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Advanced Composites and Hybrid Materials

, Volume 2, Issue 3, pp 407–422 | Cite as

Enhanced mechanical and tribological performance of PA66 nanocomposites containing 2D layered α-zirconium phosphate nanoplatelets with different sizes

  • Haoyang Sun
  • Zhengqin Fang
  • Tao Li
  • Fan Lei
  • Feng Jiang
  • Dandan Li
  • Yinghao Zhou
  • Dazhi SunEmail author
Original Research
  • 328 Downloads

Abstract

Two-dimensional layered α-zirconium phosphate (ZrP) nanoplatelets with two distinguished sizes but similar aspect ratio were directly incorporated into polyamide 66 (PA66) by simple melt processing without using any surfactants. Through the electron microscopy analysis, the large ZrP nanoplatelets with ~ 1.33 μm in size and ~ 5.8 in aspect ratio exhibit a uniform dispersion in PA66 matrices at the filler loading up to 3 wt%, while the small ZrP nanoplatelets with an average size of ~ 230 nm and aspect ratio of ~ 6.2 tend to form large-scale aggregates in PA66 even at 1 wt% loading. Tensile testing results illustrate that the large ZrP nanoplatelets exhibit a better reinforcement effect in PA66 than the small ones. With the incorporation of 3 wt% large ZrP nanoplatelets, the PA66 nanocomposites exhibit an increase of ~ 10% in tensile modulus and ~ 14% in tensile strength as compared with the pure PA66. Pin-on-disc wear tests illustrate that the nanocomposites containing large ZrP nanoplatelets have better anti-wear properties than those prepared with small ZrP nanoplatelets. In specific, the PA66 nanocomposites containing 1 wt% large ZrP nanoplatelets show a ~ 43% decrease in friction coefficient and a ~ 59% reduction in the wear rate under the test condition of 40 N in load and 0.6 m/s in velocity. The mechanisms that are responsible for the mechanical and tribological enhancements in the PA66/ZrP nanocomposites have also been discussed.

Graphical abstract

2D layered α-zirconium phosphate (ZrP) nanoplatelets with large size can effectively enhance mechanical and tribological properties of PA66 nanocomposites.

Keywords

Zirconium phosphate Nanocomposites Mechanical and tribological properties Nanolubricants 

1 Introduction

Polyamide 66 (PA66) has been increasingly used in recent years to replace the metallic materials owing to their lightweight and excellent mechanical properties [1, 2, 3]. In many applications, PA66 materials are usually subjected to various external forces and damages, which reduce their functions and esthetic performance. Consequently, to improve the mechanical and tribological performance, various fillers, such as fibers [4, 5, 6, 7], inorganic nanoparticles [8, 9, 10, 11, 12, 13, 14, 15], and solid lubricants [16, 17, 18], are commonly used to prepare high-performance PA66 composites.

Among various filler-reinforced polymer composites, polymer/layered silicate nanocomposites have received much attention due to their significant reinforcement in mechanical and tribological performance, thermal stability, barrier properties, and so on at a very low loading [19, 20, 21, 22]. Mu et al. [23] investigated the effects of modified montmorillonite (MMT) on the tribological properties of PA66. Adding less than 5 wt% of modified MMT can increase the bending strength and microhardness of PA66 and improve the wear resistance of the polymer matrix, while at higher filler loadings, the aggregation of MMT will be detrimental to the mechanical and tribological properties of PA66. Similarly, a recent study shows that only 1.5 wt% of the organo-modified clay not only can increase the hardness of ultrahigh molecular weight polyethylene (UHMWPE) but also reduce a ~ 38% coefficient of friction (COF) and a ~ 41% wear rate for the matrix owing to the load bearing capacity of rigid clay and the formation of a smooth transfer film on the sliding counterface [24].

Generally, the improvement effect of mechanical and tribological properties of the layered clay nanocomposites strongly depends on the dispersion of fillers and the interfacial interaction between the fillers and the polymer matrices [25, 26, 27]. Dasari et al. [25] prepared the well-dispersed clay-filled polyamide 6 (PA6) nanocomposites through a water-assisted process, and they found that the nanocomposites containing well-dispersed clay not only have higher mechanical performance but also show better anti-wear properties as compared with the pristine clay-filled nanocomposites. Similar results were also found in other literatures [28, 29, 30, 31]. Meanwhile, the concentration, surface characteristics, and orientation of the clay fillers can also have a significant effect on the mechanical and tribological properties of the polymer nanocomposites, which have been intensively studied before [3, 27, 32, 33]. However, owing to the limited particle sizes for the natural layered clays, there is still a lack of relevant knowledge on how the filler size affects the mechanical and tribological properties of the layered clay nanocomposites.

As compared with clay nanofillers, α-zirconium phosphate (ZrP) nanoplatelets, as one of the crystalline zirconium phosphates, exhibit excellent properties, such as good thermal stability, high aspect ratio, high purity, high ion-exchange capability, and ease to synthesize and control the size [34, 35]. As a result, numerous theoretical and experimental achievements have been made on the structure-property relationship in polymer/ZrP nanocomposites [36, 37]. These ZrP-based polymer nanocomposites also exhibit excellent mechanical performance, good fire resistance, high barrier ability, and good anti-corrosion property [38, 39, 40, 41]. Moreover, ZrP nanoplatelets have shown excellent lubrication properties owing to their 2D structure and the tendency of the adjacent platelets to slide under shear force [34]. Dong et al. [42, 43] have used ZrP as lubricant additives in mineral oil and grease. They found that layered ZrP nanoplatelets exhibit even better anti-wear properties than many traditional lubrication materials such as graphite and MoS2. Han et al. [21] have successfully synthesized silane-modified and alkyl amine–intercalated ZrP nanoplatelets. The modified ZrP nanoplatelets were dispersed in mineral oil, and they found that ZrP nanoplatelets modified by the surfactant with a longer alkyl chain and a relatively small intercalator exhibit superior lubrication and anti-wear properties in oils. Although many achievements have recently been made on the use of ZrP nanoplatelets in oil mediums, up to now, the studies on the friction and anti-wear properties of ZrP nanoplatelets in solid polymers have not been done yet.

In this study, we first synthesized ZrP nanoplatelets with different sizes through a hydrothermal method [35]. Then, the PA66 nanocomposites containing ZrP nanoplatelets of different sizes and concentrations were prepared through melt processing without using any surfactants. The dispersion of ZrP nanoplatelets in PA66 and the mechanical and tribological properties of the PA66/ZrP nanocomposites were therefore systemically studied.

2 Experimental

2.1 Materials

Commercially available PA66 (Zytel, 101L) pellets, with a density of 1.14 g/cm3 and melt temperature of 270 °C, were purchased from DUPONT Co. Zirconyl chloride (ZrOCl2·8H2O, 98%) was supplied by Sigma-Aldrich, and phosphoric acid (H3PO4, 85%) was provided by TianJin YongDa Chemical Reagent Company (Tianjin, China).

2.2 Synthesis of ZrP nanoplatelets with different sizes

ZrP nanoplatelets were synthesized using a hydrothermal method [35]. In our synthesis procedure, 15.0 g ZrOCl2·8H2O was firstly mixed with 150.0 mL of 3.0 and 12.0 M H3PO4, respectively. The mixtures were then sealed into a Teflon-lined pressure vessel and heated at 200 °C for 24 h. After reaction, the synthesized ZrP samples were washed and collected by centrifugation for at least five times and dried at 65 °C for 24 h. The ZrP products were then ground into fine powders with a mortar and pestle. The particle size of the final ZrP nanoplatelets can be tuned by the concentration of H3PO4. The small ZrP nanoplatelets obtained at 3.0 M H3PO4 was designed as 3MZrP, while the large one obtained at 12.0 M H3PO4 was designed as 12MZrP.

2.3 Preparation of PA66/ZrP nanocomposites

PA66 pellets and ZrP powders were first dried at 65 °C for 24 h before mixing. The dried PA66 pellets were then melt-mixed with ZrP nanoplatelets of different sizes and concentrations using a Haake Rheomix OS mixer. The screw speed was fixed at 50 rpm for 15 min, and the mixing temperature was set at 280 °C. After the melt blending, the products were crushed and dried again at 65 °C for 12 h. The crushed pellets were then injection molded into standard tensile testing specimens (63.50 × 3.18 × 3.20 mm3) and wear testing pin specimens with a radiused tip (ϕ6.16) according to the ASTM D638 Type V and the ASTM G99 standard, respectively, by using a Haake MiniJet II injection molding machine. The barrel temperature and nozzle temperature were fixed at 280 °C and 275 °C, respectively. An injection time of 5 s and an injection pressure of 500 bar were used. The post-time and post-pressure were set as 8 s and 550 bar, respectively. In order to eliminate the residual stress generated during the processing, these specimens were finally annealed in a vacuum oven at 135 °C for 1 h. The detailed formulations of the PA66/ZrP nanocomposites were presented in Table 1.
Table 1

Detail information of the PA66/ZrP nanocomposites

Samples

PA66 (wt%)

α-ZrP (wt%)

PA66

100

0

PA-1%(χMZrP)

99

1

PA-3%(χMZrP)

97

3

PA-5%(χMZrP)

95

5

The χMZrP means the ZrP in the matrix was synthesized by phosphoric acid with the concentration of χM (χ = 3, 12)

2.4 Characterizations

A Tescan VEGA 3 LMH scanning electron microscope (SEM) was used to investigate the morphologies of the synthesized ZrP nanoplatelets and the PA66/ZrP nanocomposites. Before the SEM observations, all the samples were coated with a thin film of platinum (Pt) to enhance the imaging contrast. To further investigate the dispersion of ZrP nanoplatelets in the matrix, transmission electron microscopy (TEM) was carried out using a Tecnai F30 microscope with an accelerating voltage of 300 kV. For TEM observations, ultrathin films with about 70 nm in thickness were microtomed at room temperature using a Leica EM UC7 instrument equipped with a diamond knife. X-ray diffraction (XRD) analysis was conducted with Bruker D8 X-ray diffractometry (Cu Kα radiation, λ = 0.154 nm, 40 kV) to study the crystal structure and the dispersion of ZrP nanoplatelets.

The mechanical properties of the PA66/ZrP nanocomposites were investigated at room temperature using a universal tensile machine (Instron 2367) with a gauge length of 35 mm. The process of a tensile test can be divided into two stages. In the first stage, the crosshead speed of 2 mm/min was applied to obtain the tensile modulus using an extensometer at the strain of 0.05 to 0.25%. In the second stage, the crosshead speed was accelerated to 20 mm/min, and the final tensile strength and elongation at break were recorded. The mean values and standard deviations of each test were reported according to at least five samples.

Friction and wear tests were conducted under dry sliding using a pin-on-disc apparatus followed the procedure of ASTM G99. The cylindrical pin with a raw surface roughness of ~ 600 nm was rotated against the standard stainless disc (E52 100 steel) with a raw surface roughness of ~ 200 nm. The diagram of the wear pairs is presented in Fig. 1. Before each test, the standard stainless disc and cylindrical pins were cleaned ultrasonically with acetone. The wear tests were carried out under normal laboratory condition with the velocity of 0.2 to 0.6 m/s and the normal load of 10 N to 40 N, which provide low and high pv values (pressure × velocity). The total sliding distance was fixed at 2000 m. The coefficient of friction (COF) was calculated by the ratio of the tangential force (Fx) and normal force (Fz). The value of COF reported in this work was determined by the mean values of COF during the test range. The specific volume wear rate of a spherical-ended pin was calculated by the following equation:
$$ \mathrm{Wear}\ \mathrm{rate}\ \left({\mathrm{mm}}^3/\mathrm{Nm}\right)=\frac{\Delta V}{Ld}\times {10}^3 $$
(1)
where ∆V is the wear volume, which is calculated from the wear scar of the pin, L is the sliding distance, and d is the load. To ensure the accuracy of the experimental results, at least three tests for each sample were conducted, and the mean values of the COF and specific wear rate were reported.
Fig. 1

Schematic diagram of the pin-on-disc apparatus used in this study (unit: mm)

3 Results and discussion

3.1 Characterizations of ZrP nanoplatelets and PA66/ZrP nanocomposites

After a 24-h reaction in 3 M and 12 M H3PO4 at 200 °C, plate-like crystals with two different particle sizes were synthesized. The morphologies of the 3MZrP and 12MZrP nanoplatelets are shown in Fig. 2a, b. The 3MZrP nanoplatelets with ~ 230 nm in size have an obvious agglomeration characteristic owing to their relatively small size and high surface energy. In contrast, 12MZrP with an average size of ~ 1.33 μm shows a more regular hexagonal shape, demonstrating their better crystallinity formed at a higher phosphoric acid concentration [44], which is in agreement with the results of the literature reported previously [35, 44]. Table 2 summarizes the structure parameters of the two types of ZrP nanoplatelets measured and calculated from the SEM results, including particle size S, thickness t, and the aspect ratio ζ (S/t). The higher phosphoric acid will result in larger and thicker ZrP nanoplatelets, while the aspect ratio of pristine 3MZrP and 12MZrP are almost equal, which are ~ 6.2 and ~ 5.8, respectively. This characteristic makes our synthesized ZrP nanoplatelets an ideal model system for studying the size effect on the mechanical and tribological properties of the polymer nanocomposites containing 2D layered nanoplatelets.
Fig. 2

SEM images of a 3MZrP (the magnified view as seen in the square region) and b 12MZrP. c XRD patterns of 3MZrP and 12MZrP and d cartoon schematic of ZrP structure

Table 2

Structure parameters of the ZrP nanoplatelets after 24-h hydrothermal reaction with different phosphoric acid concentrations

\( {C}_{{\mathrm{H}}_3{\mathrm{PO}}_4} \) (M)

S (μm)

t (μm)

ζ

3

0.23 ± 0.11

0.037 ± 0.010

6.2 ± 1.3

12

1.33 ± 0.38

0.23 ± 0.09

5.8 ± 0.6

The structure of the nanoplatelets has also been studied by XRD. Figure 2c shows the XRD pattern results of the 3MZrP and 12MZrP nanoplatelets which are consistent with the literature reported [35, 44, 45]. As compared with 3MZrP, 12MZrP shows stronger and sharper characteristic diffraction peaks, which is attributed to the fact that 12MZrP, formed in a higher-concentration phosphoric acid, has a larger size and higher crystallinity [35]. The corresponding schematic diagram of the ZrP crystal structure is shown in Fig. 2d for a better illustration.

To investigate the dispersion of ZrP nanoplatelets in the PA66 matrices, SEM observations on the cryogenically fractured surfaces of the nanocomposite samples were conducted. Figure 3 shows the typical morphologies and microstructures of the PA66/ZrP nanocomposites. The arrows and rings refer to the ZrP nanoplatelets as shown in Fig. 3. As observed in Fig. 3a, c, e, the agglomeration of 3MZrP nanoplatelets can easily be observed in all the PA66/3MZrP nanocomposites, even at only 1 wt% loading. In addition, the interface debonding in between the ZrP agglomerations and between 3MZrP and the PA66 matrices can also easily be found. The poor dispersion and debonding phenomenon of 3MZrP are attributed to the poor compatibility between 3MZrP and PA66 and the easier agglomeration of the relatively small-sized nanomaterials. On the contrary, as seen from Fig. 3b, with the incorporation of 1 wt% 12MZrP, the nanoplatelets are quite uniformly dispersed in the PA66 matrix. As shown in Fig. 3d, the SEM image of PA66/3MZrP nanocomposites containing 3 wt% 12MZrP also exhibits evenly dispersed large nanoplatelets. However, with the addition of 5 wt% 12MZrP, slight aggregations of the 12MZrP nanoplatelets can be observed in Fig. 3f, which is usually inevitable in polymer nanocomposites with a high filler loading [26, 19, 46]. In addition, no interfacial debonding between ZrP nanoplatelets and PA66 matrices occurs for all the PA66/12MZrP nanocomposites. The relatively low specific surface area for the 12MZrP nanoplatelets results in a weak interaction among these nanoplatelets. Therefore, it is easier to disperse the large 12MZrP nanoplatelets in the PA66 matrix through melt blending than the small 3MZrP nanoplatelets. Other literatures have also proven that the nanofillers with relatively large sizes would be more easily dispersed as compared with the small ones [26]. The SEM images in Fig. 3 also demonstrate that both 3MZrP and 12MZrP have maintained their original layered structure.
Fig. 3

SEM images of a PA-1%(3MZrP), bPA-1%(12MZrP), c PA-3%(3MZrP), d PA-3%(12MZrP), e PA-5%(3MZrP), and f PA-5%(12MZrP). The yellow arrows and rings refer to the ZrP nanoplatelets

TEM has been utilized to further characterize the dispersion of ZrP nanoplatelets in the PA66 matrices. Figure 4 shows the TEM images of the PA66 nanocomposites containing 3 wt% of 3MZrP and 12MZrP. The TEM image of PA-3%(3MZrP), as presented in Fig. 4a, reveals that the 3MZrP nanoplatelets tend to aggregate, which is consistent with the SEM results in Fig. 3. The inset of Fig. 4a, showing typical 3MZrP aggregates in the nanocomposites, illustrates the weak interfacial bonding between 3MZrP and the matrix and the cavities within the aggregates. Figure 4b shows the TEM image of PA-3%(12MZrP), in which the large-sized ZrP nanoplatelets are found with uniform dispersion, leading to a good interfacial bonding between the filler and the matrix.
Fig. 4

TEM images of a PA-3%(3MZrP) and b PA-3%(12MZrP). The insets in a and b are high-magnification TEM images of the 3MZrP aggregates and dispersed 12MZrP in the matrices, respectively. The yellow arrows in the inset of a refer to the weak interface bonding and cavities inside the aggregates. The circular areas in the background of both TEM images are caused by the copper meshes used for the TEM imaging

The structure and exfoliation of ZrP nanoplatelets can also be detected by XRD. Figure 5 shows the XRD patterns of the pure PA66 and the PA66/ZrP nanocomposites. The XRD patterns for the PA66/3MZrP nanocomposites exhibit a relatively broad diffraction peak at 11.7°, which is consistent with the XRD results of 3MZrP nanoplatelets. All the PA66/12MZrP nanocomposites show a strong diffraction peak at 11.7°, illustrating that those ZrP nanoplatelets in the PA66 matrices maintain their original structure. The SEM, TEM, and XRD results illustrate that the layered ZrP nanoplatelets cannot, as expected, be exfoliated in the PA66 matrix through a simple melt mixing process without using any surfactants.
Fig. 5

XRD patterns of PA66 and PA66/ZrP nanocomposites

3.2 Tensile properties and reinforcing mechanisms

Typical stress-strain curves of the pure PA66 and the PA66/ZrP nanocomposites are shown in Fig. 6. Apparent yield and post-yield characteristics can be observed for the pure PA66 and the PA66/12MZrP nanocomposites, while the PA66 nanocomposites containing 3MZrP show no yield phenomenon. This characteristic indicates that the PA66/12MZrP nanocomposites maintain the same toughness as the matrix material, while the toughness of PA66/3MZrP exhibits a significant decrease.
Fig. 6

Typical stress-strain curves for the PA66 and PA66/ZrP nanocomposites

Figure 7 shows the tensile modulus and strength of the pure PA66 and PA66 nanocomposites, and the detailed tensile results are summarized in Table 3. As seen in Fig. 7a, the tensile modulus of the PA66/3MZrP nanocomposites increases with the filler content. With the incorporation of 1 wt% 3MZrP nanoplatelets, the corresponding nanocomposites show a ~ 5% increase in tensile modulus from 3.9 GPa for the pure PA66 to 4.1 GPa for PA-1%(3MZrP). When the concentration of 3MZrP increases to 5 wt%, the tensile modulus reaches the maximum value of ~ 4.9 GPa, which is ~ 26% higher than that of the pure PA66. Incorporation of 12MZrP in the PA66 matrix also exhibits a significant increase in tensile modulus. The nanocomposites containing only 1 wt% 12MZrP show a ~ 10% increase in tensile modulus as compared with the pure PA66. With the incorporation of 3 wt% 12MZrP, the nanocomposites show a ~ 10% increase in tensile modulus. Addition of 5 wt% 12MZrP shows a modulus increase by ~ 21%. The significantly increased modulus in polymer nanocomposites also can be found by using clay particles with a filler concentration higher than 5 wt% [47]. It is also interesting to note that the modulus discrepancy between the PA66/3MZrP and PA66/12MZrP nanocomposites at the same loading is very minimal. Similar results have also been reported in other literatures [48, 49, 50]. Two possible reasons may account for the above phenomenon. First, it has been demonstrated that the effect of the filler concentration on the modulus of the polymeric matrix is more sensitive than the particle size effect [51]. Therefore, the modulus of the polymer nanocomposites, though independent on the filler size, should increase to some extent with the increase of the filler content. Second, the excellent dispersion of 12MZrP in the PA66 matrix could reduce the negative effects resulting from its large size, such as relatively low specific surface area and aspect ratio, as compared with 3MZrP.
Fig. 7

Tensile properties of the pure PA66 and the PA66/ZrP nanocomposites: a tensile modulus and b tensile strength

Table 3

Tensile properties of the pure PA66 and the PA66/ZrP nanocomposites

Samples

Elastic modulus (GPa)

Tensile strength (MPa)

Elongation at break (%)

PA66

3.9 ± 0.1

72.7 ± 0.3

28.6 ± 8.5

PA-1%(3MZrP)

4.1 ± 0.2

61.8 ± 9.0

4.3 ± 5.1

PA-1%(12MZrP)

4.3 ± 0.1

76.2 ± 2.3

16.7 ± 2.2

PA-3%(3MZrP)

4.2 ± 0.1

45.3 ± 0.8

1.6 ± 0.1

PA-3%(12MZrP)

4.3 ± 0.1

82.8 ± 1.2

11.4 ± 2.1

PA-5%(3MZrP)

4.9 ± 0.2

48.5 ± 3.8

1.6 ± 0.2

PA-5%(12MZrP)

4.7 ± 0.2

76.0 ± 1.5

12.7 ± 4.6

Before reaching the yield point, the PA66/3MZrP tensile bars broke shown in Fig. 6. Therefore, the fracture strength equals to the tensile strength for all the PA66/3MZrP nanocomposites. Figure 7b shows the tensile strength of both the PA66/3MZrP and PA66/12MZrP nanocomposites. It can be seen that the incorporation of 3MZrP into PA66 will result in a significant decrease in tensile strength. The tensile strength of PA-1%(3MZrP) shows a ~ 16% decrease as compared with that of the pure PA66 (72.7 MPa). With the further increase of the 3MZrP content, the tensile strength of PA-3%(3MZrP) and PA-5%(3MZrP) even exhibits a decrease of ~ 38% and ~ 33%, respectively, which is much lower than that of the pure PA66. On the contrary, the tensile strength for PA66/12MZrP nanocomposites shows a significant increase as compared with the pure PA66. As for the PA66 nanocomposites containing 1 wt% 12MZrP, the tensile strength shows an increase by ~ 5%. With the incorporation of 3 wt% 12MZrP, the tensile strength shows a maximum increase of ~ 14% as compared with the pure PA66. However, with the further increase of the 12MZrP content to 5 wt%, the tensile strength shows a slight decrease but still ~ 5% higher than that of the pure PA66. The discrepancy in tensile strength between the PA66/3MZrP and PA66/12MZrP nanocomposites is significant. This is because for the tensile strength measurement, there is a relatively larger displacement and load as compared with the tensile modulus measurement, which usually results in a microstructure debonding between fillers and matrices. Therefore, the tensile strength is very sensitive to the strength of interface bonding and the amount of defects in the matrix [51]. As for the pristine 3MZrP, the aggregations in the PA66 matrix are severe as seen in the SEM and TEM results, leading to a much higher density of defects and poorer adhesion between 3MZrP and the PA66 matrix, which, in turn, results in a lower tensile strength of the PA66/3MZrP nanocomposites. Compared with 3MZrP, the 12MZrP nanoplatelets show a uniform dispersion in the PA66 matrices, resulting in a relatively lower density of defects and higher adhesion strength between 12MZrP nanoplatelets and the matrices, thus increasing the tensile strength of the corresponding nanocomposites. By comparing the tensile results shown in Fig. 7 and Table 3, it can be concluded that the 12MZrP nanoplatelets can enhance the tensile modulus and strength of PA66 more effectively than 3MZrP mainly due to the better dispersion of the larger nanoplatelets in the PA66 matrix.

3.3 Friction and anti-wear properties

3.3.1 Low pv condition

The products of loading pressure (p) and velocity (v) represent different service conditions (pv conditions) of the friction parts. To investigate the tribological properties of the PA66/ZrP nanocomposites at different service conditions, low and high pv conditions were adopted in this study. Figure 8a shows the COF of the pure PA66 and the PA66/ZrP nanocomposites at the low pv condition with a load of 10 N and a sliding velocity of 0.2 m/s. The PA66 nanocomposites containing 1 wt% 3MZrP show a slight decrease in COF from ~ 0.45 to ~ 0.43. With the incorporation of 3 wt% 3MZrP, the COF of the corresponding nanocomposites reaches the lowest value of ~ 0.41, but only ~ 9% decreases as compared with the pure PA66. However, the increase of the 3MZrP content to 5 wt% leads to a slight increase in the COF of the PA66 matrix to ~ 0.47. As compared with 3MZrP, 12MZrP can reduce the COF of the PA66 matrix more effectively. The PA66 nanocomposites containing 1 wt% 12MZrP show a decrease in the COF to ~ 0.42, which is ~ 6% lower than that of the pure PA66. With the incorporation of 3 wt% 12MZrP, the corresponding nanocomposites reach the lowest COF of ~ 0.33, which is ~ 27% lower than that of the pure PA66. With the further increase of the filler content to 5 wt%, the PA66/12MZrP nanocomposites also show a slight increase in the COF of the pure PA66 from ~ 0.45 to ~ 0.46.
Fig. 8

a The COF and b wear rate of PA66/ZrP nanocomposites at a load of 10 N and a velocity of 0.2 m/s

The reduction of the COF by the incorporation of nanomaterials in polymers can be mainly attributed to the filler effects. Zhang et al. [52] prepared nano-TiO2-based epoxy nanocomposites and investigated their tribological properties. They found that the detached nano-TiO2 during the process of friction may act as a lubricant to reduce the COF by the nanoparticle rolling effect. Ali et al. [24] investigated the tribological properties of different clay-filled UHMWPE nanocomposites. They found that the nanocomposites with enhanced mechanical properties and uniform filler dispersion have lower COF. Similarly, in our study, ZrP nanoplatelets may also work as an effective friction modifier in the PA66 matrix [21]. First, during the sliding process of the PA66 nanocomposites, the incorporated ZrP nanoplatelets can be detached from the PA66 matrices and act as an effective lubricant between the friction surfaces. Second, the increase in the matrix modulus and strength, also caused by the embedded ZrP nanoplatelets, is beneficial to resist the physical deformation of the PA66 matrices and to improve their capacity of bearing loadings. However, excessive ZrP nanoplatelets in PA66, such as the case of 5 wt% ZrP concentration, form large-scale agglomerates which lower the mechanical performance and hinder the sliding movements and thus increase the COF of the nanocomposites.

Figure 8b shows the wear rate of the pure PA66 and the PA66/ZrP nanocomposites under the low pv condition. Different from the COF, the wear rate of PA66/ZrP nanocomposites is more sensitive to changes in the filler content. Similar results have been found in other PA66 composite systems [53]. The lowest wear rate for the PA66/3MZrP nanocomposites is achieved at low 3MZrP contents, such as 1 wt% and 3 wt%. However, the wear rate of the PA66 nanocomposites increases dramatically with the addition of 5 wt% 3MZrP, which is even larger than that of the pure PA66. In the case of 12MZrP, the incorporation of only 1 wt% 12MZrP can reduce the wear rate of PA66 by ~ 37% and the wear rate reaches the lowest value at 3 wt% loading of 12MZrP, which is ~ 46% lower than that of the pure PA66. However, with the incorporation of 5 wt% 12MZrP, the wear rate of the PA66 nanocomposites increases slightly but is still significantly lower than that of the pure PA66 and the PA66 nanocomposites containing 5 wt% of 3MZrP.

There are several factors influencing the wear rate of the PA66/ZrP nanocomposites under the low pv condition. First, ZrP could act as a solid lubricant in between the friction surfaces, thus reducing the wear rate of the nanocomposites. Second, the increase of the modulus and strength of the PA66 matrices by the incorporation of ZrP is beneficial to improve the wear resistance of the nanocomposites. It can be found that the PA66/12MZrP nanocomposites exhibit more significant improvement in mechanical performance compared with the PA66/3MZrP nanocomposites, thus yielding the better anti-wear properties of the PA66/12MZrP nanocomposites. The importance of excellent mechanical performance for higher anti-wear properties has also been found in other PA6 and PA66 composite systems [25, 54, 55]. Third, the presence of hard ZrP nanoplatelets in between the rubbing interface, causing two-body or three-body abrasion, may cause the increase of the wear rate [56]. In this study, the 3MZrP nanoplatelets would be easily detached from the PA66 matrix because of the easily aggregated characteristic in the PA66 matrix, which would result in a more serious nanofiller-induced abrasive wear than the 12MZrP at the same filler content, leading to a relatively higher wear rate than the large 12MZrP nanoplatelets. Zhang et al. [52] studied the tribological properties of epoxy-based nanocomposites. They also found that TiO2 nanoparticles not only act as a lubricant to reduce wear rate but also increase the abrasive wear of the polymer matrix. In addition, the detached ZrP nanoplatelets will leave cavities and cracks in the PA66 matrix, thus yielding a relatively higher wear rate [56]. Therefore, based on the above analyses and discussions, it is plausible to conclude here that the PA66/12MZrP nanocomposites exhibit better anti-wear properties than the PA66/3MZrP nanocomposites under the low pv condition.

3.3.2 High pv condition

To investigate the tribological performance of the PA66/ZrP nanocomposites under more stringent environments, a high pv test condition with the applied load of 40 N and the sliding velocity of 0.6 m/s was adopted. Figure 9a shows the COF of PA66 and PA66/ZrP nanocomposites. As compared with the pure PA66, the COF of the PA66/3MZrP nanocomposites do not show a significant change with the increase of the concentration of 3MZrP. The lowest average COF of the PA66/3MZrP nanocomposites is achieved by adding 3 wt% of 3MZrP (~ 0.42), which is lower than that of the pure PA66 (~ 0.51) by ~ 18%. As for the PA66/12MZrP nanocomposites, the COF reaches the minimum at 1 wt% of 12MZrP (~ 0.29), which shows a large decrease by ~ 43% as compared with that of PA66. With the further increase of the 12MZrP content, the COF increases to ~ 0.41 and ~ 0.49 for 3 wt% and 5 wt% of 12MZrP, respectively, but is still a little bit lower than that of the pure PA66. The above results suggest that the larger ZrP nanoplatelets are more effective in reducing the COF of the PA66/ZrP nanocomposites than the relatively smaller ones used in the current study, which may be due to the better dispersion and more enhanced mechanical properties for the PA66 matrices by using 12MZrP.
Fig. 9

a The COF and b wear rate of PA66/ZrP nanocomposites at a load of 40 N and a velocity of 0.6 m/s

Figure 9b shows the wear rate of PA66 and PA66/ZrP nanocomposites under the high pv condition. The specific wear rate of PA66 and PA66/ZrP nanocomposites under the high pv condition is lower than under the low pv condition (Fig. 8b). The phenomenon that the specific wear rate decreases with the increase of the value of pv has also been found in PA66/glass fiber composites by Cartledge and Baillie [57]. The addition of 1 wt% 3MZrP results in a wear rate decrease by ~ 46% from 7.50 × 10−5 to 4.08 × 10−5 mm3/NM. With the further increase of the content of 3MZrP to 3 wt% and 5 wt%, the wear rate of the PA66/3MZrP nanocomposites increases slightly but is still much lower than that of the pure PA66. As for 12MZrP, the wear rate of the PA66/12MZrP nanocomposites also increases with the filler content. The lowest wear rate is obtained by adding 1 wt% of 12MZrP, which is ~ 59% lower than that of the pure PA66. It is important to point out that the reduced wear rate of 1 wt% 12MZrP/PA66 nanocomposites is better than montmorillonite (MMT)–filled PA6 nanocomposites [23], nano-TiO2/PA66 nanocomposites [58], talc/PA6 composites [53], and glass fiber/PA6 composites at the same or slightly higher filler content [53, 59].

Based on the above wear test results, it can be found that the pv conditions have a significant effect on tribological properties of the PA66/ZrP nanocomposites. Under the low pv condition, both the pure PA66 and the PA66/ZrP nanocomposites can resist the external applied load during friction. Therefore, the wear resistance of the PA66/ZrP nanocomposites is not greatly improved. In contrast, at the high pv condition, the pure PA66 cannot effectively resist the pressure generated during friction because of its relatively low mechanical performance, and thus the advantages of the PA66/ZrP nanocomposites are more profound. On the other hand, it can be also found that when the test condition varies from low pv to high pv condition, the PA66/ZrP nanocomposites show the lowest wear rate at the filler content of 3 wt% and 1 wt%, respectively. This may be because of the fact that under the high pv condition, the hard ZrP caused abrasive wear and the agglomerated ZrP-induced cracks are more serious than those under the low pv condition. The abrasive wear effects and the crack density would increase with the content of ZrP nanoplatelets. Therefore, under the high pv condition, the best wear resistance is obtained at the ZrP content of 1 wt%. On the contrary, the abrasive wear effects and the wear surface damages under the low pv condition are relatively mild. In this case, the mechanical performance rather than the filler content may play as a critical factor influencing the anti-wear properties of the PA66/ZrP nanocomposites, and thus the nanocomposites containing 3 wt% ZrP nanoplatelets show the lowest wear rate. Therefore, it can be concluded that the tribological properties of the PA66/ZrP nanocomposites strongly depend on the size and dispersion of ZrP, the mechanical performance, the test conditions, and so on [25].

3.3.3 Wear mechanisms

Figure 10 shows the worn surfaces of the PA66 and the PA66/ZrP nanocomposites at a load of 10 N and a velocity of 0.2 m/s (the low pv condition). As can be seen in Fig. 10a, some pits, grooves, and plastic deformation appeared on the worn surface of the pure PA66, indicating the poor wear resistance for the unfilled polymer matrix. The adhesion and abrasive wear are the main wear mechanisms for the pure PA66. As seen from Fig. 10b, the worn surface of PA66 nanocomposites containing 1 wt% 3MZrP exhibits a relatively smooth surface with negligible grooves mainly due to the enhanced modulus of the PA66 matrix and the lubrication effect of 3MZrP. However, with the increase of the 3MZrP concentration to 3 wt%, the worn surface displays apparent cracks as observed in Fig. 10d, which may be induced by the aggregation of 3MZrP. These cracks will grow and propagate as a result of repeated friction. Eventually, the matrix material surrounded by the cracks will be lost. This process is also known as fatigue wear. In this case, the abrasive and fatigue wears are dominant. With the further increase of the 3MZrP to 5 wt%, a large amount of detached 3MZrP appears on the worn surface, demonstrating the poor combination strength between 3MZrP and PA66, which is also consistent with the SEM and TEM observations from Figs. 3 and 4. These detached 3MZrP, as the hard phase, may increase the abrasive wear. Therefore, the main wear forms are abrasive and fatigue wears. Compared with the PA66/3MZrP nanocomposites, the PA66/12MZrP nanocomposites show a smoother worn surface without the appearance of microcracks. As shown in Fig. 10c, the worn surface of PA-1%(12MZrP) exhibits a smooth surface with shallow scratches. The abrasive wear is the dominant wear form. The worn surface of PA-3%(12MZrP) shows lots of dots with a diameter of about 1 μm. These dots may be the 12MZrP nanoplatelets, which are embedded below the subsurface of the PA6 matrix, which may help the nanocomposites resist the wear damage. With the increase of the 12MZrP to 5 wt%, the apparent scratches appear, indicating the more serious abrasive wear. It can be found that the worn surface characteristics of the PA66/ZrP nanocomposites are consistent with the wear test results under the low pv condition.
Fig. 10

SEM images of the PA66/ZrP nanocomposite pin surfaces after testing under 10 N and 0.2 m/s: a PA66, b PA-1%(3MZrP), c PA-1%(12MZrP), d PA-3%(3MZrP), e PA-3%(12MZrP), f PA-5%(3MZrP), and g PA-5%(12MZrP)

The worn surfaces of PA66 and PA66 nanocomposites under the load of 40 N and velocity of 0.6 m/s (the high pv condition) are shown in Fig. 11. As seen from Fig. 11a, an apparent melting phenomenon appears on the worn surface of the pure PA66, which is more obvious than the pure PA66 testing under the low pv condition. Therefore, the adhesion wear is the main wear form for the pure PA66 under the high pv condition. This may be due to the fact that at a high pv testing condition, a large amount of heat caused by friction will soften the PA66 matrix and increase adhesion wear. Meanwhile, microcracks can be also easily observed on the PA66 matrix. Figure 11b and d show the worn surface of PA-1%(3MZrP) and PA-3%(3MZrP), respectively, with the obvious adhesion marks and microcracks, indicating that the main wear form for these two samples under the high pv condition is adhesion and fatigue wear. However, with the increase of the 3MZrP content to 5 wt%, as seen from Fig. 11f, the worn surface almost exhibits no signs of adhesion phenomenon. The grooves and microcracks are easily found in the worn surface of PA-5%(3MZrP), suggesting the abrasive wear being the dominant wear mechanism. The increased abrasive effects may be attributed to the improved stiffness of the matrix and the appearance of a large amount of rigid 3MZrP between the wear surfaces. In contrast, as can be seen from Fig. 11c, e, g, the worn surfaces of the PA66/12MZrP nanocomposites are much smoother without any microcracks as compared with the PA66 and the PA66/3MZrP nanocomposites. The apparent grooves and cloud-like fold appeared on the worn surface of PA-1%(12MZrP) and PA-3%(12MZrP), suggesting both the abrasive and adhesion wears being the main wear form. With the increase of the 12MZrP to 5 wt%, the grooves are more obvious and deeper without adhesion wear characteristic, indicating that the abrasive effect is enhanced and dominant. It can be concluded that under the high pv condition, a low content of ZrP can effectively enhance the mechanical properties and act as a lubricant, resulting in a good anti-wear performance. However, with the increase of the filler content, the detached ZrP nanoplatelets may act as rigid particles embedded in the worn surface or between the frictional couple leading to the enhanced abrasive effect. As a result, the higher the content of rigid nanoplatelets in the matrix is, the more serious the abrasive wear appears.
Fig. 11

SEM images of the PA66/ZrP nanocomposite pin surfaces after testing under 40 N and 0.6 m/s: a PA66, b PA-1%(3MZrP), c PA-1%(12MZrP), d PA-3%(3MZrP), e PA-3%(12MZrP), f PA-5%(3MZrP), and g PA-5%(12MZrP)

The wear mechanisms for the PA66/3MZrP and PA66/12MZrP nanocomposites are also schematically compared in Fig. 12. The PA66/3MZrP nanocomposites tend to appear cavities, microcracks, and excessive remained 3MZrP nanoplatelets during the sliding process. This is because that 3MZrP is poorly dispersed in the matrix, and thus microcracks are easily formed between the agglomerated particles, also resulting in the detachment of the 3MZrP nanoparticles to form cavities. At the same time, the tensile strength of PA66/3MZrP nanocomposites is even lower than that of the pure PA66, which also leads to poor wear resistance. Dasari et al. [25] also found that the high bonding strength between the PA6 matrix and clay is extremely important for the reinforcement of the wear resistance. On the contrary, as shown in Fig. 12b for PA66/12MZrP nanocomposites, the worn surface generates less microcracks and fewer cavities during the sliding process, while less 12MZrP nanoplatelets are stripped. In this case, the lubrication of detached 12MZrP nanoplatelets would be dominant and compensate the abrasive wear effect of the hard 12MZrP nanoplatelets. Moreover, the higher tensile strength, the uniform-dispersed nanoplatelets, and the stronger bonding between the filler and the PA66 matrix also contribute to the higher anti-wear properties of the PA66/12MZrP nanocomposites.
Fig. 12

Schematic of the wear mechanisms of a the polymer nanocomposites containing smaller nanofillers and b the polymer nanocomposites containing larger nanofillers

The results of the tensile and wear tests show that ZrP, as a typical 2D nanomaterial, not only can significantly improve the mechanical properties of PA66 but also yield excellent anti-wear performances at a relatively low content without using any organic surfactants. In this study, it also has been found that the large-sized 12MZrP nanoplatelets exhibit the better reinforcement effect on mechanical and tribological performance of the PA66 than small-sized 3MZrP nanoplatelets due to the strong aggregation in 3MZrP systems. The incorporation of smaller-sized nanofillers does not necessarily lead to the better performance of the corresponding nanocomposites, especially when the bonding between the matrix and the nanofiller is weak. Generally, organic modification would increase the combination and dispersion of the smaller-sized inorganic nanofillers, thus resulting in a higher reinforcement effect. However, a large number of low-molecular-weight modifiers are usually unfavorable to the reinforcement effect and thermal stability of the corresponding composites [25]. In addition, the surface modification method is more complex and expensive. Therefore, the pristine 2D nanofillers with relatively large particle sizes are more suitable for mass production of high-performance nanocomposites with ease and low cost.

4 Conclusion

The mechanical and tribological properties of PA66 nanocomposites containing different sizes and concentrations of 2D layered ZrP nanoplatelets are systematically investigated. 3MZrP and 12MZrP with the size of ~ 230 nm and ~ 1.33 μm, respectively, are synthesized by a hydrothermal method through controlling the concentration of phosphoric acid. The SEM and TEM images show that the dispersion of 12MZrP in the PA66 matrix is more uniform than the smaller 3MZrP without using any surfactants. The XRD results demonstrate that ZrP nanoplatelets still maintain the original structure in the PA66 matrix through a melting process. Both the tensile and wear test results suggest that the PA66 nanocomposites containing the large-sized 12MZrP nanoplatelets would yield a better mechanical and anti-wear performance than the small-sized 3MZrP. The optimal tensile performance occurs with the addition of 3 wt% 12MZrP into PA66, resulting in the increase of tensile modulus and strength by ~ 10% and ~ 14%, respectively. The incorporation of 3MZrP will also result in an increase in tensile modulus, while the tensile strength of PA66 will significantly decrease because of the severe aggregation of 3MZrP. Under the load of 40 N and velocity of 0.6 m/s, the optimal anti-wear properties are obtained by adding 1 wt% 12MZrP, resulting in a ~ 43% decrease in COF and a ~ 59% reduction in the wear rate as compared with the pure PA66. From the observations on the worn surfaces, it can be speculated that the detached ZrP nanoplatelets from the matrix not only act as lubricant but also increase the abrasive wear effect. In addition, it can be also found that the high pv condition leads to the higher adhesion wear, and the high ZrP content would increase the abrasive wear of the PA66/ZrP nanocomposites. Our current studies suggest that high-performance PA66 nanocomposites with highly reinforced mechanical properties and significantly improved friction and anti-wear behaviors can be achieved by using pristine 2D layered nanoplatelets with a relatively large size at low contents.

Notes

Funding information

This work was supported by the start-up funding from Southern University of Science and Technology (SUSTech), “The Recruitment Program of Global Youth Experts of China,” and the Foundation of Shenzhen Science and Technology Innovation Committee (Grant No.: JCYJ20170817110440310, KQJSCX20170726145415637, and JCYJ20160315164631204).

Compliance with ethical standards

Ethical statement

We are pleased to submit the enclosed manuscript titled “Enhanced mechanical and tribological performance of PA66 nanocomposites containing 2D layered α-zirconium phosphate nanoplatelets with different sizes,” which we wish to be considered for publication in Advanced Composites and Hybrid Materials. These data have not been published before and are not under consideration for publication elsewhere.

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval and informed consent

Since this paper does not involve any living creatures or humans, our paper conforms to the rules of the ethical approval and informed consent for this journal.

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Haoyang Sun
    • 1
  • Zhengqin Fang
    • 1
  • Tao Li
    • 1
  • Fan Lei
    • 1
  • Feng Jiang
    • 1
  • Dandan Li
    • 1
  • Yinghao Zhou
    • 1
  • Dazhi Sun
    • 1
    Email author
  1. 1.Department of Materials Science and EngineeringSouthern University of Science and TechnologyShenzhenChina

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