Significance of grain and grain boundary characteristics of ultrananocrystalline diamond films and tribological properties

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• 作者：K.J. Sankaran ; N. Kumar ; S. Dash ; H.C. Chen ; A.K. Tyagi ; N.H. Tai ; I.N. Lin
• 关键词：Ultrananocrystalline diamond films ; Microstructure ; Grain boundary ; Tribological properties
• 刊名：Surface and Coatings Technology
• 出版年：2013
• 出版时间：15 October, 2013
• 年：2013
• 卷：232
• 期：Complete
• 页码：75-87
• 全文大小：3836 K

文摘

Tribological properties of plasma chemistry dependent ultrananocrystalline diamond (UNCD) films deposited by microwave plasma enhanced chemical vapor deposition system are studied using 100Cr6 steel ball as a sliding counter element. UNCD films synthesized with 1.5 % H₂ in Ar/CH₄ plasma consist of large number of clustered diamond grains with small volume fraction of grain boundaries. Decrease in grain boundary volume fraction resulted in simultaneous decline in trans-polyacetylene (t-PA) chain and sp² bonded amorphous carbon (a-C) phase fraction. However, UNCD films grown in Ar/CH₄ plasma shows formation of UNCD grains with high volume fraction of grain boundaries, large amount of sp² bonding, a-C and network of t-PA chains. The t-PA passivates the dangling bonds and sp²/a-C phase that externally reduces sliding shear resistance, thereby, resulting in ultra-low friction behavior of UNCD films deposited from Ar/CH₄ plasma medium. Load dependent frictional behavior of these films was investigated in present study. These films tested under both low and high normal loadings showed high friction coefficient. Capillary dominated surface interaction and plastic deformation based models were found to be appropriate for describing friction behavior under low and high loads, respectively. However, at specific and intermediate normal loads, both low and ultra-low friction coefficients could be explained through elastic contact model that is accounted by the presence of lubricious phase of sp² and a-C. In addition, low friction coefficient is also governed by surface passivation mechanism. This mechanism is explained when friction test is carried out in controlled atmospheric condition. Investigating the tribological behavior of these films against steel ball is useful for implementing reliable micro-sliding based device applications.