**2.5 Conclusion**

174 Ceramic Coatings – Applications in Engineering

coatings. Furthermore not the same degree of certainty can be reached when testing coatings with the high porosity. In further workings this tool will be investigated to work out a measuring concept to classify the reliability of indentation hardness testing of thermal

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

**Coating system Feedstock grain size / Sample Mass increase on substrate** 

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

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




**Cr2O3** -25+5 µm (F1) 90 / 8

**Cr3C2-NiCr** -15+5 µm (F2) 19 / 16

**WC-CoCr** -15+5 µm (F3) 36 / 8

examined by microscopic analysis in order to determine the wear volume loss.

**1.0037 / 1.4301 [ mg]** 

sprayed coatings.

**2.4 Corrosion and wear behaviour** 

resistance of stainless steel.

spray fog.

Fine Cr2O3, Cr3C2-NiCr, and WC-CoCr feedstock with grain sizes below 25 µm were processed in order to investigate the spraying of near net shape coatings. The characteristics of the coatings based on fine powders were analysed and compared to standard coatings based on -45+5/20 µm powder fractions. Compared to standard coatings it was possible to improve the key coating characteristics porosity, surface roughness and corrosion resistance significantly. Other coating properties like hardness or wear resistance showed comparable behaviour as that of standard samples. In case of spraying cermet feedstock, especially Cr3C2-NiCr, optimized parameter sets are necessary to control decarburization and oxidation.
