Integrity of APS, HVOF and HVAF sprayed NiCr and NiCrBSi coatings based on the tensile stress-strain response

Abstract

The interlamellar cohesion of thermal spray coatings influences greatly their mechanical properties and ability to use coatings in different loading conditions and wear/erosion resistance. In the present study, micro-tensile testing of free-standing coatings was utilized to evaluate the mechanical response of thermally sprayed coatings. In addition, the longitudinal uniaxial fracture strength of free-standing coatings could be determined by a tensile test. The coating materials studied were NiCr and NiCrBSi coatings sprayed by atmospheric plasma spraying (APS), high velocity oxy-fuel (HVOF), and high velocity air-fuel (HVAF) processes. The different materials used for the coatings sprayed by different methods yield different microstructures, different stress-strain relation in tensile testing. Different tensile test response was found to be related to cohesion strength between lamellas, and thus was affecting the cavitation erosion wear. The effect of other factors such as hardness and residual stresses on cavitation resistance were also discussed. Such results are crucial to understand the suitability of microstructures obtained by TS processes for different wear conditions.