Themochemical Environmental Behaviors Of 3D SiC/SiC Composite

Fiber reinforced SiC-matrix composites (CMC-SiC), mainly referred to C/SiCand SiC/SiC, are considered as the most promising thermo-structural materials foraero-engine applications due to their high thermal stability, high specific strength,high specific modulus, good oxidation resistance, good ablation resistance, especiallyimproved flaw tolerance and noncatastrophic mode of failure. The severe serviceenvironments of hot section of aero-engine are multi parameters cooperatedenvironments, which can be divided into thermochemical environments andcomplicated stress environments. The thermochemical environments referred to hightemperature, oxidative/corrosive atmosphere have a directly effect on propertiesevolution of CMC-SiC. Stress played a role to enhance the properties evolution ofCMC-SiC. Thus, knowledge about behaviors of materials under thermochemicalenvironments without stress action is of primary importance to understand thebehaviors of them under cooperation of stress and thermochemical environments. Ithas been intensively studied and well understood that the behaviors of the C/SiCcomposites in themochemical environments. But for SiC/SiC composites, especiallyreinforced with nearly stoichiometric Hi-Nicalon fiber, there is a blank ofsystematically study on structure, key properties and their evolution inthermochemical environments. Researches on thermochemical environmentalbehaviors of SiC/SiC are very important to application of this material in hot sectionof aero-engine.In this thesis, a three-dimensional (3D) woven Hi-Nicalon fiber reinforced SiCcomposite (SiC/SiC) with PyC-interphase was prepared by chemical vapor infiltration.Key thermophysical and mechanical properties of the SiC/SiC composite weresystematically measured and analyzed. The properties and their evolution of theSiC/SiC composite in typical thermochemical environments were investigated andalso compared with those of C/SiC composite. The main contents and results are asfollows:1. The relationship between the thermal expansion coefficient and temperature ofthe composite in argon from room temperature to 1400℃was similar to that of CVDSiC and C/SiC. The thermal diffusivity of the composite in argon from roomtemperature to 1400℃was lower than those of the C/SiC and could be well fitted bya logarithmic function. The SiC/SiC composite exhibited much excellent mechanical properties than the C/SiC composite.2. Under air environments, SiC coated SiC/SiC exhibited different oxidationmechanisms and strength retention than that of SiC coated C/SiC. Below 1100℃, theoxidation of SiC/SiC was controlled by oxygen diffusion through the channelscomposed of the coating matrix defects and looped pipelines formed by the oxidationconsumption of PyC interphase. Above 1100℃, oxidation was controlled by oxygendiffusion through the SiO₂ scale and the oxidation remains superfacial for the bothcomposite. Below 1200℃, strength retention of SiC/SiC was above 80%, while thatof the C/SiC was below 60%. Above 1300℃, strength retention of the SiC/SiCdecreased rapidly. Strength retention of the SiC/SiC corresponding to 1300℃and1400℃was 65% and 48%, respe,ctively. However, strength retention of the C/SiCwas nearly kept at 80% and showed an increasing trend. Below 1100℃,a certainextent of PyC oxidation consumption was beneficial to improve interphase bonding inthe SiC/SiC, which resulted in improving of toughness without strength degradation,namely, the composite showed mechanical self-adaptability with improvedstrengthening and toughening effects.3. The oxidation of the SiC/SiC was greatly enhanced by water vapor and mainlytook place on the specimens' surface. Oxidation behavior was strongly affected bytemperature and the ratio of water to oxygen. The higher of them were, the clear ofthis effect was. After oxidation for 10h, below 1200℃, the residual flexural strengthnearly kept the same value as that of as-received specimens, while strength retentionof the C/SiC was below 70%. Above 1300℃, strength retention of the SiC/SiCdecreased with the temperature increasing in a linear mode. Strength retention of theSiC/SiC corresponding to 1300℃and 1500℃was 82% and 35%, respectively.However, strength retention of the C/SiC nearly kept at 75%.4. The corrosion behaviors and mechanisms of the SiC/SiC were investigated insimulated aero-engine combustion environments containing Na₂SO₄ vapor, oxygen,and water vapor. Below 1200℃, surface oxidation of the CVD SiC coating was thedominant mechanism. The product existed in the form of cristobalite. Above 1200℃,the specimens were covered by vitreous corrosion product due to the react betweenNa₂SO₄ and SiC. Above 1300℃, the corrosion occurred within the coating togetherwith active oxidation of CVD SiC. Oxidation was the dominant mechanism. Aftercorrosion for 10h below 1200℃, the residual flexural strength nearly kept the samevalue as that of as-received specimens. Above 1300℃, strength retention of theSiC/SiC decreased with the temperature increasing in a linear mode. Strengthretention of the SiC/SiC at 1300℃and 1500℃was 67% and 32%, respectively.Below 1300℃, strength retention of the C/SiC was below 70%. Between 1400℃and 1500℃, strength retention of the C/SiC kept at 50%. Moreover, the strength retentionof the both composites under simulated aero-engine combustion environments wasslightly small than those under water vapor/oxygen mixture environments.5. Thermal shock behavior of the C/SiC and SiC/SiC composites wasinvestigated by quenching the specimens from 1200℃to 25℃in water. Thermalshock damage of the SiC/SiC composite is obviously anisotropic. After quench for100 cycles, the SiC/SiC composite retained 80% of the original flexural strength inthe fiber woven direction while it cracked in the direction perpendicular to fiberwoven direction. The C/SiC had a better thermal shock resistance than that ofSiC/SiC in the same test conditions. After quench for 100 cycles, the C/SiC possessed83% retained strength and no breakage observed. Thermal shock behaviors differencebetween the SiC/SiC and C/SiC mainly resulted from following factors: braidstructure of the fiber preforms, difference in coefficient of thermal expansion in radialdirection between carbon fibers and Hi-Nicalon SiC fibers, as well as difference inthermal conductivity of the two fibers.6. A parameter to evaluate effects of strengthen and toughen of CMC-SiC wasproposed. The parameter was described by ratio of (W_f,c-W_m)/W_m, in which W_f,c andWm are to work of fracture and work of matrix cracking, respectively. Effects otthermochemical environments on strength and toughness of the SiC/SiC wascharacterized by the parameter.7. A factor analysis method, namely increment factor analysis method, wasintroduced and applied to analysis effects of environmental parameters on strengthdegradation of SiC/SiC composite. The effect of individual environmental parameterson strength degradation of the SiC/SiC was different, and all of them would result instrength degradation, except water. The degree of strength degradation induced by theabove environmental parameters was decreased in the order of high temperature:Na₂SO₄ and oxygen. High temperature was the most key factor to determine strengthof SiC/SiC in thermochemical environments.