Rainer Gadow

Lanthanum hexaaluminate — novel thermal barrier coatings for gas turbine applications — materials and process development

Abstract Lanthanum hexaaluminate (LHA) with a magnetoplumbite structure is a promising competitor to yttria partially stabilized zirconia (Y-PSZ) as a thermal barrier coating (TBC), since most zirconia coatings age significantly, including undesired densification at temperatures exceeding 1100 °C. The microstructure of calcined lanthanum hexaaluminate powders and thermally sprayed coatings show a platelet structure. The magnetoplumbite structure is characterized by the highly charged La 3+ cation located in an oxygen position in the hexagonal close-packed structure of oxygen ions. Ion diffusion is strongly suppressed vertical to the crystallographic c -axis, thus hindering sintering densification. In contrast to the oxygen ion conducting zirconia, lanthanum hexaaluminate permits operating temperatures above 1300 °C because of its thermal stability and electrically insulating properties. This study describes the optimization of powder preparation for thermal spraying by spray drying and the development of parameters for atmospheric plasma spraying (APS) in order to produce homogeneous crystalline coatings with controlled micro-porosity and residual stresses. The phases were characterized by X-ray diffraction (XRD).

Lanthanum hexaaluminate—a new material for atmospheric plasma spraying of advanced thermal barrier coatings

One of the main application fields of the thermal spraying process is thermal barrier coatings (TBCs). Today, partially stabilized zirconia (YSZ or MSZ) is mainly used as a TBC material. At temperatures above 1000 °C, zirconia layers age distinctively, including phenomena shrinkage and microcrack formation. Therefore, there is a considerable interest in TBCs for higher temperature applications. In this paper, lanthanum hexaaluminate, a newly developed TBC material with long-term stability up to 1400 °C, is presented. It ages significantly more slowly at these high temperatures than commercial zirconia-based TBCs. Its composition favors the formation of platelets, which prevent a densification of the coating by postsintering. It consists of La2O3, Al2O3, and MgO. Its crystal structure corresponds to a magnetoplumbite phase. Lanthanum hexaaluminate powders were produced using two different fabrication routes, one based on salts and the other one based on oxides. To optimize the granulate, various raw materials and additives were tested. The slurry was spray dried in a laboratory spray drier and calcined at 1650 °C. Using these two powders, coatings were produced by atmospheric plasma spraying (APS). The residual stresses of the coatings were measured by the hole drilling method, and the deposition process was optimized with respect to the residual stresses in the TBC. The coatings were extensively analyzed regarding phase composition, thermal expansion, and long-term stability, as well as microstructural properties.

Composite coatings with dry lubrication ability on light metal substrates

Abstract Due to the increasing importance of lightweight engineering and design driven by ecological and economical reasons, advanced composite coatings are needed to improve the surface properties of machine elements and system components made of light metals and their alloys, like magnesium and aluminum. Light metals in general exhibit very poor tribological properties in unlubricated condition resulting in severe seizing and wear. The application of thin solid lubricant coatings by PVD and CVD processes improves the tribological behavior significantly, but in many cases these coatings fail under high surface loading due to the low Youngs modulus of the light metal substrate and the mechanical incompatibility of thin solid film and substrate. In addition, the damage tolerance of the coatings is unsatisfying. This study focuses on the evaluation of composite coatings consisting of a thermally sprayed ceramic or metallurgical primary layer and a polymer or CVD/PVD deposited secondary layer. The thermally sprayed coating protects the light metal substrate by providing compressive strength, hardness and wear resistance. The polymer or CVD/PVD deposited top coating in appropriate composition on the other hand causes a low friction coefficient. For this study special polymer coatings containing microscale PTFE particles as solid lubricants as well as cathode sputtered MoS 2 coatings and pure carbon containing coatings (a-C:H) applied by plasma assisted CVD were used. The tribological performance of the composite coatings is evaluated in high cycle pin-on-disk testing.

Liquid phase coating process for protective ceramic layers on carbon fibers

The most outstanding feature of carbon fibers is their high tenacity at high temperatures; this property cannot be sustained if the unprotected fibers are exposed to an oxidative environment. Protecting the fiber surface by deposition of a ceramic layer shows the best prospects for introduction of high temperature oxidation stability. SiBCN and SiCN ceramics are the preferred coating materials due to their excellent oxidation resistance at elevated temperatures. The coating operation is carried out by a continuous liquid polymer impregnation (LPI) of carbon fibers with polyorgano(bora)silazanes. In order to reach an optimum behavior of the polymer in the LPI operation, detailed rheological studies were carried out. The viscosity of the polymer is adjusted by addition of organic solvents and other additives. In the pilot plant, the desizing and liquid phase coating of the fibers, polymerization and ceramization of the coating are subsequently carried out in a continuous process. Pilot plant construction was carried out in modular design in order to be able to modify parts of the equipment if necessary. In the first module, the fiber is desized in an organic solvent and dried. The second module is built as a vacuum chamber, the following operations of LPI, polymerization and ceramization are carried out under inert atmosphere. Before putting the plant into service, preliminary investigations were carried out. After fitting the rheological properties, the operations were optimized using a dip-coating process with subsequent batch polymerization and pyrolysis.

Suspension thermal spraying of hydroxyapatite: microstructure and in vitro behaviour.

In cementless fixation of metallic prostheses, bony ingrowth onto the implant surface is often promoted by osteoconductive plasma-sprayed hydroxyapatite coatings. The present work explores the use of the innovative High Velocity Suspension Flame Spraying (HVSFS) process to coat Ti substrates with thin homogeneous hydroxyapatite coatings. The HVSFS hydroxyapatite coatings studied were dense, 27-37μm thick, with some transverse microcracks. Lamellae were sintered together and nearly unidentifiable, unlike conventional plasma-sprayed hydroxyapatite. Crystallinities of 10%-70% were obtained, depending on the deposition parameters and the use of a TiO2 bond coat. The average hardness of layers with low (<24%) and high (70%) crystallinity was ≈3.5GPa and ≈4.5GPa respectively. The distributions of hardness values, all characterised by Weibull modulus in the 5-7 range, were narrower than that of conventional plasma-sprayed hydroxyapatite, with a Weibull modulus of ≈3.3. During soaking in simulated body fluid, glassy coatings were progressively resorbed and replaced by a new, precipitated hydroxyapatite layer, whereas coatings with 70% crystallinity were stable up to 14days of immersion. The interpretation of the precipitation behaviour was also assisted by surface charge assessments, performed through Z-potential measurements. During in vitro tests, HA coatings showed no cytotoxicity towards the SAOS-2 osteoblast cell line, and surface cell proliferation was comparable with proliferation on reference polystyrene culture plates.

Protective Multilayer Coatings for Carbon‐Carbon Composites

Abstract Carbon fiber-reinforced composites (CFCs) are well-established lightweight materials combining high specific strength and damage tolerance, especially at elevated temperatures in the range above 1000 °C. For high-temperature applications, protective coatings have to provide oxidation and corrosion resistance for the CFC, with additional thermophysical and chemical compatibility requirements. Various technologies are introduced to produce coating architectures consisting of dense ceramic top layers and porous base coatings. The top layers should provide chemically inert surfaces, which are able to act as diffusion barriers. The porous base coatings serve as a structural link with compensation ability between the brittle top coating with a low strain-to-failure and the thermophysically anisotropic carbon–carbon substrate. Furthermore, they provide the chemical compatibility between the substrate and the functional ceramic top coating.

Residual stress analysis in thermally sprayed layer composites, using the hole milling and drilling method

Residual stresses are related to the thermophysical properties of substrate and coating materials and occur after the coated component has undergone thermal spraying and machining processes. All residual stresses in layer composites result from different individual stress mechanisms occurring during the manufacturing process, mainly based on heat and mass transfer during the coating deposition. Using the hole-milling-and-drilling method, residual stress fields can be measured in a quasi-nondestructive way over the drilling depth with appropriate resolution. In several drilling and milling operations, a cylindrically shaped hole is brought step by step into the component surface. The residual stresses are locally relieved due to material removal, deform the surface around the drilled microhole, and are measured by high-resolution measurement tools (e.g., strain gages (DMS)), for every drilling step in the form of relaxed surface strains. Using calibration curves and material data (E, μ), the measured surface strains are converted into nominal strains at the bottom of the drilled hole for every drilling step. Out of the differentiated strains, in-plane stress fields can be incrementally determined by Hooke’s law. This study describes residual stress measurement features, the finite-element method (FEM) calculation, and the idealization of calibration curves, as well as the results of exemplary stress measurements.

Cyclosilazanes and borazines : polymer precursors to silicon- and boron-containing ceramics

The chemistry of cyclosilazanes and borazines is a topic of current interest in view of preceramic polymers which yield on pyrolysis various useful ceramic materials, e.g. silicon and boron carbide, silicon and boron nitride, and ternary or quaternary mixtures of these materials. These materials are hard and have high oxidative and thermal stability. Other useful properties are resistance to corrosion, thermal shock and creep, low electrical conductivity and low coefficient of thermal expansion. One of the best application prospects is the use of the preceramic polymers as protective coating material for carbon fibres.

What Do We Know, What are the Current Limitations of Suspension HVOF Spraying?

Suspension spraying has evolved during the past decades and now is at the threshold of a commercial utilization. Compared to standard powder spray methods, mainly DC plasma spraying and (high velocity) flame spraying, it is quite clear that suspension spraying will not replace these well-established technologies but can extend them by adding new coating properties. Still there remain many issues to be solved. Suspension interaction with the hot gas stream is much more complex than in ordinary powder spray processes. In case of HVOF when axial injection into the combustion chamber is used, a direct observation of the liquid flame interaction is not possible. This paper discusses the present status of suspension HVOF-spraying (high velocity suspension flame spraying) including torch concepts, torch configuration in case of a TopGun system as well as different injector concepts and their influence on suspension atomization. The role of suspensions is discussed regarding their rheological and thermodynamical properties, mainly given by the solvent type and the solid content. An overview of different available diagnostic methods and systems and the respective applicability is given. Coating properties are shown and discussed for several oxide ceramics in respect to their possible applications.

Comparison Between High-Velocity Suspension Flame Spraying and Suspension Plasma Spraying of Alumina

Two different spray processes—suspension plasma spraying (SPS) and high-velocity suspension flame spraying (HVSFS)—are under focus in the field of suspension spraying. Both techniques are suitable for manufacturing finely structured coatings. The differences in the particle velocity and temperature of these two processes cause varying coating characteristics. The high particle velocity of the HVSFS process leads to more dense coatings with low porosity values. Coatings with a higher and also homogeneous porosity, which can be generated by SPS, have also high potential, for example, for thermal barrier coatings. In this study, both the processes—SPS and HVSFS—were compared using alumina as feedstock material mixed with different solvents. Besides the characterization of the microstructure and phase composition of the applied coatings, the focus of this study was the investigation of the melting behavior of the particles in-flight and of single splat characteristics.