YPSZ等离子涂层激光重熔组织性能

采用CO2激光焊接试验系统对YPSZ(8%Y2O3-ZrO2)等离子涂层进行了激光重熔,并对涂层进行了微观组织与性能分析.结果表明,经激光重熔后YPSZ涂层气孔和微裂纹明显减少,组织均匀致密,表面粗糙度低,强度和韧度提高.涂层硬度的变化与重熔过程中涂层相成分转变有关,经激光重熔处理后YPSZ涂层中主要相成分为t-ZrO2和少量的c-ZrO2,t-ZrO2具有较高的强度和韧度是改善涂层的表面力学性能的主要因素.     

High temperature properties of thermal barrier coatings obtained by detonation spraying

NiCrAlY/YPSZ and NiCrAlY/NiAl/YPSZ thermal barrier coatings (TBCs) were successfully deposited by detonation spraying. The results indicated that the detonation sprayed TBCs included a uniform ceramic coat containing a few microcracks and a bond coat with a rough surface. The lamellar structure and the presence of cracks and impurities could reduce the thermal conductivity of the ceramic coat. Oxidation kinetics at 1000–1150 °C of detonation sprayed TBCs have been measured and discussed. The role of a Ni–Al intermediate layer in improving the oxidation resistance of duplex TBCs has also been studied.     

Thermal barrier coating experience in gas turbine engines at Pratt & Whitney

Pratt & Whitney has accumulated more than three decades of experience with thermal barrier coatings (TBCs). These coatings were originally developed to reduce surface temperatures of combustors of JT8D gas turbine engines to increase the thermal fatigue life of the components. Continual improvements in de-sign, processing, and properties of TBCs have extended their applications to other turbine components, such as vanes, vane platforms, and blades, with attendant increases in performance and component du-rability. Plasma-spray-based generation I (Gen I) combustor TBCs with 7 wt % yttria partially stabilized zirconia deposited by air plasma spray (APS) on an APS NiCoCrAlY bond coat continues to perform ex-tremely well in all product line engines. Durability of this TBC has been further improved in Gen II TBCs for vanes by incorporating low-pressure chamber plasma-sprayed NiCoCrAl Y as a bond coat. The modi-fication has improved TBC durability by a factor of 2.5 and altered the failure mode from a “black fail-ure” within the bond coat to a “white failure” within the ceramic. Further improvements have been accomplished by instituting a more strain-tolerant ceramic top layer with electron beam/physical vapor deposition (EB-PVD) processing. This Gen III TBC has demonstrated exceptional performance on rotating airfoils in high-thrust-rated engines, improving blade durability by three times through elimination of blade creep, fracture, and rumpling of metallic coatings used for oxi-dation protection of the airfoil surfaces. A TBC durability model for plasma-sprayed as well as EB-PVD systems is proposed that involves the accumulation of compressive stresses during cyclic thermal expo-sure. The model attempts to correlate failure of the various TBCs with elements of their structure and its degradation with thermocyclic exposure.     

Effect of thermal treatment on the effective thermal conductivity of YPSZ coatings

Thermally sprayed coatings present effective properties strongly different from those of the primary bulk material. In particular, the actual thermal conductivity of Yttria Partially Stabilized Zirconia (YPSZ) coatings is typically twice lower than the thermal conductivity of dense YPSZ. The architecture of the porous network plays a major role on this decrease: thin inter-lamellar cracks act as thermal resistance and contribute to decrease the effective thermal conductivity more efficiently than globular pores.From this situation, an in-house code has been developed since a few years: this code implements a finite difference method to perform calculations directly on micrographs of coating cross-sections obtained by SEM. Each pixel of the intermediate binary picture is interpreted as a cell of integration of the heat conduction equation. A thermal gradient is applied between the top and bottom edges and a system of linear equations is formed and solved, providing the thermal flux flowing through the structure and the corresponding effective thermal conductivity.In the present study, the case of YPSZ coatings before and after thermal treatment was considered. The numerical results are in rather good agreement with experimental data: the thermal treatment tends to close a part of the thinnest pores, thus providing a decrease of the pore level and an increase of the effective thermal conductivity of the produced coatings.     

Three-Dimensional Simulation of Porosity in Plasma-Sprayed Alumina Using Microtomography and Electrochemical Impedance Spectrometry for Finite Element Modeling of Properties

Moving from a 2-dimensional to a 3-dimensional (3D) approach to microstructure and properties has been expected eagerly for a long while to result in a dramatic increase in the knowledge of thermally sprayed coatings. To meet these expectations, in the present study, microtomography and electrochemical impedance spectroscopy (EIS) were carried out to simulate the microstructure of plasma-sprayed alumina. As-sprayed and excimer laser-processed deposits were studied. Some unexpected but relevant results, e.g., regarding pore orientation in the coatings, could be obtained. EIS led to the establishment of an equivalent electrical circuit representation of the microstructure which enabled modeling of the insulating properties as a function of interfaces and pore interconnection. The pore interconnection was studied by microtomography. From this 3D simulation, a finite element analysis of Young’s modulus properties was developed and compared to experiments. Using this approach, excimer laser surface processing was shown to be an innovative process to modify insulating characteristics of plasma-sprayed alumina.     

Relationships between microstructure and electrical properties of RF and DC plasma-sprayed titania coatings

Relationships between electrical properties of thermally sprayed titania coatings and their microstructures have been investigated. It is well known that the quality and properties of the plasma-sprayed coatings are strongly dependent on the in-flight particle characteristics, which directly influences the plasma-sprayed coating microstructures. As far as possible, a broad range of microstructures was produced by using various processes of plasma spraying with different powder size ranges and variation of the plasma operating parameters. The two plasma-spraying processes consisted of direct current (DC) and radio frequency (RF) plasma spraying. The major concern of the study was that plasma-sprayed parameters influence also the stoichiometry of the titania coating resulting in a large variation of their electrical properties. It is the reason why coatings with identical stoichiometry were compared. The comparison between RF and DC plasmasprayed titania coatings highlighted that electrical resistivity was directly linked to the quality of the contact between the splats and their density through the titania plasma-sprayed coatings.     

Laser glazing of plasma-sprayed zirconia coatings

A CO₂ laser with cylindrical focal lens has been used to glaze the surface layer of plasma-sprayed ZrO₂-20wt% Y₂O₃/MCrAlY coatings. Both a continuous-wave laser and a pulsed laser were used in this study. Different parameter settings for power, travel speed, and pulse frequency were used, and their effects on the melting width, melting depth, coupling efficiency, microstructure, surface roughness, and process defects have been evaluated. Results show that the melting width of the glazed track was slightly smaller than the diameter of the raw beam. The melting depth increased with increasing energy density for both a continuous-wave laser and a pulsed laser. The coupling efficiency as about 40 to 65% for a continuous-wave laser, which increased with increasing laser travel speed, but decreased with an increase in energy density. The power density has no significant effect on coupling efficiency. Defects, such as bubbles or depressions, occur easily with a continuous wave laser. A high-quality glazed layer is successfully produced using a pulsed laser. The surface roughness of the plasma-sprayed ceramic coatings was significantly improved by laser glazing. Surface roughness decreased slightly as the pulse frequency increased for the glazed surface. Based on this study, proper processing parameters have been suggested.     

Thermophysical,mechanical and microstructural characterization of aged free-standing plasma-sprayed zirconia coatings

The effect of porosity on the thermal diffusivity and elastic modulus has been studied on artificially aged, free-standing thermal barrier coatings (TBCs) produced by air plasma spray (APS). The activation energy of the sintering phenomenon was estimated from the variation in diffusivity with time and temperature. X-ray diffraction was used to evaluate the phase stability of 7 wt.% yttria partially stabilized zirconia (YPSZ) coatings. The thermal diffusivity and elastic modulus as measured by photothermal techniques and three-point bending, respectively, are reported as a function of the ageing time. Correlations between the thermal and mechanical parameters are investigated by suitable models based on the microstructural features revealed by electron microscopy. The reliability of porosity information provided by image analysis and used as input for the modelling is critically discussed.     

Comparison of a cold-sprayed and plasma-sprayed Fe25Cr20Mo1Si amorphous alloy coatings on 40Cr substrates

The purpose of this paper is to examine whether cold spraying is capable of manufacturing high-quality ferrous-based amorphous alloy coatings by comparing the performance of a cold-sprayed with a plasma-sprayed Fe25Cr20Mo1Si amorphous coating on a 40Cr substrate. The hardness, microstructure, wear resistance, and corrosion resistance of the two coatings were determined with potentiodynamic polarization curves, neutral salt spray tests, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction and their performance differences were examined. The results show that the cold-sprayed Fe25Cr20Mo1Si coating has an amorphous content of 97.63%, which is slightly higher than that for the raw powder (97.24%) and plasma-sprayed coating (96.55%). The coating hardness ranges from 720 HV to 1,030 HV, which is higher than plasma-sprayed coating (590–610 HV) and decreases the wear rate to about 2/3. The average porosity is 2.97 ± 0.59%, which is lower than that of the plasma-sprayed coating (4.95 ± 0.13%). The cold-sprayed Fe25Cr20Mo1Si coating can pass the 3,000 hr neutral salt spraying test, while the plasma-sprayed coating fails within 120 hr. The corrosion current in 3.5% NaCl solution reached to its stable value about 1.66 A/cm², which is about 1/4 of the plasma-sprayed coating (5.81 A/cm²). Upon analyzing the properties and the anticorrosion performance, it was found that there are no through-thickness pores in the cold-sprayed coating that impact its long-term anticorrosion performance. Cold spraying can be used to fabricate ferrous-based amorphous coatings instead of traditional thermal spraying technologies to obtain high-quality ferrous-based amorphous coatings.     

Residual stress evolution during the thermal cycling of plasma-sprayed zirconia coatings

X-ray diffraction has been used for the residual stress evaluation in two plasma-sprayed zirconia coatings: yttria-partially-stabilized zirconia and dysprosia-partially-stabilized zirconia. Following the evolution of these stresses with thermal cycling, an attempt at correlation between this evolution and the characteristics of the two coatings is undertaken.     

On the Self-Affine Fractal Geometry of Plasma-Sprayed Surfaces

Surface roughness strongly controls essential properties of thermally sprayed wear- and corrosion-resistant coatings including their mechanical adhesion to the substrate, tribological performance, efficient retention of lubricating materials, and also the presence of sufficient carrying surface able to support the wear couple along the line of contact expressed by the Abbott-Firestone curve. The determination of the surface fractal geometry may yield useful information on the topography of plasma-sprayed coatings beyond that provided by a single roughness parameter such as R _a or R _z. The fractal geometry of atmospheric plasma-sprayed chromium oxide coatings, deposited according to two different statistical experimental design protocols, was assessed through determination of the Hurst exponent H of fractal Brownian motion (fBM), as well as the area-scaled fractal complexity (ASFC) obtained by triangular tessellation (“patchwork” method). Attempts were made to correlate fractality with coating adhesion strength.     

TiAl合金表面激光重熔纳米陶瓷涂层

采用等离子喷涂和激光重熔复合工艺在TiA l合金表面制备了纳米A l2O3-13wt%TiO2复合陶瓷涂层。为了使重熔后的陶瓷涂层保留一定的纳米结构组织,采用相对较低的激光功率和能量密度进行重熔。用扫描电镜(SEM)和X射线衍射仪(XRD)分析了涂层形貌、微观结构和相组成。结果表明,等离子喷涂纳米陶瓷涂层由纳米颗粒完全熔化区和部分熔化区两部分组成,具有等离子喷涂态的典型层状结构;由于受到激光功率、能量密度、陶瓷材料热物性参数和涂层厚度等因素的综合影响,重熔后陶瓷涂层出现了明显的分层结构特征;依据组织形态的不同,可将其大致分为:重熔区、烧结区和残余等离子喷涂区。重熔区由致密细小的等轴晶组成,并且保留了部分来源于原等离子喷涂部分熔化区的残留纳米粒子。由于等离子喷涂过程中涂层沉积时的快速凝固作用,涂层以亚稳相-γA l2O3为主,经过激光重熔处理后,-γA l2O3又重新转变为稳定相-αA l2O3。     

Wear and erosion behavior of plasma-sprayed WC-Co coatings

Wear mechanisms of air plasma-sprayed WC-12%Co coatings were studied by using a dry sand rubber wheel (DSRW) abrasive, ring-on-square adhesive wear, and alumina particle erosion tests. Coating properties such as intersplat cohesive strength, porosity, surface roughness, hardness, and retained carbide as well as microstructures were characterized to assess their relationship on wear performance. Porosity, hardness, surface roughness, and retained carbide of the coatings are not the principal factors affecting wear performance. Intersplat cohesive strength of coatings, measured by a simple bonding test, is the most significant factor that relates to the wear rate of thermal spray coatings.     

纳米SiCw晶须/ZrO2复合热障涂层的制备与性能

运用微弧等离子喷涂制备了碳化硅晶须(SiCw)掺杂部分稳定ZrO2(YPSZ)复合热障涂层(CTBCs),对涂层进行了显微组织观察、EDS分析、XRD分析和抗热震性能试验.喷涂过程中,复合粉末里的部分SiCw在高温下分解产生的气体夹杂在熔融的颗粒内形成气孔,另一部分沉积在涂层中起到降低热应力和钉扎、桥联作用.结果表明,随着粉末中SiCw含量的增加,复合陶瓷层的孔隙率呈增大趋势;复合涂层的抗热震性能优于单纯氧化锆涂层,SiCw含量为20%的复合涂层的抗热震性能最优.     

A comparative study of titanium nitrides, TiN, TiNbN and TiCN, as coatings for biomedical applications

A strategy used to reduce wear of the ultra high molecular weight polyethylene (UHMWPE) component of orthopedic joint implants has been to coat the metallic part with a hard ceramic layer. The advantage of this procedure is to reduce both wear and ion release of the metal while keeping a high mechanical resistance. In the present study, the performance of three titanium nitride coatings: TiN, TiNbN, and TiCN for biomedical applications was assessed in terms of their surface properties and cytotoxicity. The morphology, chemical composition, and wettability were determined through atomic force microscopy (AFM) imaging, X-ray photoelectron spectroscopy (XPS) and contact angle measurement, respectively. The tribological behaviour of the coatings rubbing against UHMWPE in lubricated conditions was investigated using a pin-on-disk apparatus. Albumin adsorption on the three coatings was studied with a quartz crystal microbalance with dissipation (QCM-D) and AFM scratching. Cytotoxicity was determined both in direct or indirect contact of the cells with the coating materials. The results demonstrate that the three coatings have similar surface properties and are not cytotoxic. TiNbN seems to have the best tribological performance in the presence of albumin, although albumin adsorption is slightly higher on TiN.     

Analysis of diamond-like carbon and Ti/MoS2 coatings on Ti–6Al–4V substrates for applicability to turbine engine applications

Ti–6Al–4V substrates have been coated by diamond-like carbon (DLC) films, with no surface pretreatment, and have been coated by Ti/MoS₂ films, with a simple surface pre-cleaning. The DLC films were deposited by planar coil r.f. inductively-coupled plasma-enhanced chemical vapor deposition (r.f. ICPECVD); the Ti/MoS₂ films were deposited by magnetron sputtering. Both the DLC and Ti/MoS₂ films were characterized by pull tests, hardness tests, scanning electron microscopy (SEM), and wear tests (pin-on-disk and block-on-ring) to compare their adhesion, hardness, surface topology, and wear properties to plasma-sprayed Cu–Ni–In coating currently used for turbine engine applications. The DLC films were easily characterized by their optical properties because they were highly transparent. We used variable-angle spectroscopic ellipsometry (VASE) to characterize thickness and to unequivocally extract real and complex index of refraction, providing a rapid assessment of film quality. Thicker coatings yielded the largest hardness values. The DLC coatings did not require abrasive pretreatment or the formation of bond-layers to ensure good adhesion to the substrate. Simple surface pre-cleaning was also adequate to form well-adhered Ti/MoS₂ on Ti–6Al–4V. The results show that the DLC and Ti/MoS₂ coatings are both much better fretting- and wear-resistant coatings than plasma-sprayed Cu–Ni–In. Both show excellent adhesion to the substrates, less surface roughness, harder surfaces, and more wear resistance than the Cu–Ni–In films.     

Friction and wear mechanisms of thermally sprayed ceramic and cermet coatings

The tribological behavior of Al₂O₃/TiO₂ and WC/Co plasma-sprayed coatings was studied under dry sliding conditions. X-ray photoelectron spectroscopic analyses have shown the presence of graphite in WC/17 wt% Co coatings and a thin WO₃ layer in the contact. These phases could explain the improvement in the friction behavior of the cermet/ceramic couples versus the ceramic/ceramic couples.     

Lightweight TiC/Ti wear-resistant coatings for lightweight structural applications

Lightweight coatings based on titanium and titanium carbides produced by plasma spraying can be used to improve and modify the tribomechanical properties of aerospace structural materials. Although plasma-sprayed WC/Co coatings have been applied with success in many cases, such as primary wear-re-sistant materials, their high densities preclude their use in applications that mandate reduction in weight. In the present investigation, the sliding wear resistance of plasma-sprayed, metal-bonded TiC coatings on AI 7075 substrates was studied. Coatings containing 50, 70, and 90 vol% TiC in a Ti matrix produced from physically blended powders of Ti and TiC were compared. Metallographie evaluations showed that dense coatings with good bonding to AI 7075 substrates can be obtained. Coatings from commercial pu-rity (CP) Ti powders sprayed in air under atmospheric conditions, however, indicated considerable oxi-dation of the particles. Under dry sliding conditions, the coefficient of friction (COF) values of the Ti/TiC containing/Al 7075 substrate system were lower than high-velocity oxygen fuel (HVOF) sprayed 75% Cr₃C₂/25%NiCr coatings on steel and were comparable to coatings of WC/Co. Vacuum plasma-sprayed TiC/Ti coatings with 90 vol% TiC also exhibited better wear resistance than HVOF sprayed 75%Cr₃C₂/25%NiCr.     

Preparation of Aluminum Coatings by Atmospheric Plasma Spraying and Dry-Ice Blasting and Their Corrosion Behavior

Aluminum coating, as an example of spray coating material with low hardness, was deposited by atmospheric plasma spraying while dry-ice blasting was applied during the deposition process. The deposited coatings were characterized in terms of microstructure, porosity, phase composition, and the valence states. The results show that the APS aluminum coatings with dry-ice blasting present a porosity of 0.35 ± 0.02%, which is comparable to the bulk material formed by the mechanical compaction. In addition, no evident oxide has been detected, except for the very thin and impervious oxide layer at the outermost layer. Compared to plasma-sprayed Al coatings without dry-ice blasting, the adhesion increased by 52% for Al substrate using dry-ice blasting, while 25% for steel substrate. Corrosion behavior of coated samples was evaluated in 3.5 wt.% NaCl aqueous using electrochemistry measurements. The electrochemical results indicated that APS Al coating with dry-ice blasting was more resistant to pitting corrosion than the conventional plasma-sprayed Al coating.     

镁合金表面微弧氧化-溶胶凝胶复合膜层的结构及其耐蚀性

采用微弧氧化和溶胶凝胶技术在镁合金表面制备复合涂层,该涂层过渡层为微弧氧化膜层,外层为SiO2溶胶凝胶层。通过傅里叶红外光谱(FTIR)、X射线衍射(XRD)、扫描电镜(SEM)等分析技术对复合涂层成分、组织结构以及微观形貌进行了表征,并采用电化学测试方法综合分析了该复合涂层的耐蚀性能。研究表明,该复合涂层与基体结合较好,具有良好的高温稳定性,在质量分数为3.5%的NaCl溶液中腐蚀电位明显正移,腐蚀电流密度显著降低,表现出良好的耐蚀性。