Microstructure transmissibility in preparing SiC ceramics from natural wood

A new technique has been developed to prepare SiC ceramic products from natural wood by C–Si reaction at high temperature. Three typical types of natural wood, i.e. pine, birch and bamboo, were selected as the raw materials. XRD analysis indicated that the SiC ceramic prepared by this technique was multi-phase solid, composed of β-SiC and free-silicon. The cell–pipe–fiber structure of the original wood can be transmitted to the final SiC ceramics. The typical structure (cell, pipe or fiber) is in the scale 20–200 μm, and so the arrangement of small SiC grains in the original location of carbon induces observable microstructure transmissibility. The experiment results showed that this technique is a suitable new method to prepare ceramics with a bionics structure.     

In situ synthesis and photoluminescence of SiC nanowires by microwave-assisted pyrolysis of methane

A simple method was used to synthesize SiC nanowires directly on quartz plate by microwave pyrolysis of methane. The samples have been characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Raman spectroscopy techniques. The results showed that the nanowires are of cubic β-SiC structure with stacking faults. The diameters of SiC nanowires are in the range of 50–200 nm and their lengths up to several tens of microns. The effect of the reaction gas concentration and the reaction temperature on the morphologies of SiC nanowires was also investigated. Two broad photoluminescence peaks at the center wavelength of about 309 and 417 nm were observed at room temperature.     

B2O3 modified SiC–MoSi2 oxidation resistant coating for carbon/carbon composites by a two-step pack cementation

To protect carbon/carbon (C/C) composites against oxidation, a B₂O₃ modified SiC–MoSi₂ coating was prepared by a two-step pack cementation. The microstructure and the oxidation resistant property of the coating were studied. The results show that, the as-received coating is a dense structure, and is composed of α-SiC, β-SiC and MoSi₂. The B₂O₃ modified SiC–MoSi₂ coating has excellent oxidation resistant property, and can protect C/C composites from oxidation at 1773 K in air for more than 242 h. The failure of the coating was considered to arise from the existence of the penetration cracks in the coating during the slow cooling from 1873 to 673 K.     

SiC nanowire-toughened MoSi2-SiC coating to protect carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a SiC nanowire-toughened MoSi₂-SiC coating was prepared on them using a two-step technique of chemical vapor deposition and pack cementation. SiC nanowires obtained by chemical vapor deposition were distributed random-orientedly on C/C substrates and MoSi₂-SiC was filled in the holes of SiC nanowire layer to form a dense coating. After introduction of SiC nanowires, the size of the cracks in MoSi₂-SiC coating decreased from 18 ± 2.3 to 6 ± 1.7 μm, and the weight loss of the coated C/C samples decreased from 4.53% to 1.78% after oxidation in air at 1500 °C for 110 h.     

SiO_2-SiC基复合材料吸波性能的研究

首先利用溶胶-凝胶法制备SiO_2-SiC复合粉体,采用SEM、XRD、DSC-TG等技术对复合粉体进行表征.结果表明,溶胶-凝胶法能够制备具有核-壳结构SiO_2-SiC复合粉体.再将SiO_2-SiC复合粉体与BaTiO_3、Fe_3O_4以及环氧树脂以不同比例进行混合固化制得吸波材料样品,采用矢量网络分析仪测量样品的反射率.结果表明,SiO_2-SiC复合粉体具有一定的吸波效果,20%含量的SiO_2-SiC复合粉体样品在18 GHz时反射率达-2.07 dB,BaTiO_3、Fe_3O_4的加入实现复合吸波效果,当SiO_2-SiC:BaTiO_3:Fe_3O_4=6:2:2(体积分数,下同)时,在5.75 GHz时反射率达到-13.97 dB,合格带宽为10.08 GHz.     

Fabrication of SiC by carbothermal-reduction reactions of mountain leather asbestos

Fibrous magnesium silicate (Mg₆Si₄O₁₀(OH)₈: mountain leather asbestos) of Serbian origin was used as Si precursor for the synthesis of SiC by carbothermal-reduction process. As a reducing agent, carbon (from various precursors) was used. Formation of β-SiC was confirmed by XRD and SEM/EDS analyses. Porous SiC particles obtained at 1873 K possess longish plate-like morphology. The results show that reduction reaction is assisted by liquid phase formation, as well as vapor phase transport, and that mountain leather asbestos can be very effective mineral precursor for preparation of silicon carbide.     

Microstructure and grain refining performance of a new Al–Ti–C–B master alloy

A kind of Al–Ti–C–B master alloy with a uniform microstructure is prepared using a melt reaction method. It is found that the average grain size of α-Al can be reduced from 3500 to 170 μm by the addition of 0.2 wt.% of the prepared Al–5Ti–0.3C–0.2B and the refining efficiency does not fade obviously within 60 min. It is considered that the TiC_xB_y and TiB_2−mC_n particles found at the grain center are the effective and stable nucleating substrates for α-Al during solidification, which accounts for the good grain refining performance.     

Uniform TiO2–PANI composite capsules and hollow spheres

Here we demonstrated a simple and reliable method to prepare single or multiple core/shell structured capsules and then demonstrate how to produce single or multiple-layer hollow spheres. Those capsules or hollow spheres could be made of organic and/or inorganic functional materials depending on the experimental design. Core–shell composite capsules consisting of TiO₂–polyaniline (PANI) shell and polystyrene (PS) core were prepared by using core–shell structured sulfonated-PS spheres as templates. Aniline was polymerized in the shell of sulfonated-PS spheres. Since the PANI was in situ doped by sulfonic acid, the as-synthesized composite capsules had good conductivity. After PS cores were dissolved in solvent, hollow TiO₂–PANI spheres formed, which could be further calcined to produce mesoporous hollow TiO₂ spheres. The cavity size of the hollow spheres was uniform, at approximately 170 nm in diameter and with a shell thickness of 30 nm. The cavity size and the shell thickness can be synchronously controlled by adjusting the sulfonation reaction time of PS spheres. Scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and X-ray powder scattering were employed to characterize these as-prepared spheres.     

Changes of an Outer β-NiAl and Inner α-Cr Coating on Ni–40 at% Cr Alloy during Oxidation at 1373K in Air

Formation mechanisms of a coating with a duplex layer, outer β-NiAl(Cr) and inner α-Cr(Ni) layer structure on a Ni–40.2 at% Cr alloy were proposed and change in the coating structure was investigated during high temperature oxidation. The Ni–40.2 at% Cr alloy was electro-plated with about 12μm Ni followed by a high Al activity pack cementation at 1073K to form a coated layer with an outer δ-Ni₂Al₃ and an inner layer containing Al more than 70at% which grew with an inward diffusion of Al. The coated Ni–40.2at% Cr alloy was oxidized at 1373K in air for up to 2592ks. It was found that at the initial stage of oxidation the as-coated layer structure changed to a two-layer, outer β-NiAl(Cr) and inner α-Cr(Ni), structure. Al contents in the α-Cr(Ni) layer was less than 0.3at%. With long term oxidation an intermediate γ-Ni(Cr, Al) layer formed between the outer and inner layers, whereas the inner α-Cr(Ni) layer became thinner and then disappeared after the 2592ks oxidation at 1373K. Coating processes and changes in the coating structure during high temperature oxidation were discussed based on diffusion and composition paths plotted on a Ni–Cr–Al phase diagram     

Creep feed grinding of gamma titanium aluminide and burn resistant titanium alloys using SiC abrasive

Following a brief introduction to titanium alloys and their machinability, the cutting performance of a gamma titanium aluminide intermetallic (γ-TiAl) alloy: Ti–45Al–8Nb–0.2C wt% and a burn resistant titanium (BuRTi) alloy: Ti–25V–15Cr–2Al–0.2C wt%, is compared with creep feed grinding using SiC abrasive. The work utilised 2 separate L9 Taguchi fractional factorial arrays. Typically G-ratios were a factor of 10× greater for γ-TiAl than BuRTi, with on average 10% lower maximum power and 25% lower maximum specific energy for the γ-TiAl alloy. A combination of a moderately high wheel speed: 35 m/s, low depth of cut: 1.25 mm and low feed rate: 150 mm/min, produced the lowest average workpiece surface roughness (Ra1.4 μm). Workpiece surface integrity evaluation indicated that with lower operating parameter levels, in particular a wheel speed of 15 m/s, surfaces free of burn and cracks could be produced, while at higher wheel speeds: 35 m/s, extensive workpiece surface burn was evident, with the γ-TiAl alloy suffering extensive cracking. Microhardness measurements showed in some instances slightly increased workpiece surface hardness of around 50–60HK_0.025 for the BuRTi alloy and 200HK_0.025 for the γ-TiAl material over respective bulk hardness values of 375HK_0.025 and 400HK_0.025.     

Thermodynamic properties of the ternary system InSb–NiSb–Sb in the temperature range 640–860 K

The ternary InSb–NiSb–Sb system has been studied by X-ray diffraction and by potentiometry. The electromotive forces (EMF) have been measured in the temperature range 640<T/K<860 by using the following galvanic cell:

with x (0.075<x<0.498) and y (0<y<0.359). The investigated samples are located on the following lines of the Gibbs triangle: InSb–Ni_0.33Sb_0.66, InSb–Ni_0.48Sb_0.52, InSb–NiSb, Sb–(InSb)_0.75(NiSb)_0.25, Sb–(InSb)_{0. 5}(NiSb)_0.5, Sb–(InSb)_0.25(NiSb)_0.75. From these measurements, the values of the partial molar thermodynamic functions (Δμ°_m,In, ΔH°_m,In, ΔS°_m,In) (data at reference pressure p⁰=10⁵ Pa), for the liquid InSb alloy, for the three solid heterogeneous regions InSb–NiSb₂–Sb, InSb–NiSb_1±δ?–NiSb₂, InSb–NiSb_1±δ, for six ternary liquid–solid alloys, have been calculated.     

液相渗入-反应法制备木材结构SiC的研究

椴木经高温炭化转变为木炭，经熔融硅反应性渗入后，生成具有椴木微观结构的多孔SiC陶瓷。借助TGA(采用Rigaku Thermal型热分析仪)研究了椴木的热分解行为，用XRD和SEM研究了椴木木炭和多孔SiC陶瓷的物相组成和微观结构，还探讨了Si向木炭中渗入．反应动力学和多孔SiC的基本性能。结果显示，木炭为无定形的非晶碳材料，多孔SiC由主晶相β-SiC和少量未反应C组成的；多孔SiC具有椴木的管胞结构，与木炭相比，力学性能得到显著改善；碳纤维向SiC纤维的转化及其对裂纹萌生、扩展的抑制作用和偏转作用是提高力学性能的主要机制。     

Microstructural aspects of the corrosion of Alloy 800

Transmission electron microscopy studies on solution-annealed Alloy 800 revealed small (100–200 nm), spherical-shaped titanium carbide (face centered cubic structure) and large (200 nm–5 μm), faceted titanium nitride (hexagonal structure) particles randomly distributed in the austenite matrix. The volume fraction of former particles was found to be greater than that of the latter. Corrosion studies of the alloy in acidic, chlorides and acidic chloride environments at room temperature indicated that the passivity of Alloy 800 was adversely affected by the addition Cl⁻ ions. X-ray photoelectron spectroscopy revealed that the surface film formed on the alloy at the onset of passivity consisted of Cr³⁺ (as Cr₂O₃), without any Fe³⁺/Fe²⁺ or Ni²⁺. Scanning electron microscopy studies indicated initiation of pitting at large, faceted particles, not at small, spherical-shaped ones.     

Force control grinding of gamma titanium aluminide

This paper addresses the grinding of ordered intermetallic compounds and their brittleness at ambient temperature. The depth of plastic deformation is supposed as the measure of surface integrity. The current paper expands the work of a previously reported indentation model that correlated the depth of plastic deformation and the normal component of the grinding force. This paper studies the indentation model using force control grinding of gamma titanium aluminide (TiAl-γ). Reciprocating surface grinding is carried out for a range of normal force 15–90 N, a cutting depth of 20–40 μm and removal rate of 1–9 mm³/sec using diamond, cubic boron nitride (CBN) and aluminum oxide (Al₂O₃) abrasives. The measured depths of plastic deformation are in the range of 150–300 μm. The deviations from the indentation model are explained by changes in the ductility during the grinding process. Furthermore, a force-based model for specific energy is developed and evaluated. The measured specific energies are in the range of 40 J/mm³ (diamond) to 400 J/mm³ (CBN).     

Characterization of hot-pressed Ti3SiC2–SiC composites

The high-density Ti₃SiC₂-SiC composites with different SiC volume contents were fabricated by hot pressing technique under 35 MPa in a vacuum atmosphere at 1550 °C for 30 min. Microstructural observation showed that the distribution of SiC particulates in the Ti₃SiC₂ matrix was uniform which improved the hardness of Ti₃SiC₂–20 vol% SiC sample (13.9 GPa), compared to monolithic Ti₃SiC₂ (7.1 GPa). The sample containing 15 vol% SiC showed the highest flexural strength value, compared to the other Ti₃SiC₂-SiC samples and the monolithic Ti₃SiC₂. The fracture toughness of the Ti₃SiC₂-SiC samples was also lower than that of the monolithic Ti₃SiC₂ MAX phase.     

Vitreous bond silicon carbide wheel for grinding of silicon nitride

This study investigates the grinding of sintered silicon nitride using a SiC wheel with a fine abrasive grit size and dense vitreous bond. The difference of hardness between the green SiC abrasive and sintered Si₃N₄ workpiece (25.5 vs. 13.7 GPa) is small. Large grinding forces, particularly the specific tangential grinding forces, are observed in SiC grinding of Si₃N₄. The measured specific grinding energy is high, 400–6000 J/mm³, and follows an inverse relationship relative to the maximum uncut chip thickness as observed in other grinding studies. The SiC wheel wears fast in grinding Si₃N₄. The G-ratio varies from 2 to 12. Two unique features in SiC grinding of Si₃N₄ are the trend of increasing G-ratio at higher material removal rate and the excellent surface integrity, with 0.04–0.1 μm R_a and no visible surface damage. For a specific material removal rate, surface cracks along the grinding direction are generated on the ground surface. The problem of chatter vibration was identified at high material removal rates. Periodic and uneven wheel loading marks and clusters of workpiece surface cracks across the grinding direction could be observed at high material removal rates. This study demonstrates that the SiC grinding wheel can be utilized for precision form grinding of Si₃N₄ to achieve good surface integrity under a limited material removal rate.     

In situ formation of La containing dispersed phase on the sinter skin of La2O3 and Cr3C2 unitedly doped WC–Co cemented carbide

The as-sintered sinter skin of WC–11 wt% Co–0.71 wt% Cr₃C₂–0.06 wt% La₂O₃ cemented carbide was observed and analyzed by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. It was unexpectedly discovered that lanthanum which was added in the form of oxide could migrate directionally from the alloy to the sinter skin during the sintering process. As a result, La₂O₂S and LaCoO₃ phases were formed in situ on the sinter skin. It is significant that both phases are characterized with layered-structure related self-lubricating function and high melting point. Noteworthily, La₂O₂S grains preferentially grew up along a group of parallel faces with orientation index [0 0 1] in the WC + β matrix. It was revealed that the alloy had a diffusion-resulted dual structure (i.e., core and skin structure), i.e., WC + β + η three-phase structure in the inner part and WC + β + La₂O₂S + LaCoO₃ four-phase structure on the sinter skin. Cr₃C₂ with carbon-donation ability and the impurity elements from the sintering atmosphere with much bigger electronegative parameters than lanthanum are thought to be the driving force behind the directional migration.     

Selective laser melting of Fe-Ni-Cr layer on AISI H 13 tool steel

An attempt to fabricate Fe-Ni-Cr coating on AISI H13 tool steel was performed with selective laser melting. Fe-Ni-Cr coating was produced by experimental facilities consisting of a 200 W fiber laser which can be focused to 80 μm and atmospheric chamber which can control atmospheric pressure with N₂ or Ar. Coating layer was fabricated with various process parameters such as laser power, scan rate and fill spacing. Surface quality and coating thickness were measured and analyzed. Three different surface patterns, such as type I, type II and type III, are shown with various test conditions and smooth regular pattern is obtained under the conditions as 10 μm of fill spacing, 50–350 mm/s of scan rate and 40 μm of fill spacing, 10–150 mm/s of scan rate. The maximum coating thickness is increased with power elevation or scan rate drop, and average thickness of 10 μm fill spacing is lower than that of 40 μm fill spacing.     

Study of the formation and growth of tannic acid based conversion coating on AZ91D magnesium alloy

A tannic acid based conversion coating was formed on AZ91D magnesium alloy through a solution containing C₇₆H₅₂O₄₆ (tannic acid), NH₄VO₃, K₂ZrF₆ and H₃PO₄. The growth process and corrosion resistance of the conversion coating were investigated by SEM, FESEM, XPS, IR, potentiodynamic polarization and EIS test. The results showed that the main components of the coatings were penta-hydroxy benzamide–magnesium complex, Al₂O₃ and MgF₂. Zr(HPO₄)₂·H₂O(α-ZrP) was first formed in the time interval of 70 s–120 s, which acted as catalyst to activate the main reaction of gallic acid being oxidized into the penta-hydroxy benzamide–magnesium complex, as well as the formation of MgF₂ and Al₂O₃, which took place in the time interval of 120 s to 600 s. The thickness of coating treated for 300 s and 600 s was 1.1 μm and 1.6 μm, respectively. Polarization measurement and EIS test in 3 wt.% NaCl solution indicated that the conversion coating treated for about 300–600 s shows good corrosion resistance while an excess time treatment (720 s) induces the coating surface coarse and uneven, leading to the reduction of corrosion resistance of the conversion coating.     

Controlled synthesis of TiO2 mesoporous microspheres via chemical vapor deposition

Anatase titanium dioxide (TiO₂) mesoporous microspheres with core-shell and hollow structure were successfully prepared on a large scale by a one-step template-free chemical vapor deposition method. The effects of various reaction conditions on the morphology, composition and structure of the products were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) technique and photoluminescence (PL) method. The results indicate that the product near the source was composed of core-shell structure TiO₂ microspheres with diameters from 3 to 5 μm. With increasing the distance between the source materials and the substrate, the hollow TiO₂ spheres with 1-2 μm dominant the products. A localized Ostwald ripening can be use to explain the formation of core-shell and hollow structures, and the size of the initial TiO₂ solid nanoparticles plays an important role in determining the evacuation manner of the solid in the ripening-induced hollowing process. The surface area of TiO₂ hollow microspheres determined by the adsorption isotherms was measured to be 74.67 m²/g. X-ray photoelectron spectroscopy (XPS) analysis revealed that the O-H peaks of hollow structures have a chemical shift compared with the core-shell structures. The optical property of the products was also discussed.