Study on metal-doped diamond-like carbon films synthesized by cathodic arc evaporation

In this study, we developed a novel method of synthesizing metal-doped diamond-like carbon films (DLC) using the cathodic arc evaporation (CAE) process. Intense Cr plasma energy activated the decomposition of hydrocarbon source gas C₂H₂ to form a metal-doped amorphous carbon film on steel substrates. We deposited a Cr interlayer to prevent interdiffusion between DLC and the steel substrates. When the C₂H₂ partial pressure is higher than 1.3 Pa, the deposition reaction switched from Cr₃C₂ to DLC formation. The result is a hydrogenated DLC thin film possessing excellent microhardness as high as 3824 Hv(25g), and for which the incorporation of a Cr interface and Cr doping in the DLC matrix ensure film ductility and sufficient film adhesion. We employed Raman spectroscopy to evaluate the influences of reactive gas flow and substrate bias on the DLC composition; we carried out the microstructure and mechanical property measurements by scanning electron microscopy (SEM), X-ray diffraction (XRD), glow discharge optical spectroscopy (GDS) and wear tests.     

Preparation of diamond like carbon thin films above room temperature and their properties

Diamond like carbon (DLC) thin films were deposited on p-type silicon (p-Si), quartz and ITO substrates by microwave (MW) surface-wave plasma (SWP) chemical vapor deposition (CVD) at different substrate temperatures (RT ∼ 300 °C). Argon (Ar: 200 sccm) was used as carrier gas while acetylene (C₂H₂: 20 sccm) and nitrogen (N: 5 sccm) were used as plasma source. Analytical methods such as X-ray photoelectron spectroscopy (XPS), FT-IR and UV–visible spectroscopy were employed to investigate the structural and optical properties of the DLC thin films respectively. FT-IR spectra show the structural modification of the DLC thin films with substrate temperatures showing the distinct peak around 3350 cm^− 1 wave number; which may corresponds to the sp² C–H bond. Tauc optical gap and film thickness both decreased with increasing substrate temperature. The peaks of XPS core level C 1 s spectra of the DLC thin films shifted towards lower binding energy with substrate temperature. We also got the small photoconductivity action of the film deposited at 300 °C on ITO substrate.     

Inhibiting tribocorrosion damage of Cr/CrxN coatings by multi-layer design

The two-layer and multi-layer Cr/Cr_xN coatings were fabricated on 316 L stainless steel (316 L SS) substrates by the arc ion plating technique. The two-layer Cr/Cr_xN coating was a typical CrN coating with an adhesive Cr layer. And the multi-layer Cr/Cr_xN coating design was in two dimensions. In the first dimension, the multi-layer Cr/Cr_xN coating consisted of alternative Cr/CrN layers with the thickness ratio of 1:5; in the second dimension, the alternative Cr₂N layers with the thickness of 10 nm were inserted in CrN layers. This design was expected to increase transverse interfaces in a smaller scale. The microstructures, mechanical, corrosion and tribocorrosion performances of both Cr/Cr_xN coatings were systematically investigated. The results showed that the special multi-layer design of Cr/Cr_xN coatings improved mechanical, anti-corrosion and anti-tribocorrosion performances. Compared with the two-layer Cr/Cr_xN coating, the reduced tribocorrosion damage of the muti-layer Cr/Cr_xN coating was closely related to the inhibited synergistic effect between electrochemical corrosion and mechanical wear. In conclusion, the multi-layer Cr/Cr_xN coating was more suitable to work as the surface protective coating than the two-layer Cr/Cr_xN coating in seawater.     

Structure and mechanical properties of oxygen doped diamond-like carbon thin films

In this study, structure and mechanical properties of doped diamond-like carbon (DLC) films with oxygen were investigated. A mixture of methane (CH₄), argon (Ar) and oxygen (O₂) was used as feeding gas, and the RF-PECVD technique was used as a deposition method. The thin films were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (RS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and a combination of elastic recoil detection analysis and Rutherford backscattering (ERDA-RBS). Nano-indentation tests were performed to measure hardness. Also, the residual stress of the films was calculated by Stoney equation. The XPS and ERDA-RBS results indicated that by increasing the oxygen in the feeding gas up to 5.6 vol.%, the incorporation of oxygen into the films' structure was increased. The ratio of sp² to sp³ sites was changed by the variation of oxygen content in the film structure. The sp²/sp³ ratios are 0.43 and 1.04 for un-doped and doped DLC films with 5.6 vol.% oxygen in the feeding gas, respectively. The Raman spectroscopy (RS) results showed that by increasing the oxygen content in doped DLC films, the amount of sp² CC aromatic bonds was raised and the hydrogen content reduced in the structure. The attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) confirmed the decrease of hydrogen content and the increase the ratio of CC aromatic to olefinic bonds. Hardness and residual stress of the films were raised by increasing the oxygen content within the films' structure. The maximum hardness (19.6 GPa) and residual stress (0.29 GPa) were obtained for doped DLC films, which had the maximum content of oxygen in structure, while the minimum hardness (7.1 GPa) and residual stress (0.16 GPa) were obtained for un-doped DLC films. The increase of sp³ CC bonds between clusters and the decrease of the hydrogen content, with a simultaneous increase of oxygen in the films' structure is the reason for increase of hardness and residual stress.     

Phase formation and microstructure evolution of plasma sprayed Cr2AlC MAX phase coatings under post annealing

In this study, thick Cr₂AlC coatings were first synthesized via plasma spraying of Cr₃C₂–Al–Cr agglomerated powders and post annealing. The microstructure evolution and mechanical properties of the Cr₂AlC coatings annealed at 500–1000 °C were investigated. The as-sprayed coatings exhibited a lamellar structure, primarily consisting of Cr₂AlC, Cr₇C₃, Cr₂₃C₆, and (Cr, Al)C_x solid solutions. The short residence time during spraying led to incomplete reactions in the Cr₃C₂@Al–Cr agglomerates, resulting in the formation of (Cr, Al)C_x. Post annealing provided sufficient energy for the transition of (Cr, Al)C_x → Cr₂AlC. With an increase in the annealing temperature (<900 °C), gradual transition of the (Cr, Al)C_x phase led to a slight increase in the Cr₂AlC content, and thus, the as-annealed coatings maintained high hardness (>1000 HV_0.2) with improved fracture toughness. Higher annealing temperatures (>900 °C) promoted clear enhancement of the Cr₂AlC content, thus reducing the coating hardness. The transition phase (Cr, Al)C_x and high temperature annealing were the primary factors to promoting the formation of the Cr₂AlC phase in sprayed coatings. This study indicates that the Cr₃C₂@Al–Cr agglomerates can be effective alternatives to expensive MAX phase powders as feedstock for plasma spraying of Cr₂AlC coatings.     

Novel S,N co-doped Ba2In2-xCrxO5+y oxides: Synthesis and optical properties

The (S, N) co-doped Ba₂In_2-xCr_xO_5+y (0 ≤ x ≤ 0.5) oxides are successfully obtained by mixing the Ba₂In_2-xCr_xO_5+y oxides and thiourea through a simple ball milling method followed by sintering at 400 °C for 3 h. The colors of the compounds change from orange-brown to yellow-green after reacting with thiourea. When Cr amount is small (x = 0.1), the crystal structure of (S, N) co-doped Ba₂In_2-xCr_xO_5+y is orthorhombic Ba₂In₂O₅ phase. When x ≥ 0.3, the crystal structure of the sample is cubic BaInO_2.5 phase. And this phase transition is the same as Ba₂In_2-xCr_xO_5+y. XPS results reveal that Cr⁶⁺ in Ba₂In_2-xCr_xO_5+y (0 ≤ x ≤ 0.5) oxides are reduced to Cr³⁺ after sintering. S exists in both cation and anion forms, and N exists in substitutional forms. UV–Vis analysis indicates that the yellow-green hue comes from the d-d transition of Cr³⁺, and the doping of S, N ions leads to a red shift of the absorption edge of the samples.     

Structure and magnetic properties of Cr-Doped tin monoxide prepared by hydrothermal method

Ball- and sheet-like Sn_1-xCr_xO were successfully prepared by one-step hydrothermal method. The SEM and TEM results show that the diameter of ball-like unit is about 15 μm and the unit was assembled by several nanosheets with a thickness of about 0.5 μm. Raman pattern displays that there are two intense Raman mode E_g (~114 cm⁻¹) and A_1g (~211 cm⁻¹) in the SnO, and the A_1g peak shifted significantly. UV–Visible spectrum illustrates that the bandgap of the Cr-doped SnO firstly decreases and then increases as the Cr concentration increases, which is attributed to the sp-d exchange interactions and Burstein-Moss theory. The analysis of XPS also provides evidence for the incorporation of Cr into the system. Furthermore, it is found that oxygen vacancy (V_O) decreases with the increase of Cr content. Based on Krӧger-Vink notation system, the competition mechanism between V_O and V_Sn could explain the origin and variation of magnetism of Sn_1-xCr_xO from structural defects.     

Structure and conducting properties of La1-xSrxCoO3-δ films

La_1-xSr_xCoO_3-δ (LSCO) films have been deposited on LaAlO₃ (LAO), La_1-xSr_xGa_1-yMg_yO_3-δ/LaAlO₃ (LSGM/LAO) and yittria-stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The optimum conditions for deposition were determined for the different substrates in an ambient of 80–310 mTorr oxygen pressure and at a substrate temperature range of 450 to 750°C. The films structures were analyzed by XRD, RBS and SEM. Epitaxial LSCO films were grown with (110) preferred orientation on YSZ, and with (100) orientation on LAO and LSGM/LAO. The electrical resistivity of the epitaxial LSCO films ranged from 10⁻² to 10⁻⁴ Ω cm, depending on the deposition temperature and substrate. The ionic conducting behavior of the LSCO film on YSZ was investigated by impedance measurement.     

Piezoresistive properties of diamond like carbon films containing copper

In the present study diamond like carbon films containing copper (DLC:Cu) were deposited by reactive magnetron sputtering. Direct current (DC) sputtering and high power pulsed magnetron sputtering (HIPIMS) were used. The influence of the composition and structure on piezoresistive properties of DLC:Cu films was investigated. Structure of DLC:Cu films was investigated by Raman scattering spectroscopy and transmission electron microscopy (TEM). Chemical composition of the films was studied by using energy-dispersive X-ray spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS). Particularly analysis of XPS O1s spectra revealed oxidation of Cu nanoparticles. Piezoresistive gauge factor of DLC:Cu films was in 3–6 range and decreased with the increase of copper atomic concentration. Tendency of the decrease of the gauge factor of DLC:Cu films with the increased D/G peak area ratio (decreased sp³/sp² carbon bond ratio) was observed. It was found that resistance (R) of DLC:Cu films decreased with the increase of Cu atomic concentration by logarithmic law. It is shown that a quasilinear increase of piezoresistive gauge factor with log(R) is in good accordance with percolation theory. Temperature coefficient of resistance (TCR) of DLC:Cu films was negative and decreased with copper amount in Cu atomic concentrations ranging up to ~ 40%. Very low TCR values (zero TCR) were observed only for DLC:Cu films with low gauge factor that was close to the gauge factor of the metallic strain gauges. Role of some possible mechanisms: copper amount as well as Cu cluster size on the value of gauge factor is discussed.     

Substrate bias effect on structure of tetrahedral amorphous carbon films by Raman spectroscopy

Diamond-like carbon (DLC) films form a critical protective layer on magnetic hard disks and their reading heads. Now tetrahedral amorphous carbon films (ta–C) thickness of 2 nm are becoming the preferred means due to the highly sp³ content. In this paper, Raman spectra at visible and ultraviolet excitation of ta–C films have been studied as a function of substrate bias voltage. The spectra show that the sp³ content of 70 nm thick DLC films increases with higher substrate bias, while sp³ content of 2 nm ultra-thin films falls almost linearly with bias increment. And this is also consistent with the hardness measurement of 70 nm thick films. We proposed that substrate bias enhances mixing between the carbon films and either the Si films or Al₂O₃TiC substrate such that thin films contain less sp³ fraction. These mixing bonds are longer than C–C bonds, which inducing the hardness decreasing of ultra-thin DLC films with bias. But for 70 nm DLC, the effect of mixing layer can be negligible by compared to bias effect with higher carbon ion energy. So sp³ content will increase for thick films with substrate bias.     

The co-optimization of efficiency and emission bandwidth in GSAGG:Cr3+ NIR ceramic phosphors

The NIR phosphor-converted light-emitting diode (NIR pc-LED) is a new near-infrared light source that has been widely studied. Among various NIR phosphors, Cr³⁺ doped gadolinium aluminum gallium garnet (GAGG:Cr³⁺) ceramic phosphor has shown great potential due to its ultra-high efficiency and thermal stability. Despite its capabilities, its detection range may be limited due to a relatively narrow emission bandwidth. To make the GAGG:Cr³⁺ ceramic phosphors achieve both high efficiency and broadband emission, a series of Gd₃Al_2-x-ySc_xGa₃O₁₂:yCr³⁺ (GASGG:Cr³⁺) ceramic phosphors were prepared. Thanks to the decrease of crystal field strength with the doping of Sc³⁺, the full width at half maximum (FWHM) of GASGG:Cr³⁺ ceramic phosphors were extended from 84 nm to 117 nm, and the emission peak exhibited a red-shift of 46 nm. Meanwhile, it still retained extremely high external quantum efficiency (EQE = 47%) and excellent thermal stability (90.7%@150 °C). Then, a NIR pc-LED prototype device was fabricated by combining GASGG:Cr³⁺ ceramic phosphor with a blue LED chip. The NIR light output power and the photoelectric conversion efficiency of this device achieved 646 mW and 19.2%, respectively. Finally, the application effect in night vision and venography of this prototype device was demonstrated.     

The effect of CNT content on the surface and mechanical properties of CNTs doped diamond like carbon films

The carbon nanotubes (CNTs) doped diamond like carbon films were carried out by spinning coating multi-walled carbon nanotubes (CNTs) on silicon covered with diamond like carbon films via PECVD with C₂H₂ and H₂. The results show that the I_D/I_G and sp²/sp³ ratios are proportional to the CNT contents. For wettability and hydrogen content, the increase of CNT content results in more hydrophobic and less hydrogen for CNT doped DLC films. As for mechanical properties, the hardness and elastic modulus increases linearly with increasing CNT content. The residual stress is reduced for increasing CNT content. As for the surface property, the friction coefficient is reduced for higher CNT content. For CNT doped DLC films, the inclusion of horizontal CNT into DLC films increases the hardness, elastic modulus and reduces the hydrogen content, friction coefficient and residual stress. Like the light element and metal doping, the CNT doping has effects on the surface and mechanical properties on DLC which might be useful to specific application.     

Effect of substrate bias voltage on the properties of diamond-like carbon thin films deposited by microwave surface wave plasma CVD

Diamond-like carbon (DLC) thin films were deposited on silicon and ITO substrates with applying different negative bias voltage by microwave surface wave plasma chemical vapor deposition (MW SWP-CVD) system. The influence of negative bias voltage on optical and structural properties of the DLC film were investigated using X-ray photoelectron spectroscopy, UV/VIS/NIR spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. Optical band gap of the films decreased from 2.4 to 1.7 with increasing negative bias voltage (0 to − 200 V). The absorption peaks of sp³ CH and sp² CH bonding structure were observed in FT-IR spectra, showing that the sp²/sp³ ration increases with increasing negative bias voltage. The analysis of Raman spectra corresponds that the films were DLC in nature.     

Molecular structure of SiOx-incorporated diamond-like carbon films; evidence for phase segregation

Silicon-oxide incorporated amorphous hydrogenated diamond-like carbon films (SiO_x–DLC, 1 ≤ x ≤ 1.5) containing up to 24 at.% of Si (H is excluded from the atomic percentage calculations reported here) were prepared using pulsed direct current plasma-enhanced chemical vapour deposition (DC-PECVD). Molecular structure, optical properties and mechanical properties of these films were assessed as a function of Si concentration. The spectroscopic results indicated two structural regimes. First, for Si contents up to ~ 13 at.%, SiO_x–DLC is formed as a single phase with siloxane, O–Si–C₂, bonding networks. Second, for films with Si concentrations greater than 13 at.%, SiO_x–DLC with siloxane bonding and SiO_x deposit simultaneously as segregated phases. The variations in mechanical properties and optical properties as a function of Si content are consistent with the above changes in the film composition.     

Fast deposition of diamond-like hydrogenated carbon films

Diamond-like hydrogenated carbon films have been formed at low temperatures using methane and acetylene as precursor gases. The source used was of a cascaded arc type employing Ar and Ar/H₂ as carrier gases. Energies of ion species and ion densities in the plasma were measured with a mass energy probe and a Langmuir probe.The films produced were characterized in terms of sp³ content, refractive index, relative hydrogen content, hardness and adhesion. The variation of these parameters is presented as functions of precursor gas flow, process pressure, and surface temperature.Deposition rates up to 30 nm/s have been achieved using acetylene as precursor gas at substrate temperatures below 100 °C. Experiments with acetylene showed deposition rates seven times greater than with methane. The typical sp³ content of 55–78% in the films was determined by X-ray-Excited Auger Electron Spectroscopy (XAES) technique. The hardness and reduced modulus were determined by nanoindentation. Preliminary Atomic Force Microscopy (AFM) studies of the films showed a roughness below 3 nm (R_a).     

Structure and optical properties of Cu-DLC composite films deposited by cathode arc with double-excitation source

Copper-doped diamond-like carbon films (Cu-DLC) were fabricated on silicon and quartz substrates by cathode arc technique with direct-current and pulse double-excitation source. The microstructure, composition, morphology, hardness and optical properties of the films were studied by Raman, XPS, AFM and SEM, UV-Vis, laser ellipsometer and Vickers sclerometer. The results showed that Cu doping increases the size, ordering and amount of sp²-C clusters in the Cu-DLC composite films. The microstructure parameters enhance with increasing the Cu content to 22.4 wt%. All the films show specific nano-structural surface, however, the lower Cu content induces finer particle formation in the Cu-DLC films. When the arc current is higher than 60 A, the roughness and particle size of Cu-DLC composite films increase with increasing the Cu content. The average transmittance of the Cu-DLC films in Vis-NIR region is smaller than 40% when the Cu content exceeds 12.6 wt%. With increasing the Cu content, the optical band gap (E_g) of the films decreases from 3.54 eV to 0.25 eV. The relationships among the E_g, refractive index and extinction coefficient for the Cu-DLC films were found and indirectly revealed the correlation of microstructure and optical properties of the films with the Cu content variation.     

Structural,electrical, and magnetic transport properties of La0.72Ca0.28Mn1−xCrxO3 (0 ≤ x ≤ 0.06) ceramics

To obtain comprehensive materials with both high temperature coefficient of resistivity (TCR) and magnetoresistance (MR) at low magnetic fields, polycrystalline La_0.72Ca_0.28Mn_1?xCr_xO₃ (x = 0, 0.02, 0.04, 0.06) ceramics were prepared herein by sol–gel method. Electronic configuration of Cr³⁺ ions is similar to that of Mn⁴⁺ ions, therefore, successive substitution of Mn with Cr increases electrical resistivity and decreases metal–insulator transition temperature of ceramics, even yielding hump-like feature for Cr-rich (x = 0.06) samples. The best TCR (28.50%·K^?1) and MR (72.37%) values were obtained simultaneously at Cr dopant content of 0.02 (La_0.72Ca_0.28Mn_0.98Cr_0.02O₃). Strong response of the material to temperature and magnetic field was caused by minimal symmetry of orthorhombic structure and the most robust Jahn–Teller distortion. With increasing Cr content, Mn³⁺/Mn⁴⁺ or Mn³⁺/Cr³⁺ double exchange was diluted, and Cr³⁺/Cr³⁺ or Cr³⁺/Mn⁴⁺ superexchange was promoted. However, the internal competition effect was not conducive to the improvement of material properties.     

Substrate temperature effects on the microhardness and adhesion of diamond-like thin films

Diamond-like films were deposited on silicon substrates by r.f. plasma-enhanced chemical vapor deposition from gas methane. In this study, the substrate temperature, T_S, was varied in a wide range from 20 to 370°C while maintaining fixed other important process parameters such as r.f. power (70 W) or pressure (2.5 Pa). The increase of T_S causes an increase of the sp²/(sp²+sp³) bonded carbon ratio and a decrease of the hydrogen content. These changes produce a great modification of the mechanical properties: microhardness, friction coefficient and adhesion. The variations of mechanical properties with T_S correlate well with the sp²/(sp²+sp³) bonded carbon ratio and the hydrogen content in the films showing a gradual transformation of the diamond-like structure into a more sp²-rich one.     

Synthesis and optical properties of Cr-doped Ba2In2O5 orange-brown oxides

Cr-doped Ba₂In_2-xCr_xO_5+y (0?≤?x?≤?0.5) orange-brown inorganic oxides have been successfully synthesized by solid-state reaction. X-ray diffraction (XRD) patterns show that their crystal structures transform from orthorhombic to cubic by increasing the Cr doping amount. Field emission scanning electron microscopy (FESEM) measurements show powder particles of irregular shape with average particle size of about 10?µm. The colors of the compounds are tuned from light yellow (x?=?0) to orange (x?=?0.1) and then to dark brown (x?=?0.5). X-ray photoelectron spectroscopy and UV–Vis absorption spectroscopy demonstrate the oxidation state of Cr to be +?6 and the change in color to be due to the metal-ligand charge transfer of Cr⁶⁺-O^2-.     

CNx coatings sputtered by DC magnetron: hardness,nitrogenation and optical properties

CN_x coatings were deposited by DC magnetron sputtering at lower constant N₂ pressure 0.1 Pa. Nitrogenation of samples was controlled by increase in the DC current. It led a decrease in the nitrogen content N/C in the films, but increased their microhardness. Evaluation of optical properties combining spectroscopic ellipsometry (1.5–4 eV) and the VUV reflection spectroscopy (4–14 eV) by means of Kramers–Kronig analysis showed increase of the π-plasmon resonance peak, which indicates enhancement of amount of π-bonded electrons. It is linked with increase of sp² hybridization. The optical energy gap values indicate semimetallic properties of the CN_x films. A shift in the ε₂(ω) maximum corresponding to σ–σ1 electron transitions in carbon demonstrates deeper band structure changes in highly nitrogenated samples.