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\title{Investigation of Ceramic Dental Prostheses based on Zrsio4 -Glass Composites Fabricated by Indirect Additive Manufacturing}
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\begin{document}

             \author[1]{Marlon Wesley Machado Cunico,  Ph.D.}

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\date{\small \em Received: 7 December 2019 Accepted: 2 January 2020 Published: 15 January 2020}

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\begin{abstract}
        


Technologies for dental prosthesis and restoration have been developed during the past years. Despite the advantages of additive manufacturing, CAD/CAM technologies lead prostheses fabrication. Therefore, the main goal of this work is to investigate the fabrication of dental prosthesis based on ZrSiO4-glass composites applying the indirect fused deposition modelling. In order to achieve this goal, we used filaments filled by 90% of ZrSiO4 and 50?m glass spheres in order to fabricate prosthesis. We also applied multivariable approach to scan the feasibility of the proposed process. Holding temperature, holding time, heating rate and cooling rate were considered the control factors while shrinkage, flexural strength, process feasibility were the study responses. It was possible to see the proposal feasibility for holding temperatures between 700 and 800°C and holding time between 1 and 4 hours. Additionally, the materials flexural strength was found between 25 and 85MPa, while shrinkage fluctuated between 10 and 25%.

\end{abstract}


\keywords{FDM ceramic sintering, additive manufacturing, ceramics.}

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\let\tabcellsep& 	 	 		 
\section[{Introduction}]{Introduction}\par
uring the last years, additive manufacturing (AM) technologies started playing an important role in several segments \hyperref[b1]{(Cunico and de Carvalho, 2016)}. In addition, development of dental materials and applications points to digital fabrication whereas dental implants are majorly fabricated by CAD/CAM techniques in collaboration with 3D scanning \hyperref[b8]{(Sulaiman, 2020}\hyperref[b6]{, Li et al., 2014)}.\par
Table  {\ref 1} shows the most common techniques which are used in accordance with the type of dental prosthesis. Therefore, it is possible to see that CAD/CAM is currently the most used technique in prosthodontics \hyperref[b5]{(Karthick et al., 2019)}. On the other hand, additive manufacturing is still used for medical/dental models and temporary dentures, whereas there are not long clinical records and mechanical strength is usually lower than CAD/CAM technologies \hyperref[b8]{(Sulaiman, 2020}\hyperref[b6]{, Li et al., 2014}\hyperref[b4]{, Gali and Sirsi, 2015}\hyperref[b5]{, Karthick et al., 2019)}. On the other hand, additive manufacturing might bring the fabrication of dental prosthesis to the next level, increasing automation, flexibility, shape freedom and fabrication speed. Nevertheless, material and technology restrictions, such as mechanical strength, type of material, bio reactivity and cost are still challenges to be overcome \hyperref[b7]{(Lin et al., 2019)}. 
\section[{Investigation of Ceramic Dental Prostheses based on Zrsio4 -Glass Composites Fabricated by Indirect Additive Manufacturing}]{Investigation of Ceramic Dental Prostheses based on Zrsio4 -Glass Composites Fabricated by Indirect Additive Manufacturing}\par
For that reason additive manufacturing technologies are widely used for:\par
Provisional crown and bridge restorations, casting patterns, dental models, surgical guide and splints. On the other hand, AM technologies still need to be developed in order to apply in dental implants, crown and bridges denture and prosthetic constructions.\par
Therefore, the main goal of this work is to propose and investigate the feasibility of a novel dental prosthesis fabrication method which is based in ZrSiO4glass composite, as illustrated in Figure \hyperref[fig_0]{1}. In this method, the collapsible ZrSiO4 mould of negative of crown or bridge is fabricated by additive manufacturing based on fused filament. Therefore, the negative cavity is filled by Lanthanum glass powder in order to be subsequently sintered. By the end, the pieces are heated in order to debind the negative structure and then create sintered composite pieces.\par
In order to evaluate the feasibility of this technique, we applied multivariable techniques to investigate the main effect of control factors on responses. Holding temperature, holding time, heating rate, cooling rate and shrinkage chamber were the control factors while shrinkage, flexural strength, process feasibility were the study responses. We have also kept the fabrication parameters, materials formulation, flexural testing specimen shape and crown shape constant.\par
As a consequence, it was possible to identify whether the proposed process has potential feasibility to make dental prosthesis. 
\section[{II.}]{II.} 
\section[{Material and Methods}]{Material and Methods}\par
In order to investigate the feasibility of the proposed process in addition to the main effect of control factors on responses, we applied a 2 k multivariable methodology (full design with body central point) where: Holding temperature (T h ), holding time (t h ), heating rate (R h ) and cooling rate (R c ) were the control factors. In addition, we also defined 3 step screening in augmented design approach in order to minimize the holding time and maximize densification and mechanical properties.\par
The levels and values of each control factor is presented in Table  {\ref 2}, where it is possible to see that the values of holding temperature are between the activation temperature of glass and ZrSiO4 (700°C) and the melting temperature of glass (1078°C).\par
It is also possible to indicate that the cooling time enhance two types of heat treatment (Quenching for fast cooling and annealing for slow cooling). Therefore, it is possible to see the effect of such treatments on, crystallization level, mechanical strength and geometry distortion.\par
On the other hand, holding time and heating rate are expected to affects the sinterization parameters, such as nucleation, grain growth and diffusion. 
\section[{Table 2: Experiment Design}]{Table 2: Experiment Design}\par
For the sintering process, we used a 2000W electrical Furnace PID controller with 4 ramps curves and insulation muffle. The main control parameters are illustrated in Figure \hyperref[fig_2]{2} in addition to the schematic sintering temperature curves that we applied in this study. The flexural testing was performed in accordance with ISO 6872 in an EMIC DL10000 universal testing machine.\par
For the image processing, we used the software MATLAB, while the image acquisition was performed by the optic microscope Digital Avangard Optics AN-E500 (AVANGARD 2011). This optical microscope provides until 500x of amplification magnitude. For the gravimetric analysis (drying monitoring), we used a 0.005g error scale.\par
In order to identify the specimen dimensional distortions, we used 0.05mm calliper besides computational image processing and MATLAB software to evidence the geometrical variation of object external contour.\par
In order to measure feasibility response, we established a scale from 0 to 1. In this scale, 1 level indicates that object has no significant distortions and is feasible to be used. In addition, 0.75 levels expose minor distortion which corresponds to 5\% of distortion. Likewise, 0.5 feasibility level indicates that the object has 10\% of distortion.\par
It is important to note that although shrinkage is a sort of volumetric distortion, feasibility response only analysed non volumetric distortions.\par
For specimens fabrication, we used a FDM process and filament filled by 90\% of ZrSiO4. The main process parameters were remained constants, where extrusion temperature was 220°C, layer thickness was 0.1mm; distance between filaments was 0. nozzle diameter was 0.4mm. Additionally, we have also considered no support material and no retract to build the specimens. In all the cases, the extrusion temperature and chamber temperature were also kept constants, while no bed temperature was established. The fabrication environment was also controlled in 25°C of environment temperature and 50\% of relative humidity. 
\section[{III.}]{III.} 
\section[{Results and Discussions}]{Results and Discussions}\par
In general lines, the evaluation of concept feasibility was satisfactory, whereas a feasible process window was identified. From geometric point of view, the crown was obtained in low sintering temperatures while high levels of temperature distorted the geometry because of excessive melting and high shrinkage.\par
With regards to the main effect of control factors on the feasibility, mechanical strength and shrinkage, Figure \hyperref[fig_4]{3} indicates that holding temperature causes the strongest effect on the feasibility and flexural strength in comparison with the other control factors. On the other hand, holding time is the factor that affects shrinkage the most.\par
In this diagram, it is possible to see that holding time and holding temperatures are the most relevant factors for the augmented design. Therefore, the second screening round focused in increase the detail of feasible areas.\par
The flexural strength was also affected by heating rate, indicating that densification of material might have reduced the strength of material. With regards to the feasibility of proposed concept, the main effect diagram (Figure \hyperref[fig_5]{4}) indicated that high level of temperature implies on the unfeasibility of concept apart from time holding, cooling rate and heating rate. It is also possible to see that holding temperature is the most relevant parameter for the feasibility, being followed by holding time. In contrast, heating and cooling rates were found not to affect feasibility.\par
Continuing analysing the concept from the geometric point of view, it was possible to see that the shrinkage varied from 13.4\% to 27\% into the feasible area. The lowest value of shrinkage was found for heating rate equal to 5°C/min, holding temperature of 700°C, holding time equal to 3.25h and cooling rate of 30°C/min. Likewise, the highest value found resulted from heating rate equal to 2°C/min, holding temperature of 700°C, holding time equal to 4h and cooling rate of 2°C/min. In this case, Figure \hyperref[fig_5]{4} indicates the main effect of control parameter shrinkage. It is possible to see that holding temperature is the factor which is the most relevant for the shrinkage, while heating rate presented the smallest effect among the control parameters.\par
With respect to material mechanical strength, Figure \hyperref[fig_5]{4} also indicates the main effect of control parameter on flexural strength. It is possible to see that temperature and heating rate are the most relevant factors for the mechanical strength, while cooling rate presented the smallest effect among the control parameters.\par
The mean values of flexural strength varied from 25 to 82MPa, where the lowest values were found in low holding temperatures (700°C) and short holding time (1h). On the other hand, the highest values were obtained by long hold time (1h) and 800°C of holding temperature.\par
It is also important to indicate that this process does not evaluate the effects of neither heating treatment nor material, therefore. Further studies still need to be done in order to apply stronger materials and heating treatments in this concept.\par
Additionally, Figure \hyperref[fig_6]{5}  We also separated the feasibility in 2 areas, whereas high grain growth is obtained in low temperature (700°C) during long holding time (4h). In these areas, it is possible to see a specimen with high densification. On the other hand, low grain growth is obtained in low temperatures (700°C) during short periods of time (1h), where the specimen presented low densification. Noteworthy, the centre point of the study indicated a limit of feasibility so that the sintering process can be directed correlated to the absorbed energy as a function of time and Temperature.\par
The unfeasible area is highlighted by high densification, and excess of deformation because of viscosity decrease. In addition, this reduction implied on mould infiltration and subsequent incrustation of collapsed mould on object surface.\par
It was seen that the material obtained in high temperature and long holding time implied on high densification and grain size which are bigger than grain in material from low temperatures. This situation can be seen in Figure \hyperref[fig_7]{6}, where the comparison of material densification and grain size is presented.   
\section[{Conclusions}]{Conclusions}\par
To sum up, this work evidenced the feasibility of the glass-ceramic fabrication based on collapsible additive manufactured mould of ZrSio4. The working proof of this concept generates new perspectives to AM in dentistry, ceramics and medical applications, whereas this collapsible AM mould supports up than 2300°C.\par
This study identified that the holding temperature was the factor that mostly influences the feasibility, strength and shrinkage, being followed by holding time. In this case, holding time directly affects the material densification and grain growth. As consequence, long time and high temperatures increases the densification and soften the material so that the geometry is distorted and the process become unfeasible.\par
On the other hand, flexural strength fluctuated between 25 and 82MPa, and has been highly affected by heating rate and cooling rate. However, this study did not analyse heating treatment. Therefore, further studies are still needed to be done in order to apply new materials, heating treatments and increase mechanical and geometrical properties.\begin{figure}[htbp]
\noindent\textbf{1}\includegraphics[]{image-2.png}
\caption{\label{fig_0}Table 1 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{1}\includegraphics[]{image-3.png}
\caption{\label{fig_1}Figure 1 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{2}\includegraphics[]{image-4.png}
\caption{\label{fig_2}Figure 2 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{3}\includegraphics[]{image-5.png}
\caption{\label{fig_4}Figure 3 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{4}\includegraphics[]{image-6.png}
\caption{\label{fig_5}Figure 4 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{5}\includegraphics[]{image-7.png}
\caption{\label{fig_6}Figure 5 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{6}\includegraphics[]{image-8.png}
\caption{\label{fig_7}Figure 6 :J}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{7}\includegraphics[]{image-9.png}
\caption{\label{fig_8}Figure 7 :YearJ}\end{figure}
 			\footnote{© 2020 Global Journals} 		 		\backmatter   			 
\subsection[{Acknowledgement}]{Acknowledgement}\par
The authors would like to thank Concep3D R\&D and FAE Centro Universitário for the support and infrastructure. 			  			  				\begin{bibitemlist}{1}
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