Novel thin films and coatings find use in a broad range of applications including LEDs, solar panels, displays and semiconductors. Although several established techniques are available for determining bulk material properties, instrumented analysis is required for in-situ measurement of material properties of thin films.
Recent developments in nano and micro tribology have resulted in integrated instrumentation that uses concurrent measurements of electrical resistance, contact acoustic emission, friction force and normal load. Besides performing common mechanical tests including reciprocating wear, scratch and indentation, instruments equipped with this sophisticated feature also enables highly sensitive in-depth imaging of the coatings using integrated optical microscopy or atomic-force microscopy on the same platform. Figures 1,2 and 3 below show the scratch testing on the wafer, the micro-blade used, and the nano-indent made respectively.
Figure 1. Scratch Testing on Wafer
Figure 2. Micro-Blade
Figure 3. Nano-Indent
Nanoindentation for Optoelectronics
Modern optoelectronic devices are being built using porous and multiphase ceramic materials as passive semiconductors. Possible device malfunctions in the field can be prevented by prescreening key components for subsurface defects. A new nanoindentation-based technique has been put forward wherein shapes of loading-unloading curves are capable of describing material porosity-induced inelastic behavior and subsurface cracking in defected multiphase ceramic components. It is possible to differentiate three typical experimental curve types in illustrating multiphase porous low-k ceramics’ nanoscale contact behavior as shown in Figure 4.
- The Type I curve shape that characterizes the normal loading-unloading situation during which cracks are not seen
- Type II curve shape that characterizes a cracked contact case wherein it is possible to observe large excursions on the curve’s loading segment at which a low hardness value is produced by the resulting large displacement
- Type III load–unload curve shape that illustrates the inelastic behavior where it is possible to observe the effect of shell bending during indenting into the void
Figure 4. Shape of loading-unloading curves: Type I (a) for normal nanoindentation, Type II (b) corresponding to the crack, and Type III (c) corresponding to the inelastic response and can be attributed to the air bubble beneath the surface.
Scratch-Hardness Test of Hard Coatings
Micro-scratch-hardness test was conducted on two specimens with the help of a diamond stylus having a tip radius of 5 µm as per ASTM G171-03 altered for thin coatings. The diamond stylus within a stylus holder was positioned on a force sensor with a spring suspension. The specimen was placed on the lower linear drive’s table to enable automated lateral motion for producing several scratches on a single specimen.
Figure 5. Friction coefficient (gray) and AE (black) signals during micro-scratch tests on samples 1 and 4.
An acoustic emission sensor was mounted on the stylus holder for monitoring the high-frequency signal produced during scratching, which demonstrates the intensity of material fracture. The first step is the application of 0.4 N normal load to the stylus. The scratch was created when the stylus was dragged along the surface of the sample. The reference material for performing the scratch-hardness calculations was a polished fused quartz. Figure 6 shows 3D AFM images of scratches on two samples.
Figure 6. 3D AFM images of scratches on sample 1 (a) and sample 2 (b) at a scan size of 25 µm x 25 µm.
During the test, the indenter was made to move slowly over the coating, resulting in the removal of some amount of material. A sequence of runs with gradually increasing normal loads, which is stable in every run, was performed. The critical load attributable to the coating scratch resistance was described as the minimum load to completely cut through the coating. Micro-scratch test data are shown in Table 1.
Table 1. Micro-scratch test data with diamond stylus
Scratch-Adhesion on LCD Display Samples
Theoretically like scratch-hardness, scratch adhesion with a micro-blade or a micro-indenter is carried out by sliding under a linearly rising load. The swift change in contact acoustic emission and coefficient of friction characterizes the coating failure. The wear testing involves the reciprocation of the sample stage with a stationary indenter, causing coating wear. Coating wear resistance was described as the minimum number of cycles to completely wear through the coating. Scratch test results are shown in Table 2.
Table 2. Scratch Test Results
||Critical Load (N)
Tribological Properties of DLC Films
It is possible to increase monolithic materials’ wear resistance by introducing diamond-like carbon (DLC) thin films, which have been found useful in components like MEMS and rigid discs that require a protective coating with superior wear resistance in high velocity and light load working conditions. Wear test results are shown in Table 3.
Table 3. Wear Test Results
||Critical # Cycles, Thousands
Mechanical and Tribological Properties of Carbon Nanotube (CNT) Composite Coatings
Figure 7. Example of scratch width measurement for sample 1 (a) and sample 4 (b).
The basic test set up was utilized to measure the friction coefficients for Ni-Co-CNTs composite coating and Ni-Co coating through multiple sliding cycles. It is evident that the friction coefficient of the Ni-Co-CNTs composite coating is much lower than that of the Ni-Co alloy coating under the similar test conditions and each coating’s friction coefficient steadily increases with respect to time under test conditions. Another interesting finding is that the Ni-Co-CNTs composite coating’s friction coefficient decreases with rising normal load under the test conditions.
Figure 8. Fz, COF, and AE plots for scratch adhesion tests on LCD specimen
Capabilities of Bruker Tribo-Testers
Bruker's tribo-testers are capable of performing quite a number of tests and the aforementioned examples are some of the known applications in the field of thin films and coatings testing. A high-frequency multi-channel data acquisition system with a data sampling rate of thousands of times per second enables the detection of almost on-the-spot tiny submicro-failure and submicro-contact events in complex test sequences. The figures below show the different test results obtained.
Figure 9. Sample 1: (a) at 2N the coating was not cut, (b) at 3.5N the coating started to break, (c) at 4N the coating broke, but did not totally cut through.
Figure 10. Sample 4: (a) at 1N the coating was not broken, (b) at 2N the coating started to break, (c) at 3N the coating broke, but did not totally cut through.
Figure 11. Sample 3: (a) at 1.5N the coating started to break, (b) at 2N coating broke, but did not totally cut through, (c) at 3N the coating was totally cut through.
Figure 12. Variation of the friction coefficients with sliding time for (a) Si substrate against an Al2O3 ball and (b) DLC film about 250 nm thick against the Al2O3 ball.
Figure 13. Contact configuration of the friction pair.
Figure 14. Typical load-displacement curves of(a) Ni-Co-CNTs composite coatings.
Figure 15. Friction coefficient of (a) Ni-Co and (b) Ni-Co-CNTs composite coatings/GCr15 steel ball as a function of sliding cycles.
Bruker tribo-testers feature two easily interchangeable modules that are capable of housing either a micro-head or nano-head transducer-sensor assembly. Both heads can be utilized to make the measurement of fracture toughness, yield stress, Young’s modulus, hardness, and critical loads or contact stiffness for onset of inelastic deformation during indentation and scratch tests. The micro-head is utilized for relatively-thick films and bulk materials, while the nano-head is utilized mainly for multi-phase materials and thin films coatings.
Figure 16. Schematic of scratch/wear test setups
Both modules have an optical microscope with an option to have an in-line imaging attachment, either a 3D profiler in the case of the micro-head or an atomic force microscope (AFM) on the nano-head. The AFM measuring head comprises a 3D scanner with a laser-optic system and a probe holder for detecting probe deflection, in addition to an integrated digital optical microscope. The scanning range of this head is from 110x110x20 µm. The 3D profiler measuring head comprises a white light interferometer x-y scanner with a full-time color CCD camera and a stylus holder. The scanning range of the 3D profiler is between 10x10x10 µm and 500x500x500 µm.
Figure 17. Nano-Module (a) and Micro-Module (b) for tribo-testers.
Methodology for Scratch/Indentation Test
Bruker tribo-testers can be used to perform the following test procedure for a comprehensive analysis of the sample coatings/thin films properties:
- Slow reciprocating wear tests for coating friction and durability evaluation
- Scratch-adhesion tests under gradually rising load for the measurement of scratch toughness properties and coating adhesion
- Scratch-hardness tests under stable load for the measurement of scratch resistance and hardness
- Nano/Micro-indentation tests for evaluation of coating nano/microhardness and elastic modulus
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
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