Frequently Asked Questions
Coating quality can be checked by evaluating characteristics such as microstructure, composition and microhardness.
Coating microstructure can be observed in cross section using optical and electron scanning microscopes (SEM). These observations give indications of coating defects such as excessive porosity, high oxide content, and presence of unmelted particles. Coating composition can be determined at different points of the cross section, using energy dispersive spectrometry (EDS). X-ray mapping can be undertaken with an integrated microanalyser and a SEM, providing a chemical mapping of the coating cross section.
Tungsten carbide coatings can be etched to determine the level of tungsten carbide retention. Murakami's reactant is able to etch tungsten carbide (W 2 C) phase and some Co-W-C phases but not the tungsten carbide (WC) grains. By comparing cross section images before and after etching, the level of the different phases can be determined. X-ray diffraction methods can be used to identify and determine the proportion of different crystalline phases in the coating.
Microhardness measurements on the coating cross section can give an indication of the coating quality; such measurements can be made using Vickers or Knoop tests.
All these tests can be carried out at TWI. For further information about Materials & Corrosion Management or please contact us.