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.     

Residual stress development in cold sprayed Al,Cu and Ti coatings

Residual stresses play an important role in the formation and performance of thermal spray coatings. A curvature-based approach where the substrate–coating system deflection and temperature are monitored throughout the coating deposition process was used to determine residual stress formation during cold spray deposition of Al, Cu and Ti coatings. The effect of substrate material (carbon steel, stainless steel and aluminium) and substrate pre-treatment (normal grit blasting, grit blasting with the cold spray system and grinding for carbon steel substrate) were studied for all coating materials with optimized deposition parameters. Mainly compressive stresses were expected because of the nature of cold spraying, but also neutral as well as tensile stresses were formed for studied coatings. The magnitudes of the residual stresses were mainly dependent on the substrate/coating material combination, but the surface preparation was also found to have an effect on the final stress stage of the coating.     

Mechanical Performance of Cold-Sprayed A357 Aluminum Alloy Coatings for Repair and Additive Manufacturing

Cold-sprayed coatings made of A357 aluminum alloy, a casting alloy widely used in aerospace, underwent set of standard tests as well as newly developed fatigue test to gain an information about potential of cold spray for repair and additive manufacturing of loaded parts. With optimal spray parameters, coating deposition on substrate with smooth surface resulted in relatively good bonding, which can be further improved by application of grit blasting on substrate’s surface. However, no enhancement of adhesion was obtained for shot-peened surface. Process temperature, which was set either to 450 or 550 °C, was shown to have an effect on adhesion and cohesion strength, but it does not influence residual stress in the coating. To assess cold spray perspectives for additive manufacturing, flat tensile specimens were machined from coating and tested in as-sprayed and heat-treated (solution treatment and aging) condition. Tensile properties of the coating after the treatment correspond to properties of the cast A357-T61 aluminum alloy. Finally, fatigue specimen was proposed to test overall performance of the coating and coating’s fatigue limit is compared to the results obtained on cast A357-T61 aluminum alloy.     

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.     

Deposition of Electrically Conductive Coatings on Castable Polyurethane Elastomers by the Flame Spraying Process

Deposition of metallic coatings on elastomeric polymers is a challenging task due to the heat sensitivity and soft nature of these materials and the high temperatures in thermal spraying processes. In this study, a flame spraying process was employed to deposit conductive coatings of aluminum-12silicon on polyurethane elastomers. The effect of process parameters, i.e., stand-off distance and air added to the flame spray torch, on temperature distribution and corresponding effects on coating characteristics, including electrical resistivity, were investigated. An analytical model based on a Green’s function approach was employed to determine the temperature distribution within the substrate. It was found that the coating porosity and electrical resistance decreased by increasing the pressure of the air injected into the flame spray torch during deposition. The latter also allowed for a reduction of the stand-off distance of the flame spray torch. Dynamic mechanical analysis was performed to investigate the effect of the increase in temperature within the substrate on its dynamic mechanical properties. It was found that the spraying process did not significantly change the storage modulus of the polyurethane substrate material.     

Fatigue behavior of a 4140 steel coated with a NiMoAl deposit applied by HVOF thermal spray

The fatigue behavior of a quenched and tempered AISI 4140 steel has been investigated in three different conditions: as-polished, as-grit blasted with Al₂O₃ particles and as-coated, after grit blasting, with a deposit of Ni–Al–Mo alloy (Metco 447) of approximately 300 μm in thickness, applied by HVOF thermal spraying. It has been determined that after grit blasting with particles of 20 mesh (83 μm) at a pressure of 345 kPa, a significant decrease in the fatigue properties of the material takes place. It has also been observed that such particles, are retained at the substrate surface during blasting and become stress concentrators that enhance the nucleation of fatigue cracks. The latter give rise to a decrease in the fatigue strength of the blasted material. Further coating of the grit blasted specimens with a deposit of Metco 447 of approximately 300 μm thick, applied by HVOF thermal spraying, leads to a further reduction in the fatigue strength of the material. Under these conditions, the fatigue cracks are also nucleated at the alumina particles retained after blasting. It is believed that such a further decrease is mainly associated with two different causes. Firstly, the extensive fracture and delamination of the coating from the substrate which has been observed from the microscopic analysis. Secondly, the possible existence of tensile residual stresses in the substrate, in the vicinity of the substrate–deposit interface, which would assist in the propagation of the fatigue cracks nucleated at the alumina particles. The fatigue properties of the steel substrate in the three different conditions investigated, has been described in terms of the simple parametric relationship earlier proposed by Basquin.     

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

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

Über das korrosionsverhalten von stahlblechen,die mit verschiedenen Strahlmitteln entrostet und mit Zinkstaubanstrich versehen wurden

Corrosion behaviour of steel plate blasted with various media and coated with a zinc dust coating Steelplate which had been descaled by blasting with various blasting media was given a zinc dust coating and its corrosion behaviour was tested in salt spray and in the atmosphere. The surfaces blasted with sand, slag and corundum are superior in the salt spray test to those blasted with steel shot because the letter produces excessive roughness profiles. Under atmospheric conditions, however, corundum blasting appears to be superior. Brush application is better that spraying because the poor adhesion of spray coats facilitates underrusting.     

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.     

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).     

Examination of a grit-blasting process for thermal spraying using statistical methods

An experimental study was conducted to develop an understanding of how the grit blasting process, prior to plasma spray coating, affects various properties of the substrate and coatings. A statistical design of experiment approach was used and the results were analyzed using both the linear regression method and average response of factors calculations. The following process variables were studied: grit size (20, 36, 54), blasting pressure (20, 35, 50 psi), blasting duration (4, 6, 8 passes), blasting distance (4, 6 in.), and blasting angle (45°, 90°). Properties such as bond strength, grit contamination, surface roughness, and substrate distortion were evaluated and correlated to the process variables. Based on multiple linear regression results, it was shown that the bond strength can be improved by increasing all of the parameters within the range studied here. No relationship between the surface roughness and bond strength was observed. Grit contamination is mostly influenced by grit size, blasting pressure, and number of blasting passes. The average response method provided indications to the direction of modifying the required properties as a function of process variables. While the average response method agreed mostly with the linear regression predication, some differences are further discussed in the study.     

TiAlNb intermetallic compound coating prepared by high velocity oxy-fuel spraying

Ti_28.35Al_63.4Nb_8.25 (at.%) intermetallic compound coatings were sprayed onto 316 L stainless steel substrates by HVOF processes using various parameters. By varying the grit blasting pressure between 0.11 and 0.55 MPa, the effects of substrate roughness on the adhesion of TiAlNb thermal sprayed coatings were investigated. The microstructure, porosity and microhardness of the coatings were characterized by SEM, XRD, Image Analysis and Vickers hardness analysis. The tensile adhesion test (TAT) specified by ASTM C 633-79 was used to measure the tensile bonding strength of the coating. The results show that the coatings with substrate roughness of 8.33 μm displayed the best combined strength. TiAlNb coatings had a lamellar microstructure with different spraying parameters. The porosity, bonding strength, microhardness of coatings were assessed in relation to the spraying processes. The thickness of bond coat on the bond strength of coatings was also discussed.     

Quality Improvement of Flame Sprayed, Heat Treated, and Remelted NiCrBSi Coatings

In this study, properties of NiCrBSi coatings, produced by a two-step process of flame deposition and furnace posttreatment, are analyzed. Adhesion strength, microstructure, porosity, microhardness, chemical composition, and residual stresses were analyzed after deposition and after heat treatment; that is, remelting. Numerous specimens were made to study the adhesion strength of coatings after flame deposition. The four chosen influential factors, that is, surface roughness, preheat temperature of the substrate, distance of flame torch, and type of oxyacetylene flame, were optimized to maximize the adhesion strength, using the Taguchi parametric method. The confirmation experiment showed that the developed experimental model is suitable for optimization of flame spraying deposition process. Based on the evaluation of coating properties, the best overall quality was obtained after remelting at a peak temperature 1080 °C with 5 minutes of holding time, followed by slow air cooling.     

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.     

Peening Effect of Thermal Spray Coating Process

The present study investigated the influence of grit blasting, feedstock powder, and thermal spraying technology on performance near the surface on the substrate’s side. The experimental results show that both the grit-blasting process and thermal spraying process harden the substrate, and microhardness on or near the surface was noticeably increased. Grit blasting created deformed regions next to the surface of the substrate and interface between entrapped grits and substrate. Initial equiaxed grains in the deformed regions were elongated and spirally oriented surrounding impact spots. There were no visible changes in microstructure caused by thermal spraying, and the elongated grain regions remained in the coated substrate. Substrate hardening was attributed to grit blasting and associated heating due to flame rather than powder particle impacting during thermal spraying, thus feedstock powder and individual thermal spray technology had no influence on the hardening.     

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。     

等离子熔-喷WC-17%Co涂层工艺参数对外观形貌的影响

等离子熔-喷技术同时具备等离子喷涂、熔焊技术的综合特点,可以实现各种陶瓷材料涂层与基体的冶金结合。Co基WC具有硬度高、耐磨损、耐腐蚀、耐高温等一系列优点。采用等离子熔-喷技术在Q235钢基体表面制备WC-17％Co涂层,通过研究等离子熔-喷工艺参数（等离子熔枪和喷枪角度、熔枪和喷枪与基材表面的距离、熔-喷速度、喷涂送粉量）对涂层外观形貌的影响,获得最佳的工艺参数,制备出外观形貌良好、无缺陷的熔-喷涂层。     

Development of coating structure and adhesion during high velocity oxygen-fuel spraying of WC-Co powder on a copper substrate

This paper deals with the mathematical modeling of the development of the WC-Co coating structure and adhesion on a copper substrate during high velocity oxygen-fuel (HVOF) spraying. Two types of substrates are considered: smooth (polished) and rough (grit blasted). Variations of solidification time, solidification velocity, thermal gradient, and cooling velocity in the coating and substrate interfacial region are studied. Development of the amorphous and crystalline structures in the coating and of the crystalline structure in the substrate interfacial region is discussed. Behavior of the crystal size and intercrystalline distance with respect to the thermal spray parameters and morphology of the substrate surface is analyzed. Optimal conditions for the formation of fine and dense crystalline structure are determined. Structural changes in the solid state of the substrate occurring because of heating and rapid cooling are considered. Mechanical and thermal mechanisms of development of the substrate-coating adhesion are discussed. Results obtained agree well with experimental data.     

铝基厚梯度热障涂层制备工艺及性能研究

采用单一等离子喷涂法和超音速火焰喷涂与等离子喷涂复合喷涂法在铝质LY12基体上制蔷了总厚度分别为0.6mm、10mm和2．0mm的梯度热障涂层，并对基体界面处的涂层显微结构特征进行了金相组织和扫描电镜(SEM)观察，试验结果表明，采用复合喷涂法制备的2mm厚梯度热障涂层其抗拉强度得到显著提高．达到36MPa．其主要原因是采甩JP5000喷涂粘结底层很好的改善了铝基体与游层之间的界面结合强度。基体界面结合强度高低与两者的紧密接触程度有关。     

Pre-/During-/Post-Laser Processes to Enhance the Adhesion and Mechanical Properties of Thermal-Sprayed Coatings with a Reduced Environmental Impact

Lasers have been used to improve the ultimate performance of thermal spray coatings for specific applications, but the full potential of additional laser treatments must be further explored. Laser treatments (auxiliary processes) can be applied before, during or after thermal spraying (main process), leading to a wide range of coating improvements (microstructure, adhesion, etc.). The aim of this review is to introduce the most significant laser treatments for thermal spray applications. The potential improvements for thermal spray coatings are illustrated by a selection of representative research cases. Laser pretreatments (ablation and texturing) promote coating/substrate adhesion and are suitable to prepare the surface of sensitive substrates such as aluminum, titanium, or magnesium alloys. The use of these techniques, which leads to several benefits such as surfaces free of grit-particle inclusions, directly improves the quality of coatings. Laser treatments applied simultaneously during the spraying process deeply modify the coatings microstructure. These hybrid technologies allow in situ laser melting of coatings, resulting in improved mechanical properties and enhanced wear and corrosion behaviors. Finally, laser posttreatments can improve coatings density and adhesion, and also induce phase transformations and structure refinement. As a summary, laser treatments seem particularly promising for improving the thermal spray coating microstructure and the coating/substrate adhesion. In addition, they offer a more environmentally friendly alternative to the conventional surface preparation treatments.