Abstract In this study, a novel self-healing concept is considered in order to increase the lifetime of thermal barrier coatings (TBCs) in modern gas turbines. For that purpose, SiC healing particles were introduced to conventional 8YSZ topcoats by using various plasma spray concepts, i.e., composite or multilayered coatings. All topcoats were sprayed by SG-100 plasma torch on previously deposited NiCrAlY bondcoats produced by conventional atmospheric plasma spraying. Coatings were subjected to thermal conductivity measurements by laser flash method up to 1000°C, isothermal oxidation testing up to 200h at 1100°C and finally thermal cyclic fatigue (TCF) lifetime testing at 1100°C. Microstructural coating evaluation was performed by scanning electronic microscope (SEM), in the as-produced and post high-temperature tested states. This was done to analyze the self-healing phenomena and its influence on the high-temperature performance of the newly developed TBCs.

Abstract During the design and development of new layer coating systems, layer characteristics can be directly influenced or modified. In this case, the modification of the powder is of primary significance, i.e. factors such as grain size, grain size distribution and composition. In this paper, fine-crystalline Al2O3+SiC layer systems of differing compositions are presented. Due to their especially fine crystalline structure, as well as the imbedding of fine SiC particles in the Al2O3 matrix, these layers demonstrate improved properties compared with standard Al2O3 coatings. Additionally, improved fracture behaviour of these coatings is apparent - their behaviour can be described as "quasi-ductile". The coating process utilised was the HVOF (High Velocity Oxy-Fuel Flame Spraying) process. As the spraying system, a Top-Gun ultrasonic (airless) spraying (system) equipment was used. The coating parameters were optimised in conjunction with the ATZ Evus Company. Microscopic analyses were carried out using the SEM and EDX methods. The structural analysis of the coating was determined using X-ray diffractometer measurements. In order to determine characteristic material properties, a range of investigation methods were applied. These latter included the Palmquist tests, by which fracture toughness was quantified. Additionally, tribological investigations were performed using an abrasive disk method. In the results, the influence of the SiC content on mechanical properties was investigated. Furthermore, a comparison with standard Al2O3 coatings was made. Paper text in German.

Abstract This paper presents the metallurgical concept as well as the results for composite powder production and thermal spraying of silicon carbide (SiC) cermets. In addition to the layer characterization with regard to the SiC content and the porosity, the wear and corrosion behavior of the SiC coatings are investigated and the application potential for SiC cerments is shown. The main area of application of the high velocity oxygen fuel process is the production of wear and corrosion protection layers on the basis of cermet materials. Various investigations were carried out to apply SiC containing composites for thermal spraying. The innovative concept for the production of SiC composites shows, that adapted alloys, with a matrix alloy composition outside the primary crystallization limiting eutectic, enables the wetting of the SiC particles by reduced reactivity. Paper includes a German-language abstract.

Abstract The different HVOF processes have established themselves alongside the conventional thermal spray processes for the production of wear and corrosion protection layers. The increasing demands on functional component surfaces with regard to mechanical, chemical, and thermal loads lead to the development of novel material systems for spray application. Due to the synergy of excellent material properties, SiC is of great interest for thermal spraying. High-energy grinding is an interesting and economical alternative to the production of SiC-containing composite powders. In this paper, mechanically alloyed SiC composite powders with different metallic matrices are produced. These SiC composite powders are processed using HVOF. The sprayed layers are characterized in terms of layer morphology and wear behavior and compared with conventional cermets. Paper includes a German-language abstract.

Abstract Aluminum coatings reinforced with either Al 2 O 3 or SiC particles were deposited onto aluminum substrates and subjected to various tests. The coatings were made with mechanically alloyed powders via atmospheric plasma spraying (APS). Both types of coatings had uniformly distributed hard particles, porosities in the range of 4 to 5%, and bond strengths of around 20 MPa. The wear resistance of the SiC-reinforced coatings, however, was almost 35% higher than the coatings containing Al 2 O 3 . X-ray examination (XRD) showed that the Al 2 O 3 particles undergo partial phase transformation during spraying, making them more prone to wear.

Abstract Reusable space vehicles, which must withstand re-entry into the Earth's atmosphere, require external protection systems (TPS) which are usually in the forms of rigid surface in areas of high or moderate working temperature. High heat fluxes and temperatures related to high performance hypervelocity flights also require the use of TPS materials having good oxidation and thermal shock resistance, dimensional stability, and ablation resistance. Components by these materials are usually fabricated, starting from either billets or plate stocks, by uniaxial hot pressing, and complex parts, such as low radius edges, are then obtained by electrical discharge machining technique. This article investigates an alternative fabrication technology, based on plasma spraying, to produce near net shape components. Results of experimental activities, such as optimization of plasma spraying parameters based on a DOE approach, are reported and discussed.

Abstract Molybdenum silicides have the potential as protective coatings for high-temperature applications because of their high melting point and their high-temperature oxidation resistance. Reinforcing MoSi2 with SiC shows an improvement of its low toughness at room temperature and low creep resistance at temperatures above the brittle-ductile transition temperature of approximately 700-1000 °C. A new kind of powder processing was used to produce MoSi2 and MoSi2-SiC as a feedstock for thermal spraying. Mixtures of the elemental powders, molybdenum and silicon, were prepared by milling and subsequent heat treatment to get highly dispersed, pre-reacted powders. As high-energy milling equipment, a planetary ball mill was used to prepare the powders. In the case of reinforcement, SiC was mixed to the pre-reacted MoSi2 at the end of the milling process, that means before heat treatment. On these as-milled powders, X-ray diffraction characterization (XRD), scanning electron microscopy (SEM), electron probe micro analysis (EPMA) and determination of the oxygen level were carried out. Vacuum plasma spraying has been used to deposit the powders onto a carbon steel substrate. Evaluated coating characteristics were the microstructure (SEM), phases (XRD), EPMA, oxygen content, microhardness and surface roughness. Tests at high temperatures will be considered in future work.

Abstract In this paper a process based on both Thermal Plasma Chemical Vapor Deposition (TPCVD) and Suspension Plasma Spraying (SPS) is applied on r.f. induction thermal plasma for α/β-SiC ceramic synthesis and deposition. The starting materials are low-cost liquid disilanes. The resulting coatings are investigated by means of SEM and XRD. Results on the influence of the processing parameters (i.e. pressure, spray distance, substrate temperature, plasma gas nature and composition, precursor composition, atomization parameters) on the coating phase and microstructure are shown. Control of the microstructure (or nanostructure) as well as of the phase content, namely the ratio α/β can be achieved. A processing route presenting the elementary steps of SiC TPCVD is also proposed.

Abstract Using a DC-plasmajet amorphous and nanocrystalline Si-C-layers are synthesized from chloromethylsilanes on various substrate materials. Though most of the layers show granular morphologies with cluster diameters between 25 and 400 nm depending on the process parameters, coatings with a dense or columnar morphology and with a smooth surface can be synthesized as well. XRD analyses verify β-SiC crystals with an average diameter of 5 nm. In some samples produced from carbon rich precursors also graphite is detected. Depending on the substrate material and the process parameters deposition rates up to 1,300 µm/h are obtained. Apart from silicon and carbon the coatings convey oxygen and chlorine verified by EDX. Coatings removed from the substrate can withstand several bending cycles (45°) without any visible indication of failure. Paper text in German.

Abstract This paper reports on the synthesis of SiC material through the decomposition of silanes in a thermal high frequency (HF) plasma. The process is based on thermal plasma technology for chemical deposition from the gas phase and on suspension plasma spray technology, in which a liquid or suspension is injected axially and atomized in the plasma flame. The liquid silane then decomposes, and forms SiC with some gaseous by-products such as HCl. Various plasma parameters were varied, for example the plasma power level, the plasma gas composition, the chamber pressure, and the silane composition. The paper also presents first investigations into the elementary and phase composition as well as the morphology of the powders and coatings. Paper includes a German-language abstract.

Abstract This study was aimed at the production of SiC-MoSi2 composite powders through a high-temperature plasma reaction route. The addition of SiC appears to be the best second phase reinforcement for improving the mechanical properties of MoSi2 material for high-temperature structural application. The in-flight carbonization of MoSi2 powders was carried out in an Ar-H2-CH4 induction plasma process. Using methane served as both the powder carrier gas and the "precursor" to react with the MoSi2 powders forming the SiC phase in-situ . Under the experimental conditions employed in this investigation, up to about 8.0 wt. % of carbon was incorporated into the MoSi2 powder particles. The chemical composition, phase content and the microstructure of the composite powder products were examined by XRD, SEM, EDS etc. analysis methods. The reaction mechanisms are discussed in terms of the calculated thermodynamic equilibria.

Abstract A novel plasma spray process for producing nanostructured coatings, hypersonic plasma particle deposition (HPPD), has been experimentally investigated. In HPPD, vapor phase precursors are injected into a plasma stream generated by a DC arc. The plasma is quenched by supersonic expansion through a nozzle into a vacuum (~ 2 torr) deposition chamber. Ultrafine particles nucleated in the nozzle are accelerated in the hypersonic free jet downstream of the nozzle and inertially deposited onto a substrate. The short transit times between the nozzle and the substrate (< 50 μs) prevent inflight agglomeration, while the high particle deposition velocities result in the formation of a consolidated coating. We have investigated the production of silicon and silicon carbide coatings using SiCl 4 and CH 4 precursors. Silicon deposits analyzed by transmission electron microscopy were found to have nanostructured regions with grain sizes varying from 5-20 nm. Corresponding particle size distributions measured before deposition using an extractive aerosol probe peaked around 15 nm, suggesting negligible grain growth occurred in the samples studied. Silicon carbide particle size distributions measured at various deposition chamber pressures verify that the low residence time characteristic of the HPPD process minimizes in-flight agglomeration.

Abstract For reasons of the decrease in weight in the industry light cage design materials like aluminum alloys are frequently used. Because the wear resistance of aluminum alloys and/or aluminum generally is not sufficient, an increased wear resistance can be reached by means of particle reinforced aluminum coatings. The installation of ceramic reinforcing components (for example oxide particles) in the ductile metal matrix brings an essential improvement of the wear resistance particularly with regard to abrasion and short time fatigue wear. The results presented in the paper refer to research works concerning thermally sprayed Al - coatings with Al 2 O 3 - and SiC - particles as reinforcement components by vacuum plasma spraying.

Abstract The use of aluminum in the automobile engines and other critical parts require a superior surface property of the same. This has led to the development of plasma sprayable surface coatings in the automotive components. To impart the maximum bonding strength, along with hardness to the coatings, an aluminum based composite (Al-SiC) was chosen to be the most suitable. The presence of a hard second phase within a soft matrix improves the wear resistance of the material. The metal matrix composite powders were made by mechanical alloying of 6061 aluminum alloy (particle size 40-60 μm) along with fine SiC particles (≈ 8μm). Content of SiC was varied from 20-75vol% the balance being aluminum alloy. An organic material was used as Process Control Agent to optimize distribution of ceramic within metal matrix. The coatings obtained by plasma spraying the powders were characterized for their microstructure, adherence, wear and other physical properties.

Abstract During grinding of thermally sprayed WC-Co, the grinding ratio G ( ratio of volume of work removed to the volume of wheel consumed) is usually low and the finish produced sometimes is inadequate. Improvement in surface finish accompanies increase in grinding ratio. The objective of this investigation is to study the effect of type of abrasive, table speed, and depth of cut on the surface finish and hardness of WC-Co. Thermally sprayed WC-12 wt % Co and WC-17 wt % Co produced using the high velocity oxygen fuel (HVOF) process, have been ground using silicon carbide and diamond wheels under different operating conditions. The surface profile reveals the significant role played by the above parameters on the surface finish. The grinding ratio, G in case of diamond grinding was found to be larger than silicon carbide grinding however, the quality of the surface finish produced by silicon carbide was better than the diamond. The surface structure of the ground WC-Co was examined by SEM. Surfaces ground using a silicon carbide wheel exhibited extensive plastic flow, while surfaces ground with diamond wheels are highly fractured with localized flow which suggests two different mechanisms of material removal. The surface hardness after grinding, was found to depend on the type of abrasive and table speed. Silicon carbide grinding has shown higher hardness and better surface finish than diamond grinding.