It is well known that SiSiC ceramics in some conditions (i.e. temperatures above the melting temperature of Si) show the “sweating phenomenon”. It means that molten silicon in excess inside their microstructure comes out and gathers in drops. If you oxidize a SiSiC ceramic, sweating is not immediate though, it can happen after a long time (again depending upon temperature). The picture shows drop formation at after 40h in air @1600°C. I took a look at these droplets: they are made of silicon with a very low amount of oxygen on the surface. Pure Si expansion cannot be the cause of this massive mass transport, there must be something else.
Any opinion?
Dear Marcos,
Yours is definitely an intriguing explaination, I will go in deep into it, Have you some reference?
thanks
Alberto
Alberto, it's a very intersting phenomenon.
I'd like to suggest the following as a cause of the formation of such Si droplet.
The oxidation of SiC and some Si in SiSiC body results in weight gain and volume expansion of SiSiC body and the volume change accompanying the oxidation of SiC and Si squeezed molten Si out from the body at 1600oC and formed a droplet like that because of surface tension of molten Si.
Thanks,
Young-Wook
Dear Young-Wook,
You are right...
I always thought about some stress field which forces the molten Si out. In favour of what Marcus stated we observed, before Si exudation , silica bubble (see picture) which were blown by CO or CO2.
Another interesting thing is that, since the surface is oxidized, the Si drop doesn’t spread, as it should on pure SiC.
generally these bead are associated with low melting Fe-Si eutectics in SiC composites. source of small Fe impurity may vary widely for materials, so it may take extensive investigation.
Dear James,
I call this telepathy!
I just came out from the SEM room where I took a look at a cross section of one oxidized Si bead.
Indeed, surprisingly to me, it contains iron ! (the white dots and stripes inside the bead)
Even so, I still wonder which force drives this stuff out....
In SiC ceramics small amount of oxygen is present as oxycarbide. Oxycarbide undergo exchange reactions at high temperature and forming SiC and small amount SiO2. Which may be on cooling to form a Si bubble.
We have seen this as well in SiC containing ceramics when iron is present. Therefore, not surprised you find iron in the exuded Si ball. I think the liquid metal is driven out by pore pressure, if I remember correctly from past work.
I found that the molar volume of the condensed oxide is different than the molar volume of the substrate. In the literature for oxidation of metals, the oxide/metal volume ratio is known as the Pillings–Bedworth Ratio (PBR). When the SiC is oxidized to silicon dioxide, the molar volume of SiC (12,47 cm3/mole) is much smaller than the molar volume of SiO2 (27,43 cm3/mole). The carbon oxidizes to CO gas. The ratio of the molar volume of silica to the molar volume of SiC is the PBR of SiO2 is 2,2. It might be this compressive stress field that squeezes out what is liquid first.
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