Oxidation Behavior of SiC Coatings Modified by Zirconium-based Ceramics and SiC Nanowires

Author:Li Lu

Supervisor:li he jun


Degree Year:2018





Oxidation sensitivity is the bottleneck problem for carbon/carbon(C/C)composites,which limits their applications as high-temperature structural materials.Oxidation resistant coatings are considered to be effective methods to solve this problem and SiC coating is one of the most widely-used anti-oxidation coating systems.The oxidation behavior of SiC coating is greatly affected by different preparation methods and additives,so it is essential to do a systematic research on them and then fully realize the application potential of SiC-based coatings.In this thesis,in order to improve the oxidation resistance of SiC coatings for C/C composites,it was studied that the effects of preparation methods(pack cementation and chemical vapor deposition)and additives(ZrB2,ZrB2/ZrC and SiC nanowires)on the microstructures and oxidation behavior of SiC coatings.XRD,SEM,TEM,HRTEM,SAED,EDS,three-point bending and scratch tests were used to characterize the phase composition,microstructures,element distribution and mechanical properties of the coatings;the protective abilities of coatings were tested in various oxidation environments(high-temperature static air,broad temperature range,thermal cycles,different oxygen partial pressures)and the corresponding microstructure evolution was investigated;the oxidation protection mechanism and the failure reason of coatings was also investigated.The main contents and conclusions are listed as follows:SiC coatings were prepared on C/C composites using pack cementation(PC)and chemical vapor deposition(CVD)and their microstructures and oxygen behavior were compared.The results indicated that the PC-SiC coating was composed of hexagonal platelets ofa-SiC and a few Al2O3 impurities,while the CVD-SiC coating consisted of spherical particles of pureb-SiC.With the increase of temperature,the anti-oxidation ability of PC-SiC coating was improved and its oxidation curve transformed from linear to parabolic,suggesting that the oxidation reaction was controlled by the diffusion rate of oxygen in the glass film instead of the reactive rate between oxygen and C/C substrate;while the oxidation curve of CVD-SiC coating remained linear during the process,suggesting its anti-oxidation ability was not improved.The better oxidation resistance ability of PC-SiC coating could be attributed to two factors.On one hand,the introduction of Al3+increased the forming rate of SiO2 glass film and decreased its viscosity,resulting in the superior heal effect of the glass film.On the other hand,the coating and substrate had a good effect of reactive wetting,leading to the stronger interfacial bonding.The SiC coating modified by ZrB2 and ZrB2/ZrC possessed significantly improved oxidation resistant ability.The weight loss of SiC-ZrB2 coating after isothermal oxidation at1500℃ for 216h was 0.15%,while the weight gain of SiC-ZrB2-ZrC coating after isothermal oxidation at 1400℃ for 221h was 1.07%.A Zr-Si-O glass layer with inlaid structures was formed after the oxidation of modified SiC coatings.ZrO2 and ZrSiO4 particles could pin the glass layer and cause the microcrack deflection,improving the protective ability of coatings.The failure mechanism of coatings was governed by the formation and growth of bubbles.The rupture of bubbles generated the diffusion channels and resulted in the oxidation of lower coating.The microstructure evolution of SiC-ZrB2 and SiC-ZrB2-ZrC coatings was investigated.For the SiC-ZrB2 coating,with the increasing oxidation time,the surface of glass layer showed a transformation from discontinuous to continuous.After long-time oxidation,it exhibited a two-layered structure consisting of a ZrO2(ZrSiO4)embedded SiO2-rich glass layer and an unreacted SiC-ZrB2 layer.For the SiC-ZrB2-ZrC coating,with the increasing oxidation time,surface roughness showed a trend of“increase-decrease-increase”and the SiO2 glass transformed from amorphous to crystalline.When oxygen diffused through the glass layer gradually,ZrC was preferentially oxidized at the interface between the oxidation layer and the unreacted layer,which could lead to the rapid formation of lower Zr-Si-O glass layer and improve the long-term anti-oxidation ability of coatings.The effects of oxygen partial pressures on the isothermal oxidation behavior of the SiC-ZrB2 and SiC-ZrB2-ZrC coating were investigated.For the SiC-ZrB2 coating,SiC began active oxidation in the defects(holes and cracks)at low oxygen partial pressure and formed SiO2 whiskers.The coating surface couldn’t form dense and continuous glass layer rapidly,resulting in the partial oxidation of C/C substrate.At high oxygen partial pressure,SiC began passive oxidation and a dense Zr-Si-O glass layer was formed in a short time,which could protect the C/C substrate effectively.For the SiC-ZrB2-ZrC coating,with the increasing oxygen partial pressure,the morphology of ZrB2 and ZrC oxidation products(ZrO2)changed from the fibrous denticles to the aggregations of particles.Meanwhile,a Zr-Si-O glass layer with good heal effects was formed in a shorter time.Porous layers of SiC nanowires were constructed on the inner coating by CVD and electrophoretic deposition(EPD)to introduce SiC nanowires into SiC-ZrB2-ZrC coatings.The results indicate that the coating roughness was improved and the crack sizes were decreased,further enhancing the oxidation resistant ability of coating during thermal cycles.After 30 thermal cycles of“room temperature(?)1500℃”,the coated samples modified by EPD-SiC and CVD-SiC nanowires lost weight of 0.52% and gained weight of 2.28%,respectively.SiC nanowires in SiC-ZrB2-ZrC coatings increased the roughness of coatings by bridging and pull-out mechanism,alleviating the cracking trend of the coatings in high-low temperature cycles.