High-temperature antioxidant silicate coating of low-density Nb – Ti – Al alloy: A review

: Nb – Ti – Al base alloy is an important structural material with low density and high temperature. However, as with other niobium alloys, the weak oxidation resistance is the bottleneck of its engineering application. Surface coating technology is considered an ideal method to solve the oxidation resistance of Nb – Ti – Al-based alloys. In this article, the progress of research on high-temperature anti-oxidation silicide coatings on Nb – Ti – Al alloy in recent years is reviewed. The microstructure, phase composition, and oxidation properties of di ﬀ erent silicide coatings are analyzed. The failure mechanism and applications of Nb – Ti – Al-based alloy silicide coating are summarized. The existing problems and future development of Nb – Ti – Al-based alloy silicide coatings are analyzed and prospected.


Introduction
Aerospace technology has developed rapidly in recent years.It is imperative to develop a new type of high-temperature material beyond the traditional superalloy.Low-density niobium alloy is a type of niobium alloy with low density, high strength, and self-oxidation resistance.Tungsten, molybdenum, vanadium, titanium, aluminum, zirconium, chromium, carbon, and other elements are added to this alloy to form a niobium alloy with solid solution and precipitation strengthening [1,2] The low-density niobium alloy represented by Nb-Ti-Al is considered as one of the high-temperature structural materials with great development potential to replace nickel-based alloys because of its high melting point, good mechanical properties, good ductility, and inherent oxidation resistance below 800°C [3][4][5][6].However, the low-density niobium alloy has a significant increase in the oxidation weight after more than 900°C and will be embrittlement at more than 1,000°C due to rapid oxidation, which seriously limits its application in a high-temperature aerobic environment [7][8][9].
The surface oxidation resistance coating is an effective method to balance the mechanical properties and hightemperature oxidation resistance of the alloy [10,11].The principle is to form a dense composite film on the surface of the matrix to prevent oxygen atoms from diffusing and invading the matrix at high temperatures.There are many methods to prepare oxidation-resistant coatings on Nb alloys, such as halide-activated pack cementation (HAPC), spark plasma sintering (SPS), slurry melting process, chemical vapor deposition (CVD) and hot dip silicon-plating (HDS) [12][13][14].Among them, the coating of HAPC has good density and unifrmity, but the operation is complicated.The SPS method can realize the densification and sintering of materials at low temperatures quickly, but the equipment requirements are high.The slurry melting method has low cost and simple operation, but the coating thickness and uniformity are difficult to control.CVD has great advantages in the preparation of coatings on surfaces with complex shapes (deep and fine holes), but the reaction temperature exceeds 1,000°C, and some substrate intolerances limit its application.HDS is dense, but the cost is higher.Relevant tests show that [15][16][17][18] high-temperature ceramic, aluminide, and silicide coatings can provide good antioxidation protection effect for niobium, but high-temperature ceramic and aluminide coatings cannot meet the requirements of the new engine under complex working conditions, such as high temperature, heat wash, and pulse.Silicide coating is the most widely used coating on niobium alloy because of its stable high-temperature oxidation resistance, mature coating system, and preparation technology.In this article, the research status and future development trend of high-temperature oxidation-resistant coatings on low-density Nb-Ti-Al alloy are reviewed.

Microstructure of silicide coating of low-density Nb-Ti-Al-based alloy
The silicide coating can further improve the oxidation resistance of low-density niobium alloy (Nb-Ti-Al) while avoiding damage to its mechanical properties, which is widely used in aerospace fields at home and abroad.For example, the Si-Cr-Ti-coated R-1E(110N), R-4D(490N), and R-6C(22N) attitude control and apogee engines developed by KaiserMarquardt of the United States were successfully applied to the "Apollo" spacecraft service and lunar modules [19,20].So far, the main niobium alloys studied in the world are shown in Table 1.Ningxia Orient Tantalum Industry Co., Ltd.developed a low-density niobium alloy with a density of ≤6.0 g•m −3 , which is much lower than the density of other niobium alloys (Table 2).Hu et al. [23] prepared Si-Cr-Ti coating on niobiumtitanium alloy surface by slurry sintering method, studied the mass change rate and thermal shock resistance of the coating, and carried out microscopic analysis of the coating.The results show that the static high-temperature resistance of the coating reaches 1,200°C, and the lifetime is more than 10 h.The coating is compact and uniform and metallurgically combined with the alloy matrix.The coating sample was cycled ten times by thermal shock at a room temperature of 1,200°C, and the coating did not fall off and remained intact.The before and after thermal shock photos are shown in Figure 1.The main component of the coating is niobium silicide, and SiO 2 glass protective film is formed at high temperatures, which prevents the oxidation of the coating and the substrate.The alloying of rich Cr and Ti in the coating reduces the formation of micro-cracks, reduces the oxidation rate of the substrate, and improves the antioxidant capacity of the substrate, indicating that the coating has good antioxidant properties.
As can be seen from the surface scanning electron microscope (SEM) coating, cross-sectional microstructure of coating, and scanning of the coating section in Figure 2, the contents of Nb and Ti elements gradually increase from the outside to the inside, while the contents of Si elements continuously decrease, and the contents of Cr elements are the highest in the middle layer.The diffusion layer mainly contains Si, Ti, and Nb elements, indicating that the diffusion layer is formed by the mutual diffusion of Si and matrix elements, and the main components are (Nb, Ti) 5 Si 3 .The coating is very dense, and the existence of the diffusion layer effectively improves the bonding strength of the coating and matrix.The middle layer is relatively dense and is the main body of the coating, and the main component is (Nb, Cr) Si 2 .The outer layer of the coating is relatively loose, and the main component is (Nb, Ti, Cr) Si 2 , and the Si content is relatively high.
Zhao et al. [24] applied the slurry sintering method to coat the surface of Si-Cr-Ti composite silicide and tested the mechanical properties of the coating sample.The morphology of the coating sample is shown in Figure 3, which indicates that the overall thickness (Pa1) of the Si-Cr-Ti coating prepared by the melting method is 105 μm, showing a typical three-layer structure: the innermost layer is the  diffusion layer (Pa2), with a thickness of 11.74 μm; the middle layer is the main layer (Pa3), with a thickness of 55.82 μm; the outermost layer is the surface layer (Pa4), with a thickness of 30.15 μm; the diffusion layer and the main layer are dense, while the surface layer is relatively loose.Figure 4 shows the metallographic micromorphologies of alloy and coating samples.It can be seen from the figure that the grains of the Nb-Ti-Al base alloy are fine equiaxed, the grains are dispersed and refined, and the grain size is about grade 5.After the coating of Si-Cr-Ti, the grain of the alloy grows significantly, and macrocrystalline grains appear.The appearance of macroscopic grains indicates that the preparation temperature of the Si-Cr-Ti coating is too high and exceeds the recrystallization temperature of the Nb-Ti-Al alloy [25].The obvious increase in the grain size of the alloy will inevitably affect the strength and shape at room temperature.The results show that the mechanical properties (tensile strength, yield strength, and elongation) of the coated lowdensity niobium alloy decrease significantly at room temperature.The main reasons for the decrease in mechanical properties include the obvious growth of alloy grains after coating, the outward diffusion of strengthening element Al in the alloy, and the formation of brittle phase Nb 3 Al and the "infiltration effect" of Si-Cr-Ti coating on the alloy.High-temperature antioxidant silicate coatings of Nb-Ti-Al alloy  3 that the coating is mainly composed of dispersed and uniform equiaxed grains.Due to the difference between the thermal expansion coefficient of the coating and the substrate, there are very small cracks on the surface.Figure 5b shows the microstructure of the coating section.It can be seen from the figure that the overall thickness of the coating is about 140 μm,  and there is a diffusion layer of about 3 μm between the coating and the alloy.This layer is formed by the mutual diffusion of the coating and the alloy at high temperatures, which can improve the bonding strength of the coating and the alloy.
The diffusion layer has a dense main coating layer and a relatively surface layer structure.The formation reaction equations are as follows [26][27][28][29]: (2) (3) where M represents Nb, Ti, Cr, Al, and other elements.
It can be inferred that the Si-Cr-W coating relies on the oxidation of its own metal-based silicides to form SiO 2 and composite metal oxides to achieve antioxidation protection of the substrate.The SEM of the Si-Cr-W coating after the oxidation test for 100 h shows that the microstructure of the coating has significantly changed from uniform and dispersed equiaemic crystals to spheroidal crystals of varying sizes and some amorphous grains in Figure 5c and d.The structural porosity also increases significantly.The whole coating is transformed from the three-layer structure before the test into a four-layer structure composed of an obviously cracked, loose outermost layer, a secondary outer layer with a few small cracks and holes defects, a relatively dense innermost layer, and a significantly thickened diffusion layer.The coating reacts at high temperatures in the following ways [30][31][32]: (7) (10) Among them, M in the reaction equation refers to Nb, Cr, W, Ti, and other metal elements.At the beginning of the oxidation test, the coating reacts with oxygen ( 6) and (7).With the extension of the test time, MSi 2 is gradually consumed, and the reaction ( 8)-( 10) begins to occur and has Nb 2 O 5 , such as metal oxide formation.The volume ratio (PBR) of Nb and Nb 2 O 5 is 2.68, so there is significant volume expansion and stress when Nb 2 O 5 is formed.When the stress accumulates to a certain extent, the cracks in the surface are formed and gradually loosened.In the outermost structure in Figure 5d, oxygen permeates into the coating through cracks and other defects to form a secondary outer layer structure [33,34].With reactions ( 6)-( 10) continuing to occur, the oxidation degree of the coating will continue to deepen until the coating is completely oxidized.The results show that the coating is composed of metal silicides such as Nb, Ti, Cr, and W, and the high-temperature oxidation protection of the alloy is realized by the oxidation decomposition of metal silicides to form SiO 2 and composite metal oxides.After a 100 h of oxidation test, the outer layer of the coating was loosened due to oxidation, but the inner layer was still dense and without oxidation, indicating that the coating had good high-temperature oxidation resistance.
Cai et al. [35] prepared Si-Cr-Ti coating on the surface of the alloy by the slurry melting method and studied the oxidation behavior of the alloy and coating at 1,400°C.XRD, SEM, EDS, and EPMA were used to study the microstructure and composition distribution of the coating before and after oxidation.Figure 6 shows the SEM morphology of the surface oxide layer.It is found that the oxide layer mainly consists of small grains of 1-3 μm after 1 h of oxidation, the small-and medium-sized grains merge and grow during the oxidation process [36].At 7 h, the grain size increases to 8-10 μm, showing a lamellated structure.At the same time, the half-height and width of the diffraction peak did not change much, indicating that the crystallization of the High-temperature antioxidant silicate coatings of Nb-Ti-Al alloy  5 oxidation product was basically complete and the grain size was basically stable.
Figure 7a shows that the surface of Si-Cr-Ti prepared by the spraying method is micro-uneven, with a small amount of nodular tissue, but the coating surface is dense and uniform, without cracks.Figure 7b shows that the cross-sectional morphology coating is divided into two layers with relatively dense overall thickness of about 125 μm, of which the outer layer of the main coating is about 110 μm and the inner diffusion layer is about 15 μm.The outer layer of the main body has irregularly arranged micropores in the process of high-temperature sintering, the coating reaches a semi-molten state, and a small number of bubbles generated by high-temperature decomposition of the added binder fail to volatilize in time, and gradually form shrinkage holes with the solidification of sediments.Figure 7c shows the line scan analysis and indicates that there are a large number of matrix elements such as Nb and Al in the main layer of the coating, while the coating element Si is present in the matrix, indicating that the formation mechanism of the coating during the high-temperature melting process is as follows: the main elements such as Si in the coating diffuse into the matrix and the matrix Nb and Al.Ti, Al, and other elements are diffused into the coating at the same time, and the diffusion rate of fused Si is faster than that of matrix elements.As can be seen from Figure 7b, the outer layer of the main coating is mainly composed of phase mosaic with two different degrees.Figure 7c shows that the corresponding gray area with large component fluctuations and darker color has higher Ti and Cr contents, lower Nb and Si contents, and lower Ti and Cr contents.In the lighter white area, the content of Ti and Cr decreased significantly, while the content of Nb and Si increased.Figure 7d-f show the microstructure and composition distribution of Si-Cr-Ti coating after oxidation at 1,400°C for 11 h.It can be seen that the surface of the oxidized coating is mainly composed of a large area of semi-molten glass-like film.Compared with the original coating, the coating is more dense and flat, and the generated glass-like film effectively bridges the micro-cracks and micro-holes of the original coating, preventing oxygen from spreading inward through the defects.It is found that in the early oxidation stage, the Si element with the highest content in the outer layer of the coating is oxidized to SiO 2 .The following reaction may occur during the high-temperature oxidation process: where Me represents Nb, Ti, Cr, Al, and other elements.
A large amount of SiO 2 has a certain fluidity at high temperatures, and under the action of surface tension, it can gradually fill the coating of micro-cracks and holes, while Al 2 O 3 has a high melting point and is not easy to flow and gradually becomes the outer skeleton.In synergy with SiO 2 , the diffusion rate of O ion in SiO 2 and Al 2 O 3 is very low.The composite oxide has high enthalpy, high heat resistance, and high-temperature stability and can maximize the expansion coefficient consistent with the main body of the coating.Therefore, the formation of dense Al 2 O 3 and SiO 2 oxide layers at high temperatures can effectively block the inward diffusion of oxygen elements.With the continuous oxidation, the oxygen pressure on the surface of the coating decreases, and SiO 2 is decomposed into Si O at high temperature and vaporized and volatilized so that silicon is continuously consumed, and the main disilicide of the coating gradually changes as follows: where Me represents Nb, Ti, Cr, Al, and other elements.Under the action of temperature, surface chemical reaction driving force, and element concentration gradient, on the one hand, Nb, Ti, and Al elements in the matrix diffuse to the coating; on the other hand, the main layer of the coating continuously transforms disilicide into trisilicide.The released Si is used for the growth of SiO 2 on the surface and diffusion towards the matrix, and it reacts with the matrix elements to form low silicide.It can be seen from Figure 7e that cracks extend to the main layer of the coating.In the process of high-temperature oxidation, micro-cracks existing in the original coating diffuse into the channel as oxygen elements, and oxidation occurs preferentially around the cracks.When the crack is closed, Si in the main body of the coating spreads along the crack, and blunt oxidation of Si occurs at the crack tip, [37,38] and the crack front end is smooth, indicating that the coating has good anti-crack growth ability.The results show that the oxidation products of Nb-40Ti-7Al alloy are mainly TiNb 2 O 7 , TiO 2 , and Al 2 O 3 after 1-11 h oxidation at 1,400°C.Before oxidation, the coating is mainly composed of (Nb, Ti, Cr, Al)Si 2 main layer and (Ti, Nb, Al) 5 Si 3 transition layer.After high-temperature oxidation, the SiO 2 barrier layer containing Al 2 O 3 and TiO 2 is formed on the surface of the coating.The oxidation behavior of the alloy and the coating follows the parabolic rule.The mass increment per unit area of the alloy is 161.98 mg•cm −2 at 1,400°C for 11 h, and the mass increment per unit area decreases to 9.56 mg•cm −2 after coating.It is found that the Si-Cr-Ti coating has good oxidation resistance at high temperatures.
Chaia et al. [39] used the halide activation embedding infiltration method to prepare silicon-or aluminum-rich intermetallic compound coatings on Nb-Ti-Al alloy as a matrix.The thermodynamic calculation shows that the coating growth mechanism involves different chemical reactions.It is found that the deposited MAl 3 and MSi 2 coatings can effectively protect Nb-Ti-Al alloy.The reliability and high-temperature oxidation resistance of the coating are mainly due to the formation of Al 2 O 3 and SiO 2 to form a layer of protection on the matrix, and these oxide films grow in a very low kinetic way during the oxidation process at 900 and 1,000°C.After exposure at 1,000°C for 650 h, the mass change of the aluminum and silicon coatings was less than 1.5 mg•cm −2 .The quality change curve is shown in Figure 8.
The comparison of the antioxidant capacity of different modified element coatings is shown in Table 3 [22,23,35,39].It can be found that Nb-Ti-Al-based alloy silicide coatings have good static antioxidant properties around 1,200°C.At the same time, Nb-Ti-Al-based alloy silicide coating is suitable for service at 1,000-1,400°C.The oxidation resistance of coatings over 1,400°C remains to be studied.

Application of low-density
Nb-Ti-Al-based alloy coating In order to meet the weight reduction needs of aerospace aircraft heating part materials, domestic and foreign researchers strive to develop a new niobium alloy with low density, high strength, high toughness, and other advantages [40].At the same time, its higher specific strength can reduce the wall thickness of the component and reduce the mass of the component more [41].Nb-Ti-Al alloy has the above advantages, so it is widely used in the aerospace field.However, the alloy is easily oxidized in the atmospheric environment of more than 800°C and can be used in the atmospheric environment of more than 800°C after coating the alloy surface [24,[42][43][44][45][46].At present, the exhaust pipe of the military aircraft engine of the United States Pratt-Whitney Company, the heat exchange pipe of the Russian jet aircraft, and the thermal structure components of the new ramjet engine have been applied [47,48].
In order to further promote the rapid development of aerospace, the development of high-temperature niobium alloys with lightweight, high strength, and certain antioxidant properties and the preparation of coatings with more stable performance, better protection ability, and higher antioxidant temperature are the key to achieve the diversification of aerospace needs and the use of application conditions.The relevant research is of great significance for breaking through the engineering application of lowdensity niobium alloy and forming new high-value-added products, which will promote the rapid development of regional economy and industry.

Conclusions and prospect
Low-density Nb-Ti-Al-based alloy coating has the advantages of high-temperature resistance, oxidation resistance, and erosion resistance, and it has important applications in high-temperature thermal protection.However, with the rapid development of the science and technology industry, the use of low-density Nb-Ti-Al-based alloy environment is becoming more and more harsh in order to meet the new use requirements of the development of Si coating has become a research hotspot, but to achieve engineering applications, there is still a lot of work to be in-depth research, mainly including the following aspects: (1) At present, most of the research work focuses on static oxidation resistance, air-cooled thermal shock, watercooled thermal shock, and other laboratory properties, which need to simulate the actual application environment of the coating and carry out in-depth research on the high-temperature erosion performance of the coating, high-temperature, and high-speed gas corrosion resistance, coating application, etc. (2) Carry out research on the oxidation mechanism of lowdensity Nb-Ti-Al alloy and failure mechanism of antioxidation coating so as to evaluate the service life stability and reliability of coating under the application environment through data analysis.(3) Improve the coating preparation process and develop coating preparation technology suitable for industrial production in view of the problems existing in the coating preparation process.Most of the existing coatings are single coatings, and there are problems such as the mismatch between the thermal expansion coefficient of the substrate and the coating and the low bonding strength of the coating, which affect the performance and service life of the coating.Based on the advantages of various materials and the reasonable structural design of the coating, the preparation technology of the composite coating was studied, and the matching coating with higher temperature resistance, oxidation and long life was developed.The relevant research is of great significance for breaking through the engineering application of low-density niobium alloy and promoting the rapid development of economy and industry.

3
Oxidation behavior of silicate coating of low-density Nb-Ti-Albased alloyZhao et al.[22] prepared Nb-Ti-Al-based alloy by vacuum electron beam and electric arc furnace melting and prepared Si-Cr-W coating on the alloy surface by slurry melting.Oxidation of the coating was tested at 1,250°C.The microscopic morphology of the coating was analyzed using SEM.The microscopic morphology of the coating surface is shown in Figure5a.It can be seen from the figure

Figure 4 :
Figure 4: Metallographic microstructures of the alloy and coated samples: (a) transverse section of the alloy samples, (b) longitudinal section of the alloy samples, (c) longitudinal plane of the alloy samples, (d) transverse section of the coated samples, (e) longitudinal section of the coated samples, and (f) longitudinal plane of the coated samples [25].

Figure 5 :
Figure 5: Surface morphology of the coating before oxidation (a), section morphology of the coating before oxidation (b), surface morphology of the coating before oxidation (c), and section morphology of the coating before oxidation (d) [22].

Figure 8 :
Figure 8: Mass change versus time recorded for the aluminide and silicide coatings during oxidation tests performed at 900 and 1,000°C [39].

Table 2 :
[22]ity and mechanical properties of the Nb-Ti-Al alloy bar made by OTIC[22]

Table 3 :
Reported static oxidation resistance of different Nb-Ti-Albased alloy silicide coatings