The comparison of Brinell hardness test results obtained from neat PLA and BC/PLA composites with different BC contents is shown in Fig. 6. The figure reveals that there was an increase in Brinell Hardness Number (B.H.N.) value with the increase in BC content up to 7.5 wt.%. This is due to addition of hard filler content in the PLA which is at the surface and resists the plastic deformation of PLA. The distribution of BC particles and decrease in inter-particle distance with increasing particle loading in the matrix increase the resistance to the indentation of the samples as
the PLA and BC particles are pressed together and touch each other more tightly. Thus the interface can transfer pressure more effectively [56,57]. However, in the hardness test of 10 wt.% BC/PLA composite, a pre-cracked particle can still support compressive
traction across the crack surface, resulting in the composite not being affected by the already fractured particles. The localized pressure causes the material directly under the indented surface to support most of the load, and the stress field is predominantly
compressive [58].