Effects of potassium titanate whisker and glass fiber on tribological and mechanical properties of PTFE/PEEK blend

Abstract

Two series of 10% polytetrafluoroethylene (PTFE)/polyether ether ketone (PEEK) composites reinforced with potassium titanate whisker (PTW/PTFE/PEEK) and chopped glass fiber (GF/PTFE/PEEK) were prepared and characterized. We investigated the effects of the additives on thermal stability, tribological properties, mechanical properties, and rheological behavior. The results illustrated that the mechanical properties of 10% PTFE/PEEK blend can be dramatically improved by incorporating either PTW or GF; however, the reinforcing effect of GF was found to be superior. It was found that 1% additive resulted in blends with the best tribological properties. Compared to the unmodified blend, the friction coefficient and wear rate of the 1% PTW blend decreased by 7.2% and 21%, respectively, while the corresponding values of 1% GF blend decreased by 0.66% and 51%, respectively.

Keywords

PEEK blends thermal stability tribological properties mechanical properties

Introduction

Polyether ether ketone (PEEK) is a thermoplastic aromatic polymer material and is one of the most important engineering plastics from the polyaryl ether ketone family.^1
–3 PEEK has a lot of excellent properties, such as high temperature resistance, dimensional stability, and biocompatibility.⁴ Consequently, PEEK is widely used in many other fields such as aerospace, automotive, and medical fields.^5
–7

In recent years, researchers have found that PEEK material still has some limitations under extreme conditions. In metallurgy, automotive, aerospace and precision instruments, and other fields, wear caused by friction not only shortens the mechanical life but also limits the design and use of the equipment.⁸ For more demanding applications, pure PEEK resin may be insufficient. Therefore, modification of PEEK has been the focus of some recent polymer materials research.³ Polymer blending is a powerful route toward materials exhibiting properties and cost performances superior to those of their individual components.^5,9 Modification of PEEK may lead to PEEK products with lower cost and improved performance and further expand their application into other areas.

Since glass fiber (GF), carbon fiber, calcium carbonate whisker, and potassium titanate whisker (PTW) are excellent reinforces commonly used in engineering materials, they could be used as fillers to modify PEEK.^6,7,10

Li et al.¹¹ studied the effects of applied load and sliding time on the friction coefficient and wear loss of GF-modified PEEK (GF/PEEK). The results indicated that the friction coefficient and wear loss of the composite increased gradually and tended to a steady state with increasing applied load and sliding time.

Davim and Cardoso¹² studied the wear, coefficient of friction, and frictional force of PEEK matrix composites. It was found that PEEK with 30% by weight GF at 1500°C exhibited improved wear performance at 80 N load under dry conditions.

Burris and Sawyer¹³ prepared a PEEK-filled polytetrafluoroethylene (PTFE) composite with low friction and ultra-low wear. The wear rate of the composites for each test sample was less than that of unfilled PTFE and PEEK. For a 32 wt% PEEK-filled sample, a minimum wear rate of K = 2 × 10⁻⁹ mm³ (N·m)⁻¹ was achieved.

In this study, PTFE, as an organic lubricant material,¹⁴ was added into PEEK. It has been shown that the addition of one-component PTFE to PEEK did not meet the mechanical property requirements for practical applications.^15,16 Consequently, 10% PTFE/PEEK blend was modified by the addition of PTW and chopped GF. The effects of the reinforcing agents in PTFE/PEEK blend were studied by adjusting the loading of PTW and GF. We hope to improve the mechanical properties on the basis of keeping the inherent antifriction and wear resistance of the blend, and obtain reinforced materials that can meet the actual industrial production and application demands in both tribological and mechanical properties.¹⁷

Materials and methods

Materials

Amorphous PEEK (T _g = 151°C) powder was purchased from Changchun Jilin University SEP Co., Ltd (China). PTFE (particle size: 12.8 µm) was purchased from Solvay S.A. (Shanghai, China) Short-chopped GF (W/D ratio = 1:1) was purchased from Nittobo Co., Ltd (China). PTWs (K₂O·nTiO₂, n = 6, white powder, pH = 7–8, MP > 300°C) were purchased from Shanghai Jiachen New Material Technology Co., Ltd (China).

Sample preparation

PEEK powder, PTFE powder, and GF or PTW were first premixed in a high-speed mixer (HRS-800; Dongguan Huanxin Machinery Co., Ltd., Dongguan, China) at 18 r min⁻¹ for 15 min. The mixed powder was dried in an oven at 120°C for 8 h before extrusion. Content of PTFE was 10 wt%, and the content of PTW or GF added was varied at 1 wt%, 3 wt%, and 5 wt% of the blends. The blends were prepared using a three-screw extruder (MEDI-22/40; Guangzhou Putong Experimental Analysis Instrument Co., Ltd., Guangzhou, China) with a screw speed of 150 r min⁻¹ at a temperature range of 220–240°C. The pellets were obtained by a granulator (Magelis; Guangzhou Putong Experimental Analysis Instrument Co., Ltd.) and then shaped by an injection molding machine (TAYU-400; Hangzhou TAYU Machinery Co., Ltd., Hangzhou, China) to produce rectangular and dumbbell-shaped standard specimens. Injection molding conditions were as follows: pressure (90–120 MPa), flow rate (70–90%), barrel temperature (220–240°C), mold temperature (30–50°C), injection time (10 s), and holding time (70 s).

Thermogravimetric analysis

The weight loss was measured on a TGA 2050 thermogravimetric analyzer (USA TA Instrument company, New Castle) under nitrogen at a flow rate of 100 mL min⁻¹ and a heating rate of 10°C min⁻¹ from 100°C to 810°C.

Heat deflection temperature

The heat resistance tests were carried out on a Ceast 500 ALOXIDE high temperature thermal deflection temperature/Vicat apparatus (Instron Test Equipment Trading Co., Ltd., Shanghai, China) with a heating rate of 2°C min⁻¹ and a pressure of 1.82 MPa in accordance with GB/T 1634-2004.

Impregnation density testing

The test was performed according to GB/T 1033-86. Cylindrical pin bodies were selected as test samples, and they were cleaned with no cracks, no bubbles, and no other defects. The samples were sonicated in an acetone solution for approximately 20 min, subsequently placed in an oven to remove residual acetone and water, and allowed to cool to room temperature for testing. The density was calculated as follows

ρ_{t} = \frac{a \cdot ρ_{x}}{a - b}

where ρ_t is the density (g cm⁻³) of the sample at t (°C), a is the mass (g) of the sample in air, b is the mass (g) of the sample in the infiltrating solution, and ρ_x is the density of the infiltrating liquid (g cm⁻³). The infiltration solution used in this test was deionized water at 23°C.

Tribological properties

Tribological properties tests were performed on a friction testing machine (UMT-2; Bruker, Germany) according to ASTM G99-04. The cylindrical pin body specimens (φ6.3 × 18.8 mm²) were obtained by injection molding. The plate body was 45 # stainless steel with a dimension of φ50 × 10 mm². Pin body surface and plate surface were polished with 1000 # sandpaper. The friction and wear test was conducted for 2 h and the specimens were subjected to a load of 1 MPa at room temperature. The rotational speed of the plate was 200 r min⁻¹ and the relative distance between the pin body and the center of the plate body was 20 mm. The product of rotation speed (r min⁻¹) and friction perimeter of the plate body (mm r⁻¹) was the test distance (mm) passed through the pin body per minute.

The mass loss of the sample (Δm) was obtained by subtracting the final mass recorded after the test from the initial sample mass. The relationship between the mass loss Δm and the volume loss ΔV is as follows

Δ V = \frac{Δ m}{ρ} \times 1000

where ΔV is the volume loss (mm³), Δm is the mass loss (g), and ρ is the density of the material (g cm⁻³). The wear rate was evaluated by

W_{r} = \frac{Δ V}{F_{z} \cdot L}

where F _z is the load pressure in the vertical direction (N) and L is the total distance (m) the specimen passed on the plate surface.

Mechanical testing

Tensile test and three-point bending test were performed on a Shimadzu AG-1 universal testing machine at room temperature. For GB/T 1040.2-2006, the stretch specimens were dumbbell-shaped with dimensions of 75 × 12.5 × 2.0 mm³. For GB/T 9341-2008, the bending specimens had a size of 80 × 10 × 4 mm³. Spans for tensile and bending tests were 20 and 60 mm, and the crosshead speeds were 5 and 2 mm min⁻¹, respectively.

Impact strength was tested on a charpy impact testing machine (HIT-2492; Chengde Jinjian Detection Equipment Co., Ltd., Chengde, China) using the GB/T 9341-2000 test method. The impact speed was 2.9 m s⁻¹. The data reported were average values of five samples.

Scanning electron microscope

The morphology of wear surfaces and impact fracture surfaces were observed using a scanning electron microscope (SEM-450, FEI, the Netherlands), operating at a 15 kV accelerating voltage. The surfaces were all sprayed with gold before scanning.

Rheological behavior

The rheological behavior was measured using a LCR-7001 capillary rheometer (USA Dynisco company, Massachusetts, USA). The diameter of the capillary is 1.016 mm, and the length is 20.32 mm. The shear rate was between 10 and 1700 s⁻¹ and the temperature was constant at 240°C.

Results and discussion

Thermal stability

Thermogravimetric analysis (TGA) and heat deflection temperature (HDT) results of the different blends are presented in Table 1 and Figure 1, respectively. As shown in Table 1, T _d5 (5% weight loss temperature) and T _d10 (10% weight loss temperature) of the blends containing PTW and GF were higher than that of unmodified 10% PTFE/PEEK blend (536°C, 543°C), except for the blend with 1%PTW (532°C, 540°C). In general, T _d5 and T _d10 of both PTW/PTFE/PEEK and GF/PTFE/PEEK blends increased with the incremental increase in loading of reinforcing agents. When the loading was 1%, T _d5 and T _d10 of the GF/PTFE/PEEK blend were higher than that of the PTW/PTFE/PEEK blend. However, when the loading was above 1%, the results were opposite. The enhanced thermal stability of PTW- and GF-modified blends was also evident from the carbon residue results. The residual carbon values of all blends with reinforcing agent ranged from 42.9% to 49.0% at 800°C, which were higher than that of 10% PTFE/PEEK blend (40.9%). In other words, the incorporation of either PTW or GF had a definite impact on the thermal stability of 10% PTFE/PEEK blend, where the overall T _d5 and T _d10 remained above 532 and 540°C, respectively.

Table 1.

Thermal stability of PTW/PTFE/PEEK and GF/PTFE/PEEK blends.

Samples	T _d5 (°C)	T _d10 (°C)	R ₈₀₀ (%)	HDT (°C)
PEEK	533	540	40.0	132
10% PTFE	536	543	40.9	130
1% PTW	532	540	47.6	137
3% PTW	540	548	48.8	141
5% PTW	547	551	49.2	143
1% GF	537	544	42.9	137
3% GF	540	546	48.5	140
5% GF	543	548	49.0	143

PTW: potassium titanate whisker; PTFE: polytetrafluoroethylene; PEEK: polyether ether ketone; GF: glass fiber; T _d5: 5% weight loss temperature; T _d10: 10% weight loss temperature; R ₈₀₀: carbon residue at 800°C; HDT: heat deflection temperature.

From Figure 2, it can be seen that the addition of PTW or GF significantly increased the HDT of the blends. When the amount of reinforcing agent was 5 wt%, the HDT values of both PTW/PTFE/PEEK and GF/PTFE/PEEK blends were approximately 1.1 times higher than that of the unmodified 10% PTFE/PEEK blend. At the same loading, the HDT values of PTW system are similar to that of the GF system. PTW incorporated into the PTFE/PEEK substrate could play a supporting role in the skeleton by transferring and dispersing stress,¹⁸ and restricting movement of macromolecules, thus increasing the stiffness.

The incorporation of GF increased the difficulty of thermal motion of the molecular chains in the blends system.¹⁹ Only by increasing the temperature, the polymer chain segments could move freely again. GF also had a skeleton supporting effect, which hindered the effect of load on material under high temperature. Therefore, addition of either PTW or GF into the blends can enhance their resistance to heat and force, which can extend their use under high temperature conditions.

Tribological properties

The friction coefficient, wear amount, and wear rate of PTW system and GF system blends are listed in Table 2. According to Figure 3(a) and (b), both friction coefficient and wear rate of the blends increased slightly with increasing additive loading, but the wear rate was more susceptible. In terms of friction coefficient, the value for the 10% PTFE/PEEK blend was 0.305. Only the blends with 1% PTW, 3% PTW, and 1% GF resulted in lower values, corresponding to a reduction of 7.2%, 5.2%, and 0.66%, respectively. As for the wear rate, the value of the unmodified blend was 6.33 × 10⁻⁶ mm³ (N·m)⁻¹. Only the blends with 1% PTW and 1% GF were lower than that by 12% and 51%, respectively. Therefore, within the scope of this study, additive loading of 1 wt% gave the best results in terms of tribological properties. It should be pointed out that, at the same additive ratio, the friction coefficient of PTW series was slightly smaller than that of the GF series, but the wear rate was just the opposite. The wear rate of the PTW series was significantly higher than that of the GF series. When the content of PTW was 5 wt%, the wear rate increased by an order of magnitude compared with that of 10% PTFE/PEEK blend, indicating that the wear resistance of GF-modified blends are better than PTW blends. However, it is worth mentioning that all of the blending systems with reinforcing agents, regardless of friction coefficient or wear rate, were much lower than that of pure PEEK.

Table 2.

Tribological properties of PTW/PTFE/PEEK and GF/PTFE/PEEK blends.

Samples	Coefficient of friction	Density (g cm⁻³)	Wear amount (mg)	Wear rate (×10⁻⁶ mm³ (N·m)⁻¹)
PEEK	0.535	1.19	46.5	415
10% PTFE	0.305	1.24	0.740	6.33
1% PTW	0.283	1.28	0.600	4.97
3% PTW	0.289	1.28	1.93	16.0
5% PTW	0.310	1.29	3.09	25.5
1% GF	0.303	1.26	0.370	3.09
3% GF	0.343	1.28	0.960	7.93
5% GF	0.348	1.30	1.15	9.36

PTW: potassium titanate whisker; PTFE: polytetrafluoroethylene; PEEK: polyether ether ketone; GF: glass fiber.

Figure 4(a) is a curve of friction coefficient versus time of pure PEEK. It can be seen that the friction coefficient presented great volatility in the range of 0.460–0.570 (average: 0.535), and cannot maintain good stability. Since the thermal conductivity of PEEK is poor, friction heat increased as the sliding distance increased. As the surface temperature rose, it can lead to partial melting of the resin. This change was the main reason for the periodic change of friction coefficient and the increase of frictional resistance. The PEEK used in this study had a lower processing temperature and the contact surface was more susceptible to temperature, thus the friction coefficient cannot remain constant. Irregular ups and downs in the friction coefficient indicated that it was difficult for the material to resist the dual effects of temperature and force. The wear rate of pure PEEK was calculated to be 415 × 10⁻⁶ mm³ (N·m)⁻¹.

Figure 4(b) shows the curve of friction coefficient versus time for the 10% PTFE/PEEK blend. Compared to pure PEEK, the 10 wt% PTFE blend exhibited a lower friction coefficient during the pregrind stage (before 2000s). As the friction continued, the friction coefficient increased and then leveled off. This indicated that the blend was able to withstand the applied force and heat, which was not enough to make the contact surface undergo tremendous changes. Compared to pure PEEK, the friction coefficient of the blend improved significantly. During the friction process, the abrasive PTFE was pulled out of the crystalline zone to form a highly oriented transfer film, so that the contact interface between the material and the auxiliary pair was converted to the self-rubbing state of PTFE, resulting in a significant reduction in the friction coefficient of 10% PTFE/PEEK blend.

Figure 4(c) and (d) shows the curves of friction coefficient over time for the PTW/PTFE/PEEK blends and GF/PTFE/PEEK blends, respectively. The friction coefficient of the blends increased with increasing filler content, and the whole condition was stable. During the friction test, high PTW content made it easy to fall off and form free abrasive grains, which decreased friction stability of the blends. As for GF blends, the addition of GF destroyed the PTFE transfer film formed on the surface of the friction pair.²⁰ On the other hand, the fibers tended to form floc-shaped adsorbates in the mixing process, so that the surface roughness of the pin body was increased. With the continuous grinding of the friction pair, the pin body was subjected to more severe mechanical kneading and shearing collisions, resulting in sliding difficulty and increase in friction coefficient.

Figure 5(a) and (b) is pure PEEK wear surface SEM photographs. It can be seen that there are many fine and deep trenches along the rubbing direction, which is an indicative of abrasive wear. The wider and shallower twisted lines and stripping traces are typical morphology of adhesive wear, which indicates that plastic deformation had occurred. From the shape of the wear scar, the size of wear debris and the macroscopical smoothness of the contact surface, it can be seen that most of the debris generated during the test was flaky, which corresponded with the debris characteristics of polymer due to dry sliding friction. After repeated fatigue and cutting effects of the load, the contact surface continued to generate debris. Some debris directly transferred and adhered to the surface of the friction pair, and some accumulated around the friction path after desorption from the adhesion surface. Some debris became free abrasive grains and embedded in the friction surface of soft material to form furrows or stripping lines, and sometimes can also produce micro-cracks to form pitting (fatigue wear). In summary, the wear of pure PEEK was caused by abrasive wear, adhesive wear, and fatigue wear.^21,22

The SEM photographs of 10% PTFE/PEEK, PTW/PTFE/PEEK, and GF/PTFE/PEEK blends are shown in Figure 5(c) to (i). Compared to pure PEEK, it is quite clear that the addition of PTFE significantly decreased the wear debris on the surface of 10% PTFE/PEEK, while the furrows, stripping lines, and plastic deformation were also diminished. Increasing the loading of hard phase reinforcing fillers resulted in the thicker strip shape observed, which indicates that more fillers came into direct contact with the friction pair. The increase in filler mass fraction led to its uneven distribution in the matrix and resulted in accumulation. The aggregation of fillers and weakened PTFE/PEEK matrix made PTW and GF more prone to fall off during the friction test, which impacted the effective adhesion of PTFE transfer film between the pin body and grinding plate.^10,23 As the friction process progressed, wear debris continued to be generated, resulting in the transfer film undergoing repeated destruction–generation processes. The friction coefficient and wear amount of the blends exhibited a slight upward trend. Compared with PTW, GF seems to be better embedded in the matrix as the wear rates of the GF blends were also lower.

Dynamic mechanics analysis

The dynamic storage modulus and loss modulus of each blend are presented in Figure 6. The storage modulus values of GF system were higher than that of the PTW system. Among all blends, 5% GF/PTFE/PEEK had the highest storage modulus at the temperature region before 160°C. There was a sharp decrease in storage modulus in the range of 130–160°C. As for loss modulus, 5% GF/PTFE/PEEK exhibited the highest value, but decreased gradually as the content of GF was decreased.

Mechanical properties

As seen in Table 3 and Figure 7, the mechanical properties of PTFE/PEEK blend were improved by the addition of PTW or GF. The increase in the additive content corresponds to the increase in tensile strength, elastic modulus, bending strength, and bending modulus of the blends. Moreover, the reinforcing effect of GF was especially obvious. At 5 wt% PTW, the tensile strength and elastic modulus were higher than that of the unmodified 10% PTFE/PEEK blend by about 1.1 times and 1.2 times, respectively. Due to good interfacial adhesion between PTW and PTFE/PEEK matrix, the stress rapidly migrated from the matrix to the additive under tensile load, and PTW was subjected to the primary stress.²⁴ Similarly, when the GF content was 5 wt%, the tensile strength and elastic modulus of the blend were about 1.3 times and 1.4 times higher than that of the unmodified blend, respectively. Since the GF was well encapsulated by the PTFE/PEEK matrix,^25,26 numerous fibers were available to assume and distribute the load. As the extraction or fracture of fibers needed to impose more load, the tensile strength of the blends was enhanced. In addition, an increase in GF loading also caused an increase in the volume content, which reduced the matrix content between the fibers. The deformation of the matrix in the load direction was restrained and controlled by the fibers, leading to a substantial increase in the elastic modulus.

Table 3.

Mechanical properties of PTW/PTFE/PEEK or GF/PTFE/PEEK blends.

Samples	Tensile strength (MPa)	Elastic modulus (GPa)	Bending strength (MPa)	Bending modulus (GPa)	Impact strength (kJ m⁻²)
PEEK	63	1.0	103	2.9	4.70
10% PTFE	60	1.3	90	2.8	12.1
1% PTW	63	1.3	92	2.9	19.8
3% PTW	64	1.5	93	2.9	15.2
5% PTW	65	1.5	94	3.1	13.1
1% GF	69	1.6	102	3.2	18.5
3% GF	77	1.8	114	3.6	15.8
5% GF	79	1.9	118	3.9	15.7

PTW: potassium titanate whisker; PTFE: polytetrafluoroethylene; PEEK: polyether ether ketone; GF: glass fiber.

The use of strength reinforcing agents can effectively reduce the crack initiation of local weak bonding under bending stress and prevent the expansion of cracks. As a result, the bending strength and bending modulus of PTW and GF blends were increased. It is noteworthy that the GF system blends exhibited much better tensile and bending properties than the PTW system blends at the same loading level, indicating that GF can provide excellent load-carrying capacity and load-dispersing ability.

In terms of impact strength of the blends (Figure 7(e)), maximum impact strength was observed for PTW and GF system blends with 1% additive at 19.8 and 18.5 kJ m⁻², respectively. However, the values significantly decreased with increasing additive content but were still higher than the unmodified 10% PTFE/PEEK blend (12.1 kJ m⁻²). The impact properties were mainly associated with the inherent properties of the whiskers and polymer matrix, whisker content, geometry, arrangement, and interfacial bonding strength.^27
–29 The most important of these is the interfacial bonding strength, which affected the absorption of impact energy by affecting the fracture mode of the material. The whiskers, on the one hand, bore most of the load near the notch of the specimens and distributed the stress to a larger area. On the other hand, the breaking and pulling out of the whiskers could absorb certain impact energy, and in turn increased the impact strength of the blends. However, when the amount of reinforcing agents was too large, it was difficult to uniformly disperse the material in the matrix, while interfacial bonding may also be affected and may lead to the formation of large defects. In this case, the reinforcing agent, which originally acted as stress-bearing and stress-dispersing agent, became an impurity in the matrix and was more likely to cause the formation and propagation of cracks. Hence, the absorption of energy was reduced and the external load made the material more prone to fracture damage, which was reflected in the impact strength reduction.

Figure 8 shows the impact cross-section SEM images of PTW/PTFE/PEEK and GF/PTFE/PEEK blends. It can be seen from Figure 8(a) that the impact fracture of pure PEEK was a typical brittle fracture,³⁰ showing large radial fracture and rapid crack development. The images from Figure 8(c) to (e) show the characteristics of plastic deformation, which indicated that the PTW/PTFE/PEEK blends had undergone plastic deformation during the fracture. Some whiskers can be seen in the matrix, indicating that PTW could still be coated in the PTFE/PEEK matrix after the impact fracture with no obvious whisker debonding.^10,31,32

The effects of GF additive on the impact strength were similar to PTW. It can be seen from Figure 8(f) to (h) that the interfaces of the blends were loose and GF remained relatively intact without plenty of broken marks. As the loading of GF is increased, the inherent brittleness of GF may lead to poor adhesion to the substrate. In addition, increasing the amount of GF also increases the fiber ends, which could lead to end-induced crack growth and destruct the protective effect of GF under the action of the impact load. Hence, the impact resistance of the blends would decrease.

Rheological behavior

The apparent viscosity–shear rate curves of PTW/PTFE/PEEK and GF/PTFE/PEEK blends are shown in Figure 9. All samples showed non-Newtonian fluid and shear thinning behavior, and all curves showed a downward trend. That is, with the increase of apparent shear rate, the apparent shear viscosity of the samples decreases. At low shear rate, the apparent viscosity of PTW or GF system blends increased slightly compared to 10% PTFE/PEEK blend, but the difference became smaller with increased shear rate.

In common processing, the shear viscosity and shear rate of many polymer melts conform to the Ostwald–de Waele power law model

η = K \cdot γ^{n - 1}

where η is the shear viscosity, γ is the shear rate, K is the material parameter, and n is the non-Newtonian index. Table 4 enumerates the corresponding K and n values.

Table 4.

K and n of power law model from curves fitting of apparent shear viscosity versus apparent shear rate of PTW/PTFE/PEEK and GF/PTFE/PEEK blends.

Samples	K	n − 1	n
10% PTFE	31,653	−0.44	0.56
1% PTW	41,931	−0.48	0.52
3% PTW	42,038	−0.48	0.52
5% PTW	48,156	−0.50	0.50
1% GF	38,409	−0.47	0.53
3% GF	38,753	−0.48	0.52
5% GF	40,376	−0.49	0.51

PTW: potassium titanate whisker; PTFE: polytetrafluoroethylene; PEEK: polyether ether ketone; GF: glass fiber.

As shown in Table 4, the n values of all samples are less than 1, proving that they are all pseudoplastic fluids. With the increasing loading of the reinforcing agent, the value of K increases and the value of n decreases, which indicates that the cross-linking occurs in the melt state, resulting in the increase of the molecular weight and the decrease of the flowability.

Conclusions

In this study, PEEK was used as the resin matrix and PTFE (10 wt%) was added as an organic lubricant to form a PTFE/PEEK blend. The blend was further modified by incorporating PTW and chopped GF into the matrix. A series of PTW/PTFE/PEEK and GF/PTFE/PEEK blends containing various amounts of reinforcing additives were prepared and characterized. We investigated the thermal stability, heat resistance, tribological properties, mechanical properties, and rheological behavior of the blends. The following conclusions were made:

The incorporation of PTW or GF influenced the thermal stability of 10% PTFE/PEEK blend and significantly increased the HDT. T _d5 of all modified blends from this study were above 532°C and T _d10 were above 539°C. The residual carbon values of all modified blends ranged from 42.9% to 49.0% at 800°C, which were higher than that of the unmodified 10% PTFE/PEEK blend (40.9%).

Within the scope of this study, 1% additive loading resulted in blends with the best tribological properties. The friction coefficient of the PTW series was slightly lower than that of the GF series at the same additive ratio, while the wear rate was the opposite, indicating that the wear resistance of GF was better than PTW. All modified blend systems exhibited significantly lower friction coefficient and wear rate than that of pure PEEK.

Mechanical properties of PTFE/PEEK blend were improved dramatically by incorporating PTW or GF. Increasing the additives loading resulted in the increase in tensile strength, elastic modulus, bending strength, and bending modulus of the blends. The impact strength of the modified blends was highest when the blends contained 1% additive. Moreover, the reinforcing effect of GF was found to be superior.

Footnotes

Acknowledgements

We thank the editors and reviewers for their valuable comments throughout the review process.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

References

Seferis

. Polyetheretherketone (PEEK): processing-structure and properties studies for a matrix in high performance composites. Polym Compos 1986; 7: 158–169.

Honkhambe

Dhamdhere

Tawade

. Synthesis and characterization of poly(ether ether ketone)s and poly(ether ether ketone ketone)s containing pendant biphenyl and naphthyl groups. High Perform Polym 2012; 25: 260–267.

Ramani

Alam

. The effect of an organophosphonite antioxidant on poly(ether ether ketone)/poly(ether imide) blend as studied by quasi-isothermal modulated differential scanning calorimetry and dynamic mechanical thermal analysis methods. High Perform Polym 2013; 26: 347–356.

Small

. Outstanding physical properties make PEEK ideal for sealing applications. Seal Technol 2014; 4: 9–12.

Hayashi

Kakimoto

Imai

. Polymerization blending for compatible poly(ether sulfone)/aramid blends based on polycondensation of n-silylated m-phenylenediamine with isophthaloyl chloride in poly(ether sulfone) solution. Polymer 1995; 36: 2797–2805.

Botelho

Silva

Pardini

. A review on the development and properties of continuous fiber/epoxy/aluminum hybrid composites for aircraft structures. Mater Res 2006; 9: 247–256.

Sudheer

Prabhu

Raju

. Optimization of dry sliding wear performance of ceramic whisker filled epoxy composites using Taguchi approach. Adv Tribol 2012; 2012: 200–208.

Ueta

Sueishi

. The control of the use pattern and the life of metals for environmental risk management; the case of lead. Kyoto Univ Econ Rev 1984; 54: 13–30.

Abdeen

Ayesh

Ibrahim

. Preparation and physical characterization of conjugated polymer-polycarbonate polymer blends. J Compos Mater 2013; 48: 1947–1957.

10.

Xie

Sui

Yang

. Effects of potassium titanate whiskers and carbon fibers on the wear behavior of polyetheretherketone composite under water lubricated condition. Compos Sci Tech 2011; 71: 828–835.

11.

Guo

Wang

. Research on tribological behavior of PEEK and glass fiber reinforced PEEK composite. Physics Procedia 2013; 50: 453–460.

12.

Davim

Cardoso

. Effect of the reinforcement (carbon or glass fibres) on friction and wear behaviour of the PEEK against steel surface at long dry sliding. Wear 2009; 266: 795–799.

13.

Burris

Sawyer

. A low friction and ultra low wear rate PEEK/PTFE composite. Wear 2006; 261: 410–418.

14.

Zhu

Zhang

. Influence of polytetrafluoroethylene (PTFE) content on mechanical and tribological properties of poly(ether ether ketone)/PTFE coatings prepared by electrostatic powder spraying technique. High Perform Polym 2015; 27: 3–9.

15.

Kim

Yoon

Lee

. Mechanical properties of carbon/PEEK composites according to the fiber ply orientation and sizing removal of carbon fiber for artificial hip joint. Adv Mater Res 2013; 750-752: 164–175.

16.

Bijwe

Sen

Ghosh

. Influence of PTFE content in PEEK–PTFE blends on mechanical properties and tribo-performance in various wear modes. Wear 2005; 258: 1536–1542.

17.

Singh

Shrivastava

Singh

. Study of tribological behavior of silicon carbide based aluminum metal matrix composites under dry and lubricated environment. Adv Mater Sci Eng 2016; 2016: 11.

18.

Wang

Feng

Shi

. Effects of fibrous fillers on friction and wear properties of polytetrafluoroethylene composites under dry or wet conditions. Particuology 2007; 5: 414–419.

19.

Noda

Tsuji

Takahara

. Aggregation structure and molecular motion of (glass-fiber/matrix nylon 66) interface in short glass-fiber reinforced nylon 66 composites. Polymer 2002; 43: 4055–4062.

20.

Wang

Zhang

. Study on PEEK filled with PTFE and its composite. Eng Plast Appl 2000; 1: 40–55.

21.

Wang

. Tribological property of polyetheretherketone composites reinforced with glass fiber under water lubrication. Cailiao Gongcheng J Mater Eng 2014; 3: 77–82+89.

22.

Song

Liu

Liao

. Wear studies on ZrO₂-filled PEEK as coating bearing materials for artificial cervical discs of Ti₆Al₄V. Mater Sci Eng C Mater Biol Appl 2016; 69: 985–994.

23.

Myshkin

Petrokovets

Kovalev

. Tribology of polymers: adhesion, friction, wear, and mass-transfer. Tribol Int 2005; 38: 910–921.

24.

Hegde

Lafont

Norder

. SWCNT induced crystallization in amorphous and semi-crystalline poly(etherimide)s: morphology and thermo-mechanical properties. Polymer 2014; 55: 3746–3757.

25.

Unal

. Comparison of tribological performance of PEEK, UHMWPE, glass fiber reinforced PTFE and PTFE reinforced PEI composite materials under dry and lubricated conditions. J Polym Eng 2012; 32: 349–354.

26.

Feng

Zhu

. Comparative study of tribological properties with different fibers reinforced PTFE/PEEK composites at elevated temperatures. Tribol Tran 2009; 53: 189–194.

27.

Zhang

. Effect of surface treatment of hydroxyapatite whiskers on the mechanical properties of bis-GMA-based composites. Biomed Mater 2010; 5: 054106.

28.

Garnier

Fantozzi

Nguyen

. Influence of SiC whisker morphology and nature of SiC/Al₂ O₃ interface on thermomechanical properties of SiC reinforced Al₂O₃ composites. J Eur Ceram Soc 2005; 25: 3485–3493.

29.

Yang

Zong

Zheng

. Experimental and theoretical investigation on the compressive behavior of aluminum borate whisker reinforced 2024Al composites. Mater Charact 2014; 96: 84–92.

30.

Dwivedi

Alam

Verma

. Interrelation of scanning electron micrographs with flexural behavior of ferrite and nano-barium titanate reinforced poly-ether-ether-ketone (PEEK) composites. Polym Plast Technol Eng 2005; 44: 1235–1243.

31.

Feng

Wang

Shi

. The effects of the size and content of potassium titanate whiskers on the properties of PTW/PTFE composites. Mater Sci Eng A 2007; 448: 253–258.

32.

Wang

Zhu

Feng

. The effect of self-assembly modified potassium titanate whiskers on the friction and wear behaviors of PEEK composites. Wear 2010; 269: 139–144.