Fabrication, microstructure and tribological behavior of pulsed laser deposited a-CNx/TiN multilayer films

a-CN_x/TiN multilayer films were deposited onto high-speed steel substrates by pulsed laser ablation of graphite and Ti target alternately in nitrogen gas. The composition, morphology and microstructure of the films were characterized by energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. The tribological properties of the films in humid air were investigated using a ball-on-disk tribometer. The multilayer films consist of crystalline TiN, metallic Ti and amorphous CN_x (a-CN_x). With an increase in thickness ratio of CN_x to bilayer, the hardness of multilayer film decreases, friction coefficient decreases from 0.26 to 0.135, and wear rate increases. The film with thickness ratio of CN_x to bilayer of 0.47 exhibits a maximum hardness of 30 GPa and excellent wear rate of 2.5 × 10^− 7 mm³ N^− 1 m^− 1. The formation of tribo-layer was observed at contact area of Si₃N₄ ball. The film undergoes the combined wear mechanism of abrasion wear and adhesion wear.     

脉冲激光沉积WSx/a-C复合薄膜的结构与摩擦学性能

采用脉冲激光烧蚀石墨/WS₂组合靶,在硅基片上沉积不同碳质量分数的WSx/a-C复合膜。用能谱仪、扫描电子显微镜和X射线衍射仪对薄膜的成分、形貌和微观组织进行了表征。采用纳米压痕仪、涂层附着力划痕仪和球-盘式摩擦磨损试验机对薄膜的硬度、结合力和大气中(相对湿度50~55%)的摩擦学性能进行了测试。结果表明,薄膜的S/W比稳定在2.0左右且形成了(002)择优取向的WS₂相。随着薄膜中碳质量分数的增加,薄膜的硬度在36.1%C时出现最高值,结合力随之增大且在52.4%C时达到最高值,摩擦因数先降低后增加,在41.2%C时有最小值0.144。薄膜磨损率在(0.91~1.61)×10_-15 m₃N_-1m_-1范围内变化,36.1%C的WSx/a-C复合膜具有最佳耐磨性能。     

Effect of annealing on pulsed laser deposited zirconium oxide thin films

Zirconium oxide thin films were deposited using pulsed laser ablation from a ceramic ZrO₂ target on unheated substrates. Subsequently, the films were annealed in air in the temperature range 400-800 °C. The films were characterized by X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, and optical spectroscopy to investigate the variation of the structural, chemical, and optical properties upon annealing. As-deposited films were amorphous and had a large surface density of ablated particles. Annealing resulted in the growth of monoclinic nano-crystalline, uniform, and transparent films that were slightly sub-stoichiometric. The annealed films were compact and had high values of the refractive index. Extinction coefficients were small, and may be related to the presence of defects. The films exhibited the presence of an indirect band gap, related to defects, and a direct band gap, related to fundamental absorption.     

Characterization of calcium titanate thin films deposited on titanium with reactive sputtering and pulsed laser depositions

In the present study, characteristics of calcium titanate thin films deposited on titanium by reactive sputtering and pulsed laser deposition techniques were investigated. In both techniques, a calcium titanate target was used as a deposition source, and the titanium substrate was heated at 873 K during the deposition. The oxygen flow for the reactive sputtering was in the range of 1 to 10 sccm, and the oxygen pressure for the pulsed laser deposition was in the range of 0.13 to 13 Pa. The deposited films were crystallized into perovskite-type calcium titanate; furthermore, a titanium-dioxide layer formed in the interface between the film and substrate. In the film deposited by reactive sputtering with low oxygen flow, titanium-to-calcium ratio ([Ti]/[Ca]) is lower than that of stoichiometric calcium titanate due to the formation of calcium hydroxide. The ratio increases with an increase of oxygen flow, and the ratio of the film deposited with a 10-sccm oxygen flow was almost in accordance with that of stoichiometric calcium titanate. On the other hand, in the pulsed laser deposition, [Ti]/[Ca] ratios of the deposited film were almost in accordance with that of stoichiometric calcium titanate at the oxygen pressure under the present experimental condition. In both deposition techniques, the thickness of the titanium-oxide layer increased with an increase of the amount of oxygen gases. The results indicate that the pulsed laser deposition has an advantage for the preparation of the stoichiometric calcium titanate film without formation of a thick titanium-oxide layer.     

Effect of nitrogen pressure on structure and optical properties of pulsed laser deposited BCN thin films

Amorphous boron carbon nitride (BCN) thin films were deposited on Si (100) and quartz substrates by laser ablation of a boron carbide (B₄C) target in nitrogen atmosphere. The effects of the nitrogen pre ssure (p_N2) on the film deposition rate, composition, structure and optical properties were investigated. The film deposition rate was measured by a surface profiler, which increased from 3.4 to 6.25 nm/min at elevated p_N2. Structure and composition of the films were investigated by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy. FTIR and XPS analyses indicated that the as-deposited BCN films contained B-C, N-C and B-N chemical bonds, implying the formation of ternary BCN compounds. The nitrogen content in the films increased gradually and then saturated up to ∼ 26 at.% at 10 Pa p_N2. The optical band gap (E_g) increased from 3.78 to 3.92 eV with increasing p_N2 from 2 to 15 Pa. The evolution of E_g is in accordance with the change of film compositions and bonding states.     

Tribological performance of surface engineered tool steel at elevated temperatures

Tribological components operating at elevated temperatures can experience high wear, oxidation, thermal fatigue and changes in mechanical properties. In this work, the friction and wear characteristics of plasma nitrided and surface coated (CrN and TiAlN) tool steel during sliding against AISI52100 bearing steel have been studied at room temperature and 400 °C respectively using a ball on disc machine. Surface profiler and SEM/EDS techniques were used to characterise the surface topography and resulting surface damage of the test specimens. The results show that the friction of plasma nitrided tool steel during sliding against bearing steel ball is very high at room temperature and it drastically drops at 400 °C. The wear is mainly abrasive at room temperature and adhesive at elevated temperatures. In case of CrN coated tool steel the friction is high but its wear is negligible at room temperature. At 400 °C, the friction decreases marginally and transfer of bearing steel to the coated CrN coated disc has been observed. The TiAlN coating has shown relatively lower friction, compared to CrN and negligible wear at room temperature. At 400 °C, the friction is very high and unstable and transfer of TiAlN coating to the mating ball occurs.     

High-repetition rate pulsed laser deposition of ZrC thin films

ZrC thin films were grown on (100) Si substrates by the pulsed laser deposition (PLD) technique using a high-repetition rate excimer laser working at 40 Hz. The substrate temperature during depositions was set at 300 °C and the cooling rate was 5 °C/min. X-ray diffraction investigations showed that the films were crystalline. Films deposited under residual vacuum or 2 × 10^− 3 Pa of CH₄ atmosphere exhibited a (200)-axis texture, while those deposited under 2 × 10^− 2 Pa of CH₄ atmosphere were found to be equiaxed. The surface elemental composition of as-deposited films, analyzed by Auger electron spectroscopy (AES), showed the usual high oxygen contamination of carbides. Once the topmost − 3-5 nm region was removed, the oxygen concentration rapidly decreased, being around 3-4% only in bulk. Scanning electron microscopy (SEM) investigations showed a smooth, featureless surface morphology, corroborating the roughness values below 1 nm (rms) obtained from simulations of the X-ray reflectivity (XRR) curves. From the same simulations we also estimated films mass density values of around 6.32-6.57 g/cm³ and thicknesses that correspond to a deposition rate of around 8.25 nm/min. Nanoindentation results showed a hardness of 27.6 GPa and a reduced modulus of 228 GPa for the best quality ZrC films deposited under an atmosphere of 2 × 10^− 3 Pa CH₄.     

Tribological behaviour of borided bearing steels at elevated temperatures

Borided steels are known to exhibit excellent wear resistance at room temperature. However, the sliding wear behaviour of borided steels at high temperatures is not known. In the present study, AISI 440C and 52100 bearing steels which are extensively used in industry, were borided by pack method at 950 °C for 2 h. X-ray diffraction analysis of boride layers on the surface of steels revealed various peaks of FeB, Fe₂B and CrB. The thickness and hardness of boride layers on the 52100 and 440C steels were 56 ± 6 and 47 ± 4 μm and 1970 and 2160 HK, respectively. Dry sliding wear tests of these borided steels were performed against Si₃N₄ bearing ball at a constant sliding speed and load at elevated temperatures. The temperature changed between room temperature and 600 °C. These tests indicated that the wear rates of unborided and borided steels increase with temperature and borided 52100 and 440C steels exhibit considerably lower wear rate at all temperatures, compared with unborided steels. At temperature of 600 °C, borided 52100 and 440C steels have a wear resistance of about 3 and 2.5 times higher than that of unborided steels, respectively. Examination of the worn surface of borided steels showed that, worn surfaces were covered with a discontinuous compact layer especially above temperature of 300 °C.     

Interface growth morphologies in pulsed laser deposited, room temperature grown multilayer hard coatings

The mechanical behaviour - hardness, elasticity, and adhesion - of multilayer coatings is strongly influenced by the type of the formed interfaces between the different layers. In industrially applied tribological coatings the interface region is predominantly not a perfect sudden change of the chemical composition of the adjacent crystal planes, but a transition zone of a thickness, which is strongly dependent on the energetic conditions during deposition. Multilayer coatings grown by high-energetic deposition techniques always struggle with high atomic mixing of both adjacent coating materials due to high energetic ion implantation.One of these high-energetic deposition techniques is the Pulsed Laser Deposition (PLD), characterized by pulsed and within one pulse alternating high- and low-energetic particle fractions, hitting successively the substrate surface. Such deposition conditions were shown to be highly advantageous for low temperature deposition by the densification of the growth structures due to activated diffusion and re-sputtering, but increases the difficulty in depositing multilayer structures.The current paper addresses these specific growth conditions based on Ti/TiN and Cr/CrN multilayer coatings. High resolution transmission electron microscopy results show that the atomic mixing at the interface is not highly critical for the deposition of multilayer coatings and that extremely dense growth structures are forming even in the interface regions.     

Thickness modulation effect of CeO_2 layer for YBCO films grown by pulsed laser deposition

CeO_2 film plays an essential role in nucleation and growth of YBa_2 Cu_3 O_7-x(YBCO) films. In this work,the dependence of superconducting properties of YBCO on CeO_2 films with different thicknesses was investigated,in order to achieve fabrication of high-performance YBCO coated conductors in industrial scale. The crystalline structure and morphology of CeO2 films with thickness ranging from 21 to 563 nm were systematically characterized by means of X-ray diffraction(XRD), atomic force microscope(AFM) and reflection high-energy electron diffraction(RHEED). Additional focus was addressed on evolution of the surface quality of CeO_2 films with thickness increasing. The results show that at the optimal thickness of 221 nm, CeO_2 film exhibits sharp in-plane and out-of-plane texture with full width of half maximum(FWHM) values of 5.9° and 1.8°, respectively, and smooth surface with a mean root-mean-square(RMS) roughness value as low as 0.6 nm. Combing RHEED and transmission electron microscope(TEM) cross-sectional analysis, it is found that nucleation and growth of CeO_2 films at early stage remain in island growth mode with rougher surface,while further increasing the thickness beyond the optimal thickness leads to weak surface quality, consequently resulting in degradation of superconductor layers deposited subsequently. Eventually, a critical current density(J_c) as high as 4.6×10~6 A·cm^-2(77 K, self-field) is achieved on a YBCO film on a thickness-modulated CeO_2/MgO/Y_2 O_3/Al_2 O_3/C276 architecture, demonstrating the advantages of CeO_2 films as buffer layer in high-throughput manufacture of coated conductors.     

Influence of nitrogen background pressure on structure of niobium nitride films grown by pulsed laser deposition

Depositions of niobium nitride thin films on Nb using pulsed laser deposition (PLD) with different nitrogen background pressures (10.7 to 66.7 Pa) have been performed. The effect of nitrogen pressure on NbN formation in this process was examined. The deposited films were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM), and energy dispersive X-ray (EDX) analysis. Hexagonal β-Nb₂N and cubic δ-NbN phases resulted when growth was performed in low nitrogen background pressures. With an increase in nitrogen pressure, NbN films grew in single hexagonal β-Nb₂N phase. The formation of the hexagonal texture during the film growth was studied. The c/a ratio of the hexagonal β-Nb₂N unit cell parameter increases with increasing nitrogen pressure. Furthermore, the N:Nb ratio has a strong influence on the lattice parameter of the δ-NbN, where the highest value was achieved for this ratio was 1.19. It was found that increasing nitrogen background pressure leads to change in the phase structure of the NbN film. With increasing nitrogen pressure, the film structure changes from hexagonal to a mixed phase and then back to a hexagonal phase.     

Tribological performance of DLC films deposited on ACM rubber by PACVD

In this paper the tribological and adhesive performance of DLC films prepared by plasma assisted chemical vapor deposition on acrylic rubber (ACM) are studied. The effect of applied load and sliding velocity on the coefficient of friction and wear rate has been investigated. Effects of the rubber substrate and of ageing of the coated samples have also been explored. In addition, the adhesion of the DLC films to the rubber substrates is evaluated via stretch tests and the measured adhesion strength is larger than 40 MPa, indicating a superb adhesion to the substrate. It is shown that the tribological performance is greatly influenced by the viscoelastic properties of the substrate, and higher coefficients of friction are obtained at higher loads and velocities. The wear followed a similar trend, although very low in all the cases.     

Oriented nanocrystals in SrLaMnTiO6 perovskite thin films grown by pulsed laser deposition

We have fabricated SrLaMnTiO₆ thin films by PLD on different substrates (SrTiO₃, LaAlO₃ and Si). Their texture, width, homogeneity and morphology have been evaluated from XRD, SEM and complex impedance spectroscopy. The thickness ranged between 500 and 8800 nm depending on the synthesis conditions. The epitaxial growing could be interpreted in terms of two contributions of microstructural origin: a matrix part and some surface formations (hemi-spheres), with different texture and size distributions. The films were nanostructured and contained vertically aligned nanopores (VANPs) with a pore average size of 30-60 nm, which are very interesting for eventual SOFC anode applications. Magnetization results indicate an improved response respect to nano-sized powder samples.     

Tribological properties of nickel-graphite composite against different counterfaces at elevated temperatures

Nickel-based graphite-containing composites were prepared by powder metallurgy method. The effect of graphite addition on mechanical properties of nickel-based alloy was investigated and the tribological properties from room temperature to 600 ℃ were tested by a pin-on-disk tribometer with alumina, silicon nitride and nickel-based alloy as counterfaces. The microstructure was analyzed by X-ray diffraction（XRD） and scanning electron microscope （SEM） attached with energy dispersive spectroscopy（EDS）. The worn surfaces at high temperature were observed by optical microscope and SEM. The results show that the tensile strength and hardness of composites decrease after adding graphite, while the friction and wear properties are all improved by adding 12%（mass fraction） graphite. Compared with the counterface of alumina and silicon nitride, the friction coefficients and wear rates are lower when the composite rubs against nickel-based alloy containing molybdenum disulfide.     

Fabrication and characterization of Bi-substituted yttrium iron garnet films by pulsed laser deposition

We have systematically investigated the effects of processing parameters, including various oxygen pressures (Po₂) ranging from 200 to 1000 m Torr and substrate temperatures (T_s) ranging from 500 to 750°C, on the characteristics of Bi-substituted yttrium iron garnet (Bi:YIG) films grown on (111) gadolinium gallium gamet (GGG) substrates. Bi: YIG films were grown using a pulsed laser deposition method with an ArF excimer laser (λ=193 nm). Although the compositions of all the Bi:YIG films grown at the constant T_s of 600°C were close to the target composition irrespective of Po₂, the Bi contents were slightly increased with increasing Po₂, which was consistent with the variation in Faraday rotation angles (θ_F). In addition, the crystallinity of the Bi:YIG films was deteriorated with increasing Po₂, and their grains became irregular. At T_s above 700°C, Bi-deficient yttrium iron garnet films were grown. Consequently, high quality epitaxial Bi:YIG films exhibiting Faraday rotation angles of −1.0≈−1.5 degree/μm were successfully grown at the substrate temperatures in the range of 500–650°C when the Po₂ of 200 mTorr was used.     

The formation of diamond-like carbon film at atmospheric pressure by the pulsed laser/plasma hybrid deposition method

A pulsed laser/plasma hybrid deposition method has been developed to produce the diamond-like carbon (DLC) film at atmospheric pressure in this work. A plasma torch was used to heat up the carbon particles which were simultaneously ablated by a pulsed laser, thus the kinetic energy of the carbon particle can be increased to form the carbon atoms with amorphous bonding structure of the DLC film by this proposed hybrid deposition method.The influences of the plasma flow have been examined numerically and experimentally. In the numerical analysis, the temperatures of the plasma flow at various inlet pressures and electrode currents have been predicted. According to the experimental results of the carbon film inspected by the Raman spectroscopy, it reveals that the intensity ratio of the D-band to G-band of the carbon film can be reduced to 0.5 by the implementation of plasma flow. Therefore the DLC film was solidly formed. The adhesive strength of the DLC film was also characterized by the scratch test, it can be found that the critical loading of the film is up to 19 N.     

Microstructure and grain growth kinetics of nanocrystalline SnO2 thin films prepared by pulsed laser deposition

The isothermal grain growth of SnO₂ thin films prepared by pulsed laser deposition techniques was investigated at Si (100) substrate temperatures between 300 and 450 °C with 50 °C intervals for different annealing times. X-ray diffraction patterns proved that the average grain sizes are in the range of 2.4–27.8 nm. The grain growth data were analyzed using two different models. The first model, assuming normal grain growth as that in conventional polycrystalline materials, yields large grain growth exponent (n) and extremely low activation energy (Q). Although it can describe the evolution of grain sizes, it fails to give satisfactory physical interpretation of n and Q, both beyond the theoretical predictions. The second model is based on the structural relaxation of the interface component in nanocrystalline materials. In this case, the ordering of distorted interfaces by structural relaxation proceeds with grain growth. This structure relaxation model not only describes the evolutions of grain growth well, but also makes reasonable attribution of the low activation energy to the short-range rearrangement of atoms in the interface region as well.     

Tribological behavior of Ni-Cr-based composite at high temperatures

The wear behavior of Ni-Cr-based alloys was investigated at ambient and elevated temperatures. The wear samples were prepared by metallurgical hot pressing. Wear tests were carded out on a general purpose wear testing machine having a heating unit and pin-disc sample configuration. The counterface material was prepared from Al2O3 ceramics. The tests were carded out at room temperature （RT）, 200 ℃ and 600℃. The effects of temperatures on the tribological properties were determined by using optical microscopy, and X-ray diffractometry. The results show that at room temperature the worn surfaces of the alloys are characterized by mild scuffing and micro cracks, the action of nano-crystal structural wear debris on the worn surfaces is responsible for the reduction of friction. At 200℃, the friction coefficient is the highest. The worn surfaces of the alloys are adhesive and oxidative. At 600℃, the friction coefficient is reduced due to the effect of the oxides, tungstates and sulfides residue on the worn surface.     

Pulsed laser deposited Sb2Se3 anode for lithium-ion batteries

Sb₂Se₃ thin film has been successfully fabricated by reactive pulsed laser deposition and was investigated for its electrochemistry with lithium for the first time. The reversible discharge capacities of Sb₂Se₃/Li cells cycled between 0.3 and 2.5 V were found in the range of 530.5–660.7 mAh g⁻¹ during the first 100 cycles. By using ex situ X-ray diffraction, transmission electron microscopy, and selected-area electron diffraction measurements, both classical alloying process and the selenylation/reduction of nanosized metallic antimony were proposed in the lithium electrochemical reaction of Sb₂Se₃. Sb₂Se₃ has high reversible capacity and good cycle performance, which makes it potential anode material for future lithium-ion batteries.