Sub-Threshold Fabrication of Laser-Induced Periodic Surface Structures on Diamond-like Nanocomposite Films with IR Femtosecond Pulses

In the paper, we study the formation of laser-induced periodic surface structures (LIPSS) on diamond-like nanocomposite (DLN) a-C:H:Si:O films during nanoscale ablation processing at low fluences—below the single-pulse graphitization and spallation thresholds—using an IR fs-laser (wavelength 1030 nm, pulse duration 320 fs, pulse repetition rate 100 kHz, scanning beam velocity 0.04–0.08 m/s). The studies are focused on microscopic analysis of the nanostructured DLN film surface at different stages of LIPSS formation and numerical modeling of surface plasmon polaritons in a thin graphitized surface layer. Important findings are concerned with (i) sub-threshold fabrication of high spatial frequency LIPSS (HSFL) and low spatial frequency LIPSS (LSFL) under negligible surface graphitization of hard DLN films, (ii) transition from the HSFL (periods of 140 ± 30 and 230 ± 40 nm) to LSFL (period of 830–900 nm) within a narrow fluence range of 0.21–0.32 J/cm2, (iii) visualization of equi-field lines by ablated nanoparticles at an initial stage of the LIPSS formation, providing proof of larger electric fields in the valleys and weaker fields at the ridges of a growing surface grating, (iv) influence of the thickness of a laser-excited glassy carbon (GC) layer on the period of surface plasmon polaritons excited in a three-layer system “air/GC layer/DLN film”

Tribological Performance of Diamond-like Nanocomposite Coatings: Influence of Environments and Laser Surface Texturing

Diamond-like nanocomposite (DLN) films (a-C:H:Si:O films) are characterized by their unique structure and remarkable tribological properties to be pronounced under various environmental and surface modification conditions. In this paper, we investigated the effects of environments (humid air, water and oil lubrication, elevated temperatures) and laser surface texturing on tribological performance of DLN coatings. Femtosecond laser (wavelength 515 nm) was used for surface texturing. Comparative tests of DLN films sliding against different counterbodies (steel, Si3N4) in humid air and water demonstrated the low-friction and low-wear performance under water, in the absence of chemical interaction of water with the counterbody surface. The wear rates of the film and Si3N4 ball in water, 7.5 × 10−9 and 2.6 × 10−9 mm3/(Nm), were found to be considerably lower than the corresponding values 6.8 × 10−7 and 3.8 × 10−8 mm3/(Nm) in humid air, in spite of higher friction in water-lubricated sliding. Laser surface texturing of DLN films was performed to fabricate microcrater arrays, followed by tribological testing under oil lubrication at different temperatures, from 23 to 100 ◦C. The lubricated friction performance of laser-textured films was improved at both the room temperature and elevated temperatures. The friction coefficient was reduced from 0.1 (original film) to 0.083 for laser-textured film at room temperature, and then to 0.068 at 100 ◦C. The nano-/microfriction behavior of laser-structured surface characterized by lower friction forces than the original surface was demonstrated using friction force microscopy in ambient air. The obtained results demonstrate excellent tribological properties of DLN coatings in various environments, which can be further improved by femtosecond-laser-surface texturing

Femtosecond Laser-Induced Periodic Surface Structures in Titanium-Doped Diamond-like Nanocomposite Films: Effects of the Beam Polarization Rotation

We study the properties of laser-induced periodic surface structures (LIPSS) formed on titanium-doped diamond-like nanocomposite (DLN) a-C:H:Si:O films during ablation processing with linearly-polarized beams of a visible femtosecond laser (wavelength 515 nm, pulse duration 320 fs, pulse repetition rates 100 kHz-2 MHz, scanning beam velocity 0.05–1 m/s). The studies are focused on (i) laser ablation characteristics of Ti-DLN films at different pulse frequencies and constant fluence close to the ablation threshold, (ii) effects of the polarization angle rotation on the properties of low spatial frequency LIPSS (LSFL), and (iii) nanofriction properties of the ‘rotating’ LIPSS using atomic force microscopy (AFM) in a lateral force mode. It is found that (i) all LSFL are oriented perpendicular to the beam polarization direction, so being rotated with the beam polarization, and (ii) LSFL periods are gradually changed from 360 ± 5 nm for ripples parallel to the beam scanning direction to 420 ± 10 nm for ripples formed perpendicular to the beam scanning. The obtained results are discussed in the frame of the surface plasmon polaritons model of the LIPSS formation. Also, the findings of the nanoscale friction behavior, dependent on the LIPSS orientation relative to the AFM tip scanning direction, are presented and discussed

Raman Study of the Diamond to Graphite Transition Induced by the Single Femtosecond Laser Pulse on the (111) Face

The use of the ultrafast pulse is the current trend in laser processing many materials, including diamonds. Recently, the orientation of the irradiated crystal face was shown to play a crucial role in the diamond to graphite transition process. Here, we develop this approach and explore the nanostructure of the sp2 phase, and the structural perfection of the graphite produced. The single pulse of the third harmonic of a Ti:sapphire laser (100 fs, 266 nm) was used to study the process of producing highly oriented graphite (HOG) layers on the (111) surface of a diamond monocrystal. The laser fluence dependence on ablated crater depth was analyzed, and three different regimes of laser-induced diamond graphitization are discussed, namely: nonablative graphitization, customary ablative graphitization, and bulk graphitization. The structure of the graphitized material was investigated by confocal Raman spectroscopy. A clear correlation was found between laser ablation regimes and sp2 phase structure. The main types of structural defects that disrupt the HOG formation both at low and high laser fluencies were determined by Raman spectroscopy. The patterns revealed give optimal laser fluence for the production of perfect graphite spots on the diamond surface

Cleavage-Driven Laser Writing in Monocrystalline Diamond

The propagation of graphitization wave through the diamond bulk under multipulse laser irradiation is a largely self-guided process. This fact assists the production of graphitized wires oriented along a laser beam and greatly complicates formation of the structures oriented differently. Here, we develop new approaches to control laser graphitization that should empower the potential of 3D laser microstructuring inside a diamond crystal. Two techniques are investigated: (i) a laser seed damage of crystal with subsequent exposure at a lower laser fluence, thus restricting the propagation of the graphitization wave toward the beam and (ii) formation of a dominant microfracture perpendicular to the laser beam, thus guiding growth of the graphitized thread

Diamond Photoconductive Antenna for Terahertz Generation Equipped with Buried Graphite Electrodes

It has been shown recently that a photoconductive antenna (PCA) based on a nitrogen-doped diamond can be effectively excited by the second harmonic of a Ti:sapphire laser (λ = 400 nm). The THz emission performance of the PCA can be significantly increased if a much stronger electric field is created between the close-located electrodes. To produce a homogeneous electric field over the entire excited diamond volume, the laser fabrication of deep-buried graphite electrodes inside the diamond crystal was proposed. Several electrodes consisting of the arrays of buried pillars connected by the surface graphite stripes were produced inside an HPHT diamond crystal using femtosecond and nanosecond laser pulses. Combining different pairs of the electrodes, a series of PCAs with various electrode interspaces was formed. The THz emission of the PCAs equipped with the buried electrodes was measured at different values of excitation fluence and bias voltage (DC and pulsed) and compared with the emission of the same diamond crystal when the bias voltage was applied to the surface electrodes on the opposite faces. All examined PCAs have demonstrated the square-law dependencies of the THz fluence on the field strength, while the saturation fluence fluctuated in the range of 1200–1600 µJ/cm2. The THz emission performance was found to be approximately the same for the PCAs with the surface electrodes and with the buried electrodes spaced at a distance of 1.4–3.5 mm. However, it noticeably decreased when the distance between the buried electrodes was reduced to 0.5 mm

Effect of Substrate Holder Design on Stress and Uniformity of Large-Area Polycrystalline Diamond Films Grown by Microwave Plasma-Assisted CVD

In this work, the substrate holders of three principal geometries (flat, pocket, and pedestal) were designed based on E-field simulations. They were fabricated and then tested in microwave plasma-assisted chemical vapor deposition process with the purpose of the homogeneous growth of 100-μm-thick, low-stress polycrystalline diamond film over 2-inch Si substrates with a thickness of 0.35 mm. The effectiveness of each holder design was estimated by the criteria of the PCD film quality, its homogeneity, stress, and the curvature of the resulting “diamond-on-Si” plates. The structure and phase composition of the synthesized samples were studied with scanning electron microscopy and Raman spectroscopy, the curvature was measured using white light interferometry, and the thermal conductivity was measured using the laser flash technique. The proposed pedestal design of the substrate holder could reduce the stress of the thick PCD film down to 1.1–1.4 GPa, which resulted in an extremely low value of displacement for the resulting “diamond-on-Si” plate of Δh = 50 μm. The obtained results may be used for the improvement of already existing, and the design of the novel-type, MPCVD reactors aimed at the growth of large-area thick homogeneous PCD layers and plates for electronic applications

Combined HF+MW CVD Approach for the Growth of Polycrystalline Diamond Films with Reduced Bow

A combination of two methods of chemical vapor deposition (CVD) of diamond films, microwave plasma–assisted (MW CVD) and hot filament (HF CVD), was used for the growth of 100 µm-thick polycrystalline diamond (PCD) layers on Si substrates. The bow of HF CVD and MW CVD films showed opposite convex\concave trends; thus, the combined material allowed reducing the overall bow by a factor of 2–3. Using MW CVD for the growth of the initial 25 µm-thick PCD layer allowed achieving much higher thermal conductivity of the combined 110 µm-thick film at 210 W/m·K in comparison to 130 W/m·K for the 93 µm-thick pure HF CVD film