Grayscale-adjustable silicon nitride ceramic material and preparation method thereof

Abstract

The invention discloses a gray-scale-adjustable silicon nitride ceramic material and a preparation method thereof, the apparent density of the ceramic material is 3.0-3.6 g / cm < 3 >, the Vickers hardness is 1700-2000 HV, the bending strength is 700-1000 MPa, the fracture toughness is 6.5-10 MPa.m < 1 / 2 >, the crystal form of the ceramic material is a long columnar beta-Si3N4 crystal form, and the gray-scale-adjustable silicon nitride ceramic material comprises the following raw materials by weight percent: 85-98% of alpha-Si3N4, 5-10% of SiO2, 5-10% of SiO2, 5-10% of SiO2, 5-10% of SiO2, 5-10% of SiO2 and 5-10% of SiO2. 1-8 wt% of a sintering aid and 0-10 wt% of a coloring agent

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of silicon nitride ceramic materials, and particularly relates to a gray-scale adjustable silicon nitride ceramic material and a preparation method thereof. BACKGROUND

[0002] Silicon nitride ceramic has been widely used in metallurgy, machinery, aerospace, chemical industry, electronics and biomedical fields due to its high strength, low density, good toughness, high thermal conductivity, excellent thermal shock resistance and excellent corrosion resistance and high temperature resistance. With the progress of science and technology and the development of society, the importance of terminal communication equipment (such as smart phones and notebook computers) in daily life is increasingly prominent, and ceramic materials are gradually replacing metal parts and becoming an important choice for electronic device structural parts. Although silicon nitride ceramic has been significantly optimized in terms of mechanical properties such as hardness and fracture toughness, its color is still relatively single, mainly gray, which to some extent limits its application in appearance design sensitive fields (such as consumer electronics). Therefore, developing multi-color silicon nitride ceramic has become an important direction of current material research.

[0003] At present, zirconia ceramic is mainly used as the backboard material of smart phones, but this material has limitations such as high density, poor impact resistance and high industrial production cost. In contrast, silicon nitride ceramic not only has lower density and lighter texture, which is beneficial to the lightness and portability of equipment, but also has higher hardness and more excellent impact resistance. Its comprehensive mechanical properties are more in line with the requirements of modern electronic devices for materials and are more in line with the development trend of future communication technology. Under this background, developing a silicon nitride ceramic material with low preparation cost, excellent mechanical properties, high hardness, low density and adjustable gray scale not only has important scientific research value, but also has broad market application prospects, especially for the next generation of high-end communication equipment. SUMMARY

[0004] The present application provides a preparation method of a gray-scale adjustable silicon nitride ceramic material, comprising the following steps: wet grinding silicon nitride, sintering aids and colorants to obtain a mixed slurry; preparing a silicon nitride green body by vacuum debubbling and tape casting process of the slurry; and sequentially performing glue removal pre-sintering and vacuum constant pressure sintering on the obtained green body to finally obtain a silicon nitride ceramic material with adjustable gray scale range.

[0005] The specific technical scheme of the present application is as follows: a gray-scale adjustable silicon nitride ceramic material, the apparent density of which is 3.0-3.6 g / cm³, the Vickers hardness is 1700-2000 HV, the bending strength is 700-1000 MPa, and the fracture toughness is 6.5-10 MPa·m 1 / 2 , and the crystal form is long columnar β-Si3N4 crystal form.

[0006] The raw material composition of the gray-scale adjustable silicon nitride ceramic material is: 85-98wt% α-Si3N4, 1-8wt% sintering aid and 0-10wt% colorant.

[0007] The sintering aid is one or more of MgO, Al2O3 and Y2O3, La2O3 and Ce2O3.

[0008] Preferably, the sintering aid is Y2O3, Al2O3, MgO and La2O3, and the mass ratio is 1:1:1:1.

[0009] The colorant is one or more of Ni2O5, SiC, TiN and carbon black.

[0010] Preferably, the colorant is Ni2O5.

[0011] A method for preparing the aforementioned gray-scale adjustable silicon nitride ceramic material, comprising the following steps:

[0012] Step 1: Put the raw materials together with silicon nitride beads into a ball mill tank, add anhydrous ethanol and butanone as solvents, and then perform wet ball milling after preliminary stirring and mixing. The ball milling process parameters are set as follows: solid-liquid ratio 2:1, ball-to-material ratio 1.5:1, rotation speed 200 r / min, and ball milling time 3h.

[0013] Step 2: Casting forming, the slurry obtained in step 1 is subjected to vacuum debubbling treatment, and then the slurry is injected into a casting machine tank to form a sheet-shaped silicon nitride green body with a certain thickness.

[0014] Step 3: Put the silicon nitride green body obtained in step 2 on a boron nitride substrate and put it into a degassing furnace for pre-sintering.

[0015] Step 4: Transfer the body obtained in step 3 to a high-temperature sintering furnace and perform gas pressure sintering under N2 atmosphere. The sintering pressure is 1-6 MPa, and the temperature rising program is as follows: from room temperature to 500-1000 ℃ at a rate of 3-10 ℃ / min (low temperature section); then to 1000-1600 ℃ at a rate of 3-8 ℃ / min (medium temperature section); finally to 1600-2000 ℃ at a rate of 3-5 ℃ / min (high temperature section), and keep for 1-6 h. Then cool to room temperature with the furnace, and obtain the final product.

[0016] The raw material in step 1 is 85-98wt% α-Si3N4, and the total amount of Y2O3+Al2O3+MgO+La2O3 is 1-8wt%, and the amount of Nb2O5 is 0-10wt%.

[0017] The thickness of the sheet-shaped silicon nitride green body in step 2 is 0.3-0.9mm.

[0018] The pre-sintering process in step 3 is heating at a rate of 5 ℃ / min to 500 ℃, and holding for 2 h. Preferably, the degassing pre-sintering can be divided into 5-8 stage heating procedures, the holding temperature is 400-700 ℃, and the holding time is 0-2 h.

[0019] Preferably, the solvent used in the wet grinding process is a mixture of anhydrous ethanol and butanone, the mass ratio of which is (4-6):6, the solid-liquid ratio is (2-4):1, and the ball-material ratio is (1-3):1. The grinding rotation speed is controlled at 100-300 r / min, and the time is 2-24 h.

[0020] The application also provides a gray-tunable silicon nitride ceramic material prepared by the above method, the gray level of which can be continuously adjusted within a certain range.

[0021] The application realizes color adjustment on the basis of the original gray of the silicon nitride ceramic by introducing Ni2O5, significantly enriching the appearance color of the silicon nitride ceramic. Meanwhile, the selected sintering aids and colorants synergistically promote the densification process of the material, improve the mechanical properties of the ceramic, and make it better meet the application requirements of the terminal communication equipment backboard material. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 Scanning electron microscope images of the fracture surfaces of the silicon nitride ceramic materials prepared in Comparative Example 1 and Example 3;

[0023] Figure 2 Scanning electron microscope images of the fracture surfaces of the silicon nitride ceramic materials prepared in Comparative Example 2 and Example 3;

[0024] Figure 3 Optical photographs of the silicon nitride ceramic materials prepared in Examples 1-5; Figure 4 X-ray diffraction pattern of the silicon nitride ceramic material prepared in Example 3. DETAILED DESCRIPTION

[0025] The application provides a preparation method of a gray-tunable silicon nitride ceramic material, which comprises the following steps: wet grinding preparation raw materials including silicon nitride, sintering aids, plasticizers, binders and colorants to obtain mixed slurry. The sintering aids include main group metal oxides and rare earth metal oxides; the obtained wet mixture is further subjected to ball milling treatment to obtain uniform mixed slurry; then a green body is prepared through a flow casting process, and the green body is subjected to degassing pre-sintering and final sintering treatment to obtain the gray silicon nitride ceramic material.

[0026] Unless otherwise specified, all raw materials and components used in the application are conventional commercially available products in the art.

[0027] Preferably, the preparation raw material comprises, by mass percentage: 85-98% of silicon nitride, 1-8% of sintering aid, and 0-10% of colorant, the silicon nitride being α-Si3N4, and the particle size being preferably 0.5-1.5 μm.

[0028] The sintering aid preferably comprises one or more of Y2O3, Al2O3, MgO, and La2O3, and more preferably is a combination of Y2O3, Al2O3, MgO, and La2O3, preferably in a mass ratio of 1:1:1:1. The combined use of multiple sintering aids helps to reduce the sintering temperature and improve the material density.

[0029] The colorant is Ni2O5, and the addition of the colorant can achieve color adjustment on the basis of the original gray of the silicon nitride ceramic, and the synergistic effect of the sintering aid and the colorant helps to promote the phase transition from α to β, thereby improving the fracture toughness of the material, and has no significant adverse effect on the overall mechanical properties, meeting the application requirements of the terminal communication equipment backboard material.

[0030] In the present application, the specific implementation of the wet grinding is preferably wet ball milling. The wet grinding preferably comprises the following steps: first dissolving and mixing the binder and the plasticizer; then ball-mixing the dispersant, the ceramic powder, and the solvent; and finally secondarily ball-mixing the mixture of the above two. The ball-milling medium is a silicon nitride ball of different sizes. The solvent is preferably a mixture of anhydrous ethanol and butanone, the solid-liquid ratio is (2-4):1, the ball-milling speed is 100-300 r / min, and the ball-milling time is 2-24 h.

[0031] The forming adopts a flow casting process. The mixed slurry needs to be subjected to vacuum debubbling treatment before flow casting, and then a silicon nitride green body with a thickness of 0.3-0.8 mm is obtained through flow casting and drying. The green body thickness can be controlled by factors such as the doctor blade height, the base speed, the slurry viscosity, and the slurry tank liquid level.

[0032] The green body needs to be subjected to degassing pre-sintering before sintering, so as to remove organic matters such as the binder and the plasticizer. The degassing pre-sintering adopts a 5-8-stage temperature rising program, and the temperature is raised to 400-700 ℃ and kept for 0-2 h. Factors such as the organic matter composition, the green body morphology, the temperature rising rate, the holding time, and the atmosphere uniformity all affect the degassing effect.

[0033] The sintering process adopts vacuum sintering and gas pressure sintering. The application of 1-6 MPa gas pressure in a N2 atmosphere can inhibit material decomposition at high temperature, promote densification, and obtain high-density silicon nitride products. The entire sintering process is carried out in a graphite furnace vacuum environment to prevent oxidation and ensure uniform heating. The sintering temperature rising program is divided into three stages: from room temperature to 500-1000℃ at a rate of 3-10 ℃ / min (low temperature section) to completely remove residual organic matter; then to 1000-1600℃ at a rate of 3-8 ℃ / min (medium temperature section) to avoid defects in the green body caused by thermal stress; finally to 1600-2000℃ at a rate of 3-5 ℃ / min (high temperature section) and holding for 1-6 h to promote liquid phase formation and penetration and achieve complete densification. This staged temperature rising system helps the powder to be heated uniformly and promotes the phase transition from α to β.

[0034] After sintering, the obtained gray silicon nitride ceramic material is subjected to mechanical property testing, including Vickers hardness, bending strength and fracture toughness.

[0035] The present application successfully prepares a gray silicon nitride ceramic material with adjustable color, high density and excellent mechanical properties by optimizing the ratio of sintering aids and colorants, combining tape casting and staged sintering process. Its performance density is 3.0-3.6 g / cm³, Vickers hardness is 1700-2000 HV, bending strength is 700-1000 MPa, and fracture toughness is 6.5-10 MPa·m¹ / ², which fully meets the application requirements of high-end communication equipment backplane.

[0036] The technical solutions of the present application are further described in the following examples, but it should be understood that the examples are only exemplary and do not limit the scope of protection of the present application.

[0037] The technical solutions provided by the present application are described in detail below in conjunction with examples, but they should not be understood as limiting the scope of protection of the present application.

[0038] Comparative Example 1

[0039] (1) Raw material ratio: α-Si3N4 is used as the base material, Y2O3, Al2O3 and MgO are selected as the composite sintering aid, and Nb2O5 is used as the colorant. The mass ratio of each component is: α-Si3N4 : Y2O3 : Al2O3 : MgO : Nb2O5 = 92 : 2 : 2 : 2 : 0.3, and each raw material is accurately weighed according to the above ratio.

[0040] (2) Ball milling mixing: The weighed powder and silicon nitride milling beads were placed in a polytetrafluoroethylene ball mill tank, 20 mL of anhydrous ethanol and 30 mL of butanone were added as solvents, and after preliminary stirring and mixing, the ball mill tank was fixed on the planetary ball mill for wet ball milling. The ball milling process parameters were set as follows: solid-liquid ratio 2:1, ball-material ratio 1.5:1, rotation speed 200 r / min, and ball milling time 3 h.

[0041] (3) Tape casting: The slurry after ball milling was treated by vacuum debubbling to remove the air bubbles entrained therein. Subsequently, the slurry was injected into a tape casting tank, and a 0.6 mm thick uniform sheet of silicon nitride green body was prepared by controlling the gap between the doctor blade and the base band speed.

[0042] (4) Glue removal treatment: The green body was placed on a boron nitride substrate and put into a glue removal furnace for pre-sintering. The temperature was raised to 500 °C at a rate of 5 °C / min and kept for 2 h to completely remove the organic binder and plasticizer.

[0043] (5) Sintering densification: The glue-removed body was transferred to a high-temperature sintering furnace and sintered under N2 atmosphere by gas pressure sintering. The sintering pressure was 2 MPa, and the temperature rising program was as follows: the temperature was raised to 1000 °C at a rate of 5 °C / min, kept for 5 min; then the temperature was continuously raised to 1600 °C at the same rate, kept for 5 min; finally, the temperature was raised to 1850 °C, kept for 2 h. Subsequently, the furnace was cooled to room temperature, and a dense gray silicon nitride ceramic material was obtained.

[0044] (6) Performance test: The properties of the sintered ceramic sample were characterized, including apparent density, Vickers hardness, three-point bending strength, and fracture toughness, and the results are shown in the subsequent tables.

[0045] Comparative Example 2

[0046] (1) Raw material ratio: α-Si3N4 was used as the base material, Y2O3, Al2O3, MgO and La2O3 were selected as the composite sintering aids, and Nb2O5 was used as the colorant. The mass ratio of each component was: α-Si3N4 : Y2O3 : Al2O3 : MgO : La2O3 : Nb2O5 = 92 : 2 : 2 : 2 : 2 : 0.3, and each raw material was accurately weighed according to the above ratio.

[0047] (2) Ball milling mixing: The weighed powder and silicon nitride milling beads were placed in a polytetrafluoroethylene ball mill tank, 20 mL of anhydrous ethanol and 30 mL of butanone were added as solvents, and after preliminary stirring and mixing, the ball mill tank was fixed on the planetary ball mill for wet ball milling. The ball milling process parameters were set as follows: solid-liquid ratio 2:1, ball-material ratio 1.5:1, rotation speed 200 r / min, and ball milling time 3 h.

[0048] (3) Tape casting: The milled slurry was treated by vacuum degassing to remove the air bubbles entrapped in the slurry. Subsequently, the slurry was injected into the casting slot of a tape casting machine, and a 0.6 mm-thick green sheet of silicon nitride was cast by controlling the gap of the doctor blade and the speed of the substrate.

[0049] (4) Debinding: The green sheet was placed on a boron nitride substrate and put into a debinding furnace for pre-sintering. The temperature was raised to 500 °C at a rate of 5 °C / min and kept for 2 h to completely remove the organic binder and plasticizer.

[0050] (5) Sintering densification: The debound green body was transferred to a high-temperature sintering furnace for pressure sintering in a N2 atmosphere. The sintering pressure was 2 MPa, and the temperature program was as follows: the temperature was raised to 1000 °C at a rate of 5 °C / min and kept for 5 min; then the temperature was raised to 1600 °C at the same rate and kept for 5 min; finally, the temperature was raised to 1800 °C and kept for 2.5 h. Subsequently, the furnace was cooled to room temperature, and a dense gray silicon nitride ceramic material was obtained.

[0051] (6) Performance testing: The sintered ceramic samples were characterized for their performance, including apparent density, Vickers hardness, three-point bending strength, and fracture toughness, and the results are shown in the subsequent tables.

[0052] Example 1

[0053] (1) Raw material ratio: α-Si3N4 was used as the base material, Y2O3, Al2O3, MgO, and La2O3 were selected as the composite sintering aids, and Nb2O5 was used as the colorant. The mass ratio of each component was: α-Si3N4: Y2O3: Al2O3: MgO: La2O3: Nb2O5 = 92: 2: 2: 2: 2: 0.1, and each raw material was accurately weighed according to the above ratio.

[0054] (2) Ball milling: The weighed powder was placed in a polytetrafluoroethylene ball mill tank together with silicon nitride milling beads, 20 mL of anhydrous ethanol and 30 mL of butanone were added as solvents, and after preliminary stirring and mixing, the ball mill tank was fixed on a planetary ball mill for wet ball milling. The ball milling process parameters were set as follows: solid-liquid ratio 2:1, ball-to-material ratio 1.5:1, rotation speed 200 r / min, and ball milling time 3 h.

[0055] (3) Tape casting: The milled slurry was treated by vacuum degassing to remove the air bubbles entrapped in the slurry. Subsequently, the slurry was injected into the casting slot of a tape casting machine, and a 0.6 mm-thick green sheet of silicon nitride was cast by controlling the gap of the doctor blade and the speed of the substrate.

[0056] (4) Debinding: The green body was placed on a boron nitride substrate and put into a debinding furnace for pre-sintering. The temperature was raised to 500 °C at a rate of 5 °C / min and held for 2 h to completely remove the organic binder and plasticizer.

[0057] (5) Sintering densification: The debound body was transferred to a high-temperature sintering furnace for pressure sintering in a N2 atmosphere. The sintering pressure was 2 MPa, and the temperature program was as follows: the temperature was raised to 1000 °C at a rate of 5 °C / min and held for 5 min; then the temperature was raised to 1600 °C at the same rate and held for 5 min; finally, the temperature was raised to 1850 °C and held for 2.5 h. Subsequently, the furnace was cooled to room temperature, and a dense gray silicon nitride ceramic material was obtained.

[0058] (6) Performance testing: The sintered ceramic samples were tested for performance, including apparent density, Vickers hardness, three-point bending strength, and fracture toughness, and the results are shown in the subsequent tables.

[0059] Example 2

[0060] (1) Raw material ratio: α-Si3N4 was used as the base material, Y2O3, Al2O3, MgO, and La2O3 were selected as the composite sintering aids, and Nb2O5 was used as the colorant. The mass ratio of each component was: α-Si3N4 : Y2O3 : Al2O3 : MgO : La2O3 : Nb2O5 = 92 : 2 : 2 : 2 : 2 : 0.2, and each raw material was accurately weighed according to the above ratio.

[0061] (2) Ball milling: The weighed powder and silicon nitride milling beads were placed in a polytetrafluoroethylene ball mill tank together with 20 mL of anhydrous ethanol and 30 mL of butanone as solvents. After initial stirring and mixing, the ball mill tank was fixed on a planetary ball mill for wet ball milling. The ball milling process parameters were set as follows: solid-liquid ratio 2:1, ball-to-material ratio 1.5:1, rotation speed 200 r / min, and ball milling time 3 h.

[0062] (3) Tape casting: The milled slurry was subjected to vacuum degassing treatment to remove the air bubbles entrained therein. Subsequently, the slurry was injected into the casting machine tank, and a uniform 0.6 mm thick sheet-shaped silicon nitride green body was cast by controlling the gap between the doctor blade and the substrate speed.

[0063] (4) Debinding: The green body was placed on a boron nitride substrate and put into a debinding furnace for pre-sintering. The temperature was raised to 500 °C at a rate of 5 °C / min and held for 2 h to completely remove the organic binder and plasticizer.

[0064] (5) Sintering densification: After degreasing, the green body is transferred to a high-temperature sintering furnace for pressure sintering in a N2 atmosphere. The sintering pressure is 2 MPa, and the temperature rising program is as follows: heating to 1000 °C at a rate of 5 °C / min, holding for 5 min; continue to heat to 1600 °C at the same rate, holding for 5 min; finally heat to 1850 °C, holding for 2.5 h. Then cool down to room temperature with the furnace, and obtain a dense gray silicon nitride ceramic material.

[0065] (6) Performance test: The sintered ceramic samples are characterized for performance, including apparent density, Vickers hardness, three-point bending strength and fracture toughness, and the results are shown in the subsequent tables.

[0066] Example 3

[0067] (1) Raw material ratio: α-Si3N4 is used as the base material, Y2O3, Al2O3, MgO and La2O3 are selected as composite sintering aids, and Nb2O5 is used as a colorant. The mass ratio of each component is: α-Si3N4 : Y2O3 : Al2O3 : MgO : La2O3 : Nb2O5 = 92 : 2 : 2 : 2 : 2 : 0.3, and each raw material is accurately weighed according to the above ratio.

[0068] (2) Ball milling: The weighed powder and silicon nitride milling beads are placed in a polytetrafluoroethylene ball mill tank together with 20 mL of anhydrous ethanol and 30 mL of butanone as solvents. After preliminary stirring and mixing, the ball mill tank is fixed on a planetary ball mill for wet ball milling. The ball milling process parameters are set as follows: solid-liquid ratio 2:1, ball-to-material ratio 1.5:1, rotation speed 200 r / min, and ball milling time 3 h.

[0069] (3) Tape casting: The slurry after ball milling is treated by vacuum debubbling to remove the air bubbles entrained therein. Then the slurry is injected into the casting machine tank, and a uniform 0.6 mm thick sheet-shaped silicon nitride green body is cast by controlling the gap between the doctor blade and the base band speed.

[0070] (4) Degreasing treatment: The green body is placed on a boron nitride substrate and put into a degreasing furnace for pre-sintering. The temperature is raised to 500 °C at a rate of 5 °C / min and held for 2 h to completely remove the organic binder and plasticizer.

[0071] (5) Sintering densification: After degreasing, the green body is transferred to a high-temperature sintering furnace for pressure sintering in a N2 atmosphere. The sintering pressure is 2 MPa, and the temperature rising program is as follows: heating to 1000 °C at a rate of 5 °C / min, holding for 5 min; continue to heat to 1600 °C at the same rate, holding for 5 min; finally heat to 1850 °C, holding for 2.5 h. Then cool down to room temperature with the furnace, and obtain a dense gray silicon nitride ceramic material.

[0072] (6) Performance test: The sintered ceramic samples are characterized for performance, including apparent density, Vickers hardness, three-point bending strength and fracture toughness, and the results are shown in the subsequent tables.

[0073] Example 4

[0074] (1) Raw material ratio: α-Si3N4 is used as the base material, Y2O3, Al2O3, MgO and La2O3 are selected as composite sintering aids, and Nb2O5 is used as a colorant. The mass ratio of each component is: α-Si3N4 : Y2O3 : Al2O3 : MgO : La2O3 : Nb2O5 = 92 : 2 : 2 : 2 : 2 : 0.4, and each raw material is accurately weighed according to the above ratio.

[0075] (2) Ball milling: The weighed powder and silicon nitride milling beads are placed in a polytetrafluoroethylene ball mill tank together with 20 mL of anhydrous ethanol and 30 mL of butanone as solvents. After preliminary stirring and mixing, the ball mill tank is fixed on a planetary ball mill for wet ball milling. The ball milling process parameters are set as follows: solid-liquid ratio 2:1, ball-to-material ratio 1.5:1, rotation speed 200 r / min, and ball milling time 3 h.

[0076] (3) Tape casting: The slurry after ball milling is treated by vacuum debubbling to remove the air bubbles entrained therein. Then the slurry is injected into the casting machine tank, and a uniform 0.6 mm thick sheet-shaped silicon nitride green body is cast by controlling the gap between the doctor blade and the base band speed.

[0077] (4) Degreasing treatment: The green body is placed on a boron nitride substrate and put into a degreasing furnace for pre-sintering. The temperature is raised to 500 °C at a rate of 5 °C / min and held for 2 h to completely remove the organic binder and plasticizer.

[0078] (5) Sintering densification: After degumming, the green body is transferred to a high-temperature sintering furnace for pressure sintering in a N2 atmosphere. The sintering pressure is 2 MPa, and the temperature rising program is as follows: heating to 1000 °C at a rate of 5 °C / min, holding for 5 min; continue to heat to 1600 °C at the same rate, holding for 5 min; finally heat to 1850 °C, holding for 2.5 h. Then cool down to room temperature with the furnace, and obtain a dense gray silicon nitride ceramic material.

[0079] (6) Performance test: The sintered ceramic samples are characterized for performance, including apparent density, Vickers hardness, three-point bending strength and fracture toughness, and the results are shown in the subsequent tables.

[0080] Example 5

[0081] (1) Raw material ratio: α-Si3N4 is used as the base material, Y2O3, Al2O3, MgO and La2O3 are selected as composite sintering aids, and Nb2O5 is used as a colorant. The mass ratio of each component is: α-Si3N4 : Y2O3 : Al2O3 : MgO : La2O3 : Nb2O5 = 92 : 2 : 2 : 2 : 2 : 0.5, and each raw material is accurately weighed according to the above ratio.

[0082] (2) Ball milling: The weighed powder and silicon nitride milling beads are placed in a polytetrafluoroethylene ball mill tank together with 20 mL of anhydrous ethanol and 30 mL of butanone as solvents. After preliminary stirring and mixing, the ball mill tank is fixed on a planetary ball mill for wet ball milling. The ball milling process parameters are set as follows: solid-liquid ratio 2:1, ball-to-material ratio 1.5:1, rotation speed 200 r / min, and ball milling time 3 h.

[0083] (3) Tape casting: The slurry after ball milling is treated by vacuum debubbling to remove the air bubbles entrained therein. Then the slurry is injected into the casting machine tank, and a uniform 0.6 mm thick sheet-shaped silicon nitride green body is prepared by controlling the gap between the doctor blade and the base band speed.

[0084] (4) Degumming treatment: The green body is placed on a boron nitride substrate and put into a degumming furnace for pre-sintering. The temperature is raised to 500 °C at a rate of 5 °C / min and held for 2 h to completely remove the organic binder and plasticizer.

[0085] (5) Sintering densification: After the removal of the binder, the green body was transferred to a high-temperature sintering furnace for pressure sintering in a N2 atmosphere. The sintering pressure was 2 MPa, and the temperature program was as follows: heating to 1000 °C at a rate of 5 °C / min, holding for 5 min; continuing to heat to 1600 °C at the same rate, holding for 5 min; and finally heating to 1850 °C, holding for 2.5 h. Subsequently, the furnace was cooled to room temperature, and a dense gray silicon nitride ceramic material was obtained.

[0086] (6) Performance test: The sintered ceramic samples were subjected to performance characterization, including apparent density, Vickers hardness, three-point bending strength, and fracture toughness, and the results are shown in the subsequent tables.

[0087] The sintering aid used in Comparative Example 1 was a Y2O3 - Al2O3 - MgO ternary aid, which is a commonly used silicon nitride sintering aid system. The Y2O3 - Al2O3 - MgO - La2O3 quaternary aid was used in Examples 1-5. The crystal phase difference between the two types of aid systems focuses on the uniformity of the main crystal phase and the stability of the grain boundary. The SEM images of Comparative Example 1 and Example 3 are shown in Figure 1 Figure 1 a is the SEM image of Comparative Example 1, Figure 2 b is the SEM image of Example 3. As can be seen from the image, the ternary aid system of Comparative Example 1 has coarse grains in the main crystal phase, with a wide size distribution. In contrast, the quaternary aid system has a more uniform grain distribution and superior high-temperature stability.

[0088] Comparative Example 2 is a silicon nitride sample prepared by sintering at a temperature of 1800 °C for 2.5 h. Figure 2 The SEM images of Comparative Example 2 and Example 3 are compared in Figure 2 a is the SEM image of Comparative Example 2, Figure 2 b is the SEM image of Example 3. As can be seen from the image, the sample sintered at 1800 °C has relatively poor densification compared to the sample sintered at 1850 °C. In the image, larger pores can be seen. In contrast, the sample sintered at 1850 °C has relatively good densification, with many rod-shaped β-Si3N4 grains of large aspect ratio. The fine grains and rod-shaped grains of large aspect ratio are distributed in an alternating and uniform manner. There are also many liquid and grain boundary phases between and around the grains, with almost no bubbles, indicating complete densification.

[0089] The optical photographs of the silicon nitride ceramic materials prepared in Examples 1-5 are shown in Figure 3 ​The gray color of the samples changes continuously from light to dark with the increase of the amount of coloring agent Nb2O5, which fully proves that Nb2O5 has good gray tone control ability as a coloring agent in silicon nitride ceramics. In addition, the appearance of all samples is complete without cracks, and the surface is dense and uniform without obvious pores or defects.

[0090] Figure 4 The XRD pattern of the gray silicon nitride ceramic material prepared in Example 3 is shown in the figure. It can be seen from the figure that α-Si3N4 has been completely converted into β-Si3N4, and no other phase. It is indicated that the transformation of α phase to β phase is more complete under the selected sintering system.

[0091] Combining Figure 1 b and Figure 2 The SEM photos of the sintered samples in Example 3 are shown in Figure b. From the microstructure, it can be seen that the ceramic body has high density and few intergranular pores, indicating that the sintering process achieves good densification. The grain morphology is a typical long columnar β-Si3N4 crystal form, which is derived from the phase transformation of α phase to β phase and subsequent grain growth during sintering. Such high aspect ratio β phase grains help to improve the fracture toughness of the material through crack bridging, deflection and other toughening mechanisms. At the same time, the grain size distribution is uniform, and there is no abnormal grain growth, which indicates that the control of sintering aids and process parameters effectively promotes the uniformity of the microstructure.

[0092] The relative density, Vickers hardness, bending strength and fracture toughness of the gray silicon nitride materials prepared in Examples 1-5 and Comparative Examples 1-2 are tested, and the results are shown in Table 1:

[0093] Apparent density (g / cm 3 )]]> Vickers hardness (HV) Bending strength (MPa) fracture toughness (MPa-m 1 / 2 ) <!-- 7 -->]]> Example 1 3.22 1875 831 8.35 Example 2 3.22 1832 844 8.54 Example 3 3.23 1856 823 9.56 Example 4 3.23 1871 846 8.88 Example 5 3.23 1885 855 8.64 Comparative Example 1 3.21 1824 821 8.21 Comparative Example 2 3.21 1826 832 8.23

[0094] As can be seen from the results shown in Table 1, the gray silicon nitride ceramic material prepared by the present application has excellent mechanical properties, good hardness and fracture toughness. The bending strength is higher than 1800 MPa, which indicates that the addition of coloring agent niobium oxide (Nb2O5) does not significantly affect the mechanical properties of the ceramic. At the same time, by comparing the apparent density of the examples and the comparative examples, it is found that the apparent density of Examples 1-5 is slightly larger than that of the comparative examples, which indicates that the density of the examples is slightly better than that of the comparative examples.

[0095] The preparation method provided by the present application has the advantages of simple process, short cycle, low cost, safety and environmental protection, no pollution, and good scalability.

[0096] The above merely describes the preferred embodiments of the present application, and is not intended to limit the protection scope of the present application. Those skilled in the art should understand that various modifications and improvements can be made to the process and material ratio without departing from the principles and the essence of the present application, and these modifications and improvements shall also be considered to fall within the protection scope of the present application.

Claims

1. A silicon nitride ceramic material with adjustable grayscale, characterized in that, Its apparent density is 3.0–3.6 g / cm³, its Vickers hardness is 1700–2000 HV, its flexural strength is 700–1000 MPa, and its fracture toughness is 6.5–10 MPa·m. 1 / 2 Its crystal form is long columnar β-Si3N4.

2. The gray-scale tunable silicon nitride ceramic material according to claim 1, characterized in that, Its raw material composition is: 85-98wt% α-Si3N4, 1-8wt% sintering aid and 0-10wt% colorant.

3. The gray-scale tunable silicon nitride ceramic material according to claim 2, characterized in that, The sintering aid is one or more of MgO, Al2O3, Y2O3, La2O3, and Ce2O3.

4. The gray-scale tunable silicon nitride ceramic material according to claim 3, characterized in that, The sintering aids are Y2O3, Al2O3, MgO and La2O3, and their mass ratio is 1:1:1:

1.

5. The gray-scale tunable silicon nitride ceramic material according to claim 2, characterized in that, The colorant is one or more of Ni2O5, SiC, TiN, and carbon black.

6. A method for preparing the gray-scale tunable silicon nitride ceramic material as described in any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Place the raw material and silicon nitride milling beads together in a milling jar, add anhydrous ethanol and butanone as solvents, stir and mix initially, and then perform wet milling. The milling process parameters are set as follows: solid-liquid ratio 2:1, ball-to-material ratio 1.5:1, rotation speed 200 r / min, and milling time 3 h. Step 2: Casting and forming. The slurry obtained in Step 1 is subjected to vacuum degassing treatment, and then the slurry is injected into the casting machine trough to cast into a sheet of silicon nitride green blank of a certain thickness. Step 3: Place the silicon nitride green blank obtained in Step 2 onto a boron nitride substrate and place it in a debinding furnace for pre-sintering; Step 4: Transfer the green body obtained in Step 3 to a high-temperature sintering furnace and perform gas pressure sintering under N2 atmosphere. The sintering pressure is 1-6 MPa. The heating program is as follows: from room temperature to 500-1000 ℃ at a rate of 3-10 ℃ / min; then to 1000-1600 ℃ at a rate of 3-8 ℃ / min; finally to 1600-2000 ℃ at a rate of 3-5 ℃ / min and hold for 1-6 h. Then cool to room temperature with the furnace to obtain the final product.

7. The method for preparing tunable grayscale silicon nitride ceramic materials according to claim 6, characterized in that, The raw materials in step 1 are 85-98wt% α-Si3N4, 1-8wt% sintering aids and 0-10wt% colorants.

8. The method for preparing silicon nitride ceramic materials with tunable grayscale according to claim 6, characterized in that, In step 2, the thickness of the sheet-like silicon nitride green blank is 0.3-0.9 mm.

9. The method for preparing silicon nitride ceramic materials with tunable grayscale according to claim 6, characterized in that, The pre-sintering process in step 3 involves heating to 500 ℃ at a rate of 5 ℃ / min and holding at that temperature for 2 hours.

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