0 Introduction

Recently, the composite materials based on polymers are emerging due to their high engineering properties and high durability as the constituents are not suffering any loss in properties even in harsh environments. Owing to higher strength, economy and ease of fabrication, these polymer composite materials (PMCs) are being well accepted in several industrial applications. The applications of such composites are not limited to aerospace, automotive, sporting and home appliances, but also extend to medical applications and construction industries too. However, the manufacturing of eco-friendly sustainable materials is being now focused on by several researchers to meet the challenges of environmental concerns that associated with world community. This fact concentrates the attention of researchers on the development of new class fibers that are derived from plants, vegetables and animals. These biodegradable fibers can be used as an alternative to so called man-made fibers for preparing fiber-reinforced polymer composites (FRP). These FRPs now have numerous applications in various sectors like transport, defense, construction industries, packaging, consumer products, etc.^[1-5]. Further, it has been noticed that natural fiber composites also exhibit improved fracture resistance, superior electrical resistance, good thermal and acoustic insulating properties as well^[6-7]. Still, the use of natural fibers composites faces numerous challenges like excess of moisture absorption and poor thermal properties. The hybridization of natural fiber with synthetic fiber not only eliminates such challenges to some extent, but also enhances durability. Sanjay et al.^[8] studied the hybridization effect of jute/kenaf/glass reinforced epoxy composites. The experimental finding reveals that the hybridization of jute/kenaf fabrics with E-glass provides a suitable method to improve the mechanical impact and inter-laminar strength. In another study, Jothibasu et al.^[9] investigated the mechanical properties of Hybrid areca sheath-jute-glass epoxy composites. A significant improvement in mechanical properties is marked with composite having with jute fiber at intermittent, areca sheath fiber at core and glass fabrics at the skin of the composite. Similarly, Arpitha et al.^[10] reported that the inclusion of E-glass and Silicon carbide filler not only enhances the physical properties, but also improves the mechanical properties of composite to a greater extent. Despite several applications, the surface of these polymeric composite materials is badly affected by impacted solid particles while employed as aircraft wings, turbine blades, rotor blades, automobile bodies, wind turbine blades, etc^[11]. This erosive wear of components critically affects the performance, service life, and reliability of the system due to progressive removal of material by frequent bombardment of erodent on the surface^[12].

In fact, the erosion of hard particles is a complex process and the resistance of PMCs to erosion is very poor; its analysis is very much essential to examine the surface damage by eroding particles under various parameters like material properties, impact velocities, impingement angles, particle size, standoff distance, exposure duration, etc^[13]. Moreover, the reinforcements used for fabricating PMCs also significantly affect the erosion behavior^[14]. Patel et al.^[15] deliberated the erosive performance of jute/glass epoxy composite under various impact particle velocities, angle of impact and erodent size. The study was conducted on four layered hybrid laminates of 0°-90° oriented fiber of jute (J) and glass (G) fiber with piling order JJJJ, JJGJ, JJGG, JGJG, GJJG and JGGJ. A notable variation in erosion rate with alteration of layering order and quantity of fiber enforcement were marked. Irrespective of impact velocity, laminate with piling order JGGJ exhibited the highest erosion resistance. Moreover, a brittle property of composite was also revealed at a higher impingement angle of 90°. However, the adhesion between fiber and matrix plays a crucial role in determining the erosion efficiency. Vigneshwaran et al.^[12] argued that irrespective of material, the erosion rate was significantly affected by the interfacial bonding. Similarly, Zhang et al.^[16] reported that owing to improved interfacial bonding, the short carbon fiber reinforced composite shows an improved erosion resistance performance in comparison to others. Likewise, An et al.^[17] made a comparison of erosion rate between unreinforced and poultry feather reinforced polymer composite. Despite of reduction in mechanical properties due to insert of short poultry feathers, the erosion resistance was found enhanced. Similarly, Debnath et al.^[18] conducted erosion tests on glass, carbon, and jute fiber inserted polyester composites. Irrespective of fibers, a reduction in erosion rate was noticed with increase of standoff distance. They also concluded that, a reduction of kinetic energy of the impacted particle as the standoff distance increases reduces the erosion rate. In another study, the particle size had a greater impact on erosion wear as evidenced by Bagci et al.^[19] while examining the erosive character of glass mat-based polyester laminate. Surface cracks were also revealed on the sample specimens due to repeated bangs of solid particles. Similarly, a significant influence of impact angle and fiber orientation on the erosive resistance too was reported by Tiwari et al.^[20] through details study on erosion performance of unidirectional carbon/glass polymeric composites against high velocity solid particles. From the findings, they as well concluded that the above composite exhibits a semi-ductile nature, as a greater erosion rate was reflected at a 60^° impingement angle. Furthermore, both Harsha et al.^[21] and Tewari et al.^[20] provided a details overview on erosion characteristics against solid particle of various polyaryletherketones along with carbon and glass fiber polymer composites. Likewise, Arjula et al.^[22] reported the erosion efficiency of several polymers and their composites. While plotting the graph of erosion efficiency with respect to their hardness, a clear segregation of types of polymer and their composite was noticed. As scatter dispersion in the efficiency map was marked, the efficiency can be utilized to measure erosion resistance. Finally, three modes of erosion were suggested by Ibrahim et al.^[23] through the study of erosion performances of polyethylene against impact of solid particle. On examining the erosion rate in reference to impingement angle, three different modes such as ductile, semi-ductile and brittle mode of erosion were reported. In the ductile mode, the erosion was suggested, while a higher erosion rate was found at the impact angle ranging between 15° to 30°, whereas for the brittle mode, the same higher erosion rate was suggested at impingement angle of 90°. Similarly, irrespective of impact velocity a higher erosion rate was observed with impact angle ranging from 45°-60° as well which justifies the semi-ductile mode. Moreover, a strong impact of particle velocity and erodent size on the magnitude of erosion rate was also reported by Walley et al.^[24]

In view of literatures, it is very much clear that the importance of carrying out an erosion study is inevitable as the components are recommended for various engineering applications. Most of the works have focused to determine the association between different erosion parameters and resistance to erosion of metals, alloys and composites. Very limited research was carried out on erosion performance of polymer based hybrid laminates. However, the variations in the erosion performance of polymeric composites with process parameters to meet the industries' requirement need to be addressed.Again, suitable utilization of PPLSF for making composite to use in various engineering applications is still limited. Thus, this research aimed to provide detailed experimental insights on the erosion performances of PPLSF/Glass hybrid laminates. The influences of various testing parameters such as particle velocity and impingement angle on erosive performance of laminates were mainly intended. The influence of stacking order on erosion performance of laminate was also examined. In the current work, laminates were made by hand-lay-up method. The modes of erosion were examined through a study of wear data and evident with morphological analysis by scanning of the eroded surface by SEM.

1 Materials and Methods 1.1 Materials

The major reinforcements in the current hybrid laminates are bidirectional woven types PPLSF and E-glass fiber mats. Both fiber mats were obtained from M/s. Go Green Products Ltd, Chennai, India. An unsaturated polyester resin was used as matrix with methyl-ethyl-ketone-peroxides (MEKP) hardener. Cobalt-naphthenate was used as accelerator to promote the reaction. The chemicals were procured from Abanti Enterprises, Odisha, India. The fiber orientations in the mats were in 0°/90° pattern. The mat thickness of PPLSF and glass are 0.8 mm and 0.25 mm respectively. Properties of reinforcement and polyester resin are presented in Table 1.

1.2 Fabrication of Composites

In the current work, hand layup method was adopted to fabricate the laminates. A metallic mould of dimension 160 mm×120 mm×5 mm was used for the fabrication process. Initially silicone gel was employed on the mould surface to facilitate the removal of castings without sticking. Thin Perspex sheets were provided at both the top and bottom surfaces to get a good surface finish. A polyester mix was prepared by mixing unsaturated polyester with catalyst MEKP in 10∶1 ratio with 2% accelerator. Both the PPLSF and glass fabrics were cut to the size of mould and soaked in the polyester mix for a period of 10 min. After adequate absorption of polyester, the mats were ready for making laminates. Small amount of polyester mix was put into the mould and spread uniformly with the help of a brush. Laminate with desire pilling sequences was cast by placing the soaked mats of PPLSF and glass in accordance with stacking order. The stacking order of laminates is explained in Table 2. A thin layer of polyester mix was deployed at each interface of stacking. Each layer of soaked mats was press rolled by means of roller to remove any air entrapment during placing of mats. After placing the final layer, a small amount of polyester mix was applied. Finally, putting the Perspex sheets at the top, the cover plate mould was placed. A load of around 30 kg was applied on the cover plate to remove excess amount of polyester and air bubbles that were present in the castings. Then it was left isolated for 48 h to cure. After curing the casted laminate was taken-away from the mould and cut to the dimension for conducting different tests. Fabrication process of laminates is illustrated in Fig. 1. Again, the details of stacking order, thickness and weight percentage of fiber are presented in Table 2.

1.3 Experimental Study

After the fabrication, the test samples were cut to the size and tested as per the respective ASTM standards. For each reading, five numbers of samples of each laminate were tested and the average value of the reading was recorded.Morphological analysis is used to examine the mode failure of tested specimens.

1.3.1 Evaluation of density and void content

The density of the laminates was determined by following immersion approach, which was based on the Archimedes principle. Again, the theoretical density (ρ_ct) was evaluated by using the rule of mixture(ROM), which was represented by Eq. (1).

$ \frac{1}{\rho_{\mathrm{ct}}}=\frac{w_{\mathrm{g}}}{\rho_{\mathrm{g}}}+\frac{w_{\mathrm{p}}}{\rho_{\mathrm{p}}}+\frac{w_{\mathrm{m}}}{\rho_{\mathrm{m}}} $

(1)

where w_g, w_p, w_m, ρ_g, ρ_p and ρ_m are representing weight fractions and densities for glass, PPLSF and polyester respectively. The voids content of the laminates was evaluated by the following equation:

$ \operatorname{Void} \text { content }(\%)=\left(\frac{\rho_{\mathrm{ct}}-\rho_{\mathrm{ce}}}{\rho_{\mathrm{ct}}}\right) \times 100 $

(2)

1.3.2 Performing of micro-hardness test

The Micro-hardness was determined by performing hardness test on Vickers micro-hardness tester (Make: Leco, model: LM248AT, Beijing, China)with a pyramidal diamond indenter. The laminate specimens were cut and tested in accordance with ASTM E384 standard. The test was conducted under a load of 5gf and a dwell time of 10 s. Fig. 2 shows the hardness testing of the laminates.

1.3.3 Solid particle erosion test

In accordance with ASTM G76 standard, the test was performed on erosion wear test rig (Make-Magnum Engineering, model: TE-400-HMI, Bengaluru, India) as represented in Fig. 3. The setup was consisting of hopper for loading the erodent, conveyor belt system for feeding particle to mixing and accelerating chamber and air compressor. The silica sand should be perfectly dried before loading to the hopper. A vibrator was installed at the hopper to assure the free and continuous flow of erodent during the experiment. Dry silica sand was selected as erodent for conducting the test. The mass flow of erodent was controlled by regulating the speed of the belt. The impact velocity is controlled by regulating the supply pressure of air from compressor and its value was determined by double-disc apparatus^[28]. High velocity air mixed silica sand from mixing chamber came out from a converging nozzle of 5 mm diameter. The distance from the nozzle tip to specimen is termed as stand-off distance. The stand-off distance is adjusted by rotating the lead screw which moves the swivel table vertically up and down. For this test, 10 mm stand-off distance was fixed. The test sample was clamped to the table and different impingement angles were set by rotating the swivel table. For conducting the test, the laminates were cut to the size 25 mm×25 mm. Finally, the test was conducted at three distinct impact velocities and four different impingement angles. The various input parameters such as particle velocity, impingement angles, stand-off distance, temperature etc. are reflected in Table 3. The erosion rate was calculated by weight loss method. The weight loss of tested specimens was evaluated after proper cleaning of the test sample with a brush and acetone-soaked cotton plug. Erosion rate was then evaluated through the following equation:

$ E_{\mathrm{r}}=\frac{\Delta w}{w_{\mathrm{e}}} $

(3)

where w denotes the weight loss of the sample in g and w_e represents the weight of eroding particle (i.e. testing time×particle feed rate). A mathematical equation for calculating erosion efficiency(η) is projected by Sundararajan et al. ^[29] to differentiate ductile and brittle behavior of polymer material. The value of η can be derived as follows:

$ \eta=\frac{2 E_{\mathrm{r}} H}{\rho_{\mathrm{ce}} V^2} $

(4)

where E_r indicates the erosion rate in g/g, H represents the hardness of material, ρ_ce implies density of material in kg/m³ and V is the particle velocity in m/s. The value of this erosion efficiency will determine the mode of wear i.e. whether the material is brittle or ductile. In the case of brittle materials, the erosion efficiency must have a higher value than that of ductile material. It may so happen due to fragment breaking owing to interlinking of cracks instead of progressive removal due to recurring impingement of erodent.

1.3.4 Morphological study

The morphological study of eroded surface was carried out on SEM (Nova NANO SEM 450). Gold coating was provided to the eroded surface to increase the conductivity before scanning. Images were taken at different magnifications to study the mode of failure/fragmentation of fiber and matrix.

2 Results and Discussions 2.1 Density and Void Content

The theoretical and actual densities of all laminate samples are listed in Table 4. The percentages of variations are reflected as void content. Both the actual density and void content of laminates are simultaneously presented in Fig. 4. It is clearly noticed that the laminate C₁ (PPPP) possesses voids of about 3.716% which is 40.71% higher than that of C₅ laminate (GGGG). This signifies the poor wetting PPLSF in comparison to glass fiber which leads to increase of void/air pockets in the laminates. Again, decreasing trend of void content was also observed on successive replacement of PPLSF mat with glass. This presence of voids leads to reduction physical as well as mechanical strength of the composites^[30].

2.2 Hardness of Laminates

The micro-hardness values of tested laminates are reflected in Fig. 5. It can be marked that the hardness value of laminates is increasing with successive replacement of PPLSF layer with glass. A greater hardness is noticed in the case of C₅ (GGGG) laminate in comparison to other and it is about 131.46% higher than C₁ (PPPP) laminate. This is due to the fact of excellent interfacial joining between polyester resin and glass fiber. Thus, thehybridization of PPLSF with glass successively improved the hardness. Again, on comparing the hardness of C₃ (PGGP) laminate with C₄ (GPPG) laminate, the hardness of C₄ is found to be increased by 25.76% than that of C₃. While interchanging the position of PPLSF and glass in stacking, the variation in hardness value is observed. Similar deviation in hardness value due to the changing of stacking sequence was evident by Sanjay et al.^[31] while dealing with jute/kenaf/E-glass hybrid laminates. Thus, inclusion glass fiber and stacking order play an important role in determining the hardness of the laminates.

2.3 Erosion Rate of Laminate 2.3.1 Study of effect on erosion rate by impingement angle at constant velocity

The variation of erosion rates with respect to angle of impingement at three distinct velocities are illustrated in Fig. 6. Irrespective of particle velocity, the erosion rate of GGGG laminate is found to increase with increase in angle. The peak of erosion rate is marked at 90°. However, the erosion rate of PPPP laminate increases with the increase of the angle from 30° to 45° and then decreases with further increase of angle. Again, for all other laminates (i.e. PGPG, PGGP, GPPG), the patterns of the curves clearly indicate a peak of erosion rate at 45° angle, irrespective of the stacking sequences and velocity. Thus, the impact angle is recognized as one of the vital parameters for study of erosive performance of the materials.However, materials were characterized by ductile or brittle on the basis correlation between rate of erosion and impact angle ^{[20, 32]}. The ductile character of material was revealed when the peak value of erosion rate was noticed at low impingement angle (i.e. 15° < α < 30°). On the other hand, the brittle behavior was characterized when the peak value was exhibited at normal impact (α = 90°). Further, in the case of FRP composites, a semi-ductile behavior was characterized with a peak value at intermediate angles i.e. 45° to 60°. However, this categorization is not absolute because the constituents of the target materials and the testing parameters have a crucial effect on erosion characteristics. Thus, from the experimental finding of this case, it can be concluded that the GGGG laminate exhibited a brittle character, whereas the PPPP, PGPG, PGGP and GPPG laminates exhibited semi-ductile behavior. These facts can also be evident in the results obtained by Deo et al. ^[33] and Prakash et al. ^[34] while studying the erosion performance of epoxy composites reinforced with Lantana Camara and Rubberwood respectively. Again, irrespective of particle velocity and impact angle the erosion resistance of laminates was found to be increased with inclusion of glass mat.

2.3.2 Study of effect on erosion rate by velocity of erodent at constant impinging angle

The results of erosion rate with respect to impinging angle at three distinct impacting velocities such as 48 m/s, 70 m/s, and 82 m/s are presented in Fig. 7. Irrespective of impingement angle and pilling order, an increasing trend in erosion rate is marked with increase in particle velocity. A peak value erosion rate is marked at maximum particle velocity of 82 m/s. This has happened due to acute plastic deformation of laminates. When velocity of impact increases the rate of plastic deformation and surface crack generation also increase which leads to a higher erosion rate.

The erosion rate of GGGG laminate is found to be peak that of PPPP laminates at all impact velocities and impingement angles. The erosion rate of PGPG, PGGP and GPPG laminate shows an intermediate erosion rate in between GGGG and PPPP laminate. This reveals the fact that the PPLSF possesses higher erosive resistance than that of glass fiber. Thus, the inclusion of PPLSF by replacing the glass layer improved the erosive resistance of laminate. Again, at all impact angles, the GPPG laminate shows a higher erosion rate in comparison to PGPG and PGGP. This may so happen due to early brittle fracture of glass fiber by high velocity solid particle as it is present at the outer layer.

2.3.3 Study of Erosion efficiency of laminates

The erosion efficiency of laminates with varying stacking sequences under all testing parameters is evaluated and listed in Table 5. Fig. 8(a)-(c) illustrates the erosion efficiency of PPLSF-glass hybrid polyester laminates with various stacking sequences at impinging speeds of 48, 70 and 82 m/s.It is noticed that, at low velocity (i.e. 48 m/s), GGGG laminates exhibit a higher erosion efficiency than that of others. Whereas, an increased erosive efficiency was observed in the case of PPPP laminates at high velocities of 70 m/s and 82 m/s. Again, a variation of erosive efficiencies of hybrid laminates (i.e. PGPG, PGGP and GPPG) from 1.334% to 4.602% and 1.215% to 3.614% are noticed at two distinct impingement angles of 45° and 60° respectively.This range of erosion efficiency indicates the semi-brittle behavior. This finding can be evident in the findings of Naik et al. ^[35] whenever studied with orange peels particulate reinforced epoxy composite.

2.4 Surface Morphology

The morphology of eroded surfaces under different particle velocities and impact angles were examined through the SEM micrograph, which is presented in Fig. 9(a)-(c). De-bonding at interface, formation of craters, breakage of fibers are the few modes of failure under plastic deformation due to striking of high velocity solid particle on the laminated surface. Fig. 9(a) shows the micrograph of eroded GGGG laminate with particle velocity of 82 m/s and at 90° impact angles. Severe plastic deformation is observed due to formation of above mode of failure. The damage surface of PPLSF hybrid laminate with low impact angle of 30° and particle velocity of 48 m/s is shown in Fig. 9(b). A little fibrillation of fiber with sticking of abrasive particle is marked. Because the low kinetic energy prevented the struck particles from fibrillating the fiber sufficiently; instead, they stuck to it. This sticking of particle might be due to the absorbing capability of kinetic energy. Hence lower erosion was observed at lower particle velocity in the case of PPLSF. Fig. 9(c) shows the eroded surface of PPLSF hybrid laminate with high velocity of impact (82 m/s) at 45° of impact angle. Both micro-cutting and micro-ploughing of PPLSF fiber are marked with small crater formation in matrix. This leads to higher erosion rate. Again, no detachment of fiber matrix is visible, which is evident in the adequate bonding between fiber and matrix.

3 Conclusions

The influence of hybridization and alteration of stacking order of hybrid PPLSF/glass polyester laminates on erosion characteristics are investigated through erosion tests. Erosion wear rate and erosion efficiency are evaluated at varying impact velocities and impingement angles. The morphology of eroded surface is also studied. According to the findings of results, the following conclusions are drawn.

1) Irrespective of impact velocity and angle of impingement, laminate with PPLSF only shows maximum erosion resistance. Thus, the inclusion of PPLSF into glass has significantly enhanced the resistance to erosion of the hybrid laminates.

2) The positions of PPLSF ply in hybrid laminates play a vital role in improving the erosion resistance. The laminates with PPLSF pile at the outer layer exhibit a minimum wear rate in comparison to others.

3) Irrespective of angle of impact and reinforcement, a firm increase in erosion wear occurs with raise in velocity of solid particle. However, PPLSF laminate shows higher erosion efficiency at higher velocity than that of others except at lower velocity of 48 m/s.

4) As peak value of wear rate is observed at 45° angle of impingement under all distinct particle velocities, the brittle characteristic of glass laminates is converted into semi-ductile character due to inclusion of PPLSF.

5) The alteration of semi-ductile to semi-brittle nature is distinguished by the help of impingement angle and impact velocity of solid particles.While examining the erosion efficiency values of currently developed laminates, such alternation of behavior was also seen.

6) The mode of failure by impact of particle is supported by SEM micrograph. Formation of crater, micro-cracks, micro-cutting of fiber and fiber fibrillization are involved in erosion process. Again, good interfacial bonding between fiber and matrix was also observed.