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Review
. 2012 Jul 31;13(4):043001.
doi: 10.1088/1468-6996/13/4/043001. eCollection 2012 Aug.

Hierarchical Adaptive Nanostructured PVD Coatings for Extreme Tribological Applications: The Quest for Nonequilibrium States and Emergent Behavior

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Free PMC article
Review

Hierarchical Adaptive Nanostructured PVD Coatings for Extreme Tribological Applications: The Quest for Nonequilibrium States and Emergent Behavior

German S Fox-Rabinovich et al. Sci Technol Adv Mater. .
Free PMC article

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 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.

Keywords: adaptive materials; dissipative structures; emergent behavior; extreme applications; hierarchical materials; multi-functionality; nano-laminates; nano-structured PVD coatings; nano-tribology; self-organization; surface engineering; surface phenomena.

Figures

Figure 1
Figure 1
Fourier transforms of EELFS spectra from various microareas on the surface of worn craters of cutting tools with the TiAlCrSiYN monolayer coating (a, b) and TiAlCrSiYN/TiAlCrN multilayer coating (c, d). The nearest atomic surrounding has characteristics of sapphire (a) and mullite (b) structure; the standard interatomic bonds lengths in sapphire and mullite are presented at the top and the bottom, respectively. (Reproduced with permission from [17, 24], Elsevier, AIP.)
Figure 2
Figure 2
Beneficial heat redistribution on the surface of coated cutting tools due to formation of various tribofilms: (a) no tribofilms, (b) lubricious tribofilms and (c) protective tribofilms with the thermal barrier properties. Vc—cutting speed, m min−1; Q—heat flow: Qch—heat flow in chips; Qt—heat flow in tool, Vc1 = Vc2 = Vc3.
Figure 3
Figure 3
TEM images (cross-sectional views) of the worn coated ball-nose end mills with (a) TiAlCrSiYN monolayered and (b) TiAlCrSiYN/TiAlCrN multilayered coatings. (Reproduced with permission from [17], Elsevier.)
Figure 4
Figure 4
TEM images (a, c) with SAED patterns (b, d) of the worn TiAlCrSiYN/TiAlCrN coating (see figure 3): (a, b) 45 nm from the surface, no evidence of h-AlN; (c, d) 85 nm from the surface, small amount of the h-AlN phase. (Reproduced with permission from [240], AIP.)
Figure 5
Figure 5
Partial fluorescence yield x-ray absorption spectra of N-K edge after vacuum annealing of the TiAlCrSiYN/TiAlCrN multilayer coating at various temperatures: c-TiN and h-AlN N-K edge standards are added for comparison, to show the evolution in thermal decomposition of the TiAlCrSiYN/TiAlCrN multilayer. Features A and B of the nitrogen signal are related to the first, most prominent peaks of c-TiN and h-AlN, respectively. The inset shows the tip of the worn ball-nose tool that was cut and placed on top of a copper holder to acquire the XANES signal. (Reproduced with permission from [240], AIP.)
Figure 6
Figure 6
(a) TEM image of the Ti0.2Al0.55Cr0.2Si0.03Y0.02N/ Ti0.25Al0.65Cr0.1N coating (cross-sectional view) with SAED pattern inset, (b) HAADF-STEM image and energy-dispersive spectroscopy profile of the coating and (c) TEM image with SAED pattern inset. (Reproduced with permission from [17], Elsevier.)
Figure 7
Figure 7
TG-DTA data versus temperature in air for (a) monolayered TiAlCrSiYN coating and (b) multilayered TiAlCrSiYN/TiAlCrN coating. (Reproduced with permission from [17], Elsevier.)
Figure 8
Figure 8
Micromechanical properties of TiAlCrSiYN/TiAlCrN multilayer and TiAlCrSiYN monolayer coatings measured at room and elevated temperatures: (a) microhardness, (b) reduced elastic modulus and (c) H3/Er2 ratio. (Reproduced with permission from [240], AIP.)
Figure 9
Figure 9
Impact fatigue data of TiAlCrSiYN/TiAlCrN multilayer (b) and TiAlCrSiYN monolayer (a) coatings measured at room temperature; (c) the cross-sectional TEM image shows the propagation of nanocracks in nanomultilayered coating, which could be a possible mechanism of thermomechanical energy dissipation in the studied coating. (Reproduced with permission from [17, 70], Elsevier.)
Figure 10
Figure 10
SEM images of surface morphology versus length of cut of the worn coated ball-nose end mills with (a) TiAlCrSiYN monolayered and (b) TiAlCrSiYN/TiAlCrN multilayered coatings. Tool: carbide ball-nose end mill, D = 10 mm; workpiece-H13 tool steel, hardness HRC 53–55, speed: 500 m min−1; feed, 0.06 mm per tooth; axial depth, 5.0 mm; and radial depth, 0.6 mm. (Reproduced with permission from [17], Elsevier.)
Figure 11
Figure 11
Tool life data for adaptive TiAlCrSiYN-based coatings under strongly varying cutting conditions: (a–c) during dry machining of hardened steels. Tool, carbide ball-nose end mill, D = 10 mm; workpiece-H13 tool steel, hardness HRC 53–55; speed, 400–500 (a) and (b); 500–700 (c) m min−1; feed, 0.06 mm per tooth; axial depth, 5.0 mm; radial depth, 0.6 mm; (d) during turning of Ni-based aerospace DA 718 alloy, hardness HRC 47–48; speed, 40 m min−1; feed, 0.125 mm rev−1; depth of cut, 0.25 mm; (e) during deep hole drilling of PM hardened structural steel, grade MPIF P/F 11C60 (C: 0.4–0.8%, Cu, 1.5–2.5%, Fe-balance; HRC 30–35). Speed, 69.1 m min−1; feed rate, 625 mm min−1; depth of cut, 39 mm.
Figure 12
Figure 12
Schematic of emergent behavior of the adaptive coatings under intensifying external conditions.

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