EP2850041A1

EP2850041A1 - Silicon nitride ceramics with improved wear resistance and production method therefore

Info

Publication number: EP2850041A1
Application number: EP13724567.6A
Authority: EP
Inventors: Prof. Hasan MANDAL; Prof. Ferhat KARA; Prof. Servet TURAN; Prof. Alpagut KARA
Original assignee: MDA Ileri Teknoloji Seramikleri Sanayi Ticaret Ltd STi
Current assignee: MDA Ileri Teknoloji Seramikleri Sanayi Ticaret Ltd STi
Priority date: 2012-05-16
Filing date: 2013-05-16
Publication date: 2015-03-25
Also published as: WO2013171324A1

Abstract

The invention concerns silicon nitride ceramics, which especially can be produced by adding only the slightest amounts of sintering additives, resulting in ceramic bodies comprising very little grain boundary phase. The invention further concerns a method for producing such Si₃N₄ ceramics.

Description

Silicon nitride ceramics with improved wear resistance and production method therefore

The invention concerns silicon nitride ceramics, which especially can be produced by adding only the slightest amounts of sintering additives, resulting in ceramic bodies comprising very little grain boundary phase. The invention further concerns a method for producing such Si3N₄ ceramics.

Si3N ceramics have been used e.g. as cutting tools for many years. They are commonly produced by liquid phase sintering. Sintering additives generally used in the range of 2 to 15 Vol%, form a liquid phase during sintering and remain in the microstructure of the sintered body as a secondary phase either in the amorphous state or partially/fully crystalline. The amount of secondary phase, its composition and crystallinity severely affects the cutting performance. During cutting operations, temperatures at the cutting tip in excess of 800°C are common. Temperatures at the contact between the insert and the work piece can be even higher. Secondary phase oxidizes at these temperatures and causes wear. This effect is especially prominent during cutting operations where notch wear due to increased oxidation attack occurs.

The problem of enhancing wear resistance, i.e. enhancing heat resistance, corrosion resistance and chemical stability, is addressed in EP 2 138 252 A1 by providing Si3N ceramics with a reduced amount of grain boundary phase and a graded oxygen content decreasing from the outside of the body to its inside. These features are derived by a special composition of the sintering additives which are capable of being volatized during sintering. Sintering additives to be used according to EP 2 138 252 A1 are Mg and rare earth elements Re (Y, La, Ce, Er, Dy, Yb), wherein Mg in terms of MgO is 1 .0 to 7.0 mol%, and the content of Re in terms of an oxide thereof is 0.4 to 1 .0 mol%, and the total content of Mg and Re is from 1 .7 to less than 7.5 mol% in the sintered body, respectively. According to the teaching of this document, sufficient sinter activity is not achievable if the content of the sintering additives is less than 1 .7 mol%. If the content of sintering additives is in excess of 7.5 mol% unnecessary high amounts of secondary phase build in the ceramic which is undesired since high amounts of secondary phase reduce wear resistance.

At first sight and based on the insight disclosed in EP 2 138 252 A1 it would be obvious to try to further reduce the amount of the sintering additives in order to improve wear resistance. However, according to the EP 2 138 252 A1 sufficient sinterability cannot be obtained if the content of Mg in terms of MgO is less than 1 .0 mol%, the content of the Re element (Y, La, Ce, Er, Dy, Yb) in terms of an oxide of Re is less than 0.4 mol%, or the total content is less than 1 .7 mol% in the sintered body, respectively. Thus, enhanced wear resistance might not be achievable by further reducing the amount of sintering additives.

The object of the invention is to provide Si3N ceramics with improved wear resistance, especially with respect to oxidation processes at high temperatures as well as a method for producing such a Si3N ceramics. A further object of the invention is to provide ceramic inserts for cutting tools with an improved wear resistance and especially an improved notch wear resistance.

According to the invention, this object is achieved by a method for producing a sintered silicon nitride body according to claim 1 and a sintered silicon nitride body according to claim 8.

The method for producing a sintered silicon nitride body comprises the mixing of a Si3N powder with sintering additives, wherein the sum of the added sintering additives is between 1 .1 and 1 .5 wt% with respect to the mass of the entire mixture, and wherein the sintering additives comprise at least an oxide of at least one rare earth element with an atomic number greater than 62 in an amount between 0.6 and 0.8 wt%. A sintered silicon nitride body according to the invention comprises Si3N grains and a basically amorphous grain boundary phase, consisting of 80-95 mol% SiO₂. Generally, as known from the state of the art, amorphous grain boundary is avoided and crystalline grain boundary phase is aimed at since the crystalline grain boundary phase exhibits a clearly better wear resistance. Contrary to that, the inventors found that the state is less important than the composition of the grain boundary phase as long as low amounts are assured. A good oxidation resistance is achieved if the grain boundary phase contains as low amounts as possible in MgO and in Yb2O3, i.e. the MgO-rich crystalline phase MgYbSi2O₅N should be as low as possible. The inventors further found out that a grain boundary phase low in MgO-rich crystalline phase can be achieved if high amounts of S1O2 prevail.

According to the invention the amount of sintering additives added to the mixture for producing the sintered body is below 1 .5 wt% but above 1 .1 wt% with respect to the mass of the entire mixture which surprisingly was found to be sufficient to achieve sintering. At the same time, this minimal amount of sintering additives that accumulates in the grain boundary phase of the sintered product reduces the vulnerability of the product to wear. Especially, the resistivity of the sintered Si3N body to oxidation attack at elevated temperatures is enhanced due to its SiO2-rich grain boundary phase composition, giving rise to an extraordinary notch wear resistance. Thus, a product as claimed can advantageously be used as cutting tool, especially as an insert for a cutting tool, e.g. for cast iron machining or for use in automotive production in general .

According to a preferred embodiment of the invention the rare earth element added as sintering additive is Yb2O3. During sintering, Yb³⁺ is oxidized to Yb⁴⁺ and meanwhile more oxygen can be provided to the liquid phase which in turn can reduce the viscosity of the liquid phase. Thus, sintering is enhanced.

Advantageously, MgO and/or ZrO2 are used as sintering additives. MgO lowers the melting temperature and viscosity of the liquid phase which forms during sintering and helps to ease densification. It was found that the addition of ZrO2, even in minimal amounts, allows for the production of very dense ceramics. Relative densities in excess of 99% can be realized when ZrO2 is added as sintering additive. It is believed that the presence of ZrO₂ reduces the volatilizing tendency of the sintering additives, particularly MgO, before sintering and thus, is aiding densification. Without ZrO₂ relative densities in excess of 99 % cannot be achieved even if much higher than the claimed amounts of sintering additives are added, compare e.g. example 1 and comparative example 2 of Tab. 1 .

According to a preferred embodiment of the invention MgO is added in an amount between 0.4 and 0.5 wt% and/or ZrO₂ is added in an amount between 0.15 and 0.25 wt% with respect to the mass of the entire mixture.

A sintered silicon nitride body of the mixture described above preferably is produced by gas pressure sintering and subsequent heat treatment of the sintered body. Preferably, the gas pressure sintering comprises the steps: a) pre-sintering at 1700 to 1800°C and at 1 to 5.5 bar in nitrogen gas atmosphere for 0.5 to 1 .5 hours,

b1 ) sintering at 1900 to 1980°C at 4.5 to 5.5 bar in nitrogen gas atmosphere for 0.5 to 1 .5 hours, and

b2) sintering at 1980 to 2000°C at 90 to 1 10 bar in nitrogen gas atmosphere for 1 .5 to 2.5 hours.

During step a) substantial densification takes place reducing the porosity and surface area of the compact. This prevents/reduces the loss of the sintering additives which would otherwise take place by going straight up to the sintering temperature.

During step b1 ) pore closure due to solution precipitation occurs. In step b2) sintering is continued under high pressure leading to final pore elimination.

The subsequent heat treatment step, distinguishing this method from gas pressure sintering of Si3N ceramics known from the state of art, comprises heat treating the sintered silicon nitride body at 1700 to 1800°C at 1 bar to 100 bar in nitrogen gas atmosphere for 2-5 hours. During this step the chemistry of the amorphous grain boundary phase changes, especially near the surface of the sintered silicon nitride body. The heat treatment reduces the MgO content and enhances S1O2 content of the amorphous phase. Silica rich glasses improve the oxidation resistance of the ceramics.

The oxidation resistance of Si3N bodies is governed by the chemistry of its oxide scale which in turn depends on the composition of the grain boundary phase. If the oxide scale has a very low viscosity at oxidation temperatures then oxidation resistance is low, since the material oxidizes quickly due to high oxygen diffusivity through the scale. MgO reacts with S1O2 and forms a low viscosity glass, thus making the body vulnerable to oxidation attack. However, certain amounts of MgO are necessary to form dense sintered bodies.

The inventors realized that amounts of sintering additives above 1 .5 wt% result in the forming of YbMgSi2O₅N regions within the generally amorphous grain boundary phase. These regions reduce the oxidation resistance of the material by providing MgO to the oxide scale.

Thus, according to the invention these MgO rich, mostly crystalline, regions are avoided by generally using low amounts of sintering additives (not only MgO but the sum of sintering additives) and further reducing the amount of MgO by the post sintering step of heat treatment and at the same time enhancing the relative content of SiO2-rich amorphous intergranular phase. Thus, MgO content is preferably less than 10 mol% within intergranular phases.

A sintered silicon nitride body according to the invention comprises Si3N grains and a basically amorphous grain boundary phase, preferably containing on average 80 to 95 mol% amorphous S1O2. Due to the high silica content the claimed Si3N body displays exceptional high wear resistance.

The sintered body according to one embodiment of the invention has a relative density of at least 99%. Relative densities that high can advantageously be produced if ZrO2 as sintering additive is used. Due to the strength of Si3N ceramics as well as the extraordinary wear resistance of the ceramics according to the invention it can advantageously be used as a cutting tool, especially as insert of a cutting tool, e.g. for cast iron machining or for use in automotive production.

In the following, the invention is described in further detail with reference to Figs. 1 and 2. The figures show:

Fig. 1 : scanning electron microscopy micrograph of oxide scale thickness of heat treated Si3N₄ ceramic body of example 2;

Fig. 2: scanning electron microscopy micrograph of oxide scale thickness of heat treated Si3N ceramic body of example 1 .

Table 1 shows one example, example 1 , which is composed according to the invention, i.e. less than 1 .5 wt% of the entire mixture consist of sintering additives, wherein the sintering additives comprise at least an oxide of at least one rare earth element with an atomic number greater than 62 in an amount between 0.6 and 0.8 wt%. The mixture comprises Si3N powder and Yb2O3 as an oxide of a rare earth element with an atomic number greater than 62 as well as ZrO2 and MgO as further sintering additives. The comparative examples 2 to 4 in general comprise the same sintering additives but in different amounts or not all of them, see Tab. 1 .

All four examples were sintered at 1750°C, at 5 bar pressure in nitrogen gas atmosphere for one hour (sintering step a)), and at 1940°C at 5 bar pressure in nitrogen gas atmosphere for one hour (sintering step b1 )), and finally at 1990°C at 100 bar for two hours (sintering step b2)) in nitrogen gas atmosphere. Examples 1 and 2 were heat treated after sintering at 1750°C and 5 bar in nitrogen atmosphere for 5 hours. During sintering step a) substantial densification takes place reducing the porosity and surface area of the compact. In sintering step b1 ) first densification is achieved mainly by capillary pulling force while during sintering step b2) final pore elimination due to pressure sintering occurs. During subsequent heat treatment of the sintered bodies the mainly amorphous grain boundary phase enriches in S1O2 and depletes in MgO and Yb₂O₃.

The S1O2 content of the ceramic material results from the use of SN E10 as Si3N source (SN E10: Si3N powder from UBE Industries, Tokyo, Japan). This powder contains intrinsically 2.74 wt% S1O2 (1 .3 wt% O2) on the surfaces of Si3N particles.

Tab. 1

Example 1 and comparative example 2 achieved 99% of the theoretical density, i.e. a complete densification, whereas comparative examples 3 and 4 were not readily densified and showed only 93% and 95% of the theoretical density, respectively.

The microstructure of the sintered surface of example 1 contained -Si3N grains and only amorphous grain boundary phase. The amorphous grain boundary phase contained elements of Si, Mg, Yb, O. Microanalysis carried out by Transmission Electron Microscopy (TEM) showed that some pockets with grain boundary phase were extremely rich in silica with average compositions of:

i) 93.3 mol% SiO₂, 2.6 mol% MgO, 3.5 mol% Y₂O₃ and 0.5 mol% ZrO₂.

Some other pockets were still rich in silica but had a scattering composition ranging between:

ii) 83-90 mol% SiO₂, 4-7 mol% MgO, 5-8 mol% Yb₂O₃ and 1 -2 mol% ZrO₂.

The sintered surface microstructure of comparative example 2 contained -Si₃N grains and both, crystalline (YbMgSi₂O₅N, the presence of which was verified by X- Ray Diffraction) and amorphous grain boundary phase. Microanalysis (TEM) showed that the secondary phase located in grain boundary phase containing pockets contained generally two different chemistries with respect to the amount of SiO₂, MgO and Yb₂O₃:

i) 24 mol% MgO, 10 mol% Yb₂O₃ and 55 mol% SiO₂ which approximately corresponds to YbMgSi₂O₅N;

ii) widely varying composition amorphous phase containing 1 -7 mol% MgO, 3-20 mol% Yb₂O₃ and 78-91 mol% SiO₂. ZrO₂ was mainly observed to form zircon precipitates (ZrO₂ SiO₂) and only less than 0.5% ZrO₂ was present in the amorphous grain boundary phase containing pockets.

Although comparative example 2 exhibits very good relative density values, oxidation resistance is much lower than the one of example 1 , compare Figs. 1 and 2. This is attributed to the higher content of sinter additives (2,7 wt%) and especially the high content of Yb₂O₃, 2,0 wt%.

Comparative example 3 contained less than 1 .1 wt% sintering additives with respect to the mass of the entire mixture. Density determination revealed only 93% relative density, implying that the amount of sintering additives was not sufficient to allow for complete densification. Comparative example 4 contained no Zr0₂ as sintering additive at all but more than 5 wt% sintering additives in total. Although comparably high amounts of sintering additives were used, nevertheless, densification was incomplete and only 95% relative density of the sintered body was achieved.

Figures 1 and 2 both show scanning electron microscopy (SEM) micrographs of silicon nitride bodies referenced as example 1 and comparative example 2 which were subjected to oxidation in static air at 1400°C for 10 hours. In Fig. 1 comparative example 2 is displayed. On the left side of the micrograph a zone of 20 μιτι thickness with a different texture can be observed. This zone developed due to oxidation attack at the surface of the ceramic body.

Figure 2 shows a SEM picture of a ceramic Si3N body according example 1 which was oxidized under the same conditions as the body illustrated in Fig. 1 . The surface under oxidation attack is located at the right side. The zone which depicts a different texture due to the oxidation attack comprises only approximately 5 μιτι and hence, is 4 times smaller than the one in Fig. 1 . Thus, the depth to which the ceramic body according to the invention shows oxidation is four times smaller than the depth of a ceramic body which was sintered with 2.7 wt% of sintering additives

Higher oxidation resistance of example 1 was attributed to the predominance of the silica rich amorphous phase and the absence of MgO-rich secondary phase at the grain boundary phase containing pockets. MgO causes the formation of a low viscosity glassy phase during oxidation through which oxygen diffuses easily which is what happened in Example 2.

Comparative example 2 contains some pockets with amorphous grain boundary phase with more or less same amount of MgO as example 1 . However, comparative example 2 also has pockets with intergranular phase containing 24 mol% MgO, comprising crystalline YbMgSi₂O₅N regions. These regions reduce the oxidation resistance of the material by providing MgO to the oxide scale. According to the invention, such highly MgO containing (crystalline) regions are avoided and thus, enhanced oxidation resistance is obtained.

Claims

1 . A method for producing a sintered silicon nitride body by mixing Si3N powder with sintering additives, the sum of the added sintering additives being between 1 .1 and 1 .5 wt% with respect to the mass of the entire mixture, wherein the sintering additives comprise at least an oxide of at least one rare earth element with an atomic number greater than 62 in an amount between 0.6 and 0.8 wt%.

2. A method according to claim 1 , wherein the oxide of the rare earth element is Yb₂O₃.

3. A method according to claim 1 or 2, further comprising MgO and/or Zr0₂ as sintering additive.

4. A method according to one of the previous claim, wherein MgO is added in an amount between 0.4 and 0.5 wt% and/or Zr0₂ is added in an amount between 0.15 and 0.25 wt% with respect to the mass of the entire mixture.

5. A method according to one of the claims 1 to 4, wherein the sintered silicon nitride body is produced by gas pressure sintering and subsequent heat treatment of the sintered body.

6. A method according to claim 5, wherein the gas pressure sintering comprises the steps:

a) pre-sintering at 1700 to 1800°C and at 1 to 5.5 bar in nitrogen gas atmosphere for 0.5 to 1 .5 hours,

7. A method according to claim 5 and 6, wherein the subsequent heat treatment step comprises heat treating the sintered silicon nitride body at 1700 to 1800°C at 1 to 100 bar in nitrogen gas atmosphere for 2 to 5 hours.

8. A sintered silicon nitride body, comprising Si3N grains and a basically amorphous grain boundary phase, wherein the grain boundary phase on average contains more than 80 mol% S1O2.

9. A sintered body according to claim 8, wherein the content of MgO content is < 10 mol%.

10. A sintered body according to one of the claims 8 to 10, wherein the sintered silicon nitride body has a relative density of at least 99 %.

1 1 . A sintered body according to one of the claims 8 to 10, wherein the body is used as an insert of a cutting tool.