Partially stabilized zirconia (8Y2O3-ZrO2) coatings were studied as thick thermal barrier coatings (TTBCs) for diesel engine applications. To improve the hot corrosion resistance of TTBCs the 1 mm thick yttria stabilized zirconia coating was densified with aluminum phosphate based sealant. Combined with better hot corrosion resistance other benefits obtained with sealing treatment are improved adhesion as well as increased mechanical properties of the ceramic layer. Three aluminum phosphate based sealants were investigated with varying viscosity level. Different sealant viscosities were used to optimize the level of sealant penetration into the coating. Sealant penetration and the violence of the reaction were determined by XRD, SEM/EDS and optical microscopy. The hardness profile from bond coat to the surface of the top layer was determined. Coating microstructure and phase structure were characterized by optical microscopy and by X-ray diffraction. Microhardness and porosity were determined. Residual stress states were measured by X-ray based stress analyzer. Bond strength of the coatings was determined with tensile test equipment. To simulate the diesel engine combustion conditions, hot corrosion tests were performed for the sealed TTBCs. Hot corrosion resistance of the coating was tested in isothermal exposure of 60Na2SO4 - 40V2O5 melt for 48 hours at 600 °C.

In gas turbines and diesel engines there is a demand for Thick Thermal Barrier Coatings (TTBCs), because of the increased process combustion temperatures. Unfortunately the increased thickness of plasma sprayed TBCs normally leads to a reduced coating lifetime. So for that reason the coating structures have to be modified. When modifying the structure of TTBCs, the focus is normally set on elastic modulus reduction of the thick coating, in order to improve the coating strain tolerance. On the other hand, coating structural modification procedures, such as sealing treatments, can be performed when increased hot corrosion resistance or better mechanical properties are needed. In this paper we introduced several modified zirconia based TTBC structures and their specific microstructural properties. Coating surface sealing procedures such as phosphate sealing, laser-glazing and sol-gel impregnation were studied as potential methods in increasing the hot corrosion and erosion resistance of TTBCs. Some microstructural modifications were also made by introducing segmentation cracks into the coating structures by laserglazing and by using special spraying parameters. These last two methods were studied in order to increase the strain tolerance of TTBCs. The coating microstructures were characterized by optical microscopy, SEM, TEM, EDS analysis and X-ray diffraction. The effect of sealing procedures was studied on basic thermal and mechanical properties of the coatings. In the paper it was also presented some correlations between the coating properties and microstructures, and discussed about the advantages and drawbacks of each modification procedure.

This paper demonstrates the use of an articulated robot in a plasma spraying operation for gas turbine transition ducts. A 3D model of the coating cell and workpiece plays a key role in the application, facilitating off-line programming and the verification of process parameters prior to spraying. In the spray experiments, a YSZ coating was applied to a NiCr bondcoat and subsequently characterized based on microstructure and hardness. The results were then used to set the injection parameters and travel path for the actual component. Paper includes a German-language abstract.

The main focus of this study is sealed zirconia-based thermal barrier coatings produced by plasma spraying. AEM examinations conducted on phosphate-sealed Y 2 O 3 -ZrO 2 coatings and orthophosphoric acid sealed MgO-ZrO 2 coatings provide detailed information on the phases in the sealed coatings and the bonding mechanism of phosphate-based sealants on zirconia. In the case of aluminum phosphate sealed alumina and chromia, the primary objective is to add clarity on the bonding mechanism by means of transmission electron microscopy. Investigations of the sealing-layer interface indicate that the sealing of the layer lamellas is due to chemical reactions. Paper includes a German-language abstract.

Hot corrosion behavior of Thermal Barrier Coatings (TBCs) has been studied by comparison between double layer coatings and graded coatings. Two types of oxide ceramics, 8 mass % Y 2 O 3 -ZrO 2 (8YZ) and 2CaOSiO 2 -15mass% CaOZrO 2 (C 2 S-15 CZ), with a bond coating of NiCrAlY were applied to metallic substrates in this study. After a hot corrosion test by V 2 O 5 -Na 2 SO 4 corrosive ashes, hot corrosion behavior of TBC has been investigated by visual inspection, metallography, X-ray diffraction and EPMA. The C 2 S-15%CZ coating reacted with V 2 O 5 only where it was in direct contact with the material. The affected area from the reaction was limited to the coating surface where V 2 O 5 existed. The coating showed adequate hot corrosion-resistance. It was found on the 8YZ coating that Y 2 O 3 , the stabilizing component, particularly reacts with V 2 O 5 and loses its function; this led to partial spalling of the coating. It was observed that the durability of the double layer TBC was largely influenced by the performance of a corrosion resistant NiCrAlY undercoat which provided protection against corrosive components penetrating through the ceramic topcoat. It was observed that the graded coating degraded due to oxidation of NiCrAlY particles which independently existed near the coating surface and affected the durability of TBC.

Invar alloy (Fe – 36%Ni) is used in industrial applications which require high dimensional stability because of its exceptionally low thermal expansion coefficient. Purpose of this work is to enhance the performance of molds for the production of carbon fiber reinforced plastic (CFRP) components. Four different kinds of commercial powders were coated on an Invar substrate: Al 2 O 3 - 12TiO 2 , Cr 2 O 3 and ZrO 2 - 8Y 2 O 3 by Air Plasma Spray (APS) and WC - CoCr by High Velocity Oxygen Fuel (HVOF). Metallographic microscopy observation and SEM analysis were carried out and microhardness and fracture toughness were evaluated by means of the micro - indentation method. Friction behaviour and wear resistance were evaluated in dry sliding conditions with Pin On Disk apparatus for not coated Invar substrate and for the different coated substrates. Chromium oxide and tungsten carbide coatings exhibited higher mechanical characteristics respect to the other coatings: chromium oxide had the higher hardness value and tungsten carbide the higher fracture toughness. Tungsten carbide coating had the lower average coefficient of friction and together the chromium oxide the lower wear mass loss and wear rate. Among APS ceramic coatings, Cr 2 O 3 exhibited the best mechanical and tribological behavior while the HVOF cermet coating exhibited the best behavior among all the coatings.

The paper is devoted to thermal and thermo-mechanical analysis of the plasma sprayed thermal barrier coatings (TBC). The temperature and stress distribution using the finite element method in the microscopic scale is presented. The finite element model is based on the microscopy scan (dimension 5mmx3.5mm) and represent three levels of barrier structure: basement AMS 5599,undercoat NiCrAlY and ceramic layers of ZrO 2 ×8Y 2 O 3 , Gd 2 Zr 2 O 7 and DCL (double ceramic layer) Gd 2 Zr 2 O 7 /ZrO 2 ×8Y 2 O 3 type. The layers material properties are fully temperature dependent and coupled thermo-mechanical analysis is performed. The numerical results will be compared with the experimental one.

This study investigates the influence of plasma gas composition on deposition efficiency achieved during atmospheric plasma spraying and the properties of the resulting deposits. In the experiments, ternary mixtures of argon, hydrogen, and helium are used in different combinations and flow rates to spray Al 2 O 3 -TiO 2 and ZrO 2 -Y 2 O 3 powders on test substrates while measuring deposition efficiency. Several coating properties are measured, including porosity, hardness, and bond strength, and correlated with plasma gas ratios. Paper text in German.

It is very important to clarify the characteristic deformation behaviors of thermal barrier coatings. Monotonic and cyclic deformation behaviors of thermal barrier coatings under uniaxial compressive loading were examined. Specimens of plasma-sprayed ZrO 2 -8%Y 2 O 3 and CoNiCrAlY were fabricated to test the coating materials independent of the substrates. The specimen was fabricated by dissolving out the substrate only at the region of gauge length. Thicknesses of the coatings were 300 μm. The stress-strain response was measured using the laser speckle strain / displacement gauge (SSDG). ZrO 2 -8%Y 2 O 3 coating showed the nonlinear stress-strain response with a considerably lower elastic modulus which value was about 10% of sintering ceramics. The coating was found to leave permanent strain by compressive loading: the compliance of the coating decreased by compressive loading. The compliance was also found to decrease furthermore by undergoing cyclic compressive load. On the contrary, CoNiCrAlY coating deposited by low pressure plasma spraying was found to show the stress-strain response with insignificant nonlinearity. The compliance of the stress-strain curve didn’t also decrease with increasing the number of compressive stress cycles. It was found that coating with many defects and pores showed stress-strain response with large nonlinearity.

Three YSZ powders with different particle size distributions and an ethanol-based YSZ suspension were deposited on steel substrates using a cascaded plasma torch with a 9 mm nozzle. Coatings with dense, porous, segmented, and columnar microstructures were achieved by varying process parameters such as gun current, argon flow rate, spray distance, and suspension injection pressure. Linear relationships between in-flight particle state and process variables were observed and are shown to correlate well with coating structure and porosity.

With the modification of plasma spray parameters, porosity ratio of top coat can control along the cross-section. This improves the thermal cycle resistance and decrease the thermal conductivity. Plasma sprayed ZrO 2 /8 wt.–% Y 2 O 3 –NiCrAlY TBC systems which have different porosity (%8-12) and range of 250-350µm thicknesses of top coats, during thermal cycling tests with different hold times at 1350 °C have been performed. The main failure modes: delamination cracking, TGO growth rate and phase transformation are strongly dependent on the hold temperature and time. The correlation between TBC thermal cycle lifetimes and duration of high temperature hold time per cycle is shown and discussed with depending on thickness and porosity ratio.

An innovative plasma spraying technology has been developed with low power consumption and high deposition efficiency. The plasma torch was operated at arc currents from 50 to 150A and voltages from 50 to 80V. The powder particles were injected into the arc region from the inside of nozzle entry by the carrier gas that then transferring through the plasma flame. The ceramic coatings of Cr 2 O 3 and ZrO 2 - Y 2 O 3 were created at power consumption less than 10kW and powder feed rate of 30-50g/min. The deposition efficiency could reach 70%, higher than that of 40kW APS with an external powder feed. In addition, the enthalpy, temperature and velocity of low power plasma jet were also measured by an enthalpy probe.

This paper investigates the effect of laser post-treatment on cladded coatings deposited by different thermal spray methods. A wide range of coatings, including a VPS sprayed alloy, three APS sprayed oxide ceramics, and two composites, are treated with either a CO 2 or pulsed Nd:YAG high-power laser. The microstructure and wear resistance of the layers are examined before and after treatment and the interaction between the laser and material is modeled using Fusion-2D. Paper includes a German-language abstract.

NiCrAl/ZrO 2 -8Y 2 O 3 coatings deposited on SUS304 stainless steel and 45 carbon steel substrates were prepared by APS at different preheating temperatures, of which thickness exceeded 1mm. This study analyzed the coatings’ separation from different preheated substrates in the cooling process after spraying due to residual thermal stress. The Young’s modulus of the porous YSZ coatings was calculated and also measured by Knoop indentation methods for comparison purposes. The result indicated that the failure of porous thick YSZ coatings is mainly caused by the cracks nucleation, propagation and coalescence, which is related to the thermal-expansion coefficient difference between substrate and coatings, preheating temperature, porosity of coatings and so on. Due to their increased porosity, the porous and thick YSZ coatings had much lower calculated and measured Young’s modulus values than the sintered YSZ coatings.

Due to the superposed thermal and mechanical stress profile, thermo-mechanically coupled forming processes require tools and machine components which meet high demands. High forming forces and process temperatures in the contact zone between the tool and the workpiece limit the life span of these tools. A promising approach for protecting such tools is a combination of thermally sprayed coatings and physical vapor deposited layers. This coating system combines the characteristics of the individual layers and leads to superior mechanical, tribological as well as thermal properties under the mentioned coupled stresses. In this study thermally sprayed alumina (Al 2 O 3 ) and yttria-stabilized zirconia (ZrO 2 ) coatings were produced by atmospheric plasma spraying. Therefor different coating porosities were adjusted in order to varied the effect of thermal insulation for the substrate made of AISI H11 (1.2343). After the coating process the surface roughness of the thermal barrier coatings (TBC) were reduced by polishing process in preparation for the PVD top layer. Subsequently, wear and heat resistant hard TiAlSiN and CrAlSiN coatings were deposited on top of the polished TBCs by using magnetron sputtering process. As a reference the PVD coatings were also applied on a nitrided steel samples. Titanium and chromium interlayers were applied by PVD technique in different coating thicknesses (50 – 150 µm) between PVD and thermally sprayed coatings. Afterwards the influence of these metallic interlayers on coating adhesion of PVD coatings were analyzed by performing scratch tests. Hardness and young’s modulus of PV coatings were investigated by means of nanoindentation. The morphology and topography of the coatings were analyzed by scanning electron microscopy, light microscopy and optical three-dimensional surface analyzer. EDX analyses and X-ray diffraction were used to determine the chemical composition of the PVD coatings. Finally the wear resistant of the PVD top layers were determined at different temperatures (20°C, 500°) by using a high temperature tribometer.

Thermal barrier coatings are mainly used to protect underlying alloys from heat and chemical aggressions, especially in jet turbines and diesel engines. The challenge in TBCs’ applications is to use them to protect the upper moving turbine’s blades where their surface temperature can reach 1200°C. The limiting effect is the reliability of these coatings. The work described in this paper is a continuation of our earlier research work, which is focused on the use of Acoustic Emission Technique to assess the long-term behavior of thermal barrier coatings under thermal cycling conditions. Emphasis is placed in this presentation on the comparison of different signal processing techniques and the evaluation of their potential usefulness for the prediction of the coating behavior and failure modes. The work is carried out in parallel with a finite element modeling study of the thermal and stress distribution in the coating, which provides a valuable insight in the coating stress distribution prior to failure.

The concept, development background and the present applications of functionally gradient materials (FGMs) are introduced. Some spraying methods to fabricate FGM coatings, such as plasma spraying, flame spraying, high velocity oxy-fuel (HVOF) spraying and detonation flame spraying are reviewed. The research emphases and prospects of the technologies are put forward.

This work provides a new in-situ measurement method for the analysis of the spray-spot geometry and the thermal properties of the coating. The new approach is based on infrared detection of the thermal radiation from the coating surface combined with a subsequent automated spray-spot characterization. With this method it is possible to describe the geometry, the axis-position of the torch, the powder injection properties, and the temperature distribution in of the spray-spot. Especially for the automated production in high quantity the spray-spot analysis is a useful assistance for the operator because the detector reacts very sensitive on small changes of the process conditions. With regard on important fields of application (e.g., gas turbine production) the sensor is suitable to detect drifting spray system parameters. Also, the progression of wear at the nozzle, injector and electrode can easily be estimated. In recent research the in-situ spray spot analysis is being developed further for the characterization of multipair electrode plasma generators.

The current study has been focused on the final morphology of atmospheric plasma sprayed 8% yttria stabilized zirconia single splats. Single splats of two different sizes (-25 µm and +25/-45 µm) of ZrO 2 8Y 2 O 3 powder have been collected on polished stainless steel substrates at three different temperatures (Room, 300°C, and 600°C). The splat morphology and diameter, satellite particles, and splashing behavior were investigated using both scanning electron microscopy and image analysis software. The splat/substrate interface and splat curl up were studied from cross-sections prepared by focused ion beam milling. Results showed primarily pancake morphology and no evidence of delamination along the splat/substrate interface at 300oC substrate temperature and 100 mm spray distance. Overlapped splats showed evidence of melting (microwelding) at the splat boundaries. Splat thickness was measured to be less than 1 µm for all spray conditions. Roughness profiles of the surface of the deposited splats indicated microcracks had formed within the splats. Image analysis results exhibited a higher volume fraction of the splats relative to satellite particles at longer spray distance and higher substrate temperature. The average splat diameter increased as the substrate temperature increased.

In previous work, a thermal spray multilayer system consisting of ZrO 2 and an MCrAlY top coat showed promising results regarding oxidation behavior of the γ-TiAl substrates tested, which encouraged further research activities. Diffusion of substrate material was successfully inhibited by a ceramic ZrO 2 coating. A building up of a dense and stable oxide layer could be achieved by additional application of an MCrAlY top coat, leading to improved oxidation resistance and thus showing feasibility. In this work the main focus for development was put on enhancing adhesion and lowering residual stresses of the coatings in order to allow long term and cyclic testing without delamination taking place. Being a very brittle material, Gamma Titanium Aluminides require special surface treatment to enable roughening which is crucial for a strong mechanical bond between substrate and coating. Alternatives to conventional grit blasting as a standard preparation method were investigated. These were micro-abrasive blasting and blasting at elevated temperature (≈300-550 °C) to allow a more ductile behavior. The paper will highlight the implications by means of these measures and will also show the present development status of the multilayer system.