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

Suspension plasma spraying (SPS) process has attracted extensive effort and interest as a method to produce fine-structured and functional coatings. In particular, thermal barrier coating (TBC) applied by SPS process has gained increasing interest due to its potential for producing coatings that provide superior thermal protection of gas turbine hot-section components as compared to conventional APS-TBC and even EB-PVD TBC. The unique columnar architecture and nano- and submicron sized grains in a SPS-TBC coatings demonstrate some advantages in thermal shock durability, low thermal conductivity, and high-temperature sintering resistance. This work addresses some practical aspects of using the SPS process for TBC applications before it becomes a reliable industry method. The spray capability and applicability of SPS to achieve uniform thickness and microstructure on curved substrates was evaluated in designed spray trials to simulate industrial parts with complex configurations. The performance of SPS-TBCs in erosion, free falling ballistic impact, and indentation loading tests was evaluated to simulate SPS-TBC performance in turbine service conditions. The behaviors of SPS-TBCs in those tests were correlated to key test factors including grit incident angles, impact object sizes, indentation head shapes, and coating surface curvatures. Finally, a turbine blade was coated and sectioned to verify SPS sprayability in multiple critical sections. The SPS trials and test results demonstrate that SPS is promising for innovative TBCs, but some challenges need to be addressed before it becomes an economical and reliable industrial process, especially for gas turbine components.

Depending on the size and type defects of nickel-based alloy turbine blades two procedures are used mainly: cladding and high temperature brazing. The repair brazing of turbine blades is used to regenerate cracks and surface defects and is the focus of this work. In this contribution a two stage hybrid repair brazing process is presented which allows reducing the current process chain for repair brazing turbine blades. In the first stage of this process the filler metal (NiCrSi) then the hot gas corrosion protective coating (NiCoCrAlY) and finally the aluminium are applied in this order by atmospheric plasma spraying. In the second stage of this hybrid technology the applied coating system undergoes a heat treatment in which brazing and aluminising are combined. The temperature-time regime has an influence on the microstructure of the coating which is investigated in this work.

The Ti 2 AlNb blade is used in high pressure compressor aero-engines to provide high thrust force at relatively light weight. A series of abradability tests was carried out on CuAlNi-graphite, NiCrAl-graphite, NiCrAl-bentonite, and NiCrFeAl-hBN abradable coatings rubbed against Ti 2 AlNb dummy blades with the maximum blade-tip velocity of 300 m/s at 500 °C. In consideration of the effects of an engine’s working conditions, some tests were conducted with incursion rate as the single variable. The scratched surfaces of the samples were observed by the stereoscopic optical camera, and a ratio of the blade wear to shroud incursion depth (IDR) was evaluated to characterize the abradability of coatings. The results show that NiCrAl-graphite and NiCrFeAl-hBN abradable coatings perform very well rubbed against the Ti 2 AlNb blade, and the blade-tip wear is not obvious after abradability tests.

In this study, dense multicomponent NiCoCrAlTaY bond coats and feather-structured YSZ topcoats are deposited on DZ40M alloy vane surfaces by the PS-PVD method. Based on thickness measurements and microstructure examination, it is shown that the double vane surface was completely covered by both layers. The thickest portion of the coating was found close to the leading and trailing edges of the vane. The results show that it is possible to manufacture TBCs, including the bond coat and topcoat, on first-stage turbine blades by a single PS-PVD process.

There are different concepts for thermal barrier coating (TBC) application that vary in the method of applying the coating layers. An analysis of the existing part portfolio sprayed with different coating concepts shows that there is a 75% difference in spray performance, depending on which concept is used. Optimizing performance can significantly reduce costs and time. This paper shows the effective use of offline programming tools in combination with a detailed coating process analysis to develop a time-optimized coating concept for vanes and blades. The validation of this optimized coating concept shows an improvement of more than 75% in spray performance leading to a coating time improvement of up to 40%.

In this research project a hybrid technology is developed to repair turbine blades. This technology incorporates procedural and manufacturing aspects like raising the degree of automation or lowering the effort of machining and includes materials mechanisms (e.g. diffusion processes) as well. Taking into account these aspects it is possible to shorten the process chain for regenerating turbine blades. In this study the turbine blades of the high pressure turbine are considered and therefore nickel-based alloys are regarded. To repair or regenerate turbine blades the following methods are employed: welding and brazing and a subsequent aluminizing CVD-process. The focus in this work lies on the brazing method and the required filler-metal is applied together with the hot-gas corrosion protective coating by means of thermal spraying and represents the first stage of this hybrid technology. In the second stage of this hybrid technology the brazing process is integrated into the aluminizing CVD-process and a first effort is presented here.

For many years, the aeronautics industry has been actively engaged in the development of thermal barrier coatings (TBCs) to enhance the performance of hot section components in aerospace engines, such as turbine blades or nozzle guide vanes. The electron beam physical vapor deposition (EB-PVD) process has been widely utilized for high-performance TBCs on metallic substrates, primarily due to its extended lifespan. However, the drawbacks of EB-PVD TBCs, including their cost, relatively high thermal conductivity, and susceptibility to chemical attack, pose challenges for the next generation of turbine engines. To address these issues, suspension plasma spraying (SPS) has been investigated in this study as an alternative for TBC application. It has been demonstrated that the SPS process enables the production of a columnar microstructure that can be easily adjusted in terms of size, distribution, and morphology.

Obtaining a uniform coating on curved mechanical parts such as gas turbine blades is one of the industrial challenges in suspension plasma spraying. Through a three dimensional numerical analysis, this study is aimed at providing a better understanding of the effect of substrate curvature on in-flight particle temperature, velocity and trajectory. The high temperature and high velocity plasma flow is simulated inside the plasma torch using a uniform volumetric heat source in the energy equation. In addition, yttria stabilized zirconia (YSZ) suspension is molded as a multicomponent droplet while catastrophic breakup regime is considered for simulating the secondary break-up when the suspension interacts with the plasma flow. A two-way coupled Eulerian-Lagrangian approach along with a stochastic discrete model was used to track the particle trajectory. Particle size distribution in the vicinity of the substrate at different stand-off distances has been investigated. The results show that sub-micron particles may obtain higher velocity and temperature compared to the larger particles. However, due to the small Stokes number associated with sub-micron particles, they are more sensitive to the change of the gas flow streamlines in the vicinity of a curved substrate

This paper examines thermal barrier coating (TBC) structures, including traditional porous TBCs, dense vertically cracked TBCs, and columnar TBCs, produced by a high-power plasma torch with axial injection of feedstock. It is shown that suspension plasma sprayed columnar TBCs have properties similar to TBCs produced by electron-beam physical vapor deposition and may thus be considered a viable alternative.

This report analyzes the cost structure of the cold spray process using a generic model applicable to various systems (high pressure, low pressure, kinetic) and applications (coating, restoration, additive manufacturing, near-net forming). The cost model is relatively easy to use, yet sufficiently accurate to support decisions. It is shown that high-pressure spraying is generally favorable, that He-N2 gas blends are the most economic, and that He recovery is beneficial in high-volume production, even when He-N2 blends are used. The cost model makes it possible to determine the optimal He concentration of the propellant gas for a given application as demonstrated in a case study involving bond coats for gas turbine blades.

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

This study investigates the effect of plasma spray parameters on the erosion rate of abradable seals used for clearance control in gas turbine engines. Coating samples were sprayed using YSZ powder containing polyester that was entrapped in the deposit then subsequently removed by heating. Test results show that erosion behavior is influenced by gun voltage and spray distance. A higher voltage results in a denser coating with a lower erosion rate. Increasing spray distance, on the other hand, increases porosity, which results in a higher erosion rate. Further analysis shows that the erosion rate is proportional to the inverse square of the spray distance and the square of the gun voltage.

The aim of the research project is to combine repair brazing with protective coating against hot-gas corrosion into a common integrated process. Both the braze-metal as well as the hot-gas corrosion protection coating is applied by means of thermal spraying. The material layout is to be realized as far as possible to the near net shape by using thermal spraying. The processes are to be performed in such a way that the brazing is integrated into the CVD diffusion annealing process as a transient liquid phase bonding (TLP bonding) process which, as a consequence, can then be eliminated as a separate processing step. The thermal spraying processes of atmospheric plasma spraying (APS), high velocity oxygen fuel spraying (HVOF) and cold gas spraying (CGS) are to be qualified for this purpose. Thus the project working hypothesis is to be able to transform thermal coating and joining processes into a common integrated hybrid process and, in doing so, obtain both high-quality and economic advantages. The importance of combining these processes lies in reducing the effort of grinding as well as economizing on the vacuum brazing, which is currently a separate process step, and consequently lowering the production costs.

Gas turbine efficiency is of paramount importance in the modern carbon conscious global economy and the industry is always looking for ways to improve the efficiency of gas turbine engines. Gas bypass between the rotating turbine blade tip and the engine casing affects both the efficiency and the power output of an engine. An increase in this clearance of 125µm can result in an increase of 0.5% in specific fuel consumption. Abradable coatings have been designed to allow the turbine blade abrasive tip to cut a path into shroud abradable coating to improve the seal between the blade tip and the casing. A holistic approach to improving the abradable system – the abradable coating and the blade tip – is necessary. Better blade tips can result in use of denser, more erosion resistant abradables improving performance of the whole system. Current blade tips are limited as the matrix oxidizes at high temperature losing its ability to hold as well as protect the CBN particles. Improvement in blade tips – both in the cutting particles and the matrix which hold these particles – will therefore improve the abradable system performance, as well as allow the use of denser, more erosion resistant abradable materials. This paper represents efforts to improve the matrix oxidation resistance which holds CBN particles. The matrix is a low-aluminum MCrAlHf which is further aluminized to improve the oxidation resistance. New coatings being tested are enhanced aluminide coatings, platinum aluminide coatings and platinum chromide aluminide coatings. The results will be discussed in terms of matrix composition and microstructure as deposited and after static oxidation. The effect of matrix and its impact on the blade tip performance will also be reviewed.

Turbine blades are generally protected by thermal barrier coatings (TBCs) against high temperature oxidation and corrosion. A novel method has been developed to prepare nanostructured Al 2 O 3 powders for thermal-spraying (atmospheric plasma spray) with high flowability. In this method, nano Al 2 O 3 powders are granulated and then heat treated at 200°C which become suitable to be used in thermal spraying equipment. The normal Al 2 O 3 and granulated nano Al 2 O 3 powders were sprayed separately by using an atmospheric plasma spray device onto a typical TBC consisting of a superalloy bond coat and an YSZ top coat. Then, TBC/ normal Al 2 O 3 and TBC/ nano Al 2 O 3 coatings were oxidized at 1000°C for 24h and allowed to form the thermally grown oxide (TGO) layer onto the bond coat (NiCrAlY layer). The flowability of the granulated nano Al 2 O 3 powders was studied by using a Hall flowmeter. The microstructural characterization showed that the granulated nano Al 2 O 3 powders had very high flowability. The increased apparent density and flowability of the granulated nano Al 2 O 3 powders had substantially reduced the micro-cracks and interconnected porosities in the coating in comparison with normal Al 2 O 3 coating. The thickness of the TGO layer in YSZ/ normal Al 2 O 3 coating was higher in comparison with YSZ/ nano Al 2 O 3 coating after oxidation. Thus by using nano Al 2 O 3 as a third layer, the thickness of the TGO layer and oxidized regions inside the bond coat decreased effectively which led to less mechanical stresses and can cause the improvement of TBC life.

Oxidation behavior of NiCrAlY powder, blended with nano and micro sized Al 2 O 3 and Y 2 O 3 was studied to understand the effect of nano/micro oxide powder dispersion. The blended powders were applied on the IN 718 substrates by HVOF technique. The present work compares the oxidation behavior of IN 718 superalloy, coated with NiCrAlY powders, dispersed with nano and micro Y 2 O 3 , Al 2 O 3 oxide. Coated samples were characterized by XRD, SEM/EDAX in terms of surface composition, scale cross section and the identification of different phases. The oxidation tests were carried out at 1223K, 1323K, 1423K in air. Oxidation kinetics infer that at 1223 K and 1323 K, nano yittria addition, in fact, resulted in higher oxidation rate, while nano alumina addition resulted in lower oxidation rate. The effect was more pronounced at 1423 K, where the nano and micro size alumina, yttria addition, resulted in bringing down the oxidation rate considerably.

Oxide compounds basically composed of calcium, magnesium, aluminum and silicon cations also known as CMAS, can be deposited on the surface of thermal barrier coatings (TBC) of gas turbine blades. Under certain operation conditions these compounds have been found to aggressively degrade the TBC, hence affecting the thermo-mechanical properties of the underlying component. Detailed investigation on the interaction of CMAS and the atmospheric plasma sprayed (APS) yttria-stabilized zirconia (YSZ) TBC was performed in a burner rig test facility under thermal gradient cycling conditions and at the same time CMAS deposition. This novel and unique test approach promises a coating screening and characterization test under service conditions. Variable exposure times at approximately 1250°C/1050°C surface/substrate temperatures were applied. The lifetime of the TBC was indicated by the number of thermal cycles until significant spallation occurred. X-ray spectroscopy and microstructural analyses were conducted on the cycled samples to determine the effect of thermo-chemical interactions. It was found that with extended heating period of 10 times the standard cycle, the number of sustainable load alternations heating/cooling was reduced. Interaction of CMAS and YSZ induces formation of glassy soda-silicate phase. Thermal cycling of thermo-physically mismatched TBC and glass melt causes crack formation and coating failure.

Cavitation erosion is a common phenomenon that occurs in hydraulic turbine blades and result in mass loss. Welding is the most common technique used to recover the geometrical profile of these cavitation eroded turbine blades, however it is known that tensile residual stress can develop. The development of manufacture process that could reduce or eliminate the residual stress level will contribute for a longer service life of this component. It is aimed in this study evaluate cavitation erosion mechanism of Fe- Mn-Cr-Si-Ni arc thermally sprayed coating. Coatings were analyzed by optical and scanning electronic microscopy, microhardness, cavitation tests (ASTMG32-92) and the analysis of eroded surface areas after ultrasonic cavitation tests with DRX and SEM. The results showed that lamellae morphology, oxide volume fraction and porosity modified by changings in parameters deposition, modified cavitation mass loss mechanisms. After ultrasonic cavitation tests, it was verified that mass loss occurred by interlamellae oxide removal and splats surface deformation in initial stages, followed by rupture and finally detachment of the lamellae. Splashing droplets promote greater mass loss in some localized areas because they lower intersplat cohesion.

Agglomerated and sintered Cr 3 C 2 -25%NiCr powders possess excellent flow ability and appearance that have been extensively applied to resist abrase and erosion in high temperature applications such as power boiler and turbine blade. Microstructure of Cr 3 C 2 -25%NiCr coatings were observed through scanning electronic microscope (SEM), and bond strength and microhardness of coatings were measured by tensile shearing test and Vickers hardness test. It is indicated that ultrafine Cr 3 C 2 -25%NiCr coatings have some outstanding properties to traditional Cr 3 C 2 - 25%NiCr coatings by plasma sprayed.