Resistance of tantalum and columbium coatings to propellant gas erosion

Alloy steel tubes of 70 mm length and 20 mm bore diameter were coated with tantalum or niobium by the wire explosion technique. This process uses the energy of an electric current pulse to melt or vaporize the coating material which is in the form of a wire in the center of the tube, and to deposit it on the bore surface. For testing, these tubes were mounted as outlet nozzles on a vented vessel and subjected to propellant gas pulses of 30 ms duration and 2500 K. The pressure buildup inside the vessel was 1000 bar. After each pulse the erosive wear of the test pieces was determined and after testing the reaction zone of the coatings was sputtered to a depth of 2 μm and analyzed by AES and SIMS during the process. Both coating materials showed excellent adhesion and their erosion resistance proved superior to the unprotected alloy steel. Tantalum, however, exhibited a much better performance than niobium. This was due to the different properties of the gas/metal reaction layers formed. In both cases the bore surface was covered with propellant gas residue of 1 μm thickness, that was built up and carried out with each pulse. Underneath, a layer consisting of Ta2O5 or Nb2O5 was formed. While Ta2O5 was mechanically stable and therefore, built up to a considerable thickness providing good protection, the Nb2O5 layer was formed and removed continuously with every round. This pattern exposed the unprotected Nb to the next pulse and caused a much faster wear rate