Thin Film Coatings
The NanoTest's wide range of complementary techniques provides the most comprehensive characterization and optimization of the properties of PVD and CVD films and coatings. Taken together the results provide much greater confidence and reliability than those from nanoindentation alone:-
- The NanoTest indentation module is used to determine the hardness and modulus.
- The nano-scratch (scratch and wear test) module assesses the tribological performance of coated systems in sliding and abrasion.
- The nano-impact module assesses the toughness, fatigue wear and fracture resistance of the coating-substrate system.
Mechanical Properties vs. Depth
The variation in hardness, modulus, H/E vs. depth can be determined in a single experiment using the load partial unload method and displayed in the user-friendly NanoTest replot software. A typical result on a hard ternary nitride film on silicon is shown in the example below.
Case Study: MEMS Overcoats
With the NanoTest's high resolution NTX controller accurate measurements of hardness and elastic modulus are possible on very thin films. The example below shows the variation of elastic modulus for a range of thin films on Si being evaluated for potential MEMS overcoats. As the contact depth increases the film properties become increasingly dominated by the silicon substrate. However, the lower force indentations revealed large differences in hardness and modulus that influence their scratch and wear resistance.
Improving the durability of Diamond Like Carbon coatings
DLC films can be stressed and brittle limiting their suitability for some demanding applications. Nevertheless DLC has a wide range of attractive properties including high hardness and low friction so possible applications for this coating are being continually investigated.
Nano-impact is both an R&D and QC tool for both manufacturers and end-users of DLC coatings ensuring coating integrity and minimising costly failures. Debonding and dramatic film failure can occur at high force after only a few impacts on some DLCs (see figure). Low resistance to fatigue-induced fracture occurs when the coating is too highly stressed and/or there is poor adhesion.
In industrial research, the rapid nano-impact test is preferred to a complete engine test when evaluating new candidate coatings. For example, a major automotive parts manufacturer has been able to optimise their graded DLC coatings to reduce scuffing wear in fuel injector plungers.
Clear results with the nano-impact technique are driving the development of advanced DLC coatings
DLC-coated engine component. At or below 5 mN cohesive fracture occurs but at higher load there is a more damaging failure mechanism. Fracture over 5 mN is accompanied by coating delamination exposing the softer substrate.
Case study: optimizing coatings for Cutting tools for extended tool life
Hard coatings are often applied to components that experience dynamic loading during service such as cutting tools. The NanoTest impact testing module is used to optimize the fatigue performance of PVD coatings.
PVD coatings such as TiAlN and AlCrN are often used as cutting tools, with their good oxidation resistance and high hot hardness. The NanoTest nano-impact module is being used to measure the nanomechanical properties of these and other coatings at high strain rates and also elevated temperatures.
Conventional nanoindentation is quasi-static and it is not too surprising that it has proved difficult to optimise cutting tool coatings based on nanoindentation results. In complete contrast results with the nano-impact technique show excellent correlation with tool life-time in highly loaded interrupted cutting applications such as the end and face milling of steels and Ti alloys - but with a typical test time of only 5 minutes are much quicker!
An excellent correlation between the rapid nano-impact test and the end milling trials on structural steel is observed. The non-constant tool wear rate is well reproduced in the accelerated test
Further reading
Investigating the correlation between nano-impact fracture resistance and hardness/modulus ratio from nanoindentation at 25-500 C and the fracture resistance and lifetime of cutting tools with Ti1-xAlxN (x=0.5 and 0.67) PVD coatings in milling operations BD Beake, GS Fox-Rabinovich et al, Surf Coat Technol 201 (2007) 4585
Impact of mechanical properties measured at room and elevated temperatures on wear resistance of cutting tools with TiAlN and AlCrN coatings GS Fox-Rabinovich et al, Surf Coat Technol 200 (2006) 5738.
Influence of mechanical properties on the nanoscratch behaviour of hard nanocomposite TiN/Si3N4 coatings on Si, BD Beake, VM Vishnyakov, R Valizadeh, JS Colligon J Phys D: Appl Phys 39 (2006) 1392.
