Modification of NiCoCrAlY with Pt: Part I. Effect of Pt depositing location and cyclic oxidation performance

Pt layers of 5?μm in thickness were electroplated before or after depositing NiCoCrAlY coating by arc ion plating (AIP) aiming for identifying the effect of Pt enriching position on microstructure and cyclic oxidation behavior of Pt modified NiCoCrAlY coatings. Al-rich zones formed at the same position of Pt-rich zones for both modified coatings due to uphill diffusion of Al driven by Pt. Cyclic oxidation tests at 1000 and 1100?°C indicated that oxidation resistance of NiCoCrAlY was improved by Pt modification via different mechanisms: at surface, Pt-rich zone promoted selective oxidation of Al to form α-Al₂O₃, whilst at coating/substrate interface Pt-rich zone acted as effective diffusion barrier for titanium. Roles of Pt played in enhancing the oxidation performance of various Pt-modified NiCoCrAlY coating were investigated.     

Isothermal oxidation behaviour of EB-PVD MCrAlY bond coat

The isothermal oxidation behaviour of EB-PVD NiCoCrAlY bond coat on nickel based superalloy at different treatment conditions were studied at 1100 °C up to 100 h. The reaction products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDS). It is found that after 10 h oxidation, while a protective alumina layer was developed on pre-oxidized specimens, a mixed oxide layer was observed on non-pre-oxidized specimens. Only after 50 h, spallation was observed on pre-oxidized specimens but did occur after 10 h for a non-pre-oxidized one, in both cases it is believed that the diffusion of the substrate element Ti to the surface of the bond coat deteriorates the adhesion of protective oxide to the bond coat and hence leads to spallation. Oxide rich in Ti was found to be located on the top of column grains rather than in the column grain boundaries.     

Ellipsometry and transmission electron microscopy study of MoSi2 coatings after oxidation at high temperature in air

MoSi₂ coatings were deposited on Si (100) substrates by means of magnetron sputtering. The structural and optical properties of the coatings were then investigated through transmission electron microscopy (TEM) and UV-visible-near IR ellipsometry after heat treatments in air at two temperatures (1100 K and 1600 K). After annealing at 1100 K, no silica layer could be observed by TEM at the surface of the MoSi₂ coating, whereas annealing at 1600 K gave rise to a protective silica layer. As observed by TEM, this silica layer contained nanometric molybdenum silicide crystals. Increasing the annealing time at 1600 K from 20 min to 2 h resulted in both a higher concentration and growth of these molybdenum silicide crystals in the silica layer. An optical model, taking into account the observed microstructure and allowing a good fit of the ellipsometry data, was developed. It is also shown that the dielectric constants of the non-oxidized part of the MoSi₂ layer became modified when the annealing temperature (1100 K versus 1600 K) and the annealing time at 1600 K (20 min versus 2 h) were increased.     

The effect of yttrium content on the oxidation-induced degradation of NiCoCrAlY coatings and the influence of surface roughness on the oxidation of NiCoCrAlY and platinum aluminide coatings

Abstract

It has been found that the yttrium content of NiCoCrAlY coatings affects the useful lives of such coatings during cyclic oxidation. In particular, NiCoCrAlY coatings with 0.1wt% yttrium have more than twice the lifetime at 1100°C compared to NiCoCrAlY coatings with 0.5wt% yttrium. The mechanism by which the yttrium concentration influences the degradation of NiCoCrAlY coatings will be described. It has also been observed that the adverse effect of yttrium can be inhibited by reducing the roughness of the coating surface. The influence of surface condition on the oxidation of yttrium in NiCoCrAlY coatings will be examined in detail and the effects of surface roughness on the oxidation of NiCoCrAlY and platinum aluminide coatings will be compared. Finally the effects of yttrium in the substrate alloy on the oxidation of platinum aluminide coatings will be discussed.     

Effect of CeO2 on the microstructure and isothermal oxidation of Ni-Al alloy coatings transformed from electrodeposited Ni-Al films at 800 °C

The electrodeposited two-phase γ-Ni + γ′-Ni₃Al and single-phase γ′-Ni₃Al coatings with and without CeO₂ particles were developed by the conversion of electrodeposited Ni-Al or Ni-Al-CeO₂ films with dispersed Al microparticles in Ni matrix into γ′-Ni₃Al by vacuum annealing at 800 °C for 3 h. SEM/EDAX and TEM characterizations showed that the CeO₂-dispersed γ-Ni + γ′-Ni₃Al or γ′-Ni₃Al coatings exhibited finer grains compared to the CeO₂-free Ni-Al alloy coatings. The oxidation at 1000 °C for 20 h showed that for a given Al content the addition of CeO₂ significantly improved the oxidation resistance of the electrodeposited Ni-Al alloy coatings. The effect of CeO₂ particles on the microstructure and oxidation behavior of the electrodeposited Ni-Al alloy coating is discussed in detail.     

Hot corrosion behavior of ZrO2-MgO coatings in LiCl-Li2O molten salt

In this study, a magnesia-stabilized zirconia (MgO-ZrO₂) top coat was applied to the surface of Inconel 713LC with an aluminized NiCoCrAlY bond coat by an optimized plasma spray process. The resulting materials were tested for corrosion behavior at 948 K for 216 h in a LiCl-Li₂O molten salt under an oxidizing atmosphere. The as-coated and tested specimens were analyzed by scanning electron microscopy (SEM)/X-ray energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The bare superalloy reveals an obvious weight loss due to spalling after scale growth and thermal stress. The top coatings exhibited a superior resistance to hot corrosion in the presence of LiCl-Li₂O molten salt when compared to the bare Inconel 713LC and the aluminized bond coatings. These coatings have been found to be beneficial for improving the hot corrosion resistance of the structural materials for high-temperature lithium molten salts.     

Structure and mechanical properties of low temperature magnetron sputtered nanocrystalline (nc-)Ti(N,C)/amorphous diamond like carbon (a-C:H) coatings

This paper reports on the structure and mechanical properties of ~ 2 μm thick nanocomposite (nc-) Ti(N,C)/amorphous diamond like carbon (a-C:H) coatings deposited on 100Cr6 steel substrates, using low temperature (~ 200 °C) DC reactive magnetron sputtering. The carbon content was varied with acetylene partial pressure in order to obtain single layer coatings with different a-C:H carbon phase fractions. The nanocrystalline Ti(N,C) phase is approximately stoichiometric for all coatings and the a-C:H phase fraction increases from 31 to 47 at.% as the coatings stoichiometry changed from TiC_1.34 N_0.51 to TiC_2.48 N_0.48, respectively. TiC_1.34 N_0.51 coatings showed the highest nanoindentation hardness (H) of ~ 14 GPa and a modulus (E_r) of ~ 144 GPa; H reduced to < 6 GPa and E_r to < 70 GPa for TiC_2.48 N_0.48 coatings. nc-Ti(N,C)/a-C:H coatings are promising candidates for applications where better matching of the modulus between a relatively low modulus substrate, hard loading support layer and low modulus-high H/E ratio top layer is required.     

Influence of Zr on structure, mechanical and thermal properties of Ti-Al-N

Multinary Ti-Al-N thin films are used for various applications where hard, wear and oxidation resistant materials are needed. Here, we study the effect of Zr addition on structure, mechanical and thermal properties of Ti_1-xAl_xN based coatings under the guidance of ab initio calculations. The preparation of Ti_1-x-zAl_xZr_zN by magnetron sputtering verifies the suggested cubic (NaCl-type) structure for x below 0.6-0.7 and z ≤ 0.4. Increasing the Zr content from z = 0 to 0.17, while keeping x at ~ 0.5, results in a hardness increase from ~ 33 to 37 GPa, and a lattice parameter increase from 4.18 to 4.29 Å. The latter are in excellent agreement with ab initio data. Alloying with Zr also promotes the formation of cubic domains but retards the formation of stable wurtzite AlN during thermal annealing. This leads to high hardness values of ~ 40 GPa over a broad temperature range of 700-1100 °C for Ti_0.40Al_0.55Zr_0.05N. Furthermore, Zr assists the formation of a dense oxide scale. After 20 h exposure in air at 950 °C, where Ti_0.48Al_0.52N is already completely oxidized, only a ~ 1 μm thin oxide scale is formed on top of the otherwise still intact ~ 2.5 μm thin film Ti_0.40Al_0.55Zr_0.05N.     

Internal oxidation mechanism for Ta-Ru and Mo-Ru coatings

Ta-Ru and Mo-Ru alloy coatings with phases of intermetallic compounds are often used as protective coatings on die materials. After annealing under an oxygen containing atmosphere, the internal oxidation phenomenon resulted in the preferential oxidation of Ta or Mo in the coatings. This process created a phase separated structure consisting of continuous and alternative oxygen rich and deficient layers with a nano scale period beneath the free surface. The experiments in this study deposited Ta-Ru and Mo-Ru coatings with a Cr interlayer by direct current magnetron co-sputtering at 400 °C. Annealing treatments were conducted at 600 °C for short durations under controlled atmospheres consisting of 50 or 10,000 ppm oxygen with residual nitrogen or argon gas. The internal oxidation behavior in the initial stage was determined by evaluating the variations in crystalline structure, surface morphology and chemical composition. The laminated structure was examined by transmission electron microscopy. Results show distinct oxidation behaviors for Ta₄₇Ru₅₃ and Mo₄₆Ru₅₄ coatings. Internal oxidation phenomena appeared in Ta₄₇Ru₅₃ coatings annealed in both 50 and 10,000 ppm O₂ containing atmospheres. The Mo₄₆Ru₅₄ coatings exhibit external oxidation at 50 ppm O₂ containing atmosphere, and internal oxidation at 10,000 ppm O₂ containing atmosphere. Finally, this study proposes an internal oxidation mechanism for alloy coatings with an orientated structure.     

High temperature oxidation resistance of multilayered AlxTi1 − xN/CrN coatings

Al_xTi_1 − xN and CrN have been widely used as a protective coating material in many types of tools and mechanical components because of high wear performance and high temperature resistance. In this study, the high temperature oxidation behavior of Al_0.63Ti_0.37N and multilayered Al_0.63Ti_0.37N/CrN coatings was studied. These coatings were synthesized by a cathodic-arc deposition system with plasma enhanced duct equipment. The nanolayer thickness and alloy content of the deposited multilayered coating were correlated with the emission rate of alloy cathode materials. The multilayered Al_0.63Ti_0.37N/CrN coating revealed a laminate structure with stacking of Al_0.63Ti_0.37N and CrN layers, and the periodic thickness (Λ) was 16 nm. For the high temperature oxidation test, the coated samples were annealed in the temperature range of 700-1000 °C in air for 2 h. The multilayered Al_0.63Ti_0.37N/CrN possessed much thinner oxide layer thickness than Al_0.63Ti_0.37N. Even after oxidation at 1000 °C, the multilayered Al_0.63Ti_0.37N/CrN still retained their crystalline structure. An interface effect served as a barrier, and retarded the diffusion of oxygen into the multilayered Al_0.63Ti_0.37N/CrN. Thus, the multilayered Al_0.63Ti_0.37N/CrN showed a high temperature oxidation resistance superior to the Al_0.63Ti_0.37N.     

Radiative properties characterization of ZrB2–SiC-based ultrahigh temperature ceramic at high temperature

Experimental investigations of radiative property on pre-oxidized ZrB₂–SiC–15 vol.%–C ultrahigh temperature ceramic (ZSC UHTC) at high temperature range of 1100–1800 °C were performed. By Fourier transform infrared radiant (FT-IR) spectrometer, spectral emissivity was measured in the wavelength region between 3 and 18 μm. Total normal emissivity was calculated using spectral emissivity data via theoretical formula. It has been found that high emissivity for all the testing specimens was presented, and the total normal emissivity is between 0.65 and 0.92 with temperature range from 1100 to 1800 °C. Moreover, the total normal emissivity of pre-oxidized ZSC ceramic decreased non-monotonously as the temperature increased. The total normal emissivity decreased as the testing temperature increased from 1100 to 1800 °C, whereas the total normal emissivity at the testing temperature of 1600 °C was higher than that of 1400 and 1800 °C. Macroscopical surface morphology and microstructure were carried out before and after the testing.     

Effect of TiO2-SiO2 sol-gel coating on the cpTi-porcelain bond strength

The effects of TiO₂-SiO₂ sol-gel coating with different firing temperatures (300 °C, 500 °C, and 750 °C) on the cpTi-porcelain bond strength were investigated in the present study. Prior to applying the low-fusing dental titanium porcelain, the phase, surface morphology, surface roughness and static water contact angle of the intermediate layer were evaluated. The cpTi-porcelain bond strength was measured using the three-point flexure test according to ISO 9693 standard. Statistical analyses were made using one-way ANOVA and Dunnett-t test. Significantly higher bond strength of TiO₂-SiO₂/750 °C (specimens coated with TiO₂-SiO₂ sol-gel coating and fired at 750 °C for 1 h) when compared to the control group was observed (p < 0.05). No rutile phase was found in all the tested specimens coated with TiO₂-SiO₂ sol-gel coating. The surface morphology of the intermediate layer was apparently different with different firing temperatures. It was found that the static water contact angle of TiO₂-SiO₂/750 °C significantly decreased (p < 0.05). However, no markedly different R_a of TiO₂-SiO₂/500 °C and TiO₂-SiO₂/750 °C in comparison to that of the control group was observed (p > 0.05). The results show that the TiO₂-SiO₂ sol-gel coating fired at 750 °C for 1 h can notably improve the cpTi-porcelain bond strength and may be suitable for clinical use.     

Modification of NiCoCrAlY with Pt: Part Ⅱ. Application in TBC with pure metastable tetragonal(t’) phase YSZ and thermal cycling behavior

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4mol.% yttria stabilized zirconia(4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4 YSZ thermal barrier coatings(TBCs) with and without Pt modification was evaluated in ambient air at 1100?C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide(TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top(Ni,Co)(Cr,Al)_2O_4 spinel and underlying α-Al_2O_3, was formed at the NiCoCrAlY/4 YSZ interface with thickness of 8.4μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al_2O_3 scale with thickness of 5.6μm was formed at the interface of Pt-modified NiCoCrAlY and 4 YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al_2O_3 and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.     

High temperature oxidation interfacial growth kinetics in YSZ thermal barrier coatings with bond coatings of NiCoCrAlY with 0.25% Hf

The results of an experimental study of the high-temperature isothermal oxidation behavior and microstructural evolution in two variations of air plasma sprayed ceramic thermal barrier coatings (TBCs) are discussed in the paper. Two types of TBC specimens were produced for testing. These include a standard and vertically cracked APS. High temperature oxidation was carried out at 900, 1000, 1100 and 1200 °C. The experiments were performed in air under isothermal conditions. At each temperature, the specimens were exposed for 25, 50, 75 and 100 h. The corresponding microstructures and microchemistries of the TBC layers were examined using scanning electron microscopy and energy dispersive X-ray spectroscopy. Changes in the dimensions of the thermally grown oxide layer were determined as functions of time and temperature. The evolution of bond coat microstructures/interdiffusion zones and thermally grown oxide layers were compared in the TBC specimens with standard and vertically cracked microstructures.     

Crystallization kinetics and densification of YAG nanoparticles from various chelating agents

Yttrium aluminium garnet (YAG, Y₃Al₅O₁₂) nanoparticles were prepared using sonochemical sol-gel method with three different chelating agents and the effect of crystallization kinetics was investigated with differential scanning calorimetry-thermogravimetry (DSC-TG). The activation energy values of crystallization for the as-synthesized YAG nanoparticles using citric acid (CA), glycine (G) or a mixture of citric acid-glycine (CA-G), as chelating agents were found to be 160.5, 142.2 and 140.4 kJ mol⁻¹ and the corresponding Avarami constants were 2.2, 2.1 and 1.9, respectively. Samples produced with the mixed chelating agent under sonification, could be crystallized to single phase YAG nanoparticles (10-65 nm) after annealing at 1100 °C. Pellets made from the annealed YAG particles could be sintered to a relative density greater than 99% at 1500 °C with a grain size of 4.5 μm, made up of secondary particles formed from primary nano-crystals within the grains. Grain size and relative density increased with different chelating agents from CA to G and CA-G in the increasing order when YAG samples were sintered. Grain growth and densification occurred at a relatively low temperature of 1500 °C as compared to over 1800 °C in solid-state reactions.     

Effect of surface oxidation on thermal shock resistance of the ZrB2-SiC-ZrC ceramic

The isothermal oxidation of the ZrB₂-SiC-ZrC ceramic was carried out in static air at a constant temperature of 1000 ± 15 °C, 1200 ± 15 °C and 1400 ± 15 °C for 30 min, respectively. Compared with the original strength of 580 MPa, the strength for the specimen oxidized at 1000 °C, 1200 °C and 1400 °C for 30 min increased to 609 MPa, 656 MPa and 660 MPa, respectively, because the flaws in the surface of the specimen were sealed by the oxide layer. The thermal shock resistance of the specimens before and after the oxidation was measured by the water quenching. The measured ΔT_crit for the specimen oxidized at 1000 °C, 1200 °C and 1400 °C were 352 °C, 453 °C and 623 °C, respectively, which was obviously higher than 270 °C for the unoxidized specimen. The improvement in the thermal shock resistance was attributed to the formation the oxide layer on the surface of the specimen. The results here pointed to a promising method for improving strength and thermal shock resistance of ZrB₂-based ceramics.     

Chromizing of plasma nitrided AISI 1045 steel

In this study AISI 1045 steel specimens were plasma nitrided at 803 K for 5 h, in a gas mixture of 75% N₂ + 25% H₂. The specimens were then chromized in powder mixtures consisting of ferrochromium, ammonium chloride and alumina at 1273 K for 5 h. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis and Vickers micro-hardness test were used as characterizing techniques. The thickness of white nitrided layer was around 5 μm, which was mainly composed of iron nitrides and its hardness was around 740 HV. Chromizing of nitride layer resulted in formation of Cr₂N chromium nitride and Fe₃N iron nitrides. A significant increase was observed in hardness after chromizing of the nitrided layer. Despite its higher hardness, the post chromised specimen showed higher wear rate than single plasma nitrided specimen.     

Microstructure evolution and oxidation behaviour of Tif/TiAl3 composites

In this paper, Ti_f/TiAl₃ composites were fabricated by infiltration–in situ reaction method and its oxidation behaviours were investigated by cyclic oxidation testing at 700 °C, 800 °C and 900 °C. The microstructure evolution and oxidation of Ti_f/TiAl₃ composites were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray diffraction (EDX). The reaction between Ti₃Al particles and Al was more violent than that of Ti fibres and Al. Ti₃Al/Al reaction consumed a large amount of Al and inhibited the reaction of Ti fibres indirectly. Reactant of Ti fibres was TiAl₃ at 700 °C, and four reaction layers surrounding Ti fibre (Ti₃Al, TiAl, TiAl₂ and TiAl₃ from inner to outside) were observed above 800 °C. The thickness of the total reaction layers increased little with temperature and time, while the thickness of inner reaction layers increased remarkably. A model corresponding to the microstructure evolution process was drawn schematically. Oxidation resistance of Ti_f/TiAl₃ composites decreased with increasing of temperature, and changed from cubic law at 700 °C to parabolic law at 900 °C. The oxidation weight gain of Ti_f/TiAl₃ composite was dominated by the exposed Ti fibres. Due to outward diffusion of Ti and Al element, the oxide of Ti fibre at 900 °C changed to mushroom-shape. Fortunately, when TiAl₃ was oxidized, a thin and continuous Al₂O₃ layer was formed, protecting matrix from further oxidation.     

Flame aerosol deposition and annealing of Y3Al5O12:Tb phosphor coatings

Sub-micrometer-sized powders of Y₃Al₅O₁₂:Tb phosphor (d_SEM = 320 nm) were prepared by flame-assisted spray pyrolysis of aqueous precursors in a premixed propane/air flame and in situ deposited onto quartz substrates. Phosphor screens with densities of up to 0.7 mg cm⁻² could be produced within 20 min. As-deposited coatings were amorphous and required a thermal post-treatment. After annealing in an oven for 2 h (T ≥ 900 °C), the yttrium aluminum garnet phase (YAG:Tb) was obtained. Alternatively, the phosphor coatings were treated by an impinging flame in the same setup used for the deposition. Quasi-amorphous Y₃Al₅O₁₂:Tb coatings demonstrated bright green photoluminescence upon flame annealing at T ≈ 1100 °C for just several minutes and could outperform YAG:Tb when excited in the wavelength ranges 205–220 nm and 230–260 nm. For example, brightness of emission from the quasi-amorphous coatings was up to five times higher than that of the fully crystalline YAG:Tb phosphor at a technically important wavelength of 254 nm.     

Preparation of La1−xSrxMnO3 nanoparticles by sonication-assisted coprecipitation

La_1−xSr_xMnO₃ (x=0.3) (LSM) nanoparticles were prepared by a sonication-assisted coprecipitation method. The coprecipitation reaction is carried out with ultrasound radiation. Lower sintering temperatures are required for the sonication-assisted product. Fully crystallized LSM with an average particle size 24 nm is obtained after the as-prepared mixture is annealed at 900 °C for 2 h. Magnetic properties indicate that the transition temperature from the paramagnetic to ferromagnetic state of the sample is quite sharp and occurs at 366 K for samples annealed for 2 h at 900 and 1100 °C.