Hole exit quality and machined surface integrity of 2D Cf/SiC composites drilled by PCD tools

The brittle matrix and the anisotropic reinforcing phase of C_f/SiC composites bring great challenges to the machining process. Polycrystalline diamond (PCD) tool was used to drill 2D C_f/SiC composites. The influence of thrust force on hole exit defects was analyzed, and the transformation rule of material removal mechanism and surface generation of hole were studied. With the increase of feed rate, the thrust force increased and the hole exit defects increased. Specific drilling energy was used as an index to quantitatively describe the energy consumption of material removal. With the increase of feed rate and the decrease of cutting speed, the brittle fracture mode of carbon fibers changed from micro-brittle fracture inside carbon fiber to macro-brittle fracture. Although the machined surface of carbon fibers produced by micro-brittle fracture was composed of many micro-fracture, the hole surface was flat overall. Therefore, the hole surface roughness was small.     

Contact accuracy and orientations affecting the lapped tool sharpness of diamond cutting tools by mechanical lapping

In this paper, a brittle–ductile transition lapping mechanism is proposed for the mechanical lapping of single crystal diamond cutting tools. The critical depths of cut for brittle–ductile transition and the dynamic critical tensile stress are regarded as the references to analyze the influence of contact accuracy between the rapid rotating scaife and lapped tool surface and the effects of tool face orientation on the sharpened cutting edge radius, respectively. The experimental results indicate that the vibrations of lapping setup, surface quality of scaife, dynamic balance and motion accuracy of the spindle, lapping compression force and lapping velocity all have enormous influences on the contact accuracy so as to affect the lapped cutting edge radius of diamond cutting tools. Under the optimal settings of each influencing factor for a robust contact accuracy, a perfect diamond cutting tool is sharpened in ductile lapping mode with a cutting edge radius of 30–40 nm and a surface roughness Ra of 0.7 nm on the tool rake face. On the other side, different orientation settings of tool faces enable diamond cutting tools edge to have different micro mechanical strength. Under the same configurations of the lapping parameters, the higher the micro mechanical strength of tool cutting edge is, the smaller the sharpened cutting edge radius would be.     

Influences of clearance angle and point angle on drilling performance of 2D Cf/SiC composites using polycrystalline diamond tools

C_f/SiC composite (carbon fiber reinforced silicon carbide ceramic matrix composites) is a kind of advanced composite material constituted by SiC as matrix and carbon fiber as reinforcing phase. C_f/SiC composites are being extensively used in the modern aerospace industry owing to their excellent physical and mechanical properties. The current work aims to investigate influences of clearance angle and point angle on drilling performance of 2D C_f/SiC composites using PCD (polycrystalline diamond) tools in terms of thrust force, drilling torque, hole surface quality, and material removal mechanism. PCD drill bits with different point angles and clearance angles were used in the experiment. The obtained results indicate that the 150° point angle is beneficial to improve the hole surface quality, and larger clearance angle is helpful to reduce the damage of exits. Additionally, the variation of clearance angle has little effect on the roughness of the machined surface. During the drilling process, the dominated material removal mechanisms are matrix removal, fiber breakage, and matrix-fiber debonding. The brittle fracture mode of carbon fibers, which directly affects surface roughness, can be divided into micro-brittle fractures in carbon fiber and macro-brittle fractures. Besides, the damage identification method of hole entrance and exit based on image processing technology is helping to improve the efficiency of machining quality evaluation.     

Instrumented indentation study of materials edge chipping

The resistance to local fracture of brittle materials is currently characterized by the value of edge toughness. An “instrumented micro-edge chipping” (IMEC) is proposed and tested. This test was carried out on various brittle materials: light krone glass LK-5, polycrystalline silicon Si, partially stabilized dioxide zirconium ceramic TS and tetragonal dioxide zirconium ceramic Y-TZP. IMEC test provides a closer correlation between the edge toughness Fr values at different values of the distance L between the test specimen edge and the indentation point than with the macro-EF test. Comparison of the macro-EF and IMEC tests for studied materials showed the almost identical trend in the ratio of the edge toughness values for all the studied materials. It was found that there is a significant time from the moment of crack formation to the full realization of the edge chip. The duration of the chip process is different for the studied materials.     

Hole surface morphology and tool wear mechanisms during cutting 3D carbon/carbon composites using diamond core drill

Carbon fiber reinforced carbon matrix (carbon/carbon) composites are a new class of ceramic engineering materials and applied in aerospace structures due to high strength in non-oxidizing atmospheres at extremely high temperatures, while it is a typical difficult-to-machine material due to its anisotropy and heterogeneity. Defects such as burrs, tears and fiber pullout are likely to be produced on machined surface when inappropriate machining parameters are employed. The material removal mechanisms during mechanical cutting of carbon/carbon composites are still not fully understood. In this work, the three-dimensional braided carbon/carbon composite was employed, hole surface morphology and tool wear mechanisms during drilling with a bronze sintered diamond core tool were studied. Result showed that fiber tearing and burrs were mainly observed at hole entrance and exit under low level of feed speed down to 20 mm/min. When the feed speed increased to 40 mm/min and 60 mm/min, severe tearing, uncut fibers, and delamination appeared at hole exit. Fiber orientation showed significant influence on material removal mechanisms as well as fiber shearing/bending fracture, fiber pullout and matrix fragmentation. The ground, pullout and fracture of abrasive particles were the main tool wear mechanisms, cutting chips were attached and blocked inside the core drill probably due to high cutting temperature in dry condition, as the temperature increased to ～260 °C under the feed speed of 60 mm/min.     

Formation of high density stacking faults in polycrystalline 3C-SiC by vibration-assisted diamond cutting

Revealing the ductile deformation mechanisms of ultra-hard brittle cubic silicon carbide (3C-SiC), as well as their correlations with microstructure evolution, are crucial for facilitating the ductile machinability of the ceramic material. In the present work, we report the formation of highly oriented high density stacking faults accompanied with suppressed amorphization and cracking in polycrystalline 3C-SiC in ultrasonic elliptical vibration-assisted diamond cutting, which contributes to significantly enhanced ductile material removal of the ceramic material compared to ordinary cutting. Specifically, characterizations of Raman spectroscopy on machined surface and cross-sectional transmission electron microscopy on subsurface, as well as molecular dynamics simulations of the two kinds of cutting processes, are jointly performed to elucidate the mechanisms of phase transformation and microstructure evolution that govern the ductile material removal of polycrystalline 3C-SiC under the vibration assistance. In particular, the formation mechanisms of highly oriented high density stacking faults emitted from grain boundaries are revealed. Current findings provide insights into the ductile deformation behavior of hard brittle ceramics enhanced by field-assisted deformation process.     

Evolution of surface topography in one-dimensional laser machining of structural alumina

High energy lasers are an emerging industrial tool to fabricate complex shapes on hard and brittle structural ceramics such as alumina. The selection of laser processing parameters and the prediction of material removal rates during the laser machining are the critical issues. This paper was attempted to present the state of the art of laser machining of alumina using an integrated experimental and computational approach. A multistep computational model based on COMSOL? Multiphysics was developed to study the influence of various single-pulse laser energy densities and associated physical phenomena (recoil pressure, Marangoni convection, and surface tension) on the temperature history, fluid velocity, crater size, and surface topography. A pulsed Nd:YAG laser was employed to machine alumina under different processing conditions. The surface topography of laser machined alumina was measured by an optical profilometer and the results were compared with the computationally predicted topographic parameters with reasonably close agreement.     

Synthesis of Nd:YAG powders leading to transparent ceramics: The effect of MgO dopant

Neodymium doped yttrium aluminum garnet (Nd:YAG) ultrafine powders were synthesized by co-precipitation method using MgO as dopant. The addition of small amount of MgO can reduce the agglomeration and particle size of the produced Nd:YAG powders. The results show that pure phase YAG powders can be achieved by calcining of the precursors at 1000 °C for 2 h. The MgO doped Nd:YAG powders show better dispersion compared with the undoped powders. When the MgO content is 0.01 wt.%, well-dispersed Nd:YAG powders with spherical particles of 100 nm diameter were obtained. The transmission of the corresponding Nd:YAG ceramics is 82.6% at the wavelength of 1064 nm, which is comparable to Nd:YAG single crystals.     

Synthesis and Performance of Advanced Ceramic Lasers

This paper reports recent progress in the production of polycrystalline Nd:YAG (Y₃Al₅O₁₂), Nd:YSAG (Y₃Sc_1.0Al_4.0O₁₂), Yb:YSAG ceramics, and a Nd-doped YAG single crystal with an almost perfect pore-free structure by advanced ceramic processing. The laser conversion efficiency of pore-free polycrystalline Nd- and Yb-doped ceramics is extremely high, and their optical qualities are comparable with that of commercial high-quality Nd:YAG single crystals. We have also succeeded in the fabrication of a Nd:YAG single crystal, which can be used for laser oscillation, by the solid-state reaction method. Laser oscillation efficiency was very low when the pores remained inside the single crystal; however, the laser oscillation efficiency of the pore-free Nd:YAG single crystal was slightly higher than that of polycrystalline Nd:YAG ceramics having high optical quality. From this fact, it was recognized that optical scattering occurs mainly in the residual pores inside the Nd:YAG ceramics and the scattering at the grain boundary is very less. In addition, we confirmed that a heavily doped Nd:YAG single crystal can be fabricated by the sintering method. Moreover, we have demonstrated the fabrication of a composite ceramic with complicated structures without the need for precise polishing and diffusion bonding. Advanced ceramic processing, which enables design flexibility of the laser element, presented in this work is important in the development of a high-performance laser (high efficiency, high beam quality, and high output energy, etc.)     

Mechanisms of ductile mode machining for AlON ceramics

This paper aims to reveal the mechanisms of ductile mode machining for AlON ceramics. The removal characteristics during machining were studied through ultra-precision grinding experiments. The machined surface consists of fractured and smooth areas, which were generated by brittle and ductile removal, respectively, of the individual AlON grains. The material removal mode has a determining effect on the surface/subsurface quality. The proportion of the fractured areas on the ground surface decreased gradually with a decrease in the depth of cut. The crystal indices of the grains most prone to brittle removal on the workpiece surface were determined using micro-area X-ray diffraction (μXRD) analysis performed using a beam with 50 μm diameter. The results showed that the ductile removal of the {111} planes is critical for the ductile mode machining of AlON. Nanoindentation tests based on electron back-scattered diffraction (EBSD) indicated that AlON shows strong anisotropy in its mechanical properties and machinability. The (111) plane has the highest hardness and lowest fracture toughness, at 22.91 GPa and 1.8 MPa m^1/2, respectively. The material removal mechanism during the grinding of AlON was discussed in detail. The minimum and maximum d_c(hkl) values must be known for classifying whether the removal mode of the workpiece is brittle or ductile. A damage-free surface could be obtained during ductile mode grinding by controlling h_max to be less than d_c(111). The subsurface deformation mechanism during ductile mode grinding was analysed. An amorphous layer was observed close to the ground surface. Further, dense dislocations with no particular orientation were present beneath this amorphous layer. As the crystal structure became clearer with an increase in the depth, the plastic deformation shifted to stacking faults parallel to the {111} planes.     

Damage formation and suppression in rotary ultrasonic machining of hard and brittle materials: A critical review

Rotary ultrasonic machining (RUM) combines diamond grinding with small-amplitude tool vibration, to improve machining processes of hard and brittle materials. It has been successfully applied to the machining of a number of brittle materials from optical glasses to advanced ceramics as well as ceramic matrix composites. The emphasis of this literature review was on formation mechanism and suppression methods of machining induced damages that truly limit RUM machining efficiency improvement of brittle materials. In this review paper, material removal mechanism and cutting force modeling of RUM of brittle materials were presented, as well as all corresponding roles in the damage formation process. The critical processing capacity of RUM machine tools was described, which guarantees the RUM effectiveness and consequently constitutes the boundary condition of processing parameters determination. Formation mechanisms of edge chipping, tearing defects, subsurface damages, and their interactive effects were summarized. Advances in damage suppression methods were also described, including optimization of processing parameters, tool design of low damage, and other methods such as rotary ultrasonic elliptical machining.     

纳米结构陶瓷涂层精密磨削表面/亚表面的形貌

用扫描电镜对纳米(以n表示)结构WC／12Co(n-WC／12Co)和Al2O3／13TiO2(n-Al2O3／13TiO2)涂层经立式精密平面磨削后的磨削表面／亚表面的形貌进行观察，测量了n-WC／12Co和n-Al2O3／13TiO2涂层精密磨削的磨削力分力比、比磨削能。结果表明：在大多数磨削条件下，n-WC／12Co磨削的材料去除机理主要是非弹性变形方式，即以塑性变形为主，伴随一定的材料粉末化。材料较少以脆性碎裂去除。n-Al2O3／13TiO2陶瓷涂层磨削的材料去除机理主要是脆性去除，同时也存在一定的材料粉末化以及极少的显微塑性变形。     

Functional Properties of Nd:YAG Polycrystalline Ceramics Processed by High‐Pressure Spark Plasma Sintering (HPSPS)

High‐pressure spark plasma sintering (HPSPS) was employed to fabricate polycrystalline Nd:YAG specimens with desired functional properties. Specimens fabricated under a uniaxial pressure of 300 MPa at 1300°C at a heating rate of 50°C/min and holding time of 60 min displayed submicrometer microstructure and elevated mechanical properties, including resistance to thermal shock. Optical properties (i.e., spectral transmittance, fluorescence emission spectra and fluorescence lifetime) of the HPSPS‐processed specimens were close to those obtained with specimens fabricated by conventional sintering procedure. Specifically, remarkable differences in threshold power and laser slope efficiency were found and attributed to the variance in Nd concentration in the specimens tested. The results of this study indicate that the low cost and timesaving HPSPS process allows for the fabrication of polycrystalline Nd:YAG specimens with optical properties suitable for laser applications.     

Comparison of the Mechanical Properties of Single Crystal and Polycrystalline Yttrium Aluminum Garnet

Polycrystalline neodymium-doped yttrium aluminum garnet (YAG), Nd: Y₃Al₅O₁₂, has been produced in a transparent form and is being considered as a substitute for single crystal YAG in laser applications. To determine whether such a substitution could impact mechanical reliability, it is critical to compare the baseline mechanical properties of the single crystal and polycrystalline materials. In this study, elastic constants, hardness, fracture toughness and strength properties were measured using standard experimental procedures. In addition, fractography was used to gain insight into the fracture process and the nature of the flaws controlling the strength behavior. Overall, it was determined that the mechanical behavior of the polycrystalline YAG was very similar to that of the single crystal material. The elastic constants of polycrystalline YAG were almost the same as the single crystal and the polycrystalline material showed slight advantages in hardness and fracture toughness. Surface machining flaws were found to control the strength behavior with the single crystal material being the more sensitive to contact damage and, hence exhibiting a lower strength.     

Precision grinding of a microstructured surface on hard and brittle materials by a microstructured coarse-grained diamond grinding wheel

Microstructured surfaces on hard and brittle materials are widely used in a series of scientific and industrial applications, such as micro-electro-mechanical systems, nano-electro-mechanical systems, electronic devices, and medical products. However, the efficient precision machining of microstructured surfaces on hard and brittle materials faces great challenges. In this study, a new machining technology for high-efficiency precision fabrication of microstructured surface on hard and brittle materials was developed by a microstructured coarse-grained diamond grinding wheel. Initially, the laser microstructuring of the conditioned coarse-grained diamond grinding wheel was introduced. The influence of the laser-machined microstructure geometry on the form accuracy of the final, ground microstructured surface was theoretically analysed. Subsequently, the ductile regime grinding of the microstructured surface was examined for WC cermet and BK7 optical glass. The ground surfaces mainly under the ductile regime material removal were successfully achieved, especially in the case of WC ceramic. Finally, different linear and square microstructured surfaces with high form accuracy, sharp microstructure edge, and nanoscale surface roughness were efficiently fabricated on WC and BK7 optical glass by the method developed in the study.     

Laser-induced oxidation assisted micro milling of spark plasma sintered TiB2-SiC ceramic

In this work, a novel process of laser-induced oxidation assisted micro milling (LOMM) was proposed. TiB₂-SiC ceramic with hardness of 24.6 ± 0.8 GPa was prepared by spark plasma sintering and used as the workpiece material. The cutting force, surface quality and tool wear mechanisms were investigated. Under laser irradiation and oxygen assistance, a porous oxide layer and relatively dense sub-layer were formed. The hardness of the sub-layer was found to be 12.8 ± 0.7 GPa which was far lower than that of the substrate. Both the cutting and thrust forces increased with increasing the feed per tooth and depth of cut in micro milling of the sub-layer material. The material removal mechanism was dominated by a transition from ductile to brittle mode as the feed per tooth increased from 0.3 μm/z to 1.2 μm/z. The surface roughness Ra of 46 nm was achieved when the cutting speed, feed per tooth and depth of cut were 31.4 m/min, 0.3 μm/z and 2 μm, respectively. The tool wear mechanism was characterized by the flank wear and coating spalling. As a case study, a micro slot having width of 0.5 mm and aspect ratio of 2 was fabricated by the LOMM. For comparison, the conventional micro milling was also carried out using the same cutting parameters. The surface quality fabricated by LOMM was better than that by the conventional micro milling. The machining efficiency in LOMM was improved by 104% as compared to the conventional micro milling.     

Optical Scattering Centers in Polycrystalline Nd:YAG Laser

For the present study, 1.1-at.%-Nd-doped YAG ceramics with controlled amounts of grain-boundary phase were fabricated by a solid-state reaction method using high-purity powders. The optical scattering loss of the Nd:YAG ceramics, obtained from Fresnels equation, increased simply with increased amounts of grain-boundary phase. The continuouswave laser output power of the Nd:YAG ceramics clearly was related to the scattering loss coefficients of the specimens that, in turn, were affected by the amount of grain-boundary phase. Although the scattering loss coefficients of Nd:YAG ceramics with grain-boundary-free structure and a lower pore volume (}150 vol ppm) were almost equivalent to those of a 0.9-at.%-Nd-doped YAG single crystal grown by the Czochralski method, the laser output power of the Nd:YAG ceramics exceeded that of the Nd:YAG single crystal with increased exciting power under excitation with an 808 nm diode laser because of the large amount of neodymium additives. Lasing performance was not affected by the existence of grain boundaries in the polycrystalline specimen.     

Nano-to-microscale ductile-to-brittle transitions for edge cracking suppression in single-diamond grinding of lithium metasilicate/disilicate glass-ceramics

To suppress grinding-induced edge cracks in dental lithium metasilicate/disilicate glass-ceramics (LMGC/LDGC), this paper established a contact stress model for single-diamond grinding (SDG) to relate their crack generation and ductile-to-brittle transition (DBT) thresholds with the mechanical properties, diamond tool profiles and process variables. Nanoindentation, friction test and SDG were conducted to unravel material responses and dynamic diamond grit-workpiece interactions to determine the DBT thresholds. The nanoindentation revealed significant indentation size effects (ISEs) on the hardness and elastic moduli of the ceramics. SDG clearly elucidated their DBT behaviors, wherein edge cracks initiated at diamond peripheries when the concurrent contact stresses reached the DBT thresholds. Accordingly, the indicated critical cutting depths for DBT may be changed from the nanoscale to the microscale by increasing the tool radius and reducing the machining speed. This research contributes to edge crack suppression for ductile machining of brittle materials at large removal rates.     

Fabrication and Optical Properties of High-Performance Polycrystalline Nd:YAG Ceramics for Solid-State Lasers

Transparent polycrystalline YAG with nearly the same optical characteristics as those of a single crystal were fabricated by a solid-state reaction method using high-purity powders (>99.99 wt% purity). The average grain size and relative density of the 1.1 at.% Nd:YAG ceramics obtained were about 50 μm and 99.98%, respectively. An oscillation experiment was performed on a cw laser by the diode laser excitation system using the fabricated ceramics. The experimental results indicated an oscillation threshold and a slope efficiency of 309 mW and 28%, respectively. These values were equivalent or superior to those of the 0.9 at.% Nd:YAG single crystal fabricated by the Czochralski method.     

均相沉淀法制备掺钕钇铝石榴石纳米粉末的研究

掺钕钇铝石榴石（Nd：YAG）多晶透明陶瓷具有容易制造、成本低、光学性能好、热导率高等优点,是一种很有前途的激光工作物质。以AI（N03）·9H2O,Y2O3,Nd2O3,（NH4）2SO4和尿素为原料,正硅酸乙酯为添加剂,采用均相沉淀法制备出分散均匀、纯YAG立方晶相的Nd：YAG纳米前驱体粉末。采用XRD、FT-IR、TEM等测试手段对前驱体粉末进行表征。研究结果表明：Nd：YAG前驱体粉末在800℃时为无定型态,当温度达到890℃时析出大量的中间相YAlO3（Y=AP）和少量的Y3Al5O5（YAG）,当温度达到1000℃时就全部转化为YAG立方晶相。