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Effect of boronizing treatment on the oxidation resistance of Zr2(AlSi)4C5/ SiC composite X.P. Lua, H.M. Xiangc, P. Hana, M.S. Lib, Y.C. Zhouc,
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a
College of Materials Science & Engineering, Henan University of Technology, Zhengzhou, 450000, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China c Aerospace Research Institute of Material Processing Technology, No.1 South Dahongmen Road, 100076, Beijing, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ceramic SEM XRD Oxidation
The oxidation behavior of Zr2[Al(Si)]4C5/30 vol.%SiC composite(ZASC-30SC) at 900–1300 °C was investigated, and the effect of SiC additive on the oxidation resistance of Zr2[Al(Si)]4C5 at 1000 °C is negative. A boronizing coating containing ZrB2 and SiC was prepared through boronizing treatment. The effect of boronizing on the oxidation resistance of ZASC-30SC composite at 1000 °C and 1800 °C were explored respectively. At 1000 °C, the oxidation resistance was improved intensively as a result of a protective liquid B2O3 containing scale formed during oxidation. Besides, the oxidation properties at 1800 °C of ZASC-30SC was improved obviously because of a protective SiO2 rich layer generated.
1. Introduction Recent years, a series of layered carbides in Zr–Al(Si)–C system, such as Zr2Al3C4, Zr2[Al(Si)]4C5, Zr3Al3C5, and Zr3[Al(Si)]4C6, have been developed by adding Al and/or Si into ZrC [1–4]. These new ternary and quaternary carbides show superior oxidation resistance and fracture toughness than their binary counterpart, which render them promising high temperature structural materials [5–9]. However, the mechanical and oxidation properties of Zr–Al(Si)–C ceramics are still insufficient to meet the requirements of ultra-high temperature structural component. To further improve the properties of monolithic Zr–Al (Si)–C, SiC was introduced into Zr–Al(Si)–C system [10–13]. Compared with monophase Zr–Al(Si)–C ceramics, the Zr–Al(Si)–C/SiC composites exhibit promising mechanical properties and improved high temperature oxidation resistance [11,14]. With SiC additive, the flexural strength of Zr2[Al(Si)]4C5 increased as a result of the reduction of grain size. In addition, crack bridging and deflection around SiC particles were observed in Zr2[Al(Si)]4C5/SiC composite, which is the reason of the higher fracture toughness. The improved oxidation resistance is attributed to the formation of Si-containing oxides, such as aluminosilicate glass and mullite, in which the oxygen diffusion rate is low. Nevertheless, the oxidation of SiC below 1200 °C is unconspicuous [15], and it can be concluded that its contribution to the intermediate-temperature oxidation protection of Zr–Al(Si)–C/SiC composites is quite limited. This will lead to the poor intermediate-temperature oxidation
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resistance of the composites, and hinder their application as hightemperature structural materials. Thus effective method to improve the intermediate-temperature oxidation resistance is still urgently needed. It is well known that boron-containing materials have excellent oxidation resistance at intermediate temperatures (800 °C–1200 °C) due to the formation of liquid B2O3, which can effectively seal the cracks in oxide scale and provide good oxidation protection [16]. Mu and Shen have prepared a boride coating on CoCrMo alloy, and found the boronized CoCrMo alloy possess superior oxidation resistance at 950 °C [17]. During oxidation of B4C/ZrB2 composites, a dense and protective B2O3 layer formed at 1000–1200 °C, which can protect the substrate effectively [18]. Recent years, borides have always been combined with Si-containing materials to prepare protective coatings, such as ZrB2/ SiC, ZrB2/MoSi2 etc., which all show excellent oxidation resistance [19–21]. For these composites, below 1100 °C, a continuous liquid B2O3 layer generate. This layer of B2O3 can depress the inward diffusion of oxygen and protect the matrix effectively. While above 1100 °C, the excellent oxidation resistance come from the combined effect of both liquid SiO2 and B2O3. Introduction of Boron into matrix has been accomplished by various methods, such as compositing with borides, boron doping, and surface boronizing [22–24]. Boronizing is a thermochemical process that can use solid media, liquid media, and gaseous media [24]. Powder-pack boronizing method has been widely used in surface modification of a variety of materials because of its simplicity, good processability, cost
Corresponding author. E-mail addresses: [email protected], [email protected] (Y.C. Zhou).
https://doi.org/10.1016/j.corsci.2019.108201 Received 5 March 2019; Received in revised form 2 September 2019; Accepted 3 September 2019 0010-938X/ © 2019 Published by Elsevier Ltd.
Please cite this article as: X.P. Lu, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108201
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vertical intermediate frequency induction heating (IFIH) furnace was used to evaluate the ultra-high temperature oxidation behavior of the boronized ZASC-30SC composite in static air. The sample was placed on cylindrical graphite susceptor and heated to the testing temperature at about 200 °C/min. The duration time for oxidation was 15 min at 1800 °C. The detailed procedure of the oxidation can be found in our previous work [6]. For comparison, the same oxidation test was also conducted for ZASC-30SC composite. The phase composition and surface and cross section microstructures of the oxide scale were identified using X-ray diffractometer and the SUPRA 35 scanning electron microscope (SEM) mentioned above.
effectiveness, and relatively high quality of the boride layers produced on the surface. To control the boron source supply precisely and improve the automation of boronizing process, boriding in liquid media and gas boriding have been developed gradually. Powder-pack boriding technique has been used to prepare a boride layer on a series of ferrous and non-ferrous alloys, which exhibits improved oxidation resistance, corrosion resistance, and wear property [25,26]. Yokota et al. investigated the oxidation resistance of MoSi2, which was improved after boronizing treatment by molten salt process [27]. Kulka et al. have prepared boride layer on Ni-based alloy using gas boriding method and the boronizing efficiency was improved obviously [28]. Thus, it is expected that boriding treatment is an efficient way to improve the intermediate temperature oxidation properties of Zr–Al(Si)–C/SiC composites, while the excellent mechanical properties of the Zr–Al(Si)–C/ SiC matrix remains intact. In this work, Zr2[Al(Si)]4C5/30 vol.%SiC (shortened as ZASC-30SC) composite was selected and its oxidation resistance at 900–1300 °C was investigated. Pack boriding technique was used to prepare a boride containing coating and its effect on the oxidation resistance of ZASC30SC composites at intermediate temperature (about 1000 °C) and ultra-high temperature (1800 °C) were investigated. Our results highlight the positive effect of surface boriding on the intermediate temperature oxidation resistance of ternary carbides.
3. Results and discussion 3.1. Oxidation behavior of Zr2[Al(Si)]4C5/30 vol.%SiC composite at 900–1300 °C Fig. 1(b–d) present the weight gain per unit surface area as a function of time for ZASC-30SC composite oxidized at 900–1300 °C in flowing air. The oxidation behavior of monolithic Zr2[Al(Si)]4C5 under the same condition are also shown here as comparison. Generally, the oxidation kinetics of Zr2[Al(Si)]4C5 show parabolic model initially at each temperature, and subsequently follow linear law. The weight gain during oxidation increased with temperature increment. By comparison, the oxidation of ZASC-30SC composite is more complicated. When the oxidation temperature was 900 °C, the oxidation resistance of ZASC30SC composite was better in 8 h (the insert in Fig. 1b). When the temperature increased to 1000 °C, the oxidation procedure can be divided into two stages. In the first hour, the weight gain of ZASC-30SC was less than that of Zr2[Al(Si)]4C5. After the holding time for 1 h, the effect of SiC is negative and the the weight gain of ZASC-30SC composite exceeded that of Zr2[Al(Si)]4C5. From Fig. 1b, it can be seen that the weight gain per unit area of ZASC-30SC composite(62.5 g/m2) is much larger than that of Zr2[Al(Si)]4C5(28.5 g/m2) after 8 h, indicating hazardous effect of SiC on the oxidation of the composites. In addition, accelerated oxidation of ZASC-30SC composite and spallation of oxide scale have been observed during oxidation at 1000 °C. At 1100 °C and 1200 °C, the oxidation kinetics and weight gain are comparable with monolithic Zr2[Al(Si)]4C5 (Fig. 1c and the insert). It is noticeable that ZASC-30SC composite began to exhibit superior oxidation resistance than Zr2[Al(Si)]4C5 at about 1200 °C. As temperature increased, the effect of SiC additive on the weight gain of Zr2[Al(Si)]4C5 became more positive. At 1300 °C, the weight gain per unit area of monolithic Zr2[Al (Si)]4C5 achieved approximately 250 g/m2 after 1 h, and subsequently serious peeling of oxide scale appeared. By comparison, the weight gain of ZASC-30SC composite oxidized at 1300 °C for 1 h was 70.5 g/m2. In addition, it should be noted that the weight gain of ZASC-30SC composite at 1200 °C increased more rapidly than that at 1300 °C. In brief, SiC additive has positive effect on the oxidation resistance of Zr2[Al (Si)]4C5 above 1200 °C but negative effect at about 1000 °C, which will be discussed in detail in the following sections. To investigate the source of the abnormal oxidation kinetics of ZASC-30SC composite at 1000 °C, the phase composition and microstructure of the oxide scale were detected. Fig. 2 shows the XRD patterns of the oxide scale surfaces oxidized at different temperatures for 1 h, indicating the composition of the oxide scales. At 1000 °C, the oxide scales are mainly consisted of t-ZrO2, Al2O3, and SiC, in which t-ZrO2 and Al2O3 came from the oxidation of Zr2[Al(Si)]4C5 substrate and the SiC in the composite was almost not oxidized. The SEM micrograph of oxide scale surface of ZASC-30SC composite oxidized at 1000 °C for 8 h is shown in Fig. 3. From the figure it can be seen that the unoxidized SiC, which can be confirmed from the EDS result (the insert in Fig. 3b), was almost intact and remained in the oxide scale. As shown in Fig. 3, a lot of cracks were observed on the surface and the unoxidized SiC particles are buried in the oxide scale. Since the oxidation process of
2. Experimental The ZASC-30SC composite was fabricated by in situ hot-pressing zirconium, aluminum, silicon, and graphite powders at 1900 °C for 1 h in Ar under a pressure of 30 MPa. The preparation steps have been described in detail in the previous work [5]. The optimized molar ratio of Zr:Al:Si:C for synthesizing ZASC-30SC composite was 2:3.6:3.18:7.4. The density of sintered samples was determined by the Archimedes method, and the density of the synthesized samples is higher than 98% of the theoretical density. The samples for oxidation resistance tests and boronizing treatment with dimensions of 4 mm × 4 mm × 8 mm were cut by an electrical discharge method from an as-fabricated bulk piece. The surfaces were ground down to 1000 grade SiC paper and polished using 1.5 μm diamond paste. The specimens were ultrasonically cleaned in acetone and distilled water before boronizing treatments. Boronizing was performed by using a fluoride-activated powder pack cementation method. The boronizing media was composed of 47.5 wt.% B4C, 47.5 wt.% SiC, and 5 wt.% KBF4 (analytical pure). In this boronizing media, B4C serves as the source of boron, SiC as the diluent, and KBF4 as the active agent. The boronizing process was carried out in the following procedure: all the media powders were mixed in a polyurethane jar for 10 h, using stainless steel balls coated with a layer of polyurethane as the mixing medium. ZASC-30SC specimens were buried in the powders and packed into an Al2O3 jar, which was then placed in the furnace under a flowing argon atmosphere. The heating rate for all runs was 10 °C/min, and the boronizing treatments were carried out at 1600 °C for 2 h. After treatment, the samples were cooled to room temperature in the furnace. The phase composition of the boronizing treated specimens was identified using X-ray diffractometer. X-ray diffraction (XRD) was carried out in a D/max–2400 diffractometer (Rigaku, Tokyo, Japan) with Cu Kα□radiation. The microstructure of the polished boronizing treated samples was observed in a SUPRA 35 scanning electron microscope (SEM) (LEO, Oberkochen, Germany) equipped with an energy-dispersive spectroscopy (EDS) system. The isothermal oxidation test in flowing air was carried out using a Setsys 16/18 microbalance (SETARAM, Caluire, France). The schematic diagram of the isothermal oxidation test was shown in Fig. 1a. The samples were suspended with a Pt wire and heated to the testing temperature at a rate of 40 °C/min. The mass change was recorded continuously as a function of time during isothermal oxidation. A 2
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Fig. 1. (a) Schematic representation of the isothermal oxidation test, and weight gain per unit area versus time of ZASC-30SC composite oxidized at (b) 900 °C (insert) and 1000 °C, (c) 1200 °C and 1100 °C (insert), and (d) 1300 °C.
growth stress in the oxide scale. Under this stress, cracks easily generate at the interface between the oxides and the SiC particles. In general, this residual stress increase with the thickening of the oxide scale [32]. At 900 °C, the oxidation resistance of Zr2[Al(Si)]4C5 was improved with addition of SiC, since SiC was not oxidized at this temperature. Within 8 h at 900 °C, cracks were not formed and the oxide scale was not destroyed because the oxide scale is thin, which could not generate enough stress. So the SiC has positive effect on the oxidation resistance of Zr2[Al(Si)]4C5. It is predictable that the effect of SiC on the oxidation resistance of Zr2(AlSi)4C5at 900 °C will change when the holding time become longer and the oxide scale become thicker, which will be investigated in the future. Similar to the oxidation behavior at 900 °C, the weight gain of Zr2(AlSi)4C5in the first hour at 1000 °C is lower than that of ZASC-30SC composite when the oxide scale was not thick. After holding at 1000 °C for 1 h, cracks formed in the oxidation scale under the residual stress when the oxide scale became thicker. The cracks are the channels through which oxygen can diffuse inward to the substrate, so the oxidation resistance of Zr2[Al(Si)]4C5 at 1000 °C become even worse with SiC additive when the holding time is more than 1 h. In addition, as the oxide scale became thicker, the residual stress eventually leaded to spallation of the oxide layer (Fig. 8). At about 1200 °C, SiC begins to oxidize to form Si-containing oxides such as SiO2 and mullite (Fig. 2c), which will provide better protection by sealing cracks in the oxide scale [13]. So at about 1200 °C, the SiC additive begin to have positive effect on the oxidation resistance of Zr2[Al(Si)]4C5. But the small amounts of liquid phases is inadequate to form a protective scale, and the weight gain of ZASC-30SC and Zr2[Al (Si)]4C5 at 1200 °C is comparable. The content of the protective phases mentioned above increased with temperature. At 1300 °C, the weight gain of ZASC-30SC is much less than that of Zr2[Al(Si)]4C5, which is due to more protective silicon containing phases such as SiO2 and mullite filling the cracks in the oxides scale. Similar phenomena was reported in other SiC-containing composites, such as ZrB2/SiC and Zr2[Al (Si)]4C5/20SiC composites, in which SiC begins to have positive effect on the oxidation resistance above about 1300 °C [12,15].
Fig. 2. XRD patterns of the oxide scales on ZASC-30SC composite after exposure to air at 1000–1300 °C for 1 h.
Zr2[Al(Si)]4C5 is the inwardly diffusion of oxygen [11], the SiC particles were not covered with the oxides and can be detected using XRD method. Noticeably, most of cracks are radiate and stem from the SiC particles. The cross section morphology of the oxide scale of ZASC-30SC composite oxidized at 1000 °C for 8 h is shown in Fig. 4. The oxide scale is inhomogeneous and cracks mainly distributed in the SiC-containing region. By comparison, the oxide scale at the regions without SiC is dense without cracks. Apparently, the oxide scale with SiC existing is much thicker than that without SiC, indicating that SiC additive is harmful for the oxidation resistance of Zr2[Al(Si)]4C5 at 1000 °C. During oxidation of ZASC-30SC composite, Zr2[Al(Si)]4C5 phase began to oxidize at about 600 °C [29]. By comparison, the SiC particles did not oxidize appreciably below 1000 °C [30]. There is a volume expansion upon conversion of Zr2[Al(Si)]4C5 to ZrO2, Al2O3(Pilling–Bedworth ratio (PBR)≈1.25) [31], which will generate 3
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Fig. 3. Surface morphology of ZASC-30SC composite oxidized at 1000 °C for 8 h.
gray from original silvery white. As shown in Fig. 5b, the XRD peaks of Zr2[Al(Si)]4C5 disappear and three phases, namely ZrB2, SiC, and Al2O3 are detectable. The SEM micrograph of the as-synthesized ZASC-30SC composite was shown in Fig. 6a, from which it can be seen that the SiC particles distribute in the Zr2[Al(Si)]4C5 substrate uniformly. The surface morphology of the boronized ZASC-30SC is shown in Fig. 6b. It is seen that the Zr2[Al(Si)]4C5 in ZASC-30SC composite turned into the homogeneous mixture of ZrB2 and Al2O3, meanwhile SiC did not react with boron and spread in the boronizing coating, which is consistent with the XRD results. To investigate the structure of the boronizing coating, the cross section morphology of the ZASC-30SC composite after boronizing is shown in Fig. 7. From Fig. 7a, It can be seen that the boronizing coating (about 20μm thick) with a single layer was compact and adhered well to the substrate. No cracks or spallation are found in the coatings. From Fig. 7b, It is noticed that the boronized Zr2[Al(Si)]4C5 grains kept the same shape as the as-synthesized elongated and rod-like Zr2[Al(Si)]4C5 grains (marked with arrows). In these grains, Al2O3(black phase) and ZrB2(white phase) distribute homogeneously. In addition, the SiC particles remained unchanged and were not covered with ZrB2 and Al2O3 during boronizing treatment. During boronizing treatment, SiC can be taken as inert markers to determine the predominant diffusion process. From Fig. 7 it can be concluded that the boronizing coating grew totally by the inward diffusion of boron atoms. From the above analysis, the boronizing procedure can be described as follows. First, boron atoms were created in the boronizing agent and diffused inward to react with Zr atoms in the Zr2[Al(Si)]4C5 to form ZrB2 in situ. Meanwhile, the oxygen impurities, which maybe came from the boronizing agent or argon, also diffused inward and reacted with Al atoms to form Al2O3 simultaneously. The function of C in the Zr2[Al(Si)]4C5 during boronizing is unclear yet. According to previous work, carbon maybe reacted with boronizing media or with oxygen impurities to form gas and diffused out of the samples [33]. The detailed boronizing procedure will be investigated in the near future.
Fig. 4. Cross section morphology of ZASC-30SC composite oxidized at 1000 °C for 8 h.
3.2. Microstructure and phase composition of boronizing coatings Fig. 5 shows the XRD patterns of the ZASC-30SC surface before and after boronizing treatment. From Fig. 5a, it can be seen that the assynthesized ZASC-30SC composite is consisted of Zr2[Al(Si)]4C5 and SiC. After boronizing treatment, the surface of ZASC-30SC turned into
3.3. The effect of boronizing treatment on the oxidation resistance of Zr2[Al (Si)]4C5/SiC at 1000 °C From above analysis, the added SiC deteriorate the oxidation behavior of ZASC-30SC composite at about 1000 °C. Fig. 8 compares the weight gain per unit area of the ZASC-30SC composite before and after boronizing oxidized at 1000 °C. For comparison, the weight gain per unit area of the monolithic Zr2[Al(Si)]4C5 was also shown here. As shown in Fig. 8, the weight gain per unit area of the boronized ZASC30SC composite oxidized at 1000 °C for 7.5 h is 22.5 g/m2, which is only one third of that for the unboronized ZASC-30SC composite oxidized at the same condition(63.1 g/m2), indicating the oxidation
Fig. 5. XRD patterns of the ZASC-30SC composite (a) before, and (b) after boronizing treatment at 1600 °C for 2 h. 4
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Fig. 6. SEM photograph of ZASC-30SC composite surface (a) before, and (b) after boronizing treatment.
resistance of ZASC-30SC composite at 1000 °C was improved obviously by boronizing treatment. Fig. 9 shows the oxide scale surface of the boronized ZASC-30SC composite oxidized at 1000 °C for 7.5 h. Unlike the ZASC-30SC composite oxidized at 1000 °C mentioned above (Figs. 3 and 4), no cracks have been observed on surface of the oxide scales, which is compact and covered with molten phases during oxidation. From the cross section morphology of the oxide scale shown in Fig. 10, the thickness of the oxide scale after oxidation for 7.5 h is about 7 μm, which is about one third that of the boronizing scale. During oxidation at 1000 °C, though SiC particles in the boronizing coating were still not oxidized, no cracks or spallation appeared at the SiC containing region. The oxide layer is dense and adhered well to the unoxidized substrate, indicating that the boronized coating can protect the substrate effectively at 1000 °C. The oxidation resistance of ZASC-30SC composite was improved considerably after boronizing treatment, which is due to the formation of coating consists of ZrB2 and SiC. The oxidation behavior of ZrB2/SiC composite was investigated extensively and it has excellent oxidation resistance in a wide temperature range [34–37]. Below 1100 °C, liquid B2O3 can form a continuous passive layer as a barrier to oxygen diffusion, which results in the parabolic oxidation kinetics of ZrB2. Above 1100 °C, though the volatilization of B2O3, borosilicate layer forms and the oxygen diffusion in the oxide scale can also be prevented. As mentioned above, the oxidation resistance of ZASC-30SC is worsened compared with pure Zr2[Al(Si)]4C5 at 1000 °C, which is due to the cracks in the oxide scale. Thus during the oxidation of the boronized ZASC-30SC at about 1000 °C, which has a ZrB2 containing coating outside of the substrate, the molten phase B2O3 could be generated. These liquid B2O3 act as a barrier to the transport of oxygen at 1000 °C
Fig. 8. Weight gain per unit area versus time of Zr2[Al(Si)]4C5, ZASC-30SC composite, and boronized ZASC-30SC composite oxidized at 1000 °C.
and can protect the substrate effectively. In addition, it is reasonable to predict that the oxidation resistance of boronized ZASC-30SC above 1000 °C can also be improved. 3.4. Oxidation behavior of boronized Zr2[Al(Si)]4C5/30 vol.%SiC composite at 1800 °C Fig. 11a shows the surface morphology of the oxide scale for ZASC30SC composite oxidized at 1800 °C for 15 min. It is noticeable that many pores and bubbles spread on the oxide scale surface after oxidation. Fig. 12a shows the cross-sectional morphology and
Fig. 7. Cross section morphology of boronized ZASC-30SC composite. 5
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Fig. 9. Surface morphology of boronized ZASC-30SC composite oxidized at 1000 °C for 8 h.
(2)
SiC(s) + 3/2O2(g) → SiO2(l)+CO(g)
(3)
xZrO2(s) + SiO2(l) → xZrO2·SiO2(l)
(4)
3Al2O3(s) + 2SiO2(l) → 3Al2O3·2SiO2(l)
(5)
According to the above Eqs. (1)–(5), during oxidation of ZASC-30SC at 1800 °C, a molten scale consist of SiO2, xZrO2·SiO2 and 3Al2O3·2SiO2 formed. Meanwhile large amounts of gaseous products such as CO2 and CO (Eqs. (1)–(3)) generated and rose to the outmost layer. These gases attempt to diffuse outside of the scale, and pores and bubbles formed in the molten layer. It is noticeable that some bubbles ruptured because the internal pressure of the bubble exceeded the limit pressure of liquid bearing capacity. The volume of the bubbles increased during their rising, which is the reason of the porous outer layer in the oxide scale. In addition, cracks were also observed in oxide layer as a result of the upwelling of aggregate CO and CO2 gases. These pores and cracks will provide the channels for the diffusion of oxygen into the inner material and have harmful impact on the oxidation resistance. Fig. 11b shows the oxide scale surface of boronized ZASC-30SC composite exposed to the same oxidizing environment. It can be seen that the oxide scale surface was covered with a relatively flat and dense molten phase. The corresponding cross section morphology and the EDS line scans of the oxide scale were shown in Fig. 12b. The thickness of the oxide scale is about 45 μm, which is thinner than that of the unboronized samples. In addition, the oxide scale of boronized ZASC-30SC composite consisted of two layers: (1) one SiO2-rich layer; and (2) one SiC depleted layer, which is in agreement with the previous studies [30–34]. After boronizing treatment, Zr2[Al(Si)]4C5 in the boronizing scale
Fig. 10. Cross section morphology of boronized ZASC-30SC composite oxidized at 1000 °C for 8 h.
corresponding EDS line scans of ZASC-30SC composite oxidized at 1800 °C for 15 min. It can be seen that the oxide scale is about 60μm thick and shows a single layer structure. In addition, a relatively porous outer layer can be observed in the oxide scale. According to the relative work about oxidation of Zr–Al–Si–C system at ultra-high temperatures, the main expected reactions during oxidation process are as follows [6,11]: Zr2[Al1-x(Si)x]4C5(s) + (10+x)O2(g) → 2ZrO2(s) + 2(14x)Al2O3(s) + 2x(3Al2O3·2SiO2)(l) + 5CO2(g)
SiC(s) + 2O2(g) → SiO2(l)+CO2(g)
(1)
Fig. 11. Surface morphology of (a) unboronized ZASC-30SC, and (b) boronized ZASC-30SC composite oxidized at 1800 °C for 15 min. 6
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Fig. 12. Cross section morphology and the corresponding EDS line scans taken from the white line of (a) unboronized ZASC-30SC, and (b) boronized ZASC-30SC composite oxidized at 1800 °C for 15 min.
1800 °C, a SiO2 rich layer generated, and a low oxygen partial pressure environment formed beneath this layer. Some gases such as CO2 and SiO generated and rose to the the outmost layer. The content of these gases was not enough to destroy the liquid layer, which can provide an effective protection. By comparison, no continuous SiO2 rich layer appeared in the oxide scale of the unboronized ZASC-30SC composite. As mentioned above, large amount of gases were produced during oxidation of ZASC-30SC composite, and the liquid layer was damaged because of the diffusion of these gases. Channels to release these gases could serve as the oxygen diffusion path, which resulted in its poor oxidation resistance. Zhang et.al investigated the effect of ZrC addition on the oxidation resistance of ZrB2-SiC, and found that the SiO2 rich layer disappeared when the ZrC content increased to 20 vol% as a result of the gas byproducts [38].These results indicated that the boronizing treatment was beneficial to the oxidation resistance of ZASC-30SC composite at 1800 °C.
was mainly substituted by ZrB2 and Al2O3. During oxidation of boronized ZASC-30SC composite at 1800 °C, the main reactions of the boronizing coating with O2 can be expressed as follows: SiC(s) + 2O2(g) → SiO2(l) + CO2(g)
(6)
ZrB2(s)+5/2O2(g) → ZrO2(s) + B2O3(g)
(7)
SiO2(l) → SiO(g) + 1/2O2(g)
(8)
xZrO2(s) + SiO2(l) → xZrO2·SiO2(l)
(9)
3Al2O3(s) + 2SiO2(l) → 3Al2O3·2SiO2(l)
(10)
In this case, a SiO2-rich layer came from oxidation of SiC covered the underlying material and could, potentially, provide a barrier to oxygen diffusion. This SiO2-rich glass layer is expected to contain some B2O3, though most of B2O3 evaporated at 1800 °C. Beneath this SiO2rich layer, low oxygen partial pressure environment formed and gaseous SiO generated (Eq. (8)) because of the high vapor pressure of SiO2(l) [33]. SiO gas evaporated and transport to the outer SiO2-containing layers, which generated the SiC depleted layer. It can be seen that some bubbles also exist on the oxide scale surface of the boronizd ZASC-30SC composite, but the content and volume are smaller than that of unboronized ZASC-30SC composite. According to Fig. 12b, these bubbles almost exist at the outmost surface of oxide scale, and the SiO2 rich layer is very dense, indicating that these gases have not damaged the protective SiO2 rich layer. In summary, during oxidation of boronized ZASC-30SC composite at
4. Conclusion The oxidation resistance of Zr2[Al(Si)]4C5/30 vol.%SiC composite at intermediate(900–1300 °C) temperatures was investigated, and the oxidation at 1000 °C behaves catastrophic, which is due to the flaws such as cracks in the oxide scale. The oxidation resistance of Zr2[Al (Si)]4C5 was improved above 1200 °C with SiC additive because of the protective phases such as SiO2 and mullite generated. A ZrB2/SiC containing coating was prepared on ZASC-30SC composite by boronizing treatment, and the oxidation resistance of ZASC-30SC at 7
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1000 °C was improved obviously due to the formation of a protective liquid B2O3 layer. In addition, boronizing treatment is also beneficial for the oxidation resistance of ZASC-30SC composite at 1800 °C for a dense protective SiO2 rich layer was generated during oxidation. The effect of boronizing treatment on the oxidation resistance in the temperature range of 1000–1800 °C and higher than 1800 °C is also important for the use of ZASC-30SC composite, which will be investigated in the future.
[14] G.Q. Chen, R.B. Zhang, P. Hu, W.B. Han, Oxidation resistance of Zr2(Al(Si))4C5based composites at ultra-high temperature, Scr. Mater. 61 (2009) 697–700. [15] A. Rezaie, W.G. Fahrenholtz, G.E. Hilmas, Evolution of structure during the oxidation of zirconium diboride–silicon carbide in air up to 1500 °C, J. Eur. Ceram. Soc. 27 (2007) 2495–2501. [16] M. Kazemzadeh Dehdashti, W.G. Fahrenholtz, G.E. Hilmas, Effects of temperature and the incorporation of W on the oxidation of ZrB2 ceramics, Corros. Sci. 80 (2014) 221–228. [17] D. Mu, B.L. Shen, Oxidation resistance of boronized CoCrMo alloy, Int. J. Refract. Met. Hard Mater. 28 (2010) 424–428. [18] R.J. He, Z.L. Zhou, Z.L. Qu, X.M. Cheng, High temperature flexural strength and oxidation behavior of hot-pressed B4C-ZrB2 ceramics with various ZrB2 contents at 1000-1600°C in air, Int. J. Refract. Met. Hard Mater. 57 (2016) 125–133. [19] X.R. Ren, H.J. Li, Y.H. Chu, K.Z. Li, Q.G. Fu, ZrB2-SiC gradient oxidation protective coating for carbon/carbon composites, Ceram. Int. 40 (2014) 7171–7176. [20] Z. Wang, Y.R. Niu, C. Hu, H. Li, Y. Zeng, X.B. Zheng, M.S. Ren, J.L. Sun, High temperature oxidation resistance of metal silicide incorporated ZrB2 composite coatings prepared by vacuum plasma spray, Ceram. Int. 41 (2015) 14868–14875. [21] J. Pourasad, N. Ehsani, In-situ synthesis of SiC-ZrB2 coating by a novel pack cementation technique to protect graphite against oxidation, J. Alloys Compd. 690 (2017) 692–698. [22] L. Yu, L.M. Pan, J. Yang, Y.B. Feng, J.X. Zhang, T. Qiu, In situ synthesis, mechanical properties, and oxidation resistance of (SiC+ZrB2)/Zr3[Al(Si)]4C6 composites, Corros. Sci. 110 (2016) 182–191. [23] L. Xia, F. Jin, T. Zhang, X.T. Hu, S.S. Wu, G.W. Wen, Enhanced oxidation resistance of carbon fiber reinforced lithium aluminosilicate composites by boron doping, Corros. Sci. 99 (2015) 240–248. [24] M. Kulka, Current trends in boriding techniques, springer international publishing, Cham (2019), https://doi.org/10.1007/978-3-030-06782-3. [25] D. Mu, B.L. Shen, Oxidation resistance of boronized CoCrMo alloy, J. Refract. Met. Hard Mater. 28 (2010) 424–428. [26] J.B. Jiang, Y. Wang, Q.D. Zhong, Q.Y. Zhou, L. Zhang, Preparation of Fe2B boride coating on low-carbon steel surfaces and its evaluation of hardness and corrosion resistance, Surf. Coat. Technol. 206 (2011) 473–478. [27] H. Yokota, T. Kudoh, T. Suzuki, Oxidation resistance of boronized MoSi2, Surf. Coat. Technol. 169–170 (2003) 171–173. [28] M. Kulka, N. Makuch, A. Piasecki, Nanomechanical characterization and fracture toughness of FeB and Fe2B iron borides produced by gas boriding of Armco iron, Surf. Coat. Technol. 325 (2017) 515–532. [29] L.F. He, Z.J. Lin, Y.W. Bao, M.S. Li, J.Y. Wang, Y.C. Zhou, Isothermal oxidation of bulk Zr2Al3C4 at 500 to 1000°C in air, J. Mater. Res. 23 (2008) 359–366. [30] C.L. Hu, S.F. Tang, S.Y. Pang, H.M. Cheng, Long-term oxidation behaviors of C/SiC composites with a SiC/UHTC/SiC three-layer coating in a wide temperature range, Corros. Sci. 147 (2019) 1–8. [31] L.F. He, Y.W. Bao, M.S. Li, J.Y. Wang, Y.C. Zhou, Oxidation of Zr2[Al(Si)]4C5 and Zr3[Al(Si)]4C6, J. Mater. Res. 23 (2008) 3339–3346. [32] P. Ren, Y.F. Yang, W. Li, S.L. Zhu, F.H. Wang, Cyclic oxidation behavior of a multilayer composite coating for singlecrystal superalloys, Corros. Sci. 145 (2018) 26–34. [33] C. Li, M.S. Li, Y.C. Zhou, Improving the Surface Hardness and Wear Resistance of Ti3SiC2 by Boronizing Treatment, Surf. Coat. Technol. 201 (2007) 6005–6011. [34] C.L. Hu, S.F. Tang, S.Y. Pang, H.M. Cheng, Long-term oxidation behaviors of C/SiC composites with a SiC/UHTC/SiC three-layer coating in a wide temperature range, Corros. Sci. 147 (2019) 1–8. [35] Vinci, L. Zoli, E. Landi, D. Scit, Oxidation behaviour of a continuous carbon fibre reinforced ZrB2-SiC composite, Corros. Sci. 123 (2017) 129–138. [36] M.M. Opeka, I.G. Talmy, J.A. Zaykoski, Oxidation-based materials selection for 2000°C+ hypersonic aerosurfaces: theoretical consideration and historical experience, J. Mater. Sci. 39 (2004) 5887–5904. [37] R. Inouea, Y. Araia, Y. Kubota, Oxidation behaviors of ZrB2–SiC binary composites above 2000°C, Ceram. Int. 43 (2017) 8081–8088. [38] H.L. Liu, J.X. Liu, H.T. Liu, G.J. Zhang, Changed oxidation behavior of ZrB2-SiC ceramics with the addition of ZrC, Ceram. Int. 41 (2015) 8247–80251.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported financially by the National Natural Science Foundation of China (Nos. U1435206 and 51672064). References [1] L.F. He, Z.J. Lin, J.Y. Wang, Y.W. Bao, M.S. Li, Y.C. Zhou, Synthesis and characterization of bulk Zr2Al3C4 ceramic, J. Am. Ceram. Soc. 90 (2007) 3687–3689. [2] K. Fukuda, M. Hisamura, T. Iwata, N. Tera, K. Sato, Synthesis, crystal structure and thermoelectric properties of a new carbide Zr2[Al3.56Si0.44]C5, J. Solid State Chem. 180 (2007) 1809–1815. [3] K. Sugiura, T. Iwata, H. Yoshida, S. Hashimoto, K. Fukuda, Synthesis, crystal structure and Si solubility of new layered carbides Zr2Al4C5 and Zr3Al4C6, J. Solid State Chem. 181 (2008) 2864–2868. [4] Y.C. Zhou, L.F. He, Z.J. Lin, J.Y. Wang, Synthesis and structure-property relationships of a new family of layered carbides in Zr-Al(Si)-C and Hf-Al(Si)-C systems, J. Eur. Ceram. Soc. 33 (2013) 2831–2865. [5] L.F. He, Y.W. Bao, J.Y. Wang, M.S. Li, Y.C. Zhou, Mechanical and thermophysical properties of Zr-Al-Si-C ceramics, J. Am. Ceram. Soc. 92 (2009) 445–451. [6] L.F. He, H.B. Zhong, J.J. Xu, M.S. Li, Y.W. Bao, J.Y. Wang, Y.C. Zhou, Ultrahightemperature oxidation of Zr2Al3C4 via rapid induction heating, Scr. Mater. 60 (2009) 547–550. [7] K. Fukuda, M. Hisamura, T. Iwata, N. Tera, K. Sato, Synthesis, crystal structure and thermoelectric properties of a new carbide Zr2[Al3.56Si0.44]C5, J. Solid State Chem. 180 (2007) 1809–1815. [8] J.Y. Wang, Y.C. Zhou, Z.J. Lin, T. Liao, L.F. He, First-principles prediction of the mechanical properties and electronic structure of ternary aluminum carbide Zr3Al3C5, Phys. Rev. B 73 (2006) 134107. [9] L.F. He, Y.C. Zhou, Y.W. Bao, Z.J. Lin, J.Y. Wang, Synthesis, physical, and mechanical properties of bulk Zr3Al3C5 ceramic, J. Am. Ceram. Soc. 90 (2007) 1164–1170. [10] G.Q. Chen, R.B. Zhang, X.H. Zhang, P. Hu, W.B. Han, Oxidation resistance of Zr2Al4C5 based composites at ultra-high-temperature, Scr. Mater. 61 (2009) 697–700. [11] L.F. He, F.Z. Li, X.P. Lu, Y.W. Bao, Y.C. Zhou, Microstructure, mechanical, thermal, and oxidation properties of a Zr2[Al(Si)]4C5-SiC prepared by in situ reaction/hotpressing, J. Eur. Ceram. Soc. 30 (2010) 2147–2154. [12] G.Q. Chen, R.B. Zhang, X.H. Zhang, W.B. Han, Microstructure and properties of hot pressed Zr2[Al(Si)]4C5/SiC, J. Alloys Compd. 481 (2009) 877–880. [13] R.B. Zhang, G.Q. Chen, Y.M. Pei, D.N. Fang, Isothermal oxidation of Zr2[Al(Si)]4C5SiC composites at 1000-1300°C in air, Corros. Sci. 54 (2012) 205–211.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Effect of boronizing treatment on the oxidation resistance of Zr2(AlSi)4C5/ SiC composite X.P. Lua, H.M. Xiangc, P. Hana, M.S. Lib, Y.C. Zhouc,
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a
College of Materials Science & Engineering, Henan University of Technology, Zhengzhou, 450000, China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China c Aerospace Research Institute of Material Processing Technology, No.1 South Dahongmen Road, 100076, Beijing, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ceramic SEM XRD Oxidation
The oxidation behavior of Zr2[Al(Si)]4C5/30 vol.%SiC composite(ZASC-30SC) at 900–1300 °C was investigated, and the effect of SiC additive on the oxidation resistance of Zr2[Al(Si)]4C5 at 1000 °C is negative. A boronizing coating containing ZrB2 and SiC was prepared through boronizing treatment. The effect of boronizing on the oxidation resistance of ZASC-30SC composite at 1000 °C and 1800 °C were explored respectively. At 1000 °C, the oxidation resistance was improved intensively as a result of a protective liquid B2O3 containing scale formed during oxidation. Besides, the oxidation properties at 1800 °C of ZASC-30SC was improved obviously because of a protective SiO2 rich layer generated.
1. Introduction Recent years, a series of layered carbides in Zr–Al(Si)–C system, such as Zr2Al3C4, Zr2[Al(Si)]4C5, Zr3Al3C5, and Zr3[Al(Si)]4C6, have been developed by adding Al and/or Si into ZrC [1–4]. These new ternary and quaternary carbides show superior oxidation resistance and fracture toughness than their binary counterpart, which render them promising high temperature structural materials [5–9]. However, the mechanical and oxidation properties of Zr–Al(Si)–C ceramics are still insufficient to meet the requirements of ultra-high temperature structural component. To further improve the properties of monolithic Zr–Al (Si)–C, SiC was introduced into Zr–Al(Si)–C system [10–13]. Compared with monophase Zr–Al(Si)–C ceramics, the Zr–Al(Si)–C/SiC composites exhibit promising mechanical properties and improved high temperature oxidation resistance [11,14]. With SiC additive, the flexural strength of Zr2[Al(Si)]4C5 increased as a result of the reduction of grain size. In addition, crack bridging and deflection around SiC particles were observed in Zr2[Al(Si)]4C5/SiC composite, which is the reason of the higher fracture toughness. The improved oxidation resistance is attributed to the formation of Si-containing oxides, such as aluminosilicate glass and mullite, in which the oxygen diffusion rate is low. Nevertheless, the oxidation of SiC below 1200 °C is unconspicuous [15], and it can be concluded that its contribution to the intermediate-temperature oxidation protection of Zr–Al(Si)–C/SiC composites is quite limited. This will lead to the poor intermediate-temperature oxidation
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resistance of the composites, and hinder their application as hightemperature structural materials. Thus effective method to improve the intermediate-temperature oxidation resistance is still urgently needed. It is well known that boron-containing materials have excellent oxidation resistance at intermediate temperatures (800 °C–1200 °C) due to the formation of liquid B2O3, which can effectively seal the cracks in oxide scale and provide good oxidation protection [16]. Mu and Shen have prepared a boride coating on CoCrMo alloy, and found the boronized CoCrMo alloy possess superior oxidation resistance at 950 °C [17]. During oxidation of B4C/ZrB2 composites, a dense and protective B2O3 layer formed at 1000–1200 °C, which can protect the substrate effectively [18]. Recent years, borides have always been combined with Si-containing materials to prepare protective coatings, such as ZrB2/ SiC, ZrB2/MoSi2 etc., which all show excellent oxidation resistance [19–21]. For these composites, below 1100 °C, a continuous liquid B2O3 layer generate. This layer of B2O3 can depress the inward diffusion of oxygen and protect the matrix effectively. While above 1100 °C, the excellent oxidation resistance come from the combined effect of both liquid SiO2 and B2O3. Introduction of Boron into matrix has been accomplished by various methods, such as compositing with borides, boron doping, and surface boronizing [22–24]. Boronizing is a thermochemical process that can use solid media, liquid media, and gaseous media [24]. Powder-pack boronizing method has been widely used in surface modification of a variety of materials because of its simplicity, good processability, cost
Corresponding author. E-mail addresses: [email protected], [email protected] (Y.C. Zhou).
https://doi.org/10.1016/j.corsci.2019.108201 Received 5 March 2019; Received in revised form 2 September 2019; Accepted 3 September 2019 0010-938X/ © 2019 Published by Elsevier Ltd.
Please cite this article as: X.P. Lu, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108201
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vertical intermediate frequency induction heating (IFIH) furnace was used to evaluate the ultra-high temperature oxidation behavior of the boronized ZASC-30SC composite in static air. The sample was placed on cylindrical graphite susceptor and heated to the testing temperature at about 200 °C/min. The duration time for oxidation was 15 min at 1800 °C. The detailed procedure of the oxidation can be found in our previous work [6]. For comparison, the same oxidation test was also conducted for ZASC-30SC composite. The phase composition and surface and cross section microstructures of the oxide scale were identified using X-ray diffractometer and the SUPRA 35 scanning electron microscope (SEM) mentioned above.
effectiveness, and relatively high quality of the boride layers produced on the surface. To control the boron source supply precisely and improve the automation of boronizing process, boriding in liquid media and gas boriding have been developed gradually. Powder-pack boriding technique has been used to prepare a boride layer on a series of ferrous and non-ferrous alloys, which exhibits improved oxidation resistance, corrosion resistance, and wear property [25,26]. Yokota et al. investigated the oxidation resistance of MoSi2, which was improved after boronizing treatment by molten salt process [27]. Kulka et al. have prepared boride layer on Ni-based alloy using gas boriding method and the boronizing efficiency was improved obviously [28]. Thus, it is expected that boriding treatment is an efficient way to improve the intermediate temperature oxidation properties of Zr–Al(Si)–C/SiC composites, while the excellent mechanical properties of the Zr–Al(Si)–C/ SiC matrix remains intact. In this work, Zr2[Al(Si)]4C5/30 vol.%SiC (shortened as ZASC-30SC) composite was selected and its oxidation resistance at 900–1300 °C was investigated. Pack boriding technique was used to prepare a boride containing coating and its effect on the oxidation resistance of ZASC30SC composites at intermediate temperature (about 1000 °C) and ultra-high temperature (1800 °C) were investigated. Our results highlight the positive effect of surface boriding on the intermediate temperature oxidation resistance of ternary carbides.
3. Results and discussion 3.1. Oxidation behavior of Zr2[Al(Si)]4C5/30 vol.%SiC composite at 900–1300 °C Fig. 1(b–d) present the weight gain per unit surface area as a function of time for ZASC-30SC composite oxidized at 900–1300 °C in flowing air. The oxidation behavior of monolithic Zr2[Al(Si)]4C5 under the same condition are also shown here as comparison. Generally, the oxidation kinetics of Zr2[Al(Si)]4C5 show parabolic model initially at each temperature, and subsequently follow linear law. The weight gain during oxidation increased with temperature increment. By comparison, the oxidation of ZASC-30SC composite is more complicated. When the oxidation temperature was 900 °C, the oxidation resistance of ZASC30SC composite was better in 8 h (the insert in Fig. 1b). When the temperature increased to 1000 °C, the oxidation procedure can be divided into two stages. In the first hour, the weight gain of ZASC-30SC was less than that of Zr2[Al(Si)]4C5. After the holding time for 1 h, the effect of SiC is negative and the the weight gain of ZASC-30SC composite exceeded that of Zr2[Al(Si)]4C5. From Fig. 1b, it can be seen that the weight gain per unit area of ZASC-30SC composite(62.5 g/m2) is much larger than that of Zr2[Al(Si)]4C5(28.5 g/m2) after 8 h, indicating hazardous effect of SiC on the oxidation of the composites. In addition, accelerated oxidation of ZASC-30SC composite and spallation of oxide scale have been observed during oxidation at 1000 °C. At 1100 °C and 1200 °C, the oxidation kinetics and weight gain are comparable with monolithic Zr2[Al(Si)]4C5 (Fig. 1c and the insert). It is noticeable that ZASC-30SC composite began to exhibit superior oxidation resistance than Zr2[Al(Si)]4C5 at about 1200 °C. As temperature increased, the effect of SiC additive on the weight gain of Zr2[Al(Si)]4C5 became more positive. At 1300 °C, the weight gain per unit area of monolithic Zr2[Al (Si)]4C5 achieved approximately 250 g/m2 after 1 h, and subsequently serious peeling of oxide scale appeared. By comparison, the weight gain of ZASC-30SC composite oxidized at 1300 °C for 1 h was 70.5 g/m2. In addition, it should be noted that the weight gain of ZASC-30SC composite at 1200 °C increased more rapidly than that at 1300 °C. In brief, SiC additive has positive effect on the oxidation resistance of Zr2[Al (Si)]4C5 above 1200 °C but negative effect at about 1000 °C, which will be discussed in detail in the following sections. To investigate the source of the abnormal oxidation kinetics of ZASC-30SC composite at 1000 °C, the phase composition and microstructure of the oxide scale were detected. Fig. 2 shows the XRD patterns of the oxide scale surfaces oxidized at different temperatures for 1 h, indicating the composition of the oxide scales. At 1000 °C, the oxide scales are mainly consisted of t-ZrO2, Al2O3, and SiC, in which t-ZrO2 and Al2O3 came from the oxidation of Zr2[Al(Si)]4C5 substrate and the SiC in the composite was almost not oxidized. The SEM micrograph of oxide scale surface of ZASC-30SC composite oxidized at 1000 °C for 8 h is shown in Fig. 3. From the figure it can be seen that the unoxidized SiC, which can be confirmed from the EDS result (the insert in Fig. 3b), was almost intact and remained in the oxide scale. As shown in Fig. 3, a lot of cracks were observed on the surface and the unoxidized SiC particles are buried in the oxide scale. Since the oxidation process of
2. Experimental The ZASC-30SC composite was fabricated by in situ hot-pressing zirconium, aluminum, silicon, and graphite powders at 1900 °C for 1 h in Ar under a pressure of 30 MPa. The preparation steps have been described in detail in the previous work [5]. The optimized molar ratio of Zr:Al:Si:C for synthesizing ZASC-30SC composite was 2:3.6:3.18:7.4. The density of sintered samples was determined by the Archimedes method, and the density of the synthesized samples is higher than 98% of the theoretical density. The samples for oxidation resistance tests and boronizing treatment with dimensions of 4 mm × 4 mm × 8 mm were cut by an electrical discharge method from an as-fabricated bulk piece. The surfaces were ground down to 1000 grade SiC paper and polished using 1.5 μm diamond paste. The specimens were ultrasonically cleaned in acetone and distilled water before boronizing treatments. Boronizing was performed by using a fluoride-activated powder pack cementation method. The boronizing media was composed of 47.5 wt.% B4C, 47.5 wt.% SiC, and 5 wt.% KBF4 (analytical pure). In this boronizing media, B4C serves as the source of boron, SiC as the diluent, and KBF4 as the active agent. The boronizing process was carried out in the following procedure: all the media powders were mixed in a polyurethane jar for 10 h, using stainless steel balls coated with a layer of polyurethane as the mixing medium. ZASC-30SC specimens were buried in the powders and packed into an Al2O3 jar, which was then placed in the furnace under a flowing argon atmosphere. The heating rate for all runs was 10 °C/min, and the boronizing treatments were carried out at 1600 °C for 2 h. After treatment, the samples were cooled to room temperature in the furnace. The phase composition of the boronizing treated specimens was identified using X-ray diffractometer. X-ray diffraction (XRD) was carried out in a D/max–2400 diffractometer (Rigaku, Tokyo, Japan) with Cu Kα□radiation. The microstructure of the polished boronizing treated samples was observed in a SUPRA 35 scanning electron microscope (SEM) (LEO, Oberkochen, Germany) equipped with an energy-dispersive spectroscopy (EDS) system. The isothermal oxidation test in flowing air was carried out using a Setsys 16/18 microbalance (SETARAM, Caluire, France). The schematic diagram of the isothermal oxidation test was shown in Fig. 1a. The samples were suspended with a Pt wire and heated to the testing temperature at a rate of 40 °C/min. The mass change was recorded continuously as a function of time during isothermal oxidation. A 2
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Fig. 1. (a) Schematic representation of the isothermal oxidation test, and weight gain per unit area versus time of ZASC-30SC composite oxidized at (b) 900 °C (insert) and 1000 °C, (c) 1200 °C and 1100 °C (insert), and (d) 1300 °C.
growth stress in the oxide scale. Under this stress, cracks easily generate at the interface between the oxides and the SiC particles. In general, this residual stress increase with the thickening of the oxide scale [32]. At 900 °C, the oxidation resistance of Zr2[Al(Si)]4C5 was improved with addition of SiC, since SiC was not oxidized at this temperature. Within 8 h at 900 °C, cracks were not formed and the oxide scale was not destroyed because the oxide scale is thin, which could not generate enough stress. So the SiC has positive effect on the oxidation resistance of Zr2[Al(Si)]4C5. It is predictable that the effect of SiC on the oxidation resistance of Zr2(AlSi)4C5at 900 °C will change when the holding time become longer and the oxide scale become thicker, which will be investigated in the future. Similar to the oxidation behavior at 900 °C, the weight gain of Zr2(AlSi)4C5in the first hour at 1000 °C is lower than that of ZASC-30SC composite when the oxide scale was not thick. After holding at 1000 °C for 1 h, cracks formed in the oxidation scale under the residual stress when the oxide scale became thicker. The cracks are the channels through which oxygen can diffuse inward to the substrate, so the oxidation resistance of Zr2[Al(Si)]4C5 at 1000 °C become even worse with SiC additive when the holding time is more than 1 h. In addition, as the oxide scale became thicker, the residual stress eventually leaded to spallation of the oxide layer (Fig. 8). At about 1200 °C, SiC begins to oxidize to form Si-containing oxides such as SiO2 and mullite (Fig. 2c), which will provide better protection by sealing cracks in the oxide scale [13]. So at about 1200 °C, the SiC additive begin to have positive effect on the oxidation resistance of Zr2[Al(Si)]4C5. But the small amounts of liquid phases is inadequate to form a protective scale, and the weight gain of ZASC-30SC and Zr2[Al (Si)]4C5 at 1200 °C is comparable. The content of the protective phases mentioned above increased with temperature. At 1300 °C, the weight gain of ZASC-30SC is much less than that of Zr2[Al(Si)]4C5, which is due to more protective silicon containing phases such as SiO2 and mullite filling the cracks in the oxides scale. Similar phenomena was reported in other SiC-containing composites, such as ZrB2/SiC and Zr2[Al (Si)]4C5/20SiC composites, in which SiC begins to have positive effect on the oxidation resistance above about 1300 °C [12,15].
Fig. 2. XRD patterns of the oxide scales on ZASC-30SC composite after exposure to air at 1000–1300 °C for 1 h.
Zr2[Al(Si)]4C5 is the inwardly diffusion of oxygen [11], the SiC particles were not covered with the oxides and can be detected using XRD method. Noticeably, most of cracks are radiate and stem from the SiC particles. The cross section morphology of the oxide scale of ZASC-30SC composite oxidized at 1000 °C for 8 h is shown in Fig. 4. The oxide scale is inhomogeneous and cracks mainly distributed in the SiC-containing region. By comparison, the oxide scale at the regions without SiC is dense without cracks. Apparently, the oxide scale with SiC existing is much thicker than that without SiC, indicating that SiC additive is harmful for the oxidation resistance of Zr2[Al(Si)]4C5 at 1000 °C. During oxidation of ZASC-30SC composite, Zr2[Al(Si)]4C5 phase began to oxidize at about 600 °C [29]. By comparison, the SiC particles did not oxidize appreciably below 1000 °C [30]. There is a volume expansion upon conversion of Zr2[Al(Si)]4C5 to ZrO2, Al2O3(Pilling–Bedworth ratio (PBR)≈1.25) [31], which will generate 3
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Fig. 3. Surface morphology of ZASC-30SC composite oxidized at 1000 °C for 8 h.
gray from original silvery white. As shown in Fig. 5b, the XRD peaks of Zr2[Al(Si)]4C5 disappear and three phases, namely ZrB2, SiC, and Al2O3 are detectable. The SEM micrograph of the as-synthesized ZASC-30SC composite was shown in Fig. 6a, from which it can be seen that the SiC particles distribute in the Zr2[Al(Si)]4C5 substrate uniformly. The surface morphology of the boronized ZASC-30SC is shown in Fig. 6b. It is seen that the Zr2[Al(Si)]4C5 in ZASC-30SC composite turned into the homogeneous mixture of ZrB2 and Al2O3, meanwhile SiC did not react with boron and spread in the boronizing coating, which is consistent with the XRD results. To investigate the structure of the boronizing coating, the cross section morphology of the ZASC-30SC composite after boronizing is shown in Fig. 7. From Fig. 7a, It can be seen that the boronizing coating (about 20μm thick) with a single layer was compact and adhered well to the substrate. No cracks or spallation are found in the coatings. From Fig. 7b, It is noticed that the boronized Zr2[Al(Si)]4C5 grains kept the same shape as the as-synthesized elongated and rod-like Zr2[Al(Si)]4C5 grains (marked with arrows). In these grains, Al2O3(black phase) and ZrB2(white phase) distribute homogeneously. In addition, the SiC particles remained unchanged and were not covered with ZrB2 and Al2O3 during boronizing treatment. During boronizing treatment, SiC can be taken as inert markers to determine the predominant diffusion process. From Fig. 7 it can be concluded that the boronizing coating grew totally by the inward diffusion of boron atoms. From the above analysis, the boronizing procedure can be described as follows. First, boron atoms were created in the boronizing agent and diffused inward to react with Zr atoms in the Zr2[Al(Si)]4C5 to form ZrB2 in situ. Meanwhile, the oxygen impurities, which maybe came from the boronizing agent or argon, also diffused inward and reacted with Al atoms to form Al2O3 simultaneously. The function of C in the Zr2[Al(Si)]4C5 during boronizing is unclear yet. According to previous work, carbon maybe reacted with boronizing media or with oxygen impurities to form gas and diffused out of the samples [33]. The detailed boronizing procedure will be investigated in the near future.
Fig. 4. Cross section morphology of ZASC-30SC composite oxidized at 1000 °C for 8 h.
3.2. Microstructure and phase composition of boronizing coatings Fig. 5 shows the XRD patterns of the ZASC-30SC surface before and after boronizing treatment. From Fig. 5a, it can be seen that the assynthesized ZASC-30SC composite is consisted of Zr2[Al(Si)]4C5 and SiC. After boronizing treatment, the surface of ZASC-30SC turned into
3.3. The effect of boronizing treatment on the oxidation resistance of Zr2[Al (Si)]4C5/SiC at 1000 °C From above analysis, the added SiC deteriorate the oxidation behavior of ZASC-30SC composite at about 1000 °C. Fig. 8 compares the weight gain per unit area of the ZASC-30SC composite before and after boronizing oxidized at 1000 °C. For comparison, the weight gain per unit area of the monolithic Zr2[Al(Si)]4C5 was also shown here. As shown in Fig. 8, the weight gain per unit area of the boronized ZASC30SC composite oxidized at 1000 °C for 7.5 h is 22.5 g/m2, which is only one third of that for the unboronized ZASC-30SC composite oxidized at the same condition(63.1 g/m2), indicating the oxidation
Fig. 5. XRD patterns of the ZASC-30SC composite (a) before, and (b) after boronizing treatment at 1600 °C for 2 h. 4
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Fig. 6. SEM photograph of ZASC-30SC composite surface (a) before, and (b) after boronizing treatment.
resistance of ZASC-30SC composite at 1000 °C was improved obviously by boronizing treatment. Fig. 9 shows the oxide scale surface of the boronized ZASC-30SC composite oxidized at 1000 °C for 7.5 h. Unlike the ZASC-30SC composite oxidized at 1000 °C mentioned above (Figs. 3 and 4), no cracks have been observed on surface of the oxide scales, which is compact and covered with molten phases during oxidation. From the cross section morphology of the oxide scale shown in Fig. 10, the thickness of the oxide scale after oxidation for 7.5 h is about 7 μm, which is about one third that of the boronizing scale. During oxidation at 1000 °C, though SiC particles in the boronizing coating were still not oxidized, no cracks or spallation appeared at the SiC containing region. The oxide layer is dense and adhered well to the unoxidized substrate, indicating that the boronized coating can protect the substrate effectively at 1000 °C. The oxidation resistance of ZASC-30SC composite was improved considerably after boronizing treatment, which is due to the formation of coating consists of ZrB2 and SiC. The oxidation behavior of ZrB2/SiC composite was investigated extensively and it has excellent oxidation resistance in a wide temperature range [34–37]. Below 1100 °C, liquid B2O3 can form a continuous passive layer as a barrier to oxygen diffusion, which results in the parabolic oxidation kinetics of ZrB2. Above 1100 °C, though the volatilization of B2O3, borosilicate layer forms and the oxygen diffusion in the oxide scale can also be prevented. As mentioned above, the oxidation resistance of ZASC-30SC is worsened compared with pure Zr2[Al(Si)]4C5 at 1000 °C, which is due to the cracks in the oxide scale. Thus during the oxidation of the boronized ZASC-30SC at about 1000 °C, which has a ZrB2 containing coating outside of the substrate, the molten phase B2O3 could be generated. These liquid B2O3 act as a barrier to the transport of oxygen at 1000 °C
Fig. 8. Weight gain per unit area versus time of Zr2[Al(Si)]4C5, ZASC-30SC composite, and boronized ZASC-30SC composite oxidized at 1000 °C.
and can protect the substrate effectively. In addition, it is reasonable to predict that the oxidation resistance of boronized ZASC-30SC above 1000 °C can also be improved. 3.4. Oxidation behavior of boronized Zr2[Al(Si)]4C5/30 vol.%SiC composite at 1800 °C Fig. 11a shows the surface morphology of the oxide scale for ZASC30SC composite oxidized at 1800 °C for 15 min. It is noticeable that many pores and bubbles spread on the oxide scale surface after oxidation. Fig. 12a shows the cross-sectional morphology and
Fig. 7. Cross section morphology of boronized ZASC-30SC composite. 5
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Fig. 9. Surface morphology of boronized ZASC-30SC composite oxidized at 1000 °C for 8 h.
(2)
SiC(s) + 3/2O2(g) → SiO2(l)+CO(g)
(3)
xZrO2(s) + SiO2(l) → xZrO2·SiO2(l)
(4)
3Al2O3(s) + 2SiO2(l) → 3Al2O3·2SiO2(l)
(5)
According to the above Eqs. (1)–(5), during oxidation of ZASC-30SC at 1800 °C, a molten scale consist of SiO2, xZrO2·SiO2 and 3Al2O3·2SiO2 formed. Meanwhile large amounts of gaseous products such as CO2 and CO (Eqs. (1)–(3)) generated and rose to the outmost layer. These gases attempt to diffuse outside of the scale, and pores and bubbles formed in the molten layer. It is noticeable that some bubbles ruptured because the internal pressure of the bubble exceeded the limit pressure of liquid bearing capacity. The volume of the bubbles increased during their rising, which is the reason of the porous outer layer in the oxide scale. In addition, cracks were also observed in oxide layer as a result of the upwelling of aggregate CO and CO2 gases. These pores and cracks will provide the channels for the diffusion of oxygen into the inner material and have harmful impact on the oxidation resistance. Fig. 11b shows the oxide scale surface of boronized ZASC-30SC composite exposed to the same oxidizing environment. It can be seen that the oxide scale surface was covered with a relatively flat and dense molten phase. The corresponding cross section morphology and the EDS line scans of the oxide scale were shown in Fig. 12b. The thickness of the oxide scale is about 45 μm, which is thinner than that of the unboronized samples. In addition, the oxide scale of boronized ZASC-30SC composite consisted of two layers: (1) one SiO2-rich layer; and (2) one SiC depleted layer, which is in agreement with the previous studies [30–34]. After boronizing treatment, Zr2[Al(Si)]4C5 in the boronizing scale
Fig. 10. Cross section morphology of boronized ZASC-30SC composite oxidized at 1000 °C for 8 h.
corresponding EDS line scans of ZASC-30SC composite oxidized at 1800 °C for 15 min. It can be seen that the oxide scale is about 60μm thick and shows a single layer structure. In addition, a relatively porous outer layer can be observed in the oxide scale. According to the relative work about oxidation of Zr–Al–Si–C system at ultra-high temperatures, the main expected reactions during oxidation process are as follows [6,11]: Zr2[Al1-x(Si)x]4C5(s) + (10+x)O2(g) → 2ZrO2(s) + 2(14x)Al2O3(s) + 2x(3Al2O3·2SiO2)(l) + 5CO2(g)
SiC(s) + 2O2(g) → SiO2(l)+CO2(g)
(1)
Fig. 11. Surface morphology of (a) unboronized ZASC-30SC, and (b) boronized ZASC-30SC composite oxidized at 1800 °C for 15 min. 6
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Fig. 12. Cross section morphology and the corresponding EDS line scans taken from the white line of (a) unboronized ZASC-30SC, and (b) boronized ZASC-30SC composite oxidized at 1800 °C for 15 min.
1800 °C, a SiO2 rich layer generated, and a low oxygen partial pressure environment formed beneath this layer. Some gases such as CO2 and SiO generated and rose to the the outmost layer. The content of these gases was not enough to destroy the liquid layer, which can provide an effective protection. By comparison, no continuous SiO2 rich layer appeared in the oxide scale of the unboronized ZASC-30SC composite. As mentioned above, large amount of gases were produced during oxidation of ZASC-30SC composite, and the liquid layer was damaged because of the diffusion of these gases. Channels to release these gases could serve as the oxygen diffusion path, which resulted in its poor oxidation resistance. Zhang et.al investigated the effect of ZrC addition on the oxidation resistance of ZrB2-SiC, and found that the SiO2 rich layer disappeared when the ZrC content increased to 20 vol% as a result of the gas byproducts [38].These results indicated that the boronizing treatment was beneficial to the oxidation resistance of ZASC-30SC composite at 1800 °C.
was mainly substituted by ZrB2 and Al2O3. During oxidation of boronized ZASC-30SC composite at 1800 °C, the main reactions of the boronizing coating with O2 can be expressed as follows: SiC(s) + 2O2(g) → SiO2(l) + CO2(g)
(6)
ZrB2(s)+5/2O2(g) → ZrO2(s) + B2O3(g)
(7)
SiO2(l) → SiO(g) + 1/2O2(g)
(8)
xZrO2(s) + SiO2(l) → xZrO2·SiO2(l)
(9)
3Al2O3(s) + 2SiO2(l) → 3Al2O3·2SiO2(l)
(10)
In this case, a SiO2-rich layer came from oxidation of SiC covered the underlying material and could, potentially, provide a barrier to oxygen diffusion. This SiO2-rich glass layer is expected to contain some B2O3, though most of B2O3 evaporated at 1800 °C. Beneath this SiO2rich layer, low oxygen partial pressure environment formed and gaseous SiO generated (Eq. (8)) because of the high vapor pressure of SiO2(l) [33]. SiO gas evaporated and transport to the outer SiO2-containing layers, which generated the SiC depleted layer. It can be seen that some bubbles also exist on the oxide scale surface of the boronizd ZASC-30SC composite, but the content and volume are smaller than that of unboronized ZASC-30SC composite. According to Fig. 12b, these bubbles almost exist at the outmost surface of oxide scale, and the SiO2 rich layer is very dense, indicating that these gases have not damaged the protective SiO2 rich layer. In summary, during oxidation of boronized ZASC-30SC composite at
4. Conclusion The oxidation resistance of Zr2[Al(Si)]4C5/30 vol.%SiC composite at intermediate(900–1300 °C) temperatures was investigated, and the oxidation at 1000 °C behaves catastrophic, which is due to the flaws such as cracks in the oxide scale. The oxidation resistance of Zr2[Al (Si)]4C5 was improved above 1200 °C with SiC additive because of the protective phases such as SiO2 and mullite generated. A ZrB2/SiC containing coating was prepared on ZASC-30SC composite by boronizing treatment, and the oxidation resistance of ZASC-30SC at 7
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1000 °C was improved obviously due to the formation of a protective liquid B2O3 layer. In addition, boronizing treatment is also beneficial for the oxidation resistance of ZASC-30SC composite at 1800 °C for a dense protective SiO2 rich layer was generated during oxidation. The effect of boronizing treatment on the oxidation resistance in the temperature range of 1000–1800 °C and higher than 1800 °C is also important for the use of ZASC-30SC composite, which will be investigated in the future.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported financially by the National Natural Science Foundation of China (Nos. U1435206 and 51672064). References [1] L.F. He, Z.J. Lin, J.Y. Wang, Y.W. Bao, M.S. Li, Y.C. Zhou, Synthesis and characterization of bulk Zr2Al3C4 ceramic, J. Am. Ceram. Soc. 90 (2007) 3687–3689. [2] K. Fukuda, M. Hisamura, T. Iwata, N. Tera, K. Sato, Synthesis, crystal structure and thermoelectric properties of a new carbide Zr2[Al3.56Si0.44]C5, J. Solid State Chem. 180 (2007) 1809–1815. [3] K. Sugiura, T. Iwata, H. Yoshida, S. Hashimoto, K. Fukuda, Synthesis, crystal structure and Si solubility of new layered carbides Zr2Al4C5 and Zr3Al4C6, J. Solid State Chem. 181 (2008) 2864–2868. [4] Y.C. Zhou, L.F. He, Z.J. Lin, J.Y. Wang, Synthesis and structure-property relationships of a new family of layered carbides in Zr-Al(Si)-C and Hf-Al(Si)-C systems, J. Eur. Ceram. Soc. 33 (2013) 2831–2865. [5] L.F. He, Y.W. Bao, J.Y. Wang, M.S. Li, Y.C. Zhou, Mechanical and thermophysical properties of Zr-Al-Si-C ceramics, J. Am. Ceram. Soc. 92 (2009) 445–451. [6] L.F. He, H.B. Zhong, J.J. Xu, M.S. Li, Y.W. Bao, J.Y. Wang, Y.C. Zhou, Ultrahightemperature oxidation of Zr2Al3C4 via rapid induction heating, Scr. Mater. 60 (2009) 547–550. [7] K. Fukuda, M. Hisamura, T. Iwata, N. Tera, K. Sato, Synthesis, crystal structure and thermoelectric properties of a new carbide Zr2[Al3.56Si0.44]C5, J. Solid State Chem. 180 (2007) 1809–1815. [8] J.Y. Wang, Y.C. Zhou, Z.J. Lin, T. Liao, L.F. He, First-principles prediction of the mechanical properties and electronic structure of ternary aluminum carbide Zr3Al3C5, Phys. Rev. B 73 (2006) 134107. [9] L.F. He, Y.C. Zhou, Y.W. Bao, Z.J. Lin, J.Y. Wang, Synthesis, physical, and mechanical properties of bulk Zr3Al3C5 ceramic, J. Am. Ceram. Soc. 90 (2007) 1164–1170. [10] G.Q. Chen, R.B. Zhang, X.H. Zhang, P. Hu, W.B. Han, Oxidation resistance of Zr2Al4C5 based composites at ultra-high-temperature, Scr. Mater. 61 (2009) 697–700. [11] L.F. He, F.Z. Li, X.P. Lu, Y.W. Bao, Y.C. Zhou, Microstructure, mechanical, thermal, and oxidation properties of a Zr2[Al(Si)]4C5-SiC prepared by in situ reaction/hotpressing, J. Eur. Ceram. Soc. 30 (2010) 2147–2154. [12] G.Q. Chen, R.B. Zhang, X.H. Zhang, W.B. Han, Microstructure and properties of hot pressed Zr2[Al(Si)]4C5/SiC, J. Alloys Compd. 481 (2009) 877–880. [13] R.B. Zhang, G.Q. Chen, Y.M. Pei, D.N. Fang, Isothermal oxidation of Zr2[Al(Si)]4C5SiC composites at 1000-1300°C in air, Corros. Sci. 54 (2012) 205–211.
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