由微觀力學探討硬鍍層厚度對材料強度影響

隨著科技朝向精密、可靠與微小化的趨勢需求，在物件界面常會塗上一層硬薄膜層，達到提升抵抗變形之能力及增加抗磨耗的特性，又材料縮小至分子與巨觀尺度之間時，材料的性質都與同種物質塊材的性質會有所不同，所以了解不同尺度下材料的機械性質就顯得非常重要，而微觀下的真實接觸表面(True Contact Surface)是呈現高低起伏的，因此研究在不同曲率半徑之鍍膜粗糙峰真實接觸與壓痕量測(Indentation measurement)，以及不同薄膜厚度對於粗糙峰的壓痕硬度(Indentation Hardness)和降伏應力影響，是亟待探討的重要議題。本研究是利用非線性之有限元素法(Finite Element Method)的計算架構建立了原子模型之有限單元法(Atomic Model FEM)，探討在純銅的表面上添加一層硬度較高的鎳薄膜層時，壓痕硬度所能得到變化之趨勢以及薄膜粗糙峰的變形機制、應力分布和原子滑移向量(Slip vector)，並且剛性平板對薄膜粗糙峰進行接觸行為，可由發生初始原子滑移造成塑性變形之概念計算起始的降伏應力值，此結果將可以和壓痕硬度對照分析，而此模擬的方式屬於半靜態模擬，因此不需要考慮剛性平板與壓痕器進行壓痕時的速度問題，可以有效的減少電腦運算時所花費的時間。其結果顯示在剛性平板於小半徑之薄膜粗糙峰接觸條件下，其降伏應力因粗糙峰曲率過大其變形機制發生過快，而未有明顯的應力趨勢；將薄膜粗糙峰半徑增加後進行剛性平板接觸，發現其降伏應力隨著薄膜厚度增加達到最高降伏應力後，薄膜厚度繼續增加而應力開始呈現下降趨勢。接下來將剛性平板換成壓痕器(Indenter)進行壓痕量測硬度，發現在低薄膜厚度時粗糙峰內部原子滑移比起高薄膜厚度來的劇烈，而高密度的原子滑移過程造成能量釋放，故硬度隨著薄膜厚度增加而上升；將薄膜粗糙峰半徑增加至趨近平坦之平面進行壓痕量測，發現硬度隨著薄膜厚度的增加先上升後下降，而最高硬度值的薄膜厚度也是最大的降伏應力，意即此薄膜厚度下需要較大的應力才能使其發生塑性變形，持續增加薄膜厚度並不會增加其降伏應力值而提高硬度，故有最佳膜厚之現象產生。

關鍵字

奈米壓痕硬度；初始塑性變形之降伏應力；原子模型之有限單元法；不同半徑之薄膜粗糙峰；不同厚度之薄膜層

並列摘要

With the trend in technology toward precision, reliability and miniaturization, the object surface usually coated with hard film layer, achieve the characteristics of deformation-resistance and anti-wear. Materials down to between molecular and macro scale, the nature of materials may be different with the one of bulk materials. To understand the different scale mechanical properties of materials is very important. Because the true contact surface is rough at the microscopic, to study the indentation measurements in different curvatures of the coated asperities, the effect of film thicknesses on indentation hardness and yield stress is an urgently important issue to discuss. This study is using the architecture of nonlinear finite element method to implement the atomic simulations and explore the question of adding a hard coating of nickel on the surface of copper. The trend of indentation hardness, and deformation mechanisms, stress distribution and atomic slip vector of coated asperity are revealed in the simulations. The concept of using the initial atomic slip to calculate the initial yield stress during the contact between a rigid flat and a coating asperity can be contrast with the indentation hardness. This is a quasi-static simulation, so there is no need to consider the indentation speed of rigid flat indenter. Therefore, it can reduce the time for calculation effectively. The results show that for the contact between a rigid flat plane and a small radius coated asperity, there is no obvious variation of yield stress because the deformation mechanism is too fast to occur. For the contact between a rigid flat plane and a big radius coated asperity, we can find that the yield stress increases with the thickness up to a critical coating thickness, and then the stress began to show a downward trend with the coating thickness is increased continuously. Afterwards we use indenter to measure indentation hardness. The atomic slip in the low coating thickness is more intense than high coating thickness. High density of atomic slips causing the release of energy, the indentation hardness increases consequently with the coating thickness. As the asperity radius increased to approach a plane, the hardness increased with coating thickness revealing a trend of rise-first-then-fall, and the highest hardness value of the coating thickness is also the largest yield stress. This means that the larger coating thickness needs larger stress to make plastic deformation proceed, the yield stress value does not increase with the coating thickness accordingly. Therefore, there exists the phenomenon of optimum film thickness.

並列關鍵字

Nano-Indentation Hardness ； Yield Stress of the Initial Plastic Deformation ； Atomic Model FEM ； Different Radius of the Coating Asperity ； Different Thickness of Coating layer

參考文獻

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[7] Q. H. Tang and T. C. Wang, “Deformation twinning and its effect on crack extension”, Acta Mater., Vol. 46, No. 15, pp. 5313-5321,1998

[8] T. Nozaki, M. Doyama, Y. Kogure, and T. Yokotsuka ,“Micromachining of pure silicon by molecular dynamics”, Thin Solid Films, Vol. 334, No. 1-2, pp. 221-224, 1998

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由微觀力學探討硬鍍層厚度對材料強度影響

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