Cracking resistance of Cr3C2–NiCr and WC–Cr3C2–Ni thermally sprayed coatings under tensile bending stress

    Cracking resistance of Cr 3 C2 –NiCr and WC–Cr3 C2 –Ni thermally sprayed coatings under tensile bending stress Erwin Mayrhofer, Leo Janka, Wolfgang Peter Mayr, Jonas Norpoth, Manel Rodriguez Ripoll, Martin Gr¨oschl PII: DOI: Reference:

S0257-8972(15)30240-1 doi: 10.1016/j.surfcoat.2015.09.002 SCT 20552

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

10 March 2015 19 August 2015 1 September 2015

Please cite this article as: Erwin Mayrhofer, Leo Janka, Wolfgang Peter Mayr, Jonas Norpoth, Manel Rodriguez Ripoll, Martin Gr¨ oschl, Cracking resistance of Cr3 C2 –NiCr and WC–Cr3 C2 –Ni thermally sprayed coatings under tensile bending stress, Surface & Coatings Technology (2015), doi: 10.1016/j.surfcoat.2015.09.002

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ACCEPTED MANUSCRIPT Cracking resistance of Cr3C2-NiCr and WC-Cr3C2-Ni thermally sprayed coatings under tensile bending stress

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Erwin Mayrhofera,b , Leo Jankaa,c , Wolfgang Peter Mayrd , Jonas Norpotha,∗, Manel Rodriguez Ripolla , Martin Gr¨oschlb a

AC2T research GmbH, Viktor Kaplan-Straße 2C, A-2700 Wiener Neustadt, Austria Institute of Applied Physics, Vienna University of Technology, Wiedner Hauptstraße 8 - 10 /134, A-1040 Vienna, Austria c Department of Materials Science, Tampere University of Technology, Korkeakoulunkatu 6, FI-33101 Tampere, Finland d Voith Paper Rolls GmbH & Co. KG, Maretgasse 45, A-2632 Wimpassing, Austria

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Abstract

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The cracking behaviour of Cr3 C2 -25(Ni20Cr) and WC-20Cr3 C2 -7Ni thermally sprayed coatings during tensile load in 3-point bending tests was studied by Acoustic Emission

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(AE) monitoring and microstructure post-analysis. The AE monitoring reveals a

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superior resistance against cracking in the WC-Cr3 C2 -Ni coatings compared to

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Cr3 C2 -NiCr. The incorporation of tungsten carbides beneficially affects the residual stress state of the coatings and has an impact on the detailed fracture mode. The results hold for both as-sprayed as well as ground and polished coatings.

Keywords: Acoustic emission, HVOF thermal spray coating, Cr3 C2 , WC, Bending test, Cracking

Corresponding author Phone: +43 2622 81600 128 Fax: +43 2622 81600 99 Email address: [email protected] (Jonas Norpoth) ∗

Preprint submitted to Surface & Coatings Technology

August 19, 2015

ACCEPTED MANUSCRIPT 1. Introduction

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Resistance against cracking is a key characteristic of any coating dedicated to protect

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against mechanical wear. In the field of thermally sprayed cermet coatings,

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crack-controlled damage patterns have been reported under such different wear

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conditions like erosion [1, 2, 3], sliding [4] or fretting [5] and axial fatigue [6]. The

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fracture toughness of hardfacings basically depends on the brittleness of the hard

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phases [7], their grain size [8], the amount of binder phase [9] and the residual stress

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level [10, 11]. A peculiarity of thermally sprayed coatings in this respect is their

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inherent layered microstructure consisting of individual splats and a rather high

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density of defects like pores and oxide scale, rendering their mechanical properties

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different from those found in bulk materials [12].

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In this contribution the resistance against cracking in tensile bending conditions is

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compared for High-Velocity Oxygen-Fuel (HVOF) thermally sprayed coatings from

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Cr3 C2 -25(Ni20Cr) and WC-20Cr3 C2 -7Ni feedstock powders. While there is

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comprehensive work on the wear properties of these compositions [13, 14, 15], their

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differences in resistance against cracking are not equally well investigated. Two recent

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studies on cross-section indentation fracture toughness performed by Houdkov´a et al.

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[16] and Bolelli et al. [17] report the following order (ascending toughness):

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WC-20Cr3 C2 -7Ni < Cr3 C2 -25(Ni20Cr), 88WC-12Co < 83WC-17Co. Therein, the trend

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of a beneficial impact of a higher amount of metallic binder phase is reproduced and a

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superior toughness of WC over Cr3 C2 hard phases suggested. One shortcoming of such

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indentation tests is the significant release of residual stress during metallographical

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preparation of the coating cross-sections. We therefore used a 3-point bending test in

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combination with Acoustic Emission (AE) monitoring in order to compare the

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coatings resistance against cracking without preconditioning their microstructure.

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Furthermore, while indentation fracture basically tests some form of a complex crack

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arrest phenomenon [26] emanating from a highly deformed material volume, our

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ACCEPTED MANUSCRIPT investigations primarily address the initiation of the cracking process in a low-strain

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regime, i.e. the critical bending stresses where the first cracking events occur. The

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corresponding range of the stress-strain curves in the bending tests covers the elastic

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deformation regime of the steel substrates up to the early stages of plasticity.

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Since both as-sprayed and finished surfaces are widely used in industrial applications,

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we also included ground and polished coatings in our investigations.

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2. Materials and methods

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2.1. Coating deposition

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Coatings studied in this paper were deposited from agglomerated and sintered

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Cr3 C2 -25(Ni20Cr) and WC-20Cr3 C2 -7Ni feedstock powders with a DiamondJet

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DJ2600 (Sulzer Metco (now Oerlikon Metco), Wohlen, Switzerland) HVOF spray gun

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equipped with a 9MP-DJ powder feeder on grit-blasted, steel substrates. Both

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powders featured the same particle size distribution and were sprayed with identical

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parameters. The spray process was monitored with a flame control unit (SprayWatch

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3i, Oseir Oy, Tampere, Finland). Industrial standard spray parameters from Voith

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Paper Rolls GmbH & Co. KG with deposition efficiencies around 65% were applied.

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The detailed parameters are closely related to the recommendations of the spray gun

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manufacturer for hardmetal deposition as given in Tab. I. The thickness of the steel

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substrates was 10 mm, and the thickness of the coatings was 300 µm. For both

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materials six samples were prepared, whereof three each were ground by 100 µm and

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polished (referred to as ”finished” in the following).

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2.2. Microstructure characterization

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Microstructural characterization of the feedstock powder and sprayed coatings was

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made from ground and polished cross-sections by optical microscopy and a scanning

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ACCEPTED MANUSCRIPT electron microscope (SEM) Zeiss Supra 40VP (Oberkochen, Germany) with back

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scattered electron detector (BSE).

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The phase composition of the coatings was characterized with XPert X-ray

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diffractometer (PANAlytical, Almelo, Netherlands) with a secondary graphite

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monochromator, automatic divergence slits and a scintillation counter. The XRD

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study was done using Cu-Kα radiation in Bragg-Brentano geometry.

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Dye penetrant inspections were done on finished coatings after bending tests.

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In order to study the cracking characteristics of the coatings in detail, Focused Ion

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Beam (FIB) milling with Ga ions attached to a SEM (Zeiss 1540×B, Oberkochen,

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Germany) was applied for 3-point bent samples. This method permits to study the

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cross-section of single cracks without preparation-related carbide pull-outs. Beam

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currents of 1 nA for cutting and 100 pA for final preparation were used. Imaging of the

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FIB cuts was done by secondary electrons from excitations with a primary beam of

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either electrons (SE) or Ga ions (FIB-SE).

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2.3. 3-point bending tests

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2.3.1. Test setup

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The 3-point bending experiments were performed using a Zwick UBM (Ulm,

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Germany) with 4.5 mm diameter support pins with a span of 90 mm and a 10 mm

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diameter loading pin acting on the uncoated backside of the substrates; sample width

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and length were 20 and 100 mm, respectively. During the tests, the normal load was

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increased from a preload of 50 N to max. 13 kN at a rate of 50 N/s. The deflection of

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the sample was measured with a mechanical sensor attached to the coated sample

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surface below the loading pin.

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In addition to the coated samples, an uncoated substrate was tested as a reference as

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well.

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ACCEPTED MANUSCRIPT 2.3.2. Data analysis

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In order to provide a more universal description of the state of sample deformation

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during the bending tests, stress-strain curves were determined from the measured

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load-deflection curves. To this end, we applied finite-element (FE) modelling to

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simulate the load-deflection curve of an uncoated steel substrate, using the isotropic

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hardening plasticity model from the Solid Mechanics module of COMSOL

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Multiphysics software (Stockholm, Sweden). Satisfactory agreement with the

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experimental data was found for a Young modulus of 195 GPa, an initial yield stress of

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495 MPa and an isotropic tangent modulus of 20 GPa.

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Fig. 1 gives an overview on the load-deflection curves of all bending tests together with

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the simulated data. Obviously, there is no systematic deviation of the data from

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coated samples compared to the uncoated reference, i.e. the contribution of the

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coatings to the overall samples resistance against bending is negligible. This is

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reasonable owing to the large difference in thickness between substrate (10 mm) and

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coatings (300 or 200 µm for as-sprayed and finished types, respectively) and is

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bolstered by the observation that cracking of the coatings already starts in the elastic

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deformation regime of the substrates (see below).

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The inset of Fig. 1 shows the (von Mises) stress-strain (σ-ǫ) curve corresponding to the

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simulated bending test, calculated for the extreme fibre below the loading pin on the

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convex, i.e. tensile bent, surface of the steel substrate. The onset of plasticity is

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clearly visible as the change in the slope of σ(ǫ). Neglecting contributions from mutual

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interaction with the coatings as discussed above, this curve yields the state of

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substrate deformation ”seen” by the coatings during the bending tests. We used

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polynomial fits to the simulated data to determine both stress and strain as functions

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of sample deflection and assigned the measured deflections to them.

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ACCEPTED MANUSCRIPT 2.4. Acoustic Emission investigation

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Acoustic emission is defined as the generation of transient elastic waves originating

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from rapid release of elastic energy [18]. These waves propagate through the material

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surrounding the source as structure-borne ultrasound in the frequency range of about

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20 kHz to 1 MHz. AE monitoring proved to be a useful probe for cracking or spallation

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of thermal spray coatings during thermal or mechanical loading [27, 19, 11, 28].

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2.4.1. Measurement setup

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For measuring AE, a piezoelectric wideband transducer was used, featuring the

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maximum sensitivity in the range of 100 kHz to 4 MHz. The AE transducer was

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attached to the uncoated face of the sample substrates with a silicon grease serving as

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acoustic couplant. After preamplification of the analogue output signal at 40 dB, AE

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signals were processed using a PCI 2 AE system, supplied by Physical Acoustics

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(Princeton, USA). Preliminary data analysis was performed by the software AEWin

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from the same manufacturer. The frequency range of output signals from 20 kHz to

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1 MHz was selected by means of an analogue band-pass filter. Thus, parasitic signals

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were attenuated, both in terms of sound in the audible range generated by the test rig

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as well as regarding electromagnetic interferences in the range beyond the upper limit

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of the band-pass filter.

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2.4.2. Data analysis

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The AE energy of an AE hit (AE wave packet comprising 15 000 samples, see ref. [20]) R is defined as EAE = R1 U 2 (t)dt, with U (t) being the AE signal voltage over time, the

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integral running over the duration of the regarded AE hit, and R the resistance, which

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is set to R = 10 kΩ by convention. This arbitrary definition of R allows for comparing

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data from the same experimental setup.

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For an appropriate visualization of the AE recordings from the bending tests, the data

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were binned into sections of 15 MPa in stress and 0.02% in strain, respectively, and 6

ACCEPTED MANUSCRIPT bin bin EAE values of the corresponding AE hits were summed to yield EAE (σ) or EAE (ǫ).

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3. Results and discussion

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3.1. Microstructure of pristine coatings

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Fig. 2 shows cross-sections of as-sprayed Cr3 C2 -NiCr and WC-Cr3 C2 -Ni coatings.

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From the overview optical micrographs the samples can be classified as industrial

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standard HVOF coatings with a dense microstructures and robust mechanical binding

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to the substrates. The Cr3 C2 -NiCr coatings exhibit a slightly higher apparent porosity,

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especially in the top 100 µm of their cross-sections, most probably owing to lower

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peening forces during spraying. The BSE micrographs exhibit the principal hardmetal

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entities chromium carbides, tungsten carbides and metallic binder matrix in dark,

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bright and intermediate grey scale levels, respectively. Partial decomposition of the

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carbides during the spraying process is evident from the wavy, inhomogeneous pattern

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of the binder matrix and the rims around many carbides. The chromium carbide size

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distribution is similar for both types of coatings, while the tungsten carbides are

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typically significantly smaller.

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The XRD patterns in Fig. 3 show the phase composition of the coatings together with

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the corresponding feedstock powder materials. For the case of Cr3 C2 -NiCr, both Cr3 C2

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and Cr7 C3 carbides are present in the feedstock powder and the coatings. In the

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pattern of the coating, the peaks are broader, which indicates a certain spread of

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lattice constants from phase decomposition during spraying. Furthermore, there is

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evidence for some portion of amorphous phase in the binder phase of the coatings.

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WC-Cr3 C2 -Ni feedstock powder and coatings contain Cr3 C2 and WC carbides in a

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NiCr matrix. Additionally, complex (W,Cr,)2 C carbides, similar to those found by

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Bolelli et al. [17] and Berger et al. [21], are present. According to the related peak

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positions, the stoichiometry of these carbides is close to that of (W0.2 Cr0.8 )2 C.

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In earlier work (unpublished), residual stresses of WC-Cr3 C2 -Ni coatings, sprayed with

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ACCEPTED MANUSCRIPT the same equipment and identical parameters like in the study on hand, were

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determined by means of the hole-drilling method. The depth profiles were

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characterized by tensile residual stresses down to 200 µm below the surface of the

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coatings and a transition to compressive stresses below that depth. We argue that the

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range of tensile residual stresses extends deeper below the surface in the Cr3 C2 -NiCr

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coatings. This difference is governed by the different peening actions imparted on the

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coatings upon impact of the fast powder particles during spraying. Kuroda et al.

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demonstrated an increase of the peening intensity with the kinetic energy of the

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impacting particles in HVOF spraying [22]. The online process monitoring during the

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deposition of our coatings showed that the particle velocity distributions in the spray

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flames were identical within the precision of the flame control unit for both powders:

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given the lower apparent

density of the Cr3 C2 -NiCr particles compared to WC-Cr3 C2 -Ni (roughly 2.7 vs.

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4.6 g/cm3 ), a weaker peening effect and thus more pronounced tensile residual stresses

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are reasonable. For comparison, Poirier et al. reported that a decrease of 20% in the

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kinetic energy of the particles during HVOF spraying can alter the net residual stress

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state of Cr3 C2 -NiCr coatings from compressive to tensile [33].

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3.2. Acoustic Emission during bending tests

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Fig. 4 shows the AE recordings during the bending tests. Representative experiments

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for each type of sample are given in (a)-(d), and a comparison of all experimental data

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in (e)-(f) in stress or strain representation, respectively.

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The tests can be divided into three consecutive stages. An initial stage with only weak

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and few acoustic signals ranges up to stresses of approximately 100(200) MPa in the

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case of as-sprayed (finished) Cr3 C2 -NiCr coatings and 200(300) MPa for

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WC-Cr3 C2 -Ni. The reference experiment with the uncoated substrate yields similar

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signatures of AE over the full bending span - we therefore refer to these signals as the

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ACCEPTED MANUSCRIPT background and ascribe it basically to the friction between the samples and the

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support pins. The finished coating surfaces generate lower friction and accordingly

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lower AE activity. Afterwards, both number and energy of the recorded AE hits

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increase (stage #2) until a steady state with frequent AE hits of EAE on the order of

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106 aJ is reached around 400 MPa (ǫ = 0.2%) for the Cr3 C2 -NiCr coatings and

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500 MPa (ǫ = 0.3%) for WC-Cr3 C2 -Ni (stage #3).

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3.3. Microstructure of cracked coatings

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From the onset of stage #3, cracks become visible on the coatings surface, frequently

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accompanied by audible clicking noise. In Fig. 5 (middle rows), the surface crack

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structure on finished coatings was made visible by a dye penetrant. The comparison

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with the corresponding maps of maximum stress and strain during the preceding

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bending tests (top) shows that all of these cracks were generated in stage #3 of the

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tests. The coating cross-sections (bottom) demonstrate that the cracks nucleate at or

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near the coatings surface and eventually propagate

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towards the substrate interface. The cracks in the Cr3 C2 -NiCr coatings

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generally exhibit a straighter morphology and wider opening. Adhesive failure, or even

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coating spallation, via cracks propagating along the interface with the substrate has

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been reported as a source of strong AE signals in bending tests of HVOF WC-Co

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coatings [29, 19], but was not found in any of our samples.1

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We interprete the weaker AE signals in stage #2 as being emitted from crack

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nucleation, micro-cracking or early short-range crack propagation within surface-near

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regions of the coatings. Similarly, in a study on the AE during Vickers indentation

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testing of HVOF WC-Co coatings, Faisal et al. report that the EAE emitted by the

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initially formed minor crack network around the indents can be clearly distinguished

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from the stronger AE signals of the pronounced Palmqvist cracks later emanating from 1

Even severe bending around a mandrel (ǫ ≃ 20%) of identical coatings on 3-mm thin substrates generated no spallation.

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ACCEPTED MANUSCRIPT bin the indent corners [28]. In our experiments, log (EAE ) in the Cr3 C2 -NiCr coatings

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increases rather linearly through stage #2, whereas the WC-Cr3 C2 -Ni coatings, and

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especially the finished ones, exhibit a distinct regime of attenuated AE activity up to

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the steep transition to stage #3.

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The apparent higher resistance against cracking in the WC-Cr3 C2 -Ni coatings could be

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discussed in the framework of the coatings elastic constants. Houdkov´a et al.

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measured Youngs moduli of 140 GPa for HVOF Cr3 C2 -25NiCr coatings and 215 GPa

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for WC-20Cr3 C2 -7Ni, which is in line with our observation of a wider elastic regime

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(stage #1) in the WC-Cr3 C2 -Ni coatings. However, owing to the inherently high

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susceptibility of the elastic properties of HVOF hardmetal coatings on rather subtle

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variations of the spraying parameters [31], such cross-comparisons should not be

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stressed too much.

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Most likely, the higher cracking resistance of the WC-Cr3 C2 -Ni coatings stems to some

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extent from their higher compressive residual stresses compared to Cr3 C2 -NiCr as

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mentioned in section 3.1. Enhanced resistance against cracking in 4-point bending

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tests by an increase of the compressive stress level in HVOF WC-Co coatings was

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demonstrated by Bouaricha et al. [11]. This argument also holds for the comparison of

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as-sprayed and finished coatings. In the latter, the removal of the tensily strained, top

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100 µm by grinding renders the coatings more resistant against the early failure

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processes corresponding to stage #2 of the bending tests. Accordingly, the eventual

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release of stored mechanical energy takes place more abruptly, evident as higher EAE

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levels in stage #3 compared to the as-sprayed coatings.

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In addition to the different residual stress states of the coatings, further impact on the

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cracking resistance is introduced by their differences in composition and grain

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structure. WC-Cr3 C2 -Ni coatings are characterized by two principal types of carbides,

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whereof the tungsten carbides exhibit notably smaller average grain diameters, and a

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lower amount of binder matrix. Compared to the Cr3 C2 -NiCr coatings, this

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ACCEPTED MANUSCRIPT microstructure can be qualified as more inhomogeneous and exhibits a higher density

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of internal interfaces. Correspondingly, the detailed crack propagation characteristics

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in both types of coatings differ as shown in Fig. 6. The cracks in the Cr3 C2 -NiCr

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coatings exhibit a basically straight, vertical and unbranched morphology and they

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regularly propagate through the chromium carbides. Contrary, the fracture pattern in

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the WC-Cr3 C2 -Ni coatings is of a rather intergranular type, in which especially the

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tungsten carbides tend to resist cracking. Such crack tip branching and arresting via

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path deflection was also found by other authors in inhomogeneous and nano-scale

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microstructures of thermal spray coatings [23, 24, 25].

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Altogether, in terms of resistance against cracking, the significantly lower amount of

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ductile binder phase in the WC-Cr3 C2 -Ni coatings is outweighted by the higher density

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of internal interfaces acting as crack scattering centres, the superior toughness of WC

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hard phases and the preparation-dependent higher level of compressive residual

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stresses. The latter contribution is especially meaningful when comparing our results

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with those from indentation fracture toughness studies, that inherently ignore the

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coatings intrinsic stress states owing to the necessary metallographic sample

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preparation. This might explain the discrepancy to the results of Houdkov´a et al. [16]

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and Bolelli et al. [17], that suggest a higher fracture toughness for Cr3 C2 -25(Ni20Cr)

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compared to WC-20Cr3 C2 -7Ni.

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4. Summary

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A superior resistance against cracking of HVOF thermally sprayed WC-20Cr3 C2 -7Ni

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coatings compared to Cr3 C2 -25(Ni20Cr) was demonstrated in 3-point bending tests

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with Acoustic Emission (AE) monitoring. Formation of macroscopic cracks in the

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Cr3 C2 -NiCr coatings took already place during the regime of elastic deformation of the

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underlying steel substrate, but was delayed in the case of WC-Cr3 C2 -Ni coatings by

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approximately 100 MPa and 0.1% of applied stress and strain, respectively. The

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ACCEPTED MANUSCRIPT enhanced toughness of the WC-Cr3 C2 -Ni coatings is in part ascribed to their higher

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compressive residual stresses, as was also underlined by comparison of as-sprayed and

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ground coatings. Furthermore, microstructure post-analysis revealed differences in the

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detailed fracture modes of the two types of materials. Cracking in Cr3 C2 -NiCr

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proceeds along rather straight, vertical pathes through both binder matrix and

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chromium carbides, whereas the WC-Cr3 C2 -Ni coatings exhibit also evidence for a

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transgranular fracture mode with branched cracks and crack path deflection around

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tungsten carbides.

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Acknowledgements

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This work was co-funded by the Austrian Research Promotion Agency (FFG) under

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project no. 839126 (”EcoHardCoat”). Special thanks go to Georg Vorl¨aufer and Lukas

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Jesacher for support with the FE simulations, as well as to Walter Kantor, Claudia

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Lenauer (all AC2T research GmbH) and Paul Vetschera (Vienna University of

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Technology) for fruitful discussions.

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[17] G. Bolelli, L.M. Berger, M. Bonetti, L. Lusvarghi, Comparative study of the dry sliding wear behaviour of HVOF-sprayed WC-(W,Cr)2 C-Ni and WC-CoCr hardmetal coatings, Wear 309 (2014) 96-111.

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[15] H.M. Hawthorne, B. Arsenault, J.P. Immarigeon, J.G. Legoux, V.R. Parameswaran, Comparison of slurry and dry erosion behaviour of some HVOF thermal sprayed coatings, Wear 225 (199) 825-834.

[18] W.F. Hartman, Towards Standards for Acoustic Emission Technology, in: H. Berger (Ed.), Nondestructive Testing Standards: A Review, ASTM International, Philadelphia, MD, 1977.

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[19] J.M. Miguel, J.M. Guilemany, B.G. Mellor, Y.M. Xu, Acoustic emission study on WC-Co thermal sprayed coatings, Mater. Sci. Eng. A 352 (2003) 55-63. [20] T. Shiotani, Parameter Analysis, in: C.U. Große, M. Ohtsu (Eds.), Acoustic Emission Testing, Springer Science & Business Media, Berlin, 2008, pp. 41-51.

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[14] L.M. Berger, Coatings by Thermal Spray, in: V.K. Sarin, D. Mari, L. Llanes, C.E. Nebel (Eds.), Comprehensive Hard Materials, Vol. 1, Elsevier, Oxford, 2014, pp. 471-506.

[21] L.M. Berger, S. Saaro, T. Naumann, M. Kaˇsparova, F. Zah´alka, Microstructure and Properties of HVOF-Sprayed WC-(W,Cr)2 C-Ni Coatings, J. Therm. Spray Technol. 17 (2008) 395-403.

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[22] S. Kuroda, Y. Tashiro, H. Yumoto, S. Taira, H. Fukanuma, S. Tobe, Peening action and residual stresses in high-velocity oxygen fuel thermal spraying of 316L stainless steel, J. Therm. Spray Technol. 10 (2001) 367-374. [23] H. Luo, D. Goberman, L. Shaw, M. Gell, Indentation fracture behavior of plasmasprayed nanostructured Al2 O3 wt.%TiO2 coatings, Mater. Sci. Eng. A 346 (2003) 237-245. [24] R.S. Lima, B.R. Marple, Enhanced ductility in thermally sprayed titania coating synthesized using a nanostructured feedstock, Mater. Sci. Eng. A 395 (2005) 269280. [25] M. Gell, E.H. Jordan, Y.H. Sohn, D. Goberman, L. Shaw, T.D. Xiao, Development and implementation of plasma sprayed nanostructured ceramic coatings, Surf. Coat. Technol. 146 (2001) 48-54. [26] G.D. Quinn, R.C. Bradt, On the Vickers Indentation Fracture Toughness Test, J. Am. Ceram. Soc. 90 (2007) 673-680.

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[28] N.H. Faisal, J.A. Steel, R. Ahmed, R.L. Reuben, The Use of Acoustic Emission to Characterize Fracture Behavior During Vickers Indentation of HVOF Thermally Sprayed WC-Co Coatings, J. Therm. Spray Technol. 18 (2009) 525-535.

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[29] D. Dalmas, S. Benmedakhene, C. Richard, A. Laksimi, G. B´eranger, T. Gr´egoire, Charact´erisation par ´emission acoustique de l’adh´erence et de l’endommagement d’un revˆetement: cas d’un revˆetement WC-Co sur acier, C. R. Acad. Sci. Paris, Chimie / Chemistry 4 (2001) 345-350.

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[27] J. Voyer, F. Gitzhofer, M.I. Boulos, Study of the Performance of TBC under Thermal Cycling Conditions using an Acoustic Emission Rig, J. Therm. Spray Technol. 7 (1998) 181-190.

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[32] Sulzer Metco, Spraying Tables 56375, Issue c, 2003, p. T5-2.

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[31] M. Prudenziati, G.C. Gazzadi, M. Medici, G. Dalbagni, M. Caliari, Cr3 C2 -NiCr HVOF-Sprayed Coatings: Microstructure and Properties Versus Powder Characteristics and Process Parameters, J. Therm. Spray Technol. 19 (2010) 541-550.

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List of Figures 1

Overview of load-deflection curves of all bending tests (three samples per coating type (12 in total; overlapping curves), one uncoated reference)

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and data from the FE simulations. Inset: corresponding simulated stress-

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strain curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2

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Microstructure of pristine, as-sprayed coatings. Optical (left column) and BSE (right) micrographs of cross-sections from Cr3 C2 -NiCr (top row) and

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WC-Cr3 C2 -Ni (bottom). . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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AE recordings during bending tests. (a)-(d) Representative data for each type of coating, (e)-(f) overview of all bending tests. . . . . . . . . . . . . 21

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Post-analysis of cracked coatings of (a) Cr3 C2 -NiCr and (b) WC-Cr3 C2 -

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XRD pattern of (a) Cr3 C2 -NiCr and (b) WC-Cr3 C2 -Ni feedstock powders and coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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Ni. Top rows: stress (left) and strain (right) maps in the state of max-

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imum sample deflection during the respective preceding bending tests.

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The line features near the sample edges stem from the contacts with the

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sample supports. The colour scales given in (a) apply also for (b). Mid-

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dle: optical micrographs of cracked coating surfaces from dye penetrant

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inspections of finished coatings. Bottom: optical micrographs of cracks

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in cross-sections of as-sprayed coatings after bending tests. The assign-

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ments to the cracked surfaces in the middle rows designate the typical

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locations where the respective types of cracks are found. . . . . . . . . . 22

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6

FIB-SE (left column) and SE images (right) of cross-section FIB cuts

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through cracks in Cr3 C2 -NiCr (top row) and WC-Cr3 C2 -Ni (bottom)

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coatings after the bending tests. A cracked carbide is marked with the

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green contour line in the top right image. . . . . . . . . . . . . . . . . . . 23

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Tables and Figures

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Table I: Spray parameter recommendations for deposition of Cr3 C2 - and WC-based hardmetal coatings with DJ2600 [32].

Type

Flow (nlpm)∗

Pressure (bar)

Oxidant

O2

214

11.7

Fuel

H2

635

9.7

Cooling

Air

344

6.9

Carrier

N2

12.5

12

Feed rate (g/min)

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Process gases

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Deposition rate (µm/pass)

< 12

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normal litres per minute

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200 − 250

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Highlights

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• Superior resistance against cracking of WC-20Cr3 C2 7Ni thermally sprayed coatings compared to Cr3 C2 -25(Ni20Cr)

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• Acoustic emission monitoring to determine detailed differences in crack initiation and propagation