Purposes of this study Outline of dissertation

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1.4 Purposes of this study

According to the previous studies that have been described in Section 1.3.1, impact wear mechanisms of DLC coatings are concerned with the fracture process in the coating due to the crack propagation under severe wear conditions. Besides, nobody will use the DLC under this condition that may lead to a catastrophic failure and should be avoided in most industrial cases. However, no reports discuss about how the impact wear mechanisms of DLC coatings acts under mild wear conditions. Under impact, it is well known that plastic deformation should occur prior to the impact wear. So, the first objective of this study is to identify the most significant impact parameter that controls the deformation of DLC coating. After cyclic impacts, though under mild wear conditions, it believes that impact wear should be occurring by the phase transformation of DLC coating. Therefore, the second objective is to clarify the impact wear mechanisms of DLC coating based on its phase transformation. Finally, in order to distinguish more clearly between the plastic deformation and impact wear of DLC coating as well as to predict its transition points, a deformation-wear transition map is proposed. In summary, the objectives of this study are as follows: a To identify the governing impact parameter from the viewpoint of mechanical aspects that affects the most on the deformation of DLC coating. a To investigate the phase transformation of DLC coating under cyclic impact loading. b To propose a deformation-wear transition map of DLC coating under cyclic impact loading. 20

1.5 Outline of dissertation

This dissertation specially describes the impact wear properties of DLC coating, in terms of its wear mechanisms under cyclic impacts, up to 10 5 impact cycles. There are five chapters and briefly outlined as in Fig. 1.12. The following chapters of this dissertation are as follows: Chapter 1 is an introduction to the industrial needs and its global issues especially under impact applications. Even though an alternative solution by using DLC coatings has practically applied in the industry, the DLC coatings still require extensive future research to provide optimum combination for different applications especially under mild wear conditions. In addition to the tribological properties of DLC coating for sliding contact, this chapter also describes the differences between impact wear of metals and DLC coatings under severe wear conditions based on the previous studies . Furthermore, the purposes of this study also clearly stated. Under impact, it is well known that plastic deformation should be occurred prior to the impact wear. Therefore, Chapter 2 aims to identify the most significant impact parameter that controls the deformation of DLC coating. The impact parameters only limited to the mechanical aspects such as maximum normal impact load, absorbed energy and contact impulse. Apart from the experimental results, an analytical approximation of the residual impact crater volumedepth and absorbed energy for assigned maximum normal impact load is presented. After cyclic impacts, though under mild wear conditions, it believes that impact wear should be occurring by the phase transformation of DLC coating. Therefore, Chapter 3 is meant to clarify the impact wear mechanisms of DLC coating under cyclic impact loading based on its phase transformation. In this chapter, Raman spectroscopy is used to investigate the phase transformation of DLC coating. The relationship between the Raman properties such as I D I G ratio, FWHM G and G peak frequency with the sp 2 and sp 3 content in the DLC coating on the crater surface, wear debris as well as in the transfer layer play an important role in achieving the objective of this chapter. Additionally, in order to distinguish more clearly between the plastic deformation and impact wear of the DLC coating as well as to predict its transition points, Chapter 4 proposes a deformation-wear transition map of DLC coating under 21 cyclic impact loading based on empirical approach. This chapter goes on to describe the role that transition map plays in predicting the transition of deformation to wear of the DLC coating during impact systematically for future design purposes. Chapter 5 concludes the major findings of this work and their context in current research. 22 Fig. 1.12 Outline of dissertation.

Chapter 1 Introduction