**3.7.1 Variation of incremental mass loss and cumulative mass loss with time**

Fig. 10 shows the photographs of eroded surfaces of coating systems. In erosion test, the samples were allowed to erode by the erodent until a steady state erosion rate was attained. The mass loss of coating systems after every 5 min test is shown in Fig. 11. It is observed that mass loss is suddenly increases in the first 5min of test. After this, mass loss gradually decreases and attains a steady state. From the graphs of cumulative mass loss with time (Fig. 12), it is found that there are two distinct regions under different angles of impact such as 15, 45 and 900 for all coating systems on cast iron substrate. The first region is belongs to erosion of top coat ceramic layer. During this period the slope of erosion mass loss is high and it occurs for a period of 10 to 15 minutes from the starting of the experiment. At normal impact this slope is little higher compared to other angles. The main reason for higher slope during this period is removal of top coat ceramic material. Ceramic layer is made by brittle material which under goes brittle fracture and especially the rate of brittle fracture is high at normal angles of impact. The second region of the graph is occurred due to eroding of cermet and metallic bond layers in case of samples CI-S1, CI-S2 and CI-S3 respectively and only metallic layer in case samples CI-S4, CI-S5 and CI-S6. In this region the slope of erosion mass loss is small compared to that of first region. This region is available till the coating system reaches a steady state erosion condition.

Microhardness of the coatings increases with the decrease in their porosity. This can be explained based on the principles of microhardness measurements. During the indentation process, a complex elastic-plastic field is formed beneath the indentation. Porosity tends to reduce the effective area supporting the load and is detrimental to strength. When porosity or an equivalent defect is present in a sample, the load bearing area is reduced. It can be safely assumed that the defective region will yield first, thereby inducing strain concentration. However, voids are found to create a multiaxial stress state which can cause local strain concentrations in their vicinity. If all coating systems are considered together, it is obvious that there exists a general tendency that the microhardness decreases with increasing porosity.

**3.7.1 Variation of incremental mass loss and cumulative mass loss with time** 

Fig. 10 shows the photographs of eroded surfaces of coating systems. In erosion test, the samples were allowed to erode by the erodent until a steady state erosion rate was attained. The mass loss of coating systems after every 5 min test is shown in Fig. 11. It is observed that mass loss is suddenly increases in the first 5min of test. After this, mass loss gradually decreases and attains a steady state. From the graphs of cumulative mass loss with time (Fig. 12), it is found that there are two distinct regions under different angles of impact such as 15, 45 and 900 for all coating systems on cast iron substrate. The first region is belongs to erosion of top coat ceramic layer. During this period the slope of erosion mass loss is high and it occurs for a period of 10 to 15 minutes from the starting of the experiment. At normal impact this slope is little higher compared to other angles. The main reason for higher slope during this period is removal of top coat ceramic material. Ceramic layer is made by brittle material which under goes brittle fracture and especially the rate of brittle fracture is high at normal angles of impact. The second region of the graph is occurred due to eroding of cermet and metallic bond layers in case of samples CI-S1, CI-S2 and CI-S3 respectively and only metallic layer in case samples CI-S4, CI-S5 and CI-S6. In this region the slope of erosion mass loss is small compared to that of first region. This region is available till the coating system reaches a steady state erosion condition.

Fig. 9. Variation of hardness with porosity

**3.7 Solid particle erosion test results** 

Fig. 10. Photographs of Eroded Surfaces of Al2O3 and ZrO25CaO Coatings on Cast Iron Substrate 15, 45 and 900 Angles of Impact (Arrow Mark Indicates the Direction of Silica Sand Jet)

Erosion Behavior of Plasma Sprayed

Alumina and Calcia-Stabilized Zirconia Coatings on Cast Iron Substrate 117

Fig. 12. Erosion Rate Plots as Function of Mass of Erodent for Alumina and ZrO25CaO

Cumulative mass loss with time graphs (Fig. 12) shows that the mass loss varies with angle of impinging. According to some engineering model developed so far, for erosive wear, it has been established that the angle at which the stream of solid particles impinges the surface influences the rate at which the material is removed from the surface. This angle determines the relative magnitude of the two velocity components of the impact namely, the component normal to the surface and parallel to the surface. The normal velocity component will determine how long the impact will last and the load. The product of this contact time and the tangential velocity component determine the amount of sliding that takes place. The tangential velocity component also provides a shear loading to the surface, which is in addition to the normal load that the normal velocity component causes. Hence as

Coatings at 15, 45 and 900 Angle of Impacts

Fig. 11. Cumulative Mass Loss Plots as Function of Time for Alumina and ZrO25CaO Coatings on Cast Iron Substrates at 15, 45 and 900 Angle of Impacts

Fig. 11. Cumulative Mass Loss Plots as Function of Time for Alumina and ZrO25CaO

Coatings on Cast Iron Substrates at 15, 45 and 900 Angle of Impacts

Fig. 12. Erosion Rate Plots as Function of Mass of Erodent for Alumina and ZrO25CaO Coatings at 15, 45 and 900 Angle of Impacts

Cumulative mass loss with time graphs (Fig. 12) shows that the mass loss varies with angle of impinging. According to some engineering model developed so far, for erosive wear, it has been established that the angle at which the stream of solid particles impinges the surface influences the rate at which the material is removed from the surface. This angle determines the relative magnitude of the two velocity components of the impact namely, the component normal to the surface and parallel to the surface. The normal velocity component will determine how long the impact will last and the load. The product of this contact time and the tangential velocity component determine the amount of sliding that takes place. The tangential velocity component also provides a shear loading to the surface, which is in addition to the normal load that the normal velocity component causes. Hence as

Erosion Behavior of Plasma Sprayed

removed by subsequent impacts.

ZrO25CaO coatings.

Alumina and Calcia-Stabilized Zirconia Coatings on Cast Iron Substrate 119

impact, but become more pronounced at lower angles as seen from Fig.13. It is apparent that repeated impacts by the hard particles resulted in highly deformed platelets which are

It is well established that in bulk brittle materials such as ceramics, the ratio of particle hardness to the target hardness (Hp/Ht) has a controlling influence in the erosion mechanisms [35, 36]. When this ratio is greater than 1, the wear mechanism essentially involves indentation-induced fracture. At lower ratios cracking is suppressed and the material removal occurs by less severe micro-chipping mechanisms. In the present work, the hardness of erodent (silica) is obtained as 12000 HV. The hardness of Al2O3 and ZrO25CaO top coats are 1120-1180 and 830-850 HV respectively. Since Hp/Ht is higher than 1, top coats undergo splat ejection and indentation-induced material removal mechanism. Kingswell et al. [37] have proposed three basic mechanisms of material removal during erosion of thermal spray coatings depending on their microstructure. In poorly bonded thermal sprayed structures, material loss occurs by splat boundary fracture. As splat cohesion is improved, the dominant material removal process becomes splat fracture, microchipping and ploughing. Evidently, alumina coatings have a microstructure superior than ZrO25CaO coatings. Due to this alumina coatings have greater resistance to erosion than

this angle changes the amount of sliding that takes place also change as does the nature and magnitude of the stress system. Both of these aspects influence the way a material wears. From the graphs (Fig. 12) it can be realized that the erosion mass loss is more at 450 angle of impinging. In most of the materials, solid particle erosion behaviour can be categorized as being either brittle or ductile in nature [7]. The major differentiating characteristic of the two types of mechanism is the dependence of erosion rate on impact angle i.e. the angle between the moving erodent particle and the material surface [8]. There is a general agreement that maximum erosion occurs at a low angle (about 300) for ductile material and at 900 for brittle material. In this investigation, coating systems possess multilayer comprising a ceramic top coat and two intermediate metal and cermet bond coats in case of CI-S1, CI-S2 and CI-S3 and only metallic bond coat in case of CI-S4, CI-S5 and CI-S6. Since the erosion loss is more at 450 angle of impact, it can be realized that the coating systems behave neither as purely ductile (where the maximum loss is expected around 15-300) nor purely brittle (maximum loss is expected at 900) and has a composite behavior. However, the extent of erosion is found to be strongly dependent on impact angle.

## **3.7.2 SEM micrographs of eroded surfaces**

Fig. 13 shows the surface micrographs of worn out region of the coatings at 15, 45 and 900 impact angles. At lower impact angles (150, 450), there are evidences of grooves and ridges (indicated by1) as the material ahead of the erodent is removed by cutting action. Also material removal may occur from the ridges around the grooves by repeated impacts of erodent. The groove formation may predominantly occur within the softer binder region and this may also result in under cutting of the grains, which may get loosened and eventually pulled out. The pull-out of the grains can also be seen in some regions. At 900 impact angle, indentation impressions due to impingement of erodent on the surface are clearly seen. In ductile erosion one of the common mechanisms is the removal of material from the lips that are formed around the impact craters due to strain localization. The material removal may occur from the displaced material forming lips around the indentations as a result of repeated impacts of erodent. Thus the surface morphology shown in Fig. 13 indicates that the predominant mechanisms are grooving of binder phase, cratering and particle pull-out that are prevalent in the coatings. These mechanisms are responsible for composite erosion mode. The appearance of eroded surfaces also indicates that cracks tend to follow a variety of weak sites to produce wear debris. From the morphology of as-sprayed coatings (Fig. 3 and 4) it is observed that all coating systems possess cracks. Linkage of these pre-existing cracks with indentation cracks could have aided the material removal process. The thermal cracks normal to the surface, the interfaces between adjacent layers of splats can be identified as structural weakness for both Al2O3 and ZrO25CaO coatings, as described in [33, 34]. The erosion of plasma sprayed coating of lamellar structure occurs through spalling of surface lamella resulting from impact of abrasives. Accordingly the erosion of coating is controlled by the crack propagation along the interface, i.e., the interface bonding between lamellae. Therefore the erosion of the coatings will be dominated by interface bonding condition and lamellar thickness

The eroded surfaces showed evidence of plastic deformation. Ploughing of the surface by the impinging, sharp silica particles, resulting in groove formation, is evident at all angles of

this angle changes the amount of sliding that takes place also change as does the nature and magnitude of the stress system. Both of these aspects influence the way a material wears. From the graphs (Fig. 12) it can be realized that the erosion mass loss is more at 450 angle of impinging. In most of the materials, solid particle erosion behaviour can be categorized as being either brittle or ductile in nature [7]. The major differentiating characteristic of the two types of mechanism is the dependence of erosion rate on impact angle i.e. the angle between the moving erodent particle and the material surface [8]. There is a general agreement that maximum erosion occurs at a low angle (about 300) for ductile material and at 900 for brittle material. In this investigation, coating systems possess multilayer comprising a ceramic top coat and two intermediate metal and cermet bond coats in case of CI-S1, CI-S2 and CI-S3 and only metallic bond coat in case of CI-S4, CI-S5 and CI-S6. Since the erosion loss is more at 450 angle of impact, it can be realized that the coating systems behave neither as purely ductile (where the maximum loss is expected around 15-300) nor purely brittle (maximum loss is expected at 900) and has a composite behavior. However, the extent of erosion is

Fig. 13 shows the surface micrographs of worn out region of the coatings at 15, 45 and 900 impact angles. At lower impact angles (150, 450), there are evidences of grooves and ridges (indicated by1) as the material ahead of the erodent is removed by cutting action. Also material removal may occur from the ridges around the grooves by repeated impacts of erodent. The groove formation may predominantly occur within the softer binder region and this may also result in under cutting of the grains, which may get loosened and eventually pulled out. The pull-out of the grains can also be seen in some regions. At 900 impact angle, indentation impressions due to impingement of erodent on the surface are clearly seen. In ductile erosion one of the common mechanisms is the removal of material from the lips that are formed around the impact craters due to strain localization. The material removal may occur from the displaced material forming lips around the indentations as a result of repeated impacts of erodent. Thus the surface morphology shown in Fig. 13 indicates that the predominant mechanisms are grooving of binder phase, cratering and particle pull-out that are prevalent in the coatings. These mechanisms are responsible for composite erosion mode. The appearance of eroded surfaces also indicates that cracks tend to follow a variety of weak sites to produce wear debris. From the morphology of as-sprayed coatings (Fig. 3 and 4) it is observed that all coating systems possess cracks. Linkage of these pre-existing cracks with indentation cracks could have aided the material removal process. The thermal cracks normal to the surface, the interfaces between adjacent layers of splats can be identified as structural weakness for both Al2O3 and ZrO25CaO coatings, as described in [33, 34]. The erosion of plasma sprayed coating of lamellar structure occurs through spalling of surface lamella resulting from impact of abrasives. Accordingly the erosion of coating is controlled by the crack propagation along the interface, i.e., the interface bonding between lamellae. Therefore the erosion of the

coatings will be dominated by interface bonding condition and lamellar thickness

The eroded surfaces showed evidence of plastic deformation. Ploughing of the surface by the impinging, sharp silica particles, resulting in groove formation, is evident at all angles of

found to be strongly dependent on impact angle.

**3.7.2 SEM micrographs of eroded surfaces** 

impact, but become more pronounced at lower angles as seen from Fig.13. It is apparent that repeated impacts by the hard particles resulted in highly deformed platelets which are removed by subsequent impacts.

It is well established that in bulk brittle materials such as ceramics, the ratio of particle hardness to the target hardness (Hp/Ht) has a controlling influence in the erosion mechanisms [35, 36]. When this ratio is greater than 1, the wear mechanism essentially involves indentation-induced fracture. At lower ratios cracking is suppressed and the material removal occurs by less severe micro-chipping mechanisms. In the present work, the hardness of erodent (silica) is obtained as 12000 HV. The hardness of Al2O3 and ZrO25CaO top coats are 1120-1180 and 830-850 HV respectively. Since Hp/Ht is higher than 1, top coats undergo splat ejection and indentation-induced material removal mechanism. Kingswell et al. [37] have proposed three basic mechanisms of material removal during erosion of thermal spray coatings depending on their microstructure. In poorly bonded thermal sprayed structures, material loss occurs by splat boundary fracture. As splat cohesion is improved, the dominant material removal process becomes splat fracture, microchipping and ploughing. Evidently, alumina coatings have a microstructure superior than ZrO25CaO coatings. Due to this alumina coatings have greater resistance to erosion than ZrO25CaO coatings.

Erosion Behavior of Plasma Sprayed

Alumina and Calcia-Stabilized Zirconia Coatings on Cast Iron Substrate 121

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

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

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).

It is well understood that the erosion rates are affected by various factors [4, 41-44]. These factors can be broadly classified into three types: impingement variables describing the

**3.7.4 Effect of coating hardness on erosion rate** 

2h)α(1/KnHm) [39] where

decreases and so does hardness, which according to equation WE~(Cr

erosion rate conditions is taken for comparing the erosion of coatings.

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