Mechanical stability of boron-based coatings grown on Incoloy 909 superalloy by thermochemical diffusion

Abstract

Incoloy 909 superalloy was treated at 950°C to form coatings of different chemical composition by thermochemical diffusion. Packing of the samples in reactive powders lead to the formation of iron borides (Fe 2 B and FeB) together with nickel boride, NiB, and nickel silicide, Ni 2 Si, on the surface of the alloy depending on the exposure time. X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy analyses suggest that growth of the coatings is controlled by diffusion of reactive species into the metal structure. Microabrasion wear resistance of the coated specimens was studied at 1 N load for a fixed sliding speed of 0.11 m s −1 and it was compared against uncoated samples of the alloy. It was found that coating the alloy for 12 and 20 h increased the hardness of the alloy and therefore its wear resistance.

Investigation of mechanical, kinetic and corrosion properties of borided AISI 304, AISI 420 and AISI 430

“…The examination of the borided coating layer morphologies showed the existence of a compact layer structure and a clearly visible interface between the matrix and the borided layer is clearly visible and the layer has a compact structure. While a saw-toothed morphology was reported for the boride layer in the previous studies due to the low concentration of carbon [12,22,23], a planar morphology was observed in this study due to high alloy ratios of the materials, as shown in the literature [24,25]. As the carbon content of the matrix material and the ratio of the alloying elements increase, a more planar morphology was encountered, as the toothed structure expected to form on the matrix interface of the boride layer was suppressed [26].…”

Oxidation behaviour of borided Incoloy 909

“…It will be a good practice to remember that the coating includes the nickel silicide Ni 3 Si, nickel boride NiB, and the iron borides Fe 2 B and FeB as its main components and that were always detected after the oxidation tests (Figure 2(a–d)). From these phases, the nickel silicide Ni 3 Si is not formed during the thermochemical diffusion process and Ni 2 Si is rather expected [7]. The latter phase contains more silicon as it can be found in the Ni–Si phase diagram [9].…”

“…This was proven during the EDX analyses of the metallographic cross-sections which indicated that an iron-rich oxide was also formed (points marked as P2b in Figure 6(b)). Undoubtedly this phase results from the reaction of FeB and Fe 2 B with the oxygen present in the atmosphere which reached this location probably by travelling via through thickness cracks formed in the coating during its fabrication process and that result from growth stresses [7]. Nevertheless, and particularly for this environmental condition, the oxygen partial pressure of the gas is reduced due to the presence of water vapour [13].…”

“…The boronizing process of Fe–Ni–Co Incoloy 909 superalloy was studied in a previous work conducted by Gaona-Martínez et al [7] where the mechanical stability of the coatings was analysed, therefore, specimens of the alloy were boronized according to the same methodology for a reaction time of 12 h. Several samples were prepared for the oxidation tests that were conducted in a special reactor. This installation consists of a radiation tube furnace whose chamber includes a quartz tube where the oxidation reactions take place.…”

“…In a previous work, Gaona-Martínez et al [7] fabricated boron-based coatings on the surface of the iron–nickel Incoloy 909 superalloy by thermochemical diffusion finding that the wear resistance of the coatings under microabrasion conditions was improved. During some of the tests, the formation of a thin oxide layer was observed to occur, and such growth was undoubtedly promoted by frictional heating.…”