Sm2Fe15Ga2C3 and Sm2Fe15Ga2M0.2C3 (M = Nb, Cu, Zr) ribbons with Th2Zn17-type structure have been synthesized by melt-spinning and subsequent annealing at temperatures above 1023 K. A nanoscale 2:17 phase with grain sizes of 50-200 nm is developed. The anisotropy constants K1 and K2 are determined in the temperature range of 150-600 K by fitting the demagnetization curve of the isotropic samples. Furthermore, the intrinsic material parameters, like the Curie temperature, exchange constant, anisotropy field and critical diameter of single domain particles are given. In order to understand the magnetic hardening mechanism, the temperature dependence of the coercivity is analysed within the framework of the micromagnetic models. It is concluded that the magnetic hardening mechanism at low temperatures up to 350 K is dominated by nucleation processes in a magnetically inhomogeneous region. The average widths of the inhomogeneities of 2 nm and 3 nm for Sm2Fe15Ga2C3 and Sm2Fe15Ga2M0.2C3 (M = Nb, Cu, Zr) are determined, respectively. At high temperature the pinning of the domain walls by grain boundaries controls the coercivity mechanism.