An ultrasonic testing method for detecting delamination of sprayed ceramic coating

The adhesion strength of thermal sprayed coatings is relatively low, and they sometimes tend to delaminate from the substrate during operation. In particular, sprayed ceramic coatings for thermal barriers, such as ZrO₂, often delaminate because of thermal shock; therefore, ceramic coatings are often submitted to thermal shock tests. A nondestructive inspection method using ultrasound to detect the delamination of sprayed coating was proposed. In this study, a coating model was made with acrylic plates, and an ultrasonic test was applied to investigate the precision of detecting delamination by the ultrasonic testing method. Results indicate that delamination more than 1mm in diameter can be detected by the ultrasonic testing method. Moreover, the delaminating process of sprayed coatings under thermal shock tests can be detected by this method.     

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.     

Modeling Thermal Conductivity of Thermally Sprayed Coatings with Intrasplat Cracks

Thermal spraying is one of the most important approaches for depositing thermally insulating ceramic top coatings for advanced gas turbines due to the low thermal conductivity of the coating resulting from its lamellar structure. The thermal conductivity of the coating has been explained based on the concept of thermal contact resistance and correlated to microstructural aspects such as splat bonding ratio, splat thickness, and the size of the bonded areas. However, the effect of intrasplat cracks on the thermal conductivity was usually neglected, despite the fact that intrasplat cracking is an intrinsic characteristic of thermally sprayed ceramic coatings. In this study, a model for the thermal conductivity of a thermally sprayed coating taking account of the effect of intrasplat cracks besides intersplat thermal contact resistance is proposed for further understanding of the thermal conduction behavior of thermally sprayed coatings. The effect of the intersplat bonding ratio on the thermal conductivity of the coating is examined by using the model. Results show that intrasplat cracks significantly decrease the thermal conductivity by cutting off some heat flux paths within individual splats. This leads to a deviation from the typical ideal thermal contact resistance model which presents cylindrical symmetry. Based on the modified model proposed in this study, the contribution of intrasplat cracks to the thermal resistivity can be estimated to be 42–57 % for a typical thermally sprayed ceramic coating. The results provide an additional approach to tailor the thermal conductivity of thermally sprayed coatings by controlling the coating microstructure.     

Simultaneous determination of the coating thickness and its longitudinal velocity by ultrasonic nondestructive method

The ultrasonic reflection coefficient amplitude spectrum (URCAS) has been extended to obtain the coating thickness and its longitudinal velocity at the same time on thick substrates. A model was set up first to represent the ultrasonic waves reflected from a coating system at normal incident, and the expression of URCAS was derived in order to obtain the coating thickness and velocity. Then, an inverse algorithm based on the Gauss–Newton method was introduced to determine the thickness and velocity by comparing the theoretical and measured URCAS. Experimental validation was conducted on the homogeneous epoxy coatings on the aluminum substrates and inhomogeneous ZrO₂?7 wt%Y₂O₃ (YSZ) coatings on the superalloy substrates. The relative errors of the thickness and velocity measurement were in the ranges of 2.15–2.35% and 2.67–4.40% for epoxy coatings, and between 5.33–5.96% and 8.95–9.66% for YSZ coatings. It is concluded that the URCAS combined with inversion technique can be applied to obtain the thickness and longitudinal velocity of coatings simultaneously.     

Properties of alumina-titania coatings prepared by laser-assisted air plasma spraying

Studies have shown that microstructures formed by post-laser remelting of air plasma sprayed coatings exhibit densification but also numerous macrocracks due to the rapid cooling and thermal stresses. In laser-assisted air plasma spraying (LAAPS) process, the laser beam interacts simultaneously with the plasma torch in order to increase the temperature of the coating and possibly remelt the coating at the surface. As a result, the microstructure is partially densified and macrocracks, which are generally produced in the post-laser irradiation treatment, may be inhibited. In this paper, LAAPS was performed to improve the hardness and wear resistance of Al₂O₃-13%TiO₂ coatings. These coatings prepared by air plasma spraying (APS) are widely used to protect components against abrasive wear at low temperatures. The coating microstructure was characterized by SEM and X-ray diffraction. The mechanical characterization was done by hardness measurements, erosive wear tests and abrasion wear tests. Results showed that laser assistance may improve the microstructural and mechanical properties. Phenomena involved in LAAPS of alumina-titania coatings are discussed in this paper.     

Optimization of Air Plasma Sprayed Thermal Barrier Coating Parameters in Diesel Engine Applications

In the present paper, an optimization of thermal barrier coating parameters is performed for diesel engine applications. The substrate is A356.0-T7, a cast aluminum alloy which has a vast application in diesel engines, and the alloy is coated by plasma sprayed ZrO₂-8 wt.% Y₂O₃. Parameters including the feed rate of coating powders, the nozzle distance to specimen surfaces, and the coating thickness are optimized by thermal shock fatigue tests and bending tests. Optimum values of the feed rate and the nozzle distance are 30 g/min and 80 mm, respectively, when the objective is considered as maximizing the bending strength. Thermal shock tests demonstrate that lower thickness of coating layers has a better lifetime. By increasing the coating thickness, the thermal fatigue lifetime decreases. The reason is due to higher order of stresses near the interface of the substrate and the bond coat layer, calculated by a finite element simulation. One suggestion to improve the lifetime is to use multiple layers of coatings.     

The effects of heat treatment and gas atmosphere on the thermal conductivity of APS and EB-PVD PYSZ thermal barrier coatings

The effects of heat treatment and gas atmosphere on thermal conductivity of atmospheric plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) partially Y₂O₃ stabilized ZrO₂ (PYSZ) thermal barrier coatings (TBCs) were investigated. Two-layer samples that had an EB-PVD coating deposited on bond coated nickel-base superalloy IN625 substrates, free-standing APS and EB-PVD coatings as well as a quasi-free-standing EB-PVD PYSZ coating (coating on semitransparent sapphire) were included in the study. Thermal diffusivity measurements for determining thermal conductivity were made from room temperature up to 1150 °C in vacuum and under argon gas using the laser flash technique. To investigate the effect of heat treatment on thermal conductivity, coatings were annealed at 1100 °C in air. For both the APS and EB-PVD PYSZ coatings the first 100 h heat treatment caused a significant increase in thermal conductivity that can be attributed to microstructural changes caused by sintering processes. Compared to the measurements in vacuum, the thermal conductivity of APS coatings increased by about 10% under argon gas at atmospheric pressure, whereas for the EB-PVD coatings, the influence of gas on thermal conductivity was relatively small. The effect of gas on the thermal conductivity of APS and EB-PVD PYSZ coatings can be attributed to amount, shape, and spatial arrangement of pores in the coating material.     

Biocompatible nanostructured high-velocity oxyfuel sprayed titania coating: Deposition, characterization, and mechanical properties

Nanostructured titania (TiO₂) coatings were produced by high-velocity oxyfuel (HVOF) spraying. They were engineered as a possible candidate to replace hydroxyapatite (HA) coatings produced by thermal spray on implants. The HVOF sprayed nanostructured titania coatings exhibited mechanical properties, such as hardness and bond strength, much superior to those of HA thermal spray coatings. In addition to these characteristics, the surface of the nanostructured coatings exhibited regions with nanotextured features originating from the semimolten nanostructured feedstock particles. It is hypothesized that these regions may enhance osteoblast adhesion on the coating by creating a better interaction with adhesion proteins, such as fibronectin, which exhibit dimensions in the order of nanometers. Preliminary osteoblast cell culture demonstrated that this type of HVOF sprayed nanostructured titania coating supported osteoblast cell growth and did not negatively affect cell viability. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

The effect of Mo content in plasma-sprayed Mo-NiCrBSi coating on the tribological behavior

NiCrBSi is a material popularly used as a hard thermal sprayed coating. The coating performs well as a wear resistant coating under low stress. At higher stress in metal-to-metal sliding wear condition, however, the NiCrBSi starts to experience surface deformation, which will inevitably lead to seizure as the stress increases. In order to improve the tribological properties of the NiCrBSi plasma-sprayed coating, Mo is added to the coating to reduce the friction between the coating and other metal contacting surface, thus, improving its dry sliding wear resistance. In this study, various amounts of Mo were mixed with NiCrBSi at 0, 25, 50, 75 and 100 wt.%. The powders were sprayed using an air plasma spraying technique onto stainless steel samples to form coatings, which were ground to achieve flat surfaces and a thickness of 350-400 μm. The mechanical properties of the coatings were determined. The coating samples were then tested using a reciprocation ball-on-flat tribometer. It was found that as the Mo/NiCrBSi ratio increases, the wear mechanism changes. Coatings containing 75%Mo and 25%NiCrBSi exhibit the highest wear depths corresponding to the cracking of the thin NiCrBSi splats. On the other hand, coatings containing 25%Mo and 75%NiCrBSi possess the lowest wear depths with no surface cracks. The presence of Mo covering the coating surface hinders the metal seizure between NiCrBSi and steel counter surface.     

Improving corrosion properties of high-velocity oxy-fuel sprayed inconel 625 by using a high-power continuous wave neodymium-doped yttrium aluminum garnet laser

Thermal spray processes are widely used to protect materials and components against wear, corrosion and oxidation. Despite the use of the latest developments of thermal spraying, such as high-velocity oxy-fuel (HVOF) and plasma spraying, these coatings may in certain service conditions show inadequate performance,e.g., due to insufficient bond strength and/or mechanical properties and corrosion resistance inferior to those of corresponding bulk materials. The main cause for a low bond strength in thermalsprayed coatings is the low process temperature, which results only in mechanical bonding. Mechanical and corrosion properties typically inferior to wrought materials are caused by the chemical and structural inhomogeneity of the thermal-sprayed coating material. To overcome the drawbacks of sprayed structures and to markedly improve the coating properties, laser remelting of sprayed coatings was studied in the present work. The coating material was nickel-based superalloy Inconel 625, which contains chromium and molybdenum as the main alloying agents. The coating was prepared by HVOF spraying onto mild steel substrates. High-power continuous wave Nd:YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of power and scanning speed. The coatings as-sprayed and after laser remelting were characterized by optical microscopy and scanning electron microscopy (SEM). Laser remelting resulted in homogenization of the sprayed structure. This strongly improved the performance of the laser-remelted coatings in adhesion, wet corrosion, and high-temperature oxidation testing. The properties of the laser-remelted coatings were compared directly with the properties of as-sprayed HVOF coatings and with plasma-transferred arc (PTA) overlay coatings and wrought Inconel 625 alloy.     

电弧喷涂锡基巴氏合金涂层的组织与性能

针对电弧喷涂巴氏合金涂层组织与铸造组织的显著差异,借助于扫描电子显微镜、能谱分析和X射线衍射分析涂层的微观组织结构;测试了巴氏合金涂层在钢铁基体上的结合强度,研究了涂层与碳钢及铸铁组成摩擦副时的磨损表现.结果表明,电弧喷涂巴氏合金涂层的组织细小、均匀,SnSb和Cu6Sn5化合物的形态趋于不规则的块状或近于球形;施加过渡底层可以使涂层更可靠地与钢铁基体结合;在润滑条件下,涂层表现出比铸造合金更好的耐摩性能.     

Hybrid polytetrafluoroethylene oxide coatings on aluminum and titanium formed by plasma-electrolytic oxidation

Regularities of the formation of plasma-electrolytic oxide coatings on aluminum and titanium from an aqueous Na₂SiO₃ + NaOH electrolyte solution containing silicate-acrylate emulsion and dispersed polytetrafluoroethylene particles are studied. Coatings, the surface part of which is composed chiefly of polytetrafluoroethylene and destruction products of polytetrafluoroethylene and emulsion, are produced. The water contact angles of the coatings may be as large as 105°. The presence of polytetrafluoroethylene in the coatings increases the time of their mechanical abrasion by several exponents. The dependence of the composition and thickness of coatings and the microstructure of the coating surfaces on the content of polytetrafluoroethylene in the electrolyte is considered.     

Understanding the Formation of Limited Interlamellar Bonding in Plasma Sprayed Ceramic Coatings Based on the Concept of Intrinsic Bonding Temperature

Interlamellar bonding is an important factor controlling the mechanical, thermal and electrical properties of plasma sprayed ceramic coatings. In order to understand the formation of limited interlamellar bonding, a theoretical model is proposed based on the concept of the intrinsic bonding temperature. The numerical simulation of the interface temperature between a molten splat and underlying splats was performed for splats with uniform and non-uniform thickness, in order to reveal the conditions for the interlamellar bonding formation. The interlamellar bonding ratio was theoretically estimated based on the bonding forming conditions. The features of interlamellar bonding revealed by the simulation agree well with the experimental observations. The bonding ratio of plasma sprayed coatings is significantly influenced by the distribution of splat thickness. According to the distribution of Al₂O₃ splat thickness in the coating, the theoretical estimation of bonding ratio yielded a value of 0.41 for the plasma sprayed Al₂O₃ coating at the ambient atmosphere conditions, which is reasonably consistent with the observation value. Therefore, the limited interlamellar bonding can be reasonably explained based on the sufficient condition that the maximum interface temperature between a molten splat and underlying splats is larger than the intrinsic bonding temperature.     

Mechanical Properties of Double-Layer and Graded Composite Coatings of YSZ Obtained by Atmospheric Plasma Spraying

Double-layer and graded composite coatings of yttria-stabilized zirconia were sprayed on metallic substrates by atmospheric plasma spray. The coating architecture was built up by combining two different feedstocks: one micro- and one nanostructured. Microstructural features and mechanical properties (hardness and elastic modulus) of the coatings were determined by FE-SEM microscopy and nanoindentation technique, respectively. Additional adherence and scratch tests were carried out in order to assess the failure mechanisms occurring between the layers comprising the composites. Microstructural inspection of the coatings confirms the two-zone microstructure. This bimodal microstructure which is exclusive of the layer obtained from the nanostructured feedstock negatively affects the mechanical properties of the whole composite. Nanoindentation tests suitably reproduce the evolution of mechanical properties through coatings thickness on the basis of the position and/or amount of nanostructured feedstock used in the depositing layer. Adhesion and scratch tests show the negative effect on the coating adhesion of layer obtained from the nanostructured feedstock when this layer is deposited on the bond coat. Thus, the poor integrity of this layer results in lower normal stresses required to delaminate the coating in the adhesion test as well as minor critical load registered by using the scratch test.     

热喷涂涂层残余应力的测试与分析

利用改进的Ａｌｍｅｎ试验方法对多种热喷涂涂层的残余应力进行了测试与分析。首次提出了热喷涂涂层应变滞后于温度或外加应力这种现象的物理模型,并提出了因热喷涂涂层本身的多孔特性而引起的“气塞”作用是造成这一现象的主要原因。试验证明了热喷涂涂层的残余应力与涂层的厚度是线性关系,并且无论涂层的残余应力是压应力还是拉应力,它们都随涂层厚度的增加而增大。本文还证实了热喷涂工艺方法对涂层残余应力有着重大的影响,其     

Evaluation of wear damage in zirconia plasma-sprayed coatings using scanning white light interferometry

The mechanical and tribological properties of thermal barrier coatings (TBCs) can be improved by means of a thermal treatment. The evolution of the mechanical and tribological properties in a NiCr-ZrO₂ TBC with different times of thermal treatment has been measured. In this work, scanning white light interferometry (SWLI) is used to observe and quantify the ZrO₂ wear damage. ZrO₂ shows very poor light reflection, and a sputtering process over the coating has been made to achieve a proper light reflection and make the use of SWLI possible. It has been observed that thermal treatments at 1000 °C produce a decrease of the wear damage and an increase of hardness. The ball-on-disk test and the wear mechanisms are described and include the intersplat delamination of the main wear process in the as-sprayed coatings and thermally treated samples. The volume loss after 18 h at 1000 °C is 38% less than the as-sprayed coating. The erosion test and hardness measures show the same evolution as the ball-on-disk test.     

热喷涂钢柱10年海水腐蚀行为

采用直流电弧喷涂和火焰喷涂技术,在碳钢管表面沉积了１２种类的Ｚｎ、Ａｌ和Ｚｎ－１３Ａ１涂层在日本的千仓海岸进行海水腐蚀实验,结果表明,在５年暴露期内,所有的涂层均没有发生明显的腐蚀;经７年暴露后,未经封闭和经封闭处理的Ｚｎ涂层在浸泡区出现严重的锈蚀,电弧喷涂并经封孔处理的Ａ１涂层,火焰喷涂并经封孔处理的Ｚｎ－Ａ１涂层和火焰涂并经重涂装的Ａ１涂层显示出优良的耐蚀性能,相比之下,未经封闭处理的火焰喷涂     

The Influence of Spraying Angle on Properties of HVOF Sprayed Hardmetal Coatings

The spraying angle is one of the deposition parameters that influence the quality of thermally sprayed coatings. In theory, decreasing the spraying angle results in lower process deposition efficiency, whereas the porosity of coatings increases, becoming a cause of poorer microstructure and mechanical properties. In this study, the dependence of microstructure together with the basic mechanical properties and wear of WC-Co and Cr₃C₂-NiCr high-velocity oxyfuel (HVOF) sprayed coatings on the spraying angle was investigated. For each coating, the maximum spraying angle was determined that can be used without significantly decreasing coating quality. Based on the changes in properties of coatings and requirements for the process deposition efficiency, a maximum 30° diversion from the normal spray direction is recommended for WC-Co and 15° diversion for Cr₃C₂-NiCr coatings.     

再制造的热喷涂合金涂层的结构完整性与服役寿命预测研究

热喷涂技术是再制造工程的支撑技术,热喷涂涂层是再制造领域中常见的表面涂覆层,其初始质量和服役寿命为人关注.本文以等离子喷涂为例,研究了与工艺相关的涂层完整性和与服役条件相关的涂层寿命和失效机理.以不同H_2流量、功率和送粉量为条件,研究了工艺参数对涂层孔隙率和微观力学性能的影响.以接触疲劳过程为手段,研究了涂层寿命预测方法和寿命衰退机理.结果表明,工艺参数可以不同程度地影响涂层的结构完整性,通过优化设计可以大幅提高涂层质量;基于大样本空间建立的S-N曲线可以直观预测涂层接触疲劳寿命,机理分析表明,点蚀、剥落和分层失效诱因不尽相同,分别由粗糙接触、近表面缺陷和剪切应力导致.     

表面陶瓷层厚度对Sm2Zr2O7/8YSZ热障涂层热冲击性能的影响

稀土锆酸盐与8YSZ所组成的双陶瓷层涂层是目前热障涂层领域研究的热点,而陶瓷层厚度对其热冲击性能有着显著影响。采用有限元软件ANSYS研究了表层厚度对Sm2Zr2O7/8YSZ热障涂层淬冲击热应力的影响,并与单一Sm2Zr2O7涂层进行了比较。结果表明,在Sm2Zr2O7/8YSZ涂层的表面处具有最大的径向热冲击应力,最大轴向应力则存在于陶瓷层/金属粘结层界面处,涂层各处剪应力基本相当。涂层表面及两陶瓷层界面处的径向热应力随表层厚度的增加而减小,陶瓷层/粘结层界面处径向应力则随表层厚度增加而增大。每个界面处的轴向应力随表层厚度增加而降低,而剪应力绝对值则随表层厚度增加而增大。与单一Sm2Zr2O7涂层相比,Sm2Zr2O7/8YSZ涂层的热应力明显偏小,说明增加涂层的层数有利益改善涂层的抗热冲击性能。