­­ Microstructure and thermal conductivity of  submicron Si₃N₄ reinforced 2024Al composite

YANG Wen-shu(杨文澍), XIU Zi-yang(修子扬), CHEN Guo-qin(陈国钦), WU Gao-hui(武高辉)

1. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;

Received 10 June 2009; accepted 15 August 2009

Abstract: An 2024Al matrix composite reinforced with 36%(volume fraction) β-Si₃N₄ particles was fabricated by pressure infiltration method, and its microstructure and the effect of annealing treatment on thermo-conductivity were discussed. Si₃N₄ particles distribute uniformly without any particle clustering and no apparent particle porosity or significant casting defects are observed in the composites. The combination of particles and matrix is well. The raw Si₃N₄ particles are regular cylindrical polyhedron with flat surface and change to serrated surface in composite due to reactions during fabrication. Thermal conductivity of as-cast Si₃N_4p/2024 composite is 90.125 W/(m?K) at room temperature, and increases to 94.997 W/(m?K) after annealing treatment. The calculated results of thermal conductivity of the Si₃N_4p/Al composite by Maxwell model, H-S model and PG model are lower than experimental results while that by ROM model is higher.

Key words: composite; interface; thermal conductivity; calculation; submicron particles

1 Introduction

Silicon nitride (Si₃N₄) has a good thermal and chemical stability, high mechanical strength and hardness, and good wear, creep, and corrosion resistance. It attracts research community[1-3]. The Si₃N₄ particles reinforced aluminium matrix composites (Si₃N_4p/Al) present several advantages that are high specific strength, low thermal expansion coefficient, high thermal conductivity, high intensity and good dimensional stability, which make them very attractive for applications in aerospace, automotive, electronics, machinery manufacture industry [4-6].

However, the major obstacle for fabricating high performance Si₃N_4p/Al composite is the poor wettability of Si₃N₄ particles to liquid aluminium[7]. Fortunately, pressure infiltration method is an effective technique to fabricate poor-wetting system via forcing infiltration of molten metal alloy under high pressure, and it has the advantages of simple fabrication technology, high density and no restrictions on alloy[8]. In this work, 2024Al matrix composite reinforced with 36%(volume fraction) β-Si₃N₄ particles (about 0.2 ?m) was fabricated

by pressure infiltration method, the microstructure characteristics of Si₃N_4p/2024 composite and the interactions between Si₃N₄ and aluminium were studied, the effect of annealing treatment on thermal conductivity was discussed.

2 Experimental

The β-Si₃N₄ particles with an average particle size of 0.2 ?m, as shown in Fig.1, were used to reinforce 2024Al alloy by pressure infiltration method, and the chemical compositions of the alloy are Cu 4.79%, Mg 1.49%, Mn 0.611%, Fe 0.245%, Si 0.168%, Zn 0.068%, Cr 0.049%, Ti 0.046%, Ni 0.013% (mass fraction) and Al balance.

The microstructure analysis was conducted by Philips CM-12 and JEOL 200CX transmission electron microscope (TEM) with an accelerated voltage of 100- 120 kV and 200 kV, respectively. The thermal conductivity and specific heat capacity of composite and matrix with diameter of 12.7 mm and length of 3 mm were examined on a laser thermal conduction analyzer (JK2, Germany) from 25 to 500 ℃ with a rate of 5 ℃/min.

Fig.1 SEM image of Si₃N₄ particles

3 Results and discussion

It is well established that a dense microstructure is beneficial to thermal conductivity. The optical microstructure of the as-cast Si₃N_4p/2024 composite is shown in Fig.2. It can been seen from Fig.2 that Si₃N₄ particles distribute uniformly without any particle clustering and no apparent porosity or significant casting defects are observed in the composites.

Fig.2 Optical microstructure of as-cast Si₃N_4p/2024 composite

TEM images of Si₃N₄ particles is shown in Fig.3. It can been seen from Fig.3 that the Si₃N₄ particles in matrix alloy distribute uniformly. The distance between particles is very small due to high volume fraction, and the combination of particles and matrix is good and no separation between Al and Si₃N₄ particles is observed. Generally, high density of dislocations is always found in aluminium composites reinforced with micron particles because of large thermal mismatch stress generated by coefficient of thermal expansion (CTE) difference between the ceramic particles and matrix [9-10]. However, it is difficult to observe the dislocation in Al matrix reinforced with submicron Si₃N₄ particles although the CTE difference of them is also extreme. It accords to other researches in submicron particles reinforced composite[11-12].

It should be noted that the raw Si₃N₄ particles (see Fig.1) are regular cylindrical polyhedron with flat surface.

Fig.3 TEM image of Si₃N₄ particles

However, its surface changes to be serrated after composite fabrication (see Fig.4) and all reactions are as follows[13]:

Si₃N₄(s)+O₂(g)→3SiO₂(s)+2N₂(g)               (1)

3SiO₂(s)+4Al(l)→2Al₂O₃(s)+3Si(l)              (2)

SiO₂(s)+2Mg(l)→2MgO(s)+Si(l)                (3)

2SiO₂(s)+Mg(l)+2Al(l)→MgAl₂O₄(s)+2Si(l)     (4)

4Al(l)+Si₃N₄(s)→4AlN(s)+3Si(l)               (5)

2Mg(l)+Si(l)→Mg₂Si(s)                       (6)

The above reactions are all possible in thermodynamics and the thermodynamic stability or

Fig.4 TEM image of Si₃N₄ particle and interface (a) and electron diffraction pattern of Si₃N₄ particle (b)

kinetics needs to be in consideration for confirming the actual reaction. Unfortunately, phonons movement will be impeded by the interface and the interfacial reactant, which eventually performs negative effect on thermal conduction.

Thermal conductivity λ is calculated by following equation:

λ=α?C_p?ρ                                   (7)

where  α, ρ and c_p are referred to thermal diffusivity, density and specific heat capacity, respectively. The density of Si₃N_4p/2024 composite is 2.9 g/cm³, and test results of as-cast and as-annealing are listed in Table 1.

Table 1 Thermal conduction testing results of Si₃N_4p/2024 composite

There are two ways for heat transfer in Si₃N_4p/2024 composite, free electron in Al matrix and phonon in Si₃N₄ particles. Both movements would be scattered by interface. Therefore, heat conduction in Si₃N_4p/2024 composite depended on Al matrix, Si₃N₄ particles and their interface. The annealing treatment would release residual thermal stress in Al matrix generated in fabrication, which is beneficial to heat conduction, and then improve the thermal conductivity of Si₃N_4p/2024 composite.

One significant advantage of composites is their designable. The accurate prediction of composite performance before fabrication or control of component to meet performance requirement, which would shorten the design and fabrication cycle of composites, is attracting all researchers on composite. However, accurate prediction must be based on the accurate theoretical foundation. The main theoretical prediction models for the thermal conductivity of composite include Maxwell model, H-S model and PG model.

1) Maxwell considers the effect of matrix conduction, particle conduction and particles fraction while particle is simplified as spherical particle, and the following equation can be obtained:

           (8)

where κ_com, κ_m, κ_p are referred to thermal conductivities of composite, matrix and particle, respectively; φ_p is volume fraction of particle.

2) Based on ESHELBY’s equivalent inclusion theory[14], HATTA and TAYA[15] consider the effect of particle shape and obtained the general expression of thermal conductivity, which can be expressed as follows for particle reinforced composite:

          (9)

3) KLEMENS[16] derived the relationship among thermal conductivity of composites, thermal conductivity of matrix, thermal conductivity and volume fraction particle of as follows:

          (10)

The models mentioned above were adopted to calculate thermal conductivity of Si₃N_4p/2024 composite and the theoretical values and experimental value are listed in Table 2.

Table 2 Theoretical and experimental thermal conductivities of as-annealed Si₃N_4p/2024 (W?m^-1?K^-1)

It is obvious that the calculated values are different from values by models, and the values calculated by Maxwell model, H-S model and PG model are lower than experimental results while the value by ROM model is higher.

4 Conclusions

1) Si₃N₄ particles distribute uniformly without any particle clustering and no apparent porosity or significant casting defects are observed in the composites. The distance between particles is very small due to its high volume fraction and dispersive distribution, and the combination of particles and matrix is good.

2) The raw Si₃N₄ particles are regular cylindrical polyhedron with flat surface and its surface changes to be serrated due to reactions during composite fabric.

3) Thermal conductivity of casting Si₃N_4p/2024 composite is 90.125 W/(m?K) at room temperature, and after annealing treatment, it will improve to 94.997 W/  (m?K). The calculated values by thermal conductivity of Maxwell model, H-S model and PG model are lower than experimental result while value by ROM model is higher.

References

[1] HAN I S, SEO D W, KIM S Y, HONG K S, GUAHK K H, LEE K S. Properties of silicon nitride for aluminum melts prepared by nitrided pressureless sintering [J]. Journal of the European Ceramic Society, 2008, 28: 1057-1063.

[2] HUANG H, WINCHESTER K J, SUVOROVA A, LAWN B R, LIU Y, HU X Z. Effect of deposition conditions on mechanical properties of low-temperature PECVD silicon nitride films [J]. Mater Sci Engineering A, 2006, 435/436: 453-459.

[3] CARRASQUERO E, BELLOSI A, STAIA M H. Characterization and wear behavior of modified silicon nitride [J]. International Journal of Refractory Metals and Hard Materials, 2005, 23(4/6): 391-397.

[4] WANG Yang-wei, YU Xiao-dong, WANG Fu-chi, MA Zhang, KANG Xiao-peng. Mechanism of reaction infiltration of pressureless infiltrated Si₃N₄/Al composite [J]. Rare Metal Materials and Engineering, 2007, 36(s1): 777-780.

[5] WU Run, WANG Ya-hong. Study on material Al-Si₃N₄ infiltration technique [J]. Mining and Processing Equipment, 2007, 35(8): 122-124.

[6] WANG Yang-wei, WANG Fu-chi, YU Xiao-dong, LI Jun-tao. Study of interface reaction in the Si₃N₄/Al composite prepared by pressureless infiltration [J]. Journal of Aeronautical Materials, 2006, 26(1): 55-58. (in Chinese)

[7] de la PE?A J L, PECH-CANUL M I. Reactive wetting and spreading of Al-Si-Mg alloys on Si₃N₄/Si substrates [J]. Mater Sci and Eng A, 2008: 491(1/2): 461-469.

[8] WU Guo-hui, ZHANG Qiang, CHEN Guo-qin, JIANG Long-tao, XIU Zi-yong. Properties of high reinforcement-content aluminum matrix composite for electronic packages [J]. Journal of Materials Science Materials in Electronics, 2003, 14(1): 9-12.

[9] HONG S K, WON C W, SHIN D H. Interfacial microchemistry and microstructure of Al-Mg-Si alloy matrix composites reinforced with Al₂O₃ particulates [J]. Scripta Materialia, 1997, 36(8): 883-889.

[10] ZHANG Q, XIU Z Y, SONG M H, WU G H. Microstructure and properties of a 70vol% SiCp/Al-12Si composite for electronic packaging [J]. Materials Science Forum, 2005, 475/479: 881-884.

[11] YU Zhi-qiang, WU Gao-hui, JIANG Long-tao. Effect of surface modification of sub-micron Al₂O₃ particles by rare-earth on interfacial wettability of Al matrix composites [J]. The Chinese Journal of Nonferrous Metals, 2005, 15(7): 1087-1093. (in Chinese).

[12] JIANG Long-tao, WU Gao-hui, SUN Dong-li, ZHANG Qiang, NORIO K. Microstructural of sub-micron Al₂O₃ particles and interfacial characteristic of Al₂O₃p/1070Al composites [J]. The Chinese Journal of Nonferrous Metals, 2002, 12(2): 342-347. (in Chinese).

[13] WANG S R, WANG Y Z, WANG Y, GENG H R, CHI Q S. Microstructure and infiltration kinetics of Si₃N₄/Al-Mg composites fabricated by pressureless infiltration [J]. Journal of Materials Science, 2007, 42: 7812-7818.

[14] ESHELBY J D. The determination of the elastic field of an ellipsoidal inclusion, and related problems [J]. Proceedings of the Royal Society of London Series A, Mathematical and Physical Sciences. London: The Royal Society, 1957: 376-396.

[15] HATTA H, TAYA M. Effective thermal conductivity of a misoriented short fiber composite [J]. Journal of Applied Physics, 1985, 58(7): 2478-2486.

[16] KLEMENS P G. The thermal conductivity of dielectric solids at low temperatures (theoretical) [J]. Proceedings of the Royal Society of London Series A, Mathematical and Physical Sciences, 1951: 108-133.

(Edited by LI Yan-hong)

Foundation item: Project(2003AA305110) supported by the Hi-tech Research and Development Program of China

Corresponding author: XIU Zi-yang; Tel: +86-451-86402373-5056; E-mail:xiuzy@hit.edu.cn