**3.3 Applying orthogonal arrays for optimizing coatings**

Taguchi techniques were utilized in order to reduce the number of experiments and to evaluate and to adjust main process variables. The effectiveness of these techniques could be verified by spraying validation samples successfully. A Taguchi experimental design was used to reduce the number of coating experiments. Four main process variables, or factors, were identified and varied on three levels in an L9 orthogonal array. This matrix dictated the combination of levels, at which the factors should be set for each experiment, **Table 3**. Furthermore the process output variables, or responses, which should be optimized, were defined. For near net shape coatings a surface roughness being as low as possible is requested. So the aim of the experiments was to obtain a set of spray parameters for each material, which allows the spraying of coatings with low surface roughness under consideration of cost-effective deposition rates. It was also tried to improve coating properties like hardness and porosity. The results were analysed by means of ANOVA to determine the relative contributions of the various main factors and interactions among them. This allowed a prediction for an optimal parameter set for each investigated spraying feedstock.


Table 3. Matrix used for the Taguchi experimental designs

For validation samples were coated using the predicted optimum spray parameters shown in **Table 4**, the results are shown in **Table 5**. The values predicted and actually measured proved to be quite consistent. It can be reasoned that the validation experiments were able to confirm and to reproduce the predicted values.

While comparing validation and standard samples the latter showed higher deposition rates. Of course this has to be ascribed mainly to the fact that coarser spraying feedstock were used to spray the standard samples. The validation samples showed significant lower

noise ratio. The goal of the method is not to optimize one response regardless of other coatings criteria, but to achieve results being robust against the effect of noise factors like e.g. the wear of parts like the electrodes of the plasma gun etc. The signal factors also show effects on the results, but are kept normally constant, like e.g. the traverse speed of the gun relative to the substrate. In the following the results of applying the method are discussed.

Taguchi techniques were utilized in order to reduce the number of experiments and to evaluate and to adjust main process variables. The effectiveness of these techniques could be verified by spraying validation samples successfully. A Taguchi experimental design was used to reduce the number of coating experiments. Four main process variables, or factors, were identified and varied on three levels in an L9 orthogonal array. This matrix dictated the combination of levels, at which the factors should be set for each experiment, **Table 3**. Furthermore the process output variables, or responses, which should be optimized, were defined. For near net shape coatings a surface roughness being as low as possible is requested. So the aim of the experiments was to obtain a set of spray parameters for each material, which allows the spraying of coatings with low surface roughness under consideration of cost-effective deposition rates. It was also tried to improve coating properties like hardness and porosity. The results were analysed by means of ANOVA to determine the relative contributions of the various main factors and interactions among them. This allowed a prediction for an optimal parameter set for each investigated spraying

> **Current (APS) / Air-Fuel-Ratio (HVOF)**

For validation samples were coated using the predicted optimum spray parameters shown in **Table 4**, the results are shown in **Table 5**. The values predicted and actually measured proved to be quite consistent. It can be reasoned that the validation experiments were able

While comparing validation and standard samples the latter showed higher deposition rates. Of course this has to be ascribed mainly to the fact that coarser spraying feedstock were used to spray the standard samples. The validation samples showed significant lower

**1** 1 1 1 1 **2** 1 2 2 2 **3** 1 3 3 3 **4** 2 1 2 3 **5** 2 2 3 1 **6** 2 3 1 2 **7** 3 1 3 2 **8** 3 2 1 3 **9** 3 3 2 1

**Spray** 

**distance Powder carrier gas** 

**3.3 Applying orthogonal arrays for optimizing coatings** 

**Particle size** 

Table 3. Matrix used for the Taguchi experimental designs

to confirm and to reproduce the predicted values.

feedstock.

**Factor Experiment No.**

surface roughness. Especially the carbide based coatings showed low Ra values (about 2.7 µm) compared to the Ra values of the standard samples (near 7 µm). The hardness of the coatings did not vary much regardless of which powder fraction was processed.


Table 4. Predicted optimum spray parameters (the numbers in brackets show the corresponding parameter level)


Table 5. Predicted and measured results obtained from validation and standard samples

It can be summarized, that by applying the method of signal-to-noise ratios derived from the evaluation of orthogonal arrays, the workings could be reduced to nine experiments while investigating the effects of four quantitative parameters on three levels. The results show, that by applying this technique, reproducible forecasts regarding the optimisation of thermal spray coatings can be derived.
