Effects of heating temperature and duration on the microstructure and properties of the self-healing coatings

The present work investigates how the heating temperature and duration affect the properties of the self-healing coating on martensitic steels. The coating composed of TiC + mixture (TiC/Al₂O₃) + Al₂O₃ is fabricated by means of air plasma spraying. The thermal shock test is performed at 600 °C, 700 °C and 800 °C, respectively, to evaluate the thermal-mechanical stability of the coating. The cross-section morphology of the samples after 1 h, 9 h, 18 h and 30 h of heat treatment shows that the porosity of the coating decreases with the increase of heating duration. The evaluation of electrochemical performance by electrochemical impedance spectroscopy shows that the corrosion resistance of the coating after being heated for 18 h is much better than the other samples due to the process of the inner layer being compacted in the coating. The adhesive tensile strength test between coating and substrate shows that the adhesive strength of the coatings is higher than 9 MPa within 40 h of heat treatment at 600 °C. The residual stress reaches a minimum value after the coating was heated for 9 h at 600 °C, then increases with the heating duration after 9 h. Energy dispersive X-ray analysis at the Vickers indentation indicates that the oxygen content at the crack position increases significantly after being heated for 30 h at 600 °C. These experimental results suggest that this coating can meet the requirement of application under the actual temperature conditions.     

Preparation of epoxy microcapsule based self-healing coatings and their behavior

In this paper, epoxy microcapsules were prepared by interfacial polymerization of epoxy resin droplets with ethylenediamine (EDA), and the capsules were characterized by scanning electron microscope (SEM). Then, the coatings containing epoxy microcapsules were applied on carbon steels, and their behavior and self-healing effect were investigated by electrochemical impedance spectroscopy (EIS) technique and SEM observation. The experimental results demonstrate that the artificial scratches were successfully healed in about 4-h after made. Furthermore, coating prepared from 20 wt.% epoxy microcapsules shows the best performance among all the prepared coatings.     

On the development of polypyrrole coatings with self-healing properties for iron corrosion protection

This paper presents studies on the efficacy and on the limits of polypyrrole (Ppy) doped with either or [PMo₁₂O₄₀]³⁻ as self-healing corrosion protecting coatings. The kinetics of the cathodic delamination were studied by means of the Scanning Kelvin Probe (SKP). This method, in combination with cyclic voltammetry, UV–visible spectroscopy (UV–vis) and X-ray photoelectron spectroscopy (XPS), shows a potential driven anion release from the Ppy coating that results in an inhibition of the corrosion process taking place in the defect. Thus, an intelligent release of inhibitor occurs only when the potential at the interface decreases. Inhibitor anions are released only due to an active defect. However, the release mechanism can be easily negatively affected by the presence of small cations and/or by too high pH values at the buried interface. Hence, such a self-healing coating has to be carefully designed in order to ensure an effective performance.     

Protective coatings for very high temperature reactor applications

The future very high temperature reactors (VHTR) are nuclear systems that shall operate at a maximum temperature of about 950 °C. Primary circuit materials thus require good creep and corrosion resistance on very long time. Use of high‐strength alloys with protective coatings could significantly improve the service life of high temperature reactor components. However, coating systems are mainly designed for shorter term purposes, often under extremely aggressive atmospheres, that cannot be extrapolated to the VHTR environment. We present our first investigations on the environmental resistance of Alloy 800H coated with two different protective systems under VHTR representative conditions: NiAl(Pt)/EBPVD ZrO₂(Y) and NiCrAl(Y)/CVD ZrO₂(Y). Isothermal exposures were carried out up to 1000 h at 950 °C in impure helium. This specific atmosphere was shown to induce formation of a surface oxide scale together with carburisation of the bare Alloy 800H. After high temperature exposure to impure helium, the microstructure of the coated specimens has changed due to both thermal ageing and corrosion. Performances of the two coating systems are compared regarding the VHTR application.     

High-performance nanoscale composite coatings for boiler applications

In this article, we will show how unconventional nanoscale composite coatings can be formed using conventional wire-arc thermal spray systems. The as-sprayed SHS7170 wire-arc coatings are found to develop an amorphous matrix structure containing starburst-shaped boride and carbide crystallites with sizes ranging from 60 to 140 nm. After heating to temperatures above the peak crystalline temperature (566 °C), a solid/state transformation occurs that results in the formation of an intimate three-phase matrix structure consisting of the same complex boride and carbide phases, along with α-iron interdispersed on a structural scale from 60 to 110 nm. The nanocomposite microstructure contains clean grain boundaries, which are found to be extremely stable and resist coarsening throughout the range of temperatures found in boilers. Additionally, the properties of the coating are presented including the bond strength, hardness, bend resistance, and impact resistance. The sprayability, forgiveness, and repairability of the SHS7170 wire-arc coatings are explained in detail, with an emphasis on field applicability in boiler environments. The performance of the SHS7170 coatings in boiler environments is measured via elevated temperature-erosion experiments conducted at 300, 450, and 600 °C using bed ash from an operating circulating fluidized-bed combustor boiler, and the results are compared with those for existing boiler coatings.     

Estimating the self-healing capability of cementitious composites through non-destructive electrical-based monitoring

Self-healing evaluation of cementitious composites was made by non-destructive test (NDT) methods (electrical impedance [EI], rapid chloride permeability test [RCPT] and resonant frequency [RF]). Correlations among results obtained from different NDT methods were reported. Conclusions revealed that EI testing is easy to perform, takes very limited time and looks promising for self-healing assessment although the method itself seems to be remarkably influenced by anything modifying the ionic state of specimens. A solid exponential relationship exists between EI and RCPT measurements, but results from RF tests do not correlate with EI and RCPT results due to different parameters affecting individual tests.     

Preparation of YSZ coatings by thermal pressure and filtration of sol-gel paint (TPFSP) for thermal barrier application

Thick YSZ ceramic coatings were prepared by thermal pressure and filtration of sol-gel paint (TPFSP), a modified sol-gel composite coating technique. SEM results show that the coatings were dense and crack-free by using paints of high mass ratios of YSZ powder to Zr-Y oxide in sol and high pressures. And the cross-sectional detail of coatings exhibited that the microstructure was consisted of micro/nano-size ceramic particles and micro-pores. The thermal insulation tests indicated that mass ratio of YSZ powder to Zr-Y oxide in sol was in inversely linear relationship with temperature drop per micron thickness. The coating showed good adherence with alloy substrate and maintained structural integrity when exposed at 1050 °C for 200 h. The cyclic oxidation test also indicated that both of oxidation resistance and spallation resistance for YSZ coated specimens were greatly improved. The TPFSP process could be a promising method to prepare TBCs for wide applications.     

Structure-property relations in ZrCN coatings for tribological applications

ZrCN coatings were deposited by dc reactive magnetron sputtering with N₂ flows ranging from 2 to 10 sccm in order to investigate the influence of the nitrogen incorporation on structure and properties. Information about the chemical composition was obtained by glow discharge optical emission spectroscopy and Rutherford backscattering spectroscopy. The evolution of the crystal structure studied by X-ray diffraction revealed the formation of a face-centred cubic ZrCN phase for N₂ flows greater than 4 sccm. Additionally, the presence of an amorphous phase in the coatings deposited with the highest N₂ flows could be evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy. This phase can act as a lubricant resulting in a low coefficient of friction as shown in the conducted ball-on-disc tests. Nanoindentation measurements showed that coatings deposited with a 6 sccm N₂ flow had the maximum hardness which also revealed the best performance in the conducted dry cutting tests.     

N5基体与NiCrAlY粘结层界面元素互扩散行为研究

采用EB-PVD技术在镍基合金ReneN5基体表面沉积NiCrAlY粘结层,并对试样进行1000℃不同时间的恒温氧化处理,通过SEM、EDS、XRD研究了基体与粘接层界面元素互扩散行为和反应区的形成机制。结果表明:在1000℃条件下,NiCrAlY粘结层中的Al、Cr元素会向N5基体扩散,形成以β-NiAl和α-Cr相为主的互扩散反应区;而N5基体中的Ni元素则会向NiCrAlY粘结层扩散,形成以γ′-Ni_3Al相为主的二次反应区和以难熔金属为主的TCP相。     

Mullite-rich plasma electrolytic oxide coatings for thermal barrier applications

A study has been undertaken of the characteristics exhibited by mullite-rich plasma electrolytic oxide coatings grown on aluminium alloys by using silicate-rich electrolytes. It is found that they can be grown at a higher rate, and to a greater thickness, than alumina PEO coatings on aluminium. The thermal conductivity of these coatings has been measured using a steady-state method. It is shown to be of the order of 0.5 W m^− 1 K^− 1, which may be compared with ∼ 1.5 W m^− 1 K^− 1 for pure alumina PEO coatings and ∼ 10-15 W m^− 1 K^− 1 for dense polycrystalline mullite. Coupled with excellent substrate adhesion and good mechanical properties, this relatively low conductivity makes these coatings attractive for thermal barrier applications. Furthermore, they are shown to exhibit a relatively low global stiffness (∼ 40 GPa), which will reduce the magnitude of thermally-induced stresses and improve the resistance to spallation during temperature changes.     

Overview of thermal barrier coatings in diesel engines

An understanding of delamination mechanisms in thermal barrier coatings (TBCs) has been developed for diesel engine applications through rig tests, structural analysis modeling, nondestructive evaluation, and engine evaluation of various TBCs. This knowledge has resulted in improved TBCs that survive se-vere cyclic fatigue tests in high-output diesel engines. Although much conflicting literature now exists regarding the impact of TBCs on engine performance and fuel consumption, changes in fuel consumption appear to be less than a few percent and can be nega-tive for state-of-the-art diesel engines. The ability of the TBC to improve fuel economy depends on a num-ber of factors, including the fuel injection system, combustion chamber design, and initial engine fuel economy. Limited investigations on state-of-the-art diesel engines have indicated that surface- connected porosity and coating surface roughness may influence engine fuel economy. Current research efforts on TBCs are primarily directed at reduction of in-cylinder heat rejection, ther-mal fatigue protection of underlying metal surfaces, and possible reduction of diesel engine emissions. Significant efforts are still required to improve the plasma spray processing capability and the economics for complex-geometry diesel engine components.     

Optimised selection of new protective coatings for biofuel boiler applications

Using biofuels in power and CHP boilers can pose a challenge for materials performance. Formation of deposits containing e.g. potassium, sulphur, calcium, sodium, and chlorine can result in severe corrosion of conventional steels and alloys at relatively modest temperatures. Given suitable component design and fabrication facilities, coatings may be considered to protect the fireside surfaces. This paper aims to present a systematic approach to the design and selection criteria for protective coatings of boilers. The approach includes modelling of the process and surface conditions, optimisation of the coating process and structure, and performance validation in the laboratory and plant scales. The applied examples have included iron and nickel based HVOF and arc sprayed coatings subjected to verification field testing in boiler testing under aggressive biofuel conditions. The coatings have shown good corrosion resistance in both laboratory tests and long‐term harsh field tests. The paper discusses the used approach for finding a suitable and cost effective coating for biofuel boiler applications. The paper gives test results from microstructural, corrosion resistance and field testing experience for the selected coatings.     

Sealing procedures for thick thermal barrier coatings

Zirconia-based 8Y₂O₃-ZrO₂ and 22MgO-ZrO₂ thick thermal barrier coatings (TTBC, 1000 μm), were studied with different sealing methods for diesel engine applications. The aim of the sealing procedure was to improve hot corrosion resistance and mechanical properties of porous TBC coatings. The surface of TTBCs was sealed with three different methods: (1) impregnation with phosphate-based sealant, (2) surface melting by laser glazing, and (3) spraying of dense top coating with a detonation gun. The thicknesses of the densified top layers were 50–400 μm, depending on the sealing procedure. X-ray diffraction (XRD) analysis showed some minor phase changes and reaction products caused by phosphate-based sealing treatment and some crystal orientation changes and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values. The hot corrosion resistance of TTBCs against 60Na₂SO₄-40V₂O₅ deposit was determined in isothermal exposure at 650 °C for 200 h. Corrosion products and phase changes were studied with XRD after the test. A short-term engine test was performed for the reference coatings (8Y₂O₃-ZrO₂ and 22MgO-ZrO₂) and for the phosphate-sealed coatings. Engine tests, duration of 3 h, were performed at the maximum load of the engine and were intended to evaluate the thermal cycling resistance of the sealed coatings. All of the coatings passed the engine test, but some vertical cracks were detected in the phosphate-sealed coatings.     

三种热障涂层的组织性能

采用等离子喷涂技术制备了三种不同材料的热障涂层(TBC),对涂层进行了组织性能的分析比较.结果表明,Al-1075的TBC结合强度最高,为 24.66 MPa,具有良好的抗热震性能;KF-230的TBC结合强度最低,为 16.06 MPa;LG-210的TBC结合强度居中,抗热震性能最差.分析认为,氧化物层(TGO)在热障涂层中的失效起至关重要的作用,TGO是裂纹的产生源,是裂纹扩展的通道,是热障涂层系统中的最薄弱环节.因此抑制TGO是提高涂层结合强度、改善涂层抗热震性能的重要措施.     

石墨烯颗粒对Ni-Co-石墨烯复合镀层的表面性能影响

为了得到性能更加优异全面的复合镀层,使用复合电沉积技术制备不同石墨烯颗粒大小的Ni-Co-石墨烯复合镀层,并制备了Ni-Co合金镀层。测试镀层的表面形貌,相结构,显微硬度,耐磨性和耐蚀性能。结果显示,石墨烯在电沉积中很好的嵌入到了镀层基质中,而且石墨烯的存在并没有改变镀层基质的晶体结构;石墨烯的填加增加了复合镀层的显微硬度,最高可达805HV;降低了复合镀层的摩擦系数,在一定程度上减少了粘着磨损的面积;复合镀层的自腐蚀电流密度可以降低到1.0905×10-5A/cm2,低于Ni-Co合金镀层的自腐蚀电流密度。说明了石墨烯的添加增强了复合镀层的硬度,耐磨性和耐蚀性。     

Tantalum oxide coatings as candidate environmental barriers

Tantalum (Ta) oxide, due to its high-temperature capabilities and thermal expansion coefficient similar to silicon nitride, is a promising candidate for environmental barriers for silicon (Si) nitride-based ceramics. This paper focuses on the development of plasma-sprayed Ta oxide as an environmental barrier coating for silicon nitride. Using a D-optimal design of experiments, plasma-spray processing variables were optimized to maximize coating density. The effect of processing variables on coating thickness was also determined. X-ray diffraction (XRD) was use to ascertain that the as-sprayed coatings were comprised of α- and β-Ta₂O₅, but were fully converted to β-Ta₂O₅ after a 1200 °C heat treatment. Grain growth of the Ta₂O₅ followed a time dependence of t ^0.2 at 1200 °C.     

Technical note: Plasma-sprayed ceramic thermal barrier coatings for smooth intermetallic alloys

A durable ceramic thermal barrier coating is applied directly to a smooth, highly oxidation resistant intermetallic alloy in two layers. The first layer of ceramic is applied by low pressure plasma spraying and the second layer is applied by conventional atmospheric pressure plasma spraying. This approach would allow the use of plasma sprayed ceramic coatings in applications where a metallic bond coat is not desirable.     

Stress analysis of a barb test for thermal barrier coatings

Stress distributions in a barb pullout specimen prepared from the thermal barrier coating (TBC)/substrate structure are analyzed by the finite element method. Contribution of thermal stresses and applied stresses is included in the computations. Interfacial stress singularity at the test support edge is evaluated and effects of various Young's modulus ratios between the TBC layer and substrate are shown. The results help to understand the mechanical behavior in the barb test.     

Performance of thermal barrier coatings on γ‐TiAl

The current development of new generation gamma titanium aluminides is expected to result in alloy chemistries and microstructures capable of operating at temperatures in excess of 850 °C. Under these conditions, environmental and thermal protection becomes a concern since oxidation might eventually limit the maximum service temperatures achievable. Therefore protective coatings are necessary to exploit the full potential of gamma titanium aluminides at moderately elevated temperatures; however, as yet no coating system tested has proven sufficient performance for long‐term use in automotive and aerospace applications. Thermal barrier coatings (TBCs), typically applied to nickel‐based alloys, offer the potential to increase the service temperature of components by lowering the metal surface temperature in combination with cooling systems. The paper is focussed on development of thermal barrier coatings for gamma titanium aluminides. Different coatings were used for oxidation protection and bond coat application. Substrate specimens were either pre‐oxidized or coated with PVD‐Al₂O₃, TiAlCrYN, or diffusion aluminides. Yttria‐stabilized zirconia TBCs were deposited applying electron‐beam physical vapour deposition. Cyclic and quasi‐isothermal oxidation tests were carried out at 900 °C in air. Post‐oxidation analysis of the coating systems was performed using scanning electron microscopy with energy‐dispersive X‐ray spectroscopy. Zirconia top coats offer a promising thermal protection concept to be applied on γ‐TiAl components. However, high oxidation resistance has to be supplied by protective coatings. Diffusion layers of the TiAl₃ aluminide provided excellent environmental protection because of the formation of a continuous alumina scale. No spallation of the thermal barrier coatings was observed on aluminized specimens during 1000 1‐h cycles and 3000 h of cyclic and isothermal oxidation testing, respectively.     

Vapor deposition of platinum alloyed nickel aluminide coatings

Platinum-doped NiAl coatings are widely used to increase the oxidation resistance of superalloys. These coatings are usually synthesized by a solid state reaction-diffusion process conducted at high temperature. It requires the chemical vapor deposition of aluminum on a nickel rich superalloy substrate that has been pre-coated with several microns of electrodeposited platinum. Here, we show that an electron beam directed vapor deposition (EB-DVD) technique can be used to deposit well bonded, structurally and chemically homogeneous NiAlPt bond coats of any composition onto superalloy substrates. The approach utilized a high voltage, rapid scan frequency electron beam to independently heat elemental nickel, aluminum and platinum melt pools to create three closely spaced vapor plumes. These vapor plumes were then entrained in an inert gas jet flow, which mixed and directed them to a substrate. By adjusting the electron beam current applied to each elemental source, homogeneous, dense, Pt alloyed β-phase NiAl coatings could be synthesized at substrate temperatures of 1050 °C. The width of the substrate-coating interdiffusion zone was controlled by the deposition temperature and time.