V. Cannillo er al. / Composites: Part A 37 (2006 )23-30 image analysis of SEM acquired micrographs. The results obtained with the two techniques were consistent. The relative density has a maximum for 785C; moreover, since the temperature of 885 C was excluded due to the shrinkage behaviour, it is straightforward that the most suitable temperature is 785C. As regards the soaking time, the better results were obtained with shortest time: therefore as already pointed out for the shrinkage, the 30 min time which is the most convenient also from an economic point of view- was selected. As regards the pressure, different applied pressures give very similar densification; the treatment with an applied pressure of 28 MPa was selected because of economic reasons The effect of volume fraction seems negligible on the resulting density. Thus, different volume fractions of reinforcements could be investigated in order to provide a more complete composite characterization. Fg.5. Sample3:685°℃,2h,56MPa,20% The composites were characterized by means of X-rays diffraction, confirming that the only crystallographic phas is a-celsian, thus excluding the formation of hexacelsian disappeared, in favour of circular intergranular pores; the SEM observation were performed on the different pores increase in number at higher temperature,i.e.at 885°C. composite samples in order to support previous results and to get a deeper insight on the effect of the different A DOE approach needs a validation of the procedure, i.e parameters. Firstly, it was observed that the applied pressure an additional processing treatment has to be carried out in had a negligible effect on the microstructure. Moreover, the order to confirm the predictions which suggest the best particles appear to be well-dispersed in the matrix conditions. Conclusions drawn on the basis of the effect of Figs. 4 and 5 illustrate the microstructure of samples 1 the different factors on the shrinkage and the relative density and3, processed at 685C for 30 min and 2 h, respectively. Suggest that the optimal treatment would be at 785C for For both samples, the porosity is relevant and characterized 30 min and with an applied pressure of 28 MPa. As regards by open pores, which indicate that the densification of the the celsian volume fraction, this parameter had not a composite had not reached its final stage. Therefore, it can significant effect on the sintering behaviour. A value be concluded that the temperature of 685"C, even at of 20 vol% was selected as an average value in the range different soaking times, is not sufficient to obtain full 10-30 vol% investigated sintered composites. However, since SEM observations suggested that the Figs. 6 and 7 illustrate the processing treatment at 785 temperature of 785C could be too high, whilst 685C was and 885C, respectively. In this cases, the glassy phase wet not sufficient to obtain full densification the validation all celsian particles. In both samples, the open porosity has treatment was performed at a temperature of 735 5 un Fig4 Sample 1: 685C, 30, 28 MPa. 10%. Fg.6. Sample5:785°℃,lh,28MPa,30%image analysis of SEM acquired micrographs. The results obtained with the two techniques were consistent. The relative density has a maximum for 785 8C; moreover, since the temperature of 885 8C was excluded due to the shrinkage behaviour, it is straightforward that the most suitable temperature is 785 8C. As regards the soaking time, the better results were obtained with shortest time; therefore, as already pointed out for the shrinkage, the 30 min time— which is the most convenient also from an economic point of view- was selected. As regards the pressure, different applied pressures give very similar densification; the treatment with an applied pressure of 28 MPa was selected because of economic reasons. The effect of volume fraction seems negligible on the resulting density. Thus, different volume fractions of reinforcements could be investigated in order to provide a more complete composite characterization. The composites were characterized by means of X-rays diffraction, confirming that the only crystallographic phase is a-celsian, thus excluding the formation of hexacelsian. SEM observation were performed on the different composite samples in order to support previous results and to get a deeper insight on the effect of the different parameters. Firstly, it was observed that the applied pressure had a negligible effect on the microstructure. Moreover, the particles appear to be well-dispersed in the matrix. Figs. 4 and 5 illustrate the microstructure of samples 1 and 3, processed at 685 8C for 30 min and 2 h, respectively. For both samples, the porosity is relevant and characterized by open pores, which indicate that the densification of the composite had not reached its final stage. Therefore, it can be concluded that the temperature of 685 8C, even at different soaking times, is not sufficient to obtain full sintered composites. Figs. 6 and 7 illustrate the processing treatment at 785 and 885 8C, respectively. In this cases, the glassy phase wet all celsian particles. In both samples, the open porosity has disappeared, in favour of circular intergranular pores; the pores increase in number at higher temperature, i.e. at 885 8C. A DOE approach needs a validation of the procedure, i.e. an additional processing treatment has to be carried out in order to confirm the predictions which suggest the best conditions. Conclusions drawn on the basis of the effect of the different factors on the shrinkage and the relative density suggest that the optimal treatment would be at 785 8C for 30 min and with an applied pressure of 28 MPa. As regards the celsian volume fraction, this parameter had not a significant effect on the sintering behaviour. A value of 20 vol% was selected as an average value in the range 10–30 vol% investigated. However, since SEM observations suggested that the temperature of 785 8C could be too high, whilst 685 8C was not sufficient to obtain full densification, the validation treatment was performed at a temperature of 735 8C. Fig. 4. Sample 1: 685 8C, 300 , 28 MPa, 10%. Fig. 5. Sample 3: 685 8C, 2 h, 56 MPa, 20%. Fig. 6. Sample 5: 785 8C, 1 h, 28 MPa, 30%. 28 V. Cannillo et al. / Composites: Part A 37 (2006) 23–30