Surface Engineering

A quantitative approach, based on measurement of oxide tress evolution as a function of thermal cycling is described for identifying thermal barrier coating (TBC) damage and assessing remaining life. Residual stress in the thermally grown oxide was measured using photostimulated luminescence piezospectroscopy for electron beam physical vapour deposited TBCs, which were thermally cycled at three temperatures and over two cycling periods. The observed monotonic change in the oxide stress level with TBC life fraction is systematic with cycle time and nearly independent of exposure temperature. The temperature independence of the measured stress with life fraction would greatly simplify the use of this technology for the non-destructive evaluation of TBCs.

Low solute concentration interstitial diffusion was modelled to show the effects of an interstitial solute on flow stress, residual stress and hardness. In the present paper, the Ti–N system is used as an example to model this process. However, the concept is general and can be applied to similar systems that benefit from the hardening treatment. Profiles of solute concentration as a function of distance into the sample are shown for various combinations of pressure, temperature and time. It is shown that hardness, flow stress and residual stress all increase in magnitude with solute concentration. Large residual stresses are created in the hardened layer which are sustained owing to the corresponding increase in flow stress.

Owing to the increasing use of cyclically loaded cast aluminium components in automotive and aerospace applications, the fatigue and fatigue crack growth characteristics of aluminium castings are of great interest. Despite the extensive research efforts dedicated to this topic, a fundamental, mechanistic understanding of these alloys’ behaviour when subjected to dynamic loading is still lacking. The present research investigates the mechanisms active at the microstructure level during dynamic loading and failure of Al–Si–Mg alloys. Five model alloys were cast to isolate the contribution of constituent phases on fatigue resistance. The major constituent phases, α-Al dendrites, Al/Si eutectic phase, and Mg–Si strengthening precipitates were mechanistically investigated to relate microstructure to near-threshold crack growth D δK_th and crack propagation regimes (regions II and III) for alloys of different Si composition/morphology, grain size, (secondary dendrite arm spacing) SDAS and heat treatment. A procedure to evaluate the actual fracture toughness from fatigue crack growth data has been successfully developed based on a complex elastic plastic fracture mechanics (EPFM/J-integral) approach. Residual stress–microstructure interactions, commonly overlooked by researchers in the field, are also comprehensively defined and accounted for both experimentally and mathematically. Revisions of ASTM E647 are in progress.

The goal of this paper is to demonstrate that glow discharge plasma heating of the cathode workpiece is different from radiant heating under vacuum, given the same pressure and gas composition, and that the final temperature obtained by the workpiece and the rate of heating depend on the workpiece emissivity. Towards this goal, several glow discharge heating tests were carried out using a number of geometrically identical carbon and austenitic steel samples, each prepared so as to generate differing surface emissivity values. These tests clearly demonstrate that samples with lower emissivity heat faster and to a higher temperature than do samples with higher emissivity values. Furthermore, several observations are made about the change in workpiece emissivity. For example, in situ measurements show that emissivity increases during plasma nitriding, and the emissivity change is most rapid during ramping to final temperature. It is further observed that the effect of plasma nitriding on emissivity is stronger for austenitic stainless steel than for carbon steel, and that powder metal samples, nitrided at a higher temperature than the other samples, achieve the highest emissivity values.

The aim of the present study is to investigate the beneficial influence of an yttrium sol–gel coating on a chromia forming alloy (304 stainless steel), in air, at 1000°C. Thermal cyclic experiments were carried out in order to study the influence of the addition of yttriumon the scale adherence. The yttrium sol–gel coating plays a significant role in enhancing oxidation resistance during isothermal oxidation tests, decreasing specimen weight gain and suppressing the initial transient oxidation stage. Yttrium addition appears to promote silicon segregationat the metal/oxide interface. It is then observed that iron containing oxides are not formed. Under thermal cycling conditions, the addition of yttrium also remarkably improves oxide scale adherence.

The main purpose of this research is to obtain a hard anodic film on aluminium and to study the effects of molybdic oxyhydroxide films on the formation and properties of anodic films. The experiments begin by immersing the A1100 aluminium specimens in ammonium molybdate solution andthen anodising them in a 10 wt-% sulphuric acid solution by dc with 20 V at 20°C. Finally, the specimens are sealed in hot water at ∼95°C. The study proves that a longer immersing time, a higher immersing temperature, a higher concentration of ammonium molybdate and a pH valueof 3 result in a harder anodic film. The molybdic oxyhydroxide film (the preimmersing process) retards the formation of the anodic film, producing a harder anodic film. The SEM images of the anodic film after the preimmersing process show that the pores of the film are closely and regularlydistributed. From the EDS analysis, molybdenum is detected in the anodic films. The preimmersing process decreases the anodic film thickness. However, the anodic film with preimmersing treatment has a maximum microhardness of 601 ± 10 HV(50 g), which is 62% higher than that withoutpreimmersing.

Plasma etching of aluminium thin films is characterised using a neural network. For this, etch experiments were designed by means of a statistical experimental design. Relationships between process parameters and etch rate were captured by a back propagation neural network (BPNN). Thepredicted performance of the BPNN model was optimised as a function of training factors. Model predictions were experimentally validated. Parameter effects were examined under a variety of plasma conditions. Radio frequency (rf) power affected the etch rate in different ways, depending onthe Cl₂ flow rate. The etch rate variation with rf power (or Cl₂ flow rate) was conspicuous only at higher Cl₂ flow rates (or rf power). The noticeable effect of BCl₃ flow rate at lower N₂ flow rate was attributed to an increased concentrationof bombarding ions.

In the presence of 13–16 wt-%Cr, nitrogen enhances interstitial solubility during austenitising. Therefore, five different martensitic or ferritic–martensitic stainless steels were case hardened by solution nitriding instead of carburising. The highest compressive residualstress at the surface was obtained by a high content of δ-ferrite in the core and a low content of retained austenite in the martensitic case. The limits of alloying, the resulting microstructure and the implications for production and service are discussed.

The slurry erosion behaviour of laser clad Fe–Cr–B–Si alloy coatings was investigated at impact angles from 30° to 90° with an increment of 15°, and compared with the slurry erosion behaviour of two laser clad carbide coatings and a non-coated substratemade of AISI 4140 steel. The Fe–Cr–B–Si alloy coatings had slurry erosion resistance. Their overall slurry erosion resistance at impact angles from 30° to 90° was higher than both the carbide coatings and steel. The erosion resistance of the Fe–Cr–B–Sialloy coatings was less dependent on slurry impact angle. The Fe–Cr–B–Si alloy coatings consisted of the Fe–Cr solid solution matrix and the second phase Fe²B. The laser beam traverse rate had little effect on the erosion resistance and microstructures ofthe coatings. The liquid nitrogen cooling of the substrate resulted in a structural refinement of the deposited coating and increased the erosion resistance substantially.

A programme was conducted to evaluate the relative corrosion resistance of CoNiCrAlY and chromium modified aluminide on IN738-LC, used for turbine blades. The corrosion experiments were performed in a laboratory tube furnace. The microstructure of the coatings was characterised usingoptical electron microscopy and SEM techniques. The results indicated that at a temperature of 800°C CoNiCrAlY is more protective than a Cr–Al coating.

Many different nitriding processes have been used for reducing the wear of iron based and non-iron based materials. In the present work, the efficiency of plasma nitriding of cold work steels for reducing wear was investigated. Ball on disc tests were conducted at room temperature and200°C using austenitic stainless steel balls and different nitrided cold work steel discs. The thickness of both compound and diffusion layer and the composition of the compound layer were varied by changing the plasma nitriding parameters. Wear tracks on the discs and balls were characterisedusing both SEM and optical profiling. The results show that a thinner nitrided layer reduces wear more efficiently. Higher carbide content seems to be advantageous because the nitrided layer is mechanically better supported.