Effect of an aluminide coating on precipitate rafting in superalloys

The microstructure of single crystal superalloy SRR99 is shown to be significantly affected by the combined effect of the application of an aluminide diffusion coating and the mechanical deformation caused by thermomechanical fatigue. In the case of an R = −∞ cycle (compressive strains), plates parallel to the coating surface develop in a layer underneath the subcoating zone, markedly differing from the precipitate coalescence observed, in the bulk. This effect is absent in the R = 0 cycles (tensile strains). This phenomenon is successfully explained in the light of a criterion for the prediction of γ′ rafting, by considering the superposition of the strains produced both by the TMF cycling and by a layer of γ′ which is observed to develop during the tests with R = −∞.     

Correlation of processing technique and microstructure of platinum aluminide bond coats with performance of thermal barrier coatings deposited on nickel base superalloy

Abstract

We show that the performance of thermal barrier coating systems is critically dependent upon the processing technique and microstructure of platinum aluminides utilised as bond coats. It is demonstrated by thermal exposure tests at 1150°C in air with 24 h cycling period to room temperature that the average useful life of a coating system employing zirconia–7 wt-% yttria as top coat and alloy MAR M002DS as substrate is increased from 192 to 480 h by replacing a three-layer bond coat aluminised by conventional pack cementation with a two-layer bond coat aluminised by chemical vapour deposition. Before each aluminising process, the superalloy has been electroplated with a platinum layer about 7 μm in thickness. Microstructural characterisation using scanning electron microscopy combined with energy dispersive X-ray spectroscopy, electron-probe microanalysis, transmission electron microscopy and X-ray diffraction indicates that the superior performance provided by the two-layer bond coat is related to its higher thermal stability enhancing the adhesion of the thermally grown oxide. However, both coating systems are found to fail by the same mechanism involving loss of adhesion between the thermally grown oxide and bond coat.     

Corrosion and erosion behavior of iron aluminide (FeAl(Cr)) coating deposited by detonation spray technique

The electrochemical corrosion and erosion behavior of FeAlCr coating was reported in this article. The FeAlCr coatings were deposited by detonation spray coating system (DSC) by utilizing the gas atomized powder with a nominal composition of Fe-12Al-5Cr at two different pulse frequencies on mild steel (MS) substrate. Feedstock and coatings were characterized using XRD, SEM and elemental mapping. Nanoindentation tests using a Berkovich indenter indicate a hardness of 5.5 GPa and 4.9 GPa for the coatings deposited at 3 Hz and 6 Hz respectively as compared to 1.5 GPa of the substrate. Electrochemical corrosion tests were performed on coated samples in 3.5 wt% NaCl and 2 N H₂SO₄ media and are compared with the bulk MS substrate. Detonation pulse frequency significantly influenced the coating microstructure and corrosion performance. The coating deposited at a frequency of 6 Hz exhibited higher resistance against electrochemical corrosion in 3.5 wt% NaCl medium than the coating that was deposited at 3 Hz frequency. Solid particle erosion tests performed at room temperature (25 °C) and an impingement angle of 90° (normal incidence) using Al₂O₃ as the erodent medium demonstrate a higher erosion loss of the coatings than the bulk MS. On the contrary, at a temperature of 400 °C, the coatings exhibited notably better erosion resistance than the MS substrate, illustrating the potential of FeAlCr coatings for high temperature wear-corrosion resistant applications.     

The effect of bond coat oxidation on the microstructure and endurance of a thermal barrier coating system

Abstract

Isothermal oxidation tests have been carried out on a thermal barrier coating (TBC) system consisting of a nickel-based superalloy, CoNiCrAlY bond coat applied by HVOF and yttria-stabilised zirconia (YSZ) top coat applied by EB-PVD. Bond coat microstructure, coating cracking and failure were characterised using high resolution scanning electron microscopy complemented with compositional analyses using energy dispersive X-ray spectrometry. A protective alumina layer formed during the deposition of the YSZ top coat and this grew with sub-parabolic kinetics during subsequent isothermal oxidation at temperatures in the range 950 to 1150°C. After short exposures at 1050°C and final cooling, small sub-critical cracks were found to exist within the YSZ but adjacent to bond coat protuberances. Their formation is related to the development of local tensile strains associated with the growth of an alumina layer (TGO) on the non-planar bond coat surface. However, for the specimens examined, these cracks did not propagate, in contrast to other TBC systems, and final spallation was always found to have occurred at the bond coat/TGO interface. This shows that the strain energy within the TGO layer made a significant contribution to the delamination process.     

EB-PVD process and thermal properties of hafnia-based thermal barrier coating

Thermal barrier coatings (TBCs) are being developed for the key technology of gas turbine and diesel engine applications. In general, 8 mass% Y₂O₃–ZrO₂ (8YSZ) coating materials are used as the top coating of TBCs. The development of hafnia-based TBC was started in order to realize the high reliability and durability in comparison with 8YSZ, and the 7.5 mass% Y₂O₃–HfO₂ (7.5YSH) was selected for coating material. By the investigation of electron-beam physical vapor deposition (EB-PVD) process using 7.5YSH ceramic ingot, 7.5YSH top coating with about 200 µm thickness could be formed. The microstructure of the 7.5YSH coated at coating temperature of 850 °C showed columnars of laminated thin crystals. On the other hand, the structure of the 7.5YSH coated at coating temperature of 950 °C showed solid columnars. From the result of sintering behavior obtained by heating test of 7.5YSH coating, it was recognized that the thermal durability of 7.5YSH coating was improved up to about 100 °C in comparison with 8YSZ coating. This tendency was confirmed by the experimental result of the thermal expansion characteristics of sintered 7.5YSH and 8YSZ.

©2003 Elsevier Science Ltd. All rights reserved.     

Computational analysis of thin coating layer failure using a cohesive model and gradient plasticity

Thermal barrier coatings (TBC) are widely used to prevent transient high temperature attack and allow components high durability. Due to strong inhomogeneous material properties the TBC failure often initiates near the interface between the brittle oxide layer and the ductile substrate. A reliable prediction of the TBC failure requires detailed information about the crack tip field and the consequent fracture criteria. In the present paper both cohesive model and gradient plasticity are used to simulate the failure process and to study interdependence of the interface stress distribution with the specific fracture energies. Computations confirm that combination of the two models is able to simulate different failure mechanisms in the TBC system. The computational model has the potential to give a realistic prediction of the crack propagation process.     

Thermophysical properties of a novel high entropy hafnate ceramic

High-entropy oxides(HEOs)are considered promising thermal barrier coating(TBC)materials due to their unique thermophysical performances induced by the entropy effects.In this work,(La0.2Ce0.2Pr0.2Sm0.2Eu0.2)2Hf2O7 high entropy hafnate,as a thermal barrier coating(TBC)mate-rial,was successfully synthesized by solution combustion method for the first time.From the X-ray diffraction,scanning electron microscopy,and transmission electron microscopy results,it is confirmed that(La0.2Ce0.2Pr0.2Sm0.2 Eu0.2)2Hf2O7 has pure single-phase ordered pyrochlore structure with highly homogeneous composition at both micrometer and nanometer scales.The synthesized(La0.2Ce0.2Pr0.2Sm0.2Eu0.2)2Hf2O7 possesses excellent phase stability at 1600℃and demonstrates a low thermal conductivity(1.0-1.24 W·m-1·K-1)which is lower than those of rare earth hafnates(RE2Hf2O7,RE=La,Ce,Pr,Sm,Eu).Therefore,it provides a new perspective and potential to prompt the next gener-ation TBC materials with better performance.     

Observations of the spallation modes in an overlay coating and the corresponding thermal barrier coating system

Abstract

The oxidation dynamics of an overlay coating and the corresponding thermal barrier coating system are presented. The particular systems examined are composed of a nickel-based superalloy with an air plasma-sprayed NiCrAlY bond coat and the thermal barrier coating system consists of air plasmasprayed yttria stabilized zirconia layer. Failure can occur in these systems by crack propagation within the ceramic outer layer at the interface with the bond coat. Defects, such as microcracks and pores, are common in plasma-sprayed coatings and within the thermally grown oxide scales. These can act as initiation sites for cracks. The subsequent growth of these cracks can lead to loss of the outer protective materials. Considerable information is available by microscopic examination of sections through test specimens that have been held at temperature for varying amounts of time. By careful sample preparation it is possible to monitor the development of the oxide scale formed during high temperature testing and the sites of failure. Identification of the initiation sites and growth of cracks is important in understanding the spallation process. In this study, scanning electron microscopy is used to provide evidence of the processes involved in the two systems. A comparison of the two coating systems reveals the effect the outer ceramic layer has on the oxide scale growth, and the spallation processes crucial to the understanding of the failure mechanisms of these coating systems.     

Whey protein isolate coating on LDPE film as a novel oxygen barrier in the composite structure

To examine the feasibility of whey protein isolate (WPI) coating as an alternative oxygen barrier for food packaging, heat‐denatured aqueous solutions of WPI with various levels of glycerol as a plasticizer were applied on corona‐discharge‐treated low‐density polyethylene (LDPE) films. The resulting WPI‐coated LDPE films showed good appearance, flexibility and adhesion between the coating and the base film, when an appropriate amount of plasticizer was added to the coating formulations. WPI‐coated LDPE films showed significant decrease in oxygen permeability (OP) at low to intermediate relative humidity, with an Arrhenius behaviour and an activation energy of 50.26 kJ/mol. The OP of the coated films increased significantly with increasing relative humidity, showing an exponential function. Although the coated films showed a tendency to have less oxygen barrier and more glossy surfaces with increasing plasticizer content, differences in the OP and gloss values were not significant. Haze index and colour of the coated films were also little influenced by WPI coating and plasticizer content. The results suggest that whey protein isolate coating could work successfully as an oxygen barrier and have potential for replacing synthetic plastic oxygen‐barrier layers in many laminated food packaging structures. Copyright © 2004 John Wiley & Sons, Ltd.     

Influence of Water Vapor on the Isothermal Oxidation Behavior of Thermal Barrier Coatings

The oxidation of specimens with thermal barrier coating (TBC) consisted of nickel-base superalloy, low-pressure plasma sprayed Ni-28Cr-6AI-0.4Y (wt pct) bond coating and electron beam physical vapor deposited 7.5 wt pct yttria stabilized zirconia (YSZ) top coating was studied at 1050℃ respectively in flows of 02, and mixture of O2 and 5%H2O under atmospheric pressure. The thermal barrier coating has relatively low oxidation rate at 1050℃ in pure O2. Oxidation rate of thermal barrier coating in the atmosphere of O% and 5%H2O is increased The oxidation kinetics obeys almost linear law after long exposure time in the presence of 5% water vapor. Oxide formed along the interface between bond coat and top coat after oxidation at 1050℃ in pure O2 consisted of Al2O3, whereas interfacial scales formed after oxidation at 1050℃ in a mixture of O2 and 5%H2O were mainly composed of Ni(AI,Cr)2O4,NiO and AI2O3. It is suggested that the effect of water vapor on the oxidation of the NiCrAlY coating may be attributed     

陶瓷层与界面孔隙率对热障涂层寿命及其失效机制的影响

以NiCoCrAlY作为粘结层、8wt%Y₂O₃稳定的ZrO₂（8YSZ）为陶瓷层,利用等离子喷涂（PS）技术制备2种在陶瓷层及陶瓷层/粘结层界面处具有不同孔隙率的热障涂层（TBCs）,研究TBCs的热循环寿命差异,分析不同孔隙率TBCs的失效机制。结合有限元模拟计算了TBCs应力分布,分析了高孔隙率TBCs中重复平行裂纹形成的原因及2种TBCs剥落的失效模式。利用光学显微镜（OM）、SEM和EDX分析TBCs的断面微观结构及元素分布。结果显示:高孔隙率TBCs比致密TBCs的寿命增加了1倍。高孔隙率TBCs在陶瓷层及界面处存在更多的孔隙和微裂纹,释放了TBCs中积累的应变能,同时氧化铝层中出现的重复平行裂纹能进一步减小了陶瓷层与粘结层之间的应力,进而延长了高孔隙率TBCs的寿命。为制备长寿命TBCs奠定结构设计基础。      

Influence of Annealing on the Grain Growth and Thermal Diffusivity of Nanostructured YSZ Thermal Barrier Coating

The nanostructured zirconia coatings were deposited by atmospherically plasma spraying. Scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and X-ray diffraction were used to investigate the microstructure of the zirconia coatings. Thermal diffusivity values at normal temperatures have been evaluated by laser flash technique. Effect of annealing on the microstructure evolution of the zirconia coating has been performed. The grains and thermal diffusivity are increased with increasing annealing time and temperature. The grain growth is according to the GRIGC （the grain rotation induced grain coalescence） mechanism. The increase in thermal diffusivity is attributed to the grain growth and the decrease in porosity of nanostructured zirconia coatings.     

Low cycle fatigue behaviour of a plasma-sprayed coating material

Single crystal nickel-base superalloys employed in turbine blade applications are often used with a plasma-sprayed coating for oxidation and hot corrosion resistance. These coatings may also affect fatigue life of the superalloy substrate. As part of a larger programme to understand the fatigue behaviour coated single crystals, fully reversed, total-strain controlled fatigue tests were run on a ‘free standing’ NiCoCrAiY coating alloy, PWA 276, at 0.1 Hz. Fatigue tests were conducted at 650°C, where the NiCoCrAiY alloy has modest ductility, and at 1050°C, where it is extremely ductile, showing tensile elongation in excess of 100%. At the lower test temperature, deformation-induced disordering softened the NiCoCrAlY alloy, while at the higher test temperature cyclic hardening was observed which was linked to gradual coarsening of the two-phase microstructure. Fatigue life of the NiCoCrAlY alloy was significantly longer at the higher temperature. Further, the life of the NiCoCrAlY alloy exceeds that of coated, [001]-oriented PWA 1480 single crystals at 1050°C, but at 650°C the life of the coated crystal is greater than that of the NiCoCrAlY alloy on a total strain basis.     

Overview on the Development of Nanostructured Thermal Barrier Coatings

Thermal barrier coatings （TBCs） have successfully been used in gas turbine engines for increasing operation temperature and improving engine efficiency. Over the past thirty years, a variety of TBC materials and TBC deposition techniques have been developed. Recently, nanostructured TBCs emerge with the potential of commercial applications in various industries. In this paper, TBC materials and TBC deposition techniques such as air plasma spray （APS）, electron beam physical vapor deposition （EB-PVD）, laser assisted chemical vapor deposition （LACVD） are briefly reviewed. Nanostructured 7-8 wt pct yttria stabilized zirconia （7-8YSZ）TBC by air plasma spraying of powder and new TBC with novel structure deposited by solution precursor plasma spray （SPPS） are compared. Plasma spray conditions, coating forming mechanisms, microstructures,phase compositions, thermal conductivities, and thermal cycling lives of the APS nanostructured TBC and the SPPS nanostructured TBC are discussed. Research opportunities and challenges of nanostructured TBCs deposited by air plasma spray are prospected.     

Effect of plastic coating on the sensory properties of fibreboard-based packaging materials in microwave oven heating

The effects of the type and thickness of plastic coating on the migration of off-odours and off-flavours from fibreboard materials to certain model foodstuffs were studied in microwave oven heating. Correlations between sensory, physical and chemical properties of fibreboard materials were also determined. The plastic coating served as a very good barrier to the transfer of off-flavours into foodstuffs. Polyester coating prevented the migration of off-flavours, whereas polyethylene and polypropylene were effective only in certain cases. With the last two plastics, the thickness of the coating layer also affected the amount of off-flavours migrating. Generally, the sensory properties of the materials did not correlate with either the melting behaviour of the plastic coating or the amount of volatile compounds generated.     

界面形貌对热障涂层残余应力分布的影响

用ABAQUS有限元软件建立了热障涂层模型,计算得出,在热载荷作用下,界面形貌对热障涂层材料残余应力分布影响很大,σ22主要集中在热生长氧化层和过渡层波峰处,随着热生长氧化层变厚、波长变小、振幅变大,σ22变大,且最大值产生在热生长氧化层/过渡层界面的波峰处.     

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.     

热障涂层（TBC）稳定性的研究EI

将ＺｒＯ_２·８％Ｙ_２Ｏ_３用射频磁控溅射技术沉积在ＮｉＣｒＡｌＹ底涂层上，进行ｌｌ００℃→室温，１１００℃→冷水和９００℃→室温等热周期和熔盐作用的试验。随后进行Ｘ－射线衍射（ＸＲＤ）分析和扫描电镜（８ＥＭ）观察。结果表明，沉积态的氧化锆层主要为立方相和少量单斜相及四方相。热周期及熔盐Ｎａ_２ＳＯ_４－５％ＮａＣｌ与ＺｒＯ_２·８％Ｙ_２Ｏ_３的化学作用都促使立方相→单斜相的转变。熔盐对晶界和底涂层的化学和物理的作用是影响涂层稳定性的另一因素。     

Hot-melt coating: water sorption behavior of excipient films

Hot-melt coating allows encapsulation of water-labile, drug-laden substrates to form a barrier that resists moisture ingress. To understand the interaction of water with excipients that can form moisture-protective coatings, sorption behavior of films of lipidic (glyceryl behenate) and polymeric (polyvinyl alcohol) coating excipients was investigated. A simple and rapid method using a new, fully automated instrumental technique to investigate the sorption/desorption behavior of excipient films is reported. Further, the influence of temperature and film thickness on the sorption behavior of films is examined. Both excipient films displayed sorption isotherms that were classified as type III and demonstrated hysteresis during desorption. The sorption data for both films did not follow the Langmuir model, and the BET model could only be used restrictively. The GAB model fitted the sorption data at all conditions and over the entire range of water activity studied. The ability of the Young and Nelson model to explain the hysteresis behavior, from analytical and mechanistic perspectives, is evaluated. Temperature and film thickness were found to profoundly influence the nature of moisture interaction and distribution of moisture in the excipient films. An Arrhenius-type relationship was observed between equilibrium moisture content of excipient films and temperature at constant water activity.     

Effects of the centrifugal coating and centrifugal fluidized bed coating methods on the physicochemical properties of sustained-release microparticles using a multi-functional rotor processor

The purpose of the present study was to examine the effect of coating processes on the physicochemical properties of sustained-release microparticles prepared by centrifugal coating (CC) and centrifugal fluidized bed coating (CFC) using a multi-functional rotor processor. Acetaminophen (APAP)-loaded microparticles (DP) were coated with 30% w/w aqueous polymer dispersion of Eudragit® RS (RS) by CC or CFC methods with the apparatus until a dry polymer weight gain of 30%, 60%, 150% and 200% w/w was achieved, and these coated microparticles were abbreviated as CC-DP-RS and CFC-DP-RS, respectively. Both coated microparticles had similar physicochemical properties, but some differences in the drug dissolution behaviors of CC-DP-RS and CFC-DP-RS at lower coating levels were observed. That is, the coated microparticles prepared by CC showed faster release than that by CFC. As a result of dissolution study using Talc seal-coated microparticles and thermal study using differential scanning calorimeter, the rapid dissolution behaviors from CC-DP-RS at the lower coating levels of RS might be due to APAP migration to the coating film during coating due to the weak drying efficacy of the CC method. These findings suggest for the first time that CFC is a suitable method for the coating of functional polymers at lower polymer coating levels, whereas, for the CC method, adjustment of operational conditions (e.g., product temperature, inlet air volume and liquid flow rate) would be required.