Coatings

Abstract Many materials emit light during the application of a mechanical energy. This phenomenon is usually referred to as mechanoluminescence (ML) or triboluminescence. The more historical term is “triboluminescence.” It stands for tribo‐induced luminescence, and this was the term used for more than a century to refer to light emission induced by any type of mechanical energy. The term “mechanoluminescence” was not proposed until 1978. The prefix “mechano” is correlated to the general mechanical way used for exciting luminescence, including concepts such as deformation, piezo, tribo, stress, cutting, grinding, rubbing, and fracto. In recent years mechanoluminescence (ML) has become the preferred nomenclature. Although the transfer of mechanical stress into light radiation is very complex, successes in experimental applications suggest possible uses of the ML phenomena in stress sensors, mechanical displays, and various smart systems. In general, ML can be divided into fractoluminescence (destructive ML) and deformation luminescence (nondestructive ML); these correspond to the luminescence induced by fracture and mechanical deformation of solid, respectively. Roughly 50% of solid materials gives fractoluminescence by fracture: the well‐known materials include sugar, molecular crystals, alkali halides, quartz, silica glass, phosphors, piezoelectric complex, metals, various minerals, and biomaterials. Recently, the fractoluminescence of rare‐earth complexes was investigated in order to build smart damage sensors capable of simple real‐time detection of the magnitude and location of structural damage within materials. Deformation luminescence can be induced by mechanical deformation without fracture, and this is of interest in nondestructive evaluation. Deformation luminescence can be further divided into plasticoluminescence and elasticoluminescence. The former is produced during plastic deformation of solids, where fracture is not required, and the later is produced during the elastic deformation of solids where neither plastic deformation nor fracture is required. Nondestructive ML due to plastic deformation has been observed in several materials such as colored alkali halides, II–VI semiconductors, and rubbers. However, ML in the elastic region has been observed only for the irradiated alkali halides, and some piezoelectric materials. So far nondestructive luminescence intensities of materials have been reported to be too weak and difficult to repeat, and this has deferred any practical application of the phenomenon. For application of ML in developing new materials, repetitive ML must occur with undiminished intensity.