Mechanical behavior of a chromium coating on a zirconium alloy substrate at room temperature

The mechanical behavior of a 15 µm-thick chromium coating deposited on a zirconium alloy substrate using a particular physical vapor deposition process is studied at room temperature using several experimental techniques at different scales: biaxial tests (internal pressure + axial tension) with several stress biaxiality ratios on outer-coated tubes; in situ tensile tests in a scanning electron microscope (SEM) on coated sheet samples. The cracking and plasticity of the coating are studied using acoustic emission, digital image correlation and SEM observations. The results show that the first channeling cracks in the coating initiate at 0.3-0.4% macroscopic strain, when the substrate starts to yield. Under further loading, the crack density increases first rapidly then more slowly, before reaching saturation. Plastic strain of the non-cracked coating regions is observed, in addition to the increase of crack opening. No interface decohesion of the coating is observed. The cracks, both transgranular and intergranular, do not penetrate significantly into the substrate.