Abstract:
An abrasive grain on the basis of silicon carbide with a binder-free coating of highly dispersed silicon, aluminum or titanium oxide for preferred use in resin-bonded or ceramic-bonded abrasive wheels.

Description:
BACKGROUND OF THE INVENTION 
     1. Field Of The Invention 
     The invention relates to a surface-treated abrasive grain on the basis of silicon carbide and its use in resin-bonded or ceramic-bonded grinding wheels. 
     2. Background Art 
     Silicon carbide is one of the most important abrasives and is used especially for grinding and cutting off hard and brittle materials. It is used both for loose abrasives and for abrasives on a support and abrasive wheels such as grinding and cutting-off wheels. The grinding performance and especially the wear ratio or service life of the abrasive wheels, thus, depends not only on the properties of the abrasive grain itself but also to a considerable extent on the type and strength of the binding of the abrasive grain in the binder. Especially with resin-bonded abrasives, the adhesion of the silicon carbide grain in the binder generally is the factor determining performance and wear. Therefore, numerous attempts have been made to improve the binding by a suitable treatment of the very smooth surface of the silicon carbide. 
     A frequently used method is the production of a ceramic coating which coats the grain like a glaze. U.S. Pat. No. 1,910,444, for example, describes such a process. If a slightly meltable material in a particle size that is considerably less than the abrasive grain is added to the coating, a roughened coating is obtained, as described, for example, in U.S. Pat. No. 2,527,044. 
     But considerable drawbacks are inherent in all of such processes. They require several process steps, including a firing or sintering process and, therefore, are time- and energy-consuming. In addition, agglomerates are formed by caking of the individual grains. Such agglomerates must be removed by an additional screening process or must be mechanically destroyed, and the just applied coating is again partially lost. Below a certain grain size the agglomerate formation is so predominant that the process can no longer be used. 
     In ceramic-bonded abrasive wheels the use of ceramically coated abrasive grains is generally not advisable, since during firing of the abrasive wheels at about 1150° to 1400° C. the binder of the coating again melts and, together with the applied fine grain (e.g., Fe 2  O 3 ), penetrates into the ceramic mass and can initiate disturbing reactions there which, for example lead to pore formation or color deviation. Further, when the abrasive wheels are used, especially in dry grinding, the binder can soften by the development of heat and cause the grain to break off. 
     BROAD DESCRIPTION OF THE INVENTION 
     The object of the invention is to provide surface treatment of abrasive grains on the basis of silicon carbide, which improves the binding of the resin-bonded or ceramic-bonded abrasives, without exhibiting the described drawbacks. 
     It was surprisingly found that the object according to the invention can be acheived by coating the abrasive grain with a highly dispersed hydrophilic metal oxide without additional binder. The silicon carbide abrasive grain can, with its grain size, be both in the macrorange and microrange according to FEPA standard. Silicon dioxide, such as, Aerosil®, aluminum oxide or titanium dioxide, individually or as mixtures in each case, can be used as highly dispersed hydrophilic metal oxide. The highly dispersed hydrophilic metal oxide is suitably applied in an amount of 0.01 to 5 percent, preferably from 0.1 to 1.0 percent, relative to the weight of the abrasive grain. The primary particle size (d 50  value) of the highly dispersed hydrophilic metal oxide preferably is between 1 and 500 nm, especially preferably between 5 and 50 nm, and the specific surface (according to BET) is preferably between 5 and 500 m 2  /g, especially preferably between 50 and 400 m 2  /g. 
     The application of the highly dispersed hydrophilic metal oxide to the abrasive grain takes place preferably by mixing the abrasive grain with the highly dispersed hydrophilic metal oxide in the dry state or with a suspension of the highly dispersed hydrophilic metal oxide in a vaporizable liquid and subsequent drying. Preferably water is used as the vaporizable liquid but other polar or nonpolar liquids are also suitable, such as, lower alcohols, ketones, esters, ethers or hydrocarbons or mixtures of said liquids. The mixing especially preferably takes place in the dry state. Tumbling mixers are especially suitable as mixers, such as rotating drum mixers or double-cone mixers, or mixers with rotating mixing tools such as blade mixers or paddle mixers. 
     A preferred use of the abrasive grain according to the invention is the production of resin-bonded or ceramic-bonded abrasive wheels, such as, grinding or cutting-off wheels. 
     Of course, it is within the scope of the invention to use the abrasive grain according to the invention in combination with other abrasive grains for the production of abrasive wheels, especially with diamond abrasive grains or abrasive grains from cubic boron nitride. Here the silicon carbide abrasive grain according to the invention can be used both as support grain in the abrasive area of the abrasive wheel and for reinforcement and better heat dissipation in support foundations. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following examples illustrate embodiments of the invention and the properties of the abrasive grain according to the invention. 
     EXAMPLE 1 
     An amount of silicon carbide (Carbogran® dark F 24, LONZA-Werke GmbH, Waldshut) was divided into halves. One half was treated according to the invention of 0.5 percent by weight of hydrophilic Al 2  O 3  [primary particle size d 50  =20 nm, specific surface (BET)=100±15 m 2  /g] by dry mixing in a tumbler mixer. Cutting-off wheels were produced with these grain patterns under constant conditions as follows: 
     The initial materials [73 percent by weight of grain, 13 percent by weight of filler (cryolite), 14 percent by weight of phenolic resin powder and phenolic resin liquid], with the addition of the liquid phenolic resin portion, was worked into a homogeneous free-flowing compound and then was molded in a press mold between two outside fabrics at a pressure of 200 kg/cm 2 . Hardening of the wheels took place as usual in an oven at 190° C. 
     The abrasive performance was determined with a commercial cutting-off machine on vibration-compacted concrete bars with a cross section of 80×60 mm. For this purpose, 5 separating cuts per wheel were made, and as a measurement for the performance of the wheel, the accompanying reduction of the wheel diameter (in mm) was determined. Table 1 shows the results. 
     
                       TABLE 1______________________________________Wheel dimensions:  230 × 2.5 × 22 mmConcrete bars:     80 × 60 mm5 separating cuts              Carbogran ® F 24              Treated  Untreated______________________________________Average of breaking              160      165speed (m/s)Average of wheel   2.07     2.01density (g/cm.sup.3)Diameter reduction (mm)              11       14              10       13              13       17Average of diameter              11.3     14.7reduction (mm)Relative performance              123%     100%factor______________________________________ 
    
     EXAMPLE 2 
     Cutting-off wheels were produced and tested similarly to Example 1. The composition of the wheels: 78 percent by weight of abrasive grain, 8 percent by weight of filler (cryolite), and 14 percent by weight of phenolic resin powder and phenolic resin liquid. Table 2 shows the results. 
     
                       TABLE 2______________________________________Wheel dimensions:  230 × 3 × 22 mmConcrete bars:     80 × 60 mm5 separating cuts              Carbogran ® F 24              Treated  Untreated______________________________________Average of wheel   2.10     2.13density (g/cm.sup.3)Diameter reduction (mm)              20       26              19       24              21       25Average value (mm) 20       25Relative performance              120%     100%factor______________________________________ 
    
     EXAMPLE 3 
     A production charge of silicon carbide (Carbogran®, Gotthardwerke Bodio; granulation 0.4 to 1 mm) was divided into four parts. One part of the mixture was not treated, so as it was standard grain. One of each of the remaining three parts was surface coated according to the invention with: 
     (a) 0.5 percent by weight of hydrophilic Al 2  O 3  [d 50  =20 nm, specific surface (BET)=100±15 m 2  /g] 
     (b) 0.5 percent by weight of hydrophilic TiO 2  [d 50  =21 nm, specific surface (BET)=50±15 m 2  /g] 
     (c) 0.5 percent by weight of hydrophilic SiO 2  [d 50  =7 nm, specific surface (BET)=380±30 m 2  /g] by mixing in a tumbler mixer. The further procedure took place similarly to Example 1. Table 3 shows the results by average values. 
     
                       TABLE 3______________________________________Wheel dimensions:          230 × 2.5 × 22 mmConcrete bars: 80 × 60 mm5 separating cuts          Carabogran ® 0.4 to 1 mm                    0.5%    0.5%  0.5%          Untreated Al.sub.2 O.sub.3                            TiO.sub.2                                  SiO.sub.2______________________________________Breaking speed (m/s)          163.9     164.9   163.9 165Wheel density (g/cm.sup.3)          2.04      2.02    2.04  2.02Diameter reduction (mm)          10        8.3     8.3   8.3Relative performance          100%      117%    117%  117%factor______________________________________ 
    
     EXAMPLE 4 
     Comparative Example 
     A production charge of silicon carbide (Carbogran® F 24, LONZA-Werke GmbH, Waldshut) was divided into halves. One half was provided with a ceramically bonded coating according to the prior art. 
     Formula 
     150 kg of silicon carbide F 24 
     900 cm 3  of sodium silicate (type 37/40 van Baerle &amp; 
     Co., d 20  =1.35 g/cm 3 ) 
     1000 cm 3  of water 
     1600 g of iron oxide (Bayferrox® 180, Bayer AG.) 
     800 g of glaze frit (type A8962p, Reimbold &amp; Strick) 
     The glaze frit was worked into a homogeneous suspension with water and sodium silicate. The abrasive grain was wetted with it in a tumbler mixer, iron oxide was added in four portions and the entire mixture was homogenized in the mixer for about 10 to 15 minutes. The coating of iron oxide and binder thusly applied was sintered in a rotary tubular oven at 740° C. and an average retention time of 10 to 15 minutes. The wheel production and the wheel tests took place similarly to Example 1. 
     The results are shown in Table 4 (average values). 
     
                       TABLE 4______________________________________Wheel dimensions: 230 × 2.5 × 22 mmConcrete bars:    80 × 60 mm5 separating cuts             Carbogran ® F 24             Ceramically             Coated     Untreated______________________________________Wheel density (g/cm.sup.3)             2.04       2.02Explosion speed (m/s)             166.2      164.4Diameter reduction (mm)             12         14             13         17             12         14Average reduction (mm)             12.3       15.0Relative performance             118%       100%factor______________________________________