ICMCTF2016 Session B3-2: Deposition Technologies and Applications for Diamond-like Coatings

Nitrogen doped diamond like carbon (N-DLC) and Silicon doped DLC (Si-DLC) thin films with different concentrations were prepared on Polyethylene terephthalate substrates using ion beam deposition from an end-hall ion source. The films were characterized by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS). The results indicate a higher concentration of sp³ carbon bonding in Si-DLC than in N-DLC thin films. Furthermore, XPS C1s peak analysis show Si-C (sp³) bonds in Si-DLC and C-N (sp³) and C=N (sp²) bonds in N-DLC films. Nano-indentation tests were used to measure the hardness and Young’s modulus of the coated samples while pin-on-disk was used to evaluate the tribological properties. DLC coating hardness decreases from 11.5 GPa for pure DLC to 7 GPa for N-DLC samples whereas the hardness of Si-DLC is low at low silicon concentration but radically increases with increasing silicon content. The coefficient of friction (COF) of N-DLC is lower than pure DLC. It has been also observed that formation of silicon oxide can reduce COF of Si-DLC. The coating roughness measured by an optical profiler decreases by increasing nitrogen concentration in N-DLC. This is probably because N doping reduces residual and thus makes film dense and smooth. Optical transparency measured by UV-Vis spectroscopy is higher for N-DLC and increases with the increase of nitrogen concentration.

During the past decade, the use of plastic components in various industrial applications has rapidly grown. The constantly rising demands for fuel efficiency in the automotive industry, for example, have forced all car manufacturers to increase the number of plastic components to reduce weight. This includes both interior and exterior parts. More and more of these components are PVD-coated and need to pass industrial standards for wear and chemical resistance. A possible way to improve their performance is to deposit on them hard PVD coatings as an alternative and addition to the wet electroplating. A key issue in this process is to assure the proper adhesion between the polymer surface and the PVD coating.

In this paper, the process of activation of polymer surfaces by plasma treatment and the subsequent deposition of metal and DLC (diamond-like carbon) coatings in an industrial PVD coater is presented. A comparison of the influence of the process gases – argon, nitrogen, oxygen and their combinations is given for two sources of plasma – a mild ionic source and a 2.44 GHz microwave plasma.

The deposition of the DLC-like layers by a microwave plasma generator with different properties, such as color, hardness and friction is demonstrated. A multilayers of DLC/SiO_x and their performance is also presented. The influence of the main process parameters is studied in detaill.

Results show the advantage of the microwave technology in terms of deposition rate, temperature control and possibility to efficiently coat nonconductive substrates with DLC-like layers.

Diamond-like carbon (DLC) has been studied and applied to many industrial products as a promising coating material due to its outstanding properteis such as exellent tribological properties, chemical inertness, good hardness and good transmittance. In this study, DLC coating with nanodiamond (ND) interlayer was applied to the glass substrate and its optical and structural properteis were investigated.

ND interlayer was deposited by dipping the glass substrate into PSS (poly sodium 4-styrenesulfonate) and ND mixed solution. The PSS/ND solution was produced by milling deionized water, PSS and ND mixtures with zirconia bead (radius=0.1-0.3 mm) for 6 hr and 1000 rpm. Then, the DLC layer was deposited on the substrate using direct current (DC) magnetron sputtering method at room temperature. The target is high purity (99.9%) graphite with 2 inch diameter and bonded with copper plate. The sample was placed on the rotating substrate holder for uniform deposition. Pure argon gas was used as sputtering gas, and the base pressure was maintained below 1.0x10^-5 Torr. The flow rate of argon gas was varied to give varying working pressure with stable plasma while DC power of 430 W was applied. The working pressure was varied between 1 and 10 mTorr and the deposition time was varied between 1 and 4 hr.

The transmittance was measured using spectroscopy. Transmittance of 80% can be achieved with the DLC thickness of 60 nm in visible light region, but its thickness can be increaed upto 120 nm with similar transmittance level by using ND interlayer. Raman analysis was used to investigate the bonding structure of DLC layers, and the surface roughness was measured with atomic force mircroscopy (AFM). Both DLC and DLC/ND coated layers showed smooth surface roughness with the RMS value of 1-2 nm regardless its thickness and existance of ND interlayer, which is simliar to that of glass substrate. Hydrophobicity analysis of DLC films were performed by measureing the contact angle. The results showed that DLC films showed high hydrophobicity with contact angle of upto 75° compared to that of glass which was only 47°.

In summary, the DLC coating on glass substrate using DC magnetron sputter method can have smodth and hydrophobic surface while maintaining the transmittance of 80 %, and all the process can be performed at room temperature. Also, the addition of ND interlayer can increase the allowable thickness of DLC layer with transmittance of 80 %.

In order to investigate the effects of multilayer structure on the mechanical properties of tetrahedral amorphous carbon (ta-C) films, a series of ta-C films consisting of sp²-rich layers and sp³-rich layers with different sublayer deposition times, hence different sublayer thicknesses, were successfully deposited using filtered cathodic arc vacuum (FCVA) by altering the substrate bias voltage. The hardness and the elastic modulus of the films were measured using a nanoindenter. The microstructure of the films was characterized using Raman spectroscopy. To compare the fracture toughness of the multilayers, all the films were nano-machined in the focused ion beam (FIB) to form microcantilevers, and then the in-situ bending of the microcantilever was performed to directly determine the fracture toughness of the films. A series of videos showing the in-situ deformation at nanoscale for all the films were recorded. It has been shown that using multilayer structure it is possible to reach the thicknesses higher than 500 nm for the films with hardness ~ 46 GPa. In general, increasing the deposition time for the sublayers, hence thicker sublayers, leads to higher sp³ content of the multilayer film. Therefore, multilayers with thicker sublayers show higher hardness but lower fracture toughness.

Scandium aluminum nitride (ScAlN) thin films is a novel piezoelectric films materials for bulk acoustic wave (BAW) and layered surface acoustic wave (SAW) devices due to its high curie temperature, high acoustic velocity and high coupling coefficient (K²). Diamond-like carbon (DLC) is a very commercial material due to its excellent properties such as hardness and elastic modulus comparable with diamond.Compared to diamond, DLC has very smooth surface morphology, lower deposition temperature and possesses larger area growth. AlN/DLC are attractive composite materials apply on high frequency SAW. There are many researches study about this topic.

To our best knowledge, it is the first time to study the ScAlN/DLC structure. In this research, AlN and ScAlN thin films with different Sc concentration were deposited on DLC substrate by RF and DC reactive magnetron co-sputtering method using Al and Sc targets. The X-ray diffraction results showed that the thin films were high c-axis-oriented. With Sc concentration increase, electron probe microanalyzer (EPMA) results indicated that the Sc atoms were substituted at Al atoms positions led to a lattice distortion during the phase transition. The cross-section of SEM results showed that the ScAlN films were highly aligned columns with the c-axis perpendicular to the substrate. Then the piezoelectric coefficient (d₃₃) of ScAlN thin films were measured and the highest value, 8.25 pC/N, was achieved at Sc 50 W increasing 50%, compared with AlN/DLC, 5.5pC/N. ScAlN films on DLC structure have a great potential to be applied on high frequency SAW devices in the future.