30 May 2019

AEROSPACE MATERIALS

Carbon fiber's framework today

The demand for carbon fiber is increasing not only  in the aviation sector but in other industrial areas. By 2020, it is estimated that the annual production of CFRP will reach 140,000 tons, which implies a sales forecast of 48,700 million dollars. Specifically, the European Aeronautics and Defense Company (EADS) estimates that the demand for carbon fibre will be 20,000 tons in 2020, only for Europe.

In the aviation sector, CFRPs are typicallly reserved for aircraft structures where the temperature in service is below 150º C. The carbon fiber used to manufacture these CFRPs uses polyacrylonitrile (PAN) as the precursor because it is the best in terms of performance balance, cost, ease of use, etc. This precursor has to undergo several complex stages until it becomes CF. In short, these would be flame-proofing (or stabilization), carbonizing, graphitizing, surface treatments, sizing and finishing-packaging.

Due to the anisotropic nature of carbon fibers, the performance of these composites depends on a large extent on the quality of the interface between the matrix and the fibre. Good interfacial adhesion provides the compounds with structural integrity and efficient charge transfer between the fiber and the matrix. However, the untreated carbon fibers are extremely inert and, therefore, have a low adhesion to the resin matrices which makes the transverse and interlaminar properties relatively weak, significantly limiting the performance of the composite and its useful life. To overcome these barriers, an interfacial improvement between the matrix and the fiber is necessary, thus improve the properties of the composite material. In order to achieve this improvement of the adhesion between the matrix and the CF, what it means, to increase the energy of the surface, to equalize the energy of the polar surface of the carbon fiber and the resin as well as to increase the concentration of reactive functional groups in the interface region, countless studies have been carried out. Those found in the bibliography are based on:

  1. Changing the chemical structure of fiber surface to enhance the chemical bonding with matrix:
  • Heat treatments to increase the functionalization of the carbon fiber 
  • Oxidation process, i.e. to increase the density of COH, COR, CO and COOH groups in the fiber surface
  • Plasma surface modification of carbon fibers to improve the adhesion with polymer matrices 
  • Chemical absorption of alkanethiols to generates self-assembly monolayers which further react with epoxy resin 
  • Treatment with rare earth elements to increase the concentration of reactive functional groups 
  • Using thermoplastic-coated carbon fiber reinforcement 
  1. Increasing the carbon fiber surface roughness:
  • Whiskerization of the carbon fibres surface with SiC, SiN4, NH3, TiO2, … 
  • Growing carbon nanotubes on the carbon fibre surface to enhance interfacial area for bonds formation 
  1. Changing the chemical structure and roughness of the fibre
  • Exposition to high-energy gamma- or laser irradiation to surface roughening as well as addition of chemical groups

In these methods only the improvement of mechanical properties of the CFRPs has been sought. However, this project is based not only on the search of the improvement of the CFRPs mechanical properties, which will come in addition, but also on a significant improvement of the electrical and thermal properties of the composite material. The deposition of diamond particles shell on the carbon fiber will allow to transform the CFRP in an thermal and electrical conductive material. In addition, the diamond/carbon fiber interface properties and the increase in the crack path induced by the roughness can allow to increase the fracture toughness. To carry out this project our research group has available of a narrow relationship with aerospace companies as FIDAMC and Airbus Military since 2000’s until present through several grants, national projects as “EXPLORA” (number: ESP201791820EXP) and  industrial project thesis.

AFM characterization

To improve the mechanical, thermal and electrical properties of the carbon fibers, boron doped polycrystalline diamond crystals are grown on its surface. Nevertheless a previous carbon fiber seeding treatment is required for that. This seeding entails the use of diamond nanoparticles dispersion to create nucleation places on the carbon fiber surface. In this sense carbon fiber’s parameters could play an crucial role in it; the carbon fiber’s roughness could be the responsable of providing more o less specifical places where the functionalization process could take place. As the roughness is a key parameter for this treatment, it must be measured. In this way, techniques as atomic force microscopy (AFM) offer the posibility to characterize this latter at nanometer scale  as well as determine its electrical behaviour. 

 

Simulations of: i) mechanical behaviour ii) seeding diamond crystals and carbon fiber interface

Carbon fiber covered with boron doped polycrystalline diamond crystals can be widely used for structural applications, in particular in the aerospacial industry.  Nevertheless investigations focused on the mechanical integrity of these materials are required to quantify: i) the damages produced by the working conditions as well as ii) the early stages of dramatic failure. Therefore, numerical investigation of failure of simple carbon fiber and carbon fiber covered with boron doped polycrystalline diamond crystals must be carry out, which opens a new route to an overall microchemical analysis of CFRP materials.  
For that purpose, the parameters involved in the mechanical behaviour must be carefully identified and entered in a finite element calculation to predict the materials’s toughness for different polycrystalline diamond crystals on carbon fiber growths;  simulations have the potential  to help improve the design of B-doped CFRP materials.  

Boron doped polycrystalline diamond crystals can improve different properties such as, thermal conductivity and electrical behaviour of the several materials. The diamond nanoparticles, show different applications, typically related with the improvement of electronic and electrical properties in semiconductors materials.  As a potential application, the doped diamond can be used as carbon fiber coating.

Carbon fibers are part of a good established aerospace technology whose objective is improve the mechanical and aerodynamic behaviour. Nevertheless the use of boron doped nanocrystalline diamond crystals on carbon fiber materials is very recent technological niche. The use of doped diamond presents several key questions, among of them, those related to superficial interactions between carbon fiber and diamond seed interfaces; as they can be responsible of mechanical and electrical behaviour. An effort towards the comprension of such phenomena is taking place ultimately. In this framework the radial distribution function (RDF) and spatial distribution fuction (SDF) analysis permit to obtain structural properties of diamond seeds and carbon fiber interfaces; these functions enable to identify the places where the interactions take place.