Investigation of Failure Mechanisms in Ceramic Composites As Potential Railway Brake Disc Materials

Article Investigation of Failure Mechanisms in Ceramic Composites as Potential Railway Brake Disc Materials

Jeongguk Kim

Advanced Railroad Vehicle Division, Korea Railroad Research Institute, Uiwang 16105, Korea; [email protected]; Tel.: +82-31-460-5518 Received: 21 October 2020; Accepted: 13 November 2020; Published: 15 November 2020

Abstract: Ceramic composite materials have been efficiently used for high-temperature structural applications with improved toughness by complementing the shortcomings of monolithic ceramics. In this study, the fracture characteristics and fracture mechanisms of ceramic composite materials were studied. The ceramic composite material used in this study is Nicalon ceramic fiber reinforced ceramic matrix composites. The tensile failure behavior of two types of ceramic composites with different microstructures, namely, plain-weave and cross-ply composites, was studied. Tensile tests were performed on two types of ceramic composite material specimens. Microstructure analysis using SEM was performed to find out the relationship between tensile fracture characteristics and microstructure. It was found that there was a difference in the fracture mechanism according to the characteristics of each microstructure. In this study, the results of tensile tests, failure modes, failure characteristics, and failure mechanisms were analyzed in detail for two fabric structures, namely, plain-weave and cross-ply structures, which are representative of ceramic matrix composites. In order to help understanding of the fracture process and mechanism, the fracture initiation, crack propagation, and fracture mechanism of each composite material are schematically expressed in a two-dimensional figure. Through these results, it is intended to provide useful information for the design of ceramic composite materials based on the mechanistic understanding of the fracture process of ceramic composite materials.

Keywords: ceramic composites; mechanical properties; tensile tests; microstructural analysis; failure mode; failure mechanism

1. Introduction Ceramic composites are structural materials used at high temperatures that have been proven over the past few decades [1–4]. For this reason, it has been spotlighted as an excellent material in spacecraft insulation materials, high-temperature gas turbine rotors, and thermal management systems, and, recently, it is expanding its use as a material for brake discs for automobiles and railways [1–6]. Stadler et al. studied ceramic composite materials as friction materials [7]. In recent years, it has also been used as a braking friction material in the railroad field, increasing its utilization [6–9]. Stadler et al. proposed carbon fiber reinforced SiC composites as a railway friction material, and reported that the properties of this material show excellent corrosion and oxidation resistance, and have excellent friction and wear characteristics. It was also recommended that ceramic composites are the material that can be protected from the high temperatures occurring on the disc surface during braking [7]. In general, monolithic ceramics exhibit excellent high-temperature properties, corrosion resistance, abrasion resistance, etc., but their use is limited due to the essential characteristics of brittleness. Therefore, the development of a technology that imparts toughness to such monolithic ceramics has become important, and one solution to this is ceramic composite materials reinforced with ceramic fibers [1–5,10,11].

Materials 2020, 13, 5141; doi:10.3390/ma13225141 www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 11

Materials 2020, 13, 5141 2 of 11 such monolithic ceramics has become important, and one solution to this is ceramic composite materials reinforced with ceramic fibers [1–5,10,11]. FigureFigure 11 schematicallyschematically illustratesillustrates aa comparisoncomparison ofof thethe fracturefracture behaviorbehavior betweenbetween monolithicmonolithic ceramicsceramics and and ceramic ceramic composites. composites. As As shown shown in in Figu Figurere 1,1, monolithic ceramics show a typical brittle fracturefracture behavior behavior that that leads leads to to failure failure immediat immediatelyely after after undergoing undergoing elastic elastic deformation deformation without without a a plasticplastic deformationdeformation region.region. On On the th othere other hand, hand, in the in case the of case the ceramicof the ceramic composite composite material reinforcedmaterial reinforcedwith continuous with continuous fibers, it initially fibers, showsit initially the elasticshows behaviorthe elastic of behavior monolithic of monolithic ceramics, and ceramics, then shows and thenthe first shows matrix the crackingfirst matrix as the cracking load applied as the toload the a specimenpplied to increases.the specimen However, increases. as the However, stress increases, as the stressthe continuous increases, increasethe continuous in load increase continues in load due tocontinues complex due interactions to complex such interactions as further such matrix as further failure matrixdue to failure fibers presentdue to fibers inside presen the matrixt inside and the crack matrix deflection and crack at the deflection interface atbetween the interface the fibers between and the the fibersmatrix and (Figure the 1matrix)[ 3,4, 10(Figure–12]. After 1) [3,4,10–12]. the ultimate After tensile the stressultimate (UTS) tensil is reached,e stress the(UTS) composite is reached, shows the a compositestress-strain shows behavior a stress-strain like the shape behavior of plastic like the deformation shape of plastic exhibited deformation by a metallic exhibited material by a due metallic to the materialsequential due pullout to the of sequential the fibers untilpullout the of final the failure. fibers Dueuntil to the this final fracture failure. mechanism, Due to this it is fracture possible mechanism,to improve theit is fracturepossible toughnessto improve of the monolithic fracture toughness ceramics. of In monolithic other words, ceramics. the fracture In other toughness words, theincreases fracture by toughness the larger increases area in the by stress-strainthe larger area curve. in the In stress-strain addition, Figure curve.1 shows In addition, that interfacial Figure 1 showsbonding that between interfacial fiber bonding and matrix between material fiber is and also matrix an important materialfactor. is also Ifan there important is a strong factor. interfacial If there isbonding a strong between interfacial the fiberbonding and thebetween matrix, the the fiber fiber and reinforcement the matrix, etheffect fiber will notreinforcement be seen, and effect if there will is notproper be seen, interfacial and bondingif there betweenis proper the interfacia fiber andl bonding matrix, a between ceramiccomposite the fiber materialand matrix, with a reinforced ceramic compositetoughness material as shown with in Figure reinforced1 can toughness be expected. as shown in Figure 1 can be expected.

Figure 1. A schematic comparison of fracture behavior between monolithic ceramics and ceramic Figure 1. A schematic comparison of fracture behavior between monolithic ceramics and ceramic composites [3]. composites [3]. Figure2 schematically shows the typical failure patterns found in ceramic composites [ 3,4,10]. As theFigure load 2 increasesschematically after shows the initial the typical matrix failure cracking, patterns phenomena found in such ceramic as interfacial composites debonding [3,4,10]. Asand the sliding, load increases crack deflection, after the and initial crack matrix bridging cracking, between phenomena the fiber andsuch the as matrixinterfacial material debonding go through. and sliding,After that, crack the deflection, fiber is finally and broken,crack bridging and the between final failure the mechanismfiber and the is completedmatrix material through go extensivethrough. Afterfiber pullout.that, the fiber The fractureis finally toughness broken, and of thethe ceramicfinal failure composite mechanism material is completed is improved through by delaying extensive the fiberfinal pullout. fracture throughThe fracture those toughness energy absorption of the ceramic mechanisms composite [10]. material Marshal is et improved al. investigated by delaying the failure the finalmechanisms fracture inthrough ceramic those matrix energy composites absorption [10]. mechanisms [10]. Marshal et al. investigated the failureFigure mechanisms3 shows in an ceramic example matrix of the schematiccomposites representation [10]. shown in Figure2. It is obtained from the fractureFigure 3 surface shows ofan theexample actual of ceramic the schematic composite repres material,entation and shown it shows in Figure in detail 2. theIt isprocess obtained of fromabsorbing the fracture fracture surface energy of suchthe actual as matrix ceramic cracking composite and propagation material, and of it crack, shows interfacial in detail the sliding process and debonding between fiber and matrix, fiber bridging, and fiber pullout [10,13,14].

Materials 2020, 13, x FOR PEER REVIEW 3 of 11 Materials 2020, 13, x FOR PEER REVIEW 3 of 11 of absorbing fracture energy such as matrix cracking and propagation of crack, interfacial sliding and of absorbing fracture energy such as matrix cracking and propagation of crack, interfacial sliding and Materialsdebonding2020 ,between13, 5141 fiber and matrix, fiber bridging, and fiber pullout [10,13,14]. 3 of 11 debonding between fiber and matrix, fiber bridging, and fiber pullout [10,13,14].

Figure 2. A schematic representation of typical failure patterns of ceramic composites. Figure 2. A schematic representation of typical failure patterns of ceramic composites. Figure 2. A schematic representation of typical failure patterns of ceramic composites.

Figure 3. Typical examples of failure mechanisms in ceramic matrix composites as failure energy Figure 3. Typical examples of failure mechanisms in ceramic matrix composites as failure energy absorption mechanism. Figureabsorption 3. Typical mechanism. examples of failure mechanisms in ceramic matrix composites as failure energy absorption mechanism. Until now, there have been many studies on the mechanical properties and failure of ceramic Until now, there have been many studies on the mechanical properties and failure of composites, but relatively few studies on the fracture mechanism and detailed fracture process have ceramicUntil composites, now, there but have relatively been many few studies studies on on the the fracture mechanical mechanism properties and detailed and failure fracture of been carried out [10,14–18]. Therefore, in this study, as discussed above, the fracture behavior of the ceramicprocess composites,have been carriedbut relatively out [10,14–18]. few studies Theref on theore, fracture in this mechanism study, as anddiscussed detailed above, fracture the actual ceramic composite material was studied, and a study was conducted on the fracture mechanism processfracture havebehavior been ofcarried the actual out [10,14–18]. ceramic composite Therefore, material in this study,was studied, as discussed and a above,study wasthe that can support the fracture behavior. After tensile testing of ceramic composite materials, a study on fractureconducted behavior on the fractureof the actualmechanism ceramic that compositecan suppo rtmaterial the fracture was behavior. studied, Afterand tensilea study testing was the failure path and failure mechanism was conducted through the analysis of the specimen using an conductedof ceramic on composite the fracture materials, mechanism a thatstudy can on suppo the rtfailure the fracture path behavior.and failure After mechanism tensile testing was electron microscope. Based on this research, it is the main objective of this study to provide schematic ofconducted ceramic throughcomposite the materials, analysis ofa thestudy specimen on the using failure an pathelectron and microscope. failure mechanism Based on was this explanations of the failure mode and mechanism after analysis of the process of fracture. Therefore, conductedresearch, it through is the main the analysisobjective of of thethis specimen study to provideusing an schematic electron microscope.explanations Based of the on failure this the main objectives of this investigation are to (1) investigate the fracture mechanisms of Nicalon/SiC research,mode and it mechanismis the main afterobjective analysis of this of thestudy process to provide of fracture. schematic Therefore, explanations the main of objectives the failure of composites both plain-weave and cross-ply composites, (2) conduct microstrucural analyses in tensile modethis investigation and mechanism are to after (1) investigateanalysis of the processfracture ofmechanisms fracture. Therefore, of Nicalon/SiC the main composites objectives both of failured specimens, (3) visualize the failure mechanisms with two-dimensional schematic presentation, thisplain-weave investigation and arecross-ply to (1) investigate composites, the (2) fracture conduct mechanisms microstrucural of Nicalon/SiC analyses in composites tensile failured both andplain-weave (4) provide mechanistic and cross-ply understanding composites, of (2) failure conduct characteristics microstrucural of Nicalon analyses/SiC in composites tensile failured for the design of ceramic matrix composites.

Materials 2020, 13, x FOR PEER REVIEW 4 of 11

specimens, (3) visualize the failure mechanisms with two-dimensional schematic presentation, and (4) provide mechanistic understanding of failure characteristics of Nicalon/SiC composites for the design of ceramic matrix composites. Materials 2020, 13, 5141 4 of 11 2. Materials and Experimental Procedures 2. MaterialsThe ceramic and Experimentalcomposite material Procedures used in this study is Nicalon ceramic fiber reinforced SiC ceramicThe ceramicmatrix compositecomposite material(Nicalon/SiC). used in thisNicalo studyn/SiC is Nicalon composites ceramic are fiber representative reinforced SiC ceramic ceramic compositesmatrix composite that are (Nicalon used in/SiC). various Nicalon applications/SiC composites such as are ceramic representative rotors and ceramic heat exchangers, composites thatetc. [1– are 4].used These in various composites applications can be used such as as friction ceramic material rotors ands in heatthe future exchangers, railway etc. field, [1–4 therefore,]. These composites they were appliedcan be used in this as frictionstudy. Nicalon materials fiber in theis a future silicon railway carbide-based field, therefore, ceramic theyfiber were with applieda diameter in this of about study. 10Nicalon microns, fiber and is a the silicon Nicalon/SiC carbide-based composite ceramic was fiber fabricated with a diameterusing the of isothermal about 10 microns, chemical and vapor the infiltrationNicalon/SiC (ICVI) composite method. was fabricated using the isothermal chemical vapor infiltration (ICVI) method. Table1 1 shows shows the the mechanical mechanical properties properties data data of of Nicalon Nicalon fiber fiber and and SiC. SiC. In In this this study, study, two two types types of ofspecimens specimens were were used, used, plain-weave plain-weave Nicalon Nicalon//SiC compositeSiC composite and 0/90 and cross-ply 0/90 Nicaloncross-ply/SiC Nicalon/SiC composite. Thecomposite. fiber volume The fiber content volume was content 40 wt% was for both40 wt types% for ofboth composite types of materials. composite materials.

Table 1. The mechanical properties of Nicalon fiberfiber and SiC [[1].1]. Materials Tensile Strength (GPa) Densigy (g/cm3) Materials Tensile Strength (GPa) Densigy (g/cm3) Nicalon 200 2.5 NicalonSiC 240 200 3.21 2.5 SiC 240 3.21 Figure 4 provides a cross-sectional views of the two composite materials. Figure 5 shows the size and geometryFigure4 provides of the specimen, a cross-sectional and the views thickness of the of two the compositespecimen materials.is about 3 Figuremm. Monotonic5 shows the tensile size testand was geometry carried of out the at specimen, room temperature and the thickness using MT ofS testing the specimen equipment is about (MTS 3 mm.810, MTS, Monotonic Eden Prairie, tensile MN,test was USA). carried The tensile out at roomtest was temperature performed using under MTS disp testinglacement equipment control with (MTS a 810,cross-head MTS, Eden speed Prairie, of 0.3 mm/min.MN, USA). The The strain tensile was testmeasured was performed by attaching under an axial displacement extensometer control (MTS, with Eden a Prairie, cross-head MN, speedUSA) toof the 0.3 mmgrip/ min.part of The the strain specimen was measured (Figure 6), by and attaching the tensile an axialtest was extensometer performed (MTS, according Eden to Prairie, the ASTM MN, guidelinesUSA) to the [19]. grip Since part the of the specimen specimen used (Figure in this6), study and the is a tensile ceramic-based test was performedspecimen, an according aluminum to thetab ASTMwas attached guidelines to the [19 grip]. Since portion the specimen to prevent used the ingr thisip portion study is from a ceramic-based being distorted specimen, when the an aluminum specimen wastab was mounted attached on to the the tensile grip portion tester toas prevent shown thein gripFigu portionre 6. For from the beingtensile distorted tests, three when samples the specimen were wasprepared mounted and tested on the for tensile each testercompos asite shown material. in Figure After 6the. For tensile the tensiletest, microstructural tests, three samples analysis were was preparedperformed and using tested SEM for (S-360, each composite Cambridge material. Instrument Afters, the Cambridge, tensile test, UK). microstructural After that, analysisin order was to performedprovide information using SEM to (S-360, readers Cambridge in an easy Instruments, way, the result Cambridge, of this UK).analysis After was that, visualized in order toas provide a two- dimensionalinformation tofigure readers to express in an easy the way,final thefailure result process. of this analysis was visualized as a two-dimensional figure to express the final failure process.

(a) (b)

Figure 4. TheThe microstructures microstructures of ( a)) plain-weave plain-weave and ( b) cross-ply Nicalon/SiC Nicalon/SiC composites.

Materials 2020, 13, x FOR PEER REVIEW 5 of 11 Materials 2020, 13, x FOR PEER REVIEW 5 of 11 Materials 2020, 13, 5141 5 of 11

(a) (b)

Figure 5. The geometries of tensile(a )test specimens. All dimensions(b) in mm. The Radius of curvature in FiguretheFigure shoulder 5. 5.The The section: geometries geometries 100 mm. of of tensile tensile (a) plain-weave test test specimens. specimens. specimen. All All dimensions dimensions (b) cross-ply in in mm. mm.specimen. The The Radius Radius of of curvature curvature in in thethe shoulder shoulder section: section: 100 100 mm. mm. ( a(a)) plain-weave plain-weave specimen. specimen. ( b(b)) cross-ply cross-ply specimen. specimen.

Figure 6. A schematic illustration of mechanical testing setup with specimen. Figure 6. A schematic illustration of mechanical testing setup with specimen. 3. Results and Discussion Figure 6. A schematic illustration of mechanical testing setup with specimen. 3. ResultsFigure and7 shows Discussion the tensile test results of the Nicalon /SiC ceramic composite material. In common for3. Results bothFigure specimens, and7 shows Discussion the tensile proportional test results limit of value the Ni wascalon/SiC obtained ceramic around composite 100 MPa, material. showing In the common elastic region up to this point (Figure1). After that, as the load on the specimen increases, the load curve for bothFigure specimens, 7 shows the the proportional tensile test results limit valueof the wasNicalon/SiC obtained ceramic around composite 100 MPa, material.showing Inthe common elastic becomes increasingly nonlinear. Both composite material specimens were finally fractured near regionfor both up specimens, to this point the (Figure proportional 1). After limit that, value as the was load obtained on the around specimen 100 increases, MPa, showing the load the curveelastic the ultimate tensile stress (UTS), and the maximum tensile stresses were about 225 and 300 MPa becomesregion up increasingly to this point nonlinear. (Figure 1).Both After composite that, as materialthe load specimenson the specimen were finally increases, fractured the load near curve the for plain-weave and cross-ply composites, respectively. The tensile load of the cross-ply composite ultimatebecomes tensile increasingly stress (UTS), nonlinear. and the Both maximum composite tens materialile stresses specimens were about were 225 finally and 300 fractured MPa for near plain- the specimen was found to be higher, which is due to the microstructural influence, and it is believed weaveultimate and tensile cross-ply stress composites, (UTS), and respectively.the maximum The tens teilensile stresses load wereof the about cross-ply 225 and composite 300 MPa specimen for plain- that the plain-weave composite material is structurally more vulnerable to loads than the cross-ply wasweave found and to cross-ply be higher, composites, which is due respectively. to the microstruc The teturalnsile influence, load of the and cross-ply it is believed composite that the specimen plain- structure [1,4]. As shown in Figure1, it can be seen that the failure pattern due to plastic deformation weavewas found composite to be higher, material which is structurally is due to the more microstruc vulnerabletural to influence, loads than and the it cross-plyis believed structure that the [1,4].plain- of the metallic material according to fiber pullout after UTS was not shown. This is because, even if Asweave shown composite in Figure material 1, it can isbe structurally seen that the more failure vulnerable pattern dueto loads to plastic than deformationthe cross-ply of structure the metallic [1,4]. the two specimens are finally destroyed by fiber pullout, the energy absorption mechanism varies materialAs shown according in Figure to 1, fiberit can pulloutbe seen thatafter the UTS fail urewas pattern not shown. due to This plastic is deformationbecause, even of theif the metallic two depending on the stiffness of the fiber or the coating conditions of the fiber. In addition, it is possible to specimensmaterial according are finally todestroyed fiber pullout by fiber after pullout, UTS thewas energy not shown. absorption This mechanism is because, varies even depending if the two infer that the fiber bundles were destroyed at the same time at the final fracture. onspecimens the stiffness are offinally the fiber destroyed or the coatingby fiber conditionspullout, the of energy the fiber. absorption In addition, mechanism it is possible varies to depending infer that Figure8 shows the fracture surface in a plain-weave composite. Figure9 shows a cross-sectional theon fiberthe stiffness bundles of were the fiberdestroyed or the atcoating the same conditions time at ofthe the final fiber. fracture. In addition, it is possible to infer that view of the fracture surface of a plain-weave Nicalon/SiC composite. As shown in Figure8, the 90 the fiber bundles were destroyed at the same time at the final fracture. ◦ warps are oriented in the direction of stress application, so it can be understood that the specimen was finally ruptured through the final fiber fracture. On the other hand, in the case of 0◦ fills, since they are woven horizontally with 90◦ warps, the crack propagates at 90◦ warps after the first crack, and plays a role in giving time so that the final failure can occur at 90◦ warps. This can be inferred through the cross-sectional view in Figure9. As shown in Figures8 and9, the final failure occurred at 90 ◦ warp in

Materials 2020, 13, x FOR PEER REVIEW 6 of 11

Materials 2020, 13, 5141 6 of 11

the same orientation as the stress applied direction, and it can be seen that the 0◦ fill plays a role in supporting the final load for failure (Figure8). Figure9 also shows a fracture surface similar to that shown in Figure8. However, when viewed through the cross-sectional view, the role of the 0 ◦ fill and 90Materials◦ warp 2020 in, the13, xfracture FOR PEER process REVIEW can be clearly understood [1,2,4]. 6 of 11

Figure 7. The result of tensile test for Nicalon/SiC ceramic composite.

Figure 8 shows the fracture surface in a plain-weave composite. Figure 9 shows a cross-sectional view of the fracture surface of a plain-weave Nicalon/SiC composite. As shown in Figure 8, the 90° warps are oriented in the direction of stress application, so it can be understood that the specimen was finally ruptured through the final fiber fracture. On the other hand, in the case of 0° fills, since they are woven horizontally with 90° warps, the crack propagates at 90° warps after the first crack, and plays a role in giving time so that the final failure can occur at 90° warps. This can be inferred through the cross-sectional view in Figure 9. As shown in Figures 8 and 9, the final failure occurred at 90° warp in the same orientation as the stress applied direction, and it can be seen that the 0° fill plays a role in supporting the final load for failure (Figure 8). Figure 9 also shows a fracture surface similar to that shown in Figure 8. However, when viewed through the cross-sectional view, the role of the 0° fill and 90° warp in the fracture process can be clearly understood [1,2,4]. Figure 7. The result of tensile test for Nicalon/SiC ceramic composite. Figure 7. The result of tensile test for Nicalon/SiC ceramic composite.

Materials 2020, 13, x FORFigure PEERFigure REVIEW 8. Planar 8. Planar view view of of fracture fracture surface surface ofof plain-weave composite. composite. 7 of 11

FigureFigure 9. 9.Cross-sectional Cross-sectional view view of of fracture fracture surface surface of of plain-weave plain-weave composite. composite.

Figures 10 and 11 show more detailed fracture surfaces. In Figure 10, the 0° fill shows the crack in the SiC matrix. The matrix crack can be seen, which is the starting point of failure caused by the initial load (Figure 10). After that, the crack continues to propagate at the 0° fill (Figure 10). Subsequently, debonding occurs between the fiber and the matrix, and according to the continuously applied load, it reachesFigure 90° 8. warp.Planar viewThrough of fracture this fracture surface ofprocess, plain-weave it can composite. be seen that the fiber pullout at 90° warp and finally fiber breakage lead to final rupture. This process is illustrated schematically, which is described in detail in Figure 12 [4]. Through this process, it is possible to explain the fracture process of the plain-weave Nicalon/SiC composite material.

Figure 10. Multiple matrix cracking of 0° fill in plain-weave composite.

Figure 11. Fiber pullout of the 90° warp followed by severe matrix cracking of the 0° fill in plain- weave composite.

Materials 2020, 13, x FOR PEER REVIEW 7 of 11 Materials 2020, 13, x FOR PEER REVIEW 7 of 11

Materials 2020, 13, 5141 7 of 11

Figure 9. Cross-sectional view of fracture surface of plain-weave composite. Figure 9. Cross-sectional view of fracture surface of plain-weave composite.

FiguresFigures 10 and 10 and 11 show 11 show more more detailed detailed fracture fracture surfaces. surfaces. In Figure 10,10 ,the the 0° 0 ◦fillfill shows shows the the crack crack in the SiCin the matrix.Figures SiC matrix. The 10 and matrix The 11 matrixshow crack more crack can bedetailed can seen, be seen,fracture which which issurfaces. the is startingthe Instarting Figure point point 10, of the failureof 0°failure fill causedshows caused the by by crack the the initial in the SiC matrix. The matrix crack can be seen, which is the starting point of failure caused by the loadinitial (Figure load 10 ).(Figure After that,10). After the crack that, continuesthe crack tocontinues propagate to propagate at the 0 fillat the (Figure 0° fill 10 (Figure). Subsequently, 10). initial load (Figure 10). After that, the crack continues to propagate◦ at the 0° fill (Figure 10). debondingSubsequently, occurs debonding between the occurs fiber between and the the matrix, fiber an andd the according matrix, and to according the continuously to the continuously applied load, appliedSubsequently, load, it debonding reaches 90° occurs warp. between Through the this fiber fracture and the process, matrix, it and can accordingbe seen that to the continuouslyfiber pullout it reaches 90 warp. Through this fracture process, it can be seen that the fiber pullout at 90 warp and atapplied 90° warp◦ load, and it reachesfinally fiber 90° warp. breakage Through lead tothis final fracture rupture. process, This processit can be is seen illustrated that the schematically, fiber pullout◦ finally fiber breakage lead to final rupture. This process is illustrated schematically, which is described whichat 90° warpis described and finally in detail fiber in breakage Figure 12 lead [4]. Throughto final rupture. this process, This process it is possible is illustrated to explain schematically, the fracture in detailprocesswhich in is Figureof described the plain-weave 12[ in4]. detail Through Ni incalon/SiC Figure this 12 process, composite[4]. Through it material. is this possible process, to it explainis possible the to fractureexplain the process fracture of the plain-weaveprocess of Nicalon the plain-weave/SiC composite Nicalon/SiC material. composite material.

Figure 10. Multiple matrix cracking of 0° fill in plain-weave composite. FigureFigure 10. 10.Multiple Multiple matrix matrix cracking cracking of 0° 0◦ fillﬁll in in plain-weave plain-weave composite. composite.

FigureMaterialsFigure 11. 2020 11.,Fiber 13 Fiber, x FOR pullout pullout PEER REVIEW of the the 90° 90 ◦warpwarp followed followed by severe by severe matrix matrixcracking cracking of the 0° offill thein plain- 0◦ ﬁll8 of in 11 plain-weaveweaveFigure composite.11. composite. Fiber pullout of the 90° warp followed by severe matrix cracking of the 0° fill in plain- weave composite.

Figure 12. Failure mechanisms in plain-weave Nicalon/SiC composite; (a) matrix cracking in 0◦ fills and Figure 12. Failure mechanisms in plain-weave Nicalon/SiC composite; (a) matrix cracking in 0° fills crack propagation within 0 fills, and debonding between fiber and matrix in 0 fills, (b) delamination and crack propagation◦ within 0° fills, and debonding between fiber and matrix◦ in 0° fills, (b) in 0 fills and 90 warps and further crack propagation, and debonding between fiber and matrix in ◦ delamination◦ in 0° fills and 90° warps and further crack propagation, and debonding between fiber 90◦ warps,and matrix (c) fiber in 90° fracture warps, and (c) fiber extensive fracture fiber and pulloutextensive in fiber 90◦ pulloutwarps. in 90° warps.

Figure 13 shows the fracture surface of a cross-ply Nicalon/SiC composite. It can be seen that the 0° and 90° laminates are intersected and the final failure occurred through the 90° laminates arranged parallel to the stress applied direction (Figure 13). Figures 14 and 15 show more detailed fracture surfaces. As shown in Figure 14, as a load is applied, the matrix cracking occurs preferentially in the 0° laminate (Figure 14), and then debonding between the matrix and fibers occurs, and crack propagation occurs in the 90° laminate direction. It can be inferred that the final breakdown consists of massive fiber pullout and a final fiber breakage in the 90° laminate (Figure 15). Similar to the description of the failure process of plain-weave composites, the failure of cross-ply composites is schematically described in Figure 16. In other words, first matrix cracking starts in the 0° laminate, followed by debonding between the fiber and the matrix in the 0° laminate. After that, crack propagation continues in the 0° laminate. Thus, as the load increases, delamination occurs between the 0° and 90° laminates. Subsequently, fiber debonding and pullout are performed on a 90° laminate. The final fracture is completed with massive fiber pullout and fiber breakage in a 90° laminate [1,3].

Materials 2020, 13, 5141 8 of 11

Figure 13 shows the fracture surface of a cross-ply Nicalon/SiC composite. It can be seen that the 0◦ and 90◦ laminates are intersected and the final failure occurred through the 90◦ laminates arranged parallel to the stress applied direction (Figure 13). Figures 14 and 15 show more detailed fracture surfaces. As shown in Figure 14, as a load is applied, the matrix cracking occurs preferentially in the 0◦ laminate (Figure 14), and then debonding between the matrix and fibers occurs, and crack propagation occurs in the 90◦ laminate direction. It can be inferred that the final breakdown consists of massive fiber pullout and a final fiber breakage in the 90◦ laminate (Figure 15). Similar to the description of the failure process of plain-weave composites, the failure of cross-ply composites is schematically described in Figure 16. In other words, first matrix cracking starts in the 0◦ laminate, followed by debonding between the fiber and the matrix in the 0◦ laminate. After that, crack propagation continues in the 0◦ laminate. Thus, as the load increases, delamination occurs between the 0◦ and 90◦ laminates. Subsequently, fiber debonding and pullout are performed on a 90◦ laminate. The final fracture is completedMaterials 2020 with, 13, x massive FOR PEER fiber REVIEW pullout and fiber breakage in a 90◦ laminate [1,3]. 9 of 11

FigureFigure 13. 13.Cross-sectional Cross-sectional viewview ofof fracture fracture surface surface of of cross-ply cross-ply composite. composite.

Figure 14. Delamination between fiber and matrix in 0° laminate.

Materials 2020, 13, x FOR PEER REVIEW 9 of 11

Materials 2020, 13, 5141 9 of 11 Figure 13. Cross-sectional view of fracture surface of cross-ply composite.

Materials 2020, 13, x FOR PEER REVIEW 10 of 11 Materials 2020, 13, x FOR PEER REVIEW 10 of 11 Figure 14. Delamination between ﬁberfiber and matrix in 00°◦ laminate.

Figure 15. Final failure of the 90° laminate with fiber pullout. Figure 15. Final failure of the 90° 90◦ laminate with fiber ﬁber pullout.

Figure 16. Failure mechanisms in cross-ply Nicalon/SiC composite; (a) matrix cracking in 0◦ laminate Figure 16. Failure mechanisms in cross-ply Nicalon/SiC composite; (a) matrix cracking in 0° laminate and debonding between fiber and matrix at the 0◦ laminate, (b) further crack propagation in 0◦ laminate and debonding between fiber and matrix at the 0° laminate, (b) further crack propagation in 0° Figureand delamination 16. Failure mechanisms between 0◦ and in cross-ply 90◦ laminates, Nicalon/SiC (c) fiber composite; debonding (a and) matrix pullout cracking in 90◦ inlaminate 0° laminate and andfinallaminate debonding rupture and with delamination between extensive fiber fiberbetween and pullout. matrix 0° and at 90° the laminates, 0° laminate, (c) (fiberb) further debonding crack andpropagation pullout inin 90°0° laminatelaminate andand finaldelamination rupture with between extensive 0° and fiber 90° pullout. laminates, (c) fiber debonding and pullout in 90° 4. Conclusionslaminate and final rupture with extensive fiber pullout. 4. Conclusions In this study, tensile fracture characteristics of Nicalon ceramic fiber reinforced ceramic matrix 4.composites ConclusionsIn this study, were studied. tensile fracture It was foundcharacteristics that the compositeof Nicalon materialsceramic fiber of plain-weave reinforced ceramic and cross-ply matrix structurescompositesIn this showed study,were studied. tensile different fracture It tensile was foundcharacteristics fracture that characteristics. the ofcomposite Nicalon It ceramicmaterials was found fiber of that plain-weave reinforced the cross-ply ceramic and composite cross-ply matrix compositesmaterialstructures showed showed were studied. higherdifferent tensileIt tensilewas found strength fracture that characteristics. than the composite the plain-weave It materials was found composite, of that plain-weave the whichcross-ply and seems composite cross-ply to be structuresduematerial to the showed showed microstructural higher different tensile tensile characteristics. strength fracture than characteristics. Thethe plain-weave failure mode It was composite, andfound failure that which the characteristics cross-ply seems to composite be of due each to materialcompositethe microstructural showed system higher were characteristics. found tensile through strength The SEM failurethan microstructure the mo plain-weavede and analysis,failure composite, characteristics and the which failure seemsof mechanisms each to compositebe due were to thesummarizedsystem microstructural were schematically. found characteristics. through SEM The microstructure failure mode analysis,and failure and characteristics the failure mechanismsof each composite were systemsummarized were schematically.found through SEM microstructure analysis, and the failure mechanisms were summarizedIn the case schematically. of plain-weave composite material, it was found that the crack of the silicon carbide matrixIn thestarted case at of 0° plain-weave fill. After that, composite the crack material, continued it was to found propagate that theat 0°crack fill, ofand the then silicon debonding carbide matrixbetween started the fiber at 0° andfill. Afterthe matrix that, the occurred, crack continued and the tocrack propagate reached at 90°0° fill,warp and according then debonding to the betweencontinuously the fiberapplied and load. the Throughmatrix occurred, this fracture and pr theocess, crack it was reached found 90°that warp the fiber according pullout to at the90° continuouslywarp and finally applied fiber load.breakage Through lead tothis final fracture rupture. process, it was found that the fiber pullout at 90° warpIn and the finally case of fiber cross-ply breakage composit lead toes, final first rupture. matrix cracking begins in the 0° laminate, followed by debondingIn the casebetween of cross-ply the fiber composit and the es,matrix first inmatrix the 0° cracking laminate. begins After in that, the crack0° laminate, propagation followed occurs by debondingcontinuously between in the 0°the laminate. fiber and Thus, the matrix as the inload the increases, 0° laminate. delamination After that, occurs crack propagationbetween the occurs0° and continuously90° laminates. in Subsequently, the 0° laminate. fiber Thus, debonding as the load and increases, pullout are delamination performed onoccurs a 90° between laminate. the It 0°can and be 90°seen laminates. that the final Subsequently, rupture completes fiber debonding the failure and process pullout by are massive performed fiber pullouton a 90° and laminate. fiber breakage. It can be seenFunding: that Thisthe finalresearch rupture received completes no external the funding. failure process by massive fiber pullout and fiber breakage. Funding: This research received no external funding.

Materials 2020, 13, 5141 10 of 11

In the case of plain-weave composite material, it was found that the crack of the silicon carbide matrix started at 0◦ fill. After that, the crack continued to propagate at 0◦ fill, and then debonding between the fiber and the matrix occurred, and the crack reached 90◦ warp according to the continuously applied load. Through this fracture process, it was found that the fiber pullout at 90◦ warp and finally fiber breakage lead to final rupture. In the case of cross-ply composites, first matrix cracking begins in the 0◦ laminate, followed by debonding between the fiber and the matrix in the 0◦ laminate. After that, crack propagation occurs continuously in the 0◦ laminate. Thus, as the load increases, delamination occurs between the 0◦ and 90◦ laminates. Subsequently, fiber debonding and pullout are performed on a 90◦ laminate. It can be seen that the final rupture completes the failure process by massive fiber pullout and fiber breakage.

Funding: This research was supported by a grant from the R&D Program of the Korea Railroad Research Institute, Korea. Conﬂicts of Interest: The author declares no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

1. Chawla, K.K. Composite Materials: Science and Engineering, 3rd ed.; Springer: New York, NY, USA, 2012; ISBN 978-0-387-74365-3. 2. Krenkel, W. Ceramic Matrix Composites: Fiber Reinforced Ceramics and their Applications; WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2008; ISBN 978-3-527-31361-7. 3. Kim, J. Tensile Fracture Behavior and Characterization of Ceramic Matrix Composites. Materials 2019, 12, 2997. [CrossRef][PubMed] 4. Kim, J.; Liaw, P.K. Tensile fracture behavior of Nicalon/SiC composites. Met. Matls Trans. A 2007, 38A, 2203–2213. [CrossRef] 5. Krupka, M.; Kienzle, A. Fiber Reinforced Ceramic Composite for Brake Discs. In Proceedings of the 18th Annual Brake Colloquium & Engineering Display; SAE Technical Paper 2000-01-2761. SAE International: Warrendale, PA, USA, 2000. [CrossRef] 6. Li, W.; Yang, X.; Wang, S.; Xiao, J.; Hou, Q. Comprehensive Analysis on the Performance and Material of Automobile Brake Discs. Metals 2020, 10, 377. [CrossRef] 7. Stadler, Z.; Krnel, K.; Koamac, T. Friction and wear of sintered metallic brake linings on a C/C-SiC composite brake disc. Wear 2008, 265, 278–285. [CrossRef] 8. Bharambe, A. Medelling and Analysis of Disc Brake with Composite Material. Int. J. Sci. Res. 2016, 5, 1234–1239. 9. Benseddiq, N.; Weichet, D.; Seidermann, J.; Minet, M. Optimization of design of railway disc brake pads. J. Rail. Rapid Transit. 1996, 210, 51–61. [CrossRef] 10. Marshall, D.B.; Evans, A.G. Failure mechanisms in ceramic-fiber/ceramic-matrix-composites. J. Am. Ceram. Soc. 1985, 68, 225–231. [CrossRef] 11. Pickering, K.L.; Efendy, M.A.; Le, T.M. A review of recent developments in natural fibre composites and their mechanical performance. Compos. A Appl. Sci. Manuf. 2016, 83, 98–112. [CrossRef] 12. Tracy, J.M. Multi-Scale Investigation of Damage Mechanisms in SiC/SiC Ceramic Matrix Composites. Ph.D. Thesis, The University of Michigan, Ann Arbor, MI, USA, December 2014. 13. Ding, Z.; Li, Y.; Lu, C.; Liu, J. An investigation of fiber reinforced chemically bonded phosphate ceramic composites at room temperature. Materials 2018, 11, 858. [CrossRef][PubMed] 14. Jenkins, M.G.; Singh, R.N. Failure analysis of ceramic-matrix composites. In ASM Handbook. Vol. 21. Composites; ASM International: Materials Park, OH, USA, 2001. 15. Dowling, N.E. Mechanical Behavior of Materials, 4th ed.; Pearson Education, Inc.: Upper Saddle River, NJ, USA, 2012; ISBN 978-0-13-139506-0. 16. Morscher, G.N. Modal acoustic emission of damage accumulation in a woven SiC/SiC composite. Comput. Sci. Technol. 1999, 59, 687–697. [CrossRef] Materials 2020, 13, 5141 11 of 11

17. Kim, J.; Liaw, P.K. Characterization of fatigue damage modes in Nicalon/Calcium aluminosilicate composites. J. Eng. Mater. Technol. 2005, 127, 8–15. [CrossRef] 18. Kim, J.; Liaw, P.K. Monitoring tensile damage evolution in Nextel312/Blackglas composites. Mater. Sci. Eng. A 2005, 409, 302–308. [CrossRef] 19. ASTM. C1275-18 Standard Test Method for Monotonic Tensile Behavior of Continuous Fiber-Reinforced Advanced Ceramics with Solid Rectangular Cross-Section Test Specimens at Ambient Temperature; American Society for Testing and Materials: West Conshohocken, PA, USA, 2018.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).