Performance of cork and ceramic matrix composite joints for re-entry thermal protection structures

Significance Statement

The ability of cork-based composite materials to satisfy stringent design specifications have made them perfect candidates for aerospace applications. Cork is a natural material with superior damage tolerances, high reliability and attractive strength to weight ratios. Above all, it has excellent mechanical characteristics such as high dimensional recovery capacity, excellent compressibility without fracturing, and nonlinear elasticity. Cork displays high energy absorption capabilities making it an ideal building block for thermal insulation materials and for vibration suppression.

The ability to withstand extreme temperature during the re-entry of space vehicles has made cork widely applied for aerospace applications. Its ablative and thermo-mechanical attributes have also made cork suitable for insulation between the Kevlar case and the propellant of solid rocket boosters. Therefore, researchers led by Dr K. Mergia from the National Center for Scientific Research “Demokritos” in Greece and in collaboration with scientists from Tecnalia Research & Innovation, the German Aerospace Center and Lièges HPK S.A studied the bonded laminar structures comprised of a cork-based material and a ceramic composite. Since inorganic adhesives are ideal for joining dissimilar materials for high temperature applications, the authors used an adhesive bonding to fabricate the hybrid ceramic structure. Their work is published in peer-reviewed journal, Composites Part B.

The authors used the ceramic matrix composite and the ablative material to fabricate the joints. In order to join the two materials, the authors applied three inorganic adhesives. The initial selection of the most promising adhesives to be tested was based on the curing temperature that has to be below the temperature where ablator decomposition begins and the ability to withstand high temperatures realized during re-entry. They also realized that the presence of inorganic fillers would allow for high service temperatures of up to 1650°C. Furthermore, inorganic adhesive bonding provides uniformly distributed shear strength as compared to the counterpart mechanical bonding.

They fabricated ablator-ablator joints in a bid to analyze the interface of the selected adhesives in the material. The application was on the surfaces of both the ablative test pieces to be bonded. They then cured the joints at 93°C while applying a small load. Joints using in-situ polymerization of the cork-based material on top of the perforated ceramic matrix composite material were also fabricated and tested.

The researchers performed mechanical shear tests in liquid nitrogen and at ambient conditions. They observed that for all adhesive joints, the ultimate shear strength at room temperature ranges from 0.52-0.78 MPa. These results were also similar for the in-situ polymerized joints. For the tests done at liquid nitrogen temperature, the authors recorded an enhancement of the shear strength of up to 80%. However, the shear strain was observed to drop to 50%. They also observed a predominant gradual decohesion failure mechanism inside the adhesive, which was accompanied by tearing of the cork-based material near the adhesive.

This study managed to fabricate a hybrid thermal protection system that would be applied for aerospace applications. They used in-situ polymerization and high temperature adhesives to fabricate the joints of cork-based ablator with the ceramic matric composite. It can be safely concluded that the adhesive based on Zirconium dioxide and Zirconium silicate fillerdisplay the best bonding performance.

Reference

K. Triantou1, B. Perez2, A. Marinou1, S. Florez2, K. Mergia1, G. Vekinis1, J. Barcena2, W. Rotarmel3, C. Zuber3, and A. de Montbrun4. Performance of cork and ceramic matrix composite joints for re-entry thermal protection structures. Composites Part B, volume 108 (2017), pages 270-278.

Show Affiliations
  1. National Centre for Scientific Research “Demokritos”, Athens, Gr-15310, Greece
  2. Tecnalia Research & Innovation, Donostia-San Sebastian, E-2009, Spain
  3. German Aerospace Center (DLR), 70569, Stuttgart, Germany
  4. Lièges HPK S.A., F-47230 Lavardac, France

 

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