CN114368971A

CN114368971A - Sintering method of fully densified high-occupation-ratio covalent bond ceramic

Info

Publication number: CN114368971A
Application number: CN202210008341.8A
Authority: CN
Inventors: 赵科; 刘金铃; 刘佃光; 陈犁韬
Original assignee: Southwest Jiaotong University
Current assignee: Southwest Jiaotong University
Priority date: 2022-01-05
Filing date: 2022-01-05
Publication date: 2022-04-19

Abstract

The invention discloses a sintering method of fully densified high-occupation-ratio covalent bond ceramics, which comprises the following steps: the ceramic raw material powder is filled in a graphite mold for cold press molding, the graphite mold and the ceramic raw material powder are placed in a sintering furnace after the cold press molding, alternating pressure sintering is carried out by applying alternating pressure with variable amplitude and frequency at a certain sintering temperature, and the covalent bond ceramic with the covalent bond content of more than 85% is formed after cooling. The alternating pressure sintering of the invention can completely densify the high-occupation-ratio covalent bond ceramic at a temperature lower than 200-400 ℃ of the traditional hot-pressing sintering, and maintain smaller grain size and better tissue uniformity, so that the high-occupation-ratio covalent bond ceramic has high hardness and high fracture toughness.

Description

Sintering method of fully densified high-occupation-ratio covalent bond ceramic

Technical Field

The invention belongs to the field of superhard covalent bond ceramics and preparation thereof, and particularly relates to a sintering method of fully densified high-occupation-ratio covalent bond ceramics.

Background

The high-occupation-ratio covalent bond ceramics (such as boron carbide, silicon carbide, tungsten carbide and the like) have the advantages of high melting point, high hardness, wear resistance, high chemical stability and the like due to the fact that the high-occupation-ratio covalent bond ceramics contain a higher proportion (> 85%) of covalent bonds, and have extremely high practical value and good application prospect in the fields of aerospace, weaponry, machining and the like. However, such ceramics are difficult to fully densify due to poor sinterability, and generally contain a certain amount of pores, which causes the mechanical properties such as hardness and toughness to be drastically reduced, and the requirements of practical engineering application to be difficult to meet, thus severely limiting the application range. For example, the covalent bond occupation ratio of boron carbide is up to 94%, the common preparation methods include pressureless sintering, hot-pressing sintering, hot isostatic pressing, plasma sintering and the like, the pressureless sintering and the hot-pressing sintering are respectively possible to sinter the boron carbide to be compact at the temperature of more than 2250 ℃ and more than 2100 ℃, and crystal grains grow rapidly and even grow abnormally at the temperature of more than 2000 ℃, so that the mechanical property is reduced, and in addition, the equipment cost is increased due to the high temperature; hot isostatic pressing can be used for secondary sintering after pressureless sintering or for directly sintering powder contained in a sheath, but the process is complicated in procedure and lacks of proper sheath materials; although the plasma sintering can densify the boron carbide by firing at a relatively low temperature while maintaining a small grain size, it is difficult to prepare a large-sized sample, and the sample has structural unevenness, i.e., unevenness in density and grains on the surface and inside of the sample, and is difficult to industrially apply. Ultra-high temperature ultra-high pressure (GPa grade) sintering also faces the dilemma of difficulty in preparing large size samples. Therefore, the relative density of the boron carbide ceramic which is available in industry at present is usually below 98%, and a simple sintering process which can realize complete densification of the covalent bond ceramic with high percentage of occupation and is easy to realize industrial application is still unavailable. The existing sintering technology not only utilizes temperature to densify powder, but also applies pressure to promote sintering, but the applied pressure is static load pressure, and cannot provide enough atomic diffusion driving force to completely eliminate residual pores in the later stage of sintering.

Disclosure of Invention

In order to solve the problems of difficult complete densification, excessive sintering temperature, grain growth/abnormal growth, difficult preparation of large-size samples and the like of the high-occupation-ratio covalent bond ceramic in the prior sintering technology, thereby obtaining the covalent bond ceramic with high hardness and high toughness. The invention provides a sintering method of fully densified high-occupation-ratio covalent bond ceramic.

The invention relates to a sintering method of fully densified high-occupation-ratio covalent bond ceramic, which comprises the following steps:

step 1: and (3) filling the ceramic raw material powder into a graphite mold for cold press molding, wherein the applied pressure is 5-70 MPa, and the pressure maintaining time is 1-5 min.

Step 2: after cold press molding, placing the graphite mold and the graphite mold in a sintering furnace for alternating pressure sintering, wherein the sintering temperature is 1500-1900 ℃, and the vacuum degree is>1×10^-2Pa; the alternating pressure is applied as follows: the pressure median value is 30-100 MPa, the amplitude is 5-40 MPa, and the frequency is 2-100 Hz.

Wherein the temperature rise process is as follows: raising the temperature from the normal temperature to 900 ℃ at a speed of 10-15 ℃/min; and then raising the temperature from 900 ℃ to the sintering temperature at 1-5 ℃/min.

Wherein, the alternating pressure applying process is as follows: the prepressing pressure is kept at 5MPa between the normal temperature and the temperature 20 ℃ lower than the sintering temperature, the prepressing pressure is increased to a pressure median value at a constant speed through program setting when the temperature 20 ℃ lower than the sintering temperature is increased to the sintering temperature, and alternating pressure is applied.

And step 3: after the sintering temperature is kept for 5-300 min, reducing the sintering temperature to 1200 ℃ from 5-10 ℃/min; meanwhile, after the heat preservation is finished, the application of the alternating pressure is stopped, and the median pressure value is uniformly reduced to 0 in the temperature reduction process of the sintering temperature to 1200 ℃.

Subsequently, a covalently bonded ceramic with a covalent bond proportion of > 85% was formed with furnace cooling.

Further, before the graphite mold in the step 1 is filled with the ceramic raw material powder, a boron nitride release agent is coated, and graphite flakes with the thickness of 0.15-0.3 mm are filled between the upper and lower pressing heads and the ceramic raw material powder.

Further, the ceramic raw material powder comprises boron carbide, silicon carbide and tungsten carbide, and the sintering temperature is as follows: boron carbide: 1800-1900 ℃, silicon carbide: 1700-1800 ℃, tungsten carbide: 1700 to 1800 ℃.

Further, the sintering atmosphere was replaced with an inert gas (high purity argon).

The beneficial technical effects of the invention are as follows:

1) the invention provides a sintering method of fully densified high-occupation-ratio covalent bond ceramic, which has lower requirements on equipment than hot isostatic pressing, plasma sintering, high-temperature and high-pressure sintering and other processes, is suitable for preparing large-size (> phi 60mm) samples, and has the advantages of simple process, convenient operation, good repeatability and lower cost;

(2) compared with hot-pressing sintering with static load pressure, the method has the advantages that the sintering temperature can be reduced by 200-400 ℃ by applying alternating pressure in the sintering process, a completely compact (> 99.5%) sample is obtained, and meanwhile, the smaller grain size and the higher grain size distribution uniformity are kept;

(3) the boron carbide, silicon carbide and tungsten carbide ceramics prepared by the method have higher hardness and higher fracture toughness than other sintering methods. The prepared ceramic sample has high hardness and toughness, and the application range of the ceramic sample in the structural application field can be expanded.

Drawings

FIG. 1 shows the microstructure of the boron carbide ceramic of example 1 after sintering at different sintering temperatures and sintering methods: FIGS. a-c are the microstructures after sintering at 1800, 1850 and 1900 ℃ respectively under static load pressure; graphs d-f are microstructures after sintering at 1800, 1850 and 1900 c, respectively, under alternating pressure.

FIG. 2 is a graph showing the relative density of boron carbide ceramic at different temperatures (wherein the temperatures in the graphs a-c are 1800, 1850 and 1900 ℃ respectively) under static load pressure and alternating pressure as a function of holding time.

FIG. 3 is a graph of the rate of densification of boron carbide ceramic at different temperatures (where temperatures in graphs a-c are 1800, 1850 and 1900℃, respectively) under static load pressure and alternating pressure as a function of relative density.

FIG. 4 is a graph showing the grain size distribution of boron carbide ceramic under static load pressure and alternating pressure at various temperatures (where temperatures in graphs a-c are 1800, 1850 and 1900 ℃ respectively).

FIG. 5 is a comparison of average grain sizes of boron carbide ceramics at different temperatures.

FIG. 6 is a fracture toughness-hardness comparison of alternating pressure sintered boron carbide ceramics and conventional process sintered boron carbide ceramics.

Detailed Description

The invention is described in further detail below with reference to the figures and specific embodiments.

The sintering method of the fully densified high-occupation-ratio covalent bond ceramic applies alternating pressure with variable amplitude and frequency in the heat preservation stage in the process of sintering at the elevated temperature. Compared with static load pressure in the traditional sintering process, the alternating pressure can not only intensify the sliding and rotation of powder particles and the plastic deformation of the particle connection position in the early sintering stage so as to promote particle rearrangement, but also can circularly carry out loading-partial unloading-reloading on the powder particles in the middle sintering stage so as to stimulate the emission and movement of defects such as dislocation, stacking fault, twin crystal and the like at a sintering neck, namely promote the plastic deformation or grain boundary sliding, more importantly, in the later sintering stage, a larger stress gradient can be introduced at the sintering neck so as to provide higher diffusion driving force to promote the diffusion of atoms, so that residual nano pores are completely discharged, and complete densification is realized, and the sintering temperature is obviously reduced (200 ℃) under the action of the alternating pressure. Because the grain growth driving force is insufficient at low sintering temperature, in addition, the alternating pressure can increase the curvature radius of the grain boundary, the grain growth is slow, and finally, a sample with fine grains can be sintered.

Example 1

The particle size of the boron carbide raw material powder is 1-3 mu m, and the purity is more than 99.9%.

Sintering the boron carbide powder by adopting an alternating pressure sintering process.

Step 1: and (3) coating a boron nitride release agent on the inner wall of the graphite mold and the surface of the pressure head, and after drying, putting the boron carbide raw material powder into the graphite mold with the inner diameter of 30mm in an argon-protected glove box.

Step 2: the graphite mould filled with the powder raw material is placed in a sintering furnace to be sintered into a block sample, the sintering temperature is 1800 ℃, 1850 ℃ and 1900 ℃, the applied alternating pressure is 50 +/-10 MPa, and the frequency is 2 Hz. A comparative sample was prepared by a conventional hot press sintering method using similar sintering parameters (except that the applied pressure was 60MPa, the other process parameters were the same).

The sintered samples were subjected to density measurement using a porosity tester, and the relative densities of the alternating pressure sintered samples were 92.5, 98.6 and 99.8% at 1800 c, 1850 c and 1900 c, respectively, and further fracture observation was performed on the sintered samples, as shown in fig. 1. From the fracture morphology, it can be seen that boron carbide samples sintered under alternating pressure have fewer pores than samples sintered under static load pressure (conventional hot pressing) at the same temperature; at 1800 ℃, the static load pressure sintering sample contains a certain amount of open pores, which indicates that the sample is still in the initial sintering stage, but the pores in the alternating pressure sintering sample are completely converted into closed pores and are in the middle sintering stage; particularly, at 1900 ℃, holes are hardly observed in the alternating pressure sintered sample, which indicates that the sample is completely compact and is consistent with the relative density test result, and the static load pressure sintered sample can also observe residual nano holes, which is consistent with other literature report results (at 1900 ℃, the relative density of the traditional hot pressing sintered boron carbide sample is often lower than 98%, and the residual nano holes are difficult to discharge). Fig. 2 and 3 are graphs of densification and corresponding densification rate curves for alternating pressure and static load pressure (i.e., hot pressing) applied during sintering, respectively, and it can be seen that at any one time during the soak period, the boron carbide samples have higher relative densities and faster densification rates under the alternating pressure compared to the static load pressure.

Fig. 4 is a graph of grain sizes of alternating pressure sintered samples and static pressure sintered samples at different sintering temperatures, and fig. 5 is a comparison of average grain sizes. The grain sizes of the alternating pressure sintered samples were 2.9, 3.3 and 4 microns at 1800 c, 1850 c and 1900 c, respectively, and the grain sizes of the static load pressure sintered samples were 3.2, 3.7 and 4.5 microns, respectively. It can be seen that at any sintering temperature, the alternating pressure sintered specimens had a smaller grain size than the static load pressure sintered specimens.

The hardness of the test sample is tested by using a micro Vickers hardness tester, the load is 10N, the dwell time is 15s, the fracture toughness of the test sample can be calculated according to the crack propagation length of the indentation and the applied load, when the alternating pressure sintering test sample is sintered at 1800 ℃, 1850 ℃ and 1900 ℃, the hardness of the alternating pressure sintering test sample is respectively 21.8, 32.7 and 38.5MPa, and the fracture toughness is respectively 4.1, 4.3 and 4.7 MPa.m^1/2. When the sintering temperature is 1900 ℃, the boron carbide ceramic has high fracture toughness and high hardness, the fracture toughness-hardness is comprehensively optimal, and the boron carbide ceramic has better performance than other traditional sintering processes (pressureless sintering, hot-pressing sintering, plasma sintering and ultrahigh pressure sintering). The method provided by the invention can enable the boron carbide ceramic to obtain excellent comprehensive mechanical properties, and is expected to meet the application requirements of the boron carbide ceramic in the fields of armor, aerospace, nuclear energy and the like. The alternating pressure sintered sample has such excellent mechanical properties, and the alternating pressure can remarkably promote the progress of sintering processes such as particle rearrangement, grain boundary sliding, plastic deformation, grain boundary diffusion and the like in the sintering process, especially can provide enough diffusion driving force in the later sintering period, so that residual nano holes are completely discharged, the method is obviously superior to the traditional sintering method applying static load pressure or not loading the sintering method, the problem that the traditional sintering process is difficult to completely densify is solved, the growth of grains can be inhibited to obtain smaller grain size, and the boron carbide ceramic has high hardness and high fracture toughness due to high relative density and small grain size. In addition, the sintering method can prepare large-size samples at low cost, and is easy to realize industrial application.

As shown in fig. 6, the boron carbide ceramic prepared by the novel sintering method of the present invention has more excellent mechanical properties than the boron carbide ceramic prepared by the conventional sintering method. Wherein SPS is plasma sintering, HP is hot-pressing sintering, PS is pressureless sintering, UHPS is ultrahigh-pressure sintering, and HOP is the alternating pressure sintering method of the invention.

Example 2

The particle size of the tungsten carbide raw material powder is 100-200 nm, and the purity is more than 99.5%.

Sintering the tungsten carbide powder by adopting an alternating pressure sintering process.

Step 1: and (3) coating a boron nitride release agent on the inner wall of the graphite mold and the surface of the pressure head, and after drying, putting the tungsten carbide raw material powder into a graphite mold with the inner diameter of 30mm in an argon-protected glove box.

Step 2: and placing the graphite die filled with the powder raw material into a sintering furnace to sinter the graphite die into a block sample, wherein the sintering temperatures are 1700 ℃, 1750 and 1800 ℃, respectively, the applied alternating pressure is 70 +/-5 MPa, and the frequency is 2 Hz.

The sintered samples were subjected to density measurement using a porosity tester, and the relative densities of the alternating pressure sintered samples at 1700 ℃, 1750 ℃ and 1800 ℃ were 97.2, 98.8 and 99.5%, the Vickers hardnesses were 26.4, 27.8 and 28.5GPa, the fracture toughness was 6.3, 8.3 and 9.4 MPa.m^1/2. The general fracture toughness of the tungsten carbide prepared by the traditional sintering process is lower than 6 MPa.m^1/2The tungsten carbide prepared by the alternating pressure sintering of the invention not only has ultrahigh hardness, but also has remarkably improved fracture toughness; in addition, the traditional sintering process can sinter tungsten carbide to be compact at the temperature of more than 2000 ℃, and the alternating pressure sintering can reduce the sintering temperature and simultaneously keep smaller grain size. The benefit of the alternating pressure is that the alternating pressure can significantly promote the progress of sintering processes such as particle rearrangement, grain boundary sliding, plastic deformation and grain boundary diffusion in the sintering process, especially can provide enough diffusion driving force in the later sintering period, so that residual nano-pores are completely discharged, the grain growth can be inhibited, smaller grain size can be obtained, and the tungsten carbide ceramic has high hardness and high fracture toughness at the same time due to the high relative density and small grain size. In addition, the sintering method can prepare large-size samples at low cost, and is easy to realize industrial application.

Example 3

The particle size of the silicon carbide raw material powder is 1-3 mu m, and the purity is more than 99.5%.

Sintering the silicon carbide powder by adopting an alternating pressure sintering process.

Step 1: and (3) coating a boron nitride release agent on the inner wall of the graphite mold and the surface of the pressure head, and after drying, putting the silicon carbide raw material powder into a graphite mold with the inner diameter of 30mm in an argon-protected glove box.

Step 2: the graphite mould filled with the powder raw material is placed in a sintering furnace to be sintered into a block sample, the sintering temperature is 1700 ℃, 1750 ℃ and 1800 ℃, the applied alternating pressure is 20 +/-5 MPa, and the frequency is 2 Hz.

The sintered samples were subjected to density measurement using a porosity tester, and the relative densities of the samples sintered at 1700 ℃, 1750 ℃ and 1800 ℃ under alternating pressure were 97.2, 98.8 and 99.5%, respectively, Vickers hardnesses were 24.4, 24.8 and 26.5GPa, and fracture toughness was 3.1, 3.5 and 4.1MPa · m, respectively^1/2. The silicon carbide prepared by the alternating pressure sintering of the invention not only has ultrahigh hardness, but also has remarkably improved fracture toughness; in addition, the traditional sintering process can sinter the silicon carbide to be compact at the temperature of more than 2000 ℃, and the alternating pressure sintering can reduce the sintering temperature and simultaneously keep smaller grain size. The silicon carbide ceramic has the advantages that the alternating pressure can remarkably promote the progress of sintering processes such as particle rearrangement, grain boundary sliding, plastic deformation and grain boundary diffusion in the sintering process, especially enough diffusion driving force can be provided in the later sintering stage, so that residual nano holes can be completely discharged, the grain growth can be inhibited, smaller grain size can be obtained, and the high relative density and the small grain size can enable the silicon carbide ceramic to have high hardness and high fracture toughness at the same time. In addition, the sintering method can prepare large-size samples at low cost, and is easy to realize industrial application.

The high-occupation-ratio covalent bond ceramic sintered by the alternating pressure has high hardness and keeps good fracture toughness, and is obviously superior to the high-occupation-ratio covalent bond ceramic prepared by the traditional sintering method. The key point is that the alternating pressure not only enables the ceramic to have a faster densification rate in the early and middle stages of sintering, but also obviously improves the diffusion driving force to completely remove the residual nano holes in the later stage of sintering, so that the ceramic is completely densified at a lower temperature, in addition, the alternating pressure enables the grain boundary to be straightened, and in addition, the sintering temperature is low, the grains are difficult to grow rapidly, so that the ceramic has a smaller grain size, which is the reason for the alternating pressure sintering ceramic to obtain excellent mechanical properties. The sintering method of the invention is not dependent on the addition of sintering aids, can save natural resources and reduce cost, has low cost of required equipment, can prepare samples with large size, uniform tissue and good repeatability, and is easy to realize industrial application.

Claims

1. A sintering method of fully densified high-occupation-ratio covalent bond ceramics is characterized by comprising the following steps:

step 1: putting the ceramic raw material powder into a graphite mold for cold press molding, wherein the applied pressure is 5-70 MPa, and the pressure maintaining time is 1-5 min;

step 2: after cold press molding, placing the graphite mold and the graphite mold in a sintering furnace for alternating pressure sintering, wherein the sintering temperature is 1500-1900 ℃, and the vacuum degree is>1×10^-2Pa; the alternating pressure is applied as follows: the median pressure is 30-100 MPa, the amplitude is 5-40 MPa, and the frequency is 2-100 Hz;

wherein the temperature rise process is as follows: raising the temperature from the normal temperature to 900 ℃ at a speed of 10-15 ℃/min; raising the temperature from 900 ℃ to the sintering temperature at 1-5 ℃/min;

wherein, the alternating pressure applying process is as follows: keeping the pre-pressurizing force at 5MPa between the normal temperature and the temperature 20 ℃ lower than the sintering temperature, increasing the pre-pressurizing force to a pressure median value at a constant speed by program setting when the temperature 20 ℃ lower than the sintering temperature is increased to the sintering temperature, and applying alternating pressure;

and step 3: after the sintering temperature is kept for 5-300 min, reducing the sintering temperature to 1200 ℃ from 5-10 ℃/min; meanwhile, stopping applying the alternating pressure after the heat preservation is finished, and reducing the pressure median value to 0 at a constant speed in the temperature reduction process from the sintering temperature to 1200 ℃;

2. The method of claim 1, wherein the graphite mold in step 1 is coated with a boron nitride release agent before being filled with the ceramic raw material powder, and graphite flakes with a thickness of 0.15-0.3 mm are filled between the upper and lower indenters and the ceramic raw material powder.

3. The sintering method of claim 1, wherein the ceramic raw material powder comprises boron carbide, silicon carbide and tungsten carbide, and the sintering temperature is specifically as follows: boron carbide: 1800-1900 ℃, silicon carbide: 1700-1800 ℃, tungsten carbide: 1700 to 1800 ℃.

4. The sintering method of claim 1, wherein the sintering atmosphere is replaced by inert gas.