Microstructure and mechanical properties of WC–C nanocomposite films

WC–C nanocomposite film was prepared by using a hybrid deposition system of r.f.-PACVD and DC magnetron sputtering. W concentration in the film was varied from 5.2 to 42 at.% by changing the CH₄ fraction of the mixture sputtering gas of Ar and CH₄. Hardness, residual compressive stress and electrical resistivity were characterized as a function of W concentration. Raman spectroscopy, XRD and high resolution TEM were employed to analyze the structural change in the film for various W concentrations. In the present W concentration range, the film was composed of nano-sized WC particles of diameter less than 5 nm and hydrogenated amorphous carbon matrix. Content of the WC particles increased with increasing W concentration. However, the mechanical properties of the film increased only when the W concentration was higher than 13 at.%. Structural analysis and electrical conductance measurements evidently showed that the increase in hardness and residual stress occurred as the WC particles were in contact with each other in the amorphous carbon matrix.     

Evolution mechanism of the microstructure and mechanical properties of plasma-sprayed yttria-stabilized hafnia thermal barrier coating at 1400 °C

Yttria stabilized hafnia (Hf_0.84Y_0.16O_1.92, YSH16) coatings were sprayed by atmospheric plasma spraying (APS). The effects of thermal aging at 1400 °C on the microstructures, mechanical properties and thermal conductivity of the coatings were studied. The results show that the as-sprayed coating was composed of the cubic phase, and the nano-sized monoclinic (M) phase was precipitated in the annealed coating. The presence of M phase effectively constrained the sintering of the coating due to its superior sintering-resistance. The Young's modulus kept at a nearly same level of ~78 GPa even after annealing, and the coating annealed for 6 h yielded a maximum value of hardness but revealed a declining tendency in the Vicker's hardness with prolonged sintering time. The thermal conductivity increased from 0.8-0.95 W m^-1 K^-1 at as-sprayed state to 1.6 W m^-1 K^-1 after annealing at 1400 °C for 96 h. The dual-phase coating is promising to serve at temperatures above 1400 °C due to its excellent thermal stability and mechanical properties.     

Microstructure,kinetics and mechanisms of CO2 catalytic decomposition over freshly reduced nano-crystallite CuFe2O4 at 400–600 °C

The decomposition of CO₂ was investigated as a process of both industrial and environmental importance. Copper ferrite was obtained by the thermal decomposition of acetate precursors. CuFe₂O₄ were isothermally reduced in H₂ flow at 400–600 °C, the isothermal reduction profiles obtained in this study show that a topochemical mode of reduction is done by which the reduction process proceeds. The nano-wires metallic phase of iron (106 nm) and copper (56 nm), produced from the complete reduction of CuFe₂O₄, were subjected to the direct reoxidation in CO₂ flow at 400–600 °C. The reoxidation process was found to be controlled by both the reduction and reoxidation temperatures. CO₂ decomposes to carbon nano-tubes during the reoxidation of the freshly reduced CuFe₂O₄. The prepared, completely reduced and reoxidized CuFe₂O₄ compacts, were characterized by XRD, SEM, TEM and reflected light microscope. For the reoxidation process, it is found that at the initial stages the reaction is controlled by the interfacial chemical reaction mechanism with some contribution to the gaseous diffusion mechanism. On the other hand at the intermediate and final stages the mechanism by which the reoxidation process proceeds was found to be the solid-state diffusion.     

Microstructure and mechanical properties of Ti–C–TiN-reinforced Ni204-based laser-cladding composite coating

In situ Ti(C, N), ring phase, and multi-phase enhanced Ni204-based alloy coating were prepared by adding various Ti/C/TiN ratios particles. The effects of the reinforcement phase on the microstructure, microhardness, tribological property, and microstructure characteristics at the interface between the coating and substrate were investigated. The results show that the coatings with a 5:1 mass fraction ratio of TiN/C exhibits the highest microhardness, which is 3.78 times higher than that of the original Ni204 coating. While, the coating with 21:7:2 mass fraction ratio of TiN/Ti/C exhibits the lowest friction coefficient, which is 4.44 times smaller than that of the original Ni204 coating. The addition of Ti and C particles promotes the precipitation of ring phase and carbides, reduces ceramic agglomeration, alleviates the floating of ceramic particles, and improves the bonding strength of reinforcement phases. Owing to the good mutual solubility among Fe, Ni, and, Cr elements, the diffusion happened at the interface between the coating and substrate.     

Microstructure and mechanical properties of TiB2–B4C ceramics fabricated by hot-pressing

In this study, TiB₂–B₄C composite ceramics were prepared using Y₂O₃ and Al₂O₃ as the sintering aids. Different contents of B₄C were added to seek promoted comprehensive mechanical properties of the composites. The mixed powders were sintered at 1850 °C under a uniaxial loading of 30 MPa for 2 h via hot-pressing. Through the measurement of XRD, SEM and related mechanical properties, the influence of B₄C content on the microstructure and mechanical properties of TiB₂–B₄C composites ceramics was discussed. The experimental results show that TiB₂–B₄C composite ceramics exhibit excellent mechanical properties, which can be attributed to the dense microstructure and fine grain size. In addition, TiB₂–B₄C composite ceramic shows a relatively high comprehensive properties when the addition amount of B₄C is 20 wt%. The relative density, Vickers hardness, fracture toughness and flexural strength are measured to be 99.61%, 27.63 ± 1.73 GPa, 4.77 ± 0.06 MPa m^1/2, 612.5 ± 28.78 MPa, respectively.     

Effect of MoS2 content on friction and wear properties of Mo and S co-doped CrN coatings at 25–600 °C

With increasingly harsh working environments for mechanical systems and the rapid development of various high-tech industries, requirements for the stable operation of mechanical systems are increasing in a wide temperature range. Mo and S co-doped CrN coatings with different MoS₂ contents were prepared via unbalanced magnetron sputtering to provide better friction properties to the coatings at high temperatures. Scanning electron microscopy and nanoindentation were adopted to analyze the microstructure and mechanical performance. The mechanical performance of the coatings was enhanced by increasing the MoS₂ content, however, excessive MoS₂ reduced the mechanical properties of the coatings. Besides, the adhesion of the coatings first increased and then decreased rapidly with the increase of the MoS₂ content. In addition, the residual stress of the coating first decreased and then increased upon increasing the MoS₂ content. The high-temperature tribological behavior of the coatings was measured from room temperature (25 °C) to 600 °C. The CrN/MoS₂-0.6A coating was found to exhibit low friction and wear coefficient at room temperature and relatively good comprehensive properties at high temperature. This study provides a feasible design for engineering applications and lays the foundations for the preparation of coatings with superior high-temperature friction properties.     

Enhanced oxidation properties of ZrB2–SiC composite with short carbon fibers at 1600 °C

This study investigated the effect of short carbon fiber (C_f) on the oxidation behavior of ZrB₂–SiC composites with fiber volume fractions in the range of 0–20%. Precisely, highly dense composite compacts were manufactured by hot-press process at 2000 °C and 30 MPa for 60min. The addition of C_f increased the relative density of sintered composite The oxidation treatment at 1600 °C in air tube furnace for 0.5 h revealed that oxidation rate of ZrB₂–SiC-C_f composites decreased from 292.4 μm/h to 77.6 μm/h (almost 73.5% decline), when the content of C_f changed from 0 to 20%. Moreover, C_f played important roles in blocking and deflecting oxygen diffusion during the oxidation process, which provided a local reduction environment of oxidation products.     

Calcium–magnesium–aluminosilicate corrosion behaviors of rare-earth disilicates at 1400 °C

Environmental barrier coatings (EBCs) are used to prevent oxidation of underlying ceramic matrix composite (CMC) structural components in gas turbines. When the siliceous minerals deposit on the surface of EBCs, a glassy melt of calcium–magnesium–aluminosilicate (CMAS) will be formed, leading to the EBCs degradation. In this study, seven rare-earth disilicates (RE₂Si₂O₇, RE = Yb, Lu, La, Gd, Eu, Sc, and Y) were fabricated to analyze their CMAS corrosion behaviors. The results indicated that the RE₂Si₂O₇ could react with the CMAS in the temperature range of 1250–1350 °C. Reaction zones formed at the interfaces. For the Yb₂Si₂O₇, Lu₂Si₂O₇, La₂Si₂O₇, Eu₂Si₂O₇ and Gd₂Si₂O₇, the reaction zones dissolved into the molten CMAS and separated from the RE₂Si₂O₇. As for the Sc₂Si₂O₇ and Y₂Si₂O₇, the reaction zones could stay at the interface. They could effectively block the molten CMAS to penetrate into the RE₂Si₂O₇ and protect them from CMAS corrosion.     

Microstructure and oxidation resistant of Si–NbSi2 coating on Nb substrate at 800°C and 1000°C

The Si–NbSi₂ composite coating with a smooth surface was successfully prepared on Nb substrate by hot dip silicon-plating (HDS) technology. The composite coating is composed of Si outer layer, NbSi₂ interlayer and Nb₅Si₃ interfacial layer. And the average surface roughness (RSa) and specific surface area growth rate (Sdr) are only 0.275 μm and 2.85%, respectively. The cyclic oxidation test shows that the Si–NbSi₂ composite coating has a very excellent oxidative resistance after oxidation at 800 °C for different times. After oxidation for 40 h, the Δm/S and oxide layer thickness of the coating are only 3.72 mg/cm² and 8 μm, respectively. After oxidation at 1000 °C for 20 h, the coating surface is almost completely covered by a dense SiO₂ layer, the Δm/S and oxide layer thickness of the coating are 7.28 mg/cm² and 15 μm, respectively. The Si–NbSi₂ composite coating presents good self-healing ability and excellent oxidation resistance, which can significantly prolong the service life of bare Nb in oxidation environment.     

Enhanced mechanical,thermal and ablation properties of carbon fiber/BPR composites modified by mica synergistic MoSi2 at 1500 °C

Ablation material had been widely used in aerospace attributed to its reliability and strong adaptability but was usually accompanied by low strength at higher temperatures. Here, a high flexural strength of 66.91 MPa after ablation at 1500 °C of carbon fiber/boron phenolic resin (CF/BPR) composite modified by MoSi₂ and mica was reported. With the proportion of MoSi₂ and mica was 9:1, the formed appropriate content of the liquid phase improved the reactivity of MoSi₂ powders and provided a suitable interfacial state between CF and the matrix. The result of the oxygen-acetylene test showed that the mass ablation and the linear ablation reduced to 0.034 g/s and 0.006 mm/s, respectively. Owing to the formed liquid phase wrapping the carbon fiber and ceramic layer on the surface improved the ablation resistance. These high-strength CF/BPR composites have strong potential in new aerospace materials.     

Ablation behavior and mechanism of C/C–ZrC–SiC composites under an oxyacetylene torch at 3000 °C

C/C–ZrC–SiC composites with continuous ZrC–SiC ceramic matrix were prepared by a multistep technique of precursor infiltration and pyrolysis process. Ablation properties of the composites were tested under an oxyacetylene flame at 3000 °C for 120 s. The results show that the linear ablation rate of the composites was about an order lower than that of pure C/C and C/C–SiC composites as comparisons, and the mass of the C/C–ZrC–SiC composites increased after ablation. Three concentric ring regions with different coatings appeared on the surface of the ablated C/C–ZrC–SiC composites: (i) brim ablation region covered by a coating with layered structure including SiO₂ outer layer and ZrO₂–SiO₂ inner layer; (ii) transition ablation region, and (iii) center ablation region with molten ZrO₂ coating. Presence of these coatings which acted as an effective oxygen and heat barrier is the reason for the great ablation resistance of the composites.     

Effects of 1600 °C annealing atmosphere on the microstructures and mechanical properties of C/SiC composites fabricated by precursor infiltration and pyrolysis

Effects of 1600 °C annealing atmosphere on microstructures and mechanical properties of the C/SiC composites fabricated by PIP route were remarkable. Due to carbothermic reductions, the ratios of weight loss of the C/SiC composites were all above 7 wt% in 1 h. Consequently, the mechanical properties all had a significant drop during the first hour of annealing because of the bonding between the fibers and matrix remarkably weaken by cracks and pores. And then the flexural strengths gradually decreased with the annealing time increasing, when the flexural moduli slightly changed within the range of 44.2–49.7 GPa. However, the fracture behaviors of the C/SiC composites annealed under Ar faster became brittle than the C/SiC composites annealed under vacuum. The C/SiC composites annealed under Ar for 5 h and under vacuum for 10 h both became brittle mainly due to the sensitive to annealing of the weak carbon interphase, while the C/SiC composites annealed under Ar for 7 h became brittle mainly due to the chemical bonding between the fibers and matrix. And these phenomena were confirmed by the post densification and the stress-releasing annealing.     

On the effects of particle size and preform porosity on the mechanical properties of reaction-bonded boron carbide infiltrated with Al-Si alloy at 950 °C

The infiltration of boron carbide preforms with Al alloys at relatively low temperature prevents the formation of the undesired Al₄C₃ phase. In the present study the effect of boron carbide powder particle size on the mechanical properties and phase composition of composites infiltrated with Al-20%Si alloys at 950 °C was investigated. According to XRD analysis, the infiltrated composites contain Al₈C₇B₄, AlB₂ and AlB₁₂, as well as non-reacted Al and Si that originated from solidification of Al-Si alloy. The presence of small amounts of SiC was noted in specimens fabricated from fine boron carbide powder. No evidence for the formation of non-desired aluminum carbide phase was obtained. Infiltration of ceramic preforms with virtually the same green density generated composites with an elastic modulus and bending strength that continuously decreased from 270 GPa and 405 MPa to 195 GPa and 345 MPa for powder with 90% particles close to 3 μm and powder with 90% particles close to 180 μm, respectively. These results ambiguously confirm that boron carbide particle size strongly affects mechanical properties of reaction-bonded composites infiltrated with Al-Si alloy at 950 °C and reflect the amount of newly formed ceramic phases appearing during infiltration and the presence of defects at the metal-ceramic interface.     

Electrical and mechanical properties of BZT − xBCT lead-free piezoceramics

In this study, lead-free (1 − x)Ba(Zr_0.2Ti_0.8)O₃ − x(Ba_0.7Ca_0.3)TiO₃ compositions are synthesized via conventional solid oxide route, and the ceramics are fabricated with normal sintering in air. The effects of composition fluctuations on dielectric, piezoelectric, and mechanical properties are investigated. The phase structure and the microstructure are analyzed with X-ray diffraction and scanning electron microscopy. The best dielectric and piezoelectric properties of ε_r = 11 207 and d₃₃ = 330 pC/N were obtained for BZT−0.35BCT and BZT−0.5BCT ceramics, respectively. The mechanical behavior—in terms of Vickers hardness and compressive and flexural strengths—was investigated, and the best mechanical behavior was found in the vicinity of the phase transition boundary with x values between 0.5 and 0.6.     

Microstructure and mechanical properties of TiC–TiN–Zr–WC–Ni–Co cermets

Cermet cutting tools are widely used for semi-finishing and finishing work on steel and cast iron. However, their brittleness is still an unavoidable limitation for their utilizations. Zirconium was added to improve the fracture toughness of Ti(C, N) based cermets. The microstructure and the fracture surfaces of cermets were studied by using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The experimental results reveal that Zr dissolved and formed solid solutions during the sintering process. The amount of grains with typical core/rim structure decreases and that of coreless grains increases with increasing Zr addition. Moreover, the fracture toughness is improved clearly due to the increased amount of the coreless grains, the spinodal decomposition in cermets, as well as the crack deflection and crack branching mechanisms. Additionally, hardness and relative density were also measured, respectively.     

Characterization and properties of iron-incorporated gismondine prepared at 80°C

Iron-incorporated zeolites were successfully synthesized at a low temperature such as 80°C by choosing appropriate starting materials and characterized by inductively coupled plasma atomic emission spectrometry (ICP-AES), wide-angle X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and magnetic susceptibility. ICP-AES showed␣that Fe component can be readily incorporated␣up to a maximum extent of Fe substitution, Fe/(Fe + Al) × 100 = 22.7%. XRD measurements suggested that the zeolites obtained have a crystal structure of gismondine type. The characterizations identified that the Fe component present in the products is all incorporated into the zeolite framework. The ammonia and water desorption profiles were compared for Fe-free and 22.7% Fe-zeolites ion-exchanged for NH₄⁺ by means of TG-MS and DSC. The ammonia desorption peak temperatures considerably shifted toward lower temperatures by the introduction of Fe, suggesting decreased solid acidity. DSC thermograms of the as-synthesized gismondines revealed that they do not contain free water (i.e., water not coordinated to cations) in the pores irrespective of the Fe content. The enhanced catalytic reactivity of the Fe-incorporated gismondines was also confirmed from the decomposition of hydrogen peroxide. An apparent activation energy of 43 kJ mol⁻¹ was obtained independent of the Fe contents in zeolites. This value was much lower than 70 kJ mol⁻¹ for the same reaction in the homogeneous solution containing iron alum as a reference sample.     

Microstructure and mechanical properties of Al2O3–20 wt%Al2TiO5 composite prepared from alumina and titania nanopowders

In the present work, Al₂O₃–20 wt%Al₂TiO₅ composite was prepared from reaction sintering of alumina and titania nanopowders. The nano-sized raw powders were reconstituted into nanostructured particles by ball milling. Then, the nanostructured reconstituted powders were pressed and pressureless-sintered into bulk ceramics at 1300, 1400, 1500 °C for 2 h. The phase composition and microstructures of reconstituted powders and as-prepared ceramic composites were characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope and energy-dispersive spectrometer (EDS). The microstructural analysis of the ceramic showed that the average grain size of the alumina–aluminium titanate composite increases with increasing the temperature. Also, SEM proved the existence of a proper interface between Al₂TiO₅ and Al₂O₃ grains and preferential distribution of aluminium titanate particles in the grain boundaries. XRD analysis indicated the absence of rutile titania in the sintered composite ensuring complete formation of aluminium titanate. The hardness of the samples sintered at 1300, 1400, 1500 °C were 4.8, 6.2 and 8.5 GPa, respectively.     

Microstructure,mechanical and tribological properties of Ti–B–C–N films prepared by reactive magnetron sputtering

Quaternary Ti–B–C–N thin films are deposited on high-speed steel substrates by the reactive magnetron sputtering (RMS) technique. The microstructure, mechanical and tribological properties of Ti–B–C–N films with different carbon contents (from 28.9 at.% to 54.2 at.%) are explored systematically. The microstructure of Ti–B–C–N films deposited by RMS is consisted mainly of Ti(C, N) nano-crystals embedded into an amorphous matrix of a-C/a-CN/a-BN/a-BC. As the carbon content increases, the crystalline size of the films diminishes, but the hardness linearly increases from 14 GPa to 26 GPa. The friction coefficient of the films sliding against steel GCr15 balls in air decreases with the increase of carbon content, which shows that Ti–B–C–N films with both higher hardness and lower friction coefficient can be obtained by means of increasing the carbon concentration in the films.     

Oxidation behavior of ZrB2-SiC-ZrC at 1700 °C

The oxidation behaviors of four compositions of ZrB₂-SiC-ZrC and one composition of ZrB₂-SiC were studied at 1700 °C in air and under low oxygen partial pressure. Volatility diagrams for ZrB₂-SiC-ZrC and ZrB₂-SiC were used to thermodynamically elucidate the oxidation mechanisms. SiO₂ and ZrO₂ layers formed on the surfaces of ZrB₂-SiC-ZrC and ZrB₂-SiC oxidized at 1700 °C. A SiC-depleted layer only formed on the surface of the ZrB₂-SiC oxidized under low oxygen partial pressure. The oxide layer thickened with increasing ZrC volume content during oxidation in air and under low oxygen partial pressure. The ZrB₂-SiC-ZrC oxide surface exploded in air when the ZrC volume content was more than 50%. Under low oxygen partial pressure, the oxide surfaces of all the ZrB₂-SiC-ZrC specimens bubbled.