Surface and Coatings Technology 154 (2002) 189–193
High hardness alumina coatings prepared by low power plasma spraying Yang Gao*, Xiaolei Xu, Zhijun Yan, Gang Xin Institute of Material and Technology, Dalian Maritime University, 116026 Dalian City, PR China Received 25 June 2001; accepted in revised form 5 December 2001
Abstract Alumina coatings were prepared by low power plasma thermal spraying (2.5–4.0 kW). The powders were directly injected into the region between the cathode and the anode intake inside the plasma torch, and the carrier gases were transferred to the plasma flame. The results show that the hardness of Al2O3 coatings by low power plasma spray with an internally-fed powder system is higher than that of the coatings by APS with an externally-fed powder. The hardness of the coating increases with increasing plasma energy up to HVs1500 kgymm2 , when the plasma arc voltage and current are 80 V and 50 A, respectively, and the porosity is estimated lower than 1%. The coatings are mainly composed of g-Al2 O3 although the original powders were nearly all a-Al2O3. The amount of the g-Al2 O3 in the coating increases, and the a-Al2 O3 decreases with increasing plasma power. The grain size of the coating is 0.1–1.2 mm. The optimum spraying conditions have been established by analyzing the relation of the coating properties with spray parameters. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma spray; Power; Powders; Coating; Alumina; Hardness
1. Introduction Al2O3 ceramic coatings are widely used as wearresistant and insulating coatings. Atmosphere plasma spraying (APS) is a commonly used method for preparing alumina coatings. However, in traditional APS the powders are injected into the plasma flame outside of the nozzle, or powders are injected into the inside of the nozzle through a hole in the radius direction of the nozzle side. The porosity of Al2O3 coatings deposited by APS is usually 5–10% w1x. Some investigations showed that the hardness and porosity of the coatings are related to the type of plasma torch used. For example, the hardness of the coating is reported to be HVs1431 kgymm2 w2x using a Praxair SG-100 spraying gun under conditions of 40 kW and a gas flow rate of 47y22 lymin (AryHe), but the hardness of the coating has been reported to be as high as HVs904 kgymm2 and porosity 6.8% when performed with PT-3000S *Corresponding author. Tel. q86-411-472-6895; fax: q86-411472-6895. E-mail address:
[email protected] (Y. Gao),
[email protected] (X. Xu),
[email protected] (Z. Yan),
[email protected] (G. Xin).
equipment w3x using argon and hydrogen as plasma gases. The construction of the plasma torch strongly influences the heating and acceleration of the powder, thus forming coatings with various properties. Usually, raising the temperature and velocity of the particles is beneficial to increase the hardness and reduce the porosity of the coating. For the traditional plasma spraying technology with an externally-fed powder system, the effective energy used to heat and accelerate the powder is not only smaller (-10%), but also consumed power is greater ()40 kW), and the properties of the coatings are worse compared with plasma spraying using an internally-fed powder system. Additionally, although the velocity of the plasma flame is higher (1300 mys) at the exit of nozzle, the highest velocity of the powders measured is merely 200 mys w4x. If the powders are directly injected to a region between the cathode and anode in plasma torch, the powders are heated and accelerated more rapidly due to being at the highest temperature near the cathode. Meanwhile, the expanding plasma flame makes the velocity of the powder rise so that the hardness and the density of the coating can be improved, and the power consumed would be decreased compared with a conventional plasma spray. In the
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 7 1 1 - X
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Table 1 Process parameters of the low power plasma spray for alumina Powder Size of particle (mm) Voltage of the plasma (V) Current of the plasma (A) Amount of the feed powder (gymin) Argon flow (lymin) Hydrogen flow (lymin) Nitrogen flow (lymin) Internal diameter of nozzle (mm) Spraying distance (mm)
Al2O3 10–20 50, 60,70, 80 50, 60, 70, 80 25 30 5–10 10–20 3.2 40
(100–150 mm) in APS with an externally fed powder system at a power of 40 kW. Coatings 200–400 mm thick, were formed on the 45C carbon steel substrate sandblasted. 2.3. Phase analysis and metallography
2. Experimental details
The phases composition of the powders and the coatings were determined by X-ray diffraction (XRD) on a Rigaku Dymax-IIIA X-ray diffractometer. The radiation used was CuKa (ls0.1542 nm). The crosssectional metallographic specimens of coatings were prepared by mechanically polished on abrasive paper. The cross-sectional morphology and the porosity were observed by a Leitz MM-6 optical microscope. The Vickers microhardness profiles of the coatings were made by MH-6 micro-hardness tester at a 300 g load. The microstructure of some Al2O3 coatings was studied by TEM on an H-800 transmission electron microscope.
2.1. Plasma spray equipment and characteristics
3. Results and discussions
The low power plasma spraying equipment (PlasmaLE15) used in this experiment was developed by Dalian Maritime University w5,6x. It is comprised of a DC electrical source, gas flow control, water cooling system and powder feeder. Some technical characteristics of this system include the powder being directly fed to the region between the cathode and anode nozzle intake inside the plasma torch, and the carrier gases transformed to the plasma flame, which avoids the cooling carrier gas blowing directly into the plasma flame which result in cooling and disturbance of the plasma flame, as compared with conventional plasma spray methods. The heating effect on the powders is increased and the velocity of particles in the process is higher than that in an externally-fed powder system. With the internallyfed powder, the spraying energy is greatly decreased, especially the plasma current, which results in stabilizing the arc voltage due to the lower plasma current.
The velocity of Al2O3 particles at different distances from plasma torch was roughly measured by double rotating disk with a same rotating rate of 500 rev.ymin and space of 50 mm between the both of disk, whose distribution is shown in Fig. 1. The velocity of the Al2O3 particles is 300–350 mys, which is much higher than that by externally-fed powder gun (150–200 mys) w4 x . The cross-sectional morphology of Al2O3 coatings under different spray parameters (50 A, 50 V and 50 A, 80 V) is shown in Fig. 2. It can be seen that the porosity of coating decreases with increasing the plasma energy; the porosity of Al2O3 coatings is estimated to be lower than 1% at the higher power level. The relationships between the hardness of the coating and the plasma voltage and current are shown in Fig. 3.
present work, an energy efficient plasma torch w5,6x was used. This technology is different from the conventional plasma spray not only in the decreased power of the plasma spray, but also the internal construction of the plasma torch.
2.2. Spray material and process Angular alumina powders with the size of 10–20 mm were used for the present study and the spraying process parameters for the low power plasma are shown in Table 1. Argon as primary gas, hydrogen and nitrogen as secondary gases were used for the low power plasma. First, generating the plasma arc only in the argon gas, then changing the arc voltage by adjusting the flow of hydrogen or nitrogen, in the present experiment the voltage of 50–80 V and the current of 50–80 A were selected. The lower heat effect on work resulting from the lower plasma power during sprayed process, allows the spraying distance to be shortened. The spray distance is 40 mm in the present work, which is less than that
Fig. 1. The velocity of Al2O3 particles in the plasma jet.
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They indicate that the hardness increases with the increasing the current from 50 to 70 A but the rate of increase is less from 70 to 80 A at the voltage of 50 V (Fig. 3a). While, the hardness linearly increases and porosity decrease with arc voltage at a current of 50 A (Fig. 3b). The hardness of the coatings reaches HV0.3s 1500 kgymm2 when the current is 50 A and the arc voltage is 80 V. Increasing the ratio of H2 will raise the arc voltage and increase the hardness of the coatings. It is suggested that increasing voltage would produce a better effect than increasing the current under the same power. It should be noted that although increasing current can increase the hardness of the coating, this can result in the powders over-melting and depositing on the inside of the nozzle. The arc voltage increases with increasing gas ratio of H2 and simultaneously the arc root moves downstream, which facilitates the plasma energy to be transferred into the outside of the nozzle, and also helps the plasma energy to be transmitted into the powders. The higher arc voltage can increase the velocity of the particles, so that a coating with higher hardness can be obtained. In contrast, at lower arc voltages but higher currents the more energy is consumed inside the nozzle, so as to lead to powders to be over-melted and deposited on the inside of the nozzle. Changing the plasma current and arc voltage and gas flow rate have a lesser influence on the properties of the coatings as compared with APS using externally-fed powder feeding for both a metal powder with lower melting point or oxide ceramic powder with a higher melting point. However, for the spray with an internally fed powder plasma system, the properties of the coatings are more strongly dependent upon the process conditions, which influence the continuity and stability of the spraying process. The particles melted would easily deposit on the inside of the nozzle, which is not only related to the energy supplied to the spray torch, the type of gases and the shape of particles, but also to the
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Fig. 3. The relationship between the hardness of the coatings and the plasma voltage and current.
internal shape of the nozzle and the internal structure of the gun. At the same voltage and current, using the mixed gases of AryN2 is more favorable for preventing
Fig. 2. Morphology of Al2O3 coatings: (a) 50 A, 50 V; (b) 50 A, 80 V.
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Fig. 5. The microstructure of the Al2O3 coatings and their electron diffraction patterns (EDPs) at different spray energies: (a) 50 V, 50 A; (b) 80 V, 50 A.
Fig. 4. XRD of original Al2O3 powder and the coatings sprayed at different powers: (a) original powder; (b) 50 A, 50 V; (c) 50 A, 80 V.
powder from being over-melted and deposited on the nozzle than using mixed gases of AryH2. For APS it is widely commercialized a powder feed rate of 40–50 gy min can be attained, but a 40–50% spraying efficiency due to using an externally fed powder system makes the real deposited rate on the substrate only be 20–25 gy min at the power of 40 kW. However, in spite of the lower feed rate of 25 gymin in the present work, a 70– 80% spraying efficiency makes the real deposited rate on the substrate be 18–20 gymin, corresponding to that
of traditional plasma spraying at 40 kW, but the energy consumed is merely 1y8–1y5 of that for APS. XRD patterns of the original Al2O3 powders and the coatings at the different powers are shown in Fig. 4. The results show that the original powders are nearly all a-Al2O3 (Fig. 4a). However, the coatings consist of g-Al2O3 coexisting with a-Al2O3. g-Al2O3 would appear even at lower spray energy (Fig. 4b). The amount of g-Al2O3 increases with the plasma energy (Fig. 4c). It is suggested that under the usual spraying conditions, the undercooling of liquid droplets is such that gAl2O3 nucleates in preference to a-Al2O3 and the cooling rate after solidification is sufficiently rapid to prevent transformation g-Al2 O3 to a-Al2O3 w7,8x. From Fig. 4, it can be seen that the diffraction peak intensity of gAl2O3 increases with increasing plasma energy, in other words, the amount of the g-Al2O3 increases and aAl2O3 decreases with increasing the plasma energy. In terms of thermodynamics the crystallization of nonequilibrium g-Al2O3 is related to the cooling rate of the melted particles on the substrate sprayed. It is estimated that the solidification rate of the particles on the substrate can be above 106 8Cys in the present work without pre-heating. It is suggested that the amount of the gAl2O3 in the coating can represent the melting extent of the powders. The relation of the amount of g-Al2O3 to spraying distance was studied by Takahahi and Senda w9x, who used the SG-100 type spraying gun in their research. Their results show the amount of g-Al2O3 and melting extent of the particle increases with increasing spraying distance within the 180 mm spraying distance. The gAl2O3 amount and the hardness of the coatings by
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plasma and HVOF spraying were compared in a study by Sturgeon and Shimizu w2,10x, which indicated that the hardness increases with the amount of g-Al2O3 and the hardness by using a plasma spray is higher than with HVOF. From the results above it can be seen that the hardness of the coating would not increase by only increasing velocity of the particles. The amount of gAl2O3 represents the melting extent of the particles when the temperature of the substrate sprayed is lower. Although the plasma energy at low power is lower, the energy transferred to powders surpasses that produced by traditional plasma spray technology. The microstructure of the Al2O3 coatings and their electron diffraction patterns (EDPs) under different spray energies are shown in Fig. 5. It can be seen that the grain size of the coating at the voltage and current of 50 V, 50 A is 0.5–1.3mm (Fig. 5a), whose EDP shows that it is composed of g-Al2O3. However, at the voltage and current of 80 V, 50 A, the coating is mainly g-Al2O3 coexisting with b-Al2O3 and its grain size is smaller, 0.1–0.2mm (Fig. 5b), which result from the undercooling solidification of the particles. 4. Conclusions
1. The plasma spray energy can be reduced by an internally fed powder system. The Al2O3 powders can be sprayed at a power of 2.5 kW as shown above; the spraying efficiency corresponds to that of an externally fed powder system with a power of 40 kW. 2. The amount of g-Al2O3 and the hardness of the coatings increases with the plasma energy. Increasing plasma voltage also increases the hardness at the same power.
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3. The hardness of Al2O3 coatings is be up to 1500 kgy mm2 when energy consumption is 2.5–4.8 kW. 4. Excessively raising the plasma energy, especially the current, would result in over-melting of the powders in the inside of the nozzle. The safe spray energy for Al2O3 is -4 kW. Acknowledgments This research was sponsored by the NSCC General projects 50075011 and ministry of communications of China science project 95060221. The authors would like to thank the above for their support. References w1x P. Chraska, J. Dubsky, Alumina based plasma sprayed materials, ITSC-95, Conference Proceedings, vol. 1,, 1995, p. 495. w2x A.J. Sturgeon, M.D.F. Harvey, F.J. Blunt, S.B. Dunkerton, The influence of fuel gas on the microstructure and wear performance of alumina coatings produced by the high velocity oxyfuel (HVOF) thermal spray process, ITSC-95, Conference Proceedings, vol. 2,, 1995, p. 669. w3x K. Niemi, P. Vuoristo, E. Kumpulainen, P. Sorsa, T. Mantyla, Recent developments in the characteristics of thermally sprayed oxide coatings, ITSC-95, Conference Proceedings, vol. 2,, 1995, p. 687. w4x J.R. Fincke, W.D. Swank, D.C. Haggard, Plasma Chem. Plasma Process. 13 (4) (1993) 579. w5x Y. Gao, China Surf. Eng. 2 (1999) 24. w6x Y. Gao, Low Power Plasma Apparatus, Chinese Patent, 2000, ZL99243441.6. w7x R. McPherson, J. Met. Sci. 8 (1973) 851. w8x R. McPherson, J. Met. Sci. 15 (1980) 3141. w9x C. Takahashi, T. Senda, Microstructure measurement of plasma sprayed alumina coatings, ITSC-95 Conference Proceedings, vol. 2,, 1995, 921. w10x Y. Shimizu, K. Sugiura, K. Sakaki, An attempt to improve the deposition efficiency of Al2O3 coating by HVOF spraying, in: Proceedings of the 1st International Thermal Spray Conference, p. 181.
