| Friction is one of the most familiar physical phenomena and has been under investigation for centuries due to its importance in industry and our daily life. However, Friction phenomena are so complicated that its mechanism not properly understood till now. Friction force cannot be theoretically calculated, but has to be measured in practice. Therefore, it is imperative to establish a reasonable model to enable correctly numerical calculation to control friction. In the present paper, based on the viewpoint of quantum mechanics and thermo dynamics coupling, a quantum harmonic oscillator model is proposed for the calculation of friction with the influence of normal load, velocity and temperature rising in focus. These results will hopefully provide theoretical references to tribology design.Firstly, based on the perspective of vibration energy converting into interfacial temperature rising, a model of temperature rising calculation for the interfacial atoms has been proposed, with analyzing the microscopic mechanism of forced vibration of interfacial atoms under a contact potential field. From the research, it is shown that temperature rising of interfacial atoms is closely related to the contact potential amplitude, relative sliding velocity, atomic force constant and crystal lattice constant but atomic mass is a less important factor.Secondly, a Quantum thermodynamic model of sliding friction force calculation is proposed based on the principle that friction power dissipates into thermal vibration of interfacial atoms. Calculation for the interfacial model of the single-asperity contact indicates that the friction force increases approximately linearly to the normal road, while the rigid linear correlation with the real contact area is not held, i.e., sliding friction coefficient is not a constant as well known. The research of thermal activation effects on the sliding friction force shows that the friction force logarithmically increases with the velocity, and decreases by a less extent as temperature increases.Thirdly, a model of phonon energy dissipation is established, applying Quantum Mechanics and Crystal Lattice Dynamics. The research indicates that phonon energy dissipation takes up to 90 per cent of the interfacial energy loss, and that the phonon frequency is a decisive influencing factor for friction energy dissipation, i.e., the higher phonon frequency is the faster the energy dissipates.Fourthly, the temperature effect on vibration energy levels of interfacial atoms has been investigated, which shows that when the temperature is under 100K, the probability for a quantum harmonic oscillator being in the activated states rises with temperature, causing the friction coefficient to rise with temperature. When the temperature is around 100K, the probability for a harmonic oscillator being in the activated states reaches the peak, causing a peak friction coefficient at this point. When the temperature is above 100K, the friction coefficient decreases with the temperature. Furthermore, the thermal effect on microscopic interfacial friction, acting with temperature impact material mechanical properties, has been also discussed. Some conclusions were obtained with the other parameters fixed, while temperature rising increases the real contact area, but the friction force decreases. This implies that the critical shear strength in Bowden's adhesion theory is not a constant.In the end, AFM experiments have been carried out to verify the above theory and calculation, emphasizing on the effects of the normal load, real contact area, sliding velocity and temperature on friction force. Comparisons between theoretical calculations and experimental results indicate to be a good agreement in trends, proving the validation of theory and method presented in this dissertation. |
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