Influence of Fe impurity in nitridation of Si + B 4 C green compact
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Influence of Fe impurity in nitridation of Si 1 B4 C green compact Dangrong R. Liu and Samuel Shinozaki Ford Research Laboratory, Ford Motor Company, 20000 Rotunda Drive, Dearborn, Michigan 48121
Motoyuki Miyata and Yoshiyuki Yasutomi Hitachi Research Laboratory, Hitachi Ltd., 1-1 Omika-Cho, 7-Chome, Hitachi-shi, Ibaraki-ken, 319-12 Japan (Received 6 December 1995; accepted 10 September 1997)
The addition of B4 C powder retarded the nitridation of the silicon powder green body by the formation of a borosilicate layer in interfaces between Si grains. The viscous layers hindered the SiO formation and the Si and N diffusion. Despite the presence of borosilicate layers, the Fe impurity in the green body still promoted the Si nitridation process by the formation of fluid iron silicide and the promotion of the B4 C conversion to BN in gaps or holes in the viscous borosilicate layers. The addition of 5% H2 in the N2 atmosphere accelerated the Si 1 B4 C nitridation, where the hydrogen acted as an oxygen getter, thus reducing the amount of glassy borosilicate in the interface.
I. INTRODUCTION
When silicon is nitrided, the degree of densification of the compact is a concern because of the general difficulty in the elimination of pores between grains. Yasutomi et al.15 reported that the application of TiC additive promoted densification. The graphitic carbon released from the TiC in its nitridation process had a large specific volume. When this carbon filled pores, the amount of pores was more effectively reduced than most other additives could effect. Along the same line of thought, B4 C was chosen as the nitridation/sintering aid for Si nitridation, with the expectation that B4 C should also be nitrided to produce BN and to release graphite. As reported previously, it was indeed the case that a reasonably good densification of the sintered body was obtained (Miyata and Yasutomi9 ). However, it was also found that, even with a long nitridation time, the process did not proceed to completion, as evidenced by the amount of residual Si in the sintered body. In this paper, we will report the effect of the B4 C additive on the Si nitridation and the influence of the Fe impurity on the Si 1 B4 C green body nitridation process, as we found out that these two played competing roles in the Si nitridation process. Finally, we will see that the addition of 5% H2 in the N2 atmosphere could further improve the nitridation of the Si 1 B4 C powder mixture as it had on the nitridation of other Si powder mixture systems. II. EXPERIMENTAL A. Preparation of samples
Two types of Si starting powder were used with one having average size of 3.7 mm and the other having an average size of 1.1 mm. The B4 C powder was of the J. Mater. Res., Vol. 13, No. 2, Feb 1998
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average size of 1.3 mm. As shown in Fig. 1, each of them has a wide particle size distribution. As received, the amount of oxygen was 0.49 wt. % in the Si powder and 2.62 wt. % in B4
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