Materials and Surface Engineering in Tribology

Jamal Takadoum

• Publisher: Wiley-ISTE
• Year: 2013
• Print ISBN: 9781848210677
• eBook ISBN: 9781118623060
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This title is designed to provide a clear and comprehensive overview of tribology. The book introduces the notion of a surface in tribology where a solid surface is described from topographical, structural, mechanical, and energetic perspectives. It also describes the principal techniques used to characterize and analyze surfaces. The title then discusses what may be called the fundamentals of tribology by introducing and describing the concepts of adhesion, friction, wear, and lubrication. The book focuses on the materials used in tribology, introducing the major classes of materials used, either in their bulk states or as coatings, including both protective layers and other coatings used for decorative purposes. Of especial importance to the tribology community are sections that provide the latest information on Nanotribology, Wear, Lubrication, and Wear-Corrosion: Tribocorrosion and Erosion-Corrosion.

Chapter 1

Surfaces

1.1. Introduction

The surface of a solid delimits its volume and defines the region where interactions with its environment occur.

When considering the structure of a crystalline material, the surface corresponds to a discontinuity in the periodic arrangement of atoms. The number of nearest neighbors (8 for a body-centered cubic lattice, 12 for a hexagonal close-packed or face-centered cubic lattice) is reduced for surface atoms, so that their vibrational states, inter-atomic separations and associated electronic states are very different from those of atoms within the solid’s interior [BOI 87] (see Figure 1.1).

Figure 1.1. Centered cubic lattice showing the number of nearest neighbors for an atom: a) within a solid; b) on its surface

This difference between surface atoms and volume (or bulk) atoms allows us to introduce the notions of an ideal surface and a realistic surface. An ideal surface is defined only in terms of the topmost atomic layers spanning a dimension of only several nanometers, whereas a realistic surface describes a region that extends further below the outer surface, down to depths of several microns to several tens of microns. The mechanical, physicochemical and structural properties of this region differ noticeably from those of the material’s volume as well as those of the ideal surface [PERR 87].

Because surface atoms possess a lower number of nearest neighbors, they are involved in a smaller number of bonds and thus experience an asymmetric force field. Indeed, these atoms interact only with other surface atoms and atoms situated within the solid’s interior. This results in a certain number of dangling bonds, directed towards the exterior of the solid, which allow the solid to interact with its environment through the establishment of bonds aiming to re-establish the surface atoms’ equilibrium.

The number of dangling bonds and the nature of their interactions with atoms or molecules in the environment depend on the surface atom coordination and therefore its crystallographic orientation. Each crystallographic plane (each grain in a polycrystalline material) will therefore possess a specific reactivity and physicochemical and mechanical properties different from those of other planes. Passivation and gas adsorption kinetics, speed of corrosion, hardness, Young’s modulus, surface energy and electron work function are all amongst the properties that depend on the surface in this way [BERA 83, OUD 73].

1.2. The surface state

A surface can be characterized by its mechanical, physicochemical, topographic or structural properties. The combination of these characteristics defines what we refer to as the surface state.

1.2.1. Structural state of a surface

During shaping or machining processes, the contact between a surface and a machining tool considerably modifies the crystalline structure of the superficial surface layers through mechanical and thermal stresses.

Figure 1.2 shows the cross-section of a surface presenting different zones or layers:

(i) The first zone is a contamination zone consisting of a layer of adsorbed gases such as water vapor, hydrocarbons and other atmospheric pollutants. This surface layer extends to a depth of a few nanometers.

(ii) The second zone is made up of products arising from the interaction with the environment and generally consists of oxides whose composition depends both on the base metal and the environment.

(iii) The third zone corresponds to a material structure that has been significantly “work-hardened” and where the crystalline matrix is essentially destroyed. This layer, often referred to as the Beilby layer, extends to a depth of about one micron.

(iv) The fourth zone is one which has been mechanically deformed from the accumulation of residual stresses. Its thickness ranges from several microns to several tens of microns.

(v) The fifth zone corresponds to the unmodified structure of the original material.

Figure 1.2. Visual ...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Foreword
• Preface
• Chapter 1: Surfaces
• Chapter 2: Tribology
• Chapter 3: Materials for Tribology
• Bibliography
• Index