In order to form glass powder, we took 500 gm glass, 175 gm of SiO₂, 50 gm of P₂O₅, 85.484 gm of Na₂CO₃ (source of Na₂O), and 401.786 gm of CaCO₃ (source of CaO) and mixed them properly. After mixing the compounds the mixed batch was annealed at 1135˚C in a globular furnace (heat rate several hundred ˚C per hour) for three hours to form the desired glass-ceramic matrix structure. The glass was then crushed, and the glassy powder was prepared.

Mixing was a vital part of this experiment. Pure Al powder was mixed homogeneously with the prepared glass powder by varying its weight (wt) percentage (%) of 0%, 3%, and 6%, respectively.

A molding device was used to mold the mixture of Al powder and glass-ceramic powder. In this molding process, polyvinyl alcohol was used as the binder such that five drops of the binder were poured in three grams of the mixture. After then the mixture was pressed with sufficient load of 50 KN, applied by a Weber Pressen Hydraulic Press, to get the desired shape (60.5 mm × 5.5 mm × 4.4 - 4.6 mm) of the sample.

The samples prepared by the Weber Pressen Hydraulic Press were removed and dried properly. Finally, the dried samples were sintered at various temperatures (1050˚C, 1075˚C, 1100˚C) using high temperature furnace.

In the present work ASTM D790M, 3 points loading technique has been used to estimate the values of flexural strength or flexural stress (σ_f), flexural strain (ε_f ), and flexural or bending modulus (E_f ). In this arrangement, the flexural strength of the prepared glass-ceramic samples was determined by using the following formula [20] [21],

σ f = 3 P L 2 b d 2 ( MPa ) (1)

where P denotes the load at a given point on the load-deflection curve (expressed in N), L is the distance between the centers of the support span (expressed in mm), b is the width, and d is the depth (both measured in mm) of the specimen. The flexural strain was measured by the following expression [21],

ε f = 6 D d L 2 ( mm / mm ) (2)

where D denotes the maximum deflection of the center of the beam (measured in mm). Also, the flexural modulus of the samples was determined by using the following equation [21],

E f = L 3 m 4 b 3 d 3 ( MPa ) (3)

where m is the gradient (i.e., slope) of the initial straight-line portion of the load deflection curve (expressed in N/mm).

ASTM C135-762 technique has been used to determine the bulk density (BD) of the samples. The bulk density is given by the following equation [20],

B D = W d V ( gm / mm 3 ) (4)

where W_d determines the weight of the specimen (expressed in gm), and V represents the volume of the specimen under test (measured in mm³).

The apparent porosity (P) of the samples has been determined by employing the evacuation method. The following equation [20] was used to calculate the apparent porosity,

P = 100 ( W s − W d ) V (5)

where W_s denotes the saturated weight, and W_d denotes the dry weight of the sample (both measured in mg), V is the volume of the sample (expressed in mm³).

The water absorption is another important parameter which has been estimated by using ASTM C 67 - 91 technique. The following formula [20] provides the water absorption of the samples,

Absorption % = 100 ( W s − W d ) W d (6)

Figure 2 shows the flexural strength (σ_f) characteristic by changing sintering temperature for the glass-ceramic samples prepared with the addition of different Al powder contents: 0 wt%, 3 wt% and 6 wt%, respectively. The corresponding values of σ_f are shown in Table 1.

It is seen that the σ_f increases (5 to 9 MPa for 0 wt%, 10 to 11 MPa for 3 wt% by increasing the sintering temperature 1050˚C to 1100˚C. This cause of increasing σ_f with increasing temperature is that the strain capacity of the samples increases with the rise of temperature, and vice versa. However, σ_f decreased from 15 to 11 MPa for of Al content of 6 wt% with increasing temperature 1050˚C to 1075˚C and then little increased (14 MPa, even it is lower the value of

σ_f at 1050˚C) at high temperature of 1100˚C. It clarifies that for each temperature/percentage of oxide there is an optimum temperature/percentage of oxide amount above which the strength will be decreased. The cause of this decreasing fusion. The higher amount of Al powder lower is the fusion temperature [22].

The effect of sintering time (1.5 h and 3 h) on the σ_f for these glass-ceramic samples for different Al powder contents, 0 wt%, 3 wt% and 6 wt% as shown in Figure 3. The values of σ_f are shown in Table 2. From Figure 3, and Table 2, it could be revealed that the σ_f of the samples increases with increasing the sintering time.

The resultant change in the flexural strain (ε_f ) of the composite glass-ceramic samples with different sintering temperatures has been shown in Figure 4 and the corresponding data are shown in Table 3. We observed that by increasing the sintering temperature from 1050˚C to 1070˚C, ε_f increased from 0.0028 to 0.0036 for Al wt 0%, and 0.0031 to 0.0033 for Al wt 3%, respectively. However, both ε_f for 0 wt% and 3 wt% started to be decreased by increasing the sintering temperature to 1100˚C. For Al 6 wt%, ε_f (0.029 - 0.031) was found to be almost constant at low to high sintering temperature of 1050˚C - 1100˚C. Since stress

and strain needs to follow a proportional relationship, it is revealed that sintering temperature should be kept below 1080˚C.

Figure 5 shows the variation of flexural modulus (E_f ) with sintering temperature for the glass-ceramic composite samples prepared by adding different Al contents. The values of E_f are summarized in Table 4. It is seen that E_f for the glass-ceramics with Al wt. 6% decreased, 2700 MPa to 2200 MPa and for wt 3% decreased 2200 MPa to 1800 MPa by increasing sintering temperatures 1050˚C - 1100˚C. However, an opposite characteristic of E_f which started to be increased (1180 MPa - 1850 MPa) with increasing the sintering temperature (1080˚C - 1100˚C) was observed for the sample where no Al content was added (0 wt%). The flexural modulus of this sample with Al wt. 0% starts to increase little after a certain sintering temperature (1050˚C - 1080˚C). This discrepancy may be resolved by keeping the sintering temperature within 1080˚C which showed similar characteristics (E_f decreased by increasing temperature 1050˚C to 1080˚C) for 3 samples.

Influence of changing sintering time on E_f after adding different Al powder (wt%) is exhibited in Figure 6 and the values of E_f are shown in Table 5. The E_f

was found to be increased, i.e., for Al 3 wt% 1800 to 2000 and for Al 6 wt% 2000 to 2050 MPa at sintering time of 1.5 h and 3 h, respectively. However, for Al wt 0% it is almost constant (~1800 MPa).

Impact of sintering temperature on the bulk density of the glass-ceramic samples after the addition of different Al wt% is represented in Figure 7 and their corresponding values of bulk density (BD) are summarized in Table 6. It is observed that the bulk density increased with increasing sintering temperature for the samples prepared with the addition of 0 wt% (2.1 to 2.25 gm/mm³) and 3 wt% (2.15 to 2.5 gm/mm³) of Al powders, however for the samples with Al 6 wt% decreased at a certain minimum value (2.25 to 2.18 gm/mm³) and again increased (2.18 to 2.4 gm/mm³) by changing the sintering temperature 1050˚C to 1100˚C. The similar result was observed in the flexural strength vs sintering temperature diagram (Figure 2). The relationship between flexural strength and bulk density is shown in Figure 8. This figure specifies that the flexural strength of the sintered glass-ceramic composite samples maintains a proportional relationship (in most cases the flexural strength becomes higher by increasing the bulk density) with their bulk density [23].

Figure 9 shows how the apparent porosity of the glass-ceramic composites varies with the sintering temperature. The summarized apparent porosity values are shown in Table 7. It is seen that the apparent porosity of glass-ceramics decreased (25.5 to 16 for 0 wt%, 30 to 16.5 for 3 wt%, and 27.5 to 26 for 6 wt% with

increasing temperature (1050˚C to 1100˚C). However, the apparent porosity is found to be decreased less slowly for the sample with 6 wt% of Al powder. The reason behind the decrease of apparent porosity with sintering temperature is that the surface viscosity of particles of the samples decreases at high temperature and therefore the particles within the ceramic body bond more strongly and the space between the particles gets narrowed [24] [25].

The variation of apparent porosity with the percentage of Al powder at two different sintering times (1.5 h and 3 h) are shown in Figure 10 and Table 8. It elucidates that both higher percentage of Al powder and long sintering time influences to increase the apparent porosity of the samples.

Continuous change of water absorption with sintering temperature is represented in Figure 11 and Table 9. The water absorption curves of the composites are found to be decreased (14% to 11% for 0 wt%, 12% to 7% for 3 wt%, and 13.5% to 9% for 6 wt%) gradually with increasing sintering temperature (1050˚C to 1100˚C). This indicates that the increase of sintering temperature decreases the amount of water absorbed by the specimen, keeping the weight of the dry specimen same. Figure 12 shows the effect of sintering time on the water absorption vs percentage of Al powder characters. The corresponding water absorption values are given in Table 10. It is found that the water absorption of

the sintered glass-ceramic samples increase (7% to 9%, and 7.5% to 15%) with increasing the sintering time from 1.5 hours to 3.0 hours.