Strength, elasticity and failure of composites with pyrolyzed matrices based on polymethylsiloxane resins with optimized ratio of D and T components

Abstract

Two mixtures of T and D siloxane monomeric components labelled as TxDy (molecular ratio x:y equal 3:1 or 4:1) were chosen as matrix precursors for manufacturing Nextel720 reinforced unidirectional composites which, after pyrolysis at 1000 or 1100°C, revealed good endurance in an oxidizing environment up to 1500°C. Vickers hardness of the heat treated (1000–1500°C) samples of pyrolyzed matrices T3D1 and T4D1 are mutually similar (1100–1400 HV0.2) and reach their maximum between 1200–1300°C. Flexural strength of the pyrolyzed composites is 150–170 MPa and 170–250 MPa for T3D1 and T4D1, respectively. After annealing 3 h in air at 1200–1300°C, the strength slightly decreases but similar treatment at 1500°C yields strengths exceeding those of the pyrolyzed material. Shear modulus of the pyrolyzed T4D1 composite is roughly twice that of the T3D1 one (15 GPa vs. 8 GPa) and both increase sharply to 22–25 GPa after annealing at 1500°C, which manifests substantial improvement of the matrix properties. Fracture toughness of the composites, as measured by chevron notch test at RT, 550°C, and 1100°C, yields 4–5 MPa.m−1/2 for T3D1 and 3–4 MPa.m−1/2 for T4D1. For both composite types, the fracture toughness drops by 1 MPa.m−1/2 when measured at 550°C, which can be attributed to suppression of fibre pull-out due to stress state changes caused by the coefficient of thermal expansion (CTE) mismatch. Fracture surfaces generated during flexural tests of the annealed samples reveal decreasing occurrence of pullout towards the highest annealing temperature.