Production of LaPO4 coatings using a novel ultrasonically-assisted plating technique

Dense LaPO₄ and LaPO₄-Al₂O₃ coating films were produced on the surface of stainless steel substrates at room temperature and ambient pressure with the aid of a novel ultrasonic-based mechanical coating method, which we call UMCA. The main emphasis was on examining the conditions necessary for the successful coating operation and characterizing the as-deposited coatings for the thickness, uniformity and surface morphology. The experimental results suggested that hardness and thermal conductivity of balls and substrate are key parameters influencing the coating efficiency. The coated samples showed an improved hot corrosion resistant in Na₂SO₄-NaCl molten salts at 950 °C.     

Corrosion behavior of Cr3C2-NiCr vacuum plasma sprayed coatings

The objective of the present work is to determine the influence of the heat treatment on the corrosion resistance of a Cr₃C₂-NiCr coating of 450 μm thickness, deposited by a vacuum plasma spray process (VPS) on a steel substrate. The post-heat treatment of the as-deposited coating was carried out in Ar at 400 °C and 800 °C, respectively. The coatings were characterized by means of an electron probe micro analyzer (EPMA) with wavelength dispersive X-ray spectrometers (WDS). It was found that no significant changes were produced as a consequence of the heat treatment carried out at 400 °C. Therefore, the corrosion experiments were conducted for the substrate, the as-deposited coating and the post-heat treated coating at 800 °C. Potentiodynamic polarization showed that the annealed coating at 800 °C has a better corrosion resistance than the as-deposited coating. The corrosion current density (I_corr) of this coating was approximately 3 and 4 times smaller than that corresponding to the as-deposited coating and steel substrate, respectively. This significant improvement of the corrosion behavior of the post-heat treated coating is mainly due to both the microstructural changes that take place in the coating and the diffusion of Ni into Fe at the coating-substrate interface, which ensures the presence of a metallurgical bond.     

Cyclic oxidation and mechanical behaviour of slurry aluminide coatings for steam turbine components

The excellent steam oxidation resistance of iron aluminide coatings on ferritic steels at 650 °C has been demonstrated both by laboratory tests and field exposure. These coatings are formed by the application of an Al slurry followed by diffusion heat treatment at 700 °C for 10 h. The resulting microstructure is mostly composed of Fe₂Al₅ on top of a much thinner FeAl layer. This coating exhibits perpendicular cracks due to thermal expansion mismatch between coating and substrate. However, these stress relieving cracks do not seem to have an effect on the mechanical properties of the substrate. Cyclic oxidation, creep resistance and TMF testing of these coatings at 650 °C indicate excellent performance.     

Structure of Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy prepared by pack cementation techniques

In order to prepare Al-modified silicide coatings on an Nb-based ultrahigh temperature alloy, both a two-stage pack cementation technique and a co-deposition pack cementation technique were employed. The two-stage process included siliconizing a specimen at 1150 °C for 4 h followed by aluminizing it at 800-1000 °C for 4 h. The coating prepared by pack siliconization was composed of a thick (Nb,X)Si₂ (X represents Ti, Cr and Hf elements) outer layer and a thin (Nb,X)₅Si₃ transitional layer; after the siliconized specimens were aluminized at or above 860 °C, a (Nb,Ti)₃Si₅Al₂ phase developed at the surface of the coating, and furthermore, when aluminizing was carried out at 860 °C, a new (Nb,Ti)₂Al layer formed in the coating between the (Nb,X)₅Si₃ layer and the substrate, but when aluminizing was performed at 900-1000 °C, the new layer formed was (Nb,Ti)Al₃. The co-deposition process was carried out by co-depositing Si and Al on specimens at 1000-1150 °C for 8 h under different pack compositions, and it was found that the structure of co-deposition coatings was more evidently affected by co-deposition temperature than pack composition. An Al-modified silicide coating with an outer layer composed of (Nb,Ti)₃Si₅Al₂, (Nb,X)Si₂ and (Nb,Ti)Al₃ was obtained by co-depositing Si and Al at 1050 °C.     

Surface morphology and structure of Ni-P, Ni-P-ZrO2, Ni-W-P, Ni-W-P-ZrO2 coatings deposited by electroless method

Electroless binary Ni-P and ternary Ni-W-P alloy coatings and electroless composite (Ni-P-ZrO₂ and Ni-P-W-ZrO₂) nickel coatings were deposited. Baths with aminoacetic acid as the complexing agent were used. ICP measurements showed that the P content depending on the type of coating is in a range of 4.7-6.3 wt.% (at pH = 6, t = 75 °C). The tungsten content is around 1-2 wt.%. SEM examinations show that the electroless Ni-P coating has the most fine-grained structure. Grains in the form of microspheroids 20 μm in size are characteristic of the Ni-P-ZrO₂ coating. X-ray diffraction patterns show that for all the obtained coatings peak Ni(111) located around 2θ = 44° is the most intensive. After the coatings are heat treated at 400 °C for 1 h the peak becomes even sharper. The heat treatment results in a nearly double increase in crystallite size. The quaternary coatings' abrasion resistance is determined by the second-phase (ZrO₂) particles present in them.     

High-Temperature Oxidation of an Aluminized NiCr Alloy formed by a Magnetron-Sputtered Al Diffusion Coating

Aluminium diffusion coatings were obtained on Ni–20Cr substrate by sputtering an aluminium film, followed by a two stage diffusion treatment in an argon inert gas atmosphere (first stage at 600°C, second at 900 or 1100°C). Aluminides obtained at 900°C and 1100°C are close to those obtained by pack cementation process with high aluminium activity. These diffusion coatings are able to develop alumina scales during isothermal oxidation at high temperatures, whereas the untreated substrate had a chromia-forming behaviour. The weight gain recorded at 1100°C on coated sample is then smaller than the one of uncoated NiCr at 950°C. Presence of chromium was detected in the diffusion coating and Cr-rich precipitates were observed at the diffusion coating/substrate interface. After oxidation at 900°C and 1100°C, only α-Al₂O₃ was revealed by XRD. An intermediate scale with a “whiskered” morphology could however be observed after 48 hr oxidation at 900°C. After 100 hr of oxidation at 1100°C, the Ni_xAl_y diffusion phases were no longer detectable and the upper part of the oxide scale spalled away during cooling. Large cavities appeared at the initial location of the diffusion coating/substrate interface.     

Cyclic oxidation behavior and thermal barrier effect of YSZ-(Al2O3/YAG) double-layer TBCs prepared by the composite sol-gel method

Novel YSZ (6 wt.% yttria partially stabilized zirconia)-(Al₂O₃/YAG) (alumina-yttrium aluminum garnet, Y₃Al₅O₁₂) double-layer ceramic coatings were fabricated using the composite sol-gel and pressure filtration microwave sintering (PFMS) technologies. The thin Al₂O₃/YAG layer had good adherence with substrate and thick YSZ top layer, which presented the structure of micro-sized YAG particles embedded in nano-sized α-Al₂O₃ film. Cyclic oxidation tests at 1000 °C indicated that they possessed superior properties to resist oxidation of alloy and improve the spallation resistance. The thermal insulation capability tests at 1000 °C and 1100 °C indicate that the 250 μm coating had better thermal barrier effect than that of the 150 μm coating at different cooling gas rates. These beneficial effects should be mainly attributed to that, the oxidation rate of thermal grown oxides (TGO) scale is decreased by the “sealing effect” of α-Al₂O₃, the “reactive element effect”, and the reduced thermal stresses by means of nano/micro composite structure. This double-layer coating can be considered as a promising TBC.     

The performances of TiB2-contained iron-based coatings at high temperature

The TiB₂-contained composite Fe-B-C coatings are deposited by the plasma transferred-arc (PTA) powder surfacing process. The coating's thermal ability, arc ablation resistance and wear resistance at high temperature were analyzed. It is concluded that TiB₂-contained composite Fe-B-C coating having excellent wear resistance at 600 °C and tempering resistance at 900 °C. Furthermore, this coating can effectively resist the arc ablation (120 A arc currents) within 7 s.     

Mechanical properties of SiOx gas barrier coatings on polyester films

This paper reports the impressive mechanical properties of 1 μm thick carbon-containing SiO_x gas barrier coatings, characterised using the uniaxial fragmentation test. Such coatings have been found to act as excellent barriers to water vapour permeation partly because they can be made so thick without stress induced cracking. The impressive mechanical properties are thought to be due in part to the high amount of carbon they contain, which gives them a more organic character, as well as the fact that they are deposited as a succession of thinner layers. The adhesion of the coatings to the polyester film is good in all cases, reflecting a high density of covalent bonding at the interface. Improvement of the mechanical properties of a SiO_x/PET composite can be achieved by altering the substrate. By replacing the PET with a heat-stabilised (HS) PET film, a HS film with an acrylate layer or PEN, it is found that the coating displays improved mechanical properties and adhesive strength (as well as barrier). This is thought to be due to the superior surface thermal and mechanical properties of these substrates. Deposition temperatures are at least 80 °C, which causes molecular motion at the surface of a plain PET film and creates defects in the SiO_x coating as it grows, making it more brittle and permeable to gas flow.     

Effect of the internal stress relaxation during the post-annealing on the photo-induced properties of TiO2 coatings reactively sputtered

Photocatalyst TiO₂ coatings have been reactively sputtered at high pressure on cold glass substrates pre-coated by a SiN_x sodium diffusion barrier. The as-deposited coatings were amorphous and the TiO₂/SiN_x/glass samples were subsequently heated at different temperatures under air. The TiO₂ films crystallise in the anatase structure above temperatures of 250 °C with a [001] preferential orientation. The structural analyses have demonstrated that the crystallites are elongated following the c axis direction, perpendicularly to the surface. No modifications of grain size and texture have been observed over the complete temperature range studied (250-550 °C). However, the lattice parameters evolution shows a decrease of the tensile stress with a rise in annealing temperature. The microstructure is then completely relaxed around 400 °C and finally compressive stress is observed at higher temperature. The study of the photo-induced (photocatalytic and hydrophilic) properties shows an activity maximum at 400 °C. These results suggest that the photo-induced properties would be favoured by a relaxed microstructural state of titanium dioxide.     

Adhesion improvement of poly (vinyl alcohol) coating on silicon substrate

The adhesion of the biocompatible hydrophilic polymer, poly (vinyl alcohol) to a model substrate, silicon, was investigated. Contact angle measurements were used to reveal the effect of various substrate cleaning procedures including sonication and UV-ozone treatment prior to casting a 35 μm coating. Raman microspectrometry and X-ray reflectometry were used to characterise the composition and the thickness of PVA thin films. The use of mechanical abrasion of the substrate followed by a 131 nm primer layer of PVA in combination with vacuum treatment at temperatures higher than the glass transition temperature (T_g = 80 °C) provided the best resistance to delamination as demonstrated by visual observation during prolonged immersion of the coatings in water.     

激光原位合成TiB2-TiC颗粒增强铁基涂层

采用B4C,TiO2,石墨以及铁基粉末为激光熔覆材料,利用激光多道搭接熔覆技术在碳钢基体上制备TiB2-TiC颗粒增强铁基复合涂层．利用XRD,SEM对涂层的相结构和显微组织进行了研究．采用显微硬度计和滑动磨损试验机分别测试了涂层的硬度和耐磨性能．结果表明,激光熔覆过程B4C,TiO2和石墨反应生成了TiB2和TiC颗粒,并均匀分布在基体中．随着激光功率密度增加,涂层中TiC含量减少,甚至出现FeB脆性相．TiB2-TiC颗粒增强的涂层其硬度和耐磨性能优于基材45钢．     

Effects of low temperature thermal treatment on zinc and/or tin plated coatings of AZ91D magnesium alloy

A new method was investigated to obtain composite coatings on the AZ91D magnesium alloy by electrodeposition and low temperature thermal treatment. Zinc and tin were introduced to AZ91D Mg alloy surface by electroplating firstly. And a succedent thermal treatment was carried out at 190 ± 10 °C for 12 h. The surface and cross-section morphologies of the plated coatings with and without thermal treatment were studied by scanning electron microscopy (SEM). And the microstructure was determined by X-ray diffraction (XRD). The results reveal that it was difficult to obtain good adhesion plated Sn coating but easy to get well-adherent plated Zn coating. And the thermal treatment promoted the formation of Mg₂Sn in the plated Sn coating and the recrystallization in the plated Zn coating. The plated double Zn-Sn coating owned good adhesion and uniform surface. Furthermore, when the plated double Zn-Sn coating was treated at 190 ± 10 °C for 12 h, a three-layer structure coating was formed due to the diffusion of tin. The results of the anodic polarization behaviors in 5 wt.% NaCl solution show that the three-layer structure coating could provide better protection for AZ91D substrate than the plated Zn-Sn coating.     

Hot corrosion and oxidation behavior of a novel Pt + Hf-modified γ′-Ni3Al + γ-Ni-based coating

A new type of Pt + Hf-modified γ′-Ni₃Al + γ-Ni-based coating has been developed in which deposition involves Pt electroplating followed by combined aluminizing and hafnizing using a pack cementation process. Cyclic oxidation testing of both Pt + Hf-modified γ′ + γ and Pt-modified β-NiAl coatings at 1150 °C (2102 °F), in air, resulted in the formation of a continuous and adherent α-Al₂O₃ scale; however, the latter developed unwanted surface undulations after thermal cycling. Type I (i.e. 900 °C/1652 °F) and Type II (i.e. 705 °C/1300 °F) hot corrosion behavior of the Pt + Hf-modified γ′ + γ coating were studied and compared to Pt-modified β and γ + β-CoCrAlY coatings. Both types of hot corrosion conditions were simulated by depositing Na₂SO₄ salt on the coated samples and then exposing the samples to a laboratory-based furnace rig. It was found that the Pt + Hf-modified γ′ + γ and Pt-modified β coatings exhibited superior Type II hot corrosion resistance compared to the γ + β-CoCrAlY coating; while the Pt + Hf-modified γ′ + γ and γ + β-CoCrAlY coatings showed improved Type I hot corrosion performance than the Pt-modified β.     

Electroless ternary Ni-W-P alloys containing micron size Al2O3 particles

In the present investigation electroless ternary NiWP-Al₂O₃ composite coatings were prepared using an electroless nickel bath. Second phase alumina particles (1 µm) were used to codeposit in the NiWP matrix. Nanocrystalline ternary NiWP alloys and composite coatings were obtained using an alkaline citrate based bath which was operated at pH 9 and temperature at 88 ± 2 °C. Mild steel was used as a substrate material and deposition was carried out for about 4 h to get a coating thickness of 25 ± 3 µm. Metallographic cross-sections were prepared to find out the coating thickness and also the uniform distribution of the aluminum oxide particles in NiWP matrix. Surface analysis carried out on both the coatings using scanning electron microscope (SEM) showed that particle incorporation in ternary NiWP matrix has increased the nodularity of composite coatings compared to fine nodular NiWP deposits. Elemental analysis of energy dispersive X-ray (EDX) results showed that codeposited P and W elements in plain NiWP deposit were 13 and 1.2 wt.%, respectively. There was a decrease in P content from 13 to 10 wt.% with a marginal variation in the incorporated W (1.01 wt.%) due to the codeposition of aluminum oxide particles in NiWP matrix. X-ray diffraction (XRD) studies carried out on as-plated deposits showed that both the deposits are X-ray amorphous with a grain size of around 3 nm. Phase transformation studies carried out on both the coatings showed that composite coatings exhibited better thermal stability compared to plain NiWP deposits. From the XRD studies it was found that metastable phases such as NiP and Ni₅P₂ present in the composite coatings heat treated at major exothermic peak temperature. Annealed composite coatings at various temperatures revealed higher microhardness values compared to plain NiWP deposits.     

Thermal shock behavior of nanostructured and conventional Al2O3/13 wt%TiO2 coatings fabricated by plasma spraying

The thermal shock behavior of three kinds of Al₂O₃/13 wt%TiO₂ coatings fabricated by plasma spraying was studied in this paper. One kind of those coatings was derived from conventional fused and crushed feedstock powder available commercially; the other two kinds of coatings were derived from nanostructured agglomerated feedstock powders. These two nano coatings possess moderate pores and pre-existing microcracks, they were composed of fused structure and three-dimensional net or skeleton-like structure. For conventional coatings, the pores and pre-existing cracks were bigger, sharp-point and mostly distributed between splats. Thermal shock tests for the three coatings were performed by water quenching method. Testing result showed the two kinds of nano coatings had much higher thermal shock resistance than the conventional coatings. The improved thermal shock resistance for nano coatings could attribute to their improved microstructure and crack propagation mode. The damage evolution and failure mechanism of coatings was quite different at thermal shock temperature of 650 °C and 850 °C, which was explained by a simple model. Different crack propagating modes in nanostructured and conventional coatings during thermal shock tests were due to their different microstructures in these two kinds coatings. The stress state of coating surfaces during the thermal cycles was also discussed in this paper.     

Spinel coatings for UNS 430 stainless steel interconnects

At the usual temperature of solid oxide fuel cell (SOFC) operation, ferritic stainless steels form electrically insulating or poorly conducting oxide scales, which can cause high internal resistance losses and chromium poisoning. In an effort to avoid this problem, we applied conductive copper manganite and cobalt manganite spinel coatings, with nominal composition MnCo₂O₄ and Cu_1.4Mn_1.6O₄, which were deposited on the surface of UNS 430 stainless steel by electroplating and subsequent air annealing. Microstructural evaluation indicated that the spinel layers inhibited outward diffusion of chromium. Moreover, excellent structural and thermal stability were observed after several thermal cycles at 750 °C and for up to 28 days, and the coating layers showed good adhesion to the substrate.     

Effect of heat treatment on the structure and in vitro bioactivity of microarc-oxidized (MAO) titania coatings containing Ca and P ions

Microarc oxidized (MAO) coating containing TiO₂ and amorphous calcium phosphate was formed on Ti6Al4V in an electrolyte containing EDTA-Ca and phosphate. Subsequent heat treatment has significant effects on the structure and in vitro bioactivity of the MAO coating. After heat treatment (400-800 °C), the crystallinity of TiO₂ increases, and micropore numbers of the MAO coating decline. Moreover, Ca₃(PO₄)₂ is formed on the surfaces of the MAO coatings after heat treatment at 700 and 800 °C. The SEM and ICP-OES results indicate that the abilities of apatite-forming and Ca and P releasing of the MAO coating decrease after heat treatment. The apatite-forming ability of the MAO coating is associated with the crystallinities of titanium oxide and calcium phosphate. The MAO coating containing TiO₂ with a low crystallinity and amorphous calcium phosphate facilitates the apatite formation in vitro. In addition, the induced biomimetic apatite by the MAO coating without heat treatment exhibits carbonated structure, controllable crystallinity and pore networks on the nanometer scale.     

Molybdenum disulfide as a lubricant and catalyst in adaptive nanocomposite coatings

Nanocomposite YSZ-Ag-Mo-MoS₂ coatings with different MoS₂ additions (0-100 at.%) were deposited with a hybrid pulsed laser/magnetron sputtering/filtered cathodic arc process. Wear testing was performed from 25 to 700 °C for each of the coatings. Electron microscopy and other characterization techniques were used to examine the surfaces and wear tracks of the coatings and to determine the mechanisms resulting in the measured tribological properties. Adaptive coatings containing 8 at.% MoS₂ demonstrated a friction coefficient of 0.2 throughout the temperature range examined here, compared to 0.4 for YSZ-Ag-Mo with no MoS₂. Characterization of the YSZ-Ag-Mo-8% MoS₂ coating revealed that MoS₂ and silver provided lubrication at temperatures ≤ 300 °C, while silver molybdate phases and MoO₃ were effective lubricants at higher temperatures. Silver molybdate was not observed in the coatings containing 0% MoS₂. The role of sulfur in the formation of silver molybdate is briefly discussed.     

Low temperature growth of thin film coatings for the surface modification of dental prostheses

In this work we present some results about the low temperature, plasma-assisted growth of silicon-oxygen amorphous thin film alloys (a-SiO_x) on different types of dental materials used for the fabrication of dental prostheses. The a-SiO_x films were grown at substrate temperatures lower than 70 °C by a PECVD deposition system using silane (SiH₄) and nitrous oxide (N₂O) as precursor gases. The chemical bonding structure of the films was investigated by Fourier transform infra-red spectroscopy (FTIR), while the morphological characteristics of the dental materials were analyzed before and after the coating deposition by means of high-resolution mechanical profilometry. The surface energy of dental materials was estimated before and after the coating process by contact angle measurements, revealing that the coating produced a considerable change of surface energy in all the tested samples, evidenced by a contact angle reduction from more than 90° to less than 10°. Some tests were also performed to estimate the effect of the coating on the bacterial adhesion properties, revealing that the a-SiO_x coatings show some effectiveness in reducing the bacterial adhesion on the dental materials surface.