Diamond-like carbon (DLC) coatings have attracted significant attention in various industrial and scientific applications, including mechanical seals, biomedical implants, and microelectronic devices, owing to their exceptional combination of high hardness, superior wear resistance, and high chemical stability. These properties primarily arise from the unique hybridized carbon bonding structure (sp3/sp2 ratio) and hydrogen content within the coatings. However, the performance of DLC coatings is highly sensitive to deposition parameters, such as plasma power, substrate bias, and precursor gas composition, which influence the chemical bonding structure and microstructure of the coatings.
In this study, the effects of hydrocarbon precursor hydrogen content on the structural and mechanical properties of DLC coatings deposited by direct current pulse plasma-enhanced chemical vapor deposition (DC-PECVD) were systematically investigated. Four distinct DLC coatings were synthesized with different precursor gases: pure acetylene (DLC-C2H2), hydrogen-diluted acetylene (DLC-C2H2+H2), pure methane (DLC-CH4), and hydrogen-diluted methane (DLC-CH4+H2). The influence of precursor hydrogen content on coating thickness, chemical bonding configuration (sp3/sp2 ratio), and mechanical properties (hardness and elastic modulus) was comprehensively characterized by scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and nanoindentation techniques.
The deposition rate of DLC coatings was found to be strongly dependent on the precursor gas composition. Specifically, the DLC-C2H2 coating exhibited the highest deposition rate (0.83 μm/h), which was approximately three times greater than that of the DLC-CH4 coating (0.28 μm/h). This disparity was attributed to the higher carbon density and more efficient plasma dissociation of C2H2 compared with CH4. Interestingly, hydrogen dilution had a negligible impact (<10%) on the deposition rates of both precursor systems, suggesting that hydrogen primarily influenced the chemical bonding structure rather than the growth kinetics. Raman spectroscopy revealed a clear correlation between precursor hydrogen content and the structural evolution of DLC coatings. As the hydrogen content increased, the intensity ratio of the D peak to the G peak (ID/IG) decreased, accompanied by a redshift in the G peak position. These observations aligned with the Casiraghi model, which described the structural transition from a more graphitic (sp2-rich) to a more diamond-like (sp3-rich) configuration with increasing hydrogen incorporation. XPS analysis demonstrated that the sp3 carbon fraction in DLC coatings exhibited a positive correlation with precursor hydrogen content. The highest sp3 content (14.5%) was achieved in the DLC-CH4+H2 coating, confirming the critical role of hydrogen in stabilizing sp3-hybridized carbon bonds. In contrast, the DLC-C2H2 coating, deposited under low hydrogen conditions, exhibited the lowest sp3 content (6.1%), indicating a predominantly sp2-bonded structure. Nanoindentation measurements revealed that the DLC-C2H2 coating possessed the highest hardness (26.9 GPa) and elastic modulus (240 GPa), despite its relatively low sp3 content. This finding challenged the conventional paradigm that high hardness in DLC coatings was solely governed by a high sp3 fraction. Instead, the results suggested that a cross-linked sp2 network, formed under low hydrogen conditions, contributed significantly to mechanical reinforcement; the cognition that the sp3 fraction determined the hardness of DLC coatings originated from hydrogen-free DLC coat and was not applicable to hydrogenated DLC coatings. Furthermore, hydrogen dilution was found to generally reduce coating hardness, particularly in the CH4-based system, where a notable 18% reduction in hardness was observed.
This study reveals how the hydrogen content in precursor gases and the resulting chemical bonds affect the properties of DLC coatings. Key findings show that using C2H2 instead of CH4 speeds up coating growth due to more efficient carbon delivery. While hydrogen helps form more sp3 bonds, high hardness can also come from a dense sp2 network - not just sp3 content. This means the current model linking structure to properties needs updating. Overall, tuning the gas mixture and hydrogen level allows custom-designed DLC coatings with improved mechanical and wear performance for specific applications.
Key words
diamond-like carbon /
PECVD /
Raman /
precursor /
sp3 /
hardness /
elastic modulus
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References
[1] ERDEMIR A, DONNET C.Tribology of Diamond-Like Carbon Films: Recent Progress and Future Prospects[J]. Journal of Physics D: Applied Physics, 2006, 39(18): R311-R327.
[2] VETTER J.60years of DLC Coatings: Historical Highlights and Technical Review of Cathodic Arc Processes to Synthesize Various DLC Types, and Their Evolution for Industrial Applications[J]. Surface and Coatings Technology, 2014, 257: 213-240.
[3] BEWILOGUA K, HOFMANN D.History of Diamond- Like Carbon Films - From First Experiments to Worldwide Applications[J]. Surface and Coatings Technology, 2014, 242: 214-225.
[4] YAMAUCHI Y, SASAI Y, KONDO S I, et al.Chemical Diagnosis of DLC by ESR Spectral Analysis[J]. Thin Solid Films, 2010, 518(13): 3492-3496.
[5] TANAKA A, NISHIBORI T, SUZUKI M, et al.Tribological Properties of DLC Films Deposited Using Various Precursors under Different Humidity Conditions[J]. Diamond and Related Materials, 2003, 12(10/11): 2066-2071.
[6] RAY S C, BHATTACHARYA G, MILLER M A, et al.A Facile Method for the Deposition of Thermally Stable Diamond Like Carbon Thin Films via Carbon Dioxide Precursor Gas[J]. Diamond and Related Materials, 2017, 73: 93-98.
[7] KIM T G, OH J S, YOON H K, et al.Characteristics of Dlc Films Incorporated HMDS by RF Pecvd[J]. International Journal of Modern Physics B, 2010, 24(15/16): 2999-3004.
[8] AL MAMUN M A, FURUTA H, HATTA A. Pulsed-DC Discharge for Plasma CVD of Carbon Thin Films[J]. IEEE Transactions on Plasma Science, 2019, 47(1): 22-31.
[9] SU F H, CHEN G F, SUN J F.Synthesis of Hydrogenated DLC Film by PECVD and Its Tribocorrosion Behaviors under the Lubricating Condition of Graphene Oxide Dispersed in Water[J]. Tribology International, 2019, 130: 1-8.
[10] ANDERSSON J, ERCK R A, ERDEMIR A.Friction of Diamond-Like Carbon Films in Different Atmospheres[J]. Wear, 2003, 254(11): 1070-1075.
[11] ROBERTSON J.Diamond-Like Amorphous Carbon[J]. Materials Science and Engineering: R: Reports, 2002, 37(4/5/6): 129-281.
[12] FERRARI A C, ROBERTSON J.Interpretation of Raman Spectra of Disordered and Amorphous Carbon[J]. Physical Review B, 2000, 61(20): 14095-14107.
[13] SONG J, TIAN H P, LI J Z, et al.Effect of Methane and Acetylene Pre-Gases on the Corrosion Resistance of DLC Coatings[J]. Vacuum, 2024, 221: 112899.
[14] ERDEMIR A, ERYILMAZ O.Achieving Superlubricity in DLC Films by Controlling Bulk, Surface, and Tribochemistry[J]. Friction, 2014, 2(2): 140-155.
[15] HUANG Z H, CHEN Z J, LANG W C, et al.Adhesion Studies of CRC/a-C: H Coatings Deposited with Anode Assisted Reactive Magnetron Sputtering Combined with DC-Pulsed Plasma Enhanced Chemical Vapor Deposition[J]. Materials, 2021, 14(11): 2954.
[16] SUI X D, LIU J Y, ZHANG S T, et al.Microstructure, Mechanical and Tribological Characterization of CRN/ DLC/Cr-DLC Multilayer Coating with Improved Adhesive Wear Resistance[J]. Applied Surface Science, 2018, 439: 24-32.
[17] RITTIHONG U, AKASAKA H, EUARUKSAKUL C, et al.Synchrotron-Based Spectroscopic Analysis of Diamond- Like Carbon Films from Different Source Gases[J]. Radiation Physics and Chemistry, 2020, 173: 108944.
[18] 罗渝然. 化学键能数据手册[M]. 北京: 科学出版社, 2005.
LUO Y R.Handbook of Chemical Bond Energies[M]. Beijing: Science Press, 2005.
[19] MUTSUKURA N.Deposition of Diamondlike Carbon Film and Mass Spectrometry Measurement in CH4/N2 RF Plasma[J]. Plasma Chemistry and Plasma Processing, 2001, 21(2): 265-277.
[20] FUJIMOTO S, AKASAKA H, SUZUKI T, et al.Structure and Mechanical Properties of Diamond-Like Carbon Films Prepared from C2H2 and H2 Mixtures by Pulse Plasma Chemical Vapor Deposition[J]. Japanese Journal of Applied Physics, 2010, 49(7): 075501.
[21] PARAMANIK B, DAS D.Synthesis of Nanocrystalline Diamond Embedded Diamond-Like Carbon Films on Untreated Glass Substrates at Low Temperature Using (C2H2+H2) Gas Composition in Microwave Plasma CVD[J]. Applied Surface Science, 2022, 579: 152132.
[22] FERRARI A C, ROBERTSON J.Resonant Raman Spectroscopy of Disordered, Amorphous, and Diamondlike Carbon[J]. Physical Review B, 2001, 64(7): 075414.
[23] CASIRAGHI C.Effect of Hydrogen on the UV Raman Intensities of Diamond-Like Carbon[J]. Diamond and Related Materials, 2011, 20(2): 120-122.
[24] CASIRAGHI C, FERRARI A C, ROBERTSON J.Raman Spectroscopy of Hydrogenated Amorphous Carbons[J]. Physical Review B, 2005, 72(8): 085401.
[25] BEEMAN D, SILVERMAN J, LYNDS R, et al.Modeling Studies of Amorphous Carbon[J]. Physical Review B, 1984, 30(2): 870-875.
[26] ROBERTSON J.Mechanism of Sp3 Bond Formation in the Growth of Diamond-Like Carbon[J]. Diamond and Related Materials, 2005, 14(3/4/5/6/7): 942-948.
[27] FERRARI A C, LIBASSI A, TANNER B K, et al.Density, sp3 Fraction, and Cross-Sectional Structure of Amorphous Carbon Films Determined by X-Ray Reflectivity and Electron Energy-Loss Spectroscopy[J]. Physical Review B, 2000, 62(16): 11089-11103.
Funding
Wenzhou Major Science and Technology Innovation Project (ZG2023009)
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