**2.1 Achieving near net shape coatings**

Besides metallic feedstock for their electrical and tribological properties as well as repair purposes many ceramic materials can be sprayed. The commonly used feedstock can be

Thermal Spraying of Oxide Ceramic and Ceramic Metallic Coatings 169

The micrographs on the following page show the microstructures obtained by spraying fine feedstock with particle sizes < 25 µm (left side) and conventional fractionated feedstock (-45+5 µm in case of chromia and -45+25 µm for the cermets, right hand side). The parameter settings for the spraying experiments were investigated using methods of designed experiments (for a detailed discussion see chapter 3). After conducting tests regarding a continuous feeding of the different feedstock powders, preliminary test series were conducted to evaluate the effects of the main process parameters regarding the feedstock grain size, the amperage in case of APS and the air-fuel-ratio in case of HVOF, spraying distance and powder feed rate. The results of these experiments were investigated regarding the coatings criteria named at the beginning. For finding optimal parameter sets the economic relevant criteria deposition efficiency and surface roughness were given the highest priority as well as reaching sufficiently high indention hardness at the same time.

The microstructures of the optimum parameter sets for the fine grained feedstock compared to coatings sprayed with conventional fractionated feedstock are shown in **Figure 2**. The metallographic cross sections of the coatings showed, that the porosity of the coatings can be decreased by processing fine powders. Measurements by means of image analysis revealed, that the ratio of porosity in case of the near net shape coatings is approximately only on quarter to one third compared to the conventional coating systems reaching values of 0.1 % in case of the WC-CoCr coating. At the same time the roughness of the top layers described by the profile parameters roughness average (Ra) and height (RZ) is also considerably lower. For all fine feedstock powders Ra values in the range of 2.5 to 2.7 ± 0.1 µm of the as sprayed coatings could be reached, whereas for the usually applied powders the values were significantly higher with 4.5 ± 0.3 µm in case of chromia and 6.7 ± 0.4 µm for the cermet coatings. Furthermore the uniformity of the coatings is significantly better when spraying the fine feedstock permitting the goal of applying near net shape coatings. But these efforts are accompanied by considerably lower deposition rates caused by lower mass throughputs and the difficult heat transfer to the relative high melting NiCr-matrix in case of HVOF spraying of the Cr3C2-NiCr feedstock. On the other hand this disadvantage can be equalized by the aim of achieving coatings of lower thickness resulting in comparable times for the

Then again when spraying the finer powders in the spray process, there is also a higher risk of overheating the small spray particles. In particular the composition of the carbide based coatings can be changed because of decarburization and oxidation effects. The examination of the metallographic cross sections under this aspect showed, that especially the coatings based on fine Cr3C2-NiCr powder showed strong oxidation (see the dark-gray phases in **Figure 2b** left hand side). In order to achieve more information about these phase changes the carbide based samples were analyzed by X-ray diffraction. The obtained X-ray diffraction patterns are shown in **Figure 3**. The pattern of the Cr3C2-NiCr sample sprayed with feedstock -15+5 µm (see lower pattern in Fig. 3 a) shows noticeable Cr2O3 peaks indicating that a strong oxidation of the spray particles took place during the spray process. Furthermore decarburization effects were also stronger when using the fine powder. In the sample sprayed with the standard feedstock, the dominating carbide phase was Cr3C2, whereas the other coating was dominated by the lower carbide phase Cr23C6. For the WC-

spraying process for both the fine and the coarse fractionated feedstock.

**2.2 Comparison of microstructures, phase contents and deposition rates** 

divided into oxide ceramics and the embedding of covalent bound materials like carbides and borides in metallic binder phases (so called cermets derived from "ceramic metals"). Besides the different hardness of the hard phase and the two-phase nature of cermet coatings, the feedstock itself is manufactured by totally different production routes. For both the molten and crushed oxide ceramics and the usually agglomerated and sintered cermet powders there is the trend to use finer grain sizes to reach denser and better microstructures of the coatings on the one hand (Gell, M., et al., 2001; Tilmann et al., 2008a). On the other hand with fine feedstock powders near net shape coatings can be sprayed, showing a comparable low surface roughness, both allowing to reduce the costs of finishing workings (Matthäus, G., Wolf, J. & Ackermann, D., 2010; Tilmann et al., 2008b).

Fig. 1. Classification of Thermal Spray processes with regard to the source of energy (after DIN EN 657 "Thermal Spraying")

In the following the differences in the performance of abrasion and corrosion resistant coatings regarding the deposition efficiency, surface roughness, hardness, porosity, wear behavior and corrosion resistance will be discussed regarding the inset feedstock grain size and the resulting coatings´ microstructure. Several feedstock materials typically used for named fields of operation (WC-CoCr 86/10/4, Cr3C2-Ni20Cr 75/25 and Cr2O3) were considered for developing near net shape coatings. In contrast to grain sizes commonly used in thermal spray processes of up to appr. 50 µm, the grain sizes of all examined powders were specified with a maximum of 25 µm (-15+5 µm, -20+5 µm and - 25+5 µm). Different types of conventional and one specialized powder feeder were investigated regarding their abilities of continuous feeding. For the coating experiments the kerosene fuelled HVOF-gun K2 (GTV GmbH, Luckenbach, Germany) was used to apply the carbide based feedstock materials (WC-CoCr and Cr3C2-NiCr), whereas the conventional APS-gun F4 (Sulzer Metco AG, Wohlen, Switzerland) was used to apply Cr2O3 coatings. Compared to coatings being sprayed using conventional fractionated feedstock, the coatings based on fine feedstock showed better results concerning their key characteristics.

divided into oxide ceramics and the embedding of covalent bound materials like carbides and borides in metallic binder phases (so called cermets derived from "ceramic metals"). Besides the different hardness of the hard phase and the two-phase nature of cermet coatings, the feedstock itself is manufactured by totally different production routes. For both the molten and crushed oxide ceramics and the usually agglomerated and sintered cermet powders there is the trend to use finer grain sizes to reach denser and better microstructures of the coatings on the one hand (Gell, M., et al., 2001; Tilmann et al., 2008a). On the other hand with fine feedstock powders near net shape coatings can be sprayed, showing a comparable low surface roughness, both allowing to reduce the costs of finishing workings (Matthäus, G., Wolf, J. & Ackermann, D., 2010; Tilmann et al.,

Fig. 1. Classification of Thermal Spray processes with regard to the source of energy

In the following the differences in the performance of abrasion and corrosion resistant coatings regarding the deposition efficiency, surface roughness, hardness, porosity, wear behavior and corrosion resistance will be discussed regarding the inset feedstock grain size and the resulting coatings´ microstructure. Several feedstock materials typically used for named fields of operation (WC-CoCr 86/10/4, Cr3C2-Ni20Cr 75/25 and Cr2O3) were considered for developing near net shape coatings. In contrast to grain sizes commonly used in thermal spray processes of up to appr. 50 µm, the grain sizes of all examined powders were specified with a maximum of 25 µm (-15+5 µm, -20+5 µm and - 25+5 µm). Different types of conventional and one specialized powder feeder were investigated regarding their abilities of continuous feeding. For the coating experiments the kerosene fuelled HVOF-gun K2 (GTV GmbH, Luckenbach, Germany) was used to apply the carbide based feedstock materials (WC-CoCr and Cr3C2-NiCr), whereas the conventional APS-gun F4 (Sulzer Metco AG, Wohlen, Switzerland) was used to apply Cr2O3 coatings. Compared to coatings being sprayed using conventional fractionated feedstock, the coatings based on fine feedstock showed better results concerning their

(after DIN EN 657 "Thermal Spraying")

key characteristics.

2008b).
