Enhanced high temperature oxidation resistance for γ-TiAl alloy with electrodeposited SiO₂ film

Highlights

• SiO₂ film was prepared on TiAl alloy by electrodeposition.

• Compact sintered glass-like oxide scale emerges after thermal oxidation.

• The oxide scale formed on the SiO₂ film coated TiAl is Si- and Al-enriched.

• TiAl alloy coated with E-SiO₂ film exhibits good oxidation resistance.

Abstract

Thickness controllable SiO₂ film was prepared on γ-TiAl alloy by cathodic electrodeposition. The oxidation behaviour of the specimens at 900 °C was studied. The effects of SiO₂ electrodeposition parameters, such as electrolysis time and deposition current density, on the morphology, structure and high temperature oxidation performance of the film were investigated. Results shown during thermal oxidation, the electrodeposited SiO₂ film could react with the TiAl alloy to form an alumina- and silicon-enriched glass-like oxide scale, which can prevent the inward diffusion of oxygen from air to the film/alloy interface, resulting in the remarkable decreased oxidation rate of the alloy.

Introduction

TiAl alloys have drawn a lot of attention in aerospace and automotive industries due to their low density (3.7–3.9 g cm⁻³), high specific strength and good oxidation resistance and creep resistance up to moderately elevated temperatures [[1], [2], [3]]. However, the formation of a non-protective rutile TiO₂ or TiO₂ + Al₂O₃ mixed scale over 800 °C leads to insufficient oxidation resistance and has greatly limited their further applications [[4], [5], [6], [7]].

In recent years, a great deal of efforts based on alloying design and surface treatment have been made to improve the high temperature oxidation resistance of TiAl alloys [1,8]. Alloying design, achieved by increasing the Al content or adding foreign alloy elements, such as Mo [9], Nb [10], Si [11,12], and Y [13] is beneficial for the formation of continuous Al₂O₃ layer [14], resulting in improved high temperature oxidation resistance. However, the brittle TiAl₃ will precipitate and deteriorate its mechanical property once the Al content is too high. On the other hand, large amount of foreign elements used to improve the high temperature oxidation resistance will also weaken the mechanical properties. Alternatively, surface treatments, for example, ion implantation [15,16] and protective coating [[17], [18], [19]], are employed to enhance the high temperature oxidation resistance of the TiAl alloy to avoid greatly changing their bulk properties. By means of ion implantation, precise amount of elements, such as, halogen ions (F⁻ and Cl⁻), Al, Si, Cr, and Mo, can be doped into the TiAl alloy with good repeatability [11,15,16,20]. Whereas, the effect of ion implantation is generally limited to several hundreds nanometer depth in the top-surface of the TiAl alloy, and expensive and sophisticated equipment are required. Recently, our group proposed to anodize TiAl alloy in fluorine ion contained electrolyte, such as, 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF₆) or NH₄F to obtain aluminum and fluorine enriched surface [[21], [22], [23]]. During the anodization process, TiAl alloy was acted as anode. The derived aluminum and fluorine enriched anodic film can provide good high temperature oxidation resistance at 1000 °C. This provides a new strategy to improve the high temperature oxidation resistance of TiAl based on halogen effect. Protective coating [17,18,24], such as Al [[25], [26], [27], [28]], Al₂O₃ [29,30], nitride coatings [31], enamel coating [32], glass-ceramic coatings [[33], [34], [35]] and MCrAlY (M = Fe, Co, Ni or their combination) metallic coating [36] is another alternative approach to improve the oxidation resistance and acts as a barrier to reduce the ingress of oxygen at high temperature. Whereas, the large internal stress and big difference in thermal expansion coefficients between the enamel/ceramic coating and the substrate will deteriorate the oxidation resistance. The severe inter diffusion between the metallic coating and TiAl at elevated temperatures results in the formation of thick brittle intermetallic compound (AlCo₂Ti or AlNi₂Ti) layers and Kirkendall voids at the interface, which could greatly worsen the integral performance of the coating [36].

As one of the most abundant and eco-friendly oxides on the earth, silica has been widely used in various areas. Recently, silica-based films have been employed to improve the high temperature oxidation resistance of Ti-based alloy [[37], [38], [39], [40], [41]]. It is reported that this coating can prevent the stratification of rutile TiO₂ scale which is assumed to be responsible for the rapid penetration of oxygen [38]. Traditionally, SiO₂ films are prepared by spraying [7], dipping [38,40,41], and spinning [39] the hydrolyzed sol or magnetron-sputtering pure SiO₂ disc [37] onto the TiAl substrate. However, the derived SiO₂ films are generally full of cracks and the thickness is limited to several micrometers. Moreover, the brittleness of the SiO₂ films, poor adhesion and mismatch of thermal expansion coefficients between them and the substrates may result in cracking and spalling by traditional fabricating techniques. Thus, new technologies are emerged to fabricate SiO₂ film and further improve the high temperature oxidation resistance of titanium alloys.

In 1999, Mandler et al. proposed to fabricate sol-gel film using electrodeposition method [42]. The generation of this film is achieved by the base catalysis. In detail, a cathodic potential/current density is applied to an electrode to increase the local pH nearby the electrode and catalyze the condensation process. The properties of the sol-gel films can be easily altered by changing the electrodeposition parameters, e.g. the applied potential/current density and electrolysis time. This approach has been utilized in electroanalysis [43,44], corrosion protection of metals at room temperature [45,46], and preparation of functional materials [[47], [48], [49], [50]] and other fields.

In this work, thickness controllable inorganic SiO₂ (E-SiO₂) film was electrodeposited on γ-TiAl alloy, which was acted as cathode during the deposition process. The high temperature oxidation resistance of this E-SiO₂ film coated γ-TiAl alloy was evaluated at 900 °C. The influence of electrodeposition parameters, such as electrolysis time and electrodeposition current density, on the morphology and composition of the oxide scale was investigated systematically. Results show both the E-SiO₂ film and derived oxide scale have good adhesion to the substrate, therefore ensuring the promising high temperature oxidation resistance. After high temperature oxidation, a glass-like protective oxide scale consisting of cristobalite, α-Al₂O₃, Ti₃Al, Ti₅Si₃, and Ti₅Al₃O₂ will generate due to the reaction between the electrodeposited SiO₂ film and γ-TiAl. This alumina- and silicon-enriched oxide scale can efficiently prevent the inward diffusion of oxygen from air to the film/alloy interface and improve the high temperature oxidation performance of the γ-TiAl alloy. This is completely different to the previous works [[21], [22], [23]], where the improved high temperature oxidation resistance is achieved due to the in situ generated compact and aluminum/fluorine enriched anodic film.