The Effect of Primary and Secondary Processing on the Abrasive Wear Properties of Compocast Aluminum 6061 Alloy Matrix Composites 1

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1 ISSN , Protection of Metals and Physical Chemistry of Surfaces, 214, Vol. 5, No. 6, pp Pleiades Publishing, Ltd., 214. PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION The Effect of Primary and Secondary Processing on the Abrasive Wear Properties of Compocast Aluminum 661 Alloy Matrix Composites 1 Ali Mazahery and Mohsen Ostad Shabani Karaj Branch, Islamic Azad University, Karaj, Iran vahid_ostadshabany@yahoo.com Received August 13, 212 Abstract In this present work, the compocasting method was used to produce Aluminum 661 alloy matrix composites reinforced SiC particles with a variety of particle average sizes of 1, 5, 2 and 5 µm. The influences of particle volume fraction of metal matrix composites on the dry sliding wear behaviour have been investigated. The acceptable quality of the fabricated composites calls for a proper selection of process parameter such as pouring temperature, stirring speed, stirring time, preheat temperature of reinforcement, minimum level of the porosity and prevention of the chemical reaction between reinforcement and matrix. Experimental test was carried out on the porosity and hardness of the composites. A pin-on-disc wear test was used to assess the effect of SiC content, SiC size and secondary mechanical processing with different rolling reductions on wear characteristics of Al matrix composites. It is noted that the particle distribution in the cold rolled composites is much more uniform than the as-cast one. DOI: /S INTRODUCTION There has been considerable effort devoted to the development of ceramic particle-reinforced metal matrix composites (PMMCs). The motivation for this effort stems from the improvements in elastic properties and wear characteristics associated with the ceramic particles. This class of composite is also amenable to tailoring of thermal expansion coefficients, through appropriate selection of reinforcement type and content, making it attractive for applications involving thermal stresses [1 5]. Among the several categories of MMCs, Al based composites are finding wide spread acceptance especially in applications where weight and strength are of prime concern [6 14]. Presently, Al alloy based metal matrix composites are being used as candidate materials in several applications such as pistons, pushrods, cylinder liners and brake dics etc. Particulate-reinforced aluminum matrix composites have gained extensive applications in automotive and aerospace industries due to their specific characteristics. These include low density, high specific strength and stiffness, good fatigue properties, dimensional stability at high temperatures, and acceptable tribological properties [15 19]. In recent years, among all the Al alloys, Al Mg Si alloys are gaining much popularity as a matrix material to prepare MMCs owing to its excellent mechanical properties and good corrosion resistance. A commercial AA661 aluminum alloy is one of the Al Mg Si(Cu) 1 The article is published in the original. system alloys that can be significantly hardened by a proper heat treatment. Because of its medium strength, excellent corrosion resistance, and weldability, the AA661 alloy has been widely used as structural material for construction, transportation and sports. AA661 has one major limitation, and that is its low wear resistance. In recent years, investigators have successfully attempted to improve the tribological properties by reinforcing it with high modulus ceramic particulates such as SiC, thus making it an increasingly suitable candidate for even wider spectrum of applications. The addition of ceramic particles also has important ramifications concerning flow and fracture. Notably, the mismatch in the elastic/plastic and thermal expansion characteristics of the particles and matrix leads to inhomogeneous plastic yielding during loading, with yield initiating at stresses which are lower than that required in the matrix alloy alone. At higher strains, the mismatch results in the development of large (elastic) stresses within the particles. Provided the particles are sufficiently strong, the elevation in stress results in an enhancement in the flow stress, above that of the matrix alone. There are several manufacturing techniques for particle reinforced MMCs such as liquid metal infiltration, spray decomposition, squeeze casting, compocasting, powder metallurgy and mechanical alloying. Stir casting of MMCs is an attractive processing method for these advanced materials since it is relatively inexpensive, and offers a wide variety of material 817

2 818 ALI MAZAHERY, MOHSEN OSTAD SHABANI Disc Load Pin Fig. 1. Schematic diagram of the abrasion wear test. and processing condition options. Generally, these composites consist of a metal matrix, which is melted during casting, and ceramic reinforcement which is added to the molten matrix material by a mechanical stirrer. In this method, incorporation of the reinforcement into the melt and pouring of the composite slurry into the mould are carried out in a fully liquid state (i.e., above liquidus temperature of the matrix alloy). Different researches have shown that introduction and uniform dispersion of reinforcements into the liquid matrices is very difficult due to the poor wetting of ceramic particles by molten alloys [2, 21]. In order to overcome some of the drawbacks associated with the conventional stir casting techniques, semisolid agitation processes can be employed [2]. The benefits include reduced solidification shrinkage, lower tendency for hot tearing, suppression of segregation, settling or agglomeration and faster process cycles. These advantages are accompanied with lack of superheat (lower operating temperatures) as well as a lower latent heat which results in a longer die life together with a reduced chemical attack of the reinforcement by alloy, also a globular, non-dendritic structure of the solid phase which then explains the thixotropic behaviour of the material [2]. The sliding wear of the composite is a complex process involving not only mechanical but also thermal and chemical interaction between two surfaces in contact. The wear resistance of composite materials is of our present interest because of their potential tribological characteristics that are very well applicable in automobile such as IC engines components, brake disc etc. To fully utilize the potential of a A661/ceramic composition, the judicious selection of primary and secondary processing and heat treatment procedure becomes important. 2. EXPERIMENTAL PROCEDURE The materials studied here were based on Al661 alloy having the nominal chemical composition of.4si,.7fe,.24 Cu, Bal. Al was used as the matrix alloy. The composite was synthesized through a semisolid processing route using SiC particles with average sizes of 1, 5, 2 and 5 µm as reinforcing particles. The stir casting furnace is mounted on the floor and the temperature of the furnace is precisely measured and controlled in order to achieve sound quality composite. The process involved melting the alloy in the graphite crucible. The crucible was heated to about 68 C in a resistance furnace and after melting the aluminum, the mixture was stirred at 6 rpm using an impeller fabricated from graphite and driven by a variable ac motor. The stirring time were noted at 1 min after the addition of SiC during the process. The temperature of the furnace was gradually lowered until the melt reached a temperature in the liquid solid state (corresponding to.2 solid fraction) while stirring was continued. The casting was obtained by pouring composite slurry into steel die placed below the furnace. A continuous purge of nitrogen gas is used inside and outside of the crucible to minimize the oxidation of molten aluminum. The obtained casts were annealed at 53 C for 2 h, machined to samples of 15 mm length, 5 mm width and 1 mm thickness and cold rolled to different final reduction. Dry sliding wear tests were carried out using pinon-disc type wear tester at sliding velocity of.3 m/s within a load range of 1 to 4N (1kgf to 4kgf). Figure 1 shows Schematic diagram of the abrasion wear test. The disk was made of the steel hardened up to 63 HRC with a diameter of 5 mm and a thickness of 1 mm. The pin test sample dimensions were 6mm diameter and 25 mm length. Care should be taken to note that the test sample s end surfaces were flat and polished metallographically prior to testing. Conventional aluminium alloy polishing techniques were used to prepare the contact surfaces of the composite specimens for wear test. The procedure involves grinding of composite aluminium surfaces manually by 24, 32, 4, and 6 grit silicon carbide papers and then polishing them with 5, 1, &.5 µm alumina using low speed polishing machine. This preparation technique created considerable surface relief between hard and soft aluminium matrix. The polished surfaces were cleaned ultrasonically with acetone and methanol solutions. The counter face materials were also polished and cleaned ultrasonically before each wear test. The steel slider was polished using the above described procedure and all the tests were conducted at room temperature (21 C, relative humidity 55%). At a given load level, weight loss from the worn surfaces were found to increase linearly with sliding distance, except during the transient period at the beginning of the test. The wear rates were calculated from the slope of weight loss versus sliding distance curves determined at several applied load levels within the range of 1 to 4 N. The wear rates measured in weight units were then converted to volumetric wear rates. The experimental density of the composites was obtained by the Archimedes method of weighing composites first in air and then in water, while the theoretical density was calculated using the mixture rule according to the volume fraction of the SiC particles. Metallographic samples were prepared using standard metallographic techniques, etched with standard aluminum etching solutions and examined by a optical microscope to determine the distribution of the SiC PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No

3 THE EFFECT OF PRIMARY AND SECONDARY PROCESSING 819 Hardness (HRC) (а) 1 µm Aging Time (Min) 2 25 Fig. 2. Hardness curves for samples Solution heat treated at 57 C for 2 min and aged at 17 C. 1 µm particles. The volume fraction of SiC particles was measured by means of an image analyser system attached to the microscope. The distribution of the SiC particles within the matrix alloy was characterized by a distribution factor (DF) defined as DF = SD/Af in which Af is the mean value of the area fraction of the SiC particles measured on 1 fields of a sample and SD is its standard deviation. A nonuniform microscopic distribution of the reinforcing phase within a sample is reflected as a relatively high value of DF. 3. RESULTS AND DISCUSSION The aim of the present study was to provide preliminary information on the effect of primary and secondary processing on the microstructure and tribological properties of a SiC reinforced aluminum-based composite produced by the compocasting. The hardness curves obtained by the artificial aging procedures on the composite samples with 5% SiC is presented in Fig. 2. The strengthening mechanisms and wear behavior of Al Mg Si alloys after various working processes and heat treatments are of the interest. During the aging treatment after working and solution treatment, solute atoms form atomic clusters or G.P. zones, after which needle/lath-like precipitates nucleate and finally stable precipitates form. The main precipitates formed in Al Mg Si alloys matrix composites during aging treatment are listed in table. In general, the precipitation sequence of an Al Mg Si alloy depends on many parameters, among which (c) 1 µm Fig. 3. Optical micrograph of: (a) unreinforced Al alloy, as-cast Al 1% SiC, (c) the 9% cold rolled Al 1% SiC. the alloy composition is one of the most important factors. The Mg/Si ratio of the dominant phase in an Al Mg Si alloy is close to that of the alloy, while the heat treatment parameters (solution temperature and time, aging temperature and time) will not modify the sequence of the dominant precipitates. Microstructural characterization of the samples show that the fabrication of these composites via compocasting technique plus reduction (9%) during cold rolling process lead to reasonably uniform distribution of particles in the matrix and minimum clustering or agglomeration of the reinforcing phase and thus higher hardness and wear resistance. Typical optical microscope micrographs of the unreinforced Al alloy and the compocast SiC reinforced Al alloy composites are shown in Fig. 3. Precipitates characteristics in Al Mg Si alloys Precipitate type Spherical G.P. zones Atomic Clusters Plate-like G.P. zones Pre-β β'' Characteristics Fully coherent with Al matrix, designated as spherical G.P. zone Fine plates of a monolayer in thickness The structure is more similar to the Al matrix Streaks around the needles caused by its stress field in the Al matrix around it PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No

4 82 ALI MAZAHERY, MOHSEN OSTAD SHABANI Volume % Porosity % SiC 15% SiC 1% SiC Volume % SiC % cold rolled As-cast composite Fig. 4. The effect of the rolling process and SiC volume fraction on the porosity content of the composites Distance, cm Fig. 6. The SiC concentration as a function of the distance from the bottom of the mould (D). Distribution Factor Distribution Factor (a) 1 Volume % SiC Fig. 5. Distribution factor versus: (a) volume fraction of SiC, Reduction (%). The major reason for rolling the particulate metal matrix composites (PMMCs) is to close the pores and attain improved mechanical properties. Figure 4 shows the effect of the rolling process on the porosity content of the composites. It indicates that increasing amount of porosity is observed with increasing the volume fraction of SiC particulates. The decreased porosity of MMCs during rolling is due to the flow of the matrix alloy under the applied shear and compressive forces resulting in filling the voids. The increased rolling reduction provides easier flow of the matrix alloy and hence results in decreased porosity. It is noted that the application of the heavy reduction (9%) during cold rolling process results in approximately % volume fraction porosity. Deagglomeration phenomenon is the result of high deformation ratio applied to the aluminum matrix. Plastic flow of the aluminum matrix causes the SiC clusters to break and get separated from each other, resulting in a more uniform distribution of the particles in the matrix. During the solidification process of the composite slurries, the reinforcing particles are pushed to the interdendritic or intercellular regions and tend to segregate along the grain boundaries of matrix alloy. Figure 5 shows the gradual decrease in DF for the composites when the particle content and cold rolling reduction increased, indicating the improvement in the uniformity of the SiC particle distribution. These results can be attributed the occurrence of deagglomeration process in the cold rolled composites. A sedimentation experiment was conducted on the composites containing 1 vol % particles. Figure 6 shows the SiC concentration as a function of the distance from the bottom of the mould (D). The uniformity in distribution of particles within the sample is a microstructural feature which determines the in-service properties of particulate AMCs. A non- PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No

5 THE EFFECT OF PRIMARY AND SECONDARY PROCESSING (a) 2.5 Wear Rate, 1 12 m 3 /m Wear Rate, 1 12 m 3 /m Wear Rate, 1 12 m 3 /m % SiC 1% SiC 15% SiC % SiC 15% SiC homogeneous particle distribution in cast composites arises as a consequence of sedimentation (or flotation), agglomeration and segregation. The subject of particle distribution in particulate MMCs has been studied by several investigators either qualitatively or quantitatively. The macroscopic particle segregation due to gravity (settling) has also been studied both (c) 8 1% SiC 8 5% SiC 1% SiC 15% SiC Fig. 7. The effect of the rolling process on the wear of the composites under the applied load of: (a) 1 N, 2 N (c) 4 N. Wear Rate, 1 12 m 3 /m E+3 Unreiforced Al 1% SiC 5% SiC 15% SiC 2.E+3 3.E+3 Sliding distance, m 4.E+3 Fig. 8. Effect of the sliding distance on the wear rate of Al SiC composites under an applied load of 1 N. experimentally and theoretically, the latter of which generally involves the correlation of particle settling rate within the composite slurry with the Stocks low [29 33]. It is noted that the particle distribution in the cold rolled composites is much more uniform than the as-cast one. As can be seen, the lower parts of the ascast ingot contained a higher volume percent of particles than the upper parts, representing an uneven macroscopic particle distribution. Figure 7 shows the wear rate results of the as-cast and cold rolled composites. It is noted in Fig. 7 that the wear rate in all the samples increases marginally with applied load. The wear rate versus volume percentages Al SiC composites during wear test at an applied load of 1 N is shown in Fig. 8. It is observed that the addition of hard ceramic SiC particles increases the hardness of Al alloy. Figure 9 shows the hardness values for the unreinforced alloy and composites investigated in this study. Shabani emphasized that higher hardness of composite could be achieved by ceramic reinforced particulate (B 4 C) because B 4 C particle acts as an obstacle to the motion of dislocation. It is known that the wear loss is inversely proportional to the hardness of alloys. In case of unreinforced Al alloy, the depth of penetration is governed by the hardness of the specimen surface and applied load. But, in case of Al matrix composite, the depth of penetration of the harder asperities of hardened steel disk is primarily governed by the protruded hard ceramic reinforcement. Thus, the major portion of the applied load is carried by SiC particles. The role of the reinforcement particles is to support the contact stresses preventing high plastic deformations and abrasion between contact surfaces and hence reduce the amount of worn material. However, if the load exceeds a critical value, the particles will be fractured and comminuted, losing their role as load supporters [19]. The wear rate of the composite is found to be lower than that of the unreinforced alloy. The lowest value of PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No

6 822 ALI MAZAHERY, MOHSEN OSTAD SHABANI 12.5 (a) Hardness (HRC) Friction Coefficient, µ %15 SiC %1 SiC %5 SiC Al Volume % SiC Sliding Distance, m Fig. 9. Effect of the SiC content on the hardness of the composites. mass loss in wear test was distinct for Al 15 vol % SiC and the highest mass loss in wear test was for bare Al alloy. It has been demonstrated that the different primary processing techniques are capable of imparting totally different microstructural features and mechanical properties to a material of given composition. The microstructural and properties variation for a fixed metal/ceramic formulation can also be realized by varying the secondary processing parameters such as reduction percentage in the case of rolling. During the sliding, in fact a considerable fraction of energy is spent on overcoming the frictional force, which leads to heating of the contact surfaces. Initially, the asperities are stronger and sharper, and that is why frictional force and as a result frictional heating takes place at higher rate. After a certain period, because of the increase in flowability of the material on the specimen surface, slipping action is higher which results in reduction of frictional heating. The more possibility of adhesion between the counter surfaces leads to higher degree of friction. A number of studies have been engaged during the last twenty years on wear behaviour of Al based particulate reinforced composite. Reinforcement of Al based alloy with SiC or Al 2 O 3 are usually found to improve the wear resistance under both abrasion and lubricated sliding conditions. Y. Iwai et al. [22] found that both mild wear in the initial sliding distance and severe wear rate decreased with increasing volume fraction. Although the rate of change for the composites is smaller than that of the matrix, the wear rate of the matrix and the composites decrease with the sliding distance. It is clear that the unreinforced matrix alloy wore more rapidly than the reinforced composite materials. If the effective load on the individual particle increased above its flexural strength, the particles get fractured. Parts of the removed SiC particles are Friction Coefficient, µ Friction Coefficient, µ %15 SiC %5 SiC %1 SiC Sliding Distance, m entrapped between two partners, i.e. asperities of softer material of pin and asperities of harder material (hardened steel disk), possibly leading to three-body abrasion; then it will result in surface roughness between contacting surfaces and increase coefficient of friction. Figure 1 shows that the friction coefficients for composites containing SiC are higher than Al Sliding Distance, m (c) %15 SiC Al %1 SiC %5 SiC Fig. 1. Effect of the sliding distance on coefficient of friction of the composites under the applied load of: (a) 1 N, 2 N (c) 4 N. PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No

7 THE EFFECT OF PRIMARY AND SECONDARY PROCESSING (а) 1 µm Hardness (HRC) % SiC 1% SiC 15% SiC Fig. 12. The effect of the rolling process on the hardness of the composites. (c) 1 µm 1 µm Fig. 11. SEM worn surface micrographs of (a) unreinforced Al alloy, as cast Al 5% SiC (c) 9% cold rolled Al 5% SiC. the aluminum-based alloys while sliding under identical conditions. The higher coefficients of friction in the case of composites containing hard SiC particles are due to the formation of tribofilm at the interface between pin and disk. During sliding, frictional force acts between the counter surfaces, which caused frictional heating of them. While the counter surfaces are in relative motion, the frictional heating is continuous because of insufficient time for heat dissipation. It is reported that temperature rise is greater in the case of unreinforced alloy as compared to that in the composite irrespective of the applied load and surface conditions [16, 19]. Alpas and Zhang while investigating the wear of particle reinforced Metal Matrix Composites (MMC) under different applied load conditions identified three different wear regimes. At low load (regime I), the particles support the applied load in which the wear resistance of MMCs is better than Al alloy. At regime II, wear rates of MMCs and Al alloy were similar. At high load and transition to severe wear (regime III), the surface temperature exceeded the critical value. D.P. Mondal opinion was that the applied load affects the wear rate of alloy and composites significantly and is the most dominating factor controlling the wear behavior [23 27]. The wear surface of the unreinforced alloy under the applied load of 2 N is depicted in Fig. 11. The flow of materials along the sliding direction, generation of cavities due to delamination of surface materials and tearing of surface material is also noted in this figure. The cumulative volume loss increases with increasing applied normal load and the contact surface temperature increases as the applied load increases. P.K. Ghosh and S.C. Jain [28] found that the wear rate varies with normal load, which is an indicative of Archard s law and is significantly low in the case of composites. It is noted that the slider could penetrate and cut deeply into the surface and cause an extensive plastic deformation on the surface, resulting in a great amount of material loss. Worn surface of the as-cast and cold rolled 5 vol % composite at an applied load of 2 N is shown in Fig. 11. Worn surfaces of the composites were smoother with shallower grooves along the sliding direction. Therefore, it was reasonable that wear resistance of composites was higher than that of the unreinforced alloy. It indicates formation of continuous wear grooves, relatively smooth MML and some damaged regions. However, the degree of cracks formation on the wear surface is not much. The wear surface is characterized by the formation of parallel lips along the continuous groove marking. Unlike the worn surface of the unreinforced alloy, the number of scratches by abrasives or hard asperities was small. Figure 12 shows that the improvement in wear of the cold rolled composites which might be explained by the increase in the hardness of them. The increase PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No

8 824 ALI MAZAHERY, MOHSEN OSTAD SHABANI in the applied load leads to increase in the penetration of hard asperities of the counter surface to the softer pin surface, increase in micro cracking tendency of the subsurface and also increase in the deformation and fracture of asperities of the softer surface. 4. CONCLUSIONS In overall it can be concluded that composites exhibit better tribological properties than the unreinforced alloy. The friction coefficients for composites containing SiC are higher than the aluminum-based alloys while sliding under identical conditions. The increase in the applied load leads to increase in the penetration of hard asperities of the counter surface to the softer pin surface, increase in micro cracking tendency of the subsurface and also increase in the deformation and fracture of asperities of the softer surface. The micro structural studies revealed the more uniform distribution of the particles in the matrix system of the cold rolled samples. The lower parts of the ascast ingot contained a higher volume percent of particles than the upper parts, representing an uneven macroscopic particle distribution. Hardness and wear resistance of the composites are found to increase with the increase in rolling reduction. The porosity level of composites increased, since the contact surface area was increased. This is attributed to pore nucleation at the SiC particulate sites (porosity associated with individual particle) and to hindered liquid metal flow due to more particle clustering (porosity associated with the particle clusters). REFERENCES 1. Tseluikin, V.N., Prot. Met. Phys. Chem. Surf., 29, vol. 45, p Shabani, M.O. and Mazahery, A., Metall Mater. Trans. A, 212, vol. 43, p Barati, M. and Claytor, G., J. Mater. Sci., 27, vol. 39, p Shabani, M.O. and Mazahery, A., Trans. Indian Inst. Met., 212, vol. 65, p Pyzik, A.J. and Aksay, I.A., Proc. of the International Symposium on Advances in Processing of Ceramic and Metal Matrix Composites, New York, 1989, p Pyzik, A.J. and Beaman, D.R., J. Am. Ceram. Soc., 1995, vol. 78, p Rhee, S.K., J. Am. Ceram. Soc., 197, vol. 53, p Mazahery, A., Alizadeh, M., and Shabani, M.O., Trans. Indian Inst. Met., 212, vol. 65. p Vugt, L.V. and Froyen, L., J. Mater. Process Technol., 2, vol. 14, p Irons, G.A. and Owusu-Boahen, K., Metall Mater. Trans. B, 1995, vol. 26, p Gowri, S. and Samuel, F.H., Metall Trans. A, 1992, vol. 23, p Gupta, M., Lu, L., and Ang, S.E., J. Mater. Sci., 1997, vol. 32, p Karnezis, P.A., Durrant, G., and Cantor, B., Mater. Charact., 1998, vol. 4, p Mazahery, A. and Shabani, M.O., Trans. Indian Inst. Met., 212, vol. 65, p Quaak, C.J. and Kool, W.H., Mater. Sci. Eng. A, 1994, vol. 188, p Shabani, M.O. and Mazahery, A., Ceram. Int., 212, vol. 38, p Mazahery, A. and Shabani, M.O., JOM, 212, vol. 64, p Leon, C.A. and Drew, R.L., J. Mater. Sci., 2, vol. 35, p Davidson, A.M. and Regener, D., Compos. Sci. Technol., 2, vol. 6, p Mazahery, A. and Shabani, M.O., Compos. Part B, 212, vol. 43, p Fard, R.R. and Akhlaghi, F., J. Mater. Process Technol., 27, vols , p Shabani, M.O. and Mazahery, A., Mater. Tehnol., 212, vol. 46, p Moustafa, S.F., Badry, S.A., Sanad, A.M., and Kieback, B., Wear, 22, vol. 253, p Zhan, Y.Z. and Zhang, G., Mater. Lett., 23, vol. 57, p Mazahery, A. and Shabani, M.O., Ceram. Int., 212, vol. 38, p McKimpson, M.G. and Scott, T.E., Mater. Sci. Eng., 1989, vol. 17, p Ozdemir, I., Cocen, U., and Onel, K., Compos. Sci. Technol., 2, vol. 6, p Mazahery, A. and Shabani, M.O., J. Mater. Eng. Perform., 212, vol. 21, p Shabani, M.O. and Mazahery, A., JOM, 211, vol. 63, p Bauri, R. and Surappa, M.K., J. Mater. Process Technol., 29, vol. 29, p Mazahery, A. and Shabani, M.O., Powder Technology, 213, vol. 245, p Petalas, Y.G., Antonopoulos, C.G., Bountis, T.C., and Vrahatis, M.N., Phys. Lett., 29, vol. 373, p Tofigh, A.A. and Shabani, M.O., Ceram. Int., 213, vol. 39, p PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 5 No