For the application as a thermal barrier coating (TBC) the partially stabilized zirconia approaches some limits of performance, which hardly can be overcome in the near future. To further enhance the efficiency of gas turbines, higher temperature and a longer lifetime of the coating are needed for the next generation of TBC´s. This paper presents the development of new materials and concepts for application as TBC. Materials such as compositions with pyrochlore structure or doped zirconias are compared with new concepts like nanolayer between top- and bondcoat, metal-glass composites and double layer structures. One concept is to use new compositions in a combination with zirconia, as a double, multi or graded layer coating. In this case the benefits of zirconia will be combined with the promising properties of the new top-coating. For the concept of metal glass composites the influences of different plasma spraying processes on the microstructure are described. The quality of these coating systems are evaluated by a burner rig test.

Somewhat unconventional plasma sprayed TBC systems were produced and evaluated by interrupted or cyclic furnace oxidation life testing. Approximately 250 µm thick 8YSZ coatings were directly sprayed onto grit blasted surfaces of PWA 1484, without a bond coat, in order to take advantage of the excellent oxidation resistance of this superalloy. For nominal sulfur contents of 1 ppmw, total coating separation took place at relatively short times (200 hr at 1100oC). Reductions in the sulfur content, by melt desulfurization commercially (0.3 ppmw) or by hydrogen annealing in the lab (0.01 ppmw), improved scale adhesion and extended life appreciably, by factors of 5-10. However, edge-initiated failure persisted, producing massive delamination as one sheet of coating. To subvert this mechanism, samples surfaces of melt desulfurized PWA 1484 were EDM’ed with a grid of grooves or ribs (~250 µm wide and high), resulting in a segmented TBC surface macrostructure. Now failure only occurred as independent single segments events. For grooved samples, 1100 C segment life was extended to ~1000 hr for 5 mm wide segments, with no failure observed out to 2000 hr for segments ≤ 2.5 mm wide. Ribbed samples were even more durable, and segments ≤ 6 mm remained intact for 2000 hr. Larger segments failed by buckling at times inversely related to the segment width and decreased by oxidative effects at higher temperatures. This critical buckling size was consistent with that predicted for elastic buckling of a TBC plate subject to thermal expansion mismatch stresses. Thus, low sulfur substrates demonstrate appreciable coating lives without a bond coat, while rib segmenting extends life considerably.

Ceramic oxide coatings were made with a unique microstructure using solution plasma spray (“SPS”), a novel variant on the conventional powder plasma spray techniques. In the SPS process, a precursor solution is fed into an air plasma torch using a liquid injector, and a nanocrystalline ceramic coating is formed directly on various substrates without post heat treatment. It was found that the microstructure of the SPS coating depended on control of process parameters for liquid feed and plasma spray to a large extent. This study deals with the formation of SPS deposited yttria stabilized zirconia coatings with a well-controlled microstructure addressing porosity, cracking and adhesion. The SPS-deposited YSZ coatings have demonstrated unique microstructural characteristics including adjustable porosity, vertical microcracks and the absence of splat boundaries. Such zirconia-base coatings show great potential for the applications of high-density electrolyte layers in solid oxide fuel cells (“SOFC”s) and high porosity/low conductivity thermal barrier coatings for industrial and gas turbines.

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.

Thermal barrier coatings (TBCs) deposited either by air plasma spray (APS) or electron beam physical vapor deposition (EB-PVD) provides thermal insulation to the superalloy blades and vanes. The durability and reliability of TBCs play an important role in the service reliability and durability of hot-section components in advanced turbine engines. Development of non-destructive evaluation (NDE) techniques for quality control, lifetime monitoring and lifetime prediction of TBCs have been the focus of research for designers, manufacturers and users of the advanced turbine engines. In this work, electrochemical impedance spectroscopy (EIS) has been used as a NDE technique to evaluate the varying microstructure and chemistry of as-coated TBCs. EIS were acquired at the corrosion potential for TBCs with varying thickness, microstructure and chemistry using 0.01 M [Fe(CN)6]–3/ [Fe(CN)6]-4·3H 2 O electrolyte solution. Equivalent circuits corresponding to the multiplayer constituents of TBCs were then correlated to the experimental EIS data. Resistance and capacitance of various components in TBCs can provide useful information regarding the microstructure and chemistry as-coated TBCs.

MCrAlY bond coats were deposited on nickel base substrate by electroplated process. The bond coats were plated using ‘CrAlY’ precursor powders suspended in an electrolytic bath containing nickel and cobalt in solution. The CrAlY powders used had size in the range generally below 10 um. The as-deposited coatings were heat-treated in a vacuum at elevated temperature. The roughness of the as-deposited coatings was on the range from 2 to 4 um (average). Yttria stabilized zirconia thermal barrier coatings, 7YSZ were deposited by air plasma spray. The thickness of the both bond coats and TBCs was varied in order to determine the effects of thickness in the stability of the thermal barrier coatings. The coated samples were tested in a static furnace and also in a thermal shock test rig where the samples could be cooled rapidly from 1000°C to 100°C at a predetermined rate. The TGO formed at temperatures in the range from 800 to 1050°C was characterized by optical, scanning electron microscope and electron beam microprobe.

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 morphologies of Y-PSZ coatings remelted after (i.e., post-treated) or simultaneously (i.e., in situ) with their deposition were observed by optical and scanning electron microscopy in order to study the behavior of the coatings during laser irradiation. A change in the microstructure, from lamellar to dendritic, was observed in both cases. Moreover, cracks and delaminations are less emphasized for the coatings treated during deposition than for those treated after deposition. Finally, the pore connectivity was evaluated implementing an electrochemical test. Results clearly indicate that the coatings obtained by in situ laser remelting are significantly more impervious than as-sprayed coatings.

Yttria Partially Stabilized Zirconia (YPSZ) coatings are used as Thermal Barrier Coatings (TBCs) because of their capability to improve gas turbine performance by allowing higher turbine inlet temperature and reduced cooling air flow. Usually, YPSZ layers are applied by plasma spray on Ni-based superalloys pre-coated with a bond coat layer of MCrAlY, where M is Co, Ni, or both. In this work we used Raman spectroscopy to study the performance of TBCs during thermal cycling at two different temperature (1273K and 1373K). Raman scattering is capable of detecting monoclinic zirconia and to analyze residual stresses of the coatings with an higher sensitivity than traditional X-ray diffraction measurements. Raman spectra were analyzed by deconvolution methods in order to study the evolution of the relative intensities and position of different bands. We found that the position of the 635 cm-1 band shifts to higher values of wave number during thermal cycling and by comparing this data with those reported in literature, it is possible to retrieve the value of residual stresses in zirconia coating. We also observed a relation between the intensities of the bands at 602 cm-1 and 635 cm-1 band and the number of thermal cycles performed.

Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO 2 /Y 2 O 3 ). In this work the oxidation behavior of conventional and nanostructured HVOF NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Free-standing bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at a 1000°C for different time periods in order to form the thermally grown oxide (TGO) layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared to that of the conventional one. This behavior is a result of the formation of a continuous Al 2 O 3 layer on the top surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating.

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.

Functionally graded (FG) coatings were manufactured by High Velocity Oxy-Fuel (HVOF) thermal spraying and tested aiming for the high temperature applications. Single layers were manufactured and their elastic modulus measured by using Impulse Excitation Technique (IET). Obtained data was used for modeling of optimal gradient structure. Dual feeding hose for HVOF gun was developed. Calibration procedure for the concurrent use of two powder feeders was performed. NiCr-Al 2 O 3 coatings with coating thickness of 600 µm and 1000 µm were manufactured and tested. Promising results were obtained from high temperature corrosion tests.

For more than two decades researchers have been working on thermal barrier coatings to improve the performance of diesel engines. However, these coatings have still not achieved widespread application in conventional diesel engines. The original motivation for this work was the improvement of fuel economy, since even a few percent improvement would result in huge savings in the transportation industries, but the coatings also effect exhaust emissions, component wear, and the sensitivity of engines to fuel quality. Wear at high temperatures, where conventional lubricants are not effective, is a serious problem in low heat rejection engines. Ceramic materials such as thermal barrier coatings in cylinder liners must have an acceptable wear rate and coefficient of friction. In this work we compare the wear behaviour of nanostructured thermal spray zirconia coatings with conventional zirconia coatings. First, process parameters that allowed the nanoparticles present in the feedstock powder to be retained in the coating were found. Then pin on disc wear tests of the two types of coatings were carried out at room temperature. The coating containing retained nanoparticles exhibited a lower coefficient of friction and less wear loss under discontinuous testing than the conventional coating.

CaZrO 3 is a material for thermally sprayed ceramic coatings to which so far only a little attention was paid. This material has a high melting point, good thermal stability and a coefficient of thermal expansion close to that of steel. In this paper water stabilized plasma spraying (WSPR) and atmospheric plasma spraying (APS) were used to prepare CaZrO 3 coatings. The spraying feedstock was prepared from fine CaZrO 3 powder by agglomeration (spray drying) and sintering. Powders with three different particle sizes (- 45 + 20 µm, - 63 + 45 µm and -90+63 µm) were used in the experiments. The coarse fractions were used for WSP spraying, while the fine one was sprayed with the APS process. Plasma sprayed materials were studied from the point of view of phase changes and influence of the powder size on structure of coatings. The changes of phase composition were studied by X-ray diffraction on coatings as well as on free flight particles. Formation of a cubic phase with a reduced content of CaO in comparison to CaZrO 3 was observed. Its formation is probably connected with evaporation of CaO during spraying. This cubic phase is similar to the phase obtained by spraying of ZrO 2 +5%CaO. Plasma sprayed coatings were characterized by light and scanning electron microscopy (SEM) and by density and porosity. Coefficients of thermal expansion of plasma sprayed layers from CaZrO 3 were measured.

Superalloy substrates coated with plasma sprayed CoNiCrAlY bond coats and yttria-stabilized zirconia top coats have been subjected to a high heat flux in a controlled atmosphere chamber. The sintering exhibited by the top coat under these conditions has been studied and compared with the behavior observed during isothermal heating, both when attached to the substrate and when detached. Sintering has been characterized by (a) microstructural examinations, (b) dilatometry, in both in-plane and through-thickness directions, and (c) stiffness measurements, using both cantilever bending and nanoindentation. A numerical heat flow model has been used to explore the stress state under isothermal and thermal gradient conditions. Sintering proceeds faster at higher temperature, but is retarded by the presence of tensile stresses (from differential thermal expansion between coating and substrate) within the top coat. Sintering occurs preferentially near the free surface of the top coat under gradient conditions, not only because of the higher temperature, but also because the in-plane stress is more compressive in that region.

The effects from thermal shock loading on pre-existing microcracks within thermal barrier coatings (TBCs) have been investigated through a finite element based fracture mechanical analysis. The TBC system consists of a metallic bond coat and a ceramic top coat. The rough interface between the top and bond coats holds an alumina oxide layer. Stress concentrations at the interface due to the interface roughness as well as the effect of residual stresses were accounted for. At eventual closure between the crack surfaces, Coulomb friction was assumed. To judge the risk of fracture from edge cracks and centrally placed cracks, the stress intensity factors were continuously monitored during simulation of thermal shock loading of the TBC. It was found that fracture from edge cracks is more likely than from centrally placed cracks. It was also concluded that propagation of an edge crack is initiated already during the first load cycle whereas the crack tip position of a central crack determines whether or not propagation will occur.

Quantitative characterization of microstructures of plasma sprayed coatings, that accounts for their anisotropic and “irregular” character, is developed. An important finding is that “islands” of partial contacts along microcrack faces –even if they are very small - produce a strong effect on both elasticity and conductivity, thus reducing the “effective” microcrack density. Theoretical results are twofold: (1) conductive/elastic properties of the coatings in terms of the microstructure, and (2) conductive–elastic cross-property connections, that interrelate the anisotropic overall conductive and elastic constants. They can be utilized for “mapping” possible combinations of the two properties and, thus, for optimization of the microstructure for the combined conductive/elastic performance. Testing of the model against experimental data on YSZ coatings produces a good agreement.

Recently, thermal barrier coatings (TBCs) have been used in advanced gas turbine plants for improved performance. Usually, TBCs consist of an inner layer of metallic bond coating (MCrAlY) and an outer layer of ceramic top coating. According to several studies, the failure of the TBC is induced by thermal stresses due to the formation of thermally grown oxide (TGO) at the interface between the TBC and MCrAlY. Therefore, it is important to investigate the high temperature oxidation behavior of the interface. In this work, the TGO is characterized in detail. In particular, in order to clarify the role of the TBC top coating in regard to initiation and growth of TGO at the interface, a specimen with TBC and one without TBC were compared. In both specimens, the TGO had two different contrasting layers. One was alumina, and the other was a combination of chromium oxide, nickel oxide, cobalt oxide, and spinels (hereafter call mixed oxide). The TGO thickness of the specimen with TBC was thicker than that obtained without TBC. These specimens had different oxidation behaviors. It is thought that the reason for the difference in TGO thicknesses of both specimens is due to a difference in oxygen potential, as the oxide compositions in the mixed oxides were different. In case of the specimen with TBC, the mixed oxide consists of chromium oxide, nickel oxide, and cobalt oxide, separately. On the other hand, in case of the specimen without TBC, the mixed oxide consists mainly of spinels such as (Ni, Co)(Cr, Al) 2 O 4 .

It is generally known that the porosity of thermal barrier coatings is essential to guarantee a sufficiently high strain tolerance of the coating during thermal cycling. However, much less is known about the influence of the specific kind of porosity such as micro cracks and typically larger pores on the performance of the coatings. Both features are usually formed during plasma spraying of yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). In this investigation the influence of micro cracks on the thermal cycling behavior was studied. The amount of micro cracks within YSZ thermal barrier coatings was changed by changing the powder feed rate. However, only small changes of the total porosity were observed by changing powder feed rate. Mercury porosimetry served as a tool to investigate both the amount of micro cracks and pores in the coating. Additionally, micro crack densities were determined from metallographical investigations. A linear dependence between the amount of fine pores determined by Hg porosimetry and the crack density was obtained for one set of coatings. Thermal cycling specimens with thermal barrier coatings having different micro crack densities were produced and tested in a gas burner test facility. At high surface temperatures above 1300°C failure occurred in the ceramic close to the surface. Under these conditions the samples with increased horizontal micro crack densities showed a significant increase of thermal cycling life.

Two 7-8 wt% yttria-stabilized zirconia powders of similar size and chemistry but having different microstructure properties and manufacturing routes were studied. One significant difference was the density and internal porosity of the starting powders. Deposition efficiency (DE) of the low density (LD) powder was found to be higher and less sensitive to changes in the spray process parameters than the high density (HD) powder. Probing the in-flight particle characteristics with the DPV- 2000 made it possible to link the observed DE values with the in-flight particle temperature. For each powder, DE was found to depend mainly on a single variable, the in-flight particle temperature. DE was found to vary strongly with particle temperature for temperatures under 2700°C, whereas the dependence with particle temperature was much less important above 2700°C. Variations in DE seemed to evolve according to variations of the melted fraction of the sprayed material. Since the LD powder was found to achieve higher particle temperatures at given spray conditions, DE was found to be higher for the LD material and the range of variations in DE was found to be much less than that observed with the HD material. Examination of the coating microstructures revealed that a coating produced with the LD powder had slightly higher porosity than that produced with the HD powder at similar inflight parameters. Spraying at higher in-flight particle temperature or velocity, which resulted in higher and more robust DE values, tended to yield coatings with lower porosity, resulting in coating density exceeding the tolerance range specified by some end-users. Increasing the powder feed rate and using conditions that produced a higher in-flight particle temperature were found to increase the porosity up to an acceptable level without significantly degrading DE. Therefore, a solution was found that not only reduced the sensitivity of DE to changes in the spraying conditions but also increased the rate of production by reducing the time required to spray the part.