Polishing composition for magnetic disk substrate, and method for polishing magnetic disk substrate

Abstract

Provided is a polishing composition capable of achieving a high polishing rate without using alumina particles, a good surface smoothness, and a surface condition without adhering residual abrasive grains or polishing wastes. The polishing composition for a magnetic disk substrate includes colloidal silica having an average particle size in a range of 10 to 120nm, wet-process silica having an average particle size in a range of 200 to 600nm, a water-soluble polymer compound, a quaternary ammonium salt-type organic compound, and water, wherein the water-soluble polymer compound contains a copolymer containing at least a monomer having a carboxylic acid group and a monomer having a sulfonic acid group as essential monomers, and the quaternary ammonium salt-type organic compound has a hydrocarbon group bonded to a nitrogen atom that is a saturated aliphatic hydrocarbon group having 1 to 20 carbon atoms and / or an unsaturated aliphatic hydrocarbon group having 2 to 20 carbon atoms. [No available figure]