Ultramicrohardness cross-profiling of CVD diamond steel brazed junctions

Thick microwave plasma chemical vapour deposition (MPCVD) diamond films (24 0, 480, 1000 mu m) were grown, laser cut and vacuum brazed to steel bars wi th an Ag/Cu/Ti (100 mu m) reactive filler metal. Displacement sensing inden tation experiments using a dynamic ultramicrohardness tester on the brazing layer were performed on different samples. A calibration curve for the res idual depth values was made by linear regression to diagonal measurements o n Vickers impressions. Corrected depth values were then used to evaluate th e hardness. The thicker the films were, the higher the hardness was found t o be. Hardness changes from 1.35 to 1.72 GPa, respectively, for the thinnes t and the thicker film joints, getting an increment of ca 35%. These data i ndicate a hardening effect during the cooling stage caused by thermal expan sion coefficients mismatch. An increment of ca 25% on the flexural stresses imposed by the film/substrate curvature was theoretically evaluated. The s ame trend variation with film thickness was found for both the hardness val ues and the brazing strength from previous work. Brazing layer hardness eva luation can be regarded as a non-destructive indirect way to assess the mec hanical resistance of the diamond/steel joints. (C) 1999 Elsevier Science S .A. All rights reserved.