Surface roughness of lithium disilicate glass ceramics before and after crystallization with different milling techniques: Wet vs submerged milling / Sharon Wong Kah Khei

Sharon Wong , Kah Khei (2024) Surface roughness of lithium disilicate glass ceramics before and after crystallization with different milling techniques: Wet vs submerged milling / Sharon Wong Kah Khei. Masters thesis, Universiti Malaya.

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Abstract

This study investigates the impact of wet milling and submerged milling on the surface roughness of milled lithium disilicate glass ceramic with sequential milling and before and after crystallization process. Submerged milling is gaining traction for its potential to dissipate heat, reduce friction, remove debris more efficiently, increase milling tool longevity, and ensure accurate and efficient milling of final products. A total of 24 Hass Amber® Mill lithium disilicate blocks were divided into 2 groups according to different milling methods: wet milling group (n = 12) and submerged milling group (n = 12). The milling process was performed using a 5-axis CRAFT 5X milling machine. The milled surfaces of the lithium disilicate glass ceramic were evaluated for surface roughness (Ra) using a 3D optical non-contact surface profilometer and observed through Scanning Electron Microscopy (SEM), both before and after the crystallization process. The diamond-coated milling burs (GC 25, GC 20, GC 10) were initially observed under SEM and at 1st, 6th, and 12th milling. Overall comparisons between the surface roughness of lithium disilicate discs of the two groups were done using Paired T-test. Comparisons between the surface roughness of the disc before and after crystallization were done using Paired T-test. The comparison of surface roughness intragroup was done using one-way ANOVA. Significance was set at p value < 0.05. Results indicated that the mean surface roughness of lithium disilicate discs milled with submerged milling technique was significantly lower than that of wet milled discs, both before (p < 0.001, t = 7.093) and after crystallization (p < 0.001, t = 6.020). Within each milling group, Ra reduced after crystallization, with significant differences pre- and post-crystallization (wet: p < 0.001, t = 3.344; submerged: p < 0.001, t = 4.683). One-way ANOVA results showed that there was no significant difference in surface roughness between discs in the same group (p > 0.05 for all groups), suggesting that the 12-cycle milling sequence did not significantly affect the surface roughness. SEM analysis showed loss of abrasive particles on milling bur in both wet and submerged milling groups. This study concludes that submerged milling is more effective in reducing surface roughness compared to wet milling and that crystallization plays a crucial role in further reducing the surface roughness of lithium disilicate glass ceramics. Also, using sequential milling tool usage up to the 12th mill did not affect the surface roughness of milled lithium disilicate.

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