Phase stability,thermo-physical properties and thermal cycling behavior of plasma-sprayed CTZ,CTZ/YSZ thermal barrier coatings

ZrO₂ co-stabilized by CeO₂ and TiO₂ with stable, nontransformable tetragonal phase has attracted much attention as a potential material for thermal barrier coatings (TBCs) applied at temperatures >?1200?°C. In this study, ZrO₂ co-stabilized by 15?mol% CeO₂ and 5?mol% TiO₂ (CTZ) and CTZ/YSZ (zirconia stabilized by 7.4?wt% Y₂O₃) double-ceramic-layer TBCs were respectively deposited by atmospheric plasma spraying. The microstructures, phase stability and thermo-physical properties of the CTZ coating were examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric-differential scanning calorimeter (TG-DSC), laser pulses and dilatometry. Results showed that the CTZ coating with single tetragonal phase was more stable than the YSZ coating during isothermal heat-treatment at 1300?°C. The CTZ coating had a lower thermal conductivity than that of YSZ coating, decreasing from 0.89?W?m^?1 K^?1 to 0.76?W?m^?1 K^?1 with increasing temperature from room temperature to 1000?°C. The thermal expansion coefficients were in the range of 8.98?×?10^?6 K^?1 – 9.88 ×10^?6 K^?1. Samples were also thermally cycled at 1000?°C and 1100?°C. Failure of the TBCs was mainly a result of the thermal expansion mismatch between CTZ coating and superallloy substrate, the severe coating sintering and the reduction-oxidation of cerium oxide. The thermal durability of the TBCs at 1000?°C can be effectively enhanced by using a YSZ buffer layer, while the thermal cycling life of CTZ/YSZ double-ceramic-layer TBCs at 1100?°C was still unsatisfying. The thermal shock resistance of the CTZ coating should be improved; otherwise the promising properties of CTZ could not be transferred to a well-functioning coating.     

Evaluation of the phase stability,and mechanical and thermal properties of Ba(Sr1/3Ta2/3)O3 as a potential ceramic material for thermal barrier coatings

Ba(Sr_1/3Ta_2/3)O₃ (BST) ceramic was synthesized by a solid-state reaction method. The phase stability, microstructural evolution, and mechanical and thermal properties of the BST ceramic were investigated and characterized to evaluate the potential application of BST as a top coating material for thermal barrier coatings (TBCs). The results show that BST can maintain a stable hexagonal perovskite structure up to 1600 °C. Anisotropic growth of the grains above 1400 °C was observed. Its low elastic modulus and high fracture toughness suggest a high damage tolerance for the BST ceramic. In addition, the moderate coefficient of thermal expansion and superior heat insulation capability of the BST ceramic provide this ceramic the potential to serve as a top coating material of TBCs at higher temperature.     

Anisotropic mechanical properties and contact damage in air-plasma-sprayed thermal barrier coatings with bond coating nature and thermal exposure condition

The anisotropic mechanical properties and contact damage of air-plasma-sprayed (APS) zirconia-based thermal barrier coatings (TBCs) have been investigated using Vickers and Hertzian indentation tests as functions of the nature of the bond coating and the degree of thermal exposure. The hardness values of the TBC systems are dependent on the applied load at relatively low loads, and became saturated at a load of 30 N, independent of the nature of the bond coating or the degree of exposure. The values of the top coating obtained on the top surface from the Vickers indentation tests were higher than those on the sectional plane, indicating that there is an anisotropic strain behavior due to the microstructure. The regions near to the interface of the top coating and the thermally grown oxide (TGO) layer show higher values after thermal exposure, whereas the values of the APS bond coating increased and the indentation values of the high-velocity oxygen fuel (HVOF) sprayed bond coating slightly decreased after thermal exposure, owing to resintering and element deficiency during thermal exposure, respectively. In contact damage tests, the TBC system with the HVOF bond coating showed less damage than the TBC system with the APS bond coating. The shape of the damage was different between the two systems. After thermal exposure, the damage was reduced in both TBC systems, and the cracking or delamination formed at the regions near to the interface of the top coating and the TGO layer in both TBC systems.     

Microstructure, mechanical properties and thermal shock resistance of plasma sprayed nanostructured zirconia coatings

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.     

Thermal stability and mechanical properties of HVOF/PVD duplex ceramic coatings produced by HVOF and cathodic vacuum arc

Duplex ceramic coatings, consisting of an inner NiCr-Cr₃C₂-based coating and an outmost AlCrN film, were produced on the steel substrate in succession by velocity oxygen-fuel spraying (HVOF) and cathodic vacuum arc methods, and then isochronally annealed at annealing temperatures below 900 °C for 2 h. The thermal stability and mechanical properties of the annealed samples were systematically studied by means of X-ray diffraction, Optical microscope and transmission electron microscope, in association with mechanical property measurements. The results show that the microstructure, phase evolution and mechanical properties of duplex ceramic coatings are significantly dependent on the annealing temperature. Metastable fcc-AlCrN solid solution in AlCrN film first decomposes to rich-Al and rich-Cr domains by spinodal decomposition at 700 °C, leading to a notable increase in hardness due to its smaller grain size and high elastic strain field, and then to equiaxed hcp-AlN and Cr₂N by the nucleation and growth at 900 °C, leading to a notable decrease in hardness due to the recrystallization and the formation of hcp-AlN. Meanwhile, the both decarburization of Cr₃C₂ to Cr₇C₃ occurs at 800 °C, but becomes more intensive at 900 °C, leading to a notable loss in hardness. In addition, the dissolution of Cr₃C₂ produces high density of porosity, which also reduces the hardness. The hardness tests show the following ordering of load-bearing capacity for the duplex ceramic coatings: 700 °C>As-deposited >800 °C>900 °C. Tribological property measurements demonstrate that the wear resistance of the tested duplex ceramic coatings obeys the following ordering: 700 °C>As-deposited >800 °C>900 °C. The improved wear resistance is due to high surface hardness, load-bearing capacity and thermal stability. In addition, the wear mechanisms are shown.     

Phase stability and thermo-physical properties of ZrO2-CeO2-TiO2 ceramics for thermal barrier coatings

The properties of ZrO₂ co-stabilized by CeO₂ and TiO₂ ceramic bulks were investigated for potential thermal barrier coating (TBC) applications. Results showed that the (Ce_0.15Ti_x)Zr_0.85-xO7 (x?=?0.05, 0.10, 0.15) compositions with single tetragonal phase were more stable than the traditional 8YSZ at 1573?K. These compositions also showed a large thermal expansion coefficient (TEC) and a high fracture toughness, which were comparable to those of YSZ. However, the phase stability, fracture toughness and sintering resistance of the CeO₂-TiO₂-ZrO₂ system showed a decline tendency with the increase of TiO₂ content. The TEC of the ceramic bulks decreased with increase of TiO₂ content as well because the crystal energy was enhanced with increasing substitution of Zr⁴⁺ by smaller Ti⁴⁺. The (Ce_0.15Ti_0.05)Zr_0.8O₂ had the best comprehensive properties among the (Ce_0.15Ti_x)Zr_0.85-xO₂ compositions as well as a low thermal conductivity. Therefore, it can be explored as a TBC candidate material for high-temperature applications.     

Thermal stability and oxidation resistance of V-alloyed TiAlN coatings

V-containing nitride coatings recently attract a wide range of research interests owing to their excellent tribological properties. To evaluate their comprehensive properties, a comparative study on the intrinsic thermal stability and oxidation resistance of TiAlN and TiAlVN coatings are conducted here. Ti_0.56Al_0.44N, Ti_0.50Al_0.44V_0.06N, and Ti_0.40Al_0.50V_0.10N coatings, deposited by cathodic arc evaporation, exhibit a single-phase face-centered cubic structure with a hardness of 28.9–29.8 GPa. The V-containing coatings show a pronounced age-hardening upon annealing, which contributes to a hardness increase of 3.7 and 4.8 GPa at 800 °C for Ti_0.50Al_0.44V_0.06N and Ti_0.40Al_0.50V_0.10N, respectively, corresponding to 2.9 GPa for Ti_0.56Al_0.44N. Also, alloying with V retards the formation of wurtzite AlN upon annealing, especially in Ti_0.50Al_0.44V_0.06N, and thus contributes to a higher hardness above 30 GPa even annealing at 1100 °C, while the hardness of Ti_0.56Al_0.44N significantly reduces to 27.8 ± 0.6 GPa. However, alloying with V into TiAlN leads to an earlier formation of rutile TiO₂ and also Ti-rich oxide top-layer on the outside surface instead of dense Al₂O₃, and thus degrades the oxidation resistance. When exposed to air at 700 °C for 10 h, the Ti_0.50Al_0.44V_0.06N and Ti_0.40Al_0.50V_0.10N coatings suffer from a severe oxidation, whereas only a compact oxide scale with a thickness of ~ 80 nm for Ti_0.56Al_0.44N is formed.     

Effect of Ba concentration on phase stability and mechanical and thermal properties of La2Mo2O9

Ba-substituted La₂Mo₂O₉ ((La_1−xBa_x)₂Mo₂O_9−δ, x = 0–0.12) was prepared and the thermal and mechanical properties were evaluated. The thermal expansion coefficients (TECs) were determined from high-temperature X-ray diffraction (XRD) analysis. Phase transition in La₂Mo₂O₉ was suppressed via substitution of Ba for La, as demonstrated by differential scanning calorimetry (DSC) analysis. The mechanical properties, such as the bulk modulus, shear modulus, Young’s modulus, compressibility, and Debye temperature were evaluated from the measured sound velocities. The thermal conductivity was evaluated from the thermal diffusivity, heat capacity, and density in the temperature range from room temperature to 1073 K. The thermal conductivity decreased with increasing Ba content. Theoretical calculations based on the Klemens–Callaway model were performed to analyze the thermal conductivity, and the results suggest that the reduction of the thermal conductivity was mainly attributed to oxygen defects in the anion sublattice of La₂Mo₂O₉.     

Structural,thermal and mechanical properties of aluminum nitride ceramics with CeO2 as a sintering aid

AlN ceramics have been prepared with CeO₂ as a sintering aid at a sintering temperature of 1900 °C. The effect of CeO₂ contents on the microstructure, density, thermal conductivity and hardness was investigated. Addition of CeO₂ exerted a significant effect on the densification of AlN ceramics and hence on the microstructure. Thermal conductivity of AlN ceramics increased with CeO₂ content and was greater than that of Y₂O₃-doped AlN ceramics at a similar sintering temperature. The resulting AlN ceramics with 1.50 wt% of CeO₂ had the highest relative density of 99.94%, thermal conductivity of 156 W m⁻¹ K⁻¹ and hardness of 72.46 kg/mm².     

Effect of nitrogen content on high-temperature stability of hard and optically transparent amorphous Hf-Y-Si-B-C-N coatings

We study the effect of nitrogen content on functional properties, thermal stability and oxidation resistance of hard and optically transparent amorphous Hf-Y-Si-B-C-N coatings prepared by pulsed magnetron sputtering. Ab-initio simulations are performed to link the experimentally obtained properties with the atomic and electronic structures of the fabricated materials. It is shown that the content of N in the material, varied from subsaturation 46 at.% to saturation 51 at.%, is of significant importance for the optimization of thermal stability and tuning the refractive index and extinction coefficient. We identify an optimum N₂ content in the plasma and in turn N content in the coatings which outperform the previously introduced high-temperature material Hf₆Y₂Si₂₉B₁₂C₂N₄₅. The results constitute a progress in the efforts to combine multiple functional properties with exceptional (above 1300 °C) thermal stability and oxidation resistance.     

Structure and thermal stability of mesostructured zirconium oxophosphates

Highly ordered mesoporous zirconium oxophosphates (designated as ZOP) with hexagonal P6₃/mmc and cubic symmetries were firstly prepared by using gemini cationic surfactants as templates. It has been found that the thermal stability was elevated with the structure curvature order: cubic , hexagonal P6₃/mmc and cubic . The ZOP mesoporous materials with cubic and hexagonal P6₃/mmc structures were stable up to 800 °C, which provides a new insight into the structural factors governing self-assembly of thermally stable mesoporous materials and would open up new possibilities of porous materials for advanced applications.     

Morphology,thermal stability,and dynamic mechanical properties of atactic polypropylene/carbon nanotube composites

Nanocomposites based on atactic polypropylene (aPP) and multiwall carbon nanotubes were prepared by melt blending at 80°C with a Barabender mixer. The morphology, thermal stability, and dynamic mechanical properties of the obtained composites were studied subsequently. SEM observations indicate that the nanotubes are well dispersed in the aPP matrix. Each nanotube is covered by a layer of aPP molecules. Thermal stability of the aPP in nitrogen is found to be enhanced significantly by the addition of nanotubes. Peak temperature of the DTG curve for the nanocomposite with 5 wt % nanotube loading shows about 70°C higher than that of pure aPP. Dynamic mechanical properties of aPP are also influenced by nanotubes, as shown by the increase in the storage modulus as well as significantly broadened loss tanδ peak. These effects of nanotubes on the thermal stability and mechanical properties of aPP are explained by the adsorption effect of the aPP molecules on the nanotube surfaces in this study. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1087–1091, 2005     

The effect of alkali metal oxide on the properties of borosilicate fireproof glass: Structure,thermal properties,viscosity, chemical stability

The purpose of the current work was to research the effect of alkali metal oxide on the structure, thermal properties, viscosity and chemical stability in the glass system (R₂O–CaO–B₂O₃–SiO₂) systematically. Because the glass would emulsify when Li₂O was added to the glass batch, this article did not discuss Li₂O. The results showed that when the amount of Na₂O was less than 4 mol.%, there was a higher interconnectivity of borate and silicate sub-networks in glass, as more mixed Si–O–B bonds were present in glass. The glass samples exhibited excellent thermal properties and chemical stabilities. As the amount of Na₂O exceeded 4 mol.%, the interconnectivity of borate and silicate sub-networks was weakened. The thermal properties and chemical stabilities of the glass samples were reduced. The connectivity of the silicate sub-network was weakened slightly as the Na/K ratio varied, and the coefficient of thermal expansion (CTE) of the glass samples gradually increased, and the resistance to thermal shock (RTS) value gradually decreased. Moreover, the viscosity of the glass samples decreased with the ratio of Na/Si and Na/K increased.     

Structure and thermal properties of multilayered Laponite/PEO nanocomposite films

The structures and thermal properties of a series of nanocomposite poly(ethylene oxide)/Laponite films have been investigated by differential calorimetric and thermal analysis and complemented by microscopy and X-ray diffraction experiments. The crystalline structures of the nanocomposite multilayered films can be tuned by controlling the composition, polymer Mw and the water content. We study the concentration, polymer Mw and humidity dependence of polymer crystallinity in selected nanocomposite multilayered films. Results show that the exact sample preparation and history are important in controlling structure and properties and in developing new materials. Complementary microscopy is used to monitor the structural changes.     

Carbon content-dependent microstructures,surface characteristics and thermal stability of mechanical alloying derived SiBCN powders

Three types of SiBCN: carbon-lean, -moderate and -rich powders with the same Si/B/N mole ratio were subjected to high-energy ball milling to yield an amorphous structure. The effects of carbon content on microstructures, solid-state amorphization, surface characteristics and thermal stability of the as-milled powders were studied in detail. Results showed that the increases in carbon content can drive solid-state amorphization accompanied by strain-induced, crystallite refinement-induced and/or chemical composition-induced nucleation of nano-SiC from an amorphous body. The specific surface area increases as carbon content increases. The amorphous networks of Si–C, C–B/C–C, C–N, B–N and C–B–N bonds that compose the amorphous nature, but the species and contents of the chemical bonds are carbon content-dependent. Carbon-moderate powders possess satisfying thermal stability while carbon-rich ones perform the worst. Mechanical alloying derived SiBCN powders have outstanding oxidation resistance below 800 °C; however only carbon-moderate powders show desirable anti-oxidation ability at higher temperatures. Thus, mechanical alloying of SiBCN appears a suitable technique for developing amorphous matrix materials for practical applications.     

Characteristics and thermal cycling behavior of plasma-sprayed Ba(Mg1/3Ta2/3)O3 thermal barrier coatings

Ba(Mg_1/3Ta_2/3)O₃ (BMT) powders were synthesized by the solid state reaction method. BMT thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying (APS). The phase composition and microstructure of the BMT coatings were characterized. The thermal cycling behavior of the BMT coatings was investigated by the water quenching method from 1150 °C to room temperature. The results reveal that BMT powders have an ordered hexagonal perovskite structure, whereas the as-sprayed coating of BMT has a disordered cubic perovskite structure because of the different degree of structural order for different treatment conditions. During thermal cycling testing, the entire spalling of coatings occurred within the BMT coating near the bond coat. This is attributed to the following reasons: (1) the growth of a thermally grown oxides (TGO) layer, which leads to additional stresses in the coatings; (2) the coefficient of thermal expansion mismatch between the BMT coating and bond coat, which develops enormous stress in the coatings; (3) the precipitation of Ba₃Ta₅O₁₅ due to the evaporation of MgO during the spraying process, which changes the continuity of the coatings.     

Elasticity,hardness, and thermal properties of ZrBn (n=1, 2, 12)

Elasticity, hardness, and thermal conductivity of ZrBn (n=1, 2, 12) were studied in this paper by using the first-principles calculations of plane wave ultra-soft pseudo-potential technology based on the density functional theory (DFT). Deep analysis of elasticity and anisotropy was carried out, indicating that ZrB and ZrB₂ are soft (the most anisotropic) and superhard (the least anisotropic) respectively. The transverse wave speed (V_t), longitudinal wave speed (V_l) and the minimum thermal conductivity κ_min (both Clark and Cahill model) were calculated. Our investigations show that ZrB is a good heat insulating material and a good lubricant, ZrB₁₂ can be served as a good high-temperature-resistant material and superconducting material, and ZrB₂ is a good wear-resistant material.     

Thermal,mechanical and electrical properties of hard B4C,BCN, ZrBC and ZrBCN ceramics

We study the thermal, mechanical and electrical properties of B₄C, BCN, ZrBC and ZrBCN ceramics prepared in the form of thin films by magnetron sputtering. We focus on the effect of Zr_x(B₄C)_1−x sputter target composition, the N₂+Ar discharge gas mixture composition, the deposition temperature and the annealing temperature after the deposition. The thermal properties of interest include thermal conductivity (observed in the range 1.3–7.3 W m⁻¹ K⁻¹), heat capacity (0.37–1.6×10³ J kg⁻¹ K⁻¹ or 1.9–4.1×10⁶ Jm⁻³ K⁻¹), thermal effusivity (1.6–4.5×10³ J m⁻² s^−1/2 K⁻¹) and thermal diffusivity (0.38–2.6×10⁻⁶ m² s⁻¹). We discuss the relationships between materials composition, preparation conditions, structure, thermal properties, temperature dependence of the thermal properties and other (mechanical and electrical) properties. We find that the materials structure (amorphous×crystalline hexagonal ZrB₂-like×nanocrystalline cubic ZrN-like), more than the composition, is the crucial factor determining the thermal conductivity and other properties. The results are particularly important for the design of future ceramic materials combining tailored thermal properties, mechanical properties, electrical conductivity and oxidation resistance.     

Thermoelectric properties and thermal stability of metal-doped cuprous sulfide thermoelectrics

Mn doping and S-evaporation are strategies used to improve the thermoelectric properties and thermal stability of cuprous sulfide thermoelectric materials. Cu_1.8S and Mn-alloyed Cu_1.8S powders were prepared via ball milling, and different samples were obtained via current-assisted sintering at different times. It was found that Mn and S-evaporation optimized the carrier concentration and thus improved the figure of merit (ZT) of the samples. The introduction of pore defects induced by S-evaporation also improved the ZT. The maximum ZT of the optimized sample reached 0.89 at 500 °C. Mn in the samples reacted with oxygen to form an oxide film on the surface of the block, which inhibited the kinetic process of Cu_1.8S decomposition and improved the thermal stability of the samples. However, the reaction between Mn and oxygen led to a continuous loss of metal cations in the material, resulting in changes in the thermoelectric properties.     

Improved thermal stability of ferro/piezo-electric properties of Mn-doped Pb(In1/2Nb1/2)O3-PbTiO3 ceramics

Thermal stability of piezo-/ferro-electric properties of ferroelectrics is important for the devices working at elevated temperature. A study on thermal stability of ferroelectrics will be greatly helpful for future applications. In this work, thermal behaviors of electrical properties were studied in Mn-doped Pb(In_1/2Nb_1/2)O₃-PbTiO₃ (PINT) ceramics. The ferroelectric hysteresis loops of Mn-doped samples change anomalously with increasing temperature compared with the virgin sample. Remnant polarization of PINT ceramics with high manganese content (x?≥?0.04) exhibits increase trend as temperature increasing, leading to a negative electrocaloric effect which was reported to be beneficial to improving the cooling efficiency. For Mn-doped PINT ceramics, the reduction rate of coercive field reaches a relatively low value of 3%, indicating outstanding ability of depolarization resistance. PINT ceramics with proper dopant concentration show improved aging resistance and thermal stability of piezoelectric property after annealed. Out-of-plane domain configurations show different features at room temperature and elevated temperatures.