Experimental and Numerical Atomistic Investigation of the Third Body Formation Process in Dry Tungsten/Tungsten-Carbide Tribo Couples

Abstract

The third body in tungsten/tungsten-carbide sliding systems is studied using a combination of experiments and atomistic simulations. Ex situ X-ray photoelectron spectroscopy and focused ion beam analysis of the structural and chemical changes near the surfaces reveals that sliding of tungsten against tungsten-carbide results in plastic deformation of the W surface, leading to grain refinement, and the formation of a mechanically mixed amorphous layer on the WC counter body. Molecular dynamics simulations of W/WC sliding couples exhibit the formation of a nanoscale amorphous W/WC interface. The infrequent occurrence of atomic jamming events in the interface resulted in the emission of dislocations into the W bulk and the generation of amorphous shear bands in the WC counter body in agreement with the different third bodies observed in W and WC after the experiments.

Appendix: Screened W–C Potential

The screened interatomic W–C potential is based on the potential by Juslin et al. [29]. To facilitate screening, we replace all cut-off functions by screening functions as detailed in Ref. [24]. In addition, we modify the bond-order function in Eq. (1) from the form employed in Ref. [29].:

$$ \begin{aligned} b_{ij} = \left( {1 + \chi_{ij} } \right)^{{ - \frac{1}{2}}} \; \\ {\text{with}}\; \\ \chi_{ij} = \mathop \sum \limits_{k \ne i,j} S_{ik} \exp \left\{ {\left[ {2\mu \left( {r_{ij} - r_{ik} } \right)} \right]^{m} } \right\}g(\theta_{ijk} ) \end{aligned} $$

(2)

Here, i, j and k denote atom indices and $r_{ij}$, $r_{ik}$ are the distances between the respective atoms. $S_{ik}$ is the screening function, and $g\left( {\theta_{ijk} } \right)$ the angular contribution to the bond-order where $\theta_{ijk}$ is the angle spanned by the atoms i–j–k. In addition to the screening cutoffs and the screening function parameters (see Ref. [24]), we need to specify the parameters $\mu$ and m from Eq. (2). Unlike Ref. [24], we use a single screening function and do not distinguish between attractive/repulsive and bond-order screening. The resulting parameters are listed in Table 1. More details on the screening procedure and the rationale behind choosing a particular $\mu_{ijk}$ and $m_{ijk}$ will be presented in a future study.

Table 1 Parameters for the modified W–C potential