The scope of applications for Thin films and coatings have a wide range of applications from optical tribiological, and biomedical ones to microelectronics, packaging and decorative uses. A coating’s adhesion strength is critical to it functioning properly and can be determined through a scratch test.
When choosing the type of scratch test to use and the test parameters, the relative hardness, coating thickness and the electrical conductivity of the substrate and the coating must be considered. The application of the coated components is also important. Conducting ascratch testtest using regular scratch tools is difficult due to the inherent shortcomings of such tools. Bruker’s scratch test system, based on the Universal Materials Testing (UMT) platform, can accommodate any type of scratch test and achieve repeatable and reliable data for a wide range of test parameters.
Introduction
Coatings are used in the production of almost everything, from surgical implants to hand-held devices and computer hard disks to turbocharged internal combustion engines. The manufacturing of an automobile is incomplete without coatings, which are both decorative and functional. Some of the fundamental functions of coatings include modifying the tribiological behavior of a surface by altering the friction, enhancing the aesthetic appeal by modifying the optical properties of the surface, and minimizing surface and sub-surface damage, thereby enhancing the performance and reliability of products.
The thickness of a coating and its physical and mechanical properties depend on its intended use. Its adhesion strength is important because it determines its ability to adhere to a particular substrate and fulfil its intended purpose. General scratch testers are designed such that they can only accommodate specific scratch tests and a limited set of test parameters. This limitation can be overcome using the Bruker’s Universal Scratch Test system, which is easy to use and provides highly reliable and repeatable data. All the scratch test methods can be accommodated on this Universal Scratch test system.
Scratch Test Methods
Progressive Load Scratch
The setup for a progressive load scratch test consists of a stylus that is moved across the sample surface with a linearly increasing load up to the fracture point. A normal force (Fz) and a lateral force (Fx) are recorded with respect to the test time and the scratch distance during the test. Apart from these values, the electrical contact resistance (ECR) and the acoustic emission (AE) data are also recorded. These values are critical for the accurately determining subsurface and incipient failure. An optical microscope is used for evaluating the scratch failure mode of the coating after the test. Critical load (Lc) is defined as the normal mode under which failure occurs, and for each mode of failure there may be many critical loads. The factors that influence the critical load include the adhesion strength between the coating and the substrate, the loading rate (N.s-1), the radius of the stylus or scratch tip and the mechanical properties of the substrate and coating. The coating’s internal stress, and thickness as well as the coefficient of friction between the stylus and the coating, the flaw-size distribution in the coating and at the coating-substrate interface can also influence it. A progressive scratch test, is quicker than other tests because a single test covers the entire lod range. Therefore this test is used in the rapid assessment and QC of thin films and coatings. The linear scratch test is a type of progressively increasing load test. Figure 1 A and 1B show the schematic load and scratch-width profiles for a linear scratch test respectively.
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Figure 1. Schematics of (A) load and scratch-width (B) profiles of a linear scratch test.
It can be seen in Figure 1B that the width of the scratch increases as the linear scratch progresses with increasing normal force. The zigzag scratch is another type of progressive load scratch test. In this test, the stylus oscillates along one direction (Y) and a scratch test with increasing load is simultaneously performed in a perpendicular direction (X). Figures 2A and 2B show the load and scratch-width profiles of a zigzag scratch test.
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Figure 2. Schematics of load (A) and scratch-width (B) profiles of a zigzag scratch test.
A series of micro-scratches are produced during scratching by the oscillating stylus on the Y direction. The scratch adhesion values of soft and/or thin coatings can be determined by this test.
Constant Load Scratch
The damage levels can be differentiated more clearly using the constant load scratch test. Although this test requires a larger specimen surface and more test time, it provides statistically reliable results. This test is extensively used in research and process development of coatings. In the constant load scratch test, a constant normal load produces a series of scratches for determining the critical loads. An optical microscope is used for examining each scratch for failure of the coating and for determining the critical load. In order to confirm failure, the acoustic emission (AE) and electrical contact resistance (ECR) data may also be determined based on the electrical characteristics of the coating.
This test can be performed in a single direction or in the X-Y direction. For unidirectional scratch tests, the stylus is moved in a single direction with a constant normal force, the corresponding schematic load and scratch width profiles are shown in Figures 3A and 3B.
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Figure 3. Schematics of load (A) and scratch-width (B) profiles of a unidirectional scratch.
The stylus is moved alternately in the X and Y directions with a constant normal load for the X-Y scratch method. Figures 4A and 4B show the schematic load and scratch path. Figure 4B shows that the spacing between two consecutive scratches in the X direction is reduced gradually. The extent of flaw tolerance of the specimen can be determined by the failure mode in these tests. At the critical spacing, failure occurs due to the micro-cracks that were formed or subsurface stresses induced during earlier scratching in the X direction.
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Figure 4. Schematics of load (A) and scratch (B) paths of an scratches
X-Y scratch
The adhesion energy required for scratching micro-features off the substrate can be derived through a constant load scratch test that is used to scrape through such features.
The adhesion energy can be accurately determined by using blade-type geometry rather than sharp-tip geometry. On completing the test, the lateral force is plotted against the scratch distance. The adhesion energy that is obtained from the area under the Fx versus distance curve plot can be used for comparing the adhesion strength of these micro-features.
Bruker’s Scratch Test System
The Universal Mechanical Test (UMT TriboLab™) platform forms the basis for the Bruker’s scratch test system. This platform enables precise control of load, position and speed. Various types of scratch test capabilities over a broad range of forces and velocities are supported by this platform, thanks to its modular design. There are three main drive systems on the TriboLab, namely, Carriage, Slider and Y-stage for supporting Z-, X- and Y motions respectively. Owing to the intuitive hardware (TriboID) and the software (TriboScriptTM) the tester is extremely versatile, user friendly and productive. Automatic detection and configuration of all the components attached to the system is done by the TriboID. The scratch test sequences are compiled from the built-in test blocks by the advanced and secured scripting interface provided by TriboScript. The platform delivers high accuracy and repeatability due to the real-time control and data analysis software.
Both progressive and constant load scratch testing can be carried out on the Bruker’s scratch test system. Motion along the Z-direction is provided by the carrier for loading and displacement. The slider, consisting of a force sensor and optical microscope, is also accommodated in the carrier. Motion in the X-direction is facilitated by the slider. Motion along the Y-direction and mounting of the specimen is taken care of by the linear stage. The electrical contact resistance (ECR), acoustic emission (AE), and the electrical surface resistance (ESR) can be simultaneously measured by the scratch test system. For measuring the in-situ scratch depth profiling, a capacitance sensor or tip-displacement is used. Images of the complete scratch can be automatically captured by optical microscopy. The complete image of the scratch can be plotted along with other data like scratch distance and depth, ESR, ECR, AE, Fx and Fz using the Data Viewer software from Bruker.
The other components available for scratch testing on the Bruker’s all-inclusive scratch test system include:
- A series of force sensors: FVL: 1 to 100mN; FL: 5 to 500mN; DFM series: 0.05 to 20N; DFH series: 0.5 to 200N
- Scratching tips or styli: diamond stylus with tip radii of 2.5, 5, and 12.5µm; Rockwell indenter with 200µm tip radius and 120° cone angle; Vickers Indenter with a 4-sided 136° apex angle; Knoop Indenter with two apex angles 130° and 172°50/; microblades made of tungsten carbide and diamond
Scratch Test Results
The results of the some scratch tests performed on coatings and thin films using the Bruker’s scratch test system are presented below.
The scratch adhesion properties of paint coatings or ceramic coatings are routinely evaluated using the progressive scratch test performed on Bruker’s scratch test system. In figure 5, the Fz, COF and R (ECR) values have been plotted against the distance for the scratch test conducted on a diamond-like carbon layer (DLC) coating on a steel substrate.
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Figure 5. Fx, Fz, AE, and R versus distance (Y, mm) plot for scratch test on a paint specimen.
A tungsten carbide microblade of 400µm tip radius and a DFH-5 sensor were used to perform the scratch test. The microblade was installed under the force sensor and the coated sample was mounted on the Y-stage. An initial load of 1N was applied on the sample via the microblade, and then the specimen was moved by a distance of 5mm at a speed of 0.02mm/s. While the specimen was being moved, the vertical load was gradually increased from 1 to 45N and the corresponding values of ECR, Fx, Fz and AE were recorded. Images of the complete scratch were captured automatically after the test was completed.
It is evident from figure 5 that the control of Fz was excellent. The figure also depicts an optical micrograph of the complete scratch, where the image ruler depicts the image dimension (ΔX) between the horizontal lines segments in yellow and the related Y position. Using the image ruler, the optical image measurements are made by placing the line segments at appropriate positions with the help of a mouse pointer. At a distance of 4mm before the start of scratch, a sharp dip is seen in the electrical contact resistance plot along with a rising trend in Fx and high AE activities. At this point the optical image of the scratch demonstrated the beginning of delamination of the coating. It can be gathered from the ECR data, AE and Fx values and the optical image that the coating starts to fail at 4mm from the start of the scratch. The scratch adhesion value of the coating was derived from the Fz value at which the failure of coating occurred, which was found to be 39.6N.
Zigzag Scratch Test on a Magnetic Disk
Magnetic disk substrates have a submicron thick diamond-like carbon layer (DLC) coating with a lubricant layer on top. The disk is coated with DLC in order to protect it from corrosion and scratches by maintaining low friction. The high elasticity and hardness of the coating enhances the compatibility of the lubricant and minimizes the surface roughness. The progressive scratch method was not chosen for this test based on the mechanical properties and thickness of the film. Instead, the study of the thin and hard films was conducted using the zigzag test method.
The Fx, COF, Fz and AE were plotted against the scratch distance (Figure 6) after performing the zigzag scratch test on the magnetic disk. A Rockwell indenter of 200µm tip radius was used for the zigzag scratch test and the scratch distance was maintained as 2mm along the X direction with a velocity of 4µm/s. Over a scratch distance of 2mm along the X direction the normal load was increased linearly from 0.5 to 20N with the stylus oscillating in the Y direction. Three distinct regions are observed on the optical image shown at the top of figure 6. The first area shows the removal of the topmost lubricant layer. The second area shows the failure of DLC coating, represented in blue, and an increase in the AE and COF values at an Fz of 7.3N. The last area shows the scratching of the magnetic layer beneath the DLC and high values of COF and AE activity.
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Figure 6. Fx, COF, Fz, and AE versus distance (X) plot for a zigzag scratch on a magnetic disk.
X-Y Scratch Test on DLC Coating
Evaluation of a coating can be successfully done by performing the X-Y scratch test method on the Bruker’s scratch tester. Figures 7 and 8 represent the progress of the scratch in the X-Y plane and the corresponding scratch track on DLC coating on steel substrate, respectively. A diamond stylus with a 12.5µm radius and a FL sensor were used for the test.
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Figure 7. Trace of tip position for a X-Y scratch
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Figure 8. Optical image of the X-Y scratch on a DLC coating, as shown in Figure 7.
In the Y direction, the length of the scratch was found to be 120µm and the X-spacing between successive scratches was 60, 40, 30, 20, 15, 10, and 5µm, respectively. A constant normal load of 10µm was used for the test. Failure of the coating occurred between the sixth and the seventh scratch at a critical X- spacing of 10µm, as shown in Figure 8. The micro-cracks formed during earlier passes initiated the failure due to delamination.
Conclusions
The scratch test is an important step in the QC and R&D activities for thin films and coatings used for microelectronic, packaging, decorative, tribiological and biomedical applications. All kinds of complex scratch tests can be performed on the TriboLab universal scratch test system from Bruker. The scratch test system is equipped with automated optical imaging for comprehensive evaluation of coating and thin film materials.
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This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
For more information on this source, please visit Bruker Nano Surfaces.