Composite materials are obtained by joining two (or more) different materials. While in polymer matrix composites the reinforcement is needed to improve the mechanical strength & stiffness of the unreinforced matrix, in glass & glass-ceramic matrix composites (& generally in ceramic matrix composites) the reinforcement is needed mainly to modify the fracture behaviour; causing the composite to be less brittle than the unreinforced matrix, with secondary effects such as improvements in hardness, wear resistance, or mechanical strength at high temperatures. The enhancement of fracture toughness (the resistance to the propagation of a crack) is due to the multiphase nature of composites: cracks in the matrix come into contact with the reinforcement, which makes them deviate or branch.
