Abstract
Background:
The incorporation of digital dentistry into dental practices has greatly impacted the conventional dental workflow. This process has given clinicians access to additional treatment options, with more predictable outcomes for fabrications of prostheses by enabling a wider selection of dental materials and manufacturing techniques; one such material is cobalt chromium (CoCr). CoCr alloy is a favourable material choice due to its low cost and high physical strength, however, the outcome of traditional casting methods of CoCr alloy can be unpredictable and is heavily reliant on the skills of the technician. Computer-aided manufacturing can produce specimens with much more predictable properties, although, milling of fully sintered cobalt chromium has a high wear rate on milling components during the fabrication process due to the inherent hardness of the alloy. More recently, two techniques using CoCr powder have been developed. One involves milling of soft polymer bound CoCr (SM) which is then sintering to full density, the other is 3D printing/laser sintering (LS) of loose CoCr powder. Both of these manufacturing methods greatly reduce stress on the components of their respective machines. Published research on cobalt chromium fabricated using these two manufacturing methods, however, suffers from a lack of standardization making it difficult to draw clear conclusions on their in-vitro properties.
Aim:
To compare the properties of additive manufacturing/laser sintered cobalt chromium alloy with subtractive manufacturing/ soft milling of cobalt chromium alloy in regards to mechanical properties, bonding to porcelain, and microstructural characteristics/behaviour.
Method:
Chapter 2: Forty CoCr dumbbells were fabricated using the SM and LS manufacturing method with dimensions in accordance to the to ASTM E8 standard. The dumbbells were fractured under uniaxial tensions and calculations performed to determine the ultimate tensile strength, proof stress and elastic modulus. The fracture surfaces were examined using scanning electron microscopy. Rectangular plate specimens were also fabricated and subjected to testing using nanoindentation method to further examine elastic modulus and hardness values.
Chapter 3: Thirty-two rectangular specimens with dimensions 8 x 30 x 1.5 mm were fabricated using the methods outlined in Chapter 2. Thirty specimens were layered with porcelain and tested using 4-point strain energy release rate approach in accordance with the method by Suansuwan and Swain (1999) to measure the adhesion strength. Fracture surfaces were then visually examined and under scanning electron microscopy to determine the mode of failure. Nanoindentation was carried out on the remaining two specimens to calculate the changes in elastic modulus and hardness post-porcelain firings. The effects of porcelain firings on microstructure and phase composition of specimens were also examined using electron microscopy.
Results:
Chapter 2: The LS CoCr had a significantly higher ultimate tensile strength (1090 vs. 915.9 MPa) and proof stress (608.8 vs. 549.4 MPa) (P<0.05) than SM, while the difference in the elastic modulus values was not statistically significant (196.2 vs. 180.4 GPa). The elastic modulus calculated using nanoindentation was similar to that found using tensile testing (LS CoCr 197.0 GPa and SM CoCr 181.8 GPa). The hardness was also lower for the SM than LS CoCr (3.3 vs. 4.4 GPa). Examination of the dumbbell fracture surfaces showed uniform structure for the LS CoCr specimens whilst the SM CoCr specimens were perforated with porosities; neither the LS or SM specimens showed an obvious point of fracture.
Chapter 3: Adhesion energy of ceramic to LS CoCr was significantly higher than that of ceramic to SM CoCr (86.6 J/m2 vs. 76.9 J/m2) (P<0.05). The elastic modulus showed an increase in both specimens before and after the firing (LS 181.8 to SM 187.9 GPa vs. LS, 197 to SM 205.1 GPa). Lower hardness values of SM CoCr than LS CoCr were initially observed, however after ceramic firings, SM CoCr had a larger increase in hardness values at the interface than the LS CoCr (4.9 vs. 5.3 GPa). Electron backscatter diffraction (EBSD) mapping showed that both specimens had a regular grain structure, however SM CoCr had more localized changes in crystalline structure at the interface when compared to LS CoCr.
Conclusion:
Despite the SM CoCr having voids that directly affected the overall mechanical properties, both LS CoCr and SM CoCr results had better mechanical properties when compared to cast CoCr. The LS CoCr had a dense fine grained structure while SM CoCr had a fine grained structure. For the adhesion test, both LS and SM CoCr had higher bond strength to ceramic than the cast CoCr. An interesting relationship was found between increased hardness of interface and lower bond strength. Overall although LS CoCr had superior properties to SM CoCr, both computer-aided manufacturing methods were superior to reported properties of conventionally cast of CoCr, suggesting that both would be a suitable choice for long-span PFM prostheses or metallic frameworks for use in the mouth. Further in-vivo research is required to examine the performance of such prostheses in the oral cavity.