The evolution of tribolayers during high temperature sliding wear

The evolution of tribolayers during high temperature sliding wear

Wear 315 (2014) 1–10 Contents lists available at ScienceDirect Wear journal homepage: www.elsevier.com/locate/wear The evolution of tribolayers dur...

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Wear 315 (2014) 1–10

Contents lists available at ScienceDirect

Wear journal homepage: www.elsevier.com/locate/wear

The evolution of tribolayers during high temperature sliding wear C. Rynio a,n, H. Hattendorf b, J. Klöwer b, G. Eggeler a a b

Ruhr-Universität Bochum, Institut für Werkstoffe, Bochum, Germany Outokumpu VDM GmbH, Altena, Germany

art ic l e i nf o

a b s t r a c t

Article history: Received 26 November 2013 Received in revised form 10 March 2014 Accepted 15 March 2014 Available online 21 March 2014

High temperature reciprocating sliding wear experiments of a Ni-based superalloy pin against a cast iron disc were performed at 600 and 800 1C (load: 20 N, frequency: 20 Hz, stroke: 1 mm). The evolution of tribolayers was investigated using scanning and transmission electron microscopy (SEM and TEM) and energy dispersive X-ray spectroscopy (EDX). Four distinct subsurface zones are identified and discussed in terms of plastic strain accumulation and microstructure evolution. The development of protective nanocrystalline oxide-layers (glaze-layers) on top of the wear surfaces leads to very low wear rates due to a suppression of the direct metal–metal contact between the pin and the disc. The nanohardness, microstructure and chemical composition of the glaze-layers are reported. & 2014 Elsevier B.V. All rights reserved.

Keywords: High temperature wear Mechanically mixed layer Tribolayer Glaze layer Transmission electron microscopy

1. Introduction High temperature wear is one of the life limiting factors of internal combustion engines and hot forming tools [1,2]. It involves damage to a solid surface due to a relative motion of that surface relative to a counter body at elevated temperatures. It is a complex phenomenon which may include oxidation, creep, fatigue and frictional effects. The material's response to high temperature wear is a roughening of the surface and an evolution of the microstructure in the surface zone. Already at ambient temperature, dry sliding wear of metallic materials is strongly influenced by tribolayers, where one typically can distinguish three subsurface zones [3–8]. An upper zone 1, a middle zone 2 and a lower zone 3. The upper zone 1, a mechanically mixed layer consisting of both materials (body and counter body), often shows a nanocrystalline microstructure (grain sizeo100 nm) and differs in chemical composition from both base materials [9–12]. Similar microstructures were reported after mechanical alloying of materials by e.g. ball-milling [13,14]. The extreme microstructural refinement results from repeated welding, fracture and re-welding of contacting asperities and wear debris. Furthermore, high levels of oxygen are commonly observed in this zone. The middle zone 2, directly under the mechanically mixed layer, often shows an ultrafine grained microstructure (100 nmograin sizeo1 mm) consisting of micro grains which are elongated in sliding direction. This zone usually exhibits the same chemical composition as the base material and its formation is governed by the type of plasticity which is

n

Corresponding author. Tel.: þ 49 234 3228982. E-mail address: [email protected] (C. Rynio).

http://dx.doi.org/10.1016/j.wear.2014.03.007 0043-1648/& 2014 Elsevier B.V. All rights reserved.

observed during severe plastic deformation (SPD) processes like for example high pressure torsion (HPT) [15]. In some cases, dynamically recrystallized grains were observed [16,17]. The lower zone 3 represents deformed base material without grain refinement. To date, most studies on the evolution of subsurface microstructures during wear were performed at ambient temperature. It is well known, that higher temperatures facilitate a faster development of mechanically mixed layers [18–20]. Moreover, at higher temperatures, these layers show increased oxygen levels. They almost fully cover wear scars and show a good adherence. When the substrate provides an efficient support for these layers, a reduction in wear rate by orders of magnitude is frequently observed [21–23]. The mechanically mixed layers generated at high temperatures appear smooth and shiny in the optical microscope. They are also known as glazelayers [24]. High temperature wear of metallic materials has been studied extensively [21–35]. However, the elementary wear mechanisms which govern the evolution of the surface topography and the subsurface microstructures are not well understood. In the present work, the high temperature sliding wear of the tribosystem Alloy 80A—cast iron has been studied using a reciprocating pin-on-disc tribometer. Alloy 80A and cast iron are typical materials for valve/seat-insert tribosystems of automotive diesel engines. These tribosystems are of great economical and ecological importance and therefore a detailed understanding of the elementary wear processes which limit their service life is of utmost importance. Wear in the tribosystem valve/seat-insert occurs by two different processes, namely impact wear and sliding wear. The impact wear results from the closure of the valve and the sliding wear is a consequence of the combustion pressure. However, nowadays the impact wear can be effectively reduced by a clever design of the camshaft [36]. Therefore it seems reasonable

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to investigate only the sliding wear response of this tribosystem using a reciprocating pin-on-disc tribometer. In the present work the high temperature wear behavior of the tribosystem Alloy 80A —cast iron is investigated at 600 and 800 1C. Special emphasis is placed on a detailed analysis of subsurface zones using scanning (SEM) and transmission electron microscopy (TEM). A specific objective of the present study is to contribute to a better understanding of the formation of glaze-layers.

high angle annular dark field (HAADF) image contrasts. HAADF TEM micrographs which were taken in the scanning transmission electron microscopy (STEM) mode yield good contrast over large fields of view [40]. A MTS Nanoindenter XP was used for depth-sensing nanoindentation tests on a glaze-layer of a wear scar cross section after 10 h wear exposure at 800 1C. The penetration depth was 100 nm.

3. Results 2. Experiments

3.1. Friction coefficients and wear volumes

Reciprocating dry sliding wear tests were performed using an Optimol SRV IV tribometer with a hemispherical pin-on-disc configuration. The radius of the hemispherical pin was 5 mm. The pins were prepared from the Ni-based Alloy 80A and the discs consisted of cast iron. The microstructures of these materials are shown in Fig. 1. The microstructure of the Alloy 80A consists of globular grains with Cr-carbides on the grain boundaries, Fig. 1a. The microstructure of the cast iron disc consists of an eutectic carbide network inside of a tempered martensitic matrix, Fig. 1b. The chemical compositions are given in Table 1. Load, stroke and frequency were kept constant at 20 N, 1 mm and 20 Hz, respectively. All wear tests were performed in air with a relative humidity close to 45%. The selected type of motion (oscillation) and the sliding frequency (20 Hz) were chosen to coincide with the wear mode of real valves [37]. However, the sliding path in the real tribosystem valve/seat-insert is only about 10 mm [38]. A meaningful sliding wear experiment with a pin-on-disc tribometer is not possible for such small sliding distances. Therefore, the stroke for the pin-on-disc experiments was chosen to 1 mm. The load of 20 N was chosen with respect to the spherical pin geometry. Because of the point contact between pin and disc in the beginning of the wear test, very high contact pressures in the range of 1000 MPa occur, which decrease during further wear to values of about 13 MPa (for a spherical wear scar on the pin with a wear scar radius of 0.7 mm). Contact pressures in the real tribosystem valve/seat-insert are in the range of about 20 MPa [39]. The tribometer and the test procedure applied in the present study have been described elsewhere [35]. Wear scars were investigated using a LEO 1530 VP scanning electron microscope (SEM) equipped with a field emission gun operating in the secondary and backscatter electron modes. A FEI Quanta 200 3D dual-beam focused ion beam (FIB) instrument was used for the preparation of thin TEM-foils. Typical FIB parameters used for specimen preparation were 30 kV and 7 nA (for larger scale material removal), 0.1 nA (for precise micromachining) and 50 pA (for final polishing). TEM-analysis was carried out using a FEI Tecnai Supertwin F20 equipped with a field emission gun and operating at 200 kV. TEM work was performed exploiting bright field (BF) and

Fig. 2 shows the evolution of the friction coefficient as defined by the well-known equation m¼ FF/FN (where FF is the friction force in the sliding direction and FN represents the normal force, which Table 1 Chemical compositions of the Alloy 80A and the cast iron. Alloy 80A

Cast iron

Element

wt%

Element

wt%

Al C Cr Fe Mo Ti Ni

1.62 0.05 19.8 0.69 0.02 2.67 Bal.

C Cr S Mn Mo Si V Fe

1.5 6 0.1 1 9 1.5 3 Bal.

Fig. 2. Evolution of friction coefficients during high temperature wear. The volume losses of the pins are given in the lower right corner.

Fig. 1. Backscattered electron (BSE) micrographs showing the microstructures of the (a) Ni-based Alloy 80A pin and (b) the cast iron disc. Photographs of pin and disc are shown as inserts in the upper right corners (for details see [35]).

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pushes the pin onto the disc). The tests were performed at 600 and 800 1C and were taken through to 10 h (corresponding to 720,000 cycles). The volume loss of the pins after wear exposure for 10 h is given in the lower right corner of Fig. 2. The friction coefficients show relatively high initial values of about 0.8 during the first 200 cycles (see insert in Fig. 2) and decrease until they reach a steady state value of about 0.4. The time for reaching steady state friction at 600 1C experiments is significantly longer compared to the 800 1C experiments (about 170,000 cycles compared to about 20,000 cycles). While steady friction coefficients at 600 and 800 1C are almost similar, a higher loss of material volume at the higher temperatures is observed (600 1C: 0.03 mm3; 800 1C: 0.06 mm3).

Fig. 3. SEM results after 10 s of wear exposure at 600 1C. (a) SEM micrograph showing lumps of transferred Alloy 80A material on the cast iron disc. The white arrow points to a white square which highlights a region from where a TEM lamella was taken. (b) and (c): EDX maps of (b) Ni and (c) Fe. Bright areas correspond to high amounts of the respective element.

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3.2. Tribolayers and wear debris after 10 s of wear exposure at 600 and 800 1C Fig. 3 shows SEM results obtained for the disc wear scar, which was generated during wear exposure for 10 s at 600 1C. The SEM micrograph in Fig. 3a shows lumps of transferred material, which can be clearly distinguished from the dendritic structure of the unaffected disc (top and bottom of Fig. 3a). In Fig. 3b and c element maps for Ni and Fe are presented which were obtained using energy dispersive analysis of X-rays (EDX) for the microstructural region shown in Fig. 3a. Bright areas correspond to a high amount of the respective element. It can be observed, that the lumps are rich in Ni (bright areas in Fig. 3b) and therefore correspond to material that during high temperature wear was transferred from the Ni-based Alloy 80A pin to the cast iron disc. FIB micromachining was used to cut out the region marked with a vertical arrow pointing up and a small white square in the center of Fig. 3a. The specimen was used to investigate the resulting microstructure directly under the surface using STEM-HAADF. The corresponding STEM-HAADF micrograph is shown in Fig. 4. The view direction of the micrograph shown in Fig. 4 is indicated by the vertical arrow in Fig. 3a (perpendicular to the direction of oscillating sliding). The type of microstructure shown in Fig. 4 was observed in a subsurface region extending from 0 (specimen surface) to 12 mm. Fig. 4 shows a heavily deformed microstructure with ultra-fine grains and dislocations within the fine grains. The corresponding selected electron diffraction pattern (SADP) is shown as an insert in Fig. 4. The diameters of the rings of the SADP stem from fcc-Ni, which is confirmed by EDX measurements (not shown here). As one would expect for this small grain size, spotty ring patterns are obtained. There appears to be a preferential direction which was not considered further in the present study. A typical debris particle of Alloy 80A material (confirmed by EDX, not shown here), which detached from the pin and moved over the disc surface during the first 10 s of wear exposure at 600 1C is shown in Fig. 5. The particle was found after high temperature wear next to the wear scar on the disc. This type of particles are also observed after high temperature wear at 800 1C.

Fig. 4. STEM-HAADF micrograph of the FIB lamella which was taken out from the disc surface at the position of the small white square shown in Fig. 3a. The micrograph reveals that the Ni-rich lumps, which were transferred to the cast iron disc during high temperature wear, have an ultra-fine grained structure. The sliding direction is indicated by a white double arrow.

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These metallic particles exhibit sizes as large as 700 mm and can be observed even with the naked eye. 3.3. Tribolayers and wear debris after 10 h of wear exposure at 600 and 800 1C After wear exposure for 10 h at 600 at 800 1C, the wear scars which characterize the surfaces of pin and disc are almost fully covered with glaze-layers. The morphologies of the pin wear surfaces after 10 h of wear exposure at 600 and 800 1C are shown in Fig. 6a and b, where parallel grooves in the direction of sliding are observed. The 800 1C wear surface in Fig. 6b exhibits more surface irregularities than the 600 1C wear surface. Furthermore, sliding at 800 1C generates deeper grooves on the wear surface than sliding at 600 1C. This can be seen in the micrographs of Fig. 6c and d, which were taken from cross sections perpendicular to the wear surface. A high number of dark microcracks characterizes the glaze-layer in Fig. 6d. It cannot be excluded that some of these microcracks formed during cooling of the specimens at the end of the wear tests or during the metallographic preparation of the cross sections. Around 40 SEM images like those shown in Fig. 6c and d were used for measuring the mean glaze-layer

Fig. 5. SEM micrograph of a large metallic wear particle collected after 10 s of sliding at 600 1C close to the scar shown in Fig. 3a.

thickness across the wear scars. The mean glaze-layer thickness across the wear scars was determined as 1.5 mm and 2.5 mm after the 600 and 800 1C wear experiments, respectively. Fig. 7a and b show STEM-HAADF results after 10 h wear exposure (720,000 cycles) at 600 and 800 1C, respectively. The micrographs were taken from thin foils which were FIB micromachined from regions of the Alloy 80A pin wear surfaces parallel to the direction of sliding. A close inspection reveals that 4 distinct zones form. The outer zone 1 represents the glaze-layer. There is a thin but clearly visible white layer (zone 2) between the glazelayer and the third zone. The thickness of this thin layer is about 15 nm after wear exposure at 600 1C and 70 nm after wear exposure at 800 1C. The third zone exhibits an ultra-fine grain size. Below this third zone a zone 4 is observed which exhibits a deformed microstructure without grain refinement. Chemistry, crystallography and microstructure of the glazelayer are documented in Fig. 8. Fig. 8a shows a bright field TEM micrograph. It can be observed that the glaze-layer consists of tiny nanograins. From the corresponding SADP in Fig. 8b it is clear, that these nanograins are randomly oriented. An indexation of the dominant rings shows, that the glaze-layer mainly consists of fccNiO. About 200 grain boundaries from several images like that shown in Fig. 8a were retraced to obtain binary images, which can be evaluated using an image analysis software. The grain areas were interpreted as circles and the distribution of the corresponding circle diameters is shown in Fig. 8c. The grain sizes follow a log-normal distribution (more small than large grains), as indicated by the fitted solid curve in Fig. 8c. The mean grain size of the nanograins is about 10 nm. The glaze-layer contains the same elements in almost the same proportions as the Ni-based pin material, as can be observed in the superimposed EDX-spectra of the glaze-layer and Ni-based pin material in Fig. 8d. The only differences between the glaze-layer and the pin substrate are the high O-peak and the tiny Fe-peak in the EDX-spectrum of the glaze-layer. Fig. 9 shows results obtained using EDX-analysis in the

Fig. 6. SEM micrographs taken from the pin after 10 h (720,000 cycles) wear exposure. (a) and (b): View on wear surface. (c) and (d): View on wear surface cross section perpendicular to sliding direction. (a) Wear surface after 600 1C wear exposure. (b) Wear surface after 800 1C wear exposure. (c) Near surface cross section after 600 1C wear exposure. (d) Near surface cross section after 800 1C wear exposure.

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Fig. 7. TEM micrographs of 4 distinct microstructural zones after high temperature wear exposure for 10 h (720,000 cycles). (a) 600 1C. An EDX line scan was performed along the black reference line in the upper right corner. (b) 800 1C.

Fig. 8. TEM results for the glaze-layer which forms at the surface during high temperature wear. (a) TEM bright field micrograph of nanocrystalline grains with no obvious evidence for the presence of dislocations and twins. (b) Corresponding selected electron diffraction pattern, showing predominantly rings from fcc-NiO (c) Histogram showing the size distribution of the nanograins, which obey a log-normal distribution (fitted black line). (d) Superimposed EDX-spectra of the glaze-layer and the Ni-based pin material.

TEM. Concentration profiles of Ni, Cr, O and Fe were measured along a 900 nm reference line, which is shown as a black vertical line in the upper right of Fig. 7a. In Fig. 7a, the line starts in zone 3, crosses the thin zone 2 and its main part runs across the glazelayer in zone 1. The concentration profiles of Ni and Cr are shown in Fig. 9a. The variations of O and Fe are shown in Fig. 9b. It can be clearly seen, that there is no Fe and only little O in zone 3, where the EDX reference line starts. In the small white zone 2 a local enrichment in Cr is observed. It therefore seems reasonable to assume that the small white layer between zones 1 and 3

represents a Cr2O3 layer. The Cr-intensity abruptly decreases as the EDX reference line enters zone 1, where an increase of the Ni and O concentrations to levels which are typical for the glaze-layer can be observed. The dark inclusion in Fig. 7a represents a wear particle which during wear detached from the pin and was buried in the glaze-layer. It shows the same ultra-fine grained microstructure as the Alloy 80A substrate material in zone 3. The corresponding part of the EDX line scan indicates an increase of Ni and a decrease of O and Fe concentrations to the levels that characterize the Alloy 80A base material.

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Fig. 9. Results for the EDX line scan, which was performed across the vertical black line indicated in Fig. 7a. (a) Ni and Cr, (b) O and Fe.

The SEM micrographs in Fig. 10a and b show typical surface features of the glaze-layers generated on the pin material during wear exposure for 10 h at 600 1C. In Fig. 10a, cracking of the glazelayer is observed. In Fig. 10b, a large void in the glaze-layer can be seen. These features are found all over the pin surface and the disc surface shows similar features. Inside of the void in Fig. 10b, many small wear particles accumulate. These were not removed during ultrasonic cleaning of the specimens. In order to investigate these small particles, TEM-analysis was performed. After the wear experiments, prior to ultrasonic cleaning, these wear particles were found all over the worn surfaces. They were collected onto a thin amorphous carbon film supported by a Cu grid and investigated in the TEM. Fig. 11a shows a STEM-HAADF micrograph of a typical debris particle generated during 10 h of wear exposure at 600 1C. A comparison with Fig. 8 shows that they have a similar nanocrystalline grain structure as the glaze-layers. Fig. 11b shows that their chemical composition is also similar to that of the glazelayers (for comparison see Fig. 8d). The hardness of the glaze-layers was measured using depthsensing nanoindentation testing. Fig. 12 shows a SEM micrograph of a wear scar cross section of the Alloy 80A pin after 10 h wear exposure at 800 1C. It is difficult to recognize the two indents in the glaze-layer (marked with white circles) in the micrograph of Fig. 12. All indents in the substrate yield nanohardness values of 6.2 71 GPa, no matter whether they were taken closer or further from the glaze-layer. From the two indents in the glaze-layer, a nanohardness of 17.671 GPa was evaluated. The ultra-fine grained microstructures of zone 3 which evolve during high temperature wear loading at 600 and 800 1C are shown in the STEM-HAADF micrographs of Fig. 13a and b. The ultra-fine grains after wear exposure at 600 1C, Fig. 13a, exhibit an elongated shape. They are larger in the horizontal sliding direction than in the vertical direction and they contain a high number of dislocations. They are typically 100 nm long in sliding direction and 50 nm wide in vertical

Fig. 10. SEM micrographs of typical features of the glaze-layer after wear exposure for 10 h at 600 1C. (a) Early stage of debris generation, (b) void in glaze-layer. The micrographs in (a) and (b) were taken from different areas.

Fig. 11. (a) STEM-HAADF micrograph of a typical wear particle after wear exposure for 10 h at 600 1C. (b) EDX spectrum from the quadratic area marked in (a).

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direction. At 800 1C, larger, globular grains form (globular grain size: 300 nm), Fig. 13b. The presence of recrystallization twins suggests that the microstructure has formed as a result of dynamic recrystallization. This is also supported by a low dislocation density in the grains. The inserted electron diffraction pattern in Fig. 13b shows spotty rings, which are much less pronounced compared to the ring patterns after 600 1C wear, Fig. 13a. Fig. 14 shows a SEM micrograph of the disc after wear exposure for 10 h at 600 1C. Compared to the wear scar after sliding for 10 s, Fig. 3, the wear scar after 10 h sliding is almost fully covered with a glaze-layer. A FIB was used to cut out a TEM specimen from the region marked with a white square in Fig. 14. The corresponding STEMHAADF micrograph is shown in Fig. 15a, the viewing direction is indicated by the vertical arrow in Fig. 14. In the present work, the focus is on the evolution of tribolayers in the Alloy 80A pin material and therefore only microstructural features across the interface between glaze-layer and Fe-based disc are shown in the TEM micrograph of Fig. 15a. Four distinct subsurface zones can be distinguished. The top zone (zone 1) corresponds to the glazelayer and the lowest zone (zone 4) corresponds to the deformed Febased disc. The glaze-layer on the disc has the same chemical composition as the glaze-layer on the pin, hence no EDX spectrum of zone 1 is shown here. In between zones 1 and 4, two intermediate microstructures (zone 2 and zone 3) can be distinguished.

The results are discussed following the schematic drawings shown in Fig. 16a to e, which represent illustrations of subsequent scenarios of surface topography and subsurface microstructure evolution during high temperature wear at 600 and 800 1C. The focus of the present work was on the evolution of subsurface microstructures in the Alloy 80A pin, microstructural changes in the Fe-disc are not considered. The left and the right columns of Fig. 16 show side and top views of the pin-on-disc test at different steps of wear damage accumulation. In the side view of Fig. 16a, the

Fig. 12. SEM micrograph of nanoindents on a wear scar cross section of the pin after 10 h wear exposure at 800 1C. The white circles mark two indents in homogeneous glaze-areas.

Fig. 14. SEM micrograph of the glaze-layer on the disc after wear exposure for 10 h at 600 1C. The overexposed white areas originate through charging effects.

Zone 2, directly under the glaze-layer, corresponds to a Cr-rich oxide layer, as indicated by the EDX-spectrum in Fig. 15b. This Crrich layer is similar to the Cr-rich layer which was observed at the interface between the glaze-layer and the Alloy 80A pin, Fig. 9a. Zone 3 shows an ultra-fine grained microstructure. From the EDX spectrum of zone 3, Fig. 15c, it can be seen that this zone corresponds to Ni-based Alloy 80A material, which during wear has been transferred from the pin to the disc. The grains appear smaller compared to the grains in the transferred lumps after 10 s of wear exposure, Fig. 3. No oxide layer can be observed between zone 3 (transferred Alloy 80A material) and zone 4 (Fe-based disc).

4. Discussion

Fig. 13. STEM-HAADF micrographs showing the ultra-fine grained microstructures of zone 3 (Fig. 7) after 10 h wear exposure at (a) 600 1C and (b) 800 1C.

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Fig. 15. (a) STEM-HAADF micrograph of the FIB lamella which was taken out from the disc surface at the position of the small white square shown in Fig. 14. The micrograph reveals 4 distinct subsurface zones at the interface between glaze and disc-substrate. (b) EDX spectrum of zone 2. (c) EDX spectrum of zone 3.

globular grains of the Alloy 80A pin and the eutectic carbide network of the cast iron disc are illustrated. The carbides in the disc stick out due to the polishing of the samples prior to wear testing (material removal from matrix is faster than material removal from hard carbides). In the top view of Fig. 16a, the initial contact point between pin and disc is shown. Due to the small initial point contact, very high contact pressures arise. These facilitate the generation of high dislocation densities in the surface areas of the pin. The dislocations arrange into dislocation cells and subsequently form subgrains, illustrated as elongated rectangles in Fig. 16b to e. Such a dislocation substructure is typical for a severe plastic deformation process like high pressure torsion (HPT) [41] and has also been observed during sliding wear [4,42–44]. It was claimed that the formation of subgrains in the near-surface areas of the pin makes the material unstable to local shear events [45]. This promotes the evolution of cracks, probably along the subgrain boundaries [46] and subsequently leads to material transfer from pin to disc. The lumps of transferred Alloy 80A pin material can be observed as yellow areas on the disc surface in Fig. 16b. The preferential transfer from the Ni-based pin to the cast iron disc is facilitated by the protruding carbides, which mechanically scratch the pin surface. After around 200 cycles of wear exposure, very thick lumps of transferred pin material can be found on the disc surface, Fig. 16c. In the present work, these thick lumps are documented in the SEM micrographs of Fig. 3. Because the pin and the disc consist of chemically different elements (pin: Ni-based, disc: Fe-based), EDXelement-mappings prove that the lumps on the disc stem from the Nibased pin, Fig. 3. Next to the wear scars on the disc, very large metallic Ni-rich wear particles were found, as reported in Fig. 5. These wear particles probably arose from the detachment of the pin-material, which previously had been transferred to the disc surface. This is illustrated in Fig. 16c in form of large yellow patches, left and right from the wear surface on the disc. From Fig. 16c, it is obvious that the contact condition between pin and disc has changed from sliding a Nibased pin on a cast iron disc to sliding of a Ni-rich pin on a Ni-rich disc. This sliding situation is referred to as a like-on-like scenario. As the wear exposure proceeds, the repeated welding, fracture, re-welding and oxidation of asperities and wear particles leads to the development of glaze-layers (dark-grey areas) on both, pin and disc surfaces, Fig. 16d. Fig. 16d shows the surface zones after wear

exposure for around 10,000 cycles. Fig. 16d illustrates that the glaze-layers form as discrete islands, partially on top of the transferred Ni-based pin material. In Fig. 16e, the surface zones after around 720,000 cycles of wear exposure, corresponding to 10 h of sliding, are illustrated. It is shown that the glaze-layers now almost fully cover the wear scars on both, pin and disc surfaces. Underneath the glaze-layers of pin and disc, a new layer (blue layer) has formed, which corresponds to a Cr2O3-layer. The Cr2O3-layer results from steady state oxidation by diffusion of oxygen through the glaze-layer and reaction with the Cr-containing Ni-alloy [20,24]. Four different surface zones can be observed on the pin and on the disc in Fig. 16e. The existence of these surface zones is documented in the TEM-micrographs in Fig. 7 (pin) and Fig. 15 (disc). Starting from the pin surface, these zones are the glaze-layer (zone 1), the Cr2O3-layer (zone 2), the ultra-fine grained layer (zone 3) and the plastically deformed layer without grain refinement (zone 4). On the disc surface, these zones represent the glaze-layer (zone 1), the Cr2O3-layer (zone 2), the ultra-fine grained layer of transferred Ni-based pin-material (zone 3) and the disc substrate (zone 4). The glaze-layers on both, pin and disc, show similar microstructures and chemical compositions. Furthermore, there is no significant difference between the glazelayers generated after 600 and 800 1C wear exposure. It was reported in Fig. 8 that the glaze-layer consists of randomly oriented, nanocrystalline grains with a grain size of about 10 nm. Moreover, the glaze-layer exhibits almost the same elements in almost the same proportions as the Ni-based pin material, but with additional high amounts of O and small amounts of Fe, Fig. 8d. Obviously, the O stems from the reaction of the material with the atmosphere and the Fe stems from the Febased disc. The electron diffraction pattern of the glaze-layer indicates that the predominant phase in the glaze-layer is NiO. Similar results were obtained by Stott et al. [20], who studied the glaze-layers after like-on-like sliding of Nimonic 80A. The nanohardness of the glaze-layers has been evaluated by nanoindentation tests and it has been shown, that the glaze-layer has a hardness of about 18 GPa, which is almost three times higher than the hardness of the Ni-based pin substrate. Finally, the generation of wear particles was reported, once the wear surfaces on pin and disc are fully covered with glaze-

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Fig. 16. Schematic scenarios of the surface and subsurface microstructure evolution during high temperature wear at 600 and 800 1C. See text for details. (a) n  0, (b) n  20, (c) n  200, (d) n  10000 and (e) n  720,000. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

layers. The wear particles only emerge through delamination of the glaze-layers by the formation and growth of surface cracks, as illustrated in Fig. 10. These cracks probably grow along the interface between glaze-layer and substrate and when they reach the surface, wear particles in form of chipped-off glaze-layer areas are generated. This process of wear particle generation accounts for the formation of voids in the glaze-layer. One such void was presented in the SEM micrograph of Fig. 10b and is schematically shown in Fig. 16e. It can be observed, that inside of such voids, many small oxide wear particles agglomerate. It is suggested that during further wear exposure, these voids fill with oxide wear particles, which subsequently get compacted, sinter together and facilitate the closure of the void. This ‘self-healing’

effect results in an increase of the wear resistance, supported by the high hardness of the glaze-layer and the complete inhibition of direct metal-metal contact. The proposed mechanisms rationalize the low wear rates which are established early during high temperature wear testing.

5. Conclusions In the present study the high temperature wear behavior of a Ni-based pin on a cast iron disc was investigated. These materials are typically used in a valve/seat-insert tribosystem of automotive

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diesel engines. From the experimental results, the following conclusions can be drawn:

 During run-in, severe material transfer from the Ni-based pin to the cast iron disc takes place.

 As the tribosystem accomplishes steady-state, four distinct









subsurface zones form which can be distinguished in the TEM. The outer zone corresponds to the glaze-layer and is responsible for the low wear rates during high temperature wear. The glaze-layers develop on pin and disc and suppress the direct metal-metal contact. TEM investigations show that these layers consist of tiny nanograins. The predominant phase of the glaze-layer is NiO, although the disc was Fe-based. This is because there is only little wear on the disc and the majority of the wear debris stems from the Ni-based pin. The ultra-fine grains that result from severe plastic deformation during high temperature wear at 600 1C are mostly elongated in sliding direction and contain a high dislocation density. The ultra-fine grains after 800 1C are larger and show a more globular shape. The low dislocation density inside these grains suggests that these grains result from dynamic recrystallization. The glaze-layers benefit from a self-healing effect. Chipped-off particles account for voids in the glaze-layer, which are subsequently filled with loose oxide debris. During further sliding, compaction and sintering of this debris closes the void. Nanoindentation shows that the hardness of the glaze-layer is almost three times higher than the hardness of the Ni-based bulk material.

Based on the SEM and TEM results, detailed schematic illustrations of the surface topography and subsurface microstructure evolution during high temperature sliding wear are presented.

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