Testing the adhesion of thin film to substrate

Testing the adhesion of thin film to substrate Lilia Proskurjakov 30. 03. 2016

Adhesion • Lifetime and stability depend on a good adhesion • Weakest factor in coating • qualitative and quantitative tests

Scratch test • Quantitative method • Critical Load Lc -> minimal load for failure • Calculation of adhesion energy • Two processes: – Progressive load – Constant load https: //www. pvd-coatings. co. uk/pvd-coating-technology/testing-equipment/scratch-tester/

Critical load Intrinsic Extrinsic • Scratching speed • Substrate & coating hardness • Loading rate • Substrate and coating roughness • Diamond tip radius • Coating thickness • Friction coefficient A. J. Perry, Scratch adhesion testing of hard coatings, Thin Solid Films, 107 (1983), p. 167 -180

Scratch test - Analysis • Light microscope / scanning electron microscope (SEM) • Acoustic emission detection • Friction analysis http: //www. idinspections. com/? page_id=126

Adhesion failures • Cohesive failure: o Cracking through thin film or substrate o Tensile stress • Adhesive failure: o Failure in interface o Compressive stress Chipping Spallation http: //www. frontiersemi. com/fsm 7/Application. php? type=2

Scratch test - example Diamon-like carbon film (DLC) 3. 5 N 7. 5 N Normal force Frictional force Penetration http: //nanovea. com/wp-content/themes/wp-nanovea/Application%20 Notes/coating-failure-scratch. pdf

Conclusion • A wide spectrum of qualitative and quantitative tests available for adhesion • Comparison of results of different tests not possible • Scratch test easy to performe • But different failurs at the same time make interpretation difficult

References [1] http: //www. tau. ac. il/~chemlaba/Files/Electrodeposition/13208_03. pdf [2] K. L. Mittal, Adhesion Measurement of thin films, Electro Component Science and Technology, 3 (1976), p. 21 -42 [3] H. K. Pulker, A. J. Perry, Adhesion, Surface Technology, 14 (1981), p. 25 -39 [4] A. J. Perry, Scratch adhesion testing of hard coatings, Thin Solid Films, 107 (1983), p. 167 -180 [5] P. A. Steinmann, Y. Tardy, H. E. Hintermann, Adhesion testing by the scratch test method: the influence of intrinsic and extrinsic parameters on the critical load, Thin Solid Films, 154 (1987), p. 333 -349 [6] S. J. Bull, E. G. Berasetegui, An overview of the potential of quantitative coating adhesion measurement by scratch testing, Tribology International, 39 (2006), p. 99 -114 [7] https: //www. bruker. com/fileadmin/user_upload/8 -PDFDocs/Surface. Analysis/TMT/Webinars/Advanced-Scratch-Testing-for-Evaluation-of-Coatings. Slides. pdf [8] http: //nanovea. com/wp-content/themes/wp-nanovea/Application%20 Notes/coating-failurescratch. pdf

Definition of the problem • Coating has a important role • Lifetime and stability of the coating mostly dependend on the adhesion of thin film and substrate • Adhesion has to be stable under many different conditions • Adhesion is the weakest point in coating • Qualitative and quantitative methods for testing

Information Slide 1 – Scratch test • Quantitative method • Vertically loaded diamond stylus is moved across the surface – Progressive load: load is linear increased during movement – Constant load: stylus is moved over film several times; with every time load is increased • Applied force causes stress at interface and produce failures in the coating • Minimal load at which the first failure is detected: critical load Lc

Information slide 2 – Critical load Lc • Is influenced by intrinsic and extrinsic parameters • With increasing thickness or the coating the value for Lc is increasing • For harder materials of the thin film or substrate Lc is increasing, too • Can be determined using: – Light microscope / SEM – Accoustic emission detection – Friction analysis

Information slide 3 - Failures • Cohesive failure: occur because of tensile stress and result in cracking in the substrate or the thin film layer • Adhesive failure: occur because of compressive stress and result in failures in the interface. Determine the adhesion strenght of the coating • Type of failure that occur depends on properties of substrate and thin film • Soft substrate and soft coating: mostly plastic deformation -> hard to determine adhesion -> scratch test more effective for hard coatings, where the coating is really removed from the substrate • Three other types of failure: through thickness cracking, spallation ans chipping • Difficulties in determining the adhesion quantitative occur if several failures take place at the same time