**3.5 Adhesion test – Results**

108 Ceramic Coatings – Applications in Engineering

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

In the present work, the porosity of coatings is less than that of coating systems reported by

X-ray diffraction (XRD) patterns for the top surfaces of plasma sprayed Al2O3 and ZrO25CaO coatings are shown in Fig.6. The XRD patterns of Al2O3 coatings show the presence of γ-Al2O3 as a principal phase and α-Al2O3 as minor phase. It shows that oxidation has occurred during spraying by converting hard phase of Al2O3 into soft phase of γ-Al2O3. ZrO25CaO coatings

XRD patterns suggest that Al2O3 particles are not completely transferred into soften γ-Al2O3 phase after the plasma spray process. This is a good result for tribological behavior of coatings where the hardness plays an important role in wear resistance due to abrasion and erosion. The hardness of Al2O3 coating is lower than that of bulk alumina (HV-2045) which is mainly due to the intrinsically lower hardness of γ-Al2O3 than α-Al2O3. The indentation response of a plasma sprayed material is governed not only by the intrinsic hardness of the material, but also by the lamellar microstructure, with splat boundaries giving off under

possess tetragonal ZrO2 as a principal phase and CaZrO3 as minor phase.

Fig. 6. X-Ray Diffraction Diagrams of Al2O3 and (b) ZrO25CaO Coatings

coatings, for the reasons explained earlier.

**3.4 X-ray diffraction analysis of coatings** 

load to facilitate the indenter accommodation [25].

Portinha [24].

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


BC1/Substrate, BC3/Substrate=Adhesive Failure, BC1/BC2, BC1/TC2, BC2/TC1 and BC3/TC2 = Cohesive Failure, Glue=Failure with in Glue (Poor Test)

Table 5. Adhesion Strength and Failure Location of Coating Systems

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

Erosion Behavior of Plasma Sprayed

3, 4 of CI-S4, CI-S5, CI-S6

Alumina and Calcia-Stabilized Zirconia Coatings on Cast Iron Substrate 111

Stress-strain relations (Fig. 8) of the samples show two distinct regions. The first region is due to initial slipping of specimen from the fixtures of UTM. The second region is due to the elongation of specimen at bond coat/substrate interface. Neglecting the strain in first region, the strain percentage in the second region is found to vary between 5 to 6%. It clearly

Fig. 8. Stress v/s Strain Diagram for (a )Samples 1, 1, 4 of CI-S1, CI-S2, CI-S3 (b) Samples 3,

The amount of adhesion can be evaluated based on degree of coverage [26] of the remaining particles which are bonded after testing of bonding strength. Therefore an adhesion test which presents a singular and partial failure means that true adhesion must be evaluated from that area remaining on substrate after the test which is intact and did not detach or fail. This type of failure can be due to the problems associated with spraying process such as residual stresses, inter splats defects or related to test procedure such as sample alignment or traction speed. It is found that interlocking increases with an increase in the density of coat, the velocity of the impinging droplet and roughness of substrate surface and where as it decreases with the increase in the surface tension at substrate/droplet interface. Rebonding of partially melted particles and stress relaxation from local plastic deformation is found to influence the adhesion strength. On the other, in case of multilayered thermal barrier coatings, adhesion strength mainly depends on the proportion of bond coat, top coat and substrate material. Low bond strength is prevalent when there is a low surface

Hadad et. al [28] comparing adhesion tests found that interfacial toughness tends to increase with Ra for thin coatings (140 µm) and in their experiments, an opposite trend is seen for thicker coatings (330 µm). Since the crack propagation into a smooth interface is easier than into a rougher one, the interfacial toughness should increase with Ra and then they concluded that the residual stress effect would be dominant for thicker coatings. In the present work, all of the coating systems tested has similar thickness values as reported by Hadad et. al. and they could be considered as thick coatings. The highest roughness value is for CI-S6 (7.2 µm) that also presents one of the highest adhesion mean values (44.2 MPa).

indicates that the specimens in tensile test are ductile in nature.

**3.5.1 Factors affecting adhesion strength of coatings** 

roughness and low mechanical interlocking [27].

Fig. 7. Fractured Surfaces of Coating Systems after ASTM C633 Tensile Test- Bond Coat/ Substrate Interface

But this simple explanation did not represent the exact behavior of each distinctive coating system. A deep analysis can bring up much more information about the behavior of each individual coating system. It is observed further from Table 5, that the location of the coating failure is at the interface between bond coat and substrate. This is called as adhesion failure. It is seen that in the case of sample 4 of CI-S6, that the failure have occurred at 54 MPa along the glue line. It means that a higher value of adhesion strength could probably found for the above specified samples. Sample 3 of CI-S2, 3 of CI-S4 and 2 of CI-S5 have shown the occurrence of fractures at bond coat/ceramic or bond coat/cermet interfaces respectively. This is attributable to the defects at the bond coat/ceramic interface.

Fig. 7. Fractured Surfaces of Coating Systems after ASTM C633 Tensile Test- Bond Coat/

respectively. This is attributable to the defects at the bond coat/ceramic interface.

But this simple explanation did not represent the exact behavior of each distinctive coating system. A deep analysis can bring up much more information about the behavior of each individual coating system. It is observed further from Table 5, that the location of the coating failure is at the interface between bond coat and substrate. This is called as adhesion failure. It is seen that in the case of sample 4 of CI-S6, that the failure have occurred at 54 MPa along the glue line. It means that a higher value of adhesion strength could probably found for the above specified samples. Sample 3 of CI-S2, 3 of CI-S4 and 2 of CI-S5 have shown the occurrence of fractures at bond coat/ceramic or bond coat/cermet interfaces

Substrate Interface

Stress-strain relations (Fig. 8) of the samples show two distinct regions. The first region is due to initial slipping of specimen from the fixtures of UTM. The second region is due to the elongation of specimen at bond coat/substrate interface. Neglecting the strain in first region, the strain percentage in the second region is found to vary between 5 to 6%. It clearly indicates that the specimens in tensile test are ductile in nature.

Fig. 8. Stress v/s Strain Diagram for (a )Samples 1, 1, 4 of CI-S1, CI-S2, CI-S3 (b) Samples 3, 3, 4 of CI-S4, CI-S5, CI-S6
