Columnar Thermal Barrier Coatings Research Articles

Al-modification for PS-PVD 7YSZ TBCs to improve particle erosion and thermal cycle performances

Plasma spray-physical vapor deposition (PS-PVD) as a novel process was used to prepare feather-like columnar thermal barrier coatings (TBCs). This special microstructure shows good strain tolerance and non-line-of-sight (NLOS) deposition, giving great potential application in aero-engine. However, due to serious service environment of aero-engine, particle erosion performance is a weakness for PS-PVD 7YSZ TBCs. As a solution, an Al-modification approach was proposed in this investigation. Through in-situ reaction of Al and ZrO2, an α-Al2O3 overlay can be formed on the surface of 7YSZ columnar coating. The results demonstrate that this approach can improve particle erosion resistance since hardness improvement of Al-modified TBCs. Meanwhile, as another important performance of thermal cycle, it has a better optimization with 350-cycle water-quenching, compared with the as-sprayed TBCs.

Additive Manufacturing of Columnar Thermal Barrier Coatings by Laser Cladding of Ceramic Feedstock

AbstractThis study presents a new laser‐cladding‐based additive manufacturing technique named Clad2Z. Using a robot‐mounted confocal powder nozzle with axial infrared laser beam, ceramic columns with a diameter of 450 µm and an adjustable height are developed. Influence of laser parameters and robot movements on shape and microstructure is analyzed. As an example application, the common material yttria‐stabilized zirconia (YSZ) is used to deposit columnar‐structured thermal barrier coatings (TBCs). The excellent thermal cycling performance of the Clad2Z samples is demonstrated by burner rig tests and comparing lifetime and failure mechanism with conventional TBC systems.

Sintering behavior of columnar thermal barrier coatings deposited by axial suspension plasma spraying (SPS)

During the last decade, Suspension Plasma Spraying (SPS) attracted a lot of interest as an alternative process to produce columnar Thermal Barrier Coatings (TBCs). In this study, columnar TBCs were deposited with SPS. After spraying, samples were isothermally annealed at 1373 K for 1 h, 3 h, 10 h and 50 h, respectively. Microstructures and mechanical properties of the ceramic coatings were investigated as a function of annealing time. Annealing resulted in healing of micro-cracks, coarsening of pores, growth of domain size, companied with a decrease of porosity within columns. The change of coating microstructure led to change of mechanical properties. In addition, residual stress in SPS coatings was also investigated. Furthermore, as-sprayed coatings and pre-annealed coatings were subjected to burner rig tests. Short time pre-annealing allowed to enhance thermal cycling lifetime of such SPS coatings. The thermal cycling results were related to microstructure modifications of coatings.

Failure analysis of thermally cycled columnar thermal barrier coatings produced by high-velocity-air fuel and axial-suspension-plasma spraying: A design perspective

Axial-suspension-plasma spraying (ASPS) is a fairly recent thermal spray technology which enables production of ceramic top coats in TBCs, incorporating simultaneously the properties of both the conventional-plasma sprayed (highly insulating porous structures) and electron-beam-physical-vapor-deposited (strain-tolerant columnar structures) top coats. TBCs are required to insulate the hot components in a gas turbine engine against high temperature and harsh operating conditions. Periodic heating and cooling of turbine engines during operation can create severe thermal cyclic fatigue conditions which can degrade the performance of these coatings eventually leading to the failure. An in-depth experimental investigation was performed to understand the failure behavior of columnar TBCs subjected to thermal cyclic fatigue (TCF) test at 1100ᵒC. The study revealed that the TCF performance was influenced to an extent, by the top coat microstructure, but was primarily affected by the severity of thermally grown oxide (TGO) growth at the bond coat-top coat interface. Mixed failure modes comprising crack propagation through the bond coat-TGO interface, through TGO and within the top coat were identified. Based on the analysis of the experimental results and thorough discussion a novel design of microstructure for the high TCF performance columnar TBC is proposed.

Novel Prospects for Plasma Spray–Physical Vapor Deposition of Columnar Thermal Barrier Coatings

Plasma spray–physical vapor deposition (PS-PVD) is an emerging coating technique that can produce columnar thermal barrier coatings from vapor phase. Feedstock treatment at the start of its trajectory in the plasma torch nozzle is important for such vapor-phase deposition. This study describes the effects of the plasma composition (Ar/He) on the plasma characteristics, plasma–particle interaction, and particle dynamics at different points spatially distributed inside the plasma torch nozzle. The results of calculations show that increasing the fraction of argon in the plasma gas mixture enhances the momentum and heat flow between the plasma and injected feedstock. For the plasma gas combination of 45Ar/45He, the total enthalpy transferred to a representative powder particle inside the plasma torch nozzle is highest (~9828 kJ/kg). Moreover, due to the properties of the plasma, the contribution of the cylindrical throat, i.e., from the feed injection point (FIP) to the start of divergence (SOD), to the total transferred energy is ~69%. The carrier gas flow for different plasma gas mixtures was also investigated by optical emission spectroscopy (OES) measurements of zirconium emissions. Yttria-stabilized zirconia (YSZ) coating microstructures were produced when using selected plasma gas compositions and corresponding carrier gas flows; structural morphologies were found to be in good agreement with OES and theoretical predictions. Quasicolumnar microstructure was obtained with porosity of ~15% when applying the plasma composition of 45Ar/45He.

Deposition and CMAS Corrosion Mechanism of 7YSZ Thermal Barrier Coatings Prepared by Plasma Spray-Physical Vapor Deposition

Using agglomerated and sintered ZrO2-7wt%Y2O3(7YSZ) powders as raw materials, columnar thermal barrier coatings(TBCs) were prepared by plasma spray-physical vapor deposition(PS-PVD) at substrate temperature of 850 ℃. The microstructures of columnar coating were analyzed by field emission-scanning electron microscope(FE-SEM). Based on the theory of atoms together, the formation and growth process of 7YSZ nucleus were investigated and the deposition mechanism of columnar coating was also analyzed. Besides, under the temperature of 1200 ℃, the CMAS(CaO-MgO-Al2O3-SiO2) corrosion of columnar 7YSZ coating was examined and its failure mechanism was also discussed. During the deposition process of columnar 7YSZ coating, the 7YSZ gas molecules were firstly absorbed on substrate, then through diffusion and migration, the critical nucleus formed rapidly. Besides, the critical nucleus further grew up forming crystal island by absorbing other molecules. Then after following four steps of island formation, union formation, channel formation and continuation, the columnar coating formed eventually. When above melting temperature, the CMAS could penetrate into porous columnar coating by capillary force and react with the 7YSZ coating. Finally, the cracks appeared in the coating due to thermochemical and thermomechanical interaction.

Plasma Spray-PVD: A New Thermal Spray Process to Deposit Out of the Vapor Phase

Plasma spray-physical vapor deposition (PS-PVD) is a low pressure plasma spray technology recently developed by Sulzer Metco AG (Switzerland). Even though it is a thermal spray process, it can deposit coatings out of the vapor phase. The basis of PS-PVD is the low pressure plasma spraying (LPPS) technology that has been well established in industry for several years. In comparison to conventional vacuum plasma spraying (VPS) or low pressure plasma spraying (LPPS), the new proposed process uses a high energy plasma gun operated at a reduced work pressure of 0.1 kPa (1 mbar). Owing to the high energy plasma and further reduced work pressure, PS-PVD is able to deposit a coating not only by melting the feed stock material which builds up a layer from liquid splats but also by vaporizing the injected material. Therefore, the PS-PVD process fills the gap between the conventional physical vapor deposition (PVD) technologies and standard thermal spray processes. The possibility to vaporize feedstock material and to produce layers out of the vapor phase results in new and unique coating microstructures. The properties of such coatings are superior to those of thermal spray and electron beam-physical vapor deposition (EB-PVD) coatings. In contrast to EB-PVD, PS-PVD incorporates the vaporized coating material into a supersonic plasma plume. Owing to the forced gas stream of the plasma jet, complex shaped parts such as multi-airfoil turbine vanes can be coated with columnar thermal barrier coatings using PS-PVD. Even shadowed areas and areas which are not in the line of sight of the coating source can be coated homogeneously. This article reports on the progress made by Sulzer Metco in developing a thermal spray process to produce coatings out of the vapor phase. Columnar thermal barrier coatings made of Yttria-stabilized Zircona (YSZ) are optimized to serve in a turbine engine. This process includes not only preferable coating properties such as strain tolerance and erosion resistance but also the simultaneous coverage of multiple air foils.

Vapor Phase Deposition Using a Plasma Spray Process

Plasma spray-physical vapor deposition (PS-PVD) is a low pressure plasma spray technology recently developed by Sulzer Metco AG (Switzerland) to deposit coatings out of the vapor phase. PS-PVD is developed on the basis of the well established low pressure plasma spraying technology. In comparison to conventional vacuum plasma spraying and low pressure plasma spraying, these new processes use a high energy plasma gun operated at a work pressure below 2 mbar. This leads to unconventional plasma jet characteristics, which can be used to obtain specific and unique coatings. An important new feature of PS-PVD is the possibility to deposit a coating not only by melting the feed stock material, which builds up a layer from liquid splats, but also by vaporizing the injected material. Therefore, the PS-PVD process fills the gap between the conventional PVD technologies and standard thermal spray processes. The possibility to vaporize feedstock material and to produce layers out of the vapor phase results in new and unique coating microstructures. The properties of such coatings are superior to those of thermal spray and electron beam-physical vapor deposition (EB-PVD) coatings. In contrast to EB-PVD, PS-PVD incorporates the vaporized coating material into a supersonic plasma plume. Due to the forced gas stream of the plasma jet, complex shaped parts like multi-airfoil turbine vanes can be coated with columnar thermal barrier coatings using PS-PVD. Even shadowed areas and areas, which are not in the line-of-sight to the coating source, can be coated homogeneously. This paper reports on the progress made by Sulzer Metco to develop a thermal spray process to produce coatings out of the vapor phase. Columnar thermal barrier coatings made of yttria stabilized zircona are optimized to serve in a turbine engine. This includes coating properties like strain tolerance and erosion resistance but also the coverage of multiple air foils.

Quantitative analysis of a new model for the sintering of columnar thermal barrier coatings

Sintering of thermal barrier coatings adversely affects their long-term reliability by changing their key attributes. In recent work, we developed a novel modeling approach to study coating structure evolution due to the sintering of topcoat columns and predicted the formation of large clusters, and a network of elongated pore channels separating these [R. Krishnamurthy, D.J. Srolovitz, Acta Mater., in press]. Here we extract statistical measures, such as the pair correlation function, the in-plane porosity and pore size distribution from the predicted sintered microstructures, and systematically analyze their variation as a function of coating system parameters. The variables that have the greatest effect on these measures are the column density and the extent of the ‘feathery’ structure of the coating. At early times, low column densities, and where the ‘feathery’ protrusions extend only to a small distance, small clusters containing mainly neighboring columns are formed. On the contrary, when these variables assume large values, clusters many columns wide are formed. We also predict the formation of in-plane pores of sizes comparable to the width of the ‘mud-cracks’ seen in experiment [V. Lughi, V.K. Tolpygo, D.R. Clarke, Mater. Sci. Eng. A 368 (2004) 212–221]. Estimates for cluster size based on this pore size also agree with experiment.