Subsurface mechanical damage correlations after grinding of various optical materials

Abstract

Loose abrasive grinding was performed on a wide range of optical workpiece materials [single crystals of Al 2 O 3 (sapphire), SiC, Y 3 Al 5 O 12 (YAG), CaF 2 , and LiB 3 O 5 (LBO); a SiO 2 -Al 2 O 3 -P 2 O 5 -Li 2 O glass-ceramic (Zerodur); and glasses of SiO 2 ∶TiO 2 (ULE), SiO 2 (fused silica), and P 2 O 5 -Al 2 O 3 -K 2 O-BaO (phosphate)]. Using the magneto rheological finishing (MRF) taper wedge technique (where a wedge was polished on each of the ground workpieces and the resulting samples were appropriately chemically etched), the subsurface mechanical damage (SSD) characteristics were measured. The SSD depth for most of the workpiece materials was found to scale as E 1 1∕2 ∕H 1 , where E 1 is the elastic modulus and H 1 is the hardness of the workpiece. This material scaling is the same as that for the growth of lateral cracks, suggesting that lateral cracks are a dominant source for SSD rather than radial/median cracks, as previously proposed. Utilizing the SSD depth data from both this study and others, semiempirical relationships have been formulated, which allows for estimating the SSD depth as a function of workpiece material and important grinding parameters (such as abrasive size and applied pressure).

Lateral cracks during sliding indentation on various optical materials

“…First, this correlation suggests simple static indentation parameters can be effectively used to model and draw conclusions regarding more complex, dynamic processes involving sliding indentation such as grinding processes. In fact, this explains why previous grinding models utilizing static indentation behavior appear to do a reasonable job at predicting grinding behavior . Second, since it has been previously shown that lateral crack depth during static indentation scales as E 1 1/2 / H 1 of the workpiece material, this implies a similar scaling would apply for sliding indentation and hence grinding processes.…”

“…As discussed in the Introduction, various grinding characteristics (such as material removal rate, surface roughness, and sub‐surface mechanical damage) during optical fabrication have been shown to correlate with the characteristics and propensity of lateral cracks. Previous grinding models have quantitatively utilized the lateral crack material and load correlations (as measured by static Vickers indentation) with good success . The results shown in Figure , which illustrates that static and sliding indentation lateral crack depth behavior are directly correlated, have some important implications with respect to understanding and predicting grinding processes.…”

“…During static indentation, the lateral crack depth ( c ℓ ) has been shown to scale with the applied load ( P ) as:where s ℓ is the lateral crack slope which is a function of the material properties of the workpiece and the indenter geometry. Also, the lateral crack slope ( s ℓ ) has been shown to scale as E 1 1/2 /H 1 where E 1 is the elastic modulus of the workpiece (GPa) and H 1 is the Hardness of the workpiece (GPa) . Recent studies have quantitatively described grinding characteristics such as material removal rate, surface roughness, and even sub‐surface mechanical damage as a function of the lateral crack scaling parameters s ℓ and/or E 1 1/2 /H 1.…”

“…Recent studies have quantitatively described grinding characteristics such as material removal rate, surface roughness, and even sub‐surface mechanical damage as a function of the lateral crack scaling parameters s ℓ and/or E 1 1/2 /H 1. However, the fracturing that occurs during grinding processes is from sliding and rolling processes which are significantly more complex compared with static indentation. Some key differences, include different stress distributions, different loading times, overlapping stress distributions between neighboring abrasive particles, overlapping fracture effects, friction forces, and plastic material ploughing.…”

“…Also, the lateral crack slope (s ℓ ) has been shown to scale as E 1 1/2 /H 1 where E 1 is the elastic modulus of the workpiece (GPa) and H 1 is the Hardness of the workpiece (GPa). 1,6 Recent studies have quantitatively described grinding characteristics such as material removal rate, surface roughness, and even sub-surface mechanical damage as a function of the lateral crack scaling parameters s ℓ and/ or E 1 1/2 /H 1. 6,7 However, the fracturing that occurs during grinding processes is from sliding and rolling processes which are significantly more complex compared with static indentation.…”