On Separating Bonding Strength From Film and Substrate Properties
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JIA-WEN HE, B.C. HENDRIX, MAO-ZHONG YI, NAI-SAI HU National Laboratory for Mechanical Behavior of Metallic Materials, Xi'an Jiaotong University, Xi'an 710049, China ABSTRACT
Bonding strength is commonly measured by various kinds of single cycle tests such as scratch tests or indentation tests. These tests all reflect complex composite properties - both plastic and elastic properties of the film and the substrate - as well as the bonding strength. In many cases experimental critical value of these single cycle tests is controlled by film or substrate strength or ductility rather than bonding strength. In these cases, the single cycle test should be evaluated in terms of the specific property which controls failure and should not be taken as a measure of "bonding strength." However, the interfacial fatigue strength measured at 5X10 6 cycles is shown to be measurable over a large range of bonding strengths and composite properties. And further, this interface fatigue strength is insensitive to the composite properties. The complementary nature of these two types of tests is demonstrated for various TiN-based hard coatings on various steel and cemented carbide substrates. GENERAL TEST CHARACTERISTICS
The bonding strength between films (thin or thick) and their substrates have been measured by a number of methods which are reviewed and compared by Bull and Rickerby [1]. The pulloff and related fracture toughness tests [2] are limited by the strength of the adhesives used to
mount the sample.
The scratch test [3] slides an indentor across the coated surface, increasing the load until, at some critical load, L., debonding occurs. It has received wide-spread use because it can quickly evaluate a coating-substrate system. However, it measures a combination of the substrate properties, the film properties, film thickness, and bonding strength. It is also susceptible to small changes in the tip condition [3] and the coefficient of friction. Indentation debonding tests generally examine film debonding at the conclusion of a test to a specified load. Tests are performed at different loads and a critical load for debonding, Po, and/or crack size as a function of load are used to characterize the test. By continuously monitoring load, displacement, and acoustic emission (AE) during an indentation test, a critical load during loading, Pc1, can be obtained in a single loading cycle [4]. Although the indentation test is still sensitive to substrate properties, film properties and film thickness, it has the advantage of being insensitive to small changes in tip condition and friction coefficient [3]. This is exemplified by a series of tests where the bonding was varied by changing the roughness of the substrates shown in Figure 1. Although the indentation critical load, Pe, shows a clear increase with increasing roughness, the scratch critical load is insensitive to the change in interface condition. It is suggested that the increased interfacial strength is offset by an increase in friction coefficient. The indentation debonding
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