Parametric study of abrasive wear of Co–CrC based flame sprayed coatings by Response Surface Methodology

Tribology International 75 (2014) 39–50

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Parametric study of abrasive wear of Co–CrC based flame sprayed coatings by Response Surface Methodology Satpal Sharma School of Engineering, Gautam Buddha University, Greater Noida, Uttar Pradesh, India

art ic l e i nf o

a b s t r a c t

Article history: Received 14 January 2014 Accepted 4 March 2014 Available online 12 March 2014

Co base powder (EWAC1006 EE) was modified with the addition of 20%WC and the same was further modified by varying amounts of chromium carbide (0, 10 and 20 wt%) in order to develop three different coatings. Microstructure, elemental mapping XRD, porosity and hardness analysis of the three coatings was carried out. The effect of CrC concentration (C), load (L), abrasive size (A), sliding distance (S) and temperature (T) on abrasive wear of these flame sprayed coatings was investigated by Response Surface Methodology and an abrasive wear model was developed. A comparison of modeled and experimental results showed 5–9% error. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Coating Abrasive wear Microhardness Response Surface Methodology (RSM)

1. Introduction The progressive deterioration of metallic surfaces due to various types of wear (abrasive, erosive, adhesive, corrosive and chemical wear) in various industries (coal and hydro thermal power plants, cement, automotive, chemical and cement industry) leads to loss of plant operating efficiency and frequent breakdown of the components which in turn results in huge financial losses to the industry. The recognition of this fact has been the driving force behind the continuing development of the surface modification and surface coating technologies known as surface engineering. The properties of these surface layers may be different from those of the material as dictated by service requirements. The cobalt base alloys have found a wide variety of tribological applications for abrasive and adhesive wear resistance in many industries such as aerospace, automotive, hydro and gas turbines and cement industry. Some studies [1–6] report the effect of processing techniques, carbide additions and their distribution and post spray heat treatment on the hardness and abrasive wear resistance of Co base coatings. The abrasive wear is influenced by a number of different factors such as the properties of the materials (microstructure and hardness), the service conditions (applied load and abrasive grit size) and environment (temperature and humidity). High hardness and good resistance to abrasion of cobalt based coatings are generally attributed to the presence of high volume fraction of carbides. Increase in hardness of these alloys

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with the addition of WC and TiC has been reported [7,8]. Maiti et al. [9] reported that with addition of WC upto 20% in WC–Co–Cr coatings increases the hardness and abrasive wear résistance and further addition of WC increases hardness marginally. In the present study, the Co base alloy was modified with WC and varying amount of CrC additions (0%, 10% and 20%) to increase the hardness and abrasive wear resistance of coatings. In cement industry, various fans are used to transport alumina and silica particles of 5–50 μm size along with hot gases (temperature 393–423 K). These solid particles travel along the fan blade surface at a very low angle (o101). Abrasive wear has been reported to simulate the low angle solid particle erosion conditions [10–13]. Cement industry is trying many types of coating materials including cobalt base alloy. Therefore, in this work a cobalt base alloy was selected for study and further developed for improved abrasion and erosion performance. It has also been found from the literature that most of the research on abrasive wear behavior of Co base alloys was carried out considering single dimensional aspect of applied wear conditions such as abrasive grit size and load only. Data generated using traditional method of research using single factor effect is valuable and detailed, but fails to indicate the effect of their interactions of various test parameters on abrasive wear. Therefore, a number of statistical methods have recently been implemented in wear studies. These methods share the advantage of facilitating research into the effects of different factors and their interactions (combined effect), by limiting the number of tests. Hence in this study an attempt has been made to study the independent as well as combined effect of the factors using fractional factorial design (Response Surface Methodology).

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S. Sharma / Tribology International 75 (2014) 39–50

Based on the experimental data obtained an abrasive wear model was developed to correlate the abrasive wear of the coatings in terms of applied factors and their interactions. The validity of the abrasive wear model was evaluated under different abrasive wear conditions by comparing the experimental and modeled results.

The average of 25 areas of each coating has been used for porosity measurement. Vickers hardness of the coating was measured using a load of 5 kg and average of six readings of the coating was used for study purpose. Scanning electron microscopy of the worn surfaces of coatings was also carried out to identify the material removal mechanisms under abrasive wear conditions.

2. Experimental procedure

2.3. Factorial design of experiment

2.1. Materials and methods

The vast amounts of data have been generated by the traditional approach of experiment design in which one factor is varied at a time (load and abrasive grit size). In this approach it is difficult to evaluate the combined effects of applied factors. This is the main reason why load has always been considered first in wear research, whilst other factors, e.g. abrasive grain size, sliding distance and their 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 of the statistical method is obvious [12]. Thus RSM (Response Surface Methodology) with fractional factorial design of experiments with three levels of each factor has been used in the present study. According to Rabinowicz's classic theory [21] that claims applied load and hardness (depends upon composition) of materials are the most important factors affecting the abrasion process, therefore, both these factors were considered along with the abrasive size and sliding distance in this study. Temperature is also taken as fifth factor in this study. Thus five factors composition, load, abrasive size, sliding distance and temperature were used in the present study. These factors were designated as C (composition-% CrC concentration), L (load-N), A (abrasive size-mm), sliding distance (S) and temperature (T). The coded value of upper, middle and lower level of the three factors is designated by þ 1, 0 and  1 respectively. The actual and coded values (in parentheses) of various factors used in the present study are shown in Table 3. The experimental design matrix for different runs is shown in Table 4. The relation between the actual and coded value of a factor is shown below:

The carbon steel substrate was used for deposition of modified Co base alloy coatings. The substrate was degreased and roughened to an average surface roughness of Ra 3.15 μm (Rmax 18.2 μm). Surface roughness was measured by Mahr Perthometer (M2 409). The nominal composition of substrate and commercially available Co base powder (EWAC 1006EE) is shown in Table 1. This powder was modified by adding 20 wt% WC. Further addition of 0, 10 and 20 wt% CrC was carried out to develop three different compositions ((1006EEþ 20 wt% WCþ0 wt% CrC), (1006EEþ20 wt% WCþ10 wt% CrC) and (1006EEþ20 wt% WCþ20 wt% CrC)). In following sections these modified compositions are designated by 0, 10 and 20 wt% CrC coatings respectively. These compositions were deposited using flame spraying process by Super Jet spray torch (L & T India). The flame spraying was carried out using neutral flame of oxy-acetylene gas where the pressures of oxygen and acetylene were maintained at 0.3 MPa (3 kg f/cm2) and 0.12 MPa (1.2 kg f/cm2) respectively. The substrate was preheated to 200710 1C. The spraying parameters are shown in Table 2. 2.2. Characterization of coatings Coated samples were cut transversely for microstructural characterization (SEM, SEM-LEO-435-VP, England), porosity and hardness. The samples were polished using standard metallographic procedure and etched with a chemical mixture of 3 parts HClþ1 part HNO3. SEM micrographs were used to study microstructure and worn surfaces. The porosity was measured by the point counting method [14–20]. Table 1 Chemical composition (wt%) of substrate and surfacing powder. C Substrate 0.2–0.22 1006EE Powder 3.0–3.5

Cr

W

Si

Fe

_ _ 0.4–0.6 Balance 28–30 5–6 0.2–0.5 _

Co

Coded value ¼

Actual test conditions Mean test conditions Range of test conditions=2

2.4. Wear test Mn

_ 0.4–0.8 Balance 0.5–0.7

Table 2 Flame spray parameters. Parameters

Value

Vertical distance of spray nozzle from substrate Spraying speed Interior angle of spray nozzle with the horizontal

18 mm 120 mm/min 651

Wear behavior of flame sprayed coatings (0, 10 and 20 wt% CrC) was studied using pin on disc type wear testing unit. Coated wear pins of size 5  5  35 mm3 were held against abrasive medium under different runs. Water proof SiC abrasive papers were used as abrasive medium. Abrasive paper was mounted on a steel disc (210  20 mm2), which was rotated at 20074, 29675 and 36875 rpm (revolution per minute) corresponding to the sliding distance of 25, 55 and 85 m. The slide carrying the wear pin was moved radially to get the spiral motion under a constant increment of 0.2 mm of the wear pin. The abrasive wear pin and disc carrying the abrasive paper was enclosed in a heating chamber. Three thermocouples were used for measuring the temperature of the heating chamber. The test temperature was controlled with the

Table 3 Various factors and their levels. Factor

Designation

Lower level

Middle level

Upper level

Composition, (wt%) CrC Load (N) Abrasive size (mm) {grit size} Sliding distance (m) Temperature (1C)

C L A S T

0 (  1) 5 (  1) 20 72a {500} (  1) 25 (  1) 50 (  1)

10 (0) 15 (0) 60 74a{220} (0) 55 (0) 100 (0)

20 (þ 1) 25 (þ 1) 1007 5a {120} ( þ 1) 85 (þ 1) 150 ( þ 1)

a

As given by manufacturer.

S. Sharma / Tribology International 75 (2014) 39–50

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Table 4 Design matrix and various factors with their actual and coded values (in parentheses). Run no.

Composition (C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

0 20 0 10 10 0 10 10 20 10 20 10 0 20 20 0 0 0 10 0 10 20 20 20 20 10 0 10 10 10

(  1) ( þ1) (  1) (0) (0) (  1) (0) (0) ( þ1) (0) ( þ1) (0) (  1) ( þ1) ( þ1) (  1) (  1) (  1) (0) (  1) (0) ( þ1) ( þ1) ( þ1) ( þ1) (0) (  1) (0) (0) (0)

Load (L) 25 25 15 25 15 25 15 5 25 15 15 15 5 25 5 25 5 5 15 25 15 5 25 5 5 15 5 15 15 15

( þ1) ( þ1) (0) ( þ1) (0) ( þ1) (0) (  1) ( þ1) (0) (0) (0) (  1) ( þ1) (  1) ( þ1) (  1) (  1) (0) ( þ1) (0) (  1) ( þ1) (  1) (  1) (0) (  1) (0) (0) (0)

Abrasive size (A) 20 20 60 60 100 100 20 60 100 60 60 60 100 20 20 100 20 20 60 20 60 20 100 100 100 60 100 60 60 60

(  1) (  1) (0) (0) ( þ 1) ( þ 1) (  1) (0) ( þ 1) (0) (0) (0) ( þ 1) (  1) (  1) ( þ 1) (  1) (  1) (0) (  1) (0) (  1) ( þ 1) ( þ 1) ( þ 1) (0) ( þ 1) (0) (0) (0)

temperature controller unit (target temperature 75 1C). The tester was allowed to run idle for 2 min in order to attain the constant rpm (without reciprocating motion); afterwards load was applied and simultaneously the reciprocating unit was switched on to have a spiral motion of the wear pin. Wear tests were conducted randomly according to design matrix (Table 4) under different runs and two replications under each run were taken and average value of abrasive wear has been reported in Table 4. An electronic Mettler micro balance (accuracy 0.0001 g) was used for weighing the samples 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% chromium carbide, 10 wt% chromium carbide (not shown for brevity) and 20 wt% chromium carbide coatings are shown in Figs. 1 and 2(a–d) respectively. The microstructures were taken from the center region of the coatings. All the three coatings mainly showed eutectic (“A”), W dominated carbides (“B”) and Cr dominated carbides (“C”). The eutectic “A” is found to be composed of Co, Ni, Fe and Cr with small amount of W and C. EDAX analysis of eutectic showed 30% 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 been reported) (Fig. 1b). The W dominated carbides “B” and Cr dominated carbides “C” are present in the eutectic matrix “A”. These W and Cr dominated carbide particles primarily differ in terms of relative amounts of various elements such as W, Cr and Co etc. The EDAX analysis of W dominated carbides showed 57% W,  10% Co, 10% Cr and 10% Ni and 4% C (wt%) (Fig. 1c). The Cr dominated

Sliding distance (S) 25 85 55 55 55 85 55 55 25 55 55 55 85 25 25 25 85 25 25 85 85 85 85 85 25 55 25 55 55 55

(  1) ( þ1) (0) (0) (0) ( þ1) (0) (0) (  1) (0) (0) (0) ( þ1) (  1) (  1) (  1) ( þ1) (  1) (  1) ( þ1) ( þ1) ( þ1) ( þ1) ( þ1) (  1) (0) (  1) (0) (0) (0)

Temperature (T) 50 50 100 100 100 50 100 100 50 150 100 100 150 150 50 150 50 150 100 150 100 150 150 50 150 50 50 100 100 100

(  1) (  1) (0) (0) (0) (  1) (0) (0) (  1) ( þ1) (0) (0) ( þ1) ( þ1) (  1) ( þ1) (  1) ( þ1) (0) ( þ1) (0) ( þ1) ( þ1) (  1) ( þ1) (  1) (  1) (0) (0) (0)

Av. wt. loss (g) 0.0179 0.0209 0.0146 0.0215 0.0172 0.104 0.0061 0.0066 0.0151 0.0144 0.0147 0.0161 0.0338 0.0057 0.0028 0.0205 0.012 0.0051 0.0048 0.0473 0.0356 0.0097 0.0778 0.0237 0.0039 0.0194 0.0066 0.0151 0.0188 0.0142

carbides “C” are rich in Cr and contain 52% Cr, 15% W, 13% Co, 7% C besides small amounts of Ni and Fe ( o5%) (wt%) as shown by the EDAX analysis (Fig. 1d). The microstructures and EDAX analysis of 10 wt% chromium carbide (not shown for brevity) and 20 wt% chromium carbide coatings are shown in Fig. 2(a–d). Both these chromium carbide modified coatings exhibited features similar to that of 0 wt% chromium carbide coating except that compositions of eutectic and carbides were different. The quantitative EDAX analysis showed that the wt% of Co (E30 wt%) is same in the eutectic matrix of all the three coatings (0 wt% chromium carbide, 10 wt% chromium carbide and 20 wt% chromium carbide) and it is uniformly distributed in the eutectic matrix as shown in elemental maps (Fig. 3a2, b-2 and c-2). These results are in agreement with findings of Shetty et al. [22] as they reported that the eutectic matrix is rich in Co containing various types of carbides, which are uniformly distributed in the matrix. The other elements such as Ni, Fe and Cr are also uniformly distributed in the eutectic matrix (Fig. 3a–c). However, wt% of Cr increased from 8 to 14 wt% with the addition of chromium carbide. Some of the carbide particles appear darker in SEM micrographs as can be seen in Figs. 1 and 2. This observation is also 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 was carried out to determine the volume fraction of eutectic, W dominated and Cr dominated carbides (“A”, “B” and “C” respectively). The volume fraction of eutectic “A” was found as 72.1%, 65.7% and 46.1% respectively in 0% chromium carbide, 10% chromium carbide and 20% chromium carbide coatings. The volume fraction of W dominated carbides “B” was found as 13.8%, 17% and 27.5% respectively, whereas the Cr dominated carbides “C” was observed as 14.1%, 18.3% and 26.4% respectively in the three coatings (0 wt% chromium carbide, 10 wt% chromium carbide and 20 wt% chromium carbide).

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S. Sharma / Tribology International 75 (2014) 39–50

Cr dominated Carbides “C”

W dominated carbides “B”

Eutectic “A”

Eutectic “A”

W dominated carbides “B”

Cr dominated Carbides “C”

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 dominated carbide and (d) EDAX analysis of Cr dominated carbide.

3.2. XRD analysis

3.3. Hardness and porosity

XRD analysis of 0 wt% chromium carbide coating (Fig. 4) mainly showed Ni–Cr–Fe–C, M23C6 (M ¼Ni, Cr, Co, Fe), CoWSi, Ni4W and Fe3C phases in the coating. Cr23C6 as main carbides was found to be present in the 10 wt% chromium carbide coating (Fig. 5) besides small amount of Cr7C3, FeNi3 and Ni31Si12 were also observed in 10 wt% chromium carbide coating. Cr7C3 as the main carbides was found in the 20 wt% chromium carbide coating besides Co3W9C4, FeNi3 and Co7W6 phases (Fig. 6). These finding are in agreement with the published literature [23–27]. With the addition 10 wt% and 20 wt% chromium carbide, the carbides types were changed from M23C6 to Cr23C6 and Cr7C3 and some intemetallic compounds (Co7W6 and Co3W9C4) were also formed. The various types of carbides (M23C6, Cr3C2 and Cr7C3) are not pure phases but also contain Ni, Co, Cr and Fe as revealed by the elemental mapping (Fig. 3c-1–c-5) of the various coatings, where Ni, Co, Cr and Fe are also present in these phases of coatings. As shown by marked circle area “C” in Fig. 3c-1, c-3 and c-6, this region may correspond to chromium carbide (Cr7C3 as detected by XRD analysis (Fig. 6)). This area “C” also contains Co, Ni and Fe as shown in Fig. 3c-2, c-4 and c-5 respectively. Thus, it is inferred that these carbides are not pure phases. These results are in agreement with the findings of Chorcia et al. [27].

The Vickers hardness (Hv5) and porosity (%) of the three coatings with varying wt% of chromium carbide (0 wt% chromium carbide, 10 wt% chromium carbide and 20 wt% chromium carbide) are shown in Fig. 7(a) and (b) respectively. Vickers hardness of three coatings was measured using a normal load of 5 kg and average value of six readings of hardness of the coating cross-section has been used for study. The average Vickers hardness (Hv5) of three coatings (0 wt% chromium carbide, 10 wt% chromium carbide and 20 wt% chromium carbide) was found to be 696786 Hv5, 741795 Hv5 and 7867112 Hv5 respectively (Fig. 7a). The average hardness of 20 wt% chromium carbide coating was found higher (786 Hv5) as compared to 0 wt% chromium carbide (696 Hv5) and 10 wt% chromium carbide (741 Hv5) coatings, however, there was a more scatter in hardness of 20 wt% chromium carbide coating as compared to 0 wt% chromium carbide and 10% chromium carbide coatings may be due to higher porosity (Fig. 7b). The higher hardness of 10 wt% chromium carbide coating as compared to 0 wt% chromium carbide is due to formation of Cr23C6 carbides and intemetallic compound Co7W6 as detected by XRD analysis (Fig. 5). The highest hardness of 20 wt% chromium carbide coating as compared to other two (0 wt% chromium carbide and 10 wt% chromium carbide) is mainly due to formation of Cr7C3 carbides as detected by XRD analysis (Fig. 6). The formation

S. Sharma / Tribology International 75 (2014) 39–50

43

W dominated carbides “B” Eutectic “A”

Eutectic “A”

Cr dominated Carbides “C”

W dominated

Cr dominated

carbides “B”

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 dominated carbide and (d) EDAX analysis of Cr dominated carbide.

of Cr7C3 and Cr23C6 carbides increases the hardness of the coating owing to their high hardness. The hardness of Cr7C3 and Cr23C6 is 17.7 GPa and 9.9 GPa respectively as reported by Lebaili et al. [28]. It has also been reported [24,25] that some of the chromium may be replaced by cobalt and/or tungsten with a matrix of eutectic containing the other constituents of the alloy, thus forming intermetallic compounds. In this investigation also it has been observed that Co7W6 and Co3W9C4 intermetallic compounds were formed as found 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 higher hardness of 10 wt% chromium carbide and 20 wt% chromium carbide coatings can also be attributed to formation of these intermetallic compounds as detected by XRD analysis (Figs. 5 and 6). 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 the mathematical 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 (abrasive wear) was done by considering a second order polynomial response surface mathematical model given by: k

k

i¼1

i¼1

k1

Wr ¼ b0 þ ∑ bi xi þ ∑ bii x2i þ ∑

k

∑ bij xi xj þ εr

i ¼ 1 j ¼ iþ1

ð1Þ

This equation of abrasive wear (assumed surface) Wr contains linear, squared and cross product terms of variable xi's (C, L, A, S and 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 coefficients for the variable i (linear by linear interaction effect between the input factor xi and xi) and bij is the linear model coefficient for the interaction between factor i and j. The face centered composite design was used in this experimental study. Significance testing of the coefficients, adequacy of the model and analysis of variance was carried out to use Design Expert Software to find out the significant factors, square terms and interactions affecting the response (abrasive and erosive wear). εR is the experimental error.

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S. Sharma / Tribology International 75 (2014) 39–50

3 a-1

3 b-1

3 c-1 Area “C”

3 a-2

3 c-2

3 b-2 Area “C”

Co

3 a-3

Co

Co

3 b-3

3 c-3 Area “C”

Cr

3 a-4

Cr

Cr

3 b-4

3 c-4 Area “C”

Ni

3 a-5

Ni

Ni

3 c-5

3 b-5 Area “C”

Fe

3 a-6

Fe

Fe

3 b-6

3 c-6 Area “C”

C

C

C

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 carbide coatings.

S. Sharma / Tribology International 75 (2014) 39–50

The analysis of variance (ANOVA) is shown in Table 5. The analysis of variance (ANOVA) shows the significance of various factors and their interactions at 95% confidence interval. ANOVA shows the “Model” as “Significant” while the “Lack of fit” as “Not significant” which are desirable from a model point of view. The probability values o0.05 in the “Prob.4 F” column indicates the significant factors and interactions. The main factors and their

160

Relative Intensity

140

1- Ni-Cr-Fe-C 3- Ni W

5- Fe C

7- NiO

2- M C

6- Fe

8- Cr O

4- CoWSi

interactions are included in the final abrasive wear model while the insignificant interactions are excluded from the abrasive wear model. Composition (C), load (L), abrasive size (A) and sliding distance (S) are the significant factors while composition-load (CL), composition-temperature (CT), load-abrasive size (LA), loadsliding distance (LS) and abrasive size-sliding distance (AS) are the significant interactions. The abrasive wear model generated in terms of coded and actual factor values (Eqs. (2) and (3) respectively) is given below: Wr ¼ 0:015–4:86  10–3 C þ 0:013L þ 9:73  10–3 A þ0:016S– 2:33  10–4 T þ 9:95  10–3 S2 –3:3  10–3 CL þ4:27  10–3 CT þ 5:45  10–3 LA þ 8:13  10–3 LS

1, 3,5, 6

120

1, 2, 3 8 4

100

45

8

4

5

2

5 6

þ8:42  10–3 AS 7 εR

7

7

ð2Þ

Wr ¼ 0:053–8053–8:46  10–4 C–7:19  10–4 L– 3:47  10–4 A

80

–1:52  10–3 S–9:02  10–5 T þ 1:11  10–5 S2 –3:3

60 40

50

60

70

80

90

10–5 CL þ8:5510–6 CT þ 1:36  10–5 LA þ 2:71

100

10–5 LS þ 7:02  10–6 AS 7 εR

Diffraction angle “2θ”

ð3Þ

Fig. 4. XRD spectrum showing various phases in 0 wt% chromium carbide coating.

3.5. Validity of the abrasive wear model 180

1- Cr C

2- Cr C

3- Co W

4- WSi

The validity of the abrasive wear model was evaluated by conducting abrasive wear tests on coatings at different values of the experimental factors such as applied load (L), abrasive sizes (A), sliding distance (S) and temperature (T). The actual and coded value of various factors for confirmation tests are shown in Table 6. The variations between the experimental and the calculated values are of the order of 5–9%.

5- FeNi

Relative Intensity

160 1

140

1, 2, 3, 4,

1 1

120 1, 2, 5

100

4 1

3

80 60 40 40

50

60

70

80

90

100

Diffraction angle “2θ”

Fig. 5. XRD spectrum showing various phases in 10 wt% chromium carbide coating.

220

1- Cr C

2- Co W C

3- Ni Si

4- Fe C

5- FeNi

Relative Intensity

200 180

1, 2, 3, 4

160 140 120

3, 5

4

2

1

5

1

100 80 60 40

50

60

70

80

90

100

Diffraction angle “2θ”

800 780 760 740 720 700 680 660 640

The effect of individual factors on abrasive wear is shown in Fig. 8(a–e). The effect of composition (C), load (L), abrasive size (A), sliding distance (S) and temperature (T) and that of their interactions on abrasive wear are given in Eq. (2) which exhibits the abrasive wear in terms of coded value and Eq. (3) in terms of actual values of factors and their interactions. However, the effects of individual factors are discussed by considering Eq. (2) because all the factors are at the same level (þ 1, 0 and  1). The constant 0.015 in Eq. (2) indicates the overall mean of the abrasive wear of coatings under all the test conditions. This equation further indicates that the coefficient (  4.86  10  3) associated with composition (% CrC concentration) is negative, which signifies a decrease of abrasive wear with an increase of CrC concentration (Fig. 8a). This is attributed to the increase in hardness of the coating with increasing CrC concentration. Increase in hardness of material lowers the depth of penetration of abrasive particles,

786±112 741±95

Porosity (%)

Vickers hardness (Hv5)

Fig. 6. XRD spectrum showing various phases in 20 wt% chromium carbide coating.

3.6. Effect of individual variables on wear rate

696±86

0 wt.%

10 wt.%

20 wt.%

Wt.% Chromium carbide

10 9 8 7 6 5 4 3 2 1 0

8.6

9.2

7.7

0 wt.%

10 wt.%

20 wt.%

Wt.% Chromium carbide

Fig. 7. Effect of chromium carbide addition in 1006–20 wt%WC powder coating on (a) hardness (Hv5) and (b) porosity (%).

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Table 5 Analysis of variance (ANOVA). Source

Sum squares

Degrees of freedom

Mean square

F value

Prob.4F

Model Composition—C Load—L Abrasive size—A Sliding distance—S Temperature—T Interaction CL Interaction CT Interaction LA Interaction LS Interaction AS Residual error Lack of fit Pure error

0.013 4.25  10  4 2.850  10  3 1.703  10  3 4.431  10  3 9.800  10  7 1.742  10  4 2.92  10  4 4.752  10  4 1.056  10  3 1.136  10  3 4.477  10  4 4.358  10  4 1.189  10  5

11 1 1 1 1 1 1 1 1 1 1 18 15 3

1.21  10  3 4.25  10  4 2.850  10  3 1.703  10  3 4.431  10  3 9.800  10  7 1.742  10  4 2.92  10  4 4.752  10  4 1.056  10  3 1.136  10  3 2.487  10  5 2.906  10  5 3.963  10  6

48.45 17.10 114.58 68.48 178.12 0.039 7.00 11.76 19.11 42.46 45.66

o 0.0001 0.0006 o 0.0001 o 0.0001 o 0.0001 0.8449 0.0164 0.0030 0.0004 o 0.0001 o 0.0001

Significant

0.0632

Not significant

7.33

Table 6 Confirmations test results. Composition, C (% CrC)

Load, L (N)

Abrasive size, A (lm)

Sliding distance, S (m)

Temperature, T (1C)

Modeled abrasive wear (g)

Experimental abrasive wear (g)

Error (%)

0 (  1) 10 (0) 20 ( þ1)

15 (0) 15 (0) 15 (0)

42 72 {320} (  0.5) 42 72 {320} (  0.5) 42 72 {320} (  0.5)

70 ( þ 0.5) 70 ( þ 0.5) 70 ( þ 0.5)

100 ( þ 0.5) 100 ( þ 0.5) 100 ( þ 0.5)

0.0238 0.019 0.0141

0.0250 0.0173 0.0152

4.8 8.95 7.24

therefore, results in shallow and finer wear grooves and reduced volume of material removed. The effect of load, abrasive size, sliding distance and temperature on abrasive wear is shown in Fig. 8(b–e). The coefficient associated with load, abrasive size, sliding distance and temperature are 0.013, 9.73  10  3 0.016 and 2.33  10  4 respectively. This signifies that sliding distance has a more detrimental effect than the applied load on the abrasive wear of the coating. This is due to the fact that the load determines the depth of penetration of abrasive in the material whereas there is a prolonged interaction of abrasives at higher sliding distances. Thus, for the same load the abrasive wear increases with the increase in sliding distance as shown in Fig. 8d. The effect of abrasive size on the wear is less as compared to sliding distance and load. The abrasive wear increases with the increase in abrasive size (Fig. 8) as there is a greater tendency for large penetration of sharp abrasives with the increase of abrasive size, attributed to increase in actual contact area and hence the effective load [12]. This leads to deeper and wider grooves and finally causes more severe wear of the coating. The penetration of the small size abrasives is limited to its height of projection in the specimen surface. Thus the depth of penetration is reduced even with the increase in load on small abrasive sizes which results in reduced wear of coatings. The reduction in abrasive wear at higher temperature may be due to removal of some abrasive particles from 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-abrasive size), LS (load-sliding distance) and AS (abrasive size-sliding distance) in Eq. (2) are  3.3  10  3, 4.27  10  3, 5.45  10  3, 8.13  10  3 and 8.42  10  3 respectively showed the extent of interaction (combined) effect of different factors on abrasive wear of coatings. The effect of interactions among the different factors

on abrasive wear is almost same order as of their individual effects. The combined effect of composition -load (CL) is the lowest from all significant interactions. The combined effect of various abrasive wear test parameters on the wear behavior of coatings has been shown in the form of response surface plots (Fig. 9a–e). The combined effect of CL (composition-load) interaction can be explained by considering Eq. (2) and Fig. 9(a). The  ve sign associated with the coefficient of CL interaction shows the reduction in wear of the coating. Fig.9 (a) shows that the abrasive wear increases with the increase in load due to more penetration effect of abrasive in the coating while the wear reduces due to increase of CrC concentration from 0 to 20 wt%. The reduction in wear at high CrC concentration is due to increase in hardens of coating. The overall effect of CL interaction is to reduce the wear of the coating. The CT (composition-temperature) interaction can be explained on similar lines by considering Eq. (2) and Fig. 9(b). The combined effect of load and abrasive size (LA) on wear of coatings shows that the wear of coatings increases with an increase in both the load and abrasive size. Moreover, the effect of increase in load at high abrasive size is more predominant than at low abrasive size. Further, it can be observed from response surface plot that the effect of increase in abrasive size on wear of coatings is more at high loads than at low loads. This is attributed to the fact that at high load and large abrasive size, the depth of penetration of abrasive increases. This leads to more abrasive wear at high load and high abrasive size and vice versa. The combined effect of load-sliding distance (LS) on wear of coatings shows that the wear of coatings increases with an increase in both the load and sliding distance. Moreover, the effect of increase in sliding distance is more predominant than the increase in load on abrasive wear. However, the effect of increase in sliding distance is more predominant in the entire range of loading on abrasive wear as compared to increase in load. Further, it can be observed from response surface plot that the effect of increase in sliding distance on wear of coatings is more at high loads than at low loads.

0.104

0.0612

0.0787

0.0458

Abrasive wear, g

Abrasive wear, g

S. Sharma / Tribology International 75 (2014) 39–50

0.0534

0.0281

0.0028

0.0304

0.0150

-0.0005 0.00

5.00

10.00

15.00

5.00

20.00

10.00

15.00

20.00

25.00

Load (L), N

Composition (C), wt.%CrC

0.0612

0.104

0.0459

0.0787

Abrasive wear, g

Abrasive wear, g

47

0.0306

0.0153

0.0000

0.0534

0.0281

0.0028 20.00

40.00

60.00

80.00

100.00

25.00

40.00

55.00

70.00

85.00

Sliding Distance (S), m

Abrasive size (A), μm

Abrasive wear, g

0.104

0.0787

0.0534

0.0281

0.0028 50.00

75.00

100.00

125.00

150.00

Temperature (T),°C 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.

The combined effect of abrasive size and sliding distance (AS) on wear of coatings shows that the wear of coatings increases with an increase in both the sliding distance and abrasive size. Again

the effect of increase in sliding distance on abrasive wear is more predominant in the entire range of abrasive size. It can be observed from response surface plot that the effect of increase in

S. Sharma / Tribology International 75 (2014) 39–50

0.0612

0.0612

0.0459

0.0459

Abrasive wear, g

Abrasive wear, g

48

0.0306 0.0153 0.0000

0.0306 0.0153 0.0000

25.00

150.00 20.00

20.00

), (L 0.0612

0.0612

0.0459

0.0459

Abrasive wear, g

Abrasive wear, g

N

5.00 0.00

0.0306 0.0153 0.0000

), (C n tio 5.00 si CrC o p m t.% Co w 15.00

100.00

on iti rC 5.00 s po C m t.% o w C 10.00

10.00

re tu ra pe m ), ° C Te (T

ad Lo

15.00

20.00

125.00

), (C

15.00

10.00

75.00 50.00 0.00

0.0306 0.0153 0.0000

85.00

100.00

ze si ve m si ra ), μ Ab (A

20.00

60.00

15.00

40.00

10.00

20.00 5.00

ad Lo

), (L

N

ce an st Di g ), m in id (S Sl

25.00

80.00

25.00

70.00

20.00

55.00

15.00

40.00 25.00 5.00

10.00

ad Lo

), (L

N

Abrasive wear, g

0.0612 0.0459 0.0306 0.0153 0.0000

85.00

Sl id 70.00 in g 55.00 (S Dis ), 40.00 m tan ce 25.00

100.00 80.00

e siz e 40.00 siv m ra ), μ b 20.00 A (A 60.00

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 on abrasive wear.

sliding distance on wear of coatings is more at high abrasive size than at low abrasive size. Thus high abrasive size and high sliding distance results in severe wear of the coatings. Same

effects of load, abrasive size and sliding distance were observed in LS and AS interactions for abrasive wear of coatings as discussed above.

S. Sharma / Tribology International 75 (2014) 39–50

49

Sliding Direction Ploughing

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

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.10a–b). The worn surfaces of various coatings (0, 10 and 20 wt% CrC) mainly showed the plowing and cutting mechanisms (Fig. 10a– b). The weight loss in each coating is determined by the extent of these mechanisms. Plowing and cutting mechanism were observed in the 0% CrC coating while cutting mechanisms were observed in 10% and 20% CrC coatings. The worn grooves are wider in 0% and 10% CrC coatings as compared to 20% CrC coating. The wider grooves in 0% CrC and 10% CrC coating were due to low hardness as compared to 20% CrC coating. Due to sharp abrasive particles the width of the cutting/plowing grooves increases with the increase in depth of indentation and results in increase in wear rate of the coatings. The chromium carbide concentration increases the wear resistance of the coatings. Experimental and confirmation test results showed that the weight loss in 20% CrC coating is lowest. The weight loss of 20% chromium carbide coating is 1.5 times lower as compared to 0% chromium carbide coating. This is attributed to higher hardness of the coating.

4. Conclusions The following conclusions can be drawn from the present study: 1. The hardness increases with the increase in chromium carbide concentration. The maximum hardness was obtained with 20 wt% chromium carbide. The increase in hardness is due to formation of new phases and inetrmetallic compounds. 2. Response Surface Methodology (RSM) with fractional factorial design approach is an excellent tool, which can be successfully used to develop an empirical equation for the prediction and understanding of wear behavior of coatings in terms of individual factors (C, L, A, S and T) as well as in terms of the combined effects (CL, CT, LA, LS and AS) of various factors. 3. The load and sliding distance have a more severe effect on abrasive wear of the coating as compared to abrasive size. 4. Interaction effects of various factors on abrasive wear is almost of same order less than their main factor effects. The interaction effect of abrasive size-sliding distance (AS) is considerably higher than load-abrasive size (LA). Increasing (%) CrC concentration; reducing load, abrasive size and sliding distance minimize the abrasive wear significantly. 5. Increase in chromium carbide concentration increases the abrasive wear resistance of the coatings. Abrasive wear rate

of 20 wt% chromium carbide coating is lower as compared to 0 wt% chromium carbide coatings.

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