Diamond tools are known to wear rapidly during the machining of ferrous materials because of the catalytic effect of iron on diamond graphitization. The research presented in this dissertation is aimed at investigating three fundamental issues related to the use of two-dimensional (2D) materials for mitigating iron-induced catalytic wear of diamond. These include: (i) the influence of interstitial carbon present in ferrous workpieces on diamond wear and graphene platelet attrition; (ii) a chemistry-based understanding of the relative diamond tool wear mitigation efficacies of graphene oxide (GO), hexagonal boron nitride (hBN), tungsten disulfide (WS2), and molybdenum disulfide (MoS2); and (iii) the role played by surface-platelet interaction forces on the physical transport of 2D materials into tool-workpiece contact interface.
The influence of interstitial carbon present in ferrous workpieces on diamond wear and graphene platelet attrition is studied using a molecular dynamics (MD) model with an interatomic potential proven to capture iron-induced diamond wear. The results show that higher workpiece carbon concentration reduces both diamond tool wear and graphene platelet attrition. This suggests that surface carbonization of the workpiece, even at low concentrations can mitigate diamond tool wear. The replenishment of graphene platelets present in the cutting zone is more critical for machining ferrous alloys with lower carbon content.
On the experimental front, monocrystalline diamond turning of steel workpieces is used to compare the wear mitigation efficacy of graphene oxide (GO), hexagonal boron nitride (hBN), molybdenum disulfide (MoS2) and tungsten disulfide (WS2). Overall, graphene oxide has the highest reduction in tool wear (60%) over the baseline case. It is followed by hexagonal boron nitride, tungsten disulfide and molybdenum disulfide that offered 52%, 45% and 38% wear reduction, respectively, over the baseline case. Overall, hBN provides the best combination of low tool wear, low cutting forces and low surface roughness. The tool wear mitigation efficacy of the materials can be explained in terms of their specific chemical/physical interactions at the iron-diamond interface. X-ray Photo Spectroscopy (XPS) measurements reveal a clear correlation between the tool wear trends and strength of the graphitic carbon/carbide signals obtained on the machined surfaces. XPS measurements in the boron (B1s) and sulfur (S2p) regions shed light on the chemical reactions responsible for the performance of hBN, WS2 and MoS2. Physical transport of the platelets at the iron-diamond interface is an important pre-cursor to the experimentally observed chemical pathways responsible for mitigating tool wear. Therefore, the role played by surface-platelet interaction forces on the physical transport of 2D materials into tool-workpiece contact interface is investigated using a combination of density functional theory (DFT) calculations and molecular dynamics simulation. The DFT calculations provide additive-surface adhesion energies for the purpose of developing interatomic potentials in MD that are then able to simulate the physical transport of platelets into the iron-diamond contact. The simulation results show that platelet-iron adhesion and platelet-diamond adhesion play critical roles in successful transport of additives into contacts. The model-generated physical transport rankings are consistent with the wear mitigation trends observed in the experiments. The approach and resulting model insights have improved the contemporary understanding of solid-state cutting fluid additives, which mostly relied on qualitative descriptions of additive behaviors at tool-workpiece interface.;
August 2021; School of Engineering
Dept. of Mechanical, Aerospace, and Nuclear Engineering;
Rensselaer Polytechnic Institute, Troy, NY
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