Tribological and mechanical properties of graphene nanoplatelet/PEEK composites

TLDR: In this article, a 3-phase model of the polymer matrix structure is interpreted in terms of a 3 phase model, in which the crystalline phase fluctuates from 39 to 34% upon GNP addition, and the Raman G band shows a progressive increment proportional to the bulk GNP percentage.

About: This article is published in Carbon.The article was published on 2019-01-01 and is currently open access. It has received 118 citations till now. The article focuses on the topics: Peek & Hardness.

Figures

FIGURE 10. SEM images of the neat PEEK specimen: a) worn surfaces into the wear track after 48 h of wear process; b) a worn surface into the wear track after 48 h of wear process; and c) a zone far from the bearing contact.

FIGURE 2. XRD patterns for the neat PEEK (green) and GNP10/PEEK (black) composite. Dashed line is a plausible background of the amorphous polymer component.

FIGURE 1. Raman spectra of PEEK, GNPx/PEEK composites and GNP powder: a) baseline corrected spectra, normalized to the most intense peak, and b) insight of the C=O and C=C stretching region of PEEK.

FIGURE 11. SEM images of the surface of the tribometer Al2O3 ball counterpart: a) before any wear process, b) after the wear process on neat PEEK, and c) after the wear process on GNP10/PEEK.

FIGURE 6. SEM images of cryogenic fracture surfaces at low magnification (a-e) and high magnification (f-m) for various samples: neat PEEK (a, f, k), GNP1/PEEK (b, g), GNP3/PEEK (c, h), GNP5/PEEK (d, i, l), and GNP10/PEEK (e, j, m).

TABLE 2. Mechanical parameters obtained by uniaxial tensile stress: E = Young’s modulus; σTS = ultimate tensile strength; ε

Frequently asked questions

Q1. What are the causes of permanent deformations in pure PEEK?

Plastic mechanisms associated with crystalline deformation via rotation, shearingand fragmentation of lamellae, and with viscous flow in the amorphous phase, are the causes of permanent deformations in pure PEEK [39, 63]. 

Q2. What is the main mechanism of fatigue wear?

The combination of high hardness and low toughness favors fatigue wear as the predominant mechanism, with generation of subsurface cracks and scales. 

Q3. Why does the transfer stage not happen in the present work?

in the present work, probably due to water lubrication, the transfer stage does not happen and no adhered material can be observed on the ceramic counterpart. 

Q4. What is the effect of the introduction of GNPs on the tensile strength?

the introduction of GNPs produced a strong decrease in the elongation at break, which drastically affects the toughness (work of fracture). 

Q5. How much graphene did Liu et al. use in their experiments?

In carbon fiber/PEEK laminates, Liu et al. [55] observed that 0.7 wt. % of graphene produced a reduction in the COF from 0.48 to 0.37. 

Q6. What is the reason for the decrease in the work of fracture?

In the literature, the decrease in the work of fracture has been associated to thepresence of filler aggregates that act as stress raisers and lead to an early fracture. 

Q7. What is the effect of the presence of a rigid phase in semi-crystalline polymers?

It has also been shown that the presence of a rigid amorphous phase in semi-crystalline polymers might lower the yield stress and produce embrittlement [47, 62]. 

Q8. How much thermal conductivity improvement has been reported for graphene?

In addition, considerable thermal conductivity improvements between 20 - 70% have been reported for different polymer matrices (epoxy, PBT, PPS, silicone, PE, PP, PVA and PVDF) using graphene fillers [53]. 

Q9. How much of the amorphous phase in the glass transition is expected?

If the 53-55% of amorphous phase content from WAXD (Section 3.2) contributed to the glass transition, the expected ∆Cp would be around 0.14-0.15 J/g·K. 

Q10. What is the common method of reinforcing the polymer?

The most common method is based on reinforcing the polymer with different fillers, whose composition, size, concentration, orientation and shape are specific for the required property. 

Q11. What are some of the methods used to make PEEK?

Others are based on industrially unusual manufacturing processes, such as low-temperature sintering methods [20], or prepare complex hybrid composites with PEEK, short carbon fibers (SCFs) and PTFE [22]. 

Q12. What was the effect of assembling three small pieces to form the testing sample?

The effect of assembling three small pieces to form the testing sample was checked with poly(methyl methacrylate) (PMMA) and only a 5% increase was obtained in comparison with the measurement on a fully solid sample with the same thickness. 

Q13. What is the effect of GNPs on the deformation of fracture?

the presence of GNPs affects the deformation of fracture through a decrease in the number of deformation modes, even at the lowest graphene loadings. 

Q14. What is the effect of the decrease in the degree of crystallinity on the composite?

the decrease in the degree of crystallinity, as it is shown in Figure 4, might contribute to the degradation of heat transfer through the composite. 

Q15. What is the plot of the coefficient of friction (COF) vs. the sliding?

The plot of the coefficient of friction (COF) vs. the sliding distance reaches, afterthe running regime, a plateau for all the samples. 

Q16. What is the effect of the GNP fillers on the wear rate of graphene?

As a consequence, the increase observed in the tensile-flexural modulus and hardness with the increase in the GNP concentration seems to be related to the intrinsic mechanical properties of graphene. 

Q17. What is the trend in graphene/polymer composites?

The trend has been confirmed by experimental results in another graphene/polymer system [65], and also by a recent molecular simulation for graphene/polymer composites that predicts a 35% decrease in the indentation depth [66]. 

Q18. What is the effect of GNPs on the hardness of graphene?

the positive influence of GNPs in PEEK hardness does not appear in Kalin et al. [20], which observed a reduction of 20-25% for a composite with 2 wt. % of GNPs.