Georg Mauer

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Plasma-Sprayed Thermal Barrier Coatings: New Materials, Processing Issues, and Solutions

Growing demands on thermal barrier coatings (TBCs) for gas turbines regarding their temperature and cyclic capabilities, corrosion resistance, and erosion performance have instigated the development of new materials and coating systems. Different pyrochlores, perovskites, doped yttria-stabilized zirconia, and hexaaluminates have been identified as promising candidates. However, processing these novel TBC materials by plasma spraying is often challenging. During the deposition process, stoichiometric changes, formation of undesired secondary phases or non-optimum amorphous contents, as well as detrimental microstructural effects can occur in particular. This article describes these difficulties and the development of process-related solutions by employing diagnostic tools.

Plasma Characteristics and Plasma-Feedstock Interaction under PS-PVD Process Conditions

Plasma spray-physical vapor deposition (PS-PVD) is a novel coating process based on plasma spraying. In contrast to conventional methods, deposition takes place not only from liquid splats but also from nano-sized clusters and from the vapor phase. This offers new opportunities to obtain advanced microstructures and thus to comply with growing demands on modern functional coatings. In this study, different process conditions were investigated with regard to the application of the PS-PVD process for ceramic thermal barrier coatings. Plasma characteristics were calculated under chemical equilibrium conditions by minimizing the Gibbs energy. The plasma-feedstock interaction was modeled taking into account the particular conditions at very low pressure. Since the plasma is highly rarefied, the small feedstock particles are in the free molecular flow regime. Hence, continuum methods commonly used in fluid mechanics and heat transfer approaches with continuous boundary conditions are not appropriate; alternative methods based on the kinetic theory of gases are required. The experimental results confirm the predictions about the degree of vaporization made by such calculations. In particular, they show that the feedstock treatment mainly takes place within the very first trajectory segment between injector and jet expansion.

Improving Atmospheric Plasma Spraying of Zirconate Thermal Barrier Coatings Based on Particle Diagnostics

Lanthanum zirconate (La2Zr2O7) has been proposed as a promising material for thermal barrier coatings. During atmospheric plasma spraying (APS) of La2Zr2O7 a considerable amount of La2O3 can evaporate in the plasma flame, resulting in a non-stoichiometric coating. As indicated in the phase diagram of the La2O3-ZrO2 system, in the composition range of pyrochlore structure, the stoichiometric La2Zr2O7 has the highest melting point and other compositions are eutectic. APS experiments were performed with a TriplexPro™-200 plasma torch at different power levels to achieve different degrees of evaporation and thus stoichiometry. For comparison, some investigations on gadolinium zirconate (Gd2Zr2O7) were included, which is less prone to evaporation and formation of non-stoichiometry. Particle temperature distributions were measured by the DPV-2000 diagnostic system. In these distributions, characteristic peaks were detected at specific torch input powers indicating evaporation and solidification processes. Based on this, process parameters can be defined to provide stoichiometric coatings that show good thermal cycling performance.

Plasma Spray-PVD: Plasma Characteristics and Impact on Coating Properties

Typical plasma characteristics of the plasma spray-physical vapour deposition (PS-PVD) process were investigated by optical emission spectroscopy. Electron temperatures were determined by Boltzmann plots while temperatures of the heavy species as well as electron densities were obtained by broadening analysis of spectral lines. The results show how the plasma properties and thermodynamic equilibrium conditions are affected by the admixture of hydrogen and the ambient chamber pressure. Some experimental examples of PS-PVD coatings demonstrate the impact on feedstock treatment and deposited microstructures.

Plasma Spray Physical Vapor Deposition of La1−xSrxCoyFe1−yO3−δ Thin-Film Oxygen Transport Membrane on Porous Metallic Supports

Plasma spray physical vapor deposition (PS-PVD) is a very promising route to manufacture ceramic coatings, combining the efficiency of thermal spray processes and characteristic features of thin PVD coatings. Recently, this technique has been investigated to effectively deposit dense thin films of perovskites particularly with the composition of La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF) for application in gas separation membranes. Furthermore, asymmetric type of membranes with porous metallic supports has also attracted research attention due to the advantage of good mechanical properties suitable for use at high temperatures and high permeation rates. In this work, both approaches are combined to manufacture oxygen transport membranes made of gastight LSCF thin film by PS-PVD on porous NiCoCrAlY metallic supports. The deposition of homogenous dense thin film is challenged by the tendency of LSCF to decompose during thermal spray processes, irregular surface profile of the porous metallic substrate and crack and pore-formation in typical ceramic thermal spray coatings. Microstructure formation and coating build-up during PS-PVD as well as the annealing behavior at different temperatures of LSCF thin films were investigated. Finally, measurements of leak rates and oxygen permeation rates at elevated temperatures show promising results for the optimized membranes.

Deposition of La1−xSrxFe1−yCoyO3−δ Coatings with Different Phase Compositions and Microstructures by Low-Pressure Plasma Spraying-Thin Film (LPPS-TF) Processes

Perovskite-type materials with the general chemical formula A1−xA′xB1−yB′yO3−δ have received considerable attention as candidates for oxygen separation membranes. Preparation of La1−xSrxFe1−yCoyO3−δ (LSFC) coatings by low-pressure plasma spraying-thin film processes using different plasma spray parameters is reported and discussed. Deposition with Ar-He plasma leads to formation of coatings containing a mixture of cubic LSFC perovskite, SrLaFeO4, FeCo, and metal oxides. Coatings deposited at higher oxygen partial pressures by pumping oxygen into the vacuum chamber contain more than 85% perovskite and only a few percent Fe3−xCoxO4, and/or CoO. The microstructures of the investigated LSFC coatings depend sensitively on the oxygen partial pressure, the substrate temperature, the plasma jet velocities, and the deposition rate. Coatings deposited with Ar-rich plasma, relatively low net torch power, and with higher plasma jet velocities are most promising for applications as oxygen permeation membranes.

Plasma Spraying of Ceramics with Particular Difficulties in Processing

Abstract Emerging new applications and growing demands of plasma-sprayed coatings initiate the development of new materials. Regarding ceramics, often complex compositions are employed to achieve advanced material properties, e.g., high thermal stability, low thermal conductivity, high electronic and ionic conductivity as well as specific thermo-mechanical properties and microstructures. Such materials however, often involve particular difficulties in processing by plasma spraying. The inhomogeneous dissociation and evaporation behavior of individual constituents can lead to changes of the chemical composition and the formation of secondary phases in the deposited coatings. Hence, undesired effects on the coating characteristics are encountered. In this work, examples of such challenging materials are investigated, namely pyrochlores applied for thermal barrier coatings as well as perovskites for gas separation membranes. In particular, new plasma spray processes like suspension plasma spraying and plasma spray-physical vapor deposition are considered. In some cases, plasma diagnostics are applied to analyze the processing conditions.

Modeling precursor diffusion and reaction of atomic layer deposition in porous structures

Atomic layer deposition (ALD) is a technique for depositing thin films of materials with a precise thickness control and uniformity using the self-limitation of the underlying reactions. Usually, it is difficult to predict the result of the ALD process for given external parameters, e.g., the precursor exposure time or the size of the precursor molecules. Therefore, a deeper insight into ALD by modeling the process is needed to improve process control and to achieve more economical coatings. In this paper, a detailed, microscopic approach based on the model developed by Yanguas-Gil and Elam is presented and additionally compared with the experiment. Precursor diffusion and second-order reaction kinetics are combined to identify the influence of the porous substrates microstructural parameters and the influence of precursor properties on the coating. The thickness of the deposited film is calculated for different depths inside the porous structure in relation to the precursor exposure time, the precursor ...

Investigations on the Initial Stress Evolution During Atmospheric Plasma Spraying of YSZ by In Situ Curvature Measurement

The residual stresses within plasma-sprayed coatings are an important factor that can influence the lifetime as well as the performance in operation. The investigation of stresses evolving during deposition and post-deposition cooling for atmospheric plasma spraying of yttria-stabilized zirconia coatings using in situ measurement of the samples curvature is a powerful tool for identifying the factors that contribute to stress generation. Under various spray conditions, the first deposition pass leads to a significantly larger increase in samples curvature than the subsequent passes. It is shown in this work that the amount of curvature change at the onset of spraying is significantly influenced by the spray conditions, as well as by the substrate material. More information on the origin of this steep curvature increase at the onset of spraying was obtained by single splat experiments, which yielded information on the splat bonding behavior under various conditions. A comparison of the compressive yield strength for different substrate materials indicated the influence of substrate residual stress relaxation. Residual stress measurements using the incremental hole-drilling method and x-ray diffraction confirmed that the coating deposition affects the substrate residual stress level. The yield strength data were combined with the substrate near-surface temperature during deposition, obtained by finite element simulations, and with the measured residual stress-profile. This revealed that residual stress relaxation is the key factor for the initial curvature increase.