| The scratch adhesion test appears to be a simple and rapid method to assess qualitatively the adhesion of thin brittle coating to a substrate. Despite the widespread use of the method, it is difficult to express adhesion in a quantitative way because the cirtical load is influenced by many non-interface factors such as materials properties of coating and substrate, film thickness, interface bonding and by several parameters related to the test condition. Therefore, the stress-strain distribution of coating-substrate systems under a indenter pressure was analyzed by the establishing of hemi-spheric cavity model and the three-dimensional finite element simulation results. Then, the failure modes and failure mechanisms of coating-substrate systems in scratch adhesion tests were analysed and summarized by combining with the mechanical failure of thin brittle coatings. The results found that, the main reason for the varity of failure modes in scratch testing was the differenes in stress-strain distribution of coating-substrate systems caused by the non-interface factors such as coating and substrate properties and coating thickness.TiN thin brittle coatings with three thickness, Ti coating and Ti/TiN multilayer coating were deposited on17-4PH stainless steel and TC4alloy substrates successfully by unbalanced magnetron sputtering technique. Scratch test was used to express the adhesion performence of the coated systems. For the coating like Ti, which its shear strength was lower than the adhesion strength to the substrate, practical significance can not be obtained from the results of scratch tests. For the TiN coated systems, with different coating thickness and substrate materials, residual stress and substrate showed a great influence on the scratch failure maps and the value of critical load. And the Ti/TiN multilayer coating showed a better adhesion strength to the substrate according to the scratch testing because of the low residual stress, excellent carrying capacity and the stress-stain absorption of the Ti interlayer.Low load multiple reciprocating friction and wear test, or the friction and wear test which was carried out within the elastic range of coating/substrate system, was designed to evaluated adhesion strength of a coating to substrate, considering the influence of non-interface factors on scratch adhesion tests. In this tests, shear stress, caused by the tangential reciprocating sliding of the counterpart on the surface of coating/substrate system at the interface, could induce the initiation of cracking and coating delamination. And the coating delamination provided a gist for the qualitative analysis of coating/substrate adhesion performance. Scratch tests and low load multiple reciprocating friction and wear tests were carried out on TiN/17-4PH and TiN/TC4systems. And some similarities have been shown between the two tests’results:high residual stress showed a great influence on the adhesion of TiN coating to17-4PH stainless steel and TC4alloy. Coating delamination caused by stress releasing was observed in both two tests. For TiN coatings with two different thickness on TC4substrate, cracking initiation and growth at the interface were the main reason for coating delamination under the indenter or the counterpart. And a qualitative analysis results of coating/substrate adhesion and service performance can be achieved by analyzing the differences in coating failure modes. |
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