Improved Tribological Performance of a Polybutylene Terephthalate Hybrid Composite by Adding a Siloxane-Based Internal Lubricant

Abstract

To mitigate the environmental hazards aroused by fossil-based lubricants, the development of eco-friendly internal lubricants is imperative. Siloxane-based internal lubricants, widely applied as plasticizers in polymeric compounds, are a promising option. However, their impacts on the tribological properties of polymeric tribocomponents are still unclarified. Therefore, in the current study, a siloxane-based internal lubricant with the product name ‘EverGlide MB 1550 (EG)’ was dispersed into a polybutylene terephthalate (PBT)-based tribological composite to investigate whether the tribological properties of the composite can be optimized. A block-on-ring (BOR) test configuration was used for this purpose. It was found that the addition of EG to the composite significantly improved the tribological behavior; the improvement was particularly significant under lower load conditions (pv-product ≤ 2 MPa∙m/s). Compared to the reference PBT composite, the addition of EG reduced the friction coefficient (COF) by about 30% and the specific wear rate by about 14%. An accompanying surface analytical investigation using photoelectron spectroscopy to elucidate the effective mechanisms at the molecular level showed the availability of tribologically effective and free EG after its addition to the composite in the relevant tribocontact.

1. Introduction

With the thriving progress of aerospace, automotive, medical, and electronics technology, the demand for lightweight and durable wear-resistant materials is growing rapidly. Polymeric materials are lightweight, economical, and non-toxic, and they possess superior strength-to-weight ratios, corrosion resistance, and insulation, which are regarded as excellent alternatives to metallic materials [1,2,3,4,5,6]. In view of the lower processing temperature, as well as the flexible shape manufacturing of polymeric materials, polymeric materials had numerous advantages of production, such as lower energy consumption and costs than metallic materials, which has been widely favored in the industrial field [7,8]. Some polymeric materials demonstrate low friction noise, self-lubrication, and transfer film-forming mechanisms, which are considerable prospects to be adopted in the tribological industrial field, such as to gears, bearings, and seals [9,10]. However, the neat polymeric matrix generally presents inferior mechanical properties and load-bearing properties. The exhibition of their advantages on tribological performance relies heavily on the cooperation with reinforcing fillers [11]. For example, the fibers could be conferred superior tribological properties to make them suitable for more severe environments, which is regarded as one of the most important reinforcers for polymer materials [12,13].

As one of the most universally used engineering plastics, PBT offers excellent dimensional stability, desirable stiffness and strength, and superior heat aging behavior. It is widely utilized in industrial applications, such as in gears [14,15]. Nevertheless, PBT-based tribocomposites produced according to the traditional formulation still exhibit drawbacks of severe friction and intense wear, which cannot meet operation qualifications under some extreme/dangerous environments. Thus, to further improve the tribological performance and prolong the lifetime of the PBT-based tribological components, the proposal for an appropriate lubrication strategy is urged [16,17,18].

Traditional external lubricants utilized in metallic materials, i.e., mineral oil and mineral lubricating grease, can induce the swelling of plastic components, deteriorating their mechanical and tribological properties [19,20]. By means of this, only when choosing compatible lubricants can the performance and lifetime of plastic tribological components be effectively improved [21,22]. In actuality, internal lubricants, such as graphite-, Teflon-, and petroleum-based lubricants, have been extensively utilized in polymeric composites [23,24,25]. However, the production of these lubricants is heavily reliant on fossil fuels, and their application generates significant environmental pollution [26,27,28]. Therefore, it is imperative to explore eco-friendly alternatives to promote product innovation and industrial upgrading for traditional internal lubricants. Compared to petroleum products, siloxane internal lubricants do not contain harmful substances, such as heavy metals and polycyclic aromatic hydrocarbons. Except for elevating the tribological performance, the addition of siloxane has also proved to enhance the processability and dimensional stability of plastic tribological components, which possess intensive potential to be utilized as multifunctional additives in eco-friendly tribology components [29,30]. However, to date, there are few studies on the possibility of applying siloxane as a plasticizer/lubricant multifunctional additive, especially for PBT-based tribocomponents. The impacts of siloxane on the mechanical properties, tribological properties, and interface contribution are still unclarified. Additionally, the utilization of the combination between siloxane and the varying length scale fillers on the tribological performance of polymeric materials still exists in technical vacancy.

In this research, a siloxane-containing polymeric tribological composite was manufactured by applying a polybutylene terephthalate (PBT) matrix. The impacts of the siloxane internal lubricant on the mechanical and tribological properties of PBT composites were investigated. Accordingly, to demonstrate the potential tribological mechanisms within the introduction of siloxane, elemental compositions for the tribological transfer films of the composites were characterized by X-ray photoelectron spectroscopy (XPS). It is anticipated to provide insights into the plasticizing/lubricating effects induced by siloxane to polymeric materials, elucidate the potential mechanism of siloxane on the friction interface, and broaden the utilization of PBT-based tribocomponents in industry.

2. Experimental

2.1. Materials and Composites Manufacturing

PBT with the trade name Ultradur B 4520 was provided by BASF SE, Ludwigshafen, Germany. Short carbon fiber (C C6-4.0/240-T190) and graphite flake (RGC 39A) were kindly supported by SGL Carbon SE, Augsburg, Germany, and Superior Graphite Europe, Sundsvall, Sweden, respectively. Siloxane internal lubricant (50% siloxane dispersed in a proprietary polyethylene terephthalate resin, EverGlide (EG) MB1550, Polymer Dynamix) was kindly donated by Lehmann & Voss & Co., KG, Hamburg, Germany. In addition, different particles, i.e., submicron-sized zinc sulfide (Sachtolith HD-S, Venator Materials, Duisburg, Germany) and titanium dioxide (Kronos 2310, Kronos International, Inc., Leverkusen, Germany), as well as nano-sized silica (Aerosil R 9200, Evonik Industries, Hanau, Germany), were used as friction and wear reduction fillers within the PBT composites, which are all commercially available products. In this study, two kinds of PBT-based tribological composites were designed, namely PBT-TC and PBT-TC-EG. PBT-TC consists of short carbon fiber, graphite, zinc sulfide, titanium dioxide, and nanosilica. To study the effects of the siloxane on the friction and wear performance of this composite, EverGlide material was added to this composite, which is named PBT-TC-EG.

The designed composites were melting-compounded with the fillers by using a co-rotated twin-screw extruder (Leistritz ZSE 18 MAXX—40 D, Leistritz, Nürnberg, Germany) at a rotation speed of 200 rpm. The temperature from the feeder to the nozzle was selected between 120 and 260 °C. After extrusion, an ENGEL injection-molding machine (victory 200/80 spex, ENGEL, Schwertberg, Austria) was applied to injection-mold the composites into plates with a geometry of 50 mm × 50 mm × 4 mm, from which samples for mechanical and tribological investigations were prepared. At least five samples were tested for both characterizations in order to calculate the mean values and the standard deviations.

2.2. Mechanical Investigations

The tensile properties of PBT-TC and PBT-TC-EG were tested at room temperature (23 ± 1 °C) on a universal testing machine (RetroLine, Zwick GmbH & Co., KG, Ulm, Germany) according to DIN EN ISO 527-2:2012 within a dumbbell shape of type 1BB [31]. The measurements followed DIN EN ISO 527 using dumbbell-shaped specimens. The Young’s modulus of samples was measured by the crosshead within a speed of 1 mm/min. The tensile strength and elongation at the break of samples were determined with a tensile speed of 50 mm/min.

2.3. Tribological Studies

A block-on-ring (BOR) tribometer was adopted to evaluate the tribological properties of the composites with a sample geometry of 4 mm × 4 mm × 10 mm (apparent contact area: 4 mm × 4 mm) under dry sliding conditions at room temperature according to standard ASTM G77-17 [32]. The sliding counterbody was the standard bearing inner ring (100Cr6, INA, Schweinfurt, Germany) with a diameter of 60 mm outside, 5 mm thickness, and a surface roughness R_a = 0.2 ± 0.04 μm. The 2D force transducer was utilized to define and record the normal and frictional forces during tests. Overall, 5 input load conditions, 1 MPa and 0.5 m/s, 1 MPa and 1 m/s, 2 MPa and 1 m/s, 3 MPa and 1.5 m/s, and 4 MPa and 2 m/s were selected to comprehensively analyze the tribological performance of the composites. Steady-state coefficients of friction (COF) and specific wear rates were calculated based on at least three measurements under each load condition. The specific wear rate (w_s, mm³/Nm) was determined by the following equation:

$${\mathrm{w}}_{\mathrm{s}}=∆\mathrm{h}/\left(\mathrm{p}\times \mathrm{s}\right)$$

(1)

where Δh is the height loss measured by using a displacement transducer during the sliding test, p is the pressure, and s represents the sliding distance.

2.4. Chemical Characterization

Chemical characterization of the transfer film was carried out by X-ray photoelectron spectroscopy (XPS), using an Axis Nova small spot electron spectrometer (Kratos Analytical Ltd., Manchester, UK), equipped with 165 mm radius hemispherical analyzer, combined electrostatic and magnetic lenses, DLD detector and monochromatic Al K^α (1486.6 eV) X-ray source, working at an operating pressure lower 10⁻⁸ mbar. On each sample, three elongated measuring spots, with a size of 350 × 700 μm², were selected in the area of tribological loading and one outside. The concentrations given represent average values of the individual measurements. The detector axis was oriented parallel to the normal surface of the sample. Elemental concentrations were calculated from survey spectra (electron pass energy: 160 eV) using standard sensitivity factors given by Kratos Ltd. and Shirley shape fits for background subtraction. The binding states of carbon, oxygen, and silicon are characterized by C1s, O1s, and Si2p detailed core level spectra at an electron pass energy E_pass = 20 eV. Possible charging artifacts were compensated by shifting the binding energy scale with respect to a true C1s binding energy of 285 eV for aliphatic hydrocarbons. The information depth of this analysis is element- and material-dependent, smaller than 3–5 nm.

Chemical state analysis after subtraction of a Shirley-type background was performed by inscribing Gauss–Lorentz shaped partial peaks, localized at typical chemical shift, given by the NIST database [33].

3. Results and Discussion

3.1. Mechanical Properties

The impacts of the introduction of EG on the mechanical properties of the PBT-based material are illustrated in Figure 1. In comparison to the reference PBT-TC material, the incorporation of EG leads to a significant reduction in Young’s modulus and ultimate tensile strength. However, a notable elevation of the elongation at break for the PBT-TC-EG is observed. This can be explained by the plasticizing effect of siloxane [34,35]. According to the research by Arzhakov et al., low-thermodynamic-affinity siloxane tends to localize at the local structural regions of the boundaries between supramolecular or suprasegmental structures for polymethyl methacrylate (PMMA), changing the mobility of the macromolecules or its segments in this region; then, the mechanical behavior of PMMA is affected [36]. In their work, the elastic modulus and the yield stress behaved sharply, cutting back with the high load of the siloxane. In addition, the flexible Si-O-Si bondage enriched in siloxane also proved to contribute to the ductility enhancement of the polymer matrix [37]. Hence, the high siloxane load in this research brings about a reduced modulus, raised tensile strain, and improved processability for the PBT matrix.

3.2. Tribological Properties

The COFs of PBT-TC and PBT-TC-EG under different load conditions are elucidated in Figure 2a. With the increment of input load conditions, the COFs of the materials undergo a maximum, which is independent of the composite’s compositions. With respect to the influence of the siloxane addition on the tribological properties of PBT-TC, it is of great interest to observe that, except for extremely high load conditions (3 MPa and 1.5 m/s, 4 MPa and 2 m/s), the addition of EG could effectively reduce the COFs of the tribologically modified PBT-TC. Compared to PBT-TC, the COF reduction rates of PBT-TC-EG are 25% at 1 MPa and 0.5 m/s, 21.4% at 1 MPa and 1 m/s, and 30.6% at 2 MPa and 1 m/s, respectively. It is presumed that the introduction of EG could enhance the lubrication property of formed transfer films, which bring about lower friction forces during sliding under relatively low load conditions. However, once conducted under relatively high input load conditions (3 MPa and 1.5 m/s, 4 MPa and 2 m/s), the PBT-TC-EG presented higher real contact area between counterbody than PBT-TC as the softening due to EG addition, demonstrating higher COFs than the PBT-TC material.

The lubrication property of the EG material can also be evidenced by the specific wear rates, as demonstrated in Figure 2b. Due to the introduction of EG, the specific wear rate of PBT-TC-EG slightly decreases under the load conditions studied. In comparison with PBT-TC, the mean specific wear rate of PBT-TC-EG declines to 6.6% at 1 MPa and 0.5 m/s, 3.7% at 1 MPa and 1 m/s, 12.7% at 2 MPa and 1 m/s, 14.3% at 3 MPa and 1.5 m/s, and 13.2% at 4 MPa and 2 m/s, respectively. Although PBT-TC-EG behaves in higher real contact areas under high load conditions, its EG-contained lubricative transfer film could effectively reduce the wear of the material, contributing to a relatively lower wear. Considering the tribological results, reasonable speculation for the lubricating mechanism of EG can be summarized as follows: During the sliding process of PBT-TC-EG, EG was released from the matrix, which was further mixed under mechanical force with the decomposed debris and compressed on the counterbody to form lubricative transfer films. The enriched flexible Si-O in EG is the smooth and soft segment that demonstrated a larger bond angle and bond length than C-O and then possessed ultra-low surface energy and was easy to slide. As a benefit of this, the lubricative transfer film containing EG can continuously mitigate the severe friction of PBT-TC-EG materials. Even under a similar load condition, the addition of siloxane can bestow comparable wear properties of PBT-TC-EG than the extraordinary wear-resistant polyether ether ketone or polyimide tribocomposites [38].

3.3. Chemical Characterizations

3.3.1. Elemental Concentrations of Composite Pins

To further elucidate the lubrication mechanisms of EG, the chemical characteristics of worn surfaces on the composite pins were analyzed by XPS survey spectra. The elemental concentrations of the samples are shown in Figure 3. Predominant elements on the surface are carbon and oxygen, as expected for polybutylene terephthalate (PBT), its carbon fiber, or graphite admixtures, respectively. However, it should be noted that the dominance of C and O is not a unique feature of the compound material. Practically all samples under atmospheric conditions are covered by a thin but omnipresent hydrocarbon adsorbate film (‘adventitious hydrocarbon’) [39].

Therefore, trace elements such as Si, Ti, Zn, Ca, and N found in much smaller proportions are more interesting for clarifying the relevant mechanisms. The occurrence of Si, Ti, and Zn is in accordance with known ingredients ZnS, TiO₂, and SiO₂ of the composites. Other detected elements, like Ca and N, are included in widespread fillers of plastic material. So, the remaining most significant difference between fresh PBT-TC and PBT-TC-EG pins is a higher silicon concentration of PBT-TC-EG.

After tribological load, the detected carbon concentration decreases, and oxygen increases on the worn surface of the pin. This observation is consistent with various expected effects like enhanced oxidation processes, surface enlargement due to roughening and scoring, or removal of the adventitious hydrocarbon layer, covering the PBT bulk material with higher oxygen content. This also explains the increased concentrations of Si, Zn, and Ti at the worn surfaces of the pin and suggests their availability on the surface, with possible consequences for the tribological behavior. Additionally, transfer material from the ring counter body is detected on the worn composite pin, as evidenced by iron, and iron oxide, which is particularly significant with PBT-TC-EG. During the sliding friction process between the fiber-reinforced polymeric materials with metallic counterbody, direct contact between the end of the fiber and the metal ring engenders high flash temperature, which brings about the tribo-oxidation of the metal on the counterbody surface and leads to the formation of metal oxides on the counterface [40]. Via continuous mechanical mixing, the metal oxides can be peeled by the nanosilica and released in the interface [41]. These dissociated metal oxides could be pressed under force and embedded in the polymer matrix on the worn surfaces. As proved above, the PBT-TC-EG material possesses less modulus, indicating that it is easy to be embedded by the metal oxide, which leads to a higher iron concentration on the pin-worn surface.

But particularly striking is the significant increase in a detected silicon concentration on the friction surface for both cases (PBT-TC and PBT-TC-EG). At the same time, it is probably caused by SiO₂ in the case of PBT-TC. However, the silicon concentration on the worn surfaces of PBT-TC-EG is much more pronounced than that of PBT-TC materials. This implies the successful release of siloxane into the counterface, which may be a first hint for the availability of free, tribologically effective siloxane operated as a lubricant.

3.3.2. Elemental Concentrations of the Ring Samples

Figure 4 shows the elemental concentrations at the surface of the test rings, outside and inside the zone of tribological load. Dominant elements are carbon and oxygen, in accordance with the assumption of transfer material from the composite pins, although typical for the presence of an adventitious hydrocarbon adsorbate film, as discussed in the case of the pins.

Looking at the other elements, an almost identical element repertoire is observable like Si, F, Ca, N, Ti, Zn, and S, like in the case of the composite pins, which supports the assumption of pin transfer material. Remarkable is the detection of these elements even outside the friction track. It shows that transfer material and wear particles are not only deposited inside the zone of tribological load but also widespread in the direct surrounding.

A counterintuitive development is observed for silicon since the silicon concentration on the steel sample, in contrast to the composite sample, appears to decrease because of tribological load. Even significantly less silicon, as well as more iron, is detected on PBT-TC-EG wear tracks than on PBT-TC. A possible explanation for this is that siloxane is consumed during the process. At the composite pin, due to tribological load, continuously fresh siloxane is uncovered; at the ring, siloxane is consumed and has to be resupplied.

The assumption of siloxane consumption, even at this rather modest load (1 MPa and 0.5 m/s), may be supported by the observation that in the case of PBT-TC-EG, more iron is exposed, which is detectable by XPS. On the other hand, even in low load scenarios (1 MPa:0.5 m/s), it seems to be apparent that adding EG results in a softening and thus larger true contact area. This effect was already observed at high load conditions and resulted in a slightly higher friction coefficient.

When considering the total Si concentration of the corresponding composite pins, it can be noted that the available Si concentration PBT-TC-EG is much higher than that of PBT-TC (Cf. Figure 3 and Figure 4), which correlates with the better tribological performance of PBT-TC-EG.

3.3.3. Chemical Binding Situation at Pins and Rings in Comparison

The C1s detail spectra in Figure 5 and resulting concentrations in Figure 6, resolving the chemical bond situation, show on pins and rings typical bonds for carbon, bound in aliphatic and aromatic hydrocarbons, without and with one, respectively, two oxygen atoms as the nearest neighbor of the carbon. These findings are in accordance with the presence of polybutylene terephthalate on both types of samples, but as discussed earlier, also for the presence of an adventitious hydrocarbon adsorption film. Interestingly, the proportion of hydrocarbon on the rings shows even higher oxygen participation than on the pin material. It identified the interpretation that the transferred polymer matrix passes through the decomposition/oxidation process via abrasion during sliding friction.

The O1s detail spectra (Figure 5) suffer from a superposition of electron binding energies. Unfortunately, oxygen* in the ester group of PBT (-C-(C=O*)-O-), as well as silicon dioxide (SiO₂) and siloxanes (-O-Si-O-), show partial peaks at nearly the same electron binding energy of about E_B = 532.3 eV and cannot be distinguished in O1s details.

The Si2p detail spectra (Figure 5) also provide little help in distinguishing between SiO₂ filling material and siloxane lubricant (-O-Si-O-) since both silicon atoms have two oxygen atoms as direct binding partners, resulting in a nearly identical electron binding energy E_B = 103.5 eV. A second contribution at E_B = 102 eV is in accordance with silicon bound as hexamethyl siloxane Si(CH₃)₃-O-Si(CH₃)₃ or silanol -Si(OH)-, compounds with only one oxygen atom as the binding partner. In view of the higher silanol -Si(OH)- concentration of the pins for PBT-TC-EG (1:0.5 PBT-TC-EG) than that of the pins for PBT-TC (1:0.5 PBT-TC), it can be interpreted the silanol -Si(OH)- is the main effective segments of siloxane. In this case, the higher silanol -Si(OH)- concentration on the ring of 1:0.5 PBT-TC-EG verified the effective release of siloxane from the matrix to the counterface during sliding friction.

4. Conclusions

The present study clarifies the impacts of EG on the mechanical and tribological properties of a PBT-based polymeric composite. The lubricating effect of EG, along with its potential mechanism, is further verified via XPS analysis. The introduction of EG dramatically elevates the elongation at break with the sacrifice of Young’s modulus and ultimate tensile strength, thereby evidencing the plasticizing effect of EG on the PBT matrix. EG effectively improves the tribological properties of PBT-based materials, in particular under low load conditions (pv-product ≤ 2 MPa∙m/s). The addition of EG can cut back 21.4–30.6% of COF and 3.7–14.3% specific wear rate of the PBT-based composite studied. The XPS findings indicate the dominant lubricating mechanism, which is the higher content of silicon-contained materials at the contact interface, which leads to a better lubrication of the mating pair. According to tribological/chemical investigations, the EG lubricating mechanism has been revealed. During the sliding friction process, the EG was released from the abrasion of the PBT-TC-EG matrix. With mechanical mixing and compressing, the EG-contained transfer films were formed. The flexible Si-O bond enriched in EG conferred the transfer film’s impressive lubricating properties, which continuously serve to improve the tribological performance of PBT-TC-EG. The findings of this work can contribute to the development of siloxane-based high-performance tribological materials.