Loose abrasive lapping hardness of optical glasses and its interpretation

Abstract

We present an interpretation of the lapping hardness of commercially available optical glasses in terms of a micromechanics model of material removal by subsurface lateral cracking. We analyze data on loose abrasive microgrinding, or lapping at fixed nominal pressure, for many commercially available optical glasses in terms of this model. The Schott and Hoya data on lapping hardness are correlated with the results of such a model. Lapping hardness is a function of the mechanical properties of the glass: The volume removal rate increases approximately linearly with Young's modulus, and it decreases with fracture toughness and (approximately) the square of the Knoop hardness. The microroughness induced by lapping depends on the plastic and elastic properties of the glass, depending on abrasive shape. This is in contrast to deterministic microgrinding (fixed infeed rate), where it is determined from the plastic and fracture properties of the glass. We also show that Preston's coefficient has a similar dependence as lapping hardness on glass mechanical properties, as well as a linear dependence on abrasive size for the case of brittle material removal. These observations lead to the definition of an augmented Preston coefficient during brittle material removal. The augmented Preston coefficient does not depend on glass material properties or abrasive size and thus describes the interaction of the glass surface with the coolant-immersed abrasive grain and the backing plate. Numerical simulations of indentation are used to locate the origin of subsurface cracks and the distribution of residual surface and subsurface stresses, known to cause surface (radial) and subsurface (median, lateral) cracks.

Loose abrasive slurries for optical glass lapping

“…Relating material removal to the formation of lateral fractures, the effects of lap and glass compositions on MRR were investigated. Following the studies of Izumitani [15], Lambropoulos et al proposed an interpretation of the lapping hardness of a wide variety of optical materials, based on a material removal micromechanical model by lateral cracking [16]. More recently, Wang et al studied the effect of lapping parameters with SiC abrasive slurries on surface roughness and SSD depth [17].…”

How to break a lens

“…K I = a Yσ (2) To plug in numbers for S-FPL51, we estimate the flaw depth by assuming a 120 grit grind yielding a 9um rms surface. The max depth is typically 30-50 times the surface roughness 4 or .014 inch deep.…”

Étude comparative par indentation des caractéristiques mécaniques de verres sodo-calcique et vitrocéramique rodés et polis par des grains abrasifs liés

“…Si l'outil est neuf, les grains pénètrent profondément dans le verre et produisent de petitesécailles et des fêlures. Des composants optiques de haute précision ontété obtenus par ces méthodes.À titre d'exemple, des surfaces sphériques convexes et concaves ontété fabriquéesà l'université de Rochester (New York USA) avec un rayon de courbure de 5 mmà l'infini et des diamètres de travail de 10à 150 mm ; après moins de 5 minutes de micro-rodage, la surface spéculée résultante avait une rugosité quadratique (Rq) inférieureà 20 nm [8][9][10][11][12][13]. Les taux d'enlèvement de matière typiques sont de 6à 10 μm/min pour une taille des abrasifs en diamant de 2à 4 μm.…”