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Contact Mechanics of Layered and Functionally Graded Materials
Changing the elastic modulus of the material or – more generally – introducing material gradients can be an effective means in altering tribological behavior. So called Functionally Graded Materials (FGM) are fairly common for many biological and natural structures. They have been studied since the late 1950s, initially in context of geomechanics and later in context of various engineering applications. It has been shown that FGM can provide better mechanical properties including wear and damage resistance than homogeneous materials. Interest in FGM was enhanced by establishing new manufacturing techniques (3D printing), which allow manufacturing of materials with basically arbitrary spatial distribution of mechanical properties.
Fretting of Functionally Graded Materials (FGM)
Applications with frictional contacts under vibrations should find
an optimal path between Scylla of fretting wear and Charybdis of
fretting fatigue. Analysis of fretting of functionally graded
materials shows that, roughly speaking, fretting damage decreases
if the material is softer on the surface than in the depth.
Especially interesting is the case of the Gibson-medium which may
help avoiding both fretting wear and fretting fatigue. In this
case, no fretting happens at all, and the stress concentration at
the boarder of the contact is absent too.
Emanuel Willert, Andrey I. Dmitriev, Sergey G. Psakhie & Valentin L. Popov , Effect of elastic grading on fretting wear, Scientific Reports 9, Article number: 7791 (2019): https://www.nature.com/articles/s41598-019-44269-1 
Markus Hess, A study on gross slip and fretting wear of contacts involving a power-law graded elastic half-space, Facta Universitatis, series: Mech. Eng., 17(No1), pp. 47-64 (2019): http://casopisi.junis.ni.ac.rs/index.php/FUMechEng/article/view/4923