Enhancing tribo-mechanical properties and thermal stability of nylon 6 by hexagonal boron nitride fillers

2.1 Materials

Nylon 6 granules of 3–8 mm size, molecular weight 10,000 g/mol, and density 1.13–1.15 g/cm³ were imported from GSFC, Gujarat, India, and selected as a matrix material. h-BN microparticles with the average particle size (APS) < 15 µm, molecular weight 24.82 g/mol, Young’s modulus 19.5 GPa, and density 2.29 g/cm³ were imported from nano research elements, USA, and selected as fillers. The morphology of h-BN particles observed by SEM is shown in Figure 1. h-BN fillers were found nearly of round shape as shown at ×1,700 magnification.

Figure 1

SEM observations of h-BN microparticles: (a) at ×400 magnifications, (b) at ×1,700 magnifications, and (c) size distribution.

Fourier-transform infrared spectroscopy (FTIR) confirms the composition and form of fillers. FTIR spectrum peaks were observed at 2,919, 2,850, 2,500, 1,485, 1,108, 873, 854, and 700 cm⁻¹ due to B–N, BN–O bonding, attributed to h-BN (Figure 2) and B–OH bonding, respectively (36,37,38,39,40).

Figure 2

FTIR spectrum of h-BN.

2.2 Sample preparation

h-BN microparticles and nylon 6 granules were compounded with the help of twin-screw-extruder with wide kneading block at 250 rpm, ZV 20 model, make M/S specific engineering, and automats. The 2, 4, and 8 wt% of h-BN were used to fabricate different batches of nylon 6 composite granules. The granules of nylons were dried in the oven at 70°C for 4 h before compounding. Barrel temperatures of twin-screw-extruder were adjusted at 190°C, 200°C, 220°C, 230°C, and 240°C for zone 1, 2, 3, 4, and 5, respectively, and die temperature was adjusted at 230°C during the process (41). A water bath was used to cool the hot semi-liquid polymer composite filament. Pelletizer was used at the other end which converted the continuous filament of composites into small pellets. After completion of the extrusion process, composite pellets were dried again at 70°C for 4 h in an oven for injection molding of the specimens. Wide kneading block and low RPMs were selected for enough residence time and better dispersion quality of fillers (42). After extrusion, the polymer composite granules were injection-molded with the help of an Endura 90 injection molding machine to fabricate mechanical and tribological test specimens.

2.3 Mechanical testing

Tensile strength of nylon 6 and nylon 6 composite samples at a 50 mm/min speed was measured with the help of a universal tensile testing machine (UTM) make Tinius Olsen India Pvt. Ltd, Model L-series H50KL, 5–50 KN force measurement capacity. Modulus of elasticity and % elongation of samples before fracture were also measured at the same time. All samples were tested according to the standard ASTM D 638. Type-I sample specifications were selected with a thickness of 3.2 mm. Testing speed was selected according to the sample specifications and ASTM standards for polyamides (43,44,45).

Rockwell hardness tester make Saroj engineering, model R60 with a maximum load capacity of 60 kg was used to measure the hardness of nylon 6 and its composites. The hardness of materials was measured by the indentation of a steel ball of diameter 12.7 mm. All tests of Rockwell hardness were carried out as stated by ASTM D 785 and measured directly on R-scale. Disks of diameter 100 mm × 3 mm were the sample specifications for the test.

Izod impact tester make Ceast, model Resil impactor, 0 to 25 J capacity was used to measure the impact strength or toughness of pure nylon 6 and its composites. A pendulum from a specific height is released down to hit a sample at the notch and the energy absorbed by a specimen is calculated from the top height of the pendulum to the bottom after hitting the sample. To identify the toughness of the materials, results were shown in energy lost per unit of the thickness (J/m) at the notch of a specimen. Tests were carried out according to ASTM D 256 and the dimensions of the specimens were 63.5 mm × 12.7 mm × 3.2 mm. All mechanical tests were carried out at room temperature of 23°C with relative humidity values 60–70%.

2.4 The friction coefficient and wear testing

Pin-on-disk apparatus make DUCOM Instruments Pvt. Ltd, Model TR-20LE-PHM-200 was used to measure wear and friction coefficient of materials. A pin-on-disc tribometer as shown in Figure 3 consists of a stationary pin that is loaded against a rotating steel disc. Tests were carried out at dry conditions, i.e., no external lubrication was used in the experimentations. About 30–33 mm length with 4.1 ± 0.1 mm diameter pins and 165 mm diameter of hardened EN31 steel plate with a thickness of 8 mm was used in the experimentation. All tests were carried out at room temperatures and according to the standard ASTM G99. The 150 mm track diameter was selected for all the experiments. Surface roughness (Ra-value) of pins at the contact point with steel plate was maintained at 0.4–0.6 Ra before mounting on the tribometer.

Figure 3

Pin-on-disc tribometer.

2.5 Abrasion resistance testing

Abrasion resistance of nylon 6 and its composites was measured against the H-18 abrasive wheel on the TABER abrader make TABER Industries, model rotary abrader. Abrasion resistance was noted in % material loss at 1,000 cycles and with 1.0 kg (9.81 N) load. A 100 mm diameter disk with a 3.2 mm average width was the sample specification for abrasion testing. All abrasion tests were performed according to the standard ASTM D 1242 and at room temperature.

2.6 Thermogravimetric analysis (TGA)

TGA testing of samples was carried out on the TGA apparatus make PerkinElmer, model Pyris-1-TGA of 1,000°C temperature capacity. TGA was done to identify the degradation temperatures of materials and the filler content present in the composite samples. All samples of different batches were in the form of granules and the tests were carried out according to the standard ASTM E 1131. The heat was applied from 50°C to 850°C at 20°C/min rate.

3.1 Tensile, hardness, and impact test

Average tensile strength at break, modulus of elasticity, and % elongation data were noted from the five samples tensile testing and plotted as shown in Figure 4. Average tensile strength of 52.7, 52.8, 54.4, and 60.7 N/mm² was noticed for nylon 6 and 2, 4, and 8 wt% h-BN-reinforced nylon 6 composites, respectively. Nylon 6’s average % elongation was 266, while 2, 4, and 8 wt% composites were having 306%, 312%, and 328% elongation, respectively. Modulus of elasticity of materials was also noted during the tensile testing which is material’s resistance when deformed elastically. The elastic modulus of 420, 691, 765, and 767 were obtained for nylon 6 and 2, 4, and 8 wt% h-BN-reinforced nylon 6 composites, respectively. Tensile strength and elastic modulus were found increasing as the content of filler increasing in the nylon 6 matrix. Tensile strength of pure nylon 6 and nylon 6 with 2 wt% filler content does not show much difference, but as the filler content further increases it shows a gradual increase in the tensile strength of the composites. The maximum tensile strength of 8 wt% filler nylon 6 composite was found 15.2% higher than the pure nylon 6. Elastic modulus was found continuously increasing as the filler content increased in the composite. Much difference was found in the elastic modulus of nylon 6 and nylon 6 with 2 wt% filler which is 64.5% more than that of pure nylon 6. After that, it increased gradually but with small differences. The maximum elastic modulus of 8 wt% filler nylon 6 composite was found 82.6% more than that of the pure nylon 6 material. Percent elongation of materials was also found increasing with the increasing content of filler and was found 62% more of the nylon 6 with 8 wt% filler compared to pure nylon 6. Absorption of moistures from the environment found the probable reason for the higher % elongation as nylon 6 is sensitive to the relative humidity present in the atmosphere. The moisture acts as a plasticizer and therefore it increased the elongation up to several percentages (46,47). Pure nylon 6 testing specimens were fabricated directly with the help of an injection moulding machine, while nylon 6 composites samples were initially compounded with the help of an extruder and then injection-moulded. During the compounding process, h-BN particles were mixed with the nylon 6 granules and then the mixture was poured into the twin-screw-extruder hopper. Due to more surface area of fine microparticles powder, as the content of h-BN increased, it absorbed more moisture from the environment during mixing as well as during extrusion. Another effect of plasticizing is that it reduces the tensile strength, but due to high tensile strength and elastic modulus of h-BN, the tensile strength and elastic modulus of nylon 6/h-BN composites were found to increase as the wt% of h-BN increased in the nylon 6 matrix (48). As h-BN is a thermally stable material with good tensile properties, continuous growth in the tensile strength and elastic modulus was witnessed for the composites after passing through processing temperatures compared to the pure nylon 6 (49,50).

Figure 4

Tensile test results: (a) tensile strength, (b) modulus of elasticity, and (c) % elongation.

SEM images of the cross-section of fractured surfaces after tensile testing of nylon 6 composites are shown in Figure 5. The one end of the fractured specimen of 8 wt% filler nylon 6 composite was analyzed and it shows the stretched material with the small white dots of h-BN particles. SEM images of composite support the tensile testing results as they show the well-stretched material before the fracture which was presented in % elongation results. Also, they confirm the well-dispersed and distributed h-BN microparticles into the nylon 6 matrix. hBN microparticles were wrapped within the nylon 6 matrix which loosed out during the tensile fracture of the specimen as shown in Figure 5 (51).

Figure 5

Tensile cross-sectional SEM images of nylon 6-hBN composite.

Figure 6(a) represents the Rockwell hardness of nylon 6 and its composites. The average hardness of 116 was observed for nylon 6 on R-scale. For 2, 4, and 8 wt% h-BN-reinforced nylon 6 composites, 118, 120, and 119, respectively, was observed. Figure 6(b) represents the impact strength of nylon 6 and its composites at the notch. The observed impact strength of nylon 6 was 48.3 J/m, while 49, 51.2, and 49.2 J/m were the impact strengths of 2, 4, and 8 wt% h-BN-reinforced nylon 6 composites, respectively. Maximum hardness and impact strength were found of 4 wt% h-BN reinforcement. Both hardness and impact strength increased gradually up to the 4 wt% filler reinforcement and then started decreasing. Hardness and impact strength increased by 3.45% and 6%, respectively, of 4 wt% filler nylon 6 composite compared to pure nylon 6. Impact tests were performed by the application of sudden load to fracture the samples at their notch and fracture generally depends on the imperfection, voids, and weak spots in the samples (52). Due to these reasons sometimes, the results show some variation which is also witnessed in our impact test results. The error bars of 2 wt% and 4 wt% filler composites show the variation in impact strength results of different samples.

Figure 6

(a) Rockwell hardness (‘R’- scale); (b) impact strength (J/m).

Regression analysis of hardness and impact strength results shows good co-relationship as shown in Table 1. Multiple R and R square values are found to be 0.87 and 0.76, respectively, while the standard error found is 1.01 which shows a good co-relationship.

Regression analysis of tensile strength and hardness results shows good co-relationship up to 4 wt% filler reinforcement. Multiple R and R square values are found to be 0.89 and 0.79, respectively, while the standard error found is 0.61 which shows the good co-relationship as shown in Table 2. Beyond 4 wt% filler reinforcement, regression analysis shows weak co-relation between them as shown in Table 2. Multiple R and R square values are found to be 0.46 and 0.21, respectively, while the standard error found is 4.107 which shows the weak co-relationship between them.

For low wt% filler reinforcement, the tensile strength and hardness increase linearly and show good co-relationship, but at the higher 8 wt% filler reinforcement both show opposite behavior as hardness started decreasing.

Similarly, tensile strength and impact strength show good co-relationship up to 4 wt% filler reinforcement. Multiple R and R square values are found to be 0.98 and 0.96, respectively, while the standard error found is only 0.38 which shows an incredibly good co-relationship as shown in Table 3.

3.2 COF and wear test on pin-on-disc

Figure 7a shows the COF of nylon 6 and nylon 6 composites at different loads of 4.9, 9.81, and 14.71 N, while the sliding velocity and distance were made constant and of 1.05 m/s and 1,000 m, respectively. The 2 wt% and 4 wt% h-BN filler nylon 6 composites show less COF at all loading conditions compared to pure nylon 6 and 8 wt% h-BN filler composite. The lowest COF was found for 2 wt% filler composite at all loads which were followed by the 4 wt% filler composite. However, 8 wt% filler composite was not found much effective in reducing the COF of nylon 6. Figure 7b shows the COF of nylon 6 and nylon 6 composites at different sliding distances of 1,000, 2,000, and 3,000 m. Load and sliding velocity were made constant and of 4.9 N and 1.05 m/s, respectively. The 2 wt% and 4 wt% filler composites were found much effective in reducing COF of nylon 6, while the highest COF was found for 8 wt% filler nylon 6 composite. Figure 7c represents the COF of different materials at different sliding velocities of 1.05, 1.57, and 2.1 m/s, while load and sliding distance were made constant and of 4.9 N and 1,000 m, respectively. Again 2 wt% and 4 wt% filler composite shows the lower COF compared to the rest of the materials at all sliding velocities. The COF of 8 wt% filler composite was found much higher at all sliding velocities compared to the rest of the materials. The 2 wt% h-BN filler reduced the COF of pure nylon 6 approximately from 10% to 15% at all parameters of variable loads, sliding distances, and velocities.

Figure 7

COF vs (a) different loads, (b) sliding distances, and (c) sliding velocities.

As the load increases, the COF of all materials was also found to increase rapidly. Also, as the sliding distance and sliding velocity increase, the COF of materials increases but with little variations. It was observed that, as the load, sliding distance, and velocity increased, the dislodged debris quantity also increased which caused the increase in the friction between the contact surfaces of pins and rotating steel plate. The 2 wt% and 4 wt% h-BN nylon 6 composites formed smooth transfer layer as shown in Figure 8 during the tribological testing, while the transfer layer for 8 wt% h-BN composite was not found smooth. Lots of debris during tribological testing of 8 wt% filler nylon 6 composite were found near to the track diameter as well as the in-between pin and steel plate and that was the probable reason for the higher COF of it compared to others (53).

Figure 8

Transfer layer generation of the lubricating film on steel disc during tribological testing of 2 wt% and 4 wt% h-BN-reinforced nylon 6 composites.

Figure 9(a) shows the wear of nylon 6 and nylon 6 composites at different loads of 4.9, 9.81, and 14.71 N. Sliding velocity and distance were made constant and of 1.05 m/s and 1,000 m, respectively. The 2 and 4 wt% h-BN filler nylon 6 composites show less wear at all loading conditions compared to pure nylon 6 and 8 wt% h-BN filler composite. The lowest wear was found for 2 wt% filler composite at all loads which were followed by the 4 wt% filler composite. The 8 wt% filler composite was not found much effective in reducing the wear rate of nylon 6. Figure 9b shows the wear of nylon 6 and nylon 6 composites at different sliding distances of 1,000, 2,000, and 3,000 m. Load and sliding velocity were made constant and of 4.9 N and 1.05 m/s, respectively. The 2 wt% and 4 wt% filler composites were found much effective in reducing the wear of nylon 6 and found almost the same at different sliding distances. However, the highest wear rate was found for 8 wt% filler nylon 6 composite compared to other materials. Figure 9c represents the wear of different materials at different sliding velocities of 1.05, 1.57, and 2.1 m/s, while the load and sliding distance were made constant and of 4.9 N and 1,000 m, respectively. Again, 2 wt% and 4 wt% filler composite shows the lower wear rate compared to other materials, while the wear of 8 wt% filler composite was found higher at all sliding velocities compared to other materials. The 2 wt% h-BN filler nylon 6 composite reduced the wear rate of nylon 6 about from 8% to 10% approximately at all varying parameters.

Figure 9

Wear vs (a) different loads, (b) sliding distances, and (c) sliding velocities.

Due to an increase in friction at higher loads, sliding distances, and velocities, the wear rate was also found increasing. As the load, sliding distance, and sliding velocity increased, the contact surface temperatures between pins and rotating steel disc were also increased which resulted in the decrease in mechanical cohesion. And, that caused the erosion of the material elements and ultimately more wear (54,55).

3.3 Abrasion resistance

Pure nylon 6 weight loss due to abrasion was noticed to be 0.14% and for 2, 4, and 8 wt% nylon 6 composites, it was observed to be 0.36%, 0.49%, and 0.78%, respectively. As shown in Figure 10, pure nylon 6 was found to be better abrasion-resistant material compared to its composites. The maximum weight loss of 8 wt% filler composite was observed to be 5.5 times more compared to the pure nylon 6. h-BN solid lubricant properties were not found effective in enhancing the abrasion resistance of nylon 6. Infect it increased the weight-loss of composites due to abrasion of h-BN particles. The rough surface of the abrasive wheel plowed the nylon 6 composites surfaces and resulted in more weight loss in abrasion compared to pure nylon 6 (56).

Figure 10

Weight loss in abrasion with H-18 abrasive wheel.

3.4 TGA

Figure 11 represents the TGA plots of nylon 6 and its composites. The X-axis represents temperature in °C and Y-axis represents sample weight in percentage. Onset X temperatures were found to be 418°C, 429°C, 445°C, and 459°C of nylon 6, 2, 4, and 8 wt% h-BN-reinforced nylon 6 composites, respectively. Nylon 6 degradation started at 418°C, and at 506°C, the curve almost reached its lowest position. The 2 wt% h-BN-reinforced nylon 6 composites started its weight loss at 429°C, while 4 wt% and 8 wt% h-BN filler nylon 6 composite’s weight loss started at 445°C and 459°C, respectively. At 850°C, the sample weight of nylon 6 reached 0%, which indicates the total degradation of a material. In the case of 2, 4, and 8 wt% filler reinforcement, total sample weights were noticed to be 0.6%, 1.53%, and 4.5% at 850°C, which confirms the h-BN reinforcement presence in the composites. As h-BN is a thermally stable material, it increased the thermal stability of the nylon 6 matrix in some amount which can be identified by the weight loss starting temperatures of the materials (57).

Figure 11

TGA plots of nylon 6 and nylon 6 composites.

3.5 X-ray diffraction (XRD)

XRD analysis was carried out on Bruker D2 phaser X-ray diffractometer, Cu-Ka radiation (λ = 0.154 nm), and at a scanning speed of 1°/min at room temperature. XRD tests were done in the scanning range of 10° to 50° 2θ. In Figure 12, XRD plots of nylon 6 and its composites are shown in which the X-axis represents 2 theta values, while Y-axis represents counts per second or intensity of peaks. Pure nylon 6 is having peaks in the range of 20–25 two thetas, while nylon 6 composites are also having the same peaks in that range, but with additional peaks at 28–30 and 48–49 two thetas range due to the presence of h-BN microparticles (58). For 4 wt% filler composite, XRD peaks intensities were found low with broader hump near 21–25 two thetas probably because of change in the crystalline form of nylon 6. Nylon 6 can take two crystallographic forms α and ϒ, and this may be the reason for the drop in the intensity of peaks near 21–25 two thetas during testing (59,60,61).

Figure 12

XRD plots of nylon 6 and composites.

The crystallinities of nylon 6 and composites were calculated from XRD curves. Crystallinities were calculated from areas under the crystalline and amorphous peaks from the following Eq. 1.

Nylon 6 is a semicrystalline polymer material; its degree of crystallinity (DOC) was found to be 56.72% and the rest was the amorphous contents. For 2 wt% h-BN reinforcement, DOC was found to be 56.6% and for 4 wt% and 8 wt% reinforcement it was 55.44% and 50.92%, respectively. The lowest crystallinity was found of 8 wt% filler reinforcement and it decreased the crystallinity of pure nylon 6 by about 5.8%. The DOC was found almost the same for 2 wt% filler reinforcement and of pure nylon 6, which indicates that up to 2 wt% there was no effect of filler reinforcement on nylon 6 crystallinity.

A decrease in the DOC beyond some range may decrease the hardness of the material by some amount and is also witnessed in our hardness test results. The hardness of the composite increased up to 4 wt%, and at 8 wt% where the lowest crystallinity was found, it started decreasing. The impact strength of the material depended on the crystallinity as well as on the molecular weight of the materials. A decrease in the crystallinity increases the impact strength, while a decrease in the molecular weight decreases the impact strength. Due to this duel phenomenon combination and sudden load, some variations were noticed during the different samples test of nylon 6 and its composites in the form of error bars. Also, as per one research, other properties stay independent of molecular weight change (62).