**2. Wear-induced mild-to-severe wear transition**

#### **2.1 Experimental procedure**

**Figure 1** shows a schematic of the test rig. Semi-cylindrical upper specimen oscillatorily slides on the cylindrical specimen immersed in mineral oil [1, 2]. The lubrication condition is supposed to be boundary lubrication. This sliding motion is apparently a pure sliding but actually a rolling-sliding motion. **Figure 2** illustrates the specimen motion. The upper specimen rolls an angle of θ, while it slides a distance of R θ on the lower specimen. Note that usually the sliding direction is the same as the rolling direction in the rolling-sliding motion of a gear surface or a traction drive, but this rolling-sliding motion is opposite. Since both contact points of the upper and lower specimens move during sliding, it is more difficult to conform the sliding surface than pure sliding.

Test materials and test conditions are shown in **Tables 1** and **2**, respectively. The lower specimen was silver-plated in order to prevent the initial large contact

**259**

**Figure 2.**

**Table 1.** *Test materials [1].*

**Table 2.** *Test conditions [1].*

*Rolling-sliding motion of specimens [2].*

Upper specimen Radius; 3 mm, Width; 2 mm

Lower specimen Radius: 10 mm and width: 10 mm

Oil Mineral and synthesized mixed insulating oil Kinetic viscosity: 5.2 cSt (40°C)

Motion Oscillation

Load (N) 39, 69, 83, 98 Frequency (Hz) 2 Stroke (mm) 10

Atmosphere Laboratory air (20 ~ 25°C, RH 40 ~ 60%)

Material; 99.9% Cu (JIS C1100H, HV ~100**)**

Material: 50 μm silver electroplated 99.9% Cu (JIS C1100H, HV ~100)

to monitor the wear conditions.

**2.2 Experimental results**

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics…*

pressure due to misalignment. The load was applied by the coil spring. Loads shown in **Table 2** were those when the upper specimen was on top of the lower specimen. Therefore, the load varied during sliding by about 0 ~ −8% due to the vertical movement of the upper specimen. The horizontal force, that was the sum of the horizontal components of the friction and the load, was measured by the load cell

**Figure 3** shows the time evolution of horizontal forces. The forces shown were those of the maximum values for one oscillation. All forces indicated sudden increase except for the 39-N load. **Figure 4** indicates the trends of horizontal forces of the 69-N and the 98-N loads obtained from the tests of some oscillation numbers ((a)) and the wear depth at the center of the lower specimen wear scar of them ((b)).

*DOI: http://dx.doi.org/10.5772/intechopen.93789*

**Figure 1.** *Schematic of wear test rig [1].*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics… DOI: http://dx.doi.org/10.5772/intechopen.93789*

#### **Figure 2.**

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

**2. Wear-induced mild-to-severe wear transition**

The second one is about wear track profiles [11, 12]. When metal sliding pairs are rubbed under relatively severe conditions, streaked wear tracks are generated in many cases. By observing the wear tracks of wear specimens which we have obtained for various test purposes and those of several published papers, we noticed that the appearance of many streaked wear tracks is similar. In other words, the width of the streaks does not change much regardless of the sliding material and wear conditions. We thought this suggests the existence of a general mechanism for producing wear tracks that can suggest the way to prevent severe damage

Though some researchers seemed to have been aware of this feature, few studies were found that focused on the wear track profile characterization [13], though many studies have been conducted on the wear scar or wear track morphology

These two issues are often overlooked but we think they can be one of the key

**Figure 1** shows a schematic of the test rig. Semi-cylindrical upper specimen oscillatorily slides on the cylindrical specimen immersed in mineral oil [1, 2]. The lubrication condition is supposed to be boundary lubrication. This sliding motion is apparently a pure sliding but actually a rolling-sliding motion. **Figure 2** illustrates the specimen motion. The upper specimen rolls an angle of θ, while it slides a distance of R θ on the lower specimen. Note that usually the sliding direction is the same as the rolling direction in the rolling-sliding motion of a gear surface or a traction drive, but this rolling-sliding motion is opposite. Since both contact points of the upper and lower specimens move during sliding, it is more difficult to conform

Test materials and test conditions are shown in **Tables 1** and **2**, respectively. The lower specimen was silver-plated in order to prevent the initial large contact

**258**

**Figure 1.**

*Schematic of wear test rig [1].*

from wear.

(e.g., [14]).

aspects of wear.

**2.1 Experimental procedure**

the sliding surface than pure sliding.

*Rolling-sliding motion of specimens [2].*


#### **Table 1.**

*Test materials [1].*


**Table 2.**

*Test conditions [1].*

pressure due to misalignment. The load was applied by the coil spring. Loads shown in **Table 2** were those when the upper specimen was on top of the lower specimen. Therefore, the load varied during sliding by about 0 ~ −8% due to the vertical movement of the upper specimen. The horizontal force, that was the sum of the horizontal components of the friction and the load, was measured by the load cell to monitor the wear conditions.

#### **2.2 Experimental results**

**Figure 3** shows the time evolution of horizontal forces. The forces shown were those of the maximum values for one oscillation. All forces indicated sudden increase except for the 39-N load. **Figure 4** indicates the trends of horizontal forces of the 69-N and the 98-N loads obtained from the tests of some oscillation numbers ((a)) and the wear depth at the center of the lower specimen wear scar of them ((b)).

**Figure 3.** *Time evolution of horizontal forces for the loads tested [1].*

#### **Figure 4.**

*(a) Trends of horizontal forces of the 69-N (empty symbols) and the 98-N loads (filled symbols) and (b) wear depth of lower specimen wear scar [1].*

The wear depth gradually increased and then suddenly and rapidly increased at the oscillation numbers of sudden increase of horizontal forces (points G and D).

**Figure 5** shows the microscope images of the wear scars of the lower specimens of B and D in **Figure 4**. From **Figure 5(b)**, the wear scar was rough and indicated the traces of hard adhesion, whereas that appeared relatively smooth in **Figure 5(a)**. From the wear depth progress in **Figure 4(b)** and wear scar appearances in **Figure 5**, the wear can be referred to as mild wear and severe wear before and after the sudden increase of the horizontal force, respectively. Thus, mild-to-severe wear transition occurred above a certain load.

Traces of silver-plating were not observed in **Figure 5**. **Figure 6** indicates the Auger electron energy intensity measured in the wear scars of the lower specimen of the tests stopped before (15,600 oscillations) and after (18,600 oscillations) the point C in **Figure 4** of 69-N load. No silver was detected in **Figure 6** which indicates that the silver-plating was depleted in the early stage of wear and was not related to the wear transition. The intensity of oxygen (O1) was almost the same for two specimens in **Figure 6**. This suggests that surface products of oxide were also not related to the wear transition.

**Figure 7** shows the relations between the loads and the oscillation numbers of mild-to-severe wear transition points. It looks like the S-N curve for metal fatigue, indicating the possibility of fatigue as the cause of the wear transition. However,

**261**

**Figure 5.**

**Figure 6.**

**Figure 7.**

cylindrical sliding pairs.

of the wear transition.

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics…*

*Optical microscope images of wears of lower specimens: (a) B and (b) D in Figure 4 [1].*

*Auger electron energy intensity in wear scars of lower specimen for 69-N load [1].*

when pure sliding tests of 98-N load were carried out using the same upper specimens and the flat plates, the result is shown in **Figure 3**, no wear transition was observed. The Hertzian contact pressure calculated without considering the silver plate for the flat plate was 529 MPa, which was larger than that for cylindrical lower specimen of 69-N load: 504 MPa. The tests were conducted twice and almost the same results were obtained. This suggests that the fatigue was not the case for

*Relations between loads and oscillation numbers of mild to severe wear transition points [1].*

**Figure 8** indicates the temperature trend of the upper specimen under 69-N load. The temperature was measured with a thermocouple 1 mm above the contact point. The temperature was almost constant at around 25°C and was not the cause

*DOI: http://dx.doi.org/10.5772/intechopen.93789*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics… DOI: http://dx.doi.org/10.5772/intechopen.93789*

#### **Figure 5.**

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

The wear depth gradually increased and then suddenly and rapidly increased at the oscillation numbers of sudden increase of horizontal forces (points G and D).

*(a) Trends of horizontal forces of the 69-N (empty symbols) and the 98-N loads (filled symbols) and (b) wear* 

**Figure 5** shows the microscope images of the wear scars of the lower specimens of B and D in **Figure 4**. From **Figure 5(b)**, the wear scar was rough and indicated the traces of hard adhesion, whereas that appeared relatively smooth in **Figure 5(a)**. From the wear depth progress in **Figure 4(b)** and wear scar appearances in **Figure 5**, the wear can be referred to as mild wear and severe wear before and after the sudden increase of the horizontal force, respectively. Thus, mild-to-severe wear transition

Traces of silver-plating were not observed in **Figure 5**. **Figure 6** indicates the Auger electron energy intensity measured in the wear scars of the lower specimen of the tests stopped before (15,600 oscillations) and after (18,600 oscillations) the point C in **Figure 4** of 69-N load. No silver was detected in **Figure 6** which indicates that the silver-plating was depleted in the early stage of wear and was not related to the wear transition. The intensity of oxygen (O1) was almost the same for two specimens in **Figure 6**. This suggests that surface products of oxide were also not

**Figure 7** shows the relations between the loads and the oscillation numbers of mild-to-severe wear transition points. It looks like the S-N curve for metal fatigue, indicating the possibility of fatigue as the cause of the wear transition. However,

**260**

**Figure 3.**

**Figure 4.**

*Time evolution of horizontal forces for the loads tested [1].*

occurred above a certain load.

*depth of lower specimen wear scar [1].*

related to the wear transition.

*Optical microscope images of wears of lower specimens: (a) B and (b) D in Figure 4 [1].*

#### **Figure 6.** *Auger electron energy intensity in wear scars of lower specimen for 69-N load [1].*

#### **Figure 7.**

*Relations between loads and oscillation numbers of mild to severe wear transition points [1].*

when pure sliding tests of 98-N load were carried out using the same upper specimens and the flat plates, the result is shown in **Figure 3**, no wear transition was observed. The Hertzian contact pressure calculated without considering the silver plate for the flat plate was 529 MPa, which was larger than that for cylindrical lower specimen of 69-N load: 504 MPa. The tests were conducted twice and almost the same results were obtained. This suggests that the fatigue was not the case for cylindrical sliding pairs.

**Figure 8** indicates the temperature trend of the upper specimen under 69-N load. The temperature was measured with a thermocouple 1 mm above the contact point. The temperature was almost constant at around 25°C and was not the cause of the wear transition.

#### **2.3 Discussion**

The experimental results above indicate that the mild-to-severe wear transition was not due to the lubricant depletion, fatigue, or temperature rise. **Figure 9** shows the wear shapes of the upper and the lower specimens of 69-N load and 18,600 oscillations, measured along the circumferential direction at the center of the wear scar. The trapezoidal wear shape was generated by wear from the original circumferential shape. This peculiar wear shape was probably due to the rolling-sliding motion of the specimens, which made a contact shape inconformity and generated a large contact pressure.

To investigate the generation process of the peculiar wear shape and its effects on the contact pressure, wear simulations were conducted. Calculation model is shown in **Figure 10**. The contact pressure was approximated by the Hertzian contact pressure: P(x). The contact area was divided into discrete slices of Δx width and the upper specimen moved byΔx in one calculation step. The wear depth was calculated by KP(x)Δx, where K was a virtual specific wear rate, and the new wear shape was obtained by smoothing the calculated discrete wear shape. The detail of the simulation was described in Refs. [1, 15–18].

The results of the simulations are shown in **Figure 11**. Calculation conditions are listed in **Table 3**. Materials for both specimens were supposed to be Cu with Young's modulus of 100 GPa and Poisson's ratio of 0.3. As shown in **Figure 11(a)**, the calculated wear shape reproduced the experimental result of the trapezoidal wear shape showing that the simulation can fairly simulate the wear phenomenon.

**Figure 8.** *Temperature trend of upper specimen under 69-N load [1].*

#### **Figure 9.**

*Wear shapes of upper and lower specimens of 69-N load and 18,600 oscillations: (a) lower specimen and (b) upper specimen [1].*

**263**

**Figure 11.**

*to the oscillation numbers.*

**Figure 10.**

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics…*

*Calculation model for wear simulation: (a) the whole of the model and (b) schematic of the contact area [1].*

*Results of wear simulation for 69-N and 39-N loads: (a) specimen's shape change, (b) wear depth, (c) contact pressure on the lower specimen, and (d) time trends of the contact pressures (Pmax: maximum contact pressure, Pmean: mean contact pressure, for one oscillation) [1]. The length and the contact pressure were normalized by the upper specimen radius and hardness of the upper specimen material. Numbers correspond* 

*DOI: http://dx.doi.org/10.5772/intechopen.93789*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics… DOI: http://dx.doi.org/10.5772/intechopen.93789*

#### **Figure 10.**

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

The experimental results above indicate that the mild-to-severe wear transition was not due to the lubricant depletion, fatigue, or temperature rise. **Figure 9** shows the wear shapes of the upper and the lower specimens of 69-N load and 18,600 oscillations, measured along the circumferential direction at the center of the wear scar. The trapezoidal wear shape was generated by wear from the original circumferential shape. This peculiar wear shape was probably due to the rolling-sliding motion of the specimens, which made a contact shape inconformity and generated a

To investigate the generation process of the peculiar wear shape and its effects on the contact pressure, wear simulations were conducted. Calculation model is shown in **Figure 10**. The contact pressure was approximated by the Hertzian contact pressure: P(x). The contact area was divided into discrete slices of Δx width and the upper specimen moved byΔx in one calculation step. The wear depth was calculated by KP(x)Δx, where K was a virtual specific wear rate, and the new wear shape was obtained by smoothing the calculated discrete wear shape. The detail of

The results of the simulations are shown in **Figure 11**. Calculation conditions are listed in **Table 3**. Materials for both specimens were supposed to be Cu with Young's modulus of 100 GPa and Poisson's ratio of 0.3. As shown in **Figure 11(a)**, the calculated wear shape reproduced the experimental result of the trapezoidal wear shape showing that the simulation can fairly simulate the wear phenomenon.

*Wear shapes of upper and lower specimens of 69-N load and 18,600 oscillations: (a) lower specimen and* 

**262**

**Figure 9.**

*(b) upper specimen [1].*

**Figure 8.**

**2.3 Discussion**

large contact pressure.

*Temperature trend of upper specimen under 69-N load [1].*

the simulation was described in Refs. [1, 15–18].

*Calculation model for wear simulation: (a) the whole of the model and (b) schematic of the contact area [1].*

#### **Figure 11.**

*Results of wear simulation for 69-N and 39-N loads: (a) specimen's shape change, (b) wear depth, (c) contact pressure on the lower specimen, and (d) time trends of the contact pressures (Pmax: maximum contact pressure, Pmean: mean contact pressure, for one oscillation) [1]. The length and the contact pressure were normalized by the upper specimen radius and hardness of the upper specimen material. Numbers correspond to the oscillation numbers.*

Note that the numbers shown in the figures correspond to oscillation numbers in the calculation, not the actual ones.

**Figure 11(c)** shows the calculated largest dimensionless contact pressure for each position on the lower specimen in one oscillation of the upper specimen. The contact pressure was normalized by the specimen material hardness, which means the dimensionless contact pressure indicated 1 when the contact pressure reached the specimen material hardness. The contact pressure was "M"-shaped, and as a result, the wear depth was "W"-shaped as shown in **Figure 11(b)**. The peak contact pressure of "M"-shape increased with the number of oscillations. **Figure 11(d)** shows the time trend of the maximum and mean contact pressure. The maximum dimensionless contact pressure of 69-N load exceeded the value 1 as the oscillation number increased, while that of 39-N load never did. The mean contact pressure of both loads gradually decreased.

Since the excess of the contact pressure over the material hardness or a measure of material strength could cause the wear transition [19], the wear transition of 69-N load occurred when the maximum dimensionless contact pressure exceeded 1.

**Figure 12** shows the calculated dimensionless equivalent contact radius, normalized by the upper specimen radius, at each position on the upper specimen in one stroke. The contact point moved from the right to the left in the figure as the upper specimen moved to the right. The contact radius of the rightmost and leftmost parts of the upper specimen decreased as the number of the strokes increased, indicating


**Table 3.**

*Calculation conditions [2].*

**265**

**Figure 13.**

*contact pressure (symbols correspond to Figure 11) [1].*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics…*

*Simulation results for a pure sliding pair of cylindrical upper specimen and flat plate lower specimen of 98-N load: (a) specimen's shape change, (b) contact pressure on the lower specimen, and (c) time trends of the* 

*DOI: http://dx.doi.org/10.5772/intechopen.93789*

#### **Figure 12.**

*Calculated dimensionless equivalent contact radius [1].*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics… DOI: http://dx.doi.org/10.5772/intechopen.93789*

#### **Figure 13.**

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

the calculation, not the actual ones.

both loads gradually decreased.

Note that the numbers shown in the figures correspond to oscillation numbers in

**Figure 11(c)** shows the calculated largest dimensionless contact pressure for each position on the lower specimen in one oscillation of the upper specimen. The contact pressure was normalized by the specimen material hardness, which means the dimensionless contact pressure indicated 1 when the contact pressure reached the specimen material hardness. The contact pressure was "M"-shaped, and as a result, the wear depth was "W"-shaped as shown in **Figure 11(b)**. The peak contact pressure of "M"-shape increased with the number of oscillations. **Figure 11(d)** shows the time trend of the maximum and mean contact pressure. The maximum dimensionless contact pressure of 69-N load exceeded the value 1 as the oscillation number increased, while that of 39-N load never did. The mean contact pressure of

Since the excess of the contact pressure over the material hardness or a measure of material strength could cause the wear transition [19], the wear transition of 69-N load occurred when the maximum dimensionless contact pressure exceeded 1. **Figure 12** shows the calculated dimensionless equivalent contact radius, normalized by the upper specimen radius, at each position on the upper specimen in one stroke. The contact point moved from the right to the left in the figure as the upper specimen moved to the right. The contact radius of the rightmost and leftmost parts of the upper specimen decreased as the number of the strokes increased, indicating

Specimen radius Upper specimen 3 mm

Specific wear rate 10−1 mm3

Line load 34.5 N/mm Stroke 10 mm *Δx* 0.02 mm Number of points in moving average 17

Lower specimen 10 mm

/Nm

**264**

**Figure 12.**

*Calculated dimensionless equivalent contact radius [1].*

**Table 3.**

*Calculation conditions [2].*

*Simulation results for a pure sliding pair of cylindrical upper specimen and flat plate lower specimen of 98-N load: (a) specimen's shape change, (b) contact pressure on the lower specimen, and (c) time trends of the contact pressure (symbols correspond to Figure 11) [1].*

**Figure 14.**

*Motion and contact pressures for arc-concave lower specimen: (a) motion, (b) contact pressure (with symbols corresponding to Figure 11); concave radius: 10 mm (other conditions were the same as listed in Table 1.) [2].*

significantly smaller value than 1. This suggests that large contact pressure was generated by increasing inconformity of the contact surface shape.

As described in Section 2.1, the rolling-sliding motion of the sliding pairs in these experiments was the opposite of that often seen on machines, that is, the direction of the rolling and that of the sliding were opposite. In this rolling-sliding motion, a part that has once worn due to sliding may be returned to the contact portion again during one stroke due to the rolling motion. Perhaps this feature causes the inconformity of wear shapes leading to wear transition.

What about other shaped specimens? **Figure 13** shows the simulation results for a pure sliding pair of cylindrical upper specimen and flat plate lower specimen of 98-N load [1]. Dimensionless contact pressure exceeded 1, but its position was the end of the stroke. Therefore, the large contact pressure would have little effect for wear and caused no wear transition as shown in **Figure 2**.

**Figure 14** indicates the simulation results for the cylindrical upper specimen and the arc-concave lower specimen of 34.5 N/mm line load (corresponding to 69-N load in **Figure 2**) [2]. As the direction of the rolling and that of the sliding was the same in the rolling-sliding motion for this sliding pair, the recontact of worn portion did not occur leading to lower contact pressure.

### **3. Characteristics of wear track profiles**

#### **3.1 Experimental**

Pin-on-disk wear tests were carried out for some materials and under several conditions to obtain wear tracks to be compared and analyzed [11, 12]. The lower

**267**

**Figure 15.**

*C3604, 54 N, 20-h sliding [11].*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics…*

part of the vertical set disk was immersed in mineral oil and the contact point with the pin was wetted along with the rotation of the disk. The pin had a flat surface of 8 mm diameter. The pin and the disk surface had a roughness of Ra ~0.3 micrometer.

**Figure 15** shows the wear track photos for the stainless and the brass disks tested in wet condition. Both indicated the streaked wear track and were difficult to discern visually, despite the different materials and test conditions. The wear tracks of the other specimens also showed the similar appearances. **Figure 16** indicates the examples of the wear track profiles. The valleys with 200–500 micrometer width and 20–70 micrometer depth were prominent and these similar dimensions of the

To evaluate the geometric similarity between the wear tracks quantitatively, frequency analysis was applied to the wear track profiles. Since the characteristics of the profile shape curve seems incidental rather than periodic, discrete wavelet transform (DWT) was applied. Details of the analysis were described in Ref. [11]. **Figure 17** indicates the power spectral density (PSD) of DWT with regard to the wavelength of the roughness. Although there are some deviations, the PSD curves in the figure had almost the same shapes, that is, they had similar slopes and bended at about the same point, showing the geometrical similarity of the profiles. The profile shape curve of the wavelength at the bending point determines the rms

*Photos of typical wear tracks on disks: (a) SUS304 versus S45C, 118-N load, 8-h sliding and (b) C3604 versus* 

**(m/s)**

**Load (N)**

54, 78, 118

Wet 0.02 118 1.68, 2, 8, 40,

Dry 54, 78 2

**Sliding duration (h)**

80

20

118 1, 2, 4, 6

**Pin material Disk material Oil Sliding velocity** 

JIS SUS304 stainless steel

> JIS C3604 brass

JIS SUS304 stainless steel

JIS SUS304 stainless steel

*DOI: http://dx.doi.org/10.5772/intechopen.93789*

Test conditions and materials are shown in **Table 4**.

valleys may bring the visual similarity of the wear tracks.

**3.2 Characterization of wear track profiles**

a JIS S45C steel

b JIS C3604 brass

c JIS S45C steel

**Table 4.**

JIS S45C steel

*Test conditions and materials [11].*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics… DOI: http://dx.doi.org/10.5772/intechopen.93789*

part of the vertical set disk was immersed in mineral oil and the contact point with the pin was wetted along with the rotation of the disk. The pin had a flat surface of 8 mm diameter. The pin and the disk surface had a roughness of Ra ~0.3 micrometer. Test conditions and materials are shown in **Table 4**.

**Figure 15** shows the wear track photos for the stainless and the brass disks tested in wet condition. Both indicated the streaked wear track and were difficult to discern visually, despite the different materials and test conditions. The wear tracks of the other specimens also showed the similar appearances. **Figure 16** indicates the examples of the wear track profiles. The valleys with 200–500 micrometer width and 20–70 micrometer depth were prominent and these similar dimensions of the valleys may bring the visual similarity of the wear tracks.

#### **3.2 Characterization of wear track profiles**

To evaluate the geometric similarity between the wear tracks quantitatively, frequency analysis was applied to the wear track profiles. Since the characteristics of the profile shape curve seems incidental rather than periodic, discrete wavelet transform (DWT) was applied. Details of the analysis were described in Ref. [11].

**Figure 17** indicates the power spectral density (PSD) of DWT with regard to the wavelength of the roughness. Although there are some deviations, the PSD curves in the figure had almost the same shapes, that is, they had similar slopes and bended at about the same point, showing the geometrical similarity of the profiles. The profile shape curve of the wavelength at the bending point determines the rms


#### **Table 4.**

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

significantly smaller value than 1. This suggests that large contact pressure was

As described in Section 2.1, the rolling-sliding motion of the sliding pairs in these experiments was the opposite of that often seen on machines, that is, the direction of the rolling and that of the sliding were opposite. In this rolling-sliding motion, a part that has once worn due to sliding may be returned to the contact portion again during one stroke due to the rolling motion. Perhaps this feature causes

*Motion and contact pressures for arc-concave lower specimen: (a) motion, (b) contact pressure (with symbols corresponding to Figure 11); concave radius: 10 mm (other conditions were the same as listed in Table 1.) [2].*

What about other shaped specimens? **Figure 13** shows the simulation results for a pure sliding pair of cylindrical upper specimen and flat plate lower specimen of 98-N load [1]. Dimensionless contact pressure exceeded 1, but its position was the end of the stroke. Therefore, the large contact pressure would have little effect for

**Figure 14** indicates the simulation results for the cylindrical upper specimen and the arc-concave lower specimen of 34.5 N/mm line load (corresponding to 69-N load in **Figure 2**) [2]. As the direction of the rolling and that of the sliding was the same in the rolling-sliding motion for this sliding pair, the recontact of worn portion

Pin-on-disk wear tests were carried out for some materials and under several conditions to obtain wear tracks to be compared and analyzed [11, 12]. The lower

generated by increasing inconformity of the contact surface shape.

the inconformity of wear shapes leading to wear transition.

wear and caused no wear transition as shown in **Figure 2**.

did not occur leading to lower contact pressure.

**3. Characteristics of wear track profiles**

**266**

**3.1 Experimental**

**Figure 14.**

*Test conditions and materials [11].*

#### **Figure 15.**

*Photos of typical wear tracks on disks: (a) SUS304 versus S45C, 118-N load, 8-h sliding and (b) C3604 versus C3604, 54 N, 20-h sliding [11].*

**Figure 16.**

*Examples of combined wear track profiles for disks and pins: (a) S45C versus SUS304, 118-N load, 8 h, wet; (b) C3604 versus C3604, 54-N load, 20 h, wet; and (c) S45C versus SUS304, 118-N load, 4 h, dry [11].*

roughness [20] and was visually conspicuous because valleys (or mountains) of the roughness larger than this wavelength had comparatively small depth to their width and were recognized as waviness.

In order to indicate the bending points explicitly, PSD ratios: ln / {*P P Wn Wn* (λ λ ) ( <sup>−</sup><sup>1</sup> )} , where *PW n* (λ) denotes the PSD at *n*th wavelength in **Figure 17**,

**269**

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics…*

were calculated and shown in **Figure 18**. The PSD ratios showed sharp drops at the wavelength of 0.2–0.5 mm indicating the bending points. The PSD values at these

*Power spectral density (PSD) of DWT: (a) S45C versus SUS304, wet; (b) C3604 versus C3604, wet; and (c) S45C versus SUS304, dry, (*−*1) disks, (*−*2) pins [11]. Test conditions in (a) ~ (c) correspond to those of a ~ c in* 

valley depth of 30–80 micrometers. These results supported the above observations

Pin-on-disk tests were used to capture the moment of the streak formation. The tester used was as the same as above. JIS S45C steel pins and JIS SUS304 stainless steel disks with surface roughness of Ra ~ 0.3 micrometer are used under 118-N load and wet conditions. The pin-disk assembly is shown in **Figure 19(a)**.

The disk rotation started slowly and then maintained a rotational speed of 12–13 rpm (sliding speed of ~0.03 m/s) to observe the wear track generation process. No signs of wear or damage were observed on the disk on the first few rotations, after which a groove appeared suddenly. As soon as the groove was found, the disk rotation was stopped. Other tests continued rotating after the first groove appeared. In each test, initially only one groove appeared, and after several rotations, the second groove appeared adjacent to the first groove, and so on, and the streak pattern was generated (sometimes, new groove generated at the other place

**Figure 19(b)** and **(c)** shows the SEM images of the pin surfaces after testing. Transfer particles [21, 22] of about the same size were found on the pin surfaces that matched the number of grooves on the disks. Obviously, these transfer particles plowed the disk surface and generated the grooves of nearly the same size. The

of valleys of the roughness that characterized the wear tracks.

**3.3 Mechanism of generating wear track profiles**

•mm that were approximately equivalent to the

*DOI: http://dx.doi.org/10.5772/intechopen.93789*

wavelength were 10−5–10−4 mm2

**Figure 17.**

*Table 4.*

to become another core of the streak).

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics… DOI: http://dx.doi.org/10.5772/intechopen.93789*

**Figure 17.**

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

roughness [20] and was visually conspicuous because valleys (or mountains) of the roughness larger than this wavelength had comparatively small depth to their width

*Examples of combined wear track profiles for disks and pins: (a) S45C versus SUS304, 118-N load, 8 h, wet; (b) C3604 versus C3604, 54-N load, 20 h, wet; and (c) S45C versus SUS304, 118-N load, 4 h, dry [11].*

) denotes the PSD at *n*th wavelength in **Figure 17**,

In order to indicate the bending points explicitly, PSD ratios:

λ

**268**

**Figure 16.**

and were recognized as waviness.

 λ

) ( <sup>−</sup><sup>1</sup> )} , where *PW n* (

ln / {*P P Wn Wn* (λ

*Power spectral density (PSD) of DWT: (a) S45C versus SUS304, wet; (b) C3604 versus C3604, wet; and (c) S45C versus SUS304, dry, (*−*1) disks, (*−*2) pins [11]. Test conditions in (a) ~ (c) correspond to those of a ~ c in Table 4.*

were calculated and shown in **Figure 18**. The PSD ratios showed sharp drops at the wavelength of 0.2–0.5 mm indicating the bending points. The PSD values at these wavelength were 10−5–10−4 mm2 •mm that were approximately equivalent to the valley depth of 30–80 micrometers. These results supported the above observations of valleys of the roughness that characterized the wear tracks.

#### **3.3 Mechanism of generating wear track profiles**

Pin-on-disk tests were used to capture the moment of the streak formation. The tester used was as the same as above. JIS S45C steel pins and JIS SUS304 stainless steel disks with surface roughness of Ra ~ 0.3 micrometer are used under 118-N load and wet conditions. The pin-disk assembly is shown in **Figure 19(a)**.

The disk rotation started slowly and then maintained a rotational speed of 12–13 rpm (sliding speed of ~0.03 m/s) to observe the wear track generation process. No signs of wear or damage were observed on the disk on the first few rotations, after which a groove appeared suddenly. As soon as the groove was found, the disk rotation was stopped. Other tests continued rotating after the first groove appeared. In each test, initially only one groove appeared, and after several rotations, the second groove appeared adjacent to the first groove, and so on, and the streak pattern was generated (sometimes, new groove generated at the other place to become another core of the streak).

**Figure 19(b)** and **(c)** shows the SEM images of the pin surfaces after testing. Transfer particles [21, 22] of about the same size were found on the pin surfaces that matched the number of grooves on the disks. Obviously, these transfer particles plowed the disk surface and generated the grooves of nearly the same size. The

**Figure 18.**

*PSD ratios [11]. Notations (a-1)–(c-2) correspond to those in Figure 17.*

transfer particles included Cr, Ni and Fe according to the energy-dispersive X-ray spectroscopy analysis, suggesting that they originated in a mixture of both the pin and the disk materials and grew to and stopped growing at that size in that place.

**Figure 20** shows the surface profiles of the tested pin and the disk. The measuring directions on the pin surfaces were shown in **Figure 19**. As seen in the SEM images, the sizes of transfer particles and naturally those of the grooves were similar and were in the range of the characteristic sizes above. This caused the similar appearances of the wear tracks.

Why were the transfer particles (and their resulting grooves) about the same size? From **Figure 20**, all grooves on the disks were found to have the ridges along their sides that dug into the pin surfaces. These ridges were also found for brass material [11]. These ridges digging into the mating surfaces enclosed the transfer particles and could be assumed to terminate their growth. Also, these ridges digging into the mating disk surface seems to generate the "counter" ridges on the disk and the counter ridges grew to dig into the pin surface generating a new seed of the transfer particle, as shown in **Figure 20(b)**.

#### **3.4 Ridge formation conditions**

We assumed that the ridges along the sides of the grooves have a key role to determine the characteristic scales of the wear track streaks and conducted the

**271**

on their sides.

**Figure 19.**

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics…*

ridge formation tests. **Figure 21** shows the schematic of the test rig. Two metal plates sandwiched three hard balls and were pressed. Then one plate was rotated at an angle of 90°generating three grooves on the plate. The ball materials and the test conditions are listed in **Table 5**. Plate materials were JIS SUS304 stainless steel and JIS C3604 brass. Some tests were under oil-wetted conditions. The balls were fixed

*(b-1) and (b-2) one and 2 transfer particles adhering to pin surfaces, respectively [12].*

*Perspective of pin-disk assembly and SEM images of pin surfaces after the tests: (a) pin-disk assembly and* 

**Figure 22** shows the examples of groove profiles generated in the tests. You can see the groove with and without the ridge in the figure. **Figure 23** illustrates the groove profile. We designate 2d/λ1 in **Figure 23** as the "degree of penetration or Dp." The degree of penetration was originally defined as y/a in **Figure 24** for the wear map [23] and is approximately equivalent to 2d/λ1 in **Figure 23**. **Figure 25** shows the relations between Dp and h/d. In **Figure 25**, h/d is 0, that is, no ridge, for small Dp. h/d increased sharply when Dp exceeded about 0.05–0.1 regardless of the materials. This means that the valleys with higher aspect ratio (larger Dp) had higher ridges

From the slopes of PSD curves in **Figure 17**, PSD values were proportional to about the third power of wavelength. This relation is replaced with the relation of (depth)∝(width)1.5 in the valley profiles. Suppose the grooves grew during sliding maintaining this relation and "depth" and "width" were equivalent to d + h and λ2, the relation of d + h = 50 × (λ2/400)1.5 holds, when the typical value of d + h = 50 micrometer andλ2 = 400 micrometer were used. The solid curve in **Figure 26** indicates this relation. The groove can be considered to grow along this curve.

to one plate to prevent rolling in some cases.

*DOI: http://dx.doi.org/10.5772/intechopen.93789*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics… DOI: http://dx.doi.org/10.5772/intechopen.93789*

#### **Figure 19.**

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

transfer particles included Cr, Ni and Fe according to the energy-dispersive X-ray spectroscopy analysis, suggesting that they originated in a mixture of both the pin and the disk materials and grew to and stopped growing at that size in that place. **Figure 20** shows the surface profiles of the tested pin and the disk. The measuring directions on the pin surfaces were shown in **Figure 19**. As seen in the SEM images, the sizes of transfer particles and naturally those of the grooves were similar and were in the range of the characteristic sizes above. This caused the similar appear-

*PSD ratios [11]. Notations (a-1)–(c-2) correspond to those in Figure 17.*

Why were the transfer particles (and their resulting grooves) about the same size? From **Figure 20**, all grooves on the disks were found to have the ridges along their sides that dug into the pin surfaces. These ridges were also found for brass material [11]. These ridges digging into the mating surfaces enclosed the transfer particles and could be assumed to terminate their growth. Also, these ridges digging into the mating disk surface seems to generate the "counter" ridges on the disk and the counter ridges grew to dig into the pin surface generating a new seed of the

We assumed that the ridges along the sides of the grooves have a key role to determine the characteristic scales of the wear track streaks and conducted the

**270**

ances of the wear tracks.

**Figure 18.**

transfer particle, as shown in **Figure 20(b)**.

**3.4 Ridge formation conditions**

*Perspective of pin-disk assembly and SEM images of pin surfaces after the tests: (a) pin-disk assembly and (b-1) and (b-2) one and 2 transfer particles adhering to pin surfaces, respectively [12].*

ridge formation tests. **Figure 21** shows the schematic of the test rig. Two metal plates sandwiched three hard balls and were pressed. Then one plate was rotated at an angle of 90°generating three grooves on the plate. The ball materials and the test conditions are listed in **Table 5**. Plate materials were JIS SUS304 stainless steel and JIS C3604 brass. Some tests were under oil-wetted conditions. The balls were fixed to one plate to prevent rolling in some cases.

**Figure 22** shows the examples of groove profiles generated in the tests. You can see the groove with and without the ridge in the figure. **Figure 23** illustrates the groove profile. We designate 2d/λ1 in **Figure 23** as the "degree of penetration or Dp." The degree of penetration was originally defined as y/a in **Figure 24** for the wear map [23] and is approximately equivalent to 2d/λ1 in **Figure 23**. **Figure 25** shows the relations between Dp and h/d. In **Figure 25**, h/d is 0, that is, no ridge, for small Dp. h/d increased sharply when Dp exceeded about 0.05–0.1 regardless of the materials. This means that the valleys with higher aspect ratio (larger Dp) had higher ridges on their sides.

From the slopes of PSD curves in **Figure 17**, PSD values were proportional to about the third power of wavelength. This relation is replaced with the relation of (depth)∝(width)1.5 in the valley profiles. Suppose the grooves grew during sliding maintaining this relation and "depth" and "width" were equivalent to d + h and λ2, the relation of d + h = 50 × (λ2/400)1.5 holds, when the typical value of d + h = 50 micrometer andλ2 = 400 micrometer were used. The solid curve in **Figure 26** indicates this relation. The groove can be considered to grow along this curve.

#### **Figure 20.**

*Surface profiles of pins and disks [12]. Measuring directions of (a) and (b) correspond to those in Figure 1 (b-1) and (b-2), respectively.*

If the ridges began to form at Dp = 0.1, then d = 0.05λ1. The dashed line in **Figure 26** indicates this relation. The intersecting point of the two lines indicates the time when the ridge began to form, the groove width at that time could be read as about 70 micrometers. The time when the ridge grew and dug into the mating surface was after this. Probably the groove width grew to a few 100 micrometers at that time, which was the characteristic scale of the grooves in the streaked wear track. We, therefore, consider this mechanism causes the wear track appearance similarity.

### **4. Conclusion**

In this chapter, we have described two characteristic wear phenomena that are usually less noticeable, but we believe they characterize some aspects of wear and can serve as a background for examining various wear phenomena.

Mild-to-severe wear transition could occur for convex-shaped sliding pair due to their rolling-sliding motion which generates peculiar wear shapes leading to large contact pressure. Streaked wear track were shown to have a similar profile consisting of grooves with 200–500 micrometer width and 20–70 micrometer regardless of friction materials and conditions. This is because the transfer particles that plow the surface and produce grooves grow with sliding, but stop at about the same size, regardless of friction material and conditions.

**273**

**Table 5.**

*Ball materials and test conditions [12].*

**Figure 21.**

*trajectories [12].*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics…*

Though we have not actually verified, we can consider the methods to prevent these wear damages. One of the methods to prevent the wear-induced mild-to-severe wear transition is, for example, to reduce the curvature of the surface to reduce the rolling-sliding motion and the contact pressure. One method to prevent the wear track generation is to prevent the transfer particle generation by effective lubrication

*Figure 3 Schematic of ridge generating rig: (a) side view, (b) top view, and (c) generated groove* 

ZrO2 0.1 15

**Ball material Ball diameter (mm) Load for 3 balls (N)**

JIS SUS440C stainless steel 0.3 4, 15, 60, 100 JIS SUJ2 bearing steel 0.3 15, 60, 100

0.2 8, 30, 50 0.5 100 1 200

0.5 60, 200, 300, 500 1 20, 60, 200 1.5 500 2 500

*DOI: http://dx.doi.org/10.5772/intechopen.93789*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics… DOI: http://dx.doi.org/10.5772/intechopen.93789*

#### **Figure 21.**

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

If the ridges began to form at Dp = 0.1, then d = 0.05λ1. The dashed line in **Figure 26** indicates this relation. The intersecting point of the two lines indicates the time when the ridge began to form, the groove width at that time could be read as about 70 micrometers. The time when the ridge grew and dug into the mating surface was after this. Probably the groove width grew to a few 100 micrometers at that time, which was the characteristic scale of the grooves in the streaked wear track. We, therefore, con-

*Surface profiles of pins and disks [12]. Measuring directions of (a) and (b) correspond to those in Figure 1*

In this chapter, we have described two characteristic wear phenomena that are usually less noticeable, but we believe they characterize some aspects of wear and

Mild-to-severe wear transition could occur for convex-shaped sliding pair due to their rolling-sliding motion which generates peculiar wear shapes leading to large contact pressure. Streaked wear track were shown to have a similar profile consisting of grooves with 200–500 micrometer width and 20–70 micrometer regardless of friction materials and conditions. This is because the transfer particles that plow the surface and produce grooves grow with sliding, but stop at about the same size, regardless of

sider this mechanism causes the wear track appearance similarity.

can serve as a background for examining various wear phenomena.

**272**

**4. Conclusion**

**Figure 20.**

*(b-1) and (b-2), respectively.*

friction material and conditions.

*Figure 3 Schematic of ridge generating rig: (a) side view, (b) top view, and (c) generated groove trajectories [12].*


#### **Table 5.**

*Ball materials and test conditions [12].*

Though we have not actually verified, we can consider the methods to prevent these wear damages. One of the methods to prevent the wear-induced mild-to-severe wear transition is, for example, to reduce the curvature of the surface to reduce the rolling-sliding motion and the contact pressure. One method to prevent the wear track generation is to prevent the transfer particle generation by effective lubrication

**Figure 22.**

*Examples of measured groove profiles generated with ((a)) and without ((b)) ridges: (a) 0.3 mm in dia. SUJ-2 ball, 5 N/3 balls and (b) 2 mm in dia. SUJ-2 ball, 500 N/3 balls [12].*

**275**

**Figure 26.**

*Relations between calculated width and depth of groove [12].*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics…*

*Relations between Dp and ridge height rate h/d: (a) SUS304 plate and (b) C3604 plate [12]. Numbers, "FIX,"* 

*"OIL" in boxes indicate ball diameter, using fixed ball, oil wetted conditions, respectively.*

*DOI: http://dx.doi.org/10.5772/intechopen.93789*

**Figure 24.**

**Figure 25.**

*Degree of penetration: y/a [12].*

**Figure 23.** *Illustrated groove profile and notations of dimensions of groove and ridges [12].*

*Characteristic Aspects of Metal Wear: Wear-Induced Wear Transition and Characteristics… DOI: http://dx.doi.org/10.5772/intechopen.93789*

**Figure 24.** *Degree of penetration: y/a [12].*

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

*Examples of measured groove profiles generated with ((a)) and without ((b)) ridges: (a) 0.3 mm in dia. SUJ-2* 

*ball, 5 N/3 balls and (b) 2 mm in dia. SUJ-2 ball, 500 N/3 balls [12].*

*Illustrated groove profile and notations of dimensions of groove and ridges [12].*

**274**

**Figure 23.**

**Figure 22.**

#### **Figure 25.**

*Relations between Dp and ridge height rate h/d: (a) SUS304 plate and (b) C3604 plate [12]. Numbers, "FIX," "OIL" in boxes indicate ball diameter, using fixed ball, oil wetted conditions, respectively.*

**Figure 26.** *Relations between calculated width and depth of groove [12].*
