QI Man, YANG Wei, LI Zhuangzhuang, HUANG Zhixiong, WANG Yanbing

(Key Lab of Advanced Technology for Specially Functional Materials, Ministry of Education; School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China)

Abstract: TiSi2 reinforced boron phenolic composites (TP) and Vitreous silica fabric reinforced TiSi2/boron phenolic composites (VTP) were prepared by compression molding, and their thermal, mechanical, ablation properties were studied. TG results show that thermal stabilities and residual carbon rate of boron phenolic are improved after introducing TiSi2 particles. Compared with VTP-0 (containing 0 phr TiSi2), flexural strength of VTP-60 (containing 60 phr TiSi2) pyrolysis product increases by 29.5% at 1 200 ℃. Raman spectrum shows that TiSi2 particles promote the ordering of the glass carbon structure of VTP pyrolysis product. Compared to VTP-0, the linear and mass ablation rates of VTP-60 reduce by 32.1% and 77.5%, respectively. XRD and SEM indicate the formation of an oxide coating layer, TiO2-SiO2, integrates the bulk and protects the underlying materials from damage under high temperature oxygen-containing airstream. All these results prove that mechanical properties of pyrolysis product, thermal, and ablation resistance are improved by addition of TiSi2 particles.

Key words: TiSi2; boron phenolic resin; thermal stability; ablation; eutectic

1 Introduction

It is an immense challenge for thermal protection systems (TPS) to maintain its thermal protection function[1-3]. The heat-shield material is a key factor in the design of TPS for planetary probes and space vehicles[4-6]. Boron phenolic resin(BPR) has high carbon residual ratio and it’s widely used as the matrix of TPS materials[7-10]. When subjected to high-impact, high-shear, high temperature oxygen-containing airstream, pyrolysis product of BPR, glass carbon, is easily oxidized into CO2and CO[11,12]. During pyrolysis, a low-melting glass phase, B2O3, will be formed[13,14], leading to oxidation resistance of glass carbon. However, at high temperature, B2O3volatilizes severely, and the glass carbon will be exposed to oxygen-rich atmosphere and be oxidized, then provide no protection for underlying materials. Therefore, it’s necessary for BPR to be reinforced by fabric. Vitreous silica fabric has oxidation resistance and can melt into SiO2to cover the glass carbon at high temperature. Nevertheless, there are still lots of pores left inside pyrolysis product at high temperature.

In recent decades, researches have focusing on ceramics/phenolic composites with improving high-temperature mechanical properties[15-17]. Ding Jieet al[15]found that the flexural strength of carbon/phenolic composites was increased by 148.2% by addition of 20wt% TiB particles at 1 000 ℃. Wang Set al[16]observed that both the linear ablation rates and back-face temperatures were reduced by introducing ZrC into C/Ph composites, because of the formation of ZrO2layer on the ablation surface. Yang Weiet al[17]demonstrated that the bending strength of vitreous silica fabric/BPR composites pyrolysis product at 1 400 ℃ was enhanced by introducing MoSi2. In general, these ceramic fillers could convert into higher enthalpy of evaporation and higher viscosity phase at high temperature.

TPS materials should be equipped with high strength, low density, good thermal and ablation resistance. TiSi2particles are widely used in the fields of integrated circuit and high temperature materials for high temperature resistance and thermal stability. TiSi2particles can form SiO2and TiO2, high enthalpy and high viscosity ceramic phase at high temperature. To improve thermal stability and ablation resistance, we tried to modify BPR with TiSi2particles. The formed SiO2phase was the same as the constituent of vitreous silica fabric. Thus there would be good interface bonding between fabrics and BPR at high temperature. Then the effect of TiSi2on mechanical properties, the microstructure and ablation behavior of TP and VTP were investigated.

2 Experimental

BPR was obtained from Shanxi Taihang Flame Retardant Polymer Company. Ethanol was from Wuhan Xinshen Test Chemical Technology Co. Ltd. Vitreous silica fabric (surface density of 248.3 g·m-2, content of silica ＞96%) was used as the reinforcement of composites, manufactured by Shaanxi Huate Glass Fiber Material Group Co. Ltd. TiSi2particles (average particle size of 10 μm, purity ＞95%) were supplied by Guangzhou Nano Chemical Technology Co. Ltd.

All samples were prepared by compression molding. Fabrication process of TP was as follows. BPR and TiSi2particles were mixed in a ball grinding mill for 1 h. Then the mixture was compression molded according to the curing parameter in Fig.1.

The VTP were prepared as follows. Firstly, TiSi2particles was dispersed in BPR solution (50wt% in ethanol), then stirred by magnetic stirrer for 1 h at room temperature (formula in Table 1). Secondly, the mixed solution was evenly immersed on the vitreous silica fabric. Then the prepreg was dried at room temperature for 48 h. Thirdly, the prepreg was cut into a proper size along the warp and weft direction of the fiber fabric, stacked according to the standard thickness size, and then put into the mold. Finally, the composites were molded according to the cure parameters shown in Fig.1.

Table 1 Formula of composites

Fig.1 Curing parameters of BPR composites

The thermal stability of BPR, TP, and VTP was investigated by TG analyzer (STA 449F3, NETZSCH) at heating rate of 10 ℃/min from room temperature to 1 200 ℃ under air atmosphere. VTP were pyrolyzed in the muラe for 20 min at 600 ℃, 800 ℃, 1 000 ℃, 1 200 ℃ under air atmosphere. Then flexural strength of VTP pyrolysis product at 1 200 ℃ was tested by electronic universal testing machine at ambient temperacture(Instron 5967). And pyrolysis product of samples were analyzed by XRD (D8 advance, Bruker AXS) and Raman spectrum (InVia, RENISHAW). Raman measurements were carried out over the range 800-2 000 cm-1. Standard Gibbs free energy (ΔG) of reactions was from database of HSC chemistry and used to determine whether reactions occurred. Then ablation performance of VTP was operated at the oxyacetylene test. Fig.2 shows oxyacetylene ablation test device. The samples were cut into a size of 30 mm in diameter and 10 mm in height and exposed to the torch for 30 s. Then microstructure images of ablated samples were made by FESEM (JSM-7500F, JEOL).

Fig.2 Diagram of oxyacetylene ablation device

3 Results and discussion

3.1 Thermal gravity

Fig.3 shows TG and DTG curves of the BPR and TP under air atmosphere. In Fig.3(a), during 100-400 ℃, less than 10% weight loss of BPR and TP is mainly due to dehydration reaction[3]among the remaining monomers in BPR. This indicates that the addition of TiSi2seldom influences the initial pyrolysis process of BPR.

Fig.3 TGA (a), DTG (b) curves of BPR and TP under air atmosphere

In the period of 400-800 ℃, TP has the largest degradation rate in the whole process. And BPR also has a larger mass loss rate than before. The weight loss of the composites is mainly attributed to severe cracking and carbonization of the BPR, accompanied with the products of CO, CO2, H2O, B2O3, and glass carbon[18]. Likewise, it may be accompanied by the volatilization of B2O3with low melting point (450 ℃[14]). The carbon residual ratios (CRR) of BPR and TP is 36.93% and 62.31% at 800 ℃, respectively. Moreover, it is obvious that TiSi2particles enhance the thermal stability of TP significantly.

At 800 ℃-1 200 ℃, the CRR of BPR shows a downward trend. At 1 200 ℃, it is only 9.41%. By contrast, the trend of curve of TP weight loss is gentle, even appearing a small weight increment(4.74%). The pyrolysis product of BPR, glass carbon, oxidizes into CO2[18,19], which leads to the weight loss of BPR. However, after addition of TiSi2, TiSi2preferentially reacts with O2and forms SiO2and TiO2. That ceramic phase, SiO2and TiO2, adhere to glass carbon and provides protection. These may account for the small weight increment.

3.2 XRD analysis

Fig.4 shows XRD patterns of BPR and TP pyrolysis product at 1 200 ℃ for 20 min under air atmosphere. In Fig.4, there is no new crystal phase in BPR pyrolysis product. After adding TiSi2, besides the diffraction peaks of TiSi2, both TiO2and SiO2crystal phases appear.

Fig.4 XRD patterns of BPR and TP pyrolysis product at 1 200 ℃ under air atmosphere

Fig.5 presents XRD patterns of VTP-60 pyrolysis product at different temperatures. Both crystal phases of TiO and TiO2appear at 600 ℃. We speculate that TiSi2and O2react to produce TiO2, as shown in reaction (1), and incomplete oxidation product TiO simultaneously, as reaction (2). Or TiO is products of the oxidation-reduction reaction between TiO2and CO, the pyrolysis product of BPR (reaction (3)):

Fig.5 XRD patterns of VTP-60 pyrolysis product at different temperatures

In addition, all the diffraction peaks of decomposed composites at 800 ℃, 1 000 ℃, and 1 200 ℃ are the same,i e, TiSi2, TiO2, and SiO2. However, the diffraction peaks of TiO disappears. This is caused by TiO reacting with O2in the air and forming TiO2during 600 ℃ to 800 ℃, as shown in reaction (4):

Combined with Fig.3, there is an increment on weight of TP during 800 ℃ to 1 200 ℃. We infer that TiSi2preferentially reacts with O2. Then ceramic phase, TiO2-SiO2, is gradually formed on the surface of VTP. Then diffusion of O2is postponed. The oxidation-reduction reaction may occur between TiSi2and CO2,pyrolysis product of BPR, as reaction (5) shown. Meanwhile, more amorphous carbon is produced in the bulk:

To determine whether reactions occurred，we obtained thermodynamic parameters from the database of HSC software. Table 2 shows Gibbs free energy (ΔG) of reactions (1)-(5). Only ΔGof reaction (3) is greater than 0. It means that reaction (3) hardly proceed spontaneously and forwardly at 600 ℃, which proves that the production of TiO is the incomplete oxidation product of TiSi2. Thus reactions (1), (2), (4), and (5) can be determined.

Table 2 ΔG of reactions (1)-(5)

3.3 Mechanical properties

Fig.6 shows flexural strength of VTP. In Fig.6(a), flexural strength decreases with increasing of TiSi2content. In Fig.6(b), The flexural strength of VTP-0 and VTP-60 pyrolysis product are 12.56 MPa and 16.27 MPa at 1 200 ℃, respectively. There is a 29.5% increase compared with VTP-0. Besides, TiSi2particles improve the mechanical properties of VTP pyrolysis product at 1 200 ℃.

Fig.6 Flexural strength of VTP pyrolysis product at (a) 25 ℃ and (b) 1 200 ℃

Compared with VTP at ambient temperature, BPR decomposed at 1 200 ℃ with poor adhesion among fibers. Thus flexural strength of VTP-0 dropped drastically. When introduced TiSi2particles, the formed eutectic TiO2-SiO2at 1 200 ℃ becomes the new adhesion agent and integrates the bulk. Therefore, flexural strength is improved. To certify that, we observed surface SEM of VTP-60 pyrolysis product at different temperatures.

Fig.7 shows surface FESEM of VTP-60 pyrolysis product at different temperature. It is evident that BPR, initial covering layers of fibers, gradually decomposed into small molecules during room temperature to 600 ℃. In the meanwhile, the cracks among fibers occur and gradually extend. Until 800 ℃, ceramic phases appears and BPR disappears. After pyrolysis at 1 200 ℃, the growth of grains adhere to the surface of fibers are apparent. And most cracks among fibers were filled with TiO2-SiO2eutectic. This eutectic postponed the diffusion of oxygen and protected the underlying materials from the structural damage, leading to the improvement of flexural strength of VTP. However, combined with Fig.6, it’s easy to find that VTP-60 pyrolysis product is higher than that of VTP-80 in flexural strength at 1 200 ℃. TiSi2have good thermal resistance. When pyrolyzed at 1 200 ℃, only small parts of TiSi2particles react and form SiO2-TiO2, the remanent TiSi2particles will fill inside pores after pyrolysis. That may improve mechanical properties of VTP. However, when TiSi2content further increases, the extra TiSi2will exist among fibers and new adhesion agent, SiO2-TiO2. That may result in concentrates stress and microscopic cracks. Therefore, compared to VTP-60, flexural strength of VTP-80 decreases.

Fig.7 Surface FESEM images of VTP-60 pyrolysis product at (a) 25 ℃; (b) 600 ℃; (c) 800 ℃; (d) 1 200 ℃

3.4 Raman analysis

Fig.8 shows ID/IG values and graphite microcrystalline size (La) of VTP pyrolysis product at 1 200 ℃. The decrease of ID/IG values and increase of La mean graphitization extent of glass carbon increases.

Fig.8 Raman spectra of VTP pyrolysis product differing in TiSi2 content at 1 200 ℃

As shown in Fig.8, it is apparent that La gradually increases with TiSi2content increasing. It shows that the addition of TiSi2promotes the ordering of glass carbon structure of VTP pyrolysis product, resulting in improving thermal resistance and mechanical properties of glass carbon.

3.5 Photographs of ablated surface

Fig.9 shows photographs of the VTP after oxyacetylene torch testing for 30 s. Extensive fiber exposure was observed on the surface of VTP-0, while the surface of VTP-60 was covered by ceramic phase TiO2-SiO2. For further comparison, the linear ablation rate (LAR) and mass ablation rate (MAR) are computed after ablation. LAR and MAR can be calculated by the following formulas (6) and (7)[10]. LAR is defined as loss of thickness divided by time. In equation (6),L0means the original thickness of samples andL1means the length from the lowest point on the ablation surface to the bottom of samples.

Fig.9 Photographs of the VTP after oxyacetylene torch testing for 30 s: (a) VTP-0; (b) VTP-20; (c) VTP-40; (d) VTP-60; (e) VTP-80

Table 3 shows MAR and LAR of VTP. It indicates that both MAR and LAR of VTP-60 composites are lowest. Compared with VTP-0, MAR, and LAR of VTP-60 reduce by 77.5% and 32.1%, respectively. With TiSi2particles content further increasing, both MAR and LAR of VTP-80 increase. It means that the content of BPR reduces with TiSi2content increasing, so there is a lack of sufficient matrix bonding layer. When exposed to oxygen at high temperature, there is not enough matrix to convert into glass carbon as structural support. Likewise, the formation of TiO2-SiO2is a gradual process. When ablated for 30 s, there may be lack of glass carbon and TiO2-SiO2. As a result, partial fibers is exposed and further corroded by the hot oxygen flow. Therefore, both MAR and LAR increase. The density of VTP-0 and VTP-80 are 1.391 g/cm3and 1.703 g/cm3, respectively. Theoretically, combined with LAR, the MAR of VTP-80 should be lower than that of VTP-0. In fact, short of matrix bonding layer and protection layer, TiO2-SiO2, there might be porous structure inside VTP-80 composites after oxyacetylene torch testing. Thus the MAR of VTP-80 is similar with that of VTP-0.

Table 3 MAR and LAR of VTP

Fig.10 presents SEM images of VTP-0 and VTP-60 after oxyacetylene torch test. It is obvious that there are many pores on the surface of VTP-0, while ceramic phase fills pores on the surface of VTP-60. Combined with XRD analysis, we presume that ceramic phase of VTP-60 is a eutectic composed of TiO2-SiO2. Therefore, ablation resistance of VTP improves.

Fig.10 SEM images of VTP after oxyacetylene torch test at 2 000 ℃ for 30 s: (a) VTP-0; (b) VTP-60

4 Conclusions

TP and VTP were prepared by compression molding. Then mechanical properties, thermal and ablation resistance of them were investigated by XRD, TG, electronic universal testing machine, Raman spectrum, and oxyacetylene torch test. TG shows that the addition of TiSi2remarkably improves the thermal stability and residual carbon ratio of BPR. At 1 200 ℃, VTP can produce a visible protective layer, eutectic ceramic phase of TiO2and SiO2. The ceramic phase can protect the interior materials from structure damage. Compared to VTP-0, flexural strength of VTP-60 pyrolysis product increases by 29.5% at 1 200 ℃. Raman spectrum further confirms the ordering of glass carbon structure with TiSi2loads increasing, leading to improved mechanical properties and thermal resistance. MAR and LAR of VTP-60 reduce by 77.5% and 32.1% than them of VTP-0, respectively. In general, VTP-60 shows better performances on ablation resistance, mechanical properties than other VTP. When TiSi2content further increases, there is no significant enhancement on ablation resistance and mechanical properties, while density of composites will increase. Thus we can conclude that TiSi2can improve thermal and ablation resistance, mechanical properties of BPR.