**3.7.3 Incremental erosion rate and volume erosion of coatings**

Erosion rate is the ratio of incremental mass loss to mass of erodent per impact (5min. test is considered as one impact). Fig. 14 shows a typical plot of erosion rate as a function of cumulative erodent mass impinging the coating at 15, 45 and 900 impact angles. From the graphs it is found that a transient regime has occurred in the erosion process, during which incremental erosion rates decrease monotonically down to a constant steady state value. The starting period of erosion process is also called as incubation period. The decrease in the erosion rate with erosion time or cumulative erodent mass has been reported before [38]. They postulated that, for some brittle materials, initially the target surface is thoroughly cracked with minimum material loss. Then, significant chipping occurs, which leads to a maximum erosion rate. Further particle impact cracking proceeds, with less material removal.

Later, Levy [38] proposed that incremental erosion rate curves of brittle materials start with a high rate at the first measurable amount of erosion and that it then decrease to a much lower steady state value. Another important factor for high initial erosion rate is high surface

Fig. 13. SEM Micrographs of Eroded Surface of Alumina and ZrO25CaO at Different Angles

Erosion rate is the ratio of incremental mass loss to mass of erodent per impact (5min. test is considered as one impact). Fig. 14 shows a typical plot of erosion rate as a function of cumulative erodent mass impinging the coating at 15, 45 and 900 impact angles. From the graphs it is found that a transient regime has occurred in the erosion process, during which incremental erosion rates decrease monotonically down to a constant steady state value. The starting period of erosion process is also called as incubation period. The decrease in the erosion rate with erosion time or cumulative erodent mass has been reported before [38]. They postulated that, for some brittle materials, initially the target surface is thoroughly cracked with minimum material loss. Then, significant chipping occurs, which leads to a maximum erosion rate. Further particle impact cracking proceeds, with less material

Later, Levy [38] proposed that incremental erosion rate curves of brittle materials start with a high rate at the first measurable amount of erosion and that it then decrease to a much lower steady state value. Another important factor for high initial erosion rate is high surface

**3.7.3 Incremental erosion rate and volume erosion of coatings** 

of Impact.

removal.

roughness, where protrusions are easily knocked out from as-sprayed surface. Some insight on the reasons for the solid particle erosion transient as observed in this work can come from the current modeling of brittle erosion. According to it, debris is created due to lateral cracking and intersection between various crack types. The size of these cracks varies with load, or equivalently, impact energy. If one starts a solid particle erosion experiment with a target that has a cracking structure with dimensions lower than expected for the impact energy to be used, the incremental erosion rate should increase as the cracking dimensions increase upto a steady state. If, on the other hand, the cracking dimensions and density are higher than what would be imposed by the experiment impact energy, then the erosion rate should start high and decrease to a steady state value, as the cracking dimensions and density decrease. In the case of plasma sprayed coatings, it is possible that the near surface coating has a defect density higher than the bulk coating. With higher crack density the near surface coating toughness decreases and so does hardness, which according to equation WE~(Cr 2h)α(1/KnHm) [39] where WE , volume loss per impact, Cr , lateral crack size, K and H, coating toughness and hardness, m and n are constants, should determine a higher erosion rate than the bulk coating. Also, since solid particle impact can promote significant surface heating, it is possible that crack closing happens during erosion. The steady state erosion rate is achieved when bond layer of coating systems is exposed to erodent. The steady state erosion is almost same for the systems CI-S1, CI-S2 and CI-S3 but it is different for CI-S4, CI-S5 and CI-S6 and increases with increasing of top coat thickness. The average mass loss of the coatings under steady state erosion rate conditions is taken for comparing the erosion of coatings.

The steady state volume loss of the coatings as a result of erosion at different angles of impact of the erodent is shown in Fig. 14. From this, it is observed that the volume loss is more at 450 angle of impact. The volume erosion loss of cast iron substrate increases with increase of angle of impact showing that the erosion behaviour as brittle. The volume erosion loss of these substrates is less than that of all coating systems. The deference in deformation in uncoated substrates and coating systems can be rationalized based on the deformation response in amorphous and crystalline materials. It is known that amorphous material is prone to shear band formation [40]. Since erosion conditions involve relatively high strain rates, they are quite favorable for shear band formation. The amorphous binder in the plasma sprayed coating systems is expected to form shear bands more easily leading to higher erosion. On the other hand, the deformation in crystalline metal substrates involves strain-hardening leading to higher energy absorption resulting in lower erosion. Cast iron substrate erodes more at 900. This clearly shows that cast iron follows brittle erosion behaviour. Again, it is found that volume erosion loss of alumina coatings is more than that of ZrO25CaO coatings. The volume erosion loss of different coatings at 450 impact is 1.203, 1.23 and 1.12 x10-3cm3 for CI-S1, CI-S2 and CI-S3 (alumina coatings) 0.952, 0.9208 and 0.754 x10-3cm3 for CI-S4, CI-S5 and CI-S6 (ZrO25CaO coatings). Although the cumulative mass loss of ZrO25CaO coatings is more than that of alumina coatings, the volume erosion loss of these coatings is higher. This is mainly due to higher composite density of ZrO25CaO coatings (composite density of ZrO25CaO coatings is about 6.3 to 6.96g cm-3 where as density of alumina coatings are about 2.4 to 2.7 g cm-3).
