Showing papers by "Richard J. Day published in 2001"

Conversion of polycarbosilane (PCS) to SiC-based ceramic Part 1. Characterisation of PCS and curing products

Abstract: A commercial polycarbosilane (PCS) preceramic polymer has been characterised as-received and following curing under a variety of conditions. Elemental analysis, gel permeation chromatography (GPC), infra-red spectroscopy (FT-IR), simultaneous thermogravimetric analysis-differential thermal analysis (TG-DTA) and solid state nuclear magnetic resonance (NMR) have been employed. A number average molar mass of 1200 was found with a broad molar mass distribution ( $$\overline M _{\text{w}} $$ / $$\overline M _{\text{n}} $$ = 2.97). Elemental analysis gave an empirical formula of SiC2.2H5.3O0.3. IR and Solid state 29Si and 13C NMR spectra showed the presence of Si-O-Si, SiC4, SiC3H, Si-Si, Si-CH3 and Si-CH2 groups. Simultaneous TG-DTA performed under an argon flow showed that there was a weight gain which started at approximately 240 °C. DTA showed an exotherm starting at this temperature showing that there was oxidation of the polymer even in an inert atmosphere. This is perhaps due to the oxygen in the PCS and there may also be some impurities in the inert atmosphere. Evidently the PCS is very sensitive to oxygen. Above 500 °C, weight loss dominated although the exotherm continued to approximately 700 °C. The effect of heating rate and dwell time at 200 °C on the changes in the chemical composition during curing have been explored using IR and solid state NMR spectroscopies, and elemental analysis. The longer the cure time the higher was the weight gain and greater was the extent of the oxidation reactions. Elemental analysis showed that the ratio of H and C to Si decreased with holding time at the cure-temperature while the amount of oxygen increased. Use of a higher heating rate resulted in a lower weight gain when the same holding time was used. From this it is clear that curing starts below the holding temperature.

Conversion of polycarbosilane (PCS) to SiC-based ceramic: Part II. Pyrolysis and characterisation

Abstract: The conversion to ceramic of a commercial polycarbosilane (PCS) under various pyrolysis conditions has been investigated. The products of pyrolysis have been characterised by solid state 29Si and 13C NMR spectroscopy and X-ray diffraction (XRD). Some of the phases identified in the present study were found to differ from those reported previously, particularly in the earlier literature. Oxidation-cured PCS, when pyrolyzed up to 1400 °C in argon, generally produced silicon oxycarbide (SiO x C y ) as the second major phase with β-SiC as the major phase, and smaller amounts of free carbon. With increasing temperature above 1200 °C, the silicon oxycarbide phase decomposed to give β-SiC. Silica (SiO2) was also found to evolve from this silicon oxycarbide phase. Loss of some of the silica, probably by reaction with carbon, was found at 1400 °C, possibly yielding SiO, CO and SiC. At 1500 °C, crystalline α-cristobalite was found as a minor phase with β-SiC as the major phase and a lower amount of free carbon. Pyrolysis in vacuum leads to production and crystallization of β-SiC at a lower temperature than required if pyrolyzed in argon flow. After pyrolysis at 1600 ° in vacuum, the cured PCS converted to almost stoichiometric β-SiC.

Carbon fibre-reinforced CMCs by PCS infiltration

Abstract: C/SiC-based composites were produced by infiltration of a woven carbon fabric preform with a polycarbosilane (PCS) solution. Progress of the infiltration process up to eight infiltrations was followed by density measurements, microscopy and mechanical testing. After six infiltration cycles, the density increase tended to a plateau. Dense deposition of the matrix on the outer surface sealed off the core of the composite, which hindered successive infiltrations. With further re-infiltration cycles, the development of the matrix and the degree of bonding between the fibre and matrix increased. There was also a transition in the failure mode of the composite, when tested in 3-point bending, from shear to plain brittle tensile failure, as the number of infiltrations increased.

Thermal and mechanical characterization of epoxy resins toughened using preformed particles

Abstract: Preformed, multilayer particles have been used to toughen an epoxy resin. The particles were formed by emulsion polymn. and consist of alternate glassy and rubbery layers, the outer layer having glycidyl groups to give the possibility of chem. bonding of the particles in the cured resin. Two variants of this type of particle were used, termed GM(47/15) and GM(47/37); both types have an overall diam. of 0.5 mm, but the former have a thicker rubbery layer. For comparison, acrylic toughening particles with no surface functionality and a liq. carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber were used as toughening agents. The epoxy resin system consisted of a com. diglycidyl ether of bisphenol A (Shell Epon 828) with diamino-3,5-diethyl toluene as hardener, two com. sources of which were used, namely Ethacure-100 and DX6509 . These hardeners contain a mixt. of two isomers, namely 2,6-diamino-3,5-diethyl toluene and 2,4-diamino-3,5-diethyl toluene. Thermogravimetry in nitrogen shows that the preformed toughening particles begin to degrade at 230 DegC, whereas the cured resin begins to degrade rapidly at 350 DegC. Thus, even though the particles are less thermally stable than the cured resin, their degrdn. temp. is well above the glass transition temp. of the resin, and their use does not affect the thermal stability of the toughened materials at normal use temps. The performance of the toughening agents was compared using Ethacure-100 as the hardener. The GM(47/15) and GM(47/37) toughening particles gave rise to a greater toughening effect than the ATP and the CTBN. For example, the fracture energies were: 0.26 kJ m-2 for the unmodified resin; 0.60 kJ m-2 for the resin toughened with CTBN; and 0.69 kJ m-2 for the resin toughened with the GM(47/15) particles. The ultimate tensile stress of the unmodified epoxy resin was 43 MPa, which increased to 55 MPa when 20 wt% of GM(47/15) toughening particles were added. The toughness of resins cured with the DX6509 hardener were superior to those obtained with the Ethacure-100 hardener, most probably due to DX6509 producing a less-highly-crosslinked network. This highlights the sensitivity of the toughening process to the hardener used, even for hardeners of a similar nature. [on SciFinder (R)]