two distinct columnar-segmented and two distinct columnar-grown microstructures. The EB-PVD and APS YSZ TBCs were employed as benchmarks. The erosion performance of the different TBCs in this study was ranked based on the coating volume loss after wear testing. The TBC microstructures and phase compositions were evaluated via SEM and XRD. The erosion mechanisms of the different TBCs were compared by analyzing the cross-sectional and top surface microstructures of the as-sprayed and eroded TBCs. These are released results from the Surftec Industrial R&D Group of the National Research Council of Canada (NRC).

In this study, the mechanisms responsible for enhancing the adhesion strength of thermally sprayed metallic coatings subjected to vacuum heat treatment were investigated using atmospheric plasma sprayed (APS) CoNiCrAlY coatings as an example. The formation of metallurgical bonding between the coating and the substrate, which determined the increase in the adhesion strength of the coatings, was studied by analyzing the effect of morphological changes of the oxide film in the coating. The results showed that during the vacuum heat treatment process, the oxide film formed during the coating deposition gradually broke down and subsequently shrank into round oxides. After vacuum heat treatment, the adhesion strength of the coating improved significantly, and there was a positive nonlinear relationship between the treatment time and the adhesion strength. The increase in the adhesion strength was caused by the formation of metallurgical bonding between the coating and the substrate. However, the prerequisite for the formation of metallurgical bonding was that the oxide film had to break during the vacuum heat treatment process. A thermodynamic 2D model based on the thermal grooving theory was proposed to explore the essential conditions for the breaking and shrinking of the oxide film.

A Vickers indentation method has been applied to determine the interfacial fracture toughness of modern multilayer thermal barrier coatings. The delamination behavior of four types of coating systems will be discussed and compared with results based on modified four-point bending (4PB) tests. The investigated multi-layer coating system consists of a CoNiCrAlY-bond coat applied via low-pressure plasma spray (LPPS) on a nickel-based superalloy and an atmospheric-plasma sprayed (APS) top layer of type gadolinium zirconate (GZO) and yttria-stabilized zirconia (YSZ). A conventional YSZ mono-layer system is used for reference. The effects of GZO and YSZ microstructure were investigated using top coats with low and high porosities for both (multi- and single-layer) coating systems. Isothermal oxidation tests at 1100 °C up to 500 hours were performed to study the interaction between thermal aging and fracture behavior. Investigations of microstructure and sintering behavior show a significant influence of the annealing conditions on fracture toughness. It has been observed, that with increasing annealing time, the stiffness and thus the crack driving force of the GZO layer is increased due to sintering effects and healing of submicron defects. The lower stiffness and higher defect density of GZO seem to be the main reason for the reduced fracture toughness of the YSZ / GZO interface compared to the YSZ / CoNiCrAlY interface. As a result, the delamination of the top coat is observed to shift from the top coat / bond coat interface into the top coat double-layer.

A computer-controlled laser test rig (using a CO 2 laser) offers an interesting alternative to traditional flame-based thermal gradient rigs in evaluating thermal barrier coatings (TBCs). The temperature gradient between the top and back surfaces of a TBC system can be controlled based on the laser power and a forced air back-face cooling system, enabling the temperature history of complete aircraft missions to be simulated. An air plasma spray (APS) deposited TBC was tested and, based on experimental data available in the literature, the temperature gradients across the TBC system (ZrO 2 -Y 2 O 3 YSZ top coat / CoNiCrAlY bond coat / Inconel 625 substrate) and their respective frequencies during air-to-air combat missions of fighter jets were replicated. The missions included (i) idle/taxi on the runway, (ii) take-off and climbing, (iii) cruise trajectory to rendezvous zone, (iv) air-to-air combat maneuvering, (v) cruise trajectory back to runway and (vi) idle/taxi after landing. The results show that the TBC thermal gradient experimental data in turbine engines can be replicated in the laser gradient rig, leading to an important tool to better engineer TBCs.

Different thermal spray technologies were used to apply CoNiCrAlY bond coats to Inconel substrates. Powder compositions were the same in all cases and particle size recommendations were followed for each torch. YSZ topcoats were deposited via APS on bond coat samples selected based on roughness, porosity, residual stress, oxidation, and isothermal TGO growth. The TBCs were furnace cycle tested for 10-1400 cycles as well as to failure and changes in bond strength and TGO thickness were recorded. It was observed that bond strength values, which are relatively stable during thermal cycling, decrease significantly just before failure brought on by topcoat spall off.

MCrAlY coatings deposited by LPPS or HVOF spraying are widely used on turbine blades and vanes to mitigate the effects of oxidation and corrosion. The service life of a MCrAlY layer is dependent on the loss of aluminum, which is consumed by oxidation on the surface and by diffusion into the substrate. After long term heat treatments in air, LPPS and HVOF CoNiCrAlY coatings on IN738 and Hastelloy X substrates were examined and their interdiffusion and Al depletion layers were characterized based on the distribution of phase constituents, the amount of residual Al content, and average layer thickness. The results of the study show that differences in coating behavior with respect to Al consumption are mainly due to the oxidation of yttrium in the CoNiCrAlY powder during HVOF spraying.

This paper describes the development and verification of a thermal barrier coating (TBC) for use in class 1600 °C turbines. It explains how ceramic materials with different crystal structures were selected for the topcoat and how they were screened based on phase transformations. It discusses the method used determine a bond coat composition with high oxidation resistance and good ductility. It describes the TBC deposition process, the tuning of spray parameters, and the optimization of coating properties. It discusses the tests used to evaluate the topcoat, bond coat, and thermally grown oxide (TGO) layer and presents the results of 10,000 h of field testing in a power plant.

Previous work on thermal barrier coatings (TBCs) has shown that Ce-containing bond coats promote the formation of a wedge-like interface oxide that improves delamination resistance. The oxide was found to form at temperatures greater than 1100 °C, which in many applications, may not be reached. In this study, TBC samples consisting of a YSZ topcoat and various cold-sprayed bond coats were prepared. In order to obtain a wedge-like thermally grown oxide (TGO), pre-oxidation was carried out for 20 h at 1100 °C prior to high-temperature testing for 1000 h at 1000 °C. It was confirmed that the pre-oxidation treatment produced a wedge-like TGO that continued to grow at 1000 °C, which improved delamination resistance as four-point bend tests showed. A wedge-like oxide was also observed in some TBCs exposed to temperatures of 1000 °C, without pre-oxidation.

Thermally grown oxide (TGO) that naturally forms on bond coat surfaces plays an important role in determining the lifetime of thermal barrier coatings (TBCs). Splashed particles on thermally sprayed MCrAlY bond coat surfaces are weakly bonded to the underlying bulk coating, leading to the formation of mixed oxides that contribute to TBC failure. In this study, various heat treatments are used to modify the weakly bonded splashed particles on LPPS CoNiCrAlY bond coats in order to restrain the formation of mixed oxides and prevent associated failures.

This study compares the deposition and oxidation behavior of two oxide-dispersed CoNiCrAlY powders, one commercially obtained, the other prepared in a high-energy attrition ball mill using CoNiCrAlY and nanosize α-alumina powders. The custom powder was deposited by HVOF spraying using two sets of parameters, one optimized for CoNiCrAlY powder, the other for fine alumina. Coatings produced under the latter conditions were found to be porous, which can be attributed to a low degree of melting in the dispersed alumina. Isothermal oxidation testing at 1373 K for up to 1000 h in air caused oxidation not only at the surface, but also inside the coatings due to the movement of oxygen through the pores. The coatings deposited under the other set of parameters, i.e., at higher power levels, were free of pores. Isothermal oxidation tests were also carried out on coatings produced from the commercial powder, in this case, by HVOF and as well as vacuum plasma spraying. The coatings obtained by HVOF spraying were found to have a thinner thermally grown oxide layer than not only the VPS coatings, but also conventional metallic bond coats. Internal oxidation in the HVOF coatings is due to insufficient cohesion of the spray particles. Furnace cycling tests were conducted on specimens with an additional ceramic thermal barrier coating. Specimens with VPS bond coats produced from commercial oxide-dispersed powder achieved almost same number of cycles to delamination as specimens with conventional metal bond coats.

This study assesses the effects of dry ice blasting on the lifetime and durability of thermal barrier coatings (TBCs). Three sets of TBCs consisting of a CoNiCrAlY bond coat and YSZ topcoat were deposited by air plasma spraying, each set with a different dry ice blasting treatment. Different microstructures were obtained in both the bond coat and topcoat depending on blasting conditions. Bond coat oxidation and thermal shock lifetime of the TBC are also shown to vary with the blasting treatment. TBCs where both the bond coat and topcoat are dry-ice blasted proved to be the most durable with the biggest improvement in lifetime. They also exhibited the most regular surface roughness.

The processing conditions, microstructural and tribological characterizations of plasma sprayed CoNiCrAlY-BN high temperature abradable coatings are reported in this manuscript. Plasma spray torch parameters were varied to produce a set of abradable coatings exhibiting a broad range of porosity levels (34-62%) and superficial Rockwell hardness values (0-78 HR15Y). Abradability tests have been performed using an abradable-seal test rig capable of simulating operational wear at different rotor speeds and seal incursion rates. These tests allowed determining the rubbing forces and quantifying the blade and seal wear characteristics for slow and fast seal incursion rates. Erosion wear performance and ASTM C633 coating adhesion strength test results are also reported. For optimal abradability performance, it is shown that coating hardness needs to be lower than 70 and 50 HR15Y for slow and fast blade incursion rate conditions, respectively. It is shown that the erosion wear performance, as well as, the coating cohesive strength is a function of the coating hardness. The current results allow defining the coating specifications in terms of hardness and porosity for targeted applications.

This study investigates the effect of dry-ice blasting distance on the deposition of CoNiCrAlY coatings obtained by plasma spraying. Dry-ice blasting was used before, during, and after spraying and its effect on coating quality was measured. The results show how blasting distance influences the deposition efficiency as well as the microstructure, porosity, adhesion strength, hardness, and oxide content of plasma-sprayed coatings. The optimal dry-ice blasting distance was proposed as 25 mm.

Cold spray is a new emerging coating technology in which particles in a solid state are deposited via plastic impact on a substrate at a high velocity and a temperature that is much lower than the melting point of the starting powder. Compared to the conventional thermal spray processes, dense coatings without any degradation can be obtained by cold spray process with high deposition efficiency. CoNiCrAlY coatings are widely used for land-based gas turbines to resist high-temperature oxidation and hot corrosion. Owing to the high cost of the low-pressure plasma spray (LPPS) or some degradation in the hyper-velocity oxy-fuel (HVOF) spray process, cold spray process is a prospective candidate for coating preparation. In the current study, CoNiCrAlY coatings were prepared by cold spray and LPPS processes, and a comparison of the coating’s properties between the LPPS and cold spray process was carried out. The spray conditions of cold spray were optimized by the measurements of deposition efficiency and the observations of microstructure.

Dry-ice blasting, as an environmental-friendly method, was used to pretreat the substrate to be coated. In the present paper plasma-sprayed CoNiCrAlY splats were examined on the dry-ice blasted substrate. The cleaning effect of dry-ice blasting was demonstrated accompanying the condensation phenomenon, which is also harmful for the formation of ideal disk-like splat. A solution of ensuring the substrate temperature over dew point temperature was proposed for the proper application of dry-ice blasting during droplet flattening.

This paper presents the results of a study of the oxidation behavior of thermal barrier coating (TBC) with air plasma sprayed (APS) yttria-stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited using low pressure plasma spray (LPPS) and cold spray (CS). The TBC is subjected to isothermal oxidation and creep tests at 900 ¢XC and evaluated using scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and transmission electron microscopy (TEM). The thermally grown oxide (TGO) developed in TBC with LPPS bond coat was composed of only α-Al 2 O 3 and the TGO developed in TBC with CS bond coat is composed of α-Al 2 O 3 and γ-Al 2 O 3 . Despite the presence of this metastable γ phase, the TGO in the CS specimens exhibits a dense microstructure and lower amounts of mixed oxides. The correlation between γ-Al 2 O 3 and the formation of mixed oxides was investigated through the measurement of γ- Al 2 O 3 thickness ratio and mixed oxides coverage ratio. It was found that the mixed oxides coverage ratio is inversely proportional to the γ- Al 2 O 3 thickness ratio. Overall findings indicate that the oxidation behavior of the TBC with CS bond coat is superior compared to that of the TBC with LPPS bond coat.

In this study, the effects of bond coat compositions with and without 0.5 mass.% Y on oxidation behaviour were investigated. Oxidation behaviour was accessed in terms of The TGO structure and growth kinetics. After isothermal oxidation tests, the TGO formed on Y-free CoNiCrAl coatings was composed mainly of alumina without the presence of Y-rich oxide and showed lower growth kinetics than that formed on conventional CoNiCrAlY coating. This result indicates that potential improvements to the bond coat oxidation behaviour can be achieved using Y-free bond coat compositions.