D.K. Das

Evolution of microstructure in laser surface alloying of aluminium with nickel

Abstract Surface alloying of a commercially pure aluminium with nickel was carried out using a pulsed Nd-YAG laser. The distribution of the alloying element (nickel) in the alloyed layer was found to be highly non-uniform irrespective of the depth of alloying. The microstructure of the alloyed layer was found to consist mainly of a cellular solid solution phase of nickel in aluminium (α-Al), a lamellar eutectic of α-Al and Al 3 Ni phases, and primary Al 3 Ni dendrites. The microstructure of the alloyed layer is explained in terms of inhomogeneity of the nickel distribution. The effects on the microstructure of inherent rapid solidification under the conditions of laser surface alloying were also examined.

High emissivity coating on C-263 substrate for high temperature applications

An aluminium phosphate based high emissivity coating has been deposited on Ni based superalloy C-263 by dip coating method wherein the substrate was immersed in a chemical sol followed by curing of the green coating. Deposition of the coating by spraying the sol on the substrate surface has also been explored. The presence of carbon imparted high emissivity to the coating. Depending on the thickness of the coating, it exhibited an emissivity value of 0·6–0·9 in the wavelength range of 2.5–25 μm. The coating was found to offer good oxidation resistance to the substrate. Cyclic oxidation performance of uncoated and coated substrates has been evaluated at 800 and 1000°C for 100 h in air.

AlPO4–C composite coating for high emissivity and oxidation protection applications

Abstract AlPO4–C coating deposited by sol–gel technique on Ni base superalloy substrate Nimonic-75 has been examined for high emissivity applications. The coating showed a spectral (2–25 μm wavelength) emissivity in the range of 0·8–0·91, and the emissivity increased with coating thickness. The coating exhibited a reasonably good adhesion with the substrate, as determined from nanoscratch test. The above coating also had a good oxidation resistance in air at 800 and 900°C under cyclic heating and cooling conditions.

High temperature mechanical properties of thermal barrier coated superalloy applied to combustor liner of aero engines

High temperature load controlled fatigue, hot tensile and accelerated creep properties of thermal barrier coated (TBC) Superni C263 alloy used as a candidate material in combustor liner of aero engines are highlighted in this paper. Acoustic emission technique has been utilised to characterise the ductile-brittle transition temperature of the bond coat. Results revealed that the DBTT (ductile to brittle transition temperature) of this bond coat is around 923 K, which is in close proximity to the value reported for CoCrAlY type of bond coat. Finite element technique, used for analysing the equivalent stresses in the bond coat well within the elastic limit, revealed the highest order of equivalent stress at 1073 K as the bond coat is ductile above 923 K. The endurance limit in fatigue and the life of TBC coated composite under accelerated creep conditions are substantially higher than those of the substrate material. Fractographic features at high stresses under fatigue showed intergranular cleavage whereas those at low stresses were transgranular and ductile in nature. Delamination of the bond coat and spallation of the TBC at high stresses during fatigue was evident. Unlike in the case of fatigue, the mode of fracture in the substrate at very high stresses was transgranular whereas that at low stresses was intergranular in creep.

Prior austenite grains in steels laser surface alloyed with carbon

Prior austenite grain size is known to affect the hardness of martensite in conventional martensitic hardening of steels. These grains develop in solid state during austenitizing treatment. In the present study, however, it is pointed out that the prior austenite grains that form as a result of solidification in laser surface hardening of steels by alloying with carbon should have similar effects on the resultant martensite in the alloyed layer. A low alloy steel and a commercial pure iron have been laser surface alloyed with carbon to show the solidified austenite grains in the alloyed layer.

Microstructure and oxidation behaviour of Fe–Cr–silicide coating on a niobium alloy

The cyclic oxidation performance of a Fe–Cr-modified silicide coating on Nb-alloy C-103 was evaluated in air at temperatures between 1100 and 1500°C and the microstructural changes examined. The coating was oxidation resistant and the duration of protection increased from 60 minutes at 1100°C to 255 minutes at 1500°C. The formation of the surface glassy silica layer and its ability to heal the cracks in the coating enhanced the oxidation performance at high temperatures. The outer NbSi2 layer served as a reservoir of Si and sustained the formation of the protective silica scale. Spallation of the silica scale and the concomitant depletion of the NbSi2 layer towards the regeneration of the silica scale limit the oxidation life of the coating.

Influence of thermally grown oxide scale on fatigue resistance of a thermal barrier coated superalloy

Abstract The life of thermal barrier coating prior to spallation is dominated by micro-cracking in both the thermally grown oxide and the yttria stabilized zirconia top coat. The damage generated by this micro-cracking is expected to be a primary life limiting factor. High temperature force controlled fatigue testing of thermal barrier coated (TBC), bond coated only and bare Superni C263 superalloy was conducted in air. It was observed that the coated materials had higher endurance limits than the bare superalloy and the premature failure for these two materials is possibly due to high stress crack nucleation and growth in the TBC/bond coat layers. Oxidation is also the cause of the reduced life of the bare substrate as compared to the coated substrate while fatigue testing is carried out in an oxidizing environment. Spallation of the ceramic layer was evident at very high fatigue stress and also at low fatigue stress where the TBC composite specimen failed after 5,400,107 cycles during fatigue testing at 800 °C in air due to a continuous alumina scale growth (thickness >3 μm) at the bond coat/TBC (top coat) interface. La vie d’un revêtement thermique avant l’écaillage est dominée par la micro fissuration tant de la calamine développée thermiquement que de la couche supérieure de zircone stabilisée à l’oxyde d’yttrium. On s’attend à ce que le dommage engendré par cette micro fissuration soit un facteur primaire de limite de vie. On a effectué un essai de fatigue de force contrôlée, à haute température, à l’air, du superalliage Superni C263 revêtu d’une barrière thermique (TBC), avec seulement une couche d’ancrage et de l’alliage à nu. On a observé que les matériaux avec revêtement avaient des limites d’endurance plus élevées que le superalliage nu. La défaillance prématurée de ces deux matériaux est possiblement due à la nucléation et à la croissance élevées de fissures de contrainte dans les couches TBC/couche d’ancrage. L’oxydation est également la cause de la durée de vie réduite du substrat nu par rapport au substrat avec revêtement lorsque l’essai de fatigue est effectué dans un environnement oxydant. L’écaillage de la couche de céramique était évident à des contraintes très élevées de fatigue et également à de faibles contraintes de fatigue où l’échantillon composite TBC s’est détérioré après 5400107 cycles lors de l’essai de fatigue à 800 °C à l’air. La détérioration est attribuée à la croissance continue d’une écaille d’alumine (épaisseur > 3 μ m) à l’interface couche d’ancrage/TBC.

The influence of substrate on partially rapidly solidified Alumina-3 wt % titania nanocrystalline coatings deposited by plasma spray technique

In this paper an attempt was made to impose different degrees of rapid solidification by spraying on diverse substrates of varying thermal properties. Substrates such as Copper, Aluminum, Stainless steel, Low alloy steel substrates were used to alter the imposed cooling rate and thereby the amount of residual a phase. A start powder of 3 wt% Alumina-titania powder was Used for spraying to a thickness of 250 mu m on the different substrates specified. In all cases the rapidly solidified phases show nanocrystalline sizes with the most rapidly solidified metastable gamma phase showing finer grain size of less than 25 nm. The surface roughness of the substrate and the coating were characterized by Atomic force microscopy. In contrary to the Alumina-13 wt% titania, coupons of Alumina-3 wt% titania had shown poor indentation fracture toughness with increased amount of residual cc phase. Coupons of stainless steel and low alloy steel had shown the lowest fracture toughness when tested by Vickers type indentation at loads of 3 N and 5 N. In contrast to these results the interfacial toughness when measured by Rockwell indentation technique at loads of 150 N was found to be dependent on the elastic modulus of the substrate more than the coating hardness. The interfacial toughness was found to be lower for softer material such as aluminum and copper than stainless steel and low alloy steel.