Influence of Ceramic Particles Additives on the Mechanical Properties and Machinability of Carbon Fiber/Polymer Composites

Hybrid polymer composites have become the interest of the world, especially in mechanical and electronic applications. Recently, advanced machining techniques such as abrasive water jet machining of hybrid polymer composites have been used to solve many problems including the ability to form complex shapes, high performance, better surface features, and high levels of accuracy. In this study, the effect of ceramic particle (SiC and Al2O3) constituents on the mechanical properties and machinability of the hybrid polymer composites was investigated. The results of mechanical tests showed that an improvement in the tensile and flexural properties appeared using a hybrid polymer constituent (70 wt% Epoxy + 15 wt% CF + 10 wt% Sic + 5 wt% Al2O3). However, this hybrid polymer constituent includes the lowest value of impact strength. Also, the morphological analysis indicated the uniform distribution of particles, the best defect-free surface, and the bonding strength between the reinforcement and the epoxy matrix can be obtained using this constituent which has contributed to the improvement of the mechanical properties. The abrasive water jet automation process was also performed based on the design of experiment (Taguchi L18 design) to study the machinability the hybrid polymer composites in terms of surface roughness, hardness, and kerf width. The analysis of variance concluded that the constituent (70 wt% Epoxy + 15 wt% CF + 10 wt% Sic + 5 wt% Al2O3) has the most influential factor on the machinability of the hybrid polymer composite followed by traverse velocity, and then stand of distance that used during abrasive water jet machining. Moreover, the grey relational analysis was beneficially used to determine the multi-optimization of abrasive water jet machining parameters.


Introduction
Polymer compounds are increasingly being used, and the polymer has become an alternative to many materials, particularly the hybrid polymer loaded with fibers and particles together, where its applications have expanded to include the construction of automobiles, marine, aerospace components, military, sport and leisure industry [1,2]. Recently, the hybridization method of polymer composites recorded the enhancement of mechanical properties for different types of polymer materials [3]. Because of its numerous acceptable features such as adhesiveness and chemical and environmental inertness, epoxy resin is the typical matrix material in polymer matrix composites.
It produces no byproducts during curing and has negligible shrinkage qualities [4].The properties of the matrix material can be improved by incorporating inorganic filler particles and synthetic fibers. The simultaneous use of fibers and fillers in a polymer matrix results in the formation of a hybrid polymer composite [5]. It was observed that the existence of graphene nanoplatelets (GnPs) and Titanium dioxide (TiO 2 ) reinforcement in resulted in an improvement in the mechanical characteristics of polypropylene matrix [6]. Carbon fiber (CF) reinforcement of polymers has recently gained prominence due to its better mechanical qualities, such as strength-to-weight ratio, hardness-to-weight ratio, and chemical resistance [7]. One of the researchers employed carbon fibers to reinforce the airframe's scarf adhesive joints, as well as Sic and Al 2 O 3 nanoparticles mixed with epoxy resin. This installation produced good and great results by improving the strength of the adhesive sash joints, so prolonging their life and lowering the likelihood of failure [8]. To improve the mechanical properties of polymer composites, ceramic 1 3 particles can be used. In one of the investigations, silicon carbide was used along with sisal fibers in an epoxy matrix to fill the voids and pores that were not occupied by the fibers, as they showed superior mechanical properties [9]. Impact, hardness, and tensile properties were improved but flexural properties decreased with loading bamboo fiber and Al 2 O 3 to hybrid epoxy composites [10]. An improvement in the impact, flexural, hardness, and tensile strength characteristics were detected by adding Al 2 O 3 and SiC to the pure PA6 [11]. Silicon carbide whiskers was loaded to the carbon fiber/epoxy and the enhancement was clearly observed in the tensile modulus, tensile and flexural strength as compare to the composite without silicon carbide reinforcements [12]. The application of the compound is mainly determined by the cost of manufacture as well as the capacity and quality of the machine used for the materials used. As a result, there are many conventional CFRP treatments but they are not popular in most cases because they produce many defects, such as fiber drag, change of direction, entanglement, layer separation and use of particles that pollute the environment [13]. Furthermore, touching the tool with the workpiece causes compound damage and tool erosion, resulting in losses and low production [14]. As a result, conventional treatments have given way to unconventional ones such as laser processing, abrasive water-jet manufacturing (AWJ), abrasive air jet machining (AAJM), electrical discharge manufacturing (EDM). However, laser treatment produces a heat-affected zone, resulting in significant heat generation and workpiece damage and because electrostatic treatment is costly [15][16][17]. It is preferable to use abrasive water jets (AWJ). They are often used for hard and brittle materials because they are the most appropriate in terms of cost and give accurate dimensions and do not produce a vacuum that does not create the heat affected zone Therefore, when comparing abrasive water jet machining (AWJM) with other unconventional processes, it is the most appropriate treatment to obtain the durability, flexibility and clean environment of the polymer hybrid compound [18][19][20]. In the GRP compound that was run on an abrasive water jet machine, it was found that the stand of distance, pressure, and abrasive flow rate affect the material removal rate, hole precision, surface roughness and fiber direction, where the output parameters were improved by Taguchi design procedure in achieving the optimum input parameters [21]. Machining the hybrid glass fiber reinforced epoxy filled with powder of waste SiC grinding wheel using AWJM based Taguchi design produced the optimum material removal rate and kerf width under optimum parameters of speed, stand of distance, and water pressure [22]. using yielded to control the Moreover, in one study, low kerf taper (kt) and better average roughness (Ra) of hybrid laminated polymer composite (Carbon and S-Glass fibers/SiC nano particles/epoxy) were obtained during abrasive water jet machining. Also, the optimal combination of AWJM parameters (traverse velicity, abrasive mass flow rate, water pressure and stand-off distance) were determined [23]. High-strength epoxy composites were obtained by hybridizing it with Kevlar 49 and S-Glass fibers and silicon carbide nanoparticles and abrasive water jet machining was conducted for determining the optimum parameters (abrasive flow rate, stopping distance, and traverse velocity) that produce the maximum material removal rate and minimum Kerf width through the statistical methodology (ANOVA) [24]. The influence of fiber and fillers reinforcement on the polymer composites during conventional and nonconventional machining processes was reviewed to support the investigators in the suitable choice and loading of various types of reinforcements. The review study confirmed the need for more investigation in the field of AWJM of polymer composites and using the multi-response optimization methods to discover for enhancement of abrasive water jet machining performances on fiber-reinforced polymer composites [25][26][27].
According to the introduced review above and to promote the discovery of the influence of ceramic particles additives on polymer composites' mechanical properties, the hand layup process was used to prepare the hybrid epoxy composites then tensile, flexural, and impact strength of hybrid polymer composites have been evaluated. In addition, the possibility of machining the hybrid polymer composites and evaluating the machinability based on Taguchi design by an abrasive water jet machine was adopted. Further, the analysis of variance (ANOVA) was used to determine the effects of selected AWJM parameters such as ceramic particle constituent, standoff distance, and traverse velocity on various response variables, such as surface roughness, kerf width, and hardness. Furthermore, the grey relational analysis (GRA) technique was used to attain the multi-optimum machining responses simultaneously which helps for obtaining the best surface quality and integrity.

Materials Used
Plain weave carbon fibers (CF) with high strength to weight ratio were supplied by (MB fiberglass Company) as the primary reinforcement in a matrix of epoxy resin (Sikadur®-52 LP (US)) at high strength, low viscosity and modulus of elasticity (1′060 N/mm 2 ) purchased from.. Epoxy resin that consider compound (A) was mixed with 1 3 the hardener as compound (B) at a weight ratio (A: B) was 2:1 according to the recommendation of its manufacturer (Sika Egypt for Construction Chemicals Company).The secondary reinforcement from ceramic particles include silicon carbide(SiC) particles with a particle size of 37 μm and hard alumina (Al 2 O 3 ) particles with a size of 25 μm was used as a filler. Table 1 shows some of the typical properties of epoxy resin and 1 m wide of plain weave carbon fiber used.

Preparation of Hybrid Polymer Composites
Hybrid polymer composites were synthesized by the hybrid epoxy resin matrix containing various volume fractions of silicon carbide microparticles (10, 5%), alumina (5%, 10) and a fixed percentage of 15% carbon fiber. The weights were measured using a sensitive balance and then the hybrid mixture prepared depending on following Eqs. (1 and 2) [28,29]. Firstly, hybrid mixture (epoxy/ceramic particles) was stirred manually and then mechanically using mechanical stirrer (LS-50D) for 15 min with (50W, 220 V, 50 Hz) and 750 rpm. The mixture was placed in the ultrasonic device for 20 min to remove air bubbles that may cause defects in the mixture and prevent agglomeration of particles and to achieve a homogeneous and uniform dispersion. After mixing, the hardener was added to the mixture. As shown in Table 2 six samples of hybrid polymer composites (A1-A6) were prepared with dimensions (30 * 30 mm) at different volume portions using hand lay-up process. A uniformly distributed liquid composition of (epoxy-hardener/ Al 2 O 3 + SiC particles) was poured in the mold carefully and a plain weave carbon fiber mat (6 layers) were placed layer by layer over the poured mixture on the surface of the mold. Since:

W C
Weight percentage of composites.

W f
Weight percentage of fiber in the composite.

W p
Weight percentage of particles in the composite.

W r
Weight percentage of risen (hardener + epoxy). W C( hybrid) Weight percentage of hybrid composite.

Mechanical Properties Measurement
Tensile strength is one of the most important mechanical properties to describe, particularly in modern vehicles [30]. Samples were cut to the specified dimensions according to the ASTM E-8 M scale as shown in the Fig. 1a. Tensile test was performed on a tensile tester (Model:H50KT), brand: Tinius Olsen Company manufacture. Also, Fig. 1b reveals to the six prepared specimens according to ASTM BS 03-73B with dimensions (50 mm long and 10 mm wide) to measure the flexural strength using a flexural test apparatus (H50KT) manufactured at Tinius Olsen company and based on three-point bending method. Figure 1c shows the samples that cut with dimensions (length 55 mm and width10mm) according to the standards of the specification (Model: ASTM-D256) to measure the impact strength using the Izod test by the pendulum impact tester (Model: XJU-22) manufactured at Jinan Liangong Testing Technology Company. The fractured surface of tensile samples was subjected to scanning electron microscopy (SEM) (Model: INSPECT S50, FEI Company) with a voltage (2 kV) as shown in Fig. 1d, to investigate the morphology of the hybrid composites and to understand the mechanism of bonding between the matrix and the reinforces.

Experimental Design
Experimental design is a high-performance method. Scientists, technicians and engineers used the experimental design to facilitate and develop product design, as cost and time are reduced, which are among the most important factors affecting manufacturing process. In this study, experiments were carried out using an experimental design based of Taguchi (L 18 ) method in order to select and optimize input AWJM parameters for the fabricated samples. In the current work, three AWJM parameters (hybrid polymer constituents, stand-off distance, and traverse velocity) were selected.. The chosen input AWJM parameters and their three levels are shown in the Table 3. Taguchi technology uses the concept of signal-to-noise ratio (S/N) to determine product quality characteristics and evaluate the results. The concept of signals is an indicator of how the response diverges compared to the objective value under various noise cases. In this study, the appropriate objective for hardness was selected (larger is better) using the applicable S/N Eq. (3) while surface roughness and kerf width were calculated using (smaller is better) using the applicable S/N Eq. (4) [31][32][33]: where: S/N stands for the signal to noise ratio, which depends on the response objective (i.e. maximum or minimum), n is the number of repetitions, yi is the experimental value.

Machining Process
Abrasive water jet machining (AWJM) was conducted based of Taguchi L 18 using Minitab19 software to study the machinability of the hybrid polymer composites in terms of surface roughness (Ra), hardness (H) and kerf width (KW). The abrasive water jet machine (Model:TK-TRUMP50-G3020) with sand particles (SiO 2 ) as abrasive material at size (80mesh), pressure (1200 Kpa), and orifice (1 mm) was used for machining process. Figure 2 presents abrasive water jet AWJM setup for the machining of hybrid polymer composites. Table 4 represents the Taguchi L 18 design that considered during AWJM. The kerf width was measured by stereo microscope (Model: MEIJI TECHNO) in three places and the average values for the formed gap were taken. The shore D hardness tester (Shore D, DIN 53,505 ASTM D2240) was used to evaluate the hardness of the machined hybrid polymer surface for eighteen samples. Hardness (H) was measured three times for each sample and the averages was considered. The surface roughness test was carried out using TR200 surface roughness tester for three regions on the each machined surface then the average values for the surface roughness were taken.

Multi Optimization using Grey Relational Analysis
The Taguchi design is a strong design to detect the single optimization trouble. Grey relational analysis (GRA) is a way to take complex problems that have multiple factors and responses and combine them into one goal to get the best alternative from multiple experiences. Therefore, when determining quality based on several attributes, Taguchi design gives each response a separate attribute, and looking at the attributes of each response may lead to wrong production of the overall performance, and it cannot be obtained the specific feature. Recently GRA is widely used to obtain a specific control factor that controls the overall performance in the AWJM, which are the normalization of performance results, the calculation of the corresponding gray relational coefficients, the calculation of ANOVA to determine the importance of the parameters of AWJM, and finally the calculation of the gray relational grade and determining the multi optimization of AWJM parameters [13,[34][35][36].  1 3

Tensile Strength
The results of the tensile strength and modulus of elasticity represent in Fig. 3 The presence of ceramic particles leads to an increase in the resistance of the composites which in turn leads to an increase in the interaction between the epoxy resin and the particles. Thus, increasing the surface area of the filler material produces effective transfer of pressure between the matrix and the fibers and strong interconnection.,This points out that interfacial bonding between carbon fiber and epoxy matrix has been mended by adding ceramic particles. However, as observed in Fig. 3 (sample A5 and A6), the adding more ceramic particles may lead to slight or no more improvement in the tensile strength. This is due to the loading of more ceramic particles might lead to the formation of air bubbles during the mixing process as well as insufficient time of ultrasonication process might be lead to the creation of voids and weaknesses in the composites. In turn, this leads to inadequate bonding between the selected three various constituents (epoxy, carbon fiber, and ceramic particles), and the loads may not be conveyed efficiently from one end to another and as a result, there is a drop in tensile strength of the hybrid composites. These findings agree with previous studies [7,12,37] and also were verified by the observed morphology for the fracture surfaces of the tensile test samples.

Flexural Strength
A flexural test was conducted to study the effect of adding ceramic microparticles on the flexural strength of epoxy composites reinforced with CF. The test results in Fig. 4 showed  and increases the adhesion strength between the epoxy resin and CF, thus increasing the flexural strength as stated also in the previous studies [38][39][40][41].

Impact and Hardness Test
The results of the Izod effect test for the hybrid polymer composites were analyzed. As shown in the Fig. 5, ceramic particles additives participate in decrease the impact strength.. The lowest value of the impact force reached (1 J) in the sample A6 that contains the highest weight percentage of ceramic particles (10 SiC wt% + 10 Al 2 O 3 wt% & 15 CF wt% + 65 Epoxy wt%). The reason for this decrease is due to the brittle behavior and high hardness value of the ceramic particles, which leads to an increase in the brittleness of the compounds and a decrease in the impact strength [42]. The shore D hardness was measured for the hybrid polymer composite samples. As shown in Fig. 6, the hardness of hybrid epoxy composite was increased by adding different percentages of micro silicon carbide and alumina particles. The results show that the ceramic particles additives has a significant effect on the hardness of the hybrid epoxy composite, as the highest value of hardness was reached to 69.6 H in sample A4 and enhancement percentage 12%. This may be attributed to the high hardness nature of the ceramic particles in the produced hybrid composites. While the agglomerations of particles in samples A5 and A6 can be the reason for reducing the hardness at these weight percentages of ceramic particles.

Morphology Analysis
An SEM analysis was carried out to investigate the morphological surface and the cause of the failure of the laminated surfaces, the observed defects, comprehend the bonding mechanism between the matrix and the reinforcements, and the distribution of the reinforcements within the matrix for the fracture surfaces of the tensile test samples. Figure 7 shows the morphology of the hybrid CF/ceramic particles reinforced epoxy composites. Figure 7a shows morphological surface of sample A1 with magnification of 1400 × and 2kv. This Figure clarify the voids and the weak bond between the matrix and the fibers which leads to the great weakness in the matrix and collapse of the composites due to its inability to withstand high stress. Figure 7b illustrates morphological surface of sample A2 with magnification 500 × and 2kv. The presence of white dots indicates to the presence of ceramic particles visible on the surface of the hybrid polymer composite i.e. the heterogeneity between the matrix and the reinforcements. The fracture surface of sample A3 at magnification 80 × and 2kv is shown in Fig. 7c. As shown, the presence of the crack in the matrix and the Al 2 O 3 particles is not homogeneous with the epoxy compositeswhich cause to the failure of this composite due to the irregular distribution between the layers of carbon fibers and the presence of Al 2 O 3 particles. Figure 7d shows the morphology of sample A4 at magnification 1000 × and 2kv, and reveals to the smooth surface and good distribution of ceramic particles in the composites along with strong interconnection between the hybrid constituents (ceramic particles, CF and epoxy matrix). Thus, the uniform appearance of ceramic particles throughout the laminated in the microstructure of sample A4 and the decrement of void tenor contribute to the improvement in the mechanical properties of this hybrid polymer composite and it can be nominated to be optimal sample among the other samples. Figure 7e and f at magnification 400 × and 2kv appears to the morphology fracture surface of samples A5 and A6 respectively. The presence of agglomerations of ceramic particles inside the sample A5 and A6 is clear and this as a result of the increase the weight loading of SiC and Al 2 O 3 particles. For that reason, it might be the more loading of weight of particles is not useful in some cases, and can cause to the deterioration in the mechanical properties of hybrid polymer composites.

Machinability Evaluation of Produced Hybrid Polymer Composite
The results of the experimental work for Ra, H, and KW are presented in the Table 5. Based on F-value with P-value at level of significance equal to 5%, the ANOVA was achieved to decide the significance of AWJM parameters on machinability of the hybrid polymer composites. Figure 8 illustrates the normal probability scheme for Ra, H and KW respectively. As can be noted from this Figs that all the points of the analyzed values are located on a straight line or near it, and this indicates to the normal distribution of the obtained data.

ANOVA Analysis of AWJM Parameters
The results of ANOVA for the Ra are presented in Table 6. The main parameter affect the Ra is the constituents of hybrid polymer composite (A), where its contribution is about (51.302%). The second factor in the effectiveness is the traverse velocity (v) where its contribution is about (39.558%), while the SOD is not having remarkable effect on the surface roughness where its contribution is only (4.175%). Table 7 shows the ANOVA results of machined surface hardness (H) for hybrid polymer composites. The main factor has most influential on the hardness is the traverse velocity, where its contribution percentage is 51.67379. The second effective factor is the hybrid polymer constituents (A), where its contribution percentage is 32.71131%followed by   the SOD that has less effect than other parameters on the hardness with contribution percentage is 10.81797%. The effect of AWJM parameters on the kerf width (KW) is stated by ANOVA results in the Table 8. It was appeared that the hybrid composite constituents (A) is the most affecting parameter on the KW with contribution percentage equal to 82.98676%. Then the second parameter was the traverse velocity (v) at contribution percentage 12.49051% while the SOD has not shown significant effect on the KW of the machined hybrid polymer composites with low contribution percentage equal to 0.089879%. Figure 9 represents the contribution percentages for all AWJM parameters on the Ra, H, and KW respectively.

Regression Analysis
The final regression Eqs.

Grey Relational Analysis
The multi optimization is used to get the most out of a limited set of experiences and is an important method for decisionmaking in industry challenges. Nowadays, the application    of optimization techniques is necessary for the manufacturing unit to increase the demand for a high-quality product in the market. Grey relational analysis (GRA) is a way to take complex problems that have multiple factors and responses and combine them into one goal to get the best alternative from multiple experiences. The grey relational analysis (GRA) is used in this research to obtain the multi-optimization and ideal input parameters that give optimal values for different responses that control the overall performance in the AWJM. The GRA consists of the following main steps:

Normalization of Performance Results
In this step the original data sequence is converted into a similar, comparable sequence called normalization of performance results based on quality characteristics of responses that obtained from machining process. The performance results are normalized in a range of numerical values between 1 and 2 to obtain comparable data. The surface roughness, kerf width and hardness of the hybrid polymer composites were determined as the responses of AWJM for normalization. To normalize the results of surface roughness performance and kerf width performance, the lower is the best criterion was chosen as indicated in the Eq. (8).
However, the criterion of the higher is the best was chosen to normalize the results of hardness performance and calculated on the basis of the Eq. (9) [13,35,36]. Table 9, explains normalizing values of gray relational analysis.
W h e r e : i = 1, 2, … … … g, and z = 1, 2, … … … j. g : number of experimental trials, j: number of characteristics response. y i (z) : represents the sequence value after data processing, y 0 i (z) : original sequence value,min y 0 i (z) : minimum value of response, y 0 i (z) : maximum value of response.

Calculation of Grey Relational Coefficients
After normalizing the performance results, the gray relational coefficient is calculated to compare the relationship between the actual and ideal experimental results. The gray relational coefficient is calculated by the Eq. (10) [13,35,36]: Where: i (z) : Gray relational coefficient, : usually used 0.5 to equal weights of all responses, Δ oi (z) : deviation sequence that must be calculated, where Δ oi (z) , Δ min , Δ max can be obtained from the Eqs. (11, 12, and 13) respectively [13,35,36]. As shown in Table 10 the deviation sequence values for each response was calculated. Table 11 represents the gray relational coefficient ( i (z) ) values for each response. Calculation of Grey Relational Grade Gray relational grade (GRG) is used to determine the relation between the input AWJM parameters (A, v, SOD) and output characteristics (Ra, H, KW) inclusively, where the maximum GRG is taken to choose a multi optimal AWJM parameters as it is calculated using the Eq. (14) [36]. Gray relational grade (GRG) represents the average value of each series of the gray relational coefficient. Table 12 states the results of gray relational grade and the rank. As shown in Fig. 13, the experimental run number 12 achieves the maximum logical gray relational grade and first rank.
Where x: Number of responses to be investigated.

ANOVA for Grey Relational Grade
For further investigation, ANOVA of grey relational grade was used to obtain the multi combination effect of AWJM parameters, which were conducted by gray relational grade analysis based on Taguchi technique (TGRG). Table 13 shows the analysis of variance for grey relational grade and the importance of the AWJM parameters and their control, which greatly affects the responses. It can be concluded from Table 13 that the hybrid polymer constituents (A) is the most important parameter which greatly affects the machinability (responses) at contribution percentage equal to 79.70524%, followed by traverse velocity (v) with contribution percentage equal to 12.33802%, while SOD has lower contribution percentage equal to 6.77049%. The contribution percentages (14) of AWJM parameters base on ANOVA of grey relational grade are represented in Fig. 14.
The ideal product quality can be obtained by larger Grey relational grade. The multi optimum levels of AWJM parameters (A 4 , v 3 , SOD 3 ) that affect responses simultaneously and recorded the highest gray relational grade is shown in the Fig. 15. It was observed that the better quality characteristics and best multiple performance can be achieved using hybrid polymer constituents (70 wt% Epoxy + 15 wt% CF + 10 wt% Sic + 5 wt% Al 2 O 3 ) with high speed (v) at (300 min/mm) and high stand of distance (SOD) at (6 mm). is. As mentioned in the morphology study, these constituents gave the uniform appearance of the reinforcement (carbon fiber and ceramic particles) and provided the best improvement in the mechanical properties and in turn participated in produce optimum machined surface.. Further, the high value of traverse velocity (v) i.e. rapid transfer of the nozzle with the high stand of distance reduces the number of flowed abrasive particles (SiO 2 ) from the jet that strikes the machined surface and so contributes to ceramic particle fragmentation instead of their detachment and this participates in producing multi optimum characteristics (Ra, H, KW) as found in previous studies [23,35,[43][44][45].

Verification Test Outcome
A verification test was conducted depending on the optimum levels of AWJM parameters (A 4 v 3 SOD 3 ) that suggested based on the S/N ratio for grey relational grade. The results

Conclusion
In this study, the effect of adding ceramic particles on the mechanical properties of the hybrid polymer composites was investigated. Influencing abrasive water jet parameters on the responses (surface roughness (Ra), hardness (H), kerf width was evaluated using design of experimental based on the Taguchi L 18 design, and analysis of variance. The multi optimal abrasive water jet machining parameters was determined using grey rational analysis method. Based on the reached results in this study, the following conclusions can be drawn: 1. The hybridization by hand lay-up method using ceramic particles (SiC and Al 2 O 3 ) was demonstrated an efficient performance in enhancement of polymer composites properties. 2. Best improvement in the mechanical properties of the hybrid polymer composites (tensile strength, flexural strength, hardness) was recorded at constituents (70%wt Epoxy + 15% wt CF + 5% wt Al 2 O 3 + 10% wt SiC)., 3. The morphological analysis showed that the best defectfree surface, the bonding strength between the reinforcement and the epoxy matrix, and the uniform distribution of particles were within the hybrid polymer composites at constituents (70%wt Epoxy + 15% wt CF + 5% wt Al 2 O 3 + 10% wt SiC) 4. Based on the results of mechanical properties and the morphology analysis, it can be recommended to use this constituents of hybrid polymer composites in the different parts of automobiles and aircraft that require high hardness with high tensile and flexural strength and do not require high toughness. 5. A design of experimental based on Taguchi design is effective for analyzing results and obtaining individual optimal abrasive water jet machining parameters. 6. Based on analysis of variance for grey relational grade, the most important abrasive water jet machining parameter that affect the combined output responses is the weight percentage of ceramic particles in the hybrid polymer composites, followed by traverse velocity, and  then stand of distance. The contribution percentage for abrasive water jet machining parameters (A, v and SOD) was (79.70524%, 12.33802%, 6.77049%) respectively. 7. Grey relational analysis was effectively performed to determine the multi optimization of abrasive water jet machining parameters which are (A 4 v 3 SOD 3 ) i.e. hybrid polymer at constituents (70% wt Epoxy + 15% wt CF + 5% wt Al 2 O 3 + 10% wt SiC), traverse velocity at 300 mm/min, and stand of distance at 6 mm) that provide the best combination of the selected responses (Ra, H, and KW) to produce optimum machined surface of hybrid polymer composites.