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Ceramic thermal barrier coatings (TBCs) are potential tools to increase the durability of metal parts operating in turbine engines. In this work, partially yttria stabilized zirconia (YSZ) and lanthanum zirconate (LZ) coatings were deposited by plasma spraying on stainless steel substrates. X-Ray Diffraction (XRD) analysis revealed that plasma spraying promoted the formation of metastable phases in both the cases. Both YSZ and LZ coatings exhibited porous microstructure with a network of embedded pores and microcracks, that resulted in similar porosity values. The average microhardness of LZ coating was about 92% of that of YSZ one (5.4 against 5.9 GPa), whereas the thermal expansion coefficient (CTE) of LZ coating was about 86% of that of YSZ one. Based on the results herein discussed the development of multilayered microstructures represents a promising solution in order to develop TBCs with enhanced performance. © 2015 Elsevier Ltd and Techna Group S.r.l.

Alumina-based coatings are employed in many industrial applications, in order to protect the surface of metal components against high temperature, wear, corrosion and erosion. In this work two different alumina-based coatings were fabricated by atmospheric plasma spraying (APS), starting from powder particles composed of pure Al2O3 and Al2O3-3 wt% TiO2, respectively. Their phase composition was investigated by X-Ray Diffraction (XRD) and revealed that both the as-sprayed coatings were mainly composed of metastable γ- and α-Al2O3 phases. The γ phase recrystallized to α phase after heat treatment. The porous microstructure was analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). Thin TiO2-rich splats were observed within the microstructure of alumina-titania coatings. The pure alumina coatings exhibited similar porosity and higher microhardness than the alumina-titania ones (12.8 against 9.9 GPa). Both the coatings herein analyzed are particularly promising for high-temperature anti-wear applications, because of their enhanced mechanical properties. © 2014 Elsevier Ltd and Techna Group S.r.l.

MCrAlY coatings are usually adopted to improve the environmental resistance of Ni-based superalloy components of turbine engines against high-temperature oxidation and hot corrosion. In this work, CoNiCrAlYRe coatings were produced by atmospheric plasma spraying. The coatings exhibited relatively low oxygen content and porosity. X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy analyses revealed that the high-temperature exposure at 1383ﾠK (1110ﾠﾰC) promoted the growth of an oxide scale on the top surface being composed of a continuous and dense Al2O3 inner layer followed by an outer mixed layer (Cr2O3 and spinels). The oxide scale was mainly composed of Al2O3, while the formation of mixed oxides occurred at lesser extent. After high-temperature exposure, the formation of internal oxides in some areas reduced the inter-lamellar cohesion, so that a decrease in microhardness was found.

Rare-earth zirconates are potential materials for thermal barrier coatings. Their properties are not still well known, due to any lacks in processing and characterization techniques. To this purpose lanthanum zirconate coatings were herein manufactured by plasma spraying. The coatings exhibited high porosity, due to the presence of pores, splat boundaries and microcracks. The high-temperature evolution of mechanical properties was investigated by arranging specific bending tests up to 1500 ﾰC using SiC testing assembly which allows to reconstruct and measure the sample curvature and the deformation. At higher temperatures the coatings showed an inelastic behavior, related to their unique microstructure, as well as a toughening effect. ﾩ 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Nanostructured yttria stabilized zirconia (YSZ) coatings were deposited by Atmospheric Plasma Spraying (APS). X-ray diffraction (XRD) was used to investigate their phase composition, while scanning electron microscopy (SEM) was employed to examine their microstructure. The coatings showed a unique and complex microstructure composed of well-melted splats with columnar crystal structure, partially melted areas, which resembled the morphology of the powder feedstock, and equiaxed grains. Vickers microhardness of nanostructured zirconia coatings was similar to that of the conventional ones and strongly depended on the indentation load. Otherwise, a higher thermal shock resistance was found. This effect was addressed to the retention of nanostructured areas in coating microstructure and to the corresponding high porosity.

Yttria partially stabilized zirconia (YSZ) thick thermal barrier coatings were fabricated by Atmospheric Plasma Spraying (APS) and isothermally annealed at 1315 °C for different durations. The phase composition of as-sprayed and heat-treated free-standing coatings was investigated by X-ray Diffraction (XRD) and the Rietveld method was employed for quantitative phase analysis. High-temperature exposure of YSZ coatings produced the partial decomposition of metastable t ′ zirconia phase and the corresponding increase in the amount of stable tetragonal t , cubic c and monoclinic m phases with increasing the aging time. The thermophysical properties of as-sprayed and annealed YSZ coatings, such as thermal expansion and heat capacity, were measured. The thermal expansion coefficient kept almost constant in-plane direction after heat treatment. Otherwise, it changed in through-thickness direction due to any structural changes and high-temperature sintering of the porous microstructure. The sintering also influenced the specific heat capacity C p which increased with increasing the annealing time.

Thick mullite (3Al 2 O 3 –2SiO 2 ) coatings were fabricated by atmospheric plasma spraying (APS) in a mixture of crystalline and amorphous phases, as confirmed by X-ray diffraction (XRD) analysis. The coatings were isothermally heat treated in order to study recrystallization mechanism of the glassy phase. The morphology and the microstructure of both mullite feedstock and coatings were investigated by using scansion electron microscopy (SEM). The porosity of as-sprayed coating was in the range between 2 and 3% and substantially remained unchanged after thermal treatment. The thermal expansion of as-sprayed and annealed coatings was measured during heating up to the temperature of crystallization and the corresponding high-temperature extent of shrinkage was calculated. The differential scanning calorimetry (DSC) curves at different heating rates showed a sharp exothermic peak between 1243 and 1253 K, suggesting a rapid recrystallization of the amorphous phase. Finally, the heat capacity of recrystallized mullite coating was measured by DSC experiments. It was approximately 1.02 × 10 3 J/kg K at 373 K and increased with increasing test temperature.

Thick plasma sprayed thermal barrier coatings are suitable for thermal and hot corrosion protection of metal components in land-based turbine and diesel engines. In this work, ceria–yttria co-stabilized zirconia coatings were deposited by atmospheric plasma spraying in a mixture of non-transformable tetragonal t′ and cubic c zirconia phases. Free-standing coatings were isothermally annealed at 1315 °C for different times and their crystal structure was studied by XRD. No phase decomposition occurred. Columnar grains grew in the molten splats with increasing annealing time according to a preferential direction and, after 50 h of heat treatment, they were partially replaced by equiaxed grains. Both in-plane and out-of-plane thermal expansion coefficients (CTEs) were measured from coating expansion during heating. The CTE was slightly sensitive to thermal exposure in out-of-plane direction, whereas it kept almost constant in plane direction. The specific heat capacity Cp of annealed coatings, measured by differential scanning calorimetry (DSC), decreased in comparison with as-sprayed coating, due to high-temperature sintering.

Ceramic thermal barrier coatings (TBCs) are advanced systems for more efficient turbine engines. In this work, thick lanthanum zirconate TBCs were fabricated by Air Plasma Spraying (APS). The coatings showed a lamellar microstructure containing splat boundaries, pores and microcracks. After thermal exposure at 1200 and 1350. °C the partial sintering of the porous microstructure occurred.High-temperature evolution of the mechanical properties was investigated by arranging specific bending tests up to 1500. °C using SiC testing assembly which allowed to calculate the strain by means of curvature measurement. The mechanical properties improved with increasing the testing temperature, due to the inelastic deformation and stress relaxation which counteracted the sintering effects. The elastic modulus increased after thermal aging, but decreased for the aged coatings tested at higher temperature. The thermal expansion coefficient of as-sprayed coatings slightly increased after thermal aging, while a reduction of specific heat capacity was noticed. © 2014 Elsevier B.V.

Homogeneous, erbium-modified, non-porous silica glasses containing about 2.4% of ions compared with Si atoms have been prepared by sol–gel process. To limit the loss of the rare earth cations in the liquids, improve the uniformity of ion incorporation and to immobilize the cations in the silica matrix, an amino functionalised silane was used. It was found to be very effective also to allow foamless sintering. Structural evolution and chemical composition of the samples during the preparation process have been monitored by Fourier-Transform Infrared and Near-IR dispersion spectroscopy. The loss of the cations in the syneresis liquid and the surface area of the samples heat treated at different temperature have been evaluated. The erbium transition 4I15/2 versus 4I13/2, related electro-luminescence at 1.54 μm, was evident in gels heat treated at 1000 °C.

Bulk borosilicate glasses were prepared by the sol−gel process and doped with gold nanoparticles. First, a sol of the host glass (with nominal molar composition of 10% B2O3−90% SiO2) was prepared, and then gold was introduced into the sol by means of a solution of a Au3+ complex. The resulting gels were dried at 60 °C and then heated in air at various temperatures up to 600 °C to nucleate the gold particles and sinter the glass matrix. Various gold ligands were tested, and thiourea was found to be the most effective ligand in preventing fast gold reduction and precipitation to the gel surface during drying. A model based on the formation of insoluble gold species during drying of the gel is discussed to explain the phenomenon of gold precipitation to the sample surface. The heated gels were studied by measurements of the BET specific surface area and of the apparent density, showing the densification of the matrix during the heat treatments. UV−vis optical absorption spectroscopy was used to monitor the f...