Precautions for using ceramic hot press sintering furnace

As a high-temperature and high-pressure precision equipment, the operation of ceramic hot press sintering furnace involves knowledge from multiple fields such as materials science, thermodynamics, and safety engineering. To ensure stable operation of equipment, compliance with material performance standards, and personnel safety, strict control is required from four dimensions: operation specifications, equipment maintenance, safety protection, and process control. The following are specific precautions:

1. Preparation before operation: Details determine success or failure
Equipment inspection
Sealing of furnace body: Before use, a helium leak test must be conducted to ensure that there are no leaks in the furnace door, observation window, and gas interface.
Heating element status: Check the resistance value of the graphite heating rod with an infrared thermometer to avoid local overheating and fracture.
Pressure system calibration: Use standard pressure sensors to verify that the displayed pressure value of the hydraulic system has a small error compared to the actual value.
Material Pretreatment
Powder drying: Ceramic powder needs to be vacuum dried at 120 ℃ for more than 12 hours with low moisture content to prevent porosity during sintering.
Mold assembly standard: When using graphite molds, boron nitride (BN) release agent should be coated on the inner wall of the mold to avoid adhesion between the material and the mold.
Cold isostatic pressing preloading: For porous materials such as silicon carbide, cold isostatic pressing treatment is first performed to increase the green density close to the theoretical value.
atmosphere control
Gas purity: The purity of nitrogen (N ₂) is ≥ 99.999%, and the dew point of hydrogen (H ₂) is ≤ -70 ℃ to prevent the introduction of impurities and the deterioration of material properties.
Atmosphere replacement: Adopting a three-stage process of “vacuuming → gas filling → re vacuuming”, cycling for 3 times to ensure that the oxygen concentration in the furnace is ≤ 1 ppm.

2. Sintering process control: precise control is key
temperature control
Heating rate: segmented control according to material type (such as silicon nitride: heating rate ≤ 10 ℃/min from room temperature to 1000 ℃, 1000-1800 ℃ ≤ 5 ℃/min), to avoid cracking caused by thermal stress.
Insulation platform: Keep at the target temperature for 1-3 hours to ensure temperature uniformity (± 5 ℃) and promote element diffusion.
Cooling strategy: Adopting a “segmented cooling+atmosphere protection” approach (such as cooling with furnace from 1800 ℃ to 1000 ℃, and rapid cooling with argon gas below 1000 ℃) to prevent microcracks caused by phase transition.
pressure control
Timing of pressurization: Start pressurization when the material reaches a semi dense state to avoid deformation of the mold caused by early pressurization.
Pressure maintenance: In the later stage of sintering, the pressure should be stable within the set value range for more than 30 minutes to ensure that the material is completely dense.
Unloading rate: When the temperature drops below 800 ℃, release the pressure at a stable rate to prevent the material from breaking due to sudden pressure changes.
Dynamic adjustment of atmosphere
Nitride ceramics: In the later stage of silicon nitride sintering (1750-1800 ℃), it is necessary to gradually increase the nitrogen pressure to promote the β – →α phase transition.
Carbide ceramics: During the sintering process of silicon carbide, a small amount of methane is introduced to suppress carbon volatilization and maintain the stoichiometric ratio.

3. Security Protection: Risk Pre management
Hydrogen safety
Concentration monitoring: Install hydrogen sensors in the hydrogen usage area, automatically cut off the gas source and start exhaust when exceeded.
Explosion proof design: The furnace body adopts a double-layer water-cooled structure, and the hydrogen pipeline is equipped with a flame arrester to prevent backfire explosion.
Operation specification: Before introducing hydrogen gas, it is necessary to replace the air in the pipeline with nitrogen gas to avoid hydrogen oxygen mixing and explosion.
high-temperature protection
Personal protection: Operators must wear insulated clothing (temperature resistance ≥ 1000 ℃), heat-resistant radiation masks, and high-temperature resistant gloves (contact temperature ≤ 300 ℃).
Equipment protection: Install insulation screens around the furnace body (surface temperature ≤ 60 ℃) to prevent personnel from getting burned; The observation window adopts double-layer germanium glass.
Mechanical safety
Pressure protection: The hydraulic system is equipped with an overpressure alarm device (set pressure to 1.1 times the rated pressure), which automatically releases pressure when overpressure occurs.
Mold fixing: Adopting a dual safety structure of “bolt pre tightening+hydraulic locking” to prevent the mold from loosening and flying out under high pressure.

4. Equipment maintenance and troubleshooting: prevention is better than treatment
routine maintenance
Furnace cleaning: Use a vacuum cleaner to clean the residue inside the furnace after each sintering to avoid impurities contaminating the next experiment.
Heating element inspection: Use an endoscope to inspect the surface oxidation of the graphite heating rod every month. If the thickness of the oxidation layer is ≥ 0.5 mm, it needs to be replaced.
Sealing ring replacement: Replace the furnace door sealing ring (made of fluororubber material) every 50 sintering cycles to ensure sealing.
Common fault handling
Temperature fluctuation: If the temperature deviation is>± 5 ℃, check if the thermocouple wiring is loose or if the heating rod is aging, and replace it if necessary.
Insufficient pressure: If the pressure display value is more than 10% lower than the set value, check the hydraulic pump oil level and oil circuit for leaks, replenish hydraulic oil or replace seals.
Abnormal atmosphere: If the gas flow meter displays unstable values, check if the mass flow controller (MFC) is blocked and clean it with nitrogen back blowing.

5. Process optimization suggestion: data-driven decision-making
Orthogonal experimental design: By using L9 (3 ⁴) orthogonal table to optimize the three factors of temperature, pressure, and insulation time, the process development cycle can be shortened by more than 30%.
Online monitoring system: integrating infrared thermometer, pressure sensor, and gas analyzer, real-time data collection and feedback to the control system, achieving closed-loop control.
Simulation: Use ANSYS or COMSOL software to simulate the temperature and stress field distribution during the sintering process, guide the adjustment of process parameters, and reduce trial and error costs.