Synthesis and properties of W-Se-C coatings deposited by PVD in reactive and non-reactive processes

Transition metal dichalcogenides (TMD) are well known for their self lubricant properties, due to unique crystal structure, with low hardness which makes them inappropriate in applications requiring high load bearing capacity. Nevertheless, there is still a considerable potential for further improvements since other TMDs such as diselenides remain almost unknown. The WSeC coatings were deposited by either co-sputtering from WSe₂ and C targets or by working in reactive mode in an Ar + CH₄ atmosphere. Carbon content varied from 25 at.% up to 70 at.%, Se/W ratio of co-sputtered coatings was between 0.9 and 1.0, with no significant influence of the carbon content on their variations, while in the reactive process it reached values higher than 1.7. The hardness of the coatings was evaluated by nanoindentation, with values between 1.6 and 5.6 GPa. The structure of the coatings was analysed by X-ray diffraction in glancing mode, showing that the coatings have a quasi-amorphous structure. Finally, the selected samples were tested on pin-on-disc showing low friction properties of WSeC co-sputtered coatings.     

Failure morphologies of cyclically oxidized ZrO2-based thermal barrier coatings

Abstract

Advanced and baseline thermal barrier coatings (TBCs) were thermal cycle tested in air at 1163°C until delamination or spallation of the ceramic top coat. The top coat of the advanced TBC’s consisted of ZrO₂ with various amounts of Y₂O₃, Yb₂O₃, Gd₂O₃, or Nd₂O₃ dopants. The composition of the top coat of the baseline TBC was ZrO₂-8wt.%Y₂O₃. All top coats were deposited by air plasma spraying. A NiCrAlY or NiCoCrAlY bond coat was deposited by low pressure plasma spraying onto a single-crystal, Ni-base superalloy. The TBC lifetime for the baseline coatings was approximately 190 cycles (45 minutes at 1163°C per cycle) while the lifetime for the advanced coatings was as high as 425 cycles. The fracture surfaces and sample cross sections were examined after TBC failure by SEM and optical microscopy, and the top coats were further examined by X-ray diffraction. These post-test studies revealed that the fracture path largely followed splat boundaries with some trans-splat fracture. However, there were no obvious distinguishing features which explained the difference in TBC lifetimes between some of the advanced and baseline coatings.     

Structure and morphology of TiB2 duplex coatings deposited over X40 CrMoV 5-1-1 steel by DC magnetron sputtering

The deposition of titanium diboride (TiB₂) films over tool steel substrates (AISI H13 premium/EN X40 CrMoV 5-1-1) is being investigated due to its excellent corrosion resistance and chemical stability against liquid aluminium. The use of nitrided steels as substrates for TiB₂ deposition may contribute to increase its adhesion and the overall steel resistance in applications such as forging, extrusion and die casting of aluminium. Duplex coatings were obtained by the PVD deposition of TiB₂ films over heat treated and nitrided steel using non-reactive DC magnetron sputtering from a TiB₂ target, varying the substrate bias voltage. Well structured and crystalline TiB₂ films were obtained for the selected deposition conditions, the best crystalline coatings being obtained for the positively biased substrates. Selected films produced over die-casting pins at a bias voltage of +50 V were tested for resistance to liquid aluminium soldering by immersion tests, and compared with the nitrided steel. The duplex TiB₂ coating has a much larger chemical resistance to attack by molten aluminium alloy than the just nitrided steel. Where there is soldering, steel is rapidly attacked and a complex Al-Fe-Si intermetallic forms.     

Experimental research on residual carbon of Y211 inclusions in a melt-textured Y123/Y211 composite

The inherent doping of residual carbon during the preparation of Y₂BaCuO₅ (Y211) inclusions would degrade the physical and mechanical performance of the Y₁Ba₂Cu₃O_7-x (Y123) superconducting matrix. Y211 precursor powders were prepared by the oxalate coprecipitation process in this study. Residual carbon contents of Y211 powders under different heat treatment processes were studied by the high-frequency combustion infrared absorption method. The residual carbon content of Y211 reached the current best level ～0.012% when calcined in O₂ flow at 950°C for 20?h. Y211 powders with the lowest carbon content were used to prepare a small batch of melt-textured Y123/Y211 composites. All samples were single-domain crystals without macro defects, which were usually caused by the emission of CO₂. Among them, one sample (Ø27?mm?×?14?mm) has a maximum levitation force of 71?N (77?K, 0.5?T), with critical current density J_c of 3.2?×?10⁸ A/m² (77?K, 0?T).     

Microstructural investigations of Ni and Ni2Al3 coatings exposed in biomass power plants

Abstract

The present work investigates the corrosion resistance of Ni and Ni₂Al₃ coated austenitic stainless steel (TP347H) tubes, which were exposed in a biomass-fired boiler with an outlet steam temperature of 540 °C for 6757 h. The Ni₂Al₃ coating was produced by electroplating Ni followed by low temperature pack cementation. After exposure, microstructural investigations were performed by light optical and electron microscopy (SEM-EDS). Electroplated Ni coatings were not protective in straw firing power plants and exhibited similar corrosion morphology as uncoated tubes. For Ni₂Al₃ coatings, the nickel aluminide layer was no longer adherent to the tube and was only found within the deposit. However, Ni₂Al₃ coatings had provided some protection compared to uncoated and Ni coated tubes. The formation of nickel chloride binds aggressive chlorine and slows down the active oxidation mechanism. In local areas, sulphidation corrosion attack of Ni was detected.     

Vickers Indentation Fracture (VIF) modeling to analyze multi-cracking toughness of titania, alumina and zirconia plasma sprayed coatings

For massive brittle materials, the fracture toughness in mode I, K_IC, can be determined using various reliable techniques. Besides, Vickers Indentation Fracture (VIF) technique has been developed to locally determine fracture toughness. However, since the indentation test generates a complex three-dimensional crack system around the indent, fracture toughness, K_C, is calculated instead of K_IC. Consequently some authors rightly reject the VIF technique to determine standard fracture toughness by arguing that the literature counts numerous VIF crack equations thus revealing discrepancies of this technique. Nevertheless in some cases (e.g. brittle ceramic coatings) inclusive material techniques are not applicable since presence of the substrate and/or multi-crack network can modify the crack propagation into the coating.In this work, we employed VIF technique to study multi-cracking behavior of titania, alumina and zirconia ceramic oxide coatings obtained by plasma spraying. To calculate VIF toughness, we propose (i) to select two crack equations for radial-median and Palmqvist cracking modes respectively, (ii) to adjust the crack equation of Miranzo and Moya for intermediate cracking mode, (iii) to develop a mathematical approach to determine the cracking mode, (iv) to take into account the multi-crack network by defining an equivalent four-crack system and (v) to propose a universal crack equation applicable independently of the cracking mode.     

Creep behaviour of carbon containing Fe3Al alloy

Abstract

Tensile creep of a Fe–16 wt-%Al–0·5 wt-%C alloy was investigated over a temperature range of 773 to 873 K and stress range of 80 to 200 MPa. Creep curves exhibited very limited primary and secondary creep regimes. An extended tertiary creep regime was observed for all the test conditions. Stress dependence of minimum creep rate can be represented by a power-law equation with stress exponents being in the range 4 to 5. The activation energy for creep was found to be ～340 kJ mol^?1. The observed stress exponent and activation energy for creep suggest that creep is controlled by dislocation climb. Creep fracture in Fe₃Al–C alloy is predominantly by transgranular ductile mode by nucleation, growth and coalescence of microvoids formed at FeAlC_0·5 particle/matrix interface by decohesion as well as fracture of elongated particles. Extended tertiary creep observed in the alloy was analysed in the light of the mechanisms proposed for nickel based superalloys.     

Effect of Ti or TiN codeposition on the performance of MoS2-based composite coatings

Properties of MoS₂ coatings can be improved by the codeposition of a small amount of titanium (Ti). In this study, MoS₂-based composite coatings, which consisted of Ti or titanium nitride (TiN) and molybdenum disulfide, are synthesized by unbalanced plasma plating. Two forms of MoS₂-based composite coatings have been developed: TiN-MoS₂/Ti and TiN-MoS₂/TiN coatings. The effect of Ti or TiN and processing parameters on properties of such coatings are investigated. Scanning electron microscopy (SEM), dry drilling and turning tests are used to determine the structure, composition and tribological performance of MoS₂-based coatings. Experimental results indicate that these MoS₂-based composite coatings are considerably more wear-resistant and are less sensitive to atmospheric water vapor than pure MoS₂ coatings. These coatings show excellent results when tested for a wide range of industrial turning and drilling applications.     

ZnO复合石墨相氮化碳纳米片的制备及其光催化性能研究

为了提高石墨相氮化碳光催化性能,本文以尿素、硫脲、醋酸锌为前驱体,通过氧化热剥离与共混煅烧法分别制备g-C₃N₄纳米片和ZnO/g-C₃N₄异质结复合材料,并采用TEM、FTIR、XRD、UV-Vis DRS、BET等表征手段对制备的催化剂进行结构表征。以罗丹明、大肠杆菌为探针,考察了催化剂的光催化降解性能和抑菌活性。结果表明：以尿素和硫脲为前驱体,经过氧化热剥离处理后能得到的g-C₃N₄ 2D纳米片,其比表面积更大、光催化性能更加优异,且其对罗丹明的降解率较未剥离的g-C₃N₄提高了21.2%。在40 min氙灯照射下,纯g-C₃N₄并未表现出良好的抑菌性能,而通过ZnO复合制备的ZnO/g-C₃N₄异质结复合材料,在光催化降解率和抑菌活性方面均有很大提高,其中复合20%ZnO制得的ZnO异质结复合材料表现出最佳的光催化性能...     

Propagation of a.c. magnetic field through high-T c coatings

Studies on the propagation of AC magnetic field through plasma-sprayed superconducting Y₁Ba₂Cu₃O_7−x coatings show that complete shielding is achieved up to a certain critical magnetic field strengthH ₀. Increase in the thickness orJ _c of the specimen increases theH ₀ value. Flux-trapping occurs in the specimen at high frequencies and the frequency at which it occurs increases with increase in specimenJ _c .     

Investigation of damage threshold to TiO2 coatings at different laser wavelength and pulse duration

Laser-induced damages to TiO₂ single layers and TiO₂/SiO₂ high reflectors at laser wavelength of 1064 nm, 800 nm, 532 nm, and pulse width of 12 ns, 220 ps, 50 fs, 8 ns are investigated. All films are prepared by electron beam evaporation. The relations among microstructure, chemical composition, optical properties and laser-induced damage threshold (LIDT), have been researched. The dependence of damage mechanism on laser wavelength and pulse width is discussed. It is found that from 1064 nm to 532 nm, LIDT is mainly absorption related, which is determined by film's extinction coefficient and stoichiometric defects. The rapid decrease of LIDT at 800 nm is due to the pulse width factor. TiO₂ coatings are mainly thermally by damaged at long pulse (τ ≥ 220 ps). The damage shows ablation feature at 50 fs.     

Deformation behavior of complex carbon nitride and metal nitride based bi-layer coatings

A bi-layer coating consisting of a TiAlCrN inner layer and complex carbon nitride (CN_x) plus CrCN outer layer was deposited on to a high speed steel (M2) substrate via physical vapour deposition (PVD). Detailed microstructural analysis of the coating has been performed via transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XPS analysis indicated that the outer layer contained both C-C bonds, associated with an amorphous phase, and the presence of CrCN, which was confirmed by TEM analysis. Localized deformation of the coating was performed using nanoindentation with a spherical indenter. Force-displacement curves of the indentation tests exhibited a number of ‘pop-ins’ during the loading cycle, indicative of cracking and other deformation events. Cross-sectional analysis of the indents revealed extensive cracking in the TiAlCrN layer and shear steps at the steel-nitride interface, consistent with the events observed in the force-displacement curves. On the other hand minimal cracking was observed in the CN_x + CrCN layer. It is believed that the relatively ductile outer layer inhibits the propagation of cracks from the inner, brittle layer.     

Chemical stability of carbon nanotubes in the 2024Al matrix

Five volume percent of carbon nanotubes and 2024Al alloy powder were mixed with ball milling method, and then the composite was fabricated at 873 K by hot pressing sintering technique. The microstructure of the composite was investigated using optical microscopy, transmission electron microscopy, and X-ray diffraction. The experimental results showed that carbon nanotubes are reacted and changed into Al₄C₃. Nano-Al₄C₃ phases with needle shape are distributed mainly on Al grain boundaries; meanwhile some of them exist within Al grains. The reaction mechanism of carbon nanotubes-Al is discussed.     

C/C复合材料SiC/ZrB2-MoSi2复合涂层的抗氧化机制

采用包埋法和刷涂法在C/C复合材料基体上制备SiC/ZrB2-MoSi2抗氧化涂层,并利用SEM和XRD等测试手段对抗氧化涂层的组织结构、抗氧化性能和抗氧化机制进行了研究。研究结果表明,SiC内涂层可有效解决外涂层ZrB2-MoSi2与C/C复合材料基体间热膨胀系数差异较大的难题。刷涂法制备的ZrB2-MoSi2外涂层虽然有大量的龟裂纹,但涂层试样在1500℃空气中氧化10 h,失重率仅为3.58%,涂层具有很好的自愈合能力,表现出优异的高温抗氧化性能和抗热震性能。     

Synthesis of nano-TiC powder using titanium gel precursor and carbon particles

The paper presents a novel process for synthesis of nano-size titanium carbide by reaction between titanium bearing precursor gel and nano carbon particles derived from soot at different temperatures in the range of 1300-1580 °C for 2 h under argon cover. The HRTEM studies of TiC powder synthesized by heating at 1580 °C show the presence of cube shaped particles (~ 60-140 nm) and hollow rods (diameter ~ 30-185 nm). The average particle size of crystallites, calculated by Scherer equation is observed to be ~ 35 nm while the surface area-density measurements indicate it to be ~ 113 nm. The surface area decreases with increase in reaction temperature.     

Fracture behavior under mixed-mode loading of ceramic plasma-sprayed thermal barrier coatings at ambient and elevated temperatures

The fracture behavior under modes I and II loading of ceramic plasma-sprayed thermal barrier coatings was determined in air at 25 and 1316 °C in asymmetric four-point flexure. The mode I fracture toughness was found to be K_Ic = 1.15 ± 0.07 and 0.98 ± 0.13 MPa , respectively, at 25 and 1316 °C. The respective ‘nominal’ mode II fracture toughness values were K_IIc = 0.73 ± 0.10 and 0.65 ± 0.04 MPa . The empirical mixed-mode fracture criterion best described the coatings’ fracture behavior under mixed-mode loading. The angle of crack propagation was in reasonable agreement with the minimum strain energy density criterion.     

Effect of nano-SiO2 modified emulsion sizing on the interfacial adhesion of carbon fibers reinforced composites

The influence of nano-SiO₂ modified epoxy emulsion sizing on the interfacial adhesion properties of carbon fibers reinforced composites was investigated. The interfacial interaction between carbon fibers and the matrix was characterized by X-ray photoelectron spectrometry (XPS), scanning electron microscopy (SEM) and three-point short-beam shear testing. The results showed that the amount of hydroxyl groups was slightly increased on the carbon fibers surface after treatment with nano-SiO₂ modified sizing. Compared to the unsized composites, the interlaminar shear strength (ILSS) values for the composites with unmodified sizing and nano-SiO₂ modified sizing were increased by 9% and 14%, respectively. The holes and carbon fibers pullout were not observed in their fracture sections. Surprisingly, the fracture section of the composites with nano-SiO₂ modified sizing was more compact and the fiber debonding was more difficult.     

Formation of nanoparticles and nanorods in a N2-C2H4-H2 atmospheric pressure dielectric barrier Townsend discharge

In the present publication, the effect of the addition of H₂ in an atmospheric pressure Townsend Dielectric Barrier Discharge in an atmosphere of N₂-C₂H₄ is examined with an emphasis put on the evaluation of the surface chemistry and growth mechanisms. Scanning Electron Microscopy, Atomic Force Microscopy, X-Ray Photoelectron Spectroscopy and Fourier Transform Infrared Spectroscopy were used to characterize the coatings. For all H₂ concentrations, films obtained present high N/C ratio (∼ 0.8) with high NH_x and nitrile content. However, when H₂ is added in the discharge, nanoparticles/nanorods are formed imbedded in a smooth film. The average size of the nanorods is 100-200 nm in diameter with length from 1 to 10 μm. A growth mechanism is proposed to explain the formation of the nanorods on the surface of the coating in the presence of H₂.     

Deposition and characterization of TiAlN/Si3N4 superhard nanocomposite coatings prepared by reactive direct current unbalanced magnetron sputtering

Superhard nanocomposite coatings of TiAlN/Si₃N₄ with varying silicon contents were synthesized using reactive direct current (DC) unbalanced magnetron sputtering. The Si and TiAl targets were sputtered using an asymmetric bipolar-pulsed DC power supply and a DC power supply, respectively, in Ar+N₂ plasma. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) and nanoindentation techniques, respectively. The elemental composition of the TiAlN/Si₃N₄ nanocomposite coatings was determined using energy-dispersive X-ray analysis and the bonding structure was characterized by X-ray photoelectron spectroscopy. The surface morphology of the coatings was studied using atomic force microscopy. The XRD data showed that the nanocomposite coatings exhibited (1 1 1) and (2 0 0) reflections of cubic TiAlN phase. The broadening of the diffraction peaks with an increase in the silicon content in the nanocomposite coatings, suggested a decrease in the average crystallite size. The TiAlN/Si₃N₄ nanocomposite coatings exhibited a maximum hardness of 43 GPa and an elastic modulus of 350 GPa at a silicon concentration of approximately 11 at%. The hardness and the elastic modulus of the nanocomposite coatings decreased significantly at higher silicon contents. Micro-Raman spectroscopy was used to characterize the structural changes as a result of heating of the nanocomposite coatings in air (400-850 °C) and in vacuum (900 °C). The Raman data of the nanocomposite coatings annealed in air and vacuum showed better thermal stability as compared to that of the TiAlN coatings. Similarly, the nanocomposite coatings deposited on mild steel substrates exhibited improved corrosion resistance.