2. Specimens coated by plasma spraying
Alumina was coated by plasma spraying on the substrates made of the structural mild steel (400), size of which is 25×25 mm and 2 mm of thickness. The adapted spraying conditions are listed Table 1, where the spraying distance changes from 75 mm to 200 mm. The average diameter of Alumina powderis 25 μ m.
The SEM photographs of as-sprayed specimen surface are shown in Fig. 1 (a)-(d). The profiles of the splats which are the flattened discs after the droplet impingement are so much different depending on the spraying distance. For the short spraying distance the splats are well flattened and in some region there are splashing splats. On the other hand, the droplets are flattened like a pancake when the spraying distance is beyond 150 min. As a result their flattening degree becomes not so large and their periphery shows very smooth. This phenomena is caused by that the splats are formed by the not-well melted droplets. As soon as a small particle is induced into the plasma jet, it instantly melts and gets the maximum temperature around 100 mm from the nozzle exit. Since plasma jet temperature decreases with the distance from the nozzle exit, beyond 100 mm of distance, the molten particle is adversely cooled down by the environmental plasma gas. Eventually the flying droplet is partially solidified, which leads to the splat profile as shown in Fig. 1 (d).
3. Erosion experiments
The coated specimens are eroded by a grit blasting as shown in Fig. 2. The blasting conditions are listed in Table 2. The adapted grit material is a mixture of Al203 (75%) and ZrO2 (25%), as shown in Fig. 3 and its average diameter is 250 μ m. Blasting time depends on the specimens because they have a different wear loss rate.
Fig. 1 As-sprayed surfaces of Alumina coatings
