Measurement of the Debonding Resistance of Strongly Adherent Thick Coatings on Metals via In-plane Tensile Straining

✍ Scribed by S. Ryelandt; L. Delannay; R. Persoons; K. Van Acker; F. Delannay

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 433 KB
• Volume: 9
• Category: Article
• ISSN: 1438-1656
• DOI: 10.1002/adem.200700014

✦ Synopsis

Prevention of damage in the form of through-thickness cracking and/or interfacial debonding, leading to scaling is a key issue in the engineering of coatings. In this context, reliable testing methods are needed for the quantitative assessment of the overall quality and reliability of deposition processes. Coatings for components subject to heavy abrasive wear commonly belong to the family of thick (> 50 lm thickness), strongly adherent coatings. These coatings are usually made of intermetallic compounds, or of hard ceramic particles in a softer metallic matrix. They can be deposited by a variety of techniques. We focus in this work on the laser cladding technique, which has been shown to provide improved damage resistance with respect to thermal spraying methods. [1][2][3][4][5][6][7][8][9][10][11] A simple empirical test for assessing the damage tolerance of coatings consists in observing the crack pattern that forms under controlled straining. In such test, the coated sample is subjected to an increasing tensile strain (e.g. under uniaxial tension or beam bending), causing the film to crack and break into segments. [12] The crack initiation strain and the subsequent multiple cracking can be observed optically or by acoustical emission. [13][14][15] The crack density increases with increasing strain in a manner which is related to the overall properties of the coating and of the interface. [13] Quantitative values of interfacial properties can be inferred from appropriate models. [13,16,17] In case the coating debonds without extensive plastic yielding (both in the coating and in the substrate), a fourpoint bending test is frequently used. That test expresses adhesion in terms of the debonding toughness of the coating/ substrate interface. [e.g.18,19] The approach is based essentially on linear elastic fracture mechanics (LEFM). It has often been applied to sputtered and thermally sprayed coatings. [1,2,[20][21][22][23][24] The procedure requires a precrack both in the coating and along the interface, and, if the coating is too thin and/or too strongly adherent, the deposition of an upper layer enhancing the release of strain energy is required. This experimental procedure may prove difficult to carry out without affecting the coating properties. The reliability of the adhesion value depends on a precise knowledge of the substrate and coating elastic/yielding properties and of the internal stresses in the two materials.

LEFM-based tests such as the four point bending test fail to be applicable when the coating adhesion is high: as a matter of fact, the adhesion of anti-wear coatings is most often much higher than can be probed by such tests. Authors then tend to rely only on semi-empirical approaches based on the measurement of the debonding strength under either a tensile load normal to the interface or a simple shear load parallel to the interface. [10,11] Special test fixtures and careful specimen alignment are required. An essential condition is that failure occurs along the interface and not inside the substrate or coating.