电弧喷涂含陶瓷颗粒铝基复合涂层的微结构和性能

采用5052半硬铝带分别包覆Al_2O_3、SiC、B_4C、TiC陶瓷颗粒制备的粉芯丝材进行电弧喷涂试验,制备了含陶瓷颗粒的铝基复合涂层。利用光学显微镜、XRD分析了涂层的微观组织和相结构,测试了复合涂层的显微硬度、耐磨性及耐腐蚀性。研究结果表明,制备的铝基复合涂层中含有一定数量的未熔陶瓷颗粒,涂层较为致密,无明显缺陷。含陶瓷铝基涂层的物相主要由Al和所添加的陶瓷相构成,其中在含B_4C陶瓷涂层中还存在Al_3BC、Al_4C_3和AlB_2等新相。陶瓷颗粒的加入有利于提高铝基复合涂层的显微硬度,其中B_4C的加入使涂层中基体相显微硬度提高了1.5倍,这是由于B_4C陶瓷和Al反应生成Al_3BC、Al_4C_3和AlB_2硬质相。复合涂层的耐磨性均优于纯铝涂层,摩擦磨损的形式主要为粘着磨损。动电位极化腐蚀试验表明,含SiC和TiC陶瓷涂层具有较低的腐蚀电流,耐蚀性较好,含SiC陶瓷的复合涂层出现了明显的钝化现象。     

Wear of Plasma Sprayed Conventional and Nanostructured Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>, Based Coatings

An ever increasing demand for high-performance ceramic coatings has made it inevitable for developing techniques with precise control over the process parameters to enable the fabrication of coatings with the desired microstructure and improved structural properties. The literature on plasma sprayed nanostructured ceramic coatings such as of Al₂O₃, Cr₂O₃, and their composites obtained using reconstituted nano sized ceramic powders has been reviewed in this study. Ceramic coatings due to their enhanced properties are on the verge of replacing conventional ceramic coatings used for various applications like automotive systems, boiler components, power generation equipment, chemical process equipment, aircraft engines, pulp and paper processing equipment, land-based and marine engine components, turbine blades etc. In such cases, the advantage is greater longevity and reliability for realizing the improved performance of ceramic coatings. It has been observed that the plasma sprayed nanostructured ceramic coatings show improvement in resistance to wear, erosion, corrosion, and mechanical properties as compared to their conventional counterparts. This article reviews various aspects concerning the plasma sprayed ceramic coatings such as (i) the present understanding of formation of plasma-spray coatings and factors affecting them, (ii) wear performance of nanostructured Al₂O₃, Cr₂O₃ and their composite ceramic coatings in comparison to their conventional counterparts, and (iii) mechanisms of wear observed for these coatings under various conditions of testing.     

Experimental Investigation of Magnesium-Base Nanocomposite Produced by Friction Stir Processing: Effects of Particle Types and Number of Friction Stir Processing Passes

In this research, nanosized SiC and Al₂O₃ particles were added to as-cast AZ91 magnesium alloy, and surface nanocomposite layers with ultrafine-grained structure were produced via friction stir processing (FSP). Effects of reinforcing particle types and FSP pass number on the powder distribution pattern, microstructure, microhardness, and on tensile and wear properties of the developed surfaces were investigated. Results show that the created nanocomposite layer by SiC particles exhibits a microstructure with smaller grains and higher hardness, strength, and elongation compared to the layer by Al₂O₃ particles. SiC particles do not stick together and are distributed separately in the AZ91 matrix; however, distribution of SiC particles is not uniform in all parts of the stirred zone (SZ), which causes heterogeneity in microstructure, hardness, and wear mechanism of the layer. Al₂O₃ particles are agglomerated in the different points of matrix and create alumina clusters. However, distribution of Al₂O₃ clusters in all parts of the SZ is uniform and results in a uniform microstructure. In the specimen produced by one-pass FSP and SiC particles, the wear mechanism changes in different zones of SZ due to the nonuniform distribution of particles. However, in the specimen produced by Al₂O₃ particles, the wear mechanism in all parts of the SZ is the same and, in addition to the abrasive wear, delamination also occurs. Increasing FSP pass number results in improved distribution of particles, finer grains, and higher hardness, strength, elongation, and wear resistance.     

Correlation of microstructure and wear resistance of Al2O3-TiO2 coatings plasma sprayed with nanopowders

Correlation of microstructure and wear resistance of Al₂O₃-TiO₂ coatings plasma sprayed with nanopowders was investigated in this study. Four kinds of nanostructured Al₂O₃-13 wt pct TiO₂ coatings were fabricated by varying plasma-spraying parameters and were compared with an Al₂O₃-13 wt pct TiO₂ coating fabricated with conventional powders. The nanostructured coatings showed a bimodal microstructure composed of fully melted regions of γ-Al₂O₃ and partially melted regions, while the conventional coating mostly consisted of fully melted γ-Al₂O₃, together with some TiO₂-rich regions and unmelted Al₂O₃ powders. The wear test results revealed that the wear resistance of the nanostructured coatings was 3 or 4 times better than that of the conventional coating, because the preferential delamination seriously occurred along TiO₂-rich regions in the conventional coating. In the nanostructured coatings, TiO₂ was homogeneously dispersed inside splats and around, thereby leading to higher splat bonding strength and to better wear resistance over the conventional coating.     

基于激光重熔的纳米陶瓷颗粒改性喷涂层的耐磨性研究

介绍了基于激光重熔技术的纳米陶瓷颗粒改性热喷涂耐磨涂层复合加工方法.以SiC纳米颗粒和WC-Co及Al2O3-TiO2热喷涂涂层为研究对象,进行了涂层改性加工试验.使用扫描电镜分析了纳米颗粒改性重熔涂层微观组织结构,并分别对热喷涂涂层、激光重熔喷涂涂层和纳米颗粒改性涂层进行了耐磨性测试.结果表明:SiC纳米颗粒能有效改善热喷涂涂层组织,并能显著提高涂层耐磨损性能.     

Up‐scaling of a Low Temperature PACVD Process for the Deposition of α/γ‐Nanocrystalline Alumina Coated Tools for Thixocasting of 1.2208 steel

An industrial‐scale pulsed plasma‐assisted chemical vapor deposition (PACVD) process for crystalline alumina growth was developed. To obtain a homogeneous coating thickness distribution over complex geometries and large dimensions, a suitable gas injection system was designed. A phase formation diagram for alumina coatings as a function of pulse length and cathode voltage has been compiled, allowing for the deposition of dense α/γ‐Al₂O₃ coatings at a substrate temperature of 590 °C. Moulds coated with an α/γ‐Al₂O₃ coating were utilized in steel thixocasting at temperatures of ～1400 °C. The coatings were intact after thixocasting and showed significantly improved chemical wear resistance compared to plain steel moulds.     

Correlation of microstructure and wear resistance of ferrous coatings fabricated by atmospheric plasma spraying

The correlation of microstructure and wear resistance in ferrous coatings applicable to diesel engine cylinder bores was investigated in this study. Seven kinds of ferrous spray powders, two of which were stainless steel powders and the others blend powders of ferrous powders mixed with Al₂O₃-ZrO₂ powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the ferrous coatings showed that various Fe oxides such as FeO, Fe₂O₃, and γ-Fe₂O₃ were formed in the martensitic (or austenitic) matrix as a result of the reaction with oxygen in air. The blend coatings containing γ-Al₂O₃ and t-ZrO₂ oxides, which were formed as Al₂O₃-ZrO₂ powders, were rapidly solidified during plasma spraying. The wear test results revealed that the blend coatings showed better wear resistance than the ferrous coatings because they contained a number of hard Al₂O₃-ZrO₂ oxides. However, delamination occurred when cracks initiated at matrix/oxide interfaces and propagated parallel to the worn surface in the case of the large hardness difference between the matrix and oxide. The wear rate of the coating fabricated with STS316 powders was slightly higher than other coatings, but the wear rate of the counterpart material was very low because of the smaller matrix/oxide hardness difference due to the presence of many Fe oxides. In order to reduce the wear of both the coating and its counterpart material, the matrix/oxide hardness difference should be minimized, and the hardness of the coating should be increased over a certain level by forming an appropriate amount of oxides.     

Trying out spraying modes and properties of wear-resistant flame Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> coatings fabricated using a flexible cord material

Samples of Al₂O₃–TiO₂ coatings are fabricated by the flame spraying of a flexible cord. The influence of process parameters and composition of the sprayed material on the structure, composition, and mechanical properties of coatings is investigated. It is shown that an increase in the spraying distance and feed rate of the sprayed material leads to a decrease in their density. An increase in the concentration of the low-melting TiO₂ component predetermines a decrease in the coating porosity and has no significant effect on the coating hardness. Being subjected to measuring scratching, Al₂O₃–TiO₂ flame coatings formed with minimal porosity (Π = 3.2%) are characterized by cohesion fracture behavior and no substrate opening under an indenter load of up to 90 N. The friction factor of coatings under study varies from 0.2 to 0.78 after 2800 counterbody revolutions (44 m of the friction path). This is associated with the accumulation of fatigue cracks in the coating material and its subsequent cohesive fracture by the formation of large fragments serving as an abrasive.     

Microstructure and properties evolution of plasma sprayed Al_2O_3-Y_2O_3 composite coatings during high temperature and thermal shock treatment

Al_2O_3-Y_2O_3 composite powder with TiO_2 additive was plasma sprayed to prepare Al_2O_3-Y_2O_3 composite coatings.The micro structure and properties evolution of the Al_2O_3-Y_2O_3 coatings during high temperature and thermal shock resistance were investigated.The results show that the micro structure of the Al_2O_3-Y_2O_3-TiO_2 coating is more uniform than that of the Al_2O_3-Y_2O_3 coating.Meanwhile,amorphous phase is formed in the two coatings.The Al_2O_3-Y_2O_3(-TiO_2) coatings were heat treated for 2 h at temperatures of 800,1000 and 1200℃,respectively.It is found that the microstructure and properties of the two coatings have no obvious change at 800℃.Some of the amorphous phase is crystallized at1000℃,and meanwhile Y_2O_3 and Al_2O_3 react to form YAG phase and YAM phase.At 1200℃,all of the amorphous phases are crystallized.After heat treatment,the micro hardness of the two coatings is increased.The thermal shock resistance of the Al_2O_3-Y_2O_3 system coatings can be improved by using TC4 titanium alloy as substrate and with NiCrAlY bonding layer.Moreover,the Al_2O_3-Y_2O_3-TiO_2 coating exhibits better thermal shock resistance due to the addition of TiO_2.     

Fabrication and Characterization of Ni-SiC Nanocomposite Coatings on Al Substrates by Ball Impact Deposition Method

Micron-size Ni and SiC powder mixtures were used to prepare Ni-SiC nanocomposite coatings on an Al substrate by employing a high-energy ball milling technique. Ni:SiC weight ratio was varied over a wide range to explore the effect of the charge composition on the microstructure, composition, microhardness, and wear properties of the depositions. It was observed that the composition of the produced coating was correlated to the charge composition in a complex manner, which suggests that deposition rates for Ni and SiC particles significantly vary depending on the charge composition; SiC deposition rate was higher than that of Ni when Ni:SiC weight ratio was greater than 3:1. Diffusion of Al from the substrate into the Ni matrix provided evidence for the metallurgical bonding at the interface. Both microstructural and mechanical properties of the produced coatings were found to be crucially dependent on the charge composition. By increasing the SiC content in the charge from about 5 to 33 wt pct, the mechanical properties enhanced due to the dispersion strengthening effect of the incorporated SiC particles in the coatings and the crystallite size of the Ni matrix decreasing to the nanometer range. However, a further increase resulted in the formation of a coating with a poor degree of compaction. It was found that the composite coating with about 15 vol pct SiC, produced from the charge with Ni:SiC weight ratio of 2:1, showed a microhardness as high as 830 HV_0.05 along with excellent wear resistance. Despite the current sample size limitations for applying high-energy ball milling, the present findings demonstrate that the adopted technique holds good prospect for the synthesis of nanostructured metal matrix composite coatings with enhanced and tunable properties.

     

Tribological Behavior of A356/Al2O3 Surface Nanocomposite Prepared by Friction Stir Processing

Surface A356 aluminum alloy matrix composites containing micro and nanosized Al₂O₃ are prepared by a new approach utilizing high-velocity oxy-fuel spraying and friction stir processing (FSP). Optical and scanning electron microscopy, microhardness, and wear tests were used to characterize the surface composites. Results indicated that, the presence of Al₂O₃ in matrix can improve the mechanical properties of specimens. The microhardness of surface composites containing micro and nanosized Al₂O₃ were 89.8 ± 2.6 HV and 109.7 ± 2.5 HV, respectively, which were higher than those for the as-received (79.6 ± 1.1 HV) and the FSPed A356-T6 with no alumina powder (66.8 ± 0.9 HV). Surface composites revealed low friction coefficients and wear rates, which were significantly lower than those obtained for substrate. The wear mass losses of the as-received, the FSPed, and surface micro and nanocomposite specimens after 500-m sliding distance were 50.5, 55.6, 31, and 17.2 mg, respectively. Scanning electron microscopy tests revealed different wear mechanisms on the surface of the wear test specimens.     

Comparison in the Oxidation and Corrosion Behavior of Aluminum and Alumina-Reinforced Ni/Ni-Co Alloy Coatings

In this study, a comparison in the oxidation and corrosion behavior of Ni/Ni-Co aluminum and alumina-reinforced electrodeposited composites has been made. The developed coatings were characterized for the morphology, structure, microhardness, oxidation, and corrosion resistance. It was found that the incorporation of Al particles in NiCo matrix is higher (9 wt pct) compared to Ni matrix (1 wt pct). In the case of aluminum oxide particles, about 5 and 7 wt pct had been obtained in Ni and NiCo matrices respectively. The difference in the surface morphology was observed with respect to metallic (Al) and inert ceramic (Al₂O₃) particle incorporation. X-ray diffraction studies showed the presence of predominant Ni (200) reflection in the coatings. Also, peaks corresponding to Al and Al₂O₃ particles were present. The Ni/NiCo-Al coatings exhibited higher microhardness values at 1273 K (1000 °C) compared to alumina-reinforced coatings, indicating better thermal stability of the former coatings. The NiAl coating showed one and two orders of magnitude improved oxidation resistance compared to NiCoAl and Ni/NiCo-Al₂O₃ coatings, respectively. It was observed that the Ni-Al composite coating exhibited poor corrosion resistance in 3.5 pct NaCl solution compared to the other coatings studied.     

Relative performance of alumina coatings prepared by micro arc oxidation and detonation gun spray on AA 6063 under plain fatigue and fretting fatigue loading

The present study compares the performance of alumina coatings prepared by two different methods (micro arc oxidation (MAO) and detonation gun (D-gun) spray) on AA 6063 (Al alloy) fatigue test samples under plain fatigue and fretting fatigue loading. While MAO coating had comparable proportions of γ-Al₂O₃ and α-Al₂O₃, D-gun sprayed coating contained γ-Al₂O₃ with minimal quantities of α-Al₂O₃. MAO coating was relatively harder than D-gun sprayed coating. As both types of coated samples were ground, they exhibited almost the same surface roughness. D-gun sprayed alumina coated samples exhibited slightly higher magnitude of surface residual compressive stress compared with MAO coated specimens. Both types of coated samples experienced almost the same friction force. D-gun spray coated samples exhibited superior plain fatigue and fretting fatigue lives compared with MAO coated specimens. This may be attributed to layered structure of the D-gun sprayed coating.     

Formation and high-temperature oxidation of thermal-barrier coatings with Ti-Al-Cr binding layer

The behavior of TiAlCr detonation coatings with an outside thermal barrier layer in high-temperature oxidation in air is examined. It is established that the efficiency of thermal-barrier coatings with a zirconia external layer on substrates of Ti-rich alloys and γ-TiAl depends on the oxidation of the binding TiAlCr layer and the diffusion at the coating-substrate interface. It is shown that the presence of a ceramic layer has no fundamental effect on the oxidation of γ-TiAl-based coatings. The behavior of TiAlCr coatings in high-temperature oxidation depends on the substrate structure, which determines the nature of diffusion at the coating-substrate interface. If the substrate is made of titanium-rich alloys, there is active diffusion of Al and Cr from and of Ti into the coating. Since the Al/Ti activity ratio changes, a mixed layer of TiO₂ and Al₂O₃ forms on the surface. In case of the γ-TiAl substrate, a layer on the TiAlCr surface consists of Al₂O₃ alone.     

等离子喷涂高强韧纳米TiO2涂层的制备及其力学性能研究

为提高大气等离子喷涂（APS）TiO2陶瓷涂层的结合强度及其摩擦磨损性能,采用喷雾造粒的纳米结构TiO2粉末为原料,利用APS工艺制备出TiO2陶瓷涂层,研究了涂层的显微组织、相组成、力学性能和摩擦磨损性能,并与常见的微米级TiO2粉末制备的陶瓷涂层组织性能进行了对比。结果表明,微米TiO2粉在喷涂前后相成分从板钛矿变为主为金红石和锐钛矿的混合相;而纳米团聚的TiO2粉喷涂前后无明显的相成分变化,均以金红石相为主。纳米TiO2涂层的孔隙率为1.4%,低于微米粉涂层的3.3%。纳米TiO2涂层的力学性能优于微米涂层,微米涂层硬度为934.2 HV0.1,而纳米涂层的硬度为1 349 HV0.1;纳米和微米涂层的弹性模量分别为203.1和185.8 GPa;纳米涂层的断裂韧性为2.1 MPa?m1/2,略高于微米涂层的2.0 MPa?m1/2;纳米涂层的结合强度可达46.8 MPa,是微米涂层的3.18倍（14.7 MPa）。此外,在相同的摩擦条件下,纳米TiO2涂层的摩擦因数为0.69,比微米TiO2涂层更低,纳米涂层的磨损体积也比微米涂层更少。综合来说,纳米TiO2涂层相对于微米级TiO2涂层体现出更好的综合力学性能。     

Effects of Duty Cycle,Current Frequency,and Current Density on Corrosion Behavior of the Plasma Electrolytic Oxidation Coatings on 6061 Al Alloy in Artificial Seawater

In this study, the effects of duty cycle, current frequency, and current density on corrosion behavior of the plasma electrolytic oxidation (PEO) coatings on 6061 Al alloy in artificial seawater (3.5 wt pct NaCl solution) were investigated. To serve this purpose, the electrical parameters of a unipolar pulsed current were applied during the PEO process on 6061 Al alloy in alkaline silicate electrolyte with and without Al₂O₃ nanoparticles. The coating morphology and microstructure were characterized by the scanning electron microscopy. The corrosion behavior and electrochemical response of the specimens treated by plasma electrolytic oxidation were analyzed by the electrochemical impedance spectroscopy and the potentiodynamic polarization in artificial seawater. It was found that PEO coatings formed in the presence of Al₂O₃ nanoparticle had lower porosity and exhibited better corrosion behavior compared with the coatings formed in the absence of Al₂O₃ nanoparticle in the structure. This can be attributed to the nanoparticles’ incorporation and penetration through the PEO coatings. On the other hand, the decrease in the current density and increases in the duty cycle and frequency lead to further reduction of the nanoparticles’ incorporation and distribution on the coating surface.

     

Investigation of Tribological Behavior of Ceramic–Graphene Composite Coating Produced by Plasma Electrolytic Oxidation

Plasma electrolyte oxidation coatings were formed on AZ31 magnesium alloy in the phosphate electrolyte containing 0 and 5 g L^?1 graphenes at different process times. The composition and microstructure of the coatings were analyzed by scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer and X-ray diffraction (XRD). The SEM images showed that by increasing the coating time, the number of coating pores decreased whereas the diameter of coating pores increased. Furthermore, the diameter and number of pores related to ceramic–graphene composite coatings were lower than ceramic ones. XRD analysis indicated that major constituents of coatings were MgO and Mg₃(PO₄)₂. The pin-on-disk sliding tests revealed that the wear loss and coefficient of friction of ceramic–graphene composite coatings were lower than simple ones.     

硼化钼基涂层的制备及耐磨抗蚀性能研究

针对连续热镀锌生产线中沉没辊、轴套等零部件的腐蚀磨损问题,采用造粒烧结制备了MoB-Al2O3(MA)复合陶瓷粉末,采用等离子喷涂工艺制备了MA复合陶瓷涂层,测试了涂层的显微组织、显微硬度及耐熔融锌液腐蚀磨损性能.结果 表明,涂层显微组织均匀,显微硬度约1300HV100.采用细粒度粉末、粘结底层和封孔工艺制备的MA涂...     

Preparation and performance of electrodeposited Ni-TiB2-Sm2O3 composite coatings

Sm₂O₃ and TiB₂ were used as codeposited particles in electrodeposition Ni-TiB₂-Sm₂O₃ composite coatings to improve its performance. Ni-TiB₂-Sm₂O₃ composite coatings were electrodeposited in the nickel sulfate, hexadecylpyridinium bromide and cetyltrimethylammonium bromide solution containing TiB₂ and Sm₂O₃ particles. The content of codeposited Sm₂O₃ in the composite coating was controlled by changing the concentrations of Sm₂O₃ particles in the solution. The composite coatings were characterized with X-ray diffraction (XRD) and inductively coupled plasma-atomic emission spectrometer (ICP-AES). The effects of Sm₂O₃ content on microhardness, wear weight loss and friction coefficient of composite coatings were investigated, respectively. The microhardness of the Ni-TiB₂-Sm₂O₃ composite coatings was 19.35%, 16.58%, 2.03% higher than that of the Ni coating, Ni-Sm₂O₃ and Ni-TiB₂ composite coatings, respectively. The wear weight loss of the Ni-TiB₂-Sm₂O₃ composite coatings was 7, 2.33, 1.22 times lower than that of the Ni coating, Ni-Sm₂O₃ and Ni-TiB₂ composite coatings, respectively. The friction coefficient of the Ni coating, Ni-Sm₂O₃, Ni-TiB₂ and Ni-TiB₂-Sm₂O₃ composite coatings were 0.712, 0.649, 0.850 and 0.788, respectively. The loading-bearing capacity and the wear-reducing effect of the Sm₂O₃ particles were closely related to the content of Sm₂O₃ particles in the composite coatings.     

Wear-Resistant Layered Electrospark Coatings Based on ZrB2

We have studied the composition and tribological parameters (coefficient of friction and wear rate) of coatings obtained in electrospark alloying (ESA) of steel 45 with the composite ceramic ZrB₂ – SiC – B₄C based on zirconium diboride. We have shown that layer-by-layer electrospark alloying using Ti – Al – N composites to form an undercoat reduces the coefficient of friction of the coating down to 0.12-0.20 while maintaining a rather low wear level (7-11 μm/km). We have analyzed the effect of the composition of the secondary structures which are formed during tribo-oxidation under dry friction conditions.