A polishing agent for magnetic disk substrates: composition and method for polishing magnetic disk substrates.

Abstract

The present invention provides an abrasive composition for magnetic disk substrates that achieves a high polishing speed and reduces shallow pitting on the substrate surface without using alumina particles. [Solution] The polishing composition for magnetic disk substrates contains colloidal silica, wet silica particles, sorbitan fatty acid ester and / or sucrose fatty acid ester, acid and / or salt thereof, an oxidizing agent, and water, wherein the proportion of particles with a particle diameter of 30 to 70 nm in the colloidal silica is 10 to 90% by volume, the sorbitan fatty acid ester is at least one compound selected from sorbitan monolaurate, sorbitan monomyristate, etc., and the sucrose fatty acid ester is at least one compound selected from sucrose monolaurate, sucrose monomyristate, sucrose monopalmitate, etc.

Description

【Technical Field】 【0001】 The present invention relates to a polishing agent composition for a magnetic disk substrate and a method for polishing a magnetic disk substrate. More specifically, the present invention relates to a polishing agent composition for a magnetic disk substrate used for polishing electronic components such as magnetic recording media such as semiconductors and hard disks, and relates to a polishing agent composition for a magnetic disk substrate used for surface polishing of substrates for magnetic recording media such as glass magnetic disk substrates and aluminum magnetic disk substrates. 【0002】 In particular, the present invention relates to a polishing agent composition for a magnetic disk substrate used for surface polishing of an aluminum magnetic disk substrate for a magnetic recording medium having an electroless nickel-phosphorus plating film formed on the surface of a substrate made of an aluminum alloy, and a glass magnetic disk substrate and an aluminum magnetic disk substrate using the polishing agent composition for the magnetic disk substrate. And a method for polishing such magnetic disk substrates. 【Background Art】 【0003】 Conventionally, as a polishing agent composition for polishing the surface of an electroless nickel-phosphorus plating film of an aluminum magnetic disk substrate, a polishing agent composition in which alumina particles having a relatively large particle size, which can achieve a high polishing rate, are dispersed in water has been used from the viewpoint of productivity such as production efficiency. It was often used. 【0004】 However, since alumina particles have a higher hardness than the electroless nickel-phosphorus plating film formed on the substrate surface, the alumina particles are held in a state of piercing the substrate surface during polishing, and the alumina particles in such a state are the next step of the polishing process. There is a problem that it may affect the final polishing step (finishing polishing step). 【0005】 To solve the above problems, for example, the use of an abrasive composition in which alumina particle components and silica particle components are mixed and combined in a predetermined ratio has been proposed (see Patent Documents 1 to 4). Furthermore, a polishing method has also been proposed using an abrasive composition that contains only silica particles and no alumina particles (see Patent Documents 5 to 10). [Prior art documents] [Patent Documents] 【0006】 [Patent Document 1] Japanese Patent Publication No. 2001-260005 [Patent Document 2] Japanese Patent Publication No. 2009-176397 [Patent Document 3] Japanese Patent Publication No. 2011-204327 [Patent Document 4] Japanese Patent Publication No. 2012-43493 [Patent Document 5] Japanese Patent Publication No. 2010-167553 [Patent Document 6] Special Publication No. 2011-527643 [Patent Document 7] Japanese Patent Publication No. 2014-29754 [Patent Document 8] Japanese Patent Publication No. 2014-29755 [Patent Document 9] Special Publication No. 2003-514950 [Patent Document 10] Japanese Patent Publication No. 2012-155785 [Overview of the Initiative] [Problems that the invention aims to solve] 【0007】 However, the abrasive compositions or methods for polishing magnetic disk substrates using the abrasive compositions described in Patent Documents 1 to 10 above sometimes had the following problems. 【0008】 For example, by using abrasive compositions combining alumina particles and silica particles as shown in Patent Documents 1 to 4, it was possible to improve to some extent the penetration of alumina particles into the substrate surface of magnetic disk substrates. However, because alumina particles were included, the possibility of these particles penetrating the substrate surface still remained. In addition, because the abrasive composition contained both alumina and silica particles, the properties of the individual particle components canceled each other out, resulting in a decrease in the properties of the abrasive composition, such as polishing speed and surface smoothness. 【0009】 Therefore, polishing compositions consisting solely of silica particles without using alumina particles, and polishing methods using such polishing compositions have been proposed. For example, a combination of colloidal silica and a polishing accelerator is known (see Patent Documents 5 and 6). Furthermore, polishing methods using colloidal silica, fumed silica, surface-modified silica, and silica produced by the water glass method, as well as polishing methods using colloidal silica with special shapes, have been proposed (see Patent Documents 7 and 8). However, in the case of the above-proposed polishing methods, the polishing performance may be inferior to that of conventional polishing compositions, such as insufficient polishing speed, and further improvements have been requested. 【0010】 Furthermore, a polishing method using an abrasive composition combining colloidal silica and fumed silica has been proposed (see Patent Document 9). However, while such abrasive composition improves the polishing speed, the low bulk density of fumed silica makes it difficult to create a high-concentration slurry of the abrasive composition, which negatively impacts workability. 【0011】 Furthermore, a polishing method has been proposed that uses crushed silica particles to achieve a polishing speed close to that of alumina particles (see Patent Document 10). However, this polishing method has the problem of increasing concave defects such as pits and shallow pits and other surface defects compared to conventional abrasive compositions, resulting in a deterioration of surface smoothness, and further improvements have been needed. 【0012】 In the above, "shallow pit" refers to a type of concave defect present on the surface of a magnetic disk substrate, which is a shallower and smaller indentation than the pits previously considered problematic. It has become widely recognized, thanks to recent improvements in the measurement accuracy of surface defect measuring equipment, that such shallow pits can exist on the substrate surface even when no pits are present. 【0013】 Furthermore, with the recent increase in recording capacity due to the reduction in the unit recording area of ​​magnetic disks, shallow pits—tiny concave defects that were not previously a problem—have come to be recognized as surface defects that hinder the reduction in unit recording area. As a result, in addition to eliminating pits, reducing shallow pits has become a new challenge. 【0014】 Therefore, the present invention has been made in view of the problems of the prior art described above, and provides an abrasive composition that can achieve a high polishing speed without using alumina particles and can also achieve a substrate surface with reduced shallow pits, and a method for polishing a magnetic disk substrate using the abrasive composition. [Means for solving the problem] 【0015】 The inventors of this application, after diligently studying to solve the above problems, have found that by combining colloidal silica and wet-process silica particles, and using an abrasive composition containing sorbitan fatty acid ester and / or sucrose fatty acid ester, an acid and / or its salt, and an oxidizing agent, a magnetic disk substrate with reduced shallow pits can be obtained while maintaining the polishing speed. As a result, they have completed the abrasive composition and method for polishing magnetic disk substrates shown below. 【0016】 [1] A polishing agent composition for a magnetic disk substrate, containing colloidal silica having an average primary particle diameter of 10 to 120 nm, pulverized wet-process silica particles having an average particle diameter of 200 to 500 nm, sorbitan fatty acid ester and / or sucrose fatty acid ester, an acid and / or its salt, an oxidizing agent, and water, wherein the proportion of particles having a particle diameter of 30 to 70 nm in the colloidal silica is 10 to 90% by volume, the sorbitan fatty acid ester is at least one compound selected from sorbitan monolaurate, sorbitan monomyristate, sorbitan monopalmitate, sorbitan monostearate, and sorbitan monooleate, and the sucrose fatty acid ester is at least one compound selected from sucrose monolaurate, sucrose monomyristate, sucrose monopalmitate, sucrose monostearate, and sucrose monooleate. 【0017】 [2] The polishing agent composition for a magnetic disk substrate according to [1], wherein the mass ratio of the colloidal silica in all the silica particles composed of the colloidal silica and the wet-process silica particles is 20 to 80% by mass, and the mass ratio of the wet-process silica particles in all the silica particles is 20 to 80% by mass. 【0018】 [3] The polishing agent composition for a magnetic disk substrate according to [1], wherein the pH value (25 °C) of the polishing agent composition is in the range of 0.1 to 4.0. 【0019】 [4] The polishing agent composition for a magnetic disk substrate according to [1], wherein the sorbitan fatty acid ester is at least one compound selected from sorbitan monomyristate, sorbitan monopalmitate, sorbitan monostearate, and sorbitan monooleate. 【0020】 [5] The abrasive composition for magnetic disk substrates according to [1], wherein the sucrose fatty acid ester is at least one compound selected from sucrose monomyristate, sucrose monopalmitate, sucrose monostearate, and sucrose monooleate. 【0021】 [6] The oxidizing agent is the polishing composition for magnetic disk substrates according to [1], comprising hydrogen peroxide. 【0022】 [7] A method for polishing a magnetic disk substrate using the polishing compound composition for magnetic disk substrates described in any of [1] to [6] above, wherein the magnetic disk substrate to be polished is an aluminum alloy substrate on which a nickel-phosphorus plating film has been formed on the substrate surface, and a multi-stage polishing method is employed in which at least two polishing steps are repeated on the magnetic disk substrate, and the polishing compound composition for magnetic disk substrates is used in a polishing step prior to the final polishing step performed on the magnetic disk substrate. [Effects of the Invention] 【0023】 The present invention provides a method for polishing the surface of a magnetic disk substrate that reduces shallow pits on the substrate surface while maintaining the polishing speed, by using an abrasive composition containing two types of silica particles, sorbitan fatty acid ester and / or sucrose fatty acid ester, an acid and / or its salt, and an oxidizing agent. [Modes for carrying out the invention] 【0024】 The present invention is not limited to the following embodiments, and modifications, alterations, and improvements may be made without departing from the scope of the invention. 【0025】 1. Abrasive composition The abrasive composition of the present invention (abrasive composition for magnetic disk substrates) contains colloidal silica, wet silica particles, sorbitan fatty acid ester and / or sucrose fatty acid ester, acid and / or salt thereof, an oxidizing agent, and water as essential components. 【0026】 2. Colloidal silica The colloidal silica contained in the abrasive composition of the present invention may have an average primary particle diameter in the range of 10 to 120 nm, preferably in the range of 10 to 110 nm, and more preferably in the range of 15 to 100 nm. An average primary particle diameter of 10 nm or more for the colloidal silica can suppress a decrease in polishing speed. In other words, if the average primary particle diameter of the colloidal silica is less than 10 nm, the polishing speed of the abrasive composition may decrease, potentially affecting the work efficiency in the polishing process. On the other hand, an average primary particle diameter of 120 nm or less for the colloidal silica can suppress an increase in shallow pits and maintain good surface smoothness. In other words, if the average primary particle diameter of the colloidal silica exceeds 120 nm, there is a concern that shallow pits will increase, worsening the surface smoothness of the object to be polished. 【0027】 The proportion of particles with a particle size of 30 to 70 nm in colloidal silica is in the range of 10 to 90 volume%, preferably in the range of 12 to 80 volume%. Having a proportion of 30 to 70 nm particles within this range has the effect of suppressing the increase of shallow pits. In other words, if the proportion of particles with a particle size of 30 to 70 nm in the colloidal silica used is outside this range, it becomes difficult to suppress the increase of shallow pits, making it difficult to ensure sufficient surface smoothness. 【0028】 Colloidal silica is known to exist in various shapes, including spherical, chain-like, konpeito-shaped (particulate with protrusions on the surface), and irregular shapes, and is monodispersed in water to form a colloid. In the present invention, colloidal silica with a spherical or nearly spherical shape is particularly preferred. By using such spherical or nearly spherical colloidal silica, surface smoothness can be further improved. Colloidal silica can be formed by employing conventionally known methods such as the water glass method using sodium silicate or potassium silicate as a raw material, the alkoxysilane method in which alkoxysilanes such as tetraethoxysilane are hydrolyzed with acid or alkali, or a method in which metallic silicon and water are reacted in the presence of an alkaline catalyst to generate hydrogen and form silica particles. 【0029】 3. Wet-process silica particles The wet-process silica particles contained in the abrasive composition of the present invention are prepared from silica particles obtained as precipitated silicic acid by adding an alkaline silicate aqueous solution and an inorganic acid or an inorganic acid aqueous solution to a reaction vessel. Note that the wet-process silica particles described herein do not include the colloidal silica mentioned above. 【0030】 Examples of alkali silicate aqueous solutions used as raw materials for wet-process silica particles include sodium silicate aqueous solution, potassium silicate aqueous solution, and lithium silicate aqueous solution, with sodium silicate aqueous solution generally preferred. On the other hand, examples of inorganic acids added to the reaction vessel along with the sodium silicate aqueous solution include sulfuric acid, hydrochloric acid, and nitric acid, with sulfuric acid generally preferred. 【0031】 In the reaction vessel, various components such as an aqueous solution of alkali silicate and an inorganic acid, which are the raw materials for wet-process silica particles, are added, and the dehydration and condensation reactions of alkali silicate are carried out at a predetermined temperature. After the reaction is complete, the reaction solution is filtered and washed with water, and then dried in a dryer until the moisture content is 6% by mass or less. The dryer used here is not particularly limited, and commonly used dryers such as static dryers, spray dryers, and fluidized bed dryers can be used. After that, the mixture is crushed in a commonly used pulverizer such as a jet mill, and further classification is performed to obtain the crushed wet-process silica particles described above. 【0032】 The resulting wet-process silica particles may be subjected to further calcination treatment. For example, calcination can be performed using a general calcination apparatus such as an electric furnace or rotary kiln. In this case, the calcination temperature for the wet-process silica particles can be set in the range of 600°C to 1100°C. 【0033】 Wet-processed silica particles may undergo a pulverization process after calcination. Wet-processed silica particles pulverized in this way have a particle shape composed of multiple corners, and are expected to have higher polishing performance compared to general spherical wet-processed silica particles that have not undergone pulverization. 【0034】 The average particle size of the obtained wet-process silica particles is in the range of 200 to 500 nm. An average particle size of 200 nm or more suppresses a decrease in polishing speed. In other words, if the average particle size of the wet-process silica particles is less than 200 nm, sufficient polishing performance may not be achieved, potentially leading to a decrease in polishing speed. On the other hand, an average particle size of 500 nm or less suppresses an increase in shallow pits and a deterioration of surface roughness. In other words, if the average particle size exceeds 500 nm, shallow pits increase, making it difficult to maintain a good surface condition. 【0035】 The combined concentration of colloidal silica and wet-process silica particles, in other words, the "total silica particle concentration," is preferably in the range of 1 to 50% by mass, and more preferably in the range of 2 to 40% by mass, relative to the total weight of the abrasive composition containing other components. A total silica particle concentration of 1% by mass or more in the abrasive composition suppresses a decrease in polishing speed. In other words, if the total silica particle concentration is less than 1% by mass, it becomes difficult to polish at a sufficient polishing speed. On the other hand, a total silica particle concentration of 50% by mass or less allows for maintaining a sufficient polishing speed without using more silica particles than necessary. In other words, if the total silica particle concentration exceeds 50% by mass, further improvement in polishing speed cannot be expected, which is economically disadvantageous. 【0036】 Here, the mass ratio of colloidal silica to the total mass of all silica particles in the colloidal silica and wet-process silica particles is preferably 20 to 80% by mass. A colloidal silica mass ratio of 20% or more suppresses the increase in shallow pits and maintains a good surface condition. In other words, if the colloidal silica mass ratio is less than 20%, shallow pits increase, making it difficult to maintain a good surface condition. On the other hand, a colloidal silica mass ratio of 80% or less suppresses a decrease in polishing speed. In other words, if the colloidal silica ratio exceeds 80% by mass, there is a risk of a decrease in polishing speed. 【0037】 The mass ratio of wet-process silica particles to the total mass of colloidal silica and wet-process silica particles is preferably in the range of 20 to 80% by mass. A mass ratio of 80% by mass or less for wet-process silica particles can suppress deterioration of the substrate surface roughness and an increase in shallow pits after polishing. In other words, if the mass ratio of wet-process silica particles exceeds 80% by mass, there is a risk of deterioration of the substrate surface roughness and an increase in shallow pits after polishing. On the other hand, a mass ratio of 20% by mass or more for wet-process silica particles can suppress a decrease in polishing speed. In other words, if the mass ratio of wet-process silica particles is less than 20% by mass, it becomes difficult to polish at a sufficient polishing speed. 【0038】 4. Other silica particles Examples of silica particles contained in the abrasive composition of the present invention include colloidal silica and wet-process silica particles, as well as fumed silica. 【0039】 Fumed silica is obtained by hydrolyzing volatile silane compounds (generally silicon tetrachloride is used) in a flame of a mixed gas of oxygen and hydrogen (around 1000°C). Fumed silica consists of extremely fine and high-purity silica particles. 【0040】 Comparing the dispersion states of fumed silica and colloidal silica, colloidal silica exists as individually dispersed primary particles, while fumed silica consists of numerous primary particles that aggregate and link together in a chain-like structure, forming secondary particles. This formation of secondary particles increases the retention force on the polishing pad, allowing for a faster polishing speed compared to colloidal silica. However, fumed silica has a low bulk density, making it difficult to achieve a high particle concentration in the dispersion, which presents a problem in terms of handling. 【0041】 5. Sorbitan fatty acid esters and / or sucrose fatty acid esters Examples of sorbitan fatty acid esters contained in the abrasive composition of the present invention include sorbitan monolaurate, sorbitan monomyristate, sorbitan monopalmitate, sorbitan monostearate, and sorbitan monooleate. Preferably, sorbitan monomyristate, sorbitan monopalmitate, sorbitan monostearate, and sorbitan monooleate are included. 【0042】 It is preferable that the sorbitan fatty acid ester does not contain an ethylene oxide adduct in its molecule. When a sorbitan ester containing an ethylene oxide adduct in its molecule, such as sorbitan oleate polyoxyethylene, is present with colloidal silica, it is undesirable because the colloidal silica tends to aggregate under the acidic conditions (pH value (25°C) of 0.1 to 4.0) which are the usage conditions for the abrasive composition of the present invention. 【0043】 Sorbitan fatty acid esters are esters of sorbitan and fatty acids and are used as food emulsifiers and food additives. Sorbitan has four hydroxyl groups, and fatty acids can be esterified to these hydroxyl groups to produce monoesters to tetraesters. 【0044】 However, in actual reactions, it is not possible to precisely control the number of fatty acids bound to one sorbitan molecule, and the product is obtained as a mixture of multiple molecular species. Since fractionation of the product is not easy and is not necessary, it is sold as a mixture. 【0045】 As a result, names such as "sorbitan monolaurate" are understood as the overall molar ratio of sorbitan to fatty acid. For example, in the case of sorbitan monolaurate, it means that sorbitan and lauric acid were reacted in a molar ratio of 1:1, and does not indicate that the product is a single monoester. 【0046】 The content of sorbitan fatty acid ester in the abrasive composition of the present invention is in the range of 0.0001 to 1.0% by mass, and preferably in the range of 0.001 to 0.5% by mass. 【0047】 If the sorbitan fatty acid ester content in the abrasive composition is less than 0.0001% by mass, the shallow pit reduction effect is not observed. On the other hand, even if the content exceeds 1.0% by mass, the shallow pit reduction effect does not increase. 【0048】 Examples of sucrose fatty acid esters used in the present invention include sucrose monolaurate, sucrose monomyristate, sucrose monopalmitate, sucrose monostearate, and sucrose monooleate. Preferably, these include sucrose monomyristate, sucrose monopalmitate, sucrose monostearate, and sucrose monooleate. 【0049】 Sucrose fatty acid esters are esters of sucrose and fatty acids, and are used in dairy products, processed oils and fats, and processed foods. Sucrose has eight hydroxyl groups, and fatty acids can be esterified to these hydroxyl groups to produce monoesters to octaesters. However, in actual reactions, it is not possible to precisely control the number of fatty acids attached to a single sucrose molecule, and the product is obtained as a mixture of multiple molecular species. Since fractionation of the product is not easy and is not necessary, it is sold as a mixture. 【0050】 As a result, names such as "sucrose monolaurate ester" are understood as the overall molar ratio of sucrose to fatty acid. For example, in the case of sucrose monolaurate ester, it means that sucrose and lauric acid were reacted in a molar ratio of 1:1, and does not indicate that the product is a single monoester. 【0051】 The content of sucrose fatty acid ester in the abrasive composition of the present invention is in the range of 0.0001 to 1.0% by mass, and preferably in the range of 0.001 to 0.5% by mass. 【0052】 If the sucrose fatty acid ester content in the abrasive composition is less than 0.0001% by mass, the shallow pit reduction effect is not observed. On the other hand, even if the content exceeds 1.0% by mass, the shallow pit reduction effect does not increase. 【0053】 6. Oxidizing agent Oxidizing agents contained in the abrasive composition of the present invention include peroxides, permanganic acid or its salts, chromic acid or its salts, periodic acid or its salts, etc. Specific examples include hydrogen peroxide, sodium peroxide, barium peroxide, potassium permanganate, orthoperiodic acid, sodium metaperiodate, etc. Among these, hydrogen peroxide is preferred. 【0054】 The oxidizing agent content in abrasive compositions is typically in the range of 0.1 to 10.0% by mass. A content of 0.1% by mass or more of the oxidizing agent improves the polishing speed. On the other hand, a content of 10.0% by mass or more of the oxidizing agent does not improve the polishing speed, making it economically disadvantageous. 【0055】 7. Acids and / or salts thereof The abrasive composition of the present invention contains an acid and / or a salt thereof, and specifically may include an inorganic acid and / or a salt thereof, or an organic acid and / or a salt thereof. 【0056】 Specific examples of inorganic acids and / or their salts include sulfuric acid, hydrochloric acid, nitric acid, phosphorus-containing inorganic acids, and / or their salts. Among inorganic acids and / or their salts, it is preferable to include phosphorus-containing inorganic acids and / or their salts. 【0057】 Specific examples of phosphorus-containing inorganic acids and / or their salts include phosphoric acid, phosphonic acid, phosphinic acid, pyrophosphate, tripolyphosphate, and / or salts thereof. Specific examples of organic acids and / or their salts include organic carboxylic acids and / or their salts such as acetic acid, lactic acid, citric acid, maleic acid, malonic acid, malic acid, oxalic acid, and tartaric acid, as well as organic phosphonic acids and / or their salts. Among organic acids and / or their salts, it is preferable to include organic phosphonic acids and / or their salts. Specific examples of organic phosphonic acids and / or their salts include 2-aminoethylphosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, aminotri(methylenephosphonic acid), ethylenediaminetetramethylenephosphonic acid, diethylenetriaminepenta(methylenephosphonic acid), ethane-1,1-diphosphonic acid, and / or salts thereof. 【0058】 Phosphorus-containing inorganic acids and / or their salts can be used in combination with organic phosphonic acids and / or their salts. Furthermore, phosphorus-containing inorganic acids and organic phosphonic acids can be used in combination with nitric acid, sulfuric acid, hydrochloric acid, etc. Additionally, phosphorus-containing inorganic acids and organic phosphonic acids can be used in combination with polycarboxylic acids such as citric acid, tartaric acid, oxalic acid, and maleic acid. 【0059】 The amount of acid used can be appropriately determined by setting the pH value (at 25°C) of the abrasive composition. 【0060】 8. Water The water used in the abrasive composition of the present invention is a medium for dispersing the other components of the abrasive composition, and pure water, ultrapure water, and distilled water can be preferably used. Furthermore, a small amount of an organic medium such as alcohol may be included relative to the water in order to smoothly disperse the other components of the abrasive composition. 【0061】 9. Other ingredients The abrasive composition of the present invention may contain particles other than colloidal silica and wet-process silica. However, from the viewpoint of reducing the penetration of alumina particles into the substrate to be polished, it is preferable that the abrasive composition does not contain alumina particles. In addition, the abrasive composition may contain water-soluble polymer compounds, antifungal agents, antibacterial agents, etc., in addition to the above-mentioned components. As water-soluble polymer compounds, copolymers in which a monomer having a carboxylic acid group and a monomer having an amide group are essential monomers, and copolymers in which a monomer having a carboxylic acid group and a monomer having a sulfonic acid group are essential monomers are preferred, and it is even more preferable to use the above two copolymers in combination. 【0062】 10. Physical properties The pH value of the abrasive composition of the present invention at 25°C is preferably 0.1 to 4.0, and more preferably 0.5 to 1.8. A pH value (at 25°C) of 0.1 or higher of the abrasive composition can suppress deterioration of the surface smoothness of the substrate after polishing. On the other hand, a pH value (at 25°C) of 4.0 or lower of the abrasive composition can suppress a decrease in polishing speed. 【0063】 In polishing electroless nickel-phosphorus plated aluminum magnetic disk substrates, in which the abrasive composition of the present invention is suitably used, the electroless nickel-phosphorus plating film tends to dissolve under conditions where the pH value at 25°C is 4.0 or less. Therefore, from the viewpoint of improving the polishing speed, an abrasive composition with a pH value of 4.0 or less at 25°C is preferably used. 【0064】 11. Method for polishing magnetic disk substrates A polishing method to which the abrasive composition of the present invention can be applied includes, for example, attaching a polishing pad to the base plate of a polishing machine, supplying the abrasive composition to the surface of the object to be polished (e.g., an aluminum magnetic disk substrate) or to the polishing pad, and rubbing the surface to be polished with the polishing pad. 【0065】 Furthermore, when polishing both the front and back surfaces of an aluminum magnetic disk substrate simultaneously, a double-sided polishing machine is used, with polishing pads attached to both the upper and lower polishing plates. 【0066】 The first step involves sandwiching the aluminum magnetic disk substrate between polishing pads attached to the upper and lower polishing plates, respectively. The second step involves supplying an abrasive composition between the polishing surface and the polishing pads. The third step involves rotating the two polishing pads in opposite directions. This polishes both the front and back surfaces of the aluminum magnetic disk substrate. 【0067】 The present invention's method for polishing magnetic disk substrates is employed in a multi-stage polishing system comprising a polishing step and a final polishing step performed after the polishing step. The polishing composition of the present invention is preferably used in a polishing step (= rough polishing step) performed before the final polishing step (= finish polishing step). More preferably, the present invention's method for polishing magnetic disk substrates is used for polishing aluminum magnetic disk substrates in which an electroless nickel-phosphorus plating film is formed on the substrate surface of an aluminum alloy substrate, and more preferably, it is used in a polishing step performed before the final polishing step of an electroless nickel-phosphorus plated aluminum magnetic disk substrate. By using the polishing composition of the present invention in such a polishing step, the effects of the present invention can be fully enjoyed. 【0068】 Furthermore, the polishing pads used in the polishing process are not particularly limited, and non-woven fabric type and suede type polishing pads can be used, with suede type polishing pads being particularly commonly used. In addition, materials such as polyurethane elastomer, polystyrene, polyester, and polyvinyl chloride can be used as the material of the surface foam layer that comes into contact with the abrasive composition, with polyurethane elastomer being particularly commonly used. [Examples] 【0069】 The present invention will be described in detail below based on examples, but the present invention is not limited to these examples and can be implemented in various forms as long as they fall within the technical scope of the present invention. 【0070】 <1> Method for preparing an abrasive composition The abrasive compositions of Examples 1-17 and Comparative Examples 1-7 were prepared using the components listed in Table 1 below, and in the amounts specified in Table 1. In all Examples 1-17 and Comparative Examples 1-7, the total silica concentration in the abrasive composition was prepared to be 4.0% by mass. 【0071】 [Table 1] 【0072】 Here, in Table 1, HEDP represents 1-hydroxyethylidene-1,1-diphosphonic acid. In the following descriptions herein, abbreviations such as HEDP will be used as appropriate. 【0073】 <2> Examples 1-17, Comparative Examples 1-7 The specific preparation methods for the abrasive compositions of Examples 1-17 and Comparative Examples 1-7 are as follows. 【0074】 <2-1> Example 1 The abrasive composition of Example 1 was obtained by stirring and mixing commercially available colloidal silica I (average particle size (D50) = 75 nm, proportion of particles with particle size 30-70 nm = 35 volume%), commercially available wet-process silica I (average particle size (D50) = 300 nm), sorbitan monolaurate, sulfuric acid, and hydrogen peroxide with pure water to the contents listed in Table 1. Here, the mixing ratio (mass ratio) of commercially available colloidal silica I and commercially available wet-process silica I is 60:40, with sulfuric acid corresponding to the acid in the present invention and hydrogen peroxide corresponding to the oxidizing agent in the present invention. 【0075】 <2-2> Example 2 The abrasive composition of Example 2 was prepared in the same manner as in Example 1, except that the content of sorbitan monolaurate was changed to the content shown in Table 1. 【0076】 <2-3> Example 3 In preparing the abrasive composition of Example 2, the only difference was that commercially available colloidal silica I was replaced with commercially available colloidal silica II (average particle size (D50): 75 nm, with a proportion of 30-70 nm particles being 14% by volume). The abrasive composition of Example 3 was then prepared in the same manner as in Example 2. 【0077】 <2-4> Example 4 In preparing the abrasive composition of Example 2, the preparation was carried out in the same manner as in Example 2, except that sorbitan monomyristate was added instead of sorbitan monolaurate, to obtain the abrasive composition of Example 4. 【0078】 <2-5> Example 5 In preparing the abrasive composition of Example 1, the preparation was carried out in the same manner as in Example 1, except that sorbitan monooleate was added instead of sorbitan monolaurate, to obtain the abrasive composition of Example 5. 【0079】 <2-6> Example 6 In preparing the abrasive composition of Example 2, the preparation was carried out in the same manner as in Example 2, except that sorbitan monooleate was added instead of sorbitan monolaurate, to obtain the abrasive composition of Example 6. 【0080】 <2-7> Example 7 In preparing the abrasive composition of Example 6, the preparation was carried out in the same manner as in Example 6, except that phosphoric acid was added in the amount shown in Table 1 instead of sulfuric acid, to obtain the abrasive composition of Example 7. 【0081】 <2-8> Example 8 The abrasive composition of Example 8 was prepared in the same manner as in Example 7, except that HEDP was further added to the amount shown in Table 1. 【0082】 <2-9> Example 9 In preparing the abrasive composition of Example 1, the abrasive composition of Example 9 was obtained by adding sucrose monolaurate ester in the amount shown in Table 1, in place of sorbitan monolaurate, in the same manner as in Example 1. 【0083】 <2-10> Example 10 In preparing the abrasive composition of Example 2, the preparation was carried out in the same manner as in Example 2, except that sucrose monolaurate was added in the amount shown in Table 1 instead of sorbitan monolaurate, to obtain the abrasive composition of Example 10. 【0084】 <2-11> Example 11 In preparing the abrasive composition of Example 10, the abrasive composition of Example 11 was obtained by adding sucrose monomyristate ester in the amount shown in Table 1, instead of sucrose monolaurate ester. 【0085】 <2-12> Example 12 In preparing the abrasive composition of Example 10, the only difference was that sucrose monopalmitate was added in the same manner as in Example 10, except that it was added in the amount shown in Table 1 instead of sucrose monolaurate, to obtain the abrasive composition of Example 12. 【0086】 <2-13> Example 13 In preparing the abrasive composition of Example 10, the abrasive composition of Example 13 was obtained by adding sucrose monostearate in the amount shown in Table 1, instead of sucrose monolaurate. 【0087】 <2-14> Example 14 In preparing the abrasive composition of Example 6, the only difference was that the mixing ratio (mass ratio) of commercially available colloidal silica I and commercially available wet-process silica I was changed to 70:30 as shown in Table 1. Otherwise, the abrasive composition of Example 14 was obtained in the same manner as in Example 6. 【0088】 <2-15> Example 15 In preparing the abrasive composition of Example 14, the preparation was carried out in the same manner as in Example 14, except that sorbitan monooleate was replaced with sucrose monostearate ester, to obtain the abrasive composition of Example 15. 【0089】 <2-16> Example 16 In preparing the abrasive composition of Example 6, the only difference was that the mixing ratio (mass ratio) of commercially available colloidal silica I and commercially available wet-process silica I was changed to 50:50 as shown in Table 1. Otherwise, the abrasive composition of Example 16 was obtained in the same manner as in Example 6. 【0090】 <2-17> Example 17 In preparing the abrasive composition of Example 16, the preparation was carried out in the same manner as in Example 16, except that sorbitan monooleate was replaced with sucrose stearate ester, to obtain the abrasive composition of Example 17. 【0091】 <3> Comparative Example <3-1> Comparative Example 1 In preparing the abrasive composition of Example 2, the preparation was carried out in the same manner as in Example 2, except that sorbitan monolaurate was not added, to obtain the abrasive composition of Comparative Example 1. 【0092】 <3-2> Comparative Example 2 In preparing the abrasive composition of Example 2, the preparation was carried out in the same manner as in Example 2, except that wet-process silica I was not added and colloidal silica I was added in the amount shown in Table 1, to obtain the abrasive composition of Comparative Example 2. 【0093】 <3-3> Comparative Example 3 In preparing the abrasive composition of Example 2, the preparation was carried out in the same manner as in Example 2, except that colloidal silica I was not added and wet-process silica I was added in the amount shown in Table 1, to obtain the abrasive composition of Comparative Example 3. 【0094】 <3-4> Comparative Example 4 In preparing the abrasive composition of Example 2, the preparation was carried out in the same manner as in Example 2, except that wet-processed silica II was added in the amount shown in Table 1 instead of wet-processed silica I, to obtain the abrasive composition of Comparative Example 4. 【0095】 <3-5> Comparative Example 5 In preparing the abrasive composition of Example 2, the only difference was that colloidal silica III (average particle size (D50): 100 nm, with a proportion of particles between 30 and 70 nm being 8 vol%) was added in the same manner as in Example 2, except that it was added in the amount shown in Table 1, to obtain the abrasive composition of Comparative Example 5. 【0096】 <3-6> Comparative Example 6 In preparing the abrasive composition of Example 14, the preparation was carried out in the same manner as in Example 14, except that sorbitan monooleate was not added, to obtain the abrasive composition of Comparative Example 6. 【0097】 <3-3> Comparative Example 7 In preparing the abrasive composition of Example 16, the preparation was carried out in the same manner as in Example 16, except that sorbitan monooleate was not added, to obtain the abrasive composition of Comparative Example 7. 【0098】 <4> Measurement, conditions, and evaluation of each physical property. <4-1> Method for measuring particle size and average particle size of colloidal silica The particle size (Heywood diameter) of colloidal silica was measured by taking a photograph of the field of view at a magnification of 100,000x using a transmission electron microscope (TEM) (JEOL Ltd., Transmission Electron Microscope JEM2000FX (200kV)), and then analyzing this photograph using analysis software (Mountec Co., Ltd., Mac-View Ver.4.0) to determine the Heywood diameter (equivalent diameter of the projection area circle). 【0099】 The average particle size of colloidal silica is calculated by analyzing the particle sizes of approximately 2000 colloidal silica particles using the method described above, and then using the analysis software (Mac-View Ver. 4.0, manufactured by Mountec Co., Ltd.) to determine the particle size at which the cumulative particle size distribution (based on cumulative volume) from the smallest particle size side accounts for 50%. 【0100】 The proportion of particles with a particle size of 30-70 nm in colloidal silica can be calculated by analyzing the particle sizes of approximately 2000 colloidal silica particles using the method described above, and then calculating the percentage of the cumulative particle size distribution (based on cumulative volume) from the small particle size side at 30 nm and the percentage of the cumulative particle size distribution (based on cumulative volume) from the small particle size side at 70 nm using the above-mentioned analysis software (Mac-View Ver. 4.0, manufactured by Mountec Co., Ltd.), and then calculating it from the following formula (1). Percentage of particles with a diameter of 30-70 nm (%) = Percentage of cumulative particle size distribution at a particle size of 70 nm (%) - Percentage of cumulative particle size distribution at a particle size of 30 nm (%) ... (1) 【0101】 <4-2> Wet Method for Measuring the Average Particle Size of Silica Particles The average particle size of wet-process silica particles was measured using a dynamic light scattering particle size distribution analyzer (Microtrac UPA, manufactured by Nikkiso Co., Ltd.) for particles 400 nm or smaller, and a laser diffraction particle size distribution analyzer (SALD2200, manufactured by Shimadzu Corporation) for particles larger than 400 nm. The average particle size of wet-process silica particles is defined as the average particle size (D50) at which the cumulative particle size distribution from the smallest particle size side, based on volume, accounts for 50%. 【0102】 <4-3> Polishing conditions The object to be polished was an aluminum disk substrate with an outer diameter of 97 mm, which was an aluminum alloy substrate with electroless nickel-phosphorus plating. The aluminum disk substrate was polished under the following polishing conditions. Polishing machine: SPEEDFAM (Co., Ltd.), 9B double-sided polishing machine Polishing pad: P1 pad, manufactured by FILWEL Co., Ltd. Pad life: 50 hours or more Plate rotation speed: Upper plate - 30.0 rpm Lower surface plate 55.0rpm Abrasive composition supply rate: 45 ml / min Polishing time: Polish until the polishing amount reaches 1.0-1.5 μm per side (180-720 seconds). Processing pressure: 120kPa 【0103】 Table 2 shows the polishing results for Examples 1-13 and Comparative Examples 1-5. For Examples 1-13 and Comparative Examples 1, 4, and 5, the colloidal silica:wet-processed silica ratio is 60:40 (mass ratio). Table 3 shows the polishing results for Examples 14, 15, and Comparative Example 6. For Examples 14, 15, and Comparative Example 6, the colloidal silica:wet-processed silica ratio is 70:30 (mass ratio). Table 4 shows the polishing results for Examples 16, 17, and Comparative Example 7. For Examples 16, 17, and Comparative Example 7, the colloidal silica:wet-processed silica ratio is 50:50 (mass ratio). 【0104】 [Table 2] 【0105】 [Table 3] 【0106】 [Table 4] 【0107】 <4-4> Polishing speed ratio The polishing speed was calculated by measuring the mass of the aluminum disc substrate after polishing and using the following formula (2). Polishing speed (μm / min) = Mass loss of aluminum disc substrate (g) / Polishing time (min) / Area of ​​one side of aluminum disc substrate (cm²) 2 ) / Density of electroless nickel-phosphorus plating film (g / cm³) 3 ) / 2×10 4 ...Equation (2) (However, in formula (2) above, the area of ​​one side of the aluminum disk substrate is 69 cm².) 2 The density of the electroless nickel-phosphorus plating film is 8.0 g / cm³. 3 (Calculated as) 【0108】 The polishing speed ratios for each example in Tables 2 to 4 were calculated as relative values, with the polishing speed of the abrasive composition of Comparative Example 2, obtained using the above formula (2), set as the reference value (=1). 【0109】 <4-5> Shallow pit ratio Shallow pits were measured and evaluated using the W-ScopeED front and back defect inspection system manufactured by Wacom Denso Co., Ltd. 【0110】 • Evaluation method The evaluation method involved observing all 40 surfaces of the polished aluminum disc substrate—20 top surfaces and 20 bottom surfaces—using the inspection device described above, and evaluating the total number of shallow pits. In Table 2, the shallow pit ratio is expressed as a relative value with the total number of shallow pits in Comparative Example 1 set as the baseline value (=1). In Table 3, the shallow pit ratio is expressed as a relative value with the total number of shallow pits in Comparative Example 6 set as the baseline value (=1). In Table 4, the shallow pit ratio is expressed as a relative value with the total number of shallow pits in Comparative Example 7 set as the baseline value (=1). Comparative Example 3 was deemed "uncountable" because it had a large number of shallow pits that could not be measured. 【0111】 <5> Consideration As is clear from Table 2, compared to Comparative Example 1, which does not contain either sorbitan fatty acid ester or sucrose fatty acid ester in the abrasive composition, Examples 1-8, which contain sorbitan fatty acid ester, and Examples 9-13, which contain sucrose fatty acid ester, show a significant reduction in shallow pits. 【0112】 Compared to Comparative Example 2, which used an abrasive composition that did not contain wet-processed silica particles, Example 2, which used an abrasive composition that contained wet-processed silica particles, showed improved polishing speed. 【0113】 Comparative Example 3, which used an abrasive composition that did not contain colloidal silica, showed a significant increase in shallow pits compared to Example 2. 【0114】 Comparative Example 4, in which the average particle size of the wet-processed silica particles exceeds 500 nm, shows a significant increase in shallow pits compared to Example 2. 【0115】 Comparative Example 5, in which the proportion of particles with a particle size of 30-70 nm in the colloidal silica is less than 10 volume%, shows a significant increase in shallow pits compared to Examples 2 and 3, in which the proportion of particles with a particle size of 30-70 nm in the colloidal silica is 10 volume% or more. 【0116】 Compared to Example 1, Example 2 uses an abrasive composition with a higher sorbitan monolaurate content, resulting in an even more pronounced shallow pit reduction effect. 【0117】 Examples 4-6 show the results when the number of carbon atoms in the alkyl group of the sorbitan fatty acid ester is increased. 【0118】 Example 4 shows the results when sorbitan monomyristate was used as the sorbitan fatty acid ester, and the shallow pit reduction effect is significantly greater than that of the corresponding Example 2, where the sorbitan monolaurate content is higher. Examples 5 and 6 show the results when sorbitan monooleate was used as the sorbitan fatty acid ester, and the shallow pit reduction effect is even more significant than that of the corresponding Examples 1 and 2, where the sorbitan monolaurate content is higher. 【0119】 This indicates that the shallow pit reduction effect becomes even more pronounced as the number of carbon atoms in the alkyl group of the sorbitan fatty acid ester increases. 【0120】 Examples 6 to 8 all show results using abrasive compositions containing sorbitan monooleate. Compared to Example 6, which used sulfuric acid, Example 7, which used phosphoric acid, and Example 8, which used phosphoric acid and HEDP in combination, showed improved polishing speed and a more pronounced shallow pit reduction effect. 【0121】 Examples 9 and 10 both used abrasive compositions containing sucrose monolaurate ester, and Example 10, which has a higher sucrose monolaurate ester content, showed a more significant shallow pit reduction effect than Example 9. 【0122】 Examples 11-13 show the results when the number of carbon atoms in the alkyl group of the sucrose fatty acid ester is increased compared to Example 10. It can be seen that the shallow pit reduction effect becomes even more pronounced as the number of carbon atoms in the alkyl group of the sucrose fatty acid ester increases from myristic acid to palmitic acid to stearic acid. 【0123】 As is clear from Table 3, even when colloidal silica:wet-processed silica = 70:30 (mass ratio), compared to Comparative Example 6, which does not contain either sorbitan fatty acid ester or sucrose fatty acid ester in the abrasive composition, Example 14, which contains sorbitan fatty acid ester, and Example 15, which contains sucrose fatty acid ester, show a significant reduction in shallow pitting. Compared to Comparative Example 2, which uses an abrasive composition that does not contain wet-processed silica particles, Examples 14 and 15, which use abrasive compositions containing wet-processed silica particles, show improved polishing speed. 【0124】 As is clear from Table 4, even when colloidal silica:wet-processed silica = 50:50 (mass ratio), compared to Comparative Example 7, which does not contain either sorbitan fatty acid ester or sucrose fatty acid ester in the abrasive composition, Examples 16, which contain sorbitan fatty acid ester, and Example 17, which contains sucrose fatty acid ester, show a significant reduction in shallow pitting. Compared to Comparative Example 2, which uses an abrasive composition that does not contain wet-processed silica particles, Examples 16 and 17, which use abrasive compositions containing wet-processed silica particles, show improved polishing speed. 【0125】 From the above, it is clear that shallow pits can be reduced by using the abrasive composition of the present invention. [Industrial applicability] 【0126】 The polishing composition of the present invention can be used for polishing electronic components such as semiconductors and magnetic recording media such as hard disks. In particular, it can be used for surface polishing of substrates for magnetic recording media such as glass magnetic disks and aluminum magnetic disks. Furthermore, it can be used for surface polishing of aluminum substrates for magnetic recording media that have an electroless nickel-phosphorus plating film formed on them. In particular, it can be used in a polishing step prior to the final polishing step of aluminum substrates for magnetic recording media that have an electroless nickel-phosphorus plating film formed on them.

Claims

[Claim 1] Colloidal silica with an average primary particle size of 10 to 120 nm, Grinding wet-process silica particles with an average particle size of 200-500 nm, Sorbitan fatty acid ester and / or sucrose fatty acid ester, Acids and / or salts thereof, Oxidizing agent, Water and It contains, The proportion of particles with a particle size of 30 to 70 nm in the colloidal silica is 10 to 90% by volume. The sorbitan fatty acid ester mentioned above is It is at least one compound selected from sorbitan monolaurate, sorbitan monomyristate, sorbitan monopalmitate, sorbitan monostearate, and sorbitan monooleate. The aforementioned sucrose fatty acid ester is A polishing composition for magnetic disk substrates comprising at least one compound selected from sucrose monolaurate, sucrose monomyristate, sucrose monopalmitate, sucrose monostearate, and sucrose monooleate. [Claim 2] The mass percentage of colloidal silica in the total silica particles composed of the colloidal silica and the wet-process silica particles is 20 to 80% by mass. The polishing composition for magnetic disk substrates according to claim 1, wherein the mass ratio of the wet-process silica particles to the total silica particles is 20 to 80% by mass. [Claim 3] The polishing composition for magnetic disk substrates according to claim 1, wherein the pH value (at 25°C) of the polishing composition is in the range of 0.1 to 4.

0. [Claim 4] The sorbitan fatty acid ester mentioned above is The abrasive composition for magnetic disk substrates according to claim 1, comprising at least one compound selected from sorbitan monomyristate, sorbitan monopalmitate, sorbitan monostearate, and sorbitan monooleate. [Claim 5] The aforementioned sucrose fatty acid ester is The polishing composition for magnetic disk substrates according to claim 1, which is at least one compound selected from sucrose monomyristate, sucrose monopalmitate, sucrose monostearate, and sucrose monooleate. [Claim 6] The oxidizing agent is The polishing composition for magnetic disk substrates according to claim 1, comprising hydrogen peroxide. [Claim 7] A method for polishing a magnetic disk substrate using the polishing composition for magnetic disk substrates described in any one of claims 1 to 6, The target of polishing is a magnetic disk substrate with a nickel-phosphorus plating film formed on the surface of an aluminum alloy substrate. A multi-stage polishing method is employed, in which the magnetic disk substrate is subjected to at least two polishing steps. A method for polishing a magnetic disk substrate, wherein the polishing agent for the magnetic disk substrate is a composition used in a polishing step prior to the final polishing step performed on the magnetic disk substrate.

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