iTSSe TSS A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 2 3 7 iTSSe TSS COLD SPRAY: ADVANCED CHARACTERIZATION METHODS—MICROHARDNESS AND NANOINDENTATION This article series explores the indispensable role of characterization in the development of cold spray coatings and illustrates some of the common processes used during coating development. Dheepa Srinivasan A quick screening of the mechanical properties of a cold spray coating can be achieved by examining the microhardness—another ubiquitous tool for coating characterization to assist in optimizing the coating for a given set of parameters. Because cold spray coating thickness ranges from 0.25 to 12 mm (0.01 to 0.5 in.), Knoop and Vickers hardnesses are typically used to characterize the coating hardness, the latter being more suitable for thicker (>25 μm) coatings. Vickers microhardness is measured using loads ranging from 50 g to 1 kg (2 to 35 oz), depending on the expected coating hardness. An average of 10 measurements is recommended to span the thickness of the coating, as shown in Fig. 1a. It is also recommended that substrate hardness should be measured at a location away from the vicinity of the interface. The dwell time of the indent is typically 10 to 15 s. In the case of a softer coating, and especially a softer substrate such as pure aluminum, a lower load is used, i.e., <50 g. For harder substrates, a higher load is typically used, i.e., >100 to 500 g (3.5 to 18 oz). Vickers microhardness is calculated by using the standard formula: HV = 1.854 F/d2 where d is the mean of the two diagonals of the impression on the sample, and F is the applied force in kilogram force. It is standard practice to correlate the coating process parameters, extent of porosity, and other properties by using the coating microhardness. Due to the severe deformation that takes place in the cold spray process, a complex strain hardening is expected. Therefore, nearly all cold spray coatings tend to have 20-40% higher hardness in the as-sprayed condition than their wrought counterparts. A study on load sensitivity reveals that these coatings exhibit a behavior that is invariant under the applied load, as shown in Fig. 1b for NiCr coatings on AISI 4130 steel. Hardness does not usually vary much with coating thickness, as shown in Fig. 2a for an IN625 coating on an AISI 4130 steel substrate. Dotted lines indicate substrate hardness. With heat treatment, the hardness of many cold spray coatings increases, sometimes reaching as much as 30-40% higher than in their as-sprayed condition. This is contrary to what happens in most work-hardened materials, in which recovery processes tend to decrease the final hardness. An example is shown in Fig. 2b, taken from the IN625/NiCr cold spray coating, which shows a steady increase in hardness up to a certain temperature, above which it drops to a lower value. Coating hardness with time is shown in Fig. 2c for the same IN625 coating. Fig. 1 — (a) Schematic showing a possible method for hardness measurement in the cold spray coating and substrate; and (b) load sensitivity analysis of coating microhardness. (a) (b) FEATURE 7
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