rus
Issue #5/2021
S.V.Apresyan, M.A.Gadzhiev, K.S.Kravchuk, E.V.Gladkikh, G.Kh.Sultanova, A.A.Rusakov, A.S.Useinov
Analysis of the mechanical properties of the materials for dental structures after artificial ageing
Analysis of the mechanical properties of the materials for dental structures after artificial ageing
DOI: 10.22184/1993-8578.2021.14.5.260.269
The study of the dental structures mechanical properties behaviour after simulated ageing processing at long-term use is the main feature of the presented paper. The paper contains the test results for 3D printed materials and for the other ones manufactured by milling of workpieces. The hardness and elastic modulus were measured by the nanoindentation method, the linear wear and friction coefficients were measured by the abrasion resistance method, and the elastic modulus, strength and deformation were determined by three-point bending method.
The study of the dental structures mechanical properties behaviour after simulated ageing processing at long-term use is the main feature of the presented paper. The paper contains the test results for 3D printed materials and for the other ones manufactured by milling of workpieces. The hardness and elastic modulus were measured by the nanoindentation method, the linear wear and friction coefficients were measured by the abrasion resistance method, and the elastic modulus, strength and deformation were determined by three-point bending method.
Теги: 3d printing of biomaterials 3d-печать биоматериалов additive technologies deformation dental occlusal splints hardness modulus of elasticity strength аддитивные технологии деформация модуль упругости прочность стоматологические окклюзионные шины твердость
Analysis of the mechanical properties of the materials for dental structures after artificial ageing
INTRODUCTION
This research is sequel of the study described in [1].
It is a hard task to check the material behaviour during its regular long-term operation due to the large amount of time wasted for that. In this case, the artificial ageing can be useful [2].
That means increasing of the sample temperature relative to the operating one for a significant value (samples are placed into a climate chamber for accelerated ageing). Inside the climate chamber, the samples are submerged in water or other liquid, which is able to provide uniform heating of the samples. Such method is perfect for the materials used in the medical applications, because their operating temperature is not high, and they are often placed in a liquid medium.
The material properties, the manufacturing method and further processing substantively affect the properties of the final product. The material used for manufacturing of the dental structures can be obtained by various ways, including by milling or by the additive technologies [3]. Only experimental studies can confirm a durability of the product and its necessary technical characteristics. The nanoindentation is a widely spread method to characterize the near-surface mechanical properties [4]. A general view of the advantages of one material compared to another can be obtained in combination with the wear test [5] and bending test, which give information about bulk strength characteristics of a sample. The criteria of choosing the best material for the dental use are the high values of the mechanical properties and their preservation after ageing of samples.
RESEARCH METHODS AND MATERIALS
In this work, six groups of samples have been studied, see Table 1.
The procedure of accelerated artificial ageing of samples was performed using the KTHB-300 (NPF "Technology") climate chamber [6], see Fig.1. Each sample group was placed into a separate container) filled with distilled water. The temperature in the chamber was +80 °С. The accuracy of maintaining temperature was 0,5 °С.
This temperature was chosen due to following reasons. According to [7] dealing with the accelerated aging applied to the polymers used for medical purposes, the general approach is to assume that the rate of aging increases by factor f:
f = 2ΔT/10, (1)
here ΔT = T − Tref, Tref – reference temperature to detect the ageing effects, T – increased temperature to accelerate these effects. Temperature Tref will be equal to body temperature (37 °C) of the material implanted into a human body. It is believed in this case, that maintenance of the material for 1 month at 87 °C should be equivalent to ageing by 32 months.
In this work, the samples were maintained at +80 °C for eight days and two hours to simulate the ageing of one year.
Since liquid evaporation is more intensive at higher temperatures, the humidity in a climatic chamber was maintained at 80%. However, even under these conditions, it was necessary to add distilled water into the containers with samples every day.
Control of the sample roughness and measurements of the subsurface layers mechanical properties of samples were carried out before and after the accelerated ageing. Roughness was determined by a Sensofar S Neox optical profilometer [8], the instrumental hardness and the elastic modulus, as well as the frictional coefficients and linear wear – with a help of a "NanoScan-4D" nanohardness meter [9]. Three samples from groups mentioned in Table 1 were taken to make the experiments, and every sample was tested in three points: in a center and at a distance of 1 cm from left and right sample edges.
The hardness and elastic modulus were determined according to the Oliver and Farra method, prescribed in GOST Р 8.748 (which is the analogue of ISO 14577) [10]. Indentation was carried out with a standard Bercovich pyramid subjected to two loads, 1 and 10 mN.
A diamond semisphere dia. 130 µm was used as a counterface to measure the friction and linear wear coefficients. The standard load at wear was 250 mN, and every test consisted of one hundred cycles. The geometry of an abrasion groove is measured after an experiment which diagram is described in [11]. The conclusion about sample durability was made depending on the groove width. To measure the friction coefficient, the hard-face sphere dia. 1 mm was used besides the diamond semisphere dia. 130 µm because the big diameter sphere allows of finding this characteristic at small indentation depths. The linear wear and groove depths cannot be measured, because there is no visible destruction of the material.
Three-point bending tests of the samples were performed using Instron 5982 universal testing machine in accordance with GOST 31572-2012 [12]. The bending tests were carried out for the samples groups Vipi_original, Zirkon_oiginal, Printer3D_original (not polished). Five samples before and 5 samples after ageing (see Fig.2) have been chosen.
RESULTS AND DISCUSSIONS
3D topographic images of the polished samples relief before and after ageing are shown in Fig.3.
Table 2 indicates the following parameters of roughness: Ra – averaging roughness, Rz – roughness obtained at 10 points (average distance between 5 higher and 5 lower points on the relief image) [13, 14]. Here, Ra is the average deviation of the surface from the average level, Rz – maximum height discontinuity of the studied area. The data indicated in Table 2 is the result of averaging of nine measurements (in three areas on three samples). As the table below indicates, the ageing process leads to increasing of Rz by two or more times (see Table 2). The average roughness Ra is practically constant for non-polished samples, and grows after polishing by 40–50%.
Figure 4 indicates the hardness diagrams and the elastic moduli of the samples before ageing (columns with solid fill) and after ageing (columns with not solid fill). Scattering of values shown in the diagrams means the inaccuracy between measurements in different areas of the samples.
After ageing, polished and milled Vipi samples, as well as polished and printed on 3D printer samples did not show a significant change in the hardness and elastic module in comparison with the not aged samples. The hardness of polished and milled Zirkonzahn samples grows after ageing more than 40%, and the elastic module was stable. The highest hardness growth (by 4 times) and elastic modulus (by 2 times) were observed with the polished samples printed on a 3D printer. The abrasion test results obtained by a semisphere dia. 130 µm, were also averaged for nine measurements as shown in Fig.5. As can be seen from the diagrams, the geometric parameters of the grooves and durability are still within the measurement accuracy.
The friction coefficient measured with a sphere dia. 1 mm by the abrasion method (see Fig.6) differs significantly (higher in 2–4.5 times) practically in all aged samples, except the milled Zirconzahn samples. The sphere of a smaller diameter is indented in a sample to a greater depth with the same load, hence, it penetrates the deeper sample layers. Comparison of the results indicated in Fig.5 and Fig.6 shows that ageing changes the surface layers structures more significantly than the deeper ones. Table 3 shows the three-point bending test results before and after ageing. Table 3 indicates that after ageing the elastic modulus, which characterizes the bulk properties of samples, significantly increases. The relative deformation value decreases two times for the Zirkon sample.
CONCLUSIONS
The accelerated ageing of the occlusive splint materials simulating the processes in a human cavitas oris in time has been done. Now it is clear that a number of differences between the milled materials and the materials produced with additive technologies exist.
As to non-polished samples, the highest roughness (before and after ageing) has a material printed on a 3D printer than the milled Vipi and Zirkonzahn samples. The roughness of all polished samples turned to be higher by two or more times after ageing.
The friction coefficient measured with the use of a sphere dia. 1 mm is associated with a roughness index Rz for all tested samples. The higher the roughness growth, the greater the friction coefficient was increasing. This fact confirms a significant change in the surface relief due to the ageing observed by confocal optical microscopy.
When using a sphere dia. 130 µm at abrasive wear, the width and depth of the grooves, friction coefficient, and linear wear do not differ from the initial samples and the samples after the accelerated ageing. The Vipi samples have a high durability because of the lower values of linear wear coefficient, and depths and widths of the grooves.
The Vipi samples hardness has a stable high value before and after artificial ageing, that correlates well with the top value of durability for the tested samples groups. The greatest difference in hardness after ageing was measured for the initial sample printed on a 3D printer, while hardness before ageing (0.05 GPa) increased four times. The Zirkonzahn samples show a little hardness growth (by 25%) after the artificial ageing. The Vipi samples elastic modulus obtained on a 3D printer has not changed practically after ageing. Note, that the elastic modulus of the Zirkonzahn samples decreases by 30% both before and after polishing.
Three-point bending tests were performed on non-polished samples. Before ageing, the hardness and elastic modulus of the milled Vipi and Zirkonzahn samples were by 1.5 times greater than the corresponding values of the samples printed on a 3D printer. After ageing, the elastic modulus grows two times for the Vipi samples and 3–4 times for the Zirkonzahn and printed samples. Hence, the hardness and elastic modulus measured by an indentation method correlate with the elastic modulus value obtained in the bending tests.
Based on the obtained data. it can be concluded that the materials printed on a 3D printer, in comparison with those produced by the traditional methods, have smaller strength that can be explained by incomplte polymerization after printing.
The ageing procedure has shown, that thermal treatment of the samples produced by additive technologies can ensure achievement of the necessary mechanical properties, recommended by GOST [12].
ACKNOWLEDGEMENTS
This work was performed with the use of the CCU TISNCM equipment.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
INTRODUCTION
This research is sequel of the study described in [1].
It is a hard task to check the material behaviour during its regular long-term operation due to the large amount of time wasted for that. In this case, the artificial ageing can be useful [2].
That means increasing of the sample temperature relative to the operating one for a significant value (samples are placed into a climate chamber for accelerated ageing). Inside the climate chamber, the samples are submerged in water or other liquid, which is able to provide uniform heating of the samples. Such method is perfect for the materials used in the medical applications, because their operating temperature is not high, and they are often placed in a liquid medium.
The material properties, the manufacturing method and further processing substantively affect the properties of the final product. The material used for manufacturing of the dental structures can be obtained by various ways, including by milling or by the additive technologies [3]. Only experimental studies can confirm a durability of the product and its necessary technical characteristics. The nanoindentation is a widely spread method to characterize the near-surface mechanical properties [4]. A general view of the advantages of one material compared to another can be obtained in combination with the wear test [5] and bending test, which give information about bulk strength characteristics of a sample. The criteria of choosing the best material for the dental use are the high values of the mechanical properties and their preservation after ageing of samples.
RESEARCH METHODS AND MATERIALS
In this work, six groups of samples have been studied, see Table 1.
The procedure of accelerated artificial ageing of samples was performed using the KTHB-300 (NPF "Technology") climate chamber [6], see Fig.1. Each sample group was placed into a separate container) filled with distilled water. The temperature in the chamber was +80 °С. The accuracy of maintaining temperature was 0,5 °С.
This temperature was chosen due to following reasons. According to [7] dealing with the accelerated aging applied to the polymers used for medical purposes, the general approach is to assume that the rate of aging increases by factor f:
f = 2ΔT/10, (1)
here ΔT = T − Tref, Tref – reference temperature to detect the ageing effects, T – increased temperature to accelerate these effects. Temperature Tref will be equal to body temperature (37 °C) of the material implanted into a human body. It is believed in this case, that maintenance of the material for 1 month at 87 °C should be equivalent to ageing by 32 months.
In this work, the samples were maintained at +80 °C for eight days and two hours to simulate the ageing of one year.
Since liquid evaporation is more intensive at higher temperatures, the humidity in a climatic chamber was maintained at 80%. However, even under these conditions, it was necessary to add distilled water into the containers with samples every day.
Control of the sample roughness and measurements of the subsurface layers mechanical properties of samples were carried out before and after the accelerated ageing. Roughness was determined by a Sensofar S Neox optical profilometer [8], the instrumental hardness and the elastic modulus, as well as the frictional coefficients and linear wear – with a help of a "NanoScan-4D" nanohardness meter [9]. Three samples from groups mentioned in Table 1 were taken to make the experiments, and every sample was tested in three points: in a center and at a distance of 1 cm from left and right sample edges.
The hardness and elastic modulus were determined according to the Oliver and Farra method, prescribed in GOST Р 8.748 (which is the analogue of ISO 14577) [10]. Indentation was carried out with a standard Bercovich pyramid subjected to two loads, 1 and 10 mN.
A diamond semisphere dia. 130 µm was used as a counterface to measure the friction and linear wear coefficients. The standard load at wear was 250 mN, and every test consisted of one hundred cycles. The geometry of an abrasion groove is measured after an experiment which diagram is described in [11]. The conclusion about sample durability was made depending on the groove width. To measure the friction coefficient, the hard-face sphere dia. 1 mm was used besides the diamond semisphere dia. 130 µm because the big diameter sphere allows of finding this characteristic at small indentation depths. The linear wear and groove depths cannot be measured, because there is no visible destruction of the material.
Three-point bending tests of the samples were performed using Instron 5982 universal testing machine in accordance with GOST 31572-2012 [12]. The bending tests were carried out for the samples groups Vipi_original, Zirkon_oiginal, Printer3D_original (not polished). Five samples before and 5 samples after ageing (see Fig.2) have been chosen.
RESULTS AND DISCUSSIONS
3D topographic images of the polished samples relief before and after ageing are shown in Fig.3.
Table 2 indicates the following parameters of roughness: Ra – averaging roughness, Rz – roughness obtained at 10 points (average distance between 5 higher and 5 lower points on the relief image) [13, 14]. Here, Ra is the average deviation of the surface from the average level, Rz – maximum height discontinuity of the studied area. The data indicated in Table 2 is the result of averaging of nine measurements (in three areas on three samples). As the table below indicates, the ageing process leads to increasing of Rz by two or more times (see Table 2). The average roughness Ra is practically constant for non-polished samples, and grows after polishing by 40–50%.
Figure 4 indicates the hardness diagrams and the elastic moduli of the samples before ageing (columns with solid fill) and after ageing (columns with not solid fill). Scattering of values shown in the diagrams means the inaccuracy between measurements in different areas of the samples.
After ageing, polished and milled Vipi samples, as well as polished and printed on 3D printer samples did not show a significant change in the hardness and elastic module in comparison with the not aged samples. The hardness of polished and milled Zirkonzahn samples grows after ageing more than 40%, and the elastic module was stable. The highest hardness growth (by 4 times) and elastic modulus (by 2 times) were observed with the polished samples printed on a 3D printer. The abrasion test results obtained by a semisphere dia. 130 µm, were also averaged for nine measurements as shown in Fig.5. As can be seen from the diagrams, the geometric parameters of the grooves and durability are still within the measurement accuracy.
The friction coefficient measured with a sphere dia. 1 mm by the abrasion method (see Fig.6) differs significantly (higher in 2–4.5 times) practically in all aged samples, except the milled Zirconzahn samples. The sphere of a smaller diameter is indented in a sample to a greater depth with the same load, hence, it penetrates the deeper sample layers. Comparison of the results indicated in Fig.5 and Fig.6 shows that ageing changes the surface layers structures more significantly than the deeper ones. Table 3 shows the three-point bending test results before and after ageing. Table 3 indicates that after ageing the elastic modulus, which characterizes the bulk properties of samples, significantly increases. The relative deformation value decreases two times for the Zirkon sample.
CONCLUSIONS
The accelerated ageing of the occlusive splint materials simulating the processes in a human cavitas oris in time has been done. Now it is clear that a number of differences between the milled materials and the materials produced with additive technologies exist.
As to non-polished samples, the highest roughness (before and after ageing) has a material printed on a 3D printer than the milled Vipi and Zirkonzahn samples. The roughness of all polished samples turned to be higher by two or more times after ageing.
The friction coefficient measured with the use of a sphere dia. 1 mm is associated with a roughness index Rz for all tested samples. The higher the roughness growth, the greater the friction coefficient was increasing. This fact confirms a significant change in the surface relief due to the ageing observed by confocal optical microscopy.
When using a sphere dia. 130 µm at abrasive wear, the width and depth of the grooves, friction coefficient, and linear wear do not differ from the initial samples and the samples after the accelerated ageing. The Vipi samples have a high durability because of the lower values of linear wear coefficient, and depths and widths of the grooves.
The Vipi samples hardness has a stable high value before and after artificial ageing, that correlates well with the top value of durability for the tested samples groups. The greatest difference in hardness after ageing was measured for the initial sample printed on a 3D printer, while hardness before ageing (0.05 GPa) increased four times. The Zirkonzahn samples show a little hardness growth (by 25%) after the artificial ageing. The Vipi samples elastic modulus obtained on a 3D printer has not changed practically after ageing. Note, that the elastic modulus of the Zirkonzahn samples decreases by 30% both before and after polishing.
Three-point bending tests were performed on non-polished samples. Before ageing, the hardness and elastic modulus of the milled Vipi and Zirkonzahn samples were by 1.5 times greater than the corresponding values of the samples printed on a 3D printer. After ageing, the elastic modulus grows two times for the Vipi samples and 3–4 times for the Zirkonzahn and printed samples. Hence, the hardness and elastic modulus measured by an indentation method correlate with the elastic modulus value obtained in the bending tests.
Based on the obtained data. it can be concluded that the materials printed on a 3D printer, in comparison with those produced by the traditional methods, have smaller strength that can be explained by incomplte polymerization after printing.
The ageing procedure has shown, that thermal treatment of the samples produced by additive technologies can ensure achievement of the necessary mechanical properties, recommended by GOST [12].
ACKNOWLEDGEMENTS
This work was performed with the use of the CCU TISNCM equipment.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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