Y Liu et al. Materials Science and Engineering A 475(2008)217-223 has been demonstrated to be a very effective and matured enough machined, and cut into specimens with the same nat preparation method to fabricate SiC or Si3N4 matrix with ultra- of SiC(p)/SiC composites to further deposit Si3N4 matrix fo pure and controllable grain sizes[21-25. An alternative method 100h to fabricate particle reinforced ceramic matrix composites is to deposit SiC or Si3 N4 matrix on the internal surfaces of the 2.3. Characterization and tests porous particle preforms by CVI, similar to the fabrication of fiber reinforced SiC matrix composites. The SiC or Si3N4 matrix The macro-structure characterizations of the agglomera composites reinforced by particles can be expected to possess tions and preform were performed by a numeral camera. The excellent high temperature mechanical and chemical properties microstructure of the preform and as-infiltrated composite were if CVi is used to fabricate SiC or Si3N4 matrix with virtual examined by scanning electron microscopy (SEM)(Hitachi elimination of sintering additives In previous works [26-29]the $-4700, Japan), and transmission electron microscopy(TEM silicon carbide and silicon nitride agglomerate particle preform JEM3010) using foils prepared by ion-beam thinning. The bend specimens with dimension 3 mm x 4 mm x 40 mm were In the present work, two kinds of ceramic matrix compos- used to evaluate the 3-point flexural strength of two kinds of ites, SiCp/SiC and Si3 N4(p)/Si3 N4 composites were prepared composites with a cross head speed of 0.5 mm min-and a using particle agglomerations and chemical vapor infiltration span length of 20 mm using a SANS CMT4304 instrument rocess.The relationship between the pore structure and gas Five specimens were tested to obtain an average value. The 3- diffusion mechanism in preform were discussed. Finally, the point flexure strength of SiC(p)/SiC composites was also tested microstructure and properties of the two kinds of composites with temperature increasing from room temperature to 1600oC were reported Micro-hardness and fracture toughness were determined at roon temperature by Vickers indentation method with 196N load 2. Experiment procedure for 30s, 10 times on each of five samples per data point. The dielectric constant of the Si3N4(p/Si3 N4 composite was mea- 2.1. Preparation of the particle preform sured using Precision Impedance Analyse Instrument(Agilent 4294A, American), five times on each of three samples per data The raw silicon carbide(SiC wt %>98%)and silicon nitride point. (Si3N4 wt %>98%) particles had a mean grain size of 3. 0 um and were mixed for 5 h in a planetary milling with Al2O3 milling 3. Results and discussion balls using binder, poly-vinyl butyral(PVB), and plasticizer, dibutyl phthalate. The as-treated Sic or Si3N4 particles were 3. 1. Microstructure of particle agglomerations and preform packed in a stainless-steel die, and cold-pressed at room tem- perature and a pressure of 15 MPa or 20 MPa. The cold-press Fig. I shows typical morphologies of the particle agglom- pressure to form particle agglomeration can be named as the erations and preform. Fig. 1(a) shows that the particle first-step pressure. The as received preforms were then crushed agglomerations were grain-like shape, which were prepared and sieved to select SiC or Si3 Na particles agglomerations with through cold-press and crush process. The grain-like agglome size ranging from 0.3 to 0.6 mm. Finally, the selected particle ations would inlay in ceramic matrix when the particle preforms agglomerations were cold-pressed at room temperature and a were infiltrated. The reinforcing mechanisms of agglomerations pressure of 3.0-9.0 MPa to form the particle preform used to include micro-crack deflection, micro-crack divarication and fabricate composites. The cold-press pressure to form parti- agglomerations fracture, which were discussed in [29]. Fig. 1(b) cle preform for infiltration can be named as the second-step is the whole image of agglomerations preform prepared by cold- press process. The particle preform is uniform and porous, with a density of 1.29 g/cm, and porosity 59.6% 2. 2. CVI process Fig. 1(c)and(d) show the microstructure of the preform There are a large amount of connectable pores with a size of C matrix was deposited from MTS(content CH3 SiCl3> approximately 500-800 um inter-agglomeration in Fig. 1(c) 980wt %). Hydrogen(content H2299.99%)was a carrier gas However, the size of intra-agglomeration pore changes from 5 of MTS Argon(content H2>99.9%)was used as dilution gas. to 10 um in Fig. 1(d). The small intra-agglomeration pores are The deposition conditions were as follows: (MTS)/H2= 1/10 for also connectable basically, but they are more small and complex 400h at P=3 kPa, Ar=350 ml/min, and T=1000C. The spec- The pore size and distribution are in accordance with [29] imens were machined, and cut into specimens with dimension In CVI process, the transfer of reaction gases is very imp 3mm x4 mm x 40 mm to further deposit SiC matrix for 100h. tant to infiltration. However, the gas m In porous solids, the gas Si3N4 matrix was deposited from silicon tetrachloride size and shape of pores in the preform (SiCl4>99.99 wt %o and iron<- )and ammonia gas(con- diffusion mechanism depends on pore diameters(d), and mean tent NH3>99.99%). Hydrogen(content H2299999%) was free path of the gas molecule (1). The diffusion mechanisms a carrier gas of Sicl4. The process conditions are as fol- include Fick diffusion, Knudsen diffusion and transition diffu- lows: SiClA/NH3=1/3 for 200 h at P=2 kPa, H2=100 ml/min, sion, when d> 100A, d s01A, and d=0. 1-100A respectively. Ar=200 ml/min, and T=900C. The specimens were also In a multi-phase gas system, i can be calculated from formula218 Y. Liu et al. / Materials Science and Engineering A 475 (2008) 217–223 has been demonstrated to be a very effective and matured enough preparation method to fabricate SiC or Si3N4 matrix with ultra￾pure and controllable grain sizes[21–25]. An alternative method to fabricate particle reinforced ceramic matrix composites is to deposit SiC or Si3N4 matrix on the internal surfaces of the porous particle preforms by CVI, similar to the fabrication of fiber reinforced SiC matrix composites. The SiC or Si3N4 matrix composites reinforced by particles can be expected to possess excellent high temperature mechanical and chemical properties if CVI is used to fabricate SiC or Si3N4 matrix with virtual elimination of sintering additives. In previous works[26–29] the silicon carbide and silicon nitride agglomerate particle preform was designed. In the present work, two kinds of ceramic matrix compos￾ites, SiC(p)/SiC and Si3N4(p)/Si3N4 composites were prepared using particle agglomerations and chemical vapor infiltration process. The relationship between the pore structure and gas diffusion mechanism in preform were discussed. Finally, the microstructure and properties of the two kinds of composites were reported. 2. Experiment procedure 2.1. Preparation of the particle preform The raw silicon carbide (SiC wt.% > 98%) and silicon nitride (Si3N4 wt.% > 98%) particles had a mean grain size of 3.0m, and were mixed for 5 h in a planetary milling with Al2O3 milling balls using binder, poly-vinyl butyral (PVB), and plasticizer, dibutyl phthalate. The as-treated SiC or Si3N4 particles were packed in a stainless-steel die, and cold-pressed at room tem￾perature and a pressure of 15 MPa or 20 MPa. The cold-press pressure to form particle agglomeration can be named as the first-step pressure. The as received preforms were then crushed and sieved to select SiC or Si3N4 particles agglomerations with size ranging from 0.3 to 0.6 mm. Finally, the selected particle agglomerations were cold-pressed at room temperature and a pressure of 3.0–9.0 MPa to form the particle preform used to fabricate composites. The cold-press pressure to form parti￾cle preform for infiltration can be named as the second-step pressure. 2.2. CVI process SiC matrix was deposited from MTS (content CH3SiCl3 ≥ 98.0 wt.%). Hydrogen (content H2 ≥ 99.99%) was a carrier gas of MTS. Argon (content H2 ≥ 99.9%) was used as dilution gas. The deposition conditions were as follows: (MTS)/H2 = 1/10 for 400 h at P = 3 kPa, Ar = 350 ml/min, and T = 1000 ◦C. The spec￾imens were machined, and cut into specimens with dimension 3 mm × 4 mm × 40 mm to further deposit SiC matrix for 100 h. Si3N4 matrix was deposited from silicon tetrachloride (SiCl4 ≥ 99.99 wt.% and iron ≤ 10−5) and ammonia gas (con￾tent NH3 ≥ 99.99%). Hydrogen (content H2 ≥ 99.999%) was a carrier gas of SiCl4. The process conditions are as fol￾lows: SiCl4/NH3 = 1/3 for 200 h at P = 2 kPa, H2 = 100 ml/min, Ar = 200 ml/min, and T = 900 ◦C. The specimens were also machined, and cut into specimens with the same size as that of SiC(p)/SiC composites to further deposit Si3N4 matrix for 100 h. 2.3. Characterization and tests The macro-structure characterizations of the agglomera￾tions and preform were performed by a numeral camera. The microstructure of the preform and as-infiltrated composite were examined by scanning electron microscopy (SEM) (Hitachi S-4700, Japan), and transmission electron microscopy (TEM, JEM3010) using foils prepared by ion-beam thinning. The bend specimens with dimension 3 mm × 4 mm × 40 mm were used to evaluate the 3-point flexural strength of two kinds of composites with a cross head speed of 0.5 mm min−1 and a span length of 20 mm using a SANS CMT4304 instrument. Five specimens were tested to obtain an average value. The 3- point flexure strength of SiC(p)/SiC composites was also tested with temperature increasing from room temperature to 1600 ◦C. Micro-hardness and fracture toughness were determined at room temperature by Vickers indentation method with 196 N load for 30 s, 10 times on each of five samples per data point. The dielectric constant of the Si3N4(p)/Si3N4 composite was mea￾sured using Precision Impedance Analyse Instrument (Agilent 4294A, American), five times on each of three samples per data point. 3. Results and discussion 3.1. Microstructure of particle agglomerations and preform Fig. 1 shows typical morphologies of the particle agglom￾erations and preform. Fig. 1(a) shows that the particle agglomerations were grain-like shape, which were prepared through cold-press and crush process. The grain-like agglomer￾ations would inlay in ceramic matrix when the particle preforms were infiltrated. The reinforcing mechanisms of agglomerations include micro-crack deflection, micro-crack divarication and agglomerationsfracture, which were discussed in [29]. Fig. 1(b) is the whole image of agglomerations preform prepared by cold￾press process. The particle preform is uniform and porous, with a density of 1.29 g/cm3, and porosity 59.6%. Fig. 1(c) and (d) show the microstructure of the preform. There are a large amount of connectable pores with a size of approximately 500–800m inter-agglomeration in Fig. 1(c). However, the size of intra-agglomeration pore changes from 5 to 10m in Fig. 1(d). The small intra-agglomeration pores are also connectable basically, but they are more small and complex. The pore size and distribution are in accordance with [29]. In CVI process, the transfer of reaction gases is very impor￾tant to infiltration. However, the gas transfer mainly lies on the size and shape of pores in the preform. In porous solids, the gas diffusion mechanism depends on pore diameters (d), and mean free path of the gas molecule (λ). The diffusion mechanisms include Fick diffusion, Knudsen diffusion and transition diffu￾sion, when d ≥ 100λ, d ≤ 0.1λ, and d = 0.1–100λ respectively. In a multi-phase gas system, λ can be calculated from formula