Investigation of erosion behavior of EB-PVD-TBCs and sacrificial coatings after CMAS infiltration

Aero-engines operating in sand laden environments often encounter severe problems with thermal barrier coatings (TBCs) due to erosion damage. Since the turbine entry temperatures are raising, the life-time of TBC coatings as well as its thermal conductivity are additionally influenced by molten sand (calcium-magnesium-alumino-silicate/ CMAS). Few attempts have been made in understanding the combined impact of both erosion and CMAS effects [1,2]. Wellman and Nicholls [1] have found that a fully CMAS infiltrated electron-beam physical vapor deposited (EB-PVD) TBC behaves like a continuum during erosion and slightly improves its erosion behavior under room temperature compared to pure TBC. Development of CMAS resistant coatings has been a hot topic for the last two decades and one of the proposed method is the application of sacrificial oxide layers such as Al2O3, MgO, Sc2O3 et al. [3], on top of the TBCs. These sacrificial layers chemically react with the CMAS and modify the melting temperature or the viscosity of CMAS and thus the infiltration of CMAS into the TBC is inhibited. Since both damage mechanisms (erosion and corrosion) occur parallel and competitively in a turbine, this study focuses on deeper understanding of the erosion behavior of CMAS-infiltrated 7wt.-% yttria stabilized zirconia (7YSZ) TBCs. 400 µm thick 7YSZ coatings with two different microstructures were produced by EB-PVD. Additionally, sacrificial Al2O3 coatings were also applied on the top of 7YSZ by means of suspension plasma spraying (SPS) and suspension high velocity oxy-fuel spraying (SHVOF) using water-based suspensions. CMAS infiltration experiments were carried out at 1250 °C using different CMAS compositions and different infiltration times. Erosion tests were realized at room temperature in an in-house erosion test rig and evaluated partly by confocal microscopy. Microstructural examinations as well as crack identification before and after testing were carried out using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Infiltrated TBCs behaved as a continuum material during erosion exposure which lead mainly to surface spallation. Furthermore, the CMAS infiltration in the TBCs and partly the sintering effect at 1250 °C lead to a network of vertical cracks. These vertical cracks are weak areas where severe erosion occurs. The different TBC microstructures, infiltration times and CMAS compositions strongly influence the erosion behavior of the TBC. In case of alumina top coats the microstructure and especially the presence of porosity in the coating has strongly influenced the CMAS infiltration depth, the erosion behavior, and the stability of the entire coating system. [1] R.G. Wellman, J.R. Nicholls, Erosion, corrosion and erosion–corrosion of EB PVD thermal barrier coatings, Tribology International. 41 (2008) 657–662. doi:10.1016/j.triboint.2007.10.004. [2] S. Rezanka, D.E. Mack, G. Mauer, D. Sebold, O. Guillon, R. Vaßen, Investigation of the resistance of open-column-structured PS-PVD TBCs to erosive and high-temperature corrosive attack, Surface and Coatings Technology. 324 (2017) 222–235. doi:10.1016/j.surfcoat.2017.05.003. [3] A.K. Rai, R.S. Bhattacharya, D.E. Wolfe, T.J. Eden, CMAS-Resistant Thermal Barrier Coatings (TBC), International Journal of Applied Ceramic Technology. 7 (2010) 662–674. doi:10.1111/j.1744-7402.2009.02373.x

Investigation of CMAS resistance of SPS- and SHVOF-alumina topcoats on EB-PVD 7YSZ layers

Thermal barrier coatings (TBCs) undergo severe degradation by interaction with molten calcium-magnesium-aluminum-silicate (CMAS) minerals that are found mainly in volcanic ashes (VA) or desert sands. After the infiltration of the CMAS, chemical reactions, diffusion and phase transformation can lead to residual stress, cracks and spallation and thus significantly shorten the life-time of the components. As the state-of-the-art material 7 wt.-% Y2O3 stabilized ZrO2 (7YSZ) offers limited resistance to the CMAS attack, development of CMAS-resistant TBCs has undergone intense research during the last decades. One of the proposed approaches is the application of a sacrificial layer on top of the TBC which reacts with the molten CMAS/VA to crystalline phases and in this way inhibits further infiltration by sealing the gaps and pores. Al2O3 is one candidate for such a sacrificial layer which exhibits good CMAS resistance by formation of arresting phases. However, EB‑PVD Al2O3-topcoats suffer locally from cracks that arise from crystallization and sintering shrinkage, thereby providing only a discontinuous protection against CMAS infiltration due to their characteristic morphology. Even though the alumina is a candidate material, the coating density and the arrangement of porosity has been found to be a critical factor for restricting CMAS infiltration. In this work alumina coatings were sprayed on top of EB‑PVD 7YSZ TBCs using suspension plasma spraying (SPS) and suspension high velocity oxygen fuel spraying (SHVOF) starting from an aqueous suspension containing fine dispersed Al2O3 (d50 about 2.3 µm). The spray parameters were optimized in order to produce Al2O3 topcoats with homogeneous distributed porosity from very porous (porosity about 30 %) to denser (porosity about 10-15 %). These coatings were tested under CMAS attack by performing infiltration experiments at 1250 °C for different time intervals from 5 min to 10 hours. One Island volcanic ash from the Eyjafjallajökull volcano (IVA) and two types of synthetic CMAS compositions were tested in this study. The infiltration kinetics and reaction products were studied by SEM, energy-dispersive spectroscopy (EDS) and x-ray diffraction (XRD). It was observed that the microstructure and especially the presence of the porosity in the Al2O3 coatings strongly influenced the CMAS infiltration kinetics. Due to its high and non-uniform porosity, CMAS/VA melt infiltrated the 100 µm thick, very porous alumina SPS‑coating inhomogeneously and reached the subjacent 7YSZ layer already after one hour of annealing at 1250°C. Additionally, it was found that the infiltration kinetics varies also with the chemical composition of the CMAS/VA. Different crystalline phases such as anorthite, spinel or others were formed as reaction products of the SPS‑Alumina-TBC with the CMAS/VA-melt. The exact phases and its location depend on the used CMAS/VA composition. Furthermore, the annealing time has a major influence on the presence of the various phases. The infiltration kinetics of the SHVOF‑coatings was different due to a change in morphology. The current experiments clearly demonstrate that CMAS/VA mitigation depends on the interplay between morphology of the coating which dictates the driving force for infiltration, the reaction speed between alumina and the deposit, and the deposit chemistry

Influence of feedstock and thermal spray process on the phase composition of alumina coatings and their sliding wear.

Suspension thermal spraying is an emerging coating technology that enables the deposition of dense-structured ceramic coatings. As wear resistance is a main application field of alumina (Al2O3) coatings, this study aimed to evaluate the dry reciprocating sliding wear resistance of suspension sprayed high velocity oxy-fuel (S-HVOF) alumina coatings and to compare it with atmospheric plasma sprayed (APS) and HVOF coatings. Coatings were analyzed in the as-sprayed state and post-treated at 910 °C (hot isostatically pressed, HIPed) conditions. Wear tests were conducted using a tribometer, following the ASTM G133-02 standard and a sintered WC-6 wt.% Co ball as the counterbody. Coating characterization was done using scanning electron microscopy, X-ray diffraction and nanoindentation technique. Results indicate that the HVOF, HVOF-HIP and S-HVOF coatings had a high α-Al2O3 content, whereas the APS and APS-HIP coatings had a high γ-phase content together with high porosity. Sliding wear resistance was an order of magnitude higher for the S-HVOF and HVOF coatings than the APS and APS-HIPed coatings. This difference in wear performance was attributed to the high nanohardness, elastic modulus, dense microstructure and relatively high α-Al2O3 content in the HVOF, S-HVOF and HVOF-HIP coatings. Results are discussed in terms of the wear mechanism and structure-property relationship

Investigation of CMAS Resistance of Sacrificial Suspension Sprayed Alumina Topcoats on EB-PVD 7YSZ Layers

Molten calcium-magnesium-aluminum-silicate (CMAS) mineral particles cause significant degradation of thermal barrier coatings (TBCs) in aero-engines. One approach to protect the TBC coating against the CMAS attack is the application of a sacrificial coating on top of the TBC coating. In this work, Al2O3 coatings were deposited on EB-PVD 7YSZ layers using suspension plasma spraying (SPS) and suspension high velocity oxy-fuel spraying (SHVOF), in order to produce sacrificial topcoats with two different microstructures and porosity levels. The coating systems were tested under CMAS attack with one natural volcanic ash and two artificial CMAS powders by conducting infiltration tests at 1250 °C in the time intervals between 5 min and 10 h. It was found that the porosity and morphology of suspension sprayed alumina topcoats, the chemical composition of the deposits and the infiltration conditions strongly influence the CMAS infiltration, reaction kinetics and formation of the reaction products. While the porous SPS coatings offer limited resistance against CMAS infiltration, the dense SHVOF coatings show promising CMAS sealing behavior. Among the formed reaction products, only (Fe, Mg) Al spinel acted as an efficient barrier against CMAS infiltration. However, the formation of uniform spinel layers strongly depends on the pore morphology of the sacrificial coating and the CMAS chemistry

Comparative study of corrosion performance of HVOF-sprayed coatings produced using conventional and suspension WC-Co feedstock.

Corrosion properties of nanostructured coatings deposited by suspension high-velocity oxy-fuel (S-HVOF) via an aqueous suspension of milled WC-Co powder were compared with conventional HVOF-sprayed coatings. Microstructural evaluations of these coatings included x-ray diffraction and scanning electron microscopy equipped with an energy-dispersive x-ray spectroscopy. The corrosion performance of AISI440C stainless steel substrate and the coatings was evaluated in a 3.5 wt.% NaCl aqueous solution at ~25 °C. The electrochemical properties of the samples were assessed experimentally, employing potentiodynamic polarization and electrochemical impedance spectroscopy. The potentiodynamic polarization results indicated that coatings produced by S-HVOF technique show lower corrosion resistance compared with the coatings produced by HVOF-JK (HVOF Jet Kote) and HVOF-JP (HVOF JP5000) techniques. Results are discussed in terms of corrosion mechanism, Bode and Nyquist plots, as well as equivalent circuit models of the coating–substrate system

Düsenaufbau für das thermische Spritzen mittels einer Suspension oder einer Präcursorlösung

The invention relates to the nozzle construction for thermal spraying by means of a suspension, in which particles are contained, or a precursor solution, by means of which particles or precursor solution a layer is formed on a substrate, and which suspension or precursor solution is fed into a burner chamber or into a plasma torch, in which heating and acceleration of the particles is achieved, wherein a connection point for feeding the suspension or the precursor solution, a holder, and a nozzle insert are present. The nozzle insert has, with a tubular element arranged in the direction of the burner chamber or perpendicularly in HVOF flame or plasma torch and with an end face arranged opposite the burner chamber, a flange-shaped expanded section, which lies against a seat formed in the holder in the installed state. The contours of the flange-shaped expanded section and of the seat are complementary to each other such that the surfaces of the flange-shaped expanded section and of the seat are in direct contact with each other and an end stop and a seal are formed in this region

Microstructural characteristics and performances of Cr2O3 and Cr2O3-15%TiO2 S-HVOF coatings obtained from water-based suspensions

Cr2O3-based coatings offer high hardness, excellent sliding wear performance, and corrosion resistance. Therefore, they are widely applied in the paper industry, as well as for pumps and mechanical sealing systems. Compared to the conventional spray processes, the technology of suspension-HVOF spraying (S-HVOF) allows the production of dense, finely structured coatings with smoother surfaces and improved mechanical properties by using submicron-scaled raw materials. This work investigates the microstructure and performances of Cr2O3 and Cr2O3-15%TiO2 coatings obtained by S-HVOF starting from water-based suspensions. For the development of the suspensions with binary composition, two routes were used to produce ready-to-spray suspensions: (a) mixture of two stable suspensions in the desired ratio, and (b) dispersion of an appropriate alloyed material in the solvent. In order to evaluate the potential of suspension spraying over the conventional APS and HVOF processes, the mechanical properties, corrosion, and sliding wear resistances of the S-HVOF coatings were compared with those of the coatings produced from feedstock spray powders. From the experimental results, it was observed that, in most of the cases, the suspension-sprayed coatings showed denser microstructures, enhanced mechanical properties, wear resistance, and superior corrosion performances

Entwicklung von Cr2O3-Hochleistungsschichten durch thermisches Spritzen mit Suspensionen

Advanced coatings with improved properties have been developed by spraying water-based Cr2O3 suspensions with High Velocity Oxygen Fuel Spraying (S-HVOF). In comparison to conventionally sprayed coatings, suspension sprayed coatings show denser microstructure, reduced surface roughness, improved mechanical properties and corossion resistance, as well as better laser structuring properties. Another advantage of the S-HVOF technology is the opportunity to prepare not only thick coatings but also smooth thin ones (<20μm) and hence to extend the coatings market. Demonstrator-components for different applications have been coated; the results show the enormous potential of S-HVOF technique for applying water-based Cr2O3 suspensions

Advanced processes and system technology for high-performance laser cladding

Laser cladding technology is widely used in industry to precisely apply tailored surface coatings, as well as three dimensional deposits for repair and additive layer-by-layer fabrication of metallic parts. However, the processing of larger components, like tools for oil and gas production, is economically challenging due to the conventionally low deposition rates. Consequently, industry is requesting more powerful technologies that maintain the quality advantages of the laser technology, but also make the process more productive and time effective. The modern highest power diode lasers offer practical solutions for applying of large-area laser cladding with significantly increased productivity. Using a fiber-coupled diode laser of 20 kW power and the accordingly developed laser cladding heads, real deposition rates of metal alloys, e.g. Inconel 625, could reach 14 kg/h. With the new-developed powder nozzles with rectangular profile of the powder jet allows at a laser power of 20 kW single tracks with 45 mm-width can be produced. Besides the laser source, the processing laser head is the key parameter for a high productivity and efficiency of the whole cladding procedure. The paper presents a new generation of high-performance laser cladding heads with integrated process sensors, which guarantee a stable long-time operation at highest power levels. The deposition rates achieved with this technology are equal or even exceed typical values of the common PTA technique. Current applications are large-area coatings on power plant components, hydraulic cylinders for off-shore equipment, and large metal forming tools for automotive bodies

A Study on the Microstructural Characterization and Phase Compositions of Thermally Sprayed Al2O3-TiO2 Coatings Obtained from Powders and Water-Based Suspensions

In this work, the alumina (Al2O3) and alumina-titania coatings with different contents of TiO2, i.e., Al2O3 + 13 wt.% TiO2 and Al2O3 + 40 wt.% TiO2, were studied. The coatings were produced by means of powder and liquid feedstock thermal spray processes, namely atmospheric plasma spraying (APS), suspension plasma spraying (SPS) and suspension high-velocity oxygen fuel spraying (S-HVOF). The aim of the study was to investigate the influence of spray feedstocks characteristics and spray processes on the coating morphology, microstructure and phase composition. The results revealed that the microstructural features were clearly related both to the spray processes and chemical composition of feedstocks. In terms of phase composition, in Al2O3 (AT0) and Al2O3 + 13 wt.% TiO2 (AT13) coatings, the decrease in &alpha;-Al2O3, which partially transformed into &gamma;-Al2O3, was the dominant change. The increased content of TiO2 to 40 wt.% (AT40) involved also an increase in phases related to the binary system Al2O3-TiO2 (Al2TiO5 and Al2&minus;xTi1+xO5). The obtained results confirmed that desired &alpha;-Al2O3 or &alpha;-Al2O3, together with rutile-TiO2 phases, may be preserved more easily in alumina-titania coatings sprayed by liquid feedstocks