The development of wear-resistant steels requires an in-depth understanding of the properties of cementite (Fe3C). Previous studies on the chemical changes of steel during wear are limited (single-phase Fe3C is usually embedded in the metal matrix). This study examines the elemental and phase distributions of massive polycrystalline cementite containing small amounts of graphite, iron, and quadrensite after single-pass sliding wear. The cementite composition before and after wear was characterized by scanning transmission electron microscopy energy dispersive X-ray spectroscopy, Auger electron spectroscopy and X-ray photoelectron spectroscopy. The results show that severe plastic deformation induced by contact shear leads to partial decomposition and mechanical mixing of non-cementite inclusions into the cementite matrix, as well as partial elemental homogenization in the outermost deformation zone. Furthermore, the graphite present in the initial microstructure dissolves and carbonizes. The work was published in Acta, the top journal of metal materials, under the title "of (Fe3C) -pass wear: An by".

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Due to the high wear resistance of cementite, multiphase pearlitic, bainite and martensitic steels are commonly used in tribological applications such as bearings, rails and train wheels. The wear resistance of such steels is usually improved by strengthening with cementite phase (Fe3C). In pearlite, cementite leads to plastic stabilization in ferrite during the initial loading stage. Furthermore, the continuous microstructural refinement during deformation increases the ferrite-cementite interface density, which significantly restricts the dislocation movement. In bainitic and martensitic steels with larger spheroidized cementite particles, matrix flow is restricted by these cementite inclusions, resulting in increased local stress at the interface. Solid solution strengthening is achieved by the dissolution of cementite into the matrix phase under high strain. However, cementite dissolution during wear is also associated with the formation of brittle carbon-saturated white etch layers/areas that lead to fracture and rapid erosion of the contact area.

In order to develop steels with better wear resistance, it is necessary to fully understand the tribological behavior of the cementite phase. Since the microstructural and chemical evolution in multiphase steels depends partly on the interaction of cementite with the surrounding matrix (e.g. ferrite, martensite), it is not possible to focus on individual mechanisms in Fe3C. To this end, the wear behavior of cementite was characterized by studying bulk polycrystalline samples containing small amounts of graphite and iron oxide produced by spark plasma sintering. Current research mainly focuses on the abrasive wear properties of Fe3C. In the work of et al., the two-body wear behavior of polycrystalline cementite was compared with that of pure iron, commercial steel, and other sintered cementite-iron samples. In wear experiments under different apparent applied pressures, massive cementite exhibited the best wear resistance among all analyzed materials, with apparent applied pressures as high as 0.0, while at 1.23 MPa, 100 vol.% cementite The sample showed a phenomenon better than 75 vol.  % cementite. In three-body wear experiments of block ring geometry using SiC abrasives on pure iron and sintered iron cementite samples, et al. reported minimal loss of block cementite at an applied pressure of 0.065 MPa. Under the applied pressure of 0.098 MPa, fracture becomes the main deformation mechanism, and the amount of wear increases significantly accordingly. The calculation of the applied pressure is not described in the above studies, the principle of wear resistance and the block experiments on the ring are quite different from the microscopic wear experiments carried out in this work. Nonetheless, preliminary experimental studies have demonstrated the importance of studying the deformation behavior of cementite under various wear conditions.

Fig.1 SEM image of cementite surface after microscale sliding wear test

However, the wear behavior of cementite must be related to its underlying deformation behavior and chemical evolution during friction loading. In a previous study, the deformation mechanism of massive cementite during single-pass and multi-pass sliding wear was investigated. Cementite tends to form a layered deformed structure during sliding wear. Due to enhanced ductility, bulk polycrystalline cementite undergoes plastic deformation below the interface through dislocation slip, shear band formation, fragmentation, grain boundary sliding, and grain rotation. Furthermore, the formation of brittle fatigue cracks was observed in multi-pass experiments. The deformed microstructure leads to a significant increase in nanoindentation hardness.

Fig. 2 STEM-EDS measurement results of the cross-section of the wear track after a single wear test: (a) STEM bright-field (BF) image of the cross-section of the wear track, highlighting characteristic deformation regions (nanocrystalline regions (NR), ultrafine grain grain region (UFGR), transition region (TR)); (b) elemental distribution showing the atomic concentrations of Fe, O, and C in NR and UFGR.Marked by gray rectangle; (cd) EDS plot

Since the main purpose of the previous work was to reveal the deformation mechanism of cementite in the contact region, the effect of frictional loading on the chemical evolution was only slightly mentioned. In particular, the formation of the outermost layer of wear marks was detected. It is the most stable transitional carbide before forming a stable mixture of iron and cementite during tempering of high carbon martensite. Precipitates can coexist with cementite or grow at the expense of cementite at sufficiently high carbon concentrations. Thus, the formation of a carbon gradient in the contact zone indicates the presence of a carbon gradient, which provides the driving force for precipitation. Preliminary results suggest two possible explanations for the excess carbon that promotes the phase transformation: the decomposition of graphite particles and the oxidation of cementite during wear. However, more evidence is needed to provide conclusive support for one of these mechanisms.

This work investigates the tribological behavior of sintered polycrystalline cementite containing small amounts of graphite and iron oxide, focusing on the chemical evolution during monorail sliding wear. Elemental and phase analysis of deformed layers and undeformed regions of samples by scanning transmission electron microscopy (STEM-EDS), Auger electron spectroscopy (AES) and energy dispersive X-ray spectroscopy in X-ray photoelectron spectroscopy (XPS) Composition). By combining surface spectroscopic techniques, it aims to show the correlation between deformation-related microstructural changes and qualitative and semi-quantitative differences in elemental and phase distributions. Based on the results of this study, the proposed source of carbon enrichment in the outermost deformation zone was verified and a plausible mechanism for the cementite phase transformation was identified.

Fig.3 (a) Undeformed cementite grains and (b) Auger electron spectra obtained by sputtering on the wear track surface for 90, 120 and 210 min

Fig. 3 shows the Auger spectra obtained from undeformed cementite grains and wear track surfaces. According to the SEM images of the worn surface before and after sputtering, the depth of the transition zone reaches about 400 nm after 210 min. The carbon spectrum corresponds to the KLL transition and has a unique shape dependent on chemical bonding. The unique shape distinguishes carbides (carbon-metal bonds) from graphite (carbon-carbon bonds). The carbon spectra of undeformed cementite grains (Fig. 3a) recorded during the first two sputtering steps correspond to a combination of the two bonding types, indicating the presence of carbides and graphite. After sputtering for 210 min, the typical carbon-metal bond line shape is shown. Within the wear track, all Auger spectra of carbon consist of two carbon wire shapes after all sputtering steps: this detail confirms the presence of graphite (in addition to carbides) in the deformed region. It is worth noting that iron oxide and graphite are lighter components with lower binding energies and thus are selectively etched during sputtering compared to cementite. However, since the presence of graphitic carbon spectra within the wear track remains evident throughout the sputtering sequence, it can be concluded that the experimental results were not seriously affected by the selective sputtering of lighter compounds.

Fig. 4 XPS spectra recorded after sputtering of undeformed cementite and wear scar arrays for 126s (depth about 70nm).

Fig. 5 (a) XPS depth profile of undeformed cementite and wear scar array; (b) at.% fraction of C-carbide, C-graphite and other C-compounds in the whole XPS depth profile; (c) the whole Percent fractions of Fe metal/carbide and Fe oxide in the XPS depth profile.

To investigate the phase composition of the deformed layer, a comparative analysis of the XPS depth profiles of the undeformed cementite and worn array regions was performed. Figure 4 shows the spectra obtained after one of the initial sputtering steps (126s, 70nm depth). In this sputtering step, the composition peak is the combination of surface layer and sample volume.

Surface spectroscopic measurements of the sample composition before and after microwear experiments on polycrystalline cementite containing small amounts of iron, graphite, and iron ore revealed chemical changes induced by frictional loading. This study draws the following conclusions: