Zhu0 2 z2

To initiate crack formation in the ceramics, four Vickers indentations per specimen were performed using a load of 58.8 N for 15 s with a universal hardness testing machine (ZHU0.2/Z2.5, Zwick/Roell, Ulm, Germany; Fig. 3). The Vickers indenter was oriented to allow for a parallel alignment of the two radial cracks with the cavity edge, with each indentation centered at a distance of 570 μm from the edge of the hole (Fig. 2) [15 (link)]. The ceramic was subsequently set aside and dried in a desiccator (Duran Exsikkator, Duran Group, Mainz, Germany) at 23 °C for 24 h to allow for a slow crack growth induced by residual stress [4 (link)]. The length of the two cracks originating from each Vickers indentation was determined at a magnification of × 200 using the software of the universal hardness testing machine (TestXpert II, Version 3.2, Zwick/Roell; Fig. 4), and an average length (c) was calculated for each specimen.Fig. 3

Setup for performing the Vickers indentations

Fig. 4

Determination of the crack propagation (initial (a) and after polymerization (b) of the self-adhesive resin composite cement RelyX Unicem 2)

To analyze the Martens hardness parameter (HM in N/mm² and EIT in kN/mm²), a universal hardness testing machine (ZHU 0.2/Z2.5, Zwick Roell, Ulm, Germany) was used. Therefore, one specimen of each substrate (ZrO2, LEU, LiSi) was measured four times. Therefore, the diamond indenter pyramid (α = 136°) of the testing machine was pressed vertically into specimen surface with a load of 9.81 N for 10 s. The maximum depth of the indenter in surface was 0.01 mm. Martens hardness (HM) and indentation modulus (EIT) values were calculated (testXpert V12.3 Master, Zwick).

Ten segments (about 10 mm in length) were cut from each wire by employing orthodontic pliers and then were embedded longitudinally in epoxy resin (Epofix, Struers, Belarup, Denmark). Then, the specimens were metallographically ground with SiC water coolant paper from 400 up to 2000 grit and polished up to 1 μm alumina slurry in a grinding/polishing machine (Dap V, Struers). Finally, they were cleaned in an ultrasonic water bath for 10 min.
A universal hardness testing machine ZHU0.2/Z2.5 (Zwick Roell, Ulm, Germany) was used for IIT measurements. Force indentation depth curves were recorded by applying 9.8 N with 2 s dwell time by a Vickers indenter. Five readings were taken from the surface of each segment and the mean value was used as representative of the segment itself. (n = 10 per product). All force-indentation depth curves were recorded and Martens Hardness (HM), indentation modulus (E_IT), and elastic index (η_IT), were estimated according to the ISO 14577-1 specification [14 ]. The analytical formulas were given by the ISO 14577-1 and can also be found in recent publications [7 (link),15 (link)]. Then, Vickers hardness was determined by measuring the diagonal length of indentations.