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 $\ge 15\mathrm{MPa}$ 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.

Figure 1

aLIGO quasimonolithic fused silica suspension with hydroxide catalysis bonded ear.

Figure 2

Bond formation between fused silica. Stage 1 is hydration and etching, where ${\mathrm{OH}}^{-}$ ions form weak bonds with Si atoms on the surface. Stage 2 shows Si atoms released from the bulk to form $\mathrm{Si}(\mathrm{OH}{)}_5^{-}$ in the solution. In stage 3 the formation of siloxane chains and water occurs.

Figure 3

Hydrogen bonds between water molecules and a fused silica surface. Hydrogen bonds are indicated by a dashed line, covalent bonds by a solid line.

Figure 4

On the left, a sapphire sample with hydroxide catalysis bond in the middle is shown in the strength tester. The schematic of four-point bend test is shown on the right.

Figure 5

Strength of hydroxide catalysis bonded c plane sapphire over time (combined results by R. Douglas and M. Phelps, 2014–2017).

Figure 6

Strength of hydroxide catalysis bonded fused silicon over time (results by M. Masso Reid, 2018).

Figure 7

Strength of hydroxide catalysis bonded fused silica over time (A. A. van Veggel, V. Mangano, K. Haughian, M. Kelly, and A. Jongschaap 2009–2014 combined results).