Oxidation resistant coatings in combination with thermal barrier coatings on γ-TiAl alloys for high temperature applications

Titanium aluminide alloys based on γ-TiAl are considered of growing interest for high temperature applications due to their attractive properties. To extend the service temperatures above 750 °C, the oxidation behaviour has to be improved predominantly by protective layers. In the present study environmental and thermal protection coatings on gamma titanium aluminides were investigated. Nitride and metallic overlay coatings based on Ti-Al-Cr-Y-N and Ti-Al-Cr, respectively, were produced by magnetron sputtering techniques. Thermal barrier coatings (TBCs) of partially yttria stabilized zirconia were deposited onto Ti-45Al-8Nb, either pre-oxidized or coated with protective layers, applying electron beam physical vapour deposition (EB-PVD).Cyclic oxidation tests were performed at 900 °C and 950 °C in air. The nitride coating exhibited poor oxidation resistance when exposed at 900 °C providing no protection for γ-TiAl. The oxidation behaviour of the Ti-Al-Cr coating was reasonable at both exposure temperatures. During prolonged exposure the coating was depleted in chromium, resulting in the breakdown of the protective alumina scale. EB-PVD zirconia coatings deposited on γ-TiAl exhibited promising lifetime, particularly when specimens were coated with Ti-Al-Cr. The adherence of the TBC on the thermally grown oxide scales was excellent; failure observed was associated with spallation of the oxide scale. At 950 °C, TBCs on specimens coated with Ti-Al-Cr spalled after less than 200 thermal cycles caused by severe oxidation of γ-TiAl and reactions between the zirconia coatings and the thermally grown oxides.     

Electrochemical behaviour of oxidized NiTi shape memory alloys for biomedical applications

A new oxidation treatment (OT) for NiTi Shape Memory Alloys used in biomedical applications has been developed in a previous work to reduce Ni surface concentration and, consequently, decrease Ni ions release into the exterior medium. This OT treatment is expected to minimize adverse and toxic reactions associated to Ni ions. However, in order to assess the biocompatibility of a metallic material, its corrosion resistance has to be evaluated.In this work, the electrochemical behaviour of NiTi surfaces oxidized by the new OT treatment was compared to untreated NiTi surfaces (NT). For this purpose, tests of open-circuit potential and cyclic voltammetry were performed at 37 °C in a Hanks Balance Salt Solution.A significant increase of the corrosion (E_corr) and breakdown (E_b) potentials was observed for OT surfaces, in comparison with NT surfaces. Moreover, the qualitative potentiodynamic behaviour of OT NiTi and Ti surfaces are similar. This observation suggests that the OT treatment leads to the formation of an oxide on NiTi surface that has similar structure and electrochemical property to native Ti oxide. This new oxidation treatment is efficient to protect NiTi alloys from electrochemical degradation and, therefore, it may be an excellent candidate for biomedical applications.     

Nitrogen diffusion in medical CoCrNiW alloys after plasma immersion ion implantation

CoCr alloys are widely used for medical applications, e.g. total hip replacements or coronary stents. Nevertheless, an increase in the surface hardness and a reduction of the wear rate is still desirable to improve the biocompatibility. Plasma immersion ion implantation (PIII) at different temperatures, acceleration voltages and working pressures is used to determine the nitrogen diffusivity in the CoCr alloys SY21med, L605 and HS188. Depending on the temperature, two different treatment regimes can be distinguished, independent of the ion energy. At low temperatures, a diffusion process with an activation energy of 1.0-1.1 eV is present, indicative of interstitial nitrogen diffusion. Beyond 350 °C, a reduced activation energy of 0.5-0.7 eV is observed. Additionally, a strong dependency of the layer thickness on the working pressure in the range 0.3-0.8 Pa was observed for the temperature dependent diffusion regime, which suggests a synergistic interplay of adsorption and implantation during PIII. Below 500 °C, an increase of the diffusion coefficient by three orders of magnitude was observed for PIII, compared with pure plasma nitriding.     

Oxidation behavior of γ-TiAl coated with zirconia thermal barriers

The applicability of the thermal barrier coating concept, which is an established concept for Ni-base alloys, was investigated for the first time for γ-TiAl. As potential bond coats, the following surface treatments were applied: Al-diffusion coating, Al-diffusion coating combined with short term pre-oxidation in air, short term pre-oxidation in oxygen of a rough surface finish γ-TiAl sample without aluminide coating. For the purpose of lowering the temperature of the γ-TiAl substrate surface, and thus, to prolong the life time, a zirconia thermal barrier coating (TBC) was applied on top of the above-mentioned modified TiAl surfaces. Oxidation resistance of the modified γ-TiAl alloys/TBC systems was evaluated by isothermal oxidation tests at 900 °C for 100 h in air. Whatever the pre-treatment conditions, the TBC was adherent to the oxide scale after 100 h oxidation, which was not the case for untreated γ-TiAl.     

Surface protection of titanium and titanium–aluminum alloys against environmental degradation at elevated temperatures

Experiments have been undertaken to explore the possibility of creating an oxygen barrier coating, which is effective in preventing oxidation and oxygen embrittlement of Ti and several low-Al content Ti-base alloys during exposure to oxidizing environments at elevated temperatures. The fabrication process has involved three steps, namely co-deposition of Ti and Al by magnetron sputtering onto a substrate material to be protected, followed by vacuum annealing and plasma immersion ion implantation of fluorine. The first two steps produce an overlay of γ-TiAl while the last step provides the necessary conditions for bringing about the halogen effect upon subsequent high-temperature oxidation. Analysis techniques such as cross-sectional transmission electron microscopy (XTEM) in conjunction with electron energy loss spectroscopy (EELS), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and elastic recoil detection (ERD) have been used to study the microstructure, phase formation and depth distribution of the elements in the coating material. Following oxidation in air at 600 °C for 100 h, specimens have been prepared for metallographic analysis, and their cross sections have been characterized by scanning electron microscopy (SEM) in combination with EDX, and electron probe microanalysis (EPMA). The results obtained show that during oxidation exposure the coating is capable of forming a protective alumina-containing scale which serves as an oxygen barrier, thereby preventing oxygen embrittlement. In addition, since the only constituents of the coating are Ti and Al, it exhibits excellent chemical substrate compatibility.     

Surface protection of titanium by Ti5Si3 silicide layer prepared by combination of vapour phase siliconizing and heat treatment

In this work we proposed a new method for preparation of protective Ti₅Si₃ silicide layer. The proposed method consisted of two steps: (1) silicon electron beam evaporation and (2) heat treatment. The silicide-modified titanium was subjected to structural examination, hardness profiling and isothermal oxidation tests. The investigations revealed that the proposed surface treatment produces a layer with a thickness of 1–2 μm and hardness of about 1500 HV. The isothermal oxidation tests showed that the layer is highly resistant to oxidation in air even at 900 °C. Its oxidation rate was comparable or even lower than that of some high-temperature γ-TiAl intermetallics oxidized under similar conditions. The oxidation mechanism was discussed in terms of the internal structure and chemical composition of the scales. It was demonstrated that the silicide layer could serve as excellent protection of titanium, Ti-based alloys and intermetallics against the high-temperature oxidation and wear.     

A high-temperature oxidation-resistant Fe-Mn-Al-Si alloy

To develop satisfactory alloys without Cr or Ni for high-temperature application up to 1100C, three alloys based on Fe-10%Al-Si with differing fourth (or fifth) element additions were oxidized in air at 1100°Cfor 24 hr. A low carbon, Fe-30Mn-10Al-Si alloy exhibited excellent high-temperature oxidation resistance. The total weight gain for 24 hr oxidation in air at 1100°C was only 1.03 mg/cm ². After air oxidation for 6 days at 1100°C, no nodule formation or breakthrough oxidation occurred. Post-oxidation SEM and EDAX examination showed that a thin, compact, protective alumina scale formed on the alloy.Visiting Scientist (People's Republic of China).     

Preparation and high temperature oxidation behavior of refractory disilicide coatings for γ-TiAl intermetallic compounds

Novel refractory disilicide layers were applied to γ-TiAl to enhance oxidation resistance at 1050 °C. NbSi₂ and MoSi₂ layers were prepared by joining thin Nb and Mo foils to γ-TiAl surfaces, and siliconizing the combinations (Nb/γ-TiAl, and Mo/γ-TiAl) using molten salts. The coatings and their oxidation behavior were characterized using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy techniques. Isothermal oxidation tests showed that the oxidation resistance of uncoated γ-TiAl at 1050 °C in air was insufficient, and scale spallation occurred. NbSi₂ coatings were formed and adhered firmly to the γ-TiAl substrate, whereas Mo film detached from the substrate surface causing failure of the MoSi₂ coatings. Oxidation of the NbSi₂-coated γ-TiAl (NbSi₂/Nb/γ-TiAl) at 1050 °C in air showed improved oxidation resistance at exposure times up to 100 h. Microstructural and compositional developments of the coating at prolonged time were discussed. The NbSi₂ coatings provided sufficient oxidation resistance for γ-TiAl at 1050 °C in air, and have potential use in high temperature applications.     

Oxidation resistance of refractory γ-TiAlW coatings

The resistance to oxidation of the W-alloyed magnetron sputtered γ-TiAl fine-crystalline 4 μm thick coatings have were investigated in this work. The oxidation tests were performed in an atmosphere of pure oxygen or in the air at a temperature of 1173 K for 120 h. The resistance to high-temperature oxidation was investigated by means of micro-thermogravimetric analysis with continuous or stepwise control of the substrate weight. Before and after the oxidation the morphology of the coatings as well as their chemical and phase composition were investigated by SEM, EDS and EBSD, respectively. The results have been compared with those obtained for the uncoated γ-TiAl substrate.It was found that: (1) the W-alloyed γ-TiAl coatings have a considerably higher oxidation resistance (about four orders of magnitude) than the uncoated γ-TiAl substrates and that their resistance to oxidation increases with the concentration of alloying element in the range of the concentrations investigated in the work; (2) the high resistance to oxidation of the coatings is a result of the thin α-Al₂O₃ layer formation on the surface of the substrate during oxidation; (3) the formation of dense and uniform α-Al₂O₃ layer on the coating surface is due to a fine-crystalline structure of the magnetron deposited γ-TiAlW coating.     

The effect of long term exposure in oxidising and corroding environments on the tensile properties of two gamma-TiAl alloys

The lightweight γ-TiAl alloys offer good mechanical properties for gas turbine applications at high temperature (up to 750 °C). In order to replace the nickel-base alloys currently in use, however, their mechanical properties must be retained after exposure in a high temperature oxidising and corroding atmosphere. A complete study of the oxidation and corrosion processes of the two alloys, γ-Ti–46.5Al–4(Cr,Nb,Ta,B) and γ-Ti–45Al–X(Nb,B,C) was performed at temperatures from 700 to 800 °C. Hot corrosion tests were carried out with salt mixtures (93.7% Na₂SO₄+2.7% NaCl (wt%) or 75% Na₂SO₄+25% NaCl (wt%)) or with pure NaCl. A strong influence of the exposure on the tensile properties of these two γ-TiAl alloys has been demonstrated. Microscopic investigation showed that the bulk microstructure was stable. The main probable causes of these property reductions included the oxidation and corrosion processes and particularly the effect of niobium in the absence of chromium that induced major internal corrosion and premature failure.     

Oxidation protection of gamma-titanium aluminide using glass-ceramic coatings

γ-TiAl intermetallic alloys are presently considered an efficient structural material for advanced turbine blades and aero-engine components due to their various advantages compared to the traditionally used superalloys. However, their poor oxidation resistance at temperatures > 750 °C severely limits their wider application. The present study dealt with the improvement of oxidation resistance of this alloy by applying impervious glass-ceramic coatings by vitreous enameling technique. Results showed that MgO-SiO₂-TiO₂ glass-ceramic coating could offer excellent oxidation resistance to γ-TiAl at 800 °C even up to 100 h with negligible weight gain (~ 0.10 mg/cm²) compared to that of the bare alloy (~ 1.3 mg/cm²). The coatings those were belonging from BaO-MgO-SiO₂, ZnO-Al₂O₃-SiO₂ and BaO-SiO₂ systems also extend appreciable improvement in the oxidation resistance of the alloy at 800 °C up to 100 h. At further higher temperature such as at 1000 °C, the ABK-13 and ABK-103 glass-ceramic coatings offered significant protection to the alloy up to 25 h of exposure in air with minimum weight gain (~ 0.34 mg/cm²). However, after that the coated layers started to peel off from the alloy surface.     

Synthesis and oxidation behavior of platinum-enriched γ + γ′ bond coatings on Ni-based superalloys

Simple Pt-enriched γ + γ′ coatings were synthesized on René 142 and René N5 Ni-based superalloys by electroplating a thin layer of Pt followed by a diffusion treatment at 1150-1175 °C. The Al content in the resulting γ + γ′ coating was in the range of 16-19 at.% on superalloys with 13-14 at.% Al. After oxidation testing, alumina scale adherence to these γ + γ′ coatings was not as uniform as to the β-(Ni,Pt)Al coatings on the same superalloy substrates. To better understand the effect of Al, Pt and Hf concentrations on coating oxidation resistance, a number of Ni-Pt-Al cast alloys with γ + γ′ or β phase were cyclically oxidized at 1100 °C. The Hf-containing γ + γ′ alloys with 22 at.% Al and 10-30 at.% Pt exhibited similar oxidation resistance to the β alloys with 50 at.% Al. An initial effort was made to increase the Al content in the Pt-enriched γ + γ′ coatings by introducing a short-term aluminizing process via chemical vapor deposition or pack cementation. However, too much Al was deposited, leading to the formation of β or martensitic phase on the coating surface.     

Oxidation protection of γ-TiAl-based alloys – A review

Alloys based on γ-TiAl are lightweight materials with attractive mechanical properties at high temperatures. Although these alloys reveal a superior resistance against environmental attack compared to titanium and 2-based alloys, efficient protection is required for industrial applications at temperatures between 800 and 1050 °C. Extensive research in order to solve this problem started more than 30 years ago. This review provides a summary of the different concepts based on surface modification techniques developed for the environmental protection of γ-TiAl alloys at high temperatures, including overlay and diffusion coatings, as well as the halogen effect. The discussion includes a comparison between the most promising coating types under long-term high temperature exposure and an assessment of their processing routes from a technological point of view. Therefore, a mass gain of 1 mg/cm² after at least 1000 h of exposure was set as a benchmark to evaluate these protection systems.     

Oxidation Resistance of Ion-Implanted γ-TiAl-Base Intermetallics

A series of alloy elements, including the detrimental, neutral, and beneficial elements for bulk alloying into -TiAl base intermetallics, i.e., V, Cr, Y, Er, Nb, and W, has been implanted into a -Ti–50Al intermetallic in order to explore the mechanism of high-temperature oxidation resistance for the ion-implanted intermetallic. The oxidation resistance was investigated under cyclic-oxidation conditions at oxidation temperatures from 800 to 1000°C for 200 hr in air. At a lower oxidation temperature of 800°C, the V-ion implantation has a detrimental effect on the oxidation resistance of Ti–50Al, while a neutral and beneficial effect was observed for Er-, Y- and Cr-, Nb-, W-ion implantation, respectively. At 900°C, V-, Er-, Y-, and Cr-ion implantation all showed a neutral effect, whereas Nb- and W-ion implantation apparently improved the oxidation resistance. With increasing oxidation temperature to 1000°C, Y- and Cr-ion implantation kept the neutral effect, and the beneficial effect of Nb-ion implantation disappeared gradually. The oxidation behavior of ion-implanted -TiAl base intermetallics is different from that of bulk-alloyed materials due to the two alloying methods, although the effect of the alloy elements on the oxidation resistance has not essentially changed in the -TiAl base intermetallics.     

Study of Carbide Transformations during High-Temperature Oxidation of Nickel-Base Superalloys

Microstructure phenomena resulting from high-temperature oxidation of three nickel-base superalloys were studied by microstructure examinations. Disappearance, nature modification, volume fraction evolution or precipitation of carbides were observed in the alloys near the external surface, depending on the temperature and the chemical composition of the alloys. Thermodynamic calculations allowed to better know what happened to carbon and to quantify its new distribution. The alloys studied lost a more or less great part of their sub-surface carbon content at 1200°C while carbon seemingly diffused deeper in the alloy at 1100°C and 1000°C. The latter part of carbon promoted the coarsening of the pre-existing carbides, some modifications of their natures or the precipitation of new carbides in the matrix, then the occurrence of a new-carbides zone.     

Selective surface oxidation and nitridation of NiTi shape memory alloys by reduction annealing

The current investigation aims in understanding the effect of short-term (300 s) annealing of NiTi shape memory alloys (SMAs) in a reducing atmosphere of N₂-10% H₂. The influence of temperature on the resulting surface morphology and chemistry is elucidated. On annealing at 600 °C, the surface is covered with a thin layer of titanium oxide, which is 7.5 nm thick, while at 800 °C, the surface is covered with a golden-yellow layer of TiN of thickness more than 100 nm. The surface analysis carried out by XPS on the specimen annealed at 800 °C confirms the formation of TiN and more notably, the surface is devoid of Ni.     

Influence of a Reactive Element on the Growth of a Thermally Grown Chromia Scale: A Grazing Emission X-ray Fluorescence Study

The oxidation of 55Fe–25Cr–20Ni (wt.%) alloys, with and without added reactive element (RE) Y, were studied using grazing-emission X-ray fluorescence (GEXRF). Samples were studied after isothermal treatments at 750°C in O2 and after cyclic-oxidation treatments. In early-stage oxidation, a Ni-rich scale is formed. The distribution of this early-stage Ni deposit is studied as the scale evolves. The Ni deposit, serving as a marker, remains on the outer scale surface in Y-containing alloys, but is not detectable in scales on Y-free alloys. The results indicate that new chromia scale growth occurs at the outer surface in Y-free alloys but, for Y-containing alloys, new growth occurs away from the outer surface. Thus, a shift in the growth mode is apparently observed at 750°C, consistent with higher-temperature observations. However, unlike the high-temperature measurements, scale-growth rates are not significantly affected by the RE.     

Properties of Zr-ZrC-ZrC/DLC gradient films on TiNi alloy by the PIIID technique combined with PECVD

The Zr-ZrC-ZrC/DLC gradient composite films were prepared on TiNi alloy by the techniques combined plasma immersion ion implantation and deposition (PIIID) and plasma enhanced chemical vapor deposition (PECVD). With this method, the Zr-ZrC intermixed layers can be obtained by the ion implantation and deposition before the deposition of the ZrC/DLC composite film. In our study, an optimal gradient composite film has been deposited on the NiTi alloys by optimizing the process parameters for implantation and deposition. The surface topography was observed through AFM and the influence of the deposition voltage on the surface topography of the film was investigated. XPS results indicate that on the outmost layer, the Zr ions are mixed with the DLC film and form ZrC phase, the binding energy of C 1s and the composition concentration of ZrC depend heavily on the bias voltage. With the increase of bias voltage, the content of ZrC and the ratio of sp³/sp² firstly increases, reaching a maximum value at 200 V, and then decreases. The nano-indentation and friction experiments indicate that the gradient composite film at 200 V has a higher hardness and lower friction coefficient compared with that of the bare NiTi alloy. The microscratch curve tests indicate that gradient composite films have an excellent bonding property comparing to undoped DLC film. Based on the electrochemical measurement and ion releasing tests, we have found that the gradient composite films exhibit better corrosion resistance property and higher depression ability for the Ni ion releasing from the NiTi substrate in the Hank's solution at 37°C.     

The oxidation of two ternary Ni-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of the ternary alloys Ni-45Cu-10Al and Ni-30Cu-10Al has been studied at 800-900 °C under 1 atm O₂. The presence of 10 at.% Al reduces significantly the oxidation rate of the corresponding Cu-Ni alloys during the initial oxidation stages, even before the establishment of a complete Al₂O₃ layer. The weight of individual sample of the two ternary Ni-Cu-10Al alloys at 800 °C increases more rapidly than at 900 °C during the initial oxidation stage. As oxidation proceeds, the weight gain at 800 °C slows down to a degree that the total weight gain after 24 h oxidation at 800 °C is less than that at 900 °C. Due to a faster formation of the Al₂O₃ layer, which suppresses earlier the further oxidation of Cu and Ni, the external region of the scales grown on Ni-45Cu-10Al contain much less Cu and Ni oxides than those grown on Ni-30Cu-10Al. The transition from the internal oxidation to the selective external oxidation of the most reactive component Al in Ni-Cu-Al alloys is favored by higher values of the Al content, of temperature and of the Cu/Ni ratio.     

Oxygen adsorption on γ-TiAl surfaces and the related surface phase diagrams: A density-functional theory study

We have constructed the surface phase diagrams for oxygen adsorption on γ-TiAl low-index surfaces using density-functional theory calculations. From these surface phase diagrams, the selective oxidation behaviors of the γ-TiAl surfaces and the corresponding polycrystalline systems can be easily understood and predicted. For the (1 0 0) surface, complete selective oxidation of titanium is favored and a titanium oxide layer may be produced at the initial stage of oxidation. For the (1 1 0) and (0 0 1) surfaces, only titanium oxides may form. For the γ-TiAl polycrystalline system, O may induce complete Ti and Al surface segregations on the Ti-rich and Al-rich conditions, respectively. In addition, the microscopic oxidation mechanisms are identified and the experimental results are successfully explained. More importantly, by comparing the different TiAl surface orientations, a comprehensive surface phase diagram is constructed to study the oxidation behaviors of polycrystalline γ-TiAl. This method can also be applied to other polycrystalline materials.