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.     

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.     

A novel dual nickel coating on AZ91D magnesium alloy

Magnesium alloys covered with metal coating display excellent corrosion resistance,wear resistance,conductivity and electromagnetic shielding properties.The electroless plating Ni-P as bottom layer following the electroplating nickel as surface layer on AZ91D magnesium alloy was investigated.The coating surface morphology was observed with SEM and the structure was analyzed with XRD.Electrochemical tests and salt spray tests were carried out to study the corrosion resistance.The experimental results indi...     

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.     

Corrosion performance of laser-remelted Al-Si coating on magnesium alloy AZ91D

Laser remelting was applied to plasma-sprayed Al-Si coating on magnesium alloy AZ91D to improve corrosion performance. Both salt spray testing and potentiodynamic polarization measurement in 3.5% NaCl solution indicated that laser-remelted Al-Si coating acquired better corrosion resistance than AZ91D and plasma-sprayed Al-Si coating. The decreasing order of the corrosion rates are AZ91D base metal, sprayed Al-Si coating and laser-remelted Al-Si coating. The fine Al-Si eutectic matrix in the laser-remelted microstructure contributed to the improved corrosion performance relative to the AZ91D and the plasma-sprayed coating. The predominant corrosion mechanisms in AZ91D, plasma-sprayed coating and laser-remelted coating are intra-granular corrosion, crevice corrosion and the combined pitting and galvanic corrosion, respectively.     

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.     

热喷涂基体表面前处理技术的研究进展

热喷涂涂层与基体机械咬合的结合机理决定了基体表面前处理是热喷涂涂层中非常重要的处理工艺。文中概述了当前广泛应用的喷砂处理的工艺特点,指出砂粒易在基体表面镶嵌和对基体造成损伤是喷砂工艺的主要缺点,讨论了喷砂对高温合金单晶材料和超高强钢疲劳性能的影响,研究了软质基体表面超音速火焰喷涂WC涂层的免喷砂工艺。同时介绍了近年来其他热喷涂基体表面前处理方面的研究热点,包括高压水射流处理技术、机械粗化技术以及激光表面前处理,并重点阐述了其基本原理、特点及应用情况。     

Deposition and characterization of flame-sprayed aluminum on cured glass and basalt fiber-reinforced epoxy tubes

An oxy-acetylene flame spray torch was used to deposit thin layers of aluminum onto cured glass and basalt fiber-reinforced epoxy tubes. The composite specimens were fabricated by filament winding. Surface coatings embedded in composite laminates were produced. The composite substrates were grit blasted to promote adhesion of the molten aluminum particles. It was found that adhesion increased significantly when the composite substrate was lightly grit blasted, with no adhesion on smooth composite surfaces. The number of passes of the flame spray torch was varied to change the coating thickness and uniformity over the substrate. The electrical resistance of the coatings was measured to assess the suitability of a coating as a conductor. It was found that uniform, electrically conductive coatings were produced with a minimum of two torch passes. Optical images were captured to characterize the coating microstructure and thickness. This investigation did not reveal any visible evidence of damage to the composite substrate. To assess possible degradation effects from the grit blasting and flame spraying processes, the tube specimens were subjected to mechanical testing by applying internal pressurization with hydraulic oil. The tests indicated that the grit blasting and flame spraying processes must be carefully executed to mitigate degradation of the strength of the composite material substrate.     

AZ91D镁合金表面转化膜腐蚀及防护涂层性能研究

采用高锰酸盐、钼酸盐、锡酸盐转化液分别对AZ91D镁合金进行表面化学转化,得到三种不同的化学转化膜。分别通过SEM、EDS和全浸试验研究不同转化膜的表面微观形貌、成分和腐蚀率,通过划格法和中性盐雾试验法研究转化膜外部有机涂层的附着性能和耐蚀性能。结果表明,高锰酸盐和钼酸盐转化膜表面具有大量微细裂纹,锡酸盐转化膜表面呈鱼鳞状,均为后续涂装提供了具有一定粗糙度的表面。锡酸盐转化膜的耐蚀性最好,高锰酸盐转化后并涂层的附着力和耐蚀性能最好。     

AZ91D镁合金的化学镀镍

使用硫酸镍作为化学镀镍磷镀液的主盐,直接在AZ91D镁合金基体上化学镀镍.使用扫描电镜和X射线衍射技术分析镀层的表面形貌和组织.化学镀镍磷镀层是致密的,无明显缺陷,其磷含量约为372 mass%.化学镀镍磷镀层的显微硬度值约为660 VHN,化学镀镍的沉积速度为23 μm/h.前处理过程中的酸洗步骤使镁合金基体产生粗糙的表面,从而使镀层和基体之间起到了互锁的作用,增加了镀层的结合力.     

Role of Powder Granulometry and Substrate Topography in Adhesion Strength of Thermal Spray Coatings

APS coating is deposited with different treated surfaces to understand the effects of surface topography and particle sizes on adhesion bond strength. Grit blasting and laser surface texturing have been used to create a controlled roughness and controlled surface topography, respectively. Coating adhesion is mainly controlled by a mechanical interlocking mechanism. Fully melted Ni-Al powder fills the respected target surface with high-speed radial flow. Pores around central flattening splat are usually seen due to splash effects. Laser surface texturing has been used to study near interface coating depending on the target shape and in-contact area. Pull-off test results have revealed predominant correlation with powder, surface topography, and adhesion bond strength. Adhesion bond strength is linked to the in-contact area. So, coating adhesion might be optimized with powder granulometry. Pores near the interface would be localized zones for crack initiations and propagations. A mixed-mode failure has been reported for sharp interface (interface and inter-splats cracks) due to crack kicking out phenomena. Coating toughness near the interface is a key issue to maximize adhesion bond strength. Volume particles and topography parameters have been proposed to enhance adhesion bond strength for thermal spray process for small and large in-contact area.     

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.     

AZ91C镁合金表面Ti-TiN复合镀层的阴极多弧离子镀法制备与表征

采用阴极多弧离子镀膜技术,在AZ91C镁合金基底上首次成功镀制了结合力强的以Ti为过渡层的TiN复合膜层,并利用高分辨扫描电子显微镜(SEM)、X射线能谱仪(EDS)、显微划痕测试等技术对复合膜层的形貌、组织结构及性能进行分析研究。结果表明,采用多弧离子镀膜工艺,能在经过恰当预处理的镁合金基底上制备出性能良好的TiN膜,膜层均匀、致密,膜基结合力达130 mN以上,复合硬度达500 HV左右(AZ91镁合金基底为125 HV)。此外中性盐雾强化实验表明,经该方法处理后的镁合金在ASTM-B117标准测试条件下,腐蚀速率明显降低,经过200 h后,表面无明显腐蚀现象。真空多弧离子镀膜技术有望在镁合金表面防护领域得到应用。     

Investigation of particle flattening behaviour and bonding mechanisms of APS sprayed coatings on magnesium alloys

Magnesium alloys are promising alternatives to other lightweight materials due to their high specific strength and stiffness. However, the use of magnesium alloys is limited by their poor wear behaviour and low corrosion resistance for many industrial applications. The thermal spray technology offers a wide range of possibilities to improve the surface properties of Mg-based components. In this study, three different coating materials, namely Al, NiAl5 and Al₂O₃, were applied on AZ91 and AE42 substrates using the atmospheric plasma spray technology. The investigation was focused on the bonding strength of the coatings and the related bonding mechanisms. For a better understanding of the bonding mechanisms, the flattening behaviour of the spray particles was investigated in correlation with the substrate pre-heating temperature. It was found that NiAl5-particles could well melt the substrate at the surface and deformed it locally; Al-particles did the same but to a lower extent. The dominating bonding mechanism for NiAl5-coatings could be attributed to a metallurgical bonding. For Al-coatings, this mechanism played a more important role once the substrate pre-heating temperature was increased. Al₂O₃ particles in contrast, were less able to deform the substrate in spite of their higher thermal load and the mechanical anchoring remained the main bonding mechanism. The thermo physical properties of the Mg substrate showed also to have an influence on the adhesion of the coatings.     

The corrosion performance of anodised magnesium alloys

The corrosion performance of anodised magnesium and its alloys, such as commercial purity magnesium (CP-Mg) and high-purity magnesium (HP-Mg) ingots, magnesium alloy ingots of MEZ, ZE41, AM60 and AZ91D and diecast AM60 (AM60-DC) and AZ91D (AZ91D-DC) plates, was evaluated by salt spray and salt immersion testing. The corrosion resistance was in the sequential order: AZ91D ≈ AM60 ≈ MEZ ? AZ91D-DC ? AM60-DC > HP-Mg > ZE41 > CP-Mg. It was concluded the corrosion resistance of an anodised magnesium alloy was determined by the corrosion performance of the substrate alloy due to the porous coating formed on the substrate alloy acting as a simple corrosion barrier.     

Effects of low temperature thermal treatment on zinc and/or tin plated coatings of AZ91D magnesium alloy

A new method was investigated to obtain composite coatings on the AZ91D magnesium alloy by electrodeposition and low temperature thermal treatment. Zinc and tin were introduced to AZ91D Mg alloy surface by electroplating firstly. And a succedent thermal treatment was carried out at 190 ± 10 °C for 12 h. The surface and cross-section morphologies of the plated coatings with and without thermal treatment were studied by scanning electron microscopy (SEM). And the microstructure was determined by X-ray diffraction (XRD). The results reveal that it was difficult to obtain good adhesion plated Sn coating but easy to get well-adherent plated Zn coating. And the thermal treatment promoted the formation of Mg₂Sn in the plated Sn coating and the recrystallization in the plated Zn coating. The plated double Zn-Sn coating owned good adhesion and uniform surface. Furthermore, when the plated double Zn-Sn coating was treated at 190 ± 10 °C for 12 h, a three-layer structure coating was formed due to the diffusion of tin. The results of the anodic polarization behaviors in 5 wt.% NaCl solution show that the three-layer structure coating could provide better protection for AZ91D substrate than the plated Zn-Sn coating.     

AZ91D镁合金电偶腐蚀的扫描Kelvin探针研究

利用扫描Kelvin探针技术(SKP)研究AZ91D镁合金与316L不锈钢偶接件在盐雾试验中电偶腐蚀规律。通过在中性盐雾试验不同周期的表面腐蚀形貌的观察和伏打电位分布图的测量结果分析表明,AZ91D镁合金电偶腐蚀效应与偶接阴阳极的伏打电位差密切相关,AZ91D镁合金与316L不锈钢偶接件存在较大的电位差(约为–1.28V),其电偶腐蚀效应非常显著。在盐雾试验初始阶段,腐蚀主要发生在偶接界面AZ91D镁合金一侧,该腐蚀区域的伏打电位增加幅度较大,而316L不锈钢受到保护没有发生明显腐蚀。随着盐雾试验时间的延长,AZ91D镁合金腐蚀加快,腐蚀产物覆盖区域不断扩大,24h盐雾试验后,偶接件的平均伏打单位差由原始的–1.29V增加到–1.53V,AZ91D镁合金的电偶腐蚀效应加大。由于AZ91D镁合金在盐雾中生成的腐蚀产物对基体具有一定的保护作用,当腐蚀产物不断增加并覆盖表面,偶接件的电位差减小导致AZ91D镁合金的电偶腐蚀效应降低。     

Microstructure development in laser forming of zirconium coatings on AZ91D magnesium alloy substrates

Zr-coatings were fabricated on AZ91D magnesium alloy substrates using laser forming by means of a blow powder technique. Multi-passes were required to form a pure Zr top coating layer on the AZ91D Mg substrate. The microstructure and phases of the coating were studied using XRD, EDS and SEM. The alloy element Al in AZ91 alloy was found to be important for the formation of a metallurgically bonded interface between the Zr coating and the matrix material. The coating can be classified as a three-layer structure with a pure Zr layer at the top surface. The development of the coating microstructure is explained with the aid of a schematic solidification scheme.     

Electrodeposition of zinc on AZ91D magnesium alloy pre‐treated by stannate conversion coatings

A stannate chemical conversion process followed by an activation procedure was employed as the pre‐treatment process for AZ91D magnesium alloy substrate. Zn was electroplated onto the pre‐treated AZ91D magnesium alloy surface from pyrophosphate bath to improve the corrosion resistance and the solderability. The surface morphologies of conversion coating and zinc coating were examined with scanning electron microscope (SEM). The phase composition of conversion coating was investigated by X‐ray diffraction (XRD). The electrochemical corrosion behavior of the coatings in the corrosive solution was investigated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The experimental results showed that the activated stannate chemical conversion coating provided a suitable interface between zinc coating and the AZ91D magnesium alloy substrate. The corrosion resistance of the AZ91D substrate was improved by the zinc coating.     

Influence of Grit Blasting on the Roughness and the Bond Strength of Detonation Sprayed Coating

The process of roughening the surfaces by grit blasting prior to coating them using thermal spray techniques is very important to obtain consistently high tensile bond strength between the coating and the substrate. The available literature on the influence of grit-blasting parameters in the case of detonation spray or HVOF coatings is quite limited. The present study aims to study the influence of grit-blasting pressure and alumina grit size on the roughening of the mild steel substrate, the resulting effect on the roughness of Cu, Al₂O₃, and WC-12Co coatings deposited by detonation spray coating and also on the tensile bond strengths of these coatings. Toward the above purpose, the velocity of the alumina grits have been experimentally measured using a high-speed imaging system and the tensile bond strength of the coatings have been experimentally obtained using the pin type test. The results from the above experiments point to the importance of not only the roughness of the grit-blasted mild steel substrate but also the roughness of the coatings subsequently deposited in determining the magnitude of the bond strength.