Strength and thermal strain of carbon-filled composite materials at high temperatures. Communication 2. Nature of strain and weakening of materials when heated from 290 to 3300 K

We have investigated the physicomechanical properties of carbon-filled composite materials. We have established that with an increase in temperature up to 770 K, some weakening of the materials occurs followed by their strengthening, with maximum values for the elasticity modulus E at T=1270 K and for the compressive breaking strength σ_Bat T=2270 K. In this case, the E values are about 5% greater than the original values, and the σ_B values are more than double the original values. Further increase in temperature leads to a decrease in the strength characteristics under compression. In order to explain the reasons for weakening of composites upon heating, we have investigated the characteristic behavior of the thermal strain for carbonfilled composite materials, including in the processing stages of their fabrication. We have determined the characteristic behavior of the thermal strain for the original material: carbon-fiber reinforced plastic in a phenolformaldehyde matrix, carbonized carbon-fiber reinforced plastic, and carbon-filled composite materials densified with pyrolytic carbon. We have established that the shear stresses arising in the first annealing within carbon-fiber reinforced plastic (as a result of the action of shrinkage strain) are preserved over a broad temperature interval 570–1270 K, and are the reason for weakening of carbon-filled composite materials in the same temperature range. We have carried out extensive microstructural investigations of carbonfilled composite materials, allowing us to explain the physical processes occurring in the composites under thermal and mechanical loads.