July/August_AMP_Digital

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 | J U L Y / A U G U S T 2 0 1 8 4 4 iTSSe TSS iTSSe TSS R esidual stress builds up during the high pressure cold spray process. Characterizing this stress is paramount to assessing coating reliability during service operation. Residual stresses in cold spray coatings are caused by the plas- tic deformation of powder particles, which in turn is caused by the peening effect of the incoming high velocity particles that deform the underlying material. Because this process takes place at relatively low temperatures compared to ther- mal spray processes, it enables the deposition of ductilemetal coatings at any desired thickness. Residual stress can affect coating adhesion to the sub- strate as well as the fatigue behavior of the substrate. Be- cause the cold spray process is akin to shot peening in many ways, compressive stresses tend to develop in the coating. It is well known that compressive residual stresses are benefi- cial for fatigue behavior. Table 1 compares shot peening and cold spray coating processes. Significant peening stresses are generated during the impact of particles on the substrate and on previously deposited particles as the coating is built to the required thickness. Fatigue cracks initiate on the surface of a component, so an overall compressive stress on the surface is typically the preferred state to enhance the fatigue life of the component. It is therefore imperative to characterize the re- sidual stress of any cold spray coating, both in the as-sprayed condition and at the application temperature. Residual stresses are typically generated in two ways— as macrostresses that cover several grains and encompass the entire structure, and as more localized microstresses that cov- er individual grains or phases of the material. In both cases, residual stresses are those stresses that remain in thematerial after external forces have been removed. Compressive stress- es act by pushing thematerial together and aremathematical- ly denoted as negative, while tensile stresses act by pulling the material and are denoted as positive. MEASUREMENT TECHNIQUES Two nondestructive techniques have been used to mea- sure residual stress. One uses Stoney’s equation and deter- mines the bending radius of the substrate and the other uses x-ray diffraction (XRD). Only the latter is discussed here. The traditional XRD method uses the sin 2 Ψ technique, which is based on measuring the shift in diffraction peaks for different values of the angle Ψ between the diffracting plane normal and the specimen surface normal. In this method, a specific plane is selected and the interplanar spacing is mea- sured froma θ to 2 θ scan (froma standard Bragg-Brentano ge- ometry) of the specimen at different tilt angles Ψ . Thereafter, the residual strain canbederived fromthe slope of a linear plot between the fractional change of the place spacing (i.e., strain) and sin 2 Ψ . A biaxial stress model is then used to convert the measured strain to the stress. X-ray diffraction is by far the best and most readily avail- able, versatile technique that can provide accurate infor- mation about the through-thickness coating residual stress. Figure 1 provides a schematic that shows how layer-by-layer removal is performed to estimate the through-thickness resid- ual stress. A few reports mention the shortcomings of the XRD technique in terms of the small sampling area as well as the COLD SPRAY: ADVANCED CHARACTERIZATION METHODS—RESIDUAL STRESS 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 FEATURE TABLE I — COLD SPRAY VERSUS SHOT PEENING CHARACTERISTICS Cold spray Shot peening Surface modification process Surface modification process Very high velocity, in the range of 300-1100 m/s (1000-3600 ft/s) Shot velocity in the range of 50-100 m/s (160-330 ft/s) Particles deform plastically on impact and develop coating surface on substrate Rigid balls, do not stick to surface Diameter of particles on the order of 5-40 μ m Steel balls used, size 1-2 mm (0.04-0.08 in.) Requires process gas No process gas Additive process Nonadditive process Complex geometry can be sprayed Flat surfaces provide best results 10