Microstructure characteristics of silicon carbide coatings fabricated on C/C composites by plasma spraying technology

A functional gradient SiC coating on C/C composites has been developed using a novel process which is the combination of plasma spraying technology with reaction-formed heat-treatment. Microstructure observation and phase identification of the SiC coatings were analyzed by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Experimental results showed that a uniform silicon coating was deposited on C/C composite by plasma spraying technology. The reaction between the silicon coating and C/C substrate occurred during the heat-treatment at temperature of 1450 °C and 1600 °C in argon environment, respectively. A continuous SiC coating was formed on the surface of the C/C substrate. And a layer of SiC/C convention layer was formed on the near-surface area of the substrate, which was resulted from the molten silicon penetrating into the open pores and consequently reacting with the C/C composites. The thickness of the formed SiC coatings was closely related to the original silicon coatings.     

TEM characterization and reaction mechanism of composite coating fabricated by plasma spraying Nb–SiC composite powder

Niobium carbide composite coatings with Nb₂C, NbC, Nb₃Si as the main phases were prepared in situ on the surface of TC4 titanium alloy by plasma spraying Nb–SiC composite powder. The microstructure of the coating was characterized in detail by TEM, and the reaction mechanism of Nb–SiC was revealed. Sub-micron and nano-scale NbC grains dispersed in Nb₃Si region, nano-Nb/Nb₃Si cellular eutectic region, and equiaxed Nb₂C nanograins region were formed in the coating. The research results show that Nb and SiC reacted firstly to form cubic NbC and Nb₃Si phases during the plasma spraying process. Then, NbC with a higher melting point took the lead in crystallization during the cooling process of the coating, forming sub-micron and nano-scale NbC granular fine grains. Nb₃Si with a lower melting point crystallized around the sub-micron and nano-scale NbC granular fine grains in the subsequent cooling process. In the plasma spraying process, the molten droplets formed Nb/Nb₃Si cellular eutectic structure under large temperature gradient and extremely fast cooling rate. The remaining Nb in the raw material powder formed a diffusion couple with NbC to generate fine and dispersed nano-equiaxed Nb₂C with cubic structure. The present investigation provides a reference for the reaction synthesis of advanced nanocomposites using Nb–SiC system.     

Characterization of composite titanium nitride coatings prepared by reactive plasma spraying

Reactive plasma spraying (RPS) technology has been used to produce high thickness (>100 μm) films of composite Ti-TiN-Ti_xN_y coatings. Reactively sprayed coatings obtained from SP700 (Ti-4.5Al-3V-2Mo-2Fe) and Ti6242 (Ti-6Al-2Sn-4Zr-2Mo) powders, deposited onto flat substrates of Ti-6Al-4V, have been investigated. X-ray diffraction (XRD) and micro-hardness measurements have been used to characterize the crystallographic features and differences between the hardness of cross-section and surface. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) techniques have been used to determine the morphology and surface chemical composition of the coatings. Back-scattered electron imaging has revealed the distribution of Ti and TiN in the composite material. Obtained results evidenced a sharp interface between the coating and substrate in terms of the profile of chemical composition. The presence of unmelted Ti particles as well as the formation of nitrides was observed.     

Microstructure evolution and mechanical properties of in-situ bimodal TiC-Fe coatings prepared by reactive plasma spraying

An in-situ synthesis phenomenon of bimodal TiC-Fe coatings is discovered in the reactive thermal spraying of Fe-Ti-C composite powders. The Fe-Ti-C composite powders were prepared by a hybrid spray-drying/pyrolysis technology using a mixture of ferrotitanium (TiFe), graphite and sucrose. The microstructure evolution and mechanical properties of the coatings were investigated. The results showed that the TiC-Fe coatings exhibited an apparent bimodal particle size distribution of submicron and nano TiC particles in the α-Fe matrix. The formation of the bimodal distribution structure was revealed as following bi-precipitation mechanism: the first precipitation during the flight process to form submicron TiC particles and the second precipitation in the impact process with rapid cooling to form nano TiC particles. The bimodal TiC-Fe coatings showed high microhardness in various loads due to simultaneous hardening effect from submicron and nano TiC particles. The crack analysis showed that the in-situ bimodal TiC structure hindered the crack initiation and the crack propagation and thus improved the fracture toughness. The bimodal TiC-Fe coatings exhibited high abrasive wear resistance and the wear mechanism transformed from two-body abrasive wear to three-body abrasive wear with the increase of spray distances.     

Microstructure and wear properties of niobium carbide particulates gradient-distribution composite layer fabricated in situ

Niobium carbide (NbC) particulates gradient-distribution composite layer was successfully synthesized by in situ techniques. The formation mechanism, microstructure evolution and wear properties of the composite layer were investigated in detail. The results show that the thickness of the composite layer is about 1360 µm after the heat treatment at 1175 °C for 1 h. From the surface to the matrix, the volume fraction of NbC particulates continuously decreases while the average particulate diameter gradually increases from 213 nm to 1.46 µm. Correspondingly, the micro-hardness and relative wear resistance almost gradually decrease from the top surface to the matrix.In order to discuss the wear mechanism clearly, the composite layer was artificially divided into four regions according to the continuously variation of composition and microstructure. The wear mechanism of NbC particulates high-density zone (region A) is mainly characterized by the intergranular micro-cracks and micro-ploughing. The relative wear resistance of this region is 75 times higher than that of HT300. The wear mechanism of NbC particulates partly gathering zone (region B) is micro-ploughing and part of micro-cutting. NbC particulates dispersing zone (region C) is slight micro-cutting and a small amount broken NbC particulates. And near-matrix zone (region D) is severe micro-cutting with broken and re-embeding NbC particulates in the matrix. And the wear resistance of the four zones is higher than that of matrix.     

Properties of alumina coatings prepared on silica-based ceramic substrate by plasma spraying and sol-gel dipping methods

Silica-based ceramic cores are widely used in the manufacturing of hollow, nickel-based, superalloy turbine blades. However, elemental Hf, Ti, Al, and other active metals in the superalloy can react with silica-based ceramic cores during casting, resulting in a reduction in the quality of the turbine blades. In this study, both plasma spraying and sol-gel dipping methods were used to prepare alumina coatings on silica-based ceramic substrates to prevent the interfacial reaction. The performance of the alumina coatings prepared by both methods was evaluated by comparative analysis of the surface roughness, bonding interface morphologies, and the adhesive characteristics of the coating. The plasma-sprayed alumina coating has a roughness greater than 5 μm and peeled away from the substrate due to the difference in thermal expansion between SiO₂ and Al₂O₃ at temperatures above 1500 °C, rendering the silica-based substrate with the plasma-sprayed alumina coating unfit for the application requirements of the casting process. The alumina coating prepared by the sol-gel dipping method improved the roughness of the substrate from Ra 2.39 μm to Ra 1.83 μm, and no peeling was observed when heated to 1550 °C for 30 min due to the pinning characteristics of the coating on the substrate. Furthermore, the interfacial reaction between the DZ125 superalloy melt and the silica-based substrate coated with alumina by sol-gel dipping method were investigated. The alumina coating effectively inhibited the interfacial reaction and no reaction products were detected during the directional solidification with pouring temperature of 1550 °C and withdraw rate of 5 mm/min. While a uniform, 4–5 μm thick HfO₂ reaction layer formed between the uncoated substrate and the DZ125 alloy melt. Two dipping-drying cycles were required to ensure the alumina sol completely covered the surface of the substrate.     

SiC陶瓷基体及抗氧化涂层

介绍了5种主要SiC基体的成型方法,分别是化学气相渗透(CVI)、聚合物先驱体浸渍-裂解法(PIP)、液相硅渗透法(LSI)、反应烧结法、化学气相反应法(CVR)。阐述了各种基体的组织结构、致密效率及陶瓷基复合材料的性能,其中CVI+PIP/LSI的复合成型技术可达到优化的制备过程,提高基体的组织结构和致密化效率;C/C及C/SiC复合材料表面化学气相转换法SiC涂层及多层涂层技术是提高CMC抗氧化性能的有效途径,并已得到工程实际验证。     

Titanium oxide coatings formed by plasma spraying followed by induction heat treatment

Porous titanium-containing coatings were formed on titanium samples by atmospheric plasma spraying. The resulting samples were subjected to induction heat treatment in an oxygen-containing (air) atmosphere at normal pressure and treatment temperature of 650–1250 °C. As a result of experimental studies, it was established that thermal modification allowed an increase in the oxygen content in the coatings from 49.6 ± 9.2 to 71.7 ± 1.1 at.%. The change in the elemental chemical composition was accompanied by the formation of titanium oxide coatings, the surface of which was distinguished by the presence of acicular crystals of titanium oxide (TiO), anatase, and rutile (TiO₂) with an average length of 150–200 nm and a width of about 50–100 nm. Induction heat treatment also led to an increase in the microhardness of titanium oxide coatings from 1530 ± 55 to 1825 ± 191 HV_0.98 and an increase in adhesive strength.     

Development of YBCO coatings by atmospheric plasma spraying

Superconducting Y–Ba–Cu–O thick films were produced by the atmospheric plasma spraying method. The effect of processing parameters (powder characteristics, spraying parameters) on the coatings properties was studied. X-ray diffraction analysis, SEM studies combined with EDS microanalysis and scratch test experiments were carried out in order to characterize the adhesion of the coatings to the substrate, the coatings morphology the thickness and crystalline structure as well as the powder phase transformations during spraying. For restoring the superconducting phase after deposition, the coatings were heated in oxygen in the temperature range 750–930°C. It was shown that the quality of the coatings and the adhesion to the substrate are greatly dependent on the deposition conditions. By calcining in oxygen under the appropriate conditions coatings consisting of the pure superconducting phase can be obtained.     

粘结剂对超音速火焰喷涂碳化钨涂层磨粒磨损性能的影响

采用超音速火焰喷涂(HVOF)工艺在35钢基体上制备了WC-10Ni涂层和WC-12Co涂层,研究了镍、钴这两种粘结剂对WC涂层的显微硬度、摩擦系数和抗磨粒磨损性能的影响,采用扫描电子显微镜观察涂层磨损前后的表面形貌,探讨了WC涂层的磨粒磨损机理。结果表明,以HVOF方法制备的2种WC涂层均有较高的显微硬度,WC-10Ni涂层和WC-12Co涂层与SiC砂纸摩擦副之间的干摩擦系数相差不大。2种涂层在低载荷下均有较好的抗磨粒磨损性能,但在较高载荷下WC-12Co涂层的抗磨性明显优于WC-10Ni涂层。2种涂层的磨粒磨损形式主要为均匀磨耗磨损,磨损机理以微切削和微剥落为主。WC-12Co涂层的磨损表面损伤较轻微,综合性能优于WC-10Ni涂层。     

Synthesis and characterization of MAX phase Cr2AlC based composite coatings by plasma spraying and post annealing

In this work, a simple two-step method was developed to produce thick Cr₂AlC based coatings. Firstly, atmosphere plasma spraying was employed to deposit Cr-A-C coatings with Cr/Al/graphite mixtures. Then Ar-annealing treatment was conducted on as-sprayed coatings to in situ achieve Cr₂AlC. Microstructure evolution and mechanical performance of composite coatings was investigated. The as-sprayed coating exhibited a lamellar feature with mainly Cr₇C₃ and residual Al. With increasing temperature, the residual Al decreased and the newly formed Cr₂AlC phase increased. Especially, high temperature annealing (>700 °C) led to remarkable increasing amount of Cr₂AlC phase due to the enhanced atom diffusion. The annealing treatment enhanced both of hardness and fracture toughness of coatings due to the formation of Cr₂AlC. However, the increasing amount of Cr₂AlC phase resulted in slight decrease of hardness. Thus, the content of Cr₂AlC phase played a significant role in mechanical performance of composite coatings, which was adjusted by post-annealing.     

Microstructure and mechanical property of SiC whisker coating modified C/C composite/Ti2AlNb alloy dissimilar joints prepared by transient liquid phase diffusion bonding

SiC whisker coating was prepared on the surface of C/C composite successfully by CVD, and transient liquid phase (TLP) diffusion bonding was employed to realize the joining of SiC whisker coating modified C/C composite and Ti₂AlNb alloy using Ti–Ni–Nb foils as interlayer. The microstructure, shear strength and fracture behavior were investigated by scanning electron microscopy (SEM) with energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD) and universal testing machine. The results show that SiC has good compatibility with C/C composite, and gradient interface formed between SiC-modified C/C composite and Ti₂AlNb alloy. When the bonding experiment was carried out under bonding temperature of 1040 °C and holding time of 30min with 5 MPa pressure in vacuum, the joints formed well and no obvious defects can be observed. The typical microstructure of joints is C/C composite/SiC + TiC/Ti–Ni compounds + Ti–Ni–Nb solid solutions/residual Nb/diffusion reaction layer/Ti₂AlNb alloy. With the increasing of bonding temperature, the thickness of joining area increased due to sufficient element diffusion. However, when bonding temperature is elevated to 1060 °C, some defects such as cracks and slag inclusions exist in the interface layer between interlayer and Ti₂AlNb. The joints with maximum average shear strength of 32.06 MPa are bonded at 1040 °C for 30min. C, SiC and TiC can be found on the fracture surface of joints bonded at 1040 °C which indicated that fracture occurred at the interface layer adjacent SiC layer.     

Pressureless sintering of single-phase tantalum carbide and niobium carbide

This work presents the results of studies on the preparation of single-phase polycrystalline tantalum carbide and niobium carbide. It has been found that it is possible to obtain polycrystals with high density in the pressureless sintering process at temperatures up to 2000 °C and therefore relatively low temperatures such as for the compounds with one of the highest melting points; TaC – 3985 °C and NbC – 3600 °C. Only carbon as a sintering additive was used. The main role of carbon is to reduce of oxide contamination. It has been shown that the determination of the amount of carbon required to reduce oxide contamination is only possible through the experimental method.     

Processing and properties of plasma sprayed silicon nitride-glass composite coatings

Abstract

Abstract

Silicon nitride decomposes before it can melt, and so thermal spraying of pure silicon nitride powder is impracticable. To address this difficulty, feedstock powder for plasma spray deposition has been developed in which each particle is a composite of silicon nitride in a low temperature borosilicate glass matrix. The research showed that the silicon nitride did not decompose in the plasma because the low thermal conductivity of the glass matrix ensured a low heat transfer rate and the particle temperature remaining below the decomposition temperature. The coating density initially increased with plasma arc power because of increasing splat flow but then declined at high power levels owing to decomposition of the glass matrix. The silicon nitride dispersion substantially reduced the splat flow, particularly near the maximum packing fraction, but also had the beneficial effect of restricting crack propagation, resulting in an optimum content for wear resistance of 30?vol.-% silicon nitride.     

Structure and mechanical properties of hydroxyapatite coatings produced on titanium using plasma spraying with induction preheating

Coatings of hydroxyapatite (HAp) were prepared by plasma spraying with induction preheating of titanium substrate from 200 to 1000 °C. The combination of conventional plasma spraying and induction preheating ensured high mechanical properties of HAp coatings. The coatings produced in the temperature range 400–600 °C were characterized by homogeneous nanostructure of splats with an average grain size of 12–31 nm. According to the results of nanoindentation HAp coatings with high hardness 0.9–1.2 GPa and elastic modulus 7–16 GPa were formed on the titanium.     

Composition,structure and mechanical properties of metal oxide coatings produced on titanium using plasma spraying and modified by micro-arc oxidation

Metal oxide coatings on VT6 titanium alloy were formed by plasma spraying of aluminum oxide powder and subsequent microarc oxidation at a current density from 1 to 3?kA/m². As a result of combined treatment, metal oxide coatings consisting of a mixture of aluminum and titanium oxides were obtained on the surface of titanium samples. The most pronounced changes in the morphology of the plasma-sprayed coating were observed at the highest current density of 3?kA/m². The open porosity decreased from 56% to 38% due to the modification by micro-arc discharges, whereas the microhardness increased from 1013?±?150 HV to 1639?±?31 HV.     

Mechanical and tribological properties of nano/micro composite alumina coatings fabricated by atmospheric plasma spraying

Adding nano particles can significantly improve the mechanical properties and wear resistance of thermal sprayed Al₂O₃ coating. However, it still remains a challenge to uniformly incorporate nano particles into traditional coatings due to their bad dispersibility. In the present work, nanometer Al₂O₃ (n-Al₂O₃) powders modified by KH-560 silane coupling agent were introduced into micrometer Al₂O₃ (m-Al₂O₃) powders by ultrasonic dispersion to afford nano/micro composite feedstock, and then four resultant coatings (weight fraction of n-Al₂O₃: 0%, 3%, 5% and 10%) were fabricated by atmospheric plasma spraying. The features and constitutes of feedstock and as-sprayed coatings, as well as their porosity, bonding strength, microhardness and frictional behaviors were investigated in detail. Results show that the nano/micro composite feedstock with uniform microstructure can be better melted in the spraying process, thereby obtaining coatings with denser microstructure, higher hardness and bonding strength. Added n-Al₂O₃ has no obvious effect on the friction coefficient of composite coatings, whereas can improve their wear-resistant and reduce the worn degree of counterpart. The wear mechanism of traditional coating is brittle fracture and lamellar peeling, while that of composite coating with weight fraction of n-Al₂O₃ of 10% is adhesive wear.     

Effect of Ti particle size on mechanical and tribological properties of TiCN coatings prepared by reactive plasma spraying

Titanium carbonitride (TiCN) coatings were successfully fabricated by reactive plasma spraying (RPS) from agglomerated Ti-graphite feedstock. The effect of Ti particle size on the microstructure and phase composition of plasma sprayed TiCN coatings was investigated. The Vickers microhardness of coatings was measured by a Microhardness Test and the corresponding Weibull distribution were also analyzed. In addition, a pin-on-disk tribometer was employed to determine the trobological properties of coatings. Results show that all the coatings consist of TiC_xN_1−x (0 ≤ x ≤1) and minor Ti₂O phases, and the amount of Ti₂O increases with the increase of Ti particle size. The Weibull distribution of Vickers microhardness of all the coatings shows apparent scattering, while the coating sprayed with Ti particle size of 28 µm exhibits a relatively even distribution. Compared with the coating sprayed with Ti particle size of 14 µm or 48 µm, the coating sprayed with Ti particle size of 28 µm exhibits improved mechanical and tribological properties, which are attributed to the high microhardness and strong bonding strength.     

Influence of Ca/Al ratio on physical and dielectric properties of Al2O3/CaO-B2O3-SiO2 glass-ceramics composite coatings prepared by high enthalpy atmospheric plasma spraying

The dielectric layer plays a key role in regulating electromagnetic wave broadband scattering based on meta-surface technology. Herein, the physical properties of composite powder, prepared by spray drying of CaO-B₂O₃-SiO₂ (CBS) glass-ceramic powder and Al₂O₃ in different mass ratios, are systematically investigated. Meanwhile, a high enthalpy atmospheric plasma spraying equipment is utilized to prepare CBS/Al₂O₃ composite coatings, and the morphology, physical properties and dielectric properties of the composite coating are analyzed. The XRD and DSC data of the composite coating reveal that the crystallization behavior of β-CaSiO₃ and CaB₂O₄ gradually disappear with the increase of Al₂O₃ content. Hence, only CaAl₂Si₂O₈ phase is observed during heat treatment. The experimental results confirm that the dielectric properties of CBS/Al₂O₃ composite coating conform to the rule of mixture for composite materials. Also, the dielectric properties are affected by porosity and crystallization rate.     

Microstructure and mechanical properties of plasma sprayed TiC/Ti5Si3/Ti3SiC2 composite coatings with Al additions

The mass production of MAX phase coatings such as Ti₃SiC₂ and Ti₃AlC₂ using the plasma spraying method is highly challenging due to its ultra-high temperature and short reaction time. In this study, agglomerate powders of 3Ti/SiC/C/xAl with various Al contents (x = 0–1.5) were prepared to form TiC/Ti₅Si₃/Ti₃SiC₂ composite coatings using the plasma spraying technique. The effect of the Al addition on the microstructures and mechanical performances of the as-sprayed coatings was investigated. The addition of Al decreased the TiC content of the coatings while increasing their Ti₃SiC₂ content significantly. The addition of even small amounts of Al improved the MAX phase fraction of the coatings from 8.95 wt% (x = 0) to 34.05 wt% (x = 0.2) and 41.60 wt% (x = 0.5). Excess Al did not affect the Ti₃SiC₂ content of the coatings. The composite coatings showed a lamellar structure with pores and microcracks. With the addition of Al, the microhardness of the coatings increased slightly, while the fracture toughness improved significantly. The composite coatings with Al showed better wear resistance than those without Al. The wear mechanism of the coatings was a combination of adhesive wear, abrasive wear, and oxidative wear.