a/ß-Sialon Having Improved Sintering Activity and High Edge Strength

Abstract

The invention relates to α/β-sialon-based materials. The invention particularly relates to α/β-sialon-based materials that have an improved sintering activity and impart high edge strength to the sintered molded articles made of said materials.

Claims

1. A sintered molded article comprising a ceramic composed of α/β-sialon having a grain boundary phase, wherein the grain boundary phase contains at least one hard material formed in situ as an additional phase.

2. The sintered molded article according to claim 1, wherein the hard material is TiN.

3. The sintered molded article according to claim 1, wherein the sintered molded article has an edge strength of at least 600 N/mm, preferably of at least 680 N/mm.

4. The sintered molded article according to claim 1, wherein the sintered molded article has a final density of at least 99%.

5. The sintered molded article according to claim 1, wherein in the sintered state, in the interior of the sintered article, the proportion of α-sialon with reference to the entire sialon phase amounts to 10 to 90 vol.-%, preferably 20 to 70 vol.-%, particularly preferably 30 to 60 vol.-%, and the proportion of β-sialon amounts to 90 to 10 vol.-%, preferably 80 to 30 vol.-%, particularly preferably 70 to 40 vol.-%.

6. The sintered molded article according to claim 1, wherein the surface of the sintered article, in the sintered state, amounts to a proportion of α-sialon with reference to the entire sialon phase from 50 to 100 vol.-%, preferably from 55 to 95 vol.-%, particularly preferably from 60 to 90 vol.-%, and the proportion of β-sialon amounts to from 0 to 50 vol.-%, preferably from 5 to 45 vol.-%, particularly preferably from 10 to 40 vol.-%.

7. The sintered molded article according to claim 1, wherein the surface of the sintered molded article in the sintered state has a proportion of α-sialon with reference to the entire sialon phase that is higher by 5 to 65 vol.-%, preferably by 10 to 55 vol.-%, particularly preferably by 15 to 50 vol.-%, than the proportion of α-sialon with reference to the entire sialon phase in the interior of the sintered article.

8. The sintered molded article according to claim 1, wherein the sintered molded article has a Vickers hardness HV10 of at least 10 GPa, preferably of at least 15 GPa and/or a crack resistance K.sub.1c of at least 5 MPa*m.sup.0.5, preferably of at least 6 MPa*m.sup.0.5.

9. A method for the production of a sintered molded article composed of α/β-sialon having a grain boundary phase, composed of at least one hard material, wherein at least the following compounds are used as a starting material: Si.sub.3N.sub.4, AlN, and, if applicable, Al.sub.2O.sub.3, at least one oxide of the rare earths, and at least one oxide of the element titanium.

10. The method according to claim 9, wherein TiO.sub.2 is used as an oxide of the element titanium.

11. The method according to claim 9, wherein a ratio of AlN used to Al.sub.2O.sub.3 used is greater than 4:1.

12. The method according to claim 9, wherein the starting material comprises 70 to 96 wt.-% Si.sub.3N.sub.4, 3 to 15 wt.-% of at least one oxide of the rare earths, 1 to 15 wt.-% of an aluminum compound, comprising AlN and, if applicable, Al.sub.2O.sub.3, and 0.1 to 3 wt.-% of a titanium oxide, preferably TiO.sub.2, wherein the sum of the starting substances corresponds to 100 wt.-%.

13. The method according to claim 9, wherein the starting material comprises 78 to 95 wt.-% Si.sub.3N.sub.4, 2 to 8 wt.-% AlN, 0 to 1.2 wt.-% Al.sub.2O.sub.3, 2.5 to 6.5 wt.-% Y.sub.2O.sub.3 or 5.5 to 12 wt.-% Yb.sub.2O.sub.3, 0.08 to 0.22 wt.-% CaCO.sub.3, and 0.25 to 2.0 wt.-% TiO.sub.2, wherein the sum of the starting substances corresponds to 100 wt.-%.

14. The method according to claim 9, wherein an article molded from the starting material is sintered without pressure or with gas pressure.

15. Use of a sintered molded article according to claim 1 as a cutting tool, particularly as a cutting insert, as a wear component, particularly as a welding roll, welding centering pins, components of bearings such as roller bearings or ball bearings, components in an exhaust gas system such as exhaust gas flaps, valves or exhaust gas turbochargers.

Description

[0024] In the following, the invention will be explained in greater detail using an exemplary embodiment in comparison with a conventionally composed sintered molded article.

[0025] The starting materials, see Table 1, were mixed and a green molded article was produced. The molded article was sintered without pressure at 1725° C., for approximately 2 hours, under flowing nitrogen.

TABLE-US-00001 TABLE 1 Example A Example B Example C Starting (comparison (according to (according to material example) the invention) the invention) Si.sub.3N.sub.4 (wt.-%) 84-93 82-92 75-88 Al.sub.2O.sub.3 (wt.-%)   0-1.2   0-1.2   0-1.2 AlN (wt.-%) 3-8 3-8 3-8 Er.sub.2O.sub.3 (wt.-%) — 0-1.2 — Y.sub.2O.sub.3 (wt.-%) 4.5-6.5 —   0-1.2 Yb.sub.2O.sub.3 (wt.-%) —  8.5-12.5  8.5-12.5 CaCO.sub.3 (wt.-%) 0.08-0.22 0.08-0.22 0.08-0.22 TiO.sub.2 (wt.-%) — 0.25-2   0.25-2   % of theoretical 96.36-97.13 99.18-99.97 99.34-99.98 density Vickers 5.5 16.6 16.7 Hardness HV10 (GPa) α-sialon 77 75 86 proportion of the as fired surface of the sintered molded article (vol.-%) α-sialon 64 55 54 proportion in the interior of the sintered molded article (vol.-%) Crack resistance Cannot be evaluated, 6.6 6.5 (Palmquist) K.sub.lc because no cracks (GPa*m.sup.0.5) can be seen due to high residual porosity Edge strength Cannot be evaluated, 1054.97 946.33 R.sub.eA because parts are too (N/mm) porous and warped

[0026] The examples according to the invention and the comparison example differ only in their composition, i.e. the materials used. The examples according to the invention have a TiO.sub.2 component; the amounts of the other components were adapted accordingly. Method parameters such as shaping and sintering conditions were otherwise identical in the examples.

[0027] In all cases, an α/β-sialon sintered molded article occurred, having a grain boundary phase that has not only amorphous components but also crystalline components in the X-ray diffractogram. The sintered molded article of the examples according to the invention furthermore also contained TiN grains formed in situ.

[0028] With regard to the properties, it was shown that the examples according to the invention have a relative density that is about 3% higher than the comparison example. This high relative density is also demonstrated in the excellent results that are obtained for Vickers hardness HV10 and crack resistance. The Vickers hardness HV10 accordingly amounts to at least 10 GPa, preferably at least 15 GPa. The crack resistance according to Palmquist accordingly amounts to at least 5 MPa*m.sup.0.5, preferably at least 6 MPa*m.sup.0.5, for a sintered molded article according to the invention.

[0029] For comparison example A, it was not possible to determine the crack resistance and the edge strength because the crack progression could not be clearly recognized due to the great residual porosity. The experiment regarding edge strength therefore could not be evaluated, since the cutting inserts were too porous and furthermore warped.

[0030] In the sintered state of the material, the sialon phase of the sintered article, in the interior, consists of a proportion of α-sialon of 10 to 90 vol.-%, preferably 20 to 70 vol.-%, particularly preferably 30 to 60 vol.-%, and a proportion of β-sialon of 90 to 10 vol.-%, preferably 80 to 30 vol.-%, particularly preferably 70 to 40 vol.-% β-sialon. The proportion of α-sialon and β-sialon is determined using X-ray diffractometry images (according to Gazzara and Messier, J. Am. Ceram. Soc. Bull. 56 (1977)).

[0031] It is known that the composition of the material in the interior of a sintered molded article can be varied by means of the production parameters, such as, for example, by means of the composition of the powder mixture, the sintering conditions in the furnace, the crucible material, the type of gas, the temperature, and the sintering time. A gradient between the surface and the interior of the sintered article can be present in the sintered molded article, so that what is called the as fired surface contains up to 100% α-sialon.

[0032] The surface of the sintered molded article in the sintered state preferably has a proportion of α-sialon with reference to the entire sialon phase from 50 to 100 vol.-%, preferably from 55 to 95 vol.-%, particularly preferably from 60 to 90 vol.-%, and a proportion of β-sialon from 0 to 50 vol.-%, preferably from 5 to 45 vol.-%, particularly preferably from 10 to 40 vol.-%.

[0033] The surface of the sintered molded article in the sintered state preferably has a proportion of α-sialon with reference to the entire sialon phase that is higher by 5 to 65 vol.-%, preferably by 10 to 55 vol.-%, particularly preferably by 15 to 50 vol.-%, than the proportion of α-sialon with reference to the entire sialon phase in the interior of the sintered article.

[0034] A gradient can form in the sintered molded article under certain conditions if the surface of the sintered article cools faster than the interior or if the surface is changed in terms of its chemical composition by means of reactions with the atmosphere. An α-sialon-rich surface leads to a harder outer layer having an impact-resistant core. Thereby the hardness of the sintered molded article can be increased further at the surface, in addition to the hard material formed in situ, without reducing the high edge strength of the sintered blank, which is machined to be planar and chamfered.

[0035] The material according to the invention can be coated with known wear-reducing layers such as, for example, Al.sub.2O.sub.3, TiN, TiC or Ti(C,N), and this increases the wear resistance.

[0036] FIG. 1 shows the results of the wear test for a gas-pressure-sintered comparison example A and the embodiments B and C according to the invention, which were sintered without pressure, as described above. The width of the wear mark is plotted in millimeters as a function of the cutting length in meters during interrupted cutting of gray cast iron (GJL 150). The wear test was carried out at a cutting speed of 1000 m/min, an advance of 0.50 mm/revolution, and a cutting depth of 2 mm.

[0037] At the same cutting length, it is found that the embodiments B and C of the invention, which were sintered without pressure, have comparable or even better wear values than the comparison example A.