Wear resistance of laser remelted thermally sprayed coatings/Lasersulatatud termopinnete kulumiskindlus.
Surzhenkov, Andrei ; Kulu, Priit ; Tarbe, Riho 等
1. INTRODUCTION
In terms of product lifetime of engineering materials and machine
components, the surface is of prime concern. This involves wear
behaviour and mechanical properties such as surface fatigue [1,2].
Thermally sprayed hardmetal coatings, also often called "carbide
coatings", are used widely in many industrial applications for
wear, corrosion and high temperature protection [3]. Recent attention
has focused on reduced consumption of existing resources and materials
recycling. Therefore the application of composite powders, based on used
(recycled) hardmetals for thermal spray is topical [4,5].
Wide use of thermally sprayed coatings gives evidence of the
cost-effectiveness of self-fluxing alloys containing tungsten carbide
(WC) particles, applied by the spray and fusion methods (flame, plasma
and laser fusion). Some materials, most notably MCrSiB compositions,
where M stands for either Ni, Co or Fe, can be fused by heating them up
to the temperature of 1050[degrees]C. Due to the brittleness of tungsten
carbide, the impact wear resistance of the coatings is not high [4].
Because of their low porosity and high bond with the basic materials,
the spray-fused composite coatings, containing WC-Co hard phase, can
resist significant impact loads [6]. It has been shown that abrasive
erosive and impact wear resistance of powder materials and coatings are
not high [5,7,8]. Usage of the recycled hardmetal powder, produced by
mechanical milling, causes high iron content in the powder (up to 20%)
due to the intensive wear of the grinding media [9,10]. It is a factor,
hindering their use in nickel-based compositions.
Following from the abovementioned, this work focuses on thermal
spray-fusion for the production of high-performance surfaces, use of the
deposition of hard coatings by thermal spray and subsequent laser
treatment of powder composition, based on iron-based spray powder. To
improve the impact wear resistance of plasma sprayed coatings, the
following laser remelting was studied.
2. EXPERIMENTAL
2.1. Studied materials and coatings
For coatings, as a substrate, specimens of the size 50 x 25 x 10 mm
of plain carbon steel C45 were used. The composition and hardness of the
steel are given in Table 1.
The composite powders for spray and fused coatings contained
nickel- and iron-based powders as basic components. Table 2 shows the
chemical composition of the self-fluxing alloy powders of the powder
composites. With a spherical shape, their particle size was (+10 -45)
and (+15 -53) [micro]m for Fe- and Ni-based powders, respectively.
Nickel- and iron-based self-fluxing alloy powder compositions,
containing 25 wt% of hardmetal particles, were used. WC-Co hardmetal
powder, produced from used hardmetal by disintegrator milling, was
employed [9,10]. Chemical composition of the hardmetal powder was the
following (in wt%): WC--75.6, Co--11.5, Fe--12.9. Powders had the
particle size of (+20 -63) and -63 [micro]m. Figure 1 illustrates the
particle shape. Particles were primarily equiaxed in form and their
microstructure showed a typical tungsten carbide based hardmetal
structure.
2.2. Plasma spray and laser treatment of coatings
To depose a coating, plasma spray equipment RotAloy of Castolin
Eutectic was applied. Plasma spraying parameters of the coatings are
given in Table 3. Thickness of the coatings was about 0.2 mm. The plasma
sprayed coatings were remelted using Nd: YAG laser Haas HL 4006 D of
Trumpf with a wavelength of 1064 nm; the cross-section of the laser beam
was 8 x 5 mm, overlapping--50%. Parameters of laser remelting are
provided in Table 3.
[FIGURE 1 OMITTED]
2.3. Characterization of the coating structure, hardness and wear
resistance
2.3.1. Microstructure of coatings
Coatings were characterized both in the sprayed condition and after
laser remelting. Polished cross-sections were observed by the optical
microscope using an Omnimet image analysis system and SEM. X-ray
analysis (EDS) was performed to estimate changes in the composition of
the metal matrix.
2.3.2. Determination of hardness
To determine surface hardness, measurements were made with a
universal hardnessmeter Zwick 2.5/TS at a load from 1 to 100 N. The load
was selected to obtain the size of indents comparable with the size of
wear craters, formed by abrasive wear.
Microhardness measurements in the cross-section were carried out
using the Micromet 2001 measuring device. The applied load was equal to
0.245 N. Low loads enabled us to measure the hardness of the metallic
matrix as well as of the hardmetal particles in the matrix of the
coating.
2.3.3. Abrasive wear testing
Abrasive block-on-ring wear (ABRW) abrasion tests were carried out
using the block-on-ring rubber wheel scheme (ASTM standard G 65-94)
(Fig. 2a). The diameter of the ring was 228.6 mm, the applied force was
222 N and the speed of rotation was 200.8 1/min (linear velocity 2.4
m/s). The parameters of the wear tests are given in Table 4. Abrasive
erosive wear (AEW) and abrasive impact wear (AIW) of the coatings were
studied with the experimental centrifugal-type wear testers CAK and DESI
[7]. At AEW the velocity was 80 m/s, impact angles were 30[degrees] and
90[degrees]. By AIW tests a one-rotor system was used (Fig. 2c) [7]; the
velocity was 80 m/s and the impact angle of abrasive particles with the
specimen on the fixed pin surface was about 90[degrees]. Wear
experiments ABRW and AEW with quartzite sand of fraction 0.1-0.3 mm were
carried out. AIW tests were conducted with granite gravel of fraction
4.0-5.6 mm. Hardness of the quartzite and granite, measured at the
polished cross-section, was 11.0 and 9.28 HV 0.05 GPa, respectively.
[FIGURE 2 OMITTED]
The mass loss of the specimens was determined and the wear
coefficient at ABRW was calculated as
k = [DELTA]m/[rho]Ftvr, (1)
where [DELTA]m is mass loss (kg), [rho] is density (kg/[m.sup.3]),
F is force (N), t is time of the experiment (s), v is rotation speed
(1/min) and r is the radius of the ring (m).
At AEW and AIW the mass loss of the specimens was determined and
the volumetric wear rate [I.sub.v] was calculated as
[I.sub.v], = [DELTA]m/[rho]q, (2)
where [DELTA]m is mass loss (mg), q is quantity of the abrasive per
specimen (kg) and [rho] is sample density (mg/[mm.sup.3]).
The relative volumetric wear resistance e,, was determined for
steel C45 as follows:
[[epsilon].sub.v] = {I.sub.v]/[I.sup.C45.sub.v], (3)
where [I.sub.v] is the volumetric wear rate of the tested coating
and [I.sup.C45.sub.v] is that of the reference steel C45.
3. RESULTS AND DISCUSSION
3.1. Structure, porosity and hardness of the coatings
The cross-sections of laser remelted plasma sprayed coatings are
shown in Figs. 3 and 4.
NiCrSiB self-fluxing alloy forms a Ni-based matrix with WC hard
particles (WC-Co hardmetal particles are practically dissolved in the
Ni-based matrix). This was confirmed by the EDS analysis of the
coating--different phase distribution in the coating is shown in Fig.
3b. Retained slag nests in the Ni-based coating were observed.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
As it follows from Fig. 4, Fe-alloy based coating structure is a
typical eutectic structure and more dense than the Ni-alloy based
coating, where due to the high content of Fe (about 13%) large Fe-Cr
dendrites and smaller W-Co dendrites are formed. The rate of solution of
WC-Co in the metal matrix is higher in the Fe-alloy based
coating--practically all WC-Co particles are dissolved in the iron-based
matrix forming the (Fe-Cr)--(WC-Co) eutectic structure. The results of
hardness measurements by both methods are brought in Table 5.
3.2. Wear resistance of spray-fused coatings
Results of abrasive wear tests (abrasion, erosion and impact wear)
are given in Tables 6-8.
3.2.1. Abrasive block-on-ring wear resistance
By abrasion, the coatings studied demonstrated low wear resistance,
the relative wear resistance is lower than one (0.5-0.9) (Table 6).
Because the hardness of the abrasive is higher (about 11 GPa of the
quartz sand) than that of the coating (about 3.0-5.6 GPa), intensive
wear takes place as a result of microcutting or surface scratching. It
was confirmed by the study of the worn surfaces (Fig. 5a).
[FIGURE 5 OMITTED]
3.2.2. Abrasive erosive wear resistance
Based on the studies of wear rate and wear mechanism of the
coatings (Table 7), the wear resistance of Ni-based coatings at low
impact angles is lower than the wear resistance of reference steel C45;
Fe-based coating showed about 1.2 times higher wear resistance.
The first series (N = 1.75 kw) of the NiCrSiB-based coating
demonstrated higher relative wear resistance at straight impact angle
([alpha] = 90[degrees]). It may be explained by the high WC-Co particle
solution rate in the Ni-based metal matrix due to different parameters
of the laser remelting and the resulting lower brittleness of the
composite. Higher erosion resistance of the FeCrSiB-based coatings, in
comparison with the NiCrSiB-based coating at low impact angle ([alpha] =
30[degrees]), can be explained by the higher hardness (about 1.5 times)
and formation of a eutectic structure.
3.2.3. Abrasive impact wear
Impact wear resistance of the spray-remelted self-fluxing Ni-alloy
based coating is practically at the level of the reference
material--steel C45 (Table 8). Because of their low impact wear
resistance, The HVOF sprayed coatings, based on a Ni-based alloy, and
recycled hardmetal are not suitable for applications under impact wear
conditions [7,11,12]. Spray-fused Fe-based coatings may offer an
alternative for expensive powder steels and in some cases (under
restoration of the working elements of milling devices) for traditional
WC-Co hardmetals.
The absence of the correlation between the results of different
wear tests can be explained by different wear mechanisms, namely by
microcutting of the metal matrix in the case of abrasive wear,
microcutting of the metal matrix and direct fracture of hard particles
during abrasive erosive wear at low angles, microcutting or surface
fatigue of the metal matrix and direct fracture of hard particles at the
straight impact angle, surface fatigue of the metal matrix and direct
fracture of hard particles during abrasive impact wear.
4. CONCLUSIONS
Iron-based self-fluxing alloys are more suitable for producing
self-fluxing alloy-based composite coatings containing recycled WC-Co
hardmetal powder in comparison with the nickel-based ones. Iron-based
spray fusion coatings are of higher density and have a typical eutectic
sturcture. Due to the laser remelting, the initial WC-Co hardmetal
powder reinforcement is practically dissolved in the iron-based metal
matrix. Due to the higher hardness of iron-based coatings they are more
wear resistant by abrasion, but the iron-based coatings have lower
erosive wear resistance by the straight impact than the Ni-based
composite coatings. By abrasive impact wear, Fe-based coatings show
better performance.
doi: 10.3176/eng.2009.4.09
ACKNOWLEDGEMENTS
The authors are grateful to Petri Vuoristo from Tampere University
of Technology and to the workers of Reneko Ltd., especially to
Vyacheslav Kastyushin. This work was supported by the Estonian Ministry
of Education and Research (target-financed project SF 01400091).
Received 3 September 2009, in revised form 4 November 2009
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Andrei Surzhenkov (a), Priit Kulu (a), Riho Tarbe (a), Valdek Mikli
(b), Heikki Sarjas (a) and Jyrki Latokartano (c)
(a) Department of Materials Engineering, Tallinn University of
Technology, Ehitajate tee 5, 19086 Tallinn, Estonia;
andrei.surzenkovCcbttu.ee
(b) Centre for Materials Research, Tallinn University of
Technology, Ehitajate tee 5, 19086 Tallinn, Estonia
(c) Laser Application Laboratory, Tampere University of Technology,
Korkeakoulunkatu 10, FI-33720 Tampere, Finland
Table 1. Chemical composition and hardness of the substrate steel
Grade of the steel Composition, wt% Hardness HV1
Normalized Hardened
C45 0.45 C; 0.60 Mn; 0.30 Si 200-235 480-515
Table 2. Chemical composition and particle size of the used
self-fluxing spray powders
Composition, wt%
Type of Trade
the powder mark Cr Si B
NiCrSiB (S) 1640-02 * 7.5 3.5 1.6
NiCrSiB (H) 1660-02 * 14.8 4.3 3.1
FeCrSiB Grade 6A * 13.7 2.7 3.4
WC-Co Rec VK ** WC-75.6; Co-11.5
Composition, wt%
Type of Particle size,
the powder C Ni Fe [micro]m
NiCrSiB (S) 0.25 bal. 2.5 +15-53
NiCrSiB (H) 0.75 bal. 3.7 +15-53
FeCrSiB 2.1 6.0 bal. +10-45
WC-Co 12.9 +20-63
* Powders of Hoganas AB, Bruksgatan 35, SE-263 83 Hoganas, Sweden;
(S)-soft (380 HV), (H)-hard (780 HV).
** Experimental, TUT.
Table 3. Parameters of plasma spraying and laser remelting
Type of Power Gas flow Other
the coating rates, parameters
1/min
Plasma spraying -- Ar-135 Spray current--
[H.sub.2] 380 A, voltage
-90 --150 V, powder
feed rate--30
g/min, spray
distance--
100 mm
Laser remelting I series--1.75 kW for Ar-20 Scan speed--10
NiCrSiB compositions mm/s
II series--1.5 kW for
FeCrSiB and NiCrSiB
compositions
Table 4. Parameters of tribological tests
Type of the test Velocity, The abrasive and the Amount
m/s particle size, of the
mm abrasive,
kg
Abrasive block-on-ring
wear (ABRW) 2.4 Quartz sand 0.1-0.3 1.5
Abrasive erosive wear
(AEW) 80 Quartz sand 0.1-0.3 3
Abrasive impact wear
(AIW) 80 Granite gravel 4-5.6 6
Table 5. Hardness of spray-fused coatings on steel C45 ((WC-Co) of
fraction +20 -63 [micro]m)
Hardness HV, GPa
Composition of coatings, Thickness,
wt% [micro]m Surface (Metal matrix)/
Hv1 (hardmetal particles)
HV 0.1
NiCrSiB(S) + 25 (WC-Co) 200 3.0-3.6 2.7-4.3/5.8-18.8
FeNiCrSiB + 25 (WC-Co) 200 4.4-5.6 4.9-6.8/7.7-9.4
Table 6. Abrasive block-on-ring wear (ABRW) resistance of coatings
Type of the coating Condition and Wear coefficient K,
and metal matrix fraction, [mm.sup.3]/Nm x [10.sup.-5]
[micro]m
NiCrSiB(S) + (WC-Co) As-sprayed 36.1
Laser remelted
-63 11.1/8.7 *
+20-63 13.5
NiCrSiB(H) + (WC-Co) -63 8.4
FeCrSiB + (WC-Co) As-sprayed --
Laser remelted
-63 8.8
+20-63 7.1
Type of the coating Relative wear resistance
and metal matrix [[epsilon].sub.v]
NiCrSiB(S) + (WC-Co) 0.18
0.58/0.74 *
0.48
NiCrSiB(H) + (WC-Co) 0.76
FeCrSiB + (WC-Co) --
0.73
0.88
Table 7. Abrasive erosive wear (AEW) resistance of coatings at
different impact angles (hardmetal powder fraction +20 -63 [micro]m)
by impact angles of 30[degrees] and 90[degrees]
Type of the coating and Condition Wear rate [I.sub.v]
metal matrix [mm.sup.3]/kg
30[degrees] 90[degrees]
NiCrSiB(S) + (WC-Co) As-sprayed 447.6 --
Laser remelted 25.3/33.2 25.7/29.7*
NiCrSiB(H) + (WC-Co) Laser remelted 22.1 25.3
FeCrSiB + (WC-Co) Laser remelted 23.5 28.2
Type of the coating and Relative wear
metal matrix resistance
30[degrees] 90[degrees]
NiCrSiB(S) + (WC-Co) 0.1 --
0.7/0.8 1.6/0.7 *
NiCrSiB(H) + (WC-Co) 0.8 1.6
FeCrSiB + (WC-Co) 1.2 0.7
* I and II series.
Table 8. Abrasive impact wear (AIW) resistance of coatings
Hard phase Wear rate
Type of the coating fraction, [I.sub.v] Relative wear
[micro]m [mm.sup.3]/kg resistance
[[epsilon].sub.v]
NiCrSiB(S) + (WC-Co) -63 55.7 1.03
+20-63 54.9 1.04
FeCrSiB + (WC-Co) +20-63 43.7 1.31
