Ablation behavior of graphene reinforced SiBCN ceramics in an oxyacetylene combustion flame

Accepted Manuscript Title: Ablation behavior of graphene reinforced SiBCN ceramics in an oxyacetylene combustion flame Author: Daxin Li Zhihua Yang Dechang Jia Xiaoming Duan Peigang He Yu Zhou PII: DOI: Reference:

S0010-938X(15)30003-2 http://dx.doi.org/doi:10.1016/j.corsci.2015.07.006 CS 6399

To appear in: Received date: Revised date: Accepted date:

5-5-2015 22-7-2015 24-7-2015

Please cite this article as: Daxin Li, Zhihua Yang, Dechang Jia, Xiaoming Duan, Peigang He, Yu Zhou, Ablation behavior of graphene reinforced SiBCN ceramics in an oxyacetylene combustion flame, Corrosion Science http://dx.doi.org/10.1016/j.corsci.2015.07.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ablation behavior of graphene reinforced SiBCN ceramics in an oxyacetylene combustion flame Daxin Li, Zhihua Yang*, Dechang Jia, Xiaoming Duan, Peigang He, Yu Zhou Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150080, China Abstract

The effect of graphene on the ablation properties and microstructures of graphene reinforced SiBCN ceramics was investigated. A protective oxide layer mainly consisted of amorphous SiO2 glass and cristobalite is observed on ablation surface of all samples. Introduction of graphene provides superior thermal conductivity and a three-dimensional structure that significantly improves the ablation resistance compared to sample of SiBCN monoliths. Four ablation actions appeared to be responsible for observed ablation morphologies: oxidation of graphene and SiBCN matrix, erosion of graphene in ablation region, evaporation of gaseous products, flowing of SiO2 and B2O3 by ablation steam. Keywords: A. Ceramic matrix composites; B. SEM; B. XRD; C. High temperature corrosion. 1. Introduction SiliconBoron CarboNitride (SiBCN) ceramics have attracted considerable attention due to the stability of their amorphous structure at high temperature [1], creep resistance [2], electrical properties [3], superior mechanical properties and better resistance to high-temperature oxidation compared to SiC analogs [4–6]. Because of these outstanding properties, SiBCN ceramics have been prepared as coatings [6, 7], microelectronics [7], fibers [7-10], fiber reinforced composites [11, 12], membranes [13] and porous materials including ordered mesoporous materials or foams [14-17]. However, SiBCN ceramics properties remain insufficient for applications in the severe thermal shock environment such as the nose tip, satellite nozzles and rocket engines of hypersonic aerospace vehicles resulting from their susceptibility to brittle failure [18]. An effective way to overcome the low fracture toughness is to fabricate them as composites. Another route for improving the fracture toughness of ceramics is to prepare ceramic fiber reinforced composites or CFCs. According to the literature [19-21], polymer infiltration and pyrolysis processing can be used to prepare fiber reinforced SiBCN ceramics; however, this approach introduces microdefects such as pores and cracks because of gas evaporation during pyrolysis. These microdefects strongly reduce thermal shock and ablation resistance in severe environment. A further issue in the design and processing of optimal CFCs, is the interface which often controls the failure process. The introduction of a weak but not too weak interface is key to superior mechanical performance. Multiple efforts have been made to produce optimal CFCs. For example, carbon fibers have been used to produce CFCs [21]. However, their use in industrial applications is limited because of their *

Corresponding author. Address: Institute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology, P. O. Box 3022, No. 2, YiKuang Street, Harbin 150080, P. R. China. Tel: +86-451-86418792; Fax: +86-451-86414291 E-mail address: [email protected]

increased sensitivity to oxidative environment [22]. This does not solve the targeted high temperature regime noted above. In contrast, SiC fibers offer good high temperature performance and oxidation resistance [23-25]. Wang et al. [18] introduced BN coated SiCf into SiBCN ceramics by ball milling and investigated the ablation properties of hot pressed SiCf/SiBCN composites. They described four timedependent ablation mechanisms contributed to the various ablation regions: oxidation of SiC fibers and SiBCN matrix, erosion of SiC fibers, evaporation of SiO (g) and B2O3 (g), and high-viscosity flow of SiO2 liquid in the ablation gas-stream. But the poor or strong bonding strength between SiC fibers and ceramic matrix would lead to terrible performance of mechanical properties and thermal shock resistance. These results suggest the need for still further improvements in CFC design for SiBCN CFCs. To this end, Liang [26] investigated the ablation properties and mechanisms together with thermal shock behavior of SiCf/Cf/SiBCN ceramics. They found that the linear ablation rate obtained was desirable, but it is higher than that of SiCf/SiCBN ceramics. Thus, there is still room for further improvement perhaps by changing the nature of the reinforcing component, the basis for the current paper using graphene. Graphene has a unique atom-thick two-dimensional structure, good electronic, mechanical, optical and thermal properties [27]. Graphene also combines a large specific surface area, a high 2-D aspect ratio and outstanding mechanical properties [28] suggesting utility as a nanofiller in CMCs. Compared with carbon fibers and SiC fibers, graphene possess the largest specific surface area, highest thermal conductivity and electrical conductivity. Thus, one can envision using only small amounts of graphene to engender superior fracture toughness, thermal conductivity and electrical conductivity in the resulting CMCs. Graphene has been determined to act as an impermeable barrier to molecules offering the potential to protect underlying materials from oxidation [29]. Furthermore, graphene offers a degree of chemical inertness that is superior to carbon fibers suggesting that its use in SiBCN CMCs may reduce susceptibility to ablation [30]. Despite carbon could be oxidized easily at high temperature in oxygen containing environment, we still believe graphene incorporated into SiBCN matrix would improve the ablation properties of composites because of the effective toughening mechanisms provided by graphene and its well thermal and electrical properties; although there is no literature evidence either way. This then is the objective of the current research. In our previous research [31], graphene was prepared by a modified Hummers method and SiBCN amorphous powders were obtained by mechanical alloying. In the current, graphene was uniformly dispersed in these SiBCN powders using cetyltrimethylammonium bromide (CTAB). Subsequently, graphene reinforced SiBCN CMC discs were processed by spark plasma sintering at 1800 oC at constant uniaxial pressure of 40 MPa, with a dwell time of 5 min in a vacuum of 5 Pa. In previous studies, we described toughening mechanisms in graphene reinforced SiBCN CMCs including: pull-out, bridging, crack penetration graphene and crack deflection. In relevant studies, monolithic SiBCN exhibits three oxidation regimes [32-34]. For example, the BNC components oxidize to form a continuous passivating liquid/glassy B2O3 layer on the surface of the unoxidized matrix. With time, some of the B2O3 evaporates; however, SiC also oxidizes with time such that a borosilicate glass forms on the surface before all the B2O3 is lost [35]. A second oxidation regime occurs between 1200-1500 oC. In this temperature range, a combination of mass gain from production of SiO2 and loss from evaporation of B2O3 is observed. Oxidation kinetics clearly shows oxide layer formation that follows parabolic kinetics in this temperature

range [36]. A third oxidation regime is observed ≥ 1500 oC, where a dense and smooth layer of high-viscosity SiO2 forms. The evaporation rate of B2O3 is extremely significant at all temperatures above 900 oC. For ablation test executed in an oxyacetylene torch at ~ 3100 oC hold times of 10 s was used. Although this is a very short time; the temperature is extremely high and reflects oxidation conditions anticipated for aerospace applications in extreme environment. To date, the microstructures, mechanical properties and toughening mechanisms of graphene reinforced SiBCN ceramics have been thoroughly assessed, whereas much less is known about the effects of graphene on ablation behavior. In the present study, the influence of graphene on CMCs ablation properties was carefully evaluated based on the sample surface temperature, phase transitions and ablation morphologies. 2. Experimental procedures 2.1. Starting materials and spark plasma sintering SiBCN amorphous powder was prepared by mechanical alloying using high purity c-Si (95% in purity, 45.0 µm, Beijing Mountain Technical Development center, China), graphite (99.5% in purity, 8.7 µm, Qingdao Huatai Lubricant Sealing S&T Co. Ltd., China) and h-BN (98.0% in purity, 0.6 µm, Advanced Technology & Materials Co. Ltd., Beijing, China). According to the literature and our previous work [37-39], the chemical composition was set as a Si:BN:C=2:1:3 mole ratio. The powder mixture was milled for 40 h under argon in a planetary ball mill (P4, Fritsch GmbH, Germany) with the media to powder mass ratio set at 20:1. These mixtures were then placed in 500 mL Si3N4 vials. The vials were then loaded into the ball mill and it was rotated at 350 rpm/min with the vials rotating at 600 rpm/min in reverse. Graphene oxide was prepared by oxidizing commercially available graphite powder used above by modified Hummers method [40]. Initially, 125 ml of H2SO4 and 2.5 g solid sodium nitrate were added to a beaker containing 5 g of graphite at room temperature. After that, 15 g KMnO4 was added gradually in five separate 3 g quantities over a 5 min period. Thereafter the mixture was immersed in an ice-bath and stirred for 2 h to ensure sufficient oxidation. Thereafter, the ice-bath was removed and the mixture stirred for another 30 min at ~35 oC. At last, 250 ml distilled water was added into the mixture gradually. The suspension was cooled and H2O2 (30%) was added until gas evolution ceased. After synthesis, the resulting graphene oxide or GO suspension was washed with dilute 1 M HCl and deionized H2O repeatedly to adjust the pH to ~ 5. The resulting product was separated from the mixture by using a centrifugal machine spinning at 4000 rpm for 5 min. The yield of GO was measure to be 95% ± 2%. Graphene or RGO was obtained through the reduction of GO using hydrazine hydrate as the reducing agent [41]. GO 0.5 g was first added into a beaker filled with 250 ml distilled water. Then the solution was ultrasonicated for 1 h then, 2 ml of hydrazine hydrate was added and the mixture was heated to ≈ 100 °C in a water bath for 24 h. After vacuum filtration, the resulting solid was washed with three 100 mL portions of chloroform and then DI water. The recovered material was then dried at 90 °C/24 h/air. A uniform dispersion of graphene and SiBCN mixed powder was prepared as described elsewhere [31]. Finally, the mixed powder was loaded into a graphite die and sintered in a spark plasma sintering device (FCT-HP 25, diameter of die = 20 mm) by heating at 100 oC/min to 1800 oC. Uniaxial pressure of 40 MPa was used in parallel with increasing temperature, and kept constant through the dwell time of 5 min in a vacuum of 5 Pa. The sample surface temperature was monitored by an

optical pyrometer with the range of 1000 ~ 2500 oC. Afterward, the sample was furnace cooled to room temperature. The as-obtained samples were grinded with SiC paste (3000 mesh, LiShan Abrasive Co. Ltd., China) to a surface roughness of ~ 0.5 µm. As-obtained samples were labeled GPL X% according to the graphene volume content, where X% represents graphene volume %. The composition design and sintering parameters of the investigated materials are listed in Table 1. 2.2. Ablation test and characterization Ablation test was carried out in a flowing oxyacetylene flame environment using test samples of Φ 20 mm2 × 10 mm. The linear diameter of the oxyacetylene flame gun tip was 2 mm, and the distance between the gun tip and sample was ≈ 10 mm. The pressure of oxygen and acetylene were 0.4 and 0.085 MPa, while the gas fluxes were 1512 and 1116 L/h, respectively [18]. The torch temperature was ~ 3100 oC, and the sample was exposed for 10 s after the flame was steady. The maximum temperature of the ablation center surface reached as high as 2400 oC, as measured by optical pyrometer. After ablation, sizes of 3 mm×4 mm×10 mm bars were cut from the ablated sample across the ablation center area to investigate the morphological evolution as well as phases transformation. The structural characterization of graphene reinforced SiBCN ceramics was analyzed using X-ray diffraction (XRD, 40 kV/100 mA, D/max-γB Cukα, Rigaku Corp., Japan) method to obtain the X-ray diffraction spectra at 2θ =10o-90o with the scanning speed of 4o/min and a scanning electron microscopy (SEM, 30 kV, Quanta 200 FEG, FEI Co., USA) was employed to investigate the surface and fracture morphologies of the samples. Microstructures investigation of graphene and graphene oxide were done in a atomic force scanning probe microscope (AFM, Dimension Icon, Bruker Co., Germany). Energy dispersive spectrometer (EDS, Oxford instruments INCAx-act, Oxforshire, U.K.) was also adopted to study the elemental arrangement and phase distribution. The density of all samples was measured by Archimedes method. The mass ablation rate (the mean mass ablation rate over the entire crater area) was obtained by a mass change of unit surface area in unit time. The linear ablation rate was measured by the change of thickness of the most erosion zone on the ablation surface. The ablation rates of the ceramics were the average of three samples. The mass ablation rate and linear ablation rate were calculated by the following formula, respectively. (1)

(2) where is the mass ablation rate, mg mm-2 s-1; is the mass change of the sample before and after ablation, g; S is the ablation area of ablation surface, mm2; means the linear recession, mm/s; means the thickness change of the sample, mm; t is the ablation time, s. 3. Results and discussion 3.1. Microstructures of GO, RGO and sintered samples

To investigate the microstructures of as-prepared GO and RGO, SEM, AFM and TEM analyses were carried out prior to using as nanofillers. It is shown in Fig. 1(b) that the laminated structure of RGO is obvious after reduction reaction. TEM analyses indicate that the edge of RGO presents crimp structure and monolayer graphene is obtained. The diffraction pattern inserted in Fig. 2(d) shows that the conjugate structure of hexagonal ring has been restored. The AFM images provide the lateral dimensions and thickness of GO and RGO. As indicated, the thickness of GO and RGO are measured to be ~4.5 nm and ~0.95 nm, respectively, while showing a similar lateral dimensions in the range of 100 nm - 400 nm. Fig. 4 shows the surface and fracture morphologies of as-sintered SiBCN ceramics. Smooth surface of all investigated samples are observed after polishing and no visible cracks and pores are found on surface and fracture images. It also indicates homogenous dispersion of graphene throughout the matrix with various nanofiller loading. The conventional toughness mechanisms such as pull-out derived from fiber reinforced composites are observed. When the loading of nanofillers is low, the isolated islands of graphene are not enough for them to contact with each other in SiBCN matrix. However, situations become different when the content of graphene reaches a certain value. The nanofillers are uniformly distributed in SiBCN matrix after adequate dispersion method used, as a result, the graphene may contact with each other and form a "three dimensional structure" (not the 3-D structural graphene), as indicated in fracture morphologies. 3.2. Phases assessment and macro morphologies The XRD diffraction patterns of GPL 0%, 5% and 10% samples before and after ablation are shown in Fig. 5. The diffraction patterns before ablation are those of SiC and BNC, suggesting that Si and BN react with C during spark plasma sintering. After ablation, no obvious trace of crystalline oxide phases could be detected on the ablation surface of any samples. Only a weak cristobalite peak appears, indicating that the SiBCN oxidation mechanism is mainly active oxidation during ablation, as seen in Fig. 5. No peaks for B2O3 are seen likely because of the high B2O3 evaporation rate. Fig. 6 provides the macroscopic photographs of monolith and graphene reinforced SiBCN samples before and after ablation in an oxyacetylene combustion flame. The ablation surface has been peeled off after ablation test because of the thermal stress and gas fluxes scouring, as indicated in Fig. 6(d). However, a different morphology of GPL 5% sample could be seen in Fig. 6(e) compared with monolithic sample. There is a white and discontinuous oxide layer covering on the ablation center and no largescaled bareness and corrosion of graphene are found. The existence of oxide layer on ablation surface is consistent with a better ablation resistance. With the addition of graphene content reaching to 10 vol%, the ablation surface remains integral and keeps good adhesion to SiBCN matrix, even if the fast heating and cooling rates during ablation. During the ablation process, the width of torch is about 4-5 mm, and the temperature of different areas of ablation surface changes with different distance between the ablation zone and torch [42]. For GPL 10% sample, three distinct ablation regions (e.g., center region, transition region and heat-affect region) can be distinguished on the ablative surface. The central ablation region exhibits a black surface with ablation pits around, while the outermost layer (heat-affect area) covers an irregular white ring showing an outstanding protection for un-corrosion area. During the process of ablation and scouring, the initial oxide layer was relatively

inattentive, in that oxide particles were blown by the strong gas flow. Hence the oxides did not form a glassy and dense layer, resulting in ablation pits on the ablation surface of samples, as seen in Fig. 6. The GPL 10% samples survive at such high/cooling rate and maintain an integral structure, indicating desirable thermal shock resistance as well as ablation resistance. 3.3. Ablation morphologies The oxyacetylene torch test is very aggressive involving high temperature, high velocity gas flow and a severely oxidizing environment. The sample is vertically exposed to the flame and heated at rates of up to 500 oC s-1; moreover, the initial cooling rate is ~1000 oC s-1 on removing the flame [43]. Moreover, the temperature differences between the ablation center and edge can promote crack propagation and pores formation. Meanwhile, the ablation surface endures compressive stresses while the back of the sample will experience tensile stress that can induce catastrophic failure [18]. All the factors (operative temperature, heating/cooling rate and gas evaporation/scouring) contribute to different ablation surface and cross-section morphologies. Fig. 7 provides the surface microstructures of the monolith after ablation. Fig. 7(a) shows diverse morphological changes between ablation pit and transition area. In the ablation pit, a continuous amorphous layer forms on surface without pores or micro-cracks. This is mainly comprised of what appear to be grains 5 to 50 µm diameter. The XRD of this material shows the presence of some cristobalite; however EDS analysis of one of the grains suggests that it consists of Si, B, C, N and O elements. The formation of a dense SiO2 layer is an effective way to limit inward diffusion of oxygen to SiBCN matrix, thereby enhancing the oxidation resistance. However, when it comes to the transition area, the ablation surface is covered with rod-like structures that appear to be SiO2. The very large temperature gradient across the ablation region is likely responsible for the formation of rod-like SiO2 structures. The literature [44] indicates that temperature field distribution among other factors has a significant impact on final grain morphology, size and distribution, especially where steep temperature gradients occur across grain boundaries have an obvious influence on grain boundary migration. The temperature gradient, short ablation time and possibly B2O3-SiO2 glass viscosities all contribute to the final morphology in the transition area, all contributing to formation of the rod-like structures and also inhibit ablation resistance. Fig. 8 shows SEM cross-sections of different ablation regions. The monolithic SiBCN cross-section comprises three layers from the ablation pit to the substrate: outermost oxide layer, transition interface and SiBCN matrix. The outermost oxide layer remains relatively dense and continuous. EDS suggests an oxide layer consisting of SiO2, BN and residual carbon. However, many micro-cracks are observed on the bonding interface, caused by the thermal stress resulting from mismatch of the thermal expansion coefficients between the oxide layer and SiBCN matrix. Once a crack forms, it reduces the overall strength and stiffness of the structure and allows aggressive agents such as chlorides and oxygen to permeate faster and further into the matrix [45], thereby accelerating the erosion and reducing the utility of graphene reinforced SiBCN ceramics. The pores extant in the interface are caused by gas evolution, e.g. CO (g), SiO (g), N2 (g) and B2O3 (g), as seen in Fig. 8(c).

Fig. 9

illustrates the various surface morphologies of the GPL 5% sample in different ablation regions. Fig. 9(a) exhibits a distinct dividing line—a boundary between the ablation pit and transition area. A thin, amorphous oxide layer covers the ablation pit surface. In contrast, a highly porous layer forms in the transition area. The oxide layer formed in the ablation pit is permeated with small pores. The size, quantity and distribution of these pores vary with the distance away from the ablation pit to the transition area. According to the EDS analysis (not shown), the oxide layer on ablation pit contains Si, B, C, N and O elements, and the high content of residual B element suggests that B2O3 is still preserved after ablation test. Porous region is located around the transition area, as indicated in Fig. 9(c). This porous structure, on one hand, provides channels for oxidized gas such as O and H2O (g) to diffuse into the inner SiBCN matrix, which decreases the oxidation resistance of the sample. On the other hand, it can release the high vapor pressure gases from oxidation preventing catastrophic damage to the composites. In general, the existence of pores and cracks in inner matrix, a continuous oxidation process is inevitable. Fig. 10 presents the surface microstructures of GPL 10% sample after ablation, which exhibits a similar structure to those of GPL 5% sample. In the central ablation area, a relatively smooth and dense oxide layer with multiple spherical structures likely arising from a molten surface layer trapping gas bubbles is formed during oxidation processes. EDS studies show surfaces containing: Si, C, O and N. Combined with the XRD results, it makes clear that the spheres are composed primarily of SiO2 and small amounts of C likely from the residual SiC or partially oxidized graphene. Spherical particles form to minimize surface free energy. In details shown in Fig. 10(a), some open pores appear at the top of SiO2 spheres. It is obvious that some spheres are hollow and some holes are seen at the bottom of the spheres. The sizes of hollow spheres grow with the increasing gas pressure. When the gas pressure in SiO2 spheres exceeds the sum of ambient pressure and tensile stress of the oxide layer, the gas products could escape from the hollow spheres and gas pressure is released. Meantime, pores are formed at the top of the spheres and channels are left at the bottom, which leads to a continuous oxygen diffusion into inner matrix and decreases the anti-oxidation ability. The ablation center surface of GPL 5% samples tend to form fused and dense SiO2 layers with small scale pores uniformly distributed on surface, while a similar surface structure at the ablation center of GPL 10% sample generates spheres ranging from 20-70 µm. The different ablation center morphologies between GPL 5% and 10% may result from the different ablation surface temperatures as well as nanofiller loading. Fig. 9(c) and Fig. 10(c) show the surface morphologies of transition areas of GPL 5% and 10% samples after an oxyacetylene torch experiment for 10 s, respectively. Combination with EDS analysis, they exhibit a distinctly morphological characteristics. Fig. 9(c) shows a porous surface structure while Fig. 10(c) displays a relatively integral, dense and continuous layer with a small amount of micro pores in a range of 5-30 µm. The EDS results show that the transition areas of GPL 5% and 10% samples both contain Si, B, C, N and O elements. B2O3 and SiO2 in transition area should come from two aspects: oxidation of SiC and BNC at local area and flowing of low viscosity of B2O3 and SiO2 liquid by high speed gas scouring. As indicated in Fig. 11, the boundaries between transition area and heat-affect area of GPL 5% and 10% samples are very obvious. The transition area shows porous structure while the heat-affect area keeps smooth, dense and continuous for both

samples. For low surface temperature in heat-affect region, the oxidation process and oxidation rate in this zone should be slight and slow. As is known to all that gas evaporation at relatively low temperature and pores filled by high viscosity of SiO2 ineffectively could lead to porous morphology in heat-affect area, which is not matched with the morphologies shown above. The possible reason for this should attribute to the flowing of low viscosity of SiO2 from ablation center by high speed gas scouring, thus heals the pores impactful and covers on the heat-affect area surface. The glassy melt layer is comprised of SiO2 with few residual carbons, indicating the enrichment of SiO2 herein. Considering the morphologies and EDS results discussed above, some important conclusions can be drawn. Three distinct ablation regions (ablation center, transition region, heat-affect region) arise leading to well defined macro-morphologies. The surface morphologies of GPL 0%, 5% and 10% samples differ considerably from each other. The ablation center surface of all the samples remains relatively dense and continuous. For monolithic SiBCN ceramics, a continuous SiO2 layer forms on the ablation center surface without pores or micro-cracks, while some pores are observed in GPL 5% and 10% samples. The transition area shows porous structure while the heat-effect area keeps smooth, dense and continuous for both GPL 5% and 10% samples. An interesting result is that some rod-like structural SiO2 are formed on transition region of monolith, as shown in Fig. 7(c). The growth mechanism of rodlike morphology was likely oxide-assisted growth (OAG) and vapor-liquid-solid (VLS) mechanism [46-48]. In heat-effect region, a glassy SiO2 layer covers completely on oxide surface for graphene reinforced SiBCN ceramics. 3.4. Effect of graphene on ablation resistance From the macroscopic images of the ablation samples, we can make a conclusion that graphene nanofillers contribute to ablation resistance of SiBCN matrix. For the lateral sizes of graphene used in current work are in a range from 100 - 400 nm [31] and oxide layer covers on the ablation surface, it is difficult for us to observe whether or not graphene remains on the surface after ablation. However, the Fig. 13 SEM images reveal the presence of multilayer graphene near to the ablation surface in fracture morphologies. During the ablation process, the graphene with smaller sizes are most prone to oxidation and erosion, while larger sizes and multilayer forms are much less like to be completely destroyed. Based on these above analyses, it can be inferred that graphene plays double roles during ablation. On one hand, graphene shows a degree of chemical inertness, thus it not only protect the underlying matrix from being oxidizing, but also remains its structure unchanged depending on its location and size [30]. On the other hand, pores are left behind due to the escape of gaseous products, which may accelerate oxygen diffusion rapidly into inner matrix. Consequently, the ablation resistance and anti-oxidation ability will decrease compared with the monolith. To a certain extent, pores or channels could be healed by filling of low viscosity SiO2 and B2O3, therefore the oxidation rate of SiBCN matrix will slow down. The ablation samples were prepared by spark plasma sintering and the densification behavior of graphene reinforced SiBCN ceramics was depended on the increasing axis pressure from the graphite indenters. So the distributed orientation of graphene in SiBCN matrix was vertical to the ablation direction of the flame. In our previous research [31], the microstructures and phases are very similar for all investigated

samples. The as-prepared ceramics are composed of β-SiC, BNC and a small amount of α-SiC. The graphene mainly distributes among the BNC gathering area, which hinders the grain growth effectively. Furthermore, BNC enwraps SiC in the grain boundary, blocks the atomic diffusion and being responsible for the SiC grain growth. So graphene acts as good thermal barrier and retards heat gas flow and transfer, leading to reduce heat attack to SiBCN matrix. Graphene uniformly distributes in SiBCN matrix and forms a "three-dimensional structure" would enhance the thermal conductivity and electrical conductivity of SiBCN matrix. This would decrease the surface temperature and reduce temperature gradient in inner matrix, which increases the thermal shock resistance as well as ablation resistance. In addition, the addition of graphene improves the mechanical properties of SiBCN matrix, and pull-out, crack deflection and bridging toughening mechanisms also contribute to thermal shock resistance and ablation resistance. In comparison to Cf and SiCf nanofillers, graphene with special nanostructure is considered as most effective reinforcing materials in composites because of good connectivity with ceramic matrix. The two main factors responsible for this are as follows [49]. (1) Surface area: The nanofillers with a sheet of belt-like geometry have lager interfacial contact area with ceramic matrix than that of the nanofillers with a cylinder geometry and the same cross-sectional area. (2) Geometry: In comparison to fiber nanofillers with cylinder geometry, it is easier for ceramic matrix to adhere to nanofillers with a sheet or belt-like geometry. 3.5. Ablation mechanism During ablation, the graphene reinforced SiBCN ceramics were eroded by mechanical corrosion of high speed gas flow, chemical reaction, such as oxidation and thermal-physical corrosion, for example boiling and sublimation of component [50]. According to literature [36], graphene, SiC and BNC could react with oxygen, and the following reactions may occur during ablation test. As discussed in previous report [18], the combustion products of the oxyacetylene torch comprise of 43.02 mol% O2, 15.38 mol% CO2, 12.04 mol% CO, 10.75 mol% O, 8.45 mol% OH, 7.83 mol% H2O, 1.77 mol% H and 0.76 mol% H2. Due to the lack of relevant thermodynamic data of BNC phase which consists of t-BN, t-carbon and B doped t-carbon layer, we decide to calculate the reactions between BN, carbon and oxygen instead of BNC phase. Meanwhile, reactions between graphene and oxygen are replaced by carbon and oxygen.

The changes of Gibbs free energy and reaction enthalpy for reactions [18, 51-54] occurred during ablation test are shown in Fig. 14. All the Gibbs free energy changes of reactions displayed in Fig. 10 are negative in a range of 300 ~ 3300 K except reaction (9). We believe that the calculations of thermodynamic reactions also make contribution to explain the morphological evolution and ablation mechanisms of graphene reinforced SiBCN ceramics. The thermodynamic data of reactions taken place at 2700 K and 3300 K are summarized in Table 2. The mainly expected reactions during ablation process are reactions (4), (8), (7) and (10), indicating that carbon will act with the oxidizing species at first time, followed by SiC and BN. The solid BN starts to be oxidized and forms B2O3 layer with gaseous B2O3 and N2 evaporation at low ablation temperature according to reactions (10) and (11), meanwhile, SiC is changed into SiO2 oxide layer and CO gas on the basic of reactions (7) and (8). The exothermic reactions discussed above could release heat on ablation regions, which may weaken the corrosion resistance of SiBCN ceramics in an oxyacetylene torch environment. To clarify the ablation mechanisms of graphene reinforced SiBCN ceramics, a basic oxidation behavior consisted of phases, composition and morphologies changes should be understood. However, there will be much different between ablation process and oxidation behavior. Oxidation experiments are usually executed at relatively low temperature for long time while ablation test is a complicated procedure including thermal-physical, thermal-chemical and mechanical scouring process, during which the ablation center temperature is up to as high as ~2400 oC for short time. In the initial stage of oxidation at low temperature (~ 900 oC), BNC will be oxidized to form B2O3 on surface but is evaporated rapidly, and a small amount of SiC react with oxygen to shape SiO2. However, the SiO2 will react with B2O3 to form borosilicate glass before the end of B2O3 volatilization [36]. In this step, the oxidation of SiBCN matrix is the main ablation mechanism. Because of gaseous products, the initial surface morphology of graphene reinforced SiBCN sample is comprised of substantial pores and micro-cracks. The high viscosity of SiO2 could not fill the pores and cracks effectively. Besides, the continuous diffusion of oxygen will greatly thread the existing borosilicate layer easily [55]. Thereafter, the borosilicate structure in oxide layer would be destroyed completely with the volatilization of B2O3 at elevated temperature. With the increasing oxidation temperature, the residual B2O3 and SiO2 will be scoured by the high speed gas for their relatively low viscosities. Reactions between graphene and oxygen happen at all ablation temperatures and remain a relatively high oxidizing rate based on the thermodynamic date. With the ablation time exceeding, the surface temperature of ablation center climbs to the value of ~ 2400 oC, which is much higher than SiO2 melting point (1723 oC) [56]. The effective protection of dense and continuous SiO2 oxide layer will be weakened significantly

due to gas scouring and evaporation of SiO. However, the new oxide layer would be generated by the oxidation of inner SiBCN matrix. During the ablation process, the formation of the protective oxide layer is a dynamic equilibrium process [30]. The oxide layer plays two contributions during ablation. Contribution comes from effective retard of oxygen diffusion into matrix. Another contribution roots in heat loss by gas evaporation. However, only when the generation rate of oxide is higher than the denudation rate, the oxide layer can play a positive role during ablation. At the final ablation procedure, a relative dense SiO2 oxide layer containing a few B2O3 could be obtained. Furthermore, the introduction of graphene as nanofiller has two effective ways to enhance the ablation resistance of SiBCN marix. On one hand, the formation of a "three-dimensional structure" of graphene could result in a high thermal conductivity of ceramic composites due to its higher thermal conductivity than SiBCN matrix. Besides, the increasing thermal conductivity could reduce the surface temperature of ablation region during ablation, thereby improving the ablation resistance distinctly. On the other hand, the addition of graphene is evidently favorable to enhance the thermal shock resistance, therefore improve the configurational stability of SiBCN ceramics under extreme environment. In summary, the existence of a protective oxide layer on the ablation surface of all investigated samples is beneficial to the improvement of ablation resistance. The ablation centers of all samples are covered by a relatively dense and continuous SiO2 layer along with a few pores. A small amount of B2O3 oxide is still remained in ablation center after ablation test. For relatively low ablation temperature in transition region, B2O3 and SiO2 tend to form B2O3-SiO2 oxide layer with a great amount of pores distributed around. Therefore, a porous structure of B2O3-SiO2 oxide layer is formed due to gases evaporation in transition region. For the heat-affect area is far away from the ablation center, the ablation temperature and pressure are lowest than other regions, thus a fused and glassy SiO2 is covered integrally in heat-affect area which could protect the inner matrix from being oxidized. Base on the above analyses, it can be confirmed that the main ablation mechanisms of graphene reinforced SiBCN ceramics are co-effected by thermal physical, chemical corrosion and mechanical denudation. After ablation test, the surface microstructures of various ablation regions of graphene reinforced SiBCN ceramics can be divided into three layers: dense SiO2 layer, porous B2O3-SiO2 layer and glassy SiO2 layer as illustrated by the schematic diagram in Fig. 15. 3.6. Ablation properties The ablation properties of graphene reinforced SiBCN ceramics are listed in Table 3. It can be seen that GPL 10% sample exhibits a better ablation resistance than others. The mass and linear ablation rates of GPL 10% sample are 0.0121 ± 0.0001 mg/mm2 s and 0.0071 ± 0.0001 mm/s, respectively. Under the same ablated atmosphere, the mass and linear ablation rates of GPL 5% sample are 0.0185 ± 0.0001 mg/mm2 s and 0.0092 ± 0.0001 mm/s, respectively. The addition of graphene enhances the ablation resistance of SiBCN ceramics compared with the monolith. GPL 10% sample shows a similar contribution on ablation resistance as well as SiCf/SiBCN composites, but do much better than that of Cf/SiBCN composites. Compared with the SiCf/Cf/SiBCN ceramic composites, the linear ablation rate of GPL 10% sample is lower than the former, but the mass ablation rate shows an opposite result.

4. Conclusions Graphene reinforced SiCBN ceramics were prepared by spark plasma sintering. The phase transformation, composition changes, microstructural evolution and ablation mechanisms of graphene reinforced SiCBN ceramics were comprehensively investigated by applying in an oxyacetylene flame for 10 s. During the ablation test, a protective oxide layer consisted of SiO2 forms on the ablation center surface. A porous structure of B2O3-SiO2 oxide layer is produced on transition region due to gases evaporation, but a fused and glassy SiO2 layer spreads on heat-affect area. The incorporation of graphene into SiBCN matrix reveals a positive effect on ablation properties of ceramics. The graphene adopted in this work with small size are usually prone to be erosion and oxidation, while that with larger size in aggregated morphology may be partially ablated. The graphene with a degree of chemical inertness could protect the untouched matrix from being oxidized. Besides that, the structure of overlapped graphene near to the oxide layer maintains unchanged, indicating a relatively stable structure at such ablation condition. Because of the higher thermal conductivity and "three-dimensional structure" of graphene formed in matrix, the ablation properties of graphene reinforced SiBCN ceramics exhibit a better performance than monolith. In a whole, the graphene in SiBCN matrix acts as good thermal barrier and retards the ablation corrosion of oxyacetylene combustion flame due to its uniform distribution in matrix, size dimensions and intrinsic structure. Surface morphologies of ceramic composites after ablation test could be divided into three layers: dense SiO2 layer, porous structure consisted of SiO2-B2O3 and fused SiO2 rich layer. Four ablation actions contribute to the various ablation morphologies: (1) oxidation of graphene and SiBCN matrix; (2) denudation of graphene in ablation region; (3) evaporation of gaseous products; (4) flowing of low viscosity SiO2 and B2O3 by high speed gases scouring. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC, Grant number 51072041, 50902031 and 51021002) and the National Science Foundation for Distinguished Young Scholars of China (Grant number 51225203). The authors thank Richard M. Laine (university of Michigan, USA) for improving the language use. References [1] M.C. Bechelany, C. Salameh, A. Viard, L. Guichaoua, F. Rossignol, T. Chartier, S. Bernard, P. Miele, Preparation of polymer-derived Si–B–C–N monoliths by spark plasma sintering technique, J. Eur. Ceram. Soc. 35 (2015) 1361-1374. [2] R. Riedel, L. Ruswisch, L. An, R. Raj, Amorphous silicoboron carbonitride ceramic with very high viscosity at temperatures above 1500 oC, J. Am. Ceram. Soc. 81 (1998) 3341–3344. [3] P.A. Ramakrishnan, Y.T. Wang, D. Balzar, L. An, C. Haluschka, R. Riedel, A.M. Hermann, Silicoboron–carbonitride ceramics: a class of high-temperature, dopableelectronic materials, Appl. Phys. Lett. 78 (2001) 3076–3078. [4] M. Christ, G. Thurn, M. Weinmann, J. Bill, F. Aldinger, High temperature

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Table 1 Composition design and sintering parameters of the investigated materials.

Composition design (volume fraction) Sample

Sintering parameters (SPS) Graphene

SiBCN powder

GPL 0%

0%

100%

1800 oC/40 MPa/5 min

GPL 5%

5%

95%

1800 oC /40 MPa/5 min

GPL 10%

10%

90%

1800 oC/40 MPa/5 min

Table 2 Thermodynamic data of reactions taken place at 2700 K and 3300 K during the ablative process by Factsage. The Thermodynamic data of reactions involved O2 are in terms of kJ/mol of O2 in order to make the thermodynamics of these reactions comparable.

Gibbs free energy change, △G0

Reaction enthalpy change, △H0

(kJ/mol)

(kJ/mol)

Reaction equation

2700 K

3300 K

2700 K

3300 K

(3)

-396.16

-395.30

-398.93

-401.47

(4)

-686.99

-783.09

-249.38

-260.57

(5)

-198.20

-175.89

-299.49

-297.61

(6)

-439.96

-470.93

-297.75

-303.94

(7)

-493.41

-466.54

-615.81

-612.85

(8)

-607.27

-702.65

-172.39

-184.72

(9)

-341.58

-526.11

+1130.28

+1307.30

(10)

-462.58

-487.05

-241.07

-243.92

(11)

-306.00

-298.86

-376.93

-380.43

Table 3 Mass ablation rate and linear ablation rate of graphene reinforced SiBCN ceramics after exposing to oxyacetylene combustion flame for 10 s. The scatter bands are standard deviations calculated based on experimental results of three separate samples.

Sample

Mass ablation rate a (mg/mm2 s) Linear ablation rate a (mm/s)

a

GPL 0%

0.0326 ± 0.0003

0.0125 ± 0.0002

GPL 5%

0.0185± 0.0001

0.0092 ± 0.0001

GPL 10%

0.0121 ± 0.0001

0.0071 ± 0.0001

SiCf/SiBCN [18]

0.0134

0.0237

Cf/SiBCN [18]

0.0215

0.0455

SiCf/Cf/SiBCN [25]

0.0061

0.0523

These numbers are the mean taken over the entire crater.

Fig. 1. Surface morphologies of (a) GO and (b) RGO.

Fig. 2. TEM and HRTEM images of (a), (b) GO and (c), (d) RGO. Insets are the selected area electron diffraction (SAED) patterns corresponded to (a) and (d), respectively.

Fig. 3. Tapping mode AFM topographic images and height profiles of (a) GO and (b) RGO.

Fig. 4. SEM images of the surface and fracture morphologies of (a), (d) GPL 0% sample; (b), (e) GPL 5% sample; (c), (f) GPL 10% sample.

Fig. 5. XRD patterns of graphene reinforced SiBCN ceramics before and after ablation test.

Fig. 6. Macroscopic photographs of monolith and graphene reinforced SiCBN ceramics before and after ablation test; (a), (d) GPL 0% sample; (b), (e) GPL 5% sample; (c), (f) GPL 10% sample.

Fig. 7. Surface morphologies of monolithic SiBCN ceramics under an oxyacetylene torch experiment in different ablation regions: (a) morphology between the center area and transition area; (b) magnified SEM image of square in (a); (c) magnified SEM image of circle in (a).

Fig. 8. Cross-section morphologies of monolithic SiBCN ceramics under an oxyacetylene torch experiment in different ablation regions: (a) boundaries between ablation center A, affected region B and untouched SiBCN matrix C; (b) magnified SEM image of ablation center A in (a); (c) magnified SEM image of affected region B in (a).

Fig. 9. Surface morphologies of GPL 5% sample under an oxyacetylene torch experiment in different ablation regions: (a) morphology between the center area and transition area; (b) magnified SEM image of ablation center; (c) magnified SEM image of transition area.

Fig. 10. Surface morphologies of GPL 10% sample under an oxyacetylene torch experiment in different ablation regions: (a) morphology between the center area and transition area; (b) magnified SEM image of ablation center; (c) magnified SEM image of transition area.

Fig. 11. Surface morphologies of (a)-(c) GPL 5% and (d)-(f) 10% samples under an oxyacetylene torch experiment in different ablation regions: (a), (d) morphologies between the transition region and heat-affect region; (b), (e) magnified SEM images of transition region; (c), (f) magnified SEM images of heat-affect region.

Fig. 12. Cross-sectional SEM images and EDS results of GPL 10% sample after ablation test: (a) low magnification of SEM image; (b) large magnification image of dense SiO2 layer; (c) corresponded EDS result of area “1” in (a); (d) corresponded EDS result of area “2” in (b).

Fig. 13. Fracture morphologies of GPL 5% and 10% samples near to the oxide layer after exposing to an oxyacetylene torch experiment for 10 s: (a) GPL 5% sample; (b) GPL 10% sample.

Fig. 14. Gibbs free energy changes and enthalpy changes of reactions (3)-(11) as a function of the temperature during ablation process.

Fig. 15. Schematic diagram of graphene reinforced SiBCN ceramics during ablation process.