Polycrystalline porous Al2O3—bodies on the basis of molten aluminum oxide comprising an increased toughness and use thereof

Abstract

The present invention relates to temperature-treated polycrystalline porous Al.sub.2O.sub.3 bodies comprising an amount of aluminum oxide of more than 97% by weight, an amount of other oxide alloying components of a total of less than 3% by weight, a macroporosity of between 5 and 30% by volume, wherein the Al.sub.2O.sub.3 bodies are composed of a plurality of Al.sub.2O.sub.3 primary crystals comprising a crystallite size of between 20 and 100 m.

Claims

1. Polycrystalline porous Al.sub.2O.sub.3 bodies of molten aluminum oxide comprising an amount of a aluminum oxide of more than 97% by weight: and an amount of other oxide alloying components of a total of less than 3% by weight, wherein the Al.sub.2O.sub.3 bodies comprise a plurality of Al.sub.2O.sub.3 primary crystals having a crystalline size of between 20 and 100 m, the Al.sub.2O.sub.3 bodies exhibit a macroporosity comprising a pore volume of between 5and 30% by volume, an average diameter of the pores of between 5 and 30% by volume, an average diameter of the pores of between 20 and 60 m and a maximum pore diameter in the range of approx. 100 m, and at the boundaries of the primary crystals, the polycrystalline Al.sub.2O.sub.3 bodies comprise concentrations of individual TiO.sub.2 comprising extrinsic phases, the extrinsic phases have a diameter of less than 7 m and are distributed individually either in a punctiform manner or in lines along the primary crystal boundaries.

2. The Al.sub.2O.sub.3 bodies according to claim 1, wherein the extrinsic phases have a diameter of less than 5 m.

3. The Al.sub.2O.sub.3 bodies according to claim 1, wherein the extrinsic phases comprise beside TiO.sub.2 other oxide alloying components selected from the group consisting of Cr.sub.2O.sub.3, Fe.sub.2O.sub.3, MgO, Na.sub.2O, NiO, ZnO, CoO, ZrO.sub.2, SiO.sub.2, MnO.sub.2, or oxides of rare earths.

4. The Al.sub.2O.sub.3 bodies according to claim 1, wherein the amount of other oxide alloying components is less than 1% by weight.

5. The Al.sub.2O.sub.3 bodies according to claim 1, wherein the Al.sub.2O.sub.3 bodies are abrasive grains, which are treated and sized into defined grit sizes.

6. The Al.sub.2O.sub.3 bodies according to claim 5, wherein the abrasive grains have a bulk density of less than 175 g/cm.sub.3in the grit sizes F24-F80according to FEPA.

7. The Al.sub.2O.sub.3 bodies according to claim 5, wherein the abrasive grains have a bulk density of less than 1.70 g/cm3 in the grit sizes F24-F80 according to FEPA.

8. The Al.sub.2O.sub.3 bodies according to claim 5, wherein the abrasive grains have a bulk density of less than 1.65 g/cm3 in the grit sizes F24-F80according to FEPA.

Description

(1) The character of the present invention is additionally illustrated by means of REM images, which are enclosed to the description as FIGS. 1 to 6.

(2) FIG. 1 shows a scanning electron micrograph of a polished section of a polycrystalline abrasive grain according to the invention in 1000-fold magnification,

(3) FIG. 2 shows a scanning electron micrograph of a polished section of a polycrystalline abrasive grain according to the invention in 2000-fold magnification,

(4) FIG. 3 shows a scanning electron micrograph of a polished section of a comparative example in 1000-fold magnification,

(5) FIG. 4 shows a scanning electron micrograph of a polished section of a comparative example in 1000-fold magnification,

(6) FIG. 5 shows a scanning electron micrograph of polycrystalline abrasive grains in 100-fold magnification,

(7) FIG. 6 shows a scanning electron micrograph of compact dense monocrystalline abrasive grains in 150-fold magnification.

(8) The boundaries 3 of the primary crystals 1 can be identified in FIGS. 1 and 2 as dark border areas, which surround the individual primary crystals 1. The crystal boundaries are additionally highlighted by means of the increasingly appearing individual light extrinsic phases 2 which comprise in the present case more than 88% by weight of TiO.sub.2 (example 4). An EDX analysis of the extrinsic phases 2 thus resulted in a composition of 88.6% by weight of TiO.sub.2, 0.7% by weight of Na.sub.2O, 0.1% by weight of MgO, 0.1% by weight of SiO.sub.2 and 10.5% by weight of Al.sub.2O.sub.3. For the primary crystals 1, 99.5% by weight of Al.sub.2O.sub.3, 0.3% by weight of SiO.sub.2 and in each case 0.1% by weight of CaO and TiO.sub.2 were analyzed. For the phase boundaries 3, 5.6% by weight of Na.sub.2O, 93.4% by weight of Al.sub.2O.sub.3, 0.2% by weight of SiO.sub.2 and 0.8% by weight of TiO.sub.2 were found. The black areas, which can be identified in the image, are pores 4, which form along the primary crystalline boundaries. FIG. 2 shows a polished section of an abrasive grain in 2000-fold magnification, in the case of which in particular the TiO.sub.2-containing extrinsic phases 2, which are lined up along the crystallite boundary 3, can be seen.

(9) FIGS. 3 and 4 show a section through a base grain prior to the thermal treatment. Compared to the thermally post-treated grain according to the invention illustrated in FIG. 1 or 2, it can be seen that the quantity of the titanium-containing extrinsic phases 2 is considerably lower, the separations are coarser, and the shape of the separations is more platelet-like. It is thus assumed that the high temperature of the thermal post-treatment leads to diffusion processes, which leads to an increased separation and to a finer punctiform distribution of the extrinsic phases 2. It also appears that the temperature treatment or the diffusion effects, respectively, lead to a decrease of the thickness of the phase boundaries 3.

(10) The polycrystalline composition of the abrasive grains according to the invention, which are comprised of Al.sub.2O.sub.3 primary crystals having a size of 20-100 m and which are connected to one another, can be seen in FIG. 5 in 100-fold magnification. The fissured surface of the abrasive grains explains the good embedding thereof in a binding agent matrix of an abrasive and the good grinding performance resulting therefrom.

(11) A conventional dense compact abrasive grain, which is composed so as to be monocrystalline and which has a relatively smooth surface, can be seen in FIG. 6 in a 150-fold magnification as significant contrast to FIG. 5, so that the abrasive grain exhibits indeed high grain strength, but it is only poorly embedded in the binding agent matrix.

(12) To evaluate the quality of abrasive grains, it is essential to carry out grinding tests. Grinding tests are relative extensive and time-intensive. In the abrasive industry, it is thus common to evaluate the quality of abrasive grains in advance by means of mechanical characteristics, which can be accessed more easily and which serve as indications for the later behavior in the grinding test. In the context of the present works, the grain toughness of the abrasive grains was determined via the micro grain decomposition by milling in a ball mill.

(13) Micro Grain Decomposition (MKZ)

(14) To measure the micro grain decomposition, 10 g of corundum (of a corresponding grit size) is milled in a ball mill, which is filled with 12 steel balls (diameter 15 mm, weight 330-332 g) at 188 revolutions per minute for a predetermined period of time. The milled grain is subsequently screened in a screening machine (Haver Bcker EHL 200) for 5 minutes via a corresponding fine sieve, which is 2 classes finer than the bottom sieve, which is defined for the corresponding grit size, and the fine portion is balanced out. The MKZ value follows from:

(15) $\mathrm{MKZ}\left(\%\right)=\frac{\mathrm{sieve}\mathrm{pass}\text{-}\mathrm{through}}{\mathrm{total}\mathrm{weight}}100$

(16) In Table 1 below, several selected types of corundum are characterized, the micro grain decomposition of which, in addition to the bulk density, are then summarized in Table 2 and are compared to the abrasive grains according to the invention. Corundum from Treibacher Schleifmittel GmbH is used for the tests. For comparison, blocky and dense fused aluminum oxides (FIG. 6), which are melted in a batch process, were also used in addition to polycrystalline corundum. The temperature treatment for the polycrystalline corundum was carried out for 15 minutes at 1250 C. in a rotary kiln.

(17) TABLE-US-00001 TABLE 1 Chemical Example Type of Corundum Composition 1 White fused aluminum oxide Al.sub.2O.sub.3 99.76 (comparison) compact, dense Fe.sub.2O.sub.3 0.04 monocrystalline Na.sub.2O 0.18 SiO.sub.2 0.02 2 alloyed corundum Al.sub.2O.sub.3 99.38 (comparison) compact, dense Na.sub.2O 0.19 monocrystalline Fe.sub.2O.sub.3 0.04 3, 5, 7 alloyed corundum TiO.sub.2 0.24 (comparison) polycrystalline Cr.sub.2O.sub.3 0.09 4, 6, 8 alloyed corundum SiO.sub.2 0.06 (invention) temperature-treated polycrystalline

(18) TABLE-US-00002 TABLE 2 Micro Grain Bulk Density Decomposition Example Grit Size SD (g/cm.sup.3) MKZ (% by weight) 1 F46 1.81 12.1 2 F46 1.81 9.7 3 F46 1.64 29.6 4 F46 1.64 20.2 5 F60 1.67 18.1 6 F60 1.67 13.8 7 F80 1.63 13.4 8 F80 1.63 7.5

(19) For the blocky and dense fused corundum (example 1 and 2), the low MKZ values show a high toughness and grain strength, which is considerably higher for the alloyed corundum (example 2) than for the white fused aluminum oxide (example 1). In the case of the same chemical composition as in example 2, the MKZ value for the polycrystalline porous corundum (example 3) is considerably higher. In response to corresponding applications, this material nonetheless shows very good grinding performance, which in particular also results from the fact that, due to its porous composition, the abrasive grain can be embedded very well in the abrasive (grinding belt or grinding disc), wherein the binding agent penetrates into the outer open pores of the abrasive grain and the abrasive grain is anchored in the abrasive.

(20) After a temperature treatment at 1250 C., the same grain shows a decrease of the MKZ value or an increase of the grain toughness, respectively, by approx. 32%. Completely new application possibilities follow from this for the abrasive grain according to the invention, because the porous polycrystalline structure is now paired with relatively high grain toughness and the advantages of the good embedding can thus be combined with high grain strength.

(21) It is known that the MKZ values are a function of the grain size. Further measurements were thus carried out with finer grit sizes. In the case of examples 5-8, it also became apparent for grit sizes F60 and F80 that enormous increases of the grain toughness are obtained by means of a thermal post-treatment. A grain toughness increase of approx. 24% is thus measured for grit size F60 (examples 5 and 6), while even an increase by approx. 44% can be observed for grit size F80.

(22) Ball Mill Grain Decomposition (KMKZ)

(23) The ball mill grain decomposition is a similar method for measuring the grain strength. Because of the larger sample quantities this method is more exact and less fault-prone. 100 g of corundum (of a corresponding grit size) are milled in a ball mill, which is filled with 8 big 35 mm diameter and 40 to 45 small 14.7 mm diameter steel balls at 83 revolutions per minute for a predetermined period of time. After separating the steel balls, the milled grain is subsequently screened in a Rotap screening machine for 5 minutes via a corresponding fine sieve, which is 2 classes finer than the bottom sieve, which is defined for the corresponding grit size, and the fine portion is balanced out. The KMKZ value follows from:

(24) $\mathrm{KMKZ}\left(\%\right)=\frac{\mathrm{sieve}\mathrm{pass}\text{-}\mathrm{through}}{\mathrm{total}\mathrm{weight}}100$

(25) In the present case unalloyed polycrystalline corundum was compared with TiO.sub.2 alloyed polycrystalline corundum. The chemical compositions of both types of corundum are summarized in table 1A

(26) TABLE-US-00003 TABLE 1A Chemical Example Type of Corundum Composition 9, 13, 17 unalloyed Al.sub.2O.sub.3 99.71 (comparison) polycrystalline corundum Fe.sub.2O.sub.3 0.04 10, 14, 18 unalloyed Na.sub.2O 0.21 (comparison) polycrystalline corundum SiO.sub.2 0.04 temperature-treated 11, 15, 19 alloyed corundum Al.sub.2O.sub.3 99.37 (comparison) polycrystalline Na.sub.2O 0.20 12, 16, 20 alloyed Fe.sub.2O.sub.3 0.04 (invention) polycrystalline corundum TiO.sub.2 0.28 temperature-treated Cr.sub.2O.sub.3 0.06 SiO.sub.2 0.05

(27) TABLE-US-00004 TABLE 2A Ball Mill Grain Grain Decomposition strength Bulk Density KMKZ(% by Improvement Example Grit Size SD (g/cm.sup.3) weight) (%) 9 F46 1.61 23 10 F46 1.61 21 9 11 F46 1.63 24 12 F46 1.63 13 53 13 F60 1.60 33 14 F60 1.60 28 14 15 F60 1.62 34 16 F60 1.62 15 56 17 F80 1.66 9 18 F80 1.66 8 12 19 F80 1.66 11 20 F80 1.66 5 55

(28) The temperature treatment of the polycrystalline corundum was carried out in a rotary kiln at 1250 C. for 15 minutes. The results of the ball mill grain decomposition are summarized in table 2A. It was found out that the untreated polycrystalline corundum has comparable grain strength irrespective whether it is alloyed or unalloyed. Both types of corundum exhibit an improvement of grain strength after temperature-treatment, whereby the improvement for unalloyed polycrystalline corundum is below 20%, whereas the grain strength of the alloyed polycrystalline corundum increases more than 50%.

(29) Grinding Test

(30) To also verify the positive effect of the MKZ values for the praxis grinding praxis, additional grinding tests were carried out from the samples 1 to 4.

(31) For this purpose, cutting discs were produced in the dimensions 1251.522.23, which were then used to cut a stainless steel tube comprising the diameter of 20 mm and a thickness of 2 mm. 3 rough cuts were made initially for conditioning the disc and a total of 20 cuts were subsequently made with each disc. The grinding performance was determined via the decrease of the disc diameter (disc wear). The average values were in each case formed from 3 discs in response to the disc wear. The grinding results and the grinding conditions are summarized in Table 3 below.

(32) TABLE-US-00005 TABLE 3 Grinding machine FEIN WS14 Power 1.2 kW Speed 8.800 rpm Cutting rate 20 Workpiece tube 22/2 Material stainless steel dimension disc wear after Abrasive cutting disc 125 1.5 20 cuts 22.23 (decrease of the diameter) abrasive example 1 8.4 mm grain example 2 4.5 mm example 3 5.4 mm example 4 3.9 mm

(33) In the case of the above-listed grinding conditions, the cutting disc comprising the compact and dense white fused aluminum oxide shows the highest disc wear, which can be explained with the inferior embedding of the compact grain in the binding agent matrix and in particular with the relatively low grain toughness. Compared thereto, the cutting disc comprising the tough alloyed corundum wears considerably less (approx. 45%). The result of the cutting disc comprising polycrystalline porous corundum, which shows a disc wear, which, in spite of the very high MKZ value (example 3=29.6), is only slightly lower as compared to the compact and tough grain from example (2), is surprising and can surely be explained with the good embedding of the porous polycrystalline grain in the binding agent matrix. By tempering the abrasive grain at 1250 C. for 15 minutes, the stability of the cutting disc, based on the decrease of the diameter, can be increased by approx. 30% and thus exceeds the extremely solid compact alloyed corundum.

(34) It is important to note in this context that the above-described grinding test was mainly carried out so as to generally determine the suitability of the Al.sub.2O.sub.3 bodies according to the invention as abrasive grains and so as to show the performance increase as compared to the untreated abrasive grain. For specific applications and grinding operations, in the case of which the porosity and the relatively high grain toughness of the abrasive grain according to the invention have a particularly positive impact, further performance increases in particular as compared to the compact and dense corundum types, are to be expected. Corresponding results were found for the precision grinding or also for the high-performance grinding with ceramic-bonded abrasive grains.

(35) In particular when used in abrasive discs, which should have a defined porosity, the use of the abrasive grains according to the invention leads to improvements as compared to the state of the art, because the desired porosity is now at least partially embodied by the abrasive grain itself, which is associated with the additional advantage that the cooling lubricant can be brought directly into the abrasive contact zone. On the one hand, the cutting ability of the abrasive disc is improved and the free cutting during the operating process is supported by introducing additional porosity in response to the use of the abrasive grain according to the invention. On the other hand, the embedding of the abrasive grain in the grinding disc is furthermore improved due to the polycrystalline structure comprising a large, fissured surface, whereby the grinding performance is additionally increased.

(36) In spite of the high macroporosity, the abrasive grain is extremely stable and can also be used for grinding operations, in the case of which high contact pressures are applied. Even if, due to their composition, the polycrystalline Al.sub.2O.sub.3 bodies are predestined in particular for the use in grinding discs, they are also suitable for use as loose abrasive, for use in coated abrasives, for the production of refractory materials and for use as wear protection materials.