Cerium based abrasive material

Details Cerium based abrasive material Cerium based abrasive material

2002-07-18

2003-07-01

Marcheschi, Michael (Department: 1755)

Abrasive tool making process, material, or composition

With inorganic material

Metal or metal oxide

C051S307000, C423S263000

Reexamination Certificate

active

06585787

ABSTRACT:

TECHNICAL FIELD
This invention relates to a method of evaluating cerium-based abrasives containing cerium oxide as a main ingredient, and to a cerium-based abrasive and a method of producing the same.
BACKGROUND ART
Cerium-based abrasives containing cerium oxide (CeO
2
) as a main ingredient have been used for polishing a wide variety of glass materials, and in recent years they have been used particularly for polishing glass materials for use in electric and electronic equipment, for example, glass for magnetic recording media such as hard disks and glass substrates of liquid crystal display (LCD), and have found their application in a wider field.
The cerium-based abrasives are divided into two types: high-cerium abrasives (containing 70% or more cerium oxide) and low-cerium abrasives (containing roughly 50% cerium oxide), depending on the cerium oxide content in the total rare earth oxide content (hereinafter referred to as TREO for short); however, there is no major difference in production process between the above two types and the production is performed in the steps of: chemical treatment of the raw material (wet treatment), filtration, roasting after drying, grinding and classifying. As a raw material, a natural material, bastnasite concentrate produced by the ore dressing of rare earth ores called bastnasite, was often used; however, in recent years rare earth oxides or rare earth carbonates artificially synthesized from bastnasite ores or relatively inexpensive complex ores produced in China have come into wide use.
The reasons the cerium-based abrasives are in wide use are that the application of the cerium-based abrasives provides a polished surface of high accuracy, in addition, allows a large amount of glass materials to be polished and removed in a relatively short period of time because of their grindability. As for the polishing mechanism of the cerium-based abrasives, though there is no very clear fixed theory of it, it is considered that a fluorine component contained in the cerium-based abrasives plays an important role. Specifically, it is considered that the cerium-based abrasives have not only a mechanical polishing function, which abrasives generally have, due to the polishing particles consisting mainly of cerium oxide, but also a chemical polishing function such that the fluorine component contained in the cerium-based abrasives reacts with the glass surface and forms a fluoride, which promotes the attack on the glass surface.
As described above, the cerium-based abrasives exhibit their excellent polishing characteristics only when they can produce effects of both mechanical and chemical functions. And as for the criteria for the abrasive quality, the cerium-based abrasives are required to have the same qualities as those general abrasives are required to have; specifically, they are required to have polishing particles with a uniform particle diameter and contain no coarse particles which may cause scratches in the polished surface, and moreover, they are required to have a proper fluorine grade (concentration). In the cerium-based abrasives of the prior art, their fluorine content, particle diameter of polishing particles, roasting temperature and classifying conditions have been all properly controlled and thereby excellent abrasives have been supplied.
However, in view of the future demand for cerium-based abrasives, it is natural to desire the development of more excellent abrasives than ever. In the technical fields of hard disks, glass substrates for LCD, etc. in particular, there have been demands for the hard disks and the glass substrates to have still higher recording density. In order to meet these demands, it is considered, abrasives are required which have such higher grindability that provides a highly accurate polished surface and moreover allows speeding up of a given amount of polishing.
As for the details of the abrasives' fluorine treatment, roasting temperature, classifying conditions, etc., they have often been determined empirically. However, the correlation among them are not always simple, and it is difficult to estimate the polishing characteristics of abrasives particularly on the basis of the record during the production process, such as roasting temperature and classifying conditions. Accordingly, the evaluation of the polishing characteristics of cerium-based abrasives has been carried out by taking the trouble to actually polish glass materials and then measuring the polished values and observing the presence of scratches having occurred in a polished surface; however, the polishing test in which glass materials are actually polished is troublesome. Furthermore, it is impossible to evaluate the polishing characteristics of cerium-based abrasives simply by their fluorine content. Thus, a simpler evaluation criterion for abrasives is desired to be established, and it is considered that such a simple criterion is required particularly when a new abrasive is made by way of trial by changing the raw materials and production conditions used from the conventional ones to new ones.
This invention has been made in the light of the above circumstances; accordingly, an object of this invention is to provide a cerium-based abrasive, which enables the formation of a highly accurate polished surface and has improved grindability, and a method of producing the same. Further, another object of this invention is to provide a method of evaluating cerium-based abrasives in a relatively simple manner.
DISCLOSURE OF THE INVENTION
In order to solve the aforementioned problems, the inventors have conducted an intensive investigation thereof, resulting in directing their attention to changes in crystal structure of cerium oxide due to the behavior of fluorine during the process of producing cerium-based abrasives, in particular before and after roasting. And the inventors have come to the conclusion that the crystal structures of polishing particles contained in a cerium-based abrasive having a cerium oxide content of 40% or more (on the TREO basis) are all the same when they have been formed into an abrasive, the processes of the crystallization differ depending on the raw materials, though.
The crystal structure of the polishing particles contained in the cerium-based abrasives produced from rare earth oxides as a raw material is as follows. Cerium oxide in the raw material state before undergoing roasting exists in the form of a cerium oxide type of cubic crystal of chemical formula: Ce
x
Ln
y
O
z
(where Ln represents rare earth metal elements including cerium, and x, y and z are related to one another by the following equation: 2x≦z≦2 (x+y), hereinafter the cerium oxide type of cubic crystal shall be referred to as cerium oxide phase) where rare earth metals, such as La and Nd, exist in cerium crystal in the form of a solid solution. Fluorine in the raw material state before undergoing roasting combines with rare earth metals and exists in the form of a rare earth fluoride (LnF
3
), and this rare earth fluoride exists in a single-phase state or in the cerium oxide phase in the form of a solid solution.
When this abrasive raw material is roasted, the rare earth fluoride existing in a single-phase state is oxidized and part of or the whole of the same is changed into LnOF, and at the same time, the rare earth fluoride existing in the cerium oxide phase in the form of a solid solution is liberated from the cerium oxide phase, oxidized, and tends to be changed into LnOF. Accordingly, the abrasive having undergone roasting consists of cerium oxide phase in which rare earth fluoride exists in the form of a solid solution (the amount of the rare earth fluoride in the form of a solid solution differs depending on the roasting temperature), LnOF, and rare earth fluoride.
On the other hand, for the crystal structure of the polishing particles contained in the cerium-based abrasives produced from bastnasite concentrate or rare earth carbonate as a raw material, since the raw material before undergoing roasting is considered to consi

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