Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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.

Abstract 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.

Abstract Cold gas spraying, which has found its way into industrial production, enables the rapid production of various layer systems almost at room temperature. With this method, the errors can be avoided and layers with low porosity and a lower degree of oxidation can be produced. A novel technology of the hot injection process combines the advantages of cold gas and conventional injection processes. With this technology, the powders used are greatly accelerated and heated up to a temperature that allows good substrate-layer adhesion without the powder being melted. Hot-sprayed layers of superalloys, copper, and steel with the high frequency pulse detonation spray gun are shown in this paper. The advantages of the layers produced in this way are compared with those produced with conventional or cold spray processes. The result is interesting novel materials and surface coating options. Paper includes a German-language abstract.

Abstract This paper utilizes the chromium alloy powders of several different particle sizes with narrow size distribution to deposit the coatings and collect the powders passing through high velocity oxy-fuel (HVOF) flame, which yielded different oxide contents. The corrosion resistance of a thermally sprayed chromium alloy layer is impaired by the oxides contained in the layer. The dependence of the layer oxide content of HOVF-chromium alloy layers on the particle size is closely linked to the dependency between application behavior and particle size. The physical process concerning with the oxidization during deposition of metallic coating through thermal spray process is still much lack of well understanding. The results show that the oxide content both in the layer and in the sprayed particles is extremely dependent on the particle size. As a result, the spray particle will be likely deposited into coating without further server oxidation after it impacts on substrate surface. Paper includes a German-language abstract.

Abstract Laser beam forming is a process in which metal is deposited directly, which combines the processes of high-performance laser alloying and the advanced method of rapid prototyping in order to be able to manufacture complex three-dimensional components directly. In this article, this technique is used to process Ni-based superalloys. Simple component geometries are used in order to be able to compare them with conventional alloys. The 718 superalloy is used. The characterization of the alloys produced included a complete microstructure analysis. Paper includes a German-language abstract.

Abstract Since its commercial introduction in the early 1980s, it has been known that HVOF spraying can produce layers with low oxide levels, high toughness, and excellent adhesive strength. Compared to low-pressure plasma and vacuum plasma spraying, the HVOF process is based on different melting dynamics, which do not require the use of a controlled atmosphere. As a result, the original oxygen content of the wettable powder plays a significant role with regard to the layer quality. In this paper, the oxygen content of normal MCrAlY powder is varied and its influence on the quality of the sprayed coating is examined. It is concluded that, by controlling and limiting the initial oxygen content in the spray powder, better layer qualities with lower oxide proportions and higher density can be achieved. Paper includes a German-language abstract.

Abstract Substrates protected by thermal spray coatings are usually found intact after use, making them viable candidates for recycling and reuse. The key is to remove the coating without damaging the component. This requires a process that minimizes the development of residual stresses and the associated distortion. The purpose of this work is to determine the optimal descaling technique for Ni-base sheets with a thermal barrier coating. Test specimens were produced following industry procedures. Thin sheets (<3 mm) of Ni-base superalloy were plasma sprayed with a NiCrAlY bond coat and a Y203-stabilized ZrO2 topcoat. The coating layers were then removed using different methods, including pickling, shot blasting, and water jet descaling, and the substrates were assessed based on X-ray diffraction and chord width measurements. The findings of the study show that water jetting removes all surface materials, particularly the bond coat, without damaging the underlying surface. It also produces the least amount of stress and deformation and is relatively easy to automate.

Abstract Ni+Al, Ti+Al, NiCr+AI, and Cr+Al powders react exothermically in the heat zone of thermal spraying systems. Whether such reactions occur between aluminum and superalloy powders is the underlying question of this study. This paper describes composites of this nature and their sprayability to form adherent, metallurgically bonded deposits. Through parametric manipulation, coatings can be produced with a range of properties from dense to open (porous/abradable) structures. The paper also shows how seed particles can be clad with aluminum and sprayed to predictable property limits.

Abstract Methodology to evaluate the adhesive strength between the MCrAlY alloy coating film and the Ni-base superalloy substrate was studied and proposed. By employing the double cantilever beam specimens which were taken from the CoNiCrAlY alloy coated Ni-base superalloy, the fatigue crack propagation tests along the interface were carried out. Through the work particular attention was given to the threshold level to the fatigue crack propagation along the interface as a measure to represent the adhesive strength, based on fracture mechanics approach. The effects of temperature, the surface finishing of the substrate and the long term thermal aging on the adhesive strength were also investigated.

Abstract The feasibility of using the HVOF process for the thermal spray-forming of free-standing components has been investigated. HVOF spray forming offers a number of potential advantages compared to the established procedure of plasma forming, including increases in component density, and reduction in material decomposition during spraying. Using blends of carbide and superalloy powders in various proportions, HVOF spraying has been successfully used to form free-standing cylinders and cones of various lengths and thicknesses. Microstructural examination of the spray-formed material, using optical microscopy and scanning electron microscopy (SEM), has shown a homogeneous distribution of carbides in the superalloy matrix, with very low levels of porosity. Vickers microhardness has been measured on several sprayed forms. In order to complete the study of the different systems, abrasion (Rubber Wheel Test), friction (Ball on Disk Test) and erosion wear results have been obtained. These wear results have been used in order to evaluate the behaviour of the sprayed samples with a different powder percentage in the blends. Corrosion tests have been done to evaluate the corrosion resistance of the sprayed samples (ASTM D-1411).

Abstract The aim of this study was to investigate potential weight savings using multi-layer blade containment systems for turboengines. The association of an external ductile layer with an internal hard layer could provide a good ductility of the armor with the capability to withstand the perforation of high kinetic projectiles. Comparisons between several thick deposits obtained by the vacuum plasma spray process were performed using a Charpy impact testing machine. Mechanical and structural characterisations of these two-layer structures were performed and compared to the behavior of monolithic ones. Heat treatment effects were also considered.

Abstract Nanocrystalline Inconel 718 and Ni powders were prepared using two approaches: methanol and cryogenic attritor milling. High velocity oxy-fuel (HVOF) spraying of milled Inconel 718 powders was then utilized to produce Inconel 718 coatings with a nanocrystalline grain size. Isothermal heat treatments were carried out to study the thermal stability of the methanol milled and cryomilled Inconel 718 powders, as well as the HVOF Inconel 718 coatings. All nanocrystalline Inconel 718 powders and coatings studied herein exhibited significant thermal stability against grain growth as evidenced by a grain size around 100 nm following annealing at 1273 K for 60 min. In the case of the cryomilled nanocrystalline Ni powders, isothermal grain growth behavior was studied, from which the parameters required for the prediction of the microstructural evolution during a non-isothermal annealing were acquired. The theoretical simulation of grain growth behavior of nanocrystalline Ni during non-isothermal annealing conditions yields results that are in good correspondence with the experimental results.

Abstract Reactive plasma spraying (RPS) with hydrocarbon (HC) gas has been studied as a way to improve the mechanical properties of Ni-Cr alloys and reduce the oxygen content of MCrAlY coatings. A conventional dc plasma torch has been modified by attaching a conical graphite tube (reactor) onto the end of the gun and spraying parameters were adjusted accordingly. Significant improvements have been achieved in terms of both objectives. As test results indicate, the hardness of Ni-Cr alloys has been doubled and the oxygen content in MCrAlY coatings has been reduced by an order of magnitude.

Abstract The paper presents the influence of laser-beam remelting of Al 2O3 -Ni ceramic layers spread on creep-resisting alloy by means of plasma spray upon the quality of ceramic coatings, which form thermal barrier and high temperature corrosion shield of these alloys. The examination showed that 0.103.10 9 W/m2 power density laser-beam scanning ensures good quality of coatings at beam moving rate 1 to 2 m/min. At smaller scanning rate, laser remelted ceramic layer spalls and chips. Better quality of the ceramic coatings can be obtained by diffusive chromoaluminizing applied prior to laser remelting. Laser-beam remelting conditions of such layers, elaborated in the course of examination, ensure pores fading, decrease of remelted layer thickness and increase of base adhesion without cracks, chips and spalls. Additionally remelted zone features either strong break up of structure or the structure is amorphic with unchanged chemical composition as to the matrix chemical composition. Obtained results are the base for the elaboration of thermal barrier technology and the technology of anticorrosion shield for creep - resisting alloys applied in high temperature power engineering.

Abstract This paper explains how laser-beam remelting influences the quality of ceramic coatings on superalloy substrates. The coatings studied consist of Al2O3-Ni ceramic layers. Test results showed that a laser power density of 0.103 x 109 W/m2 is ideal for a beam rate of 1 to 2 m/min. It was also found that coating quality could be further improved by adding a diffusive Cr-Al layer prior to laser remelting. This added step reduces pore volume and layer thickness and increases adhesion without cracks, chips, or spalls. The examination also showed that, depending on cooling rate, it is possible to obtain layers that are similar in composition but different in structure or even amorphic.

Abstract Pre-alloyed and plasma spheroidized composite powders were used as the feed-stock in the plasma spraying of functionally graded ZrO 2 /NiCoCrAlY coatings. The ball milling parameters of the composite powders and the plasma spraying parameters for preparing FGM coatings were optimized in order to obtain the best performance for the thermal barrier coatings. Microstructure, physical, mechanical and thermal properties of ZrO 2 /NiCoCrAlY FGM coatings were investigated and compared with those of traditional duplex coatings. Results showed that the advantages of using prealloyed composite powders in plasma spraying were to ensure chemical homogeneity and promote uniform density along the graded layers. Microstructure observation showed the gradient distribution of ZrO 2 and NiCoCrAlY phases in the coating and no clear interface was found between two adjacent different layers. Oxidation of elements in Ni alloy occurred during plasma spray and the so-formed Al 2 O 3 and Cr 2 O 3 combined with ZrO 2 in a wide range of proportions. The bond strength of FGM coatings was about twice than that of the duplex coatings. Results also indicate that the thermal cycling resistance of FGM coating was much better than that of the duplex coating.