Mechanical and physical properties of high-velocity oxy-fuel-sprayed iron aluminide coatings

Tensile tests and thermal-expansion measurements were performed on free-standing, high-velocity oxy-fuel (HVOF)-sprayed Fe₃Al coatings produced at spray-particle velocities of 390, 560, and 620 m/s. To examine the relationship between properties and spray conditions, the microstructures of the coatings were characterized in terms of the fractions of unmelted particles, porosity, and oxide inclusions, as well as the dislocation density assessed by X-ray diffraction (XRD) line-broadening analysis. Residual coating stresses were determined as a function of coating thickness using curvature measurements. The tensile behavior was entirely brittle at room temperature; fracture strengths increased with spray-particle velocity; and the increase in fracture-strength results from decreasing fractions of microstructural defects and better interparticle bonding. The mean thermal-expansion coefficients for the coatings were lower than those for an equivalent wrought material; the differences were attributed to a 7 to 15 vol pct fraction of oxide inclusions. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Coatings of nanocrystalline metallic wires on steel substrate: mechanical characteristics of coating layer

Coatings are produced by wire arc spraying on a steel surface and the effect of coating thickness on the coating characteristics in terms of microhardness, residual stress and friction coefficient are examined. The wire used during coating is composed of FeCrBMnSi, which comprises nanocrystalline and amorphous phases. The twin wire arc spray system was incorporated to generate different coating thicknesses on steel substrate. The analytical tools including three-dimensional imaging optical and scanning electron microscopes, energy dispersive spectroscopy and X-ray diffraction (XRD) are used to assess the coating characteristics. It is found that the surface texture and surface roughness of the coatings change with coating thickness. Thus, reducing the coating thickness results in slightly increased texture height and surface roughness. Microhardness is almost 15% higher for the thin coating (300?μm) than the thick coating (550?μm). The residual stress developed in the coatings is tensile and it changes with coating thickness, such that it reduces for thick coating.     

Oxidation and corrosion resistance of TiAl3 coatings

Titanium aluminides based on TiAl, TiAl₃ and Ti₃Al are potential materials for high temperature aerospace applications. Their low density, high temperature creep resistance, high temperature strength and high oxidation resistance make them excellent coating materials. However these coatings are likely to be subjected to high temperature and corrosive environments during service. Hence it is aimed to study the oxidation and corrosion resistance of TiAl₃ coatings on various types of substrates. In the present work, TiAl₃ is coated on high speed steel, stainless steel 304, stainless steel 316, copper and aluminum substrates by physical vapor deposition technique. X-ray diffraction analysis confirms the presence of TiAl₃ phase. The hardness studies reveal that better hardness can be achieved with thick coatings. The oxidation behavior of the coatings is studied by carrying out step stress experiments at elevated temperatures. Coated samples are heated up from 400°C in the steps of 100°C for 1h in each step to 1000°C. The mass gain caused by oxidation was determined. The oxidation curve drawn as a function of mass gain versus temperature reveals that TiAl₃ film started to oxidize above 800°C, where as oxidation of the uncoated substrates began at a much lower temperature of 550°C. The excellent oxidation resistance of the coatings can be attributed to the formation of an amorphous Al₂O₃ film. Scanning electron microscope (SEM) and EDAX analysis confirm the presence of an amorphous Al₂O₃ film. The corrosion behavior of TiAl₃ coatings are investigated by the polarization resistance experiments in NaCl aqueous solution at ambient temperature. According to the Tafel plot analysis, the coatings show lower corrosion rate than the untreated substrates. The major corrosion in the coatings arose from electrolyte penetration into the pores of the coatings. In fact, a dense coating showed a high corrosion resistance in an aqueous medium.     

低压等离子喷涂耐高温涂层残余应力检测及分析

采用低压等离子工艺制备耐高温可磨耗涂层,通过光学显微镜观察了耐高温涂层的显微结构,采用逐层法与曲率法相结合的检测技术,对不同厚度涂层残余应力进行检测,形成了涂层厚度与残余应力分布及趋势图,运用热处理技术控制和消除涂层内部残余的涂层应力。结果表明,低压等离子喷涂的耐高温涂层表面残余应力随着涂层厚度的增加而增大,当涂层厚度达到0.6mm左右极限时,涂层会发生自然开裂,采用热处理的工艺方法可以减少涂层内的热应力导致的涂层残余应力。     

工艺参数对超音速火焰喷涂Fe基非晶涂层性能的影响

Fe基非晶涂层具有优异的耐磨、耐蚀性能,以及较高的性价比,适合在表面防护涂层领域广泛应用。本文通过正交试验研究了煤油流量、氧气流量、送粉速率、喷涂距离对超音速火焰喷涂制备的Fe基非晶涂层的孔隙率、硬度、耐磨性能的影响。采用图像法、显微硬度计和摩擦磨损试验机分别对Fe基非晶涂层的孔隙率、硬度、耐磨性能进行了表征。采用X射线衍射仪和扫描电镜分别对涂层的相组成和显微结构进行了表征。通过极差分析法分析得出以涂层孔隙率最低为目标的优化制备工艺,最佳喷涂工艺参数为:煤油流量0.41 L/min,氧气流量830 L/min,喷涂距离430 mm,送粉速率40 g/min。结果表明:送粉速率和氧气流量对涂层孔隙率影响较大,进而影响涂层的硬度及耐磨性能。孔隙率随着氧气流量和送粉速率的增加而增加,随着煤油流量和喷涂距离的增加而降低。制备的Fe基非晶涂层硬度达到1158.9HV0.2,孔隙率为1.22%,磨损实验的质量损失量只有316L不锈钢的一半。     

Ceramic coatings for a corrosion-resistant nuclear waste container evaluated in simulated ground water at 90 °C

Ceramic materials provide an innovative opportunity for corrosion-resistant coatings for nuclear waste containers. Their suitability can be derived from the fully oxidized state for selected metal oxides. Ceramic coatings applied to plain carbon steel substrates by several thermal spray techniques have been exposed to 90 °C simulated ground water (at 10 times typical concentration) for nearly 6 years. Thermal spray processes examined in this work included plasma spray, high-velocity oxy fuel (HVOF), and detonation gun. Some thermal spray coatings have demonstrated superior corrosion protection for the plain carbon steel substrate. In particular, the HVOF and detonation gun thermal spray processes produced coatings with low connected porosity, which limited the growth rate of corrosion products. It was also demonstrated that these coatings resisted spallation of the coating even when an intentional flaw (which allowed for corrosion of the carbon steel substrate underneath the ceramic coating) was placed in the coating. An approach for a theoretical basis for prediction of the corrosion protection provided by ceramic coatings is also presented. The theoretical development includes the effect of the morphology and amount of the porosity within the thermal spray coating and provides a prediction of the exposure time needed to produce a crack in the ceramic coating. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees.     

超音速火焰喷涂 Fe 基非晶耐磨涂层的组织与性能

采用超音速火焰喷涂 (HVOF) 在 304 不锈钢基体上制备了 FeSiBP 非晶涂层, 利用扫描电子显微镜、 X 射 线衍射仪、 维氏显微硬度计、 摩擦磨损试验机、 三维光学轮廓仪等设备对涂层的组织结构、 摩擦性能和磨损机制 进行了深入分析。 结果表明： 涂层大部分为非晶态, 非晶含量随着热量输入的增大呈先增加后减少的趋势。 当喷 涂参数为煤油流量 5.5 GPH, 氧气流量 2000 SCFH, 送粉器转速 4 RPM, 喷涂距离 360 mm 时, 涂层非晶含量可 达 83.8%, 硬度 857 HV0.2, 耐磨性是 304 不锈钢基体的 2.52 倍。     

Comparative High-Temperature Corrosion Behavior of Ni-20Cr Coatings on T22 Boiler Steel Produced by HVOF, D-Gun, and Cold Spraying

To protect materials from surface degradations such as wear, corrosion, and thermal flux, a wide variety of materials can be deposited on the materials by several spraying processes. This paper examines and compares the microstructure and high-temperature corrosion of Ni-20Cr coatings deposited on T22 boiler steel by high velocity oxy-fuel (HVOF), detonation gun spray, and cold spraying techniques. The coatings’ microstructural features were characterized by means of XRD and FE-SEM/EDS analyses. Based upon the results of mass gain, XRD, and FE-SEM/EDS analyses it may be concluded that the Ni-20Cr coating sprayed by all the three techniques was effective in reducing the corrosion rate of the steel. Among the three coatings, D-gun spray coating proved to be better than HVOF-spray and cold-spray coatings.     

High-Temperature Behavior of a High-Velocity Oxy-Fuel Sprayed Cr3C2-NiCr Coating

High-velocity oxy-fuel (HVOF) sprayed coatings have the potential to enhance the high-temperature oxidation, corrosion, and erosion-corrosion resistance of boiler steels. In the current work, 75?pct chromium carbide-25?pct (nickel-20?pct chromium) [Cr₃C₂-NiCr] coating was deposited on ASTM SA213-T22 boiler steel using the HVOF thermal spray process. High-temperature oxidation, hot corrosion, and erosion-corrosion behavior of the coated and bare steel was evaluated in the air, molten salt [Na₂SO₄-82?pct Fe₂(SO₄)₃], and actual boiler environments under cyclic conditions. Weight-change measurements were taken at the end of each cycle. Efforts were made to formulate the kinetics of the oxidation, corrosion, and erosion-corrosion. X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM)/energy dispersive spectroscopy (EDS) techniques were used to analyze the oxidation products. The coating was found to be intact and spallation free in all the environments of the study in general, whereas the bare steel suffered extensive spallation and a relatively higher rate of degradation. The coating was found to be useful to enhance the high-temperature resistance of the steel in all the three environments in this study.     

Evaluation of Microstructural Features of HVOF Sprayed Ni–20Al Coatings

The present study evaluates the microstructural features of high velocity oxy-fuel (HVOF) sprayed Ni–20Al coatings with liquefied petroleum gas (LPG) as fuel. The Ni–20Al coatings are commonly used for bond coat applications and as high temperature oxidation resistant coating. The quality of the deposited coating has been evaluated in terms of porosity, oxide content, surface roughness, micro-hardness, composition and morphology (surface and cross-sectional). The results indicate that, it is possible to develop dense Ni–20Al coatings with low oxide and porosity content along with high surface roughness using HVOF technology. Importantly, HVOF sprayed Ni–20Al coatings show better quality as compared to air plasma spray and can be used as an alternative to VPS in terms of quality to cost ratio for bond coat applications.     

超音速火焰喷涂NiCrAlY-Y2O3金属陶瓷涂层抗结瘤性能研究

以NiCrAlY和Y2O3粉末为原料通过两种工艺分别制备出团聚烧结和混合型NiCrAlY-Y2O3金属陶瓷粉末,研究了该两种用于热喷涂给料粉末的颗粒形貌及粉末性能.使用该两种粉末及一种商用CoCrAlY-Y2O3通过超音速火焰喷涂(HVOF)在不锈钢基体上制备厚度约为100um的涂层。研究了涂层的孔隙率及抗热冲击能,将四种热喷涂涂层在高温下与MnO,Fe3O4及含锰碳钢进行接触反应后对它们的抗结瘤性能进行了相对的静态比较,结果表明,团聚烧结NiCrAlY-Y2O3涂层具有较好的抗锰氧化物的结瘤,而抗铁氧化物结瘤性能差。团聚烧结金属陶瓷涂层比混合型陶瓷涂层具有更好的抗氧化物结瘤性能。     

热喷涂Fe基非晶合金涂层的微观结构与摩擦磨损行为

分别采用超音速火焰喷涂工艺和爆炸喷涂工艺,在Q235不锈钢基体上制备Fe基非晶合金涂层,对比研究这2种非晶合金涂层在室温下的干摩擦磨损特性,并探讨摩擦磨损机理.结果表明,与超音速火焰喷涂工艺制备的Fe基非晶合金涂层相比,采用爆炸喷涂工艺制备的涂层更致密,孔隙率为2.1％,显微硬度更高,平均硬度高达1 095.6 HV,且耐磨性更好;并且涂层摩擦因数增至稳定值的时间较短,具有更稳定的摩擦磨损行为.超音速火焰喷涂涂层的磨损形式主要以疲劳磨损为主,而爆炸喷涂涂层的磨损形式为粘着磨损和磨粒磨损的综合作用,并以粘着磨损为主.     

Copper coatings for minimization of retention and permeation of implanted tritium in aluminum alloy 6061

Copper coatings deposited on Al-6061 substrates by radio frequency magnetron sputtering, to prevent the retention and permeation of energetically implanted tritium in Al-6061, were evaluated by a variety of characterization techniques. The coatings, weighing in the 0.03 to 0.088 kg/m² range, were smooth and had a fine grain structure. They contained the intermetallic phases Cu₉Al₄ and CuAl₂ as well as copper. The fractions of Al and Cu in any coating increased and decreased, respectively, with increasing depth below the surface. Furthermore, the fractions of Al and Cu on the coating surface decreased and increased, respectively, with increasing coating weight. There was no texture or preferred orientation in the Cu phase of the coatings. A significant amount of oxygen was also detected at the original substrate surface. Residual stress measurements revealed that, in both Cu and CuAl₂, the stresses in the coating plane were compressive, while the stress normal to the coating plane was zero in Cu but tensile in CuAl₂. The shear-stress components were, however, negligible in both the Cu and CuAl₂ phases. In the coating plane, the residual stress in Cu was always much smaller than that in the CuAl₂ phase. Bond-strength measurements using tensile-pull testing provided a lower limit of the bond strength of about 2 MPa.     

硬质合金金刚石涂层的残余应力分析

采用有限元方法分析硬质合金金刚石涂层的热残余应力,采用轴对称二维几何模型和自由边界条件,考虑了涂层厚度及不同基体对应力场的影响。分析表明,在多晶金刚石涂层中存在压应力,在基体中出现了拉应力,这种应力结构分布对材料性能的影响是有利的。在明锐界面出现了大的剪切应力(415MPa),应力奇异场出现在明锐界面靠近自由边界处。涂层压应力随基体中钴含量的增加而增加,随涂层厚度的增加而减小。因此可以借助这两个因素来适当控制和调节材料中的应力场以得到性能优良的产品。     

制备工艺方法对 NiCrFeMo 涂层组织和性能影响研究

采用等离子喷涂、 超音速火焰喷涂和冷喷涂工艺制备了 NiCrFeMo 涂层, 并对涂层的金相性能、 不同基体 材料下涂层的结合性能、 不同温度下涂层的耐磨性能进行了检测。 实验结果表明： 采用冷喷涂工艺制备的涂层孔 隙率最低, 对应涂层的结合强度最高, 但冷喷涂工艺制备的涂层对基体材料较为敏感, 而超音速火焰喷涂和等离 子喷涂受基体材料影响不大。 在室温下, 冷喷涂涂层显示出较优的耐磨性能, 在高温下超音速火焰喷涂涂层显示 出较优的耐磨性能。     

Microstructural analysis and oxidation behavior of laser-processed Fe-Cr-AI-Y alloy coatings

Dense crack-free coatings of Fe-Cr-Al-Y quaternary alloy were produced on stainless steel 316L substrates using a continuous wave Nd-YAG solid-state laser coupled with a fiber optic beam delivery system. Experiments were performed at a laser power between 0.6 and 2.4 kW, process speed in the range 0.053 to 0.423 cm/s, powder feed rate fixed at 0.083 g/s, and focused multimode laser beam with a diameter of 0.5 cm. Various microanalysis techniques demonstrated that the coatings were metallurgically bonded to the substrate and possessed thicknesses between 0.35 and 4.64 mm, refined columnar microstructures with grain sizes of 15 to 150 μxm, increased concentration of key alloying elements, and appreciably high microhardness up to 409 kg/mm². The laser-processed microstruc-tures comprised a body-centered cubic (bcc) ferrite(α phase) crystal structure with a relatively large lattice parameter compared to α-Fe due to the enhanced dissolution of varying amounts of Cr, Al, Ni, and Y, depending on the dilution from the substrate material. Oxidation tests conducted in air at temperatures of 1100 ° to 1200 ° for 95 hours revealed the formation of an approximately 5-μm-thick dense α-Al₂O₃ oxide scale of a rhombohedral (hexagonal) crystal structure. The α-Al₂O₃ scale exhibited remarkable high-temperature resistance and strong adherence to the coating surface. Extensive oxidation of the uncoated stainless steel substrate produced a porous and heavily spalled alloy oxide scale about 60-μm thick consisting of FeCr₂O₄ and NiCr₂O₄ with cubic and tetragonal crystal structures, respectively. The retention of the bcc α phase and the insignificant grain growth after oxidation are indicative of the thermal stability of the laser-processed coating microstructures. The obtained results demonstrate that Fe-Cr-Al-Y alloy coatings exhibiting fine-grained hard mi-crostructures, high-temperature oxidation resistance, and strong adherence to stainless steel can be developed by means of laser processing.     

基于正交试验的热障涂层性能变化规律研究

热障涂层广泛应用于燃气轮机、飞机发动机等装备上,其性能直接影响装备的使用寿命。本文采用大气等离子喷涂工艺,在GH4169高温合金表面依次制备NiCoCrAlY金属粘结层和ZrO_2-8%wtY_2O_3陶瓷层。采用扫描电镜和残余应力测试仪对样品的表面形貌、孔隙率以及表面的残余应力进行测试。通过正交试验研究喷涂工艺参数对热障陶瓷层厚度、孔隙率以及残余应力的影响,实验结果表明,对陶瓷层厚度的影响因素从大到小顺序依次为氢气流量,电流,氩气流量,喷涂距离;在相同的制备时间内,陶瓷层的厚度随着电流与氢气流量的增大而明显的增大;随着氩气流量与喷涂距离的增大,陶瓷层的厚度随之减小;对陶瓷层孔隙率的影响因素从大到小顺序依次为喷涂距离,氢气流量,电流,氩气流量。当氩气流量从30 slpm/min增加到40 slpm/min时,氢气流量从5slpm/min增加到10 slpm/min时,孔隙率随之增大;当氩气流量从40 slpm/min增加到50 slpm/min时,氢气流量从10 slpm/min增加到15 slpm/min时,孔隙率随之减小。当喷涂距离从5 mm增加到10 mm时,孔隙率随之减小,而当喷涂距离从10 mm增加到15 mm时,孔隙率急剧升高,而随着电流的增大,孔隙率减小。电流、氢气流量以及氩气流量对涂层残余应力造成很大影响,随着电流以及氢气流量的增大,氩气流量的减小,涂层表面的残余应力急剧上升。     

真空冷喷涂LiNi_(0.33)Co_(0.33)Mn_(0.33)O_2涂层颗粒沉积行为研究

真空冷喷涂是一种基于室温及真空条件下超细陶瓷粉末粒子的撞击破碎实现涂层沉积的方法。目前,真空冷喷涂技术已经在微电子器件,金属防护以及新能源领域展现了良好的应用前景。本研究将目光转向锂离子电池,基于真空冷喷涂技术,在氧化铝基体上制备了锂离子电池LiNi_(0.33)Co_(0.33)Mn_(0.33)O_2(NMC)三元材料正极涂层,使用扫面电子显微镜(SEM)观察了NMC涂层的表面及截面微观形貌,使用X射线衍射(XRD)对涂层的相结构进行了测试,使用3D激光显微镜表征了涂层的表面粗糙度,系统研究了载气流量、喷涂距离、喷涂次数等沉积条件对NMC涂层微观形貌及粒子沉积行为的影响。结果表明,在真空冷喷涂NMC涂层中可以观察到明显颗粒破碎沉积现象,涂层结构致密。NMC粉末颗粒沉积方式受气流量、喷涂距离、喷涂次数等沉积条件的影响,载气流量的提高会提高粒子撞击速度,从而提高涂层沉积速率,但过高的气流量会导致粒子发生冲蚀,在涂层表面留下凹坑,致使涂层粗糙度增大。喷涂距离过大会导致NMC颗粒撞击速度减小,粒子破碎不充分,涂层呈现出类似团聚粉末堆积的疏松结构。喷涂次数影响涂层厚度,在合适的沉积参数条件下,可以通过调整喷涂次数实现涂层厚度的线性调控。     

The use of titanium aluminides to form electric-spark coatings

Titanium aluminides (TiAl₃, TiAl, Ti₃Al) fabricated by powder metallurgy were used as alloying electrodes for the formation of electric-spark coatings. Intermetallic coatings were deposited on steel substrates in argon or nitrogen. The microstructure and composition of fabricated coatings were investigated by scanning electron microscopy, X-ray structural analysis, and electron probe microanalysis. It is established that initial Ti–Al intermetallic phases are present in fabricated coatings; however, the ratio between Ti and Al concentrations is shifted to aluminum compared with the stoichiometric one. When depositing titanium aluminide in the nitrogen medium, titanium nitride is additionally formed in surface layers. Thermal and tribotechnical tests showed that the Ti₃Al coating deposited in nitrogen possesses high wear resistance and heat resistance.     

Wear-resistant coatings produced by shock-wave compaction of powders

Wear-resistant metal-matrix composite coatings with a thickness of 1.5 mm were fabricated on low-alloy steel substrates by explosively generated shock waves. Starting materials were equivolume mixtures of WC or Cr₃C₂ powder mixed with either titanium or cobalt powder as a binder phase. Three different planar geometries were used, with powder layer thicknesses varying from 1 to 3 mm. Microstructural examination showed that fully dense, crack-free coatings could be produced with a uniform distribution of the carbides within the metallic binder phase. Shear strengths in excess of 40 MPa were measured for coatings composed of equivolume powder mixtures of (Cr₃C₂ + Ti) and (WC + Ti). The weight loss of a coating produced from an equivolume (WC + Co) powder mixture measured from a two-body abrasive wear test was significantly lower than that measured for a wear-resistant tool steel used as a reference material.