Atmospheric pressure, non-thermal plasma deposition for durable protective coatings is a promising candidate to replace electroplating and revolutionize coating technologies that address Department of Defense life-cycle cost and environmental issues related to weapons systems. The objectives of this project were to investigate a novel atmospheric pressure, non-thermal High Power Impulse Plasma Source (HiPIPS) for deposition of field applicable, protective coatings and characterize the structural, mechanical, and corrosion performance of the resultant coatings.
Southwest Research Institute conducted a systematic experimental study on the HiPIPS process parameters and resultant coating properties toward development of HiPIPS deposition of cobalt-chrome (CoCr), Ti6Al4V, and TiN coatings. Using HiPIPS, Argon (Ar)-initiated metallic titanium (Ti), CoCr, or Ti-6Al-4V plasmas were generated and the plasma properties were characterized by measuring current-voltage characteristics and optical emission spectroscopy (OES). Pressure hardened 4340 steel and 7075 aluminum alloy substrate materials were evaluated. The corrosion (cyclic accelerated corrosion test, GM 14872) and mechanical performance (fatigue testing per American Society for Testing and Materials [ASTM] E466) of the HiPIPS coatings were evaluated and compared with traditional chrome plated and uncoated alloys. The chrome plating was performed by the Corpus Christi Army Depot. The microstructure and chemical composition of the resulting HiPIPS coatings were examined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, X-ray diffraction, and nanoindentation.
Altogether, the results of this project advanced the atmospheric pressure HiPIPS technique to Technical Readiness Level 4, validation in a laboratory environment to apply durable, metallic coatings. HiPIPS investigations with Ar and N2 gas and Ti, CoCr, or Ti-6Al-4V wires confirmed the hypothesis that high peak power pulses can be achieved (10-70 kW) resulting in high peak currents (100-250 A) and increased ionization and plasma density while maintaining low overall power (40W) and substrate processing temperatures (< 50 °C). OES spectral measurements confirm the presence of metal ions with high photon intensities in HiPIPS plasmas. HiPIPS CoCr, Ti-6Al-4V and TiN coatings were successfully deposited. Cross-section SEM analysis and energy dispersive spectroscopy mapping revealed uniform coatings. The nano-hardness of HiPIPS TiN, Ti6Al4V and CoCr films ranged from 11.56 to 14.09, 3.92 to 4.44, and 6.20 to 8.42 GPa, respectively. In GM 14872 accelerated corrosion test environment, the HiPIPS applied Ti6Al4V and CoCr coatings showed slightly more corrosion damage than the conventional coatings, but significantly less than that of the uncoated sample. Under fatigue testing per ASTM E466, the HiPIPS Ti6Al4V coatings had a similar stress-life response as baseline, uncoated performance and the HiPIPS CoCr coatings were approximately half the fatigue life of the baseline conditions.
The project provided vital insight needed for the development of a new deposition technology that could reduce the costs and environmental risks in maintenance and replacement of military components. The HiPIPS process is attractive for the repair or replacement of electroplated hard chromium in line-of-sight applications. HiPIPS is non-thermal, atmospheric pressure plasma capable of producing highly ionized species for deposition of durable metallic films. Additionally, HiPIPS could allow for coating removal, pre-cleaning, and coating using the same equipment and varying process parameters. A HiPIPS system is portable and can be operated in versatile environments. Further development work is required to further increase the technology readiness level and identify specific applications.