Lars Pastewka

Anisotropic mechanical amorphization drives wear in diamond

Diamond is the hardest material on Earth. Nevertheless, polishing diamond is possible with a process that has remained unaltered for centuries and is still used for jewellery and coatings: the diamond is pressed against a rotating disc with embedded diamond grit. When polishing polycrystalline diamond, surface topographies become non-uniform because wear rates depend on crystal orientations. This anisotropy is not fully understood and impedes diamonds widespread use in applications that require planar polycrystalline films, ranging from cutting tools to confinement fusion. Here, we use molecular dynamics to show that polished diamond undergoes an sp(3)-sp(2) order-disorder transition resulting in an amorphous adlayer with a growth rate that strongly depends on surface orientation and sliding direction, in excellent correlation with experimental wear rates. This anisotropy originates in mechanically steered dissociation of individual crystal bonds. Similarly to other planarization processes, the diamond surface is chemically activated by mechanical means. Final removal of the amorphous interlayer proceeds either mechanically or through etching by ambient oxygen.

Contact between rough surfaces and a criterion for macroscopic adhesion

Significance Macroscopic objects rarely stick together, yet the van der Waals interactions between surface atoms produce attractive pressures that are orders of magnitude larger than atmospheric pressure. This “adhesion paradox” has been linked to surface roughness, which reduces the area of intimate atomic contact to summits on the rough landscape. This paper presents a parameter-free theory that captures the interplay between elasticity, interatomic attraction, and surface roughness. It predicts how adhesion changes contact area and when surfaces are sticky. The results offer a simple explanation for why tape sticks to our desktops but a sheet of paper does not, and may aid in the design of adhesives and in engineering surface roughness to enhance or eliminate adhesion. At the molecular scale, there are strong attractive interactions between surfaces, yet few macroscopic surfaces are sticky. Extensive simulations of contact by adhesive surfaces with roughness on nanometer to micrometer scales are used to determine how roughness reduces the area where atoms contact and thus weakens adhesion. The material properties, adhesive strength, and roughness parameters are varied by orders of magnitude. In all cases, the area of atomic contact is initially proportional to the load. The prefactor rises linearly with adhesive strength for weak attractions. Above a threshold adhesive strength, the prefactor changes sign, the surfaces become sticky, and a finite force is required to separate them. A parameter-free analytic theory is presented that describes changes in these numerical results over up to five orders of magnitude in load. It relates the threshold adhesive strength to roughness and material properties, explaining why most macroscopic surfaces do not stick. The numerical results are qualitatively and quantitatively inconsistent with classical theories based on the Greenwood−Williamson approach that neglect the range of adhesion and do not include asperity interactions.At the molecular scale there are strong attractive interactions between surfaces, yet few macroscopic surfaces are sticky. Extensive simulations of contact by adhesive surfaces with roughness on nanometer to micrometer scales are used to determine how roughness reduces the area where atoms contact and thus weakens adhesion. The material properties, adhesive strength and roughness parameters are varied by orders of magnitude. In all cases the area of atomic contact rises linearly with load, and the prefactor rises linearly with adhesive strength for weak interactions. Above a threshold adhesive strength, the prefactor changes sign, the surfaces become sticky and a finite force is required to separate them. A parameterfree analytic theory is presented that describes changes in these numerical results over up to five orders of magnitude in load. It relates the threshold strength to roughness and material properties, explaining why most macroscopic surfaces do not stick. The numerical results are qualitatively and quantitatively inconsistent with classical theories based on the Greenwood-Williamson approach that neglect the range of adhesion and do not include asperity interactions. 1 ar X iv :1 31 1. 11 78 v1 [ co nd -m at .m tr lsc i] 5 N ov 2 01 3 Surfaces are adhesive or “sticky” if breaking contact requires a finite force. Few of the surfaces we encounter are sticky even though almost all are pulled together by van der Waals interactions at atomic scales.1 Gecko setae2,3 and engineered adhesives4 use this ubiquitous attraction to achieve pull off forces per unit area that are orders of magnitude larger than atmospheric pressure, and our world would come to a halt if these pressures operated on most macroscopic surfaces. The discrepancy between atomic and macroscopic forces has been dubbed the adhesion paradox.5 Experiments show that a key factor underlying this paradox is surface roughness, which reduces the fraction of surface atoms that are close enough to adhere.5–8 Quantitative calculations of this reduction are extremely challenging because of the complex topography of typical surfaces, which have bumps on top of bumps on a wide range of scales.9,10 In many cases they can be described as self-affine fractals from a lower wavelength λs of order nanometers to an upper wavelength λL in the micrometer to millimeter range.7,11 Here, we use an efficient Green’s function approach to calculate adhesive contact of surfaces with roughness from subnanometer to micrometer scales. Numerical results for a wide range of surfaces, adhesive interactions and material properties are presented and used to develop a simple, parameter-free equation that predicts the effect of adhesion on contact. The traditional Greenwood-Williamson (GW)12 approach for calculating contact of rough surfaces approximates their complex topography by a set of spherical asperities of radius R whose height distribution is determined from self-affine scaling. The long-range elastic interactions between different asperities are neglected. This approach is analytically tractable and provided a simple explanation for the observation that the area of contact between nonadhesive elastic surfaces is proportional to the normal force or load pushing them together. Later generalizations6,13 considered the effect of adhesion between surfaces and found that the key parameter was the ratio of the root mean squared (rms) height variation hrms to the normal displacement δc of a single asperity due to adhesion. If the work of adhesion gained per unit area of contact is w, then δ c = (3/4) 3R(πw/E∗)2 with contact modulus14 E∗ = E/(1 − ν) for an isotropic material with Young’s modulus E and Poisson ratio ν. GW based adhesion models6,13 predict that the force needed to separate surfaces drops rapidly as hrms/δc increases and is negligible for hrms/δc > 3. In the last decade, Persson has developed a scaling theory that includes an approximate treatment of asperity interactions.15,16 At the same time, large scale numerical calculations of contact between rough surfaces have become feasible.17–20 Both approaches reveal limitations in the GW treatment of nonadhesive surfaces. For example, the definition of R is ambiguous,22 the predicted

Atomic Scale Mechanisms of Friction Reduction and Wear Protection by Graphene

We study nanoindentation and scratching of graphene-covered Pt(111) surfaces in computer simulations and experiments. We find elastic response at low load, plastic deformation of Pt below the graphene at intermediate load, and eventual rupture of the graphene at high load. Friction remains low in the first two regimes, but jumps to values also found for bare Pt(111) surfaces upon graphene rupture. While graphene substantially enhances the load carrying capacity of the Pt substrate, the substrates intrinsic hardness and friction are recovered upon graphene rupture.

Screened empirical bond-order potentials for Si-C

Typical empirical bond-order potentials are short ranged and give ductile instead of brittle behavior for materials such as crystalline silicon or diamond. Screening functions can be used to increase the range of these potentials. We outline a general procedure to combine screening functions with bond-order potentials that does not require to refit any of the potentials properties. We use this approach to modify Tersoffs [Phys. Rev. B 39, 5566 (1989)], Erhart & Albes [Phys. Rev. B 71, 35211 (2005)] and Kumagai et al.s [Comp. Mater. Sci. 39, 457 (2007)] Si, C and Si-C potentials. The resulting potential formulations correctly reproduce brittle materials response, and give an improved description of amorphous phases.

Finite-size scaling in the interfacial stiffness of rough elastic contacts

The total elastic stiffness of two contacting bodies with a microscopically rough interface has an interfacial contribution K that is entirely attributable to surface roughness. A quantitative understanding of K is important because it can dominate the total mechanical response and because it is proportional to the interfacial contributions to electrical and thermal conductivity in continuum theory. Numerical simulations of the dependence of K on the applied squeezing pressure p are presented for nominally flat elastic solids with a range of surface roughnesses. Over a wide range of p, K rises linearly with p. Sublinear power-law scaling is observed at small p, but the simulations reveal that this is a finite-size effect. We derive accurate, analytical expressions for the exponents and prefactors of this low-pressure scaling of K by extending the contact mechanics theory of Persson to systems of finite size. In agreement with our simulations, these expressions show that the onset of the low-pressure scaling regime moves to lower pressure as the system size increases.

Wear, Plasticity, and Rehybridization in Tetrahedral Amorphous Carbon

Wear in self-mated tetrahedral amorphous carbon (ta-C) films is studied by molecular dynamics and near-edge X-ray absorption fine structure spectroscopy. Both theory and experiment demonstrate the formation of a soft amorphous carbon (a-C) layer with increased sp2 content, which grows faster than an a-C tribolayer found on self-mated diamond sliding under similar conditions. The faster

Experimental and numerical atomistic investigation of the third body formation process in dry tungsten/tungsten-carbide tribo couples

\hbox{sp}^{3} \rightarrow\,\hbox{ sp}^{2}

Contact area of rough spheres: Large scale simulations and simple scaling laws

sp3→sp2 transition in ta-C is explained by easy breaking of prestressed bonds in a finite, nanoscale ta-C region, whereas diamond amorphization occurs at an atomically sharp interface. A detailed analysis of the underlying rehybridization mechanism reveals that the