Evaluation of tests for porosity and defects in PVD and electroless coatings.

The porosity of electroless nickel, (TiAl)N, TiN and CrN PVD coatings on mild steel having different surface roughnesses was investigated using porosity/corrosion tests. The tests used were the neutral salt spray test, ferroxyl test, sulphur dioxide test and the copper cementation test. Additionally the effect of radii on the corrosion resistance and porosity of electroless nickel coatings was examined. Electron optics, GDOES and ICP were used to evaluate the limits of their applicability to extend porosity/corrosion tests used on coated specimen. A marking technique was developed which enabled individual features on coated surfaces to be examined by SEM before and after porosity/corrosion tests.The neutral salt spray test was used to rank electroless nickel coated mild steel specimens according to their corrosion resistance. However the test did not provide any detailed information on the porosity of the coatings. The ferroxyl test was found to be applicable to all the coatings on mild steel investigated but only macrodefects could be observed. With the help of the SO[2] test the origin of defects in electroless nickel coatings was examined using SEM techniques. Furthermore the test was used to observe the formation of corrosion products at defect sites as a function of exposure time. A ranking according to the corrosion resistance depending upon defects present was made. The SO[2] test was not applicable to (TiAl)N, TiN and CrN PVD coatings since the defect density in these coatings was too high and any corrosion products formed during the test spread over their surface thus hiding smaller defects as a result of the test's high sensitivity. In copper cementation tests on PVD coatings, copper did not preferentially precipitate at large defect sites but precipitated randomly on the coated surface. The amount of copper precipitated, its distribution and size was used to provide information concerning the defect density in PVD coatings. A mechanism has been suggested which explains how copper is deposited on the top of coatings rather than in the bottom of defects during cementation tests. The relationship between porosity, substrate roughness and coating thickness has been established, and it is suggested how the relationship might be used in industry as a quality control tool.