Application Note | Coating Wear
Gold Coating Tribology on Quartz Crystal Substrate
Friction and Wear of a QCM Gold Electrode Followed from Run-In to Substrate Exposure
Research & Experimental Testing
Duanjie Li
Visual Design & Editorial
Andrew Shore
Introduction
The Quartz Crystal Microbalance (QCM) is a piezoelectric device composed of a thin plate of quartz and two electrodes affixed to each side of the plate. It is an extremely sensitive mass sensor capable of making precise measurements of very small mass in the nanogram range. QCMs are widely applied in research and industrial instruments, such as thermogravimetry, sorption/desorption systems, and surface property instruments to detect and monitor the change of mass, adsorption, density and corrosion, etc.
Importance of Reliable Wear Evaluation for QCM
The QCM works based on the piezoelectric properties of the quartz crystal. It measures the mass change on the surface down to 0.1 nanogram during material deposition by detecting variations in the resonance frequency of the crystal. Due to the extremely sensitive and accurate characteristics of the QCM, it is critical to ensure that the two electrodes on both sides of the quartz plate possess good wear resistance. Any mass loss on the metal electrodes caused by wear can lead to significant error in the measurement. Therefore, reliable and accurate wear evaluation using a Tribometer is important for quality control and R&D of QCMs.
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Measurement Objective
In this application, we investigated the wear behaviors of a QCM sample to showcase the capacity of the NANOVEA Tribometer in performing accurate evaluation of the tribological properties of very delicate samples.
NANOVEA T2000 High Load
Pneumatic Tribometer
Test Procedure
The coefficient of friction, COF, and the wear resistance of a QCM sample were evaluated by the NANOVEA Tribometer using the linear reciprocating Wear Module. A SiN ball (6 mm diameter) was used as the counter material. The test parameters are summarized in Table 1. The wear rate, K, was evaluated using the formula K = V/(F × s), where V is the worn volume, F is the normal load, and s is the sliding distance. Wear track 3D profiles were measured by the NANOVEA Optical Profilometer, and the wear track morphology was examined using an optical microscope.
| Parameter | Value |
|---|---|
| Sample description | Quartz Crystal Microbalance |
| Normal force | 2 N |
| Amplitude | 2.5 mm |
| Speed | 60 cycles/min |
| Duration of test | 1, 2, 3, 5, 20, 30 min |
Table 1: Test parameters of the wear measurements.
Results and Discussion
The evolution of COF during the wear test of the QCM sample is plotted in Fig. 2. The QCM sample exhibits a progressively increasing COF from ~0.1 to ~0.5 in the first two minutes. It gradually decreases to ~0.4 in the third minute, and then gradually increases to above 0.6 in the following wear test.
Fig. 3 shows the development of the QCM wear tracks under the optical microscope after 1, 2, 3, 5, 20 and 30 min wear tests. The wear track evaluation is in agreement with the observation of COF evolution: the gold coating on the QCM sample has a rough surface finish, which results in a higher COF of ~0.5 in the first two minutes’ run-in period. This is followed by a slight decrease of COF in the third minute, during which the surface asperities are worn and flattened, creating a relatively smooth wear track with lower friction. However, as the wear test continues, the gold coating progressively wears out and the quartz substrate is exposed. This results in increased COF above 0.6.
Fig. 4 compares the 3D wear tracks of the QCM sample after 3, 20 and 30 min wear tests. The wear volume and wear rate calculated from the wear track profiles are summarized in Table 2. The gold coating has a wear rate of 2.26 × 10−5 mm³/N m, as calculated using the wear track after the 3 min wear test in which the wear is restricted to the gold coating. When the quartz substrate is reached, the more wear resistant quartz reduces the wear rate to ~1.5 × 10−5 mm³/N m.
This study not only investigates the wear resistance of different layers of the QCM sample, but also demonstrates the importance of the surface finish of the gold coating to its wear resistance. Smoother surface finish can reduce friction and enhance wear resistance.
| Test duration | Volume (µm³) | Wear rate (mm³/N m) |
|---|---|---|
| 3 min | 4.1E+05 | 2.26E-05 |
| 20 min | 1.8E+06 | 1.52E-05 |
| 30 min | 2.8E+06 | 1.56E-05 |
Table 2: Result summary of wear track analysis.
Conclusion
In this study, we showcased the capacity of the NANOVEA Tribometer in performing accurate evaluation of the coefficient of friction and wear rate of the Quartz Crystal Microbalance (QCM) in a controlled and monitored manner. The surface finish and homogeneity of the QCM sample play an important role in the wear behavior. A smooth surface enhances wear resistance by reducing friction.
NANOVEA Tribometers offer precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system.
Frequently Asked Questions About QCM Electrode Wear
Why does electrode wear matter on a QCM?
A QCM measures mass change down to 0.1 nanogram from shifts in the resonance frequency of the quartz; any mass lost from the metal electrodes by wear can cause significant measurement error.
How was the wear test run?
A linear reciprocating test with a 6 mm SiN ball under 2 N, 2.5 mm amplitude and 60 cycles/min, stopped after 1, 2, 3, 5, 20 and 30 minutes; the tracks were examined by optical microscope and measured with an optical profilometer.
How did friction change during the test?
The COF rose from about 0.1 to 0.5 in the first two minutes, dropped to about 0.4 in the third minute as asperities flattened, then climbed above 0.6 as the gold wore through and the quartz was exposed.
What were the wear rates?
The gold coating wore at 2.26 × 10⁻⁵ mm³/N m (3 minute track, wear within the gold); once the quartz was reached the rate fell to about 1.5 × 10⁻⁵ mm³/N m.
Does surface finish affect coating wear?
Yes. The rough gold finish raised friction during run-in; a smoother finish reduces friction and enhances wear resistance.
How is wear rate calculated?
K = V/(F × s), with V the worn volume from the 3D track profile, F the normal load and s the sliding distance.
