While sintering silicon (SPS method, T = 1900oC) carbide (99.9% β-phase) with the addition of sintering aids, namely amorphous boron and graphite, one of the sinters demonstrated the fracture surface presented in Fig 1. Has anyone seen such a fracture surface of polycrystalline silicon carbide before? I must add that the same powder was used to sinter the sample again, when it presented a completely different, “standard,” fracture surface (Fig. 2).I used the word “standard,” as it is characteristic of all of the sinters and composites based on silicon carbide. I assume that in the course of the process one of the factors changed, yet what could have caused the microstructure to look this way and what this change indicates remains unknown for me.
The microstructure on the first image is typical of molten and recrystallized materials (very large acicular crystals). Something in your powder mixture has melted during sintering. I suspect that was metallic silicon or Si-B alloy. Also, mixture B4C-B-C has a eutectic at 2075°C and can melt if your sintering temperature was too high.
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