Polishing composition for magnetic disk substrate

Abstract

A polishing composition for a magnetic disk substrate of the present invention includes water, silica particles, and at least one or more selected from an acid, a salt of the acid, and an oxidizing agent. The silica particles are observed with a transmission electron microscope to measure a maximum diameter and a projected area of each particle, and a value obtained by dividing the area of a circle whose diameter is the maximum diameter of a silica particle by the projected area of the silica particle and multiplying the result by 100, is in the range of 100 to 130.

What is claimed is: 
     
         1 . A polishing composition for a magnetic disk substrate comprising:
 water;   silica particles; and   at least one or more selected from the group consisting of an acid, a salt of the acid, and an oxidizing agent,   wherein the silica particles have a value in the range of 100 to 130, wherein the value is obtained by dividing an area of a circle whose diameter is the maximum diameter of the silica particle by the projected area of the silica particle and multiplying the result by 100, when observed with a transmission electron microscope to measure the maximum diameter and the projected area of each particle.   
     
     
         2 . The polishing composition according to  claim 1 , wherein in a graph of particle size versus cumulative volume frequency obtained by plotting a cumulative volume frequency (%) of the silica particles from a smaller particle size side against a particle size (nm) of the silica particles measured by transmission electron microscope observation, the cumulative volume frequency (V) and the particle size (R) satisfy the following formula (1):
     V≧ 0.5× R+ 40   (1)
   where R is in a range of 40 to 100 nm, and   the cumulative volume frequency of the silica particles with a particle size of 15 nm is 20% or less.   
     
     
         3 . The polishing composition according to  claim 1 , wherein the silica particles are colloidal silica. 
     
     
         4 . The polishing composition according to  claim 1 , wherein an acid value of the polishing composition is 10 mgKOH/g or less. 
     
     
         5 . The polishing composition according to  claim 1 , wherein a content of the oxidizing agent is 0.1 to 1 wt %. 
     
     
         6 . The polishing composition according to  claim 1 , wherein the silica particles have a value in the range of 100 to 130, and the value is obtained by dividing an area of a circle whose circumference is a perimeter of the silica particle by the projected area of the silica particle and multiplying the result by 100, when observed with a transmission electron microscope to measure the perimeter and the projected area of each particle. 
     
     
         7 . A method for manufacturing a magnetic disk substrate comprising:
 a step of polishing a substrate to be polished with a polishing composition comprising water, silica particles, and at least one or more selected from the group consisting of an acid, a salt of the acid, and an oxidizing agent,   wherein the silica particles have a value in the range of 100 to 130, wherein the value is obtained by dividing an area of a circle whose diameter is the maximum diameter of the silica particle by the projected area of the silica particle and multiplying the result by 100, when observed with a transmission electron microscope to measure the maximum diameter and the projected area of each particle.   
     
     
         8 . The method according to  claim 7 , wherein in a graph of particle size versus cumulative volume frequency obtained by plotting a cumulative volume frequency (%) of the silica particles from a smaller particle size side against a particle size (nm) of the silica particles measured by transmission electron microscope observation, the cumulative volume frequency (V) and the particle size (R) satisfy the following formula (1):
     V≧ 0.5× R+ 40   (1)
   where R is in a range of 40 to 100 nm, and   the cumulative volume frequency of the silica particles with a particle size of 15 nm is 20% or less.   
     
     
         9 . The method according to  claim 7 , wherein the silica particles are colloidal silica. 
     
     
         10 . The method according to  claim 7 , wherein the substrate to be polished is polished at a polishing pressure of 11.7 to 20 kPa in the polishing step. 
     
     
         11 . The method according to  claim 7 , further comprising a final polishing step of polishing the substrate to be polished at a polishing pressure of less than 11.7 kPa after polishing the substrate to be polished at a polishing pressure of 11. 7 to 20 kPa in the polishing step. 
     
     
         12 . The method according to  claim 7 , wherein the silica particles have a value in the range of 100 to 130, and the value is obtained by dividing an area of a circle whose circumference is a perimeter of the silica particle by the projected area of the silica particle and multiplying the result by 100, when observed with a transmission electron microscope to measure the perimeter and the projected area of each particle.