Evaluating the efficacy of aluminide coatings to improve oxidation resistance of high performance engine valve alloys

• Published in: Surface & coatings technology 2021-09, Vol.421 (online), p.127401, Article 127401
• Main Authors: Pillai, R., Dryepondt, S., Armstrong, B.L., Lance, M.J., Muralidharan, G.M.
• Format: Article
• Language: English
• Subjects: Alloys; Automotive engines; Chemical potential; Coupled thermodynamic-kinetic modeling; Depletion; Diffusion coatings; Engine valves; Exhaust engine valves; High temperature; Interdiffusion; Intermetallic compounds; Internal combustion engines; Iron compounds; MATERIALS SCIENCE; Materials selection; Ni-base alloy; Nickel aluminide coating; Nickel aluminides; Nickel base alloys; Nickel compounds; Operating temperature; Oxidation; Oxidation resistance; Potential gradient; Substrates; Titanium; Transportation industry
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2021.127401

Increasing peak cylinder pressures and operating temperatures of turbo-charged internal combustion engines (ICEs) engines present new challenges for existing materials employed in engine exhaust valves. Oxidation induced degradation of current Ni based alloys for these components will be a life-limiting mechanism while strict cost margins in the automotive transportation industry further limit the choice of suitable candidate materials. Metallic diffusion aluminide coatings can provide a cost-effective way to improve the oxidation resistance of underlying materials. In this study, the high temperature oxidation behavior of diffusion nickel aluminide coatings on two Ni based alloys (commercially available alloy 31V and a developmental alloy) during cyclic oxidation behavior in air+10% H2O at 900 °C was investigated. A complete depletion of the beneficial Al-rich β-(NiFe)Al phase was observed in the coating on alloy 31V while a significant fraction of this phase was retained in the coating on the second alloy. A coupled thermodynamic-kinetic model showed that the disappearance of the Al-rich phase in the coating on 31V was mainly due to the combined effect of steeper chemical potential gradients of Al and Ti between the coating and the substrate and their faster diffusivities compared to the coated developmental alloy. A higher content of Fe was shown to support the retention of β-(NiFe)Al phase in the coating on the developmental alloy while the presence of Ti in the alloy was shown to be detrimental for long-term coating performance. •Protective capability of aluminide coatings on two Ni-based alloys was investigated.•Thermokinetic modeling predicted coating degradation and lifetime.•Higher Fe content in the substrate stabilizes the Al-rich β-(NiFe)Al phase.•Faster depletion of β-(NiFe)Al in the coating on lower Ti-containing substrate•Higher Ti content in the substrate expected to impact oxidation behavior in wet air