Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process

Transcription

1 Surface & Coatings Technology 190 (2005) Effect of some parameters on microstructure and hardness of alumina coatings prepared by the air plasma spraying process Ozkan Sarikaya* Sakarya University, Faculty of Engineering, Department of Mechanical Engineering, Esentepe Campus, Sakarya, 54187, Turkey Received 8 September 2003; accepted in revised form 4 February 2004 Available online 21 August 2004 Abstract The spraying distance, substrate temperature, coating thickness and surface roughness of substrate during deposition play an important role on the plasma spray coating process and effect the final properties of the coatings. Al 2 O 3 coatings on AISI 304 L stainless steel substrate were prepared to investigate the effects on the coating of these parameters. The results indicated that the parameters such as the spraying distance, substrate temperature, coating thickness and substrate roughness were fairly effected the hardness, porosity and surface roughness of Al 2 O 3 coatings. The lowest surface roughness and the lowest porosity and the highest hardness values of Al 2 O 3 coating were obtained for the spraying distance of 12 cm and the surface roughness of 3.28 Am and the substrate temperature of 500 jc. It also found that the increases of coating thickness were lowered the hardness and enhanced the porosity and the coating roughness. D 2004 Elsevier B.V. All rights reserved. Keywords: Plasma spray; Al 2 O 3 ; Spray distance; Hardness; Porosity; Coating roughness 1. Introduction Al 2 O 3 ceramic coatings are widely used as wear-resistant and insulating coatings. Atmosphere plasma spraying (APS) is a commonly used method for preparing alumina coatings [1]. Plasma-sprayed coatings are used in a wide range of industrial applications, primarily for wear resistance, thermal barrier and corrosive environment [2,3]. A thermal spray coating is built up and the microstructure is formed, when individual, fully or partially molten particles, traveling at a particular velocity, flatten, adhere and solidify on impact with the substrate [4,5]. Thermal spraying is a highly complex deposition process with a large number of interrelated variables. Due to the high velocity and temperature gradients in the plume, even small changes in the controllable or uncontrollable parameteres can result in significant changes in the particle properties and thus in the microstructure of the coatings [6,7]. In order to achieve consistent high quality coatings to meet the more demanding performance requirements of today s applications, there is a need to put more effort into * Tel.: ; fax: address: sarikaya@sau.edu.tr (O. Sarikaya). improved control of the plasma spray process. Statical process control techniques are being widely used to improve the coating quality and to understand the relationship between the spray parameters and coating properties [8]. In this study, the coatings of Al 2 O 3 were deposited on AISI 304L stainless steel substrate. The influence on the hardness, porosity and coating roughness of the parameters such as the substrate roughness, spraying distance, substrate temperature and coating thickness was investigated. 2. Experimental details The substrates used in this study were AISI 304 L stainless steel with nominal dimensions of mm. The surface of the substrates was grit-blasted alumina, followed by ultrasonic cleaning and degreasing in a Freon solvent. Alumina powders (Metco 105 NS) containing 95 wt.% Al 2 O 3, 2 wt.% SiO 2 and 3 wt.% others were sprayed at atmospheric pressure by means of a Metco 3MB plasma spray gun, in air. The particle sizes of the Al 2 O 3 powders were in the range of 50 to + 10 Am. Powders spray parameters are given in Table /$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi: /j.surfcoat

2 O. Sarikaya / Surface & Coatings Technology 190 (2005) Table 1 Plasma spraying parameters used for different spray distances Spray parameters Al 2 O 3 Primary gas (pressure); Ar 100 psi flow rate Auxiliary gas (pressure); H 2 55 psi flow rate Arc current 500 A Arc voltage 70 V Powder feed rate 40 g/min The surface roughness was measured using a surface roughness tester (Perthometer M4P), the average roughness R a, defined as the arithmetic mean of departures of the profile from the mean line, being used to quantify the coating surface roughness. Microscopic observation of the coatings was performed using optical microscope (Olympus B 071). Each specimen was mounted in conductive resin, grinded with SiC paper and finally polished with 1 Am diamond slurry. The investigation of the amount of the porosity and the thickness of Al 2 O 3 coatings were observed by optical microscope. Average thickness and porosity of the coatings were derived from 30 measurements per sample using optical microscope. Vickers microhardness indentations (Wilson Tukon type) were made on polished sample surface using a load of 300 g at on each material. Indentation parameters were set 15 s loading time and average thickness was derived from nine measurements. The substrates were heated by the flame. Temperatures were checked in by thermocouple. Average temperature values of coatings were derived from a few measurements obtained on the coating. 3. Results and discussion 3.1. Microstructure of the coatings Fig. 1 shows a typical microstructure of the plasma sprayed Al 2 O 3 coating prepared on the under conditions Fig. 2. The surface roughness of AISI 304 L stainless steel as a function of grit-blasting pressure. (substrate roughness of 3.28 Am, spray distance of 12 cm, substrate temperature of 25 jc and coating thickness of 400 Am) by the plasma spray system. Photomicrographs shows the dense coating with homogeneously dispersed porosity. No macrocracking was evident. Pores which are shown in Al 2 O 3 coating are represented by the black areas in the microstructure. It has been reported that pore content is chanced with the surface roughness of substrate, spray distance, substrate temperature and coating thickness [8] Effect of surface roughness of substrate on the coatings Fig. 2 shows the surface roughness of substrate as a function of grit-blasting pressure with a 2, 3, 4, 5, 6, and 7 bar, an angle of 45j, a spray distance of 60 mm and a flow rate of2kgmin 1. Surface roughness (R a )ofthesubstrate increased with spray pressure 3.28 Am, 3.84 Am, 4.18 Am, 4.68 Am, 5.06 Am and 5.74 Am, respectively. As a function of substrate surface roughness, the values of porosity and coating roughness increased, while the increasing of substrate surface roughness grow up. Hardness values are also reduced relatively (Fig. 3). Tables 2 and 3 also shows hardness, porosity and coating roughness. The higher hardness, the Fig. 1. Optical microstructure of Al 2 O 3 coating (200 magnification). Table 2 Various coating characteristics values as a function of substrate surface roughness Substrate Microhardness Porosity Coating roughness (HV 0.3 ) (%) roughness R a (Am) (kg/mm 2 ) R a (Am)

3 390 O. Sarikaya / Surface & Coatings Technology 190 (2005) Fig. 3. The relationship between (a) the hardness, (b) the porosity and (c) coating roughness of the substrate roughness. Fig. 4. The relationship between (a) the hardness, (b) the porosity and (c) coating roughness of powder spray distance.

4 O. Sarikaya / Surface & Coatings Technology 190 (2005) Table 3 Various coating characteristics values as a function of powder spray distances (R a = 3.28 Am) Spray Microhardness Porosity Coating distance (HV 0.3 ) (%) roughness (cm) (kg/mm 2 ) R a (Am) lowest porosity and the lowest coating roughness were obtained at the value of substrate roughness of 3.28 Am. It has been reported that the porosity and the surface roughness of coatings as the characteristic of plasma spray process are enhanced with increasing surface roughness of substrate material and hardness is also reduced relatively [9] Effect of spray distance on the coatings The results demonstrate that spraying distance is an important parameter that influenced on the hardness, the porosity and the coating roughness of plasma spraying, significantly. The coatings were deposited on under conditions substrate roughness of 3.28 Am (the surface roughness of substrate were obtained as a function of gritblasting pressure with a 2 bar, an angle of 45j, a spray distance of 60 mm and a flow rate of 2 kg min 1 )(Fig. 3). As shown in Fig. 4, coatings of spray distance of 6 8 and cm had shown the lowest hardness while coating of spray distance of 12 cm had shown the highest. Meanwhile the lowest dense coatings are obtained at the spray distance of 6 8 and cm. This results in the increase of the porosity and coating roughness, which are deposited on the spray distance of 6 8 cm and cm (Table 3). It is clear that most of the particles sprayed at 6 8 cm and cm remain unmelted or partially melted. The spray distance of 12 cm would produce the most dense coating. In general, the particles are reaching their maximum temperature with low flow ratio at cm spray distance. It has been reported that the longer spray distance increases the dwell time in the plume and allows more thorough heating/melting of the particles. It is probable that the particle temperature will begin to decrease after a certain spray distance, as isotherms begin to decay more rapidly. Apparently, more thorough melting is occurring at a spray distance of 12.5 cm than 10 cm. This thermal processing condition results in the highest deposition efficiency and the highest hardness, while providing low levels of porosity. At shorter and longer spray distance, the particles remain unmelted or partially melted and surface roughness and porosity increases [10,11] Effect of substrate temperature on the coatings The coatings deposited under conditions such as substrate roughness of 3.28 Am, spray distance of 12 cm, Fig. 5. The relationship between (a) the hardness, (b) the porosity and (c) coating roughness of substrate temperature.

5 392 O. Sarikaya / Surface & Coatings Technology 190 (2005) Table 4 Various coating characteristics values as a function of average substrate temperatures (R a = 3.28 Am, Spray distance 12 cm) Substrate Microhardness Porosity Coating temperature (HV 0.3 ) (%) roughness (jc) (kg/mm 2 ) R a (Am) Table 5 Various coating characteristics values as a function of average coating thickness (R a = 3.28 Am, Spray distance 12 cm) Coating Microhardness Porosity Coating thickness (HV 0.3 ) (%) roughness (Am) (kg/mm 2 ) R a (Am) substrate temperatures of jc and coating thickness of 400 Am. Microhardness, porosity and coating roughness were obtained from 950 HV 0.3 to 1180 HV 0.3, from 5.8% to 8.7%, and from 6.18 Am to 6.84 Am, respectively. Fig. 5 indicates that coatings sprayed on a pre-heated substrate exhibit higher hardness and lower porosity and coating roughness. The highest coating properties are reached on the substrate temperature of 500 jc. Table 4 shows the values of hardness, porosity and coating roughness. At the coatings prepared at 500 jc, microhardness increased at 24% level, and porosity and coating roughness decreased at 33 and 1% levels. It was reported that the high temperature substrates yield well-layered lamellae whereas low temperature substrates display deposits in disarray, containing the small particles, which are the products of the fragmented splat. The fragmented particles formed on low temperature substrates evolve into structures, which display greater porosity than those deposits formed on high temperature. At higher temperature, less trapped gas in the air pocket underneath the splats (because of the smaller density) will result in less chance of splashing and due to the expansion of the air pocket upon heating by the droplet or a smaller pore when the deposit is cooled down. The same starting particle size distribution for the high substrate temperature indicates that a greater portion of the splat is intact, corresponding to a larger contiguous area. As a consequence, interlamelar porosity and surface roughness with enhanced interfacial contact are reduced. Substrate temperature has also a far more influencing effect on coating hardness. The higher substrate temperature is shown in high packed coatings compared to the lower substrate temperature and increased the hardness of coating [12,13]. Fig. 6. The relationship between (a) the hardness, (b) the porosity and (c) coating roughness of coating thickness.

6 O. Sarikaya / Surface & Coatings Technology 190 (2005) Effect of coating thickness on the coatings The effects on microhardness, porosity and coating roughness as a function of coating thickness at a substrate temperature of 25 jc, substrate roughness of 3.28 Am, spray distance of 12 cm are illustrated in Fig. 6. The microhardness of plasma sprayed coating was decreased with increased coating thickness because of increasing of the porosity. Meanwhile, coating roughness also increased owing to enhanced coating thickness. Table 5 shows the values of microhardness, porosity and coating roughness. At the coatings prepared at 500 jc, microhardness increased at 28% level, and porosity and coating roughness decreased at 86 and 36% levels. It has been reported that the thicker topcoat is mechanically weakened with increasing pores and residual stresses. The higher hardness, lower porosity and lower surface roughness can be obtained at lower coating thickness. The increase of the porosity amount will result in the decrease of the hardness of the coating [8 15]. 4. Conclusion The surface roughness of substrate increased with increasing of grit-blasted pressure. Lower and higher spray distances than 12 cm are caused on the low hardness, the high porosity and the high coating roughness. The increase of the substrate temperature enhanced the hardness and reduced the porosity and the coating roughness. The microhardness, the porosity and the roughness of the coating are decreased with increasing of the coating thickness. The highest hardness, the lowest porosity and the lowest coating roughness values are obtained at spray distance of 12 cm, substrate roughness of 3.28 Am, coating thickness of 100 Am and substrate temperature of 500 jc. Acknowledgments The author gratefully acknowledge Prof. Dr Fevzi YILMAZ, president of Plasma Laboratory of Sakarya University, and Ebubekir CEBECI, technician of this laboratory, for coating process. In addition, the author is grateful to Turkish Scientific and Industrial Research Council (Tubitak). References [1] Y. Gao, X. Xu, Z. Yan, G. Xin, Surf. Coat. Technol. 154 (2002) 189. [2] E. Celik, I.A. Sengil, E. Avci, Surf. Coat. Technol. 97 (1997) 355. [3] F. Üstel, S. Soykan, E. Cßelik, E. Avci, J. Metall. 97 (1995) 31. [4] M. Friis, C. Persson, J. Wigren, Surf. Coat. Technol. 141 (2001) 115. [5] J. Matejicek, S. Sampath, Acta Mater. 49 (2001) [6] S. Sampath, Mater. Sci. Eng. A 167 (1993) 1. [7] R. Westergard, L.C. Erickson, N. Axen, H.M. Hawthorne, S. Hogmark, Hogmark, Tribol. Int. 31 (5) (1998) 271. [8] H.M. Choi, B.S. Kang, W.K. Choi, D.G. Choi, J. Mater. Sci. 33 (1998) [9] E. Celik, A.S. Demirkan, E. Avci, Surf. Coat. Technol (1999) [10] T.J. Steeper, A.J. Rotolico, J.E. Nerz, Optimizing plasma sprayed alumina-titania coatings using statistical methods, Proceedings of the 1993 National Thermal Spray Conferance, Anaheim, CA, 7 11 June [11] Y. Li, K.A. Khor, Surf. Coat. Technol. 150 (2002) 125. [12] S. Sampath, X. Jiang, A. Kulkarni, J. Matejicek, D.L. Gilmore, R.A. Neiser, Mater. Sci. Eng. A A348 (2003) 54. [13] S. Sampath, X. Jiang, J. Matejicek, A.C. Leger, A. Vardelle, Mater. Sci. Eng. A A272 (1999) 181. [14] M.H. Staia, T. Valente, C. Bartuli, D.B. Lewis, Surf. Coat. Technol (2001) 563. [15] A. Kucuk, C.C. Berndt, U. Senturk, R.S. Lima, Mater. Sci. Eng. A 284 (2000) 41.