Sintering behaviour of α and β solid solution sialons

Abstract: Some ceramics have the ability to form solid solutions. One such group are the sialons, which contain silicon, aluminium, oxygen and nitrogen in specific proportions. Through a stabilizing process with yttrium, not only beta-sialons but also alfa-sialons can be produced. The basic difference between these two is in the two different crystal structures of the silicon nitride. This type of ceramic is usually produced by the sintering of powder mixtures at temperatures up to 1850ºC, during which complicated chemical reactions take place through a liquid phase. During sintering, the material is densified to a pore free body, characterized by high hardness, chemical inertness, high oxidation resistance and high strength at elevated temperature. Alfa- and beta-sialons with four different compositions were studied by means of high temperature sintering dilatometry. This allowed in situ studies of the densification process to be made during sintering. The compositions used werebeta-10 = Si5.23Al0.77O0.68N7.23, beta-60 = Si2Al4O4N4, alfa-33 = Y0.33Si10.51Al1.49O0.5 N15.51 and alfa-45 = Y0.45Si9.98Al2.02O0.68N15.32 .Reaction routes were determined by interrupted sintering with specimens cooled from different temperatures as well as the influence of the heating rate on the material phase composition and densification. In addition, the extent to which it is possible to control the densification by the heating rate profile was also established. Experiments were carried out in which specimens sintered with constant rate of heating were compared with specimens from sintering cycles established from a predetermined profile for the densification rate. The transient liquid which occures during the earliest stage of densification exists for only a short period of time during which only minor densification takes place. Instead, a sialon with specific composition apparently independent of the powder composition was precipitated from the liquid. This process largly absorbes the transient liquid. The composition of this first formed sialon is Si2.5Al3.5O3.5N4.5 for beta-sialon, and between Y0.40Si10.20Al1.80O0.6ON15.4 0 and Y0.50Si9.75Al2.25O0.75N15.25 for alfa-sialon. After this, the process continues by solution of the first formed sialon and alfa-Si3N4 from the powder, together with precipitation of a sialon with lower amount of additives. During this part of the reaction the main densification takes place. The densification behaviour of the high alloyed beta-60 composition differs from that of the others. Instead of densifying when the first liquid was formed, the greenbody started to expand. This is explained by the entrapment of gasses, evolved due to chemical reactions. The chemical composition of the phases formed contribute to the formation of a large amount of liquid during the early stages of sintering. The expansion continues until the material is completely transformed to beta-ss, whereafter it densifies rapidly. The densification is made possible by the completion of the transformation to beta-ss which reduces the amount of liquid phase and lets the evolved gas pass through the porous structure. The reactions taking place during sintering results in different phases being dissolved and precipitated which causes the composition of the liquid phase to change during densification. These different processes results in characteristic changes of the densification rate during sintering. Application of rate controlled sintering showed that it was possible to obtain a predetermined densification rate profile. However, depending on the composition, the densification rate profile which increases the bending strength of the beta-10 composition failed to do so for the beta-60 composition. Fractography revealed large pores to be the strength limiting defect. The results show that more than just the densification rate determine the optimum sintering programme. Depending on the composition, which leads to different reactions and structure during sintering, different sialons must be sintered with different sintering programmes to achieve optimum properties.