**3. Research materials**

The material composites were obtained from commercial steel powder AISI 316LHD manufactured by Höganäs (Belgium) sprayed with water and graphite powder Graphite FC (*fiber carbon*) manufactured by Schunk Kohlenstofftechnik GmbH (Germany). Chemical composition of the powder 316LHD is presented in Table 1.


Table 1. Chemical composition of steel powder %.

Bulk densities for the powders used in the study are contained in Table 2.


Table 2. Bulk densities for the powders used in the study.

The subject of the present study is the analysis of opportunities for the use of graphite-steel composites for components of fuel cells. The proposed composites were obtained by means of powder metallurgy. The technology for obtaining the materials used in the study allows for the determination of the effect of compaction and sintering on product properties. Finding the relationships between the technological parameters and properties of sinters allows for obtaining materials with the desired mechanical properties and resistance to corrosion. The investigations of sintered stainless steel confirmed that the use of suitable parameters of compaction pressure and sintering atmosphere ensures obtaining materials with controllable density, pore and grain size, and that suitable chemical composition of

powders allows for obtaining sinters with the desired functional properties [30-35].

Ni [%]

The material composites were obtained from commercial steel powder AISI 316LHD manufactured by Höganäs (Belgium) sprayed with water and graphite powder Graphite FC (*fiber carbon*) manufactured by Schunk Kohlenstofftechnik GmbH (Germany). Chemical

> Cr [%]

316LHD 0.025 0.005 2.2 12.3 16.7 0.9 0.1 0.30 0.06 balance

Powders Density [g cm-3] 316LHD 2.67 graphite 0.20

Si [%] Mn [%]

O [%]

N [%] Fe [%]

Fig. 3. Bipolar plate in fuel cell with channels which supply media.

composition of the powder 316LHD is presented in Table 1.

Mo [%]

Bulk densities for the powders used in the study are contained in Table 2.

S [%]

Table 1. Chemical composition of steel powder %.

Table 2. Bulk densities for the powders used in the study.

**3. Research materials** 

Powder C

[%]

Fig. 4 presents the morphology of the powders used for preparation of graphite-steel composites. The values of statistical parameters of the particles of steel and graphite powders are presented in Fig. 5 and Fig. 6 in the form of histograms. Table 3 contains statistical parameters of stereological values of the used powders.

316LHD graphite

Fig. 4. Powders morphology, magnification x500.

Fig. 5. Histograms of: a) particle surface; b) particle perimeter; c) mean particle diameter; d) roundness of the particle in 316LHD powder.

Properties of Graphite Sinters for Bipolar Plates in Fuel Cells 195

**4.1 Phase analysis** of graphite-steel composites was carried out with X-ray XRD Seifert 3003 T-T diffractometer. The investigations were carried out using cobalt lamp with the wavelength of radiation of λCoKα = 0.17902 nm. The diffractometer operated with the

**4.2 Microstructural analysis** of the obtained composites were carried out using Axiovert

**4.3 Hardness tests** for the graphite-steel composites were carried out by means of Rockwell

**4.4 Mean grain size** for the composites was determined based on comparison of microscopic photographs with the pattern scale (comparative method) according to PN-EN ISO 643 standard [36]. The investigations were also supported by the results obtained based on the

**4.5 Analysis of porosity** of graphite-steel composites was carried out using mercury porosimeter PoroMaster 33 equipped in Quantachrome Instruments software for Windows. **4.6 Analysis of wettability** of composites was carried out in a following manner: 3μl of water was dropped on the surface of material which had been previously polished with a set of abrasive papers with the finishing paper with grit designation of 2500. Before the examination, the material was degreased and left in the air until dry. The images of the material with a water drop were analyzed by a MicroCapture micro-camera which features software for image analysis. The functionality of angle analysis allowed for the

**4.7 Analysis of roughness.** In order to determine surface topography and parameters of surface geometry in the composites, the examinations using Hommel T1000 profilometer were carried out. The examinations of sinter geometry were carried out using measurement needle with the ball tip with the radius of 2.5 μm. Using the profilometer allowed for the determination of the parameters which describe height and longitudinal characteristics of

**4.8 Analysis of contact resistance.** Techniques of measurement of interfacial contact resistance have been broadly discussed in the studies [39-40]. Measurements of electrical contact resistance between the surfaces of diffusion layer (GDL, usually carbon composite) and bipolar plates (BP) were carried out according to the methodology used by Wang

100% 316L;

100% graphite.

following parameters:

optical microscope.

method in B and F scale.

determination of Θ angle.

the profile [37-38].

research using mercury porosimeter PoroMaster 33.


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

**4. Research methodology** 

Fig. 6. Histograms of: a) particle surface; b) particle perimeter; c) mean particle diameter; d) roundness of the particle in graphite powder.


Table 3. Statistical parameters of stereological values for the powders used in the study.

In order to obtain the sinters, steel and graphite powders were compacted (compaction pressure of 200 MPa), and then sintered in vacuum: sintering parameters: T=1250oC, t=30 min, cooling rate 0.5 oC/min,. Steel and graphite powders were used with the following proportions (expressed in mass percentage):

100% 316L;

194 Corrosion Resistance

Fig. 6. Histograms of: a) particle surface; b) particle perimeter; c) mean particle diameter; d)

Powders Statistic parameters Area [μm2] Perimeter [μm] Roundness

Minimum 5 13 1.081 Średnia wielkość ziaren 503 101 1.855 Maksimum 1 437 195 4.131

ziaren 346 40 0.598

Minimum 1.27 3.64 1.000 Średnia wielkość ziaren 186 40.6 1.297 Maksimum 2 285 91.5 3.456

ziaren 378 36.8 0.420

Table 3. Statistical parameters of stereological values for the powders used in the study.

In order to obtain the sinters, steel and graphite powders were compacted (compaction pressure of 200 MPa), and then sintered in vacuum: sintering parameters: T=1250oC, t=30 min, cooling rate 0.5 oC/min,. Steel and graphite powders were used with the following

roundness of the particle in graphite powder.

proportions (expressed in mass percentage):

Odchylenie standardowe wielkości

Odchylenie standardowe wielkości

316LHC

Grafit

