Filler accumulation in the tested polymers

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

Table 6.6. EDS analysis of unworn samples, contact surfaces and debris (3 MPa contact pressure and 0.1 m/s sliding speed).

Materials

Counterface material

Aluminium content Unworn

Aluminium content Worn surface

Aluminium content

Debris

Iron (Fe) content Worn surface

(%) (%) (%) (%)

PTFE/Al2O3-4 42CrMo4 2.76 5.02 0.49 9.63

PTFE/Al2O3-4 304 2.32 7.34 0.94 13.63

PTFE/Al2O3-4 34CrNiMo6 2.87 6.27 0.55 9.80

PTFE/BA80-4 42CrMo4 2.14 7.75 1.97 ~0

PTFE/BA80-4 304 2.04 8.37 2.56 ~0

PTFE/BA80-4 34CrNiMo6 1.93 7.77 1.89 ~0

Table 6.7. Aluminium content increase on the worn surfaces compared to the unworn materials (3 MPa contact pressure and 0.1 m/s sliding speed).

Materials Counterface

material Aluminium content

= worn/unworn (%) PTFE/Al2O3-4 42CrMo4 181.9

PTFE/Al2O3-4 304 316.4

PTFE/Al2O3-4 34CrNiMo6 218.5

PTFE/BA80-4 42CrMo4 362.1

PTFE/BA80-4 304 410.3

PTFE/BA80-4 34CrNiMo6 402.6

The EDS analysis of PTFE/Al2O3-4 transfer layer on the polymer sample can be seen in Figure 6.17, where the applied counterface steel was 42CrMo4. Interestingly, Fe content around ~10% was also observed on the polymer contact surface, which comes from the steel counterface in case of all three kinds of steels. Figure 6.17 (c) indicates that the lighter areas of Figure 6.17 (a) have significant Fe content. The reason for this Fe content can be that during wear the hard alumina fillers can damage and partly remove the oxide layer of the steel counterface. The measured Vickers hardness of 42CrMo4/34CrNiMo6/304 steel counterfaces were 308/363/227 (HV 10), respectively, while according to the literature the Vickers hardness of alumina is between 1400 and 1800 (HV 10) [6]. On the transfer layer of PTFE/Graphene-4, PTFE/BA80-4 and PTFE/MG70-4 polymer samples, no Fe content was registered due to the less hard filler particles and relatively high wear rate of these samples. In other words, after the wear test of PTFE/Al2O3-4 polymer, an extremely low amount of material was removed from the polymer sample. In this way, the contact layer of PTFE/Al2O3-4 polymer could collect more Fe without losing the Fe rich top layer during the wear mechanism of the polymer.

The more significant aluminium and iron content were also confirmed by FTIR spectroscopy (Figure 6.18 and 6.19). The new peaks from the iron content can only be seen in Figure 6.18 (PTFE/Al2O3-4). The peaks of aluminium bonds are more intensive in case of the transfer layer (red) than in the bulk material (blue). The hydroxyl (OH) bonds of PTFE/BA80-4 transfer layer are also more visible compared to the unworn material due to the filler accumulation.

(a) (b) (c)

Figure 6.17. EDS analysis of PTFE/Al2O3-4 transfer layer on the polymer surface (42CrMo4 steel counterface, 3 MPa contact pressure and 0.1 m/s sliding speed).

Figure 6.18. FTIR spectra of PTFE/Al2O3-4 bulk material (blue) and transfer layer (red). 304 steel counterface, 3 MPa contact pressure and 0.1 m/s sliding speed.

Figure 6.19. FTIR spectra of PTFE/BA80-4 bulk material (blue), transfer layer (red) and debris (olive). 304 steel counterface, 3 MPa contact pressure and 0.1 m/s sliding speed.

The filler content of worn surface of the polymer samples with 0.1/1/10/100/200/400 m sliding distance were analysed to collect more detailed information about the process of filler accumulation (Table 6.8 and 6.9). The filler content reached a significant increase even after 10 m sliding distance, and after this 10 m, there is only a slight increase in filler content. It means that almost the total registered accumulation occurs in the first 10 m. It indicates that a dynamic balance is reached between the detached and the eliminated wear debris, in other words, the recycled and the lost wear particles are in balance, which is in agreement with the solid third-body concept (Chapter 1.1) [7, 8].

Table 6.8. EDS analysis results of unworn samples and contact surfaces (3 MPa contact pressure and 0.1 m/s sliding speed).

Materials

Counterface material

Sliding distance

Aluminium content Unworn

Aluminium content Worn surface

Iron (Fe) content Worn surface

(m) (%) (%) (%)

PTFE/Al2O3-4 42CrMo4 0.1 2.54 3.28 0.85

PTFE/Al2O3-4 42CrMo4 1 3.06 4.23 2.59

PTFE/Al2O3-4 42CrMo4 10 3.15 9.06 4.01

PTFE/Al2O3-4 42CrMo4 100 2.91 7.89 3.85

PTFE/Al2O3-4 42CrMo4 200 3.45 10.45 5.26

PTFE/Al2O3-4 42CrMo4 400 3.40 11.31 5.71

PTFE/BA80-4 42CrMo4 0.1 2.01 2.53 ~0

PTFE/BA80-4 42CrMo4 1 2.10 2.64 ~0

PTFE/BA80-4 42CrMo4 10 1.90 6.34 ~0

PTFE/BA80-4 42CrMo4 100 1.89 7.74 ~0

PTFE/BA80-4 42CrMo4 200 2.22 7.81 ~0

PTFE/BA80-4 42CrMo4 400 1.97 7.40 ~0

Table 6.9. Aluminium content increase on the worn surfaces compared to the unworn materials (3 MPa contact pressure and 0.1 m/s sliding speed).

Materials

Counterface

material Sliding

distance Aluminium content

= worn/unworn

(m) (%)

PTFE/Al2O3-4 42CrMo4 0.1 129.1

PTFE/Al2O3-4 42CrMo4 1 138.2

PTFE/Al2O3-4 42CrMo4 10 287.6

PTFE/Al2O3-4 42CrMo4 100 271.1

PTFE/Al2O3-4 42CrMo4 200 302.9

PTFE/Al2O3-4 42CrMo4 400 332.6

PTFE/BA80-4 42CrMo4 0.1 125.9

PTFE/BA80-4 42CrMo4 1 125.7

PTFE/BA80-4 42CrMo4 10 333.7

PTFE/BA80-4 42CrMo4 100 409.5

PTFE/BA80-4 42CrMo4 200 351.8

PTFE/BA80-4 42CrMo4 400 375.6

Figure 6.20 displays the contact surfaces of PTFE/Al2O3-4 polymer samples, the applied counterface was 42CrMo4 steel.

(a) – unworn polymer (Al2O3) (b) – 0.1 m sliding distance

(c) – 1 m sliding distance (d) – 10 m sliding distance

(e) – 100 m sliding distance (f) – 200 m sliding distance

(g) – 400 m sliding distance (h) – 1000 m sliding distance Figure 6.20. Iron oxide accumulation on PTFE/Al2O3-4 polymer contact surface (42CrMo4

steel counterface, 3 MPa contact pressure and 0.1 m/s sliding speed).

The original, unworn polymer surface can be seen in Figure 6.20 (a) while the worn contact surfaces after 0.1/1/10/100/200/400/1000 m sliding distance are introduced in Figure 6.20 (b-h), respectively. In Figure 6.20 (b) it can be seen that even after 0.1 m sliding distance black spots appeared in the polymer contact surface which comes from the oxide layer of the steel counterface. In Figure 6.20 (b-d), an accumulation of the spots can be observed, which is in agreement with the results of EDS measurements (Table 6.8). After 10 m sliding distance (Figure 6.20 (d-h)) the black colour of iron oxide particles became orange.

This change in their colour shows that the removed iron oxide particles are modified during the wear process. The black colour indicates that the iron oxide particles include basically Fe3O4

(iron(II,III) oxide) molecules; in other words, the original stage is magnetite. The orange colour indicates high Fe2O3 (iron(III) oxide) content which means that the iron oxide goes through an oxidation process during wear. Focusing on Fe2O3, λ-Fe2O3 can be found in maghemite (brown colour), and α-Fe2O3 are in hematite (red colour). Equation (6.2)-(6.4) introduce the background of these stages:

4 Fe3O4 + O2 → 6 λ-Fe2O3 (200°C-400°C) (6.2)

λ-Fe2O3 → α-Fe2O3 (375°C -550°C) (6.3)

4 Fe3O4 + O2 → 6 α-Fe2O3 (from 550°C) (6.4)

These temperature values in Equation (6.2)-(6.4) indicate that the local contact temperature of the contact surfaces during the wear process reached a minimum of 375-400°C.

In case of PTFE/BA80-4 polymer samples, none of the contact surfaces included iron according to the EDS and optical measurements as well. It can have two different reasons, the first is that PTFE/BA80-4 sample had much higher wear rate compared to PTFE/Al2O3-4, in this way the top layer of the polymer with the iron content is removed. Here it is important to mention that even after 0.1/1/10 m sliding distance, no iron content was detected on the PTFE/BA80-4 polymer contact surfaces. In contrast with this, iron content accumulation was registered in the given sliding distances on the PTFE/Al2O3-4 contact surfaces. It means that even if a more significant material depth was removed from PTFE/BA80-4 samples, the worn contact surface of these polymers should have some iron content as the iron oxide is removed continuously from the counterface. In this way, the second potential explanation is that Al2O3

filler contains abrasive and hard particles. These particles can damage and remove the peaks of the steel counterface, resulting in an iron-oxide accumulation on the polymer contact surface.

Figure 6.21 and Table 6.10 shows the results of horizontal EDS mapping of PTFE/Al2O3-4 contact surface. These values indicate that the filler accumulation is independent from the sliding direction or from the position of the contact layer.

Table 6.10. EDS analysis of PTFE/Al2O3-4 contact surface, in 5 different position (42CrMo4 steel counterface, 3 MPa contact pressure and 0.1 m/s sliding speed).

Materials Location Aluminium content Worn surface

Iron (Fe) content Worn surface

(%) (%)

PTFE/Al2O3-4 Centre 11.02 4.13

PTFE/Al2O3-4 0° 11.14 3.88

PTFE/Al2O3-4 90° 11.03 3.63

PTFE/Al2O3-4 180° 10.90 4.06 PTFE/ Al2O3-4 270° 11.06 3.59

Figure 6.21. The measured five different positions of the transfer layer for EDS mapping.

Figure 6.22 indicates the aluminium content accumulation of PTFE/Al2O3-4 contact surface.

Figure 6.22 (a) introduce the unworn surface while Figure 6.22 (b) shows the contact layer of the polymer sample.

(a) (b)

Figure 6.22. EDS mapping of PTFE/Al2O3-4 polymer unworn sample (a) and contact layer (b). 1000 m sliding distance (42CrMo4 steel counterface, 3 MPa contact pressure and 0.1

m/s sliding speed).

A potential explanation for the alumina filler accumulation is that the softer PTFE particles can be torn easier from the contact surface than the hard metal oxide filler particles. In this way, the filler content of the contact surface is higher due to the still remaining fillers. Some of the torn or broken filler particles can also stick again as back transfer into the softer PTFE due to the high pressure and high temperature. This mechanism is illustrated in Figure 6.23. As less filler is removed from the contact surface of the polymer during wear, it is evident that the debris

(a) (b)

Figure 6.23. Illustration of wear mechanism and filler accumulation of the top surface.

As on the polymer surfaces of PTFE/Al2O3-4 samples iron-oxide layer was detected, it can be assumed that the hard alumina particles significantly damage the steel counterfaces. All kinds of steel surface damage have to be avoided as in general the polymer surface (e.g. bearings and seals) is the sacrificial part, the softer polymer is required to wear off instead of the steel counterfaces (e.g. shafts) as their replacement is more complex and their prices are much higher compared to the bearings or the seals. The steel counterfaces were analysed by white-light interferometry and no damages were detected on the contact surfaces. Some examples of PTFE/Al2O3-4 samples are introduced in Figure 6.24.

(a) (b)

(c) (d)

Figure 6.24. White light interferometer maps from the unworn (a), (c) and worn (b), (d) PTFE/Al2O3-4 steel counterfaces (42CrMo4), 3 MPa contact pressure, 0.1 m/s sliding speed,

1000 m sliding distance.

In document Development and Characterisation of Nanoparticle Filled Polytetrafluoroethylene for Tribological Applications Levente Ferenc Tóth (Pldal 183-191)