materials

Article Mechanical Response and Shear-Induced Initiation Properties of PTFE/Al/MoO3 Reactive Composites

Junyi Huang ID , Xiang Fang, Shuangzhang Wu, Li Yang, Zhongshen Yu and Yuchun Li * College of Field Engineering, PLA Army Engineering University, Nanjing 210007, China; [email protected] (J.H.); [email protected] (X.F.); [email protected] (S.W.); [email protected] (L.Y.); [email protected] (Z.Y.) * Correspondence: [email protected]; Tel.: +86-25-8082-1320

 Received: 18 June 2018; Accepted: 9 July 2018; Published: 12 July 2018 

Abstract: Polytetrafluoroethylene/aluminum/molybdenum oxide (PTFE/Al/MoO3) reactive composites of a volume ratio of 60:16:24 were studied in this research. Quasi-static compression, dynamic compression and drop-weight experiments were performed to explore the mechanical response and the shear-induced initiation properties of the composites. Mesoscale images of the specimens after sintering demonstrate that PTFE, Al and MoO3 powders were evenly mixed and no chemical reaction occurred after the materials were stirred, pressed and sintered. The yield stress and compressive strength of PTFE/Al/MoO3 specimens are sensitive to strain rate within the range of 10−3~3 × 103 s−1, and the yield stress shows a bilinear dependence on the logarithm values of strain rate. The established Johnson-Cook constitutive model based on the experimental data can describe the mechanical response of PTFE/Al/MoO3 material well. Drop-weight tests show that the PTFE/Al/MoO3 specimens can react violently when impacted, with the characteristic drop height (H50) calculated as 51.57 cm. The recovered specimens show that the reaction started from the outer edge of the specimen with the largest shear force and the most concentrated shear deformation, indicating a shear-induced initiation mechanism. The reaction products of PTFE/Al/MoO3 specimens were AlF3, Al2O3, Mo and C, demonstrating that redox reaction occurred between PTFE and Al, and between Al and MoO3.

Keywords: mechanical response; PTFE/Al/MoO3 materials; split Hopkinson pressure bar (SHPB); constitutive model; shear-induced initiation

1. Introduction Reactive materials (RMs) are a special type of energetic material, usually composed of at least two components which are not active on their own. These materials mainly consist of combinations of , intermetallics, metal/polymer mixtures, metastable intermolecular composites (MICs), matrix materials or hydrides [1,2]. When subjected to external loads, RMs can undergo huge exothermic reactions, with plenty of chemical energy released. Of all the different types of RMs, mixtures of PTFE (polytetrafluoroethylene) and Al have been extensively studied, because of their large heat of reaction and the excellent combinations of performance that PTFE exhibits such as low friction coefficient, high thermal stability, high electrical resistance, high chemical inertness, high melting point and easiness of forming [3]. Therefore, many scholars have carried out extensive and in-depth researches on PTFE/Al materials. Raftenberg et al. [4] studied the deformation properties and compressive mechanical responses of solid rods of PTFE/Al composites with a mass ratio of 74:26, and the experimental data were obtained by conducted quasi-static Instron compression tests and split-Hopkinson pressure bar (SHPB) experiments. Based on these data, a fit to Johnson-Cook model was determined. In their studies, finite element simulation, using the Johnson-Cook model established above, has also been

Materials 2018, 11, 1200; doi:10.3390/ma11071200 www.mdpi.com/journal/materials Materials 2018, 11, 1200 2 of 14 performed to compare with experimental values. Ge et al. [5] conducted two-dimensional microscale finite element analyses to study the mechanical behavior of PTFE/Al composites, and the real microstructure-based models and simulated microstructure models were both used. In order to improve the density and strength of the PTFE/Al materials, many researchers have tried to add (W) into the PTFE/Al composites. Cai et al. [3] performed dynamic compression tests in a drop-weight apparatus to explore the behavior of a PTFE/Al/W mixture at a high-strain and high-strain rate. They found that the initiation and propagation of cracks originate from the separation of the W particle-PTFE interface and PTFE nanofiber formed in specimens which were extensively deformed. The dynamic mechanical properties of PTFE/Al/W with different W particle sizes were also studied by Cai et al. [6], and they found that materials with fine particles have higher mechanical strength. Combining simulation and experiments, Herbold et al. [7,8] investigated the influence of particle size and sample porosity on the dynamic mechanical properties, including strength, failure and shock behavior, of the PTFE/Al/W granular composites. They discovered that force chains can be formed in the specimens with fine metallic particles under dynamic loading, which accounted for the increased strength of these specimens. Other scholars also carried out studies of the PTFE/Al/W composites from other perspectives using different techniques, including quasi-static and dynamic compression, scanning electron microscope (SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) [9–13]. However, in the above FTFE/Al/W experiments, no reaction between Al and W was observed, only a small amount of WC and W2C formed, which were activated by the high heat as a result of fluorination of Al [11]. In addition, literatures about the addition of metal oxides, such as CuO, MoO3 and Fe2O3, into the PTFE/Al are scarce. But the heat generated by the reaction between Al and metal oxides (thermites) cannot be neglected. Al/MoO3 is a kind of high , with a heat of reaction of 4.7 kJ/g and an adiabatic temperature of 3253 K, which is higher than that of other common thermites such as Al/CuO, Al/Fe2O3, Al/MnO2 and Al/Bi2O3 [14]. However, the Al/MoO3 mixture is difficult to form, and it is generally used as a powder for combustion. The addition of PTFE to Al/MoO3 can greatly increase the burning rate [15] and the PTFE/Al/MoO3 composites can be processed into a specific shape. Therefore, based on Al/MoO3, PTFE is used as a binder to prepare PTFE/Al/MoO3 multifunctional structural reactive materials, which have the dual properties of PTFE/Al and Al/MoO3. Furthermore, because PTFE is easy to form, the PTFE/Al/MoO3 materials also can be processed into a structural body with a certain strength, which greatly expands the applications of thermites. In order to ensure the stability and safety of the reactive materials in the process of manufacturing, transportation, storage, and the reliability during applications, it is necessary to understand the mechanical response and reaction performance of the reactive materials under different loading conditions. Therefore, in this research, a kind of PTFE/Al/MoO3 reactive material was prepared using cold-pressing and sintering technology. Firstly, the micro-structures of the as received raw materials and sintered PTFE/Al/MoO3 specimens were analyzed by the scanning electron microscope (SEM). Then the quasi-static and dynamic mechanical properties of the specimens at room temperature were tested using a universal testing machine and split Hopkinson pressure bar (SHPB) system. Finally, a standard drop-weight apparatus combined with a high-speed camera was utilized to explore the impact sensitivity and shear-initiation process of the PTFE/Al/MoO3 composites.

2. Experimental

2.1. Specimens Preparation

In this study, a kind of PTFE/Al/MoO3 composite was prepared with a volume ratio of 60:16:24. The ratio is based on the chemical equilibrium ratio of Al and MoO3, and PTFE is used as a binder. The original average sizes of the three raw materials were: PTFE: 25 µm (3M, Shanghai, China); Al: 1~2 µm (NAO, Shanghai, China); MoO3, 3 µm (NAO, Shanghai, China). The preparation process Materials 2018, 11, 1200 3 of 14

Materials 2018, 11, x FOR PEER REVIEW 3 of 14 Materialsof the specimens2018, 11, x FOR can PEER refer REVIEW to Nielson’s work [16], which includes powder mixture, cold isostatic3 of 14 pressing and sintering, and is shown in Figure1. The sizes of the specimens were Φ 10 mm × 10 mm, pressingpressing andand sintering,sintering, andand isis shownshown inin FigureFigure 1.1. TheThe sizessizes ofof thethe specimensspecimens werewere ΦΦ 1010 mmmm ×× 1010 mm,mm, Φ 10 mm × 5 mm, and Φ 10 mm × 3 mm, which were used for the quasi-static compression, SHPB and ΦΦ 1010 mmmm ×× 55 mm,mm, andand ΦΦ 1010 mmmm ×× 33 mm,mm, whichwhich werewere usedused forfor thethe quasi-staticquasi-static compression,compression, SHPBSHPB andand drop-weight experiments, respectively. The properties of the specimens are greatly influenced by the drop-weightdrop-weight experiments,experiments, respectively.respectively. TheThe propertipropertieses ofof thethe specimensspecimens areare greatlygreatly influencedinfluenced byby thethe sintering process [17], and the sintering process in this study is based on those described in [10,18,19]. sinteringsintering processprocess [17],[17], andand thethe sinteringsintering processprocess inin ththisis studystudy isis basedbased onon thosethose describeddescribed inin [10,18,19].[10,18,19]. The detailed process is as follows: The pressed specimens were put into a vacuum furnace, being TheThe detaileddetailed processprocess isis asas follows:follows: TheThe pressedpressed spspecimensecimens werewere putput intointo aa vacuumvacuum furnace,furnace, beingbeing heated from 25 ◦C to 360 ◦C at 90 ◦C/h, incubated for 240 min, then cooled to 325 ◦C at the rate of heatedheated fromfrom 2525 °C°C toto 360360 °C°C atat 9090 °C/h,°C/h, incubatedincubated forfor 240240 min,min, thenthen cooledcooled toto 325325 °C°C atat thethe raterate ofof 5050 50 ◦C/h, kept for 120 min, and finally cooled to 25 ◦C at the same rate. The sintering process curve is °C/h,°C/h, keptkept forfor 120120 min,min, andand finallyfinally cooledcooled toto 2525 °C°C atat thethe samesame rate.rate. TheThe sinteringsintering processprocess curvecurve isis shown in Figure2. shownshown inin FigureFigure 2.2.

FigureFigure 1. 1.The The preparationpreparation process process of of PTFE/Al/MoO PTFE/Al/MoO3 specimens.specimens. Figure 1. The preparation process of PTFE/Al/MoO3 3specimens.

FigureFigure 2. 2. SinteringSintering curve. curve.

2.2. Quasi-Static Compression Experiments 2.2.2.2. Quasi-Static Quasi-Static Compression Compression Experiments Experiments The quasi-static compression experiments were carried out by the microcomputer-controlled TheThe quasi-static quasi-static compression compression experiments experiments were were carried carried out out by the microcomputer-controlled universal testing machine (CMT5105, MTS, Eden Prairie, MN, USA) with a maximum load of 100 kN. universaluniversal testing testing machine machine (CMT5105, (CMT5105, MTS, MTS, Eden Eden Prai Prairie,rie, MN, MN, USA) USA) with with a a maximum maximum load load of of 100 100 kN. kN. The head of the testing machine travels downward at a speed of 0.6 mm/min, 6 mm/min and 60 TheThe head head of of the the testing testing machine machine travels travels downward downward at at a aspeed speed of of 0.6 0.6 mm/min, mm/min, 6 mm/min 6 mm/min and and 60 mm/min, respectively, corresponding to the strain rate of the specimen of 0.001 s−1, 0.01 s−1 and 0.1 s−1. mm/min,60 mm/min, respectively, respectively, corresponding corresponding to the to strain the strain rate rateof the of specimen the specimen of 0.001 of 0.001 s−1, 0.01 s−1 s,− 0.011 and s −0.11 and s−1. Before tests, all contact surfaces of the specimens were coated with vaseline to eliminate the effect of Before0.1 s−1 tests,. Before all tests,contact all surfaces contact surfacesof the specimens of the specimens were coated were with coated vaseline with vaselineto eliminate to eliminate the effect the of friction.friction. ThreeThree specimensspecimens werewere testedtested underunder thethe samesame coconditionsnditions toto ensureensure thethe accuracyaccuracy ofof thethe results.results.

Materials 2018, 11, 1200 4 of 14 effect of friction. Three specimens were tested under the same conditions to ensure the accuracy of the results.

2.3. SHPB Experiments

The dynamic compression properties of PTFE/Al/MoO3 specimens were tested using a split Hopkinson pressure bar (SHPB) system (AOC, Hefei, China). SHPB technology was originally proposed by Hopkinson [20] and later improved by Kolsky [21]. The SHPB technique is on the basis of two assumptions, namely (i) the one-dimensional stress wave assumption and (ii) the specimen is in force equilibrium and is deforming uniformly. The stress, strain, and strain rate in the specimen could be calculated as follows:  A0 σe = E0εt(t)  As  ε = −2 C0 R t ε (t)dt (1) e Ls 0 r  .  ε = −2 C0 ε (t) e Ls r . where σe, εe, εe are the engineering stress, engineering strain and strain rate of the specimen, εr(t) and εt(t) are reflected and transmitted strain histories sensed by strain gages. A0 is the cross-sectional area of the compression bars, E0 is the Young’s modulus of the bars, C0 is the elastic wave velocity in the bars. As and Ls are the original cross-sectional area and length of the specimen. The diagrammatic sketch and actual device of the SHPB test system are given in Figure3. The diameters of the striker bar, incident bar and transmitted bar are all 20 mm, and the lengths are 600 mm, 6000 mm, and 3500 mm, respectively. The distances of the strain gauges in the incident bar and the transmitted bar from the specimen are 3000 mm and 1300 mm, respectively. Because the mechanical impedance of PTFE/Al/MoO3 samples is relatively low, the signal in the transmitted bar is weak. So LC4 aluminum bars were used in the tests to improve the signal-to-noise ratio in the transmitted bar. If impedance-matched materials are used for the striker bar and the incident bar, the rise time of the generated pulse (possibly a square wave) will be short when the striker bar hits the incident bar, which is not conducive to facilitate stress-state equilibrium in the specimens. Therefore, during the experiments, a soft and deformable pulse shaper, such as rubber, paper, etc., can be placed on the incident end of the incident bar to increase the rise time of the generated wave. So in this study, a pulse shaper made of rubber with a thickness of 1 mm and a diameter of 10 mm was used.

2.4. Drop-Weight Experiments

The impact sensitivity of the PTFE/Al/MoO3 material was investigated using a standard drop-weight apparatus in conjunction with a high-speed camera, and the schematic illustration and the actual apparatus are presented in Figure4. The apparatus consists of a stainless steel plate (mass ca. 10 kg) which can be released from a height of 150 cm. When the drop mass was released, the high-speed camera was used to record the deformation process and reaction phenomenon of the specimen, and to determine whether the specimen was ignited based on the recorded images according to the “up-and-down technique” [22]. Once the suitable range of positive and negative reactions was found, the tests were performed at 2 cm intervals. Materials 2018, 11, 1200 5 of 14 Materials 2018, 11, x FOR PEER REVIEW 5 of 14 Materials 2018, 11, x FOR PEER REVIEW 5 of 14

(a) (a)

(b) (b) Figure 3. SHPB setup. ( a) diagrammatic sketch; (b) actual device. Figure 3. SHPB setup. (a) diagrammatic sketch; (b) actual device.

(a) (b) (a) (b) Figure 4. Drop-weight apparatus. (a) schematic illustration; (b) actual apparatus. FigureFigure 4. Drop-weightDrop-weight apparatus. apparatus. ( (aa)) schematic schematic illustration; illustration; ( (bb)) actual actual apparatus. apparatus.

Materials 2018, 11, 1200 6 of 14 Materials 2018, 11, x FOR PEER REVIEW 6 of 14 Materials 2018, 11, x FOR PEER REVIEW 6 of 14 3.3. Results Results and and Discussion Discussion 3. Results and Discussion 3.1.3.1. Mesoscale Mesoscale Characteristics Characteristics 3.1. Mesoscale Characteristics TheThe initial microstructuresmicrostructures and and element element distribution distribution of the of raw the materials raw materials and a sintered and a specimensintered specimenwereThe investigated wereinitial investigated microstructures by scanning by scanning electron and element microscopeelectron distri microscope (HITACHIbution of (HITACHI the S-4800, raw Tokyo, S-4800,materials Japan), Tokyo, and as Japan),a givensintered as in givenFigurespecimen in5. Figure It were can be 5.investigated seenIt can from be theseen by Figure scanningfrom5 a–dthe electronFigure that the 5a–d PTFEmicroscope that is soft the and (HITACHIPTFE irregular, is soft S-4800, withand irregular,an Tokyo, average Japan), with size an of as µ µ average20–25given in m,size Figure Al of particles 20–25 5. It μcan havem, Albe a particlesseen regular from sphericalhave the aFigure regular shape 5a–d spherical with that a particle the shape PTFE size with is of asoft 800 particle nmand toirregular, size 2 m,of 800 and with nm MoO toan3 particle is an irregular polyhedron with a smooth surface and a size of 2–5 µm. The PTFE/Al/MoO 2average μm, and size MoO of 320–25 particle μm, is Al an particles irregular have polyhedron a regular with spherical a smooth shape surface with aand particle a size size of 2–5 of 800 μm. nm The to3 composite material is evenly mixed, with the Al and MoO particles in good contact and scattered PTFE/Al/MoO2 μm, and MoO3 composite3 particle is material an irregular is evenly polyhedron mixed, withwith thea smooth3 Al and surface MoO3 andparticles a size in of good 2–5 μcontactm. The in the PTFE matrix. The distribution of Al, C, F, and Mo element (Figure5e–h) also confirmed the andPTFE/Al/MoO scattered in3 compositethe PTFE matrix.material The is evenly distribution mixed, of with Al, theC, F,Al and and Mo MoO element3 particles (Figure in good 5e–h) contact also uniform mixing of PTFE, Al and MoO powders. confirmedand scattered the uniformin the PTFE mixing matrix. of PTFE, The3 Aldistribution and MoO 3of powders. Al, C, F, and Mo element (Figure 5e–h) also confirmed the uniform mixing of PTFE, Al and MoO3 powders.

Figure 5. Microstructures and element distribution of the raw materials and a sintered specimen. (a) Figure 5. Microstructures and element distribution of the raw materials and a sintered specimen. (a) PTFE; PTFE;Figure (b 5.) Al; Microstructures (c) MoO3; (d) PTFE/Al/MoO and element di3; stribution(e) Al element; of the (f raw) C element; materials (g )and F element; a sintered (h) specimen.Mo element. (a ) (b) Al; (c) MoO3;(d) PTFE/Al/MoO3;(e) Al element; (f) C element; (g) F element; (h) Mo element. PTFE; (b) Al; (c) MoO3; (d) PTFE/Al/MoO3; (e) Al element; (f) C element; (g) F element; (h) Mo element. In the XRD pattern of the sintered PTFE/Al/MoO3 composite (Figure 6), only the diffraction In the XRD pattern of the sintered PTFE/Al/MoO composite (Figure6), only the diffraction peaksIn of thePTFE, XRD Al, pattern and MoO of 3the were sintered detected, PTFE/Al/MoO indicating that33 composite no chemical (Figure reaction 6), only occurred the diffraction after the peaks of PTFE, Al, and MoO were detected, indicating that no chemical reaction occurred after the materialpeaks of was PTFE, stirred, Al, and pressed MoO 3and3 were sintered. detected, indicating that no chemical reaction occurred after the material was stirred, pressedpressed andand sintered.sintered. 6 6 ■ ■ 5 ●Al ■ 5 ■●MoOAl 3 ■ ▲PTFE 4 ■MoO3 ▲PTFE 4 ■ Counts) 4 3 ■

Counts)

4 3 ▲

2 ▲ 2

Intensity(×10 1 ■ ■ ● Intensity(×10 1 ■ ■ ■ ■ ● ● ■ ■ ■ ■ ■ ■ ■ ■ ● ● 0 ■ ■ ● ■ ■ ■ ■ ■ ■ ■ ■ ● ● 0 15 30 45 60 75 15 302θ(degrees) 45 60 75 2θ(degrees) Figure 6. XRD pattern of PTFE/Al /MoO3 composite. Figure 6. XRD pattern of PTFE/Al /MoO3 composite. Figure 6. XRD pattern of PTFE/Al/MoO3 composite. 3.2. Mechanical Responses under Quasi-Static Compression 3.2. Mechanical Responses under Quasi-Static Compression Figure 7a presents the true stress-strain curves for three tests of PTFE/Al/MoO3 specimens at the strainFigure rate of 7a 0.01 presents s−1. The the results true stress-strai imply thatn curvesthe three for curvthreees tests almost of PTFE/Al/MoO overlapped together,3 specimens and at the the experimentalstrain rate of error 0.01 iss− 11.02%~3.17%,. The results implyindicating that thatthe threethe experimental curves almost data overlapped in this study together, are repeatable and the andexperimental reliable. error is 1.02%~3.17%, indicating that the experimental data in this study are repeatable and reliable.

Materials 2018, 11, 1200 7 of 14

3.2. Mechanical Responses under Quasi-Static Compression

Figure7a presents the true stress-strain curves for three tests of PTFE/Al/MoO 3 specimens at the strain rate of 0.01 s−1. The results imply that the three curves almost overlapped together, and the experimentalMaterials 2018 error, 11, x FOR is 1.02%~3.17%, PEER REVIEW indicating that the experimental data in this study are repeatable7 of 14 and reliable. TheThe true true stress-strain stress-strain curves curves of of PTFE/Al/MoO PTFE/Al/MoO3 3specimensspecimens under under quasi- quasi-staticstatic compression compression at at differentdifferent strain strain rates rates are are displayed displayed in in Figure Figure7b, 7b, and and their their corresponding corresponding parametersparameters are summarized in Tablein1 Table. The data1. The reveal data thatreveal the that yield the stress yield and stre compressivess and compressive strength strength of the specimens of the specimens both present both an increasingpresent trendan increasing as the straintrend as rate the increases. strain rate Butincrea theses. failure But the strains failure show strains the show opposite the opposite trend. However,trend. However, the elastic moduli of PTFE/Al/MoO3 specimens remain almost unchanged, demonstrating the elastic moduli of PTFE/Al/MoO3 specimens remain almost unchanged, demonstrating that the that the strain rate has a limited influence on the elastic part of the specimens. PTFE/Al/MoO3 material strain rate has a limited influence on the elastic part of the specimens. PTFE/Al/MoO material is is a typical particle-reinforced composite material whose mechanical properties mainly depend3 on a typical particle-reinforced composite material whose mechanical properties mainly depend on the the PTFE matrix and the interface formed between the matrix and the particles (Al and MoO3). In the PTFEinitial matrix stage and of theloading, interface the formedspecimens between showed the linear matrix elastic and thebehavior, particles but (Al the and entire MoO elastic3). In the initialdeformation stage of loading, stage was the short specimens and the showed corresponding linear elasticmaximum behavior, strain butwas theabout entire 0.03. elastic After deformationyielding, stagethe was specimens short and experienced the corresponding strain hardening. maximum As strain the compressive was about 0.03.load Aftercontinued yielding, to increase, the specimens the experiencedmolecular strainchains hardening. in the crystallization As the compressive zone of the load PTFE continued matrix began to increase, to slip, the resulting molecular in micro chains in thecracks, crystallization then the zonematrix of and the PTFEparticles matrix gradually began “de-bonded”. to slip, resulting When in the micro load cracks, reached then the the maximum matrix and particlesvalue, graduallythe specimens “de-bonded”. failed. When the load reached the maximum value, the specimens failed.

100 100

0.01/s-1 80 0.001/s 80 0.01/s-2 0.01/s 0.01/s-3 0.1/s 60 60

40 40 True stress/MPa True stress/MPa True

20 20

0.0 0.3 0.6 0.9 1.2 1.5 1.8 0.0 0.3 0.6 0.9 1.2 1.5 1.8 True strain True strain (a) (b)

Figure 7. True stress-strain curves of PTFE/Al/MoO3 specimens under quasi-static compression. Figure 7. True stress-strain curves of PTFE/Al/MoO specimens under quasi-static compression. (a) Strain rate 10−3 s−1; (b) Strain rate 10−3~10−1 s−1. 3 (a) Strain rate 10−3 s−1;(b) Strain rate 10−3~10−1 s−1.

Table 1. Mechanical properties parameters of PTFE/Al/MoO3 specimens. (under quasi-static Table 1. Mechanical properties parameters of PTFE/Al/MoO specimens. (under quasi-static compression). compression). 3

Strain Rate/s−1 Elastic Modulus/MPaElastic Yield Stress/MPaYield CompressionCompression Strength/MPa Failure Strain Strain Rate/s−1 Failure Strain 0.001Modulus/MPa 851 Stress/MPa 18 Strength/MPa 76 1.91 0.010.001 852851 2118 76 82 1.91 1.82 0.10.01 854852 2421 82 851.82 1.75 0.1 854 24 85 1.75 3.3. Dynamic Compression Performance 3.3. Dynamic Compression Performance The compressive mechanical properties of PTFE/Al/MoO3 specimens at high strain rates were measuredThe using compressive SHPB systemmechanical (AOC, properties Hefei, China).of PTFE/Al/MoO The true3 specimens stress-strain at high curves strain are rates presented were in Figuremeasured8, and theusing corresponding SHPB system dynamic (AOC, Hefei, mechanical China). properties The true parametersstress-strain arecurves shown are inpresented Table2. Asin is shownFigure in Figure8, and the8, the corresponding specimens dynamic are strongly mechanical sensitive properties to the strain parameters rates. are Same shown as the in Table values 2. underAs is shown in Figure 8, the specimens are strongly sensitive to the strain rates. Same as the values under quasi-static compression, the yield stress and compressive strength of PTFE/Al/MoO3 specimens underquasi-static dynamic compression, compression th alsoe yield have stress strain and rate compressiv effects. However,e strength in theof PTFE/Al/MoO elastic stage, the3 specimens stress-strain under dynamic compression also have strain rate effects. However, in the elastic stage, the stress- curves of the specimens become steeper as the strain rate increases, that is, the elastic modulus is strain curves of the specimens become steeper as the strain rate increases, that is, the elastic modulus sensitive to the strain rate, which is different from that under quasi-static compression. is sensitive to the strain rate, which is different from that under quasi-static compression.

Materials 2018, 11, x FOR PEER REVIEW 8 of 14 Materials 2018, 11, 1200 8 of 14 Materials 2018, 11, x FOR PEER120 REVIEW 8 of 14

120 -1 100 strain rate/s 700 1300 strain rate/s-1 100 1800 700 80 2300 1300 3000 1800 80 2300 60 3000

60 40 True stress/MPa True 40

True stress/MPa True 20

20

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 True strain 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Figure 8. True stress-strain curves of PTFE/Al/MoOTrue strain3 specimens under dynamic impact.

Figure 8. True stress-strain curves of PTFE/Al/MoO3 specimens under dynamic impact. TableFigure 2. Dynamic 8. True performance stress-strain parameters curves of PTFE/Al/MoO of PTFE/Al/MoO3 specimens3. (specimens under at different dynamic strain impact. rates).

TableTable 2. 2.Dynamic Dynamic performance performanceElastic parameters parameters of ofYield PTFE/Al/MoO PTFE/Al/MoO 3. .(specimens (specimensCompression at at different different strain strain rates). rates). Strain Rate/s−1 3 Critical Strain Modulus/MPa Stress/MPa Strength/MPa Elastic Yield Compression Strain700 Rate/s Rate/s −1 −1 Elastic Modulus/MPa1226 Yield Stress/MPa39.2 Compression44.9 Strength/MPa Critical Critical 0.17 StrainStrain Modulus/MPa Stress/MPa Strength/MPa 1300700 12261456 39.243.2 62.8 44.9 0.35 0.17 700 1226 39.2 44.9 0.17 13001800 14561895 43.246 76.3 62.8 0.49 0.35 18001300 18951456 4643.2 62.8 76.3 0.35 0.49 2300 2568 48 100.4 0.59 23001800 25681895 4846 76.3 100.4 0.49 0.59 3000 35693569 5252 110.2 110.2 0.60 0.60 2300 2568 48 100.4 0.59 3000 3569 52 110.2 0.60 Figure 99 presentspresents thethe relationship relationship between between the the yield yield stresses stresses of of PTFE/Al/MoO PTFE/Al/MoO3 specimensspecimens and logarithmlogarithm values values of of strain strain rate. rate. As As can can be be seen seen from from Figure Figure 99,, therethere isis aa clearclear bilinearbilinear relationshiprelationship Figure 9 presents the relationship between the yield stresses of PTFE/Al/MoO3 specimens and between the yield stresses and the logarithm values of strain rate. When When the strain strain rate rate is is greater greater than than logarithm values of strain rate. As can be seen from Figure 9, there is a clear bilinear relationship 103 ss−−1, 1the, the slope slope of ofthe the curve curve suddenly suddenly increases. increases. Walley Walley et al. et[18] al. also [18] found also found this phenomenon this phenomenon when between the yield stresses and the logarithm values of strain rate. When the strain rate is greater than studyingwhen studying the mechanical the mechanical properties properties of a series of a seriesof polymers of polymers (including (including PTFE) PTFE)at different at different strain strainrates. 103 s−1, the slope of the curve suddenly increases. Walley et al. [18] also found this phenomenon when Asrates. previously As previously discussed, discussed, the PTFE the matrix PTFE is matrix the main is thecomponent main component of the PTFE/Al/MoO of the PTFE/Al/MoO3 specimens, studying the mechanical properties of a series of polymers (including PTFE) at different strain rates.3 therefore,specimens, its therefore, mechanical its mechanical properties properties have a havegreat a greatinfluence influence on onthe themechanical mechanical responses responses of As previously discussed, the PTFE matrix is the main component of the PTFE/Al/MoO3 specimens, PTFE/Al/MoO3 specimens.specimens. therefore, its mechanical3 properties have a great influence on the mechanical responses of

PTFE/Al/MoO3 specimens.55

5055

4550

4045

3540

3035

Yield stress(MPa) 2530

Yield stress(MPa) 2025

1520 -6-4-202468 15 ln(ε ) -6-4-202468 ε  FigureFigure 9. 9. RelationshipRelationship between between yield yield ln(stre stresses)sses and logarithm train rates.

Figure 9. Relationship between yield stresses and logarithm train rates.

Materials 2018, 11, x FOR PEER REVIEW 9 of 14 Materials 2018, 11, 1200 9 of 14 A piecewise function can be used to describe the relationship between the logarithm values of strain rate and the yield stress, because there is a strain rate transition point at which the sensitivity of theA yield piecewise stress function to the strain can be rate used changes to describe grea thetly, relationshipas shown in between Figure the9. The logarithm function values can be of expressedstrain rate as: and the yield stress, because there is a strain rate transition point at which the sensitivity of the yield stress to the strain rate changes greatly, as shown in Figure9. The function can be expressed as: 3-1 σy =1.62ln(ε)+28, ε<10 s ( . . 3 −1 σy = 1.62 ln(ε) + 28, ε < 10 s (2) σ =12ln(ε)−≥ 43.9, ε 103-1 s (2)  y . . 3 −1 σy = 12 ln(ε) − 43.9, ε ≥ 10 s

σ . y ε −1 −1 22 where σy is isthe the yield yield stress, stress, MPa; MPa;ε denotes denotes strain strain rate, rate, s s. The. The correlation correlation index index RR between the experimental values and and the fitted fitted equation is 0. 0.9811998119 and 0.99939, respectively, indicating that the fittingfitting results agree well with the experimental values values.. The linear fitting fitting results of yield stresses and logarithm values of strain rate are shown in FigureFigure 1010..

52 40 3 3 ε<10 s-1 ε>10 s-1

36 Linear fit of yield strss 50 Linear fit of yield strss Yield stress Yield stress

32 48

28 46

24 44 Yield stress(MPa) Yield stress(MPa)

20 42

16 40 -6-4-202468 7.2 7.4 7.6 7.8 8.0 8.2 ε  ε ln( ) ln( ) (a) (b)

31− ε. < 3 −1 Figure 10. LinearLinear fitting fitting results results of of yield yield stresses stresses and and Logarithm Logarithm values values of of strain strain rate. rate. (a) ( a)ε <1010 ss ;; . − (b) εε>>103 s311−. (b)  10 s . 3.4. Johnson-Cook Constitutive Model 3.4. Johnson-Cook Constitutive Model The Johnson-Cook (JC) constitutive model is an empirical constitutive model based on a large The Johnson-Cook (JC) constitutive model is an empirical constitutive model based on a large number of practical applications. The data obtained in this study have been used to fit the JC number of practical applications. The data obtained in this study have been used to fit the JC constitutive equation. The specific details of the model are described in the researches of Johnson, constitutive equation. The specific details of the model are described in the researches of Johnson, Cook and Holmquist [23,24], and are briefly reviewed below. Cook and Holmquist [23,24], and are briefly reviewed below. n . . ∗m σ = (A + Bεpn*m)[1 + Cln(ε/ε0)](1−− T ) (3) σ=(A+Bεp0)[1+Cln(ε/ε )](1 T ) (3) . . where ε denotes plastic strain, ε is the strain rate, ε is the reference strain rate. A, B, n, and C are all εp ε 0 ε wherematerial constants.p denotesSince plastic the strain, tests in this is the study strain were rate, all conducted0 is the reference at room temperature, strain rate. A regardless, B, n, and ofC arethe temperatureall material softeningconstants. effectsSince ofthe the tests material, in this so study the JC were equation all conducted is simplified at to:room temperature, regardless of the temperature softening effects of the material,. .so the JC equation is simplified to: σ = (A + Bε n)[1 + Cln(ε/ε )] (4) pn 0 σ=(A+Bε )[1+Cln(ε/ε )] (4) p0 Figure9 shows that there is a bilinear relationship between the logarithm values of strain rates Figure 9 shows that there is a bilinear relationship between the logarithm values3 −of1 strain rates and yield stresses of PTFE/Al/MoO3 specimens, and the critical strain rate is 10 s . Therefore, 3 −1 andthe fittingyield processstresses wasof PTFE/Al/MoO divided into two3 specimens, parts, and and the fittingthe critical parameters strain ofrate the isJC 10 model s . Therefore, are tabulated the fittingin Table process3. was divided into two parts, and the fitting parameters of the JC model are tabulated in Table 3. From Table3, the JC constitutive equation of PTFE/Al/MoO 3 material is: From Table 3, the JC constitutive equation of PTFE/Al/MoO3 material is: ( 0.7169 . .  . 3 −1 σ = 18 + 10.23ε 1 + 0.084 ln ε/ε0 , ε < 10 s (5) 1.21353 . .  . 3 −1 σ = 43.2 + 77ε 1 + 0.2255 ln ε/ε0 , ε ≥ 10 s

Materials 2018, 11, x FOR PEER REVIEW 10 of 14 Materials 2018, 11, x FOR PEER REVIEW 10 of 14

0.7169 3 -1 σ=(18+10.23ε 0.7169)(1+0.084ln(ε/ε 0 )), ε <10 3 s -1 σ=(18+10.23ε )(1+0.084ln(ε/ε 0 )), ε <10 s (5) 1.21353 ≥ 3 -1 (5) Materials 2018, 11, 1200 σ=(43.2+77ε 1.21353)(1+0.2255ln(ε/ε 0 )), ε 10 3 s -1 10 of 14 σ=(43.2+77ε )(1+0.2255ln(ε/ε )), ε ≥ 10 s  0

TableTable 3. FittingFitting parameters parameters of the JC model. Table 3. Fitting parameters of the JC model. −−31 −1 . 31− .ε = . 31− .ε = ε <103 s−1 0 10−−−331s−1 ε >103 s−1 0 1300−s1 −1 εε << 10 31s − ,, εε0 ==1010 ss εε>>10 31s − ,, ε0ε==13001300s s  10 s , 0  10 s , 0 FittingFitting Values Values Standard Standard Error R R22 Fitting ValuesValues Standard Error Error RR2 2 Fitting Values Standard Error R2 Fitting Values Standard Error R2 AA 18 18 0 0 1 1 43.2 43.2 0 0 1 1 BA B 10.2354 10.235418 0.00492 0.00492 0 0.99633 1 77.0012 43.2 0.0062 0.0062 0 0.968420.96842 1 nB n 10.23540.7107 0.7107 0.00387 0.00492 0.00387 0.996330.99633 1.2135377.0012 0.00326 0.00326 0.0062 0.968420.968420.96842 C 0.084 0.12302 0.98039 0.2255 0.71825 0.98387 Cn 0.71070.084 0.12302 0.00387 0.980390.99633 1.213530.2255 0.71825 0.00326 0.983870.96842 C 0.084 0.12302 0.98039 0.2255 0.71825 0.98387 TheThe comparison comparison of of the the fitting fitting results results of of the the JC JC mo modeldel and and experimental experimental values values at at different different strain strain The comparison of the fitting results of the JC model and experimental values at different strain ratesrates for for PTFE/Al/MoO 3 specimens3 specimens isis presented presented in in Figure Figure 11. 11 .It It can can be be drawn drawn that that the the fitting fitting results results arearerates in in forgood good PTFE/Al/MoO agreement agreement with with3 specimens the the measured measured is presented values. values. in Th The Figuree established established 11. It can JC JC beconstitutive constitutive drawn that model model the fitting can can describe describe results are in good agreement with the measured values. The established JC constitutive model can describe thethe mechanical mechanical response of PTFE/Al/MoOPTFE/Al/MoO3 3materialmaterial well well and and can can provide provide certain certain references references for for the the practicalpracticalthe mechanical applications response of this of PTFE/Al/MoOmaterial. 3 material well and can provide certain references for the practical applications of this material. 120 -1 120 strain rate/s strain exp.0.001 rate/s-1 JC.0.001 exp.0.01 exp.0.001 JC.0.01 JC.0.001 100 exp.0.1 exp.0.01 JC.0.1 JC.0.01 exp.700 JC.700 100 exp.0.1 JC.0.1 exp.1300 exp.700 JC.1300 JC.700 exp.1800 exp.1300 JC.1800 JC.1300 exp.2300 exp.1800 JC.2300 JC.1800 80 exp.3000 exp.2300 JC.3000 JC.2300 80 exp.3000 JC.3000

60 60

40

True stress/MPa 40 True stress/MPa 20 20

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 True strain True strain FigureFigure 11. 11. ComparisonComparison of of JC JC model model to to experimental experimental data data at at different different strain strain rates. Figure 11. Comparison of JC model to experimental data at different strain rates. 3.5. Drop-Weight Tests 3.5. Drop-Weight Tests FigureFigure 12 shows a sequencesequence ofof imagesimages takentaken fromfrom aa high-speed high-speed camera camera of of the the PTFE/Al/MoO PTFE/Al/MoO3 Figure 12 shows a sequence of images taken from a high-speed camera of the PTFE/Al/MoO33 specimensspecimens under drop-weight tests (drop(drop heightheight isis 1.51.5 m).m). PTFE/Al/MoOPTFE/Al/MoO3 specimensspecimens can can react react specimens under drop-weight tests (drop height is 1.5 m). PTFE/Al/MoO33 specimens can react violentlyviolently underunder thethe impact impact of aof drop a drop hammer, hammer, with intensewith intense light and light a huge and explosiona huge explosion sound. Moreover, sound. Moreover,violently under the ignition the impact time is of about a drop 100 hammer,μs and the with burning intense of specim light anden lasts a huge approximately explosion 700sound. μs. theMoreover, ignition the time ignition is about time 100 isµ abouts and 100 the burningμs and the of specimenburning of lasts specim approximatelyen lasts approximately 700 µs. 700 μs.

Figure 12. Reaction process of PTFE/Al/MoO3 specimens under drop-weight tests. Figure 12. Reaction process of PTFE/Al/MoO3 specimens under drop-weight tests. Figure 12. Reaction process of PTFE/Al/MoO3 specimens under drop-weight tests.

Materials 2018, 11, 1200 11 of 14 Materials 2018, 11, x FOR PEER REVIEW 11 of 14

UnlikeUnlike the the reaction phenomenon of PTFE/Al, strong strong blue blue smoke smoke was was observed observed when when the the PTFE/Al/MoO specimens were impacted by the drop mass, which is probably due to the reaction of PTFE/Al/MoO3 specimens3 were impacted by the drop mass, which is probably due to the reaction of Al with MoO , while black smoke was found when PTFE/Al reacted [25]. The typical photographs of Al with MoO3, while black smoke was found when PTFE/Al reacted [25]. The typical photographs of the PTFE/Al/MoO specimens before reaction and after reaction are shown in Figure 13. From the the PTFE/Al/MoO3 specimens3 before reaction and after reaction are shown in Figure 13. From the recoveredrecovered specimen specimen (see (see Figure Figure 13b), 13b), it itcan can be be seen seen that that the the reaction reaction started started from from the the outer outer edge edge of theof thespecimen specimen with with the largest the largest shear shear force forceand th ande most the concentrated most concentrated shear deformation shear deformation [26], which [26], demonstrateswhich demonstrates a shear-induced a shear-induced initiation initiation mechanism. mechanism. Ames Ames [27], [27 Lee], Lee [28] [28 and] and Feng Feng et et al. al. [29] [29 ]also also observedobserved aa similarsimilar reaction reaction mode mode when when studying studying the reaction the reaction behaviors behaviors of PTFE/Al of PTFE/Al reactive materials.reactive materials.The reaction The then reaction propagated then propagated from the edge from to the the edge center to of the the center specimens of the and specimens quenched. and We quenched. attribute Wethe attribute quenching the to quenching the rapid densificationto the rapid densificat caused byion the caused drop mass,by the because drop mass, the confinedbecause the space confined would spacelimit thewould further limit shear the further deformation shear indeforma the centertion ofin the specimens.center of the specimens.

(a) (b) (c)

Figure 13. Typical photographs of the PTFE/Al/MoO3 specimens. (a) before reaction; (b) and (c) after Figure 13. Typical photographs of the PTFE/Al/MoO3 specimens. (a) before reaction; (b) and reaction.(c) after reaction.

Residues after reactions were collected and analyzed by XRD. Figure 14 presents the XRD Residues after reactions were collected and analyzed by XRD. Figure 14 presents the XRD pattern pattern of the reaction products of PTFE/Al/MoO3 specimens. It can be concluded from the figure that of the reaction products of PTFE/Al/MoO3 specimens. It can be concluded from the figure that the the diffraction peaks of AlF3, Al2O3, and Mo were detected in the products. Moreover, carbon black diffraction peaks of AlF , Al O , and Mo were detected in the products. Moreover, carbon black (C) is also one of the reaction3 2products3 and was not detected in the figure because of its amorphous (C) is also one of the reaction products and was not detected in the figure because of its amorphous phase. Therefore, it is probable that the initial chemical reactions of the specimens are triggered by phase. Therefore, it is probable that the initial chemical reactions of the specimens are triggered by PTFE and Al composites, followed by the reaction of Al and MoO3. Granier and Pantoya [30] found PTFE and Al composites, followed by the reaction of Al and MoO3. Granier and Pantoya [30] found that the burning velocity of Al/MoO3 thermite is strongly influenced by the preheat temperature of that the burning velocity of Al/MoO thermite is strongly influenced by the preheat temperature of the reactants. The adiabatic reaction temperature3 between PTFE and Al can even reach 4000 K [31], the reactants. The adiabatic reaction temperature between PTFE and Al can even reach 4000 K [31], which can greatly promote the reaction of Al and MoO3. Furthermore, the state of the product metal which can greatly promote the reaction of Al and MoO . Furthermore, the state of the product metal (Mo) is also important for the reaction. At the high temperature3 produced by the reaction of PTFE (Mo) is also important for the reaction. At the high temperature produced by the reaction of PTFE and Al, Mo exists in liquid form, which will also facilitate the interaction between Al and MoO3 [32]. and Al, Mo exists in liquid form, which will also facilitate the interaction between Al and MoO3 [32]. Combining the analysis above, the possible chemical reactions process of PTFE/Al/MoO3 can be Combining the analysis above, the possible chemical reactions process of PTFE/Al/MoO can be described as: 3 described as: 2 4 → 3 4Al4Al + + 3( 3(−−CC2F4−) ) → 4AlF4AlF 3+ +6C 6C (6) (6)

2Al + MoO3 → Mo + Al2O3 (7) 2Al + MoO3 → Mo + Al2O3 (7)

The sensitivity of PTFE/Al/MoO3 specimens was measured by the characteristic drop height The sensitivity of PTFE/Al/MoO3 specimens was measured by the characteristic drop height (H50), when the drop mass is released from this height, the specimen has a 50% probability of ignition. (H50), when the drop mass is released from this height, the specimen has a 50% probability of ignition. H50 is calculated by the following formula: H50 is calculated by the following formula:

∑ iCiCi 11 HABH =+=[( [A + B( i − −)] )] (8)(8) 5050 D 22 where A is the lowest height in the tests, B is the height increment, D is the number of reaction specimens, i is the order of the drop height, and Ci is the number of reaction specimens at the specified height. The drop-weight tests were performed on 20 PTFE/Al/MoO3 specimens. Figure 15 shows the

Materials 2018, 11, 1200 12 of 14 where A is the lowest height in the tests, B is the height increment, D is the number of reaction specimens, i is the order of the drop height, and Ci is the number of reaction specimens at the specified MaterialsMaterials 2018 2018, ,11 11, ,x x FOR FOR PEER PEER REVIEW REVIEW 1212 of of 14 14 height. The drop-weight tests were performed on 20 PTFE/Al/MoO3 specimens. Figure 15 shows the experimentalexperimental resultsresults recordedrecorded accordingaccording to to thethe “up-and-down”“up-and-down” methodsmethods [[21].[21].21]. The The characteristiccharacteristic dropdrop height of the PTFE/Al/MoO specimen was calculated as 51.57 cm according to Equation (8). heightheight ofof thethe PTFE/Al/MoOPTFE/Al/MoO33 3 specimenspecimen waswas calculatedcalculated asas 51.5751.57 cmcm accordingaccording toto EquationEquation (8).(8).

60006000 ■■

●Al2O3 50005000 ▲▲ ●Al2O3 ■AlF3 ■AlF3 ▲▲MoMo 40004000

▲▲ 30003000

▲▲ 2000 2000 ▲

Intensity(Counts) ▲ Intensity(Counts) ● ● ■● ■■ ● 1000 ● ■ ● 1000 ■■ ● ● ● ●●● ● ● ■■ 00 2020 40 40 60 60 80 80 2θ(Degrees) 2θ(Degrees)

Figure 14. Reaction products of PTFE/Al/MoO3 after drop-weight tests. FigureFigure 14. 14.Reaction Reaction products products of of PTFE/Al/MoO PTFE/Al/MoO3 after drop-weight tests.

6060 H =51.57cm H5050=51.57cm

5555

5050 Drop hight/cm Drop Drop hight/cm Drop

■■ reacted reacted ■■ unreacted unreacted 4545

00 2 2 4 4 6 6 8 8 10 10 12 12 14 14 16 16 18 18 20 20 TestTest No. No. Figure 15. The drop-weight tests data points. FigureFigure 15.15. TheThe drop-weightdrop-weight teststests datadata points.points.

4.4. ConclusionsConclusions 4. Conclusions Quasi-staticQuasi-static compression, compression, dynamic dynamic compression compression and and drop-weight drop-weight experiments experiments were were performed performed Quasi-static compression, dynamic compression and drop-weight experiments were performed toto study study PTFE/Al/MoO PTFE/Al/MoO33 reactive reactive composites composites prepared prepared by by cold-pressing cold-pressing and and sintering sintering technology. technology. The The to study PTFE/Al/MoO3 reactive composites prepared by cold-pressing and sintering technology. conclusionsconclusions cancan bebe drawndrawn asas follows:follows: The conclusions can be drawn as follows: (1)(1) MesoscaleMesoscale imagesimages ofof thethe specimensspecimens afterafter sinterinsinteringg obtainedobtained byby SEMSEM demonstratedemonstrate thatthat PTFE,PTFE, Al,Al, (1) Mesoscale images of the specimens after sintering obtained by SEM demonstrate that PTFE, Al, andand MoOMoO33 particlesparticles werewere evenlyevenly mixed.mixed. AndAnd nono chemicalchemical reactionreaction occurredoccurred afterafter thethe materialmaterial and MoO particles were evenly mixed. And no chemical reaction occurred after the material waswas stirred,stirred,3 pressedpressed andand sintered.sintered. was stirred, pressed and sintered. (2)(2) TheThe yieldyield stressstress andand compressivecompressive strengthstrength ofof PTFE/Al/MoOPTFE/Al/MoO33 specimensspecimens areare sensitivesensitive toto strainstrain (2) rateTherate within yieldwithin stress the the range range and compressiveof of 10 10−−33~3~3 × × 10 103 strength3 s s−−11.. The The elastic ofelastic PTFE/Al/MoO modulus modulus is is insensitive 3insensitivespecimens to to are the the sensitive strain strain rate rate to at strainat low low −3 × 3 −1 strainratestrain within rate,rate, butbut the shows rangeshows of significantsignificant 10 ~3 strainstrain10 s raterate. The dependencedependence elastic modulus atat highhigh is strainstrain insensitive rate.rate. to the strain rate at (3)(3) ThelowThe yield strainyield stress stress rate, shows butshows shows a a bilinear bilinear significant dependence dependence strain rate on on th dependencethee logarithm logarithm at values values high strainof of strain strain rate. rate, rate, and and the the slope slope hashas an an increase increase at at the the strain strain rate rate of of 10 1033 s s−−11.. The The established established JC JC constitutive constitutive model model can can describe describe the the mechanicalmechanical responseresponse ofof PTFE/Al/MoOPTFE/Al/MoO33 materialmaterial wellwell andand cancan provideprovide certaincertain referencesreferences forfor thethe practicalpractical applicationsapplications ofof thisthis material.material. (4)(4) TheThe recoveredrecovered specimensspecimens showshow thatthat thethe reactionreaction startedstarted fromfrom thethe outerouter edgeedge ofof thethe specimenspecimen withwith the the most most concentrated concentrated shear shear deformation, deformation, indicating indicating a a shear-induced shear-induced initiation initiation mechanism. mechanism.

Materials 2018, 11, 1200 13 of 14

(3) The yield stress shows a bilinear dependence on the logarithm values of strain rate, and the slope has an increase at the strain rate of 103 s−1. The established JC constitutive model can describe the mechanical response of PTFE/Al/MoO3 material well and can provide certain references for the practical applications of this material. (4) The recovered specimens show that the reaction started from the outer edge of the specimen with the most concentrated shear deformation, indicating a shear-induced initiation mechanism. The characteristic drop height of impact sensitivity (H50) of the PTFE/Al/MoO3 specimens was calculated as 51.57 cm.

(5) The reaction products of PTFE/Al/MoO3 specimens were AlF3, Al2O3, Mo and C, indicating that redox reaction occurred between PTFE and Al, and between Al and MoO3.

Author Contributions: X.F. and Y.L. conceived and designed the experiments; J.H., S.W. and Z.Y. performed the experiments; J.H. and L.Y. analyzed the data; J.H. wrote the paper. Funding: This research received financial funding from the National Natural Science Foundation of China (No. 51673213). Conflicts of Interest: The authors declare no conflict of interest.

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