Relation between microstructure and adhesion of hot dip galvanized zinc coatings on dual phase steel

The microstructure of hot dip galvanized zinc coatings on dual phase steel was investigated by electron microscopy and the coating adhesion characterized by tensile testing. The zinc coating consists of a zinc layer and columnar ζ-FeZn₁₃ particles on top of a thin inhibition layer adjacent to the steel substrate. The inhibition layer is a thin compact and continuous layer that consists of η-Fe₂Al_5–xZn_x fine and coarse particles. The coarse faceted particles are on top and fine faceted particles are at the bottom. The steel surface is covered with small fraction manganese oxides, which may impair adhesion of the zinc coating. The adhesion at various interfaces that exist in zinc-coated steel was quantitatively estimated using a so-called “macroscopic atom” model. In addition, the adhesion at the interfaces in zinc-coated steel was qualitatively assessed by examining the fracture and delamination behavior upon tensile testing. In accordance with this model, fracture along zinc grain boundaries preceded fracture along the zinc layer/inhibition layer and ζ-FeZn₁₃ particle/inhibition layer interfaces.     

Interface fracture behavior of electroplated coating on metal substrate under compressive strain

The inducement of interface fracture is crucial to the analysis of interfacial adhesion between coating and substrate. For electroplated coating/metal substrate adhering materials with strong adhesion, interface cracking and coating spalling are difficult to be induced by conventional methods. In this paper an improved bending test named as T-bend test was conducted on a model coating system, i.e. electroplated chromium on a steel substrate. After the test, cross-sections of the coated materials were prepared to compare the failure behaviors under tensile strain and compressive strain induced by T-bend test. And the observation results show that coating cracking, interface cracking and partial spalling appear step by step. Based on experimental results, a new method may be proposed to rank the coated materials with strong interfacial adhesion.     

Study of the Antibacterial Behavior of Wire Arc Sprayed Copper Coatings

Antibacterial surfaces such as silver and copper coated areas reduce risk of bacterial growth considerably. In this study, wire arc spraying has been utilized to produce an antibacterial copper coating with ultrafine microstructure on stainless steel substrate. The chemical composition, microstructure, and surface morphology of copper coatings were characterized with x-ray diffraction and scanning electron microscope. Determination of thickness and adhesion of the coating were investigated. The antibacterial property of copper coatings was analyzed by both gram negative Escherichia coli NCTC 10418 and gram positive Staphylococcus aureus NCTC 11047. The antibacterial performance of coatings was compared to that of stainless steel 316 and a micrograin structure of the commercially available copper. Results indicated that the as-sprayed copper coatings have excellent antibacterial behavior compared to stainless steel and micrograin copper, which can be attributed to the presence of the ultrafine grain size, micropores, and crystallographic defects in the microstructure.     

Composite zinc-fly ash coating on mild steel

Zinc-fly ash composite coatings were deposited on mild steel substrates with the help of electrodeposition technique. Metallographic and chemical characterization of the produced composite coatings was performed with the aid of Scanning Electron Microscopy (SEM) and Electron Dispersive X-ray Analysis (EDX). The corrosion behaviour of the composite coated mild steel substrate was studied in a 0.3 M NaCl solution (pH = 5.5) by means of anodic polarization curves. The wear of the zinc-fly ash coating was also investigated by using a pin-on-disk apparatus. The composite coating exhibited increased wear resistance, compared to the pure zinc coating and the mild steel substrate. The adhesion strength between the zinc-fly ash coating and the mild steel substrate was examined with a scratch testing apparatus. The adhesion strength between zinc-fly ash composite coating and the mild steel substrate was found to be higher than that of the pure zinc coating to mild steel.     

利用随机方法研究纳米化对纯锌点蚀行为的影响

选择柠檬酸型镀液,用脉冲电镀法成功制备纳米锌镀层;并用电化学法和随机法等研究纳米化对纯锌点蚀行为的影响.结果表明：纳米锌和铸态锌的点蚀击破电位均服从正态分布;纳米化增加了纯锌点蚀击破电位对电位扫描速度的敏感性,使纯锌的点蚀产生类型由B1（parallel）型转为B2（series）型,并能抑制纯锌的点蚀生长.     

Corrosion and Nano-mechanical Behaviors of Magnetron Sputtered Al-Mo Gradient Coated Steel

A gradient three-layer Al-Mo coating was deposited on steel using magnetron sputtering method. The corrosion and nano-mechanical properties of the coating were examined by electrochemical impedance spectroscopy and nano-indentation tests and compared with the conventional electroplated cadmium and IVD aluminum coatings. Electrochemical impedance spectroscopy was performed by immersing the coated specimens in 3.5% NaCl solution, and the impedance behavior was recorded as a function of immersion time. The mechanical properties (hardness and elastic modulus) were obtained from each indentation as a function of the penetration depth across the coating cross section. The adhesion resistance of the coatings was evaluated by scratch tests on the coated surface using nano-indentation method. The results show that the gradient Al-Mo coating exhibits better corrosion resistance than the other coatings in view of the better microstructure. The impedance results were modeled using appropriate electrical equivalent circuits for all the coated systems. The uniform, smooth and dense Al-Mo coating obtained by magnetron sputtering exhibits good adhesion with the steel substrate as per scratch test method. The poor corrosion resistance of the later coatings was shown to be due to the defects/cracks as well as the lesser adhesion of the coatings with steel. The hardness and elastic modulus of the Al-Mo coating are found to be high when compared to the other coatings.     

Characterisation of phosphate coatings obtained using nitric acid free phosphate solution on three steel substrates: An option to simulate TMT rebars surfaces

Phosphate coatings have been obtained on three steel substrates, (a) ferritic-pearlitic (F-P), (b) tempered martensitic (T-M) and (c) tempered martensitic containing oxide scale (T-M-O) at the top to simulate TMT (thermo mechanical treatment) rebar surfaces which are extensively used for composite concrete structure. Nitric acid free phosphate solution was used for the coating purpose. Scanning electron microscopy (SEM), glow discharge optical emission spectrometry (GDOES) and X-ray diffraction (XRD) techniques were used to characterise phosphate coatings. Acicular phosphate microstructure was obtained on T-M-O and T-M steel substrates, whereas coarser phosphate microstructure was obtained on F-P steel substrate. Thinner to thicker coatings were obtained on F-P, T-M and T-M-O steel substrates respectively. Oxide scale, on the T-M-O steel substrate promoted for deposition of phosphate compounds and thereby obtained thickest coating on T-M-O steel substrate. Zinc phosphate (hopeite) on T-M-O steel substrate and zinc phosphate (spencerite) on F-P steel substrate were detected as main phosphate respectively, whereas both zinc phosphates (hopeite and speccerite) on T-M steel substrate were detected in the same proportion. In addition to zinc phosphate, zinc iron phosphate (phosphophyllite) was detected on F-P and T-M steel substrates, whereas iron phosphate (beraunite) was detected on T-M-O steel substrate. A comparative performance against corrosion of all the phosphate coated steel substrates was evaluated by salt spray, Tafel and electrochemical impedance spectroscopy (EIS) studies. Test for extended exposure in open atmosphere as well as simulated highly humid condition were also conducted to identify coating performance. Phosphate coated T-M-O steel substrate showed 4-5 times improved resistance against corrosion than the phosphate coated F-P steel substrate. The bond strength of coated steel substrate with concrete was increased 2-26% for phosphate coating on T-M-O and T-M steel substrates while the same was decreased 4-12% for phosphate coating on F-P steel substrate.     

变形温度对核电用锆包壳管表面Cr涂层开裂行为的影响

Cr涂层能够有效提高核电反应堆锆包壳管的事故容错能力，但在高温下其内部可能会萌生裂纹导致涂层开裂失效，现有Cr涂层开裂行为研究多针对常温，因此研究不同温度下Cr涂层的开裂行为对于其应用具有重要的理论和工程价值。采用多弧离子镀技术在N36锆合金包壳管外表面制备厚度为14μm左右的Cr涂层，采用WDW-100C万能试验机对涂层管分别进行室温（25℃）与高温（100、200、300、400℃）拉伸试验，并通过超景深显微镜和扫描电镜（SEM）观察涂层的裂纹表面与截面形貌，对Cr涂层在不同温度下的开裂行为与开裂机理进行研究。结果表明，随着温度升高，涂层管的屈服强度从（400±5）MPa下降到（150±5）MPa，涂层管的总体塑性变化不大；室温下裂纹萌生于涂层内部，其开裂方式为脆性沿晶断裂；100℃时涂层开裂方式不变，但表面裂纹数量减少，裂纹尖端出现钝化，由V字形转变为U字形；随着温度进一步升高，涂层的塑性变形能力提高，其表面呈流线形塑性变形；200℃及以上温度下，涂层表面无明显开裂，仅出现少量微裂纹，塑性的升高导致拉伸过程中涂层的变形量与基体存在差异，裂纹开始萌生于界面处，其断裂方式也由脆性断裂...     

Effect of coating thickness on the deformation mechanisms in PVD TiN-coated steel

The deformation mechanisms of a range of TiN coatings with different thicknesses, deposited on a V820 steel substrate following nanoindentation were characterized using focused ion beam (FIB) cross-sectioning and imaging, as well as cross-sectional transmission electron microscopy (TEM) of the indented region. Four TiN coatings were examined, including a cathodic arc evaporation (CAE) coating with a thickness of ∼ 0.7 μm and low voltage electron beam (LVEB) evaporation coatings with thicknesses of ∼ 2.0, ∼ 3.7 and ∼ 4.0 μm. Based on a model developed by Xie et al., the intercolumnar shear stresses were calculated to be approximately 2.20, 3.05, 3.50 and 3.55 GPa in the ∼ 0.7, ∼ 2.0, ∼ 3.7 and ∼ 4.0 μm thick TiN coatings respectively, that is, increasing as the coating thickness increases. Columnar cracking and shear steps at the coating/substrate interface were observed more frequently in the thinner TiN coatings indicated that these coatings deformed predominantly by shear along the columnar grain boundaries. In contrast, inclined cracking was the more dominant fracture type in the thicker TiN coatings. It is suggested that increased grain boundary strength occurs together with a lack of direct crack path along the grain boundaries through the thicker coatings due to the more equiaxed grain structure. Clearly, the grain structure and/or thickness of the TiN coating play a highly significant role in the deformation mechanisms.     

The structure and deformation behavior of zinc-rich coatings on steel sheet

The deformation behavior of commercially produced hot-dipped zinc and zinc-alloy coatings on steel under simple uniaxial tension has been studied. The strong basal plane texture of the hot-dipped galvanized coatings was found to be unfavorable for plastic flow and accounted for the extensive cracking observed during deformation. The eutectic structure in Galfan coatings was found to be beneficial in suppressing the formation of microcracks at low strain levels, although when grain boundary dents were present, cracks formed easily at these locations. Microcracks were observed in Galvalume coatings at all levels of strain. A technique to measure the coating plasticity has been developed to quantify the level of cracking in coatings at varying degrees of deformation.     

Performance of Al‐rich oxidation resistant coatings for Fe‐base alloys

The oxidation resistance of Al‐rich coatings made by chemical vapor deposition and pack cementation was examined on representative ferritic‐martensitic (FM, e.g. Grade 91, Fe‐9Cr‐1Mo) and austenitic steel substrates at 650°‐800 °C. To evaluate the potential benefits and problems with these alumina‐forming coatings, oxidation exposures were conducted in a humid air environment where the uncoated substrates experience rapid oxidation, similar to steam. Exposure temperatures were increased to accelerate failure by oxidation and interdiffusion of Al into the substrate. The difference in the coefficient of thermal expansion (CTE) between coating and substrate was found to cause cracking and coating failure during rapid thermal cycling on thicker coatings with Fe‐Al intermetallic phases. Therefore, thinner coatings with less Al and a ferritic Fe(Al) structure were evaluated more extensively and tested to failure at 700° and 800 °C on FM steels. The remaining Al content at failure was measured and used to improve a previously developed coating lifetime model. At 700° and 800 °C, thin coated austenitic specimens continue to exhibit protective behavior at more than double the lifetime of a similar coating on FM steel. The longer lifetime was attributed to the ferritic coating‐austenitic substrate phase boundary inhibiting Al interdiffusion.     

Deposition effects of WC particle size on cold sprayed WC-Co coatings

The WC particle size and its influence on the deposition of Co-based cermets are examined. Micron and nanostructured powders with similar Co content were employed. Varying the WC particle size influenced significantly the deposition efficiency of the coating process. Micrometer-structured WC-Co feedstocks did not permit coating build up when processed under comparable or elevated thermal spray parameters used for the nanostructured WC-Co feedstocks. In addition, micrometer-structured WC-Co coatings exhibited a conjoint erosion and deposition effect on the surface. Fine WC particles (< 1 μm) were observed near the substrate interface and larger WC particles (1-2 μm) in the vicinity of the coating surface. These observations indicate the existence of a critical WC particle size for deposition by the cold spray method and that the size criteria arises due to the formation and cohesion mechanisms within the coating layer.Nanostructured test specimens displayed (i) a dense microstructure with little presence of porosity, (ii) a crack free interface between the coating and substrate that indicated good adhesion, and (iii) no observable phase changes. The XRD patterns of each powder and their respective coatings did not have observable peak differences but the diffraction peak broadening of coatings indicated that there was grain refinement during the coating process. Furthermore, all nanostructured as-sprayed WC-Co coatings exhibited Vickers hardness values above HV1000. The nanostructured WC-Co coatings demonstrated adhesive strengths that exceeded the limits of the glue (60 MPa).     

重载条件下钢基体表面涂层裂纹及分层失效

用有限元法分析了Hertz接触应力下钢基体表面涂层裂纹和分层失效状况,采用无预制裂纹状况下的扩展有限元(XFEM)技术和内聚力(Cohesive)模型研究了涂层的裂纹扩展和分层失效过程。分析表明:涂层的裂纹萌生于涂层表面,并向内部扩展;涂层弹性模量越大越易产生裂纹,且裂纹扩展越深。涂层的分层失效主要是由涂层-基体界面切应力造成;由于畸变应力的存在,较薄涂层(如物理气相沉积涂层),涂层越厚越易产生分层;较厚涂层(如等离子喷涂涂层),涂层越薄越易产生分层。在另一方面,涂层的裂纹和分层会相互影响,分层会使涂层更易产生裂纹,使裂纹扩展越深,影响裂纹扩展速度;涂层裂纹会使涂层更易分层,使分层区域在远离接触区域方向上不断扩展。对比分析表明,研究结果与前人的理论及实验结果吻合较好,为今后进行涂层失效数值模拟提供了依据。     

Influence of superficial coating of CeO2 on the oxidation behavior of AISI 304 stainless steel

The effect of a superficially-applied, cerium-oxide coating on the non-isothermal oxidation behavior of AISI 304 stainless steel in dry air has been investigated. The heating rate employed was 3 K/min up to a final temperature of 1423 K. The reactive oxide coating not only reduced the reaction rate but also facilitated scale adhesion to the alloy substrate. Post-oxidation analyses of the alloy/scale combination using optical microscopy, SEM, EDAX, and XRD provide evidence for a changeover in the mechanism of oxide growth from the scale/gas interface to the alloy/scale interface for the coated steel.     

T91钢表面Ni和Ni/CeO_2镀层的氧化行为研究

采用电沉积法在T91钢基体上制备Ni与Ni/CeO2镀层,研究该镀层试样在温度分别为750℃和800℃,ω（Ar）=90%、ω（H2O）=10%气氛中的氧化行为。测量试样氧化动力学曲线并利用X射线衍射仪测定样品氧化后表面产物的相,利用带能谱分析功能EDS的扫描电镜SEM观察样品表面和截面形貌。结果表明：Ni和Ni/CeO2镀层对T91钢高温水蒸气氧化具有一定防护作用;此外,Ni/CeO2镀层中纳米CeO2颗粒抑制了氧化膜的开裂,改善了氧化膜的黏附性,提高了T91钢镀Ni试样的耐高温水蒸气氧化性能。     

碳钢表面化学镀Ni-P及Ni-P-PTFE纳米非晶镀层研究

用化学镀技术在碳钢表面制备Ni-P及Ni-P-PTFE纳米非晶镀层,探索镀层制备关键技术与工艺;通过扫描电镜（SEM）、X射线能谱仪（EDS）、X射线衍射技术（XRD）进行镀层微观结构、化学组成、相组成的分析与表征;用电化学参数测试及腐蚀增重评价镀层试样防腐性能。结果表明：在碳钢表面成功制备了具有非晶特点的Ni-P及Ni-P-PTFE镀层,Ni-P镀层表面致密平整,Ni-P-PTFE 镀层具有表面孔洞的形貌特征;电化学测试及腐蚀增重分析表明,Ni-P镀层防腐性能良好,Ni-P-PTFE镀层则由于复合界面的存在而防腐性能稍逊,但二者均可实现对基材碳钢明显的防腐保护作用。     

电热爆炸定向喷涂Stellite 6合金涂层

采用电热爆炸定向喷涂工艺在45号钢基体上制备stellite6合金涂层。借助光学显微镜，扫描电镜和图像分析软件等对涂层厚度均匀性、孔隙率，显微组织、晶粒度以及涂层基体界面结合情况进行了分析。借助显微硬度仪对涂层的硬度进行了测试。对不同尺寸喷涂材料所得涂层进行了比较。结果表明，两种不同尺寸喷涂材料制备的涂层组织晶粒均大大细化，涂层孔隙率都比较低；小截面积的喷涂材料制备涂层的厚度均匀性好于大截面的喷涂层；涂层与基体界面结合良好，在界面附近发生了扩散现象；涂层硬度均远远高于原始stellite6喷涂材料硬度，最高分别达到997HV和738HV，为原始硬度的2～3倍。且小尺寸喷涂材料涂层硬度高于大尺寸喷涂材料涂层硬度。     

Ti/TiB2多层镀层的组织和性能研究

为了进一步提高超硬TiB2镀层与基体的结合力,采用磁空溅射法在高速钢表面上制备了多层相间的Ti/TiB2超硬镀层.利用多种实验方法测试和研究了镀层的组织结构,形貌,表面粗糙度,硬度和与基体的结合力.结果表明,镀层的Ti/TiB2相间层数对这些性能有很大的影响.当层数从二逐渐增加到十二,TiB2镀层的（001）织构逐渐消失而变成无择优取向,镀层表面粗糙度增加,但镀层硬度基本不变.只有当层数为十二时,镀层硬度才明显降低.结果也表明,多层镀层可有效地提高镀层与基体的结合力,并存在着一最佳的多层结合.本文也对多层结构对其它组织和性能特征的影响进行了分析.     

In-situ observation in a scanning electron microscope on the exfoliation behavior of galvannealed Zn-Fe coating layers

Exfoliation behavior of galvannealed (GA) coating layers was continuously observed in-situ under 3-point bending load in a scanning electron microscope (SEM). Two kinds of substrate, Al-killed steel and Ti-stabilized interstitial free (IF) steel, were used. For GA coating layers formed on Al-killed steel, powdering was dominant at the region under compressive stress during the bending. Exfoliation occurred at the Γ/δ₁ interface, and the amount of exfoliation decreased with an increase in Fe content in the coating layer. Meanwhile, for GA layers coated on IF steel substrate, flaking was dominant. One of the perpendicular cracks that were pre-induced during galvannealing grew and propagated up to the Γ/Fe matrix interface with increasing strain. On the other hand, the crack initiated at the Γ/Fe matrix interface grew and propagated along the interface. Then, this crack combined with a nearby perpendicular crack, resulted in flaking.     

Structure and mechanical properties of TiAlCrSiN coatings deposited on Ti(C,N)-NbC-Ni cermets with varied Mo2C contents

TiAlCrSiN coatings were deposited on Ti(C,N)-NbC-Ni cermets with varied Mo₂C contents by medium frequency reactive magnetron sputtering, and their structure, phases, mechanical and frictional properties were investigated in the present work. The results indicated that both the Mo diffusion to the coating and the Al diffusion to the substrate resulted in the decreased lattice parameter of TiAlCrSiN phase in the coating. When increasing Mo₂C contents in the substrate, the columnar crystal grains in the coating were refined, whereas the adhesion of the coating to the substrate was degraded as a result of the coating thickening and the inner stress in the coating. Apart from being influenced by grain size, the coating's hardness was also linked to the crystal plane orientation, and it declined with a decrease of relative intensity ratio of I₍₁₁₁₎/I₍₂₀₀₎ in the TiAlCrSiN coating. The elasticity modulus of the coating decreased slightly due to the grain refinement in the coating. The average friction coefficient of TiAlCrSiN coating was reduced by oxidation of the Mo diffused from the substrate, while the friction coefficient was not the predominant indicator influencing wear rate of the coated Ti(C,N)-NbC-Ni cermet.