In the automotive industry, the constant enhancement of the final performance is related to the use of top-of-the-line materials. The better the material’s response, the greater the improvement in the final part’s performance. Thus, automotive and other demanding industries have researched to develop in situ metal matrix composites (MMCs) [1]. MMCs are widely used in automotive parts due to the variety of its properties [2,3]. The artificial introduction of rigid particles (or fibers) in a ductile metal or alloy matrix can improve the final material’s properties. In addition, powder metallurgy shows remarkable advantages in production parts and components from economic and as well as environmental point of view. The reduced granulometry of the reinforcement particles makes these materials (named discontinuous reinforced materials) more cost-effective and valuable in production [4].
The tribological, thermal, and mechanical properties are critical when the material’s end-use is in the internal combustion engine. The current target of the compression piston ring investigation is improving those properties. Due to its primary role (the combustion chamber sealing), the compression ring functional surface is continuously under hard tribological and thermal conditions.
The conventional solution comprises a ceramic coating layer of a wear-resistant chemical compound deposited through a physical vapor deposition technique. However, the surface wear is not uniform along the whole surface and other relevant characteristics, such as thermal conductivity, are unsuitable for transferring the heat from the combustion chamber. According to the demanding performance, the most recent approach is to develop a multifunctional surface,
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through the incorporation on the radial surface cavities of different composite materials, such as using a wear protective material in a confined surface area [5].
Several MMCs under development allow the simultaneous improvement of the tribological and thermal performances. Copper-based composites have been selected due to their high thermal conductivity, good ductility, high wear resistance, and low cost [6]. Copper is used as a metal matrix in composites applied in plane bearings for automobiles, drive shafts and roller bearings, spherical bushings for automotive transmissions, and overhead railways for dams and flood gates.
An advantage of producing new ceramic reinforced - metal matrix composites is combining the good mechanical properties of the metals (such as ductility and toughness) with the high strength and modulus of the ceramic reinforcement. The most common reinforcement materials for those applications are graphite particles, hBN, TiC, SiC, and diamond [7–9]. Diamond offers a group of characteristics: excellent thermal conductivity, great hardness, high wear resistance associated with a low coefficient of friction (COF), and good chemical stability [10]. Micro and nanodiamond particles are embedded in matrix support in wear protection or abrasive applications.
The diamond-reinforced copper matrix composite forms a powder composite of a ductile metal shell with a wear-resistant core. The excellent thermal stability represents a fundamental property of diamond particles. However, carbon tends to degrade at high temperatures, with its conversion into a more thermodynamically stable phase (graphite) [8,11–14]. Some authors reported the surface graphitization of diamond particles in vacuum for temperatures is in the range 700C-1400C [15–17]. Most recently, Qian et al. [13] investigated the graphitization of diamond powders with sizes from 5 nm to 40 µm, at high pressure and temperature conditions and concluded that it strongly depends on diamond particle size. This effect occurs due to the large surface-to-volume ratio and high thermal conductivity of nanodiamonds. The onset temperature for nanodiamond graphitization was reported to be 666 C (both in an inert gas at atmospheric pressure), while bulk diamond begins to graphitize above 1526 C [18].
The manufacturing technology and processing techniques are limited to preserving these effective characteristics of the diamond particles. This constraint defines the upper limit to be implemented during manufacturing to avoid modifying the crystalline diamond structure. Moreover, an effective wetting of the dispersed reinforcement to the matrix prevents the failure of the composite. Conversely, particle clusters may generate high stress in those regions.
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Hot-pressing has been shown as an effective and accurate sintering method [19–23], allowing the production of near-net-shaped parts from powders. The hot-pressing is related to high compression rates at high pressures. The mixed powder is simultaneously heated and pressed, producing a final part with enhanced physical properties. The sintering process occurs at 66-75%
of the matrix melting point. The area of contact surface increases in proportion to the pressure applied. The applied external force increases the contact area and changes the shape of particles.
The consequence of growth in the contact surface is raising the product’s strength. The cohesion of particles in a powder pressing is caused by mechanical contact and interlocking among the surface ridges and irregularities of the particles. The temperature increases decrease the amount of pressure necessary for compacting the powder.
The main advantages of hot-pressing are to produce a material with a density up to 95% of the theoretical density and to reach properties close to the solid materials. Greater density is achieved with slower pressing resulting in enhanced final properties.
Few studies use exclusively diamond-reinforced Cu matrix composites without including a third element [24–27]. Some researchers reported poor interfacial bonding between Cu and diamond particles [28–33]. Some strategies to overcome this constraint have been proposed such as the coating the diamond particles or alloying of the metal matrix material. In both approaches the main purpose is the introduction of a carbide forming element (Ti [30,34,35], Cr [31,36–38]
or Mo [28,39]). The result is the formation of an interfacial carbide layer at the surface of the diamond particles, physically separating the particles from the metal matrix material. However, in applications where the thermal resistance is determinant, the presence of an interlayer and the interlayer’s properties will influence the results. In this case, the metal matrix alloying could be more effective.
To achieve an enhanced thermal conductivity, several authors studied the alloying of the Cu matrix using boron as the third element [32]. However, the interfacial bonding for alloyed metal matrix composites is not fully understood, and the interlayer’s characterization is lacking.
From the standpoint of the leading project, the main goal was to develop, through powder metallurgy technology, a multi-material surface, as illustrated in Figure 7.1. In an earlier proposal [5] illustrated in Figure 7.1a), the composite material (for tribological purposes) would be applied in a confined and isolated bore. In the most recent design represented in Figure 7.1b), the
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composite material is introduced in the textured cavities. The multi-material surface comprehends multiple regions strategically distributed along the piston ring surface, with the introduced material surrounding rectangular pillars.
Cunha et al. [40,41] explored the combination of a diamond-reinforced Cu-based alloy and a textured SS substrate with a similar distribution for tribological purposes. These authors were focused on the composite performance using different diamond particle sizes with distinct sintering techniques.
The aim of this study was to improve the surface properties of a 410 stainless steel (from now on referred to as 410 SS) piston ring by a CuCoBe-diamond composite.
In the first stage of this work, different hot-pressing parameters were used for composite optimization (mainly physical and metallurgical properties). Then, based on the results obtained, a bi-material (multifunctional surface) piston ring was produced by hot-pressing, consisting of a partial reinforcement of a previously laser-textured 410 SS by the CuCoBe-diamond composites.
Different texturing densities (surface ratios between the 410 SS and the composite) and arrangements were produced and analyzed. The friction results and the wear resistance of the developed samples are assessed and discussed.
Figure 7.1 Illustration of two different approaches for the production of piston ring multifunctional surfaces: a) early design with the proposal of the piston ring surface reinforcement and b) adapted design to the hot-pressing
manufacturing technique (adapted from [5]).
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The m is to develop different materials for distinct purposes, such as heat conduction or friction reduction, by the same methodology.