Sub-surface mechanical damage distributions during grinding of fused silica

Abstract

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.

Effect of rogue particles on the sub-surface damage of fused silica during grinding/polishing

“…5b & d. From our previous studies [14], the length of the trailing indent fracture was found to be correlated directly to the size of the abrasive leading to fracture. As shown in Table 1, the average crack length for the 15 µm loose abrasive (G2) was 3.8 µm and for the 30 µm loose abrasive was 15.2 µm.…”

“…1). Details of this characterization technique to determine SSD depth and length distributions are provided elsewhere [14][15][16]. Photographs of the grinding setup is shown in Fig.…”

“…Figure 4 shows an optical micrograph of a ground surface after the rubble-like surface has been removed, illustrating the typical cracks observed on a ground surface. The length or size of these trailing indents cracks have been found to scale with the size of the abrasive particle used for grinding [14,16]. was used instead of crack number density to describe the SSD density because the intersection of cracks near the surface causes a significant uncertainty in the crack number density.…”

Green luminescence in silica glass: A possible indicator of subsurface fracture

“…Green luminescence in silica glass: A possible indicator of subsurface fracture Jessica Fournier, 1,2 Jerome Neauport, 1,a) Pierre Grua, 1 Evelyne Fargin, 2 Vé ronique Jubera, 2 David Talaga, 3 We investigate the nature of defects triggering laser damage in fused silica in subsurface fractures in nanosecond near ultraviolet regime. Mechanical, laser induced surface flaws as well as pristine silica surface were characterized by optical microscopy and luminescence confocal microscopy before and after acid etching.…”

“…Since damage size grows exponentially when submitted to additional laser shots, the optical lifetime can become rather limited. Significant progress has been made to reduce the damage density of fused silica optics by modifying the polishing processes, 1 limiting surface impurities, 2 reducing subsurface damage, 3 or developing the surface post treatment. 4 Nevertheless, and in spite of these improvements, the real physical nature of damage precursors stays quite vague.…”