STUDY OF THE DELAMINATION OF DIAMOND COATINGS UNDER THERMAL-STRESS

The influence of the thickness of CVD diamond coatings on the adhesion to a substrate, after cooling down from deposition temperature to roo m temperature, has been studied experimentally and theoretically. Diam ond layers have been deposited at 850 degrees C on W substrates by mic rowave plasma enhanced CVD. Cooling down of the substrate-diamond coat ing system to room temperature induces thermal stresses, due to differ ent thermal expansion coefficients of coating and substrate. For thick diamond coatings a total and sudden delamination could be observed as a consequence of these stresses. On the contrary thin coatings, produ ced under identical circumstances, adhered well. These phenomena have been modelled and explained by the use of an energetic criterion for t he delamination of a two-layer system under thermal stress. From the m odel a critical thickness of the coating can be calculated. Above this critical thickness, delamination will suddenly occur. The calculation s also predict that for intermediate coating thicknesses delamination can easily be induced by external causes. (C) 1997 Elsevier Science S. A.