Nanocoatings for engine application

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Abstract

In this paper, a review of engineering coating for engine application is presented. Issues relating to dimensional stability, tribological properties, lubrication, coefficient of friction, hot hardness, amenability for honing, surface roughness and topography, residual stress, adherence, damage tolerance and resistance, pores density and conditions and cost performance are discussed. There exist advantages and limitations of conventional materials systems and techniques such as chemical-vapor-deposited diamond-like carbon (DLC) coating, plasma sprayed metal matrix composite coating, tribologically functional ceramic coatings, etc. Nano-grains of a crystalline phase hold promise to solve several such problems present in conventional coatings. In addition, surface-related problems are addressed for high performance engines and hydrogen powered automotive engines.

Introduction

Depleting fossil fuel resources, economic competitiveness and environmental concerns has compelled to explore newer avenues to improve efficiency of automotive engines. Various techniques have been adapted to achieve this goal. By allowing the engine to operate at higher temperature with reduced external cooling (heat removal), the fuel efficiency can be improved significantly. There is a need to address several materials related issues such as thermal distortion, high temperature oxidation, creep, etc. Other methods to improve fuel efficiency are to use lightweight material to reduce load, reduce heat losses due to exhaust and conduction through engine body and to reduce frictional losses. Reduction in the weight of engines is a key factor in improving the fuel efficiency. The use of lightweight materials has become more prevalent as car manufacturers strive to reduce vehicle weight in order to improve performance, lower fuel and oil consumption, and to reduce emissions [1]. Most manufacturers have replaced cast iron (density=7.8 g/cm3) engine blocks with lightweight and low-cost aluminum-silicon (density=2.79 g/cm3) crankcases. Several Al-based alloys and metal-matrix composites, such as A319Al, A356Al, A390Al and A360Al, are in use. However, inadequate wear resistance and low seizure loads have prevented their direct usage in the cylinder bores. The cylinder bores of these aluminum alloy blocks are usually made of cast iron liners because of their good operating characteristics such as wear resistance. These liners need to have a specific wall thickness, which results in a relatively large web width between the individual cylinder-bores, and increases the dimensions and weight of the engine. Moreover, mechanical friction is of another concern that needs to be addressed. Piston system is a major contributor to engine friction [2]. The cylinder bore/piston and piston ring friction constitute nearly all of the piston system's friction losses [2]. A major portion of oil consumption arises from bore distortion and poor piston ring sealing resulting from ring and bore wear. Aluminum exhibits a transition from mild to severe wear when the nominal contact stress exceeds a threshold value [3]. Presence of reinforcement particles does prevent such transition until higher threshold values. Such a situation can arise due to following factors: (1) start of ignition where oil has not spread over entire surface; (2) bore distortion. Thus, to continue using aluminum alloy engine blocks (due to lighter weight) and to improve wear resistance of the engine bore surface several techniques have been explored. These techniques are dedicated to form new composite and/or monolithic coating on the bore surface, such as, use of atmosphere plasma sprayed coating (APS) [4], [5], high velocity oxy-fuel (HVOF) coating [5], [6] and use of costlier and harder substrate materials [7], [8], etc. However, in case of automotive engine by obviating the need for liners, the engine dimension can be significantly reduced. It is estimated that direct weight savings of about 1 kg per engine can be easily achieved [8], [9]. Also, elimination of liners allows reduction in the overall dimension of engine [5], [8], [9]. Every kilogram reduction of payload is important for improvement in fuel efficiency. Reduction of about 110 kg in a typical automobile of weight 1100 kg will improve fuel economy by 7% [10]. In the lifetime of a car this reduction of engine weight is significant. Application of newer technology and/or materials is being explored to achieve this goal. By employing nanomaterial much of this objective can be achieved. Nanoscale materials have received much attention in recent years due to their outstanding properties compared to those of micron-size counterparts. The nano-world possess so much potential that Nobel laureate Dr. Richard Smalley envisions deploying nanotechnology to solve the energy problem of the tomorrow's world [11]. Employment of nanotechnology in current and future automotive, aero and other engines will go long way in solving energy crisis.

The defect density in nanoscale materials is very high, but not high enough as in amorphous. As depicted in Fig. 1, Hall–Petch relation (Hardness for a polycrystal with average grain diameter d, Hd=Hardness of single crystal, H0+kd−1/2) predicts increase of hardness and flow stress as the grain size decreases. However, as the grain size is very small (in the range of 100 nm), the deformation mechanism changes from dislocation controlled slip to grain boundary sliding increasing plasticity at the same time. When the grain size further reduces almost to become amorphous, the material behaves in visco-elastic manner. This provides a global maximum in properties such as hardness, flow stress, toughness, ductility and thermal insulation (because the conductivity of nanoscale material is much less in certain metallic system such as aluminum due to phonon scattering by high defect density) when the grain size is in nanoscale. It is, therefore, envisioned to apply nanocoating to solve the imminent problems in engines for various applications. Understanding of the general coating characteristics is necessary and is presented herein.

Section snippets

Desired characteristics of engine coating

Coatings, particularly nanocoating can help to improve performance and life of automotive engine. Higher efficiency is realized from various aspects of coatings as schematically presented in Fig. 2. In this section, desired characteristics of coatings are discussed briefly.

Coatings for engine and other automotive power systems

Cr electroplating is regularly employed to coat the piston rings in engine. Different types of ferrous-based powders, containing C, Si, Sn, Ni, Cr, Mo, Cu, Ti, V and B, etc., are also employed to coat Al alloys for diesel engine applications. APS and Laser Surface Engineering (LSE) have been explored for such coatings [4], [5], [8], [9]. Since diesel engines are exposed to sulfur containing material, corrosion resistance in sulfuric acid is a standard test for such a coating and usually

Nanocoatings in engine applications

Design for coatings with an amorphous matrix of DLC and metal nano-grains of a crystalline phase to be embedded in this matrix are being investigated [33]. These coatings have low residual stress, high hardness, and high toughness and exhibit very good tribological properties.

The nano zirconia powder (<50 nm), can be used to coat various substrate by atmospheric plasma spray [29]. Even though there is some degree of grain growth during the plasma spray, usually the nano dimension is retained.

Laser assisted reaction induced nanocoating

The ongoing efforts by the authors to synthesize a laser induced iron oxide reaction coating on aluminum alloy have shown promise in LSE for engine applications. The following section deals with such effort. Objective of this research project is to enhance the surface-related properties of aluminum alloys by synthesis of laser induced reaction nanocomposite coating suitable for auto engine applications. In this study, a popular casting alloy A319Al is chosen. A319Al [5.5–6.5 wt.% Si, 3.0–4.0

Approach and progress

A319Al can be surface melted and rapidly solidified using a high power laser. The high cooling rate causes formation of very fine and uniform microstructure in the laser-modified layer. The cellular dendrite structure with soft aluminum cells and intercellular hard Si phases provide a microstructure conducive to formation of micro-channel and thereby more effective lubrications [59]. The entire coating thickness (∼700 μm) was uniform both in microstructure and mechanical properties [54]. The

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