|Year : 2012 | Volume
| Issue : 1 | Page : 45-51
High Temperature Erosion of Flame Sprayed Coatings
School of Engineering, Gautam Buddha University, Greater Noida, U.P., India
|Date of Web Publication||24-Mar-2012|
School of Engineering, Gautam Buddha University, Greater Noida, U.P.
Source of Support: None, Conflict of Interest: None
| Abstract|| |
In the present work, Co-base powder was modified with 10%wt CrC addition in order to study the effect of CrC addition. These coatings were deposited by flame spraying process. The effect of CrC addition on Vickers hardness and erosive wear behavior of the deposited coatings was studied. The erosive wear behavior of flame sprayed coatings was investigated by response surface methodology. To investigate and develop the erosive wear model of the flame sprayed coatings four factors, namely velocity (V), impact angle (A), temperature (T), and feed rate (F), each factor at three levels, were used. Analysis of variance was carried out to determine the significant factors and interactions. Investigation showed that the composition, velocity, impact angle, temperature, and feed rate were the main significant factors while velocity-feed rate and impact angle - feed rate were the main significant interactions. Thus, an erosive 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 3-8% error. The CrC addition improves the erosive wear resistance of the coatings. This is due to an increase in hardness of the flame sprayed coatings with the addition of CrC.
Keywords: CrC, erosive wear, response surface methodology, wear resistant coatings
|How to cite this article:|
Sharma S. High Temperature Erosion of Flame Sprayed Coatings. J Eng Technol 2012;2:45-51
| 1. Introduction|| |
Surface engineering techniques are used to protect manufactured components from various types of degradation such as thermal and corrosive wear. These techniques are used to impart wear resistance and hardness to the surfaces while retaining toughness and ductility of the bulk component. It is important to remember while selecting a coating for a particular application that (1) there is no such thing as a universal coating that can be used under all conditions; (2) coatings are not inherently good or bad, and (3) all coating selection represent compromises made to satisfy all the variables involved. 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.
When solid particles strike a material surface, which are usually contained in a gas medium, the impacts cause local plastic deformation of the surface with each particle causing a small crater and local material removal by erosion. Solid-particle erosion occurs in many applications such as economizer, reheater, low temperature primary super heater, final super heater, guide vanes in ducting, and water wall tubes. These tubes are located in the bed, along the water walls, or in the convection pass of the fluidized bed combustor (FBC). Each area in which tubes are located has a different local service environment, that is, temperature, gas composition, particle velocity, impact angle, and solid (fly ash) loading (erodent feed rate), which directly relate the material wastage of tubes  . The erosivity of ash and fly ash particles depends on the conditions (impact velocity, angle of impact, and temperature) under which they strike the surface of heat exchanger tubes. Also, the different parts of a boiler are subjected to different temperature ranges.
Studies on the erosive wear behavior of Co-base coatings were carried out by various researchers ,,,,,,,,,,, . These coatings were developed by various coating techniques. These researches were carried out using single factor experiment, while the response of a material to solid-particle erosive wear is a complex process, strongly influenced by both mechanical and physical factors. These include system variables such as impact velocity and impact angle; erodent properties such as size, shape, and hardness; and target material properties such as hardness, fracture toughness, and microstructure , . The literature shows that most of the studies on erosion resistance of various coatings were carried out using single factor of experiments. But the fact is that the erosion resistance is governed by a number of factors and hence need to be considered simultaneously. Hence, in the present investigation the effect of CrC addition on the erosion resistance was studied. Five factors (composition, impact angle, velocity, feed rate, and temperature) were used to study the single and combined factor effects on erosion resistance of flame sprayed coatings using response surface methodology (RSM).
| 2. Experimental Procedure|| |
2.1 Materials and methods
The carbon steel substrate was used for various coating depositions. The substrate was degreased and roughened to an average surface roughness of Ra 4-6 μm. Surface roughness was measured by Mahr Perthometer (M 2 409). The nominal composition of commercially available Co-base powder was Cr 22-25, Si 0.2-0.5, Mn 0.5-0.7, C 2.8-3.5, Ni 15-20, W 5-6, Co-balance (wt. %). This powder was modified by adding 10 wt.% CrC in the Co-base powder. These composition/coatings were further designated as Co-base and 10wt.% CrC coatings, respectively. These compositions were deposited by flame spraying process. The substrate was preheated to 200 ± 10° C.
2.2 Selection of erosive wear parameters and their ranges
The erosivity of ash and fly ash particles depends on the conditions (impact velocity, angle of impact, and temperature) under which they strike the surface of heat exchanger tubes. These tubes are located in the bed, along the water walls, or in the convection pass of the FBC. Each area in which tubes are located has a different local service environment, that is, temperature, gas composition, particle velocity, impact angle, and solid (fly ash) loading (erodent feed rate), which directly relate the material wastage of tubes  . The various factors and their levels used for erosive wear study are shown in [Table 1].
|Table 1: Erosive wear test parameters with their actual and coded levels (within small braces) used for erosive model development of various flame sprayed coatings|
Click here to view
[Table 1] Erosive wear test parameters with their actual and coded levels (within small braces) used for erosive model development of various flame sprayed coatings.
2.3 Vickers hardness of coating
Coated samples were cut transversely for Vickers hardness. Vickers hardness of the coating was measured using a load of 5 kg, and an average of six readings of the coating was used for study purpose.
2.4 Response Surface Methodology
RSM using face centered design with three levels of each factor has been used in the present study. Four factors such as particle velocity, impact angle, flux rate (feed rate), and temperature were used in the present study. These factors were designated as V (impact velocity) (m/s), A (impact angle) (degrees), F (flux or feed rate) (g/min), and T (temperature) (°C), respectively. The coded value of upper, middle, and lower level of each factor is designated by +1, 0, and -1, respectively. The actual and coded values (in parentheses) of various factors used in the present study are shown in [Table 1]. The experimental design matrix for different runs is shown in [Table 2]. The relation between the actual and coded value of a factor is given below:
|Table 2: Design matrix showing erosive wear test parameters (factors) with their actual and coded levels (in small braces) and erosive wear test results of various flame sprayed coatings|
Click here to view
2.5 Erosive wear test
High temperature erosive wear of various flame sprayed coatings was conducted using the high temperature erosion tester. The air pressures corresponding to different particle velocities are shown in [Table 3]. Also, the feed rates corresponding to various frequencies of motor (erodent supply) shown in [Table 3]. Erosive wear tests were conducted randomly according to design matrix [Table 2] for different runs and two replications of each run were taken. The average value of erosive wear of substrate and various thermal sprayed coatings has been used for study and analysis.
|Table 3: Relation between (a) air pressure and velocity and (b) frequency and feed rate used for setting the parameters (factors) in erosive wear test.|
Click here to view
| 3. Results and Discussions|| |
3.1 Erosive wear model development
In the present work, RSM was used for developing the mathematical models in the form of multiple regression equations for the erosive wear. In applying the RSM, the dependent variable (erosive wear) is viewed as a surface to which the model is fitted. Evaluation of the parametric effects on the response (erosive wear) was done by considering a second-order polynomial response surface mathematical model given by:
This equation of erosive wear (assumed surface) EWr contains linear, squared, and cross-product terms of variable x i 's (V, A, T, and F). 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 factors x i and x i ), 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 were carried out by using Design Expert Software to find out the significant factors, square terms, and interactions affecting the response (erosive wear). εR is the experimental error.
The analysis of variance (ANOVA) is shown in [Table 4]. 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. The probability values <0.05 in the "Prob. > F" column indicates the significant factors and interactions. The main factors and their interactions are included in the final erosive wear model, while the insignificant interactions are excluded from the erosive wear model. Velocity (V), impact angle (A), temperature (T), and feed rate (F) are the significant factors, while velocity-feed rate (VF) and impact angle-feed rate (AF) are the significant interactions. The erosive wear model generated in terms of coded and actual factor values are given below:
3.2 Validation of erosive wear models of flame sprayed coatings
|Table 4: Analysis of variance for erosive wear model of flame sprayed coatings|
Click here to view
The validity of the erosive wear models of flame sprayed coatings was evaluated by conducting erosive wear tests on these coatings at different levels of the applied factors such as velocity (V), impact angle (A), temperature (T), and feed rate (F). The values of various factors for erosive wear confirmation tests are shown in [Table 5]. The experimental results of confirmation tests and the results yielded by erosive wear models (equations 3 or 4) are shown in [Table 6]. It can be observed that the variations between the experimental and the modeled values for various coatings are of the order of 3-8%.
|Table 5: Erosive wear test parameters and their levels (in actual and coded form) used for conducting the confirmation tests to validate the erosive wear models for various flame sprayed coatings.|
Click here to view
|Table 6: Erosive wear test results obtained from modeled equations and experimental confirmation erosive wear tests of 10wt.% CrC flame sprayed coating|
Click here to view
3.3 Erosive wear behavior of flame sprayed coatings
3.3.1 Single factor effects on erosive wear of flame sprayed coating
The Vickers hardness of Co-base and 10wt.% CrC coatings was found to be 435 and 568 HV 5 , respectively. The CrC addition increases the hardness of the flame sprayed coatings.
The erosive wear response surface models in terms of coded level (+1, 0, and -1) of applied factors (V, A, T, and F) and their significant interactions of flame sprayed coating is shown in equations 3-5 (substrate, Co-base, and 10wt.% CrC coating, respectively) while that in terms of actual level of applied factors and their interactions is shown in equations 6-8 (substrate, Co-base, and 10wt.% CrC coating, respectively), respectively. The effect of individual factors on erosive wear of flame sprayed coatings can be described by considering equation 5 (10wt.% CrC coating) because all the factors are at the same coded level (+1, 0, and -1). The constant 7.55 × 10 -3 in the quadratic model (equation 5) is the intercept and represents the average value of response that is erosive wear (10wt.% CrC coating) when all the factors are at the middle level (at 0 level in coded form).
Equation 5 also exhibits the effect of various applied factors such as velocity (V), impact angle (A), temperature (T), and feed rate (F) on erosive wear of flame sprayed coating. It can be observed that the coefficient associated with velocity (V), impact angle (A), temperature (T), and feed rate (F) are 5.52 × 10 -3 , 2.33 × 10 -3 , 1.59 × 10 -3 , and 5.09 × 10 -3 , respectively. The value of the constant associated with each factor shows the extent of damage caused by individual factor on erosive wear  . It can be observed from equation 5 that velocity (V) and feed rate (F) have more severe effect on erosive wear of flame sprayed coating as compared with impact angle (A) and temperature (T).
The effects of applied factors (V, A, T, and F) on erosive wear of flame sprayed coatings have been shown graphically in [Figure 1]. [Figure 1] (a) shows that there is exponential increase in erosive wear with the increase in velocity. This is attributed to the fact that the kinetic energy of the particle increases exponentially with the increase in velocity [Figure 1]a. Increase in kinetic energy of the impacting particles on the surface produces deeper craters and larger platelets  . Further impacts of the eroding particles on a crater cause strain hardening of the platelets. When strain exceeds a critical value in the material of the surface layers and platelets, the material is detached from the surface  . Divakar et al.  also reported the increase in erosion wear with the increase in velocity.
|Figure 1: Effect of various factors (a) velocity, (b) angle of impact, (c) temperature and (d) feed rate on erosive wear of flame sprayed coatings|
Click here to view
There is decrease in erosion wear of flame sprayed coating with the change of impact angles from low (30°) to high (90°) impact angles [Figure 1]b. This is due to fact that at low impact angle, the erosion is mainly caused by cutting, whereas at high impact angle (90°) the erosion is mainly caused by strain hardening. The Co-base alloys are ductile and tough, hence can absorb more kinetic energy before failure by erosion mechanism. Increase in temperature increases the erosive wear as shown in [Figure 1]c. Increase in erosion wear has also been reported by Levy  and it was attributed to softening of the target material with the increase in temperature. The erosion wear also increases with the increase in erodent feed rate [Figure 1]d. This is due to the fact that more number of eroding particles impact the target material, which results in the formation of more number of craters and platelets.
3.3.2 Interaction effects between applied factors on erosive wear of modified 1031 HVOF sprayed coating
Analysis of the erosive wear data has indicated that velocity-feed rate (VF) and impact angle-feed rate (AF) interactions are significant, while other interactions are insignificant and hence do not appear in the response surface model of flame sprayed coatings (equations 3 and 4). The magnitude of the coefficient associated with velocity-feed rate (VF) shows the extent of damage caused by velocity-feed rate (VF) interaction on erosive wear of flames sprayed coatings  . At low velocity and low feed rate, the erosive wear is low as compared with high velocity and high feed rate [Figure 2]a. The decrease in erosion wear at low velocity and low feed rate is due to low kinetic energy and less number of eroding particles striking the coatings, which results in low strain hardening and consequently low material removal of the coating material in the form of craters and platelets. The impact angle-feed rate (AF) is shown in [Figure 2]b and can be explained by considering the effect of each factor individually as explained earlier.
|Figure 2: 3D surface plots for interaction effects between various factors (a) velocity- feed rate (VF) and (b) impact angle - feed rate (AF) on erosion wear.|
Click here to view
Further, by comparing the overall coefficients of the equations 3-5, it can be concluded that the erosive wear resistance of Co-base coating and 10wt.% CrC coating is two and five times higher as compared with substrate.
| 4. Conclusions|| |
- A response surface model was developed for erosive wear study of flame sprayed coatings in terms of velocity (V), temperature (T), and feed rate (F) and their significant interaction. The developed model predicts the erosive wear rate with reasonable accuracy.
- All the applied factors (V, A, T, and F) are significant, while velocity-feed rate (VF) and impact angle-feed (AF) rate are significant interactions.
- The increase in CrC increases the hardness of the Co-base coating.
- The erosive wear resistance of the Co-base and 10wt.% CrC coating is approximately two and five times as compared with substrate, respectively.
| References|| |
|1.||J. A. Hearley, J. A. Little, and A. J. Sturgeon, "The erosion behaviour of NiAl intermetallic coatings produced by high velocity oxy-fuel thermal spraying", Wear, Vol. 233-235, pp. 328-333, 1999. |
|2.||V. H. Hidalgo, F. J. B. Varela, A. C. Menedez, and S. P. Martinez, "A Comparative study of high temperature erosion wear plasma sprayed NiCrBSiFe and 35%WC- 65% NiCrBSiFe resistance coatings under simulated coal fired boiler conditions", Tribology International, Vol. 34, pp. 161-169, 2001. |
|3.||P. J. Hoop, and C. Allen, "The high temperature erosion of commercial thermally sprayed metallic and cermet coatings by solid particles", Wear, Vol. 233-235, pp. 334-341, 1999. |
|4.||T. Hejwowski, S. Szewczyk, and A. Weronaski, "An investigation of the abrasive and erosive wear of flame-sprayed coatings", J Materials Processing Technology, Vol. 106, pp. 54-57, 2000. |
|5.||B. Wang, "Erosion-corrosion of thermal sprayed coatings in FBC boilers", Wear, Vol. 199, pp. 24-32, 1996. |
|6.||H. M. Hawthorne, B. Arsenault, J. P. Immarigeon, J. G. Legoux, and V. R. Parameswaran, "Comparison of slurry and dry erosion behavior of some HVOF thermal sprayed coatings", Wear, Vol. 225-229, pp. 825-834, 1999. |
|7.||K. J. Stein, B. S. Schorr, and A. R. Marder, "Erosion of thermal spray MCr-Cr 3 C 2 cermet coatings", Wear, Vol. 224, pp. 153-159, 1999. |
|8.||H. R. Shetty, T. H. Kosel, and N. F. Fiore, "A study of abrasive wear mechanisms using diamond and alumina scratch tests", Wear, Vol. 80, pp. 347-376, 1982. |
|9.||S. B. Mishra, K. Chandra, S. Prakash, and B. Venkataraman, "Erosion performance of coatings produced by shrouded plasma spray process on a Co- based superalloy", Surface and Coatings Technology, Vol. 201, pp. 1477-1487, 2006. |
|10.||J. Saaedi, T. W. Coyle, S. Mirdamadi, H. Arabi, and J. Mostaghimi, "Phase formation in a Ni-50Cr HVOF coating", Surface and Coatings Technology, Vol. 202, pp. 5804-5811, 2008. |
|11.||J. Vicenzi, D. L. Villanova, M. D. Lima, A. S. Takimi, C. M. Marques, and C. P. Bergmann, "HVOF-coatings against high temperature erosion (300°C) by coal fly ash in thermoelectric power plant", Materials and Design, Vol. 27, pp. 236-242, 2006. |
|12.||M. Vite, M. Castillo, L. H. Hernandez, G. Villa, I. H. Cruz, and D. Stephane, "Dry and wet abrasive resistance of Inconel 600 and satellite", Wear, Vol. 258, pp. 70-76, 2005. |
|13.||S. H. Singh, S. B. Singh, and S. Prakash, "Solid particle erosion of HVOF sprayed NiCr and Stellite-6 coatings", Surface and Coatings Technology, Vol. 202, pp. 232-238, 2007. |
|14.||ASM handbook, "Friction, wear and lubrication", Vol. 18. |
|15.||R. G. Wellman, and C. Allen, "The effects of angle of impact and material properties on the erosion rates of ceramics", Wear, Vol. 186-187, pp. 117-22, 1995. |
|16.||K. Venkateswarlu, S. Mohapatra, R. G. Rao, A. K. Ray, L. C. Pathak, and D. P. Mondal, "High abrasive wear response of diamond reinforced composite coating: A factorial design approach", Tribology Letters, Vol. 24, pp. 7-14, 2006. |
|17.||M. Divakar, V. K. Agarwal, and S. N. Singh, "Effects of material surface hardness on the erosion of AISI 316", Wear, Vol. 259, no. 1-6, pp. 110-117, 2005. |
|18.||A. J. Ninham, and A. V. Levy, "Erosion of hard material coating systems", Wear, Vol. 121, pp. 325-346, 1998. |
|19.||A. Levy, "The solid particle erosion and erosion-corrosion of materials", ASM International, materials Park, OH 44073-0002, U.S.A, 1995. |
| Authors|| |
Dr. Satpal Sharma received his B. E. and M. Tech. from NIT, Kurukshetra. He did his Ph.D. from IIT, Roorkee, Uttrakhand, India. He has a teaching experience of more than 13 years plus 1.25 years of industrial experience. Presently he is with the Department of Mechanical Engineering in School of Engineering, Gautam Buddha University, Greater Noida, U. P., India as an Assistant Professor. His area of research is tribological properties of coatings, weld surfacing and thermal spraying, welding and machining. He has published more than 11 research papers in refereed international journals and 3 papers in national and international conferences.
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]