**4.1 Tribologically induced changes in the microstructure**

The higher wear resistance of softer variants of the respective alloys was initially not expected as hardness is often associated with wear resistance. As the testing conditions were identical, we investigated the microstructure in order to examine the different wear behaviour observed. The idea was that the grain and/or defect structure evolving during the sliding contact is different for these samples. As all other factors were identical, the near surface microstructure behaviour can potentially explain the differences in the macroscopic wear behaviour as well as the observed frictional levels. The light microscopy images in **Figure 7** reveal that no resolvable changes in the microstructure beneath the contact surface occur. Only

#### **Figure 7.**

*Light microscopy images of cross sections of the cylindrical samples normal to the sliding direction. a) CuSn8 conventional b) CuSn8 MIM c) CuNi9Sn6 conventional d) CuNi9Sn6 MIM; the 50* μ*m scale applies to a) and b), the 100* μ*m scale to c) and d).*

**245**

**Figure 8.**

*the first row of grains are visible.*

*Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts…*

the MIM CuSn8 sample exhibits local increase of twins and shear bands in the first

The LF cast structure does show defects in the light microscopy images (**Figure 8**), such as bent slip lines as well as deformation twins, which are bent additionally. The deformation is very inhomogeneous and many dendrites show no or hardly any changes visible in light microscopy images. The details in **Figure 8b** illustrate pronounced effects in the vicinity of pores. If features of plastic deformation can be observed by light

The higher resolution SEM images of the cross sections shown in **Figure 7** are given in **Figure 9a** and **b** for the conventional variants and in **Figure 9c** and **d** for the MIM samples. The CuSn8 is highly twinned and some twins are bent in sliding direction. This occurs not only in the surface grains but also in grains located in deeper zones. Still, the SEM picture is not sufficient to be able to judge if these features are caused by the sliding contact as the initial structure is highly twinned already. In the higher alloyed CuNi9Sn6 the grains are larger compared to CuSn8 conventional and no features indicating deformation close to the surface are visible. The edge is very

In contrast to the two conventionally produced samples, both MIM alloys revealed the formation of a layer on top of the initial microstructure with a

*Light microscopy images of cross sections of the cylindrical CuSn12Ni2 LF samples normal to the sliding direction a) and b) show two position within the same sample, in a) slip lines and in b) deformation twins in* 

layer of grains up to a depth of 20 μm beneath the contact plane.

microscopy images, they will reach up to depths of at least 50 μm.

rounded due to polishing and etching prior to EBSD measurements.

*DOI: http://dx.doi.org/10.5772/intechopen.94210*

### *Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts… DOI: http://dx.doi.org/10.5772/intechopen.94210*

the MIM CuSn8 sample exhibits local increase of twins and shear bands in the first layer of grains up to a depth of 20 μm beneath the contact plane.

The LF cast structure does show defects in the light microscopy images (**Figure 8**), such as bent slip lines as well as deformation twins, which are bent additionally. The deformation is very inhomogeneous and many dendrites show no or hardly any changes visible in light microscopy images. The details in **Figure 8b** illustrate pronounced effects in the vicinity of pores. If features of plastic deformation can be observed by light microscopy images, they will reach up to depths of at least 50 μm.

The higher resolution SEM images of the cross sections shown in **Figure 7** are given in **Figure 9a** and **b** for the conventional variants and in **Figure 9c** and **d** for the MIM samples. The CuSn8 is highly twinned and some twins are bent in sliding direction. This occurs not only in the surface grains but also in grains located in deeper zones. Still, the SEM picture is not sufficient to be able to judge if these features are caused by the sliding contact as the initial structure is highly twinned already. In the higher alloyed CuNi9Sn6 the grains are larger compared to CuSn8 conventional and no features indicating deformation close to the surface are visible. The edge is very rounded due to polishing and etching prior to EBSD measurements.

In contrast to the two conventionally produced samples, both MIM alloys revealed the formation of a layer on top of the initial microstructure with a

#### **Figure 8.**

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

to be independent of the applied normal load.

**4.1 Tribologically induced changes in the microstructure**

The higher wear resistance of softer variants of the respective alloys was initially

not expected as hardness is often associated with wear resistance. As the testing conditions were identical, we investigated the microstructure in order to examine the different wear behaviour observed. The idea was that the grain and/or defect structure evolving during the sliding contact is different for these samples. As all other factors were identical, the near surface microstructure behaviour can potentially explain the differences in the macroscopic wear behaviour as well as the observed frictional levels. The light microscopy images in **Figure 7** reveal that no resolvable changes in the microstructure beneath the contact surface occur. Only

*Light microscopy images of cross sections of the cylindrical samples normal to the sliding direction. a) CuSn8 conventional b) CuSn8 MIM c) CuNi9Sn6 conventional d) CuNi9Sn6 MIM; the 50* μ*m scale applies to a) and* 

compared to the conventionally produced sample. Consequently, the increase in mechanical strength due to precipitations does not impair the tribological performance, as it does reduce wear and friction levels. Heat treatment reduced the scatter of the observed friction coefficient level. The results of other samples treated at other temperatures or for different times are not shown here as the wear and friction results nearly coincide for all established heat treatment cycles. In any case, the heat-treated MIM CuNi9Sn6 version offers the lowest wear and friction among the investigated materials lubricated with mineral base oil. The initial microstructure affects the measured wear volume even within systems using additivated oils, which reduces the measured wear volumes due to a tribolayer, forming also in bronze surfaces. Again, the new casting technology (LF) variant of CuSn12Ni2 shows a better performance than the continuous cast one. The effect of different material structures becomes more pronounced for higher normal loads as can be seen in **Figure 6** for 100 N and 240 N. The effects on friction levels are lower than for the systems shown in **Figure 3**, but interestingly the noisiness of the friction coefficient reduces if the structure gets coarse and more dendritic. The latter observation seems

**244**

**Figure 7.**

*b), the 100* μ*m scale to c) and d).*

*Light microscopy images of cross sections of the cylindrical CuSn12Ni2 LF samples normal to the sliding direction a) and b) show two position within the same sample, in a) slip lines and in b) deformation twins in the first row of grains are visible.*

#### **Figure 9.**

*SEM image of the edge zone on cross-sections oriented parallel to the sliding direction: a) CuSn8 conventional, b) CuNi9Sn6 conventional, c) CuSn8 MIM (mechanically mixed layer, 2 visible nano-indenter marks), CuNi9Sn6MIM (mechanically mixed layer lying and facetted grain boundary with* γ *precipitations is vertically inclined on the left side of the image).*

thickness up to 2–3 μm. This feature was only observed on the MIM microstructures of the studied alloys.

The observed layer seemed to have a patch-like structure as it did not cover the whole length of the cross section. On top views of the contact area these layers were not clearly distinguishable from the rest of the microstructure, thus their size and extent of coverage cannot be given. The EDX measurements (**Table 3**) shows increased contents of oxygen and carbon and some iron. This indicates a mixture with debris from the counter-body, even though the structure appears amorphous or extremely fine-grained. However, no grain structure was resolvable with SEM. What is observable in the SEM images are pores within this layer, which are more pronounced for the layer observed on CuNi9Sn6. In the following, we refer to it as a mechanically mixed layer in order to distinguish it from the TTL, a zone comprising of defects like the slip lines and deformation twins extending over up to 20 μm as seen in **Figure 7c**.

#### **4.2 EBSD as a tool to reveal tribologically induced changes in the microstructure**

With the EBSD technique grain orientations and local misorientations can be revealed. Thus, microstructural defects like internal stresses stored in grains or the formation of subgrains and local misorientations can be detected, which cannot be observed with SEM images [23, 24]. As the changes in wear volume were most pronounced for CuSn8, the EBSD analysis focused on this alloy.

For the conventionally produced CuSn8 the forming in the production process resulted in a highly deformed and small-grained structure and any further deformation caused due to the tribological contact is not clearly visible in the SEM

**247**

**Figure 10.**

**Table 3.**

*d) IPF image.*

*Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts…*

images. The pattern quality picture and the IPF picture of an EBSD scan with 1 μm distance to the surface are given in **Figure 10**, with the upper edge of the scan in sliding direction. The quantitative analysis of the grain boundary lengths of large angle grain boundaries (LAGB) and small angle grain boundaries (SAGB) overlaid onto the pattern quality image indicate no increase closer to the surface and so they are not shown. However, the twin density within the first 15–20 μm appears to be higher as numerous micro-twins form. Especially in the first 5 μm, the grains are strongly elongated parallel to the sliding direction. Generally, the grain size is easier to see in IPF images and they reveal high intragranular strains by different shades of colours in nearly all grains, independently of their location with respect to the surface. Hence, internal lattice rotations do not seem to increase close to the contact but are a result of the high deformation during production. Comparing the IPF image of the initial structure to the one at the surface indicates a tendency to slightly smaller grains sizes in the first 15 μm. However, the change in size is far less

Thus, the microstructure of conventional CuSn8 can hardly adapt to the stresses accumulated during sliding and has little work-hardening capability. So presumably the microstructure is just worn off soon after the initial contact during the sliding

The MIM version of CuSn8 was very large grained and therefore investigated by two closely situated EBSD scans situated close together, shown in **Figure 11**.

At % 54.58 6.16 1.07 1.90 0.62 0.31 3.33 32.03

*EDX analysis of the layer observed on CuNi9Sn6 in the MIM version as shown in* **Figure 9c***.*

**C O Si S Sn Fe Ni Cu**

*CuSn8 conventional samples with the sliding direction is parallel to the upper edge. EBSD scan at a depth of 1000* μ*m beneath the surface: a) pattern quality image with LAGB in blue, SAGB in red and b) IPF image EBSD scan at a depth of 1* μ*m beneath the surface: c) pattern quality image with LAGB in blue, SAGB in red* 

*DOI: http://dx.doi.org/10.5772/intechopen.94210*

pronounced than the change in shape.

process and cannot form a wear resistant subsurface zone.

## *Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts… DOI: http://dx.doi.org/10.5772/intechopen.94210*

images. The pattern quality picture and the IPF picture of an EBSD scan with 1 μm distance to the surface are given in **Figure 10**, with the upper edge of the scan in sliding direction. The quantitative analysis of the grain boundary lengths of large angle grain boundaries (LAGB) and small angle grain boundaries (SAGB) overlaid onto the pattern quality image indicate no increase closer to the surface and so they are not shown. However, the twin density within the first 15–20 μm appears to be higher as numerous micro-twins form. Especially in the first 5 μm, the grains are strongly elongated parallel to the sliding direction. Generally, the grain size is easier to see in IPF images and they reveal high intragranular strains by different shades of colours in nearly all grains, independently of their location with respect to the surface. Hence, internal lattice rotations do not seem to increase close to the contact but are a result of the high deformation during production. Comparing the IPF image of the initial structure to the one at the surface indicates a tendency to slightly smaller grains sizes in the first 15 μm. However, the change in size is far less pronounced than the change in shape.

Thus, the microstructure of conventional CuSn8 can hardly adapt to the stresses accumulated during sliding and has little work-hardening capability. So presumably the microstructure is just worn off soon after the initial contact during the sliding process and cannot form a wear resistant subsurface zone.

The MIM version of CuSn8 was very large grained and therefore investigated by two closely situated EBSD scans situated close together, shown in **Figure 11**.


#### **Table 3.**

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

thickness up to 2–3 μm. This feature was only observed on the MIM microstructures

*SEM image of the edge zone on cross-sections oriented parallel to the sliding direction: a) CuSn8 conventional, b) CuNi9Sn6 conventional, c) CuSn8 MIM (mechanically mixed layer, 2 visible nano-indenter marks), CuNi9Sn6MIM (mechanically mixed layer lying and facetted grain boundary with* γ *precipitations is vertically* 

The observed layer seemed to have a patch-like structure as it did not cover the whole length of the cross section. On top views of the contact area these layers were not clearly distinguishable from the rest of the microstructure, thus their size and extent of coverage cannot be given. The EDX measurements (**Table 3**) shows increased contents of oxygen and carbon and some iron. This indicates a mixture with debris from the counter-body, even though the structure appears amorphous or extremely fine-grained. However, no grain structure was resolvable with SEM. What is observable in the SEM images are pores within this layer, which are more pronounced for the layer observed on CuNi9Sn6. In the following, we refer to it as a mechanically mixed layer in order to distinguish it from the TTL, a zone comprising of defects like the slip lines and deformation twins extending over up to 20 μm as

**4.2 EBSD as a tool to reveal tribologically induced changes in the microstructure**

With the EBSD technique grain orientations and local misorientations can be revealed. Thus, microstructural defects like internal stresses stored in grains or the formation of subgrains and local misorientations can be detected, which cannot be observed with SEM images [23, 24]. As the changes in wear volume were most

For the conventionally produced CuSn8 the forming in the production process resulted in a highly deformed and small-grained structure and any further deformation caused due to the tribological contact is not clearly visible in the SEM

pronounced for CuSn8, the EBSD analysis focused on this alloy.

**246**

of the studied alloys.

*inclined on the left side of the image).*

**Figure 9.**

seen in **Figure 7c**.

*EDX analysis of the layer observed on CuNi9Sn6 in the MIM version as shown in* **Figure 9c***.*

#### **Figure 10.**

*CuSn8 conventional samples with the sliding direction is parallel to the upper edge. EBSD scan at a depth of 1000* μ*m beneath the surface: a) pattern quality image with LAGB in blue, SAGB in red and b) IPF image EBSD scan at a depth of 1* μ*m beneath the surface: c) pattern quality image with LAGB in blue, SAGB in red d) IPF image.*

**Figure 11.** *CuSn8 MIM EBSD scan.*

The initially undeformed MIM structure of CuSn8 shows some grains like the green coloured ones the left with no deformation features, but the grain on the right changes its orientation from yellow to pink which indicates a partial lattice rotation and an internal stress field. The respective stress field does not extend over the whole grain but is localised in the first 20 μm beneath the contact and does not affect the twin. In the middle, there is a pore close to the surface and the remaining grain located above this pore seems to be most suitable for storing plastic strain generated by the tribological sliding process. It has various shades of pink and is not as uniformly coloured as non-affected grains. The analysis of small- and large-angle grain boundaries showed no additional information and a figure was therefore omitted. Based on our observations there is no indication for an interrelationship between local lattice rotations and the position of the mechanically mixed layer lying on top of the tribo-contact area.

In addition to SEM and EBSD scans, some apparently affected grains were investigated with nanoindentation, the results of which are given in **Figure 12**. The defect structure shows a high local concentration of defects that lead to an increase in hardness by 1–1.5 GPa over the first 20 μm beneath the surface. However, grains that show no orientation changes in the EBSD scan also showed no hardening in the nanoindenter measurement.

The modulus of elasticity was quantified during nanoindentation and turned out not to vary with distance from the contact surface and is thus not given in **Figure 12**. Altogether, the results of the MIM sample suggest that if grains are modified due to the sliding process they have to be located directly at the surface, grains further away from the contact are unaffected in any case, but even at the surface many grains do not adapt by e.g. lattice rotations.

The cast alloy CuSn12Ni2, though tested with additivated oil, showed smaller but significant differences in the wear volumes of CC and LF cast samples. The tribo-surfaces had different characteristics, which are illustrated in **Figure 13a** and **b**. The surface exhibits grooves, presumably formed by wear debris, but resembles a

**249**

**Figure 13.**

**Figure 12.**

*Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts…*

*Location and gradient of the nano-hardness measurements performed on the MIM version of CuSn8.*

metallographic cross-section as it clearly reveals all phases. The LF surface appears even smoother than the CC surface. As the conventional cross sections indicated some edge effects, but did not enable a proper identification of the structure, only the respective FIB cuts, normal to the sliding direction, are given in **Figure 13c** and **d**. Here samples tested at a normal load of 240 N are examined as the wear volume differences are larger than for 100 N normal load. Both samples form a nanocrystalline layer during the sliding process. However, the thickness of this layer is clearly larger for the CC version with about 8 μm compared to 1.2 μm for LF. As the CC samples

*normal to the FIB cutting plane, c) conventional sample d) LF sample.*

*Wear tracks on CuSn12Ni2: Top view SEM image - sliding direction going parallel to the vertical image edge a) conventional sample, b) LF sample; sub-surface microstructure in field ion image mode - sliding direction* 

*DOI: http://dx.doi.org/10.5772/intechopen.94210*

*Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts… DOI: http://dx.doi.org/10.5772/intechopen.94210*

#### **Figure 13.**

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

The initially undeformed MIM structure of CuSn8 shows some grains like the green coloured ones the left with no deformation features, but the grain on the right changes its orientation from yellow to pink which indicates a partial lattice rotation and an internal stress field. The respective stress field does not extend over the whole grain but is localised in the first 20 μm beneath the contact and does not affect the twin. In the middle, there is a pore close to the surface and the remaining grain located above this pore seems to be most suitable for storing plastic strain generated by the tribological sliding process. It has various shades of pink and is not as uniformly coloured as non-affected grains. The analysis of small- and large-angle grain boundaries showed no additional information and a figure was therefore omitted. Based on our observations there is no indication for an interrelationship between local lattice rotations and the position of the mechanically mixed layer

In addition to SEM and EBSD scans, some apparently affected grains were investigated with nanoindentation, the results of which are given in **Figure 12**. The defect structure shows a high local concentration of defects that lead to an increase in hardness by 1–1.5 GPa over the first 20 μm beneath the surface. However, grains that show no orientation changes in the EBSD scan also showed no hardening in the

The modulus of elasticity was quantified during nanoindentation and turned out not to vary with distance from the contact surface and is thus not given in **Figure 12**. Altogether, the results of the MIM sample suggest that if grains are modified due to the sliding process they have to be located directly at the surface, grains further away from the contact are unaffected in any case, but even at the

The cast alloy CuSn12Ni2, though tested with additivated oil, showed smaller but significant differences in the wear volumes of CC and LF cast samples. The tribo-surfaces had different characteristics, which are illustrated in **Figure 13a** and **b**. The surface exhibits grooves, presumably formed by wear debris, but resembles a

lying on top of the tribo-contact area.

surface many grains do not adapt by e.g. lattice rotations.

nanoindenter measurement.

**248**

**Figure 11.**

*CuSn8 MIM EBSD scan.*

*Wear tracks on CuSn12Ni2: Top view SEM image - sliding direction going parallel to the vertical image edge a) conventional sample, b) LF sample; sub-surface microstructure in field ion image mode - sliding direction normal to the FIB cutting plane, c) conventional sample d) LF sample.*

metallographic cross-section as it clearly reveals all phases. The LF surface appears even smoother than the CC surface. As the conventional cross sections indicated some edge effects, but did not enable a proper identification of the structure, only the respective FIB cuts, normal to the sliding direction, are given in **Figure 13c** and **d**. Here samples tested at a normal load of 240 N are examined as the wear volume differences are larger than for 100 N normal load. Both samples form a nanocrystalline layer during the sliding process. However, the thickness of this layer is clearly larger for the CC version with about 8 μm compared to 1.2 μm for LF. As the CC samples

had higher wear volumes, this layer does not seem to have wear protective effect, at least under the conditions investigated here. The nanocrystalline layer is not homogeneously thick, but rather shows a wavy interface in both studied cast structures. The average grain size within the nanocrystalline layer with about 250 nm seems to be constant over the thickness and in both samples. Potentially there are even smaller grains, but they cannot properly be resolved in SEM pictures.

## **5. Discussion**

In lubricated reciprocating sliding contacts, all the studied copper alloy variants exhibited different tribological performance, if the microstructure was modified by changes in the production route. For the chosen loading conditions, the coarser and softer microstructures showed superior tribological performance compared to the finer grained and harder materials. Wear was pronouncedly reduced and friction coefficient levels of the run-in friction pairs were lower too for the softer variants of each alloy. This basic finding applies to dendritic cast structures as well as to globular structures.

In this study we compare the production routes MIM versus continuous casting followed by a massive forming step and lost foam casting versus continuous casting. MIM and LF both result in microstructures that are significantly coarser than the ones obtained through conventional processes. Thus these larger grained microstructures exhibit lower mechanical strength. Nevertheless, they showed to be more wear resistant than their smaller grained version from conventional processes. The effect was most pronounced for CuSn8 lubricated with non-additivated base oil SN150. This observation does not follow the conventional assumption that harder materials result in higher wear resistance, which follows the most common wear law of Archard and relates wear volume inversely proportional to macroscopic hardness [6]. As a consequence, small grained microstructures which result in higher tensile strength according to the Hall–Petch relation [16] are usually expected to be more wear resistant. Typically, only the initial or bulk material properties and microstructures are measured and taken into account [1].

In the present sliding systems, the detected wear volume of the initially coarsegrained structure and thus softer version of the alloy was significantly lower for both studied alloys and differed by factor of 2 for CuNi9Sn6 and by a factor of 2.5 for CuSn8. For the dendritic cast structures of the alloy CuSn12Ni2 the same effects on wear could be observed, but reduction was less pronounced. This can be attributed mainly to the use of additivated oil in order to be closer to the real application, yet, additives may contribute to a formation of a chemical wear protective tribofilm.

The analysis of the near-surface zones revealed the formation of a tribologically transformed layer and a mechanical mixed layer lying on top of the contact surface. Such a behaviour is well-known in tribology, but phenomenological descriptions of such microstructural changes often prevail [7, 25]. Few studies treat the effect of these layers on wear [26], or friction [27] More recent studies focus on the internal structure of the subsurface zone like the formation of a fibre texture [27] and/or the development of multiple grain structures, which split up in nano and micrometre sized grains [28]. Kapoor et al. [26] attribute the modified wear rates observed for different loading conditions to the different mechanical properties of these layers, independent of their genesis. Following Hall–Petch and Archard, a formation of a more fine-grained microstructure in the subsurface zone is expected to lead to local hardening and consequently to an increase of wear resistance.

However, the current study showed that the samples forming a thicker and more pronounced fine-grained zone exhibited higher wear than their chemically identical equivalents produced by MIM and LF, respectively. Typically, literature does

**251**

**Figure 14.**

*formed also in larger depths.*

*Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts…*

not cover the role of the material itself or different initial microstructures on wear and friction. As the most prominent difference between the chemically identical samples characterised here can be seen in the initial grain sizes, it was presumed to

For the MIM version of CuSn8 the EBSD scans showed that the large grains react individually to the stress imposed during sliding and some accumulate plastic strain by an extremely localised work hardening processes. The local extension of the hardening is below or equal the single grain size. According to the EBSD results, the lattice orientation of grains exhibiting hardening is substantially different from neighbouring orientations. Thus, the number of their potential gliding planes with low Schmidt factors [29] differs a lot. The number of available slip planes is regarded as the main reason for the inhomogeneity of strain accumulation along the contact surface. Grains showing internal strains in the EBSD scans are assumed to have offered more slip systems with low Schmidt factors, with respect to the sliding direction, than the grains showing no enhanced defect density. If dislocation gliding is possible, the crystallographic lattice can take up strain and will partially rotate. If not, parts of the grain will be abraded directly with hardly any defect formation. The resulting enhanced hardening over a depth of 20 μm below the surface reaching up to 1.5 GPa was proven with nano-indentation measurements (**Figure 12**) for CuSn8. The EBSD scan illustrates that some grains take up plastic strain via local lattice rotations but form no new grain boundary. Quantitative analysis of SAGB or kernel average misorientation (KAM) are not depicted, because the large grains resulting from the MIM process revealed no substructure formation during sliding. The mechanically mixed layer found in patch-like structures on the surface could not be studied by EBSD as the misorientation was too high to identify grain structures. The nature of this layer would have to be verified in detailed TEM analyses in the future. The TTL and the mechanically mixed patches are considered as two independently forming features (**Figure 14**), which both have the capability to reduce wear. The mechanically mixed layer presumably distributes the normal load and thus reduces the contact stress. The local hardening – though inhomogeneous – may postpone the detachment of wear particles as described in [30]. Surprisingly, we found no mechanical mixed layer on the small-grained structures with high initial

In contrast to the MIM version the initial grain structure of the conventional CuSn8 was fine-grained and exhibits very high twin densities. Such a highly deformed microstructure can hardly take up further plastic strain and the local work-hardening potential is low. Consequently, no TTL could be observed in the EBSD scans and the microstructure does not appear to adapt during sliding, but

The dendritic cast alloy CuSn12Ni2 forms a nanocrystalline surface layer, which is similar to the ultra-nanocrystalline layer described for a Ni-alloy by [27].

*Principal sketch of tribologically transformed zone beneath the surface a) partial grain rotation and tribolayer patches of mixed material b) formation of a thin layer of nanocrystalline grains accompanied by slip lines and twins, c) thicker nanocrystalline zone in vortex shape together with higher density of slip lines and twins* 

*DOI: http://dx.doi.org/10.5772/intechopen.94210*

twin densities (**Figure 9a**) after identical loading.

seems to be worn off immediately during sliding.

be a key influencing factor.

*Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts… DOI: http://dx.doi.org/10.5772/intechopen.94210*

not cover the role of the material itself or different initial microstructures on wear and friction. As the most prominent difference between the chemically identical samples characterised here can be seen in the initial grain sizes, it was presumed to be a key influencing factor.

For the MIM version of CuSn8 the EBSD scans showed that the large grains react individually to the stress imposed during sliding and some accumulate plastic strain by an extremely localised work hardening processes. The local extension of the hardening is below or equal the single grain size. According to the EBSD results, the lattice orientation of grains exhibiting hardening is substantially different from neighbouring orientations. Thus, the number of their potential gliding planes with low Schmidt factors [29] differs a lot. The number of available slip planes is regarded as the main reason for the inhomogeneity of strain accumulation along the contact surface. Grains showing internal strains in the EBSD scans are assumed to have offered more slip systems with low Schmidt factors, with respect to the sliding direction, than the grains showing no enhanced defect density. If dislocation gliding is possible, the crystallographic lattice can take up strain and will partially rotate. If not, parts of the grain will be abraded directly with hardly any defect formation. The resulting enhanced hardening over a depth of 20 μm below the surface reaching up to 1.5 GPa was proven with nano-indentation measurements (**Figure 12**) for CuSn8. The EBSD scan illustrates that some grains take up plastic strain via local lattice rotations but form no new grain boundary. Quantitative analysis of SAGB or kernel average misorientation (KAM) are not depicted, because the large grains resulting from the MIM process revealed no substructure formation during sliding. The mechanically mixed layer found in patch-like structures on the surface could not be studied by EBSD as the misorientation was too high to identify grain structures. The nature of this layer would have to be verified in detailed TEM analyses in the future.

The TTL and the mechanically mixed patches are considered as two independently forming features (**Figure 14**), which both have the capability to reduce wear. The mechanically mixed layer presumably distributes the normal load and thus reduces the contact stress. The local hardening – though inhomogeneous – may postpone the detachment of wear particles as described in [30]. Surprisingly, we found no mechanical mixed layer on the small-grained structures with high initial twin densities (**Figure 9a**) after identical loading.

In contrast to the MIM version the initial grain structure of the conventional CuSn8 was fine-grained and exhibits very high twin densities. Such a highly deformed microstructure can hardly take up further plastic strain and the local work-hardening potential is low. Consequently, no TTL could be observed in the EBSD scans and the microstructure does not appear to adapt during sliding, but seems to be worn off immediately during sliding.

The dendritic cast alloy CuSn12Ni2 forms a nanocrystalline surface layer, which is similar to the ultra-nanocrystalline layer described for a Ni-alloy by [27].

#### **Figure 14.**

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

grains, but they cannot properly be resolved in SEM pictures.

structures are measured and taken into account [1].

hardening and consequently to an increase of wear resistance.

**5. Discussion**

had higher wear volumes, this layer does not seem to have wear protective effect, at least under the conditions investigated here. The nanocrystalline layer is not homogeneously thick, but rather shows a wavy interface in both studied cast structures. The average grain size within the nanocrystalline layer with about 250 nm seems to be constant over the thickness and in both samples. Potentially there are even smaller

In lubricated reciprocating sliding contacts, all the studied copper alloy variants exhibited different tribological performance, if the microstructure was modified by changes in the production route. For the chosen loading conditions, the coarser and softer microstructures showed superior tribological performance compared to the finer grained and harder materials. Wear was pronouncedly reduced and friction coefficient levels of the run-in friction pairs were lower too for the softer variants of each alloy. This

In the present sliding systems, the detected wear volume of the initially coarsegrained structure and thus softer version of the alloy was significantly lower for both studied alloys and differed by factor of 2 for CuNi9Sn6 and by a factor of 2.5 for CuSn8. For the dendritic cast structures of the alloy CuSn12Ni2 the same effects on wear could be observed, but reduction was less pronounced. This can be attributed mainly to the use of additivated oil in order to be closer to the real application, yet, additives may contribute to a formation of a chemical wear protective tribofilm. The analysis of the near-surface zones revealed the formation of a tribologically transformed layer and a mechanical mixed layer lying on top of the contact surface. Such a behaviour is well-known in tribology, but phenomenological descriptions of such microstructural changes often prevail [7, 25]. Few studies treat the effect of these layers on wear [26], or friction [27] More recent studies focus on the internal structure of the subsurface zone like the formation of a fibre texture [27] and/or the development of multiple grain structures, which split up in nano and micrometre sized grains [28]. Kapoor et al. [26] attribute the modified wear rates observed for different loading conditions to the different mechanical properties of these layers, independent of their genesis. Following Hall–Petch and Archard, a formation of a more fine-grained microstructure in the subsurface zone is expected to lead to local

However, the current study showed that the samples forming a thicker and more pronounced fine-grained zone exhibited higher wear than their chemically identical equivalents produced by MIM and LF, respectively. Typically, literature does

basic finding applies to dendritic cast structures as well as to globular structures. In this study we compare the production routes MIM versus continuous casting followed by a massive forming step and lost foam casting versus continuous casting. MIM and LF both result in microstructures that are significantly coarser than the ones obtained through conventional processes. Thus these larger grained microstructures exhibit lower mechanical strength. Nevertheless, they showed to be more wear resistant than their smaller grained version from conventional processes. The effect was most pronounced for CuSn8 lubricated with non-additivated base oil SN150. This observation does not follow the conventional assumption that harder materials result in higher wear resistance, which follows the most common wear law of Archard and relates wear volume inversely proportional to macroscopic hardness [6]. As a consequence, small grained microstructures which result in higher tensile strength according to the Hall–Petch relation [16] are usually expected to be more wear resistant. Typically, only the initial or bulk material properties and micro-

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*Principal sketch of tribologically transformed zone beneath the surface a) partial grain rotation and tribolayer patches of mixed material b) formation of a thin layer of nanocrystalline grains accompanied by slip lines and twins, c) thicker nanocrystalline zone in vortex shape together with higher density of slip lines and twins formed also in larger depths.*

We again refer to it as TTL. However, under loading conditions as those in the present study, this nanocrystalline layer was thicker for the finer structure resulting from continuous casting and very thin for the softer LF structure with large dendrites ranging over several hundred μm. Therefore, the nanocrystalline layer appears to be not beneficial for higher wear resistance.

Based on these observations we set up the following hypothesis for increased wear resistance of chemically equal, but softer, microstructures. Large grains without or with low initial defect densities can work-harden if the surface grains are appropriately oriented, i.e. so that the Schmidt factor of easy slip planes is low – (111) in the cubic systems of the study. For increased wear resistance, a near-surface layer has to take up plastic strains originating from the external stress state during the sliding process. This layer can form through local rotation of surface grains, as illustrated in **Figure 14**. For large grains, such as in the present study, where the grains have an average equivalent diameter of up to 250 μm, only partial rotation of grains occurs. Although the local grain rotations are rather inhomogeneous over the contact area, as many grains do not participate in local lattice rotations, this zone is relevant for the macroscopically observed wear volume and thus referred to as TTL. If the individual grains are saturated in defect densities, the surface near zone cannot work-harden and, as a consequence, the critical stresses for delamination or detachment of wear particles are much lower, wear occurs earlier and wear rates are higher.

Classical hardening through annealing processes and spinodal precipitations, which are found in CuNi9Sn6 can further reduce the wear volume compared to the non-hardenable CuSn8 as well as compared to the MIM version as shown in **Figure 5**. In the MIM version, the grain boundaries are facetted due to γ precipitation of length up to 2 μm at the grain boundaries, which is accompanied with a matrix depletion in the vicinity of the boundary. During heat treatment these precipitations at the boundaries dissolve and only lamellar phases within the grains and small intragranular precipitates form evenly distributed in the grains. The wear and friction result indicate that the latter structure is superior to adapt to the sliding process. Yet, the precipitation structure and distribution within the grains proved to be irrelevant for wear as well as friction. This indicates that the matrix composition and its element distribution determine the ability of a material to resist abrasion.

In the case of dendritic structures, the TTL comprises a nanocrystalline zone (**Figure 14**) with grain sizes ranging from 100 nm to 300 nm. Again, the thickness of this zone is inhomogeneous with "pockets" of nanocrystalline zones with an extension of about 20 μm below the surface (**Figure 13c** and **d**). As this characteristic is the same for the large grained LF and the finer grained CC sample, the wavy interface could be either a result of the large dendrite thickness and coarse grains or an effect of a periodic pattern of the contact stress state.

Still the differences of the respective maximum thicknesses are significantly different with 8 μm for CC and 1.2 μm for LF. The formation mechanism of such nanocrystalline grains is not entirely clear. [27, 30] compare the sliding process to rolling and torsion processes and claim that the near-surface textures are typical rolling textures. As the matrix of the MIM CuSn8 as well as the CuSn12Ni2 matrix, are both bronze metal matrix lattices, the local rotation found in CuSn8 could be an early status followed by formation of nanocrystalline grains.

The nanocrystalline layer found in LF samples resembles the non-continuous layer described in [28], referred to as nanostructured mixing layer (NML) and dynamic recrystallised layer. They show that these small grains form during the sliding contact and that the kinetics are different for different initial microstructures. The coarse-grained structures show higher wear rates which is not in line with our results on copper-based alloys. We found that the nanocrystalline layer is thicker for smaller-grained initial structures. However, the idea that not the

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**Acknowledgements**

*Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts…*

hardness of the initial microstructure determines wear resistance, but the ease with which the nanocrystalline layer can be peeled-off during sliding, seems to be an appropriate explanation for the wear behaviour of the conventional CuSn12Ni2

Innovative material production processes are usually associated with lower mechanical strength and with inferior performance. They lack trust by designers as they suffer comprehensive characterisation, especially tribological performance properties. In the current study, the technology route metal-injection moulding (MIM) and lost foam casting (LF) are applied to well-known commercial bronze alloys. The alternative routes resulted in more ductile and softer materials, but proved to be superior in terms of wear and even showed tendencies for lower fric-

A detailed characterisation executed using SEM, nanointendation as well as EBSD techniques revealed that all samples formed a tribologically transformed layer (TTL) beneath the contact surface, but that the extent of this layer was pronouncedly differ-

• The thickness of the formed nanocrystalline layer depends on the material

• Thus, grain refinement as a hardening mechanism does not go along with classical Archard approaches for increasing wear resistance via increasing hardness. The found nanocrystalline layer requires lower shear forces on the grains to be removed and this results in decreased macroscopic friction coefficients.

• The tribolayer, which showed to be a presumable amorphous mechanically mixed layer, forms independently of the TTL on MIM samples and was

• Large grained initial microstructures, produced by e. g. MIM, exhibit partial lattice grain rotation, which increases hardness very locally and were only

Finally, we present a hypothesis for the formation timeline of nanocrystalline zones. Starting with local lattice grain rotation, followed by a monolayer on nanocrystalline grain layer accompanied with slip band and twin formation beneath and finally a thick nanocrystalline layer with a vortex structure. Based on the observation, we postulate that higher alloyed materials are more prone to local lattice rotation and defect formations such as twins and thus form a nanocrystalline zone

Topics presented are based on research projects with support by the Austrian COMET program (project XTribology, no. 849109, project InTribology, no. 872176)

observed on samples showing lower wear than equivalently loaded convention-

• Thicker nanocrystalline layers do not enhance wear, but increase wear.

ent. The following observations can be summarised for the current study:

*DOI: http://dx.doi.org/10.5772/intechopen.94210*

version forming thick nanocrystalline layers.

tion levels under equal configurations and loadings.

**6. Conclusions**

production route

ally produced variants.

observed on samples with lower wear.

more easily and quicker under the same loading conditions.

*Wear Protective Effects of Tribolayer Formation for Copper Based Alloys in Sliding Contacts… DOI: http://dx.doi.org/10.5772/intechopen.94210*

hardness of the initial microstructure determines wear resistance, but the ease with which the nanocrystalline layer can be peeled-off during sliding, seems to be an appropriate explanation for the wear behaviour of the conventional CuSn12Ni2 version forming thick nanocrystalline layers.
