Quantifying the inhibition chemistry and platelet-interface mechanics of two-dimensional-material additives against iron-catalyzed wear of monocrystalline diamond tools

Monocrystalline diamond tools wear rapidly when cutting steels because fresh iron surfaces catalyze conversion of the diamond's four-fold ($sp^3$) bonded carbon to graphitic three-fold ($sp^2$) carbon and absorb the carbon by diffusion. Colloidal suspensions of two-dimensional (2D) platelets — graphene oxide (GO), hexagonal boron nitride (hBN), tungsten disulfide (WS$_2$), and molybdenum disulfide (MoS$_2$) — added to the cutting fluid markedly reduce this wear, but the protection is empirical: the responsible reactions have never been quantified, and the three hypothesized inhibition pathways (physical barrier interlayer, competitive oxygen consumption, and catalytic inhibition of iron) remain unranked. The open problem aims at predictive additive design: quantify the inhibition reactions (rates, stoichiometries, conversion efficiencies) under cutting-zone-relevant conditions so that the relative contributions of the three pathways to the observed wear inhibition can be explicitly determined, and tie that attribution to the platelet–surface interactions at the iron–diamond interface that govern whether platelets enter the cutting zone and sustain a protective interlayer there.

Background

In ultra-precision diamond turning, a single-crystal (monocrystalline) diamond cutting edge removes material at nanometric depth resolution. Machining ferrous workpieces has long been known as the 'ferrous taboo': beyond mechanical abrasion, the diamond chemically degrades because chemically active iron catalyzes graphitization of the diamond surface — the conversion of stiff four-fold coordinated $sp^3$ carbon into layered $sp^2$ graphite — and carbon diffuses into the iron, so the edge wears catastrophically fast. Lane, Dow and Scattergood built a thermo-chemical wear model for single-crystal diamond tools cutting steel (Wear 300 (2013) 216–224), showing wear-side kinetics can be modeled, and a 2020 critical review by Jiang and co-workers (International Journal of Extreme Manufacturing 2 (2020) 012001) surveyed suppression strategies such as ultrasonic vibration cutting, cryogenic cooling, surface nitridation, and plasma-assisted cutting. A complementary, less mature route is to modify the cutting fluid itself with two-dimensional platelet additives. Smith et al. showed that graphene oxide colloidal suspensions mitigate carbon diffusion during diamond turning of steel (Journal of Manufacturing Processes 17 (2015) 41–47). Chu, Shi and Samuel then performed molecular dynamics (MD, atomistic simulation of motion) of a graphene platelet in nanometric cutting of steel with a diamond tool (Carbon 108 (2016) 61–71): the platelet wraps around the cutting edge as a built-up edge, acting as a physical barrier and a sacrificial carbon source, and tool wear begins only after the platelet is cleaved or slid out of the contact. Hon and Samuel (Journal of Manufacturing Processes 68 (2021) 95–103) extended this from graphene to a controlled comparison of four 2D additives (graphene oxide (GO), hexagonal boron nitride (hBN), tungsten disulfide (WS$_2$), and molybdenum disulfide (MoS$_2$), each 0.15 wt.% in a semi-synthetic cutting fluid) in diamond turning of ASTM A108 low-carbon steel, measuring projected worn area on the rake face (from 428 $\mu\mathrm{m}^2$ with the carrier fluid down to 173–265 $\mu\mathrm{m}^2$ with the additives) and analyzing the machined surfaces by X-ray photoelectron spectroscopy (XPS, a surface-sensitive technique that resolves elemental bonding states). XPS revealed additive-specific residues: boron oxide (B$_2$O$_3$) from hBN, iron sulfate (FeSO$_4$) from WS$_2$, and iron sulfide (FeS) from MoS$_2$. From these residues the authors hypothesized three co-existing inhibition pathways — (i) a physical barrier interlayer that separates diamond from iron, (ii) competitive oxygen consumption, in which additive oxidation (e.g., of hBN) consumes the dissolved oxygen that participates in oxidative wear reactions, and (iii) catalytic inhibition, in which platelet-derived surface films (e.g., FeS) passivate iron's catalysis of graphitization. Crucially, these mechanisms were inferred post hoc from residue chemistry: no reaction rates, stoichiometries, or conversion efficiencies were measured, so the relative importance of the three pathways is unknown, and the atomistic picture exists only for graphene (2016 MD), not for the other platelets, while the platelet–surface interactions that decide whether a platelet can actually penetrate the cutting zone and persist there were not characterized in that study. Density functional theory (DFT, electronic-structure computation of interfacial energies and bonding) is available in principle for these interfaces — it has been applied to related questions such as graphene adhesion and lubricity on iron oxide (Ta et al., Journal of Physical Chemistry C 125 (2021) 4310–4321) and to diamond doping for suppressing graphitization (Hao, Manzhos and Zhang, Frontiers in Materials 8 (2021) 806466) — but Hon and Samuel (2021) perform no such calculation for the platelet-loaded iron–diamond cutting interface.

Problem Statement

For monocrystalline diamond tools cutting ferrous workpieces (the configuration studied by Hon and Samuel, 2021: diamond turning of ASTM A108 low-carbon steel with 0.15 wt.% colloidal suspensions of graphene oxide, hexagonal boron nitride, tungsten disulfide, or molybdenum disulfide in a semi-synthetic cutting fluid), establish the mechanistic and quantitative basis of 2D-material wear inhibition, so that additive protection becomes predictive rather than empirical. Quantify the chemical reactions through which these additives inhibit iron-catalyzed diamond wear (e.g., hBN oxidation to B$_2$O$_3$, WS$_2$ oxidation and sulfate formation, MoS$_2$ conversion to iron sulfide, and graphene-family barrier chemistry): determine reaction rates, stoichiometries, and conversion efficiencies under cutting-zone-relevant conditions, so that the relative contributions of the three hypothesized pathways — physical barrier interlayer formation, competitive oxygen consumption, and catalytic inhibition of iron-driven graphitization — to the observed wear inhibition can be explicitly determined for each additive. Because the barrier pathway operates only where platelets actually reach and persist in the tool–workpiece contact, this attribution is to be tied to the platelet–surface interactions at the iron–diamond interface that govern whether platelets penetrate the cutting zone and sustain a protective interlayer there (including the conditions under which the interlayer fails, e.g., by cleaving or sliding-out as observed in the 2016 graphene simulations), so that the quantified chemistry yields a predictive basis for designing 2D-material additives against iron-catalyzed diamond wear. For each of the four additives this quantification comprises: the quantified reaction set (reaction rates, stoichiometries, and conversion efficiencies); the resulting relative contributions of the three pathways, with the barrier-pathway share following from the interfacial transport and persistence conditions and the oxygen-consumption and catalytic-inhibition shares from the quantified chemistry; and the interfacial conditions under which the platelet reaches and persists in, or is expelled from, the contact.

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