**3. Experimental**

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

design with a new material.

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

for limited energy and environmental resources, paired with cost aspects, innovative near net shape technologies gain interest [2]. The effects of these processing technologies on microstructural details and furthermore on friction and wear are not sufficiently understood to predict material behaviour in a sliding contact to regulate wear rates and or frictional levels. Apart from classical structural mechanical properties, these tribological characteristics have to be known for a proper

In the present chapter two alternative production routes are evaluated for three copper based alloys. The near net shape technologies metal (powder) injection moulding (MIM) and lost foam casting (LF) are described in part 2, both are known for steel [3] and aluminium [4] but have not been commercialised for copper-based alloys and lack substantial basic knowledge in published literature. Yet, near net shape technologies are especially interesting for copper-based alloys due to the high raw metal costs. Here, a special focus is put on the characterisation of wear and friction in a lubricated sliding contact of representative alloys shown in part 3.1. The chosen experimental set-up is depicted in part 3.2 and their tribological and analytical results in part 4. Based on the observations of a formed tribolayer and the nano-crystalline zones forming a tribologically transformed layer (TTL) as described in part 4. Part 5 forms the discussion that links the findings to literature and suggests a hypothesis for the formation of tribolayer and TTL and their effect on wear and friction levels. Finally, the chapter finishes with a conclusion in part 6.

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

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

**236**

**Figure 1.**

*Technological paths of the analysed materials.*

and costs.

#### **3.1 Investigated material**

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

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


#### **Table 1.**

*Characterised copper-based alloys and the grain sizes and hardness for different production routes of nominal identical alloys.*

#### **Figure 2.**

*Microstructures of studied alloys: Conventional: a) CuSn8 c) CuNi9Sn6 e) CuSn12Ni2; MIM: b) CuSn8, d) CuNi9SN6; LF: f) CuSN12Ni2.*

microscopy images, their effects on the microstructure can be observed by grain boundary faceting as well as formation of violet lamellar phases during annealing.

Alternatively, these two alloys can be produced by a metal injection moulding (MIM) process from pre-alloyed feedstocks. The MIM production route inherently lacks a forming step and thus any possibility for grain refinement. As a result, the

**239**

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

average grain size of the materials produced through MIM is much coarser than the one obtained via conventional routes. **Figure 2** shows representative microstructures of all studied materials. The MIM version of CuNi9SN6 can be heat treated in the same way as the conventional version in order to increase the mechanical

Continuous casting (CC) followed by machining is a typical production route for the cast alloy CuSn12Ni2. An alternative, innovative production route, which can help to save energy is a sand casting process called lost foam (LF) casting [4]. The dendritic structure resulting from LF casting is pronouncedly coarser than the CC variant as it is basically a sand-casting process with correspondingly long cooling

All production variants of the advanced technologies, MIM and LF, lead to larger grain sizes with hardly any defects such as twins. Therefore, these material variants are significantly softer than their counterparts from the conventional production route. As often higher wear resistance is associated with high mechanical strength, the small-grained, conventionally produced alloys are expected to exhibit better wear resistance. This assumption, which is a commonly used basis for design and material choices in mechanical engineering, is verified in the current study for three

Samples of the different alloy variants were studied in a lubricated reciprocal sliding contact with a modified SRV® test rig. The setup is described in more detail in [19–21]. **Table 2** summarises the main test conditions. Each set of parameters was repeated at least twice. The base oil SN150 was used for CuSn8 and CuNi9Sn6, for CuSn12Ni2 a fully formulated mineral oil, a commercial gear oil, was applied. Thus, one has to be aware of the effect of different viscosities. In order to increase wear of the fully formulated system, CuSn12Ni2 was also examined at a normal load of 240 N. These external conditions differ because the potential applications of the

The friction coefficients were continuously recorded at a rate of 1 Hz and the data of individual test runs were averaged. Wear scars were characterised after the tests by topographic analysis using confocal microscopy with a Leica DCM3D at 20× magnification, which allowed the measurement of the wear track width.

A wear map is used to illustrate the performance of the individual materials and their variants. It shows the wear volume measured at the end of the test run versus the coefficient of friction at a test time of 90 min, which corresponds to SRV standards [22]. This kind of diagram enables a simple but informative tribological rating, as both friction and wear behaviour are usually relevant. The desired low friction sliding material with a high lifetime can be found in the left lower corner.

In order to understand modification processes deriving from tribological interactions selected samples were investigated further with light microscopy, nanoindentation, scanning electron microscopy (SEM) and focused ion beam (FIB) cross sections or electron back scatter diffraction (EBSD). EBSD was performed on cross sections normal to the sliding plane with a Zeiss Supra 40VP instrument equipped with an EBSD system from EDAX. The samples were chemically etched prior to the scan. The step sizes (0.20 μm for CuSn8 MIM; 0.10 μm for conventional CuSn8) and the scan areas (100 × 250 μm/19 × 30 μm) were adapted to the grain

Afterwards, the wear volume was calculated as described in [20].

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

strength by precipitation hardening.

times and lacking deformation during solidification.

alloys and two innovative production routes.

two technologies are different.

**3.3 Microstructure analysis**

**3.2 Experimental tests - tribological characterisation**

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

average grain size of the materials produced through MIM is much coarser than the one obtained via conventional routes. **Figure 2** shows representative microstructures of all studied materials. The MIM version of CuNi9SN6 can be heat treated in the same way as the conventional version in order to increase the mechanical strength by precipitation hardening.

Continuous casting (CC) followed by machining is a typical production route for the cast alloy CuSn12Ni2. An alternative, innovative production route, which can help to save energy is a sand casting process called lost foam (LF) casting [4]. The dendritic structure resulting from LF casting is pronouncedly coarser than the CC variant as it is basically a sand-casting process with correspondingly long cooling times and lacking deformation during solidification.

All production variants of the advanced technologies, MIM and LF, lead to larger grain sizes with hardly any defects such as twins. Therefore, these material variants are significantly softer than their counterparts from the conventional production route. As often higher wear resistance is associated with high mechanical strength, the small-grained, conventionally produced alloys are expected to exhibit better wear resistance. This assumption, which is a commonly used basis for design and material choices in mechanical engineering, is verified in the current study for three alloys and two innovative production routes.

#### **3.2 Experimental tests - tribological characterisation**

Samples of the different alloy variants were studied in a lubricated reciprocal sliding contact with a modified SRV® test rig. The setup is described in more detail in [19–21]. **Table 2** summarises the main test conditions. Each set of parameters was repeated at least twice. The base oil SN150 was used for CuSn8 and CuNi9Sn6, for CuSn12Ni2 a fully formulated mineral oil, a commercial gear oil, was applied. Thus, one has to be aware of the effect of different viscosities. In order to increase wear of the fully formulated system, CuSn12Ni2 was also examined at a normal load of 240 N. These external conditions differ because the potential applications of the two technologies are different.

The friction coefficients were continuously recorded at a rate of 1 Hz and the data of individual test runs were averaged. Wear scars were characterised after the tests by topographic analysis using confocal microscopy with a Leica DCM3D at 20× magnification, which allowed the measurement of the wear track width. Afterwards, the wear volume was calculated as described in [20].

A wear map is used to illustrate the performance of the individual materials and their variants. It shows the wear volume measured at the end of the test run versus the coefficient of friction at a test time of 90 min, which corresponds to SRV standards [22]. This kind of diagram enables a simple but informative tribological rating, as both friction and wear behaviour are usually relevant. The desired low friction sliding material with a high lifetime can be found in the left lower corner.

#### **3.3 Microstructure analysis**

In order to understand modification processes deriving from tribological interactions selected samples were investigated further with light microscopy, nanoindentation, scanning electron microscopy (SEM) and focused ion beam (FIB) cross sections or electron back scatter diffraction (EBSD). EBSD was performed on cross sections normal to the sliding plane with a Zeiss Supra 40VP instrument equipped with an EBSD system from EDAX. The samples were chemically etched prior to the scan. The step sizes (0.20 μm for CuSn8 MIM; 0.10 μm for conventional CuSn8) and the scan areas (100 × 250 μm/19 × 30 μm) were adapted to the grain

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

treatment, chipping

debindering, sintering

treatment, chipping

debindering, sintering

Injection moulding, debindering, sintering, heat treatment

Cuni12Ni2 LF Lost foam cast, chipping Dendrite

CuSn8 conv. Continuous cast, drawing, heat

CuNi9Sn6 conv. Continuous cast, rolling, heat

CuSn8 MIM Injection moulding,

CuNi9Sn6 MIM Injection moulding,

CuNiSn6 MIM, heat treated 1 h@450°C

**Table 1.**

*identical alloys.*

**Material Technological route Grain size Hardness**

CuSn12Ni2 conv. Continuous cast, chipping ~ 50 μm 115 HB30 2.5

*Characterised copper-based alloys and the grain sizes and hardness for different production routes of nominal* 

< 20 μm 238 HV1

100–150 μm 70 HV10

~ 50/150 μm 180 HV1

~ 200 μm 171 HV10

~ 200 μm 266 HV1

90 HB30 2.5

length > 500 μm

microscopy images, their effects on the microstructure can be observed by grain boundary faceting as well as formation of violet lamellar phases during annealing. Alternatively, these two alloys can be produced by a metal injection moulding (MIM) process from pre-alloyed feedstocks. The MIM production route inherently lacks a forming step and thus any possibility for grain refinement. As a result, the

*Microstructures of studied alloys: Conventional: a) CuSn8 c) CuNi9Sn6 e) CuSn12Ni2; MIM: b) CuSn8, d)* 

**238**

**Figure 2.**

*CuNi9SN6; LF: f) CuSN12Ni2.*


**Table 2.**

*SRV test parameters.*

**241**

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

sizes of the respective samples. Scan sizes of 200 nm were chosen for CuSn8 MIM and 100 nm for conventional CuSn8. The position of the scan along the contact area was pre-selected in SEM images. Three scans were performed on a total area large enough to be representative for the largest grain sizes found in CuSn8 MIM. For microstructures like the conventionally produced CuSn8 a scan directly at the surface was not possible due to the high local deformation. A sufficient Kikuchi pattern quality was only detectable in a depth of 1 μm below the surface. The scans were analyses based on pattern quality images overlaid with the small (SAGB) and large angle grain boundaries (LAGB) as well as on the inverse pole figure (IPF) pictures with the projection direction <100>, which is the surface-normal direction to the plane of the cross section. Numerous precipitations in the MIM version of CuNi9Sn6 complicated the pattern recognition and therefore EBSD scans were omitted. FIB cuts were employed for the deeper analysis of the cast alloy CuSn12Ni2. The FIB cuts were oriented normal to the sliding direction as a cut in the sliding direction would either be located on top or at the bottom of a groove

**Figures 3** and **4** show the frictional behaviour over test time for all studied materials. All friction coefficient curves – except CuSn8 – are at their maximum during running-in, which ranges from 5 to 30 min. All curves end in an almost constant steady level for the chosen test duration and have thus reached steady-state

The observed friction coefficient values lie between 0.20 and 0.42. These values represent the arithmetic mean of the friction coefficients of different test runs. The data of each run was averaged for each recorded friction value, these mean values are shown as the friction curves in **Figures 3** and **4**. In order to add the information of the scattering of the individual runs for a better interpretation of the friction behaviour, the standard deviation of the friction signal of each run was calculated. The error bars in **Figures 3** and **4** represent the mean standard deviations of individual test runs with the same material, shown only every 5 min for the sake of

The levels of the friction coefficients as well as the friction behaviour differ distinctly for the two materials CuSn8 and CuNi9Sn6. Their sensitivity to variations

For both materials, MIM manufacturing leads to lower friction coefficients after the run-in phase. For CuSn8 the reduction is roughly 0.08, for CuNi9Sn6 it is less pronounced, namely 0.02–0.03. A heat treatment, which increases the structural strength of the CuNi9Sn6 MIM variant, further reduces the friction coefficient by

The MIM variant of CuSn8 shows a high variability throughout the whole test time and after an initial increase to 0.40 smoothly decreases to 0.30 between 30 min and 100 min. The values of the conventional CuSn8 lie within the variability of the MIM variant, but do not show the same characteristics over time. After an initial increase, the friction reaches an almost constant level at 0.40, which is on the upper bound of the scatter of the CuSn8 MIM data. As, the noise of conventional CuSn8 is much smaller than the MIM sample, the different levels of friction of the two CuSn8

The error bars largely overlap for the conventional and the MIM version of CuNi9Sn6, indicating just a small tendency for higher average friction levels for the conventional variant. However, the run-in period is distinctly different with

in the process route seems to be pronouncedly different, as well.

about 0.07 compared to the conventionally produced CuNi9Sn6.

variants are regarded as significant.

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

produced by wear debris.

conditions.

readability,

**4. Tribological behaviour**

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

sizes of the respective samples. Scan sizes of 200 nm were chosen for CuSn8 MIM and 100 nm for conventional CuSn8. The position of the scan along the contact area was pre-selected in SEM images. Three scans were performed on a total area large enough to be representative for the largest grain sizes found in CuSn8 MIM. For microstructures like the conventionally produced CuSn8 a scan directly at the surface was not possible due to the high local deformation. A sufficient Kikuchi pattern quality was only detectable in a depth of 1 μm below the surface. The scans were analyses based on pattern quality images overlaid with the small (SAGB) and large angle grain boundaries (LAGB) as well as on the inverse pole figure (IPF) pictures with the projection direction <100>, which is the surface-normal direction to the plane of the cross section. Numerous precipitations in the MIM version of CuNi9Sn6 complicated the pattern recognition and therefore EBSD scans were omitted. FIB cuts were employed for the deeper analysis of the cast alloy CuSn12Ni2. The FIB cuts were oriented normal to the sliding direction as a cut in the sliding direction would either be located on top or at the bottom of a groove produced by wear debris.
