Thesis of Celine Gharibian

Preparation and Investigation of Physical and Microstructural Properties of Si3N4/TiN Ceramic Composites

By: Celine Gharibian

Supervisor: Prof. FarhadGoelstanifard, Dr.Seyed Mohammad Mirkazemi

Iran University of Science and Technology

January 2016

Abstract: Because of their high strength, hardness and thermal stability, silicon nitride ceramics are among the most important material for structural applications.Unfortunately, the difficulties in machining and finishing of silicon nitride limit its applications. Therefore, recent developments in silicon nitride composites are focused not only on the improvements of strength and toughness, but also on different possibilities for massive production and cost reduction in manufacturing. A successful approach is to incorporate electrically conductive reinforcements into silicon nitride matrix. The incorporation of a certain content of conductive reinforcements makes the composite electrically conductive so that electrical discharge machining (EDM) can be used. TiN-reinforced silicon nitride composites has relatively high strength, low density and good electric conductivity. Si3N4/TiN composites have been sintered by pressureless sintering (PS), hot pressing (HP) and gas pressure sintering (GPS). The main strategy in manufacturing of Si3N4-TiN composites is associated with a design of the ceramics having an optimum content of TiN which distributed homogenously and provides good mechanical as well as good electrical characteristics. In the present research, titanium nitride ranging from 20 to 40 vol. % was added to silicon nitride. As sintering aids 3 wt. % Al2O3 and 6 wt. % Y2O3 were used. Pressureless sintering was done at 1650, 1700 and 1750°C. Relative density of 97.9% was achieved for the sample with 40 vol. % TiN sintered at 1750°C. This sample has the highest fracture toughness (7.1±0.3 MPa.m1/2) and the lowest electrical resistivity (2.16×10-6Ωm) among the other samples. Machining of this composite was successfully done by electrical discharge machining (EDM). For reaching a surface with minimum roughness after machining, a dense body required. Therefore the next attempt was prepare the composites by hot pressing at 1700°C. 99.8% of theoretical density was achieved for the sample with 40 vol. % TiN. This composite also showed remarkable mechanical properties such as bending strength, fracture toughness and hardness. Bending strength (904±81 MPa) and fracture toughness (7.3±0.7 MPa.m1/2) increased by increasing of the amount of TiN. In contrast, hardness decreased with increasing of TiN content. The highest hardness (15.2±0.2 GPa) was achived for the sample with 20 vol. % TiN. Electrical resistivity of the sample with 40 vol. % TiN was measured 1.9×10-5Ωm. Machining of this composite was successfully done by electrical discharge machining, as well. Investigation of surface roughness was done by profilometer.


Keywords: Silicon nitride, titanium nitride, electrical discharge machining, electrical conductivity.