GB2155007A - Ceramic material based upon silicon nitride and refractory hard principle phase - Google Patents

Abstract

In sintered ceramic materials based upon silicon nitride an essentially increased life of cutting inserts for chip forming machining of metals can be obtained by having the material contain (i) a refractory hard principle phase consisting of carbides, nitrides and/or carbonitrides of Ti, Nb, Ta, V, Cr, Mo, W, Hf and/or Zr, (ii) two different phases of Si-Al-O-N, alpha' and beta' phase and (iii) an intergranular phase. b

Description

SPECIFICATION Ceramic material based upon silicon nitride and refractory hard principle phase The present invention relates to a ceramic alloy having superior properties especially when used for cutting tools. This invention is particularly concerned with a type of cutting tool material based upon sialon in which the wear resistance has been increased by addition of refractory hard principles and the toughness behaviour has been increased by letting the sialon material consist of two different crystalline phases.

Ceramic cutting inserts based upon alumina are characterised by a superior chemical stability which allows their use in cutting operations at high cutting speeds without being worn by oxidation or chemical dissolution. A serious limitation in the use of alumina based materials is, however, the great sensitivity of these materials to mechanical and thermal shocks, which strongly reduces the applicability in intermittent cutting operations such as milling.

Materials based upon silicon nitride have during the last few years been tested in cutting tools, showing a very good toughness behaviour because of a low thermal coefficient of expansion. The applicability is limited, however, by a relatively poor resistance to wear due to the high reactivity with iron. The resistance to wear can be considerably improved by combining silicon nitride with other hard principles having greater hardness and chemical stability. Such types of materials have been earlier described in, for example, Por Met (1978):2 p 48-52, in which is mentioned a cutting material based upon silicon nitride with addition of titanium carbide.

In the same paper (1981):7 p 73-77 additions of nitrides to silicon nitride have been studied. Similar cutting materials are described in the Japanese patent application 56-32377 in which the addition is 5-40 w/o TiC, Ti(C,N) or TiN, and in the European patent applications 35777 and 95129 in which the additions are carbides and/or nitrides of metals belonging to the groups IV B, V B, and VI B of the periodical system. It is characteristic for these materials that besides the refractory carbide or nitride they consist of one crystalline phase of silicon nitride or beta'-sialon and possibly a further phase containing a large amount of sintering aids such as yttrium oxide and which can be amorphous.

According to the invention it has, quite surprisingly, been found that the cutting properties in certain cases can be considerably improved in a material essentially based upon silicon nitride, refractory carbide/nitride phase and a phase containing sintering aids if the silicon nitride base component--actually sial onis present in two crystalline modifictions: alpha' and beta'.

Alpha' is a hexagonal phase having the general formula Mx(8iAl)12(oB N).6 in which M = Li, Ca, Se, Y, La or other ianthanides and x52.

Beta' is a hexagonal phase with the general formula Si6 zAlzOzN8-z in which O < z < 4.2.

The presence of alpha'-phase increases the hardness measured at room temperature but it has also a favourable influence on the resistance to plastic deformation (see below) because hot hardness is probably also increased in a favourable direction. Plastic deformation of the cutting edge can be observed under conditions at which the edge is exposed to high temperature, i.e. at high cutting speed and/or great feed. The plastic deformation leads to an initiation of cracks in the edge of the cutting insert causing failure of the tool when the cracks grow. This type of crack formation has been described in Met. Tech 10 (1983): Dec p 482-9 (Bhattacharyya et al, Wear mechanism of syalon ceramics tools when machining nickel base materials).It has, quite surprisingly been found that the presence of alpha' sialon considerably enhances the ability of the material to resist plastic deformation, allowing higher cutting speeds and feeds without risk of catastrophic fractures.

The improvement of properties mainly manifests itself in a considerable increase of the resistance to chipping of the cutting edge in certain work piece materials. This advantage has been observed particularly in certain high temperature materials based on nickel because of the high cutting temperatures. The same phenomenon arises, however, also in other work piece materials, if certain values of cutting speeds and feeds are exceeded.

According to the present invention we provide a ceramic material based upon silicon nitride and a refractory hard principle phase comprising an alpha' phase with the formula M,(Si, Al),2 (O,N)16 in which M is one or more elements selected from lithium, calcium, scandium, ytrrium, lanthanum and other lanthanides and 0.1 < x < 2, a beta' phase with the formula Si6 zAl,O,-N8 in which O < z < 4.2, an intergranular phase containing oxides of M or other sintering aids and a refractory hard principle phase consisting of carbides, nitrides and/or carbonitrides of titanium, niobium, tantalum, vanadium, chromium, molybdenum, tungsten, hafnium, and/or zirconium, the sintered structure of which contains in percent by volume 5-60% of a first component I consisting of the refractory hard principle and the remainder essentially of a second component II consisting of 10-70% alpha' phase, 20-90% beta' phase and 0.1-20% of the intergranular phase.

Coexistence of the alpha' and beta' phase is possible at certain material compositions and has been described by H.K. Park et al in "Alpha'-sialon ceramics", Science of ceramics, Vol. 10 p.251-256, H. Hausner (Ed) and also in the British patent application 2 11 8 927A. This patent application relates to ceramic cutting materials of silicon aluminium oxynitride comprising an alpha' phase and a beta' phase of Si-AI-O-N and a glass phase.

The above mentioned material does not contain, however, any refractory carbide, nitride or carbonitride.

The material properties specific for the invention are evident from the following description. The presence of a refractory phase aims to improve the wear resistance properties. It is present in the material in the form of a fine disperse discrete phase. In the following description titanium nitride is used as refractory phase because it is a frequently used hard principle but it is obvious that similar property improvements can be obtained by other kinds of refractory hard principles such as carbides, nitrides or carbonitrides of the metals titanium, tantalum, chromium, vanadium, molybdenum, niobium, zirconium, hafnium and tungsten or mixtures thereof. The presence of oxygen in the refractory hard principles has not proved to be unfavourable but can, on the contrary, have a favourable influence on the densification during sintering.

From a manufacturing point of view, the nitrides are preferred because they make possible a densely sintered body using pressureless sintering. Using carbides the sintering can often not be performed without pressure.

The necessary presence of alpha' and beta' phase of sialon is obtained by adjusting raw materials in such a way that a relatively small amount of alumina and silicon dioxide and a greater amount of aluminium nitride and yttrium oxide (or corresponding matters) leads to an increased proportion of alpha' sialon in the structure. The presence of alpha' sialon leads to an increased hardness without any noticeable drop of the transverse rupture strength values.

Sintering aids, such as preferably yttrium oxide, have been used in making the described product, but similar results can be obtained with one or more metal oxides such as those of strontium, cerium, lanthanum and other elements of the lanthanide series.

The use of yttria as a sintering aid gives rise to an intergranular phase which essentially is glassy but which also can include other phases e.g. YAG (a cubic phase with the formula Y3AlsO,2), Y-N-alpha-wollastonite (a monoclinic phase with the formula YSiO2N), YAM (monoclinic with the formula Y4A12O9) and N-YAM (Monoclinicphase with the formula Y4Si207N2).

The product of the invention consists of several phases and can be prepared according to the following description.

Silicon nitride, aluminium nitride, alumina and silicon dioxide, where the oxides may consist of impurities in the nitrides, are brought to react with an oxide of at least one of the elements yttrium, scandium, cerium, lanthanum or another metal of the lanthanide series. At suitable compositions of the materials two phases of sialon type are obtained, first a hexagonal phase (beta' phase) which obeys the general formula Si6,AI2OzN8z in which O < z < 4.2. At high z-values (z > 1.5) a lowering of the toughness behaviour is obtained, while at low z-values the material composition can be difficult to sinter densely without pressure.

The lower limit of z is appropriately chosen so that sintering without pressure becomes possible. The exact lower limit of z, at which pressureless sintering to full density is still possible is difficult to determine because it can be dependent upon the nature of the refractory phases. Thus it has been found possible to sinter without pressure material compositions with titanium nitride as refractory hard principle at such low z-values as 0.1, an operation which is not possible without such addition. The refractory hard principles can therefore-besides giving the material combinations a high wear resistance contribute to an effective closing of pores during the sintering. The exact nature of this course of events is not known but certainly the impurities in the refractory carbides and nitrides play a certain role in this connection.

The second phase of sialon type (alpha' phase), which also is hexagonal, is represented by the general formula MX(Si, Al) ,2(0,N),6 where M can be one or more of the elements lithium, calcium, scandium, yttrium, lanthenum or other lanthanides and 0.1 < x < 2. Oxides of these metals result besides formation of the alpha' phase-also in the formation of a glass phase in the sintered product. This glass phase has been found necessary in order to make dense sintering without pressure possible. If the amount of glass phase is high it can involve a deterioration of the high temperature properties. However, the amount of glass phase can be reduced by a somewhat smaller addition of yttrium oxide (or corresponding adjustment of other compounds) or by heat treating at which a part of the glass transforms into crystalline form as, for example, YAM, YAG, N-YAM or Y-N-alpha wollastonite. The refractory hard principle phase, which consists of carbides, nitrides or carbonitrides of titanium, niobium, tantanlum, vanadium, chromium, molyb denum, tungsten, hafnium and/or zirconium is added in the form of a fine grained powder raw material in order to give a homogeneous dispersion in the above mentioned matrix.

The amount of raw material is chosen in such a way that the sintered structure contains 5-60% by volume of a refractory hard principle phase being one or more carbides, nitrides or carbonitrides of the earlier mentioned refractory metals in a matrix consisting of 10-70% by volume of an alpha' phase of sialon-type, 20-90% by volume of a beta' phase of sialon type and a glass phase being 0.1-20% by volume and which partly can be crystalline.

In general, the matrix contains at the most 40% by volume of alpha' phase. Furthermore, the contents of the intergranular phase in the matrix are normally at the most 15% by volume. The sintered structure usually contains at the most 45% by volume of hard principle phase, which as mentioned earlier consists of carbides, carbonitrides or nitrides, usually titanium nitride.

Naturally, the mentioned material structure can be obtained in another way than described above. In US Patent 4 1 27 416 there is disclosed the use of silicon aluminum oxynitrides of polytype instead of aluminium nitride for the preparation of sialon of beta' type.

Example 1 A mixture consisting of 85% Si3N4 (about 2% of which is SiO2), 1.5% Al2O3, 7% AIN(2-3% of which is AI2O3) and 6.5% Y2O- 3-making 85% of the total composition was mixed with 15% TiN, milled in propanol, dried, pressed to test bodies and sintered in nitrogen atmosphere at 1775"C. An X-ray analysis showed strong reflexes of alpha' and beta' sialon and a medium reflex of TiN. The material showed a very fine grained and homogensously dispersed Tri phase together with alpha' + beta'-phase having a certain proportion of glass phase. ( < 10%). A slight microporosity could be observed.

Example 2 The same mixture as above was used, but the sialon composition was 70% of the total weight. 30% TiN constituted the rest. Sintering was performed at 1 790 C in nitrogen for 1 hour. Uniform and finegrained structure was obtained having low microporosity. Density 3.55 g/cm3. X-ray-diffraction measurements gave strong reflexes of TiN and beta' sialon and medum reflex of alpha' sialon.

Hardness 1 550 HV1.

Example 3 62% Si3N4 (containing about 2% SiO2) was mixed with 1 5% TiN, 1 6% "home-made" polyphase of type 21 (approximate formula SiAl6O2N6, containing about 5% AIN), 5% Y203 and 2% Al2O3. The mixture was milled in a vibratory mill filled with propanol at which further 1.2% Awl209 (calculated for dried powder without pressing agent) was milled in from the milling body material. The powder was dried, pressed to test samples and sintered in the same way as in Example 1, i.e. at 1775"C. A dense and uniform material was obtained with density 3.32 g/cm3 and a hardness of about 1 560 HV1. Insignificant microporosity was obtained.X-ray examination gave strong reflexes of alpha' and beta' sialon and medium reflex of TiN.

Example 4 Materials prepared according to Examples 1 and 2 were tested in a length turning operation in steel SS (Swedish standard) 2541-03 at which flank wear and crater wear, VB(mm) and KT4tm), respectively, were measured as a function of the operating time T(min). As reference material in the test there were used two commercially available materials: Sandwik CC680 and Sandvik CC650 (Al2O3 + 30% by weight of Ti(C,N).The test was performed with the following cutting insert geometry and cutting data: Insert geometry: ISO SNGN 120416, chamfer 0.2 X 20 Cutting speed: 100 m/min Feed: 0.36 mm/rev Cutting depth: 2.0 mm In the accompanying drawings- Figures 1 and 2 are diagrams showing flank wear and crater wear of various materials as a function of time; and Figure 3 shows accumulated fracture frequencies.

The wear is shown in Figs. 1 and 2 as a mean value of two tests, Fig. 1 showing the flank wear and Fig. 2 showing the crater wear as function of the time. In Figs. 1 and 2 the various curves represent the following materials: 1 -CC680, 2-Ex 1, 3-Ex 2 and 4 rC650.

Furthermore, the toughness behaviour was tested in a special formed cast test body (SS 0125), by which the cutting material was exposed to strong mechanical and thermal shock loads.

Insert geometry: ISO SNGN 120416, chamfer 0.2 mm X 20 Cutting speed: 300 m/min Feed: 0.5 mm/rev Cutting depth: 3.0 mm In this test the wear life is defined by insert failure. The mean life, which can be obtained from the accumulated fracture frequency in Fig. 3 (designations according to Figs. 1 and 2) was determined from 1 5 tests. It was for the tested variants: CC680 7.93 min Ex 1 4.87 min Ex 2 6.00 min CC650 1.20 min It is evident from machining tests of wear resistance and toughness behaviour that the material according to the invention is characterised by the success of combining the superior wear resistance of oxide based ceramics with the superior toughness behaviour of silicon nitride based ceramics. According to Figs.

1 and 2 the wear picture agrees best with the oxide based material, while the toughness behaviour according to Fig. 3 best agrees with the silicon nitride based materials. The material according to the invention has therefore a considerably broader range of applicability than earlier known ceramic materials.

Claims (8)

1. Ceramic material based upon silicon nitride and a refractory hard principle phase comprising an alpha' phase with the formula MX(Si,AI),2(O,N),6 in which M is one or more elements selected from lithium, calcium, scandium, yttrium, lanthanum and other lanthanides and 0.1 < X < 2, a beta' phase with the formula Si6 zAl,O,N8, in which O < z < 4.2, an intergranular phase containing oxides of M or other sintering aids and a refractory hard principle phase consisting of carbides, nitrides and/or carbonitrides of titanium, niobium, tantalum, vanadium, chromium, molybdenum, tungsten, hafnium, and/or zirconium, the sintered structure of which contains in percent by volume 5-60% of a first component I consisting of the refractory hard principle and the remainder essentially of a second component Il consisting of 10-70% alpha' phase, 20-90% beta' phase and 0.1-20% of the inter-granular phase.

2. Ceramic material according to claim 1 wherein the intergranular phase comprises yttrium oxide.

3. Ceramic material according to claim 1 or claim 2 wherein the component II contains at the most 40 percent by volume of alpha' phase.

4. Ceramic material according to any of the preceding claims, wherein the component II contains at the most 45 percent by volume of the hard principle phase.

5. Ceramic material accoring to any of the preceding claims, wherein the component II contains at the most 1 5 percent by volume of the intergranular phase.

6. Ceramic material according to any of the preceding claims, wherein the hard principle phase consists of nitrides, preferably titanium nitride.

7. Ceramic material according to claim 6, wherein the hard principle phase consists of titanium nitride.

8. Ceramic material according to claim 1 substantially as described in any of the Examples.