J.S. Burnell-Gray

Air oxidation behaviour of Ti6Al4V alloy between 650 and 850

The air oxidation behaviour of Ti6Al4V alloy was studied in the temperature range 650–850°C. Continuous oxidation kinetics showed that the alloy oxidised according to a parabolic rate law at 650 and 700°C after a transient period, whilst at 750 and 800°C, linear-parabolic kinetics dominated. At 850°C, after 50 h linear-parabolic oxidation, the alloy followed a parabolic rate law. The multilayered oxide scales formed on the oxidised alloy consisted of alternate layers of Al2O3 and TiO2. The number of Al2O3 and TiO2 layers increased with increasing exposure time and temperature. The external gas/oxide interface was always occupied by an Al2O3 layer whilst TiO2 always appeared at the oxide/substrate interface.

Aluminide coating formation on nickel-base superalloys by pack cementation process

A detailed study was carried out to investigate the effects of pack powder compositions, coating temperature and time on the aluminide coating formation process on a superalloy CMSX-4 by pack cementation. With the aid of recently developed thermodynamic analytical tools, powder mixtures that are activated by a series of fluoride and chloride salts were analysed and the effectiveness of these activators in transferring and depositing Al was evaluated at a range of coating temperatures. The Al chloride vapours formed at coating temperatures from 900°C to 1100°C were also analysed thermodynamically as a function of Al concentration in the original pack for the powder mixtures activated by 4 wt% CrCl3·6H2O. Based on the thermochemical calculations, a series of coating experiments was carried out. Aluminide coatings were formed at temperatures from 850°C to 1100°C for periods varying from 4 hours to 8 hours using powder mixtures activated by NH4Cl, NaCl and CrCl3·6H2O and AlF3. The effects of changing Al concentration as well as adding small quantities of Cr in the powder mixtures on the coating formation process were also investigated. The aluminide coatings were analysed using a range of techniques including SEM, EDX and XRD. The relationships between the mass gain and coating thickness and structure were investigated. The experimental results were compared with the predictions from thermochemical calculations. Based on the understandings established, an effective approach to control the aluminide coating parameters and structures was identified, which made it possible to optimise powder mixture compositions and coating conditions for different coating requirements.

High-temperature corrosion of Ti and Ti-6Al-4V alloy

Pure titanium and Ti-6Al-4V were exposed at 750°C in an H2/H2O/H2S PO2≈10−18 Pa and PS2≈10−1 Pa), H2/H2O (PO2≈10−18 Pa) and air environments for up to 240 hr. The corrosion kinetics, obtained by the discontinuous gravimetric method, showed that the sulfidation/oxidation kinetics were linear for Ti and linear-parabolic for Ti-6Al-4V in the H2/H2O/H2S environment. Both materials obeyed parabolic rate laws in the H2/H2O atmosphere after a transient period, and linear-parabolic rate laws in air. After exposure to the H2/H2O/H2S atmosphere, the titanium specimen displayed a double scale of TiO2 with an intervening TiS2 film between the double-layered scale of TiO2 and the substrate. Ti-6Al-4V also contained a double layer of TiO2 together with a stratum consisting of Al2S3, TiS2 and vanadium sulfide at the junction of the inner TiO2 layer and substrate. Some Al2O3 precipitated in the external portion of the outer TiO2 layer. Following oxidation in the low-PO2 atmosphere a double-layered oxide of TiO2 scale formed on both Ti and Ti-6Al-4V. The scale on Ti-6Al-4V also contained an α-Al2O3 film situated between the outer and inner (TiO2) layers. For both materials, multilayered-scale formation characterized air oxidation. In detail a multilayered oxide scale of TiO2 formed on the air-oxidized Ti, while a multilayered oxide scale with alternating layers of Al2O3/TiO2 developed on Ti-6Al-4V oxidized in air.

Co-deposition of aluminide and silicide coatings on γ-TiAl by pack cementation process

Thermochemical analyses were carried out for a series of pack powder mixtures for deposition of aluminide and for co-deposition of aluminide and silicide coatings on γ-TiAl by the pack cementation process. Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures for depositing adherent and coherent aluminide and silicide coatings. Pack powder mixtures activated by 2 wt% AlCl3 was used to aluminise γ-TiAl at 1000°C. With proper control of pack compositions and coating conditions, an aluminide coating of TiAl3 with a coherent structure free from microcracking was deposited on the substrate surface via inward diffusion of aluminium. The results of thermochemical calculations indicated that co-deposition of Al and Si is possible with CrCl3 · 6H2O and AlCl3 activated pack powders containing elemental Al and Si as depositing sources. Experimental results obtained at 1100°C revealed that CrCl3 · 6H2O is not suitable for use as an activator for co-depositing aluminide and silicide coatings on γ-TiAl. It caused a significant degree of degradation instead of coating deposition to the substrate. However, adherent coatings with excellent structural integrity consisting of an outer TiSi4 layer and an inner TiAl3 layer were successfully co-deposited at 1100°C and 1000°C using pack powder mixtures activated by AlCl3. IT is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and silicon. Discussion is presented on the issues that need to be considered to ensure the deposition of aluminide and silicide coatings with coherent structure free from microcracking on γ-TiAl by the pack cementation process.

An assessment of the oxidation resistance of an iridium and an iridium/platinum low-activity aluminide/MarM002 system at 1100°C

Abstract Platinum-aluminide coatings provide excellent protection for turbine blades against the aggressive environments in which modern aero-gas turbines operate. However, there is an increasing interest in the modification of aluminide coatings with other noble metals, such as iridium. This study assessed the oxidation performance of an iridium-aluminide and an iridium/platinum-aluminide on MarM002 substrates. The coatings were isothermally soaked at 1100°C and analysed using optical, SEM and XRD techniques, with their performance being gauged against that of a low-activity platinum-aluminide. Both of the iridium systems offered a lower level of performance than that afforded by the platinum-aluminide. This paper will discuss the mechanisms of degradation of these coating systems and address the scale growth and adherence.

Effect of Nb coating on the sulphidation/oxidation behaviour of Ti and Ti-6Al-4V alloy

The environmental response of Nb-coated Ti and Ti-6Al-4V alloy was studied at 750 °C in an atmosphere of pS2 ∼ 10−1 Pa and pO2 ∼ 10 −18 Pa. By acting as a diffusion barrier and through the formation of a Nb1−xS scale the Nb coating deposited enhanced the corrosion resistance of both Ti and Ti-6Al-4V alloy. The corrosion products generated on uncoated titanium in the same environment and temperature were characterized by a double layered oxide scale of TiO2 beneath which a TiS2 layer was formed. For the Ti-6Al-4V alloy, α-Al2O3 was precipitated in the external portion of the outer-layer of TiO2 whilst a layer containing Al2S3, TiS2 and vanadium sulphide (possibly V2S3) was idenitified underlying the inner TiO2 layer. After prolonged exposure (168 h), the Nb coating deposited on Ti and Ti-6Al-4V alloy was consumed. A scale following the sequence of TiO2/TiO2+NbO2+Nb2O5/Nb1−xS/TiO2/ TiS2/(substrate) was observed on the surface of the Nb-coated Ti, whilst a scale with sequence of TiO2/V2S3/TiO2+NbO2+Nb2O5/Nb1−xS/TiO2/Al2S3+TiS2/(substrate) characterized the corrosion products formed on the Nb-coated Ti-6Al-4V alloy.

Oxidation and Sulfidation Behavior of AlTiN-Coated Ti–46.7Al–1.9W–0.5Si Intermetallic with CrN and NbN Diffusion Barriers at 850°C

Attempts have been made to improve the high-temperature corrosion behavior of an intermetallic alloy, Ti–46.7Al–1.9W–0.5Si, in an H2/H2S/H2O atmosphere at 850°C using AlTiN coating with and without CrN and NbN diffusion barriers. The oxidation and sulfidation behavior of the uncoated Ti–46.7Al–1.9W–0.5Si alloy followed protective kinetics with a parabolic rate constant of 6×10−11 g2/cm4/s. A multi-layered scale developed: an outer rutile (TiO2) layer, a continuous layer of α-Al2O3 beneath the rutile layer, and an inner TiS layer, in which pure W was scattered. Fast outward diffusion of Ti within the substrate resulted in the formation of a zone of high concentration of aluminum (TiAl3 and TiAl2) between the scale and substrate.The use of an AlTiN coating greatly increased the oxidation and sulfidation resistance of Ti–46.7Al–1.9W–0.5Si. The use of NbN and CrN diffusion barriers further enhanced its corrosion resistance. The protection of the double-layer coatings persisted even after 240 hr exposure. However the mismatch of thermal expansion coefficients between the coating and substrate led to the development of cracks in some locations within the coatings. A 2.5 μm thick AlTiN coating on the Ti–46.7Al–1.9W–0.5Si substrate with an embedded defect was modeled using the general finite element (FE) program ABAQUS. The modeling results showed rapid mode I failure of the coating at a temperature of 774°C. The through-fracture of the nitride film caused the nitride coating to shrink back leading to delamination around the crack in the nitride coating. The cracks formed acted as diffusion paths, for the ingress of oxygen and sulfur species and the outward diffusion of substrate elements, which resulted in the formation of nodular corrosion products with similar morphologies and microstructures to the uncoated alloy in those locations where cracks developed.

Use of PVD deposited TiN coating in retarding high temperature sulphidation

Abstract TiN coated Inconel 600 and Nimonic PE11 alloys were exposed to an atmosphere comprising a high sulphur potential (pS 2 ∼ 10 −1 Pa) and a low oxygen potential (pO 2 ∼ 10 −18 Pa) at 750 °C for periods up to 72 h. The sulphidation kinetics, determined by a discontinuous gravimetric method, demonstrated that the TiN coating greatly enhanced the sulphidation resistance of the substrates, particularly during the early stages of exposure. For TiN coated Inconel 600 the post-exposure analysis by scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction and glancing angle X-ray diffraction, showed the formation of an outer layer containing Ni 3 S 2 on the surface of the TiN coating while an inner layer consisting of Cr 2 S 3 developed at the coating/substrate interface. The double layered scale formed on uncoated Inconel 600 consisted of Ni 3 S 2 and Cr 2 S 3 . For the TiN coated Nimonic PE11, sulphide nodules consisting of three layers — (Fe,Ni) 9 S 8 (outmost) Cr 2 S 3 MoS 2 (innermost) were observed to develop and the TiN coating was sandwiched between (Fe,Ni) 9 S 8 and Cr 2 S 3 . The portion of the TiN coating which was enveloped by those sulphide nodules became unstable after long-term exposure and subsequently dissociated thereby causing the loss of environmental protection. Moreover, the coating was damaged mechanically by the growth of the sulphide nodules. The scale formed on the uncoated Nimonic PE11 showed the formation of a similar structure in a similar sequence.

Influence of plasma-sprayed Mo coating on sulphidation behaviour of Inconel 600 and Nimonic PE11 alloys

Abstract Sulphidation is a serious problem in many energy conversion systems. Sulphidation attack is particularly severe in environments of low oxygen ( p O2 ~10 −18 Pa) and high sulphur ( p S2 ~ 10 −1_10−3 Pa) potentials at temperatures above 700°C. Recognizing that refractory metals have a high resistance to sulphidation in reducing environments, their use as overlay coatings in sulphur-containing environments deserves serious consideration. In the adoption of such an approach, Mo has been recognized as a highly sulphidation-resistant metal with a Kp value of 10 −12 g 2 cm −4 s −1 in an atmosphere of p S2 ~ 10 −1 Pa at 750°C. In this study, Mo was deposited on two superalloys, Inconel 600 and Nimonic PE11, using air plasma spraying. The coated specimens were tested at 750°C for up to 168 h in an environment comprising p S2 ~ 10 −1 Pa and p O2 ~ 10 −18 Pa. The sulphidation kinetics (this is not real through-layer kinetics, since the specimens had open edges) were determined by a discontinuous gravimetric method. The exposed samples were characterized and analysed using SEM, EDX and XRD techniques. For uncoated Inconel 600, a duplex scale was formed which consisted of an outer Ni 3 S 2 layer and an inner Cr 2 S 3 layer. The scale developed on uncoated Nimonic PE11 comprised three sub-scale layers—an outer (Fe, Ni) 9 S 8 layer, a mid Cr 2 S 3 layer and an inner MoS 2 layer. However, after prolonged exposure, the outer layer contained both (Fe, Ni) 9 S 8 and Ni 3 S 2 on the surface of the uncoated Nimonic PE11. The kinetics results demonstrated that Mo coating enhanced the corrosion resistance of both alloys, and particularly so for Nimonic PE11. For both substrates, Mo coating led to the development of two sub-scale layers at the early stages of exposure (e.g. up to 5 h). The outer layer consisted of Ni 3 S 2 for Inconel 600 and (Fe, Ni) 9 S 8 for Nimonic PE11. A MoS 2 layer constituted the inner layer for both materials. After prolonged sulphidation, a Cr 2 S 3 layer gradually developed between the outer layer and the inner MoS 2 layer. It is apparent that the use of Mo coating hindered the formation of the Cr 2 S 3 layer.

Enhancement of oxidation/sulphidation resistance of Ti and Ti6Al4V alloy by HfN coating

Abstract An attempt has been made to enhance the oxidation/sulphidation resistance of Ti and Ti6Al4V alloy by the application of an HfN coating produced using Physical Vapour Deposition (PVD). The coated specimens were exposed to an atmosphere comprising high sulphur and low oxygen potentials ( p S 2 ∼ 10 −1 Pa and p O 2 ∼ 10 −18 Pa) at 750 °C for periods up to 240 h. The corrosion kinetics were obtained by a discontinuous gravimetric method. Linear and linear-parabolic kinetics were recorded for Ti and for Ti6Al4V alloy respectively. The HfN-coated Ti and Ti6Al4V specimens showed enhanced corrosion resistance. The scale formed on the uncoated Ti specimen consisted of a double layer of TiO 2 with an underlying TiS 2 film on the substrate. The exposed Ti6Al4V alloy also contained a double layer of TiO 2 but with Al 2 O 3 precipitated in the external portion of the outer layer of the TiO 2 , whilst a layer of Al 2 S 3 and TiS 2 with vanadium sulphide developed beneath the inner layer of TiO 2 . Application of the HfN coating suppressed the formation of the duplex oxide scales. However, extensive cracking developed either parallel or perpendicular to the sample surface, and severely compromised the protectiveness of the coating.