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C–SiC Composites in Two Heat Fluxes
Journal of Materials Science & Technology xxx (2015) 1e10
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Effect of SiC Location on the Ablation of C/CeSiC Composites in Two Heat Fluxes Lei Liu, Hejun Li*, Kui Hao, Xiaohong Shi, Kezhi Li, Chang Ni State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
a r t i c l e i n f o Article history: Received 25 May 2014 Received in revised form 23 October 2014 Accepted 17 November 2014 Available online xxx Key words: C/C composites SiC Distribution Ablation performance
How layer-segregated distribution of SiC affects the ablation of C/CeSiC composites was studied in the present work. A certain amount of SiC particles was deposited at the non-woven (C/CeSiC-1) and web (C/CeSiC-2) layer of 2D needle-punched carbon fibre fabric reinforced pyrocarbon composites, respectively. Ablation under oxyacetylene torch demonstrated that the two composites have similar ablation rates in heat flux of 2.38 MW/m2 whereas ablation rates of C/CeSiC-2 were much higher than those of C/CeSiC-1 when heat flux was 4.18 MW/m2. SiO2 covered partially the defective surface of both composites in the lower heat flux. The different SiC locations induced distinct defects and then led to the two composites' dissimilar ablation rates in the higher heat flux. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
1. Introduction Ultra high temperature ceramics (UHTC) and SiC doped[1e4] or coated[5e7] carbon/carbon (C/C) composites for the purpose of improving ablation properties have been largely reported in the last several years. Coating was considered to be a short-term protection for C/C composites against ablation while introduction of UHTC (or SiC) into C/C composites was a promising method to improve the ablation resistance thoroughly. Among these researches on the introduction of UHTC (or SiC), different contents and various kinds of UHTC (or SiC) were firstly attempted and it has been proved that C/C composites containing higher content[8e10] and appropriate kinds[11,12] of introduced phases had better ablation property. Afterward, the relative content of introduced phases was found to play a key role in the improvement[13e15]. Recently, the influences of the density of C/C composites and the architecture of fibre fabric on the following adding of UHTC (or SiC) and the ablation property were studied. Chen et al.[16] found that the pyrocarbon amount in preform was important to the densification, mechanical and ablation properties of C/ZrC composites. Zeng et al.[17] discovered that samples with chopped web needled preform displayed better ablation resistance than the samples with needle-integrated and
* Corresponding author. Prof., Ph.D.; Tel.: þ86 29 88495004; Fax: þ86 29 88492642. E-mail address: [email protected] (H. Li).
fine-weave pierced preforms in the case of being infiltrated by ZreTieC through reactive melt infiltration. Moreover, it has been proved that a moderate content of SiC interphase could improve the ablation property of C/ZrC composites while excessive SiC interphase would play an adverse role[18]. Based on these works, how to further improve the ablation resistance of C/C composites with the same fibre architecture and carbon matrix through a certain amount of UHTC should be an attractive new subject. 2D needle-punched carbon fibre fabric, fabricated by repeatedly overlapping the layers of 0 non-woven fibre cloth, chopped fibre web and 90 non-woven fibre cloth with needle-punching step by step, has been widely utilized in the investigation of UHTC (or SiC) modified C/C composites[19e21]. Since the non-woven layer and the web layer have distinct characteristics, effect of the layersegregated distribution of introduced ceramics on the ablation is necessary to be investigated. In other words, one modified C/C composite with non-woven layer or web layer containing UHTC (or SiC) should have different properties. However, to the authors' knowledge, how the location of a certain amount of UHTC (or SiC) in C/C composites affects the ablation property has not been reported up to now. In our previous work, C/C and C/CeSiC composites were ablated in different heat fluxes and the invalidation of SiC in the improvement of ablation resistance of C/C composites was ascertained to be above 3.64 ± 10% MW/m2 under oxyacetylene torch[22]. Besides, the different accumulations of ablation products on the surface of C/CeSiC composites under single/cyclic ablation modes
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in heat flux of 2.38 MW/m2 were also discussed[23]. In the present work, ablation of C/CeSiC composites with non-woven (C/CeSiC-1) and web (C/CeSiC-2) layer-segregated SiC particles were performed from the non-woven layer in two heat fluxes to investigate the influence of SiC locations on the ablation of modified C/C composites. 2. Experimental Procedure 2.1. Composites preparation As shown in Fig. 1, it was designed that introducing a certain amount of SiC particles into the two different layers of 2D needlepunched carbon fibre fabric separately and the relative composites were marked as C/CeSiC-1 (SiC in non-woven layer) and C/CeSiC-2 (SiC in web layer). Accordingly, C/CeSiC-1 was composed of two repeated layers of A and B while C/CeSiC-2 consisted of CDCDCD. Based on the different pores in non-woven and web layer of 2D needle-punched carbon fibre fabric, the two designed composites were prepared through precursor infiltration pyrolysis (PIP) and thermal gradient chemical vapour infiltration (TCVI). The precursor infiltration was performed using dimethylbenzene solution containing polycarbosilane under vacuum. Then the pyrolysis was carried out at 1400e1800 C for 2 h in a flowing Ar atmosphere. Cycles of the precursor infiltration and pyrolysis were utilized to achieve 10 wt% SiC in the composites. The TCVI processed at 950e1150 C using methane as carbon source. By controlling the concentration of polycarbosilane in solution (40 wt%), the SiC in C/CeSiC-1 was mainly deposited at the non-woven layer of carbon fibre fabric. And then the C/CeSiC-1 was obtained through TCVI. Differently, fabric of C/CeSiC-2 was firstly densified to 0.95e1.15 g/ cm3 through TCVI. As shown in Fig. 2, most of the pores in nonwoven layer were filled by pyrocarbon while the web layer was still porous. Then a certain amount of SiC particles were introduced into the low density C/C composites through PIP. Accordingly, the introduced SiC in C/CeSiC-2 concentrated on the web layer. The
composites were finally densified by TCVI. Other details of the PIP, TCVI and preparation of C/CeSiC-1 were reported[23]. The mass of SiC in C/CeSiC was calculated according to Eq. (1):
mSiC ¼ m1 m0
(1)
where m0 and m1 are the mass of preform (carbon fibre fabric of C/ CeSiC-1, low density C/C composites of C/CeSiC-2) before and after the whole PIP. Samples of 55 mm 10 mm 4 mm and 10 mm 4 mm 3 mm in dimensions and disk samples of F30 mm 10 mm in dimensions were cut from the prepared composites and lightly abraded with 80 and 400 grit SiC paper for test.
2.2. Tests and characterization Three-point bending was performed on an electronic universal testing machine (CMT 5304, Suns Co., China) to obtain the mechanical property of prepared composites with dimensions of 55 mm 10 mm 4 mm. The support span was 40 mm and the ratio of span-to-thickness was 10:1. The final value was the average of five samples tested with a crosshead speed of 0.5 mm/min. Isothermal oxidation of the two prepared composites with dimensions of 10 mm 4 mm 3 mm was carried out in static air in an electrical furnace at 1173 K for 20 min. The ablation tests were carried out under oxyacetylene torch according to GJB323A-96 with two heat fluxes of 2.38 ± 10% and 4.18 ± 10% MW/m2. The pressures of O2 and C2H2 were 0.4 and 0.095 MPa, respectively. Their rates of flow were 0.24 and 0.18 L/s respectively for heat flux of 2.38 MW/m2 and 0.42 and 0.31 L/s respectively for heat flux of 4.18 MW/m2. The inner diameter of the oxyacetylene gun tip was 2 mm and the distance between gun tip and sample which was fixed in a water-cooled concave was 10 mm. The ablation angle was 90 . As shown in Fig. 1, the ablation started from the non-woven layer for each tested sample. To evaluate the
Fig. 1. Design of the two kinds of C/CeSiC composites.
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Fig. 2. Microstructure of the low density C/C composites utilized to prepare C/CeSiC-2: (a) at low magnification; (b) magnified morphology of web; (c) magnified morphology of non-woven layer.
ablation resistance, at least one whole layer (non-woven layer þ web layer) should be ablated. Thus samples were ablated for 180 s in heat flux of 2.38 MW/m2 and for 60 s in heat flux of 4.18 MW/m2. The linear and mass ablation rates were calculated by the change of thickness and weight before and after ablation test and further details can be found[23]. The final result was the average of three samples. The surface temperature was measured by an infrared thermometer (Raytek MR1SCSF) in 2-colour mode with an error of ±0.75%. After ablation, the thickness of sample versus the distance to sample centre was measured in four different directions for each sample. The phase analysis of C/CeSiC composites before and after ablation was conducted by X-ray diffraction (XRD, X'Pert Pro MPD). Morphology and chemical composition of the prepared and ablated composites were investigated by scanning electron microscopy (SEM, JSM6460) combined with energy dispersive spectroscopy (EDS). 3. Results and Discussion 3.1. Microstructure and ablation properties of the two C/CeSiC composites Fig. 3 shows the backscattered electron morphology of the two prepared composites before and after oxidation. Relative EDS analysis attests that the white phase in Fig. 3(a,b) is SiC while the white phase in Fig. 3(c,d) is SiO2. From Fig. 3(a,b), it can be found that the microstructures of the two composites are as designed. Both oxidized composites (Fig. 3(c,d)) suggest good layer segregation of SiC. To C/CeSiC-1, the introduced SiC particles are concentrated on the non-woven layer. Oppositely, SiC particles in C/CeSiC2 are mostly located at the web layer. In accordance with the characteristics narrated above, XRD patterns (Fig. 4) obtained from different layers of the two composites display distinct relative intensity of C/SiC peaks. Although some SiC particles exist in another layer, most of the doped SiC are located at the expected layer. Thus, it is believed that the two designed C/CeSiC composites are fabricated successfully.
In the further magnified figures (Fig. 5), more details of the two prepared composites can be found. Obviously, the pores of nonwoven layer of C/CeSiC-1 are mostly infiltrated by SiC particles (white phase) and the SiC situate at the interface of fibre and matrix (Fig. 5(a)). Thus the non-woven layer of C/CeSiC-1 is mainly composed of carbon fibre, SiC and some pyrocarbon. This is different from the layer (carbon fibre reinforced pyrocarbon, Fig. 2(c)) of C/CeSiC-2. By comparing the web of these two composites, it can be found that the SiC in C/CeSiC-2 distributes in the pyrocarbon matrix and the pyrocarbon in C/CeSiC-2 can be divided into two groups: Py1: the annular pyrocarbon enwrapped the carbon fibre are prepared by the former TCVI; Py2: the pyrocarbon particles which mixed with SiC particles come from the final TCVI. Moreover, the SiC particles in C/CeSiC-2 are more agglomerated than those in C/CeSiC-1. Obviously, the agglomerated SiC particles in C/CeSiC-2 will induce higher local thermal stress than the fine SiC particles in C/CeSiC-1 during ablation as the thermal expansion coefficient of SiC is higher than carbon[24] and the agglomerated SiC is apt to have a bigger volume expansion at high temperature. Table 1 shows the bulk density and mechanical properties of the two prepared composites. The density of C/CeSiC-1 is higher than that of C/CeSiC-2. This can be explained by their fabrications. After the former densification by TCVI, most of the pores of non-woven layer in C/CeSiC-2 are filled by Py1 since the pores in non-woven layer are much smaller than those in web layer. In the subsequent PIP, many open pores are closed by SiC due to the characteristic of the process, which induces the poor infiltration of Py2 in the final TCVI. Accordingly, although the two composites have the same volume fraction of SiC, the lower density of C/CeSiC-2 leads to its slightly higher mass fraction of SiC. In the following discussion, it is assumed that the two composites are the same except for the location of SiC. Additionally, the lower bending strength of C/CeSiC-1 indicates that the continuous carbon fibres in non-woven layer are impaired since they are the main reinforcement components. Thus the pyrolysis of polycarbosilane in carbon fibre fabric is harmful to the carbon fibres. To C/CeSiC-2, the non-woven layer is densified by pyrocarbon and the fibres in web layer are protected by Py1. And then the composites display higher bending strength.
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Fig. 3. Microstructure of the two prepared composites: (a) C/CeSiC-1; (b) C/CeSiC-2; (c) oxidized C/CeSiC-1; (d) oxidized C/CeSiC-2, and EDS results for Spot A and Spot B.
Fig. 4. XRD patterns of the two prepared composites: (a) C/CeSiC-1; (b) C/CeSiC-2.
Fig. 6 shows the ablation rates of the prepared composites in the two tested heat fluxes under oxyacetylene torch. Through a simple calculation, it can be found that the ablation depth at surface centre of each tested sample is about 1 whole layer (non-woven layer þ web layer, 0.74 mm). Thus it is believed that the test can evaluate the ablation property of prepared composites. In the lower heat flux, the two C/CeSiC composites display similar ablation
resistance while C/CeSiC-1 has slightly higher linear and mass ablation rates. Considering the impaired carbon fibres of C/CeSiC-1, the ablation properties in heat flux of 2.38 MW/m2 are reasonable. However, when heat flux is raised to 4.18 MW/m2, the ablation rates of C/CeSiC-2 present a sharp rise in comparison with those of C/CeSiC-1. In other words, the two composites have different sensitivity of ablation rates to heat flux. This behaviour should be
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Fig. 5. Morphology of the two prepared composites: (a) from non-woven layer of C/CeSiC-1; (b,d) from web of C/CeSiC-2; (c) from web of C/CeSiC-1; inset in (a): cross section of fibres.
Table 1 Bulk density and mechanical properties of the two C/CeSiC composites Composites
Density (g/cm3)
Mass of SiC (wt%)
Bending strength (MPa)
Bending modulus (GPa)
C/CeSiC-1 C/CeSiC-2
1.88 1.74
10.0 10.8
61.03 100.25
15.63 8.98
attributed to the location and distribution state of SiC in the two composites. 3.2. Ablation morphologies of the two C/CeSiC composites Fig. 7 shows the macro-morphologies of the ablated samples. From the central to border region, these ablated samples display distinct ablation morphologies. Fig. 8 shows the thickness of the ablated samples, in which the data are average, indicating that the ablation mainly occurred at the centre of sample. This is corresponding with the high temperature of inner cone of oxyacetylene
torch. After ablation, centres of all the tested samples are ablated to the web layer, and the borders of tested samples still stay at the initial layer. Since the ablation stops at different layer from centre to border, several key points are marked on the curves (height versus distance to sample centre) to analysis the ablation of the two composites. The ablation rates and some of the ablated morphology of C/ CeSiC-1 have been discussed in our previous report[22,23]. It was believed that the doped SiC could act as a barrier to oxidizing species in the lower heat flux while was consumed faster than pyrocarbon in the higher heat flux. The enlarged defects (at interface of fibre and pyrocarbon) induced by SiC led to more serious ablation of C/CeSiC-1 than C/C composites in the higher heat flux. In the present work, to make a better comparison of the ablation of these two C/CeSiC, both composites are ablated from the nonwoven layer and magnified morphologies corresponding to the marked points in Fig. 8 are discussed below. Fig. 9 shows the severest ablated region of the two composites in the lower heat flux. Corresponding with the layer-segregated
Fig. 6. Ablation rates of the two composites versus heat flux: (a) linear ablation rates; (b) mass ablation rates.
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Fig. 7. Macro-morphologies of the ablated samples: (a) C/CeSiC-1 in 2.38 MW/m2; (b) C/CeSiC-2 in 2.38 MW/m2; (c) C/CeSiC-1 in 4.18 MW/m2; (d) C/CeSiC-2 in 4.18 MW/m2.
structures, the web of C/CeSiC-1 and the non-woven layer of C/ CeSiC-2 are oxidized and some eroded fibres survive from the ablation. Unlikely, a great deal of SiO2 infiltrates in the pores of C/ CeSiC-20 web while the interspaces between fibres of C/CeSiC-10 non-woven layer are filled by discontinuous SiO2. Although the SiC distributed in different layers in the two composites, it is no doubt that they played a beneficial role to ablation resistance during ablation. Additionally, the ablation rates suggest that the layer segregation of SiC has no influence on the ablation property of modified C/C composites. By strictly controlling the starting layer, the ablation centre of C/ CeSiC-1 in the higher heat flux mainly terminates at the first web below outer non-woven layer (Fig. 8(b)). As the scouring in the higher heat flux is more rigorous than that in the lower heat flux, the ablated web of C/CeSiC-1 displays a relative flat surface (Fig. 10(a)). Both fibres and pyrocarbon matrix in the web come into view. Meanwhile, some ball like SiO2 (Fig. 10(b)) adheres to the fibres of non-woven layer which is nearby the inner cone. To C/CeSiC-2 in the higher heat flux, the ablation morphologies at the central region are shown in Fig. 11. The ablated centre is also
ended at the web layer but that is the sub layer of outer web. After depletion of the doped SiC and oxidation of carbon, a porous structure comes into being (Fig. 11(a)). It is obvious that this structure is easy to be broken by the scouring of oxyacetylene torch. In the region 300 , some SiO2 is found near the porous Py2 (Fig. 11(b,c)). Based on this, oxidation of SiC and evaporation of SiO2 should be the main consumption mode of doped SiC particles. Besides, cracked particles (Fig. 11(b,d)) in this region indicates that mechanical erosion also works in the ablation of C/CeSiC-2. The ablated fibres (Fig. 11(e)) of non-woven layer present needle like shape as same as these in Fig. 9(d). Moreover, many liquid like SiO2 particles are left in the first web of ablated C/CeSiC-2 (Fig. 11(f)). In combination with Fig. 10 and reference[22], it is inferred that these doped SiC particles lost their protective effect at the centre of inner cone ablated region. However, different locations of SiC in the prepared two composites result in distinct ablation rates in the higher heat flux. Thus, the defects induced by faster ablation of SiC than pyrocarbon are important to the ablation resistance. In other words, the different defects induced by the two SiC locations determine the ablation resistance of relative composites in the higher heat flux. Fig. 12 shows the morphologies of region 5 and 50 . Because they are apart from the inner cone, the surface is covered by a great deal of SiO2. The directional distribution of SiC in C/CeSiC-1 is remained to be a compact SiO2 layer and some pores exist in the residual SiO2 of C/CeSiC-2. It is no doubt that the residual SiO2 in C/CeSiC-1 is more effective than that in C/CeSiC-2 in protecting the substrate. Since the doped SiC particles of the two composites play their roles in the web and non-woven layer separately, it is hard to make a direct comparison of their effects on the ablation from the ablated web or non-woven layer. Thus, the needle fibres (parallel to the flame, ablated by inner cone) and the matrices around are displayed in Fig. 13. In the lower heat flux, oxidized SiC (SiO2) infiltrates into the inner pores of fibre bundle in C/CeSiC-1 whereas the SiO2 fills in the pits between Py1 in C/CeSiC-2. Although defects repaired by SiO2 in the two composites are different, both SiC particles play a beneficial role during ablation. Therefore, the two composites have similar ablation rates in the lower heat flux. When heat flux is raised to 4.18 MW/m2, the two composites present distinct ablation morphologies. For C/CeSiC-1, the fibres are eroded to various states and a big gap is formed between carbon fibre and pyrocarbon matrix. However, the ablated fibre in C/CeSiC-2 is the same as that in pure C/C composites which is in the shape of needle[25]. Besides, Py1 encircles around the carbon fibre while the nearby Py2 is peeled off (Fig. 13(d)) or becomes porous (Fig. 13(e)) as same as that shown in Fig. 11(c).
Fig. 8. Thickness of the ablated samples in different heat fluxes: (a) 2.38 MW/m2; (b) 4.18 MW/m2.
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Fig. 9. Morphologies of the ablated samples obtained from the marked region in Fig. 8(a): (a) region 1; (b) region 10 ; (c) region 2; (d) region 20 .
Fig. 10. Morphologies of the ablated samples obtained from the marked region in Fig. 8(b): (a) region 3; (b) region 4.
3.3. Ablation mechanism of the two C/CeSiC composites From the ablated morphology shown above, it can be concluded that the defects on the surface of the two prepared composites determine their ablation properties. During ablation, the surface temperature rises to about 1820 C in the lower heat flux and above 2000 C in the higher heat flux for the tested two composites. Carbon and SiC may be oxidized according to reactions (2)e(5):
CðsÞ þ O2 ðgÞ/CO2 ðgÞ
(2)
2CðsÞ þ O2 ðgÞ/2COðgÞ
(3)
SiCðsÞ þ O2 ðgÞ/SiOðgÞ þ COðgÞ
(4)
2SiCðsÞ þ 3O2 ðgÞ/2SiO2 ðlÞ þ 2COðgÞ
(5)
The liquid SiO2 could fill in the pores on the surface, but is also consumed through reactions (6)e(8):
SiO2 ðlÞ þ CðsÞ/SiOðgÞ þ COðgÞ
(6)
SiO2 ðlÞ þ COðgÞ/SiOðgÞ þ 2CO2 ðgÞ
(7)
SiO2 ðlÞ/SiO2 ðgÞ
(8)
The higher surface temperature in the higher heat flux accelerates the oxidation, which leads to the increase of ablation rates. However, the different locations and distribution states of doped SiC particles in the two composites cause distinct ablation rates in the higher heat flux. Fig. 14 is the schematic of defects on the surface of the two composites during ablation. In the lower heat flux, SiO2 from oxidation of SiC covers partially the defective surface of ablated C/CeSiC, which is helpful to the ablation resistance and both composites present similar ablation rates. In the higher heat flux, the prior ablation of SiC originates many pits on the surface. It has been studied that CO and SiO would erode the C/CeSiC under oxyacetylene torch when temperature was higher than 2240 K[26]. For C/CeSiC-1, the erosion further expands the defects at the interface
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Fig. 11. Morphologies of the ablated samples obtained from the marked region in Fig. 8(b): (a) region 30 ; (b) region 300 ; (c) magnification of region A in figure (b); (d) magnification of region B in figure (b); (e) region 3000 ; (e) region 40 .
of fibre and matrix. However, for C/CeSiC-2, the mixed state of SiC and Py2 makes the Py2 become porous or peeled off with the depletion of SiC. Thus new defects are formed in composites except for these at the interface of fibre and matrix. Obviously, the new
defects are more deleterious to the ablation resistance than the enlarged interfacial defects. As a result, the web layer-segregated SiC particles are more harmful to the ablation property in the higher heat flux than these concentrated on the non-woven layer.
Fig. 12. Morphologies of the ablated samples obtained from the marked region in Fig. 8(b): (a) region 5; (b) region 50 .
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Fig. 13. Morphologies of needle fibres ablated by inner cone of oxyacetylene: (a) C/CeSiC-1 in 2.38 MW/m2; (b) C/CeSiC-2 in 2.38 MW/m2; (c) C/CeSiC-1 in 4.18 MW/m2; (d,e) C/ CeSiC-2 in 4.18 MW/m2.
Fig. 14. Defects on the surface of the two composites during ablation: (a,b,c) C/CeSiC-1; (d,e,f) C/CeSiC-2; (a,d) before ablation; (b,e) in heat flux of 2.38 MW/m2; (c,f) in heat flux of 4.18 MW/m2.
4. Conclusion A certain amount of SiC particles are successfully deposited at non-woven and web layer of 2D needle-punched carbon fibre fabric reinforced pyrocarbon composites through precursor infiltration and pyrolysis respectively. The two prepared composites are both ablated from the non-woven layer by oxyacetylene torch in two heat fluxes. Results show a similar ablation
property of the two composites in the lower heat flux and a faster increase of ablation rates of C/CeSiC-2 than that of C/CeSiC-1 when heat flux is raised from 2.38 to 4.18 MW/m2. The ablated morphology of C/CeSiC-2 in the lower heat flux indicates that the centre of front face is partially covered by SiO2, which is similar to that of C/CeSiC-1. Different defects resulted from the two SiC locations dominate their distinct ablation rates in the higher heat flux.
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Acknowledgements This work is supported by the National Natural Science Foundation of China under Grant No. 51402238 and 51221001, the Fundamental Research Foundation of Northwestern Polytechnical University under Grant No. GBKY1021, the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 25-TZ-2009) and the ‘‘111’’ Project under Grant No. B08040. References [1] S.F. Tang, J.Y. Deng, S.J. Wang, W.C. Liu, Corros. Sci. 51 (2009) 54e61. [2] E.L. Corral, L.S. Wslker, J. Eur. Ceram. Soc. 30 (2010) 2357e2364. [3] D.D. Jayaseelan, R.G. Sa, P. Brown, W.E. Lee, J. Eur. Ceram. Soc. 31 (2011) 361e368. [4] A. Paul, S. Venugopal, J.G.P. Binner, B. Vaidhyanathan, A.C.J. Heaton, P.M. Brown, J. Eur. Ceram. Soc. 33 (2013) 423e432. [5] Y.L. Wang, X. Xiong, G.D. Li, H.F. Liu, Z.K. Chen, W. Sun, X.J. Zhao, Corros. Sci. 65 (2012) 549e555. [6] Y.C. Ye, H. Zhang, Y.G. Tong, S.X. Bai, Ceram. Int. 39 (2013) 5477e5483. [7] X. Yang, W. Li, S. Wang, B.F. Zhang, Z.H. Chen, Compos. Part B-Eng 45 (2013) 1391e1396. [8] X.T. Shen, K.Z. Li, H.J. Li, Q.G. Fu, S.P. Li, F. Deng, Corros. Sci. 53 (2011) 105e112. [9] S. Wu, L. Chen, L. Qian, J.B. Zhang, J.F. Pan, C.J. Zhou, M.S. Ren, J.L. Sun, J. Chin. Ceram. Soc. 36 (2008) 973e977.
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Effect of SiC Location on the Ablation of C/CeSiC Composites in Two Heat Fluxes Lei Liu, Hejun Li*, Kui Hao, Xiaohong Shi, Kezhi Li, Chang Ni State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
a r t i c l e i n f o Article history: Received 25 May 2014 Received in revised form 23 October 2014 Accepted 17 November 2014 Available online xxx Key words: C/C composites SiC Distribution Ablation performance
How layer-segregated distribution of SiC affects the ablation of C/CeSiC composites was studied in the present work. A certain amount of SiC particles was deposited at the non-woven (C/CeSiC-1) and web (C/CeSiC-2) layer of 2D needle-punched carbon fibre fabric reinforced pyrocarbon composites, respectively. Ablation under oxyacetylene torch demonstrated that the two composites have similar ablation rates in heat flux of 2.38 MW/m2 whereas ablation rates of C/CeSiC-2 were much higher than those of C/CeSiC-1 when heat flux was 4.18 MW/m2. SiO2 covered partially the defective surface of both composites in the lower heat flux. The different SiC locations induced distinct defects and then led to the two composites' dissimilar ablation rates in the higher heat flux. Copyright © 2015, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved.
1. Introduction Ultra high temperature ceramics (UHTC) and SiC doped[1e4] or coated[5e7] carbon/carbon (C/C) composites for the purpose of improving ablation properties have been largely reported in the last several years. Coating was considered to be a short-term protection for C/C composites against ablation while introduction of UHTC (or SiC) into C/C composites was a promising method to improve the ablation resistance thoroughly. Among these researches on the introduction of UHTC (or SiC), different contents and various kinds of UHTC (or SiC) were firstly attempted and it has been proved that C/C composites containing higher content[8e10] and appropriate kinds[11,12] of introduced phases had better ablation property. Afterward, the relative content of introduced phases was found to play a key role in the improvement[13e15]. Recently, the influences of the density of C/C composites and the architecture of fibre fabric on the following adding of UHTC (or SiC) and the ablation property were studied. Chen et al.[16] found that the pyrocarbon amount in preform was important to the densification, mechanical and ablation properties of C/ZrC composites. Zeng et al.[17] discovered that samples with chopped web needled preform displayed better ablation resistance than the samples with needle-integrated and
* Corresponding author. Prof., Ph.D.; Tel.: þ86 29 88495004; Fax: þ86 29 88492642. E-mail address: [email protected] (H. Li).
fine-weave pierced preforms in the case of being infiltrated by ZreTieC through reactive melt infiltration. Moreover, it has been proved that a moderate content of SiC interphase could improve the ablation property of C/ZrC composites while excessive SiC interphase would play an adverse role[18]. Based on these works, how to further improve the ablation resistance of C/C composites with the same fibre architecture and carbon matrix through a certain amount of UHTC should be an attractive new subject. 2D needle-punched carbon fibre fabric, fabricated by repeatedly overlapping the layers of 0 non-woven fibre cloth, chopped fibre web and 90 non-woven fibre cloth with needle-punching step by step, has been widely utilized in the investigation of UHTC (or SiC) modified C/C composites[19e21]. Since the non-woven layer and the web layer have distinct characteristics, effect of the layersegregated distribution of introduced ceramics on the ablation is necessary to be investigated. In other words, one modified C/C composite with non-woven layer or web layer containing UHTC (or SiC) should have different properties. However, to the authors' knowledge, how the location of a certain amount of UHTC (or SiC) in C/C composites affects the ablation property has not been reported up to now. In our previous work, C/C and C/CeSiC composites were ablated in different heat fluxes and the invalidation of SiC in the improvement of ablation resistance of C/C composites was ascertained to be above 3.64 ± 10% MW/m2 under oxyacetylene torch[22]. Besides, the different accumulations of ablation products on the surface of C/CeSiC composites under single/cyclic ablation modes
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in heat flux of 2.38 MW/m2 were also discussed[23]. In the present work, ablation of C/CeSiC composites with non-woven (C/CeSiC-1) and web (C/CeSiC-2) layer-segregated SiC particles were performed from the non-woven layer in two heat fluxes to investigate the influence of SiC locations on the ablation of modified C/C composites. 2. Experimental Procedure 2.1. Composites preparation As shown in Fig. 1, it was designed that introducing a certain amount of SiC particles into the two different layers of 2D needlepunched carbon fibre fabric separately and the relative composites were marked as C/CeSiC-1 (SiC in non-woven layer) and C/CeSiC-2 (SiC in web layer). Accordingly, C/CeSiC-1 was composed of two repeated layers of A and B while C/CeSiC-2 consisted of CDCDCD. Based on the different pores in non-woven and web layer of 2D needle-punched carbon fibre fabric, the two designed composites were prepared through precursor infiltration pyrolysis (PIP) and thermal gradient chemical vapour infiltration (TCVI). The precursor infiltration was performed using dimethylbenzene solution containing polycarbosilane under vacuum. Then the pyrolysis was carried out at 1400e1800 C for 2 h in a flowing Ar atmosphere. Cycles of the precursor infiltration and pyrolysis were utilized to achieve 10 wt% SiC in the composites. The TCVI processed at 950e1150 C using methane as carbon source. By controlling the concentration of polycarbosilane in solution (40 wt%), the SiC in C/CeSiC-1 was mainly deposited at the non-woven layer of carbon fibre fabric. And then the C/CeSiC-1 was obtained through TCVI. Differently, fabric of C/CeSiC-2 was firstly densified to 0.95e1.15 g/ cm3 through TCVI. As shown in Fig. 2, most of the pores in nonwoven layer were filled by pyrocarbon while the web layer was still porous. Then a certain amount of SiC particles were introduced into the low density C/C composites through PIP. Accordingly, the introduced SiC in C/CeSiC-2 concentrated on the web layer. The
composites were finally densified by TCVI. Other details of the PIP, TCVI and preparation of C/CeSiC-1 were reported[23]. The mass of SiC in C/CeSiC was calculated according to Eq. (1):
mSiC ¼ m1 m0
(1)
where m0 and m1 are the mass of preform (carbon fibre fabric of C/ CeSiC-1, low density C/C composites of C/CeSiC-2) before and after the whole PIP. Samples of 55 mm 10 mm 4 mm and 10 mm 4 mm 3 mm in dimensions and disk samples of F30 mm 10 mm in dimensions were cut from the prepared composites and lightly abraded with 80 and 400 grit SiC paper for test.
2.2. Tests and characterization Three-point bending was performed on an electronic universal testing machine (CMT 5304, Suns Co., China) to obtain the mechanical property of prepared composites with dimensions of 55 mm 10 mm 4 mm. The support span was 40 mm and the ratio of span-to-thickness was 10:1. The final value was the average of five samples tested with a crosshead speed of 0.5 mm/min. Isothermal oxidation of the two prepared composites with dimensions of 10 mm 4 mm 3 mm was carried out in static air in an electrical furnace at 1173 K for 20 min. The ablation tests were carried out under oxyacetylene torch according to GJB323A-96 with two heat fluxes of 2.38 ± 10% and 4.18 ± 10% MW/m2. The pressures of O2 and C2H2 were 0.4 and 0.095 MPa, respectively. Their rates of flow were 0.24 and 0.18 L/s respectively for heat flux of 2.38 MW/m2 and 0.42 and 0.31 L/s respectively for heat flux of 4.18 MW/m2. The inner diameter of the oxyacetylene gun tip was 2 mm and the distance between gun tip and sample which was fixed in a water-cooled concave was 10 mm. The ablation angle was 90 . As shown in Fig. 1, the ablation started from the non-woven layer for each tested sample. To evaluate the
Fig. 1. Design of the two kinds of C/CeSiC composites.
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Fig. 2. Microstructure of the low density C/C composites utilized to prepare C/CeSiC-2: (a) at low magnification; (b) magnified morphology of web; (c) magnified morphology of non-woven layer.
ablation resistance, at least one whole layer (non-woven layer þ web layer) should be ablated. Thus samples were ablated for 180 s in heat flux of 2.38 MW/m2 and for 60 s in heat flux of 4.18 MW/m2. The linear and mass ablation rates were calculated by the change of thickness and weight before and after ablation test and further details can be found[23]. The final result was the average of three samples. The surface temperature was measured by an infrared thermometer (Raytek MR1SCSF) in 2-colour mode with an error of ±0.75%. After ablation, the thickness of sample versus the distance to sample centre was measured in four different directions for each sample. The phase analysis of C/CeSiC composites before and after ablation was conducted by X-ray diffraction (XRD, X'Pert Pro MPD). Morphology and chemical composition of the prepared and ablated composites were investigated by scanning electron microscopy (SEM, JSM6460) combined with energy dispersive spectroscopy (EDS). 3. Results and Discussion 3.1. Microstructure and ablation properties of the two C/CeSiC composites Fig. 3 shows the backscattered electron morphology of the two prepared composites before and after oxidation. Relative EDS analysis attests that the white phase in Fig. 3(a,b) is SiC while the white phase in Fig. 3(c,d) is SiO2. From Fig. 3(a,b), it can be found that the microstructures of the two composites are as designed. Both oxidized composites (Fig. 3(c,d)) suggest good layer segregation of SiC. To C/CeSiC-1, the introduced SiC particles are concentrated on the non-woven layer. Oppositely, SiC particles in C/CeSiC2 are mostly located at the web layer. In accordance with the characteristics narrated above, XRD patterns (Fig. 4) obtained from different layers of the two composites display distinct relative intensity of C/SiC peaks. Although some SiC particles exist in another layer, most of the doped SiC are located at the expected layer. Thus, it is believed that the two designed C/CeSiC composites are fabricated successfully.
In the further magnified figures (Fig. 5), more details of the two prepared composites can be found. Obviously, the pores of nonwoven layer of C/CeSiC-1 are mostly infiltrated by SiC particles (white phase) and the SiC situate at the interface of fibre and matrix (Fig. 5(a)). Thus the non-woven layer of C/CeSiC-1 is mainly composed of carbon fibre, SiC and some pyrocarbon. This is different from the layer (carbon fibre reinforced pyrocarbon, Fig. 2(c)) of C/CeSiC-2. By comparing the web of these two composites, it can be found that the SiC in C/CeSiC-2 distributes in the pyrocarbon matrix and the pyrocarbon in C/CeSiC-2 can be divided into two groups: Py1: the annular pyrocarbon enwrapped the carbon fibre are prepared by the former TCVI; Py2: the pyrocarbon particles which mixed with SiC particles come from the final TCVI. Moreover, the SiC particles in C/CeSiC-2 are more agglomerated than those in C/CeSiC-1. Obviously, the agglomerated SiC particles in C/CeSiC-2 will induce higher local thermal stress than the fine SiC particles in C/CeSiC-1 during ablation as the thermal expansion coefficient of SiC is higher than carbon[24] and the agglomerated SiC is apt to have a bigger volume expansion at high temperature. Table 1 shows the bulk density and mechanical properties of the two prepared composites. The density of C/CeSiC-1 is higher than that of C/CeSiC-2. This can be explained by their fabrications. After the former densification by TCVI, most of the pores of non-woven layer in C/CeSiC-2 are filled by Py1 since the pores in non-woven layer are much smaller than those in web layer. In the subsequent PIP, many open pores are closed by SiC due to the characteristic of the process, which induces the poor infiltration of Py2 in the final TCVI. Accordingly, although the two composites have the same volume fraction of SiC, the lower density of C/CeSiC-2 leads to its slightly higher mass fraction of SiC. In the following discussion, it is assumed that the two composites are the same except for the location of SiC. Additionally, the lower bending strength of C/CeSiC-1 indicates that the continuous carbon fibres in non-woven layer are impaired since they are the main reinforcement components. Thus the pyrolysis of polycarbosilane in carbon fibre fabric is harmful to the carbon fibres. To C/CeSiC-2, the non-woven layer is densified by pyrocarbon and the fibres in web layer are protected by Py1. And then the composites display higher bending strength.
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Fig. 3. Microstructure of the two prepared composites: (a) C/CeSiC-1; (b) C/CeSiC-2; (c) oxidized C/CeSiC-1; (d) oxidized C/CeSiC-2, and EDS results for Spot A and Spot B.
Fig. 4. XRD patterns of the two prepared composites: (a) C/CeSiC-1; (b) C/CeSiC-2.
Fig. 6 shows the ablation rates of the prepared composites in the two tested heat fluxes under oxyacetylene torch. Through a simple calculation, it can be found that the ablation depth at surface centre of each tested sample is about 1 whole layer (non-woven layer þ web layer, 0.74 mm). Thus it is believed that the test can evaluate the ablation property of prepared composites. In the lower heat flux, the two C/CeSiC composites display similar ablation
resistance while C/CeSiC-1 has slightly higher linear and mass ablation rates. Considering the impaired carbon fibres of C/CeSiC-1, the ablation properties in heat flux of 2.38 MW/m2 are reasonable. However, when heat flux is raised to 4.18 MW/m2, the ablation rates of C/CeSiC-2 present a sharp rise in comparison with those of C/CeSiC-1. In other words, the two composites have different sensitivity of ablation rates to heat flux. This behaviour should be
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Fig. 5. Morphology of the two prepared composites: (a) from non-woven layer of C/CeSiC-1; (b,d) from web of C/CeSiC-2; (c) from web of C/CeSiC-1; inset in (a): cross section of fibres.
Table 1 Bulk density and mechanical properties of the two C/CeSiC composites Composites
Density (g/cm3)
Mass of SiC (wt%)
Bending strength (MPa)
Bending modulus (GPa)
C/CeSiC-1 C/CeSiC-2
1.88 1.74
10.0 10.8
61.03 100.25
15.63 8.98
attributed to the location and distribution state of SiC in the two composites. 3.2. Ablation morphologies of the two C/CeSiC composites Fig. 7 shows the macro-morphologies of the ablated samples. From the central to border region, these ablated samples display distinct ablation morphologies. Fig. 8 shows the thickness of the ablated samples, in which the data are average, indicating that the ablation mainly occurred at the centre of sample. This is corresponding with the high temperature of inner cone of oxyacetylene
torch. After ablation, centres of all the tested samples are ablated to the web layer, and the borders of tested samples still stay at the initial layer. Since the ablation stops at different layer from centre to border, several key points are marked on the curves (height versus distance to sample centre) to analysis the ablation of the two composites. The ablation rates and some of the ablated morphology of C/ CeSiC-1 have been discussed in our previous report[22,23]. It was believed that the doped SiC could act as a barrier to oxidizing species in the lower heat flux while was consumed faster than pyrocarbon in the higher heat flux. The enlarged defects (at interface of fibre and pyrocarbon) induced by SiC led to more serious ablation of C/CeSiC-1 than C/C composites in the higher heat flux. In the present work, to make a better comparison of the ablation of these two C/CeSiC, both composites are ablated from the nonwoven layer and magnified morphologies corresponding to the marked points in Fig. 8 are discussed below. Fig. 9 shows the severest ablated region of the two composites in the lower heat flux. Corresponding with the layer-segregated
Fig. 6. Ablation rates of the two composites versus heat flux: (a) linear ablation rates; (b) mass ablation rates.
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Fig. 7. Macro-morphologies of the ablated samples: (a) C/CeSiC-1 in 2.38 MW/m2; (b) C/CeSiC-2 in 2.38 MW/m2; (c) C/CeSiC-1 in 4.18 MW/m2; (d) C/CeSiC-2 in 4.18 MW/m2.
structures, the web of C/CeSiC-1 and the non-woven layer of C/ CeSiC-2 are oxidized and some eroded fibres survive from the ablation. Unlikely, a great deal of SiO2 infiltrates in the pores of C/ CeSiC-20 web while the interspaces between fibres of C/CeSiC-10 non-woven layer are filled by discontinuous SiO2. Although the SiC distributed in different layers in the two composites, it is no doubt that they played a beneficial role to ablation resistance during ablation. Additionally, the ablation rates suggest that the layer segregation of SiC has no influence on the ablation property of modified C/C composites. By strictly controlling the starting layer, the ablation centre of C/ CeSiC-1 in the higher heat flux mainly terminates at the first web below outer non-woven layer (Fig. 8(b)). As the scouring in the higher heat flux is more rigorous than that in the lower heat flux, the ablated web of C/CeSiC-1 displays a relative flat surface (Fig. 10(a)). Both fibres and pyrocarbon matrix in the web come into view. Meanwhile, some ball like SiO2 (Fig. 10(b)) adheres to the fibres of non-woven layer which is nearby the inner cone. To C/CeSiC-2 in the higher heat flux, the ablation morphologies at the central region are shown in Fig. 11. The ablated centre is also
ended at the web layer but that is the sub layer of outer web. After depletion of the doped SiC and oxidation of carbon, a porous structure comes into being (Fig. 11(a)). It is obvious that this structure is easy to be broken by the scouring of oxyacetylene torch. In the region 300 , some SiO2 is found near the porous Py2 (Fig. 11(b,c)). Based on this, oxidation of SiC and evaporation of SiO2 should be the main consumption mode of doped SiC particles. Besides, cracked particles (Fig. 11(b,d)) in this region indicates that mechanical erosion also works in the ablation of C/CeSiC-2. The ablated fibres (Fig. 11(e)) of non-woven layer present needle like shape as same as these in Fig. 9(d). Moreover, many liquid like SiO2 particles are left in the first web of ablated C/CeSiC-2 (Fig. 11(f)). In combination with Fig. 10 and reference[22], it is inferred that these doped SiC particles lost their protective effect at the centre of inner cone ablated region. However, different locations of SiC in the prepared two composites result in distinct ablation rates in the higher heat flux. Thus, the defects induced by faster ablation of SiC than pyrocarbon are important to the ablation resistance. In other words, the different defects induced by the two SiC locations determine the ablation resistance of relative composites in the higher heat flux. Fig. 12 shows the morphologies of region 5 and 50 . Because they are apart from the inner cone, the surface is covered by a great deal of SiO2. The directional distribution of SiC in C/CeSiC-1 is remained to be a compact SiO2 layer and some pores exist in the residual SiO2 of C/CeSiC-2. It is no doubt that the residual SiO2 in C/CeSiC-1 is more effective than that in C/CeSiC-2 in protecting the substrate. Since the doped SiC particles of the two composites play their roles in the web and non-woven layer separately, it is hard to make a direct comparison of their effects on the ablation from the ablated web or non-woven layer. Thus, the needle fibres (parallel to the flame, ablated by inner cone) and the matrices around are displayed in Fig. 13. In the lower heat flux, oxidized SiC (SiO2) infiltrates into the inner pores of fibre bundle in C/CeSiC-1 whereas the SiO2 fills in the pits between Py1 in C/CeSiC-2. Although defects repaired by SiO2 in the two composites are different, both SiC particles play a beneficial role during ablation. Therefore, the two composites have similar ablation rates in the lower heat flux. When heat flux is raised to 4.18 MW/m2, the two composites present distinct ablation morphologies. For C/CeSiC-1, the fibres are eroded to various states and a big gap is formed between carbon fibre and pyrocarbon matrix. However, the ablated fibre in C/CeSiC-2 is the same as that in pure C/C composites which is in the shape of needle[25]. Besides, Py1 encircles around the carbon fibre while the nearby Py2 is peeled off (Fig. 13(d)) or becomes porous (Fig. 13(e)) as same as that shown in Fig. 11(c).
Fig. 8. Thickness of the ablated samples in different heat fluxes: (a) 2.38 MW/m2; (b) 4.18 MW/m2.
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Fig. 9. Morphologies of the ablated samples obtained from the marked region in Fig. 8(a): (a) region 1; (b) region 10 ; (c) region 2; (d) region 20 .
Fig. 10. Morphologies of the ablated samples obtained from the marked region in Fig. 8(b): (a) region 3; (b) region 4.
3.3. Ablation mechanism of the two C/CeSiC composites From the ablated morphology shown above, it can be concluded that the defects on the surface of the two prepared composites determine their ablation properties. During ablation, the surface temperature rises to about 1820 C in the lower heat flux and above 2000 C in the higher heat flux for the tested two composites. Carbon and SiC may be oxidized according to reactions (2)e(5):
CðsÞ þ O2 ðgÞ/CO2 ðgÞ
(2)
2CðsÞ þ O2 ðgÞ/2COðgÞ
(3)
SiCðsÞ þ O2 ðgÞ/SiOðgÞ þ COðgÞ
(4)
2SiCðsÞ þ 3O2 ðgÞ/2SiO2 ðlÞ þ 2COðgÞ
(5)
The liquid SiO2 could fill in the pores on the surface, but is also consumed through reactions (6)e(8):
SiO2 ðlÞ þ CðsÞ/SiOðgÞ þ COðgÞ
(6)
SiO2 ðlÞ þ COðgÞ/SiOðgÞ þ 2CO2 ðgÞ
(7)
SiO2 ðlÞ/SiO2 ðgÞ
(8)
The higher surface temperature in the higher heat flux accelerates the oxidation, which leads to the increase of ablation rates. However, the different locations and distribution states of doped SiC particles in the two composites cause distinct ablation rates in the higher heat flux. Fig. 14 is the schematic of defects on the surface of the two composites during ablation. In the lower heat flux, SiO2 from oxidation of SiC covers partially the defective surface of ablated C/CeSiC, which is helpful to the ablation resistance and both composites present similar ablation rates. In the higher heat flux, the prior ablation of SiC originates many pits on the surface. It has been studied that CO and SiO would erode the C/CeSiC under oxyacetylene torch when temperature was higher than 2240 K[26]. For C/CeSiC-1, the erosion further expands the defects at the interface
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Fig. 11. Morphologies of the ablated samples obtained from the marked region in Fig. 8(b): (a) region 30 ; (b) region 300 ; (c) magnification of region A in figure (b); (d) magnification of region B in figure (b); (e) region 3000 ; (e) region 40 .
of fibre and matrix. However, for C/CeSiC-2, the mixed state of SiC and Py2 makes the Py2 become porous or peeled off with the depletion of SiC. Thus new defects are formed in composites except for these at the interface of fibre and matrix. Obviously, the new
defects are more deleterious to the ablation resistance than the enlarged interfacial defects. As a result, the web layer-segregated SiC particles are more harmful to the ablation property in the higher heat flux than these concentrated on the non-woven layer.
Fig. 12. Morphologies of the ablated samples obtained from the marked region in Fig. 8(b): (a) region 5; (b) region 50 .
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Fig. 13. Morphologies of needle fibres ablated by inner cone of oxyacetylene: (a) C/CeSiC-1 in 2.38 MW/m2; (b) C/CeSiC-2 in 2.38 MW/m2; (c) C/CeSiC-1 in 4.18 MW/m2; (d,e) C/ CeSiC-2 in 4.18 MW/m2.
Fig. 14. Defects on the surface of the two composites during ablation: (a,b,c) C/CeSiC-1; (d,e,f) C/CeSiC-2; (a,d) before ablation; (b,e) in heat flux of 2.38 MW/m2; (c,f) in heat flux of 4.18 MW/m2.
4. Conclusion A certain amount of SiC particles are successfully deposited at non-woven and web layer of 2D needle-punched carbon fibre fabric reinforced pyrocarbon composites through precursor infiltration and pyrolysis respectively. The two prepared composites are both ablated from the non-woven layer by oxyacetylene torch in two heat fluxes. Results show a similar ablation
property of the two composites in the lower heat flux and a faster increase of ablation rates of C/CeSiC-2 than that of C/CeSiC-1 when heat flux is raised from 2.38 to 4.18 MW/m2. The ablated morphology of C/CeSiC-2 in the lower heat flux indicates that the centre of front face is partially covered by SiO2, which is similar to that of C/CeSiC-1. Different defects resulted from the two SiC locations dominate their distinct ablation rates in the higher heat flux.
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Acknowledgements This work is supported by the National Natural Science Foundation of China under Grant No. 51402238 and 51221001, the Fundamental Research Foundation of Northwestern Polytechnical University under Grant No. GBKY1021, the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China (Grant No. 25-TZ-2009) and the ‘‘111’’ Project under Grant No. B08040. References [1] S.F. Tang, J.Y. Deng, S.J. Wang, W.C. Liu, Corros. Sci. 51 (2009) 54e61. [2] E.L. Corral, L.S. Wslker, J. Eur. Ceram. Soc. 30 (2010) 2357e2364. [3] D.D. Jayaseelan, R.G. Sa, P. Brown, W.E. Lee, J. Eur. Ceram. Soc. 31 (2011) 361e368. [4] A. Paul, S. Venugopal, J.G.P. Binner, B. Vaidhyanathan, A.C.J. Heaton, P.M. Brown, J. Eur. Ceram. Soc. 33 (2013) 423e432. [5] Y.L. Wang, X. Xiong, G.D. Li, H.F. Liu, Z.K. Chen, W. Sun, X.J. Zhao, Corros. Sci. 65 (2012) 549e555. [6] Y.C. Ye, H. Zhang, Y.G. Tong, S.X. Bai, Ceram. Int. 39 (2013) 5477e5483. [7] X. Yang, W. Li, S. Wang, B.F. Zhang, Z.H. Chen, Compos. Part B-Eng 45 (2013) 1391e1396. [8] X.T. Shen, K.Z. Li, H.J. Li, Q.G. Fu, S.P. Li, F. Deng, Corros. Sci. 53 (2011) 105e112. [9] S. Wu, L. Chen, L. Qian, J.B. Zhang, J.F. Pan, C.J. Zhou, M.S. Ren, J.L. Sun, J. Chin. Ceram. Soc. 36 (2008) 973e977.
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