等离子喷涂纳米结构AT13基涂层硬度的双态分布

利用等离子喷涂技术制备纳米结构AT13基陶瓷涂层,通过SEM观察涂层组织结构并利用HXD-1000显微硬度计测量涂层的Vickers硬度,所得结果与对应成分的常规AT13涂层进行对比,结果表明常规涂层只含有单相层片结构,而纳米结构涂层含有双态分布(完全熔化层片结构和部分熔化颗粒结构),常规涂层的硬度平均值要低于纳米结构涂层,纳米结构涂层中存在依赖于微观结构双态分布的硬度Weibull双态分布,而且完全熔化区的硬度由于组织致密明显高于部分熔化区。采用三因素三水平对等离子喷涂纳米结构涂层工艺进行设计,得到影响涂层硬度的最主要的因素是电压,其次是电流。     

Comparison of microstructure and mechanical properties of plasma-sprayed nanostructured and conventional yttria stabilized zirconia thermal barrier coatings

The main goal of this paper was to evaluate and compare the microstructure and mechanical properties of plasma-sprayed nanostructured and conventional yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). To this end, NiCrAlY bond coat, nanostructured, and conventional YSZ coatings were deposited on Inconel 738LC substrate by atmospheric plasma spraying (APS). The mechanical properties of the coating were evaluated using nanoindentation and bonding strength tests. The microstructure and phase composition of the coating were characterized by field emission scanning electron microscopy (FESEM) and X-ray diffractometry (XRD). The nanostructured YSZ coating contained both nanosized particles retained from the powder and microcolumnar grains formed through the resolidification of the molten part of the powder, whereas the microstructure of the conventional YSZ coating consisted of columnar grain splats only. The phase composition of the as-sprayed nanostructured coating consisted of the non-transformable tetragonal phase, while the conventional coating showed the presence of both the monoclinic and non-transformable tetragonal phases. The results of nanoindentation and bonding strength tests indicated that the mechanical properties of the nanostructured coating were better than those of the conventional coating.     

Characterization of multi-scale synergistic toughened nanostructured YSZ thermal barrier coatings: From feedstocks to coatings

The nanostructured 8YSZ thermal barrier coatings were deposited by atmospheric plasma spraying onto K417 G nickel-based superalloy with high velocity oxygen fuel sprayed NiCoCrAlYCe bond-coat using as-prepared nanostructured t´-Zr_0.9Y_0.1O_1.95 feedstocks for the first time. The microstructure and mechanical properties of nanostructured and conventional 8YSZ coatings were comparatively investigated systematically. The results revealed that both coatings were composed of t´-Zr_0.9Y_0.1O_1.95 phase and the formation mechanism of t´ phase was elucidated. The nanostructured 8YSZ coatings demonstrated typical bi-modal microstructure, whereas the conventional 8YSZ coatings exhibited mono-modal microstructure. Furthermore, the bi-modal microstructure of nanostructured 8YSZ coatings was analysed by elastic modulus and nanohardness Weibull distribution plots. The high and low slopes in Weibull distribution plots corresponded to unmelted and melted regions of nanostructured 8YSZ coatings, respectively. The fracture toughness and bonding strength of nanostructured coatings were higher than that of conventional 8YSZ coatings. Finally the reasons were explained in detail.     

Microstructure analyses and thermophysical properties of nanostructured thermal barrier coatings

Nanostructured 8 wt% yttria partially stabilized zirconia coatings were deposited by air plasma spraying. Transmission electron microscopy, scanning electron microscopy, and X-ray diffraction were carried out to analyze the as-sprayed coatings and powders. Mercury intrusion porosimetry was applied to analyze the pore size distribution. Laser flash technique and differential scanning calorimetry were used to examine the thermophysical properties of the nanostructured coatings. The results demonstrate that the as-sprayed nanostructured zirconia coatings consist of the nonequilibrium tetragonal phase. The microstructure of the nanostructured coatings includes the initial nanostructure of powder and columnar grains. Moreover, micron-sized equiaxed grains were also exhibited in the nanostructured coatings. Their evolution mechanisms are discussed. The as-sprayed nanostructured zirconia coating shows a bimodal pore size distribution, and has a lower value of thermal conductivity than the conventional coating.     

Microstructure and Wear Behavior of Plasma‐Sprayed Nanostructured WC–Co Coatings

Atmospheric plasma spraying of WC–Co particles with standard gas mixtures (Ar–H₂) typically results in largely decarburized coatings with relatively low wear resistance. To fabricate cermet coatings with enhanced tribological properties, nanostructured WC–Co coatings were plasma sprayed using two different process gas mixtures. Phase composition and microstructure were investigated by X‐ray diffraction and scanning electron microscopy, respectively. Microhardness increased by increasing the amount of retained WC grains in coating microstructure. Friction and wear properties, measured under dry sliding conditions, strongly depended on the degree of decarburization. They were comparable to those of conventional coatings produced using identical conditions.     

Microstructure, mechanical properties and thermal shock resistance of plasma sprayed nanostructured zirconia coatings

Nanostructured yttria stabilized zirconia (YSZ) coatings were deposited by Atmospheric Plasma Spraying (APS). X-ray diffraction (XRD) was used to investigate their phase composition, while scanning electron microscopy (SEM) was employed to examine their microstructure. The coatings showed a unique and complex microstructure composed of well-melted splats with columnar crystal structure, partially melted areas, which resembled the morphology of the powder feedstock, and equiaxed grains. Vickers microhardness of nanostructured zirconia coatings was similar to that of the conventional ones and strongly depended on the indentation load. Otherwise, a higher thermal shock resistance was found. This effect was addressed to the retention of nanostructured areas in coating microstructure and to the corresponding high porosity.     

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.     

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.     

Synthesis and properties of ceramic-based nanocomposite layer of aluminum carbide embedded with oriented carbon nanotubes

In the present work, carbon nanotubes (CNTs) were embedded in aluminum carbide coating in desired vertical/horizontal direction in order to fabricate a nanocomposite layer with unidirectional enhanced mechanical properties. A novel method based on monopolar pulsed plasma electrolysis under magnetic field was used for this purpose. Nanostructure of the obtained nanocomposite layer was examined with high precision figure analysis of SEM, AFM and TEM nanostructures. The mechanical and tribological properties of these coatings were investigated with respect to the direction of the embedded CNTs. The coefficient of friction was lowered from 0.2 to less than 0.1 in a pin-on-disc test against steel with dramatic affected coating wear rate by a decrease to near 400% with respect to raw substrate. The lower friction is attributed to more extensive creation of amorphous carbon on the counter surface and also in the coating wear track. As a conclusion, this method is appropriate for fabrication of hard coating on the surface of low-melting-point metals and light alloys.     

Influence of original powders on the microstructure and properties of thermal barrier coatings deposited by supersonic atmospheric plasma spraying,Part I: Microstructure

In this work, two types of yttria-stabilised zirconia (YSZ) powders, a microsized powder and a reconstituted nanostructured powder, were used as the original feedstock for depositing thermal barrier coatings (TBCs) using a high-efficiency supersonic atmospheric plasma spraying (SAPS) system. The effect of the original powder on the coating microstructure was studied by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The results indicated that the microsized powder was fully melted in the plasma jet and that the as-sprayed conventional coating (named MC) was composed of regular-shaped tetragonal ZrO₂ with grain size of 200–500 nm. However, the cross-section morphology of the water-quenched powders revealed that the reconstituted nanostructured powder was partially melted during plasma spraying and that the as-sprayed nanostructured coating (named NC) exhibited a multi-modal microstructure that mainly consisted of unmelted nanoparticles (30–50 nm) and nanograins (60–110 nm), with the latter being the main microstructure of the coating. One visible polycrystalline region consisting of 10 nm grains was also found in NC. In addition, due to the full melting of the microsized powder in the plasma jet, MC exhibited a lower porosity and higher microhardness and Weibull modulus compared with those of NC. In the following paper (Part II), the thermo-mechanical properties, such as thermal shock resistance, oxidation resistance and thermal insulation performance, of the above two coating types will be further studied.     

Plasma-sprayed nanostructured YSZ thermal barrier coatings: Thermal insulation capability and adhesion strength

Adhesion strength and thermal insulation of nanostructured Yttria Stabilized Zirconia (YSZ) thermal barrier coatings (TBC) were investigated and compared with those of conventional YSZ TBCs. A Nickel based superalloy (IN-738LC) was used as the substrate with NiCrAlY bond coat, and nanostructured and conventional YSZ top coats were applied by using air plasma spray (APS). The adhesion strength of coatings was evaluated according to ASTM C633-01, and their thermal insulation capability was evaluated using a specially designed test setup at an electrical furnace. The results revealed the nanostructured YSZ coating to have a bimodal microstructure consisting of nanosized particles and microcolumnar grains. The bimodal microstructure of nanostructured coatings prevented crack propagation by splat boundaries and unmelted particles, thereby improving the bonding strength. Also, due to the presence of nano-zones in the microstructure of nano TBCs, coatings exhibited superior thermal insulation capability.     

Wear-resistant ceramic coatings deposited by liquid thermal spraying

As a surface strengthening and surface modification technology of materials, liquid thermal spray technology has been used in many fields, such as wear and friction reduction, corrosion resistance, and high-temperature oxidation resistance. This article reviews the progress of liquid thermal sprayed coating in wear resistance as well as friction reduction in recent years. The influences of microstructure, composition, phase structure and mechanical properties on the tribological properties of typical coatings (including ceramic coatings and multiphase composite coatings) are investigated. The tribological properties of the coating are determined by the coating characteristics (including microstructure, porosity, mechanical properties, etc.) and the service conditions (working temperature, lubrication state, etc.). Typical ceramic wear-resistant coatings include Al₂O₃, YSZ, HA coatings, etc. The tribological properties of the coating can be significantly improved through process optimization and heat treatment. The comparison of nanostructured and microstructured ceramic-based coating reveals that nanostructured coating reduces wear by absorbing stress. The interaction between different constituent phases improves wear resistance and reduces wear in composite coatings. Finally, various challenges faced by liquid thermal spray are pointed out, and future research focuses are proposed.     

Influence of in-flight particle state diagnostics on properties of plasma sprayed YSZ-CeO2 nanocomposite coatings

This article describes the influence of controlling in-flight hot particle characteristics on properties of plasma sprayed nanostructured yttria stabilized zirconia (YSZ) coatings. This article depicts dependence of adhesion strength of as-sprayed nanostructured YSZ coatings on particle temperature, velocity and size of the splat prior to impact on the metallic substrate. Particle temperature measurement is based on two-color pyrometry and particle velocities are measured from the length of the particle traces during known exposure times. The microstructure and adhesion strength of as-sprayed nano-YSZ coatings were studied. Field emission scanning electron microscopy results revealed that morphology of coating exhibits bimodal microstructure consisting of nano-zones reinforced in the matrix of fully melted particles. The coating adhesion strength is noticed to be greatly affected by the melting state of agglomerates. Maximum adhesion strength of 42.39 MPa has been experimentally found out by selecting optimum levels of particle temperature and velocity. The enhanced bond strength of nano-YSZ coating may be attributed to higher interfacial toughness due to cracks being interrupted by adherent nano-zones.     

Tribological Characterization of WC–Co Plasma Sprayed Coatings

Atmospheric plasma spraying of WC coatings is typically characterized by increased decarburization, with a consequent reduction of their wear resistance. Indeed, high temperature and oxidizing atmosphere promote the appearance of brittle crystalline and amorphous phases. However, by using a high helium flow rate in a process gas mixture, plasma spraying may easily be optimized by increasing the velocity of sprayed particles and by reducing the degree of WC dissolution. To this purpose, a comparative study was performed at different spray conditions. Both WC–Co powder and coating phases were characterized by X-ray difraction. Their microstructure was investigated by scanning electron microscopy. Mechanical, dry sliding friction, and wear tests were also performed. The wear resistance was highly related to both microstructural and mechanical properties. The experimental data confirmed that high-quality cermet coatings could be manufactured by using optimized Ar–He mixtures. Their enhanced hardness, toughness, and wear resistance resulted in coatings comparable to those sprayed by high velocity oxygen-fuel.     

Characterization of alumina–3 wt.% titania coating prepared by plasma spraying of nanostructured powders

Nanostructured and conventional alumina–3 wt.% titania coatings were deposited by air plasma spraying (APS). The microstructure and phase composition of the coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Mechanical properties including hardness, adhesion strength, crack extension force (G_C) and sliding wear rate were measured. Equiaxed α-Al₂O₃ grains were observed in the nanostructured Al₂O₃–3 wt.% TiO₂ coating and the diameter of α-Al₂O₃ grains were about 150 to 700 nm in size. The microhardness of both kinds of coating was similar and about 820 HV_0.2. However, the adhesion strength and crack extension force of the nanostructured coating increased by 33 and 80%, respectively, as compared with those of the conventional coating. The wear rate of the nanostructured coating was lower than that of the conventional coating. The results were explained in terms of characteristics of the powders and microstructure of the coatings.     

Studies on plasma sprayed bi-layered ceramic coating on bio-medical Ti-13Nb-13Zr alloy

Biomedical Ti alloys are prone to undergo degradation due to the combined effect of wear and corrosion. To overcome these problems, surface modification techniques are being used. In this paper, the biomedical Ti alloy Ti-13Nb-13Zr was plasma sprayed with nanostructured Al₂O₃-13 wt%TiO₂, yttria stabilized zirconia powders and bilayer containing alternate layers of the two coatings to improve the corrosion resistance and microhardness of the substrate. The plasma sprayed coatings were characterized by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The microstructure, microhardness and surface roughness of the coatings were investigated. The corrosion resistance of the coatings was studied in simulated body conditions. The results show improved corrosion resistance for the bilayered coating compared to the individual plasma sprayed coatings on biomedical Ti-13Nb-13Zr alloy substrate.     

Structure and properties of plasma sprayed BaTiO3 coatings after thermal posttreatment

Previously published results on electrical and mechanical properties of BaTiO₃ coatings prepared by atmospheric plasma spraying showed anomalies in their dielectric response. This paper provides a study of electrical and mechanical properties of BaTiO₃ coatings after thermal posttreatment. The spraying was carried out by a direct current gas-stabilized plasma gun. BaTiO₃ was fed into the plasma jet as a feedstock powder prepared by reactive sintering of micrometer-sized powders of BaCO₃ and TiO₂. In the next step the coatings were annealed in air. Microstructure and phase composition are reported and discussed in relation to electric and mechanical properties. Dielectric properties are reported for the radio frequency (RF) range.     

Mechanical properties of plasma sprayed nanostructured TiO2 coatings on mild steel

This paper discusses the effects of plasma spray parameters on the mechanical properties of nanostructured TiO₂ coatings deposited on mild steel substrates. The design of experiment method was applied to investigate the significant effects of each property and to optimize the operational spray parameters. Plasma power, powder feed rate, and stand-off distance were selected as independent variables. Agglomerated and sintered nano-TiO₂ powder was deposited on A-36 commercial mild steel. The microstructural and mechanical properties of the coatings such as porosity, microhardness, surface roughness, and wear rate were evaluated. Both plasma power and powder feed rate were found to be the main factors affecting all four responses. It was also noted that the stand-off distance was a significant factor mainly in influencing the surface roughness of the coatings. All in all, the optimized properties can be achieved by applying a plasma power of 30 KW (high level), a powder feed rate of 22 g/min (high level), and a stand-off distance of 80 mm (low level).     

Mechanical Properties of Plasma‐Sprayed Mullite‐Reinforced Titania–Bioglass Composite

Bioceramics have been extensively used for various medical applications including hip and knee prostheses, tissue engineering scaffolds, and dental implants. Bioceramics, particularly bioglass, are desired because of their bioactivity but are often limited by their inherent brittleness. To compensate, composites have been formed to obtain unique properties where both bioactivity and mechanical integrity can be achieved. Mullite‐reinforced titania–bioglass (TiO₂–BG) composites were therefore deposited using plasma spraying technique. The microstructure of the coating materials were analyzed for their morphology and microstructure using scanning electron microscopy/energy dispersive spectrometry. Mechanical properties of the coatings were tested using three‐point bend test, indentation test, and pin‐on‐disk wear test to determine their fracture strength, fracture toughness, and wear resistance, respectively. The addition of mullite fibers improved the fracture strength and wear resistance of TiO₂–BG composites while having minimal effect on fracture toughness. After the addition of mullite, failure mode was bimodal, failing intergranularly and by fiber pull‐out. Although mullite fibers have not been particularly used for medical applications, fiber reinforcement has shown efficacy in mechanically reinforcing composites of various medical applications.     

Microstructure and wear properties of plasma‐sprayed aluminum–silicon–polyester coatings

Powder coatings, which are made by plasma‐spraying processes, are being used in industrial applications because of their wear resistance, chemical resistance, and high impact strength even at low service temperatures. These factors increase the importance of plastic and plastic‐based coatings in industrial applications. In this study, an aluminum–silicon–polyester‐based composite coating was applied by plasma‐spraying processes with and without an intermediate bond coat (Ni–Al). The effects of the coating thickness, intermediate bond coat, and processes parameters on the microstructure and wear properties of the coating were studied experimentally. The wear properties of the coatings were determined according to ball‐on‐disk procedure. The microstructures of the coating were examined by optical microscopy and scanning electron microscopy. The results indicated that the plasma‐spraying current and thickness had a strong influence on the wear resistance and microstructural properties of the aluminum–silicon–polyester coating. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3609–3614, 2006