Molybdenum Crucible Usage and Maintenance in the Vacuum Coating Industry

The evaporation processes widely used for optical lenses, decorative parts and tool coatings heat a solid coating material to its evaporation temperature, and the vapour condenses on the substrate to form a film. The container holding the material may be a boat or a crucible. Molybdenum, with its high melting point and low vapour pressure, is widely used in both configurations. High-volume lines often track crucible consumption per shift, and maintenance practice directly determines the cost of this consumable.

Evaporation Boats and Crucibles Are Two Different Configurations

A boat is usually a folded molybdenum sheet that heats itself when current is passed through it, with the coating material laid directly in the recess; it suits small batches and multi-point evaporation. A crucible is heated by an external heater and does not conduct current itself; it suits processes with a large melt charge that require a stable evaporation surface. The two cannot be interchanged: using a boat as a crucible leaves both the current circuit and the thermal field wrong, while passing current directly through a crucible causes local overheating that burns through it quickly.

Selection starts with the form of the coating material. Granules roll easily in a boat and spread unevenly; powders conduct heat poorly in a crucible and need pre-melting. Flake and cylindrical charges impose dimensional requirements on the crucible bore, and after loading the evaporation surface must be level with the crucible rim, otherwise the evaporation distance changes as the charge level falls and film thickness uniformity suffers.

Stress Accumulation from Thermal Cycling

Coating is an intermittent process: one heat-up and one cool-down per run. Under repeated thermal cycling the grains in a molybdenum crucible gradually grow, high-temperature strength falls, and fine cracks appear first at the rim and at the bottom corner. Early cracks do not affect service, but they become paths for the coating material to penetrate; the penetrated material reacts with the molybdenum to form brittle phases that extend the cracks further.

Heating and cooling rates are a controllable variable. Too fast a rate creates a large temperature difference between the inner and outer wall and high thermal stress; too slow a rate costs throughput. A compromise is to reduce the rate shortly before reaching the service temperature so that wall temperature equalises, then move into the holding stage.

Wetting and Residue Cleaning

The wettability between the coating material and the crucible determines whether the residue can be removed in one piece. If wetting is too good, residue sticks to the wall and mechanical cleaning scratches the inner surface; if wetting is too poor, the material contracts into a ball inside the crucible and the evaporation surface becomes unstable. Sharp tools and hard scraping should be avoided; instead, melt a charge of the same material at low temperature to carry the residue out, then lightly blast the crucible.

Routine maintenance can follow these points:

• After each shift, let the crucible cool to a safe temperature before removal to avoid thermal shock;
• Collect residue separately by coating material to prevent cross-contamination;
• After cleaning, check the inner wall for pits, cracks and discoloured areas;
• Record the number of runs and re-measure wall thickness once the set number is reached.

Maintenance of evaporation boats follows similar but not identical logic. The boat is thin, warps easily after local overheating, and the contact points of the current circuit also heat up through oxidation. Checking the clamped ends for blackening and deformation each time a boat is replaced avoids power fluctuations caused by poor contact.

How to Set the Maintenance Rhythm

Maintenance intervals should not be set by the calendar alone but tiered by run count and by the corrosiveness of the coating material. For aggressive materials the inspection interval should be shortened. Service should stop when continuous cracks appear on the inner wall, the bottom is clearly thinned, or the rim is deformed.

At procurement, wall thickness tolerance and surface requirements can be agreed with the supplier; where a line handles several coating materials, a small trial batch is worthwhile before batch quantities are fixed against the process configuration in the application field.

Only when the crucible is managed as a process component rather than an ordinary consumable can run records, cleaning rules and replacement criteria actually take hold.