1. Introduction
Ceramic/metal composites with interconnected and interpenetrating continuous network of a ceramic and metal phase are referred to as co-continuous ceramic composites (C4) [
1]. The interlocking microstructure endows C4 with enhanced fracture toughness, superior wear resistance, higher stiffness and lower distortion with variation in operating temperature. This unique combination of desirable properties of C4 makes it suitable for applications entailing higher specific modulus, strength, creep resistance, corrosion resistance and improved wear characteristics [
2,
3]. Placing emphasis on weight reduction, numerous researchers have fabricated aluminum-based wear-resistant composite materials by reinforcing with high-strength and stiff ceramics such as B
4C, SiC, Si
3N
4, TiB
2 and TiC [
4,
5]. In C4 composites, the fraction of ceramic phase is greater than that of particulate reinforced composites. Consequently, they offer enhanced wear resistance [
6,
7]. Thus, three-dimensionally interconnected SiC porous preforms infused with Al alloys have potential in applications involving dry friction and wear [
1,
8].
Nong et al. [
8] utilized SiC
3D/Al co-continuous composites to prepare a ventilated shaft disc brake. The wear and friction performance of the C4 was found to be comparable to that of cast iron and steel. Moreover, better thermal conductivity and wear resistance was attained at half the density. Employing a digital logic approach, Maleque et al. [
9] proposed Al-Cu alloys reinforced with 20% SiC particles as good candidate materials for brake discs. These composites possessed a higher friction coefficient and lower density compared with monolithic materials such as grey cast iron and Ti-6Al-4V alloy. A composite of AA6063/SiC was fabricated by Kamboj et al. [
10] using the stir casting process and its mechanical properties were analyzed. An improvement of 47.3% in ultimate tensile strength (UTS), 50% in hardness and 60% in impact strength over the metal alloy were reported. This improvement in mechanical properties can be attributed to the presence of Mg as the principal constituent in AA6063, which acts as a surfactant and augments the wettability between Al and SiC by reducing the oxide layer on the surface of SiC [
11]. Tribological properties of AA6063/clay composites have been explored by Agbeleye et al. [
12] for brake disc rotor applications. It was reported that the sliding speed and applied load primarily influenced the wear rate. Though wear studies on AA6063/ceramic particulate composites have been reported, very few investigations on AA6063/SiC co-continuous composites exist. Therefore, to evaluate the propriety of AA6063/SiC C4 for friction and braking applications, wear studies are essential.
In order to investigate the multiple attributes associated with wear resistance of composites, numerous decision-making techniques, such as analytic hierarchy process (AHP), data envelopment analysis (DEA), grey relational analysis (GRA) and technique for order of preference by similarity to ideal solution (TOPSIS), have been suggested in literature [
13]. Among them, grey relational analysis (GRA), proposed by Deng [
14] in 1989, is one of the foremost techniques applied when the nature of information is incomplete and uncertain. Prayogo et al. [
15] employed GRA in conjunction with a Taguchi orthogonal array to determine the optimal levels of multiple process parameters of EDM. Confirmation tests showed a significant increase of 29.44% in the grey relational grade indicating that GRA was effective in improving the performance characteristics of the EDM process. A combined Taguchi L
9 orthogonal array and GRA approach has been adopted for optimization of injection molding parameters during fabrication of high-density polyethylene/TiO
2 nanocomposites by Pervez et al. [
16]. Using this approach, the optimal levels of operating variables were found to be holding time of 20 s, residence time of 30 min, TiO
2 concentration of 5 wt.% and barrel temperature of 225 °C. Utilizing an L
12 orthogonal array, Zou et al. [
17] integrated S/N ratios of the Taguchi method with GRA to identify the significant contributors to surface treatment of concrete structures. The results indicated that the water-binder ratio with 89.99% and the weight of the pozzolanic material with 2.55% were the key factors influencing concrete protection. Multi-factor experiments of dry sliding wear in AA6531 metal matrix composites (MMCs) were performed by Uthayakumar et al. [
18] using GRA and a Taguchi L
9 orthogonal array. The analysis concluded that the applied load and sliding speed were the most influential factors. Additionally, load at 19.62 N and speed at 3 m/s were estimated as the optimal levels of factors. Ghosh et al. [
19] evaluated the tribological performance of a composite comprising LM6 Al alloy with SiC
p of 37 µm mesh size using the Taguchi method and GRA. This study concluded that sliding time, with a contribution of 43.65%, significantly controlled the wear behavior of the composite. Furthermore, a 21% improvement of the grey relational grade was also realized thereby resulting in an improved design of the composite for tribological applications. In addition to this, the Taguchi method coupled with GRA has also been applied for multi-objective optimization of milling parameters [
20,
21], the turning process [
22] and the wire electrical discharge machining process [
23].
This study aims to optimize the dry sliding wear parameters to manufacture a desirable co-continuous ceramic composite with potential for friction and braking applications. Initially, a C4 of Al6063/SiC was fabricated using the gravity infiltration technique without the application of pressure [
24]. Next, an experimental design with a Taguchi L
9 (3
3) orthogonal array was developed with three controlling factors, namely, the applied load, sliding speed and sliding distance. Three distinct levels were identified for each of these three parameters. Subsequently, nine experimental runs were conducted to obtain data for the multiple responses of the friction coefficient and specific wear rate. Thereafter, S/N ratios and GRA were utilized to analyze the responses in order to obtain the optimal parameters. The most significant factors affecting the wear behavior of C4 were then identified using ANOVA. Finally, using the optimal levels of design parameters, confirmation tests were conducted to verify the improvement of the quality characteristic, namely, the grey relational grade.
4. Conclusions
The primary aim of this study was to obtain the optimal set of parameters which affect the wear performance of a co-continuous ceramic composite in the presence of multiple responses. Initially, the outcome of varying three factors—namely, applied load, sliding speed and sliding distance—on the multiple responses of the coefficient of friction and wear rate was studied using a Taguchi L9 OA and GRA approach. From the response table of the grey relational grades, the optimal set of parameters for enhanced wear performance of the C4 composite were identified to be an applied load of 60 N, a sliding speed of 1 m/s and a sliding distance of 1000 m. The ANOVA for GRG indicated that the p values of all parameters were less than 0.05 and, hence, significant. Finally, confirmation tests were performed to verify the improvement of 35.25% in GRG, from 0.5134 for the initial design parameters (L2, S3, D1), to 0.6944 for the optimal parameters (L3, S1, D1). SEM analysis of the worn surface revealed steady-state wear of C4 at optimal setting of parameters.