考慮塗層硬度與黏附效應之橢圓粗糙表面碎形微接觸模式及其碎形參數之實驗分析

Fractal Contact Properties of Ellipsoids Rough Surface with Adhesion and Coating Hardness Effect and Experimental Analysis of Parameters

古明曜(Ming-Yao Ku)

國立虎尾科技大學/工程學院/動力機械工程研究所/碩士(2006年)

摘要

大部分加工表面粗度具有加工工具的運動方向，以致在不同的方向上有不同的曲率半徑，非等向之橢圓波峰較接近實際粗糙表面。常見精密機械為減少接觸面之黏附與磨損現象，多於表面塗佈適當材料，以致元件硬度隨著厚度有所改變。本文利用表面形貌的碎形特性研究界面間之接觸性能，建立兩個新的橢圓粗糙表面微接觸模式。為說明本文模式之實際應用技巧，並利用實驗以評述模式中需要的元件表面碎形及材料參數的求法，進行實例分析。
新模式I比著名MB模式多考慮波峰彈塑性變形、橢圓波峰與塗層硬度效應。新模式II在原有的微接觸模式I中加入此種黏附效應。隨著尺度降低至奈米等級時，黏附現象顯現出對接觸界面影響的重要性，因此在精密機械或微機械之微米至奈米波峰表面時必須考量此種效應。分析結果顯示橢圓率增加、軟塗層與界面黏附力將引起真實接觸面積的增加，在黏附力越大，碎形粗度參數愈小與外力愈小時，其增加量亦增加，並且在黏附力固定時，模擬硬度曲線係數m值的減少，也將使真實接觸面積增加，另外真實接觸面積會隨著碎形維數由1.1的增加而增加，在碎形維數達到某ㄧ最大值時會達到極大值，然後隨著碎形維數的增加再減少，此臨界碎形維數受外力、碎形粗度參數、硬度對降伏應力之係數、材料特性等性質影響，不受表面能的變化而影響。此模式之分析結果比舊有模式分析結果更接近機械表面之接觸性質，可供未來進行製造與設計之分析參考。

並列摘要

Most machined surfaces are oriented with respect to the direction of the machine tools relative to workpieces. In such cases, the profile of the asperities generally contains various curvature for various directions. In accordance with these facts, anisotropic roughness must be considered. In order to minimize the adhesive and wear problems of precise machine, numerous coating methods are being designed to improve these surface properties. In this work, fractal characterization of surface topography is applied to the study of contact mechanics and developed two new elliptic contact models of rough surface. According to the experimental measurement of Silicon surface, fractal parameters were calculated to determine the contact characteristics of real surfaces.
New contact model I was presented incorporating the elastic-plastic transition deformation, elliptic summit and hardness effects in coating film from MB model. New model II also describes contact behavior between rough solids using the fractal model I that take into account the effect of adhesive properties. Adhesion occurs at the peaks of the asperities and is pronounced when the surface roughness effect is small. In order to account for the effects of asperities ranging from the nanometer to micrometer level in micro-machine or precision machine. Results show that larger real areas of contact are predicted due to the adhesive force, soft coating and high eccentricity of asperities. When the adhesive force value be fixed, the coefficient m of mimic hardness curve to reduce, will be induced the real contact area on the increase. In addition, the real contact area will increase with fractal dimension through increase of 1.1, up to some in fractal dimension Will reach maximum at the maximum, reduce with increase of fractal dimension. The critical value of fractal dimension is influenced by nondimensional external load, fractal roughness parameter, factor relating hardness to yield strength, material properties?and free of adhesive force? The analytical results of this fractal microcontact model are close to the real contact characteristics of machine surfaces. Numerical analysis and fractal parameters of experiments will be the base of manufacture and design in the future.

參考文獻

[1] Pashley, M. D., Pethica, J. B. and Tabor, D., 1980, “Adhesion and Micromechanical Properties of Metal Surfaces,” Wear, Vol. 100, pp. 7-13.

[2] Chowdhury, S. K. and Gbosh, P., 1994, “Adhesion and Adhesional Friction at the Contact Between Solids,” Wear, Vol. 174, pp. 9-19.

[3] Greenwood, J. A., 1997, “Adhesion of Elastic Spheres,” Proc. Roy. Soc. London, A453, pp. 1277-1297.

[4] Johnson, K. L. and Greenwood, J. A., 1997, “An Adhesion Map for the Contact of Elastic Spheres,” Journal of Colloidal Interface Sci., Vol.l92, pp. 326-333.

[5] Sahoo, P. and Roy Chowdhury, S. K., 1996, “A Fractal Analysis of Adhesive at The Contact Between Rough Solids,” Proc. Instn. Mech. Engrs., Vol.210, pp. 269-279.