The role of substrate plasticity on the tribological behavior of diamond-like nanocomposite coatings
Introduction
Sliding contact between ductile materials often induces large plastic strains in the near-surface regions [1], [2]. In addition, material transfer, mechanical mixing between components and reactions with the environment can occur during tribological contacts. In the case of metallic materials, the formation of metallic junctions and stick–slip behavior can further complicate tribology, specifically when sliding occurs in the absence of external fluid lubrication [3], [4], [5], [6]. Once the native surface oxide is removed, most metals exhibit high coefficients of friction (COFs), generally ranging from 0.6 to 1.2 [4]. Surface coatings are therefore employed to mitigate these tribological problems. Hard coatings such as TiC, TiN, Al2O3, TiCN are typically used to provide resistance to wear [7], [8], while solid lubricant coatings like MoS2, WS2, diamond-like carbon (DLC) and PTFE provide significant reductions in COFs as well [9], [10], [11].
In addition to the intrinsic tribological behavior of a coating, substrate deformation (elastic, plastic or elastoplastic) plays a crucial role in governing the overall frictional response of a coated system. Consequently, an understanding of the stress state within the coating and the underlying metallic substrate is required to determine the tribological reliability of a coated material. The purpose of this study is to utilize finite-element simulations as a means to determine the contact stress at which the transition from elastic to plastic deformation occurs in either a tribological coating or substrate, and relate this transition to the experimentally observed changes in the COF with contact stress determined from the normal load from ball-on-disc tribometer measurements.
There has been considerable progress in the simulation and finite-element analysis (FEA) of tribological contacts to more fully understand the deformation processes occurring underneath static and sliding contacts [12], [13], [14], [15], [16], [17], [18], [19], [20]. Although much of the early work restricted the problem by considering only elastic or plastic deformation, and generally limited the calculations to two-dimensional approximations, these relatively simplistic computational simulations have allowed for significant insight into the deformation mechanisms in tribological contacts. For the specific case of stress generation during indentation or sliding contacts, both two-dimensional (2-D) [14], [16], [19], [20] and (3-D) [15], [17] computational studies have been performed with considerable success. For example, the two-dimensional simulations by Djabella and Arnell provided insight into the contact stress development in elastic double-layer systems under normal loading conditions [14], while Tain and Saka addressed the problem of spherical contacts sliding over a layered elastic–plastic half-space [19]. A fully 3-D model for an elastic sphere sliding over an elastic–plastic substrate was subsequently developed by Kral and Komvopoulos in 1996 [17] and more comprehensive 3-D simulations for a thin hard coating on a elastic–plastic substrate were later performed by Holmberg and coworkers [15]. These studies have shown the unique flexibility of FEA in the design of tribological systems by presenting insights into how changes in material properties and interfaces under various mechanical loading situations alter the tribological response.
Model systems comprising of diamond-like nanocomposite (DLN) coating on a metallic substrate (either an electroformed nickel alloy or Inconel 718) are considered for the present study. These substrates, while of considerable interest to microsystem fabrication, form a suite of materials with a range of yield strengths from ∼500 MPa to 1 GPa. Our previous research has shown that the DLN coatings, processed from siloxane precursors by plasma-enhanced chemical vapor deposition (PECVD), can be applied conformally on the sidewalls of microsystem parts [21]. The coatings exhibit relatively high hardness and elastic modulus, typically 12–17 GPa and 88–128 GPa, respectively [24]. In the current study, FEM simulations of plastic strains generated under spherical contacts were compared with experimental frictional tests utilizing ion-channeling and electron backscatter diffraction (EBSD) data from focused ion beam (FIB) sections of the wear scars.
Section snippets
Finite-element simulations
FEM simulations were performed to evaluate the evolution of plastic strain beneath a tribological contact. Simulations were also executed to determine the mechanical properties of the nickel substrate and DLN coating, while the properties of the nickel–manganese and Inconel 718 subtrates was determined directly from uniaxial tension tests. All the simulations discussed here utilized ABAQUS/Standard version 6.4 with ABAQUS/CAE employed in post-simulation model visualization. The 2-D axisymmetric
Instrumented indentation and FEM
Typical load–displacement indentation data generated on DLN-coated nickel substrates with a 3.175 mm diameter Si3N4 ball are shown in Fig. 1. The thickness of the DLN coating was 520 nm. It should be noted that these data points correspond to the composite mechanical response of the coated system, and not of the individual coating or the substrate. The experimental load–displacement data results are binned and averaged from five separate indentation tests; the error bars are contained within the
Frictional behavior of solid lubricants
For the case of solid lubricant films where the macroscopic stresses are elastic, non-Amontonian friction behavior has been extensively explored [10], [27]. Solid lubricating films such as transition metal dichalcogenides and diamond-like carbon are known to function by forming a thin transfer film on the counterface to provide shear accommodation at the interface. For the case of a Si3N4 ball sliding on a DLN coating, the initial coefficient is between the Si3N4 surface and the coating. Once
Summary and conclusions
Finite-element simulations have been used to elucidate contact stress-induced plasticity resulting in the deviation of the COF from Hertzian models. It is proposed that at the contact stresses where plastic deformation is induced, accumulated plastic strain at the coating-substrate interface can lead to film breakdown, which results in a dramatic increase in the COF. For conditions mirroring actual tribological tests on a coated material, the load (and subsequent contact stress) at which the
Acknowledgments
The authors gratefully acknowledge Chandra Venkatraman and Cyndi Brodbeck of Bekaert Advanced Coating Technologies for supplying the diamond-like nanocomposite coatings, and Rand Garfield for performing some of the friction measurements. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy under contract DE-AC04-94AL8500.
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