Oxidation‐led decomposition of hexagonal boron nitride coatings on alloy substrates at 900 °C: chromia‐formers

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Abstract

In contrast to a mass of powder in air at 900 °C, the oxidation rate of hexagonal boron nitride does not tend to a plateau when present as a coating on austenitic stainless steel type 304 and type 310, and a real-world hot forming tool alloy. As with a mass of powder, a molten boron trioxide diffusion barrier encapsulates each hexagonal boron nitride grain around the basal plane. These subsequently merge to form a dispersion of remnant grains throughout a molten matrix. Boron trioxide is semipermeable, and as such, the substrate oxidizes concurrently. As it does so, outwardly diffusing chromium (III) ions react with boron trioxide to form chromium borate, which depletes the diffusion barrier. This causes the oxidation of hexagonal boron nitride to be sustained at an enhanced rate and the overlayer ultimately undergoes complete decomposition. During this process, the scale comprises chromium borate and chromia only. While manganese chromium spinel and iron chromium spinel form readily at the oxide-atmosphere interface of equivalently heat-treated uncoated substrates, they do not form until after the coating has completely decomposed.

Original languageEnglish
Pages (from-to)853-867
Number of pages15
JournalMaterials and Corrosion-Werkstoffe und Korrosion
Volume70
Issue number5
Early online date25 Jan 2019
DOIs
Publication statusPublished - 1 May 2019

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Keywords

  • high temperature oxidation
  • decomposition reaction
  • austenitic steels
  • ceramic platelets
  • metal forming

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