Graphitization as a precursor to wear of diamond in machining pure iron: A molecular dynamics investigation

Abstract

Abstract It is well known that although diamond is the hardest known material, it cannot be used effectively for machining pure iron or low carbon ferrous alloys due to extremely rapid tool wear. Several research groups have postulated that the mechanism for the observed wear of diamond tools involves the initial transformation of tetrahedral diamond into hcp graphite, the thermodynamically more stable form of carbon under the usual conditions of machining/grinding. The next step in the postulated mechanism involves the diffusion of graphitic carbon into the iron workpiece. In spite of the wealth of publications reporting experimental investigations of this phenomenon, this proposed mechanism still remains no more than a hypothesis, albeit, a reasonable one. The problem is that the time scales (a few seconds to a minute or more) over which the experiments are conducted are too long to permit direct observation of the diamond → graphite transformation, which occurs on a nanosecond to picosecond time scale, if it occurs at all. In this paper, we utilize molecular dynamics (MD) and realistic interaction potentials to provide the first direct evidence that the diamond → graphite transformation does occur and, therefore, could be the principal mechanism of wear of single-point diamond tools in the machining of ferrous materials. MD simulations of nanometric cutting of pure iron oriented in (1 0 0) and cut along 〈1 0 0〉 direction have been conducted in different orientations of the clearance face, namely, (1 1 0), (1 1 1), and (1 0 0) with a diamond tool at a cutting speed of 100 m s −1 to investigate the micromechanisms of diamond tool wear. A modified embedded atom (MEAM) potential was used for the Fe–Fe and Fe–C interactions, and a Tersoff potential for the C–C interactions. The computations employed the large-scale atomic/molecular massively parallel simulator (LAMMPS) software developed at the Sandia National Laboratory. The results provide the first direct evidence that as cutting commences, the structure of diamond at the cutting edge begins to transform from diamond cubic into hexagonal graphite in the presence of iron. Subsequent to this transformation, the graphitic carbon diffuses into the iron. The diamond (1 0 0) plane was found to be the most resistant and the (0 1 1) plane the least resistant to graphitization with the (1 1 1) plane showing intermediate propensity for transformation to a graphite structure. These results are in accord with reported experimental observations. Thus, the MD observations provide direct evidence supporting the wear mechanisms that have been proposed in the literature.