The effect of abrading and cutting instruments on machinability of dental ceramics

Recent advances in glass-ceramics: Performance and toughening mechanisms in restorative dentistry

The use of glass-ceramics in the medical field has grown significantly since the 1980s. With excellent aesthetic properties, semi-translucency, outstanding mechanical properties, corrosion resistance, wear resistance and great biocompatibility and workability glass-ceramics is one of the most commonly used materials in restorative dentistry and is widely used in veneers, inlays, onlays, all-ceramic crowns, and implant abutments. This review provides an overview of the research progress of glass-ceramics in restorative dentistry, focusing on the classification, performance requirements, toughening mechanisms and their association with clinical performance, as well as the manufacturing and fabrication of glass-ceramics in restorative dentistry. Finally, the developments and prospects of glass-ceramics in restorative dentistry are summarized and discussed.

Simulation of CAD/CAM milling on lithium disilicate: Mechanical and topographic analyses of surface grinding different protocols

The aim of this study was to evaluate the topography and the fatigue performance of lithium disilicate glass-ceramic after surface grinding through different laboratory protocols used to simulate the Computer-aided design/Computer-aided manufacturing (CAD/CAM) milling. Ceramic discs (IPS e.max CAD, Ø = 13.5 mm × 1.2 mm of thickness) were produced through different methodologies: milling in CAD/CAM system (CAD/CAM group); produced in-lab with a polished surface (POL group); or produced through in-lab methods and randomly distributed into five groups according to different grinding protocols to simulate the CAD/CAM milling [grinding with a CAD/CAM bur coupled to a mandrel (CAD/CAM Bur group); fine diamond bur using oscillatory movements (DBO group); fine diamond bur in x and y axes of the disc (DBXY group); #60-grit silicon carbide sandpaper (SiC group); and #60-grit wood sandpaper (WS group)]. The specimens were fatigue tested (n = 15) according to the step-stress method (initial load: 60 N; step-size: 20 N; 10,000 cycles/step; 20 Hz frequency). A roughness analysis was performed on all specimens, while fractal dimension (FD) and fractography were performed on representative samples. The Kaplan-Meier analysis showed that the POL (293.3 N) group presented better fatigue performance (higher load and number of cycles for failure) (p < 0.05) than the other groups (CAD/CAM = 222.7 N; CAD/CAM Bur = 181.3 N; DBO = 184.0 N; DBXY = 192.0 N; SiC = 182.6 N; WS = 182.6 N). For roughness, only the SiC (Ra = 1.616; Rz = 10.465) and WS (Ra = 1.673; Rz = 10.655) groups produced statistically similar Ra (μm) and Rz (μm) values to the CAD/CAM (Ra = 1.628; Rz = 9.571) group (p > 0.05). The surface created by CAD/CAM milling and POL group exhibited more complexity (FD) higher values than the experimental groups. For the ceramic surface topography images, the CAD/CAM milling visibly produced a uniform surface compared to the other groups; however, the POL group was the smoothest. The DBO, DBXY, SiC, and WS groups resulted in similar characteristics of surface topography. Therefore, although the SiC and WS groups showed similar roughness to the control group (CAD/CAM), no in-lab simulation method was fully capable to mimic the mechanical performance of the CAD/CAM-milled lithium disilicate glass-ceramic.

Effect of repeated millings on the surface integrity of diamond burs and roughness of different CAD/CAM materials

OBJECTIVES

This study evaluated the surface integrity of two types of diamond burs and their effect on the roughness of different CAD/CAM materials.

MATERIALS AND METHODS

CAD/CAM materials, Vita Mark II (VM), IPS e.max CAD (EM), Celtra Duo (CD), G-Ceram (GC), and Lava Ultimate (LU) blocks were milled with CEREC MCXL. Five pairs of pointed cylinder burs (CB) and step burs (SB) were used repeatedly. The wear rate of burs was analyzed before and after milling with a scanning electron microscope (SEM) and weighing the burs. The milling time of the blocks at different milling stages was recorded from 2 (M2) to 8 times (M8) of use. The surface roughness (Ra) of blocks was measured with a profilometer. Data from the differences in surface roughness were analyzed using univariate analyses of variance and Kruskal-Wallis tests (alpha = 0.05).

RESULTS

For M2, the highest roughness (2.12 ± 0.1 μm) was observed with the VM block. EM blocks presented the highest roughness values (1.82 ± 0.3 μm, 1.85 ± 0.1 μm, and 1.86 ± 0.04 μm) at M4, M6, and M8, respectively. While the highest Ra values were observed for VM (2.12 to 1.43 µm), LU provided the lowest mean Ra (1.62 to 1.33 µm) among the tested materials. After the repeated use of burs up to 8 times, the surface roughness of all the tested materials decreased. Milling of LU resulted in chip deposits on the CB surface in SEM images. The duration of cutting was higher for CD and EM materials than those of other materials, but the milling duration for VM, GC, and LU blocks was similar. SB burs exhibited more wear than CB after repeated milling.

CONCLUSIONS

A repeated number of millings more than 4 times decreased the surface integrity of the milling burs, increased the surface roughness of CAD/CAM blocks except for LU, and increased the milling duration. Increased material hardness was more detrimental on bur surface integrity.

CLINICAL RELEVANCE

During milling CAD/CAM blocks, clinicians should note that the surface integrity of milling burs decreases after 4 times of use as a function of material hardness where step burs wear quicker than pointed cylinder burs.

In-situ SEM cyclic nanoindentation of pre-sintered and sintered zirconia materials

Efficient diamond machining of zirconia requires a comprehensive understanding of repetitive diamond indentation mechanics. This paper reports on in-situ cyclic nanoindentations of pre-sintered and sintered zirconia materials performed inside a scanning electron microscope (SEM). In-situ SEM imaging of cyclic indentation processes and high-magnification SEM mapping of indentation imprints were conducted. The elastic and plastic behaviors of pre-sintered and sintered zirconia materials were investigated as a function of the cyclic nanoindentation number using the Sakai and Sakai-Nowak models. For pre-sintered zirconia, cyclic nanoindentation induced quasi-plastic deformation, causing localized agglomeration of zirconia crystals with microcracks and large cracking along the indentation edge. Severely compressed, fragmented, and pulverized zirconia crystals and smeared surfaces were also observed. For sintered zirconia, shear bands dominated quasi-plastic deformation with the formation of edge pile-ups and localized microfractures occurred at indentation apex and diagonals. All elastic and plastic behaviors for pre-sintered and sintered zirconia materials revealed significantly microstructure-dependent. Pre-sintered zirconia yielded significantly lower contact hardness, Young's moduli, resistance to plasticity, elastic deformation components, and resistance to machining-induced cracking, and higher elastic and plastic displacements than sintered state. Meanwhile, all the behaviors for the two materials were independent from the cyclic nanoindentation number. A model was proposed for cyclic nanoindentation mechanics, revealing their cyclic indentation-induced microstructural changes in the two zirconia materials. This study advances the fundamental understanding of nanoindentation mechanics of zirconia materials.

Microstructural responses of Zirconia materials to in-situ SEM nanoindentation

Development of optimal shaping processes for pre-sintered and sintered zirconia materials requires a fundamental understanding of damage and deformation mechanisms at small-scale contacts with diamond tools. This paper reports on responses of zirconia materials with distinct microstructures to nanoindentation associated with diamond machining using a Berkovich diamond indenter. In-situ nanoindentation was performed in a scanning electron microscope (SEM) and in-process filmed to record small contact events. Indentation morphology was SEM-mapped at high-magnifications. Although both pre-sintered porous and sintered dense zirconia materials mechanically revealed the quasi-plastic behavior in indentation, there were distinct responses of the two materials to quasi-plasticity at the microstructural level. For pre-sintered porous zirconia, the quasi-plasticity was attributed to shear faults resulting from breaking pore networks as microstructurally discrete interfaces, to lead to compression, fragmentation, pulverization and microcracking of zirconia crystals in indentation imprints. In contrast, sintered dense zirconia had shear band-induced quasi-plastic deformation, accompanied with localized tensile microfracture. A material index associated with the mechanical properties ranked the lower quasi-plasticity for pre-sintered porous zirconia than its sintered dense state, predicting more machining-induced damage in the former than the latter. Significantly higher indentation imprint volumes induced in indented pre-sintered porous zirconia than sintered dense state previses higher machining efficiency for the former than the latter. The microstructure-dependent indentation mechanisms provide the fundamental knowledge into micromechanics of abrasive machining of zirconia materials and may lead to a new microstructural design for zirconia materials to achieve a balanced machining efficiency and damage control.