An efficient protection of stainless steel against corrosion: Combination of a conversion layer and titanium dioxide deposit

In the present work, a novel process has been developed to improve the corrosion properties of ferritic stainless steels. Titanium oxide coatings have been deposited onto stainless steel by sol-gel process after a pre-functionalization of the substrate in a conversion bath. Gel titania was prepared by hydrolysis of a titanium butoxide through a sol-gel process. Duplex systems “conversion layer/uniform TiO₂ coating” have been prepared on stainless steels using a dipping technique and thermal post-treatments at 450 °C. The preparation of sol-gel coatings with specific chemical functions offers tailoring of their structure, texture and thickness and allows the fabrication of large coatings. The morphology and structure of the coatings were analysed using scanning electron microscopy with field effect gun (SEM-FEG), Mass spectroscopy of secondary ions (SIMS) and X-ray diffraction (XRD). The anticorrosion performances and the ageing effects of the coatings have been evaluated in neutral and aggressive media by using several normalized tests.The results show that the conversion layer was not sufficient to protect steel but sol-gel TiO₂ coatings, anchored on the metal substrate via the conversion layer, show good adhesion with the substrate and act as a very efficient protective barrier against corrosion. So, duplex layers with TiO₂ nanoparticle coatings on steels exhibit an excellent corrosion resistance due to a ceramic protective barrier on metal surface. Analysis of the data indicates that the films act as geometric blocking layers against exposure to the corrosive media and increase drastically the lifetime of the substrate.     

In situ IR pyrometric analysis during thermal treatment in air of TiOxNy coatings

IR pyrometry is an original diagnostic tool for in situ analysis of surface transformations of coatings or bulk materials during the thermal treatment under reactive atmosphere such as high temperature oxidation. Significant oscillations of the pyrometric signal were observed during annealing in air of TiO_1.5N_0.5 coatings in the temperature range 673-823 K. This is due to interferences resulting from multi-reflections at the interfaces of a transparent growing film. This reveals the formation of a TiO₂ thin film on the top of the TiO_1.5N_0.5 coating. Modeling of the time dependence of the IR pyrometric signal allows the determination of the oxide layer thickness, transformation rate and optical properties of the films under the growth conditions. The progressive oxidation of a compact amorphous TiO_1.5N_0.5 coating from the external surface to the substrate interface was supported by SIMS, XRD and reflectance analyses.     

Structure stability and corrosion resistance of nano-TiO2 coatings on aluminum in seawater by a vacuum dip-coating method

A stable nano-TiO₂ coating was prepared by vacuum dip-coating TiO₂ sol-gel onto the anodized aluminum surface. The effect of vacuum dip-coating method and anodization pretreatment on compactness and stability of the coatings was proved. The structure and composition of the coatings were characterized by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicate that the surface of coatings is uniform and compact, with high load capacity. The particles of TiO₂ anatase is filled into the Al₂O₃ nano-pores formed by anodization. The electrochemical measurements show that the nano-TiO₂ coatings significantly decrease the corrosion currents densities (i_corr), as simultaneously increased the values of polarization resistance (R_t) of aluminum. It indicates the nano-TiO₂ coatings exhibit excellent anticorrosion properties in sterile seawater at the room temperature.     

Laser-induced enhancement of the surface hardness of nanoparticulate TiO2 self-cleaning layer

We here report that the abrasion resistance of nanoparticulate TiO₂ self-cleaning layers can be highly enhanced without a considerable loss of photocatalytic capability. TiO₂ coating layers solution-deposited onto the glass substrate were irradiated by a pulsed ultraviolet (UV) laser at 355 nm, which modified the surface morphologies via laser-induced local melting of TiO₂ nanoparticles. The surface hardness, measured by pencil scratch test, improved with increasing laser power (P). While an unmodified TiO₂ layer revealed a hardness of 6B, it increased to 2H after the surface was irradiated at P = 0.3 W. Almost all of the stearic acid deposited on an unmodified sample disappeared after UV exposure for 12 h. The photocatalytic decomposition was slowed down on laser-irradiated TiO₂ surfaces and this is attributed to the reduction of specific surface areas as a result of the morphological modifications. However, a TiO₂ layer hardened to 2H still exhibited fairly good photocatalytic activity, decomposing more than 75% of the stearic acid after exposure for the same duration.     

Surface pretreatment of austenitic stainless steel and copper by chemical, plasma electrolytic or CO2 cryoblasting techniques for sol-gel coating

The surface pretreatments of the austenitic stainless steel and copper surfaces for the sol-gel coating were carried out by chemical, plasma electrolytic or CO₂ cryoblasting techniques. With the austenitic stainless steel the smoothest surfaces were obtained with plasma electrolytic cleaning, after which the measured contact angles of water were clearly decreased revealing improved hydrophilicity. As well with the copper samples the smooth surface and improved hydrophilicity was obtained with the plasma electrolytic cleaning, but oxide layer formed to the copper surface immediately after the treatment. CO₂ cryoblasting provided rough surface with wetting properties close to the original surface both for austenitic stainless steel and copper surfaces. CO₂ cryoblasting provided best appearance for the copper surface because no oxidation happened with that treatment. XPS and SIMS studies showed that with the plasma electrolytic treatment the surface layer of the austenitic stainless steel enriched of chromium and the oxide layer formed on the surface was less than 10 nm thick. With the chemical cleaning and CO₂ cryoblasting, the chromium enrichment to the stainless steel surface was less. However XPS and SIMS studies showed that chemical treatment provided thinner oxide layer to copper surface than plasma electrolytic treatment.     

The adhesion of SiNx thin layers on silica–acrylate coated polymer substrates

Plasma Enhanced Chemical Vapor Deposition (PECVD) was used to grow 200, 300 and 400 nm thick silicon nitride layers (SiN_x) on a high temperature aromatic polyester substrate spin coated with a silica–acrylate hybrid coating (hard coat). Layers deposited without oxygen plasma treatment remained attached to the substrate, while spontaneous buckle delamination of the layer deposited with oxygen plasma treatment was observed directly after layer deposition. This effect was investigated using Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS), Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM). SIMS analyses showed a considerable increase of silicon oxide by exposing the substrate to oxygen plasma treatment, while AFM showed an increased roughness. Bright-field transmission electron micrographs show the presence of a particulate SiO₂ layer. The oxygen plasma treatment thus removes the acrylate from the hard coat layer leaving behind the SiO₂ particles leading to lower adhesion and thus to spontaneous buckle delamination.     

Plasma pre-treatment and TiO2 coating of PMMA for the improvement of antibacterial properties

The antibacterial properties of polymethyl methacrylate (PMMA) are enhanced by coating with TiO₂ films. The transparent TiO₂ films on plasma-treated PMMA are prepared by sol-gel dip coating. The modified surfaces are characterized by XRD, AFM, ATR-FTIR, SEM, UV-vis spectroscopy, and contact angle measurements. Finally, the antibacterial properties are evaluated using the method of plate-counting of Staphylococcus aureus (gram positive) and Escherichia coli (gram negative). It is found that the anatase-TiO₂ film is well-conglutinated on PMMA surface with an average crystallite size of ca. 4 nm. The as-prepared TiO₂/PMMA exhibits excellent photoinduced antibacterial effect for the sterilization of bacteria under indoor natural light, and about 100% of both bacteria are inactivated within 2 h illumation. Compared to PMMA without any treatment, the superior anti-adhesion capability of the TiO₂/PMMA surface is also demonstrated.     

碳钢表面防腐超疏水TiO_2/PDMS涂层的制备及性能

针对碳钢腐蚀电位相对更负、更容易发生腐蚀的特点,在Q235钢表面制备超疏水TiO_2/PDMS涂层以提高其耐蚀性能。采用表面活性剂分散纳米TiO_2并进行改性,然后与PDMS混合,用溶胶凝胶法在Q235钢表面制备有聚二甲基硅氧烷(PDMS)过渡层的TiO_2/PDMS超疏水涂层。借助扫描电镜(SEM)、接触角测量仪、红外光谱(FT-IR)及X射线衍射仪(XRD)表征其表面涂层的表面形貌、化学成分及疏水性能,用电化学试验和浸泡试验测试其防腐性。结果表明:TiO_2/PDMS涂层表面具有独特的微纳结构,与水的接触角达到154.3°;其腐蚀电位由碳钢的-0.77 mV正移至超疏水涂层的-0.24 mV,腐蚀电流密度则下降两个数量级,即从5.02×10~(-6)A·cm~(-2)下降至3.95×10~(-8)A·cm~(-2);超疏水涂层的交流阻抗值高于碳钢基底3个数量级。经过7 d的3.5wt.%NaCl溶液浸泡,超疏水涂层并未发生失重。制备的TiO_2/PDMS超疏水涂层具有超疏水效果和良好的长期耐腐蚀性。     

多孔ZrO2-8 wt% Y2O3涂层的制备及性能

航空发动机的效率与转动叶片和机匣之间的间隙密切相关。为了控制转子和静子之间的间隙,需要在机匣表面制备可磨耗的封严涂层。在发动机的高温端,ZrO₂-8wt% Y₂O₃涂层是经常采用的封严涂层基体。涂层中的孔隙可以增加涂层的可磨耗性。本文利用聚苯酯(PHB)增加等离子喷涂的ZrO₂-8 wt% Y₂O₃涂层的孔隙率。为了避免聚苯酯在等离子喷涂过程中的烧损,利用溶胶-凝胶法在聚苯酯颗粒表面沉积一层TiO₂层。文中将讨论采用此方法制成的涂层的形态、孔隙率、硬度和可磨耗性。结果表明,在喷涂粉末中混合包覆型的聚苯酯后,涂层的孔隙率将会得到提升,涂层硬度将会下降。磨耗试验的结果表明涂层的磨耗深度随着涂层孔隙率的增加而增加。     

Effect of stabilising heat treatment on characteristics of electrolytic alumina coating on ferritic stainless steel

The effect of stabilising thermal treatment on the characteristics of conversion coatings modified by electrolytic alumina deposits have been studied using SIMS analysis and X‐ray diffraction in grazing incidence. Before heating, alumina deposit obtained is in form of amorphous boehmite gel. Heat treatment at 800°C in air leads to a very uniform coating morphology and induces diffusion phenomena. After heating (24 h), the coating is constituted of two layers: an alumina (δ‐Al₂O₃) outer layer and a chromium oxide (α‐Cr₂O₃) inner layer. At the coating/steel interface mixed oxide (iron, chromite) are present. The electrolytic alumina deposit blocks the chromium oxide in the deep zone.     

Characterisation and oxidation of LVOF sprayed Al2O3–40%TiO2 coating on superni 601 and superni 718 superalloys at 800 and 900°C

Failure of components due to high temperature oxidation is the major degradation mechanism in boiler and gas turbine industries. Superalloys having superior mechanical properties and creep resistance are used in these applications but lack resistance to oxidation under aggressive environments. Protective coatings are used to improve their oxidation resistance in such applications. In the present investigation, Al₂O₃–40%TiO₂ coating was deposited on superni 718 and superni 601 superalloys by low velocity oxy fuel process. The as sprayed coating was characterised for microhardness, surface roughness, scanning electron microscopy and X-ray diffraction analysis. High temperature oxidation behaviour of Al₂O₃–40%TiO₂ coated and uncoated superni 718 and superni 601 superalloys has been evaluated at the elevated temperatures of 800 and 900°C for total duration of 50 cycles under cyclic conditions. Each cycle consisted of keeping the samples for 1 h at the elevated temperature followed by 20 min cooling in ambient air. Al₂O₃–40TiO₂ coating in the as sprayed condition showed the presence of Al₂O₃–TiO₂, α-Al₂O₃, TiO₂ as the main phases. Al₂O₃–40%TiO₂ coating on superni 718 and superni 601 superalloys has shown a lower oxidation rate as compared to those of uncoated superalloys. However, the oxidation rate of the coating was not steady due to the occurrence of spallation/sputtering at various stages. The coating was found adherent on the substrate superalloys throughout the study.     

Osteoblast growth behavior on micro-arc oxidized β-titanium alloy

β-titanium (β-Ti) alloys are known for their excellent physical properties and biocompatibility, and are therefore considered as next-generation metals for orthopedics and dental implants. To improve the osseous integration between β-Ti alloys and bone, this study develops a titanium dioxide (TiO₂) coating on the surface of β-Ti alloys by using micro-arc oxidation (MAO) technique. The anatase (A) rich and rutile (R) rich TiO₂ layer, were formed on β-Ti, respectively. In vitro tests were carried out using pre-osteoblast cell (MC3T3-E1) to determine biocompatibility and bone formation performance. Biocompatibility includes cell adhesion, cell proliferation, and alkaline phosphatase (ALP) activity, while the bone formation performance contains osteopontin (OPN), osteocalcin (OCN) and calcium content. Cell morphology was also observed. In addition, raw β-Ti, A rich TiO₂ and R rich TiO₂ were implanted into the distal femora of Japanese white rabbits for 4, 8, and 12 weeks to evaluate its in vivo performance.Experimental results show that TiO₂ coating can be grown on and well-adhered to β-Ti. The anatase phase formed under a low applied voltage (350 V), while the rutile phase formed under a high applied voltage (450 V), indicating that crystal structure is strongly influenced by applied voltage. A porous morphology was obtained in the TiO₂ coating regardless of the crystal structure and exhibited superior bone formation performance than β-Ti. In vivo analysis and in vitro test show similar trends. It is also noticeable that the R rich TiO₂ coating achieved better biocompatibility, osteogenesis performance. Therefore, a MAO-treated R rich TiO₂ coating can serve as a novel surface modification technique for β-Ti alloy implants.     

纳米颗粒增强TiO_2涂层的制备及其防污性能

为提高二氧化钛涂层的防污性能,采用KH-550硅烷改性锐钛矿型TiO_2颗粒,并充分分散于二氧化钛凝胶涂层中。通过降解亚甲基蓝溶液、细菌贴附试验、藻类贴附试验,分别评价了涂层的光催化性能、抗菌性能及抗藻类附着性能,并利用激光共聚焦显微镜及扫描电子显微镜对藻类在涂层表面的附着情况进行分析。结果表明,添加TiO_2纳米颗粒涂层的防污性能较未添加TiO_2纳米颗粒涂层有较大程度的提高。添加粒径为5~10 nm TiO_2颗粒的二氧化钛涂层对小球藻、三角褐指藻及小新月菱形藻的附着降低率分别达到了92.1%、71.5%和62.1%,相较于纯二氧化钛涂层对3种藻类的附着降低率分别提高了29.7%、68.4%和43.5%。TiO_2颗粒的加入可以有效地提高涂层的光催化性能,光催化使得涂层具有亲水、抗菌及自清洁的性能进而有利于提高涂层的防污性能。     

AISI430铁素体不锈钢表面制备MnCo2O4涂层特性及内应力研究

为了探究MnCo₂O₄尖晶石涂层作为固体氧化物燃料电池的金属连接体表面涂层的性能,使用溶胶-凝胶法制备出纯净的前驱体粉末,再使用电泳沉积方法制备出致密的MnCo₂O₄尖晶石涂层,利用SEM、EDS和XRD等表征手段观察分析MnCo₂O₄尖晶石涂层的相结构和微观组织形貌。采用“四探针法”测量MnCo₂O₄尖晶石涂层800℃氧化200h前后的面比电阻使用拉拔法完成不同界面粗糙度下的涂层结合强度测试,并用有限元仿真加以验证。结果显示,MnCo₂O₄尖晶石涂层结构均匀,致密度较好。相较于AISI430不锈钢基体来说,在800℃空气中氧化200h,抗氧化性提高了接近3倍。且中温面比电阻小于SOFC金属连接体规定的极限值。此外,基体表面粗糙度可以有效的增加涂层与基体的机械咬合作用,但同时也会导致应力集中,出现缺陷,从而降低了结合强度。     

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.     

Oxidation resistance of TiAl3-Al composite coating on orthorhombic Ti2AlNb based alloy

The TiAl₃-Al composite coating on orthorhombic Ti₂AlNb based alloy was prepared by cold spray. Oxidation in air at 950 °C indicated that the bare alloy exhibited poor oxidation resistance due to the formation of TiO₂/AlNbO₄ mixture and intended to scale off at the TiO₂ rich zone. A nitride layer about 2 µm was formed under the oxide layer. The oxygen invaded deeply into the alloy and caused severe microhardness enhancement in the near surface region. The TiAl₃-Al composite coating exhibited parabolic oxidation kinetics and showed no sign of degradation after oxidized up to 1098 h at 950 °C in air under quasi-isothermal condition. No scaling of the coating was observed after oxidized at 950 °C up to the tested 150 cycles. The major oxide in the oxidized coating was Al₂O₃. The AlTi₂N, TiAl and small amount of TiO₂ were also observed in the oxidized coating. The EPMA and microhardness tests showed that inward oxygen diffusion was prevented by the interlayer, which was formed between the composite coating and the substrate during heat-treatment. Microstructure analyses demonstrated that the interlayer play a major role in protecting the substrate alloy from high temperature oxidation and interstitial embrittlement.     

Effects of protecting layer [Li,La]TiO3 on electrochemical properties of LiMn2O4 for lithium batteries

A sample of LiMn₂O₄ spinel oxide was surface-modified with lithium lanthanum titanate ([Li,La]TiO₃), which was developed as a lithium ionic conductor, by means of hydrothermal processing and subsequent heat treatment at 400 °C. The surface coating layers were analyzed by morphology observation using a transmission electron microscopy. Energy-dispersive spectrometry and X-ray photoelectron spectroscopy were used for element investigation. The surface modification effects on rate capability during cycling and capacity retention for the LiMn₂O₄ spinel oxide were confirmed. Then Mn dissolution during storage at elevated temperatures of the pristine, coated sample was characterized. The Mn dissolution characterization was based on the idea that Mn dissolution is one of the most significant reasons for capacity loss for LiMn₂O₄ spinel oxide, and this phenomenon is especially severe at elevated temperatures. Our experimental results indicate that the surface-modified sample shows much a better initial capacity and rate capability compared with the pristine sample. The [Li,La]TiO₃ coating effectively enhances the structural stability of LiMn₂O₄ at elevated temperatures, most likely because the [Li,La]TiO₃-modifying layers play a definitive role in suppressing Mn dissolution in the electrolyte during storage.     

TiO2/膨润土复合光催化材料的常温制备及性能研究

采用改进的溶胶-凝胶法,在以水为主要溶剂的反应体系中,控制钛酸四丁酯充分水解、缓慢聚合,在 常压、低温(70℃)的温和条件下制备出稳定的TiO₂纳米晶溶胶,并利用TiO₂纳米晶溶胶在膨润土表面负载,获得TiO₂/膨润土复合光催化材料。采用 X 射线衍射、扫描电镜、比表面积测定等研究手段对样品的结构形貌进行了表征,并考察了其光催化活性。结果表明：较高的水用量有利于TiO₂晶体形成,当去离子水:钛酸四丁酯摩尔比大于167:1时,在溶胶体系中出现了锐钛矿型TiO₂纳米晶体;TiO₂纳米晶主要负载于膨润土表面,并未嵌入到膨润土层间结构,但相对于单一膨润土,TiO₂负载显著提高了材料比表面积;当去离子水:钛酸四丁酯摩尔比=192:1时,在紫外光照射下,复合光催化材料表现出最高的光催化活性,对亚甲基蓝的降解率达到93.8%。     

钛基表面TiO_2-HA生物陶瓷膜层的血液相容性研究

采用微弧氧化法及微弧氧化-水热法对纯钛进行改性,制备了TiO_2与TiO_2-HA生物陶瓷膜层,通过溶血率实验、动态凝血时间实验和血小板黏附实验等方面评价其血液相容性。结果表明:各试样的溶血率都远小于5%,均符合医用材料的溶血率要求,不会产生溶血作用。与钛基TiO_2生物陶瓷膜层和钛基材相比,钛基TiO_2-HA生物陶瓷膜层的溶血率更低,动态凝血时间曲线变化更为缓慢,黏附的血小板更少,且变形程度更轻,具有更好的抑制血小板的聚集与变形的性能,血液相容性更优。     

基于溶胶-凝胶法制备PFTMS-SiO2有机-无机杂化超双疏涂层

针对超双疏涂层在建筑室内、外功能性家装领域的应用问题,采用溶胶-凝胶法,以正硅酸乙酯（TEOS）和甲基三甲氧基硅烷（MTMS）为共前驱体,结合1H,1H,2H,2H-全氟辛基三甲氧基硅烷（PFTMS）改性制备了PFTMS-SiO₂有机-无机杂化超双疏涂覆液。当改性剂与正硅酸乙酯摩尔比达到0.3时,该涂覆液在玻璃、瓷砖、石材、水泥板表面形成的涂层均呈现超双疏特性,最高水接触角157.4°,正十六烷接触角142.6°,涂层附着力达到1级,耐洗刷试验1600次后仍保持良好超双疏性能。XPS结果表明,经过改性处理后的SiO₂链段上含氟基团相对含量达到了69.53%,在玻璃表面形成的超双疏涂层含氟基团相对含量为33.75%,涂层表面的平均粗糙度达到937 nm,说明了含氟基团的成功引入及表面凹凸不平的微观形貌共同构成了涂层表面的超疏性能。