Theoretical consideration on composite oxide scales and coatings

The present paper discussed some fundamental aspects on composite oxide scales and coatings for protection of alloys from high temperature oxidation, the related thermodynamic conditions, special mechanical characteristics and a sealing mechanism. It was proposed that the oxide scales and coatings with a composite structure should possess superior mechanical properties than that with a single phase oxide. It also showed that the Al2O3 scales or coatings doped with Y2O3 and ZrO2 (or YSZ)-Al2O3 composite coatings possessed superior properties at high temperatures. In such composite oxide scales and coatings, the fracture resistance of the scales was increased by the toughening effect, the thermal stress was decreased owing to the increase of thermal-expansion coefficients, and Al2O3 phase could seal the alloy substrate well. In addition, the kinetic equation of thermal growth oxide on alloy covered with composite oxide coatings was derived.     

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.     

Reactive synthesis of <Emphasis Type="Italic">in-situ</Emphasis> ZrAl<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al composites

In this work, a reactive synthesis process is proposed to obtain ZrAl₃-Al₂O₃ particulate-reinforced aluminum matrix composites. The process involves the in-situ formation of Al₂O₃ and ZrAl₃ from Al-ZrO₂ green compacts. Upon compact heating, it is found that reduction of ZrO₂ by molten aluminum occurs at temperatures above 750 °C, leading to the development of ZrAl₃ and Al₂O₃ phases. Thermodynamically, it is found that the reduction of zirconium oxide is driven mainly by the dissolution of Zr in molten aluminum. Because the solubility of Zr in liquid aluminum is extremely small, the formation of ZrAl₃ is favored after relatively small Zr dissolutions. The first Zr-Al intermetallics to form at the lowest temperatures seem to be metastable, as infered from the measured atom ratios for Al : Zr of 2.83 : 1. At increasing temperatures, the reaction comes into completion, resulting in the formation of equilibrium intermetallic ZrAl₃ phases. The results obtained from differential scanning calorimetry (DSC) indicate that by increasing the scanning rates, both the reaction temperature and the exothermic peak intensity also increase. Alternatively, it is found that by reducing the amount of ZrO₂ in the green compact, the in-situ reaction temperatures also shift toward higher values.     

Effects of incorporation of micro and nano Al2O3 layers on thermal shock behaviour of YSZ thermal barrier coatings

In the present paper, thermal shock behaviour of usual YSZ and layered composite TBCs was evaluated. The layered composite TBCs used in this study were YSZ/micro Al₂O₃ and YSZ/nano Al₂O₃ in which Al₂O₃ was as a topcoat on the YSZ layer. In addition, coating microstructures were investigated using scanning electron microscope; EDS and X-ray diffraction techniques before and after thermal shock test. The results showed that incorporation of a nano Al₂O₃ oxygen barrier layer caused better thermal shock resistance, while micro Al₂O₃ layer on the top of YSZ sample resulted in sooner coating failure, compared to the usual TBC coating. Also, the thermal shock behaviour and failure mechanism of mentioned coatings were investigated.     

Fabrication of Al2O3-reinforced Ni3Al composites by a novel in situ route

A novel in situ technique has been used to fabricate an Al₂O₃-reinforced Ni₃Al matrix composite. The composite was prepared by first incorporating a low volume fraction of continuous Al₂O₃ fibers in a Ni₃Al alloy containing 0.34 at. pct Zr. Pressure casting was used to embed the fibers. Casting resulted in partial reduction of the Al₂O₃ fiber by the Zr present in the matrix and the formation of a layer of ZrO₂ on the surface of the fibers. The final composite was then prepared by air annealing the precursor composite at 1100 °C for 10 days. Air annealing led to the formation of networks of Al₂O₃ around the fibers. The matrix in the immediate vicinity of the networks consisted of Ni₃Al particles in a matrix of disordered α-Ni(Al). The Al₂O₃ networks raised the yield and tensile strength of the material by 35 and 18 pct, respectively. The composite had a tensile ductility of 14 pct.     

A structural study of oxidation in an Al2O3 fiber reinforced titanium aluminide composite

A composite of titanium aluminide, reinforced with ZrO₂ toughened Al₂O₃ fibers, has been produced by pressure casting. The stability of the microstructure during vacuum and air annealing was investigated by scanning and transmission electron microscopy (TEM). Processing resulted in partial dissolution of ZrO₂ of the fiber into the molten metal which solidified into a two phase lamellar α₂ + γ matrix. Vacuum annealing caused dissolution of Al₂O₃ fiber into the matrix, transformation of the matrix into γ and precipitation of ZrO₂ and Al₂Zr within γ. During air annealing diffusion of O through the fiber resulted in the formation of Al₂O₃ coated ZrO₂ nodules extending from the fiber surface into the matrix in some regions and in the other regions the growth of Al₂O₃ into the γ of the lamellar matrix. An A15 phase with the metal composition, Ti₃Al₂, was found between the external oxide scale and the metal matrix.     

Formation and properties of laser coatings based on oxynitride ceramic

The structure and properties of powder composite material coatings based on TiN-Al₂O₃ produced by plasma deposition and subsequent laser fusion, have been examined. X-ray spectral microanalysis, electron microscopy and metallographic analysis were used to establish that the coating structure with a high level of physicomechanical properties, is reinforced with Al₂O₃ fibers, and is formed under substrate irradiation conditions that are close to those for sintering and hot compaction.Translated from Poroshkovaya Metallurgiya, No. 12 (360), pp. 29–33, December, 1992.     

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.     

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.     

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。     

Corrosion Resistance of Sintered NdFeB Permanent Magnet With Ni-P/Tio22 Composite Film

The Ni-P/TiO₂ composite film on sintered NdFeB permanent magnet was investigated by X-ray diffraction (XRD), environmental scanning electron microscopy (ESEM), and energy dispersive X-ray spectrometer (EDX). The corrosion resistance of Ni-P/TiO₂ film coated on NdFeB magnet, in 0.5 mol/L NaCl solution, was studied by potentiodynamic polarization, salt spray test and electrochemical impedance spectroscopy (EIS) techniques. The self-corrosion current density (i_corr) and the polarization resistance (R_p) of Ni-P/TiO₂ film are 0. 22 μA/cm² (about 14% of that of Ni-P coating), and 120 kΩ • cm² (about 2 times of that of Ni-P coating), respectively. The anti-salt spray time of Ni-P/TiO₂ film is about 2.5 times of that of the Ni-P coating. The results indicate that Ni-P/Ti0₂ film has a better corrosion resistance than Ni-P coating, and the composite film increases the corrosion resistance of NdFeB magnet markedly.     

Investigation on strengthening and toughening mechanisms of Ce-TZP/Al2O3 nanocomposites

The strengthening and toughening mechanisms of hot-pressed Ce-TZP/Al₂O₃ ceramics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) methods. The results show that the contributions to the strength and toughness mainly come from the hindrance of the growth of Al₂O₃ grains by ZrO₂ nanoparticles and the transformation from tetragonal ZrO₂ to monoclinic ZrO₂. Additionally, TEM observations show that the dislocation structures formed both in the Al₂O₃ and on the Al₂O₃ grain boundary have a certain contribution, because the dislocation agglomeration and fixation by ZrO₂ nanoparticles could deflect cracking or stop cracking development. Meanwhile, it was found that the mechanical properties and microstructures of samples were preferential when ZrO₂ contents in the sample reached 20 pct.     

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.     

Structure and Mechanical Properties of Ni-P-Nano Al2O3 Composite Coatings Synthesized by Electroless Plating

Ni-P-nano Al2O3 composite coatings were deposited by electroless plating,and their microstructures were observed by SEM（scanning electron microscope）.The microhardness and the wear resistance of the Ni-P-nano Al2O3 composite coatings were measured using microhardness tester and block-on-ring tribometer,respectively,and the comparison with those of Ni-P coatings or Ni-P-micro Al2O3 coating was given.The influences of aging temperature on their hardness and wear resistance were analyzed.The results showed that the nano Al2O3 particles were distributed uniformly in the Ni-P-Al2O3 coatings.Among three kinds of Ni-P based coatings,the hardness and wear resistance of Ni-P-nano Al2O3 coatings were largest,and the maximum values could be obtained at 400 ℃.This indicated that the precipitation of nano Al2O3 particles would improve the hardness and wear resistance of the Ni-P coatings.     

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.     

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.     

Preparation of Al2O3/Y2O3 composite coating for deuterium permeation reduction

A tritium permeation barrier is required in fusion blankets for the reduction of fuel loss and radiological hazard. In this study, an Al₂O₃/Y₂O₃ composite coating was prepared on 316L stainless steel by radio-frequency magnetron sputtering in order to improve the tritium permeation resistance. The microstructure and the phase composition of the Al₂O₃/Y₂O₃ composite coating are observed by scanning electron microscopy, transmission electron microscopy and grazing incidence X-ray diffraction. Moreover, Auger electron spectroscopy was used to characterize the depth profiles of Al, Y and O elements. The results clearly indicate that the Al₂O₃/Y₂O₃ composite coating is fully dense and the total thickness is approximately 340 nm. The Al₂O₃/Y₂O₃ coating consists of an amorphous Al₂O₃ and the cubic Y₂O₃, in which Al, Y and O elements are homogeneously distributed in the vertical base direction. Furthermore, the deuterium permeation property of the Al₂O₃/Y₂O₃ composite coating was measured by the gas phase permeation method. The results show that the introduction of an interface and the existence of a tiny amount of micro-defects improve the deuterium resistance of the Al₂O₃/Y₂O₃ coating, and its deuterium permeation reduction factor is 536–750 at 873–973 K. Therefore, it is concluded that the Al₂O₃/Y₂O₃ composite coating as deuterium permeation barrier can significantly enhance the deuterium permeation resistance property.     

Effect of Surface Treatment with Concentrated Energy Fluxes on the Properties of Composite Materials Based on Silicon Carbide and Coatings Made from Such Composites

Based on a study of the structure and composition of the composite ceramic SiC - Al₂O₃ - ZrO₂, its tribomechanical properties and behavior in high-temperature corrosion, we recommend the material for use as sealing elements and for deposition of wear-resistant and corrosion-resistant coatings. We have studied the formation of gradient layers when the ceramic surface is modified with refractory titanium compounds TiN - TiB₂ (1:1) with an Fe(Ni) - Cr - Al undercoat using concentrated solar radiation and when the steel surface is modified with laser irradiation of the SiC - Al₂O₃ - ZrO₂ coats. We have shown that laser modification of steel by the silicon carbide-based composite increases its corrosion resistance by a factor of 4–5 at 800–900 °C. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 91–99, July–August, 2005.     

Oxidation behavior of single-crystal Al2O3-fiber-reinforced Ni3Al-based composites

A series of single-crystal Al₂O₃-fiber-reinforced Ni3Al-based intermetallic matrix composites were fabricated by pressure casting. The matrices employed were binary Ni₃Al, Ni₃Al-0.5 at. pct Cr, and Ni₃Al-0.34 at. pct Zr. The development of microstructure upon oxidation in air at either 1100 °C or 1200 °C was investigated by optical, scanning, and transmission electron microscopy. In air-oxidized binary Ni₃Al, some of the fibers were fully or partially covered with a layer of oxide. A weak fiber/matrix bond in this system, which led to fiber debonding during composite processing, is believed to be responsible for the ingress of O into the composite and oxidation of the matrix in the debonded regions at the fiber/matrix interface. Addition of Cr to Ni₃Al resulted in an almost threefold increase in fiber/matrix bond strength. No oxidation of the interface was observed. A thick layer of oxide was formed around all the fibers when the composite was thermally cycled prior to isothermal annealing. Addition of Zr to Ni₃Al resulted in the formation of a layer of ZrO₂ on the surface of the fibers during composite processing. The ZrO₂ layer provided a fast path for the diffusion of O, which led to the formation of a rootlike oxide structure around the fibers. The rootlike structure consisted of a network of Al₂O₃-covered ZrO₂.