<bold>SPS</bold>烧结制备<bold>MoSi<sub>2</sub>@Nb</bold>核壳结构材料及其表征

罗佳军; 全慈旺; 张建军; 陈水香; 张茜彤; 韩孟瑶; 梁炳亮; 陈卫华; Luo Jiajun; Quan Ciwang; Zhang Jianjun; Chen Shuixiang; Zhang Xitong; Han Mengyao; Liang Bingliang; Chen Weihua

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Preparation and Characterization of Core-Shell MoSi₂@Nb Materials Sintered by SPS PDF

- ORCID：
Luo Jiajun ¹
✉
- ORCID：
Quan Ciwang ¹
- ORCID：
Zhang Jianjun ^1,2
✉
- ORCID：
Chen Shuixiang ¹
- ORCID：
Zhang Xitong ¹
- ORCID：
Han Mengyao ¹
- ORCID：
Liang Bingliang ¹
- ORCID：
Chen Weihua ¹

1. School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang330063, China； 2. Jiangxi Provincial Engineering Research Center for Surface Technology of Aeronautical Materials, Nanchang Hangkong University, Nanchang330063, China

Updated：2024-11-18

Abstract

Core-shell MoSi₂@Nb powder was prepared by electrostatic layer self-assembly method. The surfactants SDS (CHSO₄Na) and CTAB (C₁₉H₄₂BrN) were used to modify the surface of the two particles to make them charged, and the Zeta potential of the suspension was tested by the Zeta potentiometer. Scanning electron microscope, transmission electron microscope and energy dispersive spectrometer were used to characterize the phase, morphology, microstructure and element distribution of synthetic materials. The results show that when the SDS concentration is 2 mmol/L, the CTAB concentration is 3mmol/L and the pH value of Nb suspension is 5, the coating effect is better after secondary cladding. NbSi₂ phase is found at the interface between Nb and MoSi₂ after calcination at 200 °C for 2 h in Ar atmosphere, indicating that Nb is highly active and reacts with Si. Core-shell structure is still retained in MoSi₂@Nb material after spark plasma sintering at 1450 °C for 2 h under uniaxial pressure of 40 MPa. However, it is found that Nb reacts strongly with MoSi₂, and most of the Nb phase is reacted. This issue needs to be addressed in subsequent studies. The fracture toughness (K_IC) of MoSi₂@Nb material is significantly improved to 5.75 MPa·m^0.5 compared with that of MoSi₂ material (3.32 MPa·m^0.5).

Keywords

core-shell structure; electrostatic layer self-assembly; spark plasma sintering; fracture toughness

Molybdenum disilicide, as a promising next generation high-temperature structural material, has high melting point (2030 °C), lower density (6.24 g/cm³) and excellent oxidation resistance at high temperatures^[

1–3]. MoSi₂ has a great prospect of application in the aerospace field for its high work temper-ature which is far higher than the highest service temperature of nickel-based superalloy (<1100 °C)^{[Reference 4–5}4–5]. However, its high tough-brittle transition temperature (BDTT=1000 °C) results in greater brittleness below 1000 °C. Moreover, the creep resistance is poor when work temperature is higher than 1200 °C^{[Reference 6

Baidu Scholar}6]. These deficiencies need to be addressed for broad-ening the applications of MoSi₂ materials.

Nb has the characteristics of high melting point, good high-temperature strength and low-temperature plasticity^[

7], which has been widely used in superalloys as a reinforcing element, and can be alloyed and compounded with MoSi₂. However, the toughness of MoSi₂ is limited when using Nb as the alloying enhancement element alone. When different forms of Nb and MoSi₂ are used for composite toughening, the crack will be deflected by the particle-reinforced phase, and the fracture toughness of the matrix can be increased by 20%‒30%. The fibrous ductile phase will passivate the crack tip and bridge the crack, and the toughness of the matrix will be increased by 100%‒300%. The laminate structure consisting of plastic phase and MoSi₂ can increase the fracture toughness of the matrix by 400%‒500%, so as to achieve better room temperature toughness. Studies have shown that adding the second phase particles, such as ZrO₂^{[Reference 8

Baidu Scholar}8], SiC^{[Reference 9

Baidu Scholar}9], NbSi₂^{[Reference 10

Baidu Scholar}10] and Nb^{[Reference 11

Baidu Scholar}11], can improve the microstructure, refine the grain and prevent the crack growth, thus improving the mechanical properties of MoSi₂. Dasgupta et al^{[Reference 12

Baidu Scholar}12] prepared MoSi₂-based composite ceramics doped with a certain amount of Nb and Al elements by arc melting and hot pressing sintering. They found that the mechanical properties of (Mo_0.99Nb_0.01)(Si_0.96-Al_0.04)₂ composite ceramics were significantly improved compared with that of the un-doped MoSi₂. At room temperature, the hardness and rigidity of (Mo_0.99Nb_0.01)(Si_0.96Al_0.04)₂ composite ceramics were lower. Accordingly, the fracture toughness increased from 3.33±3 MPa·m^1/2 to 4.05±0.3 MPa·m^1/2, and the fracture mode is mainly transgranular fracture. Zenk et al^{[Reference 13

Baidu Scholar}13] prepared MoSi₂-based composite ceramics doped with appropriate amount of Nb, Al and Ta elements by arc melting. From a microscopic point of view, the influence of elements on the sliding system of MoSi₂ was studied at low temperature. The results show that the micro-hardness of the composite ceramics, which was doped with Nb, Al and Ta elements, is smaller than that of the pure MoSi₂ ceramic. The dislocation lines around the micro-indentation were analyzed by transmission electron microscope (TEM), and results showed that the {110}<111> slip system was activated at room temperature due to the addition of Nb, Al and Ta elements, while the slip system could not be activated in pure MoSi₂ until above 300 °C. This is sufficient to infer that the doping of Nb can make MoSi₂ have more slip planes at room temperature, thus achieving better toughness. However, due to the limited alloying ability of MoSi₂, the improvement in toughness of fracture is restricted. Nb has a high melting point (2468 °C) and good toughness, which was used as a toughness enhancement phase to MoSi₂^{[Reference 14

Baidu Scholar}14]. It is also a means to improve the toughness of Mosi₂ at room temperature.

Nowadays, there are a large number of researches on core-shell structural materials, and it is obtained that the construction of core-shell structure is beneficial to combine the properties of various materials to achieve the additive effect. At the same time, because shell material is evenly distributed on the surface of nuclear material particles, it can protect nuclear materials. So it has a great application prospect in biomedicine, coating materials and other fields^[

15–16]. Zhu et al^{[Reference 17

Baidu Scholar}17] synthesized a new multilayer fluorescent microsphere PS@CdTe using the electrostatic layer self-assembly technique. The surface of the PS sphere is positively charged after adding cationic surfactant CTAB, which can attract CdTe quantum dots (QDs) with a negative charge. When the pH value of the buffer solution is 7.8, the negative charge on the surface of CdTe QDs reaches the maximum, which is favorable for the synthesis of electrostatic self-assembled microspheres. The fluorescence properties of PS@CdTe microspheres gradually increased with the increase in the number of coating layers, and the optimal number of coating layers was 3.

In this study, the core-shell MoSi₂@Nb powders were prepared by electrostatic layer-by-layer self-assembly method. Then, the prepared core-shell MoSi₂@Nb powder was sintered by spark plasma sintering (SPS). The phases, microstructures and mechanical properties of MoSi₂@Nb and MoSi₂ material sintered by SPS with the same process were also studied.

1 Experiment

1.1 Materials

The materials used in the preparation included commercially available MoSi₂ powder (powder size: 3 μm; purity: >99.5%; Beijing Xingrongyuan Technology Co., Ltd), sodium dodecyl sulfate SDS (CHSO₄Na; purity: 99%; Tianjin Beichen Founder Reagent Factory), commercially available spherical Nb powder (powder size: 50 nm; purity: 99.9%; Shanghai Pantian Nanotech Co., Ltd), cetyltrimethyl ammo-nium bromide CTAB (C₁₉H₄₂BrN; purity: 99%, Shanghai Macklin Biochemical Co., Ltd), dilute hydrochloric acid and sodium hydroxide (3.5wt%, prepared in the laboratory).

1.2 Preparation of MoSi₂@Nb core-shell powders and material

Raw MoSi₂ powder was modified by high-energy ball milling with an omnidirectional planetary ball mill. The carbide balls with 5 and 7 mm in diameter were used as grinding balls, and mass ratio was 4:1. The MoSi₂ and grinding balls with mass ratio of 15:1 were put into 500 mL carbide ball grinding tanks that were fed with an appropriate amount of protective Ar gas. Then, ball grinding tanks were fixed on the ball mill and MoSi₂ powders were ground at 150 r/min for 12 h. The MoSi₂ powder finished by ball milling was ground and sieved through 200# sieve.

First, 1.5 g Nb powder was weighed and dispersed in 90 mL distilled water by a constant speed mechanical stirrer at 600 r/min with an ultrasonic cleaner for 30 min. Then, 0.1006 g CTAB was added and stirred for 15 min to prepare Nb suspension for later use. The pH value of the Nb suspension was adjusted to 5 by dilute hydrochloric acid. 8.5 g ball-milled MoSi₂ powder was mixed with 255 mL distilled water and ultrasonically stirred at 600 r/min for 30 min with a constant speed mechanical stirrer and ultrasonic cleaner. 0.1522 g SDS was added to the powder and then stirred with ultrasonic stirring for 15 min. Then, the prepared Nb suspension was added into the MoSi₂ suspension followed by ultrasonic stirring for 30 min. The prepared suspension was extracted and filtered to get once-coated MoSi₂@Nb powder, which was dissolved into 255 mL distilled water again. The coating process was repeated, and then the twice-coated MoSi₂@Nb powder was prepared after extracting, filtering, vacuum drying and calcining, and extracting, filtering and vacuum drying again. Fig.1 shows a concise flow chart of the preparation process of MoSi₂@Nb powders. 4 g MoSi₂@Nb and ball-milled MoSi₂ powders were taken and sintered into MoSi₂@Nb and MoSi₂ materials, respectively by SPS (SPS-3T-3-MiNi (H)) with a ramp-up rate of 100 °C/min, and then they were ramped down to 1450 °C and held for 10 min under a uniaxial pressure of 40 MPa which was maintained during the ramp-up and holding processes.

Fig.1 Schematic diagram of electrostatic layer-by-layer self-assembly preparation of MoSi₂@Nb

1.3 Characterization

Zeta potentials of MoSi₂ suspension, Nb suspension and mixed suspension with surfactants were measured by Zeta potentiometer (NanoBrook 90Plus PALS). Particle size distribution of raw Nb and ball-milled MoSi₂ powders was measured by laser particle size analyzer (NanoBrook 90Plus PALS). X-ray diffractometer (XRD, D8, ADVANCE) was used to detect the phase of ball-milled MoSi₂ powder, MoSi₂@Nb powder and sintered samples. Scanning electron microscope (SEM, FEI Nova Nano SEM450) was used to observe the morphology of MoSi₂@Nb powders. Energy dispersive spectrometer (EDS, INCA 250 X-Max 50) was used to detect the distribution of elements and to characterize the core-shell structure. To further observe the microscopic details at the interface between Nb and MoSi₂ particles, field emission TEM (Talos F200X) was used to characterize twice-coated MoSi₂@Nb powder. The Vickers hardness of MoSi₂@Nb and MoSi₂ materials was measured with a Vickers hardness tester under a load of 1 kg for 15 s. The density of MoSi₂@Nb and MoSi₂ materials was measured by the Archimedes drainage method. The fracture toughness (K_IC) of the samples was measured by hardness test under a load of 10 kg for 15 s with crack length and calculated through the formula reported by Niihara et al^[

18–19]:

(\frac{K_{I C} Φ}{H \sqrt[]{d}}) {(\frac{H}{E Φ})}^{0.4} = 0.035 {(\frac{l}{d})}^{- 1 / 2}

(1)

where d is the diagonal half length of the indentation (mm); H is the Vickers hardness value (GPa); Φ is the constraint factor (Φ≈3); E is the elastic modulus of the composite material (GPa); l is the crack length (mm). E is obtained from Eq.(2):

E = V_{M o S i_{2}} E_{M o S i_{2}} + V_{N b} E_{N b}

(2)

where $E_{M o S i_{2}}$ and E_Nb are the elastic moduli of MoSi₂ and Nb, respectively; $V_{M o S i_{2}}$ and V_Nb are the volume fractions of MoSi₂ and Nb, respectively^[

20].

2 Results and Discussion

2.1 Morphology and Zeta potential of materials

The morphologies of raw MoSi₂ and ball-milled MoSi₂ powders are shown in Fig.2a‒2c and Fig.2d, respectively. Nb powder was not pretreated, and its morphology is shown in Fig.2e‒2f. It can be seen that MoSi₂ particles after ball milling have smooth surface, regular shape and smaller and more uniform particle size. The size of Nb particles is relatively uniform and its shape is spherical. As we all know, the surface of solid particles is easy to absorb other substances. Besides, the amount of charge of solid particle is closely related to its size and shape. When powder particles are in the liquid medium, the surface of the particle and liquid interface will produce ionization, ion adsorption and lattice substitution, so the particle will be electrically charged^[

21–22]. As shown in Fig.3a, the average particle size of the raw Nb powder is 193.89 nm. There are two peaks of 324.07 and 1409.08 nm in the particle size distribution curve of ball-milled MoSi₂ powder. Therefore, it indicates that MoSi₂ powder is refined by high energy ball milling. Moreover, the surface energy of equal mass powder will increase as the particles size decreases, which is more conducive to the adsorption of surfactant.

Fig.2 Microstructures of purchased MoSi₂ powder (a‒c), ball-milled MoSi₂ powder (d), and purchased Nb powder (e‒f)

Fig.3 Particle size distribution of raw Nb and ball-milled MoSi₂ powders (a); Zeta potential values of MoSi₂ and Nb suspensions mixed without and with surfactant (b)

Fig.3b shows the Zeta potential values of MoSi₂and Nb suspensions with surfactants. The potential of MoSi₂ particles becomes more negative after mixing with SDS, which is because the SDS molecule is negatively charged, and the Zeta potential value reaches the maximum of ‒46.28 mV when the SDS concentration is 2 mmol/L. Compared with the suspension of MoSi₂ without SDS, there is a significant increase in the Zeta potential value. Similarly, Nb particles become positively charged after mixing with CTAB possessing positive charge. The potential of Nb+CTAB suspension is significantly higher than that of Nb suspension. It is found that when the concentration of CTAB is 3 mmol/L, the maximum value of Zeta potential is 18.09 mV, and its absolute value is less than 30. For electrostatic adsorption, the absolute value of the opposite charge between particles plays a decisive role. Moreover, the absolute value of Zeta potential also reflects the dispersion degree of suspension. The larger the absolute value of potential, the better the dispersion effect, until reaching an extreme value. pH value has a significant effect on the adsorption of some surfactants on the particle surface, which directly affects the Zeta potential value^[

23–24]. Aimed on improving the surface charge of Nb particles, the effect of pH on the Zeta potential of Nb suspension is considered. A series of Nb suspensions with different pH values were modulated by dilute hydrochloric acid and sodium hydroxide, and the results are shown in Fig.3b. It can be found that the Zeta potential value of Nb suspensions with CTAB concentration of 3 mmol/L can reach 40.06 mV when the pH value is 5. Then, the mass fraction of Nb is 15wt% for primary coating, and the suspension with the same mass of Nb is prepared for secondary coating.

2.2 Core-shell structure of MoSi₂@Nb powders

Fig.4 shows the morphologies of once-coated and twice-coated MoSi₂@Nb powders. Fig.4a‒4c show the surface mor-phologies of once-coated powder. It can be observed that small spherical particles are attached to the surface of MoSi₂ particle, which correspond to the morphology of the raw Nb powder, but the amount of Nb particles is not much. The morphologies of the twice-coated powder are shown in Fig.4d‒4f. It can be seen that the surface of MoSi₂ particle is almost wrapped by spherical Nb particles. Fig.4g‒4i show EDS element mappings, which further support the above analysis. The Nb elements are mainly distributed around Mo and Si elements. Because the MoSi₂ particles are surrounded by Nb particles, and Nb element also exists in the middle of the MoSi₂ surface, the coating effect of twice-coated MoSi₂@Nb is better, and the microstructure of the interface between Nb and MoSi₂ particles should be further observed to explain the coating effect.

Fig.4 Microstructures of once-coated (a‒c) and twice-coated (d‒f) MoSi₂@Nb powders; EDS element mappings of Mo (g), Si (h) and Nb (i) corresponding to Fig.4e

Fig.5 shows TEM images and EDS analysis results of twice-coated powder. It can be seen that area 1# is mainly MoSi₂ with a small amount of Nb. This indicates that Nb is adsorbed on the particle surface, which agrees with the scanning results. From results of area 2#, it can be seen that spherical Nb particles are adsorbed on the edge of MoSi₂ particles. Fig.5e‒5g are EDS element mappings corresponding to Fig.5b, showing the distribution of Mo, Si and Nb, respectively. Mo and Si elements are uniformly distributed around the MoSi₂ particles, while Nb element is uniformly and relatively completely distributed outside the Mo and Si elements, forming a core-shell structure and achieving a better coating effect.

Fig.5 TEM images of twice-coated MoSi₂@Nb powder with core-shell microstructures (a‒b); EDS spot scanning results of area 1# (c) and area 2# (d) marked in Fig.5a; EDS element mappings of Mo (e), Si (f), and Nb (g)

As shown in Fig.6, compared to the XRD pattern of ball-milled MoSi₂ powder, not only Nb phase but also NbSi₂ phase is detected for MoSi₂@Nb powder, which may be interaction products between MoSi₂ and Nb particles during the calcination at 200 °C for 2 h. More details need to be found to prove it. Fig.7 shows the high resolution TEM (HRTEM) image of twice-coated MoSi₂@Nb powder and corresponding SEAD patterns. In Fig.7a, it can be found that in addition to Nb phase, NbSi₂ phase is also found compared with the microstructure of MoSi₂ powder after ball milling. Due to the strong chemical compatibility between Nb and MoSi₂, atoms still migrate at low temperatures, resulting in this hybrid phase. Fig.7a shows the HRTEM image of coated powder particles. Three different regions can be clearly divided at the interface between MoSi₂ particles and spherical Nb particles. Fig.7b and 7c show SAED patterns corresponding to area 1# and area 2#, respectively. Area 1# can be determined as MoSi₂ by calibration analysis and the corresponding strip axis is [110]. Area 2# is determined as Nb and the corresponding strip axis is [ $\bar{1}$ 11]. Area 3# is another substance formed by MoSi₂ and Nb. Fig.7d is inverse fast Fourier transform (IFFT) pattern of area 3# obtained by digital micrograph software. It can be determined to be NbSi₂ by XRD analysis and calibration and the corresponding strip axis is [ $7 \bar{1}$ 3].

Fig.6 XRD patterns of ball-milled MoSi₂ and MoSi₂@Nb powders

Fig.7 HRTEM image (a), SEAD patterns (b‒c), and IFFT pattern (d) of twice-coated MoSi₂@Nb powder with core-shell microstructure: (b) area 1#, (c) area 2#, and (d) area 3#

2.3 Phase and mechanical properties of MoSi₂@Nb and MoSi₂ materials

Phase analysis of the once-coated and twice-coated MoSi₂@Nb as well as the MoSi₂ materials are shown in Fig.8. We can find that the characteristic peaks of Nb phase are very low (38.4744° and 55.540°), indicating that Nb has a sharp reaction with the matrix MoSi₂. By comparing the phases of the primary coating and the secondary coating, it can be found that the primary coating material produces more products during the sintering process, so it can be inferred that the effect of the primary coating is too low to hinder the reaction between MoSi₂ and Nb. The main phases of MoSi₂@Nb materials are MoSi₂, Mo₅Si₃, Nb₅Si₃ and NbSi₂. Because the powders absorb a small amount of air and cannot be discharged, a small amount of Nb₂O₅ phase is produced after SPS. In MoSi₂material, Mo₅Si₃ and SiO₂ phases are produced, which stem from the reaction between MoSi₂ and O₂^[

25]. Therefore, there are also SiO₂ phases in MoSi₂@Nb materials.

Fig.8 XRD patterns of MoSi₂@Nb block materials

The microstructure of MoSi₂@Nb and MoSi₂ materials were further characterized by SEM, and the results are shown in Fig.9. Fig 9a‒9c show the microstructures of twice-coated MoSi₂@Nb material and Fig.9g shows the microstructure of the primary coating. It can be seen that the void of the secondary coatings is significantly lower than that of the primary coating, suggesting that the compactness of the secondary coatings is higher, which is conducive to improvement in the fracture toughness of the material. Fig.9d‒9f show the EDS element mappings corresponding to Fig.9b. It can be seen that Nb element is mainly distributed around Si element, while Mo element is distributed in a wide range. At 1450 °C, Mo, Nb and Si elements are active, and the diffusion ability of Si is stronger than that of Mo. As can be seen from the EDS results, Si element is distributed in the black structure, and there is strong mutual diffusion between Nb and Mo. As a result, Mo and Nb react sharply, and the following reactions may occur at 1450 °C^[

26–27] :

35MoSi₂+35Nb=20NbSi₂+7Mo₅Si₃+3Nb₅Si₃

(3)

xMo₅Si₃+(1‒x)Nb₅Si₃=(Mo_x, Nb_1‒_x)₅Si₃

(4)

Table 1 shows the point scanning results of spots marked in Fig.9, and three types of structures can be clearly seen from Fig.9a. Combined with the above analysis, it can be determined that grayish black structure is MoSi₂ and it is coated with grayish white structure. The composition of the grayish structure is mainly (Mo, Nb)₅Si₃, with a small number of Nb. The black structure is mainly randomly distributed in the grayish white structure. There are a small number of holes in the black structure, which may be caused by the large volume shrinkage caused by Nb reacting with MoSi₂. Excluding the pores, the composition of black structures is mainly NbSi₂ and Nb₃Si with a small amount of Si. Although Nb reacts sharply with MoSi₂, Nb and Nb compounds with Mo and Si are distributed around MoSi₂ grains. Hence, MoSi₂@Nb materials retain core-shell structure after SPS at 1450 °C for 10 min under a uniaxial pressure of 40 MPa. It can be determined from Fig.9c that the white structure is Mo₅Si₃ and the black structure is holes and Si.

Fig.9 SEM images of microstructure of twice-coated MoSi₂@Nb (a‒c); EDS element mappings corresponding to Fig.9b (d‒f); SEM image of microstructure of once-coated MoSi₂@Nb (g)

Table 1 EDS point scanning result of spots marked in Fig.9a and 9c (at%)

Spot	Mo	Si	Nb
1	30.57	69.43	-
2	20.88	44.49	34.63
3	5.31	82.62	12.07
4	34.47	65.53	-
5	60.66	39.44	-

It is known that the elastic moduli of MoSi₂, Nb, Mo₅Si₃ and NbSi₂ are 420, 105, 263 and 362 GPa, respectively. The elastic modulus values of Nb, Nb silicides and Mo silicides are lower than that of MoSi₂, indicating that the reaction products of Nb and MoSi₂ can affect the fracture toughness^[

10]. MoSi₂@Nb material has high toughness because the effect of Nb, Nb₅Si₃, Mo₅Si₃ and NbSi₂ multiphase toughening MoSi₂, and the good toughness of Nb phase can make the crack tip form a plastic deformation zone or a ductile crack bridging. However, the fracture of (Mo, Nb)₅Si₃, which is slightly tougher than that of MoSi₂, can produce a synergistic effect and play a diffusion strengthening role at the MoSi₂ grain boundary. The fracture resistance of a material needs to be measured by the fracture toughness (K_IC), so the microscopic indentations of MoSi₂ and MoSi₂@Nb materials are shown in Fig.10a‒10b, respectively, which are the source of the crack. Fig.11a shows the Vickers hardness, relative density and fracture toughness of MoSi₂ and MoSi₂@Nb materials. Fig.11b‒11c show the grain size distribution of MoSi₂ and twice-coated MoSi₂@Nb with core-shell structure, respectively. It can be found that the average grain size of MoSi₂ is reduced from 3.84 μm in MoSi₂ material to 2.61 μm in MoSi₂@Nb material, the Vickers hardness increases from 12.09 GPa to 12.52 GPa, and the fracture toughness increases from 3.32 MPa·m^1/2 to 5.75 MPa·m^1/2. For the once-coated MoSi₂@Nb with core-shell structure, the fracture toughness is 4.77 MPa·m^1/2 and the Vickers hardness is 12.19 GPa, which are not much improved. Compared to the effect of single coating, the problem of low single coating rate can be solved by secondary coating, and increasing the coating rate can better protect MoSi₂, avoid the oxidation of MoSi₂ into MoO₃, reduce the pores left by the volatilization of MoO₃ at high temperatures and improve the mechanical properties of the material. According to Table 2, it can be seen that the fracture toughness of secondary coating is greatly improved compared with that of primary coating and directly mixed MoSi₂ materials^{[Reference 28

Baidu Scholar}28]. In theory, grain refinement will improve the hardness greatly, but MoSi₂@Nb material has a small increase in hardness. This may be due to lower density compared with that of MoSi₂ material. The presence of more pores will lead to low adhesion between grains, resulting in lower hardness. while fine grains increase the number of grain boundaries in the same volume, which is not easy to produce stress

Fig.10 Vickers indentation patterns of MoSi₂ material (a), twice-coated MoSi₂@Nb material (b), and once-coated MoSi₂@Nb material (c)

Fig.11 Mechanical properties (a) and grain size distributions (b‒c) of MoSi₂ and MoSi₂@Nb materials

Table 2 Fracture toughness K_IC of MoSi₂ and MoSi₂@Nb prepared by different methods (MPa·m^1/2)

Twice-coated

MoSi₂@Nb

Once-coated

MoSi₂@Nb

Mixed MoSi₂

5.75

4.77

3.32

concentration, making the material have better toughness.

It will be beneficial to improve the K_IC value of the material while effectively preventing the crack sprouting and extension. Fig.12a‒12b and Fig.12d‒12e show the SEM images of crack extension after indentation tests for MoSi₂ and MoSi₂@Nb materials, respectively. It can be seen that the crack extension mode of MoSi₂ material is mainly transgranular fracture, while the crack extension mode of MoSi₂@Nb material is mainly intercrystal fracture. A more tortuous crack extension path is observed from Fig.12e with intergranular fracture, crack deflection and crack bridging. Compared with the MoSi₂ material, the cracks of MoSi₂@Nb material absorb more energy during the extension, so it exhibits higher fracture toughness. Fig.12c shows the fracture morphology of MoSi₂ material. After holding at 1450 °C for 10 min, the grain of pure MoSi₂ is relatively coarse, and some holes are distributed at the interface. The fracture surface is relatively flat, with obvious features of deconvolu-tion fracture and crystal penetration fracture. Fig.12f shows the fracture morphology of MoSi₂@Nb material, and the grain size of MoSi₂ is obviously reduced compared with that in Fig.12c. Besides, with the coating of Nb silicide, the fracture morphology of MoSi₂@Nb material is mainly crystalline intergranular fracture. There is also a small amount of micropore aggregation shear fracture, the generation process of which is that the dislocation slides to the front of the brittle second phase to generate dislocation plug group, and finally micropores are generated. The dimple fracture is formed through nucleation and aggregation of micropores. As previously stated, reaction products (Mo, Nb)₅Si₃ and NbSi₂ phases are uniformly wrapped around the MoSi₂ grains. In this way, the effect of Nb, Nb₅Si₃, Mo₅Si₃ and NbSi₂ multiphase toughening MoSi₂ is formed, and the great toughness of Nb phase can make the crack tip form a plastic deformation zone or a ductile crack bridging. However, different fracture forms between the phases with slightly better toughness. MoSi₂ and (Mo, Nb)₅Si₃ can produce synergistic effects and play a role in diffusion strengthening at the grain boundaries of MoSi₂. The crack tip is deflected as it expands into hard and brittle phases, resulting in a toughening effect.

Fig.12 SEM images of crack and fracture morphologies of MoSi₂(a‒c) and MoSi₂@Nb (d‒f) materials

3 Conclusions

1) When the concentration of SDS is 2 mmol/L, the concentration of CTAB is 3 mmol/L, and the pH value of Nb suspension is adjusted to 5, the twice-coated core-shell MoSi₂@Nb powder with relatively complete coating can be obtained by the electrostatic adsorption principle. The positive charge value of Nb suspension reaches the maximum Zeta potential value of 40.06 mV when the pH value is 5, which matches the Zeta potential value of ‒46.28 mV of the MoSi₂ suspension.

2) To remove the excess surfactant, the core-shell powder is calcined at 200 °C for 2 h under Ar atmosphere. NbSi₂ phase forms at the interface between MoSi₂ and Nb after calcination at 200 °C for 2 h, indicating that Nb has strong activity and reacts with Si. When twice-coated MoSi₂@Nb powder is sintered into solid materials by SPS at 1450 °C for 10 min under uniaxial pressure 40 MPa, Nb, Mo and Si ternary diffusion system is very active. Therefore, Nb and MoSi₂ react sharply to form Nb₅Si₃,NbSi₂ and Nb₃Si phases, but there are still Nb phase in the XRD pattern of MoSi₂@Nb material. The core-shell structure is preserved after sintering. Subsequent work should focus on reducing the degree of Nb reaction.

3) After SPS at 1450 °C for 10 min under uniaxial pressure of 40 MPa, fracture toughness increases from 3.32 MPa·m^1/2 of MoSi₂ material to 5.75 MPa·m^1/2 of MoSi₂@Nb material. Besides, the failure mode is brittle fracture.

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Preparation and Characterization of Core-Shell MoSi₂@Nb Materials Sintered by SPS PDF

Abstract

Keywords

1 Experiment

1.1 Materials

1.2 Preparation of MoSi₂@Nb core-shell powders and material

1.3 Characterization

2 Results and Discussion

2.1 Morphology and Zeta potential of materials

2.2 Core-shell structure of MoSi₂@Nb powders

2.3 Phase and mechanical properties of MoSi₂@Nb and MoSi₂ materials

3 Conclusions

References

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English

Preparation and Characterization of Core-Shell MoSi2@Nb Materials Sintered by SPS PDF

Abstract

Keywords

1 Experiment

1.1 Materials

1.2 Preparation of MoSi2@Nb core-shell powders and material

1.3 Characterization

2 Results and Discussion

2.1 Morphology and Zeta potential of materials

2.2 Core-shell structure of MoSi2@Nb powders

2.3 Phase and mechanical properties of MoSi2@Nb and MoSi2 materials

3 Conclusions

References

首页

Preparation and Characterization of Core-Shell MoSi₂@Nb Materials Sintered by SPS PDF

1.2 Preparation of MoSi₂@Nb core-shell powders and material

2.2 Core-shell structure of MoSi₂@Nb powders

2.3 Phase and mechanical properties of MoSi₂@Nb and MoSi₂ materials