Effect of temperature on deposition of LaPO4 coatings produced by ultrasonic-based coating process on steel substrates

High-adhesion LaPO₄ coatings were fabricated on steel substrates at temperatures of 150-400 °C after a 10 min treatment using an ultrasonic-based coating process. The principle underlying this process is the collision of ultrasonically accelerated hard balls with the substrate surface that is covered by loosely adhered LaPO₄ particles. The repeated substrate-to-ball collisions flatten the precoated LaPO₄ particles, bond them together and cold weld them to the substrate. The coating thickness, roughness and structure were found to depend on the substrate temperature. The LaPO₄ coatings produced at temperatures ranging from 150 to 250 °C exhibited a granular and porous structure. The treatment at temperatures higher than 300 °C enabled the production of rather dense coatings.     

Ultrasonic machining of Al2O3/LaPO4 composites

Al₂O₃/LaPO₄ composites of varying compositions were drilled on an ultrasonic machine with low carbon steel tools (solid and hollow), in order to evaluate the response to machining. Vickers hardness for different compositions indicate critical load dependency on LaPO₄ content. Significance of LaPO₄ content on material hardness highlights the critical content for good sinterability. X-ray diffraction was done to study the phase content. Acoustic emission (AE) signals emitted by the work piece during machining was also analyzed. Ultrascan inspection was carried out to check for any internal defects. The data presented in the paper illustrate the significance of LaPO₄ addition on machinability of Al₂O₃/LaPO₄ composites in terms of MRR, AE response and hole geometry and associated defects.     

Characterization and machining of alumina ceramic reinforced with lanthanum phosphate

Increasing demand for materials for severe working environment necessitates the use of ceramics. Fiber/whisker reinforcement in the structure can lead to certain defects such as debonding/delamination. Hence one can resort to particulate reinforcements. Of late attempts have been made to introduce dispersion strengthened/particulate reinforced ceramic composites. Addition of lanthanum phosphate (LaPO₄) and cerium phosphate (CePO₄) to alumina (Al₂O₃) matrix has been attempted. In this study Al₂O₃/LaPO₄ composites containing different LaPO₄ content have been assessed for the significance of LaPO₄ content on structure-property and consequent machinability. Ultrasonic drilling trials have been carried out. The response of the material to the machining environment has been assessed by monitoring the acoustic emission (AE) from the composites and defects induced during machining.     

Phase equilibria in the system La2O3-BaO-P2O5: The partial system LaPO4-Ba2P2O7-Ba(PO3)2

The LaPO₄-Ba₂P₂O₇-Ba(PO₃)₂ portion of the oxide La₂O₃-BaO-P₂O₅ system has been investigated. Important parts of this investigation were the determination of equilibria in the LaPO₄-Ba(PO₃)₂ subsystem and the addition of liquidus data to the partially known LaPO₄-Ba(PO₃)₂-Ba₂P₂O₇ subsystem. These data were combined with known data from the LaPO₄-Ba₂P₂O₇ subsystem and with measurements of the equilibria within the LaPO₄-Ba₃P₄O₁₃ isopleth to determine the nature of the phase equilibria in the quasi-ternary LaPO₄-Ba₂P₂O₇-Ba(PO₃)₂ system.     

Phase equilibria in the system La2O3-BaO-P2O5: The partial system LaPO4-Ba2P2O7-Ba(PO3)2

The LaPO₄-Ba₂P₂O₇-Ba(PO₃)₂ portion of the oxide La₂O₃-BaO-P₂O₅ system has been investigated. Important parts of this investigation were the determination of equilibria in the LaPO₄-Ba(PO₃)₂ subsystem and the addition of liquidus data to the partially known LaPO₄-Ba(PO₃)₂-Ba₂P₂O₇ subsystem. These data were combined with known data from the LaPO₄-Ba₂P₂O₇ subsystem and with measurements of the equilibria within the LaPO₄-Ba₃P₄O₁₃ isopleth to determine the nature of the phase equilibria in the quasi-ternary LaPO₄-Ba₂P₂O₇-Ba(PO₃)₂ system.     

Sol-gel synthesis and photoluminescence of LaPO_4:Eu~(3+) nanorods

Monoclinic LaPO4 nanostructures with uniform rod shape have been successfully synthesized by a simple sol-gel method.The procedure involves formation of homogeneous,transparent,metal-citrate-EDTA gel precursors,followed by calcination to promote thermal decomposition of the gel precursors to yield the LaPO4 nanoparticles.Their morphologies and structures were characterized by XRD,TEM,TG-DSC and HRTEM.The results indicate that single monoclinic phase LaPO4 nanorods are readily obtained at 800 ℃ within 3 h.Furthermore,photoluminescence(PL) characterization of the Eu3+-doped LaPO4 nanocrystals was carried out.The effects of calcination temperatures and Eu3+ doping content on the PL properties were elaborated in detail.Room-temperature photoluminescence(PL) characterization reveals that the optical brightness as well as the intensity ratio of 5D0-7F1 to 5D0-7F2 is highly dependent on the calcination temperature,and the Eu0.05La0.95PO4 nanophosphor shows the relatively promising PL performance with the most intense emission.     

Thermal grooving at the interface between alumina and monazite

Interfaces of Al₂O₃ (sapphire) and LaPO₄ (La-monazite) have been separated by fracture to reveal boundary grooving effects analogous to surface grooves that develop at high temperature on a polycrystalline body wherever a grain boundary intersects the surface. Atomic force microscope measurements are used to form images from matching sides of a separated interface and to compare groove profiles with the solution of Mullins.     

Improvement of the electrochemical performance of LiMn2O4 cathode active material by lithium borosilicate (LBS) surface coating for lithium-ion batteries

The LBS coating on the surface of spinel LiMn₂O₄ powder was carried out using the solid-state method, followed by heating at 425 °C for 5 h in air. The powder X-ray diffraction pattern of the LBS-coated spinel LiMn₂O₄ showed that the LBS coating medium was not incorporated in the spinel bulk structure. The SEM result showed that the LBS coating particles were homogeneously distributed on the surface of the LiMn₂O₄ powder particles. The effect of the lithium borosilicate (LBS) coating on the charge-discharge cycling performance of spinel powder (LiMn₂O₄) was studied in the range of 3.5-4.5 V at 1C. The electrochemical results showed that LBS-coated spinel exhibited a more stable cycle performance than bare spinel. The capacity retention of LBS-coated spinel was more than 93.3% after 70 cycles at room temperature, which was maintained at 71.6% after 70 cycles at 55 °C. The improvement of electrochemical performance may be attributed to suppression of Mn²⁺ dissolution into the electrolyte via the LBS glass layer.     

LaPO4:Eu, LaPO4:Ce, and LaPO4:Ce,Tb nanocrystals: Oleic acid assisted solvothermal synthesis, characterization, and luminescent properties

LaPO₄:Ln³⁺ (Ln = Eu, Ce, Tb) nanocrystals were successfully synthesized via a facile solvothermal process in the presence of oleic acid. The as-prepared crystals were well characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), Fourier transform infrared spectroscopy (FT-IR), optical spectra as well as the kinetic decay times, respectively. In the synthesis process, oleic acid as a surfactant has played a crucial role in confining the growth and size of the LaPO₄:Ln³⁺ phosphors. All the samples are well crystallized and assigned to the monoclinic monazite-type structure of the LaPO₄ phase. The prepared LaPO₄:Ln³⁺ phosphors present a narrow distribution with an average particle size of about 15 nm. Upon excitation by ultraviolet radiation, the LaPO₄:Eu³⁺ phosphors show the characteristic ⁵D₀-⁷F_1-3 emission lines of Eu³⁺, while the LaPO₄:Ce³⁺,Tb³⁺ exhibits the characteristic ⁵D₀-⁷F_3-6 emission lines of Tb³⁺. It is believed that these rare earth ion doped (Eu³⁺ ion or Ce³⁺ and Tb³⁺ ions co-doped) monoclinic monazite-type LaPO₄ nanocrystals could find potential application as future advanced optical materials.     

Development of a Pre-Oxidation Treatment to Improve the Adhesion between Thermal Barrier Coatings and NiCoCrAlY Bond Coatings

High-temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC), and a yttria (7 wt%)-stabilized zirconia (YSZ) thermal barrier coating (TBC), were thermally cycled to failure for seven different controlled pre-oxidation treatments and one commonly employed industrial pre-oxidation treatment to establish the preferred microstructures of the thermally-grown oxide (TGO) on a NiCoCrAlY bond coating after pre-oxidation. It was found that the failure of the coating system occurred along the TGO/BC interface when the TGO attained a critical thickness, except if a NiAl₂O₄ spinel layer developed contiguous to the TBC/TGO interface. Then, the coating system failed at a smaller TGO thickness along the NiAl₂O₄/α-Al₂O₃ interface. The value for the TGO thickness at failure increased for a larger area fraction of Y-rich oxide pegs at the TGO/BC interface after pre-oxidation. A desired slow-growing oxide layer on the BC surface was promoted when the presence of the oxides NiAl₂O₄, θ-Al₂O₃, Y₃Al₅O₁₂ at the TGO surface after pre-oxidation was avoided. The α-Al₂O₃ layer, which developed adjacent to the BC upon thermal cycling, grew at a low rate if the initial oxide at the onset of oxidation consisted of θ-Al₂O₃ instead of α-Al₂O₃. Based on these results a pre-oxidation treatment is proposed for which the lifetime of the entire coating system during service is enhanced.     

Microstructure and corrosion resistance of ceramic coating on carbon steel prepared by plasma electrolytic oxidation

Ceramic coating was prepared on Q235 carbon steel by plasma electrolytic oxidation (PEO). The microstructure of the coating including phase composition, surface and cross-section morphology were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscope (FTIR) and scanning electron microscopy (SEM). The corrosion behavior of the coating was evaluated in 3.5% NaCl solution through electrochemical impedance spectra (EIS), potentiodynamic polarization and open-circuit potential (OCP) techniques. The bonding strength between Q235 carbon steel substrate and the ceramic coating was also tested. The results indicated that PEO coating is a composite coating composed of FeAl₂O₄ and Fe₃O_4. The coating surface is porous and the thickness is about 100 μm. The bonding strength of the coating is about 19 MPa. The corrosion tests showed that the corrosion resistance of Q235 carbon steel could be greatly improved with FeAl₂O₄-Fe₃O₄ composite coating on its surface.     

Electrochemical performance of SiO2-coated LiFePO4 cathode materials for lithium ion battery

The surface of LiFePO₄/C particles was coated with SiO₂ via a sol-gel method, and the electrochemical performance of SiO₂-coated LiFePO₄ cathode materials at room temperature and 55 °C was investigated. Compared with pristine LiFePO₄, the structure of LiFePO₄ with SiO₂ coating had no change, the existence of SiO₂ coating effectively enhanced the cycling capacity, reduced capacity fading at high temperature and alleviated the cell impedance. The SiO₂ coating played a regulatory role for Li-ion inserting the lattice, by increasing the order of lithium ion intercalating the outer lattice of the particle. As a consequence, capacity retention improves significantly.     

Microstructure and mechanical properties of W-C:H coatings deposited by pulsed reactive magnetron sputtering

Reported are results of microstructure, mechanical and tribological properties studies for thin, amorphous hydrogenated carbon based coatings with tungsten content from 4.7 at.% up to 10.3 at.%. Studied coatings have been deposited by pulsed, reactive magnetron sputtering on substrates under planetary rotation. Resulting coatings, characterized by transmission electron microscopy (TEM) also at high resolution (HREM), show multilayer structure consisting of sub-layers of W-C:H type, with alternately high and low tungsten concentration. Thickness and number of sub-layers depend on rotation speed of planetary substrate holder. An average tungsten concentration decreases with increasing partial pressure of reactive gas (C₂H₂) during deposition. More insight into the microstructure of coatings provided HREM analysis showing crystalline precipitations of about 1-2 nm in size as well as tungsten-rich and tungsten-poor W-C:H sub-layers. Raman spectra confirm presence of amorphous, hydrogenated carbon (a-C:H) phase in the coatings. Microhardness of studied coatings depends on tungsten content and increases from 10.7 GPa to 13.7 GPa, for 5.1 at.% and 10.3 at.% of tungsten content, respectively. The highest cracking resistance and best adhesion (L_c2 = 78 N and HF1) has been achieved for coatings containing 4.9 at.% of tungsten and a sub-layer thickness of 5 nm. Tribological processes occurring in the coating-coating contact zone are dominated by graphitization and oxidation of W-C:H coating. Very low friction coefficient (0.04) and low wear rate seems to be an effect gaseous micro-bearing by tribo-generated carbon oxides and methane as well as hydrogen released from the coating. In the W-C:H-steel contact zone a tribo-layer composed of iron and tungsten oxides mixed with graphite-like products is growing at the surface of steel counterpart. This tribo-layer becomes a barrier restricting direct contact of steel with the coating and thus preventing it from further intense wear.     

Improvement of the adhesion of a ceramic coating on a ceramic substrate

The structure and adhesion of an alumina coating on a ceramic substrate with NiCrAlY alloy bond coating was investigated by heating at 1573 and 1673 K in the air. Phases of NiO, NiCrO₃, NiAl₂O₄, αAl₂O₃, and Ni were revealed in a 100 μm thick bond coating on heating at 1573 and 1673 K. A veined structure was also detected in the coating heated at 1573 K. The adhesion strength of the coating was improved and reached approximately 20 MPa on heating at 1573 and 1673 K for 14.4 ks in air although the strength of the as-sprayed coating was only 2 MPa. The improvement of adhesion strength may arise from the formation of NiAl₂O₄ with a spinel structure at the interfaces of the top coating/bond coating/substrate coating system. The adhesion strength of the coating improved on decreasing the bond coating thickness and reached approximately 45 MPa for a 20 μm thick bond coating which was heated at 1673 K. Only NiAl₂O₄ oxide was formed in the bond coating.     

Preparation and hot corrosion behaviour of an Al-gradient NiCoCrAlYSiB coating on a Ni-base superalloy

A gradient NiCoCrAlYSiB coating was prepared on a Ni-base superalloy using arc ion plating (AIP) and subsequent gaseous phase aluminisation techniques. Hot corrosion of normal NiCoCrAlYSiB and the gradient coating in pure Na₂SO₄ and Na₂SO₄/NaCl (75:25, wt./wt.) salts was performed at 900 °C in static air. The corrosion results indicated an enhanced corrosion resistance to both salts for the gradient NiCoCrAlYSiB coating, which the improved performance of it should be attributed to the β aluminide ‘‘pool” at the surface layer. By partially sacrificing Al₂O₃ (i.e. Al), the gradient NiCoCrAlYSiB coating specimen behaved excellently in the two kinds of salts. The grain growth during the gaseous phase aluminisation and the corrosion mechanism, including the role NaCl played in the mixture salt corrosion, are discussed.     

Effects of phosphate pretreatment and hot-humid environmental exposure on static strength of adhesive-bonded magnesium AZ31 sheets

An innovative phosphate–permanganate surface treatment (PPT) was developed to improve the static strength of adhesive-bonded 4 mm thick magnesium AZ31 sheets. The phosphate coating having the chemical composition of 1.43% P, 1.63% F and 0.15% Mn (in mass %) was formed after the treatment with PPT solution which has the formulation of KMnO₄, K₂HPO₄, Na₂SiO₃ and NaF. The combination of additives NaF and Na₂SiO₃ and the pH values in the range of 5–6 for a phosphate–permanganate solution was found to be the key elements for the formation of the phosphate coating. The appearance of the phosphate coating and corrosion resistance to 3.5%NaCl solution was assessed. To study the durability of the coating, the effect of an exposure in a hot-humid environment (96% R.H. at 40 °C) on the static strength of adhesive-bonded magnesium AZ31 was investigated. Test results showed that the phosphate coating improved not only the static strength of bonded magnesium AZ31 joints in an ambient condition but also the durability in a hot-humid environment. These results suggest that PPT surface pretreatment is capable of improving the static strength and thermal durability of adhesive-bonded magnesium AZ31 sheets.     

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.     

Comparison of molybdenizing and NiCrAlY coating on Ti and Ti-6Al-4V

Two surface treatments, molybdenizing and depositing NiCrAlY coating, were applied to improve the microhardness and the oxidation resistance of titanium and Ti-6Al-4V. Coupons were analyzed using optical microscopy (OM), scanning electron microscopy (SEM) with X-ray energy dispersive spectrometer (EDS), and X-ray diffraction (XRD). Vickers hardness and isothermal oxidation tests were carried out to evaluate the effects of these two surface treatments on the microhardness and oxidation resistance of the substrates. The post vacuum heat treatment of the NiCrAlY coating and the molybdenizing parameters were also discussed. It is found that molybdenizing can obviously increase the surface hardness of titanium due to the formation of β, α″, and α′ phases in the diffusion layer. As γ′ phase is formed after vacuum heat treatment, the NiCrAlY coating is effective in improving the surface hardness of Ti-6Al-4V. The NiCrAlY coating can obviously decrease the oxidation rate of Ti-6Al-4V at 700–900°C, which can be attributed to the formation of Al₂O₃ and Cr₂O₃ mixed scale during the oxidation.     

Microstructure and anti-oxidation behavior of laser clad Ni-20Cr coating on molybdenum surface

A Ni-20Cr coating was deposited on a molybdenum substrate by laser cladding. The observation of the microstructure by SEM demonstrates that the coating is free of cracks and pores, and metallurgically bonded to the substrate. XRD and EDS analysis results show that some dilution occurs at the coating/substrate interface and that Mo combines with Ni-20Cr, to form a Ni-Cr-Mo alloy coating with slight oxidation. The oxidation behavior of the coating indicates that the laser clad Ni-20Cr coating can effectively prevent oxidation of molybdenum at 600 °C in air. The oxide scale formed on the coating surface by oxidation in air is composed of NiO, Cr₂O₃ and NiMoO₄.     

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.