A novel model for coating design was proposed in this research, by considering both oxidation property and interdiffusion effect, corresponding to two factors of the Cr:Al activity ratio and the potential of Al, respectively. To verify this model, oxidation tests of coated superalloys were performed at 1000 ° C for up to 5000 h. The test results indicated a strong positive correlation between GPDZ and Al potential and a clear negative correlation between oxidation kinetics parameter k p and Cr:Al activity ratio. Our research opened up new ideas for the coating design.

A micro-plasma system was investigated for its capability in additive manufacturing (AM). Micro-plasma AM system has the advantage of lower cost and higher deposition rate over the laser-based AM systems, and generates leaner and cleaner weld deposit than other arc-based AM systems. However, the microplasma system is complex and involves a large number of process variables. In this study, the effects of two arc and wire feed modes on dimensional consistency and hardness were firstly examined. Subsequently, one set of the specimens was further subjected to oxidation tests and the results were compared to that from conventional wrought Inconel 718. It was found that all four processes could produce crack free samples without measurable distortion. Some surface discoloration was observed, ranging from light straw to a purple tint. After heat treatment, the hardness of the samples varies from 403 to 440 HV, with the transverse surface showing slightly lower hardness values. The oxidation tests at 900 °C yielded similar weight change for AM Inconel 718 and its counterpart wrought alloy; however, the rate constant for wrought alloy was slightly higher. Microstructural analysis with SEM and EDS revealed a dendritic structure in the AM Inconel 718 and the presence of Nb-rich compounds in the interdendritic region. The polycrystal grain structure was not delineated in AM material as that in wrought 718. With the increase of exposure time, the oxide layer continues to increase at a higher rate, along with a sublayer of Ni 3 Nb above the metal substrate. In addition, after 200 hours, the wrought alloy developed porous chromia, while AM material exhibited uneven oxide thickness. In consideration of all aspects of the evaluation carried out thus far, it is concluded that the AM material produced by micro-plasma process is equivalent to wrought material in mechanical properties and oxidation performance.

Anisotropy of stress-strain behavior, fracture toughness, and fatigue crack growth rate was studied for Inconel 738LC alloy built by the Dynamic Metal Deposition technique (3DMD, a high-speed Directed Energy Deposition technique). The measured quasi-static properties, i.e. stress-strain and fracture toughness showed only subtle anisotropy, with no more than 10% differences found for different orientations. The fatigue crack growth rate was influenced by the specimen orientation more significantly (30% for fatigue crack growth threshold, up to 90% for Paris exponent and coefficient). This pilot study attributes the anisotropy of fatigue crack growth properties to material texture and the columnar grain geometry resulting from directional solidification. The obtained testing results indicate that 3DMD technology can produce materials with good mechanical and fracture properties even from materials considered as non-weldable such as In 738LC. The study provides a solid experimental base for further investigation of the fatigue crack growth mechanism relation to the material texture in 3DMD In 738LC.

NiAl coating can be used as bond coats for thermal barrier coatings (TBCs) with good ductility and excellent resistance against high temperature oxidation. In this study, nickel-coated aluminum composite powders were used to prepare NiAl intermetallic compound coatings on nickel-based superalloys using an air plasma spray (APS), high-velocity oxygen-fuel (HVOF) and cold spray (CS) processes. Different spraying parameters in the HVOF and CS processes were used to make different coating microstructures, and the coating prepared by the APS technique served as a control for the HVOF and CS processes. The microstructure and phase constitution of the coatings were studied using XRD, SEM and EDS. The results indicate that the deformation behavior of the NiAl powder was different under the different spraying parameters. Less defects of oxides and inclusions were observed in the CS coatings compared with the HVOF coatings.

Additive Manufacturing (AM) processes offer geometrical freedom to design complex shaped parts that cannot be manufactured with conventional processes. This leads to new applications including aerospace propulsion systems where the Ni-superalloy based material has to withstand high operating temperatures. In this contribution suspension plasma sprayed YSZ TBC coating was applied on the spike contour of an additively manufactured aerospike engine demonstrator. The engine was designed for a hydrogen peroxide / kerosene 6 kN thrust at 2.0 MPa chamber pressure and was manufactured from nickel-based superalloy Inconel 718 powder using the laser powder bed fusion process (LPBF). Due to the novelty of the application of suspension sprayed YSZ thermal protection coatings on additively manufactured Inconel 718 components, extensive tests were necessary to characterize the interaction between the coating and the component.

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.

This paper presents a study on Cu-Ag-Zn abradable seal coatings prepared by cold spraying. The micro-morphologies of the coatings were analyzed using scanning electron microscopy and electron dispersion scanning. The hardness of the coatings was analyzed. Thermal shock tests were conducted to evaluate the combination strength between the coating and the substrate. The friction and wear properties of the coatings were also tested. Two methods, annealing the coatings in vacuum and heating the feed powders in cold spraying, were tried to improve the combination strength and quality of the coating. The results showed that Cu-Ag-Zn coatings made by cold spraying with powder heating exhibited better combination strength and anti-friction properties compared to coatings annealed in vacuum and the original samples. The paper also proposed a combination mechanism for the Cu-Ag-Zn coatings.

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.

Corrosion is a very important problem in the Municipal Solid Waste Incinerator (MSWI) superheaters. This problem causes the plant stops and tube replacements, promoting the loss of energy generation rate. The main corrosion agent is the chlorine deposits. HVOF coatings have been sprayed to improve corrosion resistance of the superheaters inside the MSWI boilers. Inconel 625 and Hastelloy C22 alloys have been sprayed as a powder feedstock material. The spray process has been analyzed by a Spray Watch system that allows carrying out the temperature and velocity measurements of the particles in flight in order to optimize the spraying process. The produced coatings have been characterized by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) to evaluate the porosity and oxidation produced in the spray process. Laboratory oxidation test have revealed the formation of Cr 2 O 3 , NiO and Fe 2 O 3 as corrosion products as well as Microanalysis by Energy dispersion Spectroscopy (EDS) composition profiles have been used to evaluate the penetration of the corrosion front inside the coating. The Nickel alloys show a good resistance in aggressive environment test and can be a good solution for the corrosion problems in the superheater tubes in MSWI.

Nickel-based superalloys can be used at temperatures up to 1050 °C in air. Superalloy open cell foam sheets with skin layers plasma sprayed on both sides can be used as high temperature heat exchangers provided that the two deposited skins are dense and well adhered to the open cell foam. In this study alloy 625 skins were deposited on each side of a sheet of metal foam by APS and HVOF to form a sandwich structure. Two densities of open cell foams, 20 and 10 pores per linear inch (ppi), were used in this study as the core. The initial Ni foam was converted to an alloy composition by plasma spraying aluminum and chromium on the foam’s struts with subsequent diffusion/solutionizing heat treatments before the alloy 625 skins were deposited. The microstructure of the coatings and the interface between the struts and skins was investigated. A layer of Ni-Al alloy was formed near the surface of the struts as a result of the heat treatment. The foam struts were imbedded more deeply into the coatings deposited by HVOF than the coatings deposited by APS.

High velocity oxy fuel thermal spray (HVOF) processing is one of the proven thermal spray technologies to develop high temperature oxidation and corrosion resistant coatings for various high temperature applications such as thermal power plants, aero turbines, and energy conversion systems. In the present investigation Nickel based coatings, viz. Ni-5Al, NiCrAl, and NiCrAlY-0.4wt%CeO 2 were sprayed on Fe based superalloy (Superfer 800) by HVOF spray process as the chosen superalloy finds extensive use steam boilers, furnace equipment, heat exchangers and piping in chemical industry The degradation behaviour of the coated superalloy was studied in the aggressive environment of 40%Na 2 SO 4 -60%V 2 O 5 at 900°C, using thermogravimetric technique, to understand the protective nature of the coatings under cyclic conditions. The coatings were found to be effective in increasing the resistance to the degradation in the given environment. Among the coatings investigated, NiCrAlY-0.4wt%CeO 2 coating was found to be more protective on the superalloy in the given aggressive environment. The corrosion mechanisms were elucidated through the X-ray diffraction (XRD) analysis and microstructural characterization techniques such as FE-SEM/EDAX analysis of the corroded products formed at high temperature.

During service damage in terms of small cracks develops in thermal barrier coatings (TBC), composed of partially yttria stabilized zirconia (PYSZ), and applied to gas turbine components made of Ni-base superalloys coated with an aluminide diffusion or MCrAlY overlay coating. Growth and coalescence of these microcracks results in cracks that run parallel to the interface with the substrate leading to failure by delamination of the TBC. A mechanism is proposed to heal the micro-cracks in a TBC by introducing MoSi 2 particles. Upon high temperature expose in air, in the range of 1200 °C, MoSi 2 forms amorphous SiO 2 that can fill micro-cracks, thereby restoring the integrity of the TBC.