Silicon Nitride Spraying Using Quasi-Steady MPD Arcjets

Transcription

1 Silicon Nitride Spraying Using Quasi-Steady MPD Arcjets IEPC Presented at the 29 th International Electric Propulsion Conference, Princeton University, Hirokazu Tahara * and Toshiaki Edaitsu + Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka , Japan Abstract: In agneto-plasa-dynaic (MPD) arcjet generators, plasa is accelerated by electroagnetic body forces. Silicon nitride reactive spraying was carried out using an MPD arcjet generator with crystal silicon rods and nitrogen gas. Because higher-velocity, higher-teperature and higher-density and larger-area plasas are produced with the MPD arcjet generator than those with conventional theral plasa torches, nitriding of silicon can be enhanced. A dense and unifor -Si 3 N coating 30 thick was fored after 200 shots at a repetitive frequency of 0.03 Hz with a discharge current of 9 ka and a substrate teperature of 700 o C. The Vickers hardness reached about Velocity and teperature histories of silicon particles generated in the MPD generator nozzle were nuerically siulated. The velocity and teperature of particles increased downstrea. The velocity reached about 1750 /s at the nozzle exit, and the particle was copletely elted when the particle diaeter was 1. This result shows that the MPD arcjet generator has high potentials for silicon nitride spraying. I. Introduction The quasi-steady agneto-plasa-dynaic (MPD) arcjet generator is a proising plasa accelerator, which has a coaxial electrode structure siilar to those of conventional plasa torches. 1 However, their acceleration echaniss are different; i.e., the MPD arcjet generator utilizes principally electroagnetic acceleration of the interaction between the discharge current of kiloaperes and the agnetic field aziuthally induced by the discharge current, although the working gas is accelerated aerodynaically through a straight or convergent-divergent nozzle in a theral plasa jet generator. In egawatt-class input-power repetitive pulsed operations, higher-velocity, higher-teperature, higher-density and larger-area plasas can be produced with the MPD arcjet generators than those with conventional plasa sources. 2,3 The discharge plasas are expected to be utilized for various aterial anufacturing processes. -8 In the present study, silicon nitride reactive spraying is carried out using an MPD arcjet generator with crystal silicon rods and nitrogen gas. The coating properties are analyzed with SEM and XRD. Vickers hardness is also easured. Furtherore, velocity and teperature histories of silicon particles generated in the MPD generator nozzle, which affect coating properties, are calculated coputationally. II. Experiental Apparatus Figure 1 shows the cross-sectional view of the coaxial MPD arcjet generator developed for silicon nitride spraying. The rod cathode 6 in diaeter is ade of thoriated tungsten. The anode nozzle ade of copper has an exit diaeter of 58 and a half angle of 20 degree. Four crystal silicon rods (cross section: 5 x 1 ; purity: %) are located in the divergent nozzle, and the distance between the edge of the silicon rods and the cathode tip is 20. Nitrogen is used as the working gas. As shown in Fig.2, a high-current arc is expected to elt the silicon rod surface, and then the ablated silicon particles react with nitrogen plasa. * Associate Professor, Departent of Mechanical Science and Bioengineering, 1-3, Machikaneyaa, E-ail: tahara@e.es.osaka-u.ac.jp,aiaa Meber. + Graduate Student, Departent of Mechanical Science and Bioengineering, 1-3, Machikaneyaa, E-ail: edaitsu@arl.e.es.osaka-u.ac.jp. 1

2 Cathode(W-ThO2) Gas Port Anode(Cu) 30 Crystal Silicon Rod Figure 1. Cross-sectional view of MPD arcjet generator for silicon nitride reactive spraying. Figure 2. Illustration of silicon nitride reactive spraying using MPD arcjet generator. The ain power-supplying pulse-foring network is capable of storing 62 kj at 8 kv and delivers a single nonreversing axiu quasi-steady current of 27 ka with a pulse width of 0.58 s. The MPD arcjet generator is installed on a stand in a vacuu tank 1 in diaeter x 1.2 in length. The tank pressure is kept at 2-5x10-2 Pa during periodical operations. Unprepared substrate plates 125 x 125 x, ade of stainless steel SS00, are placed at 150 downstrea fro the arcjet generator exit. The substrate teperature is controlled ranging fro roo teperature to 700 o C with an electrical heater behind the substrate plate. III. Silicon Particle - Plasa Flow Interaction Calculation Triple-probe easureents were ade to evaluate electron teperatures and electron nuber densities of the MPD arc plasas, and eission spectroscopic easureents were also conducted to identify excited ion and ato species in the plasas. 9 Axisyetric MPD generator flowfield was coputationally calculated in order to understand the nitrogen plasa acceleration processes. 10,11 Conservation equations of ass, oentu and energy in addition to the agnetic field equation are a group of odified Euler equations. By using all plasa diagnostic and flowfield calculated data, velocity and teperature histories of silicon particles generated in the MPD generator nozzle, which affect coating properties, are calculated. In this calculation odel, a silicon particle with an axial velocity of zero is injected at 20 downstrea fro the cathode tip on the central axis. The silicon particle is assued to be axially accelerated by the drag force fro the plasa flow and to be uniforly heated by energy transfer fro the plasa flow. Two odels of oentu and heat transfers are expressed as a function of the Knudsen nuber. 12,13 2

3 IV. Results and Discussion A. Coating Characteristics The MPD arcjet generator for silicon nitride reactive spraying was operated with 200 shots at a repetitive frequency of 0.03 Hz (1shot/30sec) for a N 2 ass flow rate of 2.5 g/s. Figure 3 shows a photograph of cross section and the XRD pattern of the coating with a substrate teperature of 700 o C at a discharge current of 9 ka. Adense and unifor coating 30 thick is fored after 200 shots. If the MPD arcjet generator is operated at a repetitive frequency of 0.05 Hz (1shot/20sec), the deposition rate is expected about 10 ties higher than those for conventional CVD and PVD ethods. As shown in Fig.3(b), peaks of Si are not identified, and the icrostructure of -Si 3 N is created. Figure shows the thickness of the coating dependent on discharge current. The thickness rapidly increases fro 5 at 6 ka to 30 at 9 ka. This is expected because an aount of silicon particles ablated by current concentration on the crystal silicon rods intensively increases as the discharge current increases. Intensity ( a.u. ) - Si3N Fe - Si3N Fe 30 u , deg. ( CuK ) (a) Cross-sectional view (b) XPD pattern Figure 3. Photograph of cross section and XRD pattern for silicon nitride coating after 200 shots with N 2 ass flow rate of 2.5 g/s at discharge current of 9 kaand substrate teperature of 700 o C. Thickness, N 2 2.5g/s Substrate Teperature Discharge Current, ka Vickers Hardness Hv 2000 substrate teperature 700 substrate teperature Discharge Current, ka Figure. Thickness vs discharge current characteristics for silicon nitride coating after 200 shots with N 2 ass flow rate of 2.5 g/s at substrate teperature of 700 o C. Figure 5. Vickers hardness vs discharge current characteristics for silicon nitride coating after 200 shots with N 2 ass flow rate of 2.5 g/s at substrate teperatures of 300 and 700 o C. 3

4 Figure 5 shows the Vickers hardness of the coating at substrate teperatures of 300 and 700 o C dependent on discharge current. The Vickers hardness increases fro 1250 at 6 ka to 1300 at 9 ka with 700 o C and fro 800 at 6 ka to 1100 at 9 ka with 300 o C. More adhesive coatings are deposited with increasing discharge current because of particles sprayed with higher-velocity and higher-teperature. At a constant discharge current, the hardness at 700 o C is higher than that at 300 o C. In XRD patterns, as the substrate teperature decreased, the -Si 3 N peak becae broad. Sprayed particles are rapidly solidified on the substrate as the substrate teperature decreases. Therefore, the icrostructure of the coating is changed fro crystal to aorphous, resulting in decreasing hardness. Accordingly, very hard coatings are deposited although depending on discharge current and substrate teperature. B. Velocity and Teperature Characteristics of Sprayed Silicon Particles The velocity and teperature histories, i.e., in-flight characteristics, of sall silicon particles ablated fro the crystal silicon rods are roughly estiated. Figure 6 shows the velocity and teperature histories of silicon particles through the divergent nozzle at a discharge current of 6 ka dependent on particle diaeter. The calculated results with odel 1 alost equaled those with odel 2. 12,13 The particle velocity gradually increases downstrea. At a constant axial position, a decrease in particle diaeter raises the velocity. The particle velocity with 1 reaches about 1750 /s at the nozzle exit. In particle teperature, as shown in Fig.6(b), the teperature rapidly increases downstrea. The particle teperatures for sall particles of 1, 5 and 10 reach the elting teperature in the divergent nozzle, and furtherore the teperature with 1 reaches the boiling one. Accordingly, high-velocity and copletely-elted sprayed particles are expected to be expanded fro the nozzle exit of the MPD arcjet generator. Particle Velocity, /s 2000 odel Axial Position, Particle Teperature, K 3000 b.p p odel Axial Position, (a) Particle velocity (b) Particle teperature Figure 6. Calculated velocity and teperature histories of silicon particles through divergent nozzle at discharge current of 6 ka dependent on particle diaeter. V. Conclusions Silicon nitride reactive spraying was carried out using an MPD arcjet generator with crystal silicon rods and nitrogen gas. A dense and unifor -Si 3 N coating 30 thick was fored after 200 shots at a repetitive frequency of 0.03 Hz with a discharge current of 9 ka and a substrate teperature of 700 o C. The Vickers hardness reached about It was found that coating thickness was highly sensitive to discharge current and that icrostructure of the coating was sensitive to substrate teperature. Velocity and teperature histories of silicon particles generated in the MPD generator nozzle, which affect coating properties, were nuerically siulated. The velocity and teperature of particles increased downstrea. The velocity reached about 1750 /s at the nozzle exit, and the particle was copletely elted when the particle diaeter was 1. This result shows that the MPD arcjet generator has high potentials for silicon nitride spraying.

5 References 1 Jahn, R.G., Physics of Electric Propulsion, McGraw-Hill, New York, 1968, Chap Tahara, H., Kagaya, Y., and Yoshikawa, T., Effects of Applied Magnetic Fields on Perforance of a Quasisteady Magnetoplasadynaic Arcjet, Journal of Propulsion and Power, Vol.11, pp , Tahara, H., Kagaya, Y., and Yoshikawa, T., Perforance and Acceleration Process of Quasisteady Magnetoplasadynaic Arcjets withapplied Magnetic Fields, Journal of Propulsion and Power, Vol.13, pp , Tahara, H., Abe, T., Tsubaki, T., Kagaya, Y., Tsubakishita, Y., Yoshikawa, T., Kuwata, M., and Ueda, Y., Applications of Quasi-Steady Magneto-Plasa-DynaicArcjets to Ceraic Coatings, Japanese Journal of Applied Physics, Vol.32, pp , Tahara, H., Kagaya, Y., and Yoshikawa, T., Developent of a Magnetoplasadynaic Arc Jet Generator for Ceraic Coatings, Proceedings of the 1 th International Theral Spraying Conference, Vol.1, pp , Tahara, H., Matsuda, T., Shibata, T., Andoh, Y., Yasui, T., Kagaya, Y., and Yoshikawa, T., Research and Developent of a Magnetoplasadynaic Arc Jet Generator for Ceraic Spray Coatings, Proceedings of the International Syposiu on Designing, Processing and Properties of Advanced Engineering Materials, pp , Tahara, H., Shibata, T., Andoh, Y., Yasui, T., Kagaya, Y., and Yoshikawa, T., Electroagnetic Acceleration Plasa Spraying for Ceraic Coatings, Proceedings of the United Theral Spray Conference, pp , Tahara, H., Shibata, T., Yasui, T., Kagaya, Y., and Yoshikawa, T., Electroagnetic Acceleration Plasa Spraying, Proceedings of the 1 th International Syposiu on Plasa Cheistry, Vol., pp Shibata, T., Tahara, H., Yasui, T., and Yoshikawa, T., Plasa Flow Characteristics of Electroagnetic Acceleration Plasa Jet Generator for Titaniu-Nitride Reactive Spray Coatings, Proceedings of the 15 th International Syposiu on Plasa Cheistry, Vol.6, pp , Tahara, H., Mitsuo, K., Kagaya, Y., and Yoshikawa, T., Magnetoplasadynaic Channel Flow Research, 35 th Joint Propulsion Conference, Los Angeles, AIAA , Tahara, H., Yasui, H., Kagaya, Y., and Yoshikawa, T., Experiental and Theoretical Researches on Arc Structure in a Self-Field Thruster, 19 th International Electric Propulsion Conference, Colorado Springs, AIAA , Chen, Xi, Heat and Moentu Transfer between a Theral Plasa and Suspended Particles for Different Knudsen Nubers, Thin Solid Fils, Vol.35, pp.10-15, Gnedovets, A.G., Kinetic Models of Plasa-Particle Charge, Moentu and Energy Transfer under Rarefied Flow Conditions, Pure & Appl. Che., Vol.68, pp ,