Abrasive tool making process – material – or composition – With inorganic material – Metal or metal oxide
Reexamination Certificate
1999-03-05
2001-04-10
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
Metal or metal oxide
C051S307000, C501S127000, C501S153000, C264S102000, C264S299000, C264S319000, C264S603000, C264S650000, C264S653000
Reexamination Certificate
active
06214069
ABSTRACT:
The invention relates to a process for the manufacture of a polycrystalline, sintered, ceramic abrasive from a powder raw material, an abrasive manufactured according to this process and grinding tools with such an abrasive.
Two types of processes have attained particular importance for the manufacture of ceramic abrasives, namely sol gel processes and powder processes.
The objective during the development of sol gel processes for the manufacture of sintered ceramic abrasives, e.g. as claimed by Cottringer in U.S. Pat. No. 4.623.364, was to achieve increased hardness and grinding performance compared to prior art fused abrasives by replacing the internal structure of the latter, which was essentially monocrystalline, with a polycrystalline, highly uniform, pore-free structure of a submicrometer character (U.S. Pat. No. 4,623,364 discloses crystallites ranging from 0.2-0.4 &mgr;m, for example). Other sol gel processes are disclosed in DE 41 13 476and EP 0 63 810.
Analogous efforts to manufacture monolithic Al
2
O
3
sintered products with grain sizes below 2 &mgr;m and improved mechanical properties from corundum/alumina powder [German: Korundpulver] (e.g. as disclosed in JP 700 35 720 B) essentially took one of two approaches, namely:
(a) the use of submicrometer or nanomicrometer powder with the finest possible grain or
(b) the addition of substances to lower the dense densification temperature.
Extremely fine grained raw materials possess the high sinter activity desired, but are also accompanied by significant shaping problems due to their poor densification. The cost-effective uniaxial dry pressing method, the most common method currently used for the manufacture of monolithic parts, results in unsatisfactory green density or inhomogeneous densities within the molded body which when sintered result in defects that reduce hardness and strength. For these reasons, various other shaping processes are used, such as cold isostatic pressing, drawing [German: Strangziehen], slip casting in porous (absorbent) molds, pressure or centrifugal slip casting or gel casting. An Al
2
O
3
grain size of 0.8 &mgr;m and a hardness of HV20=1920 was achieved by extruding ceramic bodies of submicrometer powders and sintering at 1400° C. (G. Riedel, et al., Silicates Industriels (1989) ½, 29-35); the powder used had an average grain size of 0.45 &mgr;m.
Slip casting is also used with very fine grain Al
2
O
3
powders with d
50
<0.4 &mgr;m (T.-Sh. Yeh et al., J. Am. Ceram. Soc. (1988), pp. 841-844), also in combination with the use of pressure (Pressure filtration: F. F. Lange et. al., Bull. Am. Ceram. Soc. (1987), pp. 1298-1504; Vacuum Pressure Filtration: H. Mizuta et al., J. Am. Ceram. Soc. (1992), pp. 469-473). As shown by Mizuta, for example, these complex processes can be used to achieve the heretofore best mechanical properties for pure sintered corundum, but even using hot isostatic pressing (HIP) it has not been possible to achieve a Vickers microhardness ≧2000 or bending strength ≧800 MPa.
The ability to improve the mechanical properties and grain size of Al
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O
3
sintered products manufactured from corundum powder with the additives to promote sintering is very limited. The addition of more than 1% doping agents, which form a liquid phase during sintering, reduces the dense densification temperature of submicrometer Al
2
O
3
powders to 1200° C. and lower, yet toughness remains at the usual level for traditional sintered corundums of approx. 400 MPa (L. A. Cue et al., J. Am. Ceram. Soc. (1991), pp. 2011-2013), and the grain phase boundaries that are often produced result in poor high temperature characteristics, which is undesirable in machine tools in general and especially so in abrasives.
The disadvantage of such prior art Al
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O
3
sinter products manufactured from corundum powder is that no high grade, low defect Al
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3
sinter products with high hardness and strength are known to be manufacturable.
Unaware of relevant solutions to the problems previously achieved, current developments were concentrated on, among others, two approaches thought to be promising (T. J. Carbone, p. 107 in : L. D. Hart (editor), “Alumina Chemicals Science and Technology Handbook”, The Am. Ceram. Soc., Westerville, Ohio, 1990):
(1) Improvement of sol gel technologies with nucleating additives (e.g. G. L. Messing and M. Kumagai in Bull. Am. Ceram. Soc. 73 (1194) 88-91), and
(2) the development of the use of powders with extremely uniform grain size (“monosized”, “uniform-sized”) thought to be ideal as described by K. Yamada in “Alumina Chemicals Science and Technology Handbook” (Editor: L. D. Hart, The Am. Ceram. Soc., Westerville, Ohio, 1990 p. 564).
While the first approach fails to solve the general problem of the sol/gel process (the fact that the added germs are always more coarse than the primary raw material of the sol, thus hardly allowing the formation of an extremely low defect structure), the second approach results in increasingly expensive raw materials.
Special dispersing measures in conjunction with especially low defect powder shaping processes can be used to produce monolithic sintered corundum parts such as abrasives whose Vickers hardness (at test loads ranging from 10-100 N) exceed HV=2000 if the grain size is not larger than 1.5 &mgr;m. The bending strength of pressureless sintered parts is in excess of 800 MPa. Refer to WO-95/28364A2. As disclosed in this publication, a metal removing capacity equal to 182% of that of zirconium corundum was achieved when grinding the cross-section of a steel tube using a vulcanized fiber grinding wheel to which such an abrasive had been applied. However, WO-95/28364A2 also discloses that this progress is only achieved when using powder raw materials with a limited grain size distribution.
The disadvantage that all of the powder technology approaches described above have in common is that they require shaping processes for the manufacture of unsintered raw materials which are poorly suited for the manufacture of sinterable aggregate bodies of abrasive grit from both technological and economic standpoints (e.g. uniaxial matrix pressing, cold isostatic pressing of massive, block shaped parts, casting under pressure (pressure filtration) or the use of special binders [gel casting]).
Furthermore, the casting methods used within the framework of the sol/gel process to achieve low defect, unsintered raw products with sufficiently high base density do not offer an approach to overcoming the difficulties because the networking processes which occur during the transition of the sol into the gel state are tied to special characteristics of the sols that powder suspensions simply do not possess.
The object of the invention is therefore to create a process for the cost-effective manufacture of a high grade abrasive. A further object of the invention is to create grinding tools with significantly better grinding performance than grinding tools of the prior art.
This object can be achieved by means of a process having the following steps:
Preparation of a slip largely free from polymerizing and/or coagulating additives on the basis of a powder raw material;
Pressureless shallow casting [German: flachgiessen] of the slip in a tank/vessel/container;
Degassing, drying and crushing of the cast sheet to obtain a fabricated material and
Sintering of the fabricated material to create the abrasive.
Surprisingly, this process enables the manufacture of an abrasive with which very good grinding performance can be achieved due to its extreme density and freedom from defects.
The conventional wisdom is that dense, low defect, sintered ceramic products can only be manufactured if the sinter process is performed on “green” fabricated materials of sufficient “green density” (generally>50%) in which the packing homogeneity of the powder particles is sufficiently high to prevent a spatial heterogenization [German: heterogenisierung] of the sinter shrinkage and the associated formation of coarser
Blank Paul
Krell Andreas
Fraunhoffer-Gesellschaft zur Forderung der Angewandten Forschung
Marcheschi Michael
Nils H. Ljungman & Associates
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