Electron beam-physical vapor deposition of SiC/SiO2 high emissivity thin film

When heated by high-energy electron beam (EB), SiC can decompose into C and Si vapor. Subsequently, Si vapor reacts with metal oxide thin film on substrate surface and formats dense SiO₂ thin film at high substrate temperature. By means of the two reactions, SiC/SiO₂ composite thin film was prepared on the pre-oxidized 316 stainless steel (SS) substrate by electron beam-physical vapor deposition (EB-PVD) only using β-SiC target at 1000 °C. The thin film was examined by energy dispersive spectroscopy (EDS), grazing incidence X-ray asymmetry diffraction (GIAXD), scanning electron microscopy (SEM), atomic force microscopy (AFM), backscattered electron image (BSE), electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS) and Fourier transformed infra-red (FT-IR) spectroscopy. The analysis results show that the thin film is mainly composed of imperfect nano-crystalline phases of 3C-SiC and SiO₂, especially, SiO₂ phase is nearly amorphous. Moreover, the smooth and dense thin film surface consists of nano-sized particles, and the interface between SiC/SiO₂ composite thin film and SS substrate is perfect. At last, the emissivity of SS substrate is improved by the SiC/SiO₂ composite thin film.     

SiC/SiO2 coating for improving the oxidation resistive property of carbon nanofiber

The SiC/SiO₂ deposition was performed to improve the oxidation resistive properties of carbon nanofiber (CNF) from electrospinning at elevated temperatures through sol-gel process. The stabilized polyacrylonitrile (PAN) fibers were coated with SiO₂ followed by heat treatment up to 1000 and 1400 °C in an inert argon atmosphere. The chemical compositions of the CNFs surface heat-treated were characterized as C, Si and O existing as SiC and SiO₂ compounds on the surface. The uniform and continuous coating improved the oxidation resistance of the carbon nanofibers. The residual weight of the composite was 70-80% and mixture of SiC, SiO₂ and some residual carbon after exposure to air at 1000 °C.     

Preparation and properties of electroless Ni-P-SiO2 composite coatings

Dispersible SiO₂ nanoparticles were co-deposited with electroless Ni-P coating onto AISI-1045 steel substrates in the absence of any surfactants in plating bath. The resulting Ni-P/nano-SiO₂ composite coatings were heat-treated for 1 h at 200 °C, 400 °C, and 600 °C, respectively. The hardness and wear resistance of the heat-treated composite coatings were measured. Moreover, the structural changes of the composite coatings before and after heat treatment were investigated by means of X-ray diffraction (XRD), while their elemental composition and morphology were analyzed using an energy dispersive spectrometer (EDS) and a scanning electron microscope (SEM). Results show that co-deposited SiO₂ particles contributed to increase the hardness and wear resistance of electroless Ni-P coating, and the composite coating heat-treated at about 400 °C had the maximum hardness and wear resistance.     

Influence of Co(CH3COO)2 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation

Ceramic thermal protection coatings on Ti6Al4V alloy were achieved by micro-arc oxidation (MAO) in the presence of Co(CH₃COO)₂. The morphology, crystallographic structure and chemical composition of the coating were characterized by various techniques. The thermal emission of the coating was measured by Fourier transform spectrometer apparatus. The bonding strength between the coating and substrate was studied, together with the thermal shock resistance of the coating. The results indicate that the content of Co in the coating layer significantly affects its thermal emissivity. Higher concentration of Co(CH₃COO)₂ in electrolytes leads to more Co ions into the coating, which enhances the emissivity of the coating. All the coatings show bonding strength higher than 10 MPa. In addition, the coating remains stable over 40 cycles of thermal shocking. The coating formed at 4 g/L Co(CH₃COO)₂ displays an average spectral emissivity value more than 0.9 and bonding strength about 10.4 MPa.     

Dependence of infrared radiation on microstructure of polymer derived ceramic coating on steel

High infrared emissivity ceramic coatings were prepared on 304 steel by pyrolyzing reactions with poly(hydridomethylsiloxane) (PHMS) and Al/Cr₂O₃ based mixing powders. The effects of pyrolysis temperature, testing temperature and surface roughness on infrared radiation of polymer derived ceramic coating were systematically investigated. The results indicated that the coating pyrolyzed at 800 °C exhibited a slightly higher infrared emissivity value than that of the coating pyrolyzed at 600 °C, which was attributed to the enhancing photon emission caused by the complete conversion of Al to Al₂O₃ and PHMS pyrolysis into SiO₂, together with the introduction of Cr₂O₃ based mixing powders. The emissivity value in 3–8 μm waveband of the coating was lower about 0.03 at 600 °C compared with 800 °C testing temperature, while the emissivity value was almost the same in 8–20 μm waveband. The high surface roughness of the coating led to a slightly increasing emissivity due to the enhancing infrared absorbance.     

Composite SiO2/TiO2 and amine polymer/TiO2 nanoparticles produced using plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition was used to conformally coat commercial TiO₂ nanoparticles to create nanocomposite materials. Hexamethyldisiloxane (HMDSO)/O₂ plasmas were used to deposit SiO₂ or SiO_xC_yH_z films, depending on the oxidant concentration; and hexylamine (HexAm) plasmas were used to deposit amorphous amine-containing polymeric films on the TiO₂ nanoparticles. The composite materials were analyzed using Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). These analyses reveal film composition on the nanoparticles was virtually identical to that deposited on flat substrates and that the films deposit a conformal coating on the nanoparticles. The performance of the nanocomposite materials was evaluated using UV-vis spectroscopy to determine the dispersion characteristics of both SiO_x and HexAm coated TiO₂ materials. Notably, the coated materials stay suspended longer in distilled water than the uncoated materials for all deposited films.     

Coating formation by plasma electrolytic oxidation on ZC71/SiC/12p-T6 magnesium metal matrix composite

Plasma electrolytic oxidation (PEO) of a ZC71/SiC/12p-T6 magnesium metal matrix composite (MMC) is investigated in relation to coating growth and corrosion behaviour. PEO treatment was undertaken at 350 mA cm⁻² (rms) and 50 Hz with a square waveform in stirred 0.05 M Na₂SiO₃.5H₂O/0.1 M KOH electrolyte. The findings revealed thick, dense oxide coatings, with an average hardness of 3.4 GPa, formed at an average rate of ∼1 μm min⁻¹ for treatment times up to 100 min and ∼0.2 μm min⁻¹ for later times. The coatings are composed mainly of MgO and Mg₂SiO₄, with an increased silicon content in the outer regions, constituting <10% of the coating thickness. SiC particles are incorporated into the coating, with formation of a silicon-rich layer at the particle/coating interface due to exposure to high temperatures during coating formation. The distribution of the particles in the coating indicated growth of new oxide at the metal/coating interface. The corrosion rate of the MMC in 3.5% NaCl is reduced by approximately two orders of magnitude by the PEO treatment.     

Preparation of MPS (mercaptopropyltrimethoxysilane)-coated SiO2 nanoparticle and its effect on mechanical properties of surface treated SiO2/EVA nanocomposite

SiO₂ nanoparticles were synthesized from different three precursors, namely, TEOS (tetraethyl orthosilicate), sodium metasilicate and sodium silicate. First, SiO₂ nanoparticles were prepared by a controlled hydrolysis of TEOS. In another method, SiO₂nanoparticles were prepared by precipitation in an emulsion medium from sodium metasilicate and hydrochloric acid solution. Finally, SiO₂ nanoparticles were also synthesized from sodium silicate by an emulsion method. In this study, we concentrated on dispersion and compatibility between nanosized SiO₂ particles and EVA (ethylene vinyl acetate). Therefore, surface modification of synthesized SiO₂ nanoparticles was accomplished with MPS (3-mercaptopropyl trimethoxysilane) to enhance homogeneous dispersion and compatibility between the obtained SiO₂ nanoparticles and EVA. Finally, nanocomposites of surface treated SiO₂ nanoparticles and EVA were prepared. By inserting the MPS-coated SiO₂ nanoparticles into EVA, abrasion resistance and hardness were increased remarkably. On the other hand, insertion of SiO₂ nanoparticles barely decreased original tensile strength and elongation of EVA. Consequently, MPS-coated SiO₂/EVA nanocomposite can have an improved abrasion resistance and hardness compared with raw EVA, without decrease tensile strength and elongation. The characterization of synthesized SiO₂ nanoparticles and their nanocomposite with EVA was conducted by TEM, SEM, FTIR photography and mechanical property tests such as abrasion, hardness, tensile strength and elongation.     

Ti-B-C-N纳米复合薄膜结构及力学性能研究

利用反应磁控溅射方法在单晶硅和高速钢(W18Cr4V)基片上制备出不同C含量Ti-B-C-N纳米复合薄膜. 使用X射线衍射和高分辨透射电子显微镜研究了Ti-B-C-N纳米复合薄膜的组织和微观结构,用纳米压痕仪测试了它们的硬度和弹性模量. 结果表明,利用往真空室通入C₂H₂气体的方法制备得到的Ti-B-C-N纳米复合薄膜中,在所研究成分范围内只发现TiN基的纳米晶. 当C₂H₂流量较小时,C元素的加入可以促进Ti-B-C 关键词： Ti-B-C-N薄膜 磁控溅射 微观结构 力学性能     

Physical and electrochemical characterizations of Ni-SiO2 nanocomposite coatings

Advances in materials performance often require the development of composite system. In the present investigation, SiO₂-reinforced nickel composite coatings were deposited on a mild steel substrate using direct current electrodeposition process employing a nickel acetate bath. Surface morphology, composition, microstructure and crystal orientation of the Ni and Ni-SiO₂ nanocomposite coatings were investigated by scanning electron microscope, energy dispersive X-ray spectroscopy and X-ray diffraction analysis, respectively. The effect of incorporation of SiO₂ particles in the Ni nanocomposite coating on the microhardness and corrosion behaviour has been evaluated. Smooth composite deposits containing well-distributed silicon oxide particles were obtained. The preferred growth process of the nickel matrix in crystallographic directions <111>, <200> and <220> is strongly influenced by SiO₂ nanoparticles. The average crystallite size was calculated by using X-ray diffraction analysis and it was ~23 nm for electrodeposited nickel and ~21 nm for Ni-SiO₂ nanocomposite coatings. The crystallite structure was fcc for electrodeposited nickel and Ni-SiO₂ nanocomposite coatings. The incorporation of SiO₂ particles into the Ni matrices was found to improve corrosion resistance of pure Ni coatings. The corrosion potential (E _corr) in the case of Ni-SiO₂ nanocomposite coatings had shown a negative shift, confirming the cathodic protective nature of the coating. The Ni-SiO₂ composite coatings have exhibited significantly improved microhardness (615 HV) compared to pure nickel coatings (265 HV)     

A SiC whisker-toughened SiC-CrSi2 oxidation protective coating for carbon/carbon composites

A SiC whisker-toughened SiC-CrSi₂ oxidation protective coating was prepared on the surface of C/C composites by a two-step technique of slurry and pack cementation. The oxidation protective ability and thermal stress resistance of the coating exhibit the trend of increase first and decrease afterwards as the SiCw content increases from 0 to 20 wt.%. The compound effect of SiCw and CrSi₂ on the oxidation protective ability of SiC coating is better than their individual ones.     

Fabrication and characterization of Au/SiO2 nanocomposite films grown by radio-frequency cosputtering

Au/SiO₂ nanocomposite films were prepared on Si wafers by cosputtering of SiO₂ and gold wires. Au/Si atomic ratios in Au/SiO₂ nanocomposite films were varied from 0.53 to 0.92 by controlling the length of gold wire to study the evolution of the crystallization of gold, the size of Au/SiO₂ nanocomposite particles, and the optical properties of as-deposited Au/SiO₂ nanocomposite films. An X-ray photoelectron spectroscopy reveals that Au exists as a metallic phase in the bulk of SiO₂ matrix. Dome-shaped Au/SiO₂ nanocomposite particles and both Au (1 1 1) and (2 0 0) planes were observed in a field-emission scanning electron microscopy and X-ray diffraction studies respectively. With an ultraviolet-visible, absorption peaks of Au/SiO₂ nanocomposite films were observed at 525 nm.     

Influence of FeSO4 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation

Ceramic coatings with high emission were fabricated on Ti6Al4V alloy by microarc oxidation (MAO) with additive FeSO₄ into the electrolyte. The microstructure, chemical composition and chemical state of the coatings were determined by SEM, XRD, EDS and XPS, respectively. The bonding strength between the coating and substrate was studied by tensile strength test, together with the thermal shock resistance of the coating. The results showed that Fe content in the coating layer significantly affect its thermal emissivity. The relative content of Fe in the coatings surface increased at first and then decreased with increasing the concentration of FeSO₄ in electrolytes, so does the emissivity of the coatings. The bonding strength became weaker with increasing the concentration of FeSO₄. In addition, the coating remains stable over 40 cycles of thermal shocking. The coating formed at 3 g/L FeSO₄ demonstrates the highest an average spectral emissivity value around of 0.87, and bonding strength higher than 33 MPa.     

Room temperature synthesis of water-repellent polystyrene nanocomposite coating

A stable superhydrophobic polystyrene nanocomposite coating was fabricated by means of a very simple and easy method. The coating was characterized by scanning electron microscopy and X-ray photoelectron spectrum. The wettability of the products was also investigated. By adding the surface-modified SiO₂ nanoparticles, the wettability of the coating changed to water-repellent superhydrophobic, not only for pure water, but also for a wide pH range of corrosive liquids. The influence of the drying temperature and SiO₂ content on the wettability of the nanocomposite coating was also investigated. It was found that both factors had little or no significant effect on the wetting behavior of the coating surface.     

Microstructure and infrared emissivity property of coating containing TiO2 formed on titanium alloy by microarc oxidation

Ceramic coatings containing TiO₂ were formed on Ti6Al2Zr1Mo1V alloy surface by microarc oxidation (MAO) method. The microstructure, phase and chemical composition of the coatings were analyzed by SEM, XRD and EDS techniques. The coating mainly consists of rutile TiO₂ and a small amount of anatase TiO₂. The infrared emissivity values of coated and uncoated titanium samples when exposed to 700 °C were tested. It was found that the coating exhibits a higher infrared emissivity value (about 0.9) in the wavelength range of 8–14 μm than that of the uncoated titanium alloy, although which shows a slight increase from 0.1 to 0.3 with increasing exposure time at 700 °C. The relatively high infrared emissivity value of the MAO coating is possibly attributed to the photon emission from the as formed TiO₂ phase.     

Nitric acid oxidation of Si (NAOS) method for low temperature fabrication of SiO2/Si and SiO2/SiC structures

We have developed low temperature formation methods of SiO₂/Si and SiO₂/SiC structures by use of nitric acid, i.e., nitric acid oxidation of Si (or SiC) (NAOS) methods. By use of the azeotropic NAOS method (i.e., immersion in 68 wt% HNO₃ aqueous solutions at 120 °C), an ultrathin (i.e., 1.3-1.4 nm) SiO₂ layer with a low leakage current density can be formed on Si. The leakage current density can be further decreased by post-metallization anneal (PMA) at 200 °C in hydrogen atmosphere, and consequently the leakage current density at the gate bias voltage of 1 V becomes 1/4-1/20 of that of an ultrathin (i.e., 1.5 nm) thermal oxide layer usually formed at temperatures between 800 and 900 °C. The low leakage current density is attributable to (i) low interface state density, (ii) low SiO₂ gap-state density, and (iii) high band discontinuity energy at the SiO₂/Si interface arising from the high atomic density of the NAOS SiO₂ layer.For the formation of a relatively thick (i.e., ≥10 nm) SiO₂ layer, we have developed the two-step NAOS method in which the initial and subsequent oxidation is performed by immersion in ∼40 wt% HNO₃ and azeotropic HNO₃ aqueous solutions, respectively. In this case, the SiO₂ formation rate does not depend on the Si surface orientation. Using the two-step NAOS method, a uniform thickness SiO₂ layer can be formed even on the rough surface of poly-crystalline Si thin films. The atomic density of the two-step NAOS SiO₂ layer is slightly higher than that for thermal oxide. When PMA at 250 °C in hydrogen is performed on the two-step NAOS SiO₂ layer, the current-voltage and capacitance-voltage characteristics become as good as those for thermal oxide formed at 900 °C.A relatively thick (i.e., ≥10 nm) SiO₂ layer can also be formed on SiC at 120 °C by use of the two-step NAOS method. With no treatment before the NAOS method, the leakage current density is very high, but by heat treatment at 400 °C in pure hydrogen, the leakage current density is decreased by approximately seven orders of magnitude. The hydrogen treatment greatly smoothens the SiC surface, and the subsequent NAOS method results in the formation of an atomically smooth SiO₂/SiC interface and a uniform thickness SiO₂.     

Structure and infrared emissivity of collagen/SiO2 composite

Collagen/SiO₂ composites were prepared in aqueous suspensions. Adsorption behaviors of collagen onto the surfaces of SiO₂ spheres were studied. Structure and thermal properties were measured with FTIR, SEM, TEM, and TGA-DTA. The results showed that the self-aggregation of collagen macromolecules was taken place during the adsorption of collagen on SiO₂ sphere. The morphology of collagen evolved from line to microfibrils with the increase in the concentration of collagen along with the distortion of SiO₂. Interfacial interactions of electrostatic forces and hydrogen bonding between the collagen macromolecule and SiO₂ sphere had a vital effect on the adsorption of collagen. The amount of the collagen adsorption was increased with the increase of the collagen concentration, yet decreased in increased pH value of the solution. It was found that the composites exhibited lower infrared emissivity values in the wavelength ranged from 8 to 14 μm than not only pure collagen but also SiO₂ sphere, and the value of infrared emissivity was related to the adsorption amount of collagen in the composites.     

In situ formation of Al2O3-SiO2-SnO2 composite ceramic coating by microarc oxidation on Al-20%Sn alloy

In situ formation of Al₂O₃-SiO₂-SnO₂ composite ceramic coating on Al-20%Sn alloy was successfully fabricated in aqueous Na₂SiO₃ electrolyte by microarc oxidation technology. The compositions, structure, mechanical and tribological properties of the composite coating were detailed studied by scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction, hardness tester and ball-on-disc friction tester. It is found that the species originating from the Al-20%Sn alloy substrate and the electrolyte solution both participate in reaction and contribute to the composition of the coating, which results in the generated coating firmly adherent to the substrate. The composite ceramic coating can greatly improve the microhardness and tribological property of Al-20%Sn alloy.     

InP/SiO2纳米复合膜的微观结构和光学性质

应用射频磁控共溅射方法在石英玻璃和抛光硅片上制备了InP/SiO₂复合薄膜,并在几种条件下对这些薄膜进行退火.X射线光电子能谱和卢瑟福背散射实验结果表明,复合薄膜中InP和SiO₂的化学组分都大体上符合化学计量配比.X射线衍射和激光喇曼谱实验结果都证实了复合薄膜中形成了InP纳米晶粒.磷气氛保护下的高温(520℃)退火可以消除复合薄膜中残存的In和In₂O₃并得到了纯InP/SiO₂纳米复合薄膜.实验观察到了室温下纳米复合薄膜的明显的光学吸收边蓝移现象和光学非线性的极大增强 关键词： InP 纳米晶粒 微观结构 光学性质     

Formation of atomically smooth SiO2/SiC interfaces at ∼120 °C by use of nitric acid oxidation method

Conventional thermal oxidation of SiC requires heating at ∼1100 °C. In the present study, we have developed a method of oxidizing SiC at low temperatures (i.e., ∼120 °C) to form relatively thick silicon dioxide (SiO₂) layers by use of nitric acid. When 4H-SiC(0 0 0 1) wafers are immersed in 40 wt% HNO₃ at the boiling temperature of 108 °C and the boiling is kept for 5 h after reaching the azeotropic point (i.e., 68 wt% HNO₃ at 121 °C), 8.1 nm thick SiO₂ layers are formed on the SiC substrates. High resolution transmission electron microscopy measurements show that the SiO₂/SiC interface is atomically flat and the SiO₂ layer is uniform without bunching. When SiC is immersed in an azeotropic mixture of HNO₃ with water from the first, the SiO₂ thickness is less than 0.3 nm. The metal-oxide-semiconductor (MOS) diodes with the SiO₂ layer formed by the nitric acid oxidation method possess a considerably low leakage current density.