3.1 Coating Characterization
The morphology of the three different ceramic coatings were examined with the help of the FE-SEM as shown in Fig. 5. The FESEM image of Fig. 5 (a) is Al2O3 coating, Fig. 5 (b) is Al2O3 -13% (TiO2) coating and Fig. 5 (c) is Al2O3 -40%(TiO2) coating. The examination reveals that coatings possess laminar splats, minor micro-cracks and the partially melted region. Figure 5 (a) and (b) show a few pores which are comparatively more than shown in Fig. 5 (c). Micro-pore and interconnected pores are evident as dark regions in the coatings in Fig. 5 (b). The laminar cracks got generated on the coating surface due to the high temperature flame i.e. that regions were highly exposed to the heat generated by the flame spray torch. During the flame process, powder particles are behaving differently. Some power created a completely melted region while some created partially melted, this may be due to the distribution in the powder particle and also temperature distribution in flame spray torch.
In flame sprayed, the particles were melted due to high temperature flames produced in the gun. These melted droplets accelerated and strike towards the substrate at high speed to form the micro-level laminar structured and dense coating. Due to the high speed of the flame and distance between the substrate and gun, some powder particles were not fully melted during the flying and created the partially melted region on the substrate (see in Fig. 5a). Besides, a few micro cracks and pores were also developed at the time of coating deposition. The cross sectional images of the deposited coating were also taken with FESEM micrographs and element mapping as shown in Fig. 6. The thickness of the different deposited coating ranges from 148 to 152 µm i.e. for Al2O3 coating is 148.8 µm, Al2O3-13%TiO2 is 152.1 µm and Al2O3-40% TiO2 is 151.19 µm. The element mapping reveals the evenly distribution of the powder materials on the surface of the deposited coatings.
The EDS results of the coatings are provided in Fig. 7. It shows the mass percentage of the different elements of the coatings. The XRD pattern of the different deposited coatings was shown in Fig. 8. In Al2O3 coating, the XRD pattern shows the presence of some γ-Al2O3 and alpha α-Al2O3 phases in the deposited coating. The presence of α-Al2O3 was caused by the partially melted pf the powders in the flame spray, as confirmed above in the microstructure. In the 13% TiO2 coating, the pattern shows some rutile TiO2 reacted with Al2O to form Al2TiO5 because of the high temperature during flame spraying. In 40% TiO2, the major phase was rutile tialite Al2TiO5. The tialite phase was formed as a consequence of the reaction between the spraying flame of Al2O3 and TiO2 particle.
The microhardness of the deposited coating with varying TiO2 content is tested using a micro vickers hardness tester and shown in Fig. 9. The value of the microhardness is measured at load 300 gm with a dwell time of 10 seconds. The measured value is taken at an average of five different reading with the same conditions. The microhardness values for Al2O3 is 913.30HV, Al2O3 13% TiO2 is 900.15 HV and for 40% TiO2 is 742.17 HV. The Al2O3 + 13% TiO2 has the highest average microhardness value. The surface roughness of the deposited coatings was obtained. The measured values are 5.3 µm for Al2O3, 6.3 µm for Al2O3-13%TiO2 and 6 µm for Al2O3-40%TiO2.
3.2 Specific Wear Rate
The standard wear test equation for specific wear rate (also known as wear coefficient) is often used in categorizing the resistance to contact wear [37, 38]. Expression for calculating the wear coefficient is given in Eq. 1.
Where, K - specific wear rate (mm3/Nm)
V w - Wear volume (mm3)
W- Load (N)
s- Sliding distance (m)
These wear coefficients are reflected as system dependent quantities i.e. (i) contact materials (ii) surroundings and systems and (iii) operational condition . Also, the wear behaviour of the coatings is subjective to various elements such as porosity, hardness and fracture toughness. These properties result in lower wear rate whereas lower porosity indicates the higher rate of wear of the coatings .
The specific wear rate of the deposited Al2O3, Al2O3 -13%(TiO2) and Al2O3 -40%(TiO2) ceramic coatings is taken at different temperature of 25 oC, 100 oC, 200 oC and 400 oC at constant load 40N and sliding velocity 1m/s is illustrated in Fig. 10. The measured value of specific wear rate for Al2O3 deposited coatings are 0.063684*10− 3 mm3/Nm at temperature 25oC, 0.045265*10− 3 mm3/Nm at 100 oC, 0.034228*10− 3 mm3/Nm at 200 oC. But it slightly increases to 0.072183*10− 3 mm3/Nm at 400 oC. Whereas in Al2O3-13%TiO2 coating the obtained values are 0.040595 *10− 3 mm3/Nm, 0.030219 *10− 3 mm3/Nm, 0.024618 *10− 3 mm3/Nm and 0.026756*10− 3 mm3/Nm with different temperature of 25 oC, 100 oC, 200 oC and 400 oC respectively. In case of Al2O3-40%TiO2 the values are decreasing 0.034567*10− 3 mm3/Nm to 0.014581*10− 3 mm3/Nm with temperature of 25 oC, 100 oC, 200 oC and 400 oC respectively. It is visible (Fig. 10) that the specific wear rate is continuously decreasing with the successive rise in temperature except in the case of Al2O3 coating at 400 oC. From the above discussion, For Al2O3 coating, it is clear that the wear coefficient is decreasing up to 200 oC and suddenly increases at 400 oC due to chippings or spalling from the substrate. The removed splats are broken into more modest hard particles and entrapped in the deeper crater due to continual action sliding cycles. The trapping of hard particles in crater increases the applied stress on the worn surface. It causes the dynamic of the contacting bodies to change, potentially resulting in a high specific wear rate.
It is clear from Fig. 10, at 400 oC for Al2O3 deposited coating the highest specific wear rate is observed i.e. this 0.072183*10− 3 mm3/Nm. The wear properties depend upon the debris characteristics and the contact temperature. This effect could also be explained due to the deformation of the protective layer of the coating. The deposited coating has been plastically deformed and appears to be enough ploughed. This deformation and ploughing results in a large detachment from the substrate. Similar results were also reported about the specific wear rate of Al2O3 coatings rise when the temperature is raised to 400 ◦C . Tetsuya Senda et al.  found that at elevated temperature, the grain size has a significant impact on the wear resistance behaviour of alumina coating. The larger wear grooves of nanostructure coating also confirm the increase in wear rate at 25 oC, 100 oC, 200 oC and 400 oC, respectively.
For Al2O3-13%TiO3, the decrease in the wear coefficient is observed with the increase in temperature of up to 200 oC. However, at 400 oC, Al2O3-13%TiO3 coating shows a slightly increasing trend in specific wear rate which is much lesser than pure Al2O3 coating at the same temperature. This is due to the less spalling/chilling formation in the Al2O3-13%TiO3 coating on the substrate. Whereas a similar specific wear rate trend was observed for Al2O3-40%TiO3 coating till 200 oC. However, at 400 oC Al2O3-40%TiO3 coating shows a gradually decreasing trend in specific wear rate. The addition of TiO2 content in Al2O3 composite coating results in the drop of hardness of the coating which affects the material loss of the coating . Subsequently, it results in added adhesive mode when compared with pure Al2O3. Hence, the adhesive wear mechanism is consistent and dominant as well .
All the even and compacted areas spread in the wear track are attained at 400°C and 200°C as well, but at room temperature, micro-traces of wear scars were observed. At room temperature 100°C, 200°C and 400°C, respectively, conventional wear scars of microstructured coating were formed. At 400°C, there are many smooth and compact patches distributed in the wear scars; a few patches are dispersed at 200°C and 100°C. It can be deduced that as a temperature rises, the area of this smooth and compacted zone in the wear scar grows.
3.3 Coefficient of Friction
Figure 11 depicts the evolution of the coefficient of friction (COF) trend of Al2O3 based ceramic coatings at a constant load of 40 N under the influence of four levels of variable temperatures (25°C, 100°C, 200°C and 400°C) for a sliding distance of 1000m. It can be seen from Fig. 11 that the COF of pure Al2O3 based coating is decreasing with increasing temperature from 25°C to 200°C. While COF was maximum of 0.92 at the maximum operating temperature of 400°C, this may be due to chippings or spalling of the coating from the substrate while rubbing or sliding.
For Al2O3-13%TiO2 coating the COF is decreasing with increasing the temperature i.e. the obtained values are 0.921, 0.795, 0.697 and 0.5863 with temperature 25°C, 100°C, 200°C and 400°C respectively. Similar trends were also seen for Al2O3-40%TiO2 coating where COF ranges from 0.7284 to 0.3901 and temperature ranges from 25°C to 400°C respectively. Hence, the coating Al2O3-40%TiO2 has less coefficient of friction as compared to the other two coatings i.e. Al2O3 and Al2O3-13%TiO2.
The Al2O3-TiO2 coatings and counter body react in a tribo-chemical nature which is controlled and results in the reduction of the COF. The continuous reduction in the friction coefficient can be linked to the actual contact area between the scouring surfaces. At the initial stage of the sliding, the asperities of the surface resist the sliding speed. Meanwhile, the friction causes the eradication of few asperities. The eradicated asperities would result in a change of applied stress on the worn-out surface. Thereby changing the mechanism of two or three body abrasions involved in the wear process.
The decrease in COF is due to the development of the oxide layer on the coating surface which acts as a protective layer. Therefore, the sticking tendency of wear debris and material shear strength gets reduced with the increase of the operating temperature. So, the contact temperature produces softening and melting of the wear debris. These similar trends reported about the tribological properties of Al2O3-TiO2 coatings are also highly determined by test environments, such as temperature and contact load [48, 49]. The wear of deposited Al2O3-TiO2 coatings is decreased in the presence of absorbed environmental moisture . The coefficient of friction decreases with the surge in the operating temperature due to the development of tribo-layer at higher temperatures subsequently the tribo-layer is dense and even, therefore it has a significant role in the reduction of the coefficient of friction .
3.4 Wear Mechanism
The wear behaviour of Al2O3-TiO2 coatings was mainly affected by the microstructure, porosity, micro cracks and mechanical properties like fracture toughness, hardness. Many researchers reported different wear mechanism of the alumina based composite coating. Figure 12A, D, G, J reports the wear behaviours of the Al2O3 based ceramic coatings at the temperature of 25°C, 100°C, 200°C and 400°C respectively with a constant load of 40N and sliding distance of 1000m. At 25°C the Al2O3 coating shows the abrasion wear due to the more roughness and larger surface contact between the substrate and the counter body. While temperature is increasing up to 200°C the wear track is showing a decreasing trend. This is due to the fact of higher temperature stability of the coating at 200–300°C. Another evidence for the mild wear of the coating owing to the existence of oxygen in the wear track results in oxidation of fine wear debris, their accumulation and subsequent development of tribo-film. But at 400°C Al2O3 coating showed the poor wear resistance detachment of the coating particles from the substrate. As there was a complete presence of Al2O3 content in the top layer of the deposited coating.
As per FESEM micrographs of the coatings Al2O3-13% TiO2 at 200 and 400°C, ploughing and plastic deformation marks were observed on the worn surfaces along with areas of brittle fracture. The mechanism appears to be similar for all the wear surfaces. The major difference has been observed only in the brittle fracture areas caused due to ductility and cohesion of the coating. Furthermore, in Al2O3-40%TiO2, the worn surfaces indicated that the material is frequently detached by ploughing and brittle fracture. The main type of wear in the Al2O3-TiO2 coating sliding against stainless steel is a brittle fracture and abrasive behaviour . Initially, the wear mechanism is effectively caused by abrasion and after the bond formation with the substrate, it would lose material by adhesion [14, 52, 53]. Marin et al.,  stated third body wear scars present inside the wear track visible on the surface of the coating.
Different types of morphologies can be identified on the worn surfaces of Al2O3-TiO2 coating after wear tests under variable temperature and constant load (i) material removal for delamination of the surface layers, with pits and micro-cracks randomly distributed over the wear track (Fig. 12), and (ii) metallic film deposition (Fig. 12h). In the first case, during dry sliding, micro-cracks and dislocated networks may yield wear debris as detected inside pits. The wear particles are trapped in the contact interface and are subjected to continuous fracture, distortion or chemical reaction, producing micro-sized powders.
Whereas wear mechanism in Al2O3 with varying percentage of TiO2 coating wears scars, wear track with the flattened surface, minor cracks and severe abrasion, micro brittle fracture mostly reported in the researchers[56, 57]. Oge  observed the fatigue-induced surface cracks and consequent spallation arising due to the plastic deformation of the coating. While at high temperature, Al2O3- TiO2 shows the brittle fracture during sliding wear on the worn surface of the deposited coating at a temperature varying from 25°C to 400°C. This mainly occurs due to the desorption of the moisture at elevated temperature .