Processing Effects of Chromium Carbide/Nickel Chromate on Coating Performance: An in Depth Approach by Using Process Maps and in situ Characterization

There have been enormous advances in technologies for thermal spray over the past few decades. One such application is the replacement of electroplated hard chrome for aerospace and automotive industries. Hard chrome electroplating has been a valuable surface treatment for parts in corrosion and wear applications due to its high hardness, ability to passivate, as well as its low coefficient of friction. In the past two decades, there have been concerns due to limitations in hard chrome's performance as well as environmental effects due to hexavalent chromium produced during processing. High velocity oxy-fuel (HVOF) spray processing has been developed to produce exceptional coating quality due to the very dense microstructures formed with limited porosity.;Cr3C2-NiCr has been shown to be a viable replacement to electroplated hard chrome when deposited by HVOF spray techniques. In order to produce optimized coatings with this technology, a process mapping methodology was implemented to understand the relationship between process variables. The variations of oxygen to fuel ratios as well as total volume flows of gases were examined to interrelate process variables with the particle state, stress evolution during deposition, and properties of the coatings. The performance of the coatings in aqueous corrosion and sliding wear environments were correlated back to the properties of the coatings. It has been demonstrated that monitoring the in-flight particles and evolution of stress can be directly correlated to the properties of the coating and in-directly to the coating performance.;Cr3C2-NiCr property and performance was also compared to hard chrome and WC-CoCr coatings. It was shown that the optimized coatings of Cr3C2-NiCr outperformed hard chrome in both aqueous corrosion as well as sliding wear environments. Cr3C2-NiCr was outperformed by WC-CoCr in sliding wear applications due to its lower hardness, but outperformed WC-CoCr in aqueous corrosion due to the denser microstructures that were produced.