Controlled material removal mode and depth of micro cracks in precision grinding of fused silica – A theoretical model and experimental verification

A critical function for crack propagation for the single grit scratching of fused silica is developed based on the fracture mechanics. The effects of original crack density on the surface, strain rate and grinding coolant are considered in the function. A theoretical model for controlled material removal mode and depth of micro cracks precision grinding is presented based on the critical function for crack propagation. It can be predicted by the model that the material removal mode in the grinding of fused silica with original cracks damage will change from a ductile mode to a semi-brittle mode, a full-brittle mode and a semi-brittle mode in sequence with the increasing single grit scratching depth. It was found that the micro crack damage depth of fused silica does not increase with the single grit scratching depth after a full brittle mode grinding and it is always smaller than that after a semi brittle mode grinding even with a smaller single grit scratching depth. These interesting results are explained by the fracture mechanics. The ductile mode grinding is a recognized desirable process of fabricating fused silica while the full-brittle grinding is also a feasible process for its shallow subsurface damage, high efficiency, low grinding force and energy consumption. Therefore, the depth of micro cracks after grinding can be controlled by choosing suitable grinding parameters. Grinding experiments are conducted on fused silica. The undeformed chip thickness of randomly distributed effective grits is simulated based on 3D reconstruction of wheel topography to reveal the relationship between the grinding parameters and the single grit scratching depth. Ground surface roughness, sub-surface damage (SSD) depth and grinding force are measured and discussed. It is shown that the model predictions correlate well with the experimental trend of grinding modes.     

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.     

Investigation of cohesive FE modeling to predict crack depth during deep-scratching on optical glasses

Optical glass scratching can induce various types of cracks, among which median cracks are extremely detrimental and penetrate deeply under the surface. Due to deep-scratching process complexity, it is challenging to devise a method to predict median crack depth. Indentation testing has been examined comprehensively in prior research works. It has been found that using the correlation between scratch and indentation testing can simplify predictive method development. In this research, a numerical method based on indentation testing is proposed to determine median crack depth during deep scratching. In the first step, an FE model is configured to simulate the indentation testing process and the Cohesive Zone Method is applied to describe median crack behavior. The cohesive parameters calibrated through experimental indentation testing are implemented in the FE scratch model, and the results are compared with the experimental scratch test results. According to the results, the FE scratch model was enhanced by mode II fracture energy and the modeled friction coefficient. The indentation and scratch experiments were conducted with BK7, F2, Fused silica, K5, Pyrex, Quartz, SF6, and SF19. The experimental results prove that the nonlinearity of the median crack depth curve correlates with K_Ic. A comparison of the experimental and numerical results demonstrates the model is virtually functional for materials with K_Ic below 1000?kPa?m^1/2. Comparisons between the current findings and other studies infer the model and experimental results are accurate and reliable.     

Scratch with double-tip tool: Crack behavior during simultaneous double scratch on BK7 glass

Over the past decade, successive double-scratch tests have been extensively performed to study the grinding mechanism of brittle materials. However, the grits sometimes interact with the surface simultaneously. In this study, double tips with a tip separation of 0.6–1.8 μm are fabricated by focused ion beam. Subsequently, double-scratch tests on BK7 optical glass are conducted using the double-tip scratch tool with a scratch depth of 200–600 nm. The typical crack system and its evolution mechanism for double-tip scratch are discovered, before being explained using an analytical stress model. The ductile–brittle transition and the material-removal mechanism are discussed. An influential radius for the interference between cracks and the stress field in the double scratch is obtained, which can serve as a reference for the design of textured grinding wheels. Subsequently, the advantages and disadvantages of double-tip scratches are discussed considering different applications, such as microstructure fabrication and grinding.     

Ductile to Brittle Transition Depth During Single-Grit Scratching on Alumina Ceramics

Variable-depth single-grit scratch experiments have been conducted on three different grain size alumina ceramics. The extent of induced damage as a function of depth of groove was measured. At low depth, the scratch groove appeared smooth with minimal brittle damage, indicating a ductile mode of deformation. With increased depth, brittle cracking extended beyond the scratch groove. The transition depth from the predominantly ductile mode of deformation to the predominantly brittle mode was measured and compared with an analytical model that estimates the plastic zone size surrounding a scratch in brittle materials. It was found that the ductile to brittle transition depth increases with decreasing grain size.     

On the improvement of the ductile removal ability of brittle KDP crystal via temperature effect

Brittle KH₂PO₄ (KDP) crystal is difficult-to-machine because of its low fracture resistance whereby brittle cracks can be easily introduced in machining processes. To achieve ductile machining without any cracks, this type of materials is generally processed by some ultra-precision machining techniques at ambient temperature with nanoscale material removal, yielding low machining efficiency and high processing cost. Recently, thermal-assisted techniques have been used to successfully facilitate the machining of some difficult-to-machine materials, like superalloys, but little effort has been made to explore whether the temperature effect can contribute to the ductile machinability of brittle materials yet. Thus, the aim of this study is to figure out the specific role of temperature in the deformation behaviours of brittle KDP crystal by nano indentation/scratch methods. It is found that compared with those at ambient temperature (AT, i.e. 23 °C), the hardness and Elastic modulus of KDP crystal at elevated temperature (ET, i.e. 160 °C) decrease substantially by 21.4% and 32.5%, respectively, while the fracture toughness increases greatly by 15.5%, implying a higher ability of ductile deformation at ET. Meanwhile, the scratch length within ductile removal has been identified to be extended more than 4 times by increasing temperature from AT to ET. Both the quantity and size of brittle features (e.g., cracks and chunk removal) show a reducing trend with the increase of temperature. To uncover the underlying mechanism of this phenomenon, an updated stress field model is proposed to analyze the scratch-induced stress distribution by considering the evolution of material property at various temperature. These presented results are significant for the future design of specific thermal-assisted processing techniques for machining brittle materials efficiently.     

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.     

Speed effect on the material behavior in high-speed scratching of BK7 glass

Scratch tests are often performed at a speed that is significantly lower than the real application like machining and grinding. However, brittle materials like BK7 behave very differently under high-speed conditions due to the more promising temperature and stain rate effects Therefore, it is important to study its material behavior under high-speed condition. In this study, single scratch tests and consecutive scratch tests were performed on BK7 under scratch speeds of 1, 5 and 20 m/s, which were much higher than the traditional scratch tests. The surface morphology as well as the subsurface cracks of the scratch grooves was inspected under AFM and FIB-SEM. The thermal effect that caused the changes in ductile-brittle transition (DBT) and scratch morphology was simulated and explained by a thermal-stress coupled finite element analysis. Finally, the changes in material removal behavior as well as the crack initiation mechanism due to speed effect was revealed.     

Material removal mode and friction behaviour of RB-SiC ceramics during scratching at elevated temperatures

Thermal assistance is considered a potentially effective approach to improve the machinability of hard and brittle materials. Understanding the material removal and friction behaviour influenced by deliberately introduced heat is crucial to obtain a high-quality machined surface. This paper aims to reveal the material removal and friction behaviours of RB-SiC ceramics scratched by a Vickers indenter at elevated temperatures. The material-removal mode, scratching hardness, critical depth of the ductile–brittle transition, scratching force, and friction are discussed under different penetration depths. The size effect of scratching hardness is used to assess the plastic deformation at elevated temperatures. A modified model is established to predict the critical depth at elevated temperatures by considering the changes in mechanical properties. The results reveal that the material deformation and adhesive behaviour enhanced the ductile-regime material removal and the coefficient of friction at elevated temperatures.     

Ductile-brittle transition mechanisms of amorphous glass subjected to taper grinding experiment

Taper grinding experiments were conducted in this paper to investigate the continuous and complete ductile-brittle transition process of two kinds of amorphous glass: high purity fused silica (HPFS) which is silica rich glass and soda-lime silica glass (SLSG) which is low silica glass. The grinding force, ground surface morphology, surface roughness, and subsurface damage depth induced during different stages of taper grinding were all analyzed. A mathematical model describing the cutting force of a grit and micro-crack length was established to clarify the ductile-brittle transition mechanisms of isotropic material. The model revealed that material removal mechanisms and grinding force were mainly determined by the crack equivalent length in front of the grit and its equivalent cutting force. The ground surface roughness and subsurface damage depth were mainly affected by the cutting force of the grit and length of cracks behind it. The ductile machinability of SLSG was better than that of HPFS due to the bonding of metallic atoms in SLSG with nonbridging oxygens, as well as their packing into free volume in SiO₂ network.     

Lateral cracks during sliding indentation on various optical materials

A series of static and sliding indentation (ie, scratching) was performed and characterized on a wide range of optical workpiece 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)] at various applied loads using various indenters (Vickers, 10 µm conical, and 200 µm conical). Despite having different load dependencies, the lateral crack depth formed during sliding indentation quantitatively scales with that formed during static indentation, explaining why static indentation has been historically effective in describing various grinding parameters. Depending on the indenter geometry, the amount of residual trench damage (plastic deformation and local fracturing) during sliding indentation was often enhanced by more than an order of magnitude compared with static indentation. A simple ploughing scratch model, which considers both tangential and normal stresses (where the tangential stress is amplified by relatively small tangential contact area), explains this enhancement and other observed trends. Accounting for the high correlation between residual trench depth and volumetric fracturing, the model is extended to estimate the amount of fracture damage as a function of the material properties of the workpiece, indenter geometry, and applied load. Such a model has utility in the design of optimized grinding processes, particularly the abrasive geometry. Finally, at higher loads (>1 N), lateral cracks were often observed to preferentially propagate in the forward scratching direction, as opposed to perpendicular to the scratch as typically observed. High-speed imaging of the scratch process confirms that these cracks propagate ahead of the sliding indenter during the scratching event. Finite element stress analysis suggests the ploughing frictional forces increase the mode I tensile stresses at the leading edge of the sliding indenter explaining the direction of crack propagation of such cracks.     

Scratch-induced yielding and ductile fracture in silicate glasses probed by nanoindentation

Lateral nanoindentation provides access to the scratch hardness of glass surfaces. The specific sensitivity of the scratching experiment to surface mechanical properties can be enhanced when the local load at the tip apex is reduced. Here, we report on ramp-load scratch tests on a range of silicate glasses using a sphero-conical tip shape. Similar as with regular scratching experiments using sharp indenters, such tests create a sequence of micro-ductile, micro-cracking, and micro-abrasive regimes. Detailed investigation of the indenter displacement h and of the lateral force F_L as recorded in situ, however, reveals pronounced deviations in comparison to Vickers or Berkovich scratching experiments. Most notably, this includes an abrupt increase in both h and F_L at moderate normal load, marking the onset of ductile fracture, and a yield point at the transition from fully elastic deformation to the elastic-plastic regime at low load. For the range of examined silicate glasses, we find that structural cohesion controls yielding, whereas scratch-induced fracture and micro-abrasion are dominated by the volume density of bond energy.     

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.     

Smoothed particle hydrodynamics simulation and experimental analysis of SiC ceramic grinding mechanism

Crack induced surface/subsurface damage in SiC ceramic grinding limits the industrial application. A single-grain scratching simulation based on the smoothed particle hydrodynamics (SPH) has been used to analyze the SiC grinding mechanism, including the material removal process, scratching speed effect on crack propagation, ground surface roughness, and scratching force. The simulation results showed that the material removal process went through the pure ductile mode, brittle assisted ductile mode, and brittle mode with the increase of the depth of cut. The critical depth of cut for ductile-brittle transition was about 0.35?µm based on the change of ground surface crack condition, surface roughness, and maximum scratching force. Increasing the scratching speed promoted the transformation of deep and narrow longitudinal crack in the subsurface into the shallow and wide transverse crack on the surface, which improved the surface quality. The SPH simulation results were indirectly validated by the cylindrical grinding experiments in terms of the critical single grain depth of cut for ductile-brittle transition, and the trend of ground surface roughness and grinding forces.     

Novel Combined Scratch and Nanoindentation Experiments on Soda-Lime-Silica Glass

Many advanced applications of glass demand fabrication of engineering parts of utmost dimensional precision which require very accurate grinding and polishing that involves controlled removal of glass. Despite the wealth of literature, however; the mechanism of material removal in glass grinding and polishing is still far from well understood. For instance, it is not known at all to what extent the mechanical properties are compromised inside a scratch groove so as to optimize the machining parameters. Therefore, to develop better understanding about the mechanism of material removal, a series of combined nanoindentation and single pass scratch experiments were conducted on a commercially available soda-lime-silica glass as a function of various normal loads (2–20 N) and scratch speeds (0.1–1 mm/s). It was found that the nanohardness and Young's modulus at the local microstructural length scale inside the scratch groove could decrease quite dramatically (~30% to 70%) depending on the combination of load and scratching speed. Further, the tribological properties, the severity and the spatial density of damage evolution were sensitive to the normal loads, scratching speeds, and tensile stresses. Extensive scanning electron microscopy leads to interesting observations on material removal mechanisms. These observations were explained by the theoretical predictions of a model for a brittle, microcracked solid.     

Production,properties and impact toughness of die‐drawn toughened polypropylenes

We report the solid‐state die‐drawing of polypropylene and blends of polypropylene with a polyethylene elastomer to produce highly oriented products with enhanced mechanical properties. The blends showed an improvement in the drawability compared to the polypropylene homopolymer. The tensile modulus of the polypropylene homopolymer and the blends, along the draw direction, increased with draw ratio. In the transverse direction, the modulus of the homopolymer and the blends decreased with draw ratio as a result of anisotropy along the draw direction. The impact fracture behavior of the isotropic and the oriented samples was evaluated from noteched Charpy tests at 1 m/s over a wide range of temperatures. Linear elastic fracture mechanics were used to characterize the brittle fracture and the J‐integral approach was used to characterize the semi‐brittle and ductile fracture. In the isotropic state, the inclusion of the elastomer phase in the polypropylene matrix increased the toughness and also decreased the brittle‐ductile transition temperature. The oriented sheets, drawn at 110°C to a draw ratio of 4, were tested with the initial notch parallel and perpendicular to the draw direction. When tested with the initial notch parallel to the draw direction, the toughness of the homopolymer and the blends decreased when compared with the isotropic material. The brittle‐ductile transition temperature increased as a result of anisotropy. When tested with the initial notch perpendicular to the draw direction, the blends and the homopolymer showed considerable improvement in toughness compared to the isotropic state. Fracture along this direction is complicated, with subsidiary cracks propagating perpendicular to the main crack direction (which is parallel to the draw direction).     

Morphology evolution of Vickers indentation in fused silica glass etched by atmospheric pressure plasma jet

Nondestructive machining of optical components and smoothing of surface/subsurface damage generated by pre-processing is a major challenge for ultra-tight machining. The study analyzes the crack change during smoothing by quantitative layer-by-layer removal of Vickers indentations through atmosphere pressure plasma jet; the indentation change process is modeled and analyzed using Level Set Method (LSM) simulation. The results show that plasma jet processing can smooth the subsurface damage of fused silica optical components, and the LSM simulation verifies that the processing of cracks in fused silica components by plasma jet is dominated by each isotropic etching, and there are also each anisotropic etching during the change of cracks; and the depth of adjacent cracks can be judged by the moving direction of adjacent crack boundaries.     

Brittle Fracture versus Quasi Plasticity in Ceramics: A Simple Predictive Index

Simple relations for the onset of competing brittle and quasi-plastic damage modes in Hertzian contact are presented. The formulations are expressed in terms of well-documented material parameters, elastic modulus, toughness, and hardness, enabling a priori predictions for given ceramics and indenter radii. Data from a range of selected ceramic (and other) materials are used to demonstrate the applicability of the critical load relations, and to evaluate coefficients in these relations. The results confirm that quasi plasticity is highly competitive with fracture in ceramics, over a sphere radius range 1–10 mm. Implications concerning the brittleness of ceramics in the context of indentation size effects are discussed.     

Indentation measurements using a dynamic mechanical analyzer

A dynamic mechanical analyzer equipped with a diamond indenter tip was used to measure the elastic modulus of polymeric coatings as well as various bulk materials. A fabricated indenter probe was used to indent bulk samples of aluminum and fused quartz, as well as gelatin and polystyrene films in order to compare the micron-level indentation measurements with sub-micron (nanoindentation) techniques. The measured moduli were in agreement for ductile materials and thick films (>20 μm), but limited displacement resolution, material cracking, and hydrostatic pressure effects led to diverging values for thinner coatings and more brittle materials.     

Understanding of scratch-induced damage mechanisms in polymers

Following the ASTM and ISO test standards, a series of scratch tests were carried out on four categories of polymers: I) ductile and strong, II) ductile and weak, III) brittle and weak, and IV) brittle and strong. The scratch damage features were characterized by using a desktop scanner for scratch visibility assessment, and optical and electron microscopes for detailed damage mechanisms investigation. Various scratch damage mechanisms were identified for the different categories of polymers. The effect of testing rate on possible alteration of scratch damage mechanisms was also studied. The stress fields experienced by the polymer during scratch were determined using three-dimensional finite element methods modeling. It is found that both the material characteristics and the complex stress state exerted on the scratched surface are responsible for the various scratch damage mechanisms observed. A generalized scratch damage mechanism map for polymers is presented. The usefulness of the above understanding for designing scratch-resistant polymers is also discussed.