rus
Issue #3/2015
Ya.Stanishevskiy, D.Shvitko, A.Marakhova, V.Kopylov, S.Panov
Non-destructive methods of investigation of physical and mechanical and other properties of the materials used in dentistry
Non-destructive methods of investigation of physical and mechanical and other properties of the materials used in dentistry
The study of the basic properties of materials used in the dental practice showed the feasibility of analysis of these characteristics by non-destructive methods such as nanoindentation and light microscopy.
Теги: light microscopy nanoindentation non-destructive methods of investigation наноиндентирование неразрушающие методы исследования световая микроскопия
Nanotechnologies determine future development patterns of not only chemistry, physics, biology and materials science but also the whole of science and industry in the coming decades [1]. The term "nanotechnology" was originally used only in quantum physics and electronics, and nowadays it penetrated into other industries, e.g. medicine, chemical industry, pharmacy etc. Dentistry is not an exception. The method of nano-indentation is especially widely used in the science. Indenter that has some known mechanical properties, i.e. shape, elastic modulus etc., is pressed with a predetermined force in the studied sample. Then we research the shape and size of the contact area, or the indenter’s position dependence on the load is determined.
New materials in dentistry
One of the key innovations in dentistry is the development and creation of new materials that combine high physical properties (strength, endurance) and aesthetics. The research works that are devoted to modelling the properties of materials used for sealing, restoring tooth enamel and producing coatings for implants are more commonly used in practice [2, 3].
It is extremely relevant to create composite materials without any polymeric shrinkage but this criterion is not the only one, nor the most important one. First of all, high requirements are set with regard to the filler, which ultimately determines the quality of the composite. To exercise quality control it is required to measure the flexural strength, elastic modulus, surface hardness, abrasion, shrinkage, thermal expansion and many other parameters in order to optimise them [4].
Another pressing issue of dentistry is the creation of polymer coatings for crowns. Zirconia oxide used to make them can be characterised by high hardness, but it is not sufficiently aesthetic and contributes to the pathological abrasion of the antagonist teeth, so coating applied thereto [3]. At the same time, such characteristics of coatings are taken into account as the shape and size of particles, elasticity, toughness, ductility, fatigue etc. [5].
The research aimed at studying physical, mechanical and other properties used in dental materials through scanning probe microscopy and light microscopy.
Physical and mechanical properties of the materials
used in dentistry
Research into the properties of dental materials is of great practical importance associated with the possibility of regulating them by changing the composition of materials and promoting best practices as well as the latest application technologies in various fields of dentistry. Hardness, strength, toughness and elasticity are key characteristics of the materials used in dentistry.
Hardness is the body’s ability to resist the introduction of a more solid body. Currently, the hardness of material is often determined by the methods of Vickers or Brinell, the essence of which consists in that the quadrangular diamond pyramid or beads is pressed into the tested material with a special press. By the size of the print on the surface of the sample it is possible to measure the hardness of the material. In dentistry, hardness can be regarded as a positive feature that allows using the prosthesis for a long time; however it often manifests the negative side. For example, the porcelain teeth, which have hardness two times higher than that of enamel, cause an increased blurring of natural antagonist teeth. The parts of the prosthesis made of high hard cobalt alloy are difficult to treat and polish [3].
Strength is the ability of material to resist the action of an external force, which is determined by dividing the amount of load by the value of the cross-sectional area of the test sample. Good mechanical properties are among the basic requirements for dental materials.
Elasticity is the ability of a material to change shape under pressure, and return to the original state after the termination of pressure. The maximum load at which a material can still restore the shape and dimensions is called the elastic limit. If the load exceeds the elastic limit, and the body does not return to its original position, we can speak of permanent deformation. Permanent deformation is highly undesirable in the spring elements of prosthetic appliances and devices. The material elasticity can be influenced by changing it in special ways [3].
The flexural strength and modulus of elasticity (Young’s modulus) are closely inter-related. Flexural strength indicates at what value of the applied force the test sample can be broken. The elastic modulus thus characterises how strongly a material deforms under load before breakage. Under load, a tooth is minimally deformed; therefore, the restoration should be deformed the same way as the tooth. It should be noted that a high flexural strength will not guarantee any high levels of durability without the modulus of elasticity. However a pretty high modulus of elasticity can cause "punching" the dentin below. The modulus of elasticity of the most advanced composites approaches the modulus of elasticity of dentin but it should not be higher in any way.
Most composite materials are more elastic than the hard tissues of teeth [6]. A good flexural strength of composites allows them to withstand high loads but there are horizontal tensile forces in the occlusive area that can cause chipping in the restoration of the thin walls of the cavity.
The values of the thermal expansion coefficient and polymerisation shrinkage when putting the seal are also important. High shrinkage causes internal stresses in the tooth, sensitivity enhancement and edge conformability violations leading eventually to the development of the secondary caries. Such phenomena may be caused by thermal expansion. A composite should be capable of taking more load not only during chewing but also with some changes in temperature. The physical parameter that characterizes the change in volume when the temperature of the material is changed, is called the thermal expansion coefficient. Even if the volume changes are measured in micrometers, they can have fatal consequences. The thermal expansion coefficient of a composite and polymerisation shrinkage are closely inter-related with the assessment of the clinical efficacy of a composite [5].
Here is another example confirming the need for a comprehensive research into the properties of composites. The surface quality of composites is important not only in terms of aesthetics. The surface of the composite restoration should not create more favourable conditions for colonisation by bacteria than a vital tooth. For this reason, researches into the composite surface colonisation by Streptococcus mutans bacteria are undertaken [4]. The intensity of the bacterial flora is affected by microcracks, pits and bumps thus making it necessary to study the profile of the material used.
Mechanical measurements constitute the bulk of the research of composites and coatings for dental crowns. Producers compare their new designs with other materials. Whenever possible, this is done through standard measurement methods [3]. However, many measuring tools were originally not designed for dental research but study of the materials with a homogeneous molecular structure, for example, metals, while a composite is a polymer filled with particles of different sizes. In this regard, there is a high likelihood of obtaining unreliable results, and it is relevant to search for better and more appropriate methods of analysis of materials used in dentistry.
study by nano-indentation
Nano-indentation is a method of determining functional properties of thin coatings (ISO 14577) with the NanoScan-3D Scanning nano-hardness tester (fig.1). This method can be considered conditionally non-destructive because the depth of penetration of the indenter during testing is negligible [7].
NanoScan-3D is a scanning force microscope that is operated in the open air in contact mode. The main feature of this device is the high strength (6 × 104 N/m) of the ceramic cantilever and the use of a diamond pyramid or ultra-hard fulerite as indenter. This allows not only to investigate the surface topology but also in hard contact mode evaluate the hardness and elastic modulus of super-hard materials and coatings including diamonds. The device measures the profile and mechanical properties of the microscopic surface area thus making it possible to compare the characteristics of different parts of the sample.
NanoScan-3D can be used in the indentation and scratch hardness test modes. Indentation is carried by loading the needle at a certain point on the surface, scratching – by loading and subsequent horizontal displacement of the indenter under a load. The size of indentation or scratching is determined by scanning the surface before and after pressure. Determination of hardness is based on the method of comparative sclerometry when a scratch is applied alternately to the material under study and reference specimen, the hardness of which is known. A scratch analysis consists in measuring its average width at a certain load [6, 8].
An important advantage of NanoScan-3D is the variety of measurements available on the basis of a single device. In particular, the developed method of the material wear resistance determination is based on the displacement of the tip across the surface with constant control of the normal hold-down pressure thereto and the measurement of the dependence of the tip’s deepening in the material on the time [9].
To measure nanohardness and Young’s modulus of coatings on the NanoScan-3D the following requirements to the samples should be met, i.e. roughness should be better than Class 12-14; the depth of penetration of the indenter (77 nm) should not exceed 10% of the thickness of coating. As substrates for coatings samples of silicon and glass-ceramic CT50 (Roughness Class 14) can be used [10, 11].
Fig.2 presents results of the research of a dental protective coating sample on the NanoScan-3D device.
study by light microscopy
Light microscopy can also be regarded as a non-destructive method of research. In material science, the microscope Nikon Eclipse MA 200 (fig.3), which can operate in the reflected and transmitted light, is commonly used. The device optics allow obtaining clear and high-contrast images through the brightfield and darkfield microscopy, the brightness of which is three times higher than by other models [1]. A convenient feature is to set an object on the sample stage located above the lens thus making it possible to have a better overview and allows to study an object both in a loose state and on a carrier with the weight of up to 10 kg.
The authors used extracted teeth to research ceramic inlays and the filling material. The micrographs obtained using the Nikon Eclipse MA microscope give a notion of the surface profile and allow obtaining an approximate estimate of the size of irregularities, scratches and inclusions.
From the photos presented in fig.4 and fig.5, it can be concluded that the ceramic tab has a uniform surface and its structure is more dense and non-porous. Thus diminishing the likelihood of microbial biofilm formation and of caries. On the surface of the glass-ionomer dental fillings numerous pores can be found thus promoting the bacterial adhesion and development of the periodontal pathogenic flora. In addition, the observed differences in the structure indirectly indicate greater strength and heat capacity of the ceramic material than the dental filling.
Thus, light microscopy does not replace nano-indentation but can be used for preliminary research of materials used in dentistry.
***
The conducted research suggests that for the correct comparison of dental materials it is necessary to unify methods for determining the shape and size of the particles as well as elasticity, hardness, strength, fracture toughness, plasticity and fatigue. Many measuring devices, developed for the study of materials with molecular homogeneous structure, are not suitable for composites used in dentistry. A promising method for determining the functional properties of thin coatings is the nano-indentation using the scanning nano-hardness tester NanoScan-3D. Light microscopy does not substitute nano-indentation but can be used for the preliminary research of materials used in dentistry.
New materials in dentistry
One of the key innovations in dentistry is the development and creation of new materials that combine high physical properties (strength, endurance) and aesthetics. The research works that are devoted to modelling the properties of materials used for sealing, restoring tooth enamel and producing coatings for implants are more commonly used in practice [2, 3].
It is extremely relevant to create composite materials without any polymeric shrinkage but this criterion is not the only one, nor the most important one. First of all, high requirements are set with regard to the filler, which ultimately determines the quality of the composite. To exercise quality control it is required to measure the flexural strength, elastic modulus, surface hardness, abrasion, shrinkage, thermal expansion and many other parameters in order to optimise them [4].
Another pressing issue of dentistry is the creation of polymer coatings for crowns. Zirconia oxide used to make them can be characterised by high hardness, but it is not sufficiently aesthetic and contributes to the pathological abrasion of the antagonist teeth, so coating applied thereto [3]. At the same time, such characteristics of coatings are taken into account as the shape and size of particles, elasticity, toughness, ductility, fatigue etc. [5].
The research aimed at studying physical, mechanical and other properties used in dental materials through scanning probe microscopy and light microscopy.
Physical and mechanical properties of the materials
used in dentistry
Research into the properties of dental materials is of great practical importance associated with the possibility of regulating them by changing the composition of materials and promoting best practices as well as the latest application technologies in various fields of dentistry. Hardness, strength, toughness and elasticity are key characteristics of the materials used in dentistry.
Hardness is the body’s ability to resist the introduction of a more solid body. Currently, the hardness of material is often determined by the methods of Vickers or Brinell, the essence of which consists in that the quadrangular diamond pyramid or beads is pressed into the tested material with a special press. By the size of the print on the surface of the sample it is possible to measure the hardness of the material. In dentistry, hardness can be regarded as a positive feature that allows using the prosthesis for a long time; however it often manifests the negative side. For example, the porcelain teeth, which have hardness two times higher than that of enamel, cause an increased blurring of natural antagonist teeth. The parts of the prosthesis made of high hard cobalt alloy are difficult to treat and polish [3].
Strength is the ability of material to resist the action of an external force, which is determined by dividing the amount of load by the value of the cross-sectional area of the test sample. Good mechanical properties are among the basic requirements for dental materials.
Elasticity is the ability of a material to change shape under pressure, and return to the original state after the termination of pressure. The maximum load at which a material can still restore the shape and dimensions is called the elastic limit. If the load exceeds the elastic limit, and the body does not return to its original position, we can speak of permanent deformation. Permanent deformation is highly undesirable in the spring elements of prosthetic appliances and devices. The material elasticity can be influenced by changing it in special ways [3].
The flexural strength and modulus of elasticity (Young’s modulus) are closely inter-related. Flexural strength indicates at what value of the applied force the test sample can be broken. The elastic modulus thus characterises how strongly a material deforms under load before breakage. Under load, a tooth is minimally deformed; therefore, the restoration should be deformed the same way as the tooth. It should be noted that a high flexural strength will not guarantee any high levels of durability without the modulus of elasticity. However a pretty high modulus of elasticity can cause "punching" the dentin below. The modulus of elasticity of the most advanced composites approaches the modulus of elasticity of dentin but it should not be higher in any way.
Most composite materials are more elastic than the hard tissues of teeth [6]. A good flexural strength of composites allows them to withstand high loads but there are horizontal tensile forces in the occlusive area that can cause chipping in the restoration of the thin walls of the cavity.
The values of the thermal expansion coefficient and polymerisation shrinkage when putting the seal are also important. High shrinkage causes internal stresses in the tooth, sensitivity enhancement and edge conformability violations leading eventually to the development of the secondary caries. Such phenomena may be caused by thermal expansion. A composite should be capable of taking more load not only during chewing but also with some changes in temperature. The physical parameter that characterizes the change in volume when the temperature of the material is changed, is called the thermal expansion coefficient. Even if the volume changes are measured in micrometers, they can have fatal consequences. The thermal expansion coefficient of a composite and polymerisation shrinkage are closely inter-related with the assessment of the clinical efficacy of a composite [5].
Here is another example confirming the need for a comprehensive research into the properties of composites. The surface quality of composites is important not only in terms of aesthetics. The surface of the composite restoration should not create more favourable conditions for colonisation by bacteria than a vital tooth. For this reason, researches into the composite surface colonisation by Streptococcus mutans bacteria are undertaken [4]. The intensity of the bacterial flora is affected by microcracks, pits and bumps thus making it necessary to study the profile of the material used.
Mechanical measurements constitute the bulk of the research of composites and coatings for dental crowns. Producers compare their new designs with other materials. Whenever possible, this is done through standard measurement methods [3]. However, many measuring tools were originally not designed for dental research but study of the materials with a homogeneous molecular structure, for example, metals, while a composite is a polymer filled with particles of different sizes. In this regard, there is a high likelihood of obtaining unreliable results, and it is relevant to search for better and more appropriate methods of analysis of materials used in dentistry.
study by nano-indentation
Nano-indentation is a method of determining functional properties of thin coatings (ISO 14577) with the NanoScan-3D Scanning nano-hardness tester (fig.1). This method can be considered conditionally non-destructive because the depth of penetration of the indenter during testing is negligible [7].
NanoScan-3D is a scanning force microscope that is operated in the open air in contact mode. The main feature of this device is the high strength (6 × 104 N/m) of the ceramic cantilever and the use of a diamond pyramid or ultra-hard fulerite as indenter. This allows not only to investigate the surface topology but also in hard contact mode evaluate the hardness and elastic modulus of super-hard materials and coatings including diamonds. The device measures the profile and mechanical properties of the microscopic surface area thus making it possible to compare the characteristics of different parts of the sample.
NanoScan-3D can be used in the indentation and scratch hardness test modes. Indentation is carried by loading the needle at a certain point on the surface, scratching – by loading and subsequent horizontal displacement of the indenter under a load. The size of indentation or scratching is determined by scanning the surface before and after pressure. Determination of hardness is based on the method of comparative sclerometry when a scratch is applied alternately to the material under study and reference specimen, the hardness of which is known. A scratch analysis consists in measuring its average width at a certain load [6, 8].
An important advantage of NanoScan-3D is the variety of measurements available on the basis of a single device. In particular, the developed method of the material wear resistance determination is based on the displacement of the tip across the surface with constant control of the normal hold-down pressure thereto and the measurement of the dependence of the tip’s deepening in the material on the time [9].
To measure nanohardness and Young’s modulus of coatings on the NanoScan-3D the following requirements to the samples should be met, i.e. roughness should be better than Class 12-14; the depth of penetration of the indenter (77 nm) should not exceed 10% of the thickness of coating. As substrates for coatings samples of silicon and glass-ceramic CT50 (Roughness Class 14) can be used [10, 11].
Fig.2 presents results of the research of a dental protective coating sample on the NanoScan-3D device.
study by light microscopy
Light microscopy can also be regarded as a non-destructive method of research. In material science, the microscope Nikon Eclipse MA 200 (fig.3), which can operate in the reflected and transmitted light, is commonly used. The device optics allow obtaining clear and high-contrast images through the brightfield and darkfield microscopy, the brightness of which is three times higher than by other models [1]. A convenient feature is to set an object on the sample stage located above the lens thus making it possible to have a better overview and allows to study an object both in a loose state and on a carrier with the weight of up to 10 kg.
The authors used extracted teeth to research ceramic inlays and the filling material. The micrographs obtained using the Nikon Eclipse MA microscope give a notion of the surface profile and allow obtaining an approximate estimate of the size of irregularities, scratches and inclusions.
From the photos presented in fig.4 and fig.5, it can be concluded that the ceramic tab has a uniform surface and its structure is more dense and non-porous. Thus diminishing the likelihood of microbial biofilm formation and of caries. On the surface of the glass-ionomer dental fillings numerous pores can be found thus promoting the bacterial adhesion and development of the periodontal pathogenic flora. In addition, the observed differences in the structure indirectly indicate greater strength and heat capacity of the ceramic material than the dental filling.
Thus, light microscopy does not replace nano-indentation but can be used for preliminary research of materials used in dentistry.
***
The conducted research suggests that for the correct comparison of dental materials it is necessary to unify methods for determining the shape and size of the particles as well as elasticity, hardness, strength, fracture toughness, plasticity and fatigue. Many measuring devices, developed for the study of materials with molecular homogeneous structure, are not suitable for composites used in dentistry. A promising method for determining the functional properties of thin coatings is the nano-indentation using the scanning nano-hardness tester NanoScan-3D. Light microscopy does not substitute nano-indentation but can be used for the preliminary research of materials used in dentistry.
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