Carbon Graphite is probably the best seal face you can use in most of your water and chemical applications. Its corrosion resistance and natural lubricity provides two of the features we need in a good seal face material. The question sometimes comes up about how to insert the carbon into the mechanical seal. And to make it even more confusing, the top seal companies use both versions interchangeably.
Carbon is stronger in compression than it is in tension.
Dynamic O-rings and other elastomer shapes slide on a metal holder easier than they do on carbon. Low expansion metals such as Carpenter 42 and Invar 36 are available for those applications. There are problems keeping the carbon face flat after it has been installed in the metal holder. Differential expansion is just one of these problems. Sometimes a filler material is inserted behind the carbon face between it and the metal holder to prevent pressure from penetrating in vacuum and reverse pressure applications.