f.K. Naskar et al. Ceramics international 30(2004)257-265 b) The infiltrated materials(CMCs) were charac AlO3 as the major constituents with trace impurities of terised by(i) XRD as described previously in Fe2O3, TiO2, K2O, NayO and LOI Section 2.4(a),(ii) SEM in which the fracture urfaces and the top surfaces of samples of the 3. 2. Characteristics of the infiltrates(sols) CMCs of same dimensions were examined as mentioned in Section 2. 4 (a),(iii)thermo- Table 2 summarises the characteristics of cmcs fab- gravimetry analysis(TGA)(Model: Netzsch STa ricated using sols of different viscosities in single-and 409c)from 30 to 1000C with a heating rate of bi-component oxide systems following VIT. In the pres- 10C/mm in argon atmosphere and (iv) the ent investigation, four different types of sols(Table 1) flexural strength and modulus measurement on were considered. It is to be noted that for each type of samples of dimensions 40 mmx7 mmx6 mm the sol, the higher the viscosity, the more difficult it was using three point bend test (Instron Universal to achieve good wetting of the fibres by the sol and Testing Machine, model: 5500 R)under a cross- infiltrate the sol into the interconnected pores and void head speed of 0.5 mm/min. Each strength datum [9, 10, 16]. Further, infiltration of the high viscosity sols an average over six samples gave rise to the formation of considerable amounts of (c) The stabilised zirconia powder (with the molar air bubbles during operation and deposition of frag composition of ZrO2: Y2O3 as 94: 06) was char- mented coatings on the surface of the preform after acterised by: ()XRD and (i) SEM as described drying which prevented further penetration of sols earlier and (iii) particle size analyser(Model: inside the preform during repeated infiltration. On the Autosizer IIC, Malvern Instruments) other hand, sols of low viscosity were also found to be unsuitable because in such cases even after nine cycles of infiltration, the sample exhibited considerably brittle 3. Results and discussion character. Based on the above results, in the present Inves igation, the viscosity of each type of sol was kept 3. 1. Characteristics of the precursor preforms fixed in certain optimum ranges(Table 2) which in turn were found to depend on their polymerisation beha The XRD of the precursor preforms used in the pres- viour and preparative principles. pH of the sol is also an ent investigation indicated the presence of mullite as the important point to be considered, as it affects both the only phase. The microstructural feature of the preform viscosity and stability of a sol. with the increase in pH in Fig. I indicates that the fibres are circular in diameter viscosity of a sol increased and finally caused gel for with the diameter distribution in the range 3-8 um. The mation. Thus, an optimum value of both the pH and shot-content in the fibre preform was found to be neg- viscosity of different sols are necessary for carrying out ligible. Presence of considerable amounts of inter-fibre the infiltration experiments [ll pores and voids is evident from the microstructure Infiltration of the sol in the preform is expected to fill 3.3. Characteristics of the CMCs obtained by the sol these inter-fibre pores and voids, leading to the forma- infiltration technique tion of the continuous phase, i.e. the matrix which is the primary aim of this investigation. The preforms Characteristics of the CMCs fabricated unde were found to contain 54.88 wt. SiO2 and 43.05 wt. ent experimental conditions have been presented in Table 2. as described in table 2. characteristics of the CMCs have been found to be affected by the following 3.3.1. Cracking in the matrix of the CMCs Although the sol infiltration technique of fabricating CMCS has several advantages, such as, homogeneous mixing of the multicomponent oxides, higher purity, low processing temperature, the method suffers from the serious problem of excessive shrinkage during drying due to removal of considerable amount of volatiles giving rise to extensive matrix cracking and residual fine scale porosity in composites followed by degradation of mechanical properties. In the present investigation, this 25 problem was tried to be minimised by(i) multiple infiltra tion of the infiltrated preform followed by intermediate Fig. l. SEM of the preform showing inter-fibre voids and porosities heat-treatment at the lowest possible temperature for(b) The infiltrated materials (CMCs) were charac￾terised by (i) XRD as described previously in Section 2.4 (a), (ii) SEM in which the fracture surfaces and the top surfaces of samples of the CMCs of same dimensions were examined as mentioned in Section 2.4 (a), (iii) thermo￾gravimetry analysis (TGA) (Model: Netzsch STA 409c) from 30 to 1000 Cwith a heating rate of 10 C/mm in argon atmosphere and (iv) the flexural strength and modulus measurement on samples of dimensions 40 mm
7 mm
6 mm using three point bend test (Instron Universal Testing Machine, model: 5500 R) under a cross￾head speed of 0.5 mm/min. Each strength datum is an average over six samples. (c) The stabilised zirconia powder (with the molar composition of ZrO2:Y2O3 as 94:06) was char￾acterised by: (i) XRD and (ii) SEM as described earlier and (iii) particle size analyser (Model: Autosizer IIC, Malvern Instruments). 3. Results and discussion 3.1. Characteristics of the precursor preforms The XRD of the precursor preforms used in the pres￾ent investigation indicated the presence of mullite as the only phase. The microstructural feature of the preform in Fig. 1 indicates that the fibres are circular in diameter with the diameter distribution in the range 3–8 mm. The shot-content in the fibre preform was found to be neg￾ligible. Presence of considerable amounts of inter-fibre pores and voids is evident from the microstructure. Infiltration of the sol in the preform is expected to fill these inter-fibre pores and voids, leading to the forma￾tion of the continuous phase, i.e. the matrix, which is the primary aim of this investigation. The preforms were found to contain 54.88 wt.% SiO2 and 43.05 wt.% Al2O3 as the major constituents with trace impurities of Fe2O3, TiO2, K2O, Na2O and LOI. 3.2. Characteristics of the infiltrates (sols) Table 2 summarises the characteristics of CMCs fab￾ricated using sols of different viscosities in single- and bi-component oxide systems following VIT. In the pres￾ent investigation, four different types of sols (Table 1) were considered. It is to be noted that for each type of the sol, the higher the viscosity, the more difficult it was to achieve good wetting of the fibres by the sol and infiltrate the sol into the interconnected pores and voids [9,10,16]. Further, infiltration of the high viscosity sols gave rise to the formation of considerable amounts of air bubbles during operation and deposition of frag￾mented coatings on the surface of the preform after drying which prevented further penetration of sols inside the preform during repeated infiltration. On the other hand, sols of low viscosity were also found to be unsuitable because in such cases even after nine cycles of infiltration, the sample exhibited considerably brittle character. Based on the above results, in the present investigation, the viscosity of each type of sol was kept fixed in certain optimum ranges (Table 2) which in turn were found to depend on their polymerisation beha￾viour and preparative principles. pH of the sol is also an important point to be considered, as it affects both the viscosity and stability of a sol. With the increase in pH, viscosity of a sol increased and finally caused gel for￾mation. Thus, an optimum value of both the pH and viscosity of different sols are necessary for carrying out the infiltration experiments [1]. 3.3. Characteristics of the CMCs obtained by the sol infiltration technique Characteristics of the CMCs fabricated under differ￾ent experimental conditions have been presented in Table 2. As described in Table 2, characteristics of the CMCs have been found to be affected by the following factors. 3.3.1. Cracking in the matrix of the CMCs Although the sol infiltration technique of fabricating CMCs has several advantages, such as, homogeneous mixing of the multicomponent oxides, higher purity, low processing temperature, the method suffers from the serious problem of excessive shrinkage during drying due to removal of considerable amount of volatiles, giving rise to extensive matrix cracking and residual fine scale porosity in composites followed by degradation of mechanical properties. In the present investigation, this problem was tried to be minimised by (i) multiple infiltra￾tion of the infiltrated preform followed by intermediate Fig. 1. SEM of the preform showing inter-fibre voids and porosities. heat-treatment at the lowest possible temperature for 260 M.K. Naskar et al. / Ceramics International 30 (2004) 257–265