Isothermal oxidation of air plasma spray NiCrAlY bond coatings

The isothermal oxidation behaviors of plasma-sprayed NiCrAlY bond coatings (Ni22Cr10Al1Y and Ni31Cr11Al 0.6Y) have been evaluated. Two unique microstructures, oxide stringers and improperly flattened zones, can be observed in the air plasma spray (APS) bond coatings. The structures and chemical compositions of the oxide stringers were examined by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Improperly flattened zones had a high density of open pores. These features affected the oxidation behaviors in both the transient and the steady-state stages. In the transient oxidation stage, NiO, Cr₂O₃ and -Al₂O₃ were observed on the surface simultaneously. The oxide stringers and improperly flattened zones had an especially high density of NiO and Cr₂O₃, which implied that these regions had inherently lower contents of Al. During steady-state oxidation, a continuous -Al₂O₃ layer reduced the diffusion rate of oxygen and, thus, the rate of isothermal oxidation became low. As the oxidation time was increased, depletions of Al, the formation of NiAl₂O₄ layers and the formation of Cr₂O₃ subsequently occurred. Hemispherical protrusions were also detected on the surface after cooling to room temperature.     

Surface roughness of thermal spray coatings made with off-normal spray angles

The formation of a thermal spray coating using an off-normal direction angle for the spray has been analyzed to identify the causes of the large surface roughness of the coating. In the analysis, the string method was used for modeling the formation of the coating. The method uses a string of equally spaced node points to define the shape of the coating surface and to track the change in this shape as the thermal spray mass is deposited. The method allows for the calculation of arbitrary shapes for the coating surface that may be very complex. The model simulates the stochastic deposition of a large number of thermal spray droplets. Experiments were carried out to obtain the data used in the model for the mass flux distribution on the target surface. The data show that when the thermal spray mass impinges on the target surface a large fraction of it, called overspray, splashes off the target and is redeposited with a small direction angle. This component of the deposited mass results in a large coating roughness.     

NiCrAlY涂层的表面状态对高温氧化行为的影响

将原始喷涂态和表面抛光的NiCrAlY涂层在1050℃恒温氧化300h，利用XRD、SEM，EDS方法，测定涂层的氧化物及其相转变，分析表面氧化膜的生长破坏行为，结果表明：两种涂层在1050℃保温，在150h以内均能生成α-Al2O3氧化膜，150h后，抛光态涂层保护性氧化膜被破坏，抗氧化能力下降，喷涂态涂层表面粗糙，连续Al2O3保护膜的形成较晚，氧化早期氧化膜中存在微裂纹，可释放应力，有利于氧化膜与涂层的结合。氧化动力学曲线符合抛物线规律，氧化至300h，表面氧化膜只有少量微裂纹，无剥落，说明喷涂涂层的长期恒温抗氧化能力比抛光态涂层强。     

基板温度对电热爆炸喷涂制备粘结层性能的影响

基板温度分别为室温、100和200 ℃时,采用电热爆炸喷涂技术在IC10合金表面制备NiCoCrAlY合金粘结层.利用扫描电镜(SEM)、能谱仪(EDS)和X射线衍射仪(XRD),对热循环前后的粘结层的组织形貌、化学成分及相进行分析.结果表明,基板温度对电热爆炸喷涂技术所制备NiCoCrAlY合金粘结层有明显的影响,随着基板温度的升高,所制备的粘结层与基体界面结合良好,表面粗糙度减小.在1050 ℃热循环后粘结层表面生成了Al2O3氧化层,起到了保护基体的作用.     

The influence of bond coat surface roughness on chemical failure and delamination in TBC systems

Localised loss of aluminium from the bond coat within thermal barrier coating (TBC) systems is a critical factor determining the lifetime of these coatings. In this paper, it will be demonstrated that electroplated MCrAlY bond coats with asperities of high aspect ratios can experience premature chemical failure during isothermal exposures at 1100 °C in air. This can result in the rapid conversion of the asperity into Co‐ and Cr‐rich oxides. The enhanced aluminium depletion within these asperities, which leads to chemical failure, is a consequence of their increased surface area coupled with a limited physical access to the reservoir of aluminium in the bulk of the bond coat. This process has been demonstrated using the 2‐D finite‐difference model ODIN which predicts a depletion to less than 1 at% aluminium within a typical asperity after 1 h exposure at 1100 °C. The results also show that the aluminium content of the bulk of the bond coat is sufficient to enable the formation of a continuous protective alumina underneath these regions, as observed experimentally. It is suggested that the volume increase associated with the conversion of the bond coat asperities into breakaway oxide results in out‐of‐plane tensile stress development at the oxidation temperature. These stresses are anticipated to be at a maximum between the oxide asperities and are thought to nucleate small, sub‐critical cracks at these locations at temperature. Further delamination will occur during cooling both by the extension of these cracks and by the growth of crack‐like defects within the porous breakaway oxides.     

Investigation of splat curling up in thermal spray coatings

The curling up of the edges of splats of molten metal deposited on a cold substrate was investigated both experimentally and numerically. An analytical model, based on mismatch of thermal expansion between the splat and substrate, was developed to calculate the deformation of splats after curling up. The curling-up angle was measured from both millimeter-sized splats of aluminum alloy and bismuth and plasma-sprayed nickel particles. The curling-up angles were predicted using both the analytical model and a numerical code and were found to agree reasonably well with experimental measurements. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J.Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Study of oxidation behavior of nanostructured NiCrAlY bond coatings deposited by cold spraying

Nanostructured NiCrAlY bond coating was deposited using a milled powder by cold spraying. A shot-peening treatment was then applied to the as-sprayed coating to modify the coating surface morphology. The oxidation behavior of the coating with the as-sprayed surface and shot-peened surface was investigated under isothermal oxidation at 900 °C and 1000 °C for different times. The oxidation behavior of the coating was characterized through surface morphology and cross-sectional microstructure by scanning electron microscopy. It was found that a uniform oxide layer was formed on the surface of the shot-peened nanostructured NiCrAlY coating during oxidation at temperatures of 900 °C and 1000 °C. The surface morphology of the coating has significant effect on the morphology of the oxide. The surface geometry of the cold-sprayed MCrAlY coating must be modified to promote formation of a protective oxide film during oxidation, through application of a post-treatment process such as shot-peening.     

无冷却喷涂工艺对其制备的热障涂层的裂纹系统和寿命的影响

无冷却喷涂形成的热障涂层裂纹体系,可提高陶瓷顶层应变容限.但目前缺乏对裂纹体系的系统研究,特别是横向分叉裂纹.因此,文中研究送粉率和基体预热温度对陶瓷顶层裂纹系统的定量影响,并比较不同裂纹系统的热循环寿命.结果表明,增加送粉率,垂直裂纹密度和横向分叉裂纹长度均呈现先大后小的趋势.预热温度的提高可增加涂层中垂直裂纹数量,但横向分叉裂纹长度呈现先增后降的趋势.热循环试验表明,维持一定垂直裂纹的同时,降低横向分叉裂纹可提高涂层热循环寿命.     

Substrate roughness and thickness effects on cold spray nanocrystalline Al−Mg coatings

Nanocrystalline Al−Mg coatings were produced using the cold gas dynamic-spraying technique. Unsieved Al−Mg powder of average nanocrystalline grain size in the range of 10 to 30 nm and with a particle size distribution from 10 to >100 μm was used as the feedstock powder. The resulting coatings were evaluated using scanning electron microscopy (SEM), transmission electron microscopy, as well as microhardness and nanoindentation measurements. Coating observations suggest that the wide particle size distribution of the feedstock powder has a detrimental effect on the coating quality but that it can be successfully mitigated by optimizing the spraying parameters. Nanohardness values close to 3.6 GPa were observed in both the feedstock powder and coatings, suggesting the absence of cold-working hardening effects during the process. The effects of the substrate surface roughness and thickness on coating quality were investigated. The deposited mass measurements performed on the coatings showed that the effect of using different grit sizes for the substrate preparation is limited to small changes in the deposition efficiency of only the first few layers of deposited material. The SEM observation showed that the substrate surface roughness has no significant effect on the macrostructures and microstructures of the coating. The ability to use the cold gas dynamic spraying process to produce coatings on thin parts without noticeable substrate damage and with the same quality as coatings produced on thicker substrates was demonstrated in this work. The original version of this paper was published in the CD ROM Thermal Spray Connects: Explore Its Surfacing Potential, International Thermal Spray Conference, sponsored by DVS, ASM International, and IIW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmbH, Düsseldorf, Germany.     

Porosity and surface roughness simulation of nickel-aluminum coating in plasma spray forming

As the important evaluation parameters concerning the spray qualities, the porosity and surface roughness of the coatings obtained by thermal spray forming have great influence on their forming accuracy, mechanical properties and service lifetime. But it is difficult to predict or control the two parameters for such a highly nonlinear process. A two-dimensional simulation of coating porosity and surface roughness of nickel-aluminum alloy （Ni-5%Al） in plasma spray forming was presented, which was based on the multi-dimensional statistical behaviors of the droplets as well as the simplification and digitization of the typical splat cross sections. Further analysis involving the influence of the droplet diameters and the scanning velocities of the spray gun on the two parameters was conducted. The simulation and analysis results indicate that the porosity and surface roughness are more influenced by the droplet diameters, but less influenced by the spray gun velocities. The results will provide basis for the prediction or control of coating mechanical properties by depositing parameters.     

Diffusion and phase transformation on interface between substrate and NiCrAlY in Y-PSZ thermal barrier coatings

NiCrAlY/Y₂O₃-Y-PSZ (yttria-partially stabilized zirconia) thermal barrier coatings were developed on a superalloy (Ni-10Co-9Cr-7W-5Al, wt.%) surface. The superalloys were first coated with a bond coat of Ni-19Cr-8Al-0.5Y (wt.%) alloy that was deposited by low-pressure plasma spraying and then covered with a top coat of ZrO₂-8wt.%Y₂O₃ by air plasma spraying. The microstructure near the interface was analyzed using an optical microscope, a scanning electron microscope, microhardness measurements, and x-ray diffraction, and the phases of composition were measured using an electron probe microanalyzer after exposure at 1100°C for different times in air or a vacuum. The reaction processes also were simulated using diffusion-controlled transformation (DICTRA) software in which diffusion was considered as being only the γ phase, and the γ′ phase was treated as spheroidal particles in γ. From the authors’ results, it can be concluded that a γ′-phase layer is observed at the interface between substrate and bond coat, and its thickness increases with increasing exposure times in air at 1100 °C. This layer showed good cohesion with the substrate and bond coat. It can also be concluded that the formation of the γ′-phase layer can be predicted from DICTRA simulation. The simulation also shows the same trend of the composition profiles as experimental data.     

Thermal cycling behavior of plasma-sprayed thermal barrier coatings with various MCrAlX bond coats

The influence of bond coat composition on the spallation resistance of plasma-sprayed thermal barrier coatings (TBCs) on single-crystal René N5 substrates was assessed by furnace thermal cycle testing of TBCs with various vacuum plasma spray (VPS) or air plasma-spray (APS) MCrAlX (M=Ni and/or Co; and X=Y, Hf, and/or Si) bond coats. The TBC specimens with VPS bond coats were fabricated using identical parameters, with the exception of bond coat composition. The TBC lifetimes were compared with respect to MCrAlX composition (before and after oxidation testing) and MCrAlX properties (surface roughness, thermal expansion, hardness, and Young’s modulus). The average TBC spallation lifetimes varied significantly (from 174 to 344 1 h cycles at 1150 °C) as a function of bond coat composition. Results suggested a relationship between TBC durability and bond coat thermal expansion behavior below 900 °C. Although there were only slight differences in their relative rates of cyclic oxidation weight gain, VPS MCrAlX bond coats with better oxide scale adhesion provided superior TBC lifetimes.     

Protection of carbon steel against hot corrosion using thermal spray Si- and Cr-base coatings

A Fe75Si thermal spray coating was applied on the surface of a plain carbon steel baffle plate. Beneath this coating, a Ni20Cr coating was applied to give better adherence to the silicon coating. The baffle was installed in the high-temperature, fireside, corrosion zone of a steam generator. At the same time, an uncoated 304 stainless steel baffle was installed nearby for comparison. For 13 months the boiler burned heavy fuel oil with high contents of vanadium. The samples were studied employing scanning electron microscopy, x-ray microanalysis, and x-ray diffraction techniques. After that, it was possible to inspect the structural state of the components, and it was found that the stainless steel baffle plates were destroyed almost completely by corrosion, whereas the carbon steel coated baffle plate did not suffer a significant attack, showing that the performance of the thermal spray coating was outstanding and that the coating was not attacked by vanadium salts of the molten slag.     

Sealing of thermal spray coatings by impregnation

Results from the sealing of porosity by impregnation show that below a certain wetting angle of the sealant, high penetration depths are achieved. However, only sealants with very low curing shrinkages can prevent the transport of electrolyte through the coating. Various sealant types and impregnation methods are discussed, and factors influencing impregnation and sealing ability of sealants are reviewed. Experimental results from the sealing of plasma-sprayed aluminum-oxide coatings are presented.     

Improved oxidation life of segmented plasma sprayed 8YSZ thermal barrier coatings

Unconventional plasma sprayed thermal barrier coating (TBC) systems were produced and evaluated by interrupted or cyclic furnace oxidation life testing. First, approximately 250 μm thick 8YSZ coatings were directly sprayed onto grit blasted surfaces of PWA 1484, without a bond coat, to take advantage of the excellent oxidation resistance of this superalloy. For nominal sulfur (S) contents of 1 ppmw, total coating separation took place at relatively short times (200 h at 1100°C). Reductions in the S content, by melt desulfurization commercially (0.3 ppmw) or by hydrogen (H₂) annealing in the laboratory (0.01 ppmw), improved scale adhesion and extended life appreciably, by factors of 5–10. However, edge-initiated failure persisted, producing massive delamination as one sheet of coating. Secondly, surfaces of melt desulfurized PWA 1484 were machined with a grid of grooves or ribs (∼250 μm wide and high), resulting in a segmented TBC surface macrostructure, for the purpose of subverting this failure mechanism. In this case, failure occurred only as independent, single-segment events. For grooved samples, 1100 °C segment life was extended to ∼1000h for 5 mm wide segments, with no failure observed out to 2000 h for segments ≤2.5 mm wide. Ribbed samples were even more durable, and segments ≤6 mm remained intact for 2000 h. Larger segments failed by buckling at times inversely related to the segment width and decreased by oxidation effects at higher temperatures. This critical buckling size was consistent with that predicted for elastic buckling of a TBC plate subject to thermal expansion mismatch stresses. Thus, low S substrates demonstrate appreciable coating lives without a bond coat, while rib segmenting extends life considerably.     

热喷涂合成Fe3Al基涂层的高温摩擦学特性

采用自制的新型热喷涂粉芯丝材和高速电弧喷涂技术成功制备出Fe3Al金属化合物基合金涂层，对涂层的组织、成分和相结构进行了初步分析，并测试了涂层在室温至650℃的摩擦学特性。结果表明：Fe3Al涂层的基体相由含29％Al(摩尔分数)的Fe3Al和FeAl二相混合物组成，Fe3Al／WC涂层基体为Fe-26％Al(摩尔分数)合金；添加少量WC硬质相的Fe3Al／WC涂层具有良好的高温减摩特性和耐磨性，450℃和650℃其相对耐磨性分别比20g钢高1．92倍和9．23倍；WC／W2C硬质相对Fe3Al基体的强化作用，以及涂层表面形成具有自保护作用的高硬度致密氧化膜是Fe3Al／WC涂层高温耐磨性提高的主要原因。     

Influence of surface roughness on the flattening of powder particles during thermal spraying

The time evolution of the splat thickness, radius, and rate characteristics in the process of flattening of droplets during thermal spraying is investigated taking into account the surface roughness, splat solidification, and mass loss of the droplet liquid phase. Analytical formulas describing the final values of the splat thickness, radius, and rate characteristics are found. Results agree well with the experimental data. They can be used to predict the splat flattening parameters.     

Effect of grit-blasting on substrate roughness and coating adhesion

Statistically designed experiments were performed to compare the surface roughness produced by grit blasting A36/1020 steel using different abrasives. Grit blast media, blast pressure, and working distance were varied using a Box-type statistical design of experiment (SDE) approach. The surface textures produced by four metal grits (HG16, HG18, HG25, and HG40) and three conventional grits (copper slag, coal slag, and chilled iron) were compared. Substrate roughness was measured using surface profilometry and correlated with operating parameters. The HG16 grit produced the highest surface roughness of all the grits tested. Aluminum and zinc-aluminum coatings were deposited on the grit-blasted substrates using the twin-wire electric are (TWEA) process. Bond strength of the coatings was measured with a portable adhesion tester in accordance with ASTM standard D 4541. The coatings on substrates roughened with steel grit exhibit superior bond strength to those prepared with conventional grit. For aluminum coatings sprayed onto surfaces prepared with the HG16 grit, the bond strength was most influenced by current, spray distance, and spray gun pressure (in that order). The highest bond strength for the zinc-aluminum coatings was attained on surfaces prepared using the metal grits.