What hardens arthropod exoskeletons?

Arthropod exoskeletons are naturally hard because of the composition of the protein used (a chitin composite); crustaceans further harden it using a process called biomineralization.

Chitin chemically is a long-chain polymer, a nitrogenated polysaccharide comparable to cellulose, which allows for hydrogen bonding between polymers for additional strength. By embedding in sclerotin and mineralizing it, arthropods achieve an advantage of gaining a greater toughness and less brittleness than minerals alone but being stiffer and harder than pure chitin.