Lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) possess various attractive properties such as piezo-electricity, pyro-electricity, non-linear optical behaviour, making them interesting for many electronic devices (e.g. modulators, transducers or detectors). In the smartphone market, for instance, such single crystals have found important application as surface acoustic wave (SAW) filter substrates for high frequency data transfer. The wafers are cut from a grown single crystal under specific angles given by the functional requirements of the device within a certain temperature range. Although appropriate functional characterization of these materials can be found in literature, a lack of knowledge exists regarding the mechanical properties of these single crystal components in terms of strength and fracture resistance with respect to the cutting direction and the surface quality.
In this work, the mechanical strength of macroscopic (dimensions of ~2 x 2 x 0.13 mm³) LiTaO3 and LiNbO3 single crystals has been assessed using a biaxial bending testing method, i.e. the ball-on-three-balls test (B3B). This method allows high accuracy testing of as-sintered substrates, small components, etc. in different environments. The effect of surface finishing on strength was evaluated by testing polished and roughened surfaces.
Strength results of these macroscopic experiments were interpreted using Weibull statistics and showed a strong influence of surface finishing on the strength of both materials. Interestingly, the strength of LiTaO3 crystals was significantly higher than that of LiNbO3 crystals for the same surface finishing.
For understanding of the observed macroscopic behaviour, local phenomena were investigated performing nanoindents of different sizes on the smooth as well as on the rougher side to compare the fracture patterns and identify crystallographic cleavage planes. Furthermore, notched micro cantilever samples were tested in situ in the SEM to quantify fracture toughness of the most critical cleavage planes, in order to explain the different strength between the two materials.