Cross-Linking Mechanism Of Precursor During Preparation Of Ceramic Matrix Composites By Precursor Infiltration Pyrolysis

Focusing on low cross-linking extent and low reactive ratio of active groups of ceramic precursor to prepare ceramic matrix composites (CMCs) by precursor infiltration pyrolysis (PIP), the heat cross-linking course of organic-silicon precursor was studied systemically by various instrumentation methods, including thermogravimetry (TG), infrared spectrum (IR) and scanning electron microscope(SEM),etc. Routes that were used to raise cross-linking extent of different precursor were clarified. The cross-linking process optimized was applied in polysilazane (PSZ) derived ceramic matrix composites.In order to enhance anti-oxidation and heat insulation of ceramic matrix composites surface, UV-cross-linking process of different ceramic precursor were studied firstly.The mechanism that cross-linking extent of polycarbosilane (PCS) depended on dispersing in divinyl-benzene(DVB) was proposed firstly. It was applied to improve infiltration, cross-linking in PCS derived ceramic matrix composites and increase ceramic yield validly. It showed that PCS of linear structure or branchy molecular chain could dissolve in DVB easily. However, most PCS of branchy molecular chain hardly dissolved in DVB, they formed different colloid pieces in DVB. Dispersing of PCS in DVB was the key to increase reactive ratio of Si-H in PCS, cross-linking extent and ceramic yield. Choosing high dissolving temperature, adding consistent solvent and decreasing molecular weight weremainly ways used to increasing dispersing of PCS in DVB. High dissolving temperature could dissolve more PCS of branchy structure in DVB. Adding consistent solvent could increase PCS solubility. Decreasing molecular weight could increase not only PCS solubility, but also reactive ratio of Si-H in PCS.It found firstly that different heat cross-linking rate leaded to different ceramic structure. Defects were mostly pores in ceramic structure by fast heat cross-linking rate, on the other hand, defects were mostly cracks in ceramic structure by slow heat cross-linking rate and ceramic structure was compact expect of cracks. The photographs of different ceramic structure by different heat cross-linking rate were quoted many times in others' papers and became academic evidence in damage mechanism of carbon fibers during preparation of CMCs prepared by PIP and CMCs preparation by fast pyrolysis rate.Reaction of Si-H was mainly react form in PCS, PSZ and polysiloxane(PSO) heat cross-linking. The reaction was basic route to raise ceramic yield of precursor. The influences of different precursor cross-linking condition on hydrosilylation were investigated systemically. Chloroplatinic acid active conditions in cross-linking were explored when it used as catalyst.Subsequent cross-linking course between 200~400°C after cross-linking and molding of precursor under low temperature was discussed. It showed that reactive ratio of Si-H in precursor, cross-linked extent and ceramic yield were increased in subsequent cross-linking to some extent.UV-cross-linking course of organic-silicon precursor were firstly studied. Results showed that PSZ was active and its gel content reached 98% in UV-cross-linking. UV-cross-linking form was Si-Vi aggregation itself. When adding an amount of chloroplatinic acid, some Si-H could be UV-cross-linked with Si-Vi.The key of ceramic coating preparation was sticking between ceramic coating and CMCs. sticking could be improved by CMCs surface compact treatment. SiC>2 was consistent with PSZ when it used as filling. SiO2 could be dispersedequably in PSZ and solution was transparent. By adding SiC>2 in PSZ to change viscosity of solution, hardness and thickness of ceramic coating were increased, volume shrunk was declined in ceramic coating preparation. For sticking between ceramic coating and CMCs was improved, CMCs anti-oxidation was increased accordingly.By program heat cross-linking and pyrolysis at fast rate when PSZ was used as precursor, CMCs which maximum flexural strength reached 614.0 MPa was obtained successfully. The mechanical properties of composites was fine and steady under 1500 °C.