High Quality Ceramic Coatings Sprayed by High Efficiency Hypersonic Plasma Spraying Gun Sheng Zhu, Binshi Xu and JiuKun Yao

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1 Materials Science Forum Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland High Quality Ceramic Coatings Sprayed by High Efficiency Hypersonic Plasma Spraying Gun Sheng Zhu, Binshi Xu and JiuKun Yao National Key Laboratory for Remanufacturing No. 21 Dujiakan, ChangXinDian, Beijing , China Keywords: Ceramic coatings, Hypersonic plasma spraying, Properties Abstract. This paper introduced the structure of the high efficiency hypersonic plasma spraying gun and the effects of hypersonic plasma jet on the sprayed particles. The optimised spraying process parameters for several ceramic powders such as Al 2 O 3, Cr 2 O 3, ZrO 2, Cr 3 C 2 and Co-WC were listed. The properties and microstructure of the sprayed ceramic coatings were investigated. Nano Al 2 O 3 -TiO 2 ceramic coating sprayed by using the high efficiency hypersonic plasma spraying was also studied. Compared with the conventional air plasma spraying, high efficiency hypersonic plasma spraying improves greatly the ceramic coatings quality but at low cost. Introduction Conventional air plasma spraying coating is characterized with low bond strength and high porosity, especially for ceramic coatings with high melting points. For conventional air plasma spraying guns, gas velocities of m/s and particles velocities in the range of m/s were obtained with 5-8 mm diameter nozzles and l/min total gas flow rates [1]. Additionally, the disordered plasma jet generated by conventional plasma gun has a boundary effect. As the distance from the nozzle exit increases, the enthalpy decreases and the air content drawn into the jet increases rapidly. The distribution of temperature, velocity and oxygen content in cross section of the jet are uneven. Thus the melting state, velocity and oxidation extent of the being sprayed particles near the border of the jet are out of condition compared with that in the center. In 1986 Browning Engineering Company invented a supersonic plasma spray system named PlazJet [2]. Hybridizing high voltage extended arc plasma with high gas acceleration in an extended nozzle, it leads to gas velocity in a supersonic mode at the nozzle exit with high arc power. During the last decade, several R & D works were focused on high power supersonic plasma spraying system [3,4]. The coatings quality sprayed with high power supersonic plasma spraying was improved, but the energy (up to 270 kw) and gas flow exhausts were extremely high, which leads to very high cost. In the present work, an advanced hypersonic plasma spraying system is presented with not only high ceramic coating quality, but also high efficiency and energy saved, and improved nano ceramic coating is obtained by using this hypersonic plasma spraying gun. High Efficiency Hypersonic Plasma Spraying Gun Novel hypersonic plasma spraying system is composed of plasma torch, powder feeder, gas supply, water cooling recirculator, control unit with PC interface and power supply unit [5]. The key of the system is hypersonic plasma spraying gun. The structure of the gun was specially designed to make the cooling water channels with perfect thermal pinching effect and the gases channels with strong vortex stabilization effect, so that the stabilized constricted supersonic plasma jet can be generated. Working gases are injected via gas vortex ring so that they pinch the arc and increase the gun s life. In order to heat and accelerate the powder particles adequately, the inner powder supply was injected via certain angle and position that solved the nozzle block up problem. The most important All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-19/02/16,22:13:09)

2 3982 PRICM-5 element of the gun is the anode nozzle. The anode nozzle with Laval shape was designed via CAD. Process Characteristics of Hypersonic Plasma Spraying Novel gun can generate the hypersonic plasma jet, in which 7 diamond shocks can be seen. The velocity of sprayed Al 2 O 3 particles was measured by using high speed CCD spray-watch and laser doppler velometer. The results show that the finer of the particle, the higher of its velocity. Within normal spray distance of mm, the mean velocity for 10% particles of high velocity is as high as 600 m/s. So the particle velocity of novel supersonic plasma spraying gun (mean particle velocity around 500 m/s measured 100 mm from nozzle exit) is about 2 times as high as conventional plasma spraying (particle velocity around 200 m/s measured 100 mm from nozzle exit). The spraying parameters of hypersonic plasma gun were optimized as listed in Table 1. Table 1 Optimized spray parameters of hypersonic plasma spraying gun 12Co-W C NiCr-Cr 3 C Powders sprayed Ni/Al Al 2 O 3 Cr 2 O 3 ZrO 2 2 Power (kw) Ar (m 3 h -1 ) H 2 (m 3 h -1 ) Spray distance(mm) Powder supply (kgh -1 ) Deposition efficiency (%) Improvements in Ceramic Coatings Properties of Sprayed Ceramic Coatings. Hypersonic plasma spraying system showed clearly improvements in ceramic coating qualities. As shown in Fig.1 and 2, the hypersonic plasma sprayed coatings have lower porosity and higher bond strength in comparison with typical conventional plasma spraying Metco 9M, which are almost equal to the coating quality of high power supersonic plasma spraying PlazJet (PlazJet sprayed Al 2 O 3 coating with porsity-0.81%, microhardness-hv1266 and bond strength-49.2 MPa, while novel supersonic plasma sprayed Al 2 O 3 coating with porsity-0.83%, microhardness-hv1357 and bond strength-51.1 MPa). The combination of proper microstructures and high hardness of catings enable them a better post finishing capability. The surface roughness of novel supersonic plasma sprayed ceramic coatings can be diamond ground to better than 75μm Ra. Porsity (%) Al2O3 12Co-WC NiCr-Cr3C2 Met co 9M Super soni c Bond strength (MPa) Al2O3 12Co-WC NiCr-Cr3C2 Met co 9M Super soni c Fig. 1 Comparison with coating porosity Fig. 2 Comparison with bond strength Microstructures of Sprayed Ceramic Coatings. SEM pictures of the cross-section of

3 Materials Science Forum Vols hypersonic plasma sprayed and Metco 9M sprayed ceramic coatings are shown in Fig. 3 respectively. Hypersonic plasma sprayed ceramic coatings possess much denser and more uniform microstructure than the conventional plasma sprayed coating. This is the main reason why novel supersonic plasma spraying gave rise to the improvement in coatings quality. (a) Novel supersonic plasma coating (b) Metco 9M coating Fig. 3 SEM of cross section of Al 2 O 3 coating X1500 Nano Al 2 O 3 -TiO 2 Ceramic Coating Fig. 4 shows the coating s TEM pictures of novel supersonic plasma sprayed nano Al 2 O 3 -TiO 2 ceramic powder, it can be seen that the coating consists of submicron crystal grains (grain size as 200~300nm) and nanometers crystal grains (grain size less than 50nm). Further XRD analysis shown that the coating is composed of α-al 2 O 3, brookite TiO 2 and γ -Al 2 O 3, with rather broader diffraction peaks of brookite TiO 2 andγ-al 2 O 3, which implies the existance of amorphous phases in the sprayed coating. Cost Analysis A comparison of the consumables and operating costs of novel supersonic plasma spray with Plazjet is shown in Table 2. The consumables are much less and energy saved for novel supersonic plasma spraying. So it costs only one half of the PlazJet for making coatings of the same coatings quality. a Nanometer crystal grain of the coating (DF) b Selected area electron diffraction (SAD) pattern Fig. 4 TEM of nano Al 2 O 3 -TiO 2 coating

4 3984 PRICM-5 Table 2 Comparison of consumables of novel supersonic plasma spray with Plazjet Primary gas flow (m 3 h -1 ) Secondary gas flow (m 3 h -1 ) Items PlazJet Novel supersonic plasma Ar N H Power (kw) Powder supply (kgh -1 ) Deposition efficiency (%) Costs of 1kg Al 2 O 3 coating (CNY ) Summary (1) A high efficiency hypersonic plasma spraying gun was specially designed to generate stabilized constricted supersonic plasma jet, while an inner powder supply manner was applied and a Laval nozzle was designed via CAD. With hypersonic plasma spraying gun, jet and particle velocities of 2400 and 500 m/s can be achieved respectively, 2 times as high as conventional air plasma spraying. The spraying parameters of the supersonic plasma gun were optimized. (2) The advanced hypersonic plasma spraying is able to produce ceramic coatings with groat improvement in performance and microstructure in comparison with the conventional ones. In regard to preducing coatings of the same level of quality, the production cost of the present technique is around one half of that of the high power supersonic one. (3) Nano Al 2 O 3 -TiO 2 ceramic coating sprayed via novel supersonic plasma spraying consists of grains of submicron and nanometers and amorphous phase maybe exist. Acknowledgements The financial supports for this work by the National Natural Science Foundation of China (Project No ), National Basic Research Foundation (Project No ) and Foundation of China Education Ministry are grateful acknowledged. References [1] M. Coudert, M. Planche, O. Betoule, M. Vardelle and P. Fauchais: Proc. National Thermal Spray Conf., Anadeim, CA., USA, ASM International(1993). [2] J. A. Browning: U.S. Patent No.626, 648(1986). [3] M. Breitsameter: Surface Engineering, Vol.12(1) (1996). [4] G. Irons, J. P. Poirier and R. A. Roy: Proc. 14th Int. Thermal Spraying Conf., Kobe, Japan(1995). [5] S. Zhu and B. Xu: Proc. APE2001, Warsaw, Poland (2001).

5 PRICM / High Quality Ceramic Coatings Sprayed by High Efficiency Hypersonic Plasma Spraying Gun / DOI References [5] S. Zhu and B. Xu: Proc. APE2001, Warsaw, Poland (2001) /S (00)