Theoretical analysis on thermal grease dry-out degradation in space environment

Silicone thermal grease can reduce the contact thermal resistance of thermal interfaces and has been widely used as a thermal interface material in the thermal design of spacecraft. Silicone thermal grease gradually degrades owing to the dry-out of silicone oil during a long service period and breaks the validity of the thermal interface and induces thermal overload in a spacecraft. In this study, a mechanism-based prediction method for the silicone thermal grease loss in a space environment is proposed. The prediction of long-term thermal grease loss is realised by evaluating the variation in its effective thermal conductivity. In the prediction method, an improved multi-component thermal conductivity model was established to determine the thermal conductivity of thermal grease with multiple filler sizes and a high filler volume fraction. Then, the silicone oil loss induced by complex penetration, crawl, and volatilisation were evaluated with an equivalent capillary model and a volatilisation model. The behaviour of the silicone oil loss, the primary cause of dry-out degradation, was clarified. The variation in the effective thermal conductivity of thermal grease under different contact angles, operating temperatures, and connection surface dimensions was investigated. The results show that the dominant factor gradually changes from capillary loss to volatilisation loss during dry-out degradation. The effective thermal conductivity decreased faster at higher operating temperatures, and the effective thermal conductivity decreased slower with an increase in the contact angle and connection interface dimensions. The present work provides insights for the design of silicone thermal greases for space utilization.