DE4000777A1 - Sintered silicon nitride-based body - with wear resistant surface region and tough interior region - Google Patents

Abstract

In a sintered silicon nitride-based body with integral surface and interior regions, the novelty is that the amount of silicon nitride and sialon crystal grains in the surface region is at least 30 vol. % less than that in the interior region. Also claimed is a sintered silicon nitride-based body with integral surface and interior regions, the novelty being that the ratio of the melilite content in the interior region to that in the surface region is less than 0.5, as measured by the X-ray max. intensity ratio method. USE/ADVANTAGE - The sintered bodies are useful for machine tools, wear resistant parts and relatively sliding parts. They have improved wear resistance without deletions effect on other properties such as toughness.

Description

The invention relates to a silicon nitride base material. Sintered body and in particular a silicon nitride base material sintered body, which has an improved ver has wear resistance without being essential Properties, such as toughness, to be deteriorated. The present invention can be used for machine tools, wear-resistant (wear-resistant) parts and on top of each other sliding parts are used.

In Japanese version 63-1278 there is a Described conventional silicon nitride sintered body, the a surface covering made of a ceramic material with a high hardness or an improved ver has wear resistance.

Another silicon nitride sintered body is known in which crystal phases of both α- sialon and β -sialon are homogeneously distributed over the entire sintered body (Japanese Auslegeschrift No. 63-35594 (1988), etc.). The following problems are to be solved with the invention:

With the first-mentioned sintered body, it is difficult to obtain a sufficient holding force with the substrate due to different thermal expansion coefficients cients and / or chemical affinities between the ceramic coating material and the silicon nitride sub strat are available, which result in high costs ben that are to be regarded as an economic problem. Out for this reason, the first sintered body has almost no practical application found.

The last-mentioned sintered body, which contains both crystal phases of α - and β- sialon in an even distribution over the entire sintered body, suffers from the change in its properties when their proportions change and cannot develop every property. In particular, when trying to improve one property, another of the properties is forcibly deteriorated, resulting in mediocre performance in balancing both properties.

It is therefore an object of the present invention to create a silicon nitride base sintered body in which the surface is improved by one To form surface area that a higher ver has wear resistance, the properties of the Materials in the surface area and indoors in occur satisfactorily.

As a result of research into the reason of the poor wear resistance of silicon nitride the following knowledge was gained, on which the builds the present invention.

According to a first embodiment of the present Invention can achieve the object specified above by a Silicon nitride base sintered alloy can be solved, which has a surface area and an inner area which are integrally formed with one another, where the amount of crystal grains made of silicon nitride and Sialon in the surface area by 30 volumes percent or more smaller than in the inner area is.

In a second embodiment of the invention, the task given above by a silicon nitride basic sintered body can be solved, the one upper  surface area and an inner area that are integrally formed with each other, the ratio nis the content of crystalline compounds that unite Part or all of the grain boundary phase in the inner Be form rich to that of the surface area like it by the X-ray maximum intensity ratio method can be determined is less than 0.5.

According to a third embodiment of the invention, the another improvement over the first and the offers second embodiment is the ratio of Content of crystalline compounds that are part or the whole grain boundary phase in the inner region form, to the content of silicon nitride and sialon the same area as it was by the x-rays maximum intensity ratio method can be determined, Is 0.3 or greater.

According to a fourth embodiment of the invention Melilith "the crystalline compound, the whole or to the Part of the grain boundary phase "of the surface area according to the second embodiment.

According to a fifth embodiment of the invention Melilith "the crystalline compound, the whole or to the Part of the grain boundary phase "of the surface area according to the third embodiment.

The advantages of the present invention are in the following embodiments described.

In the first embodiment, the grain boundary phase can be a glass phase or a crystalline phase in which all or part of the grain boundary phase is crystallized. The grain boundary phase of the surface area is mostly in the glass phase if Y₂O₃ does not occur (or only to a small extent). In a case where the crystallization method is not used, the glass phase will remain in the boundary phase of the surface area. The crystalline phase contained in the grain boundary phase, the basic Si₃N₄- Y₂O₃ compound (Si₃N₄ · n Y₂O₃ · m X), forms the melilite phase, the J, K, H or A phase, or a mixed phase formed therefrom . These phases can be expressed according to the following formulas:

M phase (melilite) Si₃Y₂O₃N₄ (Si₃N₄ · Y₂O₃)
J phase (mohlerite) Si₂Y₄O₇N₄ (Si₂N₂O · 2 Y₂O₃)
K phase (wollastonite) SiYO₂N (Si₃N₄ · 2 Y₂O₃ · SiO₂)
H-phase (apatite) Si₇Y₁₀O₂₃N₄ (Si₃N₄ · 5 Y₂O₃ · 4 SiO₂)
A phase (-) Si₃Y₁₀Al₂O₁₈N₄ (Si₃N₄ · 5 Y₂O₃ · Al₂O₃)

These phases can be summarized by the following formula be specified:

Si₃N₄- n Y₂O₃- m X

(n = 1-5; X = SiO₂, Al₂O₃; m = 0-4).

It was found that the workpiece or that too cutting material mostly an iron-based one Alloy is the main element for the silicon Si of silicon nitride, a high chemical affinity shows what the poor wear resistance of Silicon nitride explained. When sintering silicon nitride Attempts have been made to make a decomposition to avoid the formation and evaporation of silicon nitride.

The present invention is based on the finding that that with a sintered body made of silicon nitride substance in which the decomposition and evaporation of the Silicon nitride component in a clever way  is used and in which the size of the drop of the Concentration of silicon in the surface area predetermined value or rather in relation on the silicon content in the silicon nitride in the inner area stands, the wear resistance can be improved, without deteriorating toughness.

It is thus a feature of the first embodiment of the present invention that the silicon content in Surface area of the sintered body made of silicon nitride differs from that indoors and that the proportion of the boundary phase in the surface area increases accordingly. The steepness of the fall in silicon content is according to that shown below Formula calculated:

In the sintered body made of silicon nitride according to the present embodiment is the slope of the Falling of Si₃N₄ and / or the sialon grains in the upper area by 30 volume percent or more reduced the interior in which Si₃N₄ evaporated remains. Because of this, the essential ones Properties achieved in a satisfactory manner be one of them the higher Wearfe stability of the surface area and the other the high toughness of the interior. In other wor ten, the sintered body according to the invention does not have the Properties between these two properties lie. Accordingly, it was with the present sinter  body possible, the wear resistance through a To improve surface change without the essential properties inside the body worsen.

The surface area and the interior area scroll do not differ from each other because these areas are solid per form that together with a strong bandage held together and united by force.

The present invention also provides a method to produce the improved silicon nitride base Sintered body, which will be discussed later.

In a preferred embodiment, the Slope of the drop in concentration of silicon nitride and the sialon grains chosen so that they are not smaller than 30 volume percent is there when the above mentioned The slope of the concentration drop is not sufficient, the waste effect is insufficient, whereas if the The slope of the waste is equal to or greater than 30 volu is percent, the wear resistance improves can be without reducing the toughness that the Property of the interior is. The steepness of the Waste is preferably not less than 50 volumes percent, because then the wear resistance is clear can be improved. The silicon nitride content and even on sialon grains in the surface area be almost zero. However, in such a case, although the wear resistance is even better can be adjusted, the roughness under certain circumstances worse. For this reason, this steepness the waste in the surface area preferably in one be reduced to such an extent that a brittle connection is not generated.  

The silicon nitride base sintered body of the present invention, which mainly consists of silicon nitride, can also be made of Si-Al-ON, which is referred to as sialon. The silicon nitride and / or the sialon can be selected according to the intended purpose and field of application and can consist of the α or β type or of a mixture thereof.

The components in addition to the silicon nitride and the sialon the components forming the grain boundary phase or third components in addition to the grain boundary phase be. The components forming the grain boundary phase can generally be classified as sintering aids and can only be made from glassy Phases exist or can have different crystal phases included instead of the glassy phases. The third Components can be those components that are effective are to increase wear resistance or toughness improve. Therefore, the third components Ver Bonds include, for example carbides, nitrides or oxides of transition metals of groups IVa, Va and VIa of the international periodic table or one or several solid solutions from two or more of these Links. These connections are preferably in an amount of not more than 30% by weight in order to increase the toughness of the sintered body not to downsize. Preferred third components are TiN, ZrO₂, TiC, WC, TiB₂, HfO₂, ZrC and / or whisk ers like SiC and / or Si₃N₄, preferably with one Diameters from 0.3 to 2.0 microns and a length from 10 to 100 micrometers. Usually serve ver sprinkled particles in particle form to increase hardness increase or increase the toughness due to the grain wax improve prevention.  

The interior is the area of the sintered body excluding the surface area, i.e. the Area of the sintered body that contains the inherent eigen has properties of the sintered body. Since Si₃N₄ not through decomposition or evaporation inside the silicon ni mentioned above remains trid, the components forming the grain boundary phase and the third components as they are.

The surface area is the area of the sintered body pers, in which components other than Si₃N₄ and / or Sialon remain as much as the silicon content is reduced when evaporating Si₃N₄. So is the relative proportion of the components in addition to the Si₃N₄ and / or correspondingly in the surface area of the Sialon enriched. The thickness of the surface area depends the purpose, the use and the manufacturing method and is usually in the range of several Micrometers to 0.1 millimeters, in some cases up to to 1 millimeter. It should be noted that a sintered body in which the above compositional relationships not change abruptly, but continuously changing Border between the surface area and the interior change richly, even in the field of the present Invention falls. It is therefore sufficient that the Sintered body has a surface area that a first special relative composition and one Interior of a second special relative together has composition.

As far as the present sintered body usually with its raw sintered surface, the Si₃N₄- Surface component is evaporated, is used must pay special attention to the deterioration  tern the surface roughness by such Evaporation can be exercised. Therefore, the surface Chen evaporation can be controlled so that the surface roughness preferably not more than 12.5S according to the JIS B 0601 standard. For applications where the Surface roughness is of particular importance, it is for the components forming the grain boundary phases preferable to leave them so that they have the surface Cover the surface of the sintered body.

The following features are all embodiments of the present invention common. It is about because (i) the fraction of the grain boundary phase in Surface area much larger than that inside Range and that (ii) the degree of crystallization in Surface area is higher than the inner area. Through the synergistic effect of these two characteristics the wear resistance is significantly increased. In the the first to third embodiments listed Features determine the area sizes with an effective increase wear resistance.

Melilite is one of the crystal compounds that the Form grain boundary phase on the surface, that before in the event that the boundary phase is a crystalline one Contains phase. It was found that when the tetra gonal Si₃N₄ · Y₂O₃ compound that be with melilith is mainly crystallized in the Surface area occurs and in that area in a higher concentration than in the inner area occurs, the wear resistance increases significantly is.

It was also found that most workpieces are made of an iron-based alloy  with silicon as the main component of silicon nitride has a high chemical affinity, so that its Wear resistance is smaller and the degree of Response is reduced to such an extent that the Content of silicon reduced compared to Si₃N₄ is to ensure stability against chemicals and ab improve usage. It is critical for the kristal line grain boundary phase components in the surface rich in greater concentration and inside Area to appear in a smaller concentration since the crystallization of a higher content of the kri stallinen grain boundary phase Si₃N₄-Y₂O₃ basic compound reduced toughness in the inner region leads at room temperature.

The present invention is based on the preamble have been written. According to the first version Form is the content of crystal grains from Si₃N₄ and / or sialon in the surface area around 30 volu percent or more less than in the inner portion rich. With a smaller difference than 30 volumes percent can have a sufficient effect of the surface ver change, d. H. Increase wear resistance, not can be achieved.

According to the second embodiment, an effective one Effect of surface change not achievable, if the ratio of the content of the grain boundary phase Crystal compound in the surface area to that in the inner area, measured with the x-rays maximum intensity ratio method, 0.5 or more is.

In the third embodiment, in which the ratio the content of the crystalline mixture in the grain size  Zen phase in the surface area to that of the content Crystal grains of Si₃N₄ and / or sialon in the same Range is 0.3 or greater as determined by the procedure the detection of the maximum X-ray intensity the wear resistance can be determined even further increase.

The present raw material sintered body becomes ordinary by usual sintering under atmospheric pressure produced. However, it can also be made by sintering under pressure Gas atmosphere or according to a hot isostatic Sintering (HIP method) takes place. It is fundamental necessary that the sintering atmosphere contains nitrogen, however on the condition, e.g. B. such that the amount on silicon nitride and / or sialon grains in the surface area decreases. The sinter pressure can be between one vacuum (or reduced pressure) up to several a thousand atmospheres vary. The sintering temperature is preferably in the order of 1500 to 1800 Degrees Celsius and is particularly in the range between 1600 and 1700 degrees Celsius.

The sintered body according to the invention can essentially can be made in the following manner. Powder feed Silicon nitride and sintering aids are combined with one desired mixing ratio balanced, mixed and powdered. The sintering aids can be those those when sintering under atmospheric pressure or gas pressure or sintering according to the HIP method and preferably do not contain the element Si. Consequently The following sintering aids such as Al₂O₃, AlN, MgO, CaO, Y₂O₃ or rare earth oxides be counted. The powdery starting mass is shaped and under pressure to the desired shape  compacted and sintered. For vaporizing (or ver volatile) of Si₃N₄ on the surface during the The partial pressures of nitrogen and / or silicon, or it can be reducing atmosphere can be used.

Usually the powdered starting mass can be like be composed as follows:

The starting powder mass (mixture) has a through average particle size of preferably 5 micro meters or less, preferably 2 microns or less ger.

Thus, the manufacturing process can be as follows to get nabbed:

• (a) Prepare a starting powder mass defined Composition, including mixing (habit with simultaneous pulverization),  
• (b) making a compact of the desired Shape,
• (c) sintering the compact on the condition that existing on the surface of the sintered body whose silicon nitride can be evaporated to a Si-depleted surface area at a specific temperature.

The sintering is carried out for a certain period of time sufficient to sinter the compact, preferably 0.5 to 5 hours and most preferably 1 to 3 hours. The sintering condition is provided by a special atmosphere for sintering, with reduced partial pressures of nitrogen and / or silicon or a reducing atmosphere, such as those which (in addition to reduced partial pressures) contain CO₂ and / or Co. In general, the sintering conditions vary depending on the composition and the sintering temperature. This means that the atmosphere is set to a slightly lower partial pressure of N₂ and / or Si than those which are considered suitable for a certain composition in terms of temperature, for example according to FIG. 5. In this manner, an appropriate amount of Si and N can be decomposed and volatilized from the surface area to such an extent that no rough surface is generated.

The resulting surface area of the sintered body is mainly through the sintering aids and / or resulting compounds thereof (or other third th components) in which Si and / or N as solid solution is included. By evaporating Si and N become the remaining border phases and third Components in the surface area relative to the  Enriched inside what a change in the Density and toughness (hardness is improved) entails. The amount of silicon nitride and / or Sialon in the surface area is relatively smaller than in inner area (preferably by at least 30 vol percent, up to 100 percent by volume).

According to the fourth embodiment of the invention under the melilite content in the surface differs area of the inner area, if one Measurement like the X-ray maximum ratio method is carried out. This ratio of the Melilith-Ge hold in the inner area to that in the surface range is predetermined less than 0.5 because of a effective effect not shown at 0.5 or greater can be. The wear resistance can be increased be without the desired properties of the higher To reduce toughness in the inner region.

According to the fifth embodiment, this is the maximum X-ray intensity ratio of the Melilith Content to the silicon nitride content in the surface area not less than 0.3. In this case the melilith content will be increased during the silizi order content can be reduced to reduce wear significantly increase stability.

Although in many cases the silicon nitride-based sintered body consists essentially of silicon nitride, it can also be made of sialon, as stated above, since the purpose of the present invention is to change the surface, which is caused by the content on a grain boundary phase crystal compound, such as. B. Meli lith, is increased in the surface area compared to the corre sponding content in the inner region. Silicon nitride or sialon of the α or β type or a mixture thereof can be selected according to the purpose and the application.

According to the mixture ratio of Y₂O₃ usually between 1 and 20 weight percent and preferably between 1 and 10 weight percent in Case that the grain boundary phase the crystal stock contains parts. Y₂O₃ is used as a donor of yttrium Y preferred that an essential element of the grain size Zen phase crystal compound, such as. B. Melilith; and at least 1 percent by weight Y₂O₃ is not necessary agile. An excess of Y₂O₃ leads to a lowering generation of high temperature behavior as a result of Growth of the grain boundary phase. The element Y can also be introduced if it is not as a stock part of an oxide, but z. B. a nitride or Silicids is present. These connections are in one used amount calculated according to Y₂O₃. Al₂O₃ and / or AlN, which played a significant role in the Melilite crystal lisation play, can be fed simultaneously with Y₂O₃ will. The preferred amounts of Al₂O₃ and AlN are 1 up to 10 percent by weight or 1 to 10 percent by weight. AlN can be beige to a somewhat greater extent than Al₂O₃ be added to get a better result. In addition, conventional sintering aids can be used at normal pressure sintering of Si₃N₄ can be used, such as. B. at least one of the following compounds, MgO, SiO₂, ZrO₂ or Oxides of the rare earth elements.

Silicon nitride can also be caused by third components be replaced to increase wear resistance strength and breaking strength. In this together Menhang can use the following connections as an example  be called, one or two carbides, nitrides or Group IVa, Va and VIa transition metal oxides of the periodic table or elements, or a fixed Solution of two or more of these connections. These Replacement compounds are preferably in an amount of no more than 30 percent by weight to go on this Do not reduce the toughness of the sintered body nern.

With the inner area is the area of the silicon ni sintered body without the surface coating richly designated. The inner area does its thing Main part from, d. H. it is the area that the inherent properties of the present silicon shows nitride base material sintered body.

The surface area is according to the second embodiment to the section of the sinter body in which the relative content of Si₃N₄-Y₂O₃- Basic connection, such as B. Melilith is increased. The The thickness of the surface area is usually in the range between a few micrometers and 0.1 milli meters and sometimes ranges up to 1 millimeter, depending on the Purpose, application or production process. If a large amount of the crystals of Si₃N₄-Y₂O₃- Basic connection the entire surface of the sintered body pers covered, this layer is in a state in which contain almost no Si₃N₄ and / or sialon grains are (however, the third component = scattered Phase is included) and therefore to a 100pro percent drop in Si₃N₄ and sialon leads. The fat such a crystalline layer (100 percent State of reduction) is preferably not more than 5 microns to prevent the strength of the sintered body is reduced in its entirety.  

The present invention also includes the case where the above compositional relationship does not change abruptly, but continuously changed; it is sufficient that the sintered body has a surface area with a predetermined first composition ratio and an inner area with a predetermined second Has composition ratio.

The process for producing the sintered body in the fourth embodiment of the invention corresponds to that described above. Although sintering is preferably under Evaporation conditions for the surface area with an appropriate amount of silicon nitride the present invention is not limited to one set such procedure. The sintering is done in one Nitrogen-containing atmosphere performed and the Partial nitrogen pressure is between the vacuum and a few thousand atmospheres. Atmospheres that are more common wise when sintering silicon nitride or sialon can be used for sintering according to the present ing invention can be used.

For shaping the surface of the sintered body according to the present invention before (or after or during sintering it is preferable to use the grain boundary phases to a Si₃N₄-Y₂O₃ basic compound, such as Melilith crystallize out. For this purpose the temperature in a range between 1400 and 1700 ° Celsius and preferably between 1500 and Held at 1650 ° Celsius for a predetermined time or the cooling rate is delayed. Keeping that tempera tur can be carried out after the sintering or by reheating to the above Temperature after the usual sintering. During the Crystallization process must be particularly careful  be shown so as not to cause overheating since otherwise the Si₃N₄-Y₂O₃ basic compound, such as. B. Melilith, not only on the surface, but also on the inside Grain boundary phase of the sintered body crystallized. To Increasing the extent of melilite crystallization in the Surface area compared to that inside Area, the sintering atmosphere, such as. B. the stick substance and / or the oxygen partial pressure, correspond be checked accordingly or the surface can be covered by a connection that connects the Melilith Promotes crystallization.

The progress of the crystallization of the M, J, K, H and A phases on the surface area is essentially influenced by the setting of the sintering time and the amount of the Y₂O₃ component. According to the present invention, the composition of the surface area is subject to the complex change due to the evaporation of its components. Therefore, the resulting crystal phases are affected by the change in the atmosphere and also by the compositional secretion during cooling. As far as local occurrence is concerned, reference can be made to an SiO₂-Si₃N₄-Y₂O₃ ternary phase diagram Fig. 11 (by FF Lange et al., J. Am. Ceram. Soc. 60 (5-6), p 249-252 (1977)) or an article by Hayasi, Yoh et al. (Fig. 3 from Powder and Powder Metallurgy 34 (1), pp. 26-31).

The invention is described below with reference to the drawing described in more detail. It shows

Fig. 1 is a graphical representation to illustrate the relationship between the slope of the Si₃N₄ sialon waste in the upper surface area and the wear resistance.

Fig. 2 is a cross sectional view of the shape of a workpiece, which was used in Test 1,

Fig. 3 is a cross sectional view of the shape of a workpiece, which was used in Test 2,

Fig. 4 is a cross sectional view of the shape of a workpiece, which was used in Test 3,

Fig. 5 is a well-known phase diagram hung the Bezie between the Si₃N₄ phase and the Parti the sintering atmosphere alPress light illustrate,

Fig. 6 is a diagram for illustrating lichung the relationship between the surface area / inner area ratio for unit R₂ of the content of melilite and the wear resistance,

Fig. 7 is a diagram for illustrating the relationship between the melilite lichung / silicon nitride content ratio R₁ in the surface region and the wear resistance,

Fig. 8 is a graphical representation of the results of X-ray diffraction analysis of the surface region of the sintered body of Sample Nos. 28,

Fig. 9 is a graphical representation of the results of X-ray diffraction analysis of the inner portion of the sintered body of Sample Nos. 28,

Fig. 10 (a) and (b) are diagrams each weils of the surface area and of the area occupied ren show the structure, and

Fig. 11 is a known SiO₂-Si₃N₄-Y₂O₃ ternary phase diagram, which was generated by at 1600 to 1750 ° Celsius hot pressed patterns.

The invention is described below with reference to FIG described numerous exemplary embodiments ben.

A first embodiment relates to the first to third embodiment.

The Si content in the surface area is compared to which has been lowered in the inner area while the boundary phase crystal mixture, d. H. the Si₃N₄-Y₂O₃- Basic connection, crystalline on the surface area was identified. The investigations were on the off effects of these measures.

As powder for the starting material, powdery Si₃N₄ with an average particle size of 0.6 micrometer ( a -Si₃N₄ portion not less than 90 vol%), powdered Al₂O₃ or MgO with an average particle size of 0.5 micron, AlN powder with an average particle size of 1.3 microns, powdery Y₂O₃ with an average particle size of 1.2 microns, powdery ZrO₂ with an average particle size of 0.4 microns, powder TiN with an average particle size of 1.2 microns, powdered HfN with a average particle size of 1.7 micrometers and powdery toilet with an average particle size of 2.0 micrometers taking into account the composition ratios in the table and mixed 48 hours in a wet ball mill. A molding agent was added to the resulting mixture and it was dried.

Each of the resulting powder mixtures (starting coil dimensions) was compression molded in a metal mold and in an atmosphere that contains mainly nitrogen held under the nitrogen atmosphere from 0.7 to 10 atom (and partly also in the presence of Carbonic acid gas) and at a temperature of 1650 to Sintered at 1750 degrees Celsius to produce sintered products put. The thickness of the surface area of these Sintered products was about 0.01 to 0.1 millimeters for both cases, this by surface removal, by grinding and x-ray refraction of a small one Range (X-ray microanalyser) measured has been.

The sintered products were reworked to a size of SNMN 432 according to JIS B 4103 to form test pieces (or tips) 2. The finishing of the sintered products was carried out by grinding the upper and lower surfaces, the side surfaces of the sintered product remaining in the state of the raw sintered surfaces. The size of the test pieces was adjusted at the time of printing. Evaluation was carried out by the following three tests 1 to 3, tests 1 and 2 being used to assess wear resistance and test 3 being used to assess strength. The results are shown in Table 1 and in Fig. 1 Darge.

All examples show densities of at least 99.5 percent in terms of theoretical density. The inner area consisting of sialon was of the β type in No. 3, No. C2 and No. 4 and ( β type + α type) in No. 5-9 and No. C3, in which the α ratio 0.33, 0.28, 034, 042, 0.36 and 0.22 respectively in this order. The α ratio is defined by α / ( α + β ) measured according to the X-ray maximum intensities ratio method.

α -Sialon is a solid solution of the substitution / interstitial type, in which a part of the silicon atoms in the α -Si₃N₄ structure is replaced by aluminum and a part of the N atoms in the same structure is replaced by O, and in the elements such as Li, Na, Mg, Ca, Y or rare earth elements form a solid solution that penetrates into the interstitial space between (Si, Al) and (O, N). In general, such an α- sialon is by the formula

Mx (Si, Al) ₁₂ (O, N) ₁₆

expressed where M = Li, Na, Mg, Ca, Y or a rare earth element and 0 < x 2.

On the other hand, b- sialon is a solid solution of the substitution type, which is formed from a solid solution of Al₂O₃, AlN and SiO₂ in β- silicon nitride, that is, in which Al and O are part of the Si atoms and N atoms each in the β- silicon nitride.

This β- sialon is represented by the formula

Si₆ _{- z} Al _z O _z N₈ _{- z}

expressed with 0 < z 4.

The conditions for tests 1, 2 and 3 are as below shown, the conditions for test 2 in Brackets are shown and those for Test 3 in rectangular brackets. The durability in procedure 1  is given as the time to destruction or Failure, as described below, leads. The Destruction or failure in this case is not just a result of mechanical strength, but also the enlargement of the cutting and processing resistance due to wear and is a measure of the wear resistance used.

Shape of the workpiece (1 in the figure) Test 1

Ring tube shape with an outer diameter of 300 mm and an inner diameter of 200 mm, as shown in Fig. 2.

Test 2

(Rod shape with an outer diameter of 240 mm as shown in Fig. 3).

Test 3

[Rod shape until the occurrence of ring grooves with the groove depth of 15 mm and the outer diameter of 240 mm, as shown in Fig. 4].

The reference number 3 in the drawing relates to a tool holder. The durability or wear resistance was evaluated in test 2 by the wear on the side surfaces.

Sample Nos. 1 to 8 from Table 1 refer to the first embodiment, sample No. 9 on the second Embodiment and Sample Nos. 10 to 13 on the third embodiment. Sample No. 9 was made by Sintering of sample No. 3 obtained, which subsequently for the crystallization of a heat treatment at 1500 ° Celsius was subjected to 4 hours. Sample Nos. 12 to 14 are by sintering the sample 11 at ver different sintering temperatures, sintering times and sintering atmospheres (sintering position, e.g. arrangement in ship Chen or melting furnaces) were generated.

The composition ratio for Si₃N inclusive The Sialon is used as a function or application determined. It is therefore difficult to make all the samples compare under the same conditions. Consequently is one type of higher strength and a second more for Ni alloys more suitable type and a third more suitable type for cast products than comparison samples Nos. 1, 2 and 3 each shown. There is practically no sense getting these three types together compare, but it makes sense to compare the properties to face within each of this type. Therefore Table 1 is structured so that the properties easily within the same type, the same or similar composition of the starting material (Starting powder mass), can be compared. Results of the comparison are shown below ben. The test pieces of the comparative samples C1 and C3 were obtained by grinding those according to Sample Nos. 1 and 6 by removing the material up to one Depth of more than 2 millimeters from the surface  prepared to complete the surface area remove until the interior is exposed (determined by X-ray analysis). The test piece of the Comparative sample C2 has a composition that the is the same as the composition of Sample No. 3, and has a raw sintered surface in which the Si content has hardly been reduced on the surface.

Sample Nos. 1 and 2 were not special waiters subjected to surface crystallization processes and The boundary phase of the surface area mainly existed made of glass phases (except for the third component HfN).

A comparison of the samples according to the invention with the comparison samples with the reduced Si₃N₄ and Sialon grain content in the surface area (sample No. 1 with the comparison sample C1, sample No. 3 with the comparison sample C2 and sample numbers 6 to 8 with the comparison sample C3) shows that the wear resistance is unusually greatly improved. Although the strength is slightly reduced in any case, this does not represent a practical disadvantage. The relative Wear resistance in samples Nos. 6 to 8 and the comparative sample C3 is shown in Fig. 1, from which it can be seen that the wear resistance is greater, the greater the drop in the Si₃N₄ and Sialonkorn proportion, and that such an effect is most pronounced when the Si₃N₄ and Sialon grains drop by more than 30 percent by volume.

On the other hand, the comparison of the samples shows Nos. 2, 4 and 6 when comparing sample Nos. 1, 3 and 5 each plus the compounds of the transition metals of groups IVa and VIa (HfN, WC and TiN) with the  Samples Nos. 1, 3 and 5 that wear resistance compared to that according to sample Nos. 1, 3 and 5 is improved. However, with a larger amount of It can add the compound of the transition metal occur that the strength is significantly reduced is, as in comparative sample C4.

From the above it can be seen that the Sample Nos. 1 to 8, in which the drop of Si₃N₄- Sialon grains is at or above 30 percent by volume, the wear resistance is improved while the Strength remains unchanged and that the properties the materials of the surface and the inner Sections present themselves satisfactorily.

Sample Nos. 9 to 13 show Si₃N₄-Y₂O₃ base Kri stall grains in the surface area, which from the surface Chen crystallization treatment. These samples show a noticeable improvement in Test 1 and also satisfactory results in test 3.  

Table 1

A second embodiment relates to the fourth and fifth embodiment.

The effectiveness of the melilite phase as crystals ons grain boundary phase has been studied. Samples have been manufactured under conditions Production of the melilith phase favorable composition conditions and use sintering conditions.

As powder for the starting material were powdered miges Si₃N₄, powdered Al₂O₃ or MgO, AlN powder, powder Y₂O₃, powder ZrO₂, powder TiN, as in Example 1, and also powdery Yb₂O₃ with an average particle size of 5.1 micrometers taking into account the compositional relationships taken in Table 2 and in a wet ball mill Mixed 48 hours. The resulting mixture was a molding agent was added and it was dried.

The resulting powder mixtures were compression molded in a metal mold and sintered in various atmospheres having nitrogen pressures of 0.7 to 10 atom and at a temperature of 1650 to 1750 degrees Celsius to produce sintered body products with various melilite contents. The results are shown in Table 2. The slope of the waste of Si₃N₄ and sialon grains in the surface area for the sample nos. 27 to 29 is 100 percent by volume. The thickness of the surface area of Sample No. 27 is about 3.5 microns, whereas that of the surface area of Sample No. 28 is about 1 micron. The results of the X-ray diffraction analysis of the surface areas and the inner areas of Sample No. 28 are shown in FIGS . 3 and 4, respectively. The thickness of the surface area was measured after a surface removal by grinding with X-ray refraction of a small area (X-ray micro-analyzer) of the surface area. The melilite / silicon nitride content ratio R 1 in the surface area was calculated as shown in FIG. 8 with the formula R 1 = I _M 1 / I _S , whereas the ratio of the melilite content in the inner area to that in the surface area was determined by the ratio was determined from I _M ₂ from Fig. 4 to I _M ₁ from Fig. 3, ie by the formula R ₂ = I _M ₂ / I _M ₁.

The sintered products were reworked to a size of SNMN 432 according to JIS B 4103. The after-work of the sintered products was carried out by grinding the upper and lower surfaces, the side surfaces of the sintered product remaining in the state of the raw-sintered surfaces. The sintered bodies were used as test pieces (or tips) 2 . The size of the test pieces 2 was adjusted at the time of printing. Evaluation was carried out by the following two tests. The results are shown in Table 2 and in FIGS. 5 to 8.

The test conditions for tests 4 and 5 are below described, the Darge for test 5 in parentheses represents are. The durability in test 4 is determined by the Height of wear in millimeters and that in test 5 by the number of cut furrows (cracks) to described to break.  

Shape of the workpiece (1 in the figure) Test 4

Rod shape with an outer diameter of 240 mm, as shown in Fig. 3.

Test 5

(Rod shape with an annular groove with an outer diameter of 240 mm and with a groove depth of 15 mm, as shown in Fig. 4).

The durability for Test 4 was based on the amount assessed for wear on the side boundaries.

The composition ratios for Si₃N₄ and / or Sialon is used as a function or application determined. So there's practically no point these types with others to suit their use compare the composition, but it makes sense to mix the samples within each type to face the. The samples of the comparative samples C21 and C24 are more than 0.5 mils thick  meters from the surface have been sanded to to completely remove the surface area until the interior is exposed. These are marked with * Table 2 marked.

In the case of the sample Nos. 21 and 22 according to the invention in comparison with the comparative samples C21 and for the sample Nos. 27 to 30 in comparison with the comparison samples C22 to C24, the amount of wear was reduced and the wear resistance improved, while the number of Furrows remain unchanged until they break and so the breaking strength is retained. Sample Nos. 23, 24, 25 and 26 also showed higher wear resistance. The test results on the amount of wear are shown in Figs. 5 and 6 in comparison to the sample Nos. 27 to 30 and the comparative samples C22 and C23.

FIG. 5 shows that the amount of wear in the samples according to the invention with an internal / surface ratio of melilite content less than 0.5 is less than the corresponding value in the comparison samples, in which this ratio is not less than 0 , 5 is. The comparison between Sample Nos. 21 and 22 and Comparative Sample C21, between Sample Nos. 23 and 24 and between Sample Nos. 25 and 26 shows that the amount of wear in the first-mentioned samples is smaller than in the last-mentioned samples is. It can be seen from this that the better wear resistance is obtained when the ratio is smaller, ie when the melilite content in the surface area is higher than in the inner area.

Fig. 6 also shows that the higher the content of Meli lith in the surface area of the sample, the higher the wear resistance of the sample. This can be supported by the fact that in the comparison between Sample Nos. 21 and 22 and Comparative Sample C21, between Sample Nos. 23 and 24 and between Sample Nos. 25 and 26, the amount of wear in the the former samples with the higher value of the above ratio is smaller than in the latter samples with the smaller value of the above ratio. Fig. 7 shows that when the product of the two ratios R ₁ and R ₂ increases and the melilite content in the grain boundary phase in the inner region is increased, as in the samples Nos. 27 and 28, the number of cut Furrows are reduced and the breaking strength is lower.

The usual Sialon cutting tool of the comparison sample C25 is particularly prone to wear and tear during that commonly coated silicon nitride cutting tool Comparative sample C26 is extremely susceptible to breakage.

It can be seen from the above that in Sample Nos. 21 to 30 with the melilite / silicon nitride content ratio in the surface area between 0.37 and 4.5 and with the inside / surface area content ratio in melilith between 0.02 and 0.31 improves the wear resistance and the breaking strength is not reduced, so that the properties of the materials that form the surface and inner areas are very satisfactory. The steepness of the waste of Si₃N₄ sialon grains shows that (i) the grain boundary phase occurs more abundantly in the surface area than in the inner area. However, it cannot be said that the steepness of the drop of the Si₃N₄ sialon grains is not necessarily correlated with the Ge ratio R ₂. It is seen that (ii) crystallization occurs in the surface area rather than the inner area. Therefore, the ratio R ₂ is conceivably predetermined by the synergistic effects of (i) and (ii).

In the silicon nitride base sintered body according to the fourth and fifth embodiments come inherent properties of higher wear resistance in the Surface area and the high toughness inside full range, as a higher one Melilite content in the surface area than in that inner area occurs. Therefore occur in the present sintered body has no medium properties, by balancing two properties occur, as is the case with conventional sintered bodies is. Now it is possible with the present sintered body Lich, the wear resistance through a surface improve change without losing the inherent inherent to reduce the inner area. The waiter surface areas and the inner areas are in one piece bonded together with a large adhesive strength, so that they don't peel off each other.

In the silicon nitride base sintered body according to the fifth embodiment is the melilite content in the Surface area higher than the silicon nitride content, to ensure the permanent wear resistance most.

As can be seen from the above exemplary embodiments the improvement is significant. However, can additional coating layers or layers on the sintered bodies according to the invention are applied, wherein the coating layer made of Al₂O₃, TiC, TiN, AlON  or the like with a thickness of 0.5 to 10 micro meters is formed.

It should be noted that modifications are made can be without the core and basic idea of the present invention as set forth herein geof within the scope of the appended claims fenbt is.  

Table 2

Claims (21)

1. Sintered body based on silicon nitride with a surface area and an inner area, which are integrally formed with one another, characterized in that the amount of crystal grains of silicon nitride and sialon in the surface area is at least 30 volume percent less than that in the inner area. 2. Sintered body according to claim 1, characterized records that part or all of the grain boundary phase is crystallized in the surface area. 3. Sintered body according to claim 2, characterized records that the ratio of the salary to the Grain boundary phase in the inner area entirely or to Partial crystalline compound to that in the Surface area less than 0.5 when measured with the X-ray maximum intensity ratio method is. 4. Sintered body according to claim 2, characterized records that the ratio of the amount of the Grain boundary phase in the inner area entirely or to Part forming crystalline compound to the amount of Silicon nitride and sialon grains in the surface area range not less than 0.3 when measured with the X-ray is the maximum intensity ratio method. 5. Sintered body according to one of claims 2 to 4, characterized in that the crystalline grain boundary phase in the surface region has at least one Si₃N₄-Y₂O₃ basic compound which can be represented by Si₃N₄- n Y₂O₃- m X, where n = 1-5; X = SiO₂, Al₂O₃ and m = 0-4. 6. sintered body according to claim 5, characterized records that the Si₃N₄-Y₂O₃ basic compound a Meli lith, J, K, H or A phase or a mixture of these Phases is. 7. sintered body according to claim 2, characterized shows that the ratio of the melilite content in the inner area to the melilite content in the surface range less than 0.5 when measured with the X-ray is the maximum intensity ratio method. 8. sintered body according to claim 7, characterized records that the ratio of the melilith content to the content of silicon nitride and sialon in the surface range not less than 0.3 when measured with the X-ray is the maximum intensity ratio method. 9. Sintered body based on silicon nitride with one Surface area and an interior that is one piece are interconnected, thereby characterized in that the ratio of the Melilith content in the inner area to that in the surface area less than 0.5 when measured with the X-ray maximum intensity ratio method. 10. Sintered body according to claim 9, characterized records that the ratio of the melilith content to the content of silicon nitride and sialon in the surface range not less than 0.3 when measured with the X-ray maximum intensity ratio method.   11. Sintered body according to claim 3, characterized records that the crystalline grain boundary phase in Surface area essentially made of melilite stands. 12. Sintered body according to claim 11, characterized records that the ratio of the melilith content to the content of silicon nitride and sialon in the surface range not less than 0.3 when measured with the X-ray maximum intensity ratio method. 13. Sintered body according to claim 1, characterized records that the amount of silicon nitride and sialon grains in the surface area less than 50 volumes percent or more than the amount in the inner area is. 14. sintered body according to one of claims 1 to 13, characterized in that the sintered body third Components with a content of 30 percent by weight or less of the entire sintered body, wherein the third components connect in particular from subgroups IVa, Va and VIa des internationa len periodic table included. 15. Sintered body according to claim 14, wherein the Ver bonds oxides, carbides and nitrides as well as solid solutions against it. 16. Sintered body according to claim 15, characterized records that the compounds are ZrO₂, WC and TiN. 17. Sintered body according to claim 2, characterized records that the border phases by a sintering aid are formed.   18. Sintered body according to claim 17, characterized records that the sintering aid has at least one verb dung is the group of Al₂O₃, YN, AlN, MgO, CaO, Y₂O₃ and the rare earth oxides. 19. Sintered body according to claim 18, characterized records that the surface area is at least several Micrometer thick. 20. Sintered body according to claim 19, characterized indicates that the surface area is not thicker than Is 0.1 millimeters. 21. Sintered body according to claim 19, characterized indicates that the surface area is not thicker than 1 Is millimeters.