Hierarchical adaptive nanostructured PVD coatings for extreme tribological applications: the quest for nonequilibrium states and emergent behavior

Abstract

Adaptive wear-resistant coatings produced by physical vapor deposition (PVD) are a relatively new generation of coatings which are attracting attention in the development of nanostructured materials for extreme tribological applications. An excellent example of such extreme operating conditions is high performance machining of hard-to-cut materials. The adaptive characteristics of such coatings develop fully during interaction with the severe environment. Modern adaptive coatings could be regarded as hierarchical surface-engineered nanostructural materials. They exhibit dynamic hierarchy on two major structural scales: (a) nanoscale surface layers of protective tribofilms generated during friction and (b) an underlying nano/microscaled layer. The tribofilms are responsible for some critical nanoscale effects that strongly impact the wear resistance of adaptive coatings. A new direction in nanomaterial research is discussed: compositional and microstructural optimization of the dynamically regenerating nanoscaled tribofilms on the surface of the adaptive coatings during friction. In this review we demonstrate the correlation between the microstructure, physical, chemical and micromechanical properties of hard coatings in their dynamic interaction (adaptation) with environment and the involvement of complex natural processes associated with self-organization during friction. Major physical, chemical and mechanical characteristics of the adaptive coating, which play a significant role in its operating properties, such as enhanced mass transfer, and the ability of the layer to provide dissipation and accumulation of frictional energy during operation are presented as well. Strategies for adaptive nanostructural coating Topical Review design that enhance beneficial natural processes are outlined. The coatings exhibit emergent behavior during operation when their improved features work as a whole. In this way, as higher-ordered systems, they achieve multifunctionality and high wear resistance under extreme tribological conditions.

Improvement of Wear Performance of Nano-Multilayer PVD Coatings under Dry Hard End Milling Conditions Based on Their Architectural Development

“…Assuming that the tensor component satisfies, σ 33 = 0, σ 23 = σ 13 =0, σ 11 = σ 22 = σ, then Equation (1) becomes Equation (3).…”

“…High hardness combined with the low thermal conductivity is mandatory for continuous dry cutting conditions associated with hard turning [1]. However, when the cutting environment becomes more complex, as in interrupted cutting conditions the coating layer must achieve improved multi-functionality [3]. Modern hard PVD (Physical vapor deposition) coatings mostly have a nano-crystalline columnar structure [4].…”

“…In general nano-laminates are effective in reducing the brittleness of hard coatings [8]. The brittleness of the hard coating is a very important factor to be addressed under interrupted cutting conditions where poor low cycle fatigue performance is a parameter that limits tool life [2,3]. This characteristic of the coating could be evaluated through the nano-impact fracture resistance [9].…”

Tribofilm Formation As a Result of Complex Interaction at the Tool/Chip Interface during Cutting

“…It has been found that there are two types of tribofilms that are formed on the frictional body as a result of structural modification and interaction with the environment: (1) super-ductile and lubricating films [12]; and (2) tribo-ceramics with thermal barrier properties and increased hardness and strength [10,24,26,27,[30][31][32][33][34][35][36][37][38][39]. Tribofilms (SS) of the first type (SS-I) are observed as a result of structural activation marked by an increase in the density of atomic defects at the surface.…”

“…Various features of tribofilms (or secondary structures, i.e., generated during friction) were intensively studied [6][7][8][9][10][11][12][13][14][15][16][17]. The tribofilms generated during friction can be divided into lubricating [6] and hard ceramic tribo-phases [7].…”