Effect of Interface Design on the Mechanical Behavior of a Nicalon-Glass Composite

The objective of this study is to use an extrinsic carbon coating to tailor the interface in a unidirectional Nicalon-borosilicate glass composite for maximum strength. Three unidirectional Nicalon fiber-reinforced borosilicate glass composites were fabricated with controlled interfaces by using (1) uncoated (2) 25 nm thick coated and (3) 140 nm thick coated fibers. The tensile behaviors of the three systems differ significantly. Damage developments during tensile loading were documented by a replica technique. Fiber-matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and modulus. Specimen end-tab design was varied to obtain desirable failure mode. Interlaminar shear and splitting strengths of the composites were measured to understand the failure modes. The results demonstrate that an optimum interfacial carbon coating is necessary to prevent brittle failure and maximize the strength of the composite.