Rotational Tribology Application Notes - NANOVEA: Advanced Profilometers, Tribometers, Nanoindenters, and Scratch Testers for Materials Testing

Rock Abrasivity Testing with NANOVEA Tribometer

nanovea — Wed, 13 Sep 2023 17:07:17 +0000

ROCK TRIBOLOGY:ROCK ABRASIVITY TESTING USING NANOVEA TRIBOMETER

Prepared by

DUANJIE LI, PhD

INTRODUCTION

Rocks are composed of grains of minerals. The type and abundance of these minerals, as well as the chemical bonding strength between the mineral grains, determine the mechanical and tribological properties of the rocks. Depending on the geological rock cycles, rocks can undergo transformations and are typically classified into three major types: igneous, sedimentary, and metamorphic. These rocks exhibit different mineral and chemical compositions, permeabilities, and particle sizes, and such characteristics contribute to their varied wear resistance. Rock tribology explores the wear and friction behaviors of rocks in various geological and environmental conditions.

IMPORTANCE OF ROCK ABRASIVE TESTING

Various types of wear against rocks, including abrasion and friction, occur during the drilling process of wells, leading to significant direct and consequential losses attributed to the repair and replacement of drill bits and cutting tools. Therefore, the study of drillability, boreability, cuttability, and abrasivity of rocks are critical in the oil, gas, and mining industries. Rock tribology research plays a pivotal role in the selection of the most efficient and cost-effective drilling strategies, thereby enhancing overall efficiency and contributing to the conservation of materials, energy, and the environment. Additionally, minimizing surface friction is highly advantageous in reducing the interaction between the drilling bit and the rock, resulting in decreased tool wear and improved drilling/cutting efficiency.

MEASUREMENT OBJECTIVE

In this study, we simulated and compared the tribological properties of two types of rocks to showcase the capacity of the NANOVEA T50 Tribometer in measuring the coefficient of friction and wear rate of rocks in a controlled and monitored manner.

NANOVEA T50 Compact
Free Weight Tribometer

THE SAMPLES

TEST PROCEDURE

The coefficient of friction, COF, and the wear resistance of two rock samples were evaluated by the NANOVEA T50 Tribometer using Pin-on-Disc Wear Module. An Al2O3 ball (6 mm diameter) was used as the counter material. The wear track was examined using the NANOVEA Non-Contact Profilometer after the tests. The test parameters are summarized below.

The wear rate, K, was evaluated using the formula K=V/(F×s)=A/(F×n), where V is the worn volume, F is the normal load, s is the sliding distance, A is the cross-sectional area of the wear track, and n is the number of revolutions. Surface roughness and wear track profiles were evaluated with the NANOVEA Optical Profilometer, and the wear track morphology was examined using an optical microscope.

Please note that the Al2O3 ball as a counter material was used as an example in this study. Any solid material with different shapes can be applied using a custom fixture to simulate the actual application situation.

TEST PARAMETERS

SAMPLES	Limestone, Marble
WEAR RING RADIUS	5 mm
NORMAL FORCE	10 N
TEST DURATION	10 min
SPEED	100 rpm

RESULTS & DISCUSSION

The hardness (H) and Elastic Modulus (E) of the limestone and marble samples are compared in FIGURE 1, utilizing the Micro Indentation module of the NANOVEA Mechanical Tester. The limestone sample exhibited lower H and E values, measuring at 0.53 and 25.9 GPa, respectively, in contrast to marble, which recorded values of 1.07 for H and 49.6 GPa for E. The relatively higher variability in the H and E values observed in the limestone sample can be attributed to its greater surface inhomogeneity, stemming from its granulated and porous characteristics.

The evolution of the COF during the wear tests of the two rock samples is depicted in FIGURE 2. The limestone initially experiences a rapid increase in COF to approximately 0.8 at the beginning of the wear test, maintaining this value throughout the duration of the test. This abrupt change in COF can be attributed to the penetration of the Al2O3 ball into the rock sample, resulting from a rapid wear and roughening process occurring at the contact face within the wear track. In contrast, the marble sample exhibits a notable increase in COF to higher values after approximately 5 meters of sliding distance, signifying its superior wear resistance when compared to the limestone.

FIGURE 1: Hardness and Young’s Modulus comparison between limestone and marble samples.

FIGURE 2: Evolution of Coefficient of Friction (COF) in limestone and marble samples during wear tests.

FIGURE 3 compares cross-sectional profiles of the limestone and marble samples after the wear tests, and Table 1 summarizes the results of the wear track analysis. FIGURE 4 shows the wear tracks of the samples under the optical microscope. The wear track evaluation aligns with the COF evolution observation: The marble sample, which maintains a low COF for a longer period, exhibits a lower wear rate of 0.0046 mm³/N m, compared to 0.0353 mm³/N m for the limestone. The superior mechanical properties of marble contribute to its better wear resistance than limestone.

FIGURE 3: Cross-section profiles of the wear tracks.

TABLE 1: Result summary of wear track analysis.

FIGURE 4: Wear tracks under optical microscope.

CONCLUSION

In this study, we showcased the capacity of the NANOVEA Tribometer in evaluating the coefficient of friction and wear resistance of two rock samples, namely marble and limestone, in a controlled and monitored manner. The superior mechanical properties of marble contribute to its exceptional wear resistance. This property makes it challenging to drill or cut in the oil and gas industry. Conversely, it significantly extends its lifetime when used as a high-quality building material, such as floor tiles.

NANOVEA Tribometers offer precise and repeatable wear and friction testing capabilities, adhering to ISO and ASTM standards in both rotative and linear modes. Additionally, it provides optional modules for high-temperature wear, lubrication, and tribocorrosion, all seamlessly integrated into one system. NANOVEA’s unmatched range is an ideal solution for determining the full range of tribological properties of thin or thick, soft or hard coatings, films, substrates, and rock tribology.

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Industrial Coatings Scratch and Wear Evaluation

nanovea — Fri, 27 May 2022 22:23:41 +0000

INDUSTRIAL COATING

SCRATCH AND WEAR EVALUATION USING A TRIBOMETER

Prepared by

DUANJIE LI, PhD & ANDREA HERRMANN

INTRODUCTION

Acrylic urethane paint is a type of fast-dry protective coating widely used in a variety of industrial applications, such as floor paint, auto paint, and others. When used as floor paint, it can serve areas with heavy foot and rubber-wheel traffic, such as walkways, curbs and parking lots.

IMPORTANCE OF SCRATCH AND WEAR TESTING FOR QUALITY CONTROL

Traditionally, Taber abrasion tests were carried out to evaluate the wear resistance of acrylic urethane floor paint according to the ASTM D4060 standard. However, as mentioned in the standard, “For some materials, abrasion tests utilizing the Taber Abraser may be subject to variation due to changes in the abrasive characteristics of the wheel during testing.”1 This may result in poor reproducibility of test results and create difficulty in comparing values reported from different laboratories. Moreover, in Taber abrasion tests, abrasion resistance is calculated as loss in weight at a specified number of abrasion cycles. However, acrylic urethane floor paints have a recommended dry film thickness of 37.5-50 μm2.

The aggressive abrasion process by Taber Abraser can quickly wear through the acrylic urethane coating and create mass loss to the substrate leading to substantial errors in the calculation of the paint weight loss. The implant of abrasive particles in the paint during the abrasion test also contributes to errors. Therefore, a well-controlled quantifiable and reliable measurement is crucial to ensure reproducible wear evaluation of the paint. In addition, the scratch test allows users to detect premature adhesive/cohesive failures in real-life applications.

MEASUREMENT OBJECTIVE

In this study, we showcase that NANOVEA Tribometers and Mechanical Testers are ideal for evaluation and quality control of industrial coatings.

The wear process of acrylic urethane floor paints with different topcoats is simulated in a controlled and monitored manner using the NANOVEA Tribometer. Micro scratch testing is used to measure the load required to cause cohesive or adhesive failure to the paint.

NANOVEA T100

The Compact Pneumatic Tribometer

NANOVEA PB1000

The Large Platform Mechanical Tester

TEST PROCEDURE

This study evaluates four commercially available water-based acrylic floor coatings that have the same primer (basecoat) and different topcoats of the same formula with a small alternation in the additive blends for the purpose of enhancing durability. These four coatings are identified as Samples A, B, C and D.

WEAR TEST

The NANOVEA Tribometer was applied to evaluate the tribological behavior, e.g. coefficient of friction, COF, and wear resistance. A SS440 ball tip (6 mm dia., Grade 100) was applied against the tested paints. The COF was recorded in situ. The wear rate, K, was evaluated using the formula K=V/(F×s)=A/(F×n), where V is the worn volume, F is the normal load, s is the sliding distance, A is the cross-sectional area of the wear track, and n is the number of revolution. Surface roughness and wear track profiles were evaluated by the NANOVEA Optical Profilometer, and the wear track morphology was examined using optical microscope.

WEAR TEST PARAMETERS

NORMAL FORCE

20 N

SPEED

15 m/min

DURATION OF TEST

100, 150, 300 & 800 cycles

SCRATCH TEST

The NANOVEA Mechanical Tester equipped with a Rockwell C diamond stylus (200 μm radius) was used to perform progressive load scratch tests on the paint samples using the Micro Scratch Tester Mode. Two final loads were used: 5 N final load for investigating paint delamination from the primer, and 35 N for investigating primer delamination from the metal substrates. Three tests were repeated at the same testing conditions on each sample to ensure reproducibility of the results.

Panoramic images of the whole scratch lengths were automatically generated and their critical failure locations were correlated with the applied loads by the system software. This software feature facilitates users to perform analysis on the scratch tracks any time, rather than having to determine the critical load under the microscope immediately after the scratch tests.

SCRATCH TEST PARAMETERS

LOAD TYPE	Progressive
INITIAL LOAD	0.01 mN
FINAL LOAD	5 N / 35 N
LOADING RATE	10 / 70 N/min
SCRATCH LENGTH	3 mm
SCRATCHING SPEED, dx/dt	6.0 mm/min
INDENTER GEOMETRY	120º cone
INDENTER MATERIAL (tip)	Diamond
INDENTER TIP RADIUS	200 μm

WEAR TEST RESULTS

Four pin-on-disk wear tests at different number of revolutions (100, 150, 300 and 800 cycles) were performed on each sample in order to monitor the evolution of wear. The surface morphology of the samples were measured with a NANOVEA 3D Non-Contact Profiler to quantify the surface roughness prior to conducting wear testing. All samples had a comparable surface roughness of approximately 1 μm as displayed in FIGURE 1. The COF was recorded in situ during the wear tests as shown in FIGURE 2. FIGURE 4 presents the evolution of wear tracks after 100, 150, 300 and 800 cycles, and FIGURE 3 summarized the average wear rate of different samples at different stages of the wear process.

Compared with a COF value of ~0.07 for the other three samples, Sample A exhibits a much higher COF of ~0.15 at the beginning, which gradually increases and gets stable at ~0.3 after 300 wear cycles. Such a high COF accelerates the wear process and creates a substantial amount of paint debris as indicated in FIGURE 4 – the topcoat of Sample A has started to be removed in the first 100 revolutions. As shown in FIGURE 3, Sample A exhibits the highest wear rate of ~5 μm2/N in the first 300 cycles, which slightly decreases to ~3.5 μm2/N due to the better wear resistance of the metal substrate. The topcoat of Sample C starts to fail after 150 wear cycles as shown in FIGURE 4, which is also indicated by the increase of COF in FIGURE 2.

In comparison, Sample B and Sample D show enhanced tribological properties. Sample B maintains a low COF throughout the whole test – the COF slightly increases from~0.05 to ~0.1. Such a lubricating effect substantially enhances its wear resistance – the topcoat still provides superior protection to the primer underneath after 800 wear cycles. The lowest average wear rate of only ~0.77 μm2/N is measured for Sample B at 800 cycles. The topcoat of Sample D starts to delaminate after 375 cycles, as reflected by the abrupt increase of COF in FIGURE 2. The average wear rate of Sample D is ~1.1 μm2/N at 800 cycles.

Compared to the conventional Taber abrasion measurements, NANOVEA Tribometer provides well-controlled quantifiable and reliable wear assessments that ensure reproducible evaluations and quality control of commercial floor/auto paints. Moreover, the capacity of in situ COF measurements allow users to correlate the different stages of a wear process with the evolution of COF, which is critical in improving fundamental understanding of the wear mechanism and tribological characteristics of various paint coatings.

FIGURE 1: 3D morphology and roughness of the paint samples.

FIGURE 2: COF during pin-on-disk tests.

FIGURE 3: Evolution of wear rate of different paints.

FIGURE 4: Evolution of wear tracks during the pin-on-disk tests.

SCRATCH TEST RESULTS

FIGURE 5 shows the plot of normal force, frictional force and true depth as a function of scratch length for Sample A as an example. An optional acoustic emission module can be installed to provide more information. As the normal load linearly increases, the indentation tip gradually sinks into the tested sample as reflected by the progressive increase of true depth. The variation in the slopes of frictional force and true depth curves can be used as one of the implications that coating failures start to occur.

FIGURE 5: Normal force, frictional force and true depth as a function of scratch length for scratch test of Sample A with a maximum load of 5 N.

FIGURE 6 and FIGURE 7 show the full scratches of all four paint samples tested with a maximum load of 5 N and 35 N, respectively. Sample D required a higher load of 50 N to delaminate the primer. Scratch tests at 5 N final load (FIGURE 6) evaluate the cohesive/adhesive failure of the top paint, while the ones at 35 N (FIGURE 7) assess the delamination of the primer. The arrows in the micrographs indicate the point at which the top coating or the primer start to be completely removed from the primer or the substrate. The load at this point, so called Critical Load, Lc, is used to compare the cohesive or adhesive properties of the paint as summarized in Table 1.

It is evident that the paint Sample D has the best interfacial adhesion – exhibiting the highest Lc values of 4.04 N at paint delamination and 36.61 N at primer delamination. Sample B shows the second best scratch resistance. From the scratch analysis, we show that optimization of the paint formula is critical to the mechanical behaviors, or more specifically, scratch resistance and adhesion property of acrylic floor paints.

Table 1: Summary of critical loads.

FIGURE 6: Micrographs of full scratch with 5 N maximum load.

FIGURE 7: Micrographs of full scratch with 35 N maximum load.

CONCLUSION

Compared to the conventional Taber abrasion measurements, the NANOVEA Mechanical Tester and Tribometer are superior tools for evaluation and quality control of commercial floor and automotive coatings. The NANOVEA Mechanical Tester in Scratch mode can detect adhesion/cohesion problems in a coating system. The NANOVEA Tribometer provides well-controlled quantifiable and repeatable tribological analysis on wear resistance and coefficient of friction of the paints.

Based on the comprehensive tribological and mechanical analyses on the water based acrylic floor coatings tested in this study, we show that Sample B possesses the lowest COF and wear rate and the second best scratch resistance, while Sample D exhibits the best scratch resistance and second best wear resistance. This assessment allows us to evaluate and select the best candidate targeting the needs in different application environments.

The Nano and Micro modules of the NANOVEA Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest range of testing available for paint evaluation on a single module. The NANOVEA Tribometer offers precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system. NANOVEA’s unmatched range is an ideal solution for determining the full range of mechanical/tribological properties of thin or thick, soft or hard coatings, films and substrates, including hardness, Young’s modulus, fracture toughness, adhesion, wear resistance and many others. Optional NANOVEA Non-Contact Optical Profilers are available for high resolution 3D imaging of scratchs and wear tracks in addition to other surface measurements such as roughness.

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Sandpaper Abrasion Performance Using a Tribometer

nanovea — Mon, 01 Nov 2021 20:39:55 +0000

SANDPAPER ABRASION PERFORMANCE

USING A TRIBOMETER

Prepared by

DUANJIE LI, PhD

INTRODUCTION

Sandpaper consists of abrasive particles glued to one face of a paper or cloth. Various abrasive materials can be used for the particles, such as garnet, silicon carbide, aluminum oxide and diamond. Sandpaper is widely applied in a variety of industrial sectors to create specific surface finishes on wood, metal and drywall. They often work under high pressure contact applied by hand or power tools.

IMPORTANCE OF EVALUATING SANDPAPER ABRASION PERFORMANCE

The effectiveness of sandpaper is often determined by its abrasion performance under different conditions. The grit size, i.e. the size of the abrasive particles embedded in the sandpaper, determines the wear rate and the scratch size of the material being sanded. Sandpapers of higher grit numbers have smaller particles, resulting in lower sanding speeds and finer surface finishes. Sandpapers with the same grit number but made of different materials can have unalike behaviors under dry or wet conditions. Reliable tribological evaluations are needed to ensure that manufactured sandpaper possesses the desired abrasive behavior intended. These evaluations allow users to quantitatively compare the wear behaviors of different types of sandpapers in a controlled and monitored manner in order to select the best candidate for the target application.

MEASUREMENT OBJECTIVE

In this study, we showcase the NANOVEA T2000 High Load Pneumatic Tribometer’s ability to quantitatively evaluate the abrasion performance of various sandpaper samples under dry and wet conditions.

NANOVEA T2000 High Load
Pneumatic Tribometer

TEST PROCEDURES

The coefficient of friction (COF) and the abrasion performance of two types of sandpapers were evaluated by the NANOVEA T100 Tribometer. A 440 stainless steel ball was used as the counter material. The ball wear scars were examined after each wear test using the NANOVEA 3D Non-Contact Optical Profiler to ensure precise volume loss measurements.

Please note that a 440 stainless steel ball was chosen as the counter material to create a comparative study but any solid material could be substituted to simulate a different application condition.

TEST RESULTS & DISCUSSION

FIGURE 1 shows a COF comparison of Sandpaper 1 and 2 under dry and wet environmental conditions. Sandpaper 1, under dry conditions, shows a COF of 0.4 at the beginning of the test which progressively decreases and stabilizes to 0.3. Under wet conditions, this sample exhibits a lower average COF of 0.27. In contrast, Sample 2’s COF results show a dry COF of 0.27 and wet COF of ~ 0.37.

Please note the oscillation in the data for all COF plots was caused by the vibrations generated by the sliding movement of the ball against the rough sandpaper surfaces.

FIGURE 1: Evolution of COF during the wear tests.

FIGURE 2 summarizes the results of the wear scar analysis. The wear scars were measured using an optical microscope and a NANOVEA 3D Non-Contact Optical Profiler. FIGURE 3 and FIGURE 4 compare the wear scars of the worn SS440 balls post wear tests on Sandpaper 1 and 2 (wet and dry conditions). As shown in FIGURE 4 the NANOVEA Optical Profiler precisely captures the surface topography of the four balls and their respective wear tracks which were then processed with the NANOVEA Mountains Advanced Analysis software to calculate volume loss and wear rate. On the microscope and profile image of the ball it can be observed that the ball used for Sandpaper 1 (dry) testing exhibited a larger flattened wear scar compared to the others with a volume loss of 0.313 mm³. In contrast, the volume loss for Sandpaper 1 (wet) was 0.131 mm³. For Sandpaper 2 (dry) the volume loss was 0.163 mm³ and for Sandpaper 2 (wet) the volume loss increased to 0.237 mm³.

Moreover, it is interesting to observe that the COF played an important role in the abrasion performance of the sandpapers. Sandpaper 1 exhibited higher COF in the dry condition, leading to a higher abrasion rate for the SS440 ball used in the test. In comparison, the higher COF of Sandpaper 2 in the wet condition resulted in a higher abrasion rate. The wear tracks of the sandpapers after the measurements are displayed in FIGURE 5.

Both Sandpapers 1 and 2 claim to work in either dry and wet environments. However, they exhibited significantly different abrasion performance in the dry and wet conditions. NANOVEA tribometers provide well-controlled quantifiable and reliable wear assessment capabilities that ensure reproducible wear evaluations. Moreover, the capacity of in situ COF measurement allows users to correlate different stages of a wear process with the evolution of COF, which is critical in improving fundamental understanding of the wear mechanism and tribological characteristics of sandpaper

FIGURE 2: Wear scar volume of the balls and average COF under different conditions.

FIGURE 3: Wear scars of the balls after the tests.

FIGURE 4: 3D morphology of the wear scars on the balls.

FIGURE 5: Wear tracks on the sandpapers under different conditions.

CONCLUSION

The abrasion performance of two types of sandpapers of the same grit number were evaluated under dry and wet conditions in this study. The service conditions of the sandpaper play a critical role in the effectiveness of the work performance. Sandpaper 1 possessed significantly better abrasion behavior under dry conditions, while Sandpaper 2 performed better under wet conditions. The friction during the sanding process is an important factor to consider when evaluating abrasion performance. The NANOVEA Optical Profiler precisely measures the 3D morphology of any surface, such as wear scars on a ball, ensuring reliable evaluation on the abrasion performance of the sandpaper in this study. The NANOVEA Tribometer measures the coefficient of friction in situ during a wear test, providing an insight on the different stages of a wear process. It also offers repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear and lubrication modules available in one pre-integrated system. This unmatched range allows users to simulate different severe work environment of the ball bearings including high stress, wear and high temperature, etc. It also provides an ideal tool to quantitatively assess the tribological behaviors of superior wear resistant materials under high loads.

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Glass Coating Humidity Wear Testing by Tribometer

nanovea — Thu, 13 May 2021 19:40:10 +0000

Glass Coating Humidity Wear Testing by Tribometer

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GLASS COATING HUMIDITY

WEAR TESTING BY TRIBOMETER

Prepared by

DUANJIE LI, PhD

INTRODUCTION

Self-cleaning glass coating creates an easy-clean glass surface that prevents buildup of grime, dirt and staining. Its self-cleaning feature significantly reduces the frequency, time, energy and cleaning costs, making it an attractive choice for a variety of residential and commercial applications, such as glass facade, mirrors, shower glasses, windows and windshields.

IMPORTANCE OF WEAR RESISTANCE OF SELF-CLEANING GLASS COATING

A major application of the self-cleaning coating is the exterior surface of the glass facade on skyscrapers. The glass surface is often attacked by high-speed particles carried by strong winds. The weather condition also plays a major role in the service lifetime of the glass coating. It can be very difficult and costly to surface treat the glass and apply the new coating when the old one fails. Therefore, the wear resistance of the glass coating under
different weather condition is critical.

In order to simulate the realistic environmental conditions of the self-cleaning coating in different weather, repeatable wear evaluation in a controlled and monitored humidity is needed. It allows users to properly compare the wear resistance of the self-cleaning coatings exposed to different humidity and to select the best candidate for the targeted application.

MEASUREMENT OBJECTIVE

In this study, we showcased that the NANOVEA T100 Tribometer equipped with a humidity controller is an ideal tool for investigating the wear resistance of self-cleaning glass coatings in different humidity.

NANOVEA

T100

TEST PROCEDURES

The soda lime glass microscope slides were coated with self-clean glass coatings with two different treatment recipes. These two coatings are identified as Coating 1 and Coating 2. An uncoated bare glass slide is also tested for comparison.

NANOVEA Tribometer equipped with a humidity control module was used to evaluate the tribological behavior, e.g. coefficient of friction, COF, and wear resistance of the self-clean glass coatings. A WC ball tip (6 mm dia.) was applied against the tested samples. The COF was recorded in situ. The humidity controller attached to the tribo-chamber precisely controlled the relative humidity (RH) value in the range of ±1 %. The wear track morphology was examined under the optical microscope after the wear tests.

MAXIMUM LOAD 40 mN

RESULTS & DISCUSSION

The pin-on-disk wear tests in different humidity conditions were conducted on the coated and uncoated glass
samples. The COF was recorded in situ during the wear tests as shown in FIGURE 1 and the average COF is summarized in FIGURE 2. FIGURE 4 compares the wear tracks after the wear tests.

As shown in FIGURE 1, the uncoated glass exhibits a high COF of ~0.45 once the sliding movement begins in the 30% RH, and it progressively increases to ~0.6 at the end of the 300-revolution wear test. In comparison, the
coated glass samples Coating 1 and Coating 2 show a low COF below 0.2 at the beginning of the test. The COF
of Coating 2 stabilizes at ~0.25 during the rest of the test, while Coating 1 exhibits a sharp increase of COF at
~250 revolutions and the COF reaches a value of ~0.5. When the wear tests are carried out in the 60% RH, the
uncoated glass still shows a higher COF of ~0.45 throughout the wear test. Coatings 1 and 2 exhibit the COF values of 0.27 and 0.22, respectively. In the 90% RH, the uncoated glass possesses a high COF of ~0.5 at the end of the wear test. Coatings 1 and 2 exhibit comparable COF of ~0.1 as the wear test starts. Coating 1 maintains a relatively stable COF of ~0.15. Coating 2, however, fails at ~ 100 revolutions, followed by a significant increase of COF to ~0.5 towards the end of the wear test.

The low friction of the self-clean glass coating is caused by its low surface energy. It creates a very high static
water contact angle and low roll-off angle. It leads to formation of small water droplets on the coating surface in the 90% RH as shown under the microscope in FIGURE 3. It also results in decrease of the average COF from ~0.23 to ~0.15 for Coating 2 as the RH value increases from 30% to 90%.

FIGURE 1: Coefficient of friction during the pin-on-disk tests in different relative humidity.

FIGURE 2: Average COF during the pin-on-disk tests in different relative humidity.

FIGURE 3: Formation of small water droplets on the coated glass surface.

FIGURE 4 compares the wear tracks on the glass surface after the wear tests in different humidity. Coating 1 exhibits signs of mild wear after the wear tests in the RH of 30% and 60%. It possesses a large wear track after the test in the 90% RH, in agreement with the significant increase of COF during the wear test. Coating 2 shows nearly no sign of wear after the wear tests in both dry and wet environment, and it also exhibits constant low COF during the wear tests in different humidity. The combination of good tribological properties and low surface energy makes Coating 2 a good candidate for self-cleaning glass coating applications in harsh environments. In comparison, the uncoated glass shows larger wear tracks and higher COF in different humidity, demonstrating the necessity of self-cleaning coating technique.

FIGURE 4: Wear tracks after the pin-on-disk tests in different relative humidity (200x magnification).

CONCLUSION

NANOVEA T100 Tribometer is a superior tool for evaluation and quality control of self-cleaning glass coatings in different humidity. The capacity of in-situ COF measurement allows users to correlate different stages of wear process with the evolution of COF, which is critical in improving fundamental understanding of the wear mechanism and tribological characteristics of the glass coatings. Based on the comprehensive tribological analysis on the self-cleaning glass coatings tested in different humidity, we show that Coating 2 possesses a constant low COF and superior wear resistance in both dry and wet environments, making it a better candidate for self-cleaning glass coating applications exposed to different weathers.

NANOVEA Tribometers offer precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system. Optional 3D non-contact profiler is available for high
resolution 3D imaging of wear track in addition to other surface measurements such as roughness.

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In Situ Wear Measurement at High Temperature

Andrew Shore — Tue, 29 Dec 2020 22:20:45 +0000

IN SITU WEAR MEASUREMENT AT HIGH TEMPERATURE

USING TRIBOMETER

Prepared by

Duanjie Li, PhD

INTRODUCTION

The Linear Variable Differential Transformer (LVDT) is a type of robust electrical transformer used to measure linear displacement. It has been widely used in a variety of industrial applications, including power turbines, hydraulics, automation, aircraft, satellites, nuclear reactors, and many others.

In this study, we feature the add-ons of LVDT and high temperature modules of the NANOVEA Tribometer which allow the change of wear track depth of the tested sample to be measured during the wear process at elevated temperatures. This enables users to correlate different stages of wear process with the evolution of COF, which is critical in improving fundamental understanding of the wear mechanism and tribological characteristics of the materials for high temperature applications.

MEASUREMENT OBJECTIVE

In this study. we would like to showcase the capacity of NANOVEA T50 Tribometer for in situ monitoring the evolution of the wear process of materials at elevated temperatures.

The wear process of the alumina silicate ceramic at different temperatures is simulated in a controlled and monitored manner.

NANOVEA

T50

TEST PROCEDURE

The tribological behavior, e. g. coefficient of friction, COF, and wear resistance of alumina silicate ceramic plates was evaluated by the NANOVEA Tribometer. The alumina silicate ceramic plate was heated up by a furnace from room temperature, RT, to elevated temperatures (400°C and 800°C), followed by the wear tests at such temperatures.

For comparison, the wear tests were carried out when the sample cooled down from 800°C to 400°C and then to room temperature. An AI2O3 ball tip (6mm dia., Grade 100) was applied against the tested samples. The COF, wear depth and temperature were monitored in situ.

TEST PARAMETERS

of the pin-on-disk measurement

The wear rate, K, was evaluated using the formula K=V/(Fxs)=A/(Fxn), where V is the worn volume, F is the normal load, s is the sliding distance, A is the cross-sectional area of the wear track, and n is the number of revolution. Surface roughness and wear track profiles were evaluated by the NANOVEA Optical Profiler, and the wear track morphology was examined using an optical microscope.

RESULTS & DISCUSSION

The COF and wear track depth recorded in situ are shown in FIGURE 1 and FIGURE 2, respectively. In FIGURE 1, “-I” indicates the test performed when the temperature was increased from RT to an elevated temperature. “-D” represents the temperature decreased from a higher temperature of 800°C.

As shown in FIGURE 1, the samples tested at different temperatures exhibit a comparable COF of ~0.6 throughout the measurements. Such a high COF leads to an accelerated wear process which creates a substantial amount of debris. The wear track depth was monitored during the wear tests by LVDT as shown in FIGURE 2. The tests performed at room temperature before sample heating up and after sample cooling down show that the alumina silicate ceramic plate exhibits a progressive wear process at RT, the wear track depth gradually increases throughout the wear test to ~170 and ~150 μm, respectively.

In comparison, the wear tests at elevated temperatures (400°C and 800°C) exhibit a different wear behavior – the wear track depth increases promptly at the beginning of the wear process, and it slows down as the test continues. The wear track depths for tests performed at temperatures 400°C-I, 800°C and 400°C-D are ~140, ~350 and ~210 μm, respectively.

FIGURE 1. Coefficient of Friction during pin-on-disk tests at different temperatures

FIGURE 2. Evolution of wear track depth of the alumina silicate ceramic plate at different temperatures

The average wear rate and wear track depth of the alumina silicate ceramic plates at different temperatures were measured using NANOVEA Optical Profiler as summarized in FIGURE 3. The wear track depth is in agreement with that recorded using LVDT. The alumina silicate ceramic plate shows a substantially increased wear rate of ~0.5 mm3/Nm at 800°C, compared to the wear rates below 0.2mm3/N at temperatures below 400°C. The alumina silicate ceramic plate does not exhibit significantly enhanced mechanical/tribological properties after the short heating process, possessing a comparable wear rate before and after the heat treatment.

Alumina silicate ceramic, also knows as lava and wonderstone, is soft and machinable before heating treatment. A long process of firing at elevated temperatures up to 1093°C can substantially enhance its hardness and strength, after which diamond machining is required. Such a unique characteristic makes alumina silicate ceramic an ideal material for sculpture.

In this study, we show that heat treatment at a lower temperature that the one required for firing (800°C vs 1093°C) in a short time does not improve the mechanical and tribological characteristics of alumina silicate ceramic, making proper firing an essential process for this material before its usage in the real applications.

FIGURE 3. Wear rate and wear track depth of the sample at different temperatures

CONCLUSION

Based on the comprehensive tribological analysis in this study, we show that the alumina silicate ceramic plate exhibits comparable coefficient of friction at different temperatures from room temperature to 800°C. However, it shows a substantially increased wear rate of ~0.5 mm3/Nm at 800°C, demonstrating the importance of proper heat treatment of this ceramic.

NANOVEA Tribometers are capable of evaluating the tribological properties of materials for applications at high temperatures up to 1000°C. The function of in situ COF and wear track depth measurements allows users to correlate different stages of wear process with the evolution of COF, which is critical in improving fundamental understanding of the wear mechanism and tribological characteristics of the materials used at elevated temperatures.

NANOVEA Tribometers offer precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system. NANOVEA’s unmatched range is an ideal solution for determining the full range of tribological properties of thin or thick, soft or hard coatings, films and substrates.

Optional 3D Non-Contact Profilers are available for high resolution 3D imaging of wear tracks in addition to other surface measurements such as roughness.

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Ball Bearings: High Force Wear Resistance Study

Andrew Shore — Wed, 01 Jul 2020 18:59:15 +0000

INTRODUCTION

A ball bearing uses balls to reduce rotational friction and support radial and axial loads. The rolling balls between the bearing races produce much lower coefficient of friction (COF) compared to two flat surfaces sliding against each other. Ball bearings are often exposed to high contact stress levels, wear and extreme environmental conditions such as high temperatures. Therefore, wear resistance of the balls under high loads and extreme environmental conditions is critical for extending the lifetime of the ball bearing to cut down cost and time on repairs and replacements.
Ball bearings can be found in nearly all applications that involve moving parts. They are commonly used in transportation industries such as aerospace and automobile as well as the toy industry that manufactures items such as fidget spinner and skateboards.

BALL BEARING WEAR EVALUATION AT HIGH LOADS

Ball bearings can be made from an extensive list of materials. Commonly used materials range between metals like stainless steel and chrome steel or ceramics such as tungsten carbide (WC) and silicon nitride (Si3n4). To ensure that the manufactured ball bearings possess the required wear resistance ideal for the given application’s conditions, reliable tribological evaluations under high loads are necessary. Tribological testing aids in quantifying and contrasting the wear behaviors of different ball bearings in a controlled and monitored manner to select the best candidate for the targeted application.

MEASUREMENT OBJECTIVE

In this study, we showcase a Nanovea Tribometer as the ideal tool for comparing the wear resistance of different ball bearings under high loads.

Figure 1: Setup of the bearing test.

TESTING PROCEDURE

The coefficient of friction, COF, and the wear resistance of the ball bearings made of different materials were evaluated by a Nanovea Tribometer. P100 grit sandpaper was used as the counter material. The wear scars of the ball bearings were examined using a Nanovea 3D Non-Contact Profiler after the wear tests concluded. The test parameters are summarized in Table 1. The wear rate, K, was evaluated using the formula K=V/(F×s), where V is the worn volume, F is the normal load and s is the sliding distance. Ball wear scars were evaluated by a Nanovea 3D Non-Contact Profiler to ensure precise wear volume measurement.
The automated motorized radial positioning feature allows the tribometer to decrease the radius of the wear track for the duration of a test. This test mode is called a spiral test and it ensures that the ball bearing always slides on a new surface of the sandpaper (Figure 2). It significantly improves the repeatability of the wear resistance test on the ball. The advanced 20bit encoder for internal speed control and 16bit encoder for external position control provide precise real-time speed and position information, allowing for a continuous adjustment of rotational speed to achieve constant linear sliding speed at the contact.
Please note that P100 Grit sandpaper was used to simplify the wear behavior between various ball materials in this study and can be substituted with any other material surface. Any solid material can be substituted to simulate the performance of a wide range of material couplings under actual application conditions, such as in liquid or lubricant.

Figure 2: Illustration of the spiral passes for the ball bearing on the sandpaper.

Table 1: Test parameters of the wear measurements.

RESULTS & DISCUSSION

Wear rate is a vital factor for determining the service lifetime of the ball bearing, while a low COF is desirable to improve the bearing performance and efficiency. Figure 3 compares the evolution of COF for different ball bearings against the sandpaper during the tests. The Cr Steel ball shows an increased COF of ~0.4 during the wear test, compared to ~0.32 and ~0.28 for SS440 and Al2O3 ball bearings. On the other hand, the WC ball exhibits a constant COF of ~0.2 throughout the wear test. Observable COF variation can be seen throughout each test which is attributed to vibrations caused by the sliding movement of the ball bearings against the rough sandpaper surface.

Figure 3: Evolution of COF during the wear tests.

Figure 4 and Figure 5 compare the wear scars of the ball bearings after they were measured by an optical microscope and Nanovea Non-Contact optical profiler, respectively, and Table 2 summarizes the results of the wear track analysis. The Nanovea 3D profiler precisely determines the wear volume of the ball bearings, making it possible to calculate and compare the wear rates of different ball bearings. It can be observed that the Cr Steel and SS440 balls exhibit much larger flattened wear scars compared to the ceramic balls, i.e. Al2O3 and WC after the wear tests. The Cr Steel and SS440 balls have comparable wear rates of 3.7×10-3 and 3.2×10-3 m3/N m, respectively. In comparison, the Al2O3 ball shows an enhanced wear resistance with a wear rate of 7.2×10-4 m3/N m. The WC ball barely exhibits minor scratches on the shallow wear track area, resulting in a significantly reduced wear rate of 3.3×10-6 mm3/N m.

Figure 4: Wear scars of the ball bearings after the tests.

Figure 5: 3D morphology of the wear scars on the ball bearings.

Table 2: Wear scar analysis of the ball bearings.

Figure 6 shows microscope images of the wear tracks produced on the sand paper by the four ball bearings. It is evident that the WC ball produced the most severe wear track (removing almost all sand particle in its path) and possesses the best wear resistance. In comparison, the Cr Steel and SS440 balls left a large amount of metal debris on the wear track of the sand paper.
These observations further demonstrate the importance of the benefit of a spiral test. It ensures that the ball bearing always slides on a new surface of the sandpaper, which significantly improves the repeatability of a wear resistance test.

Figure 6: Wear tracks on the sand paper against different ball bearings.

CONCLUSION

The wear resistance of the ball bearings under a high pressure plays a vital role in their service performance. The ceramic ball bearings possess significantly enhanced wear resistance under high stress conditions and reduce the time and cost due to bearing repairing or replacement. In this study, the WC ball bearing exhibits a substantially higher wear resistance compared to the steel bearings, making it an ideal candidate for bearing applications where severe wear takes place.
A Nanovea Tribometer is designed with high torque capabilities for loads up to 2000 N and precise and controlled motor for rotational speeds from 0.01 to 15,000 rpm. It offers repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear and lubrication modules available in one pre-integrated system. This unmatched range allows users to simulate different severe work environments of the ball bearings including high stress, wear and high temperature, etc. It also acts as an ideal tool to quantitatively assess the tribological behaviors of superior wear resistant materials under high loads.
A Nanovea 3D Non-Contact Profiler provides precise wear volume measurements and acts as a tool to analyze the detailed morphology of the wear tracks, providing additional insights in the fundamental understanding of wear mechanisms.

Prepared by
Duanjie Li, PhD, Jonathan Thomas, and Pierre Leroux

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Dynamic Load Tribology

nanovea — Tue, 03 Mar 2020 21:32:52 +0000

Dynamic Load Tribology

Introduction

Wear takes place in virtually every industrial sector and imposes costs of ~0.75% of the GDP1. Tribology research is vital in improving production efficiency, application performance, as well as conservation of material, energy, and the environment. Vibration and oscillation inevitably occur in a wide range of tribological applications. Excessive external vibration accelerates the wear process and reduces service performance which leads to catastrophic failures to the mechanical parts.

Conventional dead load tribometers apply normal loads by mass weights. Such a loading technique not only limits the loading options to a constant load, but it also creates intense uncontrolled vibrations at high loads and speeds leading to limited and inconsistent wear behavior assessments. A reliable evaluation of the effect of controlled oscillation on the wear behavior of materials is desirable for R&D and QC in different industrial applications.

Nanovea’s groundbreaking high load tribometer has a maximum load capacity of 2000 N with a dynamic load control system. The advanced pneumatic compressed air loading system enables users to evaluate the tribological behavior of a material under high normal loads with the advantage of damping undesired vibration created during the wear process. Therefore, the load is measured directly with no need for buffer springs used in older designs. A parallel electromagnet oscillating loading module applies well-controlled oscillation of desired amplitude up to 20 N and frequency up to 150 Hz.

Friction is measured with high accuracy directly from the side force applied to the upper holder. The displacement is monitored in situ, providing insight into the evolution of the wear behavior of the test samples. The wear test under controlled oscillation loading can also be performed in corrosion, high temperature, humidity, and lubrication environments to simulate the real work conditions for the tribological applications. An integrated high-speed non-contact profilometer automatically measures the wear track morphology and wear volume in a few seconds.

Measurement Objective

In this study, we showcase the capacity of the Nanovea T2000 Dynamic Load Tribometer in studying the tribological behavior of different coating and metal samples under controlled oscillation loading conditions.

Test Procedure

The tribological behavior, e.g. coefficient of friction, COF, and wear resistance of a 300 µm thick wear-resistant coating was assessed and compared by the Nanovea T2000 Tribometer with a conventional dead load tribometer using a pin on disk setup following ASTM G992.

Separate Cu and TiN coated samples against a 6 mm Al₂0₃ ball under controlled oscillation were evaluated by Dynamic Load Tribology Mode of the Nanovea T2000 Tribometer.

The test parameters are summarized in Table 1.

The integrated 3D profilometer equipped with a line sensor automatically scans the wear track after the tests, providing the most accurate wear volume measurement in seconds.

Results and Discussion

Pneumatic loading system vs. Dead load system

The tribological behavior of a wear-resistant coating using Nanovea T2000 Tribometer is compared to a conventional dead load (DL) tribometer. The evolution of the COF of the coating are shown in Fig. 2. We observe the coating exhibits a comparable COF value of ~0.6 during the wear test. However, the 20 cross-section profiles at different locations of the wear track in Fig. 3 indicate that the coating experienced much more severe wear under the dead load system.

Intense vibrations were generated by the wear process of the dead load system at high load and speed. The massive concentrated pressure at the contact face combined with a high sliding speed creates substantial weight and structure vibration leading to accelerated wear. The conventional dead load tribometer applies load using mass weights. This method is reliable at lower contact loads under mild wear conditions; however, under aggressive wear conditions at higher loads and speeds, the significant vibration causes the weights to bounce repeatedly, resulting in an uneven wear track causing unreliable tribological evaluation. The calculated wear rate is 8.0±2.4 x 10-4 mm3/N m, showing a high wear rate and large standard deviation.

The Nanovea T2000 tribometer is designed with a dynamic control loading system to dampen the oscillations. It applies the normal load with compressed air which minimizes undesired vibration created during the wear process. In addition, active closed loop loading control ensures a constant load is applied throughout the wear test and the stylus follows the depth change of the wear track. A significantly more consistent wear track profile is measured as shown in Fig. 3a, resulting in a low wear rate of 3.4±0.5 x 10-4 mm3/N m.

The wear track analysis shown in Fig. 4 confirms the wear test performed by the pneumatic compressed air loading system of the Nanovea T2000 Tribometer creates a smoother and more consistent wear track compared to the conventional dead load tribometer. In addition, the Nanovea T2000 tribometer measures stylus displacement during the wear process providing further insight into the progress of the wear behavior in situ.

Controlled Oscillation on Wear of the Cu sample

The parallel oscillating loading electromagnet module of the Nanovea T2000 Tribometer enables users to investigate the effect of controlled amplitude and frequency oscillations on the wear behavior of materials. The COF of the Cu samples is recorded in situ as shown in Fig. 6. The Cu sample exhibits a constant COF of ~0.3 during the first 330-revolution measurement, signifying the formation of a stable contact at the interface and relatively smooth wear track. As the wear test continues, the variation of the COF indicates a change in the wear mechanism. In comparison, the wear tests under a 5 N amplitude-controlled oscillation at 50 N exhibits a different wear behavior: the COF increases promptly at the beginning of the wear process, and shows significant variation throughout the wear test. Such behavior of COF indicates that the imposed oscillation in the normal load plays a role in the unstable sliding state at the contact.

Fig. 7 compares the wear track morphology measured by the integrated non-contact optical profilometer. It can be observed that the Cu sample under a controlled oscillation amplitude of 5 N exhibits a much larger wear track with a volume of 1.35 x 109 µm3, compared to 5.03 x 108 µm3 under no imposed oscillation. The controlled oscillation significantly accelerates the wear rate by a factor of ~2.7, showing the critical effect of oscillation on wear behavior.

Controlled Oscillation on Wear of the TiN Coating

The COF and wear tracks of the TiN coating sample are shown in Fig. 8. The TiN coating exhibits significantly different wear behaviors under oscillation as indicated by the evolution of COF during the tests. The TiN coating shows a constant COF of ~0.3 following the run-in period at the beginning of the wear test, due to the stable sliding contact at the interface between the TiN coating and the Al₂O₃ ball. However, when the TiN coating starts to fail, the Al₂O₃ ball penetrates through the coating and slides against the fresh steel substrate underneath. A significant amount of hard TiN coating debris is generated in the wear track at the same time, turning a stable two-body sliding wear into three-body abrasion wear. Such a change of the material couple characteristics leads to the increased variations in the evolution of COF. The imposed 5 N and 10 N oscillation accelerates the TiN coating failure from ~400 revolutions to below 100 revolutions. The larger wear tracks on the TiN coating samples after the wear tests under the controlled oscillation is in agreement with such a change in COF.

Conclusion

The advanced pneumatic loading system of the Nanovea T2000 Tribometer possesses an intrinsic advantage as a naturally quick vibration damper compared to traditional dead load systems. This technological advantage of pneumatic systems is true compared to load-controlled systems that use a combination of servo motors and springs to apply the load. The technology ensures reliable and better-controlled wear evaluation at high loads as demonstrated in this study. In addition, the active closed loop loading system can change the normal load to a desired value during wear tests to simulate real-life applications seen in brake systems.

Instead of having influence from uncontrolled vibration conditions during tests, we have shown the Nanovea T2000 Dynamic-Load Tribometer enables users to quantitatively assess the tribological behaviors of materials under different controlled oscillation conditions. Vibrations play a significant role in the wear behavior of metal and ceramic coating samples.

The parallel electromagnet oscillating loading module provides precisely controlled oscillations at set amplitudes and frequencies, allowing users to simulate the wear process under real-life conditions when environmental vibrations are often an important factor. In the presence of imposed oscillations during wear, both the Cu and the TiN coating samples exhibit substantially increased wear rate. The evolution of the coefficient of friction and stylus displacement measured in situ are important indicators for the performance of the material during the tribological applications. The integrated 3D non-contact profilometer offers a tool to precisely measure the wear volume and analyze the detailed morphology of the wear tracks in seconds, providing more insight into the fundamental understanding of wear mechanism.

The T2000 is equipped with a self-tuned, high-quality, and high torque motor with a 20-bit internal speed and a 16-bit external position encoder. It enables the tribometer to provide an unmatched range of rotational speeds from 0.01 to 5000 rpm that can change in stepwise jumps or at continuous rates. Contrary to systems that use a bottom located torque sensor, the Nanovea Tribometer uses a top located high-precision load cell to accurately and separately measure friction forces.

Nanovea Tribometers offers precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes (including 4ball, thrust washer, and block on ring tests), with optional high-temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system. Nanovea T2000’s unmatched range is an ideal solution for determining the full range of tribological properties of thin or thick, soft or hard coatings, films, and substrates.

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Humidity Effect on DLC Coating Tribology

nanovea — Tue, 11 Feb 2020 14:38:38 +0000

Importance of Wear Evaluation on DLC in Humidity

Diamond-like carbon (DLC) coatings possess enhanced tribological properties, namely excellent wear resistance and a very low coefficient of friction (COF). DLC coatings impart diamond characteristics when deposited on different materials. Favorable tribo-mechanical properties make DLC coatings preferable in various industrial applications, such as aerospace parts, razor blades, metal cutting tools, bearings, motorcycle engines, and medical implants.

DLC coatings exhibit very low COF (below 0.1) against steel balls under high vacuum and dry conditions¹². However, DLC coatings are sensitive to environmental condition changes, particularly relative humidity (RH)³. Environments with high humidity and oxygen concentration may lead to significant increase in COF⁴. Reliable wear evaluation in controlled humidity simulates realistic environmental conditions of DLC coatings for tribological applications. Users select the best DLC coatings for target applications with proper comparison
of DLC wear behaviors exposed to different humidity.

Measurement Objective

This study showcases the Nanovea Tribometer equipped with a humidity controller is the ideal tool for investigating wear behavior of DLC coatings at various relative humidity.

Test Procedure

Friction and wear resistance of DLC coatings were evaluated by the Nanovea Tribometer. Test parameters are summarized in Table 1. A humidity controller attached to the tribo-chamber precisely controlled the relative humidity (RH) with an accuracy of ±1%. Wear tracks on DLC coatings and wear scars on SiN balls were examined using an optical microscope after tests.

Note: Any solid ball material can be applied to simulate the performance of different material coupling under environmental conditions such as in lubricant or high temperature.

Results and Discussion

DLC coatings are great for tribological applications due to their low friction and superior wear resistance. The DLC coating friction exhibits humidity dependent behavior shown in Figure 2. The DLC coating shows a very low COF of ~0.05 throughout the wear test in relatively dry conditions (10% RH). The DLC coating exhibits a constant COF of ~0.1 during the test as RH increases to 30%. The initial run-in stage of COF is observed in the first 2000 revolutions when RH rises above 50%. The DLC coating shows a maximum COF of ~0.20, ~0.26 and ~0.33 in RH of 50, 70 and 90%, respectively. Following the run-in period, the DLC coating COF stays constant at ~0.11, 0.13 and 0.20 in RH of 50, 70 and 90%, respectively.

Figure 3 compares SiN ball wear scars and Figure 4 compares DLC coating wear tracks after the wear tests. The diameter of the wear scar was smaller when the DLC coating was exposed to an environment with low humidity. Transfer DLC layer accumulates on the SiN ball surface during the repetitive sliding process at the contact surface. At this stage, the DLC coating slides against its own transfer layer which acts as an efficient lubricant to facilitate the relative motion and restrain further mass loss caused by shear deformation. A transfer film is observed in the wear scar of the SiN ball in low RH environments (e.g. 10% and 30%), resulting in a decelerated wear process on the ball. This wear process reflects on the DLC coating’s wear track morphology as shown in Figure 4. The DLC coating exhibits a smaller wear track in dry environments, due to the formation of a stable DLC transfer film at the contact interface which significantly reduces friction and wear rate.

Conclusion

Humidity plays a vital role in the tribological performance of DLC coatings. The DLC coating possesses significantly enhanced wear resistance and superior low friction in dry conditions due to the formation of a stable graphitic layer transferred onto the sliding counterpart (a SiN ball in this study). The DLC coating slides against its own transfer layer, which acts as an efficient lubricant to facilitate the relative motion and restrain further mass loss caused by shear deformation. A film is not observed on the SiN ball with increasing relative humidity, leading to an increased wear rate on the SiN ball and the DLC coating.

The Nanovea Tribometer offers repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional humidity modules available in one pre- integrated system. It allows users to simulate the work environment at different humidity, providing users an ideal tool to quantitatively assess the tribological behaviors of materials under different work conditions.

Learn More about the Nanovea Tribometer and Lab Service

1 C. Donnet, Surf. Coat. Technol. 100–101 (1998) 180.

2 K. Miyoshi, B. Pohlchuck, K.W. Street, J.S. Zabinski, J.H. Sanders, A.A. Voevodin, R.L.C. Wu, Wear 225–229 (1999) 65.

3 R. Gilmore, R. Hauert, Surf. Coat. Technol. 133–134 (2000) 437.

4 R. Memming, H.J. Tolle, P.E. Wierenga, Thin Solid Coatings 143 (1986) 31

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Friction Evaluation at Extreme Low Speeds

nanovea — Sun, 19 Jan 2020 18:16:06 +0000

Importance of Friction Evaluation at Low Speeds

Friction is the force that resists the relative motion of solid surfaces sliding against each other. When the relative motion of these two contact surfaces takes place, the friction at the interface converts the kinetic energy into heat. Such a process can also lead to wear of the material and thus performance degradation of the parts in use.
With a large stretch ratio, high resilience, as well as great waterproof properties and wear resistance, rubber is extensively applied in a variety of applications and products in which friction plays an important role, such as automobile tires, windshield wiper blades. shoe soles and many others. Depending on the nature and requirement of these applications, either high or low friction against different material is desired. As a consequence, a controlled and reliable measurement of friction of rubber against various surfaces becomes critical.

Measurement Objective

The coefficient of friction (COF) of rubber against different materials is measured in a controlled and monitored manner using the Nanovea Tribometer. In this study, we would like to showcase the capacity of Nanovea Tribometer for measuring the COF of different materials at extremely low speeds.

Results and Discussion

The coefficient of friction (COF) of rubber balls (6 mm dia., RubberMill) on three materials (Stainless steel SS 316, Cu 110 and optional Acrylic) was evaluated by Nanovea Tribometer. The tested metal samples were mechanically polished to a mirror-like surface finish before the measurement. The slight deformation of the rubber ball under the applied normal load created an area contact, which also helps to reduce the impact of asperities or inhomogeneity of sample surface finish to the COF measurements. The test parameters are summarized in Table 1.

The COF of a rubber ball against different materials at four different speeds is shown in Figure. 2, and the average COFs calculated automatically by the software are summarized and compared in Figure 3. It is interesting that the metal samples (SS 316 and Cu 110) exhibit significantly increased COFs as the rotational speed increases from a very low value of 0.01 rpm to 5 rpm -the COF value of the rubber/SS 316 couple increases from 0.29 to 0.8, and from 0.65 to 1.1 for the rubber/Cu 110 couple. This finding is in agreement with the results reported from several laboratories. As proposed by Grosch⁴ the friction of rubber is mainly determined by two mechanisms: (1) the adhesion between rubber and the other material, and (2) the energy losses due to the deformation of the rubber caused by surface asperities. Schallamach⁵ observed waves of detachment of rubber from the counter material across the interface between soft rubber spheres and a hard surface. The force for rubber to peel from the substrate surface and rate of waves of detachment can explain the different friction at different speeds during the test.

In comparison, the rubber/acrylic material couple exhibits high COF at different rotational speeds. The COF value slightly increases from ~ 1.02 to ~ 1.09 as the rotational speed increases from 0.01 rpm to 5 rpm. Such high COF is possibly attributed to stronger local chemical bonding at the contact face formed during the tests.

Conclusion

In this study, we show that at extremely low speeds, the rubber exhibits a peculiar frictional behavior – its friction against a hard surface increases with the increased speed of the relative movement. Rubber shows different friction when it slides on different materials. Nanovea Tribometer can evaluate the frictional properties of materials in a controlled and monitored manner at different speeds, allowing users to improve fundamental understanding of the friction mechanism of the materials and select the best material couple for targeted tribological engineering applications.

Nanovea Tribometer offers precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high-temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system. It is capable of controlling the rotational stage at extremely low speeds down to 0.01 rpm and monitor the evolution of friction in situ. Nanovea’s unmatched range is an ideal solution for determining the full range of tribological properties of thin or thick, soft or hard coatings, films, and substrates.

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Tribology of Polymers

nanovea — Fri, 01 Nov 2019 20:26:57 +0000

Introduction

Polymers have been used extensively in a wide variety of applications and have become an indispensable part of everyday life. Natural polymers such as amber, silk, and natural rubber have played an essential role in human history. The fabrication process of synthetic polymers can be optimized to achieve unique physical properties such as toughness, viscoelasticity, self-lubrication, and many others.

Importance of Wear and Friction of Polymers

Polymers are commonly used for tribological applications, such as tires, bearings, and conveyor belts.
Different wear mechanisms occur depending on the mechanical properties of the polymer, the contact conditions, and the properties of the debris or transfer film formed during the wear process. To ensure that the polymers possess sufficient wear resistance under the service conditions, reliable and quantifiable tribological evaluation is necessary. Tribological evaluation allows us to quantitatively compare the wear behaviors of different polymers in a controlled and monitored manner to select the material candidate for the target application.

The Nanovea Tribometer offers repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high-temperature wear and lubrication modules available in one pre-integrated system. This unmatched range allows users to simulate the different work environments of the polymers including concentrated stress, wear, and high temperature, etc.

MEASUREMENT OBJECTIVE

In this study, we showcased that the Nanovea Tribometer is an ideal tool for comparing the friction and wear resistance of different polymers in a well-controlled and quantitative manner.

TEST PROCEDURE

The coefficient of friction (COF) and the wear resistance of different common polymers were evaluated by the Nanovea Tribometer. An Al2O3 ball was used as the counter material (pin, static sample). The wear tracks on the polymers (dynamic rotating samples) were measured using a non-contact 3D profilometer and optical microscope after the tests concluded. It should be noted that a non-contact endoscopic sensor can be used to measure the depth the pin penetrates the dynamic sample during a wear test as an option. The test parameters are summarized in Table 1. The wear rate, K, was evaluated using the formula K=Vl(Fxs), where V is the worn volume, F is the normal load, and s is the sliding distance.

Please note that Al2O3 balls were used as the counter material in this study. Any solid material can be substituted to more closely simulate the performance of two specimens under actual application conditions.

RESULTS AND DISCUSSION

Wear rate is a vital factor for determining the service lifetime of the materials, while the friction plays a critical role during the tribological applications. Figure 2 compares the evolution of the COF for different polymers against the Al2O3 ball during the wear tests. COF works as an indicator of when failures occur and the wear process enters a new stage. Among the tested polymers, HDPE maintains the lowest constant COF of ~0.15 throughout the wear test. The smooth COF implies that a stable tribo-contact is formed.

Figure 3 and Figure 4 compare the wear tracks of the polymer samples after the test is measured by the optical microscope. The In-situ non-contact 3D profilometer precisely determines the wear volume of the polymer samples, making it possible to accurately calculate wear rates of 0.0029, 0.0020, and 0.0032m3/N m, respectively. In comparison, the CPVC sample shows the highest wear rate of 0.1121m3/N m. Deep parallel wear scars are present in the wear track of CPVC.

CONCLUSION

The wear resistance of the polymers plays a vital role in their service performance. In this study, we showcased that the Nanovea Tribometer evaluates the coefficient of friction and wear rate of different polymers in a
well-controlled and quantitative manner. HDPE shows the lowest COF of ~0.15 among the tested polymers. HDPE, Nylon 66, and Polypropylene samples possess low wear rates of 0.0029, 0.0020 and 0.0032 m3/N m, respectively. The combination of low friction and great wear resistance makes HDPE a good candidate for polymer tribological applications.

The In-situ non-contact 3D profilometer enables precise wear volume measurement and offers a tool to analyze the detailed morphology of the wear tracks, providing more insight into the fundamental understanding of wear mechanisms

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