troubleshooting, testing, & analysis

MICRO- AND NANO-INDENTATION

TESTING OF PLATING THICKNESS

BY RAHUL NAIR, FISCHER TECHNOLOGY, INC., WITH CO-AUTHORS: MATT

TAYLOR, FISCHER TECHNOLOGY, INC., AND BERND BINDER, HELMUT

FISCHER, GMBH.

Indentation Testing is the technique of using a harder material commonlyreferred to as an indenter to deform a softer material. The calculated hardness(H) is the applied force (F) divided by the corresponding area of contact (A); H =F/A. One of the first modern forms of this technique was implemented by JohanAugust Brinell in 1900 [1]. A very heavy load, up to 30,000 N, is applied througha 10mm diameter hard ball onto the test material. The hardness of the materialis calculated by measuring the diameter of the residual imprint.As materials increased in hardness over the years, new techniques had to bedeveloped to measure this property. Patented in 1914 the Rockwell Test employssmaller indenters; a diamond cone or a 1/16 inch diameter steel ball 1. A lowerfixed load in the range of 600 N to 1,500 N is applied, the penetration depthmeasured and the corresponding area of contact calculated.While the aforementioned techniques are used to measure hardness of metalsand ceramics, Durometers where developed to measure the hardness of softpolymeric materials. Developed in the 1920s, ‘Shore’ hardness of material ischaracterized through this technique using Durometers with different springconstants and a conical or spherical shaped indenter per ASTM D 2240 andISO 868.Surface treatments of soft steels like case hardening, carburizing and carbonitridingrequire the surface mechanical properties to be measured, not the bulk.In order to limit the stress field from an indent to the treated surface, lower loadshave to be applied through smaller indenters. The Vickers and Knoop hardnesswere developed in 1921 and 1939, respectively, to meet this need. Indentersused in these techniques are diamond pyramids where the four sides meet at apoint. Low loads of up to 5N are applied through these indenters, and the areaof the residual imprint is optically measured per ISO 6507-1, 2, ISO 4545-1, 2or ASTM E384.Developments in deposition technology have resulted in an increase in theuse of thin films and coatings for aesthetic, tribological as well as functionalpurposes. These materials are used for a wide range of applications like automotiveclear coatings, protective metallic coatings, cutting tools, integratedcircuits and biomaterials. While traditional indentation testing can be used tocharacterize bulk steel, micro/nano scale layers and components have broughtmore challenges.Until recently, measuring the Pencil hardness of thin films according to ISO15184 has been commonplace, especially in the automotive paint industry. Withthis method, pencils of different hardness are moved at a certain angle and witha certain force across the paint surface to be tested. The ‘pencil hardness’ of thecoating is defined by two consecutive levels of pencil hardness, where the softerpencil leaves only a writing track, whereas the harder pencil causes a tangibledeformation of the paint coating.While Pencil, Vickers and Knoop hardness are still in use, the reliability andreproducibility of these methods are contentious for reasons mentioned later in thisarticle. Due to stringent quality standards in the coating industry, it is necessary to beable to test the hardness of coatings with accuracy and repeatability. The hardness ofthin coatings on tool bits, the viscoelasticity of protective coatings on optical lenses,the low friction coatings in consumer products all require precision application ofmillinewtons of force and corresponding measurements of depth in nanometers.This has led to the development of nanoindentation.

Nanoindentation

Instrumented indentation testing, more commonly referred to as nanoindentation– or, in simpler terms, depth-sensing indentation employs high-resolutioninstrumentation to continuously control and monitor the loads and displacementsof an indenter as it is driven into and withdrawn from a material. Theanalysis of the measured force-displacement curves described in ISO 14577 isbased on work by Doerner and Nix and Oliver and Pharr 2, 3.Developed in the mid-1970s, nanoindentation is used to characterize a varietyof mechanical properties of any material that can be measured in a uniaxial tensionor compression test. While nanoindenation is most often used to measurehardness, it is also possible to calculate the modulus and creep using the datacollected in this test. Methods usingnanoindentation testers have also beendevised for evaluating the yield stress and strain-hardening characteristic ofmetals, the storage and loss modulus in polymers, and the activation energyand stress exponent for creep. The fracture toughness of brittle materials canbe estimated as well using optical measurement of the lengths of cracks thathave formed at the corners of hardness impressions made with sharp indenters.

Construction of Testing Equipment

Equipment used to perform nanoindentation consists of three basic componentsas shown in Figure 1:(a) An indenter mounted onto a rigid column(b) An actuator for applying the force(c) And a sensor for measuring the indenter displacementsSmall forces are generated either electromagnetically with a coil and magnetassembly or electrostatically using a capacitor with fixed and moving plates or withpiezoelectric actuators. Displacements may be measured by eddy current sensors,capacitive sensors, linear variable differential transducers or laser interferometers.A diamond is typically used tomake indenters because it has highhardness and elastic modulus. Thisminimizes thecontribution to themeasured displacement as comparedto those that are made of other lessstiffmaterials like sapphire or tungstencarbide in which case the elasticdisplacements of the indenter mustbe accounted for. Vickers geometryindenter, a four-sided pyramid, ismost commonly used in higherload nanoindentation tests for itsdurability. The Berkovich geometryindenter is used for measurementsof a few nanometers for two reasons; they are very sharp, thus they cause

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Figure 1: Schematic of typical nanoindentation tester

with a force actuator and displacement sensor.

plastic deformation even at very small loads, and they are easier to manufactureprecisely as they have only three sides. Cube corner indenters are even sharperthan the Berkovich, causing higher stresses and strains. They can be used to estimatefracture toughness at relatively small scales. While using spherical indentersproduces only elastic deformation at low loads, they could be used to examineyielding and work hardening, and to generate the entire uniaxial stress-straincurve 4.

Hardness, Modulus and Creep

During a nanoindentation measurement the indenter is driven into the materialas shown in Figure 2, both elastic and plastic deformation processes occur. Thisproduces an impression with a projected area Ap and surface area As of contactthat depends on the shape of the indenter to a contact depth, hc.The nanoindentation measurement includes a loading and unloading cycle.Figure 3 shows indentation load (F) plotted against the displacement (h) relativeto the surface before deformation, where the data was obtained for one completeindentation cycle. The important quantities are the maximum depth (hmax) ofpenetration, the peak load (Fmax), and the final depth after unloading (hr). Theslope of the upper portion of the unloading curve, S is known as the contactstiffness. The contact depth and stiffness are determined using the Oliver-Pharrmethod as described in ISO 14577 and ASTM E2546. The hardness and elasticmodulus are derived from these quantities.In nanoindentation the Martens Hardness is determined from the loadingportion of the load-displacement curve and includes the materials resistance toboth plastic and elastic deformation. The Martens Hardness can be plotted as a function the indentation depth. Martens Hardness is given by,

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Figure 2: Schematic of indenter (blue) deforming test

material (green).

Instrumented IndentationHardness correlates to traditionalforms of hardness as it is ameasure of the resistance to plasticdeformation. InstrumentedIndentation Hardness is given by Reduced elastic modulus, Erthat is indicative of the stiffness of the sample is given by

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Figure 3: Load-displacement curve measured on a

nanoindentation tester.

is a constant that dependson the geometry of the indenter.The reduced elastic modulus accounts for the elastic displacement that occurs in both the indenter and thesample. For a test material with elastic modulus EIT it can be calculated by Here is the Poisson’s ratio forthe test material, and Ei and ?i arethe elastic modulus and Poisson’sratio of the indenter, respectively.Creep can be used tocharacterize material behaviorat a constant load. IndentationCreep is defined as an increase inpenetration depth under constantload. As shown in Figure 4 theselected final load is kept constantfor defined time duration and theindentation depth is measured.Indentation Creep, CIT is calculated as

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Figure 4: Load-displacement curve with defined creep

period at maximum load measured on a nanoindentation

tester.

h1: indentation depth at the start of the creep test

h2: indentation depth at the end of the creep test

Comparing Traditional Hardness Testing to Nanoindentation Hardness

As hardness is already being measured for most applications it is important tounderstand the correlation between these traditional forms of hardness andInstrumented Indentation Hardness.

Vickers Hardness vs. Nanoindentation HardnessSurface hardness of hard materials is commonly measured with Vickers orKnoop indenters with traditional microhardness testers. While these tests arestill reliable to characterize the hardness of most bulk materials they are not aseffective for coatings and thin films. The loads used in traditional microhardnesstesters are usually too high and results are affected by the properties of theunderlying layer. And because the indentation is measured optically, reproducibilityand accuracy of the data collected are affected by the quality of optics anduser’s definition of the diagonals of the residual indent. In nanoindentation themeasured depth is used to calculate the area of contact. But there is still a relationshipbetween Instrumented Indentation Hardness and Vickers Hardness asa Vickers geometry indenter is used in both tests. Even the Berkovich geometryindenters that are also used in nanoindentation simulate the same strain ratesas a Vickers geometry indenter. Thus, the relationship between InstrumentedIndentation Hardness and Vickers Hardness is defined as

HV = 0.0945 HIT 5

Shore Hardness vs. Nanoindentation Hardness

A study measuring Martens hardness of ShoreA standards with the FISCHERSCOPE®HM2000 S, a nanoindentation tester shownin Figure 5, shows a very linear correlationat relatively low loads. The data in graph inFigure 6 are from indents with 50mN maximumload with loading and unloading timeof 60 seconds and a creep time of 10 seconds.These testing parameters are similar to thoseused for soft coatings and thin films whenshallow indentation depths are required toprevent substrate effects.

Pencil Hardness vs. Nanoindentation Hardness

In the following study the Martens hardnesswas measured for a set of graded pencils usedin Pencil hardness testing. The tests were carried out with the FISCHERSCOPE®HM2000 S. Figure 7 shows the results of multiple measurements on pencils ofvarious hardness levels. The large standard deviations of the individual test seriesshow the limitations of the pencil hardness method. Especially in the higherrange, the nominal hardness (B, HB, F, H, etc.) of pencils are not a dependableindicator of their actual hardness.With a nanoindentation tester the hardness of paint coatings can be measureddirectly and accurately. In addition, other characteristics can be determined, suchas creep and relaxation behavior, as well as the modulus of elasticity. All of theseparameters provide a true indication of the paint qualit

Example of Applications

Nanoindentation testersavailable in the market havea variety of features, loadand displacement ranges andresolutions. The followingexamples discuss two very differentcoatings that are commonlycharacterized with theFISCHERSCOPE® HM2000S nanoindentation tester. Keyfeatures and capabilities thatare essential for the nanoindentationtester in each applicationare described below.

Mechanical characterization oflacquer coatings in automotiveapplications

In the automotive industry,clear coatings for paint areused as protection from corrosionand external damage. These lacquers are exposed

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Figure 6: Martens Hardness (HM) of Shore A standards

performed with a FISCHERSCOPE® HM2000 S

.

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Figure 7: Comparison of the Martens Hardness of pencils of

different hardness, shown with the standard deviation of

the measurements

. to environmental influencessuch as extreme temperaturefluctuations or moisture andsalt. In addition, automotivecoatings must exhibit a certaintoughness to make themresistant to mars and scratches.This requires the right balancesbetween hardness and elasticity.A quick differentiation anddetermination of these coatingproperties is possible with thenanoindentation test.Influence from underlyinglayers or the substrate can be avoided by selecting a sufficiently low maximumload that keeps the penetration depth of the indent below 10% of the coatingthickness. At the beginning of the curing process, the clear coats are relativelysoft. One of the key features of a nanoindentation tester is a sensitive automatedsurface detection. As the measured mechanical properties polymers are influencedby rate of loading and unloading, a thermally stable nanoindentationsystem is also essential. Drift in the depth measurements caused by changes inenvironmental temperature must be avoided or accounted for.The Martens hardness (HM) and the Martens hardness after creeping (HMCR)are values which specify plastic and elastic properties of the paint coating. Theindentation hardness (HIT) considers only the plastic portion of the materialdeformation. The hardness parameters allow for better understanding of aging,curing, cross-linking, embrittlement through UV radiation, hardness changethrough temperature influences and the degree of polymerization of the lacquer.One of the most important advantages of the instrumented indentationtest is the determination of elastic properties. The indentation modulus (EIT),creep at maximum load (CIT) can be determined using this method and providesinformation regarding the visco-elastic properties of lacquer coatings. Theseproperties show the ability of the lacquer to resist weather degradation and healin case of scratches.

Nanoindentation on wear-resistant DLC coatings applied to engine components

In order to reduce emissions in combustion engines without sacrificing performance,manufacturers are continually working to improve the ability of themoving components (e.g. camshafts, valve lifters, piston rings and gears) toresist abrasion and reduce friction.Protective coatings such asdiamond-like carbon (DLC) areincreasingly used in such applications.As DLC coatings canhave a wide range of hardnessdepending on the depositionprocess it is important to measurthe fundamental mechanicalproperties of this hard, lowfriction coating.Traditional hardness measurements

would involve apply

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Figure 9: DLC-coated engine components.

Figure 10: The graph shows the depth-dependent profile of the Martens Hardness of the DLC coating.

ing a load though a sharp indenter and measuring the residual imprint under amicroscope. However, this is almost impossible due to the elastic nature and darkcolor of the DLC coating.As these coatings are only a few microns in thickness the nanoindentationtester should have high depth resolution to allow for shallow indents to beperformed, thus preventing the substrate material from influencing the measurements.And because ceramics have higher stiffness, the instrument musthave a rigid frame to eliminate instrument compliance and only deform thematerial being tested.In this example, the measurement results of a 3 μm thick DLC layer areshown. The values for indentation hardness (HIT), Martens Hardness (HM) andindentation modulus (EIT) for the coating is listed in Table 1. The convertedVickers hardness (HV) helps correlate these measurements with traditionalmicrohardness testers. The graph in Figure 10 maps the measured MartensHardness as a function of indentation depth. Minimal change in this measurementwith increasing depth indicates that even at maximum load there is noinfluence from the under lying substrate.

CONCLUSION

Improving the surface mechanical properties of materials boosts performanceand increases life cycle of products. New developments in coating and surfacetreatment technology has seen nanoindentation gain wider acceptance.Combination of ISO and ASTM standards for nanoindentation and availabilityof off-the-shelf options from different vendors has also contributed to adoptionof this technique in many industries.Given the limitations of traditional hardness testing techniques, nanoindentationtesters are viewed as tools that can give a better understanding ofthe interactions between surfaces or against abrasive elements. The wealth ofinformation about the mechanical properties derived from a nanoindentationtest defines the true strength of a material. Additionally, a single tool can beused to characterize a wide variety of materials ranging from soft polymers tohard ceramics. Most importantly, this technique removes the majority of theuser-influence and subjectivity from the test and allows one to quantitativelyanalyze a surface or coating.

REFERENCES

1. The Hardness of Metals, D. Tabor, Oxford University Press, Aug 3, 2000, ISBN0198507763, 9780198507765

2. A method for interpreting the data from depth-sensing indentation instruments,M.F. Doerner, and W. D. Nix, Journal of Materials Research, Vol. 1, No. 4,Jul/Aug 1986

3. Measurement of hardness and elastic modulus by instrumented indentation:Advances in understanding and refinements to methodology, W.C. Oliver andG.M. Pharr, , Vol. 19, No. 1, Jan 2004

4. A simple predictive model for spherical indentation, J.S. Field and M.V. Swain,Journal of Materials Research, Vol. 8, No. 2, 1993

5. The IBIS Handbook of Nanoindentation, Anthony C. Fischer-Cripps, ISBN

0 9585525 4