Plasma Sprayed Coating Using Mullite and Mixed Alumina/Silica Powders

Plasma sprayed ceramic coatings are widely used for thermal barrier coating applications. Commercially available mullite powder particles and a mixture of mechanically alloyed alumina and silica powder particles were used to deposit mullite ceramic coatings by plasma spraying. The coatings were deposited at three different substrate temperatures (room temperature, 300?°C, and 600?°C) on stainless steel substrates. Microstructure and morphology of both powder particles as well as coatings were investigated by using scanning electron microscopy. Phase formation and degree of crystallization of coatings were analyzed by x-ray diffraction. Differential thermal analysis (DTA) was used to study phase transformations in the coatings. Results indicated that the porosity level in the coatings deposited using mullite initial powder particles were lower than those deposited using the mixed initial powder particles. The degree of crystallization of the coatings deposited using the mixed powder particles was higher than that deposited using mullite powder particles at substrate temperatures of 25 and 300?°C. DTA curves of the coatings deposited using the mixed powders showed some transformation of the retained amorphous phase into mullite and alumina. The degree of crystallization of the as sprayed coatings using the mixed powder particles was significantly increased after post deposition heat treatments. The results indicated that the mechanically alloyed mixed powder can be used as initial powder particles for deposition of mullite coatings instead of using mullite powders.     

Preparation and Microstructure Characterization of Mullite Coatings Made of Mullitized Natural Andalusite Powders

Natural andalusite powders were heated at high temperature in air allowing mullitization. The mullitized powders were spray-dried and then air plasma sprayed to form mullite coatings. Microstructure and phase composition of the plasma-sprayed mullite coatings were investigated by means of scanning electron microscopy and x-ray diffraction. The effect of spraying power on the microstructure and phase composition of the mullite coatings was investigated. It has been found that spraying power has significant effect on the content of amorphous phase in the mullite coatings. At the same time, good adhesion is reached between the mullite coating and the bound coating (NiCrAlY) and between the bound coating and the substrate. The mullite coating deposited at a spraying power of 22.5 kW is free of cracks. The processes reported here are applicable to preparing mullite coatings used in high-temperature environment.     

Molybdenum and tungsten coatings for x- ray targets obtained through the low-pressure plasma spraying process

Low-pressure plasma spraying under an argon atmosphere was employed to deposit molybdenum and tungsten coatings on different metallic, ceramic, and composite substrates. Molybdenum coatings ob-tained through this technique presented a homogeneous structure with an average porosity of about 17 %. These coatings exhibited adhesion greater than 40 MPa on molybdenum and grey cast iron (FT25) substrates. No adhesion was observed on an A1N surface regardless of the preheating temperature and/or surface preparation. Adhesion on AIN-Mo (AM25) composite substrate, containing 25 % dispersed metal-lic phase by volume, showed intermediate results. Tungsten coatings exhibited porosity between 10 to 12% and a typical lamellar structure. The adhesion of tungsten coatings on molybdenum and FT25 sub-strates was around 40 MPa.     

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.     

钼及钼合金表面高温防护涂层的研究进展

系统总结了国内外关于钼及钼合金表面高温防护涂层的最新研究成果，分析了钼在不同温度区间的氧化特征，并基于涂层组织结构稳定性、涂层缺陷、涂层与基体界面结合强度、界面物理和化学相容性、氧扩散等多方面，概述了钼及钼合金表面高温防护涂层的性能要求。归纳了现阶段应用于钼及钼合金表面的高温防护涂层体系，主要包括单一硅化物涂层、改性的硅化物涂层、硅化物基梯度复合涂层、铝化物涂层、耐热合金涂层和氧化物涂层，重点讨论了涂层的成分和结构对其抗高温氧化性能的影响。同时，对比介绍了钼及钼合金表面高温防护涂层常用的制备方法，主要包括料浆烧结法、包埋渗法、等离子喷涂法、熔盐法、化学气相沉积法、磁控溅射法等。最后，对钼及钼合金表面高温防护涂层现阶段存在的问题及未来发展方向进行了展望。     

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.     

铝合金基体电热爆炸喷涂钼涂层的制备工艺与性能

采用电热爆炸喷涂技术在铝合金基体上制备钼涂层,用SEM、划痕仪、显微硬度计、表面粗糙度仪、微磨损试验机等表征了涂层性能。借助正交试验方法分析了钼涂层制备中各工艺参数对涂层质量的影响,确定了最佳工艺参数。结果表明:用电热爆炸喷涂方法制备的钼涂层均匀致密,孔隙率低;涂层与基体形成冶金结合,结合强度高;涂层硬度为基体的4.9倍,体积磨损量仅为基体的11.2%,耐磨性大幅增加。电爆炸喷涂的工艺参数中,对涂层质量和结合力影响最大的是喷涂距离,其次是喷涂次数,影响最小的是喷涂电压。     

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.     

Microstructure and adhesion of 100Cr6 steel coatings thermally sprayed on a 35CrMo4 steel substrate

Thermally sprayed of 100Cr6 steel coatings are widely used to combat degradation of components and structures due to mechanical wear. In this paper, the microstructure and adhesion energy of 100Cr6 steel coatings thermally sprayed on a 35CrMo4 steel substrate are investigated. The microstructure characteristics of the deposits are studied using the combined techniques of X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) including energy-dispersive spectroscopy (EDS). The practical work of adhesion of flame-sprayed 100Cr6 on steel substrate is determined using a four-point delamination bending test. The influence of a molybdenum bond coat on the adhesion is also studied. Microstructure suggests that the coating is mainly constructed by splats of γ-phase (fcc) and FeO. Phase analysis also confirms that during spraying process, a stable α-phase (bcc) was transformed into a new γ-phase (fcc). The highest values of the fracture energy are obtained for the 35CrMo4 substrate/100Cr6 steel deposit type samples. On the contrary, when a molybdenum bond coat is introduced (composite system 35CrMo4 substrate/Mo bond coat/100Cr6 steel deposit), the fracture energy decreases in a ratio of approximately three. So, the presence of a Mo bond coat as a barrier between the coating and the substrate has a negative role on the adhesion.     

Improvement of wear resistance of plasma-sprayed molybdenum blend coatings

The wear resistance of plasma sprayed molybdenum blend coatings applicable to synchronizer rings or piston rings was investigated in this study. Four spray powders, one of which was pure molybdenum and the others blended powders of bronze and aluminum-silicon alloy powders mixed with molybdenum powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coatings showed that the phases formed during spraying were relatively homogeneously distributed in the molybdenum matrix. The wear test results revealed that the wear rate of all the coatings increased with increasing wear load and that the blended coatings exhibited better wear resistance than the pure molybdenum coating, although the hardness was lower. In the pure molybdenum coatings, splats were readily fractured, or cracks were initiated between splats under high wear loads, thereby leading to the decrease in wear resistance. On the other hand, the molybdenum coating blended with bronze and aluminum-silicon alloy powders exhibited excellent wear resistance because hard phases such as CuAl₂ and Cu₉Al₄ formed inside the coating.     

Inspection of compositionally graded mullite coatings using laser based ultrasonics

Functionally graded materials (FGMs) have spatial variations in composition and structure resulting in corresponding changes in material properties. These materials can be developed for applications in which the functional requirements of a component are position dependent. In this paper, we evaluate the feasibility of using a laser based ultrasonic technique to determine the distribution of elastic properties in FGMs. A model for the laser generation of ultrasound in a compositionally graded coating on a semi-infinite substrate is used to explore the relationship between the acoustic wave propagation and depth dependent elastic property profile. Surface acoustic waves (SAW) generated in such a system are dispersive, and the dispersion characteristics are highly dependent on the mechanical property profile. A high-resolution laser ultrasonic system has been developed, and experimental measurements of the mechanical properties of single layer mullite coatings are presented. Using compositionally graded mullite coatings as a model system, numerical results are reported showing the sensitivity of SAW dispersion behavior to various composition profiles.     

Microstructure and thermal cycle resistance of plasma sprayed mullite coatings made from secondary mullitized natural andalusite powder

Natural andalusite powder was calcinated at a high temperature in air to realize secondary mullitization. The resultant secondary mullitized powder was spray-dried and heat-treated to improve sprayable capability. The heat-treated spherical powder was then plasma sprayed onto Ni-based high-temperature alloy (Hastelloy C-276) to form mullite coatings. The chemical composition and phase structure of the as-sprayed and thermally cycled mullite coatings were determined by means of energy dispersive X-ray fluorescence (ED-XRF) and X-ray diffraction. The microstructure of the as-sprayed coatings was analyzed by using a scanning electron microscope; and their porosity, microhardness and bonding strength were measured. Moreover, the phase transition temperature and enthalpy of the coatings were determined by means of differential scanning calorimetry; and their thermal shock resistance was evaluated as well. Results show that the spray-dried and heat-treated powder consists of mullite and a small amount of Al₂O₃; while the as-sprayed mullite coatings are composed of crystalline mullite as the major phase and a small amount of amorphous glass phase. During thermal cycle test, the amorphous glass phase is partially transformed to crystalline mullite, finally leading to failure of the coatings. Whether before or after thermal cycle, the mullite coatings experience phase transition around 980 °C, and the enthalpy of crystallization is determined to be − 141.9 × 10^− 3 J/kg and − 95.48 × 10^− 3 J/kg, respectively. The as-sprayed mullite coatings have a porosity of about 6.0 ± 0.2% and possess good thermal cycle resistance, showing promising prospect in a high-temperature application.     

微弧等离子喷涂制备莫来石/金属复合热障涂层

运用微弧等离子喷涂制备了莫来石/金属复合热障涂层.研究了涂层的微观结构、结合强度、隔热性能和抗热震性能.复合涂层结构为莫来石颗粒被"包裹"在金属层片状结构中;涂层的结合强度大于30 MPa.随着涂层中莫来石含量的增加,涂层的隔热性能有所提高,随涂层表面温度的升高,涂层的隔热温度也不断提高,涂层的最高隔热温度为125℃.1 150℃的水淬热震试验表明,基体变形是导致涂层失效的重要原因之一,随着涂层中莫来石含量的增加,涂层的抗热震次数先增加后减小,粉末中莫来石含量为40%的涂层的抗热震性能最好,抗热震次数最多为72次.     

电沉积结合包埋渗制备复合涂层的高温氧化行为

为了提高铌合金的高温抗氧化性能,首先采用阴极匀速旋转电沉积法在铌合金表面制备了钼层,然后通过包埋渗硅获得硅化物复合涂层。研究了沉积态钼层和硅化物涂层的形成和微观组织结构,对比分析了有或无涂层C103合金的高温氧化行为。结果表明,阴极匀速旋转法使沉积的电流效率提高一倍多,钼层呈胞状结构、以非晶形式存在。经包埋渗硅后,生成了表层以MoSi2为主、中间层为NbSi2的复合涂层,涂层与基体结合良好。经1200 ℃氧化后,涂层试样的氧化抛物线速率常数分别为1.83×10-2 mg2cm-4h-1、8.08 mg2cm-4h-1,与之相比,裸合金的氧化抛物线速率常数则为5.87×103 mg2cm-4h-1,而且仅氧化10 h裸合金即出现了严重粉化;在高温氧化过程中,复合涂层表面生成SiO2垢层,具有优良的抗高温氧化性能。     

A study of high-velocity combustion wire molybdenum coatings

In this paper, coatings manufactured using the high-velocity combustion wire (HVCW) spray process have been studied. Molybdenum coatings were prepared in this study, and wavelength dispersive x-ray analysis (WDX) investigations were carried out to ascertain the oxygen content of the coating and its distribution. The x-ray diffraction (XRD) analysis of the coating was also carried out to determine the phases present in the coating. Based on the above data, the authors explain the HVCW-sprayed molybdenum coating microstructure properties. These coatings were also sprayed using a modified aircap design. The parameters varied for the molybdenum coatings by HVCW and were (1) the distance of the substrate from the spray gun and (2) the wire feed rate of the gun. The wear test and coefficient of friction measurements were also carried out for the coatings. Air plasma spraying of Mo-25% NiCrBSi coatings was carried out, and these coatings were further checked for wear friction properties.     

化学气相沉积方法制备定向W涂层研究

采用化学气相沉积方法在钼基体上制备了具有[110]择优取向的W涂层,采用SEM、XRD、EBSD方法分析了涂层性能,研究表明涂层致密,法向偏离[110]晶向8°范围的晶粒占涂层面积的比例为85.32%,10°范围内[110]晶粒占全部面积的96.86%,[110]择优取向的生成受气体流量H2/Cl2,反应室压强及沉积温度的影响;对截面织构研究表明涂层形成机制为竞争生长机制。在室温～1400℃～室温20次热循环后涂层界面存在互扩散形成的孔洞,但是涂层结合良好。     

冷喷涂TC4涂层临界沉积速度计算及制备涂层性能研究

目的研究冷喷涂TC4涂层的临界沉积速度及粒子温度对临界沉积速度的影响规律,并研究气体压强对沉积涂层性能的影响规律。方法理论研究上,采用有限元LS-DYNA软件中的Johnson-Cook塑性模型,选取3D164计算单元建立模型,研究粒子在不同温度和不同速度下碰撞基体后的形貌特征,确定粒子沉积临界速度。试验研究上,采用N_2作为冷喷涂驱动气体,在TC4合金上制备TC4涂层,然后采用SEM、Image J图像分析软件、硬度计等分析已沉积涂层的孔隙率和硬度等性能。结果 25、400、500、600℃温度下,计算表明10μm的TC4合金粒子在TC4基板上的临界沉积速度分别为730、465、392、361 m/s,即随粒子温度升高,粒子临界沉积速度降低,粒子沉积成涂层更容易。采用冷喷涂工艺在TC4基板上沉积TC4涂层,在N_2温度600℃、气体压力3 MPa的条件下,制备的TC4涂层厚度约1000μm,与TC4钛合金基体结合紧密,涂层孔隙率约为6.46%。结论气体温度升高,粒子临界沉积速度降低;气体压强变大,制备的涂层厚度就大且更加致密。     

Peening action and residual stresses in high-velocity oxygen fuel thermal spraying of 316L stainless steel

316L stainless steel powder was sprayed by a high-pressure high-velocity oxygen fuel (HVOF) process. Effects of powder size and the pressure in the combustion chamber on the velocity and temperature of sprayed particles were studied by using an optical instrument, first, at the substrate position. A strong negative correlation between the particle temperature and the diameter was found, whereas the correlation between the velocity and the diameter was not significant. The pressure in the combustion chamber affected the velocity of sprayed particles significantly, whereas the particle temperature remained largely unchanged. In-situ curvature measurement was employed in order to study the process of stress generation during HVOF spraying. From the measured curvature changes, the intensity of peening action and the resultant compressive stress by HVOF sprayed particles were found to increase with the kinetic energy of the sprayed particles. The results were further used to estimate the stress distribution within the coatings. X-ray stress measurement revealed that the residual stress on the surface of the HVOF coatings is low and often in tension, but the stress inside the coatings is in a high level of compression.     

钼电极玻璃基防氧化涂层预烧工艺研究

采用料浆涂覆的方法在钼基体表面制备了玻璃基防氧化涂层;通过SEM、XRD、防氧化性能测试对涂层表面形貌、显微组织结构和氧化失重进行了研究;探讨了玻璃基涂层预烧工艺对钼电极表面的防氧化性能的影响.结果表明:涂层经1100℃保温10min的热处理后,具有最优的防氧化性能.