Polishing composition for nickel-phosphorous memory disks

1. A method of chemically-mechanically polishing a substrate comprising:

(i) contacting a substrate with a polishing pad and a chemical-mechanical polishing composition comprising:

(a) alpha alumina,

(b) fumed alumina,

(c) silica,

(d) an oxidizing agent that oxidizes nickel-phosphorous,

(e) about 0.1 wt. % to about 2 wt. % of oxalic acid,

(f) about 0.1 wt % to 0.75 wt. % tartaric acid,

(g) a nonionic surfactant,

(h) optionally, a biocide, and

(i) water,

wherein the polishing composition has a pH of about 2 to about 4,

(ii) moving the polishing pad relative to the substrate with the chemical-mechanical polishing composition therebetween, and

(iii) abrading at least a portion of the substrate to polish the substrate,

wherein the substrate comprises a nickel-phosphorous-coated aluminum disk.

2. The method of claim 1 , wherein the polishing composition comprises about 0.5 wt. % to about 2 wt. % of alpha alumina, about 0.1 wt. % to about 1 wt. % of fumed alumina, and about 0.5 wt. % to about 2 wt. % of silica.

3. The method of claim 1 , wherein the alpha alumina has an average particle size of about 300 nm to about 800 nm.

4. The method of claim 3 , wherein the alpha alumina has an average particle size of about 400 nm to about 600 nm.

5. The method of claim 1 , wherein the fumed alumina has an average particle size of about 75 nm to about 150 nm.

6. The method of claim 1 , wherein the silica has an average particle size of about 20 nm to about 120 nm.

7. The method of claim 1 , wherein the oxidizing agent is hydrogen peroxide.

8. The method of claim 1 , wherein the polishing composition comprises about 10 ppm to about 200 ppm of the nonionic surfactant.

9. The method of claim 8 , wherein the nonionic surfactant is a nonylphenol ethoxylate or a siloxane ethoxylate.

10. The method of claim 1 , wherein the pH of the polishing composition is about 2 to about 3.

11. The method of claim 1 , wherein the disk is a memory disk.