Columnar and DVC-structured thermal barrier coatings deposited by suspension plasma spray: high-temperature stability and their corrosion resistance to the molten salt

High-temperature stability of SPS YSZ coatings with the columnar and deep vertically cracked (DVC) structures and their corrosion resistance to 56 wt% V₂O₅+44 wt% Na₂SO₄ molten salt mixture were investigated. Both the columnar and DVC-structured YSZ coatings were sintered at 1000 °C, but a significant increase in porosity in combination with significant reductions in Vickers’ hardness and Young's modulus were observed at the temperatures from 1200 °C to 1400 °C. The DVC-structured YSZ coating exhibited superior corrosion resistance against the molten salt mixture attack to the columnar-structured one due to its higher density behaving as a sealing protective top layer at 950 °C.     

Liquid flame spray fabrication of WO3-reduced graphene oxide nanocomposites for enhanced O3-sensing performances

WO₃ is one of the inspiring sensing materials that show high response to O₃; an efficient fabrication of WO₃ film with incorporation of complementary additives is essential for enhanced sensitivity. Here we report film deposition by liquid flame spraying, characterization of nanostructured WO₃-reduced graphene oxide (rGO) composites and their gas-sensing activities to O₃. The starting feedstock was prepared from WC_l6 and rGO for pyrolysis synthesis by flame spraying. Nano-porous WO₃-rGO films were successfully fabricated and characterized by transmission electron microscopy, field emission scanning electron microscopy, Raman spectrometry, thermal analyses and X-ray diffraction. Nanosized WO₃ grains exhibited oriented nucleation on rGO flakes whereas rGO retained intact its nano-structural features after spraying. Constrained grain growth of WO₃ of 60–70 nm in size was realized in the rGO-containing films with as compared to ~220 nm in the pure WO₃ film. The WO₃-rGO film sensors showed quicker response to O₃ and faster recovery than rGO-free WO₃ film sensors. Addition of rGO in 1.0 wt% or 3.0 wt% in the films caused a significantly reduced effective working temperature of the film sensors from ~ 250 °C to ~ 150 °C.     

Effect of Ti particle size on mechanical and tribological properties of TiCN coatings prepared by reactive plasma spraying

Titanium carbonitride (TiCN) coatings were successfully fabricated by reactive plasma spraying (RPS) from agglomerated Ti-graphite feedstock. The effect of Ti particle size on the microstructure and phase composition of plasma sprayed TiCN coatings was investigated. The Vickers microhardness of coatings was measured by a Microhardness Test and the corresponding Weibull distribution were also analyzed. In addition, a pin-on-disk tribometer was employed to determine the trobological properties of coatings. Results show that all the coatings consist of TiC_xN_1−x (0 ≤ x ≤1) and minor Ti₂O phases, and the amount of Ti₂O increases with the increase of Ti particle size. The Weibull distribution of Vickers microhardness of all the coatings shows apparent scattering, while the coating sprayed with Ti particle size of 28 µm exhibits a relatively even distribution. Compared with the coating sprayed with Ti particle size of 14 µm or 48 µm, the coating sprayed with Ti particle size of 28 µm exhibits improved mechanical and tribological properties, which are attributed to the high microhardness and strong bonding strength.     

Fabrication of insulated metal substrates with organic ceramic composite films for high thermal conductivity

Insulated metal substrates (IMSs) were fabricated and characterized using an organic ceramic composite as a coating mixture. Organic‐inorganic sol solutions were prepared by a sol‐gel process using TEOS (tetraethylorthosilicate), MTMS (methyltrimethoxysilane) and PhTMS (phenyltrimethoxysilane). Ceramic fillers were composed of aluminum oxide (1 and 4 µm) and silicon nitride. The optimal ratio of ceramic filler in the coating mixture was found to be 70 wt%. A thermal conductivity of 3.16 W/mK and a breakdown voltage of 4 kV with a leakage current of 0.17 mA/cm² were obtained for the 122 µm-thick film. A well-networked microstructure between the sol resin and filler in the organic ceramic composite films enhanced the properties of the IMS, such as thermal conductivity and electric insulation.     

Plasma spray deposition of tri-layer environmental barrier coatings

An air plasma spray process has been used to deposit tri-layer environmental barrier coatings consisting of a silicon bond coat, a mullite inter-diffusion barrier, and a Yb₂SiO₅ top coat on SiC substrates. Solidified droplets in as-deposited Yb₂SiO₅ and mullite layers were discovered to be depleted in silicon. This led to the formation of an Yb₂SiO₅ + Yb₂O₃ two-phase top coat and 2:1 mullite (2Al₂O₃*SiO₂) coat deposited from 3:2 mullite powder. The compositions were consistent with preferential silicon evaporation during transient plasma heating; a consequence of the high vapor pressure of silicon species at plasma temperatures. Annealing at 1300 °C resulted in internal bond coat oxidation of pore and splat surfaces, precipitation of Yb₂O₃ in the top coat, and transformation of 2:1 mullite to 3:2 mullite + Al₂O₃. Mud-cracks were found in the Yb₂SiO₅ layer and in precipitated Al₂O₃ due to the thermal expansion mismatch between these coating phases and the substrate.     

Porosity analysis and oxidation behavior of plasma sprayed YSZ and YSZ/LaPO4 abradable thermal barrier coatings

In this research, the high temperature oxidation behavior, porosity, and microstructure of four abradable thermal barrier coatings (ATBCs) consisting of micro- and nanostructured YSZ, YSZ-10%LaPO₄, and YSZ-20%LaPO₄ coatings produced by atmospheric (APS) method were evaluated. Results show that the volume percentage of porosity in the coatings containing LaPO₄ was higher than the monolithic YSZ sample. It was probably due to less thermal conductivity of LaPO₄ phases. Furthermore, the results showed that the amount of the remaining porosity in the composite coatings was higher than the monolithic YSZ at 1000 °C for 120 h. After 120 h isothermal oxidation, the thickness of thermally growth oxide (TGO) layer in composite coatings was higher than that of YSZ coating due to higher porosity and sintering resistance of composite coatings. Finally, the isothermal oxidation resistance of conventional YSZ and nanostructured YSZ coating was investigated.     

Synthesis and negative thermal expansion properties of solid solutions Yb2−xLaxW3O12 (0 ≤ x ≤ 2)

A new series of rare earth solid solutions Yb_2?xLa_xW₃O₁₂ were successfully synthesized by the solid-state method. Effects of substituted ion lanthanum on the microstructures and thermal expansion properties in the resulting Yb_2?xLa_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analyzer (TGA), field emission scanning electron microscope (FESEM) and thermal mechanical analyzer (TMA). Results indicate that the structural phase transition of the Yb_2?xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phases can form in the composition range of 0 ≤ x < 0.5 with orthorhombic structure and 1.5 < x ≤ 2 with monoclinic one. High lanthanum content leads to a low hygroscopicity of Yb_2?xLa_xW₃O₁₂. Negative thermal coefficients of the Yb_2?xLa_xW₃O₁₂ (0 ≤ x ≤ 2) also vary from ?7.78 × 10^?6 K^?1 to 2.06 × 10^?6 K^?1 with increasing substituted content of lanthanum.     

Protective properties of Al2O3 + TiO2 coating produced by the electrostatic spray deposition method

Mechanical resistance of Al₂O₃ + TiO₂ nanocomposite ceramic coating deposited by electrostatic spray deposition method onto X10CrAlSi18 steel to thermal and slurry tests was investigated. The coating was produced from colloidal suspension of TiO₂ nanoparticles dispersed in 3 wt% solution of Al₂(NO₃)₃, as Al₂O₃ precursor, in ethanol. TiO₂ nanoparticles of two sizes, 15 nm and 32 nm, were used in the experiments. After deposition, coatings were annealed at various temperatures, 300, 1000 and 1200 °C, and next exposed to cyclic thermal and slurry tests. Regardless of annealing temperature and the size of TiO₂ nanoparticles, the outer layer of all coatings was porous. The first five thermal cycles caused a rapid increase of aluminum content of the surface layer to 30–37 wt%, but further increase in the number of thermal cycles did not affect the aluminum content. The oxidation rate of coating-substrate system was lower during the thermal tests than during annealing. The oxidation rate was also lower for smaller TiO₂ particles (15 nm) forming the coating than for the larger ones (32 nm). The protective properties of Al₂O₃ + TiO₂ coating against intense oxidation of substrate were lost at 1200 °C. Slurry tests showed that coatings annealed at 1000 °C had the best slurry resistance, but thermal tests had weakened this slurry resistance, mainly due to decreasing adhesion of the coating.     

Structural modifications of lanthanum silicate oxyapatite exposed to high water pressure

Dense lanthanum silicate oxyapatite samples La_9.33 + x(SiO₄)₆O_2 + 3x/2 were exposed to high water pressure in autoclave, in order to study the effects of oxygen stoichiometry and treatment duration on the protonation of these materials. TG analyses showed that protonic species were successfully introduced into the bulk of the material, especially for La_9.60(SiO₄)₆O_2.4 sample after 84 h of treatment at 550 °C under 40 bar water pressure. It was shown that the mass loss compares well with perovskite materials and increases when protonation time rises (0.66% above 600 °C for 408 h), with a time limit beyond which the microstructure is no more stable.Rietveld refinement and Raman spectroscopy studies confirmed some structural modifications which could be linked to the incorporation of protonic species. It was shown that autoclave treatment induced an increase of the cell volume related to an increase of the a and b lattice parameters (enlargement of the characteristic channels of oxyapatite).     

Eggshells: A novel bio-filler for intumescent flame-retardant coatings

The aim of this study was to develop intumescent flame-retardant coatings that incorporate chicken eggshell (CES) waste as a novel eco-friendly bio-filler. Three flame-retardant additives, namely, ammonium polyphosphate phase II, pentaerythritol and melamine were mixed with flame-retardant fillers and acrylic binder to synthesize the intumescent coatings. The fire performance of the coatings was evaluated in accordance with ‘BS 476: Part 6-Fire Propagation’ and ‘BS 476: Part 7-Surface Spread of Flame’ test standards. It was found that 4 out of 5 of the coated specimens (B, C, D and E) neither showed surface spread of flame nor any afterglow combustion upon fire exposure. The addition of 5.0 wt% and 2.5 wt% eggshell bio-filler into formulations B and E, respectively, improved fire protection due to char formation, with better morphology, height and structure of the protecting shield. The filler compositions of samples D (3.4 wt% TiO₂/3.3 wt% Al(OH)₃/3.3 wt% Mg(OH)₂) and E (2.5 wt% TiO₂/2.5 wt% Al(OH)₃/2.5 wt% Mg(OH)₂/2.5 wt% CES) applied at a thickness of 1.5 ± 0.2 mm achieved the lowest fire propagation index with a value of 4.5 and 5.0, respectively (BS 476 Part 6, Class 0 materials) which indicates excellent fire-stopping properties. The results showed that the coatings were effective in fire protection, with good qualities of water resistance, thermal stability, and adhesion strength. Significantly, coating E (with CES) has proved to be efficient in the protection of plywood against fire.     

Thermal properties of La2O3-doped ZrB2- and HfB2-based ultra-high temperature ceramics

Thermal properties of La₂O₃-doped ZrB₂- and HfB₂-based ultra high temperature ceramics (UHTCs) have been measured at temperatures from room temperature to 2000 °C and compared with SiC-doped ZrB₂- and HfB₂-based UHTCs and monolithic ZrB₂ and HfB₂. Thermal conductivities of La₂O₃-doped UHTCs remain constant around 55–60 W/mK from 1500 °C to 1900 °C while SiC-doped UHTCs showed a trend to decreasing values over this range.     

Effect of milling type on the microstructural and mechanical properties of W-Ni-ZrC-Y2O3 composites

This study reports the effect of milling type on the microstructural, physical and mechanical properties of the W-Ni-ZrC-Y₂O₃ composites. Powder blends having the composition of W-1 wt% Ni-2 wt% ZrC-1 wt% Y₂O₃ were milled at room temperature for 12 h using a Spex™ 8000D Mixer/Mill or cryomilled in the presence of externally circulated liquid nitrogen for 10 min using a Spex™ 6870 Freezer/Mill or sequentially milled at room temperature and cryogenic condition. Then, powders were compacted in a hydraulic press under a uniaxial pressure of 400 MPa and green bodies were sintered at 1400 °C for 1 h under Ar/H₂ atmosphere. Phase and microstructural characterization of the milled powders and sintered samples were performed using X-ray diffractometer (XRD), TOPAS software, scanning electron microscope/energy dispersive spectrometer (SEM/EDS), X-ray fluorescence (XRF) spectrometer and particle size analyzer (PSA). Archimedes density and Vickers microhardness measurements, and sliding wear tests were also conducted on the sintered samples. The results showed that sequential milling enables the lowest average particle size (214.90 nm) and it is effective in inhibiting W grain coarsening during sintering. The cryomilled and sintered composite yielded a lower hardness value (5.80±0.23 GPa) and higher wear volume loss value (149.42 µm³) than that of the sintered sample after room temperature milling (6.66±0.39 GPa; 102.50 µm³). However, the sequentially milled and sintered sample had the highest relative density and microhardness values of 95.09% and 7.16±0.59 GPa and the lowest wear volume loss value of 66.0 µm³.     

Lanthanum carbonate incorporated chitosan microparticles for phosphate collection

Lanthanum carbonate incorporated chitosan microparticles (LCCM) is proposed for collection of phosphate from 0.9% saline and human plasma serum samples. Loading of ～4.1 mg of La₂(CO₃)₃·3H₂O onto 0.1 g of chitosan has been achieved. The optimal glutaraldehyde (cross-linker), acetic acid concentration and lanthanum carbonate to chitosan weight ratio for preparation of LCCM were found to be 84.6–85.7%, 0.3–0.4 mol l^?1, and 1:1 respectively. Moreover, the percent collection of phosphate is higher with LCCM compared to lanthanum carbonate (LC) at all weights in a weight variation study. Under optimal conditions, the LCCM was found to be a better phosphate collector with a percent efficiency of >99% compared to 85% with lanthanum carbonate and less than 4% with glutaraldehyde cross-linked chitosan employing 0.1 g of respective materials.     

Thermal stability of nanostructured 13 wt% Al2O3–8 wt% Y2O3–ZrO2 thermal barrier coatings

Nanostructured 13 wt% Al₂O₃–8 wt% Y₂O₃–ZrO₂ (13AlYSZ) coatings were developed by atmospheric plasma spraying (APS). The phase structure and the morphology of the 13AlYSZ coatings were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). It was found that the as-sprayed coatings mainly consisted of tetragonal zirconia, with the Al element solid solution in ZrO₂. Heat treatment at 1100 °C increased the average grain size of the ZrO₂ phase from 61 to 120 nm and decreased the porosity from 23.8 to 18%. The addition of the nano-Al₂O₃ can effectively inhibit the grain growth of the zirconia phase. The mechanism on inhibiting the grain growth of nanostructured 8 wt% Y₂O₃–ZrO₂ thermal barrier coatings has been discussed in detail.     

Microstructure and oxidation behavior of Si–MoSi2 functionally graded coating on Mo substrate

The Si–MoSi₂ functionally graded coating on Mo substrate consisting of a Si–MoSi₂ layer (2.5 µm), a MoSi₂ layer (18 µm) and a Mo–Mo₅Si₃–Mo₃Si layer (2–4 µm) was prepared by a liquid phase siliconizing method. The siliconized coating has a dense layered structure and no micro-cracks and holes. The Si element mainly enriches on the surface with the highest content of about 50 wt%, and inhibits the formation of Mo₅Si₃ and volatile MoO₃ and improves the high-temperature oxidation resistance of the coating. The mass gain of Si-MoSi₂ coating is only 0.17 wt% after oxidized at 1600 ℃ for 70 h. The Si–MoSi₂ functionally graded coating exhibits better high temperature oxidation resistance than pure MoSi₂ coating.     

Preparation of nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings and their calcium-magnesium-alumina-silicate (CMAS) resistance

Nanostructured 30 mol% LaPO₄ doped Gd₂Zr₂O₇ (Gd₂Zr₂O₇-LaPO₄) thermal barrier coatings (TBCs) were produced by air plasma spraying (APS). The coatings consist of Gd₂Zr₂O₇ and LaPO₄ phases, with desirable chemical composition and obvious nanozones embedded in the coating microstructure. Calcium-magnesium-alumina- silicate (CMAS) corrosion tests were carried out at 1250 °C for 1–8 h to study the corrosion resistance of the coatings. Results indicated that the nanostructured Gd₂Zr₂O₇-LaPO₄ TBCs reveals high resistance to penetration by the CMAS melt. During corrosion tests, an impervious crystalline reaction layer consisting of Gd-La-P apatite, anorthite, spinel and tetragonal ZrO₂ phases forms on the coating surfaces. The layer is stable at high temperatures and has significant effect on preventing further infiltration of the molten CMAS into the coatings. Furthermore, the porous nanozones could gather the penetrated molten CMAS like as an absorbent, which benefits the CMAS resistance of the coatings.     

High performance electrochemical H2O2 sensor based on MWCNT thin films fabricated by novel electron beam evaporation

High quality multi-walled carbon nanotubes (MWCNT) were grown by electron beam evaporation (EBE) under a high vacuum of 10⁻⁶ mtorr. The influence of deposition thickness on the orientation, morphology and vibrational bands of MWCNT films fabricated on tantalum (Ta) substrate was discussed. XRD patterns of the film revealed the presence of (002) preferential plane of carbon. Raman spectral analysis show the G-band Raman feature corresponding to high frequency E₂g of first order mode, suggesting that CNTs were composed of crystalline carbon. SEM image of 200 nm thick MWCNT film shows well shaped homogenous fine nanotubes of length ~300 nm and diameter ~70 nm with high purity. The electrochemical performance of the MWCNTs/Ta electrodes was studied by cyclic voltammetry. The sensor prepared with optimum thickness can detect H₂O₂ in the wide range covering 5 µM to 0.025 mM, with the detection limit as low as 0.09 µM. The results demonstrate that the fabrication of MWCNTs/Ta electrode by EBE is a very interesting and useful approach, likely to be a focus of upcoming research efforts in electrochemical sensing.     

Crystallization processes,compressibility, sinterability and mechanical properties of La-monazite-type ceramics

Lanthanide orthophosphate ceramics with monazite structure gained broad interest for several industrial applications. The crystallization processes, compressibility and sinterability of monazite-type lanthanum orthophosphate powder hydrothermally synthesized at 200 °C as well as mechanical properties of the sintered compacts were investigated. Based on a combination of thermo- and surface area analyses, X-ray diffraction as well as scanning electron microscopy studies it was found that the crystallization process occurs at ∼500 °C and the final crystallization of LaPO₄ monoclinic phase takes place at 1400 °C. The sintered pellets are characterized by a density of 98% of theoretical density, a Vickers hardness of 5.7 ± 0.1 GPa and fracture toughness of 1.4 ± 0.1 MPa m^0.5.     

The effect of La2O3 on the microstructure and room temperature mechanical properties of t-ZrO2

Structural and mechanical properties of La₂O₃ added (up to 5 wt%) t-ZrO₂ compacts were examined with the aim of optimizing hardness and fracture toughness for room temperature applications. Structural examinations were performed using an X-ray diffractometer and a scanning electron microscope. Mechanical properties of the compacts were determined as modulus of elasticity, hardness and fracture toughness by conducting Vickers indentation tests under test loads of 0.5, 1 and 10 kg. Addition of 0.5 wt% La₂O₃ deteriorated the room temperature stability of t-ZrO₂ by forming m-ZrO₂ and coarse polygonal grains in the matrix. Higher concentrations (2 and 5 wt% La₂O₃) caused precipitation of La₂Zr₂O₇ at the grain boundaries, which also accompanied by a reduction in hardness. Fracture toughness increased with increasing La₂O₃ content of the compact. Finally, an optimum combination between hardness and fracture toughness was obtained for the 0.5 wt% La₂O₃ containing compact.     

Effects of B2O3 on microstructure and ionic conductivity of Li6.5La3Zr1.5Nb0.5O12 solid electrolyte

Li_6.5La₃Zr_1.5Nb_0.5O₁₂ (LLZN) garnet-type structure was synthesized at low temperature with B₂O₃ addition by solid state reaction method. The effects of B₂O₃ content on the formation, microstructure, ionic conductivity and activation energy of the LLZN solid electrolytes have been investigated by X-Ray diffraction (XRD), scanning electron microscopy (SEM) and alternate current (AC) impedance spectroscopy. The cubic LLZN phase was obtained after calcining at 850 °C for 6 h and no phase evolution was observed after sintering at 1100 °C for 6 h. The relative density and lithium ion conductivity increased first and then decreased with increasing B₂O₃ content, reaching the maximum value of 92.4% and 1.86×10⁻⁴ S cm⁻¹ respectively in the sample with 1.4 wt% B₂O₃. By contrast, the activation energy reached a minimum value of ~31.5 kJ mol⁻¹.