IV. Properties of Diamond-like Carbon (DLC) films Deposited by Pulsed Laser Deposition
4.3 Results and Analysis
4.3.3 Adhesion Strength
Indentation experiments are conducted with a load-cell typed Rockwell hardness tester (HR- 522, Mitutoyo, Japan.) Interfacial cracks are induced by loading a Rockwell C type diamond indenter with three discrete indentation loads of 600, 1000, and 1500 N. After indentations, optical micrographs of indents are taken to measure crack radii. Areas of cracked regions are measured with an image analyzer, Matrox Inspector, and then, assuming the crack is propagated in a circular shape, crack radii are calculated. As stated in section III, for more accurate results, two higher indentation loads (1000, 1500 N) are selected to calculate dP/dc. As seen in Figure 26, at the highest substrate temperature and with lower laser power, the film has the largest value of dP/dc. Values of dP/dc range from 2.04 /N mm to 8.43 /N mm. Comparing to residual stresses and sp3 contents of deposited films with variety of deposition parameters, it is shown that the adhesion strengths behave in an opposite way against residual stresses or sp3 contents. Measured values of dP/dc of DLC films are summarized in Table 9.
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Figure 26. 3D contour plot of dP/dc.
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Figure 27. Optical micrographs after Rockwell indentations. Cracks are induced with 600 N of indentation load.
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Table 7. Measured sp3 contents of DLC films. Units are %.
25 °C 317 °C 609 °C 900 °C
4 W 45.77 39.61 38.78 38.2
3 W 43.19 36.71 36.3 36.13
2 W 42.36 35.7 35.26 34.89
1 W 40.78 33.63 33.85 33.76
Table 8. Calculated compressive residual stresses of DLC films. Units are GPa.
25 °C 317 °C 609 °C 900 °C
4 W 0.299 0.245 0.242 0.234
3 W 0.268 0.228 0.226 0.224
2 W 0.262 0.159 0.104 0.078
1 W 0.252 0.104 0.100 0.045
Table 9. Measured dP/dc values of DLC films. Units are N/μm.
25 °C 317 °C 609 °C 900 °C
4 W 2.038 2.921 3.378 3.647
3 W 2.241 3.713 4.145 4.566
2 W 2.277 4.889 5.041 5.183
1 W 2.348 5.471 7.744 8.435
4.4 Conclusion
In this section, sp3 content, residual stress, and adhesion strength of DLC films deposited with different deposition parameters are examined. For this section, a summary of results is as follows:
1. To obtain the highest value of sp3 content of DLC films deposited with PLD technique, higher laser power and lower substrate temperature is required experimental conditions.
Since only 355 nm beam is used in this thesis, wavelength effect on sp3 content is unknown in this moment.
2. The compressive residual stresses behave in same way with sp3 content, but the adhesion characteristics behave in an opposite way of sp3 content.
3. When sp3 content is high, adhesion strength is weak and its compressive residual stress is large. From the results, higher sp3 content induces larger compressive residual stress inside DLC films and this will be one of the major reasons for bad adhesion of DLC films.
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ACKNOWLEDGEMENTS
I would never have been able to finish my thesis without the help from the glorious Lord, guidance of my advisor and committee members, supports from family, friends and colleagues.
First, I would like to give glory to the Lord. Without His help, completion of this thesis would not be possible.
I would like to express my deepest gratitude to my advisor, Prof. Hyungson Ki, for his excellent guidance, caring, patience, and support. His knowledge and commitment to the highest standards inspired, motivated and led me. I could not have imagined a better advisor for my M.S.
study in UNIST.
Besides my advisor, I also would like to appreciate my thesis committee members: Prof.
Jaesung Jang and Prof. Heungjoo Shin, for their insightful comments, and their encouragement.
My sincere thanks also go to my colleagues in UNIST. Especially, I really appreciate my labmates: Sangwoo So, Hongrae Cho, Jaehun Kim, Chun Deng, Sanseo Kim, Keunhee Lee and Haram Yeo, for stimulating discussions and supports.
I would like to thank my friends: Jaedoo Lee and Kwangjin Hong for their kindness, friendship and support.
Last, but by no means least, I really appreciate my parents, Kyung Sik Won and Shin Hyang Kim, and my younger brother, Yoo Jin Won. They have been always there cheering me up and stood by me through my M.S. courses.