Surface modification technologies XIII PDF

Real surface modification processes at elevated temperatures take place in reactors or furnaces, which are heated to reaction temperature during a period, which depends on the type and supplier of the reactor. In contrast to nitriding reaction conditions, heating parameters are usually not well defined. Reactions occurring during heating can influence the result of the surface modification process, especially when the process is controlled kinetically and by the composition of the furnace gas atmosphere, heating time and rate influence surface composition.' A reaction likely to occur in the presence of oxygen during heating is surface oxidation. A surface modification process, which may be influenced by prior surface oxidation, is the gas nitriding process, consisting of ammonia dissociation and nitride formation at steel surfaces. Gas nitriding usually takes place in ammonia at temperatures between 500 and 580°C.

Metals are very sensitive to surface oxidation. Nearly all metals and alloys are covered by a thin oxide layer « 1a nm) which is formed at room temperature. Metal alloys can be used as engineering materials because this oxide layer causes passivity and prevents them from further oxidation and corrosion. Oxide layers formed at steels are complex in structure and composition.

Three different iron oxides are stable at temperatures between room temperature (RT) and nitriding temperature of approximately 580°C; the Fe-II-oxide FeO (or Fe1_xO)is unstable at these temperatures and undergoes disproportionation to form Fe and Fe3 04 . Some information about the oxides is given in Table 1.2,3.4

There are two types of Fe2 03 : a-Fe2 03 , which has a corundum-type crystal structure, and y-Fe2 03 , which has a spinel-type crystal structure. Conversion between these two types and magnetite, spinel-type crystal Fe3 °4 is carried out by processes of oxidation, reduction and transition. A reversible oxidation-reduction process can be used to convert between Fe3 04 and y-Fe2 03 The a and yphases of Fe2 03 have very different crystal structure and electrical properties. When 'Y-Fe203 is heat-treated at high temperature and made to undergo the transition to a-Fe2 03 , it appears to lose almost all of its sensitivity regarding reductive gases. This is the reason for Fe2 03 not having attracted much attention as a gas sensor.'

Surface oxidation of iron occurs at elevated temperatures even at low oxygen partial pressure. Oxide layers formed at 300°C are easily one order of magnitude thicker than oxide layers formed at RT, even after short oxidation times. Real world reactors and furnaces are likely to contain sufficient oxygen for oxide layer formation, especially during heating to reaction temperature. This makes it probable that thermochemical processes at elevated temperatures have to deal with the reactivity of iron oxide layers rather than that of metal iron (steel).

In the case of gas nitriding of stainless steels, the problem of passive oxide layers is well known. The composition of high-chromium steel oxide layers is rich with chromium oxides, thus preventing not only corrosion but also the reaction with ammonia to form iron and chromium nitrides. Consequently, high-chromium steels are not easily nitrided in ammonia. On the other hand, gas nitriding (not only) of stainless steels has been reported to be possible owing to the addition of small amounts of oxygen to the nitriding atmosphere. This oxygen addition is said to be responsible for the formation of Fe3 0 4 (magnetite), which promotes nitrogen uptake and nitride formation/' See Ref. 7-10 for more and deeper information on gas nitriding and nitrocarburizing.

The influence of oxygen and of oxide layer formation on nitrogen uptake regarding·nitriding in ammonia was studied experimentally. It was the aim of the investigation to bring some light in the sometimes contradictory findings regarding the formation of iron and chromium oxides and their nitridability

Edited by: T.S. Sudarshan, K.A. Khor, M. Jeandin