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Fabrication and thermal shock resistance of multilayer γ-Y2Si2O7 environmental barrier coating on porous Si3N4 ceramic
G Model
ARTICLE IN PRESS
JECS-10238; No. of Pages 7
Journal of the European Ceramic Society xxx (2015) xxx–xxx
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Fabrication and thermal shock resistance of multilayer ␥-Y2 Si2 O7 environmental barrier coating on porous Si3 N4 ceramic Chao Wang a,b , Meng Chen a , Hongjie Wang a,∗ , Xingyu Fan a , Hongyan Xia a a b
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shannxi 710049, People’s Republic of China Huadian Electric Power Research Institute, Hangzhou, Zhejiang 310030, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 31 December 2014 Received in revised form 6 August 2015 Accepted 12 August 2015 Available online xxx Keywords: Porous Si3 N4 ceramic EBC ␥-Y2 Si2 O7 Microstructural evolution Thermal shock resistance
a b s t r a c t A dense multilayer ␥-Y2 Si2 O7 environmental barrier coating was in-situ fabricated by the liquid infiltration and filling method on porous Si3 N4 ceramic for water resistance. When the sintering temperature was 1350 ◦ C, as-prepared coating consisted of the top glass layer, the ␥-Y2 Si2 O7 interlayer and the bottom bond layer. With increasing sintering temperature from 1350 to 1450 ◦ C, the microstructure of the coating transformed from three layers into two layers including the ␥-Y2 Si2 O7 layer and the bond layer. The ␥-Y2 Si2 O7 layer in the coating consisted of ␥-Y2 Si2 O7 and Y–Si–Al–O glass, and the ␥-Y2 Si2 O7 content increased with the increase of sintering temperature. Thermal shock resistance of the coatings was tested. The results showed that the coating prepared at 1450 ◦ C displayed the better thermal shock resistance. After thermal shock for 15 times from 1000 ◦ C to room temperature, the microstructure of the coating showed little change, and water absorption increased slightly from 4.8% to 5.3%. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Porous silicon nitride (Si3 N4 ) ceramic is a promising structural/functional material for the applications of radomes because of the excellent thermomechanical behavior, low thermal conductivity and good dielectric properties [1–3]. However, the pores of porous Si3 N4 absorb moisture seriously in the practical atmosphere, leading to the decrease of dielectric properties and thermal insulation [4,5]. For practical applications, it is necessary to fabricate a dense environmental barrier coating (EBC) on the surface of porous Si3 N4 ceramic in order to enable moisture separation. Rare earth silicate (RE2 Si2 O7 and RE2 SiO5 ; RE: Yb, Y, Lu, Sc, Er) as the novel EBCs were always used to protect the dense Si-based ceramics and composites for hot section structural components [6–8]. ␥-Y2 Si2 O7 , as one of the most refractory silicates, is a very competitive candidate material for EBC to protect the porous Si3 N4 ceramic. Firstly, ␥-Y2 Si2 O7 is a high-temperature phase among the six polymorphs of Y2 Si2 O7 (y, ˛, ˇ, , ı, and probably z) and it remains stable over a wide temperature range (room temperature (RT)-1500 ◦ C) [9]. Secondly, the linear coefficient of thermal expansion (CTE) of ␥-Y2 Si2 O7 is (3.9 ± 0.4) × 10−6 /K [10], being very close to that of porous Si3 N4 ceramic (2.5 − 3.6 × 10−6 /K [11]),
∗ Corresponding author. E-mail address: [email protected] (H. Wang).
which would prevent delamination and/or cracking of EBC caused by CTE mismatch stress. Thirdly, ␥-Y2 Si2 O7 can be used in severe environments with high temperature water vapor corrosion and fast cooling-heating cycle owning to its good damage tolerance and low CTE [10,12]. Lastly, ␥-Y2 Si2 O7 has a relatively low dielectric constant (5.71) and dielectric loss (8.3 × 10−3 ) in the range of 7.3–18 GHz [12]. Up to now, several methods have been developed for fabricating rare earth silicate EBCs on the dense substrate, such as plasma spraying [8] and liquid sintering [13,14]. However, it is difficult to fabricate the dense EBCs on porous Si3 N4 ceramic using those methods. During plasma spraying, the in-flight droplets with high velocity (400–800 m/s) will generate a severe damage on the fragile porous Si3 N4 ceramic by mechanical erosion [4]. The volume of the coating will shrink greatly during liquid sintering, so stress cracks in the coating are unavoidable owing to the mismatch of shrinkage between the coating and the substrate [15,16]. However, no literature published about fabricating rare earth silicate EBCs on porous Si3 N4 ceramic. In this work, a dense multilayer ␥-Y2 Si2 O7 EBC was in-situ fabricated by the liquid infiltration and filling method on porous Si3 N4 ceramic. The double-layer powders composed of ␥-Y2 Si2 O7 inner layer and oxides (Y2 O3 –SiO2 –Al2 O3 ) outer layer were deposited on the porous Si3 N4 ceramic by the slurry spraying. During sintering, the Y–Si–Al–O liquid was firstly formed by the oxides, and infiltrated into ␥-Y2 Si2 O7 powder layer under the action of capillary and
http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.013 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: C. Wang, et al., Fabrication and thermal shock resistance of multilayer ␥-Y2 Si2 O7 environmental barrier coating on porous Si3 N4 ceramic, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.013
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gravity, forming a multilayer ␥-Y2 Si2 O7 coating. The effect of sintering temperature on the microstructural evolution and thermal shock resistance of as-received coatings were studied in details. 2. Experimental procedure Porous Si3 N4 ceramic that fabricated by the gel-casting and pressureless sintering in our previous work was employed as the substrate [2]. The pore size of porous Si3 N4 ceramic is in range of 0.5–1.2 m, and its porosity is about 52.3%. Porous Si3 N4 ceramic were machined into rectangular sample with dimensions 3 mm × 4 mm × 25 mm, then polished with a 0.25 m diamond pastes, ultrasonic cleaned in ethanol and dried in drying oven. ␥-Y2 Si2 O7 powders were synthesized in the previous work according to the process of the reference [17]. The synthesized ␥-Y2 Si2 O7 powders were mixed in ethanol to prepare the ␥Y2 Si2 O7 slurry. Y2 O3 and SiO2 powders were mixed according to the molar ratio of 1:2. In order to reduce the eutectic temperature of the Y2 O3 –SiO2 system, 5 wt% Al2 O3 powders was added in the mixtures. According to the phase equilibrium diagrams, the eutectic temperature of the Y2 O3 –SiO2 –Al2 O3 (YSA) system is about 1300 ◦ C. The mixtures of YSA were mixed in ethanol to prepare the YSA slurry. The double-layer powders were prepared on the porous Si3 N4 substrate by the slurry spraying using the atomization spray gun (Type: F-2, Green Pneumatic Co., Ltd., Taiwan, China). The parameters of spraying are: nozzle diameter: 0.5 mm, air pressure: 0.6–0.8 MPa, spraying distance: 25 cm, spraying rate: 20 cm/s. The ␥-Y2 Si2 O7 powder was deposited on the porous Si3 N4 substrate directly as the inner layer, then the YSA powder was deposited on the ␥-Y2 Si2 O7 powder layer as the outer layer. All the porous Si3 N4 with the double powder layers were sintered at 1350, 1400 and 1450 ◦ C for 1 h in N2 atmosphere. The morphologies of the coating were observed using scanning electron microscopy (SEM, Type: FEI6000, USA) and the elemental analysis was conducted by energy-dispersive spectroscopy (EDS). To confirm the reaction sequence and phase composition of each
layer in the coating, XRD study of each layer was done at progressively inner surfaces by a sequential removing of material using grounding carefully. The phase compositions of each layer in the coating were analyzed by X-ray diffraction (XRD, Type: X’Pert Pro, Holland). The volume content of ␥-Y2 Si2 O7 in the ␥-Y2 Si2 O7 layer was estimated by image analysis. The average volume content of ␥-Y2 Si2 O7 was calculated based on the image analysis results of five different regions of the ␥-Y2 Si2 O7 layer. Thermal shock test was carried out according to ASTM C1525-03 standard by quenching the samples from 1000 ◦ C into water bath (room temperature). After the furnace was heated up to 1000 ◦ C, the samples were quickly put into the furnace and maintained for 20 min, then taken out of the furnace and dropped into water parallel to their long axes, holding for 5 min. After taking the samples out of the water, removed all drops of water from the surface with a cloth, then immediately putted them into the furnace again for the next thermal cycle. After thermal shock, the water absorption of the coated porous Si3 N4 ceramics was tested by Archimedes method [4,5]. Meanwhile, the crack density of the coatings (crack number per unit length of the coating) was obtained by averaging out the number of the cracks on the whole polished cross section using SEM at 500× magnification [18]. To investigate oxidation of the coating without thermal shock, thermal treatment test was performed by heating the coated samples at 1000 ◦ C for 20 min, then furnace-varying cooling. 3. Results and discussion 3.1. Microstructure evolution of the coating 3.1.1. The overall microstructure evolution of the coating Fig. 1(a) shows the cross-section SEM image of the coating prepared at 1350 ◦ C. The coating was very dense, and no defects, including cracks, pores and delamination in the interface, were found. It is clear that the coating on the porous Si3 N4 substrate consisted of three layers. In order to investigate the microstructure of the coating, element line scanning analysis of the coating (the
Fig. 1. Cross-section SEM images and element line scanning analysis of the coating after prepared at different sintering temperatures. (a) and (d) 1350 ◦ C; (b) and (e) 1400 ◦ C; (c) and (f) 1450 ◦ C.
Please cite this article in press as: C. Wang, et al., Fabrication and thermal shock resistance of multilayer ␥-Y2 Si2 O7 environmental barrier coating on porous Si3 N4 ceramic, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.013
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Fig. 2. XRD patterns of each layer in the coatings prepared at (a) 1350 ◦ C, (b) 1400 ◦ C and (c) 1450 ◦ C.
white line in Fig. 1(a)) was employed, as shown in Fig. 1(d). It can be seen that the peak of element Y, Al, Si and O appeared in the top layer (I layer), and the peak intensity of element Al reduced sharply while the peak intensity of element Y increased clearly in the interlayer (II layer). This implied that the I layer could be the Y–Si–Al–O glass layer formed by the solidification of Y–Si–Al–O liquid and the II layer could be the ␥-Y2 Si2 O7 layer. During sintering, the Y–Si–Al–O liquid was firstly formed when the sintering temperature reached to the eutectic temperature of the YSA system (about 1300 ◦ C), then infiltrated into the ␥-Y2 Si2 O7 powder layer below it due to the action of capillary and gravity. In relatively low temperature case, the Y–Si–Al–O liquid has a relatively high viscosity, which made liquid infiltrate into the ␥-Y2 Si2 O7 powder layer incompletely. So the top Y–Si–Al–O glass layer was formed by the solidification of Y–Si–Al–O liquid on the ␥-Y2 Si2 O7 layer. In the III layer, element N was found besides element Y, Al, Si and O, indicating that Y–Si–Al–O liquid infiltrated into the porous Si3 N4 substrate through the ␥-Y2 Si2 O7 layer to form the bond layer. It is well known that the formation of the bond layer can strengthen the interfacial bonding between coating and substrate. As the sintering temperature increased to 1400 ◦ C, the microstructure of the coating transformed into two layers from three layers, as shown in Fig. 1(b). Element line scanning analysis of the coating (Fig. 1(e)) shows that the I layer consisted of element Y, Al, Si and O, and the II layer consisted of element Y, Al, Si, O and N. Compared with the morphology of the coating prepared at 1350 ◦ C, it is inferred that the I layer was the ␥-Y2 Si2 O7 layer and the II layer was the bond layer. The liquid viscosity will reduce with the increase of sintering temperature [19], which made Y–Si–Al–O liquid infiltrate into the ␥-Y2 Si2 O7 powder layer completely, thus leading to the microstructural evolution of the coating from three layers to two layers. Fig. 1(c) is the cross-section SEM image of the coating prepared at 1450 ◦ C. By analyzing element line scanning (Fig. 1(f)), it can be determined that the coating also consisted of the ␥-Y2 Si2 O7 top layer (I layer) and the bond layer (II layer). In addition, as the sintering temperature increased from 1350 to 1450 ◦ C, the thickness of the ␥-Y2 Si2 O7 layer decreased gradually from 50.4 to 23 m, and the thickness of the bond layer increased gradually from 44.3 to 200.1 m. The decrease of the thickness of the ␥-Y2 Si2 O7 layer could be attributed to two reasons: (1) the ␥-Y2 Si2 O7 particles can dissolve into Y–Si–Al–O liquid. The increasing sintering temperature can raise the consumption of ␥-Y2 Si2 O7 particles in Y–Si–Al–O liquid, thus leading to the decrease of the thickness of the ␥-Y2 Si2 O7 layer; (2) the infiltrated Y–Si–Al–O liquid can facilitate particle rearrangement of ␥-Y2 Si2 O7 during liquid sintering. The volume shrinkage caused by particle rearrangement leaded to the decrease of the thickness of the ␥-Y2 Si2 O7 layer. Meanwhile, the increasing sintering temperature can lead to the reduction of the liquid viscosity, which promoted the infiltration
of Y–Si–Al–O liquid into porous substrate. So the thickness of the bond layer increased gradually with the increase of the sintering temperature. Fig. 2(a) shows the XRD patterns of each layer in the coating prepared at 1350 ◦ C. The phase compositions of the top layer (I layer) formed by the solidification of Y–Si–Al–O liquid consisted of the primary phase -Y2 Si2 O7 and the secondary phase mullite (3Al2 O3 ·2SiO2 ) and SiO2 . In the ␥-Y2 Si2 O7 layer (II layer) and the bond layer (III layer), the phase composition was a single ␥Y2 Si2 O7 and -Si3 N4 crystalline phase, respectively, indicating that the infiltrated Y–Si–Al–O liquid existed in each layer in the form of amorphous phase. Fig. 2(b) and (c) is the XRD patterns of each layer in the coatings prepared at 1400 ◦ C and 1450 ◦ C, respectively. It can be seen that the ␥-Y2 Si2 O7 layer (I layer) and the bond layer (II layer) also consisted of the single ␥-Y2 Si2 O7 and -Si3 N4 crystalline phase, respectively.
3.1.2. The microstructure evolution of the -Y2 Si2 O7 layer in the coating Fig. 3(a) is high-magnification cross-section SEM image of the coating prepared at 1350 ◦ C showing the ␥-Y2 Si2 O7 layer and the interfaces of ␥-Y2 Si2 O7 layer with the bond layer and the top layer. It is noted that the ␥-Y2 Si2 O7 layer (II layer) exhibited a dense structure, where the white particles (Spot 1) uniformly distributed in the gray phase (Spot 2). Combined with XRD and EDS (Fig. 3(d)) analysis, it is determined that the white particle was ␥-Y2 Si2 O7 crystal and the gray phase was a glass phase including element O, Si, Y and Al. Particle rearrangement occurs during the first stage of sintering in liquid phase [20], which promotes the densification of the ␥-Y2 Si2 O7 layer in low temperature case. In addition, it can be seen that the bond layer (III layer) consisted of the dark particles (Spot 3) and the gray phase (Spot 4). It can be determined by analysis of XRD and EDS that the dark phase was -Si3 N4 and the gray phase the Y–Si–Al–O glass. Fig. 3(b) is high-magnification cross-section SEM image of the coating prepared at 1400 ◦ C. It is found that fine ␥-Y2 Si2 O7 particles grew into the coarse particles in the ␥-Y2 Si2 O7 layer compared with the coating prepared at 1350 ◦ C. Grain growth is mainly controlled solution-precipitation mechanism during liquid sintering [21]. The high sintering temperature can contribute to the solution-precipitation, thus leading to the growth of ␥-Y2 Si2 O7 particles. Meanwhile, in the interface of the ␥-Y2 Si2 O7 layer and the bond layer, the white phase stated to diffuse into the bond layer. It is inferred from EDS analysis (Spot 5) that the white phase should be ␥-Y2 Si2 O7 phase. Significantly, the interfacial diffusion of ␥-Y2 Si2 O7 can strengthen the interface bonding between the ␥Y2 Si2 O7 layer and the bond layer. Fig. 3(c) is high-magnification cross-section SEM image of the coating prepared at 1450 ◦ C, in which the growth of ␥-Y2 Si2 O7 particles and the diffusion of ␥Y2 Si2 O7 into the bond layer were intensified due to the further
Please cite this article in press as: C. Wang, et al., Fabrication and thermal shock resistance of multilayer ␥-Y2 Si2 O7 environmental barrier coating on porous Si3 N4 ceramic, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.013
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Fig. 3. High-magnification cross-section SEM images and EDS of the coatings prepared at (a) 1350 ◦ C, (b) 1400 ◦ C and (c) 1450 ◦ C; (d) EDS of each phase in the coating.
Fig. 4. Surface SEM images of the coatings prepared at different sintering temperatures. (a) 1350 ◦ C; (b) 1400 ◦ C; (c) 1450 ◦ C.
increase of sintering temperature. In addition, estimating by image analysis, it is concluded that the volume content of ␥-Y2 Si2 O7 in the ␥-Y2 Si2 O7 layer increased gradually from 72.5 to 90.2 vol% with the increase of sintering temperature from 1350 to 1450 ◦ C. This can be attributed to that (1) the grain growth of ␥-Y2 Si2 O7 leaded to the increase of the ␥-Y2 Si2 O7 content; (2) Y–Si–Al–O liquid can be induced to nucleate into ␥-Y2 Si2 O7 due to the existence of ␥Y2 Si2 O7 grain seed, which leaded to in-situ production of ␥-Y2 Si2 O7 in the ␥-Y2 Si2 O7 layer so that the ␥-Y2 Si2 O7 content increased.
3.1.3. Surface morphologies of the coating Fig. 4 shows the surface SEM images of the coatings prepared at different sintering temperatures. When the temperature was 1350 ◦ C, the surface layer of the coating should be Y–Si–Al–O glass layer. Some large pores were found on the surface of the coating. In the low temperature case, the Y–Si–Al–O liquid fluidity will be limited because the liquid viscosity was relatively high, so that large pores caused by spraying process were retained. When the sintering temperature were 1400 ◦ C and 1450 ◦ C, the Y–Si–Al–O liquid infiltrated into the ␥-Y2 Si2 O7 powder layer completely, so the surface layer of coatings should be ␥-Y2 Si2 O7 layer. It can be seen that the ␥-Y2 Si2 O7 particles bonded tightly with each other, forming the dense coating surface. In addition, the surface of coating fabricated at 1450 ◦ C was more uniform and denser than that of coating prepared at 1400 ◦ C. This is because that the viscosity of Y–Si–Al–O liquid will reduce with the increase of sintering temperature, which
can promote the transfer of ␥-Y2 Si2 O7 particles sufficiently so that the surface of coating was more uniform and denser. 3.2. Thermal shock behavior of the coating 3.2.1. Effect of thermal shock on water absorption of the coating To further investigate water resistance of the coated samples in the practical environment, thermal cycling test was performed by quenching between room temperature and 1000 ◦ C. Water absorption of the coated samples before and after thermal shock is shown in Fig. 5. Before thermal shock, it is found that the water absorption of the coated samples decreased obviously (4.8–6.3%) compared with that of porous Si3 N4 (36.3%), indicating that as-received coating sealed the surface pores of porous Si3 N4 ceramic. The surface roughness of the coating decreased gradually with increasing of sintering temperature from 1350 to 1450 ◦ C (Fig. 4), which leaded to the water absorption decreased from 6.3% to 4.8%. After thermal shock, the water absorption of all the coated samples increased with the increase of thermal cycle times. During thermal shock, a high tensile thermal stress is placed within the coating, thus cracks are initially initiated on the surface and propagate along the coating thickness [22]. Crack can provide a channel for water entering into porous substrate, so leading to the increase of water absorption. In addition, the increase trend of water absorption increased gradually as the sintering temperature increased from 1350 to 1450 ◦ C. After thermal cycles for 15 times, the water absorption of the coatings prepared at 1350, 1400 and 1450 ◦ C was 27.2%, 10% and 5.3%,
Please cite this article in press as: C. Wang, et al., Fabrication and thermal shock resistance of multilayer ␥-Y2 Si2 O7 environmental barrier coating on porous Si3 N4 ceramic, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.013
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Fig. 5. Water absorption of the coated porous Si3 N4 ceramics after thermal cycling between 1000 ◦ C and room temperature.
respectively. This implied that thermal shock resistance of coatings increased gradually with the increase of sintering temperature. 3.2.2. Effect of thermal shock on morphologies of the coating To investigate oxidation of the coating during thermal cycling, thermal treatment test was performed by heating the coated samples at 1000 ◦ C and holding for 20 min. Fig. 6 is the surface and cross-section SEM images of the coatings after oxidation without thermal cycling. It can be seen from the surface images (Fig. 6(a)–(c)) that the coating microstructures exhibited little changes compared with that of the coatings without oxidation (Fig. 4). This indicated that the influence of oxidation on the coating surface was insignificant. Fig. 6(d) is the cross-section SEM image of the coating prepared at 1350 ◦ C after oxidation. Some holes were formed within the top Y–Si–Al–O glass layer and in the interface of Y–Si–Al–O glass layer/␥-Y2 Si2 O7 layer, which can be attributed to oxidation and/or volatilization of the glass phase in the coat-
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ing. Similarly, some holes were also found in the interface of top ␥-Y2 Si2 O7 layer/bond layer and within the bond layer in the coating prepared at 1400 ◦ C (Fig. 6(e)). It can be determined by the above results that the coatings prepared at 1350–1400 ◦ C had a weak oxidation resistance. The defects, formed holes, will weaken the interface bonding between the layers and reduce thermal shock resistance inevitably. Fig. 6(f) is the cross-section SEM image of the coating prepared at 1450 ◦ C after oxidation. The test results showed that the cross-section microstructure had little changes compared with that of the coating without oxidation (Fig. 1(c)), indicating as-prepared coating had a good oxidation resistance. Fig. 7(a) and (d) shows cross-section SEM images of the coating prepared at 1350 ◦ C after thermal cycling for 1 and 15 times, respectively. It can be seen from the cross-section image (Fig. 7(a)) that thermal stress cracks were formed within the coating. The crack density of the coatings measured using SEM after thermal cycle tests for 1, 5, 10 and 15 times was about 0.25, 0.33, 0.38 and 0.42 mm−1 , respectively. The crack density increased gradually with the increase of thermal cycle times. Meanwhile, some formed cracks can propagate along the coating thickness during thermal shock, even can penetrate the coating in the subsequent process of thermal shock, as indicated by the white arrow in Fig. 7(d). The penetrating cracks can provide channels for the water entering into porous substrate. The increase of crack density and the formation of the penetrating crack leaded to an obvious increase of the water absorption with the increase of thermal cycle times. Meanwhile, some thermal damages, including peel-off and spallation, were found in the top Y–Si–Al–O glass layer. During thermal shock, some holes were easily formed within top layer and in the interface of top layer/␥-Y2 Si2 O7 layer due to oxidation and volatilization of the glass phase. The formation of holes weakened its thermal shock resistance inevitably, thus leading to the formation of thermal damages. Fig. 7(b) and (e) shows cross-section SEM images of the coating prepared at 1400 ◦ C after thermal cycling for 1 and 15 times, respectively. It is found that the top ␥-Y2 Si2 O7 layer peeled off completely after thermal cycling, only the bond layer was retained on the sur-
Fig. 6. Surface and cross-section SEM images of the coatings prepared at different temperatures after oxidation for 20 min. (a) and (d) 1350 ◦ C; (b) and (e) 1400 ◦ C; (c) and (f) 1450 ◦ C.
Please cite this article in press as: C. Wang, et al., Fabrication and thermal shock resistance of multilayer ␥-Y2 Si2 O7 environmental barrier coating on porous Si3 N4 ceramic, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.013
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Fig. 7. Cross-section SEM images of the coatings prepared at different temperatures after thermal cycling for 1 and 15 times, respectively. (a) and (d) 1350 ◦ C; (b) and (e) 1400 ◦ C; (c) and (f) 1450 ◦ C.
face of porous Si3 N4 substrate. It can be inferred from Fig. 6(e) that some holes can be formed in the interface between the ␥-Y2 Si2 O7 layer and the bond layer because of the oxidation and volatilization of the glass phase during thermal shock. The formed holes weakened the interface bonding between the ␥-Y2 Si2 O7 layer and the bond laye and reduced its thermal shock resistance inevitably. So the top ␥-Y2 Si2 O7 layer peeled off completely after thermal shock. Meanwhile, some holes caused by volatilization of glass phase also appeared in the inner of bond layer. The volatilization of the glass will increase with the increase of thermal cycle times, thus leading to the increase of the amount of hole within the bond layer (Fig. 7(e)). The formed holes can affect the ability of the coated samples to resist to water absorption, so the water absorption increased slightly with the increase of thermal cycling (Fig. 5). In addition, compared with the coating prepared at 1400 ◦ C, the coating prepared at 1350 ◦ C only generated a larger crack, without ␥-Y2 Si2 O7 layer peeling off. It can be attributed to three layers of the coating. The top Y–Si–Al–O glass layer can act as an additional thermal barrier to protect the ␥-Y2 Si2 O7 interlayer from thermal shock. Fig. 7(c) and (f) shows cross-section SEM images of the coating prepared at 1450 ◦ C after thermal cycling for 1 and 15 times, respectively. The cross-section morphologies of the coating showed little changes after thermal shock. No defects, such as peel-off, cracks and holes, were found within the coating, indicating that the coating prepared at 1450 ◦ C had an excellent thermal shock resistance. This can be attributed to the following reasons: (1) during thermal shock, a high tensile thermal stress is placed within the coating, and the stress will increase with the increase of the coating thickness [22]. The coating prepared at 1450 ◦ C had a thinner ␥-Y2 Si2 O7 layer (Fig. 1) and the stress generated by thermal shock was reduced. So the coating microstructure can improve thermal shock resistance of the coating. (2) With the increase of sintering temperature, the thickness of the bond layer increased gradually and diffusion of ␥-Y2 Si2 O7 into the bond layer intensified gradually, which will strengthen the interface bonding of coating/substrate and ␥-Y2 Si2 O7 layer/bond layer. A strong interface bonding can
strengthen the coating against thermal shock, thus contributing to thermal shock resistance. (3) The coating prepared at 1450 ◦ C exhibited a good oxidation resistance, and no thermal defects such as holes and cracks were formed in the coating. This is also a important reason that the coating prepared at 1450 ◦ C had the better resistance to thermal shock than the other two kind of coatings.
4. Conclusions A dense multilayer ␥-Y2 Si2 O7 EBC was in-situ fabricated on the porous Si3 N4 ceramic using the liquid infiltration and filling method. As the sintering temperature increased gradually from 1350 to 1450 ◦ C, the microstructures of coating transformed from three layers into double layers, and the ␥-Y2 Si2 O7 content in the ␥-Y2 Si2 O7 layer increased gradually from 72.5 to 90.2 vol%. The coating sintered at 1450 ◦ C displayed the better thermal shock resistance than the other two kind of coatings. After thermal cycle for 15 times, the microstructure of the coating showed little change, and water absorption increased slightly from 4.8% to 5.3%.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51472198, 51272206), the Ministry of Education Innovation Team Development Plan (IRT1280) and National Key Laboratory of Advanced Functional Composite (9140C560109130C56201).
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Fabrication and thermal shock resistance of multilayer ␥-Y2 Si2 O7 environmental barrier coating on porous Si3 N4 ceramic Chao Wang a,b , Meng Chen a , Hongjie Wang a,∗ , Xingyu Fan a , Hongyan Xia a a b
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shannxi 710049, People’s Republic of China Huadian Electric Power Research Institute, Hangzhou, Zhejiang 310030, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 31 December 2014 Received in revised form 6 August 2015 Accepted 12 August 2015 Available online xxx Keywords: Porous Si3 N4 ceramic EBC ␥-Y2 Si2 O7 Microstructural evolution Thermal shock resistance
a b s t r a c t A dense multilayer ␥-Y2 Si2 O7 environmental barrier coating was in-situ fabricated by the liquid infiltration and filling method on porous Si3 N4 ceramic for water resistance. When the sintering temperature was 1350 ◦ C, as-prepared coating consisted of the top glass layer, the ␥-Y2 Si2 O7 interlayer and the bottom bond layer. With increasing sintering temperature from 1350 to 1450 ◦ C, the microstructure of the coating transformed from three layers into two layers including the ␥-Y2 Si2 O7 layer and the bond layer. The ␥-Y2 Si2 O7 layer in the coating consisted of ␥-Y2 Si2 O7 and Y–Si–Al–O glass, and the ␥-Y2 Si2 O7 content increased with the increase of sintering temperature. Thermal shock resistance of the coatings was tested. The results showed that the coating prepared at 1450 ◦ C displayed the better thermal shock resistance. After thermal shock for 15 times from 1000 ◦ C to room temperature, the microstructure of the coating showed little change, and water absorption increased slightly from 4.8% to 5.3%. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Porous silicon nitride (Si3 N4 ) ceramic is a promising structural/functional material for the applications of radomes because of the excellent thermomechanical behavior, low thermal conductivity and good dielectric properties [1–3]. However, the pores of porous Si3 N4 absorb moisture seriously in the practical atmosphere, leading to the decrease of dielectric properties and thermal insulation [4,5]. For practical applications, it is necessary to fabricate a dense environmental barrier coating (EBC) on the surface of porous Si3 N4 ceramic in order to enable moisture separation. Rare earth silicate (RE2 Si2 O7 and RE2 SiO5 ; RE: Yb, Y, Lu, Sc, Er) as the novel EBCs were always used to protect the dense Si-based ceramics and composites for hot section structural components [6–8]. ␥-Y2 Si2 O7 , as one of the most refractory silicates, is a very competitive candidate material for EBC to protect the porous Si3 N4 ceramic. Firstly, ␥-Y2 Si2 O7 is a high-temperature phase among the six polymorphs of Y2 Si2 O7 (y, ˛, ˇ, , ı, and probably z) and it remains stable over a wide temperature range (room temperature (RT)-1500 ◦ C) [9]. Secondly, the linear coefficient of thermal expansion (CTE) of ␥-Y2 Si2 O7 is (3.9 ± 0.4) × 10−6 /K [10], being very close to that of porous Si3 N4 ceramic (2.5 − 3.6 × 10−6 /K [11]),
∗ Corresponding author. E-mail address: [email protected] (H. Wang).
which would prevent delamination and/or cracking of EBC caused by CTE mismatch stress. Thirdly, ␥-Y2 Si2 O7 can be used in severe environments with high temperature water vapor corrosion and fast cooling-heating cycle owning to its good damage tolerance and low CTE [10,12]. Lastly, ␥-Y2 Si2 O7 has a relatively low dielectric constant (5.71) and dielectric loss (8.3 × 10−3 ) in the range of 7.3–18 GHz [12]. Up to now, several methods have been developed for fabricating rare earth silicate EBCs on the dense substrate, such as plasma spraying [8] and liquid sintering [13,14]. However, it is difficult to fabricate the dense EBCs on porous Si3 N4 ceramic using those methods. During plasma spraying, the in-flight droplets with high velocity (400–800 m/s) will generate a severe damage on the fragile porous Si3 N4 ceramic by mechanical erosion [4]. The volume of the coating will shrink greatly during liquid sintering, so stress cracks in the coating are unavoidable owing to the mismatch of shrinkage between the coating and the substrate [15,16]. However, no literature published about fabricating rare earth silicate EBCs on porous Si3 N4 ceramic. In this work, a dense multilayer ␥-Y2 Si2 O7 EBC was in-situ fabricated by the liquid infiltration and filling method on porous Si3 N4 ceramic. The double-layer powders composed of ␥-Y2 Si2 O7 inner layer and oxides (Y2 O3 –SiO2 –Al2 O3 ) outer layer were deposited on the porous Si3 N4 ceramic by the slurry spraying. During sintering, the Y–Si–Al–O liquid was firstly formed by the oxides, and infiltrated into ␥-Y2 Si2 O7 powder layer under the action of capillary and
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gravity, forming a multilayer ␥-Y2 Si2 O7 coating. The effect of sintering temperature on the microstructural evolution and thermal shock resistance of as-received coatings were studied in details. 2. Experimental procedure Porous Si3 N4 ceramic that fabricated by the gel-casting and pressureless sintering in our previous work was employed as the substrate [2]. The pore size of porous Si3 N4 ceramic is in range of 0.5–1.2 m, and its porosity is about 52.3%. Porous Si3 N4 ceramic were machined into rectangular sample with dimensions 3 mm × 4 mm × 25 mm, then polished with a 0.25 m diamond pastes, ultrasonic cleaned in ethanol and dried in drying oven. ␥-Y2 Si2 O7 powders were synthesized in the previous work according to the process of the reference [17]. The synthesized ␥-Y2 Si2 O7 powders were mixed in ethanol to prepare the ␥Y2 Si2 O7 slurry. Y2 O3 and SiO2 powders were mixed according to the molar ratio of 1:2. In order to reduce the eutectic temperature of the Y2 O3 –SiO2 system, 5 wt% Al2 O3 powders was added in the mixtures. According to the phase equilibrium diagrams, the eutectic temperature of the Y2 O3 –SiO2 –Al2 O3 (YSA) system is about 1300 ◦ C. The mixtures of YSA were mixed in ethanol to prepare the YSA slurry. The double-layer powders were prepared on the porous Si3 N4 substrate by the slurry spraying using the atomization spray gun (Type: F-2, Green Pneumatic Co., Ltd., Taiwan, China). The parameters of spraying are: nozzle diameter: 0.5 mm, air pressure: 0.6–0.8 MPa, spraying distance: 25 cm, spraying rate: 20 cm/s. The ␥-Y2 Si2 O7 powder was deposited on the porous Si3 N4 substrate directly as the inner layer, then the YSA powder was deposited on the ␥-Y2 Si2 O7 powder layer as the outer layer. All the porous Si3 N4 with the double powder layers were sintered at 1350, 1400 and 1450 ◦ C for 1 h in N2 atmosphere. The morphologies of the coating were observed using scanning electron microscopy (SEM, Type: FEI6000, USA) and the elemental analysis was conducted by energy-dispersive spectroscopy (EDS). To confirm the reaction sequence and phase composition of each
layer in the coating, XRD study of each layer was done at progressively inner surfaces by a sequential removing of material using grounding carefully. The phase compositions of each layer in the coating were analyzed by X-ray diffraction (XRD, Type: X’Pert Pro, Holland). The volume content of ␥-Y2 Si2 O7 in the ␥-Y2 Si2 O7 layer was estimated by image analysis. The average volume content of ␥-Y2 Si2 O7 was calculated based on the image analysis results of five different regions of the ␥-Y2 Si2 O7 layer. Thermal shock test was carried out according to ASTM C1525-03 standard by quenching the samples from 1000 ◦ C into water bath (room temperature). After the furnace was heated up to 1000 ◦ C, the samples were quickly put into the furnace and maintained for 20 min, then taken out of the furnace and dropped into water parallel to their long axes, holding for 5 min. After taking the samples out of the water, removed all drops of water from the surface with a cloth, then immediately putted them into the furnace again for the next thermal cycle. After thermal shock, the water absorption of the coated porous Si3 N4 ceramics was tested by Archimedes method [4,5]. Meanwhile, the crack density of the coatings (crack number per unit length of the coating) was obtained by averaging out the number of the cracks on the whole polished cross section using SEM at 500× magnification [18]. To investigate oxidation of the coating without thermal shock, thermal treatment test was performed by heating the coated samples at 1000 ◦ C for 20 min, then furnace-varying cooling. 3. Results and discussion 3.1. Microstructure evolution of the coating 3.1.1. The overall microstructure evolution of the coating Fig. 1(a) shows the cross-section SEM image of the coating prepared at 1350 ◦ C. The coating was very dense, and no defects, including cracks, pores and delamination in the interface, were found. It is clear that the coating on the porous Si3 N4 substrate consisted of three layers. In order to investigate the microstructure of the coating, element line scanning analysis of the coating (the
Fig. 1. Cross-section SEM images and element line scanning analysis of the coating after prepared at different sintering temperatures. (a) and (d) 1350 ◦ C; (b) and (e) 1400 ◦ C; (c) and (f) 1450 ◦ C.
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Fig. 2. XRD patterns of each layer in the coatings prepared at (a) 1350 ◦ C, (b) 1400 ◦ C and (c) 1450 ◦ C.
white line in Fig. 1(a)) was employed, as shown in Fig. 1(d). It can be seen that the peak of element Y, Al, Si and O appeared in the top layer (I layer), and the peak intensity of element Al reduced sharply while the peak intensity of element Y increased clearly in the interlayer (II layer). This implied that the I layer could be the Y–Si–Al–O glass layer formed by the solidification of Y–Si–Al–O liquid and the II layer could be the ␥-Y2 Si2 O7 layer. During sintering, the Y–Si–Al–O liquid was firstly formed when the sintering temperature reached to the eutectic temperature of the YSA system (about 1300 ◦ C), then infiltrated into the ␥-Y2 Si2 O7 powder layer below it due to the action of capillary and gravity. In relatively low temperature case, the Y–Si–Al–O liquid has a relatively high viscosity, which made liquid infiltrate into the ␥-Y2 Si2 O7 powder layer incompletely. So the top Y–Si–Al–O glass layer was formed by the solidification of Y–Si–Al–O liquid on the ␥-Y2 Si2 O7 layer. In the III layer, element N was found besides element Y, Al, Si and O, indicating that Y–Si–Al–O liquid infiltrated into the porous Si3 N4 substrate through the ␥-Y2 Si2 O7 layer to form the bond layer. It is well known that the formation of the bond layer can strengthen the interfacial bonding between coating and substrate. As the sintering temperature increased to 1400 ◦ C, the microstructure of the coating transformed into two layers from three layers, as shown in Fig. 1(b). Element line scanning analysis of the coating (Fig. 1(e)) shows that the I layer consisted of element Y, Al, Si and O, and the II layer consisted of element Y, Al, Si, O and N. Compared with the morphology of the coating prepared at 1350 ◦ C, it is inferred that the I layer was the ␥-Y2 Si2 O7 layer and the II layer was the bond layer. The liquid viscosity will reduce with the increase of sintering temperature [19], which made Y–Si–Al–O liquid infiltrate into the ␥-Y2 Si2 O7 powder layer completely, thus leading to the microstructural evolution of the coating from three layers to two layers. Fig. 1(c) is the cross-section SEM image of the coating prepared at 1450 ◦ C. By analyzing element line scanning (Fig. 1(f)), it can be determined that the coating also consisted of the ␥-Y2 Si2 O7 top layer (I layer) and the bond layer (II layer). In addition, as the sintering temperature increased from 1350 to 1450 ◦ C, the thickness of the ␥-Y2 Si2 O7 layer decreased gradually from 50.4 to 23 m, and the thickness of the bond layer increased gradually from 44.3 to 200.1 m. The decrease of the thickness of the ␥-Y2 Si2 O7 layer could be attributed to two reasons: (1) the ␥-Y2 Si2 O7 particles can dissolve into Y–Si–Al–O liquid. The increasing sintering temperature can raise the consumption of ␥-Y2 Si2 O7 particles in Y–Si–Al–O liquid, thus leading to the decrease of the thickness of the ␥-Y2 Si2 O7 layer; (2) the infiltrated Y–Si–Al–O liquid can facilitate particle rearrangement of ␥-Y2 Si2 O7 during liquid sintering. The volume shrinkage caused by particle rearrangement leaded to the decrease of the thickness of the ␥-Y2 Si2 O7 layer. Meanwhile, the increasing sintering temperature can lead to the reduction of the liquid viscosity, which promoted the infiltration
of Y–Si–Al–O liquid into porous substrate. So the thickness of the bond layer increased gradually with the increase of the sintering temperature. Fig. 2(a) shows the XRD patterns of each layer in the coating prepared at 1350 ◦ C. The phase compositions of the top layer (I layer) formed by the solidification of Y–Si–Al–O liquid consisted of the primary phase -Y2 Si2 O7 and the secondary phase mullite (3Al2 O3 ·2SiO2 ) and SiO2 . In the ␥-Y2 Si2 O7 layer (II layer) and the bond layer (III layer), the phase composition was a single ␥Y2 Si2 O7 and -Si3 N4 crystalline phase, respectively, indicating that the infiltrated Y–Si–Al–O liquid existed in each layer in the form of amorphous phase. Fig. 2(b) and (c) is the XRD patterns of each layer in the coatings prepared at 1400 ◦ C and 1450 ◦ C, respectively. It can be seen that the ␥-Y2 Si2 O7 layer (I layer) and the bond layer (II layer) also consisted of the single ␥-Y2 Si2 O7 and -Si3 N4 crystalline phase, respectively.
3.1.2. The microstructure evolution of the -Y2 Si2 O7 layer in the coating Fig. 3(a) is high-magnification cross-section SEM image of the coating prepared at 1350 ◦ C showing the ␥-Y2 Si2 O7 layer and the interfaces of ␥-Y2 Si2 O7 layer with the bond layer and the top layer. It is noted that the ␥-Y2 Si2 O7 layer (II layer) exhibited a dense structure, where the white particles (Spot 1) uniformly distributed in the gray phase (Spot 2). Combined with XRD and EDS (Fig. 3(d)) analysis, it is determined that the white particle was ␥-Y2 Si2 O7 crystal and the gray phase was a glass phase including element O, Si, Y and Al. Particle rearrangement occurs during the first stage of sintering in liquid phase [20], which promotes the densification of the ␥-Y2 Si2 O7 layer in low temperature case. In addition, it can be seen that the bond layer (III layer) consisted of the dark particles (Spot 3) and the gray phase (Spot 4). It can be determined by analysis of XRD and EDS that the dark phase was -Si3 N4 and the gray phase the Y–Si–Al–O glass. Fig. 3(b) is high-magnification cross-section SEM image of the coating prepared at 1400 ◦ C. It is found that fine ␥-Y2 Si2 O7 particles grew into the coarse particles in the ␥-Y2 Si2 O7 layer compared with the coating prepared at 1350 ◦ C. Grain growth is mainly controlled solution-precipitation mechanism during liquid sintering [21]. The high sintering temperature can contribute to the solution-precipitation, thus leading to the growth of ␥-Y2 Si2 O7 particles. Meanwhile, in the interface of the ␥-Y2 Si2 O7 layer and the bond layer, the white phase stated to diffuse into the bond layer. It is inferred from EDS analysis (Spot 5) that the white phase should be ␥-Y2 Si2 O7 phase. Significantly, the interfacial diffusion of ␥-Y2 Si2 O7 can strengthen the interface bonding between the ␥Y2 Si2 O7 layer and the bond layer. Fig. 3(c) is high-magnification cross-section SEM image of the coating prepared at 1450 ◦ C, in which the growth of ␥-Y2 Si2 O7 particles and the diffusion of ␥Y2 Si2 O7 into the bond layer were intensified due to the further
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Fig. 3. High-magnification cross-section SEM images and EDS of the coatings prepared at (a) 1350 ◦ C, (b) 1400 ◦ C and (c) 1450 ◦ C; (d) EDS of each phase in the coating.
Fig. 4. Surface SEM images of the coatings prepared at different sintering temperatures. (a) 1350 ◦ C; (b) 1400 ◦ C; (c) 1450 ◦ C.
increase of sintering temperature. In addition, estimating by image analysis, it is concluded that the volume content of ␥-Y2 Si2 O7 in the ␥-Y2 Si2 O7 layer increased gradually from 72.5 to 90.2 vol% with the increase of sintering temperature from 1350 to 1450 ◦ C. This can be attributed to that (1) the grain growth of ␥-Y2 Si2 O7 leaded to the increase of the ␥-Y2 Si2 O7 content; (2) Y–Si–Al–O liquid can be induced to nucleate into ␥-Y2 Si2 O7 due to the existence of ␥Y2 Si2 O7 grain seed, which leaded to in-situ production of ␥-Y2 Si2 O7 in the ␥-Y2 Si2 O7 layer so that the ␥-Y2 Si2 O7 content increased.
3.1.3. Surface morphologies of the coating Fig. 4 shows the surface SEM images of the coatings prepared at different sintering temperatures. When the temperature was 1350 ◦ C, the surface layer of the coating should be Y–Si–Al–O glass layer. Some large pores were found on the surface of the coating. In the low temperature case, the Y–Si–Al–O liquid fluidity will be limited because the liquid viscosity was relatively high, so that large pores caused by spraying process were retained. When the sintering temperature were 1400 ◦ C and 1450 ◦ C, the Y–Si–Al–O liquid infiltrated into the ␥-Y2 Si2 O7 powder layer completely, so the surface layer of coatings should be ␥-Y2 Si2 O7 layer. It can be seen that the ␥-Y2 Si2 O7 particles bonded tightly with each other, forming the dense coating surface. In addition, the surface of coating fabricated at 1450 ◦ C was more uniform and denser than that of coating prepared at 1400 ◦ C. This is because that the viscosity of Y–Si–Al–O liquid will reduce with the increase of sintering temperature, which
can promote the transfer of ␥-Y2 Si2 O7 particles sufficiently so that the surface of coating was more uniform and denser. 3.2. Thermal shock behavior of the coating 3.2.1. Effect of thermal shock on water absorption of the coating To further investigate water resistance of the coated samples in the practical environment, thermal cycling test was performed by quenching between room temperature and 1000 ◦ C. Water absorption of the coated samples before and after thermal shock is shown in Fig. 5. Before thermal shock, it is found that the water absorption of the coated samples decreased obviously (4.8–6.3%) compared with that of porous Si3 N4 (36.3%), indicating that as-received coating sealed the surface pores of porous Si3 N4 ceramic. The surface roughness of the coating decreased gradually with increasing of sintering temperature from 1350 to 1450 ◦ C (Fig. 4), which leaded to the water absorption decreased from 6.3% to 4.8%. After thermal shock, the water absorption of all the coated samples increased with the increase of thermal cycle times. During thermal shock, a high tensile thermal stress is placed within the coating, thus cracks are initially initiated on the surface and propagate along the coating thickness [22]. Crack can provide a channel for water entering into porous substrate, so leading to the increase of water absorption. In addition, the increase trend of water absorption increased gradually as the sintering temperature increased from 1350 to 1450 ◦ C. After thermal cycles for 15 times, the water absorption of the coatings prepared at 1350, 1400 and 1450 ◦ C was 27.2%, 10% and 5.3%,
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Fig. 5. Water absorption of the coated porous Si3 N4 ceramics after thermal cycling between 1000 ◦ C and room temperature.
respectively. This implied that thermal shock resistance of coatings increased gradually with the increase of sintering temperature. 3.2.2. Effect of thermal shock on morphologies of the coating To investigate oxidation of the coating during thermal cycling, thermal treatment test was performed by heating the coated samples at 1000 ◦ C and holding for 20 min. Fig. 6 is the surface and cross-section SEM images of the coatings after oxidation without thermal cycling. It can be seen from the surface images (Fig. 6(a)–(c)) that the coating microstructures exhibited little changes compared with that of the coatings without oxidation (Fig. 4). This indicated that the influence of oxidation on the coating surface was insignificant. Fig. 6(d) is the cross-section SEM image of the coating prepared at 1350 ◦ C after oxidation. Some holes were formed within the top Y–Si–Al–O glass layer and in the interface of Y–Si–Al–O glass layer/␥-Y2 Si2 O7 layer, which can be attributed to oxidation and/or volatilization of the glass phase in the coat-
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ing. Similarly, some holes were also found in the interface of top ␥-Y2 Si2 O7 layer/bond layer and within the bond layer in the coating prepared at 1400 ◦ C (Fig. 6(e)). It can be determined by the above results that the coatings prepared at 1350–1400 ◦ C had a weak oxidation resistance. The defects, formed holes, will weaken the interface bonding between the layers and reduce thermal shock resistance inevitably. Fig. 6(f) is the cross-section SEM image of the coating prepared at 1450 ◦ C after oxidation. The test results showed that the cross-section microstructure had little changes compared with that of the coating without oxidation (Fig. 1(c)), indicating as-prepared coating had a good oxidation resistance. Fig. 7(a) and (d) shows cross-section SEM images of the coating prepared at 1350 ◦ C after thermal cycling for 1 and 15 times, respectively. It can be seen from the cross-section image (Fig. 7(a)) that thermal stress cracks were formed within the coating. The crack density of the coatings measured using SEM after thermal cycle tests for 1, 5, 10 and 15 times was about 0.25, 0.33, 0.38 and 0.42 mm−1 , respectively. The crack density increased gradually with the increase of thermal cycle times. Meanwhile, some formed cracks can propagate along the coating thickness during thermal shock, even can penetrate the coating in the subsequent process of thermal shock, as indicated by the white arrow in Fig. 7(d). The penetrating cracks can provide channels for the water entering into porous substrate. The increase of crack density and the formation of the penetrating crack leaded to an obvious increase of the water absorption with the increase of thermal cycle times. Meanwhile, some thermal damages, including peel-off and spallation, were found in the top Y–Si–Al–O glass layer. During thermal shock, some holes were easily formed within top layer and in the interface of top layer/␥-Y2 Si2 O7 layer due to oxidation and volatilization of the glass phase. The formation of holes weakened its thermal shock resistance inevitably, thus leading to the formation of thermal damages. Fig. 7(b) and (e) shows cross-section SEM images of the coating prepared at 1400 ◦ C after thermal cycling for 1 and 15 times, respectively. It is found that the top ␥-Y2 Si2 O7 layer peeled off completely after thermal cycling, only the bond layer was retained on the sur-
Fig. 6. Surface and cross-section SEM images of the coatings prepared at different temperatures after oxidation for 20 min. (a) and (d) 1350 ◦ C; (b) and (e) 1400 ◦ C; (c) and (f) 1450 ◦ C.
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Fig. 7. Cross-section SEM images of the coatings prepared at different temperatures after thermal cycling for 1 and 15 times, respectively. (a) and (d) 1350 ◦ C; (b) and (e) 1400 ◦ C; (c) and (f) 1450 ◦ C.
face of porous Si3 N4 substrate. It can be inferred from Fig. 6(e) that some holes can be formed in the interface between the ␥-Y2 Si2 O7 layer and the bond layer because of the oxidation and volatilization of the glass phase during thermal shock. The formed holes weakened the interface bonding between the ␥-Y2 Si2 O7 layer and the bond laye and reduced its thermal shock resistance inevitably. So the top ␥-Y2 Si2 O7 layer peeled off completely after thermal shock. Meanwhile, some holes caused by volatilization of glass phase also appeared in the inner of bond layer. The volatilization of the glass will increase with the increase of thermal cycle times, thus leading to the increase of the amount of hole within the bond layer (Fig. 7(e)). The formed holes can affect the ability of the coated samples to resist to water absorption, so the water absorption increased slightly with the increase of thermal cycling (Fig. 5). In addition, compared with the coating prepared at 1400 ◦ C, the coating prepared at 1350 ◦ C only generated a larger crack, without ␥-Y2 Si2 O7 layer peeling off. It can be attributed to three layers of the coating. The top Y–Si–Al–O glass layer can act as an additional thermal barrier to protect the ␥-Y2 Si2 O7 interlayer from thermal shock. Fig. 7(c) and (f) shows cross-section SEM images of the coating prepared at 1450 ◦ C after thermal cycling for 1 and 15 times, respectively. The cross-section morphologies of the coating showed little changes after thermal shock. No defects, such as peel-off, cracks and holes, were found within the coating, indicating that the coating prepared at 1450 ◦ C had an excellent thermal shock resistance. This can be attributed to the following reasons: (1) during thermal shock, a high tensile thermal stress is placed within the coating, and the stress will increase with the increase of the coating thickness [22]. The coating prepared at 1450 ◦ C had a thinner ␥-Y2 Si2 O7 layer (Fig. 1) and the stress generated by thermal shock was reduced. So the coating microstructure can improve thermal shock resistance of the coating. (2) With the increase of sintering temperature, the thickness of the bond layer increased gradually and diffusion of ␥-Y2 Si2 O7 into the bond layer intensified gradually, which will strengthen the interface bonding of coating/substrate and ␥-Y2 Si2 O7 layer/bond layer. A strong interface bonding can
strengthen the coating against thermal shock, thus contributing to thermal shock resistance. (3) The coating prepared at 1450 ◦ C exhibited a good oxidation resistance, and no thermal defects such as holes and cracks were formed in the coating. This is also a important reason that the coating prepared at 1450 ◦ C had the better resistance to thermal shock than the other two kind of coatings.
4. Conclusions A dense multilayer ␥-Y2 Si2 O7 EBC was in-situ fabricated on the porous Si3 N4 ceramic using the liquid infiltration and filling method. As the sintering temperature increased gradually from 1350 to 1450 ◦ C, the microstructures of coating transformed from three layers into double layers, and the ␥-Y2 Si2 O7 content in the ␥-Y2 Si2 O7 layer increased gradually from 72.5 to 90.2 vol%. The coating sintered at 1450 ◦ C displayed the better thermal shock resistance than the other two kind of coatings. After thermal cycle for 15 times, the microstructure of the coating showed little change, and water absorption increased slightly from 4.8% to 5.3%.
Acknowledgements This work was supported by the National Natural Science Foundation of China (51472198, 51272206), the Ministry of Education Innovation Team Development Plan (IRT1280) and National Key Laboratory of Advanced Functional Composite (9140C560109130C56201).
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Please cite this article in press as: C. Wang, et al., Fabrication and thermal shock resistance of multilayer ␥-Y2 Si2 O7 environmental barrier coating on porous Si3 N4 ceramic, J Eur Ceram Soc (2015), http://dx.doi.org/10.1016/j.jeurceramsoc.2015.08.013