Microstructure and Oxidation Behavior of Anti-Oxidation Coatings on Mo-Based Alloys through HAPC Process: A Review

: Mo and Mo-based alloys are important aerospace materials with excellent high temperature mechanical properties. However, their oxidation resistance is very poor at high temperature, and the formation of volatile MoO 3 will lead to catastrophic oxidation failure of molybdenum alloy components. Extensive research on the poor oxidation problem has indicated that the halide activated pack cementation (HAPC) technology is an ideal method to solve the problem. In this work, the microstructure, oxide growth mechanism, oxidation characteristics, and oxidation mechanism of the HAPC coatings were summarized and analyzed. In addition, the merits and demerits of HPAC techniques are critically examined and the future scope of research in the domain is outlined.


Introduction
Mo and its alloys have high melting point, excellent high-temperature mechanical properties, low thermal expansion coefficient and high conductivity and thermal conductivity, which have been widely used in high-temperature structural components in national defense industry, aerospace, and other fields, such as nozzle throat, high temperature electrode, high-temperature heating element, ray shielding material, etc. [1][2][3][4]. However, the oxidation resistance of Mo and Mo-based alloys is very poor, and they are easily oxidized to MoO 3 at a temperature of (400-800 • C) [5,6]. With the formation of MoO 3 , the volume of molybdenum alloy increases rapidly, and leads to the occurrence of low temperature pulverization phenomenon, namely "Pesting oxidation". In addition, the formation of a large amount of volatile MoO 3 will lead to the catastrophic decomposition of molybdenum and its alloys when the oxidation temperature is greater than 1000 • C [7][8][9][10]. At present, alloying and surface coating can be used to improve the oxidation resistance of Mo and its alloys. Alloying is regarded as the preferred method to improve the properties of pure Mo, and Mo-based alloys have better mechanical properties than pure Mo when used at a high temperature above 1000 • C [11][12][13][14]. The classification, preparation method, properties, and application fields of molybdenum-based alloys are shown in Table 1 [15,16]. Because of limitations of alloying capability of Mo, its high temperature oxidation resistance cannot be fundamentally improved by alloying. Therefore, surface coating technology is regarded as an ideal method to improve the high-temperature oxidation resistance of molybdenum and its alloys [17][18][19]. Among them, the HAPC technology is the most widely used. At present, there are many reports about the oxidation behavior of HAPC coatings on Mo and its alloys. However, almost no reviewing of the progress in development of oxidation resistance of Mo has been documented [20]. Electrician  [ 18] In this work, the advantages and disadvantages of the HAPC coatings are summarized and analyzed. The composition, exposure time, exposure temperature and mass change per unit area of the coatings have been given in relevant tables [21,22]. Their oxide growth mechanism and oxidation behavior are emphatically analyzed and summarized. Finally, the oxidation resistance and failure mechanism of the coatings are also summarized, aiming to provide some useful references for researchers in this field [23].

Microstructure and Growth Mechanism of HAPC Coatings
Halide activated pack cementation (HAPC) method is to embed the substrate into a mixture (Si powder, B powder, Al powder, NH 4 Cl/F, Y, NaF, Al 2 O 3 powder, etc.) with a certain particle size and proportion, and carry out thermal diffusion in vacuum or argon atmosphere to prepare diffusion coating. Figure 1 shows a schematic diagram of the HAPC reaction model. It can be seen that the Al 2 O 3 crucible is usually used as the reaction device, and the plate on the top of the crucible plays the role of isolating air during the reaction process. The device is placed in a furnace and held at a set temperature for a certain period of time to obtain a coating on the substrate surface [24][25][26][27][28][29].  [ 18] In this work, the advantages and disadvantages of the HAPC coatings are summarized and analyzed. The composition, exposure time, exposure temperature and mass change per unit area of the coatings have been given in relevant tables [21,22]. Their oxide growth mechanism and oxidation behavior are emphatically analyzed and summarized. Finally, the oxidation resistance and failure mechanism of the coatings are also summarized, aiming to provide some useful references for researchers in this field [23].

Microstructure and Growth Mechanism of HAPC Coatings
Halide activated pack cementation (HAPC) method is to embed the substrate into a mixture (Si powder, B powder, Al powder, NH4Cl/F, Y, NaF, Al2O3 powder, etc.) with a certain particle size and proportion, and carry out thermal diffusion in vacuum or argon atmosphere to prepare diffusion coating. Figure 1 shows a schematic diagram of the HAPC reaction model. It can be seen that the Al2O3 crucible is usually used as the reaction device, and the plate on the top of the crucible plays the role of isolating air during the reaction process. The device is placed in a furnace and held at a set temperature for a certain period of time to obtain a coating on the substrate surface [24][25][26][27][28][29].  Table 2 provides a summary of the process parameters, composition, and properties of HAPC coating on molybdenum-based alloys [29][30][31][32][33][34][35][36][37][38][39][40][41][42]. It can be seen that the composi-  Table 2 provides a summary of the process parameters, composition, and properties of HAPC coating on molybdenum-based alloys [29][30][31][32][33][34][35][36][37][38][39][40][41][42]. It can be seen that the composition of the mixture and the process conditions have an important influence on the microstructure, phase composition, grain size, and mechanical properties of the coating. Wang et al. [32] successfully prepared MoSi 2 -MoB coatings with an average hardness of 5.84 GPa on Mo surface, and the hardness of MoB layer is as high as 9.54 GPa. By contrast, the surface hardness of pure MoSi 2 coating is only 2.58 GPa [34]. This is attributed to the addition of appropriate amount of B element in the mixed packing, which improves the fluidity of Si and makes the coating structure more dense and uniform [43]. The growth mechanism of HAPC coatings is shown in Figure 2. A MoSi 2 layer forms on molybdenum-based alloy, due to the interdiffusion reaction between Si powder and the substrate at high temperature. In the preparation process of coating, adding an appropriate amount of beneficial components (Al, B, YSZ, ZrO 2 , Al 2 O 3 , SiC, MoB, etc.) can significantly improve the oxidation resistance and mechanical properties of the silicide coatings [44][45][46][47][48][49]. The main reaction equations involved in the above process are shown in Figure 2g. - Coatings 2021, 11, x FOR PEER REVIEW 3 of 11 tion of the mixture and the process conditions have an important influence on the microstructure, phase composition, grain size, and mechanical properties of the coating. Wang et al. [32] successfully prepared MoSi2-MoB coatings with an average hardness of 5.84 GPa on Mo surface, and the hardness of MoB layer is as high as 9.54 GPa. By contrast, the surface hardness of pure MoSi2 coating is only 2.58 GPa [34]. This is attributed to the addition of appropriate amount of B element in the mixed packing, which improves the fluidity of Si and makes the coating structure more dense and uniform [43]. The growth mechanism of HAPC coatings is shown in Figure 2. A MoSi2 layer forms on molybdenumbased alloy, due to the interdiffusion reaction between Si powder and the substrate at high temperature. In the preparation process of coating, adding an appropriate amount of beneficial components (Al, B, YSZ, ZrO2, Al2O3, SiC, MoB, etc.) can significantly improve the oxidation resistance and mechanical properties of the silicide coatings [44][45][46][47][48][49]. The main reaction equations involved in the above process are shown in Figure 2g.  The typical surface and cross-sectional morphology of the HAPC coatings are shown in Figure 3 [30]. It can be seen that some large particles are accumulated at the folds on the coating surface, which is mainly due to the large particle size of the mixture and uneven mixing, as shown in Figure 3a,c. The formation of micro-cracks is attributed to the thermal expansion mismatch between the coating and the substrate, as shown in Figure 3b,c. In addition, the addition of Al 2 O 3 , SiO 2 , and SiC has great influence on the thickness of coating and interface layer. It is shown in Figure 3e that the SiO 2 -MoSi 2 coating has a thick interface layer with a thickness of 8 to 10 µm. On the contrary, the thickness of interface layer of Al 2 O 3 -MoSi 2 coating is very thin and is only 1 to 2 µm. It is worth noting that the SiC-MoSi 2 coating has the thickest coating thickness, and the interface layer is not observed, as shown in Figure 3f. It can be seen that the addition of SiC significantly improves the deposition efficiency of the coating, which is mainly due to the physical deposition of SiC being faster than thermal diffusion deposition. observed, as shown in Figure 3f. It can be seen that the addition of SiC significantly improves the deposition efficiency of the coating, which is mainly due to the physical deposition of SiC being faster than thermal diffusion deposition.

Oxidation Behavior and Failure Mechanism of HAPC Coatings
The oxidation process parameters and oxidation properties of HAPC coatings on molybdenum and its alloy are shown in Table 3. It can be seen that the ingredients of the oxide layer mainly depend on the phase composition of the original coating. The oxide layer of pure MoSi2 coatings is mainly composed of SiO2 and Mo5Si3, however, composite coatings are mainly composed of SiO2, Al2O3, and B2O3. The thickness of the outer coating decreases gradually with the increase of oxidation time. On the contrary, the thickness of the oxide layer and interface coating increases gradually, which is mainly due to the diffusion of oxygen and the internal diffusion reaction of silicon [29][30][31][32][33][34][35][36][37][38][39][40][41][42]. Figure 4 shows the typical surface and cross-section BSE images of oxidized coatings on TZM alloy. A large number of micro-cracks and MoO3 particles are observed on the surface of the oxidized MoSi2 coating, which is mainly due to the volume expansion of MoO3, as shown in Figure  4a. However, the surface structure of Mo(Si,Al)2 coating is complete without obvious cracks and pores after oxidation. This is due to the formation of Al2O3 with small thermal expansion coefficient during oxidation, which inhibits the pulverization of MoSi2, as shown in Figure 5b [39]. The surface of the oxidized Mo(Si,Al)2-MoB coating is very smooth and the oxide layer is clearly visible. This is attributed to the fact that SiO2-Al2O3 generated in the oxidation process inhibits the volatilization of MoO3, and MoB particles are dispersed and distributed inside the MoSi2 coating, which makes the coating have a denser microstructure and better oxidation resistance, as shown in Figure 4c [38].

Oxidation Behavior and Failure Mechanism of HAPC Coatings
The oxidation process parameters and oxidation properties of HAPC coatings on molybdenum and its alloy are shown in Table 3. It can be seen that the ingredients of the oxide layer mainly depend on the phase composition of the original coating. The oxide layer of pure MoSi 2 coatings is mainly composed of SiO 2 and Mo 5 Si 3 , however, composite coatings are mainly composed of SiO 2 , Al 2 O 3 , and B 2 O 3 . The thickness of the outer coating decreases gradually with the increase of oxidation time. On the contrary, the thickness of the oxide layer and interface coating increases gradually, which is mainly due to the diffusion of oxygen and the internal diffusion reaction of silicon [29][30][31][32][33][34][35][36][37][38][39][40][41][42]. Figure 4 shows the typical surface and cross-section BSE images of oxidized coatings on TZM alloy. A large number of micro-cracks and MoO 3 particles are observed on the surface of the oxidized MoSi 2 coating, which is mainly due to the volume expansion of MoO 3 , as shown in Figure 4a. However, the surface structure of Mo(Si,Al) 2 coating is complete without obvious cracks and pores after oxidation. This is due to the formation of Al 2 O 3 with small thermal expansion coefficient during oxidation, which inhibits the pulverization of MoSi 2 , as shown in Figure 5b [39]. The surface of the oxidized Mo(Si,Al) 2 -MoB coating is very smooth and the oxide layer is clearly visible. This is attributed to the fact that SiO 2 -Al 2 O 3 generated in the oxidation process inhibits the volatilization of MoO 3 , and MoB particles are dispersed and distributed inside the MoSi 2 coating, which makes the coating have a denser microstructure and better oxidation resistance, as shown in Figure 4c [38]. 3.82 [42] In addition, Sun et al. compared the oxidation kinetics curves of the pure Mo and the MoSi 2 coatings prepared with Al 2 O 3 , SiO 2 , and SiC filler in the pack, respectively, at different temperatures, as shown in Figure 5 [30]. It can be seen that after oxidizing at 500 • C for 110h, the quality of the deposited coating only changes from −0.18%-0.09%. Among them, the quality of the deposited SiC coating has little change before and after oxidation. However, the mass variation of Mo substrate under this condition is as high as 3%, as shown in Figure 5a,b. Furthermore, in the oxidation experiments at 1200 • C, the Mo substrate rapidly failed in the initial stage of oxidation, and the quality of the deposited Al 2 O 3 , SiO 2 coatings are Increases of varying degrees, respectively. However, the mass of deposited SiC coating increases first and then decreases under the oxidation conditions. This is caused by the formation and evaporation of oxidized carbon (CO, CO 2 ) during oxidation, as shown in Figure 5c.   In addition, Sun et al. compared the oxidation kinetics curves of the pure Mo and the MoSi2 coatings prepared with Al2O3, SiO2, and SiC filler in the pack, respectively, at different temperatures, as shown in Figure 5 [30]. It can be seen that after oxidizing at 500 °C for 110h, the quality of the deposited coating only changes from −0.18%-0.09%. Among them, the quality of the deposited SiC coating has little change before and after oxidation. However, the mass variation of Mo substrate under this condition is as high as 3%, as shown in Figure 5a,b. Furthermore, in the oxidation experiments at 1200 °C, the Mo substrate rapidly failed in the initial stage of oxidation, and the quality of the deposited Al2O3, SiO2 coatings are Increases of varying degrees, respectively. However, the mass of deposited SiC coating increases first and then decreases under the oxidation conditions. This is caused by the formation and evaporation of oxidized carbon (CO, CO2) during oxidation, as shown in Figure 5c. The microstructure evolution and oxidation mechanism of the coating are shown in Figure 6. Generally, the oxidation of the coating can be divided into transient oxidation and steady oxidation [50]. At the initial stage of oxidation, Mo, Si, B, Al, and other elements are oxidized at the same time. The quality loss of the coating is faster with the formation and volatilization of MoO3, and the SiO2 layer is discontinuous at this stage. At the same time, a large number of pores were observed on the coating surface after the evaporation of MoO3. With the progress of oxidation reaction, continuous and dense oxide films (Al2O3, SiO2, B2O3-SiO2, etc.) gradually form on the coating surface. At last, a large number of pores are closed by SiO2 with a low oxidation rate, and the oxidation process changes to a steady state [51,52].  The microstructure evolution and oxidation mechanism of the coating are shown in Figure 6. Generally, the oxidation of the coating can be divided into transient oxidation and steady oxidation [50]. At the initial stage of oxidation, Mo, Si, B, Al, and other elements are oxidized at the same time. The quality loss of the coating is faster with the formation and volatilization of MoO 3 , and the SiO 2 layer is discontinuous at this stage. At the same time, a large number of pores were observed on the coating surface after the evaporation of MoO 3 . With the progress of oxidation reaction, continuous and dense oxide films (Al 2 O 3 , SiO 2 , B 2 O 3 -SiO 2 , etc.) gradually form on the coating surface. At last, a large number of pores are closed by SiO 2 with a low oxidation rate, and the oxidation process changes to a steady state [51,52]. ments are oxidized at the same time. The quality loss of the coating is faster with the formation and volatilization of MoO3, and the SiO2 layer is discontinuous at this stage. At the same time, a large number of pores were observed on the coating surface after the evaporation of MoO3. With the progress of oxidation reaction, continuous and dense oxide films (Al2O3, SiO2, B2O3-SiO2, etc.) gradually form on the coating surface. At last, a large number of pores are closed by SiO2 with a low oxidation rate, and the oxidation process changes to a steady state [51,52].

Conclusions and Prospects
In this work, the growth mechanism, oxidation behavior, and mechanism of HAPC coatings are analyzed and discussed. The process conditions and properties of the coatings are provided. During the process, due to the relatively low process temperature of HAPC methods, the deposition efficiency of the coating is low, and the preparation time is long. However, the application of the processes is not limited by the shape of the substrate, and the coatings prepared have a uniform composition and good adhesion to the substrate. Figure 7 provides a summary of the composition and oxidation characteristics of protective coatings prepared by many researchers on the surface of molybdenum and its alloys by HAPC method.
Coatings 2021, 11, x FOR PEER REVIEW 8 of 11 In this work, the growth mechanism, oxidation behavior, and mechanism of HAPC coatings are analyzed and discussed. The process conditions and properties of the coatings are provided. During the process, due to the relatively low process temperature of HAPC methods, the deposition efficiency of the coating is low, and the preparation time is long. However, the application of the processes is not limited by the shape of the substrate, and the coatings prepared have a uniform composition and good adhesion to the substrate. Figure 7 provides a summary of the composition and oxidation characteristics of protective coatings prepared by many researchers on the surface of molybdenum and its alloys by HAPC method. In addition, the addition of beneficial elements and the second phases can not only significantly improve the mechanical properties and high temperature oxidation resistance of the coating, but also can delay the diffusion of silicon to the substrate, inhibit the formation of Mo5Si3 with poor oxidation resistance, reduce the formation of volatile MoO3, and promote the rapid formation of continuous and dense anti-oxidation film on the coating surface. The addition of Al element can significantly inhibit the formation of the pulverization MoO3 and the occurrence of pulverization, which is mainly due to the formation of Al2O3 with low thermal expansion coefficient. The addition of B element can improve the oxidation resistance of the coating mainly in the following two aspects. On the one hand, in the process of high temperature oxidation (above 1400 °C), an appropri-   In addition, the addition of beneficial elements and the second phases can not only significantly improve the mechanical properties and high temperature oxidation resistance of the coating, but also can delay the diffusion of silicon to the substrate, inhibit the formation of Mo 5 Si 3 with poor oxidation resistance, reduce the formation of volatile MoO 3 , and promote the rapid formation of continuous and dense anti-oxidation film on the Coatings 2021, 11, 883 9 of 11 coating surface. The addition of Al element can significantly inhibit the formation of the pulverization MoO 3 and the occurrence of pulverization, which is mainly due to the formation of Al 2 O 3 with low thermal expansion coefficient. The addition of B element can improve the oxidation resistance of the coating mainly in the following two aspects. On the one hand, in the process of high temperature oxidation (above 1400 • C), an appropriate amount of B dissolved into the oxide layer can further improve the fluidity of SiO 2 , which promotes the healing of pores and cracks. On the other hand, MoB reacts with Si to produce Mo 5 SiB 2 with low diffusion coefficient, which delays the diffusion of Si to the substrate and improves the oxidation life of the coating. In addition, Cr and W elements can also improve the oxidation resistance of the coating, which is mainly due to the formation of Cr 2 O 3 with high oxidation resistance, and the formation of (Mo,W) Si 2 phase inhibits the internal diffusion of silicon into the substrate.
In order to improve the mechanical properties and antioxidant properties of the coating, the microstructure and phase composition of the coating must be optimized, and the addition of the second reinforcing phase also plays a positive role. Moreover, the multi-step coating preparation process can organically combine the single coating preparation technology with each other, overcome the limitations of its single application, and effectively extend the oxidation service life of the coating. This will be the direction of future research and development in this field.