Surface modification of plasma sprayed Al2O3–40 wt% TiO2 coatings by pulsed Nd:YAG laser melting

Optics & Laser Technology 48 (2013) 366–374

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Surface modification of plasma sprayed Al2O3–40 wt% TiO2 coatings by pulsed Nd:YAG laser melting Jagadeesh Sure, A. Ravi Shankar, U. Kamachi Mudali n Corrosion Science and Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 April 2012 Received in revised form 18 September 2012 Accepted 20 September 2012 Available online 7 December 2012

Plasma sprayed Al2O3–40 wt% TiO2 coatings were subjected to a pulsed Nd:YAG laser melting with power densities of 640 and 800 kW/cm2 in order to minimize the surface porosity, micro-cracks, partially melted or unmelted regions, and achieving homogeneous microstructure. As-sprayed and the laser melted surface microstructure, chemical composition, microhardness and roughness were characterized by using scanning electron microscopy-energy dispersive X-ray spectroscopy, X-ray diffraction, Vickers hardness and profilometer. Surface morphology of a laser melted coating showed that inhomogeneities are reduced, however, network of cracks were formed irrespective of power density. Columnar growth features were observed in laser melted regions when melted with 640 kW/cm2 while these were eliminated when melted with in 800 kW/cm2 power density. Predominant b-Al2TiO5 phase was observed upon both the laser melted coatings. With an increase of laser power density, laser melted coatings exhibited considerable increase in microhardness and decrease in surface roughness due to significant reduction in the surface defects. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Al2O3–40 wt% TiO2 Pulsed laser Microstructure

1. Introduction Pyrochemical reprocessing using molten chloride salt is the best option for spent metallic fuels of future fast breeder reactors in India [1,2]. For undertaking various unit operations such as salt purification and cathode processing, in the high temperature molten chloride salt environment crucibles and liners made of graphite will be utilized [3,4]. Since graphite undergoes degradation in molten salt by removal of carbon particles and molten uranium by attaching to graphite crucibles and liners [5–8], protective ceramic coating is required on graphite crucible. This is required to minimize the reaction between molten uranium and graphite and release of ingot [3] as well to restrict the molten salt attack [6,7]. After each cathode processor operation, mechanical cleaning are mandatory for graphite crucibles to reuse [3,4], and this leads to generation of solid waste. Hence, to avoid the reaction and protecting graphite from molten uranium and salt, ceramic coatings are proposed to avoid the contamination [6,7,9]. Plasma spraying is one of the extensively used processes in the group of thermal spray to improve the surface properties of the

n Correspondence to: Corrosion Science and Technology Group, Reprocessing R&D Division, Homi Bhabha National Institute, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Tel.: þ91 44 27480121; fax: þ 91 44 27480301. E-mail addresses: [email protected] (J. Sure), [email protected] (A.R. Shankar), [email protected] (U.K. Mudali).

0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.09.025

materials [10,11]. Mesrati et al. [12,13] sprayed Al2O3 and ZrO2 coatings on graphite with bond coat (SiC/Cr3C2) to increase the oxidation resistance in air. Partially stabilized zirconia (PSZ) coating provided satisfactory protection to high density (HD) graphite from molten LiCl–KCl salt up to 2000 h at 600 1C [6,7,9]. Al2O3 alloy with TiO2 ceramics are popular in thermal spray industry due to easy spray and environmental compatibility [14,15]. The composite of Al2O3 with TiO2 has better performance than that of the individual oxides. Alumina-13 wt% titania coating was vacuum plasma sprayed on graphite to protect from oxidation [16]. The presence of TiO2 in the Al2O3 powders contributes to lower porosity in the coating [17]. The microhardness of Al2O3TiO2 composite coatings depends essentially on their composition. The hardness decrease was observed linearly with either increase of TiO2 content or decrease of alumina content [18,19]. The doping of TiO2 into alumina can partially stabilize the a-phase [20]. Plasma sprayed Al2O3–40 wt% TiO2 (PSA40T) coatings are used as protective coatings against wear [18,19,21] and corrosive media [22]. For increasing adhesion strength between substrate and ceramic coating a bond coat is required. The adhesion strength between graphite and oxides like Al2O3 and ZrO2 was improved by using bond coats such as Cr3C2 or SiC [12,13]. The sprayed bond coat has to minimize the stress at the substrate-coating interface and increase the adhesion strength of the coating [23]. PSA40T coatings with bond coat of Cr3C2–25%NiCr have been proposed over the HD graphite by plasma spray process for better

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service and controlling the reactivity of graphite [24–26]. The presence of porosity, partially and unmelted particles, and microcracks are drawbacks in the sprayed coating. Generally these defects will affect the mechanical properties and decrease the corrosion resistance of top ceramic coating by penetration of molten slat. More ever, the corrosive species penetrate through these defects which lead to increase in the attack on sprayed coating as well as on substrate. Corrosion testing of Al2O3–40 wt% TiO2 (A40T) coating in molten LiCl–KCl salt at 600 1C for 500 h revealed that molten salt was penetrated through the surface connected porosity and interlamellar pores. As the exposure time increases the degradation of A40T coating increases and also the Cr3C2–NiCr bond coat oxidizes after 2000 h of exposures [25,26]. Hence, laser melting of A40T coating is an approach to reduce the penetration of salt by decreasing the surface porosity, generation of an external dense top layer and consolidation of as-sprayed microstructure for better surface properties. Lasers are powerful tools to modify the microstructures of materials for better properties [27]. Laser melting is one of the well established post spray treatments for modifying the surface properties of plasma sprayed alumina–titania based coatings [14,15,28]. The spallation of the sprayed coating exposed to molten salt has been reported due to less cohesive strength between the sprayed particles the presence of porosity, partially and unmelted particles and microcracks [29,30]. Thus, considerable interest had been shown on the application of laser technology to assist thermal-spraying process to improve the overall performance of the coatings [31]. Hence, in order to improve the properties of the PSA40T coatings, laser melting process was applied to melt the surface to achieve a homogeneous, consolidated and dense outer layer. Sreedhar et al. [28] modified the surface of alumina–titania coatings with variation of power intensity, process speed, and beam focusing distance [28]. Laser melted coatings showed improved wear [32], corrosion [33] resistance and enhancement in the microhardness [33]. Laser melting improved the durability of plasma sprayed coatings because the segmented cracks that generated during melting are beneficial for accommodating thermal stress, which resulted in two to six times higher durability [34]. Laser melting increases the spalling resistance of plasma sprayed Al2O3–TiO2 coatings to thermal shock [35]. Tomaszek et al. [14] analyzed microstructural modification of laser engraved plasma sprayed TiO2 and Al2O3 þTiO2 coatings for electron emitter applications. Studies were carried out on laser melting of PSZ coated 316L stainless steel in order to seal surface porosity and generate an external dense layer for better corrosion resistance towards molten LiCl– KCl salt [11,36–38]. Laser melting of plasma sprayed coatings showed increased corrosion resistance of the coating with low number of defects [39]. Laser treatment of coatings provides better surface structure by reducing the surface porosity and other defects in terms of shape, size and number which in turn offers a positive influence on the mechanical and corrosion properties of coatings [39,40]. Post treatments performed on plasma sprayed coatings modified the microstructure up to few microns thickness of the coating [41]. Pulsed lasers can be used for melting of very thin surface layer of ceramic coatings to form a solidified layer on coating because of high thermal gradients [41,42]. Morimoto et al. [43] performed EPMA studies on diode laser melted thermally sprayed Cr3C2–NiCr coatings to investigate the elemental distribution across the cross section of coatings after laser treatment. Limited studies have been reported on pulsed Nd:YAG laser melting of PSA40T coatings [28]. The present investigation focuses its on the application of high power pulsed mode Nd:YAG laser source to modify the surface of atmospheric PSA40T coatings on HD graphite. The aim of the present study is to characterize the pulsed laser melted surface with respect to

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process parameters like pulse energy and power density for consolidating the surface of PSA40T for better properties.

2. Experimental work 2.1. Materials, coating and laser melting HD graphite with maximum particle size 0.25 mm, apparent density 1.8 g/cm3; porosity 11% and Rockwell hardness 68 was used as substrate. Commercial alumina–40 wt% titania powder fused and crushed and the particles shapes were irregular (Fig. 1) with nominal particle size 22 þ5 mm. A METCO 3 MB type plasma gun (M/s Spraymet Surface Technologies Pvt. Ltd, Bangalore, India) was employed in plasma spraying of A40T coating on 12 mm diameter and 8 mm thick cylindrical discs of HD graphite. Prior to coating, the substrate was sand blasted to roughen the surface and cleaned well. Bond coating of Cr3C2–NiCr was initially sprayed onto the HD graphite substrate, over which A40T was coated by atmospheric plasma spraying (APS). The plasma spray process parameters for A40T coating are listed in Table 1. Laser melting of PSA40T coating on HD graphite substrates were carried out using high power micro pulsed Nd:YAG laser at RRCAT, Indore. The Nd:YAG laser beam has been generated from a cylindrical rod and has a circular spot. However, in the present investigation laser beam has been focused to a line by means of a cylindrical lens. Laser output has a circular spot of laser beam of 6 mm diameter and a full angle beam divergence of 5 mrad.

Fig. 1. Morphology of A40T powder.

Table 1 Process parameters for plasma spraying of A40T coatings. Plasma spray process parameters Plasma spraying equipment Current (A) Voltage (V) Primary gas: Ar (l/min) Secondary gas: H2 (l/min) Spray distance (mm) Powder feed rate (g/min)

METCO 9 MB 470 50 80 12 100 50

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Fig. 2. Schematic diagram of pulsed Nd:YAG laser system.

Table 2 Laser processing parameters used for melting of PSA40T coatings. Laser system

Nd:YAG

Mode of operation Laser wavelength (mm) Line of focus Repetition rate (Hz) Pulse width (msec) Pulse energy (mJ)

Pulsed mode 1.064 6 mm  150 mm length and width 20 55 320 400 5.8 7.3 640 800 1

Pulse peak power (kW) Peak power density (kW/cm2) Speed (mm/sec)

A cylindrical lens of 30 mm focal length has been used in the beam path to produce a line focus of 6 mm  150 mm size on the sample. The schematic diagram of the pulsed Nd:YAG laser system used in the present investigation is shown in Fig. 2. More details of rectangular pulsed Nd:YAG laser beam profile were discussed elsewhere [44,45]. The overall process parameters employed for melting of PSA40T coating was listed in Table 2. Argon gas was provided for shielding during the laser melting process. The coated sample was fixed to the X–Y direction movable stand. The movement of stand was controlled automatically. The argon chamber was equipped with a glass top-hat in order to allow the laser radiation to be filtered and delivered to the sample surface. Argon gas was purged into the main chamber and blown directly over the laser processing area with optimized flow rate and velocity to avoid the formation of pores during the laser melting process. The vertical and horizontal overlap of LM-1 and LM-2 is 20–30% between consecutive tracks. The melting experiments were repeated over number of samples to ensure data reliability and reproducibility. Laser melted coating with power density of 640 kW/cm2 is defined as ‘LM-1’ and with power density of 800 kW/cm2 as ‘LM-2’. 2.2. Characterization of laser melted coatings The morphology of the as-coated and laser melted coatings were characterized by field emission scanning electron microscope (FEI Quanta 200F FE-SEM). The cross sections of laser melted coatings were prepared and covered with gold coating prior to the examination with back scattered electron detector. The surface elemental composition of the coatings and elemental mapping was performed on the cross sections by energy dispersive X-ray spectroscopy (EDX). X-ray diffraction (XRD) analysis was made to identify the phases present in powder, as-sprayed and laser melted coatings using normal mode Bruker AXS D8 diffractometer (40 kV, 30 mA, Cu Ka radiation l ¼0.1542 nm).

The powder and coatings were examined in continuous scan mode 201o2y 4801 at a rate of 21 per minute. The XRD patterns obtained were analyzed using standard JCPDS database. Microhardness measurements were made on as-coated and laser melted coatings by a SHIMADZU-HMV-2 microhardness tester with a Vickers diamond indenter. The load of 200 g was applied for a holding time of 15 s. All indentations were performed on assprayed coating and laser melted coating surfaces. Hardness values were measured at 5 random locations on the sample. The average surface roughness (Ra) of the as-coated and laser melted surfaces was measured by using Taylor Hobson surtronic 3þ roughness tester.

3. Results and discussion 3.1. Morphology of as-sprayed and laser melted coatings Scanning electron microscopy (SEM) micrographs of PSA40T coating with bond coat of Cr3C2–NiCr are shown in Fig. 3. As-sprayed A40T coating microstructure (Fig. 3a) consists of completely melted splat structures, unmelted particulate regions, pores between the splats and cracks within the splats. These complex microstructures are formed due to the following reasons: (1) for very short time of residence of powders in the stream of plasma, some of the particles melt completely and deposit as splat morphologies and some particles will remain as unmelted particles; (2) stresses can be generated in the microstructure owing to the contraction of individual sprayed splats during rapid cooling (106  108 K/s) and depositing on the cooler substrate or layer of previously solidified splats and these stresses are called as ‘‘quenching stresses’’ [46,47]. These stresses have strong influence on the microstructure, performance of coatings such as adhesion, thermal cycle life and erosion resistance [46]; and (3) volume changes associated with the phase changes in the coating during spraying process also leads to generate the stresses in the coating [46]. The thickness of cross-section of top A40T coating (Fig. 3b) was approximately 100 mm. Generally plasma sprayed coatings contain up to 10% of porosity and partially or unmelted regions [48]. X-ray elemental maps from the cross section micrograph of sprayed coatings (Fig. 3c) clearly indicated the presence of Al, Ti and O elements in the top coat and furthermore, Cr, Ni and C in the bond coat. Observation at higher magnification of as-sprayed A40T coating revealed two distinct types of morphologies of unmelted regions (Fig. 4a and b). Fig. 4a shows irregular or coarse morphology and EDX analysis revealed significant titanium content. Another region identified as shown in Fig. 4b contains small size particles and the EDX analysis showed significant aluminum content. The elemental compositions of these two regions are listed in Table 3. During plasma spraying a part of titania did not react with alumina and vice

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Fig. 3. SEM micrographs of as-sprayed A40T coatings: (a) surface; (b) cross-section morphologies and (c) EDX elemental mapping of Al, Ti, O, C, Cr and Ni elements.

Fig. 4. Morphologies of unmelted/partially melted regions in as-sprayed A40T coatings: (a) titanium particles rich region and EDX spectrum and (b) aluminum particles rich region and EDX spectrum.

versa as seen in micrographs (Fig. 4). In composite coatings, some of the ceramic particles are not melted completely as these particles were entering into the lower temperature region of the plasma tail. These types of complex microstructures are eliminated by glazing with high power lasers. From the visual examination, it has been observed that the laser melted coatings changed from a light black to a shining glassy appearance. Fig. 5 showed the unmelted and laser melted

regions. High irregularities existed in sprayed coatings are minimized in the laser melted coatings. Some voids were found at the surface, located mostly over cracks, which are formed due to gas pockets and escape of entrapped gases from those gas pockets before solidification process [39,49,50]. Improvements in surface homogenization from as-coated to laser melted coatings are shown in Fig. 6. The splat morphology observed in the sprayed coating (Fig. 6a) changed to consolidated microstructure by laser

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Table 3 EDX analysis of powder, different regions in sprayed coating, LM-1 and LM-2 A40T coatings. Coatings

Powder (Fig. 1) Sprayed coating (Fig. 4a) Sprayed coating (Fig. 4b) LM-1 (Fig. 7b) LM-2 (Fig. 7d)

Elements (wt %) Al

Ti

O

37.04 01.19 58.50 33.04 41.30

27.54 70.27 03.76 38.05 22.84

35.42 28.54 37.74 28.91 35.85

Fig. 5. SEM micrographs of A40T laser melted and unmelted regions.

melting, and moreover, the surface porosity and other defects were reduced. With the increase of laser power, surface homogeneity increases, as presented in Fig. 6b and c. In laser melted coatings an external dense layer was produced owing to solidification of ceramic particles after laser processing. SEM microstructures of LM-1 (Fig. 7a and b) and LM-2 (Fig. 7c and d) showed that microcracks are nucleated during the laser melting process. It is well known that origin of such cracks in the laser melted coatings were due to high localized temperature gradients, relief of thermal induced stresses and volume change during solidification of molten ceramic particles. The formation of segmented network of cracks in laser melted coatings is beneficial to increase high thermal shock resistance and the thermal cycle life time [34]. LM-2 coatings were much denser with reduced pores and other defects on comparison to the LM-1 coatings which has some partially melted zone with growth features. These growth features are in a columnar shape (Fig. 7b). It was reported that laser melting changes the microstructure from laminar to columnar [30]. During laser melting the distribution of heat energy on the coating causes growth of unmelted and partially melted regions into different shapes, particularly in the growth direction which increases the consolidation of coating. EDX analysis of this region (Fig. 7b) revealed the high concentration of titanium as listed in Table 3. The TiO2 rich regions act as barriers for thermal energy generation during laser melting and necessarily contribute to larger loss of heat energy at the molten surface [51]. Commonly observed microstructural inhomogeneities such as unmelted and partially melted powder particles, pores and cavities, in plasmasprayed coatings were considerably diminished in LM-2 coatings. Higher magnification microstructure of LM-2 coating (Fig. 7d) did not exhibit any growth features and all the regions were melted. The EDX analysis of this region revealed more concentration of aluminum as listed in Table 3. EDX analysis on laser melted coatings (Fig. 7b and d) revealed improved homogenous elemental composition of the modified layer due to redistribution

Fig. 6. Surface morphologies of A40T (a) as- sprayed, and laser melted coatings of (b) LM-1 and (c) LM-2.

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Fig. 7. Surface morphologies and EDX spectra of laser melted A40T coatings: (a and b) LM-1 and (c and d) LM-2.

of elements by laser melting. The difference in the microstructural changes (Fig. 7b and d) in LM-1 and LM-2 coatings indicated that the surface quality and homogenization of the coatings depends on process parameters used for the laser melting. Fig. 8 depicts the cross section of LM-2 coating of A40T with Cr3C2–NiCr bond coat. It can be clearly seen that thickness of the laser-treated zone is less. Pulsed mode Nd:YAG laser reduce the thermal affected zone (small zones) in the coating because of a highly localized action and controlled energy input during the treatment [42,50]. These features will reduce undesirable side effects such as horizontal cracking of the top ceramic coating [52]. It was also reported that the absorption of the laser energy in the surface of the material depends on the thermal effects/properties of laser irradiation with different materials [53]. Chehrghani et al. [53] melted titanium surface pre-coated by graphite with pulsed Nd:YAG laser and studied the depth and width of laser treated zone along the cross section by varying the pulse duration of the laser. Their study clearly indicated that optimization of the laser parameters; alter the absorption and penetration of the laser beam into the sprayed coating. Generally vertical cracks were observed after laser melting of plasma sprayed coatings with very high power CO2 laser [36,54]. In the present study there are no cracks and delamination of coating occurred in the cross section of the laser melted coatings due to controlled supply of energy. The laser process parameters selected for melting of A40T coating avoided vertical cracks along the cross section (Fig. 8). X-ray elemental maps showed that there are no elemental depressions along the cross section of LM-2 coatings. Top coat elements Al, Ti and O were not mixed with bond coat elements at the bond coat/ top coat interface after laser melting. The top A40T coating is densified and there is no effect on bond coat as well as on HD

graphite substrate. The cross sections of LM-1 coatings showed similar characteristics. Our previous work [36,37] on laser melting of PSZ coatings for molten salt application with continuous wave mode CO2 showed vertical cracks along the cross section and also, the delamination at the bond coat-ceramic coating interface. However, in the present work the unmelted/partially melted regions present on the surface of PSA40T coatings decreases as the laser irradiation power density increased, and further, no vertical cracks along the cross section of the coatings is observed. 3.2. Phase analysis studies The crystal phases were analyzed by XRD in the following sequence: powder- sprayed coating-LM-1-LM-2. XRD identifies a significant phase change from powder to sprayed coating (Fig. 9). XRD pattern of the initial fused and crushed powder is shown in Fig. 9a, in which the peaks resulting from the primary phases of a-A12O3 and rutile (R-TiO2) were indexed. A small amount of titanium suboxides (Ti2O3 and Ti3O5) was also found in the A40T powder. The XRD pattern of the sprayed coating is shown in Fig. 9b, which proved the presence of b-Al2TiO5 phase, g-A12O3, R-TiO2 and a-A12O3. The XRD analysis clearly indicated that the a-A12O3 and R-TiO2 present in the initial powder were reacted in the plasma during spraying process and solidified as b-Al2TiO5 phase on the substrate [15]. Some of the a-A12O3 phases are changed to g-A12O3 after plasma spraying process. It is well known that g-A12O3 commonly nucleates in preference to a-Al2O3 due to lower energy of nucleation during rapid solidification of the liquid droplets [55]. In addition, titania exists in the crystallographic form of R-TiO2 phase. Fig. 9c and d show

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Fig. 8. (a) Cross section micrograph and (b) EDX elemental mapping of LM-2 A40T coating.

the XRD pattern of the LM-1 and LM-2 coatings. The metastable g-A12O3 phase in the sprayed coating transferred into stable phase of a-A12O3 because of the melting and recrystallization of the coating [56]. Al2TiO5 became more predominant phase observed in case of laser melted coatings. The beneficial Al2TiO5 has a good thermal shock resistance, wear and corrosion resistance than individual Al2O3 and TiO2 phases [22]. After laser melting stable phases of a-Al2O3, R-TiO2 and b-Al2TiO5 are solidified as a smoother surface. The variation in the pulse energy/power density of the laser melting process does not show much variation in the phase content, only the difference is that significant R-TiO2 phase was observed in LM-1 coatings compared to LM-2 coatings. 3.3. Microhardness The microhardness measurements were carried out randomly on the as-sprayed, LM-1 and LM-2 A40T coatings at different regions. The Vickers microhardness of as-sprayed, LM-1 and LM-2 coatings are shown in Fig. 10. The microhardness value measured for the assprayed coating was low compared with that of the laser melted coatings because of microstructural irregularities in the sprayed coating. Elimination of the microstructural irregularities increases hardness of laser melted coatings. The microhardness increases from as-sprayed to LM-1 and LM-2 samples were 3% and 11%, respectively. Microhardness of the coatings depends on the phases present in the coating [18]. The predominant Al2TiO5 phase formation was considered to be responsible for less percentage increase in microhardness value from as-coated to both LM-1 and LM-2 coatings. Fig. 9 clearly indicated that laser power plays a major role in the surface hardness of the A40T coating. The increasing hardness must be owing to decrease in porosity, microstructural modifications and the nucleation of a-Al2O3 phase in the laser melted coatings [18,19]. The aAl2O3 phase has highest hardness than other phases present in the coatings. In general microhardness for laser melted coatings increased

with increase in laser power density [51]. The increase in microhardness from LM-1 to LM-2 is about 8%. The significant increase in hardness in the microhardness value from LM-1 to LM-2 coating was due to significant reduction in the surface porosity, other defects and increase of microstructural homogeneity. 3.4. Surface roughness measurement The surface roughness is a measure of the average of deviation of the surface from the center line [57,58]. It gives information regarding surface smoothness, morphology and corrugation in terms of numerical value. The Ra of the as-sprayed, LM-1 and LM-2 A40T coatings is shown in Fig. 10 are measured along the center path on the surface. The decrease of Ra from as-sprayed to LM-1 and LM-2 samples is 27% and 42%, respectively. The decrease in Ra from LM-1 to LM-2 is about 21%. It is obvious that as-sprayed plasma coatings have high roughness compared to the laser melted coatings. It was observed that the surface finish improved considerably with a glazing effect upon laser melting. The surface defects present on the sprayed coating contributed to high roughness, and there were minimized by laser melting. However, specimen LM-2 presents lower roughness than LM-1 due to smoother surface caused by the large overlap of laser tracks. The partially melted regions were still present in LM-1 coating compared to LM-2. The decrease of the roughness value clearly indicated that high power density was beneficial to get more uniform microstructure (Fig. 10). The surface roughness of coatings was in the order of as-sprayed 4LM-14LM-2.

4. Conclusions Pulsed Nd:YAG laser was used for melting of plasma sprayed Al2O3–40 wt% TiO2 coatings to consolidate and generate a dense layer over the surface. The surface homogeneity of coating has improved with laser melting. The segmented crack network

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Fig. 9. XRD patterns of powder, as-sprayed, laser melted A40T coatings of LM-1 and LM-2.

Al2TiO5 phase along with stable a-Al2O3 phase were found in both the laser melted coatings. Improvement in surface microhardness and decrease of average surface roughness observed indicates that pulsed laser melted coatings exhibit uniform microstructure with increase in laser power density in the range 640–800 kW/cm2. The present work validates the deployment of that pulsed laser melting to improve or modify the surface properties of plasma sprayed Al2O3–40 wt% TiO2 coatings.

Acknowledgment

Fig. 10. Vickers microhardness and average surface roughness (Ra) of as-sprayed, laser melted A40T coatings of LM-1 and LM-2.

The authors wish to acknowledge Dr. C. Mallika, Head, Aqueous Corrosion and Surface Characterization Section (ACSCS) for fruitful technical discussion, Dr. B. N. Upadhyay of RRCAT, Indore for his kind help in technical discussion about laser system and parameters. Shri Rakesh Kaul, Shri. Ambar Choubey of RRCAT, Indore for help in laser melting operation. The authors also wish to acknowledge Prof. Ramesh Chandra of IIT Roorkee for providing FE-SEM and XRD facility for coating characterization. References

formed in the coating after laser melting was caused by shrinkage and relaxation of residual stresses during the cooling of molten ceramic coating to room temperature. The cross sections of laser melted coatings showed that the densification occurred only up to a microns thickness of the top ceramic coating. Prominent

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