A production process of high-hardness alumina ceramics

Abstract

The application discloses a production process of high-hardness alumina ceramic, which adopts yttrium oxide modified polyvinyl alcohol as an alumina ceramic binder, and the bonding performance of the polyvinyl alcohol is greatly improved compared with polyvinyl alcohol; the polyvinyl alcohol is added into the system as the binder, so that the amount of the binder is reduced, the pores and cracks formed in the glue discharging process are reduced, and the density in the sintering process is improved; on the other hand, the modification treatment can reduce the regularity of the polyvinyl alcohol molecular chain, and the adverse effect of the easy crystallization of the polyvinyl alcohol on the density of the alumina ceramic is avoided; in addition, the yttrium oxide modified polyvinyl alcohol can generate yttrium oxide in situ after the polyvinyl alcohol is removed, the pores and cracks formed due to the precipitation of the polyvinyl alcohol are filled in situ, and the density of the alumina ceramic is further improved; and the forming process disclosed by the application can obtain the alumina ceramic with high density and excellent mechanical properties.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ceramic production, in particular to a production process of high-hardness alumina ceramic. BACKGROUND

[0002] Alumina ceramic is widely used in mechanical, electronic, chemical, medical and other fields due to its high hardness, high wear resistance, good insulation and chemical stability. The hardness of alumina ceramic is affected by many factors, mainly including the following aspects: the influence of raw material powder: the most important raw material of alumina wear-resistant ceramic is high-purity alumina powder, and its performance and content have a great influence on alumina wear-resistant ceramic. The particle size of raw material powder also has a great influence on the performance of the product. Only when the raw material is fine enough, the final sintered product can form a microcrystalline structure, so that it has good wear resistance. The influence of forming method: the forming method is the key to obtain high-density ceramic green body. The forming of alumina wear-resistant ceramic generally adopts dry pressing, isostatic pressing and hot pressing casting methods. Different methods have different characteristics, and the influence on the sintering performance and microstructure of alumina ceramic will also be different. The influence of sintering: sintering is the key stage of ceramic preparation, and the sintering atmosphere has a great influence on the preparation of ceramic materials. Too high sintering temperature or too long holding time will promote the growth of crystals, so that the fine raw materials develop into coarse grains at high temperature, and the liquid phase increases. In this way, not only the strength decreases, but also the wear resistance is weakened. Too low temperature will lead to the densification of sintered body, and the wear resistance of the product is not good. Other impurities: impurities will be inevitably introduced during the preparation of ceramic powder, and the organic impurities will be burned out during the sintering process, forming irregular pores during the densification process. These microstructure defects caused by impurities have a significant influence on the densification of alumina wear-resistant ceramic, and further affect its hardness.

[0003] In summary, in order to prepare high-hardness alumina ceramic, high-purity alumina powder needs to be selected as raw material, appropriate forming method needs to be adopted, and temperature and time during sintering process need to be accurately controlled. At the same time, it is also necessary to avoid introducing too much impurity to ensure the densification and hardness of the ceramic. However, the traditional alumina ceramic production process often has problems such as low purity of raw material powder, unreasonable forming method and inaccurate sintering process control, which leads to insufficient hardness and poor wear resistance of the prepared alumina ceramic. Therefore, providing a production process of high-hardness alumina ceramic has become a problem to be solved in the current ceramic production field. SUMMARY

[0004] The application aims to provide a production process of high-hardness alumina ceramic, which can overcome the problem of insufficient bonding capacity of polyvinyl alcohol and influence on the density of alumina ceramic forming by using yttria-modified polyvinyl alcohol as the alumina ceramic binder, and can obtain the alumina ceramic with high density and excellent mechanical properties by using the forming process.

[0005] One object of the application is to provide a production process of high-hardness alumina ceramic, which is characterized by comprising the following steps:

[0006] 1) mixing and grinding alumina, sintering aids and silane coupling agent to obtain a mixture;

[0007] 2) adding the mixture obtained in step 1) into heated yttria-modified polyvinyl alcohol and mixing uniformly;

[0008] 3) forming the mixture treated in step 2) to obtain a green body;

[0009] 4) sequentially performing glue removal and sintering on the green body to obtain the alumina ceramic.

[0010] Further, the particle size of the alumina is 60-100 nm, and the purity is 96.5-99%.

[0011] Sintering is based on the migration of substances under the action of surface tension to realize the aggregation of powder, and the surface energy is the internal driving force of sintering. Compared with bulk materials, powder has a large specific surface area, and various lattice defects will occur on the surface and inside of the powder particles during the preparation process of the powder, which activates the crystal lattice. Due to these reasons, the powder has a high surface free energy. The use of monodisperse ultrafine ceramic powder with small grain size, large specific surface area and high surface activity can significantly reduce the sintering temperature.

[0012] Further, the sintering aid is one or a combination of several of magnesium oxide, titanium dioxide and manganese dioxide, and the particle size of the sintering aid is 70-90 nm.

[0013] In the application, the sintering aid can form a limited replacement solid solution with alumina. Due to the differences in coordination number, electric charge and ionic radius, the lattice distortion and cation vacancy are generated after replacing Al3+, which significantly reduces the sintering temperature and promotes sintering. In addition, due to the difference in crystal structure, their "solubility" in alumina is extremely small, and their content in the matrix is reduced by the way of impurity aggregation at the grain boundary. With the decrease of the number and area of grain boundaries during sintering, the composition of impurities at the grain boundary relatively increases, which reduces the eutectic temperature at the grain boundary. When a certain limit is reached, it becomes a liquid phase, which effectively promotes the sintering of the alumina ceramic material.

[0014] Further, the silane coupling agent is selected from any one of KH-550 and KH-560.

[0015] The silane coupling agent is added in the system, on the one hand, the silane coupling agent can tightly wrap the binder on the surface of the inorganic powder, evenly separate the inorganic powder, enhance the flowability and uniformity between the inorganic powder, and in the process of degumming, the binder can be evenly decomposed and diffused without forming or with few forming of holes and cracks; on the other hand, in the subsequent forming, degumming and sintering process, the silane coupling agent can decompose to generate silica, fill the possible holes and cracks, and increase the product density.

[0016] Further, the preparation method of the yttria modified polyvinyl alcohol is as follows:

[0017] The yttria is ground to a particle size of 500-600 nm, under the action of ultrasonic oscillation, a certain amount of water is mixed with the yttria to form a stable suspension. Then, dry polyvinyl alcohol is added and uniformly mixed, and dried to coagulation.

[0018] The coagulum of the above step is crushed into a powder of 200-300 mesh, and the powder is crushed for 6-8 h to obtain a yttria modified polyvinyl alcohol powder.

[0019] Further, the molar ratio of yttria to polyvinyl alcohol in the yttria modified polyvinyl alcohol is 2-5:1.

[0020] Further, the particle size of the yttria modified polyvinyl alcohol powder is 60-80 nm.

[0021] The yttria modified polyvinyl alcohol formed by the above method has a significantly improved bonding performance compared to polyvinyl alcohol. When it is added as a binder into the system, the amount of binder can be reduced, thereby reducing the holes and cracks formed during the degumming process, and helping to improve the density during sintering. On the other hand, the modification process can introduce yttria into the side chain of the polyvinyl alcohol polymer chain, increase the steric hindrance between the high molecular chains and the interaction force, reduce the regularity of the polyvinyl alcohol molecular chain, and avoid the adverse effects of easy crystallization of polyvinyl alcohol on the density of aluminum oxide ceramic. Furthermore, after the polyvinyl alcohol is removed, yttria can be generated in situ to fill the holes and cracks formed due to the removal of polyvinyl alcohol, further improving the density of aluminum oxide ceramic.

[0022] Further, the weight ratio of the aluminum oxide, sintering aid, coupling agent, and yttria modified polyvinyl alcohol is 90-92:15-20:3-5:1.

[0023] Further, the specific conditions of the sintering process are as follows: the sintering pressure is 80-120 MPa; and the sintering temperature is 1350-1400 DEG C.

[0024] Further, the temperature rising rate of the degumming is less than 5 DEG C / min, and the highest temperature is 350-600 DEG C.

[0025] The beneficial effects of the present application are as follows:

[0026] The present application modifies the commonly used binder polyvinyl alcohol in the field by yttria, which can significantly improve the bonding capacity of polyvinyl alcohol, and when applied to the alumina ceramic forming process, the amount of binder in the system can be reduced compared with directly using polyvinyl alcohol, thereby reducing the pores and cracks formed during the degumming process and improving the density. In addition, due to the high regularity of the molecular structure of polyvinyl alcohol, it is easy to crystallize, which has an adverse effect on the performance of alumina ceramics. The present application avoids the problem of easy crystallization of polyvinyl alcohol by modifying polyvinyl alcohol with yttria. Furthermore, using the binder described in the present application, polyvinyl alcohol is removed under the action of high temperature, and yttria is generated in situ, which can fill the pores generated due to the removal of polyvinyl alcohol in situ. In addition, due to the limitation of the solid solubility of yttria, it is free in the sintering process between the alumina particles, which hinders the grain growth, is beneficial to the escape of pores from the inside of the grain, and greatly improves the density of the alumina ceramic. The process described in the present application can obtain alumina ceramic with high hardness and excellent mechanical properties. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 SEM image of the alumina ceramic prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0028] The embodiments of the present application are described in detail below, which are intended to explain the present application and cannot be understood as a limitation of the present application. In addition, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features limited by "first", "second" can be explicitly or implicitly included at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, such as two, three, etc., unless otherwise specifically limited.

[0029] Example 1

[0030] 1) The alumina, magnesium oxide and silane coupling agent KH550 are dosed according to the mass fraction ratio of 92:20:4;

[0031] 2) Put the weighed raw materials into the ball mill tank, select zirconia porcelain balls as grinding medium, use the planetary ball mill to keep the rotation speed at 550 r / min for rapid ball milling and mixing for 1.5 h. Dry the mixed slurry at 120°C for 24 hours;

[0032] 3) Preparation of binder: grind yttria to a particle size of 600 nm, under the action of ultrasonic oscillation, mix water and yttria to form a stable suspension. Then add polyvinyl alcohol and mix uniformly, dry to solidify. Crush the solidified body into a 200-300 mesh powder, further crush the powder for 8 h to a particle size of 80 nm to obtain yttria modified polyvinyl alcohol powder.

[0033] 4) Take yttria modified polyvinyl alcohol powder according to the mass fraction ratio (yttria modified polyvinyl alcohol: mixed material in step 2) of 1:116, prepare a 5% yttria modified polyvinyl alcohol solution as binder, pour the dry mixed powder in step 2) into a ceramic mortar, then add the yttria modified polyvinyl alcohol solution, mix thoroughly and place in a sealed bag for 24 h.

[0034] 5) Screen the powder after 24 h of dampening through a 40 mesh screen, then put it into a steel mold of a hand-held tablet forming press, and press it into a tablet at a forming pressure of 120 MPa. Dry the pressed ceramic sample in an electric heating air oven at 110°C.

[0035] 6) Dry the treated sample in sequence in a high-temperature box-type resistance furnace, then sinter it at a temperature of 1400°C for 4 h. After the holding time is over, naturally cool the sintered ceramic sample to room temperature.

[0036] Example 2

[0037] 1) Alumina, magnesium oxide, silane coupling agent KH550 are prepared according to the mass fraction ratio of 92:15:4.

[0038] 2) Put the weighed raw materials into the ball mill tank, select zirconia porcelain balls as grinding medium, use the planetary ball mill to keep the rotation speed at 550 r / min for rapid ball milling and mixing for 1.2 h. Dry the mixed slurry at 120°C for 24 hours

[0039] 3) Preparation of binder: grind yttria to a particle size of 550 nm, under the action of ultrasonic oscillation, mix water and yttria to form a stable suspension. Then add polyvinyl alcohol and mix uniformly, dry to solidify. Crush the solidified body into a 200-300 mesh powder, further crush the powder for 8 h to a particle size of 80 nm to obtain yttria modified polyvinyl alcohol powder.

[0040] 4) According to the mass fraction ratio (yttria modified polyvinyl alcohol: mixed material in step 2) of 1:110, yttria modified polyvinyl alcohol powder was taken, and a 5% mass percentage concentration of yttria modified polyvinyl alcohol solution was prepared as a binder. The dry mixed powder in step 2) was poured into a ceramic mortar, then the yttria modified polyvinyl alcohol solution was added, and after mixing, it was placed in a sealed bag for 24 hours.

[0041] 5) The powder after 24 hours of dampening was sieved through a 40 mesh screen, then put into a steel mold of a hand-held tablet forming press, and pressed into shape at a forming pressure of 120 MPa. The pressed and formed ceramic sample was placed in a 110°C electric heating air oven for drying.

[0042] 6) The dried sample was placed in a high-temperature box-type resistance furnace in sequence, then sintered at a temperature of 1350°C for 4 hours. After the holding time was over, the sintered ceramic sample was cooled to room temperature with the furnace.

[0043] Example 3

[0044] 1) The alumina, manganese dioxide, and silane coupling agent KH560 were weighed and mixed according to a mass fraction ratio of 92:20:5.

[0045] 2) The weighed raw materials were put into a ball mill tank, zirconia ceramic balls were selected as the grinding medium, and a planetary ball mill was used to quickly ball mill the mixture at a speed of 550 r / min for 1.5 hours. The mixed slurry was dried at 120°C for 24 hours

[0046] 3) Preparation of binder: Yttria was ground to a particle size of 550 nm, and under the action of ultrasonic oscillation, water and yttria were mixed to form a stable suspension. Then polyvinyl alcohol was added and uniformly mixed, and dried to solidify. The solidified body was crushed into a 260 mesh powder, and the powder was further crushed for 8 hours to a particle size of 80 nm, obtaining yttria modified polyvinyl alcohol powder.

[0047] 4) According to the mass fraction ratio (yttria modified polyvinyl alcohol: mixed material in step 2) of 1:117, yttria modified polyvinyl alcohol powder was taken, and a 5% mass percentage concentration of yttria modified polyvinyl alcohol solution was prepared as a binder. The dry mixed powder in step 2) was poured into a ceramic mortar, then the yttria modified polyvinyl alcohol solution was added, and after mixing, it was placed in a sealed bag for 24 hours.

[0048] 5) The powder after 24 hours of dampening was sieved through a 40 mesh screen, then put into a steel mold of a hand-held tablet forming press, and pressed into shape at a forming pressure of 120 MPa. The pressed and formed ceramic sample was placed in a 110°C electric heating air oven for drying.

[0049] 6) The dried samples were sequentially placed in a high-temperature box-type resistance furnace, and then sintered at a temperature of 1400℃ for 4h. After the holding time ended, the sintered ceramic samples were cooled to room temperature with the furnace.

[0050] Comparative Example 1

[0051] 1) Alumina, magnesium oxide, silane coupling agent KH550, and yttrium oxide were prepared according to a mass ratio of 92:20:4:1.

[0052] 2) The weighed raw materials were placed in a ball mill jar, zirconia ceramic balls were selected as the grinding medium, and a planetary ball mill was used to rapidly ball mill the mixture at a speed of 550r / min for 1.5h. The mixed slurry was dried at 120℃ for 24 hours.

[0053] 3) A polyvinyl alcohol solution with a mass percentage concentration of 5% was prepared as a binder, the dried mixed powder in step 2) was poured into a ceramic mortar, then the polyvinyl alcohol solution was added, and after thorough mixing, it was placed in a sealed bag for 24h.

[0054] 4) The powder after 24h of soaking was sieved through a 40 mesh screen, then placed in a steel mold of a hand-held tablet forming press, and pressed into shape at a forming pressure of 120MPa. The pressed ceramic sample was dried in an electric heating air oven at 110℃.

[0055] 5) The dried samples were sequentially placed in a high-temperature box-type resistance furnace, and then sintered at a temperature of 1400℃ for 4h. After the holding time ended, the sintered ceramic samples were cooled to room temperature with the furnace.

[0056] The properties of the alumina ceramic of Example 1-3 and Comparative Example 1 were characterized, wherein the micro Vickers hardness was in accordance with international standard ISO6507 / 1-82; the bending strength was in accordance with GB / T6569-2006 standard, and the fracture toughness was calculated according to Niihara formula, and the results are shown in the following table; at the same time, the alumina ceramic prepared in Example 1 was observed by scanning electron microscope, and the morphology diagram is shown in Figure 1

[0057] Performance data Example 1 Example 2 Example 3 Comparative Example 1 Microhardness HV 2156 2133 2094 1806 Fracture toughness MPa.m1 / 2 7.2 6.9 7.3 6.1 Bending strength MPa 556 549 538 510

[0058] ​The present application adopts yttria to modify the commonly used binder polyvinyl alcohol in the art, can significantly improve the bonding capacity of polyvinyl alcohol, and can reduce the amount of binder in the system compared with directly using polyvinyl alcohol, thereby reducing the pores and cracks formed due to the degassing process and improving the density; in addition, due to the high regularity of the molecular structure of polyvinyl alcohol, polyvinyl alcohol is easy to crystallize, which has an adverse effect on the performance of alumina ceramics, the present application avoids the problem that polyvinyl alcohol is easy to crystallize by modifying polyvinyl alcohol with yttria; furthermore, by using the binder described in the present application, polyvinyl alcohol is removed under the action of high temperature, and yttria is generated in situ, which can fill the pores generated due to the removal of polyvinyl alcohol in situ; in addition, due to the limitation of the solid solubility of yttria, free yttria is present between alumina particles during sintering, which hinders the grain growth, is beneficial to the escape of pores from the inside of the grain, and greatly improves the density of alumina ceramics, and the process described in the present application can obtain alumina ceramics with high hardness and excellent mechanical properties.

[0059] It is apparent to those skilled in the art that the present application is not limited to the details of the foregoing exemplary embodiments, and that the present application can be implemented in other specific forms without departing from the spirit or essential characteristics of the present application. Therefore, the embodiments should be considered in all respects as illustrative and not restrictive, and the scope of the present application is defined by the appended claims rather than the foregoing description, and it is intended to include all changes falling within the meaning and scope of the equivalent elements of the claims.

Claims

1. A production process of high hardness alumina ceramic, characterized by, The method comprises the following steps: 1) mixing and grinding alumina, sintering aid and silane coupling agent to obtain a mixture; 2) pouring the mixture obtained in step 1) into a ceramic mortar, adding a prepared yttrium oxide modified polyvinyl alcohol solution, and mixing uniformly; 3) forming the mixture treated in step 2) to obtain a green body; 4) sequentially performing degassing and sintering on the green body to obtain an alumina ceramic; The weight ratio of the alumina, sintering aid, silane coupling agent and yttrium oxide modified polyvinyl alcohol is 90-92:15-20:3-5:1; The silane coupling agent is selected from any one of KH-550 and KH-560; The preparation method of the yttrium oxide modified polyvinyl alcohol is as follows: yttrium oxide is ground to a particle size of 500-600 nm, under the action of ultrasonic oscillation, water and yttrium oxide are mixed to form a stable suspension; then, dry polyvinyl alcohol is added and uniformly mixed, and is dried to coagulation; the coagulum is crushed into a 200-300 mesh powder, and the powder is further crushed for 6-8 h to obtain yttrium oxide modified polyvinyl alcohol powder; The molar ratio of yttrium oxide to polyvinyl alcohol in the yttrium oxide modified polyvinyl alcohol is 2-5:1; The particle size of the yttrium oxide modified polyvinyl alcohol powder is 60-80 nm; In step 1), the particle size of the alumina is 60-100 nm; the sintering aid is one or a combination of several of magnesium oxide, titanium dioxide and manganese dioxide, and the particle size of the sintering aid is 70-90 nm.

2. The production process of a high hardness alumina ceramic according to claim 1, characterized in that, In step 1), the purity of the alumina is 96.5-99%.

3. The process for producing a high hardness alumina ceramic according to claim 1, wherein In step 4), the specific conditions of the sintering process are as follows: the sintering pressure is 80-120 MPa; and the sintering temperature is 1350-1400℃.

4. The process for producing a high hardness alumina ceramic according to claim 1, wherein The degassing temperature rising rate is less than 5℃ / min, and the temperature is 350℃-600℃.

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