# Rapid Characterization Method for SMC Materials for a Preliminary Selection
## 4 Experimental Results  
### Resistivity: Organic Layer  
The pressure level increment has a high impact on the magnetic and mechanical properties of the SMCs. On the other hand, pressure level slightly affects the electrical resistivity values.  
In Figure 8, the resistivity behaviour for the SMCs with epoxy resin at 0.2 wt% was shown. The higher decrease was noticed from 400 MPa to 500 MPa (\(-\)17% in resistivity), while from 400 MPa to 900 MPa, the resistivity decreases by 23%. These results highlight that the increase in pressure up to 900 MPa causes only a slight decrease in resistivity because the higher resistivity loss already occurs for lower pressures (up to 500 MPa). The maximum resistivity value is noticed for 400 MPa (47.26 \(\mu\)m), while an anomalous value was measured for 600 MPa, although the measure was repeated. This behaviour may be due to a possible different microstructural arrangement of the insulating layer in the middle of the sample that causes an increase in the average resistivity. Furthermore, in this context, such resistivity variations did not affect the quality of the SMCs; only a reduction in about one or more orders of magnitude can be considered detrimental.  
Despite at 400 MPa the resistivity was higher, the literature demonstrated that specimens resulted in poor mechanical properties at similar compacting pressures. Moreover, the resistivity was almost constant from 700 MPa to 900 MPa. Considering these results, a compacting pressure of 800 MPa may be a good compromise between mechanical proper  
Figure 8: Resistivity variations in SMCs with epoxy resin 0.2 wt% for different pressure levels.  
ties and resistivity values. Additionally, the industrial compaction does not exceed such values (about 800 MPa).  
Figure 9 shows the resistivity as a function of wt% of resins at 800 MPa. In particular, with the increase in epoxy resin content, the resistivity has a slight increase; on the other hand, the same increment in phenolic resin content causes a significant increase in the measured resistivity. For each resin, the maximum values have been obtained for 0.75 wt%: 75.5 \(\upmu\Omega\)m for epoxy resin and 363.1 \(\upmu\Omega\)m for phenolic resin. Already from 0.075 wt% of resin content, phenolic resin results in better behaviour in terms of electrical resistivity (83.1 \(\upmu\Omega\)m). Finally, the phenolic resin showed a higher variance with respect to the epoxy resin SMCs. This phenomenon is due to the loading of the powder in the mould during the compacting process: the SMCs with phenolic resin resulted in being easily dispersible.  
As the resistivity changes slightly with the epoxy resin content, 0.2 wt% of resin may be considered a good compromise between magnetic and mechanical properties. From the comparison between Figures 8 and 9, it appears evident that for epoxy resins, resistivity is directly proportional to the resin content (as expected for the addition of insulating material) and inversely proportional to the increase in compaction pressure.  
Figure 10 shows the variation in resistivity for SMCs with phenolic resins at 0.2 wt% and 800 MPa as a function of the adopted technology processes for performing the insulating coating. With equal process parameters, turbula mixing resulted in the lower resistivity value (almost 200 \(\upmu\Omega\)m). Regarding ball milling, a significant increase from 24 h to 48 h in resistivity was noticed: 303 \(\upmu\Omega\)m and 500 \(\upmu\Omega\)m, respectively. These behaviours were due to the final ferromagnetic particle shape obtained. After turbula mixing, the SMC particles resulted in an irregular spongy shape, while after ball milling, the particles' shape resulted in a plated-like shape, and this behaviour increased with the increase in the processing time. These plate-shaped particles resulted in less thickness with respect to the spongy irregular shape, causing the increase in the measured resistivity.