Application of Taguchi techniques to study dry sliding wear behaviour of metal matrix composites

Application of Taguchi techniques to study dry sliding wear behaviour of metal matrix composites

Materials & Design Materials and Design 28 (2007) 1393–1398 www.elsevier.com/locate/matdes Short communication Application of Taguchi techniques to ...

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Materials & Design Materials and Design 28 (2007) 1393–1398 www.elsevier.com/locate/matdes

Short communication

Application of Taguchi techniques to study dry sliding wear behaviour of metal matrix composites S. Basavarajappa b

a,*

, G. Chandramohan a, J. Paulo Davim

b

a Department of Mechanical Engineering, PSG College of Technology, Coimbatore 641 004, India Department of Mechanical Engineering, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal

Received 11 July 2005; accepted 3 January 2006 Available online 3 March 2006

Abstract Aluminium metal matrix composites reinforced with SiC and graphite (Gr) particles was prepared by liquid metallurgy route. Dry sliding wear behaviour of the composite was tested and compared with Al/SiCp composite. A plan of experiments based on Taguchi technique was used to acquire the data in a controlled way. An orthogonal array and analysis of variance was employed to investigate the influence of wear parameters like as normal load, sliding speed and sliding distance on dry sliding wear of the composites. The objective was to investigate which design parameter significantly affects the dry sliding wear. It shows that graphite particles are effective agents in increasing dry sliding wear resistance of Al/SiCp composite. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Dry sliding wear; Taguchi technique; Analysis of variance

1. Introduction Metal matrix composite materials are advanced materials, which combine tough metallic matrix with a hard ceramic or soft reinforcement to produce composite materials [1,2]. These materials have superior properties compared to the monolithic materials and can be tailarable to a specific applications [3,4]. Metal matrix composite materials show advantages in a great number of specific applications (aircraft, automobile, machines) due to their high specific strength and stiffness, wear resistance and dimensional stability. The most popular hard reinforcements are silicon carbide, alumina and soft reinforcement as graphite [5–7]. These materials have shown to have different strengthening mechanisms when compared to conventional materials or continuous reinforced composites [8]. Thus, much research, both experimental and analyti-

*

Corresponding author. Tel.: +91 422 2572177/2572477; fax: +91 422 2573833. E-mail address: [email protected] (S. Basavarajappa). 0261-3069/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.01.006

cal, has been performed to gain a better understanding of mechanical behaviour of these materials and their excellent wear resistance. The presence of hard reinforcement phases, particulates, fibres or whiskers has endowed these composites with good tribological (friction and wear) characteristics. The wear resistance with good specific strength and modulus make them good candidate for many engineering situations where sliding contact is expected. Sannino et al. [9] undertook an extensive review work on dry sliding wear characteristics of aluminium alloy based composites, and abrasive wear behaviour wear by Deuis et al. [10]. In their study and discussion, the effect of reinforcement volume fraction and size, sliding distance, applied load, sliding speed, hardness of the counter face and properties of the reinforcement phase, that influences the dry sliding wear behaviour of this group of composites were examined in greater detail. The sliding wear rate and wear behaviour were reported to be influenced by several wear parameters [11–16]. Lim et al. [17] studies the tribological behaviour of Al– Cu/SiCp metal matrix composites and reported with increasing the mechanical properties, wear resistance also

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be increased drastically, and it will effect the counter face. Mohan et al. [16], under the conclusion that the incorporation of graphite after 1% slightly reduces the mechanical properties, but enhances the wear resistances of the material. The incorporation of graphite in the composite smears on the surface and forms a layer that reduces the wear. The wear resistance of the graphitic composites is more due to the inherent property of the natural lubrication. The ceramic–graphite hybrid composites demonstrated that wear resistance can substantially increases without loosing properties compared to the Al/SiCp composite [18–20]. Sahin [21] conducted an abrasive wear test on Al2011 alloy with 5–10 wt% SiCp content with 32–64 lm reinforcement size. Factorial designs of experiments were used to assess the contribution of applied load, sliding distance and particle size. The abrasive wear was the response of the material running against SiCp and Al2O3 emery papers under different sliding conditions. He concluded that the wear rate of the matrix and the composite materials increased with increasing the abrasive size, applied load and sliding distance when SiC abrasive paper was used. However, the wear rate increased with increasing abrasive size and applied load and decreased with increasing sliding distance when the Al2O3 emery paper was selected. Esteban Fernandez et al. [22] described a multi-factor based on experiments that has been applied to investigating an abrasive wear system of Ni-based alloy coatings with and without WC reinforcement. They reported that abrasive grain size exerted the greatest effect on abrasive wear followed by reinforcement. The applied load and the environment were similarly found to have minor effect. The addition of WC reinforcement particles improved wear resistance of NiCrBSi alloy coating. Increasing abrasive grain size led to obviously greater wear, especially for NiCrBSi without WC. Wear loss was increased with applied load, but showed unclear tendencies as regards the influence of environments. Mondal et al. [23] studied the two-body abrasive wear behaviour of a cast aluminium alloy – 10 wt% Al2O3 particle composite was studied at different loads (1–7 N) and abrasive sizes (30–80 lm). The wear behaviour was predicted through statistical analyses of the measured wear rate at different operating conditions. The developed model qualitatively hold good for alloy and individual variables such as load and abrasive size on the wear resistance of the composites. They concluded along with reinforcement size and the load the interaction factors also quite significant and one must take into consideration these terms for determining the wear rate of these materials. In view of the above, an attempt is made in this investigation to study the effect of applied load, sliding speed and sliding distance on dry sliding wear behaviour of the Al/ SiCp and Al/SiCp–Gr composites using Taguchi design of experiments. The analysis of variance was employed to find the percentage of influence of various factors and its interaction on dry sliding wear of the composites.

2. Taguchi techniques Taguchi technique is a powerful tool for the design of high quality systems [24–26]. It provides a simple efficient and systematic approach to optimize designs for performance, quality and cost. The methodology is valuable when design parameters are qualitative and discrete. Taguchi parameter design can optimize the performance characteristics through the setting of design parameters and reduce the sensitivity of the system performance to source of variation [26,27]. This technique is multi-step process, which follow a certain sequence for the experiments to yield an improved understanding of product or process performance. This design of experiments process made up of three main phases: the planning phase, the conducting phase and analysis interpretation phase. The planning phase is the most important phase one must give a maximum importance to this phase. The data collected from all the experiments in the set are analyzed to determine the effect of various design parameters. This approach is to use a fractional factorial approach and this may be accomplished with the aid of orthogonal arrays. Analysis of variance is a mathematical technique, which is based on a least square approach. The treatment of the experimental results is based on the analysis of average and analysis of variance [28,29]. 3. Experimental procedure 3.1. Materials Aluminium alloy 2219 was used as the matrix material in the present investigation and present following the chemical composition (%): Si = 0.2 max, Fe = 0.3 max, Cu = 5.8–6.8, Mn = 0.2–0.4, Mg = 0.02 max, Zn = 0.1 max, V = 0.05–0.15, Ti = 0.02–0.1, Zr = 0.1–0.25, Al = balance. This matrix was chosen, since it provides excellent combination of strength and damage tolerance at elevated and cryogenic temperature. Two types of composites are used, one with 15 wt% of SiCp reinforcement of size 25 lm and a second one with 15% SiCp reinforcement, 3 wt% of graphite is added with a particle size of 45 lm. Liquid metallurgy method was used to fabricate the composites which was used by the other researchers [30–32].

3.2. Plan of experiments The experiments were conducted as per the standard orthogonal array. The selection of the orthogonal array is based on the condition that the degrees of freedom for the orthogonal array should be greater than or equal to sum of those wear parameters [26–29]. In the present investigation, an L27 orthogonal array was chosen, which has 27 rows and 13 columns as shown in Table 1. The wear parameters chosen for the experiment was (i) sliding speed, (ii) load and (iii) sliding distance. Table 2, indicates the factors and their level. The experiment consists of 27 tests (each row in the L27 orthogonal array) and the columns were assigned with parameters. The first column was assigned to sliding speed (S), second column was assigned to load (L), and fifth column was assigned to sliding distance (D) and the remaining columns were assigned to their interactions. The response to be studied was the wear with the objective of smaller is the better. The experiments were conducted as per the orthogonal array with level of parameters given in each array row. The wear test results were subject to the analysis of variance.

S. Basavarajappa et al. / Materials and Design 28 (2007) 1393–1398 Table 1 Orthogonal array L27(313) of Taguchi [24] L27(313) test

1

2

3

4

5

6

7

8

9

10

11

12

13

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

1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3

1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3

1 1 1 2 2 2 3 3 3 2 2 2 3 3 3 1 1 1 3 3 3 1 1 1 2 2 2

1 1 1 2 2 2 3 3 3 3 3 3 1 1 1 2 2 2 2 2 2 3 3 3 1 1 1

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

1 2 3 1 2 3 1 2 3 2 3 1 2 3 1 2 3 1 3 1 2 3 1 2 3 1 2

1 2 3 1 2 3 1 2 3 3 1 2 3 1 2 3 1 2 2 3 1 2 3 1 2 3 1

1 2 3 2 3 1 3 1 2 1 2 3 2 3 1 3 1 2 1 2 3 2 3 1 3 1 2

1 2 3 2 3 1 3 1 2 2 3 1 3 1 2 2 2 3 3 1 2 1 2 3 2 3 1

1 2 3 2 3 1 3 1 2 3 1 2 1 2 3 1 3 1 2 3 1 3 1 2 1 2 3

1 2 3 3 1 2 2 3 1 1 2 3 3 1 2 2 3 1 1 2 3 3 1 2 2 3 1

1 2 3 3 1 2 2 3 1 2 3 1 1 2 3 3 1 2 3 1 2 2 3 1 1 2 3

1 2 3 3 1 2 2 3 1 3 1 2 2 3 1 1 2 3 2 3 1 1 2 3 3 1 2

Table 2 Process parameters with their values at three levels Level

Sliding speed, S (m/s)

Load, L (N)

Sliding distance, D (m)

1 2 3

1.53 3.06 4.59

9.81 19.6 39.2

500 1000 1500

3.3. Experimental set up and procedure A pin-on-disc test apparatus shown in Fig. 1 was used to investigate the dry sliding wear characteristics of the composite as per ASTM G9995 standards. The wear specimen with 10 mm of diameter and 30 mm height was cut from as cast samples machined and then polished metallo-

Fig. 1. The schematic view of the pin on disc apparatus used in this study.

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graphically. The block diagram shown in Fig. 2 shows the step-by-step procedure used to evaluate the dry sliding wear. The initial weight of the specimen was measured in a single pan electronic weighing machine with least count of 0.0001 g. During the test the pin was pressed against the counter part rotating against EN32 steel disc with hardness of 65 HRc by applying the load. After running through a fixed sliding distance, the specimens were removed, cleaned with acetone, dried and weighed to determine the weight loss due to wear. The difference in the weight measured before and after test gives the dry sliding wear of the composite specimen and then the volume loss was calculated. The wear of the composites was studied as a function of the sliding distance, applied load and the sliding speed.

4. Results and discussions 4.1. Statistical analysis The experiments were conducted with an aim of relating the influence of sliding speed (S), applied load (L) and sliding distance (D) with dry sliding wear of both the composites under study. On conducting the experiments as per the orthogonal array, the dry sliding wear results for various combinations of parameters were obtained and shown in Table 3. The purpose of the statistical analysis of variance (ANOVA) is to investigate which design parameter significantly affects the wear characteristic. Based on ANOVA the optimal combinations of the process parameters are predicted. This analysis is carried out for level of significance of 1% (i.e., the level of confidence 99%) [26,27]. Tables 4 and 5 show the results of ANOVA analysis for both SiCp and SiCp–Gr reinforced composite materials, respectively. It can be observed from the ANOVA analysis that the (i) sliding speed, (ii) load and (iii) sliding distance on dry sliding wear of the composite. The interaction between the above factors does not have significant influence on the wear of both the composites under study. The column 5 of the ANOVA analysis of SiCp reinforced composite (Table 4) indicates the percentage contribution (p) of each factor on the total variation indicating their degree of influence on the result. If can be observed from Table 4 that the sliding distance (p = 57.57%), load (p = 24.34%) and sliding speed (p = 6.8%). However, the interaction between sliding speed and load is (p = 2.15%) and other factors are minimum. The pooled error is 6.64%. In the case of Graphitic hybrid composite, the column 5 of the ANOVA analysis in Table 5 indicates the percentage contribution (p) of each factor on the total variation indicating their degree of influence on the result. It can be observed from Table 5 that the sliding speed (p = 57.24%) load (p = 22.58%) and sliding speed (p = 9.66%). However, the interactions between the factors are minimum. The pooled error is 5.32%. In Al/SiCp and Al/SiCp–Gr composites, the dry sliding wear parameters have statistical and physical significance. The interactions between the parameters in both the materials have statistical significance but do not have any physical significance (error > percentage contribution of interactions). The percentage of influence of each factor is more or less same and it indicates that the incorpo-

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As cast specimen

Surface preparation

Machining

Final weight

Initial weight

Test

Difference

Wear Fig. 2. Step by step procedure used to evaluate the dry sliding wear of unreinforced alloy and composites.

Table 3 Orthogonal array of Taguchi for wear Test

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

Table 4 ANOVA for wear results (SiCp)

Sliding speed, S (m/s)

Load, L (N)

Sliding distance, D (m)

Wear SiCp, WR (mm3)

Wear graphitic, WR (mm3)

1.53 1.53 1.53 1.53 1.53 1.53 1.53 1.53 1.53 3.06 3.06 3.06 3.06 3.06 3.06 3.06 3.06 3.06 4.59 4.59 4.59 4.59 4.59 4.59 4.59 4.59 4.59

9.81 9.81 9.81 19.6 19.6 19.6 39.2 39.2 39.2 9.81 9.81 9.81 19.6 19.6 19.6 39.2 39.2 39.2 9.81 9.81 9.81 19.6 19.6 19.6 39.2 39.2 39.2

500 1000 1500 500 1000 1500 500 1000 1500 500 1000 1500 500 1000 1500 500 1000 1500 500 1000 1500 500 1000 1500 500 1000 1500

1.08 1.6 2.01 1.6 2.19 3.0 1.55 2.8 3.8 0.82 1.39 1.98 1.06 1.7 2.1 1.4 2.22 2.62 0.77 1.32 2.21 1.52 2.51 3.46 1.33 2.33 3.96

0.8 1.34 1.8 1.44 2.1 2.8 1.4 2.04 3.41 0.6 1.01 1.6 0.91 1.48 1.8 1.00 1.8 2.2 0.6 1.0 2.06 1.34 2.40 3.2 1.10 2.0 3.3

ration of graphitic composite under study will influence in increasing the wear resistance compare to the SiCp reinforced composite at all conditions but it will not make any impact over the change in percentage of influence of the individual wear parameters. 4.2. Wear mechanism The asperities of pin and counter face which are in contact are subjected to relative motion under the influence of applied load. Initially both the surfaces are associated with

Source of variances

ss

Df

D L S SXL SXD LXD Pooled error

10.91 4.64 1.33 0.5 0.34 0.32 0.83

2 2 2 4 4 4 35

Total

18.87

53

Variance 5.455 2.32 0.665 0.125 0.085 0.08 0.0237

Test F 230.17 97.89 28.06 5.27 3.59 3.38

pa (%)

F b

5.27 5.27b 5.27b 5.27b 2.64c 2.64c

57.57 24.34 6.8 2.15 1.3 1.2 6.64 100

ss, sum of squares; Df, degree of freedom. a Percentage of contribution. b 99% confidence level. c 95% confidence level.

Table 5 ANOVA for wear results (SiCp–Gr) Source of variances D L S SXL SXD LXD Pooled error Total

ss 9.38 3.72 1.61 0.38 0.34 0.32 0.58

Df 2 2 2 4 4 4 35

Variance 4.69 1.86 0.805 0.095 0.085 0.08 0.016

53

Test F 293.13 116.25 50.31 5.94 5.31 5.00

pa (%)

F b

5.27 5.27b 5.27b 5.27b 5.27b 3.95c

57.24 22.58 9.66 1.94 1.69 1.57 5.32 100

ss, sum of squares; Df, degree of freedom. a Percentage of contribution. b 99% confidence level. c 95% confidence level.

a large number of sharp asperities and contact between them takes place primarily at these points. Under the influence of applied load and speed, the asperities in each surface come in contact with each other and they are either plastically deformed or remain in elastic contact. As the asperities are very sharp in nature, the effective stress on these sharp points may be more than the elastic stress and then all these sharp asperities are plastically deformed at their contact points except the partially projected points of the reinforcement. The plastically deformed surface will

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fill the valley of the material both in pin and the counter face during the course of action and there is a possibility of fracturing a few asperities on both the surfaces leading to very fine debris. The stress on the surface of the SiCp–Gr composite pin is almost uniform and the contact between them is intact, such that more surface area is in contact. The wear resistance of the graphitic composite is more than that of Al/ SiCp composite. As the sliding distance increases the wear volume loss increases which can be attributed to the ploughing ability of the fractured particles between the pin and the counter face will not decrease with increasing the sliding distance [34,35]. The dry sliding wear volume loss increases with increasing load. The SiC particles are very strong in compression than the tension. This influences the penetration ability of the fractured particles between the pin and the counter face. So the removal of material from the surface of the pin increases with increase in load [21]. The decreasing trend of the wear rate when sliding speed is increased is due to the formation of protective mechanically mixed layer (MML) between the pin and the counter face [33–35]. In Al/SiCp–Gr composites, the variation of influence of factors on the wear volume loss follows the same trend as that of the Al/SiCp composite. Along with the protecting layer of MML forms between the sliding counterparts, graphite also smears and reduces overall wear volume loss of the composite at all tested range of parameters. 5. Conclusions Taguchi’s robust design method can be used to analyze the dry sliding wear problem of the metal matrix composites as described in the paper. The following generalized conclusions can be drawn from the work. (A) The incorporation of graphite particles in the aluminium matrix as a secondary reinforcement increases the wear resistance of the material. The smearing of the graphite and formation of protecting layer between the pin and the counter face enables in reducing the wear volume loss. (B) Sliding distance is the wear factor that has the highest physical as well as statistical influence on the wear of both composites. SiCp composite present a contribution of sliding distance (57.57%), load (24.34%), and sliding speed (6.8%). SiCp–Gr reinforced composites present a contribution of sliding distance (57.24%), the load (22.58%) and sliding speed (9.66%). The interactions between the wear parameters have statistical significance but do not have any physical significance. References [1] Ibrahim A, Mohamed FA, Lavernia EJ. Metal matrix composites – a review. J Mater Sci 1991;26:1137–57.

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