Soft-brittle Crystals Research Articles

A novel water dissolution combined continuous polishing for soft-brittle crystals

A novel water dissolution combined continuous polishing for soft-brittle crystals

Machining Stability Research of Soft-Brittle Crystals Lapping by Fixed-Abrasive Pad

Fixed-abrasive (FA) lapping and polishing process is the main approach to realize the residual-free and low damage processing of soft-brittle crystals. The hardness of matrix and the abrasive micro-fragmentation are the key parameters impacting the stability of the continuous batch machining. The FA pads were prepared with hard and soft matrix, respectively, and the single diamond (SD) and the aggregated diamond (AD) were mixed in the FA pads, respectively. The FA lapping tests were carried out with LN wafers under the applied load of 10.5 kPa. The continuous batch machining stability of four pads tests were evaluated by the evaluation indexes of the material removal rate (MRR), surface roughness Ra and the material removal rate variation (MRRV). The workpiece surface morphology and abrasive wear pattern were analyzed. The results indicate that: hard matrix is not conducive to the shedding and replacement of abrasive grains because of the high holding force, causing a MRRV of 50%. The MRR of the hard matrix pad tests signally decrease from 3.71 μm/min to 1.9 μm/min. By contrast, the low hardness matrix pad can be applied to improve the machining efficiency owing to its MRRV only about 30%. The MRR of the AD pad is over 600 nm/min, which is far more than that of the SD pad, and the MRRV of the AD pad is just about 20%. While MRRV of the SD pad is as high as 75% with the remarkably decline of MRR. At the view of Ra values, the SD pad and the AD pad both attain nanoscale. The Ra values of the SD pad tests are smaller and the workpiece are superior in surface quality. In the rough lapping and fine lapping stage, the SD can be used to prepare FA pad to improve lapping stability.

Deformation behaviour of soft-brittle polycrystalline materials determined by nanoscratching with a sharp indenter

Polycrystalline zinc selenide (p-ZnSe) is a typical soft brittle material with important optical applications. In this work, single and repeated nanoscratching tests were performed using a Berkovich indenter along the face-forward (FF) and edge-forward (EF) directions. The morphological features of the scratched grooves and the subsurface microstructural changes in the material were characterised by scanning electron microscopy, Raman spectroscopy, and electron backscatter diffraction (EBSD). Material removal in the ductile mode was obtained in the EF scratching direction; this was accompanied by the slip lines, and the radial cracks generated along grain boundaries. In contrast, brittle fractures occurred in the FF scratching direction, resulting in radial and lateral cracks which are responsible for generating the peeling of the material. The EBSD results demonstrated that the {111} planes are the primary slip plane and secondary cleavage plane, whereas the {110} planes are the primary cleavage plane and secondary slip planes. Tensile residual stress was detected in the subsurface region of the grooves scratched along the FF direction, whereas compressive residual stress was detected in the EF scratching direction. Fishbone-like patterns were observed in the scratched grooves under all conditions, while no phase transformation was detected. This study provides insights into the fundamental material removal mechanisms of soft brittle crystals in various abrasive machining processes, such as grinding, lapping, and polishing.

Temperature effect on the material removal mechanism of soft-brittle crystals at nano/micron scale

Soft-brittle crystals, e.g. KH2PO4 (KDP), are difficult-to-cut due to their high brittleness which can easily generate crack during the machining process. The conventional method to machine this kind of material is by inducing ductile cutting mechanism at room temperature with ultra-precision machining, which can only remove materials at nanoscale level and hence yields very low material removal rate. While some thermal-assisted processes have been recently attempted to improve the machinability of some difficult-to-cut materials, e.g. ceramics, there is no systematic understanding of the temperature effect on material removal mechanism of soft-brittle KDP crystals yet. In this work, the temperature effect on the material removal mechanism has been investigated for the first time using nano-scratch technique. While a decreased hardness and elastic modulus have been observed with the increase of temperature, an increase of fracture toughness has been revealed with a contradictory tendency, indicating a higher capacity of plastic deformation at elevated temperature. In contrast to the almost totally brittle scratch at room temperature (RT) caused by crack propagation and edge chipping, the scratch at 170 °C can achieve more ductile-regime surfaces with a larger critical undeformed cutting depth (3.61 μm), e.g. a significant increase of 8.60 times compared with that at RT (0.42 μm). Moreover, the TEM analysis on the subsurface microstructures shows that a great number of nano grits was generated in the subsurface at RT as the result of crack propagation and interaction, while at elevated temperature some crystallographic lattice misaligned structures (LMS) and nano crystals have been brought about due to the nucleation and evolution of thermal-activated dislocations, which explains the higher plasticity of KDP at elevated temperature. The results present in this paper are of great significance for understanding the specific temperature effect on the brittle-to-ductile transition of the cutting mechanism for future designing thermal-involved processes to machine soft-brittle materials.

Research on the ploughing force in micro milling of soft-brittle crystals

The ploughing force can be assigned to supervisory control of the tool wearing and machining process on-line in micro milling, for the reason that the ploughing force is closely related with the processed surface quality and radius of cutting edge. A new theoretical model is established to determine the ploughing force in micro milling of crystals with soft-brittle characters in consideration of the cutting edge roundness, and it is verified by Finite Element Method. The ploughing force decreases quickly with an decrease of the cutting edge roundness, and it can be controlled to be less than the shearing force, which helps for obtaining a good machined surface quality, under the condition that the radius of cutting edge is less than 2.8 μm or the feed speed is larger than 5 μm/z when the radius of cutting edge is larger than 4.2 μm. Meanwhile, a large feed speed would bring heavy tool traces on the processed surface of soft-brittle crystals, destroying the processed quality. The energy consumption is researched by investigating the specific cutting force, showing that the energy consumption has a sharp increase with a small feed speed. The energy consumption is mainly caused by the ploughing effect, especially for tools with blunt cutting edge. Therefore, both the micro mill with sharp tool edge or blunt tool edge (under the cutting condition that the feed speed almost have equal value to the radius of cutting edge) are highly recommended for micro end milling of soft-brittle crystals.

Research of abrasive embedment-free lapping on soft-brittle lithium niobate wafer

The embedment of abrasives is a large trouble when lapping soft and brittle work-pieces by loose abrasive lapping, which greatly affects the surface quality of the lapped work-piece. A newly developed hydrophilic fixed abrasive (FA) pad was applied to lap lithium niobate (LN) wafers. The orthogonal test was adopted to optimize the processing parameters in the FA lapping, including platen speed, applied load, and abrasive surface. A group of comparison tests of conventional loose abrasive lapping and FA lapping on LN wafers were conducted to explore the embedded abrasive phenomenon. The topography of the lapped LN wafer was observed by Leica microscope and scanning electron microscope (SEM). The suspected points on LN wafers were detected by energy dispersive spectrometer (EDS) to deduce whether they were embedded abrasives or not. The results show that the applied load is the main factor affecting material removal rate (MRR), which is approximately proportional to the load. And the uncoated abrasive surface modification contributes to a smooth surface. When using the same abrasive grain size, the MRR of the loose abrasive lapping is higher and the surface of the FA lapping is smoother. Residual abrasives are found on the wafer surface by loose abrasive lapping, while they are not found on the wafer by FA lapping. Hydrophilic FA lapping takes advantage of abrasives embedment-free over conventional loose abrasive lapping when processing soft-brittle crystals such as LN.