Effect of Cyclic Oxidation Exposure on Tensile Properties of a Pt-Aluminide Bond-Coated Ni-Base Superalloy

The tensile behavior of a directionally solidified (DS) Ni-base superalloy, namely, CM-247LC, was evaluated in the presence of a Pt-aluminide bond coat. The effect of the thermal cycling exposure of the coated alloy at 1373 K (1100 °C) on its tensile properties was examined. The tensile properties were evaluated at a temperature of 1143 K (870 °C). The presence of the bond coating caused an approximately 8 pct drop in the strength of the alloy in the as-coated condition. However, the coating did not appreciably affect the tensile ductility of the substrate alloy. The bond coat prevented oxidation-related surface damage to the superalloy during thermal cycling exposure in air at 1373 K (1100 °C). Such cyclic oxidation exposure (up to 750 hours) did not cause any further reduction in yield strength (YS) of the coated alloy. There was a marginal decrease in the ultimate tensile strength (UTS) with increased exposure duration. Because of the oxidation protection provided by the bond coat, the drastic loss in ductility of the alloy, which would have happened in the absence of the coating, was prevented.     

High-Temperature Creep Degradation of the AM1/NiAlPt/EBPVD YSZ System

The failure mechanisms of a NiAlPt/electron beam physical vapor deposition yttria-stabilized-zirconia thermal barrier coating system deposited on the AM1 single crystalline substrate have been investigated under pure creep conditions in the temperature range from 1273 K to 1373 K (1000 °C to 1100 °C) and for durations up to 1000 hours. Doubly tapered specimens were used allowing for the analysis of different stress states and different accumulated viscoplastic strains for a given creep condition. Under such experiments, two kinds of damage mechanisms were observed. Under low applied stress conditions (i.e., long creep tests), microcracking is localized in the vicinity of the thermally grown oxide (TGO). Under high applied stress conditions, an unconventional failure mechanism at the substrate/bond coat interface is observed because of large creep strains and fast creep deformation, hence leading to a limited TGO growth. This unconventional failure mechanism is observed although the interfacial bond coat/top coat TGO thickening is accelerated by the mechanical applied stress beyond a given stress threshold.     

Hardfacing of a Modified 9Cr–1Mo Steel Component Using a Nickel-Base Alloy

A dashpot piston made of modified 9Cr–1Mo steel is hardfaced with NiCr-B alloy by the Plasma Transferred Arc (PTA) process. During initial trials, a large number of cracks were observed in the hardface deposit when hardfacing was carried out directly on the modified 9Cr–1Mo steel substrate using a preheat temperature of 723 K. Both the deposit and the martensitic structure formed in the heat affected zone of the substrate during deposition are hard and hence were unable to absorb the thermal stresses generated, leading to cracking. Subsequently, hardfacing trials carried out with an intermediate layer of 2 mm thick Inconel-625 alloy, were successful and deposits were crack-free. Use of a relatively soft Inconel-625 between the hardface deposit and the substrate reduced martensite formation in the substrate, and thus the cracking susceptibility of the deposit.     

Fatigue and creep damage resistance of a thermal barrier coated superalloy for combustor liners in aero turbines

Fatigue testing of thermal barrier coated (TBC), bond coated only and bare Superni C263 superalloy was conducted at 800°C in air. Fatigue results reveal that the endurance limits for the TBC and bond coated substrate was substantially higher than that of the base alloy, while the opposite was found for high stress, low cyclic life times. It appears that the increase in endurance limit for the TBC and bond coated superalloy is due to load shifting to the bond coat, and the premature failure for these two materials is possibly due to high stress crack imitation/growth in the TBC/bond coat layers. In addition to fatigue, accelerated creep properties of thermal barrier coated (TBC) Superni C263 were evaluated. Creep results reveal that the life of the TBC composite under accelerated creep condition is substantially high compared to that of the bare substrate. The mode of fracture in the substrate at very high stresses was transgranular whereas that at low stresses was intergranular. Delamination of bond coat, oxidation of the substrate and spallation of the ceramic layer were evident at very high stress. It was evident that the substrate has negligible estimated rupture strength after 10,000 hours of service exposure.     

Assessment of Cyclic Lifetime of NiCoCrAlY/ZrO2-Based EB-PVD TBC Systems via Reactive Element Enrichment in the Mixed Zone of the TGO Scale

The chemical composition of the alumina-zirconia mixed zone (MZ) of an electron beam physical vapor deposited thermal barrier coating (EB-PVD TBC) system is affected by service conditions and by the interdiffusion of elements from the substrate alloy below and the zirconia top coat. Three NiCoCrAlY bond-coated Ni-base substrates with YPSZ or CeSZ EB-PVD TBCs were subjected to a cyclic furnace oxidation test (FCT) at 1373 K (1100 °C) in order to provide experimental evidence of a link between chemistry of the MZ, the substrate alloy, the ceramic top coat, and the time in the FCT. Energy dispersive spectroscopy of the MZ revealed preferred accumulation of Cr, Zr, Y, and Ce. The concentration of the reactive elements (RE = Ce + Y + Zr) was related to the respective average lifetimes of the TBC systems at 1373 K (1100 °C). The RE content in the MZ turned out to be a life-limiting parameter for YPSZ and CeSZ TBC systems which can be utilized to predict their relative lifetimes on the individual substrates. Conversely, the TBC failure mechanisms of YPSZ and CeSZ TBC systems are dissimilar.     

镀铝锌彩涂板爆孔缺陷分析

对镀铝锌彩涂板生产中产生爆孔缺陷的原因进行了分析,认为爆孔缺陷主要由镀铝锌合金镀层的化学特性差异、热物性差异和镀铝锌合金层缺陷等因素造成。通过改善镀铝锌板合金镀层的表面质量、提高轧辊光整粗糙度、减少镀铝锌板表面疏松孔洞缺陷、改进彩涂固化工艺温度曲线,消除了镀铝锌彩涂板爆孔缺陷,满足了彩板用户的质量要求。     

High Temperature Oxidation and Microstructural Evolution of Modified MCrAlY Coatings

Thermal sprayed MCrAlY coatings are widely used as a bond coat in thermal barrier systems to protect the substrate from corrosion and high temperature oxidation and to improve the compatibility between the ceramic top coat and metallic substrate. In this paper, the high temperature oxidation resistance of MCrAlY coatings with modified compositions was evaluated; in particular, the effect of the addition of reactive and refractory elements (Ta, Re, Si, and Hf) was investigated. MCrAlY coatings were obtained by high velocity oxygen fuel spray and vacuum plasma spray techniques; samples were exposed to air at 1423 K (1150 °C) and the oxidation kinetics were evaluated by measuring the thickness of the thermally grown oxide (TGO) scale at several exposure times. Experimental data confirmed that the oxidation resistance of MCrAlY coatings is strictly related to the amount of the reactive and refractory elements in the starting powders and that a thorough understanding of the microstructural modifications taking place during oxidation is essential for controlling TGO growth and thermal barriers’ durability.     

Development of Functionally Graded Coating Material for Metallic Thermal Protection System of Reusable Launch Vehicle

Functionally graded coating material (FGM) based on yttria-stabilized zirconia (YSZ) and Ni–Cr–Al–Y was designed and developed for metallic thermal protection system of reusable launch vehicle (RLV). Coating was made using premixed mechanically alloyed YSZ and Ni–Cr–Al–Y powders through plasma spray technique. Thermal stress analysis was carried out, which showed significant reduction in stress in FGM coating as compared to dual coating. The phase composition of coating was found to be close to the designed one. Porosity varied in the range of 8–18%. Average emissivity of three different time exposures of 30, 60 and 90 s was found to be 0.8. Solar absorptivity was found to be 0.55. Fatigue life of FGM coating evaluated along with Inconel and Ti6Al4V metallic substrate was compared with dual coating. FGM coating could be fatigue tested to relatively higher thermal cycles as compared to dual coating on the Inconel substrate. Heat flux measured at top surface was found to be close to simulated heat flux for windward side of RLV. Top surface temperature was similar for both type of metallic substrates and was matching with predicted temperature. However, substrate temperature was higher for Ti6Al4V as compared to Inconel alloy due to higher thermal diffusivity of Ti6Al4V.     

The influence of a martensitic phase transformation on stress development in thermal barrier coating systems

Thermal barrier coatings (TBCs) provide thermal insulation and oxidation protection of Ni-base superalloys in elevated temperature turbine applications. Thermal barrier coating failure is caused by spallation, which is related to the development of internal stresses during thermal cycling. Recent microstructural observations have highlighted the occurrence of a martensitic bond coat transformation, and this finite-element analysis was conducted to clarify the influence of the martensite on the development of stresses and strains in the multilayered system during thermal cycling. Simulations incorporating the volume change associated with the transformation and experimentally measured coating properties indicate that out-of-plane top coat stresses are greatly influenced by the presence of the martensitic transformation, the temperature at which it occurs relative to the strength of the bond coat and attendant bond coat plasticity. Intermediate values of bond coat strength and transformation temperatures are shown to result in the highest top coat stresses. This article is based on a presentation in the symposium “Terence E. Mitchell Symposium on the Magic of Materials: Structures and Properties” from the TMS Annual Meeting in San Diego, CA in March 2003.     

Stability and Durability Study of Nano Pt Coated Titanium for Electrode Application

The present work involves development of noble metal nanoparticle coated Ti based electrode for application in severe corrosive environments. The etched Ti substrate was coated with noble metal nanoparticle via a seed mediated hydrothermal reduction method. Advanced surface characterization techniques elucidated that this two step synthesis method generated a uniform and highly dispersed nanoparticle coating on Ti which is essential for application in severe corrosive medium. The enhanced activity for methanol electro-oxidation compared to polycrystalline Pt proved the excellent electrocatalytic activity of as synthesized Pt nanoparticles coated Ti electrode. The adhesion strength of the coating on the Ti substrate was found to be excellent with a rank of 5A as per ASTM standard. The long term durability of Pt nanoparticles coated big cylindrical Ti mesh with 164 cm² surface area was tested by employing it as anode for electro-oxidation of Ce in 11.5 M nitric acid under an applied operational current density of 9 mA/cm² for 1000 h. Only 1 V increment was observed even after 1000 h confirming the excellent durability of this electrode. SEM and elemental mapping of the surface of coated electrode after 1000 h exposure in severe corrosive nitric acid further confirmed uniform dispersion of Pt on the surface and absence of delamination.     

Effect of Prior Oxidation on Tensile Properties of Bare and Al3Ti Diffusion Aluminide Coated Near α Titanium Alloy Titan 29A

The effect of prior oxidation, for various durations up to 2,000 h in air at 650 °C, on the room temperature tensile properties of uncoated and Al₃Ti diffusion aluminide coated near α Ti alloy, Titan 29A, has been evaluated. The tensile properties of the uncoated alloy deteriorated with oxidation. Oxidation for just 100 h caused 11–13 % decrease in yield strength (YS) and ultimate tensile strength (UTS) of the alloy. The uncoated alloy exhibited brittle fracture within the elastic regime at significantly lower stress after oxidation for 2,000 h. On the other hand, the strength of the coated alloy remained unaffected even after 2,000 h of oxidation and the YS and UTS was similar to that of the un-oxidized alloy. The ductility of the coated alloy, however, decreased with the increase in oxidation duration. Such differences in the tensile behavior of the uncoated and coated alloy can be ascribed to the beneficial effect of the Al₃Ti diffusion aluminide coating in preventing surface embrittlement in the alloy during oxidation.     

Preparation,Characterization and Mechanical Properties of Cu-Sn Alloy/Graphite Composites

Ni-B coating was prepared on the surface of graphite particles using the electroless plating method. The Ni-B coating was composed of spherical grains with average diameter of 80 nm. The phases of Ni-B coating were indexed as nanosized crystal Ni phase and amorphous Ni-B phase. Cu-Sn alloy/graphite composites with 0.5, 1.0, 1.5, and 2.0 wt pct graphite contents were synthesized by the powder metallurgy method. Ni-B coating improved the wettability and bonding strength between the Cu-Sn alloy and graphite. The composite with Ni-B coated graphite exhibited higher density, hardness, and compression strength compared with the composites with bare graphite. The crack propagation mechanism of the composites was also analyzed.     

Evaluation of Thermal Spray Alumina Coatings on Nickel Electrode Connector for Reprocessing Applications

Electro-oxidative dissolution of the spent mixed oxide nuclear fuels with high plutonium content from prototype fast breeder reactor need to be carried out in boiling 11.5 M HNO₃. Nickel electrode connectors employed in the electrolyser of the dissolver should possess good corrosion resistance as well as good electrical conductivity. Alumina coating deposited on Ni by plasma spraying was evaluated by electrochemical polarization and impedance experiments in 11.5 M HNO₃ at room temperature. In order to improve corrosion resistance, alumina coating relatively denser than plasma spray coating was deposited over Ni by detonation gun (D-gun) spray coating. This alumina coating exhibited a high insulation resistance and the weight loss of alumina coated Ni disc was only 3% compared to 29% for bare Ni disc sample when exposed to the vapour of 11.5 M boiling HNO₃ for 12 h. However, coating delamination observed at the alumina/bond coat interface was attributed to the penetration of HNO₃ vapour through the pores in the coating. Since alumina coating deposited by D-gun technique over Ni was also found to offer only short-term protection against corrosion due to HNO₃ vapour, monolithic dense alumina sleeve fabricated through powder metallurgy route was recommended instead of coating, for better corrosion protection in HNO₃ vapour compared to thermal spray coating.     

Coating of 6028 Aluminum Alloy Using Aluminum Piston Alloy and Al-Si Alloy-Based Nanocomposites Produced by the Addition of Al-Ti5-B1 to the Matrix Melt

The Al-12 pctSi alloy and aluminum-based composites reinforced with TiB₂ and Al₃Ti intermetallics exhibit good wear resistance, strength-to-weight ratio, and strength-to-cost ratio when compared to equivalent other commercial Al alloys, which make them good candidates as coating materials. In this study, structural AA 6028 alloy is used as the base material. Four different coating materials were used. The first one is Al-Si alloy that has Si content near eutectic composition. The second, third, and fourth ones are Al-6 pctSi-based reinforced with TiB₂ and Al₃Ti nano-particles produced by addition of Al-Ti5-B1 master alloy with different weight percentages (1, 2, and 3 pct). The coating treatment was carried out with the aid of GTAW process. The microstructures of the base and coated materials were investigated using optical microscope and scanning electron microscope equipped with EDX analyzer. Microhardness of the base material and the coated layer were evaluated using a microhardness tester. GTAW process results in almost sound coated layer on 6028 aluminum alloy with the used four coating materials. The coating materials of Al-12 pct Si alloy resulted in very fine dendritic Al-Si eutectic structure. The interface between the coated layer and the base metal was very clean. The coated layer was almost free from porosities or other defects. The coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages (1, 2, and 3 pct), results in coated layer consisted of matrix of fine dendrite eutectic morphology structure inside α-Al grains. Many fine in situ TiAl₃ and TiB₂ intermetallics were precipitated almost at the grain boundary of α-Al grains. The amounts of these precipitates are increased by increasing the addition of Al-Ti5-B1 master alloy. The surface hardness of the 6028 aluminum alloy base metal was improved with the entire four used surface coating materials. The improvement reached to about 85 pct by the first type of coating material (Al-12 pctSi alloy), while it reached to 77, 83, and 89 pct by the coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages 1, 2, and 3 pct, respectively.     

Effect of Mould Coatings and Pouring Temperature on the Fluidity of Different Thin Cross-Sections of A206 Alloy by Sand Casting

Thin-wall castings of the A206 alloy can pose a manufacturing problem associated with mould filling which results in fluidity defect. The mould coating generates a smooth surface, and reduces the friction between the melt and mould contact, thus reducing the heat-transfer coefficient, which in turn leads to enhancement of fluidity and mechanical properties. The fluidity of A206 alloy was observed in various cross-sections of the thin channels at three altered pouring temperatures i.e. 700, 750 and 780 °C for uncoated and coated green sand moulds. The Graphite and Soapstone powder coatings were used as sand mould coatings in the present investigation. It was found that the fluidity of aforesaid alloy was significantly increased with the Soapstone powder mould coating at pouring temperature of 750 °C. The characterization of the coating materials was performed by the X-ray diffraction analysis and scanning electron microscope test with EDAX.     

Part Load Performance Characteristics of a Low-Heat Rejection Diesel Engine Fueled with Biodiesel

This experimental study focused on improving performance characteristics of a low-heat rejection (LHR) diesel engine operating with biodiesel fuel. To ensure LHR conditions, the cylinder head and valves of the test engine were coated with a Y2O3-ZrO2 (yttria-stabilized zirconia) layer of 0.35?mm thickness over a NiCrAl bond coat of 0.15?mm thickness. The fuel used in the engine tests was produced from sunflower oil with transesterification methods. The engine test results showed that brake-specific fuel consumption (BSFC) and brake thermal efficiency were improved with biodiesel usage and thermal barrier coating (TBC) application. Further, exhaust gas temperature was decreased with use of biodiesel, while it was increased with TBC application.     

Microstructural Analysis of Multi-phase Ultra-Thin Oxide Overgrowth on Al–Mg Alloy by High Resolution Transmission Electron Microscopy

High-resolution transmission electron microscopy analyses are carried out to understand the microstructure of the ultra-thin oxide-film grown on a (native) amorphous Al₂O₃-coated Al-0.8 at.% Mg alloy substrate at T = 600 K for t = 2 h and at pO₂ of 1 × 10^?2 Pa. This oxide-film is found to be non-uniformly thick with thicknesses varying from 1.50 to 4.60 nm. Occasionally, this oxide is found to diffuse into the Al–Mg alloy substrate, forming oxide thicknesses up to 10.5 nm. Overall, this oxide-film is found to consist of a mixed amorphous, (poly) crystalline and an intermediate amorphous-to-crystalline transition regions, with crystalline regions consisting mostly of MgO and the diffused oxide regions into the Al–Mg alloy substrate coated with γ-Al₂O₃. These observations are then compared with the experimental results obtained using angle-resolved X-ray Photoelectron Spectroscopy analysis and thermodynamic predictions for the growth of an ultra-thin oxide-film due to dry, thermal oxidation of Al–Mg alloy substrates.     

Assessment of Heat Transfer During Solidification of Al–22% Si Alloy by Inverse Analysis and Surface Roughness Based Predictive Model

Heat flux transients were estimated during unidirectional downward solidification of Al?C22% Si alloy against copper, die steel and stainless steel chills. The chill instrumented with thermocouples was brought into contact with the liquid metal so as to avoid the effect of convection associated with the pouring of liquid metal. Heat flux transients were estimated by solving the inverse heat conduction problem. Higher thermal conductivity of chill material resulted in increased peak heat flux at the metal/chill interface. Peak heat flux decreased when 100???m thick alumina coating was applied on the chill surface. The lower thermal conductivity of alumina based coating and the presence of additional thermal resistance decreases the interfacial heat transfer. For uncoated chills, the ratio of the surface roughness (R_a) of the casting to chill decreased from 6.5 to 0.5 with decrease in the thermal conductivity of the chill material. However when coating was applied on the chill, the surface roughness ratio was nearly constant at about 0.2 for all chill materials. The measured roughness data was used in a sum surface roughness model to estimate the heat transfer coefficient. The results of the model are in reasonable agreement with experimentally determined heat-transfer coefficients for coated chills.     

Hot Corrosion Behaviour of HVOF Sprayed Stellite-6 Coatings on Gas Turbine Alloys

The coal burned natural gas in contact with gas turbine can contain impurities of sodium, sulfur, vanadium, silicon and possibly lead and phosphorous, induce accelerated hot corrosion during long term operation. Coatings are frequently applied on gas turbine components in order to restrict surface degradation and to obtain accurate lifetime expectancies. High velocity oxy-fuel thermal spraying has been used to deposit Stellite-6 alloy coatings on turbine alloys. Hot corrosion behavior of the coatings were investigated for 50 cycles of 1 h heating at 800 °C followed by 20 min cooling in presence of Na₂SO₄ + 50 % V₂O₅ measuring weight gain (or loss). X-ray diffraction and SEM/EDAX techniques were used to characterize the oxide scale formed. The superior performance of Stellite-6 coating can be attributed to continuous and protective thin oxide scale of CoO, Cr₂O₃ and SiO₂ formed on the surface. The coating region beneath this thin oxide scale was partially oxidized. Uncoated SuperCo-605 and MDN-121 showed less weight gain than Stellite-6 coated samples, but they showed spalling or sputtering during cyclic oxidation. Stellite-6 coating was dense and pore free even after 50 cycles, indicating that it can resist the hot corrosion cycle.     

Effects of an α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> thin film on the oxidation behavior of a single-crystal Ni-based superalloy

A ∼150-nm-thick coating layer consisting of α-Al₂O₃ as the major phase with a minute amount of Φ-Al₂O₃ was deposited on the surface of a single-crystal Ni-based superalloy by chemical vapor deposition (CVD). Within 0.5 hours of oxidation at 1150°C, the resulting thermally grown oxide (TGO) formed on the coated alloy surface underwent significant lateral grain growth. Consequently, within this time scale, the columnar nature of the TGO became established. After 50 hours, a network of ridges was clearly observed on the TGO surface instead of equiaxed grains typically observed on the uncoated alloy surface. Comparison of the TGO morphologies observed with and without the CVD-Al₂O₃ layer suggested that the transient oxidation of the alloy surface was considerably reduced. Also, the CVD-Al₂O₃ layer significantly reduced the growth rate of the TGO and improved its spallation resistance, while slowing the internal oxidation of Ta-rich areas that were present in the superalloy as-casting defects. These results demonstrated that this thin α-Al₂O₃ coating could be used as a means of favorably altering the TGO morphology and growth kinetics for no bond coat thermal barrier coating (TBC) applications. Y.-F. SU, formerly Doctoral Candidate