o Ie Ing ALEXANDER M. AGHASI R. TOROSYAN/ AND KYUNGMOK KIM' DEPARTMENT OF ENGINEERING SCIENCE, UNIVERSITY OF OXFORD, OXFORD, U.K. "INSTITUTE OF GENERAL AND INORGANIC CHEMISTRY, NATIONAL ACADEMY OF SCIENCES, YEREVAN, ARMENIA Development and Characterization of Low-Friction Coatings for Protection Against Fretting Wear in Aerospace Components (Editor's Note: This article originally appeared m Thin Solid Films) published by Elsevier B. V Korsunsky et at. 2008;516:5690-9.) F retting is a contact degradation process that arises due to recip- rocal relative displacement between the surfaces of two contacting com- ponents. Fretting damage is a com- bination of fatigue, wear, and corro- sion (chemical) damage that occurs at frictional contacts. Fretting fatigue is the phenome- non of crack initiation and propaga- tion from the region of stress con- centration in reciprocating frictional contacts. Conditions for crack initia- tion are largely determined by the shape of the contacting surfaces and the corresponding local tractions, and by the coefficient of friction. Conditions for crack propagation, on the other hand, depend on the underlying state of stress in the com- ponent, which may be determined by various other aspects of deformation state (e.g., remote loading) apart from the contact stresses. Once the crack grows to a substantial length comparable with the extent of the region of stress concentration at the contact, its further propagation is primarily controlled by the cyclically varying underlying stress state in the component. If the amplitude of this cyclic stress variation is too low, or, www.metalfinishing.com for example, the underlying remote state of stress is compressive, then propagation is arrested, and fretting fatigue does not take place. The regimes of relative displace- ment between the two contacting surfaces can be classified into full stick (no slip), partial slip (some part of the contact remains adhered, while some other pairs of contacting points experience relative slip), and gross slip (every pair of points in contact experience slip). Gross slip allows large relative displacement of con- tacting parts, and thereby leads to the reduction of the tensile stresses induced by the contact. Large ampli- tude gross slip also results in the dis- tribution of contact damage over larger surface regions of contacting bodies, thus reducing the likelihood of crack initiation. Conversely, partial slip ensures that the mating surfaces are 'locked' together over part of the contact patch, and that the alternat- ing tangential force is transmitted through the body in the form of alter- nating stress that is tensile at least over part of the cycle. Thus, partial slip gives rise to the stressing conditions and damage localization that are more dangerous from the viewpoint of promoting fretting fatigue, while gross slip leads to the relief of such stresses, and the promotion of the phenomenon of fretting wear.' 2 Under the conditions of gross slip, relative sliding of the surfaces leads to progressive surface damage and material removal. At the macroscop- ic level, frictional sliding involves shearing of the 'tops' of contacting asperities, material transfer to the opposite surface in a contacting pair, oxidation of debris, and usually an increase in the coefficient of friction. This complex phenomenon is referred to as fretting wear. It is clear that fretting wear is promoted by gross slip, and is inhibited by partial slip (since relative sliding is limited). Further, if the reciprocating sliding amplitude is substantially increased, then the wear damage is distributed over larger surface area, leading to a reduction in fretting wear. Thus, the most significant amount of fretting wear is likely to occur under gross slip at amplitudes close to the transi- tion into partial slip regime. The likelihood of crack initiation can be reduced efficiently if the coef- ficient offriction (COF) is reduced as far as possible. For this purpose a number of various anti-friction coat- ings have been proposed, often based on the incorporation of particles of dry film lubricant (DFL), such as graphite or molybdenum disulfide, in a polymer matrix. These soft coat- ings are usually based on a polymer matrix, and deposited onto the sur- face by spraying. While the coating layer is sufficiently intact to perform its COF-reducing function, crack ini- tiation is inhibited and the assembly is safe. However, under low COF con- ditions the application of remote cyclic loading usually leads to the contact undergoing cyclic gross slip, and ultimately suffering from the consequences of fretting wear. These consequences include degradation of the coating, reduction of the coating thickness, and the increase in the coefficient of friction. Eventually this may lead to the transition from gross slip to partial slip regime, and February 2009 I metalflnishlng I 45