Evolution of Lamellar Interface Cracks During Isothermal Cyclic Test of Plasma-Sprayed 8YSZ Coating with a Columnar-Structured YSZ Interlayer

The failure of plasma-sprayed thermal barrier coatings (TBC) usually occurs through spalling of ceramic coating. The crack evolution during thermal cycling of TBC is directly associated with its spalling. In this paper, the cracks in TBC along the direction of the interface between ceramic coating and bond coat were examined from cross-section of TBC experienced different numbers of thermal cycle, and crack number and the total length of cracks were measured to aim at understanding the failure mechanism. TBC consists of cold-sprayed NiCoCrAlTaY bond coat on IN738 superalloy and double layered plasma-sprayed 8YSZ with a columnar grain structured YSZ interlayer of about 20 μm thick and about 230 μm lamellar YSZ. With each isothermal cyclic test, the TBC samples were kept at 1150 °C for 26 min hold and then cooled down to a temperature less than 80 °C in 4 min by air forced cooling. Results showed that cracks propagated primarily within lamellar-structured YSZ over the columnar YSZ along lamellar interface. The measurement from the cross-section revealed that crack number and total crack length apparently increased with the increase of the number of thermal cycle. It was found that cracks with a length less than a typical size of 200 μm accounted for the majority of cracks despite the number of thermal cycle during the test. A crack initiation and propagation model for plasma-sprayed TBC is proposed with a uniform distribution of circular cracks. The propagatable cracks form homogeneously within plasma-sprayed porous YSZ coating at the early stage of thermal cycling and propagate at an identical rate during thermal cycling. Only a few of large cracks are formed before most cracks reach to the critical size for multi-cracks linking-up. The propagation of most cracks to the critical size will leads to the rapid crack bridging and subsequent spalling of top ceramic TBC.     

Effect of Thermal Exposure on Mechanical Properties of a Plasma-Sprayed Nanostructured Thermal Barrier Coating

A nanostructured thermal barrier coating (TBC) was deposited by air plasma spraying. The effect of microstructural evolution on nano-hardness and Young’s modulus has been investigated by nanoindentation technique after exposure at 1200 °C in air for different times. The results showed that the sintering process of nanostructured TBC at 1200 °C was divided into two stages. TBC completely kept the nanostructure with the grain size <100 nm at the first stage of 10 h thermal exposure. The nanostructure was lost gradually at the second stage from 10 to 200 h thermal exposure. During the first stage, nano-hardness and Young’s modulus increased rapidly for TBC densification, and Weibull bimodal distribution of both Young’s modulus and nano-hardness disappeared as grain grew and most microcracks were healed. The structure of TBC did not change basically, and nano-hardness and Young’s modulus increased slightly at the second stage.     

Plasma-Sprayed Hydroxylapatite-Based Coatings: Chemical,Mechanical, Microstructural,and Biomedical Properties

This contribution discusses salient properties and functions of hydroxylapatite (HA)-based plasma-sprayed coatings, including the effect on biomedical efficacy of coating thickness, phase composition and distribution, amorphicity and crystallinity, porosity and surface roughness, cohesion and adhesion, micro- and nano-structured surface morphology, and residual coating stresses. In addition, it will provide details of the thermal alteration that HA particles undergo in the extremely hot plasma jet that leads to dehydroxylated phases such as oxyhydroxylapatite (OHA) and oxyapatite (OA) as well as thermal decomposition products such as tri-(TCP) and tetracalcium phosphates (TTCP), and quenched phases such as amorphous calcium phosphate (ACP). The contribution will further explain the role of ACP during the in vitro interaction of the as-deposited coatings with simulated body fluid resembling the composition of extracellular fluid (ECF) as well as the in vivo responses of coatings to the ECF and the host tissue, respectively. Finally, it will briefly describe performance profiles required to fulfill biological functions of osteoconductive bioceramic coatings designed to improve osseointegration of hip endoprostheses and dental root implants. In large parts, the content of this contribution is a targeted review of work done by the author and his students and coworkers over the last two decades. In addition, it is considered a stepping stone toward a standard operation procedure aimed at depositing plasma-sprayed bioceramic implant coatings with optimum properties.     

Wear Behavior of Plasma-Sprayed Carbon Nanotube-Reinforced Aluminum Oxide Coating in Marine and High-Temperature Environments

Wear behavior of plasma-sprayed carbon nanotube (CNT)-reinforced aluminum oxide (Al₂O₃) composite coatings are investigated at room temperature (298 K), elevated temperature (873 K), and in sea water. Lowest wear volume loss was observed in the sea water as compared to dry sliding at 298 and 873 K. Relative improvement in the wear resistance of Al₂O₃-8 wt.% CNT coating compared to Al₂O₃ was 72% at 298 K, 76% at 873 K, and 66% in sea water. The improvement in the wear resistance of Al₂O₃-CNT coatings is attributed to (i) larger area coverage by protective film on the wear surface at room temperature and in sea water, (ii) higher fracture toughness of Al₂O₃-CNT coatings due to CNT bridging between splats, and (iii) anti-friction effect of sea water. The average coefficient of friction (COF) was the lowest (0.55) in sea water and the highest (0.83) at 873 K for Al₂O₃-8 wt.% CNT coating.     

Effect of Suspension Plasma-Sprayed YSZ Columnar Microstructure and Bond Coat Surface Preparation on Thermal Barrier Coating Properties

Suspension plasma spraying (SPS) is identified as promising for the enhancement of thermal barrier coating (TBC) systems used in gas turbines. Particularly, the emerging columnar microstructure enabled by the SPS process is likely to bring about an interesting TBC lifetime. At the same time, the SPS process opens the way to a decrease in thermal conductivity, one of the main issues for the next generation of gas turbines, compared to the state-of-the-art deposition technique, so-called electron beam physical vapor deposition (EB-PVD). In this paper, yttria-stabilized zirconia (YSZ) coatings presenting columnar structures, performed using both SPS and EB-PVD processes, were studied. Depending on the columnar microstructure readily adaptable in the SPS process, low thermal conductivities can be obtained. At 1100 °C, a decrease from 1.3 W m^?1 K^?1 for EB-PVD YSZ coatings to about 0.7 W m^?1 K^?1 for SPS coatings was shown. The higher content of porosity in the case of SPS coatings increases the thermal resistance through the thickness and decreases thermal conductivity. The lifetime of SPS YSZ coatings was studied by isothermal cyclic tests, showing equivalent or even higher performances compared to EB-PVD ones. Tests were performed using classical bond coats used for EB-PVD TBC coatings. Thermal cyclic fatigue performance of the best SPS coating reached 1000 cycles to failure on AM1 substrates with a β-(Ni,Pt)Al bond coat. Tests were also performed on AM1 substrates with a Pt-diffused γ-Ni/γ′-Ni₃Al bond coat for which more than 2000 cycles to failure were observed for columnar SPS YSZ coatings. The high thermal compliance offered by both the columnar structure and the porosity allowed the reaching of a high lifetime, promising for a TBC application.     

高温涂层材料物理、力学性能研究进展

高温涂层的广泛使用在很大程度上拓展了高温合金的应用领域,但随着航空发动机用涡轮叶片服役环境的日益苛刻,合理评价涂层/高温合金体系综合性能,尤其热机械疲劳、蠕变疲劳等性能备受关注.该体系综合性能的评价必然涉及到与高温涂层材料本身相关的物理、化学、力学等性能.本文将对近年来有关高温涂层材料的物理和力学等方面的性能进行综述,并据此探讨今后涂层/基体界面行为研究的新思路.     

镁合金表面等离子喷涂Al涂层及激光重熔研究

利用等离子喷涂技术在镁合金表面制备了Al涂层,并通过激光对该涂层进行重熔处理。利用SEM、金相显微镜、XRD、万能材料试验机、盐雾腐蚀试验等分析测试手段研究了该涂层在激光重熔前后的变化。结果表明:镁合金表面等离子喷涂Al涂层经激光重熔后,涂层和基体之间的结合由机械结合转变为冶金结合,结合强度由20.96MPa提高到22.13MPa;涂层相组成不变;但出现了较多的孔隙和空洞,孔隙率由4.6%增大到7.5%,涂层耐盐雾的时间由900h降低到264h。     

EN AW-4032 T6 Piston Alloy After High-Temperature Exposure: Residual Strength and Microstructural Features

This study aims to evaluate the effects of prolonged thermal exposure on both microstructural evolution and mechanical properties of the EN AW-4032 T6 piston alloy. For the purpose, the experimental activities have been carried out on samples machined from forged and heat-treated automotive pistons. The effects of overaging have been investigated in the temperature range of 140-290 °C, firstly by evaluating the time-temperature-hardness curves and then by carrying out room-temperature tensile tests on overaged samples. The material softening was substantial and extremely rapid when the soaking temperature exceeded 250 °C. During overaging, both the tensile strength and the residual hardness considerably decreased, and a relationship between these parameters has been established. The alloy behavior in the plastic field has been modeled according to the Hollomon’s equation, showing that both the strain hardening exponent and the strength coefficient are a function of the residual hardness. The results were finally related to the corresponding microstructural changes: OM and FEG-SEM metallographic and fractographic analyses on overaged samples gave evidence of coarsened precipitates along the grain boundaries.     

机械能辅助渗铝工艺及渗铝层性能研究

机械能辅助渗铝是将机械能与热能相结合一种新渗铝工艺,可以将渗铝温度降低到560～650 ℃.本文通过实验对机械能辅助渗铝工艺进行了研究,对主要工艺参数进行了优化,获得了组织致密、渗层厚度≥60 μm、以Fe2Al5相为主要组成相的渗铝层,并研究了渗铝钢的抗高温氧化性能及机械性能.     

低压等离子喷涂纳米YSZ涂层的结构及力学性能

采用低压等离子喷涂技术制备具有新型结构的纳米YSZ涂层,研究电弧电流和粉末松装密度对涂层微观结构以及力学性能的影响。结果表明,松装密度为0.95g/cm3的粉末在喷涂过程中因电流升高而引起的晶粒生长的程度远低于松装密度为1.4g/cm~3的粉体。当电流超过350A后,两种粉末所对应的涂层的Weibull模量分布均发生从二元结构向单一值的转变,但转变后涂层的结构均呈现出有别于传统涂层结构的细等轴晶结构。硬度与弹性模量的分析结果表明具有细等轴晶结构的涂层较在350A电流下形成的二元结构纳米涂层具有更高的平均硬度与弹性模量。当采用密度为1.4g/cm~3的粉末时,所对应涂层的断裂韧性Weibull曲线随电流波动较大。     

8YSZ热障涂层结构设计及其结合强度变化规律

目的 探究不同厚度的黏结层和陶瓷层对8YSZ热障涂层结合强度的变化规律。方法 采用大气等离子喷涂技术(APS)在Ti-6Al-4V合金基体表面分别制备了不同厚度的黏结层和陶瓷层等6种双层结构涂层。利用X射线衍射仪(XRD)、扫描电镜(SEM)和X射线荧光分析仪(XRF)等检测手段对喷涂粉末和涂层的相组成、微观结构及化学成分变化进行表征。借助万能材料试验机分别对6种不同厚度涂层的结合强度进行测量和评估。结果 不同厚度的8YSZ陶瓷粉末在喷涂过程中主要从单斜相(M)向四方相(T)转变。此外,不同厚度的热障涂层都呈现出典型的层状结构,涂层表面存在着完全熔融态、半熔融态和未熔态等3种复杂状态,且都存在不同程度的裂纹和孔隙。涂层结合强度随黏结层厚度的增加会有些许增大,而随陶瓷层厚度的增加逐渐下降,且陶瓷层厚度越大结合强度下降得越缓慢。在所有涂层试样中,当黏结层最厚且陶瓷层最薄时涂层结合强度最大,超过29.7 MPa;而当黏结层最薄陶瓷层最厚时涂层结合强度最低。 结论 8YSZ热障涂层的黏结层和陶瓷层厚度变化对涂层的物相组成以及化学成分无明显影响,而对涂层结合强度以及断裂方式产生显著影响。     

Improved High-Temperature Microstructural Stability and Creep Property of Novel Co-Base Single-Crystal Alloys Containing Ta and Ti

The influence of Ta and Ti additions on microstructural stability and creep behavior in novel Co-Al-W base single-crystal alloys has been investigated. Compared to the ternary alloy, the γ′ solvus temperature and γ′ volume fraction were raised by individual additions of Ta and Ti, and increased further in the quinary alloy containing both alloying additions. In contrast to ternary and quaternary alloys, an improved microstructural stability with the stable γ–γ′ two-phase microstructure and more than 60% γ′ volume fraction existed in the quinary alloy after prolonged aging treatment at 1050°C for 1000 h. The creep behavior at 900°C revealed lower creep rates and longer rupture lives in the quaternary alloys compared to the ternary alloy, whereas the quinary alloy exhibited even better creep resistance. When the creep temperature was elevated to about 1000°C, the creep resistance of the quinary alloy exceeded the previously reported Co-Al-W-base alloys and first-generation Ni-base single-crystal superalloys. The improved creep resistance at approximately 1000°C was considered to be associated with high γ′ volume fraction, γ′ directional coarsening, and dislocation substructure, which included γ–γ′ interfacial dislocation networks and the sheared γ′ precipitates containing stacking faults and anti-phase boundaries.     

铝硅合金化低碳钢多道次变形的组织演变与力学性能

利用热模拟试验研究了铝硅合金化低碳钢多道次变形及连续冷却过程的组织演变.实验结果表明,通过多道次轧制和控制冷却工艺可以得到细晶铁素体和贝氏体双相组织,此铁素体/贝氏体双相钢具有低的屈服强度、高抗拉强度、低屈强比以及连续屈服的特性.     

等离子熔覆多元镍基涂层-基体的力学性能研究

目的在FV520B不锈钢基体上,采用等离子熔覆技术制备多元镍基涂层,研究不同成分配比涂层-基体协同作用下的力学性能。方法通过扫描电子显微镜分析涂层的表面及界面形貌,并对涂层-基体进行了拉伸及高温压缩性能测试,得到各涂层的抗拉强度及高温变形抗力,对比分析组织、相分布特征及涂层成分对涂层-基体系统力学性能的影响。结果等离子熔覆涂层组织致密,界面处呈现良好的冶金结合,这种结合方式可提高涂层-基体的综合力学性能;涂层-基体协同作用可显著提高材料的抗拉强度及变形抗力,且涂层的成分、组织及相分布特征是影响涂层-基体协同形变行为的关键因素。其中,Ni60+20%Ti涂层材料的抗拉伸性能最好,抗拉强度高达921 MPa,较基体材料提高了19.6%。Ni60+30%Ti+10%WC涂层-基体的高温力学性能最好,高温变形抗力达687.87 MPa。结论等离子熔覆多元镍基涂层使基体材料的抗拉强度有所提高,且高温变形抗力提高显著。     

国产T23钢高温时效时组织和力学性能的研究

采用光学显微镜、扫描电镜、透射电镜观察和拉伸、冲击试验等方法,研究了国产T23钢在600 ℃时效时的组织变化和力学性能.结果表明,T23钢在600 ℃时效时随着时效时间的延长,T23钢中M23C6型碳化物不断粗化,粒状贝氏体形貌逐渐退化.时效时间较短时,主要是M23C6型碳化物粗化引起T23钢力学性能下降;时效时间较长时,主要是粒状贝氏体形貌的变化导致T23钢力学性能下降,同时,M23C6的球化及其向M6C的转变、亚晶的形成和W、Mo固溶强化的降低加快了T23钢力学性能的下降.     

7YSZ热障涂层高温氧化过程中的微结构演变和阻抗谱分析

在大气环境中,测试7YSZ热障涂层抗1 000℃高温氧化性能,以模拟发动机叶片的巡航飞行高温氧化过程。采用交流阻抗技术并结合扫描电镜及X射线衍射系统研究了高温氧化过程中TGO层生长与YSZ层微结构演变。结果表明:随着氧化时间的增加,TGO层厚度增加符合抛物线规律,在中频阶段的阻抗谱响应愈加显著。YSZ熔融颗粒在较低测试温度下表现为容抗特征,随测试温度升高逐步表现为感抗特征。阻抗谱分析显示YSZ晶粒的电阻及电容随氧化时间增加,均呈一定幅度波动变化,高温氧化对熔融颗粒本身的电学特征影响不明显;随着氧化时间的增加,YSZ晶界电阻值不断增加,电容值不断下降,表明YSZ内微裂纹发生了生长和扩展,最终导致YSZ层的剥落失效。     

氧化铝、硝酸铝复合YSZ的烧结性能、电性能及力学性能研究

分别以Al2O3、Al(NO3)3为铝源与氧化钇稳定氧化锆（YSZ）机械混合,经干压技术和高温烧结制得YSZ/Al2O3复合材料。通过扫描电子显微镜、电化学交流阻抗谱和三点弯曲测试等方法对样品进行分析,研究了不同铝源和添加量对YSZ电解质烧结性能、电导率及力学性能的影响。结果表明：适量的Al2O3或Al(NO3)3复合均能够促进YSZ的烧结,提高其电导率和弯曲强度;Al2O3作铝源的复合样品的烧结性能和电导率优于Al(NO3)3铝源的,但其强度低于同比例Al(NO3)3铝源。其中,加入0.5 wt.%Al2O3时,所制备的复合电解质经1350 ℃烧结10 h后的相对密度为98.8%;800 ℃时的电导率达0.0703 S·cm-1。基于该材料制备的电解质支撑型单元电池在800 ℃时以氢气为燃料的最大功率密度为308 mW·cm-2,高于纯YSZ电解质电池的,并表现出良好的稳定性。     

Effect of Submicron and Nano SiC Particles on Erosion Wear and Scratch Behavior of Plasma-Sprayed Al2O3/8YSZ Coatings

This paper reports studies into the effect of submicron and nano SiC particles on microstructure, phase composition, hardness, erosion wear, and scratch behavior of Al₂O₃-20wt.%8YSZ (ZrO₂ + 8 wt.% Y₂O₃) coatings fabricated by atmospheric plasma spraying. The failure mode of erosion wear and scratch for coatings was established and analyzed. The hardness, density, erosion wear, and anti-scratch resistance of coatings fabricated from plasma treating feedstocks were higher than that of coatings made from sintering feedstocks. The erosion wear rate of coatings with SiC was evidently decreased, and there was some small debris on worn surface with characteristic of translamellar fracture. The spallation, fracture, plough, and cracking were main failure mechanism for coatings. In the scratch process, the critical load of coating with SiC was increased. The crack growth resistance of coatings was analyzed from crack length at end of scratch test.     

累积叠轧工艺对AgSnO2复合材料组织和性能的影响

采用原位反应合成法制备AgSnO2复合材料,在温度700℃对其进行累积叠轧。利用XRD、光学显微镜等研究了AgSnO2复合材料的物相组成以及各轧制道次后样品的金相组织,结果表明,累积叠轧工艺对AgSnO2复合材料的显微组织均匀化程度影响显著。实验测试了各道次轧制后样品的硬度、密度以及电阻率与轧制道次的关系,分析主要因素对样品力学与电学性能的影响机理。     

亚音速火焰喷涂层与等离子喷涂层耐磨性的对比

利用亚音速火焰喷涂和等离子喷涂分别制备Fe-WC金属陶瓷涂层,对两种喷涂层的耐磨性进行了研究.结果表明,在耐磨性试验过程中等离子喷涂层的磨损失重变化比亚音速火焰喷涂层磨损失重变化稳定,等离子喷涂层的耐磨性明显高于火焰喷涂层的耐磨性.