V. Cannillo et al. /Composites: Part A 37 (2006 )23-30 (d)volume fraction of reinforcement. The output is analysed Table 2 in terms of shrinkage--in thickness and in diameter-and Processing conditions bulk density. In particular, since composites with different Samples Temperature Time(min)Pressure V(%) volume fraction of reinforcement were produced, the cC) (MPa) relative density (percentage of theoretical value)was determined Therefore, the materials as designed with the graeco-latin square were realized and investigated in relation to the 000000 selected output. Moreover, the composites were characterized by using 7 scanning electron microscope(SEM), image analysis and X-ray diffraction, as described in the following It should be noted that each typestyle corresponds to a 3. 1. Design of experiments actor. le The optimisation route is performed by using the graeco- ·1,2,3: temperature(685,785,885°C); latin square which allows to study four factors(k=4), each I, Il, III time(30/, 1 h, 2 h); of them assuming three possible level(m;=3). In the A, B, C: pressure(28, 42, 56 MPa) present investigation, the four factors, namely the tempera- a, B, r: celsian volume fraction(10, 20, and 30%) ture and time of thermal treatment, the pressure applied to the samples and the volume fraction of second phase, were This table is then transposed onto the experiments design assigned different values. i.e. the levels. table, in which each line correspond to one experiment. Table 2 summarizes the so-defined processing procedures temperature of thermal treatment: 685, 785, 885C time of thermal treatment: 30 min. 1 h. 2 h: 3.2. Composites preparation and characterization applied pressure: 28, 42, 56 MPa volume fraction of reinforcement: 10. 20. 30%O Celsian samples were milled in an agate mortar for 2 h and then sieved below 38 um. The commercial glass frit was The temperatures of the thermal treatment were selecte wet milled into an alumina ball mill with 50% water for 1 h taking into account the glass transition temperature of the and then sieved below 75 um. The so-obtained slurry was matrIx(T=583°C) then dried in a oven for 48 h and the powders were stored in The total number of possible experimental tests is given hermetic containers to avoid any pollution or re y Nr, where Nt=mi=81. In the present investigation humidification only selected tests were performed. The glass and celsian powders mixed with water were put The basic assumption of the approach is that the effects in an alumina ball mill for 20 min in order to homoge r the interactions of the different factors are linearly the mixtures. The slurry was then put in an heat chamber at additive. Under this hypothesis, the number of unknowns u 125C for 48 h to obtain dry composite powders.Such is equal to 9. powders were humidified at 6% of water and then Thus, the graeco-latin square should contain nine maintained in a hermetic container for 24 h to optimise cases,as illustrated in Table 1. This plan is constructed the humidification. by assigning one and only one combination of characters The samples were pressed, with a pressing cycle which to each box, where each combination is obtained by imposed a rise in pressure of 10 MPa/s and a release of circular permutation of the Latin characters located in the pressure at P/2 in order to allow evacuation of air first line of the table and by circular permutation in After drying for 24 h at 125C, the samples were heat the other direction of the Greek characters(see Table 1). treated, with a rise in temperature of 10C/min until the final temperature T, with a soaking time according to Table 2. Cooling was made in the furnace(approximately Graeco-latin square 5C/min)until 400C then in air until room temperature The composites were characterized by using different techniques. In particular, diameter and thickness linear shrinkage percentage were calculated samples before and after the heat treatment. Bulk was determined by means of a helium picnometer(Accu 1. 2. 3: temperature: I, Il, Ill: time: A. B, C: pressure: a, B, r: celsian Pyc 1330 Picnometer Micromeritics). The microstructural volume fraction orphology was investigated by using scanning electron(d) volume fraction of reinforcement. The output is analysed in terms of shrinkage—in thickness and in diameter—and bulk density. In particular, since composites with different volume fraction of reinforcement were produced, the relative density (percentage of theoretical value) was determined. Therefore, the materials as designed with the graeco-latin square were realized and investigated in relation to the selected output. Moreover, the composites were characterized by using scanning electron microscope (SEM), image analysis and X-ray diffraction, as described in the following. 3.1. Design of experiments The optimisation route is performed by using the graeco￾latin square, which allows to study four factors (kZ4), each of them assuming three possible level (miZ3). In the present investigation, the four factors, namely the tempera￾ture and time of thermal treatment, the pressure applied to the samples and the volume fraction of second phase, were assigned different values, i.e. the levels: † temperature of thermal treatment: 685, 785, 885 8C; † time of thermal treatment: 30 min, 1 h, 2 h; † applied pressure: 28, 42, 56 MPa; † volume fraction of reinforcement: 10, 20, 30%. The temperatures of the thermal treatment were selected taking into account the glass transition temperature of the matrix (TgZ583 8C). The total number of possible experimental tests is given by NT, where NTZmk i Z81. In the present investigation, only selected tests were performed. The basic assumption of the approach is that the effects or the interactions of the different factors are linearly additive. Under this hypothesis, the number of unknowns u is equal to 9. Thus, the graeco-latin square should contain nine cases, as illustrated in Table 1. This plan is constructed by assigning one and only one combination of characters to each box, where each combination is obtained by circular permutation of the Latin characters located in the first line of the table and by circular permutation in the other direction of the Greek characters (see Table 1). It should be noted that each typestyle corresponds to a factor, i.e. † 1, 2, 3: temperature (685, 785, 885 8C); † I, II, III: time (300 , 1 h, 2 h); † A, B, C: pressure (28, 42, 56 MPa); † a, b, g: celsian volume fraction (10, 20, and 30%). This table is then transposed onto the experiments design table, in which each line correspond to one experiment. Table 2 summarizes the so-defined processing procedures. 3.2. Composites preparation and characterization Celsian samples were milled in an agate mortar for 2 h and then sieved below 38 mm. The commercial glass frit was wet milled into an alumina ball mill with 50% water for 1 h and then sieved below 75 mm. The so-obtained slurry was then dried in a oven for 48 h and the powders were stored in hermetic containers to avoid any pollution or re￾humidification. The glass and celsian powders mixed with water were put in an alumina ball mill for 20 min in order to homogenise the mixtures. The slurry was then put in an heat chamber at 125 8C for 48 h to obtain dry composite powders. Such powders were humidified at 6% of water and then maintained in a hermetic container for 24 h to optimise the humidification. The samples were pressed, with a pressing cycle which imposed a rise in pressure of 10 MPa/s and a release of pressure at P/2 in order to allow evacuation of air. After drying for 24 h at 125 8C, the samples were heat treated, with a rise in temperature of 10 8C/min until the final temperature T, with a soaking time according to Table 2. Cooling was made in the furnace (approximately 5 8C/min) until 400 8C then in air until room temperature. The composites were characterized by using different techniques. In particular, diameter and thickness linear shrinkage percentage were calculated measuring the samples before and after the heat treatment. Bulk density was determined by means of a helium picnometer (Accu Pyc 1330 Picnometer Micromeritics). The microstructural morphology was investigated by using scanning electron Table 2 Processing conditions Samples Temperature (8C) Time (min) Pressure (MPa) Vf (%) 1 685 30 28 10 2 685 60 42 20 3 685 120 56 30 4 785 30 56 20 5 785 60 28 30 6 785 120 42 10 7 885 30 42 30 8 885 60 56 10 9 885 120 28 20 Table 1 Graeco-latin square I II III 1 Aa Bb Cg 2 Cb Ag Ba 3 Bg Ca Ab 1, 2, 3: temperature; I, II, III: time; A, B, C: pressure; a, b, g: celsian volume fraction. V. Cannillo et al. / Composites: Part A 37 (2006) 23–30 25