AU2020100541A4

AU2020100541A4 - Method for preparing oxidation-resistant coating for pure tungsten by modifying with rare earth element yttrium and aluminizing by embedding

Info

Publication number: AU2020100541A4
Application number: AU2020100541A
Authority: AU
Inventors: Dongguang LIU; Laima Luo; Xiaoyue Tan; Yucheng Wu; Xiang ZAN; Yuxiang Zhang; Xiaoyong ZHU
Original assignee: Hefei University of Technology
Current assignee: Hefei University of Technology
Priority date: 2019-04-16
Filing date: 2020-04-09
Publication date: 2020-05-21
Anticipated expiration: 2028-04-09
Also published as: CN109881146A

Abstract

The present invention discloses a method for preparing an oxidation-resistant coating for pure tungsten by modifying with a rare earth element (REE) yttrium (Y) and aluminizing by embedding. The surface of the tungsten is modified with the REE Y and is aluminized by embedding. The surface of the obtained coating exhibits uniform granular protrusions without obvious voids or cracks. The Y-modified and aluminized coating of the present invention effectively overcomes the easy stripping defect of the aluminide coating. The coating was tested by 1000°C cyclic oxidation. After the Y modified aluminide coating was oxidized for 10 h, an oxide film on the surface of the coating was dense and complete without stripping, showing excellent high-temperature oxidation resistance.

Description

METHOD FOR PREPARING OXIDATION-RESISTANT COATING FOR PURE TUNGSTEN BY MODIFYING WITH RARE EARTH ELEMENT YTTRIUM AND ALUMINIZING BY EMBEDDING

TECHNICAL FIELD

The present invention relates to a method for preparing an oxidation-resistant coating for pure tungsten by modifying with a rare earth element (REE) yttrium (Y) and aluminizing by embedding, and belongs to the high-temperature oxidation-resistant field of pure tungsten.

BACKGROUND

The scientific development and industrial improvement imposes higher demand for hightemperature alloys. Tungsten is a common refractory metal, which has high melting point, high strength, low thermal expansion coefficient and low tritium retention. Tungsten has a wide range of applications in key components, such as heat shields, combustion chamber liners, rocket nozzles, military heavy alloy armor-piercing projectiles, spallation materials and fusion reactors. However, tungsten oxidizes at 400°C, and the oxidation can intensify with the increase of temperature. This will affect the stability of tungsten microstructures, leading to the deterioration of high-temperature mechanical properties, and limiting the high-temperature application of tungsten.

At present, there are two main ways to improve the oxidation resistance of refractory metals at home and abroad. First, materials with high surface hardness and high toughness are prepared, or the strength and hardness of metals are improved by processes such as alloying, cold deformation and heat treatment. Second, metal surfaces are treated in various ways to prepare an oxidation-resistant coating that is well bonded with the substrate. At present, the interface between the coating obtained by the commonly used coating preparation method and the substrate does not form a good interdiffusion state, so the bonding force is weak. During the high-low temperature alternating cycle, as the coating does not match the thermal expansion coefficient of the substrate, the coating is easy to crack and strip. As a result, the substrate will still oxidize to cause the component to fail. Therefore, it is necessary to modify the coating by adding an active element to improve the oxidation resistance of the coating. Previous studies have shown that Y can improve the adhesion of the oxide film and increase the anti-stripping ability. At present, the oxidation resistance of rare earth Y-modified coatings on the surface of pure tungsten has not been thoroughly studied at home and abroad. Therefore, the research and realization of Y-modified oxidation-resistant coatings on the surface of pure tungsten is of great significance to the high-temperature application of tungsten.

SUMMARY

The present invention aims to provide a method for preparing an oxidation-resistant coating for pure tungsten by modifying with a REE Y and aluminizing by embedding, and provides a hightemperature oxidation-resistant protective coating for pure tungsten. The REE Y-modified coating i

2020100541 09 Apr 2020 has a high bonding strength with a tungsten substrate and a uniform and controllable thickness, and has no obvious crack on the surface of the coating.

The present invention provides a method for preparing an oxidation-resistant coating for pure tungsten by modifying with a REE Y and aluminizing by embedding, including the following steps:

step 1: pretreatment of substrate polishing a pure tungsten substrate sequentially by 120-grit, 240-grit, 400-grit, 600-grit and 800grit silicon carbide sandpapers, and ultrasonically cleaning and drying to obtain a pure tungsten sample with a clean and flat surface;

step 2: preparation of aluminizing agent mixing an aluminum powder (aluminum source), a REE Y (modifier), sodium fluoride (activator) and alumina (filler) uniformly in a ball mill according to a certain ratio to obtain an aluminizing agent for future use;

step 3: embedding filling the aluminizing agent prepared in step 2 into a ceramic crucible and compacting to half the depth of the ceramic crucible; then, spreading the pure tungsten in the aluminizing agent; finally, continuing to fill the ceramic crucible with the aluminizing agent and compacting until the pure tungsten is completely embedded, where the present invention has no special requirement for a ratio of the aluminizing agent to the pure tungsten, so long as the pure tungsten is completely embedded; and step 4: preparation of oxidation-resistant coating placing the tungsten-embedding ceramic crucible obtained in step 3 in a tube furnace with a highpurity argon atmosphere, and raising the temperature to 900-1100°C; holding the temperature for 36 h; then cooling to room temperature; ultrasonically cleaning with absolute ethanol for 10-20 min; drying to obtain a 45-100 pm thick oxidation-resistant coating on a surface of the pure tungsten.

In step 2, the aluminizing agent is composed of 25-35% of aluminum (Al) powder, 3-7% of sodium fluoride (NaF) powder, 3-10% of yttrium (Y) powder and 53-65% of alumina (AI2O3) powder.

A raw Al particle has an average size of 4.5 pm, a raw NaF particle has an average size of 3 pm, a raw AI2O3 particle has an average size of 5 pm, and a raw Y particle has an average size of 3 pm.

In the present invention, the REE Y is used to modify the surface of the tungsten and the aluminizing agent is used to embed the tungsten. The surface of the obtained coating exhibits uniform granular protrusions without obvious voids or cracks.

The present invention has the following beneficial effects.

The Y-modified aluminide coating improves the high-temperature oxidation resistance of the pure tungsten. The coating has a simple preparation process, a good bonding force and a uniform and dense structure. The Y-modified and aluminized coating of the present invention effectively overcomes the easy stripping defect of the aluminide coating. The coating was tested by 1000°C

2020100541 09 Apr 2020 cyclic oxidation. After the Y-modified aluminide coating was oxidized for 10 h, an oxide film on the surface of the coating was dense and complete without stripping, showing excellent high-temperature oxidation resistance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an X-ray diffraction (XRD) spectrum of a surface of a Y-modified and aluminized oxidation-resistant coating of pure tungsten. FIG. 1 shows that the surface of the Y-modified and aluminized oxidation-resistant coating of pure tungsten according to the present invention is a single phase AUW.

FIG. 2 is a field emission scanning electron microscope (FE-SEM) image of a surface of a Ymodified and aluminized oxidation-resistant coating of pure tungsten. FIG. 2 shows that there are uniform granular protrusions distributed on the surface of the coating, but no holes or cracks are seen.

FIG. 3 is a diagram showing a micro-indentation of a surface of a Y-modified and aluminized oxidation-resistant coating of pure tungsten. FIG. 3 shows that there are no obvious cracks around the micro-indentation.

FIG. 4 is an oxidation kinetic curve after 10 h oxidation of a Y-modified and aluminized oxidation-resistant coating of pure tungsten. FIG. 4 shows that after 10 h oxidation, a mass gain of the Y-modified and aluminized oxidation-resistant coating is 18.55-21.23 mg/cm², which is far lower than that of pure tungsten.

DETAILED DESCRIPTION

The present invention is further described with reference to the examples and accompanying drawings.

In order to verify the effect of the present invention, a Y-modified silicide coating was prepared on the surface of a pure tungsten substrate having a rolling ratio of 50%, and the coating was subjected to a cyclic oxidation test at 1000°C in the atmosphere for 10 h.

Example 1

1. Pretreatment of substrate: A pure tungsten substrate was polished sequentially by 120-grit, 240-grit, 400-grit, 600-grit and 800-grit silicon carbide sandpapers, and was ultrasonically cleaned and dried to obtain a pure tungsten sample with a clean and flat surface.

2. Preparation of aluminizing agent: 30% of aluminum powder (Al, aluminum source), 3% of sodium fluoride powder (NaF, activator), 3% of yttrium powder (Y, modifier) and 64% of alumina powder (AI2O3, filler) were stirred for 4 h at the speed of 300 rpm in a ball mill to obtain an aluminizing agent. A raw Al particle had an average size of 4.5 pm, a raw NaF particle had an average size of 3 pm, a raw AI2O3 particle had an average size of 5 pm, and a raw Y particle had an average size of 3 pm.

2020100541 09 Apr 2020

3. The uniformly mixed aluminizing agent and the polished tungsten were sealed in a ceramic crucible. Then the ceramic crucible was placed in a tube furnace, evacuated at room temperature, filled with argon to atmospheric pressure, and heated to 900°C, holding the temperature for 6 h.

An X-ray diffraction (XRD) analysis showed that the obtained coating was an AI4W phase. The coating was subjected to an oxidation test at 1000°C for 10 h in the atmosphere. The results showed that the oxide film on the surface was dense and complete, and had a high-temperature oxidation resistance and a mass gain of 18.55 mg/cm² after 10 h.

Example 2

2. Preparation of aluminizing agent: 25% of aluminum powder (Al, aluminum source), 5% of sodium fluoride powder (NaF, activator), 10% of yttrium powder (Y, modifier) and 60% of alumina powder (AI2O3, filler) were stirred for 4 h at the speed of 300 rpm in a ball mill to obtain an aluminizing agent. A raw Al particle had an average size of 4.5 pm, a raw NaF particle had an average size of 3 pm, a raw AI2O3 particle had an average size of 5 pm, and a raw Y particle had an average size of 3 pm.

3. The uniformly mixed aluminizing agent and the polished tungsten were sealed in a ceramic crucible. Then the ceramic crucible was placed in a tube furnace, evacuated at room temperature, filled with argon to atmospheric pressure, and heated to 1000°C, holding the temperature for 5 h.

An X-ray diffraction (XRD) analysis showed that the obtained coating was an AI4W phase. The coating was subjected to an oxidation test at 1000°C for 10 h in the atmosphere. The results showed that the oxide film on the surface was dense and complete, and had a high-temperature oxidation resistance and a mass gain of 20.39 mg/cm² after 10 h.

Example 3

2. Preparation of aluminizing agent: 35% of aluminum powder (Al, aluminum source), 7% of sodium fluoride powder (NaF, activator), 5% of yttrium powder (Y, modifier) and 53% of alumina powder (AI2O3, filler) were stirred for 4 h at the speed of 300 rpm in a ball mill to obtain an aluminizing agent. A raw Al particle had an average size of 4.5 pm, a raw NaF particle had an average size of 3 pm, a raw AI2O3 particle had an average size of 5 pm, and a raw Y particle had an average size of 3 pm.

2020100541 09 Apr 2020

3. The uniformly mixed aluminizing agent and the polished tungsten were sealed in a ceramic crucible. Then the ceramic crucible was placed in a tube furnace, evacuated at room temperature, filled with argon to atmospheric pressure, and heated to 1100°C, holding the temperature for 4 h.

An X-ray diffraction (XRD) analysis showed that the obtained coating was an AI4W phase. The coating was subjected to an oxidation test at 1000°C for 10 h in the atmosphere. The results showed that the oxide film on the surface was dense and complete, and had a high-temperature oxidation resistance and a mass gain of 21.23 mg/cm² after 10 h.

Claims

What is claimed is:

1. A method for preparing an oxidation-resistant coating for pure tungsten by modifying with a rare earth element (REE) yttrium (Y) and aluminizing by embedding, comprising the following steps:

step 1: pretreatment of substrate:

polishing a pure tungsten substrate sequentially by 120-grit, 240-grit, 400-grit, 600-grit and 800grit silicon carbide sandpapers, and ultrasonically cleaning and drying to obtain a pure tungsten sample with a clean and flat surface;

step 2: preparation of aluminizing agent:

mixing an aluminum powder (aluminum source), a REE Y (modifier), sodium fluoride (activator) and alumina (filler) uniformly in a ball mill according to a certain ratio to obtain an aluminizing agent for future use;

step 3: embedding:

filling the aluminizing agent prepared in step 2 into a ceramic crucible and compacting to half the depth of the ceramic crucible; then, spreading the pure tungsten in the aluminizing agent; finally, continuing to fill the ceramic crucible with the aluminizing agent and compacting until the pure tungsten is completely embedded; and step 4: preparation of oxidation-resistant coating:

placing the tungsten-embedding ceramic crucible obtained in step 3 in a tube furnace with a highpurity argon atmosphere, and raising the temperature to 900-1100°C; holding the temperature for 36 h; then cooling to room temperature; ultrasonically cleaning with absolute ethanol for 10-20 min; drying to obtain an oxidation-resistant coating on a surface of the pure tungsten.
2. The preparation method according to claim 1, wherein in step 2, the aluminizing agent is composed of 25-35% of aluminum (Al) powder, 3-7% of sodium fluoride (NaF) powder, 3-10% of yttrium (Y) powder and 53-65% of alumina (AI2O3) powder.
3. The preparation method according to claim 2, wherein a raw Al particle has an average size of 4.5 pm, a raw NaF particle has an average size of 3 pm, a raw AI2O3 particle has an average size of 5 pm, and a raw Y particle has an average size of 3 pm.
4. The preparation method according to claim 1, wherein the oxidation-resistant coating obtained in step 4 has a thickness of 45-100 pm.