Journal of Engineering and Technology

ARTICLE
Year
: 2014  |  Volume : 4  |  Issue : 2  |  Page : 141--146

Effect of CeO 2 Addition on Wear Behavior of Flame Sprayed Coatings


Satpal Sharma 
 Mechanical Engineering, School of Engineering, Gautam Buddha University, Greater Noida, Uttar Pradesh, India

Correspondence Address:
Satpal Sharma
Mechanical Engineering, School of Engineering, Gautam Buddha University, Greater Noida, Uttar Pradesh
India

Abstract

MEC 1240A Ni base alloys are being widely used to improve the wear resistance of various industrial components in high temperature and in corrosive environment. Addition of rare earth elements like CeO 2 further improves the wear, corrosion, and oxidation resistance of these alloys. In the present work, commercially available Ni base powder (MEC 1240 A) was modified with the optimum addition of rare earth element CeO 2 . Rare earth addition refines microstructure and increases hardness of the coatings. Abrasive wear of rare earth modified flame sprayed coating, and the effect of load, abrasive size, and sliding speed on abrasive wear of these coatings was investigated by Response Surface Methodology (RSM). For this purpose, three factors load (L), abrasive size (A) (size in ΅m), and sliding speed (S) (m/ min) with three levels of each factor were used for investigation. Analysis of variance was carried out to determine the significant factors and interactions. Investigation showed that the load, abrasive size, and sliding speed were the main significant factors, while load and abrasive size, load and sliding speed, abrasive size, and sliding speed were the main significant interactions. The interaction effects are one order less than the main factor effects. Thus, an abrasive wear model was developed in terms of main factors and their significant interactions. The validity of the model was evaluated by conducting experiments under different wear conditions. A comparison of modeled and experimental results showed 2-7% error.



How to cite this article:
Sharma S. Effect of CeO 2 Addition on Wear Behavior of Flame Sprayed Coatings .J Eng Technol 2014;4:141-146


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Sharma S. Effect of CeO 2 Addition on Wear Behavior of Flame Sprayed Coatings . J Eng Technol [serial online] 2014 [cited 2019 Dec 8 ];4:141-146
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Full Text

 1. Introduction



Surface engineering techniques are used to protect manufactured components from various types of degradation such as thermal and corrosive wear. Various types of coating techniques such as thermal spraying, hard facing, and laser cladding have been used in various industrial applications. Flame spraying is one of the cost-effective process and has been used to form dense, adherent, and homogeneous coatings to protect critical components from wear, corrosion, and oxidation in many industries [1],[2],[3],[4] . But at the same time, flame spray deposition process has a number of disadvantages compared with other spraying processes, such as plasma spray, detonation gun and HVOF, including coarse (bigger grain size) microstructure, pore size, and crack length. Some of these disadvantages can be overcome with the addition of rare earth oxides which refines microstructure, increases hardness, and improves the abrasive wear resistance of flame sprayed coating. Various researchers [5],[6],[7],[8][,[9],[10],[11],[12],[13],[14],[15],[16] reported the wear behavior of Ni base coatings with rare earth additions such as CeO 2 , La 2 O 3 , and Y 2 O 3 and have been reported that addition of rare earth oxides (CeO 2 , La 2 O 3 , and Y 2 O 3 ) increases the hardness with microstructure refinement and wear behavior of coatings. In earlier work, refinement in microstructure, increase in microhardness, and abrasive wear resistance of NiCrBSi flame sprayed coatings with the optimum addition of CeO 2 (0.8%wt.) and La 2 O 3 (1.2%) have been reported [17],[18] . In the present work, effect of CeO 2 (1%wt.) addition on hardness and abrasive wear resistance of flame sprayed coating has been reported.

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) [6] . It has also been found from the literature survey that most of the research on abrasive wear behavior of Ni 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 and combined effect of the factors using full factorial design (Response Surface Methodology [RSM]). 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.

 2. Experimental Procedure



2.1 Materials and methods

The carbon steel substrate was used for coating deposition. The normal composition of substrate is shown in [Table 1]. The substrate was degreased and roughened to an average surface roughness of Ra 3.85 μm (Rmax 18.6 μm). Surface roughness was measured by Mahr-Perthometer (M 2 409). The normal composition of the commercially available powder MEC 1240A is shown in [Table 1]. This powder was modified by adding 1% CeO 2 . In further discussion the coating without CeO 2 was designated as unmodified, while the coating with CeO 2 was designated as modified coating. These compositions were deposited using flame spraying process by Super Jet Spray Torch (L and 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 kgf/cm 2 ) and 0.12 MPa (1.2 kgf/cm 2 ), respectively. The substrate was preheated to 200 ± 10°C. The spraying parameters are shown in [Table 2]. {Table 1}{Table 2}

2.2 Vickers hardness of unmodified and modified coatings

The hardness of the unmodified and modified coatings was measured by Vickers hardness using a load of 5 kg, and average value of 10 readings have been reported in the present investigation. In the present case, the hardness (Hv 5 ) of the unmodified and modified coating with the addition of rare earths (1% CeO 2 ) was found as 328 ± 12 and 398 ± 8. Thus, there is increase in hardness with the addition of 1wt.% CeO 2 in the present case. The increase in hardness may be due to microstructure refinement as reported earlier [17],[18] .

2.3 Factorial design of experiment

The vast amounts of data have been generated by using single-factor experiment design in which one factor is varied at a time (load and abrasive grit size). The analysis of combined effects of applied factors is very difficult in this approach. This is the main reason why load has always been considered first in wear research, while other factors, e.g. abrasive grain size, sliding speed, and their combined effects (load and abrasive size, load and speed, abrasive size, and speed), which may also be important, have not been given the attention they deserve. The advantage of the statistical method is obvious [19],[20],[21] . Thus, RSM with fractional factorial design of experiments with three levels of each factor has been used in this study. In view of the fact that Rabinowicz's classic theory [22] claims that applied load and hardness (depends upon composition) of materials are the most important factors affecting the abrasion process. Thus, load, abrasive size, and sliding speed, were considered in this study. These three factors were designated as L (load in kg), A (abrasive size in μm) and sliding speed (S) respectively. The coded value of upper, middle, and lower level of these factors is designated by +1, 0, and -1, respectively. The actual and coded values (in parentheses) of various factors used in this study are shown in [Table 3]. The experimental design matrix for different runs is shown in [Table 4]. The relationship between the actual and coded value of a factor is shown below:{Table 3}{Table 4}

[INLINE:1]

2.4 Wear test

Wear behavior of rare earth unmodified and modified flame sprayed coatings was studied using pin on disc type wear testing unit. Coated wear pins of size 5 × 5 × 35 mm 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 mm), which was rotated at 200 ± 4, 285 ± 5, and 345 ± 5 rpm (revolution per minute) corresponding to the sliding speed of 25, 50, and 75 m/min. The slide carrying the wear pin was radially moved at a speed of 40, 59, and 74 mm/min to get the spiral motion under a constant increment of 0.2 mm/min of the wear pin. The tester was allowed to run idle for 2 minutes in order to attain the constant speed (without reciprocating motion); afterward load was applied and simultaneously the reciprocating unit was switched on to get the spiral motion of the wear pin. Wear tests were conducted randomly according to the 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 (g) was used as a measure of abrasive wear.

2.5 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:

[INLINE:2]

This equation of abrasive wear (assumed surface) AWr contains linear, squared, and cross-product terms of variable x i 's ( L, A, and S, ). b 0 is the mean response over all the test conditions (intercept), bi is the slope or linear effect of the input factor x i (the first-order model coefficients), bii the quadratic coefficients for the variable i (linear by linear interaction effect between the input factor x i and x i ), and bij is the linear model coefficient for the interaction between factors 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 (ANOVA) were carried out using Design Expert Software to find out the significant factors, square terms, and interactions affecting the response (abrasive wear). εR is the experimental error.

The ANOVA is shown in [Table 5]. The 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" is "Not significant" which are desirable from a model point of view. P values <0.05 in the "Prob.>F" column indicates the significant factor and interaction. The main factors and their interactions are included in the final abrasive wear model, while the insignificant interactions are excluded from the abrasive wear model. Load, abrasive size, and sliding speed are the significant factors, while load and abrasive size (LA), load and sliding speed (LS), and abrasive size and speed (AS) are the significant interactions. The abrasive wear model so generated is given below:{Table 5}

[INLINE:3]

Equations (3-5) and (6-8) repesent the abrasive wear in terms of coded and actual factor values for substrate, unmodified, and modified coating, respectively.

2.6 Validity of the abrasive wear models

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 size (A), and sliding distance (S). The actual and coded values 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 2-7%. {Table 6}

2.7 Effect of individual variables on wear rate

The effect of individual factors on abrasive wear is shown in [Figure 1] (a-c). The effects of load (L), abrasive size (A), and sliding distance(S) and that of their interactions on abrasive wear are given in equations (3-5) which exhibits the abrasive wear in terms of coded values of factor levels for substrate, unmodified coating, and modified coating, respectively. Similarly, equations (6-8) represent the abrasive wear in terms of actual values of factor levels for substrate, unmodified coating, and modified coating, respectively, and their interactions. However, the effects of individual factors are discussed by considering the equations of coded form (3-5) because all the factors are at the same levels (+1, 0, and -1). The constants 0.15, 0.04, and 0.03 in the equations (3-5) indicate the overall mean of the abrasive wear of substrate, unmodified, and modified coating under all the test conditions. Thus, it can be concluded that the modified coating shows the lowest abrasive wear and it is approximately five times lower than substrate. Further, the abrasive wear of modified coating is approximately 25% lower than the unmodified coating. This is due to higher hardness of the modified coating when compared with unmodified coating. The increase in hardness may be due to microstructure refinement of the modified coating [17],[ 18] . Increase in hardness of material lowers the depth of penetration of abrasive particles and therefore results in shallow and finer wear grooves and reduced volume of material removed, thus reducing the abrasive wear of the material.{Figure 1}

Equation (5) can be used for explaining the effect of individual factor on abrasive wear of modified coating. The equation further indicates that the coefficients 0.014, 6.29 × 10 -3 , and 0.012 are associated with load (L), abrasive size (A), and sliding speed (S), respectively, which shows their severity on the abrasive wear. Among these coefficients, load and sliding speed have the highest effect on abrasive wear. Further, the +ve sign associated with these factors signifies an increase in abrasive wear with an increase in level of these factors [Figure 1]a-c. This signifies that load has a more detrimental effect than the sliding speed 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 speed. Thus, for the same sliding speed the abrasive wear increases with the increase in load as shown in [Figure 1](a, c). The effect of abrasive size on the wear is less when compared with load and sliding speed. The abrasive wear increases with the increase in abrasive size [Figure 1]b 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 [20] . 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 effect of individual factors on abrasive wear of substrate and unmodified coating can be discussed on similar lines by considering equations 3 and 4, respectively.

2.8 Interaction effect of the different variables

The coefficients associated with the interaction terms LA (load-abrasive size), LS (load-sliding distance), and AS (abrasive size-sliding distance) in equation (5) are 5.17 × 10 -3 , 9.28 × 10 -3 , and 7.68 × 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 one order less than that of their individual effects. The combined effect of load sliding speed (LS) is the highest among the three significant interactions.

The interaction (combined) effect of various abrasive wear test parameters on the wear behavior of coatings has been shown in the form of response surface plots [Figure 2]a-c. The interaction 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. {Figure 2}

The interaction effect of load and sliding speed (LS) on wear of coatings shows that the wear of coatings increases with an increase in both the load and sliding speed. 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 speed is more predominant in the entire range of loading on abrasive wear when compared with increase in load. Further, it can be observed from response surface plot that the effect of increase in sliding speed on wear of coatings is more at high loads than at low loads.

The interaction effect of abrasive size and sliding speed (AS) on wear of coatings shows that the wear of coatings increases with an increase in both the sliding speed and abrasive size. Again the effect of increase in sliding speed 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 sliding speed on wear of coatings is more at high abrasive size than at low abrasive size. Thus, high abrasive size and high sliding speed results in severe wear of the coatings. The interaction effects of combined factors on abrasive wear of substrate and unmodified coating can be discussed on similar lines by considering equations 3 and 4, respectively.

2.9 SEM study of worn surfaces

In an attempt to identify the abrasive wear mechanism in unmodified and modified coatings, SEM images of worn surfaces were analyzed [Figure 3]a-c. The worn surfaces of both coatings (unmodified and modified coatings) mainly showed the ploughing and cutting mechanisms [Figure 3]a-c. The weight loss in each coating is determined by the extent of these mechanisms. Ploughing and cutting mechanism were observed in the unmodified coating , while cutting mechanisms were observed in modified coating. The worn grooves are wider in unmodified coating when compared with modified coating. The wider grooves in unmodified coating were due to low hardness when compared with modified coating. Due to sharp abrasive particles, the width of the cutting/ploughing grooves increases with the increase in depth of indentation and results in increase in wear rate of the coatings. The weight loss of modified coating is approximately 25% lower when compared with unmodified coating. This is attributed to higher hardness of the coating. {Figure 3}

 3. Conclusions



The following conclusions can be drawn from this study:

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 (L, A. and S) as well as in terms of the combined effects (LA, LS. and AS) of various factors. The load and sliding speed has a more severe effect on abrasive wear of the coating when compared with abrasive size. Interactions effects of various factors on abrasive wear are one order less than their main factor effects. The interaction effect of load-sliding speed (LS) is considerably higher than load and abrasive size (LA) and abrasive size-sliding speed (AS). The CeO 2 addition increases the hardness and abrasive wear resistance of the modified coating.

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