Robert Vaßen

Development of a micromechanical life prediction model for plasma sprayed thermal barrier coatings

Abstract A widely used method to produce thermal barrier coating (TBC) systems is the vacuum plasma spraying of a highly dense bondcoat layer with a defined surface roughness and the atmospheric plasma spraying (APS) of a porous (10–15%) Y 2 O 3 -stabilized zirconia top coat. In thermal cycling operation these systems often fail by crack initiation and propagation close to the bondcoat–top coat interface. This failure is attributed to stresses arising from the formation of a thermally grown oxide (TGO) layer on the rough bondcoat surface. The actual stress situation is rather complex due to TGO formation, creep effects in both bondcoat and top coat and due to the roughness of the bondcoat. All these factors have been take into account in the present work by using a finite element method (FEM) to calculate stress development during thermal loading. These results can then be introduced into a crack propagation model to estimate crack development during the thermal cycling operation. The predictions of this approach are compared to experimental results on the influence of bondcoat roughness on coating life. In these experiments TBC systems with bondcoat layers having three different levels of roughness were cycled in a gas burner rig until failure.

Stress distributions in plasma-sprayed thermal barrier coatings as a function of interface roughness and oxide scale thickness

Abstract During thermal cyclic loading, plasma-sprayed thermal barrier coatings (TBCs) often show failure within the top coat close to the interface. In all cases this results from crack propagation of pre-existing cracks near the bond coat (BC)–top coat interface. Stresses developing on a microscopic scale near the BC–TBC interface of plasma-sprayed thermal barrier coatings govern crack growth in an initial phase of the failure process. Using a finite element (FE) method the local dependence of stresses in the vicinity of this rough interface was investigated. Measurements of real roughness profiles provided geometrical parameters needed for the calculations. A significant difference in the stress distributions was found for peak and valley locations of the BC roughness profile. The effect of BC oxidation on stress development was more pronounced in the case of less roughness. Analytical fits of the FE results revealed how the parameters of roughness and the oxide thickness correlate with the stress levels. In the next stage of research these fits will serve as input data for a microstructural based lifetime model.

Influence of impurity content and porosity of plasma-sprayed yttria-stabilized zirconia layers on the sintering behaviour

Abstract Yttria-stabilized zirconia (YSZ) powders from different manufacturers have been used to prepare atmospheric plasma-sprayed (APS) ceramic coatings with different porosity levels. While the particle morphology of the different powders was similar, the amount of impurities, especially silica, was different, varying between 100 and 1500 ppm. APS coatings were removed from the substrates and the porosity distribution was measured by mercury porosimetry. Typically porosity levels between 10 and 15% have been used. Free-standing coatings were investigated in the dilatometer during long-term (>50 h) annealing at 1200°C. Additionally, the coefficient of thermal expansion was determined from expansion during heating. It turned out that the higher porosity levels, as well as the high impurity levels, led to a significant increase in the sintering rate of the coating during high-temperature annealing. A linear relationship between the total shrinkage after 60 h and the porosity, as well as the silica content, was used to describe the influence of the investigated parameters on the sintering behaviour in a more quantitative way.

Sintering and creep processes in plasma-sprayed thermal barrier coatings

During operation at elevated temperatures, sintering processes can significantly influence the mechanical properties of thermal barrier coatings (TBCs) by increasing Young’s modulus and reducing strain tolerance. These changes of the mechanical response of TBCs were investigated using free-standing plasma-sprayed TBCs in a thermomechanical analysis (TMA) facility. The time-dependent change of Young’s modulus was determined in situ in a flexure mode at different annealing temperatures. In addition, relaxation processes during loading and unloading were monitored. The time-dependent deformation behavior of the TBC sample can be described by a simple viscoelastic approach (Burgers model). Viscosity data are determined as a function of annealing temperature and time.

A life time model for ceramic thermal barrier coatings

The life time of Y2O3 stabilized ZrO2 thermal barrier coatings (TBC) has been modelled using a rather simple fracture mechanical approach. The basis of the model, is a finite element analysis of the thermal stresses and approximate assumptions of crack growth along the bond coat (BC)-ceramics interface. The FE calculations show the influence of several microstructural features of the TBC system, as profile of the BC TBC interface, thickness of thermally grown oxide formed during thermal cycling and others, on the stress state. From these results, a specific way of crack growth is predicted and included into the model. The modelling results are compared to life times obtained from thermal cycling experiments. An analysis of the location of failure within the samples, as well as the influence of a variation of the roughness of the BC–ceramics interface on life time are presented. Both are in reasonable agreement with the modelling results. Finally, the shorter life times, which are predicted for samples exposed to an additional compressive mechanical strain, are discussed.

Processing and properties of nanophase non-oxide ceramics

Abstract The present paper describes recent activities in the development of non-oxide nanophase ceramics. Examples on the processing of TiN, TiC, Si3N4, SiC and others will be given. Only a few of the described activities ended up with both high densities (i.e. >95% of the theoretical density (TD)) and grain sizes below 100 nm. A possible reason for the behavior of non-oxide ceramics is the contamination of the surface by oxygen. For SiC, it is shown that the oxide layer on the surface hinders densification. It favors non-densifying sintering mechanisms and hence coarsening of the microstructure. A processing route was developed which effectively reduces the oxide layer. Subsequent Hot Isostatic Pressing (HIP) of these SiC samples led to densities above 97%TD and grain sizes below 100 nm. Finally, a short overview of properties (hardness, fracture toughness, thermal diffusivity, behavior under irradiation) of nanophase non-oxide ceramics will be given. Due to the fact that the manufacture of ceramics with grain size below 100 nm turns out to be difficult also results of samples with slightly larger grain sizes are included.

Densification of ultrafine SiC powders

Recent results on the densification behaviour of ultrafine SiC powders (below 20 nm) are presented and compared with results on the densification of ultrafine silicon-based ceramic powders given in the literature. A study of different powder processing routes and their influence on the pore-size distribution is given. Pressureless sintered green bodies having pore sizes of about 20 nm show extreme coarsening without significant densification. The results indicate a significant influence of green density on shrinkage. Encapsulated hot isostatic pressing (HIPing) led to a reduction of pore size and to considerable density increase at temperatures below 1600 °C. But even then full density without extensive grain growth was difficult to achieve. The applied method to determine grain sizes (X-ray diffraction measurements, XRD, using the Scherrer formula, scanning electron microscopy, SEM, and transmission electron microscopy, TEM) gave similar results for TEM and SEM but lower values for XRD. A possible explanation is presented. Density and grain growth both during pressureless sintering and HIPing showed significant differences between samples with and without sintering additives (B and C). Whether or not the use of sintering agents is favourable in reaching high densities and fine grain sizes, is discussed. HIP densification was modelled assuming diffusion to be the dominant mechanism. Grain growth according to a t1/4 dependence and an activation energy of 6.8 eV was introduced into the model. Results on the properties (hardness, also at elevated temperatures, fracture toughness, bending and compression tests, thermal conductivity) of the hot isostatically pressed samples, are presented.

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.

Correlation between spraying conditions and microcrack density and their influence on thermal cycling life of thermal barrier coatings

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 morphology of porosity, such as microcracks 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 microcracks on the thermal cycling behavior was studied. The amount of microcracks within YSZ thermal barrier coatings was changed by changing the powder-feeding rate. Only small changes of the total porosity were observed. Mercury porosimetry served as a tool to investigate both the amount of microcracks and pores in the coating. Additionally, microcrack 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 TBC specimens with different microcrack 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 microcrack densities showed a significant increase of thermal cycling life.

Processing and properties of nanophase ceramics

Abstract An ideal processing of nanophase powders should lead to a pure nanophase ceramic, i.e. a material, in which the major phase (or at least one constituent) has a grain size in the nanometre range. However, nanophase powders with their promising properties such as high specific surface area or high sintering activity often require the development of new and elaborate processing techniques to achieve these aims. Contamination levels can be reduced by using processing under UHV or inert gas conditions. In SiC nanoceramics low oxygen contents ( Conventional sintering of most nanophase green bodies is accompanied by fast grain growth when the density exceeds 90% of the theoretical density (TD). Pressure-assisted densification or a surface modification of the powders are possible methods to optimise the porosity distribution of the green bodies and reduce grain growth. Examples will be given. The improvement of mechanical properties has been demonstrated for different nanophase ceramics. Increasing hardness with decreasing grain size is observed in non-oxide ceramics and in cermets (WC/Co). Improved thermal shock resistance was found in nanophase SiC, although the thermal conductivity was reduced. High possible deformations even under tensile loadings are possible in nanophase ZrO 2 . This superplastic behaviour offers interesting applications with respect to the near-net-shape forming of ceramic parts at moderate temperatures.