Characterization of Residual Stresses in Al and Al/Al2O3 Cold Spraied Coatings

The residual stresses build up in cold spray deposition represent a key parameter to develop compact and well-adherent coatings. The physical and mechanical effects affecting the coating stress are numerous and various. They range from the peening effect due to the high velocity impact to the cooling effect due to the different thermal properties between coating and substrate materials. As a consequence, the resulting coating stress could be vary, both on macroscopic and microscopic scales, influencing several coating properties such as adhesion, structure, grain size, toughness, hardness and then reflecting on the macroscopic chemical and mechanical behaviour of the coating. Different techniques are devoted to investigate stresses in different scales; for example curvature method is typically used to estimate the macroscopic mean stress, while XRD-diffraction give more information about micro-stress and stress distribution. In the present work pure Al and Al/Al₂O₃ composites coatings have been deposited by coldspray using CGT-Kinetic4000 system equipped with special polymeric nozzle. The impact velocity and the substrate temperature have been evaluated using an in-flight particle sensor and a thermo camera analyser respectively. Deposition efficiency and microhardness have been determined and related to process’ parameters, while the microsctructure and the coating porosity have been observed by optical and electronic microscopy. Macroscopic and microscopic stresses have been evaluated using three different techniques: the macroscopic stress has been determined by means of a curvature method on calibrated Almen strips. On the other side, microstress and stress distribution have been determined both by XRD and Modified Layer Removal Method. The effect of the introduction of aluminium oxide inside the coating has been evaluated and discussed. The stress data emerging from different analysis have been compared then among each others and connected to the process parameters.