Silicon nitride (Si3N4) ceramics have been produced by a number of manufacturing processes for use as bearings, gas turbine components, and other high temperature environment applications. Compared with other oxide ceramics such as Al2O3 and ZrO2, Si3N4 is less dense and exhibits lower thermal expansion co-efficient, a greater strength retention at high temperature, and good corrosion resistance.
Yian-Feng Yang and colleagues at the Changsa University of Science & Technology, Changsa, and Tsinghua University in Beijing, have been investigating the use of Ceramic Injection Moulding (CIM) to produce high precision and complex functional shapes having high surface quality from ultra-fine Si3N4 powder. The authors report the results of their investigation in the International Journal of Minerals, Metallurgy and Materials (Vol. 22, No. 6, June 2015). They state that although CIM is successfully used for some oxide ceramics, it has not yet been extensively used for Si3N4 because of the difficulty in producing a CIM feedstock with a sufficiently high powder loading and low viscosity. This is especially the case for sub-micron Si3N4 powders.
The researchers used Si3N4 powder having a median particle size (d50) of 0.34 micron where the α-Si3N4 phase content was greater than 92 wt%. This was ball milled in ethanol together with a sintering additive (6 wt% Y2O3 + 2 wt% La2O3) and the resulting composite slurry was dried for use in the CIM feedstock. The organic binders used for the feedstock included polypropylene (PP), polyethylene (PE) and wax (PP:PE:W = 10:10:80 by mass). Additionally, stearic acid was used as a surfactant. Spherical parts and test bars were produced from the feedstock by injection moulding followed by solvent debinding at 40°C, thermal debinding at 900°C, and gas pressure sintering at 1795°C for 3 hrs under N2 pressure of 8 MPa.
The results show that a solid loading of less than 50vol% and the surfactant mass fraction of 6wt% gave perfect flowability of the Si3N4 feedstock making it suitable for CIM. Defects detected during production are traced to improper moulding parameters, mould design, debinding parameters, residual stress, or inhomogeneous composition distribution in the green parts.Gas pressure sintered properties of Si3N4 included density of 3.2 g/cm3, Vickers hardness of 16.5 GPa, and fracture toughness of 7.2 MPa.m1/2.