열피로하중이 알루미늄·알루미나 코팅층의 열차폐 성능에 미치는 영향

Abstract

열차폐 코팅은 상부 코팅 즉, 탑코팅과 층간 접착, 즉 본드코팅으로 구성되어 있으며, 낮은 열전도도로 인해 외부로부터 전달되는 열을 감소시켜 모재를 보호하는 역할을 한다. 그러나 이와 같이 상이한 재료의 사용에 따라 층간 열팽창계수의 차이가 발생하고 이에 의해 유발된 열응력은 열차폐 코팅의 파손을 초래하게 된다.

본 논문에서는 열피로 하중에 의해 열화된 알루미늄·알루미나 코팅층의 열차폐 성능을 확인하기 위하여 두께 300 ㎛로 알루미나 코팅된 알루미늄을 모델링 하였다. 엔진부의 가혹 온도조건을 고려하여 온도를 400 ℃까지 상승시킨 다음 120 ℃까지 하강시키는 사이클을 반복 부여하였다. 탄-소성 모델과 복합 박리모드를 적용한 해석적 분석과 함께 산화반응에 기반된 화학적 접근 및 코팅부의 열차폐 성능검증 연구를 병행, 수행하였다. 실험적 검증을 위하여 두께 300 ㎛로 알루미나 코팅된 알루미늄 시편을 준비하였고, 120 ℃에서 400 ℃를 반복 부여할 수 있는 열피로 모사시험기를 제작하였다. 화학적 영향인 열생성 산화물을 확인하기 위하여 미시조직 분석을 수행하였으며, 열팽창계수 차이에 의한 실질적인 알루미늄·알루미나 코팅층의 박리를 확인하기 위하여 C-scan 이미지를 촬영하였다. 화학적 반응에 의해 탑코팅과 본드코팅 사이에서 생성되는 열생성 산화물은 발견되지 않았으며, 열팽창계수의 차이에 의해 본드코팅와 모재 사이에서 박리가 발생하였다. 열차폐 사이클 증가에 따라 박리면적이 증가함을 확인하였으며, 이에 따라 열차폐 성능이 감소함을 정량적으로 분석하였다. 이상과 같은 결과를 바탕으로 120 ℃에서 400 ℃의 반복하중을 받는 환경에서, 열피로 사이클 증가에 따른 열차폐 성능 감소률에 대한 경험식을 제안하였다.|The thermal barrier coating (TBC) consists of an upper coat, that is top coat and interlayer cohesion in other words bond coat. Due to the low thermal conductivity of top coat, it serves to protect the substrate by reducing the temperature transferred from the external environment. However, the thermal stress induced by the differences in coefficient of thermal expansion(CTE) of different materials brings about the failure in TBC.

In this thesis, Al alloy specimen coated with the alumina layer having a thickness of 300 ㎛ was modeled investigate on the thermal barrier performance of the aluminum·alumina coating layer degraded by thermal fatigue loads. Considering the severe temperature condition in an engine part, the specimen was repeatedly loaded by heating up to 400 ℃ and then cooling down to 120 ℃. An elasto-plastic model and a mixed mode decohesion model were applied in a numerical analysis. Chemical approaches based on the oxidation kinetics were performed along with the validation study of thermal barrier performance For the experimental verification, Al alloy specimen coated with the alumina layer with a thickness of 300 ㎛ was prepared and a thermal fatigue simulator capable of providing thermal cycle from 120 ℃ to 400 ℃ was made. The microstructure was analyzed to confirm the TGO induced by chemical effect. The C-scan image was taken to determine the effective delamination of the aluminum·alumina coating layer due to the CTE differences. TGO produced by the chemical reactions were not observed between the top coat and the bond coat. The delamination was confirmed between the bond coat and the substrate due to different CTE. The delamination area increased with increasing the number of thermal cycles, thereby the decrease in the performance of thermal barrier was analyzed quantatively. Based on above mentioned results, under the cyclic loading condition from 120 ℃ to 400 ℃, an empirical equation describing a relation the thermal cycle and the thermal barrier performance was suggested.; The thermal barrier coating (TBC) consists of an upper coat, that is top coat and interlayer cohesion in other words bond coat. Due to the low thermal conductivity of top coat, it serves to protect the substrate by reducing the temperature transferred from the external environment. However, the thermal stress induced by the differences in coefficient of thermal expansion(CTE) of different materials brings about the failure in TBC.

In this thesis, Al alloy specimen coated with the alumina layer having a thickness of 300 ㎛ was modeled investigate on the thermal barrier performance of the aluminum·alumina coating layer degraded by thermal fatigue loads. Considering the severe temperature condition in an engine part, the specimen was repeatedly loaded by heating up to 400 ℃ and then cooling down to 120 ℃. An elasto-plastic model and a mixed mode decohesion model were applied in a numerical analysis. Chemical approaches based on the oxidation kinetics were performed along with the validation study of thermal barrier performance For the experimental verification, Al alloy specimen coated with the alumina layer with a thickness of 300 ㎛ was prepared and a thermal fatigue simulator capable of providing thermal cycle from 120 ℃ to 400 ℃ was made. The microstructure was analyzed to confirm the TGO induced by chemical effect. The C-scan image was taken to determine the effective delamination of the aluminum·alumina coating layer due to the CTE differences. TGO produced by the chemical reactions were not observed between the top coat and the bond coat. The delamination was confirmed between the bond coat and the substrate due to different CTE. The delamination area increased with increasing the number of thermal cycles, thereby the decrease in the performance of thermal barrier was analyzed quantatively. Based on above mentioned results, under the cyclic loading condition from 120 ℃ to 400 ℃, an empirical equation describing a relation the thermal cycle and the thermal barrier performance was suggested.