Material based on SiAlONs

Abstract

Sialon materials contain HFO.sub.2 in a maximum of 1 mass-% as a sintering additive, methods of producing them and methods of using them an /-SiAlON material with 5 mass % to 50 mass %, /(/) RE--SiAlON wherein RE stands for at least one cation selected from the group consisting of Y, Sc, Lu, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Mg or Ca, and 95 mass % to 50 mass %, /(/) -SiAlON and of an Hf-containing amorphous or partially crystalline grain-boundary phase with a proportion with respect to the overall material is below 10 vol %, wherein the Hf content of the sintered material is 0.2 mass % to 1.0 mass %, and of a dispersion phase comprising globular particles with a mean particle size of from 0.2 m to 15 m, containing at least one hard material selected from the group consisting of SiC, TiN, TiC, Ti(C,N), carbides of further elements of groups IVb, Vb and VIb of the periodic system, nitrides of further elements of groups IVb, VB and VIb of the periodic system, scandium carbide and scandium oxycarbide, which are contained in the sintered compact in a proportion from 5 vol % to 30 vol %.

Claims

1. A sintered material based on SiAlONs, comprising an /-SiAlON material with 5 mass % to 50 mass %, /(+) RE--SiAlON wherein RE stands for at least one cation selected from the group consisting of Y, Sc, Lu, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, and additionally at least one cation selected from the group consisting of Mg and Ca, and 95 mass % to 50 mass %, /(+) -SiAlON and Hf-containing amorphous or partially crystalline grain-boundary phase, wherein said sintered material has an Hf content of from 0.2 mass % to 1.0 mass %, and a dispersion phase comprising globular particles with a mean particle size of from 0.2 m to 15 m, containing at least one hard material selected from the group consisting of SiC, TiN, TiC, Ti(C,N), scandium carbide and scandium oxycarbide, which are contained in the sintered compact in a proportion from 5 vol % to 30 vol %, wherein the globular particles do not contain HfN or HfC.

2. A sintered material according to claim 1, wherein the Hf content ranges from 0.4 mass % to 0.6 mass %.

3. A sintered material according to claim 2, wherein the Si.sub.3N.sub.4 powder exhibits a specific surface area of 10 m.sup.2/g.

4. A sintered material according to claim 1, wherein the hard-material particles have a grain size between 0.2 m and 15 m.

5. A sintered material according to claim 4, containing SiC having a grain size of 0.6 m.

6. A sintered material according to claim 1, wherein the theoretical density is greater than 99%.

7. A sintered compact comprising material according to claim 1 in sintered form, wherein the sintered compact is a cutting tool.

8. A process for producing a sintered material based on SiAlONs according to claim 1 comprising the steps of: axial pressing a binder-containing pressed granular material at 140 MPa to 200 MPa, debinding at a temperature matched to the binder, and subsequently sintering at a temperature between 1750 C. and 2000 C. to yield a sintered compact comprising the sintered material.

9. A process according to claim 8, wherein a raw-material mixture of the /-SiAlONSiC material of the composition /-SiAlON and /(+) RE--SiAlON comprising Si.sub.3N.sub.4, Al.sub.2O.sub.3, AlN, MgO, Y.sub.2O.sub.3, HfO.sub.2 and hard-material particles of the SiC, TiN, TiC, Ti(C,N), carbides in a grain size from 0.2 m to 15 m with a proportion from 5 vol % to 30 vol %, and having a proportion of HfO.sub.2 from 0.2 mass % to 1.0 mass %, is produced, wherein the atomic % ratio of Y to Mg is 7.0 to 10.0, and wherein the Si.sub.3N.sub.4 powder has a grain size of D501 m and a specific surface area 10 m.sup.2/g.

10. A process according to claim 8, wherein the mixture is subjected to gas-pressure sintering at 1930 C. and at 100 bar gas pressure in a dwell-time of 3 hours.

11. A sintered compact, produced by a process according to claim 8, wherein the sintered compact is a cutting tool.

Description

(1) The object of the present invention is to make available a wear-resistant SiAlON material that, despite low proportions of sintering aids, in particular Hf, can also be densified by more economical gas-pressure sintering instead of elaborate hot pressing (HP) or hot isostatic pressing (HIP).

(2) In the case of the material according to the invention, based on SiAlONs, a smaller addition of Hf than in the case of conventional SiAlON materials already suffices in order to obtain good sintering properties and improved wear resistance in the course of machining. The material according to the invention is sintered at temperatures from 1750 C. to 2000 C. It can be densified by gas-pressure sintering to >99% theoretical density.

(3) The /-SiAlON material according to the invention contains 5 mass % to 50 mass %, preferably 5 mass % to 30 mass %, /(+) RE--SiAlON, where RE stands for at least one cation selected from Y, Sc, Lu, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Mg or Ca, preferably at least one cation selected from Y, Sc, Lu, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and additionally at least one cation selected from Mg or Ca, and also 95 mass % to 50 mass %, preferably 95 mass % to 70 mass %, /(+) -SiAlON and of an Hf-containing amorphous or partially crystalline grain-boundary phase with a proportion with respect to the overall material amounting to below 10 vol %. The Hf content of the sintered material amounts to 0.2 mass % to 1.0 mass %, preferably 0.3 mass % to 0.8 mass %, particularly preferably 0.4 mass % to 0.6 mass %. The dispersion phase contains globular particles with mean particle sizes from 0.2 m to 15 m, preferably 0.4 m to 10 m, consisting of at least one hard material selected from SiC, TiN, TiC, Ti(C,N), carbides and/or nitrides of further elements of groups IVb, Vb and VIb of the periodic system and also scandium carbide and/or scandium oxycarbide or mixtures of the listed hard materials, which are contained in the sintered compact in a proportion from 5 vol % to 30 vol %, preferably 7.5 vol % to 20 vol %, particularly preferably 10 vol % to 15% vol %.

(4) The raw-material mixture of the /-SiAlONSiC material contains the components Si.sub.3N.sub.4, Al.sub.2O.sub.3, AlN, MgO, Y.sub.2O.sub.3, HfO.sub.2 and SiC, the proportion of SiC amounting to 5 vol % to 30 vol %, preferably 7.5 vol % to 20 vol %, particularly preferably 10 vol % to 15 vol %, the proportion of HfO.sub.2 amounting to 0.2 mass % to 1.0 mass %, preferably 0.3 mass % to 0.9 mass %, particularly preferably 0.4 mass % to 0.8 mass %, the total additive proportion amounting to 6.0 to 10.0, preferably 6.5 to 9.0, particularly preferably 7.0 to 8.5, and the atomic % ratio of Y to Mg amounting to 7.0 to 10.0, preferably 7.5 to 9.0, particularly preferably 8.0 to 8.5.

(5) In the melt phase of the other sintering additives HfO.sub.2 possesses a low solubility, which depends on the composition of the initial mixture. In the course of cooling, the dissolved Hf partially precipitates in the grain-boundary phase as a finely distributed crystalline Hf phase. The addition of a small quantity of Hf oxide accordingly increases the quantity of melt phase during the sintering operation but does not lead to more vitrified grain-boundary phase in the product. Hence there is a possibility to reduce the quantity of the other sintering additives without impairing the sinterability. Excess HfO.sub.2 which is unable to dissolve in the liquid phase can be converted into dispersely distributed Hf nitrides by means of an N.sub.2-rich atmosphere in the course of sintering.

(6) Only through the addition of HfO.sub.2 with, at the same time, slightly reduced quantity of other additives can a noticeable improvement in the wear behaviour be established, as will be demonstrated on the basis of exemplary embodiments (see Table 2). An addition of more than 1 mass % HfO.sub.2, on the other hand, impairs the properties of the material, as will be shown on the basis of examples. The simultaneous use of sintering additives with different cationssuch as, for example, Y and Mg in the exemplary embodimentspositively influences the sintering behaviour in addition to the addition of Hf. It is presumed, however, that also in the case of raw-material mixtures with only one cation an additional Hf addition brings about the advantages that have been described.

(7) Rather than in the form of oxide, the Hf can also be introduced in another form, as an organic or inorganic compound. If the Hf is introduced in the form of pulverulent compound, for example as HfO.sub.2, the size of the powder particles should amount to only a few m. If the powder particles are too coarse, they dissolve only slowly during the sintering, and hence barely contribute to the increase of the liquid phase, bringing about no improvement of the sintering properties.

(8) After sintering, no indications, or only very weak indications, of the presence of a crystalline Hf-oxide phase can be detected with conventional radiographic analytical methods, for example XRD, since the contents thereof are too small, i.e. less than approximately 1 mass %. Depending on the sintering conditions, however, small quantities of Hf-oxynitride or Hf-nitride phases can be detected, which in the case of high Hf contents are even visible in the SEM (scanning electron microscope) as disperse particles with a diameter of approximately 0.5 m.

(9) Besides SiC, all other hard-material particles that do not enter into reactions with the other components of the material according to the invention are accordingly possible. However, the size of the admixed hard-material particles is to be borne in mind. If these are too small, below 0.2 m, on account of the large surface area of the powder a lot of glass phase is needed for the purpose of wetting, which is not present in sintering. If the hard-material particles are too coarse, approximately in the region of over 15 m, an impediment to sintering results.

(10) The invention will be elucidated in more detail on the basis of exemplary embodiments. Three groups of materials were formed, which are listed in the following Tables 1 and 2. From the respective materials, sintered compacts were produced in the form of cutting tools. The effect of the addition of HfO.sub.2 to YMg alpha/beta SiAlON materials of varying composition on the sintering behaviour and on the wear in the course of machining was compared.

(11) For the purpose of producing a SiAlON material according to the invention as used in the exemplary embodiments, fine or finely ground Si.sub.3N.sub.4 powder with a grain size of D501 m and with a specific surface area 10 m.sup.2/g and also SiC in the form of hard-material particles with a grain size D50 of approximately 0.6 m with addition of the remaining pulverulent raw materials and of known binding agents were mixed and axially pressed the granular material at 140 MPa to 200 MPa.

(12) After debinding, sintering was effected. The exemplary embodiments were all produced by means of gas-pressure sintering at 1930 C. and at a gas pressure of 100 bar. The dwell-time amounted to three hours.

(13) From the material, cutting tools were produced, with which machining tests were carried out on brake discs consisting of the material GG15. In these tests, the brake discs were turned at a cutting speed of vc=1000 m/min, with a feed of f=0.50 mm/rev, with a depth of cut of ap=2.0 mm, and with a setting angle of =85.

(14) TABLE-US-00001 TABLE 1 Exemplary embodiments: data of the raw-material mixture Mixture of raw materials Total additive Si.sub.3N.sub.4 AlN Atomic % ratio content Hf SiC Group No. [mass %] [mass %] Y:Mg [mass %] .sup.(1) [mass %] [vol. %] A 1 89.5 5.00 7.5 10.5 (Ref.) A 2 88.5 5.00 7.5 11.5 0.85 A 3 85.5 5.00 7.5 14.5 3.39 B 4 66.0 4.14 9.0 9.3 25 (Ref.) B 5 65.0 4.14 9.0 10.3 0.85 25 B 6 65.3 4.14 9.0 10.0 0.42 25 C 7 80.8 4.14 9.0 9.3 10 (Ref.) C 8 81.1 4.14 8.5 9.0 0.85 10 C 9 81.3 4.14 8.3 8.8 0.76 10 C 10 81.7 4.14 8.3 8.4 0.69 10 C 11 82.4 4.14 8.2 7.7 0.58 10 C 12 83.2 4.14 9.2 6.9 0.46 10 .sup.(1) Corresponds to the quantity of AlN + Al.sub.2O.sub.3 + MgO + Y2O.sub.3 + HfO.sub.2, the substances that form the liquid phase in the course of sintering (oxide impurity of the Si.sub.3N.sub.4 or AlN disregarded). The mass % ratio of Al.sub.2O.sub.3 to MgO always amounts to 2.53.

(15) TABLE-US-00002 TABLE 2 Exemplary embodiments: properties Properties of the material Alpha- Toughness Density SiAlON Hardness KIC Wear w.r.t. Group No. [% th.] [mass %] .sup.(2) [HV10] [MPam1/2] reference A 1 99.8 42 1782 6.3 1 (Ref.) A 2 99.8 38 1697 5.9 *.sup.) A 3 99.7 19 1582 6.3 *.sup.) B 4 99.8 25 2053 6.3 1 (Ref.) B 5 99.8 33 1897 5.8 1.06 B 6 99.8 41 1893 6.1 1.03 C 7 >99.8 30 1746 6.3 1 (Ref.) C 8 >99.8 17 1707 5.8 0.96 C 9 >99.8 20 1758 6.0 0.94 C 10 >99.8 20 *.sup.) *.sup.) *.sup.) C 11 >99.8 18 1744 5.8 0.81 C 12 88 *.sup.) *.sup.) *.sup.) *.sup.) *.sup.) not determined .sup.(2) In the sintered components; proportion of alpha-SiAlON, relative to the total quantity of alpha- and beta-SiAlON, i.e. /( + ) in mass %.
Group A (Nos. 1 to 3):

(16) The influence of the addition of HfO.sub.2 to an /-SiAlON composition: the hardness diminishes, the toughness is not changed significantly. Machining tests were not carried out, on account of the low hardness. An additional addition of Hf to a conventional SiAlON composition accordingly impairs the hardness, which would have an immediate effect on a higher wear in the course of machining.

(17) Group B (Nos. 4 to 6):

(18) The influence of the addition of HfO.sub.2 to an SiC-containing /-SiAlON: here, despite the high proportion of SiC, the hardness likewise diminishes noticeably.

(19) The wear is increased in comparison with the Hf-free composition. The sinterability of the SiAlON containing a high quantity of hard material is not noticeably improved by the small addition of Hf.

(20) Group C (Nos. 7 to 12):

(21) The influence of the addition of HfO.sub.2 to an SiC-containing SiAlON in which the total additive content, i.e. the sum of all the additives of the raw-material mixture that form a melt, is not increased, despite additional addition of HfO.sub.2: the conventional quantity of sintering additive is reduced and is added as compensating HfO.sub.2, which, as described above, increases the quantity of the liquid phase in the course of sintering but crystallises in the course of cooling and brings about a high-temperature-stable grain-boundary phase. The compensating quantity of HfO.sub.2 can even be lower than the reduced quantity of the remaining sintering additives. The hardness is not noticeably influenced; the wear, however, is lowered. The better wear behaviour is caused in this case by the lower content of amorphous grain-boundary phase, bringing about a better high-temperature hardness and slighter chemical reactions with the material of the workpiece.