Hardness of multilayered ceramics

WJ CLEGG, F GIULIANI, Y LONG, S J LLOYD, University of Cambridge UK and JM MOLINA-ALDAREGUIA, Centro de Estudios e Investigaciones Tecnicas de Gipuzkoa (CEIT), Spain 

1 The variation of hardness with multilayer wavelength in a range of different types of structures. These include multilayers of (a) isostructural transition metal nitrides and carbides, which show the greatest hardening (b) nonisostructural multilayer materials, where slip cannot occur by the movement of dislocations across the planes of the composition modulation, because the slip systems are different in the two materials and (c) materials where different crystal structures are stabilized at small layer thicknesses, such as AIN deposited onto TiN. 

The use of indentation complicates the interpretation of the measurements. However, relationships between hardness and flow stress exist for monolithic materials and these have been used to obtain information in the multilayers. When an indenter is pressed into the surface of a material, the material that is displaced must be accommodated either by material being pushed out of 

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It can be seen that ceramic multilayer structures have been produced with increments of the hardness of up to 60 GPa, increasing the hardness by up to a factor of almost 3. Initial work in this area has developed a number of ideas, such as the effect of modulus mismatch, which in some cases give good agreement with the models suggested but in many others do not. It is suggested that at least some of this discrepancy can be accounted for by differences in the microstructure and residual stress-state of the film, both of which are often poorly characterized. Furthermore there is very little direct evidence about how these structures deform and in particular about how different layers must be strained in order to accommodate the indenter when it is pressed into the sample. Further advances in this area will require the greater use of numerical techniques to analyse the complex stress and strain behaviour under the indentation, coupled with the use of recently developed techniques that allow the localized deformation behaviour to be observed in detail. 

In electronics, platinum, palladium and ruthenium are used for computer hard disks and for multilayer ceramic capacitors. Platinum and rhodium are used in sensor applications, for instance in equipment for measuring temperature. Palladium and platinum are used for dental purposes. As platinum and iridium are very resistant to chemical and high-temperature attack they are widely used for equipment in chemical industry and in laboratories. Compounds of platinum and osmium have been used in medicine for anti-cancer drugs and implanted sensors. Platinum and to some extent also palladium are used as jewelry and for investment items, such as coins and bars. 

Recent applications of e-beam and HF-plasma SNMS have been published in the following areas aerosol particles [3.77], X-ray mirrors [3.78, 3.79], ceramics and hard coatings [3.80-3.84], glasses [3.85], interface reactions [3.86], ion implantations [3.87], molecular beam epitaxy (MBE) layers [3.88], multilayer systems [3.89], ohmic contacts [3.90], organic additives [3.91], perovskite-type and superconducting layers [3.92], steel [3.93, 3.94], surface deposition [3.95], sub-surface diffusion [3.96], sensors [3.97-3.99], soil [3.100], and thermal barrier coatings [3.101]. 

In conclusion, one should choose an appropriate multilayer system for different application purposes. For the case of fatigue wear, multilayer films consisting of two hard materials with different shear modulus, such as DLCAVC multilayer film [115], would satisfy the requirement for wear resistance. While for abrasive wear, multilayer films consisting of hard ceramic layers and soft metal layers, such as TiN/Ti and CrN/Cr [116,117] multilayer films are more competent. 

Rahmoun K et al (2009) A multilayer model for describing hardness variations of aged porous silicon low-dielectric constant thin films. Thin Solid Films 518 213-221 Rice RW (ed) (1998) Porosity of ceramics. Marcel Dekker, New York... 

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