Method for eliminating internal stress of large-scale polycrystalline diamond block

Abstract

The invention discloses a method for eliminating internal stress of a large-scale polycrystalline diamond block material. The method comprises the following steps that the block material is placed in a vacuum heat treatment furnace; carrying out vacuumizing and gas washing operation on the furnace chamber so as to remove residual active gas; introducing high-purity protective gas into the furnace and maintaining positive pressure; executing a stepped heating and heat preservation process: firstly, heating to a first heat preservation temperature T1 at a first heating rate and preserving heat, and then heating to a higher second heat preservation temperature T2 at a second heating rate and preserving heat; and after heat preservation is completed, the controlled cooling process is executed till the furnace temperature is reduced to the room temperature. A strategy of'stepped heating + long-time heat preservation 'is adopted, firstly, the stress of a part of a high-energy area is preliminarily relaxed at a low temperature T1, and cracking caused by stress concentration due to rapid heating is avoided; then, at a higher temperature T2, sufficient energy is provided for atomic diffusion and grain boundary slippage, and deep and uniform release of the internal stress is achieved. And the method is particularly suitable for large-scale blocks.

Description

Technical Field

[0001] This invention belongs to the field of superhard material processing technology, specifically relating to a method for eliminating internal stress in large-scale polycrystalline diamond blocks. Background Technology

[0002] Large-scale polycrystalline diamond bulk materials are advanced materials directly synthesized from graphite under ultra-high pressure (14–18 GPa) and high temperature (2300–2600 K). Due to the extreme pressure and temperature changes involved in their synthesis, they exhibit enormous internal stress. This internal stress poses a serious threat to the material's performance and subsequent applications. 1. Cracking during processing and use: When cutting, grinding, welding or used as a tool / drill bit, the superposition of external load and internal stress can easily cause the material to crack or chip instantly, resulting in a high product scrap rate.

[0003] 2. Reduced performance and lifespan: Internal stress reduces the effective strength and toughness of the material, leading to premature tool failure and decreased wear resistance.

[0004] 3. Affects dimensional stability: Unstable internal stresses can be released over time or with changes in ambient temperature, causing slight changes in the dimensions of precision products, which may not meet the requirements of high-precision applications.

[0005] 4. Damage to optical performance: For applications such as optical windows and lenses, internal stress can cause birefringence, which can damage optical uniformity.

[0006] Currently, heat treatment (annealing) is a common method for eliminating internal stress. However, for large-scale (typically referring to polycrystalline diamond blocks with any dimension exceeding 6 mm) polycrystalline diamond, conventional annealing processes face significant challenges. If the temperature is too low or the time is insufficient, stress release will be incomplete; if the temperature is too high or the time is too long, graphitization transformation can easily occur on the diamond surface and even inside, severely damaging the material's properties. Therefore, developing a dedicated method that can efficiently and safely eliminate internal stress in large-scale polycrystalline diamond while precisely suppressing its graphitization has become a pressing technical challenge in this field. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for eliminating internal stress in large-scale polycrystalline diamond bulk materials. This method, through the design of specific temperature profiles, precise control of heating and cooling rates, and execution under an ultra-pure protective atmosphere, can completely release internal stress while maximally suppressing diamond graphitization, thus ensuring the integrity, dimensional stability, and overall performance of the bulk material.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A method for eliminating internal stress in large-scale polycrystalline diamond bulk materials, comprising the following steps: (1) Loading: Place the large-scale polycrystalline diamond block to be processed into the graphite mold, and place the graphite mold on the tantalum support frame of the vacuum hot pressing sintering furnace or special annealing furnace. (2) Vacuuming and gas washing: Close the furnace door, perform vacuuming and gas washing operations in the furnace cavity, introduce high-purity protective gas into the furnace and maintain positive pressure; (3) Stepwise heating and holding: a. First stage heating: At the first heating rate, the furnace temperature is raised from room temperature to the first holding temperature T1 and held at T1 for 1 to 3 hours, where T1 is 650℃ to 750℃; b. Second stage heating: At the second heating rate, the furnace temperature is raised from T1 to the second holding temperature T2 and held at T2 for 3 to 8 hours, where T2 is 850℃ to 1000℃; The insulation time is adjusted according to the size and thickness of the block material; the larger the size and the thicker the material, the higher the insulation time will be.

[0009] (4) Controlled cooling: After the second stage of heat preservation is completed, heating is stopped and the furnace is cooled. The cooling rate is controlled to not exceed 80°C / h. The protective atmosphere can be turned off only after the furnace temperature naturally drops below 300°C. The furnace is then cooled to room temperature before being removed from the furnace.

[0010] Further, in step (2), the furnace cavity is evacuated to 1.0×10⁻² Pa ~ 5.0×10⁻³ Pa; the gas washing operation is to fill the furnace with high-purity argon gas (purity ≥99.999%) to 0.05MPa~0.08MPa, and then evacuate again to 1.0×10⁻² Pa ~ 5.0×10⁻³ Pa; this is repeated 2~3 times to fully replace and remove residual oxygen and water vapor in the furnace.

[0011] Furthermore, in step (2), the high-purity protective gas is argon with a purity of not less than 99.999%, and positive pressure refers to the furnace pressure of 0.1 ~ 0.5 MPa.

[0012] Furthermore, in step (3), the first heating rate is 100 ~ 150℃ / h.

[0013] Furthermore, in step (3), the second heating rate is 50 ~ 80℃ / h.

[0014] Furthermore, in step (1), the large-scale polycrystalline diamond bulk material refers to polycrystalline diamond material synthesized by high temperature and high pressure method, and whose size in any dimension is not less than 6mm.

[0015] The present invention has the following beneficial effects 1. Highly efficient deep stress release: Employing a "step-up heating + long-term heat preservation" strategy, the stress in some high-energy regions is initially relaxed at a lower temperature T1 to prevent stress concentration and cracking caused by rapid heating. Subsequently, at a higher temperature T2, sufficient energy is provided for atomic diffusion and grain boundary slip, achieving deep and uniform release of internal stress. This is particularly suitable for large-scale bulk materials.

[0016] 2. Effectively inhibits graphitization: By employing ultra-high vacuum pretreatment and dynamic protection with high-purity argon, the oxidation and catalytic graphitization effects of oxygen and water vapor are completely eliminated. Simultaneously, the precisely controlled maximum temperature T2 (≤1000°C) and holding time remain within the safe range of diamond's thermodynamic stability, fundamentally preventing material performance degradation.

[0017] 3. Prevent secondary damage: The slow heating and cooling rates effectively avoid new thermal stress caused by thermal shock, ensuring that the blocks will not crack or deform during the stress relief process, resulting in a high yield.

[0018] 4. Stable process and strong applicability: The process parameters of this method are clear and the repeatability is good. It can be applied to large-scale polycrystalline diamond blocks of different sizes and shapes, such as blanks for cutting tools, optical windows, corrosion-resistant parts, etc., and has good industrial application prospects. Attached Figure Description

[0019] Figure 1 This is a process flow diagram of the present invention; Figure 2 This is a schematic diagram of the temperature-time curve of the heat treatment process described in this invention; Figure 3 The observation of the polycrystalline diamond block material before processing in Example 1 under cross-polarized light; Figure 4 The polycrystalline diamond block material was observed under cross-polarized light after being treated according to Example 1. Figure 5 To observe the condition of the polycrystalline diamond block material of Comparative Example 1 under cross-polarized light after processing; Figure 6 To illustrate the observation of polycrystalline diamond blocks after treatment under cross-polarized light in Comparative Example 2; Figure 7 This is a scanning electron microscope image of the surface of the block material after polishing, as shown in Comparative Example 2. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto.

[0021] Example 1 Taking a polycrystalline diamond block with dimensions of 6mm × 6mm × 1mm synthesized by a high-temperature and high-pressure method as an example, the stress relief treatment is performed using the method of this invention, such as... Figure 1 As shown, the process is as follows: Loading: Place the block material into a graphite mold inside a vacuum hot pressing sintering furnace.

[0022] Vacuuming and gas purging: Evacuate the furnace cavity to 5.0×10⁻³ Pa, then fill it with high-purity argon gas to 0.05MPa, and then evacuate again; repeat this process 3 times.

[0023] Establish a protective atmosphere: Continuously introduce high-purity argon gas into the furnace and control the furnace pressure to be stable at 0.3 MPa.

[0024] Stepped heating and heat preservation, such as Figure 2 As shown: Heat to 700℃ (T1) at a rate of 120℃ / h and hold for 2 hours.

[0025] Then, the temperature is increased to 950℃ (T2) at a rate of 60℃ / h and held for 6 hours.

[0026] Controlled cooling: Stop heating and allow the furnace to cool at a controlled rate of approximately 60℃ / h. Once the temperature drops to 250℃, stop the argon gas supply and allow it to cool naturally to room temperature before removing it. The treated blocks are then inspected using the following methods: Polarized light detection: such as Figure 3 and Figure 4 As shown, when observed under cross-polarized light, compared with the polycrystalline diamond block before treatment, the interference fringes (stress manifestation) inside the block are significantly reduced and the color becomes more uniform, indicating that the macroscopic stress has been largely eliminated.

[0027] X-ray diffraction stress measurement: The residual stress on the surface was measured using the XRD sin²ψ method. The results showed that the stress value dropped from 380 MPa before treatment to below 50 MPa.

[0028] Raman spectroscopy analysis: Multiple points were detected on the surface of the bulk material. A sharp diamond characteristic peak was found only at 1332 cm⁻¹. No obvious D peak and G peak (approximately 1350 cm⁻¹ and 1580 cm⁻¹) were found, proving that no graphitization occurred.

[0029] Processing test: Laser cutting of this block material reduced the edge chipping rate by more than 80% compared to untreated similar blanks.

[0030] Comparative Example 1: Comparison of the effects of insulation temperature and time This comparative example aims to demonstrate the importance of a suitable insulation temperature (T2) and a sufficiently long insulation time for fully releasing the internal stress of polycrystalline diamond blocks.

[0031] Sample: 1 piece of polycrystalline diamond ingot of the same batch and specifications (6mm × 6mm × 1mm) as in Example 1.

[0032] Treatment method (changing only the parameters of the insulation stage): The steps of charging, vacuuming / washing, and establishing a protective atmosphere are exactly the same as in Example 1.

[0033] Stepped heating and heat preservation: The temperature is increased to 700℃ (T1) at a rate of 120℃ / h and held for 2 hours. (This step is the same as in Example 1) Then, the temperature is increased to 850℃ (T2') at a rate of 60℃ / h and held for 2 hours (t2').

[0034] (Note: T2' < 800℃ in Example 1; t2' < 6 hours in Example 1) Controlled cooling: Stop heating and control the cooling rate to approximately 60℃ / h. After the temperature drops to 250℃, stop the argon gas supply and allow it to cool naturally to room temperature before removing it.

[0035] Figure 5 To illustrate the effect of processing Example 1 under cross-polarized light, the color distribution of the interference fringes (stress manifestation) is compared with... Figure 3 Compared to the past, there has been almost no change; Residual stress level: Testing revealed that the residual stress in the comparative sample was significantly higher than that in Example 1. The lower temperature (850°C) was insufficient to allow sufficient migration and rearrangement of amorphous carbon or impurity phases at the grain boundaries of the polycrystalline diamond bulk, while the shorter holding time (2 hours) also failed to allow the thermally activated stress release process to proceed sufficiently.

[0036] Macroscopic integrity: The risk of microcracks appearing on the surface or inside the sample is higher than that of the sample in Example 1, indicating that the stress relief is incomplete and brittle fracture is more likely to occur during subsequent processing or use.

[0037] Raman spectroscopy analysis of Comparative Example 1 (low temperature / short time treatment): The spectrum still has 1332 cm⁻¹ as the main peak, but there may be a slight, broadened scattering background on the low or high wavenumber side of the 1332 cm⁻¹ peak, suggesting that a very small amount of disordered carbon is produced or that the peak broadening is caused by incomplete stress release, but there is usually no obvious D / G separation peak.

[0038] Comparative Example 2: The Effects of Heating and Cooling Rates This comparative example aims to demonstrate the crucial role of slow and controlled heating and cooling rates in preventing thermal stress and achieving uniform and stable elimination of residual stress.

[0039] Sample: 1 piece of polycrystalline diamond ingot of the same batch and specifications (6mm × 6mm × 1mm) as in Example 1.

[0040] Treatment method (changing only the heating / cooling rate): The steps of charging, vacuuming / washing, and establishing a protective atmosphere are exactly the same as in Example 1.

[0041] Stepped heating and heat preservation: Heat to 700℃ at a rate of 300℃ / h (R1') and hold for 2 hours.

[0042] (Note: R1'>120℃ / h in Example 1) Then, the temperature is increased to 950℃ at a rate of 150℃ / h (R2') and held for 6 hours.

[0043] (Note: R2'>60℃ / h in Example 1) Controlled cooling: After heating is stopped, allow the furnace to cool naturally (initial rate >200℃ / h).

[0044] Figure 6 To compare the results of the treatment in Example 2 under cross-polarized light, the interference fringes (stress manifestation) increased significantly and the colors became disordered, indicating that excessively rapid heating and cooling rates can negate or even destroy the stress relief effect, leading to product failure.

[0045] Figure 7 The cracks and chipping that appeared on the block material after polishing in Comparative Example 2 caused the sample to fail.

[0046] Raman spectroscopy analysis of Comparative Example 2 (rapid heating and cooling treatment): In addition to the 1332 cm⁻¹ peak, which may be significantly broadened or shifted due to thermal stress, it is highly likely that distinguishable broad peaks or humps will appear near 1350 cm⁻¹ and 1580 cm⁻¹, indicating that rapid thermal shock has led to local graphitization or amorphization of the surface or grain boundaries, and the material has undergone phase transformation damage.

[0047] (Note: The cooling rate is much greater than the controlled 60°C / h in Example 1) Residual stress measurement results (XRD sin²ψ method) Comparative Example 1 demonstrates that the combination of 950°C and holding for 6 hours in Example 1 of this invention is a necessary condition to ensure that the internal stress of polycrystalline diamond is fully and effectively released. Lowering the temperature or shortening the time will result in incomplete stress relief. The sample treated in Comparative Example 2 not only showed a less effective initial stress relief than that in Example 1, but may also have introduced new thermal stress. Excessively rapid heating (300°C / h and 150°C / h) and cooling (natural cooling) can cause huge temperature gradients between the sample's interior and surface, and between the core and edges, thereby triggering harmful thermal stress.

[0048] Macroscopic integrity: The probability of cracking and breakage in the sample was significantly higher than in Example 1. This is a direct consequence of rapid thermal shock.

[0049] Conclusion: This comparative example demonstrates that the stepped, gentle heating (120℃ / h, 60℃ / h) and controlled cooling (~60℃ / h) employed in Example 1 of this invention are key to avoiding the introduction of secondary thermal stress and achieving uniform and stable elimination of the original residual stress. Excessively rapid heating and cooling rates can negate or even destroy the stress elimination effect, leading to product failure.

[0050] In summary, the method of the present invention successfully provides a safe, efficient, and reliable stress relief solution for large-scale polycrystalline diamond blocks.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the specific implementation of the present invention and not to limit it. Those skilled in the art should understand that any equivalent substitutions or obvious modifications made to the implementation of the present invention without changing its performance or use, without departing from the spirit of the present invention, should be covered within the scope of protection claimed by the present invention.

Claims

1. A method for eliminating internal stress in large-scale polycrystalline diamond blocks, characterized in that, Includes the following steps: (1) Place the block material in a vacuum heat treatment furnace, perform vacuuming and gas washing operations on the furnace cavity, introduce high-purity protective gas into the furnace and maintain positive pressure; (2) Perform a stepped heating and holding process: First, heat the temperature to 650℃~750℃ at the first heating rate and hold for 1~3 hours, then heat the temperature to 850℃~1000℃ at the second heating rate and hold for 3~8 hours; (3) After the heat preservation is completed, the controlled cooling process is carried out until the furnace temperature drops to room temperature.

2. The method according to claim 1, characterized in that, The vacuuming refers to evacuating the furnace to a vacuum level of 1.0×10⁻² Pa ~ 5.0×10⁻³ Pa; the gas washing operation involves first filling the furnace with high-purity protective gas to 0.05MPa~0.08MPa, then evacuating to a vacuum level of 1.0×10⁻² Pa ~ 5.0×10⁻³ Pa, and repeating this step 2~3 times.

3. The method according to claim 1 or 2, characterized in that, The high-purity protective gas is argon with a purity of not less than 99.999%, and positive pressure refers to the pressure inside the furnace being 0.1 ~ 0.5 MPa.

4. The method according to claim 1, characterized in that, The first heating rate is 100 ~ 150℃ / h.

5. The method according to claim 1 or 4, characterized in that, The second heating rate is 50 ~ 80℃ / h.

6. The method according to claim 1, characterized in that, The cooling rate of the controlled cooling process shall not exceed 80°C / h, and the protective gas shall be turned off only after the furnace temperature drops below 300°C.

7. The method according to claim 1, characterized in that, In step (1), the large-scale polycrystalline diamond bulk material refers to polycrystalline diamond material synthesized by high temperature and high pressure method, and whose size in any dimension is not less than 6mm.

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