Modification of the oxidation behaviour of Si3N4–TiB2 composites by PECVD alumina coatings

A nearly fully dense (>98%) electroconductive silicon nitride—35 vol.% titanium diboride composite was obtained by hot isostatic pressing (HIP) in presence of a low content of sintering aids (0.5 wt.% Y2O3 + 0.25 wt.% Al2O3). To improve the oxidation behaviour of this composite material, a 3-μm thick protective coating of aluminium oxide was deposited on cubic samples (4 mm × 4 mm × 4 mm) by microwave plasma-enhanced chemical vapor deposition (PECVD) using an oxygen plasma and an organometallic precursor (trimethylaluminium). SEM images demonstrated that the coating was homogeneously distributed on the external surface of the specimens. othermal and isothermal oxidation tests were carried out with a Setaram Microbalance under pure flowing oxygen (10 L/h) on both uncoated and coated Si3N4–TiB2 samples. In the case of non-isothermal oxidation of a substrate without coating, the reaction started at 600 °C. Between 1100 and 1350 °C, a plateau was observed and above 1350 °C the weight gain increased significantly. In presence of an Al2O3 coating, the composite started to oxidize at higher temperature (1200 °C). Isothermal kinetics recorded for 24 h, at 1350 and 1400 °C, revealed that the presence of the Al2O3 coating improved drastically the oxidation resistance and changed the shape of the curves from globally parabolic to almost logarithmic. An explanation of this protective behaviour, based on the characterization by XRD, SEM and EDS of the reaction products, is proposed.