## 1 Introduction
In the case of titanium (Ti) alloys bonded to stainless steel, the use of copper (Cu), nickel (Ni) and silver (Ag) interlayers has been shown to reduce the incidence of iron-titanium (FeTi) and iron-chromium-titanium (Fe-Cr-Ti) phases at the interface [12]. In addition to the process conditions described above, the conditions of the surfaces being bonded, most notably cleanliness, surface roughness and the presence of any passivation layers, can also influence the state of the bond [3]. Of these, the roughness of the two surfaces has also been found to exert a significant influence on the strength of the bond formed. [PERSON] et al. [15] showed that the tensile strengths of oxygen-free high-conductivity Cu increased with decreasing surface roughness. Examination of the fracture surfaces of the tensile specimens revealed the fraction of the interface that was bonded. The Cu specimens with an initial ten-point height roughness (\(R_{z}\)) of 90 nm that were bonded at 300 \({}^{\circ}\)C exhibited tensile strengths of between approximately 20 and 40 MPa. In contrast, the tensile strengths at 300 \({}^{\circ}\)C of the rougher specimens (which had \(R_{z}\) values prior to bonding of between 1.5 \(\upmu\)m and 14 \(\upmu\)m) were negligible. Furthermore, even at a bonding temperature of 700 \({}^{\circ}\)C, tensile strengths of only 10-20 MPa were recorded on the rougher specimens. This was attributed to greater contact area in the smoother specimens, the effect of which was to reduce the voids at the interface and facilitate greater inter-diffusion. In contrast, for rougher surfaces higher pressures are needed for plastic deformation of the asperities and fracture of the oxide layers that are needed for intimate contact between the two surfaces [8]. Upon completion of the bonding process, it is important to ascertain the state of that bond and demonstrate that it is of the required quality. However, analysis of diffusion-bonded interfaces has shown that the character of the near-interfacial regions can be markedly different from the parent materials. This is often due to the presence of intermetallic compounds formed from the two parent metals during the bonding process. Such differences may also be reflected in the mechanical properties of these near-interfacial regions, which can be distinctly different from the materials being bonded. One such example is TiAl\({}_{3}\) that was formed at the interface of Ti bonded to Al and which increasedin thickness with increasing annealing temperature and time [16]. The Vickers microhardness (50 g. load) of the TiAl\({}_{3}\) interfacial region was approximately 300 HV\({}_{0.05}\) (2.94 GPa), which was significantly higher than that of either the Al (\(\sim\)50 HV\({}_{0.05}\) (0.49 GPa)) or the Ti (\(\sim\)150 HV\({}_{0.05}\) (1.47 GPa)) parent metals. In tensile tests of the bonded joints, failure occurred by intercrystalline fracture of the TiAl\({}_{3}\). It should be noted that in some of the references cited in this review, hardness values are quoted in Vickers hardness (HV), while others are in GPa; for reasons of consistency, the former have been converted to the latter by this author. The interfacial regions that are formed in diffusion-bonded systems are often only a few micrometres in width, which can complicate the task of obtaining their mechanical properties. Although some studies have used tensile or shear tests to evaluate the strength of the bond, this is not always practicable. Indeed, in some cases the process of test specimen manufacture can be detrimental to that bond and, when tested, may lead to unrepresentative bond strengths. These difficulties in specimen production have prompted the search for other tests that can provide an understanding of the bond that has formed. One such technique by which this can be performed is by instrumented indentation. The purpose of this review is to examine the use of instrumented indentation to evaluate diffusion-bonded materials.