[Darokthar (OP)]

If two galaxies collide it may happen, that their black holes unite. It is possible, that the black holes break into smaller pieces during such a process?

If one thinks about the earth being hitten by a big piece of matter there will surely be some smaller pieces. As black holes have some very different physics my question would be, if it is possible for black holes to be broken into pieces.

[DoctorBeaver]

Very doubtful due to the enormous gravity involved. You cannot think of a BH in the same way that you think of a rocky planet such as Earth. If light cannot escape a BH then I don't see how 1 could fragment. Physics goes a bit doolally inside a BH, and what's inside one may not even be matter as we currently understand it.

However, as the event horizons approach & converge, there may be some interesting effects. If the BHs are of different size, material from the accretion disk of the smaller hole would probably be pulled across into the larger one. The orbital mechanics could be interesting too.

The stars in the galaxies would have fun. Many would be caught up in the scrap between the BHs and nosedive into one or the other never to be seen again. Others could be flung out of the galaxies where they would live out their lives as Billy-no-mates, isolated in the vast regions of intergalactic space like a halitosis-ridden Pariah.

[Soul Surfer]

In general black holes can only grow and merge.  The concentration of energy required to make a black hole gets greater as the black hole you try to make gets smaller.

[qazibasit]

what i think is yes just like liquid mercury the black hole can also shatter into pieces.

[DoctorBeaver]

what i think is yes just like liquid mercury the black hole can also shatter into pieces.

Can you explain why you think that?

[Bored chemist]

for a black hole to break in 2 one half would have to leave the other half.
One of the things about black holes is that nothing leaves them. I can't see how they could possibly split.