A. Maciejny

Structure and resistance to oxidation of an Al-Si diffusion coating deposited by Arc-PVD on a TiAlCrNb alloy

Abstract This article presents research results of isothermal and cyclic oxidation of TiAlNbCr intermetallic with and without an Al–Si coating. The coating was deposited by the arc-PVD method in two steps. Firstly, the AlSi layer was deposited then the samples were heated in a vacuum chamber to form a diffusion TiAlSi coating. After coating deposition the samples were heat-treated in vacuum at a temperature of 950 °C for a period of 2 h. Cyclic oxidation tests were carried out at 950 °C. The heating and cooling time was 5 min (3 °C/s). Mass changes of the specimen were recorded every 100 cycles. A total of 2400 cycles were performed. An analysis of the results obtained from cyclic oxidation was performed. The analysis of the Si effect on the structure of the layers and on the oxidation mechanism of the TiAlNbCr alloy was performed. Phase composition, morphology and the distribution of elements were defined by EDX, XRD and SEM in AlSi layers as well as in the scale. The results showed that Al–Si coatings (thickness approx. 40 μm) were effective in reducing the oxidation rate of γ-TiAl. The oxide formed on the surface of the Ti–48Al–2Cr–2Nb coating after oxidation is composed of a mixture of a large portion of Al 2 O 3 and a small portion of TiO 2 .

Influence of coatings obtained by PVD on the properties of aircraft compressor blades

Abstract In this paper, the results of investigations to increase aircraft compressor blade durability were presented. The blades were made of the martensitic steels. The coatings were obtained by the physical vapour deposition (PVD) method. TiN and (Ti, Al)N coatings on a titanium nitride base and CrN and CrCrN multilayer coatings on a chromium nitride base were used in the tests. Low corrosion resistance of the TiN coatings was shown in laboratory tests. The (Ti,Al)N coatings, particularly CrN and CrN modifications, behaved better. Coating density and chemical interaction of the coating and steel in the corrosive medium were shown to have an influence on the blade properties. A significant increase in the corrosion resistance was achieved by using the diffusion and chemical treatment of coatings obtained by the PVD method. Investigations of coatings in a humid atmosphere containing SO2 showed very low resistance of the steel. The resistance of the coatings was increased significantly by additional chemical treatment.

Use of quantitative metallography in the evaluation of hydrogen action during martensitic transformations

Experiments based on image analysis method and electrochemical potentiostatic tests were performed with particular attention to the influence of hydrogen on the stabilisation of austenite of Fe–32 wt.%Ni alloy. This alloy exhibits great hydrogen absorption capacity (25 wt. ppm) and this presence modifies the martensitic transformation essentially at temperatures near MS (−40°C). In the whole range of the quenching temperatures, the difference between the volume fraction of martensite, measured on specimens with hydrogen or without hydrogen, is constant. It was found that the value of the interfacial area of the martensite plate groups SV is the best parameter describing the interfacial area between martensite and austenite. It was also found that there is a general relationship, expressed by a linear function, between this morphological parameter and an anodic current I.

Corrosion damage and regeneration of aluminide coatings on aircraft turbine blades

Abstract The corrosion-induced microstructure changes associated with the successive stages of damage of the protective layers on nickel-base alloys have been studied up to complete layer penetration. The microanalytical investigations performed on the distribution of the alloying elements show increased contents of chromium and sulphur in the corrosion attack front zone. However, the microstructure of the diffusion zone, enriched with molybdenum, tungsten and chromium, remains practically unchanged. Attempts at regeneration of the corrosion-worn protective layers of the blades by removing the corrosion products and remnants of the coating with a subsequent new layer deposition have been also described. Two methods of blade recoating are presented in the paper: 1. (a) chemical-vapour-deposited chromoaluminizing either in low- or high-activity processes with the analysis of the effect of processing parameters on the layer microstructure and properties and 2. (b) diffusion annealing of the sprayed silicoaluminizing suspension. The purpose of regeneration is to regain the initial properties of the blades after a period of service. Some of blades formerly rejected for their inadequate structure or defects in the protective layer would be also regenerated in this way. The criteria of surface quality assessment and verification of the blades in successive stages of surface coating regeneration have been worked out.

Microstructure wear resistance and erosion resistance of plasma-sprayed boride coatings

Abstract The paper presents both material and technological concepts of coatings containing selected borides (FeB, CrB2, (TiCr)B2) and carbides (B4C, B4C−CrB). The selected borides and powder compositions were prepared by diffusion synthesis (chemically) and then were electrolytically covered with a nickel layer. The plasma spray parameters were determined. X-ray analysis was carried out to investigated the chemical compositions of the coatings. From the X-ray analysis results, we determined the phase transformations in the coatings after plasma spraying. Both the wear and erosion resistances of the coatings obtained were determined and compared with those of WC–Co, NiCrSiB and 45 VFNS (Metco) coatings. The results obtained in the tribological tests were applied to a selection of highly wear-resistant boride coatings.

Microstructure and resistance to cracking of modified Al-Si and Al-Cr diffusion coatings on ŻS6K nickel-based superalloys

Abstract The subject of the investigations was a superalloy with the following chemical composition: 0.14 wt.% C, 10.2 wt.% Cr, 5.3 wt.% Al, 2.8 wt.% Ti, 3.7 wt.% Mo, 5.2 wt.% W, 4.7 wt.% Co, 1.5 wt.% Fe and the balance nickel. A pack cementation technique of coating was applied. Acoustic emission signals were recorded as a function of specimen elongation (Σ N - e ). The values of elongation ( e 0 ) at the moment of the first crack appearance and at the beginning of rapid fracture were measured. The crack resistances of the aluminide, silicoatuminide and chromoaluminide diffusion coatings were the main subject of the studies performed. If we take the e 0 parameter as a crack resistance criterion it was found that, when compared with the aluminide coatings, Al-Si and Al-Cr coatings are less resistant. The crack morphologies of aluminium and Al-Cr coatings were similar in character. Among the typical features were cracks that did not spread through the entire layer thickness and discontinuous cracks with branches. In the depleted zone of the coatings, cracks developed along the NiAl phase grain boundaries.