Ceramic Spraying using Electromagnetically Accelerated Plasma

In magneto-plasma-dynamic (MPD) arcjet generators, plasma is accelerated by electromagnetic body forces. The MPD arcjet generator can produce higher-velocity, higher-temperature, higher-density and larger-area plasmas than those of conventional thermal plasma torches. Two types of MPD arcjet generator were developed for applications to ceramic spray coatings. One generator was installed with a cathode covered with mullite or zirconia ceramics and the other with a titanium cathode. The former was operated with Ar for mullite or zirconia coating by an ablation process of the cathode cover and the latter with N₂ for titanium nitride coating by a reactive process between ablated titanium particles and nitrogen plasma. Silicon nitride reactive spraying was also carried out. The MPD spray process could successfully form dense, uniform and hard ceramic coatings. In titanium nitride reactive spraying, plasma diagnostic measurement and flowfield analysis were conducted. A large amount of N and N⁺ was expected to be exhausted with a high velocity from the MPD generator. Both the electron temperature and the electron number density were kept high at a substrate position compared with those for conventional low-pressure thermal sprayings. A chemically active plasma with excited particles of N⁺, Ti, Ti⁺ and Ti²⁺ was considered to contribute to better titanium nitride coatings. All coating characteristics showed that the MPD arcjet generators had high potentials for ceramic spray coatings.