Novel oxidation protection system for carbon-carbon composites at high temperature
Carbon-carbon composite materials have been identified as one of the most potential materials for light weight and high temperature applications. Mechanical properties of carbon-carbon composites do not degrade even at temperature as high as 2000°C. However the main problem in their use in high temperature oxidizing environment is their tendency to oxidize at temperatures of 400°C and above. Therefore some oxidation protection mechanism is mandatory to make these materials available for high temperature applications. It is the purpose of the current research to develop a viable high temperature oxidation protection system for carbon-carbon composites. It has been shown that such a coating system must have at least two layers; a gradient porous SiC layer aimed to redistribute the stress produced due to CTE (coefficient of thermal expansion) mismatch and a dense top layer of a suitable material meant to protect carbon-carbon composite substrate from oxidation. Materials for the top layer experimented during the current research were SiC, ShN4 and HfC. Pack cementation technique was used to develop the gradient SiC layer while top dense layers were deposited by using the reactive sputtering technique. To improve the oxidation protection and crack resistance of the top dense coating multilayering approach was adopted. During the current research basically four different coating systems were produced, characterized and then tested at high temperature for their oxidation performance. These coating systems were, gradient SiC layer plus dense sputtered SiC layer, gradient SiC layer plus dense sputtered ShN 4 layer, gradient SiC layer plus dense sputtered SiC/ShN 4 layers, gradient SiC layer plus dense sputtered SiClHfC layers. Oxidation testing of these coatings in atmosphere showed that these coatings are thermodynamically stable at all test temperatures studied (1300-1575°C), except coatings with a ShN 4 layer. ShN4 becomes thermodynamically unstable at 1575°C. These coatings remained mechanically stable (no spallation) except the coatings with HfC layers. Coatings with HfC layers spalled off at all temperatures. Investigation into the causes of spallation indicated that the thickness (20-25 flm) of the converted SiC gradient layer was insufficient, plus the processing conditions during the deposition of HfC were the main causes for their failure.