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Research Papers

# Correlation of Thermal Conduction Properties With Mechanical Deformation Characteristics of a Set of $SiC–Si3N4$ Nanocomposites

[+] Author and Article Information
Vikas Tomar1

School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47906tomar@purdue.edu

Vikas Samvedi

School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47906

1

Corresponding author.

J. Eng. Mater. Technol 133(1), 011013 (Dec 02, 2010) (7 pages) doi:10.1115/1.4002646 History: Received March 05, 2010; Revised June 23, 2010; Published December 02, 2010; Online December 02, 2010

## Abstract

New developments in high temperature ceramic materials technology have focused on obtaining nanocomposite materials with nanoscale features for an optimal control of thermal and mechanical properties. One example is the silicon carbide (SiC)–silicon nitride $(Si3N4)$ nanocomposites with nanosized SiC particles placed either in microsized $Si3N4$ grains or along $Si3N4$ grain boundaries (GBs). This work focuses on analyzing the influence of GBs, interfaces, and impurities on thermal and mechanical properties of a set of $SiC–Si3N4$ nanocomposites at three different temperatures (300 K, 900 K, and 1500 K). Nanocomposite thermal conductivity values predicted in this study are smaller in comparison to the bulk $Si3N4$ values $(∼30 W/m K)$. Even with the volume fraction of SiC phase being limited to maximum 40%, it is shown that the thermal conductivity values could be reduced to less than those of the bulk SiC phase $(∼3 W/m K)$ by microstructural feature arrangement. Nanocomposite phonon spectral density values show a short rage structural order indicating a high degree of diffused phonon reflection. Visual analyses of the atomistic arrangements did not reveal any loss of crystallinity in the nanocomposites at high temperatures. This indicates that structural arrangement, not the phase change, is a factor controlling thermal conduction as a function of temperature. The nanocomposite deformation mechanism is a trade-off between the stress concentration caused by SiC particles and $Si3N4–Si3N4$ GB sliding. The temperature increase tends to work in favor of GB sliding leading to softening of structures. However, microstructural strength increases with increase in temperature when GBs are absent. GBs also contribute to reduction in thermal conductivity as well as increase in fracture strength. Replacement of sharp GBs by diffused GBs having C/N impurities, lowered thermal conductivity, and increased fracture strength. Decrease in $SiC–Si3N4$ interfaces by removal of SiC particles tends to favor an increase in thermal conductivity as well as fracture resistance. Overall, it is shown that for high temperature mechanical strength improvement, judicious placement of SiC particles and optimal control of GB atomic volume fraction are the main controlling factors.

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## Figures

Figure 1

Set of Si–C–N atomistic microstructures analyzed

Figure 2

(a) The thermal conductivity calculation setup with a plot showing convergence in thermal conductivity value for microstructure S1 with time, (b) a comparison of thermal conductivity of microstructures S1, S2, and S4 at 300 K, 900 K and 1500 K, and (c) a comparison of thermal conductivity of microstructures S1, S12, S2, S22, S4, and S42 at 300 K

Figure 3

Phonon density of states of (a) Si–Si (in Si3N4) bonds in microstructure S1 and S2 at 300 K, (b) Si–Si (in Si3N4) bonds (normalized) and combined C–C, N–N, Si–Si (in SiC) bonds in microstructures S1, S2, and S4 at 300 K, and (c) Si–Si (in Si3N4) bonds (normalized) and combined C–C, N–N, Si–Si (in SiC) bonds in microstructures S12, S22, and S42 at 300 K

Figure 4

A comparison of the phonon density of states of combined C–C, N–N, and Si–Si (in SiC) bonds (a) in microstructures S1 and (b) in microstructure S2 at 300 K, 900 K, and 1500 K

Figure 5

A comparison of virial stress in loading direction as a function of true strain at (a) 300 K and (b) 1500 K for microstructures shown in Fig. 1

Figure 6

A comparison of deformation mechanism in microstructures (a) S1 and (b) S4 at 300 K and 1500 K

Figure 7

A comparison of deformation mechanism in microstructures (a) S12 and (b) S42 at 300 K

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