Thermal Expansion Behavior of Hot-Pressed Engineered Matrices

Advanced engineered matrix composites (EMCs) require that the coefficient of thermal expansion (CTE) of the engineered matrix (EM) matches those of the fiber reinforcements as closely as possible in order to reduce thermal compatibility strains during heating and cooling of the composites. The present paper proposes a general concept for designing suitable matrices for long fiber reinforced composites using a rule of mixtures (ROM) approach to minimize the global differences in the thermal expansion mismatches between the fibers and the engineered matrix. Proof-of-concept studies were conducted to demonstrate the validity of the concept. Powder mixtures of seven engineered matrices were formulated based on ROM calculations and hot-pressed for thermal expansion measurements: 10(vol.%)CrSi270%SiC-20%Si3N4 (CrSi2-EM); 10(vol.%)CrMoSi-60%SiC-30%Si3N4 (CrMoSi-EM); 10(vol.%)MoSi270%SiC-20%Si3N4 (MoSi2-EM); 10(vol.%)TiSi2-70%SiC-20%Si3N4 (TiSi2-EM); 10(vol.%)WSi270%SiC-20%Si3N4 (WSi2-EM); 50(vol.%)MoSi2-50%Si3N4 and 20(vol.%)TiSi2-80%Si3N4. Density measurements conducted on the hot-pressed specimens revealed that the volume fractions of total porosity varied between 36 and 43%. Thermal expansion measurements were conducted between room temperature and 1523 K during three heat-cool cycles. The corrected thermal expansion, (∆L/L0)thermal, varied with the absolute temperature, T, as (∆L/L0)thermal = A(T – 293)3 + B(T – 293)2 + C(T – 293) + D where, A, B, C and D are regression constants. The magnitudes of (∆L/L0)thermal for the 50(vol.%)MoSi250%Si3N4 and 20(vol.%)TiSi2-80%Si3N4 specimens increased with increasing thermal cycle due to an increase in the global residual tensile stresses. In contrast, excellent reproducibility was observed for the other five engineered matrix compositions containing SiC after the first heat-up cycle had relieved residual stresses due to hot-pressing. The experimental (∆L/L0)thermal data on these engineered matrices are shown to be close to those for SiC thereby proving the validity of the proposed concept.