Structure and high-temperature stability of compositionally graded CVD mullite coatings

Dense, uniform and crack-free mullite (3Al₂O₃·2SiO₂) coatings were deposited on SiC by chemical vapor deposition. The coatings were compositionally graded, with the Al/Si ratio increasing towards the outer surface of the coatings for improved corrosion resistance. The coatings were found to start out as a nanocrystalline layer, which is an intimate mixture of γ-Al₂O₃ nanocrystallites imbedded in a vitreous silica-rich matrix at the substrate/coating interface. Mullite grains nucleated when the surface composition of the growing coating was in a narrow range close to that of stoichiometric mullite. The phase transformations occurring in these coatings during high-temperature anneals in the range 1100–1400 °C were studied. These phase transformations, which include a tetragonal-to-orthorhombic transformation, mullitization and devitrification of silica in the nanocrystalline layer, and -alumina precipitation and twinning of the alumina-rich mullite, are discussed in light of the adhesion and corrosion resistance of the coatings.     

Cross-sectional nanoindentation and nanoscratch of compositionally graded mullite films

Nanoindentation and nanoscratch have become well-established techniques for measuring mechanical properties of thin films. Conventionally, these tests are performed on the surface of the films to evaluate their mechanical integrity: elastic modulus, hardness and adhesion strength. However, in complex systems such as compositionally graded thin films, small spatial variations in mechanical properties are difficult to distinguish using this approach. In this work, the evaluation of the above parameters was conducted on cross-sections of compositionally graded mullite coatings, chemical-vapor deposited on silicon carbide substrates. To assess the intrinsic mechanical properties and their spatial variation, nanoindentation tests were carried out on mullite coatings with constant and graded Al/Si ratios. Additionally, transverse nanoscratch tests to evaluate the cohesive and adhesive resistance of the coatings as well as the coating/substrate systems, respectively, were performed. Different damage morphologies were identified within the coating and at the interface by using complementary characterization techniques. In the case of functionally graded coatings a gradual rise in the hardness and elastic modulus with increasing distance from the coating/substrate interface was observed. Nanoscratch tests on the cross-sections allowed determining the critical loads for cohesive and adhesive damage by following this approach. Compositionally graded mullite coatings exhibited the best combination of hardness/stiffness and cohesive/adhesive scratch strength.     

Direct laser cladding of Co on Ti–6Al–4V with a compositionally graded interface

In the present study, attempts have been made to fabricate Co layers on the surface of Ti–6Al–4V substrate with a compositionally graded interface by direct laser cladding. Laser processing is carried out by pre-placing the powder (or powder blends) on the substrate, and melting it using a high power continuous wave CO₂ laser with Ar as shrouding gas. A compositionally graded interface is developed by applying powder blends of Ti to Co at a ratio of 90:10 near to Ti–6Al–4V substrate to 10:90 prior to development of Co layer. A defect-free microstructure is developed with the presence of Ti₂Co and TiCo and Co₂Ti at the interface. The volume fraction of individual phase was found to vary with the depth from the Co-clad zone. A significant improvement in microhardness is achieved at the interfacial region. Uniform corrosion resistance increases along the graded interface, but the pitting corrosion resistance is marginally deteriorated. Direct laser clad layer possesses a better biocompatibility than that of as-received Ti–6Al–4V sample.     

Corrosion study of Ni/Zn compositionally modulated multilayer coatings using electrochemical impedance spectroscopy

Ni/Zn compositionally modulated multilayer (CMM) coatings were deposited using dual bath technique. Coatings corrosion performance was evaluated using electrochemical impedance spectroscopy (EIS) during extended immersion times up to 48 h. The results of electrochemical impedance spectroscopy showed that Ni/Zn CMM coatings had better corrosion resistance compared to that of the zinc single layer coating. The modified corrosion product which is formed on the Ni/Zn CMM coatings during extended exposure times and also a good barrier effect of the nickel layer against aggressive species in these coatings can be two important reasons for high corrosion performance and so protection performance of the Ni/Zn CMM coatings.     

Thickness and grain size monitoring in seamless tube-making process using laser ultrasonics

The seamless tube-making process often causes wall-thickness variations generally in a helical pattern along the tube length. A laser-ultrasonic system installed immediately after the final operation in tube making provides process monitoring. Tube wall thickness and temperature measurements guide the mill adjustments to achieve the desired tolerances. Using the same ultrasonic signals, additional functionality provides the ability to measure the size of austenite grains. A signal processing approach based on a single echo analysis is used for determining wall thickness and austenitic grain size in relatively thick materials. Discussions review challenges specific to on-line conditions such as limited signal-to-noise ratio. A statistical comparison with metallographic results shows that the laser-ultrasonic grain sizes determined on-line have at least the same accuracy.     

Thermal shock characteristics of plasma sprayed mullite coatings

Commercially available mullite (3Al₂O₃·2SiO₂) powders containing oxides of calcium and iron as impurities, have been made suitable for plasma spraying by using an organic binder. Stainless steel substrates covered with Ni-22Cr-10Al-1.0Y bond coat were spray coated with mullite. The 425 μm thick coatings were subjected to thermal shock cycling under burner rig conditions between 1000 and 1200 °C and less than 200 °C with holding times of 1, 5, and 30 min. While the coatings withstood as high as 1000 shock cycles without failure between 1000 and 200 °C, spallation occurred early at 120 cycles when shocked from 1200 °C. The coatings appeared to go through a process of self erosion at high temperatures resulting in loss of material. Also observed were changes attributable to melting of the silicate grains, which smooth down the surface. Oxidation of the bond coat did not appear to influence the failure. These observations were supported by detailed scanning electron microscopy and quantitative chemical composition analysis, differential thermal analysis, and surface roughness measurements.     

Improvement of YAG laser weldability using graded index optical fibres

The article analyses the possibilities of post-accident repairs to vehicle bodies by means of welding. Basic connection of vehicle body components during post-accident repairs by means of spot welding and shielding gas coverage methods were discussed. Studies were also made on the results of the mechanical properties of the welded joints.     

A kinetic investigation of CVD mullite coatings on Si-based ceramics

Chemically vapor deposited (CVD) mullite coatings were deposited on SiC substrates using the AlCl₃,SiCl₄,CO₂,H₂ system in a hot-walled vertical type reactor. An empirical kinetic model was developed indicating that an intermediate gaseous reaction was kinetically significant to the growth rate of mullite on SiC. The kinetically limiting equation was defined between a temperature range of 673–1327 K and 30–400 mbar where temperature was defined as a function of reactor position. The growth rate and crystal structure of mullite were dependent upon temperature, pressure, reactant concentration, and reactant ratios.     

Effect of substrate temperature on microstructures and dielectric properties of compositionally graded BST thin films

Compositionally graded Ba1-xSrxTiO3 （BST） （x = 0-0.3） thin films were prepared on Pt/Ti/SiO2/Si substrate at different substrate temperatures ranging from 550 ℃ to 650 ℃ by radio-frequency （rf） magnetron sputtering. The effect of substrate temperature on the preferential orientation, microstructures and dielectric properties of compositionally graded BST thin films was investigated by X-ray diffraction, scanning electron microscopy and dielectric frequency spectra, respectively. As the temperature increases, the preferential orientation evolves in the order： randomly orientation→ （111） → highly oriented （111） （α（111）= 60.2%）. The surface roughness of the graded BST thin films varies with the substrate temperatures. No visible internal interface in the compositionally graded thin films can be observed in the cross-sectional SEM images. The graded BST thin films deposited at 650 ℃ possess the highest dielectric constant and dielectric loss, which are 408 and 0.013, respectively.     

Effect of space charge on the polarization hysteresis characteristics of monolithic and compositionally graded ferroelectrics

A non-linear thermodynamic analysis of ferroelectric systems with localized space charges for monolithic and compositionally graded materials is described wherein the electrostatic interlayer interactions are specifically accounted for. The electrostatic coupling is established through the built-in polarization due to the space charges and the intrinsic polarization variations between the ferroelectric layers. The findings show that the polarization hysteresis response of monolithic stress-free barium strontium titanate (BST) ferroelectrics with asymmetrically distributed space charges result in a displacement of the hysteresis loop along the applied electric field axis. In compositionally graded BST multilayers, the hysteresis response is characterized by off-sets along both the polarization and the electric field axes, yet with magnitudes of displacement that are markedly larger than those for monolithic ferroelectrics.     

Formation of mullite coatings on silicon-based ceramics by chemical vapor deposition

Dense, uniform, mullite coatings have been deposited by chemical vapor deposition on SiC substrates, using a AlCl₃-SiCl₄-CO₂-H₂ system. The typical coating microstructure consisted of a thin layer of nanocrystallites of γ-Al₂O₃ in vitreous silica at the coating-substrate interface, with columnar mullite grains over this interfacial layer. The composition of the coating was graded such that the outer surface of the coating was highly alumina rich. The changes in the coating microstructure with processing parameters are discussed. The ability of mullite to incorporate such large composition variations is discussed in the light of vacancy formation as theAl/Si ratio is increased and the ordering of these vacancies leads to changes in lattice parameters. The formation of domains was studied by measuring the spacing of superlattice spots in electron diffraction patterns and the relationship between domain size andAl/Si ratio is discussed.     

基于激光视觉的焊缝特征信息检测技术研究

对于基于激光视觉的焊缝自动跟踪及参数自适应控制系统,从焊缝激光条纹中实时、快速、精确地检测焊缝几何特征信息是前提.文中利用模块化开发工具包(MDK,modular development kit)构建的焊缝特征检测系统,实现了对焊缝特征信息,如焊枪跟踪点位置和焊缝几何特征等的快速精确提取.文中主要从系统构成、工作原理、软件实现等几个方面进行了详细阐述,并通过实际的检测,验证了系统的精确性和稳定性.试验结果表明,系统具有较高的精度和稳定性,能够满足焊缝跟踪及参数自适应控制对焊缝特征信息检测的要求.     

基于激光视觉传感的角焊缝外形尺寸检测

针对目前角焊缝外形尺寸手工检测的缺点,采用了一种基于激光视觉传感的角焊缝外形尺寸检测方法.该方法利用基于三角形测量原理的激光传感器,采集反映焊缝几何形状特征的激光条纹图像,利用中值滤波、二值化、中心线提取以及特征识别等算法,从而获得了角焊缝的外形尺寸信息.结果表明,与传统的手工检测方法相比,该方法具有检测准确度高、可靠性好、耗时短、检测内容丰富、使用方便等特点,并且消除了人为主观判断的影响.此外,检测的数据可长久保存在计算机中,便于后续的统计分析.     

Microstructure of cobalt based alloy coatings produced by laser deposit welding

Abstract

Coatings of cobalt based alloy HMSP 2528, deposited on a nickel based superalloy using a transverse flow CO₂ laser, are investigated. Sections of the coatings have been examined to reveal their microstructure and phase composition using an Olympus PME–3 type optical microscope, scanning electron microscope, transmission electron microscope, and X-ray diffractometer. The results showed that primary phase (γ-Co) and eutectic (γ-Co + Cr₂₃C₆) were present in the coatings. Rapid directional solidification was found at the interface, where the growth direction was perpendicular to the interface. Multidirectional growth and growth parallel to the scanning direction were also found in the central region and near the top surface, respectively. At the remelted boundary between two tracks, a novel microstructure was found in which the crystallisation direction of the first track was transmitted to the second track. As more tracks were deposited, coarsening dendrites could be found at the interfaces.     

Design and fabrication of compositionally graded inorganic oxide thin films: Mechanical,optical and permeation characteristics

Different types of inorganic oxide films composed of a chemical composition gradient single layer were designed, fabricated and characterized. Compositionally graded thin films were created by power-controlled co-sputtering of alumina (Al₂O₃) and silica (SiO₂) at room temperature, allowing the structural design of the film to be tailored at the nanometer scale. Two distinct graded thin films were fabricated: one with a compositionally asymmetric structure consisting of a SiO₂-rich bottom interface and a Al₂O₃-rich top surface, and the other with a compositionally balanced sandwich structure consisting of both the top surface and bottom interface rich in SiO₂ and a core rich in Al₂O₃ (referred to as SGS for ‘sandwich graded structure’). Smoothly graded thin films without interfacial boundaries were verified by Auger electron spectroscopy profiles. X-ray photoelectron spectroscopy demonstrated that the Al₂O₃/SiO₂ graded structures consisted of Si–O and Al–O bonds, as well as Al–O–Si bonds in the transition layer. Neat Al₂O₃ or SiO₂ and their graded ones were all investigated for their mechanical, optical and permeation properties. A SGS thin film presented the best mechanical stability (i.e., about three times improved film toughness of a neat Al₂O₃ single layer), demonstrating that balanced internal stresses and alternating bonding structures, achieved via a graded structure without interfaces, are crucial for enhancing mechanical stability. Furthermore, neat and graded thin films exhibited the similar level of optical transmittance and the permeation properties for the graded films were well matched with the behaviors of mechanical stability.     

Synthesis of composite coatings using rapid laser sintering of metallic powder mixtures

Presented here is a brief review of research of an authors’ collective dealing with laser treatment of materials, sintering metallic powders, and multiscale simulation. A theoretical analysis of the processes of structure formation upon the rapid laser synthesis of composite coatings has been performed. The experimentally obtained structural and phase characteristics of the sintered layers have been explained based on an analytical and numerical simulation of the dynamics of thermal fields in the zone of treatment, processes of melting, and subsequent solidification of porous materials. Upon rapid sintering and solidification, the effect of impurity trapping has been taken into account, which determines the chemical composition of the powders under nonequilibrium conditions of their formation. It has been shown that rapid laser treatment retains the composite structure of the powder layer due to the high rates of local heating/cooling and high rate of solidification comparable with the rate of diffusion of chemical components. The results obtained are applicable in the development of a wide class of functional-gradient composite materials.     

Protective Al-rich mullite coatings on Si- based ceramics against hot corrosion at 1200 °C

The hot-corrosion behavior of uncoated SiC, bulk mullite and CVD grown mullite coatings in contact with Na₂SO₄ were investigated at 1200 °C. The range of thermodynamic activity of Na₂O (10^− 4 to 10^− 6) simulated in this study makes the oxide very basic leading to high reaction rates with SiO_2. Uncoated SiC corroded severely, forming various Na₂O·SiO₂ compounds with a significant weight gain. Even though the thermodynamic activity of silica is reduced in mullite, several compounds in Na₂O·SiO₂·Al₂O₃ system were formed in case of bulk mullite. CVD based mullite coatings with high alumina content at the top surface of the coating, and therefore reduced SiO₂ activity, offered protection to the underlying SiC in corrosive environments. As predicted correctly by thermodynamic calculations, the trend in weight gain with the coated SiC samples followed the theoretically calculated SiO₂ activity in Al-rich mullite.     

激光能量输入对直接激光沉积莫来石陶瓷组织性能的影响

研究了激光能量密度对直接激光沉积熔体自生莫来石陶瓷孔隙/密度、微观组织和力学性能的影响.结果表明,较低激光能量密度(15 J/mm2)所制备的陶瓷样件边缘分布有尺寸较大的气孔,使得样件整体孔隙率较高,且表面粘粉严重,这与成形扫描速度大和硅酸盐熔体粘度高有关.而较高激光能量密度(90 J/mm2)虽然可以获得表面光滑、莫...     

Non-destructive evaluation of plasma sprayed functionally graded thermal barrier coatings

Acoustic emission (AE) as a non-destructive evaluation technique has recently been used in a number of studies to investigate the performance and failure behavior of plasma sprayed thermal barrier coatings. The mechanism of coating failure is complex, especially when considering the composite nature of the coating. In the present paper, the thermal shock tests with in situ acoustic emission are used to study the cracking behavior of plasma sprayed functionally graded thermal barrier coatings. Each thermal cycle consists of 8 min heating in the furnace at 1000°C and 8 min cooling from 1000°C to the room temperature by a compressed air jet. The AE signals are recorded during the quench stage. Three, four and five layer functionally graded coatings have been tested. The results show that the five layer functionally graded coatings appear to have the best thermal shock resistance in the specimens tested, because of the gradual changes in material properties. Higher AE energy counts and cumulative counts recorded by the tests are associated with the macro-crack initiation and growth. On the other hand, micro cracking and phase transformation only give rise to lower AE signals.