TRIBOLOGICAL AND DECORATIVE COATINGS
While protective and decorative coatings serve different functions, they share similar sets of material and deposition requirements critical for specialty optics. Decorative coatings also provide protection to softer substrate materials against wear and corrosion, enabling lower weight products based on coated plastics to replace metal substrates. Hard wear-resistant coatings are metallic in appearance and are based on compounds of carbides, nitrides, carbonitrides, borides and oxides. They are generally opaque as opposed to optical interference coatings. Different functional applications require specific deposition conditions that include layer composition, thickness and substrate temperature tolerance. The high energy aggressive cathodic arc process produces µm-thick dense compositions with no color shift. Enhanced PVD processes such as High-Power Impulse Magnetron Sputtering (HIPIMS) continue to show promise of high performance at a fraction of the coating thickness. A large variety of sputter tools involve reactive gas control and custom target and geometrical considerations. Applications of decorative and tribological coatings range from consumer to military, space and sea environments.
Tribological coatings are used in applications that are exposed to severe contact forces under which normal thin-film coatings would fracture and lose adherence, thereby forfeiting their protective function. The reduction of abrasive and adhesive wear between contacting surfaces requires low friction coefficients, high hardness to resist abrasive wear that can result in stress cracking, erosion and fatigue . Wear occurs is several forms: adhesive welding between metals, shearing, abrasive, high-speed particle impact, erosive, corrosive, and high temperature induced chemical interaction . Welding does not occur between ceramic and ceramic-metal surfaces because ceramic materials are harder and less reactive than metals. Examples of coatings that are subject to these forms of wear include: metal cutting tools, mechanically contacting surfaces operating at high temperatures, optical surfaces exposed to high-velocity sand and water impact, and surfaces exposed to contact abrasion and corrosive liquids. Due to their increased durability properties, tribological coatings are employed in applications ranging from machining and medical implants, to military air and sea vehicles, commercial decorative metals and polymer surfaces.
An example of opti-tribo coatings currently used in commercial applications has been adapted from previous military technological requirements. Supersonic aircrafts encounter high impact velocities of sand, dust and rain, any of which can degrade the optical transmission by erosion or micro-fracturing. Thermal IR nighttime imagery is available on high-end cars in the consumer market. The imagers require a Germanium IR window that is protected with a DLC layer against erosion from blowing sand and rain. Environmental protection for military operations in harsh environments has been provided by DLC, GeC, BN, and other coatings. Previous issues of CMN have addressed some of these topics .
Click here to access the full technical paper, Tribological and Decorative Coatings.