Microstructure and Properties of Polytetrafluoroethylene Composites

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Article Microstructure and Properties of Polytetraﬂuoroethylene Composites Modiﬁed by Carbon Materials and Aramid Fibers

Fubao Zhang *, Jiaqiao Zhang , Yu Zhu, Xingxing Wang and Yuyang Jin

School of Mechanical Engineering, Nantong University, Nantong 226019, China; [email protected] (J.Z.); [email protected] (Y.Z.); [email protected] (X.W.); [email protected] (Y.J.) * Correspondence: [email protected]; Tel.: +86-13646288919

Received: 12 October 2020; Accepted: 16 November 2020; Published: 18 November 2020

Abstract: Polytetrafluoroethylene (PTFE) is polymerized by tetrafluoroethylene, which has high corrosion resistance, self-lubrication and high temperature resistance. However, due to the large expansion coefficient, high temperature will gradually weaken the intermolecular bonding force of PTFE, which will lead to the enhancement of permeation absorption and the limitation of the application range of fluoroplastics. In order to improve the performance of PTFE, the modified polytetrafluoroethylene, filled by carbon materials and aramid fiber with different scales, is prepared through the compression and sintering. Moreover, the mechanical properties and wear resistance of the prepared composite materials are tested. In addition, the influence of different types of filler materials and contents on the properties of PTFE is studied. According to the experiment results, the addition of carbon fibers with different scales reduces the tensile and impact properties of the composite materials, but the elastic modulus and wear resistance are significantly improved. Among them, the wear rate of 7 µm carbon fiber modified PTFE has decreased by 70%, and the elastic modulus has increased by 70%. The addition of aramid fiber filler significantly reduces the tensile and impact properties of the composite, but its elastic modulus and wear resistance are significantly improved. Among them, the wear rate of the modified composite material with 3% alumina particles and 5% aramid pulp decreased by 68%, and the elastic modulus increased by 206%.

Keywords: polytetrafluoroethylene; carbon material; aramid fiber; material modification

1. Introduction

n PTFE is polymerized by tetrafluoroethylene, and its molecular chain is -[–CF2–CF2–]- . In detail, the conformation of PTFE molecular chain presents a spiral structure, and the C–F bond is one of the strongest chemical bonds, making the main chain structure of PTFE very stable [1]. PTFE has incomparable corrosion resistance, self-lubrication and high and low temperature resistance, which are unmatched by metal materials [2]. Thus, it is often used on the surface of some objects, such as textiles [3,4], valves [5], pipes [6], etc., through the fluorine lining process. Especially when PTFE is used as a sealing material, it is necessary to increase the elastic modulus of PEFT to prevent creep phenomenon [7], which will lead to leakage. Therefore, how to improve the corrosion resistance and elastic modulus of PTFE and expand the application range of PTFE on the surface of engineering objects has become a research hotspot [8]. The filling is one of the simplest and most effective modification methods for fluoroplastics [9,10]. More specifically, by adding fillers to the fluoroplastic matrix, the crystal structure of the fluoroplastic is changed, thereby improving and overcoming the original defects of the fluoroplastic [11]. The materials

Coatings 2020, 10, 1103; doi:10.3390/coatings10111103 www.mdpi.com/journal/coatings Coatings 2020, 10, 1103 2 of 20

filled with modified PTFE mainly include carbon materials and aramid fibers [12,13]. In the modification of PTFE-filled fiber, the fiber mainly plays the role of bearing, and the matrix material plays the role of bonding the fibers firmly to each other and transferring the load to the fibers. Carbon fiber (CF) is a new type of fiber material with the carbon content of more than 95% after carbonization and graphitization of organic fibers [14,15]. The CF has excellent corrosion resistance and high temperature resistance, which is widely used as a high-quality filler or performance improver for rubber, plastic and various composite materials. Multiwalled carbon nanotube (MWCNT) is a quantum material with a special structure, which has good mechanical properties and more stable structure than polymers [16]. It is commonly used as the enhancer to improve the strength, elasticity and fatigue resistance of composite materials. Kevlar poly-p-phenylene terephthamide (PPTA) is a synthetic fiber with ultra-high strength and abrasion resistance [17]. PPTA has a high elastic modulus and excellent dimensional stability, and is widely used in the filling modification of polymer materials [18]. Al2O3 is a kind of metal aluminum oxide [19–21], it is often used as an inorganic filler to improve the hardness and wear resistance of modified PTFE materials. Due to the lack of necessary affinity between the inorganic filler and the PTFE interface, the combination of the two is poor, resulting in a decrease in the tensile strength, elongation, impact performance and compression rate of the modified PTFE material. Scholars have conducted many studies on modification experiments of PTFE, and certain properties of PTFE have been improved. A.P. Vasilev studied the effect of composite fillers on the properties and structure of PTFE, and confirmed that the introduction of composite fillers significantly improved the wear resistance of PTFE composites [22]. A.A. Ohlopkova studied the effect of carbon fiber on the performance and structure of polytetrafluoroethylene. It is proved that carbon fiber can improve the tribological properties of PTFE [23]. Prateek Saxena et al. used a variety of filler materials to improve the frictional mechanical properties of cyanate ester, and proved that graphite produced the best effect among all fillers used, offering a lower specific wear rate, a lower friction value and a higher tensile strength and tensile modulus [24]. In addition, Prateek Saxena’s team has developed a new type of testing device to identify the friction characteristics of carbon fiber reinforced polymer composites under high surface pressure, which is conducive to the performance testing of polymer materials [25]. Fuchuan Luo et al. used Na1/2Sm1/2TiO3 and glass fiber to fill PTFE to improve the electrical conductivity of PTFE composites [26]. YingYuan et al. prepared a kind of PTFE composite material filled with Si3N4. As the content of Si3N4 increases, the electrical conductivity and thermal conductivity of the PTFE composite material are improved [27]. L.J. van Rooyen et al. used graphene oxyfluoride to fill PTFE, thereby reducing the permeability of PTFE. The incorporation of the oxyfluorinated and commercial grade graphene into the PTFE reduced the helium gas permeability by 96% at 4 vol.% and by 88% at 7 vol.% nanofiller contents, respectively [28]. Compared with previous studies, this article innovatively uses carbon nanotubes and aramid fibers to fill modified PTFE, thereby improving the wear resistance and elastic modulus of the composite material, making it more suitable for engineering object surfaces and seals. Starting from the modification process of fluoroplastics, this paper studies the influence of different types of filler materials on the properties of PTFE, and analyzes the mechanism of the influence of filler content on the properties of PTFE. Our research has effectively improved the wear resistance and elastic modulus of PTFE materials, which is conducive to the use of PTFE on the surface and seals of engineering objects, and provides scientific basis and reference for the research and practical application of new PTFE composite materials.

2. Materials and Instruments

2.1. Experimental Materials The main reagents and materials used in the experiment are shown in Table1. Coatings 2020, 10, 1103 3 of 20

Table 1. Main reagents and materials used in the test.

Reagent Name Reagent/Material Manufacturer Description of Product Codes Japan Daikin Fluorochemical (China) PTFE Particle size of 20 µm Co., Ltd. (Changshu, China) Chuangjia Welding Material Co., Ltd. Length of 50 µm, monofilament 7 µm carbon fiber (Qinghe, China) diameter of 7 µm Shanghai Alighting Biochemical Length of 5–50 µm, outer diameter of 200 nm carbon fiber Technology Co., Ltd. (Shanghai, China) 200–600 nm Carboxylation, length of 50 µm, inner Shanghai McLean Biochemical Multi-walled carbon nanotubes diameter of 3–5 nm, outer diameter of Technology Co., Ltd. (Shanghai, China) 8–15 nm, –COOH content of 2.6 wt.% Dupont China Group Co., Ltd. Aramid Pulp Kevlar 1F1710, length of 0.7–1.6 mm (Shenzhen, China) Mingshan New Materials Co., Ltd. Al O Powder, particle size of 30–40 nm 2 3 (Laiwu, China) Ding Hai Plastic Chemical Co., Ltd. Titanate coupling agent KH-792 (Dongguan, China) Sinopharm Chemical Reagent Co., Ltd. Absolute ethanol Analytical reagent (Shanghai, China) Jiangsu Qiangsheng Functional Acetic anhydride Analytical reagent Chemical Co., Ltd. (Changshu, China) Shanghai Lingfeng Chemical Reagent Acetone Analytical reagent Co., Ltd. (Shanghai, China)

2.2. Experimental Instruments The main performance test instruments used to complete all the tests are shown in Table2.

Table 2. Main test and analysis instruments used in the experiment.

Device Name Device Model Vendor Jinan Dongfang Experimental Instrument Co., Ltd. Compressive and ﬂexural testing machine YAW-300C (Jinan, China) Nanjing Suce Measuring Instruments Co., Ltd. Shore Hardness Tester LX-D type (Nanjing, China) Scanning electron microscope S-3400N Hitachi Manufacturing Co., Ltd. (Tokyo, Janpan) Transmission electron microscope JEM-2100hr Japan Electronics Corporation (Tokyo, Japan) MTS Industrial Systems Co., Ltd. Pendulum impact sample machine E45.105 (Shenzhen, China) MTS Industrial Systems (China) Co., Ltd. Universal testing machine CMT5105 (Shenzhen, China) American Rtec Equipment Co., Ltd. Friction and wear tester MFT-V (San Jose, CA, USA)

2.3. Performance Testing and Organization Analysis Methods The hardness testing shall be conducted in accordance with GB/T2411-2008 [29] Plastics and Ebonite Determination of Indentation Hardness (Shore hardness) using a Durometer. The LX-D plastic shore hardness tester was used to measure the shore hardness of friction and wear samples, using a D type indenter with a total test force of 588.4 N. Each sample measured 5 points, and the results are represented by the arithmetic mean of the 5 measured values. The tensile testing was carried out in accordance with GB/T1040-2006 [30] Determination of Tensile Properties of Plastics. According to the standard, the sample was processed into a rectangular spline of 80 mm 10 mm 4 mm, and then tested on the electronic tensile testing machine of CMT5504 model × × at a beam speed of 10 mm/min. Three samples were prepared for each test, and the final results were Coatings 2020, 10, 1103 4 of 20 taken as the tensile strength, elastic modulus and elongation at break according to the average value of the three samples. The impact testing shall be conducted in accordance with GB/T1043-2008 [31] Determination of Impact Performance of Plastic Simply Supported Beams. According to the standard, the sample was processed into 80 mm 10 mm 4 mm with a depth of 2 mm A-shaped notch, and then the × × notched sample impact strength was measured on the combined impact tester of ZBC3000 model. The maximum impact energy of the pendulum was 7.5 J. Three samples were prepared for each test, and the final result is the average value of the three samples to express its impact toughness. The friction and wear test was performed on the friction and wear tester of MFT-V model. The complex friction test method was adopted. The specific steps were to fix the sample on the chassis and reciprocate with the chassis. The grinding parts were made of cylindrical Cr15 bearing steel with a diameter of 5 mm, the friction stroke was set to 20 mm, the acceleration time was set to 1 s, and the reciprocating frequency was set to 2 Hz. The experiment was carried out under the load of 98 N, and the wear time was 30 min. Impact section and wear surface morphology were performed by the following steps: first, the cross-section of the impact sample and the surface wave of the friction and wear sample were sprayed with gold to increase the conductivity of the composite material. Then, S-3400N scanning electron microscope (SEM) was used to observe the impact section and wear surface morphology under high magnification.

3. Preparation of Samples

3.1. Preparation of Carbon Material Modiﬁed PTFE

3.1.1. Surface Treatment of Carbon Fiber

Carbon fiber and nano Al2O3 ceramic particles are inorganic particles, which have poor binding properties with PTFE. Therefore, it is necessary to modify the surface of carbon fiber by the titanate coupling agent [32,33]. More specifically, the titanate coupling agent is coupled by its alkoxy group directly with the trace carboxyl groups or hydroxyl groups adsorbed on the surface of the filler or pigment to couple, thereby effectively improving the bonding strength between carbon fiber and PTFE.

3.1.2. Preparation of Modified PTFE First of all, the filling modification needs to solve the problem of the dispersibility of the filler in PTFE, because the dispersion of the filler in the PTFE matrix will directly affect the performance of the composite material. In this paper, the dry–wet mixing method was used to mix the composite materials [34], and the SEM micrograph of the carbon fiber used is as shown in Figure1. First, the pretreated PTFE powder and carbon materials were weighed according to the mass percentage. Then, a certain amount of ethanol solution was poured into it and ultrasonic dispersion was performed for 30 min. In the next step, the PTFE powder after being stirred by a high-speed mixer and sieved was slowly poured into the stirring ethanol solution. It should be noted that the solution was stirred at high speed under the condition of heating in a water bath until the mixture shows a jelly without stratification. It is necessary to prevent the delamination of the solution during mixing. Then, the solution was placed in a drying cabinet at 80 ◦C for drying. The dried mixture was crushed by a high-speed crusher for 5 min and then sieved with a 100-mesh sieve to further ensure the homogeneity of the mixture. Then, the powder was placed in a mold for cold pressing to obtain a preformed sample. Next, the preformed sample was sintered in a box-type electric furnace to obtain a composite sintered sample. Finally, the test sample was cut with a punching knife for testing. The modified PTFE test sample is shown in Figure2. Coatings 2020, 10, 1103 5 of 20 Coatings 2020, 10, x FOR PEER REVIEW 5 of 19

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FigureFigure 1. 1.TheThe SEM SEM micrograph micrograph ofof the the carbon carbon ﬁber. fiber. Figure 1. The SEM micrograph of the carbon fiber.

FigureFigureFigure 2. 2.Test TestTest sample sample of of carbon carbon fiber ﬁber (CF)(CF) modified modiﬁed modified PTFE PTFE composite composite. composite. .

3.2. Preparation of the Aramid Modified PTFE Material 3.2. Preparation3.2. Preparation of the of the Aramid Aramid Modified Modified PTFE PTFE MaterialMaterial 3.2.1. Surface Treatment of the Aramid Fiber 3.2.1. Surface Treatment of the Aramid Fiber 3.2.1. Surface Treatment of the Aramid Fiber The surface of the product often produces some defects during processing [35]. In order to ensure The surface of the product often produces some defects during processing [35]. In order to the cleanliness of the PPTA surface, it is necessary to clean the PPTA first. The detailed cleaning method ensureThe surface the cleanliness of the product of the PPTA often surface, produces it is some necessary defects to clean during the processing PPTA first. [The35]. detailedIn order to is to soak PPTA in acetone for 1 h, then boil it with absolute ethanol and distilled water, and then ensurecleaning the cleanliness method is to of soak the PPTA PPTA in surface,acetone for it is1 h, necessary then boil it to with clean absolute the PPTA ethanol first. and Thedistilled detailed filter the mixture and put it in an oven to dry for later use. The wastewater is treated in a harmless cleaningwater, method and then is filter to soak the mixturePPTA in and acetone put it in for an 1oven h, then to dry boil for it later with use. absolute The wastewater ethanol isand treated distilled manner [36,37]. The SEM morphology of the PPTA surface is shown in Figure3a. It can be seen that water,in aand harmless then filter manner the [36,37 mixture]. The and SEM put morphology it in an oven of the to PPTAdry for surface later use.is shown The inw astewaterFigure 3a. It is can treated there are more attachments on the surface of PPTA. The SEM morphology of PPTA after cleaning with in a harmlessbe seen that manner there are [36,37 more]. attachmentsThe SEM morphology on the surface of of the PPTA. PPTA The surface SEM morphology is shown in of Figure PPTA after3a. It can acetone is shown in Figure3b. It can be seen that the surface of PPTA fiber was smoother after cleaning, be seencleaning that there with acetoneare more is attachments shown in Figure on the 3b. surface It can beof PPTA. seen that The the SEM surface morphology of PPTA offiber PPTA was after and there was no obvious adhesion remaining. Therefore, the latter was more conducive to increasing cleaningsmoother with after acetone cleaning, is shown and there in Figurewas no 3obviousb. It can adhesion be seen remai thatning. the Therefore,surface of the PPTA latter fiber was was cleaningthe bonding with acetone force between is shown the fiber in Figure and the 3 resinb. It matrix. can be seen that the surface of PPTA fiber was smoothermore conduciveafter cleaning, to increasing and there the bonding was no force obvious between adhesion the fiber remai and thening. resin Therefore, matrix. the latter was more conducive to increasing the bonding force between the fiber and the resin matrix.

(a) (b)

Figure 3. SEMSEM micrographs micrographs of of aramid aramid fiber:fiber: ( (aa)) untreated untreated aramid aramid fiber fiber and and ( (bb)) a aramidramid fiberfiber after cleaning with acetone acetone.. (a) (b)

Figure 3. SEM micrographs of aramid fiber: (a) untreated aramid fiber and (b) aramid fiber after cleaning with acetone.

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Coatings 2020In ,order 10, x FOR to further PEER REVIEW improve the bonding performance of PPTA and PTFE, acetic anhydride was6 of 19 used to modify the surface of PPTA to increase the contact area between PPTA and PTFE matrix [38]. Coatings 2020, 10, 1103 6 of 20 TheIn order treatment to further process improve was that the pour bonding the acetic performance anhydride of solution PPTA andwith PTFE, a purity acetic specification anhydride of was usedanalytical to modify reagent the surface into a beaker,of PPTA and to thenincrease the PPTAthe contact was immersed area between in the PPTAsolution. and Place PTFE the matrix beaker [38] . in a water bathIn order and to heat further to improve70 °C and the bondingkeep it performancefor 1 h. Then of PPTA, the andPPTA PTFE, was acetic cleaned anhydride with was ultrapure The treatmentused process to modify wasthe surface that ofpour PPTA the to increase acetic the anhydride contact area between solution PPTA with and PTFEa purity matrix specification [38]. of analyticalwater andreagentThe dried treatment into for use.a process beaker, The was SEM and that morphology pourthen thethe acetic PPTA of anhydride PPTA was fiberimmersed solution surface with in after a the purity acetic solution. speciﬁcation anhydride Place of thesurface beaker in a treatmentwater bathanalytical is shown and reagent heat in Figure intoto 70 a beaker, 4°.C and and thenkeep the it PPTA for was1 h. immersed Then, inthe the PPTA solution. was Place cleaned the beaker with in a ultrapure water bath and heat to 70 ◦C and keep it for 1 h. Then, the PPTA was cleaned with ultrapure water and water and drieddried forfor use. use. The The SEM SEM morphology morphology of PPTA ﬁber of surfacePPTA afterfiber acetic surface anhydride after surface acetic treatment anhydride is surface treatment isshown shown in Figurein Figure4. 4.

Figure 4. SEM image of aramid fiber after surface treatment with acetic anhydride for 1 h.

It can be seen from Figure 4 that there were obvious etching marks on the surface of PPTA after modification with acetic anhydride. The surface roughness and the specific surface area of PPTA after Figure 4. SEMFigure image 4. SEM of image aramid of aramid fiber fiber after after surface surface treatment treatment with with acetic acetic anhydride anhydride for 1 h. for 1 h. etching increased. Therefore, during the molding process of the composite material, PTFE was filled into the cavitiesIt can on be the seen surface from Figure of PPTA4 that therefiber, were which obvious caused etching uneven marks and on theinterlaced surface of interfaces PPTA after between theIt canfiber bemodification and seen the from matrix with Figure acetic material anhydride. 4 that after there The cooling. surfacewere roughnessobviousThis not andetchinonly the effectively specificg marks surface on increased areathe ofsurface PPTA the aftermechanical of PPTA after etching increased. Therefore, during the molding process of the composite material, PTFE was filled modificationchimerism with with acetic the PTFE anhydride matrix, .but The also surface improved roughness the bonding and the strength specific between surface the area interfaces. of PPTA after into the cavities on the surface of PPTA fiber, which caused uneven and interlaced interfaces between etching increased. Therefore, during the molding process of the composite material, PTFE was filled the fiber and the matrix material after cooling. This not only effectively increased the mechanical into3.2.2. the cavities Preparationchimerism on the withof surfaceAramid the PTFE ofFiber matrix, PPTA and but fiber, Nano also improved which Al2O3 Modifiedcaused the bonding uneven PTFE strength Material and between interlaced the interfaces. interfaces between the fiber and the matrix material after cooling. This not only effectively increased the mechanical The3.2.2. blending Preparation method of Aramid adopts Fiber the andmixing Nano method Al2O3 Modified combining PTFE Materialdry and wet methods. The detailed chimerism with the PTFE matrix, but also improved the bonding strength between the interfaces. steps are as follows.The blending First method of all, adopts the pretreated the mixing powder method combining and reinforcer dry and were wet methods. weighed The according detailed to the mass percentage,steps are as and follows. a certain First of amount all, the pretreated of ethanol powder solution and reinforcer was poured were weighed into a accordinghigh-speed to the mixer for 3.2.2.mixing. Preparation Then,mass percentage, the of sievedAramid and PTFE aFiber certain powder and amount Nano was of slowly ethanolAl2O3 poured solutionModified wasinto pouredPTFE the stirring Material into a high-speedethanol solution, mixer for and then mixing. Then, the sieved PTFE powder was slowly poured into the stirring ethanol solution, and then placedThe blending in a drying method oven at adopts 80 °C for the drying mixing and method cold-pressed combining into preformed dry and wetsamples. methods. The preformed The detailed sample wasplaced put in ainto drying a box oven-type at 80 ◦electricC for drying furnace and cold-pressed for sintering into to preformed form a samples.composite The preformedmaterial sample. steps are as samplefollows. was First put into of all, a box-type the pretreated electric furnace powder for sintering and reinforcer to form a composite were weighed material sample. according to the Finally, the sample was cut into test specimens with a punching knife for testing. Test samples of mass percentage,Finally, theand sample a certain was cut amount into test of specimens ethanol with solution a punching was knife poured for testing. into Testa high samples-speed of mixer for composite materials are shown in Figure 5. mixing. Then,composite the sieved materials PTFE are shownpowder in Figure was5 slowly. poured into the stirring ethanol solution, and then placed in a drying oven at 80 °C for drying and cold-pressed into preformed samples. The preformed sample was put into a box-type electric furnace for sintering to form a composite material sample. Finally, the sample was cut into test specimens with a punching knife for testing. Test samples of composite materials are shown in Figure 5.

Figure 5.Figure Test 5. sampleTest sample of PPTA of PPTA and and nano nano Al Al22O33 ﬁlledfilled modiﬁed modified PTFE PTFE composite composite material. material.

4. Experimental Results and Discussion

4.1. The Structure and Properties of Carbon Material Modified PTFE Composite

4.1.1. HardnessFigure 5. Test sample of PPTA and nano Al2O3 filled modified PTFE composite material. The impact of different types and contents of carbon materials on the hardness of filled with 4. Experimental Results and Discussion modified PTFE is shown in Figure 6. During the experiment, we tested each set of the data five times,

4.1. The Structure and Properties of Carbon Material Modified PTFE Composite

4.1.1. Hardness The impact of different types and contents of carbon materials on the hardness of filled with modified PTFE is shown in Figure 6. During the experiment, we tested each set of the data five times,

Coatings 2020, 10, 1103 7 of 20

4. Experimental Results and Discussion

4.1. The Structure and Properties of Carbon Material Modiﬁed PTFE Composite

Coatings 20204.1.1., 10 Hardness, x FOR PEER REVIEW 7 of 19 The impact of different types and contents of carbon materials on the hardness of filled with and calculated the average and standard deviation. It can be seen from Figure 5 that the hardness of modified PTFE is shown in Figure6. During the experiment, we tested each set of the data five times, PTFE decreasedand calculated slightly the averageafter adding and standard different deviation. carbon It canmaterials. be seen from Among Figure them,5 that thethe hardness hardness of of PTFE compositesPTFE filled decreased with slightly MWCNT after decreased adding different most carbon significantly. materials. AmongWhen them,the filling the hardness amount of PTFE of MWCNT was 1%, compositesthe hardness filled of with the MWCNT composite decreased material most decreased significantly. from When 62 the to filling52, which amount was of MWCNT15.3%. With the increase wasof the 1%, content the hardness of MWCNT, of the composite the hardness material decreased rebound froms. When 62 to 52, the which filling was amount 15.3%. With of the200 nm CF increase of the content of MWCNT, the hardness rebounds. When the filling amount of 200 nm CF was was 1%, the hardness of the composite material dropped from 62 to 58.2, with a decrease of 4.8%. 1%, the hardness of the composite material dropped from 62 to 58.2, with a decrease of 4.8%. When the When thefilling filling amount amount of 7 µ ofm CF7 μm was CF 0.5%, was the 0.5%, hardness the ofhardness the composite of the material composite dropped material from 62 todropped 59.6, from 62 to 59.6,with with a decrease a decrease of 5.9%. of 5.9%.

Figure 6.Figure Hardness 6. Hardness of micro/nano of micro/nano CF CF and and multiwalled multiwalled carbon carbon nanotube nanotube (MWCNT) (MWCNT) filled modified filled modified PTFE composites. PTFE composites. 4.1.2. Impact Performance 4.1.2. ImpactThe Performance impact toughness of PTFE composites filled with micro/nano CF and MWCNT was tested. TheThe impact impact toughness performance of of PTFE PTFE modifiedcomposites by di fffillederent carbonwith micro/nano materials is shown CF and in Figure MWCNT7. It can was be tested. seen from the figure that the addition of CF and MWCNT reduced the impact strength of the composite The impactmaterial. performance Among them, of PTFE when modified the filling amountby different of MWCNT carbon was materials 1%, the impact is shown toughness in Figure of the 7. It can be seen compositefrom the materialfigure that decreased the addition from 17.89 of to 8.73CF and kJ/m 2MWCNT, with a decrease reduced rate ofthe 51%. impact Subsequently, strength of the compositewith material. the increase Among of the them, filling amount,when the the filling impact amount toughness of tended MWCNT to stabilize. was 1%, When the theimpact 200 nm toughness of the compositeCF filling amount material was 0.5%, decreased the impact from toughness 17.89 of to the 8.73 composite kJ/m2 material, with decreased a decrease from rate 17.89 of 51%. 2 Subsequently,to 9.5 kJ with/m , withthe increase a decrease of rate the of filling 47%. Subsequently, amount, the as impact the filling toughness amount increased, tended to the stabilize. impact When toughness tended to stabilize. When the filling amount of 7 µm CF was 1%, the impact toughness of the 200 nmthe compositeCF filling material amount decreased was 0.5%, from 17.89the impact to 8.61 kJ toughness/m2, with a decrease of the ratecomposite of 55%. Subsequently,material decreased 2 from 17.89as the to filling9.5 kJ/m amount, with increased, a decrease the impact rate toughness of 47%. slowlySubsequently, rose. The reasonas the is filling that the amount mechanical increased, the impactchimerism toughness between tended the matrix to stabilize. material and When the modified the filling material amount lacked of su ffi7 cientμm flexibility,CF was which1%, the is impact toughnessprone of the to brittle composite fracture material when bearing decreased an impact. from In addition, 17.89 to the 8.61 bonding kJ/m2 strength, with a between decrease CF rate and of 55%. SubsequentMWCNTly, as and the the filling matrix amount material wasincreased, poor, and the the impact energy of toughness an impact fracture slowly cannot rose. be The effectively reason is that transferred between the composite materials. the mechanical chimerism between the matrix material and the modified material lacked sufficient flexibility, which is prone to brittle fracture when bearing an impact. In addition, the bonding strength between CF and MWCNT and the matrix material was poor, and the energy of an impact fracture cannot be effectively transferred between the composite materials.

Figure 7. Impact properties of micro/nano CF and MWCNT modified PTFE.

Coatings 2020, 10, x FOR PEER REVIEW 7 of 19 and calculated the average and standard deviation. It can be seen from Figure 5 that the hardness of PTFE decreased slightly after adding different carbon materials. Among them, the hardness of PTFE composites filled with MWCNT decreased most significantly. When the filling amount of MWCNT was 1%, the hardness of the composite material decreased from 62 to 52, which was 15.3%. With the increase of the content of MWCNT, the hardness rebounds. When the filling amount of 200 nm CF was 1%, the hardness of the composite material dropped from 62 to 58.2, with a decrease of 4.8%. When the filling amount of 7 μm CF was 0.5%, the hardness of the composite material dropped from 62 to 59.6, with a decrease of 5.9%.

Figure 6. Hardness of micro/nano CF and multiwalled carbon nanotube (MWCNT) filled modified PTFE composites.

4.1.2. Impact Performance The impact toughness of PTFE composites filled with micro/nano CF and MWCNT was tested. The impact performance of PTFE modified by different carbon materials is shown in Figure 7. It can be seen from the figure that the addition of CF and MWCNT reduced the impact strength of the composite material. Among them, when the filling amount of MWCNT was 1%, the impact toughness of the composite material decreased from 17.89 to 8.73 kJ/m2, with a decrease rate of 51%. Subsequently, with the increase of the filling amount, the impact toughness tended to stabilize. When the 200 nm CF filling amount was 0.5%, the impact toughness of the composite material decreased from 17.89 to 9.5 kJ/m2, with a decrease rate of 47%. Subsequently, as the filling amount increased, the impact toughness tended to stabilize. When the filling amount of 7 μm CF was 1%, the impact toughness of the composite material decreased from 17.89 to 8.61 kJ/m2, with a decrease rate of 55%. Subsequently, as the filling amount increased, the impact toughness slowly rose. The reason is that the mechanical chimerism between the matrix material and the modified material lacked sufficient flexibility, which is prone to brittle fracture when bearing an impact. In addition, the bonding strength between CF and MWCNT and the matrix material was poor, and the energy of an impact Coatings 2020, 10, 1103 8 of 20 fracture cannot be effectively transferred between the composite materials.

CoatingsCoatings 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 88 ofof 1919 FigureFigure 7. Impact 7. Impact properties properties of ofmicro/nano micro/nano CFCF and and MWCNT MWCNT modiﬁed modified PTFE. PTFE. 4.1.3. Tensile Performance 4.1.3. Tensile4.1.3. TensilePerformance Performance

The tensileThe tensile strength, strength, tensile tensile elongation at break at break and elastic and modulus elastic of modulus CF and MWCNT of CF modiﬁed and MWCNT The tensile strength, tensile elongation at break and elastic modulus of CF and MWCNT modifiedmodifiedPTFE PTFEPTFE composites compositescomposites with diwithwithﬀerent differentdifferent contents contentscontents are shown areare in Figures shownshown8– 10inin. FiguresFigures 88––1010..

FigureFigure 8.Figure8. TensileTensile 8. Tensile strengthstrength strength ofof micro/nanomicro/nano of micro/nano CFCF andand MWCNT MWCNTMWCNT modiﬁed modifiedmodified PTFE PTFEPTFE composites. compositescomposites..

Figure 9. Tensile elongation at break of micro/nano CF and MWCNT modiﬁed PTFE composites. FigureFigure 9.9. TensileTensile elongationelongation atat breakbreak ofof micro/nanomicro/nano CFCF andand MWCNTMWCNT modifiedmodified PTFEPTFE compositescomposites..

FigureFigure 10.10. ElasticElastic modulusmodulus ofof micro/nanomicro/nano CFCF andand MWCNTMWCNT modifiedmodified PTFEPTFE compositescomposites..

ItIt cancan bebe seenseen fromfrom FigureFiguress 88 andand 99 thatthat thethe tensiletensile strengthstrength andand elongationelongation atat breakbreak ofof PTFEPTFE werewere significantlysignificantly reducedreduced afterafter addingadding CFCF andand MWCNT.MWCNT. AmongAmong them,them, whenwhen thethe additionaddition amountamount ofof 77 μmμm CFCF waswas 0.5%,0.5%, thethe tensiletensile strengthstrength ofof thethe PTFEPTFE compositecomposite materialmaterial decreasedecreasedd fromfrom 27.927.9 toto 21.8721.87 MPa,MPa, whichwhich waswas aboutabout 20%,20%, andand thethe tensiletensile elongationelongation atat breakbreak decreasedecreasedd fromfrom 810%810% toto 541%.541%. WithWith thethe increaseincrease ofof CFCF toto 1%,1%, thethe tensiletensile strengthstrength ofof thethe compositecomposite materialmaterial increasedincreased toto 23.6423.64 MPa,MPa, andand thethe tensiletensile elongationelongation atat breakbreak alsoalso roserose toto thethe originaloriginal levellevel.. Then,Then, asas thethe CFCF increaseincreasedd,, thethe tensiletensile strengthstrength slowlyslowly decreasedecreasedd.. ComparedCompared withwith CF,CF, whenwhen thethe additionaddition amountamount ofof MWCNTMWCNT waswas 0.5%,0.5%, thethe tensiletensile strengthstrength andand tensiletensile elongationelongation atat breakbreak ofof PTFEPTFE compositescomposites dropdroppedped significantlysignificantly toto 11.1911.19 MPaMPa andand 111%,111%, respectivrespectively,ely, withwith aa decreasedecrease ofof aboutabout 60%60% andand 87%.87%.

Coatings 2020, 10, x FOR PEER REVIEW 8 of 19

4.1.3. Tensile Performance The tensile strength, tensile elongation at break and elastic modulus of CF and MWCNT modified PTFE composites with different contents are shown in Figures 8–10.

Figure 8. Tensile strength of micro/nano CF and MWCNT modified PTFE composites.

Coatings 2020, 10, 1103 9 of 20 Figure 9. Tensile elongation at break of micro/nano CF and MWCNT modified PTFE composites.

Figure Figure10. Elastic 10. Elastic modulus modulus of ofmicro/nano micro/nano CF andand MWCNT MWCNT modiﬁed modified PTFE PTFE composites. composites.

It can be seen from Figures8 and9 that the tensile strength and elongation at break of PTFE were It cansignificantly be seen from reduced Figure after addings 8 and CF 9 andthat MWCNT. the tensile Among strength them, whenand elongation the addition amountat break of of 7 µ PTFEm were significantlyCF was reduced 0.5%, the after tensile adding strength CF of theand PTFE MWCNT. composite Among material them, decreased when from the 27.9 addition to 21.87 MPa,amount of 7 μm CF waswhich 0.5%, was aboutthe tensile 20%, and strength the tensile of elongationthe PTFE at composite break decreased material from 810%decrease to 541%.d from With 27.9 the to 21.87 MPa, whichincrease was of about CF to 1%, 20%, the and tensile the strength tensile of elongation the composite at materialbreak decrease increasedd to from 23.64 MPa,810% and to 541%. the With tensile elongation at break also rose to the original level. Then, as the CF increased, the tensile strength the increase of CF to 1%, the tensile strength of the composite material increased to 23.64 MPa, and slowly decreased. Compared with CF, when the addition amount of MWCNT was 0.5%, the tensile the tensilestrength elongation and tensile at elongationbreak also at rose break to of PTFEthe original composites level dropped. Then, significantly as the CF to increase 11.19 MPad and, the tensile strength111%, slowly respectively, decrease withd. Compared a decrease of with about CF, 60% when and 87%. the addition amount of MWCNT was 0.5%, the tensile strengthIt can and be seentensile from elongation Figure 10 thatat break the elastic of PTFE modulus composites of the composite dropped can significantly be effectively to 11.19 MPa andimproved 111%, respectiv by addingely, CF andwith MWCNT. a decrease Among of them,about the 60% addition and 87%. of 7 µ m and 200 nm CF increased the elastic modulus of PTFE by about 70% to 471.95 MPa and 137% to 655 MPa from the original

276.8 MPa. Then, as the filler increased, the elastic modulus of the composite material tended to stabilize. In contrast, MWCNT had a greater impact on the elastic modulus of PTFE composites. The addition of 1% MWCNT increased the elastic modulus of PTFE from 276.8 to 1075 MPa, with the increase of about 388%. Then, as the filler content increased, the change in the elastic modulus was smaller By comparing the tensile properties of 200 nm CF filled composites and 7 µm CF filled composites, it can be seen that the elastic modulus of the former had doubled that of the latter, but there was no significant difference between the tensile properties and the tensile fracture productivity. Therefore, it could be concluded that nano-level CF could effectively improve the stiffness of PTFE. When the addition amount of nano CF reached 1.5%, the elastic modulus of the composite material began to decrease, indicating that the excessive CF had a negative effect on the elastic modulus of PTFE. The reason is that with the increase of CF content, CF agglomerates become an impurity point of the composite, which can not further improve its mechanical properties. Compared with the composite filled with CF and MWCNT, it can be seen that MWCNT can significantly enhance the elastic modulus of the composite, but its tensile properties and tensile elongation at break were significantly reduced. This may be because compared with CF, MWCNT had a smaller diameter and more uniform dispersion and larger contact area in PTFE matrix materials. These conditions make it easier to restrict the slippage of PTFE macromolecules, which can effectively increase the elastic modulus of the composite material. However, because the C–C bond in PTFE is wrapped by F atoms, and the F atoms are relatively stable, CF and MWCNT cannot generate a strong chemical bond link with the PTFE matrix, but only through simple physical bonding. During the stretching process, the stress on the PTFE matrix material cannot be transferred to the filler material, which leads to the decrease of the tensile strength of the composite. In order to further study the influence of different carbon materials and different scale carbon materials on the properties of PTFE composites and the strengthening mechanism, the tensile section Coatings 2020, 10, x FOR PEER REVIEW 9 of 19

It can be seen from Figure 10 that the elastic modulus of the composite can be effectively improved by adding CF and MWCNT. Among them, the addition of 7 μm and 200 nm CF increased the elastic modulus of PTFE by about 70% to 471.95 MPa and 137% to 655 MPa from the original 276.8 MPa. Then, as the filler increased, the elastic modulus of the composite material tended to stabilize. In contrast, MWCNT had a greater impact on the elastic modulus of PTFE composites. The addition of 1% MWCNT increased the elastic modulus of PTFE from 276.8 to 1075 MPa, with the increase of about 388%. Then, as the filler content increased, the change in the elastic modulus was smaller By comparing the tensile properties of 200 nm CF filled composites and 7 μm CF filled composites, it can be seen that the elastic modulus of the former had doubled that of the latter, but there was no significant difference between the tensile properties and the tensile fracture productivity. Therefore, it could be concluded that nano-level CF could effectively improve the stiffness of PTFE. When the addition amount of nano CF reached 1.5%, the elastic modulus of the composite material began to decrease, indicating that the excessive CF had a negative effect on the elastic modulus of PTFE. The reason is that with the increase of CF content, CF agglomerates become an impurity point of the composite, which can not further improve its mechanical properties. Compared with the composite filled with CF and MWCNT, it can be seen that MWCNT can significantly enhance the elastic modulus of the composite, but its tensile properties and tensile elongation at break were significantly reduced. This may be because compared with CF, MWCNT had a smaller diameter and more uniform dispersion and larger contact area in PTFE matrix materials. These conditions make it easier to restrict the slippage of PTFE macromolecules, which can effectively increase the elastic modulus of the composite material. However, because the C–C bond in PTFE is wrapped by F atoms, and the F atoms are relatively stable, CF and MWCNT cannot generate a strong chemical bond link with the PTFE matrix, but only through simple physical bonding. During the stretching process, the stress on the PTFE matrix material cannot be transferred to the filler material, which leads to the decrease of the tensile strength of the composite. CoatingsIn order2020 to, 10 further, 1103 study the influence of different carbon materials and different scale10 of carbon 20 materials on the properties of PTFE composites and the strengthening mechanism, the tensile section of theof sample the sample was was observed observed by by a ascanning scanning electron electron microscope.microscope. In In this this study, study, gold gold was was sprayed sprayed on on the sectionthe section of the of thetensile tensile specimen specimen and and the the surface surface of thethe frictionfriction and and wear wear specimen, specimen, and and then then the the impactimpact section section and and the the wear wear morphology surface surface were were observed observed under under high magniﬁcation high magnification using SEM. using SEM.The The SEM SEM of of the the tensile tensile fracture fracture surface surface is shown is shown in Figure in Figure11. 11.

(a) (b)

Figure 11. SEM image of the 1% 7 µm CF-filled PTFE tensile section: (a) the magnification 2000 ; Figure 11. SEM image of the 1% 7 μm CF-filled PTFE tensile section: (a) the magnification 2000××; (b) (b) the magnification 600 . the magnification 600×. × It can be seen from Figure 11 that there was CF perpendicular/parallel to the tensile fracture plane. It can be seen from Figure 11 that there was CF perpendicular/parallel to the tensile fracture Furthermore, there were certain gaps between the CF and the matrix. These voids easily caused stress planeconcentration,. Furthermore, which there induced were certain and propagated gaps between cracks. the Then, CF and the the cracks matrix. propagated These voids along easily with thecaused Coatings 2020, 10, x FOR PEER REVIEW 10 of 19 stressinterface, concentration, causing which the interface induced to beand damaged, propagated and cracks. the CF wasThen, pulled the cracks out of propagated the matrix material, along with the interface,and finally causing the tensile the propertiesinterface to of be the damaged, composite and material the CF were was reduced. pulled The out fracture of the matrix mode of material the , the interfaceand ficompositenally bonding the material tensile performance wasproperties the fiber of/ matrixdetermines the composite debonding the material fiberstress pull-out. tranweresfer Underreduced. effect the actionThe between fracture of external the mode fiber force, of andthe the matrix,composite whichthe interface material in turn bonding was affected the performance fiber/matrix the failure determines debonding mechanism the fiber stress pull of transfer the-out . composite eUnderffect between the action material, the of fiber external and the ultimately force, affected matrix,the macroscopic which in turn mechanical affected the failure properties mechanism of the of the composite composite material. material, and ultimately affected the macroscopic mechanical properties of the composite material. 4.1.4. Friction and Wear Performance 4.1.4. Friction and Wear Performance The studyThe te studysted testedthe friction the friction and andwear wear of micro/nano of micro/nano CF CF and and MWCNT MWCNT filledfilled modifiedmodified PTFE composites.PTFE The composites. friction Thecoefficient friction coeandffi cientwear and rate wear of the rate composite of the composite material material are shown are shown in Figures in 12 and 13. Figures 12 and 13.

Figure 12.Figure Friction 12. Friction coefficient coefficient of micro/nano of micro/nano CF CF and MWCNT MWCNT filled filled modified modified PTFE composites.PTFE composites.

Figure 13. Wear rate of micro/nano CF and MWCNT filled modified PTFE composites.

It can be seen from Figure 12 that the addition of different carbon materials slightly increased the friction coefficient of the composite material. It can be seen from Figure 13 that the strength and load-bearing capacity of the composite material were improved, indicating that the ability of the composite material to resist plastic deformation was significantly improved, and the ploughing effect of the grinding parts on the surface of the composite material was reduced. The addition of CF and MWCNT has greatly improved the wear resistance of composite materials. Under the experimental conditions, as the content of different types of carbon materials increased, the wear of composite materials decreased significantly. Among them, when 7 μm CF was used as the modified filler and the mass fraction was 1.5%, the wear rate of the composite material dropped from 7.8463 × 10−4 to 2.5124 × 10−4 mm3/Nm, a decrease of 58%. Subsequently, the wear rate of the composite material increased slightly. When 200 nm CF was used as a modified filler and the mass fraction was 1%, the wear rate of the composite material decreased from 7.8463 × 10−4 to 3.9038 × 10−4 mm3/Nm, a decrease of 50.3%. Then the wear rate gradually increased. When MWCNT was used as a modified filling and the mass fraction was only 0.5%, the wear rate of the composite material decreased from 7.8463 × 10−4 to 2.5897 × 10−4 mm3/Nm, a decrease of 57%. Subsequently, the wear rate increased slowly, but the wear rate remained below 50% of the unmodified PTFE.

Coatings 2020, 10, x FOR PEER REVIEW 10 of 19 the interface bonding performance determines the stress transfer effect between the fiber and the matrix, which in turn affected the failure mechanism of the composite material, and ultimately affected the macroscopic mechanical properties of the composite material.

4.1.4. Friction and Wear Performance The study tested the friction and wear of micro/nano CF and MWCNT filled modified PTFE composites. The friction coefficient and wear rate of the composite material are shown in Figures 12 and 13.

Coatings 2020, 10, 1103 11 of 20 Figure 12. Friction coefficient of micro/nano CF and MWCNT filled modified PTFE composites.

Figure 13.Figure Wear 13. Wearrate of rate micro/nano of micro/nano CF CF and and MWCNTMWCNT ﬁlled filled modiﬁed modified PTFE PTFE composites. composites.

It can be seen from Figure 12 that the addition of different carbon materials slightly increased It can be seen from Figure 12 that the addition of different carbon materials slightly increased the friction coefficient of the composite material. It can be seen from Figure 13 that the strength and the frictionload-bearing coefficient capacity of the of composite the composite material material. It were can improved, be seen from indicating Figure that 13 the that ability the ofstrength the and load-bearingcomposite capacity material of to the resist composite plastic deformation material was were significantly improved, improved, indicating and the thatploughing the ability effect of the compositeof thematerial grinding to parts resist on plastic the surface deformation of the composite was significantly material was reduced. improved, The additionand the ofploughing CF and effect of the grindingMWCNT parts has greatly on the improved surface the of wear the resistancecomposite of compositematerial was materials. reduced. Under The the experimentaladdition of CF and conditions, as the content of different types of carbon materials increased, the wear of composite MWCNT has greatly improved the wear resistance of composite materials. Under the experimental materials decreased significantly. Among them, when 7 µm CF was used as the modified filler and conditions,the massas the fraction content was 1.5%,of different the wear types rate of of the carbon composite materials material droppedincreased, from the 7.8463 wear10 of4 compositeto × − materials2.5124 decreased10 4 mm significantly.3/Nm, a decrease Among of 58%. them, Subsequently, when 7 μm the CF wear was rate used of the as composite the modified material filler and × − the massincreased fraction slightly. was 1.5%, When the 200 wear nm CF rate was of used the as composite a modified material filler and thedropped mass fraction from was7.8463 1%, × 10−4 to the wear rate of the composite material decreased from 7.8463 10 4 to 3.9038 10 4 mm3/Nm, 2.5124 × 10−4 mm3/Nm, a decrease of 58%. Subsequently, the × wear− rate of the× composite− material a decrease of 50.3%. Then the wear rate gradually increased. When MWCNT was used as a modified increased slightly. When 200 nm CF was used as a modified filler and the mass fraction was 1%, the filling and the mass fraction was only 0.5%, the wear rate of the composite material decreased from −4 −4 3 wear rate7.8463 of the 10composite4 to 2.5897 material10 4 mm decreased3/Nm, a decrease from 7.8463 of 57%. × Subsequently, 10 to 3.9038 the × wear10 mm rate increased/Nm, a decrease × − × − of 50.3%.slowly, Then butthe the wear wear rate rate gradually remained below increased. 50% of theWhen unmodified MWCNT PTFE. was used as a modified filling and the mass fractionIn order was to furtheronly 0.5%, study the mechanismwear rate ofof filler the materialcomposite on the material wear performance decreased of from composite 7.8463 × 10−4 to 2.5897materials, × 10−4 mm the3 wear/Nm, surface a decrease was tested of 57%. by scanning Subsequently, electron microscopy. the wear The rate surface increased morphology slowly, of but the different carbon materials after wear is observed, as shown in Figure 14. wear rate remained below 50% of the unmodified PTFE. According to Figure 14, there was no obvious crack on the surface of the composite material. However, there was adhesive wear of the PTFE matrix. In addition, no obvious dents left by carbon CF exposure or peeling of carbon fiber were seen on the PTFE surface. Due to its good self-lubricity, PTFE is easy to form a thin crystalline layer. The crystalline layer and the amorphous layer were alternately arranged in a strip structure. In addition, because of the low friction coefficient, the amorphous partial layer was easy to slide. Since PTFE has a strong intramolecular structure and weak intermolecular binding force, macromolecules are prone to slippage under external force. The essence of PTFE wear is that the macromolecules slip and break under the action of external force, so that the material is pulled out of the crystallization zone and transferred to the surface of the mating part in a sheet shape, resulting in adhesive wear. The addition of CF in PTFE can improve its compressive strength. When the composite is in the process of reciprocating friction and wear, CF mainly plays the role in bearing and limiting the PTFE macromolecular sliding, so it effectively improves the wear resistance of the composite. Coatings 2020, 10, x FOR PEER REVIEW 11 of 19

In order to further study the mechanism of filler material on the wear performance of composite materials,Coatings the 2020wear, 10, surface 1103 was tested by scanning electron microscopy. The surface morphology12 of 20 of different carbon materials after wear is observed, as shown in Figure 14.

(a) (b)

(c)

Figure 14.Figure SEM 14. diagramSEM diagram of PTFE of PTFE wear wear surfaces surfaces filled filled withwith di differentfferent carbon carbon materials: materials: (a) 1% 7(aµ)m 1% CF; 7 μm CF; (b) 1% 200(b) 1%nm 200 CF nm and CF ( andc) 1% (c) MWCNT 1% MWCNT.. 4.2. Structure and Properties of the Aramid Modified PTFE Composite According to Figure 14, there was no obvious crack on the surface of the composite material. However,4.2.1. there Hardness was adhesive wear of the PTFE matrix. In addition, no obvious dents left by carbon CF exposureThe or hardnesspeeling testof carbon of PPTA fiber and nano were Al 2seeO3 filledn on modifiedthe PTFE PTFE surface. was conducted. Due to its The good hardness self of-lubricity, PTFE is theeasy composite to form material a thin is showncrystalline in Figure layer. 15. It The can becrystalline seen from the layer figure and that the after amorphous PPTA was added, layer were the hardness of PTFE decreased slightly. When the amount of PPTA added was 1%, the shore hardness alternatelyof the arranged composite materialin a strip dropped structure. from 62 toIn 59.2, addition, a decrease because of 4.5%. Subsequently, of the low the friction hardness coefficient,of the the amorphouscomposite partial material layer slowlywas easy increased. to slide. When Since the content PTFE ofhas PPTA a strong reached intramolecular the peak at 7%, the structure hardness ofand weak intermolecularthe composite binding was force, 61.2, which macromolecules was similar to the are hardness prone to of unmodifiedslippage under PTFE. Whenexternal 3% nanoforce. Al 2TheO3 essence of PTFE particleswear is andthat PPTA the macromolecules were compounded andslip modified and break into u PTFE,nder thethe hardness action of theexternal PTFE composite force, so that the materialmaterial is pulled increased out of slightly the crystallization from 62 to 64.2 degrees, zone and an increase transferred of 4.8%, to indicating the surface that ceramic of the particles mating part in could effectively increase the hardness of PTFE. By comparing the two materials, it can be seen that a sheet shape, resulting in adhesive wear. The addition of CF in PTFE can improve its compressive the hardness of the composite material increased first and then decreased. Three percent nano Al2O3 strength.particles When couldthe composite increase the hardnessis in the of process the composite of reciprocating material on the friction basis of and only fillingwear, PPTA. CF mainly plays the role in bearing and limiting the PTFE macromolecular sliding, so it effectively improves the wear resistance of the composite.

4.2. Structure and Properties of the Aramid Modified PTFE Composite

4.2.1. Hardness

The hardness test of PPTA and nano Al2O3 filled modified PTFE was conducted. The hardness of the composite material is shown in Figure 15. It can be seen from the figure that after PPTA was added, the hardness of PTFE decreased slightly. When the amount of PPTA added was 1%, the shore hardness of the composite material dropped from 62 to 59.2, a decrease of 4.5%. Subsequently, the hardness of the composite material slowly increased. When the content of PPTA reached the peak at 7%, the hardness of the composite was 61.2, which was similar to the hardness of unmodified PTFE. When 3% nano Al2O3 particles and PPTA were compounded and modified into PTFE, the hardness of the PTFE composite material increased slightly from 62 to 64.2 degrees, an increase of 4.8%,

Coatings 2020, 10, x FOR PEER REVIEW 12 of 19 indicating that ceramic particles could effectively increase the hardness of PTFE. By comparing the two materials, it can be seen that the hardness of the composite material increased first and then Coatings 2020, 10, x FOR PEER REVIEW 12 of 19 decreased. Three percent nano Al2O3 particles could increase the hardness of the composite material on the basis of only filling PPTA. indicating that ceramic particles could effectively increase the hardness of PTFE. By comparing the two materials, it can be seen that the hardness of the composite material increased first and then decreased. Three percent nano Al2O3 particles could increase the hardness of the composite material Coatings 2020, 10, 1103 13 of 20 on the basis of only filling PPTA.

Figure 15. The hardness of PPTA and nano Al2O3 particles filled modified PTFE composite material.

4.2.2. Impact Performance

TheFigure impact 15.Figure The properties 15.hardnessThe hardness ofof PPTA modified of PPTA and and nano PTFE nano Al Alfilled22OO33 particlesparticles with PPTA filled filled modified modifiedand nano PTFE PTFE compositeAl2O composite3 particles material. material were tested.. The impact4.2.2. performance Impact Performance of the composite material is shown in Figure 16. It can be seen from the 4.2.2.figure Impact that the P erformanceaddition of PPTA significantly reduced the impact toughness of PTFE. When the filling The impact properties of modified PTFE filled with PPTA and nano Al O particles were tested. amount of PPTA was 5%, the impact toughness of the composite material2 decreased3 from the original TheThe impact impact properties performance of of modified the composite PTFE material filled is with shown PPTA in Figure and 16 .nano It can Al be2 seenO3 particles from the figure were tested. 17.89 to 4.46 kJ/m2, a decrease of 75%, which was due to the poor bonding force between PPTA and The impactthat theperformance addition of PPTA of the significantly composite reduced material the impact is shown toughness in ofFigure PTFE. When16. It the can filling be amountseen from the Al2O3 andof PPTAthe PTFE was 5%, matrix. the impact In addition, toughness the of the impact composite performance material decreased of composite from the originalmaterials 17.89 was to mainly figure that the addition2 of PPTA significantly reduced the impact toughness of PTFE. When the filling affected 4.46by fibers, kJ/m , a because decrease offibers 75%, whichwould was significantly due to the poor affect bonding the force failure between mode PPTA of andcomposite Al2O3 and materials amount of PPTA was 5%, the impact toughness of the composite material decreased from the original during thethe PTFEimpact matrix. process. In addition, The impact the impact strength performance was of sensitive composite to materials the notch was mainly of the a ffsample,ected by and all 17.89 to 4.46fibers, kJ/m because2, a decrease fibers would of significantly75%, which aff wasect the due failure to the mode poor of compositebonding materialsforce between during thePPTA and impact energy was absorbed in the notch. Therefore, the impact strength had higher requirements Al2O3 andimpact the PTFE process. matrix. The impact In addition, strength was the sensitive impact toperformance the notch of the of sample, composite and all materials impact energy was mainly for the interface bonding performance of composite materials. However, the impact properties of affected wasby fibers, absorbed because in the notch. fibers Therefore, would thesignificantly impact strength affect had the higher failure requirements mode of for composite the interface materials PPTA composites decreased significantly. The impact strength after adding Al2O3 was similar to that during thebonding impact performance process. ofThe composite impact materials. strength However, was sensitive the impact to properties the notch of PPTAof the composites sample, and all of PPTA.decreased significantly. The impact strength after adding Al2O3 was similar to that of PPTA. impact energy was absorbed in the notch. Therefore, the impact strength had higher requirements for the interface bonding performance of composite materials. However, the impact properties of PPTA composites decreased significantly. The impact strength after adding Al2O3 was similar to that of PPTA.

Figure 16.Figure Impact 16. Impact performance performance of PPTA of PPTA and and nano nano AlAl2O2O3 3particles particles filled filled modified modified PTFE composites.PTFE composites. 4.2.3. Tensile Performance 4.2.3. Tensile Performance Tensile properties of PPTA and nano Al2O3 particles filled modified PTFE were tested. The tensile TensileFigurestrength, 16. properties Impact tensile performance elongation of PPTA at ofand break PPTA nano and and elasticAl nano2O3 modulusAlparticles2O3 particles of filled PPTA filled modifiedmodified modified PTFE PTFE PTFE are were showncomposites tested. in . The tensile strength,Figures 17 tensile–19. elongation at break and elastic modulus of PPTA modified PTFE are shown 4.2.3.in Figures Tensile 17 –P1erfo9. rmance

Tensile properties of PPTA and nano Al2O3 particles filled modified PTFE were tested. The tensile strength, tensile elongation at break and elastic modulus of PPTA modified PTFE are shown in Figures 17–19.

Coatings 2020, 10, 1103 14 of 20 Coatings 2020, 10, x FOR PEER REVIEW 13 of 19 Coatings 2020,, 10,, xx FORFOR PEERPEER REVIEWREVIEW 13 of 19

Figure 17. Tensile strength of PPTA and nano Al2O3 filled modified PTFE composite. 2 3 FigureFigure 17. Tensile 17. Tensile strength strength of of PPTA PPTA and and nanonano Al Al2O2O3 ﬁlled3 filled modiﬁed modified PTFE PTFE composite. composite.

Figure 18. Tensile elongation at break of PPTA and nano Al2O3 filled modified PTFE composites. Figure 18. Tensile elongation at break of PPTA and nano Al2O32 ﬁlled modiﬁed PTFE composites. Figure 18. Tensile elongation at break of PPTA and nano Al2O3 filled modified PTFE composites.

Figure 19. Elastic modulus of PPTA and nano Al O filled modified PTFE composite. Figure 19. Elastic modulus of PPTA and nano Al2 2O3 3 filled modified PTFE composite. 2 3 Figure 19. Elastic modulus of PPTA and nano Al2O3 filled modified PTFE composite. It can be seen from Figures 17 and 18 that the tensile strength and elongation at break of PTFE It werecan be significantly seen from reduced Figures by adding17 and PPTA.18 that When the thetensile PPTA strength content wasand 1%, elongation the tensile at strength break ofof PTFE It can be seen from Figures 17 and 18 that the tensile strength and elongation at break of PTFE were significantlythe composite reduced material dropped by adding from PPTA the original. When 27.9 the to 18.6PPTA MPa, content with a decreasewas 1%, rate the of tensile 33%. At strength this of were significantly reduced by adding PPTA. When the PPTA content was 1%, the tensile strength of the compositetime, the tensilematerial elongation dropped at breakfrom decreasedthe original from 27.9 the to original 18.6 MPa, 810–520%, with which a decrease was only rate 64% of of33%. At the composite material dropped from the original 27.9 to 18.6 MPa, with a decrease rate of 33%. At this time,that the of unmodified tensile elongation PTFE at break. at break When decrease 3% nanod Al from2O3 particlesthe original and PPTA 810–520%, were compounded which wasand only 64% this time,modified the tensile into PTFE, elongation the composite at break tensile decrease strengthd furtherfrom the dropped original to 13.2 810 MPa,–520%, which which was onlywas 50% only 64% of that of unmodified PTFE at break. When 3% nano Al2O3 particles and PPTA were compounded 2 3 of that of unmodified PTFE at break. When 3% nano Al2O3 particles and PPTA were compounded and modified into PTFE, the composite tensile strength further dropped to 13.2 MPa, which was only and modified into PTFE, the composite tensile strength further dropped to 13.2 MPa, which was only 50% of the tensile strength of unmodified PTFE, while the tensile elongation at break decreased to 50% of the tensile strength of unmodified PTFE, while the tensile elongation at break decreased to 215%, which was only 26% of unmodified PTFE tensile elongation at break. With the increase of the 215%, which was only 26% of unmodified PTFE tensile elongation at break. With the increase of the PPTA content, the tensile strength of the composite material did not change significantly, while the PPTA content, the tensile strength of the composite material did not change significantly, while the tensile elongation at break further decreased. When the PPTA content was 9%, the tensile elongation tensile elongation at break further decreased. When the PPTA content was 9%, the tensile elongation at break of the composite material was only 16%. It can be seen from Figure 19 that PPTA and nano at break of the composite material was only 16%. It can be seen from Figure 19 that PPTA and nano Al2O3 particles rapidly increased the elastic modulus of PTFE. When PPTA was added at 5%, the 2 3 Al2O3 particles rapidly increased the elastic modulus of PTFE. When PPTA was added at 5%, the

Coatings 2020, 10, 1103 15 of 20 Coatings 2020, 10, x FOR PEER REVIEW 14 of 19 of the tensile strength of unmodified PTFE, while the tensile elongation at break decreased to 215%, elastic moduluswhich was reached only 26% the of unmodifiedhighest point PTFE of tensile 519 MPa, elongation which at break.was 1.8 With times the increasethat of ofunmodified the PPTA PTFE. With thecontent, further the increase tensile strength of PPTA of the content, composite the material elastic didmodulus not change of the significantly, composite while material the tensile gradually decreased.elongation When at3% break nano further Al2 decreased.O3 particles When were the PPTAcompounded content was and 9%, themodified tensile elongation with PPTA, at break the elastic modulusof o thef the composite composite material material was only was 16%. further It can improved be seen from on Figure the basis 19 that of PPTA only and PPTA. nano Among Al2O3 them, particles rapidly increased the elastic modulus of PTFE. When PPTA was added at 5%, the elastic when themodulus addition reached amount the highest of nano point Al of2 519O3 MPa,particles which and was 1.8PPTA times was that of3%, unmodified the elastic PTFE. modulus With the reached the highestfurther point increase 595 MPa. of PPTA content, the elastic modulus of the composite material gradually decreased. WithWhen the 3%increase nano Al of2O PPTA,3 particles PPTA were compoundedand PTFE in and the modified composite with PPTA,material the elastic weremodulus entangled of the with each other, resultingcomposite in material a rapid was increase further in improved the elastic on the modulus basis of only of the PPTA. composite Among them, material. when theWhen addition the amount of PPTAamount added ofwas nano 5%, Al 2theO3 particles elastic modulus and PPTA wasbega 3%,n to the decrease elastic modulus gradually. reached This the is highest because point when the 595 MPa. content of PPTAWith thewas increase high, ofthe PPTA, PTFE PPTA matrix and PTFEmaterial in the could composite not completely material were wrap entangled the PPTA with fibers, resultingeach in other, contact resulting between in a rapid some increase PPTA in the fibers elastic and modulus PPTA of fibers. the composite However, material. there When was the only an electrostaticamount force of PPTA between added PPTA was 5%, and the elastic PPTA modulus as the began binding to decrease force, gradually. which led This to is the because decrease of compositewhen properties. the content It of can PPTA be wasseen high, from the the PTFE tensile matrix strength material couldthat the not completelyaddition of wrap PPTA the PPTAsignificantly reduced fibers,the tensile resulting properties in contact betweenof PTFE, some because PPTA the fibers C and–C bond PPTA fibers.on the However, PTFE surface there was was only wrapped an by electrostatic force between PPTA and PPTA as the binding force, which led to the decrease of composite F atoms,properties. and the F It atoms can be were seen from relatively the tensile stable. strength Thus, that the the PPTA addition fiber of PPTA and significantlythe PTFE matrix reduced interface could notthe form tensile a strong properties bonding of PTFE, force, because only the the C–C interaction bond on the of PTFE physical surface meshing was wrapped and byelectrostatic F atoms, force was generated.and the FMoreover, atoms were the relatively bonding stable. force Thus, was the relatively PPTA fiber weak, and the resulting PTFE matrix in the interface gradual could decline of the tensilenot properties form a strong of bonding the material. force, only the interaction of physical meshing and electrostatic force was In ordergenerated. to further Moreover, study the the bonding influence force was of PPTA relatively onweak, the properties resulting in of the PTFE gradual and decline the strengthening of the tensile properties of the material. mechanism, the tensile and impact sections of the sample were observed by SEM. The SEM images In order to further study the influence of PPTA on the properties of PTFE and the strengthening of the tensilemechanism, fracture the tensilesurface and and impact the sectionsimpact of fracture the sample surface were observed are shown by SEM. in Figure The SEM 20 images. It can of be seen from thethe figure tensile that fracture the surfacePPTA andfiber the and impact PTFE fracture matrix surface material are shown were in Figure entangled 20. It can with be seen each from other. The impact fracturethe figure surface that the PPTApresents fiber ductile and PTFE fracture, matrix material and the were interface entangled bond with eachlack other.ed flexibility. The impact It can be seen fromfracture the fracture surface presentssurface ductile that the fracture, PPTA and thread the interface that was bond obviously lacked flexibility. drawn It from can be the seen other from fracture the fracture surface that the PPTA thread that was obviously drawn from the other fracture surface surface protrudes from the fracture surface, indicating that the bonding performance between PPTA protrudes from the fracture surface, indicating that the bonding performance between PPTA and the and the PTFE matrix matrix was was poor. poor. When When impact impact fracture fracture or tensile fractureor tensile occurred, fracture there occur was ared clear, there boundary was a clear boundarylayer layer between between PPTA PPTA and PTFE. and Due PTFE. to the Due small to interaction the small between interaction PTFE matrix between and PPTA, PTFE PPTA matrix and PPTA, PPTAwill pull will out pull from out PTFE from and PTFE eventually and lead eve tontually the fracture lead ofto the the composite. fracture of the composite.

(a) (b)

Figure 20.Figure SEM 20. imageSEM image of cross of cross section section of PPTAPPTA modiﬁed modified composite composite material: material: (a) stretched (a) section stretched view section view andand (b ()b impact) impact section section. .

4.2.4. Friction and Wear Performance

Coatings 2020, 10, 1103 16 of 20

4.2.4. Friction and Wear Performance

In the study, the friction and wear properties of PPTA and nano Al2O3 filled modified PTFE were tested. The friction coefficient and wear rate of the composite material are shown in Figures 21 and 22. It can be seen from Figure 21 that the addition of PPTA and nano-Al2O3 particles improved the friction coefficient of the composite material. It can be seen from Figure 22 that the wear rate of the composite CoatingsCoatings 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 1515 ofof 1919 material was significantly improved. When the PPTA filling amount was 1%, the wear rate of the composite material dropped from the original 7.85 10 4 to 5.18 10 4 mm3/Nm, a decrease of 35%. It can be found material was reduced. The addition of ×PPTA− could× effectively− reduce material loss and improve materialthat was the reduced. ability of the The composite addition material of toPPTA resist plasticcould deformation effectively was reduce significantly material improved, loss and and the improve frictionfriction andandploughing wearwear effect performance.performance. of the grinding WithWith parts thethe on increaseincrease the surface inin of thethe the compositemassmass fractionfraction material ofof was PPTA,PPTA, reduced. thethe Thefrictionfriction addition coefficientcoefficient ofof thethe compositecompositeof PPTA could materialmaterial effectively increasedincreased reduce slightlyslightly material bylossby 66 and––12%,12%, improve maimainlynly friction becausebecause and wear thethe performance.compositecomposite materialmaterial With the formedformed aa thickerthickerincrease transfertransfer in the filmfilm mass duringduring fraction thethe of PPTA, adhesiveadhesive the friction wearwear coefficient process.process. of theWithWith composite thethe additionaddition material increased ofof PPTA,PPTA, slightly thethe frictionfriction torque increaseby 6–12%,d mainlyand the because friction the composite coefficient material also formed increase a thickerd. In transfer addition, film during with the the adhesive increase wear of PPTA torque increaseprocess. Withd and the the addition friction of PPTA, coefficient the friction also torque increase increasedd. and In theaddition, friction coefficient with the also increase increased. of PPTA content, the wear rate of the composite material decreased first and then rose. Among them, when content, Inthe addition, wear withrate theof increasethe composite of PPTA content, material the wear decrease rate ofd the first composite and then material rose decreased. Among first them, and when −4 thethe contentcontentthen ofof rose. PPTAPPTA Among waswas them, 5%,5%, when thethe the wearwear content raterate of of PPTAof thethe was compositecomposite 5%, the wear materialmaterial rate of the waswas composite thethe lowestlowest material atat was 2.472.47 ×× 1010−4 mm33/Nm,the with lowest a decrease at 2.47 10 rate4 mm of3/ Nm,68%. with a decrease rate of 68%. mm /Nm, with a decrease× −rate of 68%.

2 3 FigureFigure 21.21.Figure WearWear 21. coefficientcoefficientWear coefficient ofof modifiedmodified of modified PTFEPTFE PTFE compositecomposite composite filled filledfilled with withwith PPTA PPTAPPTA and nano andand Alnanonano2O3 particles. AlAl2OO3 particlesparticles..

Figure 22. Wear rate of PPTA and nano Al22O3 particles ﬁlled modiﬁed PTFE composite. FigureFigure 22.22. WearWear raterate ofof PPTAPPTA andand nanonano AlAl2OO3 particlesparticles filledfilled modifiedmodified PTFEPTFE compositecomposite.. When the content of PPTA exceeded a certain amount, the wear rate of the composite material WhenWhenpresented thethe contentcontent an instantaneous ofof PPTAPPTA surge. exceededexceeded This is aa mainly certaincertain because amount,amount, the composite thethe wearwear material raterate ofof mainly thethe compositecomposite adhered to materialmaterial presentedpresented anan instantaneousinstantaneous surge.surge. ThisThis isis mainlymainly becausebecause thethe compositecomposite materialmaterial mainlymainly adheredadhered toto eacheach otherother throughthrough thethe PTFEPTFE matrixmatrix andand transferredtransferred thethe loadload toto thethe fibers.fibers. DueDue toto thethe largelarge specificspecific surfacesurface areaarea ofof PPTA,PPTA, whenwhen PPTAPPTA increasesincreases toto aa certaincertain content,content, ththee PTFEPTFE matrixmatrix willwill notnot bebe ableable toto bondbond withwith PPTAPPTA well.well. However,However, thethe interactioninteraction betweenbetween PPTAPPTA isis mainlymainly throughthrough anan electrostaticelectrostatic force,force, soso thethe fibersfibers areare easyeasy toto bebe peeledpeeled offoff andand pulledpulled out,out, whichwhich makesmakes thethe overalloverall performanceperformance ofof thethe compositecomposite materialmaterial drodropp sharply.sharply. InIn orderorder toto furtherfurther studystudy thethe wearwear mechanismmechanism ofof thethe composites,composites, thethe wearwear surfacesurface waswas testedtested byby SEMSEM andand thethe surfacesurface morphologymorphology afterafter wearwear waswas observed.observed. TheThe wearwear morphologymorphology ofof thethe compositecomposite materialmaterial underunder drydry frictionfriction conditionsconditions isis shownshown inin FigureFigure 2233..

Coatings 2020, 10, 1103 17 of 20 each other through the PTFE matrix and transferred the load to the fibers. Due to the large specific surface area of PPTA, when PPTA increases to a certain content, the PTFE matrix will not be able to bond with PPTA well. However, the interaction between PPTA is mainly through an electrostatic force, so the fibers are easy to be peeled off and pulled out, which makes the overall performance of the composite material drop sharply. In order to further study the wear mechanism of the composites, the wear surface was tested by SEM and the surface morphology after wear was observed. The wear morphology of the composite material under dry friction conditions is shown in Figure 23. Coatings 2020, 10, x FOR PEER REVIEW 16 of 19

(a) (b)

Figure 23. SEM image of the wear surface of 5% PPTA filledfilled PTFE:PTFE: (a)) thethe magnificationmagnification 200200×;;( (b) the × magnificationmagnification 10001000×.. × It can be seen from from Figure Figure 2233 thatthat there there were obvious PPTA PPTA fibers fibers distributed, and a little little PPTA PPTA pulled out out on on the the wear wear surface. surface The. The wear wear surface surface of the of composite the composite material material was rough. was PTFE rough. exhibited PTFE exhibitedlarge flakes, large without flakes, obvious without adhesive obvious wear.adhesive The wear. PPTA The fiber PPTA was exposedfiber was on exposed the wear on surfacethe wear of surfacethe composite of the comp materialosite andmaterial had poorand had bonding poor bonding performance performance with the with matrix the material.matrix material. When When PPTA wasPPTA added was added into PTFE, into PTFE, PPTA mainlyPPTA mainly played play theed role the of role bearing of bearing and restraining and restraining the slippage the slippage of PTFE of PTFEmacromolecules, macromolecules, so as toso e asffectively to effectively improve improve the wear the resistancewear resistance of the compositeof the composite material. material.

5. Conclusions Conclusions (1) ThroughThrough the the ultrasonic ultrasonic dispersion dispersion of CF of and CF MWCNT and MWCNT and various and various processes processes (crushing, (crushing, stirring volatilization,stirring volatilization, crushing crushing and sieving) and sieving) of PTFE of materials, PTFE materials, the effective the eff ective dispersion dispersion of CF of and CF MWCNTand MWCNT in the in compositethe composite material material and and the the stability stability of of the the composite composite material material performance obtainedobtained an an effective effective guarantee. guarantee. (2) ThroughThrough testing testing the the hardness hardness and and impact impact pe performancerformance of CF CF and and MWCNT MWCNT filled filled modified modified PTFE composites,composites, it it was was found found that that various various carbon carbon materials would cause the hardness and impact toughnesstoughness of of PTFE PTFE composites composites to to decrease decrease to varying degrees. Among Among them, them, the the hardness of MWCNTMWCNT-filled-filled PTFE PTFE composites composites decreased most obviously. When the content of MWCNT was 1%,1%, the impactimpact hardnesshardness of of the the composite composite material material dropped dropped from from 62 to 62 52, to with 52, awith decrease a decrease of 15.3%. of (3) 15.3%.By testing the tensile properties of CF and MWCNT filled modified PTFE composites, it was (3) Byfound testing that the additiontensile properties of various of carbon CF and materials MWCNT would filled significantly modified reducePTFE composites, the tensile strength it was foundand elongation that the addition at break of of various PTFE,but carbon greatly materials enhanced would the significantlyelastic modulus. reduce Among the tensile them, strengththe addition and ofelongation 7 µm CF reducedat break theof PTFE, tensile but strength greatly of enhanced the PTFE compositethe elastic materialmodulus from. Among 27.9 them,MPa bythe about addition 20%, of and 7 μm the CF tensile reduced elongation the tensile at strength break decreased of the PTFE by aboutcomposite 30%. mater The additionial from 27.9of MWCNT MPa by about significantly 20%, and reduced the tensile the elongation tensile strength at break and decreased tensile elongation by about 30%. at break The addition of PTFE ofcomposites, MWCNT significantly by 60% and 87%reduced respectively. the tensile Among strength them, and the tensile addition elongation of 7 µm at andbreak 200 of nm PTFE CF composites,increased the by elastic 60% modulusand 87% ofrespectively. PTFE by about Among 70% them, to 471.95 the MPa addition and 137% of 7 μm to 655 and MPa 200 from nm theCF increasedoriginal 276.8 the elastic MPa. MWCNTmodulus of had PTFE a greater by about impact 70% on to the 471.95 elastic MPa modulus and 137% of PTFEto 655 composites. MPa from the original 276.8 MPa. MWCNT had a greater impact on the elastic modulus of PTFE composites. The addition of 1% MWCNT increased the elastic modulus of PTFE from 276.8 to 1075 MPa, with an increase of 388%. (4) Through the friction and wear test of 200 nm and 7 μm CF and MWCNT filled PTFE composite materials, it was found that when 7 μm CF was used as the modified filler and the mass fraction was 1.5%, the composite wear rate decreased from 7.8463 × 10−4 to 2.5124 × 10−4 mm3/Nm, with a decrease rate of 58%. Then the wear rate of the composite material increased slightly. When 200 nm CF was used as the modified filler and the mass fraction was 1%, the wear rate of the composite material decreased from 7.8463 × 10−4 to 3.9038 × 10−4 mm3/Nm, with a decrease rate of 50.3%. Then the wear rate gradually increased. When MWCNT was used as a modified filling

Coatings 2020, 10, 1103 18 of 20

The addition of 1% MWCNT increased the elastic modulus of PTFE from 276.8 to 1075 MPa, with an increase of 388%. (4) Through the friction and wear test of 200 nm and 7 µm CF and MWCNT filled PTFE composite materials, it was found that when 7 µm CF was used as the modified filler and the mass fraction was 1.5%, the composite wear rate decreased from 7.8463 10 4 to 2.5124 10 4 mm3/Nm, × − × − with a decrease rate of 58%. Then the wear rate of the composite material increased slightly. When 200 nm CF was used as the modified filler and the mass fraction was 1%, the wear rate of the composite material decreased from 7.8463 10 4 to 3.9038 10 4 mm3/Nm, with a decrease × − × − rate of 50.3%. Then the wear rate gradually increased. When MWCNT was used as a modified filling and the mass fraction was only 0.5%, the wear rate of the composite material decreased from 7.8463 10 4 to 2.5897 10 4 mm3/Nm, with a decrease rate of 57%. × − × − (5) Through the hardness and impact toughness test of PPTA and nano Al2O3 particle composite modified PTFE, it was found that the increase of PPTA and nano Al2O3 particles reduced the hardness and impact toughness of PTFE. When the PPTA content was 1%, the shore hardness of the composite material dropped from 62 to 59.2, with a decrease rate of 4.5%. When the content of PPTA was 5%, the impact hardness of the composite material dropped from the original 17.89 to 4.46 kJ/m2, with a decrease of 75%.

(6) Through the tensile performance test of PPTA and nano Al2O3 particles composite modified PTFE, it was found that the increase of PPTA and nano Al2O3 particles could effectively increase the elastic modulus of PTFE. Among them, PPTA composite modification had a more obvious increase in the elastic modulus of PTFE. As the content of PPTA increased, the elastic modulus of the composite material first rose and then fell. When PPTA was added at 5%, the elastic modulus reached the highest value of 519 MPa. With the increase of filler content, the tensile strength and tensile elongation of PTFE composite material modified by PPTA fiber and nano-Al2O3 decreased. (7) By testing the friction and wear properties of PPTA and nano-Al2O3 composite modified PTFE, it was found that the addition of PPTA and nano-Al2O3 particles could significantly increase the wear rate of the composite. When the PPTA filling amount was 1%, the wear rate of the composite material dropped from the original 7.85 10 4 to 5.18 10 4 mm3/Nm, with a decrease of 35%. × − × −

Author Contributions: Conceptualization, F.Z. and Y.Z.; validation, J.Z. and Y.J.; formal analysis, F.Z. and X.W.; resources, F.Z.; writing—original draft, J.Z.; writing—review and editing, F.Z.; visualization, J.Z.; supervision, F.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by A Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); Engineering Research Center for Harmless Treatment and Resource Utilization of Aluminum Ash and Slag Solid Waste in Jiangsu Province; Nantong Applied Research Project, grant number JC2018115 and JC2019129; Ministry of Education’s Cooperative Education Project, grant number 201901189019. Acknowledgments: Sincere thanks go to the anonymous reviewers for their valuable comments and good suggestions to improve this article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Sato, Y.; Murahara, M. Protein adsorption on PTFE surface modified by ArF excimer laser treatment. J. Adhes. Sci. Technol. 2004, 18, 1545–1555. [CrossRef] 2. Liu, Y.; Xu, N.; Wang, Y.; Yao, Y.; Xiao, H.; Jia, J.; Lv, H.; Zhang, D. Preparation and tribological properties of hybrid PTFE/Kevlar fabric self-lubricating composites. Surf. Coat. Technol. 2019, 361, 196–205. [CrossRef] 3. Ashrafi, F.; Lavasani, M.R.B. Improvement of the mechanical and thermal properties of polyester nonwoven fabrics by PTFE coating. Turk. J. Chem. 2019, 43, 760–765. [CrossRef] 4. Villalobos, G.J.; Dow, N.; Milne, N.; Zhang, J.; Naidoo, L.; Gray, S.; Duke, M. Membrane distillation trial on textile wastewater containing surfactants using hydrophobic and hydrophilic-coated polytetrafluoroethylene (PTFE) membranes. Membranes 2018, 8, 31. [CrossRef] Coatings 2020, 10, 1103 19 of 20

5. He, G.; Zeng, F.; Lei, K.; Xia, S.; Deng, L.; Zhang, C.; Liu, D. Handle operation lug type ptfe lined butterfly valve (TD71X). Eur. J. Hosp. Pharm. 2017, 24, 162. [CrossRef] 6. Haque, R.I.; Briand, D. Triboelectric freestanding flapping film generator for energy harvesting from gas flow in pipes. Smart Mater. Struct. 2019, 28, 085002. [CrossRef] 7. Feng, S.; Zhong, Z.; Wang, Y.; Xing, W.; Drioli, E. Progress and perspectives in PTFE membrane: Preparation, modification, and applications. J. Membr. Sci. 2018, 549, 332–349. [CrossRef] 8. Yanhai, C.; Lu, R.; Xianliang, M.; Jinyong, Y.; Xianhua, T. The effect of PTFE addition on mechanical and anti-corrosion properties of coating of heat exchangers. Mater. Res. Express 2019, 6, 085207. [CrossRef] 9. Wang, J.; Wu, D.; Li, X.; Zhang, M.; Zhou, W. Poly (vinylidene fluoride) reinforced by carbon fibers: Structural parameters of fibers and fiber-polymer adhesion. Appl. Surf. Sci. 2012, 258, 9570–9578. [CrossRef] 10. Luo, F.; Tang, B.; Fang, Z.; Yuan, Y.; Li, H.; Zhang, S. Polytetrafluoroethylene based, F8261 modified realization

of Li2SnMg0.5O3.5 filled composites. Appl. Surf. Sci. 2020, 503, 144088. [CrossRef] 11. Shi, Y.; Feng, X.; Wang, H.; Lu, X. The effect of surface modification on the friction and wear behavior of carbon nanofiber-filled PTFE composites. Wear 2008, 264, 934–939. [CrossRef] 12. Gürgen, S.; Çelik, O.N.; Ku¸shan,M.C. Tribological behavior of UHMWPE matrix composites reinforced with PTFE particles and aramid fibers. Compos. B Eng. 2019, 173, 106949. [CrossRef] 13. Yao, S.S.; Jin, F.L.; Rhee, K.Y.; Hui, D.; Park, S.J. Recent advances in carbon-fiber-reinforced thermoplastic composites: A review. Compos. B. Eng. 2018, 142, 241–250. [CrossRef] 14. Ni, H.; Zhang, J.; Lv, S.; Wang, X.; Pei, Y.; Li, F. Coating process parameters and structural properties of the tubular electrodes of fuel cells based on a self-made coating device. Coatings 2020, 10, 830. [CrossRef] 15. Subramanian, R.; Shanmugam, K.; Marappan, S. Fabrication of robust superhydrophobic coatings using PTFE-MWCNT nanocomposite: Supercritical fluid processing. Surf. Interface Anal. 2018, 50, 464–470. [CrossRef] 16. Xu, F.; Fan, W.; Zhang, Y.; Gao, Y.; Jia, Z.; Qiu, Y.; Hui, D. Modification of tensile, wear and interfacial properties of Kevlar fibers under cryogenic treatment. Compos. B Eng. 2017, 116, 398–405. [CrossRef] 17. Liu, K.; Yang, W.; Peng, T.; Wang, F. Progress in Surface Modification of Aramid Fiber (II)—Chemical Modification. Synth. Fiber China 2011, 40, 26–31. (In Chinese) [CrossRef] 18. Gonzalez-Canche, N.G.; Flores-Johnson, E.A.; Carrillo, J.G. Mechanical characterization of fiber metal laminate based on aramid fiber reinforced polypropylene. Compos. Struct. 2017, 172, 259–266. [CrossRef] 19. Ni, H.; Zhang, J.; Lv, S.; Gu, T.; Wang, X. Performance optimization of original aluminum ash coating. Coatings 2020, 10, 831. [CrossRef] 20. Ni, H.; Zhang, J.; Lv, S.; Wang, X.; Zhu, Y.; Gu, T. Preparation and performance optimization of original aluminum ash coating based on plasma spraying. Coatings 2019, 9, 770. [CrossRef] 21. Shuaishuai, L.; Jiaqiao, Z.; Hongjun, N.; Xingxing, W.; Yu, Z.; Tao, G. Study on preparation of aluminum ash coating based on plasma spray. Appl. Sci. 2019, 9, 4980. 22. Vasilev, A.P.; Okhlopkova, A.A.; Struchkova, T.S. Investigation of the influence of complex fillers on the properties and structure of polytetrafluoroethylene. J. Frict. Wear 2018, 39, 427–432. [CrossRef] 23. Ohlopkova, A.A.; Struchkova, T.S.; Vasilev, A.P. Studying the properties and structure of polytetrafluoroethylene filled with Belum modified carbon fibers. J. Frict. Wear 2016, 37, 529–534. [CrossRef] 24. Bajpai, A.; Saxena, P.; Kunze, K. Tribo-mechanical characterization of carbon fiber-reinforced cyanate ester resins modified with fillers. Polymers 2020, 12, 1725. [CrossRef] 25. Saxena, P.; Schinzel, M.; Andrich, M. Development of a novel test-setup for identifying the frictional characteristics of carbon fibre reinforced polymer composites at high surface pressure. IOP Conf. Ser. Mater. Sci. Eng. 2016, 149, 012124. [CrossRef]

26. Luo, F.; Tang, B.; Yuan, Y. Microstructure and microwave dielectric properties of Na1/2Sm1/2TiO3 ﬁlled PTFE, an environmental friendly composites. Appl. Surf. Sci. 2018, 436, 900–906. [CrossRef]

27. Yuan, Y.; Li, Z.; Cao, L. Modification of Si3N4 ceramic powders and fabrication of Si3N4/PTFE composite substrate with high thermal conductivity. Ceram. Int. 2019, 45, 16569–16576. [CrossRef] 28. Van Rooyen, L.J.; Bissett, H.; Khoathane, M.C. Gas barrier properties of oxyfluorinated graphene filled polytetrafluoroethylene nanocomposites. Carbon 2016, 109, 30–39. [CrossRef] 29. GB/T 2411-2008. Plastics and Ebonite-Determination of Indentation Hardness by Means of a Durometer (Shore Hardness); China National Standardization Management Committee: Beijing, China, 2008. (In Chinese) 30. GB/T1040-2006. Determination of Tensile Properties of Plastics; China National Standardization Management Committee: Beijing, China, 2006. (In Chinese) Coatings 2020, 10, 1103 20 of 20

31. GB/T1043-2008. Determination of Impact Properties of Plastic Simply Supported Beams; China National Standardization Management Committee: Beijing, China, 2008. (In Chinese) 32. Jiang, B.; Zhu, A.; Zhang, C.; Li, Y. Interface enhancement between polytetrafluoroethylene and glass fibers modified with a titanate coupler. J. Appl. Polym. Sci. 2017, 134. [CrossRef] 33. Lv, S.; Zhang, J.; Ni, H.; Wang, X.; Zhu, Y.; Chen, L. Study on the coupling relationship of low temperature fluidity and oxidation stability of biodiesel. Appl. Sci. 2020, 10, 1757. [CrossRef] 34. Paul, R.; Bhowmik, S. Tribological Behavior of micro coir filler reinforced polymer composite under dry, wet, and heated contact condition. J. Nat. Fibers 2020, 1–16. [CrossRef] 35. Zhang, J.; Kang, X.; Ni, H.; Ren, F. Surface defect detection of steel strips based on classification priority YOLOv3-dense network. Ironmak. Steelmak. 2020, 1–12. [CrossRef] 36. Ni, H.; Wang, K.; Lv, S.; Wang, X.; Zhuo, L.; Zhang, J. Effects of concentration variations on the performance and microbial community in microbial fuel cell using swine wastewater. Energies 2020, 13, 2231. [CrossRef] 37. Ni, H.; Wang, K.; Lv, S.; Wang, X.; Zhang, J.; Zhuo, L.; Li, F. Effects of modified anodes on the performance and microbial community of microbial fuel cells using swine wastewater. Energies 2020, 13, 3980. [CrossRef] 38. Wang, L.; Duan, Y.; Zhang, Y.; Huang, R.; Dong, Y.; Huang, C.; Zhou, B. Surface modification of poly-(p-phenylene terephthalamide) pulp with a silane containing isocyanate group for silicone composites reinforcement. Wuhan Univ. J. Nat. Sci. 2016, 21, 505–511. [CrossRef]

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