CN111393170A

CN111393170A - Method for preparing high-density silicon nitride ceramic through multi-factor optimization and prepared silicon nitride ceramic

Info

Publication number: CN111393170A
Application number: CN202010198103.9A
Authority: CN
Inventors: 管晶; 李世佳; 管甲锁; 宋索成
Original assignee: Xi'an Aoqin New Material Co ltd
Current assignee: Jiayuguan sailite New Material Co.,Ltd.; Jiayuguan Sanwei Iron Alloy Smelting Co.,Ltd.; XI'AN AOQIN NEW MATERIAL Co.,Ltd.
Priority date: 2020-03-19
Filing date: 2020-03-19
Publication date: 2020-07-10

Abstract

The invention provides a method for preparing high-density silicon nitride ceramic through multi-factor optimization and the prepared silicon nitride ceramic, wherein test parameters such as cold isostatic pressing, presintering, hot isostatic pressing sintering processes, components and total content of a binder and a sintering aid are optimized, multi-factor optimization is adopted, the density of a silicon nitride product is improved, the problem that the existing silicon nitride ceramic product is difficult to obtain a high-density (99.5%) part is solved, the produced silicon nitride product not only has high density (99.5%), but also has extremely high strength, and the excellent performances determine that the high-density silicon nitride ceramic can be more widely applied to the fields of aviation, aerospace, precise high-speed bearings, electronic packaging and the like.

Description

Method for preparing high-density silicon nitride ceramic through multi-factor optimization and prepared silicon nitride ceramic

Technical Field

The invention belongs to the technical field of silicon nitride ceramic products, and relates to a method for preparing high-density silicon nitride ceramic through multi-factor optimization and the prepared silicon nitride ceramic.

Background

Because of the characteristic of self-toughening of β rod-shaped crystal, the fracture toughness of the silicon nitride ceramic can reach 7 Mpa.m^1/2However, the staggered structure of the silicon nitride β rod-shaped crystals also makes the compactness difficult to improve and easily generates air-closed holes which are difficult to eliminate.

At present, the approaches for improving the compactness of silicon nitride ceramics mainly comprise improving the content of sintering aids, hot-pressing sintering, SPS sintering or increasing the nitrogen pressure during atmospheric pressure sintering, and the like. These methods have mainly the following disadvantages:

1) the mechanical property is integrally reduced due to excessive glass phases;

2) the anisotropic ceramic is subjected to unidirectional pressure in the sintering process to generate anisotropy;

3) only products with simple shapes can be produced under the influence of the die;

4) the sintering temperature is too high, so that the internal structure is not uniform, and internal defects are generated;

5) the grain growth is too coarse, which seriously affects the mechanical properties.

Disclosure of Invention

The invention aims to provide a method for preparing high-density silicon nitride ceramic through multi-factor optimization and the prepared silicon nitride ceramic, and solves the problem that the existing silicon nitride ceramic product is difficult to obtain a high-density (99.5%) product.

The invention is realized by the following technical scheme:

a method for preparing high-density silicon nitride ceramic through multi-factor optimization comprises the following steps:

step 1, preparation of mixture

(1) Weighing the composite powder: the composite powder comprises, by mass, 84-94% of silicon nitride powder and 6-16% of sintering aid; the sintering aid is a mixture of lutetium oxide, lanthanum oxide and aluminum oxide;

(2) wet grinding and mixing: putting the composite powder into a ball mill, adding a wet grinding medium, a dispersing agent and a binder, and carrying out wet grinding and mixing to obtain a mixture; the mass of the binder is 2-5% of that of the composite powder;

step 2, spray granulation and cold isostatic pressing

(1) Carrying out spray granulation on the mixture;

(2) putting the powder after spray granulation into a mould, and obtaining a biscuit through cold isostatic pressing; wherein the pressure adopted by the cold isostatic pressing is between 180 and 200 MPa;

step 3, degreasing-air pressure presintering and hot isostatic pressing densification

(4) Drying and degreasing the pressed biscuit;

(5) carrying out air pressure pre-sintering on the degreased biscuit, wherein the air pressure pre-sintering temperature is 1630-1680 ℃;

(6) and carrying out hot isostatic pressing sintering on the silicon nitride ceramic subjected to air pressure pre-sintering, wherein the hot isostatic pressing sintering temperature is 1730-1830 ℃.

Preferably, in step 1, the mass ratio of lutetium oxide, lanthanum oxide and aluminum oxide is 2:1: 1.

Preferably, in step 1, the dispersant is a sodium polyacrylate solution, and the organic binder is a 2130 type phenolic resin.

Preferably, in the step 3, in the air pressure pre-sintering process, the temperature rise rate is 5 ℃/min before the air pressure pre-sintering temperature reaches 1550 ℃, and the temperature rise rate is maintained between 3 and 5 ℃/min after the pre-sintering temperature reaches 1550 ℃.

Preferably, in the step 3, the highest pressure in the hot isostatic pressing sintering process is 150 MPa.

The high-density silicon nitride ceramic prepared by the preparation method.

Preferably, the compactness is more than 99.5 percent and the bending strength is more than 1000 MPa.

Compared with the prior art, the invention has the following beneficial technical effects:

the invention optimizes the cold isostatic pressing forming, presintering and hot isostatic pressing sintering processes and test parameters such as the components and the total content of the binder and the sintering aid, adopts multi-factor optimization, and improves the density of the silicon nitride product. According to the invention, through the adjustment of the types of the sintering aids of silicon nitride, the combination of common aluminum oxide and yttrium oxide is changed into the ternary aid combination of lutetium oxide, lanthanum oxide and aluminum oxide, the viscosity of a liquid phase during liquid phase sintering is regulated and controlled, the mass transfer process is favorably carried out at a proper speed, and the presintered sample can further realize a densification process in the hot isostatic pressing sintering process; the formability of the biscuit and the density of the pressed green body are improved by optimizing the content of the binder 2130 type phenolic resin; the preparation of a part of complex silicon nitride ceramic parts can be realized by cold isostatic pressing, the pressure of the cold isostatic pressing is optimized, and a high-density silicon nitride green body is obtained (the improvement is 8.7%); after the surface layer densification is realized through pre-sintering, the mass transfer process is accelerated and the defects of internal pores and the like are reduced by utilizing the isotropic pressure sintering of hot isostatic pressing; by optimizing the total content of the sintering aid, the liquid phase content during sintering is properly increased, the densification degree during hot isostatic pressing sintering is improved, and finally, the preparation of the silicon nitride ceramic product with uniform structure, excellent mechanical property and extremely high density (> 99.5%) is realized. The produced silicon nitride product not only has high density (> 99.5%), but also has extremely high strength, and the excellent performances determine that the silicon nitride ceramic with high density can be more widely applied in the fields of aviation, aerospace, precise high-speed bearings, electronic packaging and the like.

Furthermore, the temperature rising system before reaching the pre-sintering temperature can also influence the mass transfer and densification process of the sample, and the invention obtains the proper temperature rising system before reaching the pre-sintering temperature.

Drawings

FIG. 1 is a thermogravimetric plot of a biscuit;

FIG. 2 is a degreasing process of the present invention;

FIG. 3 is a gas pressure pre-sintering process of the present invention;

FIG. 4 is a hot isostatic pressing process according to the invention;

FIG. 5 is the flexural strength and density data for example 13 and examples 15-19;

fig. 6 is a gas pressure sintering process in comparative example 2.

Detailed Description

The present invention will now be described in further detail with reference to specific examples, which are intended to be illustrative, but not limiting, of the invention.

The invention provides a method for preparing high-density silicon nitride ceramic through multi-factor optimization, which mainly comprises the steps of pressing composite powder into a blank through a cold isostatic pressing process, and performing aftertreatment through degreasing, air pressure presintering and hot isostatic pressing sintering.

The preparation process comprises the following steps:

step 1, preparation of mixture

Composite powders in which nano-and submicron-sized powders do not agglomerate and the respective kinds of powders are uniformly distributed are the first to produce excellent ceramic products. The specific mixing process of the silicon nitride ceramic powder, the sintering aid, the dispersing agent and the organic binder comprises the following steps:

(1) weighing the composite powder. The composite powder comprises, by mass, 84-94% of silicon nitride powder and 6-16% of sintering aid. The sintering aid is a mixture of lutetium oxide, lanthanum oxide and aluminum oxide, wherein the mass ratio of lutetium oxide to lanthanum oxide to aluminum oxide is 2:1: 1.

(2) Wet grinding and mixing. Taking a silicon nitride grinding ball as a ball milling medium, mixing the weighed composite powder with alcohol, and putting the mixture into a planetary ball mill, wherein the ball-material ratio is 2:1, the rotating speed is 240r/min, and the ball milling time is 24 hours; before the ball milling starts, a dispersing agent sodium polyacrylate and a binder 2130 phenolic resin are added, and the addition amounts of the dispersing agent and the binder are respectively 2% and 2-5% of the total mass of the composite powder according to the mass percentage.

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

(2) Putting the granulated powder into a mould, vacuumizing, and pressing into a biscuit by a cold isostatic press; and (3) keeping the highest pressure of the pressing at 180-200 MPa for 2min, releasing the pressure to 140MPa, keeping the pressure for 1min, releasing the pressure to 70MPa, and keeping the pressure for 40s to obtain the biscuit.

1) And (6) degreasing. Determining the heat preservation temperature of degreasing according to a comprehensive thermogravimetric curve (TG-DSC), selecting a temperature area with rapid mass loss and violent heat release in the curve as the degreasing temperature, and setting the cracking temperature interval of the 2130 type phenolic resin to be 300-500 ℃. And (3) putting the pressed sample into a graphite crucible at room temperature, and putting the graphite crucible into a hearth, wherein the blank is kept at 110 ℃ for 10min because some crystal water can remain in the blank. In order to uniformly carry out the degreasing process, a gradient heating mode is adopted, the heating speed is controlled at 10 ℃/min before the degreasing temperature reaches 300 ℃, and the heating speed is controlled at 2 ℃/min when the degreasing temperature reaches 300 ℃; finally, after heat preservation is carried out for 1h at 500 ℃, the temperature is reduced to room temperature for sampling, and the specific degreasing process is shown in figure 1.

2) And (5) presintering under air pressure. In order to make the densification process of hot isostatic pressing sintering smoothly proceed, the air pressure pre-sintering should form a layer of compact structure on the surface of the blank, and the inside is in a pre-sintered state. Therefore, the pre-sintering temperature is not suitable to be too high, the heat preservation time is not too long, and the gas pressure is moderate. The air pressure sintering process is shown in figure 2, the room temperature is increased to 500 ℃ within 100min, the temperature is increased to 0.3Mpa at 500 ℃ and maintained for 30min, the temperature is increased to 1200 ℃ within 140min, the temperature is maintained at 1200 ℃ for 30min and increased to 1MPa, the temperature is maintained at 1200 ℃ for 30min, the temperature is increased to 1550 ℃ within 70min, and the temperature increase rate is maintained between 3 ℃/min and 5 ℃/min after the temperature reaches 1550 ℃. And (3) reaching the pre-sintering temperature of 1630-1680 ℃, pressurizing to 3Mpa, keeping the temperature and pressure for 1h, cooling to room temperature, deflating and sampling.

3) And hot isostatic pressing sintering densification. In order to ensure the sufficient mass transfer of the liquid phase and realize densification, the temperature should be slowly increased when the temperature is close to the sintering temperature, the heat preservation time should be properly prolonged when the sintering temperature is reached, and the densification degree is further improved by adding equivalent pressure, so that the defects of air holes and the like are eliminated. The hot isostatic pressing sintering process is shown in figure 3, the room temperature is heated to 500 ℃ after 100min, the temperature is kept at 500 ℃ for 30min and pressurized to 5MPa, the temperature is heated to 1000 ℃ after 100min, the temperature is kept at 1000 ℃ for 30min and pressurized to 100MPa, the temperature is heated to 1650 ℃ after 130min, the temperature is kept at 1650 ℃ for 30min and pressurized to 120MPa, the heating speed is kept at 2 ℃/min after the sintering temperature reaches 1650 ℃, the pressure is pressurized to 150MPa when the sintering temperature reaches 1730-1830 ℃, the temperature is kept and pressurized for 90min, the temperature is reduced to the room temperature, and unloading and sampling are carried out.

The invention adopts the composite effect of optimizing the type and the content of the sintering aid, optimizing the content of the binder, optimizing the cold isostatic pressing process and optimizing the two-step sintering process to improve the compactness of the silicon nitride ceramic to the maximum extent. The produced silicon nitride product not only has high density (> 99.5%) but also has extremely high strength (>950Mpa), and the excellent performances determine that the silicon nitride ceramic with high density can be more widely applied in the fields of aviation, aerospace, precise high-speed bearings, electronic packaging and the like.

The binder addition serves to give the green cold isostatic pressed product a higher density and thus to increase the density of the finished sintered product, and the invention has been carried out in three examples (examples 1-3) with respect to the determination of the optimum amount of binder addition.

Example 1

Step 1, mixing of the mixture

(1) Weighing the composite powder. The raw materials of the composite powder comprise 94% of silicon nitride powder and 6% of sintering aid by mass percentage. The sintering aid is a mixture of lutetium oxide, lanthanum oxide and aluminum oxide, wherein the mass ratio of lutetium oxide to lanthanum oxide to aluminum oxide is 2:1: 1.

(2) Wet grinding and mixing. Taking a silicon nitride grinding ball as a ball milling medium, mixing the weighed composite powder with alcohol, and putting the mixture into a planetary ball mill, wherein the ball-material ratio is 2:1, the rotating speed is 240r/min, and the ball milling time is 24 hours; before the ball milling starts, a dispersing agent sodium polyacrylate and a binder 2130 phenolic resin are added, and the addition amounts of the dispersing agent and the binder are respectively 2% and 2% of the total mass of the composite powder according to the mass percentage.

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

(2) Putting the granulated powder into a mould, vacuumizing, and pressing into a biscuit by a cold isostatic press; and (3) keeping the highest pressure of the pressing at 180MPa for 2min, releasing the pressure to 140MPa, keeping the pressure for 1min, releasing the pressure to 70MPa, and keeping the pressure for 40s to obtain the biscuit.

Step 3, calculating the density of the green compact

The pressed green body was weighed, measured for length, width and height, and its density was calculated to be 1.81g/cm³。

Example 2

Step 1, mixing of the mixture

(2) Wet grinding and mixing. Taking a silicon nitride grinding ball as a ball milling medium, mixing the weighed composite powder with alcohol, and putting the mixture into a planetary ball mill, wherein the ball-material ratio is 2:1, the rotating speed is 240r/min, and the ball milling time is 24 hours; before the ball milling starts, a dispersing agent sodium polyacrylate and a binder 2130 phenolic resin are added, and the addition amounts of the dispersing agent and the binder are respectively 2% and 3% of the total mass of the composite powder according to the mass percentage.

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

Step 3, calculating the density of the green compact

The pressed green body was weighed, measured for length, width and height, and its density was calculated to be 1.84g/cm³。

Example 3

Step 1, mixing of the mixture

(2) Wet grinding and mixing. Taking a silicon nitride grinding ball as a ball milling medium, mixing the weighed composite powder with alcohol, and putting the mixture into a planetary ball mill, wherein the ball-material ratio is 2:1, the rotating speed is 240r/min, and the ball milling time is 24 hours; before the ball milling starts, a dispersing agent sodium polyacrylate and a binder 2130 phenolic resin are added, and the addition amounts of the dispersing agent and the binder are respectively 2% and 5% of the total mass of the composite powder according to the mass percentage.

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

Step 3, calculating the density of the green compact

By 3 examples, the optimum amount of binder addition can be determined to be 3%.

The advantage of isostatic cool pressing is that the stress is uniform in all directions, so that the green body is molded uniformly, the molding pressure of isostatic cool pressing is a key parameter, the molding density of the green body is influenced, and the invention carries out two embodiments (embodiments 4-5) aiming at the optimal molding pressure of isostatic cool pressing.

Example 4

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

(2) Putting the granulated powder into a mould, vacuumizing, and pressing into a biscuit by a cold isostatic press; and (3) keeping the highest pressure of the pressing at 190MPa for 2min, releasing the pressure to 140MPa, keeping the pressure for 1min, releasing the pressure to 70MPa, and keeping the pressure for 40s to obtain the biscuit.

Step 3, calculating the density of the green compact

Weighing the pressed green body, measuring length, width and height, and calculating to obtain a density of 2.0g/cm³。

Example 5

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

(2) Putting the granulated powder into a mould, vacuumizing, and pressing into a biscuit by a cold isostatic press; and (3) keeping the highest pressure of the pressing at 200MPa for 2min, releasing the pressure to 140MPa, keeping the pressure for 1min, releasing the pressure to 70MPa, and keeping the pressure for 40s to obtain the biscuit.

Step 3, calculating the density of the green compact

The pressed green body was weighed, measured for length, width and height, and its density was calculated to be 1.96g/cm³。

By comparing the two examples with example 2, the optimum cold isostatic pressure of 190MPa can be determined.

The function of presintering is to make the green body preliminarily compact to form a compact layer on the surface, so that the pressure is fully applied to the green body during hot isostatic pressing sintering, the maximum sintering temperature influences the uniformity of mass transfer and the densification degree, and 3 examples (examples 6-8) are carried out according to the determination of the maximum presintering temperature.

Example 6

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

Step 3, degreasing-air pressure presintering

1) And (6) degreasing. Determining the heat preservation temperature of degreasing according to a comprehensive thermogravimetric curve (TG-DSC), selecting a temperature region with rapid mass loss and violent heat release in the curve as the degreasing temperature, and setting the degreasing temperature range of the 2130 type phenolic resin at 300-500 ℃. The degreasing process is shown in fig. 2.

2) And (5) presintering under air pressure. Opening the upper cover of the air pressure furnace, putting the degreased biscuit into a graphite crucible, and carrying out air pressure sintering process as shown in figure 3, wherein the room temperature is 100min to 500 ℃, the pressure is increased to 0.3Mpa at 500 ℃ and the temperature is kept for 30min, the temperature is increased to 1200 ℃ after 140min, the temperature is kept for 30min at 1200 ℃, the pressure is increased to 1MPa, the temperature is kept for 30min at 1200 ℃, the temperature is increased to 1550 ℃ after 70min, and the temperature increasing rate is kept at 3 ℃/min. Reaching the pre-sintering temperature of 1630 ℃, pressurizing to 3Mpa, keeping the temperature and pressure for 1h, cooling to room temperature, deflating and sampling.

And 4, calculating the density of the sample by an Archimedes drainage method to be 97.15%.

Example 7

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

Step 3, degreasing-air pressure presintering

2) And (5) presintering under air pressure. Opening the upper cover of the air pressure furnace, putting the degreased biscuit into a graphite crucible, and carrying out air pressure sintering process as shown in figure 3, wherein the room temperature is 100min to 500 ℃, the pressure is increased to 0.3Mpa at 500 ℃ and the temperature is kept for 30min, the temperature is increased to 1200 ℃ after 140min, the temperature is kept for 30min at 1200 ℃, the pressure is increased to 1MPa, the temperature is kept for 30min at 1200 ℃, the temperature is increased to 1550 ℃ after 70min, and the temperature increasing rate is kept at 3 ℃/min. And when the pre-sintering temperature is reached to 1650 ℃, pressurizing to 3Mpa, preserving heat and pressure for 1h, cooling to room temperature, deflating and sampling.

And 4, calculating the compactness of the sample to be 97.43% by an Archimedes drainage method.

Example 8

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

Step 3, degreasing-air pressure presintering

2) And (5) presintering under air pressure. Opening the upper cover of the air pressure furnace, putting the degreased biscuit into a graphite crucible, and carrying out air pressure sintering process as shown in figure 3, wherein the room temperature is 100min to 500 ℃, the pressure is increased to 0.3Mpa at 500 ℃ and the temperature is kept for 30min, the temperature is increased to 1200 ℃ after 140min, the temperature is kept for 30min at 1200 ℃, the pressure is increased to 1MPa, the temperature is kept for 30min at 1200 ℃, the temperature is increased to 1550 ℃ after 70min, and the temperature increasing rate is kept at 3 ℃/min. And (3) reaching the pre-sintering temperature of 1680 ℃, pressurizing to 3Mpa, preserving heat and pressure for 1h, cooling to room temperature, deflating and sampling.

And 4, calculating the density of the sample by an Archimedes drainage method to be 97.62%.

By comparison of 3 examples, the optimum pre-sintering temperature was determined to be 1680 ℃.

The temperature raising schedule before reaching the pre-sintering temperature also influences the mass transfer and densification process of the sample, and 2 examples (examples 9-10) are carried out by the invention aiming at the determination of the temperature raising schedule before reaching the pre-sintering temperature.

Example 9

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

Step 2, pressing of composite powder

And adding the composite powder into a special mould, adjusting the highest cold isostatic pressing pressure to be 190Mpa, and keeping the pressure for 120s to obtain a biscuit with high green density.

2) And (5) presintering under air pressure. Opening the upper cover of the air pressure furnace, putting the degreased biscuit into a graphite crucible, and carrying out air pressure sintering process as shown in figure 3, wherein the room temperature is 100min to 500 ℃, the pressure is increased to 0.3Mpa at 500 ℃ and the temperature is kept for 30min, the temperature is increased to 1200 ℃ after 140min, the temperature is kept for 30min at 1200 ℃, the pressure is increased to 1MPa, the temperature is kept for 30min at 1200 ℃, the temperature is increased to 1550 ℃ after 70min, and the temperature increasing rate is kept at 4 ℃/min. And (3) reaching the pre-sintering temperature of 1680 ℃, pressurizing to 3Mpa, preserving heat and pressure for 1h, cooling to room temperature, deflating and sampling.

Step 4, calculating the density of the sample by an Archimedes drainage method to be 97.53 percent

Example 10

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

Step 3, degreasing-air pressure presintering

2) And (5) presintering under air pressure. Opening the upper cover of the air pressure furnace, putting the degreased biscuit into a graphite crucible, and carrying out air pressure sintering process as shown in figure 3, wherein the room temperature is 100min to 500 ℃, the pressure is increased to 0.3Mpa at 500 ℃ and the temperature is kept for 30min, the temperature is increased to 1200 ℃ after 140min, the temperature is kept for 30min at 1200 ℃, the pressure is increased to 1MPa, the temperature is kept for 30min at 1200 ℃, the temperature is increased to 1550 ℃ after 70min, and the temperature increasing rate is kept at 5 ℃/min. And (3) reaching the pre-sintering temperature of 1680 ℃, pressurizing to 3Mpa, preserving heat and pressure for 1h, cooling to room temperature, deflating and sampling.

And 4, calculating the compactness of the sample to be 97.32% by an Archimedes drainage method.

By comparing the two examples with example 8, the temperature rise rate before the optimum pre-sintering junction temperature is reached is determined to be 3 ℃/min.

The purpose of hot isostatic pressing sintering is to accelerate the mass transfer process of a presintered product under the action of high pressure and high temperature, so as to realize high densification. The present invention has been carried out in 4 examples (examples 11-14) with respect to the determination of the optimum sintering temperature in hot isostatic pressing sintering.

Example 11

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

3) And hot isostatic pressing sintering densification. The silicon nitride ceramic burned by air pressure is put into a crucible, the crucible is put on a heating body, a heat insulation cover is sleeved on the crucible, and the crucible is hung into a furnace chamber by a crane. And turning on the power supply and cooling water to start heating. The sintering process is as shown in figure 4, room temperature is raised to 500 ℃ after 100min, heat preservation is carried out for 30min at 500 ℃ and pressurization is carried out to 5Mpa, temperature is raised to 1000 ℃ after 100min, heat preservation is carried out for 30min at 1000 ℃ and pressurization is carried out to 100Mpa, temperature is raised to 1650 ℃ after 130min, heat preservation is carried out for 30min at 1650 ℃ and pressurization is carried out to 120Mpa, after the sintering temperature reaches 1650 ℃, the temperature raising speed is kept at 2 ℃/min, the pressurization is carried out to 150Mpa when the sintering temperature reaches 1730 ℃, the temperature is kept and the pressure is kept for 90min, then the temperature is reduced to.

And 4, testing that the density of the sample is 99.41% and the bending strength is 951Mpa according to an Archimedes drainage method.

Example 12

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

3) And hot isostatic pressing sintering densification. The silicon nitride ceramic burned by air pressure is put into a crucible, the crucible is put on a heating body, a heat insulation cover is sleeved on the crucible, and the crucible is hung into a furnace chamber by a crane. And turning on the power supply and cooling water to start heating. The sintering process is as shown in figure 4, room temperature is raised to 500 ℃ after 100min, heat preservation is carried out for 30min at 500 ℃ and pressurization is carried out to 5Mpa, temperature is raised to 1000 ℃ after 100min, heat preservation is carried out for 30min at 1000 ℃ and pressurization is carried out to 100Mpa, temperature is raised to 1650 ℃ after 130min, heat preservation is carried out for 30min at 1650 ℃ and pressurization is carried out to 120Mpa, after the sintering temperature reaches 1650 ℃, the temperature raising speed is kept at 2 ℃/min, the pressurization is carried out to 150Mpa when the sintering temperature reaches 1750 ℃, the temperature is kept and the pressure is kept for 90min, then the temperature is reduced to.

And 4, testing that the density of the sample is 99.47% and the bending strength is 940MPa according to an Archimedes drainage method.

Example 13

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

3) And hot isostatic pressing sintering densification. The silicon nitride ceramic burned by air pressure is put into a crucible, the crucible is put on a heating body, a heat insulation cover is sleeved on the crucible, and the crucible is hung into a furnace chamber by a crane. And turning on the power supply and cooling water to start heating. The sintering process is as shown in figure 4, room temperature is raised to 500 ℃ after 100min, heat preservation is carried out for 30min at 500 ℃ and pressurization is carried out to 5Mpa, temperature is raised to 1000 ℃ after 100min, heat preservation is carried out for 30min at 1000 ℃ and pressurization is carried out to 100Mpa, temperature is raised to 1650 ℃ after 130min, heat preservation is carried out for 30min at 1650 ℃ and pressurization is carried out to 120Mpa, after the sintering temperature reaches 1650 ℃, the temperature raising speed is kept at 2 ℃/min, the pressurization is carried out to 150Mpa when the sintering temperature reaches 1780 ℃, the temperature is kept and the pressure is kept for 90min, then the temperature is reduced to.

And 4, testing that the density of the sample is 99.53% and the bending strength is 1013Mpa according to an Archimedes drainage method.

Example 14

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

3) And hot isostatic pressing sintering densification. The silicon nitride ceramic burned by air pressure is put into a crucible, the crucible is put on a heating body, a heat insulation cover is sleeved on the crucible, and the crucible is hung into a furnace chamber by a crane. And turning on the power supply and cooling water to start heating. The sintering process is shown in figure 4, room temperature is raised to 500 ℃ after 100min, heat preservation is carried out for 30min at 500 ℃ and pressurization is carried out to 5Mpa, temperature is raised to 1000 ℃ after 100min, heat preservation is carried out for 30min at 1000 ℃ and pressurization is carried out to 100Mpa, temperature is raised to 1650 ℃ after 130min, heat preservation is carried out for 30min at 1650 ℃ and pressurization is carried out to 120Mpa, after the sintering temperature reaches 1650 ℃, the temperature raising speed is kept at 2 ℃/min, the pressurization is carried out to 150Mpa when the sintering temperature reaches 1830 ℃, the temperature is kept and the pressure is maintained for 90min, then the temperature is reduced to room.

And 4, testing that the density of the sample is 99.50% and the bending strength is 1010Mpa according to an Archimedes drainage method.

By comparison of the four examples, the optimal hot isostatic pressing sintering temperature was determined to be 1780 ℃.

On the basis of determining the optimal binder content, the optimal cold isostatic pressure, the optimal pre-sintering system and the optimal hot isostatic pressure sintering temperature, the total content of the sintering aid is a key factor influencing the compactness of a finished product. The invention proceeds through 5 examples (15-19) to determine the optimum amount of sintering aid.

Example 15

Method for preparing high-density silicon nitride ceramic through multi-factor optimization

Step 1, mixing of the mixture

(1) Weighing the composite powder. The raw materials of the composite powder comprise, by mass, 92% of silicon nitride powder and 8% of sintering aid. The sintering aid is a mixture of lutetium oxide, lanthanum oxide and aluminum oxide, wherein the mass ratio of lutetium oxide to lanthanum oxide to aluminum oxide is 2:1: 1.

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

And 4, testing the density of the pre-sintered sample to be 97.80 percent according to an Archimedes drainage method, the density of the sample after hot isostatic pressing sintering to be 99.60 percent and the bending strength to be 1020 Mpa.

Example 16

Step 1, mixing of the mixture

(1) Weighing the composite powder. The raw materials of the composite powder comprise 90% of silicon nitride powder and 10% of sintering aid by mass percentage. The sintering aid is a mixture of lutetium oxide, lanthanum oxide and aluminum oxide, wherein the mass ratio of lutetium oxide to lanthanum oxide to aluminum oxide is 2:1: 1.

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

And 4, testing the density of the pre-sintered sample to be 98.10% according to an Archimedes drainage method, the density of the sample after hot isostatic pressing sintering to be 99.51% and the bending strength to be 1001 Mpa.

Example 17

Step 1, mixing of the mixture

(1) Weighing the composite powder. The raw materials of the composite powder comprise 88 mass percent of silicon nitride powder and 12 mass percent of sintering aid. The sintering aid is a mixture of lutetium oxide, lanthanum oxide and aluminum oxide, wherein the mass ratio of lutetium oxide to lanthanum oxide to aluminum oxide is 2:1: 1.

(2) Wet grinding and mixing. Taking a silicon nitride grinding ball as a ball milling medium, mixing the weighed composite powder with alcohol, and putting the mixture into a planetary ball mill, wherein the ball-material ratio is 2:1, the rotating speed is 240r/min, and the ball milling time is 24 hours; before the ball milling starts, a dispersing agent sodium polyacrylate and a binder 2130 phenolic resin are added, and the addition amounts of the dispersing agent and the binder are respectively 2% and 3% of the composite powder by mass percent.

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

And 4, testing the density of the sample after pre-sintering to be 98.32% according to an Archimedes drainage method, and testing the density of the sample after hot isostatic pressing sintering to be 99.40% and 970 Mpa.

Example 18

Step 1, mixing of the mixture

(1) Weighing the composite powder. The raw materials of the composite powder comprise 86% of silicon nitride powder and 14% of sintering aid by mass percentage. The sintering aid is a mixture of lutetium oxide, lanthanum oxide and aluminum oxide, wherein the mass ratio of lutetium oxide to lanthanum oxide to aluminum oxide is 2:1: 1.

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

And 4, testing the density of the sample after pre-sintering to be 98.46 percent according to an Archimedes drainage method, and testing the density of the sample after hot isostatic pressing sintering to be 99.10 percent and 971 Mpa.

Example 19

Step 1, mixing of the mixture

(1) Weighing the composite powder. The raw materials of the composite powder comprise 84 percent of silicon nitride powder and 16 percent of sintering aid by mass percentage. The sintering aid is a mixture of lutetium oxide, lanthanum oxide and aluminum oxide, wherein the mass ratio of lutetium oxide to lanthanum oxide to aluminum oxide is 2:1: 1.

Step 2, spray granulation and pressing of composite powder

(1) Spray granulating the composite powder added with dispersant and binder

And 4, testing the density of the sample after pre-sintering to be 98.50% according to an Archimedes drainage method, and testing the density of the sample after hot isostatic pressing sintering to be 99.0% and 939 Mpa.

As shown in fig. 5, the optimum sintering aid addition amount was determined to be 8% by comparing five examples with example 13.

Comparative example 1

Step 1, mixing of the mixture

(1) Weighing the composite powder. The raw materials of the composite powder comprise, by mass, 92% of silicon nitride powder and 8% of sintering aid. The sintering aid is a mixture of yttrium oxide and aluminum oxide, wherein the mass ratio of yttrium oxide to aluminum oxide is 3:1 (example 15 is compared with a comparative example, the rare earth oxide is replaced by lutetium oxide and lanthanum oxide in a ratio of 2: 1).

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

And 4, testing the density of the pre-sintered sample to be 98.4% according to an Archimedes drainage method, the density of the sample after hot isostatic pressing sintering to be 99.03%, and the bending strength to be 796 Mpa.

The comparative example 1 adopts the same preparation process as the invention, under the condition that the total content of the sintering aids is 8 percent, the conventional sintering aids combined with yttrium oxide and aluminum oxide (3:1) are adopted in the comparative example 1, the novel sintering aids combined with lanthanum oxide, lutetium oxide and aluminum oxide (2:1:1) are adopted in the example 15, and the total percentage of the rare earth sintering aids is also the same, so that the preparation process ensures that the sintering aids are prepared by the method

Comparative example 1 is comparable to example 15. Example 15 the final density and flexural strength were improved by 0.58%, 28.1% respectively compared to comparative example 1.

Comparative example 2

Step 1, mixing of the mixture

Step 2, spray granulation and pressing of composite powder

(1) Spraying and granulating the mixture

Step 3, degreasing-air pressure sintering

2) And (5) sintering under air pressure. Opening the upper cover of the air pressure furnace, putting the degreased biscuit into a graphite crucible, wherein the air pressure sintering process is shown in figure 2, the room temperature is 100min to 500 ℃, the temperature is increased to 0.3Mpa at 500 ℃ for 30min, the temperature is increased to 1200 ℃ after 140min, the temperature is increased to 1200 ℃, the temperature is increased to 30min at 1200 ℃, the pressure is increased to 1Mpa, the temperature is maintained at 1200 ℃ for 30min, the temperature is increased to 1550 ℃ after 70min, the temperature rate is maintained at 3 ℃/min, the pressure is increased to 3Mpa when the temperature reaches 1680 ℃, after the pressure is stable, the biscuit is heated to the sintering temperature 1780 ℃ at the speed of 3 ℃/min, the temperature is maintained for 2.5h, the biscuit is cooled to the room temperature.

And 4, testing the density of the sample after air pressure sintering to be 98.1% according to an Archimedes drainage method, wherein the bending strength is 836 Mpa.

The comparison example 2 adopts the same optimal proportion and total amount of the sintering aids as the invention, and does not adopt the hot isostatic pressing method which is a method for improving the densification degree, so that the air pressure sintering time is the same as the total time of the air pressure presintering sintering of the example 15, thereby leading the comparison example 2 to be comparable with the example 15. Example 15 has a 1.52% and 22.0% increase in final density and flexural strength, respectively, over comparative example 2.

Claims

1. A method for preparing high-density silicon nitride ceramic through multi-factor optimization is characterized by comprising the following steps:

step 1, preparation of mixture

step 2, spray granulation and cold isostatic pressing

(1) Carrying out spray granulation on the mixture;

(1) Drying and degreasing the pressed biscuit;

(2) carrying out air pressure pre-sintering on the degreased biscuit, wherein the air pressure pre-sintering temperature is 1630-1680 ℃;

(3) and carrying out hot isostatic pressing sintering on the silicon nitride ceramic subjected to air pressure pre-sintering, wherein the hot isostatic pressing sintering temperature is 1730-1830 ℃.

2. The method for preparing high-density silicon nitride ceramic through multifactor optimization according to claim 1, wherein in the step 1, the mass ratio of lutetium oxide, lanthanum oxide and aluminum oxide is 2:1: 1.

3. The method for preparing high-density silicon nitride ceramic through multifactor optimization according to claim 1, wherein in the step 1, the dispersant is sodium polyacrylate solution, and the organic binder is phenolic resin 2130.

4. The method for preparing high-density silicon nitride ceramic through multifactor optimization according to claim 1, wherein in the step 3, in the air pressure pre-sintering process, the temperature rise rate is 5 ℃/min before the air pressure pre-sintering temperature reaches 1550 ℃, and the temperature rise rate is maintained between 3 ℃/min and 5 ℃/min after the pre-sintering temperature reaches 1550 ℃.

5. The method for preparing high-density silicon nitride ceramic through multifactor optimization according to claim 1, wherein in the step 3, the highest pressure is 150MPa in the hot isostatic pressing sintering process.

6. The high-density silicon nitride ceramic obtained by the preparation method of any one of claims 1 to 5.

7. The method for preparing high-density silicon nitride ceramic through multifactor optimization according to claim 6, wherein the density is more than 99.5%, and the bending strength is more than 1000 MPa.