Adhesion Strength of HVOF Sprayed IN718 Coatings

The adhesion strength of high-velocity oxyfuel thermally sprayed coatings is of prime importance when thick coatings are to be sprayed in repair applications. In this study, relationships between process parameters, particle in-flight characteristics, residual stresses, and adhesion strength were explored. The most important process parameters that influence HVOF sprayed IN718 coating adhesion strength on IN718 substrate material were identified. Residual stress distributions were determined using the modified layer removal method, and adhesion strength was measured using an in-house-developed tensile test. Relationships between process parameters, particle in-flight characteristics, coating microstructure, and adhesion strength were established. Particle temperature, particle velocity, substrate preparation, and deposition temperature were identified as critical parameters to attain high adhesion strength. Controlling these parameters can significantly improve the adhesion strength, thus enabling thick coatings to be sprayed for repair applications.     

Relationships Between Process Parameters, Microstructure, and Adhesion Strength of HVOF Sprayed IN718 Coatings

Fundamental understanding of relationships between process parameters, particle in-flight characteristics, and adhesion strength of HVOF sprayed coatings is important to achieve the high coating adhesion that is needed in aeronautic repair applications. In this study, statistical Design of Experiments (DoE) was used to identify the most important process parameters that influence adhesion strength of IN718 coatings sprayed on IN718 substrates. Special attention was given to the parameters combustion ratio, total gas mass flow, stand-off distance and external cooling, since these parameters were assumed to have a significant influence on particle temperature and velocity. Relationships between these parameters and coating microstructure were evaluated to fundamentally understand the relationships between process parameters and adhesion strength.     

Bending behavior of HVOF produced WC-17Co coating: Investigated by acoustic emission

A four-point bend test using acoustic emission (AE) was used to compare coating properties under mechanical solicitation, mainly the toughness and spalling behavior. Coatings are made from the same material; Sulzer-Metco (Westbury, NY) 2005NS (WC-17Co) sprayed with an HVOF gun with different spray parameters. Coatings deposited on thin rectangular substrates were first bent in tension then in compression. AE features like the event number, energy per event, and cumulative energy were used to assess the damages in the coatings. The results are analyzed in relation to the coating microstructure.     

Effect of particle state on the adhesive strength of HVOF sprayed metallic coating

NiCrBSi and Ni-50Cr coatings were deposited using the high velocity oxygen fuel (HVOF) spray process under different spray parameters with two powders of different sizes to clarify the influence of the melting state of spray particles on the adhesive strength of the coating. The adhesive strength of the coating was estimated according to the American Society for Testing and Materials (ASTM) C633-79. The melting state of the spray droplet was examined from the coating microstructure. It was found that the melting state of spray particles had a significant effect on the adhesive strength of HVOF sprayed Ni-based coatings. The significant melting of the spray particle did not contribute to the increase in the adhesion of HVOF metallic coatings. On the other hand, the deposition of a partially melted large particle contributed to the substantial improvement of adhesive strength of the HVOF coating. The subsequent coating presented a dense microstructure and yielded an adhesive strength of more than 76 MPa, which was double that of the coating deposited with completely molten particles. It can be suggested that the good melting of the spray particle is mainly related to the mechanical interlocking effect, which reaches the limited and approximately defined adhesive strength up to 40–50 MPa.     

Investigation on Interface Structure and Wear-resistant Properties of HVOF Sprayed Carbides Coating onto Copper Substrate

SURFACE TREATMENT has been widely used allover the world as a way of improving materials'properties and prolonging its service life.High velocityoxy-fuel(HVOF)spray is a new process of surfacetreatment developed in recent20years.Because ofgood bond strength and high hardness of HVOFsprayed coating,the applications of HVOF sprayincreasingly take place of the traditional process,suchas electro-plate,thermal spray,etc.in the field ofmetallurgy,manufacture,mining and so on[1,2].Because …     

镁合金表面冷喷涂纳米WC-17Co涂层及其性能

采用冷喷涂和超音速火焰喷涂(HVOF)在AZ80镁合金表面制备了纳米WC-17Co涂层。利用SEM分析了原始粉末形貌、喷涂粒子沉积行为及涂层显微结构,并采用球盘式摩擦磨损实验机考察了涂层的摩擦磨损性能。结果表明:采用冷喷涂工艺可在AZ80镁合金基体上制备出高质量的WC-17Co涂层,涂层的显微硬度为(1 380±82)HV,磨损率为9.1×10-7 mm3/Nm,其耐磨性较HVOF制备的WC-17Co涂层提高了1倍,较镁合金基材提高了3个数量级。研究表明,冷喷涂WC-17Co涂层在不对镁合金基体产生热影响的情况下,可以显著提高镁合金的表面性能,是一种新型镁合金表面强化工艺。     

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.     

Processing optimization,surface properties and wear behavior of HVOF spraying WC–CrC–Ni coating

In this work, the optimal coating process (OCP) designed by Taguchi program for high velocity oxy-fuel (HVOF) thermal spraying WC–CrC–Ni powder on Inconel 718 substrate (IN 718) is obtained by optimizing hardness (38 FMR oxygen flow rate, 53 FMR hydrogen flow rate, 25 g/min powder feed rate and 7 in. spray distance). Oxygen flow rate affects hardness mostly. The surface properties such as microstructure, crystalline phase, hardness, and porosity of WC–CrC–Ni coating have been investigated. The phase of coating has been changed during the OCP spraying because a portion of carbides, such as WC, Cr₇C₃, Ni₃C decomposes to W₂C, Cr, Ni and free carbon. Hardness (1150 ± 50 Hv) and porosity (1.2 ± 0.2%) of the OCP coating have been improved by optimization. The friction and wear behaviors of the WC–CrC–Ni coating, electrolytic hard chrome (EHC) plating and IN 718 have been studied comparatively. The lubrication due to free carbon and metal oxide debris results in a decrease of friction coefficients of the WC–CrC–Ni, compared to EHC and IN 718 at both 25 and 450 °C. It is concluded that HVOF WC–CrC–Ni coating performs more excellent anti-wear than others at both temperatures.     

Adhesion evaluation of multilayered based WC-Co-Cr thermally sprayed coatings

This paper describes the adhesion evaluation of different interlayers such as Co-Cr, Ni-Cr 80-20 HVOF (High Velocity Oxy-Fuel) thermally sprayed coatings and Ni-plating between the cermet based WC-Co-Cr coatings. Three adhesion measurement methods for these different multilayered based thermal spray coatings, namely tensile adhesive strength (according to EN 582), interfacial indentation and solid impact tests were conducted. The distinguished coating properties include: i) the adhesive strength, ii) the interfacial toughness, iii) the depth of impact. The metallographic and experimental results show that the electrochemically deposited interlayer Ni-plating provides the highest adhesion to cermet coating within the multilayered structured coatings. This is not only due to the chemical affinity between the Ni-plating and the cermet coating, but also to its homogeneous microstructure, since the electrochemically deposition does not provide splat formation.     

The Influence of Spraying Angle on Properties of HVOF Sprayed Hardmetal Coatings

The spraying angle is one of the deposition parameters that influence the quality of thermally sprayed coatings. In theory, decreasing the spraying angle results in lower process deposition efficiency, whereas the porosity of coatings increases, becoming a cause of poorer microstructure and mechanical properties. In this study, the dependence of microstructure together with the basic mechanical properties and wear of WC-Co and Cr₃C₂-NiCr high-velocity oxyfuel (HVOF) sprayed coatings on the spraying angle was investigated. For each coating, the maximum spraying angle was determined that can be used without significantly decreasing coating quality. Based on the changes in properties of coatings and requirements for the process deposition efficiency, a maximum 30° diversion from the normal spray direction is recommended for WC-Co and 15° diversion for Cr₃C₂-NiCr coatings.     

热处理对选择性激光熔炼IN718显微组织和力学性能的影响

选择性激光熔炼(SLM)建立在激光熔覆/沉积基础上,能够由粉末直接制备或修复近成形高性能部件。选择性激光熔炼部件优异的力学性能是保证其用于航空发动机产品的先决条件。镍基高温合金IN718广泛用于制备航空发动机中的高性能部件。在过去的研究中,利用预合金化IN718合金粉末,通过选择性激光熔炼制备出增材制造部件。通过优化激光沉积过程试验参数,以最大限度地降低气孔率。对沉积态、直接时效态、固溶时效态、均匀化后固溶时效态四种状态激光沉积IN718合金的显微组织和力学性能进行了对比分析。拉伸试验结果显示,直接时效态合金强度最高,均匀化后固溶时效态合金塑性最好。综合考虑三种热处理状态的室温和高温拉伸试验结果,均匀化后固溶时效态试样不仅具有优于锻态AMS标准的强度,而且有很好的塑性。因此,选择均匀化后固溶时效处理作为选择性激光熔炼IN718合金的热处理方式。考察了该种热处理状态合金的650_oC/700MPa和725MPa持久性能和455_oC低周疲劳性能,并与锻态IN718进行了对比。     

微束等离子喷涂制备羟基磷灰石涂层微观结构的特征研究

目的 通过与大气等离子喷涂和超音速火焰啧涂的对比,研究微束等离子喷涂制备的羟基磷灰石涂层的微观组织特点.方法 以高结晶度的羟基磷灰石粉末为原料,采用三种不同的喷涂方法(微束等离子喷涂、大气等离子喷涂和超音速火焰喷涂),在Ti-6Al-4V基体上制备羟基磷灰石(HA)涂层.利用冷场发射扫描电子显微镜和X射线衍射仪,对三种涂层的形貌、相组成和择优取向进行分析.结果 与大气等离子喷涂及超音速火焰喷涂制备的涂层相比,应用该设备制备的羟基磷灰石涂层表面平整致密,无大量的气孔存在;涂层截面呈典型的层状结构,在近三分之一表面处观察到柱状晶;涂层中仅有少量的非晶相及分解相,结晶度高达90％以上.这些特征均有利于羟基磷灰石涂层在体液环境中的稳定性.结论 比较三种喷涂方法,采用微束等离子喷涂制备的羟基磷灰石涂层致密,结晶度高,杂相少,且存在择优取向的柱状晶.     

Increased Lifetime for Biomass and Waste to Energy Power Plant Boilers with HVOF Coatings: High Temperature Corrosion Testing Under Chlorine-Containing Molten Salt

Heat exchanger surfaces of waste to energy and biomass power plant boilers experience often severe corrosion due to very aggressive components in the used fuels. High velocity oxy-fuel (HVOF) coatings offer excellent protection for boiler tubes against high temperature corrosion due to their high density and good adherence to the substrate material. Several thermal spray coatings with high chromium content were sprayed with HVOF technique. Their mechanical properties and high temperature corrosion resistance were tested and analyzed. The coating materials included NiCr, IN625, Ni-21Cr-10W-9Mo-4Cu, and iron-based partly amorphous alloy SHS9172 (Fe-25Cr-15W-12Nb-6Mo). High temperature corrosion testing was performed in NaCl-KCl-Na₂SO₄ salt with controlled H₂O atmosphere at 575 and 625 °C. The corrosion test results of the coatings were compared to corrosion resistance of tube materials (X20, Alloy 263 and Sanicro 25).     

Indentation size effect on mechanical properties of HVOF sprayed WC based cermet coatings for a roller cylinder

The present paper concerns the determination of mechanical properties such as hardness, elastic modulus and yield strength of WC-based cermet coatings for a roller cylinder. With this regard, Co and Ni containing WC-based coatings were sprayed on Ni-Al deposited 316 L stainless steel substrates by using High Velocity Oxygen Fuel (HVOF) technique. These HVOF sprayed coatings were analyzed by Scanning Electron Microscopy (SEM) with an Energy Dispersive Spectroscopy (EDS) system attachment. Mechanical properties of the coatings were examined by Shimadzu Dynamic Ultra-micro hardness test machine in order to determine the Young's modulus through load-unload sensing analysis. In addition to mechanical investigation, hardness-depth and hardness-force curves of WC-based coatings were investigated. It was found that both of these characteristics exhibit significant peak load dependency. Experimental indentation studies were carried out to determine load-unload curves of WC-Co and WC-Ni based coatings under 300 mN, 350 mN, 400 mN and 450 mN applied peak loads. Hardness and Young's modulus of WC-based coatings were calculated from experimental indentation test data of samples. It has been observed that the hardness and Young's modulus of the coating depends on the contact area and indentation size. The originality of this study is to determine the indentation size effect and contact area variations on mechanical properties of HVOF sprayed WC-based coatings.     

燃料类型及喷涂参数对HVOF喷涂WC10Co4Cr涂层的组织及力学性能的影响

超音速火焰喷涂（HVOF）制备的WC基金属陶瓷涂层广泛应用于金属构件的磨损、腐蚀及空蚀防护。分别采用氢气燃料及煤油液体燃料HVOF喷涂设备分别在9种不同的工艺条件下制备了WC10Co4Cr涂层,研究了燃料类型对涂层的组织、残余应力及力学性能的影响规律。在两种燃料HVOF工艺各自优化的喷涂参数条件下,通过对基体曲率的原位监测对比测试了涂层中的平均残余应力;利用显微维氏硬度、压痕法（断裂韧性）和球盘摩擦磨损对比研究了涂层的力学性能。结果表明：液体燃料（LF）HVOF焰流中粒子的温度更低,速度更高。LF-HVOF喷涂的WC10Co4Cr涂层内的残余压应力更高且涂层致密度更高,而气体燃料（GF）HVOF喷涂的WC10Co4Cr涂层内为残余拉应力。LF-HVOF涂层（1280 HV_0.3, 7.3 MPa·m^0.5）比GF-HVOF涂层（1032 HV_0.3, 4.5 MPa·m^0.5）具有更高的硬度和断裂韧性,LF-HVOF涂层的耐磨性约为GF-HVOF涂层的1.7倍。     

超音速火焰喷涂Cr3C2-NiCr涂层磨粒磨损行为

采用橡胶轮磨损实验机，对不同工艺条件下三种类型粉末制备的HOVF Cr3C2-25％NiCr涂层进行了磨粒磨损实验，发现该涂层的磨损失重量与磨程基本呈现线性关系，磨损率远低于低碳钢。氧气流量，燃气流量适中的条件下制备的涂层磨损率较低，用团聚致密化工艺制备的粉末沉积的涂层耐磨粒磨损性能较好，涂层的磨损机制主要为先期的粘结相优先切削和随后的碳化物剥落，其中碳化物的剥落对磨损过程起制约作用。     

Microstructural characterization and abrasive wear performance of HVOF sprayed Cr3C2–NiCr coating

Cr₃C₂–NiCr coatings were deposited by high velocity oxy-fuel (HVOF) process with different spray parameters to examine dominant microstructural factors in abrasive wear of the coatings. The microstructure of the HVOF sprayed Cr₃C₂–NiCr coatings was characterized by scanning electron microscopy and transmission electron microscopy (TEM). The apparent average size and volume fraction of carbide particles in the coatings were estimated through a quantitative imaging analysis. The formation of carbide phases in the coating was discussed based on the TEM observation results. The abrasive wear behavior of the coating was evaluated by the dry rubber wheel abrasive wear test and the wear mechanisms were elucidated. Influences of apparent size and volume fraction of carbide particles on the abrasive wear weight loss were examined through correlating the proposed relation with the experimental results. Results showed that Cr₃C₂ particle size was significantly reduced after the spraying and Cr₇C₃ carbide was present around Cr₃C₂ particles, and Cr₂₃C₆ carbide was dispersed in NiCr alloy matrix with a nano-crystalline structure. The three carbides were formed in the coating through different mechanisms. The removal of carbide particles in the coating was mainly responsible for the abrasive wear of the coating. The content and particle size of the Cr₃C₂ carbides were the two key factors controlling the abrasive wear of the HVOF sprayed Cr₃C₂–NiCr coatings.     

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.     

Properties of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings

TiO₂ coatings were manufactured by the High Velocity Suspension Flame Spraying (HVSFS) technique using a nanopowder suspension. Their microstructure, nanohardness, tribological properties and photocatalytic activity were studied and compared to conventional atmospheric plasma sprayed (APS) and HVOF-sprayed TiO₂ coatings manufactured using commercially available feedstock. The HVSFS process leaves a fairly large freedom to adjust coating properties (thickness, porosity, anatase content, hardness, etc…) according to the desired objective. Layers with higher anatase content and higher porosity can be produced to achieve higher photocatalytic efficiency, better than conventional APS and HVOF TiO₂. Alternatively, dense protective layers can be deposited, possessing lower porosity and pore interconnectivity and better wear resistance than as-deposited APS and HVOF layers. In all cases, HVSFS-deposited layers are thinner (20 µm-60 µm) than those which can be obtained by conventional spraying processes.     

Characterization of thermal sprayed nanostructured WC-Co coatings derived from nanocrystalline WC-18wt.%Co powders

Nanostructured WC-Co coatings were synthesized using high velocity oxygen fuel (HVOF) thermal spray. The nanocrystalline feedstock powder with a nominal composition of WC-18 wt.%Co was prepared using the novel integrated mechanical and thermal activation (IMTA) process. The effects of HVOF thermal spray conditions and powder characteristics on the microstructure and mechanical properties of the as-sprayed WC-Co coatings were studied. It was found that the ratio of oxygen-to-hydrogen flow rate (ROHFR) and the starting powder microstructures had strong effects on decarburization of the nano-coatings. Decarburization was significantly suppressed at low ROHFR and with the presence of free carbon in the powder. The level of porosity in the coatings was correlated with the powder microstructure and spray process conditions. The coating sprayed at ROHFR=0.5 exhibited the highest microhardness value (HV_300g=1077), which is comparable to that of conventional coarse-grained coatings.