MCrAlX powder compositions (M=Ni, Co and X=Y, Hf, Si or combination) are often thermally sprayed (TS) via vacuum plasma spray (VPS), low pressure plasma spray (LPPS) or high velocity oxy-fuel (HVOF) to produce high temperature oxidation and hot corrosion resistant bond coats (BC) for thermal barrier coatings (TBCs). Cold spray (CS) technology is currently considered as a promising alternative to the traditional TS solutions having the advantage of delivering oxide-free and very dense metallic coatings at relatively lower costs compared to VPS and LPPS. Here, we first present high-pressure CS deposition of NiCoCrAlY and NiCoCrAlYHfSi and discuss the influence of feedstock properties on the deposited BCs. CFD numerical simulation is employed to tailor the spray conditions based on the feedstock characteristics. Secondly, we present the laser assisted cold spray (LACS) deposition of NiCoCrAlYHfSi BCs using a low-pressure CS system. We show that LACS can be successfully used to deposit this particular powder while eliminating nozzle erosion and low deposition efficiency disadvantages observed during conventional CS. Lastly, high temperature isothermal oxidation of a TBC architecture having LACS BC is presented.

The hot-section components of modern gas turbines (e.g., turbine blades and vanes) are typically manufactured from Ni-base superalloys. To develop the γ/γ' microstructure that imparts superior thermomechanical and creep properties, Ni-base superalloys usually require three distinct heat treatments: first a solution heat treatment, followed by primary aging, and finally secondary aging. To achieve oxidation resistance, MCrAlY coatings are applied on the superalloy components as either environmental coatings or bond coats for thermal barrier coatings. In this study, the effects of different processing sequences on MCrAlY coating characteristics and short-term isothermal oxidation performance were investigated. Specifically, cold spray deposition of NiCoCrAlTaY coatings was carried out on single-crystal Ni-base superalloy substrates that underwent various degrees of the full heat treatments prior to being coated. The remaining required heat treatments for the superalloy substrates were then performed on the coated samples after the cold spray deposition. The microstructures of the CMSX-4 substrates and NiCoCrAlTaY coatings were characterized after each heat treatment. Isothermal oxidation performance of the coated samples prepared using different sequences was evaluated at 1100°C for 2 hours. The results suggested a promising procedure of performing only solution heat treatment on the superalloy substrate before coating deposition and then primary aging and secondary aging on the coated samples. This processing sequence could potentially improve the oxidation performance of MCrAlY coatings, as the aging processes can be used to effectively homogenize coating microstructure and promote a thin thermally grown oxide (TGO) scale prior to actual isothermal oxidation.

As a critical technology, thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades, however, the most commonly used MCrAlY bond coats which control air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. This motivates a reconsideration of development of MCrAlY at a fundamental level to understand why the huge efforts in the past three decades has so little impact on industrial application of MCrAlY alloys. Detailed examination of crack trajectories of thermally cycled samples and statistic image analyses of fracture surface of APS TBCs confirmed that APS TBCs predominately fails in top coat. Cracks initiate and propagate along splat boundaries next to interface area. TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in top coat or both. By focusing on the reduction of tensile stress in top coats, three new bond coat alloys have been designed and developed, and the significant progress in TBC lifetime have been achieved by using new alloys. Extremely high thermal cycle lifetime is attributed to the unique properties of new alloys, such as remarkably lower coefficient of thermal expansion (CTE) and weight fraction of β phase, absence of mixed / spinel oxides, and TGO self repair ability, which cannot be achieved by the existed MCrAlY alloys.

In this paper, a diffusion kinetic model was applied to simulate the microstructure development in a MCrAlY-superalloy system at high temperatures. Both simulation and experimental results showed that γ+γ’ microstructure was obtained in the coatings due to Al depletion after oxidation. With the help of the modelling, the mechanism of the formation of the diffusion zones in the single crystal (SC) superalloy can be also analyzed. The results revealed that the inward diffusion of Al from coating affected the depth of secondary reaction zone (SRZ) with the precipitation of TCP phases while the depth of inter-diffusion zone (IDZ) was decided by the inward diffusion of Cr.

In this work, Inconel 718 gas-atomized powder was successfully heat treated over the range of 700-900°C. As-atomized and as-heat treated powders were cold sprayed with both nitrogen and helium gasses. Cold spray of high strength materials is still challenging due to their resistance to particle deformation affecting the resulting deposit properties. Powder heat treatment to modify its deformation behavior has recently been developed for aluminum alloy powders, however, there is no literature reported for Inconel 718 powders. The microstructural evolution of the powder induced by the heat treatment was studied and correlated with their deformation behavior during the cold spray deposition. Deposits sprayed with heat-treated powders at 800 and 900 °C and nitrogen showed less particle deformation and higher porosity as compared to as-atomized deposit associated to the presence of delta phase in the powders precipitated by the heat treatment. In contrast, deposits sprayed with helium using both powder conditions, as-atomized and as heat-treated powders, showed high particle deformation and low porosity indicating that the type of gas has a greater effect on the particle deformation than the delta phase precipitated in the heat-treated powders. These results contribute to understanding the role of powder microstructure evolution induced by heat treatment on the cold spray deposits properties.

Residual stress can be developed in most thermally sprayed coatings due to the momentum of molten particles during impact, and heat transfer during solidification of the splats. Another reason for residual stress built-up in thermally sprayed coatings is due to splat curl-up during solidification and the differences in thermal expansion coefficients between the coating and the substrate. However, in the cold spraying process, it is believed that the main reason for residual stress formation is plastic deformation during impact and flattening of solid particles. Residual stresses can drastically influence coating quality and reduce its service time. In this study, residual stress is measured for two well-known nickel based super alloys (Inconel 625 and Inconel 718) deposited on 7074 aluminum alloy substrates by the cold spraying technique. Residual stress in Inconel 625 was found to be highly tensile on the surface and compressive on the subsurfaces. After heat treatment the residual stress was relieved and was compressive in nature. Whereas for Inconel 718, residual stress was compressive on the surface and tensile on the subsurfaces in the as-sprayed condition. After heat treatment, the residual stress was compressive with increased magnitude. The heat treatment at 800°C made the residual stress more compressive. The porosities of both Inconel 625 and Inconel 718 were reduced after heat treatment.

This study assesses the viability of using nitrogen instead of helium to cold spray NiCoCrAlTaY coatings onto single-crystal superalloy substrates. The process, though feasible, has a low deposition efficiency, leading to a high level of deformation that affects the microstructure of both the coating and substrate. SEM and TEM analysis revealed metallurgical and mechanical bonding at the interface and grain refinement in the coating. A fine grain structure that developed in the substrate after deposition was also observed possibly caused by dynamic recrystallization during the deposition process. Evidence of element segregation in the substrate, identifiable as zones with a deformed γ/γ’ structure, was found as well.

This study demonstrates a two-step laser cladding process for copper substrates in which cold spraying is used as a powder preplacing method to overcome problems associated with the high laser reflectivity of copper as well as the effects of high-temperature oxidation. In the first step of the process, Inconel powders are cold sprayed onto pure copper, producing a layer with a thickness of about 250 μm and a porosity of 0.88%. This is followed by a 3.5 kW laser remelting treatment using a 1030 nm laser with a spot size of 2.5 mm. Examination and testing of the as-sprayed and remelted layers show how the laser treatment improves coating microstructure, hardness, density, and metallurgical bonding.

Sand blasting and high-velocity thermal spray processes can produce residual stresses in superalloy substrates that can significantly influence microstructure development. To investigate this effect, single-crystal superalloy substrates were sand blasted using different levels of force (zero, light, and heavy) and then coated with a MCrAlY layer by HVOF spraying. Cross-sectional analysis of an as-sprayed sample revealed a subsurface depletion zone with a composition rich in Mo nano precipitates. Cross-sectional examinations after vacuum heat treating and at various points during oxidation testing showed that elemental interdiffusion occurred between the coating and substrate and that sand blasting intensity has a major influence on the depth of the interdiffusion zones.

Repairing of Ni-alloy components using cold spray is being increasingly considered as an option in the aerospace industry. To further the understanding of the microstructure of Ni-alloy coatings and the bonding mechanism, gas atomised alloy 718 was sprayed onto carbon steel substrates to form 0.5mm thick coatings and single particle impacts. Spray trials were performed with different process parameters to compare the splat and coating morphology/microstructure and to optimise the parameters. The powder consumable, single particle impacts and coatings were characterised using SEM, EBSD, TEM and nanoscale XRF and XRD. Four-point bend tests were performed to test strength, ductility, cracking and de-bonding. Fine grains were observed in the substrate-particle interfaces caused by particle fragmentation, deformation and dynamic recrystallisation. Low angle grain boundaries and sub-grains form in the substrate due to strain induced by high energy impacts. The deposition efficiency, thickness, porosity, hardness and surface roughness of the coatings were measured and compared across all parameters. The porosity decreases notably (1.2% to 0.25%) and the hardness increases (410HV to 465 HV) with the increase in gas temperature and pressure. The results indicate that temperature has a larger effect on the coating properties compared to the pressure and that deformation has an important role in bonding.

A method measuring the thermal conductivity and the interfacial thermal resistance of thermal barrier coatings (TBCs) which consist of metallic bond-coats (BCs) and ceramics top-coats (TCs) on superalloys was newly developed. It was based on the areal heat diffusion time method analysing the heat diffusion across multilayers. The developed method was experimentally verified using the BC and the TBC specimens coated by APS. It was found that there were the interfacial thermal resistance not only between the TC and the BC but also between the BC and the substrate. Furthermore, the thermal conductivities of the BC and the TC obtained from the BC and the TBC specimens by this method considering the interfacial thermal resistance were in good agreement with those measured from the free-standing specimen of each coating. Thus, it was confirmed that the newly developed method is effective to evaluate the thermal conductivity and the interfacial thermal resistance of the TBC.

In this work, interdiffusion between different nickel-based superalloys and two MCrAlY bond coats, containing different chemical compositions, is investigated. To determine the influence of the coating deposition process, the MCrAlY bond coats were applied using two different spraying processes, high velocity oxygen fuel spraying (HVOF) and low-pressure plasma spraying (LPPS). Of primary interest is the evolution of Kirkendall porosity, which can form at the interface of substrate and bond coat and depends largely on the chemical compositions of the coating and substrate. Experimental evidence suggested also a dependence on the coating deposition process. Formation of porosity at the interface causes a degradation of the bonding strength between substrate and coating, with functional breakdown of the coating system as a worst result. After coating deposition, the samples were annealed at 1050 °C for varying test periods up to 2000 hours. Microstructural and compositional analyses were performed to determine and to evaluate the Kirkendall porosity. The results reveal a strong influence of both the spraying process and the chemical composition. The amount of Kirkendall porosity formed, as well as the location of appearance and the shape, is largely influenced by the coating deposition process. In general, samples with bond coats applied by means of HVOF show accelerated element diffusion. It is hypothesized that recrystallization of the substrate material, as a consequence of the surface treatment prior to coating deposition, is the main root cause for these observations.

Ti-Al and Al-Cr metallic coatings were deposited on Superfer800H (Fe-based superalloy) through a plasma spray process. Then the gas nitriding of the coatings was done in the lab and the parameters were optimized after conducting several trials on plasma sprayed coated specimens. Characterization and high temperature corrosion behaviour of coatings after exposure to air and molten salt at 900°C were studied under cyclic conditions. Techniques like XRD, SEM/EDAX and EPMA analysis have been used for characterization of the coatings and to analyze the oxide scale. Both the coatings have successfully protected the substrate and were effective in decreasing the corrosion rate when subjected to cyclic oxidation at 900°C for 50 cycles in air and molten salt. The coatings subjected to cyclic oxidation in air have shown relatively high weight gains in the early cycles of the exposure. Uncoated Superfer800H (Fe-based superalloy) showed very poor resistance to hot corrosion in molten salt environment.

A thermal cycling test was carried out in an EB-PVD MCrAlY – superalloy system, the result of which showed that a continuous β-NiAl layer formed in the MCrAlY coating near the coating–superalloy interface. Since β phase can be as a reservoir of Al, the formation of the β layer, in which much Al is reserved, is probably beneficial to the coating’s life. An oxidation-diffusion model was adapted to simulate the development of the microstructures in the coating-superalloy system. The simulation results indicate that the formation of the β layer was strongly related to the high Al activity in the substrate; if the Al activity of the substrate was high enough, a β layer could be built up in the coating near the coating-substrate interface.

The stiffness and thermal conductivity of thermal barrier coatings (TBCs) are inevitably changed by healing up of intersplat pores and intra-splat cracks during high temperature exposure, which results in less compliance and thermal insulating performance. However most publications on sintering of plasma sprayed TBCs are based on free-standing coatings, which ignore residual stress and the stress produced by the mismatch of thermal expansion coefficient between substrate and ceramic top coatings. In this paper, individual splat of YSZ and YSZ coatings have been sprayed on substrate of YSZ and Ni-based superalloy. Evolution of healing and morphology of 2D cracks and some properties, such as hardness and thermal conductivity, have been revealed during thermal exposure. Results showed that, during heating stage, the shear stress coming from substrate caused some tearing of bonding area tips and narrowing of inter-splat pores. Some recoverable and unrecoverable widening on intra-splat cracks occurred also due to shear stress. During annealing stage, compared with free-standing coating, the surface hardness of the coating deposited onto the Ni-based superalloy showed enhanced increasing due to the faster healing of inter-splat pores by narrowing down, and the hardness in cross-section presented retarding increasing due to the widening of cracks in out-plane direction leading to slower healing. The case of YSZ substrate fell between free-standing case and Ni-based superalloy. This would benefit the further in-depth understanding of the thermal cycling failure mechanism of plasma sprayed TBCs.

Cu-Ag-Zn abradable seal coatings were prepared on superalloy DZ445 by cold spraying. The micro-morphologies of Cu-Ag-Zn powders and coatings were analyzed by scanning electron microscopy (SEM) and electron dispersion scanning (EDS). The hardness of Cu-Ag-Zn coatings was analyzed by Vickers hardness of Rockwell. Thermal shock test was done to evaluate the combination strength between Cu-Ag-Zn coating and the substrate. In addition, the friction and wear properties of Cu-Ag-Zn coatings were also tested on a disc sliding friction machine sales. In order to improve the combination strength and the quality of the coating, two methods including annealing the coatings in vacuum and heating the feed powders in cold spraying were tried, respectively. It is shown that Cu-Ag-Zn coatings made by cold spraying with powder heating exhibit better combination strength and anti-friction properties, comparing with coatings annealed in vacuum and the original samples. In addition, the combination mechanism of Cu-Ag-Zn coatings was also proposed.

In this paper, the development of surface oxide scale and the evolvement of spallation mechanism of Fe-21Cr-5.6Al super alloy were investigated at 1200°C and 1300°C. The oxidation kinetic curves were obtained by isothermally measuring the weight gain of alloy oxidized with various time durations. The morphologies of oxide scale and grain structures were observed by SEM/EDX, and the phase structure was analyzed by XRD. The results show that the oxidation processes follow the parabolic law and the oxidation rate is higher at 1300°C than 1200°C. Though the FeCrAl alloy shows capabilities against oxidation even at a high temperature of 1300°C, the oxidation behavior and mechanism are distinct from those at moderate temperatures (<1000°C). Different morphologies and phase structure were found in oxide scales generated at different temperatures within the same time duration. Typical buckling was observed in the super alloy when it was subjected to 1200°C. Equiaixed grains with multiple voids were found near the alloy surface. At 1300°C, a flat and thicker oxide layer was formed. The grains were stretched vertically against the alloy and presented as coarse and compact near the interface. The vertically stretching of grain was triggered by fast element transportation inside the alloy. The differences in grain morphologies among the different test temperatures demonstrated that although the super alloy followed parabolic law at both test temperatures, the oxidation processes were different due to the evolvement of grain morphologies and oxide scale structures caused by exposure to high temperature.

Presently, highly stressed components in gas turbines are mainly made of single crystal nickel based alloys and the maximum application temperature (without coatings) is typically limited to 1100°C. Superalloys are now reaching limits posed by their melting temperatures. Increasing the substrate temperature beyond 1200°C will increase the efficiency of the turbine significantly. A new generation high temperature Co-Re alloys are aimed for use at +100°C above present single crystal nickel-superalloys. The substrates will be protected against the higher gas temperatures by thermal barrier coatings. For Co-Re alloy substrates CoReCrSi is a promising bond-coat material. CoReCrSi is thermo-chemically compatible to Co-Re due to the very similar mechanical and chemical properties. The oxide formation and the adhesion of the top coat are being investigated by studying a simplified coating system. The coating system consists of a CoReCrSi bond coat bulk material, and an yttria-stabilised zirconia top coat. The system was tested under cyclic conditions at 1200°C. This study provides a first insight into the TGO growth, the basic failure mechanism of the top coat, and the diffusion processes at the top coat/bond coat interface. It is shown that CoReCrSi with 2 at.% silicon promotes a good adhesion of the top coat by forming a dense chromium oxide layer. The critical TGO thickness beyond which the TGO fails by spallation was determined to be 25 microns and is roughly 2.5 times the critical thickness in MCrAlY based system in nickel-alloys.

In twin wire arc spraying process it is possible to use feedstock wires of two different compositions at the same time. As a result of this procedure it can be achieved composite coatings called also as pseudo alloys with modified physical properties. In this study nickel and cobalt based super alloy materials were arc sprayed with pure molybdenum wire to tailor corrosion and wear resistance of the coatings. Coatings for the tests were sprayed using two different twin wire Sulzer Metco arc-spraying units, Smart Arc and OSU 300, operating with suitable spray parameters to produce coatings of good quality. It was already known that these twin wire configurations are producing coatings with differing microstructures. Coating sprayed with the OSU system was clearly finer in structure and one purpose of this study was to measure the effect of the micro structural size on the corrosion and wear properties of the final coatings. Microstructures of the coating materials were studied and analyzed from cross-sectional specimens. Volume fraction of pure molybdenum in the coating matrices was evaluated with simple line method and according to the results volume fraction of pure molybdenum metal is over 50 volume-% in all of these tested composite coatings and higher in materials sprayed with OSU unit. Also the microstructure of the coatings was seen to be finer when OSU was used as was expected. Wear resistance was measured with modified ASTM G65 rubber wheel sand abrasion wear test and corrosion resistance was tested in low pH values and chlorine containing environment according to the ASTM G48 corrosion testing standard. Corrosion testing was carried out at room temperature 22°C and also at higher 50°C temperature. Molybdenum addition is clearly improving the abrasion wear resistance of the tested coating systems. At room temperature also the corrosion resistance is getting better with molybdenum addition but at higher temperature this effect is not so clear.

Fundamental understanding of relationships between process parameters, particle in-flight characteristics and adhesion strength of HVOF sprayed coatings is important to achieve the high coating adhesion that is needed in aeronautic repair applications. In this study statistical Design of Experiments (DoE) was utilized to identify the most important process parameters that influence adhesion strength of IN718 coatings sprayed on IN718 substrates. Special attention was given to the parameters combustion ratio, total gas mass flow, spray distance and external cooling, since these parameters were assumed to have a significant influence on particle temperature and velocity. Relationships between these parameters and coating microstructure were evaluated to fundamentally understand the relationships between process parameters and adhesion strength.