Microstructures of Metallic NiCrBSi Coatings Manufactured via Hybrid Plasma Spray and In Situ Laser Remelting Process

This paper deals with coating alternatives to hard chromium plating. Thermal spraying is already used in industry, but results are not always satisfactory for reasons of porosity and microstructures. In this study, atmospheric plasma spraying (APS) and in situ laser irradiation by diode laser processes were combined to modify the structural characteristics of thick NiCrBSi alloy layers. The microstructure evolution was studied, and results show that in situ laser remelting induced the growth of a dendritic structure, which strongly decreased the porosity of as-sprayed coatings and increased the adhesion on the substrate. Moreover, no phase transition after laser treatment was observed. Lastly, a mechanical investigation demonstrated that the combination between plasma spray and in situ melting with a diode laser could result in very good mechanical properties. The increase of the laser incident power involved an increase of the mean contact pressure, along with coating hardness. The hybrid process appears to be a possible alternative to hard chromium plating, in order to protect mechanical parts, because of the improved mechanical properties of the NiCrBSi layer.     

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.     

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.     

激光表面重熔对热喷涂铝涂层微观结构及其阻氢性能的影响

采用Ｘ射线衍射仪、扫描电镜及热充氢等方法,研究了激光重熔对不锈钢表面热喷涂铝涂层的微观结构及其阻氢性能的影响。结果表明,激光重熔后涂层组织均匀、致密,主要由ＡｌＦ３（Ｎｉ,Ｃｒ）固溶体、ＣｒＦｅＮｉ奥氏体等相组成,而且涂层与基体形成了良好的冶金结合。此外,激光表面重熔后涂层的阻氢性能亦得到改善。     

时效对Fe基合金激光重熔层组织和性能的影响

利用场发射扫描电镜、透射电镜、X射线衍射仪和电化学工作站等,对比分析了Fe基合金原始热喷焊层、在500 ℃经不同时间时效处理后激光重熔层的组织和性能。结果表明,火焰喷涂后的热喷焊层组织较粗大,经激光重熔后组织明显细化,由较细小的树枝晶间均匀分布着层片状共晶组织构成;随时效时间的增加,重熔层中距离共晶组织较近的柱状晶球化,基体中Cr₇C₃、Cr₂₃C₆型碳化物进一步析出,弥散分布在枝晶间层片状共晶组织上;时效6 h所获得的激光重熔层组织硬度最大,可达507 HV0.5,随时效时间继续增加,硬度有所下降,但仍高于未经时效处理时的激光重熔层组织硬度;热喷焊层经激光重熔,耐腐蚀能力显著提高到最强,时效6 h的激光重熔层耐腐蚀性降低不明显,时效48 h的激光重熔层的耐腐蚀性则明显降低。     

不同参数下激光重熔Cr_3C_2-NiCr涂层冲蚀磨损性能研究

采用超音速火焰喷涂技术在低碳钢表而喷涂Cr_3C_2-NiCr涂层,然后在不同工艺参数下对其进行激光重熔处理.考察不同重熔参数下涂层的冲蚀性能.并与喷涂层对比.结果表明:在激光功率3.0 kW,扫描速率50mm/s的工艺参数下激光重熔.涂层的冲蚀失重最小.所形成的涂层表面均匀性较好;重熔层的致密度要优于喷涂层,重熔后涂层由机械结合转变为冶金结合;南于表面氧化层的存在和内部冶金结合的形成,重熔层硬度较喷涂层高.     

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.     

Microstructures and characterization of zirconia-yttria coatings formed in laser and hybrid spray process

Hybrid plasma spraying combined with yttrium-aluminum-garnet laser irradiation was studied to obtain optimum zirconia coatings for thermal barrier use. Zirconia coatings of approximately 150 μm thickness were formed on NiCrAlY bond coated steel substrates both by means of conventional plasma spraying and hybrid plasma spraying under a variety of conditions. Post-laser irradiation was also conducted on the plasma as-sprayed coating. The microstructure of each coating was studied and, for some representative coatings, thermal barrier properties were evaluated by hot erosion and hot oxidation tests. With hybrid spraying, performed under optimum conditions, it was found that a microstructure with appropriate partial densification and without connected porosity was formed and that cracks, which are generally produced in the post-laser irradiation treatment, were completely inhibited. In addition, hybrid spraying formed a smooth coating surface. These microstructural changes resulted in improved coating properties with regard to hardness, high temperature erosion resistance, and oxidation resistance. This paper originally appeared in Thermal Spray: Meeting the Challenges of the 21st Century; Proceedings of the 15th International Thermal Spray Conference, C. Coddet, Ed., ASM International, Materials Park, OH, 1998. This proceedings paper has been extensively reviewed according to the editorial policy of the Journal of Thermal Spray Technology.     

钢结构热喷涂长效防腐蚀技术的研究与发展

综述了热喷涂技术在钢结构长效防腐蚀方面的研究与发展概况。对锌、铝及锌铝合金涂层的耐蚀机理和防护特点作了比较,从国内外对长效防腐蚀涂层的耐蚀性研究、热喷涂防腐蚀材料的开发、防腐蚀喷涂工艺等方面阐述了该项技术的发展,指出了热喷涂长效防腐蚀技术存在的问题并对其发展前景作了展望。     

冷喷涂Cu402F涂层激光重熔表面改性后摩擦学行为

目的研究激光重熔后冷喷涂Cu402F涂层在腐蚀介质中的摩擦学行为。方法采用冷喷涂技术在镍铝青铜9442合金上制备了厚度约为882.11μm的Cu402F涂层,并使用激光重熔技术对冷喷涂涂层进行表面改性。使用OM、SEM观察涂层截面与表面的微观形貌;使用XRD、X射线残余应力测试仪、多功能表面性能测试仪、多频直线往复磨蚀实验机,重点表征测试了涂层的组织、表面残余应力、摩擦学性能与磨蚀行为。结果激光重熔后的涂层分为表面重熔层、多孔的重熔过渡层以及冷喷涂遗传层。激光重熔前后,涂层的物相、残余应力均未发生较大变化。在20、50、100 N条件下,激光重熔态涂层的平均摩擦系数呈递增趋势,磨损率分别为1.01×10~(-2)、1.17×10~(-2)、1.34×10~(-2) mm~3/(N·m);由磨蚀实验可知,磨蚀是涂层表面产生钝化膜与钝化膜被破坏的此消彼长的过程,反映着摩擦与腐蚀的协同作用,涂层在磨蚀过程中,开路电位与摩擦系数呈正相关,说明激光重熔态涂层表面钝化膜产生的速度快于钝化膜被坏的速度。结论激光重熔保留了冷喷涂技术制备的Cu402F涂层的优点,同时激光重熔涂层上产生钝化膜的速度更快,有利于提高涂层的耐磨蚀性能。     

热喷涂涂层孔隙率的降低方法

简述了解决热喷涂涂层多孔率问题的几种途径：工艺改进、重熔处理、新热喷涂材料的开发和封孔处理。     

激光重熔等离子喷涂Al2O3–13 %TiO2涂层的组织结构

采用等离子喷涂方法制备Al2O3–13%TiO2涂层,对涂层进行激光重熔处理。利用电子扫描显微镜(SEM)观察涂层断口组织,金相截面组织和金相表面组织形貌,分析激光重熔处理后涂层的凝固过程。结果表明:激光重熔处理,使涂层由块状结构转变为平行排列,垂直于基体方向生长的柱状晶和柱状枝晶结构。由于金属基体温度低、散热快,使得陶瓷涂层上下温差大,诱发了陶瓷晶粒的定向生长,这是使陶瓷晶粒垂直于基体生长的主要原因。     

激光表面重熔对ZrO_2-8wt%Y_2O_3热障涂层组织与抗热震性能的影响

采用等离子喷涂设备在H13热作模具钢表面制备氧化钇部分稳定的氧化锆(ZrO2-8 wt%Y2O3)热障涂层,并用CO2横流激光器对热障涂层进行表面重熔处理,并采用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、热震试验等手段研究激光重熔前后热障涂层的微观结构及其抗热震性能的变化。结果表明,重熔前后涂层均由四方结构钇锆氧化物和立方相的氧化锆组成,重熔后涂层结晶度增加,晶粒有长大现象。激光重熔后涂层产生明显分层,表层组织孔隙和裂纹明显减少,裂纹呈网状且沿晶界分布,重熔涂层内部仍保持等离子喷涂典型结构。激光重熔后涂层孔隙率降低了67%,涂层的抗热震性能也显著提高。     

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.     

涂层结构对Cr2O3涂层组织和性能的影响

采用等离子喷涂技术制备了3种结构的Cr2O3陶瓷涂层，即双层涂层，3层涂层和5层梯度涂层。探讨了涂层结构对涂层组织、抗拉结合强度、抗热震性和耐磨损性能的影响。结果表明，在涂层总厚度相同的条件下，采用多层复合涂层可提高Cr2O3涂层的结合强度、耐磨性和抗热冲击性，其中，5层结构涂层的综合性能最佳。涂层微观组织观察和显微硬度测试结果发现，5层结构涂层从基体到陶瓷层，涂层成分逐渐变化，具有梯度材料的特征。试验表明采用等离子喷涂技术可以制备梯度涂层。     

镁合金AZ91HP表面激光重熔Al2O3涂层的组织及性能

采用等离子喷涂和激光重熔复合工艺在AZ91HP镁合金表面制备了Al2O3陶瓷涂层.结果表明,由于受激光作用区温度场分布、陶瓷材料热物性参数和涂层厚度等因素的综合影响,激光重熔Al2O3涂层呈现出明显的分层结构特征.依据组织结构不同,可将其分为:由α-Al2O3柱状晶构成的表面熔凝区、具有团絮状形貌特征的烧结区以及保持原喷涂态结构特征的残留等离子喷涂层.由于激光重熔陶瓷涂层表面单相α-Al2O3柱状晶的形成,使其硬度及耐磨、耐蚀性均明显优于等离子喷涂Al2O3涂层和原始镁合金.     

Properties of alumina-titania coatings prepared by laser-assisted air plasma spraying

Studies have shown that microstructures formed by post-laser remelting of air plasma sprayed coatings exhibit densification but also numerous macrocracks due to the rapid cooling and thermal stresses. In laser-assisted air plasma spraying (LAAPS) process, the laser beam interacts simultaneously with the plasma torch in order to increase the temperature of the coating and possibly remelt the coating at the surface. As a result, the microstructure is partially densified and macrocracks, which are generally produced in the post-laser irradiation treatment, may be inhibited. In this paper, LAAPS was performed to improve the hardness and wear resistance of Al₂O₃-13%TiO₂ coatings. These coatings prepared by air plasma spraying (APS) are widely used to protect components against abrasive wear at low temperatures. The coating microstructure was characterized by SEM and X-ray diffraction. The mechanical characterization was done by hardness measurements, erosive wear tests and abrasion wear tests. Results showed that laser assistance may improve the microstructural and mechanical properties. Phenomena involved in LAAPS of alumina-titania coatings are discussed in this paper.     

An investigation on the microstructure and oxidation behavior of laser remelted air plasma sprayed thermal barrier coatings

NiCrAlY bond-coat was coated on Inconel 718 substrate by air plasma spraying (APS) followed by APS ZrO₂-8 wt.%Y₂O₃ as top-coat. Using CO₂ laser of different energy densities, ceramic top-coat surface was remelted. Laser remelting with high energy density (4 J/mm²) produced a dense microstructure over the whole thickness of top-coat, while low energy density (0.67 J/mm²) laser remelting produced a ~ 50 μm thick dense layer on the top-coat surface. It was found that the volume fraction of monoclinic phase decreased from 9% in as-sprayed coating to 4% and 3% after laser remelting with high and low energy density respectively. After isothermal oxidation at 1200 °C for 200 h, the thickness of oxide layer (TGO) in the sample produced by low energy density laser remelting was ~ 5.6 μm, which was thinner than that of oxide layer in as-sprayed (~ 7.6 μm) and high energy density laser remelted (~ 7.5 μm) samples. A uniform and continuous oxide layer was found to develop on the bond-coat surface after low energy density laser remelting. Thicker oxide layer containing Cr₂O₃, NiO and spinel oxides was observed in both as-sprayed and high energy density laser remelted coatings. After cyclic oxidation at 1200 °C for 240 h, the weight gain per unit area of as-sprayed coating was similar to that of high energy density laser remelted coating while a significantly smaller weight gain was found in low energy density laser remelted coating.     

激光与氩弧重熔热喷涂涂层的表面性能

采用电弧喷涂方法制备了合金涂层,然后使用氩弧和激光重熔技术分别获得了具有冶金结合的耐磨涂层。对涂层进行了硬度试验、高温冲蚀磨损试验和显微组织试验,并进行了分析和比较。结果表明,熔覆层表面硬度高达1200HV,是20G钢的6～7倍,组织均匀,与基体之间冶金结合,耐高温冲蚀性能为20G钢的2.5～4倍。对两种熔覆技术进行了比较。     

等离子喷涂ZrO2层经激光再熔后的组织变化

本文研究了等离子喷涂ZrO_2陶瓷层的激光再熔化处理经激光再熔化处理的ZrO_2涂层致密、无孔隙,但熔化层有裂纹在ZrO_2中添加适量的SiO_2,降低了熔化层在冷却时的热应力,有效抑制裂纹的产生经激光再熔化处理后的陶瓷展和粘结层之间的结合为冶金结合;粘结尾和基材的冶金结合层厚度明显增加;随着激光功率的增加,陶瓷层、粘结层以及一定厚度的基材发生熔化,陶瓷层和粘结层之间产生Zr_6Fe_3O反应相