**4. Tribological behaviour**

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

**Cylinder**

mm ∅ 4.2

l = 12

**Table 2.**

*SRV test parameters.*

**Disc** mm ∅ 24

3

30

100 or

240

l = 7.8

mm

Hz

N

**Stroke**

**Frequency**

**Normal force**

**Test duration**

h 2

**Sliding distance**

m

1296

—

oil drop

**Lubrication**

**Disc temperature**

°C

50

**240**

**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 conditions.

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 readability,

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

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 about 0.07 compared to the conventionally produced CuNi9Sn6.

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 variants are regarded as significant.

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

**Figure 3.** *Friction coefficient of CuSn8 and CuNi9Sn6 both conventionally produced and via the MIM route.*

#### **Figure 4.**

*Friction coefficient of CuSn12Ni2 produced via conventional casting (CC) and via lost foam cast (LF) for 100 N and 240 N normal load.*

a pronounced increase for the MIM version, but lasting only 30 min, whereas the conventional material increases and decreases smoothly, reaching a steady state level just before the test ends. The characteristics of the friction curve over time seems to be unaffected by the microstructural changes during heat treatment, as there is again a steep but short increase during run-in. However, the scatter of the friction data is reduced by the heat treatment.

The averaged friction values at a test time of 90 min are depicted in **Figure 5** together with the averaged wear volume at the end of the test in a wear map. The error bars of the coefficient of friction are equivalent to those in the friction curves (**Figure 3**), the uncertainty of the wear volume is the standard deviation of the wear volume at the end of the test. The measured uncertainty is in some cases so small that it is nearly invisible in an appropriately scaled wear map.

The results illustrate a pronounced decrease of wear when the CuSn8 is produced via MIM instead of conventionally. There is also a reduction in wear for CuNi9Sn6 when comparing conventional to MIM in the heat-treated condition, but this decrease is far less pronounced. Between MIM and conventional samples there is no significant change in wear results.

**243**

**Figure 6.**

**Figure 5.**

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

the LF microstructure shows significantly lower wear volume.

For CuSn12Ni2 the friction coefficient levels are lower, ranging between 0.11 and 0.14 (**Figure 4**). This can mainly be attributed to the fully formulated gear oil used in this study, which was chosen because the tribosystem should be as close to the real application as possible. Therefore, a direct comparison between all the materials discussed before is not permissible. The differences in terms of friction between continuous and lost foam cast variant are little and judged to be irrelevant for applications. Still, the level of coefficient of friction is significantly lower for LF if the contact pressures are increased. Nevertheless, none of the two variants seems to be more sensitive to normal pressure changes than the other. More pronounced differences can be observed in the wear behaviour (**Figure 6**) of CuSn12Ni2, where

For all investigated materials, the innovative production routes lead to lower wear despite weaker mechanical properties, certainly the degree of improvement depends on the alloy, **Figures 5** and **6**. There are large differences in the final wear volume between the two production routes for CuSn8, which exceeds by far the measurement uncertainty of wear. For CuNi9Sn6 the effect of the production route is much smaller but the heat-treated MIM version shows a distinct wear reduction

*Wear map of CuSn12Ni2 produced via conventional casting (CC) and via lost foam cast (LF). The depicted friction coefficient was taken at test time of 90 min and the error bars are identical to those depicted in Figure 4,* 

*Wear map of CuSn8 and CuNi9Sn6 in two production variants: Conventional and MIM. The depicted friction coefficient was taken at a test time of 90 min and the error bars are identical to those depicted in Figure 3, the wear volume represents the value at the end of the test and the standard deviation of the measured volume.*

*the wear volume represents the value at the end of the test and its standard deviation.*

*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*

For CuSn12Ni2 the friction coefficient levels are lower, ranging between 0.11 and 0.14 (**Figure 4**). This can mainly be attributed to the fully formulated gear oil used in this study, which was chosen because the tribosystem should be as close to the real application as possible. Therefore, a direct comparison between all the materials discussed before is not permissible. The differences in terms of friction between continuous and lost foam cast variant are little and judged to be irrelevant for applications. Still, the level of coefficient of friction is significantly lower for LF if the contact pressures are increased. Nevertheless, none of the two variants seems to be more sensitive to normal pressure changes than the other. More pronounced differences can be observed in the wear behaviour (**Figure 6**) of CuSn12Ni2, where the LF microstructure shows significantly lower wear volume.

For all investigated materials, the innovative production routes lead to lower wear despite weaker mechanical properties, certainly the degree of improvement depends on the alloy, **Figures 5** and **6**. There are large differences in the final wear volume between the two production routes for CuSn8, which exceeds by far the measurement uncertainty of wear. For CuNi9Sn6 the effect of the production route is much smaller but the heat-treated MIM version shows a distinct wear reduction

#### **Figure 5.**

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

a pronounced increase for the MIM version, but lasting only 30 min, whereas the conventional material increases and decreases smoothly, reaching a steady state level just before the test ends. The characteristics of the friction curve over time seems to be unaffected by the microstructural changes during heat treatment, as there is again a steep but short increase during run-in. However, the scatter of the

*Friction coefficient of CuSn12Ni2 produced via conventional casting (CC) and via lost foam cast (LF) for* 

*Friction coefficient of CuSn8 and CuNi9Sn6 both conventionally produced and via the MIM route.*

The averaged friction values at a test time of 90 min are depicted in **Figure 5** together with the averaged wear volume at the end of the test in a wear map. The error bars of the coefficient of friction are equivalent to those in the friction curves (**Figure 3**), the uncertainty of the wear volume is the standard deviation of the wear volume at the end of the test. The measured uncertainty is in some cases so small

The results illustrate a pronounced decrease of wear when the CuSn8 is produced via MIM instead of conventionally. There is also a reduction in wear for CuNi9Sn6 when comparing conventional to MIM in the heat-treated condition, but this decrease is far less pronounced. Between MIM and conventional samples there

friction data is reduced by the heat treatment.

is no significant change in wear results.

that it is nearly invisible in an appropriately scaled wear map.

**242**

**Figure 3.**

**Figure 4.**

*100 N and 240 N normal load.*

*Wear map of CuSn8 and CuNi9Sn6 in two production variants: Conventional and MIM. The depicted friction coefficient was taken at a test time of 90 min and the error bars are identical to those depicted in Figure 3, the wear volume represents the value at the end of the test and the standard deviation of the measured volume.*

#### **Figure 6.**

*Wear map of CuSn12Ni2 produced via conventional casting (CC) and via lost foam cast (LF). The depicted friction coefficient was taken at test time of 90 min and the error bars are identical to those depicted in Figure 4, the wear volume represents the value at the end of the test and its standard deviation.*

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 to be independent of the applied normal load.
