In the context of the race to develop lighter, high-performance materials for cryogenic applications, particularly in the space sector, developing lightweight composite structures that are resistant to cryogenic fluids is a major challenge. Composite materials with an organic matrix and continuous carbon fibers offer an excellent compromise between mass and mechanical properties; however, their behavior at low temperatures remains limited by interlaminar brittleness and sensitivity to delamination, leading to materials that are no longer gas-tight. The integration of nanometric reinforcements, such as carbon nanotubes (CNTs), into a LOx (liquid oxygen)- compatible matrix represents a promising avenue for enhancing the performance of these composites in extreme environments.
This thesis, the result of a collaborative project between the CEA and the CNES, focuses on the integration of carbon nanotubes into laminated composites for use in cryogenic tanks. Its objective is to compare the mechanical and crack resistance properties of three combining methods for different forms of CNTs in laminated composites based on a matrix of interest: cyanate ester and carbon fibres. The first approach consists of growing CNTs aligned on CF plies by CVD before infusing the plies with resin. The second approach involves growing VACNT mats on a flexible aluminum substrate to transfer the VACNT mat to the interlayer between pre-impregnated plies using a hot pressing protocol. The third approach involves dispersing carbon nanotubes in the resin using a hot-mixing under vacuum process before infusing the CF plies. Three CNT synthesis processes were adapted to meet the requirements of each integration approach, enabling the production of pre-materials on a scale suitable for the final composite test specimens. In each of the three approaches, several CNT lengths and concentrations in the matrix were compared in order to determine the influence of these variables on the mechanical properties of the composites.
Firstly, protocols for producing laminated composite test specimens were developed on-site at the industrial partner, CMP Composites, for each of the considered approaches, with the aim of preserving the aligned or dispersed morphology of the NTCs according to the approach. The mechanical behaviour of the materials obtained was characterised by uniaxial tensile tests assisted by optical microscopy, an experimental protocol developed by the teams at I2M Bordeaux. This approach enabled the in situ observation of damage mechanisms at the microscopic scale and provided a deeper understanding of the influence of NTC characteristics on critical areas of crack initiation and propagation.
The obtained results highlight the role of CNTs in improving interlaminar cohesion, depending on their length and integration path, as well as the redistribution of stresses within the laminate. This work thus opens up prospects for designing nanostructured composites with enhanced mechanical performance and resistance to severe cryogenic conditions, making them suitable for liquid propellant storage applications.
Keywords: nanostructured composites, CVD growth, mechanical properties, carbon nanotubes, cryogenic environment