Hardness and stress

Analysis of Table II shows discrepancies in the hardness and stress behavior of a-C(N) H films. Although all the works reported a clear stress reduction upon nitrogen incorporation, the hardness sometimes is quoted as almost constant, or on the other hand clearly decreasing. In addition to the possible effect of different deposition methods and conditions, it can be easily seen that the differences in hardness testing methods are the major source for discrepancies. Constant hardness behavior is only reported with the use microindentation methods, like Vickers and Knoop microhardness. On the other hand, the use of low-load nanoindentation methods always led to a nitrogen-induced decrease in hardness. This is basically the consequence of two factors. The first one is the higher penetration... 

Density, Hardness, and Stress Data for a Range of Gold and Gold Alloy Electrodeposits... 

An alloy ideally should be homogeneous, but in practise it can contain segregations, for example hard alpha in titanium. Beeause of their different mechanieal properties sueh segregations can be the origin of eracks when the component is operated near to its temperature and stress limits. 

Fig. 4. Types of stress—strain curves (a) soft and weak (b) hard and brittle (c) soft and tough (d) hard and strong and (e) hard and tough.

The resulting films are usually hard and scratch-resistant but often show considerable stress (152). Such films can be used as corrosion-resistant coatings on metals or as membranes in gas sensor devices (153,154)... 

An intrinsic surface is built up between both phases in coexistence at a first-order phase transition. For the hard sphere crystal-melt interface [51] density, pressure and stress profiles were calculated, showing that the transition from crystal to fluid occurs over a narrow range of only two to three crystal layers. Crystal growth rate constants of a Lennard-Jones (100) surface [52] were calculated from the fluctuations of interfaces. There is evidence for bcc ordering at the surface of a critical fee nucleus [53]. 

It is not surprising that hardness is important because the mechanical toughness can be expected to decrease with increasing hardness, and the level of residual stress present will also depend on the hardness of the steel, especially for welded components. Thus, the important role of the microstructure in influencing susceptibility to stress-corrosion cracking is consistent with the observation that hardness levels are a good guide to stress-corrosion resistance, but they should not be used universally without due consideration of the specific alloy and the environment in which it is to be used. 

In using metallic Pb as an anode the formation and maintenance of a hard layer of PbOj is essential, since it is the PbOj that is the actual inert anode, the Pb acting both as a source of PbOj and an electrical conductor. PbOj is relatively insoluble in seawater and its dissipation is more usually associated with mechanical wear and stress than electrochemical action. 

---