High temperature characteristics of Ir–Ta coated and aluminized Ni-base single crystal superalloys

High temperature oxidation and hot corrosion properties of Ir–Ta coated and aluminized Ni-base superalloys are presented. An Ir–Ta binary alloy, proposed as a novel metallic bond coat material, was coated on a Ni-base single crystal superalloy TMS-75 using electron beam physical vapor deposition, followed by a conventional low activity Al pack cementation process. Cyclic oxidation tests and hot corrosion tests revealed that these Ir–Ta coated and aluminized specimens showed reasonably good oxidation and hot corrosion resistance. In addition, it was found that the formation of TCP phases is suppressed by the presence of the Ir–Ta enriched layer. These results indicated that the Ir–Ta alloy system is promising as a new metallic bond coat material for high temperature structural materials.     

Failure mechanism of a thermal barrier coating system on a nickel-base superalloy

An investigation was carried out to determine the failure mechanism of a thermal barrier coating system on an Ni-base superalloy. The coating system consisted of an outer layer of yttria-stabilized zirconia (top coat), and an inner layer of Pt-aluminide (bond coat). Specimens were exposed at 1010 and 1150 °C with a 24-h cycling period to room temperature. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used in microstructural characterization. Spallation of the oxide scale developed by the bond coat was found to be the mode of failure. Experimental results indicated that the breakdown of oxide was affected by internal oxidation of Hf diffusing from the alloy substrate into the bond coat surface developing localized high levels of stress concentration at the oxide–bond coat interface. It was concluded that the cause of failure was degradation of thermal stability of the bond coat accelerating its oxidation rate and permitting outward diffusional transport of elements from the substrate.     

Hot corrosion behavior of ZrO2-MgO coatings in LiCl-Li2O molten salt

In this study, a magnesia-stabilized zirconia (MgO-ZrO₂) top coat was applied to the surface of Inconel 713LC with an aluminized NiCoCrAlY bond coat by an optimized plasma spray process. The resulting materials were tested for corrosion behavior at 948 K for 216 h in a LiCl-Li₂O molten salt under an oxidizing atmosphere. The as-coated and tested specimens were analyzed by scanning electron microscopy (SEM)/X-ray energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The bare superalloy reveals an obvious weight loss due to spalling after scale growth and thermal stress. The top coatings exhibited a superior resistance to hot corrosion in the presence of LiCl-Li₂O molten salt when compared to the bare Inconel 713LC and the aluminized bond coatings. These coatings have been found to be beneficial for improving the hot corrosion resistance of the structural materials for high-temperature lithium molten salts.     

Microstructural changes to metal bond coatings on gas turbine alloys with time at high temperature

Complex coating systems are required to protect nickel-based super alloys from high temperature oxidation and corrosion. Industrial gas turbine blades and heat shields are generally plasma sprayed with a metal bond coating containing nickel, chromium, cobalt, aluminium and yttrium, and then an external thermal barrier coating of yttria-stabilised zirconia is applied. In this study, samples of an IN939 alloy heat shield with both a metal bond coat and a ceramic thermal barrier coating have been heated in air at high temperature for up to 2000 hours to assess the long term stability of the metal bond coat. Polished sections of the heat treated samples were examined by SEM and EDX to determine microstructural changes. The Ni-Cr-Co-Al-Y coating was found to be a very effective barrier against oxidation; the only apparent oxidation being the growth of an alumina layer between the bond coat and ceramic thermal barrier coating. With time, the growth of the Ni₃Al phase in the metallic bond coat was observed, with extensive diffusion of other elements to and from the bond coat.     

Influence of surface modification and long-term exposure on mechanical creep properties of γ-TiAl G4

Abstract

An investigation of the corrosion processes were performed for coated and uncoated γ-TiAl G4, an alloy designed to work in the temperature range 750 – 800°C, where oxidation and corrosion phenomena occur. An aluminising pack cementation treatment was used to improve the oxidation resistance of this γ-TiAl G4 alloy. Cyclic corrosion tests were performed at 800°C in air for up to 800 1-hour cycles with a Na₂SO₄/NaCl mixture. The influence of both aluminisation and the corrosion phenomena on the creep behaviour was investigated. The cyclic corrosion resistance of the coated γ- TiAl G4 was shown to be improved by aluminising. The pack cementation treatment had no detrimental effect on the creep behaviour. Moreover, neither is creep affected by the corrosion of coated specimens. As corroded uncoated specimen exhibited good creep behaviour, it can be concluded that this alloy is suitable, even without coating, for turbine applications in hot corrosion atmospheres at least up to 800°C.     

Adhesion of thermally sprayed hydroxyapatite–bond-coat systems measured by a novel peel test

Ti6Al4V foils, 100 m thick, were coated with thin (10–15 m) bond coats based on titania and zirconia, and subsequently coated with a thick (100–120 m) hydroxyapatite layer, using atmospheric plasma spraying. Peel adhesion tests of the coating systems performed on the foils showed that titania, and mixed titania/non-stabilized zirconia bond coats improved the adhesion of the ceramic layers to the metallic substrate in a statistically significant way, while a partially CaO-stabilized zirconia bond coat led to a decrease of the peel adhesion strength when compared to hydroxyapatite coatings without a bond coat.     

Development of metallic closed cellular materials containing polymers

A closed cellular material containing polymers for intelligent materials has been developed. Polystyrene powder particles coated with a nickel–phosphorus alloy layer by electroless plating were sintered at high temperature. A metallic closed cellular material containing polystyrene was then fabricated. Scanning electron microscope indicated that polystyrene remains in the cells after heat treatment. The compressive tests of this material show a low Young’s modulus and high-energy absorption. These results indicate that this metallic closed cellular material can be used for the energy absorbing systems.     

Fine structures in Fe3Al alloy layer of a new hot dip aluminized steel

The fine structure in the Fe-Al alloy layer of a new hot dip aluminized steel (HDA) was examined by means of X-ray diffractometry (XRD), electron diffraction technique, etc. The test results indicated that the Fe-Al alloy layer of the new aluminized steel mainly composed of Fe₃Al, FeAl and α-Fe (Al) solid solution. There was no brittle phase containing higher aluminum content, such as FeAl₃ (59.18% Al) and Fe2Al7 (62–93% Al). The tiny cracks and embrittlement, formerly caused by these brittle phases in the conventional aluminum-coated steel, were effectively eliminated. There was no microscopic defect (such as tiny cracks, pores or loose layer) in the coating. This is favourable to resist high temperature oxidation and corrosion of the aluminized steel.     

High temperature behavior of diffusion aluminide coating on alloy 600 superalloy

Diffusion coatings are well developed for protection of nickel-based superalloys in hot and corrosive atmospheres. In the present study, aluminide coatings are produced on an alloy 600 substrate via dipping in molten aluminium. The coatings have a two-layered structure containing a dense and compact inner layer across with a porous outer layer. Thermal oxidation and hot corrosion tests showed increased resistance by production of diffusion coatings on the superalloy. Increasing the dipping time had a positive effect on the high temperature resistance of the coated alloy.     

Bioactive double glass coatings for Co-Cr-Mo alloy

Glass compositions for double coatings for a Co-Cr-Mo alloy were developed. The glass compositions were chosen to fulfil such requirements as matching thermal expansion, low glass transition temperature and moderate solubility. For the ground coat a fairly high durability is required, whereas the cover coat must be bioactive, i.e. become attached to living bone by a chemical bond. Two compositions of each type were developed by computer-aided optimization. The glasses were chosen in the Na₂O–CaO–B₂O₃–Al₂O₃–SiO₂–P₂O₅ system. The bioactivity was tested in vitro by immersion in a simulated body fluid. The double coatings on Co–Cr–Mo alloy released hexavalent chromium into the solution as detected by yellow colouration and spectrophotometry. This colouration was strong at the margin between coated and uncoated metal and may be explained by oxidation of trivalent chromium of the alloy in the presence of glass. The released chromium did not have any notable effect on the calcium phosphate formation. After replensihing the solution no coloration was observed. This suggests that the chromate is easily dissolved and that it may be possible to wash it out prior to implantation.     

热浸镀铝钢的性能及用途

为了便于研究人员和用户了解、掌握热浸镀铝钢的性能及其用途,进一步推动我国在这方面的大力发展,作者综述了热浸镀铝钢镀层的机械性能、耐腐蚀性能、抗高温氧化性能和其他性能及其在各工程领域上的应用情况,列出了部分性能指标。     

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.     

Improving thermal barrier coatings by laser remelting

Thermal barrier coatings are extensively used to protect metallic components in applications where the operating conditions include aggressive environment at high temperatures. These coatings are usually processed by thermal spraying techniques and the resulting microstructure includes thin and large splats, associated with the deposition of individual droplets, with porosity between splats. This porosity reduces the oxidation and corrosion resistance favouring the entrance of aggressive species during service. To overcome this limitation, the top coat could be modified by laser glazing reducing surface roughness and sealing open porosity. ZrO2(Y2O3) top coat and NiCrAlY bond coating were air plasma sprayed onto an Inconel 600 Ni base alloy. The top coat was laser remelted and a densified ceramic layer was induced in the top surface of the ceramic coating. This layer inhibited the ingress of aggressive species and delayed bond coat oxidation.     

Oxidation and Hot Corrosion of Gadient Thermal Barrier Coatings Prepared by EB—PVD

The performances of gradient thermal barrier coatings (GTBCs)produced by EB-PVD were evaluated by isothermal oxidation and cyclic hot corrosion(HTHC) tests.Compared with conventional two-layered TBCs, the GTBCs exhibite better resistance to not only oxidation but also hot-corrosion.Adense Al2O3 layer in the GTBCs effectively prohibites inward diffusion of Oand S and outward diffusion of Al and Cr during the tests.On the other hand ,an “inlaid“ interface ,resulting from oxidation of the Al along the columnar grains of the bond coat,enhances the adherence of Al2O3 layer.Failure of the GTBC finally occurred by cracking at the interface between the bond coat and Al2O3 layer, due to the combined effect of sulfidation of the bond coat and thermal cycling.     

Oxidation and Hot Corrosion of Gradient Thermal Barrier Coatings Prepared by EB-PVD

The performances of gradient thermal barrier coatings (GTBCs) produced by EB-PVD were evaluated by isothermal oxidation and cyclic hot corrosion (HTHC) tests. Compared with conventional two-layered TBCs, the GTBCs exhibite better resistance to not only oxidation but also hot-corrosion. A dense Al2O3 layer in the GTBCs effectively prohibites inward diffusion of O and S and outward diffusion of Al and Cr during the tests. On the other hand, an "inlaid" interface, resulting from oxidation of the Al along the columnar grains of the bond coat, enhances the adherence of AI2O3 layer. Failure of the GTBC finally occurred by cracking at the interface between the bond coat and AI2O3 layer, due to the combined effect of sulfidation of the bond coat and thermal cvcling.     

Hot Corrosion of Protective Coatings

Improvement in efficiencies of gas turbine engines requires a significant increase of gas inlet temperatures. This results in an increased service temperature for blade materials and consequently in enhanced oxidation and hot corrosion attack of the blade coatings, which are usually of MCrAlY type where M is Ni, Co or NiCo. This type of coating can provide protection against oxidation and hot corrosion and act as a bond coat for thermal barrier coating systems. In both cases slow growth rates and optimum adherence of the alumina scales forming on the MCrAlY coatings during high temperature exposure are significant for component life. The above mentioned properties for the alumina scales strongly depend on the coating base composition as well as on the presence of minor alloying elements. In the present paper the performance of existing superalloys during hot corrosion is briefly described followed by the results obtained on hot corrosion of MCRAlY type coatings explaining the effect of trace elements on the life of coatings in the presence of NaCl and vanadium containing environments. Optimum thickness to improve the life of superalloys with NiCoCrAlY as a bond coat and yttria stabilized zirconia thermal barrier coatings has been identified. Based on the results, an electrochemical mechanism is proposed and shows that hot corrosion of protective coatings is an electrochemical phenomenon. Hence electrochemical techniques appear to be quite useful in evaluating the coatings for hot corrosion resistance.     

Effect of superalloy substrate composition on the performance of a thermal barrier coating system

An investigation was carried out to determine the performance of a thermal barrier coating system consisting of (ZrO₂-8% Y₂O₃)/(Pt) on two single-crystal Ni-base superalloys. Coating/alloy behavior was studied with reference to: (i) initial microstructural features, (ii) oxidation properties, (iii) thermal stability characteristics, and (iv) failure mechanism. All thermal exposure tests were carried out at 1150°C in still air with a 24-h cycling period to room temperature. Failure of the coating system was indicated by macroscopic spallation of the ceramic top coat. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used to characterize the microstructure.Decohesion between the thermally grown oxide and bond coat was found to be the mode of failure of the coating system for both alloys. This was correlated with the formation of Ti-rich and/or Ti+Ta-rich oxide particles near the oxide-bond coat interface degrading the adherence of the thermally grown oxide. However, the thickening rate of the oxide had very little or no effect on the relative coating performance. It was concluded that the coating performance is critically dependent on alloy substrate composition particularly the concentration of elements, which could have adverse effects on oxidation resistance such as Ti.     

Microstructure and mechanical properties of cold-rolled TiNbTaZr biomedical β titanium alloy

The influence of the plasma-sprayed coatings and of the atmosphere on creep of the Ti–6Al–4V alloy was investigated. Yttria partially stabilized zirconia (YSZ) with CoNiCrAlY bond coat was atmospherically plasma sprayed on Ti–6Al–4V substrates. Constant load creep tests were conducted on a standard creep machine in air and nitrogen atmospheres on uncoated samples and in air on coated samples, at stress levels of 520 MPa at 500 °C, 319 MPa at 600 °C and 56 MPa at 700 °C. Results indicated that the creep rates in nitrogen and of the coated alloy were lower than those of the uncoated in air.     

Bonding strength of Al/Mg/Al alloy tri-metallic laminates fabricated by hot rolling

One of major drawbacks of magnesium alloy is its low corrosion resistance, which can be improved by using an aluminized coating. In this paper, 7075 Al/Mg-12Gd-3Y-0·5Zr/7075 Al laminated composites were produced by a hot roll bonding method. The rolling temperature was determined based on the flow stresses of Mg-12Gd-3Y-0·5Zr magnesium alloy and 7075 Al alloy at elevated temperature. The bonding strength of the laminate composites and their mechanism were studied. The effects of the reduction ratio (single pass), the rolling temperature, and the subsequent annealing on the bonding strength were also investigated. It was observed that the bonding strength increased rapidly with the reduction ratio and slightly with the rolling temperature. The bonding strength increases with the annealing time until the annealing time reaches 2 h and then decreases. The mechanical bond plays a major role in the bonding strength.     

Novel hydroxyapatite/tantalum surface coating for metallic dental implant

The aim of this study was to design and produce a novel surface composite coating on metallic substrate in order to improve the biocompatibility of metallic dental implant and the bone osteointegration simultaneously.Stainless steel 316L (SS) was used as a metallic substrate and a novel double-layer hydroxyapatite/tantalum (HA/Ta) coating was prepared on it. Tantalum coating was made using physical vapor deposition process and HA coating was produced using plasma-spraying technique on it. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were utilized to investigate the coating characterization. Electrochemical polarization tests were performed in two types of physiological solutions at 37 ± 1 °C in order to determine the corrosion behavior of the coated and uncoated specimens as indication of biocompatibility.The results indicated that the decrease in corrosion current density was significant for HA/Ta coated specimens and was much lower than the value obtained for uncoated 316L SS. The novel double-layer HA/Ta composite coating could improve the corrosion resistance and thus the biocompatibility of 316L SS dental implant.