rus
Issue #7-8/2019
K.S.Kravchuk, E.V. Gladkikh, A.V.Morozov
Nano-dynamic mechanical properties investigation of automotive tread rubber at temperatures from –60 to 60 °C using nano-hardness tester "NanoScan-4D"
Nano-dynamic mechanical properties investigation of automotive tread rubber at temperatures from –60 to 60 °C using nano-hardness tester "NanoScan-4D"
We investigated rubber viscoelastic properties at near surface layer using nano-hardness tester "NanoScan-4D" placed in climatic chamber. The same temperature of sample and the device made it possible to minimize the measurement errors arising due to thermal drifts.
Теги: climatic chamber indentor temperature drift thermo-mechanical properties индентор климатическая камера температурный дрейф термомеханические свойства
We investigated rubber viscoelastic properties at near surface layer using nano-hardness tester "NanoScan-4D" placed in climatic chamber. The same temperature of sample and the device made it possible to minimize the measurement errors arising due to thermal drifts.
INTRODUCTION
The development of methods for studying the mechanical properties of a material surface layer in the temperature range from –60 to 60 °С is especially relevant for the elastomers used in friction joints.
Nano-hardness meters are the standard equipment to study the physical and mechanical properties. The key method applied in these instruments is based on indention of a hard tip (indenter) in the studied sample surface simultaneously with measuring of the indenter displacement and the applied force. This method makes it possible to measure hardness and elasticity modulus. The measurement area of mechanical properties is the surface layer of the sample which volume is determined by the depth of the indenter penetration. The method of instrumental indention is regulated by GOST Р 8.748-2011. The elasticity modulus of the rubber near-surface layer that has passed wear tests may be significantly different from the value obtained in the volume of the material [1]. One of the problems reflecting the topicality of the presented paper is a study of viscoelastic properties of rubber and their changes by depth from the surface of the worn-out material.
As a rule, measurements last tens of seconds, which corresponds to the quasi-static loading mode [2]. The possibilities of the method may be much more expanded in the dynamic mode, when periodical loading is applied to the sample, and some time-dependent properties maybe calculated.
Usually, special cells [3] are used to perform the tests at different temperatures by hardness meters in order to maintain the necessary temperature in measurement area. A small sample and an indenter are placed inside the cell. The measuring sensors are placed outside the instrument at room temperature and are connected to the tip by a long inflexible rod. Such a design usually does not allow of the uniform heating of the sample, and the movement of the indenter and its contact with the sample lead to a change in the temperature gradient along the rod that connects the indenter with the force and displacement sensors. The temperature changes make it difficult to accurately measure the tip penetration depth into the sample. Besides, such type of a design, as a rule, significantly limits the sample dimensions to a few millimeters and does not allow of comprehensive studies of the mechanical properties for the wide range of objects.
In the presented paper we propose to maintain the same temperature in a sample and in the measuring sensor during the temperature tests. The measuring equipment with samples to be tested is placed in a chamber where the necessary temperature is maintained. An electronic unit and a computer for processing the hardness meter data are placed outside of the temperature chamber.
Studies of the mechanical properties of the motor vehicle tread rubber tires in a wide temperature range have been carried out.
METHODS OF MEASURING
Dynamical mechanical analysis
Dynamic mechanical analysis [5, 6] means the testing method and the instrument for measuring the mechanical and viscoelastic properties of rubbers (real and imaginary part of the complex elasticity modulus, mechanical loss tangent, etc.) and enables to obtain frequency-temperature dependences of relaxation processes in rubber at deformation under periodic loading. A spherical ceramic tip (0.5 mm, made of silicon nitride) was penetrated into the sample until achievement of the maximum load of 100 mN. The harmonic oscillations of 25 mN amplitude in a frequency range from 0.01 to 80 Hz during 5 min were applied to the indenter at a constant average loading force F. During the tests the tip displacement amplitude, related to the contact rigidity (Scont) of a tip with a sample, and a phase difference (δ) between the displacement signal and loading force were measured. The equations describe the force applied to the indenter and its displacement as follows:
F = Fquasi–static + F0eiωt ,
h = hquasi–static + h0ei(ωt+δ).
The real and imaginary parts of the complex elasticity modulus are determined by the following relationships:
where А – contact area, – contact rigidity, where С – coefficient of viscous friction.
SAMPLES AND EQUIPMENT
DESCRIPTION OF THE SAMPLES
DM tests were performed on two samples. The first sample was made of a tread rubber of mainline all-steel (AST) tyre. The second one was made of a tread rubber of summer tyre. The samples to be studied were measured using tribometer where the counterbody was a disk with the glued abrasive paper made of silicon carbide with a grain size of 120 µm [7]. The depth of the disturbed layer of the studied surface is comparable with the abrasive grains and is equal to about 10 µm (See Fig.1). The result of the dry friction was a high roughness of the surface (Ra – 30 µm). It was measured using a 3D-profilometer S neox (Sensofar).
"NanoScan-4D" nano-hardness meter
The tests were performed using a "NanoScan-4D" [4]. The portable model of this device does not exceed 300 mm in size and it was entirely placed into the climate chamber. The highest reliability of design of the device allows of using it in a wide temperature range. The portable design of the device has an indentation modulus and two motorized translators to provide the mutual movement of the indenter and a sample. The design of the translators enables to use them in the temperature range from –60 to 60 °С without parasitic effects such as blocking, loss of rigidity, play, etc. The indentation modulus permits to perform the standard tests by nano-indention method in the dynamic and quasi-static modes. In any test method, a tip of an arbitrary shape can be selected. The standard tip for mechanical testing presents an indentor of the Berkovich type. In this work we used the spherical tip in order to reduce the plasticity deformation of the sample.
КТХВ-300 climate chamber
The temperature was maintained in the range of –60…60 °С with the aid of КТХВ-300 climate chamber. It is possible to maintain air humidity in the range of 20…98% to prevent frost on the moving parts when the temperature drops. Accuracy of temperature control in the chamber was 0.5 °С. To minimize a temperature drift and vibrations, a protective box was used, which inner surface was coated with acoustic foaming plastic and the joints were glued with a GERLEN sealing tape. We used the passive vibration-isolating platform. Air temperature was controlled by sensors built into the climate chamber. The temperature of the device case was controlled using an LT-300 electronic thermometer with a digital resolution of 0.01 °C.
RESULTS
The climate chamber compressors are the main source of noise in the described experiment design. The use of vibration isolation made it possible to significantly reduce the noise level when measuring depth during indentation of the tip. The temperature drift of the system was no more than 0.5 nm/s. Indention depth of the tip was in the range of about 1–20 µm. As a result of the tests, the temperature dependences of the elasticity modulus and the mechanical loss tangent were constructed for two rubber samples (See Fig.2). The test results are consistent with DMA data performed on bulk samples on a METRAVIB dynamomechanical analyzer.
CONCLUSIONS
A scheme for conducting dynamic mechanical testing of rubbers using a "NanoScan-4D" while maintaining the same temperature on the sample and on the indentation measuring modulus is proposed, which ensures uniformity of the temperature of the sample and minimizes temperature drifts during testing. The use of a small radius tips allows of measuring physical properties within a small surface volume. With a decrease in temperature, a significant increase in the elastic modulus of the sample was observed on the studied rubber samples. The temperature dependence of the phase shift is nonmonotonic and has the maximum that corresponds to the maximum ratio of the loss modulus to the elasticity modulus of the material. ■
This work was financially supported by the RFBR grant 18-08-00558-a "Improving probe methods for studying the surfaces of tribotechnical materials based on contact interaction mechanics".
INTRODUCTION
The development of methods for studying the mechanical properties of a material surface layer in the temperature range from –60 to 60 °С is especially relevant for the elastomers used in friction joints.
Nano-hardness meters are the standard equipment to study the physical and mechanical properties. The key method applied in these instruments is based on indention of a hard tip (indenter) in the studied sample surface simultaneously with measuring of the indenter displacement and the applied force. This method makes it possible to measure hardness and elasticity modulus. The measurement area of mechanical properties is the surface layer of the sample which volume is determined by the depth of the indenter penetration. The method of instrumental indention is regulated by GOST Р 8.748-2011. The elasticity modulus of the rubber near-surface layer that has passed wear tests may be significantly different from the value obtained in the volume of the material [1]. One of the problems reflecting the topicality of the presented paper is a study of viscoelastic properties of rubber and their changes by depth from the surface of the worn-out material.
As a rule, measurements last tens of seconds, which corresponds to the quasi-static loading mode [2]. The possibilities of the method may be much more expanded in the dynamic mode, when periodical loading is applied to the sample, and some time-dependent properties maybe calculated.
Usually, special cells [3] are used to perform the tests at different temperatures by hardness meters in order to maintain the necessary temperature in measurement area. A small sample and an indenter are placed inside the cell. The measuring sensors are placed outside the instrument at room temperature and are connected to the tip by a long inflexible rod. Such a design usually does not allow of the uniform heating of the sample, and the movement of the indenter and its contact with the sample lead to a change in the temperature gradient along the rod that connects the indenter with the force and displacement sensors. The temperature changes make it difficult to accurately measure the tip penetration depth into the sample. Besides, such type of a design, as a rule, significantly limits the sample dimensions to a few millimeters and does not allow of comprehensive studies of the mechanical properties for the wide range of objects.
In the presented paper we propose to maintain the same temperature in a sample and in the measuring sensor during the temperature tests. The measuring equipment with samples to be tested is placed in a chamber where the necessary temperature is maintained. An electronic unit and a computer for processing the hardness meter data are placed outside of the temperature chamber.
Studies of the mechanical properties of the motor vehicle tread rubber tires in a wide temperature range have been carried out.
METHODS OF MEASURING
Dynamical mechanical analysis
Dynamic mechanical analysis [5, 6] means the testing method and the instrument for measuring the mechanical and viscoelastic properties of rubbers (real and imaginary part of the complex elasticity modulus, mechanical loss tangent, etc.) and enables to obtain frequency-temperature dependences of relaxation processes in rubber at deformation under periodic loading. A spherical ceramic tip (0.5 mm, made of silicon nitride) was penetrated into the sample until achievement of the maximum load of 100 mN. The harmonic oscillations of 25 mN amplitude in a frequency range from 0.01 to 80 Hz during 5 min were applied to the indenter at a constant average loading force F. During the tests the tip displacement amplitude, related to the contact rigidity (Scont) of a tip with a sample, and a phase difference (δ) between the displacement signal and loading force were measured. The equations describe the force applied to the indenter and its displacement as follows:
F = Fquasi–static + F0eiωt ,
h = hquasi–static + h0ei(ωt+δ).
The real and imaginary parts of the complex elasticity modulus are determined by the following relationships:
where А – contact area, – contact rigidity, where С – coefficient of viscous friction.
SAMPLES AND EQUIPMENT
DESCRIPTION OF THE SAMPLES
DM tests were performed on two samples. The first sample was made of a tread rubber of mainline all-steel (AST) tyre. The second one was made of a tread rubber of summer tyre. The samples to be studied were measured using tribometer where the counterbody was a disk with the glued abrasive paper made of silicon carbide with a grain size of 120 µm [7]. The depth of the disturbed layer of the studied surface is comparable with the abrasive grains and is equal to about 10 µm (See Fig.1). The result of the dry friction was a high roughness of the surface (Ra – 30 µm). It was measured using a 3D-profilometer S neox (Sensofar).
"NanoScan-4D" nano-hardness meter
The tests were performed using a "NanoScan-4D" [4]. The portable model of this device does not exceed 300 mm in size and it was entirely placed into the climate chamber. The highest reliability of design of the device allows of using it in a wide temperature range. The portable design of the device has an indentation modulus and two motorized translators to provide the mutual movement of the indenter and a sample. The design of the translators enables to use them in the temperature range from –60 to 60 °С without parasitic effects such as blocking, loss of rigidity, play, etc. The indentation modulus permits to perform the standard tests by nano-indention method in the dynamic and quasi-static modes. In any test method, a tip of an arbitrary shape can be selected. The standard tip for mechanical testing presents an indentor of the Berkovich type. In this work we used the spherical tip in order to reduce the plasticity deformation of the sample.
КТХВ-300 climate chamber
The temperature was maintained in the range of –60…60 °С with the aid of КТХВ-300 climate chamber. It is possible to maintain air humidity in the range of 20…98% to prevent frost on the moving parts when the temperature drops. Accuracy of temperature control in the chamber was 0.5 °С. To minimize a temperature drift and vibrations, a protective box was used, which inner surface was coated with acoustic foaming plastic and the joints were glued with a GERLEN sealing tape. We used the passive vibration-isolating platform. Air temperature was controlled by sensors built into the climate chamber. The temperature of the device case was controlled using an LT-300 electronic thermometer with a digital resolution of 0.01 °C.
RESULTS
The climate chamber compressors are the main source of noise in the described experiment design. The use of vibration isolation made it possible to significantly reduce the noise level when measuring depth during indentation of the tip. The temperature drift of the system was no more than 0.5 nm/s. Indention depth of the tip was in the range of about 1–20 µm. As a result of the tests, the temperature dependences of the elasticity modulus and the mechanical loss tangent were constructed for two rubber samples (See Fig.2). The test results are consistent with DMA data performed on bulk samples on a METRAVIB dynamomechanical analyzer.
CONCLUSIONS
A scheme for conducting dynamic mechanical testing of rubbers using a "NanoScan-4D" while maintaining the same temperature on the sample and on the indentation measuring modulus is proposed, which ensures uniformity of the temperature of the sample and minimizes temperature drifts during testing. The use of a small radius tips allows of measuring physical properties within a small surface volume. With a decrease in temperature, a significant increase in the elastic modulus of the sample was observed on the studied rubber samples. The temperature dependence of the phase shift is nonmonotonic and has the maximum that corresponds to the maximum ratio of the loss modulus to the elasticity modulus of the material. ■
This work was financially supported by the RFBR grant 18-08-00558-a "Improving probe methods for studying the surfaces of tribotechnical materials based on contact interaction mechanics".
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