Mechanical properties of the plasma-sprayed Al2O3/ZrSiO4 coatings

The mechanical properties of plasma-sprayed Al₂O₃/ZrSiO₄ coatings were investigated by indentation-based techniques. Two types of feedstock were used to prepare the coatings: spray-dried powders and plasma-spheroidized powders. A 100-kW direct current (d.c.) thermal plasma system was employed. The values obtained were found to exhibit a close relationship with the microstructure of the as-sprayed coatings, which composed of zircon, alumina, amorphous silica and tetragonal zirconia. The coatings produced with plasma-spheroidized powders had higher microhardness, Young's modulus and fracture toughness than that produced with the spray-dried powders. The coatings produced with plasma-spheroidized powders by a 100-kW computerized system at 15 kW of net plasma energy had the best mechanical properties, while those deposited at 19 kW of net plasma energy had the worst properties due to the high density of cracks in the coatings.     

Microstructure and tribological behavior of suspension plasma sprayed Al2O3 and Al2O3-YSZ composite coatings

Several alumina and alumina-zirconia composite coatings were manufactured by suspension plasma spraying (SPS), implementing different operating conditions in order to achieve dense and cohesive structures. Temperatures and velocities of the in flight particles were measured with a commercial diagnostic system (Accuraspray®) at the spray distance as a function of the plasma operating parameters. Temperatures around 2000 °C and velocities as high as 450 m/s were detected. Hence, coatings with high amount of α-alumina phase were produced. The microstructure evolution according to the spray parameters was studied as well as the final tribological properties showing efficient wear resistance.     

Microstructural analysis of YSZ and YSZ/Al2O3 plasma sprayed thermal barrier coatings after high temperature oxidation

Air plasma sprayed TBCs usually include lamellar structure with high interconnected porosities which transfer oxygen from YSZ layer towards bond coat and cause TGO growth and internal oxidation of bond coat.The growth of thermally grown oxide (TGO) at the interface of bond coat and ceramic layer and internal oxidation of bond coat are considered as the main destructive factors in thermal barrier coatings.Oxidation phenomena of two types of plasma sprayed TBC were evaluated: (a) usual YSZ (yttria stabilized zirconia), (b) layer composite of (YSZ/Al₂O₃) which Al₂O₃ is as a top coat over YSZ coating. Oxidation tests were carried out on these coatings at 1100°C for 22, 42 and 100h. Microstructure studies by SEM demonstrated the growth of TGO underneath usual YSZ coating is higher than for YSZ/Al₂O₃ coating. Also cracking was observed in usual YSZ coating at the YSZ/bond coat interface. In addition severe internal oxidation of the bond coat occurred for usual YSZ coating and micro-XRD analysis revealed the formation of the oxides such as NiCr₂O₄, NiCrO₃ and NiCrO₄ which are accompanied with rapid volume increase, but internal oxidation of the bond coat for YSZ/Al₂O₃ coating was lower and the mentioned oxides were not detected.     

等离子喷涂Al2O3/TiO2纳米陶瓷复合涂层的微观结构及性能

采用等离子喷涂技术制备了不同成分的Al2O3/TiO2纳米陶瓷复合涂层,并利用X射线衍射仪(XRD)、扫描电镜(SEM)、显微硬度计和摩擦磨损试验机等分析测试手段研究了纳米陶瓷复合涂层的微观结构与性能.结果表明:纳米陶瓷复合涂层中Al2O3以α-Al2O3和γ-Al2O3两相共存的形式存在,且γ-Al2O3的含量随喷涂功率的增加而增加,而涂层中的TiO2则以金红石型存在;其微结构为完全熔化区的片状微观组织和部分熔化区的纳米级颗粒共存的组织;等离子喷涂功率和TiO2含量对涂层的硬度和耐磨性能均有显著的影响.     

Microstructure, spallation and corrosion of plasma sprayed Al2O3–13%TiO2 coatings

The electrochemical corrosion behaviours of the steel substrates coated with three different plasma sprayed Al₂O₃–13%TiO₂ coatings were studied in this paper. The three kinds of Al₂O₃–13%TiO₂ coatings were conventional ME coating, nanostructured NP coating and NS coating. There were micro cracks, laminar splats and straight columnar grains in ME coating. For the two nanostructured coatings, the laminar microstructure and columnar grains were not obvious. The NP coating had the highest hardness and spallation resistance. Electrochemical corrosion behaviour of the three coatings was mainly investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in aqueous Na₂SO₄ solution.     

等离子喷涂Al_2O_(3p)/NiCrBSi涂层优化设计

利用二次回归正交组合设计法优化喷涂工艺参数,建立了等离子喷涂Al2O3p/NiCrBSi复合材料涂层冲蚀磨损失重率的二次回归方程.研究了等离子喷涂参数对涂层冲蚀磨损失重率的影响.结果表明,利用Matlab软件编程计算,优化等离子喷涂参数,明显提高了涂层的抗冲蚀磨损性能.为等离子喷涂制备以氧化铝(Al2O3)颗粒增强金属基(NiCrBSi)复合材料涂层(Al2O3p/NiCrBSi)的实际应用提供必要的理论依据和技术数据.     

Toughening and strengthening mechanism of plasma sprayed nanostructured Al2O3–13 wt.%TiO2 coatings

The starting materials of Al₂O₃, TiO₂, ZrO₂ and CeO₂ nanoparticles were agglomerated into sprayable feedstock powders and plasma sprayed to form nanostructured coatings. There were net structures and fused structures in plasma sprayed nanostructured Al₂O₃–13 wt.%TiO₂ coatings. The net structures were derived from partially melted feedstock powders and the fused structures were derived from fully melted feedstock powders. The nanostructured Al₂O₃–13 wt.%TiO₂ coatings possessed higher hardness, bonding strength and crack growth resistance than conventional Metco 130 coatings which were mainly composed of lamellar fused structures. The higher toughness and strength of nanostructured Al₂O₃–13 wt.%TiO₂ coatings were mainly related to the obtained net structures.     

Microstructure and mechanical properties of Al2O3-Cr2O3 composite coatings produced by atmospheric plasma spraying

Al₂O₃, Al₂O₃-Cr₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying. Phase composition of powders and as-sprayed coatings was determined by X-ray diffraction. Electron probe microanalyzer was employed to investigate the polished and fractured surface morphologies of the coatings. Mechanical properties including microhardness, fracture toughness and bending strength were evaluated. The results indicate that the addition of Cr₂O₃ is conducive to the stabilization of α-Al₂O₃. Compared with the pure Al₂O₃ and Cr₂O₃ coatings, Al₂O₃-Cr₂O₃ composite coatings show lower porosities and denser structures. Heterogeneous nucleation of α-Al₂O₃ occurs over the isostructural Cr₂O₃ lamellae and partial solid solution of Al₂O₃ and Cr₂O₃ might be occurring as well. Furthermore, grain refining and solid solution strengthening facilitate the mechanical property enhancement of Al₂O₃-Cr₂O₃ composite coatings.     

Effects of alloying elements on microstructure and protective properties of Al2O3 coatings formed on aluminum alloy substrates by plasma electrolysis

Hard alumina coatings were formed on three typical Al alloys using a plasma electrolytic oxidation (PEO) process. The microstructure and protective behavior of the coatings were characterized in the context of the elements alloyed in the substrates. The substrates investigated were three commercial aluminum alloys (Al 6061, 2024, 7075) and various Al − x–Mg binary alloys with x = 0.6–4.6 wt.%. The major portion of all coating films consisted of a mixture of γ- and α-alumina on top of a very thin amorphous substrate-based layer. Examination of the coatings on the commercial alloys reveals that the ratio of α- to γ-alumina is inversely proportional to the total concentration of residual Mg, Cu, and Zn ions in the coatings. Additional experiments on Al − x–Mg binary alloy substrates suggest that Mg ions play the most important role in suppressing the transition of γ- to α-alumina. Our results suggest that Mg content ≥ 3 wt.% yields only the γ-alumina. We propose a mechanism that correlates the residual elements in the coatings to the alumina phase transition. A higher content of the dense α-alumina in the coatings yielded greater hardness as well as improved wear tolerance and crack resistance of the coating layers.     

Analysis of phase transformation in plasma sprayed alumina coatings using Rietveld refinement

During plasma spraying of alumina with the stable α phase in the starting powder, metastable phases tend to form in the final coating. This is attributed to the rapid quenching associated with the process. In this paper the weight fraction of metastable phase formed, i.e., stable phase retained, and has been estimated using Rietveld refinement of X-ray diffraction data. This weight fraction depends on the process parameters like standoff distance, primary and secondary gas flow rate, nozzle size, etc., which in turn control particle melting. Under favourable melting conditions the weight fraction of the metastable phases approaches 1.     

Microstructure and properties of CVD γ-Al2O3 coatings

This work deals with the microstructures and wear properties of chemical vapour deposited γ-Al₂O₃. The γ-Al₂O₃ coatings were deposited at 800 °C on TiN and Ti(C,N) pre-coated cemented carbide substrates. The microstructures developed in the γ-Al₂O₃ coatings and the influence of the nucleation surface on the growth of γ-Al₂O₃ were characterised using transmission electron microscopy, electron energy-loss spectroscopy and X-ray diffraction. The γ-Al₂O₃ coatings were fine-grained with a high density of {1 1 1} growth twins and contained some residual sulphur. γ-Al₂O₃ was found to grow epitaxially on the investigated substrates. The mechanical properties were evaluated in metal cutting and were compared with those of κ-Al₂O₃ coated tools. As compared with the κ-Al₂O₃ coatings, the γ-Al₂O₃ coatings exhibited slightly worse adhesion and tendency for edge chipping. However, the γ-Al₂O₃ coatings showed better crater wear resistance on the rake face than κ-Al₂O₃ coatings.     

Thermal shock behavior of nanostructured and conventional Al2O3/13 wt%TiO2 coatings fabricated by plasma spraying

The thermal shock behavior of three kinds of Al₂O₃/13 wt%TiO₂ coatings fabricated by plasma spraying was studied in this paper. One kind of those coatings was derived from conventional fused and crushed feedstock powder available commercially; the other two kinds of coatings were derived from nanostructured agglomerated feedstock powders. These two nano coatings possess moderate pores and pre-existing microcracks, they were composed of fused structure and three-dimensional net or skeleton-like structure. For conventional coatings, the pores and pre-existing cracks were bigger, sharp-point and mostly distributed between splats. Thermal shock tests for the three coatings were performed by water quenching method. Testing result showed the two kinds of nano coatings had much higher thermal shock resistance than the conventional coatings. The improved thermal shock resistance for nano coatings could attribute to their improved microstructure and crack propagation mode. The damage evolution and failure mechanism of coatings was quite different at thermal shock temperature of 650 °C and 850 °C, which was explained by a simple model. Different crack propagating modes in nanostructured and conventional coatings during thermal shock tests were due to their different microstructures in these two kinds coatings. The stress state of coating surfaces during the thermal cycles was also discussed in this paper.     

TiN/Cr/Al2O3 and TiN/Al2O3 hybrid coatings structure features and properties resulting from combined treatment

New experimental results are presented on the structure and the elemental and phase composition of hybrid coatings, which were deposited on a substrate of AISI 321 stainless steel using a combination of plasma-detonation, vacuum-arc and subsequent High-Current Electron Beam (HCEB) treatment. We found that an increase in energy density intensified mass transfer processes and resulted in changes in aluminum oxide phase composition (γ → α and β → α). Also we observed the formation of a nanocrystalline structure in Al₂O₃ coatings. Electron beam treatment of a hybrid coating surface induced higher adhesion, decreased the intensity of surface wear and increased corrosion resistance in a sulphuric acid solution. The corrosion resistance of the coatings was studied in several electrolytic solutions (0.5 M H₂SO₄, 1 M HCl, 0.75 M NaCl) using electrochemical techniques. In most cases the corrosion resistance was improved, except those in NaCl solutions. The nano-hardness of the protecting coating was 13 GPa before electron beam melting and 9 GPa after it (as a result of TiN and Al₂O₃ sub-layers mixing).     

In-situ TiB2 and Al2O3 formation by DC plasma spraying

In-situ plasma spraying (IPS) is a promising process to fabricate composite coatings with in-situ formed thermodynamically stable phases. In the present study, mechanically alloyed Al-12Si, B₂O₃ and TiO₂ powder was deposited onto an aluminum substrate using atmospheric plasma spraying (APS). It has been observed that, during the coating process, TiB₂ and Al₂O₃ are in-situ formed through the reaction between starting powders and finely dispersed in hypereutectic Al-Si matrix alloy. Also, obtained results demonstrate that in-situ reaction intensity strongly depends on spray conditions.     

Hot corrosion behaviour of plasma sprayed YSZ/Al2O3 dispersed NiCrAlY coatings on Inconel-718 superalloy

Oxide dispersed NiCrAlY bond coatings have been developed for enhancing thermal life cycles of thermal barrier coatings (TBCs). However, the role of dispersed oxides on high temperature corrosion, in particular hot corrosion, has not been sufficiently studied. Therefore, the present study aims to improve the understanding of the effect of YSZ dispersion on the hot corrosion behaviour of NiCrAlY bond coat. For this, NiCrAlY, NiCrAlY + 25 wt.% YSZ, NiCrAlY + 50 wt.% YSZ and NiCrAlY + 75 wt.% YSZ were deposited onto Inconel-718 using the air plasma spraying (APS) process. Hot corrosion studies were conducted at 800 °C on these coatings after covering them with a 1:1 weight ratio of Na₂SO₄ and V₂O₅ salt film. Hot corrosion kinetics were determined by measuring the weight gain of the specimens at regular intervals for a duration of 51 h. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques were used to determine the nature of phases formed, examine the surface attack and to carry out microanalysis of the hot corroded coatings respectively. The results show that YSZ dispersion causes enhanced hot corrosion of the NiCrAlY coating. Leaching of yttria leads not only to the formation of the YVO₄ phase but also the destabilization of the YSZ by hot corrosion. For the sake of comparison, the hot corrosion behaviour of a NiCrAlY + 25 wt.% Al₂O₃ coating was also examined. The study shows that the alumina dispersed NiCrAlY bond coat offers better hot corrosion resistance than the YSZ dispersed NiCrAlY bond coat, although it is also inferior compared to the plain NiCrAlY bond coat.     

Microstructure and adhesion of Cr3C2-NiCr vacuum plasma sprayed coatings

Vacuum plasma spraying (VPS) was used to spray a Cr₃C₂-NiCr coating of ∼ 150, 300 and 450 μm in thickness onto a plain carbon steel substrate, employing a commercially available Cr20Ni9.5C powder. The splat microstructures observed in the coating were found to consist of a NiCr matrix with a predominant Cr₃C₂ phase, besides Cr₇C₃ and Cr₂O₃. The adhesion of the coating to the substrate was evaluated by means of interfacial indentation techniques. It has been found that the interfacial toughness value changes from 7.6 to 10.1 MPa m^1/2 when the thickness increases from 150 to 450 μm. Also, it has been found that the parameter Kca_o, determined by linear regression from the Kca versus 1 / e² curve by means of the interfacial indentation model advanced by Chicot et al., has a value of ∼ 9.8 MPa m^1/2.     

In-situ TiB2-Al2O3 formed composite coatings by atmospheric plasma spraying: Influence of process parameters and in-flight particle characteristics

In the present study, mechanically alloyed Al-12Si, B₂O₃ and TiO₂ powder was deposited onto an aluminum substrate using atmospheric plasma spraying (APS). The effects of mechanical alloying (MA) time and plasma parameters (arc current and primary/secondary/carrier gas flow rate) on in-situ reaction intensity and in-flight particle characteristics (temperature and velocity) have been investigated. It has been observed that MA time has a remarkable effect on powder morphology and relative amount of in-situ formed TiB₂ and γ-Al₂O₃. In-flight particle diagnostic measurements demonstrate that among the plasma parameters arc current has the strongest effect on in-flight particle velocity and temperature. Also, results indicate that in-flight particle velocity is more dominant than temperature on the relative amount of in-situ formed phases.     

Thermal cycle behavior of plasma sprayed La2O3, Y2O3 stabilized ZrO2 coatings

The effects of La₂O₃ addition on thermal conductivity, phase stability and thermal cycle life of Y₂O₃ stabilized ZrO₂ plasma sprayed coatings were investigated. Although low thermal conductivity as well as high resistance to sintering was achieved by La₂O₃ addition, it tended to also result in lower phase stability and thermal cycle life of the coatings. Optimization of the composition and structure of the coatings improved these properties, and the optimized coatings showed prolonged thermal cycle life.     

Mechanical and tribological performance of Al2O3-TiO2 coatings elaborated by flame and plasma spraying

Mechanical properties and wear rates of Al₂O₃-13 wt.% TiO₂ (AT-13) and Al₂O₃-43 wt.% TiO₂ (AT-43) coatings obtained by flame and atmospheric plasma spraying were studied. The feed stock was either ceramic cords or powders. Results show that the wear resistance of AT-13 coatings is higher than that of AT-43 and it seems that the effect of hardness on wear resistance is more important than that of toughness. Additionally, it was established that, according to conditions used to elaborate coatings and the sliding tribological test chosen, spray processes do not seem to have an important effect on the wear resistance of these coatings.     

Corrosion behavior of Cr3C2-NiCr vacuum plasma sprayed coatings

The objective of the present work is to determine the influence of the heat treatment on the corrosion resistance of a Cr₃C₂-NiCr coating of 450 μm thickness, deposited by a vacuum plasma spray process (VPS) on a steel substrate. The post-heat treatment of the as-deposited coating was carried out in Ar at 400 °C and 800 °C, respectively. The coatings were characterized by means of an electron probe micro analyzer (EPMA) with wavelength dispersive X-ray spectrometers (WDS). It was found that no significant changes were produced as a consequence of the heat treatment carried out at 400 °C. Therefore, the corrosion experiments were conducted for the substrate, the as-deposited coating and the post-heat treated coating at 800 °C. Potentiodynamic polarization showed that the annealed coating at 800 °C has a better corrosion resistance than the as-deposited coating. The corrosion current density (I_corr) of this coating was approximately 3 and 4 times smaller than that corresponding to the as-deposited coating and steel substrate, respectively. This significant improvement of the corrosion behavior of the post-heat treated coating is mainly due to both the microstructural changes that take place in the coating and the diffusion of Ni into Fe at the coating-substrate interface, which ensures the presence of a metallurgical bond.