**5.2 Microstructural examinations, density and hardness of sinters**

Fig. 9 presents microstructures in graphite-steel composites.

**100% 316L 80% 316L + 20% graphite**

Properties of Graphite Sinters for Bipolar Plates in Fuel Cells 201

Table 5 contains mean cross-sectional surface area, mean number of grains per mm2 of the surface and mean number of grains per mm3 of graphite-steel composites. Based on the data contained in the table, one should note that no effect of chemical composition of the sinter on mean grain size is observed. Mean grain diameter varies from 48 to 68 mm, whereas the greatest grain diameters are observed for the sinter with 50% proportion of graphite. The

100% 316L 0.055 0.00346 227 3 633 80% 316L + 20% grafit 0.048 0.00195 512 11 585 50% 316L + 50% grafit 0.068 0.00427 280 4 492 20% 316L + 80% grafit 0.055 0.00346 227 3 633 100% grafit 0.051 0.01275 210 3 369

Fig. 10. Relationship between mean grain diameter in graphite-steel composite and

proportion of graphite in the composite.

Mean surfach of grains [mm2]

Mean number of grains per 1mm2

Mean number of grains per 1mm3

Mean grains diameter [mm]

Table 5. Mean values of grain parameters in graphite-steel composites.

**5.3 Stereology of composite grain size** 

data are also presented in Fig. 10.

Composites

**50% 316L + 50% graphite 20% 316L + 80% graphite** 

**100% graphite** 

Fig. 9. Microstructures of graphite-steel composites, magnitude 100 x.

Table 4 contains the values of density for 316L steel sinter and graphite-steel composites. Addition of graphite to the material reduces density from the level of 7.16 g cm-3 for steel to the level of 2.35 g cm-3 for the sinter modified with 80% of graphite. Modification of steel sinter 316L with graphite allows for obtaining materials with reduced density. With respect to future applications of these materials for bipolar plates in fuel cells, the use of light materials will allow for achievement of one of the most essential goals of hydrogen technologies, i.e. reduction in generator weight. Addition of graphite to steel sinter impacts also on material hardness. Change in sinter hardness with concentration of graphite in the composite is presented in Table 4.


Table 4. Density and hardness of graphite-steel composites.
