Studies on broad spectrum corrosion resistant oxide coatings

The corrosion resistant oxide coatings, developed and applied by the conventional vitreous enamelling techniques, showed superior resistance to a range of mineral acids at various strengths and temperatures, alkaline solutions, boiling water and chrome plating solutions. These coatings possess considerable abrasion and impact resistance as well as high thermal shock resistance. The properties of the coating system have been studied in detail and found to be strongly dependent on composition and processing parameters. These coatings have been characterized by X-ray diffraction analysis and SEM studies. Some of the coating materials have been found to be biocompatible.     

Effects of nano-Fe2O3 powder on thermal emission property of microarc oxidation coating on titanium alloy

Abstract

A ceramic coating was formed on the titanium alloy by microarc oxidation in an electrolyte containing nano-Fe₂O₃, emulsifier OP-10 and sodium phosphate. The composition, surface and cross-sectional morphology and the element compositions of the coatings were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis system. The spectral emissivity of the coatings was measured by a Fourier transform spectrometer apparatus. The bonding strength between the coating and the titanium alloy was studied by tensile strength test. The thermal shock resistance of the coatings was also evaluated. The results showed that nano-Fe₂O₃ was incorporated into the coating, and the coating had high emission at the wavelength range of 3–20 μm. The bonding strength was 33·2 MPa, and after being subjected to severe thermal shocking for 50 cycles, little peeling-off of the coating occurred.     

Application of a UV curable hard coating containing κ-aluminum oxide particles on poly (vinyl chloride) plastic tiles

In this study, ultraviolet (UV) curable hard coatings, containing various κ-aluminum oxide (κ-Al₂O₃) contents, have been used to improve the abrasion resistance of poly (vinyl chloride) (PVC) plastic tiles. This developed UV curable hard coating can form a protection film on the surface of the PVC plastic tile. Thus, the abrasion resistance of the PVC plastic tile can be improved. After coated by the UV curable hard coating containing 10 wt% κ-Al₂O₃, the abrasion resistance of the PVC plastic tile can be improved up to 16.6%. Besides, the transparent UV curable hard coatings showed excellent adhesion on the PVC plastic tiles. Scanning electron microscopy (SEM) images showed that most of κ-Al₂O₃ particles were well dispersed in the UV curable hard coating film.     

Preparation of YSZ/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite coatings via electrophoretic deposition of nanopowders

Using electrophoretic deposition (EPD), we have produced YSZ individual ceramic coatings and YSZ/Al₂O₃ composite coatings for a wide range of applications in modern materials research. YSZ and Al₂O₃ nanopowders were prepared by high-energy physical dispersion techniques, namely, by a laser evaporation–condensation process and electroexplosion of wire, respectively. Stable nonaqueous suspensions for the EPD process have been prepared using YSZ and Al₂O₃ nanopowders with an average particle size of 11 and 22 nm, respectively. The YSZ/Al₂O₃ composite coating produced by sintering at 1200°C has been shown to have higher density in comparison with the YSZ individual coating produced at the same temperature. X-ray diffraction characterization showed that the YSZ/Al₂O₃ composite coating consisted of two crystalline phases: α-Al₂O₃ (corundum) (42 wt %) and cubic ZrO₂〈Y₂O₃〉 (58 wt %). Quantitative analysis of electron micrographs of the surface of the films showed that the YSZ individual coating produced by sintering at 1200°C had a loose structure and contained pores (9%), as distinct from the composite coating, which had a dense, porefree grain structure.     

Effect of laser glazing on the thermo-mechanical properties of plasma-sprayed LaTi2Al9O19 thermal barrier coatings

Conventional duplex (DL) and functionally graded (FG) LaTi₂Al₉O₁₉ (LTA) coatings were deposited over C263 nickel alloy by air plasma spray (APS) and compared with subsequent laser glazing processes. The effect of laser glazing on adhesion strength and thermal barrier performance was investigated. The thermal barrier effect was measured using the temperature difference technique involving infrared (IR) rapid heater and the adhesion strength was measured using the scratch tester. The surface morphology and microstructure were analyzed by optical microscopy (OM), Scanning Electron Microscope (SEM) and 3D profilometer. Based on the experimental results, the laser glazing showed a remarkable temperature drop after IR rapid heating. The changes in porosity and grain refinement make more contributions to the temperature drop of the laser-glazed coatings than that of as-sprayed coatings. The temperature drop is about 110°C for laser-glazed LTA FG coating after 100?s of IR flash, while the drop in DL as-sprayed coating is 60°C compared to the base material.     

Improvement of adhesion of plasma-sprayed Al2O3 coatings by using dry-ice blasting

Dry-ice blasting, as an environment-friendly method, was introduced for the first time into atmospheric plasma spraying for improving properties of Al₂O₃ coatings. The tensile adhesion of the coating was examined. The microstructure of the coating was characterized using scanning electron microscopy. The temperature evolutions during the spraying were measured using an infrared pyrometer measurement system. The adhesive strength of Al₂O₃ coating deposited with dry-ice blasting exceeded 60 MPa, which was nearly increased by 30% compared with that of the coating deposited with conventional air cooling. The comparison of adhesions and microstructures of Al₂O₃ coatings plasma-sprayed with dry-ice blasting and with air cooling revealed that dry-ice blasting can optimize the coated substrate besides a cooling effect, and consequently resulted in the improved adhesion of plasma-sprayed Al₂O₃ coatings.     

Corrosion behavior of plasma sprayed ceramic and metallic coatings on carbon steel in simulated seawater

Al₂O₃, ZrO₂ and Ni60 coatings were produced on carbon steels by plasma spray. Ni60 was used as the bond coat in all the cases. The microstructure of these coatings was analyzed by scanning electron microscopy (SEM). The corrosion behavior of the plasma spray coated samples as well as uncoated samples was evaluated by open circuit potential (OCP) measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy (EIS) in simulated seawater. The results showed that Ni60 coating protected carbon steels against the corrosion and plasma spraying ceramic powders on metallic coating improved the corrosion resistance of the coatings further. The corrosion resistance of the Al₂O₃ coating was superior to that of the ZrO₂ coating due to the relatively few defects in Al₂O₃ coating.     

Influence of Zn doping on electrical and optical properties of multilayered tin oxide thin films

In this study, the electrical and optical properties of Zn doped tin oxide films prepared using sol-gel spin coating process have been investigated. The SnO² : Zn multi-coating films were deposited at optimum deposition conditions using a hydroalcoholic solution consisting of stannous chloride and zinc chloride. Films with Zn doping levels from 0–10 wt% in solution are developed. The results of electrical measurements indicate that the sheet resistance of the deposited films increases with increasing Zn doping concentration and several superimposed coatings are necessary to reach expected low sheet resistance. Films with three coatings show minimum sheet resistance of 1–479 kΩ/ in the case of undoped SnO₂ and 77 kΩ/ for 5 wt% Zn doped SnO₂ when coated on glass substrate. In the case of single layer SnO₂ film, absorption edge is 3.57 eV and when doped with Zn absorption edge shifts towards lower energies (longer wavelengths). The absorption edge lies in the range of 3.489-3.557 eV depending upon the Zn doping concentration. The direct and indirect transitions and their dependence on dopant concentration and number of coatings are presented.     

Effects of TIG surface treating on microstructural characteristics and mechanical properties of Al2O3–TiB2 coating by APS

Abstract

The pulsed DC tungsten inert gas (TIG) method was employed to post-spray treat an electroconductive Al₂O₃–TiB₂ coating by atmosphere plasma spraying (APS) Al₂O₃–30 wt-%TiB₂ powder. The microstructure and mechanical properties of the coatings before and after treatment were comparatively investigated by scanning electron microscopy, laser scanning confocal microscopy, X-ray diffraction, microhardness tester and block on ring wear tester. It was detected that the treated coating presented a two layer structure consisting of the remelted zone and the sintered zone, which was comprised of TiB₂ and single α-Al₂O₃; surface roughness of the treated coating exhibited a remarkable decrease while microhardness and wear resistance showed a significant increase. These experimental results could be ascribed to the effect of high heat input and discharge plasma during the pulsed DC TIG treating.     

Corrosion of thermal barrier coatings by vanadium and sulfur compounds

Abstract

Hot corrosion studies of two plasma-sprayed coatings, yttria-stabilized zirconia and calcium silicate, were undertaken in order to compare the performance of these materials for use as thermal barrier coatings in high-temperature combustion environments. The coatings were tested in contact with vanadium pentoxide at 1,000°C and, also, under conditions in which they were exposed to sulfur-containing compounds at 900°C or 1,000°C. The samples were subsequently characterized by scanning electron microscopy and X-ray diffraction analysis to identify the effects of these tests on the microstructure and composition. The results indicate that reactions with V₂O₅ lead to a disruptive phase transformation in zirconia that rapidly degrades the coating. For calcium silicate, the reactions with V₂O₅ appear to be more limited and less disruptive so that the coating is much more slowly degraded by the vanadium compounds. Exposure to SO_x and sulfate salts at high temperature caused rapid degradation of the calcium silicate coatings through a reaction involving the formation of CaSO₄. Under similar conditions, the yttria-stabilized zirconia coatings experienced much less attack.     

Alumina coating formed on medical NiTi alloy by micro-arc oxidation

Al₂O₃ coatings were prepared on NiTi alloy by micro-arc oxidation in an aluminate solution. Thin-film X-ray diffraction (TF-XRD) indicated that the coating consisted of only Al₂O₃ crystal phase. Energy dispersive X-ray spectrometer (EDS) showed that there was about 2.53 at.% Ni in the surface layer, which was greatly lower than that of NiTi substrate. Scanning electron microscopy (SEM) showed that the coating exhibited a typical porous surface and excellent adhesive interface between the coating and the substrate. Direct pull-off test showed that the coating had a mean coating-substrate bonding strength of 28 ± 2 MPa. The results of electrochemical impedance spectroscopy (EIS) study and potentiodynamic polarization test indicated that the corrosion resistance of the coated sample was increased by two orders of magnitude compared with uncoated sample.     

Processing and Characterization of Plasma Spray Coatings of Glass Microspheres Premixed with Al2O3 Particles

Various ceramics and metals are being deposited as functional, protective, and near-homogenous coatings on engineering components by exploiting the characteristic properties of plasma medium. Such coatings are known to exhibit improved wear, thermal, and corrosion resistance. Although a lot of studies have been reported on coatings made up of a large number of metals and ceramic particles, hardly any effort is made to coat glass microspheres on metals despite their high hardness. In view of this, the present work was undertaken to study the preparation and characterization of a new class of coatings made up of borosilicate glass microspheres (BGM) premixed with micro-sized aluminum oxide (Al₂O₃) in different proportions. Deposition of these BGM and BGM/Al₂O₃ coatings is carried out at five different levels of torch input power. Coatings are characterized in terms of their thickness, hardness, adhesion strength, and porosity. The coatability of BGM and the BGM/Al₂O₃ mixture on metallic substrates is assessed by evaluating the coating deposition efficiency. This work reveals that the torch input power and the Al₂O₃ content in the feedstock affect the major coating characteristics, and premixing of Al₂O₃ with BGM results in better coating properties.     

Microstructure and wear resistance of electrodeposited RE-Ni-Mo-P-B4C-PTFE composite coating

Abstract

The structure, hardness and wear resistance of RE-Ni-Mo-P-B₄C-PTFE composite coating have been studied by means of X-ray diffraction, scanning electron microscopy, abrasion testing and microhardness testing. The results show that the structure of RE-Ni-Mo-P-B₄C-PTFE composite coating experiences a transformation from amorphous via a mixture to crystalline as the heat treatment temperature is increased. Crystalline particles in the coating become gradually finer with the addition of B₄C, B₄C plus polytetrafluoroethylene (PTFE) and B₄C plus PTFE plus rare earths (RE), in that order. The hardness and wear resistance of the RE-Ni-Mo-P-B₄C-PTFE composite increase with increasing temperature, reaching peak values at 400°C and 300°C, respectively. The wear resistance of the composite coating is greatly superior to that of other traditional coatings.     

Preparation of nanocrystalline ferroelectric BaNb2O6 by citrate gel method

A gel was formed when a aqueous solution of BaCl₂, NbF₅ and citric acid in stoichiometric ratio is heated on a water bath. This gel on decomposition at 600°C yielded the nano crystallites of BaNb₂O₆, as confirmed by X-ray diffraction study (XRD). This is a much lower temperature as compared to that prepared by traditional solid state method (1000°C) as reported for the formation of BaNb₂O₆. Transmission electron microscopic (TEM) investigations revealed that the average particle size is 50 nm for the calcined powders. The room temperature dielectric constant at 1 kHz is found to be 1000. The ferroelectric hysteresis loop parameters of these samples were also studied.     

Formation of alkali resistant PDMS-TiO2-SiO2 hybrid coatings

PDMS-TiO₂-SiO₂ hybrid coatings have been prepared on glass substrates via sol-gel method. The structural evolution of the coatings containing different PDMS amounts was examined by TG-DTA and FTIR. It was found that the hybrid character of coatings can be reserved after heating at 200 °C. The alkali resistance of hybrid coatings in 1 N NaOH solution at 50 °C was investigated by X-ray fluorescence analysis, weight loss and scanning electron microscopy. The results showed that the coating could resist effectively the alkali attack for 24 h. Such alkali resistance depended on the added amount of PDMS and the coating with 20 wt.% PDMS showed a better alkali resistance.     

The microstructure and wear resistance characteristics of electroformed nickel and partially stabilized zirconia composite coatings

Ni-PSZ composite coatings with various PSZ particle content were prepared by the electroforming technique. The microstructure and surface components of the coatings have been examined by optical microscopy, electron microscopy and X-ray photoelectron spectroscopy analysis and the wear properties of the coatings tested on a reciprocating wear test machine. The results show that the PSZ particles are uniformly dispersed in the coatings and thus increase the wear resistance of the coatings by inhibiting plastic deformation of the nickel matrix. The co-deposition of the PSZ particles in the electrolyte is mainly in the form of agglomeration and is accompanied by the incorporation of Ni(OH)₂. When the PSZ content in a coating is higher than a critical value, the wear resistance of the coating could deteriorate because of the decrease in the integrity of the nickel matrix. After heat-treatment at high temperature, Ni(OH)₂ in the coating is turned into Ni₂O₃ and NiO which can wet the PSZ particles and increase the bonding strength between the PSZ and nickel. In addition, the agglomerated PSZ particles are sintered when heat-treated. These are all beneficial to increasing the wear resistance of the coating.     

Role of rare earth oxide coatings on oxidation resistance of chromia-forming alloys

Rare earths (RE) have been used to increase high temperature oxidation resistance of chromia and alumina forming alloys. The RE can be added as elements (or oxides) to the alloys or applied as oxide coatings to the alloy surface. This paper presents the effect of different RE oxide coatings and lanthanum chromite coatings on the high temperature oxidation behavior of Fe20Cr and Fe20Cr4Al alloys. The oxidation resistance of the Fe20Cr alloy increased with increase in ionic radius of the RE element in the coating. The RE oxides decreased chromia growth rate more than alumina growth rate. In extended cyclic oxidation tests that were carried out from peak temperatures of 900 °C, 1,000 °C and 1,100 °C to room temperature at cooling rates of 300 °C/s and 1,000 °C/s, the La₂O₃ coating increased cyclic oxidation resistance of the Fe20Cr alloy significantly more than the Pr₂O₃ coating. The role of RE in increasing overall oxidation resistance of chromia forming alloys is discussed.     

Preparation and tribological properties of polyurethane/α-aluminum oxide hybrid films

This study focused on the preparation and tribological properties of polyurethane/α-aluminum oxide (PU/α-Al₂O₃) hybrid films. PU/α-Al₂O₃ hybrid films containing various nanoscaled α-Al₂O₃ contents were prepared by an effectively mechanical stirring method. The tribological properties of PU/α-Al₂O₃ hybrid films were investigated by a TABER type abrasion tester after 2000 cycles. The results of abrasion tests showed the abrasion resistance of the PU/α-Al₂O₃ hybrid film was increased as the α-Al₂O₃ content was increased. The abrasion resistance of the PU/α-Al₂O₃ hybrid film was significantly improved up to 27.4% by adding 2 wt.% nanoscaled α-Al₂O₃ particles. The surface morphologies of PU/α-Al₂O₃ hybrid films, before and after abrasion tests, were examined by scanning electron microscopy (SEM). For the loading of 2 wt.% α-Al₂O₃ particles, the SEM image of the worn surface of the PU/α-Al₂O₃ hybrid film showed much smoother than those of pure PU film and other PU/α-Al₂O₃ hybrid films.     

Plasma-sprayed wear-resistant ceramic and cermet coating materials

Some wear-resistant ceramic and cermet coatings, such as Al₂O₃, TiO₂, Al₂O₃-TiO₂, Cr₂O₃, Cr₃C₂-NiCr and cobalt-cladded WC, were prepared by the arc plasma spraying technique. In this paper optimized spraying parameters of various coatings are briefly described. Some physical properties of these coatings materials were measured. Wear tracks and damage forms were studied by microscopy and scanning electron microscopy techniques. Wear damage mechanisms of these coatings are discussed together with the relationship between wear damage of the coatings and their microstructure, physical properties and especially their thermal diffusivity. The higher the thermal diffusivity of a coating, the better is its wear resistance. The wear resistance of coatings has no obvious relationship with many other physical properties. The pore size, shape and distribution as well as the crack texture in the coatings have important bearings on their wear resistance. Approaches to improve wear-resistant coatings are presented.     

A coprecipitation technique to prepare Sro.5Bao.5Nb206

An aqueous mixture of ammonium oxalate and ammonium hydroxide was used to coprecipitate barium and strontium ions as oxalates and niobium ions as hydroxide under basic conditions. This precursor on calcining at 750°C yielded Sr_0.5Ba_0.5Nb₂O₆ phase. This is a much lower temperature than that prepared by traditional solid state method (1000°C) as reported for the formation of Sr_0.5Ba_0.5Nb₂0₆ (SBN). Transmission electron microscopic (TEM) investigations revealed that the average particle size was 80 nm for the calcined powders. The room temperature dielectric constant at 1 kHz was found to be 1100. The ferroelectric hysteresis loop parameters of these samples were also studied.