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Surface Characterization chap01.qxd 6/1/01 5:52 PM Page 1 © 2001 ASM International. All Rights Reserved. www.asminternational.org Surface Wear: Analysis, Treatment, and Prevention (#06034G) CHAPTER 1 Surface Characterization THE SURFACES OF A SOLID are the free bounding faces forming interfaces with the surrounding environment. The bulk of the material constitutes the bulk phase, while the free bounding face is known as the surface phase. The formation of a solid surface from liquid is accompanied by heat extraction at the freezing temperature. The latent heat of fusion is the quantity of heat to be extracted for liquid to transform to solid. The sur- face of the solid, or the surface phase, retains sufficient free energy in order to remain in equilibrium with its surroundings. The microstructural features on the solidified surface depend on composition and rate of cool- ing. The subsequent forming and thermal processes alter the surface mor- phology. Finally, the roughness of the surface or surface texture is depend- ent on the forming and finishing processes. The tendency for surface free energy of solids to decrease leads to an adsorption of molecules from the interfacing environment at the surface. The adsorbed layer is probably not more than a single molecule in thick- ness due to the rapid fall in intermolecular forces with distance. The nature of bonding may be physical or chemical and, accordingly, is termed van der Waal adsorption or chemisorption. The adsorbed product on the surface may be quite stable (e.g., the self-replenishing layer of chromium oxide in stainless steels) or unstable (e.g., loose oxide scale on mild steel), thus affording protection to the surface or leading to loss in material from the surface. The loss in material due to interaction with the environment has been termed wear. The surface properties can be altered suitably in order to reduce surface wear. The energy, morphology, and composition of the surface phase play a significant role in surface wear. Surface hardness has been widely used as a rough indicator of resistance to wear. While the intrinsic properties of the material mainly control the behavior of the bulk phase, the surface characteristics of the material to a large extent determine wear of the sur- face phase. chap01.qxd 6/1/01 5:52 PM Page 2 © 2001 ASM International. All Rights Reserved. www.asminternational.org Surface Wear: Analysis, Treatment, and Prevention (#06034G) 2 / Surface Wear: Analysis, Treatment, and Prevention Surface Energy An important quality of a material is its surface free energy at different conditions and temperatures. The amount of free energy at the surface of a material depends on various factors such as crystal structure, alloy con- tent, temperature, and interfacing phases. The surface of liquid or solid has excess free energy, a result of which causes a tendency for free energy to decrease in order to attain the stable state. In the case of liquid, the state of strain on the surface is known as surface tension. The surface free en- ergy of solid, depending on the interfacing environment, can be that γ γ between solid-vapor ( sv) or solid-liquid ( sl). The grain boundary, stack- ing fault, and twin boundary represent solid-solid interfaces. The free energy at these interfaces is the surface energy value expressed for grain γ γ γ boundaries ( gb), stacking faults ( sf), and twin boundaries ( tb). Beyond free energy, residual stresses develop at the surface during deformation, thermal shrinkage, and transformation processes. The effect of residual stress on wear depends on the type and magnitude of the stress. Free Energy at the Surface The free energy at the surface of a solid depends on the solid itself and the interfacing material. The interfacing phase may be liquid, vapor, or solid. Accordingly, the interfacial energy is expressed as the free energy of γ γ γ solid-liquid ( sl), solid-vapor ( sv), and solid-solid ( ss) interfaces, respec- tively. γ Free Energy at Solid-Liquid Interface ( sl). The total free energy change (∆F) in creating a new solid surface from the liquid is the sum of the decrease in volume free energy and increase in surface free energy (Ref 1): 4 ¢F ϭ πr3¢F ϩ 4πr2g (Eq 1.1) 3 v sl where 4⁄3 πr3 is the volume of a spherical embryo of volume free energy ∆ π 2 γ Fv,4 r is the surface area of the sphere, and sl is the surface free en- ergy. When the embryo reaches the critical size of r = r*, the stable nucleus is in equilibrium with the liquid, and the change in free energy with respect to nucleus size is zero, or otherwise (Ref 1): 4 d πr3¢F ϩ 4πr2g d ¢F a3 v slb 1 2 ϭ ϭ 0 dr dr π 2¢ ϩ π ϭ 4 r* Fv 8 r*gsl 0 (Eq 1.2) Ϫ¢ Fvr* g ϭ sl 2 chap01.qxd 6/1/01 5:52 PM Page 3 © 2001 ASM International. All Rights Reserved. www.asminternational.org Surface Wear: Analysis, Treatment, and Prevention (#06034G) Surface Characterization / 3 γ γ sv sv Ω Vapors Vapor Grain A Grain B Solid Solid γ gb γ γ Ω Fig. 1.1 Free energy at surface ( sv) and grain boundary interface ( gb). s, dehedral angle ∆ ∆ The volume free energy, Fv, is related to latent heat of fusion, H,the ∆ freezing temperature, Tm, and the degree of undercooling, T, as follows: (F ϭ ¢H ؒ ¢T T (Eq 1.3¢ v > m ∆ Substituting Fv from Eq 1.2 in Eq 1.3: *g ϭ ¢H ؒ ¢T T ؒ 2r sl > m The surface free energy at solid-liquid interface increases with increasing degree of undercooling and latent heat of fusion. Some typical values are given in Table 1.1. Solid-Solid and Solid-Vapor Interfacial Free Energy. The solid-vapor γ surface energy sv at the free surface is the summation of energy vectors γ due to vapor/solid interface and that of solid-solid at grain boundary ( gb), γ γ stacking fault ( sf), and twin boundary ( tb) (Fig. 1.1). Measurement of Free Surface Energy Some of the important techniques for measurement of free surface ener- gy of solid materials are described here. The zero-creep method consists of determining free energy of the sur- face from the stress that just fails to produce creep in a wire loaded at ele- vated temperature. Simultaneously, the grain boundary free energy can be found from the dihedral angle measurement of thermally etched grain boundary grooves of the same specimen (Ref 2). γ Principle (Ref 2, 3). The equation correlating surface free energy sv, γ the grain boundary free energy gb, and the load for zero creep W0 is as follows: n W ϭ πr g Ϫ g r 0 c sv gb a l bd (Eq 1.4) πrg ϭ W ϩ πr2g n l sv 0 gb1 > 2 chap01.qxd 6/1/01 5:52 PM Page 4 © 2001 ASM International. All Rights Reserved. www.asminternational.org Surface Wear: Analysis, Treatment, and Prevention (#06034G) 4 / Surface Wear: Analysis, Treatment, and Prevention where r is the radius of the wire, and n/l the average number of grains per unit length. At zero creep, the free energy on any section of the surface of the wire is equal to energy due to grain boundaries plus the load at zero creep. According to Smith (Ref 3), after prolonged annealing (i.e., at equi- Ω librium), the dihedral angle ( s) is a function of the surface and grain boundaries involved. At the equilibrium configuration (Fig. 1.1) the dihe- dral angle is related to surface and grain boundary energies by the follow- ing equation: g ϭ 2 g cos ⍀ 2 (Eq 1.5) gb sv 1 s> 2 γ Substituting gb from Eq 1.4 by Eq 1.5: W g ϭ 0 sv πr 1 Ϫ 2 n l rcos ⍀ 2 (Eq 1.6) 3 1 > 2 1 s> 24 The load at zero creep, average number of grains (n) present in the section, Ω and the dihedral angle ( s) between the grains are required to be deter- γ mined (Eq 1.6) in order to find the free energy at the surface ( sv). Method (Ref 2). In a thin wire (0.127 mm or 0.005 in., diameter) test specimen, several knots are made at 50 mm (2.0 in.) intervals. Each knot serves as a gage marker, and the weight of the knot acts as applied load. A large knot of the same wire serves the purpose of the rest of the load (Fig. 1.2). The total load to be applied is estimated from the surface tension of the liquid metal. The assembly is preannealed at or near test temperature, after which the initial gage measurement is carried out. The whole assembly is heated in a controlled-atmosphere furnace. During the creep anneal, the added weight has a tendency to allow the wire to elongate, while the surface free energy tends to shorten the wire. At the zero-creep stage, the tensile loads balanc- ing the opposing surface forces lead to neither elongation nor contraction of the wire. The gage length measurements are made on each segment before and after creep by a vertical measurement microscope, which is capable of measuring ±0.00127 mm (50 µin.). The number of grains per unit length is counted and the average value of (n/l) found. The strain for each segment and effective load are measured at midpoints of the segments (Fig. 1.2). The balance load for zero creep, W0, is found graphically from the least-square line of the plot of load versus strain as shown in Fig.
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