Thin Solid Films 420 – 421 (2002) 318–323
`
`Investigation of the residual stresses and mechanical properties of
`(Cr,Al)N arc PVD coatings used for semi-solid metal (SSM) forming
`dies
`
`E. Lugscheider, K. Bobzin, Th. Hornig, M. Maes*
`
`Materials Science Institute, Aachen University of Technology, Augustinerbach 4-22, 52056 Aachen, Germany
`
`Abstract
`
`In many cases, high compressive stresses are an unwanted side effect of deposited PVD coatings, because they are known to
`reduce the adhesive strength of the coating on the substrate. However, in some applications a main focus of the PVD coatings
`consists of bringing the surface of a substrate into a compressive state. A surface being in a compressive state is more likely to
`withstand thermal and mechanical alternating stresses within the surface and has a higher resistance against forming cracks and
`increases the life span of semi-solid metal forming (SSM) dies. Arc ion plating is a PVD process, which is known to cause high
`compressive stresses in coatings due to its high ionisation rate and the applied bias voltage to the substrate. Therefore, the arc
`ion plating process is suitable for bringing a surface of a substrate into a compressive state. The investigated (Cr,Al)N coatings
`were deposited in such an arc ion plating PVD process and the thickness varies from 2.7 to 17 mm. The correlation of thickness
`vs. residual stresses of these coatings was investigated. In order to determine these residual stresses a stripe bending test is backed
`up and compared with a XRD stress analysis. Additionally, the coatings were exposed to impact tests to determine the influence
`of compressive stresses on the wear behaviour caused by alternating stresses.
`䊚 2002 Elsevier Science B.V. All rights reserved.
`
`Keywords: Residual stress analysis; Semi-solid metal forming; Chromium based coating
`
`1. Introduction
`
`The ternary coating (Cr,Al)N combines, compared to
`other hard materials, a good chemical resistance with a
`high hardness and melting point. The combination of
`these properties makes (Cr,Al)N coatings suitable for
`application on dies for semi-solid metal forming (SSM).
`SSM is a fairly new metal forming process, which
`combines the advantages of forging and die casting in a
`single process. The process is based on the specific
`rheological behaviour first discovered at the MIT in the
`early 1970s by Flemming and Spencer w1,2x. Related
`areas like pressure die casting of aluminium have already
`shown promising results concerning lifespan increases
`due to use of hardcoatings on dies w3,4x. An additional
`high residual compressive stress within the coating or
`surface in a die is known to enhance the resistance
`against crack initiation and propagation caused by alter-
`nating mechanically- or
`thermally-induced stresses.
`*Corresponding author. Tel.: q49-241-809-5577; fax: q49-241-
`809-2264.
`E-mail address: maes@msiww.rwth-aachen.de (M. Maes).
`
`Increasing the compressive stress within a coating is
`therefore a focus of this paper w5x.
`
`2. Experimental
`
`2.1. Deposition process
`
`The investigated coatings were deposited in a Multi
`Arc PVD 20 system, equipped with two random arc
`sources, containing a pure chromium and a pure alumin-
`ium target each. A single batch contained a H11
`(1.2343) hot working steel, a cemented carbide and an
`annealed stress free bending stripe (1.4310) sample. The
`last mechanical preparation step consists of polishing
`the substrates with a 6 mm diamond suspension followed
`by an ultrasonic cleaning.
`The film thickness was varied by the length of
`deposition time, which resulted in coatings varying from
`2.7 to 17 mm with an average deposition rate of 1.1–
`1.4 nmys. All other deposition parameters were then
`kept constant at all batches.
`
`0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved.
`PII: S 0 0 4 0 - 6 0 9 0Ž 0 2. 0 0 8 3 1 - 3
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`
`319
`
`by X-ray diffraction. After determining the angle areas
`(theta and omega) in which a peak occurs, these same
`areas were then scanned at tilted angles (f and c).
`After removing the background counts and smoothing
`the reading, the shift of the peaks’ centre of gravity
`position is then calculated by a Pseudo-Voigt fit. It
`should be mentioned that XRD stress measurements
`with rather low energetic Co radiation are limited to the
`near-surface-regions of the coating due to the low X-ray
`penetration in chromium based PVD coatings, which
`could cause a problem at higher film thicknesses.
`
`Fig. 1. Co-ordinate system used in X-ray stress analysis.
`
`2.2. Determination of mechanical properties
`
`3. Theoretical basis
`
`A common way to adjust the amount of compressive
`residual stress can be achieved by varying the bias
`voltage at the substrate, since the arc ion plating process
`provides a high ionisation rate. The upper limit of the
`bias voltage is given by spalling of a coating near the
`edges of the substrate, due to the inhomogenous distri-
`bution of the residual stress. A second method of varying
`the amount of stress can be achieved by changing the
`total pressure within the vacuum chamber, which signif-
`icantly changes the mean free path of an atom. A
`reduction of the mean free path thereby increases the
`chance of a collision with a second atom and thus
`changes the average kinetic energy of the ionised parti-
`cles, which is a main cause of lattice elongation and
`growth-induced stresses within the coating. A lower
`limit of the pressure is given by an insufficient nitrida-
`tion (in case a reactant is nitrogen) of the coating. A
`third method of adjusting residual stresses within a
`coating can be done by varying the film thickness,
`which is the main focus of this paper.
`s s
`q s
`qs
`residual
`growth
`thermomechanical
`mechanical
`Residual stress within a coating can be divided into
`three separate classes:
`● thermomechanically-induced stresses,
`are present when
`– the application temperature differs from the depo-
`sition temperature and
`– CTE (coefficients of thermal expansion) values
`between coating and substrate do not match
`● mechanically-induced stresses exist when the sub-
`strate is prestressed during deposition
`● growth-induced stresses are caused during deposition,
`when the lattice spacing of the coating is distorted
`by the high kinetic energy during ion implantation
`
`these stresses
`
`s
`
`4. Results
`
`4.1. Film thickness
`
`All coatings show linear growth in film thickness.
`The only exception is batch 5, which had a layer
`
`The mechanical coating properties were analysed
`using tests such as scratch test, microhardness, nanoin-
`w6x. Additionally,
`dentation and surface roughness
`impact tests were used to simulate alternating stresses
`within the surface. The impact tester is a device which
`offers the possibility to evaluate the resistance of (coat-
`ed) surfaces w7x against impulsive strain by means of
`impacting a cemented carbide ball onto a coated surface
`with a predefined load. The coatings’ morphology and
`impact craters were investigated by means of scanning
`electron microscopy (SEM).
`
`2.3. Determination of
`bending stripe method
`
`intrinsic stresses by means of
`
`Due to the importance of compressive stresses within
`the coating, three different ways of determining residual
`stresses are compared in this paper. First, by using the
`Stoney w8x, secondly by Senderhoff’s equation w9x and
`thirdly, by means of X-ray diffraction techniques.
`In order to minimise failure using the bending stripe
`method, the bent beam has to be clamped in a specific
`way. The construction of the substrate holder allowed
`the substrate to expand in the lateral direction, which
`restricts the substrate to be bent during deposition.
`Special demands concerning material properties of
`the bent beam are met using a spring steel stripe
`(X12CrNi17-7 or AISI 302) with the dimensions 70
`mm=7 mm=0.2 mm,
`that has little or no plastic
`deformation in the required temperature range. A bent
`beam was then mounted on the substrate holder and
`single sided coated. After cooling the coated bending
`stripe to room temperature, it should show a perfect
`bending radius caused by the residual stress.
`
`2.4. Determination phases and intrinsic stresses by
`means of X-ray diffraction
`
`The in-plane surface residual stress in the coatings
`were also determined by X-ray diffraction using the
`sin R method (Fig. 1).
`rior to the stress analysis, phases were determined
`
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`E. Lugscheider et al. / Thin Solid Films 420 – 421 (2002) 318–323
`
`Fig. 2. Nanoindentation, showing Young’s moduli and nano hardness.
`
`thickness of 12.2 mm. When comparing batch 5 and 6,
`the layer thicknesses of both batches are almost equal,
`although the deposition time of batch 6 was 30 min
`longer then batch 5. The different fibre texture of this
`coating due to small differences in deposition parame-
`ters, could be responsible for this behaviour. Although
`a different fibre texture was found in XRD analysis, no
`conclusions about the lattice (bcc, fcc or hcp), which
`could influence the deposition rate can be made with
`the acquired data.
`
`4.2. Scratch test
`
`loads were obtained for cemented carbide
`Highest
`samples with a film thickness of 8.9 mm, which showed
`no signs of any adhesive or cohesive failure. A further
`increase of the film thickness proved to be useless on a
`cemented carbide, since maximum scratch loads were
`lower at higher film thicknesses and showed cohesive
`failure within the coating, mainly caused by impurities
`within the coatings’ surface.
`Caused by the weaker substrate, an increase in film
`thickness on a H11 or 1.2343 hot working steel proved
`to have a positive effect in terms of resistance against
`higher scratch loads. In the latter case the coating’s
`thickness provides a self-supporting layer, which restricts
`the coating to give way against the high pressure caused
`by the scratch diamond. In case of the cemented carbide
`substrate, this self-supporting behaviour is not required
`because the substrate already has a higher basic hardness
`(cemented carbide 1600 HV) compared to the much
`softer hot working steel (HV 600 near surface).
`
`4.3. Hardness
`
`The indentation hardness was measured by nanoin-
`dentation. The nanoindents were taken in plane and at
`constant depth of 300 nm. Results of these nano-
`indentations are shown in Fig. 2, which shows that
`Young’s modulus and hardness of most samples stay
`within a certain range. The only exception is sample 6,
`which shows a different behaviour concerning the
`Young’s modulus and hardness. The cause of this behav-
`iour was again linked to a different fibre texture meas-
`ured with XRD phase analyses.
`
`4.4. X-Ray analysis
`
`A phase analysis was carried out prior to the X-ray
`stress analysis. The investigated coatings showed a
`strong fibre texture, and most of the coatings showed a
`(111) texture, see Fig. 3. Batch 6 showed in many ways
`a different behaviour, concerning film thickness, residual
`stress and spalling during impact tests. Phase analysis
`proved that this sample had a strong (220) texture. By
`investigating the coatings’ protocols a distinct difference
`in arc current could be found at the chromium target.
`This property would have a profound impact on the
`chromium ion flux, and would lead to the fewer chro-
`mium atoms being incorporated in the coating. EDS-
`Analysis showed that except for batch 6, all batches had
`an Al-content in the range of 25% and a Cr content of
`63% and 11% nitrogen. Batch 6 had an Al-content of
`16.24% and a Cr content of 78.93% and 4.82% nitrogen.
`
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`
`321
`
`residual stress coating
`
`s
`
`s
`
`2
`h
`substrate substrate
`h
`
`substrate
`
`coating
`
`y
`
`B
`C
`D
`
`1
`r
`
`b
`
`E
`F
`G
`
`1
`r
`
`a
`
`(1)
`
`E
`6 1yn
`.
`Ž
`To ensure that no thermomechanical or mechanical
`stresses are present within the bending stripe before
`deposition, the bending stripes were stress-free annealed.
`A more accurate stress analysis can be determined by
`the bending stripe method using Senderhoff’s equation,
`which additionally makes use of the coating’s Young’s
`modulus.
`
`sresidual stress coating
`B
`C
`h
`substrate
`D
`6 1yn
`Ž
`
`E
`
`s
`
`q
`
`substrate
`
`E
`E
`
`substrate
`h
`
`substrate
`
`.
`
`coating
`
`Ž
`
`substrate
`
`coating
`
`h
`
`coating
`
`E3
`F
`G
`
`.
`.
`
`1yn
`1yn
`Ž
`h
`coating substrate
`
`Fig. 3. Phase analysis and fibre texture of batch 5 and 6.
`
`The low amounts of nitrogen are due to the resolution
`limits of the EDS detector for low atomic numbers.
`
`4.5. Residual stress
`
`A common and easy to use method for determining
`the residual stress of PVD coatings is given by the
`Stoney equation. The major advantage of this method is
`that in order to calculate the amount of residual stress
`within a coating, the Young’s Modulus of the film is
`not
`required, provided that
`the PVD coating is in
`comparison to its
`thickness
`relatively small, e.g.
`h <h
`ratio of 1:100.
`coating
`substrate
`
`B
`1
`= y
`C
`r
`D
`
`b
`
`1
`r
`
`a
`
`E
`F
`G
`
`(2)
`The Young’s moduli needed for this equation were
`separately determined by nanoindentation for each single
`batch. This ensures that differences between batches
`concerning deposition parameters are compensated.
`In order to back up this acquired data from the
`bending stripe method, an X-ray stress analysis was
`carried out for some of the samples. As mentioned
`earlier this theory of stress analysis is based on a shift
`in peak position, when a sample is tilted. For initial
`measurements, reflexes did not occur in the expected
`angle ranges. To determine any inconsistencies within
`the coating’s texture a pole figure was then created. The
`graph of this pole (Fig. 4) shows that the pole of the
`
`Fig. 4. Pole figure of textured sample from batch 5.
`
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`
`Fig. 5. Residual stress of the investigated coatings.
`
`thereby
`
`shifted
`
`and
`
`intensity was
`coating’s
`asymmetrical.
`Along with this acquired data, a shift of the sample
`could be determined and taken into account when using
`an X-ray stress analysis. A possible cause of this shift
`might be a result of the spatial orientation of the targets
`within the vacuum chamber, since all samples were
`coated with a static arrangement.
`In order to compensate this asymmetrical property the
`sample has to be pre-tilted and rotated along the f and
`c axis.
`Intrinsic stresses could then be calculated using the
`following equation:
`
`s ys s
`
`33
`
`11
`
`1
`≠D
`E
`D 1qn ≠sin c
`Ž
`.
`2
`where s is a tension component in lateral direction
`11
`and s in the in-plane direction, E is given by the
`33
`
`Young’s modulus, n the Poisson’s ratio and D the
`0
`lattice spacing of a stress free sample.
`The results of the determination of the residual stress
`are shown in the Fig. 5. The validity of Stoney equation,
`with respect to film and substrate thickness ratio (1:100)
`was not met when the coatings’ thickness exceeds 2
`mm. Therefore, the error is still small when calculated
`in the thin film range, but
`tends to get
`larger with
`increasing film thickness. As far as the validity of
`Senderhoff’s equation is concerned, a certain length-to-
`width ratio has to be met w9x.
`
`4.6. Impact tests
`
`All impact tests were carried out on a hot working
`steel substrate. Although increased film thickness leads
`to a higher scratch resistance. These coatings showed
`that increasing film thickness does not lead to improved
`results when exposed to a high impulsive strain. The
`samples with a thickness larger than 9 mm and an
`alternating force of 700 N, all lead to massive spalling
`within the impact crater (Fig. 6). The cause of this
`behaviour can be linked to a lack of plasticity within
`the coating. A thinner coating is better able to follow
`the impact of a ball and will therefore less likely lead
`to spalling.
`
`5. Conclusions
`
`Results show that the amount of compressive residual
`stress increases with increasing film thickness, provided
`that all deposition parameters, except deposition time
`are kept constant. Compressive residual stress must be
`developed by Ion Peening w10,11x. When the deposition
`process is started, the growing film is still able to relax,
`since the substrate does not inhibit plastic deformation
`of the coating. With increasing layer thickness,
`the
`
`0
`
`(3)
`
`Fig. 6. Impact crater after 100 000 impacts at 700 N, left (12.2 mm), and right (8.9 mm).
`
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`E. Lugscheider et al. / Thin Solid Films 420 – 421 (2002) 318–323
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`323
`
`influence of the substrate becomes smaller and thus
`plastic deformation is inhibited by the already existing
`hard coating.
`Desirable properties of coatings for semi-solid metal
`forming applications are given by a high compressive
`stress. These stresses superimpose the smaller alternating
`tensile stresses caused during forming operations and
`considerably reduce heat checking. PVD-coatings could
`be deposited up to 17 mm. The intrinsic stress which
`could lead to spontaneous spalling, since film thickness
`was very high, was not reached. These thick coatings
`even showed an exceptional behaviour during scratch
`tests on a hot working steel. However, during impact
`tests, which applied an impulsive strain in the samples’
`surface, these same thick coatings (film thickness )9
`mm) lead to intensive spalling and are unable to with-
`stand the impact force, even at low forces (300 N).
`Conclusively,
`it can be stated,
`that optimal CrAlN-
`coatings thicknesses on hot working steel substrates, are
`typically in the range of 8–10 mm, when besides wear
`protection, protection against
`impulsive
`strain is
`required.
`Finally, it can be stated that an accurate determination
`of the exact amount of residual stress within a PVD
`coating remains difficult. However, the bending stripe
`method can provide an easy to use, quantitative stress
`indication determined by Stoney or Senderhoff’s equa-
`tion, assuming that
`the ranges of validity of these
`equations are taken into consideration. The disagreement
`between Senderhoff’s and X-ray analysis is caused by
`the lack of material properties concerning CrAlN coat-
`
`ings in a stress-free state environment. This missing data
`could be gained by the bending stripe method using the
`Senderhoff’s equation.
`
`Acknowledgments
`
`The authors gratefully acknowledge the financial sup-
`port of the Deutsche Forschungsgemeinschaft (DFG)
`within the Collaborative Research Center (SFB) 289
`‘Forming of metals in the semi-solid state and their
`properties’.
`
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