Experimental investigation and first-principles calculations of Nb and W alloying effects on the microstructure and properties of MoSi2 coatings fabricated via arc cladding

MoSi2 is one of the most promising refractory metal silicide materials, but its further use as a structural material is limited by its drawbacks such as poor room-temperature toughness and low high-temperature strength. The work performed a comprehensive investigation combining first-principles calculations and arc cladding experiments to explore the effects of Nb and W doping on the mechanical properties and electronic structure of MoSi2 coatings. The first-principles calculations revealed that Nb addition improved the B/G value and Poisson’s ratio of MoSi2, indicating enhanced ductility. W addition yields the opposite effect and led to a higher elastic modulus and improved hardness. Experimental results demonstrated that the arc-cladding MoSi2 coating mainly consisted of MoSi2 and Mo5Si3 phases with a dendritic microstructure. Upon doping with Nb and W, additional t-(Mo,Nb)Si2 and t-(Mo,W)Si2 phases were formed, which resulted in a denser and finer microstructure. Nb addition contributed to the solid-solution toughening of the coating, while W addition enhanced hardness but reduced toughness. Remarkably, the synergistic alloying of Nb and W significantly increased the hardness and fracture toughness of the coating by 30.7 and 70.7%, respectively, compared to pure MoSi2. The strengthening mechanism of the coating was attributed to solid-solution softening and fine-grain strengthening, while the crack extension mechanism involved the crack deflection and bridging. Furthermore, the coatings doped with 2% Nb and 4% W exhibited the lowest wear weight loss and superior wear resistance. The dominant wear mechanisms were oxidation wear and abrasive wear.


Introduction
The demand for materials with enhanced abrasion resistance, corrosion resistance, and high-temperature strength has escalated significantly with the advancement of modern science and technology, particularly in aerospace and advanced weaponry.Traditional high-temperature structural materials [1] are inadequate in meeting the requirements of modern applications.Consequently, surface protective coating technology has emerged for a balance between the mechanical and chemical properties of materials.Researchers have investigated silicide coatings since the 1950s [2].Notably, modified coatings predominantly composed of MoSi 2 have been utilized to protect superalloys, including Ni-based and Co-based alloys.For instance, Simanjuntak et al [3] demonstrated good oxidation resistance at 1100 °C for Al/MoSi 2 composite coatings prepared using the high-energy milling method on a 316 stainless steel substrate.Similarly, Mao et al [4] exhibited the successful development of high-density ZrB 2 /MoSi 2 reinforced composite coatings on the surface of the graphite substrate through the low-pressure plasma-spraying method.the structural relaxation calculations until Hellman-Feynman force on each atom converged to within 0.02 eV Å.
The test specimens were prepared using industrial pure Mo plates as the base material.The plates were cut into 20 × 20 × 5 mm specimens using a wire-cutting machine.The substrate surface was then ground with 400grit sandpaper to remove the oxidation layer.Then ultrasonic cleaning was performed to ensure a clean surface.A planetary ball mill was employed to prepare the MoSi 2 composite powders.Pure MoSi 2 powders and MoSi 2 composite powders with different compositions were used, including 2 wt% Nb, 4 wt% W, 10 wt% W, 2 wt% Nb + 4 wt% W, and 2 wt% Nb + 10 wt% W [26]. Powders were ball milled with ZrO 2 ceramic beads in a ball-topowder ratio of 1 : 1.1 at a ball milling speed of 450 rpm.The total ball milling time was 10 h, which resulted in an average particle size of composite powders ranging from 5 to 10 microns.Ball-milled powders were then applied onto the substrate surface [27] with the thickness of 1-2 mm.The powder was compacted and bonded using alcohol, and the material was subsequently dried in an oven at 120 °C for 20 min.
The MoSi 2 coating was fabricated on a Mo substrate using Tungsten Inert Gas (TIG) arc cladding.The cladding process was performed at a cladding current of 185 A, a cladding speed of 1.5 mm s −1 , and an argon flow rate of 10 l min −1 .
Figure 1 shows the arc cladding operation.The tungsten electrode on the welding torch radiates a highenergy arc.Alloy powders and the substrate surface melt rapidly to form a molten pool at the high temperature of the arc.Alloying elements in the molten pool undergo sufficient diffusion and condensation under the heat and force sources.Finally, metallurgical bonding is realized.
The microstructure of the specimen cross-section was examined using an optical metallographic microscope.The hardness of the specimen cross-section was measured using a Vickers hardness tester.The Vickers hardness test was conducted with a load of 1,000 g applied for 15 seconds.Ten regular indentations were made on the fused layer of each specimen, and the crack half-lengths were measured.The average value of the crack half-lengths was used to calculate Vickers hardness.The fracture toughness of the coating was determined at room temperature using the indentation method, and the K IC value was calculated based on the formula.A reciprocating friction and wear tester was utilized to evaluate the performance of the melted layer surface.Si 3 N 4 ceramic balls were used as the friction pair.The load applied during the test was 15 N, with a frequency of 5 Hz  and friction durations of 8, 16, 24, and 32 min.The morphological characteristics and elemental distribution of the coating were observed using a Sigma500 field emission scanning electron microscope.Additionally, the coatings were qualitatively analyzed using a D8 Advance x-ray diffractometer.

Results and analysis
3.1.Density functional theory (DFT) calculation of the effect of Nb and W doping on the mechanical properties of MoSi2 Table 1 shows the elastic constants of C11 b -type MoSi 2 doped with Nb and W. The non-zero independent elastic constants, determined by crystal symmetry, are specific to the crystal system.There exist six independent matrix elements (C 11 , C 12 , C 13 , C 33 , C 44 , and C 66 ) that represent the material's resistance to stress along different axes of its crystal structure in the tetragonal system.The elastic constants satisfy the Born stability criterion [28] for tetragonal crystalline systems, which confirms the stability of MoSi 2 and the doped systems.Note that C 33 for both doped and undoped systems is higher than C 11 , indicating greater resistance to the deformation along the caxis compared to the a-axis.This observation highlights the strong bonding of the Mo-Si bonds in the c-axis direction [29].Calculated data demonstrate that the elastic constants of the non-zero matrix elements (except for C 11 ) increase with Nb addition and decrease with W addition.This further supports the notion that the incorporation of Nb and W enhances the plasticity and hardness of the material.Table 2 shows the mechanical properties of C11 b -type MoSi 2 doped with Nb and W. B H , G H , E, H V , and ν stand for bulk modulus, shear modulus, Young's modulus, Vickers hardness, Poisson's ratio, and the ratio of Young's modulus to the shear modulus, respectively.The shear modulus and Young's modulus decrease with Nb doping and increase with W doping.On the other hand, the bulk modulus increases for both doping cases.Nb addition decreases hardness, while the inclusion of W increases hardness.Poisson's ratio and B/G values [30] allow the ductility of MoSi 2 to be assessed.Lower B/G values and Poisson's ratios indicate higher stress concentration at the root of cracks produced by stress on the material, which makes plastic flow relatively difficult and limits the ability of the material to deform plastically.
The Poisson's ratio of MoSi 2 , as well as Nb-and W-doped MoSi 2 , is below 0.26 and the B/G values are below 1.75, indicating the inherent brittleness of these compounds.Note that the Poisson's ratio and B/G of MoSi 2 increase with added Nb and decrease with added W, indicating that MoSi 2 doped with Nb exhibits enhanced ductility.Waghmare et al [31] investigated the effect of substitution alloying on the ductility of MoSi 2 using firstprinciples total energy calculations.The substitution of Mo with Nb is particularly beneficial in improving ductility.Peng et al [32] analyzed the valence electron structure of MoSi 2 -based solid solution alloys using the average atom model based on the empirical electron theory (EET) of solids and molecules.The valence electron structure of MoSi 2 -based solid solution alloys is analyzed using the average atom model.W addition can increase hardness and decrease the fracture toughness of the solid solution.These conclusions are in agreement with our calculations.
Figure 2 shows the differential charge densities of MoSi 2 , (Mo 0.9167 Nb 0.0833 )Si 2 , and (c) (Mo 0.9167 W 0.0833 )Si 2 .The plot illustrates the distinct atomic orbital profiles surrounding Mo and Si, which arise from the hybridization effect between the Mo-4d and Si-3p orbitals [33].This hybridization leads to the dense distribution of electron orbitals and the formation of strong covalent bonds.The red regions in the figure represent electron localization and electron accumulation.The doping of Nb and W changes the charge density of MoSi 2 .
Figure 1 reveals that Nb addition increases the charge accumulation between Mo and Si, whereas W exhibits minimal impact on charge accumulation.These findings align with the calculated elastic modulus values presented in table 2. MoSi 2 with Nb addition demonstrates a lower elastic modulus, while the opposite trend is observed for W addition.
Figure 3 shows the state density of MoSi 2 , (Mo 0.9167 Nb 0.0833 )Si 2, and (Mo 0.9167 W 0.0833 )Si 2 .The bonding peaks of MoSi 2 and Nb shift towards the Fermi energy level.The DOS at the Fermi energy level is higher compared to other alloys, indicating a stronger metallic character.MoSi 2 and MoSi 2 with W addition exhibit higher stability in comparison to MoSi 2 with Nb addition.Figure 3(b) reveals that the state density in the crystal is primarily contributed by Nb 4d, Mo 4d, and Si 3p orbitals.Furthermore, some Nb 4d orbital antibonding states appear below the Fermi energy level, which reduces the energy gap between its bonding and antibonding states [34].The presence of Nb can lower energy required for dislocation slip and enhance the ductility of the material.These results align with calculated B/G and Poisson's ratio presented in table 2.

Effect of Nb and W elements on the morphology and microstructure of MoSi 2 coatings
Figure 4 shows the microstructure of the MoSi 2 coating with different ratios.The coating layer predominantly exhibits dendrites, which primarily grow in the direction opposite to the flow of the molten heat flux.The formation of columnar dendrites is primarily attributed to a significant component subcooling zone ahead of the solid-liquid interface during the alloy's solidification process.Initial cellular growth becomes skewed towards a specific preferred orientation within the subcooling zone, which causes secondary branches on the cell flanges.These secondary branches further develop into columnar dendrites over time.[35].
Figure 5(b) shows that the primary diffraction peaks of MoSi 2 in all specimens, except for pure MoSi 2 , exhibit a shift towards lower diffraction angles to varying degrees.This phenomenon can be attributed to the larger atomic radii of Nb and W compared to Mo.When Nb and W substitute Mo' position, the lattice constant of MoSi 2 increases, which results in lattice distortion and the generation of tensile stress [36].Consequently, the  main diffraction peaks of MoSi 2 exhibit a shift towards smaller angles.Tensile stress significantly affects the mechanical properties of coatings [37] as well as the crystal structure and interactions of the material.Thus, the stability of the material is affected.
Figure 6 shows the average grain size of MoSi 2 coatings with different ratios.The average grain size of the pure MoSi 2 coating was determined to be approximately 38.2 μm using the x-ray diffraction (XRD) full spectrum fitting Rietveld method and MDI Jade software.The introduced small amounts of Nb and W exhibit a modest grain refining effect.However, the grain size of the coating continues to decrease with the synergistic addition of Nb and W. The addition of 10% W significantly reduces the average grain size of MoSi 2 in the coatings to 28.2 μm.The coating with 2% Nb and 10% W exhibits the smallest grain size (approximately 14.1 μm).It represents a reduction of 63.0% compared to the grain size of the pure MoSi 2 coating.
Figure 7 shows the EDS element distribution of the pure MoSi 2 coating.The high-temperature arc heat source rapidly heats metal powders above its melting point during the process of arc melting and cladding, which results in a liquid state.Therefore, Mo and Si in the molten pool uniformly diffuse throughout the entire coating under heat flow.This phenomenon enhances the metallurgical bonding between the coating and the substrate.
Figure 8 shows the EDS element distribution of MoSi 2 coating with 2% Nb and 4% W. Table 3 shows the distribution of EDS elements in different areas.Mo exhibits uniform distribution throughout the coating, while Si, Nb, and W appear to be segregated.This segregation phenomenon can be attributed to the rapid cooling rate during the arc cladding process, which results in a large temperature gradient.This non-uniform diffusion of solid components in the liquid state, coupled with non-equilibrium crystallization below the solid-phase line, forms microscopic segregation.Table 3 shows that Si, W, and Nb are interwoven with the Mo-rich phase in the coating, which impedes its growth.Based on the EDS point scan data, the ratio of Mo to Si atoms in the light gray area of the coating (figure 8(a)) is approximately 5:3.The predominant organization in this region is identified as Mo 5 Si 3 .This phenomenon arises from the selective oxidation of silicon elements in molybdenum disilicide,  accompanied by the incorporation of a minor amount of oxygen elements from the arc welding process.It results in the formation of SiO 2 .Consequently, the Mo 5 Si 3 phase emerges within the MoSi 2 matrix, which is enriched with molybdenum and comparatively deficient in silicon.The combined sum of Mo and Nb atoms, along with the atomic ratio of Si, approaches 1 : 2 in the dark gray area of the figure.Similarly, the atomic ratio of the sum of Mo and W to Si is also close to 1 : 2. Further analysis indicates that the prevailing organization in this region comprises (Mo, Nb)Si 2 and (Mo, W)Si 2 .A small quantity of Nb and W is attributed to their solid solution within the lattice of MoSi 2 .They replace some Mo atoms to form a solid solution.The above processes can be described through the following chemical reaction equations.

Effect of Nb and W on the microhardness of MoSi 2 coatings
Figure 9 shows the microhardness curves of MoSi 2 coatings with different ratios.The hardness curves exhibit three distinct regions: the coating, the transition layer, and the substrate.In, it is evident that the hardness values within the coating regions remain stable and significantly higher than those of the substrate (figure 9(a)).However, a sharp decrease in microhardness is observed during approaching the transition layer.This drop can  be attributed to the substantial dilution of alloying elements by the substrate.Figure 9(b) presents the average hardness of MoSi 2 coatings with different ratios.Nb addition decreases coating hardness, with an average hardness reduction of 2.2% compared to pure MoSi 2 .This decrease can be attributed to the softening effect from the incorporation of small amounts of Nb as a solid solution [38].W addition significantly increases in average coating hardness.In particular, the inclusion of 10% W increase average hardness by 20.1% compared to pure MoSi 2 .This enhancement is primarily attributed to the substitution of Mo by W, which significantly strengthens W-Si bonding energy in comparison to Mo-Si bonding [39].Consequently, the macroscopic performance of the coatings is improved.The synergistic alloying of Nb and W substantially increases average hardness.The coating exhibits the average hardness of 1,183.3HV, indicating a 30.7% increase compared to pure MoSi 2 .The significant hardness improvement is attributed to the combined effect of fine-grain strengthening from Nb and W as well as robust chemical bonding contributed by W.

Effect of Nb and W on the wear resistance of MoSi 2 coatings
Figure 10 shows the microscopic abrasion marks of the MoSi 2 coating with different ratios; tables 4 and 5 list the EDS point scan data of the abrasion marks on the surface of the pure MoSi 2 and MoSi 2 coatings with 2% Nb and 4% W, respectively.The pure MoSi 2 coating exhibits numerous pits and abrasive adherence on the surface (figure 10(a)), indicating a brittle fracture mechanism of the coating.Additionally, the formation of an oxide film on the MoSi 2 surface, influenced by the high temperature and oxygen, contributes to the adhesion of oxides to the coating surface.As a result, the wear mechanism of the pure MoSi 2 coating is predominantly abrasive wear and oxidation adhesion wear.Figure 10(b) illustrates the MoSi 2 coating with 2% Nb addition, demonstrating a  significant reduction in surface defects.We can only observe a few cracks, oxidized adhesions, and abrasive chip.This improvement is attributed to the self-lubricating effect of plastic-phase Nb, which alters the friction coefficient between MoSi 2 and abrasive grains.The MoSi 2 coating with 4% W addition exhibits more severe brittle peeling on the surface (figure 10(c)).Peeled-out abrasive chips adhere to the coating and form turtle crack plates.Tortoise crack plates are fragmented structures designed to mimic the intricate texture of a tortoise shell.Figure 10(d) shows that the surface quality of the coating further deteriorates with the addition of 10% W, which is characterized by large craters, numerous abrasive chips adhering to the coating surface, and dense cracks.Brittle-phase W increases coating hardness, which intensifies cracking and peeling under the applied load.
Consequently, the dominant wear mechanisms are peeling wear and adhesive wear.Figure 10(e) presents the MoSi 2 coating with 2% Nb and 4% W, with alleviated surface defects and a notable increase in the white oxide layer.It is consistent with the findings in table 5.This phenomenon can be attributed to the toughening and reinforcing effects of the solid solution formed by Nb and W as well as the lubricating effect of the generated oxide layer.These factors contribute to the reduced wear of the coating, with minor abrasive wear and oxidation wear as the prevailing wear mechanisms.Figure 10(f) exhibits the MoSi 2 coating with 2% Nb and 10% W, with a significant number of oxide layers adhering to the surface.The high content of brittle-phase W results in severe brittle peeling of the coating.Tables 4 and 5 show that the coatings with 2% Nb and 4% W exhibit higher oxygen content compared to the pure MoSi 2 coating.Small amounts of Nb and W can accelerate the formation of oxides on the surface of the MoSi 2 coating.The generation of the white oxide layer on the coating surface is primarily attributed to amorphous SiO 2 .The following is the equation of the chemical reaction that occurs during oxidation.

MoSi
Calculating the wear weight loss rate of a material is a common method for evaluating wear.The mass reduction of various specimens is individually measured under the same load.The formula for determining the wear-weight loss rate is as follows: ẃhere W Q is the mass of the specimen before wear, g; W H is the mass of the specimen after wear, g.  Figure 11 shows the wear-weight loss rate of the MoSi 2 coating with different ratios at different friction times.The wear-weight loss curves in the figure demonstrate that the MoSi 2 coating with 10% W addition exhibits the most pronounced slope, indicating a higher wear rate of approximately 1.0% after a 32-minute abrasive wear test.The lowest wear-weight loss rate observed is approximately 0.1% after a 16-minute frictional wear test.

Effects of Nb and W on the fracture toughness of MoSi 2 coatings
Figure 12 shows a standard indentation measured by a Vickers hardness tester.c is the crack half-length, mm; l is the indentation diagonal half-length, mm.
Since MoSi 2 exhibits both metallic and ceramic properties, the determination of its room-temperature fracture toughness is typically performed using the direct indentation method due to its simplicity and convenience.The semilunar cracking system equation proposed by Anstis [40] is commonly utilized for fracture toughness evaluation among the various calculation equations available.The calculation equation is expressed by where K IC is the fracture toughness value; δ is dimensionless-constant empirical-correction (0.016); E is the elasticity modulus, GPa; H is the Vickers hardness of the material, GPa; P is the indentation load, N; C is the average half length of the indentation crack, mm.
Table 6 presents the fracture toughness values of MoSi 2 coatings with different ratios.The addition of 2% Nb significantly increases the fracture toughness by 34.4% compared to pure MoSi 2 .On the other hand, the coating with 10% W exhibits the lowest fracture toughness at 2.1 MPa•m 1/2 .However, the synergistic effect of 2% Nb and 4% W leads to the highest fracture performance (6.4 MPa•m 1/2 ), which represents a substantial increase of 70.7% compared to pure MoSi 2 .The incorporation of Nb and the solid solution effect facilitated by an appropriate amount of W reduce coating hardness (figure 9(b)) and enhance its plasticity.Therefore, the toughness value is optimized.However, the addition of a high mass fraction of W results decreases coating toughness.
Figure 13(a) depicts the indentation morphology of pure MoSi 2 , which reveals the main crack propagating outward along the indentation angle with minimal deflection and bending.The fracture mechanism predominantly involves transcrystalline fractures.While microcracks appear around the non-indentation angle, which consumes a portion of primary stress energy.Indentation morphology with the addition of 2% Nb demonstrates crack deflection around the indentation angle, which weakens crack propagation (figure 13(b)).Figures 13(c) and (d) display indentation morphology with the inclusion of 4% W and 10% W, respectively.The indentation cracks exhibit similarities to those of pure MoSi 2 .The addition of brittle-phase W renders the crystal structure more stable and resistant to destruction, which results in longer crack propagation paths and increases crack formation.The indentation patterns of 10% W even exhibit deformations in some indentations under high stress due to excessive brittle phases.Based on table 6, the indentation profiles of Nb and W composite coatings in figures 13(e) and (f) exhibit the lowest fracture-toughness values due to crack deflection and bridging during crack expansion.Note that the coating with the addition of 2% Nb and 4% W exhibits the most pronounced limiting effect on crack instability, which aligns with the calculated fracture-toughness values.
The room-temperature fracture toughness of MoSi 2 is significantly improved by the co-alloying of Nb and W compared to pure MoSi 2 .This enhancement can be attributed to the following main reasons.(1) Synergistic strengthening mechanism: The combination of solid solution softening and fine grain strengthening contributes to improved fracture toughness.MoSi 2 exhibits a large c/a ratio, which results in significant anisotropy in grain stress after the solid solution.The lower solvation energy of MoSi 2 [41] after the solid solution leads to radial deflection and separation disconnection of advancing cracks under dual actions.Furthermore, the appropriate addition of Nb and W reduces grain size and improves fracture behavior and uniform grain deformation, which reduces stress concentration and crack formation.Previous studies on (Mo,W)Si 2 -SiC composites by Wu et al [42] have shown that W doping reduces the grain size of (Mo,W)Si 2 .Similarly, Harada et al [26] investigated MoSi 2 materials with different alloying elements and found that Nb has a solid solution softening effect.(2) Toughening mechanism: The crack deflection and crack bridging formation contribute to improved toughness.The mismatch in the elastic modulus and thermal expansion coefficient among Nb, W, and MoSi 2 results in radial tensile stress and tangential compressive stress within MoSi 2 .The combination of residual internal stress and external stress modifies the fracture surface energy of the material.Macroscopically, it leads to crack bending along the propagation path, which consumes crack extension energy and improves fracture performance.Cracks induce a bridging effect on the surface of the reinforced phase, which impedes the extension of the cracks.Energy generated by stress concentration at the main crack can be dissipated through the mechanisms of crack deflection and crack bridging [43].It reduces stress concentration at the crack tip and  decelerates the crack-propagation rate.Consequently, the fracture toughness of the coating is improved.Zhu et al [44] studied MoSi-MoB-ZrO 2 composite coatings, indicating that fracture toughness is influenced by changes in the crack extension paths and the formation of microcracks.The addition of the reinforcing phase leads to more complex crack extension paths, which enhances fracture toughness.The incorporation of the reinforcing phase results in a more intricate crack propagation path and allows the crack to absorb more energy during extension.Therefore, fracture toughness is improved.These findings align with the conclusions we have drawn.
Figure 14 illustrates the crack extension mechanism in Nb-and W-doped MoSi 2 coatings.Microcracks initiate at the tip of the main crack, and the solid solution of (Mo,Nb)Si 2 and (Mo,W)Si 2 particles hinder the microcrack growth under the surrounding stress field.As a result, the microcrack grows perpendicular to the main stress axis.It leads to sawtooth-shaped crack morphology that enhances surface energy and blunts the crack tip.Consequently, stress concentration at the crack tip is relieved to ultimately overcome crack extension [45].When the crack reaches a grain boundary, it becomes blocked.Some cracks are forced to pass through the grain, which bridges cracks.The resulting closed stress field impedes further crack propagation for toughening.

Conclusion
(1) First-principles calculations demonstrated the structural stability of Nb-and W-doped MoSi 2 .Nb addition enhanced the ductility of MoSi 2 , while W addition primarily improved its elastic modulus, which resulted in higher hardness.
(2) Arc cladding-prepared MoSi 2 coatings consisted of dendrites, with MoSi 2 and Mo 5 Si 3 phases as the primary constituents.Nb and W addition led to grain refinement, and the solid solution substitution of Nb and W doping produced C11 b -type (Mo,Nb)Si 2 and (Mo,W)Si 2 phases.
(3) The hardness of the MoSi 2 coating increased significantly with the synergistic alloying of Nb and W. Specifically, the hardness of the MoSi 2 coating with 2% Nb and 4% W addition increased by 21.7% compared to pure MoSi 2 .Moreover, the MoSi 2 coating with 2% Nb and 4% W had the lowest wear-weight loss and the smoothest weight loss.The wear mechanism of this coating involved oxidation wear and abrasive wear.
(4) The fracture toughness of the MoSi 2 coating with 2% Nb and 4% W reached 6.4 MPa m 1/2 , which was 70.7% higher than that of the pure MoSi 2 coating.The strengthening mechanisms mainly involved solid solution softening and fine grain strengthening.The toughening mechanisms mainly included crack deflection and crack bridging.
(5) Considering the combined effects of hardness, wear resistance, and fracture toughness, the MoSi 2 coating with 2% Nb and 4% W exhibited optimal mechanical properties.

Figure 5 (
a) shows the x-ray diffraction patterns of MoSi 2 coatings with different ratios, and figure 5(b) shows the enlarged view of zone A in figure 5(a).The diffraction lines exhibit strong intensity in the MoSi 2 spectra with added alloying elements and sharp and symmetrical diffraction peaks.The system has undergone the full reaction, and the synthesized products are well crystallized through arc cladding.The pure MoSi 2 coating primarily consists of the MoSi 2 phase with a small amount of the Mo 5 Si 3 phase.Following the synergistic alloying of Nb and W, Nb and W occupy the Mo lattice positions through solid solution substitution, which forms C11 b -type (Mo,Nb)Si 2 and (Mo,W)Si 2 solid solutions.No other Nb-Si and W-Si phases are detected in the XRD diffraction pattern, indicating that Nb and W are within the solid solution range of MoSi 2

Figure 5 .
Figure 5. (a) X-ray diffraction patterns of MoSi 2 coatings with different ratios; (b) Enlarged view of zone A.

Figure 6 .
Figure 6.Average grain size of MoSi 2 coatings with different ratios.

Figure 9 .
Figure 9. Microhardness of MoSi 2 coatings with different ratios: (a) Microhardness curves of MoSi 2 coating systems with different ratios; (b) Average microhardness of MoSi 2 coatings with different ratios.

Figure 11 .
Figure 11.Friction loss and weight loss rates of MoSi 2 coatings at different ratios and friction times.

Figure 14 .
Figure 14.Crack extension mechanism of MoSi 2 coatings with Nb and W.

Table 1 .
Elastic constants (GPa) of C11 b MoSi 2 doped with Nb and W.

Table 2 .
Mechanical properties of C11 b MoSi 2 doped with Nb and W.

Table 3 .
Distribution of EDS elements in different areas of MoSi 2 coatings with 2% Nb and 4% W (at%).

Table 4 .
EDS point scan data (at.%) on the abrasive surface of the pure MoSi 2 coating.

Table 5 .
EDS point scan data (at.%) on the abrasive surface of the MoSi 2 coating with 2% Nb and 4%W.

Table 6 .
Fracture toughness values of MoSi 2 coatings with different ratios.