Chemical-mechanical polishing (CMP) compositions containing chemical additives and methods of using the CMP compositions are disclosed. The CMP composition comprises abrasive; chemical additive; liquid carrier; optionally an oxidizing agent; a pH buffering agent and salt; a surfactant and a biocide. The CMP compositions and the methods provide enhanced removing rate for “SiC”, SiN” and “SiCxNy” films; and tunable removal selectivity for “SiC” in reference to SiO2, “SiN” in reference to SiO2, “SiC” in reference to “SiN”, or “SiCxNy” in reference to SiO2; wherein x ranges from 0.1 wt % to 55 wt %, y ranges from 0.1 wt % to 32 wt %.