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Structure–property differences between supersonic and conventional atmospheric plasma sprayed zirconia thermal barrier coatings
Surface & Coatings Technology 205 (2011) 3833–3839
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Structure–property differences between supersonic and conventional atmospheric plasma sprayed zirconia thermal barrier coatings Y. Bai a, Z.H. Han a,⁎, H.Q. Li a, C. Xu a, Y.L. Xu a, C.H. Ding b, J.F. Yang a a b
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China School of Aerospace, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China
a r t i c l e
i n f o
Article history: Received 20 October 2010 Accepted in revised form 25 January 2011 Available online 31 January 2011 Keywords: Supersonic atmospheric plasma spraying ZrO2 Thermal barrier coatings Thermal cycling lives Thermally grown oxide
a b s t r a c t Yttria-stabilized zirconia (YSZ) based thermal barrier coatings (TBCs) were deposited by high efficiency supersonic atmospheric plasma spraying (SAPS) system. The microstructure and thermal shock resistance of the SAPS-TBCs were investigated. As compared to conventional atmospheric plasma sprayed TBCs (APS-TBCs) with the same composition, the microstructure of SAPS-TBCs was much finer. It was found that the thickness of lamellar structure consisted of columnar crystals in the SAPS- and APS-coatings was in the range of 1–4 μm and 2–8 μm, respectively. Besides, the statistical results revealed that the average thickness of the lamellar structure in SAPS-coating was 2.5 ± 0.6 μm, while that of APS-coating was 5.3 ± 0.9 μm. The desirable structure was attributed to higher impact velocity of in-flight particles during SAPS process, which resulted in the improvement of flattening degree of molten particles after impinging on the target. The well-adhered fine lamellar structures, fine micro-cracks and lower growth rate of thermally grown oxide (TGO) appeared to be responsible for greatly improved thermal cycling lives of SAPS-TBCs as compared to their conventional plasma sprayed counterparts. The results of water-quenching test from 1100 °C into room temperature showed that SAPS-TBCs presented high thermal shock resistance, only 10% coating area spalled after 265 thermal cycles, about 90% higher than that of APS-TBCs. The SAPS method, which offered some unique advantages over the conventional plasma spraying process, is expected to be potentially used to deposit high-performance TBCs at lower cost. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Thermal barrier coatings (TBCs), serving as heat-insulating layer on the surfaces of metallic components, have been widely used in the field of gas-turbine engines for aircraft propulsion and power generation. Using TBCs on the surface of superalloy components, the surface temperature of the superalloy can be reduced as much as 100– 300 °C. This has enabled modern gas-turbine engines to operate at gas temperatures well above the melting temperature of the superalloy (1300 °C), thereby improving engine efficiency and performance [1]. A typical layered TBC system consists of a thermally insulating ceramic top coating applied over an oxidation-resistant metallic bond coating. 6–8 wt.% yttria partially stabilized zirconia (PSZ) coatings were commonly used as top coating due to their high temperature stability and low thermal conductivity. MCrAlY (M = Ni, and/or Co) is often used as a bond coat to provide a good thermal expansion match between the top coat and substrate and to prevent oxidation of the substrate [2,3].
⁎ Corresponding author. Tel.: + 86 2982668614; fax: + 86 29 82663453. E-mail address: [email protected] (Z.H. Han). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.01.056
However, a major problem in TBCs applied to gas turbine components is the spallation of ceramic coat under thermal cycling processes. Thus, thermal shock behavior is an important indication of TBCs performance. The failure mode during the thermal cycling is always associated with the initiation and propagation of cracks in the top PSZ coating and at the interface between top coat and thermally grown oxide (TGO), typically α-Al2O3, formed at the interface of the bond coat and top ceramic coat during high-temperature exposure [4,5]. The two commercial processes used for the deposition of TBCs are air plasma spraying (APS) and electron-beam physical-vapor deposition (EB-PVD). Compared with EB-PVD process, plasma spraying is a low-cost method and results in ubiquitous “splat” boundaries and cracks running parallel to the bond coat/top coat interface in the as-sprayed coatings. These cracks parallel to the bond coat/top coat interface can effectively reduce the high-temperature thermal conductivity (0.8–1.7 W/m K), however, leave the top YSZ coat relatively shorter thermal-cycling lives [1,6]. So, since the 1980s, great attention has been given to optimize the microstructure of plasma-sprayed TBCs in order to improve their thermal durability and thermal-cycling lives [7,8]. Recently, high efficiency supersonic atmospheric plasma spray (SAPS) process has been successfully developed to deposit ceramic and metallic coatings with excellent performance [9–12]. For
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conventional air plasma spray (APS) guns, the gas velocities of 300– 800 m/s and velocities of the particles in the range of 130–220 m/s were obtained with 5–8 mm diameter nozzles and 50–75 L/min total gas flow rates [10,13]. In contrast, using the SAPS gun, the velocities of the plasma jet and particles of 2400 m/s and 500 m/s can be achieved at the normal distance of 100 mm from the nozzle exit [10,14]. Another important plasma spraying process is vacuum plasma spraying (VPS) and the stream of high velocity molten particles (typically two to three times as high as that in APS [15]) can also be obtained during VPS. However, the low pressure produces significant expansion of the isotherms and velocity contours of the plasma jet, due to the increased mean free path between the ions and electrons in the plasma [16]. As compared with APS, VPS requires higher power and longer spray distances for the varied materials to ensure complete melting of particles. In the process of SAPS, the stream of high velocity molten particles can be obtained in the highenergy plasma jet, which is exposed to the atmosphere. The high-energy plasma jet results from the structural design of SAPS gun with a Laval nozzle. The SAPS gun makes the cooling water channels with good thermal pinching effect and the working gases are injected into the nozzle via gas vortex ring in order to pinch the arc and increase the life of plasma gun [14]. Besides, in order to heat and accelerate the powder particles adequately, the powder was injected into the plasma jet via an internal injection port (positioned inside the nozzle). So, as compared with VPS, SAPS is expected to deposit some high-performance coatings in the atmosphere, which could reduce the cost effectively. Since the molten particles with higher kinetic energy will form thinner flakes when they are quenched onto the substrate, the coatings deposited by SAPS system may have finer multilayered microstructure, higher adhesive strength and lower porosity as compared to those of coatings deposited by conventional plasma spraying system. In this work, YSZ-based TBCs were deposited by SAPS system and the microstructure and thermal shock resistance were investigated.
2. Experimental
Fig. 1. Starting powders used for plasma spraying: (a) CoNiCrAlY powder, (b) ZrO2–8wt.% Y2O3 (YSZ) powder.
2.1. Materials 2.3. Particle in-flight properties A nickel-base super alloy, GH3030, previously sand-blasted with alumina particles, was used as the substrate with diameter of 20 mm and thickness of 10 mm. A commercially available CoNiCrAlY powder (AMDRY 995M, Sulzer Metco Inc. USA) with nominal composition of Ni-32, Cr-21, Al-8, Y-0.4 Co-balance (wt.%) was used for spraying the bond coat of TBCs. The spherical powders were gas atomized and the particle size was about 65 μm, as shown in Fig. 1a. The ceramic powder used for top coat was commercially spray dried and sintered ZrO2–8wt.%Y2O3 with diameter ranging from 40 to 110 μm, as shown in Fig. 1b.
During spraying, a commercially available Spray Watch 2i system (made by Osier, Finland) was used to monitor the velocity and surface temperature of in-flight particles for each operating conditions. In this system, velocity of individual particle was measured by time-of-flight method, ranging from 10 to 1000 m/s with the resolution 0.5–5 m/s. Particle surface temperature was measured by two-color pyrometry, ranging from 1000 to 4000 °C with the resolution about 5 °C. The number of detected particles was about 2000 for one measurement independently and the final value was the average of five measurements. It should be noted that for the determination of the particle
2.2. Plasma spraying process The thermal barrier coatings, composed of a bond CoNiCrAlY coat (about 60 μm thick) and a top coat (about 250 μm thick) , were deposited by a Metco 9M air plasma spraying (APS) system and high efficiency supersonic atmospheric plasma spraying (SAPS) system, respectively. The advanced SAPS system was developed by the National Key Laboratory for Remanufacturing, China, which consisted of plasma torch, powder feeder, gas supply and water cooling circulator. The key advantage of this system was a novel SAPS gun with a Laval nozzle. The length and exit diameter of nozzle were 44 mm and 6 mm, respectively. The powder was injected into the plasma jet by an internal injection port with inlet diameter of 2 mm, which was inside the nozzle and directed perpendicular to the plasma jet. The detailed information about SAPS system can be found elsewhere [17]. The spraying parameters are presented in Table 1.
Table 1 Spraying parameters for SAPS and APS top coat and bond coat. Spraying parameters
Gun nozzle inner diameter (mm) Powder injection port inlet diameter (mm) Current (A) Voltage (V) Primary gas Ar (slpm) Second gas H2 (slpm) Carrier argon gas flow rate (slpm) Gun traverse speed (mm/s) Powder feed rate (g/min) Spray distance (mm) a b
SAPS
APS
YSZ
CoNiCrAY YSZ
6 2a 405 160 60 17 7.5 800 40 100
6 2a 363 124 65 8 8 800 40 100
Powder injection port is inside the nozzle. Powder injection port is outside the nozzle.
CoNiCrAlY
7 7 2b 2b 650 500 74 66 45 80 12 6 8 8.5 800 800 40 40 100 100
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temperature diagnostic systems were restricted to the analysis of surface temperature of the particles and the details about testing principle were reported in other reference [18].
2.4. Specimens characterization Crystal structures of the as-sprayed coatings were determined by means of X-ray diffraction (XRD) on a Rigaku X-RAY diffractometer with Cu Kα radiation. The microstructure and chemical composition of samples were examined by TESCAN-VEGAII XUM scanning electron microscope (SEM) equipped with INCA-AE350 energy dispersive spectrometer (EDX). Before specimen sectioning, the surface of top YSZ coat was coated by two-part thermosetting resin in order to protect the microstructure of samples from damage during cutting afterwards. After the resin was cured at 120 °C for 1 h, cross-sections were produced by wet diamond sawing at lower speed in order to minimize damage. One side was ground on 600-grit silicon carbide papers, followed by sequential polishing on soft cloths until no obvious scratches were observed by optical microscopy. At present, the two most common methods used to evaluate the porosity value in the coating are mercury intrusion porisimetry (MIP) and image analysis (IA) [17]. MIP is a quantitative porosity measurement method but it is very difficult to determine the true porosity value due to the presence of closed pores in the coating. IA is somewhat subjective, but it is suitable for a relative comparison among different coatings [17,19,20]. In this work, porosity of the coatings was estimated by Archimedean (water displacement) method and quantitative image analysis (IA) using a picture analysis system in scanning electron microscopy. In the Archimedean method, the top YSZ coat was firstly separated from the metallic substrate by submerging the samples in 40% hydrochloric acid until the metallic bond coat was dissolved. Then the free-standing YSZ top coat was cleaned and dried. Before measurement, the samples were boiled in distilled water for 2 h to facilitate pore penetration. A detailed description of Archimedean technique used to determine the porosity of coating was available in the literature [21]. In the image analysis, to get a representative porosity value, a series of SEM images with a magnification of 1000× were obtained for individual specimens. The resolution of image was 600 dpi and the minimum area of detectable voids was about 0.1 μm2. Twenty micrographs were randomly taken from cross-section of each coating. In addition, the thickness of splats in the as-sprayed coatings was measured by software (Image-Pro Plus). Twenty SEM images of fractured cross section with a magnification of 4000× and resolution of 600 dpi were randomly taken from the as-sprayed coatings, and the final value was the average of sixty measurements.
2.5. Bonding strength and thermal shock test The bonding strength of the as-sprayed coatings was measured using a materials tester (Instron1196, USA) in accordance with ASTM C 633-79 standard. A rod made of Nickel-base superalloy, GH3030, was used as the substrate with diameter of 25.4 mm. Film epoxy adhesive (FM-1000, USA) with tensile fracture strength more than 60 MPa was applied. The final value represented the average value of 5 samples sprayed at the same parameters. Thermal shock tests were conducted by using a muffle furnace. When the temperature inside the furnace reached to 1100 °C, the samples were pushed into the furnace. The holding time at the temperature was 5 min, then the samples were directly quenched into water, the temperature of the water throughout the cycling was between 20 and 30 °C. More than 10% of the spalled region of the surface of the top coating (quantitatively calculated by image analysis software Image Tool 3.0) was adopted as criteria for the failure of the coating.
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3. Results and discussion 3.1. Microstructure of as-sprayed coating Fig. 2 shows fractured and polished cross-sectional micrographs for as-sprayed top YSZ coatings fabricated by SAPS and conventional APS, respectively. As seen from Fig. 2a and b, typical lamella structure and columnar structure within the individual splats were visible in both coatings, since rapid nucleation occurred at the cooler surface of the flattened droplet at large under-cooling and crystals grew rapidly in the opposite direction to the heat flow forming a columnar grain structure. However, as compared Fig. 2a with Fig. 2b, it was found that the thickness of lamellar structure consisted of columnar crystals in the SAPS- and APS-coating was in the range of 1–4 μm and 2–8 μm, respectively. Besides, the statistical results revealed that the average thickness of the lamellar structure in SAPS-coating was 2.5 ± 0.6 μm, while that of APS-coating was 5.3 ± 0.9 μm. Moreover, a large amount of coarse inter-lamellar cracks were observed in Fig. 2b, while only a few fine cracks were found in Fig. 2a. The above results indicated that the SAPS-coating had a finer microstructure as compared with the APS-coating. Besides, as shown in Fig. 2c and d, for SAPS-coating, micro-cracks (as marked by black arrows shown in Fig. 2c) and fine pores were homogeneously distributed in the top YSZ coat. Whereas, a large amount of coarse pores and cracks were observed in the APScoating which could be rarely found in the SAPS-coating, further indicating that the SAPS-coating had a finer microstructure as compared with its APS counterpart. In plasma spraying, the microstructure of the coatings was strongly dependent on processing conditions. The splats were separated by inter-lamellar pores that resulted from rapid solidification of the lamellae, pores formed by incomplete contact between the splats or around unmelted particles, and cracks arising from the relaxation of thermal or tensile quenching stresses. The microstructure differences between SAPS- and APS-coatings can be explained by the following model. During the plasma spraying process, the powders were injected into the plasma jet, where they should be melted (droplet) and accelerated against the substrate to form lamella structure. The flattening degree, which is defined as the ratio of the diameter of the lamella to the diameter of starting droplet, has a close relationship with the particle impact velocity. The following formula was used to describe this relationship [22–24]. b
ξ = aRe
ð1Þ
Re ¼ρdv = η
ð2Þ
where ξ is the flattening degree, Re is the Reynolds number, a and b are coefficients, and ρ, d, v and η are the density, diameter, velocity and viscosity of the molten droplet, respectively. Table 2 summarizes the coefficients reported by different researchers. As shown in Table 2, although the values of a and b provided by many investigators were different, the flattening degree of lamella increased with the rise of particle impact velocity. The velocity and surface temperature of inflight particles measured by Spray watch 2i system for each operating conditions were shown in Table 3. As seen from Table 3, the average in-flight velocity of YSZ particles in SAPS was 424 m/s, much higher than that in APS 180 m/s. Moreover, the surface temperature of inflight particles in SAPS was 3089 °C, a little higher than that in APS, 2814 °C. The great improvement of velocity was attributed to the structural optimization of SAPS gun and internal powder injection, which made in-flight particles acquire the higher energy and momentum in plasma jet. So, the SAPS-coating had a higher flattening degree of lamella as compared with the APS-coating. The bonding strength between top coat and bond coat and the porosity of top coat of as-sprayed SAPS- and APS-coating are listed in Table 4. As shown in it, the average bonding strength of SAPS-coating
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Fig. 2. Fractured and polished cross-sectional micrographs for as-sprayed top YSZ coatings fabricated by SAPS and conventional APS process: (a) fractured cross section of SAPScoating, (b) fractured cross section of APS-coating, (c) polished cross section of SAPS-coating, (d) polished cross section of APS-coating.
was 47 MPa, about 31% higher than that of APS-coating. Meanwhile, as seen from Table 4, the porosity of SAPS-coating measured by image analysis and Archimedean principle was about 5% and 8% respectively, whereas the porosity of APS-coating obtained from the above two methods was about 9% and 13% respectively. So, as compared with APS-coating, SAPS-coating was much denser. In addition, the porosity measured by Archimedean method was higher than that obtained from the image analysis. A similar phenomenon was also found elsewhere [25]. This may be attributed to the fact that some micro cracks or fine pores were not discernable by image analysis. For SAPScoating, the increase of bonding strength and decrease of porosity are ascribed to the rise of flattening degree. The rise of flattening degree led to the increase of effective bonding surface between the lamella structure and the under bond coat (CoNiCrAlY), and the decrease of inter-lamellar pores resulted from the poor wetting/adhesion between the lamellae as they accumulate to form the coating. The result was that the SAPS-coating had a higher bonding strength and lower porosity. Generally, the mechanical properties of coating are degraded
Table 2 Coefficients reported for relationshipξ = aReb by different investigators. Coefficient (a)
Coefficient (b)
Methods
References
1
0.2
[22]
0.925
0.2
1.04
0.2
Simulated by mathematical modeling Simulated by finite-element method Numerical simulation
[23] [24]
by the porosity, for example, brittle fracture is extremely sensitive to defect size and the strength is determined by the largest crack-like defect [26]. On the other hand, some studies revealed that the thermal conductivity of coating was reduced by the presence of pores, which could reduce the effective cross-sectional area of the materials [26,27]. So, as compared with APS-coatings, the SAPS-coatings with lower porosity may have higher toughness, strength, and higher thermal conductivity. This needs to be further studied. 3.2. Thermal shock resistance The thermal shock resistance of SAPS- and APS-coating was estimated by water-quenching cycling number. Fig. 3 shows the surface morphology of SAPS- and APS-coatings after different thermal cycles. As seen from Fig. 3a, the evidently visible spalled area on the fringe of SAPS coating was found after 180 cycles (noted by an arrow), and sustained up to 265 cycles, about 10% of surface area spalled. While for APS-TBCs, as shown in Fig. 3b, after 84 cycles, some white spots were found on the center of the APS-samples, which may be
Table 3 Average velocity and surface temperature of in-flight YSZ particles measured by Spray watch 2i system. In-flight properties Average velocity (m/s) Average surface temperature (°C)
SAPS
APS
424
180
3089
2814
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Table 4 Porosity of top YSZ coat and bonding strength between top YSZ coat and underlying bond CoNiCrAlY coat. Methods
SAPS APS
Porosity of top YSZ coat (%) Image analysis
Archimedean principle
5±2 9±3
8±3 13 ± 4
Bonding strength (MPa) 47 ± 7 36 ± 5
attributed to a small mount of YSZ top coat delaminated and spalled from the surface of samples. As the thermal cycles increased, the area of white spots on the center of the sample continuously increased, and after 100 cycles, obvious surface spallation of the coating, as noted by an arrow, can be easily observed on the right edge of Fig. 3b, and about 10% of surface area spalled after 141 cycles. It should be noted here that for convenient comparison of thermal shock resistance of SAPSand APS-coatings, the statistical spalled area (measured by software) referred to YSZ top coat fully spalled from the surface of samples, which was always observed on the fringe of samples due to the extreme heating and cooling conditions encountered at the edges, as shown in Fig. 3a and b, respectively. The above results demonstrated that, compared with conventional APS-TBCs, the thermal cycling lives of SAPS-TBCs were improved dramatically, about 90% higher than that of APS-TBCs. Meanwhile, as compared with the reported work of other authors, the thermal cycling lives of SAPS-TBCs also greatly improved [5,28]. Fig. 4 shows the SEM micrographs for polished cross-section of SAPSand APS-coating after 100 thermal cycles. As shown in Fig. 4a, after being quenched from 1100 °C to water at room temperature for 100 times, only one obvious vertical crack (normal to the surface of SAPS-sample) and fine horizontal cracks were observed and the vertical crack did not penetrate the whole top coat and stopped in the area above the top coat/ bond coat interface, whereas as seen from Fig. 4b, after the same thermal cycles, a large amount of coarse cracks parallel to the surface of APSsample were found. These cracks were fatal to the coating during thermal cycles, since they led to the delamination and spallation of top YSZ coat, as shown in Fig. 4b, but no obvious separation along the interface between the top coat and bond coat was observed. Figs. 3 and 4 demonstrated that the spallation process of coating during thermal cycles was progressive and the surface of the top coat continuously delaminated and spalled during the course of thermal cycling. Damage progression was by micro-cracking extension followed by linking-up, and then large crack propagation until a large surface area of coating spalled. As mentioned above, as compared with conventional APS-TBCs,
Fig. 4. SEM micrographs for polished cross-section of SAPS- and APS-coatings after 100 thermal cycles: (a) SAPS-coating, (b) APS-coating.
the thermal cycling lives of SAPS-TBCs were improved dramatically. It was well known that the thermal shock resistance of the as-sprayed coating is related to the microstructures. Homogeneous distribution of fine pores or pre-existing cracks could act as the tensile stress release center, making the coating more tolerant against stress, remarkably
Fig. 3. Surface morphology of SAPS- and APS-coatings after different thermal cycles: (a) SAPS-coatings, (b) APS-coatings.
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reducing the driving force for horizontal crack extension in the coatings [29,30]. As discussed above, comparing the microstructure of the two assprayed coatings, it can be found that the SAPS-TBCs had thinner welladhered lamellar structures and homogeneously distributed finer preexisting cracks and pores, as shown in Fig. 2, the presence of such a microstructure can effectively increase the strain tolerance and enhance the thermal shock resistance for TBCs during thermal cycles [1]. Meantime, the rise of flattening degree led to the increase of effective bonding surface between splats, preventing the delamination of lamella structures and premature failure on the poorly bonded regions (observed in Fig. 4b). As a result, the thermal shock resistance of SAPS-TBCs was improved greatly.
3.3. Oxidation behavior of coating during thermal cycles The CoNiCrAlY bond coat is an important layer of TBC systems. It enhances the adhesion of the YSZ top coat to the substrate and also provides oxidation protection to the Ni-based substrate. At elevated temperatures, however, oxidation of the bond coat results in the formation of a thermally grown oxide (TGO) layer at the original top/ bond coat interface [31,32]. Fig. 5 shows cross-sectional backscattered electron images for the morphology and thickness of TGO of SAPS- and APS-coatings after different thermal cycles, respectively. As shown in Fig. 5a and b, the TGO layer for SAPS-sample was compact and consisted of two layers, the top gray layer and the underlying black one. EDX analysis showed that the main constituent oxide of the black layer was Al2O3 and the gray layer was composed of mixed oxides, and these mixed oxides mainly included NiO and Ni(Cr,Al)2O4 spinel, resulted from the diffusion of Ni and Cr atoms from the bond coat to the top of the TGO layer and the formation of some simple and/or complex oxides, such as spinel phases at high temperature [31–33]. Meanwhile, the measured mean thickness of the TGO layer was 1.4 μm after 100 thermal cycles and it increased to 1.9 μm after 265 thermal cycles, the average growth rate was about 3 nm/cycles. As compared with SAPSsample, the TGO for APS-sample was comparatively loose and ununiform, and the measured mean thickness was 1.6 μm after 100 thermal cycles, slightly higher than that of SAPS-sample after the same cycles. But the mean thickness of TGO increased dramatically to about 2.4 μm after 141 cycles, and the average growth rate was about 20 nm/cycles. The above results indicated that, as compared with APScoatings, the SAPS-coatings had a lower oxidation rate. It is well known that the zirconia helps to transport oxygen from outside to the bond coat by two mechanisms: ionic transportation through the lattice by reverse movement of oxygen ion vacancies, and gaseous diffusion along the networks of interconnected cracks and pores [34]. SAPS process was characterized by higher velocity of particles, which produced a comparatively denser YSZ top coat than the APS-coatings. So, at the high temperature, the SAPS-coating with lower porosity and fine lamellar structures, as shown in Fig. 2, could play an important role in decreasing the diffusive channels for oxygen and reducing the oxygen pressure at the top/bond coat interface, thus hindering the diffusion of oxygen into the bond coat and reducing the growth rate of TGO [35]. Moreover, it is well accepted that the morphology and the composition of the top TGO layer are crucial for the performance of TBC coatings and the onset of the growth of NiO and spinel phases (Ni (Cr,Al)2O4) is more damaging because these exhibit a very high growth rate, which rapidly increased the volume of the bond coat [32,36]. In addition, some studies revealed that extensive crack initiation and propagation along TGO and bond coat interface could lead to the failure of the TBCs when the TGO attained a critical thickness [37–39]. So, the lower growth rate of TGO also helped to improve the thermal shock resistance of TBCs system and the result was that the SAPS-coatings had much higher thermal cycling lives, about 90% higher than the APS-coatings.
Fig. 5. Cross-sectional backscattered electron images for the morphology and thickness of TGO layer between top coat and bond coat by the SAPS and APS after different thermal cycles: (a) SAPS-coating after 100 thermal cycles, (b) SAPS-coating after 265 thermal cycles, (c) APS-coating after 100 thermal cycles, (d) APS-coating after 141 thermal cycles.
3.4. Phase change of coating after thermal cycles X-ray phase analysis carried out on the coatings surface after different thermal cycles is shown in Fig. 6. As seen from it, both the SAPS- and APS-coatings showed a predominant, non-transformable, tetragonal phase structure (t′) as a result of rapid solidification. Furthermore, as seen from Fig. 6a, the SAPS-coatings with only a small amount of calcium zirconium oxide (CaZrO3) was found after 265 thermal cycles while the APS-coatings showed very small percentage of CaZrO3 the after 95 thermal cycles and it increased its percentage (by comparing the intensity of peak at approximately 45.7°) after 141 cycles when about 10% surface area spalled from the edge of samples, as shown in Fig. 6b. This result can be explained by that the calcium which was present in the tap water reacted with zirconium during the quenching operation, and formed CaZrO3. This CaZrO3 formed at the surface of the coating is orthorhombic in structure, and
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thickness of the lamellar structure in SAPS-coating was 2.5 ± 0.6 μm, while that of APS-coating was 5.3 ± 0.9 μm. The desirable structure for SAPS-samples was attributed to higher impact velocity of in-flight particles during SAPS process, resulting in the improvement of flattening degree of molten particles. The well-adhered fine lamellar structures, fine micro-cracks and lower growth rate of TGO appeared to be responsible for greatly improved thermal cycling lives of SAPS-TBCs as compared to their conventional plasma sprayed counterparts. The results of water-quenching test from 1100 °C into room temperature showed that SAPS-TBCs presented high thermal shock resistance, only 10% coating area spalled after 265 thermal cycles, about 90% higher than that of APS-samples. During the thermal cycles, the transformation from tetragonal to monoclinic ZrO2 was not observed. Acknowledgements This work was supported by the National Basic Research Program (Grant No. 2007CB707702). The authors would like to thank Prof. B.J. Ding (Xi'an Jiaotong University, China) and technician Y.M. Qiang (Xi'an Jiaotong University, China) for their suggestions and help. References
Fig. 6. X-ray spectra of coatings after different thermal cycles: (a) SAPS-coatings, (b) APScoatings.
it played a role in the delamination of the surface of the top coat [5]. In general, there are two types of tetragonal phase, such as a high stabilizer non-transformable tetragonal phase (t′) and a low stabilizer transformable tetragonal phase (t). After long-term thermal cycling, the metastable non-transformable tetragonal phase may decompose into the low stabilizer tetragonal phase by diffusion of the stabilizer such as Y2O3, and the low stabilizer tetragonal phase may transform to monoclinic phase induced by thermal stress. The presence of the monoclinic phase may reduce the useful life of TBCs owing to the large volume increase of about 3–4% associated with the tetragonal– monoclinic phase transformation [33]. However, in this work, no peak for monoclinic phase was observed in both SAPS- and APS-coatings, indicating that the phase transformation from tetragonal to monoclinic ZrO2 did not occur. 4. Conclusions The microstructure and thermal shock resistance of the ZrO2–Y2O3 thermal barrier coatings (TBCs) fabricated by high efficiency supersonic atmospheric plasma spraying (SAPS) and conventional atmospheric plasma spraying (APS), respectively, were comparatively studied in the present work. As compared to APS-TBCs with the same composition, the microstructure of SAPS-TBCs was much finer. It was found that the thickness of lamellar structure consisted of columnar crystals in the SAPS- and APS-coating was in the range of 1–4 μm and 2–8 μm, respectively. Besides, the statistical results revealed that the average
[1] N.P. Padture, M. Gell, E.H. Jordan, Science 296 (2002) 280. [2] C. Zhou, N. Wang, Z.B. Wang, S.K. Gong, H.B. Xu, Scr. Mater. 51 (2004) 945. [3] N.P. Padture, K.W. Schlichting, T. Bhatia, A. Ozturk, B. Cetegen, E.H. Jordan, M. Gell, S. Jiang, T.D. Xiao, P.R. Strutt, E. Garcia, P. Miranzo, M.I. Osendi, Acta Mater. 49 (2001) 2251. [4] P.L. Ke, Q.M. Wang, J. Gong, C. Sun, Y.C. Zhou, Mater. Sci. Eng. A 435–436 (2006) 228. [5] A.N. Khan, J. Lu, Surf. Coat. Technol. 166 (2003) 37. [6] A. Rabiei, A.G. Evans, Acta Mater. 48 (2000) 3963. [7] G. Johner, K.K. Schweitzer, Thin Solid Films 119 (1984) 301. [8] S. Rangaraj, K. Kokini, Acta Mater. 52 (2004) 455. [9] X.C. Zhang, B.S. Xu, Y.X. Wu, F.Z. Xuan, S.T. Tu, App. Surf. Sci. 254 (2008) 3879. [10] X.C. Zhang, B.S. Xu, S.T. Tu, F.Z. Xuan, H.D. Wang, Y.X. Wu, App. Surf. Sci. 254 (2008) 6318. [11] L.Z. Du, B.S. Xu, S.Y. Dong, W.G. Zhang, J.M. Zhang, H. Yang, H.J. Wang, Surf. Coat. Technol. 202 (2008) 3709. [12] Z.H. Han, B.S. Xu, H.J. Wang, S.K. Zhou, Surf. Coat. Technol. 201 (2007) 5253. [13] J.F. Coudert, M.P. Planche, O. Betoule, M. Vardelle, P. Fauchais, Proceedings of the 1993 National Thermal Spray Conference, Anaheim, CA, June 7–11, 1993, ASM International, Metals Park, OH, 1993, p. 19. [14] S. Zhu, B.S. Xu, J.K. Yao, Mater. Sci. Forum 475–479 (2005) 3981. [15] D.J. Varacalle Jr., L.B. Lundberg, H. Herman, G. Bancke, Surf. Coat. Technol. 86–87 (1996) 70. [16] J.R. Davis, Introduction to thermal spray processing, Handbook of thermal technology, , 2004, p. 71. [17] X.C. Zhang, B.S. Xu, F.Z. Xuan, H.D. Wang, Y.X. Wu, S.T. Tu, J. Alloy. Compd. 467 (2009) 501. [18] J.C. Fang, W.J. Xu, Z.Y. Zhao, H.P. Zeng, Surf. Coat. Technol. 201 (2007) 5671. [19] H. Du, J.H. Shin, S.W. Lee, J. Therm. Spray Technol. 14 (2005) 453. [20] A.D. Jadhav, N.P. Padture, E.H. Jordan, M. Gell, P. Miranzo, E.R. Fuller Jr., Acta Mater. 54 (2006) 3343. [21] N. Berger-Keller, G. Bertrand, C. Filiatre, C. Meunier, C. Coddet, Surf. Coat. Technol. 168 (2003) 281. [22] G. Trapaga, J. Szekely, Metall. Trans. B 22 (1991) 901. [23] M. Bertagnolli, M. Marchese, G. Jacucci, J. Therm. Spray Technol. 4 (1995) 41. [24] H. Liu, E.J. Lavemia, R.H. Rangel, J. Therm. Spray Technol. 2 (1993) 369. [25] S. Yugeswaran, V. Selvarajan, D. Seo, K. Ogawa, Surf. Coat. Technol. 203 (2008) 129. [26] R. Mcpherson, Surf. Coat. Technol. 39/40 (1989) 173. [27] A. Kulkarni, A. Vaidya, A. Goland, S. Sampath, H. Herman, Mater. Sci. Eng. A. 359 (2003) 100. [28] P.L. Ke, Y.N. Wu, Q.M. Wang, J. Gong, C. Sun, L.S. Wen, Surf. Coat. Technol. 200 (2005) 2271. [29] B. Liang, C.X. Ding, Surf. Coat. Technol. 197 (2005) 185. [30] B. Zhou, K. Kokini, Acta Mater. 52 (2004) 4189. [31] Z.M. Li, S.Q. Qian, W. Wang, J.H. Liu, J. Alloy. Compd. 505 (2010) 675. [32] L. Ajdelsztajn, J.A. Picas, G.E. Kim, F.L. Bastian, J. Schoenung, V. Provenzano, Mater. Sci. Eng. A 338 (2002) 33. [33] C.H. Lee, H.K. Kim, H.S. Choi, H.S. Ahn, Surf. Coat. Technol. 124 (2000) 1. [34] A. Bennett, Mater. Sci. Technol. 2 (1986) 257. [35] Y.N. Wu, F.H. Wang, W.G. Hua, J. Gong, C. Sun, L.S. Wen, Surf. Coat. Technol. 166 (2003) 189. [36] R.D. Maier, C.M. Scheuermann, C.W. Andrews, Am. Ceram. Soc. Bull. 60 (5) (1981) 555. [37] W.R. Chen, X. Wu, B.R. Marple, P.C. Patnaik, Surf. Coat. Technol. 201 (2006) 1074. [38] M. Subanovic, P. Song, E. Wessel, R. Vassen, D. Naumenko, L. Singheiser, W.J. Quadakkers, Surf. Coat. Technol. 204 (2009) 820. [39] F. Cao, B. Tryon, C.J. Torbet, T.M. Pollock, Acta Mater. 57 (2009) 3885.
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Structure–property differences between supersonic and conventional atmospheric plasma sprayed zirconia thermal barrier coatings Y. Bai a, Z.H. Han a,⁎, H.Q. Li a, C. Xu a, Y.L. Xu a, C.H. Ding b, J.F. Yang a a b
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China School of Aerospace, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China
a r t i c l e
i n f o
Article history: Received 20 October 2010 Accepted in revised form 25 January 2011 Available online 31 January 2011 Keywords: Supersonic atmospheric plasma spraying ZrO2 Thermal barrier coatings Thermal cycling lives Thermally grown oxide
a b s t r a c t Yttria-stabilized zirconia (YSZ) based thermal barrier coatings (TBCs) were deposited by high efficiency supersonic atmospheric plasma spraying (SAPS) system. The microstructure and thermal shock resistance of the SAPS-TBCs were investigated. As compared to conventional atmospheric plasma sprayed TBCs (APS-TBCs) with the same composition, the microstructure of SAPS-TBCs was much finer. It was found that the thickness of lamellar structure consisted of columnar crystals in the SAPS- and APS-coatings was in the range of 1–4 μm and 2–8 μm, respectively. Besides, the statistical results revealed that the average thickness of the lamellar structure in SAPS-coating was 2.5 ± 0.6 μm, while that of APS-coating was 5.3 ± 0.9 μm. The desirable structure was attributed to higher impact velocity of in-flight particles during SAPS process, which resulted in the improvement of flattening degree of molten particles after impinging on the target. The well-adhered fine lamellar structures, fine micro-cracks and lower growth rate of thermally grown oxide (TGO) appeared to be responsible for greatly improved thermal cycling lives of SAPS-TBCs as compared to their conventional plasma sprayed counterparts. The results of water-quenching test from 1100 °C into room temperature showed that SAPS-TBCs presented high thermal shock resistance, only 10% coating area spalled after 265 thermal cycles, about 90% higher than that of APS-TBCs. The SAPS method, which offered some unique advantages over the conventional plasma spraying process, is expected to be potentially used to deposit high-performance TBCs at lower cost. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Thermal barrier coatings (TBCs), serving as heat-insulating layer on the surfaces of metallic components, have been widely used in the field of gas-turbine engines for aircraft propulsion and power generation. Using TBCs on the surface of superalloy components, the surface temperature of the superalloy can be reduced as much as 100– 300 °C. This has enabled modern gas-turbine engines to operate at gas temperatures well above the melting temperature of the superalloy (1300 °C), thereby improving engine efficiency and performance [1]. A typical layered TBC system consists of a thermally insulating ceramic top coating applied over an oxidation-resistant metallic bond coating. 6–8 wt.% yttria partially stabilized zirconia (PSZ) coatings were commonly used as top coating due to their high temperature stability and low thermal conductivity. MCrAlY (M = Ni, and/or Co) is often used as a bond coat to provide a good thermal expansion match between the top coat and substrate and to prevent oxidation of the substrate [2,3].
⁎ Corresponding author. Tel.: + 86 2982668614; fax: + 86 29 82663453. E-mail address: [email protected] (Z.H. Han). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.01.056
However, a major problem in TBCs applied to gas turbine components is the spallation of ceramic coat under thermal cycling processes. Thus, thermal shock behavior is an important indication of TBCs performance. The failure mode during the thermal cycling is always associated with the initiation and propagation of cracks in the top PSZ coating and at the interface between top coat and thermally grown oxide (TGO), typically α-Al2O3, formed at the interface of the bond coat and top ceramic coat during high-temperature exposure [4,5]. The two commercial processes used for the deposition of TBCs are air plasma spraying (APS) and electron-beam physical-vapor deposition (EB-PVD). Compared with EB-PVD process, plasma spraying is a low-cost method and results in ubiquitous “splat” boundaries and cracks running parallel to the bond coat/top coat interface in the as-sprayed coatings. These cracks parallel to the bond coat/top coat interface can effectively reduce the high-temperature thermal conductivity (0.8–1.7 W/m K), however, leave the top YSZ coat relatively shorter thermal-cycling lives [1,6]. So, since the 1980s, great attention has been given to optimize the microstructure of plasma-sprayed TBCs in order to improve their thermal durability and thermal-cycling lives [7,8]. Recently, high efficiency supersonic atmospheric plasma spray (SAPS) process has been successfully developed to deposit ceramic and metallic coatings with excellent performance [9–12]. For
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conventional air plasma spray (APS) guns, the gas velocities of 300– 800 m/s and velocities of the particles in the range of 130–220 m/s were obtained with 5–8 mm diameter nozzles and 50–75 L/min total gas flow rates [10,13]. In contrast, using the SAPS gun, the velocities of the plasma jet and particles of 2400 m/s and 500 m/s can be achieved at the normal distance of 100 mm from the nozzle exit [10,14]. Another important plasma spraying process is vacuum plasma spraying (VPS) and the stream of high velocity molten particles (typically two to three times as high as that in APS [15]) can also be obtained during VPS. However, the low pressure produces significant expansion of the isotherms and velocity contours of the plasma jet, due to the increased mean free path between the ions and electrons in the plasma [16]. As compared with APS, VPS requires higher power and longer spray distances for the varied materials to ensure complete melting of particles. In the process of SAPS, the stream of high velocity molten particles can be obtained in the highenergy plasma jet, which is exposed to the atmosphere. The high-energy plasma jet results from the structural design of SAPS gun with a Laval nozzle. The SAPS gun makes the cooling water channels with good thermal pinching effect and the working gases are injected into the nozzle via gas vortex ring in order to pinch the arc and increase the life of plasma gun [14]. Besides, in order to heat and accelerate the powder particles adequately, the powder was injected into the plasma jet via an internal injection port (positioned inside the nozzle). So, as compared with VPS, SAPS is expected to deposit some high-performance coatings in the atmosphere, which could reduce the cost effectively. Since the molten particles with higher kinetic energy will form thinner flakes when they are quenched onto the substrate, the coatings deposited by SAPS system may have finer multilayered microstructure, higher adhesive strength and lower porosity as compared to those of coatings deposited by conventional plasma spraying system. In this work, YSZ-based TBCs were deposited by SAPS system and the microstructure and thermal shock resistance were investigated.
2. Experimental
Fig. 1. Starting powders used for plasma spraying: (a) CoNiCrAlY powder, (b) ZrO2–8wt.% Y2O3 (YSZ) powder.
2.1. Materials 2.3. Particle in-flight properties A nickel-base super alloy, GH3030, previously sand-blasted with alumina particles, was used as the substrate with diameter of 20 mm and thickness of 10 mm. A commercially available CoNiCrAlY powder (AMDRY 995M, Sulzer Metco Inc. USA) with nominal composition of Ni-32, Cr-21, Al-8, Y-0.4 Co-balance (wt.%) was used for spraying the bond coat of TBCs. The spherical powders were gas atomized and the particle size was about 65 μm, as shown in Fig. 1a. The ceramic powder used for top coat was commercially spray dried and sintered ZrO2–8wt.%Y2O3 with diameter ranging from 40 to 110 μm, as shown in Fig. 1b.
During spraying, a commercially available Spray Watch 2i system (made by Osier, Finland) was used to monitor the velocity and surface temperature of in-flight particles for each operating conditions. In this system, velocity of individual particle was measured by time-of-flight method, ranging from 10 to 1000 m/s with the resolution 0.5–5 m/s. Particle surface temperature was measured by two-color pyrometry, ranging from 1000 to 4000 °C with the resolution about 5 °C. The number of detected particles was about 2000 for one measurement independently and the final value was the average of five measurements. It should be noted that for the determination of the particle
2.2. Plasma spraying process The thermal barrier coatings, composed of a bond CoNiCrAlY coat (about 60 μm thick) and a top coat (about 250 μm thick) , were deposited by a Metco 9M air plasma spraying (APS) system and high efficiency supersonic atmospheric plasma spraying (SAPS) system, respectively. The advanced SAPS system was developed by the National Key Laboratory for Remanufacturing, China, which consisted of plasma torch, powder feeder, gas supply and water cooling circulator. The key advantage of this system was a novel SAPS gun with a Laval nozzle. The length and exit diameter of nozzle were 44 mm and 6 mm, respectively. The powder was injected into the plasma jet by an internal injection port with inlet diameter of 2 mm, which was inside the nozzle and directed perpendicular to the plasma jet. The detailed information about SAPS system can be found elsewhere [17]. The spraying parameters are presented in Table 1.
Table 1 Spraying parameters for SAPS and APS top coat and bond coat. Spraying parameters
Gun nozzle inner diameter (mm) Powder injection port inlet diameter (mm) Current (A) Voltage (V) Primary gas Ar (slpm) Second gas H2 (slpm) Carrier argon gas flow rate (slpm) Gun traverse speed (mm/s) Powder feed rate (g/min) Spray distance (mm) a b
SAPS
APS
YSZ
CoNiCrAY YSZ
6 2a 405 160 60 17 7.5 800 40 100
6 2a 363 124 65 8 8 800 40 100
Powder injection port is inside the nozzle. Powder injection port is outside the nozzle.
CoNiCrAlY
7 7 2b 2b 650 500 74 66 45 80 12 6 8 8.5 800 800 40 40 100 100
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temperature diagnostic systems were restricted to the analysis of surface temperature of the particles and the details about testing principle were reported in other reference [18].
2.4. Specimens characterization Crystal structures of the as-sprayed coatings were determined by means of X-ray diffraction (XRD) on a Rigaku X-RAY diffractometer with Cu Kα radiation. The microstructure and chemical composition of samples were examined by TESCAN-VEGAII XUM scanning electron microscope (SEM) equipped with INCA-AE350 energy dispersive spectrometer (EDX). Before specimen sectioning, the surface of top YSZ coat was coated by two-part thermosetting resin in order to protect the microstructure of samples from damage during cutting afterwards. After the resin was cured at 120 °C for 1 h, cross-sections were produced by wet diamond sawing at lower speed in order to minimize damage. One side was ground on 600-grit silicon carbide papers, followed by sequential polishing on soft cloths until no obvious scratches were observed by optical microscopy. At present, the two most common methods used to evaluate the porosity value in the coating are mercury intrusion porisimetry (MIP) and image analysis (IA) [17]. MIP is a quantitative porosity measurement method but it is very difficult to determine the true porosity value due to the presence of closed pores in the coating. IA is somewhat subjective, but it is suitable for a relative comparison among different coatings [17,19,20]. In this work, porosity of the coatings was estimated by Archimedean (water displacement) method and quantitative image analysis (IA) using a picture analysis system in scanning electron microscopy. In the Archimedean method, the top YSZ coat was firstly separated from the metallic substrate by submerging the samples in 40% hydrochloric acid until the metallic bond coat was dissolved. Then the free-standing YSZ top coat was cleaned and dried. Before measurement, the samples were boiled in distilled water for 2 h to facilitate pore penetration. A detailed description of Archimedean technique used to determine the porosity of coating was available in the literature [21]. In the image analysis, to get a representative porosity value, a series of SEM images with a magnification of 1000× were obtained for individual specimens. The resolution of image was 600 dpi and the minimum area of detectable voids was about 0.1 μm2. Twenty micrographs were randomly taken from cross-section of each coating. In addition, the thickness of splats in the as-sprayed coatings was measured by software (Image-Pro Plus). Twenty SEM images of fractured cross section with a magnification of 4000× and resolution of 600 dpi were randomly taken from the as-sprayed coatings, and the final value was the average of sixty measurements.
2.5. Bonding strength and thermal shock test The bonding strength of the as-sprayed coatings was measured using a materials tester (Instron1196, USA) in accordance with ASTM C 633-79 standard. A rod made of Nickel-base superalloy, GH3030, was used as the substrate with diameter of 25.4 mm. Film epoxy adhesive (FM-1000, USA) with tensile fracture strength more than 60 MPa was applied. The final value represented the average value of 5 samples sprayed at the same parameters. Thermal shock tests were conducted by using a muffle furnace. When the temperature inside the furnace reached to 1100 °C, the samples were pushed into the furnace. The holding time at the temperature was 5 min, then the samples were directly quenched into water, the temperature of the water throughout the cycling was between 20 and 30 °C. More than 10% of the spalled region of the surface of the top coating (quantitatively calculated by image analysis software Image Tool 3.0) was adopted as criteria for the failure of the coating.
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3. Results and discussion 3.1. Microstructure of as-sprayed coating Fig. 2 shows fractured and polished cross-sectional micrographs for as-sprayed top YSZ coatings fabricated by SAPS and conventional APS, respectively. As seen from Fig. 2a and b, typical lamella structure and columnar structure within the individual splats were visible in both coatings, since rapid nucleation occurred at the cooler surface of the flattened droplet at large under-cooling and crystals grew rapidly in the opposite direction to the heat flow forming a columnar grain structure. However, as compared Fig. 2a with Fig. 2b, it was found that the thickness of lamellar structure consisted of columnar crystals in the SAPS- and APS-coating was in the range of 1–4 μm and 2–8 μm, respectively. Besides, the statistical results revealed that the average thickness of the lamellar structure in SAPS-coating was 2.5 ± 0.6 μm, while that of APS-coating was 5.3 ± 0.9 μm. Moreover, a large amount of coarse inter-lamellar cracks were observed in Fig. 2b, while only a few fine cracks were found in Fig. 2a. The above results indicated that the SAPS-coating had a finer microstructure as compared with the APS-coating. Besides, as shown in Fig. 2c and d, for SAPS-coating, micro-cracks (as marked by black arrows shown in Fig. 2c) and fine pores were homogeneously distributed in the top YSZ coat. Whereas, a large amount of coarse pores and cracks were observed in the APScoating which could be rarely found in the SAPS-coating, further indicating that the SAPS-coating had a finer microstructure as compared with its APS counterpart. In plasma spraying, the microstructure of the coatings was strongly dependent on processing conditions. The splats were separated by inter-lamellar pores that resulted from rapid solidification of the lamellae, pores formed by incomplete contact between the splats or around unmelted particles, and cracks arising from the relaxation of thermal or tensile quenching stresses. The microstructure differences between SAPS- and APS-coatings can be explained by the following model. During the plasma spraying process, the powders were injected into the plasma jet, where they should be melted (droplet) and accelerated against the substrate to form lamella structure. The flattening degree, which is defined as the ratio of the diameter of the lamella to the diameter of starting droplet, has a close relationship with the particle impact velocity. The following formula was used to describe this relationship [22–24]. b
ξ = aRe
ð1Þ
Re ¼ρdv = η
ð2Þ
where ξ is the flattening degree, Re is the Reynolds number, a and b are coefficients, and ρ, d, v and η are the density, diameter, velocity and viscosity of the molten droplet, respectively. Table 2 summarizes the coefficients reported by different researchers. As shown in Table 2, although the values of a and b provided by many investigators were different, the flattening degree of lamella increased with the rise of particle impact velocity. The velocity and surface temperature of inflight particles measured by Spray watch 2i system for each operating conditions were shown in Table 3. As seen from Table 3, the average in-flight velocity of YSZ particles in SAPS was 424 m/s, much higher than that in APS 180 m/s. Moreover, the surface temperature of inflight particles in SAPS was 3089 °C, a little higher than that in APS, 2814 °C. The great improvement of velocity was attributed to the structural optimization of SAPS gun and internal powder injection, which made in-flight particles acquire the higher energy and momentum in plasma jet. So, the SAPS-coating had a higher flattening degree of lamella as compared with the APS-coating. The bonding strength between top coat and bond coat and the porosity of top coat of as-sprayed SAPS- and APS-coating are listed in Table 4. As shown in it, the average bonding strength of SAPS-coating
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Fig. 2. Fractured and polished cross-sectional micrographs for as-sprayed top YSZ coatings fabricated by SAPS and conventional APS process: (a) fractured cross section of SAPScoating, (b) fractured cross section of APS-coating, (c) polished cross section of SAPS-coating, (d) polished cross section of APS-coating.
was 47 MPa, about 31% higher than that of APS-coating. Meanwhile, as seen from Table 4, the porosity of SAPS-coating measured by image analysis and Archimedean principle was about 5% and 8% respectively, whereas the porosity of APS-coating obtained from the above two methods was about 9% and 13% respectively. So, as compared with APS-coating, SAPS-coating was much denser. In addition, the porosity measured by Archimedean method was higher than that obtained from the image analysis. A similar phenomenon was also found elsewhere [25]. This may be attributed to the fact that some micro cracks or fine pores were not discernable by image analysis. For SAPScoating, the increase of bonding strength and decrease of porosity are ascribed to the rise of flattening degree. The rise of flattening degree led to the increase of effective bonding surface between the lamella structure and the under bond coat (CoNiCrAlY), and the decrease of inter-lamellar pores resulted from the poor wetting/adhesion between the lamellae as they accumulate to form the coating. The result was that the SAPS-coating had a higher bonding strength and lower porosity. Generally, the mechanical properties of coating are degraded
Table 2 Coefficients reported for relationshipξ = aReb by different investigators. Coefficient (a)
Coefficient (b)
Methods
References
1
0.2
[22]
0.925
0.2
1.04
0.2
Simulated by mathematical modeling Simulated by finite-element method Numerical simulation
[23] [24]
by the porosity, for example, brittle fracture is extremely sensitive to defect size and the strength is determined by the largest crack-like defect [26]. On the other hand, some studies revealed that the thermal conductivity of coating was reduced by the presence of pores, which could reduce the effective cross-sectional area of the materials [26,27]. So, as compared with APS-coatings, the SAPS-coatings with lower porosity may have higher toughness, strength, and higher thermal conductivity. This needs to be further studied. 3.2. Thermal shock resistance The thermal shock resistance of SAPS- and APS-coating was estimated by water-quenching cycling number. Fig. 3 shows the surface morphology of SAPS- and APS-coatings after different thermal cycles. As seen from Fig. 3a, the evidently visible spalled area on the fringe of SAPS coating was found after 180 cycles (noted by an arrow), and sustained up to 265 cycles, about 10% of surface area spalled. While for APS-TBCs, as shown in Fig. 3b, after 84 cycles, some white spots were found on the center of the APS-samples, which may be
Table 3 Average velocity and surface temperature of in-flight YSZ particles measured by Spray watch 2i system. In-flight properties Average velocity (m/s) Average surface temperature (°C)
SAPS
APS
424
180
3089
2814
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Table 4 Porosity of top YSZ coat and bonding strength between top YSZ coat and underlying bond CoNiCrAlY coat. Methods
SAPS APS
Porosity of top YSZ coat (%) Image analysis
Archimedean principle
5±2 9±3
8±3 13 ± 4
Bonding strength (MPa) 47 ± 7 36 ± 5
attributed to a small mount of YSZ top coat delaminated and spalled from the surface of samples. As the thermal cycles increased, the area of white spots on the center of the sample continuously increased, and after 100 cycles, obvious surface spallation of the coating, as noted by an arrow, can be easily observed on the right edge of Fig. 3b, and about 10% of surface area spalled after 141 cycles. It should be noted here that for convenient comparison of thermal shock resistance of SAPSand APS-coatings, the statistical spalled area (measured by software) referred to YSZ top coat fully spalled from the surface of samples, which was always observed on the fringe of samples due to the extreme heating and cooling conditions encountered at the edges, as shown in Fig. 3a and b, respectively. The above results demonstrated that, compared with conventional APS-TBCs, the thermal cycling lives of SAPS-TBCs were improved dramatically, about 90% higher than that of APS-TBCs. Meanwhile, as compared with the reported work of other authors, the thermal cycling lives of SAPS-TBCs also greatly improved [5,28]. Fig. 4 shows the SEM micrographs for polished cross-section of SAPSand APS-coating after 100 thermal cycles. As shown in Fig. 4a, after being quenched from 1100 °C to water at room temperature for 100 times, only one obvious vertical crack (normal to the surface of SAPS-sample) and fine horizontal cracks were observed and the vertical crack did not penetrate the whole top coat and stopped in the area above the top coat/ bond coat interface, whereas as seen from Fig. 4b, after the same thermal cycles, a large amount of coarse cracks parallel to the surface of APSsample were found. These cracks were fatal to the coating during thermal cycles, since they led to the delamination and spallation of top YSZ coat, as shown in Fig. 4b, but no obvious separation along the interface between the top coat and bond coat was observed. Figs. 3 and 4 demonstrated that the spallation process of coating during thermal cycles was progressive and the surface of the top coat continuously delaminated and spalled during the course of thermal cycling. Damage progression was by micro-cracking extension followed by linking-up, and then large crack propagation until a large surface area of coating spalled. As mentioned above, as compared with conventional APS-TBCs,
Fig. 4. SEM micrographs for polished cross-section of SAPS- and APS-coatings after 100 thermal cycles: (a) SAPS-coating, (b) APS-coating.
the thermal cycling lives of SAPS-TBCs were improved dramatically. It was well known that the thermal shock resistance of the as-sprayed coating is related to the microstructures. Homogeneous distribution of fine pores or pre-existing cracks could act as the tensile stress release center, making the coating more tolerant against stress, remarkably
Fig. 3. Surface morphology of SAPS- and APS-coatings after different thermal cycles: (a) SAPS-coatings, (b) APS-coatings.
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reducing the driving force for horizontal crack extension in the coatings [29,30]. As discussed above, comparing the microstructure of the two assprayed coatings, it can be found that the SAPS-TBCs had thinner welladhered lamellar structures and homogeneously distributed finer preexisting cracks and pores, as shown in Fig. 2, the presence of such a microstructure can effectively increase the strain tolerance and enhance the thermal shock resistance for TBCs during thermal cycles [1]. Meantime, the rise of flattening degree led to the increase of effective bonding surface between splats, preventing the delamination of lamella structures and premature failure on the poorly bonded regions (observed in Fig. 4b). As a result, the thermal shock resistance of SAPS-TBCs was improved greatly.
3.3. Oxidation behavior of coating during thermal cycles The CoNiCrAlY bond coat is an important layer of TBC systems. It enhances the adhesion of the YSZ top coat to the substrate and also provides oxidation protection to the Ni-based substrate. At elevated temperatures, however, oxidation of the bond coat results in the formation of a thermally grown oxide (TGO) layer at the original top/ bond coat interface [31,32]. Fig. 5 shows cross-sectional backscattered electron images for the morphology and thickness of TGO of SAPS- and APS-coatings after different thermal cycles, respectively. As shown in Fig. 5a and b, the TGO layer for SAPS-sample was compact and consisted of two layers, the top gray layer and the underlying black one. EDX analysis showed that the main constituent oxide of the black layer was Al2O3 and the gray layer was composed of mixed oxides, and these mixed oxides mainly included NiO and Ni(Cr,Al)2O4 spinel, resulted from the diffusion of Ni and Cr atoms from the bond coat to the top of the TGO layer and the formation of some simple and/or complex oxides, such as spinel phases at high temperature [31–33]. Meanwhile, the measured mean thickness of the TGO layer was 1.4 μm after 100 thermal cycles and it increased to 1.9 μm after 265 thermal cycles, the average growth rate was about 3 nm/cycles. As compared with SAPSsample, the TGO for APS-sample was comparatively loose and ununiform, and the measured mean thickness was 1.6 μm after 100 thermal cycles, slightly higher than that of SAPS-sample after the same cycles. But the mean thickness of TGO increased dramatically to about 2.4 μm after 141 cycles, and the average growth rate was about 20 nm/cycles. The above results indicated that, as compared with APScoatings, the SAPS-coatings had a lower oxidation rate. It is well known that the zirconia helps to transport oxygen from outside to the bond coat by two mechanisms: ionic transportation through the lattice by reverse movement of oxygen ion vacancies, and gaseous diffusion along the networks of interconnected cracks and pores [34]. SAPS process was characterized by higher velocity of particles, which produced a comparatively denser YSZ top coat than the APS-coatings. So, at the high temperature, the SAPS-coating with lower porosity and fine lamellar structures, as shown in Fig. 2, could play an important role in decreasing the diffusive channels for oxygen and reducing the oxygen pressure at the top/bond coat interface, thus hindering the diffusion of oxygen into the bond coat and reducing the growth rate of TGO [35]. Moreover, it is well accepted that the morphology and the composition of the top TGO layer are crucial for the performance of TBC coatings and the onset of the growth of NiO and spinel phases (Ni (Cr,Al)2O4) is more damaging because these exhibit a very high growth rate, which rapidly increased the volume of the bond coat [32,36]. In addition, some studies revealed that extensive crack initiation and propagation along TGO and bond coat interface could lead to the failure of the TBCs when the TGO attained a critical thickness [37–39]. So, the lower growth rate of TGO also helped to improve the thermal shock resistance of TBCs system and the result was that the SAPS-coatings had much higher thermal cycling lives, about 90% higher than the APS-coatings.
Fig. 5. Cross-sectional backscattered electron images for the morphology and thickness of TGO layer between top coat and bond coat by the SAPS and APS after different thermal cycles: (a) SAPS-coating after 100 thermal cycles, (b) SAPS-coating after 265 thermal cycles, (c) APS-coating after 100 thermal cycles, (d) APS-coating after 141 thermal cycles.
3.4. Phase change of coating after thermal cycles X-ray phase analysis carried out on the coatings surface after different thermal cycles is shown in Fig. 6. As seen from it, both the SAPS- and APS-coatings showed a predominant, non-transformable, tetragonal phase structure (t′) as a result of rapid solidification. Furthermore, as seen from Fig. 6a, the SAPS-coatings with only a small amount of calcium zirconium oxide (CaZrO3) was found after 265 thermal cycles while the APS-coatings showed very small percentage of CaZrO3 the after 95 thermal cycles and it increased its percentage (by comparing the intensity of peak at approximately 45.7°) after 141 cycles when about 10% surface area spalled from the edge of samples, as shown in Fig. 6b. This result can be explained by that the calcium which was present in the tap water reacted with zirconium during the quenching operation, and formed CaZrO3. This CaZrO3 formed at the surface of the coating is orthorhombic in structure, and
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thickness of the lamellar structure in SAPS-coating was 2.5 ± 0.6 μm, while that of APS-coating was 5.3 ± 0.9 μm. The desirable structure for SAPS-samples was attributed to higher impact velocity of in-flight particles during SAPS process, resulting in the improvement of flattening degree of molten particles. The well-adhered fine lamellar structures, fine micro-cracks and lower growth rate of TGO appeared to be responsible for greatly improved thermal cycling lives of SAPS-TBCs as compared to their conventional plasma sprayed counterparts. The results of water-quenching test from 1100 °C into room temperature showed that SAPS-TBCs presented high thermal shock resistance, only 10% coating area spalled after 265 thermal cycles, about 90% higher than that of APS-samples. During the thermal cycles, the transformation from tetragonal to monoclinic ZrO2 was not observed. Acknowledgements This work was supported by the National Basic Research Program (Grant No. 2007CB707702). The authors would like to thank Prof. B.J. Ding (Xi'an Jiaotong University, China) and technician Y.M. Qiang (Xi'an Jiaotong University, China) for their suggestions and help. References
Fig. 6. X-ray spectra of coatings after different thermal cycles: (a) SAPS-coatings, (b) APScoatings.
it played a role in the delamination of the surface of the top coat [5]. In general, there are two types of tetragonal phase, such as a high stabilizer non-transformable tetragonal phase (t′) and a low stabilizer transformable tetragonal phase (t). After long-term thermal cycling, the metastable non-transformable tetragonal phase may decompose into the low stabilizer tetragonal phase by diffusion of the stabilizer such as Y2O3, and the low stabilizer tetragonal phase may transform to monoclinic phase induced by thermal stress. The presence of the monoclinic phase may reduce the useful life of TBCs owing to the large volume increase of about 3–4% associated with the tetragonal– monoclinic phase transformation [33]. However, in this work, no peak for monoclinic phase was observed in both SAPS- and APS-coatings, indicating that the phase transformation from tetragonal to monoclinic ZrO2 did not occur. 4. Conclusions The microstructure and thermal shock resistance of the ZrO2–Y2O3 thermal barrier coatings (TBCs) fabricated by high efficiency supersonic atmospheric plasma spraying (SAPS) and conventional atmospheric plasma spraying (APS), respectively, were comparatively studied in the present work. As compared to APS-TBCs with the same composition, the microstructure of SAPS-TBCs was much finer. It was found that the thickness of lamellar structure consisted of columnar crystals in the SAPS- and APS-coating was in the range of 1–4 μm and 2–8 μm, respectively. Besides, the statistical results revealed that the average
[1] N.P. Padture, M. Gell, E.H. Jordan, Science 296 (2002) 280. [2] C. Zhou, N. Wang, Z.B. Wang, S.K. Gong, H.B. Xu, Scr. Mater. 51 (2004) 945. [3] N.P. Padture, K.W. Schlichting, T. Bhatia, A. Ozturk, B. Cetegen, E.H. Jordan, M. Gell, S. Jiang, T.D. Xiao, P.R. Strutt, E. Garcia, P. Miranzo, M.I. Osendi, Acta Mater. 49 (2001) 2251. [4] P.L. Ke, Q.M. Wang, J. Gong, C. Sun, Y.C. Zhou, Mater. Sci. Eng. A 435–436 (2006) 228. [5] A.N. Khan, J. Lu, Surf. Coat. Technol. 166 (2003) 37. [6] A. Rabiei, A.G. Evans, Acta Mater. 48 (2000) 3963. [7] G. Johner, K.K. Schweitzer, Thin Solid Films 119 (1984) 301. [8] S. Rangaraj, K. Kokini, Acta Mater. 52 (2004) 455. [9] X.C. Zhang, B.S. Xu, Y.X. Wu, F.Z. Xuan, S.T. Tu, App. Surf. Sci. 254 (2008) 3879. [10] X.C. Zhang, B.S. Xu, S.T. Tu, F.Z. Xuan, H.D. Wang, Y.X. Wu, App. Surf. Sci. 254 (2008) 6318. [11] L.Z. Du, B.S. Xu, S.Y. Dong, W.G. Zhang, J.M. Zhang, H. Yang, H.J. Wang, Surf. Coat. Technol. 202 (2008) 3709. [12] Z.H. Han, B.S. Xu, H.J. Wang, S.K. Zhou, Surf. Coat. Technol. 201 (2007) 5253. [13] J.F. Coudert, M.P. Planche, O. Betoule, M. Vardelle, P. Fauchais, Proceedings of the 1993 National Thermal Spray Conference, Anaheim, CA, June 7–11, 1993, ASM International, Metals Park, OH, 1993, p. 19. [14] S. Zhu, B.S. Xu, J.K. Yao, Mater. Sci. Forum 475–479 (2005) 3981. [15] D.J. Varacalle Jr., L.B. Lundberg, H. Herman, G. Bancke, Surf. Coat. Technol. 86–87 (1996) 70. [16] J.R. Davis, Introduction to thermal spray processing, Handbook of thermal technology, , 2004, p. 71. [17] X.C. Zhang, B.S. Xu, F.Z. Xuan, H.D. Wang, Y.X. Wu, S.T. Tu, J. Alloy. Compd. 467 (2009) 501. [18] J.C. Fang, W.J. Xu, Z.Y. Zhao, H.P. Zeng, Surf. Coat. Technol. 201 (2007) 5671. [19] H. Du, J.H. Shin, S.W. Lee, J. Therm. Spray Technol. 14 (2005) 453. [20] A.D. Jadhav, N.P. Padture, E.H. Jordan, M. Gell, P. Miranzo, E.R. Fuller Jr., Acta Mater. 54 (2006) 3343. [21] N. Berger-Keller, G. Bertrand, C. Filiatre, C. Meunier, C. Coddet, Surf. Coat. Technol. 168 (2003) 281. [22] G. Trapaga, J. Szekely, Metall. Trans. B 22 (1991) 901. [23] M. Bertagnolli, M. Marchese, G. Jacucci, J. Therm. Spray Technol. 4 (1995) 41. [24] H. Liu, E.J. Lavemia, R.H. Rangel, J. Therm. Spray Technol. 2 (1993) 369. [25] S. Yugeswaran, V. Selvarajan, D. Seo, K. Ogawa, Surf. Coat. Technol. 203 (2008) 129. [26] R. Mcpherson, Surf. Coat. Technol. 39/40 (1989) 173. [27] A. Kulkarni, A. Vaidya, A. Goland, S. Sampath, H. Herman, Mater. Sci. Eng. A. 359 (2003) 100. [28] P.L. Ke, Y.N. Wu, Q.M. Wang, J. Gong, C. Sun, L.S. Wen, Surf. Coat. Technol. 200 (2005) 2271. [29] B. Liang, C.X. Ding, Surf. Coat. Technol. 197 (2005) 185. [30] B. Zhou, K. Kokini, Acta Mater. 52 (2004) 4189. [31] Z.M. Li, S.Q. Qian, W. Wang, J.H. Liu, J. Alloy. Compd. 505 (2010) 675. [32] L. Ajdelsztajn, J.A. Picas, G.E. Kim, F.L. Bastian, J. Schoenung, V. Provenzano, Mater. Sci. Eng. A 338 (2002) 33. [33] C.H. Lee, H.K. Kim, H.S. Choi, H.S. Ahn, Surf. Coat. Technol. 124 (2000) 1. [34] A. Bennett, Mater. Sci. Technol. 2 (1986) 257. [35] Y.N. Wu, F.H. Wang, W.G. Hua, J. Gong, C. Sun, L.S. Wen, Surf. Coat. Technol. 166 (2003) 189. [36] R.D. Maier, C.M. Scheuermann, C.W. Andrews, Am. Ceram. Soc. Bull. 60 (5) (1981) 555. [37] W.R. Chen, X. Wu, B.R. Marple, P.C. Patnaik, Surf. Coat. Technol. 201 (2006) 1074. [38] M. Subanovic, P. Song, E. Wessel, R. Vassen, D. Naumenko, L. Singheiser, W.J. Quadakkers, Surf. Coat. Technol. 204 (2009) 820. [39] F. Cao, B. Tryon, C.J. Torbet, T.M. Pollock, Acta Mater. 57 (2009) 3885.