Nanotribology and Nanomechanics of Thin Films Including Material Characterization, Mechanical Wear, Adhesion and Lubrication
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The present work is dedicated to addressing nanotribological issues of ultra-thin (sub-10 nm) films at contacting interfaces. In devices such as micro-electro-mechanical systems (MEMS), thin films are deposited for specific functions. In some occasions, mechanical durability of the thin films is also important. Magnetic storage hard disk drives (HDD) are a good example where nanotribology at the head-disk interface (HDI) is extremely important. Especially in recent years, where the areal density increases exponentially and the write/read head has been brought as close to as less than 10 nm to the disk surface. As a result, direct contact is possible to occur at such small distance and such unfavorable contact will cause mechanical wear and demagnetization. Nanometer thick diamond like carbon (DLC) and lubricant films provide important protection and study of their failure mechanisms is necessary. The present thesis has conducted research to understand the nanotribology of thin films in the multilayered system used in HDDs. The majority of the work is measurement of the nanomechanical and nanotribological properties of the solid thin films with thickness of less than 20 nm. A method combining finite element analysis (FEA) and nanoindentation was proposed to extract nanomechanical properties from nanoindentation data for multilayered samples. A highly sensitive nanomechanical transducer was introduced to perform sub-5 nm shallow nanoindentation experiments on thin films deposited at different conditions. To study the tribological performance of DLC films at high temperatures up to 300 °C, the present work performs nanoscratch and nanowear tests on a 3-nm thick DLC film. The results show the wear rate of DLC films begin to increase abruptly at around 200°C and this degradation of wear resistance is irreversible. The present thesis also proposes a mathematical model to quantitatively predict the hydrodynamic lubrication effects of the molecularly thin lubricant between the head and the disk surfaces. After considering the nanorheological behavior of the lubricant, the model is able to make predictions of contacting forces and pressures and explain the tribological role of the lubricant in terms of continuum mechanics. Lastly, present thesis proposed a model considering Van der Waals forces between lubricants on the disk and on the head. The proposed model provides stricter criterion for onset of adhesion induced lubricant-transfer between the two wet surfaces and is in better agreement with Molecular Dynamics simulations than conventional models. In summary, the findings above center about nanomechanics and nanotribology at the interfaces of the magnetic storage hard disk. However, these findings can also extend their applications to other MEMS devices where tribology issues are of important concerns. The shallow nanoindentaton instrument and FEA-based characterization method can be applicable any other solid thin films. The high-temperature tribological properties of a ultra-thin DLC films utilize a unique test rig but the findings are generally instructive in understanding behaviors of DLC at high temperature. The nano-lubrication model for a lubricated single asperity can be an addition of current contact mechanics which usually neglects the presence of lubricants.
Zhang, Youfeng (2017). Nanotribology and Nanomechanics of Thin Films Including Material Characterization, Mechanical Wear, Adhesion and Lubrication. Doctoral dissertation, Texas A&M University. Available electronically from