Thermal Spray Coating Surface Preparation Before Application

Surface preparation for Thermal Spray Coating is more important than the coating itself. If each stage of surface preparation is conducted correctly then the sprayed coating will perform as intended. However, if surface preparation is conducted incorrectly then the coating will fail early, regardless.

Degrease the Substrate First

Clean the substrate of all oil, grease, cutting fluids and machining residues. Solvent wipe using Acetone or Isopropyl Alcohol will suffice for most metal surfaces. However, in extreme cases of contamination, alkaline degrease followed by a water wash may be required prior to surface preparation. The key here is to remove all contamination from the substrate surface prior to grit blasting.

Grit Blast to the Right Profile

Use angular grit to create a mechanical key for the thermal spray coating to bond to. A rounded shot will not create the same mechanical key as angular grit. A number of different types of grit can be used to create the required mechanical key. These include: Aluminium Oxide (AA) grit; Chilled Iron (CI) grit; and others. The surface roughness, in terms of Ra (Arithmetical Mean Deviation), will typically be in the range of 50 to 100 microns (0.05 to 0.1 mm).

Check for Contamination After Blasting

On completion of the blast cleaning, inspect the surface for smears, shadow areas and glossy areas which could indicate the surface has been contaminated with oil. Conduct a water-break test on the surface to determine if the surface is clean. If the water breaks evenly and does not form beads on the surface, then the surface is clean. Any contaminated areas must be cleaned with solvents and re-blasted to the required standard.

Mask Areas That Must Stay Uncoated

Masking areas to be left uncoated – e.g. threads, bearing seats and precision bores – can be achieved by the use of a number of different solutions. High-temperature masking tape, metal plugs, and even custom-turned soft metal caps are all common practice. Ensure that masking edges are tight to prevent subsequent overspray, leading to adhesion problems during subsequent finishing operations.

Control the Time Between Blasting and Spraying

A surface that has been blasted will rapidly oxidize and begin to pick up moisture from the atmosphere. In most cases surfaces will need to be coated within 2 to 4 hours of the surface preparation having been carried out. Some thermal spray coatings however may require coating within hours.

Final Inspection Before the Gun Starts

Replica Tape Measurements – Confirm surface profile after blasting, check temperature of part to ensure above dew point to prevent condensation and complete written inspection records prior to coating application. Brief but vital for quality and future cost avoidance.

For Thermal Spray Coating, visit https://www.poeton.co.uk/surface-treatments/thermal-metal-sprays.

Thorough preparation is not a preamble to the real work – it is most of the work.