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In nuclear fusion reactors, the first wall is the name given to the surface which is in direct contact with the plasma. A part of it is the divertor which is a device that removes fusion products from the plasma and impurities that have entered into it from the vessel lining. It is covered with water cooled tiles which have to withstand high temperatures and high heat fluxes. Moreover, resistance to neutron bombardment, low tritium absorption and low hydrogen permeation are additional demands. One materials concept under research is the application of a Reduced Activation Ferritic Martensitic Steel (RAFM) as a structural material with a tungsten protective coating. Since there is a considerable thermal mismatch between, a functional graded materials (FGM) concept was proposed. As the formation of undesired intermetallic Fe-W phases as well as oxidation should be avoided, cold gas spraying was chosen as manufacturing process. Two powder blends of EUROFER97 RAFM steel and a fine tungsten powder cut on the one hand and a coarser one on the other hand were tested in different ratios. The coatings were characterized with respect to their porosity and surface structure. Furthermore, the deposition efficiencies for steel and tungsten were determined each. It turned out, that the deposition process is a complex mixed situation of bonding and erosion mechanisms as the deposition windows of these very different materials obviously diverge. Thus, a lower working gas temperature and pressure was advantageous in some cases. Unexpectedly, the coarser tungsten powder in general enabled to achieve better results.

Functionally graded (FG) coatings were manufactured by High Velocity Oxy-Fuel (HVOF) thermal spraying and tested aiming for the high temperature applications. Single layers were manufactured and their elastic modulus measured by using Impulse Excitation Technique (IET). Obtained data was used for modeling of optimal gradient structure. Dual feeding hose for HVOF gun was developed. Calibration procedure for the concurrent use of two powder feeders was performed. NiCr-Al 2 O 3 coatings with coating thickness of 600 µm and 1000 µm were manufactured and tested. Promising results were obtained from high temperature corrosion tests.

The application of FGMs is quite difficult, but thermal spray processes like Plasma spray have demonstrated their unique potential in producing graded deposits, where researchers have used twin powder feed systems to mix different proportions of powders. FGMs vary in composition and/or microstructure from one boundary (substrate) to another (top service surface), and innovative characteristics result from the gradient from metals to ceramics or non-metallic to metals. The present study investigates an innovative modification of a HVOF (High Velocity Oxy- Fuel) thermal spray process to produce functionally graded thick coatings. In order to deposit thick coatings, certain problems have to be overcome. Graded coatings enable gradual variation of the coating composition and/or microstructure, which offers the possibility of reducing residual stress build-up with in coatings. In order to spray such a coating, modification to a commercial powder feed hopper was required to enable it to deposit two powders simultaneously which allows deposition of different layers of coating with changing chemical compositions, without interruption to the spraying process. Various concepts for this modification were identified and one design was selected, having been validated through use of a process model, developed using ANSYS Flotran Finite Element Analysis. In the current research the mixing of different proportions of powders were controlled by a computer using LabVIEW software and hardware, which allowed the control and repeatability of the microstructure when producing functionally graded coatings.

Functionally graded coatings with continuous changes of microstructures and properties across the material are expected to have low residual and thermal stresses and improved bonding strength between base materials and the ceramic coatings. This paper presents a technique to produce high-performance graded coatings in which the mix of components in the coating changes continuously from the base materials out. A hybrid HVOF-Arc spray gun has been employed to create such coatings. The metallic matrix material is utilized in the form of wires that are fused by arcing process. A high velocity combustion jet carrying the ceramic particles atomizes the molten material and mixes the ceramic particles with the matrix material. The feed rate of the matrix material and the reinforcing material are controlled together giving a systematic variation of the reinforcement phase. Two material systems; WC-16%Cr 3 C 2 -Ni-5%Al and B 4 C-Al(5%Si) have been investigated. The in-flight particle characteristics of the process have been characterized. The resulting microstructures and process capabilities are discussed.

Functionally graded material (FGM) designs aim to combat challenges posed by harsh and conflicting operational requirements. There is a great demand for manufacturing technologies that can fabricate net-shape components incorporating the FGM design concepts. The focus of this paper is on the development of net-shape components incorporating functionally gradient material designs. This novel manufacturing scheme combines the use of re-useable mandrels to generate complex shapes with a unique thermal spray process called “hybrid spray” that can deposit functionally designed materials. This generic hybrid spray process combines arc spraying with either high velocity oxy-fuel (HVOF) or plasma spraying (APS). The resulting benefits are; ability to achieve continuous composition variation, high deposition rates combined with the high coating densities. The multi-material component designs aim to provide cost as well as performance advantages. The geometric resolution and dimensional accuracy of these functional components are presented. This paper also reports results on material compatibility and process parameter development tests. Functional properties such as high temperature resistance and thermal fatigue performance are also reported.

In this paper, WC-Co reinforced polymer matrix coatings are sprayed on preheated steel and carbon-fiber reinforced substrates, producing relatively dense, adherent coatings. The particle morphology of the feedstock materials and the microstructure of the HVOF sprayed coatings are characterized and the thermal properties of the polymer powder and coatings are compared. It was found that the deposition and build-up of the polymer coating was only successful when substrates were preheated to the curing temperature of the thermosetting polyimide powder used. Layered coatings of varying polyimide and WC-Co content have been successfully deposited, showing that it is possible to produce graded composite coatings consisting of pure polymer at the substrate and pure WC-Co on the surface. Paper includes a German-language abstract.

Functionally graded coatings exhibit outstanding thermomechanical performances that meet economic demands. By influencing the microstructural formation of the layers of the coated tool, the distribution and transfer of heat from the impact zone into the forming process is determined. Thus, largely affects the tool’s life cycle and microstructure in the surface area of the parts to be formed. With respect to graded coatings, examinations on simple substrates have already shown the applicability of different microstructure formations of coatings. In these approaches, varied parameter settings were investigated to identify the most significant factors and their effects on the formation of the microstructure. Furthermore, using different grain-sized powder materials and utilizing other thermal spraying methods, it is possible to generate either porous or dense coatings according to their requirement profile. The utilization of different thermal spray methods particularly leads to high costs caused by increased setup times. As opposed to traditional thermal spray deposition process parameters for graded coatings, generated by an in-situ process, need to become coordinated with the movements of the robot. In this study, we investigated the dependencies between the spray parameters of Atmospheric Plasma Spraying, such as spray distance and spray angle, and the formation of microstructures. The investigation focusses on the utilization of Alumina (Al 2 O 3 ) which is widely used as a wear and corrosion resistant coating.

The one problem of zirconia (ZrO 2 ) coatings is control of the porosity, because the functionality of wear resistance, corrosion resistance, and so on are required for their application in the extreme environment. In this study, a high hardness and dense ZrO 2 coatings is formed by a gas tunnel type plasma spraying under the condition of a short spraying distance. The properties of this ZrO 2 coating formed are investigated about coating qualities, such as Vickers hardness characteristics, wear characteristic, etc. The graded functionality of this coating is clarified by the coating qualities.

In this study, two types of thermal barrier coatings (TBC); duplex and functionally graded coatings were deposited on superalloy Nimonic 263 substrates using air plasma spray process. The duplex coating consists of YSZ top coat and NiCrAlY bond coat. The functionally graded coating consists of five layers with GZ as top layer, GZ+YSZ and YSZ+NiCrAlY as intermediate layers. The TBC samples were subjected to isothermal heat treatment at 1100 °C for 100 hours before undergoing thermal cyclic tests at 1200 °C up to 20% spallation to evaluate the oxidation and thermal fatigue resistance of the coatings. Results indicate that the functionally graded GZ TBC has a better cyclic life than the duplex YSZ TBC after isothermal heat treatment. The isothermal heat treatment also improved the thermal cyclic lifetime of the functionally graded GZ TBC by more than threefold in comparison to the as-sprayed GZ TBC.

The use of polymer matrix composites [PMC's] in the gas flow path of advanced turbine engines offers significant benefits for aircraft engine performance, but their useful lifetime is limited by their poor erosion resistance. HVOF and flame sprayed polymer/cermet functionally graded coatings based on a polyimide matrix filled with varying volume fractions of WC-Co are being investigated to improve the erosion and oxidation resistance of polymer matrix composites. A study of the coating's effectiveness as erosion barriers was accomplished through a statistical analysis of the results of solid particle erosion testing of coated and uncoated PMC samples using a design of experiments [DoE] approach. Three coating systems and a control were evaluated in a randomized test matrix. The coatings were tested at room temperature and 250 °C, using an alumina erodent impacting the coatings at a speed of 100 m/s at angles of 20° and 90°. Erosion volume loss at 250 °C was approximately twice than at room temperature, but the maximum erosion volume loss did not exceed 0.30 mm 3 at the elevated temperature. In general, as the angle of incidence of the eroding material increased from 20 degrees to 90 degrees the volume loss increased.

Functionally graded coatings (FGC), in which the composition and properties vary gradually from the bond layer to the top layer, are introduced in this study. Using a mixing nozzle, a graded coating was generated employing twin wire spraying (TWAS) process. The deposition started with a base layer of massive metallic wires for better adhesion to the substrate. At following top layers hard material, in form of powders, was injected to the massive wires to enhance the wear protection. The results show that microstructure, porosity, and compositions are gradually varied in the coatings. This is a clear indication for the better performance in as-sprayed FGC than coatings sprayed by means of cored wires. The approach was to grade the coating composition from pure metallic to composite with higher content of hard material particles. The goal of the study is to articulate the needed coating performances by customizing the layers deposited regarding to their position.

Thermal barrier coating (TBC) structures composed of Al 2 O 3 and ZrO 2 with different chemical compositions on the NiCoCrAlY bondcoat are proposed to improve the oxidation resistance of TBC systems. The concept of functionally graded materials (FGM) was applied to manage residual stresses due to sharp interface between dissimilar materials that can lead to a premature failure of the TBC system. A numerical study using finite element analysis was performed to investigate the effects of system architecture on the residual stresses developed in Functionally Graded - Thermal Barrier Coatings (FG-TBCs) and in a typical duplex TBC comprising of NiCoCrAlY bondcoat and ZrO 2 topcoat. The effects of different cooling rates and substrate preheating process on the residual stress distribution were also studied. The results showed that lower cooling rate and substrate preheating process reduce stresses within the duplex coating. In addition, the incorporation of Al 2 O 3 interlayer results in a manageable level of residual stress. Stresses at critical locations are reduced, and hence contributing to an increase in resistance to the interfacial crack. The possibility of surface cracks is also reduced since the radial and tangential stresses within the FGTBC system are lower than those of the duplex system. It was found that the thickness of Al 2 O 3 layer and the number of graded layers introduced between Al 2 O 3 and ZrO 2 do not significantly affect the stress distribution. To provide adequate comparison to the computational results. X-ray diffraction was used to assess the in-plane residual stresses on the coating surface.

Atmosphere plasma spray and robot digital fabricating were integrated and presented to prepare the planar PEN and MOLB-type PEN SOFCs. On the basis of the spraying conditions optimized previously and the self-developed functionally graded powder feed system, two kinds of PEN cells were prepared. Then the microstructure and material components of the PEN cells were analyzed. The results show that graded layers were formed between the electrodes and electrolyte. Moreover, the material components and the porosity of the graded layers vary gradually. In particular, the porosities of the anode and cathode reach 32.74%, 32.24%, respectively. Using the AC complex impedance technology, the conductivity of the MOLB-type composite electrode is tested. As a comparison, the electrical conductivity of the MOLB-type composite electrode with the graded layers is larger than that without the graded layers.

Functionally graded thermal barrier coatings (FG-TBCs) with a gradual composition change from the MCrAlY bond coat (BC) layer to the yttria stabilized zirconia (YSZ) TBC layer is one way to decrease the residual stresses at the BC/TBC interface during a thermal cycling process. At high temperatures, the oxidation resistance of TBCs is also important which would affect the cyclic life of the coatings. In this paper, two YSZ materials were used to make FG-TBCs and conventional duplex TBCs by applying air-plasma spray (APS) technique. The coated samples were thermal cycled with 24-hours heating at 1100 °C followed by air cooling. The results showed that the FG-TBCs performed earlier failure than the duplex ones. Microstructure study indicated that the poor oxidation resistance of the graded mixed layer was responsible for the coating failure. In addition, the influence of spraying parameters on the coating life was discussed.

This paper presents the results of long-term thermal cycling tests on plasma-sprayed thermal barrier coatings, including coating samples produced with functionally graded materials. The role of oxidation is also considered based on the results of elemental analysis. The authors explain how the coatings were produced and tested and present and analyze the test results. The thermal barrier coatings formed with functionally graded materials were found to be relatively unaffected after the long-term thermal cycling test and showed no signs of oxidation. Paper includes a German-language abstract.

The use of polymer matrix composites (PMC's) in the gas flow path of advanced turbine engines offers significant benefits for aircraft engine performance but their useful lifetime is limited by their poor erosion resistance. High velocity oxy-fuel (HVOF) sprayed polymer/cermet functionally graded (FGM) coatings are being investigated as a method to address this technology gap by providing erosion and oxidation protection to polymer matrix composites. The FGM coating structures are based on a polyimide matrix filled with varying volume fractions of WC-Co. The graded coating architecture was produced using a combination of internal and external feedstock injection, via two computer-controlled powder feeders and controlled substrate preheating. Porosity, coating thickness and volume fraction of the WC-Co filler retained in the coatings were determined using standard metallographic techniques and computer image analysis. The pull-off strength (often refered to as the adhesive strength) of the coatings was evaluated according to the ASTM D 4541 standard test method, which measured the greatest normal tensile force that the coating could withstand. Adhesive/cohesive strengths were determined for three different types of coating structures and compared based on the maximum indicated load and the surface area loaded. The nature and locus of the fractures were characterized according to the percent of adhesive and/or cohesive failure, and the tested interfaces and layers involved were analyzed by Scanning Electron Microscopy.

Mullite (Al 6 Si 2 O 13 ) is the basis of efficient environmental barrier coatings (EBCs) for protecting Si-based ceramic matrix composites (CMCs) selected to replace specific hot-section metallic components in advanced gas turbines. Furthermore, YSZ-mullite multilayer architectures with compositional grading between the bond coat and YSZ top coat were envisioned as solutions to ease their coefficient of thermal expansion (CTE) mismatch induced stress. Consequently, a proper understanding of the mechanical properties such as the elastic modulus, hardness or plastic/elastic recovery work serve for an efficient design of such refractory oxide multilayers. In this work, three different mullite powder morphologies (fused and crushed, spray-dried and freeze-granulated) were employed. Using depth-sensing indentation with loads in the range 100 – 500 mN, the role of the microstructure and morphology of the powder feedstock on the mechanical behaviour of air plasma sprayed mullite bond coats deposited on SiC Hexoloy substrates was investigated. Fully crystalline as-sprayed mullite coatings were engineered under controlled deposition conditions. Mechanical properties were measured for the as-sprayed coatings as well as for coatings heat-treated at 1300°C, in water vapour environment, for periods up to 500 h. Both E and H values of the coatings are found to be highly dependent on the morphology of the starting powders.

The aim of this study is to produce free-standing functionally-graded structures in which density varies continuously from 2.2 to 17.3 g/cm3 through a total thickness of 4.5 mm. In order to optimize material performance, it is necessary to account for the different combinations or ratios of materials (i.e., tungsten and aluminum) in the plasma jet when determining mixture laws. A relationship based on the deposition efficiency, powder feed rate, and density of individually sprayed materials has been established and was used to predict the density, thickness, and deposition efficiency of the combined materials. The mixture laws were found to be in good agreement with experimental results, making it possible to build up coatings with a parabolic density profile and uniform layer thickness.

Pre-alloyed, plasma spheroidized powders were used as feedstock in the plasma spraying of functionally graded ZrO 2 /NiCrAlY coatings. The advantage of using pre-alloyed powders was to ensure chemical homogeneity and promote uniform density along the graded layers, and these pre-alloyed powders could be successfully used to prepare the different inter-layers of functionally graded coatings. The microstructure, density and microhardness changed gradiently in the ZrO 2 /NiCrAlY coatings. The bond strength of ZrO 2 /NiCrAlY coatings with different graded layers was measured. Results showed that for as-sprayed coatings with the same thickness, the bond strength increased with the number of graded layers. The bond strength of the coatings with five graded layers was about twice as high as that of the duplex coatings because of the significant reduction of the residual stress in the coatings. Experimental results also showed that the bond strength of as-sprayed coating increased significantly after hot isostatic press (HIP) and vacuum heat treatments, and the reason can be attributed to the densification of the microstructure, the decrease of defects in the coatings, inter-diffusion between layers and further reduction in the residual thermal stress.

A functionally graded thermal barrier coating (FG-TBC) of CeO 2 -Y 2 O 3 -ZrO 2 /NiCoCrAlY was prepared using a recently developed supersonic plasma spray (S-PS) system. The system had dual powder feed ports through which the metal alloy powders were fed into the lower temperature region of the plasma plume, to prevent over oxidation, and the ceramic powders were fed into the high temperature region, to produce complete melting. Such an approach enabled fine configurations having a continuously graded composition transition to be obtained. The thermal shock testing of the deposited samples with 1mm thick FG-TBC system on Ni-based alloy substrates was performed using an in-house-designed multi-functional rotational thermal shock tester. In this tester, the heating-cooling curves and surface morphology of tested specimens can also be observed by a microscope with a CCD (charge-coupled device) camera and recorded on line by means of accessorial computer system to evaluate the thermal shock resistance of tested samples while they were heated by a high heat flux of oxygen-acetylene flame to 1200° C in a time interval of 15~20s followed by water-quenched to ambient temperature. The temperature fields and relevant thermal stresses distribution through the thickness of disk samples were calculated by means of ANSYS finite element method. The numerical approach shows that the maximal tensile stress occur at the C-YSZ top coating at the center of disc samples at the start of rapid cooling by water-quenched, where small reticulated surface cracks were observed, which then propagated perpendicularly about 350µm deep to near the interface between the pure C-YSZ coating and the FGMs layer transition between the C-YSZ and the NiCoCrAlY coatings. These vertical cracks appeared to be arrested without any delamination after 200 thermal shock cycles.