FY07 LDRD Final Report A Fracture Mechanics and Tribology Approach to Understanding Subsurface Damage on Fused Silica during Grinding and Polishing

Abstract

The objective of this work is to develop a solid scientific understanding of the creation and characteristics of surface fractures formed during the grinding and polishing of brittle materials, specifically glass. In this study, we have experimentally characterized the morphology, number density, and depth distribution of various surface cracks as a function of various grinding and polishing processes (blanchard, fixed abrasive grinding, loose abrasive, pitch polishing and pad polishing). Also, the effects of load, abrasive particle (size, distribution, foreign particles, geometry, velocity), and lap material (pitch, pad) were examined. The resulting data were evaluated in terms of indentation fracture mechanics and tribological interactions (science of interacting surfaces) leading to several models to explain crack distribution behavior of ground surfaces and to explain the characteristics of scratches formed during polishing. This project has greatly advanced the scientific knowledge of microscopic mechanical damage occurring during grinding and polishing and has been of general interest. This knowledge-base has also enabled the design and optimization of surface finishing processes to create optical surfaces with far superior laser damage resistance.There are five major areas of scientific progress as a result of this LDRD. They are listed in Figure 1 and described briefly in this summary below. The details of this work are summarized through a number of published manuscripts which are included this LDRD Final Report.In the first area of grinding, we developed a technique to quantitatively and statistically measure the depth distribution of surface fractures (i.e., subsurface damage) in fused silica as function of various grinding processes using mixtures of various abrasive particles size distributions. The observed crack distributions were explained using a model that extended known, single brittle indentation models to an ensemble of loaded, sliding particles. The model illustrates the importance of the particle size distribution of the abrasive and its influence on the resulting crack distribution. The results of these studies are summarized in references 1-7.In the second area of polishing, we conducted a series of experiments showing the influence of rogue particles (i.e., particles in the polishing slurry that are larger than base particles) on the creation of scratches on polished surfaces. Scratches can be thought of a as a specific type of sub-surface damage. The characteristics (width, length, type of fractures, concentration) were explained in terms of the rogue particle size, the rogue particle material, and the viscoelastic properties of the lap. The results of these studies are summarized in references 6-7.In the third area of etching, we conducted experiments aimed at understanding the effect of HF:NH 4 F acid etching on surface fractures on fused silica. Etching can be used as a method: a) to expose sub-surface mechanical damage, b) to study the morphology of specific mechanical damage occur...

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“…For 3u laser fusion optics, a link between surface flaws such as scratches, fractures and digs and degraded laser resistance has been clearly established [20][21][22]. To minimize these flaws, polishing research has focused on the following: minimization of grinding-induced damage, determination of adequate material removal between manufacturing steps to remove subsurface damage, shear polishing, elimination of rogue particles during polishing, post-surface treatment to eliminate absorption centres and mitigation strategies to arrest damage growth.…”

“…What might appear at first to be conflicting requirements, the quality of the surfaces needed to improve to reduce the number of nano-absorbing defects that initiate laser damage, while the manufacturing time needed to be reduced to lower costs, but, in reality, better process control and increased determinism for faster fabrication reduces the surface flaws leading to high laser-resistant optics. For 3u laser fusion optics, a link between surface flaws such as scratches, fractures and digs and degraded laser resistance has been clearly established [20][21][22]. To minimize these flaws, polishing research has focused on the following: minimization of grinding-induced damage, determination of adequate material removal between manufacturing steps to remove subsurface damage, shear polishing, elimination of rogue particles during polishing, post-surface treatment to eliminate absorption centres and mitigation strategies to arrest damage growth.…”