A Conformable High Temperature Nitride Coating for Ti Alloys

: There are many applications including aeroengine design where one would like to operate Ti or its alloys at higher temperatures, but the threat of oxidation or fire remains a longstanding challenge. Here, we have designed a bilayer nitride coating for Ti and its alloys produced by magnetron sputter deposition of a SiAlN coating (1 .2 μ m thick) with a Mo interlayer. We have taken advantage of interdiffusion and inter-reaction at the interface during cyclic oxidation at 800°C to form a layered nitride coating system comprising: a SiAlN top layer, a TiN 0.26 and Ti 5 Si 3 mixed phase interlayer, and a Ti-Mo solid solution. The novel TiN 0.26 interlayer exhibits adaptive conformability via mechanical twinning, thereby accommodating the thermal mismatch strain between the coating and substrate. This, along with high adhesion, confers excellent thermal cycling life with no cracking, spallation and oxidation of the coating evident after hundreds of hours of cyclic oxidation (>40 cycles) in air at 800°C. This work provides a design pathway for a new family of coatings displaying excellent adhesion, adaptive conformability and superior environmental protection for Ti alloys at high temperature. fitted with Super-X-EDS system. The phase distribution across the interlayer and diffusion layer was mapped using transmission Kikuchi diffraction (TKD) performed on an FEI Magellan HR scanning electron microscope. The 3D distribution of phases and voids across the interdiffusion zone was studied by FIB/SEM slice-and-view. A dual beam workstation (FEI, Quanta 3D) equipped with a FIB column employing a Ga liquid metal ion source,


Introduction
Ti and Ti alloys are widely used in the aeroengine gas turbine (e.g. fan blade, compressor, etc.) and automotive industries as well as for medical implants due to their low density, high specific strength and excellent corrosion resistance [1][2][3][4][5][6][7][8]. Increasing the gas turbine inlet temperature can increase engine efficiency and thereby increase fossil fuel efficiency [3]. However, Ti alloys have 2 inadequate oxidation resistance during high temperature exposure, which restricts their application.
Degradation is characterized by the formation of a rapidly growing, less protective scale of rutile TiO2 or TiO2+Al2O3 mixtures and a brittle oxygen-rich sublayer (oxygen absorbed layer with enhanced propensity of cracking) beneath the oxide scale, especially at temperatures above 500°C [9][10][11][12][13]. In order for Ti alloys to be safely applied at higher temperature, various techniques have been used to improve the oxidation resistance by bulk alloying, surface treatments and coating technologies. Of these, coating methods have been found to be the most effective way to improve the oxidation resistance of Ti alloys [9,[13][14][15][16][17].
Coatings of several types of material have been applied, including aluminides, silicides, glass-ceramics and nitrides [12,[18][19][20][21][22][23]. Among the numerous oxidation resistance materials, aluminide and silicide coatings are widely used to provide high temperature protection due to their propensity for forming protective oxide films at the surface. Nevertheless, the aluminide and silicide coatings can degrade by depletion of aluminium and silicon, which are consumed by oxide scale formation and inward diffusion into the Ti substrate during long time exposure at elevated temperatures [9,13,15]. Due to the chemical incompatibility between these types of coatings and Ti or Ti alloy substrates, a brittle phase can form along the coating/substrate interface and, thus, degrade the mechanical properties of the interface [15]. Glass-ceramic (MgO-SiO2-TiO2), nitride (TiAlN) and MAX phase (Cr2AlC) coatings have also attracted considerable interest because of their good chemical stability and excellent hightemperature oxidation resistance [16,21,24]. However, the inherent low ductility of ceramic coatings, poor adhesion and thermal mismatch between the coating and the substrate, especially during inservice cyclic oxidation [25], are areas of great concern restricting their application. All of the above factors mean that, despite the number of candidate coatings for the protection of Ti-alloys at high temperature, it is worthwhile exploring the potential of new coating systems with the aim of achieving a combination of good adhesion, good spallation resistance, controllable interdiffusion and good oxidation resistance. 3 Previous studies have shown that SiAlN exhibits excellent oxidation resistance and good thermal stability at high temperature [26][27][28], but the challenge is to engineer good interfacial performance.
Traditional routes for improving the adhesion between physical vapour deposited coatings and the substrate have included optimizing the surface of the substrate, applying a bias potential during deposition, or inserting an interlayer and developing a multilayer concept, etc. [29][30][31][32][33][34]. Annealing or thermal treatments, on the other hand, have been reported to deteriorate the adhesion of coatings owing to interdiffusion, which leads to the formation of brittle phases and voids at interfaces, thermal expansion mismatches, which initiate surface cracks and phase transitions that result in a weak interface [35][36][37][38][39][40].
There are many reasons to think that Mo might act as a good interlayer between Ti and SiAlN. Its coefficient of thermal expansion is between that of Ti and SiAlN [41,42], Ti-Mo can form an infinite solid solution, and it has a relatively high diffusivity in Ti at high temperature [43,44]. Thus, employing Mo as an interlayer may alleviate the thermal mismatch between the Ti substrate and the nitride coating, while interdiffusion of Mo into Ti may enhance the adhesion of the coating.
In this study, we have deposited SiAlN coatings (1.2 μm thick) with a Mo interlayer (300 nm) on Ti and Ti6Al4V alloy by magnetron sputtering. We have taken advantage of interdiffusion between Ti and Mo, and inter-reaction between Ti and Si3N4 from the SiAlN coating at the interface during thermal exposure, aiming to build a new generation of coating systems for Ti alloys at high temperature exhibiting strong diffusion bonding and superior oxidation resistance. Uniquely these coatings appear to be highly conformable and thus resistant to spalling during thermal cycling.

Experiments and methods
The coatings were deposited on one side of commercially pure Ti and Ti alloy (Ti-6wt%Al-4wt%V) coupons (50×50×3 mm 3 ), which were ground, polished (surface roughness, Ra 60 nm) and ultrasonically pre-cleaned in acetone. Deposition took place by reactive sputtering in a Teer Coatings UDP 350 magnetron sputtering system, described in detail elsewhere [45]. Three vertically opposed 4 unbalanced magnetrons (300 x 100 mm 2 ) were installed through the chamber walls surrounding a centrally mounted rotating unheated substrate holder. The 99.5% pure Si, Al and Mo targets were fitted to the magnetrons. Prior to deposition, the chamber was evacuated to a base pressure of lower than 1x10 -3 Pa. The substrates were sputter cleaned by Ar+ ions at a bias voltage of -600 V DC for 15 mins prior to the deposition. The Si, Al and Mo targets were each powered by Advanced Energy Pinnacle Plus power supplies operating in pulsed DC mode at a powers of 700, 300 and 500 W, respectively, and a pulse frequency of 100 kHz (duty cycle = 60%) and a bias of -30 V was applied to the substrate throughout coating deposition. First the pure molybdenum layer was deposited in an argon (～2.1×10 -3 mbar, 0.21 Pa) only atmosphere prior to the deposition of the SiAlN coating. The argon to nitrogen gas ratio (Ar: 20 sccm, N2:60 sccm) during the reactive sputter deposition of SiAlN was controlled using mass flow controllers (MKS Ltd.) and by monitoring the partial pressures (～ 3.0×10 -3 mbar, 0.3 Pa) of the introduced gases. The substrate was rotated at a speed of 5rpm during deposition of Mo and SiAlN. For tribological and environmental protection applications, thin coatings of a few micrometres thickness have been commonly used, with the coating thickness being controlled by the coating deposition rate. The deposition rate of the SiAlN coating is 9 nm/minute. For practical purposes in these experiments, a maximum run time of 130 minutes was adopted. This gave a thickness of 1200 nm for SiAlN is and 300 nm for Mo. Whilst thicker coatings may be desired for commercial applications, from the perspective of developing and characterising new coatings, this was deemed to be sufficient. [26,[46][47][48]. The SiAlN-coated coupons (50 ×50 × 3 mm) were cut into smaller rectangular (15×15×3mm) pieces using a SiC abrasive cutting blade in a precision cut-off machine (Accutom 5, Struers). These were used for scratch tests and oxidation tests after cleaning with soapy water and acetone.
For coatings having a thickness around 1 μm thick or less, the nano-scratch test shows relatively higher reproducibility and lower sensitivity to testing parameters (loading rate, radius etc) than the standard scratch test. The nano-scratch test was used to evaluate coating adhesion, with 5 scratches made on 5 each sample. Prior to scratch testing the as-deposited samples were isothermal annealed in a static air furnace at 800°C for 0.5 h and 3 h, respectively. The coated surface was scratched with a spheroconical diamond indenter with a tip radius of 5 μm and a cone angle of 90°. The normal load was continuously increased linearly, from 0 to 450 mN, within a distance of 300 μm and the profiling load was 50 μN. Cyclic oxidation tests were conducted to evaluate the oxidation resistance and adhesion of the coatings. The tests were performed in a furnace with an accuracy ±1°C in static air.
The as-deposited samples were directly placed inside the furnace when the temperature stabilised at the targeted temperature of 800°C and the test duration was 5 hours. After this, the samples were taken out directly from the furnace at 800°C, cooled for 10 minutes at ambient temperature, then returned for the next cycle, continuing up to 200 h (40 cycles).
The composition of the as-deposited SiAlN coating was analysed by focused ion beam --X-ray photoelectron spectroscopy (FIB-XPS, Kratos AXIS Supra). The sample was depth profiled using 5 kV Ar+ ions to sequentially sputter remove surface material. A 2 mm x 2 mm etch crater was created with a 110 μm analysis area in the centre of the etch crater. The cross-sections of the oxidized SiAlN coating samples were investigated by scanning electron microscopy (SEM, FEI, Quanta 650, Magellan HR) fitted with an energy dispersive X-ray spectroscopy (EDS) system, coupled with a focused ion beam (FIB, FEI, Quanta 3D, Helios 660), described in detail elsewhere [45,48]. To observe the microstructure of the as-deposited coatings and oxidized samples in greater detail, thin lamellae of the cross-sections of the coatings were prepared by FIB using the lift-out technique and then examined by transmission electron microscopy (TEM, FEI, Tecnai G2; FEI, Tecnai T30) fitted with an energy dispersive X-ray spectroscopy (EDS) system and advanced TEM (FEI, Talos, F200A) fitted with Super-X-EDS system. The phase distribution across the interlayer and diffusion layer was mapped using transmission Kikuchi diffraction (TKD) performed on an FEI Magellan HR scanning electron microscope. The 3D distribution of phases and voids across the interdiffusion zone was studied by FIB/SEM slice-and-view. A dual beam workstation (FEI, Quanta 3D) equipped with a FIB column employing a Ga liquid metal ion source, 6 combined with a high resolution field emission scanning electron microscope was used to perform serial cross-sections through the selected volume (10x8x5 μm). The milling current was 0.3 nA and the voxel size was 80 nm, which set the distance between individual cross-sections imaged by SEM. The stack of cross-sectional SEM micrographs (2048 x 1768) was reconstructed into 3D volume images using Avizo 9.4 software.

As-deposited SiAlN/Mo coating
The as-deposited SiAlN coating is smooth and about 1.2 μm thick, there is no significant porosity or cracking and the 300 nm molybdenum layer deposited between the SiAlN coating and Ti substrate is fully dense, as shown in Figure 1 a. The selective area diffraction (SAD) pattern of the SiAlN coating shows a diffraction halo, as shown in Figure 1 a inset, which indicates that this coating is amorphous.
The top surface of the as-deposited SiAlN coating was milled by Ar+ ions and the XPS quantification is

Adhesion and oxidation resistance
The as-deposited SiAlN coatings with a Mo interlayer on Ti or Ti6Al4V alloy were exposed to air at  The critical load of the coating annealed at 800°C for 3 h is around 3 times higher than that of the asdeposited coating. This indicates that the adhesion between the SiAlN coating and Ti substrate has been improved significantly after thermal exposure.
The oxidation protection and resistance to degradation under thermal cycling of the as-deposited coating is demonstrated in Figure 2  These scratch and oxidation tests suggest that the adhesion between the SiAlN coating and Ti substrate can progressively strengthen. It shows a crack-free structure, free from spallation, with negligible interdiffusion, and no weight gain (no observed oxide scale) after hundreds of hours of cyclic 8 oxidation. This is better than any reported coatings for the protection of Ti alloy. By comparison, SiO2-Al203-glass, Cr2AlC, TiAlN, MgO-SiO2-TiO2-glass, TiAl, Al-Si, Cr-Al, TiAlCrY, NiCrCoAl/Al, Al/NiCrAlY, coatings deposited on pure Ti or Ti alloys were tested in static air or by cyclic oxidation from 700°C to 900°C [10, 12, 14, 15, 18, 19, 21-23, 32, 51]. All these coatings demonstrated cracking, spallation or interdiffusion or weight gain problems. For instance; 13 μm interdiffusion thickness and cracking for the glass composite coating after cycling to 800°C for 100 h [12]; microcracks and spallation for the

Coating/substrate interface
To examine the interface strengthening of the SiAlN coating caused by annealing, cross-sections of asdeposited and annealed (at 800°C for 0.5 h and 3 h in air) coatings have been observed by STEM in  Video 1 and 2). Thus, the interdiffusion and interreaction of Ti/Mo/SiAlN avoids the accumulation of pores and intermetallic compounds at the coating/substrate interface.

Layered nitride coating
To examine the superior environmental protection and the adaptive deformation of the coating layers, cross sections of the SiAlN coatings after cyclic oxidation were milled by FIB and imaged by SEM, which could provide accurate microstructure details without damaging the coating surface. The cross section of a SiAlN coating/ Mo interlayer on Ti after oxidation at 800°C for 100 h is shown in Figure 5 a. It is evident that due to interdiffusion and inter-reaction, the original coating/substrate interface has transformed to a complex layered structure. To view this in finer detail, thin lamellas of the cross- TiNX extends to a maximum value of x=0.2 such that TiN0.26 nitride belongs to an hcp structure. The c/a value of TiN0.26 is higher than that of TiN0.2 as the incorporation of nitrogen atoms caused a continuous increase in c/a value [54]. Similar phases at the interface between Ti and Si3N4 have been reported previously [52,[55][56][57]. It is noteworthy that the TiN0.26 layer between the substrate and SiAlN coating 10 may serve as a diffusion barrier to mitigate further inter-reaction between Ti and SiAlN, which maintains the chemical and structural compatibility of SiAlN. It can be confirmed that the remnant SiAlN coating (about 450 nm thick) is still amorphous after oxidation at 800°C for 100 h as shown in the inset image in Fig.6 a and HRTEM image in Fig.6 b. During the Ti/Si3N4 reaction, a discontinuous

Mechanical twinning
To understand why this novel coating structure is able to adapt to the changes that take place during many thermal cycles without debonding, the microstructure of the coating after cyclic oxidation at 800°C for 100 h has been characterised in finer detail, as shown in Figure 8.

Interface strengthening
We have seen that the adhesion of the coating after annealing at 800°C in air for 0.5 h and 3 h is better than that of the as-deposited coating. The primary mechanism of adhesion can be simply divided into

Adaptive conformability via mechanical twinning
During cyclic thermal oxidation, compressive stresses develop from thermal mismatch between the SiAlN (Si3N4=3.2 μm/(m•K)) coating and the Ti substrate (8.6 μm/(m•K)) during cooling [41,57]. In addition, after extended periods at high temperature, the growth of the solid reaction layers (TiN0.26, Ti5Si3) between the SiAlN layer and the Ti substrate results in a volume expansion. Therefore, it is essential to alleviate this stress/strain to maintain the coating system integrity and avoid cracking and spallation. In our layered nitride coating system, the deformable TiN0.26 layer helps to overcome this problem via mechanical twinning and stacking faults (Figure 8). Such adaptive deformability during thermal exposure is able to mediate and release the stress/strain, and thereby effectively reduce the debonding susceptibility and prolongs the cyclic lifetime during oxidation tests at high temperatures.

Oxidation protection mechanism
The conformal complex layered nitride coating appears to provide excellent protection for the Ti and Ti alloy substrate at 800°C with no oxide observed by SEM and STEM, no cracks and no spallation, 13 confirmed by Figure 2, Figure 3 and Figure 5. It is well known that the protection mechanism of a metallic substrate by thin coatings against oxidation at high temperature can be affected by the intrinsic oxidation resistance of the coating, the chemical compatibility of the coating (e.g. no interdiffusion with underneath substrate) and the coating-substrate integrity (no cracks and no spallation) [26,63] SiAlN and the substrate, which could change the chemical composition and degrade the structure of SiAlN and degrade the oxidation resistance of the coating. Indeed, J. Musil [26] had reported that the diffusion elements from the underlying substrate to the coating can stimulate the crystallization of amorphous nitride coatings, and thereby decrease the oxidation resistance during thermal exposure.
To maintain the chemical compatibility of coating, the interdiffusion should be avoided or mitigated.
In our case this is achieved by inserting an interlayer of Mo, since Mo is known to be able to reduce the activity of Ti and also slow down the interfacial reaction [68,69]. Even if the reaction between Ti and SiAlN occurs, the reaction products, TiN0.26, are mainly distributed between the substrate and SiAlN coating and are capable of serving as a diffusion barrier to mitigate further inter-reaction between Ti and SiAlN, which maintains the chemical and structural compatibility of SiAlN. It is confirmed that no diffusion from underlying Ti into the SiAlN coating happens and the remnant SiAlN coating (about 450 nm thick) is still amorphous after oxidation at 800°C for 100 h, as shown in Fig.5 c and Fig.6, respectively. This explains why our coating displays good thermal stability with no oxide 14 scale formed on the coating even after hundreds of hours at 800°C in air (as confirmed by Figure 2 b, c, Figure 3 and Figure 5). The integrity of the coating-substrate system also benefits from mechanical twinning in the TiN0.26 interlayer which mediates and releases the coating stress , and thereby greatly reduces the debonding susceptibility. Thus, the layered nitride coating system is effective because it develops an enhanced bonding interlayer by interdiffusion, a transition layer with adaptive deformability and controllable interdiffusion, while the SiAlN layer provides oxidation resistance at the free surface. Overall, this appears to deliver excellent protection for Ti and its alloys for up to 200 hours at 800°C.

Conclusion
In conclusion, we have designed a bilayer nitride coating for Ti and its alloys produced by magnetron sputtering a SiAlN coating with a Mo interlayer. We have taken advantage of interdiffusion and interreaction at the interface during cyclic oxidation at 800°C to form a complex layered nitride coating