For the engines used in small turboprop aircrafts, the introduction of abradable coatings represents a feasible way to reach higher levels of overall engine efficiency, specifically by improving the fuel consumption and increasing the inter turbine temperature margin. Abradable coatings on seals also contribute to improved hot restarts capability of an engine and lead to substantial extension of service life of the rotating counter bodies. In our contribution, we concentrate on flame sprayed nickel graphite abradable coating that can be used in turboprop engines both for seals and clearance control. The focus is the impact of spraying parameters on the physical and function properties of the abradable coating.

The current investigation focuses on understanding the influence of a columnar microstructure and a sealing layer on the corrosion behavior of suspension plasma sprayed (SPS) thermal barrier coatings (TBCs). Two different TBC systems were studied in this work. First is a double layer made of a composite of gadolinium zirconate + yttria stabilized zirconia (YSZ) deposited on top of YSZ. Second is a triple layer made of dense gadolinium zirconate deposited on top of gadolinium zirconate + YSZ over YSZ. Cyclic corrosion tests were conducted between 25 °C and 900 °C with an exposure time of 8h at 900 °C. 75 wt. % Na 2 SO 4 + 25 wt.% NaCl were used as the corrosive salts at a concentration of 6 mg/cm 2 . Scanning electron microscopy analysis of the samples’ cross-sections showed that severe bond coat degradation had taken place for both TBC systems and the extent of bond coat degradation was relatively higher in the triple layer system. It is believed that the sealing layer in the triple layer system reduced the number of infiltration channels for the molten salts which resulted in overflowing of the salts to the coating edges and caused damage to develop relatively more from the edge.

A novel composite MCrAlY coating with a fine dispersion of sub-micron Al 2 O 3 particles were sprayed by high velocity oxygen fuel (HVOF) thermal spraying for high temperature oxidation protections. The presence of Al 2 O 3 could reinforce the metallic MCrAlY coatings, in a similar way to ODS alloys (oxide dispersion strengthening). It could also enhance the oxidation resistance owing to the dispersion of Al 2 O 3 on the surface as nucleation sites, which promotes the early formation of a coherent α-Al 2 O 3 scale. This is essential for an effective protection against oxidation attack, especially for these applications at relatively lower temperatures (<900 °C) with slower growing Al 2 O 3 . In this study, a suspension route was employed to achieve a uniform dispersion of 0-10 wt.% Al 2 O 3 particles with commercial MCrAlY powders. A liquid fueled HVOF spray gun (MetJet IV) was used to deposit the composite MCrAlY-Al 2 O 3 coatings onto 304 stainless steels substrates. The composite coatings were examined thoroughly by field emission gun scanning electron microscopy (FEGSEM) with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. Isothermal oxidation at 900 °C was carried out to study the effect of Al 2 O 3 on the growth of oxide on the coatings surface. The composite coatings exhibited superior oxidation behavior against the conventional metallic coatings with the formation of nearly exclusively Al 2 O 3 on the coatings surface and inhibited NiO growth.

The plasma torch design affects the particle-plasma interaction, in-flight properties and the coating microstructure. When spraying metallic powders, the in-flight oxidation as well as the particle velocity and temperature determine the mechanical, corrosion and oxidation properties, which have a major impact on the in-service degradation of bond coats. This study aims to determine the microstructural and mechanical properties of as-sprayed CoNiCrAlY coatings deposited on the Inconel 718 alloy. Depositions were made using a High Velocity Plasma Spray Process (HVPS), which is based on a special plasma torch design. In-flight particle characteristics were determined to elucidate the kinetic and thermal regime of HVPS process.

Thermal barrier coatings (TBCs) with high thermal strain tolerance and erosion resistance are commonly applied onto the inner and outer diameters of hot sections of gas turbine engine components. In this work, strain tolerant, segmented TBCs with a variety of crack densities and porosities were developed using the SinplexPro cascaded torch. Design of experiments were carried out to study the effect of process variables such as plasma power, powder feeding rate, spraying distance and surface speed on the coating microstructure and properties. Optimized process parameters for the segmented coating microstructures at shorter spray distance (<75mm) and longer spray distance (>114mm) are achieved, which are targeted for spraying inner diameter and outer diameter engine components, respectively. The plasma torch hardware life was evaluated by torch cycle duration runs. Examples of highly strain tolerant TBCs onto the ID and OD engine components were demonstrated, highlighting the wide versatility and process range of the SinplexPro.

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.

As a candidate material against plasma etching, yttrium oxide has been coated onto etching chamber by plasma spray technique. However, the plasma spray technique introduces undesirable coating properties such as porous structure and deleterious thermal effects. To reduce the disadvantage of thermal impact, cold spray was used as an alternative technology to deposit thick and dense yttrium oxide coatings. Primary nanoscale Y 2 O 3 were used as the original powder, for the ceramic materials are intrinsic brittle and are difficult to be deposited by cold spray. The nano-powder were first agglomerated by hydrothermal treatment with addition of inorganic salt to acquire suitable powder for cold spray, and then deposited on aluminum alloy 6061 substrates by cold spray process with compressed air as propellant gas. About 200μm yttrium oxide coatings were formed on the substrate alloy. Different processing parameters were employed to optimize microstructure and properties of the coating.

Wear and corrosion of aircraft actuator parts and helicopter gearbox rotating shafts and also wear of seals working against these parts can lead to oil leaks, which require expensive maintenance and increase the risk of failure. The Hardide nanostructured Chemical Vapor Deposition (CVD) Tungsten Carbide coating can help reduce oil leakage from gearboxes and actuators and increase maintenance intervals. The coating protects metal pistons and shafts from abrasion and corrosion, keeping their surface roughness parameters within optimum ranges for longer; this reduces seal wear and makes the whole unit more durable and reliable. The 50-100 microns thick CVD Hardide coating can be applied uniformly on internal and external surfaces and has enhanced fatigue and anti-galling properties. The coating has enhanced wear resistance outperforming Hard Chrome by 14 times and Thermal Spray WC-Co (12%) by 3 times. The fine-grain coating nanostructure wears uniformly so even worn Hardide coating shows no hard micro-grain asperities which are abrasive for seals. The coating is free from porosity and from Cobalt binder and is an excellent barrier against corrosion. As a result the coating keeps the optimal seal-friendly surface finish for longer even in abrasive and corrosive environments. The coating was qualified by Airbus as an environmentally-friendly replacement for Hard Chrome plating.

Nondestructive Evaluation and Testing (NDE&T) techniques have been played vital roles in property characterization, process development and quality control of various thermal spray coatings. Besides conventional NDE&T lab methods such as eddy current test (ECT) for thickness measurement and fluorescent penetrant inspection (FPI) for cracking detection, some latest NDE techniques have been developed, demonstrated and applied to evaluate and characterize thermal sprayed coatings recently. The improved and innovative NDE methods provide more capable and accurate measurement to inspect on surface morphology, 2D and 3D coating porosity, oxide content, interface debonding, as well as other types of coating features, defects or specific properties. In this work, some non-contact NDE techniques and their applications were investigated and discussed based on several case studies of thermal sprayed coatings. Laser confocal microscopy had been used for characterizing surface morphologies and roughness profiles of HVOF WC-based coatings with 2D and 3D mapping methods. In particular, thermal wave imaging and ultrasonic micro imaging methods were used to detect the suspicious existence of lateral coating separation within or at the MCrAlY coating-substrate interfaces. Laser dimension sensoring method exhibited the extended capability of in-situ coating thickening measurements on turbine blade and vane. The latest non-contact NDE techniques demonstrated their unique and strong capability for in-situ and ex-situ coating characterization, process and quality control and coating failure analysis.

The metal finishing process of electrolytic hard chrome (EHC) plating has been identified as a source of environmental pollution in most industrialized countries like Australia, Europe and USA. The key driver for the technology replacement is that the EHC plating process uses hexavalent chromium, which is a known carcinogen. Our previous research has identified that cold spray nanostructured tungsten carbide cobalt (WC-Co) coatings can be a suitable alternative to provide a functional coating in wear applications. This work explores at another similar technology- Kinetic Metallization for deposition of WC-Co coatings. In this work, the objective is to characterize the residual stress profile of these WC-Co coatings that are deposited by the latest KM systems. These coating systems are used in critical applications such as landing gear pistons and axle journals, hydraulic rods, engine shaft journals, and numerous other external surfaces that operate under high cyclic loading conditions. As such, the residual stress developed during the KM coating process has a significant influence on the fatigue properties of the components. Thus, knowledge of stresses and their linkage with other properties and production parameters is essential for the quality control of these critical structures.

Aluminum silicon hexagonal boron nitride abradable seal coatings were deposited using the atmospheric plasma spraying (APS) process under several processing conditions. In the present study, the effects of process parameters, such as spray power, feed rate, and spray distance, on the microstructure and properties of coatings have been investigated. The microstructure, hardness, bonding strength, chemical composition, and online monitoring of these coatings and flying particles process of the powers were characterized. The results showed that the temperature and speed of the flying particles increased with the increase of power, and the porosity and BN content of the coating were reduced, Therefore, the hardness and bonding strength were improved. Compared with the spraying power, the effect of the feed rate was opposite. With the increase of spraying distance, the influence of particle flight velocity was greater than that of temperature, which resulted in the porosity of the coating and the increase of BN content. The hardness and bonding strength of the coatings were consequently reduced.

50 vol. % La 2 Ce 2 O 7 (LC)/yttria partially stabilized zirconia (YSZ) composite thermal barrier coating (TBC) was deposited by supersonic atmospheric plasma spraying (SAPS). The mixture of LC and YSZ can effectively eliminate the sudden decrease of thermal expansions coefficients of LC. The results of CMAS corrosion tests indicated that the LC/YSZ composite coating reveals high resistance to the penetration of CMAS with reprecipitation of La-Ce apatite, CaAl 2 Si 2 O 8 , MgAl 2 O 4 and t-ZrO 2 . Furthermore, compared to YSZ coating, the LC/YSZ composite coating can obviously improve the thermal cycling lives under CMAS corrosion.

Thermal barrier coatings (TBCs) play a vital role in allowing the gas turbine engines to operate at high temperatures. With higher operating temperatures (>1200°C), the standard TBC material, 7-8wt. % Yttria Stabilized Zirconia (YSZ), is susceptible to CMAS (Calcium Magnesium Alumino Silicates) degradation and undesirable phase transformation. New TBC materials such as gadolinium zirconate (GZ) have shown to be capable of overcoming the challenges faced by YSZ. However, GZ has inferior fracture toughness relative to YSZ. In this work, three double layered TBC variations with different GZ and YSZ thickness respectively (400GZ/100YSZ, 250GZ/250YSZ and 100GZ/400GZ respectively, where the prefix numbers represent thickness in ìm) were produced by suspension plasma spray (SPS) process. In all the three double layered TBC variations, the overall TBC thickness with GZ as the top layer and YSZ as the base layer was kept the same (500 μm). The objective was to investigate the influence of YSZ thickness on the thermal cyclic fatigue performance of GZ/YSZ double layered TBC. The as sprayed TBCs were characterized by SEM, XRD and porosity measurements and later subjected to thermal cyclic fatigue test at 1100°C. It was observed that the GZ/YSZ double layered TBC with lowest YSZ thickness (400GZ/100YSZ) showed higher thermal cyclic lifetime whereas the TBC with thicker YSZ layer (100GZ/400YSZ) showed lowest thermal cyclic fatigue lifetime. The failure analysis of the thermally cycled TBCs revealed similar failure modes, i.e. spallation of the top coat due to horizontal crack propagation within the thermally grown oxide (TGO). Furthermore, the ceramic top coats in all the three TBC variations after failure showed the widening of column gaps.

Fabrication of Thermal Barrier Coatings (TBCs) with higher lifetime and relatively cheaper processes is of particular interest for gas turbine applications. Suspension Plasma Spray (SPS) is capable of producing coatings with porous columnar structure, and it is also a much cheaper process compared to the conventionally used Electron Beam Physical Vapor Deposition (EB-PVD). Although TBCs fabricated using SPS have lower thermal conductivity as compared to other commonly used processes, they are still not commercialized due to their poor lifetime expectancy. Lifetime of TBCs is highly influenced by the top coat microstructure. The objective of this work was to study the TBCs produced using axial SPS with different process parameters. The bond coat was deposited using High Velocity Air Fuel (HVAF) spray. Influence of the microstructure on lifetime of the coatings was of particular interest and it was determined by thermal cyclic fatigue testing. Thermal conductivity of the coatings was determined by laser flash analysis. The results show that axial SPS could be a promising method of producing TBCs for high temperature gas turbine applications.

The erosion behaviour at room temperature (RT) of as-deposited SPS, EB-PVD and APS YSZ-based TBCs was investigated. All coatings were deposited on Inconel 625 alloy coupons. The same APS CoNiCrAlY bond coat was employed for all SPS and APS TBCs. The erodent material was 50 μm alumina and the impact angles were 15° and 90°. A total of 4 different types of SPS YSZ-based TBCs were tested, which consisted of two distinct columnar-segmented and two distinct columnar-grown microstructures. The EB-PVD and APS YSZ TBCs were employed as benchmarks. The erosion performance of the different TBCs in this study was ranked based on the coating volume loss after wear testing. The TBC microstructures and phase compositions were evaluated via SEM and XRD. The erosion mechanisms of the different TBCs were compared by analyzing the cross-sectional and top surface microstructures of the as-sprayed and eroded TBCs. These are released results from the Surftec Industrial R&D Group of the National Research Council of Canada (NRC).

In this work, one new technique named as supersonic suspension plasma spraying (SSPS) is applied to deposit quasi-columnar scandia-yttria co-doped zirconia (ScYSZ) and yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). The phase composition, microstructural evolution, fracture toughness and failure behavior of both TBCs before and after thermal cycling tests at 1300 °C were systematically studied. It was found that both as-sprayed TBCs were fully non-transformable tetragonal (t’) phase. After the thermal cycling test, tetragonal (t) phase and cubic (c) phase formed for the SSPS-YSZ TBC, while single t’ phase retained for the SSPS-ScYSZ coating. The fracture toughness of the ScYSZ coating was comparable or superior to that of the YSZ coating. As for the thermal cycling behavior, the lifetime of the ScYSZ coating was better than that of the YSZ coating, which confirmed that ScYSZ was a promising alternative material for YSZ.

Ba(Mg 1/3 Ta 2/3 )O 3 (BMT), a high melting point refractory oxide, is envisioned as a thermal barrier coating material. In this study, six chemical reagents combinations are investigated as BMT coating precursors: one BMT powder suspension and five Ta 2 O 5 suspensions in nitrate solutions or acetate solutions. A hybrid suspension / sol plasma spray process is designed to axially inject these precursors into a RF thermal plasma torch to synthesize BMT and to deposit nanostructured coatings. X-ray photoelectron spectroscopy (XPS) was used to evaluate the element evaporation during plasma spraying. Thermogravimetric analysis and differential thermal analysis (TG/DTA) are applied to investigate the BMT formation. Parameters such as precursor chemistry and proportion, plasma power, spray distance and substrate preheating are studied with regards to the coating phase structure. The results indicate that the combination of twice the Mg stoichiometric amount with a power of 50 kW shows the best results when using nanocrystalline Ta 2 O 5 as Ta precursor. When choosing nitrates as Ba and Mg precursors, predominant crystalized BMT can be obtained at lower plasma power (45 kW) when compared to acetates (50 kW). BaTa 2 O 6 , Ba 3 Ta 5 O 15 , Ba 4 Ta 2 O 9 , Mg 4 Ta 2 O 9 are the main secondary phases during BMT preparation process. Because of the complicated acetate decomposition, the coating deposition rate from nitrate precursors is higher than that from acetate ones.

Thermal barrier coatings (TBCs) are one of the most promising applications for suspension thermal spraying. By using a liquid feedstock, coating morphologies close to the columnar-like structure of electron-beam physical vapor deposition (EB-PVD) are possible. Additionally, vertically cracked TBC coatings (VC-TBC) have shown a great potential for industrial applications. In this work, we will analyze the process window for TBCs produced by using suspension spraying. Commercially available alcohol- and water-based YSZ suspensions have been processed using suspension atmospheric plasma spraying (S-APS) and suspension high-velocity oxy-fuel (S-HVOF) to achieve both columnar-like and VC morphologies. The coatings have been analyzed and compared in terms of microstructure, phase composition, bond strength and thermal cycling performance. Additionally, the suitability of S-APS and S-HVOF TBCs for industrial applications will be discussed, as well as which actions are needed to increase its competitiveness.

The main purposes of applying the coatings on the surface of different components include the improvement of their functional and decorative properties. The functional properties of coating are strongly dependent on its microstructure. The main goal of the paper is to show the differences of two microstructures that can be obtained using suspension plasma spraying technology: (i) columnar one; and, (ii) lamellar, two-zone microstructure. Initially, the optimization of spray parameters was made and then the microstructural studies were performed. The work was focused on zirconia stabilized by yttria (YSZ) and both by yttria and ceria (YCSZ) which are most frequently used as thermal barrier coatings (TBC). Moreover, two types of microstructure were achieved using two different plasma torches, namely SG-100 of Praxair and Triplex of Sulzer Metco. After optimizing the spray parameters the microstructure of prepared coatings was analyzed using electron microscopy (SEM). The conventional SEM microscopy with secondary electrons detector (SE) and back scattered electrons one (BSE) were used. The energy dispersive spectroscopy (EDS) was performed to analyze the chemical composition. Finally, by electron backscatter diffraction (EBSD) grain shape, size and texture were determined for discussion of the coatings growth mechanism.

The presence of defects such as voids, inter-lamellar porosities or cracks, provides a decrease of the effective thermal conductivity of plasma sprayed coatings as well as a decrease of the corresponding mechanical properties such as the Young’s modulus. In general, effective properties of thermal spray coatings are thus strongly different from that of the bulk material and have thus to be quantified to validate their in service performances. A complementary approach allowing understanding the relationships between the microstructure of a coating and its macro-properties is the use of Finite Element Modeling. The case of composite coatings is still more complicated due to the presence of different materials. In the present study, thermo-mechanical properties of a plasma sprayed composite coating were estimated by numerical modeling based on FEM. The applied method uses directly cross-sectional micrographs without simplification using a one-cell per pixel approach. Characteristics such as the thermal conductivity, the Young’s modulus, the Poisson ratio and the dilatation coefficient were considered. The selected example was an AlSi/polyester coating used as abradable seal in the aerospace industry.