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Boron Nitride Sputtering Measurements

        Ion sputtering is a primary life-limiting mechanism in electric propulsion thrusters used for satellite and space exploration. We are interested in the sputtering of boron nitride (BN) because of its widespread use as as an acceleration channel wall material in stationary plasma thrusters (SPTs). Erosion of the insulator channel is the dominant thruster life-limiting mechanism in SPTs. Furthermore, deposition of the sputtered BN can contaminate spacecraft surfaces (e.g. solar panels or thermal control surfaces).  
Hall Thruster
        Despite the importance of BN erosion there is a lack of basic sputtering data on BN. Numerical modeling of thruster erosion shows that the ions most critical to the erosion process have relatively low energy (<~100 eV). Measurements in this energy range are challenging since the low sputter yields can lead to signals below the detection sensitivity of the measurement system; additionally, it can be difficult to obtain collimated monoenergetic ion beams at these energies. The goal of this study is to contribute towards filling this gap.
Boron Nitride under SEM
      A high sensitivity quartz crystal microbalance (QCM) based sputter measurements system is used in this study. A DC Kaufman ion source is used to produce collimated ion beams at the low ion energies of interest to this work (20 - 500 eV) and a QCM deposition sensor is used to measure differential and total sputter yields of BN. The ion source and QCM are housed within a vacuum chamber equipped with a high vacuum pumping system. The temperature-controlled sample holder allows us to study temperature dependence of the sputter yield. Using this system differential and total sputter yields for several grades of BN at ion energies down to 60 eV were measured.
Yield Results Test Results Plotted Over Hemisphere
Ion Thruster in Operation Hall Thruster in Operation
 
 
References:  
  • J. Yim, "Computational Modeling of Hall Thruster Channel Wall Erosion", Ph.D. Dissertation, University of Michigan (2008)
     
  • J. Topper, B. Rubin, C.C. Farnell, A.P. Yalin, "Preliminary Results for Low Energy Sputter Yields of Boron Nitride due to Xenon Ion Bombardment", 44th AIAA Joint Propulsion Conference, AIAA-2008-5092, Hartford, CT (2008)
     
  • B. Rubin, J.L. Topper, A.P. Yalin, "Total and Differential Sputter Yields of Boron Nitride Measured by Quartz Crystal Microbalance", J. Phys. D: Appl. Phys., 42, 205205 (2009)
       

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