Qualification of thermal barrier coating for semi-cryogenic engine thrust chamber through strain-controlled fatigue tests

The combustion chamber of semi-cryogenic engines experiences elevated temperatures which are beyond the capability of most structural materials. To solve this issue, an effective cooling mechanism to be implemented. In one of the developmental launch vehicles of Indian Space Research Organisation, the wall temperature in the combustion chamber is reduced by means of employing regenerative cooling along with thermal barrier coating applied on the inner wall made of special grade copper alloy. This special coating made of layers of Nickel and Nickel-Chromium also protect the thrust chamber from oxidation. The coating forms an integral part of the inner wall and are subject to a complex pattern of loads involving both thermal and pressure loads. This paper presents details about the qualification process used to evaluate integrity of the coating. Firstly, the stresses and strains on the inner wall during the engine operating conditions are arrived at based on detailed finite element analysis. Test specimens are fabricated from copper alloy and the thermal barrier coating as in the actual thrust chamber is applied on it. The specimens are tested under tensile and compressive loading that will induce the expected strains obtained from finite element analysis. The tests are done by means of strain-controlled fatigue tests at elevated temperature in tensile and compressive strain conditions. The integrity of the coating is assessed by means of Non-Destructive Test after each cycle of test and based on these, modifications were made for better performance. The modified coating was again tested for the same loading conditions and evaluated the coating integrity.