Multilayer protective polymeric coatings: Providing for their continuity

It has been shown that, owing to the application of a multilayer design (construction) and the correct choice of technology and materials, the following results on enhancement of the continuity of polymeric protective coatings (PCs) may be achieved: (1) a reduction in the technological defectiveness to a preassigned level due to overlapping of flaws, even at a high specific defectiveness of the materials used in the PC layers; (2) an increase in the resistance to cracking of rigid PC layers and hard substrates because of the redistribution of internal stresses in the substrate-multilayer PC system; and (3) an enhancement in PC resistance to cracking on the opening-up of a crack in the substrate under tension of bending. Original Russian Text ? V.A. Golovin, 2007, published in Korroziya: Materialy, Zashchita, 2006, No. 2, pp. 28–36.     

Ni-Al基涂层制备工艺及其抗氧化性能

通过调整固体粉末渗铝法工艺在镍基高温合金DZ417G基体上制备了Ni-Al二元成分区间内组织可控的γ′-Ni3Al、γ′-Ni3Al+β-NiAl、β-NiAl、β-NiAl+δ-Ni2Al3和δ-Ni2Al3涂层,采用光学显微镜、X射线衍射仪和扫描电镜对涂层的结构、断面形貌以及高温氧化后的表面形貌进行观察和分析。900℃和1 100℃空气恒温氧化实验结果表明:涂层能显著提高材料的抗氧化性能,在涂层厚度相同的情况下,NiAl涂层抗氧化性能最好。     

电火花沉积MCrAlY涂层及其高温氧化行为

采用DZ-1600型电火花沉积设备在不锈钢表面电火花沉积MCrAlY涂层,研究了工艺参数对沉积效率的影响以及涂层的组织结构和抗氧化性能之间的关系.结果表明,沉积效率与输出功率成正比,频率对沉积效率几乎无影响.涂层表面呈"泼溅状"形貌,整体呈现纳米级微晶堆垛结构,反映了沉积过程的快速冷凝机制.XRD分析结果显示,高温氧化后在涂层表面形成的氧化膜主要由Al2O3组成.在1000℃空气中氧化200h的结果表明,获得的MCrAlY涂层具有优异的抗高温氧化与抗剥落性能.     

Diamond coatings on tungsten carbide and their erosive wear properties

Diamond coatings up to ∼60-μm thick have been grown by microwave plasma CVD (MPCVD) on sintered tungsten carbide (WC) substrates, and their erosive wear properties are investigated under high velocity air–sand erosion testing. Two different sintered tungsten carbide (WC) substrates have been investigated and compared, the binder being either 6% Co or 5% Ni by weight. Significant differences in morphology, residual stress, adhesion and erosion performance are seen as a function of pre-deposition treatment, deposition conditions and the source of the substrates. Adherent coatings could be deposited to a thickness of ∼35 μm. They offer significantly better erosion resistance compared to uncoated substrates, with the erosion rate being lowered by up to a factor between ∼5 and 20 for particle test velocities of 148 and 63 m s⁻¹, respectively. The steady-state erosion rates of the coatings are a function of a gradual micro-chipping mechanism. However, the life of the coating is dependent on the progression of sub-surface damage promoted by sub-surface shear stresses associated with the particle impacts. It is thought that the coating debonding is driven by the shear stresses interacting with the grain boundary porosity at the substrate/coating interface.     

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.     

不同工艺制备的Ni-Cr-Al-Y合金涂层及性能

采用低压等离子喷涂（LPPS）、常规超音速火焰喷涂（HVOF）和低温超音速火焰喷涂（LT-HVOF）三种工艺在铸造镍基高温合金K438基体上制备了Ni-Cr-Al-Y涂层,并对三种工艺制备涂层的显微结构、元素分布、相组成和基本性能进行对比研究。结果表明：LPPS涂层致密无层状结构,含氧量微小,具有较高的显微硬度和好的抗氧化性;HVOF涂层为典型的层状结构,含氧量高,显微硬度低,抗氧化性差;LT-HVOF涂层含氧量大大降低,具有优良的抗氧化性;三种涂层都发生了元素扩散。     

Formation of microplasma coatings on high-silicon aluminum alloy and their characteristics

This work presents the results of studying the electrochemical microplasma formation of coatings based on the AK21 alloy of hypereutectic composition in silicate-alkali electrolyte. It is found that the boundary size of primary silicon inclusion in the alloy and additional cathodic surface treatment affect significantly the shape of voltage chronograms and growth dynamics of the coating thickness. It is shown that thick (more than 180 μm) layered coatings with a mosaic inner layer structure formed by fragments on the basis of the aluminum forms (amorphous Al(OH)₃, α-, δ-, γ-Al₂O₃) in the matrix of amorphous SiO₂ are formed on this alloy at all the applied treatment modes and silicon crystal sizes. It is found that the coating inner layer structure is regularly related to the limiting dimensions of primary silicon crystals. In addition, introduction of an additional cathodic component in the high-voltage polarization modes of the electrolytecoating-alloy system yielded a pronounced positive effect in the pattern of inner layer microhardness distribution that also characteristically depends on the limiting dimensions of primary silicon crystals in the initial samples.     

Atomic structure studies of chrome alloy coatings and their abrasion resistance

In this study, thin chrome alloy reflective coatings of less than 50 nm are prepared by magnetron co-sputtering chrome and a dopant material onto coated polymeric substrates. This report shows that the lattice parameter of the alloy of chrome and dopant can be used to control the macroscopic abrasion resistance of the coating. These alloy structures are not predicted by thermodynamic equilibrium phase diagrams, illustrating the ability of physical vapour deposition to generate novel coatings. As these coatings are deposited on substrates having a glass transition temperature below 150 °C, the structure after deposition but without post processing is critical because conventional post processing (high temperature annealing) cannot be applied.     

Disperse-strengthening by nanoparticles advanced tribological coatings and electrode materials for their deposition

Two types of electrode materials were developed using self-propagating high-temperature synthesis (SHS) and powder metallurgy: 1- composite with nanosized additives; 2- nanostructured cemented carbide WC-Co. Electrospark deposition (ESD) was applied to produce tribological coatings which were disperse-strengthened by incorporation of nanosized particles. Nanostructured electrodes of cemented carbides WC-8% Co provide increasing density, thickness, hardness, Young's modulus, and wear resistance of ESD-coatings. Positive effects of nanostructural state of the electrodes on the deposition process and structure/properties of the coatings are discussed. In that case the tungsten carbide phase becomes predominant in the coatings. A mechanism of the dissolution reaction of WC in Ni at the contact surface of the electrode was proposed. It was shown that formation of the coating structure is initiated on the electrode and accomplished on the substrate.     

Recent progress in thermal/environmental barrier coatings and their corrosion resistance

Thermal/environmental barrier coatings(T/EBCs)play important roles in jet and/or gas turbine engines to protect the Ni-based superalloys and/or ceramic matrix composite substrates from the high-temperature airflow damage.Great efforts have been contributed to searching for enhanced T/EBC materials to improve the efficiency of the engines,which is the key of improving thrust-to-weight ratio and energy saving.The practical candidates,rare earth-contained materials,are widely used for T/EBCs in gas turbines due to their excellent properties such as low thermal conductivity,high melting point,hightemperature strength and durability as exhibited in yttriastabilized zirconia,pyrochlore oxides and rare earth silicates.In addition to the intrinsic properties,the microstructures obtained by different synthesis processes and the service performances,as well as the underlying failure mechanism,are also significant to this specific application.However,the main challenges for T/EBCs developments are T/EBC materials selection with balanced properties and their anti-corrosion performances at higher operating temperature.In this review,we summarized the progress in their fabrication techniques and mechanical/thermal properties of typically rare earth-contained T/EBCs,together with their anti-corrosion performance under the condition of molten salts or oxides(such as Na_2SO_4,V_2O_5and NaVO_3),calcium–magnesium–alumina–silicate(CMAS)and high-temperature water vapor.     

Effect of technological parameters of applying yttrium-containing coatings on their structure and properties

The effect on the structure and properties of ion-plasma yttrium-containing coatings applied to VKh2K alloy of the technological parameters of their deposition is studied.     

Thermal stresses in diamond coatings and their influence on coating wear and failure

Thermal stresses in diamond coatings deposited onto cemented carbide substrates are calculated using the finite element method. The thermo-elastic stress fields for some coating-substrate geometries are presented. The results are compared with experimental data on the tribological behaviour of diamond coatings. Residual stresses can explain many of the observed patterns of coating wear and failure. A model for the abrasive wear of brittle coatings under large compressive biaxial stresses is described. These stresses prevent cracks initiated at the surface to propagate towards the interface and may promote crack paths parallel to the interface, thus causing the formation of a smooth coating surface. Once the smooth appearance is reached it will become extremely hard to initiate and propagate cracks into the coating and consequently the wear rate becomes very low. Thus, large compressive residual stresses increase the already high wear resistance of diamond coatings. When diamond coatings are deposited onto substrate edges, intense concentrations of normal and shear stresses may lead to coating failure by interfacial spalling. These stresses are lowered by increasing the ratio r/h, where r is the edge radius and h is the coating thickness.     

Exfoliation corrosion of aluminum alloys and their protection by coatings with metal-filled primers

Exfoliation corrosion (EC) is shown to represent a special type of stress corrosion which develops under the surface of aluminum high-alloyed alloys like 16T and B95T1. Protection against EC by using common paint coatings (PCs) is ineffective. Indoor tests demonstrate that EC can be almost completely suppressed by using PCs with zinc-filled primers (ZFPs). After 2-year tests in sea tropics, common PCs do not prevent the appearance and development of EC. At the same time, samples protected by PCs that involve ZFP EP-057 demonstrate the total absence of EC including bare surface areas up to 5 and even 10 mm wide where the coating has failed. Similar results are obtained in 6-year tests in north sea atmosphere. Potentiodynamic studies of different protection schemes confirm the electrochemical protection mechanism of aluminum alloys against EC by using ZFPs. It is shown that ZFPs can also be used during maintenance works. The advantages of metal-filled primers based on polyurethane over those based on epoxy resin are shown.Translated from Zashchita Metallov, Vol. 41, No. 1, 2005, pp. 40–51.Original Russian Text Copyright © 2005 by Sinyavskii, Kalinin, Gladyshev, Yakimova.     

Modeling of the anisotropic elastic properties of plasma-sprayed coatings in relation to their microstructure

The transversely isotropic elastic moduli of plasma-sprayed coatings are calculated in terms of microstructural parameters. The dominant features of the porous space are identified as strongly oblate pores, that tend to be either parallel or normal to the substrate. “Irregularities” in the porous space geometry—the scatter in pore orientations and the difference between pore aspect ratios of the two pore systems—are shown to have a pronounced effect on the effective moduli. They may be responsible for the “inverse” anisotropy (Young’s modulus in the direction normal to the substrate being higher than the one in the transverse direction) and for the relatively high values of Poisson’s ratio in the plane of isotropy. The analysis utilizes results of Kachanov et al. (Appl. Mech. Rev., 1994, 47, 151) on materials with pores of diverse shapes and orientations.     

Structure of duplex CrN/NbN coatings and their performance against corrosion and wear

In tribological applications the coating-substrate combination can be considered as a system, since both greatly influence the properties of that affect the tribological performance. Further, it is often desirable that both high wear resistance and corrosion resistance can be achieved even when low cost and easily machineable substrate materials are considered. Duplex surface treatment combining pulse plasma nitriding and PVD coating can provide solution for excellent wear and corrosion resistance for low alloy and constructional steels.In this work three different pulse plasma nitriding processes were carried out prior to the CrN/NbN PVD coating to attain high surface hardness and enhanced load bearing behaviour for S154 high strength construction steel. The phase composition of the compound layer, formed in the nitriding process, was found to greatly affect the tribological properties of the duplex system. The compound layer with high amount of ?-phase contributed to superior corrosion and wear resistance, whereas the ductile γ'-phase compound layer provided better impact resistance and enhanced. The best duplex treated S154 samples had wear resistance comparable to that of similarly coated HSS. The corrosion resistance was also improved by duplex process. If anodic current at + 500 mV vs. SCE is considered as criteria, the best system has almost 3 orders of magnitude lower corrosion current than with the PVD coating alone.