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1.
X.H Wang 《Corrosion Science》2003,45(5):891-907
The isothermal oxidation behavior of bulk Ti3AlC2 has been investigated at 1000-1400 °C in air for exposure times up to 20 h by means of TGA, XRD, SEM and EDS. It has been demonstrated that Ti3AlC2 has excellent oxidation resistance. The oxidation of Ti3AlC2 generally followed a parabolic rate law with parabolic rate constants, kp that increased from 4.1×10−11 to 1.7×10−8 kg2 m−4 s−1 as the temperature increased from 1000 to 1400 °C. The scales formed at temperatures below 1300 °C were dense, adherent, resistant to cyclic oxidation and layered. The inner layer of these scales formed at temperatures below 1300 °C was continuous α-Al2O3. The outer layer changed from rutile TiO2 at temperatures below 1200 °C to a mixture of Al2TiO5 and TiO2 at 1300 °C. In the samples oxidized at 1400 °C, the scale consisted of a mixture of Al2TiO5 and, predominantly, α-Al2O3, while the adhesion of the scales to the substrates was less than that at the lower temperatures. Effect of carbon monoxide at scale/substrate was involved in the formation of the continuous Al2O3 layers.  相似文献   

2.
Air plasma sprayed TBCs usually include lamellar structure with high interconnected porosities which transfer oxygen from YSZ layer towards bond coat and cause TGO growth and internal oxidation of bond coat.The growth of thermally grown oxide (TGO) at the interface of bond coat and ceramic layer and internal oxidation of bond coat are considered as the main destructive factors in thermal barrier coatings.Oxidation phenomena of two types of plasma sprayed TBC were evaluated: (a) usual YSZ (yttria stabilized zirconia), (b) layer composite of (YSZ/Al2O3) which Al2O3 is as a top coat over YSZ coating. Oxidation tests were carried out on these coatings at 1100°C for 22, 42 and 100h. Microstructure studies by SEM demonstrated the growth of TGO underneath usual YSZ coating is higher than for YSZ/Al2O3 coating. Also cracking was observed in usual YSZ coating at the YSZ/bond coat interface. In addition severe internal oxidation of the bond coat occurred for usual YSZ coating and micro-XRD analysis revealed the formation of the oxides such as NiCr2O4, NiCrO3 and NiCrO4 which are accompanied with rapid volume increase, but internal oxidation of the bond coat for YSZ/Al2O3 coating was lower and the mentioned oxides were not detected.  相似文献   

3.
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.  相似文献   

4.
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.  相似文献   

5.
Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La2(Zr0.7Ce0.3)2O7 (LZ7C3) and yttria stabilized zirconia (YSZ) were deposited by electron beam-physical vapor deposition (EB-PVD). The thermal cycling test at 1373 K in an air furnace indicates the DCL coating has a much longer lifetime than the single layer LZ7C3 coating, and even longer than that of the single layer YSZ coating. The superior sintering-resistance of LZ7C3 coating, the similar thermal expansion behaviors of YSZ interlayer with LZ7C3 coating and thermally grown oxide (TGO) layer, and the unique growth modes of columns within DCL coating are all very helpful to the prolongation of thermal cycling life of DCL coating. The failure of DCL coating is mainly a result of the reduction-oxidation of cerium oxide, the crack initiation, propagation and extension, the abnormal oxidation of bond coat, the degradation of t′-phase in YSZ coating and the outward diffusion of Cr alloying element into LZ7C3 coating.  相似文献   

6.
Oxide dispersed NiCrAlY bond coatings have been developed for enhancing thermal life cycles of thermal barrier coatings (TBCs). However, the role of dispersed oxides on high temperature corrosion, in particular hot corrosion, has not been sufficiently studied. Therefore, the present study aims to improve the understanding of the effect of YSZ dispersion on the hot corrosion behaviour of NiCrAlY bond coat. For this, NiCrAlY, NiCrAlY + 25 wt.% YSZ, NiCrAlY + 50 wt.% YSZ and NiCrAlY + 75 wt.% YSZ were deposited onto Inconel-718 using the air plasma spraying (APS) process. Hot corrosion studies were conducted at 800 °C on these coatings after covering them with a 1:1 weight ratio of Na2SO4 and V2O5 salt film. Hot corrosion kinetics were determined by measuring the weight gain of the specimens at regular intervals for a duration of 51 h. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques were used to determine the nature of phases formed, examine the surface attack and to carry out microanalysis of the hot corroded coatings respectively. The results show that YSZ dispersion causes enhanced hot corrosion of the NiCrAlY coating. Leaching of yttria leads not only to the formation of the YVO4 phase but also the destabilization of the YSZ by hot corrosion. For the sake of comparison, the hot corrosion behaviour of a NiCrAlY + 25 wt.% Al2O3 coating was also examined. The study shows that the alumina dispersed NiCrAlY bond coat offers better hot corrosion resistance than the YSZ dispersed NiCrAlY bond coat, although it is also inferior compared to the plain NiCrAlY bond coat.  相似文献   

7.
The effects of K2O and Li2O-doping (0.5, 0.75 and 1.5 mol%) of Fe2O3/Cr2O3 system on its surface and the catalytic properties were investigated. Pure and differently doped solids were calcined in air at 400-600 °C. The formula of the un-doped calcined solid was 0.85Fe2O3:0.15Cr2O3. The techniques employed were TGA, DTA, XRD, N2 adsorption at −196 °C and catalytic oxidation of CO oxidation by O2 at 200-300 °C. The results revealed that DTA curves of pure mixed solids consisted of one endothermic peak and two exothermic peaks. Pure and doped mixed solids calcined at 400 °C are amorphous in nature and turned to α-Fe2O3 upon heating at 500 and 600 °C. K2O and Li2O doping conducted at 500 or 600 °C modified the degree of crystallinity and crystallite size of all phases present which consisted of a mixture of nanocrystalline α- and γ-Fe2O3 together with K2FeO4 and LiFe5O8 phases. However, the heavily Li2O-doped sample consisted only of LiFe5O8 phase. The specific surface area of the system investigated decreased to an extent proportional to the amount of K2O and Li2O added. On the other hand, the catalytic activity was found to increase by increasing the amount of K2O and Li2O added. The maximum increase in the catalytic activity, expressed as the reaction rate constant (k) measured at 200 °C, attained 30.8% and 26.5% for K2O and Li2O doping, respectively. The doping process did not modify the activation energy of the catalyzed reaction but rather increased the concentration of the active sites without changing their energetic nature.  相似文献   

8.
The oxidation behavior of hot-pressed Al2O3–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al2O3–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al2O3–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.  相似文献   

9.
Crystalline PVD Al2O3 coatings offer great potential for their use in cutting operations. They promise high hot hardness and high oxidation resistance at elevated temperatures. Alumina exists in different crystallographic phases. α-Al2O3 appears to be the only thermodynamically stable phase at all common temperatures and pressures. Today there are many efforts to generate α-Al2O3 by means of physical vapour deposition. In this regard one problem is the high deposition temperature, which does not allow the deposition on temperature-sensitive materials.Another promising candidate is γ-Al2O3 which is more fine-grained than α-Al2O3 and can be deposited at lower temperatures. At high temperatures γ-Al2O3 might be transformed into α-Al2O3, which could limit the application temperature. But until now it is not clearly proved, up to which temperatures γ-Al2O3 thin films are stable and which mechanisms influence the stability. In the present work different (Ti,Al)N/γ-Al2O3 coatings are deposited on cemented carbides by means of Magnetron Sputter Ion Plating (MSIP). The (Ti,Al)N bond coat was employed to improve adhesion of γ-Al2O3 on the substrate. It could be shown that the γ-phase is stable in vacuum up to 1200 °C. In the atmosphere the formation of α-Al2O3 begins at 900 °C and it is influenced by the choice of transition zone between the (Ti,Al)N interlayer and γ-Al2O3. The results show that the thermal stability of the γ-phase and therefore the application temperature of the coating can be enhanced by the choice of interlayer.  相似文献   

10.
A novel YSZ?CYSZ/Al2O3 (YSZ means 6 wt% yttria partially stabilized zirconia) double-layer thermal barrier coating was fabricated using composite sol?Cgel and pressure filtration microwave sintering (PFMS) technologies. In this double-layer coating, the top layer was YSZ ceramics with a thickness of about 150 ??m and the bottom layer was composed of micro-sized YSZ particles packed by nano-sized ??-Al2O3 films and had a thickness of about 10 ??m. Cyclic oxidation tests indicated that this coating possessed superior properties to resist oxidation of alloy and spallation of coating. These beneficial effects could be mainly attributed to that, the alloy substrate could be sealed completely by ??-Al2O3 phase and the thermal stress could be decreased by means of better thermal matching and nano/micron structure in YSZ/Al2O3 layer. Moreover, thermal insulation capability tests indicated that the thermal barrier effect was improved due to the application of YSZ/Al2O3 layer. YSZ/Al2O3 layer could be considered as a promising bond coat in TBCs.  相似文献   

11.
The high-temperature oxidation behaviour of pure Ni3Al alloys in air was studied above 1000°C. In isothermal oxidation tests between 1000 and 1200°C, Ni3Al showed parabolic oxidation behavior and displayed excellent oxidation resistance. In cyclic oxidation tests between 1000 and 1300°C, Ni3Al exhibited excellent oxidation resistance between 1000 and 1200°C, but drastic spalling of oxide scales was observed at 1300°C. When Ni3Al was oxidized at 1000°C, Al2O3 was present as -Al2O3 in a whisker form. But, at 1100°C the gradual transformation of initially formed metastable -Al2O3 to stable -Al2O3 was observed after oxidation for about 20 hr. After oxidation at 1200°C for long times, the formation of a thick columnar-grain layer of -Al2O3 was observed beneath a thin and fine-grain outer layer of -Al3O3. The oxidation mechanism of pure Ni3Al is described.  相似文献   

12.
We attempted the room-temperature fabrication of Al2O3-based nanodiamond (ND) composite coating films on glass substrates by an aerosol deposition (AD) process to improve the anti-scratch and anti-smudge properties of the films. Submicron Al2O3 powder capable of fabricating transparent hard coating films was used as a base material for the starting powders, and ND treated by 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) was added to the Al2O3 to increase the hydrophobicity and anti-wear properties. The ND powder treated by PFOTES was mixed with the Al2O3 powder by ball milling to ratios of 0.01 wt.%, 0.03 wt.%, and 0.05 wt.% ND. The water contact angle (CA) of the Al2O3-ND composite coating films was increased as the ND ratio increased, and the maximum water CA among all the films was 110°. In contrast to the water CA, the Al2O3-ND composite coating films showed low transmittance values of below 50% at a wavelength of 550 nm due to the strong agglomeration of ND. To prevent the agglomeration of ND, the starting powders were mixed by attrition milling. As a result, Al2O3-ND composite coating films were produced that showed high transmittance values of close to 80%, even though the starting powder included 1.0 wt.% ND. In addition, the Al2O3-ND composite coating films had a high water CA of 109° and superior anti-wear properties compared to those of glass substrates.  相似文献   

13.
The oxidation behaviour of an intermetallic alloy, Ti-46.7Al-1.9W-0.5Si, was studied in air and Ar-20%O2 atmospheres at 750, 850 and 950 °C. Oxidation of the alloy followed a parabolic rate law at low temperature (750 °C) in both environments. The alloy oxidised parabolically in air and at a slower rate in Ar-20%O2 at 850 °C. Following a parabolic oxidation for a relatively short exposure period (72 h) at 950 °C, the oxidation rate was reduced after prolonged exposure (up to 240 h) in air. The alloy oxidised in a slower manner in the Ar-20%O2 atmosphere at 950 °C. Higher oxidation rates were observed in air than in Ar-20%O2 at all three experimental temperatures. Multi-layered scales developed in both environments. The scale formed in air consisted of TiO2/Al2O3/TiO2/TiN/TiAl2 layers, ranging from the surface to the substrate—whilst the scale developed in the Ar-20%O2 atmosphere comprised of the sequence TiO2/Al2O3/TiO2/Al2O3/Ti3Al/substrate. The two layers of Al2O3 in Ar-20%O2 were more effective in providing protection of the substrate against high temperature corrosion than the single layer of Al2O3 formed in air.  相似文献   

14.
In this paper, the effect of α-Al2O3 on in situ synthesis low density O′-sialon multiphase ceramics was investigated. Thermodynamics analysis was used to illustrate the feasibility of synthesizing O′-sialon at a low temperature of 1420 °C. The crystalline phase and microstructure were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The actual substitution parameter x value of O′-sialon was estimated via lattice correction. The results showed that, O′-sialon multiphase ceramics with different x values could be synthesized successfully through varying α-Al2O3 content. Bulk densities of samples ranging from 1.64 to 2.11 g cm−3 were adjusted with the percentage of α-Al2O3 increasing from 5.21 wt.% to 15.62 wt.%. Formation of nearly single-phase O′-sialon was obtained in the sample containing 10.42 wt.% α-Al2O3. The actual substitution parameter x increased with the increase of α-Al2O3, whereas it was lower than the original designation, and the O′-sialon with a low x value was achieved.  相似文献   

15.
Graphite is one of the candidate materials proposed for application in pyrochemical reprocessing plants involving aggressive molten chloride environment. Post treatments are promising techniques for the improvement of properties of thermal spray coatings for different industrial applications. In the present work, the effect of post treatments like vacuum annealing (VA) and laser melting (LM) on the microstructure and chemical modification of plasma sprayed Al2O3-40 wt.% TiO2 coatings over high density (HD) graphite substrates has been investigated. When compared with sprayed coatings (SC), VA coatings showed cluster morphology and LM coatings exhibited homogenous microstructure. On laser melted surfaces networks of cracks were observed. XRD studies showed that the metastable γ-Al2O3 phase present in the SC is transformed to stable α-Al2O3 after post treatments. In LM coatings Al2TiO5 phase was more predominant in contrast to SC and VA coatings. The microhardness enhancement was observed in case of LM coating compared to the VA and SC. Due to elimination of coating defects in LM samples, there is a considerable reduction in the surface roughness.  相似文献   

16.
A new type of Pt + Hf-modified γ′-Ni3Al + γ-Ni-based coating has been developed in which deposition involves Pt electroplating followed by combined aluminizing and hafnizing using a pack cementation process. Cyclic oxidation testing of both Pt + Hf-modified γ′ + γ and Pt-modified β-NiAl coatings at 1150 °C (2102 °F), in air, resulted in the formation of a continuous and adherent α-Al2O3 scale; however, the latter developed unwanted surface undulations after thermal cycling. Type I (i.e. 900 °C/1652 °F) and Type II (i.e. 705 °C/1300 °F) hot corrosion behavior of the Pt + Hf-modified γ′ + γ coating were studied and compared to Pt-modified β and γ + β-CoCrAlY coatings. Both types of hot corrosion conditions were simulated by depositing Na2SO4 salt on the coated samples and then exposing the samples to a laboratory-based furnace rig. It was found that the Pt + Hf-modified γ′ + γ and Pt-modified β coatings exhibited superior Type II hot corrosion resistance compared to the γ + β-CoCrAlY coating; while the Pt + Hf-modified γ′ + γ and γ + β-CoCrAlY coatings showed improved Type I hot corrosion performance than the Pt-modified β.  相似文献   

17.
The present paper focuses on the investigation of the relationship between microstructure of Fe3Al prepared by hot isostatic pressing (HIP) and kinetics of alumina layer formation during oxidation at 900 °C, 1000 °C and 1100 °C. As prepared HIPed Fe3Al sample reveals lamellar microstructure with inhomogeneous Al distribution which originates from the preliminary mechanical activation of Fe-Al mixture. At 900 °C, Fe3Al oxidation is characterized by selective growth of very rough alumina layer containing only transient aluminium oxides. In addition to these transient oxides, α-Al2O3 stable phase is formed at 1000 °C. At the highest temperature (1100 °C), continuous and relatively smooth alumina layer mainly contains fine crystallites of α-Al2O3. The initial lamellar structure and phase inhomogeneity in as-HIPed Fe3Al samples are supposed to be the main factors that determine observed peculiarities after Fe3Al oxidation at 900 °C and 1000 °C.  相似文献   

18.
DZ40M alloy is a new Co-base superalloy, which is suitable for the blade material of gas turbines. In this paper, isothermal oxidation of an aluminide coating on this alloy was examined at 900–1100°C in air. It was observed that the weight gain at lower temperatures (900 and 1000°C) was greater than that at the higher temperature (1050°C), which was due to the formation of both -Al2O3 and -Al2O3 at 900 and 1000°C but only -Al2O3 at 1050 and 1100°C.  相似文献   

19.
Face centered cubic (Al0.32Cr0.68)2O3 thin films have been annealed in the temperature range of 500–1000 °C during 2–8 h. The fcc structure of the film remains intact when annealed at temperatures up to 700 °C for 8 h. X-ray diffraction and transmission electron microscopy show the onset of phase transformation to corundum phase alloys in the sample annealed at 900 °C for 2 h, where annealing at 1000 °C for 2 h results in complete phase transformation to α-(Al0.32Cr0.68)2O3. In-plane and out-of-plane line scans performed in EDX TEM and θ/2θ XRD patterns did not show any phase separation into α-Cr2O3 and Al2O3 prior and after the annealing. The apparent activation energy of this process is 380–480 kJ/mol as determined by the Johnson–Mehl–Avrami model.  相似文献   

20.
The TiAl3-Al composite coating on orthorhombic Ti2AlNb based alloy was prepared by cold spray. Oxidation in air at 950 °C indicated that the bare alloy exhibited poor oxidation resistance due to the formation of TiO2/AlNbO4 mixture and intended to scale off at the TiO2 rich zone. A nitride layer about 2 µm was formed under the oxide layer. The oxygen invaded deeply into the alloy and caused severe microhardness enhancement in the near surface region. The TiAl3-Al composite coating exhibited parabolic oxidation kinetics and showed no sign of degradation after oxidized up to 1098 h at 950 °C in air under quasi-isothermal condition. No scaling of the coating was observed after oxidized at 950 °C up to the tested 150 cycles. The major oxide in the oxidized coating was Al2O3. The AlTi2N, TiAl and small amount of TiO2 were also observed in the oxidized coating. The EPMA and microhardness tests showed that inward oxygen diffusion was prevented by the interlayer, which was formed between the composite coating and the substrate during heat-treatment. Microstructure analyses demonstrated that the interlayer play a major role in protecting the substrate alloy from high temperature oxidation and interstitial embrittlement.  相似文献   

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