Effect of hBN Content on Mechanical and Tribological Properties of hBN/Polyurethane Acrylate Composites
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摘要:
采用硅烷偶联剂KH550对六方氮化硼（hBN）纳米颗粒进行表面改性处理,利用立体光刻技术制备不同质量分数（0.05%,0.10%,0.15%,0.20%）hBN改性聚氨酯丙烯酸酯（hBN/PUA）复合材料,研究了hBN含量对复合材料力学性能和干摩擦磨损性能的影响。结果表明：随着hBN含量的增加,复合材料的拉伸和弯曲强度均先增再降,稳定摩擦因数和磨损质量损失均先减后增；当hBN的质量分数为0.10%时,复合材料的拉伸强度和弯曲强度最大,相较于纯PUA分别提高35.87%和36.92%,稳定摩擦因数和磨损质量损失均最小,与纯PUA相比分别降低31.03%和45.83%,此时复合材料的力学性能和耐磨性能最佳。当hBN的质量分数不高于0.10%时,hBN颗粒均匀地分布在PUA基体中,阻碍裂纹扩展,因此复合材料强度和耐磨性提高；当hBN质量分数大于0.10%时,hBN颗粒发生团聚,导致应力集中,强度和耐磨性能降低。
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关键词:
- hBN/PUA复合材料 /
- 拉伸性能 /
- 弯曲性能 /
- 干摩擦磨损性能
Abstract:The hexagonal boron nitride (hBN) nanoparticles were surface modified by silane coupling agent KH550. The different mass fractions (0.05%, 0.10%, 0.15%, 0.20%) of hBN modified polyurethane acrylate (hBN/PUA) composites were prepared by stereolithography technology. The effect of hBN content on the mechanical properties and dry friction and wear properties of the composites was studied. The results show that with the increase of hBN content, the tensile and bending strengths of the composites first increased and then decreased, while the stable friction coefficient and wear mass loss first decreased and then increased. When the mass fraction of hBN was 0.10%, the tensile strength and bending strength of the composites were the highest, which increased by 35.87% and 36.92% compared with those of pure PUA, respectively; the stable friction coefficient and wear mass loss were the smallest, which decreased by 31.03% and 45.83% compared with those of pure PUA, respectively. At this time, the mechanical and wear resistance of the composites were the best. When the mass fraction of hBN was not larger than 0.10%, hBN particles were evenly distributed in the PUA matrix, hindering crack propagation, resulting in improvement of strength and wear resistance. When the mass fraction of hBN was larger than 0.10%, the hBN particles agglomerated, resulting in stress concentration, and the strength and wear resistance decreased.
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0. 引言
聚氨酯丙烯酸酯（PUA）是一种广泛使用的光敏聚合物材料,兼具聚氨酯（PU）和聚丙烯酸酯（PA）的特点[1],具有良好的耐腐蚀性能、高弹性[2]和化学稳定性[3],但由于其本身热稳定性和力学性能较差[4],制成的零件在摩擦、撞击、高温等工况下的服役寿命短,更换频繁,造成材料和资源的浪费。研究[5-6]表明,可以选择添加无机纳米粒子对PUA进行增韧改性以提高其性能,同时无机纳米粒子的加入也可以大大降低材料成本。
纳米六方氮化硼（hBN）具有良好的力学、润滑和抗氧化性能[7],常用作填料来增强聚合物的力学性能和耐磨性能[8-10]；填料的分散性及其与聚合物基体之间的界面相容性决定着复合材料的性能[11]。hBN纳米颗粒在PUA中的分散性较差[12],一般采用硅烷偶联剂对hBN纳米颗粒进行表面改性处理来解决此问题[13]。KIM等[14]研究发现,使用硅烷偶联剂对hBN进行改性处理可以增强其与PUA基体之间的界面黏附性。HONG等[15]研究发现,hBN的添加可提高PUA的热导率,当hBN的质量分数为5%时,复合材料的热导率相比于PUA提高了180%。目前,有关hBN/PUA复合材料的研究主要集中在导热特性方面,有关hBN含量对其力学性能和摩擦学性能影响方面的研究较少。研究[16]表明,过量的hBN添加到PUA基体中,易发生团聚,导致hBN/PUA复合材料的拉伸强度降低。为此,作者以硅烷偶联剂改性的hBN纳米颗粒为填料,利用立体光刻技术制备不同质量分数hBN改性PUA复合材料（hBN/PUA复合材料）,研究了hBN含量对复合材料的拉伸性能、弯曲性能和摩擦磨损性能的影响,以期为综合性能优异的聚氨酯丙烯酸酯复合材料的研制提供试验参考。
1. 试样制备与试验方法
试验原料包括：聚氨酯丙烯酸酯,惠州华鑫新材料有限公司提供；hBN粉末,粒径为100 nm,纯度99%,济南至鼎焊材有限公司提供；氢氧化钠（NaOH）,分析纯,国药集团化学试剂有限公司提供；硅烷偶联剂KH550,纯度大于99%,东菀康锦新材料科技有限公司提供；无水乙醇,分析纯,广东林氏化学试剂有限公司提供；盐酸,分析纯,广州化学试剂厂提供；去离子水,杭州蒸馏水厂提供。
改性hBN的制备：取5 g hBN粉末加入到400 mL NaOH溶液（5 mol·L−1）中,置于CR-009S型超声清洗机中分散10 min,然后将混合物加热至80 ℃,连续搅拌12 h；使用去离子水洗涤5~6次至中性,放入DZF-6020A型真空干燥箱中在100 ℃下干燥10 h；取3 g干燥后的粉末加入到80 g无水乙醇溶液中,并缓慢加入8 g硅烷偶联剂KH550,再滴入盐酸溶液（0.1 mol·L−1）,将混合体系的pH调节至4,在80 ℃下超声加热搅拌8 h；用无水乙醇洗涤5~6次至中性,在100 ℃下真空干燥10 h,取出研磨成粉末。
取适量的PUA置于烧杯中,按照hBN的质量分数分别为0,0.05%,0.10%,0.15%,0.20%称取改性hBN粉末,然后加入到烧杯中,使用搅拌器以100 r·min−1的转速搅拌30 min；将混合物倒入小方S130型光固化3D打印机的树脂盒中,按照图1所示的拉伸、弯曲和摩擦磨损试样尺寸进行打印,试样厚度均为4 mm,打印精度为每层0.1 mm。打印结束后,用无水乙醇清洗试样表面,再放入紫外线固化机中固化3 min,取出待用。
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图 1 拉伸、弯曲和摩擦磨损试样的形状和尺寸Figure 1. Shape and size of tensile (a), bending (b), and frictional wear (c) specimens利用Nicolet iS50型傅里叶变换红外光谱仪对复合材料和改性前后hBN的化学结构进行表征。分别按照GB/T 1040.2—2022和GB/T 9341—2008,采用CTM8050型微机控制电子万能材料试验机对复合材料进行室温拉伸和弯曲试验,拉伸和弯曲速度均为1 mm·min−1。使用MMW-1型微机控制立式万能摩擦磨损试验机对复合材料进行干摩擦磨损试验,主轴转速为200 r·min−1,法向载荷为40 N,试验时间为30 min,对磨件为45钢环,硬度为40~45 HRC,表面粗糙度Ra为0.4 μm。使用精度为0.1 mg的JJ224BC型电子天平称取试验前后试样的质量,计算磨损质量损失。利用FlexSEm1000型扫描电子显微镜（SEM）观察拉伸断口形貌和磨损表面形貌。
2. 试验结果与讨论
2.1 化学结构
由图2可知,硅烷偶联剂改性前后hBN均在波数1 361 cm−1和766 cm−1处存在2个强吸附峰,分别与B―N键的面内拉伸振动和面外弯曲振动相关[17],改性hBN在波数1 100 cm−1附近出现1个新的吸附带,这与硅烷偶联剂中Si―O键的拉伸振动有关,表明硅烷偶联剂KH550成功接枝到了hBN表面。
由图3可知,不同质量分数hBN改性PUA试样的红外光谱相似,波数3 324 cm−1处的吸收峰由N―H键的拉伸振动引起[18],波数2 920 cm−1附近的吸附带与C―H键的反对称和对称拉伸有关,波数1 720 cm−1的吸收峰与羰基（C=O）的拉伸振动有关,对比纯PUA（hBN的质量分数为0）,hBN/PUA复合材料还在波数1 361 cm−1和766 cm−1处出现由hBN中B―N键拉伸振动引起的吸收峰。
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图 3 不同质量分数hBN改性PUA试样的红外光谱Figure 3. Infrared spectra of different mass fractions of hBN modified PUA samples2.2 拉伸性能
由图4可知,不同质量分数hBN改性PUA试样在拉伸过程中均出现了明显的屈服现象,同时还存在真应力保持不变,而真应变随时间延长而增大的现象,说明试样发生了蠕变。添加改性hBN纳米颗粒后,复合材料的拉伸强度和拉伸模量较纯PUA均有所提高,当hBN的质量分数为0.10%时,复合材料的拉伸强度和拉伸模量均达到最大,分别为72.157 MPa和0.419 GPa,较纯PUA分别提高35.87%和69.61%。随着hBN质量分数的增加,拉伸强度呈先增后降的趋势。当在PUA中添加少量的改性hBN纳米颗粒时,hBN纳米颗粒能够均匀地分散在PUA中,可以起到分散载荷、传递应力的作用,从而提高复合材料的拉伸强度；添加过量时,hBN纳米颗粒易发生团聚[19-20],分散度降低,导致拉伸时应力分布不均匀,从而降低复合材料的拉伸强度。
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图 4 不同质量分数hBN改性PUA试样的拉伸性能Figure 4. Tensile properties of different mass fractions of hBN modified PUA samples: (a) tensile true stress-true strain curves and (b) tensile strength and tensile modulus由图5可知：纯PUA的拉伸断口相对光滑,表面起伏较小,出现较浅的河流状裂纹,属于典型的脆性断裂形貌[21]；当hBN的质量分数为0.05%和0.10%时,复合材料拉伸断口出现明显的褶皱,表面较为粗糙,起伏较大,裂纹相对不规则,呈韧性断裂形貌[22],这是因为在复合材料拉伸时,hBN颗粒能很好地阻碍裂纹扩展,消耗裂纹的边缘能量,从而提高拉伸强度；当hBN的质量分数为0.15%和0.20%时,复合材料拉伸断口出现明显的hBN颗粒团聚现象,这是因为过量的hBN纳米颗粒发生团聚,导致拉伸过程中出现应力集中,从而降低拉伸强度。
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图 5 不同质量分数hBN改性PUA试样的拉伸断口形貌Figure 5. Tensile fracture morphology of different mass fractions of hBN modified PUA samples2.3 弯曲性能
由图6可知,不同质量分数hBN改性PUA试样在弯曲过程中都出现了明显的屈服和蠕变现象。随着hBN质量分数的增加,复合材料的弯曲模量增大,而弯曲强度则呈先升高后降低的趋势。当hBN的质量分数为0.10%时,复合材料的弯曲强度最大,较纯PUA提高36.92%；当hBN的质量分数为0.20%时,复合材料的弯曲强度最低,但弯曲模量达到最大,较纯PUA提高73.55%。过量的hBN易发生团聚形成应力集中区,导致弯曲强度降低。
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图 6 不同质量分数hBN改性PUA试样的弯曲性能Figure 6. Bending properties of different mass fractions of hBN modified PUA samples: (a) bending true stress-true strain curves and (b) bending strength and bending modulus2.4 摩擦磨损性能
由图7可知,不同质量分数hBN改性PUA试样的摩擦磨损过程可分为磨合期和稳定期2个阶段。磨合期摩擦因数随时间延长不断增加,经过一段时间的磨合后进入到稳定期,摩擦因数保持相对平稳,波动较小。纯PUA的稳定摩擦因数和磨损质量损失均最大,在PUA中添加hBN纳米颗粒后稳定摩擦因数和磨损质量损失降低；随着hBN含量的增加,复合材料的稳定摩擦因数和磨损质量损失均先降后增。当hBN的质量分数为0.10%时,稳定摩擦因数和磨损质量损失均最小,分别为0.10和1.3 mg,较纯PUA减少31.03%和45.83%。
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图 7 不同质量分数hBN改性PUA试样的摩擦磨损性能Figure 7. Friction and wear properties of different mass fractions of hBN modified PUA samples: (a) friction coefficient curves and (b) wear mass loss由图8可知,纯PUA的磨损表面存在许多微观裂纹,同时出现了树脂脱落和磨损碎屑,说明在摩擦磨损过程中表面发生较严重的塑性变形以及疲劳磨损。当hBN的质量分数为0.05%时,磨损表面出现了树脂脱落,当hBN的质量分数为0.10%时,磨损表面较为平坦。适量hBN纳米颗粒在PUA基体中分散良好,经硅烷偶联剂改性后hBN与PUA基体产生很强的界面结合,促进从PUA基体到hBN纳米颗粒的有效应力传递[23]。在摩擦磨损过程中,部分hBN会从基体中剥离出来,填充对磨件凹面并将铁元素转移到磨损表面,在树脂的润滑作用下,转移的铁元素发生氧化和化学转化形成转移膜[24],隔离摩擦副之间的直接接触,产生较强的自润滑作用,从而提高复合材料的摩擦磨损性能。当hBN的质量分数为0.15%和0.20%时,hBN纳米颗粒在PUA基体中出现团聚现象,导致形成的转移膜不连续,因此复合材料的摩擦磨损性能降低。
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图 8 不同质量分数hBN改性PUA试样的磨损形貌Figure 8. Wear morphology of different mass fractions of hBN modified PUA samples3. 结论
（1）适量添加硅烷偶联剂改性hBN（hBN质量分数不高于0.10%）,hBN颗粒均匀地分布在PUA基体中,起到阻碍裂纹扩展作用,复合材料拉伸强度和弯曲强度随hBN含量增加而升高；但当hBN过量时（hBN质量分数高于0.10%）,hBN颗粒发生团聚,导致应力集中,强度降低。当hBN的质量分数为0.10%时,拉伸强度和弯曲强度最大,分别为72.157,117.689 MPa,较纯PUA分别提高35.87%和36.92%。与纯PUA相比,复合材料的拉伸断口更粗糙,呈典型的韧性断裂形貌。
（2）随着hBN含量的增加,复合材料的稳定摩擦因数和磨损质量损失均先减后增；当hBN的质量分数为0.10%时,稳定摩擦因数和磨损质量损失均最小,分别为0.10和1.3 mg,与纯PUA相比分别降低31.03%和45.83%。
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图 1 拉伸、弯曲和摩擦磨损试样的形状和尺寸
Figure 1. Shape and size of tensile (a), bending (b), and frictional wear (c) specimens
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图 3 不同质量分数hBN改性PUA试样的红外光谱
Figure 3. Infrared spectra of different mass fractions of hBN modified PUA samples
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图 4 不同质量分数hBN改性PUA试样的拉伸性能
Figure 4. Tensile properties of different mass fractions of hBN modified PUA samples: (a) tensile true stress-true strain curves and (b) tensile strength and tensile modulus
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图 5 不同质量分数hBN改性PUA试样的拉伸断口形貌
Figure 5. Tensile fracture morphology of different mass fractions of hBN modified PUA samples
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图 6 不同质量分数hBN改性PUA试样的弯曲性能
Figure 6. Bending properties of different mass fractions of hBN modified PUA samples: (a) bending true stress-true strain curves and (b) bending strength and bending modulus
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图 7 不同质量分数hBN改性PUA试样的摩擦磨损性能
Figure 7. Friction and wear properties of different mass fractions of hBN modified PUA samples: (a) friction coefficient curves and (b) wear mass loss
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