Predictive modeling and experimental study of generated surface-profile for ultrasonic vibration-assisted polishing of optical glass BK7 in straight feeding process

Optical glass BK7 is widely used in optical lenses. It is a kind of hard and brittle material and thus difficult to machine. To solve the problems it has encountered in terms of polishing efficiency and quality, the axial ultrasonic vibration assisted polishing (UVAP) method is adopted in this study. A mathematical-prediction-model, firstly considering the effects of periodically changing pressure caused by ultrasonic vibration, is developed to predict the generated surface-profile. And for the first time, the concept of “Preston-coefficient distribution-function” is proposed to describe the effect of the abrasive-particle distribution on material-removal distribution-function. A series UVAP experiments on optical glass BK7 at different polishing parameters are conducted to validate the polishing efficiency and feasibility of the developed prediction model and proposed concept. The results confirm that UVAP method can realize a higher material removal rate (MRR) and a better surface quality. Compared to conventional method, the predictions are in good agreement with the experiments. The comparison of the obtained surface profiles proves the benefits of ultrasonic vibration as well. Moreover, the research results can be used to guide the adjustment of polishing parameters for deterministic material removal in straight feeding process of UVAP, and provide a valuable basis for trajectory planning of the global polishing of workpiece surfaces.     

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.     

Study on the effect of ultrasonic vibration-assisted polishing on the surface properties of alumina ceramic

Due to the excellent biocompatibility and chemical durability, alumina ceramics are widely used in biomedical fields such as dentistry. Meanwhile, the high hardness and brittleness of the alumina ceramic causes difficulties in ensuring the processing quality of conventional machining methods. Ultrasonic vibration-assisted polishing (UVAP) can effectively improve the surface quality of hard and brittle materials as a novel machining method. The surface properties of the alumina ceramic for different ultrasonic amplitudes are obtained in this study. The evaluation of the surface properties includes microscopic topography, frictional property, surface hardness, microscopic strain, and surface roughness. In addition, a surface roughness model of the alumina ceramic is built based on the UVAP machining mechanism. The model considers the effect of different types of abrasive particles (free and fixed abrasive particles) on the material removal mechanism. The experimental results show that the errors of the predicted model are less than 20%. This work will provide some ideas for the subsequent UVAP processing.     

Parameter optimization of ultrasonic vibration polishing K9 optical glass based on ultrasonic atomization

To further improve the surface finish and processing efficiency of optical components, ultrasonic vibration technology is frequently combined with conventional processing and the process parameters that play a critical role in this composite processing are identified. This research proposes an ultrasonic vibration polishing method based on ultrasonic atomization (UA-UVP). The polishing performance of K9 optical glass is increased by ultrasonic atomization (UA) assisted by polishing solvent for ultrasonic vibration polishing (UVP). Orthogonal experiments are used to study the effects and variation laws of the flow rate of ultrasonic atomization (Q), the gap distance between the polishing tool and workpiece (G), ultrasonic electro spindle speed (W), abrasive particle size (D) and ultrasonic amplitude (A) on surface roughness (SR) and material removal rate (MRR), respectively. When these two polishing characteristics are considered together, the optimization of polishing parameters becomes complicated. Therefore, the principal component analysis (PCA) and grey relational analysis (GRA) methods were employed to the optimal experimental combination as Q:18 ml/min, G: 5 μm, W: 4000 r/min, D: 0.5 μm, A: 8 μm. The experimental results showed that Ra and MRR were measured as 10.466 nm and 0.473*10^8 μm³/min, respectively. Compared with the best experimental combination of orthogonal experiments, the improvement rates of SR and MRR were 26.65% and 25.80%, respectively. Overall, the application of ultrasonic vibration technology contributes to enhancing the uniform distribution of polished abrasive particles and improving the polishing characteristics.     

Mechanism and Simulation of Removal Rate and Surface Roughness During Optical Polishing of Glasses

Glass optics with ultra‐low roughness surfaces (<2 Å rms) are strongly desired for high‐end optical applications (e.g., lasers, spectroscopy, etc.). The complex microscopic interactions that occur between slurry particles and the glass workpiece during optical polishing ultimately determine the removal rate and resulting surface roughness of the workpiece. In this study, a comprehensive set of 100 mm diameter glass samples (fused silica, phosphate, and borosilicate) were polished using various slurry particle size distributions (PSD), slurry concentrations, and pad treatments. The removal rate and surface roughness of the glasses were characterized using white light interferometry and atomic force microscopy. The material removal mechanism for a given slurry particle is proposed to occur via nano‐plastic deformation (plastic removal) or via chemical reaction (molecular removal) depending on the slurry particle load on the glass surface. Using an expanded Hertzian contact model, called the Ensemble Hertzian Multi‐gap (EHMG) model, a platform has been developed to understand the microscopic interface interactions and to predict trends of the removal rate and surface roughness for a variety of polishing parameters. The EHMG model is based on multiple Hertzian contacts of slurry particles at the workpiece–pad interface in which the pad deflection and the effective interface gap at each pad asperity height are determined. Using this, the interface contact area and each particle's penetration, load, and contact zone are determined which are used to calculate the material removal rate and simulate the surface roughness. Each of the key polishing variables investigated is shown to affect the material removal rate, whose changes are dominated by very different microscopic interactions. Slurry PSD impacts the load per particle distribution and the fraction of particles removing material by plastic removal. The slurry concentration impacts the areal number density of particles and fraction of load on particles versus pad. The pad topography impacts the fraction of pad area making contact with the workpiece. The glass composition predominantly impacts the depth of plastic removal. Also, the results show that the dominant factor controlling surface roughness is the slurry PSD followed by the glass material's removal function and the pad topography. The model compares well with the experimental data over a variety of polishing conditions for both removal rate and roughness and can be extended to provide insights and strategies to develop practical, economic processes for obtaining ultra‐low roughness surfaces while simultaneously maintaining high material removal rates.     

High-rate ultrasonic polishing of polycrystalline diamond films

We report on fast polishing of polycrystalline CVD diamond films by ultrasonic machining in a slurry with diamond particles. The material removal mechanism is based on diamond micro-chipping by the bombarding diamond particles subjected to action of an ultrasonic radiator. The treated samples were characterized with optical profilometry, SEM, AFM and micro-Raman spectroscopy. The developed method demonstrates the polishing rate higher than those known for mechanical or thermo-mechanical polishing, particularly, the surface roughness of 0.5 mm thick film can be reduced in a static regime from initial value Ra ≈ 5 μm to Ra ≈ 0.5 μm for the processing time as short as 5 min. No appearance of amorphous carbon on the lapped surface was revealed, however, formation of defects in a sub-surface layer of a few microns thickness was deduced using Raman spectroscopy. The polishing of a moving workpiece confirmed the possibility to treat large-area diamond films.     

Efficient and slurryless ultrasonic vibration assisted electrochemical mechanical polishing for 4H–SiC wafers

This paper proposes a slurryless, highly efficient polishing method called ultrasonic vibration assisted electrochemical mechanical polishing (UAECMP) to realize 4H–SiC wafers with subnanometer surface roughness. UAECMP involves using ultrasonic vibration to simultaneously assist anodic oxidation of the SiC surface and mechanical removal of the generated oxide layer. The performance of UAECMP was evaluated by experiments and theoretical analyses. For a 4H–SiC (0001) surface, UAECMP achieved a material removal rate (MRR) of 14.54 μm/h, which was 4.5 times greater than that of ordinary electrochemical mechanical polishing (ECMP) and 290 times greater than that of mechanical polishing. Ultrasonic vibration increased the anodic oxidation rate by introducing local transient strain to the SiC surface and increasing the temperatures of the polishing area and electrolyte. The effect increased with the amplitude of the ultrasonic vibration. However, increasing the ultrasonic vibration amplitude also increased the surface roughness due to the large fluctuations of polishing marks caused by the grinding stone and SiC surface impact and the increasing residual oxide. Therefore, we propose a high-efficiency and -quality polishing process for SiC wafers that combines UAECMP and ECMP. The proposed polishing process may help simplify the existing manufacturing process for SiC wafers.     

超精密表面研抛磨粒的研究进展

半导体材料的超精密加工是一种获得高表面质量和表面完整性的加工技术,研抛磨粒是实现半导体材料超精密加工的关键耗材之一。从研抛磨粒的组成方式和结构特点,概述了研抛磨粒的研究现状和发展趋势。首先,构建了研抛界面内半导体材料工件-研抛磨粒-研抛垫的接触模型,讨论了研抛磨粒的材质、形状、浓度、粒径等因素对半导体材料研抛质量和研抛效率的影响;其次,从材质和粒径等方面介绍了混合磨粒的研抛性能,以及相应的研抛机理;然后,从材料结构和化学作用等角度总结了复合磨粒在超精密加工技术中的应用;最后,展望了研抛磨粒未来的研究方向。     

Microscopic Removal Function and the Relationship Between Slurry Particle Size Distribution and Workpiece Roughness During Pad Polishing

Various ceria and colloidal silica polishing slurries were used to polish fused silica glass workpieces on a polyurethane pad. Characterization of the slurries' particle size distribution (PSD) (using both ensemble light scattering and single particle counting techniques) and of the polished workpiece surface (using atomic force microscopy) was performed. The results show the final workpiece surface roughness is quantitatively correlated with the logarithmic slope of the distribution function for the largest particles at the exponential tail end of the PSD. Using the measured PSD, fraction of pad area making contact, and mechanical properties of the workpiece, slurry, and pad as input parameters, an Ensemble Hertzian Gap (EHG) polishing model was formulated to estimate each particle's penetration, load, and contact zone. The model is based on multiple Hertzian contact of slurry particles at the workpiece–pad interface in which the effective interface gap is determined through an elastic load balance. Separately, ceria particle static contact and single pass sliding experiments were performed showing ~1‐nm depth removal per pass (i.e., a plastic type removal). Also, nanoindentation measurements on fused silica were made to estimate the critical load at which plastic type removal starts to occur (P_crit~5 × 10^?5 N). Next the EHG model was extended to create simulated polished surfaces using the Monte Carlo method where each particle (with the calculated characteristics described above) slides and removes material from the silica surface in random directions. The polishing simulation utilized a constant depth removal mechanism (i.e., not scaling with particle size) of the elastic deformation zone cross section between the particle and silica surface, which was either 0.04 nm (for chemical removal) at low loads (<P_crit) or 1.0 nm (for plastic removal) at intermediate loads (>P_crit). The simulated surfaces quantitatively compare well with the measured rms roughness, power spectra, surface texture, absolute thickness material removal rate, and load dependence of removal rate.     

Suppression of diamond tool wear in machining of tungsten carbide by combining ultrasonic vibration and electrochemical processing

As an important ceramic material, tungsten carbide (WC) is utilized as the typical mold in precision glass molding, which has replaced conventional grinding and polishing to provide a highly replicative process for mass manufacturing of optical glass components. Ultra-precision grinding, which is time consuming and has low reproducibility, is the only method to machine such WC molds to high profile accuracy. Although diamond turning is the most widely used machining method for fabrication of optical molds made of metals, diamond turning of WC is still considered challenging due to fast abrasive wear of the diamond tool caused by high brittleness and hardness of WC. Ultrasonic vibration cutting has been proven to be helpful in realizing ductile-mode machining of brittle materials, but its tool life is still not long enough to be utilized in practical diamond turning of optical WC molds. In the current study, a hybrid method is proposed to combine electrochemical processing of WC workpiece surface into the diamond turning process. Cutting tests on WC using poly-crystalline diamond tools were conducted to evaluate its effect on improvement of tool wear and surface quality. Validation cutting tests using single crystal diamond tools has proven that the proposed hybrid method is able to significantly reduce the diamond tool wear and improve the surface quality of machined ultra-fine grain WC workpiece compared to ultrasonic vibration cutting without electrochemical processing.     

Investigation on ultrasonic vibration-assisted femtosecond laser polishing of C/SiC composites

This paper proposed a novel ultrasonic vibration-assisted femtosecond laser polishing method for C/SiC composites. The effect of near-field convection enhancement of ultrasonic vibration can improve the cooling of ablated particles and reduce their tendency of bonding to the material surface, reducing surface oxidation and improving the machined surface quality, removal depth and material removal rate. Through optimizing defocusing distance and scanning speed, a specific relationship between ultrasonic amplitude, pulse energy density, and spot overlap rate was established, obtaining a smooth and flat surface without defects. The residual stress of carbon fibers was investigated, and found that the coupling effect of ultrasonic energy and laser energy fields can enhance the residual compressive stress of carbon fibers. The formation of typical features of fiber fracture and pulling-out, banded pits, voids and deposition, was explained. This paper proposes new research ideas for better understanding of the removal mechanism of C/SiC composites using ultrasonic vibration-assisted femtosecond laser.     

Mechanistic difference between Si-face and C-face polishing of 4H–SiC substrates in aqueous and non-aqueous slurries

The traditional aqueous-based polishing slurries have been extensively used in the ultra-precision machining process of SiC substrates, but their processing efficiency remains a major challenge in making SiC wafers with high surface quality. SiC polishing slurries based on non-aqueous solvents have been explored and reported, however, the mechanism for the accelerated SiC material removal rate (MRR) remains unknown. In this work, the Si-face and C-face of the SiC wafer were polished with water and methanol as polishing liquid carriers, respectively. The MRR of Si-face using the methanol-based slurry, can reach 260.9 nm/h, and the polished Si-face surface roughness Ra reduces to 0.150 nm. In contrast, the MRR of Si-face by using the aqueous-based slurry, is 66.8 nm/h, the polished Si-face surface roughness Ra is 0.691 nm. However, the results of MRR and Ra for C-face are opposite. The reaction between the polishing liquid carriers and the atomic structures of Si-face and C-face lead to differences of the MRRs by analyzing contact angle, XPS, and molecular dynamics (MD) simulation results. The newly revealed polishing mechanisms shined light for speeding up the development of SiC polishing slurries based on the specific aspects of the polishing surface of SiC.     

Influence of Temperature and Material Deposit on Material Removal Uniformity during Optical Pad Polishing

The effects of temperature and material deposit on workpiece material removal spatial uniformity during optical pad polishing are described. Round and square‐fused silica workpieces (25–265 mm in size) were polished on a polyurethane pad using ceria slurry under various conditions. Using a nonrotated workpiece on a rotating lap, elevated temperatures (as measured by IR imaging), due to frictional heating at the workpiece–lap interface, were observed having a largely radial symmetric profile (relative to the lap center) on both the workpiece and lap with a peak temperature corresponding to the workpiece center. A 3D steady‐state thermal model of the polishing process, which accounts for the frictional heating and effective heat transfer from various surfaces, quantitatively describes the observed thermal profiles. The temperature spatial uniformity, which affects the material removal spatial uniformity, can be significantly improved using a rotated workpiece and a specially designed compensating septum during polishing. Next, using a rotating workpiece and lap, the workpiece surface develops two types of mid‐range structure: (1) fine ripples (sub‐mm scale length) that run circumferentially with respect to the lap, which have been attributed to microscopic islands of slurry on the lap leading to radial material removal nonuniformities; and (2) a center depression (cm scale length) which has been attributed to nonlinear slurry & glass products buildup at a specific radial lap location. A polishing simulator model (called Surface Figure or SurF), which accounts for workpiece wear, pad wear, and now deposition on the pad, correctly simulates the preferential material deposit on the pad and the center depression structure developed on the workpiece. Strategies, such as time averaging through kinematics and diamond conditioning, for preventing both these nonuniformities are demonstrated.     

Diamond polishing

The empirical know-how of single crystalline diamond polishing has been developed over centuries in the diamond gem cutting industry. Since the 1950s new and varied uses and potential applications for synthetically produced diamond have been consistently proposed and developed. This innovation process continues with the availability of ever better, more specialized and less costly single crystalline and polycrystalline diamond materials. Yet, the potential exploitation of this hardest of materials is still in its infancy. Polishing is a critical and limiting step for advancing diamond applications in terms of cost effective processing and the achievable material surface finish. The current state-of-the-art of polishing single crystalline and polycrystalline diamond materials is reviewed based on the published literature. The material removal process during traditional mechanical polishing using diamond grit and polishing wheels is strongly anisotropic and depends upon crystal planes and polishing directions. Wear debris analyses and molecular dynamic simulations led to the understanding that this anisotropy is primarily caused by a mechanically induced transition from diamond to an amorphous carbon phase rather than by microchipping as previously thought. Mechanical polishing also leads to subsurface damage and limits the achievable surface finish for single crystalline diamond. Advanced techniques are discussed to improve the polished crystal's surface quality. Mechanical polishing of polycrystalline diamond films and freestanding plates is particularly slow due to the intrinsic structure variations in such materials. To overcome these limitations faster polishing techniques have been developed and are reviewed and compared. These techniques introduce additional chemical and physical means of material removal extending the capabilities of mechanical polishing. There is no single method that can address all requirements, but the available variety affords the careful selection of an optimal process for a given task. Finally, while diamond polishing is a subject of interest since centuries, it still remains a very important research area required to unfold the promise of diamond as a technical material.     

Synthesis and polishing characteristics of a novel green GO/diamond hybrid slurry under ultrasonic technology

Ultrasonic polishing (UP), as an efficient and clean precision machining process, is increasingly applied in the hard and brittle material processing. To minimize the surface damage and polishing expenses, this paper aims to prepare novel graphene oxide/diamond hybrid slurries (GDS) by ultrasonication, and to investigate the UP mechanisms for silicon carbide (SiC) ceramics on UP and tribological experiments. Findings showed that GDS exhibited superior polishing performance compared to diamond polishing slurries. Moreover, the promotion effect of GDS was more obvious under UP. The surface roughness was improved by 31.62% after UP using GDS with a graphene oxide (GO) content of 0.1 wt%. On the one hand, the lubrication of GO reduce the wear and coefficient of friction. On the other hand, ultrasonic vibration induces stronger impact kinetic energy of abrasives to remove material and allows for a better uniform distribution of GDS to further enhance the lubrication characteristics of GO nanosheets. The application of GO in UP is a processing technology penetrating the manufacturing industry and even human life, laying the foundation for SiC nano-polishing.     

Influence of partial charge on the material removal rate during chemical polishing

A partial charge-based chemical polishing model has been developed, which can serve as metric for describing the relative polishing material removal rate for different combinations of slurries and workpieces. A series of controlled polishing experiments utilizing a variety of colloidal polishing slurries (SiO₂, CeO₂, ZrO_2, MgO, Sb₂O₅) and optical materials [single crystals of Al₂O₃ (sapphire), SiC, Y₃Al₅O₁₂ (YAG), CaF₂, and LiB₃O₅ (LBO); a SiO₂-Al₂O₃-P₂O₅-Li₂O glass ceramic (Zerodur); and glasses of SiO₂:TiO₂ (ULE), SiO₂ (fused silica), and P₂O₅-Al₂O₃-K₂O-BaO (Phosphate)] was performed and its material removal rate was measured. As previously proposed by Cook (J Non-Cryst Solids. 1990;120:152), for many polishing systems, the removal rate is governed by a series of chemical reactions which include the formation of a surface hydroxide, followed by condensation of that hydroxyl moiety with the polishing particle, and a subsequent hydrolysis reaction. The rate of condensation can often be the rate limiting step, thus it can determine the polishing material removal rate. By largely keeping the numerous other factors that influence material removal rate fixed (such as due to particle size distributions, interface interactions, pad topography, kinematics, and applied pressure), the material removal rate is shown to scale exponentially with the partial charge difference (δ_wp-s) between the workpiece and polishing slurry particle for many of the slurry-workpiece combinations indicating that condensation rate is the rate limiting step. The partial charge (δ) describes the equilibrium distribution of electron density between chemically bonded atoms and is related to the electronegativity of the atoms chemically bonded to one another. This partial charge model also explains the age-old experimental finding of why cerium oxide is the most effective polishing slurry for chemical removal of many workpieces. Some of the slurry-workpiece combinations that did not follow the partial charge dependence offer insight to other removal mechanisms or rate limiting reaction pathways.     

Chemistry and Formation of the Beilby Layer During Polishing of Fused Silica Glass

The chemical characteristics and the proposed formation mechanisms of the modified surface layer (called the Beilby layer) on polished fused silica glasses are described. Fused silica glass samples were polished using different slurries, polyurethane pads, and at different rotation rates. The concentration profiles of several key contaminants, such as Ce, K, and H, were measured in the near surface layer of the polished samples using Secondary Ion Mass Spectroscopy (SIMS). The penetration of K, originating from KOH used for pH control during polishing, decreased with increase in polishing material removal rate. In contrast, penetration of the Ce and H increased with increase in polishing removal rate. In addition, Ce penetration was largely independent of the other polishing parameters (e.g., particle size distribution and the properties of the polishing pad). The resulting K concentration depth profiles are described using a two‐step diffusion process: (1) steady‐state moving boundary diffusion (due to material removal during polishing) followed by (2) simple diffusion during ambient postpolishing storage. Using known alkali metal diffusion coefficients in fused silica glass, this diffusion model predicts concentration profiles that are consistent with the measured data at various polishing material removal rates. On the other hand, the observed Ce profiles are inconsistent with diffusion based transport. Rather we propose that Ce penetration is governed by the ratio of Ce–O–Si and Si–O–Si hydrolysis rates; where this ratio increases with interface temperature (which increases with polishing material removal rate) resulting in greater Ce penetration into the Beilby layer. Calculated Ce surface concentrations using this mechanism are in good agreement to the observed change in measured Ce surface concentrations with polishing material removal rate. These new insights into the chemistry of the Beilby layer, combined together with details of the single particle removal function during polishing, are used to develop a more detailed and quantitative picture of the polishing process and the formation of the Beilby layer.     

A super-high speed polishing technique for CVD diamond films

A new set-up for polishing of CVD diamond films on a high speed rotating titanium plate has been developed. The influence of polishing pressure on the surface character, roughness and material removal rate have been studied by using scanning electron microscopy, stylus profilometer, X-ray photoelectron spectroscopy and Raman spectroscopy before and after polishing, respectively. The results showed that the material removal mechanism is mainly the chemical reaction between carbon and titanium and the diffusion of carbon atoms into the polishing plate during the super-high speed polishing. The current method exhibits a high polishing rate in only a few hours. This preliminary result reveals a great potential for commercializing.     

Convergent Pad Polishing of Amorphous Silica

A new method of optical polishing termed “Convergent Polishing” is demonstrated where a workpiece, regardless of its initial surface figure, will converge to the lap shape in a single iteration. This method of polishing is accomplished by identifying the phenomena that contribute to non-uniform spatial material removal, and mitigating the non-uniformity for each phenomenon (except for the workpiece-lap mismatch due to the workpiece surface shape). The surface mismatch at the interface between the workpiece and lap causes a spatial and time varying pressure differential which decreases with removal, thus allowing the workpiece to converge to the shape of the lap. In this study, fused (amorphous) silica workpieces are polished using ceria slurry on various polyurethane pads. Polishing parameters were systematically controlled to prevent various sources of non-uniform material removal which include: (i) moment force, (ii) viscoelastic lap relaxation, (iii) kinematics, (iv) pad wear, and (v) workpiece bending. The last two are described herein. With these mitigations, removal uniformity has been demonstrated to within 1.0 μm over the surface after 83 μm of material removal corresponding to a within workpiece non-uniformity (WIWNU) of <1.2%. Also, convergence has been demonstrated down to 0.18 ± 0.04 μm peak-to-valley flatness on 100 mm-sized workpieces.     

Nanoscale Friction and Wear of Phosphate Laser Glass and BK7 Glass Against Single CeO2 Particle by AFM

CeO₂ (ceria) particles are considered as a type of ideal polishing particle used to polish glass substrate. The friction and wear of glass substrates caused by a single CeO₂ particle is the origin of material removal in polishing, but this has not been well‐understood in previous research. In this investigation, the nanoscale friction and wear behaviors of the Nd‐doped phosphate laser glass and the BK7 optical glass were quantitatively studied against a single CeO₂ particle by an atomic force microscopy in humid air. The investigations on the phosphate laser glass indicate directly that this type of glass cannot resist the wear when it rubs against the single CeO₂ particle in the elastic contact in humid air. During the test, high friction coefficient and severe material removal were observed in the friction process. The chemical activity of the CeO₂ particle was proved to be a cause that induces the tribochemical wear of the phosphate laser glass since the tribochemical wear cannot occur when a chemically inert diamond tip was used. On the other hand, the BK7 glass presented a much better wear‐resistance, where the friction coefficient is relatively lower and the expected tribochemical wear cannot occur in the same stress condition as compared with that for the phosphate laser glass, and the damage of the BK7 glass is more like the mechanical peeling of the asperities on the glass surface. The results provide new insights into single‐asperity friction and wear of glass materials, which would be useful in understanding the mechanisms of friction and material removal in polishing glass materials with ceria slurry.