**3.2 Morphology of coatings**

104 Ceramic Coatings – Applications in Engineering

incremental erosion rate is defined as the steady-state erosion rate. On each coating system,

It is evident that the average roughness values of alumina coatings cast iron substrates vary between 3.5 and 5.5 m and it is between 4.5 and 7.2 m in case of ZrO25CaO coatings. Top coat of test samples such as CI-S4, CI-S5, CI-S6 possess mounds of molten and unmolten particles contribute to the increase in roughness. Flowability of ZrO25CaO is less compared to alumina and this contributes to the formation of mounds and affects the quality of the surface texture of the coating. Increase in porosity as well as the coating thickness enhances the roughness of top coat. Coating roughness also increases with enhanced coating thickness. Similar observations regarding to the effect of coating thickness

Fig.2 shows the roughness profiles of few coating systems (CI-S2 and CI-S6) with average

three tests were conducted. The test parameters are given in Table 3.

Impact Angle (°) 15, 45 and 90 Test Temperature Room Temperature Test Time (min) 5 minutes Cycles Sample Size (mm) 30 x 30 x 5 Nozzle Diameter (mm) 4.5 Stand-off Distance (mm) 10

Average roughness of different coatings is indicated in Table 4.

on roughness were reported by O. Sarikaya [17].

roughness (Ra) as the main parameter.

Fig. 2. Roughness profiles of coating systems

Erodent Size (μm) 150-300 Particle Velocity (m/s) 40 Erodent Feed Rate (g/min) 4.3

Table 3. Erosion Test Parameters

**3. Results and discussion 3.1 Surface texture of coatings** 

Erodent Material Silica Sand (Angular)

Al2O3 coated test samples viz., CI-S1, CI-S2, CI-S3 are characterized by their disc shaped grains (Fig.3). These grains are found to be the flattened solidified droplets of the coating material. The molten particles are found to be distributed more or less evenly producing a smooth coating surface. Enlarged view of marked region of CI-S1 sample (Fig.3) indicates a network of microcracks. Cracks are also observed on the surface of flattened droplets. This may be possibly due to the presence of residual stresses introduced by thermal shocks resulted during the spraying process. Changing the thickness of top coat appears to have no significance on the microstructure as in the case with test samples CI-S2, CI-S3 (Fig.3).

Enlarged View of the Region Marked in CI-S1 Showing Micro-cracks

Fig. 3. Topology of Al2O3 Coatings Cast Iron

Erosion Behavior of Plasma Sprayed

the coating.

**3.3 Coating thicknesses and porosity** 

Alumina and Calcia-Stabilized Zirconia Coatings on Cast Iron Substrate 107

It is observed that the variation in coating thickness (Fig.5) is about ±25 µm from the actual required thickness. This is attributable to the variations in speed of the gun during plasma spraying process. This variation can be minimized by applying Robotic Plasma spraying. Sample polishing technique is also believed to contribute to the variations in the thickness of

Sub-Substrate BC-Bond Coat (BC1+BC2/ BC3+BC2), TC1-Top Coat 1

The porosity of Al2O3 (CI-S1, CI-S2 and CI-S3) coatings are in the range of 5.7 to 6.4% in case of bond coat and it varies between 6.4 to 7.1% in case of top coat. ZrO25CaO coated samples such as CI-S4, CI-S5, CI-S6 also shows pores. The porosity of these coatings varies between 6.3 and 6.8% in case of bond coat and 8.2 to 9.4% in case of top coat. Porosity is high, due to formation of rounded pores which are produced by unmelted particles, splats stacking faults and gas entrapment. Porosity of coatings is found to increase with increase in the

Porosity formation is due to residual stresses present in coatings. It is found to influence the tendency of the coating to de-bond from the substrate [18-23]. Residual stresses are

Fig. 5. SEM Cross-sections of Al2O3 and ZrO25CaO Coatings

thickness of top coat.

ZrO25CaO coated test samples such as CI-S4, CI-S5, CI-S6 exhibit a dense undulated structure (Fig.4). The enlarged view of marked region of CI-S6 sample indicates a network of microcracks (Fig. 4). The sizes of these microcracks appear to be slightly larger than that observed with Al2O3 coated test samples, possibly due to the large difference in the magnitude of thermal conductivity between the substrate and coated material. The thermal conductivity of ZrO25CaO is found between 2 to 4 Wm-1K-1, where as it varies from 33 to 37 Wm-1K-1 for alumina. Since the difference in the magnitude of thermal conductivity between cast iron (50 to 55 Wm-1K-1) substrate and Al2O3 coating is less, heat is transferred more or less effectively through the coating system, resulting lower level of thermal stresses which in turn producing smaller size microcracks. On the other hand, the difference in thermal conductivity of alumina and cast iron substrate and ZrO25CaO is large, higher level of thermal stresses will be developed resulting larger size microcracks. Further, the splats in the coatings are separated by inter-lamellar pores resulting from rapid solidification of the lamellae and very fine void are being formed due to incomplete inter-splat contact in and around un-melted particles.

Enalarged View of the Region Marked in CI-S6 Showing Micro-cracks Fig. 4. Topology of ZrO25CaO Coatings on Cast Iron Substrate
