Silicon carbide
Nikasil is short for Nickel Silicon Carbide. Silicon carbide is a very hard ceramic (much harder than steel) that can be dissolved in nickel. The nickel solution can then be electroplated onto the aluminium cylinder bore. The piston rings will then rub off the exposed nickel, leaving a very hard layer of silicon carbide to prevent the cast iron/steel piston rings directly contacting the aluminium cylinder. With this setup, the engine tolerances can be much tighter for better performance. The cylinder must be re-plated after it is re-bored, but Nikasil is extremely durable, so the cylinder does not need to be reworked as often as an iron or chrome cylinder.