Thermally Sprayed CaZrO3 Coatings

A comparative study of CaZrO₃ coatings prepared by air plasma and flame spray processes is presented. The microstructural characteristics and phases were analyzed and discussed as a function of the spraying temperature achieved with each technique. Vickers hardness tests performed on polished cross sections of the different coatings were used to estimate their porosity. Thermal diffusivity values measured on freestanding thick coatings (500 μm) using the laser flash technique were very low and depended strongly on the pore shape. Crystallization was promoted by treating the coatings at 1200 °C, and the results are explained with aid of the ZrO₂-CaO equilibrium phase diagram.     

Anti-icing Behavior of Thermally Sprayed Polymer Coatings

Surface engineering shows an increasing potential to provide a sustainable approach to icing problems. Currently, several passive anti-ice properties adoptable to coatings are known, but further research is required to proceed for practical applications. This is due to the fact that icing reduces safety, operational tempo, productivity and reliability of logistics, industry and infrastructure. An icing wind tunnel and a centrifugal ice adhesion test equipment can be used to evaluate and develop anti-icing and icephobic coatings for a potential use in various arctic environments, e.g., in wind power generation, oil drilling, mining and logistic industries. The present study deals with evaluation of icing properties of flame-sprayed polyethylene (PE)-based polymer coatings. In the laboratory-scale icing tests, thermally sprayed polymer coatings showed low ice adhesion compared with metals such as aluminum and stainless steel. The ice adhesion strength of the flame-sprayed PE coating was found to have approximately seven times lower ice adhesion values compared with metallic aluminum, indicating a very promising anti-icing behavior.     

Colour Matching in Decorative Thermally Sprayed Glass Coatings

Coloured coatings were obtained on steel by plasma spraying without severe in-flight alteration of pigments, taking profit of the low thermal conductivity of the glassy matrix of glaze particles. Colour matching was studied by mixing 3 different glazes, comparing Grassmann and Kubelka-Munk based algorithms. Results suggest that the latter method should be preferred upon Grassmann method, particularly when the light absorption/dispersion ratios of coloured feedstocks are very different.     

Surface Pre-treatment for Thermally Sprayed ZnAl15 Coatings

Pre-treatment of substrates is an important step in thermal spraying. It is widely accepted that mechanical interlocking is the dominant adhesion mechanism for most substrate-coating combinations. To prevent premature failure, minimum coating adhesion strength, surface preparation grades, and roughness parameters are often specified. For corrosion-protection coatings for offshore wind turbines, an adhesion strength ≥ 5 MPa is commonly assumed to ensure adhesion over service lifetime. In order to fulfill this requirement, Rz > 80 µm and a preparation grade of Sa3 are common specifications. In this study, the necessity of these requirements is investigated using the widely used combination of twin-wire arc-sprayed ZnAl15 on S355J2 + N as a test case. By using different blasting media and parameters, the correlation between coating adhesion and roughness parameters is analyzed. The adhesion strength of these systems is measured using a test method allowing measurements on real parts. The results are compared to DIN EN 582:1993, the European equivalent of ASTM-C633. In another series of experiments, the influence of surface pre-treatment grades Sa2.5 and Sa3 is considered. By combining the results of these three sets of experiments, a guideline for surface pre-treatment and adhesion testing on real parts is proposed for the considered system.     

Sealing HVOF Thermally Sprayed WC-CoCr Coatings by Sol-Gel Methods

The offshore industry faces challenges with corrosion resistance in hydraulic cylinders used in marine environments. Thermal spray coating appears to be a promising technique for replacing electroplated coating in many applications, but the performance of thermal spray coatings in highly corrosive environments must be improved. In the present work, a new sealing method for HVOF WC-CoCr coatings for their application on hydraulic cylinders for marine environments has been tested. The method consists of applying sol-gel solution that can penetrate, fill, and thus seal the pores and cracks in the coating. The sealed coatings have been tested with and without posttreatment and compared with as-sprayed coatings. Open-circuit potential (OCP) and tribocorrosion tests in 3.4% NaCl were performed to evaluate the performance of the sealing method. Both tests showed that the sealed coatings had the best performance.     

Thermally Sprayed Coatings as Interlayers for DLC-Based Thin Films

This article examines the usefulness of a thick thermally sprayed interlayer (plasma-sprayed Ni-50%Cr, plasma-sprayed Al₂O₃-13%TiO₂, or high-velocity oxygen-fuel-sprayed WC-17%Co) for enhancing the wear resistance and the corrosion protectiveness of a diamond-like carbon (DLC)-based thin film deposited onto a carbon steel substrate. Scratch tests indicate that the Al₂O₃-13%TiO₂ and WC-17%Co interlayers definitely increase the critical spallation load of the thin film, but the Al₂O₃-13%TiO₂ interlayer itself undergoes brittle fracture under high-contact loads. Accordingly, during ball-on-disk tests at room temperature, no cracking and spallation occur in the DLC-based film deposited onto the WC-17%Co interlayer, whereas the one onto the Al₂O₃-13%TiO₂ interlayer is rapidly removed because the interlayer itself is fractured. At 300 °C, by contrast, the DLC-based film on the Al₂O₃-13%TiO₂ interlayer offers the best tribological performance, possibly thanks to the increased toughness of the ceramic interlayer at this temperature. Electrochemical polarization tests indicate that the thin film/WC-Co systems possess the lowest corrosion current density.     

Thermally Sprayed SiC Coatings for Offshore Wind Turbine Bearing Applications

Tribological tests were conducted on thermally sprayed silicon carbide (SiC) coatings to investigate its potential on reducing wear in offshore wind turbine bearings. The tests were carried out under dry conditions, 3.5 wt.% NaCl solution, and polyalfaolefin (PAO)-lubricated conditions. In order to obtain good quality SiC coatings, it is compulsory to modify the feedstock to limit SiC decomposition during atmospheric spraying process. The SiC feedstock used in this research has been modified with yttrium aluminum garnet (Y₃Al₅O₁₂) oxide additives that originated from its metal salt precursors. High-frequency pulse detonation (HFPD) technique has been utilized to produce coatings of around 100 μm in thickness. The sliding tests have recorded the lowest coefficient of friction (COF) of 0.15 in PAO condition and the highest COF of 0.50 in dry sliding. The wear tracks morphology show that during dry sliding test, the coatings experience abrasive wear accompanied by tribo-oxidation reaction that initiates crack formation along the splat boundaries. On the other two sliding test conditions (NaCl and PAO), polishing of asperities and some grain plowing from the splats were observed in the wear tracks. Tribochemical wear was found to be the main mechanism producing smooth surfaces. Nevertheless, in all cases, the wear losses were negligible.     

Quantitative Topographical Characterization of Thermally Sprayed Coatings by Optical Microscopy

Topography measurements and roughness calculations for different rough surfaces (Rugotest surface comparator and thermally sprayed coatings) are presented. The surfaces are measured with a novel quantitative topography measurement technique based on optical stereomicroscopy and a comparison is made with established scanning stylus and optical profilometers. The results show that for most cases the different methods yield similar results. Stereomicroscopy is therefore a valuable method for topographical investigations in both quality control and research. On the other hand, the method based on optical microscopy demands a careful optimization of the experimental settings like the magnification and the illumination to achieve satisfactory results.     

Thermophysical and Microstructural Studies on Thermally Sprayed Tungsten Carbide-Cobalt Coatings

The development of new hardmetal coating applications such as fatigue-loaded parts, structural components, and tools for metal forming is connected with improvement of their performance and reliability. For modelling purposes, the knowledge of thermophysical, mechanical, and other material data is required. However, this information is still missing today. In this study, the thermophysical data of a WC-17Co coating sprayed with a liquid-fuelled HVOF-process from a commercial agglomerated and sintered feedstock powder from room temperature up to 700 °C was determined as an example. The dependence of the heat conductivity on temperature was obtained through measurement of the coefficient of thermal expansion, the specific heat capacity, and the thermal diffusivity. Heat conductivities ranging from 29.2 W/(mK) at 50 °C to 35.4 W/(mK) at 700 °C were determined. All measurements were performed twice (as-sprayed and after the first thermal cycle) to take into account the structural and compositional changes. Extensive XRD and FESEM studies were performed to characterize the phase compositions and microstructures in the as-sprayed and heat-treated states. Bulk samples obtained by spark plasma sintering from the feedstock powder were studied for comparison.     

Modeling Thermal Conductivity of Thermally Sprayed Coatings with Intrasplat Cracks

Thermal spraying is one of the most important approaches for depositing thermally insulating ceramic top coatings for advanced gas turbines due to the low thermal conductivity of the coating resulting from its lamellar structure. The thermal conductivity of the coating has been explained based on the concept of thermal contact resistance and correlated to microstructural aspects such as splat bonding ratio, splat thickness, and the size of the bonded areas. However, the effect of intrasplat cracks on the thermal conductivity was usually neglected, despite the fact that intrasplat cracking is an intrinsic characteristic of thermally sprayed ceramic coatings. In this study, a model for the thermal conductivity of a thermally sprayed coating taking account of the effect of intrasplat cracks besides intersplat thermal contact resistance is proposed for further understanding of the thermal conduction behavior of thermally sprayed coatings. The effect of the intersplat bonding ratio on the thermal conductivity of the coating is examined by using the model. Results show that intrasplat cracks significantly decrease the thermal conductivity by cutting off some heat flux paths within individual splats. This leads to a deviation from the typical ideal thermal contact resistance model which presents cylindrical symmetry. Based on the modified model proposed in this study, the contribution of intrasplat cracks to the thermal resistivity can be estimated to be 42–57 % for a typical thermally sprayed ceramic coating. The results provide an additional approach to tailor the thermal conductivity of thermally sprayed coatings by controlling the coating microstructure.     

Nonlinear Behavior in Compression and Tension of Thermally Sprayed Ceramic Coatings

Mechanical properties of thermally sprayed coatings, especially of ceramics, are strongly influenced by a high density of mesoscopic defects, microcracks of dimensions between fractions of μm up to tens of μm. The anisotropic linear elastic stress–strain relations are valid only at very low deformations, e.g., |e| < 0.05%, with small values of Young’s moduli due to elastic openings and elastic partial closings of microcracks. At higher deformations, e.g., 0.05% < |e| < 0.4%, the stress–strain relations are strongly nonlinear. Under compressive stresses, elastic closing of microcracks leads to a gradual decrease of the microcrack density and to an increase of Young’s modulus in compression. Under tensile stresses, the microcracks slightly grow by inelastic processes; the microcrack density gradually increases and effective Young’s modulus in tension decreases. A two-parametric equation containing linear and quadratic terms is used to describe the nonlinear stress–strain curves of plasma-sprayed ceramic coatings. The effect of nonlinearity on the bending of beams with coatings and the nonlinear combination of external and residual stresses are discussed. The fracture of coatings at higher tensile stresses due to coalescence of the microcracks is mentioned.     

Finite Element Simulation of Residual Stress Development in Thermally Sprayed Coatings

The coating buildup process of Al₂O₃/TiO₂ ceramic powder deposited on stainless-steel substrate by atmospheric plasma spraying has been simulated by creating thermomechanical finite element models that utilize element death and birth techniques in ANSYS commercial software and self-developed codes. The simulation process starts with side-by-side deposition of coarse subparts of the ceramic layer until the entire coating is created. Simultaneously, the heat flow into the material, thermal deformation, and initial quenching stress are computed. The aim is to be able to predict—for the considered spray powder and substrate material—the development of residual stresses and to assess the risk of coating failure. The model allows the prediction of the heat flow, temperature profile, and residual stress development over time and position in the coating and substrate. The proposed models were successfully run and the results compared with actual residual stresses measured by the hole drilling method.     

Microstructural Analysis and Transport Properties of Thermally Sprayed Multiple-Layer Ceramic Coatings

Multilayer, graded ceramic/metal coatings were prepared by an air plasma spray method on Ti-6Al-4V, 4140 steel and graphite substrates. The coatings were designed to provide thermal barriers for diesel engine pistons to operate at higher temperatures with improved thermal efficiency and cleaner emissions. A systematic, progressive variation in the mixture of yttria-stabilized zirconia and bondcoat alloys (NiCoCrAlYHfSi) was designed to provide better thermal expansion match with the substrate and to improve thermal shock resistance and cycle life. Heat transfer through the layers was evaluated by a flash diffusivity technique based on a model of one-dimensional heat flow. The aging effect of the as-sprayed coatings was captured during diffusivity measurements, which included one heating and cooling cycle. The hysteresis of thermal diffusivity due to aging was not observed after 100-h annealing at 800 °C. The measurements of coatings on substrate and freestanding coatings allowed the influence of interface resistance to be evaluated. The microstructure of the multilayer coating was examined using scanning electron microscope and electron probe microanalysis.     

Fabrication and Properties of Thermal Sprayed AlSi-Based Coatings from Nanocomposite Powders

AlSi-based nanocomposite powders (where nanoparticles were TiO₂, ZrO₂, and Al₂O₃ and the amount of reinforcement was 2.5, 5, and 10 wt.%) were made by ball milling and then thermal sprayed using low velocity oxy-fuel technique. The AlSi-based nanocomposite powders had nanosized ceramic reinforcement adhered to the surface of the powders after ball milling. The AlSi-based coatings had the typical thermal spray microstructure where lamellae, oxide layers, unmelted particles, and pores could be seen. Submicron second phase in the form of agglomerates, molten splats, or unmelted particles between AlSi lamellae could be observed as well. Hardness and porosity of the coatings increased when more ceramic second phase particles (harder than AlSi) were added. Sliding wear tests were carried out in pin-on-disk geometry. The wear tracks of AlSi and AlSi-based coatings show plastic deformation as the main material removal mechanism during the sliding wear test. The sliding wear rate of the coatings decreased as more second phase ceramic particles were added. It was due to an increase in the hardness and a decrease in the friction coefficient of the coatings.     

Fast Regime Fluidized Bed Machining (FR-FBM) of Thermally Sprayed Coatings

Finishing of thermally sprayed metallic, ceramic, and cermet coatings is required to meet tolerances and requirements on surface roughness in most industrial applications. Conventional machining is a costly and time-consuming process, and is difficult to automate. Therefore, this study investigates and develops a new technique highly amenable for automation: fast regime—fluidized bed machining (FR-FBM). Atmospheric plasma sprayed TiO₂, Cr₂O₃, and HVOF-sprayed WC-17%Co and Tribaloy-800 coatings, deposited on AISI 1040 steel substrates, were subjected to FR-FBM treatment. The effects of the leading operational parameters, namely, abrasive size, jet pressure, and processing time, were evaluated on all coatings by using a two/three-levels full factorial design of experiments. The FR-FBM treated surfaces were observed by FE-SEM and their surface finishing was evaluated by contact profilometry. Significant improvements in surface finishing of all the machined thermally sprayed coatings can always be detected, with FR-FBM being able to guarantee the precision and to ensure the closest geometrical tolerances.     

Microstructure Control of Thermally Sprayed Co-Based Self-Fluxing Alloy Coatings by Diffusion Treatment

This article describes microstructure control aimed for wear-resistance improvement of Co-based (Co-Cr-W-B-Si) self-fluxing alloy coating by diffusion treatment. The diffusion treatments of thermally sprayed Co-based self-fluxing alloy coating on steel substrate were carried out at 1370-1450 K for 600-6000 s under an Ar gas atmosphere. Microstructural variations of the coating and the interface between the substrate and the coating were investigated in detail. A proper diffusion treatment precipitates two kinds of fine compounds in Co-based matrix. X-ray diffractometer (XRD) and electron probe microanalyzer (EPMA) analysis revealed these precipitates to be a chromium boride dissolving cobalt and a tungsten boride containing cobalt and chromium. The size of each precipitate became larger with increasing treatment temperature and time. A coating with the proper size borides showed a superior wear-resistance. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Comparison of the Microstructural Characteristics and Electrical Properties of Thermally Sprayed Al2O3 Coatings from Aqueous Suspensions and Feedstock Powders

In this work the microstructural characteristics and electrical insulating properties of thermally sprayed alumina coatings produced by suspension-HVOF (S-HVOF) and conventional HVOF spray processes are presented. The electrical resistance at different relative air humidity (RH) levels (from 6 to 97% RH) and values of dielectric strength were investigated by direct current electrical resistance measurements, electrochemical impedance spectroscopy, and dielectric breakdown tests. Relationships between electrical properties and coating characteristics are discussed. At low humidity levels (up to 40% RH) the electrical resistivities of S-HVOF and HVOF coatings were on the same order of magnitude (10¹¹???·m). At a very high humidity level (97% RH) the electrical resistivity values for the S-HVOF coatings were in the range 10⁷-10¹¹???·m, up to five orders of magnitude higher than those recorded for the HVOF coating (orders of magnitude of 10⁶???·m). The better electrical resistance stability of the suspension-sprayed Al₂O₃ coatings can be explained by their specific microstructure and retention of a higher content of ??-Al₂O₃. The dielectric strength E _d of suspension-sprayed coatings was found to be 19.5-26.8?kV·mm^?1 for coating thicknesses ranging from 60 to 200???m. These values were slightly lower than those obtained for conventional HVOF coatings (up to 32?kV·mm^?1). However, it seemed that the dielectric strength of conventionally sprayed coatings was more sensitive to the coating thickness (when compared with the values of E _d determined for S-HVOF coatings) and varied to a greater extent (up to 10?kV·mm^?1) when the coating thickness varied in the range 100-200???m.     

热喷涂NiCoCrAlYTa+7YSZ热障涂层颗粒沉积行为

分别采用低压等离子喷涂和大气等离子喷涂在K4169基体上收集了NiCoCrAlYTa颗粒沉积物及涂层,并对颗粒沉积物的形貌及涂层性能进行了观察分析。结果表明:低压等离子喷涂收集到的单个NiCoCrAlYTa扁平颗粒主要呈圆盘状,涂层致密且氧含量低。而大气等离子喷涂收集到的扁平颗粒主要呈溅射状,涂层孔隙率和氧含量均较高。又在经镜面抛光的NiCoCrAlYTa涂层和K4169基体上分别收集了7YSZ颗粒沉积物,并对其沉积形貌进行了观察分析,结果表明:在K4169基体上收集到的7YSZ颗粒沉积物主要呈圆盘状,表面存在大量的网状微裂纹及宏观环状贯通裂纹。在镜面抛光的NiCoCrAlYTa涂层表面收集的7YSZ颗粒沉积物,周围有少量的指状溅射物,中心部存在一定数量的网状微裂纹,但宏观环状裂纹消失。     

Modeling of Thickness and Profile Uniformity of Thermally Sprayed Coatings Deposited on Cylinders

In thermal spraying processes, kinematic parameters of the robot play a decisive role in the coating thickness and profile. In this regard, some achievements have been made to optimize the spray trajectory on flat surfaces. However, few reports have focused on nonholonomic or variable-curvature cylindrical surfaces. The aim of this study is to investigate the correlation between the coating profile, coating thickness, and scanning step, which is determined by the radius of curvature and scanning angle. A mathematical simulation model was developed to predict the thickness of thermally sprayed coatings. Experiments were performed on cylinders with different radiuses of curvature to evaluate the predictive ability of the model.     

纳米稀土改性热喷涂WC/12Co涂层的摩擦磨损性能研究

利用超音速火焰喷涂技术在45#钢表面制备了不同稀土含量的WC/12Co涂层.在HV-5型小负荷维氏硬度计上测定了涂层的显微硬度,在WTM-2E微型摩擦磨损试验仪上测定了涂层的摩擦磨损性能.结果表明:适量稀土的加入使WC/12Co涂层的显微硬度提高,耐磨性增强.当稀土含量在1.5%时,涂层的硬度提高42%,磨损体积最小.