Polysilazane Modification And Application In Joining Of High-temperature Resistant Stainless Steel

High-temperature resistant stainless steel is widely used in the petrochemical,transportation, construction, electronics industries and other fields because of itsoutstanding high-temperature resistance. Joining techniques of stainless steel aregenerally needed for machining complex steel connections and sealing stainless-steelparts. Preceramic joining technique, simple and innovative, is mainly used inhigh-temperature joining of ceramics and ceramic matrix composite. In this work, theprecursor polymer, having better adhesion properties, and being suitable forhigh-temperature resistant stainless steel connection, is prepared by modifyingpolysilazane (PSN) precursor. On this basis, we adopted the modified precursors tojoin high-temperature resistant stainless steel.(1) PSN was modified by tetramethyl tetravinylcyclotetrasiloxane (D4Vi), gettingthe new preceramic polymer-Vinyl Polysilazane (VPSN). The introduction ofCH=CH2bonds in D4Vi into PSN by the substantial cross-linking reaction betweenthe abundant Si–H bonds in PSN and the CH=CH2bonds in D4Vi, lead to the increaseof CH=CH2bonds. The amorphous Si-C-N ceramics can be obtained by the pyrolysisof VPSN under a nitrogen atmosphere. VPSN was adopted to join high-temperatureresistant stainless steels, a maximum bending strength of115.8MPa can be obtainedby this connection type. The bonding layer is uniform and basically compact, andthere are few cracks, holes and other defects at the interface. The connection betweenVPSN and substrate is good.(2) Modification on PSN was realized with borane dimethyl sulfide (C2H9BS).The B element of C2H9BS was introduced into the PSN through reaction of the CH=CH2bonds in PSN with the B-H bonds in C2H9BS, getting the new preceramicpolymer-poly boron silazane (PSNB). Si-B-C-N ceramic can be obtained by thepyrolysis of PSNB at high temperature under nitrogen atmosphere. PSNB was used tojoin high-temperature resistant stainless steels, a maximum bending strength of116.1MPa can be obtained. The pyrolysis products of the fracture surfaces for stainlesssteel joints contain Cr3C2, NiCr2O4, Cr2B3and Cr2N. The chemical bonding forceexisted between the connection layer and the stainless steel surface, and produced high strength joints.(3) PSN was modified by ferrocene (Fe(C5H5)2). The modified precursor ironpolysilazane (PFSN) was obtained by the cross-linking reaction between the Si-Hbonds of PSN and Fe(C5H5)2. The introduction of iron element increased theferromagnetism of precursor. The pyrolysis products of PFSN at high temperatureunder nitrogen atmosphere contain α-Fe, SiC and Si3N4crystallites. The α-Fecrystallites dispersed uniformly in the Si-C-N matrix. PFSN was used to joinhigh-temperature stainless steels, a maximum bending strength of107MPa can beobtained. PFSN has ferromagnetism and its component is similar to thermal resisitantstainless steel, thus the combination of both is good.