Property Evolution in Amorphous Steel Coatings by Different Thermal Spray Processes

Amorphous/nanocrystalline coatings are very useful for high strength, wear and corrosion-resistant applications, and these coatings are usually produced in thermal spray processes because of rapid cooling rates. Here we report, characterization of SHS7170 coatings developed by APS, high-velocity oxy-fuel (HVOF), TWA, and a novel hybrid spray technique. Phase and microstructural analysis of the optimized SHS7170 coatings revealed an amorphous structure with very fine nanocrystals embedded in the HVOF and APS coatings, and crystalline structure with grain size in the range of several nanometers in the TWA and hybrid spray coatings. Mechanical test data obtained from shear punch and impact tests illustrated brittle behavior of the amorphous coatings (APS and HVOF coatings) with lower strength compared to the nanocrystalline coatings (TWA and hybrid coatings). Corrosion tests showed the influence of microstructure and porosity of the coatings on polarization curves obtained. Wear and coefficient of friction data indicated similarities between the HVOF, APS and hybrid coatings, albeit the microstructure of hybrid coatings was noticeably different from the other two. TWA sprayed coatings exhibited higher coefficient friction with higher weight loss compared to the remaining three coatings. Erosion tests indicated higher weight loss for APS and TWA coatings at two different chosen angles and erosion behavior was observed to follow the hardness data of the coatings.