Preparation And Performance Of High-temperature Resistant Polyborosilazane Adhesive

In the view of the urgent demand of high-temperature resistant organic adhesives for aviation and spaceflight,the preparation and properties of polyborosilazane adhesives were studied.Liquid polyborosilazanes(B-PMS)were successfully synthesized by choosing polymethylsilane(PMS)and borazine(BZ)as the raw materials.By adding fillers to B-PMS,the mass loss and volume shrinkage of the adheisves during the heat-treatment process was controlled,the densification of the bonding layer and join force at the interface was improved,and the high performance polyborosilazane adhesives were successfully prepared.Polymethylsilane(PMS)and Borazine(BZ)were chosen as the raw materials,and a yellow sticky liquid preceramic polymer B-PMS,which could be used as a adhesive directly,was sucessfully synthesized by changing the proportion of the raw materials,reaction temperation,and holding time to control the degree of the polymerization.The optimal synthesis condition was found to perform at 100℃ for 7h with the mass ratio of PMS:BZ=10:1.The as-synthesized B-PMS was characterized by FTIR,GPC,NMR,TG,SEM and rotating viscosity meter.The results showed that B-PMS was a yellow viscous liquid.The viscosity of B-PMS was 2470mPa·s,the number-average molecular weight and the weight average molecular weight of B-PMS were 3000 and 4927,respectively.The main chain of B-PMS was mostly made up of Si-Si,Si-N bonds and B3N3 hexagons,the side chains were Si-H and methyl bonds.The ceramic yields of the achieved B-PMS treated at 1000℃ under Ar was 92wt%.B-PMS can be cured at 200℃ for 2h under Ar atmosphere.The gel content of the cured B-PMS was 97.4wt%.The ceramic yield of the product was 89.4wt% at 1200℃ under Ar atmosphere.In an inert atmosphere,the thermal decomposition of B-PMS was divided into three stages.From room temperature to 300℃ was the first stage.B-PMS was an organic state and exhibited a weight loss of about 0.1wt%,which indicated that B-PMS showed no obvious changes.In the second stage(300℃~800℃),B-PMS exhibited a weight loss of about 7.1wt%.With the increase of temperatures,the characteristic peaks of Si-CH3 and Si-H gradually decreased,and the polymer was changed form organic state to inorganic state.In the third stage(800℃~1000℃),B-PMS exhibited a weight loss of about 0.6wt%,and amorphous SiBNC ceramic was finally achieved.When the temperature increased to 1200℃,the ceramic product began to appear the peaks of β-SiC.When the temperature increased to 1600℃,the ceramic product began to appear the peaks of h-BN.The hert-resistant property and bond property of B-PMS were investigated by joining sintered SiC ceramics.The results revealed that the room-temperature shear strengths of the joints treated at 200℃,600℃,1000℃ under Ar were 5.1MPa,3.8MPa and 9.2MPa,respectively.When the joints were treated at 1600℃,the adhesive sample was directly cracking due to the volume shrinkage during the heat treatment.BP-N was preparad by adding Ni powder to B-PMS.The preferable technics for preparing the adhesive was performed under the mass ratio of B-PMS:Ni=10:2.The room-temperature shear strengths of the SiC joints bonded by BP-N treated at 200℃,600℃,1000℃ and 1600℃ under Ar were 8.8MPa,2.5MPa,5.3MPa and 1.6MPa,respectively.BP-A was prepared by adding Al powder to B-PMS.The preferable technics for preparing the adhesive was performed under the mass ratio of B-PMS:Al=10:1.The room-temperature shear strengths of the SiC joints bonded by BP-A treated at 200℃,600℃,1000℃ and 1600℃ under Ar were 8.8MPa,9.8MPa,11.4MPa and 7.9MPa,respectively.At low temperature,the aluminum powder is mainly acted as inert fillers,which can reduce the volume shrinkage during heat treatment,which definitely improves the densification of the bonding layer.When the treated temperatures increased,the reactions between aluminum and the pyrolysis products of B-PMS happened.This reactions can improve the densification of the bonding layer and enhance the interface strength between the adhesive layer and the substrate,which can improve the bonding property and high-temperature resistance of the obtained adhesive.