Research of brittle-plastic behavior of SiCp/Al composites based on nano-indentation/scratch

Objectives: SiCp/Al composite is a kind of particle-reinforced metal matrix composite, which offers high specific strength and low density. It is widely used in electronic packaging, aerospace and automobile manufacturing. Although the presence of a large number of SiC particles improves the mechanical properties of the material, it also presents significant challenges in processing. As the volume fraction of SiCp/Al composite material increases, its mechanical parameters such as hardness improve, but a large number of surface defects appear during processing.To improve the surface machining quality and select better machining parameters, it is crucial to investigate the mechanical properties and the removal mechanism of SiCp/Al composites with medium or high volume fraction. Methods: Nanoindentation is a commonly used method for testing hardness, which can quantitatively characterize the hardness, elastic modulus and other mechanical parameters of materials. It provides a theoretical basis for predicting machined surface roughness. The load-depth curves of SiCp/Al composites under specific indentation load and rate can be obtained through nanoindentation experiments. The hardness and elastic modulus of SiCp/Al composites are determined using the Oliver-Pharr method. Due to the presence of a large number of SiC particles in the SiCp/Al composite, different failure behaviors are observed when the diamond indenters applies loads to the SiC particles, Al matrix, and two-phase interfaces, resulting in significant differences in measured hardness and elastic modulus. After the indentation experiment, scanning electron microscope (SEM) is used to observe the indentation surface morphology, and the ABAQUS finite element software is used to simulate the indentation process of SiCp/Al composites. The reasons for the differences in mechanical properties are then analyzed based on the finite element simulation results and the experimental indentation surface defects. Results: It is found that when the diamond indenter acts on SiC particles, the experimental values of hardness and elastic modulus of the material are the largest, with average values of 22.75 GPa and 190.78 GPa, respectively. When the indenter acts on the matrix phase, the average values of hardness and elastic modulus are 1.39 GPa and 66.52 GPa, respectively. For the interface between the two phases, the average hardness and elastic modulus measured are 4.62 GPa and 84.38 GPa, respectively. Additionally, the nanoscratch experiment simplifies the complex interaction between abrasive particles and the workpiece during grinding. It explores the brittle-plastic transformation behavior and potensial surface defects of the material surface by applying loads to the workpiece. This is an effective method for studying the material removal form. The hardness and elastic modulus values obtained from the nanoindentation experiment are introduced into the scratch finite element simulation. A variable load of 0 to 400 mN is applied to the SiCp/Al composite, with the scratch speed fixed at 0.05 mm/s. The results show that the removal form of the material changes with the load during the scraping, ploughing and cutting stages. The matrix phase undergoes plastic plastic flow, causing plastic ridge accumulation with coating phenomenon, while the SiC particles are removed by brittle mechanisms such as debonding, breaking, and pulling out. Conclusions: In the indentation experiment, secondary indentation of SiC particles in SiCp/Al composites results in significant differences between the mechanical properties of SiC particles and the theoretical mechanical properties of SiC crystals. Due to fracture and breakage of the particles during the loading process of the diamond indenter, the test results tend to be exaggerated. As the scratch load increases, the removal form of SiCp/Al composites with a volume fraction of 45% replies more on the plastic removal of the matrix phase, while the removal form of SiC particles is mainly brittle. The brittle-plastic behavior of the material surface during machining, as analyzed by the nanoindentation/scratch experiments, provides a theoretical basis for predicting the material's surface quality during machining.