Structure, mechanical and tribological properties of diamond-like carbon films on aluminum alloy by arc ion plating

The low hardness and poor tribological performance of aluminum alloys restrict their engineering applications. However, protective hard films deposited on aluminum alloys are believed to be effective for overcoming their poor wear properties. In this paper, diamond-like carbon (DLC) films as hard protective film were deposited on 2024 aluminum alloy by arc ion plating. The dependence of the chemical state and microstructure of the films on substrate bias voltage was analyzed by X-ray photoelectron spectroscopy and Raman spectroscopy. The mechanical and tribological properties of the DLC films deposited on aluminum alloy were investigated by nanoindentation and ball-on-disk tribotester, respectively. The results show that the deposited DLC films were very well-adhered to the aluminum alloy substrate, with no cracks or delamination being observed. A maximum sp³ content of about 37% was obtained at −100 V substrate bias, resulting in a hardness of 30 GPa and elastic modulus of 280 GPa. Thus, the surface hardness and wear resistance of 2024 aluminum alloy can be significantly improved by applying a protective DLC film coating. The DLC-coated aluminum alloy showed a stable and relatively low friction coefficient, as well as narrower and shallower wear tracks in comparison with the uncoated aluminum alloy.     

Structural and mechanical properties of diamond-like carbon films deposited by an anode layer source

An anode layer source is a special ion gun, which can be fed with carbon precursors like acetylene to deposit hard and highly defect-free hydrogenated diamond-like carbon films at room temperature. The present study focuses on the influence of the process parameters — discharge voltage, process pressure and acetylene flow — on structure and mechanical properties of the deposited films. Raman spectra show that an increased discharge voltage yields decreased structural disorder, i.e. a lower C-C sp³ hybridised fraction of carbon atoms in the films. By an elevation of the discharge voltage from 1 to 3 kV the full width at half maximum of the G-band decreases from 194 ± 0.2 cm^− 1 to 183 ± 0.7 cm^− 1. Films deposited at the lowest discharge voltage show in accordance to the spectroscopic data the highest nanohardness (36 ± 1 GPa), stress (− 2.34 ± 0.2 GPa) and reduced elastic modulus (180 ± 4 GPa).     

Deposition and microstructure of Ti-containing diamond-like carbon nanocomposite films

Ti-containing diamond-like carbon (DLC) films were deposited by plasma decomposition of CH₄/Ar gas mixtures with an introduction of tetrakis(dimethylamino)titanium (TDMAT, Ti[(CH₃)₂N]₄), which was used as a precursor of titanium. The films deposited were found to be nanocomposite coatings consisting of TiN nanocrystalline clusters and amorphous hydrocarbon (a-C:H), indicating that the nanocrystalline clusters were embedded in the DLC matrix. The crystallinity of TiN clusters, as well as the Ti atomic concentrations in the films, increased with an increase of substrate temperature. The substrate temperature applied to form a crystalline phase in the DLC matrix induced a graphitization of amorphous hydrocarbon matrix. The increase of volume fraction of TiN nanocrystalline clusters in the DLC matrix enhanced the mechanical properties of nanostructured coatings, although the graphite-like structural transition of DLC matrix happened due to the applied heating.     

Studies of diamond-like carbon films prepared by ion beam-assisted deposition

Ion beam-assisted deposition offers a novel and unique process to prepare diamond-like carbon (DLC) films at room temperature, with particularly good interface adhesion. This advantage was explored in this study to deposit highly wear-resistant coating on bearing 52100 steel. Both dual ion beam sputtering and ion beam deposition were employed. Various bombarding species and energy were investigated to optimize the process. Raman, X-ray photoelectron and Auger electron spectroscopy were used to characterize the bonding structure of DLC. Extensive experiments were carried out to examine the tribological behaviour of the DLC/52100 system. A metal intermediate layer can help tremendously in wear resistance. The results are optimistic and may lead to useful applications.     

衬底负偏压对线性离子束DLC膜微结构和物性的影响

采用一种新型线性离子束PVD技术制备出大面积类金刚石薄膜(DLC膜),研究了衬底负偏压对薄膜微结构和物性的影响.结果表明:制备出的类金刚石薄膜在300 mm×100 mm范围内纵向厚度均方差约10-12 nm,横向薄膜厚度均方差约2-4 nm.随着衬底偏压的提高,薄膜中sp~3键的含量先增加后减小,在衬底偏压为-100 V时sp~3键的含量最大;DLC膜的残余应力、硬度和弹性模量与sp~3键的含量呈近似线性的关系,在衬底偏压为-100 V时其最大值分别为3.1 GPa、26 GPa和230 GPa.DLC薄膜的摩擦学性能与薄膜中sp~3碳杂化键的含量密切相关,但是受衬底偏压的影响不大,其摩擦系数大多小于0.25.偏压对磨损的影响很大,在偏压比较低(0～-200 V)时,薄膜的磨损率约为10~(-8)mm~3/N·m,偏压升高到300 V磨损率急剧提高到10~(-7)mm~3/N·m.     

In-situ investigation on the deformation and damage behaviour of diamond-like carbon coated thin films under uniaxial loading

In the present work, the failure behaviour of diamond-like carbon (DLC) coatings on thin steel substrate under uniaxial tensile loading is analyzed in-situ in scanning electron microscopy as well as ex-situ using focused ion beam cross section and transmission electron microscopy. Aim of the work is to find correlations between the failure behaviour of the coating system, the adhesion and the stress-strain behaviour of a DLC coating system under tensile loadings conditions. Therefore thin amorphous DLC films were coated onto thin stainless steel foils using a plasma assisted chemical vapour deposition technique. It is found from the in-situ investigations that at increasing strains cracks were formed in the coating, with decreasing spacing at higher strains. By comparing uncoated steels foils with coated systems the stress-strain behaviour of a DLC coating was determined. The DLC coating, although already strongly cracked, bears loads up to a total strain of 15%. Cross section analyses with a focused ion beam and microscopy techniques supported these investigations. During straining the formation of two deformation bands adjacent to the Cr adhesion layer was observed. This deformation bands also indicate a high interfacial adhesion.     

Effect of coating thickness on the deformation behaviour of diamond-like carbon-silicon system

The effect of coating thickness on the deformation behaviour of diamond-like carbon (DLC) coatings on silicon substrates was investigated. Following nanoindentation of a 0.6 µm thick DLC coating, the subsurface microstructures were characterized and the data was compared to prior studies on a similar, but thicker coating. Indentation resulted in localized plastic compression in the coating without any through-thickness cracking. It was shown that the discontinuities in the load-displacement curves appeared at lower loads for the thinner coating. Accordingly, the silicon substrate exhibited cracking, plastic deformation and phase transformation at significantly lower loads than in the case of the thicker coating. Further, the widths, parallel to the interface, over which slip and the phase transformation zone are spread out in the substrate, was found to increase with the thickness of the coating. The mechanism responsible for the first pop-in was found to change from phase transformation in uncoated silicon to dislocation nucleation in the presence of the coating.     

Investigation of the properties of diamond-like carbon thin films deposited by single and dual-mode plasma enhanced chemical vapor deposition

In this study diamond-like carbon (DLC) films were deposited by a dual-mode (radio frequency/microwave) reactor. A mixture of hydrogen and methane was used for deposition of DLC films. The film structure, thickness, roughness, refractive index of the films and plasma elements were investigated as a function of the radio frequency (RF) and microwave (MW) power, gas ratio and substrate substance. It was shown that by increasing the H₂ content, the refractive index grows to 2.63, the growth rate decreases to 10 (nm/min) and the surface roughness drops to 0.824 nm. Taking into consideration the RF power it was found that, as the power increases, the growth rate increases to 11.6 (nm/min), the variations of the refractive index and the roughness were continuously increasing, up to a certain limit of RF power. The Raman G-band peak position was less dependent on RF power for the glass substrate than that of the Si substrate and a converse tendency exists with increasing the hydrogen content. Adding MW plasma to the RF discharge (dual-mode) leads to an increase of the thickness and roughness of the films, which is attributed to the density enhancement of ions and radicals. Also, optical emission spectroscopy is used to study the plasma elements.     

Structure and mechanical properties of W-Se-C/diamond-like carbon and W-Se/diamond-like carbon bi-layer coatings prepared by pulsed laser deposition

Bi-layer W-Se-C/diamond-like carbon (DLC) and WSe_x/DLC coatings were obtained by standard and shadow-masked pulsed laser co-deposition from WSe₂ and graphite targets. W-Se-C coatings appeared as nanocomposites containing quasi-amorphous WSe₂, WC, spherical β-W nanocrystalline particles encapsulated in WSe₂ amorphous shell, and amorphous carbon phases. In WSe_x/DLC coatings, the formation of chemical bonds between W and C atoms was noticed at the interface. An increase of the C concentration over 40 at.% increases hardness and elasticity (up to 2 times at ~ 60 at.%C), and the Se/W ratio was always close to 1.4. The use of shadow-masked configuration avoids the deposition of micro- and nanoparticles. However, this method leads to a substantial increase of the Se content (Se/W ≥ 4), and the coatings became softer.     

Mechanical properties and thermal stability of nitrogen incorporated diamond-like carbon films

The nitrogen incorporated diamond-like carbon films were deposited on Si (100) substrates by arc ion plating (AIP) under different N₂ content in the gas mixture of Ar and N₂. The influence of N₂ content on the film microstructure and mechanical properties was studied by atomic force microscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and nanoindentation. It was found that the hardness (H), elastic modulus (E), elastic recovery (R) and plastic resistance parameter (H/E) decrease with increasing the nitrogen content. The decrease of mechanical properties of DLC films resulted from nitrogen incorporation was associated with total sp³ carbon bond content and N-sp³C bond content. The structural modification as well as mechanical properties of the annealed nitrogen incorporated diamond-like carbon films was investigated as a function of annealing temperature. Raman spectra indicate that the I_D/I_G ratio starts to increase and G peak position shifts upward at the annealing temperature over 500 °C. The hardness and elastic modulus of thermally annealed nitrogen incorporated DLC films decreased slightly at lower annealing temperature and then significantly decreased at higher annealing temperature. The strong covalent bonding between C and N atoms is expected to be effective on their thermal stability enhancement.     

Microstructure and property evolution of Cr-DLC films with different Cr content deposited by a hybrid beam technique

Cr-containing diamond-like carbon (Cr-DLC) films was deposited on silicon wafers by a hybrid beams system, which consists of a DC magnetron sputtering and a linear ion source. The chromium content in the films was adjusted by varying the fraction of Ar in the Ar and CH₄ gas mixture. The composition, microstructure, surface morphology, mechanical properties and tribological behavior of the films were investigated by XPS, TEM, AFM, SEM, nano-indentation and tribological tester as a function of Cr content. It is shown that, as the Cr content increased from 1.49 to 40.11 at.%, the Cr-DLC films transfer from amorphous DLC with dispersed metallic-like Cr to composite DLC with carbide phases embedding in the DLC matrix, and the film surface morphology also evolve from flat surface into rough surface with larger hillocks. The amorphous Cr-DLC films exhibit a low friction coefficient and wear rate as pure DLC, while the composite Cr-DLC films show a higher friction coefficient and wear rate, although they possess a relatively high hardness.     

Influence of hydrogen on the mechanical properties and microstructure of DLC films synthesized by r.f.-PECVD

This paper aims to investigate the influence of hydrogen on the variation of mechanical properties and microstructure of diamond-like carbon (DLC) films synthesized by radio frequency plasma chemical vapor deposition (r.f.-PECVD). The DLC films were deposited on a silicon substrate (p-type). The reactant gases employed in this paper are a mixture of acetylene and hydrogen. The ratio of hydrogen in the gas mixture was successively varied to clarify its influence on the roughness, thickness, microstructure, hardness, modulus, residual stress and wear depth for the DLC films. The results reveal that increasing the concentration of hydrogen decreases thickness and roughness. Meanwhile, increasing the hydrogen concentration causes the decrease of sp³ ratio, hardness as well as modulus. Finally, wear behavior is correlated to the surface morphology and hydrogen concentration for deposition with hydrogen-containing reactant gas.     

The effect of carbon content on the microstructure of hydrogen-free physical vapour deposited titanium carbide films

Titanium carbide (TiC) coatings for tribological applications were deposited on high speed steel. Several coatings with different titanium to carbon ratio were deposited by means of physical vapour deposition in which titanium was evaporated and carbon was sputtered. The coatings were characterised using analytical electron microscopy. It was observed that the change in titanium to carbon ratio significantly changed the microstructure of the coatings. The low carbon containing coatings consisted of columnar grains exhibiting a preferred crystallographic orientation whereas the coating with highest carbon content consisted of randomly ordered TiC grains in an amorphous carbon matrix. Energy filtered transmission electron microscopy revealed a change in Ti/C ratio as the distance from the substrate increased. The titanium to carbon ratio was observed to increase with distance from the substrate until a stable level was reached. This is due to a variation in the titanium evaporation during the early stages of film growth. This change of the titanium to carbon ratio affected the columnar growth in the initial stage of coating growth for the coatings with low carbon content.     

The influence of fluorine doping on the optical properties of diamond-like carbon thin films

Optical properties of fluorine doped diamond-like carbon (F:DLC) films deposited by the direct current plasma enhanced chemical vapor deposition (PECVD) technique were studied in detail. Surface morphologies of the F:DLC films were studied by an atomic force microscope, which indicated surface roughness increased with increase in at.% of F in the films. The chemical binding was investigated by X-ray photoelectron spectroscopic studies. Fourier transformed infrared spectroscopic studies depicted the presence of CF_x (x = 1,2,3) and CH_n (n = 1,2) bonding within the F:DLC films. Optical transparency and the optical band gap decreased with the fluorine incorporation in the DLC film. Optical band gap calculated from the transmittance spectra decreased from 2.60 to 1.95 eV with a variation of 0-14.8 at.% of F concentration in the diamond-like carbon films. Urbach parameter determined from the band tail of the transmittance spectra showed that it increased with the doping concentration.     

Comparison of microstructure and mechanical properties of chromium nitride-based coatings deposited by high power impulse magnetron sputtering and by the combined steered cathodic arc/unbalanced magnetron technique

Sliding, abrasive, and impact wear tests were performed on chromium nitride (CrN)-based coatings deposited on mirror-polished M2 high speed steel substrates by the novel high power impulse magnetron sputtering (HIPIMS) utilising high peak cathode powers densities of 3000 W cm⁻². The coatings were compared to single layer CrN and multilayer superlattice CrN/NbN coatings deposited by the arc bond sputtering (ABS) technique designed to improve the coating substrate adhesion by a combined steered cathodic arc/unbalanced magnetron (UBM) sputtering process. The substrates were metal ion etched using non-reactive HIPIMS or steered cathodic arc at a substrate bias voltage of −1200 V. Subsequently a 2- to 3-μm thick CrN or CrN/NbN coating was deposited by reactive HIPIMS or UBM. No bias was used during the HIPIMS deposition, while the bias during UBM growth was in the range 75-100 V. The ion saturation current measured by a flat electrostatic probe reached values of 50 mA cm⁻² peak for HIPIMS and 1 mA cm⁻² continuous during UBM deposition. The microstructure of the HIPIMS coatings observed by transmission electron microscopy was fully dense in contrast to the voided columnar structure observed in conventional UBM sputtered CrN and CrN/NbN. The sliding wear coefficients of the HIPIMS CrN films of 2.3×10⁻¹⁶ m³ N⁻¹ m⁻¹ were lower by a factor of 4 and the roughness of the wear track was significantly reduced compared to the UBM-deposited CrN. The abrasive wear coefficient of the HIPIMS coating was 2.2×10⁻¹³ m³ N⁻¹ m⁻¹ representing an improvement by a factor of 3 over UBM deposited CrN and a wear resistance comparable to that of the superlattice CrN/NbN. The adhesion of the HIPIMS deposited CrN was comparable to state-of-the-art ABS technology.     

Surface characterization and nanomechanical properties of diamond-like carbon films synthesized by RF plasma enhanced chemical vapor deposition

Diamond-like carbon (DLC) films were synthesized by RF plasma enhanced chemical vapor deposition using acetylene as the carbon source and the effects of acetylene/nitrogen ratio in the reaction atmosphere, deposition pressure, and plasma post-treatment using different atmospheres on the surface roughness and mechanical properties of DLC films were investigated. Although the surface roughness, characterized by AFM, decreased as the acetylene/nitrogen ratio in the reaction atmosphere decreased, the hardness of DLC films measured by nanoindentation also decreased with the decrease of the acetylene/nitrogen ratio, which is consistent with the Raman results of the I_D/I_G ratio. Rougher films with higher residual stress were obtained when using a deposition pressure higher than 40.0 Pa (0.3 torr). For the effect of plasma post-treatment using different atmospheres, surface smoothing was found for the hydrogen plasma post-treatment, whereas nitrogen and argon plasma post-treatments resulted in surface roughening. Hydrogen plasma post-treatment was found to lower the surface roughness without significantly sacrificing the hardness.     

Influence of the microstructure on the mechanical and tribological behavior of TiC/a-C nanocomposite coatings

The performance of protective thin films is clearly influenced by their microstructure. The objective of this work is to study the influence of the structure of TiC/a-C nanocomposite coatings with a-C contents ranging from ~ 0% to 100% on their mechanical and tribological properties measured by ultramicroindentation and pin-on-disks tests at ambient air, respectively. The microstructure evolves from a polycrystalline columnar structure consisting of TiC crystals to an amorphous and dense TiC/a-C nanocomposite structure when the amount of a-C is increased. The former samples show high hardness, moderate friction and high wear rates, while the latter ones show a decrease in hardness but an improvement in tribological performance. No apparent direct correlation is found between hardness and wear rate, which is controlled by the friction coefficient. These results are compared to the literature and explained according to the different film microstructures and chemical bonding nature. The film stress has also been measured at the macro and micro levels by the curvature and Williamson-Hall methods respectively. Other mechanical properties of the coating such as resilience and toughness were evaluated by estimating the H³/E?² and H/E? ratios and the percentage of elastic work (W_e). None of these parameters showed a tendency that could explain the observed tribological results, indicating that for self-lubricant nanocomposite systems this correlation is not so simple and that the assembly of different factors must be taken into account.     

Relative performance of hydrogenated, argon-incorporated and nitrogen-incorporated diamond-like carbon coated Ti-6Al-4V samples under fretting wear loading

Hydrogenated diamond-like carbon (DLC) (H-DLC), argon-incorporated DLC (Ar-DLC) and nitrogen-incorporated DLC (N-DLC) coatings were deposited on flat rectangular Ti-6Al-4V samples. The DLC coatings were characterised by Raman spectroscopy and nanoindentation. Fretting wear tests were conducted on uncoated and DLC coated samples with an alumina ball as the counterbody. As the Ar-DLC and N-DLC coatings had relatively more sp² network compared to the H-DLC coating, they exhibited lower values of hardness and elastic modulus. At both loads of 4.9 N and 14.7 N, all DLC coated specimens showed lower values of tangential force coefficient (TFC), wear volume and specific wear rate compared to the uncoated samples. While the Ar-DLC coated sample exhibited the lowest TFC, wear volume and specific wear rate at 4.9 N load, the N-DLC coated specimen exhibited the lowest TFC, wear volume and specific wear rate at 14.7 N load.     

Evaluation of microstructures and mechanical properties of diamond like carbon films deposited by filtered cathodic arc plasma

Diamond-like carbon (DLC) films were deposited by a cathodic arc plasma evaporation (CAPD) process, using a mechanical shield filter combined with a magnetic filter with enhanced arc structure at substrate-bias voltage ranging from − 50 to − 300 V. The film characteristics were investigated using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HRTEM). The mechanical properties were investigated by using a nanoindentation tester, scratch test and ball on disc wear test. The Raman spectra of the films showed that the wavenumber ranging from 900 to 1800 cm^− 1 could be deconvoluted into 1140 cm^− 1, D band and G band. The bias caused a significant effect on the sp³ content which was increased with the decreasing of I_D/I_G ratio. The XPS spectra data of the films which were etched by H⁺ plasma indicated the sp³ content are higher than those of the as-deposited DLC films. This implied that there is a sp²-rich layer present on the surface of the as-deposited DLC films. The nanoindentation hardness increased as the maximum load increased. A 380 nm thick and well adhered DLC film was successfully deposited on WC-Co substrate above a Ti interlayer. The adhesion critical load of the DLC films was about 33 N. The results of the wear tests demonstrated that the friction coefficient of the DLC films was between 0.12 and 0.2.     

Annealing effect on the structural, mechanical and electrical properties of titanium-doped diamond-like carbon films

Titanium-doped diamond-like carbon (Ti-doped DLC) films with a Ti content of 1.1 at.% were synthesized on a Si substrate by a process that involves filtered cathodic vacuum arc (FCVA) and metal vapor vacuum arc (MeVVA) systems. The effect of annealing temperature on the microstructure, surface roughness, hardness and electrical resistivity of the resulting films was evaluated in this study. The Raman spectra revealed that the degree of graphitization of the Ti-doped DLC thin films was increased from 25 to 600 °C and the microstructure of the films is converted to a nano-crystalline graphite structure. The resulting films maintain a smooth surface after the annealing process. The hardness of the Ti-doped DLC films increases as the annealing temperature increases up to 400 °C because the induced defects and the inter-atomic bonds are repaired after the annealing process. But the hardness decreases at the higher temperature due to the increase of number and size of the nano-crystalline graphitic domains. Since the degree of graphitization of the thin films increases, the electrical resistivity of the Ti-doped DLC thin films decreases from 0.038 to 0.006 Ω cm.