1

Introduction

Due to the high esthetic appearance, glass-ceramics are often used—primarily in the anterior region [ ]. The atoms or oxide molecular groups in ceramic materials are linked by covalent or by ionic bonds. Covalent bonds, which were separated by overuse, cannot be re-formed (except for additional ceramic firing >700 °C). This means that a microcrack in the ceramic cannot close at oral temperatures. In general, ceramics are brittle and not resistant to pulling. At any load that exceeds a critical value, they will slowly but steadily increase in size by rupturing further atomic bonds, spreading in the workpiece to a fracture. The mechanical properties of glass-ceramics can be increased by incorporating small crystalline particles with a high bond to the glass in the glass matrix [ , ]. Cracks, which are relatively easy to move through the glass contents of the ceramic, are stopped or redirected on such crystals, thereby slowing the progression of cracks and toughening the material [ ]. The higher the fracture toughness of a ceramic, the more the fracture is delayed and the restoration shows higher long-term stability.

Lithium-X-silicate ceramics include lithium disilicate, lithium silicate, and lithium aluminosilicate ceramics. Compared to leucite ceramics, these are ceramics with about 2–3 times higher mechanical properties [ , ]. In the case of lithium disilicate and lithium silicate ceramics, lithium silicate oxide crystals are generated in the glass phase by crystallization. The molar ratio between LiO ₂ and SiO ₂ in the glass phase leads to the formation of lithium silicate or lithium disilicate crystals. Lithium disilicate (Li ₂ [Si ₂ O ₅ ]) ceramic was one of the first lithium-X-silicate ceramics. Here, about 70% platelet-shaped lithium disilicates in the size between 3 and 6 microns were crystallized in the glass phase. The lithium silicate ceramics consist of lithium metasilicate (Li ₂ SiO ₃ ), lithium orthophosphate (Li ₃ PO ₄ ), and lithium disilicate (Li ₂ Si ₂ O ₅ ). The nucleating agents used are cerium oxide (CeO ₂ ) and phosphorus pentoxide (P ₂ O ₅ ). These ceramics (Celtra Press) can already be used adhesively directly after pressing. However, by an additional Power Firing, the strength of the ceramic increases. The indications for these ceramics are crowns, abutment crowns, 3-unit bridges to the second premolars, inlays, onlays, and veneers [ ]. In lithium aluminosilicate ceramics, co-crystallization takes place between lithium disilicate and lithium aluminosilicate. These ceramics are used immediately after grinding without additional crystallization firing and must, therefore, be well polished to close the surface defects. They show lower mechanical properties compared to lithium disilicate or lithium silicate ceramics. Therefore, lithium aluminosilicate ceramics are indicated for crowns, abutment crowns, partial crowns, inlays, onlays, and veneers.

Conventionally, pressed ceramics are available as monochrome, industrially produced pellets in various sizes. In addition to many advantages in terms of fit accuracy, marginal edge quality, low shrinkage, low porosity, and high mechanical properties, there is also the disadvantage that perfect esthetics with color-graded layering is no longer possible [ , , ]. Individualization of the restoration can only be performed by glazing. Pressing lithium-X-silicate ceramics is a simple and cost-effective production method. Pressed lithium disilicate ceramics result in superior fracture toughness compared to milled ceramics [ , ]. These observations might be traced back to different heating parameters that are known to possibly upset the driving force for growing lithium disilicate crystals and alter the overall percentage of residual glasses, which in turn might adversely impact several material properties including load-bearing capacity and fracture toughness [ ].

Wear behavior is an important aspect of all dental restorative materials. Ideally, restorative materials should show wear behavior comparable to that of natural dentition, with clinical wear rates in the range of 15–29 μm per year [ ]. Patients with parafunctional habits show increased tooth wear of more than 140 μm per year [ ]. The in-vitro wear tests observed small material loss for glass ceramics compared to polymer-based materials [ ]. However, it is not known if there are differences between the lithium-X-silicate ceramics with respect to wear behavior. In vitro assessment of wear in a dental material requires the digitization of specimen surfaces, wear quantification, and statistical analysis of wear measurements.

The present investigation included the above mentioned standardized mechanical tests to evaluate the flexural strength, fracture toughness, wear, and Martens parameters of five lithium-X-silicate ceramics. The null hypothesis stated that the tested materials result in a comparable outcome for four-point flexural strength ( FS ), fracture toughness ( K _IC ), ACTA three-body wear ( 3BW ), and Martens parameters ( HM and E _IT ).

2

Material and methods

This study tested the mechanical and wear properties of five lithium-X-silicate ceramics ( Table 1 , Fig. 1 ). In addition, one ceramic (Celtra Press, CP) was heat-treated according to the manufacturer instructions and tested only on FS and Martens parameters for comparison with the non-heat-treated stage. All specimens (N = 330; N = 90 per ceramic for FS and HM / E _IT ; N = 75 per ceramic for 3BW and K _IC , see Fig. 1 ) were milled from wax blanks (ProArt CAD Wax blue, Ivoclar Vivadent, Liechtenstein) using a five-axis milling machine (Ceramill Motion 2, Amann Girrbach, Koblach, Austria) ( Fig. 2 ). The wax specimens were embedded using the manufacturers’ recommended investment materials ( Table 1 ) and preheated in a furnace (Typ 5635, Kavo Ewl, Biberach, Germany) at 850 °C for 60 min. Specimens were pressed in a ceramic press furnace (Austromat 654 Press-i-dent, Dekema, Freilassing, Germany) according to the manufacturer´s instructions ( Table 2 , Figs. 3 and 4 ). After cooling down to room temperature, all specimens were divested and cleaned by air-particle abrasion (Sandmaster FG3-92, Sandmaster AG, Zofingen, Switzerland) using nutshells (Hasenfraz, Aßling, Germany) for rough devesting and 50 μm/0.2 MPa glass beads for fine divesting (Rolloblast, Renfert, Hilzingen, Germany) ( Fig. 5 ). The specimens were polished with P4000 grit silicon carbide grinding paper (SiC) using a water-cooled polishing machine at 150 rpm (Abramin, Struers, Ballerup, Denmark) ( Fig. 6 ). The four-point FS (crosshead speed: 1 mm/min) and K _IC (crosshead speed: 0.5 mm/min) were measured in universal testing machine 1445 (ZwickRoell, Ulm, Germany) until fracture ( Fig. 7 ). HM and E _IT were measured using testing machine ZHU 2.5 (ZwickRoell). Each subgroup included 15 specimens per ceramic and per test method.

Study design.

Bar-shaped specimens milled from a wax-blank using a five-axis milling machine.

Preparations for pressing bar-shaped specimens.

Pressed bar-shaped specimen in ceramic press furnace.

Cleaned bar-shaped specimen after air-particle abrasion using nutshells.

Specific polishing device.

FS (left) and K _IC (right) measurements using a universal testing machine.

For FS (N = 90, n = 15/ceramic ) and K _IC (N = 75, n = 15/ceramic ), the final dimension of the bar-shaped specimens was 30.0 mm (length) × 4.0 ± 0.2 mm (width) × 3.0 ± 0.2 mm (thickness). For FS , the wide side of the specimen was placed in the adapted specimen holder onto 2 steel rolls 20 mm apart and loaded until fracture ( Fig. 7 ). Force was exerted by the plunger apparatus with 2 steel rolls 12 mm apart. The four-point FS was calculated using [ ]:

where σ is the flexural strength (MPa), F is the fracture load (N), L ₀ is the distance between the outer steel rolls (mm), L _i is the distance between the inner steel rolls (mm), b is the specimen width (mm), and h is the specimen height (mm).

For the single edge V-notched Beam (SEVNB) K _IC , 15 specimens were placed on the narrow side (3.0 ± 0.2 mm), fixed upright, side by side and centered in an adapted specimen holder ( Fig. 7 ). A saw cut was inserted in the center using a universal cutting machine (Secotom-50; Struers) with a diamond charged cut-off wheel (127 mm dia. × 0.4 mm, Diamond cut-off wheel M1D13; Struers). The depths of the saw cuts were more than 0.5 mm according to the standard. After that, the specimen holder was placed in a specially constructed notching machine (SD Mechatronik, Feldkirchen-Westerham, Germany). The specimens were notched and sharpened using a razor blade (0.4 mm blades, Lux Tools, Wermelskirchen, Germany) with 3 μm polishing diamond paste (MetaDi diamond paste; Buehler, Esslingen am Neckar, Germany) ( Fig. 8 ). Following the standard, the depth of the saw cut together with the depth of the notching was between 0.8 mm and 1.2 mm. The movement cycles of the machine varied as well as the pressing force of the movement via weights. Specimens were ultrasonically cleaned (Sonorex RK102 H; Bandelin electronic, Berlin, Germany) in 80% alcohol. The saws and notches were measured using a microscope (Zwick/Roell Z 2.5; Zwick). Specimens were placed into the same adapted specimen holder as for FS , but this time lying on the narrow side with the notched surface pointing downwards and loaded until fracture ( Fig. 7 ). The K _IC was calculated according to the formula [ ]:

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