TWI Frequently asked questions
The liquid displacement method to evaluate bulk material density (ASTM B328) can be readily adapted to thermally sprayed coatings. An alternative liquid displacement method consists of producing coatings on a thin mild steel strip,after which the steel strip is dissolved away by immersion in an acid solution. The remaining strip is used for density measurement. The strip is accurately weighed prior to being coated with a lacquer of known density, and again aftercoating. The coated strip is suspended in a beaker of demineralised and deaerated water and weighed while immersed. The coating density can then been calculated.
It is often necessary to measure the porosity levels in a coating as well as the density. A cross section of the coated test piece is prepared (mounted and polished) and observed using an optical or scanning electron microscope(SEM). Pores and oxide area are determined using an image analyser, and the level of porosity calculated as volume fractions.
The porosity level can also be determined using commercially available mercury intrusion porosimetry (MIP). The method consists of evacuating the sample and immersing in mercury at low pressure. The mercury is then pressurised;corresponding volumes of intruded mercury are measured at a series of pressures. The pore diameter can be calculated. This method enables the total porosity and pore distribution to be determined.
ASTM B328-96 Standard test method for density, oil content and interconnected porosity of sintered metal structural parts and oil-impregnated bearings
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