Study on surface integrity and ductile cutting of PV polycrystalline silicon and wear mechanisms of electroplated diamond wire

This study aimed to evaluate and better understand the mechanical and crystalline responses of polycrystalline silicon sawn by diamond wire sawing. To simplify the multi-wire sawing kinematic, an endless wire saw with a single looped diamond wire welded was used. The wire cutting speed and feed rate were varied in order to evaluate the characteristics of surface morphology, surface roughness, and subsurface damage. The analysis of brittle-ductile transition and residual stress of the sawn surface and silicon chips were performed with Raman spectroscopy. The wear and failure mechanism of the diamond wire were analyzed. The results showed that sawn surface is composed of brittle and ductile regions and the predominance of one of these directly affected the surface roughness R_a. The ductile cutting mode induced the predominance of microgrooves and ploughing over the sawn surface and led to formation of an amorphous layer with residual compressive stress around of − 192.3 MPa. Micro-cracks in subsurface were identified and it reached a minimum depth of 7.2 ± 1.6 µm. Chip fragments and elongated chips were observed and latter is practically amorphous. The wire wear analysis indicated that during the cutting there is deformation of the Ni-layer, exposed grits, and grit pullout. The main wear mechanisms are Ni-matrix removal and abrasive wear of the diamond grit. A better surface quality of polycrystalline silicon was obtained on increasing wire cutting speed and decreasing feed rate. Thus, the looped diamond wire sawing allows to reach a high surface quality of the silicon wafer, since the material removal occurs more in ductile cutting mode, generating a smooth surface with shallow subsurface damage.