Stability Of Diamond-like Carbon Films Research Articles

Influence of Ag doping on the microstructure, mechanical properties, and adhesion stability of diamond-like carbon films

Silver (Ag)-doped diamond-like carbon (Ag-DLC) films were deposited on Si wafer and Co-Cr-Mo alloy substrates using a hybrid deposition technique that combined high-power pulsed magnetron sputtering (HPPMS) and high-power pulsed plasma-enhanced chemical vapor deposition (HPP-PECVD). The Ag concentration (0.0–10.0 at.%) in Ag-DLC films was controlled by adjusting the number of Ag rods in the mosaic silver-graphite target. The effects of Ag doping on the microstructure, chemical bonding, mechanical properties, and adhesion stability of DLC films were systematically investigated. The results demonstrated that Ag doping could refine the columnar structure in DLC films and change the shape and size of DLC surface hillocks. The residual stress in the DLC films decreased as the Ag concentration increased, which effectively improved the adhesion between the films and substrates. The fraction of sp3 bonds in the carbon structure decreased with increasing Ag concentration, which resulted in a reduction in the film hardness when the Ag concentration was higher than 3.2 at.%. Ag doping could improve the wear performance of DLC films, and the Ag-DLC film with 3.2 at.% Ag had excellent wear resistance. Compared with pure DLC films, the Ag-DLC films had better adhesion stability in physiological solutions, which is beneficial for the long-term service of DLC films in vivo applications.

A study of surface swelling caused by graphitisation during pulsed laser ablation of carbon allotrope with high content of sp3 bounds

Experiments and theory are employed to investigate the laser ablation of boron doped diamond and tetrahedral amorphous carbon using nanosecond laser pulses. For a single laser pulse at low values of fluence, the laser induces a swelling of the surface due to graphitisation, whilst a high level of fluence leads to recession of the surface due to vaporization. To understand and investigate the underlying phenomena during the diamond-laser interaction, a model has been developed to reliably and quickly predict the behaviour of the surface and the thickness of the heat affected zone. The model is based on conservation of heat and mass during the laser-workpiece interaction. It consists of a one-dimensional system of non-linear equations that models the material heating, evaporation, graphitisation and plasma shielding. There is excellent agreement between numerical and experimental results for the position of the interfaces up to a high laser fluence. This model is the first to investigate the ablation of diamond that is able to capture surface swelling due to the graphitisation of the diamond layer, the graphite thickness and the amount of ablated material within a single framework. Furthermore, the model provides a novel methodology to investigate the thermal stability of diamond-like carbon films. The activation energy for tetrahedral amorphous carbon is obtained using the model with an accuracy of eV.

Mechanical stability of the diamond-like carbon film on nitinol vascular stents under cyclic loading

The mechanical stability of diamond-like carbon (DLC) films coated on nitinol vascular stents was investigated under cyclic loading condition by employing a stent crimping system. DLC films were coated on the vascular stent of a three dimensional structure by using a hybrid ion beam system with rotating jig. The cracking or delamination of the DLC coating occurred dominantly near the hinge connecting the V-shaped segments of the stent where the maximum strain was induced by a cyclic loading of contraction and extension. However the failures were significantly suppressed as the amorphous Si (a-Si) buffer layer thickness increased. Interfacial adhesion strength was estimated from the spalled crack size in the DLC coating for various values of the a-Si buffer layer thickness.

Investigation of Mechanical Stability of Thermally Aged a-c:H DLC Films with Different Precursor Gases

A-C: H (Hydrogenated amorphous carbon) diamond like carbon(DLC) film is the low stabilities of chemical and mechanical properties at high temperatures. Some researchers reported that Raman spectra shown significant conversion of DLC films to nano-crystalline graphite on heating in ambient air at temperatures above 300°C and conversion to nano-crystalline graphite was completed by 450~600°C In this study, DLC films(thickness : 1.5-m) were deposited on p-type (100) silicon films by using C6H6 and C6H14 plasma produced with a 13.6MHz RF source. To investigate the mechanical stability of DLC films below 250°C, we thermally aged the films for different aging temperatures (150°C, 200°C, 250°C) and times (0: virgin, 120, 240, 360 hours). By using indentation test and Raman spectra analyses, it has been found that the H3/E2 decreases with increasing I(D)/I(G) ratio. Also we confirmed that sample-2(precursor gas: C6H14) had better mechanical characteristic and thermal stability than the sample-1(precursor gas: C6H6).

Investigation of fracture characteristics in thermally aged thin films of diamond-like carbon

To investigate the chemical and mechanical stability of diamond-like carbon films at 300 °C, we thermally aged the films for different aging times (0: virgin, 24, 48 h). From the Raman spectra analyses, the indentation tests, and the total work fracture data, we confirmed that the aged sample-2(precursor gas: C 6H 14) had better mechanical characteristics and thermal stabilities than the aged sample-1(precursor gas: C 6H 6). Also, the fracture toughness is found to decrease with increasing ratio of the intensities of the D- and G-peaks in Raman spectroscopy.

Thermal stability of diamond-like carbon films deposited by plasma based ion implantation technique with bipolar pulses

Silicon incorporated diamond-like carbon (DLC) films were deposited using a bipolar-type plasma based ion implantation technique, and the effect of the positive pulse voltage on the thermal stability of the DLC films was investigated. The positive pulse voltage was varied from 2.0 to 6.3 kV while the negative pulse voltage was maintained at − 5.0 kV. The deposited DLC films were annealed in air for 1 h at constant temperatures of 500 and 600 °C. As a result, an optimum positive pulse voltage exists for the high deposition rate and thermal stability of the DLC films, which correlates with the surface temperature during the film deposition. The DLC film deposited at a positive pulse of 4.0 kV, whose surface temperature during deposition was 300 °C, showed a typical DLC Raman spectrum even though the annealing temperature increased to 600 °C, and exhibited good friction properties. This high thermal stability is attributed to the effect of pre-annealing during the deposition and the formation of a stable and thick silicon oxide layer on the DLC surface when annealed in air. On the other hand, the thermal stability of the DLC film deposited at 2.0 kV, whose surface temperature during deposition was 150 °C, quickly deteriorated when the film was annealed at 600 °C.

Mechanical properties and thermomechanical stability of diamond-like carbon films synthesized by pulsed vacuum arc plasma deposition

The higher sp 3 bonding fraction hydrogen-free diamond-like carbon (DLC) films were deposited on Si(1 0 0) and Cr17Ni14Cu4 stainless steel substrates by pulsed vacuum arc plasma deposition as a function of bias substrate voltage. The results show that the sp 3 content increases slightly with higher substrate bias and the effective microhardness only increases slightly. The wear resistance of the as-deposited DLC films decreases with increasing substrate bias. The effective hardness of the annealed DLC films diminishes by approximately 20% and the wear resistance decreases after annealing at 400 °C due to local adhesion decreases, even though the wear resistance of the annealed DLC films is still much better than that of the Cr17Ni14Cu4 substrate. The thermomechanical stability of hydrogen-free DLC films synthesized by pulsed vacuum arc plasma deposition is better than the DLC films containing hydrogen synthesized by PECVD.

Thermal stability of diamond-like carbon films with added silicon

Abstract Diamond-like carbon (DLC) films with added silicon content from 0 to 19.2 at.% were deposited using r.f. PECVD (radio frequency plasma enhanced chemical vapor deposition). Fourier transform IR (FTIR) spectrometry, Raman spectrometry and X-ray photoelectron spectrometry (XPS) were used to determine the structural change of the annealed DLC films in ambient air. By increasing the annealing temperature the CHn and Si–H groups in the FTIR spectra decrease because of hydrogen evolution, whereas the intensities of CO and Si–O peaks increase owing to oxidation. From Raman spectra, the integrated intensity ratio ID/IG of the pure DLC films and the silicon-doped films increases at 300 and 400 °C, respectively, whereas the observable shoulder of the D band occurs at 400 and 500 °C, respectively, which indicates that the addition of silicon improves the thermal stability of DLC films. Using XPS analysis, a surface reaction for the annealed films is investigated.

Thermal stability of diamondlike carbon films

IR and UV-VIS optical spectroscopy have been used to study the transformations of the properties of diamondlike carbon films following isothermal anneals from Ta=300 to 650 °C. Several processes due to the annealing have been observed: (1) the increase of the absorption due to vibrations of unsaturated bonds of C(sp2) atoms at ∼1600 cm−1 for Ta≥350 °C, (2) the decrease of the absorption due to C(sp3)–H bonds at Ta≥350 °C, and (3) the reduction of the optical energy gap. Analysis of the kinetics has shown that the dehydrogenation of the alloys and the formation of unsaturated bonds may proceed independently. The reduction of the energy gap is related to the formation of C(sp2) atoms with unsaturated bonding which occurs mostly in hydrogen-free regions. Intensive graphitization of the films occurs above Ta=650 °C. Transformations of C–H bonds are proposed to occur via fast rearrangement in stressed regions leading to formation of new C(sp2)–H bonds and formation of methane molecules as the most important product of the anneals inside the polymeric highly hydrogenated regions in the alloys. It appears that both bond removal and reordering have taken place as a result of annealing.