Trans. Nonferrous Met. Soc. China 22(2012) 111-116

Effect of MoSi₂ content on dielectric and mechanical properties of MoSi₂/Al₂O₃ composite coatings

WU Zhi-hong, ZHOU Wan-cheng, LUO Fa, ZHU Dong-mei

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China

Received 27 December 2010; accepted 19 September 2011

Abstract: Molybdenum disilicide (MoSi₂) sheath and aluminum oxide (Al₂O₃) core blended powders were fabricated by spray drying. A derived coating material was produced for the application as microwave absorbers using the as prepared powders by atmospheric plasma spray (APS) technology. The effects of MoSi₂/Al₂O₃ mass ratio on the dielectric and physical mechanical properties of the composite coatings were investigated. When the MoSi₂ content of the composites increases from 0 to 45%, the flexure strength and fracture toughness improve from 198 to 324 MPa and 3.05 to 4.82 MPa·m^1/2 then decline to 310 MPa and 4.67 MPa·m^1/2, respectively. The dielectric loss tangent increases with increasing MoSi₂ content, and the real part of permittivity decreases conversely over the frequency range of 8.2-12.4 GHz. These effects are due to the agglomeration of early molten MoSi₂ particles and the increase of the electrical conductivity with increasing MoSi₂ content.

Key words: MoSi₂/Al₂O₃ composite coating; atmospheric plasma spraying; mechanical properties; dielectric properties

1 Introduction

Radar absorbing materials (RAMs) are widely used in commercial and military applications. RAMs are made of compounds with high loss energy, which enables them to absorb the incident radiation in synchronized frequencies and dissipate it as heat [1]. At present, the most cost effective means of shielding radar radiation, controlling electromagnetic interference and dissipating electrostatic charge is to use magnetic or dielectric fillers [2] or intrinsically conducting polymers [3, 4]. Composite materials with different types of ferromagnetic or ferroelectric fillers have attracted much attention. MoSi₂ has potential applications varying from matrix material to reinforcing second phase for various high temperature structural applications due to its high modulus of elasticity, high melting point (2030 °C), superior oxidation resistance [5, 6]{Wu, 2011 #2675}, relatively low density (6.31 g/cm³) , reasonably good electrical and thermal conductance (52 W/(mK)) [7, 8]. On the other hand, Al₂O₃ also has good oxidation and corrosion resistance because of its α-Al₂O₃ main crystalline phase, high melting point (2050 °C), and low density (3.7 g/cm³). As the typical representative of low dielectric permittivity microwave dielectric ceramic, Al₂O₃ has perfect dielectric permittivity property, especially its low dielectric loss. ALFORD and PENN [9] and HUANG et al [10] reported the dielectric properties of the single phase Al₂O₃. When conductive inter-metallic compound MoSi₂ particles are introduced into Al₂O₃ matrix, not only the mechanical properties of Al₂O₃ composites can be enhanced, but also the dielectric loss and dielectric permittivity of the composites can be modified. Consequently, it is possible to provide the MoSi₂/Al₂O₃ composites with electromagnetic attenuation, but little attention is paid to its microwave absorbing properties. Because coating is the general form of RAM, efforts were focused on the preparation and dielectric properties of the MoSi₂/Al₂O₃ coatings by APS in this work. The complex permittivity, physical mechanical properties and possible mechanisms of the sprayed coatings were reported and discussed.

2 Experimental

Raw powders with different MoSi₂/ Al₂O₃ ratios (alumina produced by Hengjitianli Chemical Industry Co., Ltd. of Zibo，Shandong, China, Al₂O₃ ≥99.99%, average particle size of 40 μm; molybdenum disilicide produced by Dengfeng Electric Heating Industry Co., Ltd. of Henan, China, average particle size of 0.5 μm) were mixed in water with the addition of the organic binder system (poly vinyl alcohol, poly ethylene glycol, surfactant: Zschimmer & Schwartz Dolpapix CE64). The alumina mass fractions in these samples were 90%, 85%, 75%, 65% and 55%, marked as M1, M2, M3, M4 and M5, respectively.

After it was spray-dried, the composite powders of Al₂O₃ and MoSi₂ were obtained. Granularity analysis shows that the diameter of the composite powders is 45-75 μm. SEM photographs indicate that the dried powders have a MoSi₂-rich sheath and Al₂O₃-rich core. Figure 1 shows the typical microstructures of the composites with 15% (mass fraction) MoSi₂ after spray drying. Table 1 summarizes the compositions of slurries for spray drying.

Air plasma spray depositions of the processed powders were made on rough-surfaced graphite sheet substrates in order to easily get free-standing coatings. Plasma spraying was done with a non-transferred arc plasma torch operated at different power levels. The powders were fed at 16 g/min using argon as carrier gas. The plasma spraying parameters are listed in Table 2.

The density, microstructure, mechanical and dielectric properties were determined all on the free-standing coatings. The densities were measured by Archimedes’ method. The immersing liquid for density measurements was water, relative densities were calculated based on the rule of mixture, the bulk densities of Al₂O₃ and MoSi₂ were 4.0 and 6.2 g/cm³, respectively. The flexure strength was measured with the three-point bending method. The fracture toughness was calculated with the indentation strength method.

Fig. 1 SEM images of composite powders M2 after spray drying (a) and typical partied (b)

Table 1 Physical parameters of raw materials

Table 2 Operating parameters of plasma spraying

Surface morphology was carried out by scanning electron microscopy (JSM-5610LV). The phase compositions were analyzed by full-automatic X-ray diffractometer (XRD). The dielectric parameters were carried out in the frequency range of 8.2-12.4 GHz by a network analyzer (Agilent Technology E8362B), which requires specimens are in dimensions of 10.16 mm× 22.86 mm×2 mm.

3 Results and discussion

3.1 Physical mechanical properties

The density and flexure strength of the free-standing coatings with different MoSi₂ contents are shown in Fig 2. For comparison, samples with no MoSi₂ are also detected.

Figure 2 indicates that the relative densities of the MoSi₂/Al₂O₃ coatings decrease dramatically compared with the pure Al₂O₃ coating, which means that the addition of MoSi₂ into Al₂O₃ is detrimental to the sintering of the coatings. But with further increasing MoSi₂ content, the relative density of the MoSi₂/Al₂O₃ coatings increases gradually. It can be ascribed to the lower melting point and the higher thermal coefficient of MoSi₂ than those of Al₂O₃, which improves the molten state of the dried grains during flight dwell and enhances the densities of the coatings.

Fig. 2 Mechanical properties of MoSi₂/Al₂O₃ composition

The flexure strength and fracture toughness increase with the increase of MoSi₂ when the content is less than 35%. The reasons of the flexure strength variation with MoSi₂ content are summarized as follows. MoSi₂ particles can impede the growth of Al₂O₃ grains. High density of the coatings results from better melting state of sprayed particles; the grain-boundary energy and interfacial energy can be improved obviously by adding MoSi₂, and as the content increases, the effect becomes more significant [11]. The fracture toughness increase with MoSi₂ content is resulted from finer Al₂O₃ grains and microcracks between Al₂O₃ and MoSi₂ grains due to their different thermal expansion coefficients [12]. When the MoSi₂ content reaches 45%, the flexure strength and fracture toughness all decline, because the effect of Al₂O₃ matrix is weakened.

3.2 Phase of composite coatings

The XRD patterns of the free-standing coatings are shown in Fig. 3.

Fig. 3 XRD patterns of MoSi₂/Al₂O₃ composite with different MoSi₂ contents

It is seen that the dominating phases are MoSi₂ and Al₂O₃. In addition, new AlN phase is found. This is because nitrogen (N₂) is used as shielding gas and reaction gas during plasma spraying process. It reacts with Al₂O₃ on the surface of graphite substrate to form AlN. The possible reaction to produce AlN is [13, 14]

=

In this case, it can be found that AlN is produced by Al₂O₃, C (from the graphite substrate) and N₂ (spraying atmosphere). Considering the small amount of AlN, the effect of this third phase on the dielectric performance of the composite studied in this work should be negligible.

3.3 Microstructure of composite coating

Figure 4 shows the typical microstructures of the composites with 10%, 15% and 25% (mass fraction) MoSi₂ after polishing the surface of the samples.

Fig. 4 SEM images of coatings after APS with different MoSi₂ contents: (a) M1; (b) M2; (c) M3

From Fig. 4, it is apparent that the composites contain some black matrix materials and some speckled black phases. The XPS X-ray photoelectron spectroscopy result indicates that the black phase contains 65.3% Si (molar fraction) and 32.5% Mo. Hence, it is molybdenum disilicide. It can be found from Fig. 4 that MoSi₂ particles are homogeneously distributed in the Al₂O₃ matrix and along with the increase of the MoSi₂ content, this distribution is more apparent. But at the same time, there are some conglobation and adhesion of MoSi₂ particles, which is mainly due to two aspects. On one hand, some of the MoSi₂ particles on the surface of Al₂O₃ particles can congregate during spray drying granulation. On the other hand, in the plasma spray process, the MoSi₂ particles melt more completely than Al₂O₃ particles. The molten MoSi₂ is inclined to form partial clustering by capillary forces.

3.4 Dielectric properties

The real parts of permittivity (ε′) and dielectric loss tangent properties (tan δ) of the MoSi₂/Al₂O₃ coatings sprayed using different amounts of MoSi₂ powders are shown in Figs. 5(a) and (b), respectively. From Fig. 5(a), it can be seen that the ε′ of the MoSi₂/Al₂O₃ composites decreases with the increase of MoSi₂ content across the whole frequency range. The ε′ presents an obvious decrease from 8-11.3 to 5-8.4 in the tested frequency range when MoSi₂ content increases from 10% to 45%. With the same MoSi₂ content, ε′ also slightly decreases with increasing frequency which indicates that the composite material has some frequency dispersion. It is generally known that ε′ reflects the capability of store charge of materials. According to the principle of equivalent circuit, the real parts of MoSi₂/Al₂O₃ permittivity vary directly with the surface area of MoSi₂ particles. As mentioned before, increased MoSi₂ content might promote the clustering of MoSi₂ particles in the MoSi₂/Al₂O₃ composites (as shown in Fig. 4), expanding the MoSi₂ particles surface. When the content of MoSi₂ increases from 10% to 45%, the specific surface area decreases, the real parts of permittivity descend. From Fig. 5(b), it can be seen that the value of loss factor in MoSi₂/Al₂O₃ composites increases with increasing MoSi₂. This is because the dielectric loss of MoSi₂/Al₂O₃ composite relates to its electrical conductivity. As known, MoSi₂ has high electrical conductivity. When MoSi₂ particles are dispersed in Al₂O₃ matrix, a MoSi₂ conductive network can be formed [15]. When the interparticle distance is shorter than the gap width that quantum tunneling effect permits, conductive networks form. For composites blending with dispersed spherical particles, MARGOLINA and WU [16] assumed a simple cubic lattice to calculate the surface-to-surface interparticle distance as:

                           (1)

where δ is the average surface-to-surface interparticle distance; D is the particle diameter and φ is the volume fraction of particles. Equation (1) shows that the δ of the MoSi₂ particles is dependent on the volume fraction as the particle diameter is constant. This can be observed in the SEM images (Fig. 4) of the composites with different MoSi₂ contents. The distances between the MoSi₂ particles are shortened by increasing MoSi₂ content. At the same time, the MoSi₂ agglomerations also increase due to the increasing possibility of the contact between MoSi₂ particles as the proportion of MoSi₂ increases. The schematic of MoSi₂/Al₂O₃ composites with different MoSi₂ contents is shown in Fig. 6. The average surface-to-surface interparticle distance of filled particles in high MoSi₂ content composites is relatively short. This means that the high MoSi₂ content is more likely to reach the critical surface-to-surface interparticle distance (i.e. the electron hopping gap width) based on Eq. (1). The MoSi₂ particles in high MoSi₂ content composites are more prone to form conductive networks. Therefore, with increasing MoSi₂ content, the electrical conductivity and tan δ both increase.

Fig. 5 Complex permittivity (a) and dielectric loss tangent (b) vs frequency of different MoSi₂ composite coatings

The real permittivity and dielectric loss tangent value show an obvious fluctuation between 10 and   10.5 GHz among the lines λ₁ and λ₂, as shown in Fig. 5. This phenomenon can be interpreted as the correlated relationship between the real permittivity (polarization) and imaginary permittivity (electric loss), i.e., energy storage (capacitor) ? energy dissipation (resistor). Similar results were reported in the other filled composites [17, 18].

Fig. 6 Schematic diagrams of different MoSi₂/Al₂O₃ composites: (a) Low content of MoSi₂; (b) High content of MOSi₂

4 Conclusions

1) The spray drying method can effectively let MoSi₂ cover the surface of the Al₂O₃ core particles and isolate its contact with each other.

2) The MoSi₂ particles are homogeneously distributed in the Al₂O₃ matrix, but the appearance of some conglobation occurs with increasing MoSi₂ contents. These are due to the congregate of MoSi₂ particles on the surface of Al₂O₃ core particles during spray drying granulation and partial clustering by capillary forces during APS.

3) The decrease of the real parts (ε′) of permittivity is mainly caused by the agglomeration of early molten MoSi₂ particles, and the increase of the dielectric loss tangent properties (tan δ) of the MoSi₂/Al₂O₃ coatings is due to the increase of the electrical conductivity with increasing MoSi₂ content.

4) The mechanical properties can be tailored by choosing a suitable MoSi₂ content. MoSi₂/Al₂O₃ composite is a promising material in both the electro- magnetic absorption characteristics and the load-bearing capacity aspects.

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MoSi₂含量对MoSi₂/Al₂O₃复合涂层介电与力学性能的影响

武志红，周万城，罗 发，朱冬梅

西北工业大学 凝固技术国家重点实验室，西安 710072

摘  要：通过喷雾干燥法制备MoSi₂包覆Al₂O₃的壳核结构混合粉，利用该混合粉以等离子喷涂技术制备MoSi₂/Al₂O₃复合涂层材料。研究MoSi₂/Al₂O₃质量比涂层材料的力学和介电性能的影响。结果表明：随着MoSi₂含量从0增加到45%，复合材料的抗弯强度和断裂韧性分别从198 MPa和3.05 MPa·m^1/2增加到324 MPa和4.82 MPa·m^1/2，随后又降到310 MPa和4.67 MPa·m^1/2。在8.2-12.4 GHz微波频率波段内，随着MoSi₂含量的增加，复合材料的介电损耗增加，而介电常数的实部却呈减小趋势。这主要是由于MoSi₂颗粒熔化后的凝聚及导电网络结构的形成导致电导率的增加引起的。

关键词：MoSi₂/Al₂O₃复合涂层；大气等离子喷涂；物理力学性能；介电性能

(Edited by FANG Jing-hua)

Foundation item: Project (50572090) supported by the National Natural Science Foundation of China; Project (KP200901) supported by the States Key Laboratory of Solidification Processing in NWPU, China

Corresponding author: WU Zhi-hong; Tel: +86-29-82202789, E-mail: wuzhihong@msn.com

DOI: 10.1016/S1003-6326(11)61148-5