**6. Conclusions**

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

appears to be not beneficial for higher wear resistance.

an effect of a periodic pattern of the contact stress state.

early status followed by formation of nanocrystalline grains.

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

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

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

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

**252**

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 friction levels under equal configurations and loadings.

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


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 more easily and quicker under the same loading conditions.
