Constituent development for higher temperature capable ceramic matrix composites

Two of the highest capability priorities for the Air Force, energy-efficient turbine engines and long-range precision strike require high-temperature CMCs to enable increased turbine engine efficiency and thermal protection of hypersonic vehicles. Ceramic-matrix composites (CMCs) currently lack the temperature capability and durability required for long-life at the highest temperatures desired. This presentation highlights research that is addressing the need for improved high-temperature-capable CMCs, with a focus on CMC constituents and an understanding of their processing, microstructure, and behavior in relevant service environments. The most pervasive lifetime and temperature limitations for SiC/SiC CMCs are related to oxidation, creep and stress rupture of the fibers, oxidation-induced instability of the fiber-matrix interface, and instability of the matrix at temperatures \u3e1400°C. Consequently, we are addressing these shortcomings by developing technologies to enable higher-temperature capable SiC fibers, oxidation-resistant fiber-matrix interfaces, and improvements in processing of refractory matrices for both turbine engine and hypersonic applications

Processing, performance and process modeling of preceramic polymers

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Processing and evaluation of UHTC loaded composites

The ceramics composite research team at the Air Force Research Laboratory (AFRL) has been investigating the impact of adding UHTC fillers to traditional SiC/SiC and C/SiC matrices. This talk will highlight these efforts and their corresponding results. A more thorough review of a SiC/SiC-UHTC composite will be presented to discuss processing development and the evaluation regiment. Ceramic matrix composites with BN/SiC coated Hi Nicalon STM SiC fibers and matrices derived from a combination of polymer-derived SiC ceramic and powders of SiC and HfB2 were prepared. Flat plates and leading edges were processed using a wet layup of slurry infiltrated 8HS fabric. The processing was designed to result in a graded microstructure where the bulk of the sample was SiC/SiC and the outer layers to be exposed to the high heat flux environment were SiC/SiC-HfB2. In addition, a HfB2-rich coating was applied using a novel processing method amenable to preceramic polymer infiltration and pyrolysis (PIP) processing. The samples were tested for oxidation resistance using two methods: (1) arcjet and (2) laser heating. Arcjet testing was performed at the Italian Aerospace Research Centre (CIRA) using a mixed Air/Ar plasma in the Ghibli facility. Surface temperatures were set between 1500 and 2200oC by controlling the current. After high temperature exposure, an adherent oxide scale formed on the surface of the composite that limited further damage to the interior of the CMC. A rapid heating occurred near 1750oC that may be related to active oxidation. To further elucidate oxidation mechanisms, samples were also tested in the Laser Hardened Materials Evaluation Laboratory (LHMEL) at Wright Patterson AFB. The samples were subjected to gas flows using mixtures of N2 and air to control the environment while the sample was heated using a laser. As seen in Figure 1, damage was shown to be intensified by exposure to a flowing oxygen deficient atmosphere. The oxidation behavior of both the arcjet and laser heated samples were compared using SEM/EDS analysis of cross sections. Results will be discussed to elucidate oxidation mechanisms of a refractory loaded SiC/SiC in different environments. Please click Additional Files below to see the full abstract

Modeling environmental effects in MeB2/SiC UHTCs: Oxidation by oxygen and water vapor

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Modeling environmental degradation in SiC/BN/SiC CMCs

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Effect of Oxidation on Mechanical Properties of Fibrous Monolith Si 3 N 4 /BN at Elevated Temperatures in Air

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65838/1/j.1151-2916.2002.tb00597.x.pd

Evaluation of ceramic matrix composite leading edge samples under simulated hypersonic flight conditions

Sharp leading edge (LE) samples, with tip radius of curvature of 0.76mm, fabricated from three different Ceramic Matrix Composites CMCs were exposed to simulated hypersonic flight conditions using a direct-connect scramjet rig and their thermal and erosion responses measured. UHTC (20vol%SiC-HfB2) LE samples were used in the same run as controls. The simulated flight conditions approached equivalent free flight conditions of about Mach 7 at 22km altitude, for short durations (~1 min), in combination with prolonged exposure (several minutes to an hour) at Mach 5 flight conditions at the same altitude. All the CMC samples survived the thermal, acoustic and other mechanical shock but exhibited erosion to various extents. In comparison the UHTC samples suffered no significant erosion, but some samples cracked from thermal shock after repeated exposure cycles. The matrix chemistry played a significant role, while the fiber orientation had no significant effect, with damage being dominated by matrix degradation kinetics. A SiC fiber reinforced hybrid UHTC matrix composite leading edge with a 2.5 mm radius of curvature, survived for short duration at Mach 7 condition without significant erosion or cracking

High-temperature ceramic matrix composites using microwave enhanced chemical vapor infiltration

To deliver the next generation of aerospace propulsion systems, major modifications to the materials used and their manufacture are required. High-temperature ceramic fibre reinforced ceramic matrix composites (HT-CMCs), specifically SiCf/SiC, have been identified as potential candidates to operate in the hostile aero-thermo-chemical environments experienced in service without compromising structural integrity, whilst keeping mass at a premium. Presently a lack of notably higher temperature properties and durability compared to Ni-super alloys, combined with high manufacturing costs, is preventing widespread utilisation of these composites. Current advanced manufacturing techniques are able to produce these HT-CMCs, which are starting to come into service but all of these techniques introduce compromising features, such as a residual silicon phase, thermal stresses or micro cracking in the matrix microstructure. One of these advanced methods, chemical vapour infiltration (CVI), is an effective manufacturing route capable of creating near fully dense components with an extremely refined microstructure with little or no preform degradation and minimal residual stresses. CVI’s challenges, however, are three fold; i) processing uses isothermal heating rates so batch production times are typically 2 – 3 months; ii) premature pore closure results in a need for repeated machining stages to re-open the closed channels, which reduces process efficiency to between 5-10%; iii) as a consequence of the previous two points, associated costs are very high and the product expensive. Microwave energy (MCVI) has been proposed as a potential solution to heat the SiC fibre preform for CVI; it produces a favourable inverse temperature profile, meaning the temperature is hottest at the centre of the component in contrast to conventional CVI. This inverse profile initiates densification at the centre of the sample, thus avoiding surface porosity closure. It is expected that the use of a microwave-enhanced CVI processing routes could yield near fully dense products in as little as 72 – 96 hours. This poster presents an update on the forming and characterising of the SiC matrix inside the SiC fabric preform (the latter made of Tyranno ZMI, UBE industries) using the MCVI technique. Kinetics, composition, densification profile, morphology and mechanism of growth of the SiC matrix have all been observed and analysed using a suite of characterisation techniques to see the effect of changing the processing variables. Transmission electron microscopy (TEM) and high resolution scanning electron microscopy (SEM) have been used to observe the degree of crystallinity of the resulting SiC and more specifically the grain growth mechanism and thus the resulting morphology. Wave dispersive spectroscopy (WDS) and Raman has been used to determine the (consistently near stoichiometric) Si to C ratio with an accuracy of ±2% due to a small contribution from traces of oxygen present, the results corroborating the data obtained using the TEM. Raman identified the deposit as ß-SiC and, after further analysis, a number of common polytopes were found including 3C, 6H/15R and 4H. Presented results suggest MCVI is a viable method of producing SiC composites that are potentially suitable for the next generation of aerospace material, though a better understanding of the extent to which full densification can be achieved is still required