Shunyan Tao

Wear characteristics of plasma-sprayed nanostructured yttria partially stabilized zirconia coatings

Reconstituted nanostructured and conventional yttria partially stabilized zirconia coatings were deposited by atmospheric plasma spray. The tribologic properties of the coatings against 100C6 steel were evaluated with a ball-on-disc configuration under dry friction conditions at room temperature. Microstructure and the phase composition of the powders and the coatings were examined using a scanning electron microscope, optical microscope, and x-ray diffraction. Microhardness and the Young’s modulus of coatings were measured by indentation testing. Results showed that the wear resistance of the coatings produced using the nanostructured powder is improved compared with the coating produced using the conventional powder. The wear rates of nanostructured zirconia coatings are about four-fifths of those of conventional counterparts under a load of 5 N. The wear mechanism is also discussed.

Evaluating microhardness of plasma sprayed Al2O3coatings using Vickers indentation technique

In this work, the microhardness of plasma sprayed Al2O3 coatings was evaluated using the Vickers indentation technique, and the effects of measurement direction, location and applied loads were investigated. The measured data sets were then statistically analysed employing the Weibull distribution to evaluate their variability within the coatings. It was found that the Vickers hardness (VHN) increases with decreasing applied indenter load, which can be explained in terms of Kicks law and the Meyer index k of 1.93, as well as relating to the microstructural characteristics of plasma sprayed coatings and the elastic recovery taking place during indentation. In addition, VHN, measured on the cross section of coatings, was obviously higher than that on its top surface. The obtained Weibull modulus and variation coefficient indicate that the VHN was less variable when measured at a higher applied load and on the cross section of coating. The obvious dependence of the VHN on the specific indentation location within through-thickness direction was also realized. These phenomena described above in this work were related to the special microstructure and high anisotropic behaviour of plasma sprayed coatings.

Sliding Wear Performance of Plasma-Sprayed Al2O3-Cr2O3 Composite Coatings Against Graphite under Severe Conditions

Al2O3, Cr2O3, and Al2O3-Cr2O3 composite coatings were produced by plasma spraying. Their tribological properties were evaluated at high load conditions. The average friction coefficients, wear rates, and worn surface temperatures of the coating/graphite pairs were measured. Compared with the single coating/graphite pairs, the friction coefficients of composite coating/graphite pairs are more stable. The corresponding wear rates and worn surface temperatures are lower, which may be conducive to the formation of more effective and stable graphite transfer film on the surface of the coating subjected to abrasion. Especially, 10wt.%Al2O3-90wt.%Cr2O3 (AC90) composite coating shows better anti-wear performance, which may be attributed to its higher thermal conduction.

Design and optimization of coating structure for the thermal barrier coatings fabricated by atmospheric plasma spraying via finite element method

Abstract The first prerequisite for fabricating the thermal barrier coatings (TBCs) with excellent performance is to find an optimized coating structure with high thermal insulation effect and low residual stress. This paper discusses the design and optimization of a suitable coating structure for the TBCs prepared by atmospheric plasma spraying (APS) using the finite element method. The design and optimization processes comply with the rules step by step, as the structure develops from a simple to a complex one. The research results indicate that the suitable thicknesses of the bond-coating and top-coating are 60–120 μm and 300–420 μm, respectively, for the single ceramic layer YSZ/NiCoCrAlY APS-TBC. The embedded interlayer (50 wt.%YSZ + 50 wt.%NiCoCrAlY) will further reduce the residual stress without sacrificing the thermal insulation effect. The double ceramic layer was further considered which was based on the single ceramic layer TBC. The embedded interlayer and the upper additional ceramic layer will have a best match between the low residual stress and high thermal insulation effect. Finally, the optimized coating structure was obtained, i.e., the La2Ce2O7(LC)/YSZ/Interlayer/NiCoCrAlY coating structure with appropriate layer thickness is the best choice. The effective thermal conductivity of this optimized LC/YSZ/IL/BL TBC is 13.2% lower than that of the typical single ceramic layer YSZ/BL TBC.

Microstructural Characterization and Strengthening-Toughening Mechanism of Plasma-Sprayed Al2O3-Cr2O3 Composite Coatings

In this study, Al2O3, Cr2O3, and Al2O3-Cr2O3 coatings were fabricated by plasma spraying. X-ray diffraction was employed to determine the phase composition of powders and coatings. The morphologies and microstructures of the coatings were characterized using electron probe microanalyzer and transmission electron microscopy. Vickers hardness, fracture toughness, and bending strength of the coatings were measured. Al2O3-Cr2O3 composite coatings show better comprehensive mechanical properties than the individual Al2O3 and Cr2O3 coatings, which are attributed to the formers larger intersplat adhesion or interlamellar cohesion and lower porosity. Solid solution strengthens the phase interfaces and grain boundaries, which is beneficial to improve the mechanical performance of the composite coatings.

Influence of “Island-Like” Oxides in the Bond-Coat on the Stress and Failure Patterns of the Thermal-Barrier Coatings Fabricated by Atmospheric Plasma Spraying During Long-Term High Temperature Oxidation

Thermal-barrier coatings (TBCs) are very important ceramic-coating materials due to their excellent performance at high temperature. The inner zone of the bond-coat is often easily endured oxidized (internal oxidation) in the process of thermal spraying and the long-time exposure to the high temperature, and the “island-like” oxides can be formed. Especially, when the bond-coat was fabricated by atmospheric plasma spraying (APS), this trend is more evident. In this paper, the stress distribution around the thermally grown oxide (TGO) has been calculated by the finite element method when the “island-like” oxides have been considered. The simulation results indicate that the maximum tensile stress and compressive stress existed in the TGO, and the existence of the “island-like” oxides will further decrease the maximum tensile stress level in the TGO. While the “island-like” oxides in the bond-coat will decrease the effective thickness of the TGO at the metallic layer/ceramic layer interface due to the oxidation of the metallic elements in the bond-coat. The crack propagation equation has been established and the failure mechanism of the TBC due to the formation and growth of the TGO has also been discussed in detail. The lifetime of the TBCs which have experienced high temperature oxidation has been predicted and the theoretical results agreed well with the experimental data.

Investigation of Crack Propagation Behavior of Atmospheric Plasma-Sprayed Thermal Barrier Coatings under Uniaxial Tension Using the Acoustic Emission Technique

Uniaxial tension is a common technique to characterize the adhesive strength of plasma-sprayed thermal barrier coatings (TBCs). In this work, the crack initiation, growth, and propagation behavior of atmospheric plasma-sprayed TBCs during uniaxial tension testing was investigated using the acoustic emission (AE) technique, x-ray diffraction analysis, scanning electron microscopy, and the finite-element method (FEM). The experimental results indicated that the position of crack initiation was usually located within the ceramic layer, and the crack tended to propagate along the tension direction, with some key horizontal cracks reaching the metallic layer/ceramic layer interface, after which vertical cracks initiating at the middle and lower segments of the horizontal cracks propagated along the interface. When some critical cracks were formed at the interface and a series of assembled splats separated from the coating, the coating failed completely. The AE signal could be divided into three typical stages, corresponding to the three stages of the stress–stain curve under uniaxial tension. Detailed analysis of the AE signal associated with the failure behavior was performed. The dynamic propagation patterns of the key cracks in the ceramic layer during the tension process were simulated using the FEM, whose results further confirmed the conclusions drawn from the experimental results.

Mechanism in reactive plasma spraying synthesis of TiC–TiB2 composite coating

Abstract Self-propagating high-temperature synthesis reaction in the LaMgAl11O19–Ti–B4C system was quenched in the glove box and during plasma spraying, respectively, in order to clarify the formation mechanism of in situ TiC–TiB2 composite coating. Microstructure of the quenched samples was investigated by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectrometer. The results showed that the formation mechanism of TiC and TiB2 during reactive plasma spraying is same as that in the glove box, namely the Ti–B–C melt is first formed by B4C particles dissolving into the Ti–B eutectic liquid as well as the molten Ti, and then TiC and TiB2 are formed and precipitated from the saturated Ti–B–C melt.

Effect of the Bond Coating Surface Morphology on Ceramic Splat Construction

In this work, yttria-stabilized zirconia splats deposited onto different bond coatings were characterized. The influence of both substrate topography and temperature on splat morphologies was investigated. The substrate surface was described using a method characterized by multi-level roughness. On non-heated substrate, the peak-valley structure in scale of micrometer contributed to the formation of crater-like holes on splats. The amount of such holes was reduced with the increase of substrate temperature. It was also observed that on micro-smooth surface, island-like fragments in the center of splat would warp and their edges could be detached from the underlying substrate, especially when the substrate was heated. The obtained results reveal that the influence of substrate pre-heating on the splat formation is realized via thinning the air film near substrate surface.

Evolution and Prospect of Thermal Spraying Technique: Evolution and Prospect of Thermal Spraying Technique

TiB 2 , ZrB 2 , HfB 2 , B 4 C及BN为代表的硼化物陶瓷具有优异的物理化学性能, 在超高温、超硬以及超疏水等极限条件下有广阔的应用前景, 但材料的烧结致密化困难、断裂韧性低等问题制约了它们更为广泛的应用. 本文针对无压烧结在材料制备过程中的优势, 探讨了影响硼化物陶瓷无压烧结的主要因素, 总结了以“除氧”机制为代表的硼化物陶瓷无压烧结技术; 针对硼化物陶瓷韧性低的不足, 介绍了以“板晶增韧”、“纳米相增强”为代表的硼化物陶瓷微结构调控手段和强韧化措施. 最后, 文章还对硼化物陶瓷的织构化设计、制备方法与性能提升进行了简要介绍.TiB 2 , ZrB 2 , HfB 2 , B 4 C及BN为代表的硼化物陶瓷具有优异的物理化学性能, 在超高温、超硬以及超疏水等极限条件下有广阔的应用前景, 但材料的烧结致密化困难、断裂韧性低等问题制约了它们更为广泛的应用. 本文针对无压烧结在材料制备过程中的优势, 探讨了影响硼化物陶瓷无压烧结的主要因素, 总结了以“除氧”机制为代表的硼化物陶瓷无压烧结技术; 针对硼化物陶瓷韧性低的不足, 介绍了以“板晶增韧”、“纳米相增强”为代表的硼化物陶瓷微结构调控手段和强韧化措施. 最后, 文章还对硼化物陶瓷的织构化设计、制备方法与性能提升进行了简要介绍.