Moisture induced crack filling in barrier coatings containing montmorillonite as an expandable phase

In order to explore the option of healing surface scratches in coatings using an expandable phase, the crack filling behavior of transparent, multi layer coating consisting of a polysiloxane film on top of a thin montmorillonite interlayer was studied. The filling process was monitored using confocal and SEM microscopy. Filling of cracks in the polymeric topcoat occurred primarily within the first two hours of exposure to moisture saturated air and resulted in a good restoration of the surface flatness. Beyond this 2 h healing period, further expansion of the clay layers resulted in a “pushing-up” effect of the swollen clay in proximity of the damaged coating. This paper attempts to quantify the crack closure of the clay-containing coating by monitoring changes in scratch width and area as a function of time.     

Fatigue behavior of duplex-treated samples coated with Cr(C,N) film

The effect of a Cr(C,N) PVD coating on the fatigue behavior of 42CrMo4 steel grade was investigated both in a quenched and tempered condition and after nitriding treatment. The latter, so-called “duplex process”, combines a nitriding pre-treatment in order to obtain a hard and stiff substrate and a PVD coating in order to reduce the friction coefficient and improve the wear resistance of tools and mechanical components.Prior to fatigue testing, the surface modifications were characterized by means of X-ray diffraction (XRD), residual stress distribution, Vickers nano-hardness measurements and scanning electron microscopy (SEM) analysis.For the quenched and tempered steels, the PVD coating acts as a high compressive residual stressed zone able to increase the fatigue resistance of the coated component. On the contrary, for the duplex-treated samples, a limited increase in the fatigue limit was detected with respect to uncoated nitrided samples. This result was interpreted by means of the residual stress gradient which does not show high differences for nitrided coated and uncoated samples.     

Preparation of Al2O3/DLC/Au/MoS2 chameleon coatings for space and ambient environments

We have investigated the tribological properties of nanocomposite “chameleon” coatings, which adapt their low friction behavior with the surrounding environmental humidity and temperature. The material system of interest included alumina (Al₂O₃) in an Au matrix with diamond-like carbon (DLC) and MoS₂ nanoparticle inclusions. The coating design included formation of nanocrystalline hard oxide particles for wear resistance, embedding them into an amorphous matrix for toughness enhancement, and inclusion of nanocrystalline and/or amorphous solid lubricants for friction adaptation to different environments.Chemical analysis was used to ascertain a correlation between chemical bonding of species and frictional properties. Friction measurements were studied in cycling between humid air and dry nitrogen conditions at room temperature and during heating in air to 500 °C. It was observed that both graphitic carbon and MoS₂ worked together to give low friction in variable humidity environment, while Au was valuable for a low friction at elevated temperatures. Friction coefficients were found to be 0.02-0.03 in dry nitrogen, 0.1-0.15 in humid air, and 0.1 in air at 500 °C. Thus the tribological property results have shown that the system provides “chameleon” type adaptation behavior in different environments relevant for aerospace systems.     

Silver-palladium alloy deposited by DC magnetron sputtering method as lubricant for high temperature application

The silver-palladium(Ag-Pd) alloy coating as a solid lubricant was investigated for its application to the high temperature stud bolts used in nuclear power plants. A hex bolt sample was prepared in the following steps: 1) bolt surface treatment using alumina grit blasting for cleaning and increasing the surface area; 2) nickel(Ni) film coating as a glue layer on the surface of the bolt; and 3) Ag-Pd alloy coating on the Ni film. The films were deposited by using a direct current(DC) magnetron sputtering system. The thickness and composition of the Ag-Pd alloy film have effect on the friction coefficient, which was determined using axial force measurement. A 500 nm-thick Ag-Pd (80:20, molar ratio) alloy film has the lowest friction coefficient of 0.109. A cyclic test was conducted to evaluate the durability of bolts coated with either the Ag-Pd (80:20) alloy film or N-5000 oil. In a cycle, the bolts were inserted into a block using a torque wrench, which was followed by heating and disassembling. After only one cycle, it was not possible to remove the bolts coated with the N-5000 oil from the block. However, the bolts coated with the Ag-Pd (80:20) alloy could be easily removed up until 15 cycles.     

冷镦钢产品顶锻开裂的机理分析及应用

冷镦钢是用于制造螺栓、螺母、铆钉、插销等紧固件的一种重要材料,冷镦性能是材料性能的重要指标.冷镦钢在冷顶锻检验时,顶锻开裂的现象可以归纳为周面脆性开裂、塑性斜向开裂、纵向断续微裂、纵向通长开裂4类主要开裂现象.本文根据断裂力学理论,结合顶锻试样开裂的不同表现,对试样不同开裂情况进行了理论研究,分析了各种开裂现象的产生机理,并据此制定了冷镦钢产品的生产过程中提高冷顶锻性能的相关措施.     

可降解镁合金骨螺钉表面微弧电泳水热复合涂层的制备与性能

目的在医用镁合金骨螺钉表面构建羟基磷灰石涂层,有效控制其降解速率。方法利用微弧电泳/水热复合方法,在形貌复杂的骨螺钉表面制备涂层。该方法首先利用电解抛光对骨螺钉表面进行表面预处理,采用微弧电泳技术在其表面制备羟基磷灰石涂层,再利用水热合成对微弧电泳涂层进行封孔。利用XRD、SEM、AFM等分析手段对涂层显微结构进行分析,利用体外浸泡实验和电化学实验对涂层耐腐蚀性能及其对钙磷盐的诱导特性进行了评价。结果在电解抛光电流0.14 A、抛光时间2 min的工艺条件下进行电解抛光预处理,可以提高基体和涂层的结合性能。由于骨螺钉的特殊形状,在微弧电泳电解液中添加丙三醇,并通过调整电解液中丙三醇含量优化微弧电泳工艺(电压155 V,反应时间20 min),能有效抑制尖端放电现象,防止膜层组织疏松和大量的氧化物堆积,以及涂层剥落甚至基体烧蚀的现象。再优化水热合成工艺参数(处理液p H值8.5,反应时间1.5 h,反应温度393 K)对微弧电泳涂层进行封孔,得到微弧电泳/水热复合涂层。结论微弧电泳/水热复合涂层表面形貌为菜花状结构,由纳米棒状羟基磷灰石组装而成,均匀致密,结晶性好。电化学腐蚀测试表明,制备复合涂层后,骨螺钉的腐蚀电流密度降低了一个数量级。在模拟体液中浸泡6天,骨螺钉的形貌依然完整,说明水热复合涂层在改善生物相容性的同时,提高了骨螺钉的耐腐蚀性能。但微动摩擦磨损测试显示,水热复合封孔处理后磨损性能下降。     

Surface modification of indium tin oxide films with Au ions implantation: Characterization and application in bioelectrochemistry

Indium tin oxide (ITO) thin film coated glass substrates have been implanted with 21 keV Au ions at a fluence of 1.0 × 10¹⁷ ions/cm² at room temperature. The resulting gold film was characterized with X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis spectra and electrochemical methods. The results reveal that the implanted Au atoms were in the zero-valent metallic state and these Au atoms precipitated to form nanoclusters on the ITO surface whose average radius was estimated between 2 and 5 nm. The preferentially growing orientation was Au (111) plane during the formation process of gold film and the value of gold active surface area normalized by the geometric electrode areas was 0.48 for Au ion implanted ITO (Au/ITO) electrode. The potential utility of Au/ITO films was investigated. The Au/ITO electrode exhibited effective catalytic responses towards biomolecules such as ascorbic acid (AA) oxidation and lowered oxidation potential of AA by 0.6 V when compared with the bare ITO electrode. Myoglobin (Mb) was also successfully immobilized on the Au/ITO electrode and the direct electron transfer between proteins and electrode surface was realized. It was demonstrated that the Au/ITO film offered a favorable microenvironment for the orientation of biomolecules. New biomaterials with specific electrocatalytic and electrochemical features could be fabricated using this method.     

Inert anode composed of Ni–Cr alloy substrate, intermediate oxide film and α-Al2O3/Au (Au–Pt, Au–Pd, Au–Rh) surface composite coating for aluminium electrolysis

An inert anode composed of alloy substrate, intermediate oxide film and surface composite coating for aluminium electrolysis has been fabricated. The intermediate oxide film (ZrO₂/Y₂O₃) provides good adhesion and mutual diffusion resistance between the substrate and the surface coating which consists of α-Al₂O₃ particles embedded in Au (Au–Pt, Au–Pd, Au–Rh) matrix. The results of electrolysis test revealed that aluminium with high purity (>99.999%) can be produced. It is demonstrated that the inert anode exhibits superior erosion and corrosion resistance during aluminium electrolysis, especially in low-temperature (800 °C) electrolytes.     

Characterization of corrosion of X65 pipeline steel under disbonded coating by scanning Kelvin probe

Corrosion of X65 pipeline steel under a disbonded coating was studied by scanning Kelvin probe measurements. Three types of specimen were designed and prepared to investigate the effects of immersion time, oxygen concentration and wet-dry cycle on Kelvin potential profile and thus corrosion behavior of the steel. Kelvin potential measured on “intact” area is shifted negatively with time, indicating an increasing water uptake under the “intact” coating. With the increase of the amount of solution, it is expected that the electrolyte concentration and electrochemical reaction rate change, resulting in a significant decrease of interfacial potential. Moreover, there is a more negative Kelvin potential on disbonded area than that on “intact” area. The negative shift of Kelvin potential is attributed to corrosion reaction of steel occurring under the disbonded coating. Due to the narrow geometry of coating disbondment, an oxygen concentration difference exists along the depth of the disbondment. The corrosion behavior under disbonded coating strongly depends on the oxygen partial pressure and local geometry. With continuous purging of nitrogen and removing of oxygen, Kelvin potential tends to be identical throughout the disbonded area. During wet-dry cycle, the thickness of solution layer trapped under disbonded coating decreases due to evaporation of water. With the decrease of solution layer thickness, the measured Kelvin potential decreases, indicating that the effect associated with the reduction of oxygen solubility in the concentrated solution during drying of electrolyte is favored over that related to the enhanced oxygen diffusion and reduction. There exists a critical thickness of solution layer, below which the oxygen solubility is sufficiently low to support the electrochemical corrosion reaction of steel.     

Application of PEO113-b-PS218 nano-aggregates for improved protective characteristics of composite zinc coatings in chloride-containing environment

This study reports on the influence of a very low concentration of polyethylene oxide-b-polystyrene nano-aggregates (PEO₁₁₃-b-PS₂₁₈ micelles of 0.5 g/l in the starting electrolyte) on the corrosion behavior and surface characteristics of zinc coatings in chloride-containing environment (5% NaCl). Both zinc (Zn) and composite Zn-polymer (ZnP) coatings were electrodeposited from slightly acidic electrolytes with a coating thickness of approximately 8 μm. The involved experimental techniques were electrochemical methods (potentio-dynamic (PD) polarization, EIS and SVET) and surface analysis techniques (SEM, EDX, XRD and XRF). The morphology, distribution and size of the PEO₁₁₃-b-PS₂₁₈ micelles were investigated in demi-water by TEM and dynamic light scattering (DLS), prior to their co-deposition within the metallic zinc matrix.The electrochemical behavior was studied on certain intervals (from initial measurements after OCP stabilization up to 480 h of immersion in 5% NaCl). The recorded parameters account for higher corrosion resistance of the nano-composite (ZnP) coating, compared to galvanic Zn, especially after prolonged periods of treatment. The superior performance of the composite ZnP coating in the very aggressive medium of 5% NaCl is denoted to the combination of increased barrier effects (in the presence of the nano-aggregates) and “self-healing” mechanisms (as a result from a reversible shrinkage and swelling of the polymer shells in the presence of chlorides).     

A novel surface activation method for Ni/Au electroless plating of acrylonitrile-butadiene-styrene

Direct electroless metallization on acrylonitrile-butadiene-styrene (ABS) plastic with Pd-free activation method could reduce the cost of production. A novel surface activation method with the immobilization of Ni(0) nanoparticles by chitosan (CTS) film on ABS and then the deposition of Ni and Au on ABS were investigated in this paper. X-ray photoelectron spectroscopy (XPS), fourier transform infrared (FTIR) spectra and scanning electron microscopy (SEM) data revealed the related interfacial reaction mechanism in activation process. The Ni(0) nanoparticles immobilized by the CTS films were effective auto-catalysts in the nickel electroless plating process. The formations of Ni plating layers at different deposition time were observed by SEM. The x-ray diffraction (XRD) patterns revealed the Ni layer at 30 min was in an amorphous phase. Au was successfully plated on the Ni layers in a new, stable non-cyanide Au electroless plating bath with different reducing agents. The chemical compositions of Ni/Au layers were analyzed by inductive couple plasmas (ICP) and ion chromatography (IC) measurements. Inorganic element (P or S) from reducing agents made the surface morphology of Au layers different in SEM images. However, they did not change the Au crystallization phase at all based on XRD patterns.     

The effect of Mg alloy substrate on “electroless” E-coating performance

The corrosion performance of “electroless” E-coating pre-film on eight different Mg alloys is compared in a 5 wt.% NaCl. The results show that the alloys have different levels of surface alkalization effect, resulting in different thickness of films formed on the alloys. The alloying elements in a Mg alloy do not directly influence the film formation and corrosion performance. Instead, the corrosion resistance of substrate has a significant effect on the degradation or corrosion process of the coated Mg alloy. The corrosion resistance of the substrate Mg alloy can influence the porosity of pre-film during “electroless” E-coating deposition, and it is also a critical factor determining the film corrosion degradation.     

Characteristics of blueing as an alternative chemical conversion treatment on carbon steel

Alternative pretreatments are proposed in order to avoid the environmental aggressive impact produced by using surface finishing processes based on Cr(VI) compounds. In this work, blueing method is proposed as chemical conversion treatments (CCTs) in AISI 1010 commercial steel. CCTs were carried out by the so-called “Swiss blueing” method using a solution with 20 g HNO₃ + 5 g CH₃CH₂OH + 5 g CuSO₄·5H₂O + 160 g FeCl₃ dissolved in water (1:1). This solution was added onto steel sheets using spray impregnation technique. Anticorrosion protection and adhesion properties were evaluated using polarization resistance (Rp), ac impedance and pull-off test, respectively. The thickness, morphology and microstructure of the protective films have been examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Although, no-treated specimens showed the highest polarization resistance (Rp) and impedance values, the oxide compounds are porous and unstable, while samples with the CCTs increased anticorrosive protection and favoured a strong adhesion between substrate and top coating. Continuous immersion values obtained by ac impedance showed remarkable variations in the coating resistance. SEM micrographs indicate that chemical treatment increased the surface porosity-roughness. XRD patterns showed some iron oxide compounds: mainly magnetite, and a small quantity of hematite, which have influenced the electrochemical behavior and adhesion. Pull-off tests showed good adhesion in the previously treated samples. These results indicate that obtained roughness with one CCTs using “Swiss blueing” method increased anticorrosion protection and adhesion properties.     

Super-hydrophobic surface treatment as corrosion protection for aluminum in seawater

“Underwater super-hydrophobic” surface applied in the corrosion protection was prepared by melting myristic acid (CH₃(CH₂)₁₂COOH) adsorbed onto the anodized aluminum. The static contact angle for seawater on the surface was measured to be 154°. The surface structure and composition were then characterized by means of scanning electron microscopy (SEM) with energy dispersive X-ray spectrum (EDS) and atomic force microscope (AFM). The electrochemical measurements showed that the super-hydrophobic surface significantly improved the corrosion resistance of aluminum in sterile seawater. In addition, the mechanism of the underwater super-hydrophobic surface applied in the corrosion resistance was discussed using a schematic.     

Scratch resistance of platinum-vanadium single and multilayer systems

The effects of annealing on coating morphology and scratch resistance have been studied in several single and multilayered Pt-V coated systems. The changes in coating morphology included the formation of distinctive “square shape precipitates”, increased surface roughness, coating thickening and cracking. The results show that scratch resistance was affected by coating thickness, the sequence of deposited layers and Pt-V phases induced by annealing. The scratch resistance was improved for all coated systems by annealing. However, there is an indication that a large volume fraction of PtV₃ formed at 900 °C/45 min significantly increased the critical load at which the coating failed. This result will be the base for further study on the development of targeted phases to improve the surface characteristics for specific coating applications.     

Electroless synthesis of nano-structured gold particles using conducting polymer nanoparticles

In this study, gold nanoparticles were synthesized by electroless recovery of [AuCl₄]⁻ from an acidic aqueous solution using nano-structured conducting polymer, polypyrrole nanoparticles, as active surface. The formation of gold nanoparticles was confirmed by TEM, SEM and EDX measurements. The effects of the initial Au(III) concentration on the gold uptake was examined. The recovery capability and gold particle morphology prepared from polypyrrole nanoparticle were compared to that from cast PPy film counterpart.     

Novel model for film growth based on surface temperature developing during magnetron sputtering

New physical phenomenon consisting in development of the surface temperature T_surf, which being equal to the substrate temperature T_s at the beginning of deposition, steeply increases and becomes several times higher than T_s at the end of the process, is revealed by means of IR-camera and new calorimetric method during sputter deposition of metal films. The reason for the phenomenon is the formation of a liquid-like layer on the growth surface with extremely low (∼10⁹ times lower than for metals) thermal conductivity. Variation in the film structure along thickness correlates with the variation in T_surf. To explain these effects we developed a model according to which film grows by “gas → liquid → solid” rather than “gas → solid” mechanism which is realized provided that the film grows from energetic atoms.     

Effect of deposition processes on residual stress profiles along the thickness in (Ti,Al)N films

The effect of deposition processes on the distribution of residual stresses in the thickness of the (Ti,Al)N films prepared by arc ion plating (AIP) was investigated in the present work, which indicates that the stress distribution exhibits a “bell” shape and the maximum compressive stress appears in the layer near the surface. The residual stress increases with the thickness of a film and the substrate bias voltage, respectively. The stress distribution can be altered, and the adhesion of the film/substrate can be improved by optimizing the deposition parameters. Finally, a film with a thickness of 7.57 μm was successfully directly deposited on the substrate through optimizing the bias voltage.     

Bio-mimetic surface structuring of coating for tribological applications

A novel multilayered coating was developed for applications associated with friction reduction and wear resistance improvement. The nano-engineered coating integrates a soft lubricating layer, consisting of MoS₂-PTFE, onto hard load-supporting layers, with controlled surface morphology (roughness and patterning) of cBN-TiN. The coating was synthesized by sequential procedures including electrostatic spray deposition of cBN particles with different average particle sizes, chemical vapor deposition of TiN, deposition of nano- and micro-sized MoS₂ dispersed in PTFE, and curing. The effect of cBN particle size (with different combinations of particle size) and deposition parameters (specifically electrical voltage) on the cBN-TiN surface morphology were studied experimentally and optimized. SEM characterization of the as-synthesized cBN-TiN coating shows surface features similar to that of colocasia esculenta, with alternating nano- and micro-sized domes and “pockets”; the MoS₂-PTFE top layer has MoS₂ particles retained in the pockets by a basket structure formed during PTFE curing. Tribological and scratch tests were carried out for the as-prepared cBN-TiN and cBN-TiN/MoS₂-PTFE multilayered coatings. Sliding test results demonstrate significantly lower friction coefficient for the multilayered coating, showing that the unique integration of soft lubricating layer and biomimetically structured hard layer can effectively improve tribological performance. It is suggested that lubrication at the frictional contacts was realized by continuous release of the lubricants, MoS₂ and PTFE, from the pockets.     

Tribological performance of hybrid filtered arc-magnetron coatings. Part II: Tribological properties characterization

Nano-structured coating architectures were developed to provide a best blend of corrosion and wear resistance for high chromium content steels used in aerospace bearing and gear applications. A hybrid filtered arc-magnetron deposition process was employed to deposit functionally graded, multilayered and nanocomposite TiCrN/TiCrCN + TiBC cermet coatings on carburized steel substrates. Coatings exhibited excellent adhesion to the carburized surfaces and had hardness in the range of 23-25 GPa. Tribological properties of the coatings were characterized by: pin-on-disk COF, lubricated sliding, reciprocating sliding, and 3 ball half thrust bearing tests in dry and lubricated environments at high contact stresses. Both polyester and perfluoropolyalkylethers (PFPAE) based lubricants were used to evaluate coating performance with neutral and chemically aggressive lubrication. Sliding friction and reciprocating sliding wear tests were performed using modified disk-on-ring and point-on-disk arrangements, respectively. Contact stresses were estimated using the Hertzian contact formula (sliding friction), and through direct measurements of contact areas by SEM (reciprocating sliding). Low-speed thrust bearing high load rolling contact was evaluated at 350 °C, using Si₃N₄ balls and PFPAE-based lubricant, at contact stresses of ∼ 3.2 GPa. Aggressive corrosion testing was performed on coated samples using MIL-STD-810F “salt-fog” testing. Wear and corrosion behavior was investigated using SEM/EDS, EDX, AFM, profilometry, and optical microscopy. The influence of coating architecture on wear properties was investigated. Multifold improvements in the surface dry and lubricated wear life, reduction of the dry friction coefficient, prevention of corrosion attack from the products of PFPAE lubricant degradation, and improvement of salt-fog corrosion resistance are demonstrated.