11
Morphological Examination of Worn out Surfaces of Basalt Fiber‐PEI Composites with Varying Loading Conditions
Kalimuthu Mayandi1Subramanian Karthikeyan2Nagarajan Rajini1 and Azeez B. Alavudeen1
1 Kalasalingam University, Department of Mechanical Engineering, Anand Nagar, Krishnankoil, 626126, Tamil Nadu, India
2 Kalasalingam University, Department of Automobile Engineering, Anand Nagar, Krishnankoil, 626126, Tamil Nadu, India
11.1 Introduction
The use of polymer composite materials (PCMs) is increasing in various application areas such as aerospace, marine, construction field, and sports due to its attractive properties. Properties such as weight‐to‐strength ratio, Young’s modulus, elongation, and corrosion resistance are high in PCM compared to traditional materials. Moreover, the fabrication methods and materials handling of PCMs are easy compared to traditional materials. In addition, PCM has light weight and is easily transferable [1–4]. This work studies the basalt fiber wear characteristics for low loading application areas. The basalt fiber withstands high temperatures up to 1700 °C. It is one of the natural resource materials obtained from the lava of volcanic rocks. Among the various natural materials, basalt has high tensile strength and modulus when compared to other natural resource materials such as banana, sisal, pineapple, and bamboo. Glass fibers lead to irritation to the human skin during direct handling. This does not happen with the basalt fiber and it is easy to handle during fabrication [5, 6]. Moreover, large‐scale industries consider basalt fibers for reinforcement in polymer matrix, which can find application in heavy‐duty friction units requiring high strength. Further, basalt can replace the carbon fiber in areas of high temperature application due to economic considerations and availability [7].
In an earlier study, the mechanical performance of basalt fiber reinforced PCMs was found to be superior to that of glass fiber reinforced composites materials [8]. Tapi et al. [9] investigated that basalt with thermoplastic has improved mechanical and wear properties. Polyetherimide (PEI) is one of the thermoplastic materials. It has good tensile strength and modulus properties. It can be used in various engineering application areas such as electrical and electronics parts, microwave applications, and automobile industries. PEI is one of the newest engineering thermoplastics materials [10]. Tiwari et al. [11] developed the composites materials with polyetherimide as a matrix for a study of the tribological and mechanical properties. They concluded that the composite materials developed with PEI had better properties in all aspects. Polytetrafluroethylene (PTFE) is one of the filler materials for polymer matrix composite materials. It has high mechanical performance and tribological properties. The PTFE filled materials have better improvement in wear resistance compare to unfilled composite materials [12]. The recently developed jute fiber hybrid composites are filled with PTFE as the filler material. It was observed that the addition of PTFE in polymer composites showed better performance in high loading wear application areas [12, 13]. The influence of reinforcement with some synthetic fibers such as glass, carbon, and kevlar on PTFE filled composites has been studied for various tribological aspects with respect to different testing parameters [14–16]. Similarly, there are many reports on the tribological behavior of PTFE filled composites with carbon fiber as reinforcement in various thermoplastic systems that were found to exhibit better frictional and wear behavior.
This chapter discusses the friction and wear properties of basalt/PEI composites with and without the addition of PTFE and comparison is made with the results for glass/PEI composites. The same tribology testing conditions for glass/PEI composite have been used in the case of basalt/PEI seen in literature. A hot compression technique has been used for the fabrication of basalt/PEI composites.
11.2 Materials Used
Plain woven basalt fabric was used as the primary reinforced material. It was procured from the Nickunj Eximp Enterprises Pvt Ltd, Chennai, India. The fabric was obtained in the form of a roll. A solution of PEI and dichloromethane was used as the matrix material. These materials were obtained from Sigma Aldrich (P) Ltd, Bangalore, India. In addition, PTFE was selected as filler material to reduce the coefficient of friction (CoF).
11.3 Fabrication of the Composite Materials
Untreated basalt fabric, PEI, and PTFE were used to fabricate the composite materials. First, the basalt fabric was cut into the required size of 100 mm ×100 mm and the PEI solution was prepared by adding 25 wt% of dichloromethane. The prepared solution was kept in a container vessel. Then, the basalt fabric was immersed in the container PEI solution for 1 h followed by drying at atmospheric room temperature. Twenty one prepergs were prepared by using the PEI solution. These prepergs were used to fabricate a composite plate of 3 mm thickness. All the prepergs were placed in the cavity of a bottom mold. The top cover of the mold was kept closed and placed in the hot compression machine and the compression pressure was set at 15 MPa and temperature at 350 °C. A second specimen was also fabricated in the same manner with the addition of 10 wt% of PTFE as filler material in PEI solution.
11.4 Testing of Composite Materials
11.4.1 Density Test
The density of the prepared composite was measured using Mettler Toledo densitometer. It was used with water as the immersing liquid. Five samples were tested for each condition in the ASTM D792 standard. The average density of the basalt/PEI composite was 1.0052 g cm−3. The density of the same composite with the addition of PTFE filler increased to 1.0086 g cm−3.
11.4.2 Hardness Test
Hardness measurements were carried out on 3 mm thick specimens as per ASTM D2240 standards on a shore D scale. Indentations were made at several locations for each specimen and the average hardness value was calculated. At least five specimens were tested and the average value of the basalt/PEI composite was measured as 98.06. The addition of filler materials to the composite caused a marginal improvement in the hardness up to 99.82.
11.4.3 Wear Test
In the present work, the friction and dry sliding wear behavior of basalt/PEI and basalt/PEI/PTFE composite samples have been studied in terms of the coefficient of friction and specific wear rate. A pin‐on‐disk setup (as per ASTM G‐99 standard, Make: Magnum Engineers, Bangalore) was used for the sliding wear and two‐body wear tests. The surface of an 8 mm diameter composite specimen, glued to a pin of 10 mm width and 50 mm length, comes in contact with a hardened alloy steel disk with hardness value of 62 HRC and surface roughness (Ra) of 0.54 μm. The test was conducted on a track of 50 mm diameter for a specified test duration, load, and velocity. The specimen was initially weighed using a digital electronic balance (0.1 mg accuracy). The test was carried out by applying normal load (70, 80, 90, 100 N) and run for a constant sliding distance of 1413 m at 150 rpm. At the end of the test, the sample was again weighed in the same balance. The difference between the initial and final mass was used as a measure of wear loss. The sample was placed so that the specimen was perpendicular to the steel disk and parallel with respect to the abrading direction. All the tests were conducted at ambient temperature. The specific wear rate was calculated using the following relation:
where Ko is the specific wear rate, ΔV is the volume loss, L is the load, and D is the sliding distance.
11.5 Results and Discussion
11.5.1 Wear Performance of Basalt Fiber Reinforced Thermoplastic Composite
The coefficient of friction of basalt/PEI and basalt/PEI/PTFE composite for different loading and sliding conditions was plotted as a function of test duration. It was observed that the CoF (μ) increased slightly with increasing load for the basalt/PEI composite. Figures 11.1 and 11.2 clearly show the appearance of these variations in wear within the range and also the positive behavior of the material toward wear resistance. It was found that almost both the composite materials attained a stable state of friction in 15 min of sliding.
Figure 11.1 Coefficient of friction of basalt/PEI.
Figure 11.2 Coefficient of friction of basalt/PEI/PTFE.
Inclusion of PTFE in the basalt reinforced composite, however, affects the CoF (μ) in an unfavorable way. A coefficient of friction (μ) value as low as 0.14 was reached in the basalt/PEI composite, but, in the case of the PTFE filled composite, it was 0.284. However, for 80 and 90 N loading conditions, there was no significant difference in friction characteristics between the two composite materials. It was clearly noticed that with the increase in sliding duration and loading conditions, μ changed very slowly. The frictional value of basalt fabric is compared with that of glass fiber from the literature. The μ value of the both the composites is found to be very low when compared to glass/PEI and PTFE filled glass/PEI composite. The addition of PTFE in PEI changes the contact condition with the counter surface and increases the overall friction value slightly from 0.23 to 0.3. The basalt/PEI composite shows better performance in terms of CoF.
Selection of the same tribological test configuration and load from the existing literature allows comparison of the present result with the reported data. It is evident from Figures 11.3 and 11.4 that there is a significant improvement in the friction characteristics of the basalt‐PEI composite when compared with the glass fiber PEI composite. Similar results are observed when comparing the influence of the PTFE addition to the same composite, where the basalt fiber composite with PTFE additives has lower friction values compared to the counterparts in the literature. Both the composites exhibit wear rates in the order of 10−15 m3 N−1 m−1. A similar tendency in the friction characteristics is observed for both the basalt‐PEI and basalt‐PEI‐PTFE composites. At higher loads, the PTFE composites showed increased wear rate.
Figure 11.3 Comparison of CoF of glass/PEI and basalt/PEI.
Figure 11.4 Comparison of CoF of basalt/PEI/PTFE and glass/PEI/PTFE.
Generally, increase in load can increase the magnitude of friction force and therefore the contact temperature could also increase at the contact surface. After a certain temperature, depletion of materials takes place and it adheres to the counter surface. Moreover, a continuous increase in temperature can remove the matrix component as much as possible. It implies that the glass transition temperature could have appeared in the contact surface, so that molecule disorder can be initiated in the PEI molecular structure and propagate further with increasing temperature. At the same time, an almost constant specific wear rate was observed for both the composites with the effect of varying load. However, a significant reduction in specific wear rate was noticed for basalt/PTFE composites. The results show the removal of PTFE to be higher in the case of glass/PEI composition. It could happen due to the non‐occurrence of re‐adhesion of PTFE sample. But in the case of basalt/PTFE composite, the adhesion of PTFE on the counterface could be strong enough to ensure smooth running action. Further, the surface of PTFE transfer layer on the counter plate can be the reason for achieving this lower specific wear rate.
Figure 11.3 shows the comparative results of CoF for glass/PEI and basalt/PEI composites with the application of varying loads. They reveal almost consistent CoF in the case of glass/PEI composites. On the other hand, an irregular variation in CoF was noticed in basalt/PEI composites. However, the value of CoF is always lower than the values of glass/PEI composites. It was realized from the results that the compatibility of PEI with glass and basalt fiber may vary due to different morphologies of the fiber surface.
The results show that the degree of adhesion between the basalt and PTFE is higher than that of glass/PEI composition. This is confirmed from the morphological analysis of basalt/PEI worn out samples. This is discussed in the forthcoming sections. The removal of PEI matrix after the running‐in period of testing condition can expose the fiber to contact with the counter surface.
A similar kind of variation in CoF was observed in both the composites with the addition of PTFE filler as seen in Figure 11.4. CoF was seen decreasing from 70 N load to 80 N load, clearly indicating the transfer of material occurring from the sample to the counterface at higher loading conditions. Later, the adhesion and re‐adhesion could have happened with increasing load.
11.5.2 Morphological Analysis of Worn out Samples
Microscopic analysis was performed for two different extreme loading conditions at 70 and 100 N with and without basalt/PTFE addition for examining the wear behavior of basalt/PTFE composites as shown in Figure 11.5a,b. Figure 11.5a clearly shows the morphology of PTFE at 70 N load. The wear track along the direction of friction force clearly indicates the absence of any material removal. However, a smooth surface was found with the partially melted and scratched surface and re‐adhered particle. In the case of PTFE coated material (Figure 11.5b), large removal of PTFE was seen clearly in a few thin patches, which led to the discontinuous surface causing an increase in the friction force. As expected, the detached particles from the samples were found to be adhered firmly to the counter surface. Hence, a steady state friction was achieved in the case of 100 N loading condition after a certain period of time.
Figure 11.5 (a) SEM micrograph of basalt/PEI composite and (b) with PTFE.
Figure 11.6a,b shows the topography of basalt/PEI worn out composites with and without PTFE addition at lower loading (70 N) conditions. Figure 11.6a shows only a straight longitudinal scratch on the surfaces of uncoated composites. Further, the pressure of wear debris with small dimensions is also seen from the surface. This is responsible for the scratch along the normal to the frictional direction. In Figure 11.6b the removal of PTFE is noticed on the surface, indicating the insufficient bonding of PTFE particle with the highly viscous thermoplastic PEI. However, the tribo lubricant transfer to the counterpart can reduce the magnitude of frictional force until the existence of transfer layer on the counter surface.
Figure 11.6 (a) SEM micrograph of basalt/PEI composite at 70× and (b) with PTFE at 70×.
The higher magnified image was compared at 500× magnification for basalt/PEI composites under 70 N loading conditions to get a close look at the worn out surfaces. Figure 11.7a shows that the depleted PEI matrix occupies the space between the fibers and forms the smooth and even surface of the samples. This can minimize the changes of asperity content between the materials. At the same time, the larger gap between the fibers shown in Figure 11.7b indicates the adherence of PTFE coated matrix on the counter plate. Further, the exposure of fiber seems to be dominant compared to the previous case.
Figure 11.7 (a) SEM micrograph of basalt/PEI composite at 500× and (b) with PTFE at 500×.
At a higher loading condition, large quantity matrix removal was noticed. This is shown in Figure 11.8a. Further, fiber breakage, interfacial debonding, and separation of loosely bonded wear debris with small particle size were observed at the interface between the fibers. It implies that the increase in friction force due to the application of higher load ensures adherence and re‐adherence of the matrix on the counter plate. In Figure 11.8b, micro‐ploughing with deeper grooves is seen in the case of PTFE coated composites. It confirms the irregular adhesion of PTFE coating on the counter surface and thus it creates a higher magnitude of asperity region at the contact surface.
Figure 11.8 (a) SEM micrograph of basalt/PEI composite and (b) with PTFE.
11.6 Conclusions
An attempt has been made in the research work to introduce PEI basalt fiber composites. From the investigation on mechanical and tribological testing the following conclusions are drawn.
- The hardness of the basalt‐PEI composites remained the same with the addition of PTFE.
- CoF values are lower for basalt PEI composite when compared with the basalt PEI‐PTFE composite.
- However, at 80 and 90 N loading condition no significant change was observed in the friction characteristics.
- The wear rate showed similar characteristics as the friction behavior where the basalt/PEI composite has better wear resistance when compared with the basalt PEI‐PTFE composite.
- Morphological analysis confirmed that after wear test the basalt/PEI composite sample has better smooth surfaces while the worn out surfaces of basalt/PEI/PTFE composites materials have more rough surfaces.
- On comparison with the existing literature, the newly developed polymer stands out as a suitable candidate material for tribological applications.
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