Thermal conductivity of EB-PVD ZrO2-4 mol% Y2O3 films using the laser flash method

The variation in thermal conductivity and thermal diffusivity of ZrO₂-4 mol% Y₂O₃ coatings deposited onto Inconel substrates by EB-PVD is examined as a function of coating thickness using the laser flash method. The coatings are found to consist of columnar grains with a feather-like microstructure. The thermal conductivities of the coatings are calculated using two methods: the first involves separating the coating from the substrate and measuring the thermal diffusivity directly; the second uses thermal diffusion results from coatings still attached to the substrate and is based on the response function method. The results of both methods are in excellent agreement, and show that the thermal conductivities of the coatings increase with increasing coating thickness. The results also confirm that the double layer method can be used successfully to calculate the thermal conductivities of thin film coatings.     

Thermal shock resistance of FeMnCrAl/Cr3C2-Ni9Al coatings deposited by high velocity arc spraying

FeMnCrAl/Cr₃C₂ and FeMnCrAl/Cr₃C₂-Ni9Al coatings were deposited onto low-carbon steel substrates by high velocity arc spraying. The cross-section and interface microstructures of the coatings were analyzed by optical microscopy (OM). The thermal shock resistance of the coatings was investigated. The characteristics of the coatings after the thermal cycling test were studied by OM, field emission scanning electron microscopy, and energy dispersion spectrometry. The results show that laminated structures with pores, oxide phases, and unmelted particles were found on all the prepared coatings. The FeMnCrAl/Cr₃C₂ coating with a Ni9Al interlayer registered the best thermal shock resistance, which may be attributed to the interdiffusion between the low-carbon steel substrates and the Ni9Al arc-sprayed coating that converted the mechanical bond between the substrates and the coatings to a metallurgical one.     

The effects of heat treatment and gas atmosphere on the thermal conductivity of APS and EB-PVD PYSZ thermal barrier coatings

The effects of heat treatment and gas atmosphere on thermal conductivity of atmospheric plasma sprayed (APS) and electron beam physical vapor deposited (EB-PVD) partially Y₂O₃ stabilized ZrO₂ (PYSZ) thermal barrier coatings (TBCs) were investigated. Two-layer samples that had an EB-PVD coating deposited on bond coated nickel-base superalloy IN625 substrates, free-standing APS and EB-PVD coatings as well as a quasi-free-standing EB-PVD PYSZ coating (coating on semitransparent sapphire) were included in the study. Thermal diffusivity measurements for determining thermal conductivity were made from room temperature up to 1150 °C in vacuum and under argon gas using the laser flash technique. To investigate the effect of heat treatment on thermal conductivity, coatings were annealed at 1100 °C in air. For both the APS and EB-PVD PYSZ coatings the first 100 h heat treatment caused a significant increase in thermal conductivity that can be attributed to microstructural changes caused by sintering processes. Compared to the measurements in vacuum, the thermal conductivity of APS coatings increased by about 10% under argon gas at atmospheric pressure, whereas for the EB-PVD coatings, the influence of gas on thermal conductivity was relatively small. The effect of gas on the thermal conductivity of APS and EB-PVD PYSZ coatings can be attributed to amount, shape, and spatial arrangement of pores in the coating material.     

Deposition and characterization of non-isostructural (Ti0.7Al0.3N)/(Ti0.3Al0.7N) multilayers

Non-isostructural Ti_0.7Al_0.3N(cubic B1)/Ti_0.3Al_0.7N(hexagonal B4) nanoscale multilayers were deposited by dc magnetron sputtering on steel substrates, with nominal periods of 6, 10, 20 and 40 nm. The structure, composition, periodicity, and interface abruptness of the samples were characterized by X-ray diffraction, glow discharge optical spectroscopy, medium energy ion scattering, and narrow resonant nuclear reaction profiling. The nanohardness and elastic modulus of the samples were determined, revealing superhardness of up to 57 GPa for the lowest nanoscale multilayer period. The H³/E² ratios were found to be superior to those of most metal nitride multilayers commonly used as protective coatings, which indicates a superior wear resistance for the present nanostructured coating. The results are discussed in terms of the present stage of understanding of nanoscale multilayer effects on the tribological properties of protective coatings.     

Oxidation behavior of arc-sprayed FeMnCrAl/Cr3C2-Ni9Al coatings deposited on low-carbon steel substrates

FeMnCr/Cr₃C₂ and FeMnCrAl/Cr₃C₂ coatings, using Ni9Al arc-sprayed coating as an interlayer on low-carbon steel substrates, were deposited by high velocity arc spraying (HVAS) on the cored wires. The high temperature oxidation behavior of the arc-sprayed FeMnCrAl/Cr₃C₂-Ni9Al and FeMnCr/Cr₃C₂ coatings on the low-carbon steel substrates was studied during isothermal exposures to air at 800 °C. The surface and interface morphologies of the coatings after isothermal oxidation after 100 h were observed and characterized by optical microscopy, field emission scanning electron microscope, energy dispersion spectrum, and X-ray diffraction. The results showed that the oxidation weight gains of the coatings were significantly lower than that of the low-carbon steel substrate. Moreover, the FeMnCrAl/Cr₃C₂-Ni9Al coating registered the lowest oxidation rate. This favorable oxidation resistance is due to the Al and Cr contents of the aforementioned coating that inhibits the generation of Fe and Mn oxides. This is attributed to the interdiffusion between the substrates and the Ni9Al arc-sprayed coating, which can convert the mechanical bonding between substrates and coatings into a metallurgical one, thereby inhibiting the oxidation of interface between the low-carbon steel and the coating.     

Thermal shock behavior of nanostructured and conventional Al2O3/13 wt%TiO2 coatings fabricated by plasma spraying

The thermal shock behavior of three kinds of Al₂O₃/13 wt%TiO₂ coatings fabricated by plasma spraying was studied in this paper. One kind of those coatings was derived from conventional fused and crushed feedstock powder available commercially; the other two kinds of coatings were derived from nanostructured agglomerated feedstock powders. These two nano coatings possess moderate pores and pre-existing microcracks, they were composed of fused structure and three-dimensional net or skeleton-like structure. For conventional coatings, the pores and pre-existing cracks were bigger, sharp-point and mostly distributed between splats. Thermal shock tests for the three coatings were performed by water quenching method. Testing result showed the two kinds of nano coatings had much higher thermal shock resistance than the conventional coatings. The improved thermal shock resistance for nano coatings could attribute to their improved microstructure and crack propagation mode. The damage evolution and failure mechanism of coatings was quite different at thermal shock temperature of 650 °C and 850 °C, which was explained by a simple model. Different crack propagating modes in nanostructured and conventional coatings during thermal shock tests were due to their different microstructures in these two kinds coatings. The stress state of coating surfaces during the thermal cycles was also discussed in this paper.     

Thermal conductivity and thermal cycle life of La2O3 and HfO2 doped ZrO2-Y2O3 coatings produced by EB-PVD

The effects of La₂O₃ and HfO₂ addition on thermal conductivity and thermal cycle life of EB-PVD YSZ coatings were investigated. La₂O₃ and HfO₂ were selected as additives, because they significantly suppress the sintering of YSZ. The developed coating showed low thermal conductivity as well as high resistance to sintering. Burner rig tests confirmed that the developed coating have a superior thermal insulating effect and have a longer life than that of a coating with conventional composition.     

Preparation of a-C:H/a-C:H:Si:O and a-C:H/a-C:H:Si multilayer coatings by PACVD

Carbon based multilayer coatings were prepared by plasma assisted chemical vapor deposition (PACVD) using methane (CH₄) and hexamethyldisiloxane (HMDSO) or methane and tetramethylsilane (TMS) as precursors. These coatings were deposited in a modified plasma nitriding plant operated at relatively high working pressures of 20 Pa. The multilayer design consisted of a-C:H and a-C:H:Si:O or a-C:H and a-C:H:Si single layers, respectively. The number of single layers and the material of the top layer were varied at constant total coating thicknesses. These multilayer coatings were investigated with regard to their morphology and composition as well as indentation hardness, abrasive wear, lubricant free friction and wetting behavior via contact angle measurements. The multilayer coatings exhibited lower wear rates and higher hardness values than a-C:H:Si:O or a-C:H:Si single layers and lower friction coefficients than pure a-C:H coatings under unlubricated test condition. Utilizing duplex processes, combining plasma nitriding pre-treatment and a following coating deposition, the adhesion of the multilayer coatings on high speed and cold working steel substrates could be considerably improved.     

Corrosion protection of electrodeposited multilayer nanocomposite Zn-Ni-SiO2 coatings

Multilayer nanocomposite coatings of Zn-Ni-SiO₂ were deposited galvanostatically on mild steel (MS) from Zn-Ni bath, having Zn⁺² and Ni⁺² ions and uniformly dispersed nano-SiO₂ particles. The corrosion characteristics and properties of multilayered nanocomposite (MNC) coatings were evaluated by electrochemical polarization and impedance methods. Such deposition conditions as, bath composition, cyclic cathode current densities (CCCD’s) and number of layers were optimized for peak performance of coatings against corrosion. A significant improvement in the corrosion performance of MNC coatings was observed when a coating was changed from a monolayer to multilayer type. Corrosion rate (CR) of MNC coating decreased progressively with number of layers up to an optimal level, and then started increasing. The increase of CR at a higher degree of layering is attributed to diffusion of layers due to a very short deposition time, failing to give the enhanced corrosion protection. The formation of layers, inclusion of silica particle in MNC coating matrix were confirmed by SEM and XRD study. At optimal current densities, i.e. at 3.0–5.0 A/cm², the Zn-Ni-SiO₂ coating having 300 layers, represented as (Zn-Ni-SiO₂)_30/5.0/300 is found to be about 107 times more corrosion resistant than a monolayer Zn-Ni-SiO₂ coating, developed from the same bath for the same time. The reasons responsible for the extended corrosion protection of MNC Zn-Ni-SiO₂ coatings, compared to corresponding monolayer Zn-Ni and (Zn-Ni-SiO₂) coatings were analyzed, and results were discussed.     

Synthesis and characterization of Y3Al5O12 and ZrO2-Y2O3 thermal barrier coatings by combustion spray pyrolysis

Y₃Al₅O₁₂ and ZrO₂-Y₂O₃ (8 mol% YSZ) coatings for potential application as thermal barrier coatings were prepared by combustion spray pyrolysis. Thermal cycling of as deposited coatings on stainless steel and FeCrAlY bond coat substrates was carried out at 1000 °C and 1200 °C to determine the thermal fatigue response. Structural and morphological studies on Y₃Al₅O₁₂ and 8 mol% YSZ coatings before and after thermal cycling have been carried out. It has been noted that the coatings on FeCrAlY substrates remain intact after 50 cycles between room temperature and 1200 °C, whereas the coatings on stainless steel show some minor damage such as peeling off near the periphery after 50 cycles at 1000 °C. Thermal diffusivity values of Y₃Al₅O₁₂ and 8 mol% YSZ films were measured by using photo thermal deflection spectroscopy and the values are lower than those of coatings produced by conventional techniques such as EBPVD and APS.     

Preparation and spray drying of Al2O3-TiO2 nanoparticle suspensions to obtain nanostructured coatings by APS

Al₂O₃-TiO₂ coatings were deposited on austenitic stainless steel coupons from nanostructured powders by atmospheric plasma spraying (APS). Commercial suspensions of nanosized Al₂O₃ and TiO₂ particles were used as starting materials. Mixtures of these suspensions and of more concentrated suspensions of Al₂O₃ and TiO₂ were then agglomerated into plasma sprayable feedstock. Agglomeration was performed by spray drying, followed by consolidation thermal treatment.These powders were successfully deposited, yielding coatings that were well bonded to the substrates. The coating microstructure thus consisted of semi-molten feedstock agglomerates surrounded by fully molten particles that acted as binders. Agglomerates from suspensions with higher solids contents yielded coatings with lower porosity and fewer semi-molten areas.     

Influence of Deposition Temperature on the Microstructures and Properties of Plasma-Sprayed Al2O3 Coatings

Al₂O₃ coatings were deposited on 1Cr13 substrates by atmospheric plasma spraying at different deposition temperatures of 140, 275, 375, 480, 530, and 660 °C to investigate the effect of coating surface temperature on the lamellar bonding formation. The fractured cross section morphology was characterized by scanning electron microscopy to reveal the lamellar interface bonding. X-ray diffraction was used to characterize the phase contents in the coating. Micro-hardness, Young??s modulus, and thermal conductivity of the deposits were measured for examining the dependency of coating properties on its microstructure. The results show that the interface area bonded through columnar grain growth across splat-splat interfaces was increased with increasing deposition temperature. Moreover, micro-hardness, Young??s modulus and thermal conductivity were increased with the increase of deposition temperature. However, the phase structure of the coating changed little with deposition temperature. The results evidently indicate that the apparent bonding ratio and properties of deposits can be significantly changed in a wider range through controlling the deposition temperature.     

基于分形方法的YSZ热障涂层有效热导率分析

陶瓷基热障涂层具有优异的阻热性能、耐热腐蚀性能以及热稳定性能,在航空发动机热端部件中广泛使用。 利用大气等离子喷涂方法制备 ZrO₂-8%Y₂O₃(YSZ)涂层,利用聚苯酯(PHB)调节涂层的孔隙形态和含量,利用扫描电镜(SEM)和图像软件分析涂层的截面形貌,计算了孔隙的分形维数,建立了基于分形维数的有效热导率计算方法,优化了热导率与涂层孔隙的定量关系。 同时利用导热仪测量面层的热导率,对有效热导率计算结果进行验证。 结果表明: YSZ 粉末中混合聚苯酯粉末可增加涂层中的孔隙含量;当 PHB 的质量含量达到 15%时,涂层孔隙率可增加至 30%左右。 较高的喷涂功率会形成扁平化的孔隙,孔隙分形维数的取值在 1~ 2,并且孔隙越扁平取值越大。 孔隙含量越大、形态越趋近于扁平化,涂层有效热导率越小。 把孔隙的分形维数引入到有效热导率的计算中,使得计算结果更加趋近于实测结果。     

Hot corrosion behavior of detonation gun sprayed Cr3C2–NiCr coatings on Ni and Fe-based superalloys in Na2SO4–60% V2O5 environment at 900 °C

The present work investigates the hot corrosion resistance of detonation gun sprayed (D-gun) Cr₃C₂–NiCr coatings on Superni 75, Superni 718 and Superfer 800 H superalloys. The deposited coatings on these superalloy substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 0.8%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of bare and Cr₃C₂–NiCr coated superalloys in molten salt environment (Na₂SO₄–60% V₂O₅) at high temperature 900 °C for 100 cycles. The corrosion products of the detonation gun sprayed Cr₃C₂–NiCr coatings on superalloys are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for elucidating the corrosion mechanisms. It is shown that the Cr₃C₂–NiCr coatings on Ni- and Fe-based superalloy substrates are found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. Particularly, the coating deposited on Superfer 800 H showed a better hot corrosion protection as compared to Superni 75 and Superni 718. The coatings serve as an effective diffusion barrier to preclude the diffusion of oxygen from the environment into the substrate superalloys. It is concluded that the hot corrosion resistance of the D-gun sprayed Cr₃C₂–NiCr coating is due to the formation of desirable microstructural features such as very low porosity, uniform fine grains, and the flat splat structures in the coating.     

Novel thermal barrier coatings based on La2Ce2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Novel thermal barrier coatings based on La₂Ce₂O₇/8YSZ double-ceramic-layer (DCL) systems, which were deposited by electron beam physical vapor deposition (EB-PVD), were found to have a longer lifetime compared to the single layer La₂Ce₂O₇ (LC) system, and even much longer than that of the single layer 8YSZ system under burner rig test. The DCL coating structure design can effectively alleviate the thermal expansion mismatch between LC coating and bond coat, as well as avoid the chemical reaction between LC and Al₂O₃ in thermally grown oxide (TGO), which occurs above 1000 °C as determined by differential scanning calorimetry (DSC) analysis. The failure mechanism of LC/8YSZ DCL coating is mainly due to the sintering of LC coating surface after long-term thermal cycling.     

Structure and properties of multi-component and multilayer TiCrBN/WSex coatings deposited by sputtering of TiCrB and WSe2 targets

Single-layer (SL), double-layer and multilayer (ML) TiCrBN/WSe_x coatings were deposited by sputtering of TiCrB and WSe₂ targets in a gaseous mixture of argon and nitrogen. To enhance film adhesion, ion implantation was employed at the initial stage of deposition for 5 min. The structure and chemical composition of the coatings were studied by means of X-ray diffraction, scanning and transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The coatings were characterized in terms of their adhesion, hardness, elastic modulus, elastic recovery, internal stress, corrosion resistance, friction and wear. All coatings showed a dense structure free of columnar grains. The ML TiCrBN/WSe_x coating demonstrated a sequence of nanocrystalline TiCrBN and amorphous WSe_x layers with various thicknesses. The SL TiCrBN/WSe_x coating deposited at magnetron current of 2 A displayed completely amorphous structure. As the magnetron current was decreased to 1 A, the coating consisted of mixture of face centred cubic (Ti,Cr)(B,N), WSe₂, and amorphous a-phases. This multi-component TiCrBN/WSe_x coating showed almost the same hardness of 30 GPa as the SL TiCrBN coating. The incorporation of WSe_x into TiCrBN coating was shown to decrease the friction coefficient in air from 0.5 to 0.2-0.25 with only little influence on the wear resistance and electrochemical behavior. In the case of SL TiCrBN/WSe_x coatings, the friction curves were typically flat with a very short running-in stage which did not exceed 50 m, whereas in the case of ML TiCrBN/WSe_x coating, the fluctuations and spikes of friction coefficient were observed. It has been confirmed by Raman spectroscopy that the existence of WSe₂ phase in sliding contact during the tribological test provides low friction. To evaluate thermal stability, the SL TiCrBN/WSe_x coating deposited at magnetron current of 1 A was annealed in vacuum at 550 °C for 1 hour. The superior tribological performance of the SL TiCrBN/WSe_x coating after annealing can be explained by the presence of two constituents, of which one is a hard and oxidation resistant (Ti,Cr)(B,N) phase with a grain size of a few nanometers and the other is a mixture of WSe₂ + a-WSe_x solid lubricant grains and/or inter-granular phases.     

High temperature oxidation resistance of FeCrAl alloys covered with ceramic SiO2–Al2O3 coatings deposited by sol–gel method

One-, three- and five-layer SiO₂–Al₂O₃ coatings were deposited on a FeCrAl alloy basis by the sol–gel method. Sols in which the molar ratio of tetraethoxysilan to aluminium tri-sec-butoxide was 1:1 and 1:3 were used.As the samples were being soaked at T = 1200 °C for t = 700 h the mass of the samples increased. Thermal shock (T = 1200 °C, 10,000 cycles) causes greater degradation of the surface than soaking at a constant temperature. The XPS and EDS results show that the composition of the top layer of the coatings changes during high temperature oxidation and thermal shock. The outward movement of aluminium cations results in surface enrichment with aluminium, particularly for the single-layer coatings. The measured energies of bonds Si 2p and Al 2p in the multilayer coatings indicate that a structure of aluminosilicates with a composition between that of mullite and that of sillimanite forms during sintering.     

Evolution of thermal properties of EB-PVD 7YSZ thermal barrier coatings with thermal cycling

During high-temperature exposure, the microstructure of thermal barrier coatings evolves, leading to increased thermal conductivity. We describe the evolution in the thermal properties of a 7 wt.% Y₂O₃ stabilized ZrO₂ electron beam-physical vapor deposited (EB-PVD) thermal barrier coating with thermal cycling between room temperature and 1150 °C until failure. The thermal diffusivity and conductivity of the coating were evaluated non-destructively based on the analysis of its photothermal infrared emission. Although the coating density does not increase significantly with thermal cycling, the thermal diffusivity and conductivity of the coating increased substantially, particularly during the first 20 1 h cycles. The values then approach a limiting value. Complementary Raman spectroscopy suggests that the increase is accompanied by a reduction in the defect concentration in the coating and that there is also a correlation between the width of the Raman lines and the thermal conductivity.     

Thermo-physical and thermal cycling properties of plasma-sprayed BaLa2Ti3O10 coating as potential thermal barrier materials

Increased turbine inlet temperature in advanced turbines has promoted the development of thermal barrier coating (TBC) materials with high-temperature capability. In this paper, BaLa₂Ti₃O₁₀ (BLT) was produced by solid-state reaction of BaCO₃, TiO₂ and La₂O₃ at 1500 °C for 48 h. BLT showed phase stability between room temperature and 1400 °C. BLT revealed a linearly increasing thermal expansion coefficient with increasing temperature up to 1200 °C and the coefficients of thermal expansion (CTEs) are in the range of 1 × 10^− 5–12.5 × 10^− 6 K^− 1, which are comparable to those of 7YSZ. BLT coatings with stoichiometric composition were produced by atmospheric plasma spraying. The coating contained segmentation cracks and had a porosity of around 13%. The microhardness for the BLT coating is 3.9–4.5 GPa. The thermo-physical properties of the sprayed coating were investigated. The thermal conductivity at 1200 °C is about 0.7 W/mK, exhibiting a very promising potential in improving the thermal insulation property of TBC. Thermal cycling result showed that the BLT TBC had a lifetime of more than 1100 cycles of about 200 h at 1100 °C. The failure of the coating occurred by cracking at the thermally grown oxide (TGO) layer due to severe oxidation of bond coat. Based on the above merits, BLT could be considered as a promising material for TBC applications.     

Structure and mechanical properties of TiAlSiN/Si3N4 multilayer coatings

TiAlSiN/Si₃N₄ multilayer coatings which have different separate layer thicknesses of TiAlSiN or Si₃N₄ were deposited onto glass sheets, single-crystal silicon wafers and polished WC-Co substrates by reactive magnetron co-sputtering. The morphology, crystalline structure and thickness of the as-prepared multilayer coatings were characterized by TEM, SEM, XRD and film thickness measuring instrument. The mechanical properties of the coatings were evaluated by a nanoindenter. The effects of monolayer thickness on the microstructure and properties of TiAlSiN/Si₃N₄ multilayer coatings were explored. The coatings showed the highest hardness when the thickness of Si₃N₄ and TiAlSiN monolayers was 0.33 nm and 5.8 nm, respectively. The oxidation characteristics of the coatings were studied at temperatures ranging from 700 °C to 900 °C for oxidation time up to 20 h in air. It was found that the coatings displayed good oxidation resistance.