The researchers studied how the quality of the bond between carbon fibers and thermoplastic materials affects the strength of composite materials. They treated the fiber surfaces and added epoxy sizing to improve the bond. They found that changing the interphase improved the strength of some composites, but not all. The AS4 and AS4CGP fibers showed better strength than the AU4 fibers. The way the fibers and matrix interact affects the overall performance of the composite materials.