**3.3 Coating thicknesses and porosity**

106 Ceramic Coatings – Applications in Engineering

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

Enalarged View of the Region Marked in CI-S6 Showing Micro-cracks

Fig. 4. Topology of ZrO25CaO Coatings on Cast Iron Substrate

around un-melted particles.

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 the coating.

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

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

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 thickness of top coat.

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

Erosion Behavior of Plasma Sprayed

**3.5 Adhesion test – Results** 

Failure Location

> BC3/S BC3/S BC3/S BC3/S

Strength MPa

> 24.8 20.4 18.8 20.8

Cohesive Failure, Glue=Failure with in Glue (Poor Test)

Table 5. Adhesion Strength and Failure Location of Coating Systems

substrate. Fig.7 shows the fracture of samples at substrate/bond coat interface.

Average Thickness (µm)

Coating Type

Samples

Mean Strength MPa

Strength MPa

> 20.4 18.2 22.0 15.8

Alumina and Calcia-Stabilized Zirconia Coatings on Cast Iron Substrate 109

BC TC1/TC2 BC1 BC2 TC1/

CI-S1 80 105 5.7 6.2 6.5 4.8 5.8 3.5

CI-S2 100 275 6.1 6.8 6.9 5.3 5.7 5 CI-S3 95 360 6.4 7.0 7.1 5.1 5.5 5.5 CI-S4 100 100 6.3 ---- 8.2 5.4 ---- 6.2 CI-S5 85 270 6.5 ---- 9.0 5.6 ---- 6.8 CI-S6 85 380 6.4 ---- 9.4 5.5 ----- 7.2

CI-S1 CI-S2 CI-S3 CI-S4 CI-S5 CI-S6

19.1 21.2 28.7 35.9 38.87 44.2

BC1/Substrate, BC3/Substrate=Adhesive Failure, BC1/BC2, BC1/TC2, BC2/TC1 and BC3/TC2 =

The results indicate that the mean values of adhesion for test samples CI-S1to CI-S6. The bond strength is found to increase with the increase in the thickness of the top coat. Analysis of the chemical composition of CI substrate and that of the bond coat layer (BC3) for these samples indicate that the bond coating material consists of as high as 52% of Fe. This will influence the possibility of fusing Fe into the cast iron substrate since the time of exposing the substrate to the plasma spray gun is more in case of samples CI-S2, CI-S3, CI-S5, CI-S6. This probably explains the reason for high adhesion strength of samples coated on cast iron

Strength MPa

> 37.5 34.2 32 35.4

Failure Location

BC3/S BC3/S BC3/TC 2 BC3/S

Strength MPa

> 39.8 34.5 37.6 38.2

Failure Location

> BC3/S BC3/ TC2 BC3/S BC3/S

Strength MPa

> 42.8 46.7 54 45.2

Failure Location

> BC3/S BC3/S Glue BC3/S

Failure Location

> BC3/S BC3/S BC3/S BC3/S

Table 4. Thicknesses, Porosity and Average Surface Roughness of Coatings

Strength MPa

> 30.8 27.4 27 29.2

The location of coating failures during the test is described in Table.5.

Failure Location

BC3/S BC3/S BC2/TC 1 BC3/S

Average Porosity (%)

Avg. Surface Roughness (µm)

> TC1/ TC2

TC2 BC1 BC2

introduced into the coatings when the molten particles are quenched upon impact causing a difference in the coefficients of thermal expansion between the coating and the substrate. Residual stresses are also indirectly affected by the pore structure since the stresses depend upon the elastic modulus and magnitude of strain as well. Porosity of ZrO25CaO coatings is slightly higher than that of Al2O3 coatings which is due to the larger difference in thermal conductivity between the substrate and top coat in comparison with that found with Al2O3 coatings, for the reasons explained earlier.

In the present work, the porosity of coatings is less than that of coating systems reported by Portinha [24].
