摘要:
本文采用分子动力学计算方法和Tersoff作用势研究了无定型碳(amorphous carbon, a-C)涂层厚度对SiC纳米纤维/SiC纳米复合材料断裂方式及力学性能的影响.分析结果发现,随着涂层厚度的增加,纳米纤维的平均应力集中系数下降,即足够厚度涂层可以同时起到增强和补韧的作用.当a-C涂层厚度t 60.3 nm时,裂纹直接穿透纤维,纳米复合材料表现出典型的脆性断裂方式； t =4.0 nm时,裂纹发生偏转, SiC纳米纤维发生拔出现象,此时纳米复合材料的拉伸强度约为无涂层纳米复合材料的4倍,断裂能则提高一个数量级.计算结果表明, a-C涂层的厚度是SiC纳米纤维/SiC纳米复合材料中产生韧性机理的重要因素,即传统微米级陶瓷基复合材料的增韧理论在纳米复合材料中仍适用.研究结果可望为设计同时具有高强度、高韧性的陶瓷基纳米复合材料提供理论基础.
Abstract:
Fracture behavior and mechanical properties of SiC nanofiber (SiCNF) reinforced SiC nanocomposites as influenced by the thickness of amorphous carbon (a-C) coatings are studied via molecular dynamics simulations using Tersoff potential. To simulate the condition that a matrix crack arrives at the interface between matrix and coating, a pre-setting matrix crack is created. Results show that the tensile stress-strain curve of nanocomposites without and/or with thin a-C coatings (e.g., t 6 0.3 nm) demonstrates an abrupt drop after achieving a maximum value, while nonlinear tails appear in the curves of nanocomposites with thick a-C coatings (e.g., t > 2.0 nm). It is demonstrated that the SiCNF is penetrated by the matrix crack when it is uncoated and/or coated by a thin a-C layer (t 6 0.3 nm) and the nanocomposite fails in a typical brittle mode;whereas the crack deflection path changes and the SiCNF is pulled out from the matrix when the a-C coatings are thick enough (e.g., 4 nm), showing a different fracture mode in nanocomposites. Compared to nanocomposites without an a-C coating, the tensile strength of nanocomposites with a-C coating of 4.0 nm thickness is about four times higher, and the fracture energy increases around an order of magnitude. Furthermore, the average stress concentration factor for SiCNF in nanocomposites, defined as the ratio of tensile strength of single SiCNF to the average stress of the nanofiber in the composite when it is broken, is extracted and shows a decreasing trend with increasing coating thickness, indicating that a-C coating can therefore be expected to simultaneously enhance the tensile strength and fracture energy of the SiCNF/SiC nanocomposites. This work sheds light on the toughening mechanism in SiCNF/C/SiC nanocomposites where a-C coating plays a significant role, indicating that the toughening mechanism in conventional ceramic matrix composites on a microscale is still valid on a nanoscale. Simulation results suggest that coating thickness in material design is e?cient for engineering SiCNF/SiC nanocomposites with high strength and toughness.
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