R.R. Naslain et al/ Solid State ionics 141-142(2001)541-548 steps is repeated vs time. Each sequence comprises: BX -NH, ( with X= F, CD), respectively. Under low i the evacuation of the deposition chamber, (i) the mperature(T)and pressure(P), the process yields injection of the gaseous reactant(s)and (iii)a hold- a deposit of high microstructural quality and the ing period during which deposition occurs. P-CVI preform porosity can be almost totally filled with the has been first introduced as a way to shorten the deposit [ densification [8-11]. Then, it has been shown that In P-CVI, the gaseous precursor is periodically P-CVI could be used to control the maturation of the injected in the deposition chamber as short pressure gas phase and, hence, the microtexture of, e.g. pyro- pulses(Fig. 1). Each pressure pulse corresponds to carbon deposits [12-14]. More recently, P-CVI has the following sequence of events. First, the deposi been used to tailor the composition and microstruc- tion chamber is evacuated by pumping. Then, the ture of multilayered interphases and matrices gaseous precursor is injected in the reactor and its [6,7,15-17] internal pressure reaches almost instantaneously a The aim of the present contribution is to show preset value. In a third step, deposition occurs during how P-CVI can be used to produce CMCs with a given time(the residence time, tg). This elemen ilored interphases at the nanometer scale, as well as tary sequence is repeated automatically vs. time, the self-healing matrices combining crack arrestor and inlet and outlet valves ng and closing being glass-former layers controlled with a computer. If T, P, tg are low enough, deposition can occur atomic layer by atomic layer. Finally, if one periodically switches from one 2. Experimental given precursor(C,H, for carbon) to a different precursor (MTS-H,, for SiC), a multilayered de In conventional CVI, a porous fiber substrate(the yC-SiC), is formed, in which the thickness preform) is set in a hot-wall deposition chamber of the sublayers can be as low as a few nanometers fed with a gaseous precursor flowing under a con- Different substrates have been used including: (i) stant pressure. Carbon, SiC and BN are deposited model pores open at both ends(length: 20 mm) with from hydrocarbons C,H,, CH; SiCI,(MTS)-H2 and rectangular cross sections(60X 2000, 120X 2000 depositi O: chamber 128 furnace Pumps MrS Fig. I. P-CVI apparatus for the deposition of carbon, silicon carbide or(PyC-SiC)n multilayers(1: sample, 2: electrical furnace; 3: precursors; 4: MTS vaporization chamber; 5: liquid nitrogen traps; 6: evacuated tank; 7: rotary vacuum pump)542 R.R. Naslain et al.rSolid State Ionics 141–142 2001 541–548 ( ) steps is repeated vs. time. Each sequence comprises: Ž. Ž . i the evacuation of the deposition chamber, ii the injection of the gaseous reactant s and iii a hold- Ž. Ž . ing period during which deposition occurs. P-CVI has been first introduced as a way to shorten the densification 8–11 . Then, it has been shown that w x P-CVI could be used to control the maturation of the gas phase and, hence, the microtexture of, e.g. pyro￾carbon deposits 12–14 . More recently, P-CVI has w x been used to tailor the composition and microstruc￾ture of multilayered interphases and matrices w x 6,7,15–17 . The aim of the present contribution is to show how P-CVI can be used to produce CMCs with tailored interphases at the nanometer scale, as well as self-healing matrices combining crack arrestor and glass-former layers. 2. Experimental In conventional CVI, a porous fiber substrate the Ž preform is set in a hot-wall deposition chamber . fed with a gaseous precursor flowing under a con￾stant pressure. Carbon, SiC and BN are deposited from hydrocarbons C H , CH SiCl MTS –H and Ž . x y 33 2 BX –NH with X 3 3 Ž . sF, Cl , respectively. Under low temperature Ž. Ž . T and pressure P , the process yields a deposit of high microstructural quality and the preform porosity can be almost totally filled with the deposit 1 . w x In P-CVI, the gaseous precursor is periodically injected in the deposition chamber as short pressure pulses Fig. 1 . Each pressure pulse corresponds to Ž . the following sequence of events. First, the deposi￾tion chamber is evacuated by pumping. Then, the gaseous precursor is injected in the reactor and its internal pressure reaches almost instantaneously a preset value. In a third step, deposition occurs during a given time the residence time, Ž . t . This elemen- R tary sequence is repeated automatically vs. time, the inlet and outlet valves opening and closing being controlled with a computer. If T, P, tR are low enough, deposition can occur atomic layer by atomic layer. Finally, if one periodically switches from one given precursor C H for carbon to a different Ž . x y precursor MTS–H , for SiC , a multilayered de- Ž . 2 posit PyC–SiC is formed, in which the thickness Ž .n of the sublayers can be as low as a few nanometers. Different substrates have been used including: iŽ . model pores open at both ends length: 20 mm with Ž . rectangular cross sections 60 Ž =2000, 120=2000 Fig. 1. P-CVI apparatus for the deposition of carbon, silicon carbide or PyC–SiC multilayers 1: sample; 2: electrical furnace; 3: Ž. Ž n precursors; 4: MTS vaporization chamber; 5: liquid nitrogen traps; 6: evacuated tank; 7: rotary vacuum pump