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The distribution and characteristics of surface cracking (i.e. sub-surface damage or SSD) formed during standard grinding processes has been investigated on fused silica glass. The SSD distributions of the ground surfaces were determined by: 1) creating a shallow (18-108 μm) wedge/taper on the surface by magneto-rheological finishing; 2) exposing the SSD by HF acid etching; and 3) performing image analysis of the observed cracks from optical micrographs taken along the surface taper. The observed surface cracks are characterized as near-surface lateral and deeper trailing indent type fractures (i.e., chatter marks). The SSD depth distributions are typically described by a single exponential distribution followed by an asymptotic cutoff in depth (c max ). The length of the trailing indent is strongly correlated with a given process. Using established fracture indentation relationships, it is shown that only a small fraction of the abrasive particles are being mechanically loaded and causing fracture, and it is likely the larger particles in the abrasive particle size distribution that bear the higher loads. The SSD depth was observed to increase with load and with a small amount of larger contaminant particles. Using a simple brittle fracture model for grinding, the SSD depth distribution has been related to the SSD length distribution to gain insight into 'effective' size distribution of particles participating in the fracture. Both the average crack length and the surface roughness were found to scale linearly with the maximum SSD depth (c max ). These relationships can serve as useful rules-of-thumb for nondestructively estimating SSD depth and to identify the process that caused the SSD. In certain applications such as high intensity lasers, SSD on the glass optics can serve as a reservoir for minute amounts of impurities that absorb the high intensity laser light and lead to subsequent laser-induced surface damage. Hence a more scientific understanding of SSD formation can provide a means to establish recipes to fabricate SSD-free, laser damage resistant optical surfaces.

We report a method for fabricating optical quality silica and silica-titania glasses by additive manufacturing, or 3D printing. Key to this success was the combination of sol-gel derived silica and silica-titania colloidal feedstocks, 3D direct ink writing (DIW) technology, and conventional glass thermal processing methods. Printable silica and silica-titania sol inks were prepared directly from molecular precursors by a simple one-pot method, which was optimized to yield viscous, shear-thinning colloidal suspensions with tuned rheology ideal for DIW. After printing, the parts were dried and sintered under optimized thermal conditions to ensure complete organic removal and uniform densification without crystallization.Characterizations of the 3D-printed pure silica and silica-titania glasses show that they are This article is protected by copyright. All rights reserved. 2 equivalent to commercial optical fused silica (Corning ® 7980) and silica-titania glasses (Corning ULE ® 7972). More specifically, they exhibit comparable chemical composition, SiO 2 network structure, refractive index, dispersion, optical transmission, and coefficient of thermal expansion. 3D printed silica and silica-titania glasses also exhibited comparable polished surface roughness and meet refractive index homogeneity standards within range of commercial optical grade glasses. This method establishes 3D printing as a viable tool to create optical glasses with compositional and geometric configurations that are inaccessible by conventional optical fabrication methods. † denotes value determined by LA-ICP-MS; a-SiO 2 used to represent amorphous SiO 2

molding have been demonstrated for fabricating complex glass structures. [11][12][13][14][15][16][17][18] However, there are limitations to these methods. In binder jetting, the sintered glasses can be fragile and appear opaque due to incomplete densification. Methods to print glass directly (e.g., fused deposition or filament feed) require high temperatures to melt the silica feedstock, resulting in filaments that are potentially vulnerable to thermal stresses and unable to completely merge into the desired structure, which may limit the printing speed and resolution of the printed parts. Soft replication molding cannot be used to produce gap-spanning features, and pseudo-3D objects (i.e., stacked assemblies of thin molded sheets) can only be formed by precisely aligning layers that must then be bonded during heat treatment. None of these methods have been shown to produce glasses that are simultaneously transparent, free form, and 3D with sub-millimeter features.We have developed a two-part process (forming and sintering) which uses direct ink writing (DIW) for the 3D printing of optically transparent glass structures with sub-millimeter features. DIW is a layer-by-layer assembly technique in which shear-thinning inks are extruded through a nozzle in a programmable pattern, upon which the inks rapidly solidify via gelation, evaporation, or temperature-induced phase change. [19] DIW has been used in a wide range of applications such as polymeric optical wave guides, complex scaffolds, 3D periodic graphene aerogels, and self-healing materials. [6,[20][21][22][23][24] Our process first relies on DIW printing of colloidal silica suspensions to form silica green bodies (porous, low density structures) of the desired shape. A key feature of this process is the ability to control yield stress and shear thinning to obtain ink properties best suited for specific applications of the printed glass. Second, the printed structures are dried and heated to temperatures below the melting point of silica to sinter the green body into a fully dense, amorphous, transparent solid structure (Figure 1). In contrast to direct 3D printing of molten glass, this two-step approach does not require high temperatures during printing and allows for higher resolution features, due to both the ability to extrude thinner filaments and the shrinkage that occurs during the densification stage.The critical challenges in formulating a suitable ink for formation of the silica green bodies toward glass structures are: i) the ink must possess the desired rheological behavior for printing and shape retention, and ii) the ink must be able to dry without cracking while still maintaining open porosity that Silica inks are developed, which may be 3D printed and thermally processed to produce optically transparent glass structures with sub-millimeter features in forms ranging from scaffolds to monoliths. The inks are composed of silica powder suspended in a liquid and are printed using direct ink writing. The printed structures are then dried and sintered ...

Several silylated coumarin dyes with different linkages and degrees of functionality have been synthesized and incorporated in both SiO 2 xerogels and various solvent hosts. The absorption and fluorescence spectra were examined to explore selected structural and environmental effects on the optical properties of these dyes. Silylated dyes (also referred to as grafted or functionalized dyes) are dye molecules that have been chemically altered to provide alkoxysilane functionality allowing the active molecule to be covalently bonded to the host. Silylation of dye molecules had little effect on the absorption and fluorescence spectra in neutral solvent environments. The optical spectra of silylated dyes compared to those of their conventional counterpart were less influenced by the local chemical environment (e.g., pH) and thus allow for greater control and stability of the fluorescent properties of the dyes in different host environments. Spectroscopy during the drying/gelling of a derCoum xerogel film containing a silylated coumarin dye illustrate changes in the chemical forms of the dye molecules associated with changes in the local chemical environment of the dye.

The effect of various HF‐based etching processes on the laser damage resistance of scratched fused silica surfaces has been investigated. Conventionally polished and subsequently scratched fused silica plates were treated by submerging in various HF‐based etchants (HF or NH4F:HF at various ratios and concentrations) under different process conditions (e.g., agitation frequencies, etch times, rinse conditions, and environmental cleanliness). Subsequently, the laser damage resistance (at 351 or 355 nm) of the treated surface was measured. The laser damage resistance was found to be strongly process dependent and scaled inversely with scratch width. The etching process was optimized to remove or prevent the presence of identified precursors (chemical impurities, fracture surfaces, and silica‐based redeposit) known to lead to laser damage initiation. The redeposit precursor was reduced (and hence the damage threshold was increased) by: (1) increasing the SiF62− solubility through reduction in the NH4F concentration and impurity cation impurities, and (2) improving the mass transport of reaction product (SiF62−) (using high‐frequency ultrasonic agitation and excessive spray rinsing) away from the etched surface. A 2D finite element crack‐etching and rinsing mass transport model (incorporating diffusion and advection) was used to predict reaction product concentration. The predictions are consistent with the experimentally observed process trends. The laser damage thresholds also increased with etched amount (up to ∼30 μm), which has been attributed to: (1) etching through lateral cracks where there is poor acid penetration, and (2) increasing the crack opening resulting in increased mass transport rates. With the optimized etch process, laser damage resistance increased dramatically; the average threshold fluence for damage initiation for 30 μm wide scratches increased from 7 to 41 J/cm2, and the statistical probability of damage initiation at 12 J/cm2 of an ensemble of scratches decreased from ∼100 mm−1 of scratch length to ∼0.001 mm−1.

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 3 10 À5 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 ). 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.

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