Patent ID: 8404009

Claim:
A polishing composition for a hard disk substrate comprising: alumina particles; silica particles; water; an acid and/or its salt; and an oxidizing agent; wherein a volume median diameter of secondary particles of the alumina particles measured by a laser beam diffraction method is 0.1 to 0.8 μm, a volume median diameter of primary particles of the silica particles measured by transmission electron microscope observation is 45 to 100 nm, a standard deviation in number-basis particle size of the primary particles of the silica particles measured by the transmission electron microscope observation is 11 to 25 nm, the silica particles include silica particles with a particle size of 20 to 120 nm in an amount of 40% or more by volume of a total amount of the silica particles, silica particles with a particle size of 20 to 40 nm in an amount of 1 to 30% by volume of the total amount of the silica particles, silica particles with a particle size of 60 to 80 nm in an amount of 10 to 77% by volume of the total amount of the silica particles, and silica particles with a particle size of 100 to 120 nm in an amount of 0 to 30% by volume of the total amount of the silica particles, wherein the volume percentage of the silica particles with a particle size of 60 to 80 nm is the largest among the volume percentages of the particle size ranges of 20 to 40 nm, 60 to 80 nm, and 100 to 120 nm, wherein the polishing composition further comprises a copolymer and/or its salt having a constitutional unit expressed as the following formula (I) and a constitutional unit derived from a hydrophobic monomer whose solubility in 100 g of water at 20° C. is 2 g or less; wherein the formula (I). R 1 is a hydrogen atom or a methyl group, R 2 is a hydrogen atom or a C 1 to C 4 alkyl group, AO is a C 2 to C 8 oxyalkylene group, p is 0 or 1, n is a total average number of mols of AO added and ranges from 9 to 250, and the proportion of an oxyethylene group in (AO) n is 80 mol% or more, and wherein pH of the composition is less than 7.