Vitrified bonded abrasive article

Abstract: Vitrified bonded grinding articles which contain a mixture of sintered sol gel aluminous abrasive and silicon carbide abrasive provide surprisingly good performance, in their ability to abrade certain metals.

Description: The invention relates to a vitrified bonded abrasive article. The abrasive grain portion of the majority of vitreous-bonded articles exemplified as vitreous-bonded grinding wheels in commercial use is composed of either fused aluminum oxide or silicon carbide. Those skilled in the art of grinding select silicon carbide as the preferred abrasive when the object to be ground is titanium, a super alloy, a low-tensile ferrous alloy, or a nonmetallic such as stone, ceramic, or glass. Similarly, those skilled in grinding select fused aluminum oxide for grinding steels, hardened or soft. The use of mixtures of silicon carbide abrasive grains with fused aluminum oxide abrasive grains in the manufacture of grinding wheels and other abrasive bodies is known in the art and produces a product with properties intermediate to either abrasive type used singly. Such mixed abrasive type wheels are used where a single wheel must grind a wide variety of materials and need not be optimum for any single material. The grinding performance is usually intermediate between that of either abrasive type used alone. US-A-4,800,685 discloses that sol-gel process alumina abrasive grain provides superior performance in bonded abrasives for grinding cast iron. It is furthermore described in this document that bonded abrasives of sol-gel process alumina abrasives in combination with other abrasive grains such as those of silicon carbide and fused alumina also provide superior grinding of cast iron. The present invention provides a vitrified bonded abrasive article according to independent claim 1. Further advantageous details and features of the invention are evident from the dependent claims, the description and examples. The invention provides vitrified bonded grinding wheels containing a mixture of a seeded sol gel aluminous abrasives and silicon carbide. The present invention provides a vitreous-bonded mixed abrasive grinding article in which the abrasive grit portion is composed of silicon carbide grains plus seeded sintered sol-gel alumina grains. The invention grinding articles provide substantial and unexpected benefits in the grinding of certain hard-to-grind metals over and above the results obtained using either type of abrasive grain alone. The present invention also provides a seeded sol gel alumina based vitreous bonded grinding wheel that is well adapted for use in a wide range of applications including those for which alumina based wheels have usually been considered unsuitable. The present invention further provides an abrasive article comprising a seeded sintered sol gel aluminous abrasive and a vitreous bond which demonstrates excellent grinding performance that is quite unexpected in the circumstances. The present invention resides in the discovery that when seeded sol gel aluminous abrasives are blended with silicon carbide in certain proportions in a vitrified bond, a product results which is not inferior to a grinding wheel wherein the abrasive is all seeded sintered sol gel aluminous abrasive and in fact in some cases, produces a product the grinding properties of which are actually superior to the product which contains 100% of the seeded sintered sol gel aluminous abrasive. Novel and useful grinding wheels result from replacing from 10 to 90 %, preferably 20 to 80 %, more preferably 30 to 70 % by volume of the seeded sintered sol gel aluminous abrasive with silicon carbide. The invention is concerned with vitrified bonded abrasive articles, preferably grinding wheels, which contain seeded sintered sol gel aluminous abrasives blended with silicon carbide in certain proportions. There are several sintered aluminous abrasives currently in existence including sintered bauxite based abrasives and co-sintered alumina-zirconia. Sol gel aluminous abrasive grits are taught, for example, in U.S. Patent 4,314,827. However the abrasives that are used in the present invention are the newer sol gel types of alumina based abrasives taught by U.S. Patent 4,623,364. The grain is made by preparing an aqueous gel of submicron sized hydrated alumina particles with an effective amount of submicron seed crystals, drying the gel, crushing the dried gel to a desired size, and firing the green grains at between about 1100°C and 1500°C. Alternatively, the gel can be extruded in the form of filaments, dried, broken to the desired size and fired to form a microcrystalline abrasive grit with a substantially constant cross-section and an aspect ratio greater than 1. The resulting abrasive is such that each abrasive grain is made up of numerous submicron sized alpha alumina crystals and has a hardness of at least 16 and preferably at least 18 GPa. Such abrasives are commonly referred to as "seeded gel abrasives". The seed material can also be any other material such as ferric oxide or any other material that is substantially isostructural with alpha alumina under the reaction conditions following known principles of crystal propagation and growth. The silicon carbide component of the abrasive blend can be, for example, the grit available as Crystolon@ from Norton Company. The grit can be either the green or the black silicon carbide varieties. Preferably the grit size of both abrasive components is from about 0.811 mm (20 mesh) to about 0.06 mm (240 mesh) such as from about 0.59 mm (30 mesh) to about 0.092 mm (160 mesh) and most conveniently from about 0.42 mm (40 mesh) to about 0.125 mm (120 mesh). The two abrasives may each be present in the article or the wheel in quantities about 1% to 55% by volume, preferably of about 10% to about 40% by volume, based on the total volume of the wheel. The articles or wheels themselves are usually made up of from about 30% to about 56% by volume of combined abrasive, about 2.5% to about 50% by volume of vitrified bond, and about 5% to about 65% by volume of pores though proportions outside these ranges can be used if desired in special applications. As pointed out in U.S. Patent 4,543,107 sintered sol gel aluminous abrasives, unlike the non-sintered type, can be susceptible to damage by inorganic glassy bonds at elevated temperatures. Therefore the bonds employed in the present invention should preferably be formulated so that they mature at 1220° C or lower and even more preferably at 1100° C or lower. In addition to the usual inorganic materials that go into vitrified or inorganic glassy bonds for grinding wheels, the bond may also incorporate therein beneficial fillers such as mullite, kyanite, bubble alumina, and the like. According to a further aspect, in the vitrified bonded abrasive article of the present invention the sintered seeded sol gel aluminous abrasive is substantially calcium ion and alkali metal ion free and has a substantially homogenous microcrystalline structure comprising a modifying component in a dominant continuous alumina phase comprising alpha alumina, said modifying component, on a volume percent of fired solids of said sintered seeded sol gel aluminous abrasive, being selected from (i) at least 10% of zirconia, hafnia, or a combination of zirconia and hafnia, (ii) at least 1% of a spinel derived from alumina and at least one oxide of a metal selected from cobalt, nickel, zinc, or magnesium, or (iii) 1 to 45% of said zirconia, hafnia, or the combination of zirconia and hafnia and at least 1% of said spinel. While the discussion and data thereafter is directed to grinding wheels, it should be understood that abrasive articles such as sharpening stones, segments, and the like, are within the scope of the invention. EXAMPLE I A series of grinding tests were performed to illustrate the advantageous properties of the abrasive grit blends of the invention. In each case the grits were bonded with a bond, which comprised 30 wt % ball clay and 70 wt % of a glass frit with a silica content of from 40 to 65% weight, which had no deleterious effect on the grain when fired to a working temperature of 1100°C or lower. The mixture was formed into a 12.7 cm wheel with a thickness of 0.64 cm. Each wheel after firing contained 48% by volume of total abrasive grit. The sintered sol-gel aluminous grits used in the following Examples were seeded sol-gel grits made by Norton company having a grit size of 60 and a hardness in excess of 18 GPa. The green or black silicon carbide used also had a grit size of 60. The amount of bond in each case represented 11.55% of the total volume of the wheel. The wheels were tested in an oil cooled plunge grinding rig on four different metal billets: titanium 6Al-4V; stainless steel 440C; stainless steel 302 and Titanium metal. Wheel rotation was 4970 rpm and the table speed was 15.4 m/min. The results on the four metals are set out in the following Tables: Titanium Alloy Grits MRR cc/min.cm WWR c/min.cm G Ratio Power Watts/cm A. 100% SiC Green 1.74 0.65 2.66 2088 50% SiC/50% SG 1.63 0.85 1.92 1753 100% SG 1.55 1.05 1.48 2479 B. 100% SiC Black 1.74 0.68 2.58 2044 50% SiC/50% SG 1.62 0.82 1.96 1871 100% SG 1.55 1.05 1.48 2479 C. 100% SiC Green 3.17 2.05 1.54 2637 50% SiC/50% SG 2.98 2.53 1.18 2209 100% SG 3.02 2.40 1.26 3034 D. 100% SiC Black 3.25 1.95 1.66 2684 50% SiC/50% SG 3.10 2.35 1.32 2276 100% SG 3.02 2.40 1.26 3034 A and C used green SiC and B and D used black SiC. A and B used a unit feed advance of 0.013 mm and a total depth of cut of 1.31 mm. C and D used a unit feed advance of 0.026 mm and a total depth of cut of 1.3 mm. 440C Stainless Steel Grits MRR cc/min.cm WWR c/min.cm G Ratio Power Watts/cm A. 100% SiC Green 1.95 0.085 22.86 4814 50% SiC/50% SG 1.95 0.008 254.09 1319 100% SG 1.97 0.010 191.42 1689 B. 100% SiC Black 1.94 0.085 22.8 4732 50% SiC/50% SG 1.97 0.010 192.63 1228 100% SG 1.97 0.010 191.42 1689 C. 100% SiC Green 3.83 0.262 14.63 6602 50% SiC/50% SG 3.93 0.030 128.74 1730 100% SG 3.93 0.025 154.37 2590 D. 100% SiC Black 3.83 0.332 11.53 6479 50% SiC/50% SG 3.91 0.025 154.86 1809 100% SG 3.93 0.025 154.37 2590 A and C used green SiC and B and D used black SiC. A and B used a unit feed advance of 0.013 mm and a total depth of cut of 1.3 mm. C and D used a unit feed advance of 0.026 mm and a total depth of cut of 2.54 mm. 302 Stainless Steel Grits MRR cc/min.cm WWR c/min.cm G Ratio Power Watts/cm A. 100% SiC Green 3.89 0.255 15.28 9366 50% SiC/50% SG 3.90 0.055 70.84 2737 100% SG 3.89 0.055 69.78 2652 B. 100% SiC Black 3.93 0.323 12.18 9636 50% SiC/50% SG 3.93 0.040 98.68 2464 100% SG 3.89 0.055 69.78 2652 C. 100% SiC Green 7.11 1.783 3.98 11084 50% SiC/50% SG 7.86 0.495 15.87 5096 100% SG 7.74 0.211 36.69 8200 D. 100% SiC Black 7.20 1.827 3.94 11915 50% SiC/50% SG 7.70 0.511 15.08 5222 100% SG 7.74 0.211 36.69 8200 A and C used green SiC and B and D used black SiC. A and B used a unit feed advance of 0.025 mm and a total depth of cut of 2.54 mm. C and D used a unit feed advance of 0.051 and a total depth of cut of 2.54 mm. TITANIUM - OIL GRITS MRR cc/min. cm WWR cc/min. cm G-RATIO POWER watts/cm 100% SiC green 1.79 0.539 3.32 2122 50% SiC/ 50% SG 1.75 0.679 2.58 1739 30% SiC green/70%SG 1.75 0.777 2.25 1890 100% SG 1.74 0.699 2.49 1985 100% SiC green 4.27 5.755 0.74 2836 50% SiC/ 50% SG 4.32 5.339 0.81 2130 30% SiC/ 70% SG 4.44 4.818 0.92 2596 100% SG 4.56 4.465 1.02 2765 The above data demonstrate clearly that although, as might be anticipated, the G-ratio of the mixture is generally intermediate between the extremes represented by the component grits used alone, or else very close to that of the premium SG grit figure, the corresponding power drawdown to achieve that G ratio is significantly lower than with either component grit alone. This is an unexpected result that could not have been predicted on the basis of the known behavior of the component grits. In an efficient grinding operation it is important to have a wheel with a long life that remains sharp that uses the lowest possible power drawdown to achieve the desired cut. The data clearly shows that the grit combinations of the present invention meet this requirement, especially in the context of grinding stainless steel. The beneficial results detailed above are most surprising in view of the fact that they are secured by the incorporation of a grit that would normally be expected to have a deleterious effect on the metals ground. Indeed the data for SiC grit wheels indicates that their performance on stainless steels is poor and the performance of aluminous wheels on titanium alloys is likewise relatively inferior. Yet the performance of the blend is superior on all of these. EXAMPLE II The procedures of Example I were repeated except that the wheels were tested using a coolant that was a water soluble oil. The results are reported in Tables V to VII below. As can be seen, the pattern of results largely duplicates that found in Example I. Titanium Alloy Grits MRR cc/min.cm WWR c/min.cm G Ratio Power Watts/cm A. 100% SiC Green 1.61 0.843 1.91 1850 50% SiC/50% SG 1.59 0.944 1.68 1492 100% SG 1.68 0.798 2.11 1939 B. 100% SiC Black 1.70 0.857 1.98 1676 50% SiC/50% SG 1.61 0.921 1.74 1522 100% SG 1.68 0.798 2.11 1689 C. 100% SiC Green 3.19 2.182 1.46 2276 50% SiC/50% SG 2.94 2.624 1.12 1618 100% SG 3.17 1.920 1.65 2645 D. 100% SiC Black 3.11 2.355 1.32 2072 50% SiC/50% SG 3.02 2.477 1.22 1857 100% SG 3.17 1.920 1.65 2645 A and C used green SiC and B and D used black SiC. A and B used a unit feed advance of 0.013 mm and a total depth of cut of 1.3 mm. C and D used a unit feed advance of 0.026 mm and a total depth of cut of 1.3 mm. 440C Stainless Steel Grits MRR cc/min.cm WWR c/min.cm G Ratio Power Watts/cm A. 100% SiC Green 1.86 0.205 9.01 5040 50% SiC/50% SG 1.93 0.031 61.42 1875 100% SG 1.95 0.027 72.02 2161 B. 100% SiC Black 1.83 0.312 5.85 4012 50% SiC/50% SG 1.94 0.029 67.34 1783 100% SG 1.95 0.027 72.02 2161 C. 100% SiC Green 3.51 0.932 3.76 4914 50% SiC/50% SG 3.89 0.068 57.63 2245 100% SG 3.85 0.029 130.75 3213 D. 100% SiC Black 3.50 0.995 3.51 4429 50% SiC/50% SG 3.84 0.078 49.16 2387 100% SG 3.85 0.029 130.75 3213 A and C used green SiC and B and D used black SiC. A and B used a unit feed advance of 0.013 mm and a total depth of cut of 1.3 mm. C and D used a unit feed advance of 0.026 mm and a total depth of cut of 2.54 mm. 302 Stainless Steel Grits MRR cc/min.cm WWR c/min.cm G Ratio Power Watts/cm A. 100% SiC Green 3.21 1.649 1.95 4104 50% SiC/50% SG 3.26 1.510 2.16 1998 100% SG 3.42 1.123 3.05 2852 B. 100% SiC Black 3.23 1.690 1.91 3942 50% SiC/50% SG 3.32 1.415 2.35 2087 100% SG 3.42 1.123 3.05 2853 C. 100% SiC Green 5.62 5.284 1.06 4104 50% SiC/50% SG 5.22 6.184 0.84 1815 100% SG 5.96 4.253 1.40 3379 D. 100% SiC Black 5.72 4.830 1.18 3824 50% SiC/50% SG 5.26 5.863 0.90 1857 100% SG 5.96 4.253 1.40 3379 A and C used green SiC and B and D used black SiC. A and B used a unit feed advance of 0.025 mm and a total depth of cut of 2.54 mm. C and D used a unit feed advance of 0.051 mm and a total depth of cut of 2.54 mm. EXAMPLE III In this Example. Titanium Alloy (6AC-4V) and Titanium metal were evaluated in the manner described above in the previous Examples. The results are set forth in Tables VIII, IX and X. TITAMIUM ALLOY A. GRITS MRR CC/MIN. CM WWR CC/MIN. CM G-RATIO POWER WATTS/ CM 100% SiC Green 1.64 .836 1.96 1721 70% SiC/30% SG 1.63 .865 1.89 1689 50% SiC/50% SG 1.61 .894 1.77 1513 100% SG 1.67 .784 2.13 1963 B. 100% SiC Green 3.10 2.237 1.39 2107 70% SiC/30% SG 3.02 2.322 1.30 2123 50% SiC/50% SG 2.90 2.515 1.16 1827 100% SG 3.17 1.906 1.66 2529 A = Used a downfeed of 0.013 mm. B = Used a downfeed of 0.0254 mm. The data shown above are averages of two data points. The cooling oil used was a water soluble oil called Trim VHPE 300 (at 5% concentration) from Master Chemical Company. TITANIUM ALLOY A. GRITS MRR CC/MIN. CM WWR CC/MIN. CM G-RATIO POWER WATTS/C M 100% SiC Green 1.86 .521 3.57 2291 70% SiC/30% SG 1.80 .430 4.19 2100 50% SiC/50% SG 1.78 .488 3.65 2088 100% SG 1.71 .590 2.90 6115 B. 100% SiC Green 3.41 1.283 2.65 2834 70% SiC/30% SG 3.41 2.287 1.49 2165 50% SiC/50% SiC 3.33 2.351 1.42 2411 100% SG 3.44 1.309 2.63 7160 A used a 0.013 mm downfeed. B used a 0.0254 mm downfeed. The cooling oil was W&3B Oil 1572. TITANIUM METAL A. GRITS MRR CC/MIN. CM WWR CC/MIN. CM G-RATIO POWER WATTS/ CM 100% SiC Green 1.79 0.539 3.32 2122 50% SiC/50% SG 1.75 0.679 2.58 1739 30% SiC/70% SG 1.75 0.777 2.25 1890 100% SG 1.74 0.699 2.49 1985 B. 100% SiC Green 4.27 5.755 0.75 2836 50% SiC/50% SG 4.32 5.339 0.81 2130 30% SiC/70% SG 4.44 4.818 0.92 2596 100% SG 4.56 4.465 1.02 2765 A used a 0.013 mm downfeed. B used a 0.0254 mm downfeed. The cooling oil was W&B 1572.

Claims: A vitrified bonded abrasive article for grinding hard-to-grind metals comprising a mixture of from 10 to 90% by volume of sintered seeded sol gel aluminous abrasive grains comprising submicron sized alpha alumina crystals and 90 to 10% by volume of silicon carbide grains and an inorganic glassy bond therefor, wherein said inorganic glassy bond is matured at 1200°C or below. The vitrified bonded abrasive article of claim 1, wherein the mixture comprises from 30 to 70% by volume of sintered seeded sol gel aluminous abrasive grains and 70 to 30% by volume of silicon carbide grains. The vitrified bonded abrasive article of claim 1 or 2, wherein said abrasive grains are present in an amount of from 30% to 56% by volume of said article, said glassy bond is present in an amount of from 2.5% to 50% by volume of said article and the article includes 5% to 65% by volume pores. The vitrified bonded abrasive article of one of the preceding claims wherein said abrasive grains are made up of sintered seeded sol gel aluminous abrasive grains in an amount of from about 1% to 55% by volume of said article and said silicon carbide grains are present in an amount of 1% to 55% by volume of said article. The vitrified bonded abrasive article according to one of the preceding claims, wherein each sintered seeded sol gel aluminous abrasive grain has a hardness of at least 16 GPa. The vitrified bonded abrasive article of claim 5, wherein each sintered seeded sol gel aluminous abrasive grain has a hardness of at least 18 GPa. The vitrified bonded abrasive article according to one of the preceding claims, wherein said silicon carbide particles have a mesh size of from 0.811 mm to 0.06 mm (20 to 240 mesh). The vitrified bonded abrasive article according to one of the preceding claims, wherein the sintered seeded sol gel aluminous abrasive is substantially calcium ion and alkali metal ion free and has a substantially homogenous microcrystalline structure comprising a secondary phase of crystallites comprising a modifying component in a dominant continuous alumina phase comprising alpha alumina, said modifying component, on a volume percent of fired solids of said sintered seeded sol gel aluminous abrasives, being selected from: (i) at least 10% of zirconia, hafnia, or a combination of zirconia and hafnia, (ii) at least 1% of a spinel derived from alumina and at least one oxide of a metal selected from cobalt, nickel, zinc, or magnesium or (iii) 1-45% of said zirconia, hafnia, or the combination of zirconia and hafnia and at least 1% of said spinel. The vitrified bonded abrasive article according to one of the preceding claims, which includes a filler selected from the group consisting of mullite, kyanite, and mixtures thereof. The vitrified bonded abrasive article according to one of the preceding claims, wherein said article is a grinding wheel.

Inventors: LEE KEN, RUE CHARLES V, LEE, KEN, RUE, CHARLES V.
Applicants: NORTON CO, NORTON COMPANY
Application year: 1991
Main classification: B24D3
IPC codes: C09K3, C04B35, B24D3, C03C14