Metal-ceramic bond strength of a cobalt chromium alloy for dental prosthetic restorations with a porous structure using metal 3D printing

Highlights

• Selective laser sintering (SLS) leads to a new type of Additive Manufacturing (AM) in dental restoration.

• The bond strength of simple shape porous Co–Cr alloy can achieve the ISO standard.

• During the SLS process, balling phenomenon tends to cause porosity and even delamination induced by poor inter-layer bonding.

Abstract

Selective laser sintering (SLS) is a new type of additive material manufacturing technology. The development of precise 3D metal printing technology has enabled the printing of complicated metal structures, particularly in the medical field. Finding a way to integrate new technologies with Co–Cr alloys for the precision manufacturing of dental restoration materials and a way to improve the metal-ceramic bonding strength of the materials have become a key focus of dental restoration clinical trials. The purpose of this study is to evaluate bonding strength and ceramic adhesion between metal and ceramics using Co–Cr specimens with different porous structures manufactured using SLS technology. According to the international standard ISO9693:1999, we printed three sets of 10 rectangular Co–Cr alloy test specimens of the same size (25 × 3 × 0.5 mm) using an SLS-3D metal printer and fused a ceramic layer (8 x 3 × 1.1 mm) to the center of the Co-Cy alloy test specimen. Before testing, we conducted stress and fracture simulation analysis on three specimen types (no holes, circular-shaped holes, and rhombic-shaped holes), using ABAQUS results to predict the results of three-point bending tests. These simulation results were then compared with the experimental data. We used three-point bending tests to assess the bonding strength of the fabricated metal-ceramic bonding surface. We also used a digital microscope (100× and 200×) to observe the surface conditions of the samples. Finally, we analyzed the results using one-way analysis of variance. The ABAQUS bending simulations indicated that the bending energy decreased sequentially for the hole-free, circular-hole and rhombic-hole specimens. Similarly, when the three types of test specimen were manufactured using SLS, significant differences in bending energy were observed between the rhombic-hole specimens and both the hole-free (P < 0.05) and circular-hole specimens (P < 0.05). In addition, the bond strength for all groups was higher than the international minimum standard of 25 MPa (33.36 ± 3.17 MPa). In this research, the bond strength of all three metal-ceramic test specimens was higher than the international minimum standard of 25 MPa set by ISO1999/9693. However, the circular porous design did not show previous diversity with other porous shape design. In addition, due to limitations in the accuracy of 3D printing using SLS, the structural advantages our proposed specimen design are difficult to verify. Therefore, we plan to develop new structural designs to improve the bonding strength of metal-ceramic structures in future work.

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.