Evaluation of mixed oxide formation and sintering behavior in thermal barrier coatings on nickel‐based superalloy

Thermal barrier coatings are widely used in aircraft turbines to protect nickel‐based superalloys from the effect of high temperature oxidation and hot corrosion. In this study, both NiCrAlY bond coat and yttria‐stabilized zirconia top coat were deposited using atmospheric plasma spray technique. After coating production, specimens were exposed to oxidation in air atmosphere at 900 °C, 1000 °C and 1100 °C for different periods of time up to 50 h. Microstructural transformations in the ceramic top coat and growth behavior of the thermally grown oxide layer were examined using scanning electron microscopy, porosity calculation, elemental mapping and hardness measurement. Formation of different types of oxides in the thermally grown oxide layer shows that this process strongly depends on deposition technique as well as on oxidation time and temperature. Hardness values of the top coat increased with a decrease in the porosity of the top coat. Uniformity and homogeneity of the thermally grown oxide layer and densification of the top coat were evaluated in terms of the structural durability of thermal barrier coating systems.     

Role of nickel in the oxidation of Fe–Cr–Ni alloys in air–water vapour atmospheres

Abstract

A series of experimental austenitic alloys has been produced in which the nickel content ranges from 14 to 43%, with constant levels of 20%Cr, 1%Mn and 0.5%Si. A combination of isothermal, discontinuous and cyclic oxidation testing has been used to elucidate the performance in dry air and in air with 10%, 45% or 62% water vapour at 700°C and 1000°C. Evaluation was by means of thermogravimetry, surface analysis with glow discharge optical emission spectroscopy and scanning electron microscopy.

Nickel is shown to have several roles: it accelerates the kinetics of chromia formation yet suppresses chromia spallation at 700°C. At 1000°C, it strongly decreases the breakaway oxidation and spalling associated with iron oxide formation. This effect is particularly marked in environments containing water vapour, where the material loss may be decreased 10-fold by an increase in the nickel content. Results correlate to thermodynamic and kinetic data which show nickel to increase the chromium activity and diffusivity in the alloy.     

High temperature oxidation behaviour of directionally solidified nickel base superalloy CM–247LC

Abstract

The present paper describes the isothermal and cyclic oxidation behaviour of the technologically important nickel base directionally solidified superalloy CM-247LC in air in the temperature range 1000-1200°C. This superalloy behaves as a transition nickel base alloy under isothermal oxidation conditions and exhibits a fairly long transient oxidation period (~20 h at 1100°C). Irrespective of the temperature of exposure and nature of oxidation (isothermal or cyclic), a composite oxide scale develops on CM-247LC. While the outer portion of the oxide scale consists of either spinel (NiAl₂O₄) or a mixture of spinel and NiO, depending on oxidation temperature, the inner portion is always constituted of alumina. Beyond the transient period, the alloy is found to follow parabolic oxidation kinetics. The oxide layer that forms is invariably very non-uniform in thickness, and is dispersed with two types of oxide particles. While tantalum rich oxide particles are found scattered in the outer zone of the oxide layer, hafnium rich oxide particles lie close to the oxide/metal interface. Results also reveal that the nature of oxidation associated with the CM-247LC superalloy causes entrapment of metal islands in the oxide layer.     

Aluminium and silicon base coatings for high temperature alloys—Process development and comparison of properties

The paper is concerned with systematic studies on the formation of overlay coatings on nickel and iron base superalloys to improve their resistance against high temperature oxidation and hot corrosion. In contrast to the simpler case of aluminizing nickel base alloys, the problems arising in aluminizing iron base superalloys and in siliconizing nickel base alloys have not yet been solved.A new and economical coating procedure is presented, which involves a reaction sintering process of unalloyed powder mixtures to obtain overlay coatings. The influence of the compositions of the layer and the substrate on the chemical compatibility of the whole layer composite is described in detail. It is shown that overlay coatings containing high concentrations of silicon can only be applied on nickel base superalloys if elements that are able spontaneously to form reaction barriers are present within the substrate alloy (e.g. aluminium) or in the as-preformed interlayer. Refractory metals have proved to be the most advantageous.To obtain a ductile coating, silicon-rich donor phases in the form of isolated precipitates were incorporated into a matrix containing low concentrations of silicon. These coating systems can also be applied to improve the aluminization of iron base alloys. The high temperature oxidation and corrosion behaviours of the coated samples were tested in burner gas or air at 1000°C and in molten salts at 900°C.     

The influence of platinum on the oxidation and sodium sulfate induced hot corrosion of NiAl diffusion coatings

Abstract

Oxidation and corrosion properties at 900°C of two coatings, one containing platinum and the other platinum-free, on IN792 were investigated. During the corrosion exposures, sodium sulfate salt was situated in the furnace together with the coated specimens. The temperature of the salt was kept above its melting point but lower than the temperature of the coating i.e. 900°C. The exposure times ranged from 100 to 1000 h. The formed oxide scales were studied by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy and scanning transmission electron microscopy techniques. It is shown that the presence of Pt improves the protective properties of the coating against corrosion. The propagation stage of corrosion on the platinum-free coating is reached already after 100 h of exposure. On the other hand the scale formed on the platinum-rich samples still appeared to be protective even after 500 h. The details concerning structure and chemistry of the scales formed are presented and discussed.     

Oxidation behavior and electrochemical corrosion performance of Ti<Subscript>3</Subscript>Al alloy with TiAl coating

Magnetron-sputter deposition was used to produce a Ti–48Al–8Cr–2Ag (at. %) coating on a Ti–24Al– 17Nb–0.5Mo (at. %) alloy substrate. Oxidation behavior was studied in air at 900–1000°C. The results indicated that the oxidation rate of sputtered Ti–48Al–8Cr–2Ag nanocrystalline coating was lower than that of the Ti₃Al alloy at 900°C. The former formed a scale of merely Al₂O₃, and the latter formed a scale of TiO₂. However, the Ti–48Al–8Cr–2Ag nanocrystalline coating showed a little bit higher oxidation rate than Ti₃Al alloy at 1000°C because the outer TiO₂ scale formed and columnar boundaries of the coating gave a larger actual oxidation area than the original alloy. The electrochemical corrosion behavior was investigated in a 3.5% NaCl solution at room temperature. The coating showed excellent electrochemical corrosion resistance in 3.5% NaCl solution because it exhibited stable passive polarization behavior without any overpassivation phenomena.     

The halogen effect for improving the oxidation resistance of TiAl-alloys

Abstract

Alloys based on TiAl intermetallics are potential candidates for high temperature applications in e.g. aero engines or automotive engines because of their low specific weight and good high temperature strength. To improve their oxidation resistance at temperatures up to 1000°C the halogen effect offers an innovative and cost-effective way. The addition of small amounts of halogens into the surface leads to the preferential formation of gaseous aluminium halides which are oxidised to aluminium oxide during their outward migration forming a dense, protective and slowly growing alumina scale on the surface. In this paper two methods were used to apply halogens to the surface, ion implantation (F and Cl) and a liquid phase process (F). Ion beam analysis with detection limits in the ppm-range was applied to quantify the needed amount of halogens to achieve the halogen effect. Thermocyclic oxidation experiments at 900°C were performed in laboratory air and wet air. Depth concentration profiles of fluorine were measured by PIGE within the first 1.4 μm without destruction of the sample before and after oxidation. Furthermore, the loss of fluorine during heating up and oxidation was measured characterising the stability of the effect. Simultaneous RBS-measurements of the O-, Al- and Ti-depth profiles prove the formation and growth of an almost pure alumina scale. Correlation with the fluorine profiles validates the proposed model for the halogen effect. Furthermore, metallographic methods, REM, EPMA, AES and the proton micro beam (PIXE) were applied to study cross-sections. A virtually pure alumina scale was found after F-treatment and oxidation up to 1500 hours at 900–1000°C in air. The fluorine depth profiles after ion implantation and liquid phase treatment, respectively, show similar levels for both methods before and after oxidation. The development of the fluorine interfacial concentration underneath the oxide scale as a function of oxidation time and temperature was recorded. The results are discussed in the light of the existing model considerations on the halogen effect and with regards to differences in the behaviour between F- and Cl-doping.     

FEG-SEM examination of alumina scales formed on FeCrAl alloys

Abstract

Field emission gun scanning electron microscopy (FEGSEM) with electron backscattered diffraction (EBSD) has been used to investigate the microstructure and the oxidation behaviour of ultra-high purity Fe–20Cr–5Al model alloys and a commercial Fe–20Cr–5Al alloy. The model alloys contain controlled additions of phosphorus and carbon impurities and increased levels of more beneficial elements including yttrium, hafnium and titanium. The samples studied were oxidised at 800°C and 1200°C in humidified air for up to 3100 h, and 900°C and 1000°C for 1 h in laboratory air. At the higher temperature, well-adhered, compact and highly protective α-alumina scales formed, whereas at the lower temperature the scales formed were a less protective type of metastable alumina.

Preliminary examination showed that the texture of the formed alumina scale was unaffected by the texture of the underlying substrate and the substrate compositions. At the higher temperature, the study revealed that the alumina scale comprised two distinct regions; the outer region at the scale/gas interface contained small, equiaxed (0.5–1 micron) grains and the inner region at the scale/metal interface contained, columnar grains that are 2–3 times larger than the equiaxed ones. However, at the lower temperature these two distinct regions were not apparent. Instead, grains of predominantly metastable alumina were observed. The links between texture morphology and oxide growth mechanisms will be discussed in this paper.     

Effects of water vapour on the oxidation of a nickel-base 625 alloy between 900 and 1,100 °C

The effect of water vapour was studied on a nickel-based SY 625 alloy oxidized at 900, 1000 and 1100 °C under dry and wet conditions. It appears that H₂O has little effect on the oxidation rate and scale composition after 48 h. The outer scale is composed of chromia Cr₂O₃. At 900 and 1,000 °C, NbNi₄ and Ni₃Mo intermetallics are found at the oxide/alloy interface. At 1,100 °C, the scale is composed of an outer chromia scale and an internal CrNbO₄ subscale. At this temperature the oxide scale morphology differs between dry and wet conditions. Under dry conditions the oxide scale appears to be compact but the external part of the scale partially spalled of during cooling. The oxide scales formed under wet conditions show porosities spread inside the scale and the chromia grain size is smaller. At 1,100 °C scale spallation is observed under dry conditions due void accumulation in the middle part of the scale. Under wet conditions the uniform distribution of the porosities inside the scale leads to a better scale adherence.     

Characterisation of oxide scales formed on nickel superalloy using impedance spectroscopy

Abstract

Impedance spectroscopy has been used to measure the electrical properties of oxide scales formed from oxidation of IN738LC superalloy at high temperature. Electrical resistance and capacitance of the oxide scales were obtained from the simulation of the measured impedance diagrams based on the equivalent circuit model, which represents the features of the oxide scales. For oxidation of IN738LC superalloy, the electrical resistance of oxide scales increased with increasing oxidation time for the specimens exposed to air at 900°C. However, for the specimens oxidised at 1,200°C, the oxide scales showed very low electrical resistance, which indicated that cracking and spallation in oxide scales occurred continuously. By using scanning electron microscopy and X-ray diffraction techniques, the composition and microstructure of the oxide scales were examined. It was found that electrical properties were determined, not only by the microstructure of oxide scales, but also by the composition of the oxide scales. By determining the relationship between electrical properties, microstructure and composition of oxide scales, impedance spectroscopy could be used as a non-destructive technique for monitoring the oxidation of metallic alloys at high temperature.     

Mechanical properties of a spinel-ferrite-based cermet: effects of temperature and oxidation

Thermomechanical properties of a spinel-ferrite-based cermet, envisaged to built inert anodes for the Hall–Heroult processing of aluminium, have been intensively investigated before and after full oxidation under air. At low temperatures, the behaviour is purely linear, elastic and brittle. Above 900 °C, the material creeps and exhibits a very large ductility for a ceramic-based material. Elasticity, fracture and viscoplasticity have been characterised up to 1000 °C using compression and bending tests. A non-linear isotropic strain hardening creep law has been identified using an optimisation procedure coupled with a finite element analysis of the bending creep tests. Microstructural, physical and mechanical changes induced by full oxidation have been characterised.     

Coal-ash deposit corrosion of some steels and nimonic alloys

The corrosion behaviour of some commercial austenitic steels and nimonic aalloys has been studied at 650, 800, 900 and 1000°C in air and in the presence of ash residues obtained from coals of Indian origin. The ash is non-aggressive at temperature s where formation of corrosion-producing alkali iron trisulphates is normally expected; on the contrary, the silicates present in the ash provide protection against corrosron. A.t higher temperatures the ash is corrosive due to onset of sulphidation and molten alhali sulphate attack. In general, the corrosion rates of high-ash coated alloys are much higher than those coated with low ash. The higher corrosion rates in the former have been attributed to a higher level of free silica in the ash which undergoes slag-type reactions. Both types of ash exhibit two different and distinct scale morphologies.     

High temperature cyclic oxidation and hot corrosion behaviours of superalloys at 900°C

Oxidation and hot corrosion are serious problems in aircraft, marine, industrial, and land-base gas turbines. It is because of the usage of wide range of fuels coupled with increased operating temperatures, which leads to the degradation of turbine engines. To obviate these problems, superalloys, viz. Superni 75, Superni 718 and Superfer 800H superalloys (Midhani grade), are the prominent materials for the high temperature applications. It is very essential to investigate the degradation mechanism of superalloys due to oxidation and hot corrosion and substantiate the role of alloying elements for the formation of protective oxide films over the surface of the superalloys. Therefore, the present work investigates the oxidation and hot corrosion behaviour of superalloys exposed to air and molten salt (Na₂SO₄–60% V₂O₅) environment, respectively, at 900°C under cyclic conditions. The weight change measurements made on the superalloys during the experiments are used to determine the kinetics of oxidation and hot corrosion. X-ray diffraction (XRD), X-ray mapping and field emission scanning electron microscope (FESEM, FEI, Quanta 200F company) with EDAX Genesis software attachment, made in Czech Republic are used to characterize the corroded products of the superalloys. It is observed that the formation of scale rich in Cr₂O₃, NiO and spinel NiCr₂O₄ has contributed for the better oxidation and hot corrosion resistance of Superni 75; whereas relatively lesser hot corrosion resistance of Superfer 800H is due to the formation of non-protective oxides of iron and sulphides of iron and nickel. The parabolic rate constants calculated for the superalloys show that the corrosion rate is minimum in air as compared to molten salt environment.     

High temperature oxidation of high purity nickel: oxide scale morphology and growth kinetics

Abstract

The oxidation of high purity nickel was studied between 600 and 1200°C for scale thickness between 1 and 30 µm. At or above 1100° C, the scale growth kinetics are strictly parabolic. The scales are then compact with columnar and facetted NiO grains. A more complex behaviour is observed below 1000°C: (i) for test temperatures between 1000 and 800°C, mass gain curves cannot be fitted to a parabola, (ii) different scale morphologies and microstructures are observed depending on scale thickness and temperature, (iii) a duplex scale is formed below 800°C. In addition to the possible effect of grain-boundary diffusion, the departure of growth kinetics from simple pure parabolic kinetics could be also related to the complex scale microstructure and its large evolution during scale growth. In situ oxidation of nickel specimens in an environmental SEM equipped with a hot stage specimen holder permitted to follow the morphological evolution of NiO scales. In situ grown NiO scales show the same microstructural features as observed on Ni specimens oxidised for longer duration in pure oxygen at atmospheric pressure.     

Techniques for High Temperature Fatigue Testing

The influence of small concentrations of SO₂, HCl or Na₂SO₄ vapour in air on the corrosion of the uncoated nickel-based superalloys IN 792 DS + Hf, CMSX-6 and MA 760 ODS and the coatings RT 22 and LCO 22 has been investigated at 1000 and 1100°C. A level of 1% SO₂ in air somewhat increased the oxidation of IN 792 DS + Hf and favoured the scale spallation of CMSX-6 at 1000°C. Some precipitates of Ti sulphide were found in the subsurface zones of both alloys. No influence was observed at 1100°C. The oxidation of MA 760 ODS and the two coatings was not affected by SO₂ at 1000 and 1100°C. The addition of 100ppm HCl to air favoured the spallation of the scales of IN 792 DS + Hf and CMSX-6 at 1000°C. Again, no influence could be observed with these alloys at 1100°C nor with MA 760 ODS or the coatings at either of these temperatures. Contrary to expectations, no synergistic effect was found in air with 1% SO₂ and 100ppm HCl, but SO₂ reduced the negative effect of HCl. Catastrophic corrosion occurred with IN 792 DS + Hf and CMSX-6 in the presence of Na₂SO₄ vapour in air at 1000 and 1100°C after an incubation period of more than 100 h. The mechanism of this rapid corrosion could not be clarified and it is still an open question whether the corrosion is caused by acidic fluxing of a liquid Na₂SO₄?MoO₃ film or by the reaction between Na₂SO₄ vapour and the scale, thus altering the transport properties of the oxides. MA 760 ODS and the coatings were not tested. Constant extension rate tests with IN 792 DS + Hf and CMSX-6 at 1000°C did not reveal any influence of 100 ppm HCl on the mechanical properties at strain rates of 1 × 10^?6 and 3 × 10^?8 S^?1.     

The effect of carbon on the high temperature oxidation of Fe-31 Mn-9Al-0.87C alloy

The alloy with the composition Fe-31Mn-9Al-0.87C was employed to investigate the effects of carbon on the oxidation behaviour at 800, 900 and 1000° C in dry air. Electron and optical microscopy were applied to examine the morphology and elemental redistribution in the oxide scale. Oxidation kinetics of the alloy oxidized at 800 and 900° C exhibited three-stage and two-stage parabolic rate laws, respectively. For the alloy oxidized at 1000° C, a carbon-induced breakaway three-stage oxidation mechanism developed. The carbon addition had a detrimental effect on the oxidation resistance and resulted in a porous initial oxide layer, which was favourable to the oxidation of manganese as well as the formation of a uniform and bulky oxide. As the oxidation temperature was increased, the diffusion rates of the metallic elements and the healing ability of oxide scales were enhanced. However, when the carbon content in alloy was above the saturation value, a breakaway scaling may have occurred due to the carbon-induced oxidation.     

Simulation of road salt corrosion in austenitic alloys for automotive exhaust systems

Abstract

Cyclic oxidation tests in air with intermittent salt spraying have been performed to simulate the conditions of road salt (NaCl–CaCl₂) enhanced corrosion in automotive exhaust systems.

Tests were carried out at 600 and 700°C on three austenitic alloys, including two stainless steels currently employed for exhaust components (AISI 316 Ti and AISI 302B) and a higher nickel heat resisting alloy.

The presence of salt causes internal corrosion, both along a regular front beneath an outer oxide scale and down alloy grain boundaries. An increase in temperature accelerates the corrosion rate and particularly enhances intergranular penetration.

The results of micrographic and microanalytical investigations are in general agreement with an active oxidation mechanism in which Na_xCl_x vapour species, and not only chlorine, appear to play an important role. The regions directly affected are depleted in chromium and iron and enriched in nickel.

Although internal oxidation of silicon is observed, a high silicon content (2%) does not necessarily ensure effective protection against this type of attack.     

Thermal stability and oxidation behavior of Al-containing nanocrystalline powders produced by cryomilling

Al-containing nanostructured coatings provide excellent protection from high temperature corrosion. Aluminum oxide scales generally provide better oxidation resistance and yield lower oxidation rates than other oxide scale compositions. In this study, nanocrystalline 316L stainless steel containing 6 wt.% Al was synthesized using cryogenic milling (cryomilling). Complete alloying was obtained after 32 h of milling and the average grain size was found to be 7 nm. High temperature thermal stability and oxidation kinetics of the alloyed powders were examined. The powder demonstrated good grain growth stability at 500 °C, at which point, the powders had been heat treated for 120 h and the average grain size was found to be 11.4 nm. The oxidation kinetics of the powder were studied for 48 h at 500, 800, and 1,000 °C, respectively. For comparison, conventional 316LSS powder was also tested. Nanocrystalline 316LSS-6 wt.% Al showed lower weight gain than the conventional 316LSS powders. During the oxidation of nanocrystalline 316LSS-6 wt. % Al at 500 °C, protective aluminum oxide scale formed at the surface. At 800 °C and 1,000 °C, most of the nanocrystalline 316LSS-6 wt.% Al particles showed completed outer aluminum oxide scale. However, at 800 and 1,000 °C, some particles showed growth of chromium oxide scale underneath the aluminum oxide scale. In those samples, Al depletion was also observed due to a non-homogenous distribution of Al during cryomilling. The activation energy of the oxidation reaction was calculated and was found to be affected by the enhancement of the grain boundary diffusion in nanostructured particles.     

Effect of microalloying with silicon on high temperature oxidation resistance of novel refractory high-entropy alloy Ta-Mo-Cr-Ti-Al

Abstract

The effect of 1 at.% Si addition to the refractory high-entropy alloy (HEA) Ta–Mo–Cr–Ti–Al on the high temperature oxidation resistance in air between 900 °C and 1100 °C was studied. Due to the formation of protective chromia-rich and alumina scales, the thermogravimetric curves for Ta–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al–1Si showed small mass changes and low oxidation rates which are on the level of chromia-forming alloys. The oxide scales formed on both alloys at all temperatures are complex and consist of outermost TiO₂, intermediate Al₂O₃, and (Cr, Ta, Ti)-rich oxide at the interface oxide/substrate. The Si addition had a slightly detrimental effect on the oxidation resistance at all temperatures primarily as a result of increased internal corrosion attack observed in the Si-containing HEA. Large Laves phase particles distinctly found in the Si-containing alloy were identified to be responsible for the more rapid internal corrosion.     

Effects of water vapor on corrosion behaviors of C/SiC in oxidizing atmosphere containing Na2SO4 vapor

Corrosion of a C/SiC composite has been investigated in the atmosphere containing oxygen, water vapor and sodium sulfate vapor at the temperatures range from 1000 to 1500 °C. The effect of water vapor on the corrosion mechanism of C/SiC were discussed based on the weight change, the residual strength change, the microstructure and calculated results from FactSage. The corrosion of C/SiC is attributed to (i) the permeation of gas through the SiO₂ film below 1300 °C, (ii) the diffusion of oxidant through pores caused by bubbles broken in the SiO₂ film above 1300 °C. The water vapor does not change the corrosion mechanism of C/SiC composite but the temperature range in which the corrosion mechanism works by accelerating the oxidation of SiC and the corrosion of SiO₂.