Blasting and Polishing Treatments for Molybdenum Surfaces

The same molybdenum part presents a uniform matte surface after blasting and approaches a mirror finish after polishing; the two conditions behave differently in terms of outgassing in a vacuum furnace and adhesion on a plating line. Surface treatment is not a case of the smoother the better, nor of the coarser the less trouble; define the application first, then settle the process.

Degrease before treatment. Where cutting fluid or rust-preventive oil is present, grit impact presses the oil into the pits and later cleaning struggles to remove it completely, so alkaline cleaning or solvent degreasing before blasting is the more reliable route.

Establish the Roughness Target First

The roughness requirement comes from downstream: where coating adhesion is needed, a moderately rough surface provides mechanical interlocking; for vacuum chamber components or electrode contact faces, an excessively coarse surface increases the actual contact resistance and the outgassing area; for appearance parts, uniformity matters more than the numerical value itself. Only by matching the application to a roughness range do the process parameters have a basis. The link between surface condition and vacuum applications is covered further in the Process Technology section.

Blasting Media and Pressure

Media hardness, grit size and shape together determine removal efficiency and surface morphology. Alumina grit has a strong cutting action and suits removal of oxide scale and burrs; glass beads act gently and produce uniform dimples; stainless steel shot is mostly used for surface peening. Excessive pressure deforms thin parts and over-rounds edges, while insufficient pressure makes uniform cleaning difficult. Nozzle distance and angle affect coverage, so keep the movement steady during operation and avoid dwelling in one spot.

Nozzle wear on the blasting equipment changes the blast angle and flow rate, so the pressure gauge and the condensate drain on the air receiver also need daily checks. When the media absorb moisture and cake, blasting becomes uneven and patches appear on the surface.

Polishing Methods and Their Risks

• Mechanical polishing: high efficiency, but pressure and speed must be controlled to prevent surface overheating and abrasive embedding.
• Electropolishing: suits complex shapes and gives good leveling; the electrolyte and temperature must be managed.
• Chemical polishing: simple to operate, but surface uniformity is affected by the material microstructure.
• Barrel polishing: suits small parts in batches, and edges and corners are rounded.

Over-polishing weakens thin walls and sharp corners and can also smear surface defects flat so that later inspection misses them. If the blasting media contain iron or silicon, embedding in the molybdenum surface can become a contamination source at high temperature; for clean applications, select compatible media and follow with post-cleaning.

The pairing of buffing wheel and polishing compound affects surface morphology; within one product it is unwise to keep switching between consumables of different grit numbers, and cleaning between operations must keep pace. Dust and noise protection are basic items of site management, and exhaust ventilation and filter replacement in the blasting room should follow a fixed cycle. For thin-walled and slender parts, confirm the clamping and support method before blasting and polishing to avoid deformation under load; if a part cannot move directly to the next operation after treatment, apply temporary protection.

Two Steps That Cannot Be Skipped After Treatment

1. Cleaning: remove residual grit, polishing compound and metal chips; ultrasonic cleaning with a suitable solvent.
2. Drying and packaging: avoid water marks and fingerprints; use cleanroom packaging for clean parts and shorten the storage period.

The effect of surface treatment needs to be confirmed by sampling: roughness meter measurement points must cover different areas, appearance is compared under uniform lighting, and adhesion or outgassing tests are run where necessary.