Silicon carbide ceramic composite material based on 3D printing and preparation method thereof

Abstract

The invention discloses a silicon carbide ceramic composite material based on 3D printing and a preparation method thereof.The preparation method comprises the following steps that S1, SiC powder, Al2O3 powder and BN powder are taken, mixed to be uniform and then added into a ball milling tank, ball milling and sieving are conducted, and mixed powder is obtained; s2, the mixed powder and a binder are mixed and then subjected to ball milling, drying and sieving again, and a composite powder system is obtained; deionized water and a water-based binder are stirred to be uniform, and a printing binder system is obtained; s3, the printing binder system is added into a 3D printer to print the composite powder system layer by layer, and a green body is obtained; s4, the green body is subjected to high-temperature degreasing, and a degreased green body is obtained; and S5, uniformly covering the surface of the degreased green body with silicon powder, and then carrying out vacuum sintering to obtain the silicon carbide ceramic composite material. The low-shrinkage and high-density silicon carbide ceramic composite material can be prepared, and the preparation method is simple, short in period and high in preparation efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon carbide ceramic materials, and in particular to a silicon carbide ceramic composite material based on 3D printing and a preparation method thereof. Background Art

[0002] The primary challenge of binder jet 3D printing of SiC-based ceramic composite powders is that the preparation process of composite powders is complex and has limited adaptability. In traditional methods, SiC powders need to be surface modified or sintered with additives (such as Y 2 O 3 -Al 2 O 3 Encapsulation) to improve its compatibility with the binder, but such processes not only require precise control of the uniformity and thickness of the coating layer, but may also cause a decrease in powder fluidity or the introduction of impurities, limiting the flexibility of the material formulation and large-scale application. In addition, the existing technology has strict requirements on the physical properties of the powder (such as sphericity and particle size distribution). For example, spherical powders with high fluidity and low hydrophilicity are required to ensure the uniformity of powder spreading. However, in actual production, the preparation cost of such powders is high and it is difficult to adapt to the needs of different printing equipment. Some existing technologies attempt to optimize the uniformity of powders and binders through mixing devices, but there are still problems such as powder sinking to the bottom and low mixing efficiency during mechanical stirring, which further increases the complexity of the process.

[0003] Secondly, the existing binder jetting technology generally faces the problem of high shrinkage and insufficient density during the degreasing and sintering process of silicon carbide ceramics. Since the printed blank contains a large amount of organic binder, pores and cracks are easily caused in the degreasing stage. Although the subsequent high-temperature sintering (such as reaction sintering or chemical vapor infiltration) can improve the density, it will cause the part shrinkage rate to be as high as 8%-15%, which significantly affects the dimensional accuracy and structural integrity. For example, although the process of melt infiltration or reaction sintering can achieve partial densification, it will introduce heterogeneous phases (such as free silicon) and reduce the high-temperature stability of the material. Although some studies have tried to reduce the sintering temperature by chemical vapor infiltration (CVI) to control shrinkage, this method has extremely high requirements on equipment accuracy and process parameters (such as gas flow rate, temperature gradient), and it is difficult to achieve uniform penetration of complex structures. The density can usually only reach about 90%, and the porosity is still maintained at 8%-10%. In addition, the existing binder system (such as phenolic resin) may leave carbon impurities when burned, further weakening the material performance. Summary of the invention

[0004] In view of the above problems, the present invention aims to provide a silicon carbide ceramic composite material based on 3D printing and a preparation method thereof.

[0005] The technical solution of the present invention is as follows:

[0006] On the one hand, a method for preparing a silicon carbide ceramic composite material based on 3D printing is provided, comprising the following steps:

[0007] S1: Take SiC powder, Al 2 O 3 After the powder and BN powder are evenly mixed, they are added to a ball mill, and ball milled for 4-12 hours at a ball-to-material ratio of 1, and the ball milling medium is continued to be added to a ball-to-material ratio of 2, wherein the ball-to-material ratio 2 is greater than the ball-to-material ratio 1, and after ball milling for 4-12 hours, the mixed powder is sieved to obtain a mixed powder;

[0008] S2: mixing the mixed powder with the binder and then ball milling again, first ball milling in a clockwise or counterclockwise direction for 12-24 hours, resting for 1-2 hours, and then ball milling in a reverse direction for 12-24 hours, drying, and sieving to obtain a composite powder system;

[0009] Mixing deionized water and a water-based binder evenly to obtain a printing binder system;

[0010] S3: adding the printing binder system into a 3D printer to print the composite powder system layer by layer to obtain a green body;

[0011] S4: placing the green body in a vacuum sintering furnace for high-temperature degreasing to obtain a degreased body;

[0012] S5: evenly covering the surface of the degreased green body with silicon powder, and then performing vacuum sintering to obtain the silicon carbide ceramic composite material.

[0013] Preferably, in step S1, the SiC powder, Al 2 O 3 The mass ratio of powder and BN powder is 27:3:1-32:6:1; in step S2, the amount of the binder is 3-7% of the mass of the mixed powder; the concentration of the water-based binder in the printing binder system is 20-30%; in step S3, the amount of the printing binder system is 2-4% of the mass of the composite powder system; in step S5, the amount of the silicon powder is 1.2-1.7 times the mass of the degreased body.

[0014] Preferably, in step S1, the SiC powder, Al 2 O 3 The mass ratio of powder and BN powder is 30:5:1;.

[0015] Preferably, in step S1, the ball-to-material ratio one is 4:1, and the ball-to-material ratio two is 6:1.

[0016] Preferably, in step S1 and step S2, ball milling is performed at a rotation speed of 350-450 rad / min.

[0017] Preferably, in step S1, a 200-300 mesh sieve is used for sieving; in step S2, a 300-400 mesh sieve is used for sieving.

[0018] Preferably, in step S2, the binder is any one or more of polyvinyl pyrrolidone, stearic acid, polymethyl methacrylate, polyvinyl alcohol, and polyacrylonitrile; and the water-based binder is acetylene glycol and / or 2-butoxyethanol.

[0019] Preferably, in step S3, when printing is performed, the printing speed is 100 mm / s, the roller speed is 20 mm / s, the layer thickness is 0.03-0.09 mm, the drying speed is 25 mm / s, and the reference voltage is 80-120V.

[0020] Preferably, in step S4, when high temperature degreasing is performed, the vacuum degree is 10 -3 Pa, high temperature degreasing at 900-1100°C for 2-4h; in step S5, vacuum sintering is performed at a vacuum degree of 10 -3 Pa, the temperature was raised to 800°C at 10°C / min, then to 1400°C at 5°C / min, and finally to 1500°C at 3°C / min. Finally, it was kept at 1500°C for 1h and then cooled to below 100°C with the furnace.

[0021] On the other hand, a 3D printing-based silicon carbide ceramic composite material prepared by using any of the above-mentioned methods for preparing a 3D printing-based silicon carbide ceramic composite material is also provided.

[0022] The beneficial effects of the present invention are:

[0023] 1. The preparation process of the present invention is simple: a simple ball milling process is used to replace the traditional slurry granulation preparation process, thereby reducing the preparation steps, shortening the preparation cycle, and improving the preparation efficiency.

[0024] 2. The present invention can improve material adaptability: the formula adopted by the present invention can be applicable to binder jet 3D printing technology, enhance the printing adaptability of powder, and ensure the stability of the printing process and the mechanical properties of the finished product.

[0025] 3. The present invention can produce low-shrinkage and high-density silicon carbide ceramic composite materials: by precisely controlling the raw material preparation and sintering parameters, the shrinkage rate of the printed ceramic parts is reduced, the dimensional stability and structural integrity of the parts are ensured, and at the same time the densification degree of the material is improved, and the mechanical strength and wear resistance are enhanced.

[0026] 4. The present invention can improve dimensional accuracy: by optimizing various parameters in the 3D printing process, such as printing speed, roller speed, layer thickness, drying speed and reference voltage, the dimensional accuracy of printed parts can be significantly improved to meet high-precision manufacturing requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0028] Figure 1 It is a schematic flow chart of a method for preparing a silicon carbide ceramic composite material based on 3D printing of the present invention;

[0029] Figure 2 This is a schematic diagram of the scanning electron microscope results of the silicon carbide ceramic composite material of Example 4;

[0030] Figure 3 This is a schematic diagram of the scanning electron microscope results of the silicon carbide ceramic composite material of Comparative Example 1;

[0031] Figure 4 This is a schematic diagram of the scanning electron microscope results of the silicon carbide ceramic composite material of Comparative Example 3;

[0032] Figure 5 This is a schematic diagram of the scanning electron microscope results of the silicon carbide ceramic composite material of Comparative Example 4. DETAILED DESCRIPTION

[0033] The present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, in the absence of conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other. It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those generally understood by those of ordinary skill in the art to which this application belongs. The words "including" or "comprising" and the like used in the disclosure of the present invention mean that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0034] On the one hand, if Figure 1 As shown, the present invention provides a method for preparing a silicon carbide ceramic composite material based on 3D printing, comprising the following steps:

[0035] S1: Take SiC powder, Al 2 O 3After the powder and BN powder are evenly mixed, they are added to a ball mill, and ball milled for 4-12 hours at a ball-to-material ratio of 1, and the ball milling medium is continued to be added to a ball-to-material ratio of 2, wherein the ball-to-material ratio 2 is greater than the ball-to-material ratio 1, and after ball milling for 4-12 hours, the mixed powder is sieved to obtain a mixed powder;

[0036] S2: mixing the mixed powder and the binder and then ball milling again, first ball milling in a clockwise or counterclockwise direction for 12-24 hours, resting for 1-2 hours, and then ball milling in a reverse direction for 12-24 hours, drying, sieving, and obtaining a composite powder system; mixing deionized water and a water-based binder evenly to obtain a printing binder system;

[0037] S3: adding the printing binder system into a 3D printer to print the composite powder system layer by layer to obtain a green body;

[0038] S4: placing the green body in a vacuum sintering furnace for high-temperature degreasing to obtain a degreased body;

[0039] S5: evenly covering the surface of the degreased green body with silicon powder, and then performing vacuum sintering to obtain the silicon carbide ceramic composite material.

[0040] In the above embodiment, by adopting a simple ball milling process to replace the original slurry granulation preparation process in the binder jet 3D printing of silicon carbide ceramics, not only the overall preparation process is greatly simplified, but also the convenience and efficiency of the preparation process are significantly improved, so that the entire cycle from raw material processing to the final product is greatly shortened.

[0041] In the present invention, by adopting the specific ball milling process of step S1, SiC powder, Al 2 O 3 By adding a binder in the secondary ball milling in step S2, the SiC powder, Al powder and BN powder can be mixed to form a mixed powder with a mosaic effect and excellent particle size distribution. 2 O 3 The raw materials such as powder and BN powder are further mixed evenly to reduce the agglomeration of particles, which is helpful to form a more uniform microstructure in the subsequent sintering process and improve the mechanical properties and density of the material. The powder after the second ball milling can better form a dense microstructure during the sintering process, reduce the porosity, and improve the strength of the material. At the same time, the addition of the binder enhances the fluidity of the composite powder, the powder spreading effect is better during the printing process, and the dimensional accuracy of the material is increased. By adopting a water-based binder, the strength and forming effect of the embryo can be guaranteed, and it has a good surface roughness. High-temperature degreasing can effectively reduce the porosity of the embryo, laying a good foundation for subsequent siliconization sintering. Finally, vacuum sintering and siliconization treatment make the silicon carbide ceramic composite material prepared by the present invention have low shrinkage and high density performance.

[0042] In a specific embodiment, the SiC powder, Al 2 O 3 The mass ratio of SiC powder to BN powder is 27:3:1-32:6:1; in step S2, the amount of the binder is 3-7% of the mass of the mixed powder; the concentration of the water-based binder in the printing binder system is 20-30%; in step S3, the amount of the printing binder system is 2-4% of the mass of the composite powder system; in step S5, the amount of silicon powder is 1.2-1.7 times the mass of the degreased blank. Optionally, the SiC powder, Al 2 O 3 The mass ratio of powder and BN powder is 30:5:1;.

[0043] In the above embodiment, the present invention precisely controls SiC powder, Al 2 O 3 By adjusting the ratio of the Ag powder to the BN powder and combining the specific ball milling process adopted in the present invention, a mixed powder with a mosaic effect and excellent particle size distribution can be successfully prepared.

[0044] In a specific embodiment, in step S1, the ball-to-material ratio one is 4:1, and the ball-to-material ratio two is 6:1; in step S1 and step S2, ball milling is performed at a rotation speed of 350-450 rad / min; in step S1, 200-300 mesh sieve is used for sieving; in step S2, 300-400 mesh sieve is used for sieving.

[0045] In a specific embodiment, in step S2, the binder is any one or more of polyvinyl pyrrolidone, stearic acid, polymethyl methacrylate, polyvinyl alcohol, and polyacrylonitrile; and the water-based binder is acetylene glycol and / or 2-butoxyethanol.

[0046] In a specific embodiment, in step S3, when printing is performed, the printing speed is 100 mm / s, the roller speed is 20 mm / s, the layer thickness is 0.03-0.09 mm, the drying speed is 25 mm / s, and the reference voltage is 80-120V.

[0047] In the above embodiment, by adopting precise printing parameters, the stability of the printing process and the dimensional accuracy of the finished product can be ensured.

[0048] In a specific embodiment, in step S4, when high temperature degreasing is performed, the vacuum degree is 10 -3 Pa, high temperature degreasing at 900-1100°C for 2-4h; in step S5, vacuum sintering is performed at a vacuum degree of 10 -3Pa, the temperature was raised to 800°C at 10°C / min, then to 1400°C at 5°C / min, and finally to 1500°C at 3°C / min. Finally, it was kept at 1500°C for 1h and then cooled to below 100°C with the furnace.

[0049] In the above embodiments, the silicon carbide ceramic composite material finally obtained has low shrinkage and high density properties through the preferred high-temperature debinding and sintering parameters.

[0050] On the other hand, the present invention also provides a 3D printing-based silicon carbide ceramic composite material prepared by using any one of the above-mentioned methods for preparing a 3D printing-based silicon carbide ceramic composite material.

[0051] Example 1

[0052] A silicon carbide ceramic composite material based on 3D printing is prepared by the following steps:

[0053] (1) Take SiC powder, Al 2 O 3 The powder and BN powder were mixed evenly in a ratio of 28:3:1 and placed in a ball mill, and ball milled for 6 hours at a ball-to-material ratio of 4:1; then, ball milling media was added to make the ball-to-material ratio reach 6:1, and the ball milling was continued for 8 hours, and then passed through a 250-mesh sieve to obtain a mixed powder;

[0054] (2) mixing the mixed powder with polyvinyl pyrrolidone accounting for 5% by weight of the mixed powder and then ball milling again, the specific operation is ball milling in a clockwise direction for 12 hours, resting for 2 hours, and then ball milling in a counterclockwise direction for 12 hours; drying and passing through a 350-mesh sieve after the ball milling to obtain a composite powder system with fluidity and non-adhesive particles;

[0055] (3) The composite powder system is placed in the loading bin of a BJAM printer, and the printing parameters are set as follows: printing speed 100 mm / s, roller speed 20 mm / s, layer thickness 0.6 mm, drying speed 25 mm / s, reference voltage 100 V, and 15% acetylene glycol and 8% 2-butoxyethanol (water accounts for 77%) are used as the printing binder, and then printing is started until the end to obtain a green body;

[0056] (4) heating the green body to 1000° C. at a heating rate of 5° C. / min and keeping the temperature for 2 h to perform high-temperature degreasing to obtain a degreased body;

[0057] (5) Cover the degreased body with silicon powder 1.4 times its mass and evacuate to 10 -3Pa, then heated to 800°C at a heating rate of 10°C / min, then heated to 1400°C at a heating rate of 5°C / min, and finally heated to 1500°C at a heating rate of 3°C / min. After being kept at 1500°C for 1 hour, the temperature was cooled to below 100°C with the furnace, and the sample was taken out to obtain the silicon carbide ceramic composite material.

[0058] Example 2

[0059] The difference from Example 1 is that in step (1) of this example, SiC powder and Al 2 O 3 The mass ratio of powder to BN powder is 29:4:1.

[0060] Example 3

[0061] The difference from Example 1 is that in step (1) of this example, SiC powder and Al 2 O 3 The mass ratio of powder to BN powder is 29:5:1.

[0062] Example 4

[0063] The difference from Example 1 is that in step (1) of this example, SiC powder and Al 2 O 3 The mass ratio of powder to BN powder is 30:5:1.

[0064] Example 5

[0065] The difference from Example 1 is that in step (1) of this example, SiC powder and Al 2 O 3 The mass ratio of powder and BN powder is 30:6:1.

[0066] Example 6

[0067] The difference from Example 1 is that in step (1) of this example, SiC powder and Al 2 O 3 The mass ratio of powder to BN powder is 31:6:1.

[0068] Comparative Example 1

[0069] Different from Example 4, this comparative example does not include step (2), and the mixed powder obtained in step (1) is directly placed in the loading bin of the BJAM printer to carry out step (3).

[0070] Comparative Example 2

[0071] Different from Example 4, the polyvinyl pyrrolidone is not added in step (2) of this comparative example, that is, the secondary ball milling is performed without adding a binder.

[0072] Comparative Example 3

[0073] Different from Example 4, this comparative example also added 5% polyvinyl pyrrolidone in step (1).

[0074] Comparative Example 4

[0075] Different from Example 4, only SiC powder is added in step (1) of this comparative example, and the amount used is the total mass of the three powders in Example 4.

[0076] Comparative Example 5

[0077] Different from Example 4, in step (1) of this comparative example, no BN powder was added, that is, only SiC powder and Al powder were mixed in a ratio of 30:5. 2 O 3 pink.

[0078] Test Example 1

[0079] The morphology of each embodiment and each comparative example was observed by scanning electron microscope, and the results of embodiment 4, comparative example 1, comparative example 3 and comparative example 4 were as follows: Figure 2-Figure 5 As shown. Figure 2 It can be seen that the silicon carbide ceramic composite material prepared by the present invention has a dense microstructure, a relatively complete crystal morphology, and the particles are closely embedded. Figure 3 It can be seen that the finished product structure obtained without step (2) has larger pores and no mosaic between particles. Figure 4 It can be seen that in step (1), the finished product structure obtained by adding a binder has serious particle agglomeration and large pores. Figure 5 It can be seen that the finished product structure without BN powder has more voids and the bonding between particles is seriously loose.

[0080] Test Example 2

[0081] The compactness and shrinkage properties of each embodiment and each comparative example were tested, and the results are shown in Table 1:

[0082] Table 1 Test results of compactness and shrinkage properties

[0083] performance Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Density 97.18％ 97.88％ 98.28％ 98.79％ 97.68％ 97.98％ Shrinkage 1％ 1％ 0.9％ 0.8％ 1％ 0.9％ performance Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Density 98.79％ 84.56％ 86.84％ 90.13％ 80.21％ 87.68％ Shrinkage 0.8％ 7.8％ 6.5％ 4.3％ 9.8％ 6.1％

[0084] It can be seen from Table 1 that the silicon carbide ceramic composite material prepared by the present invention has low shrinkage (≤1%) and high density (97-99%) properties, while the silicon carbide ceramic composite material prepared by other methods cannot achieve the performance that can be achieved by the present invention.

[0085] It should be noted that the above embodiments are only some embodiments of the present invention. The silicon carbide ceramic composite materials prepared by the preparation method of the present invention by changing the binder type, printing parameters, degreasing temperature, degreasing time and other parameters all have similar properties.

[0086] In summary, the present invention can produce a silicon carbide ceramic composite material with low shrinkage and high density. Compared with the prior art, the present invention has significant progress.

[0087] The above description is only a representative embodiment of the present invention and does not limit the present invention in any form. Any technician familiar with the profession, without departing from the scope of the technical solution of the present invention, uses the above-disclosed technical content to make some changes or modifications to the embodiment is an equivalent embodiment of the present invention. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a silicon carbide ceramic composite material based on 3D printing, characterized in that: The following steps are involved: S1: Take SiC powder, Al2O3 powder and BN powder, mix them evenly and add them to a ball mill, ball mill them for 4-12 hours according to the ball-to-material ratio of 1, continue to add ball milling media to the ball-to-material ratio of 2, the ball-to-material ratio of 2 is greater than the ball-to-material ratio of 1, and sieve after ball milling for 4-12 hours to obtain a mixed powder; S2: mixing the mixed powder with the binder and then ball milling again, first ball milling in a clockwise or counterclockwise direction for 12-24 hours, resting for 1-2 hours, and then ball milling in a reverse direction for 12-24 hours, drying, and sieving to obtain a composite powder system; Mixing deionized water and a water-based binder evenly to obtain a printing binder system; S3: adding the printing binder system into a 3D printer to print the composite powder system layer by layer to obtain a green body; S4: placing the green body in a vacuum sintering furnace for high-temperature degreasing to obtain a degreased body; S5: evenly covering the surface of the degreased green body with silicon powder, and then performing vacuum sintering to obtain the silicon carbide ceramic composite material.

2. The method for preparing a silicon carbide ceramic composite material based on 3D printing according to claim 1, characterized in that: In step S1, the mass ratio of the SiC powder, Al2O3 powder and BN powder is 27:3:1-32:6:1; in step S2, the amount of the binder is 3-7% of the mass of the mixed powder; the concentration of the water-based binder in the printing binder system is 20-30%; in step S3, the amount of the printing binder system is 2-4% of the mass of the composite powder system; in step S5, the amount of the silicon powder is 1.2-1.7 times the mass of the degreased body.

3. The method for preparing a silicon carbide ceramic composite material based on 3D printing according to claim 2, characterized in that: In step S1, the mass ratio of the SiC powder, Al2O3 powder and BN powder is 30:5:1;.

4. The method for preparing a silicon carbide ceramic composite material based on 3D printing according to claim 1, characterized in that: In step S1, the ball-to-material ratio 1 is 4:1, and the ball-to-material ratio 2 is 6:

1.

5. The method for preparing a silicon carbide ceramic composite material based on 3D printing according to claim 1, characterized in that: In step S1 and step S2, ball milling is performed at a rotation speed of 350-450 rad / min.

6. The method for preparing a silicon carbide ceramic composite material based on 3D printing according to claim 1, characterized in that: In step S1, a 200-300 mesh screen is used for sieving; in step S2, a 300-400 mesh screen is used for sieving.

7. The method for preparing a silicon carbide ceramic composite material based on 3D printing according to claim 1, characterized in that: In step S2, the binder is any one or more of polyvinyl pyrrolidone, stearic acid, polymethyl methacrylate, polyvinyl alcohol, and polyacrylonitrile; and the water-based binder is acetylene glycol and / or 2-butoxyethanol.

8. The method for preparing a silicon carbide ceramic composite material based on 3D printing according to claim 1, characterized in that: In step S3, when printing is performed, the printing speed is 100 mm / s, the roller speed is 20 mm / s, the layer thickness is 0.03-0.09 mm, the drying speed is 25 mm / s, and the reference voltage is 80-120V.

9. The method for preparing a silicon carbide ceramic composite material based on 3D printing according to any one of claims 1 to 8, characterized in that: In step S4, when high temperature degreasing is performed, the vacuum degree is 10 -3 Pa, high temperature degreasing at 900-1100°C for 2-4h; in step S5, vacuum sintering is performed at a vacuum degree of 10 -3 Pa, the temperature was raised to 800°C at 10°C / min, then to 1400°C at 5°C / min, and finally to 1500°C at 3°C / min. Finally, it was kept at 1500°C for 1h and then cooled to below 100°C with the furnace.

10. A silicon carbide ceramic composite material based on 3D printing, characterized in that: The silicon carbide ceramic composite material is prepared by the preparation method based on 3D printing as described in any one of claims 1 to 9.

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