Anti-sticking coating of graphite boat plate for sintering hard alloy as well as preparation method and application of anti-sticking coating

Abstract

The invention discloses an anti-sticking coating of a graphite boat plate for hard alloy sintering as well as a preparation method and application of the anti-sticking coating, the anti-sticking coating comprises zirconium oxide, rare earth oxide and a cobalt corrosion resistant phase, and the ratio of the zirconium oxide to the rare earth oxide to the cobalt corrosion resistant phase in the anti-sticking coating is (10-20%): (60-80%): (10-20%); the rare earth oxide can maintain zirconium oxide in a stable tetragonal phase in the sintering process and the cooling process, it is guaranteed that a sample does not have phase change, the cobalt corrosion resisting phase can resist corrosion of cobalt steam in the compact sintering process, the anti-sticking coating can balance the difference value of the thermal expansion coefficient of the coating and the thermal expansion coefficient of the graphite surface, and therefore the quality of the compact is improved. Stress generated in the heating or cooling process is reduced, the coating is prevented from cracking, and the service life of the boat is prolonged; and the coating structure is fixed, powder does not need to be replaced in the spraying process, and the spraying efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of cemented carbide sintering technology, specifically to an anti-stick coating for graphite boat plates used in cemented carbide sintering, its preparation method, and its application. Background Technology

[0002] Cemented carbide possesses properties such as high hardness, high strength, high wear resistance, good thermal stability, and red hardness, making it widely used in the manufacture of cutting tools, mining tools, and wear-resistant parts. The main production processes for cemented carbide include raw material preparation, compact forming, and sintering. The sintering process involves placing the cemented carbide compact on a graphite boat and reacting it at high temperatures to produce a dense cemented carbide product. Various influencing factors in the sintering process have a significant impact on the quality of the final product.

[0003] The binder phases cobalt and nickel in cemented carbide impart toughness and strength. Under high-temperature sintering conditions, they exhibit good wettability with graphite. Liquid cobalt and nickel readily interact with carbon, resulting in mutual diffusion and migration between the sintered cemented carbide and the graphite boat. This leads to cemented carbide products exhibiting boat adhesion and carburization at the bottom of the contact boat, ultimately causing deformation of the contact layer and substandard performance in the cemented carbide products.

[0004] To address the aforementioned issues, the industry commonly uses an anti-stick coating to coat the surface of the graphite boat. However, ordinary brush-applied anti-stick coatings have poor adhesion to the graphite boat, typically requiring only one sintering cycle, resulting in a short boat lifespan. Furthermore, the volatile substances in the anti-stick coating may affect the porosity of the cemented carbide product during sintering. Therefore, researchers have improved upon brush-applied coatings by attempting to replace brush-applied methods with plasma spraying technology. For example, Chinese patent CN110643929A discloses a method for spraying an anti-stick coating onto the surface of a boat. This method involves spraying a silicon carbide / alumina layer as a transition layer and a rare earth oxide layer as the working layer. However, this multi-layer coating structure requires multiple sprayings, leading to low production efficiency. Moreover, due to the large coefficient of thermal expansion between the oxide coating and the graphite boat, an excessively thick oxide coating can easily cause cracking and peeling, allowing graphite to penetrate into the working layer. Over time, this can also lead to sticking to the boat, further affecting its lifespan.

[0005] In summary, it is necessary to develop a non-stick coating that requires only one coating layer to further improve the service life of graphite boat plates. Summary of the Invention

[0006] The main objective of this invention is to provide an anti-stick coating for graphite boat plates used in cemented carbide sintering, as well as its preparation method and application, aiming to improve the service life of graphite boat plates.

[0007] To achieve the above objectives, this invention provides an anti-sticking coating for graphite boat plates used in cemented carbide sintering. The anti-sticking coating comprises zirconium oxide, rare earth oxides, and a cobalt-resistant phase, wherein the ratio of zirconium oxide:rare earth oxides:cobalt-resistant phase in the anti-sticking coating is 10-20%:60-80%:10-20%. In the anti-sticking coating prepared within the above ratio range, the rare earth oxides can maintain zirconium oxide as a stable tetragonal phase at room temperature to 2000℃, ensuring that no phase transformation occurs during cemented carbide sintering at 1350-1600℃, preventing mutual diffusion and migration between the zirconium oxide and the graphite boat. Furthermore, the cobalt-resistant phase resists cobalt vapor erosion during compact sintering. This allows the anti-sticking coating to balance the difference in thermal expansion coefficients between the coating and the graphite surface, reducing stress generated during heating or cooling, preventing coating cracking, extending the service life of the boat, and ensuring a fixed coating structure, eliminating the need for powder replacement during spraying and improving spraying efficiency.

[0008] Furthermore, the cobalt-resistant phase is a powder with a particle size of 1–10 μm. The particle size of the cobalt-resistant phase powder of the present invention is controlled within 1–10 μm, which can improve the flowability of the powder during plasma spraying. The finer powder melts more fully during spraying, thereby reducing the porosity of the coating and contributing to the formation of a denser coating.

[0009] Furthermore, the zirconium oxide and rare earth oxides are powders with a particle size of 15–45 μm. In this invention, the particle size of the zirconium oxide powder and rare earth oxide powder is controlled to be 15–45 μm. A multi-level particle size filling structure is formed between the main components of zirconium oxide or rare earth oxides and the cobalt-resistant fine particles. Large zirconium oxide or rare earth oxide particles form the skeleton, while the cobalt-resistant fine particles fill the gaps, alleviating stress concentration and preventing premature detachment of the anti-stick coating from the graphite surface, thereby improving the service life of the sintered boat plate.

[0010] Furthermore, the thickness of the anti-stick coating is in the range of 0.4 to 0.7 mm. Through extensive experimental research, the inventors of this application have discovered that an anti-stick coating thickness in the range of 0.4 to 0.7 mm yields the best application effect. This thickness is neither too thin to allow for multiple sintering cycles, nor too thick to pose a risk of cracking. A preferred thickness is 0.45 mm. This improves the high-temperature stability and isolation ability of the coating, thereby extending its service life.

[0011] Furthermore, the rare earth oxide is at least one of yttrium oxide, lanthanum oxide, and cerium oxide, and the cobalt-resistant phase is Al2O3. In a cobalt-containing environment, Al2O3, as a continuous and dense oxide film, can effectively isolate cobalt from the external environment, preventing further reaction between cobalt and the graphite boat plate, thereby providing protection. This further improves the high-temperature stability and isolation capability of the coating, and extends the service life of the coating.

[0012] Furthermore, the anti-stick coating comprises 13% zirconium oxide, 70% rare earth oxides, and 17% cobalt-resistant phase by weight percentage. This ratio is optimal, and the coating has a service life of up to 25 applications. After use, the coating does not peel off, stick to the boat, or carburize.

[0013] Another aspect of the present invention provides a method for preparing an anti-stick coating for a graphite boat plate used in cemented carbide sintering, the method comprising:

[0014] The zirconium oxide, rare earth oxide, and cobalt-resistant phase are mixed to form a spray powder;

[0015] The powder is applied to the graphite boat plate using plasma spraying.

[0016] The plasma spraying technology in this invention involves ionizing gas at high temperatures and ejecting it from the spray gun nozzle to form a high-temperature, high-speed plasma jet. Powder entering the plasma jet is rapidly heated to a molten or semi-molten state, forming a sprayed plasma beam that successively impacts the pre-treated substrate surface to form a coating. This eliminates the need to wait for the coating to dry before use, resulting in high production efficiency. It is understood that the preparation method of this invention can achieve the desired coating thickness by increasing the spraying time or slowing down the spraying speed in a single application, but this takes longer than direct two-coat applications and results in a slightly lower powder coverage rate.

[0017] Furthermore, prior to the plasma spraying step, the surface of the graphite boat plate is subjected to sandblasting. Sandblasting further increases the adhesion between the anti-stick coating and the graphite substrate, preventing peeling.

[0018] Furthermore, the process parameters for plasma spraying are: current 40±1A, voltage 590±5V, robotic arm feed rate 16%~20%, spraying angle 80°~90°, and spraying distance 100mm.

[0019] Beneficial effects:

[0020] The purpose of this invention is to provide an anti-sticking coating for graphite boat plates used in cemented carbide sintering. The raw materials for preparing the anti-sticking coating include zirconium oxide, rare earth oxides, and a cobalt-resistant phase. In the anti-sticking coating prepared with a ratio of zirconium oxide: rare earth oxides: cobalt-resistant phase of 10-20%: 60-80%: 10-20%, the rare earth oxides can maintain zirconium oxide as a stable tetragonal phase at room temperature to 2000℃, ensuring that no phase transformation occurs during the sintering of cemented carbide at 1350-1600℃. Furthermore, the cobalt-resistant phase can resist the erosion of cobalt vapor during the pressing and sintering process. This allows the anti-sticking coating to balance the difference in thermal expansion coefficients between the coating and the graphite surface, reducing the stress generated during heating or cooling, preventing coating cracking, and extending the service life of the boat. Moreover, the coating structure is fixed, requiring only one powder preparation, and no powder replacement is needed during spraying, thus improving spraying efficiency. Detailed Implementation

[0021] Unless otherwise specified, the experimental methods described in the following embodiments of the present invention are generally performed under conventional conditions or as recommended by the manufacturer. All commonly used chemical reagents used in the embodiments are commercially available products.

[0022] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention.

[0023] The following embodiments further describe the present invention, but these embodiments are not intended to limit the scope of protection of the present invention.

[0024] Unless otherwise specified, all percentages (%) below are by weight.

[0025] The specific implementation method is as follows:

[0026] Example 1

[0027] A method for preparing an anti-stick coating for a graphite boat plate used in cemented carbide sintering includes the following steps:

[0028] (1) Pre-treatment of graphite boat plate: High-purity graphite boat plate is sandblasted with corundum sand;

[0029] (2) Preparation of anti-stick coating powder: 13% ZrO2, 70% Y2O3 and 17% Al2O3 are mixed in proportion according to mass percentage to obtain an anti-stick coating powder with ZrO2:Y2O3:Al2O3 = 13%:70%:17%; wherein, the particle size of ZrO2 powder is 15-45μm, the particle size of Y2O3 powder is 15-45μm and the particle size of Al2O3 powder is 1-10μm.

[0030] (3) Preparation of anti-stick coating: Plasma spraying powder is performed on the pretreated graphite boat plate. After the first plasma spraying is completed, the program is set to automatically adjust the nozzle angle of the plasma spray gun while other parameters remain unchanged. The second spraying is performed to obtain an anti-stick coating with a thickness of 0.45 mm.

[0031] The graphite boat plate with anti-stick coating prepared in Example 1 was used to sinter cemented carbide products. The cemented carbide products could be sintered continuously for 25 times without any peeling, boat sticking, or carburization. At the same time, it did not affect the performance of the cemented carbide products.

[0032] Example 2

[0033] Unlike Example 1, the composition of the spray powder used to prepare the anti-stick coating and the spraying process are different. In Example 2, 20% 35μm ZrO2, 70% 25μm Y2O3 and 10% 5μm Al2O3 are mixed into a spray powder, and the coating is deposited in a single operation.

[0034] The graphite boat plate with anti-stick coating prepared in Example 1 was used to sinter cemented carbide products. The cemented carbide products could be sintered continuously for 15 times without any peeling, boat sticking, or carburization. At the same time, it did not affect the performance of the cemented carbide products.

[0035] Example 3

[0036] Unlike Example 1, the composition of the spray powder used to prepare the anti-stick coating is different. In Example 3, 17% 20μm ZrO2, 65% 35μm Y2O3 and 18% 8μm Al2O3 are mixed to form the spray powder.

[0037] The graphite boat plate with anti-stick coating prepared in Example 1 was used to sinter cemented carbide products. The cemented carbide products could be sintered continuously for 16 times without any peeling, boat sticking, or carburization. At the same time, it did not affect the performance of the cemented carbide products.

[0038] Comparative Example 1

[0039] The difference between this comparative example and Example 1 is that the composition ratio of the anti-stick coating in Comparative Example 1 is: ZrO2:Y2O3:Al2O3 = 25%:70%:5%. The boat plate with the anti-stick coating prepared in Comparative Example 1 was sintered into a cemented carbide product. The cemented carbide product could be sintered continuously for 14 cycles. After the sintering, coating peeling and carburization occurred, but no boat sticking occurred. It is speculated that this is because the increased amount of zirconium oxide and the decreased amount of cobalt-resistant phase made the anti-stick coating unable to resist cobalt vapor erosion during sintering, resulting in poor thermal stability and the occurrence of peeling and carburization.

[0040] Comparative Example 2

[0041] The difference between this comparative example and Example 1 is that the composition ratio of the anti-stick coating in Comparative Example 2 is: ZrO2:Y2O3:Al2O3 = 13%:70%:17%, and the coating thickness is 0.9mm.

[0042] The boat-shaped cemented carbide product with an anti-stick coating prepared in Comparative Example 2 was sintered 12 times consecutively. After the sintering, coating peeling and carburization occurred, but no sticking to the boat was observed. It is speculated that this is because the coating is thicker. The thicker the coating, the greater the thermal stress generated during high-temperature sintering and cooling, which is more likely to cause coating cracking or delamination. In addition, the internal stress distribution of the thicker coating is more uneven. Especially after multiple thermal cycles, microcracks will expand and eventually cause large-area peeling of the coating, exposing the substrate and causing failure.

[0043] Comparative Example 3

[0044] The only difference between this comparative example and Example 1 is that the composition ratio of the anti-stick coating in Comparative Example 1 is: ZrO2:Y2O3:Al2O3 = 20%:50%:30%.

[0045] The boat-shaped plate with the anti-stick coating prepared in Comparative Example 3 was sintered into a cemented carbide product. The cemented carbide product could be sintered 10 times. After the sintering, the coating peeled off and carburized, but no sticking to the boat occurred. The inventors speculate that this may be because the proportion of rare earth oxides added is relatively low, and ZrO2 is relatively easy to undergo phase transformation during high-temperature sintering, leading to graphite diffusion and transfer to the coating surface.

[0046] Comparative Example 4

[0047] The only difference between this comparative example and Example 1 is that no anti-cobalt phase was added, and 20% ZrO2 and 80% Y2O3 were mixed as a spray powder.

[0048] The boat plate with anti-stick coating prepared in Comparative Example 4 was sintered into a cemented carbide product. The cemented carbide product was sintered 4 times. After the sintering, the coating peeled off and carburized. There was no sticking to the boat. It is speculated that the reason is that no anti-cobalt corrosion phase was added. During the sintering process, the coating was eroded by cobalt vapor.

[0049] Comparative Example 5

[0050] The only difference between this comparative example and Example 1 is that the particle size of the spraying powder is relatively coarse. Specifically, the particle size of ZrO2 powder is 60μm, the particle size of Y2O3 powder is 60μm, and the particle size of Al2O3 powder is 30μm. During spraying, clumping occurs, making spraying difficult, resulting in low powder application rate and substandard spraying thickness.

[0051] The boat plate with the anti-stick coating prepared in Comparative Example 5 was sintered into a cemented carbide product. The cemented carbide product could be sintered five times, after which the coating peeled off, carburized, and stuck to the boat appeared. The particle size of the sprayed powder was relatively coarse, which significantly reduced its fluidity during spraying, making it difficult for the powder to completely melt. The coating surface had pores, resulting in poor application performance.

[0052] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. An anti-stick coating for a graphite boat plate used in cemented carbide sintering, characterized in that, The anti-stick coating comprises zirconium oxide, rare earth oxides, and a cobalt-resistant phase, wherein the ratio of zirconium oxide: rare earth oxides: cobalt-resistant phase in the anti-stick coating is 10-20%: 60-80%: 10-20%.

2. The anti-stick coating of the graphite boat plate for cemented carbide sintering according to claim 1, characterized in that, The cobalt-resistant phase is a powder with a particle size of 1–10 μm.

3. The anti-stick coating of the graphite boat plate for cemented carbide sintering according to claim 1, characterized in that, The zirconium oxide and rare earth oxides are powders with a particle size of 15–45 μm.

4. The anti-stick coating of the graphite boat plate for cemented carbide sintering according to claim 1, characterized in that, The thickness of the anti-stick coating is in the range of 0.4 to 0.7 mm.

5. The anti-stick coating of the graphite boat plate for cemented carbide sintering according to claim 1, characterized in that, The rare earth oxide is at least one of yttrium oxide, lanthanum oxide, and cerium oxide, and the cobalt-resistant phase is aluminum oxide.

6. The anti-stick coating of the graphite boat plate for cemented carbide sintering according to claim 1, characterized in that, The anti-stick coating comprises 13% zirconium oxide, 70% rare earth oxides and 17% cobalt-resistant phase by weight percentage.

7. A method for preparing an anti-stick coating for a graphite boat plate for cemented carbide sintering as described in claims 1-6, characterized in that, The preparation method includes: The zirconium oxide, rare earth oxide, and cobalt-resistant phase are mixed to form a spray powder; The powder is applied to the graphite boat plate using plasma spraying.

8. The method for preparing the anti-stick coating of the graphite boat plate for cemented carbide sintering according to claim 7, characterized in that, Prior to the plasma spraying step, the surface of the graphite boat plate is sandblasted.

9. The method for preparing the anti-stick coating of the graphite boat plate for cemented carbide sintering according to claim 7, characterized in that, The plasma spraying process parameters are: current 40±1A, voltage 590±5V, robotic arm feed rate 16%~20%, spraying angle 80°~90°, and spraying distance 100mm.

10. A graphite boat plate for sintering cemented carbide, characterized in that, The method includes an anti-stick coating for a graphite boat plate for sintering hard alloys as described in any one of claims 1 to 6, and further includes a graphite boat plate, wherein the anti-stick coating is applied to the surface of the graphite boat plate.