- Email: [email protected]
MoSi2-borosilicate glass coating on fibrous ceramics prepared by in-situ reaction method for infrared radiation
Materials and Design 103 (2016) 144–151
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
MoSi2-borosilicate glass coating on fibrous ceramics prepared by in-situ reaction method for infrared radiation Xin Tao a, Xiaojing Xu b, Linlin Guo a, Wenhu Hong b, Anran Guo a, Feng Hou a, Jiachen Liu a,⁎ a b
School of Materials Science and Engineering, Key Lab of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, P. R. China Research & Development Center of China Academy of Launch Vehicle Technology, Beijing 100076, P. R. China
a r t i c l e
i n f o
Article history: Received 16 March 2016 Received in revised form 18 April 2016 Accepted 19 April 2016 Available online 20 April 2016 Keywords: MoSi2-glass coating Infrared radiation Impact resistance Thermal shock resistance Thermal endurance
a b s t r a c t MoSi2-borosilicate glass coating with MoSi2 as emittance agent was prepared on mullite fibrous ceramics for enhanced superficial infrared radiation via an in-situ reaction method. The phase and structure evolutions during sintering were studied in detail. The infrared radiating property, impact resistance, thermal shock resistance and thermal endurance of the coating were investigated comprehensively. The results show that the coating was dense and flat, and it tightly adhered to the substrate by mechanical interlocking. During the sintering process, the low-temperature oxidation of MoSi2 was restrained due to the package of binder phase formed below 500 °C, and the coating was fully densified via the viscous flow of molten Si. The total emissivity of the asprepared coating was higher than 0.85 and the emissivity at low wavelength was higher than 0.9. The impact resistance was preferable attributed to the reinforced transition layer at the interface. The coating went through 30 thermal cycles between 1500 °C and 25 °C without cracking and spalling. The total emissivity declined merely by 4% after oxidation at 1500 °C for 50 h, which was influenced by the MoSi2 content and the surface roughness of the coating. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Thermal protection systems can maintain the structural temperature of reusable launch vehicles within acceptable limits during reentry flights [1–3]. One of the most common thermal protection systems is high-temperature reusable surface insulation, which has attracted considerable interest due to its low density, low thermal conductivity, reusability, excellent thermal radiation ability, and moderate stiffness under high temperatures [4–6]. The high-temperature reusable surface insulation mainly composes of a fibrous ceramic tile and a coating on the surface. The fibrous ceramic tile with low thermal conductivity is applied for thermal insulation [7,8], while the coating with high emissivity is designed to withstand high heat load and reject the most aerodynamic heating by radiation [5,9,10]. The aforementioned infrared radiating coating must meet many structural requirements in addition to high emissivity. Firstly, the coating needs eminent temperature tolerance and thermal shock resistance to keep stable performance at temperature shifts up to 1500 °C [3,11]. Secondly, the coating must be chemically and physically compatible with the substrate so that no devitrification or cracking would occur [11]. Thirdly, because the thermal protection systems is highly exposed to space environmental hazards like solid particle impacts, the coating urgently needs enhanced impact resistance [4,5,12]. Finally, the coating ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (J. Liu).
http://dx.doi.org/10.1016/j.matdes.2016.04.065 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
must be waterproof to avoid moisture into the internals of the system [11]. For over three decades, glass coatings consisting of glass as matrix and emittance agent dispersed therein have been the mainstay of the infrared radiating coatings in high-temperature reusable surface insulation [5]. Reaction Cured Glass coating (RCG), the pioneer, was thin and dense, in which silicon tetraboride (SiB4) was used as emittance agent, and the highest operating temperature was 1260 °C [11,13]. To improve the impact resistance, a new porous coating called Toughed Uni-piece Fibrous Insulation (TUFI) was developed by promoting the coating penetration into the tile. The infiltration area formed a fiber reinforced composite, which significantly enhanced the impact resistance of the coating. Besides, the emittance agent in TUFI was molybdenum disilicide (MoSi2), which had better oxidation resistance than SiB4. However, the quality of the coating increased to 0.14 g/cm2 accordingly, about twice of that in the RCG system [5,10,14]. In order to protect the nose caps and the leading edges of wings in reusable launch vehicles, the heat resistance of the coating needed great improvement. Therefore, a Toughened Uni-piece Fibrous Reinforced Oxidation Resistant Composite (TUFROC) was developed for high-temperature applications [5, 15–17]. It had higher tolerable temperature (1650 °C) than TUFI did (1260 °C) because tantalum silicide (TaSi2) replaced part of MoSi2 as the emittance agent and the content of the refractory emittance agents was nearly doubled. However, the quality of TUFROC greatly increased (0.21 g/cm2), which limited the widespread use of TUFROC over the fuselage.
X. Tao et al. / Materials and Design 103 (2016) 144–151
145
Table 1 Physical properties of the fibrous ceramics. Density (g/cm3)
Porosity (%)
Compressive strength (MPa)
Elastic modulus (MPa)
Thermal conductivity W/(m·K)
CTE (/K)
0.40
87.34%
0.84
20
0.0995
4.10 ∗ 10−6
A new glass coating was reported in this work, which exhibited much better heat resistance (long-term use at 1500 °C) than RCG and TUFI, and the coating quality per unit area (0.1 g/cm2) was much lower than TUFROC. Borosilicate glass with low thermal expansion coefficient, good thermal stability, tailorable softening temperature and thermal expansion coefficient [9,18], was chosen as the matrix of the coating. Among the three emittance agents applied in hightemperature reusable surface insulation, SiB4 oxidizes fastest at high temperatures [12], and TaSi2 suffers from a thermal mismatch with the fibrous substrate [15]. Therefore, MoSi2 was chosen as the emittance agent for its excellent oxidation resistance at high temperatures [19–21] and high infrared emissivity [10]. Most of the glass coatings are prepared by a two-step method, in which glass powders with specific compositions are firstly prepared by a melt-quench method, and then the as-prepared glass powders are used as raw materials for the coating preparation [22–24]. However, the MoSi2-glass coatings prepared by the two-step method were commonly faced with the oxidation of MoSi2 during the coating preparation [9,25–27]. A large proportion of MoSi2 was deteriorated during the coating preparation due to the low-temperature oxidation of MoSi2 at 400–700 °C. Therefore, an insitu reaction method was employed for the coating preparation in this study, in which silicon and boron oxide were used as the raw materials in order to form a binder phase at temperatures below 500 °C and protect MoSi2 from oxidation, and the borosilicate glass was obtained through the in-situ reaction of Si, B2O3 and O2 during the sintering process. In this work, MoSi2-borosilicate glass coating was prepared on mullite fibrous ceramics via an in-situ reaction method. The microstructure, phase composition and interfacial bonding were characterized. The formation process of the coating was studied, including the phase and structure evolutions during sintering. The infrared radiating property, impact resistance, thermal shock resistance and thermal endurance were investigated comprehensively.
The coating was fabricated by a slurry technique, which included slurry preparation, brushing and sintering. Firstly, stable slurry was prepared. Silicon powder (Si, 1 μm, Jinan Tianqin Silicon Industry, Shandong, China), boron oxide (B2O3, 200 mesh, Guangfu chemical company, Tianjin, China) and boron carbide (B4C, 1500 mesh, Mudanjiang Chenxi Boron Carbide Company, Heilongjiang, China) were mixed (mass ratio of 8.5:1.2:1) and dry-milled for 4 h (ball percentage: 70 wt%). The as-received powders were sieved to 300 mesh, mixed with MoSi2 (3 μm, Eno Material, Hebei, China) and water with mass ratio of 5:3:8, and milled for 2 h (ball percentage: 20 wt%) to form a homogeneous slurry. Secondly, the slurry was deposited on the substrates by brushing with a dosage of 0.15 g/cm3, after which the coated specimens were dried at 50 °C for 10 h in a drying oven. Finally, the fully dried specimens were sintered at 1500 °C for 1 h and later taken out of the furnace for rapid cooling to room temperature. The rapid cooling after sintering is required to minimize the as-processed residual tensile strain on the coating and inhabit quartz crystallization.
2.2. Characterization Phases of the coating were analyzed via X-ray Diffraction (XRD, D/ Max-2500 Rigaku, Japan) with filtered Cu-Kα radiation. Scanning Electron Microscope (SEM, S-4800, Hitachi, Tokyo, Japan) equipped with an Energy Dispersive Spectrometer (EDS) was used for the microstructural and elemental analysis of the specimens. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) of raw powders was performed using a combined TGA/DSC instrument (STA-449C, Netzsch, Bavaria, Germany), and the volume changes were measured by a dilatometry (DIL-402C, Netzsch, Bavaria, Germany), in which both of the samples were heated up to 1500 °C with a speed of 10 °C/ min in static air. Microstructure and phase evolutions of the coating were studied by heating the dried specimens at 5 °C/min to 300, 500, 700, 900, 1100, 1300, and 1500 °C, and holding the temperatures for 20 min.
2. Materials and methods 2.1. Coating preparations
2.3. Performance testing
The substrates were commercially available mullite fibrous ceramics, and their basic properties were listed in Table 1, in which CTE is short for Coefficient of Thermal Expansion. Specimens in size of 50 mm × 50 mm × 10 mm were cut down from a big threedimensional block, then polished to a 1200 grit finish on the surfaces, and finally cleaned by a vacuum sweeper.
The infrared emissivity of the MoSi2-borosilicate glass coating was measured by a Thermal Handheld Emissometer (ET-100, Surface Optics Corp., America) at room temperature, in which directional emissivities at six bands were measured in the thermal infrared spcetral region from 1.5 μm to 21 μm at two incidence angles, 20° and 60°. In theory, the total directional emissivity (ε) was calculated according to Eq. 1
Fig. 1. SEM images of the surface (a) and the cross section (b) of the coating.
146
X. Tao et al. / Materials and Design 103 (2016) 144–151
impact resistance was defined as the maximum impact energy when the failure area was invisible. The thermal shock behavior was evaluated by a water-quench method to accelerate the fracture of the glass layer. The specimen was held at 1500 °C for 15 min in a chamber electric furnace, then dropped into a water bath maintained at 25 °C, and put into the furnace again 15 min later. These operations were repeated until the destruction of the coating was observed. Isothermal oxidation test was carried out in an electric furnace at 1500 °C for 50 h to evaluate the thermal endurance of the coating. The phase composition, microstructure and emissivity of the coating were analyzed during the oxidation.
Fig. 2. XRD pattern of the coating, with the standard pattern of MoSi2 (PDF#74-1148) presented together.
[28–30]: Z ε¼
λ2 λ1
Z
ε ðλÞ dλ ð1Þ
λ2
dλ λ1
where ε(λ) was the spectral emissivity and was unavailable by ET-100; therefore, the total emissivity was obtained approximatively in accordance with Eq. 2 in practice, denoted as ε⁎, X
εi Δλi ε ¼ Xi Δλi i
ð2Þ
where εi and Δλi were the emissivity at a certain band and the band width, respectively. The impact resistance of the MoSi2-borosilicate glass coating was measured by a custom-made vertical drop device with steel balls of 5, 8, 10, 15, 20, 35 and 50 g as impactors. Impactor drop height was varied to simulate the impact energy in joules (height × weight) [10,31]. The
3. Results and discussion 3.1. Characteristics 3.1.1. Morphology and phase composition Fig. 1a shows the surface of the MoSi2-borosilicate glass coating and the contrast of the picture had been increased for better observation of the details. The coating surface was dense and flat, and no penetrated cracks or holes were observed. The coating was water-impervious, confirmed by a simple immersion test, in which the mullite fibrous block with the coating deposited all around floated on water all the time, while the uncoated one submerged in water. Fig. 1b is the crosssectional image of the coating and the fibrous substrate, exhibiting that the coating was compact with a uniform thickness of 300 μm. Fig. 2 shows the XRD pattern of the MoSi2-borosilicate glass coating. The broad hump between 15° and 30° was related to borosilicate glass, and the main crystalline phase in the coating was MoSi2 (JCPDS#659392). Apart from that, Mo2B5 (JCPDS#65-4029) and Cristobalite (JCPDS#77-1316) were also detected. Mo2B5 was generated from the reaction of MoSi2 and B2O3, and would benefit the thermal endurance of the coating for its high melting point (2280 °C). In addition, Si was scarcely detected in the coating, suggesting its complete oxidation into borosilicate glass. EDS mapping was used to further investigate the element distribution on the surface of the MoSi2-borosilicate glass coating. The SEM
Fig. 3. SEM image (a) and EDS mappings (white = Mo (b), red = O (c), and green = Si (d)) of the coating surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
X. Tao et al. / Materials and Design 103 (2016) 144–151
image of the coating surface was shown in Fig. 3a and EDS mappings in Fig. 3b–d (with colors indicate white = Mo, red = O, and green = Si). Fig. 3a shows that dispersion phases (bright regions) evenly distributed in the dark regions. The dispersion phase with higher Mo content, lower Si and O content, was supposed to be MoSi2 or Mo2B5 particles, on the basis of the XRD analysis in Fig. 2. The continuous phase with relatively higher Si and O content was the borosilicate glass matrix. Therefore, the coating was a composite with borosilicate glass as matrix and MoSi2 particles as main dispersion phase. 3.1.2. Interfacial adhesion Element line scanning analysis (Fig. 4a) illustrates the composition profile of Si, Al, and Mo across the interface between the coating and substrate. The dense coating was a composite with borosilicate glass as matrix and MoSi2 as dispersion phase, thus containing dominant Si element and minor Mo element. The mullite fibrous substrate mainly contained Si, O and Al elements. A transition layer was recognized
147
near the interface, where both Mo and Al were detected. The magnifying micrograph of the transition layer was shown in Fig. 4b and the EDS analysis of region “c” was displayed in Fig. 4c. In the transition layer, mullite fiber cross-sections were observed in the coating, as confirmed by EDS analysis. Therefore, the transition layer was a fiber reinforced composite, resulted from the infiltration of coating into the fibrous substrate. In fact, the transition layer mattered a lot. On one hand, the fibers in the coating acted as reinforcement to improve the coating strength; on the other hand, the mechanical lock effect in the transition layer contributed to the tight bond between the coating and substrate. 3.2. Formation process In the presenting coating, Si and B2O3 were used as raw materials, and the glass coating was prepared via the in-situ reacting of Si, B2O3 and O2. The formation process of the MoSi2-borosilicate glass coating
Fig. 4. Fracture morphology of the coating and EDS line profile of the interface (a). Magnifying micrograph of region “b” (b) and EDS analysis of region “c” (c).
148
X. Tao et al. / Materials and Design 103 (2016) 144–151
was investigated in detail. Based on the TGA/DSC analysis and volumetric change (Fig. 5a), the sintering process consisted of four stages. Combined with the XRD patterns (Fig. 5b) and micrographs (Fig. 5c) of the coatings calcined at different temperatures, the phase transformation and microstructure evolution at each stage were discussed. The first stage (RT-390 °C) was the linear thermal expansion stage. The major transformation was water evaporation, corresponding to the mass decrease in the TGA curve and the endothermic peak in the DSC curve at 107 °C. At this stage, particles accumulated irregularly in the coating without binder phase. The second stage (390–490 °C) was the volume compensation stage. In this stage, mass and volume increased with an exothermic peak in the DSC curve at 450 °C, which was connected to the oxidation of B4C. The oxidation can be described by reactions 1 and 2, where (s) and (g) denote the solid and gas phase, respectively. B4C was oxidized to B2O3 accompanied with a 250% volume increase, which partially compensated the sintering shrinkage of the coating [32,33]. Meanwhile, the oxidation products (B2O3) melted at 445 °C, impregnated into pores and cracks in the coating, and covered the surface of MoSi2 particles. B4 CðsÞ þ 3O2 ðgÞ ¼ 2B2 O3 ðsÞ þ CðsÞ
ð1Þ
B4 CðsÞ þ 4O2 ðgÞ ¼ 2B2 O3 ðsÞ þ CO2 ðgÞ
ð2Þ
of MoSi2 and Si into SiO2 (reactions 3 and 4) was ongoing and meanwhile B2O3 partially volatilized in the process of heating up. The generating SiO2 might be amorphous and thereby was not obvious in the XRD patterns. The molten binder phase promoted particle rearrangement and local densification in the coating, leading to the volume decrease. The transformation of MoSi2 in this stage took place as follows. A spot of MoSi2 was oxidized to MoO3, as indicated by the XRD pattern of 700 °C and corresponding to the exothermic peaks in the DSC curve at around 600 °C. As temperature increased, the existence of reductant and the decreasing oxygen partial pressure in the system prompted to generate the products of low-valence Mo, such as MoO2 and Mo, which can be described by reactions 5 and 6. In addition, MoxBy was detected in the coating calcined at 1300 °C, which was generated from the reaction of MoSi2 and B2O3 (reaction 7). 2MoSi2 ðsÞ þ 7O2 ðgÞ ¼ 2MoO3 ðsÞ þ 4SiO2 ðsÞ
ð3Þ
SiðsÞ þ O2 ðgÞ ¼ SiO2 ðsÞ
ð4Þ
CðsÞ þ 2MoO3 ðsÞ ¼ 2MoO2 ðsÞ þ CO2 ðgÞ
ð5Þ
CðsÞ þ MoO2 ðsÞ ¼ MoðsÞ þ CO2 ðgÞ
ð6Þ
xMoSi2 þ ð0:5yÞB2 O3 þ ð2 −0:75yÞO2 ¼ Mox By þ 2 SiO2
ð7Þ
−
The third stage (490 1390 °C) was the elementary densification stage, in which the molten binder phase protected MoSi2 from oxidation and promoted the coating densification. At low temperature, the binder phase with much B2O3 and little SiO2 melted, packed the MoSi2 particles and controlled inward oxygen diffusion [34]; therefore, the lowtemperature oxidation of MoSi2 was restrained. We inferred that the SiO2 content in the binder phase was increasing, because the oxidation
The last stage (1390− 1500 °C) stood for the further densification of the coating via melting of Si related to the sharp endothermic peak in the DSC curve at 1390 °C. Thereafter, the mass decreased faster, the quality increased faster, and the content of Si sharply decreased suggested by the XRD patterns of the coatings calcined at 1300 °C and 1500 °C. Consequently, the melting of Si significantly promoted the
Fig. 5. TGA/DSC analyses and volume change (a) of the coating from RT to 1500 °C; XRD patterns (b) and SEM images (c) of the coatings after calcination at different temperature.
X. Tao et al. / Materials and Design 103 (2016) 144–151
149
Table 2 Thermal shock tests of the coatings.
Fig. 6. Emissivity of the coating at six bands from 1.5 μm to 21 μm at two incidence angles, 20° and 60°.
densification process and the oxidation of Si in the last stage, and the coating after this stage was smooth and flat without visible pores.
3.3. Performance 3.3.1. Infrared radiating property The infrared emissivity of the MoSi 2 -borosilicate glass coating was shown in Fig. 6, and the total emissivities at 20° and 60° were 0.867 and 0.849 respectively, according to Eq. 2, which satisfied the requirement for infrared radiating coatings (ε ∗ N0.80) [11]. Because directional emissivities at all wavelength decreased with the increasing exitance angle [35], the emissivity at 20° was always higher than that at 60°. According to the Planck black-body radiation law and Wien's displacement law, the black-body radiation at high temperatures was mostly in the short wavelength [28]. For example, when the ambient temperature is 1300 °C, the maximum radiation intensity appears at 1.84 μm and the radiation energy at 1.5–3.5 μm is 86.9% of that in the full wavelength. Therefore, high emissivity at short wavelength was required for high-temperature infrared radiation. The emissivity of the presenting coating at 1.5–3.5 μm was higher than 0.9, which would greatly contribute to the radiation at high temperatures [28,29]. A final note about this result was that, the relatively low emissivity at 5.5–10 μm was attributed to the asymmetrical Si\\O stretching vibration absorptions of [SiO4] tetrahedra [36], which occurred in many silicate glass and centered at 9.08 μm.
Fig. 7. Impact resistance of the coatings with different thickness and quality. The schematic diagram of the impact-induced failure mode was presented at the top-left.
Thermal shock cycles
Appearance of the coatings
10 20 23 31
Unchanged Unchanged Gloss decreased Locally spalled on the edges
3.3.2. Impact resistance The impact resistance of the MoSi2-borosilicate glass coating as a function of coating thickness was shown in Fig. 7. The impact energy multiplied exponentially with the increasing coating thickness; thus thick coating was required for better impact resistance. However, thick coating would increase the weight of the whole TPS, and finally the coating of 300 μm was employed for other performance tests. RCG was the typical glass coating for fibrous ceramics made by NASA and it had the similar structure to the presenting coating: a dense thin glass coating of about 300 μm [4,5]. The impact energy of RCG was marked out in Fig. 7 [10], which was half of that of the presenting coating (0.07 J). The better impact resistance of the presenting coating was closely related to the higher coating strength and tighter interface bonding, resulted from the fiber reinforced transition layer near the interface. The impact-induced failure of the glass coating-fibrous ceramics system included two aspects. One was cracking of the coating along the circumference and the other was compressive deformation of the substrate due to the fracture of fibers and debonding at the bonding points [8].
3.3.3. Thermal shock resistance Thermal shock test was conducted on the MoSi2-borosilicate glass coating at 1500 °C, and the results were listed in Table 2. During the first 20 thermal shock cycles, no visible change was observed on the coating. With the increase of thermal cycles, gloss reduced on the coating surface, but the coating was still compact and well adhered to the substrates without cracking or bubbling. Spalling of the coating was first observed on the edges in the 31st thermal cycle and the crosssectional morphology was presented in Fig. 8. Interfacial fractures decreased the bonding strength of the coating, and was the main thermal shock damage form of the integrative glass coating and fibrous ceramic. We assume that mullite fibers at the interface would deform to relieve the thermal stress during each thermal cycles, and after 30 thermal cycles, fatigue fracture occurred to the fibers at the interface. In consideration of the great temperature gradient (1475 °C) and rapid temperature change rate (a water-quench method) in the thermal shock test, as well as the larger quantity of thermal cycles before damage (30 cycles), the MoSi2-borosilicate glass coating was demonstrated to have good thermal shock resistance, which was attributed to several reasons. Firstly, the CTEs of the coating and substrate were similar
Fig. 8. Cross-sectional morphology of the coating and substrate after 31 thermal shock cycles.
150
X. Tao et al. / Materials and Design 103 (2016) 144–151
Fig. 9. Total emissivity as a function of oxidation time during the isothermal oxidation test at 1500 °C (a), XRD patterns (b) and surface SEM images of the coatings tested for 35 h (c), 40 h (d) and 50 h (e).
(3.41 × 10− 6/K and 4.10 × 10−6/K, respectively), so minor thermal stress was produced during thermal cycles. Secondly, the tight integration at the interface through mechanical interlocking prevented the coating from spalling. Last but not the least, much of the thermal stress was absorbed by the deformation of the fibrous ceramics with relatively low modulus and high fracture strain. 3.3.4. Thermal endurance For long-term use at high temperatures, chemical reactions or matter volatilizations may take place in the coating and result in degraded infrared radiating performance. Hence the thermal endurance of the MoSi2-borosilicate glass coating was evaluated by the emissivity change during isothermal oxidation at 1500 °C (Fig. 9a). The total emissivity declined merely by 4% after oxidation for 50 h and was always higher than 0.8. Therefore, the coating can be used as long-term infrared radiating coating at temperatures up to 1500 °C. The total emissivity at 20° declined slightly with the oxidation time. By contrast, the emissivity at 60° was declining in overall trend, but it increased when the oxidation time increased from 35 h to 40 h. In order to explain the emissivity changes, the XRD patterns and SEM images of the tested samples were shown in Fig. 9b–e. With the extension of oxidation time, the content of MoSi2 decreased (Fig. 9b), while that of MoxBy gradually increased. Though MoSi2 was oxidation resistant at high temperature due to the passivation layer of SiO2, part of MoSi2 was oxidized in the presenting coating as a result of the following three reasons. Firstly, fine MoSi2 powders with larger specific surface area and more defects were less oxidation resistant than bulk MoSi2 [37]. Secondly, the oxygen diffusion coefficient in borosilicate glass at higher temperatures was higher and the glass cannot isolate MoSi2 particles from oxygen [34]. Besides, the B2O3 in the coating destroyed part of the SiO2 passivation layers around MoSi2 particles and reacted with MoSi2 to generate MoxBy. Therefore, MoSi2 was gradually oxidized with the extension of oxidation time, resulting in the decreased emissivity at 20° and 60°. The changing regularity of the total emissivity during the oxidation test was also related to the surface roughness. The coating tested for b35 h was flat and smooth, and showed no significant change in appearance (Fig. 9c). When the oxidation time reached 40 h, matte surface was observed on a macro level. Bare particles were observed in the
micrograph of the coating (Fig. 9d), and the surface roughness increased correspondingly. As the surface roughness slightly increased, the emissivity rose due to the enhanced inter-reflection of incident light in the grooves [29]. Therefore, the total emissivity at 60° increased when the oxidation time increased from 35 h to 40 h. When the oxidation time reached 50 h, the coating surface became macroscopic rough; holes and cracks were seen in the micrograph (Fig. 9e).
4. Conclusion MoSi2-borosilicate glass infrared radiating coating was prepared on mullite fibrous ceramics via an in-situ reaction method. The coating was dense and uniform with borosilicate glass as matrix and MoSi2 as dispersion phase. Tight interfacial bonding via mechanical interlock was observed in the transition layer. The formation process of the coating included four stages. The sintering shrinkage was reduced via the addition of B4C in the second stage, the low-temperature oxidation of MoSi2 was restrained via the package of borosilicate glass in the third stage, and the fully densification of the coating was based on the melting of Si in the last stage. The total emissivity of the as-prepared coating was higher than 0.85 and the emissivity at shorter wavelength was higher than 0.9. The impact resistance of the coating was improved owing to the enhanced transition layer at the interface. The coating could went through 30 thermal cycles between 1500 °C and 25 °C without cracking and spalling; spalling on the edges was observed in the 31st thermal cycles, and the main failure mode was interfacial fracture, resulted from the fracture of mullite fibers at the interface. The total emissivity of the coating declined merely by 4% after oxidation at 1500 °C for 50 h. The decrease of emissivity in overall trend was attributed to the oxidation of MoSi2 and the emissivity rose to some extent when the oxidation time increased from 35 h to 40 h due to the increased surface roughness of the coating during oxidation.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Project No. 51572298 and Project No. 51372164).
X. Tao et al. / Materials and Design 103 (2016) 144–151
References [1] G.N. Xie, W. Qi, W.H. Zhang, B. Sunden, G. Lorenzini, Optimization design and analysis of multilayer lightweight thermal protection structures under aerodynamic heating conditions, J. Thermal Sci. Eng. Appl. 5 (2013) 011011. [2] K. Wei, R.J. He, X.M. Cheng, R.B. Zhang, Y.M. Pei, D.N. Fang, A lightweight, high compression strength ultra high temperature ceramic corrugated panel with potential for thermal protection system applications, Mater. Des. 66 (2015) 552–556. [3] B. Pan, L.P. Yu, D.F. Wu, Thermo-mechanical response of superalloy honeycomb sandwich panels subjected to non-steady thermal loading, Mater. Des. 88 (2015) 528–536. [4] J.E. Miller, W.E. Bohl, E.L. Christiansen, B.A. Davis, Ballistic performance of porousceramic, thermal protection systems, Int. J. Impact Eng. 56 (2013) 40–46. [5] T.E. Steyer, Shaping the future of ceramics for aerospace applications, Int. J. Appl. Ceram. Technol. 10 (2013) 389–394. [6] M. Ferraiuolo, O. Manca, Heat transfer in a multi-layered thermal protection system under aerodynamic heating, Int. J. Therm. Sci. 53 (2012) 56–70. [7] X. Dong, G.F. Sui, Z.Q. Yun, M.C. Wang, A.R. Guo, J. Zhang, J.C. Liu, Effect of temperature on the mechanical behavior of mullite fibrous ceramics with a 3D skeleton structure prepared by molding method, Mater. Des. 90 (2016) 942–948. [8] X. Dong, G.F. Sui, J.C. Liu, A.R. Guo, S.E. Ren, M.C. Wang, H.Y. Du, Mechanical behavior of fibrous ceramics with a bird's nest structure, Compos. Sci. Technol. 100 (2014) 92–98. [9] G.F. Shao, X.D. Wu, Y. Kong, S. Cui, X.D. Shen, C.R. Jiao, J. Jiao, Thermal shock behavior and infrared radiation property of integrative insulations consisting of MoSi2/borosilicate glass coating and fibrous ZrO2 ceramic substrate, Surf. Coat. Technol. 270 (2015) 154–163. [10] D.B. Leiser, R. Churchward, V. Katvala, D. Stewart, Advanced porous coating for lowdensity ceramic insulation materials, J. Am. Ceram. Soc. 72 (1989) 1003–1010. [11] H.E. Goldstein, D.B. Leiser, V. Katvala, Reaction cured borosilicate glass coating for low-density fibrous silica insulation, series Materials Science Research in Borate Glasses (1978) 623-634. [12] D.A. Stewart, D.B. Leiser, Characterization of the thermal conductivity for advanced toughened uni-piece fibrous insulations, AlAA 28th Thermophysics Conference 1993, pp. 1–18 Washington, D.C. [13] J.C. Fletcher, Reaction cured glass and glass coatings, U. S. patent. 4,093,771, 1978. [14] D.B. Leiser, Toughened uni-piece firous insulation, U. S. patent. 5,079,082, 1992. [15] D.A. Stewart, High efficiency tantalum-based ceramic composite structures, U. S. patent. 7,767,305 B1, 2010. [16] D.A. Stewart, Toughened uni-piece fibrous reinforced oxidization-resistant composite, U. S. patent. 7,314,648 B1, 2008. [17] S.M. Johnson, M.J. Gasch, D. Leiser, D. Stewart, Development of new TPS at NASA Ames Research Center, 15th AIAA International Space Planes and Hypersonic Systems and Technologies Conference 2008, pp. 1–10 Dayton. [18] J. Li, R.Y. Luo, Y.P. Chen, Q. Xiang, C. Lin, P. Ding, N. An, J.W. Cheng, Oxidation behavior and kinetics of SiC/alumina-borosilicate coating for carbon-carbon composites, Appl. Surf. Sci. 255 (2008) 1967–1974. [19] S.J. Park, M.K. Seo, The effects of MoSi2 on the oxidation behavior of carbon/carbon composites, Carbon 39 (2001) 1229–1235.
151
[20] S. Knittel, S. Mathieu, M. Vilasi, The oxidation behaviour of uniaxial hot pressed MoSi2 in air from 400 to 1400 degrees C, Intermetallics 19 (2011) 1207–1215. [21] Q.G. Fu, H.J. Li, K.Z. Li, K. Tong, A Si-Mo-W coating to protect SiC-coated carbon/carbon composites against oxidation, J. Am. Ceram. Soc. 92 (2009) 2132–2135. [22] F. Smeacetto, M. Salvo, M. Ferraris, Oxidation protective multilayer coatings for carbon-carbon composites, Carbon 40 (2002) 583–587. [23] M.H. Chen, W.B. Li, M.L. Shen, S.L. Zhu, F.H. Wang, Glass coatings on stainless steels for high-temperature oxidation protection: mechanisms, Corros. Sci. 82 (2014) 316–327. [24] M.L. Shen, S.L. Zhu, M.H. Chen, F.H. Wang, The oxidation and oxygen permeation resistance of quartz particle-reinforced aluminosilicate glass coating on titanium alloy, J. Am. Ceram. Soc. 94 (2011) 2436–2441. [25] X.L. Li, C. Zhang, M. Wang, Y.C. Qiao, H.J. Tang, Preparation and thermal expansion coefficient control of MoSi2-Al2O3-SiO2 coating with high temperature resistance and emissivity, Rare Met. Mater. Eng. 44 (2015) 200–203. [26] G.F. Shao, X.D. Wu, Y. Kong, X.D. Shen, S. Cui, X. Guan, C.R. Jiao, J. Jiao, Microstructure, radiative property and thermal shock behavior of TaSi2\\SiO2-borosilicate glass coating for fibrous ZrO2 ceramic insulation, J. Alloys Compd. 663 (2016) 360–370. [27] M. Wang, X.L. Li, D. Su, H.M. Ji, H.J. Tang, Z.H. Zhao, J. He, Effect of glass phase content on structure and properties of gradient MoSi2-BaO-Al2O3-SiO2 coating for porous fibrous insulations, J. Alloys Compd. 657 (2016) 684–690. [28] X.D. He, Y.B. Li, L.D. Wang, Y. Sun, S. Zhang, High emissivity coatings for high temperature application: progress and prospect, Thin Solid Films 517 (2009) 5120–5129. [29] D.B. Mahadik, S. Gujjar, G.M. Gouda, H.C. Barshilia, Double layer SiO2/Al2O3 high emissivity coatings on stainless steel substrates using simple spray deposition system, Appl. Surf. Sci. 299 (2014) 6–11. [30] H. Tanaka, S. Sawai, K. Morimoto, K. Hisano, Evaluation of hemispherical total emissivity for thermal radiation calorimetry, Int. J. Thermophys. 21 (2000) 927–940. [31] C. Zhang, H. Zhou, L. Liu, Laminar Fe-based amorphous composite coatings with enhanced bonding strength and impact resistance, Acta Mater. 72 (2014) 239–251. [32] M.E. Westwood, J.D. Webster, R.J. Day, F.H. Hayes, R. Taylor, Oxidation protection for carbon fibre composites, J. Mater. Sci. 31 (1996) 1389–1397. [33] M.C. Wang, J.C. Liu, H.Y. Du, F. Hou, A.R. Guo, S. Liu, X. Dong, Joining of C/C composites by using B4C reinforced phosphate adhesive, Ceram. Int. 40 (2014) 11581–11591. [34] L.S. Walker, E.L. Corral, Self-generating high-temperature oxidation-resistant glassceramic coatings for C-C composites using UHTCs, J. Am. Ceram. Soc. 97 (2014) 3004–3011. [35] A.E. Wald, J.W. Salisbury, Thermal infrared directional emissivity of powdered quartz, J. Geophys. Res. 100 (1995) 24665–24675. [36] B.R. da Luz, J.K. Crowley, Spectral reflectance and emissivity features of broad leaf plants: prospects for remote sensing in the thermal infrared (8.0–14.0 mu m), Remote Sens. Environ. 109 (2007) 393–405. [37] K. Kurokawa, H. Houzumi, I. Saeki, H. Takahashi, Low temperature oxidation of fully dense and porous MoSi2, Mat. Sci. Eng., A 261 (1999) 292–299.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
MoSi2-borosilicate glass coating on fibrous ceramics prepared by in-situ reaction method for infrared radiation Xin Tao a, Xiaojing Xu b, Linlin Guo a, Wenhu Hong b, Anran Guo a, Feng Hou a, Jiachen Liu a,⁎ a b
School of Materials Science and Engineering, Key Lab of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, P. R. China Research & Development Center of China Academy of Launch Vehicle Technology, Beijing 100076, P. R. China
a r t i c l e
i n f o
Article history: Received 16 March 2016 Received in revised form 18 April 2016 Accepted 19 April 2016 Available online 20 April 2016 Keywords: MoSi2-glass coating Infrared radiation Impact resistance Thermal shock resistance Thermal endurance
a b s t r a c t MoSi2-borosilicate glass coating with MoSi2 as emittance agent was prepared on mullite fibrous ceramics for enhanced superficial infrared radiation via an in-situ reaction method. The phase and structure evolutions during sintering were studied in detail. The infrared radiating property, impact resistance, thermal shock resistance and thermal endurance of the coating were investigated comprehensively. The results show that the coating was dense and flat, and it tightly adhered to the substrate by mechanical interlocking. During the sintering process, the low-temperature oxidation of MoSi2 was restrained due to the package of binder phase formed below 500 °C, and the coating was fully densified via the viscous flow of molten Si. The total emissivity of the asprepared coating was higher than 0.85 and the emissivity at low wavelength was higher than 0.9. The impact resistance was preferable attributed to the reinforced transition layer at the interface. The coating went through 30 thermal cycles between 1500 °C and 25 °C without cracking and spalling. The total emissivity declined merely by 4% after oxidation at 1500 °C for 50 h, which was influenced by the MoSi2 content and the surface roughness of the coating. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Thermal protection systems can maintain the structural temperature of reusable launch vehicles within acceptable limits during reentry flights [1–3]. One of the most common thermal protection systems is high-temperature reusable surface insulation, which has attracted considerable interest due to its low density, low thermal conductivity, reusability, excellent thermal radiation ability, and moderate stiffness under high temperatures [4–6]. The high-temperature reusable surface insulation mainly composes of a fibrous ceramic tile and a coating on the surface. The fibrous ceramic tile with low thermal conductivity is applied for thermal insulation [7,8], while the coating with high emissivity is designed to withstand high heat load and reject the most aerodynamic heating by radiation [5,9,10]. The aforementioned infrared radiating coating must meet many structural requirements in addition to high emissivity. Firstly, the coating needs eminent temperature tolerance and thermal shock resistance to keep stable performance at temperature shifts up to 1500 °C [3,11]. Secondly, the coating must be chemically and physically compatible with the substrate so that no devitrification or cracking would occur [11]. Thirdly, because the thermal protection systems is highly exposed to space environmental hazards like solid particle impacts, the coating urgently needs enhanced impact resistance [4,5,12]. Finally, the coating ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (J. Liu).
http://dx.doi.org/10.1016/j.matdes.2016.04.065 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
must be waterproof to avoid moisture into the internals of the system [11]. For over three decades, glass coatings consisting of glass as matrix and emittance agent dispersed therein have been the mainstay of the infrared radiating coatings in high-temperature reusable surface insulation [5]. Reaction Cured Glass coating (RCG), the pioneer, was thin and dense, in which silicon tetraboride (SiB4) was used as emittance agent, and the highest operating temperature was 1260 °C [11,13]. To improve the impact resistance, a new porous coating called Toughed Uni-piece Fibrous Insulation (TUFI) was developed by promoting the coating penetration into the tile. The infiltration area formed a fiber reinforced composite, which significantly enhanced the impact resistance of the coating. Besides, the emittance agent in TUFI was molybdenum disilicide (MoSi2), which had better oxidation resistance than SiB4. However, the quality of the coating increased to 0.14 g/cm2 accordingly, about twice of that in the RCG system [5,10,14]. In order to protect the nose caps and the leading edges of wings in reusable launch vehicles, the heat resistance of the coating needed great improvement. Therefore, a Toughened Uni-piece Fibrous Reinforced Oxidation Resistant Composite (TUFROC) was developed for high-temperature applications [5, 15–17]. It had higher tolerable temperature (1650 °C) than TUFI did (1260 °C) because tantalum silicide (TaSi2) replaced part of MoSi2 as the emittance agent and the content of the refractory emittance agents was nearly doubled. However, the quality of TUFROC greatly increased (0.21 g/cm2), which limited the widespread use of TUFROC over the fuselage.
X. Tao et al. / Materials and Design 103 (2016) 144–151
145
Table 1 Physical properties of the fibrous ceramics. Density (g/cm3)
Porosity (%)
Compressive strength (MPa)
Elastic modulus (MPa)
Thermal conductivity W/(m·K)
CTE (/K)
0.40
87.34%
0.84
20
0.0995
4.10 ∗ 10−6
A new glass coating was reported in this work, which exhibited much better heat resistance (long-term use at 1500 °C) than RCG and TUFI, and the coating quality per unit area (0.1 g/cm2) was much lower than TUFROC. Borosilicate glass with low thermal expansion coefficient, good thermal stability, tailorable softening temperature and thermal expansion coefficient [9,18], was chosen as the matrix of the coating. Among the three emittance agents applied in hightemperature reusable surface insulation, SiB4 oxidizes fastest at high temperatures [12], and TaSi2 suffers from a thermal mismatch with the fibrous substrate [15]. Therefore, MoSi2 was chosen as the emittance agent for its excellent oxidation resistance at high temperatures [19–21] and high infrared emissivity [10]. Most of the glass coatings are prepared by a two-step method, in which glass powders with specific compositions are firstly prepared by a melt-quench method, and then the as-prepared glass powders are used as raw materials for the coating preparation [22–24]. However, the MoSi2-glass coatings prepared by the two-step method were commonly faced with the oxidation of MoSi2 during the coating preparation [9,25–27]. A large proportion of MoSi2 was deteriorated during the coating preparation due to the low-temperature oxidation of MoSi2 at 400–700 °C. Therefore, an insitu reaction method was employed for the coating preparation in this study, in which silicon and boron oxide were used as the raw materials in order to form a binder phase at temperatures below 500 °C and protect MoSi2 from oxidation, and the borosilicate glass was obtained through the in-situ reaction of Si, B2O3 and O2 during the sintering process. In this work, MoSi2-borosilicate glass coating was prepared on mullite fibrous ceramics via an in-situ reaction method. The microstructure, phase composition and interfacial bonding were characterized. The formation process of the coating was studied, including the phase and structure evolutions during sintering. The infrared radiating property, impact resistance, thermal shock resistance and thermal endurance were investigated comprehensively.
The coating was fabricated by a slurry technique, which included slurry preparation, brushing and sintering. Firstly, stable slurry was prepared. Silicon powder (Si, 1 μm, Jinan Tianqin Silicon Industry, Shandong, China), boron oxide (B2O3, 200 mesh, Guangfu chemical company, Tianjin, China) and boron carbide (B4C, 1500 mesh, Mudanjiang Chenxi Boron Carbide Company, Heilongjiang, China) were mixed (mass ratio of 8.5:1.2:1) and dry-milled for 4 h (ball percentage: 70 wt%). The as-received powders were sieved to 300 mesh, mixed with MoSi2 (3 μm, Eno Material, Hebei, China) and water with mass ratio of 5:3:8, and milled for 2 h (ball percentage: 20 wt%) to form a homogeneous slurry. Secondly, the slurry was deposited on the substrates by brushing with a dosage of 0.15 g/cm3, after which the coated specimens were dried at 50 °C for 10 h in a drying oven. Finally, the fully dried specimens were sintered at 1500 °C for 1 h and later taken out of the furnace for rapid cooling to room temperature. The rapid cooling after sintering is required to minimize the as-processed residual tensile strain on the coating and inhabit quartz crystallization.
2.2. Characterization Phases of the coating were analyzed via X-ray Diffraction (XRD, D/ Max-2500 Rigaku, Japan) with filtered Cu-Kα radiation. Scanning Electron Microscope (SEM, S-4800, Hitachi, Tokyo, Japan) equipped with an Energy Dispersive Spectrometer (EDS) was used for the microstructural and elemental analysis of the specimens. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) of raw powders was performed using a combined TGA/DSC instrument (STA-449C, Netzsch, Bavaria, Germany), and the volume changes were measured by a dilatometry (DIL-402C, Netzsch, Bavaria, Germany), in which both of the samples were heated up to 1500 °C with a speed of 10 °C/ min in static air. Microstructure and phase evolutions of the coating were studied by heating the dried specimens at 5 °C/min to 300, 500, 700, 900, 1100, 1300, and 1500 °C, and holding the temperatures for 20 min.
2. Materials and methods 2.1. Coating preparations
2.3. Performance testing
The substrates were commercially available mullite fibrous ceramics, and their basic properties were listed in Table 1, in which CTE is short for Coefficient of Thermal Expansion. Specimens in size of 50 mm × 50 mm × 10 mm were cut down from a big threedimensional block, then polished to a 1200 grit finish on the surfaces, and finally cleaned by a vacuum sweeper.
The infrared emissivity of the MoSi2-borosilicate glass coating was measured by a Thermal Handheld Emissometer (ET-100, Surface Optics Corp., America) at room temperature, in which directional emissivities at six bands were measured in the thermal infrared spcetral region from 1.5 μm to 21 μm at two incidence angles, 20° and 60°. In theory, the total directional emissivity (ε) was calculated according to Eq. 1
Fig. 1. SEM images of the surface (a) and the cross section (b) of the coating.
146
X. Tao et al. / Materials and Design 103 (2016) 144–151
impact resistance was defined as the maximum impact energy when the failure area was invisible. The thermal shock behavior was evaluated by a water-quench method to accelerate the fracture of the glass layer. The specimen was held at 1500 °C for 15 min in a chamber electric furnace, then dropped into a water bath maintained at 25 °C, and put into the furnace again 15 min later. These operations were repeated until the destruction of the coating was observed. Isothermal oxidation test was carried out in an electric furnace at 1500 °C for 50 h to evaluate the thermal endurance of the coating. The phase composition, microstructure and emissivity of the coating were analyzed during the oxidation.
Fig. 2. XRD pattern of the coating, with the standard pattern of MoSi2 (PDF#74-1148) presented together.
[28–30]: Z ε¼
λ2 λ1
Z
ε ðλÞ dλ ð1Þ
λ2
dλ λ1
where ε(λ) was the spectral emissivity and was unavailable by ET-100; therefore, the total emissivity was obtained approximatively in accordance with Eq. 2 in practice, denoted as ε⁎, X
εi Δλi ε ¼ Xi Δλi i
ð2Þ
where εi and Δλi were the emissivity at a certain band and the band width, respectively. The impact resistance of the MoSi2-borosilicate glass coating was measured by a custom-made vertical drop device with steel balls of 5, 8, 10, 15, 20, 35 and 50 g as impactors. Impactor drop height was varied to simulate the impact energy in joules (height × weight) [10,31]. The
3. Results and discussion 3.1. Characteristics 3.1.1. Morphology and phase composition Fig. 1a shows the surface of the MoSi2-borosilicate glass coating and the contrast of the picture had been increased for better observation of the details. The coating surface was dense and flat, and no penetrated cracks or holes were observed. The coating was water-impervious, confirmed by a simple immersion test, in which the mullite fibrous block with the coating deposited all around floated on water all the time, while the uncoated one submerged in water. Fig. 1b is the crosssectional image of the coating and the fibrous substrate, exhibiting that the coating was compact with a uniform thickness of 300 μm. Fig. 2 shows the XRD pattern of the MoSi2-borosilicate glass coating. The broad hump between 15° and 30° was related to borosilicate glass, and the main crystalline phase in the coating was MoSi2 (JCPDS#659392). Apart from that, Mo2B5 (JCPDS#65-4029) and Cristobalite (JCPDS#77-1316) were also detected. Mo2B5 was generated from the reaction of MoSi2 and B2O3, and would benefit the thermal endurance of the coating for its high melting point (2280 °C). In addition, Si was scarcely detected in the coating, suggesting its complete oxidation into borosilicate glass. EDS mapping was used to further investigate the element distribution on the surface of the MoSi2-borosilicate glass coating. The SEM
Fig. 3. SEM image (a) and EDS mappings (white = Mo (b), red = O (c), and green = Si (d)) of the coating surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
X. Tao et al. / Materials and Design 103 (2016) 144–151
image of the coating surface was shown in Fig. 3a and EDS mappings in Fig. 3b–d (with colors indicate white = Mo, red = O, and green = Si). Fig. 3a shows that dispersion phases (bright regions) evenly distributed in the dark regions. The dispersion phase with higher Mo content, lower Si and O content, was supposed to be MoSi2 or Mo2B5 particles, on the basis of the XRD analysis in Fig. 2. The continuous phase with relatively higher Si and O content was the borosilicate glass matrix. Therefore, the coating was a composite with borosilicate glass as matrix and MoSi2 particles as main dispersion phase. 3.1.2. Interfacial adhesion Element line scanning analysis (Fig. 4a) illustrates the composition profile of Si, Al, and Mo across the interface between the coating and substrate. The dense coating was a composite with borosilicate glass as matrix and MoSi2 as dispersion phase, thus containing dominant Si element and minor Mo element. The mullite fibrous substrate mainly contained Si, O and Al elements. A transition layer was recognized
147
near the interface, where both Mo and Al were detected. The magnifying micrograph of the transition layer was shown in Fig. 4b and the EDS analysis of region “c” was displayed in Fig. 4c. In the transition layer, mullite fiber cross-sections were observed in the coating, as confirmed by EDS analysis. Therefore, the transition layer was a fiber reinforced composite, resulted from the infiltration of coating into the fibrous substrate. In fact, the transition layer mattered a lot. On one hand, the fibers in the coating acted as reinforcement to improve the coating strength; on the other hand, the mechanical lock effect in the transition layer contributed to the tight bond between the coating and substrate. 3.2. Formation process In the presenting coating, Si and B2O3 were used as raw materials, and the glass coating was prepared via the in-situ reacting of Si, B2O3 and O2. The formation process of the MoSi2-borosilicate glass coating
Fig. 4. Fracture morphology of the coating and EDS line profile of the interface (a). Magnifying micrograph of region “b” (b) and EDS analysis of region “c” (c).
148
X. Tao et al. / Materials and Design 103 (2016) 144–151
was investigated in detail. Based on the TGA/DSC analysis and volumetric change (Fig. 5a), the sintering process consisted of four stages. Combined with the XRD patterns (Fig. 5b) and micrographs (Fig. 5c) of the coatings calcined at different temperatures, the phase transformation and microstructure evolution at each stage were discussed. The first stage (RT-390 °C) was the linear thermal expansion stage. The major transformation was water evaporation, corresponding to the mass decrease in the TGA curve and the endothermic peak in the DSC curve at 107 °C. At this stage, particles accumulated irregularly in the coating without binder phase. The second stage (390–490 °C) was the volume compensation stage. In this stage, mass and volume increased with an exothermic peak in the DSC curve at 450 °C, which was connected to the oxidation of B4C. The oxidation can be described by reactions 1 and 2, where (s) and (g) denote the solid and gas phase, respectively. B4C was oxidized to B2O3 accompanied with a 250% volume increase, which partially compensated the sintering shrinkage of the coating [32,33]. Meanwhile, the oxidation products (B2O3) melted at 445 °C, impregnated into pores and cracks in the coating, and covered the surface of MoSi2 particles. B4 CðsÞ þ 3O2 ðgÞ ¼ 2B2 O3 ðsÞ þ CðsÞ
ð1Þ
B4 CðsÞ þ 4O2 ðgÞ ¼ 2B2 O3 ðsÞ þ CO2 ðgÞ
ð2Þ
of MoSi2 and Si into SiO2 (reactions 3 and 4) was ongoing and meanwhile B2O3 partially volatilized in the process of heating up. The generating SiO2 might be amorphous and thereby was not obvious in the XRD patterns. The molten binder phase promoted particle rearrangement and local densification in the coating, leading to the volume decrease. The transformation of MoSi2 in this stage took place as follows. A spot of MoSi2 was oxidized to MoO3, as indicated by the XRD pattern of 700 °C and corresponding to the exothermic peaks in the DSC curve at around 600 °C. As temperature increased, the existence of reductant and the decreasing oxygen partial pressure in the system prompted to generate the products of low-valence Mo, such as MoO2 and Mo, which can be described by reactions 5 and 6. In addition, MoxBy was detected in the coating calcined at 1300 °C, which was generated from the reaction of MoSi2 and B2O3 (reaction 7). 2MoSi2 ðsÞ þ 7O2 ðgÞ ¼ 2MoO3 ðsÞ þ 4SiO2 ðsÞ
ð3Þ
SiðsÞ þ O2 ðgÞ ¼ SiO2 ðsÞ
ð4Þ
CðsÞ þ 2MoO3 ðsÞ ¼ 2MoO2 ðsÞ þ CO2 ðgÞ
ð5Þ
CðsÞ þ MoO2 ðsÞ ¼ MoðsÞ þ CO2 ðgÞ
ð6Þ
xMoSi2 þ ð0:5yÞB2 O3 þ ð2 −0:75yÞO2 ¼ Mox By þ 2 SiO2
ð7Þ
−
The third stage (490 1390 °C) was the elementary densification stage, in which the molten binder phase protected MoSi2 from oxidation and promoted the coating densification. At low temperature, the binder phase with much B2O3 and little SiO2 melted, packed the MoSi2 particles and controlled inward oxygen diffusion [34]; therefore, the lowtemperature oxidation of MoSi2 was restrained. We inferred that the SiO2 content in the binder phase was increasing, because the oxidation
The last stage (1390− 1500 °C) stood for the further densification of the coating via melting of Si related to the sharp endothermic peak in the DSC curve at 1390 °C. Thereafter, the mass decreased faster, the quality increased faster, and the content of Si sharply decreased suggested by the XRD patterns of the coatings calcined at 1300 °C and 1500 °C. Consequently, the melting of Si significantly promoted the
Fig. 5. TGA/DSC analyses and volume change (a) of the coating from RT to 1500 °C; XRD patterns (b) and SEM images (c) of the coatings after calcination at different temperature.
X. Tao et al. / Materials and Design 103 (2016) 144–151
149
Table 2 Thermal shock tests of the coatings.
Fig. 6. Emissivity of the coating at six bands from 1.5 μm to 21 μm at two incidence angles, 20° and 60°.
densification process and the oxidation of Si in the last stage, and the coating after this stage was smooth and flat without visible pores.
3.3. Performance 3.3.1. Infrared radiating property The infrared emissivity of the MoSi 2 -borosilicate glass coating was shown in Fig. 6, and the total emissivities at 20° and 60° were 0.867 and 0.849 respectively, according to Eq. 2, which satisfied the requirement for infrared radiating coatings (ε ∗ N0.80) [11]. Because directional emissivities at all wavelength decreased with the increasing exitance angle [35], the emissivity at 20° was always higher than that at 60°. According to the Planck black-body radiation law and Wien's displacement law, the black-body radiation at high temperatures was mostly in the short wavelength [28]. For example, when the ambient temperature is 1300 °C, the maximum radiation intensity appears at 1.84 μm and the radiation energy at 1.5–3.5 μm is 86.9% of that in the full wavelength. Therefore, high emissivity at short wavelength was required for high-temperature infrared radiation. The emissivity of the presenting coating at 1.5–3.5 μm was higher than 0.9, which would greatly contribute to the radiation at high temperatures [28,29]. A final note about this result was that, the relatively low emissivity at 5.5–10 μm was attributed to the asymmetrical Si\\O stretching vibration absorptions of [SiO4] tetrahedra [36], which occurred in many silicate glass and centered at 9.08 μm.
Fig. 7. Impact resistance of the coatings with different thickness and quality. The schematic diagram of the impact-induced failure mode was presented at the top-left.
Thermal shock cycles
Appearance of the coatings
10 20 23 31
Unchanged Unchanged Gloss decreased Locally spalled on the edges
3.3.2. Impact resistance The impact resistance of the MoSi2-borosilicate glass coating as a function of coating thickness was shown in Fig. 7. The impact energy multiplied exponentially with the increasing coating thickness; thus thick coating was required for better impact resistance. However, thick coating would increase the weight of the whole TPS, and finally the coating of 300 μm was employed for other performance tests. RCG was the typical glass coating for fibrous ceramics made by NASA and it had the similar structure to the presenting coating: a dense thin glass coating of about 300 μm [4,5]. The impact energy of RCG was marked out in Fig. 7 [10], which was half of that of the presenting coating (0.07 J). The better impact resistance of the presenting coating was closely related to the higher coating strength and tighter interface bonding, resulted from the fiber reinforced transition layer near the interface. The impact-induced failure of the glass coating-fibrous ceramics system included two aspects. One was cracking of the coating along the circumference and the other was compressive deformation of the substrate due to the fracture of fibers and debonding at the bonding points [8].
3.3.3. Thermal shock resistance Thermal shock test was conducted on the MoSi2-borosilicate glass coating at 1500 °C, and the results were listed in Table 2. During the first 20 thermal shock cycles, no visible change was observed on the coating. With the increase of thermal cycles, gloss reduced on the coating surface, but the coating was still compact and well adhered to the substrates without cracking or bubbling. Spalling of the coating was first observed on the edges in the 31st thermal cycle and the crosssectional morphology was presented in Fig. 8. Interfacial fractures decreased the bonding strength of the coating, and was the main thermal shock damage form of the integrative glass coating and fibrous ceramic. We assume that mullite fibers at the interface would deform to relieve the thermal stress during each thermal cycles, and after 30 thermal cycles, fatigue fracture occurred to the fibers at the interface. In consideration of the great temperature gradient (1475 °C) and rapid temperature change rate (a water-quench method) in the thermal shock test, as well as the larger quantity of thermal cycles before damage (30 cycles), the MoSi2-borosilicate glass coating was demonstrated to have good thermal shock resistance, which was attributed to several reasons. Firstly, the CTEs of the coating and substrate were similar
Fig. 8. Cross-sectional morphology of the coating and substrate after 31 thermal shock cycles.
150
X. Tao et al. / Materials and Design 103 (2016) 144–151
Fig. 9. Total emissivity as a function of oxidation time during the isothermal oxidation test at 1500 °C (a), XRD patterns (b) and surface SEM images of the coatings tested for 35 h (c), 40 h (d) and 50 h (e).
(3.41 × 10− 6/K and 4.10 × 10−6/K, respectively), so minor thermal stress was produced during thermal cycles. Secondly, the tight integration at the interface through mechanical interlocking prevented the coating from spalling. Last but not the least, much of the thermal stress was absorbed by the deformation of the fibrous ceramics with relatively low modulus and high fracture strain. 3.3.4. Thermal endurance For long-term use at high temperatures, chemical reactions or matter volatilizations may take place in the coating and result in degraded infrared radiating performance. Hence the thermal endurance of the MoSi2-borosilicate glass coating was evaluated by the emissivity change during isothermal oxidation at 1500 °C (Fig. 9a). The total emissivity declined merely by 4% after oxidation for 50 h and was always higher than 0.8. Therefore, the coating can be used as long-term infrared radiating coating at temperatures up to 1500 °C. The total emissivity at 20° declined slightly with the oxidation time. By contrast, the emissivity at 60° was declining in overall trend, but it increased when the oxidation time increased from 35 h to 40 h. In order to explain the emissivity changes, the XRD patterns and SEM images of the tested samples were shown in Fig. 9b–e. With the extension of oxidation time, the content of MoSi2 decreased (Fig. 9b), while that of MoxBy gradually increased. Though MoSi2 was oxidation resistant at high temperature due to the passivation layer of SiO2, part of MoSi2 was oxidized in the presenting coating as a result of the following three reasons. Firstly, fine MoSi2 powders with larger specific surface area and more defects were less oxidation resistant than bulk MoSi2 [37]. Secondly, the oxygen diffusion coefficient in borosilicate glass at higher temperatures was higher and the glass cannot isolate MoSi2 particles from oxygen [34]. Besides, the B2O3 in the coating destroyed part of the SiO2 passivation layers around MoSi2 particles and reacted with MoSi2 to generate MoxBy. Therefore, MoSi2 was gradually oxidized with the extension of oxidation time, resulting in the decreased emissivity at 20° and 60°. The changing regularity of the total emissivity during the oxidation test was also related to the surface roughness. The coating tested for b35 h was flat and smooth, and showed no significant change in appearance (Fig. 9c). When the oxidation time reached 40 h, matte surface was observed on a macro level. Bare particles were observed in the
micrograph of the coating (Fig. 9d), and the surface roughness increased correspondingly. As the surface roughness slightly increased, the emissivity rose due to the enhanced inter-reflection of incident light in the grooves [29]. Therefore, the total emissivity at 60° increased when the oxidation time increased from 35 h to 40 h. When the oxidation time reached 50 h, the coating surface became macroscopic rough; holes and cracks were seen in the micrograph (Fig. 9e).
4. Conclusion MoSi2-borosilicate glass infrared radiating coating was prepared on mullite fibrous ceramics via an in-situ reaction method. The coating was dense and uniform with borosilicate glass as matrix and MoSi2 as dispersion phase. Tight interfacial bonding via mechanical interlock was observed in the transition layer. The formation process of the coating included four stages. The sintering shrinkage was reduced via the addition of B4C in the second stage, the low-temperature oxidation of MoSi2 was restrained via the package of borosilicate glass in the third stage, and the fully densification of the coating was based on the melting of Si in the last stage. The total emissivity of the as-prepared coating was higher than 0.85 and the emissivity at shorter wavelength was higher than 0.9. The impact resistance of the coating was improved owing to the enhanced transition layer at the interface. The coating could went through 30 thermal cycles between 1500 °C and 25 °C without cracking and spalling; spalling on the edges was observed in the 31st thermal cycles, and the main failure mode was interfacial fracture, resulted from the fracture of mullite fibers at the interface. The total emissivity of the coating declined merely by 4% after oxidation at 1500 °C for 50 h. The decrease of emissivity in overall trend was attributed to the oxidation of MoSi2 and the emissivity rose to some extent when the oxidation time increased from 35 h to 40 h due to the increased surface roughness of the coating during oxidation.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Project No. 51572298 and Project No. 51372164).
X. Tao et al. / Materials and Design 103 (2016) 144–151
References [1] G.N. Xie, W. Qi, W.H. Zhang, B. Sunden, G. Lorenzini, Optimization design and analysis of multilayer lightweight thermal protection structures under aerodynamic heating conditions, J. Thermal Sci. Eng. Appl. 5 (2013) 011011. [2] K. Wei, R.J. He, X.M. Cheng, R.B. Zhang, Y.M. Pei, D.N. Fang, A lightweight, high compression strength ultra high temperature ceramic corrugated panel with potential for thermal protection system applications, Mater. Des. 66 (2015) 552–556. [3] B. Pan, L.P. Yu, D.F. Wu, Thermo-mechanical response of superalloy honeycomb sandwich panels subjected to non-steady thermal loading, Mater. Des. 88 (2015) 528–536. [4] J.E. Miller, W.E. Bohl, E.L. Christiansen, B.A. Davis, Ballistic performance of porousceramic, thermal protection systems, Int. J. Impact Eng. 56 (2013) 40–46. [5] T.E. Steyer, Shaping the future of ceramics for aerospace applications, Int. J. Appl. Ceram. Technol. 10 (2013) 389–394. [6] M. Ferraiuolo, O. Manca, Heat transfer in a multi-layered thermal protection system under aerodynamic heating, Int. J. Therm. Sci. 53 (2012) 56–70. [7] X. Dong, G.F. Sui, Z.Q. Yun, M.C. Wang, A.R. Guo, J. Zhang, J.C. Liu, Effect of temperature on the mechanical behavior of mullite fibrous ceramics with a 3D skeleton structure prepared by molding method, Mater. Des. 90 (2016) 942–948. [8] X. Dong, G.F. Sui, J.C. Liu, A.R. Guo, S.E. Ren, M.C. Wang, H.Y. Du, Mechanical behavior of fibrous ceramics with a bird's nest structure, Compos. Sci. Technol. 100 (2014) 92–98. [9] G.F. Shao, X.D. Wu, Y. Kong, S. Cui, X.D. Shen, C.R. Jiao, J. Jiao, Thermal shock behavior and infrared radiation property of integrative insulations consisting of MoSi2/borosilicate glass coating and fibrous ZrO2 ceramic substrate, Surf. Coat. Technol. 270 (2015) 154–163. [10] D.B. Leiser, R. Churchward, V. Katvala, D. Stewart, Advanced porous coating for lowdensity ceramic insulation materials, J. Am. Ceram. Soc. 72 (1989) 1003–1010. [11] H.E. Goldstein, D.B. Leiser, V. Katvala, Reaction cured borosilicate glass coating for low-density fibrous silica insulation, series Materials Science Research in Borate Glasses (1978) 623-634. [12] D.A. Stewart, D.B. Leiser, Characterization of the thermal conductivity for advanced toughened uni-piece fibrous insulations, AlAA 28th Thermophysics Conference 1993, pp. 1–18 Washington, D.C. [13] J.C. Fletcher, Reaction cured glass and glass coatings, U. S. patent. 4,093,771, 1978. [14] D.B. Leiser, Toughened uni-piece firous insulation, U. S. patent. 5,079,082, 1992. [15] D.A. Stewart, High efficiency tantalum-based ceramic composite structures, U. S. patent. 7,767,305 B1, 2010. [16] D.A. Stewart, Toughened uni-piece fibrous reinforced oxidization-resistant composite, U. S. patent. 7,314,648 B1, 2008. [17] S.M. Johnson, M.J. Gasch, D. Leiser, D. Stewart, Development of new TPS at NASA Ames Research Center, 15th AIAA International Space Planes and Hypersonic Systems and Technologies Conference 2008, pp. 1–10 Dayton. [18] J. Li, R.Y. Luo, Y.P. Chen, Q. Xiang, C. Lin, P. Ding, N. An, J.W. Cheng, Oxidation behavior and kinetics of SiC/alumina-borosilicate coating for carbon-carbon composites, Appl. Surf. Sci. 255 (2008) 1967–1974. [19] S.J. Park, M.K. Seo, The effects of MoSi2 on the oxidation behavior of carbon/carbon composites, Carbon 39 (2001) 1229–1235.
151
[20] S. Knittel, S. Mathieu, M. Vilasi, The oxidation behaviour of uniaxial hot pressed MoSi2 in air from 400 to 1400 degrees C, Intermetallics 19 (2011) 1207–1215. [21] Q.G. Fu, H.J. Li, K.Z. Li, K. Tong, A Si-Mo-W coating to protect SiC-coated carbon/carbon composites against oxidation, J. Am. Ceram. Soc. 92 (2009) 2132–2135. [22] F. Smeacetto, M. Salvo, M. Ferraris, Oxidation protective multilayer coatings for carbon-carbon composites, Carbon 40 (2002) 583–587. [23] M.H. Chen, W.B. Li, M.L. Shen, S.L. Zhu, F.H. Wang, Glass coatings on stainless steels for high-temperature oxidation protection: mechanisms, Corros. Sci. 82 (2014) 316–327. [24] M.L. Shen, S.L. Zhu, M.H. Chen, F.H. Wang, The oxidation and oxygen permeation resistance of quartz particle-reinforced aluminosilicate glass coating on titanium alloy, J. Am. Ceram. Soc. 94 (2011) 2436–2441. [25] X.L. Li, C. Zhang, M. Wang, Y.C. Qiao, H.J. Tang, Preparation and thermal expansion coefficient control of MoSi2-Al2O3-SiO2 coating with high temperature resistance and emissivity, Rare Met. Mater. Eng. 44 (2015) 200–203. [26] G.F. Shao, X.D. Wu, Y. Kong, X.D. Shen, S. Cui, X. Guan, C.R. Jiao, J. Jiao, Microstructure, radiative property and thermal shock behavior of TaSi2\\SiO2-borosilicate glass coating for fibrous ZrO2 ceramic insulation, J. Alloys Compd. 663 (2016) 360–370. [27] M. Wang, X.L. Li, D. Su, H.M. Ji, H.J. Tang, Z.H. Zhao, J. He, Effect of glass phase content on structure and properties of gradient MoSi2-BaO-Al2O3-SiO2 coating for porous fibrous insulations, J. Alloys Compd. 657 (2016) 684–690. [28] X.D. He, Y.B. Li, L.D. Wang, Y. Sun, S. Zhang, High emissivity coatings for high temperature application: progress and prospect, Thin Solid Films 517 (2009) 5120–5129. [29] D.B. Mahadik, S. Gujjar, G.M. Gouda, H.C. Barshilia, Double layer SiO2/Al2O3 high emissivity coatings on stainless steel substrates using simple spray deposition system, Appl. Surf. Sci. 299 (2014) 6–11. [30] H. Tanaka, S. Sawai, K. Morimoto, K. Hisano, Evaluation of hemispherical total emissivity for thermal radiation calorimetry, Int. J. Thermophys. 21 (2000) 927–940. [31] C. Zhang, H. Zhou, L. Liu, Laminar Fe-based amorphous composite coatings with enhanced bonding strength and impact resistance, Acta Mater. 72 (2014) 239–251. [32] M.E. Westwood, J.D. Webster, R.J. Day, F.H. Hayes, R. Taylor, Oxidation protection for carbon fibre composites, J. Mater. Sci. 31 (1996) 1389–1397. [33] M.C. Wang, J.C. Liu, H.Y. Du, F. Hou, A.R. Guo, S. Liu, X. Dong, Joining of C/C composites by using B4C reinforced phosphate adhesive, Ceram. Int. 40 (2014) 11581–11591. [34] L.S. Walker, E.L. Corral, Self-generating high-temperature oxidation-resistant glassceramic coatings for C-C composites using UHTCs, J. Am. Ceram. Soc. 97 (2014) 3004–3011. [35] A.E. Wald, J.W. Salisbury, Thermal infrared directional emissivity of powdered quartz, J. Geophys. Res. 100 (1995) 24665–24675. [36] B.R. da Luz, J.K. Crowley, Spectral reflectance and emissivity features of broad leaf plants: prospects for remote sensing in the thermal infrared (8.0–14.0 mu m), Remote Sens. Environ. 109 (2007) 393–405. [37] K. Kurokawa, H. Houzumi, I. Saeki, H. Takahashi, Low temperature oxidation of fully dense and porous MoSi2, Mat. Sci. Eng., A 261 (1999) 292–299.