Surface preparation and thermal spray in a single step: The PROTAL process—Example of application for an aluminum-base substrate

Thermal spray techniques can fulfill numerous industrial applications. Coatings are thus applied to resist wear and corrosion or to modify the surface characteristics of the substrate (e.g., thermal conductivity/thermal insulation). However, many of these applications remain inhibited by some deposit characteristics, such as a limited coating adhesion or pores or by industrial costs because several nonsynchronized and sequential steps (that is, degreasing, sand blasting, and spraying) are needed to manufacture a deposit. The PROTAL process was designed to reduce the aforementioned difficulties by implementing simultaneously a Q-switched laser and a thermal spray torch. The laser irradiation is primarily aimed to eliminate the contamination films and oxide layers, to generate a surface state enhancing the deposit adhesion, and to limit the contamination of the deposited layers by condensed vapors. From PROTAL arises the possibility to reduce, indeed suppress, the preliminary steps of degreasing and grit blasting. In this study, the benefits of the PROTAL process were investigated, comparing adhesion of different atmospheric plasma spray coatings (e.g., metallic and ceramic coatings) on an aluminum-base substrate. Substrates were coated rough from the machine shop, for example, manipulated barehanded and without any prior surface preparation. Results obtained this way were compared with those obtained using a classical procedure; that is, degreasing and grit blasting prior to the coating deposition.     

Post-treatment of thermal spray coatings on magnesium

Magnesium alloys have a beneficial combination of high strength to weight ratio, good machinability and high recycling potential. Despite this, the application of magnesium still is behind that of other constructive materials mainly due to low wear and corrosion resistance. For more demanding applications, a large amount of surface treatment methods are developed to overcome this problem. Thermal spraying is an efficient and flexible method of coating deposition and is widely used for protection of different materials against corrosion and wear. Nevertheless, the bonding of thermal spray coatings on magnesium alloys is not sufficient, so the following post-treatment processes are needed. One of such possibilities is high energy beam treatment of thermally sprayed coatings. During the heat treatment of magnesium substrates with coating the remelting of coating and a thin surface layer of substrate occurs. Depending on the combination of applied coating system and treatment method, different processes can be realised in modified layers: the alloying of magnesium substrate with other elements to improve corrosion properties, redistribution of hard particles from composite coating and new phases formation during the processing to improve the wear resistance of magnesium alloys. In the present work some examples concerning the laser and electron beam treatment of aluminium based composite coatings as well as infra red irradiation of zinc based coatings are described. Coatings are deposited on magnesium substrates (AM20, AZ31, AZ91) by arc spraying with Zn, ZnAl4 and ZnAl15 solid wires and cored wires in aluminium core with powder filling containing different hard particles, such as boron, silicon and tungsten carbide or titanium oxide. Remelting of thermal spray coatings is carried out by means of continuous irradiation of СО₂-laser in nitrogen or argon atmosphere, electron beam in vacuum and focused tungsten halogen lamp line heater in atmosphere. Microstructure of sprayed coatings as well as that of modified surface layers is investigated by metallographic methods. Corrosion properties are estimated by electrochemical measurements. Abrasion wear resistance of the modified layers is determined by scratch test, corundum grinding disk test and Rubber wheel test. It is shown that all methods applied for processing of thermal spray coatings lead to formation of modified surface layers in magnesium substrate with improved wear and corrosion properties. Different mechanisms of microstructure formation such as redistribution of chemical composition of composite coating components, partial remelting of hard phase particles, and new phases formation are discussed. Electrochemical behaviour of modified surface layers is mostly improved due to alloying, homogenization of element distribution and strong decrease of as-sprayed coating porosity. Abrasion wear resistance of processed magnesium substrates strongly depends on the microstructure and usually is 5 to 20 times higher compared with base material.     

Effect of Pulsed Laser Ablation and Continuous Laser Heating on the Adhesion and Cohesion of Cold Sprayed Ti-6Al-4V Coatings

The individual and cumulative effects of in situ pulsed laser ablation and continuous laser pre-heating on adhesion and cohesion strength of cold sprayed Ti-6Al-4V coatings are investigated. Laser beams were coupled to a cold spray gun in order to ablate and pre-heat the substrate surface a few milliseconds prior to the impact of the spray particles. Cohesion and adhesion strength were evaluated by scratch test, standard ASTM C633 pull test and laser shock (LASAT) technique. The effects of laser ablation before and during cold spray operations were investigated. Results demonstrate that laser ablation of the substrate before cold spraying led to a smooth surface which improved adhesion strength. However, when laser ablation was maintained throughout the cold spray process, i.e., in between the coating layers, a reduction of cohesion and adhesion was observed. These negative effects were circumvented when laser ablation and laser pre-heating were combined.     

纯铜SHS反应热喷涂Al2O3基复合陶瓷涂层的性能研究

    采用SHS(自蔓延高温合成)反应火焰喷涂工艺,将Al-CuO铝热反应体系引入到喷涂陶瓷材料中,在纯铜表面制备Al₂O₃基复合陶瓷涂层.结果表明,SHS反应热喷涂层与基体的结合好于常规热喷涂,辅以Ni-Al合金打底,复合涂层500度下热震循环40次时仍完好无损.复合涂层的XRD图谱表明,在层间及涂层内部生成的NiCu及Al_xCu_y化合物有助于增强涂层的性能,同时Al的适当过量可以起到弥补喷涂过程中Al的损失并为体系提供良好的液相环境的作用,提高反应转化率,降低孔隙率,同时复合涂层具有较好的耐磨性及抗氧化性.     

Cold Gas-Sprayed Deposition of Metallic Coatings onto Ceramic Substrates Using Laser Surface Texturing Pre-treatment

Cold spraying enables a variety of metals dense coatings onto metal surfaces. Supersonic gas jet accelerates particles which undergo with the substrate plastic deformation. Different bonding mechanisms can be created depending on the materials. The particle–substrate contact time, contact temperature and contact area upon impact are the parameters influencing physicochemical and mechanical bonds. The resultant bonding arose from plastic deformation of the particle and substrate and temperature increasing at the interface. The objective was to create specific topography to enable metallic particle adhesion onto ceramic substrates. Ceramic did not demonstrate deformation during the impact which minimized the intimate bonds. Laser surface texturing was hence used as prior surface treatment to create specific topography and to enable mechanical anchoring. Particle compressive states were necessary to build up coating. The coating deposition efficiency and adhesion strength were evaluated. Textured surface is required to obtain strong adhesion of metallic coatings onto ceramic substrates. Consequently, cold spray coating parameters depend on the target material and a methodology was established with particle parameters (diameters, velocities, temperatures) and particle/substrate properties to adapt the surface topography. Laser surface texturing is a promising tool to increase the cold spraying applications.     

Ablation laser and heating laser combined to cold spraying

Cold spraying is particularly suitable for the elaboration of coatings sensitive to heat and oxidation. As spraying particles are not subjected to melting, the adhesion and formation of the coating is due to the kinetic energy transmitted to the particles by accelerating gas. Bonding mechanisms are not only strongly dependent on the particle velocity but also on the state of the substrate surface. The presence of surface pollutants inhibits the coating elaboration. Surface modifications are necessary to reach a high adhesion between the coating and the substrate. A laser ablation and a laser heat treatment are proposed to prepare the substrate prior to cold spraying. Ablation laser is used to eliminate adsorbed pollutant molecules and heating laser is employed to improve the contact between substrate and particles and coating substrate adherence. The bonding of aluminium coating on aluminium 2017 alloy was evaluated by tensile adhesion tests and demonstrated the strong influence of the laser treatments in comparison with conventional processes (grit blasting and degreasing).     

Temperature measurements and adhesion properties of plasma sprayed thermal barrier coatings

Metal-ceramic coatings have been widely used for industrial applications, mainly in the gas turbine and diesel engine industries as thermal barrier coatings (TBCs). Conventional thermal barrier coatings consist of a metallic bond coat and an insulating ceramic topcoat. Temperatures and temperature gradients in the coating during plasma spraying play an important role on the final coating quality, especially the temperature of the particles just hitting the substrate surface. In this work, metal-ceramic coatings were applied on nickel-superalloy substrates. The temperatures of both the coating surface and substrate were measured during spraying. The adhesion of the coatings was determined using ASTM C 633 and correlated with the measured temperatures. Optical pyrometry and thermocouples were used to measure the interfacial and substrate temperatures, respectively. Temperature was shown to have a significant influence where lower interfacial temperatures were found to result in lower adhesion values.     

Laser texturing as an alternative to grit blasting for improved coating adhesion on AZ91D magnesium alloy

Substrate preparation plays an important role in the performance of thermal spray coating, especially on softer materials like magnesium and aluminium alloys. Conventional substrate preparation methods such as grit blasting may not be the most suitable choice due to grit embedding, lower coating adhesion strength and environmental concerns. Laser texturing can be an attractive alternative to the grit blasting method for such materials. AZ91D substrate was prepared for thermal spray coating using grit blasting and laser texturing techniques. WC-12Co powder was thermally sprayed on AZ91D magnesium alloy using the high-velocity oxygen fuel technique. The adhesion strength of the coating, thus produced, was determined using the ASTM 633C adhesion strength test. Scanning electron microscopy was used to investigate substrate morphology and to qualitatively analyse substrate and coating interface. X-ray diffraction was used to identify phase compositions. The coating was characterised for roughness, porosity, micro-hardness and fracture toughness. Laser texturing as a substrate preparation technique has been able to produce well-adhered coatings, with adhesion strength of 45.6?MPa, and comparable coating characteristics with those of the grit blasting technique.     

NiAl/NiCoCrAlY/8YSZ复合喷涂层的微观结构与性能研究

目的提高金属/陶瓷隔热涂层体系在海洋环境下的耐腐蚀性能。方法利用冷喷涂方法制备NiAl复合打底层和Ni CoCrAlY粘结层,与等离子喷涂制备的8YSZ陶瓷层构成适用于海洋环境的多层结构耐蚀隔热涂层体系。利用FE-SEM分别观察喷涂态粘结层和陶瓷层的表面、横截面形貌,通过EDS分析涂层元素分布;利用XRD分析表征涂层的物相组成;借助万能材料试验机,采用拉伸法测试涂层结合强度;利用热循环试验和焰流冲刷试验测试涂层的耐高温性能。结果微观分析表明,冷喷涂制备的NiAl复合打底层和Ni CoCrAlY粘结层形貌致密,涂层材料未发生明显氧化,颗粒变形程度不一,粘结层与基体间的结合强度约为18.4 MPa,粘结层与8YSZ陶瓷层界面结合紧密。陶瓷层物相结构和成分稳定,涂层经12次热震循环和1000个周期的高温焰流冲击后,表面未出现开裂、起皮和脱落。结论采用冷喷涂法和等离子喷涂法联合制备的耐蚀隔热复合涂层体系具备良好的耐热性和耐腐蚀性。冷喷涂制备的金属涂层结构致密,孔隙率低,与陶瓷层结合良好,能够有效提高涂层体系在腐蚀性环境中的耐蚀性能。NiAl复合涂层可以缓解Ni CoCrAlY粘结层和铝合金基材间的热匹配问题,增强涂层的结合性能。     

Corrosion behavior of HVOF-sprayed and Nd-YAG laser-remelted high-chromium,nickel-chromium coatings

Thermal spray processes are widely used to deposit high-chromium, nickel-chromium coatings to improve high temperature oxidation and corrosion behavior. However, despite the efforts made to improve the present spraying techniques, such as high-velocity oxyfuel (HVOF) and plasma spraying, these coatings may still exhibit certain defects, such as unmelted particles, oxide layers at splat boundaries, porosity, and cracks, which are detrimental to corrosion performance in severe operating conditions. Because of the process temperature, only mechanical bonding is obtained between the coating and substrate. Laser remelting of the sprayed coatings was studied in order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties. The coating material was high-chromium, nickel-chromium alloy, which contains small amounts of molybdenum and boron (53.3% Cr, 42.5% Ni, 2.5% Mo, 0.5% B). The coatings were prepared by HVOF spraying onto mild steel substrates. A high-power, fiber-coupled, continuous-wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF-sprayed coating using different levels of scanning speed and beam width (10 or 20 mm). Coating that was remelted with the highest traverse speed suffered from cracking because of the rapid solidification inherent to laser processing. However, after the appropriate laser parameters were chosen, nonporous, crack-free coatings with minimal dilution between coating and substrate were produced. Laser remelting resulted in the formation of a dense oxide layer on top of the coatings and full homogenization of the sprayed structure. The coatings as sprayed and after laser remelting were characterized by optical and electron microscopy (OM, SEM, respectively). Dilution between coating and substrate was studied with energy dispersive spectrometry (EDS). The properties of the laser-remelted coatings were directly compared with properties of as-sprayed HVOF coatings.     

Multiple-Approach Evaluation of WSP Coatings Adhesion/Cohesion Strength

Adhesion/cohesion testing represents one of the most common methods for benchmarking and optimization of thermal spray coatings. However, owing to the inhomogeneous coating microstructure, such testing may be quite troublesome. In this study, adhesion/cohesion strength of representative metallic and ceramic coatings deposited by water-stabilized plasma (WSP) spraying was evaluated by four different methods: tensile adhesion test, pin test, tubular coating tensile test, and shear test. Combination of various methods enabled the evaluation of the coating adhesion/cohesion strength under different loading conditions. Limitations and benefits of each method for testing of WSP coatings are demonstrated. Dominating failure micromechanisms were determined by supplementary fractographic analysis.     

热处理对冷喷涂Fe涂层组织与性能影响研究

冷喷涂作为一种新型的涂层技术,在制备大部分金属涂层、金属陶瓷复合涂层方面有着巨大的潜力。本文采用冷喷涂在Al基体上制备了Fe涂层,并结合喷涂后热处理研究了涂层组织与性能特点。结果表明,所得Fe涂层的内部组织比较致密;受到喷涂过程中空气的影响,喷涂射流呈现亮流,所制备涂层表面有较大、较深气孔。在较低温度下热处理后Fe涂层的显微组织变化不明显,显微硬度明显降低;Fe涂层与Al基体之间形成约10μm厚度的金属间化合物层。     

Effect of substrate materials on rutile crystalline orientation in plasma-sprayed TiO2 coatings

TiO2 coatings are of technical importance owing to their promising applications to photocatalytical, electrical, optical and tribological coatings. Thermal spraying process has been widely used to deposit both metallic and nonmetallic coatings. During thermal spraying, spray particle at fully or partially melted condition is projected to a substrate and subsequently flattens, rapidly cools and solidifies. Therefore, a coating in lamellar structure is usually formed as a quenched microstructure. TiO2 coatings were deposited on different substrates through plasma spraying with fused-crushed powder in rutile phase as feedstock to reveal the crystalline orientation in the coatings. XRD results show that the coatings consist of rutile phase with a fraction of anatase phase, and the rutile phase presents a preferable crystalline orientation along [101] direction. It is found that the orientation factors of rutile phase in the thin coatings are significantly influenced by substrate materials. The thick coatings yield the same orientation factors of 0.22 to 0.23 on all substrates in spite of substrate materials. It is considered that the thermal properties of substrate materials are the dominant factors for the preferable crystalline orientation in rutile phase within plasmasprayed TiO2 coating.     

Improving corrosion properties of high-velocity oxy-fuel sprayed inconel 625 by using a high-power continuous wave neodymium-doped yttrium aluminum garnet laser

Thermal spray processes are widely used to protect materials and components against wear, corrosion and oxidation. Despite the use of the latest developments of thermal spraying, such as high-velocity oxy-fuel (HVOF) and plasma spraying, these coatings may in certain service conditions show inadequate performance,e.g., due to insufficient bond strength and/or mechanical properties and corrosion resistance inferior to those of corresponding bulk materials. The main cause for a low bond strength in thermalsprayed coatings is the low process temperature, which results only in mechanical bonding. Mechanical and corrosion properties typically inferior to wrought materials are caused by the chemical and structural inhomogeneity of the thermal-sprayed coating material. To overcome the drawbacks of sprayed structures and to markedly improve the coating properties, laser remelting of sprayed coatings was studied in the present work. The coating material was nickel-based superalloy Inconel 625, which contains chromium and molybdenum as the main alloying agents. The coating was prepared by HVOF spraying onto mild steel substrates. High-power continuous wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of power and scanning speed. The coatings as-sprayed and after laser remelting were characterized by optical microscopy and scanning electron microscopy (SEM). Laser remelting resulted in homogenization of the sprayed structure. This strongly improved the performance of the laser-remelted coatings in adhesion, wet corrosion, and high-temperature oxidation testing. The properties of the laser-remelted coatings were compared directly with the properties of as-sprayed HVOF coatings and with plasma-transferred arc (PTA) overlay coatings and wrought Inconel 625 alloy.     

Laser Patterning Pretreatment before Thermal Spraying: A Technique to Adapt and Control the Surface Topography to Thermomechanical Loading and Materials

Coating characteristics are highly dependent on substrate preparation and spray parameters. Hence, the surface must be adapted mechanically and physicochemically to favor coating–substrate adhesion. Conventional surface preparation methods such as grit blasting are limited by surface embrittlement and produce large plastic deformations throughout the surface, resulting in compressive stress and potential cracks. Among all such methods, laser patterning is suitable to prepare the surface of sensitive materials. No embedded grit particles can be observed, and high-quality coatings are obtained. Finally, laser surface patterning adapts the impacted surface, creating large anchoring area. Optimized surface topographies can then be elaborated according to the material as well as the application. The objective of this study is to compare the adhesive bond strength between two surface preparation methods, namely grit blasting and laser surface patterning, for two material couples used in aerospace applications: 2017 aluminum alloy and AISI 304L stainless steel coated with NiAl and YSZ, respectively. Laser patterning significantly increases adherence values for similar contact area due to mixed-mode (cohesive and adhesive) failure. The coating is locked in the pattern.     

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.     

Influence of process conditions in laser-assisted low-pressure cold spraying

A laser-assisted low-pressure cold spraying (LALPCS) is a one-step coating process in which the laser irradiation interacts simultaneously with the spraying spot on the substrate or deposited coating surface in order to improve coating properties. It is expected that the LALPCS could be an effective method to improve a low-pressure cold sprayed coating deposition efficiency and denseness. The purpose of the additional energy from the laser beam is to create denser and more adherent coatings, enhance deposition efficiency and increase the variety of coating materials.In this study copper and nickel powders with additions of alumina powder were laser-assisted low-pressure cold sprayed on carbon steel. Coatings were sprayed using air as process gas. A 6 kW continuous wave high power diode laser and a low-pressure cold spraying unit were used in the experiments. The influence of laser energy on coating microstructure, density and deposition efficiency was studied. The coatings were characterized by optical microscopy and SEM. The coating denseness was tested with open cell potential measurements. Results showed that laser irradiation improved the coating denseness and also enhanced deposition efficiency.     

钛合金表面激光重熔NiCrBSi+TiN涂层的组织研究

对TC4合金喷涂NiCrBSi+20vol%TiN涂层进行激光重熔试验,利用SEM、TEM等手段对熔覆层的组织进行了分析。结果表明,激光重熔层组织消除了喷涂层的组织缺陷,其相组成为(Cr-Ni-Fe),TiNi,NiB,Cr2Ti,Ti2Ni等,在熔覆区主要以富铬相和富镍相为主,稀释区内的黑色和白色衬度的杆状组织是Ti2Ni型的金属间化合物。     

Laser cycling and thermal cycling exposure of thermal barrier coatings on copper substrates

Thermal barrier coatings (TBCs) are successfully applied in turbines and could also protect combustion chambers in rocket engines. Apart from different loading conditions, the main difference between these applications is the substrate material, which is nickel-based for turbines and copper-based for rocket engines. To optimize the coating system, more knowledge of possible failure modes is necessary.In this work a standard coating system was applied by atmospheric plasma spraying to copper specimens. These specimens were exposed to thermal cycling with different cooling rates and to laser shock testing. A laser-cycling set-up was developed to qualify different coating systems. This set-up consists of a high-power diode laser (3 kW) which provides high heating rates to up to 1500 °C. Laser shock testing has proven to be a suitable alternative to burner rig testing.The results were different to the common failure modes for TBCs on nickel substrates as the coatings system does not fail at the interface between top coat and bond coat, but at the interface between substrate and bond coat. Two failure modes were observed: copper oxide was undermining the coatings at the substrate/bond coat-interface in the case of thermal cycling experiments, and complete delamination occurred at the same interface in the case of laser shock testing. Consequently, this interface is critical in the investigated material system.     

铝硅合金表面激光Cr／WC梯度层组织及抗微动磨损性能研究

利用5kWCO2激光器对铸造铝硅合金表面进行两次激光辐照,先后将不同成分的预置合金粉末和基体材料一起熔化后迅速凝固,在其表面获得Cr/WC激光表面梯度层,并对它们的显微组织进行分析,发现在表面梯度层中存在大量的Al/Cr化合物Al9Cr4。在激光表面梯度层中自表及里显微硬度基本呈连续变化趋势,而单次激光表面合金化层与基体之间的显微硬度值有一突变,在基体和激光表面改性层的微动磨损试验中发现,激光改性后的表面抗微动磨损的能力增强,磨损机制不同于铝硅合金基体,而表现出较好的抗粘着损伤性能。