Chapter 6 Tribological characterisation of mono-filled PTFE composites
6.3. Results and discussion
6.3.2. Transfer layer analysis and wear mechanism
6.3.2.5. Wear-induced crystallisation
The wear-induced crystallisation of the unfilled/filled PTFE materials was investigated by DSC (Protocol 1, Chapter 3.2.4). Table 5.5 introduces the DSC results of the unworn samples (Chapter 5.3.6); these samples were later tested against 42CrMo4 steel discs. Table 6.3 shows the DSC results of the polymer debris (42CrMo4 counterface).
Table 6.3. DSC results of the filled/unfilled PTFE debris (Phase 1, Chapter 3.3.6.1), applying Protocol 1 for the DSC analysis (Chapter 3.2.4). Here the enthalpy of fusion at the first heating cycle was evaluated between 300°C and 370°C.
Polymer debris (42CrMo4)
First heating Initial
temperature of melting (°C)
Melting peak temperature
(°C)
Enthalpy of
fusion (J/g) Degree of crystallinity (%)
PTFE 327.6 334.2 54.55 79.1
PTFE/graphene-0.25 327.3 332.9 53.21 77.3
PTFE/graphene-1 327.0 332.6 53.96 79.0
PTFE/graphene-4 322.6 327.8 55.87 84.3
PTFE/graphene-8 --- --- --- ---
PTFE/graphene-16 --- --- --- ---
PTFE/Al2O3-1 317.2 327.2 51.39 75.2
PTFE/Al2O3-4 --- --- --- ---
PTFE/BA80-1 326.1 333.5 52.95 77.5
PTFE/BA80-4 323.8 333.8 52.79 79.7
PTFE/BA80-8 321.9 326.4 49.65 78.2
PTFE/BA80-16 321.1 326.9 46.45 80.1
PTFE/MG70-1 326.0 334.0 47.96 70.2
PTFE/MG70-4 325.9 334.9 56.06 84.6
Second heating
PTFE 324.9 327.7 46.58 67.5
PTFE/graphene-0.25 325.1 327.9 48.64 70.7
PTFE/graphene-1 325.3 327.9 49.11 71.9
PTFE/graphene-4 324.7 327.2 52.89 79.8
PTFE/graphene-8 --- --- --- ---
PTFE/graphene-16 --- --- --- ---
PTFE/Al2O3-1 323.7 326.8 56.67 83.0
PTFE/Al2O3-4 --- --- --- ---
PTFE/BA80-1 324.9 327.6 45.80 67.0
PTFE/BA80-4 325.7 327.9 53.31 80.5
PTFE/BA80-8 324.8 326.7 51.10 80.5
PTFE/BA80-16 324.8 326.7 51.47 88.8
PTFE/MG70-1 323.1 327.3 42.59 62.3
PTFE/MG70-4 324.6 327.7 49.34 74.5
First cooling Initial
temperature of crystallisation
(°C)
Crystallisation peak temperature
(°C)
Enthalpy of crystallisation
(J/g)
Degree of crystallinity (%)
PTFE 314.8 311.2 44.81 64.9
PTFE/graphene-0.25 315.5 311.9 46.12 67.0
PTFE/graphene-1 316.0 313.3 49.95 73.1
PTFE/graphene-4 320.2 317.7 46.57 70.3
PTFE/graphene-8 --- --- --- ---
PTFE/graphene-16 --- --- --- ---
PTFE/Al2O3-1 314.2 312.0 53.25 78.0
PTFE/Al2O3-4 --- --- --- ---
PTFE/BA80-1 314.8 311.7 44.85 65.7
PTFE/BA80-4 315.8 313.5 51.73 78.1
PTFE/BA80-8 317.4 315.0 50.30 79.2
PTFE/BA80-16 317.7 316.1 47.89 82.6
PTFE/MG70-1 315.4 311.6 39.20 57.4
PTFE/MG70-4 316.7 313.0 49.27 74.4
The analysed unworn specimens and the wear debris come from the same polymer sample.
The DSC specimens of the unworn material were cut from the opposite (unworn) side of the tested sample. Table B.19 and B.20 refer to polymer samples tested against 304 and 34CrNiMo6 counterface material. The DSC analysis did not include the debris of PTFE/graphene-8, PTFE/graphene-16 and PTFE/Al2O3-4 polymers as these materials had low wear rate; in this way, the amount of the formed debris was not sufficient for DSC analysis. In case of 34CrNiMo6 steel counterface, PTFE/graphene-4 debris was neither analysed due to its low wear rate.
The initial temperature of melting and the melting peak temperature of debris were higher both in the first and second heating cycle compared to the unworn material (Table 5.5, 6.3, B.18, B.19 and Figure 6.16). The debris also had a higher initial temperature of crystallisation and crystallisation peak temperature compared to the unworn materials. It means that the crystallisation process is initiated sooner in case of the debris compared to the original unworn samples (Table 5.5, 6.3, B.18, B.19).
In the first heating cycle, the degree of crystallinity of the debris increased between 23% and 42% compared to the unworn samples (Table 6.4). This increasing can come from the different thermal and mechanical antecedents. The surface temperature during wear tests can be so high which can affect the morphological structure of PTFE materials. Furthermore, the applied pressure and the sliding motion can also align the molecular chains due to the high shear stress during the wear process.
The increase of the degree of crystallinity was also confirmed by the results of the second heating cycle where, after the first melt, all the analysed samples had the same thermal history.
In this way, the thermal history or the molecular chain aligning of the debris cannot be the reason for this significant increase of the degree of crystallinity. In the second heating cycle, the debris had 24-49% higher degree of crystallinity than the unworn samples (Table 6.4). The degree of crystallinity evaluated from the enthalpy of crystallisation also had an increase between 20% to 49% compared to the unworn samples (Table 6.4).
From these results, it can be concluded that this significant increase in the degree of crystallinity comes from the influence of high shear stress during the wear process. It is well known from the literature [1, 2, 5] that during wear process, the PTFE molecular chains undergo mechanical chain scission. This mechanical chain scission can cause a significant molecular length decrease in the formed debris. These shorter molecular chains of the debris can more efficiently reach an aligned arrangement than the longer chains of the unworn materials. In this way, the degree of crystallinity will be higher in both of the first and the second heating cycle.
(a)
(b) (c)
Figure 6.16. DSC results of the filled/unfilled PTFE materials and debris (Phase 1, Chapter 3.3.6.1), applying Protocol 1 for the DSC analysis (Chapter 3.2.4). Neat PTFE (a), PTFE with 1 wt% filler content (b) and PTFE with 4 wt% filler content (c), first heating cycle.
Table 6.4. DSC comparison of unworn polymer samples with their debris (Protocol 1, Chapter 3.2.4). Polymer samples were tested against 42CrMo4/304/34CrNiMo6 counterface material (Phase 1 and 2, Chapter 3.3.6.1 and 3.3.6.1).
Materials Counterface material
Degree of crystallinity – increase
= debris (%) – unworn (%)
First heating First cooling Second heating
(%) (%) (%)
PTFE 42CrMo4 25.7 19.7 24.1
PTFE/graphene-0.25 42CrMo4 30.1 24.1 30.4
PTFE/graphene-1 42CrMo4 27.8 29.0 28.0
PTFE/graphene-4 42CrMo4 34.4 29.6 38.2
PTFE/graphene-8 42CrMo4 --- --- ---
PTFE/graphene-16 42CrMo4 --- --- ---
PTFE/Al2O3-1 42CrMo4 23.3 36.6 41.6
PTFE/Al2O3-4 42CrMo4 --- --- ---
PTFE/BA80-1 42CrMo4 30.7 27.1 27.3
PTFE/BA80-4 42CrMo4 32.7 35.7 39.8
PTFE/BA80-8 42CrMo4 27.5 33.4 37.5
PTFE/BA80-16 42CrMo4 32.2 39.9 48.5
PTFE/MG70-1 42CrMo4 30.1 21.6 26.7
PTFE/MG70-4 42CrMo4 35.1 30.3 31.4
PTFE 304 37.1 29.1 28.1
PTFE 34CrNiMo6 36.6 24.9 29.2
PTFE/graphene-4 304 36.5 36.6 34.5
PTFE/graphene-4 34CrNiMo6 --- --- ---
PTFE/Al2O3-4 304 --- --- ---
PTFE/Al2O3-4 34CrNiMo6 --- --- ---
PTFE/BA80-4 304 41.3 48.8 49.1
PTFE/BA80-4 34CrNiMo6 35.4 41.2 44.7
Table 6.5 introduces the calculated molecular weight of the unworn unfilled/filled PTFE samples and their debris. The molecular weight and in this way, the length of the PTFE molecular chains in the debris decreased by 1-2 orders of magnitude compared to the unworn materials. From the results, it can be seen that filled PTFE had a more significant decrease in molecular weight than the reference PTFE. The increase of filler content further decreased the molecular weight due to the hard particles which damaged with a higher rate the molecular chains. The details about the molecular weight calculation method can be found in Chapter 3.2.4. In general, the filler content also influences the calculated degree of crystallinity but here the difference between the filler content of the debris and the unworn material is negligible compared to the significant increase of the degree of crystallinity (Chapter 6.3.2.6).
From the presented results, it can be concluded that the degree of crystallinity and the morphological structure of the tested materials changed during the wear process. It can influence the mechanical features as well. In case of a sliding bearing, as an example, after the first running period, the introduced characteristics of the PTFE top surface will change. It means that for the second running period the contact surface will have different morphological structure and mechanical features, which can influence the further tribological performance.
Table 6.5. The calculated molecular weight of unworn polymer samples compared to their debris. Polymer samples were tested against 42CrMo4/304/34CrNiMo6 counterface material (Phase 1 and 2, Chapter 3.3.6.1 and 3.3.6.2).
Materials Counterface material
First cooling Molecular
weight - unworn
Molecular weight - debris
Molecular weight
= debris/unworn
(g/mol) (g/mol) (%)
PTFE 42CrMo4 6.57E+06 1.02E+06 15.6
PTFE/graphene-0.25 42CrMo4 8.78E+06 8.84E+05 10.1
PTFE/graphene-1 42CrMo4 8.03E+06 5.90E+05 7.4
PTFE/graphene-4 42CrMo4 1.47E+07 8.74E+05 5.9
PTFE/graphene-8 42CrMo4 1.82E+07 --- ---
PTFE/graphene-16 42CrMo4 2.32E+07 --- ---
PTFE/Al2O3-1 42CrMo4 1.11E+07 4.24E+05 3.8
PTFE/Al2O3-4 42CrMo4 8.89E+06 --- ---
PTFE/BA80-1 42CrMo4 1.60E+07 1.03E+06 6.4
PTFE/BA80-4 42CrMo4 1.19E+07 5.08E+05 4.3
PTFE/BA80-8 42CrMo4 1.03E+07 6.13E+05 5.9
PTFE/BA80-16 42CrMo4 2.58E+07 8.65E+05 3.3
PTFE/MG70-1 42CrMo4 2.36E+07 2.06E+06 8.7
PTFE/MG70-4 42CrMo4 9.75E+06 6.53E+05 6.7
PTFE 304 1.27E+07 7.53E+05 5.9
PTFE 34CrNiMo6 8.86E+06 8.27E+05 9.3
PTFE/graphene-4 304 1.12E+07 4.56E+05 4.1
PTFE/graphene-4 34CrNiMo6 2.12E+07 --- ---
PTFE/Al2O3-4 304 1.12E+07 --- ---
PTFE/Al2O3-4 34CrNiMo6 1.18E+07 --- ---
PTFE/BA80-4 304 1.73E+07 2.70E+05 1.6
PTFE/BA80-4 34CrNiMo6 1.23E+07 3.64E+05 3.0