COATING POROSITY ANALYSIS

Pores and pinhole defects in coatings can create paths for corrosive fluids or aggressive chemicals to attack and damage the substrate material, quickly leading to critical component failure and substantial repair and downtime costs.

A low porosity coating is essential for gas and fluid control valve components, where a diffusion of gases through a porous coating can result in an explosive gaseous mixture in the pipeline. Porosity can also affect the performance of coated parts in cyclic fatigue conditions where defects can become stress concentrators which initiate mechanical failures.

Hardide coatings are effectively pore-free as they have a volume porosity of <0.5% as measured in accordance with the widely recognised ASTM E2109-01 standard for measuring coating porosity. Under this this method, a cross-section of coating is inspected under high magnification and compared against a series of standard images representing various levels of porosity.

The ASTM E2109-01 standard image below on the left represents the lowest area porosity level of 0.5%. When compared to this, the Hardide-T coating cross-section on the right does not have this level of porosity (in fact the image does not show any visible porosity at all), thus in accordance with ASTM E2109-01 this Hardide coating sample has less than 0.5% porosity.

This exceptionally low porosity is a result of the CVD (chemical vapour deposition) process where the coating is crystallised atom-by-atom from the gas phase. The atoms have high surface mobility and are attracted to the areas of lowest energy such as holes or pores. Any defects are filled as the coating grows. This produces a very dense and impervious coating layer which protects the substrate from corrosion and aggressive chemicals.