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Effect of heat treatment on microstructure and property of plasma-sprayed lanthanum hexaaluminate coating
Accepted Manuscript Effect of heat treatment on microstructure and property of plasma-sprayed lanthanum hexaaluminate coating Junbin Sun, Jinshuang Wang, Shujuan Dong, Yu Hui, Lifen Li, Longhui Deng, Jianing Jiang, Xin Zhou, Xueqiang Cao PII:
S0925-8388(17)34562-0
DOI:
10.1016/j.jallcom.2017.12.347
Reference:
JALCOM 44433
To appear in:
Journal of Alloys and Compounds
Received Date: 12 October 2017 Revised Date:
27 December 2017
Accepted Date: 29 December 2017
Please cite this article as: J. Sun, J. Wang, S. Dong, Y. Hui, L. Li, L. Deng, J. Jiang, X. Zhou, X. Cao, Effect of heat treatment on microstructure and property of plasma-sprayed lanthanum hexaaluminate coating, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.347. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Effect of heat treatment on microstructure and property of plasma-sprayed
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lanthanum hexaaluminate coating
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Junbin Suna, Jinshuang Wanga, Shujuan Donga, Yu Huib, Lifen Lia, Longhui Denga,
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Jianing Jianga, Xin Zhoua, ∗, Xueqiang Caoa,* a
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State Key Laboratory of Silicate Materials for Architectures (Wuhan University of
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Technology), China.
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b
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Sciences, China.
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Abstract: Amorphous phase is commonly found in LaMgAl11O19 (LMA) coating
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prepared by atmospheric plasma spraying. In order to improve their performance, the
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as-sprayed LMA thermal barrier coatings on a nickel-based DZ125 substrate were
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heat-treated at 900, 1000 and 1100°C for 5~20 h, respectively. The effect of heat
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treatment on the microstructure, porosity, phase composition, and crystallization
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behavior of the LMA coatings was investigated using scanning electron microscopy,
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X-ray diffraction, differential scanning calorimetry analysis. Samples before and after
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heat treatment for 20 h were thermally cycled at 1127°C. Results indicated that the
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annealing of the as-sprayed LMA coating prior to thermal cycling is preferable to
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promote the crystallization of amorphous phase. Variations in microstructure, phase
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composition and thermal cycling lifetime of LMA coatings have been correlated to
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the heat treatment. Heat treatment at 900°C improved the lifetime of LMA coatings,
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whereas their thermal cycling performance did not significantly improved when the
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Atmospheric Environment Research Center, Shenyang Academy of Enviromental
Corresponding author. Tel/Fax: +86-27-87651856. E-mail: [email protected]. (X. Zhou), [email protected].(X. Cao) *
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ACCEPTED MANUSCRIPT aging temperature increased to 1000 or 1100°C. The corresponding mechanism had
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been explored by tracing the microstructure and thermo-physical properties evolutions
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during thermal annealing process.
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Keywords: Heat treatment; Lanthanum hexaaluminate; Amorphous phase; Thermal
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cycling lifetime; Plasma spraying; Thermal barrier coatings
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1 Introduction
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Thermal barrier coatings (TBCs) have exhibited an increasing potential for
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improving the durability and efficiency of gas turbine engines by allowing an increase
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in turbine inlet temperature and reducing the amount of cooling air required by the
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hot-section components [1]. TBCs are usually made up of a two-layered structure: a
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metallic bond coat for oxidation/corrosion resistance and a ceramic topcoat for
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thermal protection [2, 3]. MCrAlY (M=Ni, and/or Co) is used as a bond coat to
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provide a good thermal expansion match between the topcoat and substrate and to
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inhibit oxidation of the substrate [4]. The current state-of-the-art of topcoat is zirconia
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partially stabilized with 7-8 wt.% yttria (7-8 YSZ) with high coefficient of thermal
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expansion (CTE) (10.5~11.5×10-6 K-1, 20–1200°C), fracture toughness (~2 MPa • m1/2)
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as well as low thermal conductivity (2.1~2.2 W • m-1 • K-1), which are always
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deposited onto the components either by electron beam-physical vapor deposition
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(EB-PVD) or by atmospheric plasma spraying (APS) [5-7]. YSZ as the commonly
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used TBCs material performs quite well up to 1200°C. When the material is used at
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higher application temperature over 1200°C for a long time, it exhibits a poor service
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lifetime due to the phase transformation [8, 9], and distinct sintering in combination
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ACCEPTED MANUSCRIPT with the increase of Youngꞌs modulus [10-13]. As a result, developing new TBCs
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materials has been a hot spot of present research to improve the temperature capability
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and durability of the deposited coatings. New TBCs candidate materials such as
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oxides with pyrochlore (La2Zr2O7, LZ) [14], fluorite (La2Ce2O7) [15], perovskite
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(SrZO3) [16], magnetoplumbite (LaMgAl11O19, LMA) [10, 13] structures have been
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researched and developed.
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Among these new TBCs materials, LMA has essential characteristics of low
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thermal conductivity (0.8~2.6 W • m-1 • K-1), high fracture toughness (~3.59 MPa •
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m1/2), high CTE (9~11×10-6 K-1, 20–1200°C) and outstanding thermal stability up to
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1600°C, owing to the magnetoplumbite structure in which spinel blocks are separated
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by mirror planes containing the large La3+ cation [17]. With the superior structural and
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thermochemical stability as well as at least equivalent thermophysical properties with
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YSZ, LMA has been proposed as one of promising candidates for the next generation
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TBCs to overcome above problems associated with the use of YSZ at higher
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temperature [18, 19]. However, as a new TBCs candidate material, some problems are
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urgent to be solved before it can be widely used. For instance, a large amount of
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amorphous phase often presents in the plasma-sprayed LMA coating due to the rapid
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quenching from the molten state [20-22]. After the LMA coating is heated to a high
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temperature for the first time, it suffers large volume shrinkage, and plane tensile
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stress will be developed in LMA coating due to the restriction of the superalloy
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substrate. Besides, the recrystallization can give rise to a sudden reduction in CTE
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value of the LMA coating [17]. Then, thermal expansion mismatches between
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ACCEPTED MANUSCRIPT substrate and coating materials result in the development of residual stresses during
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thermomechanical loading, which will lead to the spallation and may put questions on
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the reliability of the plasma-sprayed LMA coating [17, 23]. It is obvious that the
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negative effect of amorphous phase crystallization must be removed to improve the
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reliability and service lifetime of LMA coatings. As we all known, transition from
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amorphous phase to crystalline phase occurs as the temperature higher or close to its
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crystallization temperature [24]. Therefore, the negative effect of amorphous phase
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crystallization on the LMA coating can be removed by heat treatment prior to its using.
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On the one hand, heat treatment will induce the crystallization of the amorphous
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phase and avoid the strong thermal stresses caused by fierce heating and cooling cycle
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alternately. One the other hand, heat treatment also leads to the relaxation of stress
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formed during the plasma spraying process. These are very helpful to the
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improvement of the thermal cycling lifetime of the LMA coating in theory. Actually,
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to the best of our knowledge, no works have been carried out on the thermal cycling
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behavior of the LMA coatings after heat treatment.
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Since the crystallization temperature of amorphous LMA is between 800 and
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1200°C [17] and the upper limit temperature of superalloy DZ125 is about 1200°C,
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therefore, the as-sprayed LMA coatings were heat-treated at 900, 1000 and 1100°C for
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5~20 h, respectively. The effects of heat treatment on the microstructure and thermal
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cycling behavior of the coatings were studied, and failure mechanisms of the coatings
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were also analyzed.
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2 Experimental
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2.1 Preparation of powder and coating As the chemicals in this work, the powders of La2O3, MgO and Al2O3 in proper
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molar ratio of 0.5:1:5.5 were mixed with deionized water for 24 h by ball-milling
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and then dried later at 100°C for 48 h. In order to synthesize the LMA powder, the
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obtained powder mixture of La2O3, MgO and Al2O3 was heated at 1450°C for 12 h
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and this process was replicated for two times. The as-synthesized LMA powder was
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mixed with deionized water, Gum Arabic (adhesion agent), and ammonium citrate
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(dispersing agent), followed by ball-milling (72 rpm) using zirconia balls for 72 h
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and then the slurry was spray-dried (SFOC-16, Shanghai-Ohkawara Dryers Co.,
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Ltd.). The free-flowing powders (see Fig. 1) with particle size between 20 and 125
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µm were collected and used directly for plasma spraying without other treatments.
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LMA coating were produced by APS with a Multicoat Plasma Spray Unit
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(Oerlikon Metco, Switzerland). Argon was used as primary plasma gas and powder
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carrier gas and hydrogen was used as a secondary gas to adjust the arc voltage [25,
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26]. The substrate was heated to 120°C using plasma flame prior to the coating
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deposition. LMA coatings with thickness ~200 µm for thermal cycling tests were
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prepared by APS on the DZ125 superalloy substrate (30×10×2 mm3) with NiCrAlY
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bond coat. In addition, LMA coatings with thickness 1000~1200 µm for thermal
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expansion test and thermal analysis were made by APS on graphite substrate without
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bond coat. Plasma spraying parameters were listed in Table 1.
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2.2 Heat treatment and thermal cycling test
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The as-sprayed LMA coatings were heat-treated in a tube furnace. The pressure 5
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heat treatment process was composed of heating of the coatings from room
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temperature to 900, 1000 and 1100°C with a heating rate of 5°C/min, respectively,
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and holding at the temperature for 5, 10, 15 and 20 h, respectively. The as-sprayed
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LMA coating and coatings after heat treatment at 900, 1000 and 1100°C for 20 h
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were named as C0, C900, C1000 and C1100, respectively. Thermal cycling
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experiments, to determine TBCs cyclic lifetime, were performed in a
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specially-designed, automated furnace. A full 35-min thermal cycle consisted of
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rapid heating of the samples to 1127°C and holding at temperature (30 min),
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followed by conventional cooling to near-room temperature (5 min). Each specimen
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was examined periodically. A specimen was considered to have failed when the
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topcoat failure area (spallation plus delamination) reached ~10% of the total topcoat
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area. The specimen was then removed from the furnace and the number of thermal
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cycles at which it failed was recorded. Three specimens of every kind of coating
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were tested simultaneously and the thermal cycling lifetime was the average value of
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the three specimens. Coatings of C0, C900, C1000 and C1100 after thermal cycling
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failures were named as FC0, FC900, FC1000 and FC1100, respectively.
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2.3 Characterization
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Phase composition of the samples was identified by X-ray diffraction (XRD,
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D/MAX-RB RU-200B, Cu-Kα radiation, λ=0.15406 nm) over 2θ values of 15–65°
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with a scanning rate of 4°·min−1 by a step width of 0.02°. In addition,
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microstructural studies in this work were performed using a field emission-scanning 6
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X-ray spectroscopy (EDS). All the coatings for SEM cross-section analysis were
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embedded in a transparent epoxy resin and polished with diamond pastes, while the
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fractured cross-sections were directly analyzed without other treatments. Coating
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samples for thermal analyses and thermal expansion tests were prepared by abrading
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the graphite substrate. Differential scanning calorimetry (DSC) of the LMA coatings
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was performed on a thermoanalyzer (Schleibinger CDF Test) in air atmosphere with
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a heating rate of 20°C·min−1. Coating samples with dimensions of (23~25)
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mm×(5~8) mm×(1~1.2) mm were selected for thermal expansion measurements.
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Thermal expansion of the coatings was determined between room temperature (RT)
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and 1300°C using the Netzsch 402C high-temperature dilatometer. Amorphous
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phase contents of the coatings were based on their amorphous indexes, which were
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the ratios of the amorphous hump areas to the total area of the crystalline peaks and
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humps in XRD patterns in the 2θ range between 15 and 65° [27, 28]. Porosity of the
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coatings was evaluated by image analyses. Average thickness of coatings before and
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after heat treatment was analyzed using SEM micrographs. About 50 measurements
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were taken for each specimen in the thickness analysis.
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3 Results and discussion
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3.1 Crystallization of amorphous phase
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Fig. 2 exhibits XRD patterns of LMA coatings heated at 900, 1000 and 1100°C
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for 0~20 h, respectively. Characteristic peaks of LMA are observed obviously in these
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four coatings, and a broad hump corresponding to amorphous phase [23, 25] is also 7
ACCEPTED MANUSCRIPT noticed (2θ=20-40°) in the XRD patterns of the as-sprayed LMA coating. With the
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heat treatment time increasing from 5 to 20 h, the broad hump gradually flattens. As
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shown in Fig. 2c, the broad hump seems to disappear after heat treatment at 1100°C
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for 5 h. However, for the coating heated at 900°C, the broad hump still can be
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observed obviously after heat treatment for 20 h (see Fig. 2a). In addition, some weak
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peaks ascribed to LaAlO3, which cannot be observed in the as-sprayed coating, also
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presents in the patterns of annealed LMA coatings. In fact, the LaAlO3 may be
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covered by the high amorphous phase content in the as-sprayed coating. After heat
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treatment, amorphous phase decreases by crystallization and LaAlO3 is detected in the
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patterns. The amorphous phase contents of LMA coatings heated 900, 1000 and
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1100°C for 0~20 h are presented in Fig. 3. As shown in Fig. 3, the amorphous phase
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content decreases from 59 to 33% after heat treatment at 900°C for 20 h. However,
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for the coating heated at 1100°C, the amorphous phase content suddenly decreases
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from 59 to 8% after heat treatment for 5 h and then almost has no obvious changes
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with the increase of time. It is obvious that the crystallization rate of amorphous phase
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increases with the increase of temperature from 900 to 1100°C. In order to better
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describe the crystallization rate, Avrami equation was adopted in this work as follows
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[29]:
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φ = 1 − ݁ ି௧ (1)
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where k is the crystallization rate factor, n is the Avrami exponent, t is heat treatment
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time and ϕ is the crystallization fraction. The relationship between the crystallization
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fraction ϕ and the amorphous phase content can be described as follows:
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φ=
ூబ ିூ ூబ
× 100%(2)
where I0 is the amorphous phase content of the as-sprayed LMA coating and It is the
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amorphous phase content of the coating heated in different time. Fig. 4 shows a plot
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of ϕ as the function of heat treatment time at different temperatures, which is
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simulated according to Avrani equation. It is obvious that crystallization rate factor k
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of the LMA coating heated at 1100°C (1.779) is far larger than that of the coatings
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heated at 900°C (0.017) and 1000°C (0.012). In fact, crystallization rate factor k has
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dependent on the temperature following the Arrhenius law,
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kሺTሻ = A݁ ି (3)
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where A and activation energy, Ea, are kinetic parameters which are not dependent on
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temperature. R is the ideal gas constant and T is the temperature. Therefore, k
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increases with the increase of heat treatment temperature from 900 to 1100°C. The
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larger crystallization rate factor k at 1100°C also can be used to explain the suddenly
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decrease of amorphous phase after heat treatment at 1100°C for 5 h.
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In order to further investigate the crystallization process of the amorphous phase
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in the LMA coatings, DSC analyses of LMA coatings heated at different temperatures
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for 20 h were carried out. As shown in Fig. 5a, there are two sharp exothermal peaks
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at about 900 and 1170°C in the curve of C0. After heat treatment at 900°C for 20 h,
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the first sharp peak disappears from the DSC curve of C0. As the heat treatment
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temperature increases successively, the exothermal peak at about 1170°C gradually
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gets flat in the DSC curves of the LMA coating, especially in the curve of C1100,
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only a small exothermal peak is observed (see the inset of Fig. 5a). As discussed
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heat treatment temperature between 900 and 1100°C. For these reasons, two sharp
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exothermal peaks can be due to the crystallization of the amorphous phase. In fact, the
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same viewpoint was also proposed by Friedrich [17]. Although the second exothermal
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peak is at about 1170°C, when the as-sprayed LMA coating is heat-treated at 1100°C
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for 20 h, it also can accelerate the transformation to the stable phase transition [24].
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Besides, compared with the areas of the two peaks, it can be found that the area of
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peak at 900°C is almost 1.5 times the peak at 1170°C, which shows that the
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crystallization of amorphous phase mostly occurs at about 900°C [30]. Although
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about 28% amorphous phase is remained in the coating after heat treatment at 1000°C
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for 20 h, it is hard to decrease the amorphous phase content by prolong the heat
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treatment time owing to the kinetic limitations of crystallization [31]. Therefore, in
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this work, LMA coatings were heated at different temperatures for 20 h before their
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thermal cycling tests.
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3.2 Heat capacity and thermal expansion
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The heat capacity (Cp) curves of these coatings are illustrated in Fig. 5b. Molar
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specific heat capacity of the as-sprayed LMA coating suffers two sharp decreases
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around 900 and 1170°C, respectively. The sudden changes in Cp values are believed
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to be caused by the discrete change of the heat resulted from the phase transitions of
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the as-sprayed LMA coating during heating [23]. Besides, the crystallization of the
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amorphous phase will induce the volume shrinkage of the coating, leading to the
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mismatch of CTE between the ceramic coat and the bond coat. As shown in Fig. 6, C0
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shrinkages ascribed to the crystallization of amorphous LMA phase [17]. The
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temperature ranges where shrinkages occur are consistent with the results of thermal
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analyses as discussed above. However, the first shrinkage disappears from the thermal
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expansion curve of the coating C900. In addition, LMA coating almost show a linear
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expansion from RT to 1300°C after aging at 1100°C for 20 h. In comparison with the
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low CTE value (4.68×10-6 K-1) of C0, there is a dramatically growth in CTE value of
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the coating after heat treatment (see Table 2). For instance, CTE value of C1100
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reaches to 7.64×10-6 K-1. Furthermore, the first shrinkage of 2.3% is higher than the
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second shrinkage of 0.99%, because the crystallization of the amorphous phase in the
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LMA coating mainly occurs at about 900°C. Results show that the adverse effect of
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the sudden change of Cp and the volume shrinkage on the as-sprayed LMA coating
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can be removed by heat treatment prior to its using.
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3.3 Microstructure
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Fig. 7 shows the fractured cross-sections SEM images of LMA coatings before
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and after heat treatment at 900, 1000 and 1100°C for 20 h, respectively. As shown in
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Fig. 7a, there is a large amount of amorphous phase in the as-sprayed LMA coating,
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which is consistent with the previous report [23]. After heat treatment at 900°C for 20
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h, some plate-like crystals (see Fig. 7b), with the different micromorphology in
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comparison with the amorphous phase, are embedded in the coating matrix. As the
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fractured cross-section SEM micrographs shown in Fig. 7c and Fig. 7d, it can be
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observed that the grain growth process has been significantly accelerated and many
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ACCEPTED MANUSCRIPT plate-like grains present consequently in C1000 and C1100. Fig. 8 presents EDS
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spectra and chemical compositions acquired from plate-like crystals (see Fig. 7(c, d))
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of the LMA coatings C1000 and C1100. It is obvious that the plate-like crystals are
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composed of La2O3-MgO-Al2O3. Besides, in our previous work [23], the
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crystallization process was confirmed by high-resolution transmission electron
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microscopy examination (HR-TEM). The amorphous phase as well as the crystalline
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phase was present in the as-sprayed LMA coating. After heat treatment, the HR-TEM
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image exhibited a typical lattice configuration of the hexaaluminate with the
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magnetoplumbite-type structure. It indicated that a transformation of disorder to order
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occurred to the as-sprayed LMA coating during the heat treatment process. Therefore,
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it can be concluded that the platelet crystals are formed by the crystallization of
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amorphous phase and the crystallization level shows a growing tendency with the
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temperature increasing, which agrees well with the XRD analysis (see Fig. 2).
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Cross-sectional SEM micrographs of the LMA coatings before and after heat
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treatment for 20 h are shown in Fig. 9. In the four TBCs systems, the LMA topcoat
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bonds well with the bond coat. However, in comparison with the microstructure of the
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as-sprayed coating (see Fig. 9a), coatings after heat treatment show a porous
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microstructure with more cracks. As shown in Fig. 9b, some fine vertical cracks start
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to be observed in C900. After increasing the heat treatment temperature further, the
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vertical-crack tendency is more obvious. Besides, horizontal cracks also appear in
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C1000 and C1100 (see Fig. 9(c, d)). As shown in Fig. 10, the porosity of the coating
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increases with the heat treatment temperature increasing. On the one hand, large
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treatment, which is a contribution to the improvement of the porosity. On the other
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hand, the in-plane tensile stress, generated because of thermal expansion mismatch
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between the ceramic topcoat and the bond coat or metallic substrate, can lead to the
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formation of vertical cracks [32]. As discussed above, the as-sprayed LMA coating
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suffers two sudden volume shrinkages by crystallization of the amorphous phase and
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the average CTE value is about 4.68×10-6 K-1. However, the average CTE value of the
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nickel-base superalloy substrate is about 14×10-6~16×10-6 K-1 (20~1000°C) [33]. The
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higher CTE of the substrate ensures that the topcoat is under significant tension at
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high temperature [34]. For the reason, the number and size of the crack all increase
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with the heat treatment temperature increasing. Besides, there is an obvious reduction
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in thickness of the coating after heat treatment compared with that of the as-sprayed
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coating. As shown in Fig. 10, coatings heated at different temperatures show different
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thickness such that C900＞C1000＞C1100. The decrease in thickness is in good
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agreement with the thermal expansion analysis of the LMA coatings.
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3.4 Thermal cycling behavior
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Thermal cycling lifetimes of these four coatings are compared in Fig. 11. C900
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has the longest thermal cycling lifetime (126 cycles), which has been improved by
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25% compared with C0 (100 cycles). C1000 and C1100 (102 cycles) have similar
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lifetime with C0. Thermal cycling results indicate that the heat treatment temperature
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plays an important role in the thermal cycling lifetime of the LMA coatings.
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Fig. 12 shows the surface morphology of the coatings after thermal cycling 13
ACCEPTED MANUSCRIPT failure. As seen from FC0, visible cracks are found evidently (noted by the black
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frame). In addition, due to the extreme heating and cooling conditions encountered at
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the edges, obvious surface spallation is observed on the edge of the sample and more
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than 10% of surface area spalled after failure. The surface photo indicates the failure
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of the LMA coatings starts at the edge, and the spallation seems to occur instantly
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during cycling.
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Fig. 13 exhibits XRD patterns of the coatings after thermal failures. Compared
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with coatings before thermal cycling tests (see Fig. 2), the broad hump
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corresponding to amorphous phase has disappeared in the XRD patterns of all these
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coatings, especially in the pattern of FC0. Although the LMA coatings were
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heat-treated at different temperatures for 20 h, the crystallization of the amorphous
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phase still occurs during thermal cycling. Besides, LaAlO3 phase disappears from
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the XRD patterns of coatings after thermal cycling tests, indicating that LaAlO3
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phase may re-form to LMA accompanying the crystallization of the amorphous
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phase during the thermal cycling.
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Fig. 14 shows cross-sectional SEM images of coatings after thermal cycling
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failures. In comparison with the microstructures of coatings before thermal cycling
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tests, large vertical and horizontal cracks are observed in the failed coatings. From
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Fig.14, the failure of these four LMA coatings occurred by the formation of large
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horizontal crack in the ceramic top coat which was close to the interface of ceramic
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top coat and bond coat. In addition, some dark substances with the thickness of 3~5
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µm are found at the interface between the ceramic coat and the bond coat of FC900
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TGO. It is well known that TGO is the result of selective oxidation of elements
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within the bond coat. The oxidation of the bond coat is conclusively a result of
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oxygen transport through the interconnected pores within the porous plasma-sprayed
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ceramic topcoat during thermal cycling tests. In order to demonstrate the existence of
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TGO, elemental distributions by map scanning at the topcoat-bond coat interface of
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FC1100 is determined and their results are shown in Fig. 15(b-g). Cr and Al should
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be uniformly distributed throughout the bond coat. However, after thermal cycling
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failure there are high levels of Cr (see Fig. 15f) and Al (see Fig. 15c) at the interface
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between the bond coat and the ceramic topcoat. From the EDS results, it is obvious
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that TGO has formed in C900 and C1100 during thermal cycling, and its
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composition may be Cr2O3 and Al2O3.
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Generally, stresses caused by the thermal expansion mismatch and the growth of
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thermally grown oxides (TGO) lead to the failure of TBCs during thermal cycling. As
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discussed above, the as-sprayed LMA coating with large amount of amorphous phase
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has the lowest CTE (4.68×10-6 K-1). Therefore, it suffers the largest thermal stress
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caused by mismatch of CTEs between the ceramic topcoat and the substrate, which is
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enough to lead the failure of C0. For the heat-treated LMA coatings, the amorphous
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phase content decreases dramatically during heat treatment. LMA coatings of C900,
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C1000 and C1100 have the larger CTEs (between 7.02 and 7.64×10-6 K-1) than that of
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C0. This effectively mitigates the thermal expansion mismatch stress level. Besides,
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since the annealing process is preferable to promote the crystallization of amorphous
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recrystallization and the platelet-like LMA grain growth can be avoided. Those can
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well explain the improvement of thermal durability for C0. However, the emergency
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of the large cracks in C1000 and C1100 decreases the bond strength of the coating and
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promote the growth of TGO, which is adverse to the improvement of thermal shock
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resistance of the coating. Stresses caused by TGO growth also contribute to the failure
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of these coatings. For these reasons, only the thermal cycling lifetime of C900 has
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been improved dramatically, and C1000 and C1100 have no obvious improvement in
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thermal cycling lifetime compared with C0.
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4. Conclusions
LaMgAl11O19 (LMA) coatings were prepared by plasma spraying and then were
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heat-treated at 900, 1000 and 1100°C for 0~20 h, respectively. With the heat
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treatment increasing from 900 to 1100°C, the crystallization rate of the amorphous
14
phase increases. During the heat treatment process, the crystallization of the
15
amorphous phase induces the volume shrinkage, the increase of porosity, and the
16
change in heat capacity (Cp) value of the LMA coating. The changes are more obvious
17
by increasing the heat treatment temperature. Heat treatment prior to use of the
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plasma-sprayed LMA coating at 900°C weakens the negative effect of the amorphous
19
phase and improves the thermal shock resistance of the coating. In comparison with
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thermal cycling lifetime (100 cycles) of C0, C900 has an obvious improvement of
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thermal cycling lifetime (126 cycles). However, C1000 and C1100 with the same
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thermal cycling lifetime (102 cycles) are similar to that of C0.
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and the LMA topcoat leads to the failure of the as-sprayed LMA coating (C0). For
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coatings heated at 900 (C900), 1000 (C1000) and 1100 (C1100), the mismatch of
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CTE is decreased effectively by recrystallization. The growth of TGO and the weak
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bond strength lead to the failure of these coatings.
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Acknowledgement
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The authors thank Dr. Zhitao Shan for the DSC measurements. This work was
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supported by the National Natural Science Foundation of China (No. 51501137,
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51702244) and the Natural Science Foundation of Hubei Province (No.
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2017CFB285).
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ACCEPTED MANUSCRIPT References
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[1] C.G. Zhou, N. Wang, H.B. Xu, Comparison of thermal cycling behavior of plasma-sprayed nanostructured and traditional thermal barrier coatings, Mater. Sci. Eng. A, 452 (2007) 569-574. [2] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas-turbine engine applications, Science, 296 (2002) 280-284. [3] L.H. Gao, H.B. Guo, L.L. Wei, C.Y. Li, S.K. Gong, H.B. Xu, Microstructure and mechanical properties of yttria stabilized zirconia coatings prepared by plasma spray physical vapor deposition, Ceram. Int., 41 (2015) 8305-8311. [4] C.G. Zhou, N. Wang, Z.B. Wang, S.K. Gong, H.B. Xu, Thermal cycling life and thermal diffusivity of a plasma-sprayed nanostructured thermal barrier coating, Scripta Mater., 51 (2004) 945-948. [5] L. Wang, Y. Wang, X.G. Sun, J.Q. He, Z.Y. Pan, C.H. Wang, Microstructure and indentation mechanical properties of plasma sprayed nano-bimodal and conventional ZrO2–8wt%Y2O3 thermal barrier coatings, Vacuum, 86 (2012) 1174-1185. [6] W.G. Mao, J.M. Luo, C.Y. Dai, Y.G. Shen, Effect of heat treatment on deformation and mechanical properties of 8 mol% yttria-stabilized zirconia by Berkovich nanoindentation, Appl. Surf. Sci., 338 (2015) 92-98. [7] K.D. Bouzakis, A. Lontos, N. Michailidis, Determination of mechanical properties of electron beam-physical vapor deposition-thermal barrier coatings (EB-PVD-TBCs) by means of nanoindention and impact testing, Surf. Coat. Technol., 163 (2003) 75-80. [8] R. Vaßen, F. Cernuschi, G. Rizzi, A. Scrivani, N. Markocsan, L. Östergren, A. Kloosterman, R. Mevrel, J. Feist, J. Nicholls, Overview in the Field of Thermal Barrier Coatings Including Burner Rig Testing in the European Union, Adv. Eng. Mater., 43 (2008) 371-382. [9] E. Withey, C. Petorak, R. Trice, G. Dickinson, T. Taylor, Design of 7wt.% Y2O3–ZrO2/mullite plasma-sprayed composite coatings for increased creep resistance, J. Eur. Ceram. Soc., 27 (2007) 4675–4683. [10] H. Lu, C.A. Wang, C. Zhang, S. Tong, Thermo-physical properties of rare-earth hexaaluminates LnMgAl11O19 (Ln: La, Pr, Nd, Sm, Eu and Gd) magnetoplumbite for advanced thermal barrier coatings, J. Eur. Ceram. Soc., 35 (2015) 1297-1306. [11] Z.G. Liu, J.H. Ouyang, Y. Zhou, J. Li, X.L. Xia, Influence of ytterbium- and samarium-oxides codoping on structure and thermal conductivity of zirconate ceramics, J. Eur. Ceram. Soc., 29 (2009) 647-652. [12] Z.G. Liu, J.H. Ouyang, Y. Zhou, X.L. Xia, Hot corrosion behavior of V2O5-coated Gd2Zr2O7 ceramic in air at 700–850°C, J. Eur. Ceram. Soc., 66 (2009) CD008046-CD008046. [13] X. Chen, Y. Zhao, X. Fan, Y. Liu, B. Zou, Y. Wang, H. Ma, X. Cao, Thermal cycling failure of new LaMgAl11O19/YSZ double ceramic top coat thermal barrier coating systems, Surf. Coat. Technol., 205 (2011) 3293-3300. [14] R. Vaßen, F. Traeger, D. Stöver, New Thermal Barrier Coatings Based on Pyrochlore/YSZ Double-Layer Systems, Int. J. Appl. Ceram. Technol., 1 (2004) 18
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351–361. [15] X.Q. Cao, R. Vassen, W. Fischer, F. Tietz, W. Jungen, D. Stoever, Lanthanum–Cerium Oxide as a Thermal Barrier‐Coating Material for High‐ Temperature Applications, Adv. Mater., 15 (2003) 1438-1442. [16] W. Ma, D.E. Mack, R. Vassen, D. Stöver, Perovskite-Type Strontium Zirconate as a New Material for Thermal Barrier Coatings, J. Am. Ceram. Soc., 91 (2008) 2630–2635. [17] C. Friedrich, R. Gadow, T. Schirmer, Lanthanum hexaaluminate—a new material for atmospheric plasma spraying of advanced thermal barrier coatings, J. Therm. Spray Technol., 10 (2001) 592-598. [18] R. Gadow, M. Lischka, Lanthanum hexaaluminate — novel thermal barrier coatings for gas turbine applications — materials and process development, Surf. Coat. Technol., s 151–152 (2002) 392-399. [19] P. Jana, P.S. Jayan, S. Mandal, K. Biswas, Microstructural design of neodymium-doped lanthanum–magnesium hexaaluminate synthesized by aqueous sol–gel process, J. Mater. Sci., 50 (2015) 344-353. [20] X.Q. Cao, Y.F. Zhang, J.F. Zhang, X.H. Zhong, Y. Wang, H.M. Ma, Z.H. Xu, L.M. He, F. Lu, Failure of the plasma-sprayed coating of lanthanum hexaluminate, J. Eur. Ceram. Soc., 28 (2008) 1979-1986. [21] X.L. Chen, Y.F. Zhang, X.H. Zhong, Z.H. Xu, J.F. Zhang, Y.L. Cheng, Y. Zhao, Y.J. Liu, X.Z. Fan, Y. Wang, H.M. Ma, X.Q. Cao, Thermal cycling behaviors of the plasma sprayed thermal barrier coatings of hexaluminates with magnetoplumbite structure, J. Eur. Ceram. Soc., 30 (2010) 1649-1657. [22] R. Vaßen, M.O. Jarligo, T. Steinke, D.E. Mack, D. Stöver, Overview on advanced thermal barrier coatings, Surf. Coat. Technol., 205 (2010) 938-942. [23] X.L. Chen, Y. Zhao, W.Z. Huang, H.M. Ma, B.L. Zou, Y. Wang, X.Q. Cao, Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating, J. Eur. Ceram. Soc., 31 (2011) 2285-2294. [24] V.M. Fokin, E.D. Zanotto, N.S. Yuritsyn, J.W.P. Schmelzer, Homogeneous crystal nucleation in silicate glasses: A 40 years perspective, J. Non-Cryst. Solids, 352 (2006) 2681-2714. [25] H.Z. Liu, J.H. Ouyang, Z.G. Liu, Y.M. Wang, Microstructure, thermal shock resistance and thermal emissivity of plasma sprayed LaMAl11O19 (M = Mg, Fe) coatings for metallic thermal protection systems, Appl. Surf. Sci., 271 (2013) 52-59. [26] J.S. Wang, J.B. Sun, H. Zhang, S.J. Dong, J.N. Jiang, L.H. Deng, X. Zhou, X.Q. Cao, Effect of spraying power on microstructure and property of nanostructured YSZ thermal barrier coatings, J. Alloy Compd., 730 (2018) 471-482. [27] F. Tarasi, Suspension plasma sprayed alumina-yttria stabilized zirconia nano-composite thermal barrier coatings: formation and roles of the amorphous phase, in, Concordia University, 2010. [28] F. Tarasi, M. Medraj, A. Dolatabadi, R.S. Lima, C. Moreau, Thermal Cycling of Suspension Plasma Sprayed Alumina- YSZ Coatings Containing Amorphous Phases, J. Am. Ceram. Soc., 95 (2012) 2614-2621. [29] E.B. Peixoto, E.C. Mendonça, S.G. Mercena, A.C.B. Jesus, C.C.S. Barbosa, C.T. 19
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Meneses, J.G.S. Duque, R.A.G. Silva, Study of the dynamic of crystallization of an amorphous Fe40Ni40P14B6 ribbon through Johnson-Mehl-Avrami model, J. Alloy Compd., 731 (2018) 1275-1279. [30] L.L. Huang, H.M. Meng, J. Tang, Crystallization behavior of plasma-sprayed lanthanide magnesium hexaaluminate coatings, Int. J. Miner. Metall. Mater., 21 (2014) 1247-1253. [31] A. Rosenflanz, M. Frey, B. Endres, T. Anderson, E. Richards, C. Schardt, Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides, Nature, 430 (2004) 761. [32] A.C. Fox, T.W. Clyne, Oxygen transport by gas permeation through the zirconia layer in plasma sprayed thermal barrier coatings, Surf. Coat. Technol., 184 (2004) 311-321. [33] X.Q. Cao, R. Vassen, D. Stoever, Ceramic materials for thermal barrier coatings, J. Eur. Ceram. Soc., 24 (2004) 1-10. [34] Y. Bai, L. Zhao, J.J. Tang, S.Q. Ma, C.H. Ding, J.F. Yang, L. Yu, Z.H. Han, Influence of original powders on the microstructure and properties of thermal barrier coatings deposited by supersonic atmospheric plasma spraying, part II: Properties, Ceram. Int., 39 (2013) 5113–5124.
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Figures
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Fig. 1. XRD pattern and SEM image of the starting powder.
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Fig. 2. XRD patterns of LMA coatings before and after heat treatment at different temperatures: (a) 900°C; (b) 1000°C; (c) 1100°C. A, B, C, D, E represent the heat treatment time of 0, 5, 10, 15, 20 h, respectively.
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Fig. 3. Amorphous phase content of LMA coatings heated at 900, 1000 and 1100°C in different time.
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Fig. 4. Crystallization fraction of LMA coatings heated at 900, 1000 and 1100°C in different time.
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Fig. 5. DSC curves of coatings (a) and heat capacities (Cp) curves (b) of the LMA coating before and after heat treatment at different temperatures. The insets in (a) and (b) are the DSC and Cp curves of C1100 with magnification, respectively.
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Fig. 6. Thermal expansion curves of the LMA coatings before and after heat treatment at different temperatures.
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Fig. 7. SEM micrographs of the fractured cross-sections of coatings: (a) C0; (b)-(bꞌ) C900 with the different magnifications of 5000× and 20000×; (c) C1000; (d)-(dꞌ) C1100 with the different magnifications of 5000× and 20000×.
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Fig. 8. EDS spectra acquired from plate-like crystals of the LMA coating after heat treatment at different temperatures for 20 h: (a) 1000°C; (b) 1100°C.
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Fig. 9. Cross-sectional SEM micrographs of the LMA coating: (a) C0; (b) C900; (c) C1000 and (d) C1100.
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Fig. 10. Porosity and thickness of the LMA coatings before and after heat treatment at different temperatures.
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Fig. 11. Thermal cycling lifetime of LMA coatings before and after heat treatment at different temperatures.
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Fig. 12. Surface photos of LMA coatings before and after thermal cycling tests at 1127°C.
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Fig. 13. XRD patterns of LMA coatings after thermal cycling failure.
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Fig. 14. Cross-sectional SEM micrographs of LMA coatings after thermal cycling failure: (a) FC0; (b) FC900; (c) FC1000; (d) FC1100.
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Fig. 15. SEM images of the elemental maps for FC1100.
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ACCEPTED MANUSCRIPT List of Table Table 1 Plasma spraying parameters Current
Power
Plasma gas,
Carrier gas,
Powder feeding rate
(mm)
(A)
(kW)
(SLPM*)
Ar (SLPM)
(%)
100
620
42
35/12
3.2
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*SLPM: standard liter per minute.
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Spray distance
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C0
4.68 (RT to 736°C)
C900
7.02 (RT to 1014°C)
C1000
7.08 (RT to 1096°C)
C1100
7.64 (RT to 1224°C)
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Coating
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ACCEPTED MANUSCRIPT Highlights • LaMgAl11O19 (LMA) coatings were pretreated at different temperatures. • Microstructures, porosities, and thermal properties of the coatings were studied. • Large cracks were observed in coatings after heat treatment at 1000 and 1100°C.
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• Thermal cycling lifetime of the coating heated at 900°C was increased by 25%.
S0925-8388(17)34562-0
DOI:
10.1016/j.jallcom.2017.12.347
Reference:
JALCOM 44433
To appear in:
Journal of Alloys and Compounds
Received Date: 12 October 2017 Revised Date:
27 December 2017
Accepted Date: 29 December 2017
Please cite this article as: J. Sun, J. Wang, S. Dong, Y. Hui, L. Li, L. Deng, J. Jiang, X. Zhou, X. Cao, Effect of heat treatment on microstructure and property of plasma-sprayed lanthanum hexaaluminate coating, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.347. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Effect of heat treatment on microstructure and property of plasma-sprayed
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lanthanum hexaaluminate coating
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Junbin Suna, Jinshuang Wanga, Shujuan Donga, Yu Huib, Lifen Lia, Longhui Denga,
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Jianing Jianga, Xin Zhoua, ∗, Xueqiang Caoa,* a
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State Key Laboratory of Silicate Materials for Architectures (Wuhan University of
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Technology), China.
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b
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Sciences, China.
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Abstract: Amorphous phase is commonly found in LaMgAl11O19 (LMA) coating
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prepared by atmospheric plasma spraying. In order to improve their performance, the
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as-sprayed LMA thermal barrier coatings on a nickel-based DZ125 substrate were
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heat-treated at 900, 1000 and 1100°C for 5~20 h, respectively. The effect of heat
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treatment on the microstructure, porosity, phase composition, and crystallization
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behavior of the LMA coatings was investigated using scanning electron microscopy,
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X-ray diffraction, differential scanning calorimetry analysis. Samples before and after
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heat treatment for 20 h were thermally cycled at 1127°C. Results indicated that the
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annealing of the as-sprayed LMA coating prior to thermal cycling is preferable to
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promote the crystallization of amorphous phase. Variations in microstructure, phase
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composition and thermal cycling lifetime of LMA coatings have been correlated to
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the heat treatment. Heat treatment at 900°C improved the lifetime of LMA coatings,
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whereas their thermal cycling performance did not significantly improved when the
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Atmospheric Environment Research Center, Shenyang Academy of Enviromental
Corresponding author. Tel/Fax: +86-27-87651856. E-mail: [email protected]. (X. Zhou), [email protected].(X. Cao) *
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ACCEPTED MANUSCRIPT aging temperature increased to 1000 or 1100°C. The corresponding mechanism had
2
been explored by tracing the microstructure and thermo-physical properties evolutions
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during thermal annealing process.
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Keywords: Heat treatment; Lanthanum hexaaluminate; Amorphous phase; Thermal
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cycling lifetime; Plasma spraying; Thermal barrier coatings
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1 Introduction
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Thermal barrier coatings (TBCs) have exhibited an increasing potential for
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improving the durability and efficiency of gas turbine engines by allowing an increase
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in turbine inlet temperature and reducing the amount of cooling air required by the
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hot-section components [1]. TBCs are usually made up of a two-layered structure: a
11
metallic bond coat for oxidation/corrosion resistance and a ceramic topcoat for
12
thermal protection [2, 3]. MCrAlY (M=Ni, and/or Co) is used as a bond coat to
13
provide a good thermal expansion match between the topcoat and substrate and to
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inhibit oxidation of the substrate [4]. The current state-of-the-art of topcoat is zirconia
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partially stabilized with 7-8 wt.% yttria (7-8 YSZ) with high coefficient of thermal
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expansion (CTE) (10.5~11.5×10-6 K-1, 20–1200°C), fracture toughness (~2 MPa • m1/2)
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as well as low thermal conductivity (2.1~2.2 W • m-1 • K-1), which are always
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deposited onto the components either by electron beam-physical vapor deposition
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(EB-PVD) or by atmospheric plasma spraying (APS) [5-7]. YSZ as the commonly
20
used TBCs material performs quite well up to 1200°C. When the material is used at
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higher application temperature over 1200°C for a long time, it exhibits a poor service
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lifetime due to the phase transformation [8, 9], and distinct sintering in combination
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ACCEPTED MANUSCRIPT with the increase of Youngꞌs modulus [10-13]. As a result, developing new TBCs
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materials has been a hot spot of present research to improve the temperature capability
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and durability of the deposited coatings. New TBCs candidate materials such as
4
oxides with pyrochlore (La2Zr2O7, LZ) [14], fluorite (La2Ce2O7) [15], perovskite
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(SrZO3) [16], magnetoplumbite (LaMgAl11O19, LMA) [10, 13] structures have been
6
researched and developed.
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Among these new TBCs materials, LMA has essential characteristics of low
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thermal conductivity (0.8~2.6 W • m-1 • K-1), high fracture toughness (~3.59 MPa •
9
m1/2), high CTE (9~11×10-6 K-1, 20–1200°C) and outstanding thermal stability up to
10
1600°C, owing to the magnetoplumbite structure in which spinel blocks are separated
11
by mirror planes containing the large La3+ cation [17]. With the superior structural and
12
thermochemical stability as well as at least equivalent thermophysical properties with
13
YSZ, LMA has been proposed as one of promising candidates for the next generation
14
TBCs to overcome above problems associated with the use of YSZ at higher
15
temperature [18, 19]. However, as a new TBCs candidate material, some problems are
16
urgent to be solved before it can be widely used. For instance, a large amount of
17
amorphous phase often presents in the plasma-sprayed LMA coating due to the rapid
18
quenching from the molten state [20-22]. After the LMA coating is heated to a high
19
temperature for the first time, it suffers large volume shrinkage, and plane tensile
20
stress will be developed in LMA coating due to the restriction of the superalloy
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substrate. Besides, the recrystallization can give rise to a sudden reduction in CTE
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value of the LMA coating [17]. Then, thermal expansion mismatches between
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thermomechanical loading, which will lead to the spallation and may put questions on
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the reliability of the plasma-sprayed LMA coating [17, 23]. It is obvious that the
4
negative effect of amorphous phase crystallization must be removed to improve the
5
reliability and service lifetime of LMA coatings. As we all known, transition from
6
amorphous phase to crystalline phase occurs as the temperature higher or close to its
7
crystallization temperature [24]. Therefore, the negative effect of amorphous phase
8
crystallization on the LMA coating can be removed by heat treatment prior to its using.
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On the one hand, heat treatment will induce the crystallization of the amorphous
10
phase and avoid the strong thermal stresses caused by fierce heating and cooling cycle
11
alternately. One the other hand, heat treatment also leads to the relaxation of stress
12
formed during the plasma spraying process. These are very helpful to the
13
improvement of the thermal cycling lifetime of the LMA coating in theory. Actually,
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to the best of our knowledge, no works have been carried out on the thermal cycling
15
behavior of the LMA coatings after heat treatment.
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Since the crystallization temperature of amorphous LMA is between 800 and
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1200°C [17] and the upper limit temperature of superalloy DZ125 is about 1200°C,
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therefore, the as-sprayed LMA coatings were heat-treated at 900, 1000 and 1100°C for
19
5~20 h, respectively. The effects of heat treatment on the microstructure and thermal
20
cycling behavior of the coatings were studied, and failure mechanisms of the coatings
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were also analyzed.
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2 Experimental
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2.1 Preparation of powder and coating As the chemicals in this work, the powders of La2O3, MgO and Al2O3 in proper
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molar ratio of 0.5:1:5.5 were mixed with deionized water for 24 h by ball-milling
4
and then dried later at 100°C for 48 h. In order to synthesize the LMA powder, the
5
obtained powder mixture of La2O3, MgO and Al2O3 was heated at 1450°C for 12 h
6
and this process was replicated for two times. The as-synthesized LMA powder was
7
mixed with deionized water, Gum Arabic (adhesion agent), and ammonium citrate
8
(dispersing agent), followed by ball-milling (72 rpm) using zirconia balls for 72 h
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and then the slurry was spray-dried (SFOC-16, Shanghai-Ohkawara Dryers Co.,
10
Ltd.). The free-flowing powders (see Fig. 1) with particle size between 20 and 125
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µm were collected and used directly for plasma spraying without other treatments.
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LMA coating were produced by APS with a Multicoat Plasma Spray Unit
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(Oerlikon Metco, Switzerland). Argon was used as primary plasma gas and powder
14
carrier gas and hydrogen was used as a secondary gas to adjust the arc voltage [25,
15
26]. The substrate was heated to 120°C using plasma flame prior to the coating
16
deposition. LMA coatings with thickness ~200 µm for thermal cycling tests were
17
prepared by APS on the DZ125 superalloy substrate (30×10×2 mm3) with NiCrAlY
18
bond coat. In addition, LMA coatings with thickness 1000~1200 µm for thermal
19
expansion test and thermal analysis were made by APS on graphite substrate without
20
bond coat. Plasma spraying parameters were listed in Table 1.
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2.2 Heat treatment and thermal cycling test
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The as-sprayed LMA coatings were heat-treated in a tube furnace. The pressure 5
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heat treatment process was composed of heating of the coatings from room
3
temperature to 900, 1000 and 1100°C with a heating rate of 5°C/min, respectively,
4
and holding at the temperature for 5, 10, 15 and 20 h, respectively. The as-sprayed
5
LMA coating and coatings after heat treatment at 900, 1000 and 1100°C for 20 h
6
were named as C0, C900, C1000 and C1100, respectively. Thermal cycling
7
experiments, to determine TBCs cyclic lifetime, were performed in a
8
specially-designed, automated furnace. A full 35-min thermal cycle consisted of
9
rapid heating of the samples to 1127°C and holding at temperature (30 min),
10
followed by conventional cooling to near-room temperature (5 min). Each specimen
11
was examined periodically. A specimen was considered to have failed when the
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topcoat failure area (spallation plus delamination) reached ~10% of the total topcoat
13
area. The specimen was then removed from the furnace and the number of thermal
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cycles at which it failed was recorded. Three specimens of every kind of coating
15
were tested simultaneously and the thermal cycling lifetime was the average value of
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the three specimens. Coatings of C0, C900, C1000 and C1100 after thermal cycling
17
failures were named as FC0, FC900, FC1000 and FC1100, respectively.
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2.3 Characterization
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Phase composition of the samples was identified by X-ray diffraction (XRD,
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D/MAX-RB RU-200B, Cu-Kα radiation, λ=0.15406 nm) over 2θ values of 15–65°
21
with a scanning rate of 4°·min−1 by a step width of 0.02°. In addition,
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microstructural studies in this work were performed using a field emission-scanning 6
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2
X-ray spectroscopy (EDS). All the coatings for SEM cross-section analysis were
3
embedded in a transparent epoxy resin and polished with diamond pastes, while the
4
fractured cross-sections were directly analyzed without other treatments. Coating
5
samples for thermal analyses and thermal expansion tests were prepared by abrading
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the graphite substrate. Differential scanning calorimetry (DSC) of the LMA coatings
7
was performed on a thermoanalyzer (Schleibinger CDF Test) in air atmosphere with
8
a heating rate of 20°C·min−1. Coating samples with dimensions of (23~25)
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mm×(5~8) mm×(1~1.2) mm were selected for thermal expansion measurements.
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Thermal expansion of the coatings was determined between room temperature (RT)
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and 1300°C using the Netzsch 402C high-temperature dilatometer. Amorphous
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phase contents of the coatings were based on their amorphous indexes, which were
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the ratios of the amorphous hump areas to the total area of the crystalline peaks and
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humps in XRD patterns in the 2θ range between 15 and 65° [27, 28]. Porosity of the
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coatings was evaluated by image analyses. Average thickness of coatings before and
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after heat treatment was analyzed using SEM micrographs. About 50 measurements
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were taken for each specimen in the thickness analysis.
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3 Results and discussion
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3.1 Crystallization of amorphous phase
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Fig. 2 exhibits XRD patterns of LMA coatings heated at 900, 1000 and 1100°C
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for 0~20 h, respectively. Characteristic peaks of LMA are observed obviously in these
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four coatings, and a broad hump corresponding to amorphous phase [23, 25] is also 7
ACCEPTED MANUSCRIPT noticed (2θ=20-40°) in the XRD patterns of the as-sprayed LMA coating. With the
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heat treatment time increasing from 5 to 20 h, the broad hump gradually flattens. As
3
shown in Fig. 2c, the broad hump seems to disappear after heat treatment at 1100°C
4
for 5 h. However, for the coating heated at 900°C, the broad hump still can be
5
observed obviously after heat treatment for 20 h (see Fig. 2a). In addition, some weak
6
peaks ascribed to LaAlO3, which cannot be observed in the as-sprayed coating, also
7
presents in the patterns of annealed LMA coatings. In fact, the LaAlO3 may be
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covered by the high amorphous phase content in the as-sprayed coating. After heat
9
treatment, amorphous phase decreases by crystallization and LaAlO3 is detected in the
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patterns. The amorphous phase contents of LMA coatings heated 900, 1000 and
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1100°C for 0~20 h are presented in Fig. 3. As shown in Fig. 3, the amorphous phase
12
content decreases from 59 to 33% after heat treatment at 900°C for 20 h. However,
13
for the coating heated at 1100°C, the amorphous phase content suddenly decreases
14
from 59 to 8% after heat treatment for 5 h and then almost has no obvious changes
15
with the increase of time. It is obvious that the crystallization rate of amorphous phase
16
increases with the increase of temperature from 900 to 1100°C. In order to better
17
describe the crystallization rate, Avrami equation was adopted in this work as follows
18
[29]:
19
φ = 1 − ݁ ି௧ (1)
20
where k is the crystallization rate factor, n is the Avrami exponent, t is heat treatment
21
time and ϕ is the crystallization fraction. The relationship between the crystallization
22
fraction ϕ and the amorphous phase content can be described as follows:
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φ=
ூబ ିூ ூబ
× 100%(2)
where I0 is the amorphous phase content of the as-sprayed LMA coating and It is the
3
amorphous phase content of the coating heated in different time. Fig. 4 shows a plot
4
of ϕ as the function of heat treatment time at different temperatures, which is
5
simulated according to Avrani equation. It is obvious that crystallization rate factor k
6
of the LMA coating heated at 1100°C (1.779) is far larger than that of the coatings
7
heated at 900°C (0.017) and 1000°C (0.012). In fact, crystallization rate factor k has
8
dependent on the temperature following the Arrhenius law,
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kሺTሻ = A݁ ି (3)
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ಶೌ
where A and activation energy, Ea, are kinetic parameters which are not dependent on
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temperature. R is the ideal gas constant and T is the temperature. Therefore, k
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increases with the increase of heat treatment temperature from 900 to 1100°C. The
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larger crystallization rate factor k at 1100°C also can be used to explain the suddenly
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decrease of amorphous phase after heat treatment at 1100°C for 5 h.
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In order to further investigate the crystallization process of the amorphous phase
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in the LMA coatings, DSC analyses of LMA coatings heated at different temperatures
17
for 20 h were carried out. As shown in Fig. 5a, there are two sharp exothermal peaks
18
at about 900 and 1170°C in the curve of C0. After heat treatment at 900°C for 20 h,
19
the first sharp peak disappears from the DSC curve of C0. As the heat treatment
20
temperature increases successively, the exothermal peak at about 1170°C gradually
21
gets flat in the DSC curves of the LMA coating, especially in the curve of C1100,
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only a small exothermal peak is observed (see the inset of Fig. 5a). As discussed
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ACCEPTED MANUSCRIPT above, the crystallization level of the amorphous phase increased by increasing the
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heat treatment temperature between 900 and 1100°C. For these reasons, two sharp
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exothermal peaks can be due to the crystallization of the amorphous phase. In fact, the
4
same viewpoint was also proposed by Friedrich [17]. Although the second exothermal
5
peak is at about 1170°C, when the as-sprayed LMA coating is heat-treated at 1100°C
6
for 20 h, it also can accelerate the transformation to the stable phase transition [24].
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Besides, compared with the areas of the two peaks, it can be found that the area of
8
peak at 900°C is almost 1.5 times the peak at 1170°C, which shows that the
9
crystallization of amorphous phase mostly occurs at about 900°C [30]. Although
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about 28% amorphous phase is remained in the coating after heat treatment at 1000°C
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for 20 h, it is hard to decrease the amorphous phase content by prolong the heat
12
treatment time owing to the kinetic limitations of crystallization [31]. Therefore, in
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this work, LMA coatings were heated at different temperatures for 20 h before their
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thermal cycling tests.
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3.2 Heat capacity and thermal expansion
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The heat capacity (Cp) curves of these coatings are illustrated in Fig. 5b. Molar
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specific heat capacity of the as-sprayed LMA coating suffers two sharp decreases
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around 900 and 1170°C, respectively. The sudden changes in Cp values are believed
19
to be caused by the discrete change of the heat resulted from the phase transitions of
20
the as-sprayed LMA coating during heating [23]. Besides, the crystallization of the
21
amorphous phase will induce the volume shrinkage of the coating, leading to the
22
mismatch of CTE between the ceramic coat and the bond coat. As shown in Fig. 6, C0
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2
shrinkages ascribed to the crystallization of amorphous LMA phase [17]. The
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temperature ranges where shrinkages occur are consistent with the results of thermal
4
analyses as discussed above. However, the first shrinkage disappears from the thermal
5
expansion curve of the coating C900. In addition, LMA coating almost show a linear
6
expansion from RT to 1300°C after aging at 1100°C for 20 h. In comparison with the
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low CTE value (4.68×10-6 K-1) of C0, there is a dramatically growth in CTE value of
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the coating after heat treatment (see Table 2). For instance, CTE value of C1100
9
reaches to 7.64×10-6 K-1. Furthermore, the first shrinkage of 2.3% is higher than the
10
second shrinkage of 0.99%, because the crystallization of the amorphous phase in the
11
LMA coating mainly occurs at about 900°C. Results show that the adverse effect of
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the sudden change of Cp and the volume shrinkage on the as-sprayed LMA coating
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can be removed by heat treatment prior to its using.
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3.3 Microstructure
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Fig. 7 shows the fractured cross-sections SEM images of LMA coatings before
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and after heat treatment at 900, 1000 and 1100°C for 20 h, respectively. As shown in
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Fig. 7a, there is a large amount of amorphous phase in the as-sprayed LMA coating,
18
which is consistent with the previous report [23]. After heat treatment at 900°C for 20
19
h, some plate-like crystals (see Fig. 7b), with the different micromorphology in
20
comparison with the amorphous phase, are embedded in the coating matrix. As the
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fractured cross-section SEM micrographs shown in Fig. 7c and Fig. 7d, it can be
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observed that the grain growth process has been significantly accelerated and many
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ACCEPTED MANUSCRIPT plate-like grains present consequently in C1000 and C1100. Fig. 8 presents EDS
2
spectra and chemical compositions acquired from plate-like crystals (see Fig. 7(c, d))
3
of the LMA coatings C1000 and C1100. It is obvious that the plate-like crystals are
4
composed of La2O3-MgO-Al2O3. Besides, in our previous work [23], the
5
crystallization process was confirmed by high-resolution transmission electron
6
microscopy examination (HR-TEM). The amorphous phase as well as the crystalline
7
phase was present in the as-sprayed LMA coating. After heat treatment, the HR-TEM
8
image exhibited a typical lattice configuration of the hexaaluminate with the
9
magnetoplumbite-type structure. It indicated that a transformation of disorder to order
10
occurred to the as-sprayed LMA coating during the heat treatment process. Therefore,
11
it can be concluded that the platelet crystals are formed by the crystallization of
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amorphous phase and the crystallization level shows a growing tendency with the
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temperature increasing, which agrees well with the XRD analysis (see Fig. 2).
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Cross-sectional SEM micrographs of the LMA coatings before and after heat
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treatment for 20 h are shown in Fig. 9. In the four TBCs systems, the LMA topcoat
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bonds well with the bond coat. However, in comparison with the microstructure of the
17
as-sprayed coating (see Fig. 9a), coatings after heat treatment show a porous
18
microstructure with more cracks. As shown in Fig. 9b, some fine vertical cracks start
19
to be observed in C900. After increasing the heat treatment temperature further, the
20
vertical-crack tendency is more obvious. Besides, horizontal cracks also appear in
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C1000 and C1100 (see Fig. 9(c, d)). As shown in Fig. 10, the porosity of the coating
22
increases with the heat treatment temperature increasing. On the one hand, large
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ACCEPTED MANUSCRIPT numbers of plate-like crystalline grains (see Fig. 7) exist in the coatings after heat
2
treatment, which is a contribution to the improvement of the porosity. On the other
3
hand, the in-plane tensile stress, generated because of thermal expansion mismatch
4
between the ceramic topcoat and the bond coat or metallic substrate, can lead to the
5
formation of vertical cracks [32]. As discussed above, the as-sprayed LMA coating
6
suffers two sudden volume shrinkages by crystallization of the amorphous phase and
7
the average CTE value is about 4.68×10-6 K-1. However, the average CTE value of the
8
nickel-base superalloy substrate is about 14×10-6~16×10-6 K-1 (20~1000°C) [33]. The
9
higher CTE of the substrate ensures that the topcoat is under significant tension at
10
high temperature [34]. For the reason, the number and size of the crack all increase
11
with the heat treatment temperature increasing. Besides, there is an obvious reduction
12
in thickness of the coating after heat treatment compared with that of the as-sprayed
13
coating. As shown in Fig. 10, coatings heated at different temperatures show different
14
thickness such that C900＞C1000＞C1100. The decrease in thickness is in good
15
agreement with the thermal expansion analysis of the LMA coatings.
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3.4 Thermal cycling behavior
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Thermal cycling lifetimes of these four coatings are compared in Fig. 11. C900
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has the longest thermal cycling lifetime (126 cycles), which has been improved by
19
25% compared with C0 (100 cycles). C1000 and C1100 (102 cycles) have similar
20
lifetime with C0. Thermal cycling results indicate that the heat treatment temperature
21
plays an important role in the thermal cycling lifetime of the LMA coatings.
22
Fig. 12 shows the surface morphology of the coatings after thermal cycling 13
ACCEPTED MANUSCRIPT failure. As seen from FC0, visible cracks are found evidently (noted by the black
2
frame). In addition, due to the extreme heating and cooling conditions encountered at
3
the edges, obvious surface spallation is observed on the edge of the sample and more
4
than 10% of surface area spalled after failure. The surface photo indicates the failure
5
of the LMA coatings starts at the edge, and the spallation seems to occur instantly
6
during cycling.
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Fig. 13 exhibits XRD patterns of the coatings after thermal failures. Compared
8
with coatings before thermal cycling tests (see Fig. 2), the broad hump
9
corresponding to amorphous phase has disappeared in the XRD patterns of all these
10
coatings, especially in the pattern of FC0. Although the LMA coatings were
11
heat-treated at different temperatures for 20 h, the crystallization of the amorphous
12
phase still occurs during thermal cycling. Besides, LaAlO3 phase disappears from
13
the XRD patterns of coatings after thermal cycling tests, indicating that LaAlO3
14
phase may re-form to LMA accompanying the crystallization of the amorphous
15
phase during the thermal cycling.
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Fig. 14 shows cross-sectional SEM images of coatings after thermal cycling
17
failures. In comparison with the microstructures of coatings before thermal cycling
18
tests, large vertical and horizontal cracks are observed in the failed coatings. From
19
Fig.14, the failure of these four LMA coatings occurred by the formation of large
20
horizontal crack in the ceramic top coat which was close to the interface of ceramic
21
top coat and bond coat. In addition, some dark substances with the thickness of 3~5
22
µm are found at the interface between the ceramic coat and the bond coat of FC900
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2
TGO. It is well known that TGO is the result of selective oxidation of elements
3
within the bond coat. The oxidation of the bond coat is conclusively a result of
4
oxygen transport through the interconnected pores within the porous plasma-sprayed
5
ceramic topcoat during thermal cycling tests. In order to demonstrate the existence of
6
TGO, elemental distributions by map scanning at the topcoat-bond coat interface of
7
FC1100 is determined and their results are shown in Fig. 15(b-g). Cr and Al should
8
be uniformly distributed throughout the bond coat. However, after thermal cycling
9
failure there are high levels of Cr (see Fig. 15f) and Al (see Fig. 15c) at the interface
10
between the bond coat and the ceramic topcoat. From the EDS results, it is obvious
11
that TGO has formed in C900 and C1100 during thermal cycling, and its
12
composition may be Cr2O3 and Al2O3.
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Generally, stresses caused by the thermal expansion mismatch and the growth of
14
thermally grown oxides (TGO) lead to the failure of TBCs during thermal cycling. As
15
discussed above, the as-sprayed LMA coating with large amount of amorphous phase
16
has the lowest CTE (4.68×10-6 K-1). Therefore, it suffers the largest thermal stress
17
caused by mismatch of CTEs between the ceramic topcoat and the substrate, which is
18
enough to lead the failure of C0. For the heat-treated LMA coatings, the amorphous
19
phase content decreases dramatically during heat treatment. LMA coatings of C900,
20
C1000 and C1100 have the larger CTEs (between 7.02 and 7.64×10-6 K-1) than that of
21
C0. This effectively mitigates the thermal expansion mismatch stress level. Besides,
22
since the annealing process is preferable to promote the crystallization of amorphous
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2
recrystallization and the platelet-like LMA grain growth can be avoided. Those can
3
well explain the improvement of thermal durability for C0. However, the emergency
4
of the large cracks in C1000 and C1100 decreases the bond strength of the coating and
5
promote the growth of TGO, which is adverse to the improvement of thermal shock
6
resistance of the coating. Stresses caused by TGO growth also contribute to the failure
7
of these coatings. For these reasons, only the thermal cycling lifetime of C900 has
8
been improved dramatically, and C1000 and C1100 have no obvious improvement in
9
thermal cycling lifetime compared with C0.
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4. Conclusions
LaMgAl11O19 (LMA) coatings were prepared by plasma spraying and then were
12
heat-treated at 900, 1000 and 1100°C for 0~20 h, respectively. With the heat
13
treatment increasing from 900 to 1100°C, the crystallization rate of the amorphous
14
phase increases. During the heat treatment process, the crystallization of the
15
amorphous phase induces the volume shrinkage, the increase of porosity, and the
16
change in heat capacity (Cp) value of the LMA coating. The changes are more obvious
17
by increasing the heat treatment temperature. Heat treatment prior to use of the
18
plasma-sprayed LMA coating at 900°C weakens the negative effect of the amorphous
19
phase and improves the thermal shock resistance of the coating. In comparison with
20
thermal cycling lifetime (100 cycles) of C0, C900 has an obvious improvement of
21
thermal cycling lifetime (126 cycles). However, C1000 and C1100 with the same
22
thermal cycling lifetime (102 cycles) are similar to that of C0.
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2
and the LMA topcoat leads to the failure of the as-sprayed LMA coating (C0). For
3
coatings heated at 900 (C900), 1000 (C1000) and 1100 (C1100), the mismatch of
4
CTE is decreased effectively by recrystallization. The growth of TGO and the weak
5
bond strength lead to the failure of these coatings.
6
Acknowledgement
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The authors thank Dr. Zhitao Shan for the DSC measurements. This work was
8
supported by the National Natural Science Foundation of China (No. 51501137,
9
51702244) and the Natural Science Foundation of Hubei Province (No.
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Meneses, J.G.S. Duque, R.A.G. Silva, Study of the dynamic of crystallization of an amorphous Fe40Ni40P14B6 ribbon through Johnson-Mehl-Avrami model, J. Alloy Compd., 731 (2018) 1275-1279. [30] L.L. Huang, H.M. Meng, J. Tang, Crystallization behavior of plasma-sprayed lanthanide magnesium hexaaluminate coatings, Int. J. Miner. Metall. Mater., 21 (2014) 1247-1253. [31] A. Rosenflanz, M. Frey, B. Endres, T. Anderson, E. Richards, C. Schardt, Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides, Nature, 430 (2004) 761. [32] A.C. Fox, T.W. Clyne, Oxygen transport by gas permeation through the zirconia layer in plasma sprayed thermal barrier coatings, Surf. Coat. Technol., 184 (2004) 311-321. [33] X.Q. Cao, R. Vassen, D. Stoever, Ceramic materials for thermal barrier coatings, J. Eur. Ceram. Soc., 24 (2004) 1-10. [34] Y. Bai, L. Zhao, J.J. Tang, S.Q. Ma, C.H. Ding, J.F. Yang, L. Yu, Z.H. Han, Influence of original powders on the microstructure and properties of thermal barrier coatings deposited by supersonic atmospheric plasma spraying, part II: Properties, Ceram. Int., 39 (2013) 5113–5124.
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Figures
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Fig. 1. XRD pattern and SEM image of the starting powder.
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Fig. 2. XRD patterns of LMA coatings before and after heat treatment at different temperatures: (a) 900°C; (b) 1000°C; (c) 1100°C. A, B, C, D, E represent the heat treatment time of 0, 5, 10, 15, 20 h, respectively.
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Fig. 3. Amorphous phase content of LMA coatings heated at 900, 1000 and 1100°C in different time.
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Fig. 4. Crystallization fraction of LMA coatings heated at 900, 1000 and 1100°C in different time.
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Fig. 5. DSC curves of coatings (a) and heat capacities (Cp) curves (b) of the LMA coating before and after heat treatment at different temperatures. The insets in (a) and (b) are the DSC and Cp curves of C1100 with magnification, respectively.
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Fig. 6. Thermal expansion curves of the LMA coatings before and after heat treatment at different temperatures.
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Fig. 7. SEM micrographs of the fractured cross-sections of coatings: (a) C0; (b)-(bꞌ) C900 with the different magnifications of 5000× and 20000×; (c) C1000; (d)-(dꞌ) C1100 with the different magnifications of 5000× and 20000×.
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Fig. 8. EDS spectra acquired from plate-like crystals of the LMA coating after heat treatment at different temperatures for 20 h: (a) 1000°C; (b) 1100°C.
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Fig. 9. Cross-sectional SEM micrographs of the LMA coating: (a) C0; (b) C900; (c) C1000 and (d) C1100.
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Fig. 10. Porosity and thickness of the LMA coatings before and after heat treatment at different temperatures.
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Fig. 11. Thermal cycling lifetime of LMA coatings before and after heat treatment at different temperatures.
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Fig. 12. Surface photos of LMA coatings before and after thermal cycling tests at 1127°C.
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Fig. 13. XRD patterns of LMA coatings after thermal cycling failure.
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Fig. 14. Cross-sectional SEM micrographs of LMA coatings after thermal cycling failure: (a) FC0; (b) FC900; (c) FC1000; (d) FC1100.
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Fig. 15. SEM images of the elemental maps for FC1100.
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Power
Plasma gas,
Carrier gas,
Powder feeding rate
(mm)
(A)
(kW)
(SLPM*)
Ar (SLPM)
(%)
100
620
42
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3.2
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*SLPM: standard liter per minute.
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C0
4.68 (RT to 736°C)
C900
7.02 (RT to 1014°C)
C1000
7.08 (RT to 1096°C)
C1100
7.64 (RT to 1224°C)
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Coating
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ACCEPTED MANUSCRIPT Highlights • LaMgAl11O19 (LMA) coatings were pretreated at different temperatures. • Microstructures, porosities, and thermal properties of the coatings were studied. • Large cracks were observed in coatings after heat treatment at 1000 and 1100°C.
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• Thermal cycling lifetime of the coating heated at 900°C was increased by 25%.