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.

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.

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.

There are different concepts for TBC coating application that varies 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. This could lead to major savings regarding costs and time. This paper shows the effective use of offline programming tools in combination with a detailed coating process analysis. Based on this a time optimized coating concept for vanes and blades was developed. The validation of this optimized coating concept shows an improvement of more than 7 5 % 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.

Over many years, the aeronautic industry has been involved in Thermal Barrier Coatings (TBCs) development to increase the performances of the hot section components of aero engines like turbine blades or nozzle guide vanes. The Electron Beam Physical Vapor Deposition (EB-PVD) process has been widely used for high performances TBC’s on metallic substrate mainly due to enhanced life time performances. Nevertheless, the cost, a rather high thermal conductivity and a poor resistance against chemicals aggressions of TBCs performed by EB-PVD are real drawbacks for the next generation of turbine engines. For this purpose, Suspension Plasma Spraying (SPS) has been employed in this study for TBC. It is showed that SPS process allows to performing columnar microstructure easily tunable 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

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.

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.

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.

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.

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.

Applications such as landing gears and turbine blades place new demands on near-net coating technologies. Such demands include the replacement of traditional grinding, finishing and grit basting techniques with better, more efficient methods. A method is described for near-net-shape spraying of complex internal and external geometries which eliminates the need for grinding. This is achieved by combining automatic, mass-flow controlled HVOF grit blasting with Nano-HVOF methods. The resulting coating displays an as-sprayed surface roughness of less than 2 μm Ra and a tight control over coating thickness and distribution. By carefully controlling the coating thickness and surface properties, it is possible to hone the required dimension and surface roughness.

Erosion-resistant coatings on high-temperature polymer matrix composites are of great interest for turbine blade applications. This study evaluates the erosion resistance of thermal spray coatings using conventional weight loss methods in order to compute net erosion volume loss and assess thermal cycling durability. During erosion tests, coated polymer composite coupons were subjected to runway sand and aluminum oxide erodent at different temperatures and angles of incidence. Erosion test data are reported along with the results of coated polymer matrix composite blades.

The most commonly used structural materials for blades and other high temperature components of gas turbines are nickel base superalloys. A TBC protection coating system consists of a top coat of yttria partially stabilized zirconia and an underlying bond coat, usually MCrAlY (where M stands for Ni, Co or a combination of both). MCrAlY is normally deposited by the thermal spray processes: air plasma spray (APS), vacuum plasma spray (VPS/LPPS) or high velocity oxygen fuel (HVOF). The adhesion between the bond coat and the substrate, and therefore of the whole thermal barrier system, strongly depends upon the surface roughness. A high level of roughness generally denotes better adhesion, especially with the HVOF thermal spray process, where it is a necessity. Generally the roughness is reached by means of grit blasting with an abrasive media; this results in a certain level of surface contamination due to the entrapment of abrasive particles. The aim of this work was to set up a new surface preparation process in order to obtain a completely clean surface with a suitable roughness, which can be coated afterwards with HVOF or VPS/LPPS thermal spray technology. The tests carried out by this process on turbine blades, coated with a HVOF system, led to obtaining a coating/base material interface without any contamination caused by the surface preparation.

MCrAlY materials are widely used as bond coats for thermal barrier coatings on turbine blades. The aim of this work is to improve mechanical properties and wear resistance of thermal sprayed NiCoCrAlY-coatings by strengthening the coating with hard phase particles. In order to retain the effect of the dispersion reinforcement at high temperatures, the use of temperature-stable oxide hard phases such as ZrO 2 is necessary. To realise this new material structure, the high energy ball milling process is applied and analysed. With this process it is possible to achieve a homogeneous distribution of the oxide hard phases in the NiCoCrAlY matrix. The mixture ratio between NiCoCrAlY and ZrO 2 was varied between 5 wt-% and 10 wt-% ZrO 2 . The influences of the milling time of the high energy ball milling process on the distribution of the hard phases in the metal matrix were analysed. After spraying with a HVOF system the mechanical properties of the coatings are measured and compared with conventional NiCoCrAlY coatings.

The deposition of cavitation-resistant materials coatings in turbine blades is an important way to reduce cavitation damage. Fe-Cr-Mn-Si is a cavitation-resistant steel with many deoxidation elements, which can be important for arc thermal spraying materials. The influence of air pressure, arc tension, and chemical composition on the microstructure, area fraction oxide, porosity, microhardness, and cavitation resistance were studied. Microstructures and properties were investigated by XRD, optical and electronic microscopy, microhardness testing, and ultrasonic cavitation testing per ASTM G32-93. Chromium addition promotes an increase in area fraction oxide, and reduces the porosity, changing the microhardness. An increase in air pressure raised the oxide fraction in the SMA_A and 2 alloys. The SMA_A mass loss rates were 31.8, 25.8, and 37.2 mg/h, respectively, for the samples with 280, 410, and 550 kPa of air pressure. For the SMA_3 samples, the increase in the arc voltage reduces the oxide fraction, changing the mass loss rate to 43.8, 32.4, and 29.4 mg/h for 25, 30, and 35 V, respectively. Phase transformations occurred in the arc thermal spray, for all coatings, during cavitation tests. The SEM analysis verified that the mass loss in arc thermal spray coatings occurred because of the oxide fracture and delamination of the splats.