Factors Affecting Fracture Toughness of 4H-SiC

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SiC has the advantages of high hardness, good thermal stability, and strong chemical stability, making it of significant application value in the power electronics, aerospace, and semiconductor industries. However, its inherent brittleness and relatively low fracture toughness pose challenges to precision manufacturing processes, especially in the process of laser cutting wafers.

1. What Is Fracture Toughness?

Fracture toughness, as a key ability of materials to resist crack propagation, reflects the characteristic of absorbing energy during the crack propagation process. According to different crack conditions, fracture toughness can be divided into the following two types: long crack fracture toughness (KIc) refers to the crack resistance of materials under long crack conditions, usually measured using the single-sided precast crack beam (SEPB) method; The short crack fracture toughness corresponds to the short crack condition and is often measured by indentation method. This method requires measuring the length and depth of the crack generated by indentation and calculating it using a specific formula. The fracture toughness of 4H-SiC is an important indicator for evaluating its performance and application, which is of great significance for material selection, design, and application. Research has shown that by regulating the microstructure and chemical composition of materials, their fracture toughness can be significantly improved.

2. Factors Affecting the Fracture Toughness of 4H–SiC Wafers

2.1 Grain Size and Microstructure

Grain size is a key factor, and typically the smaller the grain size, the higher the fracture toughness. This is because small grain boundaries can effectively hinder crack propagation, forcing cracks to deflect or bifurcate, thereby increasing the energy required for crack propagation. Secondly, grain shape and orientation are equally important. For example, elongated grains or oriented grains can lead to directional dependence of crack propagation paths, resulting in anisotropic fracture toughness (i.e. differences in crack resistance in different directions).

In addition, the texture (orientation and arrangement pattern of grains) further enhances this anisotropy – in the microstructure of oriented arrangement, cracks may preferentially propagate along specific crystal directions, leading to a decrease in toughness in specific directions. For example, the atomic bond strength in the {111} direction is higher than that in the {110} and {100} directions, resulting in higher fracture toughness in the {111} direction. The secondary phase and impurities at the grain boundary directly affect the grain boundary bonding strength. Certain secondary phases (such as strengthening particles) can enhance the grain boundary bonding strength and improve toughness; Brittle impurities or harmful precipitates may weaken grain boundaries and become the source of crack initiation, thereby damaging fracture toughness.

2.2 Fracture Modes of Materials

Transgranular fracture refers to the direct propagation of cracks through the interior of grains, which typically corresponds to higher fracture toughness. This is mainly due to the need for cracks to overcome strong covalent bonds within grains, consuming more energy. On the contrary, intergranular fracture (i.e. crack propagation along grain boundaries) typically results in lower fracture toughness due to relatively weak bonding strength at grain boundaries (especially when impurities or weakened phases are present), making it easier for cracks to propagate along grain boundaries. Therefore, the dominant fracture mode of the material can serve as an important basis for evaluating its crack resistance performance.

2.3 Toughening Mechanism

Crack deflection: Cracks deflect when encountering grain boundaries, secondary phases, or other microstructural defects, thereby increasing the energy required for crack propagation. The unbroken grains near the crack tip can bridge the crack surface, thereby preventing further crack propagation. In some cases, limited plastic deformation occurs near the crack tip, which absorbs energy and improves fracture toughness.

2.4 Temperature

Temperature has a decisive impact on the toughening mechanism. At low temperatures, the vibration of SiC atoms weakens, and the resistance to crack propagation mainly depends on the strength of lattice bonding, while the fracture toughness remains relatively stable; In high-temperature environments, the intensification of atomic vibration significantly activates the plastic deformation mechanism, making it easier for energy dissipation processes such as dislocation slip to occur near the crack tip, resulting in a significant increase in fracture toughness.

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