ASPÈCTOS METODOLOGICOS DE LA INVESTIGACION

When considering the reinforcement phase, materials that introduce desirable properties, such as high hardness, high thermal conductivity, etc., are often selected. For this reason, materials with extreme properties such as diamond are often attractive. However, naturally occurring diamond is subject to some degree of property variation, and more importantly is extremely high cost. Therefore, the advent of reliable methods for synthetic diamond production was a vital enabling technology for the advent of diamond- based MMCs.

26 Originally, the word ‘diamond’ is a derivative from the Greek word ‘adámas’, which means unbreakable or adamant in the English language. It is usually identified as the hardest material, with its hardness located at the top of Moh’s hardness scale (based on the relative measurement of materials’ ability to scratch each other), equalling 10 on the scale. Although some other mechanical properties of the diamond are not as good as its hardness (for example, diamond tends to be entirely brittle during fracture [60]) the Young’s modulus is also very high, about 1000-1075 GPa [60], [61]. These properties make diamond as a theoretical engineering material potentially suitable for a wide range of applications, although for cost and processing reasons the actual applications are more limited. For example, one area of use is where there is a demand for high hardness and high abrasive resistance such as grinding wheels and cutting tools and so on. Diamond applications in different fields can be simply shown in Figure (2-8).

Furthermore, diamond has particular optical properties such as its unique luminescence; its ability to absorb and reflect light with several colours. These give it a particular visual appearance which make it attractive for jewellery [62]. Although mechanically resistant, diamond is thermally degradable into graphite or oxidized form at elevated temperature, around 700℃ and above, especially in the presence of oxygen [32], [60].

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Figure (2-8): The general areas of application for diamond applications [61].

There are two main classes of diamond, which are natural and synthetic. Natural diamond forms naturally from carbon in the mantle of the earth under very high pressure and temperatures [62], while the synthetic diamond is a man-made product which is produced during exposure of carbon to similarly extreme conditions of temperatures and pressure. Diamond can be classified depending on impurities into two main types which are type I and type II. Then these types are also classified into sub-types according to the co- existence of atoms of impurities inside the crystal structure [63] as shown in Figure (2-9) below, which represents the two dimensional drawing of tetrahedral bonding of carbon

Elecroresistance Thermal conductivity

28 atoms in the diamond structure. Where either nitrogen or boron atoms replace the C atoms the diamond types are delineated [63].

Figure (2-9): Diamond types and classification depending on impurities occupying carbon atom sites [63].

The existence of defects and impurities in the crystal lattice of diamond is a significant factor in the change of the diamond colour as shown in Figure (2-10). Therefore, many attempts have been made to recognize the diamond type from observation of its colour [63]. But these efforts have not succeeded, because the colour of diamond is derived from the elements present, while the position of these atoms relative to the carbon atoms determines the diamond type.

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Figure (2-10) : Colour variety of diamonds depending on impurities [63].

Manufacturing of synthetic diamond started in the late 1950s when the first successful synthetic diamond was made [60]. The manufacturing processes of synthetic diamond can be divided into two main production technologies which are the High Pressure- High Temperature process (HPHT) and Chemical Vapour Deposition (CVD) [64].

Diamond production was started by Swedish and American scientists using HPHT. In this technique, a carbon substrate is exposed to temperature of about 1300 – 1800°C and 50 to 70 kbar pressure with some molten metals as a catalyst to obtain an effective growth of diamond [65]. Suitable catalysts include iron, nickel [60], FeCo, FeNi or Al, Ti, Zr for getting nitrogen free diamond [65]. The HPHT process, which is schematically shown in Figure (2-11 a), is considered quite expensive because it requires high pressure and temperature, which makes the equipment costly.

In contrast the CVD technique Figure (2-11 B) requires less demanding conditions. Typically in this method CH4 gas is passed through a plasma at about 10-200 Torr pressure and 700 -1000°C temperature [65]. In CVD the diamond produced is often polycrystalline and much purer than the HPHT diamond because the latter often contains some inclusion or residual metal from the metal catalysis as shown in Figure (2-11 C). Another important aspect to mention is that both techniques have to use a diamond seed to grow a new diamond from graphite as shown in Figure (2-11 D) [65].

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Figure (2-11) : A, B schematic drawings of HPHT and CVD techniques, respectively, C- inclusion and metallic resides in HPHT diamond, and D- growth of diamond on seed plates [65].

A - B -

D -

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Chapter 3 Metal Matrix-Diamond Composites (MMDCs)

Should a diamond-reinforced MMC be desired, the choice of which metal to select as the matrix remains. There are many metals which have been used as matrices for MMCs in general, including aluminium (Al), titanium (Ti), magnesium (Mg), copper (Cu), silver (Ag), lead (Pb), iron (Fe), zinc (Zn), tin (Sn) and nickel (Ni) [12]. These metals have been used with different kinds of reinforcement. Especially, silver, copper and aluminium as a matrix are reported where diamond is used as reinforcement phase [66]. This is particularly the case for thermal management applications such as high heat flux conductors and heat sinks in electronic devices [67], [68]. The selection of these metals has evidently been made from the point of view of maximizing the overall conductivity, with the ease of processing a secondary consideration.

These classes of composites are attractive for use with microelectronics and semiconductors because of their exceptional thermal properties (their thermal conductivity reaches up to 900 W m-1 K-1 and they have very low thermal expansion [69]). There are many studies which have dealt with these elements as metal matrices with diamond particle reinforcement for thermal applications, and some of these studies have measured their mechanical properties, which will be discussed in the sections below.