A downhole tool can be advanced downhole a wellbore. The downhole tool can include an electromagnetic source, a directional antenna, and a casing. The casing can either completely or partially consist of ceramic materials. The directional antenna can be oriented to direct electromagnetic radiation generated by the electromagnetic source towards the ceramic materials of the casing. In response to receiving the electromagnetic radiation, the ceramic materials can absorb the electromagnetic radiation, which can cause the ceramic materials to rapidly increase in temperature.