Lafad‐Assisted Plasma Surface Engineering Processes for Wear and Corrosion Protection: A Review

Abstract

The large area filtered arc deposition (LAFAD) process is characterized for deposition of various metal, ceramic and cermet coatings having different architectures: from monolithic ceramic to nano-microlaminated, nanolayered and sub-stoichiometric coatings. Duplex processes, consisting of a bottom ionitrided segment followed by coating layers, have been tested for application in forming tools, dies and molds. The barrier properties of LAFAD coatings are mostly due to its nearly defect-free morphology and smoothness as it was demonstrated in die casting die applications and in the application of protective coatings for solid oxide fuel cell (SOFC) metallic interconnects, operating in oxidizing environment at 800 C. Ultra-thick ceramic and cermet coatings with thicknesses ranging from 30 to 120 μm have been deposited by one unidirectional dual arc LAFAD vapor plasma source onto substrates installed on a rotating turntable 0.5 m in diameter in an industrial coating system with deposition rates exceeding 5 μm/hr. The operating pressure range of the LAFAD process allows it to be used in plasma-assisted hybrid technologies with conventional magnetron sputtering and electron beam evaporation processes. The industrial applications of LAFAD wear and corrosion resistant coatings; their performance in different applications; and a commercialization strategy for LAFAD technology will be discussed. INTRODUCTION The contemporary surface engineering technology has reached the ability to grow layered structures in a controlled way, leading ultimately to a new generation of electronic and optical devices, cutting and forming tools operating at higher speeds, protecting the turbomachinery components against erosion and corrosion in a harsh environment, widening the horizons of new energy related applications from fuel cells to power stations, to automotive and aircraft components operating at high contact stress, among many others. In order to meet requirements for coatings operating in extreme conditions such as high contact stress, hostile environments, high temperatures and vibration, coating properties such as adhesion and cohesion toughness, fracture resistance, corrosion and high temperature oxidation resistance must be improved compared to the present state-of-the-art coatings. It is well established that assistance of the coating deposition process with bombardment by energetic particles, especially energetic metal ions, can dramatically improve coatings by densifying the depositing materials, reducing the grain size and reducing or completely eliminating the growth defects. It is also a mechanism for improving coating adhesion and cohesion toughness by mixing the substrate atoms with the atoms of the depositing coating, or mixing the atoms of the neighbor sublayers in laminated coating architectures by ion-bombardment assisted deposition processes. In coating processes assisted with ion bombardment, the surface layer is affected by a high rate of bombardment by energetic ions, which affects the mobility of depositing metal vapor atoms and in many cases creates metastable structures with unique functional properties. In addition, the ion bombardment of the coating surface influences gas adsorption behavior by increasing the sticking coefficient of gases such as nitrogen and changing the nature of adsorption sites from lower energy physisorption sites to higher energy chemisorption sites. This approach is especially productive in deposition of nanostructured nanocomposite coatings with ultrafine or glass-like amorphous structure. The ion-to-atoms arrival ratio represents one of the most important characteristics of such processes. It is essential that this ratio is calculated at the atom Figure 1. Schematic illustration of IBAD