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

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.

Oxidation‐led decomposition of hexagonal boron nitride coatings on alloy substrates at 900 °C: Ti‐6Al‐4V

“…This causes the initially discrete entities to agglomerate into a compact bulk that impedes further oxygen from reaching reaction sites. This effect was evident in the thermogravimetric analysis (TGA) of h‐BN powder that was described in the companion piece to this present study . In the first 24 h at 900 °C in dry air, 43.77% of the starting material reacted; in the next 48 h only a further 7.95% reacted.…”

“…The absence of alumina peaks in diffractograms taken both during and subsequent to the decomposition of h‐BN and boron trioxide, however, suggests that an alternative reaction mechanism took place in this system. This is analogous to the case of heat‐treated boron trioxide on SS304 where manganese borate but no manganese oxides were detected . Whilst it is possible that the rate of reaction to form the borate species was exceptionally fast compared with the rate of reaction to form the oxide species, it is thought extremely unlikely that the second stage reaction could happen so fast and in such a complete manner as to not leave any XRD detectable trace of the intermediate oxide.…”

“…The resultant trace is shown in Figure , along with that from the early stages of exposure of equivalently heat‐treated uncoated Ti‐6Al‐4V (the reader is referred to Figure for the full version of this trace). The inset to Figure shows the initial mass gain undergone by the coated Ti‐6Al‐4V system alongside the full plot from isolated coating exposed to the same conditions, which was described previously …”

“…At this point, it is pertinent to comment on the finding from the analogous chromia‐former experiments that appreciable manganese and iron were present at the scale‐film interface, but no manganese or iron oxides . Based on this, it was proposed that the overlayer inhibited the formation of these oxide species.…”

“…Tool materials are typically chromia-former alloys, and the 900°C ambient atmosphere fate of h-BN coatings on examples of these substrates was reported previously by the same author. [7] In this complementary body of work, the chemical reactions undergone by an h-BN coating on the archetypal SPF titanium alloy, Ti-6Al-4V, under these process conditions are elucidated.…”