The Physical Phenomena of Wettability in Particulate Reinforced Metal Matrix Composites

By
Dr Thoguluva Raghavan Vijayaram
BE (Mechanical Engineering, Madurai Kamaraj University, India),
ME (Metallurgical Engineering, Bharathiyar University, India),
PhD (Mechanical Engineering, Universiti Putra Malaysia, Malaysia),
Rector Grant Researcher in Metallurgy (Genoa University, Italy),
Chartered Engineer (M123412-3, IIE, Calcutta, India)
MIIF, MISTE, MIIPE, MIE (Calcutta, India)
Senior Lecturer in Manufacturing Engineering and Researcher in Metallurgy,
Department of Manufacturing Process and System, Faculty of Manufacturing Engineering, UTeM, Universiti Teknikal Malaysia Melaka, Ayer Keroh, 75450 Melaka, Malaysia.
E-mail: thoguluva@utem.edu.my

Composites technology requires interaction of several engineering disciplines such as structural design and analysis, materials science, mechanics of materials, mechanical engineering, manufacturing and materials process engineering. The tasks of composite materials research are to investigate the basic characteristics of the constituents, optimize the material for service conditions, develop effective and efficient fabrication procedures and understanding their effect on material properties and to determine material properties. An important task is the non-destructive evaluation of material integrity, structural reliability, durability assessment, flaw criticality, and life prediction. Various types of metal matrix composites are continuous fiber reinforced, dispersoid reinforced, monofilament reinforced, particulate reinforced, short fiber reinforced, whisker reinforced, hybrid composite, cermets, functionally graded aluminium base matrix composites, flake composites, and laminar composites.

In recent years, the interest in materials which have light weight, high resistance and hardness, are continuously displacing traditional engineering materials. Composite materials are continuously displacing traditional engineering materials because of their advantages of high stiffness and strength over homogeneous materials formulations. Especially metal matrix composites such as silicon carbide reinforced aluminium matrix, have been used at aerospace, automobile and ceramic industries.

The major methods to produce aluminium metal matrix composites are: stir casting: vortex mixing method, powder metallurgy, liquid metal infiltration, squeeze casting, rheocasting, and spray deposition technique. Liquid infiltration is a common process to produce metal infiltration, which involves a melt liquid infiltration into porous perform. However, the major problem for the production of these materials is to obtain the wetting of reinforcement by the liquid metal, which is very poor and is favored by strong chemistry bonding at the interface. The poor wetting is because of the presence of a film oxide at the surface of the aluminium. The wettability is a complex phenomenon that depends on factors such as geometry of interface, process temperature, soaking time, and it determines the quality of bonding among the systems. Research works show that the contact angles decrease with the increase of the metal liquid temperature, addition of alloy elements, such as magnesium, calcium, titanium or zirconia. An addition of silicon is assigned for the infiltration, because it affects the aluminium alloys fluidity positively, lowers the melting point and avoids the weak phase development.

Conventional stir casting technology has been employed for producing particulate reinforced metal matrix composites for decades. The casting methods and associated techniques used to fabricate composites based on aluminium alloys have been amply studied. The major problem in this technology is to obtain sufficient wetting of dispersoid by the liquid metal and to get a homogeneous dispersion of the ceramic particles.

In any type of the fabrication method used, wettability and distribution of the reinforcement material in the alloy matrix are among the main problems. Many methods have been proposed to overcome this situation. However ideas normally suitable for the preparation of materials and their use may not be suitable for different approaches. In general stir casting of metal matrix composites involves producing a melt of selected matrix material followed by the introduction of reinforcement material into the melt and the dispersion of the reinforcing material through stirring. Stirring is carried out vigorously to form a vortex where the reinforcing particles are introduced through the side of the vortex. The formation of the vortex will drag not only the reinforcement particles into the melt, but also all impurities which are formed on the surface of the melt. The vortex will also entrap air into the mould which is extremely difficult to remove as the viscosity of the slurry increase. In this approach of fabricating cast metal matrix composites, magnesium was used as a wetting agent and two stirring steps in which the composite slurry in the semi solid condition was applied in order to enhance wettability between the silicon carbide particles and matrix alloy. The emphasis was on the wettability and chemical reaction between the substances.

Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting or wettability is determined by a force balance between adhesive and cohesive forces. Wetting is important in the bonding or adherence of two materials. Wetting and the surface forces that control wetting are also responsible for other related effects, including capillary effects.

Wettabilty is defined as the extent to which a liquid will spread over a solid surface. Interfacial bonding is due to the adhesion between the reinforcement phase and the matrix. For adhesion to occur during the manufacture of a composite, the reinforcement and the matrix must be brought into an intimate contact. During a stage in composite manufacture, the matrix is often in a condition where it is capable of flowing towards the reinforcement and this behavior approximates to that of the flow of a liquid.

Many technological processes require control of liquid spreading over solid surfaces. When a drop is placed on a surface, it can completely wet, partially wet, or not wet the surface. By reducing the surface tension with surfactants, a non-wetting material can be made to become partially or completely wetting. A key concept in this contact is wettabilty. Once the matrix wets the reinforcement particle, and thus the matrix is in intimate contact with the reinforcement, which causes the bonding to occur.

Different types of bonding will occur and the type of bonding varies from system to system and it entirely depends on the details such as the presence of surface contaminants. The different types of bonding observed are mechanical bonding, electrostatic bonding, chemical bonding, and inter diffusion bonding. The bonding strength can be measured by conducting the tests like single particle test, bulk specimen test, and micro-indention test. Poor wettability of most ceramic particulates with the molten metals is a major barrier to processing of these particulate reinforced metal matrix composites by liquid metallurgy route. The characterization and enhancement of wettability is therefore, of central importance to successful composite processing. Wettability is shown in Figure-1 and it is customarily represented in terms of a contact angle.

Figure-1 A sessile drop to the left is an example of poor wetting (q>90°) and the sessile drop to the right is an example of good wetting (q<90°)

It is defined by the Young-Dupre equation which is expressed as follows:

Where γSV = Solid/Vapor surface energy, γSL = Solid/Liquid surface energy and γLV = Liquid/Vapor surface energy.

The sessile drop technique is a method used for the characterization of solid surface energies, and in some cases, aspects of liquid surface energies. The main premise of the method is that by placing a droplet of liquid with a known surface energy, the shape of the drop, specifically the contact angle, and the known surface energy of the liquid are the parameters which can be used to calculate the surface energy of the solid sample. The liquid used for such experiments is referred to as the probe liquid, and the use of several different probe liquids is required.

The contact angle is defined as the angle made by the intersection of the liquid/solid interface and the liquid/air interface. It can be alternately described as the angle between solid sample’s surface and the tangent of the droplet’s ovate shape at the edge of the droplet. A high contact angle indicates a low solid surface energy or chemical affinity. This is also referred to as a low degree of wetting. A low contact angle indicates a high solid surface energy or chemical affinity, and a high or sometimes complete degree of wetting. For example, a contact angle of zero degrees will occur when the droplet has turned into a flat puddle; this is called complete wetting. Figure-2 shown below illustrates the degrees of wetting and corressponding contact angles.

Figure-2 A sketch of three degrees of wetting and the corresponding contact angles

The wetting behavior of a liquid on a solid can be characterized by the wetting or contact angle that is formed between the liquid and the solid substrate. A “sessile drop” is a continuous drop of liquid on a flat, solid surface under steady-state conditions. To neglect the effects of gravity, the gravitational forces should be small compared to the surface tension of the drop. If this condition is satisfied, the drop will approach a hemispherical shape which represents its smallest area and lowest surface free energy. The sessile drop is placed on the solid substrate and the angle between the solid surface and the tangent to the liquid surface at the contact point is measured. This is known as the contact angle or wetting angle. The contact angle can vary between 0 and 180° and is a measure of the extent of wetting. The conditions of good wetting (q<90°) and partial wetting (q>90°) are illustrated in Figure-1. Complete wetting (also referred to as spreading) is obtained at an angle of 0° and complete non-wetting occurs at an angle of 180°. The contact angle is the vector sum of the interfacial surface energies between the solid/liquid (γSL), liquid/vapor (γLV), and solid/vapor (γSV) phases. Young’s equation represents a steady-state condition for a solid/liquid interface in stable or metastable thermodynamic equilibrium.

Temperature changes have been shown to affect the contact angle of many different systems. The temperature effect, in most cases, can be explained by a reaction at the liquid/solid interface. Thermally activated reactions can occur because many systems are not at chemical equilibrium. The reactions that contribute to wetting (decrease of the contact angle) are those that increase the driving force for wetting (γSV - γSL), which is acting at the surface of the liquid drop and the solid substrate. The reactions that contribute to the driving force for wetting are the ones in which the composition of the substrate changes by dissolution of a component of the liquid. On the contrary, if the reaction results in a change of the liquid’s composition by dissolution of the solid substrate, but with no change in the composition of the substrate, there is no contribution to the driving force for wetting.

As mentioned above, if the solid substrate is an active participant in the reaction, the free energy of the outer surface of the liquid drop will contribute to the driving force for wetting. As the drop expands on the substrate, the perimeter remains in contact with unreacted solid and thus the reaction continues to contribute to the driving force for wetting. Examination of phase diagrams representing the interaction between the constituents of the liquid and solid surfaces can help to predict the wetting behavior of a system.

Moreover, measurement of wettability of powders consisting of irregular and polysized particles is extremely difficult. Several techniques have been proposed in the thermodynamic literature to measure wettabilty, however, these techniques have been applied mostly to non-metallic liquids and their application to metal ceramic systems with reference to pressure casting of composites has been quite limited. The engineering approaches to increasing wettability can be broadly classified into two categories. One method is the surface modification of the reinforcement phase and the other technique is melt treatment. Surface modifications of reinforcements include heat treatment of the particulates to determine surface gas desorbtion, surface oxidation and coating of particles with materials that react with the matrix. Melt treatment is usually done either to promote reactivity between the metal and the particulate surface. The wetting reaction must be constrained to prevent reinforcement degradation during the fabrication of subsequent utilization.

About the Author
Dr.Thoguluva Raghavan Vijayaram is currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published research papers in reputed International journals, National journals, International conference proceedings, on-line journals and in the broadsheets. He has a wide range of work experience, both in academics and as well as in industry, consultancy, teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering. His special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations. He is a reviewer of several international journals in UK which includes Elsevier Science Direct.

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