**2.4 Corrosion and wear behaviour**

The corrosion resistance of the coatings was determined with salt spray tests according to DIN EN ISO standard 9227. For this purpose mild and stainless steel substrates were coated and were exposed for 240 hours to a corroding atmosphere produced by spraying a sodium chloride solution. The appearance of corrosion products was evaluated every 24 hours. In addition the samples were weighed before and after the test period to determine mass increasing effects caused by formation of corrosion products. During the testing period the mass of the samples increased because of the formation of corrosion products, **Table 2**. In the case of the carbide based coatings the use of the fine fractionated feedstock lead to a considerable improvement in terms of corrosion resistance, the samples sprayed with fine powders showed significant less mass increases than the standard fractionated samples. The Cr2O3 coatings showed a quite contrary behaviour. This is due to the fact, that the chromia coatings received no sealing treatment leaving, so that the salt media could reach the substrate through the thin coating more easily compared to the thicker conventional sample. The coatings on stainless steel substrates showed the same behaviour like the coatings on mild steel substrates. But of course the actual values were lower due to the higher corrosion resistance of stainless steel.


Table 2. Results of the corrosion tests: mass increase of coated samples exposed 240 h in salt spray fog.

The wear resistance of the coatings was evaluated by ball-on-disk wear tests according to ASTM standard G 99. The ball-on-disk test is a model test for determining friction and wear of two solid surfaces being in sliding contact (ball against coated disk). A sintered WC6Co ball (10 mm in diameter) fixed into a steady ball holder was pressed against the coated and polished sample disk (105 mm in diameter) with a normal load of 40 N. The disk rotated 2500 cycles with a linear speed of 0.1 m/s. After the experiments the wear track was examined by microscopic analysis in order to determine the wear volume loss.

The results of the wear tests after 2500 cycles showed different results for each spray feedstock material. The Cr2O3 coatings regardless whether based on fine or standard powder fractions showed almost no volume loss. According to optical micrographs of the wear scars a tribofilm was formed consisting of plastically deformed debris and splats. This tribofilm was smoother than the original surface and was placed slightly above the mean line of the unworn surface protecting the surface from further wear, see **Figure 6a**. During the experiments of Cr3C2-NiCr samples measurable wear scars were formed, **Figure 6b**. The volume loss was higher on the coating based on the fine powder (0.33 mm3) whereas the wear rate of the standard sample was a bit lower (0.24 mm3). Apparently the above-mentioned phase changes which occurred while processing the fine Cr3C2-NiCr powder influenced the wear behaviour negatively. On the contrary the WC-CoCr coatings regardless whether based on fine or standard powder fractions did not suffer a measurable volume loss. In fact the sintered WC6Co ball was abraded instead of the coatings. This is probably due to the surface finish of the coatings as fine and carbide rich hard grooves (see **Figure 3c)** abraded the Co-matrix of the ball.

a) Cr2O3 validation sample (E1) b) Cr3C2-NiCr validation sample (E2) c) WCCoCr validation sample (E3)

Fig. 6. Optical micrographs of wear scars after 2500 cycles in ball on disk-tests
