Quality Considerations for the Evaluation of Thermal Spray Coatings

Revealing the true structural and mechanical properties is of utmost importance for the optimized use of thermal sprayed coatings. Only the true properties can be expected to correlate to the spray parameters. During the recent decade, the gas turbine industry has experienced a focus on the laboratory procedures being the weakest link in a frozen and robust process. This article will show several results indicating that the laboratory procedures are more essential to the evaluation results than the spray parameters themselves. With new and robust laboratory techniques, the true properties of thermal spray coatings are revealed, causing a major problem with respect to the quality standards developed 30-40 years ago. In many cases, these old specifications need updates, which is a difficult task from a cost, time, and quality perspective for OEM’s. Coatings that have been successfully used for almost half a century no longer conform to the specification they were optimized to, because of these new appropriate laboratory techniques and procedures. What is actually meant when stating the following? (1) The coating has 5% porosity; (2) No cracks are allowed; (3) Tensile bond is 50 Mpa; (4) Hardness is 1000 HV; and (5) Coating thickness is 100 μm. This article also initiates a discussion on the measurement inaccuracies, for testing of thermally sprayed coatings, with respect to the commonly used general international standards (such as QS9000, ISO17025, AS9003, and ISO10012), as well as with respect to recommendations from the Six Sigma methodology.     

Hot Corrosion Behavior of HVOF Sprayed Coatings on ASTM SA213-T11 Steel

Cr₃C₂-NiCr, NiCr, WC-Co and Stellite-6 alloy coatings were sprayed on ASTM SA213-T11 steel using the HVOF process. Liquid petroleum gas was used as the fuel gas. Hot corrosion studies were conducted on the uncoated as well as HVOF sprayed specimens after exposure to molten salt at 900 °C under cyclic conditions. The thermo-gravimetric technique was used to establish the kinetics of corrosion. XRD, SEM/EDAX and EPMA techniques were used to analyze the corrosion products. All these overlay coatings showed a better resistance to hot corrosion as compared to that of uncoated steel. NiCr Coating was found to be most protective followed by the Cr₃C₂-NiCr coating. WC-Co coating was least effective to protect the substrate steel. It is concluded that the formation of Cr₂O_3, NiO, NiCr₂O₄, and CoO in the coatings may contribute to the development of a better hot-corrosion resistance. The uncoated steel suffered corrosion in the form of intense spalling and peeling of the scale, which may be due to the formation of unprotective Fe₂O₃ oxide scale.     

Formation of Solid Splats During Thermal Spray Deposition

This paper reviews the findings of recent research on the formation of solid splats by the impact of thermal spray particles on solid substrates. It discusses methods of describing the substrate, by characterizing both chemical (oxide layers) and physical (surface topography, adsorbed and condensed contaminants) aspects. Recent experiments done to observe impact of thermal spray particle are surveyed and techniques used to photograph particle impact and measure cooling rates described. The use of numerical modeling to simulate impact and deformation of impacting particles is appraised. Two different break-up mechanisms are identified: solidification around the edges of splats; and perforations in the interior of thin liquid films created by droplet spreading without solidification. These two modes can be reproduced in numerical models by varying the value of thermal contact resistance between the splat and substrate. A simple criterion to predict the final splat shape is presented.     

Evidence of mechanical interlocking of NiCr particles thermally sprayed onto Al substrates

Ni-chrome alloy particles were thermally sprayed onto aluminum substrates using the high-velocity air fuel technique. The particle substrate interface was investigated with focused ion beam microscopy, cross-sectional scanning electron microscopy, and cross-sectional transmission electron microscopy. No evidence of melting or chemical bonding was found in the samples. Instead, evidence of mechanical bonding was found that had been predicted by a previous theoretical study by Grujicic et al. At locations where the particle and substrate are in intimate contact, the interface exhibited interlocking features. These features are caused by the effects of turbulence due to interfacial instability and mixing at the interface during the coating process, resulting in a strong particle-substrate bond. Conversely, separated interfaces exhibited smooth surfaces, suggesting insignificant bonding between the particle and the substrate. The discovery of these interfacial formations, together with no evidence of chemical bonding across the particle-substrate interface indicate that mechanical interlocking is the dominant bonding mechanism.     

膜基结合强度评定方法的探讨―划痕法、压入法、接触疲劳法测定的比较

薄膜（镀层）与基体之间的结合强度是评价薄膜质量的重要性能指标。目前使用最广泛的是划痕法和压入法，这些都属于一次性加载方式。一种较好的评定硬质膜膜基结合强度的方法是采用以临界剪切应力幅△τc作为判据的滚动接触疲劳法。通过与划痕法，压入法的对比可知表征结合强度大小的剪切应力幅只对界面因素敏感，对非界面因素不敏感，能真实反映膜基结合状态，是一个较为合理的动态结合强度判据。     

Technical and Economical Aspects of Current Thermal Barrier Coating Systems for Gas Turbine Engines by Thermal Spray and EBPVD: A Review

The most advanced thermal barrier coating (TBC) systems for aircraft engine and power generation hot section components consist of electron beam physical vapor deposition (EBPVD) applied yttria-stabilized zirconia and platinum modified diffusion aluminide bond coating. Thermally sprayed ceramic and MCrAlY bond coatings, however, are still used extensively for combustors and power generation blades and vanes. This article highlights the key features of plasma spray and HVOF, diffusion aluminizing, and EBPVD coating processes. The coating characteristics of thermally sprayed MCrAlY bond coat as well as low density and dense vertically cracked (DVC) Zircoat TBC are described. Essential features of a typical EBPVD TBC coating system, consisting of a diffusion aluminide and a columnar TBC, are also presented. The major coating cost elements such as material, equipment and processing are explained for the different technologies, with a performance and cost comparison given for selected examples.     

Optimization of Atmospheric Plasma Spray Process Parameters using a Design of Experiment for Alloy 625 coatings

Alloy 625 is a Ni-based superalloy which is often a good solution to surface engineering problems involving high temperature corrosion, wear, and thermal degradation. Coatings of alloy 625 can be efficiently deposited by thermal spray methods such as Air Plasma Spraying. As in all thermal spray processes, the final properties of the coatings are determined by the spraying parameters. In the present study, a D-optimal experimental design was used to characterize the effects of the APS process parameters on in-flight particle temperature and velocity, and on the oxide content and porosity in the coatings. These results were used to create an empirical model to predict the optimum deposition conditions. A second set of coatings was then deposited to test the model predictions. The optimum spraying conditions produced a coating with less than 4% oxide and less than 2.5% porosity. The process parameters which exhibited the most important effects directly on the oxide content in the coating were particle size, spray distance, and Ar flow rate. The parameters with the largest effects directly on porosity were spray distance, particle size, and current. The particle size, current, and Ar flow rate have an influence on particle velocity and temperature but spray distance did not have a significant effect on either of those characteristics. Thus, knowledge of the in-flight particle characteristics alone was not sufficient to control the final microstructure. The oxidation index and the melting index incorporate all the parameters that were found to be significant in the statistical analyses and correlate well with the measured oxide content and porosity in the coatings.

Influence of substrate properties on the impact resistance of WC cermet coatings

This research delivers a generic understanding of the design and integrated performance of the coating-substrate systems under impact loading, and comprehends the understanding of underpinning failure mechanisms. Repeated severe impacts to the coatings often result in poor performance by cracking and delamination from the coating-substrate interface. The durability of coatings thus depends on the choice of coating and substrate materials, coating deposition process, and service conditions. The design of thermal spray coatings thus requires an optimization of these parameters. This investigation provides insight into the role of coating and substrate properties on the impact resistance of coated materials, and maps the relationship between the impact resistance of WC cermet coatings on a variety of substrates. Results indicate that the delamination resistance of the coating during impact loading not only depends upon the hardness and roughness of the substrate material, but, more importantly, substrates with a higher work-hardening coefficient indicate a higher delamination resistance. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

热喷涂涂层结合强度的冲击测量法

讨论了一种新型快速定性评价热喷涂层结合强度的冲击测试方法,采用自由落体的落锤冲击设备对涂层进行冲击试验,并采用声发射技术进行动态监测,以定性分析涂层结合强度。首先用超音速等离子喷涂技术分别制备了组织致密,性能良好的Ni Cr金属、Cr_2O_3陶瓷涂层。采用90°和120°金刚石压头、120°及Ф2 mm球形硬质合金压头试验,其中120°硬质合金压头更适合冲击法测试涂层结合强度。随着冲击能量的提高,声发射的能量计数也不断提高,但是落锤高度超过400 mm后,能量计数变化不大,说明在400 mm时涂层发生了破坏。对基体金属、Ni Cr涂层、以及有无Co Cr Al Y为结合层的Cr_2O_3陶瓷涂层进行对比测试,发现声发射能量计数峰值随着涂层结合强度的增强而降低。     

Measuring Substrate Temperature Variation During Application of Plasma-Sprayed Zirconia Coatings

Substrate temperature variation was measured during plasma spraying of ZrO₂ 7% Y₂O₃ powder using fast-response thermocouples embedded in the stainless steel surface. Coatings were deposited with both stationary and moving torches. The substrate was either kept at room temperature at the start of coating deposition or pre-heated to 270-300 °C. Peak temperature during spraying reached 450 °C for a surface initially at room temperature, and 680 °C for a surface preheated to 300 °C before coating deposition. Preheating the substrate reduced coating porosity by approximately 40%. The porosity at the center of the deposit was significantly lower than that at its periphery since particle temperature and velocity were lower at the edges of the plasma plume than along its axis. When a coating was applied with a moving torch the substrate temperature did not increase above 450 °C, at which temperature heat losses to the ambient equalled the heat supplied by the plasma plume and particles. Coating porosity decreased with distance from the substrate. As sequential layers of coating are applied surface temperature increases and roughness decreases. Both of these factors suppress break-up of particles landing on the substrate and thereby reduce coating porosity.     

������������������������ȷ��������

 以Zwick Z600E电子式拉力试验机测定H型钢翼缘拉伸性能为实例，对拉伸试验中常用技术指标的测量不确定度来源进行分析，并作出评定，为金属材料拉伸试验测量不确定度的评定提供了一种简单可行的模式。     

基体激光淬火对镀铬层界面剪切强度的影响

针对激光淬火基体后再镀铬复合工艺提高镀铬身管寿命的实际工程问题，采用多裂纹拉伸技术证明该复合镀铬工艺可以提高界面剪切强度，并从材料学角度给予解释。离子冲击技术对界面强度的定性分析表明：激光淬火基体在消除铬层和基体之间过渡层的同时也提高了界面附近材料的强度。化学腐蚀去基体法研究表明，铬层界面材料强度的提高是晶粒细化的结果。因此得出激光淬火基体提高镀铬层界面剪切强度的原因是激光淬火消除了铬层和基体之间的过渡层和增强了界面附近材料的硬度和强度。     

不同厚度8YSZ热障涂层的结构及性能表征

目的探究厚度变化对8YSZ热障涂层结构、力学性能以及抗热震性能的影响。方法通过超音速火焰喷涂技术(HVOF)和大气等离子喷涂技术(APS)分别制备了Ni CoCrAlTaY粘结层和厚度为500μm、1.0mm、1.5mm的8YSZ陶瓷涂层,采用扫描电子显微镜(SEM)、光学显微镜和X射线衍射仪(XRD)对喷涂粉末和涂层的形貌、物相进行了表征,借助显微硬度计和万能材料试验机分别考察了涂层的硬度和结合强度,最后采用水淬法对涂层的抗热震性能进行了测试。结果不同厚度的8YSZ涂层均由非平衡的四方相(t′-YSZ)组成,且断面呈现出明显的层状结构。随着厚度的增加,涂层中逐渐产生了明显的网状纵向裂纹和边缘界面裂纹。涂层的表面和截面显微硬度都不随厚度的增加而发生显著变化,并且所制备的涂层在整个截面上的显微硬度都比较均匀。涂层的结合强度随着涂层厚度的增加而显著降低。热震试验过程中,三种厚度涂层皆以界面开裂的形式失效,且厚度越大的涂层热震寿命越短。结论 8YSZ热障涂层的厚度变化对其微观形貌、结合强度以及抗热震性能皆有显著影响,而对涂层的物相组成以及显微硬度无明显影响。     

YG6金刚石涂层刀片衬底真空渗硼预处理新技术研究

本文研究了真空渗硼预处理硬质合金基体的表面组织、形貌、粗糙度，并在处理过的YG6刀片基体上，用强电流直流伸展电弧等离子体CVD法沉积金刚石薄膜涂层。结果表明，真空渗硼预处理不仅可以有效的消除或控制钴在金刚石沉积时的不利影响，而且，还显著粗化硬质合金基体表面。因此，提高了金刚石薄膜的质量和涂层的附着力。     

球形压痕法评价(Ti,Nb)N膜层断裂性能

采用球形压痕试验，在高速钢基体上对电弧离子镀工艺沉积制备的(TixNb)N膜层断裂性能进行评价。在600N载荷下，在压痕的周围只产生径向裂纹；统计分析压痕周围产生的裂纹，采用裂纹密度函数β，对膜层的断裂性能进行了分析。结果表明：(TixNb)N均质膜层在显微硬度提高的同时，裂纹密度β增加，膜层的脆性增大，断裂韧度下降。TixNb1-xN梯度膜具有较高膜层显微硬度，仍然具有较好的断裂韧度，预渗氮处理的膜层裂纹密度较小，膜层的断裂韧度最好。采用裂纹密度函数卢统计分析，通过对(TixNb)N膜层压痕试验，可以很好的定量和半定量地比较膜层的断裂性能。     

改善SiO2涂层与聚酰亚胺基体界面结合性能的研究

目的 利用硅烷偶联剂(SCA)有机官能基对有机物具有反应性或相容性的特点,探讨通过SCA改善SiO2涂层与聚酰亚胺(PI,Kapton)基体界面粘附力的实验方法,提高SiO2涂层与PI基体的结合强度,同时削弱因温度变化而引起的内应力.方法 水热条件下,用低浓度碱液对PI表面进行处理,选取γ-氨丙基三乙氧基硅烷(KH-550)、γ-(2,3-环氧丙氧)丙基三甲氧基硅烷(KH-560)和 γ-甲基丙烯酰氧基丙基三甲氧基硅烷(KH-570)等三种不同的SCA,在溶剂热环境中与碱处理后的PI表面进行作用.用水汽辐照试验评价SCA与PI的结合状况.样品表面形貌用SEM扫描电镜和材料分析显微镜表征,表面润湿性用接触角测量仪测定,透光率用紫外可见分光光度计表征.结果 经0.1 mol/L NaOH水热处理后,Kapton表面的水接触角由77°下降为53°.KH-550溶剂热环境下,在Kapton表面形成均匀的水解层.水汽辐照试验后,该水解层依然保持完整,透光率没有下降.但KH-560和KH-570水解层出现脱落,透光率分别下降了4.4％和9.3％.结论 SCA虽然可以有效改善聚合物基体的润湿性,提高无机涂层与基体的界面粘附性,但在SCA选取上,必须考虑其与聚合物表面的相互作用.若SCA与聚合物基体的结合不牢固,反而更易造成涂层开裂、脱落,成为潜在的不稳定因素.     

AISI 316L表面氮化复合类金刚石涂层的相结构及摩擦学性能

为提高类金刚石涂层与奥氏体不锈钢之间的结合强度,利用等离子体增强化学气相沉积技术分别在未处理和氮化处理的AISI 316L表面沉积类金刚石（DLC）涂层,研究不同沉积温度下DLC及氮化复合DLC涂层的相结构与摩擦学性能。采用X射线衍射仪（XRD）、拉曼光谱仪（Raman）表征涂层的相结构;采用扫描电子显微镜（SEM）观察截面形貌,并用EDS测量氮、碳元素的深度分布;采用纳米压痕仪、摩擦磨损试验机、超景深显微镜、划痕仪检测DLC涂层的摩擦学性能。结果表明：氮化复合DLC涂层的结合力和耐磨性优于DLC涂层;其中100 ℃时,硬度和结合力分别提高25%和175%,综合性能最好。沉积DLC涂层的过程中,氮化层中氮原子因扩散而重新分布,使氮化层的厚度增加,硬度梯度减缓,更有利于基体与DLC涂层间的过渡。     

Evaluation of the Corrosion Resistance of Thermal-Spray Coatings Under Oxidant Atmosphere in a Fluid Catalytic-Cracking Unit

The effectiveness of several thermal-spray coatings for improving the corrosion resistance of a low-alloy steel was evaluated at 650°C under two conditions: an oxidizing atmosphere in a fluid catalytic-cracking regenerator of a petrochemical unit and a simulated laboratory atmosphere. The high porosity present in all coatings studied in the present work, inherent to the thermal-spray technique, allowed the penetration of gaseous species from the atmosphere into the substrate, leading to the formation of nonprotective oxides and sulfides, as well as internal oxidation, sulfidation, and nitridation. A protective alumina and/or chromia layer did not form, probably due to the relatively low temperature used in both the real and simulated conditions. Characterization of the phases present in the oxidized layer was carried out by X-ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy (SEM) with X-ray energy-dispersive analysis (EDS).     

滚动接触疲劳法评定硬质薄膜的结合强度

工件大多在交变应力下服役,薄膜的剥落是一个长时间高周次过程,其失败过程属疲劳失效。采用高周次界面疲劳强度作为膜基结合强度的判据,以膜基界面处的切应力△τ表征膜基界面疲劳强度。采用该方法对气相沉积硬质薄膜的结合强度进行了评定,并与划痕法比较。结果表明,薄膜在膜基界面处剥落,该方法对同因素十分敏感,而对非界面因素不敏感,因而是一个有效的测量膜基界面结合强度的方法。可用来比较不同方法制备的不同成分和硬度     

A Study of Interaction of Cobalt Sulphide Coatings with Cu(II) Ions

Cobalt sulphide coatings formed by two cycles (one cycle includes the treatment of the surface with the solution of Co(II) ammoniate complex; hydrolysis of the adsorbed Co(II) compounds; and sulphidation (in Na₂S solution) of the products of the hydrolysis) contain ～15% CoS, which in 0.1 M KClO₄ solution is reduced in the region of potentials of hydrogen evolution, and Co hydroxosulphide. Reduction of the latter occurs at the potentials from -0.4 to -0.8 V in 0.1 M KClO₄ solution and in the potential region 0.2-0.3 V in 0.05 M H₂SO₄, The reduction occurs in the solid phase and is reversible. After treating such a coating with a solution of Cu(II) ions, only CoS interacts with Cu(II), giving, as a result of redox and exchange processes, Cu_2–xS.