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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.04.015
Open Access
Study of the surface properties of the epoxy/quasicrystal composite
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Thayza Pacheco dos Santos Barrosa, Danielle Guedes de Lima Cavalcanteb,
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danielleguedes02@gmail.com

Corresponding author.
, Danniel Ferreira de Oliveirab, Rafael Evaristo Caluêtea, Severino Jackson Guedes de Limaa
a Department of Mechanical Engineering, Federal University of Paraíba, João Pessoa, PB, Brazil
b Department of Materials Engineering, Federal University of Paraíba, João Pessoa, PB, Brazil
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Table 1. Surface energy values.
Abstract

This work consists of the study of the surface and Hardness properties of epoxy/quasicrystal (QC) composites. The composites had volumetric proportions of 1%, 10%, 20% and 30%, where the comparison was made between the composites and the pure epoxy resin. Techniques of Shore D Hardness, Thermogravimetric Analysis Techniques, Roughness Test, Contact Angle Analysis and Scanning Electron Microscopy (SEM) were used. Wettability test allowed to analyze the surface of the composite under the influence of two liquids: saline water and paraffinic oil. Regarding the composite and the pure resin, the contact angle increased when the liquid used in the test was saline water and decreased when exposed to paraffinic oil. It could be ensured that this small variation of the contact angles in relation to the increase of the composition occurred due to the influence of the roughness. In relation to saline water, the roughness has operated in order to prevent the liquid from spreading on the composite. As for paraffinic oil, it influenced in an opposite way, absorbing the oil and reducing the angle formed. When analyzing the pure quasicrystal, the effect was contrary to that of the composite: when it was exposed to saline water the contact angle decreased, on the other hand when exposed to paraffinic oil the resulting angle increased. This effect is related to the low polarity of the Quasicrystal, which when exposed to an apolar liquid tends to repel, thus forming a greater contact angle. Through this study it is concluded that it was possible to obtain a composite QC/Epoxy with greater hardness and still maintain its surface characteristics.

Keywords:
Quasicrystal
Composite
Angle of contact
Roughness
Wettability
Shore D Hardness
Full Text
1Introduction

Quasicrystals belong to a new class of materials. Comparing to crystalline and amorphous materials, they lay in between. While crystalline materials have their atoms organized in crystalline structures and these same structures repeat themselves periodically filling the entire space, quasicrystalline materials present atomic organization in structures that would be impossible for the crystalline ones, but they repeat themselves aperiodically in a long range. This characteristic is enough for not classifying them as amorphous materials.

In addition to all their odd characteristics when analyzing their atomic structure and way of spatial organization, quasicrystals present unique and distinct properties compared to crystalline metallic materials.

Although they are metallic materials, their mechanical properties are very close to another type of material: the ceramics. Properties such as high hardness and brittleness, low thermal and electrical conductivity, make these materials totally different from the well-known crystalline metallic materials. The surface characteristics of quasicrystals are of great interest to engineering because of its low coefficient of friction and low surface energy; approximately three times smaller than that of aluminum.

Due to their unique properties, their main applications and potentials are wide. Ranging from aerospace to medical areas.

Although quasicrystals have odd properties, their high hardness and brittleness limits them when considering applications equivalent to those of other metals. So, their application is conditioned to metallurgy powder, applied by thermal spray or inserted in ductile matrices so as to form the composite material.

As the surface properties of quasicrystals are of great interest for technological application, this work studies the surface characteristics of epoxy/quasicrystalline composites, specifically roughness and contact angle.

1.1Composite epoxy/quasicrystal

Quasicrystals have unusual mechanical and optical properties, such as high hardness, low coefficient of friction and low surface energy, although they are fragile. The main applications of quasicrystals are for superficial coatings and composites with ductile matrix. These properties encouraged the research, development, and application of these materials as a composite having polymer matrix.

In the last 40 years there has been a rapid increase in the production of synthetic composites, incorporating reinforcements in several plastic materials (polymers) that dominate the market. One of the main factors was the obtaining of new materials that have excellent mechanical resistance with regard to their strength-weight and stiffness-weight ratios. Forecasts indicate that demand for composite materials will continue rising [1].

This work will deal with a composite polymer matrix material together with the particulate reinforcement of the quasicrystalline alloy. Currently, much has been studied about this composite material because it has been very promising in the scope of industrial applications.

In the particular case of the epoxy/QC composite, Bloom [2] show that quasicrystal reduces the wear resistance of epoxy resin when compared to pure epoxy and other elements such as aluminum, copper and iron. The decrease in wear resistance in combination with its low abrasiveness is attributed to the unique properties of the quasicrystalline AlCuFe alloy.

Altidis [3] evaluated the effect of adhesion on quasicrystalline composites having epoxy matrix. The results showed that although the improvement in adhesion did not occur linearly, samples with 25% QC showed a significant improvement in adhesion. Due to the low wettability and brittle nature, a decrease in the adhesion with the addition of quasicrystals was expected. However, the results indicated that it is possible to produce composites with good adhesion.

1.2Wettability

Wettability is the easiness of a solid being in contact with a fluid. The balance of the surface as well as the interfacial forces determines this easiness or difficulty of wetting.

According to Anderson [4], wettability is defined as the tendency of a fluid to spread or adhere to a solid surface. In another words, wettability is the process through which a liquid spreads or wets a solid substrate or surface.

Depending on the nature of the binding forces between the solid/liquid interfaces, the wetting can be divided into two classes: physical wetting and chemical wetting [5].

In physical wetting, the wettability is determined by the contact angle formed by the liquid in the solid. Equation (1) that describes the energy balance that governs the contact angle of the liquid on a surface is called the Young's equation [6]

where γ is called the free surface energy and the subscripts “S” and “L” indicate the solid and liquid, respectively. Fig. 1 shows a schematic diagram showing the contact angle (θ) made by a drop of a liquid on a flat and smooth surface. Wettability studies generally involve the measurement of the contact angles that indicate the degree of wetting in the interaction between a solid and a liquid. Small contact angles (˂90°) correspond to high wettability, while large contact angles (˃90°) correspond to low wettability [7]. Fig. 2 shows the phenomenon of hydrophobicity of rainwater in a plant leaf. Wettability is favored when the surface energy of the substrate is high and the surface tension of the liquid is low, thus the low surface energy polymers have difficult solubility [8]. Some factors may affect the behavior of the wettability of a solid by a liquid, such as surface roughness and heterogeneity, liquid–solid reactions, test atmosphere, time and temperature [5].

Fig. 1.

Illustration of contact angles formed by a liquid on a homogeneous and smooth solid surface. Wetting improves from right to left [7].

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Fig. 2.

Phenomenon of hydrophobicity of rainwater in a vegetal leaf (Personal Archive).

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1.3Relation between roughness and wettability

Roughness is defined as the “set of microgeometric deviations, characterized by small 24 protrusions and recesses present on a surface” [9]. The wettability property studied by surface engineering is used in many industrial applications, such as printing, painting, coating, adhesives, etc. This property is strongly affected by roughness when exposed to static or dynamic wettability [10]. The relationship of wettability and roughness must be studied because in the real scope of deposition of a liquid on a solid surface such surface by better prepared and polished will not be perfect, therefore, there will still be microscopic defects that will directly influence the result of the wettability. Fig. 3 expresses the difference between an ideal and actual solid surface. This relationship was defined in 1936 by Wenzel, who stated that wettability can be improved with addition of surface roughness. This situation is described in Equation (2).

where θw is the Wenzel angle and r is the roughness factor which is related to the actual surface area.

Fig. 3.

Relation between the contact angle formed by the deposition of a liquid and the ideal and real surfaces [11].

(0.11MB).

However, this Wenzel equation is based on the assumption that the liquid penetrates completely into the roughness grooves. This wetting situation on rough surfaces is called “Homogeneous Wetting” and under some rough conditions, especially when the roughness is high, this case cannot be represented by the equation, since air bubbles can be trapped in the grooves of the roughness under the liquid, and thus generating a heterogeneous wetting.

In the case of homogeneous wetting, the greater the roughness, the greater the wettability. As a consequence of the heterogeneous wetting, the roughness helps to decrease the wettability, because in this case the imperfections present on the surface of the material act to mechanically prevent the liquid from spreading.

Depending on the type of roughness present on superhydrophobic surfaces, it may hinder wettability due to the air concentrated in the spaces between the micro-holes, causing the drop to hang over these structures as a consequence of the surface tension effect. In the particular case of water, it can present an angle of contact with air of 180° or an angle superior to this when exposed to surface irregularities [12].

Nicolaiewsky and Fair [13] studied the influence of surface treatment on metallic plates and smooth and rough ceramic. They observed that for all liquids used the higher roughness of the surface of the material, the lower the contact angle and consequently the greater the wettability.

In his work, Souza [14] states that there are controversies in relation to roughness with wettability, because while some authors maintain that there is a relationship between these properties, others believe that good wettability can not be attributed solely to surface roughness, but also to the surface characteristics of solids.

1.4Surface energy

Surface energy can be termed as the difference between the energies of the species (solid or liquid) inside the material. For example, atoms and molecules of a liquid when under the surface of a material tend to move in such a way in order to occupy a lower energetic position, whereas the particles on the surface of a material exert only force toward the liquid, thus with which surfaces always become regions of lower energy. [15]

According to Nossa [16], there are several methods to calculate the surface energy of solids as a function of Young's equation. One of the most used being Owens-Wendt's theory. From this method, the polar components (dipole–dipole interactions, hydrogen bonds) and non-polar (dispersive forces) for the surface energy are combined according to Equation (3).

where WSL is the adhesion work between solid and liquid, and γSD and γSP are the dispersive and polar components of the surface energy of the solid, while γLd and γLp are the dispersive and polar components of the liquid.

2Scientific methodology2.1Obtention of the alloy and manufacture of quasicrystalline powder

The alloy with atomic composition Al59.2Cu25.5Fe12.3B3 was produced in an induction furnace under argon atmosphere. It was then treated thermally at a temperature of 720°C for 12h in a controlled atmosphere under argon gas. A Planetary Ball Mill was used for the production of the powder. The grinding took place through steel balls of different sizes and at a speed of 200rpm for 2h.

2.2Manufacture of epoxy/quasicrystal composite

The composites were manufactured in volumetric proportions of 0%, 1%, 10%, 20% and 30% QC. The resin was initially weighed to subsequently add the quasicrystalline powder, which was mixed for about 10min in such a way as to approximate the homogeneity of the system. Then the hardener was added to the mixture so that it was mixed for approximately 5min. Finally, the blend was cast in a PVC mold containing dimensions of 40mm in diameter and 15mm in height under a teflon plate so as to make it possible to detach the epoxy resin. The cure occurred at room temperature for 48h. After curing, the composites were machined so that their final dimensions were 35mm in diameter and 7mm in height. Fig. 4 shows the final result of the epoxy/quasicrystal composite.

Fig. 4.

Example of epoxy/quasicrystal samples after machining.

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2.3Characterization of the epoxy/quasicrystal2.3.1Scanning electron microscopy

The Scanning Electron Microscopy (SEM) technique was used to analyze the distribution of the particles along the polymer matrix as well as the apparent adhesion of the system. In the Thermogravimetric Analysis Techniques, the sample was placed in an alumina pan with nitrogen flow of 50ml/min to avoid oxidation. The composite samples were heated at a rate of 10° C/min in the range of 26°C to 500°C.

2.3.2Hardness and roughness test

In relation to the hardness test, it was performed on a portable Kori-branded durometer with a scale of 0–100 Shore D. Seven indentations were randomly performed in each sample in order to extract the arithmetic mean. This essay was performed at the UFRN Tribology Laboratory. For the surface analysis the roughness test (Ra) was used.

2.3.3Wettability test

The Wettability test was performed at the UFRN Tribology Laboratory. The equipment used is composed of a volume adjustable pipette, fixed light and a sample apparatus. Two measurements were performed on each test specimen. The liquids used were Saline Water and Paraffin Oil.

The drop used was approximately 3mL and the average weights are 0.024g for saline water and 0.017g for paraffinic oil. After being deposited on the surface of the specimens, a time of 5s was allowed to stabilize the drop on the surface and then the image was recorded using the Sony H×300 Full Hd Zoom 50× 20.4 MP camera.

The images were analyzed for the measurement of the contact angles in Surftens software version 4.5. The measurement of the contact angle was given by measurements of the diameter of the base of the drop and the height of the drop, where each drop was measured seven times.

3Results3.1SEM

The reinforcement was well distributed throughout the matrix although the presence of some agglomerates was evident. This was due to the low granulometry of the particles which facilitate agglomeration. Another factor that contributed to the presence of agglomerates (Fig. 5, (a)–(e)) was the increase in the volumetric fraction of the quasicrystalline, resulting also in the increase of the viscosity of the system which caused a greater difficulty to mix the quasicrystal with the matrix. In addition to the agglomerates, microbubbles were detected along the composite (Fig. 6). These bubbles come from gases that have failed to surface, since the curing process abruptly increases the viscosity of the resin, making it difficult or impossible to move.

Fig. 5.

Scanning electron microscopy of the composite containing (a) 0%, (b) 1%, (c) 10%, (d) 20% and (e) 30% by volume of QC with the presence of agglomerates.

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Fig. 6.

Scanning electron microscopy of the composite containing 30% by volume of QC with the composite containing 30% by volume of QC presence of agglomerates.

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Another factor to be observed by SEM was the wetting of the resin in the quasicrystal particles. Although mixing materials of a distinct nature results in a poor or even non-existent interface, the quasicrystals particles have been continuously enveloped by the polymer matrix as shown in Fig. 7.

Fig. 7.

Scanning electron microscopy of the composite containing 30% by volume of QC showing good wetting between the polymer and the metal. It is possible to observe the continuity of the interface, which presents itself in a minimal and well defined way.

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Such good wetting between the matrix and the reinforcement is an initial requirement for achieving a good interface and consequently an excellent transmission of forces between the particles and the matrix.

3.2Thermogravimetric analysis

In the thermogravimetric analysis the mass variation in relation to the temperature is observed. This variation can be related to the chemical reactions of loss or mass gain when exposing the material to certain temperatures. Fig. 8 shows the curves obtained by the thermogravimetry technique.

Fig. 8.

Curves obtained by the thermogravimetry technique.

(0.06MB).

It is observed that all the composites lost mass. This loss of mass is related to the degradation of the polymer matrix, considering that the pure quasicrystal does not present significant loss of mass until the temperature of 500°C. In addition, the polymeric materials have a higher thermal sensitivity when compared to metallic materials through the nature of their chemical bonds. This decrease in composite mass resulted from thermal degradation reactions of the epoxy resin and occurs predominantly at two levels. The first at approximately 180° C and the second at about 360°C, these values set the temperature of a possible application to at most 180°C.

The mass loss was reduced according to the increase in the quasicrystal volume fraction in the composite, this is due to the fact that the density of the quasicrystal is on the order of approximately four times the value of the density of the resin, resulting in a greater percentage of mass than the volumetric one. For example, although the composite has only 30% quasicrystal volume, in mass percentage it has about 38% resin and 62% quasicrystal, thus making the quasicrystal in mass percentage, the major element. As the resin is more thermally sensitive, the composites that present the highest percentage of resin mass will also present the greatest mass loss when exposed to high temperatures, which leads us to conclude that the higher the amount of quasicrystal, the lower the mass loss of the composite due to thermal stability of the QC overlapping of the epoxy resin.

3.3Shore D hardness

Fig. 9 shows a boxplot-type graph for shore D hardness as a function of the volumetric fraction of quasicrystal added to the polymer matrix (Epoxy). A significant increase in average shore D hardness was observed with the increase in the content (up to 20% QC) of quasicrystal added to the matrix. With increasing quasicrystal content from 20% to 30%, the average shore D hardness showed a slight increase (from 66 to 67.5), this result is lower than that presented when the QC content is changed from 10% to 20%, that is, the average shore D hardness increases from 58 to 66, respectively. It is also possible to observe that the greatest dispersion of data concerning shore D hardness was verified in the sample with 10% QC. This dispersion is associated with the presence of non-uniformly distributed agglomerates in the polymer matrix. The data set for the other QC fractions presented a low dispersion, in addition, the data were asymmetric negative, that is, presenting a median line equal to the third quartile.

Fig. 9.

Shore D hardness.

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3.4Roughness

Fig. 10 shows the roughness behavior Ra as a function of the volumetric fraction of quasicrystal added to the polymer matrix. The increase in the volumetric fraction of QC causes a slight increase in the average value of the roughness Ra. The sample with 10% QC showed a distribution of the data with negative asymmetric behavior and the sample with 30% QC had an asymmetric positive behavior. For the contents of 1% QC and 20% QC the data set behaved symmetrically, that is, the median line was in the center of the rectangle. The highest dispersion of the data was presented by the sample with 30% QC content. It is also observed in Fig. 10 that the increase in the average value of roughness Ra was higher between the composites with 1% of QC and 10%. The increase in the roughness between them was of 0.0467μm, which represents approximately 54% of the total gain. It was concluded that, of the total net increase of 30% of the roughness (between pure resin and the composite with maximum volumetric fraction), more than half of the roughness characteristics acquired in the product occurred up to 10% of the second phase, while the remainder was distributed among the other composites. In summary, it is assumed that the increase in roughness was more expressive up to 10% of QC, sequentially there was a tendency to the stability of the roughness value.

Fig. 10.

Result of the roughness (Ra) test.

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3.5Contact angle and wettability

For a better visualization of the contact angles obtained, the values are shown in Fig. 11 (a) and (b)

Fig. 11.

Comparative result between the contact angles obtained through the wettability test for: (a) saline water and (b) paraffinic oil.

(0.12MB).

When analyzing the effect of increasing the volume fraction of QC on the epoxy resin, in the presence of saline water, it is possible to notice that as the amount of quasicrystal increased, the contact angle formed between the solid and the saline water increased, thus making the composite material more hydrophobic in the presence of this fluid. The same did not happen when the composite was exposed to paraffinic oil, that is, it presented opposite behavior, it became more oleophilic. Consequently the wettability decreased in the presence of saline water and increased to paraffinic oil.

In relation to the pure quasicrystal, the observed effect acted in an inverse way, that is, the oil showed less wettability while the saline water presented greater wettability. This was due to the surface behavior of the quasicrystal, according to Dubois [17] under atmospheric conditions, the surface of the quasicrystal behaves more like a covalent surface, for example, like Teflon, than properly as a metal, thus being the polar component of the surface energy is very low, which suggests low surface polarity, i.e., the formation of a non-polar surface. Thus, unlike the composite that behaved like polar surface due to the polarity of the epoxy resin, the quasicrystal behaves apolar, resulting in the greater attraction of saline water and greater repulsion of paraffinic oil.

There are two phenomena that can justify the behavior of the composites presented in the curve (Fig. 11 a and b). The first is the influence of the energy of the components. Given different characteristics of each liquid (polar—saltwater, apolar—paraffinic oil), these react differently when in contact with the solid. Considering that the epoxy resin has a highly polar character, it is suggested that its characteristics overlapped the surface characteristics of the quasicrystal, which justifies the adhesion of the oil and the repulsiveness of the water.The second phenomenon that probably occurred was the influence of the roughness with the increase of the second quasicrystalline phase. In the case of the solid exposed to saline water, the contact angle increased and in relation to the paraffin oil this angle decreases. It is known that the roughness can act in two ways, depending on the type of wetting that can be homogeneous or heterogeneous. According to Burkarter [12], in the case of homogeneous wetting, the greater the roughness, the greater the wettability. As a consequence of the heterogeneous wetting, the roughness helps to decrease the wettability because the liquid does not penetrate the entire surface. This theory may justify the antagonistic behavior of the two liquids. In the case of saline water, the behavior presented was heterogeneous wetting, that is, the liquid could not wet all the surface and expel the air present between the roughness, being forced upwards which brought on the largest contact angle. As for paraffin oil, the situation was opposite to that of water, that is, its wetting was homogeneous, increasing with the increase of the second phase in the composite, this may have occurred due to the higher viscosity of the oil which through the gravitational force was able to overcome the barrier formed by the roughness and wet the entire surface, thus being absorbed in its entirety.

Fig. 12, column (a) shows the images of counted angles formed between solids and saline water as a function of the QC added.

Fig. 12.

Column (a) shows the images of counted angles formed between solids and saline water as a function of the QC added; column (b) shows the images of the counting angles formed between the solid and paraffinic oil as a function of the QC added.

(0.16MB).

Fig. 12, column (b) shows the images of the counting angles formed between the solid and paraffinic oil as a function of the QC added.

3.6Surface energy

Table 1 presents the estimated results of the surface energy values of the pure resin and composites, obtained through Equation (3).

Table 1.

Surface energy values.

Volume fraction (%)  Surface energy of composite and pure QC (mN/m)
  γSP  γSD  γS  WSL
        Saline water  Paraffinic oil 
3.643  14.727  18.370  30.88  4.44 
2.612  16.546  19.158  33.82  3.37 
10  2.309  16.991  19.300  34.89  2.76 
20  1.936  18.959  19.959  36.24  2.22 
30  0.660  21.103  21.763  43.34  0.97 
100  20.977  6.3695  27.340  12.75  24.28 

From the values expressed in Table 1, it is observed that there was no significant change in the surface energy of the composites that justifies the changes presented in the values of the contact angles corresponding to each composition. This result suggests that the changes that occurred in the wettability behavior are associated to the increase of the roughness and/or microporosity presented by the composites, according to data presented in Fig. 10, and not by the change in the surface energies of the composites formed. This result agrees with Burkarter [12], which suggests that depending on the type of roughness present on hydrophobic surfaces, this can hinder wettability due to the air concentrated in the spaces between the micro-holes, causing the drop to hang on these structures as a consequence of surface tension effect. In the particular case of water, it may have an angle of contact with air of 180° or an angle greater than this when exposed to surface irregularities. What explains the fact that even the composite presenting increase in its surface energy, as it is added a second phase it presents decrease in its wettability.

For the data presented in Table 1, the calculated result of the pure quasicrystal energy corroborates the data presented in the literature. According to Dubois et al. [17] the icosahedral Al–Cu–Fe alloy presents surface energy under atmospheric air between 24 and 25mN/m.

4Conclusions

The composites presented good distribution of the particles as well as the presence of microbubbles and agglomerates. The good wettability of the epoxy matrix in the quasicrystalline particles was achieved and could be observed via SEM through the matrix/quasicrystalline powder interface.

With the wettability test it was possible to analyze the contact angle and consequently the wettability of the composite. Comparing the composite and the pure resin, the contact angle increased when the liquid used in the test was the saline water and decreased when exposed to the saline oil. It can be concluded that probably this small variation of the contact angles in relation to the increase of the composition occurred under the influence of the roughness. In relation to saline water, the roughness has operated in order to prevent the liquid from spreading on the composite. As for paraffinic oil, it influenced in an opposite way, absorbing the oil and reducing the angle formed. When analyzing the pure quasicrystal, the effect was contrary to that of the composite, i.e., when it was exposed to saline water the contact angle decreased, already in relation to the paraffinic oil the resulting angle increased. This effect is related to the low polarity and/or apolarity of the QC that when exposed to an apolar liquid (paraffinic oil) tends to repel, thus forming a greater contact angle.

Through the calculation of surface energy it is possible to estimate the values of surface energies. It is concluded that, there was no significant change in the surface energy of the composites that justifies the changes presented in the values of the contact angles corresponding to each composition. This result suggests that the changes that occurred in the wettability behavior are associated to the increased roughness and/or microporosity presented by the composites. Regarding pure QC, the value found through the Surface Energy Equation was in agreement with the literature.

In general, the production of this composite is viable due to its reinforcing constituent, quasicrystal, being a low cost material when compared to other reinforcements already used, such as Alumina.

Conflict of interest

The author declares no conflicts of interest

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