Solution precursor plasma sprayed tungsten oxide particles and coatings

Solution precursor plasma spray process was used to deposit single particles and coatings of tungsten oxide (WO₃), and the microstructures of single particles and coatings were characterized by field emission scanning electron microscopy. The effects of substrate temperature and spraying distance on the microstructure of single particles and coatings were studied. In the case of WO₃ particles, the particle spheroidization degree became better as the increase in substrate temperature. When the substrate temperature increased up to 200°C, bubble-like morphologies appeared. For the deposited WO₃ coatings, a highly porous structure was obtained when a 100?mm spraying distance was used. Besides, the grain size of coatings decreased through increasing the spraying distance from 60 to 100?mm.     

Mikrostrukturieren von thermisch gespritzten Mo‐Schichten für Anwendungen im Bereich hoher Reib‐ und Verschleißbeanspruchungen

The properties of thermally sprayed coatings are dependent on many parameters such as the spraying material, substrate properties, and the injection parameters. In this study, the influence of two variable spray parameters (spraying distance and current) were investigated on molybdenum‐containing thermal spray coatings. Particularly, materials and surface characterizing properties were analyzed, and the dependence on each other was examined. The important surface parameters studied in this case are the porosity and the pore depth of the coatings. Following the correlation between spray parameters and coating properties, the influence of surface properties on the tribological behavior will be discussed, in comparison to an uncoated steel surface.     

Theoretical and experimental approaches of bacteria‐biomaterial interactions

About the influence of substrate roughness and spray particle size on the adhesion of thermal spray coatings Bacterial adhesion to surfaces is a complicated process influenced by many factors including the bacteria (surface energy and charge, molecular details), the substratum surface (chemical composition, roughness, configuration, surface energy and charge) and environmental factors (serum proteins, flow conditions, temperature, bacterial concentration, time of exposure, antibiotics). The models that have been proposed for the quantitative prediction of bacteria‐material interactions are based on colloidal theories and macromolecular binding considerations. Two categories of techniques used in calculating bacterial adhesion strength and bacteria‐material interactions have been proposed: those that utilize fluid flowing against adherent bacteria and those that manipulate single bacteria (atomic force microscopy and optical tweezers), and are concisely reviewed.     

Development of dense corrosion resistant coatings by an improved HVOF spraying process

For 6 years, we have developed corrosion resistant coatings to protect steel structures in the marine environment by using a thermal spray technique. This paper summarizes the major developments and results obtained. Such a coating requires primarily impermeability and secondarily homogenous and clean microstructure. In order to make denser and highly corrosion resistant coatings, we selected spray materials and improved fabrication processes. HastelloyC was a suitable material for High Velocity Oxy-Fuel (HVOF) spraying to form corrosion resistant coatings because of its high resistance against thermal oxidation as well as seawater corrosion, especially crevice and pitting corrosion. An inert gas shroud system was attached with a commercial HVOF apparatus and this attachment increased the in-flight velocity of spray particles over 750 m s^?1 and simultaneously suppressed oxidation significantly. In addition, some new methods were designed to evaluate the sprayed particle’s state and the coating properties with high accuracy and sensitivity. Thermal energy of in-flight spray particles was revealed by molten fraction of spray particles, determined by quantitative analysis of melted and unmelted particles captured in an agar gel. Through-porosity of the coatings with open porosity below 0.1% was determined by using Inductively Coupled Plasma analysis of dissolved substance from substrate through the penetrating path of the coatings. The coating of HastelloyC nickel base alloy by the HVOF spraying with the gas shroud attachment had zero through-porosity and 0.2 mass% of oxygen content. The laboratory corrosion tests showed that the on-shroud HastelloyC coating was comparable to the bulk material of HastelloyC in terms of corrosion resistance. This coating, formed on steel, demonstrated an excellent protective performance over 10 months in the marine exposure test.     

Grenzflächenphänomene und Haftung bei thermisch gespritzten Verbundwerkstoffen

In this paper. results from investigations on the bonding mechanism between thermally sprayed metal and ceramic coatings and different substrates are presented. The materials were sprayed by atmospheric arc (Ni, Mo und Cu) and plasma spraying techniques (NiCr-Al, Al₂O₃, ZrO₂-7Y₂O₃) on iron (Armco-Fe) and steel (St37) substrates. The morphology, microstructure and impact behaviour of sprayed particles on the polished substrates have been analysed as a function of particle properties (temperature, heat content, size and velocity, surface tension, viscosity) and substrate properties (composition, temperature, surface structure). The results of the influence of investigated properties on the adherence mechanism of sprayed particles can be also directly correlated to the experimentally measured bond strength of are sprayed nickel coatings.     

Verarbeitung von feinen Spritzwerkstoffen zur Verbesserung von Korrosions‐ und Verschleißschutzeigenschaften von thermisch gespritzten Schichten

About the application of fine spray materials for improvements in terms of corrosion and wear protection of thermal spray coatings Within a research project the spray materials Cr₂O₃, Cr₃C₂ NiCr and WCCoCr were considered. Process parameters were determined, optimized and validated allowing the application of very fine spray material fractions (< 25 μm) with plasma and high velocity oxy‐fuel spray systems. In addition to improved coating properties like low porosity and improved corrosion resistance the application of near net shape coatings was enabled. The subsequent reduced effort for after‐treatment of the coatings can contribute to further rationalization of thermal spray processes.     

热喷涂制备二氧化钛涂层中锐钛矿相调控的研究进展

二氧化钛(TiO_2)涂层在许多领域具有重要的应用价值,如光催化降解和光电池,近年来引起了相关研究者的广泛关注。在众多涂层制备技术中,热喷涂技术可以快速、高效、大面积、大批量地制备TiO_2涂层,且得到的涂层力学性能良好,喷涂成本较为低廉,因而使热喷涂TiO_2涂层的应用更具前景。TiO_2涂层的晶相组成是影响涂层最终性能的一个重要因素,而控制涂层中晶相组成的关键是调控亚稳态的锐钛矿相含量。综述了近些年来国内外制备TiO_2涂层常用的热喷涂技术,如传统粉末热喷涂、液料热喷涂以及冷喷涂等,分别阐述了不同热喷涂技术中通过调节一些重要参数达到调控涂层中锐钛矿相的方法,并讨论了材料掺杂对TiO_2涂层中锐钛矿相的影响,指出了目前调控涂层中锐钛矿相存在的问题和后续的研究方向。     

Microstructure and bonding mechanisms in cold spray coatings

Cold spray additive manufacturing is a solid state deposition process with applications in surface coatings, manufacture of near net shape parts and component repairs. The technology can be used to spray a wide range of metallic powders onto varying substrates to produce the desired properties for application. This is a review of the progress in cold spray technology with particular interest in the bonding behaviour and deformation microstructure. Comparisons between cold spray and other thermal spray technologies such as plasma, wire arc, flame and high velocity oxyfuel spraying are discussed. The key advantage of cold spray over thermal spray is the reduced heat input and absence of melting. This drastically changes the mechanisms in which coatings are formed and bonded to the substrate, thus material properties and particle velocity have a large influence on the mechanical and microstructural properties of the coating. Fundamental understanding of the cold spray process is the key to its rapid uptake into the industry.

This review was submitted as part of the 2018 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

Computational simulation of thermally sprayed WC–Co powder

WC–Co cemented carbides are a class of hard composite materials of great technological importance. They are widely used as tool materials in a large variety of applications that have high demands on hardness and toughness, including mining, turning, cutting and milling. The HVOF (high velocity oxygen fuel) technology has been very successful in spraying wear resistant WC–Co coatings with higher density, superior bond strengths and less decarburization than many other thermal spray processes, attributed mainly to its high particle impact velocities and relatively low peak particle temperatures. The degree of decomposition and bond strength is directly related to relevant particle parameters such as velocity, temperature and state of melting or solidification. These are consecutively related to process parameters such as powder particle size distribution, carrier gas flow rate, and fuel type employed. To obtain detailed particle data important for thermal spraying, mathematical models are developed in the present paper to predict the particle dynamic behavior in a liquid fuelled HVOF thermal spray gun. The particle transport equations are coupled with the three-dimensional, chemically reacting, turbulent gas flow, and solved in a Lagrangian manner. The melting and solidification within the particles as a result of heat exchange with the surrounding gas flow is solved numerically. The in-flight characteristics of WC–Co particles are studied and the effects of carrier gas parameters on particle behavior are examined. The results demonstrate that WC–Co particles smaller than 5 μm in diameter undergo melting and solidification prior to impact while most particles never reach liquid state during the HVOF thermal spraying. The flow rate of carrier gas has considerable influence on particle dynamics as well as deposition on substrate. At higher flow rate the powder particles are redirected further away from the substrate center, while smaller flow rate results in better heating, higher impact velocity and deposition closer to the substrate center.     

Detonation spraying behaviour of refractory metals: Case studies for Mo and Ta-based powders

In thermal spraying of refractory metal powders, two major issues need to be solved: particles of materials having high melting temperatures should be heated to reach a semi-molten/molten state or temperatures close to the melting point, while oxidation of the metals should be prevented. It has long been believed that it is rather difficult, if not impossible, to produce high-quality refractory metal coatings by detonation spraying. In this work, we demonstrated the capability of the detonation spraying method to produce tantalum-based and molybdenum coatings of low porosity. Using a computer-controlled detonation spray (CCDS2000) facility, the detonation spraying behaviour of a molybdenum powder and a partially oxidized tantalum powder was studied. Spraying was conducted onto steel substrates using an acetylene-oxygen mixture with O₂/C₂H₂?=?1.1. The spraying process was studied by means of analyzing the splat morphology and calculating the particle temperatures and velocities. According to the X-ray diffraction phase analysis, the metals did not experience oxidation during the deposition. Rather, partial reduction of the oxide phase contained in the Ta-based powder occurred during spraying.     

基于粒子飞行特性及铺展行为的WC-10Co4Cr涂层孔隙形成机理研究

采用超音速等离子喷涂设备制备WC-10Co4Cr涂层, 通过调整喷涂功率得到了具有不同孔隙结构特征的涂层, 以液氮为冷却介质收集不同熔化状态下的飞行粒子, 并通过镜面钢收集熔滴撞击铺展之后所形成的扁平粒子, 采用扫描电子显微镜观察涂层、收集的飞行粒子及扁平粒子形貌, 采用透射电镜观察涂层显微组织结构。结果表明: 随着喷涂功率的增大, 涂层的大孔隙含量逐渐降低, 而当喷涂功率上升到60 kW时, 涂层内部出现较多的热裂纹, 涂层的显微硬度则随着功率的增加而先增大后减小, WC扁平粒子主要有四种类型, 包括熔化不佳、破碎型、气泡型及花瓣型, 孔隙的形成主要是由于“遮蔽效应”、微区气体作用、熔滴不充分润湿及气孔迁移等。     

Ar + H2 plasma etching for improved adhesion of PVD coatings on steel substrates

Ar + H₂ plasma cleaning has been described for the surface modification of the steel substrates, which removes oxides and other contaminants from substrate surface effectively leading to a better adhesion of the physical vapor deposited (PVD) coatings. Approximately 1.1-1.3 μm thick TiAlN coatings were deposited on plasma treated (Ar and Ar + H₂) and untreated mild steel (MS) substrates. A mechanism has been put forward to explain the effect of plasma treatment on the substrate surface based upon the data obtained from X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The XPS measurements on untreated and Ar + H₂ plasma etched MS substrates indicated that the untreated substrate surface mainly consisted of Fe₃O₄, whereas, after etching the concentration of oxides decreased considerably. The FESEM and the AFM results showed changes in the surface morphology and an increase in the substrate roughness as a result of Ar + H₂ plasma etching. Removal of oxide/contaminants, formation of coarser surface and increased substrate surface roughness as a result of Ar + H₂ plasma etching facilitate good mechanical interlocking at the substrate surface, leading to a better adhesion of the deposited PVD coatings. The adhesion of TiAlN coating could be increased further by incorporating a very thin Ti interlayer.     

Influence of temperature on the bonding mechanism of plasma-sprayed coatings

Although considerable work has been carried out on the bonding mechanism of sprayed coatings, the relationship between the temperatures of substrate and particle and the adhesion of the coatings was not clearly pointed out. The purpose of this paper is to analyse the influence of temperature on the adhesive mechanism of thermally sprayed coatings. In this paper the importance of the temperature of both the substrate and the particle in the adhesive mechanism is emphasized. In order to clarify the adhesive mechanism at the boundary between the coating and the spreads on it is made. The model of a uniform layer of liquid (the particle) at initial temperature T_p which is suddenly brought into contact at time zero with a substrate at initial temperature T_s is considered. The diameter of the splat particle is assumed to be much larger than its thickness; edge effects are neglected and the problem is then considered as one-dimensional heat flow. Two phase changes are under consideration: the solidification of the spray droplet and the melting of the substrate.By analogy with the method of Weiner who obtained the solution for transient heat conduction in a multiphase media, the exact solution for this solidification- melting problem is found. The solution gives the pertinent information on the thermal behaviour of both the particle and the substrate. Moreover it gives the relationship between the initial molten droplet temperature and the initial solid substrate temperature for the onset of melting in the solid.The results of the analysis indicate the importance of physical properties of both the spray droplet and the substrate in the incipient melting of the substrate and emphasize the importance of the substrate because the temperature of the particle is usually difficult to control.Obviously, a particle-substrate system where the solidification-melting conditions are easily realized has greater bond strength than a system where these conditions are difficult to meet. Thus the reason why some materials stick generally to all surfaces is clarified, particularly if the solidification-melting conditions are favourable to the creation of a metallurgical bond.     

Plasma spray synthesis of TiB2-Fe coatings

A limiting feature of the plasma spray process is the need for the powder to melt during its passage through the plasma flame. It is quite impossible to obtain coatings with materials that are difficult to melt. However, metal borides, particularly titanium boride, are attractive. Because of their high melting point, satisfactory coatings based on these materials have not been achieved.To overcome this problem, a process for making TiB₂-Fe coatings was studied. The TiB₂-based coatings were produced by the reaction of ferrotitanium with boron. The TiB₂ formation was first studied by thermal differential analysis and X-ray diffraction. It was observed that TiB₂ is formed at low temperature by an exothermic reaction. The characteristics of the reaction products obtained at different reaction temperatures are described.Agglomeration techniques were used to prepare the reagents: fine powders of ferrotitanium alloy and boron. TiB₂-Fe coatings were produced by plasma spraying the agglomerated powders. The influence of the plasma spray process parameters and the powder preparation techniques on the coating microstructure is discussed.Thick hard coatings comprising compounds of the reagent materials are produced during spraying by this synergetic process. Such coatings may be suitable for wear resistance applications.     

Fe3Al-Al2O3陶瓷梯度涂层性能研究

利用等离子喷涂制备了不同成分设计的多种Fe3Al-Al2O3陶瓷复合涂层，采用金相显微镜、SEM、XRD及电子探针等手段，研究了涂层的微观组织及成分分布，并对涂层的结合强度、显微硬度及抗热震性能进行了试验研究。结果表明，Fe3Al过渡层的引入可有效地改善涂层的质量，Fe3Al金属间化合物是钢基体上制备陶瓷涂层较为理想的过渡材料。涂层成分的梯度化有利于涂层结合强度和抗热震性能的提高。     

Applikation ausscheidungshärtender Bronzeschichten mittels Hochgeschwindigkeitsflammspritzen

Application of age hardenable‐copper‐based‐coatings trough HVOF‐spraying Thermal spraying of age hardenable copper based alloys, enables the producing of wear resistant coatings with high conductivity. With high velocity flame spraying it is possible to create dense coatings with good adhesion to the substrate. The coatings are produced in two steps the thermal spray process itself and the followed heat treatment. The characterisation of the produced specimen shows promising results. The achieved wear resistance of the coatings is similar to bulk material and better than pure copper.     

Effect of parameters on adhesion strength for slurry spray coating technique

In this article, slurry spray technique (SST) has been adopted for depositing mullite–nickel based environmental barrier coatings (EBCs) on some ASTM 1018 low-carbon steel. Considerable value of adhesion strength of a deposited cermet is critical for the coating to comply with service condition. The effect of identified process parameters like stamping pressure, fly ash content, sintering additive, sintering time, and sintering temperature on optimizing adhesion strength is evaluated. Parametric assessment of the developed coatings is done utilizing Taguchi L₁₈ orthogonal array and analysis of variance (ANOVA). Based on the analytical study of the experimentation, the dependence of adhesion strength of slurry sprayed coatings on the process parameters has been studied. Maximum adhesion strength value of 18.15?MPa was attained during experimentation within the range of the selected control parameters. The enhanced adhesion strength was found for increased sintering temperature up to 950°C which may be attributed to the improved sintering of mullite mixed fly ash due to lowering of secondary mullitization temperature. Furthermore, it has been proven experimentally that the quality of coatings achieved in this work is acceptable and approaching to the quality of thermal coatings manufactured with commercially available fabrication methods.     

Role of process type and process conditions on phase content and physical properties of thermal sprayed TiO<Subscript>2</Subscript> coatings

Thermal spray represents an advantageous technique for depositing large-area titanium dioxide coatings that are of interest for both traditional wear-resistant coatings as well as functional applications such as photo-induced decontamination surfaces. Numerous past studies have examined the phase evolution and properties of TiO₂ coatings using different thermal spray processes or parameters. In this paper, an integrated study of thermal sprayed TiO₂ was conducted with different thermal spray devices and process parameters for a single feedstock powder comprising the metastable anatase phase. The aforementioned variables are correlated with in-flight particle state (particle temperature and velocity), phase evolution, and coating physical properties. The results are represented through the framework of process maps which connect process parameters with material properties. Based on the phase characterization, an initial exploration of the metastable phase evolution during thermal spray deposition of TiO₂ is proposed. Furthermore, the sprayed TiO₂ coatings show varying degrees of electrical conductivity associated with process-induced stoichiometric changes (vacancy generation) in the TiO₂. The effects of these stoichiometric changes as well as extrinsic microstructural attributes (pores, cracks, interfaces), contribute to the complex electrical response of the coatings. This integrated study provides insights into the process–microstructure–property relationship with the ultimate goal of tailoring the functionality of spray deposited oxide thick films.     

High Velocity Oxy Fuel Spraying of Cold Work Tool Steels‐ A Novel Approach to Thick Coatings for Wear Protection Applications

Within this work, HVOF sprayed coatings based on X220CrVMo13‐4 cold work steel were applied to a S235JR construction steel substrate. The investigations focus on the influence of particle size and spray parameters on the coating microstructure, analyzed by means of optical microscope (OM) and scanning electron microscopy (SEM). Additional XRD measurements and micro hardness plots across the interface between substrate material and coating were carried out. Furthermore, the influence of particle size on the detected phases and coating porosity was studied. The results were compared with an X220CrMoV13‐4 reference sample produced by HIP.     

Characterization and comparison between APS coatings prepared from ball milled and plasma processed nickel–aluminium powders

Plasma spraying has wide range of applications which include corrosion, thermal and abrasion resistance coatings. In the present work, nickel and aluminium powders were ball milled and the same were thermal plasma processed to produce spherical nickel alumindes particles. Both ball milled and plasma processed powders were spray deposited on stainless steel (SS 304) substrate using atmospheric plasma spray technique (APS). The experiments were carried out for different plasma input power levels, torch to base distances and coating thicknesses. Microstructure, micro hardness, adhesive strength, and porosity of the coatings are reported and discussed. Effect of plasma processing parameters and plasma spheroidization of powders on coating properties has been evaluated and reported. High plasma power, low torch to base distance lead to high temperature supplied to in-flight particles which correspond to high hardness, low porosity and high adhesion. Spherical morphology and formation of nickel aluminide intermetallic were achieved by plasma spheroidization. Coatings prepared from plasma processed powders enhance the coating properties positively.