Microstructure and property of Al 2 O 3 coating microplasma-sprayed using a novel hollow cathode torch

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1 Materials Letters 58 (2003) Microstructure and property of Al 2 O 3 coating microplasma-sprayed using a novel hollow cathode torch Chang-Jiu Li*, Bo Sun Key State Laboratory for Mechanical Behavior of Materials, Welding Research Institute, School of Materials Science and Engineering, Xi an Jiaotong University, Xian, Shaanxi , PR China Received 9 December 2002; received in revised form 28 April 2003; accepted 2 May 2003 Abstract Al 2 O 3 coating is deposited using a novel microplasma torch of a hollow cathode through axial powder injection under low power, up to several kilowatts. The microstructure of the coating is characterized using optical microscopy, scanning electron microscopy, and X-ray diffraction analysis. The property of the coating is characterized by dry rubber wheel abrasive wear test. The velocity of in-flight particles is measured using a velocity/temperature measurement system for spray particles based on thermal radiation. The effect of plasma power on particle velocity during spraying, and the microstructure and property of the coating are examined. The comparison of the velocity of in-flight particles, and microstructure and property of the coating deposited by the microplasma spraying with those of conventional plasma spraying is carried out. The results reveal that Al 2 O 3 coating, of comparable performance to that deposited by conventional plasma jet under a power of about 40 kw, can be deposited by the present microplasma spray torch. D 2003 Elsevier B.V. All rights reserved. Keywords: Microplasma spray; Plasma torch; Al 2 O 3 coating; Ceramics; Abrasive wear 1. Introduction Plasma spraying has been widely used to deposit various coatings of different performances [1 3]. The features of high temperature and energy density of plasma jet make spray refractory materials possible, which are difficult to melt by other conventional thermal spray methods. Velocity and temperature of spray particle are the main factors influencing the microstructure and properties of plasmasprayed coating. It is considered that an increase in plasma power will lead to the increase in both temperature and velocity of particle. Consequently, the dense coating with good cohesion between flattened particles in the coating and good adhesion between the coating and the substrate will be deposited. Accordingly, the power input to plasma torch for thermal spray has increased remarkably in the last decades [4 6]. A recent study on the lamellar bonding of plasmasprayed Al 2 O 3 coating suggested that the cohesion between the flattened particles in the coating will determine the * Corresponding author. Tel.: ; fax: address: licj@mail.xjtu.edu.cn (C.-J. Li). properties of plasma-sprayed coatings [7]. On the other hand, the lamellar bonding of plasma-sprayed Al 2 O 3 coating determined by using copper electroplating technique revealed that the effort to increase the bonding ratio between lamellar particles through the increase in plasma power is limited [8]. This effect is attributed to the fact that particle temperature plays a dominant role in the formation of the cohesion between flattened particles in the coating [9 11]. With an increase in plasma power, both the temperature and velocity of plasma jet are increased simultaneously. Such effect leads to an increase in particle velocity and a decrease in the dwelling time of the particle in plasma jet inevitably. Therefore, the increase in heating effect owing to jet temperature increase can only compensate for the shortening of dwelling time of particles in plasma jet [12]. To obtain higher particle temperature and, subsequently, an improved lamellar cohesion, the concept of low-velocity plasma spraying was proposed, and the plasma torch with a nozzle of a larger diameter than that of conventional plasma torch was developed [13]. The experiment using such nozzle showed that a more uniform heating of particle was achieved and, consequently, the deposition efficiency as well as the cohesion of the coating were improved [13] X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi: /s x(03)

2 180 C.-J. Li, B. Sun / Materials Letters 58 (2003) The features of the microplasma spray torch Fig. 1 shows a schematic illustration of the microplasma spraying torch used in the present study. A vortex gas flow is established through gas passing through eight holes drilled tangentially to the insulator wall to stabilize the cathode arc attachment, and continuously moving anode arc spot to improve anode heating load. The hollow cathode made of two tungsten rings was soldered on a water-cooled electrical conducting rod of copper. The tungsten ring in larger diameter works as the cathode and plasma arc burn on the surface of ring, while the inner tungsten ring works as a Fig. 1. Schematic diagram of the microplasma torch. Accordingly, it can be proposed that coating with identical microstructures and properties can be deposited, provided that particle condition parameters are identical. To improve the heating of spray particles, the most effective is to inject the powder along the axis of the plasma jet, which will lead to the increase in the dwelling time of particles in the hottest zones of the plasma jet [14]. Following such concept, high-power plasma torches using axial powder injections have been developed [15,16]. Injection of particles through the central hole of a hollow cathode is generally regarded as a simple way in which the effective heating zone to spray particles expands from the plasma jet to the whole plasma arc and plasma jet. However, problems including the instability of hollow cathode plasma arc, the shortness of cathode life, and the adhesion of spray powder to the electrode at the exit of the cathode limit the practical application of hollow cathode plasma torch [17]. In this paper, a novel hollow cathode microplasma spray torch is developed and refractory material Al 2 O 3 is deposited to examine the feasibility of the hollow cathode operating at a low-power level. Fig. 2. Effect of plasma power on particle velocity. Fig. 3. Typical microstructure of Al 2 O 3 coatings deposited under different plasma powers: (a) 2.2 kw; (b) 3.0 kw; (c) 3.9 kw.

3 powder injection port. The difference between the front planes of two rings along the axis ensures that the plasma arc burns at the outer tungsten ring. Consequently, the adhering of spray powder onto the wall of the hollow tungsten can be eliminated through the easy control of particle trajectory and the low temperature of the inner tungsten, resulting from the cooling by powder carrier gas flow. The stability of the plasma arc can be improved because the disturbance of the carrier gas on the plasma arc can be controlled to a low level. C.-J. Li, B. Sun / Materials Letters 58 (2003) Material and experimental procedures Powder used was commercially available pure Al 2 O 3, which had a particle size range from 10 to 20 Am. A mild Fig. 5. XRD patterns of Al 2 O 3 coatings deposited under different plasma powers: (a) 2.2 kw; (b) 3.9 kw. Fig. 4. Typical SEM microstructure of Al 2 O 3 coatings deposited under different plasma powers: (a) 2.2 kw; (b) 3.0 kw; (c) 3.9 kw. steel plate was used as substrate, which was sandblasted prior to spraying. Argon was used as both plasma-operating and powder carrier gas. The pressures of the plasma gas and carrier gas were operated at 0.5 and 0.3 MPa during spraying, respectively. The flows of Ar as plasma and powder carrier gases were kept at 35 and 10 l/min during spraying, respectively. Al 2 O 3 coatings were deposited at a spray distance of 10 mm under different plasma powers. The velocity of in-flight particle was measured using a velocity/ temperature measurement system developed based on the thermal radiation of spray particle. The system setup and detailed descriptions were given elsewhere [18]. The melting state of spray particle prior to impact on the substrate was examined based on the examination of the microstructure of the coating and phase constitution of Al 2 O 3 coating. The cross-sectional microstructure of the Al 2 O 3 coating was examined by optical microscopy and scanning electron microscopy (SEM). The phase structure of the coating was characterized by X-ray diffraction (XRD) analysis. The property of the coating was characterized by abrasive wear weight loss using dry rubber wheel test according to ASTM-G6-91. The test was carried out under a normal load of 13 N to the rubber wheel with a diameter of 220 mm at a rotation speed of 50 rpm. The abrasives used were 100-mesh Al 2 O 3 grit fed at a rate of 70 g/min. The test

4 182 C.-J. Li, B. Sun / Materials Letters 58 (2003) was carried out for 15 min. Prior to the test, the surface of the coating was polished. 4. Results and discussion Fig. 6. XRD patterns of Al 2 O 3 powder. Fig. 2 shows the influence of plasma power on particle velocity. It can be recognized that Al 2 O 3 particles achieved a velocity of about m/s during microplasma spraying. Those velocities are comparable to those of Al 2 O 3 particles found in conventional plasma spraying [12]. Moreover, it can be seen that the velocity was not influenced significantly by the increase in plasma power. This may be due to much less intensive electromagnetic pinching effect on plasma jet, resulting from much lower plasma power during microplasma spraying, which is only about 1/10 1/6 that of the conventional plasma. Fig. 3 illustrates a typical microstructure of Al 2 O 3 coatings sprayed at a gas flow of 35 l/min and spray distance of 10 mm under different plasma powers. Although the coating deposited at high power presented a relatively apparent Fig. 8. Microstructure of Al 2 O 3 coating deposited by conventional plasma jet operated at a power of 38 kw. dense microstructure, no significant effect of plasma power on the microstructure of the deposited coating was recognized. SEM examination of the deposited coating, as illustrated by SEM images of the coating shown in Fig. 4, revealed that the coating mainly consisted of well-flattened Al 2 O 3 particles. It can also be found that the splat thickness of the flattened particles decreased slightly with the increase in plasma power. XRD patterns of the Al 2 O 3 coatings deposited under plasma powers of 2.2 and 3.9 kw, as shown in Fig. 5, revealed that Al 2 O 3 coatings mainly consisted of g-al 2 O 3 compared with a-al 2 O 3 in powder, as shown in Fig. 6. This fact suggests that spray particles have reached sufficient melting prior to impact on the substrate. Fig. 7 shows the effect of plasma power on the abrasive wear weight loss of the Al 2 O 3 coating deposited by microplasma spray. It can be seen that the abrasive wear weight loss decreased significantly with the increase of plasma power from 2.2 to 3.0 kw. However, no significant change in the abrasive wear was recognized with the further increase in plasma power. The increase in plasma power results in a plasma jet of higher temperature. On the other hand, the particle velocity was not increased significantly with the increase in plasma power. Therefore, it can be considered that an improved heating to spray particle can be achieved by increasing plasma power, which leads Fig. 7. Effect of plasma power on the abrasive wear weight loss of Al 2 O 3 coating. Fig. 9. SEM microstructure of Al 2 O 3 coating deposited by conventional plasma jet operated at a power of 38 kw.

5 C.-J. Li, B. Sun / Materials Letters 58 (2003) to more sufficient melting as confirmed by SEM micrographs in Fig. 4 and XRD patterns in Fig. 5. Such effect contributed to the improvement of the cohesion between flattened particles in the coating [19]. Accordingly, the abrasive wear performance of the coating was improved at high plasma power. For comparison, Al 2 O 3 coating was also deposited using a conventional plasma spray torch (GP-80; Jiujiang) operating at an arc power of 38 kw (700 A/55 V). The typical microstructures of the Al 2 O 3 coating observed by optical microscopy and SEM are shown in Figs. 8 and 9, respectively. It can be seen that the microstructure of the coating is similar to that of the microplasma-sprayed Al 2 O 3 coating. The coating was also composed of well-flattened lamellas as revealed by SEM observation. The abrasive wear test of the conventional plasma-sprayed Al 2 O 3 coating yielded a weight loss of 24.0 mg. This value is comparable to that of the microplasma-sprayed Al 2 O 3 coatings deposited at a plasma power from 3 to 3.9 kw. Therefore, it is clear that an Al 2 O 3 coating of comparable wear performance to that deposited by conventional plasma spraying can be deposited by microplasma spray under much lower power level. Taking into account the velocity and melting state of spray particle prior to deposition during both microplasma spraying and conventional plasma spraying, the present results clearly revealed that the coating of identical microstructure and performance can be deposited despite plasma power, provided that spray particles achieve identical velocity and melting state. 5. Conclusions With a plasma torch of a novel hollow cathode, the effective heating of Al 2 O 3 powder as a typical refractory material to sufficient melting was realized at a plasma power level up to 4 kw. The deposited Al 2 O 3 coating presented a comparable abrasive wear resistance to that deposited by conventional plasma spray operated at an arc power of 38 kw. The present results clearly confirmed that the coating of the identical property can be deposited independent of the plasma power level, provided that spray particles reach identical conditions. References [1] W. Haessler, R. Thielsch, N. Mattern, Mater. Lett. 24 (1995) 387. [2] K.A. Khor, Z.L. Dong, Y.W. Gu, Mater. Lett. 38 (1999) 437. [3] R. Montanari, B. Riccardi, R. Volterri, L. Bertamini, Mater. Lett. 52 (2002) 100. [4] T. Morishitai, in: S. Blum-Sandmeier (Ed.), Proceedings of the 2nd Plasma Technik Symposium, Plasma-Technik, 1991, p [5] P. Chraska, M. Hrabosky, in: C.C. Berndt (Ed.), Proceedings of the International Thermal Spray Conference and Exposition, ASM International, Materials Park, OH, 1992, p. 81. [6] J.M. Houben, Proceedings of the 9th International Thermal Spraying Conference, Nederlands Instituut Voor Lastechniek, 1980, p [7] C.-J. Li, A. Ohmiri, J. Therm. Spray Technol. 11 (2002) 365. [8] A. Ohimori, C.-J. Li, Y. Arata, Trans. JWRI 19 (2) (1990) 99. [9] M. Prystay, P. Gougeon, C. Moreau, J. Therm. Spray Technol. 10 (1) (2001) 67. [10] C.-J. Li, A. Ohimori, Surf. Coat. Technol. 82 (1996) 254. [11] L. Pejryd, J. Wigren, P. Gougeon, C. Moreau, in: C. Coddet (Ed.), Proceedings of the 15th International Thermal Spray Conference, ASM International, Materials Park, OH, 1998, p [12] M. Vardelle, A. Vardelle, P. Fauchais, J. AIChE 29 (1983) 236. [13] M.P. Planche, O. Betoule, J.F. Coudert, A. Grimaud, M. Vardelle, P. Fauchais, in: C.C. Berndt, T.F. Bernecki (Eds.), Proceedings of the 5th National Thermal Spray Conference, ASM International, Materials Park, OH, 1993, p. 81. [14] M. Vardelle, A. Vardelle, P. Fauchais, Mater. Manuf. Process. 9 (4) (1994) 735. [15] D.R. Marantz, in: T.F. Bernecki (Ed.), Proceedings of the 3rd National Thermal Spray Conference, ASM International, Materials Park, OH, 1990, p [16] A.W. Burgess, Vancouver, in: E. Lugscheider (Ed.), Proceedings of the International Thermal Spray Conference, ASM International, Materials Park, OH, 2002, p [17] A. Notomi, Y. Takeda, in: A. Ohmori (Ed.), Proceedings of the 14th International Thermal Spray Conference, High Temperature Society of Japan, 1995, p [18] T. Wu, C.-X. Li, C.-J. Li, Mater. Prot. 32 (10B) (1999) 124. [19] E. Lugsheider, A. Fischer, D. Koch, N. Papenfuß, in: C.C. Berndt (Ed.), Proceedings of the International Thermal Spray Conference, ASM International, Materials Park, OH, 2001, p. 751.