1-s2.0-S0301679X14000917-main.pdf

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).

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Tribology International

http://dx.doi.org/10.1016/j.triboint.2014.03.0040301-679X/& 2014 Elsevier Ltd. All rights reserved.

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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

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

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


Eutectic A


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.


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



Eutectic A



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.


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.


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


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


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 (%).


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


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.


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.


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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Ploughing

Sliding Direction

Fig. 10. SEM micrographs of worn surfaces (a) 0 wt% chromium carbide, (b) 10 wt% chromium carbide and (c) 20 wt% chromium carbide.

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