Erosive and abrasive wear resistance of overlay coatings

In the paper, the erosion and abrasion resistance of PTA, TIG and flame deposited coatings was investigated. Hardness of coatings has almost no effect on erosion resistance and incubation period. Microstructure of coatings has significant effect on erosive wear of coatings. No significant correlation was found between results of abrasive and erosive tests. Statistically significant correlation was found between erosive wear intensities determined in tests carried out at similar angles, the total content of B and C correlates with mass loss in abrasion test and erosive wear intensity at normal incidence. Laboratory tests with model abrasives cannot be used as a guide for material selection in industry.     

Solid particle erosion of SiC-Al2OC ceramics

Erosion rates of SiC-Al₂OC ceramics, with Al₂OC content varying from 5 to 75wt%, were assessed using 240-grit alumina abrasive particles accelerated to a velocity estimated at 120msec^–1 and impacting the target at normal incidence. The target ceramics varied in hardness from 27.1 GPa for SiC-5wt% Al₂OC to 10.8 GPa for SiC-75wt% Al₂OC, but the fracture toughness was essentially independent of composition (K _lc ã 3.5 MPa m^1/2). The erosion weight loss varied linearly with the test duration for all the ceramics and the erosion rate decreased systematically with increasing target hardness; the hardness dependence of the erosion rate was, however, much greater than the predictions of the currently available erosion models.     

Research on cavitation erosion and wear resistance performance of coatings

Depending on the nature of the working medium and working conditions, corrosive and cavitation damage shall arise to pump’s components. In industrial applications the corrosion-reducing coatings are sprayed on hydraulic components. But it is questionable whether such products actually do help under wear or cavitation loads or not. Abrasive jet wear tests were carried out to determine the wear resistance of coating materials: polymers and ceramics, cast iron, and steel of various types. The samples were loaded for five hours, and finally the wear depth was measured as a determining indicator of the sample’s wear resistance. Results of investigation on anti-erosion performance of epoxy resin, ceramic and Polyurethane (PU) coatings brushed on alloy steel surface were also presented. Cavitation erosion tests were performed on the ultrasonic rig. The mass loss and surface morphology of the specimens were examined by balance analysis and 3-D laser microscopy, respectively. The investigations showed excellent wear-resisting performance of ceramic coatings, which is better than wear-resistance of stainless steel, cast iron and high chrome alloy steel. But the excellent wear-resisting performance could not guarantee a good erosion-resisting performance. The ceramic coatings’ anti-erosion performances were inferior to that of gray cast iron, and hardly comparable to those of stainless steels. The basic factors that influenced coating’s cavitation erosion endurance were adhesion and thickness of coatings. Analysis of coating’s degradation mechanism showed that PU coatings could withstand longer incubation period thus enhancing the materials’ cavitation erosion resistance. Several practical cases were analyzed, showing some guidance for coatings’ application.     

材料表面抗空蚀涂层的研究进展

空蚀现象广泛存在于海洋平台、船舶机械和能源发电等领域.这种腐蚀现象不仅造成了巨大的经济损失,也成为相关从业人员的安全隐患.本文概述了抗空蚀涂层材料技术的研究进展,重点介绍了抗空蚀金属涂层技术和抗空蚀聚合物涂层技术.最后对目前抗空蚀材料存在的问题及未来发展进行了展望.     

Cyclic crack resistance of ferritic-pearlitic steels and their weld joints

The cyclic crack resistance of ferritic-pearlitic steels and their weld joints is determined. On the basis of an analysis of our own and literature data a method of evaluation of the endurance of welded structures in the area of multicycle fatigue is proposed. The endurance limit _e, the parameter K_th ^*, and other parameters have been introduced into this method. The concepts of repairless structures and safely damaged structures are proposed.Translated from Problemy Prochnosti, No. 7, pp. 25–30, July, 1991.     

Microsegregation and solidification cracking of an HR-160 weld overlay

The solidification behaviour (microsegregation, secondary phase formation, solidification temperature range) of a Haynes HR-160 weld overlay deposited on 2.25 Cr-IMo steel by gas metal arc welding has been examined. Results of differential thermal analysis and various microscopy techniques conducted on an all-weld-metal sample indicate the overlay terminates solidification at ≈1290°C by the formation of a (Ti-S)-rich second phase distributed semi-continuously along grain boundaries. The semi-continuous morphology of the second phase, combined with an increase in the solidification temperature range induced by the second phase, promoted solidification cracking in the overlay deposit. Electron probe microanalysis (EPMA) was used to reveal patterns of elemental segregation within the weld overlay. The EPMA data was used with basic solidification theory to estimate values for equilibrium distribution coefficients of major alloying elements and the values are found to be similar to results previously reported for Incoloy 909. This revised version was published online in November 2006 with corrections to the Cover Date.     

Modeling degradation and failure of Ni-Cr-Al overlay coatings

Degradation of an Ni-16at.%Cr-25at.%Al-0.06at.%Zr overlay coating on an Ni-22at.%Cr substrate was examined after oxidation accompanied by thermal cycling. Concentration-distance profiles were measured in the coating and substrate after various numbers of 1 h cycles at 1150°C. A numerical model was developed to simulate coating degradation by simultaneously oxidation and coating- substrate interdiffusion. The validity of the model was confirmed by comparison of predicted and measured concentration—distance profiles. The ability of the model to identify critical system parameters was demonstrated for the case of the initial aluminum and chromium content of the coating and substrate.     

Analysis of steady-state ductile crack growth along a laser weld

The fracture toughness in an elastic-plastic material joined by a laser weld is analyzed for steady-state crack growth along the weld. The analysis is performed for laser welds in steel. Laser welding gives high mismatch between the yield stress within the weld and that in the base material, due to the fast thermic cycle that the material undergoes in welding. The material is described by J ₂-flow theory, and the analysis is performed using a special numerical algorithm, in which the finite element mesh remains fixed relative to the tip of the growing crack, so that the material moves through the mesh. Fracture is modelled by using a cohesive zone criterion in front of the crack tip along the fracture zone. It is found that in general a thinner laser weld gives a higher interface toughness. Furthermore, it is shown that the preferred path of the crack is in the base material slightly outside the weld; a phenomenon also observed in experiments.     

反应磁控溅射制备的SiO2防护涂层抗原子氧侵蚀性能研究

在原子氧侵蚀地面模拟设备中对Kapton和利用反应磁控溅射制备的SiO2涂层进行了原子氧暴露实验，并采用XPS和SEM等分析手段对暴露前后试样表面的物理和化学变化进行了研究．结果表明，Kapton试样遭受了严重的侵蚀，质量损失较大；SiO2涂层质量变化很小，对基体提供了良好的保护作用．XPS分析结果表明，反应溅射的SiO2涂层是富Si的，初始暴露时由于氧化反应而质量有少许增加，随时间延长，涂层变得完全符合化学计量．SiO2涂层在原子氧暴露后涂层的太阳吸收率、辐射率和反射系数均没有发生明显的变化．SiO2涂层较脆，易产生裂纹，原子氧会通过缺陷位侵蚀下面的基体材料，严重影响飞行任务的正常进行。     

Investigation of equilibrium and phase stability in the liquid/solid state in nickel-based wrought superalloys

Isothermal holding experiments between the liquidus temperatures, T _L, and the solidus temperatures, T _S, were carried out on nickel-based wrought superalloys. It was found that during solidification the elements aluminium, cobalt and tungsten tend to dissolve in the matrix whereas titanium, tantalum and molybdenum tend to segregate into the liquid. Molybdenum and titanium cause the formation of brittle and phases, respectively, after prolonged holding at elevated temperatures, in accordance with the results of New-PHACOMP (new phase computation).     

Modelling of the scratch resistance of particle filled sol-gel coatings on aluminium

Sol-gel derived coatings with 12 nm and 0.5-10 μm silica fillers were prepared on aluminium to evaluate the effect of particle size and filler content on coating properties The measured maximum crack-free thickness and hardness strongly depended on the type of particles used and the filler volume fraction. The scratch resistance primarily depended on coating thickness and much less on the mechanical properties of the coating. This finding was interpreted via modelling of the stresses under the scratching stylus. It was established that the initial yield occurred in the substrate for most of the coatings and the load needed for this initial yield had a relatively small dependence on the coating properties but it was strongly influenced by the coating thickness.     

Femtosecond laser machining of single-crystal superalloys through thermal barrier coatings

Femtosecond laser machining of single-crystal superalloys coated with thermal barrier coatings (TBCs) has been investigated. The investigations were carried out in air using a titanium:sapphire laser system (λ = 780 nm) operating at a repetition rate of 1 kHz and delivering individual pulses of 150 fs in duration. The ablation threshold of 7 wt.% yttria stabilized zirconia (7YSZ) has been measured to be 1.52 ± 0.21 J/cm². Microstructural investigations indicated a complete absence of conventional processing defects such as recast layers and microcracking in the vicinity of the machining area. The absence of machining-induced melting or delamination along interfaces of the TBC system demonstrates a significant advantage in comparison with conventional laser machining.