Micro Scratch Testing CrN Coating on Titanium Alloy - Nanovea

MICRO SCRATCH TESTING OF CrN ON TITANIUM ALLOY

Prepared by Pierre Leroux

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INTRO Titanium alloys are known for very high tensile strength and toughness, light weight, corrosion resistance and the ability to withstand extreme temperatures. The superior and unique ability results in a high cost of both raw materials and processing, limiting use to military applications, aircraft, automotive, spacecraft, medical devices and consumer electronics. Titanium alloy is used in high stress environments due to its durable properties. Titanium is a strong, light metal, as strong as steel but 45% lighter. It is also twice as strong as aluminum but only 60% heavier. IMPORTANCE OF MICRO SCRATCH TESTING FOR QUALITY CONTROL Specific Coatings are used to further increase performance of titanium alloy parts at elevated temperatures. More specifically, Chromium Nitride (CrN) Coatings are designed to perform under conditions of high load and temperature. By using scratch testing it is possible to detect premature adhesive failure of the CrN coating in real-life applications. The test was performed at standard temperature but the test could also be performed at high temperature to better simulate the conditions to which the coating is subjected. MEASUREMENT OBJECTIVE The process of scratching is simulated in a controlled and monitored manner to observe adhesive or cohesive failures. In this application, the Nanovea Mechanical Tester in its micro scratch testing mode is used to measure the load required to cause failure to a CrN coating on a cylindrical Titanium Alloy substrate. A 20µm spherical diamond stylus is used at progressive load ranging from 0.1N to 8N to scratch the coating. The points where the coating fails by cracking are taken as critical failure points.

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MEASUREMENT PRINCIPLE: The scratch testing method is a very reproducible quantitative technique in which critical loads at which failures appear are used to compare the cohesive or adhesive properties of coatings or bulk materials. During the test, scratches are made on the sample with a sphero-conical stylus (tip radius ranging from 1 to 20 m) which is drawn at a constant speed across the sample, under a constant load, or, more commonly, a progressive load with a fixed loading rate. Sphero-conical stylus is available with different radii (which describes the “sharpness” of the stylus). Common radii are from 20 to 200 m for micro/macro scratch tests, and 1 to 20 m for nano scratch tests. When performing a progressive load test, the critical load is defined as the smallest load at which a recognizable failure occurs. In the case of a constant load test, the critical load corresponds to the load at which a regular occurrence of such failure along the track is observed. In the case of bulk materials, the critical loads observed are cohesive failures, such as cracking, or plastic deformation or the material. In the case of coated samples, the lower load regime results in conformal or tensile cracking of the coating which still remains fully adherent (which usually defines the first critical load). In the higher load regime, further damage usually comes from coating detachment from the substrate by spalling, buckling or chipping.

Figure 1 : Principle of scratch testing

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Comments on the critical load The scratch test gives very reproducible quantitative data that can be used to compare the behavior of various coatings. The critical loads depend on the mechanical strength (adhesion, cohesion) of a coating-substrate composite but also on several other parameters: some of them are directly related to the test itself, while others are related to the coating-substrate system. The test specific parameters include: Loading rate Scratching speed Indenter tip radius Indenter material

The sample specific parameters include: Friction coefficient between surface and indenter Internal stresses in the material For bulk materials Material hardness and roughness For coating-substrate systems Substrate hardness and roughness Coating hardness and roughness Coating thickness

Means for critical load determination Microscopic observation This is the most reliable method to detect surface damage. This technique is able to differentiate between cohesive failure within the coating and adhesive failure at the interface of the coating-substrate system. Tangential (frictional) force recording This enables the force fluctuations along the scratch to be studied and correlated to the failures observed under the microscope. Typically, a failure in the sample will result in a change (a step, or a change in slope) in coefficient of friction. Frictional responses to failures are very specific to the coating-substrate system in study. Acoustic emission (AE) detection Detection of elastic waves generated as a result of the formation and propagation of microcracks. The AE sensor is insensitive to mechanical vibration frequencies of the instrument. This method of critical load determination is mostly adequate for hard coatings that crack with more energy. Depth Sensing Sudden change in the depth data can indicate delimitation. Depth information pre and post scratch can also give information on plastic versus elastic deformation during the test. 3D Non-Contact imaging such as white light axial chromatism technique and AFM’s can be useful to measure exact depth of scratch after the test.

Test parameters Load type

Progressive

Initial Load

0.10 N

Final Load Loading rate Scratch Length Scratching speed, dx/dt Indenter geometry

8.00 N 8.00 mN/min 5 mm 5.063 mm/min Rockwell (120° cone)

Indenter material (tip

Diamond

Indenter tip radius

20 m

Cone angle Tip Radius

Figure 1: Sphero-conical indenter

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Results This section presents the data collected on the failures during the scratch test. Detailed results – Sample #1

Sample #1 (GOOD)

Type of Scratch

Load (N)

Initial Coating Deformation First Crack Initial Chipping Continuous Chipping Initial Delamination Complete Delamination

0.611 1.186 1.853 2.405 2.405 5.403

Micrograph of Initial Coating Deformation 500x magnification (image width 0.086mm)

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Micrograph of First Crack 500x magnification (image width 0.086mm)

Micrograph of Initial Chipping 500x magnification (image width 0.086mm)

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Micrograph of Continuous Chipping 500x magnification (image width 0.086mm)

Micrograph of Initial Delamination 500x magnification (image width 0.086mm)

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Micrograph of Complete Delamination 500x magnification (image width 0.086mm)

Load vs. Displacement Graph

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Detailed results – Sample #2

Sample #2 (GOOD)

Type of Scratch

Load (N)


0.516 0.795 1.693 1.954 5.542 6.820


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Sample #3 (GOOD)

Type of Scratch

Load (N)

Initial Coating Deformation First Crack Initial Chipping Initial Delamination Complete Delamination

0.306 0.794 1.257 1.376 5.733


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Micrograph of Complete Delamination 500x magnification (image width 0.086mm) 15



Sample #4 (BAD)

Type of Scratch

Load (N)


0.438 0.762 1.168 1.864 2.126 6.941

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Sample #5 (BAD)

Type of Scratch

Load (N)

Chipping and Initial Delamination

0.205

First Crack Continuous Delamination Complete Delamination

0.588 0.906 4.049

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Micrograph of Chipping and Delamination 500x magnification (image width 0.086mm)


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Discussion

The main failure that clearly relates to real-life performance of the CrN coatings is the chipping delamination that is seen on the side of the scratches on the parts that have performed poorly. This behavior was not seen on “GOOD” parts. It is to be noted that the other failures could prove critical for other aspects or if the coatings were used in other applications. It must be noted that changing the diameter of the diamond used during the test could emphasize other aspects of the use of these coatings. For example, a smaller radius might simulate a grain of sand being squeezed on the surface and creating local cracking while larger tip might indicate pressure from wider contact. Sometime a good coating for one application might not be right for another and this is also valid for the test used to qualify them.

Conclusion

Nanovea Mechanical Tester, during Micro Scratch Tester Mode, is a superior tool for quality control of CrN coatings on Titanium Alloy parts. Scratch testing can detect adhesion problems in coating process before parts are actually put to use. By applying loads in a controlled and closely monitored fashion, the tool allows to identify quantitative and reproducible critical load failures. Chipping delamination on the side of the scratch proved to be related to premature failure of the coatings. This type of information can help manufacturers improved the quality of their coatings.

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