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Pore structure of reactively synthesized nanolaminate Ti3SiC2 alloyed with Al
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Pore structure of reactively synthesized nanolaminate Ti3SiC2 alloyed with Al Zhonghe Wanga, Yao Jianga,∗, Xinli Liub, Yuehui Hea a b
State Key Laboratory for Powder Metallurgy, Central South University, Changsha, 410083, China School of Materials Science and Engineering, Central South University, Changsha, 410083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: MAX phase Reactive synthesis Porous material Ti3SiC2 Tortuosity factor Pore structure
Ti3SiC2 has the unique properties integrating the advantages of metals and ceramics, and good open pore structure when alloyed with Al. In this work, porous Ti3SiC2 compounds with different Al/Si atom ratios were prepared through the reactive synthesis of elemental powders at 1300 °C. The results indicate that the phase compositions are determined by Al element mole number, and that the pore structure can be controlled through varying Ti particle size. The MAX phase transits from Ti3SiC2 with Al element mole number no more than 0.6 to Ti3AlC2 with Al element mole number in the range of 0.8–1.2. When Al element mole number is 0.6, the porous compound has a single MAX phase of Ti3SiC2 with uniform microporous structure and high bending strength. Porous Ti3SiC2 alloyed with 0.6Al has a slow linear increase rate of 0.0083%/μm in open porosity with increasing Ti particle size, and a strict linear relationship between the maximum aperture and Ti particle size with the increase rate of 0.0342 μm/μm. The pore structure formed by the phase transition mechanism for porous MAX phase has the smallest tortuosity factor compared with that formed by the clearance mechanism and the Kirkendall effect.
1. Introduction Ti3SiC2 MAX phase has attracted much attention because of its unique material properties of the combination of metals and ceramics [1–3], such as satisfactory mechanical properties [4], machinability [5], good thermal shock resistivity [6], and severe rugged environment corrosion resistivity [7], which is mainly attributed to the mixed covalent and metallic bonds and the nanolaminated microstructure. Recently, porous Ti3SiC2 compound has been developed through a simple fabrication method of reactive synthesis of elemental powders [6–8]. The excellent structure stability and material properties make the material good replacement for traditional porous materials, especially in the filtration fields of harsh environment and process industries [3,7]. Many studies have been focused on the alloying of Ti3SiC2 compound with Al element [2,3,8–10], which can improve the purity of MAX phase by reducing the content of TiC impurity. In the high-temperature synthesis process of Ti3SiC2 compound, the addition of Al can enhance the interdiffusion of elements and promote the formation of final phase at relatively low temperatures. More importantly, the fast diffusion behavior of Al element is conducive to the formation of open pore structure due to the Kirkendall effect [8,11], and the resultant can
∗
still keep the MAX phase of Ti3SiC2 when Al/Si atom ratio rise to 1:1 [8,9]. To the best of our knowledge, further investigations on the characteristic and control of pore structure of porous Ti3SiC2 compound alloyed with Al have not been observed in the literatures. In this work, we demonstrate that series of nanolaminate Ti3SiC2 compounds alloyed with Al element with open pore structure can be fabricated through the reactive synthesis of elemental powders. The changes of phase composition with different Al/Si atom ratios and the pore structure through different initial Ti powder particle sizes were investigated and discussed in details. 2. Experimental procedure A typical powder metallurgy process was used to prepare the porous Ti3SiC2 compounds alloyed with Al. The starting materials include Ti, Si, graphite and Al elemental powders with the purities over 99.5%. To investigate the effect of Al content on the phase composition and determine the appropriate ingredient, a series of material compositions with different Al/Si ratios were designed. The element mole number ratio of Ti: Si and Al: C is 3: 1.2: 2, in which Al element mole numbers are 0, 0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 in order. Compared with the stoichiometric composition of Ti3SiC2 compound, excess Si and Al content
Corresponding author. E-mail address: [email protected] (Y. Jiang).
https://doi.org/10.1016/j.ceramint.2019.09.005 Received 17 July 2019; Received in revised form 19 August 2019; Accepted 1 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhonghe Wang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.005
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of 20% was utilized to compensate for the evaporation losses during the high-temperature synthesis procedure [8]. The particle sizes of Si, Al and graphite elemental powders are 8 μm, 15 μm and 10 μm, respectively. The particle sizes of Ti powder are in the range of 10–75 μm to investigate the effects of powder size on pore structure parameters including porosity, pore size and permeability. The elemental powders were mixed in a V-type mixer for 48 h to obtain a uniform mixture, which was then cold pressed into disc compacts through a double-action pressing method under the pressure of 120 MPa using stearic acid as the molding agent. The green compacts were sintered in a vacuum environment with the pressure in the range of 1.0–5.0 × 10−3 Pa. In order to avoid the possible strenuous reactions and keep the compacts in good shape, a stepped heating method was used in the sintering process, which can promote the formation of the pre-reaction layer [12]. The compacts were finally sintered at 1300 °C for 3 h. The phase characteristics of the synthesized compounds were revealed by an X-ray diffraction analyzer (XRD: Dmax 2500VB) using a Cu Kα source. The pore structure morphology was observed using a field-emission scanning microscopy (SEM, FEI Nova Nano 230). The microstructure was observed by SEM and transmission electron microscopy (TEM, JEM 2100F, 200 kV). The maximum pore size and gas permeability were measured through a bubble point method [11]. The open porosity was measured using the Archimedes method [6]. 3. Results 3.1. Effect of Al element mole number on the phase composition Fig. 1a shows the effect of Al element mole number on the phase composition of the synthesized porous compounds. The sizes of elemental powders remain constant with Ti particle size of 38 μm. When Al element mole number is no more than 0.6, the phases of the sintered compacts are mainly composed of Ti3SiC2 MAX phase and TiC impurity, which indicates that the alloyed Al atoms partially substitute Si atoms
Fig. 1. XRD patterns of porous MAX compounds with different Al element mole numbers (a) and the portion pattern with the diffraction angle from 38° to 44° for the comparison of the strongest peaks of Ti3SiC2, Ti3AlC2 and TiC (b).
Fig. 2. SEM images (a–g), bending strength (h) and open porosity (i) of the synthesized porous MAX compounds with the element mole number ratio of Ti: Si and Al: C equal to 3: 1.2: 2 and Al element mole numbers of (a) 0, (b) 0.2, (c) 0.4, (d) 0.6, (e) 0.8, (f) 1.0 and (g) 1.2. 2
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Fig. 2. (continued)
change of the MAX phase crystal structure parameters with Al element mole number, it can be inferred that a similar continuous solid solution can be formed between Ti3SiC2 and Ti3AlC2 phases [14]. However, the position of the strongest peak of TiC phase basically remains unchanged with changing Al element mole number.
in Ti3SiC2 lattice [8]. However, when Al element mole number is greater than 0.6, that is, the element mole number ratio of Al: Si is greater than 1:1, the phases of the sintered compacts are mainly composed of TiC impurity and Ti3AlC2 instead of Ti3SiC2, which implies a solid solubility behavior of Al in the Ti3SiC2 lattice. The Al/Si atom ratio determines the phase composition of the Ti–Si–Al–C quaternary system. Fig. 1b shows the angle variation of the strongest diffraction peaks of MAX phase and TiC impurity phase with Al element mole number. When Al element mole number increases from 0 to 1.2, the strongest peak of MAX phase gradually shifts to the small angle. This is mainly because the introduction of Al element gradually replaces Si element in the lattice of MAX phase with Al atom radius of 143pm greater than Si atom radius of 117pm. When Al element mole number ranges from 0 to 0.6, the main peaks of MAX phase are basically consistent with those of Ti3SiC2 with the content of TiC impurity phase in the range of 0.8–7.8 vol% according to the standard additive method [13]. When Al element mole number ranges from 0.8 to 1.2, the main peaks of MAX phase are consistent with Ti3AlC2. It's worth noting that the strongest peak of MAX phase is obviously between Ti3SiC2 and Ti3AlC2 phases with Al element mole number of 0.6. Therefore, considering the gradual
3.2. Effect of Al element mole number on the pore structure and the bending strength Fig. 2 shows the influence of Al element mole number on the microstructure, the bending strength and the open porosity of the porous material. All the synthesized MAX compounds show well-developed microscopic pore structure with the pore size in the range of a few microns to a dozen microns. A large number of pores are generated along the resultant particles. Investigation on the open porosity measured by the Archimedes method is shown in Fig. 2i. When Al element mole number increases from 0 to 1.2, the open porosity of the material is in the range of 40%–65%, indicating a characteristic of high open porosity, which is in accord with the results shown in Fig. 2a-g. Fig. 2h shows the change behavior of the bending strength of series of porous MAX compounds with different Al element mole numbers. When Al 3
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Fig. 3. SEM images of porous Ti3SiC2 alloyed with 0.6Al prepared with Ti particle sizes of (a) 75 μm, (b) 48 μm, (c) 38 μm, (d) 32 μm, (e) 20 μm, (f) 10 μm.
introduction of Al element can reduce the sintering temperature of porous MAX phase and accelerate the diffusion of elements, thus strengthen the binding strength between the particles of the resultants. At the same time, Fig. 2b–g shows that the sizes of the large particle clearance pores decrease, and the open porosity decreases to the range of 40%–50% (Fig. 2i), which is conducive to the improvement of the bending strength. When Al element mole number is 0.6, the bending strength of the material reaches the maximum value of 42.5 MPa with good size uniformity of the resultant particles in the microstructure. When Al element mole number further increases from 0.6, Al element is easy to evaporate and oxidize at high temperature in the synthesis procedure of MAX phase [2,15], which leads to the decrease of the purity of the synthesized MAX phase and the degradation of the size uniformity of the resultant particles. When Al element mole number reaches 1.2, namely, the material composition is composed of Ti, Al and C, the XRD results show that the material contains obvious TiC impurity phase, which results in the rapid decrease of the bending strength of the porous material to ca. 25 MPa. Therefore, the synthesized MAX phase compound shows high purity, uniform pore microstructure and the
element mole number increases from 0 to 1.2, the bending strength of the material is in the range of 5–45 MPa with the maximum value of 42.5 MPa at Al element mole number of 0.6. The strength of the porous MAX compounds is closely related to the phase composition, pore structure, diffusion behavior and process parameters during the sintering procedure. When Al element mole number is 0, that is, the material composition is composed of Ti, Si and C components, the bending strength of the material is the lowest, which is only 5–10 MPa. Fig. 2a reveals that there are a few fine particles adjacent to larger ones with obvious large particle clearance pores. Meanwhile, as shown in Fig. 2i, the open porosity is high and close to 65%. This suggests that, the MAX phase can be formed through the reactive synthesis of Ti, Si and C elemental powders at 1300 °C without the addition of Al element, but it is difficult to realize the complete surface and volume diffusion during the sintering procedure at the relatively low temperature, which leads to the weak metallurgical combination between the synthesized particles. In fact, the sintering temperature of at least 1350 °C is required for Ti–Si–C systems [6,7]. When Al element mole number increases from 0.2 to 1.0, the bending strength of the material rapidly increases to 35–45 MPa, indicating that the
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Fig. 6. The maximum pore size of the porous Ti3SiC2 alloyed with 0.6Al prepared with different Ti powder sizes.
Fig. 4. SEM (a) and TEM (b) images of the microstructure of the synthesized porous Ti3SiC2 alloyed with 0.6Al.
Fig. 7. Gas permeability of the porous Ti3SiC2 alloyed with 0.6Al prepared with different Ti powder sizes.
Fig. 5. Open porosity of the porous Ti3SiC2 alloyed with 0.6Al prepared with different Ti powder sizes.
maximum strength with the Al element mole number of 0.6. 3.3. Effect of Ti particle size on the pore microstructure Fig. 8. Bending strength of the porous Ti3SiC2 alloyed with 0.6Al prepared with different Ti powder sizes.
Fig. 3 shows the effect of Ti particle size on the pore microstructure of the synthesized porous Ti3SiC2 alloyed with 0.6Al. The particle size 5
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where, kp is the proportional coefficient and Cp is a constant. kp reflects the rate at which the open porosity increases with the particle size, with a value of 0.0083%/μm. When Ti particle size further decreases, the open porosity is close to the constant Cp with the value of 43.97%, which is the intercept of the trend line on the porosity coordinate axis. The coefficient of determination R2 is 0.9853, indicating a strict linear relationship between them. In the synthesis process of porous MAX phase compound, the total pores are mainly composed of the particle clearance pores in the compact and the reaction pores formed in the subsequent reactive synthesis process [6–8]. The clearance pores are mainly related to the powder particle size and cold forming pressure [16]. The reaction pores are mainly related to the raw material particle size and the sintering process [7,8,11]. The influences of powder particle size on the clearance porosity and the reaction one are inconsistent. When the powder particle size decreases, under the same pressure condition, due to the poor compression performance of fine powder compared with coarse powder, the relative density of green compact decreases, and the clearance porosity will increase to a certain extent. However, in the subsequent sintering process, the fine powder has a larger specific surface area, which enables the material to have a more sufficient surface diffusion behavior in the sintering process, promotes the densification of the material, and reduces the reaction porosity [17,18]. As a result of the combination of the two factors, the porosity reduced by surface diffusion is slightly dominant, and the overall porosity of the material decreases slowly with decreasing the powder particle size. Fig. 6 shows the influence of Ti particle size on the maximum aperture of the porous Ti3SiC2 alloyed with 0.6Al. The maximum aperture dm of porous material increases with the increase of Ti particle size dp. They satisfy the following relation:
Fig. 9. Tortuosity factors of the porous Ti3SiC2 alloyed with 0.6Al prepared with different Ti powder sizes.
of all the resultants is uniform, which decreases gradually with the decrease of Ti particle size. However, there is a certain degree of inhomogeneity in the pore size, which is improved with the decrease of Ti particle size. When Ti particle size is 75 μm, the clearance pores formed among the original powder particles are significantly larger than the pores formed through the phase transition procedure in the sintering process [7,8]. The coarse clearance pores are continuously refined with decreasing Ti particle size. When Ti particle size reaches 10 μm, the size of clearance pores is basically equivalent to that of the pores formed during the sintering procedure. Fig. 4 exhibits the microstructure of the synthesized porous Ti3SiC2 alloyed with 0.6Al. The pore micromorphology shows a characteristic of curved surface with layered structure, as shown in Fig. 4a, which is consistent with the typical microstructure of the reactively synthesized porous MAX phase compounds [6–8]. This particular pore microstructure is helpful to increase the specific surface area and reinforce the surface adsorption and activity. A further investigation on the microstructure of porous Ti3SiC2 alloyed with 0.6Al is revealed by TEM, as shown in Fig. 4b. The microstructure of nanolaminate, which reflects the typical structure characteristic of MAX phase, is verified in the synthesized particle, which consists of grains with different orientation. The laminate structure with the lamellar spacing of nanoscale is clearly observed, as indicated by the arrow mark in Fig. 4b, which imparts porous Ti3SiC2 compound good machinability. There is no obvious presence of secondary phases, which is in accordance with the results revealed by the XRD patterns as shown in Fig. 1.
dm = km·dp + Cm
where, km is the proportional coefficient and Cm is a constant. km reflects the increase rate of the maximum pore size with the particle size, which is 0.0342 μm/μm. When the particle size is further reduced, the pore size tends to be the constant Cm with the value of 4.01 μm. The coefficient of determination R2 is 0.9858, indicating a strict linear relationship between them. Different from the influence rule of powder particle size on the clearance and reaction porosities, the influence rule of particle size on the clearance and reaction pore size is consistent. When the particle size decreases, the sizes of the clearance pores amongst the powder particles and the reaction pores formed in the subsequent synthesis process decrease accordingly. Therefore, the pore size of the synthesized porous material decreases rapidly with decreasing the powder particle size. It is worth noting that when Ti particle size decreases, the particle sizes of Al, Si and graphite powders remain unchanged. Therefore, when Ti particle size is close to that of other components, the maximum aperture of the material is correlated with the coarse particle size of the components. As a result, the maximum aperture finally approaches to the fixed value, that is, the intercept value of the trend line on the aperture coordinate axis. Fig. 7 shows the influence of Ti particle size on the permeability of porous Ti3SiC2 alloyed with 0.6Al. With the increase of Ti powder size, the permeability of porous materials increases. The increase rate of permeability is accelerated with the coarsening of Ti powder size. In fact, permeability of porous materials is related to open porosity and
3.4. Effect of Ti particle size on the pore structure parameters Fig. 5 shows the influence of Ti particle size on the open porosity of porous Ti3SiC2 alloyed with 0.6Al. As Ti particle size dp increases, the open porosity θ increases slowly. The 2 parameters satisfy the following equation.
θ = kp·dp + Cp
(2)
(1)
Table 1 Comparison of tortuosity factors of 3 typical alloy & compound porous materials with similar powder size. Reference
Material type
Materials
Powder size/μm
Tortuosity factor
Main pore type
[20] [20] Present work
Alloy Intermetallic compound MAX compound
stainless steel FeAl Ti3(Si,Al)C2
3–8 3–5 8–20
2.20–4.33 1.87–2.90 1.40–1.45
Particle clearance pore Kirkendall pore Phase transition pore
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particle size of Ti powder is close to other components with the size of 10–20 μm. The relationship between the tortuosity factor and Ti powder size dp is as follows:
pore size. In general, the Hagen-Poiseuille equation will be followed [19,20]. When Ti powder is relatively coarse, the permeability of the material rapidly decreases with decreasing Ti particle size, which is mainly caused by the rapid reduction of the pore size of the material. The variation rate of permeability slows down with Ti particle size close to that of other components. Fig. 8 reveals the influence of Ti particle size on the bending strength of porous Ti3SiC2 alloyed with 0.6Al. The bending strength of porous Ti3SiC2 increases rapidly with the decrease of Ti particle size. Through data fitting, the parameters approximately satisfy the following equation:
σ = k m·d pλ
τ = kt ·dp + Ct
where, kt is the proportional coefficient with the value of 0.0187/μm, which reflects the change rate of the tortuosity factor with Ti particle size; Ct is the constant with the value of 1.18. The coefficient of determination R2 is 0.979, indicating that on the premise that the particle size of elemental powders other than Ti powder size remains unchanged, the relationship between the tortuosity factor and Ti particle size is strictly linear. Table 1 shows the comparison of tortuosity factors of 3 kinds of porous materials. In the preparation process of common metal or compound porous materials, the formation mechanisms of pores mainly include 3 types: (1) the clearance pores amongst powder particles, such as porous stainless steel and titanium [25–27]; (2) the Kirkendall pores caused by asymmetric diffusion of elements in a multi-element system with significant difference in diffusion rates, such as TiAl and FeAl intermetallic compounds [11,20]; (3) the phase transition pores due to the volume shrinkage of the resultant particles caused by the large density difference between the resultant and the reactant in the sintering or reactive synthesis process, such as porous MAX phase [6–8]. It can be seen from Table 1 that the tortuosity factor of the phase transition pores is the smallest with the value about 49% and 63% of the tortuosity factors of the clearance pores and the Kirkendall ones, respectively. In the process of pore formation by phase transition, pores are mainly derived from in-situ shrinkage of particles. The generation of a large number of shrinkage channels along particles is conducive to the connection of initial clearance pores in the compact, and greatly reduces the pore path. The effectiveness of pore connectivity induced by particle shrinkage is higher than that of gradual connectivity induced by the formation of the Kirkendall pores based on atomic diffusion. In the process of particle clearance pore formation, pore connectivity is mainly dependent on the distribution of the original particle clearance and the minor diffusion adjustment of particle surface in the subsequent sintering process, resulting in the maximum tortuosity factor.
(3)
where, σ and dp are the bending strength of porous Ti3SiC2 and Ti particle size, respectively. Km and λ are the proportional coefficient and exponential constant with the values of 121.66 and −0.216, respectively. The coefficient of determination R2 is 0.8095, indicating that the relationship between the bending strength and Ti particle size approximately follows a power function equation. The relationship between the bending strength of porous Ti3SiC2 and Ti particle size is similar to the Hall-Petch equation [21] reflecting the relation between the material mechanical properties and the grain size. In fact, as Ti particle size decreases, both the pore size (Fig. 6) and the grain size of the porous MAX compound decrease accordingly [22]. Under the premise of approximately equivalent porosity, the decrease of pore size means the decrease of structural defect scale, which is beneficial to disperse stress and reduce the stress concentration. The decrease of grain size, according to the Hall-Petch relation, will result in the increase of the strength and hardness of the material [21]. Therefore, as Ti particle size decreases, the changes of grain size and aperture of porous Ti3SiC2 are conducive to the improvement of material strength, which leads to the rapid increase of the bending strength. 4. Discussion The synthesized MAX compound with high purity, uniform microstructure and high material strength can be obtained with appropriate addition of Al. By adjusting the particle size of Ti powder, the porous material shows a strict linear variation of pore size and porosity with the powder size. This means that in the preparation process of Ti3SiC2 alloyed with Al by reactive synthesis of elemental powders, the pore structure can be regulated by adjusting Al element mole number and the particle size of principal component powder. In the study on pore structure of porous material, tortuosity factor, that is, the ratio of the actual length of the pores to the apparent length, can be used to characterize the degree of tortuosity of pore structure [20,23,24]. The smaller the tortuosity factor, the shorter the pore path is along the flow direction, which is beneficial to improving filtration flux of porous material. At the same time, the tortuosity factor reflects the pore formation mechanism and growth behavior in the process of pore formation to a certain extent. After determining the pore structure parameters, such as pore size d, porosity θ, and permeability K, the tortuosity factor τ can be calculated through the Hagen-Poiseuille equation [20].
K=
d2⋅θ 32η⋅L⋅τ
(5)
5. Conclusion (1) Reactively synthesized porous MAX compound has a characteristic of single Ti3SiC2 main phase with Al element mole number no more than 0.6 in Ti–Si–Al–C quaternary system, which is converted to Ti3AlC2 phase with Al element mole number in the range of 0.8–1.2. (2) The synthesized MAX compound shows a characteristic of continuous solid solution when Al element mole number increases from 0.2 to 1.0. (3) Porous Ti3SiC2 alloyed with 0.6Al has a slow linear increase rate in open porosity with increasing Ti particle size, and a strict linear relationship between the maximum pore size and Ti particle size. The permeability of porous Ti3SiC2 alloyed with 0.6Al increases with increasing Ti particle size, and the rate increases with the roughening of Ti powder. (4) The pores formed by the phase transition mechanism for porous MAX compound have the smallest tortuosity factor compared with the pores derived from particle clearance and the ones formed through the Kirkendall effect.
(4)
where, η and L are the dynamic viscosity of the fluid and the sample thickness, respectively. Fig. 9 shows the tortuosity factors of the porous MAX compounds prepared through different Ti particle sizes. When Ti particle size ranges from 10 to 75 μm, the tortuosity factors of the porous compounds range from 1.40 to 2.56. Considering that the particle sizes of Si, Al and graphite elemental powders are 8 μm, 15 μm and 10 μm, respectively, the tortuosity factors are in the range of 1.40–1.45 when the
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Pore structure of reactively synthesized nanolaminate Ti3SiC2 alloyed with Al Zhonghe Wanga, Yao Jianga,∗, Xinli Liub, Yuehui Hea a b
State Key Laboratory for Powder Metallurgy, Central South University, Changsha, 410083, China School of Materials Science and Engineering, Central South University, Changsha, 410083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: MAX phase Reactive synthesis Porous material Ti3SiC2 Tortuosity factor Pore structure
Ti3SiC2 has the unique properties integrating the advantages of metals and ceramics, and good open pore structure when alloyed with Al. In this work, porous Ti3SiC2 compounds with different Al/Si atom ratios were prepared through the reactive synthesis of elemental powders at 1300 °C. The results indicate that the phase compositions are determined by Al element mole number, and that the pore structure can be controlled through varying Ti particle size. The MAX phase transits from Ti3SiC2 with Al element mole number no more than 0.6 to Ti3AlC2 with Al element mole number in the range of 0.8–1.2. When Al element mole number is 0.6, the porous compound has a single MAX phase of Ti3SiC2 with uniform microporous structure and high bending strength. Porous Ti3SiC2 alloyed with 0.6Al has a slow linear increase rate of 0.0083%/μm in open porosity with increasing Ti particle size, and a strict linear relationship between the maximum aperture and Ti particle size with the increase rate of 0.0342 μm/μm. The pore structure formed by the phase transition mechanism for porous MAX phase has the smallest tortuosity factor compared with that formed by the clearance mechanism and the Kirkendall effect.
1. Introduction Ti3SiC2 MAX phase has attracted much attention because of its unique material properties of the combination of metals and ceramics [1–3], such as satisfactory mechanical properties [4], machinability [5], good thermal shock resistivity [6], and severe rugged environment corrosion resistivity [7], which is mainly attributed to the mixed covalent and metallic bonds and the nanolaminated microstructure. Recently, porous Ti3SiC2 compound has been developed through a simple fabrication method of reactive synthesis of elemental powders [6–8]. The excellent structure stability and material properties make the material good replacement for traditional porous materials, especially in the filtration fields of harsh environment and process industries [3,7]. Many studies have been focused on the alloying of Ti3SiC2 compound with Al element [2,3,8–10], which can improve the purity of MAX phase by reducing the content of TiC impurity. In the high-temperature synthesis process of Ti3SiC2 compound, the addition of Al can enhance the interdiffusion of elements and promote the formation of final phase at relatively low temperatures. More importantly, the fast diffusion behavior of Al element is conducive to the formation of open pore structure due to the Kirkendall effect [8,11], and the resultant can
∗
still keep the MAX phase of Ti3SiC2 when Al/Si atom ratio rise to 1:1 [8,9]. To the best of our knowledge, further investigations on the characteristic and control of pore structure of porous Ti3SiC2 compound alloyed with Al have not been observed in the literatures. In this work, we demonstrate that series of nanolaminate Ti3SiC2 compounds alloyed with Al element with open pore structure can be fabricated through the reactive synthesis of elemental powders. The changes of phase composition with different Al/Si atom ratios and the pore structure through different initial Ti powder particle sizes were investigated and discussed in details. 2. Experimental procedure A typical powder metallurgy process was used to prepare the porous Ti3SiC2 compounds alloyed with Al. The starting materials include Ti, Si, graphite and Al elemental powders with the purities over 99.5%. To investigate the effect of Al content on the phase composition and determine the appropriate ingredient, a series of material compositions with different Al/Si ratios were designed. The element mole number ratio of Ti: Si and Al: C is 3: 1.2: 2, in which Al element mole numbers are 0, 0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 in order. Compared with the stoichiometric composition of Ti3SiC2 compound, excess Si and Al content
Corresponding author. E-mail address: [email protected] (Y. Jiang).
https://doi.org/10.1016/j.ceramint.2019.09.005 Received 17 July 2019; Received in revised form 19 August 2019; Accepted 1 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhonghe Wang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.005
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of 20% was utilized to compensate for the evaporation losses during the high-temperature synthesis procedure [8]. The particle sizes of Si, Al and graphite elemental powders are 8 μm, 15 μm and 10 μm, respectively. The particle sizes of Ti powder are in the range of 10–75 μm to investigate the effects of powder size on pore structure parameters including porosity, pore size and permeability. The elemental powders were mixed in a V-type mixer for 48 h to obtain a uniform mixture, which was then cold pressed into disc compacts through a double-action pressing method under the pressure of 120 MPa using stearic acid as the molding agent. The green compacts were sintered in a vacuum environment with the pressure in the range of 1.0–5.0 × 10−3 Pa. In order to avoid the possible strenuous reactions and keep the compacts in good shape, a stepped heating method was used in the sintering process, which can promote the formation of the pre-reaction layer [12]. The compacts were finally sintered at 1300 °C for 3 h. The phase characteristics of the synthesized compounds were revealed by an X-ray diffraction analyzer (XRD: Dmax 2500VB) using a Cu Kα source. The pore structure morphology was observed using a field-emission scanning microscopy (SEM, FEI Nova Nano 230). The microstructure was observed by SEM and transmission electron microscopy (TEM, JEM 2100F, 200 kV). The maximum pore size and gas permeability were measured through a bubble point method [11]. The open porosity was measured using the Archimedes method [6]. 3. Results 3.1. Effect of Al element mole number on the phase composition Fig. 1a shows the effect of Al element mole number on the phase composition of the synthesized porous compounds. The sizes of elemental powders remain constant with Ti particle size of 38 μm. When Al element mole number is no more than 0.6, the phases of the sintered compacts are mainly composed of Ti3SiC2 MAX phase and TiC impurity, which indicates that the alloyed Al atoms partially substitute Si atoms
Fig. 1. XRD patterns of porous MAX compounds with different Al element mole numbers (a) and the portion pattern with the diffraction angle from 38° to 44° for the comparison of the strongest peaks of Ti3SiC2, Ti3AlC2 and TiC (b).
Fig. 2. SEM images (a–g), bending strength (h) and open porosity (i) of the synthesized porous MAX compounds with the element mole number ratio of Ti: Si and Al: C equal to 3: 1.2: 2 and Al element mole numbers of (a) 0, (b) 0.2, (c) 0.4, (d) 0.6, (e) 0.8, (f) 1.0 and (g) 1.2. 2
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Fig. 2. (continued)
change of the MAX phase crystal structure parameters with Al element mole number, it can be inferred that a similar continuous solid solution can be formed between Ti3SiC2 and Ti3AlC2 phases [14]. However, the position of the strongest peak of TiC phase basically remains unchanged with changing Al element mole number.
in Ti3SiC2 lattice [8]. However, when Al element mole number is greater than 0.6, that is, the element mole number ratio of Al: Si is greater than 1:1, the phases of the sintered compacts are mainly composed of TiC impurity and Ti3AlC2 instead of Ti3SiC2, which implies a solid solubility behavior of Al in the Ti3SiC2 lattice. The Al/Si atom ratio determines the phase composition of the Ti–Si–Al–C quaternary system. Fig. 1b shows the angle variation of the strongest diffraction peaks of MAX phase and TiC impurity phase with Al element mole number. When Al element mole number increases from 0 to 1.2, the strongest peak of MAX phase gradually shifts to the small angle. This is mainly because the introduction of Al element gradually replaces Si element in the lattice of MAX phase with Al atom radius of 143pm greater than Si atom radius of 117pm. When Al element mole number ranges from 0 to 0.6, the main peaks of MAX phase are basically consistent with those of Ti3SiC2 with the content of TiC impurity phase in the range of 0.8–7.8 vol% according to the standard additive method [13]. When Al element mole number ranges from 0.8 to 1.2, the main peaks of MAX phase are consistent with Ti3AlC2. It's worth noting that the strongest peak of MAX phase is obviously between Ti3SiC2 and Ti3AlC2 phases with Al element mole number of 0.6. Therefore, considering the gradual
3.2. Effect of Al element mole number on the pore structure and the bending strength Fig. 2 shows the influence of Al element mole number on the microstructure, the bending strength and the open porosity of the porous material. All the synthesized MAX compounds show well-developed microscopic pore structure with the pore size in the range of a few microns to a dozen microns. A large number of pores are generated along the resultant particles. Investigation on the open porosity measured by the Archimedes method is shown in Fig. 2i. When Al element mole number increases from 0 to 1.2, the open porosity of the material is in the range of 40%–65%, indicating a characteristic of high open porosity, which is in accord with the results shown in Fig. 2a-g. Fig. 2h shows the change behavior of the bending strength of series of porous MAX compounds with different Al element mole numbers. When Al 3
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Fig. 3. SEM images of porous Ti3SiC2 alloyed with 0.6Al prepared with Ti particle sizes of (a) 75 μm, (b) 48 μm, (c) 38 μm, (d) 32 μm, (e) 20 μm, (f) 10 μm.
introduction of Al element can reduce the sintering temperature of porous MAX phase and accelerate the diffusion of elements, thus strengthen the binding strength between the particles of the resultants. At the same time, Fig. 2b–g shows that the sizes of the large particle clearance pores decrease, and the open porosity decreases to the range of 40%–50% (Fig. 2i), which is conducive to the improvement of the bending strength. When Al element mole number is 0.6, the bending strength of the material reaches the maximum value of 42.5 MPa with good size uniformity of the resultant particles in the microstructure. When Al element mole number further increases from 0.6, Al element is easy to evaporate and oxidize at high temperature in the synthesis procedure of MAX phase [2,15], which leads to the decrease of the purity of the synthesized MAX phase and the degradation of the size uniformity of the resultant particles. When Al element mole number reaches 1.2, namely, the material composition is composed of Ti, Al and C, the XRD results show that the material contains obvious TiC impurity phase, which results in the rapid decrease of the bending strength of the porous material to ca. 25 MPa. Therefore, the synthesized MAX phase compound shows high purity, uniform pore microstructure and the
element mole number increases from 0 to 1.2, the bending strength of the material is in the range of 5–45 MPa with the maximum value of 42.5 MPa at Al element mole number of 0.6. The strength of the porous MAX compounds is closely related to the phase composition, pore structure, diffusion behavior and process parameters during the sintering procedure. When Al element mole number is 0, that is, the material composition is composed of Ti, Si and C components, the bending strength of the material is the lowest, which is only 5–10 MPa. Fig. 2a reveals that there are a few fine particles adjacent to larger ones with obvious large particle clearance pores. Meanwhile, as shown in Fig. 2i, the open porosity is high and close to 65%. This suggests that, the MAX phase can be formed through the reactive synthesis of Ti, Si and C elemental powders at 1300 °C without the addition of Al element, but it is difficult to realize the complete surface and volume diffusion during the sintering procedure at the relatively low temperature, which leads to the weak metallurgical combination between the synthesized particles. In fact, the sintering temperature of at least 1350 °C is required for Ti–Si–C systems [6,7]. When Al element mole number increases from 0.2 to 1.0, the bending strength of the material rapidly increases to 35–45 MPa, indicating that the
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Fig. 6. The maximum pore size of the porous Ti3SiC2 alloyed with 0.6Al prepared with different Ti powder sizes.
Fig. 4. SEM (a) and TEM (b) images of the microstructure of the synthesized porous Ti3SiC2 alloyed with 0.6Al.
Fig. 7. Gas permeability of the porous Ti3SiC2 alloyed with 0.6Al prepared with different Ti powder sizes.
Fig. 5. Open porosity of the porous Ti3SiC2 alloyed with 0.6Al prepared with different Ti powder sizes.
maximum strength with the Al element mole number of 0.6. 3.3. Effect of Ti particle size on the pore microstructure Fig. 8. Bending strength of the porous Ti3SiC2 alloyed with 0.6Al prepared with different Ti powder sizes.
Fig. 3 shows the effect of Ti particle size on the pore microstructure of the synthesized porous Ti3SiC2 alloyed with 0.6Al. The particle size 5
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where, kp is the proportional coefficient and Cp is a constant. kp reflects the rate at which the open porosity increases with the particle size, with a value of 0.0083%/μm. When Ti particle size further decreases, the open porosity is close to the constant Cp with the value of 43.97%, which is the intercept of the trend line on the porosity coordinate axis. The coefficient of determination R2 is 0.9853, indicating a strict linear relationship between them. In the synthesis process of porous MAX phase compound, the total pores are mainly composed of the particle clearance pores in the compact and the reaction pores formed in the subsequent reactive synthesis process [6–8]. The clearance pores are mainly related to the powder particle size and cold forming pressure [16]. The reaction pores are mainly related to the raw material particle size and the sintering process [7,8,11]. The influences of powder particle size on the clearance porosity and the reaction one are inconsistent. When the powder particle size decreases, under the same pressure condition, due to the poor compression performance of fine powder compared with coarse powder, the relative density of green compact decreases, and the clearance porosity will increase to a certain extent. However, in the subsequent sintering process, the fine powder has a larger specific surface area, which enables the material to have a more sufficient surface diffusion behavior in the sintering process, promotes the densification of the material, and reduces the reaction porosity [17,18]. As a result of the combination of the two factors, the porosity reduced by surface diffusion is slightly dominant, and the overall porosity of the material decreases slowly with decreasing the powder particle size. Fig. 6 shows the influence of Ti particle size on the maximum aperture of the porous Ti3SiC2 alloyed with 0.6Al. The maximum aperture dm of porous material increases with the increase of Ti particle size dp. They satisfy the following relation:
Fig. 9. Tortuosity factors of the porous Ti3SiC2 alloyed with 0.6Al prepared with different Ti powder sizes.
of all the resultants is uniform, which decreases gradually with the decrease of Ti particle size. However, there is a certain degree of inhomogeneity in the pore size, which is improved with the decrease of Ti particle size. When Ti particle size is 75 μm, the clearance pores formed among the original powder particles are significantly larger than the pores formed through the phase transition procedure in the sintering process [7,8]. The coarse clearance pores are continuously refined with decreasing Ti particle size. When Ti particle size reaches 10 μm, the size of clearance pores is basically equivalent to that of the pores formed during the sintering procedure. Fig. 4 exhibits the microstructure of the synthesized porous Ti3SiC2 alloyed with 0.6Al. The pore micromorphology shows a characteristic of curved surface with layered structure, as shown in Fig. 4a, which is consistent with the typical microstructure of the reactively synthesized porous MAX phase compounds [6–8]. This particular pore microstructure is helpful to increase the specific surface area and reinforce the surface adsorption and activity. A further investigation on the microstructure of porous Ti3SiC2 alloyed with 0.6Al is revealed by TEM, as shown in Fig. 4b. The microstructure of nanolaminate, which reflects the typical structure characteristic of MAX phase, is verified in the synthesized particle, which consists of grains with different orientation. The laminate structure with the lamellar spacing of nanoscale is clearly observed, as indicated by the arrow mark in Fig. 4b, which imparts porous Ti3SiC2 compound good machinability. There is no obvious presence of secondary phases, which is in accordance with the results revealed by the XRD patterns as shown in Fig. 1.
dm = km·dp + Cm
where, km is the proportional coefficient and Cm is a constant. km reflects the increase rate of the maximum pore size with the particle size, which is 0.0342 μm/μm. When the particle size is further reduced, the pore size tends to be the constant Cm with the value of 4.01 μm. The coefficient of determination R2 is 0.9858, indicating a strict linear relationship between them. Different from the influence rule of powder particle size on the clearance and reaction porosities, the influence rule of particle size on the clearance and reaction pore size is consistent. When the particle size decreases, the sizes of the clearance pores amongst the powder particles and the reaction pores formed in the subsequent synthesis process decrease accordingly. Therefore, the pore size of the synthesized porous material decreases rapidly with decreasing the powder particle size. It is worth noting that when Ti particle size decreases, the particle sizes of Al, Si and graphite powders remain unchanged. Therefore, when Ti particle size is close to that of other components, the maximum aperture of the material is correlated with the coarse particle size of the components. As a result, the maximum aperture finally approaches to the fixed value, that is, the intercept value of the trend line on the aperture coordinate axis. Fig. 7 shows the influence of Ti particle size on the permeability of porous Ti3SiC2 alloyed with 0.6Al. With the increase of Ti powder size, the permeability of porous materials increases. The increase rate of permeability is accelerated with the coarsening of Ti powder size. In fact, permeability of porous materials is related to open porosity and
3.4. Effect of Ti particle size on the pore structure parameters Fig. 5 shows the influence of Ti particle size on the open porosity of porous Ti3SiC2 alloyed with 0.6Al. As Ti particle size dp increases, the open porosity θ increases slowly. The 2 parameters satisfy the following equation.
θ = kp·dp + Cp
(2)
(1)
Table 1 Comparison of tortuosity factors of 3 typical alloy & compound porous materials with similar powder size. Reference
Material type
Materials
Powder size/μm
Tortuosity factor
Main pore type
[20] [20] Present work
Alloy Intermetallic compound MAX compound
stainless steel FeAl Ti3(Si,Al)C2
3–8 3–5 8–20
2.20–4.33 1.87–2.90 1.40–1.45
Particle clearance pore Kirkendall pore Phase transition pore
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particle size of Ti powder is close to other components with the size of 10–20 μm. The relationship between the tortuosity factor and Ti powder size dp is as follows:
pore size. In general, the Hagen-Poiseuille equation will be followed [19,20]. When Ti powder is relatively coarse, the permeability of the material rapidly decreases with decreasing Ti particle size, which is mainly caused by the rapid reduction of the pore size of the material. The variation rate of permeability slows down with Ti particle size close to that of other components. Fig. 8 reveals the influence of Ti particle size on the bending strength of porous Ti3SiC2 alloyed with 0.6Al. The bending strength of porous Ti3SiC2 increases rapidly with the decrease of Ti particle size. Through data fitting, the parameters approximately satisfy the following equation:
σ = k m·d pλ
τ = kt ·dp + Ct
where, kt is the proportional coefficient with the value of 0.0187/μm, which reflects the change rate of the tortuosity factor with Ti particle size; Ct is the constant with the value of 1.18. The coefficient of determination R2 is 0.979, indicating that on the premise that the particle size of elemental powders other than Ti powder size remains unchanged, the relationship between the tortuosity factor and Ti particle size is strictly linear. Table 1 shows the comparison of tortuosity factors of 3 kinds of porous materials. In the preparation process of common metal or compound porous materials, the formation mechanisms of pores mainly include 3 types: (1) the clearance pores amongst powder particles, such as porous stainless steel and titanium [25–27]; (2) the Kirkendall pores caused by asymmetric diffusion of elements in a multi-element system with significant difference in diffusion rates, such as TiAl and FeAl intermetallic compounds [11,20]; (3) the phase transition pores due to the volume shrinkage of the resultant particles caused by the large density difference between the resultant and the reactant in the sintering or reactive synthesis process, such as porous MAX phase [6–8]. It can be seen from Table 1 that the tortuosity factor of the phase transition pores is the smallest with the value about 49% and 63% of the tortuosity factors of the clearance pores and the Kirkendall ones, respectively. In the process of pore formation by phase transition, pores are mainly derived from in-situ shrinkage of particles. The generation of a large number of shrinkage channels along particles is conducive to the connection of initial clearance pores in the compact, and greatly reduces the pore path. The effectiveness of pore connectivity induced by particle shrinkage is higher than that of gradual connectivity induced by the formation of the Kirkendall pores based on atomic diffusion. In the process of particle clearance pore formation, pore connectivity is mainly dependent on the distribution of the original particle clearance and the minor diffusion adjustment of particle surface in the subsequent sintering process, resulting in the maximum tortuosity factor.
(3)
where, σ and dp are the bending strength of porous Ti3SiC2 and Ti particle size, respectively. Km and λ are the proportional coefficient and exponential constant with the values of 121.66 and −0.216, respectively. The coefficient of determination R2 is 0.8095, indicating that the relationship between the bending strength and Ti particle size approximately follows a power function equation. The relationship between the bending strength of porous Ti3SiC2 and Ti particle size is similar to the Hall-Petch equation [21] reflecting the relation between the material mechanical properties and the grain size. In fact, as Ti particle size decreases, both the pore size (Fig. 6) and the grain size of the porous MAX compound decrease accordingly [22]. Under the premise of approximately equivalent porosity, the decrease of pore size means the decrease of structural defect scale, which is beneficial to disperse stress and reduce the stress concentration. The decrease of grain size, according to the Hall-Petch relation, will result in the increase of the strength and hardness of the material [21]. Therefore, as Ti particle size decreases, the changes of grain size and aperture of porous Ti3SiC2 are conducive to the improvement of material strength, which leads to the rapid increase of the bending strength. 4. Discussion The synthesized MAX compound with high purity, uniform microstructure and high material strength can be obtained with appropriate addition of Al. By adjusting the particle size of Ti powder, the porous material shows a strict linear variation of pore size and porosity with the powder size. This means that in the preparation process of Ti3SiC2 alloyed with Al by reactive synthesis of elemental powders, the pore structure can be regulated by adjusting Al element mole number and the particle size of principal component powder. In the study on pore structure of porous material, tortuosity factor, that is, the ratio of the actual length of the pores to the apparent length, can be used to characterize the degree of tortuosity of pore structure [20,23,24]. The smaller the tortuosity factor, the shorter the pore path is along the flow direction, which is beneficial to improving filtration flux of porous material. At the same time, the tortuosity factor reflects the pore formation mechanism and growth behavior in the process of pore formation to a certain extent. After determining the pore structure parameters, such as pore size d, porosity θ, and permeability K, the tortuosity factor τ can be calculated through the Hagen-Poiseuille equation [20].
K=
d2⋅θ 32η⋅L⋅τ
(5)
5. Conclusion (1) Reactively synthesized porous MAX compound has a characteristic of single Ti3SiC2 main phase with Al element mole number no more than 0.6 in Ti–Si–Al–C quaternary system, which is converted to Ti3AlC2 phase with Al element mole number in the range of 0.8–1.2. (2) The synthesized MAX compound shows a characteristic of continuous solid solution when Al element mole number increases from 0.2 to 1.0. (3) Porous Ti3SiC2 alloyed with 0.6Al has a slow linear increase rate in open porosity with increasing Ti particle size, and a strict linear relationship between the maximum pore size and Ti particle size. The permeability of porous Ti3SiC2 alloyed with 0.6Al increases with increasing Ti particle size, and the rate increases with the roughening of Ti powder. (4) The pores formed by the phase transition mechanism for porous MAX compound have the smallest tortuosity factor compared with the pores derived from particle clearance and the ones formed through the Kirkendall effect.
(4)
where, η and L are the dynamic viscosity of the fluid and the sample thickness, respectively. Fig. 9 shows the tortuosity factors of the porous MAX compounds prepared through different Ti particle sizes. When Ti particle size ranges from 10 to 75 μm, the tortuosity factors of the porous compounds range from 1.40 to 2.56. Considering that the particle sizes of Si, Al and graphite elemental powders are 8 μm, 15 μm and 10 μm, respectively, the tortuosity factors are in the range of 1.40–1.45 when the
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