Neutron Diffraction Study of Macrostress and Microstress in Al-Al2O3-Based Corrosion Protection Coating Obtained by Cold Spray (Dynamic Metallization)

Abstract

Protective coatings based on an Al-Al 2 O 3 metal matrix composite (MMC) were sprayed using dynamic metallization (DM), a low-pressure cold spray variant. A series of samples approximately 1 mm in thickness were sprayed using different spray process parameters (temperature, velocity) and different feedstock powder compositions (Al, Zn, Al 2 O 3 ). This resulted in MMCs of different phase compositions and slightly different physical conditions of coating formation. The through-thickness residual stresses that accumulate in coatings during the spray process were studied using neutron diffraction in all phases comprising the MMCs. The overall residual stress in the coating (macrostress) was compressive, which is in good agreement with the data on residual stress observed in other cold spray coatings, accumulating as a result of the peening process. However, due to the slightly elevated spray temperature characteristic of DM in comparison with other cold spray variants, thermal stresses are also present and play an equally important role in the accumulation of residual stress in each phase. Because of the multi-phase composition and thermal mismatch between the metal and ceramic components of the MMC, inter-phase microstresses also accumulate. A micro-mechanical explanation of the observed tensile microstress in Al/Zn versus compressive stress in Al 2 O 3 is proposed.

Measuring Residual Strain and Stress in Thermal Spray Coatings Using Neutron Diffractometers

“…As will be seen through examples below, neutron sources have been regularly used to study residual strains in thermally sprayed and cold sprayed coatings. For coated samples -SANS QUOKKA (monochromatic) § -USANS KOOKABURRA (monochromatic) § Ni, Ni-29%Cr, WC-17%Co [47]; Inconel 625 [48]; Ni-914-3 [50]; AA-6061-T6 [51]; Ti [52]; Inconel®718 [53]; Cu, Al [62]; YSZ [72] 0.2 × 0.2 × 15 mm 3 (and 0.3 mm) [47]; 0.2 × 0.2 × 20 mm 3 [48,50]; 0.5 × 0.5 × 18 mm 3 [51]; 0.5 × 0.5 × 20 mm 3 [52]; 1 × 1 × 1 mm 3 Ni + 5%Al [58]; IN625 alloy [59]; Al, Si, Mo 2 C-Mo [60]; Mo [61]; Cu, Al [62]; Al [63]; Al-Al 2 O 3 [64]; 0.5 × 0.5 × 7 mm 3 [39]; 1 × 1 × 7 mm 3 [58]; 2 × 2 × 7 mm 3 [59]; 0.5 × 0.5 × 18 mm 3 [62]; 0.5 × 0.5 × 7 mm 3 of various thicknesses, investigators have used various gauge volumes with slit widths (0.2 mm, 0.3 mm, 0.5 mm) in through thickness dimension. Interestingly, different strategies were considered to obtain strain-free lattice parameter ( d 0 hkl ) for the coating and substrate part.…”

“…Using the BT8 instrument at NIST, Luzin et al [64] investigated residual stresses in low-pressure cold sprayed coatings (e.g., Al-Al 2 O 3 metal matrix composite) of 1 mm thickness onto 3 mm thick A16061 aluminium substrate. A gauge volume 0.5 × 0.5 × 20 mm 3 was used and applied equal-biaxial stress approach to obtain residual stresses.…”

Measuring Residual Strain and Stress in Thermal Spray Coatings Using Neutron Diffractometers

“…Using the BT8 instrument at NIST, Luzin et al [55] investigated residual stresses in low-pressure cold sprayed coatings (e.g., Al-Al2O3 metal matrix composite) of 1 mm thickness onto 3 mm thick A16061 aluminium substrate. A gauge volume 0.5 x 0.5 x 20 mm 3 was used and applied equal-biaxial stress approach to obtain residual stresses.…”

“…In the example shown, it was observed that the overall residual stress in the coating was compressive. Also, due to the elevated spray temperature characteristic of low-pressure cold sprayed coatings (dynamic metallization), thermal stresses were also present, and because of the multi-phase composition and thermal mismatch between the metal and ceramic components of the metal matrix composite (MMCs), inter-phase micro-stresses also accumulated, leading to overall compressive residual stresses [55].…”

Measuring Residual Strain and Stress in Thermal Spray Coatings Using Neutron Diffractometers

“…It was proposed that a deconvolution procedure (i.e., modelling the Gaussian-like distribution of the intensity through the gauge volume may increase the resolution in the interface region. In another example, BT8 instrument at NIST in partnership with KOWARI instrument at ANSTO[55] studied residual stress using BT8 instrument and texture using KOWARI instrument. Such collaboration is good for informed test measurement and guidance along with potential to open new avenue of future research.Neutrons diffraction techniques complement other characterisation techniquesand number of neutron facilities worldwide are considered the most useful for materials of today as well for future new materials.…”