Coatings by arc spraying of nanocrystalline metallic wires on carbon steel surfaces: surface hardness,hydrophobicity, and residual stress

Wire arc spraying of two different metallic wires composed of nanocrystalline and amorphous phases onto carbon steel sheets is carried out. Coating characteristics, including morphological and metallurgical changes, microhardness, residual stress, and surface hydrophobicity are examined using analytical tools. It is found that microhardness increases in the surface region of the coating because of oxidation and high cooling rates. Nanoparticles in the coating act like grain arresting centres contributing to higher residual stress levels. The surface texture of the coating, composed of micro/nanopores, enhances surface hydrophobicity. The coating produced by the 140MXC wire has higher surface hardness than that of the 95MXC wire, resulting in a correspondingly lower friction in the 140MXC wire. However, the residual stress formed in the coating is higher for 140MXC wire than that corresponding to 95MXC wire. In addition, both coatings produced by two different wire sprayings have similar hydrophobic characteristics at the surface.     

Residual stresses in high-velocity oxy-fuel metallic coatings

X-ray based residual stress measurements were made on type 316 stainless steel and Fe₃Al coatings that were high-velocity oxy-fuel (HVOF) sprayed onto low-carbon and stainless steel substrates. Nominal coating thicknesses varied from 250 to 1500 μm. The effect of HVOF spray particle velocity on residual stress and deposition efficiency was assessed by preparing coatings at three different torch chamber pressures. The effect of substrate thickness on residual stress was determined by spraying coatings onto thick (6.4 mm) and thin (1.4 mm) substrates. Residual stresses were compressive for both coating materials and increased in magnitude with spray velocity. For coatings applied to thick substrates, near-surface residual stresses were essentially constant with increasing coating thickness. Differences in thermal expansion coefficient between low-carbon and stainless steels led to a 180 MPa difference in residual stress for Fe₃Al coatings. Deposition efficiency for both materials is maximized at an intermediate (∼600 m/s) velocity. Considerations for X-ray measurement of residual stresses in HVOF coatings are also presented.     

Elemental composition,topography, and wettability of the surface of graphite modified by ion-assisted deposition of chromium coatings

Elemental composition, roughness, and wettability of a graphite surface modified by ion-assisted deposition of a Cr coating ～300–1000 nm thick are investigated using the methods of Rutherford backscattering, atomic force microscopy, and contact angle measurements. It is established that, in addition to chromium, coatings involve hydrogen, carbon, and oxygen from residual gases of a vacuum chamber because of their incomplete pumping by vacuum pumps and 1–2 at % silicon diffusing from the graphite. It is determined that, as the coating thickness increases, the surface roughness of the modified graphite increases and becomes the predominant factor in increasing its hydrophobicity.     

Effect of Thin Polymer Coatings on the Mechanical Properties of Steel Plates

The influence of powder coatings based on an epoxide polyester mixture on the mechanical properties of the plates made of rolled sheet steel is studied. The influence of 100-μm-thick coatings on the mechanical properties of specimens 0.7–1.5 mm thick is considered. The bending and tensile tests performed at room and elevated temperatures show that the thin coatings weakly affect the mechanical properties of the plates. In particular, the stiffness of the coated plates during tension and bending remains almost the same despite the fact that the elastic modulus of the coated plates is always slightly lower than Young’s modulus of steel because of an increased specimen thickness determined with allowance for the presence of a coating. The influence of the coatings is substantial in compressive stability tests. Under supercritical deformation conditions, the bearing capacity of the coated plates decreases substantially, and their critical stability load on the loss of stability falls by 20–30% (depending on the substrate thickness). This effect can be explained by the influence of the residual thermal stresses that appear in the specimens during coating deposition.     

Influence of Surface Roughness of Galvanized Steel Sheet on Self-lubricated Coating

Three different thicknesses of self-lubricated coatings were prepared on galvanized steel sheets with different surface roughness(Ra).Performances of the coatings were evaluated by various laboratory tests.Scanning electron microscopy(SEM),atomic force microscope(AFM),neutral salt spraying test(NSST),reciprocating friction and wear test were taken in order to characterize the coatings.Results show that the self-lubricated coating has good corrosion resistance and lubricating property.In 0.857-1.629μm of Ravalue,the relationship between friction coefficients of the self-lubricated steel sheets and the surface roughness of the galvanized steel sheets likes a parabolic curve,and has a peak value of friction coefficient in certain range of Ra.Affected by the hereditary surface topography of the galvanized steel sheet,dents on the surface of self-lubricated steel become deeper and larger with the increase of the surface roughness of the galvanized steel sheets.The influence of the surface roughness of the galvanized steel sheet on the corrosion resistance of the self-lubricated coating steel is similar to that on friction coefficient of self-lubricated coatings,like a parabolic curve.     

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。     

Assessment of Heat Transfer During Solidification of Al–22% Si Alloy by Inverse Analysis and Surface Roughness Based Predictive Model

Heat flux transients were estimated during unidirectional downward solidification of Al?C22% Si alloy against copper, die steel and stainless steel chills. The chill instrumented with thermocouples was brought into contact with the liquid metal so as to avoid the effect of convection associated with the pouring of liquid metal. Heat flux transients were estimated by solving the inverse heat conduction problem. Higher thermal conductivity of chill material resulted in increased peak heat flux at the metal/chill interface. Peak heat flux decreased when 100???m thick alumina coating was applied on the chill surface. The lower thermal conductivity of alumina based coating and the presence of additional thermal resistance decreases the interfacial heat transfer. For uncoated chills, the ratio of the surface roughness (R_a) of the casting to chill decreased from 6.5 to 0.5 with decrease in the thermal conductivity of the chill material. However when coating was applied on the chill, the surface roughness ratio was nearly constant at about 0.2 for all chill materials. The measured roughness data was used in a sum surface roughness model to estimate the heat transfer coefficient. The results of the model are in reasonable agreement with experimentally determined heat-transfer coefficients for coated chills.     

Effect of a thin prior-metallic coating on the morphology of hot-dip Zn coating

Copper has been investigated as a potential alloying element in the molten Zn bath and was found to have significant effects in phase formation kinetics as well as corrosion performance. In the present work, the effect of a prior-Cu coating process on hot-dip Zn coating morphology and thickness has been investigated. A thin layer of Cu was coated on steel by a displacement reaction having a thickness of 70–90?nm. The Cu and Zn coatings were characterised using glow discharge optical emission spectroscope, scanning electron microscope and confocal laser scanning microscope. The prior-Cu coating acted as a barrier to Fe dissolution and altered the substrate surface roughness as well as the interfacial energies resulting in modification of nucleation and growth of Fe–Zn intermetallic phases.     

Electrophoretically applying chromium carbide hard alloy to various steel grades

The microstructure and hardness of electrophoretic chromium carbide coatings on certain structural and tool steel grades have been examined under the initial surface roughness. X-ray spectral studies on the coating and a transition zone have been carried out. It can be shown that the roughness and hardness of chromium carbide coatings are only slightly dependent on the initial steel substrate roughness. It has also been shown that the coating hardness is essentially dependent on the steel chemical composition, and is governed by the solubility and diffusion interchange differences between the coating and the substrate during liquid phase sintering of the coating.Translated from Poroshkovaya Metallurgiya, No. 12 (360), pp. 25–28, December, 1992.     

Structural changes in the surface layer of plasma coatings with ultrasonic treatment by steel balls

Conclusions Ultrasonic treatment with metal balls provides a reduction in the surface roughness of plasma coatings with a mcirohardness of 5630–7890 MPa by a factor of one and a half, and with a thickness of deposited layer less than 4·10^–4 m the time required in order to achieve the minimum roughness depends considerably on the magnitude of residual compressive stresses and the properties of the base.The shock effect of balls changes the size of mosaic blocks, dislocation density, and the magnitude of microstresses in the surface layer of a coating, and the treatment time required in order to achieve the maximum changes is reduced with a smaller deposited layer thickness.Ultrasonic treatment of plasma coatings provides formation at their surface layer of structures which improve the wear resistance of deposited coatings under fretting conditions by a factor of two to four.Translated from Poroshkovaya Metallurgiya, No. 2(350), pp. 23–28, February, 1992.     

Numerical Investigation of Residual Stress in Thick Titanium Alloy Plate Joined with Electron Beam Welding

A finite-element (FE) simulation process integrating three dimensional (3D) with two-dimensional (2D) models is introduced to investigate the residual stress of a thick plate with 50-mm thickness welded by an electron beam. A combined heat source is developed by superimposing a conical volume heat source and a uniform surface heat source to simulate the temperature field of the 2D model with a fine mesh, and then the optimal heat source parameters are employed by the elongated heat source for the 3D simulation without trial simulations. The welding residual stress also is investigated with emphasis on the through-thickness stress for the thick plate. Results show that the agreement between simulation and experiment is good with a reasonable degree of accuracy in respect to the residual stress on the top surface and the weld profile. The through-thickness residual stress of the thick plate induced by electron beam welding is distinctly different from that of the arc welding presented in the references.     

Effect of Electron-Beam Treatment on Wear-Resistant Coatings Applied by Electroexplosive Sputtering

TiC–Mo, TiC–Ni, TiB₂–Mo, and TiB₂–Ni coatings applied to the surface of Hardox 450 steel by electroexplosive sputtering are subjected to electron-beam treatment, After electroexplosive application, the surface relief of the coatings includes features such as deformed solidifying microglobules, buildup, microcraters, microcracks, and peeling. After electron-beam treatment, the microglobules, buildup, microcraters, and microcracks disappear from the coating surface. A polycrystalline structure containing cellular elements is formed. After electron-beam treatment, the surface roughness is 1.1–1.2 μm. The thickness of the layers modified by the electron beam in the electroexplosive coatings depends linearly on the surface energy density. The greatest coating thickness is observed when using the TiB₂–Mo system; the coating thickness is least for the TiC–Ni system. That may be attributed to the thermophysical properties of the coatings. The following substructures are observed in the coatings: cellular, striated, fragmented, and subgranular. Grains with chaotically distributed dislocations and reticular dislocations are also observed. Electron-beam treatment leads to the formation of composite filled structure over the whole cross section of the remelted layer. The structure formed in this layer is more disperse and uniform than in coatings formed without electron-beam treatment. The inclusions of titanium carbide or titanium diboride in the molybdenum or nickel matrix are 2–4 times smaller than immediately after electroexplosive sputtering. Within the molybdenum or nickel grains and at their boundaries, rounded particles of secondary phase (titanium carbide or titanium diboride) are observed. They may be divided into two classes by size: particles of the initial powder (80–150 nm) that have not dissolved on irradiation; and particles formed on solidification of the melt (10–15 nm). In the electroexplosive powder coatings, the structure is mainly formed by dynamic rotation of the sprayed particles, which form a vertical structure both in the coating and in the upper layers of the substrate. The coatings have excellent operational properties: nano- and microhardness, elastic modulus of the first kind, and wear resistance in dry slipping friction.     

32Cr3MoVE轴承钢旋弯疲劳特性及裂纹萌生扩展行为

摘要：采用光滑漏斗状试样对32Cr3MoVE轴承钢进行旋转弯曲疲劳测试，研究了32Cr3MoVE轴承钢旋转弯曲疲劳性能及裂纹萌生扩展行为。采用升降法测得其疲劳极限为860MPa，疲劳断口SEM观察并统计破断试样结果表明：疲劳破坏68.7%是由于非金属夹杂起裂，18.8%由表面加工缺陷起裂，125%为表面粗糙度起裂。当加载应力低于980MPa时，疲劳断裂主要是由于内部非金属夹杂引起的，高于980MPa时，疲劳断裂主要是由于表面粗糙度引起的。表面加工缺陷和表面粗糙度引起的最大应力强度因子分别为3.05和2.97MPa·m1/2，容易引发疲劳裂纹。非金属夹杂物尺寸在5.30～5.90μm范围内，局部应力从859.35MPa升至977.75MPa时，疲劳寿命从1.96×105降低到1.58×105；非金属夹杂物局部应力在840～900MPa范围内，夹杂物尺寸从2.28μm升至5.83μm时，疲劳寿命从1.10×106降低到1.96×105。     

Erosion-resistant coatings for gas turbine compressor blades

The effect of ion-plasma coatings made from high-hardness metal compounds on the erosion and corrosion resistance and the mechanical properties of alloy (substrate) + coating compositions is comprehensively studied. The effects of the thickness, composition, deposition conditions, and design of coatings based on metal nitrides and carbides on the relative gas-abrasive wear of alloy + coating compositions in a gas-abrasive flux are analyzed. The flux contains quartz sand with an average fraction of 300–350 μm; the abrasive feed rate is 200 g/min; and the angles of flux incidence are 20° (tangential flow) and 70° (near-head-on attack flow). Alloy + coating compositions based on VN, VC, Cr₃C₂, ZrN, and TiN coatings 15–30 μ m thick or more are shown to have high erosion resistance. A detailed examination of the coatings with high erosion resistance demonstrates that a zirconium nitride coating is most appropriate for protecting gas turbine compressor blades made of titanium alloys; this coating does not decrease the fatigue strength of these alloys. A chromium carbide coating is the best coating for protecting compressor steel blades.     

High-Cycle Fatigue Behavior of Microarc Oxidation Coatings Deposited on a 6061-T6 Al Alloy

The objective of this article is to evaluate the influence of microarc oxidation (MAO) coatings on the high-cycle rotating bending fatigue behavior of the 6061-T6 aluminum alloy. Toward this purpose, the influence of the MAO coating process parameter (current density) and coating thickness on the fatigue life of the 6061-T6 Al alloy has been evaluated in the present study. In addition, the influence of the coating roughness on the fatigue life of the MAO-coated 6061-T6 Al-alloy sample has also been investigated. The results indicate that the high-cycle fatigue life of the 6061-T6 Al alloy is substantially degraded due to the presence of MAO coatings, especially at lower alternating stress values and for thicker coatings. Surface roughness, altered by polishing, does not have any effect on fatigue life. An examination of coated samples interrupted at various fractions of fatigue life leads to the conclusion that the crack propagates from the coating surface to the coating-substrate interface very rapidly and thus fatigue life is largely controlled by the propagation of the crack into the substrate.     

钨铜合金表面粗糙度对抗电弧烧蚀性能的影响

将纯W、纯Cu和W70Cu30合金分别进行机械磨光、电解抛光和机械抛光后获得3种不同的表面状态,在专有设备上模拟电触头材料的电弧烧蚀过程,通过扫描电子显微镜观察首击穿烧蚀形貌。结果表明:通过机械抛光获得的表面粗糙度最小,对于W70Cu30合金可达到0.044μm,电解抛光次之,机械磨光最大;随表面粗糙度降低,W70Cu30合金的击穿场强逐渐增大,烧蚀区域趋于规整化,烧蚀产物增加,烧蚀坑的分布更加集中,纯W、纯Cu也表现出相同的现象;在本实验条件下材料表面粗糙度在0.2~0.3μm时,其抗电弧烧蚀性能最好。     

Properties of nanolayer erosion-resistant coatings based on metal carbides and nitrides

Vacuum-arc ion-assisted deposition is used to form nanolayer protective 2D coatings based on the nitrides or carbides of titanium and chromium, vanadium carbide, and aluminum nitride with a layer thickness of 5–80 nm and a total thickness up to 25 μm. The phase composition of the coatings is studied after deposition and tests. Titanium alloy VT1-0 (EP866 steel)-nanolayer coating compositions are subjected to hot-strength and rapid cyclic corrosion tests, and the erosion resistance of the 2D nanolayer coatings in a dust-air flux (the average fraction of quartz sand is 300–350 μm) is studied. Among the 2D nanolayer coatings on titanium and steel substrates, a composition of VT1-0 alloy with a TiN/CrN coating at a nanolayer thickness of 60–70 nm and a total thickness of 19 μm has the maximum erosion resistance. The erosion resistance of the TiN/CrN coating is shown to decrease with decreasing nanolayer thickness, and it has a high thermal stability after holding at 700°C for 100 h.     

Effect of Surface Mechanical Attrition Treatment on Fatigue Lives of Alloy 718

The effect of surface mechanical attrition treatment (SMAT) and its duration on fatigue lives of alloy 718 has been studied. The SMAT process was carried out in vacuum (?0.1?MPa) with SAE 52100 steel balls of 5?mm diameter for 30 and 60?min at a vibrating frequency of 50?Hz. SMAT resulted in surface nanocrystallization, higher surface roughness, higher surface hardness, higher compressive residual stress, higher tensile strength, reduced ductility and superior fatigue lives. The enhancement in the fatigue lives of treated samples can be attributed to the positive influence of nanostructured surface layer, compressive residual stress and work hardened layer, which surpassed the negative effect of increased surface roughness. There was no significant difference between the fatigue lives of samples treated for 30 and 60?min.     

织构对纳米晶纯镍镀层耐蚀性能的影响

采用脉冲电镀法在黄铜表面制备出具有(111)、(200)和(220)晶面择优生长织构的纳米晶纯镍镀层.采用扫描电镜对镀层的显微形貌进行观察,采用X射线衍射对不同晶面织构的择优性进行表征,并对镀层在3.5%NaCl溶液中的动电位极化曲线和交流阻抗谱进行了测试,研究不同织构镀层的耐蚀性能.不同织构镀层的耐蚀性能存在显著差异:具有(220)强织构的镀层耐蚀性最差,其自腐蚀电流密度最大,为1.23μA·cm-2,镀层的电荷转移电阻为2.09kΩ·cm2;具有(200)强织构的镀层耐蚀性能最佳,镀层的电荷转移电阻为27.32kΩ·cm2,自腐蚀电流密度为0.15μA·cm-2;具有(111)织构镀层的耐蚀性居中.认为织构引起的表面胞状物的差别是造成纯镍镀层耐蚀性能不同的原因.      

铝镁及稀土铝镁电弧喷涂涂层的耐腐蚀性能研究

采用电弧喷涂技术在Q235钢表面制备了Al-Mg和Re-Al-Mg涂层,分别研究了这两种涂层的耐腐蚀性能,利用中性盐雾试验及电化学测试等试验方法研究了两种涂层的防腐蚀性能并通过扫描电子显微镜分析了Al-Mg和Re-Al-Mg涂层腐蚀前后的微观组织形貌,并讨论了涂层的结构及耐蚀机理,结果表明两种涂层在极化过程中都出现了明显的钝化现象,两种涂层在腐蚀过程中表面都会生成一层氧化膜,起到了物理屏蔽作用延缓了涂层的腐蚀速率,但稀土铝镁合金涂层的防腐蚀性能明显优于铝镁合金涂层,这是由于稀土元素的加入,降低了涂层的孔隙率,使涂层具有了良好的耐腐蚀性能。稀土铝镁合金涂层在中性盐雾试验中表现出的腐蚀速率是最慢的,电化学试验过程中其表现出钝化区域最大,具有良好的腐蚀与防护性能。