Graduation Semester and Year

2021

Language

English

Document Type

Thesis

Degree Name

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

First Advisor

Efstathios I Meletis

Abstract

Diamond like carbon (DLC) films have been extensively used in a wide range of applications over the last decade due to their excellent mechanical and tribological properties. Metallic elements are used to dope DLC films in an effort to overcome some DLC limitations and make it compatible to use it in larger variety of applications. This study focuses on the effect of different processing parameters on DLC deposition, and the effect of Mo content on microstructure, mechanical and tribological properties. Plasma Enhanced Chemical Vapor Deposition (PECVD) technique coupled with magnetron sputtering is used in chamber atmosphere composed of CH4 and Ar to synthesize DLC films with various Mo content. Mo-DLC films were deposited using pulsed DC bias on substrate with DC power on target, and DC bias on substrate with pulsed DC power on target. These DLC and Mo-DLC films were characterized by optical profilometer, Scanning Electron Microscopy (SEM), Electron Dispersive Spectroscopy (EDS), Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), and Raman spectroscopy. Nano-indentation tests were carried out to assess the hardness of the Mo-DLC films. Tribological tests were conducted with and without oil to study the steady state Coefficient of Friction (COF) and wear rate of DLC and Mo-DLC films as a function of Mo content. Film thickness, topography, and Mo content results from profilometry, and SEM/EDS study revealed that only deposition rate is affected by target power type, and neither surface finish nor Mo content are affected. The maximum deposition rate of 33 nm/min and 27 nm/min were achieved for pulsed DC power and DC power on the Mo target, respectively. Residual stress measurements showed a significant reduction with increasing Mo content from 1.5 GPa in pure DLC to 0.3 GPa for 16.9 at.% Mo-DLC. AFM measurements showed very smooth film surfaces with roughness values around 14 nm and 17 nm for 4.2 at.% Mo and 11.8 at.% Mo, respectively. TEM observation and SAD pattern analysis revealed that Mo is present as nano particles that were distributed uniformly in amorphous DLC matrix forming a cubic MoC phase. With increase in Mo content from 4.2 at.% to 11.8 at.% the MoC grain size increased from 2.20 nm to 3.10 nm while the MoC clusters remained surrounded by graphitic boundaries. Diffraction peak position and FWHM of peaks from XRD study also confirmed the formation of cubic MoC with grain size 2.89 nm and 3.05 nm for 7.5 at.% Mo and 11.8 at.% Mo, respectively. The Raman spectra revealed that sp2 content in DLC film increases with increase in Mo content. With doping 4.2 at.% Mo in DLC, hardness decreases from 14.20 GPa to 12.60 GPa which remains almost the same with further increase in Mo content. The lowest steady state COF of Mo-DLC films for dry sliding was 0.115 while that for DLC was 0.1. The lowest specific wear rate in dry sliding it was observed for 6 at.% Mo-DLC (24*10⁻⁸ mm³/N-m) that compared well with that of pure DLC (7*10⁻⁸ mm³/N-m). This behavior is consistent with higher sp² content resulting from Mo doping. In oil lubricated sliding, low COF were observed for both Mo-DLC and DLC (0.065 and 0.085, respectively) with no signs of wear even after testing for 3000 m sliding distance.

Keywords

Mo-Diamond like carbon films, Wear, Microstructure

Disciplines

Engineering | Materials Science and Engineering

Comments

Degree granted by The University of Texas at Arlington

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