Influence of FeSO4 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation

Ceramic coatings with high emission were fabricated on Ti6Al4V alloy by microarc oxidation (MAO) with additive FeSO₄ into the electrolyte. The microstructure, chemical composition and chemical state of the coatings were determined by SEM, XRD, EDS and XPS, respectively. The bonding strength between the coating and substrate was studied by tensile strength test, together with the thermal shock resistance of the coating. The results showed that Fe content in the coating layer significantly affect its thermal emissivity. The relative content of Fe in the coatings surface increased at first and then decreased with increasing the concentration of FeSO₄ in electrolytes, so does the emissivity of the coatings. The bonding strength became weaker with increasing the concentration of FeSO₄. In addition, the coating remains stable over 40 cycles of thermal shocking. The coating formed at 3 g/L FeSO₄ demonstrates the highest an average spectral emissivity value around of 0.87, and bonding strength higher than 33 MPa.     

Tribological behavior of microarc oxidation coatings formed on titanium alloys against steel in dry and solid lubrication sliding

The coatings mainly composed of nanostructured TiO₂ were deposited on Ti6Al4V alloy by microarc oxidation (MAO). The duplex coatings of microarc oxidation combined with spraying graphite process were fabricated for the antifriction purpose. The tribological properties of unpolished, polished and duplex coating against steel under dry friction conditions were examined. It is found that antifriction property of the polished microarc oxidation coating is superior to that of the unpolished one. The improvement is attributed to the low surface roughness and the nanocrystalline structure of coatings. The duplex coating exhibits best antifriction property, registering a lower and steady friction coefficient of ≈0.12 than that of the polished microarc oxidation coating sliding in the similar condition. The good tribological property is attributed to the specially designed duplex structure, the coating adhering strongly to the substrate and serving as the load-supporting underlayer and the graphite layer on top of it working as solid lubricant.     

Characteristic of microarc oxidized coatings on titanium alloy formed in electrolytes containing chelate complex and nano-HA

Microarc oxidized (MAO) TiO₂-based coatings containing Ca and P on titanium alloy were formed in electrolytes containing nano-hydroxyapatite (nano-HA), calcium and phosphate salts. The effects of HA concentration on the thickness, micropore size and number of the MAO coatings were not pronounced. However, the surfaces of the MAO coatings become rough and the crystallinity of anatase increases with increasing HA concentration. In addition, the Ca and P concentrations on the surfaces of the MAO coatings decrease, since the chelate complex of CaY²⁻ (Y = [₂(OOC)NCH₂CH₂N(COO)₂]⁴⁻) and phosphate ions are hindered to be incorporated into the MAO coatings by HA. In vitro experiments indicate that the apatite-forming abilities of the MAO coatings decrease with increasing HA concentration. Furthermore, with increasing HA concentration, the solubility of Ca and P of the MAO coatings decreases, which could lower the supersaturation of the SBF with respect to apatite near the surfaces of the MAO coatings, further leading to the decreased apatite-forming ability. The results indicate that the HA addition in the electrolytes has an important effect on the structure and in vitro bioactivity of the MAO coatings.     

Fretting wear behaviour of microarc oxidation coatings formed on titanium alloy against steel in unlubrication and oil lubrication

Ceramic coatings were formed on Ti6Al4V alloy surface by microarc oxidation (MAO) in a Na₂SiO₃ system solution. Unlubricated, smear oil and oil bath lubricated fretting tests were performed on MAO coatings against 52100 steel on a fretting wear tester. Microstructural investigation of the worn surfaces was performed and the wear mechanisms were studied. The results show that the coatings are mainly composed of rutile and a small amount of anatase TiO₂, both in nano grain structure. Friction coefficient of microarc oxidation coatings under oil bath lubrication was significantly reduced, favorable stable at 0.15, which indicates that the coatings with oil lubricated lowered the shear and adhesive stresses between contact surfaces, consequently alleviating the possibility of initiation and propagation of cracks in the inner layer of the coating or titanium alloy substrate.     

Influence of Co(CH3COO)2 concentration on thermal emissivity of coatings formed on titanium alloy by micro-arc oxidation

Ceramic thermal protection coatings on Ti6Al4V alloy were achieved by micro-arc oxidation (MAO) in the presence of Co(CH₃COO)₂. The morphology, crystallographic structure and chemical composition of the coating were characterized by various techniques. The thermal emission of the coating was measured by Fourier transform spectrometer apparatus. The bonding strength between the coating and substrate was studied, together with the thermal shock resistance of the coating. The results indicate that the content of Co in the coating layer significantly affects its thermal emissivity. Higher concentration of Co(CH₃COO)₂ in electrolytes leads to more Co ions into the coating, which enhances the emissivity of the coating. All the coatings show bonding strength higher than 10 MPa. In addition, the coating remains stable over 40 cycles of thermal shocking. The coating formed at 4 g/L Co(CH₃COO)₂ displays an average spectral emissivity value more than 0.9 and bonding strength about 10.4 MPa.     

Structure and infrared emissivity of collagen/SiO2 composite

Collagen/SiO₂ composites were prepared in aqueous suspensions. Adsorption behaviors of collagen onto the surfaces of SiO₂ spheres were studied. Structure and thermal properties were measured with FTIR, SEM, TEM, and TGA-DTA. The results showed that the self-aggregation of collagen macromolecules was taken place during the adsorption of collagen on SiO₂ sphere. The morphology of collagen evolved from line to microfibrils with the increase in the concentration of collagen along with the distortion of SiO₂. Interfacial interactions of electrostatic forces and hydrogen bonding between the collagen macromolecule and SiO₂ sphere had a vital effect on the adsorption of collagen. The amount of the collagen adsorption was increased with the increase of the collagen concentration, yet decreased in increased pH value of the solution. It was found that the composites exhibited lower infrared emissivity values in the wavelength ranged from 8 to 14 μm than not only pure collagen but also SiO₂ sphere, and the value of infrared emissivity was related to the adsorption amount of collagen in the composites.     

Chemical treatment of TiO2-based coatings formed by plasma electrolytic oxidation in electrolyte containing nano-HA, calcium salts and phosphates for biomedical applications

TiO₂-based coatings were formed on titanium alloy by plasma electrolytic oxidation (PEO) in an electrolyte containing nano-HA, calcium salts and phosphates. Bioactive surface was formed after chemical treatment (NaOH aqueous solution) of the PEO coating. The surface of the PEO coating was mainly composed of Ti, O, Ca and P showing anatase and rutile; while that of the chemically treated PEO (CT-PEO) coating mainly contains Ti, O, Ca and Na showing anatase, rutile and amorphous phase. And the chemically treated surface exhibits dissolution of P and introduction of Na during the chemical treatment process. The chemical treatment has no effect on the chemical states of Ca and Ti of the PEO coating. In addition, the surface constituents of the CT-PEO coating show a uniform distribution near its surface with increasing depth. When incubated in a simulated body fluid for 7 and 14 days, the PEO coating does not exhibit apatite-forming ability; however, apatite was successfully deposited on the CT-PEO coating after 7 days probably due to the formation of hydroxyl functionalized surface, enhancing the heterogeneous nucleation of apatite. The addition of nano-HA in the electrolyte has effects on the surface character and apatite-forming ability of the PEO coating; however, it has no obvious influence on those of the CT-PEO coatings.     

Interface adhesion properties of functional coatings on titanium alloy formed by microarc oxidation method

Three functional coatings (namely Al-C, Si-P-Al and P-F-Al coating) were fabricated by microarc oxidation method on Ti6Al4V alloy in different aqueous solutions. The microstructure, phase and chemical composition of coatings were investigated using scanning electron microscope, X-ray diffraction and energy dispersive spectroscopy. The interface adhesion failure mode of the coating is revealed by shear, tensile and thermal shock methods. The coatings exhibit high adhesion strength by the quantitative shearing test, registering as 110, 70, and 40 MPa for Al-C, Si-P-Al and P-F-Al coating, respectively. The tensile test of the coated samples shows that microarc oxidation treatment does not significantly deteriorate mechanical properties of substrate titanium alloy. The observations of the coating failure after subjected to the identical tensile elongation of 3.0% are well in agreement with those results of the shear test. The thermal cycle test indicates that all the coatings have good anti-thermal shocking properties.     

Influence of electrolyte temperature on properties and infrared emissivity of MAO ceramic coating on 6061 aluminum alloy

6061 aluminum alloy was treated by MAO at various temperatures of the alkali silicate electrolyte using pulsed bipolar current mode for ten minutes. The surface microstructures and properties were studied using SEM, EDX, and XRD. The infrared emissivities of the MAO ceramic coatings were measured at the 70 °C using FTIR spectrometer. The electrolyte temperature strongly affected all the surface properties. The MAO alumina ceramics prepared in cold electrolytes have volcano-like and accumulated particles microstructures, while those prepared in hot electrolytes were: rougher, thinner and contained grainy spherical hollow bulgy microstructures with more pore density and more sillimanite and cristobalite phases which enhanced the IR emissivity. Also, the increment of sillimanite and cristobalite phases moved the apparent peaks toward longer wavelengths, and broadened the opaque region of the IR spectra. As a result, the increment of electrolyte temperature from 12.3 °C to 90.5 °C increased the average of LWIR emissivity from 80.4% to 94.4%, respectively, for the MAO ceramic coatings.     

Microstructure and corrosion behavior of coated AZ91 alloy by microarc oxidation for biomedical application

Magnesium and its alloy currently are considered as the potential biodegradable implant materials, while the accelerated corrosion rate in intro environment leads to implant failure by losing the mechanical integrity before complete restoration. Dense oxide coatings formed in alkaline silicate electrolyte with and without titania sol addition were fabricated on magnesium alloy using microarc oxidation process. The microstructure, composition and degradation behavior in simulated body fluid (SBF) of the coated specimens were evaluated. It reveals that a small amount of TiO₂ is introduced into the as-deposited coating mainly composed of MgO and Mg₂SiO₄ by the addition of titania sol into based alkaline silicate electrolytic bath. With increasing concentration of titania sol from 0 to 10 vol.%, the coating thickness decreases from 22 to 18 μm. Electrochemical tests show that the E_corr of Mg substrate positively shifted about 300500 mV and i_corr lowers more than 100 times after microarc oxidation. However, the TiO₂ modified coatings formed in electrolyte containing 5 and 10 vol.% titania sol indicate an increasing worse corrosion resistance compared with that of the unmodified coating, which is possibly attributed to the increasing amorphous components caused by TiO₂ involvement. The long term immersing test in SBF is consistent with the electrochemical test, with the coated Mg alloy obviously slowing down the biodegradation rate, meanwhile accompanied by the increasing damage trends in the coatings modified by 5 and 10 vol.% titania sol.     

Growth of TiO2 coating on wood surface using controlled hydrothermal method at low temperatures

This paper examines the growth of anatase TiO₂ coating on a wood surface through the hydrolysis of tetrabutyl orthotitanate (TBOT) in different conditions, using a controlled hydrothermal method at low temperatures. Energy disperse X-ray analysis and Fourier transform infrared spectroscopy analysis confirm that the growth of TiO₂ coating on a wood surface is bonded to hydrocarbon chains. Several reaction factors that influence the morphologies and amount of TiO₂ present on wood surface were also investigated. As observed from the scanning electron microscopy images, the morphology and content of TiO₂ grown on a wood surface could be controlled under appropriate reaction conditions. Approximately 32.6% TiO₂ content on a wood surface could be obtained when specific conditions are applied.     

In situ formation of Al2O3-SiO2-SnO2 composite ceramic coating by microarc oxidation on Al-20%Sn alloy

In situ formation of Al₂O₃-SiO₂-SnO₂ composite ceramic coating on Al-20%Sn alloy was successfully fabricated in aqueous Na₂SiO₃ electrolyte by microarc oxidation technology. The compositions, structure, mechanical and tribological properties of the composite coating were detailed studied by scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction, hardness tester and ball-on-disc friction tester. It is found that the species originating from the Al-20%Sn alloy substrate and the electrolyte solution both participate in reaction and contribute to the composition of the coating, which results in the generated coating firmly adherent to the substrate. The composite ceramic coating can greatly improve the microhardness and tribological property of Al-20%Sn alloy.     

Characteristic and microstructure of the microarc oxidized TiO2-based film containing P before and after chemical- and heat treatment

Chemical- and heat treatment was performed to modify the surface of the microarc oxidized TiO₂-based (TOB) film containing P to produce nano-scale compounds containing Na, Ti and O elements. In the TOB film, anatase and rutile nanocrystals were randomly distributed in P-doped matrix. On the surface of the chemically treated TOB (C-TOB) film, amorphous titanium oxide containing Na shows nano-scale ribbonlike morphology. Na, Ti and O show uniform distribution in the outer layer of the C-TOB film along surface depth. Chemical treatment did not alter the surface roughness of the TOB film obviously; however, it improved its hydrophilic property. Heat treatment has no influence on the chemical states of Ti, Na and O, as well as wetting ability, elemental composition and atomic concentration in the outer layer of the C-TOB film. However, the phase compositions and surface morphology of the C-TOB film after heat treatment are dependent on the heat treatment temperature.     

Evolution of zirconia coating layer on rutile TiO2 surface and the pigmentary property

Zirconia-coated rutile TiO₂ composites were prepared by the chemical liquid deposition method starting from rutile TiO₂ and ZrOCl₂. The amorphous zirconia coating layers were anchored at the TiO₂ surface via Zr-O-Ti bond. The formation of continuous and dense zirconia coating layers was dependent on the pH value of the reaction solution and the mole ratio of ZrOCl₂ to TiO₂. As compared to the naked rutile TiO₂, the water dispersibility, whiteness, brightness, and relative light scattering index of the zirconia-coated rutile TiO₂ composites were increased.     

Microstructure and shear fracture characteristics of porous anodic TiO2 layer before and after hot water treatment

Porous TiO₂ layer was fabricated on the surface of commercially pure titanium using an anodic spark oxidation technique for biomedical application, and subsequent hot water treatment was performed to modify the resultant oxide layer. The microstructure features and shear fracture characteristics of anodic oxide layer before and after water treatment were investigated. Results show that before water treatment, the oxide layer exhibited a porous surface with few nanometer features and consisted of poorly crystallized oxides, and an inner layer containing numerous cavities was observed near the oxide-substrate interface. After water treatment, the crystallinity degree of oxide layer was increased significantly and a nanostructured surface layer was obtained. The shear fracture characteristics of oxide layer were greatly influenced by its microstructure features. Before water treatment, the shear fracture took place primarily within the cavity-containing layer, resulting in a pitted fracture surface on the substrate side. However, after water treatment, the shear fracture occurred mainly along the bottom surface of nanostructured surface layer and the shear strength of oxide layer decreased obviously.     

Fabrication of nitrogen-doped TiO2 layer on titanium substrate

In this paper, macropores TiO₂ layer was fabricated on titanium substrates based on plasma based ion implantation (PBII). In order to increase the photodegradation efficiency of fabricated TiO₂ layer, two approaches are used: (1) preparation of macropores on TiO₂ layer to increase the total photodegradation area and (2) nitrogen doping (N-doping) to increase light absorption efficiency. The fabrication process of the N-doped macropores TiO₂ layer comprises four steps: firstly, helium plasma based ion implantation (He-PBII) is employed to generate He bubbles in substrate; secondly, oxygen plasma based ion implantation (O-PBII) and a followed annealing in air are executed to obtain rutile and anatase mixture TiO₂ phases; thirdly, He bubbles are exposed to the surface via an Ar ion sputter process; lastly, the samples are doped by nitrogen PBII (N-PBII). The photodegradation of Rhodamine B solution under Xe lamp indicates that the TiO₂ layer with surface macropores and N-doping has higher light photocatalysis efficiency.     

Corrosion and wear properties of PEO coatings formed on AM60B alloy in NaAlO2 electrolytes

Coatings with a thickness of 22-32 μm were formed on an AM60B magnesium alloy by plasma electrolytic oxidation (PEO) in electrolytes containing 12.0-24.0 g/l NaAlO₂ and other additives. SEM analyses of the coated samples showed that the coatings were compact with relatively low porosity. X-ray diffraction revealed that the coatings consisted of mainly MgAl₂O₄ and MgO phases. The relative amount of MgAl₂O₄ in the coating increased with increasing NaAlO₂ concentration. The relatively compact and thick coatings provide good corrosion protection for magnesium, as indicated by the results of potentiodynamic polarization tests. In addition, the PEO treatment also significantly improved the wear resistance of the alloy. Pin-on-disk wear tests showed that the PEO treatment reduced the wear volume loss by a factor of 10.     

Formation and structure of sphene/titania composite coatings on titanium formed by a hybrid technique of microarc oxidation and heat-treatment

Sphene/titania composite coatings were fabricated on titanium by a hybrid technique of microarc oxidation (MAO) and heat treatment. The high-applied voltages promote the formation of sphene in the MAO coatings after heat-treatment. Heat treatment could change the surface morphology of the MAO coatings such as roughness, macropores size and density and the thickness of the MAO coatings. Increasing the heat-treatment temperature decreased the atomic concentration ratios of Ti/Si and Ti/Ca of the MAO coatings. The chemical states of Ti⁴⁺, Ca²⁺, Si²⁺ and O²⁻ were observed on all the coatings. Additionally, Ti²⁺ was detected in the MAO and heat-treated MAO coatings at 600 and 700 °C. The heat-treatment has obvious effect on the chemical states of Si, Ti and O elements due to the formation of sphene and oxidation of TiO phase of the MAO coating, but did not affect that of Ca. In the heat-treated MAO coatings at 800 °C (MAO-H8), the titanium surface shows a MAO top layer and oxidized interior layer. A concentration gradient in components in the MAO layer of the MAO-H8 coating was formed.     

Microstructure and anti-oxidation property of CrSi2-SiC coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation, a new type of oxidation protective coating has been produced by a two-step pack cementation technique. XRD and SEM analysis show, the coating obtained by the first step pack cementation was a porous β-SiC structure, and a new phase of CrSi₂ was generated in the porous SiC coating after heat-treatment according to the second step pack cementation process. Oxidation test shows that, the weight loss of the SiC coated C/C is up to 11.26% after 5 h oxidation in air at 1773 K, and the weight loss of the CrSi₂-SiC coated C/C composites is only 4.15% after oxidation in air at 1773 K for 34 h. The oxidation of C/C composites was primarily due to the reaction of C/C matrix and oxygen diffusing through the penetrable cracks in the coating.     

Effect of microarc oxidation coating on fatigue performance of Ti-Al-Zr alloy

Ceramic coatings of different thickness were fabricated on Ti6Al2Zr1Mo1V alloy by microarc oxidation (MAO), and the effect of the coating on fatigue life was evaluated by 810 Material Test System. The microstructure, phase and chemical composition of the coatings were determined by SEM, XRD and EDS techniques. The coating mainly consists of rutile and a small amount of anatase TiO₂. With oxidation time ranging from 10 to 30 min, the coating thickness increases from 13 to 25 μm, while the interface between coating and substrate becomes more zigzag, characterized by increasing overgrowth regions of coating into substrate. Under the same cyclic stress of 750 MPa, the fatigue life decreases from 2.08 × 10⁶ cycles for uncoated specimen to about 3 × 10⁴ cycles for microarc oxidized specimen. Under the cyclic stress, the thicker the coating, the more cracks initiate in the overgrowth regions of coating into substrate near the interface, which are considered as the notch sites of stress concentration to induce the crack initiation, also is the key factor to cause the facture.