NL2028306B1

NL2028306B1 - Method for preparing binderless wc-y2o3 cemented carbide by pressure-assisted cold and hot sintering

Info

Publication number: NL2028306B1
Application number: NL2028306A
Authority: NL
Inventors: Liu Menghui; Wu Mingwei; Tu Zhibiao; Lin Xiao; Wang Ronghui; Lin Qiqun; Wu Yinan; Zhu Haifeng; Wang Jinfang; Zhu Liu; Shao Jiandi
Original assignee: Taizhou Product Quality And Safety Testing Res Institute; Lifeng Prec Tools Zhejiang Co Ltd; Univ Taizhou
Priority date: 2020-07-29
Filing date: 2021-05-27
Publication date: 2022-07-13
Also published as: CN111778436A; CN111778436B; NL2028306A

Abstract

The present disclosure relates to the technical field of preparation of binderless cemented carbides, in particular to a method for preparing a binderless WC—Yﬂh cemented carbide by pressure—assisted cold and hot 5 sintering. The method includes the following steps: ball— milling WC powder and Yxh powder to obtain WC—Yﬂh powder; adding a saturated oxalic acid solution to the WC—Yﬂh powder until the powder is completely wet, and subjecting the wet WC—Yﬂh powder to pressure—assisted cold sintering under 200— 10 400 MPa to obtain a sintered body, where a temperature rising procedure for the pressure—assisted cold sintering is as follows: first heating up to lOO—lSOOC for l h, and then heating' up to ZOO—300°C for 1—2 h; and subjecting the sintered body to pressure—assisted hot sintering to obtain 15 a.binderless WC—Yﬂh cemented carbide. The present disclosure can significantly reduce the sintering temperature of the binderless WC—based cemented carbide, and improve the density and comprehensive mechanical properties of the alloy. Fig.6

Description

METHOD FOR PREPARING BINDERLESS WC-Y203 CEMENTED CARBIDE BY PRESSURE-ASSISTED COLD AND HOT SINTERING

TECHNICAL FIELD The present disclosure relates to the technical field of preparation of binderless cemented carbides, in particular to a method for preparing a binderless WC-Y:03 cemented carbide by pressure-assisted cold and hot sintering.

BACKGROUND ART Tungsten carbide (WC)-based cemented carbides are widely used in cutting tools, molds and wear-resistant and high- pressure parts due to their high strength, high hardness and high Young's modulus. As pure WC has a high melting point (2785°C), its sintering features high temperature, poor densification and poor fracture toughness. To solve these problems, cobalt (Co), nickel (Ni) and other metal binder phases (M) are usually added to prepare WC-M cemented carbides. These metal binder phases are more susceptible to oxidation, corrosion and wear failure than the WC hard phase under high-speed, high-efficiency dry cutting, which limits the uses of cemented carbides in difficult-to-machine materials such as titanium alloys and high-temperature alloys. The metal phase in the WC-M cemented carbide mold is easy to oxidize and diffuse into glass at high temperatures, which has become a bottleneck in the manufacture of high-precision optical glass molds. In addition, the metal binder phases such as Co and Ni will enter the environment in the form of dust and steam along with the wear of tools and parts, posing a great threat to workers! health. Therefore, it is necessary to develop a technology for preparing a binderless WC-based cemented carbide with high hardness, high fracture toughness and better wear resistance, corrosion resistance and high temperature oxidation resistance than a traditional WC-M alloy. This will be of great significance to the development of cutting tool engineering and green manufacturing.

At present, binderless WC-based cemented carbides have become a research hotspot in the field of cemented carbides worldwide. However, due to the high sintering temperature of 2785°C, the prepared binderless WC-based cemented carbides have poor density and comprehensive mechanical properties, which affects their uses. The spark plasma sintering (SPS) technique can be used to greatly reduce the sintering temperature to prepare a binderless WC-based cemented carbide with excellent comprehensive properties. However, it is not conducive to the industrial promotion and application of the binderless WC-based cemented carbide due to the expensive equipment, high preparation cost and limited sintering size.

SUMMARY An objective of the present disclosure is to provide a method for preparing a binderless WC-Y;03 cemented carbide by pressure-assisted cold and hot sintering. The present disclosure can significantly reduce the sintering temperature of the binderless WC-based cemented carbide, improve the density and comprehensive mechanical properties of the alloy, and has low cost.

To achieve the objective of the present disclosure, the present disclosure provides the following technical solutions.

The present disclosure provides a method for preparing a binderless WC-Y203 cemented carbide by pressure-assisted cold and hot sintering, including the following steps: ball-milling WC powder and Y:03 powder to obtain WC-Y203 powder; adding a saturated oxalic acid solution to the WC-Y:03 powder until the powder is completely wet, and subjecting the wet WC-Y.0; powder to pressure-assisted cold sintering under 200- 400 MPa to obtain a sintered body, where a temperature rising procedure for the pressure-assisted cold sintering is as follows: first heating up to 100-150°C for 1 h, and then heating up to 200-300°C for 1-2 h; and subjecting the sintered body to pressure-assisted hot sintering to obtain a binderless WC-Y;0; cemented carbide. Preferably, a ratio of the WC-Y;0; powder to the saturated oxalic acid solution may be 10 g:(1-1.5) mL.

Preferably, the Y;0; powder may account for 1-3% of a total mass of the WC powder and the Y:03 powder.

Preferably, an average grain size of the WC powder may be 200-400 nm.

Preferably, an average grain size of the Y:0:3 powder may be 50-200 nm.

Preferably, a temperature rising procedure for the pressure- assisted hot sintering may be as follows: holding 150°C for min, heating up to 250°C for 20 min, then heating up to 800°C for 60 min, and finally heating up to 1400-1650°C for 90 min.

Preferably, the pressure-assisted hot sintering may be conducted under 40 MPa.

20 Preferably, each heat-up in the pressure-assisted cold and hot sintering may be conducted independently at 10-15°C/min. Preferably, the ball-milling may be conducted at a ball-to- powder ratio (BPR) of (5-15):1 at 200-250 r/min for 6-12 h with the aid of anhydrous ethanol.

Preferably, the method may further include vacuum-drying the ball-milled powder after the ball-milling.

The present disclosure provides a method for preparing a binderless WC-Y:03 cemented carbide by pressure-assisted cold and hot sintering, including the following steps: ball- milling WC powder and Y:03 powder to obtain WC-Y:03 powder; adding a saturated oxalic acid solution to the WC-Y:0: powder until the powder is completely wet, and subjecting the wet WC-Y20:3 powder to pressure-assisted cold sintering under 200- 400 MPa to obtain a sintered body, where a temperature rising procedure for the pressure-assisted cold sintering is as follows: first heating up to 100-150°C for 1 h, and then heating up to 200-300°C for 1-2 h; and subjecting the sintered body to pressure-assisted hot sintering to obtain a binderless WC-Y:03 cemented carbide. In the present disclosure, the WC powder and the Y:0: powder are ball-milled, such that the Y:0: powder is fully dispersed in the WC powder. Then a saturated oxalic acid solution, in which Y203 is slightly soluble, is added to the WC-Y:03 powder until it is completely wet. Under the pressure and temperature of the pressure-assisted cold sintering, Y:0: begins to partially dissolve. As the temperature rises, the oxalic acid solution evaporates and Y:03 precipitates. Because of the dissolution-precipitation of the trace Y:0s in the oxalic acid solution, gaps of the WC-Y,0; powder reach a supersaturated state, and a chemical potential in a particle contact zone is higher than that of a crystal. At this time, a dissolved atom or ion cluster is precipitated at the crystal, thereby promoting the densification of WC- Y203. In this way, a sintered body of the binderless WC-Y20: cemented carbide with a relative density of more than 70% is formed at a low temperature (100-300°C). This greatly reduces the temperature of the subsequent traditional sintering process (pressure-assisted hot sintering) in preparing the binderless WC-based cemented carbide. In addition, the prepared binderless WC-Y:03 cemented carbide has high density and excellent comprehensive properties. The results of examples show that the method of the present disclosure can reduce the temperature of the traditional pressure-assisted hot sintering to below 1650°C, and the prepared binderless WC-based cemented carbide has a density as high as 87.87-100%, and has excellent comprehensive properties, including a hardness of 2357-2500 HV30 and a fracture toughness of 8.2-9.3 MPaem?/2,

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a scanning electron microscope (SEM) morphology image of a material before and after ball-milling according to Example 1.

FIG. 2 is an SEM fracture morphology image of a sintered body obtained by pressure-assisted cold sintering according to Example 1.

FIG. 3 is an SEM fracture morphology image of a binderless 5 WC-1wt® Y:03 cemented carbide prepared by Example 1.

FIG. 4 is an SEM fracture morphology image of a binderless WC-2wt% Y203 cemented carbide prepared by Example 2.

FIG. 5 is an SEM fracture morphology image of a binderless WC-3wt% Y:03 cemented carbide prepared by Example 3.

FIG. 6 is an SEM fracture morphology image of a binderless WC-3wt% Y203 cemented carbide prepared by Example 4, FIG. 7 is an SEM fracture morphology image of a binderless WC-2wt% Y:03 cemented carbide prepared by a comparative example.

DETAILED DESCRIPTION OF THE EMBODIMENTS The present disclosure provides a method for preparing a binderless WC-Y203 cemented carbide by pressure-assisted cold and hot sintering, including the following steps: Ball-mill WC powder and Y203 powder to obtain WC-Y.0; powder. Add a saturated oxalic acid solution to the WC-Y:03 powder until the powder is completely wet, and subject the wet WC- Y203 powder to pressure-assisted cold sintering under 200- 400 MPa to obtain a sintered body, where a temperature rising procedure for the pressure-assisted cold sintering is as follows: first heating up to 100-150°C for 1 h, and then heating up to 200-300°C for 1-2 h.

Subject the sintered body to pressure-assisted hot sintering to obtain a binderless WC-Y203 cemented carbide.

In the present disclosure, unless otherwise specified, all raw materials used are commercially available products well known to those skilled in the art.

The present disclosure ball-mills WC powder and Y:03 powder to obtain WC-Y203 powder. In the present disclosure, an average grain size of the WC powder is preferably 200-400 nm, more preferably 200-300 nm, and specifically 200 nm in an example of the present disclosure. In the present disclosure, an average grain size of the Y:03 powder is preferably 50-200 nm, more preferably 100-150 nm, and specifically 50 nm in an example of the present disclosure.

In the present disclosure, the Y,0;3 powder accounts for preferably 1-32, more preferably 1.5-2.5% of a total mass of the WC powder and the Y;03 powder.

In the present disclosure, the ball-milling is preferably conducted at a ball-to-powder ratio (BPR) of (5-15):1 at 200-250 r/min for 6-12 h with the aid of anhydrous ethancl.

Further, the ball-milling is more preferably conducted at a BPR of 10:1 at 210-230 r/min for 8-10 h. In the present disclosure, directions preferably change every 5 min during ball-milling. In order to avoid impurities generated in the ball-milling process, the present disclosure preferably adopts a cylinder and balls made of cemented carbide. The present disclosure preferably further includes: vacuum-dry the ball-milled powder at 60°C for 24 h after the ball- milling. By ball-milling, the present disclosure fully disperses the Y:03 powder in the WC powder.

After obtaining the WC-Y20:3 powder, the present disclosure adds a saturated oxalic acid solution to the WC-Y203 powder until the powder is completely wet, and subjects the wet WC- Y203 powder to pressure-assisted cold sintering to obtain a sintered body.

In the present disclosure, in order to completely wet the WC-Y;03 powder, a ratio of the WC-Y:03 powder to the saturated oxalic acid solution is preferably 10 g:{1-1.5) mL, more preferably 10 g:1 mL.

In the present disclosure, the pressure-assisted cold sintering is conducted under 200-400 MPa, more preferably 250-350 MPa. A temperature rising procedure for the pressure-assisted cold sintering is preferably: first heat up to 100-150°C for 1 h, and then heat up to 200-300°C for 1-2 h; more preferably: first heat up to 150°C for 1 h, and then heat up to 250°C for 2 h. In the present disclosure, the temperature rise preferably starts from room temperature. In the present disclosure, each heat-up in the pressure-assisted cold sintering is preferably conducted independently at 10-15°C/min.

In the present disclosure, the WC-Y.0; powder is preferably placed in a metal mold which is externally provided with a heating coil.

A pressure is applied on the metal mold by an upper punch.

The heating coil heats up according to the above temperature rising procedure to conduct the pressure-assisted cold sintering.

In the present disclosure, a saturated oxalic acid solution, in which Y203 is slightly soluble, is added to the WC-Y20; powder until it is completely wet.

Under the pressure and temperature of the pressure-assisted cold sintering, Y:0s begins to partially dissolve.

As the temperature rises, the oxalic acid solution evaporates and Y:03 precipitates.

Because of the dissolution-precipitation of the trace Y.0: in the oxalic acid solution, gaps of the WC-Y:0; powder reach a supersaturated state, and a chemical potential in a particle contact zone is higher than that of a crystal.

At this time, a dissolved atom or ion cluster is precipitated at the crystal, thereby promoting the densification of WC- Yz03. In this way, a sintered body of the binderless WC-Y:03 cemented carbide with a relative density of more than 70% is formed at a low temperature (100-300°C). This greatly reduces the temperature of the subsequent traditional sintering process (pressure-assisted hot sintering) in preparing the binderless WC-based cemented carbide.

In addition, the prepared binderless WC-Y,0; cemented carbide has high density and excellent comprehensive properties.

After obtaining the sintered body, the present disclosure subjects the sintered body to pressure-assisted hot sintering to obtain a binderless WC-Y,0:; cemented carbide.

In the present disclosure, a temperature rising procedure for the pressure-assisted hot sintering is preferably: hold 150°C for 20 min, heat up to 250°C for 20 min, then heat up to 800°C for 60 min, and finally heat up to 1400-1650°C for 90 min.

The pressure-assisted hot sintering is preferably conducted under 40 MPa.

The present disclosure preferably places the sintered body in a graphite mold for pressure-

assisted hot sintering. In the present disclosure, during the pressure-assisted hot sintering process, the residual oxalic acid solution further evaporates for precipitation, and WC grains are further rearranged, effectively reducing pores and grain boundaries, thereby further improving the densification of the sintered body. In the present disclosure, the temperature rise preferably starts from room temperature. In the present disclosure, each heat-up in the pressure-assisted hot sintering is preferably conducted independently at 10-15°C/min. The method for preparing a binderless WC-Y;203 cemented carbide by pressure-assisted cold and hot sintering provided by the present disclosure is described in detail below with reference to the examples, but the examples may not be construed as a limitation to the protection scope of the present disclosure. Example 1

1. Ball-milling WC powder and lwt3 of Y:0;:3 powder with an average grain size of 200 nm and 50 nm each were ball-milled. By using a cylinder and balls made of cemented carbide, the ball- milling was conducted at a BPR of 10:1 at 200 r/min for 12 h with the aid of anhydrous ethanol. During ball-milling, directions changed every 5 min. After ball-milling, the powder was placed in a vacuum drying oven at 60°C for 24 h to obtain WC-1wt% Y203 powder. The morphologies of the powder before and after ball-milling are shown in FIG. 1, where a shows the Y:03 powder, b shows the WC powder, and c shows the WC-1wt% Y:03 powder after ball-milling. It can be seen from FIG. 1 that the powder is evenly mixed and there is no local enrichment of the Y:0:3 powder.

2. Pressure-assisted cold sintering 30 g of WC-lwt& Y»03 powder was weighed, and 3 mL of saturated oxalic acid solution was added until the powder was completely wet. Then the wet powder was placed in a metal mold which had a diameter of 20 mm and was externally provided with a heating coil. A pressure of 400 MPa was applied to the metal mold by an upper punch. The heating coil heated up to 150°C for 1 h and then 250°C for 2 h at a rate of 10°C/min to obtain a sintered body. FIG. 2 shows a fracture morphology of a sintered body sample. It can be seen from the figure that although there was only a low sintering temperature of 250°C, there was diffusion between WC grains under 400 MPa in the presence of the saturated oxalic acid solution. The sample had a density of 70%.

3. Pressure-assisted hot sintering The sintered body prepared by pressure-assisted cold sintering was put into a graphite mold for pressure-assisted hot sintering under 40 MPa by heating at a rate of 10°C/min. During the sintering process, 150°C was held for 20 min, 250°C for 20 min, 800°C for 60 min, and 1600°C for 90 min. In this way, a binderless WC-based cemented carbide doped with 1lwt® Y203 was obtained. A sample had a relative density of 97.87%, a hardness of 2399 HV30 and a fracture toughness of 8.2 MPaem'/2., FIG. 3 shows a fracture morphology of the binderless WC-based cemented carbide doped with 1wt% Y;03 in this example. The figure shows that the WC grains are tightly bonded, and there are pores in some zones. Example 2

1. Ball-milling WC powder and 2wt% of Y203 powder with an average grain size of 200 nm and 50 nm each were ball-milled. By using a cylinder and balls made of cemented carbide, the ball- milling was conducted at a BPR of 10:1 at 200 r/min for 12 h with the aid of anhydrous ethanol. During ball-milling, directions changed every 5 min. After ball-milling, the powder was placed in a vacuum drying oven at 60°C for 24 h to obtain WC-2wtk® Y,0; powder.

2. Pressure-assisted cold sintering 30 g of WC-2wt® Y203 powder prepared by ball-milling was weighed, and 3 mL of saturated oxalic acid solution was added until the powder was completely wet. Then the wet powder was placed in a metal mold which had a diameter of 20 mm and was externally provided with a heating coil. A pressure of 400 MPa was applied to the metal mold by an upper punch. The heating coil heated up to 150°C for 1 h and then 250°C for 2 h at a rate of 10°C/min to obtain a sintered body. The sintered body had a relative density of 73%.

3. Pressure-assisted hot sintering The sintered body prepared by pressure-assisted cold sintering was put into a graphite mold for pressure-assisted hot sintering at 1600°C under 40 MPa, by heating at a rate of 10°C/min. During the sintering process, 150°C was held for 20 min, 250°C for 20 min, 800°C for 60 min, and 1600°C for 90 min. In this way, a binderless WC-based cemented carbide doped with 2wt& Y:03 was obtained.

A sample had a relative density of 99.962, a hardness of 2500 HV30 and a fracture toughness of 9.1 MPaemi/2, FIG. 4 shows a fracture morphology of the binderless WC-based cemented carbide doped with 2wt% Y:03 in this example. The figure shows that the WC grains are tightly bonded, and there are basically no pores, close to a fully dense state. Example 3

1. Ball-milling WC powder and 3wt% of Y;03 powder with an average grain size of 200 nm and 50 nm each were ball-milled. By using a cylinder and balls made of cemented carbide, the ball- milling was conducted at a BPR of 10:1 at 200 r/min for 12 h with the aid of anhydrous ethanol. During ball-milling, directions changed every 5 min. After ball-milling, the powder was placed in a vacuum drying oven at 60°C for 24 h to obtain WC-3wt4 Y,0: powder.

2. Pressure-assisted cold sintering 30 g of WC-3wt® Y;03 powder prepared by ball-milling was weighed, and 3 mL of saturated oxalic acid solution was added until the powder was completely wet. Then the wet powder was placed in a metal mold which had a diameter of 20 mm and was externally provided with a heating coil. A pressure of 400 MPa was applied to the metal mold by an upper punch. The heating coil heated up to 150°C for 1 h and then 250°C for 2 h at a rate of 10°C/min to obtain a sintered body. The sintered body had a relative density of 73%.

3. Pressure-assisted hot sintering The sintered body prepared by pressure-assisted cold sintering was put into a graphite mold for pressure-assisted hot sintering under 40 MPa by heating at a rate of 10°C/min. During the sintering process, 150°C was held for 20 min, 250°C for 20 min, 800°C for 60 min, and 1600°C for 90 min. In this way, a binderless WC-based cemented carbide doped with 3wt3 Y203 was obtained. A sample had a relative density of 99.87%, a hardness of 2357 HV30 and a fracture toughness of 8.8 MPasm+#2, FIG. 5 shows a fracture morphology of the binderless WC-based cemented carbide doped with 3wt% Y:03 in this example. The figure shows that the WC grains are tightly bonded, and there are only a few pores. Example 4

1. Ball-milling WC powder and 3wt® of Y,0; powder with an average grain size of 200 nm and 50 nm each were ball-milled. By using a cylinder and balls made of cemented carbide, the ball- milling was conducted at a BPR of 10:1 at 200 r/min for 12 h with the aid of anhydrous ethanol. During ball-milling, directions changed every 5 min. After ball-milling, the powder was placed in a vacuum drying oven at 60°C for 24 h to obtain WC-3wt% Y203 powder.

2. Pressure-assisted cold sintering g of WC-3wt% Y;0; powder prepared by ball-milling was 30 weighed, and 3 mL of saturated oxalic acid solution was added until the powder was completely wet. Then the wet powder was placed in a metal mold which had a diameter of 20 mm and was externally provided with a heating coil. A pressure of 400 MPa was applied to the metal mold by an upper punch. The heating coil heated up to 150°C for 1 h and then 250°C for 2 h at a rate of 10°C/min to obtain a sintered body. The sintered body had a relative density of 73%.

3. Pressure-assisted hot sintering The sintered body prepared by pressure-assisted cold sintering was put into a graphite mold for pressure-assisted hot sintering under 40 MPa by heating at a rate of 10°C/min. During the sintering process, 150°C was held for 20 min, 250°C for 20 min, 800°C for 60 min, and 1650°C for 90 min. In this way, a binderless WC-based cemented carbide doped with 3wt% Y;0; was obtained. A sample had a relative density of 100%, a hardness of 2460 HV30 and a fracture toughness of 9.3 MPaem!/2. FIG. 6 shows a fracture morphology of the binderless WC-based cemented carbide doped with 3wt% Y;0; in this example. The figure shows that the WC grains are tightly bonded without pores. Comparative Example

1. Ball-milling WC powder and 2wt% of Y:03 powder with an average grain size of 200 nm and 50 nm each were ball-milled. By using a cylinder and balls made of cemented carbide, the ball- milling was conducted at a BPR of 10:1 at 200 r/min for 12 h with the aid of anhydrous ethanol. During ball-milling, directions changed every 5 min. After ball-milling, the powder was placed in a vacuum drying oven at 60°C for 24 h to obtain WC-2wt% Y,03 powder.

2. Pressure-assisted hot sintering g of WC-2wt% Y203 powder prepared by ball-milling was weighed, and placed in a metal mold which had a diameter of 20 mm. A pressure of 400 MPa was applied to the metal mold by an upper punch for 10 min to obtain a pressed body of WC- 2wt? Y203. The pressed body was taken out of the metal mold, 30 and put into a graphite mold for pressure-assisted hot sintering under 40 MPa by heating at a rate of 10°C/min. During the sintering process, 150°C was held for 20 min, 250°C for 20 min, in 800°C for 60 min, and 1600°C for 90 min. In this way, a binderless WC-based cemented carbide doped with 2wt% Y,0; was obtained. A sample had a relative density of 96.65%, a hardness of 2156 HV30 and a fracture toughness of 8.2 MPaemi/2., FIG. 7 shows a fracture morphology of the binderless WC-based cemented carbide doped with 2wt% Y:03 in this example. The figure shows that the WC grains are not well bonded, and there are many pores.

The above examples show that the method for preparing a binderless WC-Y:03 cemented carbide by pressure-assisted cold and hot sintering provided by the present disclosure can significantly reduce the sintering temperature of the binderless WC-based cemented carbide, improve the density and comprehensive mechanical properties of the alloy, and has low cost. The above descriptions are merely preferred implementations of the present disclosure. It should be noted that a person of ordinary skill in the art may further make several improvements and modifications without departing from the principle of the present disclosure, but such improvements and modifications should be deemed as falling within the protection scope of the present disclosure.

Claims

CONCLUSIONS

A process for preparing a binderless WC-Y:03 hard metal by pressure-assisted cold and hot sintering, comprising the steps of: ball milling WC powder and Y:03 powder to give WC-Y:0:- to obtain powder; adding a saturated oxalic acid solution to the WC-Y20:3 powder until the powder is completely wet, and subjecting the wet WC-Y;:03 powder to pressure-assisted cold sintering under 200-400 MPa to obtain a sintered body wherein a temperature rise procedure for the pressure assisted cold sintering is as follows: first heating to 100-150°C for 1 hour, and then heating to 200-300°C for 1-2 hours; and subjecting the sintered body to pressure-assisted hot sintering to obtain a binderless WC-Y:0: carbide.

A method according to claim 1, wherein a ratio of the WC-Y,Os i powder to the saturated oxalic acid solution is 10 g: (1-1.5) mL.

The method of claim 1, wherein the Y:0:3 powder constitutes 1-3% of a total mass of the WC powder and the Y2 O3 powder.

The method of claim 1, wherein an average grain size of the WC powder is 200-400 nm.

The method of claim 1, wherein an average grain size of the Y:0 i powder is 50-200 nm.

The method of claim 1, wherein a temperature rise procedure for the pressure assisted hot sintering is as follows: holding at 150°C for 20 min, heating to 250°C for 20 min, then heating to 800°C for 60 min, and finally heating to 1400-1650 °C for 90 min.

The method of claim 6, wherein the pressure-assisted hot sintering is performed below 40 MPa.

The method of claim 1, wherein each heating in the pressure-assisted cold and hot sintering is performed independently at 10-15°C/min.

The method of claim 1, wherein the ball milling at a ball-to-powder ratio (BPR) of (5-15):1 at 200-250 r/min for 6-12 h with the aid of anhydrous ethanol is performed.

The method of claim 1, wherein the method further comprises vacuum drying the ball-milled powder after the ball-milling.

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