Surface Engineering Techniques

Introduction

Properties of materials may be classified as those pertaining to the bulk and those that result from the characteristics of the surface. As seen in previous chapters the function of modern objects depends, to a great extent, on surface characteristics. Moreover, if those characteristics do not meet functional demands of the object, the structural integrity of the bulk material may be compromised. It is often the case that the ideal surface properties for a particular application are not satisfied by using certain material. To meet functional constraints, deliberate, often calculated, alteration of the properties of the surface has to take place. Alteration is realized through several routes and may include changing surface topography, heat treating of surface and subsurface layers, alteration of the chemical composition of the surface, altering the microstructure of subsurface layers, etc.

The process of deliberate change, or customization, of surface and subsurface properties is known as “surface engineering”. Such a process stands core of preventing catastrophic failure of engineering components. Although the scientific analysis of such a practice intensified around the early 1980s, the practice of customizing of surface properties has been around since antiquity.

Surface engineering has been practiced since the dawn of age when man learned to make and use tools, figure 1 (Frere, 1800)

Figure 1.

A typical Acheulean handaxe; this example is from the Douro valley, Zamora, Spain. The small chips on the edge are from reworking

The fundamental techniques of surface modification comprise adsorption of small or large molecules onto surfaces through incorporation of a layer forming additive in a fluid layer above the surface, deliberate roughening (i.e. engineering of texture), or morphological modification of the surface. It is interesting that these techniques, in principle, have been practiced since antiquity. In ancient Egypt black patinated copper alloy objects has been produced as early as the middle Kingdom Period (2040 to 1782 BC). Production continued until disappearing after the fall of the Roman Empire (Giumlia-Mair. 1996, 2005). Studies on objects from the Egyptian Museum in Cairo (Rehren, et al. 2013), showed that the black patina was induced by thermal treatment of the surface and by application of animal glue as a surface coating (Johnson et al., 2013). Debated evidence of smelting iron from meteorites, have also been identified in tombs of Egyptian royalty (Mohamed and Darweesh, 2012). Surface engineering of sword blades is perhaps a well-known practice. It is synonymous with Damascus steel (Kennedy, D. M., 2008), and samurai-sword steels (Ranganathan and Srinivasan, 2006).

Bell (1990), reports that surface engineering has been practiced in Europe for several thousand years. The first written reference to thermal quench-hardening for treatment the weapons was referred to in 880 BC in Homer’s Odyssey (Bell, 1990). Carburizing case hardening, which is an of austenitic thermochemical process, was documented in Latin by the German monk Theophilus in his “schedule diversarum artium”. This 11th century manuscript clearly mentions the word “temper”, which is a direct translation from the Latin “temperament”. The usage of this word is equivalent to the modern usage of “quench and self-temper”,which is necessity for adequate mass transfer of carbon and nitrogen from the nitrogenous compounds used at the time-urine in the case of Theophilus, to enhance surface hardness and hardenability. Hardening of steel through using organic material was common practice for quite sometimes. In medieval times many references mentioned a wide variety of organic nitrogenous compounds used in hardening of weapons and agriculture tools. The practices of nitriding by using organic compounds were not confined to the Middle East nor to Europe. It perhaps originated in Asia. Archaeological mythological studies in China (Scott, 1907-a), revealed that in the early Chinese dynasty the strength of steel swords was improved by extraction of soybean proteins.