F.H. Stott

The influence of alloying elements on the development and maintenance of protective scales

Some of the important principles that determine the establishment, growth and long-term maintenance of protective Cr2O3, Al2O3 and SiO2 scales on hightemperature iron-, nickel- and cobalt-base alloys are reviewed and discussed. Emphasis is placed on the effects of alloying elements and other additions, such as third elements and reactive elements or oxide dispersions, on each of these processes. Particular attention is paid to transport processes in the scales and the importance of short-circuit paths. Some of the important parameters that influence the long-term mechanical stability of such scales are considered and evaluated.

The role of triboparticulates in dry sliding wear

In this paper, wear processes and mechanisms for wear transitions with sliding time and temperature during sliding of a nickel-based alloy, N80A, in oxygen at temperatures to 250°C are discussed. Transitions in wear from high rates to low rates with sliding time were always observed at all the temperatures investigated. The transitions in wear were usually accompanied by transitions in contact resistance between the rubbing surfaces from nearly zero to positive high values. It was found that wear debris particles were heavily involved in the wear processes. The transitions in wear and contact resistance with sliding time mainly resulted from the development of wear-protective layers following the compaction of wear debris particles on the rubbing surfaces. The adhesion of triboparticulates to each other and to the rubbing surfaces played an important role in the rapid decrease in wear rate with sliding time and with increase in temperature. Processes involved in the development of the wear-protective particle layers and mechanisms for the wear transitions have been described on the basis of experimental observations. The importance of triboparticulates in wear and its implications for wear protection are discussed.

High-temperature sliding wear of metals

Temperature can have a considerable effect on the extent of wear damage to metallic components. During reciprocating sliding, under conditions where frictional heating has little impact on surface temperatures, there is generally a transition from severe wear to mild wear after a time of sliding that decreases with increase in ambient temperature. This is due to the generation and retention of oxide and partially-oxidized metal debris particles on the contacting load-bearing surfaces; these are compacted and agglomerated by the sliding action, giving protective layers on such surfaces. At low temperatures, from 20 to 200°C, the layers generally consist of loosely-compacted particles; at higher temperatures, there is an increase in the rates of generation and retention of particles while compaction, sintering and oxidation of the particles in the layers are facilitated, leading to development of hard, very protective oxide ‘glaze’ surfaces. This paper reviews some of the main findings of extensive research programmes into the development of such wear-protective layers, including a model that accounts closely for the observed effects of temperature on wear rates during like-on-like sliding.

The structure and mechanism of formation of the ‘glaze’ oxide layers produced on nickel-based alloys during wear at high temperatures

Abstract The structure and composition of the worn surfaces, and in particular of the tribologically important ‘glaze’ region, formed on Nimonic 75, Nimonic C263, Nimonic 108 and Incoloy 901 after sliding in air at elevated temperatures (150–800°C) have been determined. A typical wear scar is comprised of a number of areas, some having a thin, physically homogeneous surface ‘glaze’ layer, the rest having no ‘glaze’ surface. The ‘glaze’ layer lies on top of a region either of highly compacted oxide particles above a growing, steady-state oxide layer, or of alloy, deformed to varying degrees, depending on time of sliding, ambient temperature and the relative strength of the alloy. Electron diffraction shows the surface ‘glazes’ to consist of simple oxides after sliding at temperatures up to 400°C, viz. NiO in that on all four alloys, CoO on N108 and C263, and FeO on Incoloy 901. At the higher temperatures, NiCr 2 O 4 is observed in the ‘glaze’ on all the alloys, with NiO and Cr 2 O 3 on N75, Cr 2 O 3 and probably CoO on C263 and N108 and α-Fe 2 O 3 , FeO and Fe 3 O 4 on Incoloy 901. The ‘glaze’ and underlying, wear-affected, oxidized regions are shown by ion microprobe mass spectrometry, electron spectroscopy and electron probe microanalysis to consist of oxides, containing all the alloying elements, approximately in the same average proportions as in the alloys. Three possible mechanisms for the formation of the observed structure of the ‘glaze’ regions are proposed. It is concluded that high-strength properties and relatively rapid transient oxidation rates at elevated temperatures are desirable qualities in alloys employed under high temperature sliding conditions.

Oxidation of alloys

AbstractKey principles of alloy oxidation are discussed, with emphasis on Cr2O3 and Al2O3 formation on nickel-, cobalt, and iron-base alloys. The various special cases of alloy oxidation, which are quantifiable to varying degrees, are presented schematically. The important competition between surface scale development and internal oxidation is emphasized and extended to explain transient oxidation. The ability to measure and model the distribution of alloying elements in steady-state scale and substrate is described. The priority now is to understand further alloying element and defect segregation and transport in grain boundaries, and also other short-circuit paths including pores, in Cr2O3 and Al2O3 scales.Interaction between alloy depletion, void formation, and phase-boundary oxidant transport in single- and multi-phase alloy substrates requires further elucidation. Brief consideration of ternary and quaternary alloy oxidation illustrates the, ability partially to explain complex alloy behaviour. The r...

A mathematical model for sliding wear of metals at elevated temperatures

Abstract The transition in wear rate from a high value to a low value for metals after some time of sliding is a well known phenomenon. However, few models have been presented to account for such a transition. In this paper, a mathematical model, based on experimental observations that the transition is caused by the development of wear protective layers on the rubbing surfaces, is proposed. The protective layers are developed mainly from accumulated wear debris particles retained within the wear tracks; these can have various characteristics, depending on the experimental conditions and the properties of the metal, particularly the oxidation conditions and the contact between the rubbing surfaces. There is broad agreement between reported experimental observations and calculated predictions based on this model. For example, the development of protective layers occurs very quickly once the transition time/distance has been attained; whether or not ‘glaze’ layers develop on top of the compact particle layers depends on the sliding temperature, leading to the concept of a transition temperature. Wear debris particle size plays an important role in determining the wear transition; if the particles are too large and/or are difficult to fragment, such as those generated when the load or speed are high, they are more likely to be removed from the wear tracks and the severe to mild wear transition becomes difficult, or even impossible. The model is applicable to both room temperature and elevated temperature sliding wear.

Some frictional features associated with the sliding wear of the nickel-base alloy N80A at temperatures to 250 °C

Abstract The time-dependent variations of friction coefficient and the contact resistance of a nickel-base high-temperature alloy, N80A, during like-on-like sliding in pure oxygen at temperatures of 20–250 °C were simultaneously recorded and the tribological behaviour correlated with the nature of the sliding contact. A transition to a positive contact resistance always occurred after some time of sliding in the temperature range investigated. However, the time-dependent variations of coefficient of friction showed quite different features at the various temperatures. Corresponding to the times of the transitions in contact resistance, at 20 °C, the friction coefficient increased to a higher value from the initial value while, at 250 °C, it decreased to a lower value; however, at 150 °C, the coefficient of friction remained unchanged after the transition. Scanning electron microscopy observations have shown that the predominant factor for such different tribological responses at the various temperatures is the nature of the contact between the surfaces. At 20 °C, the real contact areas mainly consisted of loosely compacted particles while, at 250 °C, the load-bearing areas were smooth wear-protective oxide layers. At the intermediate temperature, 150 °C, the load-carrying areas comprised both types of contact. A mathematical model has been proposed and is used to relate the frictional behaviour to morphological features of the wear surfaces developed at the various temperatures.

The early stages of development of ?-Al2O3 scales on Fe-Cr-Al and Fe-Cr-Al-Y Alloys at high temperature

The development ofthe oxides on Fe-14%Cr-4%Al, Fe-27%Cr-4%Al, and similar alloys containing 0.008% Y, 0.023% Y, and 0.8% Y has been investigated during the early stages of oxidation in 1 atm oxygen at 1000 and 1200°C. In all cases, a steady-state α-Al2O3layer is established rapidly, after some initial formation of transient oxides rich in iron and chromium. For the yttrium-free alloys the steady-state situation is achieved more rapidly for the higher chromium-containing alloy and at the higher temperature. The amount of transient oxide formed is also determined by the specimen surface topography since the development of the α-Al2O3 layer is less rapid at the base of alloy asperities than at a flat alloy-oxide interface. Following establishment of the complete α-Al2O3layer, the oxide develops a convoluted oxide morphology at temperature, due to high compressive growth stresses in the oxide. These arise following reaction between oxygen ions diffusing inward down the oxide grain boundaries and aluminum ions diffusing outward through the bulk oxide. This results in lateral growth of the oxide and plastic deformation and movement of the alloy in a direction parallel to the alloy-oxide interface. The addition of yttrium to the alloys promotes the selective oxidation of aluminum. Also, the yttrium is incorporated into the growing oxide where it changes the mechanism of growth, reducing the production of the high compressive growth stresses and thus the development of the convoluted oxide morphology.

Surface treatment of alumina-based ceramics using combined laser sources

Abstract Alumina-based refractory materials are extensively used as linings in incinerators and furnaces. These materials are subject to molten salt corrosion and chemical degradation because of the existence of porosity and material inhomogeneity. Efforts to improve the performance of these materials have so far concentrated mainly on the optimisation of the manufacturing processes (e.g. producing denser refractory bricks) and in-service monitoring. Laser surface treatment has also been used to improve performance. The main problem identified with laser surface treatment is solidification cracking due to the generation of very large temperature gradients. The aim of this paper is to investigate the surface modification of alumina-based ceramics by using two combined laser sources in order to control the thermal gradients and cooling rates during processing so that crack formation can be eliminated. The material under investigation is 85% alumina refractory ceramic, used as lining material in incineration plants. The surface morphology and cross-section of the treated samples are analysed using optical and scanning electron microscopy (SEM) and compared with single laser beam treated samples.

Degradation of Thermal-Barrier Coatings at Very High Temperatures.

Thermal-barrier coatings are finding increasing use in engineering applications, particularly in gas turbines. Such coatings, consisting of ceramic insulating layers bonded to the superalloy substrate by oxidation-resistant alloy coatings, are deposited onto components to reduce heat flow through the cooled substrate and to limit operating temperature. They have been used effectively on static components such as combustor cans, flare heads, hot gas seal segments, fuel evaporators, and deflector plates, giving considerable improvements in component life. They have been used successfully on vane platforms. In recent years, the emphasis has shifted toward the development of coatings for high-risk areas, such as turbine blades. A ceramic thermal-barrier coating needs to be refractory and chemically inert, and to have low thermal conductivity. However, it also needs to possess a high thermal expansion coefficient of ~11 × 10 −6 K −1 , to match the nickel-base superalloy substrate. The latter specification has focused attention on ZrO 2 . However, ZrO 2 is polymorphic and undergoes two phase changes, cubic to tetragonal at 2350°C and tetragonal to monoclinic at 1170°C. The latter transformation is accompanied by a 5% volume increase which means that ZrO 2 has to be alloyed to stabilize one of the high-temperature phases. Early systems in the 1970s consisted of ZrO 2 stabilized with MgO, but this has been shown to be a metastable system. Present-day commercial thermal-barrier coatings consist of a plasma-sprayed yttria- or magnesiastabilized zirconia layer on top of an M-Cr-Al-Y bond coat. The latter plays a very important role by helping to key the ceramic to the alloy substrate and to accommodate the mechanical strains arising because of differences in thermal expansion coefficients and elastic moduli between the ceramic and the substrate.