The development and implementation of a wide range of innovative nanomechanical test techniques to solve tribological problems in surface engineered systems are described in this review. By combining results with several different nanomechanical techniques, predictive design rules based on the elastic and plastic deformation energies involved in contact are proposed to optimise mechanical properties in the various contact situations that occur for different applications. Results are presented with the NanoTest platform for applications in biomedical devices, surface engineering of lightweight alloys, wear resistance of physical vapour deposition and chemical vapour deposition coatings as well as fracture fatigue resistance of diamond-like carbon coatings. Surface engineering to increase the ratio of hardness to elastic modulus (H/E) can be beneficial in a range of applications but care should be taken that, first, it be done without introducing too large intrinsic stress or stress discontinuities in mechanical contact loading, second, the severity of the contact results in high stresses and there is a requirement for some plasticity in contact to avoid fracture.

The nanoindentation creep behaviour of several different polymers has been investigated. The extent of creep ε is represented by the Chudoba and Richter equation: ε = εeln(εrt + 1), where t is the loading time and εe and εr are material constants. Creep was determined in this way for a variety of polymers at Texper = 301.7 K. Some of the materials studied were far above, some far below and some near their glass transition temperatures Tg. The creep rate εr was plotted as a function of y = (Tg − Texper); a single curve was obtained in spite of a large variety of chemical structures of the polymers. The εr = εr(y) diagram can be divided into three regions according to the chain mobility. At large negative y values, the creep rate is high due to the liquid‐like behaviour. At large positive y values in the glassy region, the creep rate is higher than that in the negative y‐value region; the creep mechanism is assigned to material brittleness and crack propagation. In the middle y range there is a minimum of εr. These results can be related to glassy and liquid structures represented by Voronoi polyhedra and Delaunay simplices. The latter form clusters; in the glassy material there is a percolative Delaunay cluster of nearly tetrahedral high‐density configurations. The creep mechanism here is related to crack propagation in brittle solids. In the liquid state there is a different percolative Delaunay cluster formed by low‐density configurations, which, as expected, favour high creep rates. Copyright © 2007 Society of Chemical Industry

Mechanical properties and creep behaviour of an atactic-polypropylene (aPP) have been studied in the vicinity of its glass transition temperature (-18ºC) via a nanoindentation platform integrated with a sub-ambient temperature capability. All low temperature tests were validated by measurements on a fused silica reference sample from 25ºC to -30ºC. The fused silica results showed virtually invariant elastic modulus with temperature over this range consistent with literature measurements by sonic resonance. Hardness and elastic modulus of aPP increased as the test temperature decreased and the amorphous regions went through the glass transition. The creep behaviour was analysed using two approaches: (i) a logarithmic method, and (ii) the Boltzmann integral method. The results showed that the creep extent decreased as the temperature was reduced, and for the time constants obtained there were upper-limit values at -10ºC, about 8ºC above the quoted glass transition temperature. The strain rate sensitivity obtained by the logarithmic method also showed a maximum at -10ºC.

A nanoindentation system fitted with a fluid cell has been used to probe the influence of water on the nanoindentation creep response of commercial Nylon-6 samples. Measurements on samples taken while immersed in deionized water were compared with measurements under usual ambient ($ 50% relative humidity) conditions. Water absorption reduces hardness by around 50% and elastic modulus by around 65%. The dimensionless creep parameter, A/d(0), where A is a constant and d(0) is the initial penetration at the start of the creep-hold period, is a measure of the proportion of time-dependent deformation compared with the total deformation. This parameter decreases significantly in water. We have suggested previously that A/d(0) correlates with tan d. The observed reduction in A/d(0) when wet is consistent with a decrease in the tan d peak due to a shift in the glass transition temperature when wet.