Growth Of Cnts On SiO_2f/SiO₂and The Process And Mechanism Of Brazing With TC4Alloy

Quartz fiber reinforced silica composite (SiO_2f/SiO₂) is an advanced aerospacematerial that owns excellent wave transmissivity, and heat protection properties. TC4alloy is also a widely used high-temperature material in aerospace industry due to itsheat-resistance property. Brazing composite and TC4alloy could be meaningful and hasa lot of practical values. Poor wettability of silica matrix composite and mismatch ofCTE between joined material are the two important factors that weaken the brazed joint.In this paper, we propose a new method that grow a layer of directional carbon nanotubes（CNTs） to assist brazing process. Grown CNTs could improve the wettability ofcomposite by AgCuTi alloy, and lower the CTE of braze seam. An integrated, highstrength braze joint could be achieved through this method.PECVD was occupied to grow CNTs on SiO_2f/SiO₂composite. The technology ofgrowing CNTs on SiO_2f/SiO₂composite with PECVD was studied. The influence ofdifferent parameters in PECVD process was carefully investigated. An optimized growthparameter was selected. The height of aligned CNTs array we got could be2μm.After the growth of CNTs, the shear strength of joint reached36.88MPa, which ishigher than the shear strength5.8MPa of joint without CNTs. The typical interface ofjoint brazed with AgCuTi filler alloy was SiO_2f/SiO₂composite/TiSi2/Cu4Ti3+Cu3Ti3O/Ag（s,s）+Cu（s,s）/TiCu/Ti₂Cu/α,β-Ti/TC4. Increasing brazing temperature or extendingholding time would lead to the formation of （Ti,Cu） compound in joint and the increaseof thickness of reaction layers. Also, the shear strength exhibits a tendency of firstincrease then decrease.Two effects of addition of CNTs were studied namely improvement in wettability ofliquid AgCuTi alloy and lowering the CTE of brazing seam were observed. Improvementin wettability should be subscribed to the fact that CNT could be wet by liquid AgCuTialloy. Besides, the nanoscale roughness of aligned CNTs array further promotes thespread of liquid alloy. CNTs existing in the brazing seam could effectively lower the CTEof brazing seam, which could finally modify the stress distribution of joint and decreasethe residual stress at the interface of composite and brazing seam. The interfacephenomenon between CNT and AgCuTi alloy was studied by evaporating alloy onto thesurface of CNT. Results showed that Ti could promote the wetting of AgCu alloy onCNT surface. After heat treatment at850℃and900℃, interfacial reaction between Tiand CNT could happen and TiC was observed.