Effect of spraying parameter and injector angle on the properties of in-flight particles and alumina coatings on Al alloy with PA-HT

Bonding strength is one of the most important properties of plasma sprayed coatings, especially ceramic coatings on complexly shaped light alloys, which is mainly determined by the properties of in-flight particles and their flattening behaviors on substrate surface. Consequently, the influence of current, voltage, primary gas flow rate and injector angle on temperature and velocity of in-flight alumina particles and morphology of splats on Al alloy with plasma arc-heat treatment (PA-HT) were investigated in detail by DPV Evolution, optical microscope and 3D non-contact surface mapping profile. The bonding strength, hardness and wear behavior of corresponding alumina coatings were measured by universal testing machine, nano-indentation test and wear test. Results showed that particle temperature and velocity increased with increase of input current and voltage, while their trends were reverse with increasing primary gas flow rate. The droplets could melt Al alloy surface and penetrate into its interior that greatly affected the morphology and corresponding shape factor (SF) of splats. Moreover, reducing injector angle caused droplets to glide on substrate surface and then to form “sole-like” splats. The morphology change of these splats obviously influenced the interfacial bonding strength, compactness, hardness and wear resistance of alumina coatings.     

激光重熔NiCrAlY涂层研究

采用空气等离子喷涂技术(APS)将NiCrAlY粉末作为粘结层材料喷涂在IN718镍基合金上,再用5 kW CO2连续激光器对其进行激光重熔处理,利用扫描电镜(SEM)、能谱分析(EDX)和X射线衍射(XRD)等手段对等离子喷涂层和激光重熔层的微观组织与成分进行了比较分析.结果表明:应用优化激光重熔工艺进行重熔后,涂层...     

Correlation of morphology and barrier properties of thin microwave plasma polymer films on metal substrate

The barrier properties of thin model organosilicon plasma polymers layers on iron are characterised by means of electrochemical impedance spectroscopy (EIS). Tailored thin plasma polymers of controlled morphology and chemical composition were deposited from a microwave discharge. By the analysis of the obtained impedance diagrams, the evolution of the water uptake ?, coating resistance and polymer capacitance with immersion time were monitored and the diffusion coefficients of the water through the films were calculated. The impedance data correlated well with the chemical structure and morphology of the plasma polymer films with a thickness of less than 100 nm. The composition of the films were determined by means of infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The morphology of the plasma polymer surface and the interface between the plasma polymer and the metal were characterised using atomic force microscopy (AFM). It could be shown that, at higher pressure, the film roughness increases which is probably due to the adsorption of plasma polymer nanoparticles formed in the plasma bulk and the faster film growth. This leads to voids with a size of a few tens of nanometers at the polymer/metal interface. The film roughness increases from the interface to the outer surface of the film. By lowering the pressure and thereby slowing the deposition rate, the plasma polymers perfectly imitate the substrate topography and lead to an excellent blocking of the metal surface. Moreover, the ratio of siloxane bonds to methyl-silyl groups increases which implies that the crosslink density is higher at lower deposition rate. The EIS data consistently showed higher coating resistance as well as lower interfacial capacitance values and a better stability over time for the film deposited at slower pressure. The diffusion coefficient of water in thin and ultra-thin plasma polymer films could be quantified for the smooth films. The measurements show that the quantitative evaluation of the electrochemical impedance data requires a detailed understanding of the film morphology and chemical composition. In addition, the measured diffusion coefficient of about 1.5×10⁻¹⁴ cm² s⁻¹ shows that plasma polymers can act as corrosion resistant barrier layers at polymer/metal interfaces.     

Novel, water-based fluorinated polymers with excellent antigraffiti properties

In this article we describe novel, water-based, crosslinkable fluorinated polymers that form coatings with excellent antigraffiti properties. The synthesis of the binders and the surface and bulk properties of their coatings are discussed. The surface properties of these coatings are characterized in terms of their surface-free energy, as calculated from static contact angle measurements. The distribution of the fluorine atoms throughout the coating is measured by X-ray photoelectron spectroscopy (XPS). The bulk properties are studied by determining the crosslink density through dynamic mechanical thermal analysis (DMTA), and the effect of the crosslinking conditions on the crosslink density and the antigraffiti properties is discussed. The results indicate that a combined action of surface and bulk properties gives these coatings their excellent antigraffiti properties. The applicability of these polymers as protective coatings for metal and concrete surfaces are demonstrated. Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology, October 27–29, 2004, in Chicago, IL     

Production of Re doped La2Zr2O7 based TBCs and numerical analysis of their use on IC engine piston surface

In this study, first of all, a metallic bond layer was coated on the metal substrate using the HVOF method. Then, Gd and Yb doped La₂Zr₂O7 powders, which were specially produced to obtain a low thermal conductivity value, were coated on the metallic bond layer by atmospheric plasma spraying method. The coatings were produced in single-layer and double-layer designs using YSZ as the buffer layer. In the microstructure analysis, it was observed that the coatings exhibited the characteristic microstructure properties of the materials produced by atmospheric plasma spraying method. In the phase analysis, it was found that the Gd and Yb doped La₂Zr₂O₇ was in the form of defect fluorite type structure after plasma spraying. The thermal conductivity of the YSZ coating ranged from 0.88 to 1.00 W/mK, while the thermal conductivity of the doped La₂Zr₂O₇ coatings was measured between 0.38 and 0.68 W/mK. Especially, the lowest thermal conductivity values were obtained in the double-layer Gd doped coating. As a result of modeling these coatings on the piston surface of a diesel engine using the finite element method, it was found that the maximum and minimum surface temperatures of the pistons increased by 69% and 60%, respectively. There was also a reduction of up to 6.5% in the temperature of the piston substrate surface.     

Quantitative model for the viscous flow and composition of two-phase silicon nitride-based particles in plasma-spray deposition

Silicon nitride does not melt but decomposes at 1900 °C and so thermal spraying of pure silicon nitride powder is impracticable. However, the use of silicon nitride and other non-oxide ceramics as thick, thermally sprayed coatings has considerable engineering potential owing to their unique combination of properties. This research shows that embedding fine silicon nitride particles within an oxide matrix to form composite feedstock particles enables the formation of silicon nitride composite coatings with little decomposition of the silicon nitride. Successful deposition of the coatings depends critically on the flow of the feedstock particles on impact with the substrate. This paper concerns the design of oxide matrix systems for the deposition of silicon nitride composite coatings by thermal spraying. A quantitative model is developed for the viscous flow of two-phase feedstock particles at impact. A number of matrix systems are investigated, including a series of yttria–alumina and yttria–alumina–silica compositions. The research shows that certain oxide matrices can provide the required viscous flow and protect the silicon nitride from decomposition.     

Effects of Yttria-Stabilized Zirconia on Plasma-Sprayed Hydroxyapatite/Yttria-Stabilized Zirconia Composite Coatings

The low bonding strength between hydroxyapatite (HA) and the metal substrate interface of plasma-sprayed HA coating has been a point of potential weakness in its application as a biomedical prosthesis. In the present study, yttria-stabilized (8 wt%) zirconia (YSZ) has been used to enhance the mechanical properties of HA coatings. The effects of YSZ additions (in the range 10–50 wt%) on the phase composition, microstructure, bond strength, elastic modulus, and fracture toughness of plasma-sprayed HA/YSZ composite coatings have been studied. The results indicated that decomposition of HA during plasma spraying was reduced significantly with the addition of zirconia. The higher the zirconia content, the lower the amount of calcium oxide, tricalcium phosphate, and tetracalcium phosphate formed in the coatings. In addition, there was a trace of calcium zirconate formed when less than 30 wt% zirconia was present. A solid solution of HA mixed with YSZ formed during plasma spraying; however, the amount of unmelted particles increased as the zirconia increased. The mechanical properties of the HA/YSZ composite coatings, such as bond strength, elastic modulus, and fracture toughness, increased significantly as the contents of zirconia increased.     

Effect of a moving flame on the temperature of polymer coatings and substrates

A computational model is developed to predict the temperature profile over an organic coating on a metal surface as a result of the action of a moving flame. The deflection of the flame as it impinges on the surface is simulated and its consequent heat transfer to the polymer is determined. The scanning action of the flame across the substrate is quantified and the temperature profiles within the polymer are calculated. The results show a substantial build up of temperature at the surface and large temperature gradients throughout the thickness, which are due to the low thermal conductivity of polymers. This can be particularly detrimental for polymers owing to their low softening and decomposition temperatures. The model can be applied to flame impingement on a bulk polymer or on an organic coating on a metal substrate. The research shows the risks of a moving flame overheating a polymer surface and indicates remedial measures.     

Macrostructure and environment-influenced surface layer in epoxy polymers

Several epoxy polymers were shown to be two-phase systems; roughly spherical floccules arranged in layers in an interstitial fluid resembling the starting materials. The size of the floccules was found to be dependent on the initial rate of cure of a given polymer. The density, hardness, glass transition temperature, etching rate, and dielectric strength were related to the floccule size. The surface layer in the epoxy polymers and in several thermoplastic polymers was found to be different from the bulk material. The properties of the surface layer are dependent on the surface energy of the mold material and on the atmospheric environment. A gradient in properties was found to extend from the polymer surface several hundred microns into the bulk.     

Structure and electrical properties of yttrium oxide sprayed by plasma torches from powders and suspensions

Yttrium oxide was sprayed by a plasma torch using a coarse thermal spray powder, which must be in size range of tens of micrometers to ensure good penetration into the plasma stream. Thick coatings on steel substrates were produced with two sprays systems facilitating gas stabilized plasma (GSP) and hybrid water-argon stabilized plasma (WSP–H) techniques. Additionally, an ultra-fine yttrium oxide powder was sprayed from a suspension. Hybrid water-argon stabilized plasma system was used for this purpose. Markedly thinner compact coatings were produced this way. All three sorts of plasma sprayed deposits were studied by the same methods. Dielectric properties were studied in a broad range of frequencies and temperatures. The microstructure aspects as well as crystallite size were analyzed and discussed in relation to electrical properties. All coatings exhibited stable dielectric parameters versus changing frequency and temperature, comparable with literature values for various samples. Concerning sintered bulks, and especially their thermal stability of capacitance, the plasma sprayed coatings were slightly worse. However due to shape and size variability of the plasma spraying are yttria coatings prospective for technical applications.     

金属催化剂在聚合物降解成炭阻燃中的研究进展

介绍了抗熔滴阻燃聚合物的研究现状,综述了金属催化剂在催化聚合物降解成炭方面的应用,主要包括铁系、钼系、镍系、锌系化合物及部分稀土材料等作为催化剂在聚合物阻燃中催化成炭抗熔滴的研究现状,尤其重点阐述了铁系催化剂的主要作用.最后,对金属催化剂在聚酯抗熔滴中的应用进行了展望.     

Interface interaction and structural transformation in particles of ceramic and cermet composite powders in flame spraying

The behavior of particles of Al₂O₃, TiO₂, Al₂O₃ - TiO₂, Al₂O₃ - Ni, ZrO₂ - Ni, Al₂O₃ - Ni- Ti powders in flame spraying has been studied. The interaction between the surface tension forces and the wetting forces has been found to produce either a melt shell around the particles core or drops of the second component on the surface of the core. Subsequently a total or partial capture of drops of the second component melt by the core melt or their separation during the movement within the plasma jet volume are possible. At high particle heating and cooling rates, polymorphous transformations, higher - to - lower oxide transformations and metastable phase fixations in the coating take place. These effects influence the conditions of ceramic powder plasma coating formation and also the properties of the coatings.     

Visualisation of the local electrochemical activity of thermal sprayed anti-corrosion coatings using scanning electrochemical microscopy

Scanning electrochemical microscopy was used to study the electrochemical activity of anti-corrosion coatings formed from Inconel 625, a Ni-Cr-Mo alloy commonly used in engineering applications. The coatings were formed using a high velocity oxygen fuel thermal spraying technique. Upon spraying the alloy onto mild steel substrates, clear splat boundaries were formed at the interface between adjacent droplets as they cooled on the substrate surface. Scanning electrochemical microscopy in the feedback mode, employing ferrocenemethanol as redox mediator, was used to study the local electrochemical activity of samples of the wrought alloy, the sintered alloy and the thermal sprayed coating. The wrought and sintered materials showed responses typical of that expected for a purely insulating material. However, SECM approach curve data showed that the electrochemical activity of the thermal sprayed material was higher than that of the bulk alloy. Local variations in the coating's electrochemical activity were then visualised using SECM imaging, which appear to be related to the splat boundaries formed during the thermal spray process.     

聚烯烃接枝的研究进展

介绍了以过氧化物为中间产物的聚烯烃本体接枝、链转移反应、多单体熔融接枝的进展情况,重点阐述了表面接枝中的化学方法、利用辐照和光照技术对聚烯烃的表面改性、等离子体工艺及相关的聚丙烯表面改性剂。聚烯烃接枝是制备功能性聚烯烃的主要方法,也是高分子与工程材料领域最活跃的研究热点之一。     

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.     

Adhesion at copper- polyamide 11- copper and aluminum-poly amide 11-aluminum laminate interfaces: influence of metal surface oxidation

Adhesion at copper-polyamide 11-copper and at aluminum-polyamide 11-aluminum laminate interfaces was studied. Metal-polymer-metal laminates were prepared by compression molding using processing conditions similar to the normal melt processing of polyamide 11. The results show that the time of contact at the molding temperature required to reach a constant level of adhesion is significant. Mild oxidation of the metal prior to molding improves the adhesion of polyamide 11 to aluminum; with copper, a monotonic slow decrease in adhesion with the oxidation time is observed. The presence of a metal surface affects the crystallization behavior of polyamide. With a cooling rate of 40-50°C/min, an approximately 15 μm transcrystalline polymer layer is formed with a degree of crystallinity that is almost 10% higher than the material away from the interface. The metal substrate surface oxidation prior to molding does not change the crystallinity profile of the polymer in the bulk. The polymer surface crystallinity is also a function of the time of contact with the metal substrate. The cooling rate and the metal substrate structure and its nucleating activity are responsible for the surface/bulk crystallinity ratio. Although the highly-crystalline polymer surface layer improves the adhesion to some extent, the formation of active species on the polymer surface which are able to react with the metal surface is mostly responsible for the increase of adhesion with time and its ultimate strength.     

Frictional behavior of solid polymers on a metal surface at processing conditions

The frictional coefficients of three glassy polymers (polystyrene, polycarbonate, and polymethylmethacrylate) and three crystalline polymers (high density polyethylene, low density polyethylene and polypropylene) on a highly polished steel surface were measured at high temperatures, high pressures, and high speeds, all comparable to actual processing conditions. The frictional behavior of these polymers was found to depend on temperature, pressure-and speed in a very complicated manner. There appears to exist inter-relationships among the temperature, pressure and speed dependences of the frictional coefficients. The frictional coefficients of ductile, crystalline polymers as a function of temperature appear to undergo two distinct transitions: one associated with yielding and the other associated with melting. The frictional coefficients of glassy polymers go through only one transition, associated with the glass transition. The friction-generated heat at high pressures and high speeds can increase the sliding interface temperature of a polymer to values much greater than the metal surface temperature, and thus the polymer can start to melt (or plasticate) at metal surface temperatures appreciably below its thermodynamic melting (or glass transition) temperature.     

Polymer structure and properties of heterophase and self-stratifying coatings

Phase separations in bulk and during the film forming process on a substrate, accompanied with the evaporation of solvents and chemical reactions of curing, have been studied in incompatible polymer blends (combinations of solid epoxy resin with selected thermoplastic resins, priorly dissolved in organic solvents) in order to be able to control the heterophase polymer structure of coatings. The resultant polymer/ polymer heterogeneous and sometimes non-homogeneous-in-layer (ultimately double-layer) polymer structures of coatings were evaluated by scanning electron microscopy (SEM). A few driving forces for self-stratification and the formation of double-layer coating structures, where each layer is enriched with a certain polymer component, are discussed as possibly associated with selective wetting of a substrate, phase contraction, interfacial tension gradients, etc. Examples are given to illustrate non-additive changes of structure-dependent coating properties with the composition of the epoxy/thermoplastic resin blends, which is characteristic for polymer/polymer heterophase materials. Depending on the phase state of the binder in the paint (formulated with the use of a selected incompatible polymer blend, combination of solvents and sometimes with an additive, controlling the phase separation process) and conditions of application to a substrate and film formation, various polymer/polymer heterophase or self-stratifying one-coat coatings can be obtained. Some of them, particularly those that combine partial stratification with polymer/polymer heterogeneity, are capable of meeting the demands for advanced performance due to improved combinations of bulk and surface properties.     

The fracture toughness of alumina coatings plasma-sprayed at different in situ temperatures

Alumina coatings were prepared by atmospheric plasma spraying through controlling the surface temperature of the coatings during spraying. Both the polished and fractured cross-section microstructures of the coatings were characterized by scanning electron microscopy (SEM). The phase structures of the coatings and the feedstock were analyzed by X-ray diffraction technique (XRD). The microstructure and phase structure of the coatings prepared at different substrate temperatures were examined. SEM observations show that the intersplat bonding within the coatings was significantly improved by increasing the substrate temperature. The fracture toughness of the deposits was measured by indentation methods. For the coatings prepared at low substrate temperatures, the fracture toughness increased with the substrate temperature due to the improvement in the intersplat bonding. However, a significant decrease in the fracture toughness was found for the coatings prepared at high substrate temperatures. The change in phase structure of the coatings suggested that the residual tensile stress mainly resulted from phase transformation from γ-alumina to α-alumina at high substrate temperature should answer for the decline in the fracture toughness.