Effect of alloying element Nb on microstructure, mechanical and tribological properties of SiMoV compacted graphite cast iron (CGI)

In order to improve the friction properties of compacted graphite cast iron (CGI), trace amounts of Nb have been used to tune the microstructure of CGI, and as-cast high strength CGI with different Nb contents (0.046–0.22wt %) have been newly developed. The microstructure under the influence of niobium and its tribological behavior under different sliding speeds and loads were investigated by systematic friction and characterization experiments, and wear mechanisms and contact stresses were discussed. The results show that the niobium content affects the solidification process, thus the matrix morphology and precipitated phases. Under the influence of Nb, the graphite and matrix morphology was refined, the pearlite content increased, and the tensile strength increased. Moreover, the lattice constant of a-Fe matrix increases, and the phase hardness is improved. However, a critical value exists for the amount of Nb added. The specimens were subjected to abrasive, adhesive, and oxidative wear at low speeds and loads. With increasing speed or load, the main wear mechanisms change to oxidative and abrasive wear. The three-body oxide abrasive generated by oxide layer crushing significantly affects the wear rate. At high speeds, Nb caused the wear of CGI first to decrease and then increase. While at high loads, the wear rate decreases with increasing Nb content due to larger normal and shear contact stresses at the friction interface. The present work contributes to the understanding of the tribological properties of Nb-CGI under different sliding conditions.