Sintering simulation of nickel and alumina composite using discrete element method

Composite is a class of material made by the combination of two or more materials, which produces a third one with unique characteristics. For this reason, composites have a wide range of engineering applications, such as spacecrafts and sports’ equipment. Composite materials can be suitably produced by Powder Metallurgy. In this manufacturing process, the blend of different powders is shaped and later sintered at high temperatures for consolidation of the part. Thereby, sintering is considered a densification process, which is responsible for providing strength and stiffness to the material or composite. Moreover, its control is essential to reach the desired properties of the final part. In addition, numerical simulations of the sintering process represent an alternative procedure in relation to the lengthy and costly physical experiments. A well-known simulation technique is the Discrete Element Method (DEM). In contrast to continuum methods, DEM considers every particle of the system as a single element and it is recommended to simulate granular media, such as sintering. Thus, the general purpose of this project is to simulate and analyze the solid-state sintering process of composite materials when both materials are sintering using DEM. The software used is the MUSEN system, developed at TUHH – Germany. The materials chosen for the composite are nickel (metal) and alumina (ceramic). Specifically, the present work aims to investigate the influence of varying contents of metal/ceramic in monosized samples during sintering. These contents range from metal volume fraction of 0.9 to 0.1, and include pure metal and ceramic bodies. Furthermore, the effect of larger metallic particles in the sample is also investigated for a constant metal volume fraction of 0.6. Among the analyses carried out, the contact size growth was evaluated considering the interfaces metal-metal, ceramic-ceramic and metalceramic. The coordination number of the particles within these three contacts is also analyzed. Finally, the influence of the varied parameters on the densification behavior is investigated and correlated with the contact size growth and coordination number evolution. The results have shown that the special modeling was capable to simulate sintering of composites even in case of interpenetrating structures. The simulation results of the monosized packing can be divided in three different sintering behaviors: metal-controlled, ceramic-controlled and interpenetrating structures. The metal-controlled samples have shown the highest densification rates and relative density evolution, as one might expect. The nickel particles have higher potential to sinter faster than alumina due to their kinetic parameters and surface energy. Hence, metal particles induce high forces to shrink the system and indirectly transfer forces to the sintering of ceramic phase. Interpenetrating structures have shown the lowest overall densification due to independent densification of metal and ceramic phase. It has led to large cracks through the samples and the initial structure has been lost. The results of bimodal packings have shown a slower growing of the contact radius for larger nickel particles, as expected. However, the global densification has been higher for samples with larger nickel particles. This unexpected behavior can be explained due to the particle configuration and distribution of forces in the interpenetrating structures. Smaller particles induce higher forces, quickly agglomerate themselves and are not capable to drive a global densification. On the other hand, larger particles induce weaker sintering forces. Thereby, the resistance force between nickel-alumina contacts is high enough to keep these contacts attached and, consequently, a global densification can be observed.