Additive manufacturing of lithium disilicate glass-ceramic by vat polymerization for dental appliances

Abstract

Objectives: The objectives of this study were to evaluate the mechanical properties of lithium disilicate components produced by additive manufacturing (AM) and to assess the effect of build orientation on the resistance to fracture.

Methods: Oversized bars were printed with a glass-filled photoactive resin using a digital light processing technique. After sintering and post-processing, flexure and chevron notch fracture toughness bars were obtained in three principal orientations (0°, 45°, and 90°) with respect to the build direction. Mechanical properties were obtained according to the relevant ASTM standards. The hardness, indentation fracture resistance, and elastic modulus were measured for each orientation, and a Weibull analysis was conducted with the flexure responses. Fractography of the fracture surfaces was performed to identify the failure origins.

Results: The 0° orientation exhibited characteristic strength, Weibull modulus, and elastic modulus of 313 MPa, 4.42, and 168 ± 3 GPa, respectively, which are comparable to lithium disilicate materials from traditional processes. However, build orientation contributed significantly to the flexure strength, elastic modulus, and Weibull modulus; the characteristic strengths for the 45° and 90° build orientations were 86 MPa and 177 MPa, respectively. The primary contribution to the orientation dependence was the number of residual build layer-related flaws from incomplete union between printed layers. Of note, hardness and the fracture toughness were not dependent on build orientation.

Significance: AM of lithium disilicate materials can achieve the mechanical properties of materials produced by traditionally processing. Thus, while further process development is warranted, the outlook for dentistry is promising.

Keywords: Additive manufacturing; Defect analysis; Digital light projection; Lithium disilicate glass ceramics.

Figure 1:

The three build orientations investigated in this study. Bars show build orientation (inset parallel lines representing the layer boundaries) with respect to loading schema for the flexure specimens. Chevron notch specimens were loaded following the same schema. However, the geometry of the specimens was slightly modified as required by the appropriate standard.

Figure 2:

Comparison of Vicker’s hardness values for each build orientation. There was no significant difference in hardness between the three orientations (α=0.05).

Figure 3:

Comparison of indentation fracture resistance measurements for each build orientation. * indicates groups that are significantly different (α=0.05).

Figure 4:

Comparison of fracture toughness (K_Ic) for each build orientation as measured using the chevron notch method (n=10 for each orientation). These values were not significantly different (α=0.05).

Figure 5:

Weibull plot of all sample groups tested. There is a substantial influence of build orientation on the strength distributions as evident in this figure. Due to premature failure of specimens that occurred during net-shape machining, only nine specimens of the 90° orientation were available for testing.

Figure 6:

Elastic moduli of each orientation group. * indicates groups that are significantly different (α=0.05). The 0° and 90° groups are not significantly different from one another.

Figure 7:

Results of fractography for the 0° orientation group. (a) censored Weibull plot, (b-d) examples of fracture surfaces that exhibited layer line flaws. In all fracture surface images, both halves of the specimens are shown with the tensile surfaces back-to-back in the center to make identification of a mirror easier and the origin is indicated with a white triangle pointer. Other fracture features of interest (e.g. layer line) are indicated with an arrow.

Figure 8:

Results of fractography for the 45° orientation group. (a) censored Weibull plot, (b) example of fracture surface that exhibited a layer-line flaw. (c-d) side view of specimens that failed due to layer-line flaws. In all specimen images, both halves of the specimens are shown with the tensile surfaces back-to-back in the center to make identification of the origin easier. When appropriate, the origin is indicated with a white triangle pointer. Other fracture features of interest are indicated with an arrow.

Figure 9:

Results of the fracture surface examination for the 90° orientation group. (a) censored Weibull plot (b-c) examples of fracture surfaces that exhibited layer line flaws. (d) side view of a specimen that failed due to layer line flaw. Both halves of the specimens are shown in all images with the tensile surfaces back-to-back in the center to make identification of the origin easier. When appropriate, the origin is indicated with a white triangle pointer. Other fracture features of interest are indicated with an arrow.

Figure 10:

Representative image of heterogenous microstructure of the lithium disilicate that was observed on all indentation surfaces.