Mechanical and tribological performance of Al2O3-TiO2 coatings elaborated by flame and plasma spraying

Mechanical properties and wear rates of Al₂O₃-13 wt.% TiO₂ (AT-13) and Al₂O₃-43 wt.% TiO₂ (AT-43) coatings obtained by flame and atmospheric plasma spraying were studied. The feed stock was either ceramic cords or powders. Results show that the wear resistance of AT-13 coatings is higher than that of AT-43 and it seems that the effect of hardness on wear resistance is more important than that of toughness. Additionally, it was established that, according to conditions used to elaborate coatings and the sliding tribological test chosen, spray processes do not seem to have an important effect on the wear resistance of these coatings.     

The effect of plasma spray parameters on the cavitation erosion of Al2O3-TiO2 coatings

This paper reports a study of how the choice of plasma spray parameters, used during deposition of Al₂O₃-13%TiO₂ coatings on carbon steel, influences the cavitation erosion properties of such coatings. The parameters studied are the power feeding rate and hydrogen flow rate. The surface and cross section of coatings before and after cavitation were also observed by scanning electron microscopy (SEM). The phases present in the coatings were characterized by X-ray diffraction method (XRD). The microscopic observations were used to study the inter-lamellar connection, porosity, unmelted particles and so on inside the coating. We also measured the roughness, microhardness, adhesion strength and cavitation erosion of the coatings. The XRD results showed that the coating includes different allotropes of Al₂O₃ such as α and γ. The cavitation erosion studies of the coatings were conducted by ultrasonic cavitation testing on the basis of ASTM G32 standard. It was found that cavitation erosion is accelerated around the unmelted particles and porosities. The results reveal that the cavitation resistance of the coating is determined by its microstructure and that increasing discontinuities (inside the coating) reduce its cavitation resistance. We have found that the coating obtained at hydrogen gas flow rate of 16 L/min and powder feeding rate of 20 g/min has the best cavitation resistance.     

Microstructure and mechanical properties of Al2O3-Cr2O3 composite coatings produced by atmospheric plasma spraying

Al₂O₃, Al₂O₃-Cr₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying. Phase composition of powders and as-sprayed coatings was determined by X-ray diffraction. Electron probe microanalyzer was employed to investigate the polished and fractured surface morphologies of the coatings. Mechanical properties including microhardness, fracture toughness and bending strength were evaluated. The results indicate that the addition of Cr₂O₃ is conducive to the stabilization of α-Al₂O₃. Compared with the pure Al₂O₃ and Cr₂O₃ coatings, Al₂O₃-Cr₂O₃ composite coatings show lower porosities and denser structures. Heterogeneous nucleation of α-Al₂O₃ occurs over the isostructural Cr₂O₃ lamellae and partial solid solution of Al₂O₃ and Cr₂O₃ might be occurring as well. Furthermore, grain refining and solid solution strengthening facilitate the mechanical property enhancement of Al₂O₃-Cr₂O₃ composite coatings.     

Fe3O4/SiO2/Bi2WO6磁性复合光催化剂的制备及其光催化性能

钨酸铋(Bi₂WO₆),结构最简单的Aurivillius相化合物,是近期受到研究者关注的新型光催化材料。然而,光催化剂粉末在反应介质中难被回收,工业化应用成本较高。本文用三步方法合成了可回收的Fe₃O₄/SiO₂/Bi₂WO₆磁性复合光催化剂,通过溶剂热法合成具有磁性的Fe₃O₄,用溶胶凝胶法在Fe₃O₄表面覆盖SiO₂层,后将磁性颗粒与Bi₂WO₆纳米片相结合。光催化剂的形貌结构及性能通过XRD、SEM、PL、UV-vis进行表征测试。结果表明,直径约500 nm的Fe₃O₄微球附着在边长约500 nm的Bi₂WO₆纳米片的表面,SiO₂在两者之间起到了粘连作用。光催化剂Fe₃O₄/SiO₂/Bi₂WO₆对于罗丹明B的光降解活性较好,且有一定磁性,可以通过外加磁场将其从溶液中分离,有较大的应用潜力。     

Improved dielectric and non-ohmic properties of Ca2Cu2Ti4O12 ceramics prepared by a polymer pyrolysis method

Non-ohmic and dielectric properties of Ca₂Cu₂Ti₄O₁₂ (CaCu₃Ti₄O₁₂/CaTiO₃ composite) ceramics prepared by a polymer pyrolysis method (PP-ceramic samples) are investigated. The PP-ceramics show a highly dense structure and improved non-ohmic and dielectric properties compared to the ceramics obtained by a solid state reaction method (SSR-ceramic samples). ?′ (tan δ) of the PP-ceramic samples is found to be higher (lower) than that of the SSR-ceramic samples. Interestingly, the PP-ceramic sintered at 1050 °C for 10 h exhibits the high ?′ of 2530 with weak frequency dependence below 1 MHz, the low tan δ less than 0.05 in the frequency range of 160 Hz-177 kHz, and the little temperature coefficient, i.e., |Δ?′| ≤ 15 % in the temperature range from −55 to 85 °C. These results indicate that the CaCu₃Ti₄O₁₂/CaTiO₃ composite system prepared by PP method is a promising high-?′ material for practical capacitor application.     

Microstructures and electrical responses of pure and chromium-doped CaCu3Ti4O12 ceramics

Pure and chromium-doped CCTO (CaCu₃Ti₄O₁₂) ceramics were prepared by a conventional solid-state reaction method, and the effects of chromium doping on the microstructures and electrical properties of these ceramics were investigated. Efficient crystalline phase formation accompanied by dopant-induced lattice constant expansion was confirmed through X-ray diffraction studies. Scanning electron microscopy (SEM) results show that doping effectively enhanced grain growth or densification, which should increase the complex permittivity. The dielectric constant reached a value as high as 20,000 (at 1 kHz) at a chromium-doping concentration of 3%. The electrical relaxation and dc conductivity of the pure and chromium-doped CCTO ceramics were measured in the 300-500 K temperature range, and the electrical data were analyzed in the framework of the dielectric as well as the electric modulus formalisms. The obtained activation energy associated with the electrical relaxation, determined from the electric modulus spectra, was 0.50-0.60 eV, which was very close to the value of the activation energy for dc conductivity (0.50 ± 0.05 eV). These results suggest that the movement of oxygen vacancies at the grain boundaries is responsible for both the conduction and relaxation processes. The short-range hopping of oxygen vacancies as “polarons” is similar to the reorientation of the dipole and leads to dielectric relaxation. The proposed explanation of the electric properties of pure and chromium-doped CCTO ceramics is supported by the data from the impedance spectrum.     

国外多孔基体氧化铝/氧化铝复合材料工程应用进展

氧化铝/氧化铝复合材料(Al2O3/Al2O3)是20世纪90年代兴起的一类连续陶瓷纤维增强陶瓷基复合材料,已经发展为与SiC/SiC、C/SiC等非氧化物陶瓷基复合材料并列的一类陶瓷基复合材料。与非氧化物陶瓷基复合材料相比,Al2O3/Al2O3具有长时抗氧化、高温耐腐蚀、低成本等独特优势,已经在航空发动机、地面燃气轮机等军民两用热结构材料领域展现出广阔的应用前景。本文从材料应用的角度出发,系统分析阐述了目前在Al2O3/Al2O3占主导地位的多孔基体Al2O3/Al2O3(P-Al2O3/Al2O3)的增韧机制、成型工艺和性能特点,重点归纳了国外近年来P-Al2O3/Al2O3的工程化应用进展及前景,最后指出了P-Al2O3/Al2O3存在的局限性并展望了未来发展方向,旨在为国内Al2O3/Al2O3体系发展提供借鉴和参考。     

Thermal shock fatigue behavior of TiC/Al2O3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt.% TiC/Al₂O₃ composites were studied. The effect of TiC and Al₂O₃ starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference (ΔT) and number of thermal cycles (N) on fatigue crack growth (Δa). The mechanical properties and thermal fatigue resistance of TiC/Al₂O₃ composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al₂O₃ composites is due to a “true” cycling effect (thermal fatigue). Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.     

Piezoelectric and dielectric properties of Dy2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ + x wt% Dy₂O₃ with x = 0-0.3 ceramics were synthesized by conventional solid-state processes. The effects of Dy₂O₃ on the microstructure, the piezoelectric and dielectric properties were investigated. X-ray diffraction pattern confirmed that the coexistence of tetragonal and rhombohedral phases in the (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ composition was not changed by adding 0.05-0.3 wt% Dy₂O₃. SEM images indicate that all the ceramics have pore-free microstructures with high density, and that doping of Dy₂O₃ inhibits the grain growth of the ceramics. The addition of Dy₂O₃ shows the double effects on decreasing the piezoelectric and dielectric properties for 0 < x < 0.15 when Dy³⁺ ions substitute B-site Ti⁴⁺ ions, and increasing the properties for 0.15 < x < 0.3 when Dy³⁺ ions enters into A-site of the perovskite structure. The optimum electric properties of piezoelectric constant d₃₃ = 170 pC/N and the dielectric constant ?_r = 1900 (at a frequency of 1 kHz) are obtained at x = 0.3.     

Dielectric relaxation behaviors of pure and Pr6O11-doped CaCu3Ti4O12 ceramics in high temperature range

Pure and Pr₆O₁₁-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were prepared by conventional solid-state reaction method. The compositions and structures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influences of Pr-ion concentration on dielectric properties of CCTO were measured in the ranges of 60 Hz-3 MHz and 290-490 K. The third phase of Ca₂CuO₃ was observed from the XRD of CCTO ceramics. From SEM, the grain size was decreased obviously with high valence Pr-ion (mixing valence of Pr³⁺ and Pr⁴⁺) substituting Ca²⁺. The room temperature dielectric constant of Pr-doped CCTO ceramics, sintered at 1323 K, was an order of magnitude lower than the pure CCTO ceramics due to the grain size decreasing and Schottky potential increasing. The dielectric spectra of Pr-doped CCTO were flatter than that of pure CCTO. The loss tangent of Pr-doped CCTO ceramics was less than 0.20 in 2 × 10²-10⁵ Hz region below 440 K. The complex impedance spectra of pure and Pr-doped CCTOs were fitted by ZView. From low to high frequency, three semicircles were observed corresponding to three different conducting regions: electrode interface, grain boundary and grain. By fitting the resistors R and capacitors C, the activation energies of grain boundary and electrode contact were calculated. All doped CCTOs showed higher activation energies of grain boundary and electrode than those of pure CCTO ceramics, which were concordant with the decreasing of dielectric constant after Pr₆O₁₁ doping.     

Effects of K2O-Li2O doping on surface and catalytic properties of Fe2O3/Cr2O3 system

The effects of K₂O and Li₂O-doping (0.5, 0.75 and 1.5 mol%) of Fe₂O₃/Cr₂O₃ system on its surface and the catalytic properties were investigated. Pure and differently doped solids were calcined in air at 400-600 °C. The formula of the un-doped calcined solid was 0.85Fe₂O₃:0.15Cr₂O₃. The techniques employed were TGA, DTA, XRD, N₂ adsorption at −196 °C and catalytic oxidation of CO oxidation by O₂ at 200-300 °C. The results revealed that DTA curves of pure mixed solids consisted of one endothermic peak and two exothermic peaks. Pure and doped mixed solids calcined at 400 °C are amorphous in nature and turned to α-Fe₂O₃ upon heating at 500 and 600 °C. K₂O and Li₂O doping conducted at 500 or 600 °C modified the degree of crystallinity and crystallite size of all phases present which consisted of a mixture of nanocrystalline α- and γ-Fe₂O₃ together with K₂FeO₄ and LiFe₅O₈ phases. However, the heavily Li₂O-doped sample consisted only of LiFe₅O₈ phase. The specific surface area of the system investigated decreased to an extent proportional to the amount of K₂O and Li₂O added. On the other hand, the catalytic activity was found to increase by increasing the amount of K₂O and Li₂O added. The maximum increase in the catalytic activity, expressed as the reaction rate constant (k) measured at 200 °C, attained 30.8% and 26.5% for K₂O and Li₂O doping, respectively. The doping process did not modify the activation energy of the catalyzed reaction but rather increased the concentration of the active sites without changing their energetic nature.     

SNDC/La2Mo2O9复合电解质材料的制备及性能表征

本文采用甘氨酸-硝酸盐法(GNP)和溶胶凝胶法分别合成了Sm_0.1Nd_0.1Ce_0.8O_1.9(SNDC) 和La₂Mo₂O₉(LAMOX)粉末,并用常压烧结的方法制备了不同比例的SNDC和LAMOX的复合材料,通过XRD和SEM等手段表征了不同复合比例样品的物相和表面形貌并测试了烧结样品的电导率。结果表明,复合样品的电导率在相变点前后随着复合量增加变化趋势相反,其中LAMOX含量为20mol%的样品在550℃时的电导率能达到0.01S/cm,高于同温度下SNDC电导率。     

三元共晶成分Al2O3/YAG/ZrO2粉体及陶瓷的制备与组织性能研究

本文采用醇水共沉淀法制备了三元共晶成分Al₂O₃/YAG/ZrO₂粉体,在600-1350_oC温度范围煅烧后研究其物相转变过程。经1300_oC煅烧后Al₂O₃/YAG/ZrO₂共晶成分粉体的物相由α-Al₂O₃、c-ZrO₂和YAG构成,且具有α-Al₂O₃相包裹c-ZrO₂相的特殊结构。将煅烧粉体在1550_oC下热压烧结,制备具有内晶型结构的共晶成分Al₂O₃/YAG/ZrO₂复相陶瓷,其致密度、室温抗弯强度、断裂韧性和高温(1000_oC)抗弯强度分别为98.8%、420 MPa、3.69 MPa.m_1/2和464 MPa,并对复相陶瓷组织结构的形成机理进行了探讨。     

Effect of Bi2O3 on structure and wetting studies of Bi2O3-ZnO-B2O3 glasses

Glasses with different Bi₂O₃ contents (37-42 mol%) have been prepared by conventional melt quench technique. The IR and Raman studies indicate that these glasses are made up of [BiO₆], [BiO₃], [BO₃] and [BO₄] basic structural units. The vibrations of [BiO₃] and [BO₃] become stronger as the content of Bi₂O₃ increases, which makes glass structure loosened. Viscosity of the glasses was measured by using a Rheotronic III paralleled plate rheometry, which shows that the viscosity of glass samples decreased when the content of Bi₂O₃ increased at the same temperature (400-460 °C). The temperature range which suits for glasses sealing was calculated by using the approximation of Arrhenian behaviour. The wetting performance of Bi₂O₃-ZnO-B₂O₃ glasses was described by using high-temperature microscope, which also proves that the structure of investigated Bi₂O₃-ZnO-B₂O₃ glasses become loosened due to the increasing of the content of Bi₂O₃.     

Microstructure and characterization of La2O3 and CeO2 doped diamond-like carbon nanofilms

Seven kinds of hydrogen-free La₂O₃ and CeO₂ doped DLC films with thickness of 220-280 nm were deposited on Si (100) substrates by unbalanced magnetron sputtering. Nanoparticles with diameter of 20-30 nm are formed on the surface of films. The surface roughness Ra of films is in the range of 1.5-2.0 nm. C, La, Ce and O elements distribute uniformly along the depth direction, and C, La, and Ce elements diffuse into the Si substrate at the interface. X-ray photoelectron spectroscopy confirms that the La₂O₃ and CeO₂ form within the DLC amorphous films, and Raman spectra indicate the obvious amorphous characteristics of DLC films. High-resolution transmission electron microscopy shows the nanocrystallines structure with diameter of 2-3 nm of 16% La₂O₃ and 10% CeO₂ doped DLC films, and Fourier transformation spectroscopy also exhibits the obvious crystalline characteristics. In this work, the microstructure of two kinds of rare earth oxides doped DLC composite films is measured and analyzed.     

Optical floating zone method growth and photoluminescence property of MgNb2O6 crystal

MgNb₂O₆ single crystals are grown by the optical floating zone method. The as-grown crystals are dark brown and have dimensions of Ø 4-6 mm × L 87 mm, with the largest crystal domain being Ø 5 mm × L 32 mm. After being annealed, the crystals fade to light brown. The powder X-ray diffraction analysis shows that the crystals have a columbite-type MgNb₂O₆ structure. The crystal grows along the c-axis and the cleavage plane is the (0 1 0) plane. Transmission polarized light microscopy measurements show that the crystal is free of low-angle grain boundaries and inclusions. The crystals have been characterized by Raman scattering, which reveals the change of Nb-O band in annealed and non-annealed samples. The photoluminescence spectra exhibit a broad and strong blue emission band centered at 435 nm.     

Influences of annealing temperature on microstructure and mechanical properties of Mo-La2O3

Influences of annealing temperature on the microstructure and mechanical properties of Mo-La₂O₃ were investigated. Effects of annealing temperature on tensile properties, fracture toughness, and microhardness are discussed. Microstructure and fracture morphology of Mo alloys are observed by optical microscope, SEM, and TEM. The results indicate that grain size increased while tensile strength, fracture toughness, and microhardness decreased with increasing annealing temperature. Larger La₂O₃ particles are located at grain boundaries or sub-boundaries, while the majority of smaller La₂O₃ particles are located within the grain. The strengthening effect is quantitatively assessed, which yielded a predicted yield strength in good agreement with measurements.     

Surface integrity and material removal mechanisms associated with the EDM of Al2O3 ceramic composite

Electric discharge machining (EDM) has been proven as an alternate process for machining complex and intricate shapes from the conductive ceramic composites. The performance and reliability of electrical discharge machined ceramic composite components are influenced by strength degradation due to EDM-induced damage. The success of electric discharge machined components in real applications relies on the understanding of material removal mechanisms and the relationship between the EDM parameters and formation of surface and subsurface damages. This paper presents a detailed investigation of machining characteristics, surface integrity and material removal mechanisms of advanced ceramic composite Al₂O₃–SiC_w–TiC with EDM. The surface and subsurface damages have also been assessed and characterized using scanning electron microscopy (SEM). The results provide valuable insight into the dependence of damage and the mechanisms of material removal on EDM conditions.     

La2O3含量对Al2O3/Cu基复合材料组织与性能的影响

以La₂O₃粉、Al粉、CuO粉为反应物原料、纯铜为基体,采用原位合成技术和近熔点铸造法制备颗粒增强Cu基复合材料,研究La₂O₃对Al-CuO体系制备的Cu基复合材料组织及性能的影响。结果表明：添加La₂O₃可获得纳米Al₂O₃颗粒,且弥散分布于Cu基体中,制备的材料组织更加细小、均匀,其材料的电导率及摩擦磨损性能明显提高。当添加0.6%wtLa₂O₃,复合材料的电导率达到90.2%IACS,磨损量达到最小,相比未添加La₂O₃,其导电率提高10.1%,磨损量减小36.6%。     

Role of titanium valence states in optical and electronic features of PbO–Sb2O3–B2O3:TiO2 glass alloys

PbO–Sb₂O₃–B₂O₃ glasses mixed with different concentrations of TiO₂ (ranging from 0 to 1.5 mol.%) were synthesized. The samples are characterized by X-ray diffraction, scanning electron microscopy and DSC techniques. A variety of properties, i.e. optical absorption, photoluminescence, infrared, ESR spectra, magnetic susceptibility, photo-induced birefringence (PIB) and dielectric properties (constant ′, loss tan δ, a.c. conductivity σ_ac over a wide range of frequency and temperature) of these glass–ceramics have been explored. The analysis of these results indicated that Ti ion surrounding ligands play principal role in the observed PIB and the sample crystallized with 0.8 mol.% of TiO₂ is the most suitable for the applications in non-linear optical devices.