In-situ Fabrication of (Ti, Mo)Si

Abstract

(Ti, Mo)Si₂/MoSi₂ composite coatings were prepared on Mo substrate by the continuous deposition pack cementation method. The X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and thermody-namic calculation were used to characterize the composite coatings and to analyze the formation mechanism. The results show that the co-deposition process cannot achieve the titanium deposition effectively. The titanium-modified MoSi₂ coatings can be prepared by a two-step deposition process of titanizing and siliconizing. The coatings contain three layers: the outer layer is (Ti, Mo)Si₂ ternary compound layer; the second layer is MoSi₂ layer; the layer between the MoSi₂ and Mo substrate is the Mo₅Si₃ transition layer. The siliconizing temperature shows negligible effect on coating structure. The growth rate of titanium-modified MoSi₂ coating is slightly lower than that of single MoSi₂ coating. The growth of (Ti, Mo)Si₂/MoSi₂ composite coating is dominated by the inward diffusion of Ti and Si. Ti is concentrated on the outer layer of the coating. Si diffuses through the (Ti, Mo)Si₂ compound layer and interacts with the substrate to form the MoSi₂ layer and Mo₅Si₃ transition layer. In the titanizing process, the free state AlF₃ is introduced by the reaction among pack mixtures. In the subsequent siliconizing process, a trace amount of Al in free state is precipitated in the form of Al₃Mo phases in the (Ti, Mo)Si₂ layer near the upper interface of MoSi₂ layer. During the cyclic oxidation tests at 1200 °C, the (Ti, Mo)Si₂/MoSi₂ composite coatings do not lose mass obviously after exposure in oxidation atmosphere for 180 h. A dense composite oxide layer consisting of SiO₂ and TiO₂ can be formed by the oxidation of (Ti, Mo)Si₂ phase. This composite oxide layer can fill the surface cracks of the coating and continuously block the oxygen diffusion, so the oxidation resistance of (Ti, Mo)Si₂/MoSi₂ composite coating in the periodic oxidation environment is far superior to that of the single MoSi₂ coating.

Keywords

pack cementation; molybdenum disilicide; titanium modification; anti-oxidation

Science Press

Molybdenum has been widely used in electronic devices, heater elements, glass fiber processing, thermometry protective tubes, and aero-space crafts due to its excellent high-temperature properties. However, the poor antioxidation ability restricts the further application of molybdenum under high temperature atmosphere^[1-3]. The silicide coating prepared on Mo substrate has been extensively researched, because MoSi₂ can form the dense SiO₂ layer on the coating surface, which can hinder the oxygen diffusion into the matrix and fill the cracks, holes, and other defects caused by thermal expansion during the high temperature oxidation^[4-6]. However, the strength and creep resistance of the MoSi₂ coating at high temperature is poor, and it is easy to cause the spalling of SiO₂ under thermal cycle process, which reduces the anti-oxidation life of the coating^[7,8]. The oxidation resistance of MoSi₂ can be effectively improved and the pesting oxidation can be inhibited by adding other elements into MoSi₂, especially the ones with greater affinity to oxygen than to Si. The addition of alloying elements can form the protective oxide film during the oxidation process^[9,10]. Yanagihara et al^[8] showed that adding Ti in MoSi₂ can produce selective oxidation at MoSi₂ grain boundary, thus inhibiting the pesting effect. Boettinger et al^[11] reported that titanizing of MoSi₂ can form the (Ti, Mo)Si₂ phase with C40 structure, and the duplex phase structure composed of C11b MoSi₂ and C40 (Ti, Mo)Si₂ performs good thermal stability and ductility at high tempera-tures^[12]. Li et al^[13] prepared (Ti, Mo)Si₂/MoSi₂ composite coat-ing by slurry sintering method. The (Ti, Mo)Si₂ is dispersed in MoSi₂, and this coating can be oxidized at 1500 °C for 40 h without mass loss. Thus, the synthesis of (Ti, Mo)Si₂/MoSi₂ coating on the Mo surface can improve the oxidation resistance.

The methods used to prepare element-modified MoSi₂ coating include pack cementation, chemical vapor deposition, thermal spraying, slurry, etc^[14-17]. Compared with other methods, the pack cementation method has been widely used for its advantages of simple process, low cost, no restriction of specimen shape, and high bonding strength between the coating and the matrix^[18]. However, the preparation of titanium-modified MoSi₂ coating by pack cementation has rarely been reported. Therefore, the titanium-modified MoSi₂ coatings by pack cementation method were prepared in this research. The influence of different pack cementation methods on coating structure was investigated and the effective synthetic route was developed. The influence of siliconizing temperature and siliconizing time after titanizing of the (Ti, Mo)Si₂/MoSi₂ composite coatings was investigated, and the formation mechanism of (Ti, Mo)Si₂/MoSi₂ composite coatings formed by deposition of Ti and Si was studied. Finally, the cyclical oxidation resistance of the composite coatings at 1200 °C was also evaluated.

1 Experiment

The molybdenum rods (hot working state, purity of 99.5%) were used as the substrates for coating experiment. Cylindrical specimens with the dimension of Φ12 mm×6 mm were obtained by electric-discharge machining. After polishing by 800# SiC paper, the specimens were cleaned ultrasonically in ethanol and then dried. Two kinds of pack cementation methods were employed to prepare (Ti, Mo)Si₂/MoSi₂ composite coatings. (1) Co-deposition method: prepare the coatings at 1000 °C for 5 h in Ar atmosphere with the pack mixture containing different contents of Ti and Si. The final product prepared by co-deposition method is named as xTi-ySi-10NH₄F-50Al₂O₃. (2) Two-step deposition method (titanizing- siliconizing): firstly prepare the Ti-modified layer at 1000 °C for 5 h in Ar atmosphere with the pack mixture and obtain the intermediate product of 40Ti-10NH₄F-50Al₂O₃ (wt%); then process the intermediate product at 900~1100 °C for 1~9 h. The specimens with single MoSi₂ layer were also prepared at 1000 °C for 1~9 h as the control group. The effect of depo-sition kinetic parameters of coating preparation on oxidation resistance of coatings was investigated, as shown in Table 1.

Table 1 Process parameters of coating preparation

Method		Temperature/°C	Time/h	Composition of pack mixture/wt%
				Ti	Si	NH4F	Al2O3
Co-deposition cementation		1000	5	20	20	10	50
				15	25
				10	30
Two-step pack cementation	Titanizing	1000	5	40	-	10	50
	Siliconizing	900	5	-	40
		1000	1, 3, 5, 7, 9	-	40
		1100	5	-	40
Single-layer cementation		1000	1, 3, 5, 7, 9	-	40	10	50

The pack mixtures consisted of NH₄F (activator), Al₂O₃ (filler), and Si/Ti (Si source/Ti source), which were mixed in a ball mill for 2 h. The specimens were embedded in cylindrical alumina crucibles sealed with alumina lids using silica sol binder. The sealed crucibles were then inserted into a horizontal tube furnace with a flowing Ar atmosphere (purity of 99.999%, 30 mL/min). After the Ar atmosphere was established, the furnace was heated up to the setting temperature for coating preparation.

Periodic oxidation tests were conducted in a horizontal tube furnace from room temperature to 1200 °C. The (Ti, Mo)Si₂/MoSi₂ composite coatings prepared by the two-step pack cementation method and the single MoSi₂ coating were separately placed in the alumina crucibles of high-purity and then the alumina crucibles were put into the horizontal tubular furnace. The furnace was heated from room temperature to 1200 °C at a heating rate of 20 °C/min in the atmosphere for 10 h, and then the crucibles were taken out for air-cooling to room temperature, which was recorded as an oxidation cycle. After each oxidation cycle, the mass change of specimens was measured by an electronic balance (balance precision of 0.1 mg). The mass changes of the specimens with increasing the oxidation time were also recorded.

Scanning electron microscopy (SEM, TESCAN VEGA3) was used to observe the surface and cross section morphologies of the coatings. The chemical composition of the coatings was analyzed by energy dispersive spectroscopy (EDS, TESCAN VEGA3). X-ray diffraction (XRD, X'pertpro MPD) was used to analyze the phases of coatings and pack mixtures after pack cementation.

2 Results and Discussion

2.1 Phase composition and microstructure of coatings

Fig.1 shows the coating phase component, the backscattered electron (BSE) image of the cross section of coatings, and EDS element line scanning result of the coating prepared by co-deposition method at 1000 °C for 5 h. The diffraction peaks in Fig.1a correspond to the (Ti, Mo)Si₂ and MoSi₂ phases, which indicates that the addition of Ti modifies the phases of coatings. The coatings display a two-phase structure with C40 (Ti, Mo)Si₂ and C11b MoSi₂^[19]. Meanwhile, with decreasing the Ti content in pack mixtures, the diffraction peaks of (Ti, Mo)Si₂ phase are weakened, suggesting that the content of (Ti, Mo)Si₂ phase is also decreased. Fig.1b shows the BSE image and elemental line distribution of the cross section of 20Ti-20Si-10NH₄F-50Al₂O₃ coating. The discon-tinuous (Ti, Mo)Si₂ layer with a thickness of 3~5 μm in the outer layer can be observed, and the second layer is the single MoSi₂ layer. According to the element line scanning result of the cross section of the coating, three elements Si, Mo, and Ti are all distributed in a diffusion gradient. The growth of (Ti, Mo)Si₂/MoSi₂ composite coating is dominated by the inward diffusion of Ti and Si. Ti is only concentrated in the outer layer of coating; Si is distributed in the whole coating and becomes less near the substrate; Mo exists steadily in the coating and is concentrated in the substrate. The above results show that when Si:Ti=20:20 in the pack mixtures, the Ti-modified (Ti, Mo)Si₂ coating can be formed on the surface of MoSi₂ coating, but the (Ti, Mo)Si₂ layer is not continuous. As the Ti content in pack mixtures is further reduced, the intensity of diffraction peaks of (Ti, Mo)Si₂ phase is significantly weakened. So the co-deposition method cannot achieve the Ti deposition effectively.

Fig.2 presents the coating phase component, BSE image of the cross section of coating, and EDS element line scanning result of the intermediate product coating after titanizing at 1000 °C for 5 h. Fig.2a shows the intermediate coating consists of AlMoTi₂ and AlMo₃Ti_x phases. Fig.2b shows the BSE image and elemental line scanning result of the intermediate coating after titanizing. It can be seen that the Ti-modified layer is uneven. The outer layer is AlMoTi₂ phase and the transition layer between the AlMoTi₂ phase and the substrate is AlMo₃Ti_x phase. The EDS element line scanning result in the cross section presents that the distribution of Al and Ti in the coating is relatively stable. The content of Al and Ti gradually reduces while the Mo content increases near the substrate. The element distribution shows that the formation of alloy phase in the coating is caused by the diffusion of Al and Ti elements. Because the metal bond between the metal elements has no saturation state, a gradient distribution of elements in the coating appears.

Fig.3 presents the coating phase component, BSE image of the cross section of coating, and EDS element line scanning result of coating prepared by two-step pack cementation, i.e., titanizing and siliconizing. Fig.3a shows that the (Ti, Mo)Si₂ and MoSi₂ phases exist in the coating after siliconizing. In addition, the weak diffraction peaks of TiSi and Al₃Mo phases can also be observed. The existence of Al is due to the introduction of trace Al in the titanizing process, and consequently Al is dispersed in the (Ti, Mo)Si₂ layer. BSE image and EDS element line scanning result of the coating prepared by two-step pack cementation are shown in Fig.3b. The coating structure and element line scanning result are similar to those of the coating prepared by co-deposition method. The outer layer consists of continuous (Ti, Mo)Si₂ phase with the thickness of 7~10 μm, and the second layer is the single MoSi₂ layer. The above results show that the Ti-modified MoSi₂ coating can be effectively prepared by the two-step pack cementation method.

2.2 Effect of siliconizing parameters on (Ti, Mo)Si₂/MoSi₂ composite coating prepared by two-step deposition method

After titanizing at 1000 °C, the specimens were treated through siliconizing process at 900~1100 °C. BSE images of cross-section of coating and element line scanning results are shown in Fig.4. It can be seen that with increasing the siliconizing temperature, the coating structure does not change significantly. The coatings contain three layers: the outer layer is (Ti, Mo)Si₂ ternary compound; the second layer is MoSi₂; the layer between the MoSi₂ and Mo substrate is the Mo₅Si₃ transition layer. Furthermore, with increasing the siliconizing temperature, the thickness of (Ti, Mo)Si₂ ternary compound layer does not increase obviously. When the siliconizing temperature reaches 1100 °C, the thickness of MoSi₂ and Mo₅Si₃ transition layer increases significantly. Moreover, a certain amount of Al₃Mo precipitate can be observed in the (Ti, Mo)Si₂ layer near the interface of MoSi₂ layer. The siliconizing temperature exerts negligible effect on the coating structure. The growth rate of titanium-modified MoSi₂ coating is slightly lower than that of single MoSi₂ coating.

EDS analysis was conducted to determine the (Ti_x, Mo_1-x)Si₂ layer after siliconizing at 900~1100 °C, and the results are shown in Table 2. The (Ti_x, Mo_1-x)Si₂ ternary compound layer can be identified as (Ti_0.71, Mo_0.29)Si₂, (Ti_0.64, Mo_0.36)Si₂, and (Ti_0.28, Mo_0.72)Si₂ in the coatings after siliconizing at 900, 1000, and 1100 °C, respectively. The Ti content in (Ti_x, Mo_1-x)Si₂ phase ranges from 26at% to 75at%, which fits the (Ti_x, Mo_1-x)Si₂ phase with C40 structure^[13]. Moreover, with increasing the siliconizing temperature, the proportion of Ti in (Ti, Mo)Si₂ is decreased and the proportion of Mo is increased, suggesting that Ti is gradually homogenized in the ternary alloy layer.

Table 2 Results of EDS analysis for (Ti_x, Mo_1-x)Si₂ ternary compound layer

Siliconizing temperature/°C	Composition/at%			Phase component
	Si	Mo	Ti
900	64.73	12.17	23.10	(Ti0.71, Mo0.29)Si2
1000	64.88	14.22	20.9	(Ti0.64, Mo0.36)Si2
1100	70.41	19.57	10.02	(Ti0.28, Mo0.72)Si2

The effect of siliconizing time on the growth of modified silicide coating is shown in Fig.5. It can be seen that the thickness of single MoSi₂ coating and (Ti, Mo)Si₂/MoSi₂ composite coating shows a nearly linear relationship with the square root of siliconizing time, which indicates that the growth of coatings conforms to the parabolic law h=(K_pt)^1/2 (h is the total thickness of the coating, t is the siliconizing time, K_p is the parabolic rate constant). According to the fitting equation in Fig.5, the parabolic rate constant K_p of single MoSi₂ coating and (Ti, Mo)Si₂/MoSi₂ composite coating is 3.52×10² and 4.20×10² μm²/h, respectively, suggesting that the growth rate of Ti-modified silicide coating is slightly lower than that of single siliconized coating. The above results show that the outer (Ti, Mo)Si₂ layer has the diffusion blocking effect which can slow down the inner diffusion of Si.

2.3 Mechanism analysis of (Ti, Mo)Si₂/MoSi₂ composite coating formed by deposition of Ti and Si

Fig.6 shows XRD patterns of coatings with different Si/Ti contents after co-deposition. The diffraction peaks of Si and Al₂O₃ can be observed. Meanwhile, the diffraction peaks of TiSi₂ also exist, which indicates that TiSi₂ is formed by the reaction of Si, Ti, and activator during co-deposition reaction. In addition, when the Si content in the pack mixtures is further increased, the intensity of diffraction peaks of Si is increased gradually, which means that part of unreacted Si remains in the pack mixtures. These results show that TiSi₂ is generated by the reaction of Si, Ti, and activator, leading to the insufficient Si and Ti sources (SiF_x and TiF_x) deposited on the surface of Mo substrate and the low deposition efficiency of coating prepared by the co-deposition method.

The chemical reactions in the co-deposition process are similar to those in the single siliconizing process, including the formation of volatile halide vapors of the master alloy elements, the deposition of gaseous halides in substrate surface, and the reactions of deposition elements with substrate^[17,20,21]. In addition, the master elements possibly interact with each other. The main reactions occurring in the co-deposition process can be summarized as follows.

(1) The decomposition of halide activator:

NH4F (s)→NH3 (g)+HF (g)

(1)

(2) The formation of volatile halide vapors of the master alloy elements:

Si (s)+xHF (g)→SiFx (g)+$$\frac{x}{\mathrm{2}}$$H2 (g)

(2)

Ti (s)+xHF (g)→TiFx (g)+$$\frac{x}{\mathrm{2}}$$H2 (g)

(3)

where x ranges from 1 to 4.

(3) The deposition of gaseous halides in substrate surface:

(x+1)SiFx (g)→Si (s)+xSiFx+1 (g)

(4)

(x+1)TiFx (g)→Ti (s)+xTiFx+1 (g)

(5)

TiFx (g)+2SiFx (g)→TiSi2 (s)+$$\frac{\mathrm{3}x}{\mathrm{2}}$$F2 (g)

(6)

2Si (s)+Ti (s)→TiSi2 (s)

(7)

Since SiF₄ and TiF₄ are relatively stable, x ranges from 1 to 3.

(4) The reactions of deposition elements with substrate:

Si (s)+Ti (s)+Mo (s)→(Ti, Mo)Si2

(8)

Si (s)+Mo (s)→MoSi2

(9)

Fig.7a shows the Gibbs free energy ∆_rG-temperature T curves of the formation of gaseous halides of master alloy elements. It can be seen that the Gibbs free energies of the chemical reaction for generating SiF and TiF is all positive in the temperature range from 1100 K to 1500 K, indicating that the formation of SiF and TiF during the formation of gaseous halides is not supported in thermodynamics. In addition, the Gibbs free energy of formation of TiF_x (x=2~4) is lower than that of the formation of SiF_x (x=2~4) with the same x value, which indicates that the Ti-halides are more stable than Si-halides. In the reaction process, Ti is easier to react with activator than Si for forming gaseous halides.

The gaseous halides are diffused from the pack powder to the surface of Mo substrate and deposited at the interface between the gas phase and the matrix. Yang^[19] and Majumdar^[22] et al considered that the migration of difluoride controls the diffusion and deposition of gaseous halides to the substrate surface. The thermodynamic analysis results of possible reactions about the difluoride deposition and the interaction of Si and Ti are shown in Fig.7b. It presents that the Gibbs free energies of the deposition reactions of SiF₂ and TiF₂ are all negative, i.e., the deposition of SiF₂ and TiF₂ is spontaneously. Meanwhile, the Gibbs free energy of the interaction between the gas phase SiF₂ and TiF₂ is positive, which indicates the reaction to form solid phase TiSi₂ by the gas phases is not supported in thermodynamics. The TiSi₂ can be formed by interaction between Si and Ti, because the Gibbs free energy of the interaction is negative at 1100~1500 K. However, due to the filler Al₂O₃, the reaction between silicon powder and titanium powder is inhibited. TiSi₂ is generated mainly through the gaseous halides containing Si and Ti deposited on the surface of Ti or Si powder, which explains the existence of a strong TiSi₂ diffraction peak and the fact that no diffraction peak of co-existence of silicon and titanium appears in the pack mixtures after packing process.

The XRD pattern of pack cementation components after titanizing process is shown in Fig.8a. The filler Al₂O₃ and titanium-rich oxides can be observed. Fig.8b shows the Gibbs free energy ∆_rG-temperature T curves of possible reactions of TiF₂, Ti, and filler Al₂O₃. It can be seen that the reaction of TiF₂ and Al₂O₃ for formation of titanium-rich oxide (TiO) and free state AlF₃ is a spontaneous process. The inflection point appears at 1200~1300 K due to the transformation of TiO from β phase to α phase, according to the Ti-O phase diagram^[23]. The Ellingham diagram shows that Al₂O₃ is more stable than TiO₂, and the reactions are spontaneous because F element shows more electronegativity than O element does, which makes F atom occupy the position of O atom and form AlF₃. The related chemical reaction is also verified in thermodynamics, as shown by the red line in Fig.8b. In the low oxygen partial pressure environment, Ti combines with free O to form various titanium-rich oxides, as shown in Fig.8a. In addition, the liquid phase AlF₃ contacts with Mo matrix in the form of free Al, which makes the free state Al and titanium-fluoride deposited on the Mo surface and diffused into the matrix to form a ternary alloy layer containing Al, Mo, and Ti.

In the process of siliconizing after titanizing, Si reacts with Ti-modified alloy layer to form (Ti, Mo)Si₂ layer, and then the Si element is diffused through the compound layer and interacts with the substrate forming the MoSi₂ layer. With the increase of siliconizing temperature, Si element is diffused along the substrate direction continuously and forms the Mo₅Si₃ transition layer between MoSi₂ and the substrate. The thickness of Mo₅Si₃ transition layer is increased gradually with increasing the siliconizing temperature. Ti element is only concentrated on the outer layer of coating due to the solid solution formed by Ti and Mo. When the titanium-fluoride is not deposited on the coating surface, the driving force of Ti diffusion to substrate is insufficient, and the asymmetric diffusion of Ti in the Mo substrate occurs^[24,25], resulting in the fact that Ti element cannot be further diffused to the substrate. In the siliconizing process, a trace amount of Al introduced during the titanizing process can be diffused along the substrate direction. Since the diffusion rate of Al is faster than that of Si^[10], the trace amount of Al₃Mo phase is precipitated in the (Ti, Mo)Si₂ layer near the interface of MoSi₂ layer.

2.4 Effects of Ti on oxidation resistance of coating

Cyclic oxidation tests of (Ti, Mo)Si₂/MoSi₂ composite coatings prepared by two-step method were conducted at 1200 °C. The oxidation performance of single MoSi₂ coating was also tested for comparison. The relationship between the mass gain per unit area of specimens and oxidation time is shown in Fig.9. It can be seen that the oxidation process includes three stages of coating mass: linear increase, parabolic change, and increase with a decreased rise rate. At the initial stage of coating oxidation (<10 h), there is no dense oxide film on coating surface, and oxygen can react with the

Fig.9 Mass change of single MoSi₂ coating and (Ti, Mo)Si₂/MoSi₂ composite coating after cyclic oxidation at 1200 °C in air

coating to form a dense oxide protective film. So the oxidation kinetics of the initial stage is a linear process^[26,27]. As the oxide film covers the whole coating, a dense oxide layer is formed on the surface of the coating. After oxidation for 20 h, the oxidation rate in this stage is controlled by the internal diffusion of oxygen, so the relationship between the mass gain per unit area of the coating and oxidation time shows a parabolic change trend. For the single MoSi₂ coating, the rapid mass loss can be observed after exposure for 120 h, and the coating completely fails at 140 h. However, the (Ti, Mo)Si₂/MoSi₂ composite coating does not lose mass obviously after exposure for 180 h, which indicates that Ti element can effectively improve the oxidation resistance and prolong the oxidation resistance life of MoSi₂ coating.

Fig.10a and 10b show the BSE images of surface and cross-section of MoSi₂ coating after periodic oxidation at 1200 °C for 120 h. According to Fig10a, the MoO₃ phase exists in sheet form on the surface cracks of the coating. In Fig.10b, the

coating consists of a complete MoSi₂ layer and a Mo₅Si₃ diffusion layer. A large number of Mo₅Si₃ phases formed by the decomposition of MoSi₂ into SiO₂ and Mo₅Si₃ during the oxidation process are distributed in the outer layer of the coating. During the periodic oxidation of the MoSi₂ coating at 1200 °C, the cracks in the coating are formed by thermal stress, which become the diffusion channels and make oxygen further react with the substrate. Although MoSi₂ layer is not completely transformed into Mo₅Si₃ layer, the hot cracks formed in the periodic oxidation process cannot be completely filled by SiO₂. When the cracks continue to expand due to thermal stress, the volatilization of MoO₃ is intensified, and then the protective coating fails. Fig.10c and 10d show the BSE images of surface and cross-section morphologies of (Ti, Mo)Si₂/MoSi₂ compound coating after periodic oxidation at 1200 °C for 120 h. Fig.10c shows that there is no MoO₃ phase volatilizing along the cracks on the coating surface, because the (Ti, Mo)Si₂ layer improves the high temperature stability of the coating and slows down the thermal stress impact rate to the coating in the periodic oxidation process. Fig.10d presents a dense composite oxide layer consisting of SiO₂ and TiO₂ which is formed by the oxidation of (Ti, Mo)Si₂. This composite oxide layer can fill the surface cracks of the coating, thus inhibiting the pesting effect and significantly improving the oxidation resistance of the coating^[28]. Moreover, during the oxidation process, the composite oxide layer can continuously block the oxygen diffusion, prevent the coating from being the oxygen diffusion channel due to hot cracks, and avoid the direct contact reaction of oxygen with the substrate. Therefore, the Ti-modified silicide coating prepared by the two-step deposition method shows significantly better oxidation resistance in the periodic oxidation environment than the single silicide coating does.

3 Conclusions

1) The co-deposition process cannot achieve the titanium deposition effectively. The titanium-modified MoSi₂ coatings contain three layers: the outer layer is (Ti, Mo)Si₂ ternary compound; the second layer is MoSi₂; the layer between the MoSi₂ and the Mo substrate is the Mo₅Si₃ transition layer.

2) The siliconizing temperature exerts negligible effect on coating structure. The growth rate of titanium-modified MoSi₂ coating is slightly lower than that of the single MoSi₂ coating.

3) The growth of (Ti, Mo)Si₂/MoSi₂ composite coating is dominated by the inward diffusion of Ti and Si. Ti is only concentrated on the outer layer of coating. Si is diffused through the (Ti, Mo)Si₂ compound layer and interacts with the substrate to generate the MoSi₂ layer and Mo₅Si₃ transition layer. In the titanizing process, the free state AlF₃ can be introduced by the reaction among pack mixtures. In the subsequent siliconizing process, a trace amount of Al is precipitated in the form of Al₃Mo phases in the (Ti, Mo)Si₂ layer near the interface of MoSi₂ layer.

4) During the cyclic oxidation tests at 1200 °C, the

(Ti, Mo)Si₂/MoSi₂ composite coatings do not lose mass obvi-ously after exposure for 180 h. A dense composite oxide layer consisting of SiO₂ and TiO₂ can be formed by the oxidation of (Ti, Mo)Si₂. This composite oxide layer can fill the surface cracks of the coating and continuously block the oxygen diffusion, so the oxidation resistance of (Ti, Mo)Si₂/MoSi₂ composite coating in the periodic oxidation environment is far superior to that of the single silicide coating.