纳尺度摩擦润滑机理的研究
发布时间：2018-10-13 11:17
【摘要】：微纳机电系统(M/NEMS)技术的快速发展促进了对原子级摩擦力的研究。作为M/NEMS基础的摩擦学问题使人们渴望对摩擦机理有更加深入的了解并尝试去控制这些设备的摩擦特性。本文在开放的环境条件下应用理论和实验方法且在超高真空(UHV)条件下进行分子动力学模拟(MD)来共同研究原子级摩擦性能。首先,运用原子力显微镜(AFM)在正常环境中研究了不同的相对湿度(RH)下由机械振动所引起的流体润滑,热润滑和动态润滑。实验结果显示：低温状况下高湿度会增强流体润滑效果而高温状况下湿度高则会由于形成流体桥接来降低热润滑效果。无论高温还是低温,动态润滑对于摩擦力的影响保持一致,即无论在任何RH值下增加针尖振动的振幅都会使摩擦力减小。有趣的是对于动态润滑作用来讲,在高温状况下,湿度越高摩擦力越大而低温状况下湿度越低摩擦力越大。本文还提出了考虑湿度的二维动态模型以进一步说明自然环境下的摩擦机理。其次,应用分子动力学模型本文还模拟了硅探针在金刚石基底上滑动来研究摩擦时效。模拟实验表明：真空中接触加强主要是由表面二聚作用引起的。随着温度升高,由于晶体序列由(1×1)转换到(2×1),针尖和基底之间的接触也由非公度转变为公度。接触增强和热润滑效果的结合导致平均摩擦力随着温度的升高呈非单调性变化。然而,由表面二聚作用引起的摩擦力增强趋势可能会在很大程度上受到结构润滑作用抑制.第三点,本文在超真空环境下,通过向MD模拟的探针施加正弦变化的法向作用力来仿真原子力显微镜中的动态润滑作用并对其进行研究。为了找出随着振荡增加使摩擦力明显减小的频率,作者还计算了声子态密度(DOS)。最先发现该频率可能位于探针和样品态密度曲线的狭小交汇处,但是THz的高频会略微地减小摩擦力,因为作用力相互排斥的情况下这种高频在应用的幅值范围内会促使针尖快速振动。因此,作者暂时得出以下结论：此现象是之前提到的高频下摩擦力增强的原因。此外,将频率降低到GHz接近扫描频率,即使在很低的振荡幅值下由于针尖脱离接触面依然会造成摩擦力急剧下降。在大的振幅下,探针在排斥和吸引两种作用下都会振动。最后,振荡频率在GHz情况下,结果显示在一个往复周期内针尖从样品撤回的过程中,滑动的势垒增高而且振幅越高要跨越的能量势垒越高；另一方面,在针尖接触样品的过程中,振幅增强使得针尖转变从而导致能量势垒降低。本文试图回答微纳摩擦学领域的一些关键问题来帮助理解原子级摩擦的某些方面。
[Abstract]:The rapid development of micro-nano-electromechanical system (M/NEMS) technology has promoted the study of atomic-level friction. Tribology, which is the basis of M/NEMS, makes people eager to have a deeper understanding of the friction mechanism and try to control the friction characteristics of these devices. In this paper, the atomic friction properties are studied in open environment by using theoretical and experimental methods and molecular dynamics simulation (MD) under ultra-high vacuum (UHV) conditions. Firstly, the fluid lubrication, thermal lubrication and dynamic lubrication caused by mechanical vibration at different relative humidity (RH) in normal environment were studied by atomic force microscope (AFM). The experimental results show that high humidity at low temperature will enhance the lubricating effect of fluid, and high humidity at high temperature will reduce the effect of thermal lubrication because of the formation of fluid bridging. The effect of dynamic lubrication on friction force is consistent with that of high temperature or low temperature, that is to say, increasing the amplitude of tip vibration at any RH value will decrease the friction force. It is interesting that for dynamic lubrication, the higher the humidity is, the greater the friction force is at high temperature, and the higher the friction force is at low temperature. A two-dimensional dynamic model considering humidity is also proposed to further explain the friction mechanism in natural environment. Secondly, the molecular dynamics model is used to simulate the sliding of silicon probe on diamond substrate to study the friction aging. The simulation results show that the contact strengthening in vacuum is mainly caused by surface dimerization. With the increase of temperature, the contact between the tip and the substrate is changed from incommensurate to incommensurate because the crystal sequence changes from (1 脳 1) to (2 脳 1). The combination of contact reinforcement and thermal lubrication results in the non-monotonic variation of the average friction force with the increase of temperature. However, the increasing trend of friction induced by surface dimerization may be restrained to a large extent by structural lubrication. Thirdly, the dynamic lubrication in atomic force microscope (AFM) is simulated and studied by applying sinusoidal force to the probe simulated by MD in the hypervacuum environment. In order to find out the frequency at which the friction force decreases obviously with the increase of oscillation, the density of phonon states (DOS). Is also calculated. It was first found that the frequency may be located at the narrow junction of the density of states curve between the probe and the sample, but the high frequency of THz reduces the friction slightly. Because the force repel each other, the high frequency will cause the tip to vibrate rapidly in the range of the applied amplitude. Therefore, the author draws the following conclusion temporarily: this phenomenon is the reason of friction enhancement at high frequency mentioned earlier. In addition, reducing the frequency to GHz close to the scanning frequency, even at very low oscillatory amplitude, the friction force will decrease sharply because the tip of the needle detaches from the contact surface. At large amplitudes, the probe vibrates under both repulsive and attractive effects. Finally, when the oscillation frequency is in the case of GHz, the results show that during a reciprocating period the tip of the needle retracts from the sample, the sliding barrier increases and the amplitude increases, the higher the amplitude, the higher the energy barrier; on the other hand, In the process of needle contact, the increase of amplitude makes the tip change and lead to the decrease of energy barrier. This paper attempts to answer some key questions in the field of micro-nano tribology to help understand some aspects of atomic level friction.
【学位授予单位】：东南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TH117.2
，
本文编号：2268417
[Abstract]:The rapid development of micro-nano-electromechanical system (M/NEMS) technology has promoted the study of atomic-level friction. Tribology, which is the basis of M/NEMS, makes people eager to have a deeper understanding of the friction mechanism and try to control the friction characteristics of these devices. In this paper, the atomic friction properties are studied in open environment by using theoretical and experimental methods and molecular dynamics simulation (MD) under ultra-high vacuum (UHV) conditions. Firstly, the fluid lubrication, thermal lubrication and dynamic lubrication caused by mechanical vibration at different relative humidity (RH) in normal environment were studied by atomic force microscope (AFM). The experimental results show that high humidity at low temperature will enhance the lubricating effect of fluid, and high humidity at high temperature will reduce the effect of thermal lubrication because of the formation of fluid bridging. The effect of dynamic lubrication on friction force is consistent with that of high temperature or low temperature, that is to say, increasing the amplitude of tip vibration at any RH value will decrease the friction force. It is interesting that for dynamic lubrication, the higher the humidity is, the greater the friction force is at high temperature, and the higher the friction force is at low temperature. A two-dimensional dynamic model considering humidity is also proposed to further explain the friction mechanism in natural environment. Secondly, the molecular dynamics model is used to simulate the sliding of silicon probe on diamond substrate to study the friction aging. The simulation results show that the contact strengthening in vacuum is mainly caused by surface dimerization. With the increase of temperature, the contact between the tip and the substrate is changed from incommensurate to incommensurate because the crystal sequence changes from (1 脳 1) to (2 脳 1). The combination of contact reinforcement and thermal lubrication results in the non-monotonic variation of the average friction force with the increase of temperature. However, the increasing trend of friction induced by surface dimerization may be restrained to a large extent by structural lubrication. Thirdly, the dynamic lubrication in atomic force microscope (AFM) is simulated and studied by applying sinusoidal force to the probe simulated by MD in the hypervacuum environment. In order to find out the frequency at which the friction force decreases obviously with the increase of oscillation, the density of phonon states (DOS). Is also calculated. It was first found that the frequency may be located at the narrow junction of the density of states curve between the probe and the sample, but the high frequency of THz reduces the friction slightly. Because the force repel each other, the high frequency will cause the tip to vibrate rapidly in the range of the applied amplitude. Therefore, the author draws the following conclusion temporarily: this phenomenon is the reason of friction enhancement at high frequency mentioned earlier. In addition, reducing the frequency to GHz close to the scanning frequency, even at very low oscillatory amplitude, the friction force will decrease sharply because the tip of the needle detaches from the contact surface. At large amplitudes, the probe vibrates under both repulsive and attractive effects. Finally, when the oscillation frequency is in the case of GHz, the results show that during a reciprocating period the tip of the needle retracts from the sample, the sliding barrier increases and the amplitude increases, the higher the amplitude, the higher the energy barrier; on the other hand, In the process of needle contact, the increase of amplitude makes the tip change and lead to the decrease of energy barrier. This paper attempts to answer some key questions in the field of micro-nano tribology to help understand some aspects of atomic level friction.
【学位授予单位】：东南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TH117.2
，
本文编号：2268417
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