This invention relates to coated abrasive grains and to a manufacturing method therefor, and particularly to coated abrasive grains obtained by an immersion method. More specifically, it relates to coated abrasive grains and a manufacturing method therefor in which a molten salt bath is employed, and the abrasive grains are coated with a layer of a metal compound such as at least one substance selected from metallic carbides, metallic borides, and metallic nitrides.
Abrasive grains such as diamond abrasive grains and hard BN abrasive grains are referred to as ultrahard abrasive grains and are widely used as starting materials for abrasive articles or as abrasives. In particular, abrasive grains comprising natural or synthetic diamonds are bonded together with a binder in the form of a metal, a synthetic resin, or a vitreous inorganic material to form abrasive articles, such as grinding wheels. These abrasive articles have a wide range of uses including the grinding of carbide tools and hard materials such as ceramics, ferrite, and glass, and the cutting of stones and concrete. As a result of the spectacular development of ceramics in recent years, diamond wheels and hard BN wheels, which enable high-precision grinding of ceramics, have been used in large quantities.
However, presently-used abrasive grains have the following problems.
For example, in the case of resin-bonded wheels or metal-bonded wheels which have diamond abrasive grains as their main component, in order to increase the bond strength between the abrasive grains and the resin or metal which serves as a binder, the surfaces of the abrasive grains are coated with a metal such as nickel. This metal coating is said to have the effect of preventing the deterioration of the resin due to heat which is generated along the edges of the abrasive grains during use of a grinding wheel. However, the metal coating has the problem that it causes the clogging of the wheel.
On the other hand, vitrified-bonded wheels which also have diamond abrasive grains as their main component use a ceramic as a binder. As these wheels are manufactured by firing at high temperatures, the bonding strength between the abrasive grains and the binder is high due to a solid phase reaction. However, during the high-temperature firing, the diamond abrasive grains undergo thermal corrosion, and as a result manufacture is difficult.
Japanese Published Unexamined Patent Application No. 55-162499 (1980) disclosed a method of coating the surface of diamond abrasive grains with Ti(C,N,O) by chemical vapor deposition in order to prevent oxidation of the abrasive grains during manufacture of wheels, improve the adhesion to coatings, and prevent the clogging of the wheel during grinding. However, the adhesion of a coating formed by this method is said to be inadequate.
Thus, as the advantages of coated abrasive grains have been recognized, various methods have been proposed for improving the properties thereof. However, as all of these methods employ CVD or other vapor phase deposition processes, they are unsuitable for mass production, and they require large and expensive equipment in order to be carried out.
The following U.S. patents are issued regarding coated diamond abrasive grains of the prior art: U.S. Pat. Nos. 3,465,416; 3,520,667; 3,617,346; 3,650,714; 3,826,630; 3,924,031; 3,929,432; 4,063,907; 4,220,455; 4,417,906; and 4,606,738.
Accordingly, in order to take advantage of the above-described excellent properties of coated abrasive grains, there is a need for a method which can easily and greatly improve the adhesion to coatings of coated abrasive grains and which at the same time is inexpensive.