Voids from incomplete filling between splats, gas trapped during solidification, shrinkage and unmelted particles. It is measured by image analysis of a polished cross section, typically giving under 1 percent for HVOF carbides, a few percent for good plasma metals, 10 to 20 percent for thermal barriers and 20 to 35 percent for porous ceramic abradables. Connected porosity lets corrosive fluids reach the substrate, which is why coatings are sealed.
Porosity on the mount is 4 percent, spec max is 2, so that's a coupon fail on the whole lot.
Bad polishing manufactures porosity: pulled-out particles look like pores, so a surprising number should send the mount back before the parts get rejected.
Also heard as
- pores
- voids
- void content
Related terms
Splat
A single flattened, frozen droplet: the basic building block of a thermal spray coating.
Sealer
A thin liquid resin brushed or soaked into a sprayed coating to fill its connected porosity.
Unmelts
Lab shorthand for particles that hit the surface without fully melting and kept their round shape.
Pullout
Particles torn out of a coating during polishing, leaving holes that look like porosity in the mount.
Mount
A polished cross section of a coated coupon set in resin, used to check structure under the microscope.