The influence of sintering temperature on the density of silicon nitride ceramic balls

Silicon nitride is a strong covalent bond compound. The self-diffusion coefficients of nitrogen and silicon atoms are low, resulting in low rates of volume diffusion and grain boundary diffusion required for densification, as well as insufficient sintering driving force, which leads to poor sinterability. Various defects are prone to form during the preparation process, thereby reducing material properties and limiting the application of Si₃N₄ ceramic ball bearings. At present, gas pressure sintering is commonly used in China to prepare Si₃N₄ ceramic balls, with the sintering mechanism being liquid-phase sintering. The grains achieve densification through dissolution, precipitation and recrystallization. Due to the low sintering driving force, the sintering temperature of silicon nitride ceramic balls is generally not lower than 1700℃. Meanwhile, to prevent the decomposition of silicon nitride at high temperatures, nitrogen (N₂) is used as the pressurizing medium to inhibit its decomposition. In this experiment, the gas pressure sintering method was adopted to investigate the effects of sintering temperature on the properties of Si₃N₄ ceramic balls, such as density, crush load ratio, porosity, hardness and fracture toughness.

A fundamental prerequisite for rolling bearing components is to possess sufficient fatigue life. However, due to the high elastic modulus of Si₃N₄ materials, ceramic ball bearings are subjected to higher contact stresses (i.e., Hertzian contact stresses) during high-speed operation. Therefore, for Si₃N₄ ceramic ball materials, internal defects such as pores, inclusions, high additive content and segregation, uneven microstructure, and cracks will significantly shorten their rolling contact fatigue life. Thus, ceramic materials are required to not only have excellent mechanical and physical properties but also high densification degree and low levels of material defects.

As the internal porosity of silicon nitride materials increases, their properties such as strength and fracture toughness decrease accordingly. Free silicon, additives, pores, inclusions and other components in the ceramic ball material exert a significant impact on the density of ceramic balls, which in turn directly affects the performance of ceramic ball bearings—particularly their fatigue life and load-bearing capacity. Free silicon, additives and inclusions will increase the density of ceramic balls, while internal defects such as pores and cracks will reduce their density. Therefore, the densification degree and purity of ceramic balls can be evaluated by measuring the density of ceramic green balls.

The Archimedes drainage method was used to measure the density of sintered ceramic balls in the experiment. For each sintering temperature, no fewer than 5 groups of density tests were conducted, and the average value was taken as the density at that temperature. Figure 1 shows the density variation trend of ceramic balls at different temperatures. It can be seen from the figure that the density of Si₃N₄ ceramic balls is the lowest at 1710℃, reaching 3.166 g/cm³. With the increase of temperature, the density increases sharply, reaching 3.235 g/cm³ at 1740℃. As the temperature continues to rise, the density increases slowly, peaking at 3.271 g/cm³ at 1810℃.