Chameleon Coatings: Adaptive Surfaces to Reduce Friction and Wear in Extreme Environments

Abstract

Adaptive nanocomposite coating materials that automatically and reversibly adjust their surface composition and morphology via multiple mechanisms are a promising development for the reduction of friction and wear over broad ranges of ambient conditions encountered in aerospace applications, such as cycling of temperature and atmospheric composition. Materials selection for these composites is based on extensive study of interactions occurring between solid lubricants and their surroundings, especially with novel in situ surface characterization techniques used to identify adaptive behavior on size scales ranging from 10 −10 to 10 −4 m. Recent insights on operative solid-lubricant mechanisms and their dependency upon the ambient environment are reviewed as a basis for a discussion of the state of the art in solid-lubricant materials. 297 Click here for quick links to Annual Reviews content online, including: • Other articles in this volume • Top cited articles • Top downloaded articles • Our comprehensive search Further ANNUAL REVIEWS Tribology: the study of surfaces in relative motion, from the Greek word tribos, meaning rubbing Friction coefficient: the ratio of tangential force to applied normal force, or the ratio of shear strength to pressure if both forces are divided by contact area Abrasive wear: the plowing away of softer material by harder particles at contact interfaces Adhesive wear: asperities from both surfaces in contact adhere, and material from the softer surface is sheared away as the counterface moves Fatigue wear: the removal of material due to crack growth and propagation over repeated load cycles Wear rate: the rate of material loss from a surface due to sliding contact, often reported in units of volume lost per unit sliding distance per unit of applied normal force (e.g., mm 3 m −1 N −1) FUNDAMENTALS OF SOLID LUBRICATION IN DIVERSE ENVIRONMENTS A spray of liquid lubricant from a can available at the hardware store solves most familiar friction and wear problems on Earth under ordinary conditions of ambient pressure, humidity, and temperature. When any one of those operating parameters reaches the limits of advanced engineering materials, however, the availability of suitable lubricants is diminished significantly. Moreover, if operation throughout a broad range of any one of these environmental factors is desired, the number of choices becomes exactly zero; tribologists have found that no single surface can lubricate effectively outside a narrow span of ambient humidity or temperature (1-3). In fact, one of the primary technological barriers for the development of new jet engines for sustained highmach-number (>5) flights and other key aerospace advances is the ability to lubricate mechanical assemblies from ambient temperature to >1000 • C. Only recently have discoveries of self-adaptive mechanisms and subsequent developments in tribological coatings, made possible with modern instrumentation and pioneering efforts to observe phenomena at atomic through microscopic size scales at contact ...

Combined reactive/non-reactive DC magnetron sputtering of high temperature composite AlN–TiB2–TiSi2

“…The changes in the functional properties of gradient coatings are achieved through continuous variation of elemental composition and microstructure with the depth of the coating. The main ways of practical application of the gradient coatings are materials and tools which undergo high gradients of mechanical forces as well as temperatures [15]. Therefore, it is possible to combine complex functional properties, such as high hardness, corrosion resistance, high-temperature oxidation resistance and wear resistance, in one coating using the concept of gradient coatings.…”

Self-Lubricity of WSex Nanocomposite Coatings

“…The improved performance of this tailored WSe x lubricant material is highlighted and discussed for tribological applications. 5 Tribological tests have been carried out in the CATRI © IK4-TEKNIKER HV tribometer with a 3-mm 100Cr6 ball with variable surrounding atmosphere (atmospheric pressure to 10 -8 mbar). The WSe x samples deposited onto a silicon substrate were supported in a vertical oscillating rod that moves in the sliding direction.…”

Oxidation and diffusion processes during annealing of TiSi(V)N films

“…Traditionally, oils and other liquid lubricants have been used to reduce the friction between the cutting tool and piece primarily by shearing the oil molecules across the solid-liquid-solid interface. However, most of the liquid lubricants volatilize at high temperature, which leads to the dry sliding and consequent failure of the cutting tools resulting in the increasing machine down times, higher process instability, poor product quality and higher costs [1]. To face these problems, a wide range of solid lubricant coatings, such as WC/C, MoS 2 , diamond-like carbon (DLC), h-BN as well as their combinations in nanocrystalline or multilayer structures, have been developed in the last decades and successfully applied in order to improve the tribological behaviour under dry machining conditions [2][3][4][5].…”