rus
Issue #3-4/2021
M.A.Gadzhiev, K.S.Kravchuk, E.V.Gladkikh, G.Kh.Sultanova, A.A.Rusakov, A.S.Useinov, S.V.Apresyan
Comparative study of volumetric and surface mechanical properties of the materials for dental constructions obtained using additive technologies
Comparative study of volumetric and surface mechanical properties of the materials for dental constructions obtained using additive technologies
DOI: 10.22184/1993-8578.2021.14.3-4.196.204
This paper presents the results of testing the instrumental hardness and elastic modulus, linear wear and friction coefficients of the original and polished samples for dental structures obtained using additive technologies. In addition to the study of local mechanical properties, volumetric characteristics – elastic modulus, strength and deformation – were measured using the three-point bending method. The surface of the samples subjected to the bending test did not undergo sample preparation.
This paper presents the results of testing the instrumental hardness and elastic modulus, linear wear and friction coefficients of the original and polished samples for dental structures obtained using additive technologies. In addition to the study of local mechanical properties, volumetric characteristics – elastic modulus, strength and deformation – were measured using the three-point bending method. The surface of the samples subjected to the bending test did not undergo sample preparation.
Теги: 3d printing of biomaterials 3d-печать биоматериалов deformation dental occlusal splints hardness modulus of elasticity strength деформация модуль упругости прочность стоматологические окклюзионные шины твердость
INTRODUCTION
Materials science is the area of knowledge that is widely used not only for technical applications, but also in medicine. In particular, modern dentistry is based on improving biomaterials, methods of their obtaining and processing [1]. The materials for manufacture of dental structures must have a number of characteristics depending on the location and the functions performed [2].
In the case of the temporomandibular joint pathologies, a splint therapy is one of the common treatment methods [3, 4]. Occlusal splints are manufactured by various methods: thermal and press, cold and hot polymerization. Special attention is drawn to the possibility of manufacturing such elements with additive technologies [5]. 3D printing in combination with modern software and computed tomography seems to be a very promising direction in order to easily get elements possessing unique and specific geometry suitable for a patient [6]. However, not only the method of manufacturing should be taken into account, but the material itself, because during an operation, the dental structures are experiencing a large load and a rational choice of material and the method of manufacture directly affect the quality of treatment [7].
It is possible to decide on the use of a particular material and method of its manufacture by applying the quantitative criteria for estimating the strength characteristics of samples which volume and surface can be studied. The methods of measuring the mechanical properties of the surface include the hardness tests based on indenting a hard tip (instrumental indenting) [8], as well as abrasion by repeating passage passes (wear test) [9]. The instrumental indenting allows of getting the values of the values of hardness and elastic moduli, wear tests – the linear wear coefficient and the friction coefficient. A comparison of the data obtained according to the described methods can be useful to understand the applicability of materials as declared structural elements for dentistry.
RESEARCH METHODS
Images were printed on the Phrozen Sonic 3D printer from low-irritating photopolymer. Roughness of the samples is measured on the S Neox optical profilometer. The surface relief measurement mode with the aid of the optical confocal microscopy was used. The monochrome radiation was provided by a green LED. The scan field is 340 × 280 microns. Roughness measurements using a confocal optical profilometer were carried out on three samples from each group in three regions (two near the edge and one in the center).
Measurement of hardness and elastic module of samples were carried out using a NanoScan-4D nano-hardness tester [10]. There were two groups of samples to be studied: three initial ones printed on a 3D printer and three additionally polished samples. On each sample three areas were measured: in the centre of the sample and at a distance of 1 cm from the sample edge. In each area two series of tests were conducted with various loads according to the recommendations of GOST R 8.748-2011 standard [8]. The maximum loading force equaled 1 and 10 mN (the number of indenters in each series was at least 20). The indenter was a pyramidal triangular diamond tip of Berkovich type. Loading and unloading times were equal to 10 seconds and exposure time – 2 sec. Calibration of the tip shape and rigidity of the device was carried out on the melted quartz. Indentation depths at light loads are comparable to the average surface roughness. With light loads, a thin near-surface layer of the sample and its mechanical properties were measured, their mechanical properties were dependent on the method of obtaining material and mechanical surface treatment. The data obtained by the indentation method in case of a high surface roughness leads to a large scatter of the values.
For high loads, the effect of roughness decreases, but the effect of material properties in a volume increases.
Measurements of the friction coefficient and the linear wear coefficient are also carried out on three samples from each group in three areas (two near the edge and one in the center). The number of wear cycles in each test is 100, the normal load is 250 mN. The tests were carried out using a NanoScan-4D, but the tip was replaced with a diamond hemisphere of 130 μm diameter (Fig.1). The load and the diameter of the tip are selected in such a way that the penetration depth during the test was small, but there was a plastic or fragile destruction of the sample, that is, there was an abrasive mechanism of material deterioration.
The test is carried out as follows: The spherical shape tip of a hard material is pressed against the surface of the sample with a constant normal force and performs a reciprocal movements along a straight line. During the test the lateral loading force is measured (along the surface of the sample) simultaneously with the depth of the tip penetration into the sample surface. After testing, the geometry of the abrasion groove is measured. Three-dimensional image of the surface relief is obtained at S Neox optical profilometer.
As a result of the abrasion test, the following parameters are calculated: linear wear, width and depth of groove after wear testing, and friction coefficient. The linear wear reflects the average change in the groove depth for one test cycle and indicates the material wear rate, and the friction coefficient shows the ratio of the lateral and normal force during the slip on the material. The linear wear and the friction coefficient are measured on the area between 50 and 100 wear cycles. At the beginning of the test, the sample is rubbed in as the surface is irregular. Wear parameters are measured in situ after the abrasion mode testing become stable.
Test samples on a three-point bend were conducted on the Instron 5982 universal testing machine according to GOST 31572-2012 [11]. There were prepared 5 strips made of the studied material and the absence of porosity was checked. The strip dimensions were: length 64 mm, width (10 ± 0.2) mm and height (3.3 ± 0.2) mm. The height and width of the finished strips are measured three times along the longitudinal axis using the caliper.
A device for a three-point bending test consists of a central loading plunger and two cylindrical supports with polished surfaces dia. 3.2 mm and a minimum length of 10.5 mm. The supports are located in parallel with a deviation not exceeding 0.1 mm and perpendicular to the longitudinal center line. The distance between the centres of supports – (50 ± 0.1) mm; the loader plunger is located in the center between the supports, and permissible deviation from the center is 0.1 mm.
Before proceeding to bending, the samples are kept in water of temperature (37 ± 1) °C for (50 ± 1) hour. Before testing, the strip is removed from the water and immediately put on the supports of the testing device (Fig.2). The load applied to the plunger is evenly increased at a constant speed (5 ± 1) mm/min until the sample is destructed.
The bending strength σ is calculated by the formula:
, (1)
where F – load at sample destruction, l – distance between the supports, b – sample width, h – sample height.
The elastic modulus at bending E, MPa, is calculated by the formula:
, (2)
where F1 – the load in the field of elastic deformation of the sample, selected on the diagram "Load – deformation" straight line, d – deformation at load F1.
RESULTS AND DISCUSSION
Figure 3 shows the three-dimensional topographic images of the surface relief of the measured samples. The initial samples have a strongly developed surface relief. A periodic structure associated with the print mode of the material is visible.
Table 1 shows two parameters of roughness: Ra is an arithmetic-mean roughness, Rz – roughness in 10 points (average distance between 5 the highest and 5 the lowest areas on the surface relief image). Ra shows the average surface deviation from the middle level, Rz shows the maximum surface differential drop on the measured area. The data provided reflects the result of nine averaged measurements.
Figure 4 shows the dependences of the hardness and elastic modules of the samples on depth and indicates the measurement error (the scatter is shown between measurements on different areas).
The hardness and elastic modulus of the material on the surface of the initial samples is very lower than the material with the treated surface. The difference between the properties of the samples is reducing with the depth increase. It can be assumed that there is an area on the printed samples surface, where the full polymerization of the material and polishing has not taken place, polishing allows of removing this layer from the surface.
The abrasion test conducted on the NanoScan-4D, indicated the results given in Table 2, also averaged for nine dimensions.
The results of the abrasive wear test show a correlation between the wear resistance and the hardness of the materials. The higher the hardness, the higher the wear resistance and less the area of destruction during wear tests. The hardness of the material of the original samples is smaller on the surface and increases with depth, approaching the hardness of the sample after polishing. The values of the linear wear and the friction coefficient of the samples have close values, as they are measured in the temporary area, when the tip destroyed the surface layer and deeply penetrated into the material. The measurements of the wear groove size show a smaller wear resistance of the initial material, which is associated with a soft layer of material on the surface.
Three-point test results showed that the deformation of the samples until destruction is (2.7 ± 0.6) %, the strength is (45 ± 10) MPa, the elastic modulus – (1.59 ± 0.14) of the GPa. Materials have less rigidity and bending strength than when testing by the indentation method, which is the closest to the tests for monoaxial compression.
CONCLUSIONS
In this paper, tests of hardness, elastic modulus, abrasive wear as well as the strength of the material obtained by additive technologies in order to analyze their applicability for the manufacture of occlusal splints were conducted.
The initial sample compared to the polished one showed a developed relief of a periodic structure. The surface layer of the material in the initial sample has smaller hardness and wear resistance than the material in the volume. Polishing of the sample allows to remove the soft layer on the surface and reduce surface roughness. ■
The work was performed using the CUC 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.
Materials science is the area of knowledge that is widely used not only for technical applications, but also in medicine. In particular, modern dentistry is based on improving biomaterials, methods of their obtaining and processing [1]. The materials for manufacture of dental structures must have a number of characteristics depending on the location and the functions performed [2].
In the case of the temporomandibular joint pathologies, a splint therapy is one of the common treatment methods [3, 4]. Occlusal splints are manufactured by various methods: thermal and press, cold and hot polymerization. Special attention is drawn to the possibility of manufacturing such elements with additive technologies [5]. 3D printing in combination with modern software and computed tomography seems to be a very promising direction in order to easily get elements possessing unique and specific geometry suitable for a patient [6]. However, not only the method of manufacturing should be taken into account, but the material itself, because during an operation, the dental structures are experiencing a large load and a rational choice of material and the method of manufacture directly affect the quality of treatment [7].
It is possible to decide on the use of a particular material and method of its manufacture by applying the quantitative criteria for estimating the strength characteristics of samples which volume and surface can be studied. The methods of measuring the mechanical properties of the surface include the hardness tests based on indenting a hard tip (instrumental indenting) [8], as well as abrasion by repeating passage passes (wear test) [9]. The instrumental indenting allows of getting the values of the values of hardness and elastic moduli, wear tests – the linear wear coefficient and the friction coefficient. A comparison of the data obtained according to the described methods can be useful to understand the applicability of materials as declared structural elements for dentistry.
RESEARCH METHODS
Images were printed on the Phrozen Sonic 3D printer from low-irritating photopolymer. Roughness of the samples is measured on the S Neox optical profilometer. The surface relief measurement mode with the aid of the optical confocal microscopy was used. The monochrome radiation was provided by a green LED. The scan field is 340 × 280 microns. Roughness measurements using a confocal optical profilometer were carried out on three samples from each group in three regions (two near the edge and one in the center).
Measurement of hardness and elastic module of samples were carried out using a NanoScan-4D nano-hardness tester [10]. There were two groups of samples to be studied: three initial ones printed on a 3D printer and three additionally polished samples. On each sample three areas were measured: in the centre of the sample and at a distance of 1 cm from the sample edge. In each area two series of tests were conducted with various loads according to the recommendations of GOST R 8.748-2011 standard [8]. The maximum loading force equaled 1 and 10 mN (the number of indenters in each series was at least 20). The indenter was a pyramidal triangular diamond tip of Berkovich type. Loading and unloading times were equal to 10 seconds and exposure time – 2 sec. Calibration of the tip shape and rigidity of the device was carried out on the melted quartz. Indentation depths at light loads are comparable to the average surface roughness. With light loads, a thin near-surface layer of the sample and its mechanical properties were measured, their mechanical properties were dependent on the method of obtaining material and mechanical surface treatment. The data obtained by the indentation method in case of a high surface roughness leads to a large scatter of the values.
For high loads, the effect of roughness decreases, but the effect of material properties in a volume increases.
Measurements of the friction coefficient and the linear wear coefficient are also carried out on three samples from each group in three areas (two near the edge and one in the center). The number of wear cycles in each test is 100, the normal load is 250 mN. The tests were carried out using a NanoScan-4D, but the tip was replaced with a diamond hemisphere of 130 μm diameter (Fig.1). The load and the diameter of the tip are selected in such a way that the penetration depth during the test was small, but there was a plastic or fragile destruction of the sample, that is, there was an abrasive mechanism of material deterioration.
The test is carried out as follows: The spherical shape tip of a hard material is pressed against the surface of the sample with a constant normal force and performs a reciprocal movements along a straight line. During the test the lateral loading force is measured (along the surface of the sample) simultaneously with the depth of the tip penetration into the sample surface. After testing, the geometry of the abrasion groove is measured. Three-dimensional image of the surface relief is obtained at S Neox optical profilometer.
As a result of the abrasion test, the following parameters are calculated: linear wear, width and depth of groove after wear testing, and friction coefficient. The linear wear reflects the average change in the groove depth for one test cycle and indicates the material wear rate, and the friction coefficient shows the ratio of the lateral and normal force during the slip on the material. The linear wear and the friction coefficient are measured on the area between 50 and 100 wear cycles. At the beginning of the test, the sample is rubbed in as the surface is irregular. Wear parameters are measured in situ after the abrasion mode testing become stable.
Test samples on a three-point bend were conducted on the Instron 5982 universal testing machine according to GOST 31572-2012 [11]. There were prepared 5 strips made of the studied material and the absence of porosity was checked. The strip dimensions were: length 64 mm, width (10 ± 0.2) mm and height (3.3 ± 0.2) mm. The height and width of the finished strips are measured three times along the longitudinal axis using the caliper.
A device for a three-point bending test consists of a central loading plunger and two cylindrical supports with polished surfaces dia. 3.2 mm and a minimum length of 10.5 mm. The supports are located in parallel with a deviation not exceeding 0.1 mm and perpendicular to the longitudinal center line. The distance between the centres of supports – (50 ± 0.1) mm; the loader plunger is located in the center between the supports, and permissible deviation from the center is 0.1 mm.
Before proceeding to bending, the samples are kept in water of temperature (37 ± 1) °C for (50 ± 1) hour. Before testing, the strip is removed from the water and immediately put on the supports of the testing device (Fig.2). The load applied to the plunger is evenly increased at a constant speed (5 ± 1) mm/min until the sample is destructed.
The bending strength σ is calculated by the formula:
, (1)
where F – load at sample destruction, l – distance between the supports, b – sample width, h – sample height.
The elastic modulus at bending E, MPa, is calculated by the formula:
, (2)
where F1 – the load in the field of elastic deformation of the sample, selected on the diagram "Load – deformation" straight line, d – deformation at load F1.
RESULTS AND DISCUSSION
Figure 3 shows the three-dimensional topographic images of the surface relief of the measured samples. The initial samples have a strongly developed surface relief. A periodic structure associated with the print mode of the material is visible.
Table 1 shows two parameters of roughness: Ra is an arithmetic-mean roughness, Rz – roughness in 10 points (average distance between 5 the highest and 5 the lowest areas on the surface relief image). Ra shows the average surface deviation from the middle level, Rz shows the maximum surface differential drop on the measured area. The data provided reflects the result of nine averaged measurements.
Figure 4 shows the dependences of the hardness and elastic modules of the samples on depth and indicates the measurement error (the scatter is shown between measurements on different areas).
The hardness and elastic modulus of the material on the surface of the initial samples is very lower than the material with the treated surface. The difference between the properties of the samples is reducing with the depth increase. It can be assumed that there is an area on the printed samples surface, where the full polymerization of the material and polishing has not taken place, polishing allows of removing this layer from the surface.
The abrasion test conducted on the NanoScan-4D, indicated the results given in Table 2, also averaged for nine dimensions.
The results of the abrasive wear test show a correlation between the wear resistance and the hardness of the materials. The higher the hardness, the higher the wear resistance and less the area of destruction during wear tests. The hardness of the material of the original samples is smaller on the surface and increases with depth, approaching the hardness of the sample after polishing. The values of the linear wear and the friction coefficient of the samples have close values, as they are measured in the temporary area, when the tip destroyed the surface layer and deeply penetrated into the material. The measurements of the wear groove size show a smaller wear resistance of the initial material, which is associated with a soft layer of material on the surface.
Three-point test results showed that the deformation of the samples until destruction is (2.7 ± 0.6) %, the strength is (45 ± 10) MPa, the elastic modulus – (1.59 ± 0.14) of the GPa. Materials have less rigidity and bending strength than when testing by the indentation method, which is the closest to the tests for monoaxial compression.
CONCLUSIONS
In this paper, tests of hardness, elastic modulus, abrasive wear as well as the strength of the material obtained by additive technologies in order to analyze their applicability for the manufacture of occlusal splints were conducted.
The initial sample compared to the polished one showed a developed relief of a periodic structure. The surface layer of the material in the initial sample has smaller hardness and wear resistance than the material in the volume. Polishing of the sample allows to remove the soft layer on the surface and reduce surface roughness. ■
The work was performed using the CUC 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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