Defects extending through the coating (open pores and cracks, grain boundaries) result from the coating The 
itself, from the deposition process and from poor surface quality of the substrates and are detrimental to with 
the intended functioning of coated materials. These openings provide paths for the corrodant to reach the hold 
substrate-coating interface, thereby attack the substrate and promote spalling off the coating. Open defects 
not only hinder the use of these films as corrosion-protective coatings but also create spalling and 
staining problems in other fields such as cutting [4] or decorative applications [5]. 
In the recently published DIN Fachbericht 39 and in the European Pre-Norm (CEN TC 184 WG 5) 
recommended practices are given for the determination of porosity in thin ceramic films [6]. The 
techniques in these practices cover optical and scanning electron microscopic cross sectional 
investigations, electrographic printing and infrared microscopy. With respect to hard, wear-resistant 
ceramic coatings, there are also several other approaches in the literature to determine those defects 
extending through the entire coating: 
Electrochemical techniques [7-9] are used to estimate porosity from the different electrochemical 2.1. 
behaviour of the coating and the substrate in aqueous solutions. In this test the sources of porosity are left 
unidentified. Another approach [10] is to estimate the area of visible defects such as "pores, cracks and The 
nodular cracks" on coated samples by observations using scanning electron microscopy (SEM) without Be 
taking into account the invisible pores or pinholes. In ferroxyl test (similar to electrographic printing) [11] pase 
filter paper wetted by corrosive chloride solution and indicators for iron ions is put on the coated sample fo 
and the number of coloured stains formed on the filter paper were counted; the number of stain spots proc 
gives a rough estimation of open porosity. 
The decoration of the defects extending through the coatings and passive surface layers by noble metals 
such as platinum, gold and copper is valuable in making the defect sites visible [12-16]. Recently, this Bete 
technique has been applied to hard, wear-resistant coatings [17-21]. The examination of the decorated 50% 
samples by SEM can give a detailed picture of the sources of porosity in hard and wear-resistant then 
coatings. the 
meth 
In this investigation defects extending through the coating on PVD TiN, Zr-B and Zr-B-N coated carbon 
steels was evaluated by using copper decoration technique. The capability and limitations of this 
technique is discussed. > 
3.1. 
2. Experimental 
The 
Carbon steel was used as substrate material. Zr-B and Zr-B-N coatings were produced by non-reactive gets 
and reactive dc magnetron sputtering, respectively, using a ZrB, target. TiN coatings were obtained by arc the | 
PVD technique. Coating parameters and the thicknesses of the coatings are given in Table 1. bias 
444  Prakt. Met. Sonderbd. 26 (1995)