Optimizing Silicon Nitride Ceramics
Silicon nitride (Si3N4) plays a dominant role among nitride ceramics, primarily due to its good thermal shock resistance and high corrosion resistance. It also provides extremely high stability, high viscosity, excellent abrasion resistance, low thermal expansion, medium thermal conductivity and very good chemical resistance. This combination of properties yields a ceramic that withstands extreme application conditions and is ideally suited for machinery components with very high requirements for reliability and mechanical loads.
For the manufacture of dense Si3N4, diverse sintering additives (aluminum oxide [Al2O3], yttrium oxide [Y2O3], magnesium oxide [MgO], etc.) are generally added to the original powder. After the shaping process, the green body is sintered at temperatures between 1950 and 2200 K (3050 and 3500°F). In many production processes, an increased nitrogen pressure is frequently used in the sintering furnace above 2000 K to counteract the decomposition of the silicon nitride and considerably reduce the pore size in the ceramic. However, few manufacturers understand what happens to silicon nitride during the various stages of the sintering process; as a result, it can be difficult to make changes to further optimize the results.
State-of-the-art thermal analysis and thermophysical properties testing instruments can be used to overcome this challenge. By obtaining accurate measurements of properties such as thermal expansion, density change, phase transitions, specific heat, thermal diffusivity and thermal conductivity throughout the sintering process, such instruments enable manufacturers to confidently adjust their processes to maximize product quality.
A Sample AnalysisTo prove the instruments' capabilities, researchers at NETZSCH-Gerätebau GmbH analyzed samples of debinded silicon nitride throughout the sintering process. The sintering powder consisted of fine-grained silicon nitride, with an average grain size of 0.7 mm. Al2O3 and Y2O3 were added at 5 mass% each as sintering additives, and the raw material still contained small amounts of silicon dioxide contamination on the Si3N4 powder surfaces. The green body particles were mainly from the a-phase (trigonal structure), which transfers into the b-phase (hexagonal structure) during sintering. In actual production processes, the ceramic material is sintered up to ~1973 K under normal pressure in a nitrogen atmosphere. To reduce the pore sizes, the pressure is increased to approximately 1 Mpa, and the temperature is increased to a maximum of 2023 K. The influence of the pressure increase was not examined in this project.
To determine the thermal expansion and density change of the material, the samples were analyzed with a high-temperature dilatometer1 in a nitrogen atmosphere. The system was equipped with a graphite furnace and sample holder system and allowed measurements to be carried out between room temperature and 2250 K. The schematic design of the instrument used for the tests is shown in Figure 1. To reduce the potential for the samples to react with the graphite, the samples were contained in protective aluminum nitride tubes. A W/Re thermocouple was used to measure the temperature at the sample.
A high-temperature differential scanning calorimetry (DSC) instrument2 was used to characterize the phase transitions and measure specific heat. The measurements were carried out at 20 K/min between room temperature and 1773 K in a nitrogen atmosphere.
To measure the thermal diffusivity of small sample dimensions at high temperatures, the laserflash method was used.3 In this technique, a plan-parallel sample is heated in a furnace to the required measuring temperature. After temperature equilibration, the front surface of the sample is heated by a short laser pulse (generally <1 ms in length). The energy/temperature rise penetrates the sample and leads to a temperature rise on the rear surface of the sample, which is measured with an infrared detector vs. time. From the temperature vs. time behavior, the thermal diffusivity can be determined.4
Thermal Expansion and Density ChangeThe results for the length change and rate of length change at a heating rate of 5 K/min are shown in Figure 2. The measurement was conducted from room temperature to 2150 K. At 1553 K (extrapolated onset), a first shrinkage step of 1.12% was obtained due to the formation of a new liquid phase within the sintering body. The liquid phase was formed by the sintering additives and the silicon dioxide that was present as contamination on the silicon nitride particles.
Figure 4 shows the length change (linear thermal expansion) and average expansion coefficient (reference temperature: 298 K) of a silicon nitride specimen sintered at 1975 K. The measurement was carried out at 5 K/min to 1650 K. It can be seen that the sample length increased with an increasing rate of expansion vs. temperature; therefore, the coefficient of thermal expansion also increased with temperature. A change in the expansion rate was determined above 1200 K. Since pure silicon nitride should show no transitions in this range, this effect may be due to the influence of additives. The decrease in the rate of length change above 1600 K might be caused by post-sintering.
Specific Heat and Transformation EnergeticsThe measured specific heat flow rate between 700 and 1750 K on the green material is shown in Figure 5. At 1285 K, an exothermal reaction with an enthalpy of -16.4 J/g was detected. The peak temperature was at 1419 K. The explanation for this effect is the formation of new phases from the sintering additives and silicon dioxide impurities. Above 1500 K, an endothermal reaction with an enthalpy of 18.4 J/g was detected. The peak temperature was 1591 K, and a further endothermal reaction started at 1710 K, possibly due to phase transitions in the additives and silicon nitride.
Thermal DiffusivityThe laser flash measurements were carried out between room temperature and 1973 K in steps of approximately 100 K. Five individual measurements were conducted at each temperature, again in a nitrogen atmosphere. The investigated green body had a thickness of 3.28 mm, a diameter of 18 mm and an initial density of 1.816 g/cm3. The green body therefore had a slightly higher (>3.7%) initial density than the dilatometer sample. At 1973 K, the temperature was kept constant for one hour. The sample was then cooled and again measured up to 1973 K. The changes in the sample thickness were taken into consideration for the analysis of the results.
Figure 9 shows the thermal diffusivity for the green body, along with the values of the Si3N4 material sintered at 1973 K. As expected, the sintered material shows a considerably higher thermal diffusivity. The measured data continuously decrease with an increasing temperature. Due to the phase transitions (DSC results), a few (weak) steps can also be seen in the sintered ceramic above 1200 K. Approximately the same values for the sintered material were determined in the range of the maximum temperature as for the green body.
Thermal ConductivityFigure 10 shows the thermal conductivity values for the silicon nitride (both the green body and sintered material), which were calculated by multiplying the density, specific heat and thermal diffusivity. In the temperature range up to 1000 K, only slight changes in the thermal conductivity were obtained for the green material. Between 1000 and 1500 K, the values increased with the temperature due to the increasing density and thermal diffusivity, while a considerable increase in the thermal conductivity was determined in the sintering range. However, the measured values of the sintered material continuously decreased with the increasing temperature.
A thermal conductivity value of 12.8 W/(m•K) was determined at room temperature. While this is below the values (16 W/(m•K)) that are typically found in literature for corresponding materials,8 it can be explained by the low sintering temperature, as well as the fact that the sample was not sintered at high temperatures under pressure (1 MPa), as is usually the case in a production process. The temperature dependence of the thermal conductivity followed the course as expected according to the phonon model. Between 1100 and 1700 K, smaller overlapping steps occurred due to the phase transitions mentioned.
With the help of the measured data, a simulation of the temperature gradient in a silicon nitride green body was carried out through finite element calculation using ANSYS‘ simulation software.9 An infinitely long cylinder with an initial diameter of 30 mm was used for the product geometry, and a constant heating rate of 5 K/min for the outer surface area was used for simulation. The heat input was adjusted in such a way that this heating rate was constant over the simulation range between room temperature and 2000 K.
Upon initial inspection, it seems unusual that the temperature differences between the center and surface increase from 374 K to 749 K. However, this can be explained with the decrease in the thermal diffusivity and conductivity of the material vs. the temperature in the lower temperature range. In the sintering range above 1400 K, a considerable change in the temperature gradients was found. Above 1850 K, the temperature gradients were reduced to values around 1 K.
Optimized ManufacturingThe experiments described in this article show how state-of-the-art thermal analysis and thermophysical properties testing instruments can be used to analyze silicon nitride during sintering. The resulting values for the density change, specific heat, thermal diffusivity and thermal conductivity allow detailed insights into the sintering process and related phase transitions. The results can also be used to determine temperature gradients occurring in a sintering part during the production process.
By using these instruments in production operations, manufacturers of Si3N4 ceramics can truly optimize the sintering process to achieve higher quality final products.
For more information about thermal analysis and thermophysical properties testing, contact
NETZSCH-Gerätebau GmbH, Wittelsbacherstr. 42 95100 Selb/Bavaria, Germany; (49) 9287-881-49; fax (49) 9287-881-44; e-mail firstname.lastname@example.org ; or visit www.ngb.netzsch.com. (U.S. contact: NETZSCH Instruments, Inc., 37 North Ave., Burlington, MA 01803; (781) 272-5353; fax (781) 272-5225; e-mail email@example.com ; www.e-Thermal.com.)