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Industrial Coatings Scratch and Wear Evaluation

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.

Compact Pneumatic Tribometer T100
NANOVEA T100

The Compact Pneumatic Tribometer

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NANOVEA PB1000

The Large Platform Mechanical Tester

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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 TYPEProgressive
INITIAL LOAD0.01 mN
FINAL LOAD5 N / 35 N
LOADING RATE10 / 70 N/min
SCRATCH LENGTH3 mm
SCRATCHING SPEED, dx/dt6.0 mm/min
INDENTER GEOMETRY120º cone
INDENTER MATERIAL (tip)Diamond
INDENTER TIP RADIUS200 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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Scratch Hardness Measurement using Mechanical Tester

SCRATCH HARDNESS MEASUREMENT

USING A MECHANICAL TESTER

Prepared by

DUANJIE LI, PhD

INTRODUCTION

In general, hardness tests measure the resistance of materials to permanent or plastic deformation. There are three types of hardness measurements: scratch hardness, indentation hardness and rebound hardness. A scratch hardness test measures a material’s resistance to scratch and abrasion due to friction from a sharp object1. It was originally developed by German mineralogist Friedrich Mohs in 1820 and is still widely used to rank the physical properties of minerals2. This test method is also applicable to metals, ceramics, polymers, and coated surfaces.

During a scratch hardness measurement, a diamond stylus of specified geometry scratches into a material’s surface along a linear path under a constant normal force with a constant speed. The average width of the scratch is measured and used to calculate the scratch hardness number (HSP). This technique provides a simple solution for scaling the hardness of different materials.

MEASUREMENT OBJECTIVE

In this study, the NANOVEA PB1000 Mechanical Tester is used to measure the scratch hardness of different metals in compliance with ASTM G171-03.

Simultaneously, this study showcases the capacity of the NANOVEA Mechanical Tester in performing scratch hardness measurement with high precision and reproducibility.

NANOVEA

PB1000

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nanoindenter and scratch tester Nanovea PB1000

TEST CONDITIONS

The NANOVEA PB1000 Mechanical Tester performed scratch hardness tests on three polished metals (Cu110, Al6061 and SS304). A conical diamond stylus of apex angle 120° with tip radius of 200 µm was used. Each sample was scratched three times with the same test parameters to ensure reproducibility of the results. The test parameters are summarized below. A profile scan at a low normal load of 10 mN was performed before and after the scratch test to measure the change in the surface profile of the scratch.

TEST PARAMETERS

NORMAL FORCE

10 N

TEMPERATURE

24°C (RT)

SLIDING SPEED

20 mm/min

SLIDING DISTANCE

10 mm

ATMOSPHERE

Air

RESULTS & DISCUSSION

The images of the scratch tracks of three metals (Cu110, Al6061 and SS304) after the tests are shown in FIGURE 1 in order to compare the scratch hardness of different materials. The mapping function of the NANOVEA Mechanical Software was used to create three parallel scratches tested under the same condition in an automated protocol. The measured scratch track width and calculated scratch hardness number (HSP) are summarized and compared in TABLE 1. The metals show different wear track widths of 174, 220 and 89 µm for Al6061, Cu110 and SS304, respectively, resulting in a calculated HSP of 0.84, 0.52 and 3.2 GPa.

In addition to the scratch hardness computed from the scratch track width, the evolution of coefficient of friction (COF), true depth and acoustic emission were recorded in situ during the scratch hardness test. Here, the true depth is the depth difference between the penetration depth of the stylus during the scratch test and the surface profile measured in the pre-scan. The COF, true depth and acoustic emission of Cu110 are shown in FIGURE 2 as an example. Such information provides insight into mechanical failures taking place during scratching, enabling users to detect mechanical defects and further investigate the scratch behavior of the tested material.

The scratch hardness tests can be finished within a couple of minutes with high precision and repeatability. Compared to conventional indentation procedures, the scratch hardness test in this study provides an alternative solution for hardness measurements, which is useful for quality control and the development of new materials.

Al6061

Cu110

SS304

FIGURE 1: Microscope image of the scratch tracks post test (100x magnification).

 Scratch track width (μm)HSp (GPa)
Al6061174±110.84
Cu110220±10.52
SS30489±53.20

TABLE 1: Summary of scratch track width and scratch hardness number.

FIGURE 2: The evolution of coefficient of friction, true depth and acoustic emissions during the scratch hardness test on Cu110.

CONCLUSION

In this study, we showcased the capacity of the NANOVEA Mechanical Tester in performing scratch hardness tests in compliance to ASTM G171-03. In addition to coating adhesion and scratch resistance, the scratch test at a constant load provides an alternative simple solution for comparing the hardness of materials. In contrast to conventional scratch hardness testers, NANOVEA Mechanical Testers offer optional modules for monitoring the evolution of coefficient of friction, acoustic emission and true depth in situ.

The Nano and Micro modules of a NANOVEA Mechanical Tester include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest and most user-friendly range of testing available in a single system. NANOVEA’s unmatched range is an ideal solution for determining the full range of mechanical 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.

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Titanium Nitride Coating Scratch Test

TITANIUM NITRIDE COATING SCRATCH TEST

QUALITY CONTROL INSPECTION

Prepared by

DUANJIE LI, PhD

INTRODUCTION

The combination of high hardness, excellent wear resistance, corrosion resistance and inertness makes titanium nitride (TiN) an ideal protective coating for metal components in various industries. For example, the edge retention and corrosion resistance of a TiN coating can substantially increase the work efficiency and extend the service life of machine tooling such as razor blades, metal cutters, injection molds and saws. Its high hardness, inertness and non-toxicity make TiN a great candidate for applications in medical devices including implants and surgical instruments.

IMPORTANCE OF TiN COATING SCRATCH TESTING

Residual stress in protective PVD/CVD coatings plays a critical role in the performance and mechanical integrity of the coated component. The residual stress derives from several major sources, including growth stress, thermal gradients, geometric constraints and service stress¹. The thermal expansion mismatch between the coating and the substrate created during coating deposition at elevated temperatures leads to high thermal residual stress. Moreover, TiN coated tools are often used under very high concentrated stresses, e.g. drill bits and bearings. It is critical to developing a reliable quality control process to quantitatively inspect the cohesive and adhesive strength of protective functional coatings.

[1] V. Teixeira, Vacuum 64 (2002) 393–399.

MEASUREMENT OBJECTIVE

In this study, we showcase that the NANOVEA Mechanical Testers in Scratch Mode are ideal for assessing the cohesive/adhesive strength of protective TiN coatings in a controlled and quantitative manner.

NANOVEA

PB1000

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nanoindenter and scratch tester Nanovea PB1000

TEST CONDITIONS

The NANOVEA PB1000 Mechanical Tester was used to perform coating scratch tests on three TiN coatings using the same test parameters as summarized below:

LOADING MODE: Progressive Linear

INITIAL LOAD

0.02 N

FINAL LOAD

10 N

LOADING RATE

20 N/min

SCRATCH LENGTH

5 mm

INDENTER TYPE

Sphero-Conical

Diamond, 20 μm radius

RESULTS & DISCUSSION

FIGURE 1 shows the recorded evolution of penetration depth, coefficient of friction (COF) and acoustic emission during the test. The full micro scratch tracks on the TiN samples are shown in FIGURE 2. The failure behaviors at different critical loads are displayed in FIGURE 3, where critical load Lc1 is defined as the load at which the first sign of cohesive crack occurs in the scratch track, Lc2 is the load after which repeated spallation failures take place, and Lc3 is the load at which the coating is completely removed from the substrate. The critical load (Lc) values for the TiN coatings are summarized in FIGURE 4.

The evolution of penetration depth, COF and acoustic emission provides insight into the mechanism of the coating failure at different stages, which are represented by the critical loads in this study. It can be observed that Sample A and Sample B exhibit comparable behavior during the scratch test. The stylus progressively penetrates into the sample to a depth of ~0.06 mm and the COF gradually increases to ~0.3 as the normal load increases linearly at the beginning of the coating scratch test. When the Lc1 of ~3.3 N is reached, the first sign of chipping failure occurs. This is also reflected in the first large spikes in the plot of penetration depth, COF and acoustic emission. As the load continues to increase to Lc2 of ~3.8 N, further fluctuation of the penetration depth, COF and acoustic emission takes place. We can observe continuous spallation failure present on both sides of the scratch track. At the Lc3, the coating completely delaminates from the metal substrate under the high pressure applied by the stylus, leaving the substrate exposed and unprotected.

In comparison, Sample C exhibits lower critical loads at different stages of the coating scratch tests, which is also reflected in the evolution of penetration depth, coefficient of friction (COF) and acoustic emission during the coating scratch test. Sample C possesses an adhesion interlayer with lower hardness and higher stress at the interface between the top TiN coating and the metal substrate compared to Sample A and Sample B.

This study demonstrates the importance of proper substrate support and coating architecture to the quality of the coating system. A stronger interlayer can better resist deformation under a high external load and concentration stress, and thus enhance the cohesive and adhesive strength of the coating/substrate system.

FIGURE 1: Evolution of penetration depth, COF and acoustic emission of the TiN samples.

FIGURE 2: Full scratch track of the TiN coatings after the tests.

FIGURE 3: TiN coating failures under different critical loads, Lc.

FIGURE 4: Summary of critical load (Lc) values for the TiN coatings.

CONCLUSION

In this study, we showcased that the NANOVEA PB1000 Mechanical Tester performs reliable and accurate scratch tests on TiN-coated samples in a controlled and closely monitored manner. Scratch measurements allow users to quickly identify the critical load at which typical cohesive and adhesive coating failures occur. Our instruments are superior quality control tools that can quantitatively inspect and compare the intrinsic quality of a coating and the interfacial integrity of a coating/substrate system. A coating with a proper interlayer can resist large deformation under a high external load and concentration stress, and enhance the cohesive and adhesive strength of a coating/substrate system.

The Nano and Micro modules of a NANOVEA Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest and most user-friendly range of testing available in a single system. NANOVEA’s unmatched range is an ideal solution for determining the full range of mechanical 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.

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Fractography Analysis Using 3D Profilometry

FRACTOGRAPHY ANALYSIS

USING 3D PROFILOMETRY

Prepared by

CRAIG LEISING

INTRODUCTION

Fractography is the study of features on fractured surfaces and has historically been investigated via Microscope or SEM. Depending on the size of the feature, a microscope (macro features) or SEM (nano and micro features) are selected for the surface analysis. Both ultimately allowing for the identification of the fracture mechanism type. Although effective, the Microscope has clear limitations and the SEM in most cases, other than atomic-level analysis, is unpractical for fracture surface measurement and lacks broader use capability. With advances in optical measurement technology, the NANOVEA 3D Non-Contact Profilometer is now considered the instrument of choice, with its ability to provide nano through macro-scale 2D & 3D surface measurements

IMPORTANCE OF 3D NON-CONTACT PROFILOMETER FOR FRACTURE INSPECTION

Unlike an SEM, a 3D Non-Contact Profilometer can measure nearly any surface, sample size, with minimal sample prep, all while offering superior vertical/horizontal dimensions to that of an SEM. With a profiler, nano through macro range features are captured in a single measurement with zero influence from sample reflectivity. Easily measure any material: transparent, opaque, specular, diffusive, polished, rough etc. The 3D Non-Contact Profilometer provides broad and user-friendly capability to maximize surface fracture studies at a fraction of the cost of an SEM.

MEASUREMENT OBJECTIVE

In this application, the NANOVEA ST400 is used to measure the fractured surface of a steel sample. In this study, we will showcase a 3D area, 2D profile extraction and surface directional map of the surface.

NANOVEA

ST400

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RESULTS

TOP SURFACE

3D Surface Texture Direction

Isotropy51.26%
First Direction123.2º
Second Direction116.3º
Third Direction0.1725º

Surface Area, Volume, Roughness and many others can be automatically calculated from this extraction.

2D Profile Extraction

RESULTS

SIDE SURFACE

3D Surface Texture Direction

Isotropy15.55%
First Direction0.1617º
Second Direction110.5º
Third Direction171.5º

Surface Area, Volume, Roughness and many others can be automatically calculated from this extraction.

2D Profile Extraction

CONCLUSION

In this application, we have shown how the NANOVEA ST400 3D Non-Contact Profilometer can precisely characterize the full topography (nano, micro and macro features) of a fractured surface. From the 3D area, the surface can be clearly identified and subareas or profiles/cross-sections can be quickly extracted and analyzed with an endless list of surface calculations. Sub nanometer surface features can be further analyzed with an integrated AFM module.

Additionally, NANOVEA has included a portable version to their Profilometer line-up, especially critical for field studies where a fracture surface is immovable. With this broad list of surface measurement capabilities, fracture surface analysis has never been easier and more convenient with a single instrument.

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Fiberglass Surface Topography Using 3D Profilometry

FIBERGLASS SURFACE TOPOGRAPHY

USING 3D PROFILOMETRY

Prepared by

CRAIG LEISING

INTRODUCTION

Fiberglass is a material made from extremely fine fibers of glass. It is used as a reinforcing agent for many polymer products; the resulting composite material, properly known as fiber-reinforced polymer (FRP) or glass-reinforced plastic (GRP), is called “fiberglass” in popular usage.

IMPORTANCE OF SURFACE METROLOGY INSPECTION FOR QUALITY CONTROL

Although there are many uses for Fiberglass reinforcement, in most applications it is crucial that they are as strong as possible. Fiberglass composites have one of the highest strength to weight ratios available and in some cases, pound for pound it is stronger than steel. Aside from high strength, it is also important to have the smallest possible exposed surface area. Large fiberglass surfaces can make the structure more vulnerable to chemical attack and possibly material expansion. Therefore, surface inspection is critical to quality control production.

MEASUREMENT OBJECTIVE

In this application, the NANOVEA ST400 is used to measure a Fiberglass Composite surface for roughness and flatness. By quantifying these surface features it is possible to create or optimize a stronger, longer lasting fiberglass composite material.

NANOVEA

ST400

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MEASUREMENT PARAMETERS

PROBE 1 mm
ACQUISITION RATE300 Hz
AVERAGING1
MEASURED SURFACE5 mm x 2 mm
STEP SIZE5 µm x 5 µm
SCANNING MODEConstant speed

PROBE SPECIFICATIONS

MEASUREMENT RANGE1 mm
Z RESOLUTION 25 nm
Z ACCURACY200 nm
LATERAL RESOLUTION 2 μm

RESULTS

FALSE COLOR VIEW

3D Surface Flatness

3D Surface Roughness

Sa15.716 μmArithmetical Mean Height
Sq19.905 μmRoot Mean Square Height
Sp116.74 μmMaximum Peak Height
Sv136.09 μmMaximum Pit Height
Sz252.83 μmMaximum Height
Ssk0.556Skewness
Ssu3.654Kurtosis

CONCLUSION

As shown in the results, the NANOVEA ST400 Optical Profiler was able to accurately measure the roughness and flatness of the fiberglass composite surface. Data can be measured over multiple batches of fiber composites and or a given time period to provide crucial information about different fiberglass manufacturing processes and how they react over time. Thus, the ST400 is a viable option for strengthening the quality control process of fiberglass composite materials.

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Polymer Belt Wear and Friction using a Tribometer

POLYMER BELTS

WEAR AND FRICTION USING a TRIBOMETER

Prepared by

DUANJIE LI, PhD

INTRODUCTION

Belt drive transmits power and tracks relative movement between two or more rotating shafts. As a simple and inexpensive solution with minimal maintenance, belt drives are widely used in a variety of applications, such as bucksaws, sawmills, threshers, silo blowers and conveyors. Belt drives can protect the machinery from overload as well as damp and isolate vibration.

IMPORTANCE OF WEAR EVALUATION FOR BELT DRIVES

Friction and wear are inevitable for the belts in a belt-driven machine. Sufficient friction ensures effective power transmission without slipping, but excessive friction may rapidly wear the belt. Different types of wear such as fatigue, abrasion and friction take place during the belt drive operation. In order to extend the lifetime of the belt and to cut the cost and time on belt repairing and replacement, reliable evaluation of the wear performance of the belts is desirable in improving belt lifespan, production efficiency and application performance. Accurate measurement of the coefficient of friction and wear rate of the belt facilitates R&D and quality control of belt production.

High Load Pneumatic Tribometer

MEASUREMENT OBJECTIVE

In this study, we simulated and compared the wear behaviors of belts with different surface textures to showcase the capacity of the NANOVEA T2000 Tribometer in simulating the wear process of the belt in a controlled and monitored manner.

NANOVEA

T2000

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TEST PROCEDURES

The coefficient of friction, COF, and the wear resistance of two belts with different surface roughness and texture were evaluated by the NANOVEA High-Load Tribometer using Linear Reciprocating Wear Module. A Steel 440 ball (10 mm diameter) was used as the counter material. The surface roughness and wear track were examined using an integrated 3D Non-Contact profilometer. 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 a smooth Steel 440 ball counterpart was used as an example in this study, any solid material with different shapes and surface finish can be applied using custom fixtures to simulate the actual application situation.

RESULTS & DISCUSSION

The Textured Belt and Smooth Belt have a surface roughness Ra of 33.5 and 8.7 um, respectively, according to the analyzed surface profiles taken with a NANOVEA 3D Non-Contact Optical profiler. The COF and wear rate of the two tested belts were measured at 10 N and 100 N, respectively, to compare the wear behavior of the belts at different loads.

FIGURE 1 shows the evolution of COF of the belts during the wear tests. The belts with different textures exhibit substantially different wear behaviors. It is interesting that after the run-in period during which the COF progressively increases, the Textured Belt reaches a lower COF of ~0.5 in both the tests conducted using loads of 10 N and 100 N. In comparison, the Smooth Belt tested under the load of 10 N exhibits a significantly higher COF of~ 1.4 when the COF gets stable and maintains above this value for the rest of the test. The Smooth Belt tested under the load of 100 N rapidly was worn out by the steel 440 ball and formed a large wear track. The test was therefore stopped at 220 revolutions.

FIGURE 1: Evolution of COF of the belts at different loads.

FIGURE 2 compares the 3D wear track images after the tests at 100 N. The NANOVEA 3D non-contact profilometer offers a tool to analyze the detailed morphology of the wear tracks, providing more insight in fundamental understanding of wear mechanism.

TABLE 1: Result of wear track analysis.

FIGURE 2:  3D view of the two belts
after the tests at 100 N.

The 3D wear track profile allows direct and accurate determination of the wear track volume calculated by the advanced analysis software as shown in TABLE 1. In a wear test for 220 revolutions, the Smooth Belt has a much larger and deeper wear track with a volume of 75.7 mm3, compared to a wear volume of 14.0 mm3 for the Textured Belt after a 600-revolution wear test. The significantly higher friction of the Smooth Belt against the steel ball leads to a 15 fold higher wear rate compared to the Textured Belt.

 

Such a drastic difference of COF between the Textured Belt and Smooth Belt is possibly related to the size of the contact area between the belt and the steel ball, which also leads to their different wear performance. FIGURE 3 shows the wear tracks of the two belts under the optical microscope. The wear track examination is in agreement with the observation on COF evolution: The Textured Belt, which maintains a low COF of ~0.5, exhibits no sign of wear after the wear test under a load of 10 N. The Smooth Belt shows a small wear track at 10 N. The wear tests carried out at 100 N create substantially larger wear tracks on both the Textured and Smooth Belts, and the wear rate will be calculated using 3D profiles as will be discussed in the following paragraph.

FIGURE 3:  Wear tracks under optical microscope.

CONCLUSION

In this study, we showcased the capacity of the NANOVEA T2000 Tribometer in evaluating the coefficient of friction and wear rate of belts in a well-controlled and quantitative manner. The surface texture plays a critical role in the friction and wear resistance of the belts during their service performance. The textured belt exhibits a stable coefficient of friction of ~0.5 and possesses a long lifetime, which results in reduced time and cost on tool repairing or replacement. In comparison, the excessive friction of the smooth belt against the steel ball rapidly wears the belt. Further, the loading on the belt is a vital factor of its service lifetime. Overload creates very high friction, leading to accelerated wear to the belt.

The NANOVEA T2000 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 tribocorrosion 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.

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Fossil Microstructure Using 3D Profilometry

FOSSIL MICROSTRUCTURE

USING 3D PROFILOMETRY

Prepared by

DUANJIE LI, PhD

INTRODUCTION

Fossils are the preserved remains of traces of plants, animals and other organisms buried in sediment under ancient seas, lakes and rivers. The soft body tissue usually decays after death, but the hard shells, bones and teeth fossilize. Microstructure surface features are often preserved when mineral replacement of the original shells and bones takes place, which provides an insight into the evolution of weather and the formation mechanism of fossils.

IMPORTANCE OF A 3D NON-CONTACT PROFILOMETER FOR FOSSIL EXAMINATION

3D profiles of the fossil enable us to observe the detailed surface features of the fossil sample from a closer angle. The high resolution and accuracy of the NANOVEA profilometer may not be discernible by the naked eye. The profilometer’s analysis software offers a wide range of studies applicable to these unique surfaces. Unlike other techniques such as touch probes, the NANOVEA 3D Non-Contact Profilometer measures the surface features without touching the sample. This allows for the preservation of the true surface features of certain delicate fossil samples. Moreover, the portable model Jr25 profilometer enables 3D measurement on fossil sites, which substantially facilitates fossil analysis and protection after excavation.

MEASUREMENT OBJECTIVE

In this study, the NANOVEA Jr25 Profilometer is used to measure the surface of two representative fossil samples. The entire surface of each fossil was scanned and analyzed in order to characterize its surface features which include roughness, contour and texture direction.

NANOVEA

Jr25

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BRACHIOPOD FOSSIL

The first fossil sample presented in this report is a Brachiopod fossil, which came from a marine animal that has hard “valves” (shells) on its upper and lower surfaces. They first appeared in the Cambrian period, which is more than 550 million years ago.

The 3D View of the scan is shown in FIGURE 1 and False Color View is shown in FIGURE 2. 

FIGURE 1: 3D View of the Brachiopod fossil sample.

FIGURE 2: False Color View of the Brachiopod fossil sample.

The overall form was then removed from the surface in order to investigate the local surface morphology and contour of the Brachiopod fossil as shown in FIGURE 3. A peculiar divergent groove texture can now be observed on the Brachiopod fossil sample.

FIGURE 3: False Color View and Contour Lines View after form removal.

A line profile is extracted from the textured area to show a crossectional view of the fossil surface in FIGURE 4. The Step Height study measures precise dimensions of the surface features. The grooves possess an average width of ~0.38 mm and depth of ~0.25 mm.

FIGURE 4: Line profile and Step Height studies of the textured surface.

CRINOID STEM FOSSIL

The second fossil sample is a Crinoid stem fossil. Crinoids first appeared in the seas of the Middle Cambrian Period, about 300 million years before dinosaurs. 

 

The 3D View of the scan is shown in FIGURE 5 and False Color View is shown in FIGURE 6. 

FIGURE 5: 3D View of the Crinoid fossil sample.

The surface texture isotropy and roughness of the Crinoid stem fossil are analyzed in FIGURE 7. 

 This fossil has a preferential texture direction in the angle close to 90°, leading to texture isotropy of 69%.

FIGURE 6: False Color View of the Crinoid stem sample.

 

FIGURE 7: Surface texture isotropy and roughness of the Crinoid stem fossil.

The 2D profile along the axial direction of the Crinoid stem fossil is shown in FIGURE 8. 

The size of the peaks of the surface texture is fairly uniform.

FIGURE 8: 2D profile analysis of the Crinoid stem fossil.

CONCLUSION

In this application, we comprehensively studied the 3D surface features of a Brachiopod and Crinoid stem fossil using the NANOVEA Jr25 Portable Non-Contact Profilometer. We showcase that the instrument can precisely characterize the 3D morphology of the fossil samples. The interesting surface features and texture of the samples are then further analyzed. The Brachiopod sample possesses a divergent groove texture, while the Crinoid stem fossil shows  preferential texture isotropy. The detailed and precise 3D surface scans prove to be ideal tools for palaeontologists and geologists to study the evolution of lives and the formation of fossils.

The data shown here represent only a portion of the calculations available in the analysis software. NANOVEA Profilometers measure virtually any surface in fields including Semiconductor, Microelectronics, Solar, Fiber Optics, Automotive, Aerospace, Metallurgy, Machining, Coatings, Pharmaceutical, Biomedical, Environmental and many others.

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Sandpaper Abrasion Performance Using a Tribometer

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.

High Load Pneumatic Tribometer

MEASUREMENT OBJECTIVE

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

NANOVEA

T2000

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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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Processed Leather Surface Finish using 3D Profilometry

PROCESSED LEATHER

SURFACE FINISH USING 3D PROFILOMETRY

Prepared by

CRAIG LEISING

INTRODUCTION

Once the tanning process of a leather hide is complete the leather surface can undergo several finishing processes for a variety of looks and touch. These mechanical processes can include stretching, buffing, sanding, embossing, coating etc. Dependent upon the end use of the leather some may require a more precise, controlled and repeatable processing.

IMPORTANCE OF PROFILOMETRY INSPECTION FOR R&D AND QUALITY CONTROL

Due to the large variation and unreliability of visual inspection methods, tools that are capable of accurately quantifying micro and nano scales features can improve leather finishing processes. Understanding the surface finish of leather in a quantifiable sense can lead to improved data driven surface processing selection to achieve optimal finish results. NANOVEA 3D Non-Contact Profilometers utilize chromatic confocal technology to measure finished leather surfaces and offer the highest repeatability and accuracy in the market. Where other techniques fail to provide reliable data, due to probe contact, surface variation, angle, absorption or reflectivity, NANOVEA Profilometers succeed.

MEASUREMENT OBJECTIVE

In this application, the NANOVEA ST400 is used to measure and compare the surface finish of two different but closely processed leather samples. Several surface parameters are automatically calculated from the surface profile.

Here we will focus on surface roughness, dimple depth, dimple pitch and dimple diameter for comparative evaluation.

NANOVEA

ST400

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RESULTS: SAMPLE 1

ISO 25178

HEIGHT PARAMETERS

OTHER 3D PARAMETERS

RESULTS: SAMPLE 2

ISO 25178

HEIGHT PARAMETERS

OTHER 3D PARAMETERS

DEPTH COMPARATIVE

Depth distribution for each sample.
A large number of deep dimples were observed in SAMPLE 1.

PITCH COMPARATIVE

Pitch between dimples on SAMPLE 1 is slightly smaller
than SAMPLE 2, but both have a similar distribution

 MEAN DIAMETER COMPARATIVE

Similar distributions of mean diameter of dimples,
with SAMPLE 1 showing slightly smaller mean diameters on average.

CONCLUSION

In this application, we have shown how the NANOVEA ST400 3D Profilometer can precisely characterize the surface finish of processed leather. In this study, having the ability to measure surface roughness, dimple depth, dimple pitch and dimple diameter allowed us to quantify differences between the finish and quality of the two samples that may not be obvious by visual inspection.

Overall there were no visible difference in the appearance of the 3D scans between SAMPLE 1 and SAMPLE 2. However, in the statistical analysis there is a clear distinction between the two samples. SAMPLE 1 contains a higher quantity of dimples with smaller diameters, larger depths and smaller dimple-to-dimple pitch in comparison to SAMPLE 2.

Please note that additional studies are available. Special areas of interest could have been further analyzed with an integrated AFM or Microscope module. NANOVEA 3D Profilometer speeds range from 20 mm/s to 1 m/s for laboratory or research to meet the needs of high-speed inspection; can be built with custom sizing, speeds, scanning capabilities, Class 1 clean room compliance, indexing conveyor or for in-line or online integration.

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Mechanical Properties of Hydrogel

MECHANICAL PROPERTIES OF HYDROGEL

USING NANOINDENTATION

Prepared by

DUANJIE LI, PhD & JORGE RAMIREZ

INTRODUCTION

Hydrogel is known for its super absorbency of water allowing for a close resemblance in flexibility as natural tissues. This resemblance has made hydrogel a common choice not only in biomaterials, but also in electronics, environment and consumer good applications such as contact lens. Each unique application requires specific hydrogel mechanical properties.

IMPORTANCE OF NANOINDENTATION FOR HYDROGEL

Hydrogels create unique challenges for Nanoindentation such as test parameters selection and sample preparation. Many nanoindentation systems have major limitations since they were not originally designed for such soft materials. Some of the nanoindentation systems use a coil/magnet assembly to apply force on the sample. There is no actual force measurement, leading to inaccurate and non-linear loading when testing soft materials. Determining the point of contact is extremely difficult as the depth is the only parameter actually being measured. It is almost impossible to observe the change of slope in the Depth vs Time plot during the period when the indenter tip is approaching the hydrogel material.

In order to overcome the limitations of these systems, the nano module of the NANOVEA Mechanical Tester measures the force feedback with an individual load cell to ensure high accuracy on all types of materials, soft or hard. The piezo-controlled displacement is extremely precise and fast. This allows unmatched measurement of viscoelastic properties by eliminating many theoretical assumptions that systems with a coil/magnet assembly and no force feedback must account for.

MEASUREMENT OBJECTIVE

In this application, the NANOVEA Mechanical Tester, in Nanoindentation mode, is used to study the hardness, elastic modulus and creep of a hydrogel sample.

NANOVEA

PB1000

nanoindenter and scratch tester Nanovea PB1000
TEST CONDITIONS

A hydrogel sample placed on a glass slide was tested by nanoindentation technique using a NANOVEA Mechanical Tester. For this soft material a 3 mm diameter spherical tip was used. The load linearly increased from 0.06 to 10 mN during the loading period. The creep was then measured by the change of indentation depth at the maximum load of 10 mN for 70 seconds.

APPROACH SPEED: 100 μm/min

CONTACT LOAD
0.06 mN
MAX LOAD
10 mN
LOADING RATE

20 mN/min

CREEP
70 s
RESULTS & DISCUSSION

The evolution of the load and depth as a function of time is shown in FUGURE 1. It can be observed that on the plot of the Depth vs Time, it is very difficult to determine the point of the change of slope at the beginning of the loading period, which usually works as an indication where the indenter starts to contact the soft material. However, the plot of the Load vs Time shows the peculiar behavior of the hydrogel under an applied load. As the hydrogel begins to get in touch with the ball indenter, the hydrogel pulls the ball indenter due to its surface tension, which tends to decrease the surface area. This behavior leads to the negative measured load at the beginning of the loading stage. The load progressively increases as the indenter sinks into the hydrogel, and it is then controlled to be constant at the maximum load of 10 mN for 70 seconds to study the creep behavior of the hydrogel.

FIGURE 1: Evolution of the load and depth as a function of Time.

The plot of the Creep Depth vs Time is shown in FIGURE 2, and the Load vs. Displacement plot of the nanoindentation test is shown in FIGURE 3. The hydrogel in this study possesses a hardness of 16.9 KPa and a Young’s modulus of 160.2 KPa, as calculated based on the load displacement curve using the Oliver-Pharr method.

Creep is an important factor for the study of a hydrogel’s mechanical properties. The close-loop feedback control between piezo and ultrasensitive load cell ensures a true constant loading during the creep time at the maximum load. As shown in FIGURE 2, the hydrogel subsides ~42 μm as a result of creep in 70 seconds under the 10 mN maximum load applied by the 3 mm ball tip.

FIGURE 2: Creeping at a max load of 10 mN for 70 seconds.

FIGURE 3: The Load vs. Displacement plot of the hydrogel.

CONCLUSION

In this study, we showcased that the NANOVEA Mechanical Tester, in Nanoindentation mode, provides a precise and repeatable measurement of a hydrogel’s mechanical properties including hardness, Young’s modulus and creep. The large 3 mm ball tip ensures proper contact against the hydrogel surface. The high precision motorized sample stage allows for accurate positioning of the flat face of the hydrogel sample under the ball tip. The hydrogel in this study exhibits a hardness of 16.9 KPa and a Young’s modulus of 160.2 KPa. The creep depth is ~42 μm under a 10 mN load for 70 seconds.

NANOVEA Mechanical Testers provide unmatched multi-function Nano and Micro modules on a single platform. Both modules include a scratch tester, hardness tester and a wear tester mode, offering the widest and the most user friendly range of testing available on a single
system.

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