Strength of hydroxide catalysis bonds between sapphire, silicon, and fused silica as a function of time

Hydroxide catalysis bonds have formed an integral part of ground-based gravitational wave (GW) observatories since the 1990s. By allowing the creation of quasimonolithic fused silica mirror suspensions in detectors such as GEO600 and Advanced LIGO, their use was crucial to the first ever direct detection of gravitational waves. Following these successes, this bonding technique has been included in advanced next generation cryogenic detector designs. Currently, they are used to create quasimonolithic crystalline sapphire suspensions in the KAGRA detector. They are also planned for use in silicon suspensions of future detectors such as the Einstein Telescope. In this paper we report how the strength of hydroxide catalysis bonds evolves over time, and compare the curing rates of bonds as they form between fused silica substrates to those between sapphire to sapphire and silicon to silicon substrates. For bonds between all three types of substrate material we show that newly formed bonds exhibit slightly higher breaking stresses than bonds cured for longer periods of time. We find that the strength stabilizes at $\ensuremath{\ge}15\text{ }\text{ }\mathrm{MPa}$ for bonds cured for up to 30 weeks (7 months). This finding is important to future cryogenic GW detector design as it is crucial to ensure the long term integrity of the suspension interfaces. Monitoring the strength of bonds that have been allowed to cure for shorter lengths of time can also shed light on the chemistry of bond formation.