The microhardness-nanostructure correlation of a series of silica/silicon oxycarbide porous composites has been investigated, as a function of pyrolysis temperature, T p . The pyrolyzed products have been studied by means of scanning electron microscopy, mercury porosimetry, chemical analysis, solid-state 29 Si-NMR, X-ray diffraction, Raman spectroscopy, and microindentation hardness. Two distinct regimes are found for the microhardness behavior with T p . In the lowtemperature regime (1000°C < T p < 1300°C), the material response to indentation seems to be dominated by the large amount of pores present in the samples. In this T p range, low microhardness values, H, are found (<110 MPa). Above T p ‫؍‬ 1300°C, a conspicuous H increase is observed. In this hightemperature regime (T p ‫؍‬ 1300 -1500°C), microhardness values are shown to notably increase with increasing pyrolysis temperature. The H behavior at T p ‫؍‬ 1300 -1500°C is discussed in terms of (i) the volume fraction of pores and the average pore size, (ii) the bond density of the oxycarbide network, and (iii) the occurrence of a nanocrystalline SiC phase.

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Organically modified silicates (Ormosils) appear as precursors to prepare silicon oxycarbide based materials. These Ormosils were prepared from tetraethoxysilane (TEOS), silanol terminated polydimethylsiloxane (PDMS) and fumed silica, in different ratios by using the sol-gel method, and then pyrolysed in nitrogen atmosphere up to 1500ºC in order to obtain silicon oxycarbide materials. It was found that Ormosils are thermally stable in these conditions up to approximately 400ºC. The transition between the hybrid material and the inorganic material occurs between approximately 600 and 1000ºC. At 1000ºC, silicon oxycarbide network is formed by [C 2 SiO 2 ], [CSiO 3 ] and [SiO 4 ] sites and a polyaromatic free carbon phase. As the pyrolysis temperature increases, the amorphous silicon oxycarbide network and free carbon phase undergo structural rearrangements. At temperatures of 1400 and 1500ºC, the network of pyrolysed materials is formed mainly by [SiC 4 ] and [SiO 4 ] sites and a nanocrystalline graphite-like phase. ExperimentalThree different precursor compositions with inorganic (TEOS, Aerosil 200)/organic (PDMS M.W. 4200) weight ratio were prepared 80/20, 70/30 and 60/40. Experimental details of the synthesis and the pyrolysis are described in a previous paper [2]. FT-IR spectra in KBr were obtained using a Perkin-Elmer 1760X spectrometer. Thermogravimetric analyses were carried out with a Perkin-Elmer Pyris 1 equipment at 5ºC/min in flowing nitrogen (50 ml/min). 29 Si MAS-NMR spectra were collected with a Bruker MSL-400 spectrometer at 79.49 MHz, in zirconia rotors spun at 4 kHz, with a spectral width of 50 000 Hz, a pulse with of 2.5 µs and relaxation delays of 60 s. Raman spectra were obtained using a Renishaw Raman Microscope System RM1000. Total carbon and hydrogen content was determined using elemental analyzers LECO CS-200 and CARLOERBA EA1108. Silicon content was evaluated using gravimetric techniques. Oxygen was estimated by difference. Results and DiscussionThe FT-IR spectrum of the material pyrolysed at 400ºC do not show any significant difference as compare with the spectrum of the precursor hybrid material, as Fig. 1a shows for the 70/30 sample. This fact indicates the stability of the Ormosil up to this temperature in the pyrolysis conditions. It can be observed that the intensity of the band located at 1262 cm -1 , associated to C-H deformation in Si(CH 3 ) 2 units of the PDMS [3], decreases between 400 and 600ºC. The little bands located at 2964 and 2906 cm -1 and associated to C-H asymmetric and symmetric stretching of the methyl