M. Pons

Oxidation of ion-implanted metals☆

Abstract The mechanisms involved in the modification of the thermal oxidation of metals and alloys by ion implantation are reviewed and discussed. Radiation damage is thought to have some influence, but chemical effects always seem to be of greater importance. These can be closely related to the oxidizability of the implanted species. In the very early stages of oxidation, oxidizable elements can act as nucleation promoters and allow the development of more coherent scales. The solubility of the implants in the matrix is an important parameter in the discussion of the blocking of material transport along fast diffusion paths. The presence of particles in the matrix or in the oxide scale is also thought to be of importance either because the particles act as sinks for flowing defects or because they modify the relief of the mechanical stresses resulting from the scale growth.

The high temperature oxidation of aluminium-implanted iron

Abstract Aluminium was implanted into iron at different doses between 1 × 10 14 and 1.5 × 10 17 atm cm −2 . Oxidation tests were carried out in oxygen during 100–120 h in the temperature range 720–1020 K. Doses of 5 × 10 16 atm cm −2 had to be achieved to observe a notable inhibition of the oxidation process. In these conditions, the formation of two ternary oxides was detected but the spinel phase Fe Al 2 O 4 seemed to be responsible for the observed blocking effect.

Laser surface alloying of Ti-6Al-4V with silicon for improved hardness and high-temperature oxidation resistance

Abstract The surface of Ti-6Al-4V precoated with silicon powders was irradiated by a YAG: Nd 3+ laser to produce a homogeneous and uniform alloy layer. The surface hardness measured was 730 HV 0.1 and the oxidation resistance was greatly improved. This improvement is mostly due to the diminution of the inward diffusion of O 2− and the outward diffusion of Ti 4+ by the presence of silica and a little alumina on the top surface of the oxide scale.

Ion implantation into metals to prevent high temperature oxidation

Abstract The conditions leading to a complete inhibition of the oxidation of implanted metals are discussed using iron and titanium as test metals. This inhibition is thought to be possibly observed with metals oxidizing by outward cation diffusion. In this case, thermodynamic oxidability of the implants is the most important parameter. In the case of metals oxidizing mainly by inward oxygen transport, ion implantation is believed to be unsuited for completely inhibiting the oxidation process.

Oxidation of ion-implanted titanium in the 750–950 °C temperature range

Abstract Ion implantation of various species into titanium is shown to have a strong influence on the oxidation kinetics of this metal in pure oxygen at temperatures between 750 and 950 °C. Phosphorus is particularly active and reduces the parabolic rate constant by a factor of 2 according to the Wagner-Hauffe rules. This element also reduces oxygen dissolution in α-Ti and inhibits stress relaxation in the metal, leading to the formation of highly stratified rutile scales. It is shown to migrate with the metal-oxide interface and therefore its influence is long lasting.

Numerical modelling for CVD simulation and process optimization : coupled thermochemical and mass transport approaches

Abstract It is now well established that the properties of films grown by chemical vapour deposition are strongly determined by the interaction of transport phenomena and homogeneous and heterogeneous chemical reactions in the reactor. The main objective of the modelling work presented is to relate deposition performance (deposition rate and uniformity, film composition and selectivity) to reactor geometry and process conditions (pressure, temperature, reactant concentrations and gas flow rate). Owing to the lack of kinetic data, the joint use of thermodynamic databases, thermochemical equilibrium modelling and transport phenomena modelling could be an original way to offset partially the incomplete knowledge of chemical kinetics. This coupled approach is restricted to equilibrium conditions (high temperature and/or high deposition pressure). A description is given for test cases of how thermochemical and mass transport simulations can help advances in materials and process engineering.

High temperature oxidation of bismuth-implanted iron

Abstract Bismuth implantation has been shown to reduce drastically the parabolic rate constant of iron oxidation, but only at temperatures below 600°C. This result is explained in terms of bismuth segregation blocking fast diffusion paths for transport of Fe ions through iron oxides, while bulk diffusion is unaffected. Complete oxidation inhibition was not observed, contrarily to boron or aluminium implantation. Difference in thermodynamic affinities for implants oxidation can account for this result.

Catalytic reactions at an artificial fractal interface: Simulation with the ‘Devil's Comb’

Abstract We have proposed the model of a 2-D fractal objects exhibiting an important contour length within a finite area. This structure offers a large internal interface can be used as a catalytic support. In the case of a single first-order reaction A → R at the interface, numerical simulations show evidence for a new diffusional regime due to the fractality of the support. In this regime, the rate of formation of product R no longer depends on the kinetic rate constant of the reaction.

Influence of laser surface boronizing on the high-temperature oxidation of nickel

Abstract Laser nickel boronizing was performed by simultaneous melting of a predeposited boron powder and the nickel substrate and was shown to lead to the formation of a Ni 3 B-rich coating. During oxidation, after a period during which B 2 O 3 forms and evaporates, a protective scale of Ni 3 B 2 O 6 appears, inhibiting the formation of NiO.

The behaviour in SO2 at high temperatures of FeB coatings on Fe obtained by ion beam, laser or pack-cementation techniques

Abstract Boron was incorporated in the near surface region of iron samples by three different methods: ion implantation, laser alloying and pack cementation. The behaviour of these different surface alloys was studied thermogravimetrically between 400 and 600°C in static SO 2 atmospheres at the pressure of 133 hPa. Implanted samples were obtained near room temperature at a dose of 10 17 B + cm −2 . The boron amount (∼ 2 μ g cm −2 ) and the thickness involved (∼ 300 nm) although small, induced a noticeable reduction of the corrosion rate. At low temperatures (T ≤ 450°C), the rate law was paralinear and the only product formed was iron (II) sulphate in good agreement with thermochemical provisions. In this temperature range, boron was thought to have a catalytic effect as, when absent, iron (II) sulphate did not appear. At higher temperatures a duplex layer containing iron oxide and sulphide grew parabolically. Surface alloys obtained by laser alloying were 50–100 μm in thickness and exhibited compositions lying in the FeFe 2 B region of the phase diagram. Different microstructures were observed, depending on the irradiation conditions. The kinetic laws were parabolic or cubic, showing a greater protection when compared to implanted samples. Eutectic surface alloys exhibited the best behaviour for high temperatures or long times of oxidation. Pack-boronization at 950°C for 2 h in a powder mixture containing boron, ammonium fluoride and alumina led to a layer consisting mainly of Fe 2 B. Kinetic experiments showed that such a coating exhibited the best protection at the lowest temperatures.