**2. Copper-based alloys as tribomaterials**

Copper alloys are materials with a good track record for tribological applications comprising pronounced sliding. However, the demand of alloys is limited by high raw material costs and traditional energy intensive production routes; such as melt metallurgy, casting, hot and/or cold forming; the subsequent machining processes lead to large amounts of chips that have to be collected and re-melted to be recycled. As consequence, the production of components made of copper-based alloys demand excessive energy and consequently result in high ecological impact and costs.

Energy-efficient technologies offer economical production methods and additionally large options for complex component shapes. Currently, additive manufacturing and near net shape manufacturing are the main avenues towards achieving these goals. **Figure 1** depicts a schematic of the processes involved: MIM and LF

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

production technologies; the basic principles are described in [5]. Such innovative production technologies usually result in different material properties compared to traditional production methods; the functionality of the component produced by alternative processes are often perceived to be inferior. For a more widespread application of new production technologies, a prediction of the target properties would be necessary. Since a correlation between the material structure and the resulting properties are not trivial, the material selection process is still mainly based on empirical knowledge of engineers. For tribologically loaded components the material selection usually relays on bulk material properties and the assumption that the harder material is the more wear resistant, which is conformed in many cases. This material rating is based on the well-known Archard's wear law [6], which

Tribological studies in the nineties, have revealed that ductile alloys form an altered, fine-grained microstructure in the volume just beneath the sliding contact area as published by Rigney in [7]. Following classical approaches of material hardening, the material increases in strength as the grain size decreases and according to

Most published tribological sliding experiments studying material changes were carried out on pure metals, such as Au or Cu, in a ball on plate configuration in dry contact [8–10]. They show the formation of a microstructure with much finer grain sizes compared to the initial grain size, which is only preserved in the bulk. Some studies can be found on alloys, e.g. steel [11, 12] or Co-based alloys [13, 14], which revealed grain refinement beneath the surface, too. Recent tribological studies focus on the evolution of microstructural changes such as [9, 13], but correlations of grain-size to properties e. g. strength or work-hardening are treated by material science studies: Studies on work-hardening behaviour due to grain size changes, as described for copper by [15], do not deal with grain refinement processes exhibited by tribological contacts. Besides, the strength increasing effect of refinement according to Hall–Petch relations was reported to apply until a gain size of approxi-

Usually, superior wear resistance is attributed to those newly formed structures [13, 14], but often no explicit correlation to wear or friction is given [9]. Thus, the effect of microstructural changes in subsurface regions in a sliding contact on wear and also friction levels is not really understood and often underrated. Classical material selection strategies follow the idea that material with higher strength

The current study compares three copper-based alloys, each produced via two different process technologies – one conventional technology route via casting, forming and machining and a near net shape new technology. The alloys are listed in **Table 1** together with their hardness values and the range of grain sizes observed in the cross sections. The microstructures resulting from different production routes are shown in **Figure 2**. The wrought alloys CuSn8 and CuNi9Sn6 are typically continuously cast, followed by forming (pressing, drawing, rolling) and, for CuNi9Sn6, heat treatment through spinodal decomposition [17]. Apart from the spinodally forming precipitations, which cannot be observed in SEM images, this alloy also forms γ precipitations (in different crystallographic structures as described in [18]) that are several μm large and are visible in SEM pictures. In light

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

correlates wear volume inversely with hardness.

Archard's wear law this results in lower wear.

mately 10 nm is reached for copper [16].

exhibit higher wear resistance.

**3. Experimental**

**3.1 Investigated material**

**Figure 1.** *Technological paths of the analysed materials.*

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

production technologies; the basic principles are described in [5]. Such innovative production technologies usually result in different material properties compared to traditional production methods; the functionality of the component produced by alternative processes are often perceived to be inferior. For a more widespread application of new production technologies, a prediction of the target properties would be necessary. Since a correlation between the material structure and the resulting properties are not trivial, the material selection process is still mainly based on empirical knowledge of engineers. For tribologically loaded components the material selection usually relays on bulk material properties and the assumption that the harder material is the more wear resistant, which is conformed in many cases. This material rating is based on the well-known Archard's wear law [6], which correlates wear volume inversely with hardness.

Tribological studies in the nineties, have revealed that ductile alloys form an altered, fine-grained microstructure in the volume just beneath the sliding contact area as published by Rigney in [7]. Following classical approaches of material hardening, the material increases in strength as the grain size decreases and according to Archard's wear law this results in lower wear.

Most published tribological sliding experiments studying material changes were carried out on pure metals, such as Au or Cu, in a ball on plate configuration in dry contact [8–10]. They show the formation of a microstructure with much finer grain sizes compared to the initial grain size, which is only preserved in the bulk. Some studies can be found on alloys, e.g. steel [11, 12] or Co-based alloys [13, 14], which revealed grain refinement beneath the surface, too. Recent tribological studies focus on the evolution of microstructural changes such as [9, 13], but correlations of grain-size to properties e. g. strength or work-hardening are treated by material science studies: Studies on work-hardening behaviour due to grain size changes, as described for copper by [15], do not deal with grain refinement processes exhibited by tribological contacts. Besides, the strength increasing effect of refinement according to Hall–Petch relations was reported to apply until a gain size of approximately 10 nm is reached for copper [16].

Usually, superior wear resistance is attributed to those newly formed structures [13, 14], but often no explicit correlation to wear or friction is given [9]. Thus, the effect of microstructural changes in subsurface regions in a sliding contact on wear and also friction levels is not really understood and often underrated. Classical material selection strategies follow the idea that material with higher strength exhibit higher wear resistance.
