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Crystallization mechanism of plasma-sprayed LaMgAl11O19 coating
Applied Surface Science 504 (2020) 144509
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Crystallization mechanism of plasma-sprayed LaMgAl11O19 coating a,⁎
c
b
b
Junbin Sun , Yu Hui , Jianing Jiang , Longhui Deng , Xueqiang Cao
b,⁎
T
a College of Chemistry and Material Science, Hengyang Normal University, Hunan Province Universities Key Laboratory of Functional Organometallic Materials, Key Laboratory of Functional Metal-Organic Compounds of Hunan Province, Hengyang 421008, PR China b State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, PR China c Atmospheric Environment Research Center, Shenyang Academy of Environmental Sciences, Shenyang 110016, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Crystallization Al2O3 phase transition Amorphous LaMgAl11O19 Plasma spraying Thermal barrier coatings
Amorphous phase is often detected in the plasma-sprayed LaMgAl11O19 (LMA) coating, the crystallization of which can cause the volume reduction of the coating, resulting in the failure of the coating. However, it is still not clear for the crystallization mechanism of the amorphous phase. In this paper, the coating samples were deposited using atmospheric plasma spraying. Adequate evidence showed that the crystallization of amorphous LMA occurred in two stages at about 900 and 1170 °C, respectively, by studying the phase transition of Al2O3 and the synthesis process of LMA. A novel crystallization mechanism on the amorphous LMA coating was also proposed. Results showed that highly charged La3+ in LMA magnetoplumbite structure controls the crystallization rate. The higher the La content in the composition the harder the crystallization occurs.
1. Introduction Thermal barrier coatings (TBCs) are often coated on the surface of the high-temperature components in aero engine to increase the temperature in the combustion chamber [1–3]. TBCs usually include two layers, ceramic topcoat for thermal insulation and MCrAlY bond coat for oxidation or corrosion resistance [4–6]. To be a TBCs ceramic material, it should have good properties during high-temperature service, such as a high melting point, high thermal expansion coefficient (TEC), high fracture toughness and low thermal conductivity [7]. For instance, yttria-stabilized zirconia (YSZ) has been used as the classic ceramic topcoat material in TBCs for server years because it has good properties [8,9]. However, the YSZ can just be used as the TBCs materials at operating temperature below 1200 °C. When the temperature reaches above 1200 °C, the phase transformation from t-ZrO2 to m-ZrO2 with a volume variation of 4–6% can cause the generation of high stress, leading to the failure of the coating. It is obvious that YSZ has failed to be a TBCs ceramic material in high-temperature (≥1200 °C) thermal insulation. Therefore, researches on new ceramic TBCs materials are extremely urgent to reach the requirements of the TBCs at higher temperature application. LaMgAl11O19 (LMA) with a magnetoplumbite structure shows good mechanical and thermal physical properties, such as excellent hightemperature thermal stability, high fracture toughness (~3.59 MPa⋅m1/ 2 ), high TEC (9–11 × 10−6 K−1, 20–1200 °C) and low thermal
⁎
conductivity (0.8 ~ 2.6 W⋅m−1⋅K−1) [7]. Therefore, LMA has been a candidate topcoat material to replace YSZ for the next generation TBCs [7,10,11]. For example, LMA coatings have the same thermal cycling lifetime with YSZ as tested with a gas burner at 1250 ± 30 °C [12]. However, there are some problems in LMA material to be solved prior to its application in thermal insulation. Firstly, amorphous phase was formed easily in the plasma-sprayed LMA coating, which can seriously influence the reliability of the coating during thermal cycling service. Secondly, amorphous LMA has the weaker corrosion resistance of molten salt than the crystalline phase [6,13,14]. Besides, in previous studies [5,15–17] we found that if the coating had the higher amorphous phase, it always suffered a larger volume reduction ratio during high temperature service and showed a lower TEC. The sudden volume reduction can lead to the failure of the TBCs directly. There are two volume reduction processes between 800–900 °C and 1100–1200 °C in the linear TEC curve of the coating from room temperature (RT) to 1400 °C, respectively [5,15–17]. For the volume variation between 800–900 °C, previous works all regarded it as a result of the transition from amorphous LMA to LMA crystals. However, for the volume variation between 1100–1200 °C, researches on the cause were far from conclusive. For instance, Friedrich et al. [7] thought that the shrinkage between 1100–1200 °C might be also owing to the crystallization of amorphous LMA. Besides, in the process of thermal spraying, partial LMA powders are decomposed to LaAlO3, MgAl2O4 and γ-Al2O3. Therefore, some researchers [10,12] pointed out that the second
Corresponding authors. E-mail addresses: [email protected] (J. Sun), [email protected] (X. Cao).
https://doi.org/10.1016/j.apsusc.2019.144509 Received 19 July 2019; Received in revised form 17 September 2019; Accepted 25 October 2019 Available online 05 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 504 (2020) 144509
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shrinkage was ascribed to the transition from γ to α-Al2O3. Additionally, LaAlO3, MgAl2O4 and γ-Al2O3 starts to react to form the LMA phase at about 1200 °C. Therefore, the volume variation between 1100–1200 °C might be ascribed to the reaction of the decomposed Al2O3, LaAlO3, and MgAl2O4 [18]. No matter what the causes of the shrinkages are in the coating, they are all adverse to the prolongation of the service lifetime for the coating [19]. Therefore, it is a matter of great significance to find the cause of the volume variation of the LMA coating during high-temperature service. The focus of this work was to research the cause of the volume variation in the LMA coating and explained the crystallization mechanism of the amorphous LMA. All the coating samples were prepared by atmospheric plasma spraying (APS). The LMA coating was used to explore the crystallization process. Al2O3 coating was also prepared to investigate the phase transition of Al2O3. Mixture (LaAlO3 + MgAl2O4 + Al2O3) coating was deposited to study the reaction process of LaAlO3 with MgAl2O4 and Al2O3.
Table 1 Plasma spraying parameters. Coating sample
Spray distance (mm)
Current (A)
Power (kW)
Plasma gas (SLPM*)
Carrier gas Ar (SLPM)
LMA (22) LMA (27) LMA (32) LMA (37) LMA (42) Al2O3 coating Mixture coating
100 100 100 100 100 100 100
290 380 450 530 620 620 620
22 27 32 37 42 42 42
32/12 32/12 32/12 32/12 32/12 32/12 32/12
3.2 3.2 3.2 3.2 3.2 3.2 3.2
* SLPM: standard liter per minute.
with varied spraying powers to investigate the effect of the amorphous phase content on the crystallization process. Besides, the LMA coating was named according to its spraying power. For example, if the LMA coating was prepared at the power of 22 kW, it would be named LMA (22). Coatings deposited with the mixture powders and α-Al2O3 powders were prepared at 42 kW. Spraying parameters of these coatings were listed in Table 1. The Al2O3 coating and the mixture coating were heated at predetermined temperatures for 20 h prior to their physicochemical researches.
2. Experimental 2.1. Powder preparation La2O3, MgO, and α-Al2O3 as the raw materials were mixed with each other and then sintered to synthesis the LMA ceramic powders at 1600 °C for 12 h by the solid state reaction. The as-synthesized ceramic LMA powders were spray-dried (SFOC-16, Shanghai-Ohkawara Dryers Co., Ltd.) to get the particles between 20 and 125 μm for plasma spraying. The process of the spray-drying was described in great detail in our previous work [17]. La2O3 and α-Al2O3 were used to synthesize LaAlO3. MgO and αAl2O3 were selected for the synthesis of MgAl2O4. LaAlO3 and MgAl2O4 were all synthesized at 1200 °C for 12 h by solid-state reaction. LaAlO3, MgAl2O4, and α-Al2O3 were mixed with the molar ration of 1:1:4 to hold the same compositions of LaMgAl11O19 (LaAlO3·MgAl2O4·4Al2O3). The mixture powders were also spray-dried with the similar way of LMA powders. In addition, α-Al2O3 powder was spray-dried in the same way. The X-ray diffractometer patterns of the LMA powder and the mixture powders are shown in Fig. 1.
2.3. Characterization Phase analysis was performed by an X-ray diffractometer (XRD, D/ MAX-RB RU-200B) with Cu-Kα radiation (λ = 0.15406 nm) to scan the diffraction angles (2θ) between 15° and 65° in rate of 4°⋅min−1. The coatings for cross-section analysis were mounted in epoxy resin followed by fine polishing with a diamond paste, then the microstructure of the coatings were analyzed by a field emission-scanning electron microscope (SEM, QUANTA FEG 450). Element analysis of the LMA crystal was carried out using an energy dispersive X-ray spectroscopy (EDS) equipped on the SEM. A thermal analyzer (Netzsch STA 449 F1) was used for differential scanning calorimetry (DSC) analysis from RT to 1250 °C. A Netzsch 402C high-temperature dilatometer was used for thermal expansion test from RT to 1400 °C.
2.2. Coating preparation and heat treatment 3. Results and discussion A Multicoat Plasma Spray unit (Oerlikon Metco, Switzerland) was used to deposit the coating samples using Ar-H2 as plasma gases. All coatings were deposited on graphite substrate directly and then were stripped from the substrate by abrading. LMA coatings were prepared
3.1. Crystallization process The crystallization of the amorphous phase is accompanied by the exothermic event [20], which can be proved using thermal analysis. Fig. 2 shows the DSC curves of the LMA coatings prepared by varied spraying powers. As shown in Fig. 2a, two remarkable exothermal peaks present in the curves at the place of 900 °C (the first exothermal peak, P1) and 1170 °C (the second exothermal peak, P2), respectively. As shown in Fig. 2b and Fig. 2c, the intensity of the two exothermal peaks strengthens from LMA (22) to LMA (42). As reported in previous studies [10,12], the first peak at about 900 °C was owing to the transition from amorphous phase to crystal phase. Naturally, coatings with high amorphous phase content will generate large thermal effect. Therefore, the peak with larger area in the DSC curve means the higher amorphous phase content in the corresponding LMA coating. In fact, fully melted particles can form the amorphous phase easily as impacting on the substrate. High spraying powers can increase the plasma flame temperature and then the particles are melted well, resulting in the high amorphous phase content in the depositions. Peak areas of the two peaks are calculated by integration method and compared in Fig. 3. No matter what the cause of P1 or P2 is, their areas all increase by increasing the spraying power. On the basis fact of that P1 is induced by the transition of amorphous phase to crystal phase, while the area of P2 exhibits the same tendency with that of P1. Therefore, it is reasonable
Fig. 1. XRD patterns of the starting powders: (a) LMA powders synthesized at 1600 °C; (b) mixture powders (LaAlO3 + MgAl2O4 + α-Al2O3). 2
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Fig. 2. (a) DSC curves of LMA coatings deposited with different spraying powers; (b) and (c) are the selected areas in figure (a), respectively.
Fig. 3. Area of the two exothermal peaks in DSC curves of the coatings deposited with different spraying powers. Fig. 4. XRD patterns of (a) the as-sprayed Al2O3 coating and after heat treatment at different temperatures for 20 h: (b) 900 °C; (c) 1000 °C; (d) 1100 °C and (e) 1200 °C.
to speculate that P2 is as well as caused by the phase transition from the amorphous LMA to LMA crystals. However, other works on the phase transition of γ to α-Al2O3 and the reaction of LaAlO3 with MgAl2O4 and Al2O3 should be excluded to explain the crystallization peak of P2 adequately.
after heat treatment present in Fig. 4. Although the feedstock is composed of α-Al2O3 (see Fig. 1), γ-Al2O3 is observed in the as-sprayed Al2O3 coating (Fig. 4a). In fact, γ-Al2O3 was formed by the melt during APS [22]. The relative content of γ-Al2O3 for about 30 mol.% is determined by normalization method of the diffraction peak intensity. Before a complete development of the α-Al2O3 (Fig. 4e), only one intermediate phase of δ* [23] is observed (Fig. 4b). With the temperature increasing from 900 to 1100 °C, there is considerably more δ*-Al2O3 present, but no discernable increase in α-Al2O3 (Fig. 4 (b-d)). This indicates that γ-Al2O3 with 30 mol.% content has transformed into δ*Al2O3. The results show that there is an intermediate phase of δ* between the phase transitions of γ to α-Al2O3. Consideration the fact that
3.2. Phase transition of Al2O3 In TBCs system, alumina as the product by the oxidation of bond coat is often observed at the interface in the TBCs systems. Once the Al2O3 has formed during thermal cycling, thermal expansion mismatch and the volume shrinkage caused by the phase transition of Al2O3 induce the failure of TBCs. However, in the LMA TBCs, Al2O3 can be the product of the decomposition of the LMA powder during the process of spraying [21]. The diffraction patterns of Al2O3 coatings before and 3
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Fig. 5. Thermal expansion curves of Al2O3 bulk, the as-sprayed Al2O3 coating and LMA (42) coating. Fig. 6. XRD patterns of (a) the as-sprayed mixture coating and after heat treatment at different temperatures for 20 h: (b) 900 °C; (c) 1000 °C; (d) 1100 °C; (e) 1200 °C.
phase transition can induce the volume variation, therefore, thermal expansion test of the Al2O3 bulk (α-Al2O3) and as-sprayed Al2O3 coating is carried out and the results are shown in Fig. 5. It can be seen that the Al2O3 coating shows approximately linear expansion from room temperature to 1194.5 °C, followed by a rapid shrinkage for about 0.71%. As the temperature increasing to about 1300 °C, a little further shrinkage is observed. However, no obvious shrinkage at about 1200 °C is observed and only a little shrinkage at about 1300 °C is observed in the Al2O3 bulk. According to the XRD analyses results, the rapid shrinkage at about 1200 °C in the Al2O3 coating should be caused by the phase transitions from δ* to α-Al2O3. The little shrinkage both in Al2O3 bulk and Al2O3 coating should be caused by the sintering. In this paper, the ideal density of α-Al2O3 is taken to be 3.987 g/cm3 (JCPDS card 46–1212) and δ*-Al2O3 is taken to be 3.665 g/cm3 (JCPDS card 46–1215) [23]. There should be linear shrinkage of about 2.7% from δ* to α-Al2O3. The lower shrinkage than the expected could be a result of the lower content of δ*-Al2O3 in the Al2O3 coating (Fig. 4d). The results also indicate that δ*-Al2O3 transits to α-Al2O3 at about 1200 °C. However, no obvious shrinkage is observed at about 900 °C where γ-Al2O3 transforms into δ*-Al2O3. As reported in previous works [22,24,25], the γ-Al2O3 transforms continuously via the short range diffusion and reordering of vacancies and cations to δ*-Al2O3. Besides, the as-sprayed Al2O3 coating is mainly composed of α-Al2O3. For these reasons, no obvious volume change can be observed at about 900 °C in the linear thermal expansion curve. In addition, the linear thermal expansion curve of LMA (42) coating is also shown in Fig. 5. There are two shrinkages of about 2.3% between 800–900 °C and 0.99% between 1100–1200 °C which coincides with previous works reported [5,7,15–17]. The second shrinkage between 1100 °C and 1200 °C in LMA (42) coating is distinctly different with the shrinkage at 1200 °C of the as-sprayed Al2O3 coating in temperature range and shrinkage level. Even if the second shrinkage in the LMA coating was caused by the change from γ-Al2O3 to α-Al2O3, it is impossible to induce such a larger linear shrinkage of 0.99% with the lower content of γ-Al2O3 (< 5 mol. %) in the LMA (42) coating[17], because the shrinkage in the assprayed Al2O3 coating with 30 mol.% γ-Al2O3 (see Fig. 4a) is only 0.71%. Therefore, the second shrinkage between 1100 °C and 1200 °C in the plasma-sprayed LMA coating has no any relations with the change from γ-Al2O3 to α-Al2O3. As a result, P2 in the DSC curves of LMA coatings cannot be owing to the change from γ-Al2O3 to α-Al2O3.
reaction, MgO reacts with Al2O3 to synthesize MgAl2O4 in the temperature of about 700 °C, and then LaAlO3 is synthesized in the temperature of about 900 °C by the reaction between La2O3 and Al2O3, finally, LMA starts to form at approximately 1200 °C by the further reaction of LaAlO3 with Al2O3 and MgAl2O4 [26]. However, a temperature of more than 1800 °C is required to hold the pure LMA within 24 h [27]. Therefore, some intermediate products such as LaAlO3 and MgAl2O4 always exist in the LMA powders synthesized by solid-state reaction at the synthesis temperature below to 1800 °C (see Fig. 1). Besides, partial LMA decomposed during the process of plasma spraying. During high-temperature service, LaAlO3, MgAl2O4 and Al2O3 retained in the coating reacted with each other to resynthesize LMA [18]. In order to prove it and find the reaction temperature, mixture coatings using mixture powders of LaAlO3·MgAl2O4·4Al2O3 with the same element compositions of LaMgAl11O19 were deposited by APS and heat-treated at predetermined temperatures for 20 h, respectively. The diffraction patterns of the as-deposited mixture coatings and the heated are shown in Fig. 6. Compared with the diffraction pattern of the mixture powder (Fig. 1), some characteristic peaks ascribed to LaAlO3, MgAl2O4 and α-Al2O3 are also observed in the primitive mixture coating (Fig. 6a), the difference is that the peak intensity decreases dramatically. Besides, a broad hump and some small peaks ascribed to LMA and γ-Al2O3 are also observed. These show that the amorphous LMA and the LMA crystal are formed during APS. As the coating heated at 900 °C, many more LMA characteristic peaks exhibit and the broad hump gradually flattens (Fig. 6b). In addition, δ*-Al2O3 is also observed. The formation of δ*-Al2O3 during APS has been discussed in Section 3.2. Before heat treatment at 1100 °C, the main phase in the XRD patterns is LaAlO3 (Fig. 6(a–c)). Once the mixture coating was heated at 1100 °C for 20 h, the peak intensity of LaAlO3 has a sudden reduction and LMA become the main phase in the XRD pattern, besides, MgAl2O4 and δ*-Al2O3 disappear from the pattern (Fig. 6d). When the coating was heat-treated at 1200 °C, LMA characteristic peaks become sharp and the broad humps disappear from the XRD pattern (Fig. 6e). Fig. 7 shows the TG-DSC curves of the mixture coating. As shown in Fig. 7, two exothermal peaks are also observed as well as the LMA coating. Besides, a relatively flat peak is also observed at about 1100 °C (Fig. 7b), which is out of the temperature range of P2 at about 1170 °C. According to the XRD analyses results (Fig. 6), the flat exothermal peak at about 1100 °C should be due to the reaction of LaAlO3, MgAl2O4 and Al2O3. The synthesis of LMA is diffusion-controlled and does not have the obvious thermal effect as the crystallization process, so the reaction process is shown in the DSC curve by a flat peak. Besides, there is lower
3.3. Reformation of LMA La2O3, MgO and Al2O3 are often selected as the raw materials to prepare LMA by solid-phase synthesis. The synthesis mechanism of LMA can be summarized as follows: during the first part of the overall 4
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Fig. 7. (a) TG-DSC curves of the mixture coating and (b) the selected area of Fig. 7a with magnification.
results of the plate-like LMA crystals (Fig. 8(a, b)) [15], which are formed by the crystallization at different temperatures during heat treatment. The chemical compositions of the crystals are listed in Table 2. It can be seen that plate-like crystals are composed of La2O3MgO-Al2O3. As listed in Table 2, there is a slight deviation in stoichiometry ratio of the crystals, and crystals crystallized at 900 °C have higher La content than that crystallized at 1200 °C. However, there are no any other phases in the coatings heat-treated at 900 °C and 1200 °C compared with the ideal magnetoplumbite-type LMA phase [15]. The ideal composition of LaMgAl11O19 can be expressed as the formula La2O3–xMgO–yAl2O3 (x = 2 and y = 11). However, Gadow et al. [29] pointed out that a little deviation in the composition of the ternary system La2O3–MgO–Al2O3 was allowed to be the magnetoplumbite phase. For the formula La2O3–xMgO–yAl2O3, if the value of x was among the range of 0.2–3.3 and the value of y was among the range of 10.0–13, the magnetoplumbite phase would be formed by the reaction of La2O3, MgO and Al2O3. As show in Table 2, the compositions of the plate-like crystals crystallized at 900 °C and 1200 °C are within the varied composition range. For these reasons, the retained LaAlO3, Al2O3 and MgAl2O4 also can react with each other to form the LMA with a
content of LaAlO3, MgAl2O4 and Al2O3 in the as-deposited coating, therefore, the thermal effect is hard to be detected by the thermal analyzer and no obvious exothermal peaks at about 1100 °C can be observed in the DSC curves (Fig. 2). Therefore, it can be concluded that LaAlO3, MgAl2O4 and Al2O3 retained in LMA coatings react completely with each other to synthesis LMA at 1100 °C, and the second exothermal peak at about 1170 °C in the LMA coating cannot be caused by the reformation of LMA.
3.4. Crystallization mechanism As discussed above, it can be concluded that partial amorphous LMA crystallizes at 900 °C firstly and then the remained crystallizes at 1170 °C. The mechanism of the crystallization for amorphous LMA phase will be discussed in this section. As shown in Fig. 2 and Fig. 3, although LMA coatings deposited with different amorphous phase contents, crystallization peaks of P1 and P2 in the DSC curves of theses coatings are all at about 900 °C and 1170 °C. It indicates that the difference in amorphous phase content cannot cause the change of the crystallization temperature [28]. Fig. 8 shows the element analysis
Fig. 8. EDS spectra acquired from plate-like crystals of the LMA coating after heat treatment at different temperatures for 20 h: (a, c) 900 °C; (b, d) 1200 °C. 5
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Table 2 EDS analyses of plate-like crystals in LMA (42) after heat treatment at different temperatures. Plate-like crystals
La (wt.%)
Mg (wt.%)
Al (wt.%)
Chemical formula
Heat treatment at 900 °C Heat treatment at 1200 °C
17.26 20.35
3.38 3.25
42.79 44.14
La2O3·2.24MgO·12.78Al2O3 La2O3·1.82MgO·11.18Al2O3
Fig. 9. DSC curves of LMA (42) coating with heating rates of 10°/min, 20°/min, 30°/min, 40°/min and the crystallization activation energy of the two exothermal peaks: (a)-(b) P1, at 900 °C; (c)-(d) P2, at 1170 °C.
crystallization kinetics factor k was a more reasonable criterion than the existing criterion, activation energy of crystallization E. Modified Johnson-Mehl-Avrami (JMA) equation were also presented by Hu et al. to calculate the kinetics factor, as follows:
Table 3 Crystallization kinetics factors of LMA (42) coating at 900 °C and 1170 °C. α (K/ min)
P1 Tp (K)
10 20 30 40
1164 1171 1173 1179
P2 E (kJ/ mol)
ν (min−1)
1072
1.28 × 1048
Tp (K)
1432 1451 1458 1466
E (kJ/ mol)
ν (min−1)
697
7.98 × 1024
2
E E ⎛ Tp ⎞ ln ⎜ ⎟ = + ln ⎛ ⎞ − ln ν . RT R α ⎝ ⎠ p ⎝ ⎠
(1)
where Tp is the crystallization peak temperature, K; E is the apparent activation energy, J/mol; R (8.3144 J/mol·K) is the ideal gas constant; α is the heating rate, K/min; ν is the frequency factor, min−1. Besides, the relationship between the crystallization rate factor k and the temperature T is as follows:
slight deviation in stoichiometry ratio and matches well with the XRD pattern of the ideal magnetoplumbite-type LMA phase (Fig. 6d). Therefore, different crystallization temperatures of P1 and P2 can be caused by the different chemical compositions of the amorphous phase. Generally, high crystallization temperature of P2 than that of P1 shows the poor crystallization ability of the amorphous LMA at 1170 °C. The crystalline ability of the amorphous phase is a key index for the properties of the amorphous LMA coating. In fact, a series of theories on the crystallization of glass were proposed, which are also suited to the crystallization process of the amorphous LMA. Activation energy E was used to evaluate the crystallization ability. Kissinger method and Ozawa method [30] were commonly used to study the apparent activation energy E. It was considered that the larger E value means glasses crystallize more difficult. Nevertheless, more and more contradictions were found in these two methods. Hu et al. [31] reported that the
E ⎞. k(T) = ν exp ⎛− ⎝ RT ⎠
(2)
According to Eq. (1), Tp can be obtained by adjusting the heating Tp2
rate of DSC. Eq. (1) is used to plot ln ⎛ α ⎞ versus 1 for various heating Tp ⎝ ⎠ rates. It will generally lead to a straight line of practically equal slope E , R and then the E and ν can be obtained, as shown in Fig. 9 (b, d). Values of E and ν obtained from Equation (1) are also listed in Table 3. Fig. 10 shows a plot of k as the function of temperature for the two crystallization peaks in the DSC curve of the LMA coating. It can be seen that the value of k at 900 °C are much higher than that at 1170 °C. Although the activation energy of P1 is larger than that of P2, the crystallization kinetics factor k takes into account both the activation energy E and the 6
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Fig. 10. Plot of crystallization rate factor, k, as function of temperature for two exothermal peaks: (a) P1, at 900 °C; (b) P2, at 1170 °C.
frequency factor ν, so it is believed to be a good criterion for the evaluation of glass stability. The higher value of k at 900 °C means the higher crystalline ability [32,33]. LMA with a magnetoplumbite structure consists of two close-packed blocks of Al and O atoms separated by a mirror plane where the highly charged La3+ lies [34,35]. This layered structure has fully occupied mirror planes because of the highly charged La3+ and causes effective suppression of diffusion processes vertically to the crystallographic caxis [7]. It means that the La3+ ion strongly affect the crystallization rate of the amorphous LMA coating [12]. For amorphous LMA, the crystallization rates are diffusion-controlled at 900 °C and 1170 °C, the activation energies E (see Table 3) is related to the viscosity of the amorphous LMA [36]. As temperature increases, the viscosity of the amorphous LMA decreases. The change of viscosity may be the reason that activation energy E at 900 °C is higher than that at 1170 °C. In fact, a small amount of amorphous phase remains in the coating after heat treatment at 900 °C [15]. Higher temperature is needed to cause the complete crystallization of the amorphous LMA. As listed in Table 2, plate-like crystals in Fig. 8b have the higher La content than that in Fig. 8a. Furthermore, high La content will decreases the crystallization rates of the amorphous LMA coating. For this reason, the crystallization kinetic factor k (Fig. 10) at 1170 °C is lower than that at 900 °C. According to the element compositions analyses of the plate-like crystals and the crystallization kinetics studies, the mechanism of the amorphous LMA coating can be concluded as follows: La3+ plays a decisive role to control the amorphous LMA crystallization, plate-like crystals originated from the crystallization of the amorphous phase with the higher La content often correspond to the higher crystallization temperature. Amorphous LMA crystallizes at about 900 °C and 1170 °C, respectively for the difference in element compositions.
Acknowledgement This work was financially supported by National Natural Science Foundation of China (No. 51902092 and No. 51902279), the Foundation of Hengyang Normal University (No. 18D07) and the Foundation of Key Laboratory of Functional Metal-Organic Compounds of Hunan Province (No. MO19K07). References [1] D. Clarke, C. Levi, Materials design for the next generation thermal barrier coatings, Annu. Rev. Mater. Res. 33 (2003) 383–417. [2] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas-turbine engine applications, Science 296 (2002) 280–284. [3] D.R. Clarke, S.R. Phillpot, Thermal barrier coating materials, Mater. Today 8 (2005) 22–29. [4] X. Zhong, H. Zhao, C. Liu, L. Wang, F. Shao, X. Zhou, S. Tao, C. Ding, Improvement in thermal shock resistance of gadolinium zirconate coating by addition of nanostructured yttria partially-stabilized zirconia, Ceram. Int. 41 (2015) 7318–7324. [5] J. Sun, J. Wang, X. Zhou, S. Dong, L. Deng, J. Jiang, X. Cao, Microstructure and thermal cycling behavior of plasma-sprayed LaMgAl11O19 coatings, Ceram. Int. 44 (2018) 5572–5580. [6] S. Tsukada, S. Kuroda, M. Nishijima, H. Araki, A. Yumoto, M. Watanabe, Effects of amorphous phase on hot corrosion behavior of plasma-sprayed LaMgAl11O19 coating, Surf. Coat. Technol. 363 (2019) 95–105. [7] 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. [8] D. Stöver, G. Pracht, H. Lehmann, M. Dietrich, J.-E. Döring, R. Vaßen, New material concepts for the next generation of plasma-sprayed thermal barrier coatings, J. Therm. Spray Technol. 13 (2004) 76–83. [9] X.Q. Cao, R. Vassen, D. Stoever, Ceramic materials for thermal barrier coatings, J. Eur. Ceram. Soc. 24 (2004) 1–10. [10] R. Gadow, M. Lischka, Lanthanum hexaaluminate — novel thermal barrier coatings for gas turbine applications — materials and process development, Surf. Coat. Technol. 151-152 (2002) 392–399. [11] P. Jana, P.S. Jayan, S. Mandal, K. Biswas, Microstructural design of neodymiumdoped lanthanum–magnesium hexaaluminate synthesized by aqueous sol–gel process, J. Mater. Sci. 50 (2015) 344–353. [12] X. Chen, Z. Yu, W. Huang, H. Ma, B. Zou, W. Ying, X. Cao, Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating, J. Eur. Ceram. Soc. 31 (2011) 2285–2294. [13] J. Zeng, J. Sun, H. Zhang, X. Yang, F. Qiu, P. Zhou, W. Niu, S. Dong, X. Zhou, X. Cao, Lanthanum magnesium hexaluminate thermal barrier coatings with pre-implanted vertical microcracks: thermal cycling lifetime and CMAS corrosion behaviour, Ceram. Int. 44 (2018) 11472–11485. [14] J. Zeng, J. Sun, P. Liang, X. Yang, S. Dong, J. Jiang, L. Deng, X. Zhou, X. Cao, Heattreated lanthanum magnesium hexaaluminate coatings exposed to molten calciummagnesium-alumino-silicate, Ceram. Int. 45 (2019) 11723–11733. [15] 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, J. Alloys Compd. 739 (2018) 856–865. [16] J. Sun, J. Wang, Z. Hao, J. Yuan, S. Dong, J. Jiang, L. Deng, Z. Xin, X. Cao, Preparation, structure, mechanical properties and thermal cycling behavior of porous LaMgAl11O19 coating, J. Alloys Compd. 750 (2018) 1007–1016. [17] J. Sun, J. Wang, X. Zhou, Y. Hui, S. Dong, L. Li, L. Deng, J. Jiang, X. Cao, Thermal cycling behavior of the plasma-sprayed coating of lanthanum hexaaluminate, J. Eur. Ceram. Soc. 38 (2018) 1919–1929. [18] L.L. Huang, H.M. Meng, J. Tang, Crystallization behavior of plasma-sprayed lanthanide magnesium hexaaluminate coatings, Int. J. Miner. Metal. Mater. 21 (2014)
4. Conclusions In the present work, some disputes on the cause of the second exothermal peak (P2) at about 1170 °C in the DSC curves of the plasmasprayed LaMgAl11O19 (LMA) coating are clarified. The phase transition of γ to α-Al2O3 in plasma-sprayed Al2O3 coating starts at about 1200 °C and induces the shrinkage of about 0.71%. The reaction to reform LMA with the retained of LaAlO3, MgAl2O4 and Al2O3 in the as-sprayed LMA coating occurs at about 1100 °C, which are out of the temperature range of P2. The results show that P2 is also owing to the crystallization of amorphous LMA, which can induce the linear shrinkage of about 0.99% of the coating. The difference in the element compositions leads to the crystallization process of amorphous LMA in two stages. Plate-like crystals crystallized at 900 °C have the lower La content than that crystallized at 1170 °C. La3+ ion strongly affect the crystallization rates of the amorphous LMA coating. The higher the La content in the composition the harder the crystallization occurs. 7
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1247–1253. [19] C.C. Berndt, Thermal spray: surface engineering via applied research, in: proceedings of the 1st Intenational Thermal Spray Conference, 8-11 May, 2000, Montreal, Canada, ASM International; 2000. [20] N. Karpukhina, R.G. Hill, R.V. Law, Crystallisation in oxide glasses - a tutorial review, Chem. Soc. Rev. 43 (2014) 2174–2186. [21] X. Chen, Y. Zhang, X. Zhong, Z. Xu, J. Zhang, Y. Cheng, Y. Zhao, Y. Liu, X. Fan, Y. Wang, 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.J. Damani, P. Makroczy, Heat treatment induced phase and microstructural development in bulk plasma sprayed alumina, J. Eur. Ceram. Soc. 20 (2000) 867–888. [23] D. Fargeot, D. Mercurio, A. Dauger, Structural characterization of alumina metastable phases in plasma sprayed deposits, Mater. Chem. Phys. 24 (1990) 299–314. [24] I. Levin, L.A. Bendersky, D.G. Brandon, M. Rühle, Cubic to monoclinic phase transformations in alumina, Acta Mater. 45 (1997) 3659–3669. [25] Y.G. Wang, P.M. Bronsveld, J.T.M. DeHosson, B. Djuričić, D. McGarry, S. Pickering, Ordering of Octahedral Vacancies in Transition Aluminas, J. Am. Ceram. Soc. 81 (1998) 1655–1660. [26] J. Sun, J. Wang, W. Huang, Y. Hui, X. Zhou, L. Li, J. Jiang, L. Deng, Y. Niu, S. Dong, Influence of synthesis temperatures on the crystalline grain growth and morphology of lanthanum magnesium hexaaluminate, J. Rare Earths 35 (2017) 1226–1232. [27] R.R. Rao, L. Mariappan, Combustion synthesis and characterisation of lanthanum hexa-aluminate, Adv. Appl. Ceram. 104 (2005) 268–271. [28] F. Tarasi, M. Medraj, A. Dolatabadi, J. Oberste-Berghaus, C. Moreau, High-
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Full Length Article
Crystallization mechanism of plasma-sprayed LaMgAl11O19 coating a,⁎
c
b
b
Junbin Sun , Yu Hui , Jianing Jiang , Longhui Deng , Xueqiang Cao
b,⁎
T
a College of Chemistry and Material Science, Hengyang Normal University, Hunan Province Universities Key Laboratory of Functional Organometallic Materials, Key Laboratory of Functional Metal-Organic Compounds of Hunan Province, Hengyang 421008, PR China b State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, PR China c Atmospheric Environment Research Center, Shenyang Academy of Environmental Sciences, Shenyang 110016, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Crystallization Al2O3 phase transition Amorphous LaMgAl11O19 Plasma spraying Thermal barrier coatings
Amorphous phase is often detected in the plasma-sprayed LaMgAl11O19 (LMA) coating, the crystallization of which can cause the volume reduction of the coating, resulting in the failure of the coating. However, it is still not clear for the crystallization mechanism of the amorphous phase. In this paper, the coating samples were deposited using atmospheric plasma spraying. Adequate evidence showed that the crystallization of amorphous LMA occurred in two stages at about 900 and 1170 °C, respectively, by studying the phase transition of Al2O3 and the synthesis process of LMA. A novel crystallization mechanism on the amorphous LMA coating was also proposed. Results showed that highly charged La3+ in LMA magnetoplumbite structure controls the crystallization rate. The higher the La content in the composition the harder the crystallization occurs.
1. Introduction Thermal barrier coatings (TBCs) are often coated on the surface of the high-temperature components in aero engine to increase the temperature in the combustion chamber [1–3]. TBCs usually include two layers, ceramic topcoat for thermal insulation and MCrAlY bond coat for oxidation or corrosion resistance [4–6]. To be a TBCs ceramic material, it should have good properties during high-temperature service, such as a high melting point, high thermal expansion coefficient (TEC), high fracture toughness and low thermal conductivity [7]. For instance, yttria-stabilized zirconia (YSZ) has been used as the classic ceramic topcoat material in TBCs for server years because it has good properties [8,9]. However, the YSZ can just be used as the TBCs materials at operating temperature below 1200 °C. When the temperature reaches above 1200 °C, the phase transformation from t-ZrO2 to m-ZrO2 with a volume variation of 4–6% can cause the generation of high stress, leading to the failure of the coating. It is obvious that YSZ has failed to be a TBCs ceramic material in high-temperature (≥1200 °C) thermal insulation. Therefore, researches on new ceramic TBCs materials are extremely urgent to reach the requirements of the TBCs at higher temperature application. LaMgAl11O19 (LMA) with a magnetoplumbite structure shows good mechanical and thermal physical properties, such as excellent hightemperature thermal stability, high fracture toughness (~3.59 MPa⋅m1/ 2 ), high TEC (9–11 × 10−6 K−1, 20–1200 °C) and low thermal
⁎
conductivity (0.8 ~ 2.6 W⋅m−1⋅K−1) [7]. Therefore, LMA has been a candidate topcoat material to replace YSZ for the next generation TBCs [7,10,11]. For example, LMA coatings have the same thermal cycling lifetime with YSZ as tested with a gas burner at 1250 ± 30 °C [12]. However, there are some problems in LMA material to be solved prior to its application in thermal insulation. Firstly, amorphous phase was formed easily in the plasma-sprayed LMA coating, which can seriously influence the reliability of the coating during thermal cycling service. Secondly, amorphous LMA has the weaker corrosion resistance of molten salt than the crystalline phase [6,13,14]. Besides, in previous studies [5,15–17] we found that if the coating had the higher amorphous phase, it always suffered a larger volume reduction ratio during high temperature service and showed a lower TEC. The sudden volume reduction can lead to the failure of the TBCs directly. There are two volume reduction processes between 800–900 °C and 1100–1200 °C in the linear TEC curve of the coating from room temperature (RT) to 1400 °C, respectively [5,15–17]. For the volume variation between 800–900 °C, previous works all regarded it as a result of the transition from amorphous LMA to LMA crystals. However, for the volume variation between 1100–1200 °C, researches on the cause were far from conclusive. For instance, Friedrich et al. [7] thought that the shrinkage between 1100–1200 °C might be also owing to the crystallization of amorphous LMA. Besides, in the process of thermal spraying, partial LMA powders are decomposed to LaAlO3, MgAl2O4 and γ-Al2O3. Therefore, some researchers [10,12] pointed out that the second
Corresponding authors. E-mail addresses: [email protected] (J. Sun), [email protected] (X. Cao).
https://doi.org/10.1016/j.apsusc.2019.144509 Received 19 July 2019; Received in revised form 17 September 2019; Accepted 25 October 2019 Available online 05 November 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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shrinkage was ascribed to the transition from γ to α-Al2O3. Additionally, LaAlO3, MgAl2O4 and γ-Al2O3 starts to react to form the LMA phase at about 1200 °C. Therefore, the volume variation between 1100–1200 °C might be ascribed to the reaction of the decomposed Al2O3, LaAlO3, and MgAl2O4 [18]. No matter what the causes of the shrinkages are in the coating, they are all adverse to the prolongation of the service lifetime for the coating [19]. Therefore, it is a matter of great significance to find the cause of the volume variation of the LMA coating during high-temperature service. The focus of this work was to research the cause of the volume variation in the LMA coating and explained the crystallization mechanism of the amorphous LMA. All the coating samples were prepared by atmospheric plasma spraying (APS). The LMA coating was used to explore the crystallization process. Al2O3 coating was also prepared to investigate the phase transition of Al2O3. Mixture (LaAlO3 + MgAl2O4 + Al2O3) coating was deposited to study the reaction process of LaAlO3 with MgAl2O4 and Al2O3.
Table 1 Plasma spraying parameters. Coating sample
Spray distance (mm)
Current (A)
Power (kW)
Plasma gas (SLPM*)
Carrier gas Ar (SLPM)
LMA (22) LMA (27) LMA (32) LMA (37) LMA (42) Al2O3 coating Mixture coating
100 100 100 100 100 100 100
290 380 450 530 620 620 620
22 27 32 37 42 42 42
32/12 32/12 32/12 32/12 32/12 32/12 32/12
3.2 3.2 3.2 3.2 3.2 3.2 3.2
* SLPM: standard liter per minute.
with varied spraying powers to investigate the effect of the amorphous phase content on the crystallization process. Besides, the LMA coating was named according to its spraying power. For example, if the LMA coating was prepared at the power of 22 kW, it would be named LMA (22). Coatings deposited with the mixture powders and α-Al2O3 powders were prepared at 42 kW. Spraying parameters of these coatings were listed in Table 1. The Al2O3 coating and the mixture coating were heated at predetermined temperatures for 20 h prior to their physicochemical researches.
2. Experimental 2.1. Powder preparation La2O3, MgO, and α-Al2O3 as the raw materials were mixed with each other and then sintered to synthesis the LMA ceramic powders at 1600 °C for 12 h by the solid state reaction. The as-synthesized ceramic LMA powders were spray-dried (SFOC-16, Shanghai-Ohkawara Dryers Co., Ltd.) to get the particles between 20 and 125 μm for plasma spraying. The process of the spray-drying was described in great detail in our previous work [17]. La2O3 and α-Al2O3 were used to synthesize LaAlO3. MgO and αAl2O3 were selected for the synthesis of MgAl2O4. LaAlO3 and MgAl2O4 were all synthesized at 1200 °C for 12 h by solid-state reaction. LaAlO3, MgAl2O4, and α-Al2O3 were mixed with the molar ration of 1:1:4 to hold the same compositions of LaMgAl11O19 (LaAlO3·MgAl2O4·4Al2O3). The mixture powders were also spray-dried with the similar way of LMA powders. In addition, α-Al2O3 powder was spray-dried in the same way. The X-ray diffractometer patterns of the LMA powder and the mixture powders are shown in Fig. 1.
2.3. Characterization Phase analysis was performed by an X-ray diffractometer (XRD, D/ MAX-RB RU-200B) with Cu-Kα radiation (λ = 0.15406 nm) to scan the diffraction angles (2θ) between 15° and 65° in rate of 4°⋅min−1. The coatings for cross-section analysis were mounted in epoxy resin followed by fine polishing with a diamond paste, then the microstructure of the coatings were analyzed by a field emission-scanning electron microscope (SEM, QUANTA FEG 450). Element analysis of the LMA crystal was carried out using an energy dispersive X-ray spectroscopy (EDS) equipped on the SEM. A thermal analyzer (Netzsch STA 449 F1) was used for differential scanning calorimetry (DSC) analysis from RT to 1250 °C. A Netzsch 402C high-temperature dilatometer was used for thermal expansion test from RT to 1400 °C.
2.2. Coating preparation and heat treatment 3. Results and discussion A Multicoat Plasma Spray unit (Oerlikon Metco, Switzerland) was used to deposit the coating samples using Ar-H2 as plasma gases. All coatings were deposited on graphite substrate directly and then were stripped from the substrate by abrading. LMA coatings were prepared
3.1. Crystallization process The crystallization of the amorphous phase is accompanied by the exothermic event [20], which can be proved using thermal analysis. Fig. 2 shows the DSC curves of the LMA coatings prepared by varied spraying powers. As shown in Fig. 2a, two remarkable exothermal peaks present in the curves at the place of 900 °C (the first exothermal peak, P1) and 1170 °C (the second exothermal peak, P2), respectively. As shown in Fig. 2b and Fig. 2c, the intensity of the two exothermal peaks strengthens from LMA (22) to LMA (42). As reported in previous studies [10,12], the first peak at about 900 °C was owing to the transition from amorphous phase to crystal phase. Naturally, coatings with high amorphous phase content will generate large thermal effect. Therefore, the peak with larger area in the DSC curve means the higher amorphous phase content in the corresponding LMA coating. In fact, fully melted particles can form the amorphous phase easily as impacting on the substrate. High spraying powers can increase the plasma flame temperature and then the particles are melted well, resulting in the high amorphous phase content in the depositions. Peak areas of the two peaks are calculated by integration method and compared in Fig. 3. No matter what the cause of P1 or P2 is, their areas all increase by increasing the spraying power. On the basis fact of that P1 is induced by the transition of amorphous phase to crystal phase, while the area of P2 exhibits the same tendency with that of P1. Therefore, it is reasonable
Fig. 1. XRD patterns of the starting powders: (a) LMA powders synthesized at 1600 °C; (b) mixture powders (LaAlO3 + MgAl2O4 + α-Al2O3). 2
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Fig. 2. (a) DSC curves of LMA coatings deposited with different spraying powers; (b) and (c) are the selected areas in figure (a), respectively.
Fig. 3. Area of the two exothermal peaks in DSC curves of the coatings deposited with different spraying powers. Fig. 4. XRD patterns of (a) the as-sprayed Al2O3 coating and after heat treatment at different temperatures for 20 h: (b) 900 °C; (c) 1000 °C; (d) 1100 °C and (e) 1200 °C.
to speculate that P2 is as well as caused by the phase transition from the amorphous LMA to LMA crystals. However, other works on the phase transition of γ to α-Al2O3 and the reaction of LaAlO3 with MgAl2O4 and Al2O3 should be excluded to explain the crystallization peak of P2 adequately.
after heat treatment present in Fig. 4. Although the feedstock is composed of α-Al2O3 (see Fig. 1), γ-Al2O3 is observed in the as-sprayed Al2O3 coating (Fig. 4a). In fact, γ-Al2O3 was formed by the melt during APS [22]. The relative content of γ-Al2O3 for about 30 mol.% is determined by normalization method of the diffraction peak intensity. Before a complete development of the α-Al2O3 (Fig. 4e), only one intermediate phase of δ* [23] is observed (Fig. 4b). With the temperature increasing from 900 to 1100 °C, there is considerably more δ*-Al2O3 present, but no discernable increase in α-Al2O3 (Fig. 4 (b-d)). This indicates that γ-Al2O3 with 30 mol.% content has transformed into δ*Al2O3. The results show that there is an intermediate phase of δ* between the phase transitions of γ to α-Al2O3. Consideration the fact that
3.2. Phase transition of Al2O3 In TBCs system, alumina as the product by the oxidation of bond coat is often observed at the interface in the TBCs systems. Once the Al2O3 has formed during thermal cycling, thermal expansion mismatch and the volume shrinkage caused by the phase transition of Al2O3 induce the failure of TBCs. However, in the LMA TBCs, Al2O3 can be the product of the decomposition of the LMA powder during the process of spraying [21]. The diffraction patterns of Al2O3 coatings before and 3
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Fig. 5. Thermal expansion curves of Al2O3 bulk, the as-sprayed Al2O3 coating and LMA (42) coating. Fig. 6. XRD patterns of (a) the as-sprayed mixture coating and after heat treatment at different temperatures for 20 h: (b) 900 °C; (c) 1000 °C; (d) 1100 °C; (e) 1200 °C.
phase transition can induce the volume variation, therefore, thermal expansion test of the Al2O3 bulk (α-Al2O3) and as-sprayed Al2O3 coating is carried out and the results are shown in Fig. 5. It can be seen that the Al2O3 coating shows approximately linear expansion from room temperature to 1194.5 °C, followed by a rapid shrinkage for about 0.71%. As the temperature increasing to about 1300 °C, a little further shrinkage is observed. However, no obvious shrinkage at about 1200 °C is observed and only a little shrinkage at about 1300 °C is observed in the Al2O3 bulk. According to the XRD analyses results, the rapid shrinkage at about 1200 °C in the Al2O3 coating should be caused by the phase transitions from δ* to α-Al2O3. The little shrinkage both in Al2O3 bulk and Al2O3 coating should be caused by the sintering. In this paper, the ideal density of α-Al2O3 is taken to be 3.987 g/cm3 (JCPDS card 46–1212) and δ*-Al2O3 is taken to be 3.665 g/cm3 (JCPDS card 46–1215) [23]. There should be linear shrinkage of about 2.7% from δ* to α-Al2O3. The lower shrinkage than the expected could be a result of the lower content of δ*-Al2O3 in the Al2O3 coating (Fig. 4d). The results also indicate that δ*-Al2O3 transits to α-Al2O3 at about 1200 °C. However, no obvious shrinkage is observed at about 900 °C where γ-Al2O3 transforms into δ*-Al2O3. As reported in previous works [22,24,25], the γ-Al2O3 transforms continuously via the short range diffusion and reordering of vacancies and cations to δ*-Al2O3. Besides, the as-sprayed Al2O3 coating is mainly composed of α-Al2O3. For these reasons, no obvious volume change can be observed at about 900 °C in the linear thermal expansion curve. In addition, the linear thermal expansion curve of LMA (42) coating is also shown in Fig. 5. There are two shrinkages of about 2.3% between 800–900 °C and 0.99% between 1100–1200 °C which coincides with previous works reported [5,7,15–17]. The second shrinkage between 1100 °C and 1200 °C in LMA (42) coating is distinctly different with the shrinkage at 1200 °C of the as-sprayed Al2O3 coating in temperature range and shrinkage level. Even if the second shrinkage in the LMA coating was caused by the change from γ-Al2O3 to α-Al2O3, it is impossible to induce such a larger linear shrinkage of 0.99% with the lower content of γ-Al2O3 (< 5 mol. %) in the LMA (42) coating[17], because the shrinkage in the assprayed Al2O3 coating with 30 mol.% γ-Al2O3 (see Fig. 4a) is only 0.71%. Therefore, the second shrinkage between 1100 °C and 1200 °C in the plasma-sprayed LMA coating has no any relations with the change from γ-Al2O3 to α-Al2O3. As a result, P2 in the DSC curves of LMA coatings cannot be owing to the change from γ-Al2O3 to α-Al2O3.
reaction, MgO reacts with Al2O3 to synthesize MgAl2O4 in the temperature of about 700 °C, and then LaAlO3 is synthesized in the temperature of about 900 °C by the reaction between La2O3 and Al2O3, finally, LMA starts to form at approximately 1200 °C by the further reaction of LaAlO3 with Al2O3 and MgAl2O4 [26]. However, a temperature of more than 1800 °C is required to hold the pure LMA within 24 h [27]. Therefore, some intermediate products such as LaAlO3 and MgAl2O4 always exist in the LMA powders synthesized by solid-state reaction at the synthesis temperature below to 1800 °C (see Fig. 1). Besides, partial LMA decomposed during the process of plasma spraying. During high-temperature service, LaAlO3, MgAl2O4 and Al2O3 retained in the coating reacted with each other to resynthesize LMA [18]. In order to prove it and find the reaction temperature, mixture coatings using mixture powders of LaAlO3·MgAl2O4·4Al2O3 with the same element compositions of LaMgAl11O19 were deposited by APS and heat-treated at predetermined temperatures for 20 h, respectively. The diffraction patterns of the as-deposited mixture coatings and the heated are shown in Fig. 6. Compared with the diffraction pattern of the mixture powder (Fig. 1), some characteristic peaks ascribed to LaAlO3, MgAl2O4 and α-Al2O3 are also observed in the primitive mixture coating (Fig. 6a), the difference is that the peak intensity decreases dramatically. Besides, a broad hump and some small peaks ascribed to LMA and γ-Al2O3 are also observed. These show that the amorphous LMA and the LMA crystal are formed during APS. As the coating heated at 900 °C, many more LMA characteristic peaks exhibit and the broad hump gradually flattens (Fig. 6b). In addition, δ*-Al2O3 is also observed. The formation of δ*-Al2O3 during APS has been discussed in Section 3.2. Before heat treatment at 1100 °C, the main phase in the XRD patterns is LaAlO3 (Fig. 6(a–c)). Once the mixture coating was heated at 1100 °C for 20 h, the peak intensity of LaAlO3 has a sudden reduction and LMA become the main phase in the XRD pattern, besides, MgAl2O4 and δ*-Al2O3 disappear from the pattern (Fig. 6d). When the coating was heat-treated at 1200 °C, LMA characteristic peaks become sharp and the broad humps disappear from the XRD pattern (Fig. 6e). Fig. 7 shows the TG-DSC curves of the mixture coating. As shown in Fig. 7, two exothermal peaks are also observed as well as the LMA coating. Besides, a relatively flat peak is also observed at about 1100 °C (Fig. 7b), which is out of the temperature range of P2 at about 1170 °C. According to the XRD analyses results (Fig. 6), the flat exothermal peak at about 1100 °C should be due to the reaction of LaAlO3, MgAl2O4 and Al2O3. The synthesis of LMA is diffusion-controlled and does not have the obvious thermal effect as the crystallization process, so the reaction process is shown in the DSC curve by a flat peak. Besides, there is lower
3.3. Reformation of LMA La2O3, MgO and Al2O3 are often selected as the raw materials to prepare LMA by solid-phase synthesis. The synthesis mechanism of LMA can be summarized as follows: during the first part of the overall 4
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Fig. 7. (a) TG-DSC curves of the mixture coating and (b) the selected area of Fig. 7a with magnification.
results of the plate-like LMA crystals (Fig. 8(a, b)) [15], which are formed by the crystallization at different temperatures during heat treatment. The chemical compositions of the crystals are listed in Table 2. It can be seen that plate-like crystals are composed of La2O3MgO-Al2O3. As listed in Table 2, there is a slight deviation in stoichiometry ratio of the crystals, and crystals crystallized at 900 °C have higher La content than that crystallized at 1200 °C. However, there are no any other phases in the coatings heat-treated at 900 °C and 1200 °C compared with the ideal magnetoplumbite-type LMA phase [15]. The ideal composition of LaMgAl11O19 can be expressed as the formula La2O3–xMgO–yAl2O3 (x = 2 and y = 11). However, Gadow et al. [29] pointed out that a little deviation in the composition of the ternary system La2O3–MgO–Al2O3 was allowed to be the magnetoplumbite phase. For the formula La2O3–xMgO–yAl2O3, if the value of x was among the range of 0.2–3.3 and the value of y was among the range of 10.0–13, the magnetoplumbite phase would be formed by the reaction of La2O3, MgO and Al2O3. As show in Table 2, the compositions of the plate-like crystals crystallized at 900 °C and 1200 °C are within the varied composition range. For these reasons, the retained LaAlO3, Al2O3 and MgAl2O4 also can react with each other to form the LMA with a
content of LaAlO3, MgAl2O4 and Al2O3 in the as-deposited coating, therefore, the thermal effect is hard to be detected by the thermal analyzer and no obvious exothermal peaks at about 1100 °C can be observed in the DSC curves (Fig. 2). Therefore, it can be concluded that LaAlO3, MgAl2O4 and Al2O3 retained in LMA coatings react completely with each other to synthesis LMA at 1100 °C, and the second exothermal peak at about 1170 °C in the LMA coating cannot be caused by the reformation of LMA.
3.4. Crystallization mechanism As discussed above, it can be concluded that partial amorphous LMA crystallizes at 900 °C firstly and then the remained crystallizes at 1170 °C. The mechanism of the crystallization for amorphous LMA phase will be discussed in this section. As shown in Fig. 2 and Fig. 3, although LMA coatings deposited with different amorphous phase contents, crystallization peaks of P1 and P2 in the DSC curves of theses coatings are all at about 900 °C and 1170 °C. It indicates that the difference in amorphous phase content cannot cause the change of the crystallization temperature [28]. Fig. 8 shows the element analysis
Fig. 8. EDS spectra acquired from plate-like crystals of the LMA coating after heat treatment at different temperatures for 20 h: (a, c) 900 °C; (b, d) 1200 °C. 5
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Table 2 EDS analyses of plate-like crystals in LMA (42) after heat treatment at different temperatures. Plate-like crystals
La (wt.%)
Mg (wt.%)
Al (wt.%)
Chemical formula
Heat treatment at 900 °C Heat treatment at 1200 °C
17.26 20.35
3.38 3.25
42.79 44.14
La2O3·2.24MgO·12.78Al2O3 La2O3·1.82MgO·11.18Al2O3
Fig. 9. DSC curves of LMA (42) coating with heating rates of 10°/min, 20°/min, 30°/min, 40°/min and the crystallization activation energy of the two exothermal peaks: (a)-(b) P1, at 900 °C; (c)-(d) P2, at 1170 °C.
crystallization kinetics factor k was a more reasonable criterion than the existing criterion, activation energy of crystallization E. Modified Johnson-Mehl-Avrami (JMA) equation were also presented by Hu et al. to calculate the kinetics factor, as follows:
Table 3 Crystallization kinetics factors of LMA (42) coating at 900 °C and 1170 °C. α (K/ min)
P1 Tp (K)
10 20 30 40
1164 1171 1173 1179
P2 E (kJ/ mol)
ν (min−1)
1072
1.28 × 1048
Tp (K)
1432 1451 1458 1466
E (kJ/ mol)
ν (min−1)
697
7.98 × 1024
2
E E ⎛ Tp ⎞ ln ⎜ ⎟ = + ln ⎛ ⎞ − ln ν . RT R α ⎝ ⎠ p ⎝ ⎠
(1)
where Tp is the crystallization peak temperature, K; E is the apparent activation energy, J/mol; R (8.3144 J/mol·K) is the ideal gas constant; α is the heating rate, K/min; ν is the frequency factor, min−1. Besides, the relationship between the crystallization rate factor k and the temperature T is as follows:
slight deviation in stoichiometry ratio and matches well with the XRD pattern of the ideal magnetoplumbite-type LMA phase (Fig. 6d). Therefore, different crystallization temperatures of P1 and P2 can be caused by the different chemical compositions of the amorphous phase. Generally, high crystallization temperature of P2 than that of P1 shows the poor crystallization ability of the amorphous LMA at 1170 °C. The crystalline ability of the amorphous phase is a key index for the properties of the amorphous LMA coating. In fact, a series of theories on the crystallization of glass were proposed, which are also suited to the crystallization process of the amorphous LMA. Activation energy E was used to evaluate the crystallization ability. Kissinger method and Ozawa method [30] were commonly used to study the apparent activation energy E. It was considered that the larger E value means glasses crystallize more difficult. Nevertheless, more and more contradictions were found in these two methods. Hu et al. [31] reported that the
E ⎞. k(T) = ν exp ⎛− ⎝ RT ⎠
(2)
According to Eq. (1), Tp can be obtained by adjusting the heating Tp2
rate of DSC. Eq. (1) is used to plot ln ⎛ α ⎞ versus 1 for various heating Tp ⎝ ⎠ rates. It will generally lead to a straight line of practically equal slope E , R and then the E and ν can be obtained, as shown in Fig. 9 (b, d). Values of E and ν obtained from Equation (1) are also listed in Table 3. Fig. 10 shows a plot of k as the function of temperature for the two crystallization peaks in the DSC curve of the LMA coating. It can be seen that the value of k at 900 °C are much higher than that at 1170 °C. Although the activation energy of P1 is larger than that of P2, the crystallization kinetics factor k takes into account both the activation energy E and the 6
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Fig. 10. Plot of crystallization rate factor, k, as function of temperature for two exothermal peaks: (a) P1, at 900 °C; (b) P2, at 1170 °C.
frequency factor ν, so it is believed to be a good criterion for the evaluation of glass stability. The higher value of k at 900 °C means the higher crystalline ability [32,33]. LMA with a magnetoplumbite structure consists of two close-packed blocks of Al and O atoms separated by a mirror plane where the highly charged La3+ lies [34,35]. This layered structure has fully occupied mirror planes because of the highly charged La3+ and causes effective suppression of diffusion processes vertically to the crystallographic caxis [7]. It means that the La3+ ion strongly affect the crystallization rate of the amorphous LMA coating [12]. For amorphous LMA, the crystallization rates are diffusion-controlled at 900 °C and 1170 °C, the activation energies E (see Table 3) is related to the viscosity of the amorphous LMA [36]. As temperature increases, the viscosity of the amorphous LMA decreases. The change of viscosity may be the reason that activation energy E at 900 °C is higher than that at 1170 °C. In fact, a small amount of amorphous phase remains in the coating after heat treatment at 900 °C [15]. Higher temperature is needed to cause the complete crystallization of the amorphous LMA. As listed in Table 2, plate-like crystals in Fig. 8b have the higher La content than that in Fig. 8a. Furthermore, high La content will decreases the crystallization rates of the amorphous LMA coating. For this reason, the crystallization kinetic factor k (Fig. 10) at 1170 °C is lower than that at 900 °C. According to the element compositions analyses of the plate-like crystals and the crystallization kinetics studies, the mechanism of the amorphous LMA coating can be concluded as follows: La3+ plays a decisive role to control the amorphous LMA crystallization, plate-like crystals originated from the crystallization of the amorphous phase with the higher La content often correspond to the higher crystallization temperature. Amorphous LMA crystallizes at about 900 °C and 1170 °C, respectively for the difference in element compositions.
Acknowledgement This work was financially supported by National Natural Science Foundation of China (No. 51902092 and No. 51902279), the Foundation of Hengyang Normal University (No. 18D07) and the Foundation of Key Laboratory of Functional Metal-Organic Compounds of Hunan Province (No. MO19K07). References [1] D. Clarke, C. Levi, Materials design for the next generation thermal barrier coatings, Annu. Rev. Mater. Res. 33 (2003) 383–417. [2] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gas-turbine engine applications, Science 296 (2002) 280–284. [3] D.R. Clarke, S.R. Phillpot, Thermal barrier coating materials, Mater. Today 8 (2005) 22–29. [4] X. Zhong, H. Zhao, C. Liu, L. Wang, F. Shao, X. Zhou, S. Tao, C. Ding, Improvement in thermal shock resistance of gadolinium zirconate coating by addition of nanostructured yttria partially-stabilized zirconia, Ceram. Int. 41 (2015) 7318–7324. [5] J. Sun, J. Wang, X. Zhou, S. Dong, L. Deng, J. Jiang, X. Cao, Microstructure and thermal cycling behavior of plasma-sprayed LaMgAl11O19 coatings, Ceram. Int. 44 (2018) 5572–5580. [6] S. Tsukada, S. Kuroda, M. Nishijima, H. Araki, A. Yumoto, M. Watanabe, Effects of amorphous phase on hot corrosion behavior of plasma-sprayed LaMgAl11O19 coating, Surf. Coat. Technol. 363 (2019) 95–105. [7] 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. [8] D. Stöver, G. Pracht, H. Lehmann, M. Dietrich, J.-E. Döring, R. Vaßen, New material concepts for the next generation of plasma-sprayed thermal barrier coatings, J. Therm. Spray Technol. 13 (2004) 76–83. [9] X.Q. Cao, R. Vassen, D. Stoever, Ceramic materials for thermal barrier coatings, J. Eur. Ceram. Soc. 24 (2004) 1–10. [10] R. Gadow, M. Lischka, Lanthanum hexaaluminate — novel thermal barrier coatings for gas turbine applications — materials and process development, Surf. Coat. Technol. 151-152 (2002) 392–399. [11] P. Jana, P.S. Jayan, S. Mandal, K. Biswas, Microstructural design of neodymiumdoped lanthanum–magnesium hexaaluminate synthesized by aqueous sol–gel process, J. Mater. Sci. 50 (2015) 344–353. [12] X. Chen, Z. Yu, W. Huang, H. Ma, B. Zou, W. Ying, X. Cao, Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating, J. Eur. Ceram. Soc. 31 (2011) 2285–2294. [13] J. Zeng, J. Sun, H. Zhang, X. Yang, F. Qiu, P. Zhou, W. Niu, S. Dong, X. Zhou, X. Cao, Lanthanum magnesium hexaluminate thermal barrier coatings with pre-implanted vertical microcracks: thermal cycling lifetime and CMAS corrosion behaviour, Ceram. Int. 44 (2018) 11472–11485. [14] J. Zeng, J. Sun, P. Liang, X. Yang, S. Dong, J. Jiang, L. Deng, X. Zhou, X. Cao, Heattreated lanthanum magnesium hexaaluminate coatings exposed to molten calciummagnesium-alumino-silicate, Ceram. Int. 45 (2019) 11723–11733. [15] 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, J. Alloys Compd. 739 (2018) 856–865. [16] J. Sun, J. Wang, Z. Hao, J. Yuan, S. Dong, J. Jiang, L. Deng, Z. Xin, X. Cao, Preparation, structure, mechanical properties and thermal cycling behavior of porous LaMgAl11O19 coating, J. Alloys Compd. 750 (2018) 1007–1016. [17] J. Sun, J. Wang, X. Zhou, Y. Hui, S. Dong, L. Li, L. Deng, J. Jiang, X. Cao, Thermal cycling behavior of the plasma-sprayed coating of lanthanum hexaaluminate, J. Eur. Ceram. Soc. 38 (2018) 1919–1929. [18] L.L. Huang, H.M. Meng, J. Tang, Crystallization behavior of plasma-sprayed lanthanide magnesium hexaaluminate coatings, Int. J. Miner. Metal. Mater. 21 (2014)
4. Conclusions In the present work, some disputes on the cause of the second exothermal peak (P2) at about 1170 °C in the DSC curves of the plasmasprayed LaMgAl11O19 (LMA) coating are clarified. The phase transition of γ to α-Al2O3 in plasma-sprayed Al2O3 coating starts at about 1200 °C and induces the shrinkage of about 0.71%. The reaction to reform LMA with the retained of LaAlO3, MgAl2O4 and Al2O3 in the as-sprayed LMA coating occurs at about 1100 °C, which are out of the temperature range of P2. The results show that P2 is also owing to the crystallization of amorphous LMA, which can induce the linear shrinkage of about 0.99% of the coating. The difference in the element compositions leads to the crystallization process of amorphous LMA in two stages. Plate-like crystals crystallized at 900 °C have the lower La content than that crystallized at 1170 °C. La3+ ion strongly affect the crystallization rates of the amorphous LMA coating. The higher the La content in the composition the harder the crystallization occurs. 7
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