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.     

Effect of Atmospheric Plasma Spraying Power on Microstructure and Properties of WC-(W,Cr)2C-Ni Coatings

WC-(W,Cr)₂C-Ni coatings were prepared by atmospheric plasma spraying (APS) with different spraying powers. The effect of spraying power on microstructure, phase composition, hardness, fracture toughness, and oscillating dry friction and wear behaviors of the coatings were studied. Simultaneously, the microstructure and properties of the as-sprayed coatings were compared with those of WC-17Co coating prepared under the optimal spraying power. It was found that spraying power had significant effect on the molten degree of feedstock powder and phase composition as well as microstructure and properties of WC-(W,Cr)₂C-Ni coatings. WC-(W,Cr)₂C-Ni coating deposited at a moderate spraying power of 22.5?kW had the highest fracture toughness and the best wear resistance. WC-17Co coating obtained under the moderate spraying power had poor fracture toughness and wear resistance. Moreover, the four kinds of coatings were all dominated by subsurface cracking and removal of materials when sliding against Si₃N₄ ball under unlubricated conditions.     

Surface modification of atmospheric plasma sprayed cermet coatings by plasma transferred arc method

ABSTRACT

AISI 8620 steel substrates were coated with WC–Co and Cr₃C₂–NiCr by using the atmospheric plasma spraying (APS) method. Subsequently, surface melting of samples coated with APS was performed at different current values using the plasma transfer arc (PTA) method. Microstructure, microhardness, and wear properties of as-sprayed and surface-modified coatings were investigated. The microstructure of the APS-coated surface had some voids, cracks and nonhomogeneous areas. These defects were eliminated with the PTA surface modification process and microstructural properties of coatings were improved. The wear resistance of PTA modified coatings was also increased. The highest wear resistance and microhardness were obtained in WC–Co coating modified by PTA at a current of 80?A. The wear resistance of this coating was 8.5 times higher than that of the substrate. The coating hardness reached values as high as 980?HV_0,1 in this coating.     

Study of plasma- and detonation gun-sprayed alumina coatings using taguchi experimental design

Atmospheric plasma spraying (APS) is a most versatile thermal spray method for depositing alumina (Al₂O₃) coatings, and detonation gun (D-gun) spraying is an alternative thermal spray technology for depositing such coatings with extremely good wear characteristics. The present study is aimed at comparing the characteristics of Al₂O₃ coatings deposited using the above techniques by using Taguchi experimental design. Alumina coating experiments were conducted using a Taguchi fractional-factorial (L₈) design parametric study to optimize the spray process parameters for both APS and D-gun. The Taguchi design evaluated the effect of four APS and D-gun spray variables on the measured coating attributes. The coating qualities evaluated were surface roughness, porosity, microhardness, abrasion, and sliding wear. The results show that the coating quality is directly related to the corresponding coating microstructure, which is significantly influenced by the spray parameters employed. Though it is evident that the D-gun-sprayed coatings consistently exhibit dense and uniform microstructure, higher hardness, and superior tribological performance, the attainment of suitable plasma-sprayed coatings can be improved by employing the Taguchi analysis.     

Superior performance of high-velocity oxyfuel-sprayed nanostructured TiO<Subscript>2</Subscript> in comparison to air plasma-sprayed conventional Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-13TiO<Subscript>2</Subscript>

Air plasma-sprayed conventional alumina-titania (Al₂O₃-13wt.%TiO₂) coatings have been used for many years in the thermal spray industry for antiwear applications, mainly in the paper, printing, and textile industries. This work proposes an alternative to the traditional air plasma spraying of conventional aluminatitania by high-velocity oxyfuel (HVOF) spraying of nanostructured titania (TiO₂). The microstructure, porosity, hardness (HV 300 g), crack propagation resistance, abrasion behavior (ASTM G65), and wear scar characteristics of these two types of coatings were analyzed and compared. The HVOF-sprayed nanostructured titania coating is nearly pore-free and exhibits higher wear resistance when compared with the air plasma-sprayed conventional alumina-titania coating. The nanozones in the nanostructured coating act as crack arresters, enhancing its toughness. By comparing the wear scar of both coatings (via SEM, stereoscope microscopy, and roughness measurements), it is observed that the wear scar of the HVOF-sprayed nanostructured titania is very smooth, indicating plastic deformation characteristics, whereas the wear scar of the air plasma-sprayed alumina-titania coating is very rough and fractured. This is considered to be an indication of a superior machinability of the nanostructured coating.     

A comparative study of tribological behavior of plasma and D-gun sprayed coatings under different wear modes

In recent years, thermal sprayed protective coatings have gained widespread acceptance for a variety of industrial applications. A vast majority of these applications involve the use of thermal sprayed coatings to combat wear. While plasma spraying is the most versatile variant of all the thermal spray processes, the detonation gun (D-gun) coatings have been a novelty until recently because of their proprietary nature. The present study is aimed at comparing the tribological behavior of coatings deposited using the two above techniques by focusing on some popular coating materials that are widely adopted for wear resistant applications, namely, WC-12% Co, A1₂O₃, and Cr₃C₂-MCr. To enable a comprehensive comparison of the above indicated thermal spray techniques as well as coating materials, the deposited coatings were extensively characterized employing microstructural evaluation, microhardness measurements, and XRD analysis for phase constitution. The behavior of these coatings under different wear modes was also evaluated by determining their tribological performance when subjected to solid particle erosion tests, rubber wheel sand abrasion tests, and pin-on-disk sliding wear tests. The results from the above tests are discussed here. It is evident that the D-gun sprayed coatings consistently exhibit denser microstructures and higher hardness values than their plasma sprayed counterparts. The D-gun coatings are also found to unfailingly exhibit superior tribological performance superior to the corresponding plasma sprayed coatings in all wear tests. Among all the coating materials studied, D-gun sprayed WC-12%Co, in general, yields the best performance under different modes of wear, whereas plasma sprayed Al₂O₃ shows least wear resistance to every wear mode.     

Mechanical property of Fe-base metallic glass coating formed by gas tunnel type plasma spraying

Metallic glass has excellent functions such as high toughness and corrosion resistance. Therefore it is one of the most attractive materials, and many researchers have conducted various developmental research works. However, the metallic glass material is expensive and a composite material is preferred for the industrial application. Thermal spraying method is one of potential candidates to produce metallic glass composites. The gas tunnel type plasma system, which has high energy density and efficiency, is useful for smart plasma processing to obtain high quality ceramic coatings such as alumina (Al₂O₃) and zirconia (ZrO₂) coatings. Also, the gas tunnel type plasma spraying can produce metallic glass coatings. In this study, the Fe-base metallic glass coatings were formed on the stainless-steel substrate by the gas tunnel type plasma spraying, and the microstructure and mechanical property were investigated. The Fe-base metallic glass coatings of about 200 μm in thickness were dense with a Vickers hardness of about Hv = 1100 at plasma current of 300 A. The abrasive wear resistance of Fe-base metallic glass coating was higher than the SUS substrate.     

Improvement of wear resistance of plasma-sprayed molybdenum blend coatings

The wear resistance of plasma sprayed molybdenum blend coatings applicable to synchronizer rings or piston rings was investigated in this study. Four spray powders, one of which was pure molybdenum and the others blended powders of bronze and aluminum-silicon alloy powders mixed with molybdenum powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coatings showed that the phases formed during spraying were relatively homogeneously distributed in the molybdenum matrix. The wear test results revealed that the wear rate of all the coatings increased with increasing wear load and that the blended coatings exhibited better wear resistance than the pure molybdenum coating, although the hardness was lower. In the pure molybdenum coatings, splats were readily fractured, or cracks were initiated between splats under high wear loads, thereby leading to the decrease in wear resistance. On the other hand, the molybdenum coating blended with bronze and aluminum-silicon alloy powders exhibited excellent wear resistance because hard phases such as CuAl₂ and Cu₉Al₄ formed inside the coating.     

Mechanical and Thermal Transport Properties of Suspension Thermal-Sprayed Alumina-Zirconia Composite Coatings

Micro-laminates and nanocomposites of Al₂O₃ and ZrO₂ can potentially exhibit higher hardness and fracture toughness and lower thermal conductivity than alumina or zirconia alone. The potential of these improvements for abrasion protection and thermal barrier coatings is generating considerable interest in developing techniques for producing these functional coatings with optimized microstructures. Al₂O₃-ZrO₂ composite coatings were deposited by suspension thermal spraying (APS and HVOF) of submicron feedstock powders. The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to unique and novel fine-scaled microstructures. The suspensions were injected internally using a Mettech Axial III plasma torch and a Sulzer-Metco DJ-2700 HVOF gun. The different spray processes induced a variety of structures ranging from finely segregated ceramic laminates to highly alloyed amorphous composites. Mechanisms leading to these structures are related to the feedstock size and in-flight particle states upon their impact. Mechanical and thermal transport properties of the coatings were compared. Compositionally segregated crystalline coatings, obtained by plasma spraying, showed the highest hardness of up to 1125 VHN_{3 N}, as well as the highest abrasion wear resistance (following ASTM G65). The HVOF coating exhibited the highest erosion wear resistance (following ASTM G75), which was related to the toughening effect of small dispersed zirconia particles in the alumina-zirconia-alloyed matrix. This microstructure also exhibited the lowest thermal diffusivity, which is explained by the amorphous phase content and limited particle bonding, generating local thermal resistances within the structure.     

Deposition and properties of high-velocity-oxygen-fuel and plasma-sprayed Mo-Mo2C composite coatings

Molybdenum thermal-spray coatings, dispersion strengthened by molybdenum oxides and molybdenum carbides, play an important role in industrial tribological applications. Traditionally, they have been prepared by plasma and wire flame spraying. High porosity and lower cohesion strength limit their application in situations where both galling and abrasion wear is involved. In this study, high-velocity-oxygen-fuel (HVOF) deposition of molybdenum and molybdenum carbide coatings was attempted. Deposition was achieved for all powders used. Composition, microstructure, mechanical, and wear properties of the HVOF synthesized coatings were evaluated and compared with plasma-sprayed counterparts. The HVOF coatings possessed a very good abrasion resistance, whereas plasma deposits performed better in dry sliding tests. Measurements showed a close relationship between the coating surface hardness and its abrasion resistance. Results also suggested correlation between molybdenum carbide distribution in the molybdenum matrix and the sliding friction response of Mo-Mo₂C coatings.     

Spraying TiN by a Combined laser and low-pressure plasma spray system

The formation of a TiN-Ti composite coating by thermal spraying of titanium powder with laser processing of the subsequent coating in a low-pressure N₂ atmosphere was examined. A low-pressure plasma spray system was used in combination with a CO₂ laser. First, the coating was plasma sprayed onto a mild steel substrate using a N₂ plasma jet and titanium powder in a controlled low-pressure N2 atmosphere. The coating was then irradiated with a CO₂ laser beam in a N₂ atmosphere, and the coating was heated with a N₂ plasma jet. The amount of TiN formed in the coating was characterized by X-ray diffraction analysis. The influence of plasma spraying conditions such as plasma power, flow of plasma operating gases, chamber pressure, and laser irradiating conditions on the formation of TiN was investigated. The effect of TiN formation in the titanium coating on Vickers hardness of the coatings was examined. It was evident that coating hardness increased with an increase in TiN content in the coating and that a TiN-Ti composite coating with a hardness of more than 1200 H V can be obtained with the use of laser irradiation processing.     

温度对WC-WB-CoCr涂层高温摩擦磨损性能的影响

采用超音速火焰喷涂(HVOF)制备了WC-WB-CoCr涂层,研究了温度对WC-WB-CoCr涂层高温摩擦磨损性能的影响。通过SEM、XRD和显微硬度仪对涂层的微观组织、相结构和力学性能进行表征。通过摩擦磨损试验机和拉曼光谱仪研究了WC-WB-CoCr涂层的高温摩擦学性能和氧化产物,采用台阶仪扫描磨痕形貌并计算WC-WB-CoCr涂层的磨损率。结果表明：WC-WB-Co-Cr涂层主要由WC和CoW₂B₂组成,涂层结构致密,与基体结合紧密;随着磨损试验温度升高,涂层的摩擦系数从0.66降低到0.57,涂层的磨损率随着温度的升高而升高,但是其磨损率增长程度随着温度的升高而降低。在高温磨损过程中,磨痕表面的氧化膜主要由WO₃和CoWO₄组成,且CoWO₄比WO₃表现出更好的耐高温磨损性能。涂层的主要磨损机制为氧化磨损、疲劳磨损和粘着磨损。     

Preparation and characterization of nanostructured Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-13wt.%TiO<Subscript>2</Subscript> ceramic coatings by plasma spraying

Nanostructured and conventional Al₂O₃-13wt.%TiO₂ ceramic coatings were prepared by plasma spraying with nanostructured agglomerated and conventional powders, respectively. The microstructure and microhardness of the coatings were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and microhardness measurement. Meanwhile, the friction and wear behaviors were analyzed and compared using a ball-on-disk tribometer. The results show that the conventional coating has lamellar stacking characteristic and has some pores. However, the nanostructured coating shows a bimodal microstructure, which is composed of both fully melted regions and partially melted regions. According to the microstructural difference, the partially melted regions can be divided into liquid-phase sintered regions (a three-dimensional net or skeleton-like structure: Al₂O₃-rich submicron particles embedded in the TiO₂-rich matrix) and solid-phase sintered regions (remained nanoparticles). The microstructural characteristics of the liquid-phase sintered region are formed due to the selective melting of TiO₂ nanoparticles during plasma spraying. On the other hand, the TiO₂ and Al₂O₃ nanoparticles of the solid-phase sintered regions are all unmelted during plasma spraying. Due to the existence of nanostructured microstructures, the nanostructured coating has a higher microhardness, a lower friction coefficient, and a better wear resistance than the conventional coating.     

原位生成 WC-B4C 增强镍基激光熔覆层及其性能研究

目的通过激光熔覆技术,在Q235钢表面原位生成WC-B4C增强镍基熔覆层。方法以WO3,B2O3,C和Ni60混合粉末为预涂原料,采用激光熔覆技术原位生成WC-B4C增强镍基熔覆层,对熔覆层的显微组织和物相构成进行分析,研究其摩擦磨损性能。结果采用合适的工艺参数,通过原位生成WC-B4C形成的增强镍基涂层形貌良好,与基材呈现较好的冶金结合。熔覆层平均硬度1200HV0.3,摩擦磨损失重仅为纯Ni60熔覆层的1/3。结论熔覆层硬度较高,耐磨性很好。大量原位生成的WC-B4C增强相及其均匀分布是熔覆层硬度和耐磨性提高的原因。     

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。     

等离子喷涂氧化铝基复合涂层研究进展

随着等离子喷涂技术的发展,等离子喷涂氧化铝基复合涂层在防腐蚀、耐磨损和航天航空等领域得到了广泛应用。首先简要介绍了新型等离子喷涂技术(激光等离子喷涂、悬浮液等离子喷涂和超音速等离子等)和主要喷涂工艺参数(喷涂功率、送粉方式和喷涂距离等),然后从改善涂层耐腐蚀性能的角度出发,阐述了第二相、喷涂工艺参数和后处理工艺对涂层气孔率的影响及与涂层耐腐蚀性能的关系。重点分析了硬度、喂料特征和激光熔覆技术对氧化铝基复合涂层耐磨损性能的影响,详述了影响硬度的因素,以及喷涂粉末特征和激光熔覆处理对复合涂层微观结构的影响。在电磁波吸收性能研究方面,论述了吸收剂含量、涂层厚度和多种电磁波吸收剂匹配以及喷涂参数的调整对等离子喷涂氧化铝基复合涂层吸波性能的影响。最后对以等离子喷涂技术制备性能更加优异的氧化铝基复合涂层提出了展望。     

复合制备Ni基合金涂层的组织结构及性能演变特征

采用超音速等离子喷涂技术在45#钢基体上制备Ni60合金涂层,对预制的涂层分别进行高频感应重熔和感应重熔+强制冷却处理。借助OM、SEM、XRD、显微硬度计和销盘式摩擦磨损试验机对3种涂层的组织结构、显微硬度分布及摩擦磨损性能进行分析,研究Ni60合金涂层组织结构及其性能的演变特征。结果表明:3种涂层组织结构差异较大,单纯感应重熔涂层使喷涂涂层组织结构细密化,感应重熔+强制冷却的涂层形成了外延型生长的定向晶结构。喷涂涂层硬度自内向外呈明显下降趋势,而后续处理的2种涂层均表现为自内向外略为增加趋势,导致喷涂涂层尽管有较高的平均硬度,但表层硬度低于其他2种涂层。3种涂层均有很好的耐磨性能,但后续处理使涂层的摩擦系数明显增大,耐磨性能显著增强,尤其附加强制冷却的涂层表现出更加优异的耐磨性能,其平均磨损率分别低于喷涂涂层约8.5倍和单纯感应重熔涂层约2倍。     

碳化硼对电弧喷涂涂层性能的影响

将碳化硼(B4C)陶瓷粉末和其它合金元素与304L不锈钢带轧制成粉芯丝材,采用电弧喷涂技术制备金属陶瓷复合涂层.研究了B4C在电弧喷涂中的应用.利用XRD,SEM对涂层的形貌、相组成和磨损表面进行了分析.利用自行设计的高温磨粒磨损装置和高温冲蚀设备分别评价了B4C对涂层耐高温磨粒磨损性能和耐高温冲蚀性能的影响.结果表明,粉芯丝材喷涂工艺良好,B4C陶瓷与粉芯中其它组分反应,可以形成含Fe3B,CrB,FexN i23-xB6,Fe23(C,B)6,(Cr,Fe)7C3和Fe3C等硬质相的复合涂层,大幅度提高了涂层的硬度和耐磨耐冲蚀性能.     

激光重熔纳米Al2O3-13%TiO2陶瓷涂层组织及性能

为了进一步提高等离子喷涂纳米Al2O3-13%TiO2(质量分数, 下同)复合陶瓷涂层的性能,在γ-TiAl基体材料表面采用激光重熔工艺对涂层进行处理,研究了激光重熔对涂层微观组织和性能的影响.用扫描电镜(SEM)和显微硬度计分析了涂层形貌、微观结构和显微硬度,同时对涂层的磨损特性进行了考察.结果表明,等离子喷涂纳米陶瓷涂层由纳米颗粒完全熔化区和部分熔化区两部分组成,仍然具有等离子喷涂态的典型层状结构.经过激光重熔后,形成了致密细小的等轴晶重熔区、烧结区和残余等离子喷涂区,由于激光快速加热和快速冷却加工特点,在重熔区仍保留了部分来源于原等离子喷涂部分熔化区的残留纳米粒子.与常规等离子喷涂陶瓷涂层相比,纳米结构涂层可在一定程度上提高其硬度和耐磨性,经过激光重熔后其硬度和耐磨性进一步提高.     

喷涂距离对等离子喷涂WC-12Co涂层抗冲蚀磨损性能的影响

为提高WC-12Co涂层抗冲蚀磨损性能,在Q235钢基体上采用大气等离子喷涂(APS)方法制备WC-12Co涂层,研究了喷涂距离对粒子温度与速度、涂层组织结构、力学性能及抗冲蚀磨损性能的影响。结果表明:喷涂距离对涂层质量影响较为明显,喷涂距离为130 mm时涂层质量较好,粒子速度与温度达到较好的配合,涂层抗冲蚀磨损能力较强。喷涂距离为120 mm与140 mm时涂层抗冲蚀磨损能力较差。550μm(30目)沙粒直径对涂层冲蚀磨损量大,沙粒速度为15.68 m/s比13.33 m/s沙粒速度冲蚀磨损量大;冲蚀角为60°时冲蚀磨损量最大,30°冲蚀磨损量最小。