Nickel-based superalloy DZ125 was first sprayed with a NiCrAlY bond coat and followed with a nanostructured 2 mol% Gd2O3−4.5 mol% Y2O3-ZrO2 (2GdYSZ) topcoat using air plasma spraying (APS). Hot corrosion behavior of the as-sprayed thermal barrier coatings (TBCs) were investigated in the presence of 50 wt% Na2SO4 + 50 wt% V2O5 as the corrosive molten salt at 900 °C for 100 h. The analysis results indicate that Gd doped YVO4 and m-ZrO2 crystals were formed as corrosion products due to the reaction of the corrosive salts with stabilizers (Y2O3, Gd2O3) of zirconia. Cross-section morphology shows that a thin layer called TGO was formed at the bond coat/topcoat interface. After hot corrosion test, the proportion of m-ZrO2 phase in nanostructured 2GdYSZ coating is lower than that of nano-YSZ coating. The result reveals that nanostructured 2GdYSZ coating exhibits a better hot corrosion resistance than nano-YSZ coating.

To investigate the interdiffusion behavior of Ge-modified silicide coatings on an Nb–Si-based alloy substrate, the coating was oxidized at 1250°C for 5, 10, 20, 50, or 100 h. The interfacial diffusion between the (Nb,X)(Si,Ge)2 (X = Ti, Cr, Hf) coating and the Nb–Si based alloy was also examined. The transitional layer is composed of (Ti,Nb)5(Si,Ge)4 and a small amount of (Nb,X)5(Si,Ge)3. With increasing oxidation time, the thickness of the transitional layer increases because of the diffusion of Si from the outer layer to the substrate, which obeys a parabolic rate law. The parabolic growth rate constant of the transitional layer under oxidation conditions is 2.018 μm·h−1/2. Moreover, the interdiffusion coefficients of Si in the transitional layer were determined from the interdiffusion fluxes calculated directly from experimental concentration profiles.

•Mo–Si–B coating was prepared through depositing Mo followed by Si/B co-deposition.•Microstructural evolution of the coating during the pack cementation is elucidated.•The obtained coating exhibits good oxidation resistance.•The good oxidation resistance is due to the formation of the borosilicate scale.Mo–Si–B was coated on Nb–Si based alloys using d-gun spraying of Mo followed by Si and B co-deposition. Microstructure evolution and the oxidation behavior of the coating were studied. The coating consists of a MoSi2 layer with dispersed MoB particles during the initial co-deposition. A continuous MoB layer forms beneath the MoSi2 layer after a prolonging time. The growth rate of the coating follows a parabolic law. The mass gain of Mo–Si–B coating is only 0.92 mg/cm2 after oxidation at 1250 °C for 100 h. The good oxidation resistance results from a protective borosilicate scale formed on the coating.

•Dy-Co-modified aluminide coating was prepared on the nickel base superalloy.•The microstructure and hot corrosion behavior of the coating were investigated.•Dy-Co-modified aluminide coating exhibits good hot corrosion resistance.•Role of Dy in the improvement of the hot corrosion resistance has been revealed.Dy-Co-modified aluminide coating was deposited onto Ni-based superalloy DZ125 using pack cementation method. The microstructure and hot corrosion behavior of the coating were investigated. The results show that as-deposited coating has a two-layer structure. The outer layer of the coating is composed mainly of Al0.9Ni1.1 with dispersed Dy2Hf2O7. The mass gain of the coating is 0.23 mg/cm2 after exposure to 75 wt.% Na2SO4 + 25 wt.% NaCl at 900 °C for 100 h. The corrosion product formed on the coating is characterized as Al2O3. Role of Dy in the improvement of the hot corrosion resistance has been discussed.After hot corrosion at 900 °C for 100 h, the oxide scale is continuous and compact and no voids and spallation are observed. The mass gain of the coating is 0.23 mg/cm2 which exhibits good hot corrosion resistance.

•A B-modified silicide coating was prepared on the Nb-Si based alloy.•Formation mechanism for the microstructure of the coating has been revealed.•The B-modified silicide coating exhibits good oxidation resistance.•Oxidation-resistant mechanism of the coating has been proposed.A B-modified silicide coating was prepared on the Nb-Si based alloy by Si-TiB2 co-deposition at 1300 °C for 10 h. The results show that the two-layer coating is composed of a (Nb, X)Si2 (X represents Al, Cr, Ti and Hf) + NbTiB4 outer layer and a (Nb, X)Si2 + Cr2Nb inner layer. Static oxidation tests indicate that the mass gain of B-modified silicide coating was 2.39 mg/cm2 after oxidation at 1250 °C for 100 h. The good oxidation resistance is attributed to the formation of a continuous and dense scale mainly consisting of glassy borosilicate.After oxidation, dense borosilicate scale forms on the B-modified silicide coating. The mass gain of the coating is 2.39 mg/cm2 which exhibits good oxidation resistance of the coating.

Three types of carbon nanofiber (CNF): filamentous CNF, chain-like CNF and thick CNF were successfully synthesized from three hydrocarbon precursor gases of ethylene (C2H4), n-hexane (n-C6H14) and n-dodecane (n-C12H26) on iron catalyst at 1073 K. A detailed study implied that the nucleation of iron catalyst in the incubation period varied with the type of the precursor gas, which intensively affected the morphology, structure and growth kinetics of the CNFs. A thermodynamic basis of carbon diffusion in iron particle for the catalyst activity was derived. Various characterizations demonstrated that the typical filamentous CNFs (∼50 nm), developed from C2H4 with relatively large aspect ratios and highly ordered graphene layers, possessed the highest field emission enhancement factor (β). By contrast, the chain-like CNFs (∼80 nm), developed from n-C6H14 with non-sequence graphene layers, and the thick CNFs (∼500 nm), developed from n-C12H26 with numerous micropores and mesopores in the disordered graphene layers, possessed the lower values of β.

The nanostructured 4–8 mol% Gd2O3−4.5 mol% Y2O3-ZrO2 (4–8 mol% GdYSZ) coatings were developed by the atmospheric plasma spraying technique. The microstructure and thermal properties of plasma-sprayed 4–8 mol% GdYSZ coatings were investigated. The experimental results indicate that typical microstructure of the as-sprayed coatings were consisted of melted zones, nano-zones, splats, nano-pores, high-volume spheroidal pores and micro-cracks. The porosity of the 4, 6 and 8 mol% GdYSZ coatings was about 9.3%, 11.7% and 13.3%, respectively. It was observed that the addition of gadolinia to the nano-YSZ could significantly reduce the thermal conductivity of nano-YSZ. The thermal conductivity of GdYSZ decreased with increasing Gd2O3 addition. And the reduction in thermal conductivity is mainly attributed to the addition of Gd2O3, which results in the increase in oxygen vacancies, lattice distortion and porosity.

B–Y modified silicide coatings were prepared on Nb–Si based alloy by pack cementation at 1300 ℃ for 10 h. The effect of Y2O3 content in the pack mixtures on microstructure and oxidation resistance of the coatings was investigated. The results show that the four coatings have similar structures, which possess a (Nb,X)Si2 outer layer and a (Nb,X)5Si3 transitional layer. Y2O3 content in the pack mixtures has an obvious effect on the Si content in the coating. The mass gains of the coatings prepared with 0.5, 1, 2 and 3 wt% Y2O3 in pack mixtures are 2.33, 1.96, 2.05 and 2.86 mg/cm2 after oxidation at 1250 ℃ for 100 h, respectively. The coating prepared with 1 wt% Y2O3 exhibits the best oxidation resistance due to the formation of a dense glass-like borosilicate scale.

The microstructure and hot corrosion behavior of a Si–Y–Co-modified aluminide coating prepared on a nickel base superalloy DZ125 by the pack cementation process was studied. The Co–Al–Si–Y coated specimen had a mass gain of only 0.25 mg/cm2 after being exposed to NaCl + Na2SO4 salt for 100 h at 1173 K. Thus the addition of Si to a Y–Co-modified aluminide coating increased its hot corrosion resistance by 40 %. The improved hot corrosion resistance of the Co–Al–Si–Y coating was mainly attributed to the formation of an Al2O3 scale with SiO2 on the surface of the coating during the hot corrosion test. SiO2 can prohibit the high fluxing rate of dissolution of the protective Al2O3 scale and prevent rapid corrosion attack owing to its low solubility over a wide range of salt acidity and its weak oxygen permeability.

4 wt% CuO–96 wt% TiO2 granules were prepared by a spray drying process. The microstructure and optical property of CuO–TiO2 granules were studied. The results indicate that copper existed in the form of CuO. The spray dried granules possess spherical geometry and smooth surface with grain size in the range of 40–80 μm. CuO–TiO2 has a relatively smaller Eg value (2.85 eV) than TiO2 (3.17 eV). The photocatalytic property of CuO–TiO2 granules was investigated by degradation of a model pollutant (the azo dye methyl orange) under the irradiation of the xenon lamp equipped with a band pass filter of 365 nm. The CuO–TiO2 spray-dried granules degrade about 10% more MO than TiO2 spray-dried granules under UV irradiation within the same time. The XPS spectra suggested that Cu2+ and reduced copper species were coexistent in reacted CuO–TiO2 photocatalyst. The improvement of photocatalytic activity for CuO–TiO2 was mainly attributed to effective separation of photo-generated electron–hole pairs in the presence of CuO.

Pure Al coating was prepared by a detonation gun (D-gun) spraying process to protect sintered NdFeB magnets. The detonation gun sprayed coating is very uniform and has a low porosity of 0.77%. The thickness of the Al coating is approximately 16 μm. The corrosion current density for the coated sample was 1.30 × 10−5 A/cm2 immediately after immersion in 3.5% NaCl solution, compared to 6.54 × 10−5 A/cm2 for the uncoated sample. X-ray photoelectron spectrometry results indicate that the formation of Al2O3 film contributes to the increased corrosion resistance of Al coating. Meanwhile, electrochemical impedance spectroscopy with equivalent electrical circuit was used to ascertain the corrosion process of the Al coatings. Results show the corrosion procedure consists of two stages which agree with the potentiodynamic polarization test. It can be concluded that the Al coating deposited by the D-gun spray process can improve the corrosion resistance of sintered NdFeB.

•A suitable condition for codeposition of Co, Al and Hf was determined.•A Co–Al–Hf coating with high Co and minute Hf content was obtained.•The Co–Al–Hf coating had excellent cyclic oxidation resistance due to improved adhesion.The codeposition of Co, Al and Hf on nickel base superalloys by pack cementation in a single-step process was investigated in this work. Thermochemical analyses were applied to search for suitable conditions including pack composition and deposition temperature. Co, Al, Hf, NH4Cl, NH4I and Al2O3 made up the pack powder mixture. According to a series of thermochemical calculations, the pack powder mixture of 20Co–10Al–2Hf–4NH4Cl–4NH4I–60Al2O3 (wt.%) was adopted. Further experimental results demonstrated that the codeposition of Co, Al and Hf could be achieved practically. The coating consisted of a diffusion zone and an outer layer. The outer layer was mainly composed of Al0.9Ni1.1 where a part of Ni was replaced by Co or Hf. The trace element Hf was enriched in the interface between the outer layer and the diffusion zone. The Co–Al–Hf coating exhibited excellent cyclic oxidation resistance due to improvement in adhesion between the oxide scale and the coating.

A protective Al coating was achieved on the sintered NdFeB magnet by cold spray. The sprayed Al particles generate plastic deformation and hang together. The thickness of the coating is about 170 μm. The corrosion currents of Al coating and NdFeB without immersion tested by potentiodynamic polarization in 3.5 wt.% NaCl solutions are 1.350 × 10−6 and 4.361 × 10−6 A/cm2, respectively. X-ray photoelectron spectrometry results confirm that the oxide film is Al2O3 and the corrosion process can be derived into two different stages. The Al coating can provide long-term protection for NdFeB effectively.

In order to identify suitable halide activators and pack compositions for codepositing Cr and Si to form diffusion coatings on Nb-base in situ composites by the pack cementation process, thermochemical calculation was taken to analyze the vapor pressure of halide species generated at high temperatures. NH4Cl, NaF and CrCl3·6H2O were selected as the halide salts. The results of thermochemical calculations suggested that the pack powder mixtures, which contained Cr, Si, halide salts and Al2O3, may be activated by NH4Cl and NaF. According to the thermochemical calculations, the pack powder mixture of 12Cr–6Si–5NH4Cl–77Al2O3 (wt%) activated by NH4Cl was formulated and coating deposition experiments were carried out at 1200 and 1300 °C. With adequate control of pack compositions and deposition conditions, it was found that codeposition of Cr and Si could indeed be achieved at these temperatures. The coating has a three-layer structure, of which was mainly composed of Cr2(Nb,X) (X represents Ti and Hf elements), Nb5Si3 and (Nb,Cr)3Si. Then the kinetics of coating growth process affected by temperature was studied. The experimental results of the oxidation showed that the coating can efficiently prevent substrate from oxidizing.

The magnetic properties, band structure, density of states (DOS) and optical properties of ferromagnetic Fe2TiO5 were calculated by a plane wave pseudopotential method based on the local spin density approximation (LSDA) and the LSDA plus Hubbard U (LSDA+U) theory and compared with the known experimental and theoretical results. By choosing the opportune Hubbard U parameter 4 eV, LSDA+U gives magnetic moments of 3.80 µB/Fe (3d orbital), which contribute the main magnetic moments to Fe2TiO5. The inclusion of U changes the band gap of Fe2TiO5 and gives a value in better agreement with previous experiment. We find that strong correlations dramatically change the density of states and band structures. A detailed analysis shows that the LSDA+U method provides the better description of electronic structures like this system. Series optical properties were explored also. There is no ab-initio study related to Fe2TiO5 at present. These findings provide good theoretical understanding for Fe2TiO5.

First principles calculations were performed to investigate the elastic, electronic and thermal properties of 14% cubic yttria-stabilized zirconia (YSZ) using the pseudo potential plane-wave method within the gradient generalized approximation (GGA) for the exchange and correlation potential. Computed lattice constant parameters are in good agreement with the available experimental results. The three independent elastic constants were computed by means of the stress–strain method, indicating that 14% cubic YSZ is a mechanically stable structure. From the knowledge of the elastic constants, a set of related properties, namely bulk, shear modulus, Young’s modulus, sound velocity, Debye temperature, thermal capacity and minimum thermal conductivity are numerically estimated in the frame work of the Voigt–Reuss–Hill approximation for YSZ polycrystalline. The calculated bulk modulus, shear modulus, Young’s modulus, sound velocity, Debye temperature, thermal capacity and minimum thermal conductivity are in reasonable agreement with the available experimental and theory data. Density of states, charge density and Mulliken population analysis show that the 14% cubic YSZ is covalent and possess ionic character.

NiCrAlY bond coat was prepared by HVOF (high-velocity oxygen fuel) spray on nickel-based superalloy. Surface treatments like grit-blasting, shot-peening and vacuum treatment methods were carried out in order to study the effects of surface modification on thermal cycling lifetime of TBCs. The surface-modified TBCs exhibited better thermal shock resistance. Failure of TBCs with the as-sprayed bond coat occurred within the top coat and at the interface between spinels and the top coat, while that of after shot-peening, grit-blasting and vacuum treatment occurred mainly within the top coat. TGO (thermally grown oxide) formed on as-sprayed bond coat was composed of a Ni(Al,Cr)2O4 spinels outer layer and a Al2O3 inner layer. But, a continuous and uniform Al2O3 formed after surface modification. Formation of the mixed oxides (spinels) on the as-sprayed bond coat accelerated the failure of TBCs.

A nanostructured thermal barrier coating (TBC) was deposited by air plasma spraying. The effect of microstructural evolution on nano-hardness and Young’s modulus has been investigated by nanoindentation technique after exposure at 1200 °C in air for different times. The results showed that the sintering process of nanostructured TBC at 1200 °C was divided into two stages. TBC completely kept the nanostructure with the grain size <100 nm at the first stage of 10 h thermal exposure. The nanostructure was lost gradually at the second stage from 10 to 200 h thermal exposure. During the first stage, nano-hardness and Young’s modulus increased rapidly for TBC densification, and Weibull bimodal distribution of both Young’s modulus and nano-hardness disappeared as grain grew and most microcracks were healed. The structure of TBC did not change basically, and nano-hardness and Young’s modulus increased slightly at the second stage.

NiCrAlY coatings were deposited on Ni-based superalloy by high-velocity oxygen-fuel spraying (HVOF). Surface modification by means of grit-blasted, shot-peened and ground methods was used in order to study the effect of surface conditions on the isothermal oxidation behavior of HVOF-sprayed NiCrAlY coatings at 1 050 °C. The results showed that surface modification had an obvious effect on the isothermal oxidation behavior of the coatings. There was a large decrease in growth rate compared with the as-sprayed coating. The scale formed on the grit-blasted and shot-peened coatings was a mixture of Al2O3 and NiCr2O4, while the oxide formed on the ground coating was composed mainly of Al2O3. After surface modification, the content of NiCr2O4 spinels decreased compared with the as-sprayed coating.

AbstractThe phase equilibria, diffusion growth and diffusivities in the Ni-Al-Pt system at 1 150, 1 200 and 1 250 °C were studied using Pt/β-NiAl diffusion couples. Based on the measured concentration profiles coupled with the local equilibrium hypothesis, the tie-lines between neighboring phases were determined. Two intermediate phases, Pt3Al and α-NiPt(Al), are found to develop between the Pt and β-NiAl couples. The thicknesses of Pt3Al and α-NiPt(Al) layers varies linearly with the square of annealing time, indicating that the growth of Pt3Al and α-NiPt(Al) are diffusion-controlled. The calculated Arrhenius equations of the parabolic growth constants, k, for the Pt3Al and β-NiPt(Al) phases are obtained, respectively. The average ternary inter-diffusivities of the Pt3Al and β-NiAl phases at different temperatures are determined from an integration of inter-diffusion fluxes calculated directly from the experimental concentration profiles.

Thermal Barrier Coatings with HVOF NiCrAlY Bond Coat is prepared on nickel-based superalloy substrates. The lifetime of the coating is about 1630 h for 1-h cycle at 1050 °C. Growth of the TGO (thermally grown oxide) approximately follows a parabolic kinetics, and the TGO presents a bi-layered structure. Failure of the coating occurs near the interface between the mixed oxides layer of TGO and top coat. A finite element method is employed to analyze the stress distribution in the coating. The results show that maximum stresses occur at the top coat/TGO interface near the edge of the coating. The maximum radial stress for TGO consisting of spinel and Al2O3 is about five times larger than that of Al2O3, while the maximum axial stress is about ten times larger. The mixed oxide layer of TGO plays an important role in the premature failure of TBCs.

TiO2–Fe3O4 coatings on transparent glasses were prepared by atmospheric plasma spraying (APS). As-sprayed TiO2–Fe3O4 coatings consist of anatase TiO2, rutile TiO2, Fe3O4 and FeTiO3. The grain size, rate of porosity and fractions of the anatase and TiFeO3 phases in APS coatings were dependent on the process parameters. With an increase in plasma power, the content of anatase TiO2 and the rate of porosity in the coatings were decreased while the content of the resultant FeTiO3 phase and the grain size in the coating were increased. The coating condition of 500 A has the best photocatalytic efficiency and 600 A has the worst. The photocatalytic property of the APS TiO2–Fe3O4 coatings was primarily dependent on synergistic effect of the fractions of FeTiO3 and anatase phases.TiO2–Fe3O4 coatings on transparent glasses were prepared by atmospheric plasma spraying (APS). As-sprayed TiO2–Fe3O4 coatings consist of anatase TiO2, rutile TiO2, Fe3O4 and FeTiO3. The grain size, rate of porosity and fractions of the anatase and TiFeO3 phases in APS coatings were dependent on the process parameters. With an increase in plasma power, the content of anatase TiO2 and the rate of porosity in the coatings were decreased while the content of the resultant FeTiO3 phase and the grain size in the coating were increased. The coating condition of 500 A has the best photocatalytic efficiency and 600 A has the worst. The photocatalytic property of the APS TiO2–Fe3O4 coatings was primarily dependent on synergistic effect of the fractions of FeTiO3 and anatase phases.

A Ni–24Cr–6Al–0.7Y (NiCrAlY) coating was deposited on a nickel-base superalloy by low-pressure plasma spraying, and the top coating, ZrO2 partially stabilized with Y2O3 (7.5 wt%), was deposited on the NiCrAlY coating by air-plasma spraying. The cyclic-oxidation behavior of the NiCrAlY + YSZ coating exposed to NaCl vapor was investigated under atmospheric pressure at 1,050 °C, 1,100 °C and 1,150 °C. The cyclic-oxidation life of the NiCrAlY + YSZ coating in the presence of NaCl vapor was shortened compared with that in air. The higher the temperature is, the shorter the cyclic oxidation life. The oxide scale formed at the interface between the bond coat and the ceramic layer after exposure to NaCl vapor consisted of voluminous and non-protective NiO, Al2O3 and NiCr2O4 spinel. The failure of the TBC exposed to NaCl vapor occurs within the top coat and close to the YSZ/thermal growth oxide interface. The failure mechanism has been discussed based on the experimental results and thermodynamics.

Nanostructured and traditional thermal barrier coatings have been prepared by atmospherical plasma spraying (APS) on NiCrAlY-coated superalloy substrates. Nanostructured thermal barrier coating has relatively longer lifetime than the common coating after cyclic testing at 1050, 1100 and 1150 °C. A transient thermal structural finite element solution was employed to analyze the stress distribution in the coatings. The reasons why the two kinds of the coatings have different lifetimes have been explored. The results indicate that the stresses (Sx) within the nanostructured yttria stabilized zirconia (YSZ) ceramic layer in the axial (x) direction is about 67% that of the traditional YSZ. The stress (Sy) in the radial (y) direction at the vicinity of the interface between the thermally grown oxide scale (TGO) and the YSZ topcoat at the edge of the sample is only about 73% that of the traditional YSZ. So it could be concluded that the decrease of the stress in the nanostructured ceramic layers is a fundamental reason, resulting in longer thermal cycling lifetime of the nanostructured thermal barrier coating.

The wear-resistant AlCuFe quasicrystalline coating was fabricated on substrate of titanium alloy by low-pressure plasma-spraying (LPPS) method. The LPPS AlCuFe quasicrystalline coating has a rapidly solidified lamellar microstructure consisting of mainly icosahedral phase and small amount of cubic phase peaks. The results showed that AlCuFe quasicrystalline coating improved the wear resistance of titanium-based alloys under dry sliding wear test conditions. The excellent wear resistance may be attributed to the high hardness of AlCuFe quasicrystal and the formation of the tribofilm.

NiCrAlY bond coat was prepared by HVOF (high-velocity oxygen fuel) spray on nickel-based superalloy. Surface treatments like grit-blasting, shot-peening and vacuum treatment methods were carried out in order to study the effects of surface modification on thermal cycling lifetime of TBCs. The surface-modified TBCs exhibited better thermal shock resistance. Failure of TBCs with the as-sprayed bond coat occurred within the top coat and at the interface between spinels and the top coat, while that of after shot-peening, grit-blasting and vacuum treatment occurred mainly within the top coat. TGO (thermally grown oxide) formed on as-sprayed bond coat was composed of a Ni(Al,Cr)2O4 spinels outer layer and a Al2O3 inner layer. But, a continuous and uniform Al2O3 formed after surface modification. Formation of the mixed oxides (spinels) on the as-sprayed bond coat accelerated the failure of TBCs.