Hardness
Hardness is an important material property of concern in films utilized for electronic and optical as well as mechanically functional applications. It affects wear resistance and plays an important role in the friction and lubrication of surface films in contact. Hardness, a complex property related to the strength of interatomic forces, apparently depends on more than one variable. Hard materials are generally modeled by a deep potential energy well with steep walls. These characteristics imply a combination of high cohesive energy and short bond length. Measures of the cohesive energy are the heat of sublimation and enthalpy of compound formation (A//298). If some account is taken of the bond length by dividing the cohesive energy by the molar volume of the material, then the correlations with hardness shown in result. It is apparent that the hardest materials are covalently bonded, and that increasing the ionic character of the bond leads to reduced hardness. At a microscopic level, directional bonds more readily resist distortion and rupture by concentrated loads than do ionic bonds.
The hardness of a material is usually defined as its resistance to local plástic deformation. Since the hardness test consists of pressing a hard indenter into the surface, it is equivalent to performing a highly localized compression test. A correlation between hardness H and yield strength ay is therefore expected. Typically H « 3ay in bulk metáis, but the correlation has not been directly verified in films or hard coatings. It is well known that ay for bulk materials decreases with temperature, a fact that facilitates hot mechanical forming operations. Therefore, it can be anticipated that the hot hardness of coatings will be lower than that at ambient temperatures for which valúes are usually quoted. It is these hot hardness valúes that are importam in thermal coatings and in applications such as machining.
Hardness Testing
Hardness testing of films and coatings is a relatively simple (though decep-tively so) measurement to perform. The most frequently employed methods are modifications of techniques having long standing in the metallurgical commu-nity. The Vickers hardness test, also known as the diamond pyramid hardness (DPH) test, employs an indenter consisting of a square-based diamond pyramid ground to have a face angle of 136°. Hv, the Vickers hardness number, is obtained as the ratio of the applied load L to the surface área of the resulting indentation.
Effect of Microstructure on Hardness
A fundamental cornerstone of materials science is the relationship between microstructure and properties. Following Sundgren and Hentzell connections between the hardness of films and coatings and various microstruc-tural characteristics will be discussed.
What does apparently affect the hardness and strength of refractory com-pounds is the perfection of the grain boundaries. Porosity and fine microcracks are very deleterious to such coatings and lower their strength and hardness significantly. Metal films are, however, somewhat more tolerant of such defects, whose influence can be blunted by plástic deformation effects. Increas-ing the substrate temperature is the simplest and most common way to reduce the grain-boundary defect structure and enhance the hardness of these com-pounds. The effect can be rather dramatic. For example, an increase in substrate temperature from 100 to 600 °C raised the hardness of magnetron-sputtered TiN from 1300 to 3500 kg/mm2. Elevated temperatures eliminate void networks and evidently promote the strengthening of grain boundaries. The grain size increases as does hardness, contrary to the Hall-Petch prediction.
Metastable Structures.
Metastable phases are frequently ob-
served in refractory compound films. As in the case with metastable metal
alloy films, high deposition rates and low substrate temperatures are conducive
to the formation of nonequilibrium structures and a fine grain size. Manifesta-
tions of the metastability are the incorporation of C and N in interstitial lattice
sites and the generation of supersaturated solid solutions. This generally occurs
during PVD rather than CVD, which is usually carried out under conditions
closer to thermodynamic equilibrium. The incorporated interstitials tend to
distort the lattice and the subsequent difficulty in initiating dislocation motion
is reflected in increased hardness. The effect can be quite large. Hardnesses of
2500 to 3500 kg/mm2 are generally found in reactively sputtered HfN films
compared to 1600 in CVD-grown films. Apparently the bombardment of the
growing HfN film by the sputtering gas forces N into tetrahedral interstitial
positions. This creates a high compressive stress in the plañe of the film and
thus a higher hardness.
In general, metastable phases and structures can be frozen in up to tempera-tures of 0.37^ (TM is the melting point). As a consequence, metastable hard coating systems can be used at temperatures up to 550-800 °C (for TM = 2500-3300 °C)
Impurities.
Since hard PVD coatings are generally grown in médium vacuum or under even higher pressure ambients, the incorporation of noble gases, C, N, and O from residual gases, and impurities from chamber hardware and walls is not uncommon. The deposit impurities are located in both substitutional, interstitial as well as grain-boundary sites at total levéis up to a few atomic percent. Even at such low concentrations, the effect on hardening can be pronounced. The mechanism of hardening due to impurities apparently involves the electrostatic attachment of the latter to charged disloca-tions in ionic materials and to dangling dislocation bonds in covalent com-pounds. Such interactions limit dislocation mobility by pinning effects.
Film Texture.
By texture we mean the preponderance of one (or more) crystallographic planes oriented parallel to the film surface compared with the case of randomly oriented planes. In the latter, isotropic behavior may be expected. Films grown by PVD and CVD techniques usually display a preferred oríentation, however, with low índex planes lying parallel to the substrate surface, creating a texture that is strongly dependent on virtually all deposition and process variables. Factors of as much as 2 in hardness have been observed in cubic coatings (e.g., TiC, TiN, ZrC) as a function of the preferred oríentation plañe [e.g., (111), (100) and (110)]; similar hardness anisotropy on basal (0001) and prísmatic (1100) planes of hexagonal materials (e.g., WC, SiC) has been reponed.
Fracture
It is a fact of nature that materials that are extremely hard are simultaneously brittle and prone to fracture. The phenomenon of fracture is of cardinal concern in a great many materials engineering applications; once fracture of a component or structure occurs, other issues quickly assume secondary impor-tance. Films and coatings are no exception. For example, with the exception of oxidation wear, all wear mechanisms are based on some sort of crack development that creates new surfaces, from which particles can be detached by fracture processes. In a similar vein, high-temperature fracture or spalling of coatings leaves the underlying substrate unprotected and at the mercy of harsh corrosive atmospheres.
At the outset it is important to distinguish between brittle and ductile fracture. The latter occurs after the material has undergone some plástic deformation. Metáis tend to undergo ductile fracture upon overloading. Brittle fracture, on the other hand, occurs rapidly, wíthout warning and in such a way that the broken pieces can usually be neatly fitted together. Generally, brittle fracture occurs more readily in materials having small tensile strengths com¬pared with their compressive strengths. Thus, glasses, ceramic oxides, and covalent, as well as hard metal compounds, to a lesser extent, are particularly prone to brittle fracture.
Two approaches to fracture of coatings will be presented next. The paramount assumption is that the materials involved possess an intrinsic collection of structural flaws distributed laterally as well as through the coating thickness. Voids, porosity, interconnected voids or porosity, voided or grooved grain boundaries, local regions of de-adhesion, etc. may be viewed, in a broader context, as flaws and even incipient cracks. Under either external or residual internal stressing, each flaw will locally concéntrate stress and the surrounding material will tend to deform. If the stresses are sufficiently large, they can ultimately destroy the coating by crack propagation if tensile, and by wrinkling or buckling if compressive.
Ronellys Flores---CRF---libro the materials science of thin films
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