Investigation on high-shear and low-pressure grinding characteristics for zirconia ceramics using newly developed flexible abrasive tool

Zirconia ceramics is widely employed in medical, chemical and aerospace domain due to its excellent combination of physical and chemical attributes. However, high hardness and brittleness property have limited its application in manufacturing industries. Therefore, the present work aimed at development of a flexible abrasive tool based on concept of liquid body armor with large ratio of tangential grinding force to normal grinding force in order to enhance the surface processing quality of zirconia ceramics. A flexible body-armor-like abrasive tool was developed. Fundamental dynamic simulation and micro material removal processing at high-shear and low-pressure grinding conditions was analyzed. A multi-group of grinding experiments were carried out on an industrial robot platform for zirconia ceramic workpieces to validate the grinding performance of flexible body-armor-like abrasive tool. The surface roughness, surface morphology, grinding force and grinding temperature were investigated. At the optimal grinding condition, the surface roughness (Ra) of the workpiece was reduced by 91.8% and decreased from 110 nm to 9 nm. The scratches from the surface of the workpiece vanished and uniform grinding textures were left. The experimental results revealed that the developed flexible body-armor-like abrasive tool could achieve ultra-precision grinding of zirconia ceramics.     

Study on mechanism of chip formation of grinding plasma-sprayed alumina ceramic coating

The mechanisms of ductile–brittle transition and surface/subsurface crack damage during the grinding of plasma–sprayed alumina ceramic coatings were investigated in an experiment and simulation on single diamond abrasive grain cutting. We observed that the brittle damage modes of alumina ceramic include boundary cracks, median cracks and lateral fractures. The normal force of the abrasive grain results in the initiation of median cracks, whereas the tangential force of the abrasive grain results in the propagation of median cracks in the direction of the abrasive grain cutting. Some cracks propagate downward to form machined surface cracks, whereas others propagate to the unmachined surface of the workpiece to produce brittle removal. Owing to the alternating tensile and compressive stresses, the material in contact with the top of the abrasive grain fractures continuously, forming the main morphology of the machined surface. The geometry and cutting depth of the abrasive grain have a significant influence on the ductile–brittle transition, whereas the cutting speed of the abrasive grain have no significant influence. On one hand, the stress concentration at the pore defects result in crack propagation to the deep layer; on the other hand, it reduces the local strength of the surface material, produces brittle fracturing, and interrupts crack propagation. The pores exposed on the machined surface and the broken morphology around them are important factors for reducing the surface roughness. Experimental observations show that the machined surface morphology of the alumina ceramic coating is composed of brittle fracturing, ductile cutting and plowing, cracks, original pores, and unmelted particles.     

Experimental study on the mechanism of strain rate on grinding damage of zirconia ceramics

To study the mechanism of strain rate on the grinding damage of zirconia ceramics (ZrO₂), a ZrO₂ surface grinding experiment was conducted to analyse the effect of strain rate on the grinding force, surface morphology and subsurface cracks. As the strain rate increased, the grinding force decreased significantly; the grinding scratches gradually became continuous and clear, and the grinding surface gradually became smooth. Increasing the strain rate can greatly reduce the length of subsurface cracks; however, the length of subsurface cracks drops slowly and tends to stabilise when the strain rate reaches a certain value. The direction of subsurface cracks is related to the internal defects of the material, and there is a trend of approaching the surrounding cracks and defects. When the internal defects were stimulated by the impact stress wave, the cracks started to grow and expand from the tip, and the expanding direction remained at approximately 125° in the direction of the defect length. The results show that an increase in the grinding strain rate can effectively improve the grinding quality of zirconia ceramics, which has guiding significance for practical production.     

Effect of zirconia content and particle size on the properties of 3D-printed alumina-based ceramic cores

Alumina-based ceramic cores are used to manufacture the internal structures of hollow alloy blades, requiring both high precision and moderate properties. In this work, zirconia is regarded as a promoter to improve the mechanical properties of sintered ceramic. The effect of zirconia content and particle size on the microstructure and mechanical properties of ceramics was evaluated. The results indicate that the flexural strength of sintered ceramics reached the maximum of 14.5 ± 0.5 MPa when 20 wt% micron-sized (10 μm) zirconia (agglomerate size, consistent with the alumina particle size) was added, and 26.5±2.5 MPa when 15 wt% 0.3 μm zirconia was added. Zirconia with submicron-sized (0.3 μm) particles effectively filled the pores between alumina particles, thus leading to the maximum flexural strength with a relatively low content. The corresponding sintered ceramics had a bulk density of 2.0 g/cm³ and open porosity of 59.6%.     

Damage formation mechanisms of sintered silicon carbide during single-diamond grinding

In this research, a single-diamond grinding test was performed on sintered silicon carbide (SSiC) to explore the damage formation mechanism. A scanning electron microscope and a transmission electron microscope (TEM) were used to examine the surface and subsurface morphologies of the grinding groove, respectively. The characteristics of the ground surface morphologies reveal that the single-diamond grinding process of SSiC can be classified into purely ductile, primarily ductile, primarily brittle, and purely brittle stages. Based on the high-resolution TEM (HRTEM) images and the corresponding Fast Fourier transform images of the near-surface region, results reveal that the high density of dislocations and amorphization of SiC grains are responsible for the plastic deformation of SSiC. Most of the cracks congregate on the top grains of the ground surface due to the distinct obstruction of the grain boundary on the cracks propagation, and the cracks generated at the grain boundaries emit into the top grain interiors and go up toward the exposed surface for the distortedly deformed region with higher strain energy; Furthermore, stress concentration caused by the dislocation pileups at grain boundaries represents the crack initiation mechanisms for SSiC. Finally, based on the dislocations pile-up theory, a critical undeformed chip thickness model for boundary crack system nucleation is established, which considers the cutting-edge radius, grinding wheel speed, material properties, and grain size of ceramics.     

Enhancing ductile removal of Cf/SiC composites during abrasive belt grinding using low-hardness rubber contact wheels

C_f/SiC composites are used as advanced thermal protection and friction materials. However, machining these materials is difficult because of their hard, brittle, anisotropic, and heterogeneous characteristics. This study investigated the removal behavior and surface integrity of C_f/SiC composites during abrasive belt grinding using rubber contact wheels of various hardness. Additionally, detailed analysis was performed on their thermal-mechanical coupling characteristics, surface integrity (that is, surface roughness, surface micro morphology, and subsurface damages), and the grinding chips produced. Results revealed that with decreasing hardness of the contact wheel, the surface roughness in all directions, grinding force, and temperature decreased significantly. Moreover, the surface removal morphology of the C_f/SiC composites changed from macro-fracture to micro-fracture, and the subsurface morphology changed from SiC matrix cracking and carbon fibers pull-out to matrix plastic flow and fiber micro-fracture, respectively. Furthermore, strip chips with plastically squeezed and cut surfaces were visible in the grinding chips obtained under the 40-HA contact wheel. Therefore, the ductile removal behavior of the C_f/SiC composites was enhanced, and the surface quality in abrasive belt grinding with low-hardness contact wheels was markedly improved.     

Ceramic Grinding Temperatures

Workpiece and abrasive-tip temperatures are measured during the grinding of silicon nitride, zirconia, sapphire, and Ni-Zn ferrite with diamond abrasives. The measurements are carried out using a multiple-element infrared sensor having a time constant of ∼7 μs and a spot size as small as 30 μm in diameter; the temperature measurements were made for both single-point and full-wheel grinding. A simple analytical model is used to predict the abrasive-tip and work-surface temperatures, the subsurface temperature of the workpiece, and the cooling of the abrasive grains between cutting. The predictions of the model agree reasonably well with experiments. The model requires as inputs only the grinding forces, grinding process variables, and the physical properties of the abrasive and workpiece materials. It is shown that, in view of the excellent thermal properties of the diamond abrasive, the surface temperature of the ceramic workpieces is essentially independent of the thermal properties of the ceramics.     

四方氧化锆多晶瓷的磨料磨损

四方氧化锆多晶瓷在实验条件下，由于磨损表层发生了由四方氧化锆转变为单斜氧化锆的马氏体相变，其磨料磨损率随施加载荷的增加反而减小，耐磨料磨损性能优于氧化锆增韧氧化铝瓷、普通氧化铝瓷。实验用陶瓷材料的表面磨损破坏主要为显微断裂脱落和由于晶粒部分破碎而形成的粉末刻划式磨痕。尽可能提高材料的断裂韧性并适当改善其硬度，将提高材料的磨料靡损性能。     

Material removal mechanism of laser-assisted grinding of RB-SiC ceramics and process optimization

Laser-assisted grinding (LAG) is a promising method for cost-effective machining of hard and brittle materials. Knowledge of material removal mechanism and attainable surface integrity are crucial to the development of this new technique. This paper focusing on the application of LAG to Reaction Bonded (RB)-SiC ceramics investigate the material removal mechanism, grinding force ratio and specific grinding energy as well as workpiece surface temperature and surface integrity, together with those of the conventional grinding for comparison. Response surface method and genetic algorithm were used to optimize the machining parameters, achieving minimum surface roughness and subsurface damage, maximum material removal rate. The experiments results revealed that the structural changes and hardness decrease enhanced the probability of plastic removal in LAG, therefore obtained better surface integrity. The error of 3-D finite element simulation model that developed to predict the temperature gradient produced by the laser radiation is found to be within 2.7%–15.8%.     

Grinding of hard and brittle ceramic coatings: Force analysis

In this work, a grinding force model has been proposed and a correlation between the analytical model and the experimental grinding forces obtained during finishing of plasma sprayed ceramic coatings has been investigated. Thermally sprayed ceramic coatings have low fracture toughness and develop micro brittle fracture under moderate mechanical loading. In this investigation, grinding of air plasma sprayed alumina, zirconia, and titania coatings were studied theoretically and experimentally. It was observed that the ground surface contained micro-cracks and debris of irregular fragmented chips owing to the micro-brittle fracture. A grinding force model is proposed to incorporate the fracture behavior of the ceramic coating. This was substantiated through experimental values showing low grinding forces.     

Investigation on preparation of composite vitrified bonding ultrafine diamond grinding wheel and machining of PcBN inserts

The grinding of polycrystalline cubic boron nitride (PcBN) is hard owing to its high hardness and superior wear-resistance capacity. Machining of PcBN tools via vitrified diamond grinding wheels with a size above 10 μm may lead to brittle fracture instead of a ductile machining because of the poor toughness of cubic boron nitride. A uniformly dispersed M0.5/1.5 diamond grinding wheel with a composite vitrified bonding was fabricated to improve the surface roughness of PcBN inserts. It is demonstrated that the preparation of composite vitrified bonding with various additions of vitrified bonding produced by the melting-quenching technique (VB-MQ) has little effect on the performance of vitrified bonding, such as bending strength, CTE and phase and achieves the uniform dispersion of M0.5/1.5 diamond as the addition of VB-MQ is no greater than 50%. Both the grinding ratios and the surface roughness of PcBN inserts are enhanced.     

Nano-scale surface of ZrO2 ceramics achieved efficiently by peanut-shaped and heart-shaped SiO2 abrasives through chemical mechanical polishing

Zirconia ceramics are regarded as the best development target for 5G mobile phone rear covers. However, it is necessary and urgent to improve the surface quality and processing efficiency of zirconia ceramics. Non-spherical silica abrasives were prepared by the KH550 induction method and were used in chemical mechanical polishing (CMP) of zirconia ceramics for the first time. While achieving low surface roughness of 1.9 nm, it has an efficient polishing rate of 0.31 μm/h which is superior to conventional abrasives. Silica particles are peanut-shaped and heart-shaped in the scanning electron microscopy image, and its distinctive morphology provides the possibility of its excellent polishing performance. X-ray photoelectron spectroscopy analysis shows that during the CMP process, silica abrasives and zirconia ceramic undergo a solid phase chemical reaction to form ZrSiO₄. At the same time, the contact wear model established in combination with the coefficient of friction indicates that the two-dimensional surface contact mode of non-spherical silica abrasives on the surface of zirconia ceramics greatly improves its mechanical effect.     

Experimental investigations on enhanced alternating-magnetic field-assisted finishing of stereolithographic 3D printing zirconia ceramics

Zirconia ceramic components have great applications in the fields of medical, aerospace, and energy. Stereolithographic (SLA) 3D printing technology is widely employed for zirconia ceramic fabrication. However, the surface quality of manufactured components by direct SLA 3D printing is hard to meet stringent requirements of industrial application. In this work, an enhanced alternating-magnetic field-assisted finishing (A-MFAF) method was proposed for SLA printed zirconia ceramics. The A-MFAF was achieved using a flexible alternating-magnetic-field generator, integrating a rectangular magnetic pole and radial magnetic column. The novel finishing tool was fabricated to regulate finishing media behaviors for ensuring desirable finishing force. The unique construction of the magnetic field generator provided a controllable alternating magnetic field in the finishing zone. The magnetic control characteristics were investigated with finite element analysis (FEA). A serial of finishing experiments were carried out to verify the feasibility of the proposed A-MFAF method for SLA printed zirconia ceramics. The finishing efficiency with the developed magnetorheological shear thickening finishing (MSTF) media was improved by over 24% compared to that with the conventional magnetorheological finishing (MRF) fluids. The variation of surface roughness was qualitatively evaluated under different finishing conditions. The surface roughness of 89 nm was obtained from the initial 1.79 μm at 0.6 mm working gap and 700 r/min spindle rotational speed. Digital microscope, optical profiler and surface hydrophobicity measuring instrument were employed to investigate the surface characteristics of the finished SLA printed zirconia ceramics. Ultra-smooth surface with slight defects and deformations was obtained. The feasibility of A-MFAF method for the ultra-precision finishing of SLA printed zirconia ceramics was verified.     

Effect of abrasive grinding on the strength of Y-TZP

The residual stress and flaw size in yttria-stabilized zirconia (Y-TZP) before and after abrasive grinding are examined in the present study. As long as the distribution of flaw size remains the same after grinding, the strength of zirconia increases with increasing residual stress. For the grinding conditions employed, a compressive residual stress as high as 1 GPa is introduced on the surface. Such stress is large enough to induce lattice distortion. The strength of Y-TZP is consequently increased by 190 MPa. The compressive residual stress can be partly removed by subsequent polishing or annealing treatment.     

Fractography of self-glazed zirconia with improved reliability

The fractography of a new grade of zirconia ceramics, known as self-glazed zirconia, was investigated. The as-sintered intact top surface was made with superior smoothness that mimicked the optical appearances of the natural teeth enamel. The beneath surface opposite to this was made hierarchically rough with microscopic pits of the size up to 60 μm together with grain-level roughness of about 2 μm. The three-point bending test of the samples made with the hierarchically rough surface being tensile one demonstrated an average bending strength of 1120 ± 70 MPa and a Weibull modulus of as high as 18 ascribed to the improved structural homogeneity. Surface topography was found the main origins of crack initiation leading to fracture. The observed unusually predominant transgranular fracture mode of submicron-sized grains disclosed a possible toughening mechanism of disassembling of mesocrystalline grains that differs significantly from the commonly quoted phase transformation toughening of this category of ceramics.     

Study on the subsurface damage mechanism of optical quartz glass during single grain scratching

The single grain scratching SPH simulation model was established to study the subsurface damage of optical quartz glass. Based on the analysis of the stress, strain and scratching force during scratching, the generation and propagation of subsurface cracks were studied by combining with the scratch elastic stress field model. The simulation results show that the cracks generate firstly at the elastic-plastic deformation boundary in front of the grain (φ = 28°) due to the influence of the maximum principal tensile stress. During the scratching process, the median crack closes to form the subsurface damage by extending downward, the lateral crack promotes the brittle removal of the material by extending upward to the free surface, and microcracks remain in the elastic-plastic boundary at the bottom of the scratch after scratching. The depth of subsurface crack and plastic deformation increases with rising scratching depth. The increase of scratching speed leads to the greater dynamic fracture toughness, accompanied by a significant decrease of the maximum depth of subsurface crack and the number of subsurface cracks. The subsurface residual stress is concentrated at the bottom of the scratch, and the residual stress on both sides of the scratch surface would generate and propogate the Hertz crack. When the scratching depth is less than 1.5 μm or the scratching speed is greater than 75 m/s, the residual stress value and the depth of residual stress are relatively small. Finally, the scratching experiment was carried out. The simulation analysis is verified to be correct, as the generation and propagation of the cracks in the scratching experiment are consistent with the simulation analysis and the experimental scratching force indicates the same variation tendency with the simulation scratching force. The research results in this paper could help to restrain the subsurface damage in grinding process.     

Improving wettability,antibacterial and tribological behaviors of zirconia ceramics through surface texturing

3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) ceramics are promising restorative materials being extensively used for fabricating dental prosthodontics. However, peri-implant inflammation and the severe abrasive wear on occlusive natural teeth are two critical problems in the clinical application of zirconia dentures. The paper aims to improve the antibacterial and tribological performance of 3Y-TZP ceramics through laser surface texturing. Three types of surface textures, including micro-honeycombs, micro-composite grids, and micro-grooves, were fabricated onto the zirconia specimens. The effects of different microtextures on the surface behaviors, including wettability, bacteria adhesion, and wear behavior, of 3Y-TZP ceramics were rigorously studied. The results indicate that the introduction of microtextures can change the solid-liquid contact of the zirconia surface, thus affecting its wettability. Wettability is a decisive factor that determinines the antibacterial behavior of textured zirconia ceramics. A hydrophobic surface is more conducive to inhibiting the adhesion, extension, reproduction of bacteria and thus achieves a superior antibacterial performance. The examined microtextures yield the improvement of wear resistance for the zirconia ceramics, but their performances depend on the texture density and the structural strength. The results obtained can provide technical guidance for the design and application of microtextures in the restorative dental fields.     

Experimental investigation of ultrasonic-vibration polishing of K9 optical glass based on ultrasonic atomization

K9 optical glass has an important position in the field of optical material because of its excellent chemical stability and optical projection. The hard and brittle characteristics of K9 optical glass make conventional processing difficult and time-consuming. A non-conventional hybrid polishing system combining ultrasonic atomization (UA) spraying method and axial ultrasonic vibration was developed for processing K9 optical glass. This system utilizes the high-frequency vibration characteristics of ultrasonic vibration technology: On the one hand, the ultrasonic atomization spraying method is used to generate evenly distributed atomized droplets for polishing, on the other hand, the axial ultrasonic vibration of the polishing tool provides impact kinetic energy for the free abrasive particles. Mechanical polishing (MP), ultrasonic-assisted polishing (UVP), mechanical polishing under ultrasonic atomization spraying (UA-MP) and ultrasonic vibration polishing under ultrasonic atomization spraying (UA-UVP) were carried out on K9 optical glass. The material removal rate (MRR), material removal depth (MRD), surface quality and surface micromorphology of the polished workpieces were also analyzed and compared. The experimental results showed that the best surface was obtained at UA-UVP (A_Ⅰ = 9 μm) with MRR of 0.0994 mm³/min, material removal depth of 26.816 μm, the Ra and Sa values were 0.028 μm and 0.033 μm respectively. Meanwhile, no obvious pits and scratches were observed on the micromorphological surface. Ultrasonic atomization contributes to even material removal from the polished surface and axial ultrasonic vibration of the polishing tool has a significant effect in improving the polishing characteristics, which provides the experimental basis for applying ultrasonic vibration technology in polishing.     

Effect of Microstructure on Material-Removal Mechanisms and Damage Tolerance in Abrasive Machining of Silicon Carbide

Effects of microstructural heterogeneity on material-removal mechanisms and damage-formation processes in the abrasive machining of silicon carbide are investigated. It is shown that the process of material removal in a conventional silicon carbide material with equiaxed-grain micro-structure and strong grain boundaries consists of the formation and propagation of transgranular cracks which results in macroscopic chipping. However, in a silicon carbide material, containing 20 vol% yttrium aluminum garnet (YAG) second phase, with elongated-grain micro-structure and weak grain boundaries, intergranular micro-cracks are formed at the interphase boundaries, leading to dislodgment of individual grains. These different mechanisms of material-removal affect the nature of machining-induced damage. While in the conventional silicon carbide material the machining damage consists of transgranular median/radial cracks, in the heterogeneous silicon carbide material, abrasive machining produces interfacial micro-cracks distributed within a thin surface layer. These two distinct types of machining damage result in a different strength response in the two forms of silicon carbide materials. In the case of the conventional silicon carbide, grinding damage results in a dramatic decrease in strength relative to the as-polished specimens. In contrast, the ground heterogeneous silicon carbide specimens show no strength loss at all.     

Predicting subsurface damage in silicon nitride ceramics subjected to rotary ultrasonic assisted face grinding

Silicon nitride is an advanced ceramic used in high performance applications. One of the main problems in machining of brittle materials such as silicon nitride is subsurface damage (SSD). On the other hand, rotary ultrasonic assisted face grinding (RUAFG) is considered as state of the art machining process for brittle and hard to machining materials such as ceramics and optical glasses. In this research, a new study on SSD generation in RUAFG by establishing both ductile deformation and brittle fracture conducted. To achieve this goal, initially single diamond grit cutting force based on Vickers hardness correlation and indentation fracture mechanics established and placed in crack propagation formulas to anticipate SSD. Verification tests performed and average 8% error detected. Moreover, RUAFG depicted up to 30% SSD reduction in comparing to conventional face grinding (CFG). Besides, scanning electron microscope utilized to investigate cracks morphology.