Effect of the oxidization of Si3N4 powder on the microstructural and mechanical properties of hot isostatic pressed silicon nitride

Abstract

The effect of nanosized oxidized silicon nitride powder particles on the microstructural and mechanical properties of hot isostatic pressed silicon nitride was studied. The starting <alpha> Si 3 N 4 powders were oxidized for 10 and 20 h at 1000 ˚C in ambient air environment. An amorphous oxide layer was observed on the surface of Si 3 N 4 powder particles using high resolution transmission electron microscopy (HRTEM) whose thickness increased with the oxidation time. The powders were hot isostatic pressed at 1500 ˚C and 1700 ˚C in nitrogen gas environment under 20 Mpa for 3h. Complete <alpha> to <beta> transformation was observed in Si 3 N 4 samples sintered at 1700 C under 20 MPa for 3 h compared to Si 3 N 4 samples sintered at 1500 ˚C. Silicon oxynitride (Si 2 N 2 O) was found in sintered samples whose amount increased with the oxidation time. The <beta> phase decreased with the increase of Si 2 N 2 O phase in the sintered samples. The flexural strength of samples sintered at 1700 ˚C is higher due to the higher amount of <beta>-Si 3 N 4 phase than for samples sintered at 1500 ˚C. Decreasing flexural strength was observed with the oxidation time. Higher amount of oxygen in the base powders resulted in a higher Si 2 N 2 O content in the sintered Si 3 N 4 .

Properties of MWCNTs added Si3N4 composites processed from oxidized silicon nitride powders

“…1. In the case of the sample prepared with 20 h oxidized α-Si 3 N 4 powder, five times higher atomic percent of oxygen was detected than that of the sample prepared with un-oxidized powder [23]. In this current work, no structural peaks of any oxygen-containing phase (including SiO 2 ) were detected by XRD after oxidation.…”

“…The low rpm was used to avoid the damaging of MWCNTs in the powder. A similar powder preparation process was used in previous works [23,24]. The drying and sieving process with 150 µm mesh number were done after the milling process.…”

“…The heat treatment of green samples was done at 400°C in the muffle furnace to eliminate the PEG prior to the sintering process. After the heat treatment of the green samples, the samples were sintered at 1700°C and 20 MPa in nitrogen (N 2 ) gas environment for 3 h as a holding time with the heating rate of 25°C/min, similar as in our previous work [23].…”

“…Si 2 N 2 O was supposed to be formed during sintering at high temperature because of the presence of oxygen content in starting powders. Previously, Si 2 N 2 O was formed by the oxidization of the starting powders and its effects on the mechanical properties were observed [23]. The reason might be the reaction occurring between the surface oxygen on the silicon nitride and carbon nanotubes.…”

“…High resolution electron microscopy (HREM) images of Si 3 N 4 powders without SiO 2 layers (a) and with SiO 2 layers formed after the oxidation of powders during 10 h (b) and 20 h (c), based on[23] shape and homogeneity of the powders and MWC-NTs were examined by scanning electron microscopy (SEM, LEO 1540 XB). Energy dispersive X-ray spectroscopy (EDS) was carried out for the qualitative analysis of the base powders (un-oxidized and oxidized Si 3 N 4 powders) for the quantification of elemental compositions.…”