Nanoindentation behavior of a two-dimensional carbon-carbon composite for nuclear applications Original Research Article

Nanoindentation tests on a carbon fiber and carbon matrix composite (C/C composite) for nuclear applications were carried out over a load range from 50 μN to 20 mN on two different crosssections, normal and parallel to the fiber axis. For reference, an isotropic nuclear graphite was also examined. Both the composite and graphite revealed a large amount of elastic recovery on the first loading-unloading indentation curves and inelastic hysteresis on the subsequent cycling curves. The analysis of mean indentation pressure data on the basis of the Weibull statistics indicates that the data on the C/C composite are separated into two groups with different Weibull moduli; the group of the lower modulus with higher mean pressure is attributed to the fiber, and the higher modulus with lower mean pressure to the matrix. The average of mean indentation pressure is obtained as 101 MPa for the fiber and 35.5 MPa for the matrix, while graphite has an intermediate value of 52.2 MPa. There is a slight dependence of mean stress pm on the applied load P in all the materials. However, the trend appears to be different between the two materials; pm decreases with increasing P for the C/C composite, while the reverse is true in graphite. Youngʹs moduli of the graphite and the two constituents of the C/C composite are estimated from the stiffness of the unloading indentation curves at 10.7 GPa for the graphite as 6.71 GPa for the fiber and 1.97 GPa for the matrix. The values estimated for the graphite and the fiber are comparable to those of the bulk graphite and C/C composite, respectively.