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

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

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-

Thermal Spraying of Oxide Ceramic and Ceramic Metallic Coatings 171

One characteristic criterion determining the wear resistance of thermal sprayed coatings is the hardness, which is usually measured by indentation techniques. The Vickers hardness indentation test is well-established both in the course of the quality management of job shops as well as in the characterization of coatings reported in literature. Another technique is the superficial Rockwell hardness testing, by means of that the coatings can be analysed without metallographic preparation. To investigate the suitability of both methods and the influences on the measurement results, a cause-and-effect diagram was established for the indentation testing of thermal spray coatings (see **Figure 4**). The goal of the workings was the reduction of the variability of the measuring results to enhance the

Fig. 4. Cause-and-effect diagram of the indentation hardness measurement of coatings

Fig. 3. X-Ray diffraction patterns of Cr3C2-NiCr (a) and WC-CoCr (b) coatings

**2.3 Indentation hardness** 

comparability.

CoCr samples the effect of decarburization was examined by determining the intensity ratio of the strongest WC in relation to the W2C peak (IW2C(100)/IWC(100), see Fig. 3 b). Similar values of 0.22 in case of the fine and 0.17 for the coarser fractionated feedstock were obtained indicating a stronger decarburization when spraying the fine feedstock. Compared to the Cr3C2-NiCr samples these effects of phase chances were quite low.

Fig. 2. Comparison of achievable microstructures using fine powder feedstock < 25 µm grain size (left) and conventional more coarsely fractionated feedstock -45+5/20 (right) for Cr2O3 (a), Cr3C2-NiCr (b) and WC-CoCr (c)

Fig. 3. X-Ray diffraction patterns of Cr3C2-NiCr (a) and WC-CoCr (b) coatings
