Papers by Author: O. Addemir

Abstract: The overall mechanical characterization of hot-pressed hexagonal boron nitride (h-BN) based ceramics was performed. Hardness, flexure strength and fracture toughness (KIC) measurements were carried out using Vickers indentations, 3-point bending tests and SEVNB method, respectively. The fracture characteristics of hot-pressed h-BN ceramics were investigated by analyzing fracture surfaces using scanning electron microscope (SEM). The hardness, ranged from HV = 0.20GPa to HV = 0.25GPa, which was higher in the surfaces parallel to the hot-press direction due to the perpendicular alignment of the basal planes of the h-BN platelets to the hotpress direction. The fracture toughness and flexure strength values were determined to be 2.6 MPa.m1/2 and 0.14 GPa, respectively. A single KIC value was obtained since a slight variation of the fracture toughness was observed with the orientation of h-BN. The microstructural observations of h-BN grains indicated completely brittle fracture with intergranular voids and microcracks revealing a mixed mode of fracture.

Abstract: Hexagonal boron nitride (h-BN) based ceramics have been fabricated by hot-pressing in a nitrogen atmosphere at 1900°C for 60 min. under a pressure of 50 MPa with B2O3 as sintering additive. The phase composition was confirmed by XRD. The microstructural properties of h-BN powders and hot-pressed surfaces were evaluated by SEM-EDS analysis. The formation of the secondary phase was also investigated with particular interest on grain boundaries. The sintered densities were also determined by Archimedes’ method. The maximum relative density was obtained to be about 92 %. The effect of the existence of boron oxide on the boron nitride grain growth behaviour was discussed on the basis of microstructural features.

Abstract: Silicon nitride based composites have been fabricated by carbon addition. Carbon black nanograins and graphite micrograins were used as second phase additions. Alumina and yttria were used as sintering aids. Mixing of powders was performed in a ball mill and for comparison in a high efficient attritor mill. For sintering the hot pressing technique has been applied. Experiments at 2 MPa uniaxial pressure have been performed. The Si/N mass fraction after sintering were determined by prompt gamma activation analysis (PGAA). The amount of carbon black and graphite introduced in the silicon nitride matrix increased the porosity and decreased the hardness and bending strength of composites. Lower modulus, and lower strength was obtained for composites with carbon black addition in comparison to graphite added samples. The microstructure of composites consisted mainly of alpha and beta silicon nitride. The formation of silicon carbide was observed only at 10 wt% carbon black addition.