The Influence of Preparation Conditions on the Structural Properties and Hardness of Diamond-Like Carbon Films, Prepared by Plasma Source Ion Implantation

At a Glance

Metadata	Details
Publication Date	2020-04-06
Journal	Coatings
Authors	R. Hatada, S. Flege, Muhammad Naeem Ashraf, Arne Timmermann, Christoph Schmid
Institutions	Technical University of Darmstadt
Citations	21
Analysis	Full AI Review Included

Executive Summary

This analysis summarizes the investigation into the influence of Plasma Source Ion Implantation (PSII) preparation conditions on the properties of hydrogenated Diamond-Like Carbon (DLC) films.

Core Method: DLC films were synthesized using unassisted PSII (plasma generated solely by the high voltage bias) with ethylene (C₂H₄) precursor gas, varying voltage type (DC/Pulsed), pressure, Argon addition, and sample holder geometry (plate/grid).
Key Finding (Counterintuitive): Films prepared under conditions that resulted in a lower sp² content (more DLC-like structure) exhibited both higher hydrogen content (up to 35 at.%) and higher hardness.
Structural Dominance: The study demonstrated that the specific carbon bonding structure (sp³ fraction) is a more critical determinant of film hardness than the absolute hydrogen concentration, contradicting typical expectations for a-C:H films.
Maximum Hardness: The highest hardness achieved was 22.4 GPa, placing these a-C:H films at the upper limit of the typical 10-20 GPa range.
Voltage Influence: DC voltage preparation generally yielded films with higher hardness and higher hydrogen content compared to pulsed high voltage preparation.
Tribological Performance: Most films exhibited excellent tribological properties, achieving very low steady-state friction coefficients, with the minimum recorded value being 0.049.

Technical Specifications

Parameter	Value	Unit	Context
Maximum Hardness	22.4	GPa	Achieved using DC voltage preparation.
Lowest Friction Coefficient	0.049	N/A	Steady-state value, primarily achieved with DC voltage films.
Hydrogen Content Range	22.7 to 35	at.%	Estimated empirically via Raman spectroscopy (log(N/S)).
Film Thickness Range	100 to 700	nm	Resulting from process times of 45-150 min.
DC Voltage Bias Range	-1.5 to -2.5	kV	Used for preparation of the hardest films.
Pulsed Voltage Bias Range	-10 to -18	kV	Pulse length: 40 µs; Repetition rate: 250 Hz.
Process Pressure Range	0.65 to 0.8	Pa	Total pressure maintained during deposition.
Ethylene (C₂H₄) Flow	6	sccm	Constant flow rate of the precursor gas.
Argon (Ar) Flow Rate	0.3 or 0.6	sccm	Added to C₂H₄ plasma gas to influence dissociation.
Maximum Deposition Rate	~10	nm/min	Achieved at -2.5 kV DC, 0.8 Pa, using the grid-type holder.
Raman Laser Wavelength	633	nm	Used for structural analysis (D-peak ~1330 cm^-1, G-peak ~1550 cm^-1).

Key Methodologies

The DLC films were prepared and characterized using the following methods:

Plasma Source Ion Implantation (PSII): Films were deposited in a homemade PSII setup utilizing an unassisted discharge. The high negative voltage applied to the sample holder served as the sole plasma source, coupling plasma generation and film deposition.
Precursor Gas Control: Ethylene (C₂H₄) was used as the primary precursor (6 sccm flow). Argon (Ar) was introduced (0.3 or 0.6 sccm) to study its effect on hydrocarbon dissociation and film properties, while maintaining constant total process pressure (0.65-0.8 Pa).
Bias Variation: Two distinct voltage regimes were explored: low DC voltage (-1.5 to -2.5 kV) and high pulsed voltage (-10 to -18 kV, 40 µs pulse length).
Sample Holder Geometry: Experiments compared a standard plate-type holder against a grid-type holder (92 mm diameter) known to increase deposition rate and lower ignition voltage.
Structural Analysis (Raman Spectroscopy): The relative sp³/sp² bonding ratio was assessed by fitting the Raman spectra (633 nm laser) to determine the I(D)/I(G) ratio, G peak position, and Full Width at Half Maximum (FWHM(G)).
Hydrogen Content Estimation: Hydrogen concentration (at.%) was empirically derived from the Raman spectra by calculating the ratio of the photoluminescence background (N) to the G peak maximum intensity (S), log(N/S).
Hardness Measurement (Nanoindentation): Hardness was measured using the continuous stiffness measurement (CSM) technique, with evaluation performed at 10% of the film thickness to minimize substrate influence.
Friction Testing (Ball-on-Disk): The friction coefficient was determined using a ball-on-disk tribometer (Tungsten Carbide ball, 1 N force) under ambient conditions (room temperature, 25% relative humidity).

Commercial Applications

The resulting hard, low-friction hydrogenated DLC (a-C:H) films are highly relevant for demanding engineering applications:

Tribological Coatings: Used extensively in components requiring low friction and high wear resistance, such as seals, bearings, and sliding surfaces.
Automotive Industry: Coating engine parts (e.g., piston rings, tappets, fuel injection components) to reduce friction losses and improve fuel efficiency.
Precision Manufacturing: Enhancing the durability and performance of cutting tools, molds, and dies, particularly where high surface smoothness is required.
Medical Devices: Applications in surgical tools and implants, benefiting from the low friction, high hardness, and potential biocompatibility of a-C:H films.
Microelectromechanical Systems (MEMS): Providing protective, low-wear coatings for microscopic moving parts where minimal adhesion and friction (0.049) are critical.
Protective Barriers: Utilizing the dense structure and good adhesion (promoted by the keV-range ion implantation during PSII) for corrosion and erosion protection on various substrates.

View Original Abstract

Diamond-like carbon (DLC) films were prepared from a hydrocarbon precursor gas by plasma source ion implantation (PSII), in which the plasma generation and the film deposition were coupled; i.e., the plasma was generated by the applied voltage and no additional plasma source was used. Several experimental parameters of the PSII process were varied, including the sample bias (high voltage, DC or pulsed), gas pressure, sample holder type and addition of argon in the plasma gas. The influence of the deposition conditions on the carbon bonding and the hydrogen content of the films was then determined using Raman spectroscopy. Nanoindentation was used to determine the hardness of the samples, and a ball-on-disk test to investigate the friction coefficient. Results suggest that films with a lower sp2 content have both a higher hydrogen content and a higher hardness. This counterintuitive finding demonstrated that the carbon bonding is more important to hardness than the reported hydrogen concentration. The highest hardness obtained was 22.4 GPa. With the exception of a few films prepared using a pulsed voltage, all conditions gave DLC films having similarly low friction coefficients, down to 0.049.

Tech Support

Original Source

DOI: https://doi.org/10.3390/coatings10040360

References

1997 - Thick stress-free amorphous-tetrahedral carbon films with hardness near that of diamond [Crossref]
1997 - Mechanical properties and Raman spectra of tetrahedral amorphous carbon films with high sp3 fraction deposited using a filtered cathodic arc [Crossref]
2013 - Effect of sp2/sp3 bonding ratio and nitrogen content on friction properties of hydrogen-free DLC coatings [Crossref]
1995 - Tribology of carbon coatings: DLC, diamond and beyond [Crossref]
2002 - Diamond-like amorphous carbon [Crossref]
2001 - Differentiating the tribological performance of hydrogenated and hydrogen-free DLC coatings [Crossref]
2012 - Properties of hydrogenated DLC films as prepared by a combined method of plasma source ion implantation and unbalanced magnetron sputtering [Crossref]
2001 - The role of hydrogen in tribological properties of diamond-like carbon films [Crossref]
2018 - Anode layer source plasma-assisted hybrid deposition and characterization of diamond-like carbon coatings deposited on flexible substrates [Crossref]