The Ballistic Performance of Laminated Sic Ceramics for Body Armor and the Effect of Layer Structure on It

applied sciences

Article The Ballistic Performance of Laminated SiC Ceramics for Body Armor and the Effect of Layer Structure on It

Yuzhe Hong 1,2,*, Fangmin Xie 2, Lijun Xiong 2, Mingliang Yu 2, Xiangqian Cheng 2, Mingjie Qi 2, Zhiwei Shen 3, Guoping Wu 2, Tian Ma 3 and Nan Jiang 1

1 Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China; [email protected] 2 Ningbo FLK Co., Ltd., Ningbo 314000, China; [email protected] (F.X.); [email protected] (L.X.); [email protected] (M.Y.); [email protected] (X.C.); [email protected] (M.Q.); [email protected] (G.W.) 3 Institute of Quartermaster Engineering Technology, PLA Academy of Military Science, Beijing 100010, China; [email protected] (Z.S.); [email protected] (T.M.) * Correspondence: [email protected]

Abstract: Laminated ceramics with weak interface layers have been proven to be effective in toughening ceramics. The energy absorption ability of laminated ceramics may also beneﬁt their ballistic performance. However, the effect of the layer structure on the ballistic performance of laminated ceramics has not been studied. Focusing on the application for body armor, this paper studied the effect of the different layer structures on the ballistic performance of laminated SiC ceramics. The laminated SiC ceramics with different layered structures were designed and prepared by tape-casting and hot-pressing. When used for the ‘in conjunction with’ armor system, the laminated SiC ceramics with a gradual-layered structure had the backface signature depth of 30% less than the laminated Citation: Hong, Y.; Xie, F.; Xiong, L.; Yu, M.; Cheng, X.; Qi, M.; Shen, Z.; SiC with no interface structure and 50% less than the commercial solid-state sintered SiC. However, Wu, G.; Ma, T.; Jiang, N. The Ballistic when used stand-alone, the laminated SiC had a similar ballistic performance regardless of the layer Performance of Laminated SiC structure, which was likely due to the weak back support. In conclusion, the ballistic performance of Ceramics for Body Armor and the the laminated ceramics was related to the back support of the armor system. When used for the ‘in Effect of Layer Structure on It. Appl. conjunction with’ armor system, the laminated SiC had a better ballistic performance than that of the Sci. 2021, 11, 6145. https://doi.org/ solid-state sintered SiC. 10.3390/app11136145 Keywords: laminated ceramics; tape-casting; ballistic performance Academic Editor: Yuzo Nakamura

Received: 20 May 2021 Accepted: 26 June 2021 1. Introduction Published: 1 July 2021 With the development of military technology, armor protection for personnel is being

Publisher’s Note: MDPI stays neutral increasingly emphasized. Body armor needs to resist the penetration of the bullet as well with regard to jurisdictional claims in as absorb the energy of the bullet, preventing lethal damage to the personnel. Additionally, published maps and institutional affil- to effectively protect the person from auto weapons, multi-hit resistance is also required. iations. Ceramics, compared to metals, have superior hardness, good strength, and low density, making them an attractive armor material. The ceramic armor plate has become an essential part of personal armor to protect from rifles or higher-level thread [1]. However, ceramics are brittle in nature with low toughness and are sensitive to defects [2]. Defects in the ceramics can facilitate crack propagation and effectively reduce the ballistic performance Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. of the ceramics [3]. This article is an open access article The defects are often introduced to the ceramic green body during fabrication and distributed under the terms and are not eliminated during sintering. The ceramics prepared by colloid processing have conditions of the Creative Commons fewer defects in the final products compared to the ceramics made by the conventional Attribution (CC BY) license (https:// dry-pressing route [4]. Laminated ceramics, which are prepared by stacking ceramic creativecommons.org/licenses/by/ green tapes and sintering, allow for fabrication from colloid processing [5,6] and would be 4.0/). advantageous over conventional dry-pressed ceramics in armor application.

Appl. Sci. 2021, 11, 6145. https://doi.org/10.3390/app11136145 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 6145 2 of 11

The introduction of the weak interface layer to the laminated ceramics has also been proven to be effective in toughening the ceramics and thus enhancing the energy absorption of the ceramics. The toughening mechanism of the laminated ceramics was proposed by Cook [7] in 1964 in that crack deflection would occur by introducing the weak interface in the composite. In 1990, Clegg [8] first prepared the SiC/C laminated composite using this strategy and reported that the material had several times higher toughness and 100 times higher work of fracture than that of the bulk SiC. Since then, many studies have been carried out on laminated ceramics to develop high toughness in ceramics. The layer-by- layer stacking method provided laminated ceramics many design variables, such as the layer composition, layer thickness, stacking sequence for layers with different compositions, etc. The effects of these designing factors were extensively studied to optimize mechanical performance. The strength of the matrix layer was found to be important to the final mechanical properties of the laminated ceramics [9]. Toughening the matrix layer of the laminated ceramics would provide more percentage of improvement to the overall toughness than toughening the monolithic ceramics [10]. An optimal range of the bonding strength of the interface layer also exists in which the toughness of the laminated ceramics reaches the maximum [6,10]. However, it is still in doubt whether the toughening mechanism of the layered ceramics for static mechanical loading is effective for the ballistic impact. The ballistic impact is a complex dynamic event that is very different from static mechanical loading [11]. Due to the difficulty in theoretical study and analysis, no conclusion has been drawn until now. The study for the ballistic performance of laminated ceramics has been very limited. The only study, carried out by Li et al. [12], reported that the laminated Si3N4/BN would remain a whole piece after ballistic impact, while the monolithic Si3N4 broke into pieces, but the penetration resistance of the laminated ceramics was decreased. Introducing the weak interface layer only at the backside of the ceramic plate would mitigate the penetration resistance decrease. The laminating structure is an important factor in the performance of laminated ceramics. Previous studies in the AlN/polymer laminated composite found that the composite would have a depth of penetration (DOP) greater or less than the monolithic AlN, depending on the layer structure [13,14]. However, such an effect on ballistic performance has not been studied for laminated ceramics. This study aimed at the application of laminated ceramics as a body armor material. Based on this purpose, the effect of structure on the ballistic performance of laminated ceramics was studied. It was found that the laminated SiC with interface layers could provide good energy absorption ability under the ballistic impact when used in conjunction with a flexible armor jacket. The laminated SiC ceramics with differently layered structures were prepared by tape-casting and hot-pressing. The effects of layer structure on mechanical performance and ballistic performance were discussed. The results showed that when used for the ‘in conjunction with’ armor system, the laminated SiC ceramics with a gradual-layered structure had the backface signature depth of 30% less than the laminated SiC with no interface structure and 50% less than the commercially available solid-state sintered SiC.

2. Materials and Methods The laminated ceramics were prepared by tape-casting followed by hot-press sintering. The ceramic slurry was prepared by ball-milling α-SiC (average particle size of 0.5 µm, North High-tech Industry Co., Ltd., Ningxia, China), Y2O3 (purity > 99%, average particle size of 1 µm, Shanghai Shuitian technology Co., Ltd., Shanghai, China), Al2O3 (purity > 99.8%, average particle size of 1 µm, Shanghai Shuitian technology Co., Ltd., Shanghai, China), BN (purity > 99.9%, average particle size of 0.2 µm, Shanghai Shuitian technology Co., Ltd., Shanghai, China), triethylphosphate, and ethanol for 24 h. The polyvinyl butyral, dioctyl phthalate, and glycerol were then added to the slurry and ball-milled for another 24 h. After de-gassing for 30 min, the slurry was tape-casted to tapes with the desired thickness. Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 12

Appl. Sci. 2021, 11, 6145 3 of 11 milled for another 24 h. After de-gassing for 30 min, the slurry was tape-casted to tapes with the desired thickness. TwoTwo types of ceramic ceramic green green tapes, tapes, the the matrix matrix tapes tapes and and the theinterface interface tapes tapes were were pre- preparedpared for forthe the lamination. lamination. The The matrix matrix tape tape consisted consisted of of a a92 92 wt.% wt.% αα-SiC-SiC and and 8 wt.%wt.% AlAl22O3/Y2O2O3 mixture3 mixture (the (the molar molar ratio ratio of ofAl Al2O23 Oto3 Yto2O Y32 wasO3 was 3:2) 3:2)of total of total ceramic ceramic powder. powder. The Theinterface interface tape tapeconsisted consisted of a 46 of wt.% a 46wt.% α-SiC,α -SiC,46 wt.% 46 wt.%h-BN h-BNand 8 andwt.% 8 Al wt.%2O3/Y Al22OO3 3mixture/Y2O3 mixtureof total ceramic of total ceramicpowder. powder.The thickness The thickness of the matrix of the tape matrix and tape interface and interfacetape was tape approxi- was approximatelymately 0.25 mm 0.25 and mm 0.05 and mm, 0.05 respectively. mm, respectively. TheThe greengreen tapestapes werewere thenthen cutcut intointo 130130× × 130130 piecespieces andand stackedstacked asas designed.designed. TheThe uniform-layereduniform-layered samplesample hadhad aa uniformlyuniformly layeredlayered structure.structure. InIn eacheach repeatingrepeating unit,unit, thethe numbernumber ofof thethe matrixmatrix layerlayer toto thethe numbernumber ofof thethe interfaceinterface layerlayer waswas 1010 toto 1.1. The gradual- layeredlayered samplesample was was designed designed as as a a gradually gradually layered layered structure, structure, i.e., i.e., the the number number of theof the matrix ma- layertrix layer between between two interfacetwo interface layers layers was gradually was gradually changed. changed. The number The number of the matrixof the matrix layers usinglayers theusing following the following sequence sequence from strike from facestrike to face the back:to the 10,back: 10, 10, 5, 2,10, and 5, 2, 2. and The 2. layer The structurelayer structure for different for different sample sample types istypes listed is inlisted Table in1 Table. 1.

TableTable 1.1.The The layerlayer structurestructure forfor differentdifferent sample sample types. types.

SampleSample Type Type Layer LayerStructure Structure Uniform-layeredUniform-layered 10 matrix 10 matrixlayers layersfollowed followed by 1 byinterface 1 interface layer layer Gradually layered,Gradually the layered, number the of number the matrix of the layer matrix followed layer Gradual-layeredGradual-layered by followed1 interface by 1layer interface was layer10, 10, was 5, 10,2, 2 10, 5, 2, 2 Pure-matrixPure-matrix Matrix Matrixlayers layerswithout without any interface any interface layers layers

TheThe ceramicceramic greengreen bodiesbodies werewere thenthen preparedprepared byby pressingpressing thethe stackedstacked tapestapes underunder ◦ thethe pressurepressure ofof 80 MPa. Before Before sintering, sintering, th thee green green bodies bodies were were de-binded de-binded at at 850 850 °CC for for 4 4h hin in nitrogen. nitrogen. The The de-binded de-binded samples samples were were th thenen sintered at temperaturetemperature rangingranging fromfrom ◦ ◦ 18401840 °CC toto 19001900 °CC for 1 h under the pressure ofof 4040 MPaMPa inin argon.argon. AfterAfter ﬁring,firing, thethe sinteredsintered ceramicceramic platesplates werewere cut cut and and polished polished into into the the dimension dimension of of 120 120× ×120 120× × 5.5 mm,mm, asas shownshown inin FigureFigure1 1..

FigureFigure 1. TheThe image image of the of the120 120× 120× × 1205.5 mm× 5.5 laminate mm laminatedd ceramic prepared ceramic preparedin this study in this(gradual study (graduallayered).layered).

TheThe ballisticballistic performanceperformance waswas carriedcarried outout followingfollowing thethe ChineseChinese nationalnational militarymilitary standard,standard, GJB4300-2002.GJB4300-2002. The 5.85.8 mmmm ×× 42 mm type-87 steel core bulletbullet waswas usedused inin thisthis study.study. TheThe bulletbullet hadhad aa fullfull metalmetal jacketjacket andand thethe massmass ofof thethe bulletbullet waswas 4.154.15 g.g. TheThe velocityvelocity ofof thethe bulletbullet waswas measuredmeasured usingusing anan infraredinfrared tachometer.tachometer. TheThe infraredinfrared tachometertachometer hadhad twotwo consecutiveconsecutive screensscreens soso thatthat itit couldcould recordrecord thethe timetime forfor thethe bulletbullet thatthat passedpassed throughthrough itit andand toto calculatecalculate thethe velocityvelocity ofof thethe bullet.bullet. TheThe gunpowdergunpowder forfor thethe bulletbullet waswas adjustedadjusted based on the velocity measured several times to guarantee that the velocity of the bullet was within the range of 900 to 920 m/s. The armor or armor system was mounted on the Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 12 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 12

Appl. Sci. 2021, 11, 6145 based on the velocity measured several times to guarantee that the velocity of the bullet 4 of 11 was withinbased the on range the velocity of 900 to measured 920 m/s. Theseveral armor times or toarmor guarantee system that was the mounted velocity on of the the bullet armor carrier.was within The armorthe range carrier of 900 was to filled 920 m/s. with The clay armor as the or backface armor systemsignature was (BFS). mounted The on the armor carrier. The armor carrier was filled with clay as the backface signature (BFS). The configurationarmor carrier.for the Theballistic armor testing carrier is wasshown filled in Figure with clay 2a. asThe the BFS backface depths signature were meas- (BFS). The configuration for the ballistic testing is shown in Figure 2a. The BFS depths were measured forconfiguration the shot that for was the ballisticpartially testing penetrated. is shown After in Figureballistic2a. impact, The BFS irregular depths were bulges measured ured for the shot that was partially penetrated. After ballistic impact, irregular bulges form aroundfor the the shot indent. that was To avoid partially the penetrated.interference Afterof the ballistic bulge, the impact, depth irregular was counted bulges form form around the indent. To avoid the interference of the bulge, the depth was counted from thearound original the flat indent. clay plane To avoid to the the maximum interference depth of of the the bulge, indent the after depth ballistic was impact, counted from from the original flat clay plane to the maximum depth of the indent after ballistic impact, as illustratedthe original in Figure flat 2b. clay Two plane conditions to the maximum were performed depth ofin thethis indent study. afterIn one ballistic condition, impact, as as illustrated in Figure 2b. Two conditions were performed in this study. In one condition, the armorillustrated plate was in Figuretested 2inb. conjunction Two conditions with were a flexible performed armor in jacket, this study. named In the one CA condition, test. the the armor plate was tested in conjunction with a flexible armor jacket, named the CA test. In this test,armor the plate armor was plate tested was in inserted conjunction into the with flexible a flexible armor armor jacket jacket, as a removable named the unit CA test. In In this test, the armor plate was inserted into the flexible armor jacket as a removable unit to enhancethis the test, ballistic the armor performance plate was of inserted the armor into system. the flexible The armorarmor jacketplate took as a removableone shot unit due to totheto enhanceenhance limited thethespace ballisticballistic of the performanceperformance plate. In the ofofother thethe armorarmorcondition, system.system. the ThearmorThe armorarmor plate plateplate was tooktooktested oneone shotshot stand-alonedue totoand thethe the limitedlimited armor spacespace plates ofof took thethe threeplate.plate. sh InInots. theth eThis otherother condition condition,condition, was thethe named armorarmor the plateplate SA test. waswas testedtested stand-alonestand-alone andand thethe armorarmor platesplates tooktook threethree shots.shots. ThisThis conditioncondition waswas namednamed thethe SASA test.test.

(a) (b) (a) (b) Figure 2. The illustration of ballistic testing: (a) the configuration; (b) the measurement for the BFS depth. FigureFigure 2.2. TheThe illustrationillustration ofof ballisticballistic testing:testing: ((aa)) thethe configuration;configuration; ((bb)) thethe measurementmeasurement forfor thethe BFSBFS depth.depth. The armor plate was prepared in Zhejiang Light-Tough Composite Co., Ltd. For both the CA and TheSA tests, armor the plate plate ceramic was was prepared plate prepared was in first in Zhejiang Zhejiang bonded Light-Tough with Light-Tough ultra-high-molecular Composite Composite Co., Co., Ltd.poly- Ltd. For both For ethyleneboththe (UHMWPE) CA the and CA SA and fabrics tests, SA tests,the layer ceramic the by ceramic layer. plate The platewas overall first was bonded first areal bonded density with withultra-high-molecular of the ultra-high-molecular UHMWPE poly- fabrics polyethylenewasethylene 7.3 kg/m (UHMWPE) (UHMWPE)2. The armor fabrics fabricsplate layer for layer theby byCAlayer. layer. test The used The overall overallone piece areal areal of density densitythe ceramic of of the plate UHMWPE 2 and thefabricsfabrics dimension waswas 7.37.3 was kg/mkg/m 1202 .. ×The The120 armor armor× 5.5 platemm. plate Thefor for the thearmor CA CA test testplate used used for one the pieceCA test of thethewas ceramicceramic then plateplate × × attachedand to a thethe flexible dimensiondimension armor waswasjacket 120120 made × 120 by ×UHMWPE 5.55.5 mm. mm. The Thefabrics armor armor with plate an areal for thedensity CA testoftest 5.77 waswas thenthen 2 attached to a a flexible flexible armor armor jacket jacket made made by by UHMWPE UHMWPE fabrics fabrics with with an areal an areal density density of 5.77 of kg/m (FDF-15, Zhejiang2 Light-Tough Composite Co., Ltd., Huzhou, China). The armor plate for5.77kg/m the kg/m 2SA (FDF-15, test(FDF-15, used Zhejiang four Zhejiang pieces Light-Tough of Light-Tough the ceramic Composit Compositeplatee toCo., provide Ltd., Co., Huzhou, Ltd.,a dimension Huzhou, China). of China).240 The × armor The armor plate for the SA test used four pieces of the ceramic plate to provide a dimension 240 × 5.5plate mm. for The the SAcommercially test used four available pieces solid-stateof the ceramic sintered plate SiCto provide ceramic a dimensionplate (SSIC of 240 × of 240 × 240 × 5.5 mm. The commercially available solid-state sintered SiC ceramic plate plate, Ningbo240 × 5.5 FLK mm. Co., The Ltd., commercially Ningbo, China) available was also solid-state used to prepare sintered an SiC armor ceramic plate plateas a (SSIC (SSIC plate, Ningbo FLK Co., Ltd., Ningbo, China) was also used to prepare an armor plate reference.plate, Figure Ningbo 3 illustrates FLK Co., how Ltd., the Ni ngbo,armo rChina) or armor was system also used was to prepared prepare an for armor the CA plate as a as a reference. Figure3 illustrates how the armor or armor system was prepared for the CA and SA reference.tests. Figure 3 illustrates how the armor or armor system was prepared for the CA and SA tests.tests.

(a) (b) (a) (b) Figure 3.Figure The illustration 3. The illustration of the armor of the or armor armor or system armor prepared system prepared for (a) the for CA (a) test the CA(b) the test SA (b) test. the SA test. Figure 3. The illustration of the armor or armor system prepared for (a) the CA test (b) the SA test. The strength of the samples was measured using the three-point bending test. The specimens were cut into 3 × 4 × 35 mm in size. The fracture toughness of the samples was Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 12

Appl. Sci. 2021, 11, 6145 The strength of the samples was measured using the three-point bending test. The5 of 11 specimens were cut into 3 × 4 × 35 mm in size. The fracture toughness of the samples was measured using the single-edge notched beam (SENB) method. The Vickers hardness was tested using a microhardness machine with a load of 1 kg. The surface and cross-section of themeasured samples were using studied the single-edge using scanning notched electron beam (SENB) microscopy method. (SEM). The Vickers hardness was tested using a microhardness machine with a load of 1 kg. The surface and cross-section of 3. Resultsthe samples and Discussion were studied using scanning electron microscopy (SEM). To3. Resultsoptimize and the Discussion sintering temperature of the laminated ceramics, pure-matrix samples wereTo sintered optimize at thedifferent sintering sintering temperature temperatures. of the laminated The relative ceramics, density pure-matrix of the pure- samples matrixwere ceramic sintered at different at different sintering sintering temperatur temperatures.es is shown The relativein Figure density 4. The of relative the pure-matrix density increasedceramic atfrom different 95.7% sinteringto 98.1% and temperatures the temperature is shown increased in Figure from4 .1840 The °C relative to 1880 density °C and thenincreased slightly from decreased 95.7% to to 98.1% 97.2% and at 1900 the temperature°C. The decrease increased of the from density 1840 at◦ aC higher to 1880 ◦C temperatureand then was slightly likely decreaseddue to the toevaporation 97.2% at 1900 of the◦C. sintering The decrease additives of the [6,15]. density The atliquid a higher phasetemperature overflow was was also likely observed due to at the 1900 evaporation °C. The highest of the relative sintering density additives of the [6, 15ceramics,]. The liquid 98.1%,phase was overﬂowachieved wasat the also sintering observed temper at 1900ature◦C. Theof 1880 highest °C. The relative samples density prepared of the ceramics, for ballistic98.1%, testing was were achieved sintered at theat this sintering temperature. temperature of 1880 ◦C. The samples prepared for ballistic testing were sintered at this temperature.

100

Relative Density (%) 85

80 1840 1860 1880 1900 Temperature (°C)

Figure 4. The relative density of the sintered uniform-layered sample under different temperatures. Figure 4. The relative density of the sintered uniform-layered sample under different temperatures. The X-ray spectrum of the pure-matrix sample is shown in Figure5. α-SiC was the main phase in the ﬁnal product. YAG phase was also observed, which was formed from Thethe additivesX-ray spectrum during of sintering the pure-matrix following sample the reaction: is shown in Figure 5. α-SiC was the Appl. Sci. 2021, 11, x FOR PEER REVIEWmain phase in the final product. YAG phase was also observed, which was formed6 of from 12

the additives during sintering following3Al2O 3the+ 5Yreaction:2O3 → 2Y3Al5O12 (1)

This result was consistent3Al withO +5 otherYO research→2YAl [6O]. (1)

This result was consistent with other research [6].

Figure 5.Figure The X-ray 5. The spectrum X-ray spectrum of the sintered of the sinteredlaminated laminated SiC ceramics. SiC ceramics.

The mechanical performance of the samples is shown in Table 2. The flexural strength of the uniform-layered, gradual-layered, and pure-matrix sample was 348 MPa, 396 MPa, and 640 MPa, respectively. The toughness of the uniform-layered sample was the highest at 7.4 MPa∙m1/2. The toughness of the gradual-layered and the pure-matrix sample was 6.8 and 6.9 MPa∙m1/2, respectively. The hardness of the matrix layer for the laminated SiC was 22 GPa. The SSIC sample had a flexural strength of 414 MPa and a toughness of 4.2 MPa∙m1/2. The hardness of the SSIC was 26 GPa, higher than that of the laminated SiC.

Table 2. Mechanical performance of the samples.

Flexural Strength 1 Toughness 2 Vickers Hardness 3 Sample Type (MPa) (MPa∙m1/2) (GPa) Uniform-layered 348 7.4 22 (martix) Gradual-layered 396 6.8 22 (martix) Pure-matrix 640 6.9 22 SSIC 414 4.2 26 1: data obtained by three-point bending test; 2: data obtained by the single-edge notched beam method; 3: the testing load was 1 kg.

The microstructures of the laminated ceramics are shown in Figure 6. The thickness of the matrix layer in the uniform-layered sample was approximately 800 μm and the thickness of the interface layer was approximately 20 μm after sintering. The matrix layer thickness in the gradual-layered sample was gradually changed: from the bottom (i.e., the strike face in the ballistic test) to the top were 800 μm, 450 μm, and 250 μm, respectively. The interface layer of the gradual-layer sample was also 20 μm. The detail of the interface layer is shown in Figure 6c and the SiC particles and the BN plates were observed. The fracture surface of the matrix layer is shown in Figure 6d. The fracture mode was a combination of inter-granular and trans-granular fractures. The grain was fine with a size of 1–3 μm. Appl. Sci. 2021, 11, 6145 6 of 11

The mechanical performance of the samples is shown in Table2. The ﬂexural strength of the uniform-layered, gradual-layered, and pure-matrix sample was 348 MPa, 396 MPa, and 640 MPa, respectively. The toughness of the uniform-layered sample was the highest at 7.4 MPa·m1/2. The toughness of the gradual-layered and the pure-matrix sample was 6.8 and 6.9 MPa·m1/2, respectively. The hardness of the matrix layer for the laminated SiC was 22 GPa. The SSIC sample had a ﬂexural strength of 414 MPa and a toughness of 4.2 MPa·m1/2. The hardness of the SSIC was 26 GPa, higher than that of the laminated SiC.

Table 2. Mechanical performance of the samples.

Flexural Strength 1 Toughness 2 Vickers Hardness 3 Sample Type (MPa) (MPa·m1/2) (GPa) Uniform-layered 348 7.4 22 (martix) Gradual-layered 396 6.8 22 (martix) Pure-matrix 640 6.9 22 SSIC 414 4.2 26 1: data obtained by three-point bending test; 2: data obtained by the single-edge notched beam method; 3: the testing load was 1 kg.

The microstructures of the laminated ceramics are shown in Figure6. The thickness of the matrix layer in the uniform-layered sample was approximately 800 µm and the thickness of the interface layer was approximately 20 µm after sintering. The matrix layer thickness in the gradual-layered sample was gradually changed: from the bottom (i.e., the strike face in the ballistic test) to the top were 800 µm, 450 µm, and 250 µm, respectively. The interface layer of the gradual-layer sample was also 20 µm. The detail of the interface layer is shown in Figure6c and the SiC particles and the BN plates were observed. The fracture Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 12 surface of the matrix layer is shown in Figure6d. The fracture mode was a combination of inter-granular and trans-granular fractures. The grain was ﬁne with a size of 1–3 µm.

(a) (b)

FigureFigure 6. The 6. The microstructure microstructure of theof the laminated laminated ceramics: ceramics: (a )(a uniform-layered;) uniform-layered; ( b(b)) gradual-layered; gradual-layered; ((cc)) highhigh magniﬁcationmagnification of the interfaceof the interface layer; layer; (d) the (d fracturing) the fracturing surface surface of the of pure-matrixthe pure-matrix laminated laminated ceramic. ceramic.

The laminated SiC without the interface layer (pure-matrix) showed a high strength as well as toughness compared to the solid-state sintered SiC, SSIC. Previous studies [4,16] have shown that tape-casting is a colloid process that could leave fewer defects in the green body compared to the conventional dry-press method because the colloid process allows particles to be well-dispersed to avoid agglomerates. Since ceramics are a brittle material that is sensitive to defects, this would improve the strength of the final products. For liquid-phase-sintered SiC, the inter-granular crack deflection was the main toughening mechanism [17,18]. The phase formed in the grain boundary of the liquid phase sintering SiC (in this study, the phase was YAG) was weaker than SiC, so the cracks tended to propagate along the grain boundary rather than through the grain [15]. Moreover, the residual stress generated by the thermal mismatch of the phase in the grain boundary further weakened the grain boundary, facilitating the inter-granular crack deflection [15,16]. The crack deflection elongated the cracking path, leading to the high toughness of the liquid phase sintered SiC, compared to that of the solid-state sintered SiC. The degree of the inter-granular fracture reflected the effect of the inter-granular crack deflection. A higher inter-granular fracture indicated a weaker grain boundary, and the liquid phase sintered SiC with a higher degree of the inter-granular fracture tended to have higher toughness but lower strength and lower Vickers hardness, compared to the SiC with a lower degree of the inter-granular fracture [19,20]. In this study, the laminated SiC showed a low degree of inter- granular fracture, so the strength and Vickers hardness of the pure-matrix sample remained high among the liquid phase sintered SiC (approximately 450–650 MPa in strength and Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 12 18–20 GPa in Vickers hardness). The price for the high strength and hardness of the pure- matrix SiC was the reduced toughness. The toughness of the pure-matrix SiC (with 8 wt.% 1/2 Al2O3-Y2O3) was 6.9 MPa·m , lower than that of the SiC prepared by She [15]. Their SiC withprepared 5–10 wt.%by She Al [15].2O3-Y Their2O3 additivesSiC with was5–10 sinteredwt.% Al pressureless2O3-Y2O3 additives but reached was sintered a toughness pres- ofsureless 7.0–7.5 but MPa reached·m1/2. However,a toughness the of toughness 7.0–7.5 MPa of∙ them1/2 pure-matrix. However, the SiC toughness was still higher of the pure- than thematrix SSIC SiC due was to thestill crack higher deflection than the effect.SSIC due to the crack deflection effect. TheThe introductionintroduction ofof thethe interfaceinterface layerlayer toto thethe laminatedlaminated ceramics ceramics in in this this study study was was to to promotepromote thethe energyenergy absorptionabsorption abilityability ofof thethe material.material. AccordingAccording toto Sun’sSun’s studystudy [6[6],], thethe interfaceinterface layerlayer withwith aa combinationcombination ofof SiCSiC andand BNBN (approximately (approximately 1:11:1 in in weight) weight) would would facilitatefacilitate the the crack crack kinking kinking and and delamination delamination cracking cracking in in the the interface interface layer layer during during fractur- frac- ingturing and and the the crack crack would would propagate propagate step-like. step-like. In In this this study, study, the the uniform-layered uniform-layered sample sample ((1010matrix matrixlayers layersto to 1 1 interface interface layer) layer) had had improved improved toughness toughness compared compared to theto the pure-matrix pure-ma- sample.trix sample. A step-like A step-like cracking cracking path path is observed is observed in Figure in Figure7, showing 7, showing strong strong delamination delamina- crackingtion cracking and crack and kinkingcrack kinking effect. effect. The gradient-layered The gradient-layered structure structure sample, sample, however, however, did not havedid not this have effect this and effect had and similar had toughness similar toughness as the pure-matrix as the pure-matrix sample. sample.

FigureFigure 7. 7. OpticalOptical image image of the side-view of the side-viewof the gradua ofl-layered the gradual-layered sample after the sample three-point-bend- after the three-point-bendinging test. test.

TheThe ballisticballistic resultsresults areare shownshown inin TableTable3 .3. In In the the CA CA test, test, all all of of the the samples samples resisted resisted thethe shotshot andand werewere notnot completelycompletely penetrated penetrated by by thethe bullets.bullets. TheThe BFSBFS depthdepth ofof thethe uniform-uniform- layered, gradual-layered, pure-matrix, and SSIC samples was 7.2 mm, 5.6 mm, 9.1 mm, and 11.2 mm, respectively. All three types of the laminated SiC had decreased BFS depths compared to the SSIC sample. The BFS depth of the gradual-layered sample after ballistic impact, which was the least in this study, was 50% less than that of the SSIC. In the SA test, all three laminated ceramics resisted three hits and were not completely penetrated, showing good multi-hit resistance. The maximum BFS depth of the uniform-layered sample was 16.4 mm, slightly higher than that of gradual-layered and pure-matrix samples (both were 15.7 mm). A typical front and side view of the sample after ballistic impact was taken for the pure-matrix sample after the SA test and is shown in Figure 8.

Table 3. Ballistic test results of the laminated SiC ceramics tested in conjunction with a flexible armor jacket (the CA test) and stand-alone (the SA test).

CA Test Results SA Test Results Sample Type BFS Depth of Hit Result(s) The Maximum Hit Result Hits (mm) 1st 2nd 3rd BFS Depth (mm) Uniform-layered PP a 7.2 PP PP PP 16.4 Gradual-layered PP 5.6 PP PP PP 15.7 Pure-matrix PP 9.1 PP PP PP 15.7 SSIC PP 11.2 - - - - a: The bullet was partially penetrated. -: Not measured Appl. Sci. 2021, 11, 6145 8 of 11

layered, gradual-layered, pure-matrix, and SSIC samples was 7.2 mm, 5.6 mm, 9.1 mm, and 11.2 mm, respectively. All three types of the laminated SiC had decreased BFS depths compared to the SSIC sample. The BFS depth of the gradual-layered sample after ballistic impact, which was the least in this study, was 50% less than that of the SSIC. In the SA test, all three laminated ceramics resisted three hits and were not completely penetrated, showing good multi-hit resistance. The maximum BFS depth of the uniform-layered sample was 16.4 mm, slightly higher than that of gradual-layered and pure-matrix samples (both were 15.7 mm). A typical front and side view of the sample after ballistic impact was taken for the pure-matrix sample after the SA test and is shown in Figure8.

Table 3. Ballistic test results of the laminated SiC ceramics tested in conjunction with a ﬂexible armor jacket (the CA test) and stand-alone (the SA test).

CA Test Results SA Test Results Sample Type Hit Result(s) Hit Result BFS Depth of Hits (mm) The Maximum BFS Depth (mm) 1st 2nd 3rd Uniform-layered PP a 7.2 PP PP PP 16.4 Gradual-layered PP 5.6 PP PP PP 15.7 Pure-matrix PP 9.1 PP PP PP 15.7 Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 12 SSIC PP 11.2 ---- a: The bullet was partially penetrated. -: Not measured

(a) (b)

FigureFigure 8. 8. TheThe images images for for the the pure-matrix pure-matrix sample sample after after the the SA SA test; test; ( (aa)) front front view view (the (the ballistic ballistic hit- hitting tingsequence sequence was was labeled); labeled); (b) ( sideb) side view. view.

Generally,Generally, three three stages occuroccur duringduring the the projectile projectile impact impact [21 [21]:]: shattering, shattering, erosion, erosion, and andcatching. catching. Shattering Shattering is the is the primary primary stage stage in defendingin defending the the hard-core hard-core projectile. projectile. During During the theshattering shattering stage, stage, the the projectile projectile impacts impacts the the ceramic ceramicsurface. surface. A shock wave wave is is generated generated andand reflected reflected from from the the back back of of the the ceramic/ba ceramic/backingcking interface. interface. The The reflected reflected tensile tensile wave wave initiatesinitiates the the damage damage accumulation accumulation and and cracki crackingng in in the the ceramic. ceramic. The The interaction interaction between between projectileprojectile and and ceramic ceramic also also results results in in deforming, deforming, shattering, shattering, or or eroding eroding of of the the projectile. projectile. DuringDuring the the erosion erosion stage, stage, the the ceramic ceramic is alread is alreadyy cracked cracked but but still still prevents prevents the theprojectile projectile or fragmentsor fragments from from penetration penetration by the by erosion the erosion mech mechanism.anism. In the In last the catching last catching stage, stage, the ce- the ramicceramic and and its back-plate its back-plate stops stops the theprojectile projectile and and fragments fragments by bymomentum momentum transfer. transfer. TheThe interaction interaction between between projectile projectile and and ceramic ceramic that that results results in in the the deforming deforming or or shat- shat- teringtering of of the the projectile projectile is is a akey key process process in in defeating defeating the the projectile projectile because because the the deformed, deformed, oror even even shattered, shattered, projectile projectile dissipates dissipates its its kinetic kinetic energy energy to to a alarger larger area area and and reduces reduces the the stressstress generated. generated. To To effectively effectively deform deform or or shatter shatter the the projectile, projectile, the the ceramic ceramic needs needs to to have have sufficientsufficient hardness. hardness. Flinders Flinders et et al. al. [22] [22 ]repo reportedrted that that the the hardness hardness was was negatively negatively related related to the depth of penetration (DOP) for several SiC ceramics, including solid-state sintered SiC and liquid-phase sintered SiC. Huang et al. [12] reported a decrease in the penetration resistance of the laminated ceramics with low-hardness interface layers and found that introducing the interface layer at the backside affected penetration resistance less. The interface layer in this study had very low Vickers hardness due to the 50 wt.% of BN, thus the interface layer may not provide an effective projectile-fracturing effect. To guarantee the projectile-fracturing effect of the laminated ceramics, no interface layer was introduced to the first 0.8 mm in the strike face side. As a result, the uniform-layered and gradual-layered samples both successfully defeated the 5.8 mm bullet, indicating a sufficient hardness of the sample to deform or shatter the bullet. In this study, the laminated SiC was found to provide better energy absorption ability under the ballistic impact compared to the solid-state sintered SiC when used in conjunction with a flexible armor jacket. The BFS depths of the four types of the samples in the CA test were in the following sequences: gradual-layered < uniform-layered < pure-matrix < SSIC. The laminated SiC, even with no interface layer, had a BFS depth of 20% less than that of the SSIC. As can be seen in Table 2, the pure-matrix sample had a higher strength and a higher toughness but a slightly lower hardness than that of SSIC. Flinders et al. [22] reported that a higher hardness would benefit the ballistic performance when using 7.62 mm M993 (WC-Co core, designed to target light armor vehicles). However, in this study, the type-87 5.8 mm steel core is not designed as armor-piercing (AP) ammunition and the steel core had a much lower hardness than that of the WC-Co core. The hardness of the Appl. Sci. 2021, 11, 6145 9 of 11

to the depth of penetration (DOP) for several SiC ceramics, including solid-state sintered SiC and liquid-phase sintered SiC. Huang et al. [12] reported a decrease in the penetration resistance of the laminated ceramics with low-hardness interface layers and found that introducing the interface layer at the backside affected penetration resistance less. The interface layer in this study had very low Vickers hardness due to the 50 wt.% of BN, thus the interface layer may not provide an effective projectile-fracturing effect. To guarantee the projectile-fracturing effect of the laminated ceramics, no interface layer was introduced to the first 0.8 mm in the strike face side. As a result, the uniform-layered and gradual-layered samples both successfully defeated the 5.8 mm bullet, indicating a sufficient hardness of the sample to deform or shatter the bullet. In this study, the laminated SiC was found to provide better energy absorption ability under the ballistic impact compared to the solid-state sintered SiC when used in conjunction with a flexible armor jacket. The BFS depths of the four types of the samples in the CA test were in the following sequences: gradual-layered < uniform-layered < pure-matrix < SSIC. The laminated SiC, even with no interface layer, had a BFS depth of 20% less than that of the SSIC. As can be seen in Table2, the pure-matrix sample had a higher strength and a higher toughness but a slightly lower hardness than that of SSIC. Flinders et al. [22] reported that a higher hardness would benefit the ballistic performance when using 7.62 mm M993 (WC-Co core, designed to target light armor vehicles). However, in this study, the type-87 5.8 mm steel core is not designed as armor-piercing (AP) ammunition and the steel core had a much lower hardness than that of the WC-Co core. The hardness of the matrix SiC layer for the laminated ceramics, 22 GPa, was likely sufficient, and further increasing the hardness of the ceramic might not contribute to the ballistic performance [23]. High flexural strength would be beneficial to resist the tensile wave generated by the impact and the toughness would resist the penetration through energy absorption [24]. With a higher flexural strength and a higher toughness, the pure-matrix sample would have better energy absorption ability and thus less BFS depth compared to the SSIC sample. However, the mechanical performance of the laminated ceramics was not directly related to their ballistic performance of the CA test. The least BFS depth was obtained by the gradual-layered sample which had a lower strength and a close toughness compared to the pure-matrix sample. This indicated that the energy absorption effect by the layered structure was the dominating factor. Due to different mechanical properties between the matrix layer and the interface layer, the stress wave in the laminated ceramics or composites would be reflected at the interface and would be attenuated [14,25,26]. Li et al. [25] showed that laminated ceramics could have better impact resistance due to the multi-reflection and attenuation of the stress wave at the interfaces. The reflection and attenuation of the stress wave also reduced the wave propagation speed within the material and thus enhanced penetration resistance [14,26]. The gradual-layered sample, with more interface layers at the back face side, had a decreased BFS depth compared to the uniform-layered sample. This was in agreement with the studies mentioned above in that the introduction of the interface layers helps to improve the ballistic performance. The ballistic performance of the laminated ceramics with interface layers not only depended on the layer structure of the ceramics itself but was also highly dependent on the testing conditions. The BFS depths of the uniform-layered, gradual-layered, and pure- matrix samples were similar in the SA test, while these ceramics had different BFS depths in the CA test. Of note is that the ballistic performance was highly related to the testing conditions as well as the armor itself, e.g., the back support [27], the confinement [28], or the pre-stress applied to the armor plate [29,30]. In the CA test, the armor plate was attached to a flexible armor jacket which provided stronger back support than the armor plate for the SA test. The stress wave generated by the ballistic impact was reflected at the ceramic/backing interface due to the mechanical impedance [2]. A theoretical study by Gour [27] also showed that a thicker back-plate would help to offset the tensile stress at the ceramics and support the ceramic for a longer time, preventing the premature failure of the ceramics. Compared to the laminated SiC ceramics for the CA test, the laminated Appl. Sci. 2021, 11, 6145 10 of 11

SiC ceramics in the SA test had a weaker back support, thus they likely failed before the laminated structure could play a role. As a result, the BFS depths of the three laminated SiC ceramics in the SA test were close. It is still unclear whether there is a threshold for the laminated ceramics with interface layers to be effective, e.g., the critical impact energy or velocity of the wave. Additional experiments, as well as theoretical studies, are needed to explain this phenomenon.

4. Conclusions The three types of laminated SiC ceramics, the uniform-layered, gradual-layered, and pure-matrix, were designed and prepared by tape-casting and hot-pressing. When tested in conjunction with a ﬂexible armor jacket, the BFS depths of the samples were in the following sequences: gradual-layered < uniform-layered < pure-matrix and all the laminated SiC obtained less BFS depth than the solid-state sintered SiC. The layer structure was the main reason for the differences. The gradual-layered sample had the back face signature depth of 30% less than the laminated SiC with no interface structure and 50% less than the solid-state sintered SiC. However, the ballistic performance of the laminated ceramics with interface layers not only depended on the layer structure of the ceramics itself but was also highly dependent on the back support for the ceramics. When tested stand-alone, the laminated SiC armor plates had a similar ballistic performance regardless of the layer structure, which was likely because the armor plate for the stand-alone test had weaker back support compared to the armor plates tested in conjunction with a ﬂexible armor jacket. All of the laminated SiC resisted three shots of the bullet in the stand-alone test, showing good multi-hit resistance. In conclusion, the ballistic performance of the laminated ceramics was related to the back support of the armor system. When used in the in conjunction armor system, the laminated SiC had a better ballistic performance than that of the solid-state sintered SiC, making the laminated ceramic a potential armor ceramic.

Author Contributions: Conceptualization, Y.H. and F.X.; Data curation, Y.H. and L.X.; Formal analysis, M.Y. and X.C.; Funding acquisition, G.W.; Investigation, M.Y., X.C. and Z.S.; Methodology, L.X.; Project administration, F.X. and G.W.; Resources, L.X. and M.Q.; Supervision, F.X. and T.M.; Validation, F.X.; Visualization, Y.H.; Writing—original draft, Y.H.; Writing—review and editing, Z.S., F.X., T.M. and N.J. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key R&D Program of China, grant number 2017YFB1103503. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors would like to thank the Institute of Quartermaster Engineering Technology of the PLA Academy of Military Science for the support of ballistic testing. Conﬂicts of Interest: The authors declare 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.

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