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Parametric study of abrasive wear of CoCrC based flame
sprayedcoatings by Response Surface Methodology
Satpal SharmaSchool of Engineering, Gautam Buddha University,
Greater Noida, Uttar Pradesh, India
a r t i c l e i n f o
Article history:Received 14 January 2014Accepted 4 March
2014Available online 12 March 2014
Keywords:CoatingAbrasive wearMicrohardnessResponse Surface
Methodology (RSM)
a b s t r a c t
Co base powder (EWAC1006 EE) was modified with the addition of
20%WC and the same was furthermodified by varying amounts of
chromium carbide (0, 10 and 20 wt%) in order to develop three
differentcoatings. Microstructure, elemental mapping XRD, porosity
and hardness analysis of the three coatingswas carried out. The
effect of CrC concentration (C), load (L), abrasive size (A),
sliding distance (S) andtemperature (T) on abrasive wear of these
flame sprayed coatings was investigated by Response
SurfaceMethodology and an abrasive wear model was developed. A
comparison of modeled and experimentalresults showed 59% error.
& 2014 Elsevier Ltd. All rights reserved.
1. Introduction
The progressive deterioration of metallic surfaces due tovarious
types of wear (abrasive, erosive, adhesive, corrosive andchemical
wear) in various industries (coal and hydro thermalpower plants,
cement, automotive, chemical and cement industry)leads to loss of
plant operating efficiency and frequent breakdownof the components
which in turn results in huge financial losses tothe industry. The
recognition of this fact has been the driving forcebehind the
continuing development of the surface modificationand surface
coating technologies known as surface engineering.The properties of
these surface layers may be different from thoseof the material as
dictated by service requirements.
The cobalt base alloys have found a wide variety of
tribologicalapplications for abrasive and adhesive wear resistance
in manyindustries such as aerospace, automotive, hydro and gas
turbinesand cement industry. Some studies [16] report the effect
ofprocessing techniques, carbide additions and their
distributionand post spray heat treatment on the hardness and
abrasive wearresistance of Co base coatings. The abrasive wear is
influenced by anumber of different factors such as the properties
of the materials(microstructure and hardness), the service
conditions (appliedload and abrasive grit size) and environment
(temperature andhumidity). High hardness and good resistance to
abrasion of cobaltbased coatings are generally attributed to the
presence of highvolume fraction of carbides. Increase in hardness
of these alloys
with the addition of WC and TiC has been reported [7,8]. Maitiet
al. [9] reported that with addition of WC upto 20% in
WCCoCrcoatings increases the hardness and abrasive wear rsistance
andfurther addition of WC increases hardness marginally. In
thepresent study, the Co base alloy was modified with WC andvarying
amount of CrC additions (0%, 10% and 20%) to increasethe hardness
and abrasive wear resistance of coatings.
In cement industry, various fans are used to transport
aluminaand silica particles of 550 m size along with hot gases
(tempera-ture 393423 K). These solid particles travel along the fan
bladesurface at a very low angle (o101). Abrasive wear has
beenreported to simulate the low angle solid particle erosion
conditions[1013]. Cement industry is trying many types of coating
materialsincluding cobalt base alloy. Therefore, in this work a
cobalt basealloy was selected for study and further developed for
improvedabrasion and erosion performance. It has also been found
from theliterature that most of the research on abrasive wear
behavior of Cobase alloys was carried out considering single
dimensional aspect ofapplied wear conditions such as abrasive grit
size and load only.Data generated using traditional method of
research using singlefactor effect is valuable and detailed, but
fails to indicate the effectof their interactions of various test
parameters on abrasive wear.Therefore, a number of statistical
methods have recently beenimplemented in wear studies. These
methods share the advantageof facilitating research into the
effects of different factors andtheir interactions (combined
effect), by limiting the number oftests. Hence in this study an
attempt has been made to studythe independent as well as combined
effect of the factorsusing fractional factorial design (Response
Surface Methodology).
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/triboint
Tribology International
http://dx.doi.org/10.1016/j.triboint.2014.03.0040301-679X/&
2014 Elsevier Ltd. All rights reserved.
E-mail address: [email protected]
Tribology International 75 (2014) 3950
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Based on the experimental data obtained an abrasive wear
modelwas developed to correlate the abrasive wear of the coatings
interms of applied factors and their interactions. The validity of
theabrasive wear model was evaluated under different abrasive
wearconditions by comparing the experimental and modeled
results.
2. Experimental procedure
2.1. Materials and methods
The carbon steel substrate was used for deposition of modifiedCo
base alloy coatings. The substrate was degreased and roughenedto an
average surface roughness of Ra 3.15 m (Rmax 18.2 m).Surface
roughness was measured by Mahr Perthometer (M2 409).The nominal
composition of substrate and commercially availableCo base powder
(EWAC 1006EE) is shown in Table 1. This powderwas modified by
adding 20 wt% WC. Further addition of 0, 10 and20 wt% CrC was
carried out to develop three different compositions((1006EE20 wt%
WC0 wt% CrC), (1006EE20 wt% WC10 wt%CrC) and (1006EE20 wt% WC20 wt%
CrC)). In following sectionsthese modified compositions are
designated by 0, 10 and 20 wt%CrC coatings respectively. These
compositions were deposited usingflame spraying process by Super
Jet spray torch (L & T India). Theflame spraying was carried
out using neutral flame of oxy-acetylenegas where the pressures of
oxygen and acetylene were maintainedat 0.3 MPa (3 kg f/cm2) and
0.12 MPa (1.2 kg f/cm2) respectively. Thesubstrate was preheated to
200710 1C. The spraying parametersare shown in Table 2.
2.2. Characterization of coatings
Coated samples were cut transversely for microstructural
charac-terization (SEM, SEM-LEO-435-VP, England), porosity and
hardness.The samples were polished using standard metallographic
procedureand etched with a chemical mixture of 3 parts HCl1 part
HNO3. SEMmicrographs were used to study microstructure and worn
surfaces.The porosity was measured by the point counting method
[1420].
The average of 25 areas of each coating has been used for
porositymeasurement. Vickers hardness of the coating was measured
using aload of 5 kg and average of six readings of the coating was
used forstudy purpose. Scanning electron microscopy of the worn
surfaces ofcoatings was also carried out to identify the material
removalmechanisms under abrasive wear conditions.
2.3. Factorial design of experiment
The vast amounts of data have been generated by the
traditionalapproach of experiment design in which one factor is
varied at atime (load and abrasive grit size). In this approach it
is difficult toevaluate the combined effects of applied factors.
This is the mainreason why load has always been considered first in
wear research,whilst other factors, e.g. abrasive grain size,
sliding distance andtheir combined effects (load and abrasive size,
load and speed,abrasive size and sliding distance), which may also
be important,have not been given the attention they deserve. The
advantage ofthe statistical method is obvious [12]. Thus RSM
(Response SurfaceMethodology) with fractional factorial design of
experiments withthree levels of each factor has been used in the
present study.According to Rabinowicz's classic theory [21] that
claims appliedload and hardness (depends upon composition) of
materials are themost important factors affecting the abrasion
process, therefore,both these factors were considered along with
the abrasive size andsliding distance in this study. Temperature is
also taken as fifthfactor in this study. Thus five factors
composition, load, abrasivesize, sliding distance and temperature
were used in the presentstudy. These factors were designated as C
(composition-% CrCconcentration), L (load-N), A (abrasive size-mm),
sliding distance(S) and temperature (T). The coded value of upper,
middle and lowerlevel of the three factors is designated by 1, 0
and 1 respectively.The actual and coded values (in parentheses) of
various factors usedin the present study are shown in Table 3. The
experimental designmatrix for different runs is shown in Table 4.
The relation betweenthe actual and coded value of a factor is shown
below:
Coded value Actual test conditionsMean test conditionsRange of
test conditions=2
2.4. Wear test
Wear behavior of flame sprayed coatings (0, 10 and 20 wt%
CrC)was studied using pin on disc type wear testing unit. Coated
wearpins of size 5535 mm3 were held against abrasive mediumunder
different runs. Water proof SiC abrasive papers were used
asabrasive medium. Abrasive paper was mounted on a steel disc(21020
mm2), which was rotated at 20074, 29675 and36875 rpm (revolution
per minute) corresponding to the slidingdistance of 25, 55 and 85
m. The slide carrying the wear pin wasmoved radially to get the
spiral motion under a constant incrementof 0.2 mm of the wear pin.
The abrasive wear pin and disc carryingthe abrasive paper was
enclosed in a heating chamber. Threethermocouples were used for
measuring the temperature of theheating chamber. The test
temperature was controlled with the
Table 1Chemical composition (wt%) of substrate and surfacing
powder.
C Cr W Si Fe Co Mn
Substrate 0.20.22 _ _ 0.40.6 Balance _ 0.40.81006EE Powder
3.03.5 2830 56 0.20.5 _ Balance 0.50.7
Table 2Flame spray parameters.
Parameters Value
Vertical distance of spray nozzle from substrate 18 mmSpraying
speed 120 mm/minInterior angle of spray nozzle with the horizontal
651
Table 3Various factors and their levels.
Factor Designation Lower level Middle level Upper level
Composition, (wt%) CrC C 0 (1) 10 (0) 20 (1)Load (N) L 5 (1) 15
(0) 25 (1)Abrasive size (mm) {grit size} A 2072a {500} (1) 60
74a{220} (0) 10075a {120} (1)Sliding distance (m) S 25 (1) 55 (0)
85 (1)Temperature (1C) T 50 (1) 100 (0) 150 (1)
a As given by manufacturer.
S. Sharma / Tribology International 75 (2014) 395040
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temperature controller unit (target temperature 75 1C). The
testerwas allowed to run idle for 2 min in order to attain the
constantrpm (without reciprocating motion); afterwards load was
appliedand simultaneously the reciprocating unit was switched on to
havea spiral motion of the wear pin. Wear tests were
conductedrandomly according to design matrix (Table 4) under
different runsand two replications under each run were taken and
average valueof abrasive wear has been reported in Table 4. An
electronic Mettlermicro balance (accuracy 0.0001 g) was used for
weighing thesamples after washing in acetone before and after
abrasive wear.Weight loss was used as a measure of abrasive wear
(g).
3. Results and discussion
3.1. Microstructure
The microstructures and EDAX analysis of 0 wt% chromiumcarbide,
10 wt% chromium carbide (not shown for brevity) and20 wt% chromium
carbide coatings are shown in Figs. 1 and 2(ad)respectively. The
microstructures were taken from the centerregion of the coatings.
All the three coatings mainly showedeutectic (A), W dominated
carbides (B) and Cr dominatedcarbides (C).
The eutectic A is found to be composed of Co, Ni, Fe and Crwith
small amount of W and C. EDAX analysis of eutectic showed30% Co,
24% Ni and 15% Fe (wt%) and other elements such as 8% Cr,6% W, 5% C
(wt%) (average of six readings in each case has beenreported) (Fig.
1b). The W dominated carbides B and Cr domi-nated carbides C are
present in the eutectic matrix A. These Wand Cr dominated carbide
particles primarily differ in terms ofrelative amounts of various
elements such as W, Cr and Co etc. TheEDAX analysis of W dominated
carbides showed 57% W, 10% Co,10% Cr and 10% Ni and 4% C (wt%)
(Fig. 1c). The Cr dominated
carbides C are rich in Cr and contain 52% Cr, 15% W, 13% Co, 7%
Cbesides small amounts of Ni and Fe (o5%) (wt%) as shown by theEDAX
analysis (Fig. 1d).
The microstructures and EDAX analysis of 10 wt% chromiumcarbide
(not shown for brevity) and 20 wt% chromium carbidecoatings are
shown in Fig. 2(ad). Both these chromium carbidemodified coatings
exhibited features similar to that of 0 wt%chromium carbide coating
except that compositions of eutecticand carbides were different.
The quantitative EDAX analysis showedthat the wt% of Co (E30 wt%)
is same in the eutectic matrix of allthe three coatings (0 wt%
chromium carbide, 10 wt% chromiumcarbide and 20 wt% chromium
carbide) and it is uniformly dis-tributed in the eutectic matrix as
shown in elemental maps (Fig. 3a-2, b-2 and c-2). These results are
in agreement with findings ofShetty et al. [22] as they reported
that the eutectic matrix is rich inCo containing various types of
carbides, which are uniformlydistributed in the matrix. The other
elements such as Ni, Fe andCr are also uniformly distributed in the
eutectic matrix (Fig. 3ac).However, wt% of Cr increased from 8 to
14 wt% with the addition ofchromium carbide. Some of the carbide
particles appear darker inSEM micrographs as can be seen in Figs. 1
and 2. This observation isalso in line with the findings of Shetty
et al. [22].
Image analyses of three coatings viz. 0 wt% chromium carbide,10
wt% chromium carbide and 20 wt% chromium carbide wascarried out to
determine the volume fraction of eutectic, Wdominated and Cr
dominated carbides (A, B and C respec-tively). The volume fraction
of eutectic A was found as 72.1%,65.7% and 46.1% respectively in 0%
chromium carbide, 10% chro-mium carbide and 20% chromium carbide
coatings. The volumefraction of W dominated carbides B was found as
13.8%, 17% and27.5% respectively, whereas the Cr dominated carbides
C wasobserved as 14.1%, 18.3% and 26.4% respectively in the
threecoatings (0 wt% chromium carbide, 10 wt% chromium carbideand
20 wt% chromium carbide).
Table 4Design matrix and various factors with their actual and
coded values (in parentheses).
Run no. Composition (C) Load (L) Abrasive size (A) Sliding
distance (S) Temperature (T) Av. wt. loss (g)
1 0 (1) 25 (1) 20 (1) 25 (1) 50 (1) 0.01792 20 (1) 25 (1) 20 (1)
85 (1) 50 (1) 0.02093 0 (1) 15 (0) 60 (0) 55 (0) 100 (0) 0.01464 10
(0) 25 (1) 60 (0) 55 (0) 100 (0) 0.02155 10 (0) 15 (0) 100 (1) 55
(0) 100 (0) 0.01726 0 (1) 25 (1) 100 (1) 85 (1) 50 (1) 0.1047 10
(0) 15 (0) 20 (1) 55 (0) 100 (0) 0.00618 10 (0) 5 (1) 60 (0) 55 (0)
100 (0) 0.00669 20 (1) 25 (1) 100 (1) 25 (1) 50 (1) 0.0151
10 10 (0) 15 (0) 60 (0) 55 (0) 150 (1) 0.014411 20 (1) 15 (0) 60
(0) 55 (0) 100 (0) 0.014712 10 (0) 15 (0) 60 (0) 55 (0) 100 (0)
0.016113 0 (1) 5 (1) 100 (1) 85 (1) 150 (1) 0.033814 20 (1) 25 (1)
20 (1) 25 (1) 150 (1) 0.005715 20 (1) 5 (1) 20 (1) 25 (1) 50 (1)
0.002816 0 (1) 25 (1) 100 (1) 25 (1) 150 (1) 0.020517 0 (1) 5 (1)
20 (1) 85 (1) 50 (1) 0.01218 0 (1) 5 (1) 20 (1) 25 (1) 150 (1)
0.005119 10 (0) 15 (0) 60 (0) 25 (1) 100 (0) 0.004820 0 (1) 25 (1)
20 (1) 85 (1) 150 (1) 0.047321 10 (0) 15 (0) 60 (0) 85 (1) 100 (0)
0.035622 20 (1) 5 (1) 20 (1) 85 (1) 150 (1) 0.009723 20 (1) 25 (1)
100 (1) 85 (1) 150 (1) 0.077824 20 (1) 5 (1) 100 (1) 85 (1) 50 (1)
0.023725 20 (1) 5 (1) 100 (1) 25 (1) 150 (1) 0.003926 10 (0) 15 (0)
60 (0) 55 (0) 50 (1) 0.019427 0 (1) 5 (1) 100 (1) 25 (1) 50 (1)
0.006628 10 (0) 15 (0) 60 (0) 55 (0) 100 (0) 0.015129 10 (0) 15 (0)
60 (0) 55 (0) 100 (0) 0.018830 10 (0) 15 (0) 60 (0) 55 (0) 100 (0)
0.0142
S. Sharma / Tribology International 75 (2014) 3950 41
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3.2. XRD analysis
XRD analysis of 0 wt% chromium carbide coating (Fig. 4)mainly
showed NiCrFeC, M23C6 (MNi, Cr, Co, Fe), CoWSi,Ni4W and Fe3C phases
in the coating. Cr23C6 as main carbides wasfound to be present in
the 10 wt% chromium carbide coating(Fig. 5) besides small amount of
Cr7C3, FeNi3 and Ni31Si12 werealso observed in 10 wt% chromium
carbide coating. Cr7C3 as themain carbides was found in the 20 wt%
chromium carbide coat-ing besides Co3W9C4, FeNi3 and Co7W6 phases
(Fig. 6). Thesefinding are in agreement with the published
literature [2327].With the addition 10 wt% and 20 wt% chromium
carbide, thecarbides types were changed from M23C6 to Cr23C6 and
Cr7C3 andsome intemetallic compounds (Co7W6 and Co3W9C4) were
alsoformed.
The various types of carbides (M23C6, Cr3C2 and Cr7C3) are
notpure phases but also contain Ni, Co, Cr and Fe as revealed by
theelemental mapping (Fig. 3c-1c-5) of the various coatings,
whereNi, Co, Cr and Fe are also present in these phases of
coatings. Asshown by marked circle area C in Fig. 3c-1, c-3 and
c-6, thisregion may correspond to chromium carbide (Cr7C3 as
detected byXRD analysis (Fig. 6)). This area C also contains Co, Ni
and Fe asshown in Fig. 3c-2, c-4 and c-5 respectively. Thus, it is
inferred thatthese carbides are not pure phases. These results are
in agreementwith the findings of Chorcia et al. [27].
3.3. Hardness and porosity
The Vickers hardness (Hv5) and porosity (%) of the three
coatingswith varying wt% of chromium carbide (0 wt% chromium
carbide,10 wt% chromium carbide and 20 wt% chromium carbide)
areshown in Fig. 7(a) and (b) respectively. Vickers hardness of
threecoatings was measured using a normal load of 5 kg and
averagevalue of six readings of hardness of the coating
cross-section hasbeen used for study. The average Vickers hardness
(Hv5) of threecoatings (0 wt% chromium carbide, 10 wt% chromium
carbide and20 wt% chromium carbide) was found to be 696786
Hv5,741795 Hv5 and 7867112 Hv5 respectively (Fig. 7a). The
averagehardness of 20 wt% chromium carbide coating was found
higher(786 Hv5) as compared to 0 wt% chromium carbide (696 Hv5)
and10 wt% chromium carbide (741 Hv5) coatings, however, there was
amore scatter in hardness of 20 wt% chromium carbide coating
ascompared to 0 wt% chromium carbide and 10% chromium
carbidecoatings may be due to higher porosity (Fig. 7b).
The higher hardness of 10 wt% chromium carbide coating
ascompared to 0 wt% chromium carbide is due to formation ofCr23C6
carbides and intemetallic compound Co7W6 as detectedby XRD analysis
(Fig. 5). The highest hardness of 20 wt% chromiumcarbide coating as
compared to other two (0 wt% chromiumcarbide and 10 wt% chromium
carbide) is mainly due to formationof Cr7C3 carbides as detected by
XRD analysis (Fig. 6). The formation
W dominated carbides B
Cr dominated Carbides C
Cr dominated Carbides C
Eutectic A
W dominated carbides B
Eutectic A
Fig. 1. Microstructure and EDAX analysis of 0 wt% chromium
carbide coating (a) microstructure of coating, (b) EDAX analysis of
eutectic, (c) EDAX analysis of W dominatedcarbide and (d) EDAX
analysis of Cr dominated carbide.
S. Sharma / Tribology International 75 (2014) 395042
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of Cr7C3 and Cr23C6 carbides increases the hardness of the
coatingowing to their high hardness. The hardness of Cr7C3 and
Cr23C6 is17.7 GPa and 9.9 GPa respectively as reported by Lebaili
et al. [28]. Ithas also been reported [24,25] that some of the
chromium may bereplaced by cobalt and/or tungsten with a matrix of
eutecticcontaining the other constituents of the alloy, thus
forming inter-metallic compounds. In this investigation also it has
been observedthat Co7W6 and Co3W9C4 intermetallic compounds were
formed asfound in the XRD analysis of 10 wt% chromium carbide and
20 wt%chromium carbide coating (Figs. 5 and 6). Otterloo et al.
[24,25]reported that the intermetallic compounds (Co7W6 and
Co3W9C4)also increase the hardness of Co-base alloys. Thus, the
higherhardness of 10 wt% chromium carbide and 20 wt%
chromiumcarbide coatings can also be attributed to formation of
theseintermetallic compounds as detected by XRD analysis (Figs. 5
and6). The porosity of all the three coatings was found to be 7.7%,
8.6%and 9.2% respectively (Fig. 7b).
3.4. Abrasive wear model
In the present work RSM was applied for developing
themathematical models in the form of multiple regression
equations
for the abrasive wear. In applying the RSM the dependent
variable(abrasive wear) is viewed as a surface to which the model
is fitted.Evaluation of the parametric effects on the response
(abrasivewear) was done by considering a second order
polynomialresponse surface mathematical model given by:
Wr b0 k
i 1bixi
k
i 1biix2i
k1
i 1k
j i1bijxixjr 1
This equation of abrasive wear (assumed surface) Wr
containslinear, squared and cross product terms of variable xi's
(C, L, A, Sand T). b0 is the mean response over all the test
conditions(intercept), bi is the slope or linear effect of the
input factor xi(the first-order model coefficients), bii the
quadratic coefficientsfor the variable i (linear by linear
interaction effect between theinput factor xi and xi) and bij is
the linear model coefficient for theinteraction between factor i
and j. The face centered compositedesign was used in this
experimental study. Significance testing ofthe coefficients,
adequacy of the model and analysis of variancewas carried out to
use Design Expert Software to find out thesignificant factors,
square terms and interactions affecting theresponse (abrasive and
erosive wear). R is the experimental error.
Eutectic A
W dominated carbides B
Cr dominated Carbides C
Eutectic A
W dominated carbides B
Cr dominated Carbides C
Fig. 2. Microstructure and EDAX analysis of 20 wt% chromium
carbide coating (a) microstructure of coating, (b) EDAX analysis of
eutectic, (c) EDAX analysis of W dominatedcarbide and (d) EDAX
analysis of Cr dominated carbide.
S. Sharma / Tribology International 75 (2014) 3950 43
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3 a-1 3 b-1 3 c-1
Area C
3 a-2 3 b-2
Area C
3 c-2
Co Co Co
3 a-3 3 b-3
Area C
3 c-3
Cr Cr Cr
3 a-4 3 b-4
Area C
3 c-4
Ni Ni Ni
3 a-5 3 b-5
Area C
3 c-5
Fe Fe Fe
3 a-6 3 b-6
Area C
3 c-6
C CC
Fig. 3. Elemental maps showing the distribution of Co, Cr, Ni,
Fe, and C in (a) 0 wt% chromium carbide, (b) 10 wt% chromium
carbide and (c) 20 wt% chromium carbidecoatings.
S. Sharma / Tribology International 75 (2014) 395044
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The analysis of variance (ANOVA) is shown in Table 5.
Theanalysis of variance (ANOVA) shows the significance of
variousfactors and their interactions at 95% confidence interval.
ANOVAshows the Model as Significant while the Lack of fit as
Notsignificant which are desirable from a model point of view.
Theprobability values o0.05 in the Prob.4F column indicates
thesignificant factors and interactions. The main factors and
their
interactions are included in the final abrasive wear model
whilethe insignificant interactions are excluded from the abrasive
wearmodel. Composition (C), load (L), abrasive size (A) and
slidingdistance (S) are the significant factors while
composition-load (CL),composition-temperature (CT), load-abrasive
size (LA), load-sliding distance (LS) and abrasive size-sliding
distance (AS) arethe significant interactions. The abrasive wear
model generated interms of coded and actual factor values (Eqs. (2)
and (3) respec-tively) is given below:
Wr 0:0154:86 103C0:013L9:73 103A0:016S 2:33 104T9:95 103S23:3
103CL4:27 103CT5:45 103LA8:13 103LS8:42 103AS7R 2
Wr 0:05380538:46 104C7:19 104L 3:47 104A1:52 103S9:02 105T1:11
105S23:3105CL8:55106CT1:36 105LA2:71105LS7:02 106AS7R 3
3.5. Validity of the abrasive wear model
The validity of the abrasive wear model was evaluated
byconducting abrasive wear tests on coatings at different values
ofthe experimental factors such as applied load (L), abrasive
sizes(A), sliding distance (S) and temperature (T). The actual and
codedvalue of various factors for confirmation tests are shown in
Table 6.The variations between the experimental and the calculated
valuesare of the order of 59%.
3.6. Effect of individual variables on wear rate
The effect of individual factors on abrasive wear is shown
inFig. 8(ae). The effect of composition (C), load (L), abrasive
size (A),sliding distance (S) and temperature (T) and that of their
interac-tions on abrasive wear are given in Eq. (2) which exhibits
theabrasive wear in terms of coded value and Eq. (3) in terms
ofactual values of factors and their interactions. However, the
effectsof individual factors are discussed by considering Eq. (2)
becauseall the factors are at the same level (1, 0 and 1). The
constant0.015 in Eq. (2) indicates the overall mean of the abrasive
wear ofcoatings under all the test conditions. This equation
furtherindicates that the coefficient (4.86103) associated
withcomposition (% CrC concentration) is negative, which signifies
adecrease of abrasive wear with an increase of CrC
concentration(Fig. 8a). This is attributed to the increase in
hardness of thecoating with increasing CrC concentration. Increase
in hardness ofmaterial lowers the depth of penetration of abrasive
particles,
40 50 60 70 80 90 100
60
80
100
120
140
160 1- Ni-Cr-Fe-C
1, 3,5, 6
2- M C
1, 2, 3 2
3- Ni W
4
4- CoWSi
4
6- Fe
7
56
5- Fe C 7- NiO
8- Cr O
58 78
Rel
ativ
e In
tens
ity
Diffraction angle 2
Fig. 4. XRD spectrum showing various phases in 0 wt% chromium
carbide coating.
40 50 60 70 80 90 100
40
60
80
100
120
140
160
180 5- FeNi1- Cr C 2- Cr C 3- Co W 4- WSi
1, 2, 3, 4,
1, 2, 531
4
11
1
Rel
ativ
e In
tens
ity
Diffraction angle 2
Fig. 5. XRD spectrum showing various phases in 10 wt% chromium
carbide coating.
40 50 60 70 80 90 100
60
80
100
120
140
160
180
200
220
1, 2, 3, 4
1- Cr C 2- Co W C 3- Ni Si 5- FeNi4- Fe C
3, 51
2 15
4
Rel
ativ
e In
tens
ity
Diffraction angle 2
Fig. 6. XRD spectrum showing various phases in 20 wt% chromium
carbide coating.
640660680700720740760780800
Vick
ers
hard
ness
(Hv5
)
Wt.% Chromium carbide
0123456789
10
0 wt.% 10 wt.% 20 wt.% 0 wt.% 10 wt.% 20 wt.%
Poro
sity
(%)
Wt.% Chromium carbide
69686
74195
786112 7.7
8.6 9.2
Fig. 7. Effect of chromium carbide addition in 100620 wt%WC
powder coating on (a) hardness (Hv5) and (b) porosity (%).
S. Sharma / Tribology International 75 (2014) 3950 45
-
therefore, results in shallow and finer wear grooves and
reducedvolume of material removed. The effect of load, abrasive
size,sliding distance and temperature on abrasive wear is shown
inFig. 8(be). The coefficient associated with load, abrasive
size,sliding distance and temperature are 0.013, 9.73103 0.016
and2.33104 respectively. This signifies that sliding distance has
amore detrimental effect than the applied load on the abrasivewear
of the coating. This is due to the fact that the load determinesthe
depth of penetration of abrasive in the material whereas thereis a
prolonged interaction of abrasives at higher sliding
distances.Thus, for the same load the abrasive wear increases with
theincrease in sliding distance as shown in Fig. 8d. The effect
ofabrasive size on the wear is less as compared to sliding
distanceand load. The abrasive wear increases with the increase in
abrasivesize (Fig. 8) as there is a greater tendency for large
penetration ofsharp abrasives with the increase of abrasive size,
attributed toincrease in actual contact area and hence the
effective load [12].This leads to deeper and wider grooves and
finally causes moresevere wear of the coating. The penetration of
the small sizeabrasives is limited to its height of projection in
the specimensurface. Thus the depth of penetration is reduced even
with theincrease in load on small abrasive sizes which results in
reducedwear of coatings. The reduction in abrasive wear at
highertemperature may be due to removal of some abrasive
particlesfrom the abrasive paper.
3.7. Interaction effect of the different variables
The coefficients associated with the interaction terms
CL(composition-load), CT (composition-temperature), LA
(load-abra-sive size), LS (load-sliding distance) and AS (abrasive
size-slidingdistance) in Eq. (2) are 3.3103, 4.27103,
5.45103,8.13103 and 8.42103 respectively showed the extent
ofinteraction (combined) effect of different factors on abrasive
wearof coatings. The effect of interactions among the different
factors
on abrasive wear is almost same order as of their
individualeffects. The combined effect of composition -load (CL) is
thelowest from all significant interactions.
The combined effect of various abrasive wear test parameterson
the wear behavior of coatings has been shown in the form ofresponse
surface plots (Fig. 9ae). The combined effect of
CL(composition-load) interaction can be explained by consideringEq.
(2) and Fig. 9(a). The ve sign associated with the coefficientof CL
interaction shows the reduction in wear of the coating. Fig.9(a)
shows that the abrasive wear increases with the increase inload due
to more penetration effect of abrasive in the coatingwhile the wear
reduces due to increase of CrC concentration from0 to 20 wt%. The
reduction in wear at high CrC concentration isdue to increase in
hardens of coating. The overall effect of CLinteraction is to
reduce the wear of the coating. The CT (composi-tion-temperature)
interaction can be explained on similar lines byconsidering Eq. (2)
and Fig. 9(b).
The combined effect of load and abrasive size (LA) on wear
ofcoatings shows that the wear of coatings increases with
anincrease in both the load and abrasive size. Moreover, the
effectof increase in load at high abrasive size is more predominant
thanat low abrasive size. Further, it can be observed from
responsesurface plot that the effect of increase in abrasive size
on wear ofcoatings is more at high loads than at low loads. This is
attributedto the fact that at high load and large abrasive size,
the depth ofpenetration of abrasive increases. This leads to more
abrasive wearat high load and high abrasive size and vice
versa.
The combined effect of load-sliding distance (LS) on wear
ofcoatings shows that the wear of coatings increases with an
increasein both the load and sliding distance. Moreover, the effect
of increasein sliding distance is more predominant than the
increase in load onabrasive wear. However, the effect of increase
in sliding distance ismore predominant in the entire range of
loading on abrasive wear ascompared to increase in load. Further,
it can be observed fromresponse surface plot that the effect of
increase in sliding distanceon wear of coatings is more at high
loads than at low loads.
Table 5Analysis of variance (ANOVA).
Source Sum squares Degrees of freedom Mean square F value
Prob.4F
Model 0.013 11 1.21103 48.45 o0.0001 SignificantCompositionC
4.25104 1 4.25104 17.10 0.0006LoadL 2.850103 1 2.850103 114.58
o0.0001Abrasive sizeA 1.703103 1 1.703103 68.48 o0.0001Sliding
distanceS 4.431103 1 4.431103 178.12 o0.0001TemperatureT 9.800107 1
9.800107 0.039 0.8449Interaction CL 1.742104 1 1.742104 7.00
0.0164Interaction CT 2.92104 1 2.92104 11.76 0.0030Interaction LA
4.752104 1 4.752104 19.11 0.0004Interaction LS 1.056103 1 1.056103
42.46 o0.0001Interaction AS 1.136103 1 1.136103 45.66
o0.0001Residual error 4.477104 18 2.487105Lack of fit 4.358104 15
2.906105 7.33 0.0632 Not significantPure error 1.189105 3
3.963106
Table 6Confirmations test results.
Composition,C (% CrC)
Load,L (N)
Abrasive size, A (lm) Sliding distance,S (m)
Temperature,T (1C)
Modeled abrasivewear (g)
Experimental abrasivewear (g)
Error(%)
0 (1) 15 (0) 4272 {320} (0.5) 70 (0.5) 100 (0.5) 0.0238 0.0250
4.810 (0) 15 (0) 4272 {320} (0.5) 70 (0.5) 100 (0.5) 0.019 0.0173
8.9520 (1) 15 (0) 4272 {320} (0.5) 70 (0.5) 100 (0.5) 0.0141 0.0152
7.24
S. Sharma / Tribology International 75 (2014) 395046
-
The combined effect of abrasive size and sliding distance (AS)on
wear of coatings shows that the wear of coatings increases withan
increase in both the sliding distance and abrasive size. Again
the effect of increase in sliding distance on abrasive wear is
morepredominant in the entire range of abrasive size. It can
beobserved from response surface plot that the effect of increase
in
0.00 5.00 10.00 15.00 20.00
0.0028
0.0281
0.0534
0.0787
0.104
Composition (C), wt.%CrC
Abr
asiv
e w
ear,
g
25.00 40.00 55.00 70.00 85.00
0.0028
0.0281
0.0534
0.0787
0.104
Sliding Distance (S), m
Abr
asiv
e w
ear,
g
20.00 40.00 60.00 80.00 100.00
0.0000
0.0153
0.0306
0.0459
0.0612
Abrasive size (A), m
Abr
asiv
e w
ear,
g
5.00 10.00 15.00 20.00 25.00
-0.0005
0.0150
0.0304
0.0458
0.0612
Load (L), N
Abr
asiv
e w
ear,
g
50.00 75.00 100.00 125.00 150.00
0.0028
0.0281
0.0534
0.0787
0.104
Temperature (T),C
Abr
asiv
e w
ear,
g
Fig. 8. Effects of individual factors such as (a) %
CrC-concentration, (b) load, (c) abrasive size (d) sliding distance
and (e) temperature on abrasive wear.
S. Sharma / Tribology International 75 (2014) 3950 47
-
sliding distance on wear of coatings is more at high abrasive
sizethan at low abrasive size. Thus high abrasive size and
highsliding distance results in severe wear of the coatings.
Same
effects of load, abrasive size and sliding distance were
observedin LS and AS interactions for abrasive wear of coatings
asdiscussed above.
0.0000
0.0153
0.0306
0.0459
0.0612
0.00
5.00
10.00
15.00
20.00
5.00
10.00
15.00
20.00
25.00
Abr
asiv
e w
ear,
g
Load (L), N
0.0000
0.0153
0.0306
0.0459
0.0612
0.00
5.00
10.00
15.00
20.00
50.00
75.00
100.00
125.00
150.00
Abr
asiv
e w
ear,
g
Temperature
(T),CCo
mpos
ition (
C),
wt.%C
rC
Comp
ositio
n (C),
wt.%C
rC
0.0000
0.0153
0.0306
0.0459
0.0612
5.00
10.00
15.00
20.00
25.00
20.00
40.00
60.00
80.00
100.00
Abr
asiv
e w
ear,
g
Abrasive size
(A), m
0.0000
0.0153
0.0306
0.0459
0.0612
5.00
10.00
15.00
20.00
25.00
25.00
40.00
55.00
70.00
85.00
Abr
asiv
e w
ear,
gSliding Distance
(S), m
0.0000
0.0153
0.0306
0.0459
0.0612
20.00
40.00
60.00
80.00
100.00
25.00
40.00
55.00
70.00
85.00
Abr
asiv
e w
ear,
g
Sliding Distance
(S), m
Load
(L), N
Load
(L), N
Abras
ive siz
e
(A), m
Fig. 9. Effects of interactions (a) composition-load, (b)
composition-temperature, (c) load-abrasive size, (d) load-sliding
distance and (e) abrasive size and sliding distance onabrasive
wear.
S. Sharma / Tribology International 75 (2014) 395048
-
3.8. SEM study of worn surfaces
In an attempt to identify the abrasive wear mechanism in 0%,
10%and 20% CrC coatings; SEM images of worn surfaces were
analyzed(Fig.10ab). The worn surfaces of various coatings (0, 10
and 20 wt%CrC) mainly showed the plowing and cutting mechanisms
(Fig. 10ab). The weight loss in each coating is determined by the
extent ofthese mechanisms. Plowing and cutting mechanism were
observedin the 0% CrC coating while cutting mechanisms were
observed in10% and 20% CrC coatings. The worn grooves are wider in
0% and 10%CrC coatings as compared to 20% CrC coating. The wider
grooves in0% CrC and 10% CrC coating were due to low hardness as
comparedto 20% CrC coating. Due to sharp abrasive particles the
width of thecutting/plowing grooves increases with the increase in
depth ofindentation and results in increase in wear rate of the
coatings.
The chromium carbide concentration increases the wear
resis-tance of the coatings. Experimental and confirmation test
resultsshowed that the weight loss in 20% CrC coating is lowest.
Theweight loss of 20% chromium carbide coating is 1.5 times lower
ascompared to 0% chromium carbide coating. This is attributed
tohigher hardness of the coating.
4. Conclusions
The following conclusions can be drawn from the
presentstudy:
1. The hardness increases with the increase in chromium
carbideconcentration. The maximum hardness was obtained with20 wt%
chromium carbide. The increase in hardness is due toformation of
new phases and inetrmetallic compounds.
2. Response Surface Methodology (RSM) with fractional
factorialdesign approach is an excellent tool, which can be
successfullyused to develop an empirical equation for the
prediction andunderstanding of wear behavior of coatings in terms
of indivi-dual factors (C, L, A, S and T) as well as in terms of
the combinedeffects (CL, CT, LA, LS and AS) of various factors.
3. The load and sliding distance have a more severe effect
onabrasive wear of the coating as compared to abrasive size.
4. Interaction effects of various factors on abrasive wear is
almostof same order less than their main factor effects. The
interac-tion effect of abrasive size-sliding distance (AS) is
considerablyhigher than load-abrasive size (LA). Increasing (%) CrC
concen-tration; reducing load, abrasive size and sliding distance
mini-mize the abrasive wear significantly.
5. Increase in chromium carbide concentration increases
theabrasive wear resistance of the coatings. Abrasive wear rate
of 20 wt% chromium carbide coating is lower as compared to0 wt%
chromium carbide coatings.
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study of abrasive wear of CoCrC based flame sprayed coatings by
Response Surface MethodologyIntroductionExperimental
procedureMaterials and methodsCharacterization of coatingsFactorial
design of experimentWear testResults and
discussionMicrostructureXRD analysisHardness and porosityAbrasive
wear modelValidity of the abrasive wear modelEffect of individual
variables on wear rateInteraction effect of the different
variablesSEM study of worn surfacesConclusionsReferences