【摘要】：微納機(jī)電系統(tǒng)(M/NEMS)技術(shù)的快速發(fā)展促進(jìn)了對原子級摩擦力的研究。作為M/NEMS基礎(chǔ)的摩擦學(xué)問題使人們渴望對摩擦機(jī)理有更加深入的了解并嘗試去控制這些設(shè)備的摩擦特性。本文在開放的環(huán)境條件下應(yīng)用理論和實(shí)驗(yàn)方法且在超高真空(UHV)條件下進(jìn)行分子動力學(xué)模擬(MD)來共同研究原子級摩擦性能。首先,運(yùn)用原子力顯微鏡(AFM)在正常環(huán)境中研究了不同的相對濕度(RH)下由機(jī)械振動所引起的流體潤滑,熱潤滑和動態(tài)潤滑。實(shí)驗(yàn)結(jié)果顯示：低溫狀況下高濕度會增強(qiáng)流體潤滑效果而高溫狀況下濕度高則會由于形成流體橋接來降低熱潤滑效果。無論高溫還是低溫,動態(tài)潤滑對于摩擦力的影響保持一致,即無論在任何RH值下增加針尖振動的振幅都會使摩擦力減小。有趣的是對于動態(tài)潤滑作用來講,在高溫狀況下,濕度越高摩擦力越大而低溫狀況下濕度越低摩擦力越大。本文還提出了考慮濕度的二維動態(tài)模型以進(jìn)一步說明自然環(huán)境下的摩擦機(jī)理。其次,應(yīng)用分子動力學(xué)模型本文還模擬了硅探針在金剛石基底上滑動來研究摩擦?xí)r效。模擬實(shí)驗(yàn)表明：真空中接觸加強(qiáng)主要是由表面二聚作用引起的。隨著溫度升高,由于晶體序列由(1×1)轉(zhuǎn)換到(2×1),針尖和基底之間的接觸也由非公度轉(zhuǎn)變?yōu)楣取＝佑|增強(qiáng)和熱潤滑效果的結(jié)合導(dǎo)致平均摩擦力隨著溫度的升高呈非單調(diào)性變化。然而,由表面二聚作用引起的摩擦力增強(qiáng)趨勢可能會在很大程度上受到結(jié)構(gòu)潤滑作用抑制.第三點(diǎn),本文在超真空環(huán)境下,通過向MD模擬的探針施加正弦變化的法向作用力來仿真原子力顯微鏡中的動態(tài)潤滑作用并對其進(jìn)行研究。為了找出隨著振蕩增加使摩擦力明顯減小的頻率,作者還計算了聲子態(tài)密度(DOS)。最先發(fā)現(xiàn)該頻率可能位于探針和樣品態(tài)密度曲線的狹小交匯處,但是THz的高頻會略微地減小摩擦力,因?yàn)樽饔昧ο嗷ヅ懦獾那闆r下這種高頻在應(yīng)用的幅值范圍內(nèi)會促使針尖快速振動。因此,作者暫時得出以下結(jié)論：此現(xiàn)象是之前提到的高頻下摩擦力增強(qiáng)的原因。此外,將頻率降低到GHz接近掃描頻率,即使在很低的振蕩幅值下由于針尖脫離接觸面依然會造成摩擦力急劇下降。在大的振幅下,探針在排斥和吸引兩種作用下都會振動。最后,振蕩頻率在GHz情況下,結(jié)果顯示在一個往復(fù)周期內(nèi)針尖從樣品撤回的過程中,滑動的勢壘增高而且振幅越高要跨越的能量勢壘越高；另一方面,在針尖接觸樣品的過程中,振幅增強(qiáng)使得針尖轉(zhuǎn)變從而導(dǎo)致能量勢壘降低。本文試圖回答微納摩擦學(xué)領(lǐng)域的一些關(guān)鍵問題來幫助理解原子級摩擦的某些方面。
[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.
【學(xué)位授予單位】：東南大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：TH117.2
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本文編號：2268417