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The influence of micro- and nano-scaled SiC on the phase formation, the structural properties, and the superconductivity of MgB2 bulk samples was studied. Mechanical alloying was applied to prepare a nanocrystalline precursor powder with SiC additions up to 10 wt.%. The data implies that, in contrast to previously reported results in literature, the use of SiC as a dopant in MgB2 shows a conflicting effect. Although a certain carbon substitution was observed and, therefore, H c2 was enhanced, an often reported increase of J c was only obtained for low SiC concentrations. Beyond that, a strong decline of J c was observed, coming from an accumulation of SiC and Mg2Si nanoparticles at the grain boundaries which decouples the grains and thereby decreases the connectivity and J c.