Preparation of polyimide-graphite composite and evaluation of its friction behavior at elevated temperatures

2.1 Materials

Polyamide acid (PAA) (10 wt%) was provided by the Hubei Institute of Chemistry (Wuhan, China). GR powder was purchased from Dengfeng Chemicals Company Limited (Tianjin, China). GR powder was sieved using a 200-mesh sieve (particle size <75 μm). All other chemical reagents were used as received without further purification.

2.2 Preparation of the PG composite

The typical procedure for preparing PG composite was as follows. First, 0.2 g of GR powder was added into 2 g of PAA solution with stirring for 30 min. Then the mixture of PAA-GR dispersion was cast onto a glass slide and then gradually heated to 320°C in a furnace for 2 h to allow imidization of PAA and to generate PG composite (here the mass ratio of PI to GR was 1:1).

By contrast, PI was also prepared with the same preparation processes as that of PG composite except that no GR powder was introduced.

2.3 Characterization of GR, PI, and PG composite

Phase analysis of samples was performed by XRD (X’Pert PRO, PANalytical B.V., Holland) at a scan step size of 0.0167° in the scan range of 2θ=10°–35°. The chemical structure of samples was analyzed by FTIR (Equinox 55, Bruker Corporation, Germany) with a resolution of 0.4 cm^-1. Thermogravimetric and differential thermal analyses (TG/DTA, Diamond TG/DTA, PerkinElmer Instruments, USA) were conducted with a thermal analysis system in air at a heating rate of 5°C/min.

2.4 Friction test

Friction tests were conducted on an MG-2000B type high-temperature friction and wear tester produced by Xuanhuakehua testing machine manufacturing company limited (Zhangjiakou, China). A schematic diagram of the ring-on-ring friction and wear tester is shown in Figure 1. The friction tester is connected to a computer through a group of sensors so that the temperature inside the furnace, the sliding velocity, the load, and the friction torque can be automatically monitored and displayed on the monitor. The frictional pair comprises self-mated AISI-321 steel upper ring (inner diameter, 54 mm; outer diameter, 66 mm) and lower ring (inner diameter, 52 mm; outer diameter, 67 mm). Before the ring-on-ring friction tests, solid lubricants (GR, PI, and PG composite) were deposited on the lower steel ring. The deposition process of PI and PG composite is discussed in Section 2.2, where the substrate was AISI-321 steel. GR was deposited on the steel substrate as follows. First, the GR dispersion was prepared by homogeneously mixing GR powder, methylcellulose, and distilled water. Then the above-mentioned dispersion was cast onto the substrate and then gradually heated to 180°C in a furnace for 2 h.

Figure 1:

Schematic diagram of the ring-on-ring friction and wear tester.

The ring-on-ring friction tests were conducted under elevated temperatures of 300–600°C for a period of 5.2 min at a rotary velocity of 200 rpm, normal load ranging from 300 to 700 N. Friction coefficient is automatically provided by the computer, and the sliding time at which the friction coefficient sharply rises is recorded as the antiwear life of tested lubricating materials.

3.1 Characterizations of the as-received GR, PI, and PG composite by XRD

The XRD patterns of the as-received GR, PI, and PG composite are shown in Figures 2 and 3, respectively. Figure 2A shows that the as-received GR contained both virgin GR and Ga-sulfanilamide. GR is a kind of layered material characterized by covalent bonds between the carbon layers and the van der Waals interaction between successive carbon layers. This makes it feasible to obtain expandable GR (e.g., Ga-sulfanilamide), which can be doped by atoms or molecules between the carbon sheets [27, 28]. PI showed amorphous structure according to the XRD pattern (Figure 2B). Moreover, the PG composite showed a similar XRD pattern (Figure 3) to that of the as-received GR, but the peaks of Ga-sulfanilamide in PG composite shifted toward low diffraction angles compared with those in the as-received GR, which is possibly attributed to some kinds of interactions between the GR and the PI matrix [29]. Furthermore, we calculated the relative peak intensity and characteristic d-spacings of the two types of GR in the as-received GR and in PG composite according to the Scherra equation. The corresponding results are listed in Table 1. It shows that the lattice spacing of Ga-sulfanilamide in PG composite tended to increase to some extent compared with the lattice spacing of Ga-sulfanilamide in the as-received GR, and the maximum variation of 0.05 Å appeared around 2θ=12.4°. This illustrates that there must be some interaction between the as-received GR and the PI matrix, leading to changes in the microstructure of PG composite.

Figure 2:

XRD patterns of (A) the as-received GR and (B) PI.

Figure 3:

XRD patterns of the PG composite (line style: solid) and the as-received GR (line style: dot).

3.2 Thermal stability of PI and PG composite

The TGA/DTA curves of PI and PG composite are shown in Figure 4. It shows that PI and PG composite had the first stage decomposition temperature (T_d) of 550°C and 501°C, respectively. Here, PG composite showed a lower decomposition temperature than PI, which might be due to the interaction between GR and PI matrix [30, 31]. Besides, when the temperature was higher than 600°C, PI was almost completely decomposed and oxidized, whereas PG composite only lost 30% of its weight at 600°C. This illustrated that PG composite exhibits better thermal stability than PI as a result of the interaction between the GR and the PI matrix. This also can be confirmed by the FTIR spectra of PI and PG composite shown as follows.

Figure 4:

TGA/DTA curves of (A) PI and (B) PG composite.

Figure 5 shows the FTIR spectra of PI and PG composite, where the FTIR spectra of heat-treated PI and PG composite (500°C for 30 min; corresponding samples are denoted as PI-500 and PG-500) were also provided for comparisons. The peaks at 1350, 1500, 1720, and 1780 cm^-1 were assigned to the characteristic functional groups of PI [32], and they disappeared after heat treatment at 500°C. This means PI was completely decomposed after it was heated at 500°C for 30 min. In the meantime, the peak of PI at 1010 cm^-1 also disappeared after 30 min of heat treatment at 500°C, but PG composite retained this peak even after 30 min of heat treatment at 500°C. This implies that the PG composite exhibits better thermal stability than PI. As a result, as what can be expected and will be discussed later, changes was caused in the friction behavior of PG composite as compared with that of PI.

Figure 5:

FTIR spectra of the as-prepared PI and PG composite as well as heat-treated PI and PG composite.

3.3 Friction behavior of PI and PG composite

Figure 6 shows the effect of temperature on the friction coefficients of PI, GR, and PG composite at 300 N. It is seen that in the selected range of test temperatures, PI always exhibited the highest friction coefficient and the largest friction coefficient fluctuation among the three kinds of tested materials (the friction coefficient of PI at 600°C could not be measured because of its nearly complete pyrolysis at this temperature), whereas PG composite always had the lowest friction coefficient and the minimum friction coefficient fluctuation. Generally, as the temperature rises, the lubricants will tend to gradually fail owing to their decomposition and/or oxidation at elevated temperatures. As shown in Figure 6, at 300°C and 400°C, the lowest values of friction coefficients of PI were <0.08. However, its friction coefficient fluctuation at various temperatures was too large, and it almost failed completely at above 500°C, owing to its susceptibility to degradation and decomposition at above 500°C. In the whole range of test temperatures, PG composite exhibited good friction-reducing ability. Namely, its average friction coefficient in the temperature range of 300–500°C was <0.10, its minimum friction coefficient was always around 0.05, and its friction coefficient is lower than 0.13 even at 600°C. The friction coefficient of GR, regardless of test temperatures, is moderate and lies in between. This implies that GR and PI matrix seem to exhibit some kind of synergistic friction-reducing effect, which might be attributed to the interactions between the GR and the PI matrix.

Figure 6:

Friction coefficients of GR, PI, and PG composite at various temperatures under the applied load of 300 N.

Figure 7 shows the variation of the friction coefficients of PI, GR, and PG composite with sliding time at 400°C and applied loads of 300 N. The friction coefficients at 400°C were discussed as representative examples because the three kinds of tested materials exhibited the lowest friction coefficients at this temperature. PI exhibited lower friction coefficient than GR during the initial 2 min of sliding, but its friction coefficient began to increase after sliding for approximately 2.5 min, and it sharply increased after sliding for approximately 3.5 min, which is possibly due to accelerated pyrolysis of PI at extended sliding duration under elevated temperature (400°C). GR exhibits lower and more stable friction coefficient than PI, which is attributed to its inherent good solid lubricity. Advantageous over PI and GR, PG composite exhibits the lowest and most stable friction coefficient in the whole sliding duration. This means that PG composite may have integrated the advantages of both PI and GR, thereby showing the best friction-reducing abilities.

Figure 7:

Variation of the friction coefficients of GR, PI, and PG composite with sliding time at 400°C under the applied load of 300 N.

Figure 8 shows the variation of the friction coefficients of PI and PG composite with sliding time under the temperature of 400°C (A: 500 N applied load; B: 700 N applied load). GR lubricant failed too soon (several seconds) after sliding under 500 N and 700 N; thus, its friction coefficient-sliding time curve was not shown here. However, PI failed after sliding for approximately 1.75 min under 500 N, and it virtually failed very soon after sliding under 700 N. Interestingly, PG composite retained very low and had stable friction coefficient in the whole sliding process under both 500 N and 700 N, which means it possesses good load-carrying capacity as well as excellent friction-reducing ability and could be use as an advanced high-temperature solid lubricant. Generally, the friction coefficient is highly related to the contact stress, and the continuous sliding of the frictional pair would result in temperature increase in the sliding surfaces, thereby accelerating the thermal degradation and/or decomposition of polymer matrix even below the pyrolysis temperature. In addition, the greater the contact stress, the larger the friction force and friction-induced heat. As a result, the friction-reducing abilities of PI tend to decline with the increasing applied load. However, such a frcition-induced heat effect associated with rising normal load has insignificant influence on the tribological properties of PG composite because of its better thermal stability than PI as a result of interactions between the GR and the PI matrix. In other words, under elevated temperature, the PI matrix and the GR may undergo chemical reactions to generate new species that may increase the thermal stability of PG composite, thereby leading to reduced friction and increased friction-reducing ability as well as load-carrying capacity of PG composite.

Figure 8:

Variation of the friction coefficients of PI and PG composite with sliding time under the temperature of 400°C (A: 500 N applied load; B:700 N applied load).