Journal of Ceramics

Volume 2014 (2014), Article ID 370851, 6 pages

http://dx.doi.org/10.1155/2014/370851

Research Article

Preparation of Stable ZrB₂-SiC-B₄C Aqueous Suspension for Composite Based Coating: Effect of Solid Content and Dispersant on Stability

Mehri Mashadi,¹ Mohsen Mohammadijoo,² Alireza Honarkar,³ and Zeinab Naderi Khorshidi²

¹Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University, P.O. Box 1411713116, Tehran, Iran

²Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada, T6G 2V4

³School of Engineering, Shahid Rajaee University, P.O. Box 16785-136, Tehran, Iran

Received 26 February 2014; Revised 5 August 2014; Accepted 24 August 2014; Published 7 September 2014

Academic Editor: Keizo Uematsu

Copyright © 2014 Mehri Mashadi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

ZrB₂-SiC-B₄C aqueous suspension has been prepared using poly(ethyleneimine) as a dispersant. Since increasing the solid content of suspension leads to high compaction and consequently low porosities through final coat, the effect of solid content has been studied. The dispersant and solid content were changed in the range of 0.3–1.5 wt.% and 45–55 vol.%, respectively, to assess the optimal conditions effect on stability and characteristics of suspension. Results of zeta potential measurements and rheological analysis at pH 7.8 showed that the composite suspension including 45 vol.% solid content and 1.5 wt.% dispersant was in stable state.

1. Introduction

The interest in ultrahigh temperature ceramics (UHTCs) has increased significantly in recent years [1–3] because of their remarkable properties, such as high melting point, high thermal conductivities, excellent corrosion resistance, and good oxidation resistance [4, 5], which make them promising candidates for high temperature structural applications. Among the UHTCs, zirconium diboride (ZrB₂) is a material of particular interest owing to the excellent and unique combination of high melting point, high electrical and thermal conductivity, good thermal shock and wear resistance, and chemical inertness [6]. These properties make it an attractive candidate for ultrahigh temperature applications where corrosion-wear-oxidation resistance is demanded [6, 7].

In spite of the excellent high temperature and mechanical properties of ZrB₂-based ceramics, their rather low fracture toughness (3-4 MPa m1/2) has limited the sample size because of the low reliability and, thus, has reduced the chances for application of boride ceramics [8].

Several studies have demonstrated that the addition of SiC could improve the oxidation resistance and mechanical properties of ZrB₂ ceramics, so a lot of works have been carried out on ZrB₂-SiC ceramics [9–16]. Nowadays, colloidal processing such as tape casting, slip casting [17, 18], and dip coating method, which can produce a more homogeneous green microstructure, is becoming more and more important in the fabrication of advanced ceramics because it offers the potential to produce reliable ceramic films and bulk forms through careful control of initial suspension “structure” and its evolution during fabrication [19].

For the fabrication of highly dense ceramic composites, the preparation of well-dispersed and stable ceramic suspensions is one of the most important issues in order to guarantee a homogeneous filling of the interstices among ceramic powders [8]. However, dispersion behaviors of aqueous ZrB₂-SiC-B₄C slurries have been rarely analyzed [17, 18] and reports on the successful preparation of highly concentrated aqueous suspensions have not been available. To maintain the stability through an aqueous suspension, it is needed to prevent particles to (i) stick when they collide and (ii) sediment when they are introduced in a colloidal system. This can be achieved by enhancing the charge associated with the particles, that is, zeta potential [20]. Also, the proper viscosity of suspension causes a slower settling velocity and therefore better stability of suspension.

In the present research, the dispersion behaviors of ZrB₂-SiC-B₄C (ZSB) composite in aqueous medium are investigated with the application of a dispersant (poly(ethylenimine), PEI) for the preparation of highly concentrated aqueous ZSB suspensions.

2. Experimental Procedure

2.1. Aqueous Suspension Preparation

Commercially available ZrB₂ powder (initial particle size ~6 μm), SiC powder (average particle size 1.5 μm), and B₄C powder (average particle size as 3 μm) were used as raw materials. Deionized water and PEI (molecular weight (): 2,000; 50 wt.% in H₂O; Sigma-Aldrich, Belgium) were used for the preparation of ZSB aqueous suspension.

The as-received ZrB₂ powder was milled using a planetary mill with ethanol, a WC ball (diameter: 10 mm), and a stainless steel (coated by WC) jar at 150 revolutions per minute (rpm) for 2 h. The average size of ZrB₂ powders was reduced to 3–5 μm after milling (Figure 1). A mixture of 80 vol.% zirconium diboride and 20 vol.% silicon carbide powders was selected as starting materials. 3 wt.% boron carbide was also used as sintering aid which causes a better sintering of the ceramic coating through next step of preparation.

Figure 1: Size distribution of ZrB₂ milled powders.

To produce slurries, powders were mixed with various amounts of PEI (0.3–1.5 wt.%) as a dispersant. The pH of slurries was also set 7.8. The initial ratio of solid to liquid was set as 45 vol.% according to previous researches [17–19]. To investigate the effect of solid content on ZSB suspension properties, slurries with 45, 46, 47.5, 50, 52.5, and 55 vol.% solid contents were prepared. Table 1 shows the composition of slurries.

Table 1: Characteristics of slurries.

Solid content
PEI

Investigation of effect of PEI on slurry properties

45 vol.%
0.3 wt.%

0.5 wt.%

0.7 wt.%

1 wt.%

1.5 wt.%

Investigation of effect of solid content on slurry properties

45 vol.%
1.5 wt.%

46 vol.%

47.5 vol.%

50 vol.%

52.5 vol.%

55 vol.%

45 vol.%
1 wt.%

46 vol.%

2.2. Characterization

Size distribution of milled ZrB₂ powders was measured by zeta size analyzer (ZEN3600, England). Rheological behavior and viscosity of slurries were characterized by rheometer apparatus (Physica CPR300, Japan). Ultrasonic vibrator (Hielscher-UP200H, Germany) was used to disperse suspensions prior to zeta potential analyzing. The zeta potential analyzer (ZEN3600, England) was also used to measure the zeta potential of slurries.

3. Results and Discussion

Prior to any investigation of a colloidal system, it is necessary to bring a proper understanding of a stable suspension to the system, for example, how could a colloidal stability be achieved? The force balance associated with the particles in the suspension is simply demonstrated in two terms [21] (1): (i) gravitational forces (numerator in (1)) and (ii) Brownian forces (denominator in (1)). Consider

where , , and are the force balance, particles size, and density difference of particles and continuous phase, respectively. and are Boltzmann constant and temperature, respectively. In submicron colloidal systems, the Brownian motion is usually significant to overcome the effect of gravity. To maintain stability through Brownian motion, it is necessary to prevent particles sticking when they collide. This can be achieved by increasing the charge associated with the particles, that is, zeta potential of particles. Figure 2 indicates the zeta potential of a sphere particle within an aqueous medium.

Figure 2: Electrostatic double layer surrounding sphere particles in an aqueous medium.

By increasing the zeta potential (over ±30 mV), the significance of long range electrostatic double layer surrounding particles increases which leads to a repulsion between particles in suspension [22]. Figure 3 depicts the schematic view of the effect of zeta potential on dispersion and coagulation of particles in a suspension.

Figure 3: Schematic view of the particles interactions in an aqueous suspension [22].

Zeta potential () of slurries including 45 vol.% solid content and different amounts of dispersant is shown in Figure 4. pH of slurries was set as 7.8. As it is indicated, the zeta potential of slurry without PEI is in unstable area . Increasing PEI amount, 0.3 wt.%, led to the zeta potential falling in stable area ( and ). A slight increase occurred in the zeta potential while PEI amount increased from 0.3 to 1.5 wt.%. This increase could be explained in terms of adsorption of PEI molecules on surface of particles [23]. Although the data concerning the reaction between ZrB₂-SiC-B₄C particles and PEI are currently unavailable, Wang and Gao [24] have reported that the adsorption of PEI on ZrB₂ is of a high affinity type, and hydrogen bonding was proposed to be the predominant mechanism between PEI and ZrB₂ under both acidic and basic conditions.

Figure 4: Zeta potential of ZSB slurry with various amounts of PEI dispersant.

Protons are adsorbed on PEI molecules when PEI is dissolved in a neutral solution which results in the protonation of the amine group in the molecule [23]. So, the adsorption of positively charged PEI on the surface of the powder is increased. Therefore, as stated earlier, the higher the charge density on particles is, the higher the zeta potential and repulsive force on particles would be [22]. Consequently, it eventually leads to stability of suspension. On the other hand, viscosity of suspension plays a significant role in the stability of suspension. The settling velocity of suspension could be explained by Stokes equation (2) for concentrated colloidal systems. Consider

where , , and are the density difference of particles and continuous phase, particles size, and continuous phase viscosity, respectively. is the phase volume percent which adversely affects settling velocity. Although higher solid phase volume percent decreases the velocity of sedimentation and increases viscosity, it causes a higher chance of coagulation of particles. Therefore, a proper value of solid content is necessary to be estimated in a colloidal system.

Rheological behavior of slurries including different amounts of PEI is showed in Figure 5(a). The viscosity of the concentrated slurry increased with the addition of PEI amount up to 1.5 wt.% which was due to very high charge absorbed by PEI molecules on particles surface. Thus, high charge density helps slow down sedimentation of suspension. According to the rheological behavior and viscosity of slurries (Figure 5(b)), as well as zeta potential of slurries, it seems slurries with 1 and 1.5 wt.% PEI would have appropriate stability and viscosity depending on coating process. To fabricate composite based coating by plasma spray, slip casting, and tape casting methods [17–19], the stable slurry with low viscosity is suitable. On the other hand, the stable slurry with absolute proper viscosity (almost in the range of 0.8–1.0 Pa·s) is definitely necessary to fabricate the coat by dip coating.

(a)

(b)

Figure 5: Effect of dispersant content on (a) rheological behavior of 45 vol.% ZSB slurries and (b) the viscosity at 96.2 s⁻¹ (pH = 7.8).

Previous researches [17–19, 25, 26] show that the optimum slurry which was used for coating includes 40–45 vol.% solid content and different amounts of 1–1.5 wt.% of various dispersants (e.g., poly(acrylamine), Dolapix, and Duramax). It is obviously supposed that increasing the solid content of suspension leads to high homogeneity and compaction and subsequently low porosities of coat. Accordingly, the effect of solid content has been studied to determine proper suspension to fabricate ZSB composite based coating. The changes of zeta potential of slurries with various amounts of solid content are depicted in Figure 6.

Figure 6: Zeta potential of ZSB slurries with 1.5 wt.% PEI and different amounts of solid content.

According to the zeta potential and viscosity of slurries with different amounts of PEI, 1.5 wt.% PEI was a proper amount to prepare the stable slurry for dip coating. Hence, solid content of slurries with 1.5 wt.% PEI was changed to determine the proper slurry for subsequent step, fabrication of ZSB based coating by dipping method. By increasing the solid content from 45 to 52.5 vol.%, the zeta potential of slurries was in stable area; however, increasing to 55 vol.% caused slight diminution of zeta potential falling into the unstable state.

Figure 7 depicts the rheological behavior of ZSB slurries. It indicates that increasing the solid content of slurry, up to 47.5 vol.%, leads to the high viscosity; therefore, slurry with solid content higher than 47.5 vol.% is not suitable for coating step by dip coating method. On the other hand, the high viscose suspension is not suitable for coating by slip casting or tape casting. Table 2 shows the viscosity of slurries. As it seems, the viscosity of slurries with 46 and 47.5 vol.% solid content and 1.5 wt.% PEI is very high. The rheological test was used to estimate the viscosity of slurry including 46 vol.% solid content and 1 wt.% PEI to improve solid content from 45 to 46 vol.%, but unfortunately the viscosity was high again. Indeed, increasing the solid content is very critical since it enhances the chance of the particles agglomeration and consequently increases the particle size. According to (1) and (2), increasing the particle size leads to increasing the gravitation forces and also higher settling velocity. Although increasing in (2) causes lower settling velocity () and higher viscosity (), its effect on agglomeration and increasing particle size is more dominant than that on and . Therefore, the slurry with optimal condition was obtained including 45 vol.% solid content along with 1 and 1.5 wt.% PEI to be used as composite based coating by slip or tape casting and dip coating method.

Table 2: Specification of slurries.

Solid content/vol.%
PEI/wt.%
Viscosity/Pa·s (at 96.2 s⁻¹)

45
1.5
0.82

45
1
0.53

46
1.5
1.64

46
1
1.84

47.5
1.5
1.82

Figure 7: Rheological behavior of ZSB slurries including different amounts of PEI and solid content (pH = 7.8).

4. Conclusion

ZSB composite aqueous suspension was prepared at pH 7.8, adjacent to the neutral state, using PEI as a dispersant. Different values of PEI (0.3–1.5 wt.%) and different amounts of solid content (45–55 vol.%) were used to investigate their effect on stability of aqueous suspensions. A slight increase occurred in the zeta potential while PEI amount increased from 0.3 to 1.5 wt.%. Increasing the solid content from 45 to 52.5 vol.% led to being the zeta potential of slurries in stable state (); however, increasing till 55 vol.% caused the sudden decrease to the unstable state. Viscosity of slurry would be low or in a proper amount depending on coating method. Incorporating zeta potential and viscosity showed that the stable composite suspension including 45 vol.% solid content as well as 1 and 1.5 wt.% PEI is an appropriate slurry to produce ZSB composite coat by slip and tape casting and dip coating methods, respectively.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

References

S. R. Levine, E. J. Opila, M. C. Halbig, J. D. Kiser, M. Singh, and J. A. Salem, “Evaluation of ultra-high temperature ceramics for aeropropulsion use,” Journal of the European Ceramic Society, vol. 22, no. 14-15, pp. 2757–2767, 2002.

M. M. Opeka, I. G. Talmy, and J. A. Zaykoski, “Oxidation-based materials selection for 2000°C + hypersonic aerosurfaces: Theoretical considerations and historical experience,” Journal of Materials Science, vol. 39, no. 19, pp. 5887–5904, 2004.

D. M. van Wie, D. G. Drewry Jr., D. E. King, and C. M. Hudson, “The hypersonic environment: required operating conditions and design challenges,” Journal of Materials Science, vol. 39, no. 19, pp. 5915–5924, 2004.

S. Kumar, “Self-propagating high temperature synthesis of refractory nitrides, carbides and borides,” Key Engineering Materials, vol. 56, pp. 183–188, 1991.

K. Upadhya, J.-M. Yang, and W. P. Hoffman, “Materials for ultrahigh temperature structural applications,” The American Ceramic Society Bulletin, vol. 76, no. 12, pp. 51–56, 1997.

M. M. Opeka, I. G. Talmy, E. J. Wuchina, J. A. Zaykoski, and S. J. Causey, “Mechanical, thermal, and oxidation properties of refractory hafnium and zirconium compounds,” Journal of the European Ceramic Society, vol. 19, no. 13-14, pp. 2405–2414, 1999.

Y. Yan, H. Zhang, Z. Huang, J. Liu, and D. Jiang, “In situ synthesis of ultrafine ZrB2-SiC composite powders and the pressureless sintering behaviors,” Journal of the American Ceramic Society, vol. 91, no. 4, pp. 1372–1376, 2008.

S.-H. Lee, Y. Sakka, and Y. Kagawa, “Dispersion behavior of ZrB2 powder in aqueous solution,” Journal of the American Ceramic Society, vol. 90, no. 11, pp. 3455–3459, 2007.

H. Zhang, Y. J. Yan, Z. Huang, X. Liu, and D. Jiang, “Pressureless sintering of ZrB₂-SiC ceramics incorporating sol-gel synthesized ultra-fine ceramic powders,” Key Engineering Materials, vol. 434-435, pp. 193–196, 2010.

D. W. Ni, G. J. Zhang, Y. M. Kan, and Y. Sakka, “Highly textured ZrB2-based ultrahigh temperature ceramics via strong magnetic field alignment,” Scripta Materialia, vol. 60, no. 8, pp. 615–618, 2009.

F. Monteverde and A. Bellosi, “Development and characterization of metal-diboride-based composites toughened with ultra-fine SiC particulates,” Solid State Sciences, vol. 7, no. 5, pp. 622–630, 2005.

Y. Yan, Z. Huang, S. Dong, and D. Jiang, “Pressureless sintering of high-density ZrB2-SiC ceramic composites,” Journal of the American Ceramic Society, vol. 89, no. 11, pp. 3589–3592, 2006.

S. Zhu, W. G. Fahrenholtz, and G. E. Hilmas, “Influence of silicon carbide particle size on the microstructure and mechanical properties of zirconium diboride-silicon carbide ceramics,” Journal of the European Ceramic Society, vol. 27, no. 4, pp. 2077–2083, 2007.

S. S. Hwang, A. L. Vasiliev, and N. P. Padture, “Improved processing and oxidation-resistance of ZrB2 ultra-high temperature ceramics containing SiC nanodispersoids,” Materials Science and Engineering A, vol. 464, no. 1-2, pp. 216–224, 2007.

J. Han, P. Hu, X. Zhang, and S. Meng, “Oxidation behavior of zirconium diboride-silicon carbide at 1800°C,” Scripta Materialia, vol. 57, no. 9, pp. 825–828, 2007.

F. Monteverde, “Beneficial effects of an ultra-fine α-SiC incorporation on the sinterability and mechanical properties of ZrB2,” Applied Physics A: Materials Science and Processing, vol. 82, no. 2, pp. 329–337, 2006.

Z. Lü, D. Jiang, J. Zhang, and Q. Lin, “Processing and properties of ZrB₂-SiC composites obtained by aqueous tape casting and hot pressing,” Ceramics International, vol. 37, no. 1, pp. 293–301, 2011.

X. G. Wang, J. X. Liu, Y. M. Kan, G. J. Zhang, and P.-L. Wang, “Slip casting and pressureless sintering of ZrB2-SiC ceramics,” Journal of Inorganic Materials, vol. 24, no. 4, pp. 831–835, 2009.

F. F. Lange, “Powder processing science and technology for increased reliability,” Journal of the American Ceramic Society, vol. 72, no. 1, pp. 3–15, 1989.

J. N. Israelachvili, Intermolecular and Surface Forces, Academic Press, New York, NY, USA, 2009.

E. J. W. Verwey and J. T. G. Overbeek, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, The Netherlands, 1948.

R. J. Hunter, Zeta Potential in Colloid Science: Principles and Applications, Academic Press, London, UK, 1988.

X. Zhu, F. Tang, T. S. Suzuki, and Y. Sakka, “Role of the initial degree of ionization of polyethylenimine in the dispersion of silicon carbide nanoparticles,” Journal of the American Ceramic Society, vol. 86, no. 1, pp. 189–191, 2003.

J. Wang and L. Gao, “Surface properties of polymer adsorbed zirconia nanoparticles,” Journal of Nanostructured Materials, vol. 11, no. 4, pp. 451–457, 1999.

Z. Lü, D. Jiang, J. Zhang, and Q. Lin, “Aqueous tape casting of zirconium diboride,” Journal of the American Ceramic Society, vol. 92, no. 10, pp. 2212–2217, 2009.

Z. Lü, D. Jiang, J. Zhang, and Q. Lin, “Microstructure and mechanical properties of zirconium diboride obtained by aqueous tape casting process and hot pressing,” Journal of the American Ceramic Society, vol. 93, no. 12, pp. 4153–4157, 2010.