A chemical mechanical polishing (CMP) composition for planarizing a nickel phosphorus (NiP) substrate comprises a suspension of colloidal silica particles and fused silica particles in an acidic aqueous carrier containing hydrogen peroxide, in which the concentration of the fused silica particles is less than or equal to the concentration of the colloidal silica particles. In some embodiments, the CMP composition includes a primary complexing agent, a secondary complexing agent, and a metal ion such as ferric ion, which is capable of reversible oxidation and reduction in the presence of hydrogen peroxide and NiP.