In the manufacturing process of molybdenum crucibles, preventing oxidation is a key link to ensure material performance and product quality. The following systematically explains the specific measures to prevent oxidation from three dimensions: process design, environmental control and post-processing:
- Process design stage
- Powder metallurgy process optimization
Raw material selection: Use high-purity molybdenum powder (≥99.95%) to reduce oxidation catalysis caused by impurities (such as Fe, Ni).
Molding protection:
Cold isostatic pressing: carried out under the protection of inert gas (such as argon) to avoid powder oxidation.
Rubber sheath seal: prevents oxygen from penetrating during the molding process.
Sintering environment: Vacuum sintering (vacuum degree ≤ 10 ⁻ ³ Pa) or hydrogen protection sintering (H ₂ purity ≥ 99.999%) is used to inhibit the formation of MoO ₃.
- Machining protection
Tool selection: Use carbide tools (such as K10 / K20) or CBN tools to avoid the reaction between the tool and molybdenum at high temperatures.
Coolant: Use oil-based cutting fluid or emulsion to reduce cutting temperature and prevent oxidation.
Processing environment: Process in argon atmosphere to reduce air contact.
- Environmental Control
- Inert gas protection
Application scenarios:
The entire process of powder processing, molding, sintering and machining.
Argon purity: ≥99.996%, dew point ≤-60℃ (to prevent moisture from causing oxidation).
Equipment: Equipped with a gas circulation system and an oxygen content monitor (real-time display of O₂ content ≤10 ppm).
- Vacuum environment
Sintering furnace: Use vacuum hot pressing furnace or hot isostatic pressing furnace, and the vacuum degree must be stable at 10 ⁻ ³ ~10 ⁻⁵ Pa.
Inspection: Calibrate the vacuum gauge regularly to ensure that the vacuum degree meets the standard.
- Post-processing and packaging
- Surface passivation treatment
method:
Chemical passivation: Soak in dilute hydrochloric acid (10%) or hydrofluoric acid (5%) for a short time to remove the surface oxide layer.
Electrochemical passivation: Anodic oxidation is carried out in a phosphate solution to form a dense oxide film (thickness ≤ 1μm).
Effect: Significantly reduce surface activity and slow down oxidation rate.
- Anti-oxidation coating
Coating Material:
Molybdenum carbide /nitride (such as Mo₂C, MoN): prepared by physical vapor deposition (PVD) or chemical vapor deposition (CVD).
Ceramic coating (such as Al₂O₃, ZrO₂): resistant to high temperatures (≥1800℃) and isolated from oxygen.
Thickness: 5-20μm, ensuring the coating is dense and free of pores.
- Vacuum packaging
Packaging Materials:
Aluminum foil composite film: vacuumized and heat-sealed to isolate oxygen and moisture.
Glass/ceramic sealed jar: built-in desiccant (such as molecular sieve) for long-term storage.
Detection: After packaging, helium mass spectrometry leak detection is performed to ensure that the leak rate is ≤ 10 ⁻⁹ Pa · m ³ /s.
- Typical application cases
Molybdenum crucible for sapphire single crystal growth furnace
Process Control:
Powder purity ≥ 99.98%, sintering temperature 1900-2100℃ (vacuum degree 10 ⁻³ Pa).
Surface roughness Ra≤0.4μm, coating thickness 10μm.
Performance: Service life ≥ 100 times, oxidation weight gain ≤ 0.1wt %.
Molybdenum crucible for nuclear fusion device
Process Control:
Powder particle size D50 = 2-5μm, density after sintering ≥ 98%.
The coating adopts Mo₂C +ZrO₂ composite layer, and the thermal shock resistance is ≥10 cycles.
Performance: Oxidation rate ≤ 0.01mg /cm²·h at 1200℃.
- Notes
Operation Specifications:
Avoid direct contact between molybdenum products and oxygen-containing tools (such as alumina crucibles).
Wear rubber gloves when handling powder to prevent oxidation caused by sweat.
Equipment maintenance:
Clean the inner wall of the sintering furnace regularly to remove residual oxides.
Check the vacuum pump oil level to prevent oil vapor contamination.
Quality Inspection:
The oxygen content of raw materials is tested when they enter the factory ( ICP -OES method).
Finished product testing must include oxidation weight gain test (1000℃/ 24h ) and metallographic analysis.
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