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Characterization of thermal barrier coatings with a gradient in porosity
Surface & Coatings Technology 195 (2005) 245 – 251 www.elsevier.com/locate/surfcoat
Characterization of thermal barrier coatings with a gradient in porosity A. Portinhaa, V. Teixeiraa,*, J. Carneiroa, J. Martinsb, M.F. Costac, R. Vassend, D. Stoeverd a
GRF-Functional Coatings Group, Physics Department, University of Minho, Campus de Azure´m, 4800 Guimara˜es, Portugal b Mechanical Engineering Department, University of Minho, Campus de Azure´m, 4800 Guimara˜es, Portugal c Physics Department, University of Minho, Campus de Gualtar, 4700 Braga, Portugal d Institute for Materials and Processes in Energy Systems 1, Forschungszentrum Julich GmbH, D-52425 Julich, Germany Received 11 February 2004; accepted in revised form 8 July 2004 Available online 11 September 2004
Abstract A major problem in thermal barrier coatings (TBC) applied to gas turbine components is the spallation of ceramic coating under thermal cycling processes. In order to prevent spallation and improve the thermomechanical behaviour of the TBC, graded ceramic coatings can be produced. For this purpose we are developing a new concept of Thermal Barrier Coating (TBC) that consist of a conventional NiCoCrAlY bond coat and an atmospheric plasma sprayed ZrO2–8 wt.%Y2O3 top coat graded in porosity on an Inconel 738 LC substrates. The aim of this work is to produce coatings with low thermal conductivity and better thermomechanical behaviour due to the gradient in porosity which reflects a gradient in the elastic properties. Absolute porosity was measured with a mercury porosimetry and by image analysis. The second technique was also used to estimate the porosity variation along the cross-section. Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) were used to observe the morphology and coating microstructure. The microhardness was measured with a Vickers indenter and 0.981 N load. The microhardness has been evaluated for coatings in as-sprayed condition and after annealing at 1100 8C during 100 h. The results show a fast increase of the hardness after annealing. After thermal shock heating at 1000 8C, 1 h and quickly cooling in water no spallation was observed for 100 cycles. D 2004 Published by Elsevier B.V. Keywords: Stabilised zirconia coatings; Thermal barrier coatings; Graded in porosity; Microhardness
1. Introduction Ceramic coatings of engineering materials such as zirconia partially or totally stabilized are used for a variety of technological applications requiring thermal insulation, wear and erosion resistance or protection from oxidation, sulfidation and hot corrosion. These kinds of coatings have been applied as Thermal Barrier Coatings (TBCs) for protection of metallic components in gas turbines (vanes, blades, shrouds, etc.) and diesel engines, and improve performance at high temperatures [1–3]. The TBC concept allows increase in operating temperature and/or reducing the cooling systems due to the temperature gradient across the thick ceramic coating, that permit better thermodynamic * Corresponding author. Tel.: +351 253510465/400; fax: +351 253510401. E-mail address: [email protected] (V. Teixeira). 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.07.094
performance, lower emissions without requiring major alloy development. Zirconia coatings are very interesting materials because of their outstanding mechanical, thermal, optical and electrical properties. Zirconia has a high melting point, high resistance to oxidation, low thermal conductivity, high hardness, and high coefficient of thermal expansion. These ceramic coatings are widely use in many technological applications such in components at high temperature and adverse corrosive environments, oxygen sensors [4], optical coatings [5], etc. TBCs, traditionally, consist in a thick partially stabilized ZrO2 top coating commonly deposited by atmospheric plasma spraying (APS) on superalloys precoated with a metallic bond coat (NiCoCrAlY) produced by vacuum plasma spraying (VPS) [6]. The partially stabilized ZrO2 top coat has a porous and laminar structure and consists of splats with cracks perpendicular to the surface, this porous structure allows the increase in the thermal isolation and the
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cracks permit better stresses accommodation [1,7–9]. The metallic bond coat reduces the diffusion of contaminants and the mismatches of the thermal expansion between top coat and the substrate and this leads to an increase of the lifetime of operation for the components. Zirconia (ZrO2) crystallizes in three distinct polymorphs according to the temperature and pressure conditions. The main phases of ZrO2 are the monoclinic phase (m) stable at room temperature, the tetragonal phase (t) stable between 1170 and 2370 8C and cubic phase (c) stable from 2370 to 2680 8C [10–14]. When the applications for these coatings involve operation in range of temperatures that involve phases transformations, it is necessary to stabilize the high temperature phases at room temperature because the t Z m phase transformation is accompanied by a 3–5% volume expansion, and this volume expansion can cause high residual stresses and microcracks leading to delamination and spallation of coatings. To prevent this transformation, it is necessary to stabilize the high temperature phases at room temperature. For this purpose, these phases can be stabilized by doping ZrO2 with other oxides such as Y2O3, CeO2, MgO, CaO, Gd2O3 or Al2O3 [10–17]. The tetragonal phase of zirconia also can be stabilized at room temperature by decreasing the grain size for a few nanometers (about 6 nm) and it can be obtained producing nanolayered coatings of ZrO2/Al2O3 and ZrO2/TiO2 [13,16]. These coatings with nanometer grain size and nanolayered structures also lead the decrease in thermal conductivity. Nowadays, higher operation temperatures are required and in order to obtain systems of coatings that allow its range of temperatures we need to develop new materials for coatings or new architectures for the existing materials. These new concepts of TBCs should have lower thermal conductivity, and be more stable at higher temperatures than the 7–8 wt.% yttria-stabilized zirconia (YSZ) [6,18]. In addition, these new materials should have other properties comparable to the YSZ like thermal expansion coefficient, corrosion resistance. It is known that increasing the porosity, it will reduce the thermal conductivity; however, above certain values, this increase can degrade the mechanical integrity due to the decrease in cohesion between lamellas. Producing coatings with porosity variable, increasing to the surface we probably can have coatings with higher porosity content with the same or better thermomechanical behavior because the adherence and residual stresses are maintained at the interface but increasing the porosity towards the surface a reduction both in elastic modulus is expected and the level of residual stresses. These properties can be also controlled with better rigor if we will control the pore geometry [19]. New materials, stable at high temperatures and with lower thermal conductivity are under development in order to allow the use of high inlet temperatures [6,17,20]. In addition, multilayer systems with different functions are in study: layers for chemical insulation, with an intermediate
Fig. 1. Model of TBC graded in porosity along cross section. The graded coating was divided in four layers. The porosity was increasing from layer 1 to 4.
conventional zirconia partially stabilized layer and a new material top layer or a graded structures changing the chemical composition from interface with bond coat to the surface using 100% YSZ at interface and then reducing it contents in substitution with the new materials like lanthanides [6]. New dopants have also presented good results, applied alone or joint with the yttria (Y2O3) that have substantial reductions in thermal conductivity reaching 40% for coatings codoped with Y2O3 and Gd2O3 [17,20,21]. In order to obtain better thermal insulation, in this contribution we present a new concept of TBC. It consists in a conventional bond coat and a graded ZrO2–8 wt.%Y2O3 top coat that is graded in the porosity (see Fig. 1). In this paper, we report on the study of structural properties of ZrO2Y2O3 multilayered coatings focusing on the porosity of the microlayers. In order to increase the efficiency of the thermal barrier, different layers with different porosities increasing towards the surface were studied. The SEM images are processed using dedicated routines, in order to measure the porosity of the coatings [22,23]. Not only the porosity values for each layer were obtained but also it was evaluated the way the porosity changes along the coatings cross-section. To obtain the total porosity, we also determine the porosity by mercury intrusion [23]. The microhardness was measured with a Vickers indenter in as-sprayed condition and after annealing. The values of microhardness were thus related with the deposition conditions, heat treatment and porosity variation [24,25].
2. Experimental setup Thermal barrier coatings (TBCs) investigated were produced by plasma spraying. A Sulzer Metco AG vacuum plasma spraying (VPS) system was used to deposit a NiCoCrAlY bond coat (Ni 192-8 powder by Praxair Surface Technologies, Indianapolis, IN) on square plates of a nickel superalloy Inconel 738 LC previously sand-blasted with alumina particles which are 0.71-Am average size and ultrasonically cleaned in acetone. The substrate dimensions were 40403 mm. The ceramic powder used for top coats was ZrO2–8 wt.%Y2O3 (Metco 204 NS Sulzer Metco GmbH, Germany) and was sprayed by atmospheric plasma spraying (APS)
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Table 1 Deposition parameters Deposition parameters
APS
All samples
HP
GPI
GPII
GPIII
VPS
Power (kW) Int. of current (A) Gas plasma (slpm) Gas transport (Ar) (slpm) Diameter of nozzle (mm) Flux of powder (g/min) Distance of gun to substrate (mm) Pressure in the chamber (mbar) Substrate temperature (K) Thickness of the total layer (Am) Gun speed (mm/s)
14.5 240 20.1/13.1 (Ar/He) 1.5 10 8.5 90 atm 423 300 500
20.5 to 14.5 300 to 240 20.1/13.1 (Ar/He) 1.5 10 8.5 90 atm 473–423 265 500
20 to 15 300 to 240 20.1/13.1 (Ar/He) 1.5 10 8.5 120 atm 453 305 500
20.5 to 14.5 300 to 240 20.1/13.1 (Ar/He) 1.5 10 8.5 90 to 120 atm 423 200 500
49.4 733 50.6/9.1 ( Ar/H2 ) 1.7 7 40 275 60 1073–1093 140 440
using a Sulzer Metco Triplex gun. Deposition conditions are presented in Table 1. For characterization of the as-sprayed condition of the top coat, steel substrates were coated at the same time. The samples were annealed at 1100 8C for 100 h in air, and were made a thermal shock heating at 1000 8C during 1 h and after cooling in water for 100 cycles. The microhardness was measured with a Vickers indenter with 0.981 N load and was measured along the whole cross-section. Total porosity levels were evaluated in as-sprayed condition by mercury intrusion after removal of the steel substrate from the coating with hydrochloric acid. The morphology of the coatings was analyzed by optical and scanning electron microscope (SEM). Micrographs with two magnifications (400 and 500) from polished cross-
sections were used for image analysis for the determination of the total porosity and the porosity profile through the cross-section.
3. Results and discussion 3.1. Microstructural characterization In Fig. 2 is presented SEM cross-sectional micrographs for thermal barrier coatings showing the microstructure before and after annealing. The thickness was determined by SEM analysis (see Table 1) of all coatings, and combining this analysis with deposition parameters was estimated the thickness of each microlayer for the different porosities along the cross-section. The coatings present a porous and
Fig. 2. SEM micrographs showing microstructure and porosity on ZrO2–8 wt.%Y2O3 coatings produced by APS. (a) Typical lamellar microstructure with columnar structure in each lamella, (b) cross-section after annealing in which the Thermal Grown Oxide (TGO) is clearly visible, (c) cross-section of as-sprayed top coat where the increase in porosity towards the surface is visible.
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lamellar structure (see Fig. 2a) which is characteristic for this kind of coatings [26]. In plasma spraying, the microstructure of the coatings is strongly dependent on processing conditions. The splats are separated by interlamellar pores resulting from rapid solidification of the lamellae, very fine voids formed by incomplete intersplat contact or around unmelted particles, and cracks due to thermal stresses and tensile quenching relaxation stresses. The presence of cracks also increases the strain tolerance and enhances the thermal shock resistance for TBCs in service. From the SEM micrographs it is possible to observe the variation of the porosity for different coatings and also its variation from the interface with bond coat to the surface as is demonstrated in Fig. 2c, the quantification of this variation along crosssection has been measured by image analysis. Additionally, small microcracks with diameters of about 200 nm are observed. These cracks are originated from the thermal stresses which arise from the rapid cooling during the spray process (quenching stresses). After annealing in air, all coatings present a sintered structure and consequently a reduction in the porosity levels which is in agreement with other studies [27]; these sintering effects will promote the increase in thermal conductivity, in elastic modulus and a loss of the strain tolerant behavior. A thermally grown oxide (TGO) observed between bond coat and top coat for the annealed samples is shown in (Fig. 2b) which have 5 Am after 100 h at1100 8C in air. Energy dispersion X-ray spectrometry (EDX) analysis reveals that the TGO is predominantly aluminum oxide. The aluminum growth oxide at the interface between bond coat and top coat results from the diffusion of the aluminum present in the bond coat to it surface, this oxide protects the alloy from oxidation. However, when it reaches certain thickness and due the interface irregularities develops high residual stresses that promote microcracking which are responsible for the spallation of top coat and system failures. After thermal shock in all samples, no spallation was observed and only it has seen some densification at the surface and the appearance of small cracks perpendicular to the coating plane that improve the strain accommodation. This behavior is important and means that our coatings show an excellent thermal shock resistance. In our opinion, the gradation in porosity improves this resistance because the coatings have better accommodation of thermal stresses during the quenching period. 3.2. Porosity measurements Porosity of TBCs can be characterized qualitatively by microstructure observation and quantitatively by mercury intrusion porosimetry (MIP) technique besides coating density measurement. The direct examination of coatings microstructure from cross-section of coatings using a (SEM) gives comparative information about porosity for the different coatings. In conjunction with a backscattered electron (BSE) detector, the chemical composition of the
microstructure is represented in the images by gray level variation. Pores appear very dark, which permit them to be distinguished and quantified by image analysis. By this method, we can’t obtain information about the 3-D pore network or connectivity between them [22,23]. For the analyzed coatings, two series of images were acquired, one with 400 magnification and the other with 500 magnification. Using the MIP, it is possible to obtain measurements of total porosity for open pores and the evaluation of pore size distribution. MIP does provide information about the connectivity of the pores and microscopy reveals information about pore geometry, so there is interest in combining these two techniques for a more complete analysis. Mercury intrusion porosimetry is based on the premise that a nonwetting liquid (one having a contact angle greater than 908) will only intrude capillaries under pressure. Mercury must be forced using pressure into the pores of a material. The pore size distribution is determined from the volume intruded at each pressure increment. Total porosity is determined from the total volume intruded. The digital micrographs were evaluated on a Matrox II program for image analysis. The pores were identified by thresholding the brightness of the pores to produce a binary image, after the dark area fraction in the binary image was evaluated and the percentage determined. The corresponding porosity values for the different coatings are presented in Table 2. We can see in Table 2 a considerable difference between the measured Hg porosities and the porosities evaluated by image analysis. In addition, a reduction was found in porosity values after annealing for all samples, its reduction is mainly due to the sintering effects. While the Hg porosimetry gave reliable results for small pores and microcracks, it failed for pores with radius larger than 80 Am. For large pores the mercury fills it without any external applied pressure (because of the weight of the mercury and the size of the pores) and was therefore not measured. Contrarily, the image analysis is a technique to analyse porosity in which the contribution of small pores and small microcracks between and through the lamellas within the plasma-sprayed coatings is difficult to measure in contrast to the large pores. This microcracked microstructure leads to relatively low thermal conductivity values in APS TBCs and Table 2 Coating porosity measured by image analysis and Hg porosimetry Samples
HP
GPI
GPII
GPIII
Hg porosity (%) Image analysis (%)a Image analysis (%)b Image analysis after annealing (%)a Image analysis after annealing (%)b
14.75 11.79 11.15 8.44 6.94
15.31 13.08 10.76 8.33 7.99
15.29 15.48 12.73 9.90 7.40
13.38 13.34 9.34 10.57 8.27
a b
Porosity with small cracks and ribbons. Porosity without ribbons.
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Fig. 3. Porosity distribution along the cross section determined by image analysis: (b, d, f) porosity with small cracks and ribbons; (a, c, e) porosity without ribbons.
enhances the strain tolerance in service. In agreement with SEM analysis and deposition parameters the thickness of each microlayer was estimated in the graded coatings. Using image analysis the porosity values of each layer was determined. In Fig. 3, the porosity variation along the cross-section for these three kinds of coatings is presented. The porosity increases from the bond coat interface towards the surface of the top coat. In as-sprayed condition the absolute porosity variation ranges from 3% to 5% from interface to top coat surface and in annealed condition we observe a significant reduction in its variation (from 1% to 2%). This reduction is due the sintering effects at high temperature [27,28]. The pore size distribution determined by Hg porosimetry can be seen in the Fig. 4 for all coatings. In this graph is represented the cumulative porosity function of the pore sizes and a typical bimodal distribution for the pore radii is found. All coatings have almost the same behavior in terms of the pore size distribution, although being the profile of porosity gradation through the coatings thickness different for each. The nongraded coatings (HP) present more pores in the 0.04–0.4 Am radius range than the graded samples. The GPII samples have a further contribution of pores with
radii lower than 0.008 Am for total porosity that implies more small cracks or interlamellar pores. One fraction of porosity represents microcracks and the other, larger one opens pores. The pores with radii lower than 0.2 Am have lower contribution to the total porosity and its amount is about 1% to 2%. Pores with radii bigger than 1 Am have also for all coatings one contribution of 2.5% for the total porosity. The porosity with radii in the range 0.1–1 Am has the most influence in the total porosity which is responsible for about 8.5%. The fraction under 0.2 Am represents the microcracks through the lamellas and between them that are very important for the strain accommodation. Fig. 5 shows the relation between the porosity and the deposition parameters. We observe that porosity increase when increase the working distance and decrease significantly for higher power for the plasma gun. These results are due the less velocity of the melted particles in the first case when they reach to the coating in growth and in the second case the particles have more velocity and higher temperature. 3.3. Microhardness measurements Fig. 6(a1) and (a2) shows the microhardness distribution of the ceramic top coatings for as-sprayed condition. It can
Fig. 4. Pore size distribution of as-sprayed coatings obtained by Hg intrusion.
Fig. 5. Porosity evolution: with power of the plasma gun and with working distance.
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Fig. 6. Microhardness measurements in atmospheric plasma sprayed coatings, along the cross section; (a1) and (a2) in as sprayed condition; (b1) and (b2) after annealing at 1100 8C during 100 h.
be observed that the microhardness decrease towards the surface in graded samples (GPI, GPII and GPIII) and slightly increased in case of the constant porosity. For the sample with constant deposition parameters, we observe a little decrease in the porosity values towards the surface that might be explained by the increase of the surface temperature during deposition, and justifies the small microhardness increase on sample HP. Not only can the reduction in the porosity contribute to an increase of the hardness but also the thermal residual stresses within coatings are important. The successive material arriving to the substrate solidify on a surface at lower temperature that rises with the coating growth which slight decrease the cooling velocity of splats. With this effect, the layers more close to the final coating top surface should present a more dense structure that has higher elastic properties, which is favorable to the formation of higher residual stresses and also present more hardness. The decrease in microhardness for the graded samples is due the increase in porosity along the crosssection, this variation can be observed in the Fig. 7 and it is clear a reduction on the harness values for the higher porosities. Sample GPII has less decrease because the variation in porosity is minor too. In addition, it is observed that the microhardness values have a considerable standard deviation, corresponding to the inhomogeneous, porous
microstructure present in thermal barrier coatings obtained by plasma spraying. After annealing, all coatings show higher values for the microhardness (Fig. 6 (b1) and (b2)) and maintain gradients from the bond coat interface to the surface. In addition, the microhardness of the bond coat was measured and gave constant values along the cross-section (about 450 HV). The higher values for the annealed coatings are due the sintering effects at high temperature [26,27]. 3.4. Structural analysis and phase transformation The structural analysis was performed to determine the structure for all coatings and to determine the volume of phase modification. For the as-sprayed coatings, all present a polycrystalline structure in the tetragonal phase and the main peak diffraction is for (111) planes. After annealing and thermal shock a very small amount of monoclinic phase was detected for the main diffraction peaks m(111) and m( 111). It is known that the presence of monoclinic phase in TBC’s is not wanted because reduce the TBC’s lifetime due the volume increase during the tetragonal to monoclinic transformation. It is well observed that a small shift in the peak positions that can be explained by the increase in thermal stresses after annealing and after thermal shock.
4. Conclusions
Fig. 7. Relation between hardness and porosity (as-sprayed coatings).
With the modification of deposition parameters, we can get a thermal barrier coatings graded in porosity along the cross-section, and this way we improve the thermal shock resistance and can be an important factor for decrease the thermal conductivity. Improving the thermal conductivity and thermal shock resistance, it is possible to increase the inlet temperatures in gas turbines and their performance. In the as-deposited condition, all coatings have a lamellar structure which is a characteristic for this type of deposition technique. After annealing and thermal shock a thermal grown oxide with about 5 Am was observed as coatings sintering effects.
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Small cracks and interlamellar defects gave a rather small contribution to the total porosity in image analysis measurements, in contrast to Hg-intrusion investigations. All coatings present a bimodal pore size distribution. The porosity increases towards the surface in graded coatings, decreases with increasing plasma gun power and increases with increasing working distance. The microhardness decreases from the interface to the surface and increase after annealing. As it was expected, the microhardness decreases when the porosity increases. Nongraded, as-sprayed HP sample showed a slightly increase in the hardness from the interface to the surface which was explained by the increase of surface temperature during deposition that implies a densification of the structure with lower porosity and higher thermal residual stress within the coating. The annealed coatings showed reduced porosity levels due the sintering effects. After heat treatments, it was observed a very small amount of monoclinic phase.
Acknowledgments This work was financially supported by FCT-Portuguese Foundation for Science and Technology under the project POCTI/EME/39316/2001: bPVDCOAT-Composite and multilayered protective coatings for efficient energy systemsQ. The cooperative work is also supported by German–Portuguese Cooperative Programme ICCTIDAAD and European Commission-DG-XII under contracts: ICCTI-DAAD/ 423/2000, bComposite Coatings for high temperature applicationsQ and COST 522, WP2/SP21999/01: bResidual stresses and failure in multilayered and functionally graded coatings for advanced energy systemsQ (projects leader: V. Teixeira). A. Portinha is grateful for the Research Grants supported by F.C.T.-Portuguese Foundation for Science and Technology. The authors acknowledge to Alcino Monteiro for some hardness measurements.
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Characterization of thermal barrier coatings with a gradient in porosity A. Portinhaa, V. Teixeiraa,*, J. Carneiroa, J. Martinsb, M.F. Costac, R. Vassend, D. Stoeverd a
GRF-Functional Coatings Group, Physics Department, University of Minho, Campus de Azure´m, 4800 Guimara˜es, Portugal b Mechanical Engineering Department, University of Minho, Campus de Azure´m, 4800 Guimara˜es, Portugal c Physics Department, University of Minho, Campus de Gualtar, 4700 Braga, Portugal d Institute for Materials and Processes in Energy Systems 1, Forschungszentrum Julich GmbH, D-52425 Julich, Germany Received 11 February 2004; accepted in revised form 8 July 2004 Available online 11 September 2004
Abstract A major problem in thermal barrier coatings (TBC) applied to gas turbine components is the spallation of ceramic coating under thermal cycling processes. In order to prevent spallation and improve the thermomechanical behaviour of the TBC, graded ceramic coatings can be produced. For this purpose we are developing a new concept of Thermal Barrier Coating (TBC) that consist of a conventional NiCoCrAlY bond coat and an atmospheric plasma sprayed ZrO2–8 wt.%Y2O3 top coat graded in porosity on an Inconel 738 LC substrates. The aim of this work is to produce coatings with low thermal conductivity and better thermomechanical behaviour due to the gradient in porosity which reflects a gradient in the elastic properties. Absolute porosity was measured with a mercury porosimetry and by image analysis. The second technique was also used to estimate the porosity variation along the cross-section. Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) were used to observe the morphology and coating microstructure. The microhardness was measured with a Vickers indenter and 0.981 N load. The microhardness has been evaluated for coatings in as-sprayed condition and after annealing at 1100 8C during 100 h. The results show a fast increase of the hardness after annealing. After thermal shock heating at 1000 8C, 1 h and quickly cooling in water no spallation was observed for 100 cycles. D 2004 Published by Elsevier B.V. Keywords: Stabilised zirconia coatings; Thermal barrier coatings; Graded in porosity; Microhardness
1. Introduction Ceramic coatings of engineering materials such as zirconia partially or totally stabilized are used for a variety of technological applications requiring thermal insulation, wear and erosion resistance or protection from oxidation, sulfidation and hot corrosion. These kinds of coatings have been applied as Thermal Barrier Coatings (TBCs) for protection of metallic components in gas turbines (vanes, blades, shrouds, etc.) and diesel engines, and improve performance at high temperatures [1–3]. The TBC concept allows increase in operating temperature and/or reducing the cooling systems due to the temperature gradient across the thick ceramic coating, that permit better thermodynamic * Corresponding author. Tel.: +351 253510465/400; fax: +351 253510401. E-mail address: [email protected] (V. Teixeira). 0257-8972/$ - see front matter D 2004 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2004.07.094
performance, lower emissions without requiring major alloy development. Zirconia coatings are very interesting materials because of their outstanding mechanical, thermal, optical and electrical properties. Zirconia has a high melting point, high resistance to oxidation, low thermal conductivity, high hardness, and high coefficient of thermal expansion. These ceramic coatings are widely use in many technological applications such in components at high temperature and adverse corrosive environments, oxygen sensors [4], optical coatings [5], etc. TBCs, traditionally, consist in a thick partially stabilized ZrO2 top coating commonly deposited by atmospheric plasma spraying (APS) on superalloys precoated with a metallic bond coat (NiCoCrAlY) produced by vacuum plasma spraying (VPS) [6]. The partially stabilized ZrO2 top coat has a porous and laminar structure and consists of splats with cracks perpendicular to the surface, this porous structure allows the increase in the thermal isolation and the
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cracks permit better stresses accommodation [1,7–9]. The metallic bond coat reduces the diffusion of contaminants and the mismatches of the thermal expansion between top coat and the substrate and this leads to an increase of the lifetime of operation for the components. Zirconia (ZrO2) crystallizes in three distinct polymorphs according to the temperature and pressure conditions. The main phases of ZrO2 are the monoclinic phase (m) stable at room temperature, the tetragonal phase (t) stable between 1170 and 2370 8C and cubic phase (c) stable from 2370 to 2680 8C [10–14]. When the applications for these coatings involve operation in range of temperatures that involve phases transformations, it is necessary to stabilize the high temperature phases at room temperature because the t Z m phase transformation is accompanied by a 3–5% volume expansion, and this volume expansion can cause high residual stresses and microcracks leading to delamination and spallation of coatings. To prevent this transformation, it is necessary to stabilize the high temperature phases at room temperature. For this purpose, these phases can be stabilized by doping ZrO2 with other oxides such as Y2O3, CeO2, MgO, CaO, Gd2O3 or Al2O3 [10–17]. The tetragonal phase of zirconia also can be stabilized at room temperature by decreasing the grain size for a few nanometers (about 6 nm) and it can be obtained producing nanolayered coatings of ZrO2/Al2O3 and ZrO2/TiO2 [13,16]. These coatings with nanometer grain size and nanolayered structures also lead the decrease in thermal conductivity. Nowadays, higher operation temperatures are required and in order to obtain systems of coatings that allow its range of temperatures we need to develop new materials for coatings or new architectures for the existing materials. These new concepts of TBCs should have lower thermal conductivity, and be more stable at higher temperatures than the 7–8 wt.% yttria-stabilized zirconia (YSZ) [6,18]. In addition, these new materials should have other properties comparable to the YSZ like thermal expansion coefficient, corrosion resistance. It is known that increasing the porosity, it will reduce the thermal conductivity; however, above certain values, this increase can degrade the mechanical integrity due to the decrease in cohesion between lamellas. Producing coatings with porosity variable, increasing to the surface we probably can have coatings with higher porosity content with the same or better thermomechanical behavior because the adherence and residual stresses are maintained at the interface but increasing the porosity towards the surface a reduction both in elastic modulus is expected and the level of residual stresses. These properties can be also controlled with better rigor if we will control the pore geometry [19]. New materials, stable at high temperatures and with lower thermal conductivity are under development in order to allow the use of high inlet temperatures [6,17,20]. In addition, multilayer systems with different functions are in study: layers for chemical insulation, with an intermediate
Fig. 1. Model of TBC graded in porosity along cross section. The graded coating was divided in four layers. The porosity was increasing from layer 1 to 4.
conventional zirconia partially stabilized layer and a new material top layer or a graded structures changing the chemical composition from interface with bond coat to the surface using 100% YSZ at interface and then reducing it contents in substitution with the new materials like lanthanides [6]. New dopants have also presented good results, applied alone or joint with the yttria (Y2O3) that have substantial reductions in thermal conductivity reaching 40% for coatings codoped with Y2O3 and Gd2O3 [17,20,21]. In order to obtain better thermal insulation, in this contribution we present a new concept of TBC. It consists in a conventional bond coat and a graded ZrO2–8 wt.%Y2O3 top coat that is graded in the porosity (see Fig. 1). In this paper, we report on the study of structural properties of ZrO2Y2O3 multilayered coatings focusing on the porosity of the microlayers. In order to increase the efficiency of the thermal barrier, different layers with different porosities increasing towards the surface were studied. The SEM images are processed using dedicated routines, in order to measure the porosity of the coatings [22,23]. Not only the porosity values for each layer were obtained but also it was evaluated the way the porosity changes along the coatings cross-section. To obtain the total porosity, we also determine the porosity by mercury intrusion [23]. The microhardness was measured with a Vickers indenter in as-sprayed condition and after annealing. The values of microhardness were thus related with the deposition conditions, heat treatment and porosity variation [24,25].
2. Experimental setup Thermal barrier coatings (TBCs) investigated were produced by plasma spraying. A Sulzer Metco AG vacuum plasma spraying (VPS) system was used to deposit a NiCoCrAlY bond coat (Ni 192-8 powder by Praxair Surface Technologies, Indianapolis, IN) on square plates of a nickel superalloy Inconel 738 LC previously sand-blasted with alumina particles which are 0.71-Am average size and ultrasonically cleaned in acetone. The substrate dimensions were 40403 mm. The ceramic powder used for top coats was ZrO2–8 wt.%Y2O3 (Metco 204 NS Sulzer Metco GmbH, Germany) and was sprayed by atmospheric plasma spraying (APS)
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Table 1 Deposition parameters Deposition parameters
APS
All samples
HP
GPI
GPII
GPIII
VPS
Power (kW) Int. of current (A) Gas plasma (slpm) Gas transport (Ar) (slpm) Diameter of nozzle (mm) Flux of powder (g/min) Distance of gun to substrate (mm) Pressure in the chamber (mbar) Substrate temperature (K) Thickness of the total layer (Am) Gun speed (mm/s)
14.5 240 20.1/13.1 (Ar/He) 1.5 10 8.5 90 atm 423 300 500
20.5 to 14.5 300 to 240 20.1/13.1 (Ar/He) 1.5 10 8.5 90 atm 473–423 265 500
20 to 15 300 to 240 20.1/13.1 (Ar/He) 1.5 10 8.5 120 atm 453 305 500
20.5 to 14.5 300 to 240 20.1/13.1 (Ar/He) 1.5 10 8.5 90 to 120 atm 423 200 500
49.4 733 50.6/9.1 ( Ar/H2 ) 1.7 7 40 275 60 1073–1093 140 440
using a Sulzer Metco Triplex gun. Deposition conditions are presented in Table 1. For characterization of the as-sprayed condition of the top coat, steel substrates were coated at the same time. The samples were annealed at 1100 8C for 100 h in air, and were made a thermal shock heating at 1000 8C during 1 h and after cooling in water for 100 cycles. The microhardness was measured with a Vickers indenter with 0.981 N load and was measured along the whole cross-section. Total porosity levels were evaluated in as-sprayed condition by mercury intrusion after removal of the steel substrate from the coating with hydrochloric acid. The morphology of the coatings was analyzed by optical and scanning electron microscope (SEM). Micrographs with two magnifications (400 and 500) from polished cross-
sections were used for image analysis for the determination of the total porosity and the porosity profile through the cross-section.
3. Results and discussion 3.1. Microstructural characterization In Fig. 2 is presented SEM cross-sectional micrographs for thermal barrier coatings showing the microstructure before and after annealing. The thickness was determined by SEM analysis (see Table 1) of all coatings, and combining this analysis with deposition parameters was estimated the thickness of each microlayer for the different porosities along the cross-section. The coatings present a porous and
Fig. 2. SEM micrographs showing microstructure and porosity on ZrO2–8 wt.%Y2O3 coatings produced by APS. (a) Typical lamellar microstructure with columnar structure in each lamella, (b) cross-section after annealing in which the Thermal Grown Oxide (TGO) is clearly visible, (c) cross-section of as-sprayed top coat where the increase in porosity towards the surface is visible.
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lamellar structure (see Fig. 2a) which is characteristic for this kind of coatings [26]. In plasma spraying, the microstructure of the coatings is strongly dependent on processing conditions. The splats are separated by interlamellar pores resulting from rapid solidification of the lamellae, very fine voids formed by incomplete intersplat contact or around unmelted particles, and cracks due to thermal stresses and tensile quenching relaxation stresses. The presence of cracks also increases the strain tolerance and enhances the thermal shock resistance for TBCs in service. From the SEM micrographs it is possible to observe the variation of the porosity for different coatings and also its variation from the interface with bond coat to the surface as is demonstrated in Fig. 2c, the quantification of this variation along crosssection has been measured by image analysis. Additionally, small microcracks with diameters of about 200 nm are observed. These cracks are originated from the thermal stresses which arise from the rapid cooling during the spray process (quenching stresses). After annealing in air, all coatings present a sintered structure and consequently a reduction in the porosity levels which is in agreement with other studies [27]; these sintering effects will promote the increase in thermal conductivity, in elastic modulus and a loss of the strain tolerant behavior. A thermally grown oxide (TGO) observed between bond coat and top coat for the annealed samples is shown in (Fig. 2b) which have 5 Am after 100 h at1100 8C in air. Energy dispersion X-ray spectrometry (EDX) analysis reveals that the TGO is predominantly aluminum oxide. The aluminum growth oxide at the interface between bond coat and top coat results from the diffusion of the aluminum present in the bond coat to it surface, this oxide protects the alloy from oxidation. However, when it reaches certain thickness and due the interface irregularities develops high residual stresses that promote microcracking which are responsible for the spallation of top coat and system failures. After thermal shock in all samples, no spallation was observed and only it has seen some densification at the surface and the appearance of small cracks perpendicular to the coating plane that improve the strain accommodation. This behavior is important and means that our coatings show an excellent thermal shock resistance. In our opinion, the gradation in porosity improves this resistance because the coatings have better accommodation of thermal stresses during the quenching period. 3.2. Porosity measurements Porosity of TBCs can be characterized qualitatively by microstructure observation and quantitatively by mercury intrusion porosimetry (MIP) technique besides coating density measurement. The direct examination of coatings microstructure from cross-section of coatings using a (SEM) gives comparative information about porosity for the different coatings. In conjunction with a backscattered electron (BSE) detector, the chemical composition of the
microstructure is represented in the images by gray level variation. Pores appear very dark, which permit them to be distinguished and quantified by image analysis. By this method, we can’t obtain information about the 3-D pore network or connectivity between them [22,23]. For the analyzed coatings, two series of images were acquired, one with 400 magnification and the other with 500 magnification. Using the MIP, it is possible to obtain measurements of total porosity for open pores and the evaluation of pore size distribution. MIP does provide information about the connectivity of the pores and microscopy reveals information about pore geometry, so there is interest in combining these two techniques for a more complete analysis. Mercury intrusion porosimetry is based on the premise that a nonwetting liquid (one having a contact angle greater than 908) will only intrude capillaries under pressure. Mercury must be forced using pressure into the pores of a material. The pore size distribution is determined from the volume intruded at each pressure increment. Total porosity is determined from the total volume intruded. The digital micrographs were evaluated on a Matrox II program for image analysis. The pores were identified by thresholding the brightness of the pores to produce a binary image, after the dark area fraction in the binary image was evaluated and the percentage determined. The corresponding porosity values for the different coatings are presented in Table 2. We can see in Table 2 a considerable difference between the measured Hg porosities and the porosities evaluated by image analysis. In addition, a reduction was found in porosity values after annealing for all samples, its reduction is mainly due to the sintering effects. While the Hg porosimetry gave reliable results for small pores and microcracks, it failed for pores with radius larger than 80 Am. For large pores the mercury fills it without any external applied pressure (because of the weight of the mercury and the size of the pores) and was therefore not measured. Contrarily, the image analysis is a technique to analyse porosity in which the contribution of small pores and small microcracks between and through the lamellas within the plasma-sprayed coatings is difficult to measure in contrast to the large pores. This microcracked microstructure leads to relatively low thermal conductivity values in APS TBCs and Table 2 Coating porosity measured by image analysis and Hg porosimetry Samples
HP
GPI
GPII
GPIII
Hg porosity (%) Image analysis (%)a Image analysis (%)b Image analysis after annealing (%)a Image analysis after annealing (%)b
14.75 11.79 11.15 8.44 6.94
15.31 13.08 10.76 8.33 7.99
15.29 15.48 12.73 9.90 7.40
13.38 13.34 9.34 10.57 8.27
a b
Porosity with small cracks and ribbons. Porosity without ribbons.
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Fig. 3. Porosity distribution along the cross section determined by image analysis: (b, d, f) porosity with small cracks and ribbons; (a, c, e) porosity without ribbons.
enhances the strain tolerance in service. In agreement with SEM analysis and deposition parameters the thickness of each microlayer was estimated in the graded coatings. Using image analysis the porosity values of each layer was determined. In Fig. 3, the porosity variation along the cross-section for these three kinds of coatings is presented. The porosity increases from the bond coat interface towards the surface of the top coat. In as-sprayed condition the absolute porosity variation ranges from 3% to 5% from interface to top coat surface and in annealed condition we observe a significant reduction in its variation (from 1% to 2%). This reduction is due the sintering effects at high temperature [27,28]. The pore size distribution determined by Hg porosimetry can be seen in the Fig. 4 for all coatings. In this graph is represented the cumulative porosity function of the pore sizes and a typical bimodal distribution for the pore radii is found. All coatings have almost the same behavior in terms of the pore size distribution, although being the profile of porosity gradation through the coatings thickness different for each. The nongraded coatings (HP) present more pores in the 0.04–0.4 Am radius range than the graded samples. The GPII samples have a further contribution of pores with
radii lower than 0.008 Am for total porosity that implies more small cracks or interlamellar pores. One fraction of porosity represents microcracks and the other, larger one opens pores. The pores with radii lower than 0.2 Am have lower contribution to the total porosity and its amount is about 1% to 2%. Pores with radii bigger than 1 Am have also for all coatings one contribution of 2.5% for the total porosity. The porosity with radii in the range 0.1–1 Am has the most influence in the total porosity which is responsible for about 8.5%. The fraction under 0.2 Am represents the microcracks through the lamellas and between them that are very important for the strain accommodation. Fig. 5 shows the relation between the porosity and the deposition parameters. We observe that porosity increase when increase the working distance and decrease significantly for higher power for the plasma gun. These results are due the less velocity of the melted particles in the first case when they reach to the coating in growth and in the second case the particles have more velocity and higher temperature. 3.3. Microhardness measurements Fig. 6(a1) and (a2) shows the microhardness distribution of the ceramic top coatings for as-sprayed condition. It can
Fig. 4. Pore size distribution of as-sprayed coatings obtained by Hg intrusion.
Fig. 5. Porosity evolution: with power of the plasma gun and with working distance.
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Fig. 6. Microhardness measurements in atmospheric plasma sprayed coatings, along the cross section; (a1) and (a2) in as sprayed condition; (b1) and (b2) after annealing at 1100 8C during 100 h.
be observed that the microhardness decrease towards the surface in graded samples (GPI, GPII and GPIII) and slightly increased in case of the constant porosity. For the sample with constant deposition parameters, we observe a little decrease in the porosity values towards the surface that might be explained by the increase of the surface temperature during deposition, and justifies the small microhardness increase on sample HP. Not only can the reduction in the porosity contribute to an increase of the hardness but also the thermal residual stresses within coatings are important. The successive material arriving to the substrate solidify on a surface at lower temperature that rises with the coating growth which slight decrease the cooling velocity of splats. With this effect, the layers more close to the final coating top surface should present a more dense structure that has higher elastic properties, which is favorable to the formation of higher residual stresses and also present more hardness. The decrease in microhardness for the graded samples is due the increase in porosity along the crosssection, this variation can be observed in the Fig. 7 and it is clear a reduction on the harness values for the higher porosities. Sample GPII has less decrease because the variation in porosity is minor too. In addition, it is observed that the microhardness values have a considerable standard deviation, corresponding to the inhomogeneous, porous
microstructure present in thermal barrier coatings obtained by plasma spraying. After annealing, all coatings show higher values for the microhardness (Fig. 6 (b1) and (b2)) and maintain gradients from the bond coat interface to the surface. In addition, the microhardness of the bond coat was measured and gave constant values along the cross-section (about 450 HV). The higher values for the annealed coatings are due the sintering effects at high temperature [26,27]. 3.4. Structural analysis and phase transformation The structural analysis was performed to determine the structure for all coatings and to determine the volume of phase modification. For the as-sprayed coatings, all present a polycrystalline structure in the tetragonal phase and the main peak diffraction is for (111) planes. After annealing and thermal shock a very small amount of monoclinic phase was detected for the main diffraction peaks m(111) and m( 111). It is known that the presence of monoclinic phase in TBC’s is not wanted because reduce the TBC’s lifetime due the volume increase during the tetragonal to monoclinic transformation. It is well observed that a small shift in the peak positions that can be explained by the increase in thermal stresses after annealing and after thermal shock.
4. Conclusions
Fig. 7. Relation between hardness and porosity (as-sprayed coatings).
With the modification of deposition parameters, we can get a thermal barrier coatings graded in porosity along the cross-section, and this way we improve the thermal shock resistance and can be an important factor for decrease the thermal conductivity. Improving the thermal conductivity and thermal shock resistance, it is possible to increase the inlet temperatures in gas turbines and their performance. In the as-deposited condition, all coatings have a lamellar structure which is a characteristic for this type of deposition technique. After annealing and thermal shock a thermal grown oxide with about 5 Am was observed as coatings sintering effects.
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Small cracks and interlamellar defects gave a rather small contribution to the total porosity in image analysis measurements, in contrast to Hg-intrusion investigations. All coatings present a bimodal pore size distribution. The porosity increases towards the surface in graded coatings, decreases with increasing plasma gun power and increases with increasing working distance. The microhardness decreases from the interface to the surface and increase after annealing. As it was expected, the microhardness decreases when the porosity increases. Nongraded, as-sprayed HP sample showed a slightly increase in the hardness from the interface to the surface which was explained by the increase of surface temperature during deposition that implies a densification of the structure with lower porosity and higher thermal residual stress within the coating. The annealed coatings showed reduced porosity levels due the sintering effects. After heat treatments, it was observed a very small amount of monoclinic phase.
Acknowledgments This work was financially supported by FCT-Portuguese Foundation for Science and Technology under the project POCTI/EME/39316/2001: bPVDCOAT-Composite and multilayered protective coatings for efficient energy systemsQ. The cooperative work is also supported by German–Portuguese Cooperative Programme ICCTIDAAD and European Commission-DG-XII under contracts: ICCTI-DAAD/ 423/2000, bComposite Coatings for high temperature applicationsQ and COST 522, WP2/SP21999/01: bResidual stresses and failure in multilayered and functionally graded coatings for advanced energy systemsQ (projects leader: V. Teixeira). A. Portinha is grateful for the Research Grants supported by F.C.T.-Portuguese Foundation for Science and Technology. The authors acknowledge to Alcino Monteiro for some hardness measurements.
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