There are two ways to improve the strength of expanded graphite sheets:
First, increase the force required for interlayer sliding of the graphite layers.
Second, the defects in the graphite crystal structure and the porous network characteristics cause stress concentration within the layers; therefore, efforts should be made to reduce this stress.
The reason for this problem lies in the fact that the bond energy of expanded graphite sheets is of a mixed type:
Within the plane, the atoms are connected by covalent bonds, resulting in relatively strong bonding. Between the planes, the bonding is through weaker molecular bonds. There are two situations for crystal layer fracture: one is the separation of atoms, causing brittle fracture, mostly along the cleavage planes.
Another type of failure is shear fracture at the crystal level, where plastic deformation and fracture occur due to relative sliding. Expanded graphite often falls into this latter category.
The problem of stress concentration caused by defects and pores on graphite flakes can be observed under a high-powered microscope. Cracks appear where there are circular holes, and parallel lines, or stress lines, appear under tensile stress. Stress lines are more concentrated around the holes. Significant stress concentration occurs at the sharp edges of cracks or at the corners of square holes. Therefore, reducing the structural defects and cracks in the material and minimizing the tendency of crystal layer slippage can improve strength. In practical production, increasing the amount of vermicular graphite and reducing the amount of dendritic graphite can solve some of these problems.
Adding borides can create obstacles to layer slippage, making it more difficult for layers to slide. The amount of boron added is related to the strength of the expanded graphite; for a given process and formula of expanded graphite sheets, there is an optimal amount of boron to add. Even a small amount of boron can significantly increase strength. In addition to boron, carbon-philic substances can also be added to form "nail-like" compounds in the carbon matrix. Inserting several "nails" into the carbon network structure can prevent shear stress caused by interlayer slippage, thus increasing strength.