Metaserv 250

The 24 fully sintered Y-TZP cubes (10 mm length, 10 mm width, and 10 mm height) (LUXEN, DentalMax, Seoul, Korea) were prepared to analyze the surface energy and 160 fully sintered Y-TZP discs (5 mm diameter, 3 mm height) were prepared for shear bond strength. For all disc type specimens, all surfaces except for the top were embedded into an acrylic resin (Orthodontic resin, Dentsply, Konstanz, Germany) to be treated (Figure 1a). All specimens were polished to 600 grit and 800 grit silicon carbide paper under water cooling using a grinding machine (MetaServ 250, Buehler, Lake Blu, IL, USA).

Histological sections of teeth were prepared in our laboratory following standard procedures^{8 (link)}. In the case of extant species, teeth were firstly extracted from the mandible and dehydrated using different concentrations of alcohol for a total period of 72 hours (70, 96 and 100%; 24 h in each). With both extant and fossil samples, each tooth was then embedded in epoxy resin (Araldite 2020) and longitudinally sectioned at the level of the protoconid in the bucco-lingual plane using a low-speed diamond saw (IsoMet, Buehler). The cut surface was later polished using a Metaserv®250 (Buehler) and fixed to a frosted glass with ultraviolet-curing adhesive (Loctite 358). Each sample was then cut and ground with a diamond saw (PetroThin, Buehler) up to a thickness of 150 μm and polished again to obtain a final thickness of approximately 120 μm. Finally, the thin sections obtained were dehydrated again in increasing concentrations of alcohol, immersed in a histological clearing agent (Histo-Clear II) and mounted using DPX medium (Scharlau) to improve visualisation of the dental microscopic features. Due to the high height of equids’ crowns, most of the samples had to be mounted on two separate slides^{10 (link)}. The identification of incremental markings enabled confirmation of both slides being cut from the same plane (Fig. 1a).

All specimens were polished according to the following process: using silicon carbide papers with 320-, 400-, 600-, and 1200 grit for 5 min under continuous water cooling (MetaServ 250, Buehler; Illinois, USA). Then, specimens were cleaned with distilled water using an ultrasonic cleaner (PowerSonic 405, Hwashin; Seoul, South Korea), for 5 min.
The CAD/CAM milled specimens were polished before sintering. Therefore, they received no further polishing treatment following sintering. FDM-printed specimens were fully sintered then polished using the above-mentioned procedure. Because these specimens were fully sintered, it took significantly longer time and more effort to reach the fully polished surface.

30 bars (2 × 5 × 27 mm) for the flexural strength test and 20 discs (12 × 1.5 mm thickness) for the Vickers microhardness test were milled out of pre-sintered zirconia blocks (Ceramill ZI, AMANNGIRRBACH AG., Herrschaftswiesen 1, 6842 Koblach, Austria). Specimens then were polished using silicon carbide abrasive papers with 320-, 400-, 600-, and 1200 grit for 10 s under water cooling (MetaServ 250, Buehler; Illinois, USA). Then, specimens were cleaned with distilled water using an ultrasonic cleaner (PowerSonic 405, Hwashin; Seoul, South Korea), for 5 min.

The aluminum dolly measuring 14 mm in diameter was polished with sandpaper of decreasing roughness (P400–P600–P1200–P2500) using a rotational polisher (Buehler Metaserv 250) at 400 rpm until an even appearance was achieved. The dolly was then dipped in the prepared adhesive and fixated to the bottom of the glass substrates. Curing was performed overnight, at 70 °C in the case of the reference polyurethane adhesive or at 90 °C in the case of the epoxy adhesives.

A glass substrate was provided by Steklarna Hrastnik (model glass Tina 25, Hrastnik, Slovenia). To ensure the repeatability of pull-off tests, the glass substrates were firstly polished with P60 sandpaper in order to remove irregularities, followed by polishing with sandpaper of decreasing roughness (P240–P400–P600–P1200–P2500) to achieve a smooth surface. Polishing was performed using a rotational polisher (Buehler Metaserv 250, Buehler, Lake Bluff, IL, USA) at 400 rpm with the glass in alternating positions (four quadrants). The glass and sandpaper were rinsed with water following each change in polishing direction. Lastly, the substrates were polished using a 3 µm diamond suspension.