Metal-ceramic bond strength of a cobalt chromium alloy for dental prosthetic restorations with a porous structure using metal 3D printing
Introduction
Additive manufacturing (AM), which is generally known as 3D printing (3DP) or rapid prototyping (RP), was first introduced in the late 1980s [[1], [2], [3]]. Since then, 3DP technology [4] has been used to produce accurate one-off, complex 3D geometrical structures from digital data using a variety of materials. Compared with conventional manufacturing methods, this technology is particularly useful for dentistry applications, particularly considering the advances that have been made in 3D imaging and modelling technologies. As such, 3DP has been used to overcome limitations inherent to the processing of materials for various applications.
Over the past few years, 3DP technology has developed rapidly. Due to the recent advances in multi-material printing methods, selective laser sintering (SLS) [5,6] technology is now able to handle a wide range of materials (such as wax, cermet, ceramics, and metal-polymers) for the creation of solid structures, in which a laser is aimed automatically at points in space defined by a 3DP model. The SLS method [7] allows parts to be fabricated without requiring any part-specific tools, meanwhile, it also can shortens the design and production cycle, and promises to revolutionize traditional manufacturing processes by significantly reducing time and costs.
The ability to produce high-quality AM products illustrates that SLS with 3DP offers significant technical advantages compared with traditional casting and CAD/CAM techniques in the manufacture of dental restorations [[8], [9], [10]]. As a consequence, the development provides a new direction for the establishment of medical models and progress in clinical trials in dental medicine. Porcelain-fused-to-metal (PFM) [[11], [12], [13]], which is used to provide strength to a crown or bridge, has a translucency that mimics natural tooth enamel, a characteristic that is particularly desirable. Because metals possess excellent compressive strength and high fatigue resistance, porous metallic scaffolds made of titanium (Ti) and tantalum (Ta) and biocompatible alloys such as Co–Cr alloys have been proposed as teeth replacement materials. These restorations are very strong, durable, and resistant to wear because the combination of porcelain and metal creates a stronger restoration than porcelain alone.
The improvement in the precision of metal printing has led to advantages for computerized dentistry. However, in stomatology, dental restoration is a precision technology that requires not only post-processing but also a higher printing precision than in other fields. Repairing teeth is still expensive and time-consuming because the cermet frequently fractures after teeth restoration. Therefore, in order to prolong the life of a porcelain-fused-metal (PFM) crown, sufficient bonding strength between the metal substrate and the porcelain veneer is crucial.
In this respect, SLS printing still struggles to accurately print complex and highly developed architectural structures, such as complex hive structures, due to the porous surface of metal alloys [14,15]. In response to this, dental researchers have developed a method of sealing the porous surface by applying a coating to increase the bond strength between the Co–Cr alloy substrate and the overlying porcelain [[16], [17], [18]]. However, little relevant data regarding the success of this approach is available for different types of Co–Cr alloy substrate manufactured using SLS printing technology.
Therefore, the goal of this study is to evaluate whether the nature of the porous structure of the metal substrate has a positive effect on the bond strength between it and the ceramic layer. We use SLS with 3DP to manufacture Co–Cr alloy substrates with different porous structures to determine the effect of these structures in dental restorations. We also employ computer simulations to predict the bonding strength of our designed porous structures with the ceramic layer using ABAQUS software.
Section snippets
Materials & methods
We first designed three types of Co–Cr alloy porous specimen (hole-free, circular-hole, and rhombic-hole specimens) and fabricated 30 samples (10 samples per type for a total of 30) in a stereolithography (STL) file using Solidworks® software (Fig. 1). We then used ABAQUS to simulate the bonding strength of the three specimen types with the ceramic layer. In the three-point bending simulations, we set the distance from the probe head to the plate top surface at 1 mm. The loading force was
Results and discussion
In the present research, we used SLS printing technology with a Co–Cr metal alloy to print three different structures (hole-free, circular-hole, and rhombic-hole specimens) and evaluate the bonding force between these specimens and a ceramic layer. Unlike the simulations results, the experimentally obtained bonding strength indicated that the metal-ceramic combination has no obvious advantages over other combination types using most conventional processes [20,21]. However, for SLS metal
Conclusion
In this research, three types of metal-ceramic specimen with different structures were fabricated using SLS, and it was found that the bond strengths of the three designs were all higher than the international minimum standard of 25 MPa set by ISO1999/9693. Therefore, these designs can be applied to teeth restoration. the roughness of the specimens produced with SLM method was considerably higher than that for SLS-derived specimens, resulting in a higher bonding strength for SLM over SLS. And
Acknowledgements
This work was supported by the Technology Development Program of Ministry of SMEs and Startups (MSS) [C0511440], the Technology Innovation Program funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) [10073062] and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) [2018R1A5A7023490].
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