Composites Science and Technology 70(2010)678-684 Contents lists available at Science Direct Composites Science and Technology ELSEVIER journalhomepagewww.elsevier.com/locate/compscitech Effects of temperature and stress on the oxidation behavior of a 3D C/SiC composite in a combustion wind tunnel Xin gang Luan,, Laifei Cheng, Jun Zhang, Jianzhang Li, Litong Zhai Key laboratory of therme chnical University, XI'an, Shaanxi 710072, PR China b School of Materials Science and Engineering Xi' an Shiyou University, Xi'an, Shaanxi 710065, PR China ARTICLE INFO A BSTRACT High-temperature oxidation of a 3D C/Sic loads in a combustion wind tunnel at 1200-1500oC. The effects of rature and stress on the oxida- Received in revised form 24 December 2009 pted 27 December 2009 tion behavior were evaluated according to length change, lifetime and morphology of the specimens. The Available online 4 January 2010 damage mechanisms of the composite are changed from superficial oxidation to non-uniform even uni form oxidation by a tensile stress. The stressed oxidation process is controlled by a normalized threshold stress(NTS), which is increased with rising temperature. When the normalized stress(NS)is below the A Ceramic-matrix threshold value, the oxidation of carbon fibers is controlled by the in-crack diffusion, starts windward and develops region by region along the combustion gas flow. The specimen displays C Damage mechanics D Scanning electron microscopy(SEM) each reep behavior because the applied tensile load is borne by several load-bearing regions in t Oxidation to it. When NS is above NTS, the oxidation of carbon fibers is limited by the boundary layer diffusion, and the specimen exhibits a typical creep behavior. e 2009 Elsevier Ltd. All rights reserved. 1 Introduction pended on NS which was the ratio of the tensile stress to the mate rial ultimate tensile strength [10]. The behaviors were similar to Ceramic-matrix composites(CMCs)have been highly expected those of 2D C/SiC composites when NS was above NTS of 0.4 serve as gas turbine hot section components because of their However, they were stepwise when NS was below NTS. specific weight and high specific strength over a large temper Temperature and stress almost the most important ature range compared to current nickel base superalloys, their parameters in engine environments. The aims of this paper are to great damage tolerance compared to monolithic ceramics and clarify the effects of temperature and stress on the oxidation the great potential reducing component weights and cooling flow behaviors and the nts of a 3D C/SiC composite in a combustion requirements [1, 2]. wind tunnel by investigating the length change, the rate of High-speed oxidizing gases, high temperature and various loads strength change and the microstructure of fracture section. coexist in engine environments. As a promising CMC, it is neces- ary to investigate the degradation behavior and mechanisms of arbon fiber reinforced silicon carbide(C/sic)composite in engine 2. Experimental environments. The oxidation behaviors of C/SiC composites with- out external stress in a high speed combustion gas have been 2.1. Specimen preparations investigated [3-6]. The degradation of the composite was attrib- uted to the oxidation of the interlayer and the fibers. the cree Fibrous preform was prepared by the three-dimensional braid behavior of 2D C/SiC composites in static high temperature oxidiz- method and supplied by the Nanjing Institute of Glass Fiber. Car- n fiber(t-300 Japan Toray) was employed. The volume fraction that the oxidation of the carbon fibers was accelerated by the of fibers in the preform was 40-45% The composite was prepared external tensile load due to the reopening of microcracks pre isted in the matrix and the coating. The creep behaviors of 3D preform was deposited with pyrolytic carbon(Pyc) as interlayer SiC composites in a high speed combustion gas at 1300 c de- using butane and densified with Sic as matrix using methyltrichlo- rosilane(mrS). The interlayer of Pyc was deposited for I h at 870C. The conditions for deposition of the Sic matrix were as fol- Eomes ondres. 02988494620 lows: the temperature was 1100C, the time was 2 h, the flow of ahoo. com. cn(x luan). H2 was about 150 ml/min, and the molar ratio of H2 and MTs ront matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016 ech2009.12025
Effects of temperature and stress on the oxidation behavior of a 3D C/SiC composite in a combustion wind tunnel Xin’gang Luan a,*, Laifei Cheng a , Jun Zhang b , Jianzhang Li a , Litong Zhang a aNational Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, PR China b School of Materials Science and Engineering, Xi’an Shiyou University, Xi’an, Shaanxi 710065, PR China article info Article history: Received 18 September 2009 Received in revised form 24 December 2009 Accepted 27 December 2009 Available online 4 January 2010 Keywords: A. Ceramic–matrix composites (CMCs) B. Creep C. Damage mechanics D. Scanning electron microscopy (SEM) Oxidation abstract High-temperature oxidation of a 3D C/SiC composite has been conducted under various tensile creep loads in a combustion wind tunnel at 1200–1500 C. The effects of temperature and stress on the oxidation behavior were evaluated according to length change, lifetime and morphology of the specimens. The damage mechanisms of the composite are changed from superficial oxidation to non-uniform even uniform oxidation by a tensile stress. The stressed oxidation process is controlled by a normalized threshold stress (NTS), which is increased with rising temperature. When the normalized stress (NS) is below the threshold value, the oxidation of carbon fibers is controlled by the in-crack diffusion, starts from the windward and develops region by region along the combustion gas flow. The specimen displays a multiple creep behavior because the applied tensile load is borne by several load-bearing regions in turn and each region manifests a typical creep behavior after the tensile load transferred from an oxidized region to it. When NS is above NTS, the oxidation of carbon fibers is limited by the boundary layer diffusion, and the specimen exhibits a typical creep behavior. 2009 Elsevier Ltd. All rights reserved. 1. Introduction Ceramic–matrix composites (CMCs) have been highly expected to serve as gas turbine hot section components because of their low specific weight and high specific strength over a large temperature range compared to current nickel base superalloys, their great damage tolerance compared to monolithic ceramics and the great potential reducing component weights and cooling flow requirements [1,2]. High-speed oxidizing gases, high temperature and various loads coexist in engine environments. As a promising CMC, it is necessary to investigate the degradation behavior and mechanisms of carbon fiber reinforced silicon carbide (C/SiC) composite in engine environments. The oxidation behaviors of C/SiC composites without external stress in a high speed combustion gas have been investigated [3–6]. The degradation of the composite was attributed to the oxidation of the interlayer and the fibers. The creep behavior of 2D C/SiC composites in static high temperature oxidizing atmospheres was similar to that of metals [7–9]. It was found that the oxidation of the carbon fibers was accelerated by the external tensile load due to the reopening of microcracks preexisted in the matrix and the coating. The creep behaviors of 3D C/ SiC composites in a high speed combustion gas at 1300 C depended on NS which was the ratio of the tensile stress to the material ultimate tensile strength [10]. The behaviors were similar to those of 2D C/SiC composites when NS was above NTS of 0.4. However, they were stepwise when NS was below NTS. Temperature and stress were almost the most important parameters in engine environments. The aims of this paper are to clarify the effects of temperature and stress on the oxidation behaviors and the NTS of a 3D C/SiC composite in a combustion wind tunnel by investigating the length change, the rate of strength change and the microstructure of fracture section. 2. Experimental 2.1. Specimen preparations Fibrous preform was prepared by the three-dimensional braid method and supplied by the Nanjing Institute of Glass Fiber. Carbon fiber (T-300 Japan Toray) was employed. The volume fraction of fibers in the preform was 40–45%. The composite was prepared by a low-pressure chemical vapor infiltration (LPCVI) process. The preform was deposited with pyrolytic carbon (PyC) as interlayer using butane and densified with SiC as matrix using methyltrichlorosilane (MTS). The interlayer of PyC was deposited for 1 h at 870 C. The conditions for deposition of the SiC matrix were as follows: the temperature was 1100 C, the time was 2 h, the flow of H2 was about 150 ml/min, and the molar ratio of H2 and MTS 0266-3538/$ - see front matter 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.12.025 * Corresponding author. Fax: +86 029 8849 4620. E-mail address: xingangluan@yahoo.com.cn (X. Luan). Composites Science and Technology 70 (2010) 678–684 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
X Luan et aL/ Composites Science and Technology 70(2010)678-68 679 130010966 △1500 403.4 Fig. 1. Specimen drawing of the standard 3D C/SiC composite for stressed oxidation y2=6.37-608X was 10. The dog bone shaped specimens, as shown in Fig. 1, were ut from the prepared composite plates and a two-layer Sic 7.4 vas deposited to seal the open ends of the fibers and open (5.85±0.32)+(6.59±0.69)x or a standard 3D C/Sic composite, the thickness of Pyc int was 200 nm and the thickness of Sic coating was 40 um. 0.10.203040.50.60.7 2. 2. Oxidation tests Fig. 2. Lifeti andard 3D C/SiC oxidized in high temperatu ure wind tunnel The high-temperature oxidation was carried out under a tensile creep load in a wind tunnel until the specimen was failed The oxi- dizing gases were the products of the aircraft fuel burnt in air. The According to the previous results 10, for the 3D C/SiC compos te with a thin interlayer of 50 nm, the bigger scatter of lifetime at air-fuel ratio was 27.8, the total pressure was 1 atm and the gas 1300 C under NS of 0. 42 was attributed to the change of oxidation velocity was 240 m/s. The gas flow was directly impinging on the specimen. The diameter of the heated zone was 30 mm. The tem- mechanism because NS of 0. 42 was close to the ntS of oxidation peratures of the gas flow were 1200C, 1300%C and 1500 C which mechanism transformation. The bigger scatter under the three were detected by a platinum-rhodium thermocouple nearby conditions for the standard 3D C/SiC composite may result from pecimen during the tests. The tensile creep load, perpendicular the same reason Fig 3 shows the morphologies of fracture sections of the 3D C/ 8872). Several external stress levels were selected and the corre- SIC composite oxidized at 1200"C under the condition of NS of 0.32 sponding NS were listed in Table 1 or 193 min(the result belongs to range D). Four regions could be distinguished by the extent of oxidation of carbon fibers in Fig 3a. The most serious oxidation of carbon fibers occurs in the re- 23 Measurements and observatio gion closest to the windward, namely region L, in which almost all The length changes of the specimens were calculated based on out except those at the center of each bundle. As shown in Fig. 3b the displacement of loading cell recorded by INSTRON 8872. The those survived carbon fibers are oxidized uniformly Almost no car bon fiber is oxidized in the region closest to the leeward, namely scanning electron microscope(FE-SEM, JSM 6700). region IV. The fiber pull-out morphology in region IV is consisten with the fracture section morphology of as-received composite 3. Results and discussion after a tensile strength testing. In region Ill, only superficial fibers of each bundle are oxidized non-uniformly as shown in Fig 3c. 3.1. Effects of stress on the oxidation behavior The morphologies indicate that the oxidation is quite different from each region, but the oxidation of fiber bundles in every single The oxidation lifetimes of the standard 3D C/Sic composite at region is same. According to the oxidation extent of carbon fibers different temperatures under selected stress levels in a combustion it can be sure that the oxidation of the composite starts from re- wind tunnel are shown in Fig. 2. The results indicate that the data gion I and progresses gradually region by region along the gas flow scatter of lifetime is small under most of given conditions except The pull-out of fibers suggests that region Iv is not destroyed by the following:1200°cNs0.265,1200°CNs0.320and1300°/ NS 0.425. The data scatters under the three conditions are similar that the oxidation of region ll is controlled by two different re- but much larger than the others. The results can be divided into gimes in turn from the coexistence of the non-uniform oxidation two ranges. In range I, the lifetimes are longer than 90 min. In of the whole region and the uniform oxidation of survived carbon range ll, the lifetimes are shorter than 54 min. The natural loga- fibers in region ll. The coexistence of uniform and non-uniform fi- rithm of the lifetime decreases with NS linearly under all given ber oxidation can be observed more clearly in region I as shown conditions. The decrease rates in range I are faster than those i range ll, however, they are independent on temperature in each formly. however, the typical non-uniform oxidation morphologies range. This indicates that the composite is oxidized by different such as the oxidation gradient and the cone-shaped fibers still can mechanisms in two ranges and the effects of stress on the lifetime be distinguished. The non-uniform oxidation of carbon fiber in re- are more obvious in range I than in range IL. e 1 376±15 Stress(MPa 0.16±0.006 0265±001 0.32±0012 0425±0015 064±0025
was 10. The dog bone shaped specimens, as shown in Fig. 1, were cut from the prepared composite plates and a two-layer SiC coating was deposited to seal the open ends of the fibers and open pores. For a standard 3D C/SiC composite, the thickness of PyC interlayer was 200 nm and the thickness of SiC coating was 40 lm. 2.2. Oxidation tests The high-temperature oxidation was carried out under a tensile creep load in a wind tunnel until the specimen was failed. The oxidizing gases were the products of the aircraft fuel burnt in air. The air–fuel ratio was 27.8, the total pressure was 1 atm and the gas velocity was 240 m/s. The gas flow was directly impinging on the specimen. The diameter of the heated zone was 30 mm. The temperatures of the gas flow were 1200 C, 1300 C and 1500 C which were detected by a platinum–rhodium thermocouple nearby the specimen during the tests. The tensile creep load, perpendicular to the flame, was supplied by a hydraulic servo frame (INSTRON 8872). Several external stress levels were selected and the corresponding NS were listed in Table 1. 2.3. Measurements and observations The length changes of the specimens were calculated based on the displacement of loading cell recorded by INSTRON 8872. The microstructure of fracture section was observed by field-emission scanning electron microscope (FE-SEM, JSM 6700). 3. Results and discussion 3.1. Effects of stress on the oxidation behavior The oxidation lifetimes of the standard 3D C/SiC composite at different temperatures under selected stress levels in a combustion wind tunnel are shown in Fig. 2. The results indicate that the data scatter of lifetime is small under most of given conditions except the following: 1200 C/NS 0.265, 1200 C/NS 0.320 and 1300 C/ NS 0.425. The data scatters under the three conditions are similar but much larger than the others. The results can be divided into two ranges. In range I, the lifetimes are longer than 90 min. In range II, the lifetimes are shorter than 54 min. The natural logarithm of the lifetime decreases with NS linearly under all given conditions. The decrease rates in range I are faster than those in range II, however, they are independent on temperature in each range. This indicates that the composite is oxidized by different mechanisms in two ranges and the effects of stress on the lifetime are more obvious in range I than in range II. According to the previous results [10], for the 3D C/SiC composite with a thin interlayer of 50 nm, the bigger scatter of lifetime at 1300 C under NS of 0.42 was attributed to the change of oxidation mechanism because NS of 0.42 was close to the NTS of oxidation mechanism transformation. The bigger scatter under the three conditions for the standard 3D C/SiC composite may result from the same reason. Fig. 3 shows the morphologies of fracture sections of the 3D C/ SiC composite oxidized at 1200 C under the condition of NS of 0.32 for 193 min (the result belongs to range I) . Four regions could be distinguished by the extent of oxidation of carbon fibers in Fig. 3a. The most serious oxidation of carbon fibers occurs in the region closest to the windward, namely region I, in which almost all fibers are burn out. In region II, most of the carbon fibers are burn out except those at the center of each bundle. As shown in Fig. 3b, those survived carbon fibers are oxidized uniformly. Almost no carbon fiber is oxidized in the region closest to the leeward, namely region IV. The fiber pull-out morphology in region IV is consistent with the fracture section morphology of as-received composite after a tensile strength testing. In region III, only superficial fibers of each bundle are oxidized non-uniformly as shown in Fig. 3c. The morphologies indicate that the oxidation is quite different from each region, but the oxidation of fiber bundles in every single region is same. According to the oxidation extent of carbon fibers, it can be sure that the oxidation of the composite starts from region I and progresses gradually region by region along the gas flow. The pull-out of fibers suggests that region IV is not destroyed by the oxidation of carbon fibers but the tensile load. It is deduced that the oxidation of region II is controlled by two different regimes in turn from the coexistence of the non-uniform oxidation of the whole region and the uniform oxidation of survived carbon fibers in region II. The coexistence of uniform and non-uniform fi- ber oxidation can be observed more clearly in region I as shown in Fig. 4. All fibers in region I are consumed back into the holes uniformly, however, the typical non-uniform oxidation morphologies such as the oxidation gradient and the cone-shaped fibers still can be distinguished. The non-uniform oxidation of carbon fiber in region III means that the oxidation of the region is controlled by just Fig. 1. Specimen drawing of the standard 3D C/SiC composite for stressed oxidation test (all dimensions in mm). Fig. 2. Lifetimes of standard 3D C/SiC oxidized in high temperature wind tunnel under different temperatures and NS. Table 1 Stress levels and corresponding NS for oxidation of 3D C/SiC composites. UTS (MPa) 376 ± 15 Stress (MPa) 60 100 120 160 240 NS 0.16 ± 0.006 0.265 ± 0.01 0.32 ± 0.012 0.425 ± 0.015 0.64 ± 0.025 X. Luan et al. / Composites Science and Technology 70 (2010) 678–684 679
X Luan et aL /Composites Science and Technology 70(2010)678-684 a I 300kv120mm×35sE(M c 300k∨120mmx150SE(M) Fig. 3. Morphologies of 3D C/SiC composite oxidized in high temperature wind tunnel at 1200.C under NS of 0.32 with lifetime of 193 min: (a)fraction section;(b) magnification of frame in belt ll and (c)magnification of frame in belt lll. one regime all the time, and the region is the last oxidized region ing area closed to the windward are oxidized simultaneously be- before the specimen broken. cause oxidizing gases are pressed into the cracks by the high The formation of different regions in the fracture section prob- speed gas flow, however, the oxidation h bundle is non-uni- ibly results from the uneven load-bearing of carbon fibers. For the form, as shown in Fig 3c. Once the load-bearing area can not bear 3D C/SiC composite, the fibrous tightness is different from the out- the external load due to the oxidation of the carbon fibers, the side to the inside The outside fibers will tension prior to the inside cracks will spread into the next region. Then, the region become fibers during a tensile process. when the load is small, the limited to the new load-bearing area, in which the fiber bundles are oxi- external load could be borne by the prior tensioned fibers because dized as mentioned above. The survived carbon fibers in the ex the strength of the carbon fiber is higher than the composite. The load-bearing region are oxidized uniformly, as shown in Fig. 3b, be- load-bearing area, in which the prior load-bearing fibers existed, cause the cracks are wide enough and full of o includes two oxidation regions, first region I and Iv, next region Fig 5 shows the morphologies of fracture section of the 3D C/ and Iv, then region lll and Iv All fiber bundles in the load-bear- Sic composite oxidized at 1200C under NS of 0.32 with lifetime
one regime all the time, and the region is the last oxidized region before the specimen broken. The formation of different regions in the fracture section probably results from the uneven load-bearing of carbon fibers. For the 3D C/SiC composite, the fibrous tightness is different from the outside to the inside. The outside fibers will tension prior to the inside fibers during a tensile process. When the load is small, the limited external load could be borne by the prior tensioned fibers because the strength of the carbon fiber is higher than the composite. The load-bearing area, in which the prior load-bearing fibers existed, includes two oxidation regions, first region I and IV, next region II and IV, then region III and IV. All fiber bundles in the load-bearing area closed to the windward are oxidized simultaneously because oxidizing gases are pressed into the cracks by the high speed gas flow, however, the oxidation of each bundle is non-uniform, as shown in Fig. 3c. Once the load-bearing area can not bear the external load due to the oxidation of the carbon fibers, the cracks will spread into the next region. Then, the region become to the new load-bearing area, in which the fiber bundles are oxidized as mentioned above. The survived carbon fibers in the exload-bearing region are oxidized uniformly, as shown in Fig. 3b, because the cracks are wide enough and full of oxidizing gases. Fig. 5 shows the morphologies of fracture section of the 3D C/ SiC composite oxidized at 1200 C under NS of 0.32 with lifetime Fig. 3. Morphologies of 3D C/SiC composite oxidized in high temperature wind tunnel at 1200 C under NS of 0.32 with lifetime of 193 min: (a) fraction section; (b) magnification of frame in belt II and (c) magnification of frame in belt III. 680 X. Luan et al. / Composites Science and Technology 70 (2010) 678–684
X Luan et aL/Composites Science and Technology 70(2010)678-684 oxidation mechanisms of the composite depend on NS. When NS is close to the nTs, a small fluctuation of stress will change the deg adation mechanism and result in remarkable differences of the morphologies and length changes. In this work, the fluctuation of 0 MPa or 200 N is acceptable after taking into account the multi le effects of the difference of specimens, the fluctuation of tem perature of the combustion gas and the load controlling precision of the frame 3. 2. Effects of temperature on the oxidation behavior oxidation behavior [11]. Temperature affects the stressed oxida- C The first aspect is fiber strength. It was found that the tensile strength of carbon fiber increase linearly with temperature at 1200-1500C even if the fiber was exposed tures for several seconds [12]. In term of this reason, the lifetime Fig 4. Mor of uniform/non-uniform fiber oxidation coexistence of 3D C/Sic of the composite will increase linearly with increasing omposite oxidized in high temperature wind tunnel at 1200 oc under NS of 0.32 temperature with lifetime of 193 min The second aspect is crack width. Without the external load the cracks in the 3D C/Sic composite were closed above 900C due to of 43 min(the result belongs to range If). The morphologies are dif- the thermal expansion of matrix [13, 14. However, the cracks were ferent from that shown in Fig 3. The fracture section is oxidized as reopened by the applied tensile load during the stressed oxidation a whole. The destruction of the specimen should be attributed to tests. the crack width mainly depends on the differences of ther the external tensile load mainly rather than the oxidation of the mal expansion between free carbon fiber and free silicon carbide arbon fibers because lots of carbon fibers are pulled out, mean- matrix resulted from the interlayer debond by the crack propaga- while, a few carbon fibers are oxidized(see Fig 5a). As shown in tion. the bonded fiber and matrix does not contribute to the crack Fig 5b and c, regardless near the windward or in the center of width because they respond to the stress and the temperature as a the cross section, the oxidation morphologies of the carbon fibers whole. The crack width decreases with rising temperature because tre similar. The cross section of each bundle can be divided into the coefficient of thermal expansion of Sic is bigger than carbon fi two parts from the corners to the center: the central part with fiber ber. The increase of NTS from 0.265 to 0.32 at 1200C to 0.425 remaining and the surrounding part with fiber burning out. It is 1300C is attributed to the decrease of crack width with increasing illuminated by the non-uniform oxidation morphologies that the temperature carbon fibers in the surrounding part are oxidized during the test. The third aspect is oxidation rate of carbon fiber. The oxidation however, the carbon fibers in the central part are oxidized after the of carbon fiber was depended on the supply of oxidizing gase specimen failed because the oxidation of the carbon fibers is above 600C[8]. For the carbon fiber composite, the supply of oxi dizing gases was controlled by two regimes: in-pore(or in-crack he morphology of fracture section of the 3D C/Sic composite diffusion and boundary layer diffusion[15]. when the oxidation oxidized under NS of 0.64 with lifetime of 4.5 min(belongs to of carbon fibers is controlled by the former, the oxidation lifetime range Il) is shown in Fig. 6. Comparing to Fig 5a, it is found that of the 3D C/Sic composite will increase with rising temperature be- the morphologies of fracture section of the two specimens are sim- cause the diffusion rate decreases with decreasing crack width ilar. It is illuminated that the specimens whose lifetimes belonged When the oxidation of carbon fibers is controlled by the latter, to the same range have the same degradation mechanism. Con- the oxidation lifetime will decrease with increasing temperature versely, the specimens whose lifetimes belong to the different because the boundary layer diffusion coefficient increases with o nges have a different degradation mechanism according to the temperature to the 1.5 power [16]. difference of Figs. 3 and 5 The effects of temperature on the oxidation lifetime of the 3D C/ There are distinct effects of degradation mechanisms on the SiC composite under creep stresses in a combustion wind tunnel progresses of length change of the 3D C/SiC composite during the depend on the oxidation regime of carbon fibers. As shown in stressed oxidation As shown in Fig. 7, when the composite are oxi- Fig 8, the longer lifetime and the faster and faster increase of life- dized region by region and the morphologies are non-uniform, the time are displayed if the oxidation of carbon fibers is controlled by eral typical creep curves in tandem. It is suggested that each region NS of 0.265. The regime results in the non-uniform morphologies keep its as-received mechanical properties until it become the of the load-bearing region(see Fig 3). In this condition, the degra- load-bearing region. When the morphologies of the oxidize dation mechanism of each load-bearing area is similar to that of 2D bon fibers are uniform, the length of the specimen changes follow- C/SiC above 1100C [17] and the shrinking core oxidation patte g a typical creep behavior Under the condition of NS of 0.64, just of each bundle in the load-bearing area is consisted with that of 2D the first phase of typical creep curve is kept due to the quick failure C/Sic [18, 19 With the increase of applied tensile load, more and of the carbon fibers resulted from the coupled damage of oxidation more combustion gas go into the inside of the specimens due to and overload. The jump of the line in the box(see Fig. 7)indicates the crack widening, then all exposed fibers are immersed in the that the carbon fibers do not bear the load as a whole. They are di- same combustion gas and oxidized following the ne of bound- vided into two parts to bear the load in turn. However, the two ary layer diffusion. A shorter lifetime and a slower and slower in- load-bearing areas are difficult to distinguish in Fig. 6 due to the crease of lifetime under the condition of NS of 0.64(see Fig. 8) quick failure of carbon fiber e resulted from this regime, as well as a uniform morphology The differences of the morphologies and the length change con- of the carbon fibers in the surrounding part of each bundle(see rm that the nts is between 0. 265 and 0.320 at 1200C and the Fig 5b)
of 43 min (the result belongs to range II). The morphologies are different from that shown in Fig. 3. The fracture section is oxidized as a whole. The destruction of the specimen should be attributed to the external tensile load mainly rather than the oxidation of the carbon fibers because lots of carbon fibers are pulled out, meanwhile, a few carbon fibers are oxidized (see Fig. 5a). As shown in Fig. 5b and c, regardless near the windward or in the center of the cross section, the oxidation morphologies of the carbon fibers are similar. The cross section of each bundle can be divided into two parts from the corners to the center: the central part with fiber remaining and the surrounding part with fiber burning out. It is illuminated by the non-uniform oxidation morphologies that the carbon fibers in the surrounding part are oxidized during the test, however, the carbon fibers in the central part are oxidized after the specimen failed because the oxidation of the carbon fibers is uniform. The morphology of fracture section of the 3D C/SiC composite oxidized under NS of 0.64 with lifetime of 4.5 min (belongs to range II) is shown in Fig. 6. Comparing to Fig. 5a, it is found that the morphologies of fracture section of the two specimens are similar. It is illuminated that the specimens whose lifetimes belonged to the same range have the same degradation mechanism. Conversely, the specimens whose lifetimes belong to the different ranges have a different degradation mechanism according to the difference of Figs. 3 and 5. There are distinct effects of degradation mechanisms on the progresses of length change of the 3D C/SiC composite during the stressed oxidation. As shown in Fig. 7, when the composite are oxidized region by region and the morphologies are non-uniform, the length change of the specimen is not a typical creep curve, but several typical creep curves in tandem. It is suggested that each region keep its as-received mechanical properties until it become the load-bearing region. When the morphologies of the oxidized carbon fibers are uniform, the length of the specimen changes following a typical creep behavior. Under the condition of NS of 0.64, just the first phase of typical creep curve is kept due to the quick failure of the carbon fibers resulted from the coupled damage of oxidation and overload. The jump of the line in the box (see Fig. 7) indicates that the carbon fibers do not bear the load as a whole. They are divided into two parts to bear the load in turn. However, the two load-bearing areas are difficult to distinguish in Fig. 6 due to the quick failure of carbon fibers. The differences of the morphologies and the length change con- firm that the NTS is between 0.265 and 0.320 at 1200 C and the oxidation mechanisms of the composite depend on NS. When NS is close to the NTS, a small fluctuation of stress will change the degradation mechanism and result in remarkable differences of the morphologies and length changes. In this work, the fluctuation of 20 MPa or 200 N is acceptable after taking into account the multiple effects of the difference of specimens, the fluctuation of temperature of the combustion gas and the load controlling precision of the frame. 3.2. Effects of temperature on the oxidation behavior Temperature was shown to have a profound influence on the oxidation behavior [11]. Temperature affects the stressed oxidation behavior of the 3D C/SiC composite by three aspects. The first aspect is fiber strength. It was found that the tensile strength of carbon fiber increase linearly with temperature at 1200–1500 C even if the fiber was exposed to the high temperatures for several seconds [12]. In term of this reason, the lifetime of the composite will increase linearly with increasing temperature. The second aspect is crack width. Without the external load, the cracks in the 3D C/SiC composite were closed above 900 C due to the thermal expansion of matrix [13,14]. However, the cracks were reopened by the applied tensile load during the stressed oxidation tests. The crack width mainly depends on the differences of thermal expansion between free carbon fiber and free silicon carbide matrix resulted from the interlayer debond by the crack propagation. The bonded fiber and matrix does not contribute to the crack width because they respond to the stress and the temperature as a whole. The crack width decreases with rising temperature because the coefficient of thermal expansion of SiC is bigger than carbon fi- ber. The increase of NTS from 0.265 to 0.32 at 1200 C to 0.425 at 1300 C is attributed to the decrease of crack width with increasing temperature. The third aspect is oxidation rate of carbon fiber. The oxidation of carbon fiber was depended on the supply of oxidizing gases above 600 C [8]. For the carbon fiber composite, the supply of oxidizing gases was controlled by two regimes: in-pore (or in-crack) diffusion and boundary layer diffusion [15]. When the oxidation of carbon fibers is controlled by the former, the oxidation lifetime of the 3D C/SiC composite will increase with rising temperature because the diffusion rate decreases with decreasing crack width. When the oxidation of carbon fibers is controlled by the latter, the oxidation lifetime will decrease with increasing temperature because the boundary layer diffusion coefficient increases with temperature to the 1.5 power [16]. The effects of temperature on the oxidation lifetime of the 3D C/ SiC composite under creep stresses in a combustion wind tunnel depend on the oxidation regime of carbon fibers. As shown in Fig. 8, the longer lifetime and the faster and faster increase of lifetime are displayed if the oxidation of carbon fibers is controlled by the in-crack diffusion of the combustion gas under the condition of NS of 0.265. The regime results in the non-uniform morphologies of the load-bearing region (see Fig. 3). In this condition, the degradation mechanism of each load-bearing area is similar to that of 2D C/SiC above 1100 C [17] and the shrinking core oxidation pattern of each bundle in the load-bearing area is consisted with that of 2D C/SiC [18,19]. With the increase of applied tensile load, more and more combustion gas go into the inside of the specimens due to the crack widening, then all exposed fibers are immersed in the same combustion gas and oxidized following the regime of boundary layer diffusion. A shorter lifetime and a slower and slower increase of lifetime under the condition of NS of 0.64 (see Fig. 8) are resulted from this regime, as well as a uniform morphology of the carbon fibers in the surrounding part of each bundle (see Fig. 5b). Fig. 4. Morphology of uniform/non-uniform fiber oxidation coexistence of 3D C/SiC composite oxidized in high temperature wind tunnel at 1200 C under NS of 0.32 with lifetime of 193 min. X. Luan et al. / Composites Science and Technology 70 (2010) 678–684 681
X Luan et aL /Composites Science and Technology 70(2010)678-684 b c logies of 3D c/sic composite oxidized in high temperature wind tunnel at 1200"C under NS of 0.32 with lifetime of 43 min: (a)fraction section;(b) agnification of left frame and (c) magnification of right fram As shown in Figs. 9 and 10, the model of length change has no limited by in-crack diffusion of oxidizing gases. When NS is above relationship with the temperature once the damage mechanism NTS, the load-bearing region can not be distinguished clearly from is fixed. However, the multiple creep behavior become more and the morphologies of fracture section. The oxidation of the exposed more clearly with increasing temperature when the oxidation of fibers is controlled by the diffusion of oxidizing gases through the load-bearing region is controlled by in-crack diffusion as shown boundary layer. The nts increases with temperature due to the difference of thermal expansion between the free carbon fibers and the free sic 4. Conclusions matrix. The multiple creep behavior become more and more clearly with the increase of temperature when the oxidation of the load- The damage mechanisms of the standard 3D C/Sic composite in bearing region is controlled by in-crack diffusion of oxidizing gase combustion wind tunnel depended on NTS. When NS is below NTS, however, the models of length change depend on the damage mech- the composite is damaged region by region which is named the anism of the composite and do not change with temperature under load-bearing region. The oxidation of the load-bearing region is the same damage mechanism whatever what it is
As shown in Figs. 9 and 10, the model of length change has no relationship with the temperature once the damage mechanism is fixed. However, the multiple creep behavior become more and more clearly with increasing temperature when the oxidation of load-bearing region is controlled by in-crack diffusion as shown in Fig. 9. 4. Conclusions The damage mechanisms of the standard 3D C/SiC composite in combustion wind tunnel depended on NTS. When NS is below NTS, the composite is damaged region by region which is named the load-bearing region. The oxidation of the load-bearing region is limited by in-crack diffusion of oxidizing gases. When NS is above NTS, the load-bearing region can not be distinguished clearly from the morphologies of fracture section. The oxidation of the exposed fibers is controlled by the diffusion of oxidizing gases through the boundary layer. The NTS increases with temperature due to the difference of thermal expansion between the free carbon fibers and the free SiC matrix. The multiple creep behavior become more and more clearly with the increase of temperature when the oxidation of the loadbearing region is controlled by in-crack diffusion of oxidizing gases, however, the models of length change depend on the damage mechanism of the composite and do not change with temperature under the same damage mechanism whatever what it is. Fig. 5. Morphologies of 3D C/SiC composite oxidized in high temperature wind tunnel at 1200 C under NS of 0.32 with lifetime of 43 min: (a) fraction section; (b) magnification of left frame and (c) magnification of right frame. 682 X. Luan et al. / Composites Science and Technology 70 (2010) 678–684
X Luan et aL/Composites Science and Technology 70(2010)678-684 可品一 Duration 00mm Fig. ology of fracture section of 3D oxidized in high e mperature wind tunnel at 1200 C under NS of 0.64 with lifetime of 4.5 min. 0.16 Duration/min 0.12 255075100125150175200 1500°C uniform oxidation Fig. 10. Effects of temperature on length change of 3D C/sic composite during uniform oxidation Acknowled tunnel at 1200"C under different stresses or degradation mechanisms. The authors acknowledge the financial support of Natural Sci- ence Foundation of China(No. 90405015 and No 50820145202 and program for Yazijiang Scholars and Innovative Research Team. 1000g References [11 Lachapelle DG, Noe /G, Stegemiller H. Steibel. 100 [21 Beyer S, Sc “ pplications. American Institute of Aeronautics and s. AIAA 2004- [3] Cheng LF, Xu YD, Zhang LT, Yin XW. Oxidation behavior of three dimensional C/ [4] Yin xW. Cheng LF, Zhang LT, Xu YD, Li JZ Oxidation behavior of 3D C/Sic D NS-0 265 non-uniform oxidation [5] Filsinger D, Munz S, Schulz A, Wittig S, Anrees G. Experimental assessment of rced ceramics for combustor walls. J Eng Gas Turb Power 1200125013001350140014501500 awa H. Environmental Ceram Eng Sci Proc Fig 8. Effect of tempe on stressed oxidation lifetime of 3D C/siC composite con carbide composite in a high-temperature combustion environment. in high temperature wind tunnel under different NS. Am Ceram Soc2008:91(1):291-5
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