**3.5.1 Factors affecting adhesion strength of coatings**

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 roughness and low mechanical interlocking [27].

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).

Erosion Behavior of Plasma Sprayed

Table 6. Hardness of Coating Systems

Alumina and Calcia-Stabilized Zirconia Coatings on Cast Iron Substrate 113

780 to 820 HV respectively. It is also observed that for the first three series coating systems, the BC1 thickness is about 50µm and for the next three coatings it is about 100 µm. From this data it can be realized that the microhardness of BC1 decreases as its thickness increases. Similarly, the hardness of TC1 and TC2 decreases with the increase in their thicknesses. Further, the hardness of Al2O3 coatings is found to be more than that of ZrO25CaO coatings. It is also evident that the micro hardness measurements exhibit a wide dispersion. Such dispersion in the microhardness values of the coatings is a typical characteristic of APS

**Substrate BC1 BC2 TC1/TC2** 

ceramic coatings clearly attributable to their microstructural heterogeneity [31].

**CI-S1** 355 150 135 1190 **CI-S2** 340 148 130 1158 **CI-S3** 330 140 140 1110 **CI-S4** 330 160 ----- 820 **CI-S5** 342 145 ----- 800 **CI-S6** 328 150 ----- 780

Hardness of the coating is a measure of the resistance to plastic deformation. It is widely recognized that the hardness increases with the increase in the density, i.e. by decreasing the number of pores and micro-cracks. Therefore the hardness of the top coat is a measure of the amount of sintering and integrity and can provide information about the temperature

Microhardness measurements of coatings have specific implications with regard to their basic science and technological applications. The effective hardness of a microvolume of a material depends on the cooling rate, phase structure, crack size and distribution, residual stress and strain of the local environment as well. Thus by examining the variation of microhardness within the coatings ensures avenues to understand the processing, structure and property relationships of coatings. Hardness tests may be related to the tensile adhesion tests since both these measurements rely on deformation under stress. Moreover, microhardness studies can give the variation of strength and the flaw distribution throughout the specimen, whereas the

Portinha [32] has reported about the decrease in microhardness towards the surface in variable porosity samples and slightly increase in microhardness in case of samples with constant porosity. The decrease in microhardness for the graded samples is attributed to the increase in porosity along the cross section. Samples with constant deposition parameters have exhibited only marginal porosity towards the surface with the increase in the surface temperature during deposition process which also contributes to the enhanced hardness. The variation in microhardness within the given thickness of the coating is due to the variation in local structure which is attributable to the pores or lamellar boundaries. In the present investigation, it is observed that the porosity of Al2O3 and ZrO25CaO increases with the increase in coating thickness of the samples. From the graphs (Fig. 9) it is seen that the hardness of top coat decreases with the increase of porosity. It shows that the coating systems used in the present

**Samples Hardness HV0.3**

history of the top coat. Bond coat hardness has no effect on life of TBC.

strength tests yield the strength of the weakest link of the system.

investigation are in good agreement with the results reported by Portinha.

Another issue to be observed is that the roughness just after bond coat application. From Table.4, it is evident that bond coat roughness increases with increasing the thickness. According to Khan et. al. [29] the adhesion of the coating increases with the increase of substrate roughness or bond coat surface roughness up to certain limits (about 5 µm) and then decreases. In case of CI-S2, CI-S3, CI-S5, CI-S6, the adhesion increases with increase in bond coat roughness. With the increase of bond coat roughness there is an increase in interfacial toughness due to high compressive stresses associated with high rough surfaces but further temperature and pressure from the spray process affect the residual stress profile and thus the interfacial toughness of the coatings.

Limarga et al. [27] have carried out investigations on multilayered thermal barrier coatings in which they have obtained adhesion strengths between 5 and 23 MPa depending on the proportion of bond coat, Al2O3 and ZrO2 in the coating systems. In their tests the majority of failures are found to have occurred inside the ceramic layer. They have registered highest values of adhesion strength when the interfacial bond coat/ceramic failure has occurred. According to the authors, the very low bond strength exhibited by some coating systems is due to the low surface roughness of the sprayed ceramics, where as the mechanical interlocking is negligible. Further, they have found that the low surface roughness correspond to the small particle size of materials used in plasma spraying has affected the bond strength. Using the same analogy in the present work and by considering the top ceramic layer only, it can be observed from Table 4 that the highest roughness values are found with CI-S4, CI-S5 and CI-S6 with ZrO25CaO top coat. These coating systems have highest values of adhesion. The grain size of ZrO25CaO (-53+11m) powder is greater than that of Al2O3 (-31+3.9m) powder which is used as top coat in case of CI-S1, CI-S2, CI-S3 coating systems. This is partially in confirming with that observed by earlier investigator [29]. That is, higher the adhesion strength, higher would be grain size as well as surface roughness.

Lima and Trevisan [30] while working with graded TBCs have found that not only by increasing the thickness, coating adhesion can be decreased but also by increasing the number of coating layers for the same thickness. They have reported that increasing the number of layers has indicated a greater interruption time for the spraying of the subsequent layer due to the time required for making necessary arrangements. In the present work, the only difference with the tested coating systems is the greater number of passes required to deposit Al2O3 on CI-S2, CI-S3 and ZrO25CaO on CI-S5, CI-S6 systems. 40 to 50 % higher number of passes as employed in this investigation implies that an increased number of ceramic interfaces as well as more homogeneous ceramic coating with thinner intermediate layers. This would ensure that the specimen would fracture only at ceramic interfaces in tensile adhesion strength giving higher magnitudes of cohesion.
