Structure and Tribological Performance of diamond-like carbon based coatings for aerospace component processing

Abstract

Conventional electroplating processes, such as hard chromium plating, are extensively used on aircraft components to improve their functionality and reduce wear. While hard chrome plating offers certain benefits, it also has some categorical disadvantages. Being a known carcinogen and respiratory toxicant, chromium exposure poses a significant occupational health hazard. [1,2] Chromium can also be released to the environment via accidental spillage, spray and disposal of waste plating solution where it is biomagnified in terrestrial and aquatic food chains. In many cases the coating has to be removed at every overhaul interval, the components have to be reworked to remove defects and new coating applied in order to recover assembly design dimensions. Clearly, the availability of non-toxic, high-performance coatings would benefit aircraft companies by enabling them to reduce electroplating emissions and implement best environmental practices. Previous research and development efforts have established that diamond-like carbon (DLC) coatings have the potential of minimising the friction and wear of various moveable devices [3-6]. DLC coatings possess many attractive properties such as high hardness, low friction and high chemical stability and have been successfully used in automotive, medical and manufacturing industries. DLC coating is produced when carbon is deposited under energetic conditions by plasma enhanced chemical vapour deposition (PECVD). In contrast to other coating systems no external heating is applied therefore temperature sensitive aircraft materials can be coated. High internal stresses during plasma-deposition can limit DLC coating thickness to less than 10m thus limiting their practical application, particularly in maintenance and overhaul of aircraft components. In order to produce a thick coating with optimum wear properties a different approach is required. A new coating system, based on a layered structure of epoxy interlayer with DLC applied for the surface finish, has a great potential in restoring worn or damaged aircraft components in a more efficient and, on balance, more environmentally-friendly manner.