Study on the Effect of Carburizing on the Microstructure and High-Temperature Oxidation Properties of Hot-Dip Aluminum Coating on Titanium Alloy

Abstract

A Ti–Al alloy phase layer/Ti–Al carburizing composite coating was prepared on the surface of titanium alloy by the stepwise coating method of hot-dip aluminizing and then carburizing. The weight gain results of the composite coating showed that the titanium alloy coated with the composite coating had long-term stability (≥16 days) at 800 °C. The microstructure, phase structure, and composition of the composite coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The composite coating is composed of an alloy phase layer and a carburized layer. The natural transition of four phases (Ti₃Al/TiAl/TiAl₂/TiAl₃) in the alloy phase layer significantly improves the interfacial bonding between the coating and the substrate and slows down the propagation of microcracks through the coating. Al₂O₃, TiC, and C in the carburizing layer improve the surface hardness of the coating, and TiAl₂ and Al₂O₃ also have excellent oxidation resistance at high temperature.

1. Introduction

The aero-engine is known as the “heart” of aircraft, and the progress of its design, material, and manufacturing technology plays a key role in the development of the aviation industry. Advanced aero-engines are developing towards high turbine front temperatures, high thrust-to-weight ratios, long lifespans, and low fuel consumption. In addition to advanced design technology, the improvement of engine performance strongly depends on the development of advanced materials and manufacturing technology.

Titanium alloy, known as “space metal”, is widely used in the aerospace field because of its advantages, such as low density, high specific strength, corrosion resistance, and stable performance at low temperature [1,2,3,4]. However, titanium alloy’s poor high-temperature oxidation resistance [5], low surface hardness, high friction coefficient, and poor wear resistance seriously restrict its service life and application field [6]. In order to effectively improve the properties of titanium alloy, the most widely used method is to prepare aluminum coating with excellent oxidation resistance on the surface of titanium alloy.

The formation of a Ti–Al diffusion coating on titanium alloy is a common method to improve its high-temperature oxidation resistance [7,8]. The Ti–Al diffused coating can be prepared by different processes, such as thermal spraying [9], aluminizing [10], laser surface alloying [11], the sol-gel method [12], and hot-dip aluminizing [7]. For example, Li, Z.W. et al. prepared Ti₃Al (O)-Al₂O₃ composite coating on the surface of titanium alloy by thermal spraying. The composite coating showed good oxidation resistance and excellent spalling resistance in the temperature range of 700~900 °C. Chen, Y. et al. prepared a Ti–Ni alloy composite coating on the surface of Ti-6Al-4V by laser surface modification. By adjusting the process parameters, different combinations of ductile TiNi and hard Ti₂Ni intermetallic phases can be synthesized on the Ti-6Al-4V surface to obtain adjustable surface properties. Hot-dip aluminizing is a mature and extensible process [13,14]. Wang, Z. et al. [14] prepared a Ti–Al–Si gradient coating based on silicon concentration gradients and added Ce to modify the coating. The coating has a dense Ti(Al,Si)₃ phase layer, which can effectively improve the high-temperature oxidation resistance of titanium alloys. The addition of cerium can effectively inhibit the formation of the τ₂: Ti(Al_xSi_1−x)₂ phase in a certain heat infiltration time, so as to form a continuous dense Al₂O₃ layer, and further improve the oxidation resistance of the coating. The hot-dip aluminizing process includes surface pretreatment of the sample and holding in a molten aluminum pool for a certain time. This method was first applied to stainless steel, carbon steel, and alloy steel, and is often used to improve the corrosion resistance, wear resistance, and high-temperature oxidation resistance [15,16,17]. TiAl₃ phase +L-(Ti,Al) and L-(Ti,Al) layers are formed on the surface of titanium alloy after hot-dip aluminizing. Due to the brittleness of TiAl₃ phase and the mismatch of thermal expansion coefficients between TiAl₃ phase and titanium alloy substrate [18], the mechanical properties of the substrate are reduced and cracks are easy to propagate. Therefore, proper heat treatment is required after hot-dip aluminizing to make elements diffuse between the coating and substrate, so as to produce metallurgical bonding between the coating and substrate [19]. The L-(Ti,Al) layer on the surface is mainly aluminum, and its wear resistance and hardness are insufficient. Surface modification of the L-(Ti,Al) layer to improve its wear resistance and surface hardness and reduce its friction coefficient is an urgent requirement to improve its application in various fields.

In the process of carburizing, carbon atoms penetrate into the surface of titanium alloy through a concentration gradient at a high temperature to form the corresponding carbide (TiC) gradient coating. The advantage of carburizing on titanium alloy is to effectively improve the surface hardness and wear resistance of the material without changing the substrate properties and macroscopic size. For example, Duan, H.q. et al. adopted solid carburizing technology to improve the hardness of Ti-6Al-4V alloy and (TiB + La₂O₃)/Ti composite [20]. They used graphite powder as a carbon source, avoiding hydrogen embrittlement. The gradient structure coating is grown by in situ extension, which significantly improves the interface bonding between the coating and the substrate and eliminates the mismatch of the thermal expansion coefficient and weak adhesion [21]. However, the thickness of the coating prepared by this method is limited, generally only a few microns to tens of microns.

We combine the hot-dip aluminizing process with the carburizing process to give full play to the advantages of both and achieve the optimization effect of “1 + 1 > 2”. When the titanium alloy after hot-dip aluminizing is carburizing, the surface L-(Ti,Al) layer reacts with active carbon atoms to form a Ti–Al carburizing layer with good wear resistance and high hardness. The aluminum element in the TiAl₃ phase continues to diffuse to the substrate, and Al reacts with Ti to form more Ti–Al alloy phase layers. Finally, the Ti–Al alloy phase layer/Ti–Al carburizing layer composite coating is formed. The composite coating has the advantages of close bonding with the substrate and excellent oxidation resistance at high temperatures.

In this paper, we studied the process of hot-dip aluminizing and carburizing of Ti65 samples and analyzed the high temperature and oxidation resistance of the obtained samples. We studied the microstructure and growth kinetics of the coating before and after thermal exposure tests. The internal reasons for the excellent high temperature and oxidation resistance of titanium alloy after hot-dip aluminizing and carburizing were discussed.

2. Materials and Methods

2.1. Experimental Material

High purity aluminum ingots (99.999%, Nantong Taide electronic Material Technology Co., Ltd., Rugao City, China), carburizing agents (C, NaCO₃, BaCO₃, CaCO₃) (Changge Tanerno catalytic Technology Co., Ltd., Changge City, China and Sinopharm Group Chemical reagent Co., Ltd., Shanghai, China), and Ti65 alloy (Dongguan Xinrui Metal material Co., Ltd., Dongguan City, China) were selected in the experiment. The composition of Ti65 alloy is shown in

Table 1. The composition of Ti65 alloy.

Element	Ti	Al	Sn	Zr	Mo	Nb	Ta	Si	W	C
Content (wt.%)	82.55	5.9	4.0	3.5	0.3	0.3	2.0	0.4	1.0	0.05

. The sample was linearly cut to 15 mm (length) × 10 mm (width) × 3 mm (thickness). All samples were polished with 400#, 600#, and 800# sandpaper to ensure a smooth surface.

2.2. Hot-Dip Aluminizing

After several Ti65 samples were cleaned with acetone for 5 min, washed with water, and cleaned with alcohol for 15 min, they were immersed in pure aluminum liquid protected by argon at 760 °C for 5 min, 10 min, 15 min, and 20 min, respectively. Then, the Ti65 samples were slowly extracted and the excess aluminum liquid on their surface was removed.

2.3. Carburization

The hot-dip-aluminized Ti65 samples were placed in a corundum crucible equipped with a solid carburizing agent and sealed with a mixture of calcined kaolin and sodium silicate. Then, the sealed crucible was dried in an oven for 2 h and then put into a box-type resistance furnace (Xiangtan Samsung Instrument Co., Ltd., Xiangtan, China) for carburizing. The samples were kept at 1050 °C for 2 h, 4 h, 6 h, and 8 h, respectively, and then removed after cooling to room temperature in the furnace.

2.4. Characterization Test

The phase of the coating was analyzed by X-ray diffraction (XRD) using an Ultima IV (Ultima IV, Rigaku Co., Tokyo, Japan) diffractometer with Cu Ka radiation (λ = 0.15406 nm). The working voltage of the X-ray tube was 40 kV and the working current was 40 mA, and the XRD spectra obtained were in the range of 10°–90°. The angle step was 0.02° and the scanning speed was 10°min⁻¹.

Scanning electron microscopy (SEM) (ZEISS EVO MA10, ZEISS, Jena, Germany) and energy dispersion spectroscopy (EDS) (OXFORD X-MAXN, ZEISS, Jena, Germany) systems were used for the analysis of the coating morphology and element distribution.

The hardness of the uncoated Ti65 sample, the hot-dip-aluminized (760 °C × 10 min) Ti65 sample, and the hot-dip-aluminized (760 °C × 10 min) and carburized (1050 °C × 4 h) Ti65 sample was measured using a hardness tester (SHYCHVT-30, Laizhou Huayin Hardness Meter Factory, Lai Zhou, China). These samples were observed under the Vickers hardness tester microscope and the samples were measured three times; the macro hardness value was obtained by taking the average value. The penetrator type was a diamond cone penetrator with a force of 3 kg and a load holding time of 10 s.

2.5. Thermal Exposure Test

For comparison, uncoated Ti65 samples, hot-dip-aluminized (760 °C × 10 min) Ti65 samples, and hot-dip-aluminized (760 °C × 10 min) and carburized (1050 °C × 4 h) Ti65 samples (represented by group A, B and C below) were placed in static air at 800 °C at the same time for the thermal exposure test. The duration varied from 2 to 16 days, with 2 day intervals, and 3 samples were consumed for each thermal exposure time. Before and after the thermal exposure test, an electronic balance was used to weigh and record (the measurement accuracy of weight is ±10⁻⁴ g), so as to calculate the weight gain per unit area of the sample at different times and obtain the high-temperature oxidation weight gain curve. The microstructure, phase composition, and element distribution of the coating were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS).

3. Results and Discussion

3.1. Characterization of the Coating Microstructure

3.1.1. Microstructure of the Hot-Dip Aluminizing Coating

We found that the TiAl₃ phase layer can be formed by hot-dip aluminizing at 760 °C for 5–20 min, but its thickness is different. When the hot-dip aluminizing time is less than 10 min, the thickness of the TiAl₃ phase layer is low, which is not conducive to the formation of an appropriate thickness Ti–Al alloy phase layer in the subsequent carburizing process, while the hot-dip aluminizing time is too long, which is not suitable for industrial production and is more likely to lead to the generation and expansion of micro-cracks in the TiAl₃ phase layer. Therefore, we chose to analyze the structure and properties of the coating obtained by hot-dip aluminizing for 10 min. Figure 1 shows the microstructure of Ti65 after 10 min of hot-dip aluminizing.

We found that three new Ti–Al alloy phase layers (Ti₃Al, TiAl, TiAl₂) can be formed by carburizing at 1050 °C for 2 to 4 h, but the thickness is different, and the Ti–Al carburizing layer contains Al₂O₃, TiAl₂, C, and TiC. When the carburizing time is less than 4 h, the thickness of the Ti–Al carburizing layer and the three Ti–Al alloy phase layers (TiAl₂, TiAl, Ti₃Al) is small. When the carburizing time is too long, the surface brittleness of the coating increases, and the carburizing layer is easy to peel off. Therefore, we chose to analyze the structure and properties of the coating obtained by carburizing for 4 h.

3.1.2. Microstructure of the Composite Coating

Figure 2 shows the cross-section microstructure of the Ti–Al alloy phase layer/Ti–Al carburizing composite coating obtained by hot-dip aluminizing (760 °C × 10 min) and carburizing (1050 °C × 4 h). It can be observed that the coating has obvious stratification, consisting of two regions: one is the alloying phase layer region and the other is the carburizing layer region.

EDS data of different regions are shown in

Table 2. EDS data (at.%) and phase composition in different regions.

Point	Ti	Al	C	O	Ba	Compounds
1#	71.61	28.39	0	0	0	Ti3Al
2#	48.38	51.62	0	0	0	TiAl
3#	39.08	60.92	0	0	0	TiAl2
4#	27.37	72.63	0	0	0	TiAl3
5#	24.94	61.62	0	13.43	0	L-(Ti,Al), Al2O3
6#	4.85	25.92	14.11	55.12	0	Al2O3, TiAl2, C, TiC
7#	2.78	10.62	21.88	53.30	11.01	BaAl2O4, BaTiO3, C

. EDS data of the alloy phase layer region indicate that four Ti–Al diffusion layers are formed between the coating and substrate. In addition, some voids were observed in the coating. The atomic ratios of Ti and Al at positions 1, 2, 3, and 4 are 3:1, 1:1, 1:2, and 1:3, respectively. Combined with the XRD pattern shown in Figure 3, the four phases are Ti₃Al, TiAl, TiAl₂, and TiAl₃, respectively.

One of the causes of the holes in the coating is the Kirkendall effect. Al [22] in TiAl₃ diffuses with Ti in the substrate, and the diffusion rate of Al is much higher than that of Ti [23], so the net diffusion is manifested as Al in TiAl₃ diffusing into the Ti65 substrate, resulting in the vacancy at the edge of the TiAl₃ phase [24,25]. The accumulation of vacancy leads to the formation of the Kirkendall void [26]. Another reason is the brittleness and large thermal expansion coefficient of TiAl₃ phase, which leads to the formation of cavities in the cooling process [27].

Figure 4 shows that C and O elements are mainly distributed in the carburized layer. EDS results and XRD patterns of the carburized layer indicate that the region is composed of Al₂O₃, TiAl₂, TiC, and free C. In Figure 2a, the white part at the top of the coating is BaAl₂O₄, BaTiO₃, and free C. The BaAl₂O₄ phase and BaTiO₃ phase are generated by the reaction of the energizer BaCO₃ with Al and Ti atoms in the process of carburizing, while the free C is formed by the carbon in the carburizing agent remaining in the sag on the surface of the sample.

3.1.3. Growth Kinetics of the TiAl₃ Phase

After hot-dip aluminizing, the coating formed on the surface of the Ti65 sample is composed of the TiAl₃ phase +L-(Ti,Al) and L-(Ti,Al) layer. The relationship between the thickness and time of the TiAl₃ phase can be expressed by the empirical formula [28,29]:

d = ktⁿ

(1)

where d is the thickness of the TiAl₃ phase (μm), t is the hot-dip aluminizing time (min), k is the rate constant, and n is the kinetic index. Kinetic index values of n < 0.5 represent diffusion-controlled growth, while values of n > 0.5 represent reaction-controlled growth.

The thickness of the TiAl₃ phase is represented by d_{TiAl${}_3$}, and the growth kinetics curve of the TiAl₃ phase was fitted by Equation (1), as shown in Figure 5:

$$d_{{\mathrm{TiAl}}_3}={12.066t}^{0.755}$$

(2)

The results show that the growth of the TiAl₃ phase is reaction-controlled growth.

3.1.4. Growth Kinetics of the Ti–Al Alloy Phase Layer

d₅, d₁₀, d₁₅, and d₂₀ are used to represent the thickness of the Ti–Al alloy phase layer in the process of carburizing after 5 min, 10 min, 15 min, and 20 min of hot-dip aluminizing, respectively. The growth kinetics curve of the Ti–Al alloy phase layer was fitted by Formula (1), as shown in Figure 6:

d₅ = 35.725t^0.443

(3)

d₁₀ = 56.503t^0.441

(4)

d₁₅ = 76.160t^0.358

(5)

d₂₀ = 114.050t^0.487

(6)

The results show that the growth of the Ti–Al alloy phase layer is controlled by diffusion.

3.1.5. Formation Mechanism of the Composite Coating

During hot-dip aluminizing of titanium alloy, the liquid aluminum and the base metal interdiffuse and the TiAl₃ phase is formed on the titanium alloy. The diffusion reaction is

Ti + 3Al → TiAl₃

(7)

Since the titanium atoms in the substrate diffuse into the liquid aluminum, which contains titanium atoms, and there are voids in the TiAl₃ phase, the liquid aluminum-containing titanium atoms will partially fill the voids around TiAl₃ during hot-dip aluminizing, forming a mixed-phase layer composed of the TiAl₃ phase and the L-(Ti,Al) phase. When the titanium alloy sample is extracted from the aluminum pool, the liquid aluminum-containing titanium atoms will adhere to the outside of the mixed-phase layer and solidify into the outermost layer of the hot-dip-aluminized coating. Therefore, the coating after hot-dip aluminizing is composed of a TiAl₃ phase +L-(Ti,Al) mixed-phase layer and a L-(Ti,Al) layer.

When the hot-dip-aluminized Ti65 sample is carburized, the aluminum element in the TiAl₃ phase continues to diffuse to the substrate, while the Ti element in the substrate diffuses to the coating. During the diffusion process, Al and Ti react to form three Ti–Al alloy layers (Ti₃Al, TiAl, and TiAl₂). Kattner et al. calculated the Gibbs free energy of the three products as follows:

ΔGf (TiAl₂) = −43858.4 + 11.02077T (K)

(8)

ΔGf (TiAl) = −37445.1 + 16.79376T (K)

(9)

ΔGf (Ti₃Al) = −29633.6 + 6.70801T (K)

(10)

At 1050 °C, the Gibbs free energy of the three phases is less than zero, namely, ΔGf < 0, indicating that the three compounds can be formed thermodynamically. The formation process of the alloy phase layer is

2TiAl₃ + 4Ti → Ti₃Al + TiAl + 2TiAl₂

(11)

The natural transition of the three newly formed phases (Ti₃Al, TiAl, and TiAl₂) and TiAl₃ not only ensures the high-temperature oxidation resistance of the composite coating, but also enhances the interface bonding between the coating and the substrate and slows down the propagation of microcracks through the coating.

The surface L-(Ti,Al) layer reacts with active carbon atoms to form a Ti–Al carburizing layer, and finally forms a Ti–Al alloy phase layer/Ti–Al carburizing layer composite coating.

The residual small amount of O₂ in the corundum crucible will cause the incomplete combustion of C to produce CO, and the energizer BaCO₃ will also produce CO during the decomposition process. Because there is a certain carbon concentration gradient and oxygen concentration gradient between the surface layer and the inner layer of the coating, the temperature is relatively high, and the thermal vibration of the atoms is relatively severe, CO, C, and O₂ will continue to diffuse inward through the L-(Ti,Al) layer, and the following reactions will occur in the L-(Ti,Al) layer and at the interface between it and the TiAl₃ phase:

4Al + 3O₂ → 2Al₂O₃

(12)

Ti + C → TiC

(13)

2TiAl₃ + 3CO → Al₂O₃ + 2TiAl₂ + 3C

(14)

The newly generated C and the C diffused from the carburizing agent will be polarized at the crystal defects with disordered atomic arrangements, such as those at the boundary, the grain boundary, and the dislocation of the L-(Ti,Al) layer and the TiAl₃ phase, thus further distorting the interface, dislocation, and other defects. These distortion regions will become the rapid diffusion channels of CO, C, and O₂. The Al in the L-(Ti,Al) layer is further oxidized into a continuous dense Al₂O₃ layer distributed in the Ti–Al carburizing layer outside the TiAl₃ phase, and TiAl₃ is further consumed to produce Al₂O₃ and TiAl₂. Since the Ti content in the L-(Ti,Al) layer is low and dispersing in Al, TiC generated during carburizing process will disperse in Ti–Al carburizing layer.

In the process of carburizing, the energizer BaCO₃ will react with Al and Ti atoms in the L-(Ti,Al) layer to form BaAl₂O₄ and BaTiO₃ at the top layer of the sample. The reaction may be

2BaCO₃ + Ti + 2Al + O₂ → BaAl₂O₄ + BaTiO₃ + C + CO

(15)

3.2. High-Temperature Oxidation Resistance of the Coating

3.2.1. Microstructure of Composite Coatings after the Thermal Exposure Test

The interfacial reaction of the composite coating in the thermal exposure test is studied by observing the cross-section morphology, EDS pattern, and XRD pattern of the hot-dip-aluminized (760 °C × 10 min) and carburized (1050 °C × 4 h) Ti65 samples after different thermal exposure times. Figure 7 shows the sectional morphology of the composite coating obtained by hot-dip aluminizing (760 °C × 10 min) and carburizing (1050 °C × 4 h) after thermal exposure to static air at 800 °C for 2 days and 16 days. After the thermal exposure test, micro-cracks appear in the coating [30]. As can be seen from Figure 7b, micro-cracks do not penetrate the entire coating, but mainly distribute in the TiAl₃ phase layer with the largest thickness. This is because the natural transition of the four Ti–Al alloy phase layers significantly improves the interface bonding between the coating and the substrate and slows down the further propagation of microcracks through the coating.

By comparing Figure 7d with Figure 2b, it can be found that the thickness of the Ti₃Al, TiAl, and TiAl₂ phases increases after the thermal exposure test, while the thickness of the TiAl₃ phase decreases. The portion of L-(Ti,Al) around the TiAl₃ phase is oxidized into Al₂O₃ and distributed between the TiAl₃ phase and L-(Ti,Al). The Al₂O₃ content of samples under heat exposure for 2 days is small and distributed outside L-(Ti,Al) in a fine network, while the Al₂O₃ content of samples under heat exposure for 16 days is massive and significantly more than that shown by residual heat exposure for 2 days. Due to the increase in Ti content, L-(Ti,Al) near the TiAl₂ phase has enough Ti atomic weight to react with Al in the TiAl₂ phase to form TiAl₂. Therefore, a mixed-phase layer of TiAl₂ and Al₂O₃ appears in the upper part of the TiAl₂ phase. After 16 days of thermal exposure, a relatively continuous and dense TiAl₂ and Al₂O₃ layer was formed above the TiAl₃ phase layer. These changes in thermal exposure tests can effectively prevent oxygen from entering the substrate.

Figure 8 shows the EDS element maps of the composite coating after 16 days of thermal exposure. The results show that the oxygen content from the coating surface to the substrate shows a decreasing trend and that oxygen is mainly distributed in the Ti–Al carbon layer. Therefore, the composite coating can effectively prevent oxygen from entering the substrate and protect the substrate from oxidation for a long time at 800 °C. Figure 9a–c are XRD patterns before thermal exposure testing for 2 days and for 16 days, respectively. Figure 9b adds Ti₂AlC and Al₄C₃ compared with Figure 9a. In Figure 9c, compared with Figure 9b, the contents of TiAl₃, C, and Ti₂AlC decrease, while Al₄C₃, TiC, and BaTiO₃ disappear, and TiO₂ and BaAlTi₅O₁₄ appear.

3.2.2. High-Temperature Oxidation Weight Gain Curve

Figure 10 shows the oxidation kinetics curves of the uncoated Ti65 sample, the hot-dip-aluminized (760 °C × 10 min) Ti65 sample, and the hot-dip-aluminized (760 °C × 10 min) and carburized (1050 °C × 4 h) Ti65 sample (represented by groups A, B, and C below) in the air at 800 °C. The weight of all samples increased with the increase in thermal exposure time, and the weight of group A was significantly higher than that of group B or C.

The oxidation kinetics curve of group A samples can be fitted as follows:

Δm = 14.702t^0.7

(16)

where Δm is the oxidation weight gain (mg/cm²) of group A samples and t is the oxidation time (days).

The oxidation kinetics curve of group B samples can be fitted as follows:

Δm′ = 1.745t^0.4

(17)

where Δm′ is the oxidation weight gain (mg/cm²) of group B samples and t is the oxidation time (days).

The oxidation kinetics curve of group C samples can be fitted as follows:

Δm″ = 1.010t^0.25

(18)

where Δm″ is the oxidation weight gain (mg/cm²) of group C samples and t is the oxidation time (days).

The oxidation of group A samples, namely uncoated Ti65 substrate samples, is the most serious, indicating that the oxidation layer [5] (mainly TiO₂) of uncoated Ti65 substrate samples cannot inhibit the further oxidation of the substrate at 800 °C. After 16 days of thermal exposure, the weight gain of group A was 102.390 mg/cm², that of group B was 5.29 mg/cm², and that of group C was 2.044 mg/cm². The oxidation weight gain of group A is about 50 times that of group C, and the oxidation weight gain of group B is about 2.6 times that of group C, which means that the composite coating significantly improves the oxidation resistance of Ti65 alloy at high temperature.

In the process of thermal exposure testing of group A samples, a large amount of oxide peel was shed [5], while in the whole process of thermal exposure testing of group B samples, no oxide peel was shed and the surface of group B was kept smooth; in the process of thermal exposure testing of group C samples, no obvious oxide peel was shed, but white spot-like substances appeared on the coating surface. One of the reasons for the peeling of oxide skin is that the TiO₂-rich oxide skin formed at 800 °C has a loose structure and weak adhesion with the substrate. Another reason is the mismatch of thermal expansion coefficients between the oxide layer and the substrate.

Ebach-Stahl, A. et al. deposited intermetallic Ti–Al–Cr–Y coatings on the surface of Ti-17 and Ti-6242 titanium alloys by magnetron sputtering technology. The adhesion of the coating to the substrate was good. Compared with the bare substrate material, the mass gain of the coated sample was significantly reduced at 600 °C and 700 °C exposure, indicating that the Ti–Al–Cr–Y layer has a higher oxidation resistance [31]. Zhao, P. et al. prepared an AlNbTaZrx high-entropy alloy coating by laser cladding technology in Ti6Al4V, which improved the wear resistance and high-temperature oxidation resistance [32].

3.2.3. Mechanism of Oxidation Resistance at High Temperature

The process of thermal exposure is also a process of atomic diffusion. At high temperatures, atomic diffusion intensifies, and each element diffuses in the direction of the decreasing concentration gradient. For example, Al, O, and C elements diffuse to the inner layer and Ti elements diffuse to the outer layer. Ti₂AlC and Al₄C₃ appeared in the coating after 2 days of thermal exposure, indicating that the binding energy of Ti₂AlC and Al₄C₃ is large; this diffusion cannot occur during the 4 h carburizing process. It can form under the condition of high temperature and sufficient time only after sufficient energy is obtained. The formation process can be expressed as:

2Ti + Al + C → Ti₂AlC

(19)

4Al + 3C → Al₄C₃

(20)

With the increase in thermal exposure time, oxygen continues to diffuse inward. The oxygen reacts with the residual carbon in the depression of the coating surface to produce CO₂ and then reacts with the osmotic agent products BaAl₂O₄ and BaTiO₃. The Ti atom in the Ti–Al carburizing layer diffuses outwards and is oxidized by oxygen into TiO₂. TiO₂ and the oxidation products of BaAl₂O₄ and BaTiO₃ form the mixture BaAlTi₅O₁₄. The reaction equation of this interface may be

C + O₂ → CO₂

(21)

Ti + O₂ → TiO₂

(22)

BaTiO₃ + BaAl₂O₄ + 2Al + 9TiO₂ + 3O₂ → 2BaAlTi₅O₁₄ + Al₂O₃

(23)

Then, oxygen will enter the Ti-Al carburizing layer, continue to react with the unoxidized Al in the carburizing process, and react with Ti [5] atoms, TiC [33], Ti₂AlC [34], and Al₄C₃ [35]. At the same time, TiC [36] will react with Al and Ti. The reaction equation of this interface may be

Ti + O₂ → TiO₂

(24)

4Al + 3O₂ → 2Al₂O₃

(25)

TiC + 2O₂ → TiO₂ + CO₂

(26)

4Ti₂AlC + 15O₂ → 8TiO₂ + 4CO₂ +2Al₂O₃

(27)

Al₄C₃ + 6O₂ → 2Al₂O₃ + 3CO₂

(28)

TiC + Ti + Al → Ti₂AlC

(29)

The structure of TiO₂ generated by oxidation in the Ti–Al carburizing layer is loose, and the escape of CO₂ will leave a void in the layer. The occurrence of these defects will destroy the continuity of the Al₂O₃ phase layer and promote the further inward diffusion of oxygen. After 16 days of thermal exposure, the contents of C and Ti₂AlC are reduced due to oxidation, TiC is oxidized to TiO₂ and converts to Ti₂AlC, and Al₄C₃ is completely oxidized and disappears.

When oxygen passes through the Ti–Al carburizing layer to the TiAl₃ phase layer, it will react with TiAl₃, and the resulting oxidation products will form a relatively continuous and dense TiAl₂ and Al₂O₃ phase layer distributed on the interface between the TiAl₃ phase layer and Ti–Al carburizing layer [37]. The reaction is as follows:

4TiAl₃ + 3O₂ → 4TiAl₂ + 2Al₂O₃

(30)

Oxygen penetrating into the TiAl₃ phase layer will react preferentially with Al atoms in L-(Ti,Al) around the TiAl₃ phase to generate Al₂O₃ distributed between the TiAl₃ phase and L-(Ti,Al), namely,

4Al + 3O₂→ 2Al₂O₃

(31)

The atomic weight of Ti in L-(Ti,Al) is too small to react, and L-(Ti,Al) near the TiAl₂ phase layer can react with the Al of L-(Ti,Al) to form TiAl₂ due to the complementary source of Ti atoms in the TiAl₂ phase:

Ti + 2Al → TiAl₂

(32)

Therefore, a mixed-phase layer of TiAl₂ and Al₂O₃ appears in the upper part of the TiAl₂ phase.

During the thermal exposure test, the aluminum element in the TiAl₃ phase continues to diffuse to the substrate, and Al and Ti continue to react, resulting in an increase in the thickness of the TiAl₂, TiAl, and Ti₃Al phase layers, while the thickness of the TiAl₃ phase layer decreases due to consumption. The increase or decrease in each alloy phase layer thickness can be obtained by observing and comparing Figure 1b and Figure 6d.

3.3. Hardness and Wear Resistance of the Coating

Figure 11 and Figure 12 show the indentation patterns and surface hardness values of the uncoated Ti65 sample, the hot-dip-aluminized (760 °C × 10 min) Ti65 sample, and the hot-dip-aluminized (760 °C × 10 min) and carburized (1050 °C × 4 h) Ti65 sample, respectively. From Figure 12, we find that the surface hardness of the composite coating is better than that of the titanium alloy substrate, and the problem of the sharp decline of the hardness of the single aluminized coating is avoided. In general, the greater the hardness of the material, the better its wear resistance, and the smaller its coefficient of friction. The reason why the hardness of the Ti65 sample increased significantly after hot-dip aluminizing (760 °C × 10 min) and carburizing (1050 °C × 4 h) is that the carbide particles formed in the Ti-Al carburizing layer played a strengthening role.

4. Conclusions

In this study, a Ti–Al alloy phase layer/Ti–Al carburizing composite coating was formed on the surface of titanium alloy by the stepwise coating method of hot-dip aluminizing and then carburizing. The microstructure, formation mechanism, and growth kinetics of the composite coating were systematically studied. At 800 °C, the oxidation behavior of the uncoated Ti65 sample, the hot-dip-aluminized (760 °C × 10 min) Ti65 sample, and the hot-dip-aluminized (760 °C × 10 min) and carburized (1050 °C × 4 h) Ti65 sample over 2–16 days was observed and analyzed. The conclusion is as follows:

(1)

During hot-dip aluminizing of the Ti65 sample, solid-liquid diffusion of Al and Ti atoms occurs on the surface of titanium alloy to form a TiAl₃ phase +L-(Ti,Al) mixed-phase layer and a L-(Ti,Al) layer. After carburizing, the surface L-(Ti,Al) layer reacts with active carbon atoms to form a Ti–Al carburizing layer. The aluminum element in the TiAl₃ phase layer continues to diffuse to the substrate, and the titanium element in the substrate continues to diffuse to the coating. Al and Ti continue to react to form a series of Ti–Al alloy phase layers (TiAl₂, TiAl, Ti₃Al) from outside to inside. Finally, the Ti–Al alloy phase layer/Ti–Al carburizing layer composite coating is formed.

(2)

After 800 °C × 16 days of thermal exposure testing, the oxidation kinetics curves of the three groups of samples all conform to the parabola law. In addition, the oxidation gain of the uncoated Ti65 is about 50 times that of the hot-dip-aluminized and carburized Ti65, and the oxidation gain of the hot-dip-aluminized Ti65 is about 2.6 times that of the hot-dip-aluminized and carburized Ti65. Therefore, the composite coating has excellent oxidation resistance at high temperatures.

(3)

During the oxidation process, TiAl₃ is consumed by oxidation, but the loss of TiAl₃ has no obvious effect on the oxidation rate. This is because the mixed-phase layer of TiAl₂ and Al₂O₃ formed during the oxidation process is relatively continuous and dense and has good oxidation resistance at high temperatures, which can effectively prevent the further diffusion of oxygen elements to the substrate. This indicates that the Ti65 samples coated with the composite coating have long-term stability in 800 °C air (≥16 days).

(4)

The surface hardness of the composite coating is better than that of the titanium alloy substrate, and the problem of the sharp decline of the hardness of the single aluminized coating is avoided.