Oxide Scale Spallation Research Articles

418 鋼球衝突の角度依存性に基づく酸化スケール剥離の評価

418 鋼球衝突の角度依存性に基づく酸化スケール剥離の評価

419 各種球体衝突による鋼材上の酸化皮膜の剥離観察

419 各種球体衝突による鋼材上の酸化皮膜の剥離観察

Oxidation behavior of nickel coating on ferritic stainless steel interconnect for SOFC application

The oxidation behavior of nickel coated ferritic stainless steel SS441 has been investigated. A nickel coating layer is deposited on the steel which is employed in a solid oxide fuel cell stack as interconnect. The nickel film is about 8 μm thick and is topped by an additional 4 μm thick La0.67Sr0.33MnO3 (LSM) film on the interconnect cathode-contacting surface for the prevention of chromium evaporation from the steel substrate. A 10,000-h 800 °C isothermal ageing on 10 × 10 mm2 steel coupons shows a continuous growth of oxide scales up to ∼200 μm in thickness on the surface, consisting of a 100 μm thick iron oxide layer followed by a complex Fe–Ni–Cr spinel structure. A single-cell stack is tested at 800 °C for up to 1226 h and an average degradation rate of 7.5% kh−1 is observed. Oxidation characteristics of the coating system are analyzed after testing. A Fe–Ni spinel phase is found covering most of the surface area. This is attributed to the intensive interdiffusion of iron and nickel during the stack operation and the high intersolubility of the two elements. In both the tests of the steel coupons and the stack, LSM film structures are damaged by the thermally grown Fe–Ni oxides, and the expected Cr-preventing function is limited. The Fe–Ni spinel layer initially forms an effective obstacle against Cr out-migration. However, the increasing content of iron in the spinel phase induces oxide scale spallation afterwards. Though the fast grown Fe–Ni oxide scale can serve as an effective barrier against chromium out-migration, the iron-enriched scale structure is susceptible to corrosion attacks after an extended stack operation period.

Oxidation resistance of novel multilayer coatings for gas turbine components

This article presents a study of the oxidation behaviour of two multilayer coatings, denoted as MC I and MC II, respectively, which are designed with a three-layer architecture: a Cr–Si barrier layer on the IN738 substrate, a NiCrAlY intermediate layer, and an aluminised top layer. The emphasis of this research is on the total amount of oxide scales formed and the residual oxide scales on the coatings with various combinations among the three layers to investigate the effects of each layer and the interactions between the layers on the oxidation resistance of the coatings. The difference between the two multilayer coatings is that the Al content in the top layer of MC II is twice as much as that in the top layer of MC I. The coatings are fabricated through a combination of plasma spray and pack cementation processes, and then experience oxidation test at 1050°C for 1000 h. The average thicknesses of the oxide scales are around 15 and 45 μm for MC I and MC II, respectively. The experimental results are modelled using the analysis of variance. Two models for the volume of total oxide scales and the ratio of the volume of spalled oxide scales to the volume of total oxide scales, are proposed to study the effects of the coating layers on the formation and spallation of oxide scales, which directly affect the oxidation resistance of the coatings. The experimental and analytical results demonstrate that the Cr–Si layer and its interactions with the other coating layers affect the spallation of oxide scales the most among the three layers. It prevents Al and Cr from diffusing into the substrate. The interaction between the Cr–Si layer and the NiCrAlY layer, and the increase in Al/Ni ratio of the aluminide top layer, promote the formation of oxide scales and also minimise the spallation of oxide scales.

Long-Term Oxidation of Candidate Cast Iron and Stainless Steel Exhaust System Alloys from 650 to 800 °C in Air with Water Vapor

The oxidation behavior of candidate cast irons and cast stainless steels for diesel exhaust systems was studied for 5,000 h at 650–800 °C in air with 10 % H2O. At 650 °C, Ni-resist D5S exhibited moderately better oxidation resistance than did the SiMo cast iron. However, the D5S suffered from oxide scale spallation at 700 °C, whereas the oxide scales formed on SiMo cast iron remained relatively adherent from 700 to 800 °C. The oxidation of the cast chromia-forming austenitics trended with the level of Cr and Ni additions, with small mass losses consistent with Cr oxy-hydroxide volatilization for the higher 25Cr/20–35Ni HK and HP type alloys, and transition to rapid Fe-base oxide formation and scale spallation in the lower 19Cr/12Ni CF8C plus alloy. In contrast, small positive mass changes consistent with protective alumina scale formation were observed for the cast AFA alloy under all conditions studied. Implications of these findings for exhaust system components are discussed.

Effect of a grain-refined microalloyed steel substrate on the formation mechanism of a tight oxide scale

The formation mechanism of tight oxide scale on the microalloyed steel was investigated at temperatures of 550–850°C in dry air. Microstructural characterisations reveal that the spallation of oxide scale dominates at the centre of coarse grains on the oxidation initiation. The fine-grained steel improves the adhesive properties of oxide scale by enhanced grain-boundary diffusion. The lower activation energy and higher oxidation rate accelerate cation/anion migration along grain boundaries, leading to high magnetite content in the oxide scale. The approach by grain refinement at initial oxidation has been proposed to generate the pickle-free tight oxide scale.

Influence of yttrium addition on high temperature oxidation resistance of single crystal superalloy

Two experimental single crystal superalloys Y-free alloy and Y-containing alloy were cast in the directionally solidified furnace, while other alloying element contents were basically kept unchanged. The isothermal oxidation behavior of two single crystal superalloy was studied at 1100 °C in ambient atmosphere. Morphology of oxides was examined by scanning electron microscopy (SEM) and their compostion was analyzed by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The effect of yttrium on the high temperature oxidation resistance of the single crystal superalloy was investigated. The results showed that the oxidation resistance of Ni-base single crystal superalloy was substantially improved by adding a little amount of rare earth yttrium. Yttrium could promote the oxide of Al formation and decreased the proportion of NiO. Yttrium increased the coherence between the oxide scale and the alloy substrate to decrease the spallation of oxide scale. Yttrium reduced the oxide grain size and the thickness of oxide layer.

Effect of Cr and Re on the Oxidation Resistance of Ni<sub>3</sub>Al-Base Single Crystal Alloy IC21 at 1100

The oxidation kinetic curves of three Ni3Al-based single crystal alloys (IC21, IC21C and IC21CR) with different Cr and Re content were examined at 1100. The microstructures and element distributions of the oxide scales on these alloys were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectrometry (EDS). Results revealed that the oxidation resistance of these alloys was improved in the order of IC21 < IC21C < IC21CR. The oxide products of IC21 mainly consisted of NiO, α-Al2O3, NiAl2O4 and a small amount of NiMoO4 and MoO2. The volatilization of Mo oxides led to the oxide scale spallation from IC21C, thus deteriorated the oxidation properties. While for IC21C and IC21CR, the oxidation resistance was significantly improved. The Mo oxides in the oxide layer were greatly reduced and a continuous α-Al2O3 layer was formed.

High Temperature Performance of TiAlON Thin Films

In this paper, the thermal oxidation behavior, adhesion and tribological properties of TiAlON films coated on hastelloyX substrate, typically used for fuel nozzle in gas turbine engine application, have been studied. The uncoated and coated samples were heated to different temperatures, i.e. 950, 1050 and 1150 °C in the controlled atmosphere. The surface appearance, microstructure, chemical composition and adhesion of films were investigated. The thermal oxidations were observed in all testing conditions showing thicker oxide film at higher temperature. However, spalling of oxide scales was found in hastelloyX and TiAlON coated at 1150°C suggesting the maximum working temperature of < 1150 °C. The critical loads corresponding to the full delamination of the thermal oxidation coated specimens were found to be higher than the non-thermal oxidation specimens. The effect of thermal oxidation on damage patterns during scratch tests, i.e. less chipping and cracking for thermal oxidation specimen, were also observed. The tribological properties were also investigated under different load under room temperature and 600 and 1000°C. The results suggested significant improvement in wear resistance of coated sample especially at low load at all temperatures.

Degradation behaviour of sputtered Co–Al coatings on superalloy

Degradation behaviour of sputtered Co–Al coatings on Superni-718 substrate has been investigated. Cyclic high temperature oxidation tests were conducted on uncoated and coated samples at peak temperatures of 900 °C for up to 100 thermal cycles between the peak and room temperatures. The results showed that a dense scale formed on the coated samples during thermal cycling at the peak temperature of 900 °C. The external scale exhibited good spallation resistance during cyclic oxidation testing at both temperatures. The improvement in oxide scale spallation resistance is believed to be related to the fine-grained structure of the coating. Nanostructured Co–Al coatings on Superni-718 substrate were deposited by DC/RF magnetron sputtering. FE-SEM/EDS, AFM, and XRD were used to characterize the morphology and formation of different phases in the coatings, respectively. The Co–Al coating on superalloy substrate showed better performance of cyclic high temperature oxidation resistance due to its possession of β-CoAl phase as Al reservoir and the formation of Al2O3 and spinel phases such as CoCr2O4 and CoAl2O4 in scale. The oxidation results confirmed an improved oxidation resistance of the Co–Al coating on superalloy as compare to bare substrate in air at 900 °C temperature up to 100 cycles.

High temperature degradation behavior of sputtered nanostructured Co–Al coatings on superalloy

Microstructure and cyclic high temperature oxidation behavior of nanostructured Co–Al coatings on Superni-718 substrate have been investigated. Cyclic high temperature oxidation tests were conducted on uncoated and coated samples at peak temperatures of 800°C and 900°C for up to 100 thermal cycles between the peak and room temperatures. The results showed that bare substrate has higher oxidation resistance at 800°C as compared to 900°C and coated sample has slightly higher oxidation resistance at 900°C. The external scale of both coated sample exhibited good spallation resistance during cyclic oxidation testing at both temperatures. The improvement in oxide scale spallation resistance is believed to be related to the fine-grained structure of the coating. Nanostructured Co–Al coatings on Superni-718 substrate were deposited by DC/RF magnetron sputtering. FE-SEM/EDS and XRD were used to characterize the morphology and formation of different phases in the coatings, respectively. The Co–Al coating on superalloy substrate showed better performance of cyclic high temperature oxidation resistance due to its possession of β-CoAl phase as Al reservoir and the formation of Al2O3 and spinel phases such as CoCr2O4 and CoAl2O4 in the scale. The oxidation results confirmed an improved oxidation resistance of the Co–Al coating on superalloy as compared to bare substrate up to 100 cycles.

Thermal stability of nanostructured iron–chromium alloys for interconnect application of solid oxide fuel cells

The thermal expansion mismatch between a metallic substrate and its external oxide scale generates a strain on cooling that is the primary cause of spallation of protective oxide scales. This study investigates the thermal stability, by means of thermal expansion and oxidation behaviour, of the nanostructured FeCr alloy prepared by different consolidation techniques by means of the spark plasma sintering (SPS) method and to compare with conventional sintering technique by means of hot pressing (HP). This has potential application as interconnect in solid oxide fuel cell. Commercially available ferritic steel is chosen as a comparison of another high Cr ferritic model alloy. The beneficial effect of the reactive element by means of lanthanum onto the alloy surface that is introduced using ion implantation is also explored. The specific aspects studied were the effects of nanocrystalline structure, influenced by sintering method and surface treatment through La ion implantation of chromia forming alloys, which may improve their high thermal stability. Oxidation testing was conducted at 900–1100°C for 100 h in laboratory air. Characterisations using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were carried out before and after each route or process to investigate the microstructure, phase change and formation of the oxide layer. The results revealed that the FeCr alloy prepared by SPS were more effective to retain nanocrystalline and better properties than those prepared by HP and the commercially available ferritic alloy. For all types of specimens, the presence of La had no detectable effect on thermal expansion but a major effect on scale adherence. The coefficients of thermal expansion for the alloy prepared by SPS were lower than those prepared by HP, and the scale adherence to the La implanted alloy was generally superior. The results consistently showed that a better reduction in oxidation resistance corresponds to excellent nanostructured alloy and La implantation.

The effect of superalloy substrate on the behaviour of high-purity low-density air plasma sprayed thermal barrier coatings

Several superalloy-bond coat couples were prepared without ceramic topcoat layers to better understand the effects of superalloy substrate on the oxidation behaviour of NiCoCrAlY bond coats. The same composition NiCoCrAlY bond coats were deposited on three superalloy substrates (Inconel 718, Haynes 188 and Rene’ N5) via argon-shrouded plasma spraying. The specimens were exposed to cyclic oxidation in laboratory air at 1100°C in a bottom loading furnace. Scaling behaviour and rate of aluminum depletion were compared between the various specimens. The bond coats on all three superalloys experienced some form of chemical failure after an extended number of cycles. The number of cycles until chemical failure was shortest for the IN718 specimen followed by the HA188 specimen, both of which experienced complete bond coat chemical failure, and then the Rene’ N5 specimen, which experienced localized chemical failure. The cycles to chemical failure coincide with the cycles to thermal barrier coating (TBC) spallation from previous work, indicating chemical failure of the bond coat is a critical event in the lifetime of TBCs. The effect of bond coat surface finish and porosity on the scaling behaviour has been investigated using specimens with the same superalloy substrate but with different bond coat surface finishes and porosity levels which were produced by utilizing two separate sized starting bond coat metallic powders. Bond coats with minimal porosity and smooth surface finishes did not experience chemical failure, at least in the time frame they were tested; however, oxide scale spallation was more apparent in the smooth bond coats as compared to the specimens with the rough surface finishes and high levels of porosity.

Cyclic oxidation behavior of plasma surface chromising coating on titanium alloy Ti–6Al–4V

The cyclic oxidation behavior of plasma surface chromising coating on titanium alloy (Ti–6Al–4V) was researched in air at 650°C, 750°C and 850°C. A NiCrAlY coating was prepared by multi-arc ion plating as a comparison. The surface morphologies, microstructures and phases of both coatings before and after oxidation were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffractometry (XRD). The results show that the chromising coating consisted of an outer layer of loose Cr deposition, an intermediate layer of compact Cr deposition and an inner Ti–Cr mutual diffusion layer. The multilayer oxide scales formed in the oxidation process, which has the better cyclic oxidation resistance compared with NiCrAlY thermal barrier coating. However, the brittleness of Ti(Cr, Al)2 laves phase resulted in spallation of oxide scales at 750°C and 850°C.

Detection of Crack and Spallation of Oxide Scales using Acoustic Emission Technique

Detection of Crack and Spallation of Oxide Scales using Acoustic Emission Technique

High temperature oxidation behaviour of Cr/Co–Al coated superalloy

The high temperature oxidation behaviour of nanostructured Cr/Co–Al coatings on Superni-718 substrate, deposited by sputtering, has been investigated. Cyclic high temperature oxidation tests were conducted on bare and coated samples at 900°C for up to 100 cycles. The results showed that a dense scale formed on the coated samples during thermal cycling at the peak temperature of 900°C. The external scale exhibited good spallation resistance during cyclic oxidation testing. The improvement in oxide scale spallation resistance is believed to be the fine grained structure of the coating. The Cr/Co–Al coating on the superalloy substrate showed better performance of cyclic high temperature oxidation resistance due to its possession of β-CoAl phase as Al reservoir and the formation of Al2O3 and spinel phases, such as CoCr2O4 and CoAl2O4, in the scale.

Effect of Sulfur Partial Pressures on Oxidation Behavior of Fe–Ni–Cr Alloys

Fe–Ni–Cr alloys containing different contents of Si with and without pre-formed oxide scale at the surface were tested in oxidation environments at 1,050 °C with varied sulfur partial pressures. The oxide-scale growth on Fe–Ni–Cr alloys was accelerated by increasing sulfur partial pressures in the oxidizing-carburizing environments. This accelerated oxidation was characterized by the formation of plate-shaped MnCr2O4 spinel crystallites and the nodular clusters at the site of scale spallation. Pre-oxidized Fe–Ni–Cr alloys generally did not suffer from sulfur attack because of excellent protection of pre-formed oxide scale. Scale spallation and sulfur attack were found only on high-Si alloy subjected to the maximum sulfur potential, which was attributed to accelerated oxidation and selective oxidation and sulfidation at the sites where oxide scale spallation had occurred. For bare alloys in absence of pre-formed oxide layers, scale spallation was found to occur at lower level of sulfur potential on low-Si alloy than on high-Si alloy. A higher content of Si is necessary for the formation of protective silica sub-layer, which is believed to be the main cause of the difference in scale spallation observed.

Thermal Oxidation Behavior of TiAlCrSiN and AlCrTiN Films on HastelloyX

In this paper, the thermal oxidation behavior of TiAlCrSiN and AlCrTiN films coated on hastelloyX substrate, typically used for fuel nozzle in gas turbine engine application, have been studied. The thermal oxidation behavior at 950, 1050 and 1150 °C in controlled atmosphere were investigated. The surface appearance, microstructure, chemical composition and adhesion of films were investigated. The thermal oxidations were observed in all testing conditions showing oxide films at the surface with thicker oxide film at higher temperature. However, spalling of oxide scales was found in both coated and uncoated specimens at 1150°C suggesting the maximum working temperature of < 1150 °C for turbine engine applications. The critical loads corresponding to the full delamination of the thermal oxidation coated specimens were found to be higher than the non-thermal oxidation specimens. The effect of thermal oxidation on damage patterns during scratch tests, i.e. less chipping and cracking for thermal oxidation specimen, were also observed.

Effect of cerium addition on oxidation behavior of 25Cr20Ni alloy under low oxygen partial pressure

The influence of Ce addition on the oxidation behavior of 25Cr20Ni alloy at 950 °C under low oxygen partial pressure was investigated. The oxidized samples were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and scratch tester to obtain the oxide phases, morphology, thickness, composition and adhesion property of the oxide scales. The experiment results indicated that a small amount of Ce addition (0.02 wt.% or 0.05 wt.%) promoted oxidation resistance and inhibited the growth of the needlelike oxide. The Ce addition also decreased the formation of MnCr2O4 but promoted the SiO2 formation underneath the Cr2O3, which largely contributed to the improvement of oxide scale spallation resistance. For the sample with 0.3 wt.% Ce addition, the oxidation rate significantly increased and the spallation resistance of the oxide scale decreased.

Isothermal oxidation behavior of Ni-Al-Co-Cr-Mo-Ti alloy at 850°C and 950°C

The oxidation behavior of a superalloy with the chemical composition (wt.%) of Al 5.5-Ti 2.6-Co 4.8-Mo 3.9-W 5.0-Cr 11.0-Ni bal at 850°C and 950°C in air has been investigated. The kinetics curves of the oxidation were measured by thermal gravimtric analysis (TGA), and the microstructure and the elements distribution in the different regions of the specimens after oxidation were analysed by XRD and SEM/EDAX. The experimental results showed that the oxidation rate in the primary stage was high, then the mass gaining rate decreased gradually with the oxidation time, and the oxidation kinetics obeys parabolic law. A small amount of oxide scale spallation happened during the oxidation at 950°C. The analysis of the specimens after oxidized at 950°C revealed that three oxide layers formed, including the outer layer consisting of TiO2 and CrTiO3, the middle layer consisting of Cr2O3, Ni(Cr2O4) and TiO2 phases, and internal oxidizing layers identified as Al2O3 phase